Meetings & Events

fall 1997 logo1997 MRS Fall Meeting & Exhibit

December 1 - 5, 1997 | Boston
Meeting Chairs:
 Harry A. Atwater, Peter F. Green, Dean W. Face, A. Lindsay Greer 

Symposium H—Materials and Devices for Silicon-Based Optoelectronics



Salvatore Coffa, CNR
John Cunningham, Bell Labs, Lucent Technologies
Albert Polman, FOM Inst
Richard Soref, USAF Rome Laboratory

Symposium Support 

  • SGS-Thomson Microelectronics
  • Texas Instruments, Inc.

Proceedings published as Volume 486 
of the Materials Research Society 
Symposium Proceedings Series.

* Invited paper

Chairs: John E. Cunningham and Richard Soref 
Monday Morning, December 1, 1997 
Simmons (M)

8:30 AM *H1.1 
SILICON-BASED OPTOELECTRONICS. Chris Buchal, Institut für Schicht- und Ionentechnik (ISI), Forschungszentrum Jülich, Jülich, GERMANY.

Silicon dominates microelectronics, but also carries significant potential for the integration of several optoelectronic functions. The actual device fabrication comprises interesting challenges for materials science. We discuss aspects of the following devices: Si-based light-sources, detectors, waveguides, modulators and an optical amplifier. We will show, that for some functions Si-based optoelectronics offers realistic solutions already now. The compatibility between different functions will be discussed.

9:00 AM *H1.2 
PRACTICAL NANOSCALE SILICON LIGHT EMITTERS. Philippe M. Fauchet, Department of Electrical Engineering, University of Rochester, Rochester, NY.

Impressive advances have been made over the last few years in teaching silicon how to emit light. Recently, light-emitting devices made of porous silicon and other forms of nanoscale silicon have been demonstrated with specifications that are starting to make them attractive for commercial applications. This talk will review the state-of-the-art in the materials science and device properties of nanoscale silicon-based LEDs, including their integration with microelectronic circuits.

9:30 AM H1.3 
850 nm VERTICAL CAVITY SURFACE EMITTING LASERS FOR LAN APPLICATIONS. J.E. Cunningham, A. Krishnamoorthy, K.W. Goossen, J.E. Ford, J.A. Walker, W.Y. Jan, W.H. Knox, G. Zhang, Lucent Technologies, Holmdel, NJ; and B. Tseng, J. Lothian, G. Chu, L. Chirovsky and W.T. Tsang, Lucent Technologies, Murray Hill, NJ.

We are evaluating device designs and material growth for fabricating 850 nm Vertical Cavity Surface Emitting Laser (VCSEL) designs for Gbit LAN applications. Our device is intended to produce lasing at very low threshold current (<1mA) with drive voltages of order 3-4 V. and hence compatible with Si based CMOS circuits. The advantages of VCSEL integration to CMOS is that systems can be engineered such that a 2-D array of light beams can be electronically controlled at the chip level. This could enable future systems that can bring fiber directly to the desktop. Unfortunately, state of the art VCSEL devices have not yet been bonded successfully to Si. Nor has anyone accomplished devices with low enough power dissipation to avoid burnout at the Si CMOS level because of integrating large 2D arrays of VCSELs to Si. To achieve this goal we are developing a VCSEL based on oxidized top mirrors and current apertures. Individual devices have previously been demonstrated on GaAs. To enable arrays and flip chip bonding such devices to Si we are developing a coplanar contact scheme to the VCSEL based on a selective ohmic contact method. The results thus far are that VCSEL devices incorporating all the above innovations have successfully been processed into arrays at the pitch required for flip chip bonding to Si. The oxidized mirror has reflectivity, current apertures of 5 microns in diameter, series resistances of 400 ohms and record resonantly enhanced LED emission of 60uW/sr. ( for devices without the top oxide mirror).

9:45 AM H1.4 
MATERIALS ISSUES IN INTEGRATION OF GERMANIUM PHOTODETECTORS ON SILICON SUBSTRATES. Srikanth B. Samavedam, Matthew T. Currie, Thomas A. Langdo, Steve M. Ting, Eugene A. Fitzgerald, MIT, Dept. of Materials Science and Engineering, Cambridge, MA.

Germanium (Ge) photodiodes are capable of high quantum yields and can operate at gigahertz frequencies in the 1-1.6 m wavelength regime. The compatibility of Si-Ge alloys with Si substrates makes Ge a natural choice for photodetectors in Si-based optoelectronics applications. The large lattice mismatch (4%) between Si and Ge, however, leads to the formation of a high density of misfit and associated threading dislocations when uniform Ge layers are grown on Si substrates. High quality Ge layers were grown on relaxed graded SiGe/Si layers by ultra high vacuum chemical vapor deposition (UHVCVD). The threading dislocation density in the Ge layers determined through plan view transmission electron microscopy (TEM) and etch pit density (EPD) was found to be in the range 7-13 x 106/cm2. As the Ge concentration in the graded layers increases, strain fields from underlying misfit dislocations result in increased surface roughness and the formation of dislocation pile-ups. In this study these effects were minimized by growing on miscut (001) substrates. The thermal mismatch between Si and Ge results in unwanted residual tensile stresses and surface micro-cracks when the substrates are cooled from the growth temperature. Various growth modifications have been incorporated into the graded layers to overcome the thermal mismatch problem, resulting in crack-free relaxed Ge on Si. In addition, the growth modifications have eliminated dislocation pile-ups, decreased gas-phase nucleation of particles, and eliminated the increase in threading dislocation density that occurs when grading to Ge concentrations greater than 70% Ge. Ge diodes were fabricated to assess the electronic quality and prove feasibility of high quality photodetectors on Si substrates.

Chairs: Salvatore Coffa and Albert Polman 
Monday Morning, December 1, 1997 
Simmons (M)

10:30 AM *H2.1 
RESONANTLY EXCITED LUMINESCENCE OF Si NANOSTRUCTURES. Y. Kanemitsu and S. Okamoto, Graduate School of Materials Sciencer, Nara Institute of Science and Technology, Nara, JAPAN.

The goal of achieving efficient visible photoluminescence in Si materials has stimulated considerable research in understanding the optical properties of Si nanostructures. In this work, we report luminescence properties of H-terminated and SiO2-capped Si nanocrystals and discuss the effect of the surface structure on optical properties of non polar semiconductor Si nanostructures. Surface-oxidized Si nanocrystals were fabricated by laser breakdown of SiH4 and microwave plasma decomposition of SiH4. H-terminated Si nanocrystals were fabricated by electrochemical etching of Si wafers without air exposure. As in many inhomogeneously broadened systems, site-selective excitation of Si nanocrystals results in fluorescence line narrowing, displaying a well-resolved phonon progression. The TO-phonon related structure is clearly observed in H-terminated Si nanocrystals at 2 K. However, the c-Si phonon-related structure in the PL spectra is not observed in Si/SiO2 systems. Instead, the strong coupling of excitons and surface silicon oxide vibrations cause fine structures in the PL spectra in nanoscale Si/SiO2 systems. Resonant luminescence properties of SiO2-capped Si nanocrystals are completely different Eom those of H-terminated Si nanocrystals. H-terminated Si nanocrystals show their crystalline nature, while SiO2-capped Si nanocrystals show their disorder nature. The understanding of electronic structures nanometer c-Si/SiO2 systems is an inevitable step toward realization of high-performance nanoscale semiconductor devices.

11:00 AM H2.2 
TUNING THE EMISSION WAVELENGTH OF Si NANOCRYSTALS IN SiO2 BASED WAVEGUIDES ON Si. M.L. Brongersma(a), K.S. Min(b), E. Boer(b), H.A. Atwater(b), and A. Polman(a) (a)FOM Institute for Atomic and Molecular Physics, Amsterdam, NETHERLANDS; (b) Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA.

In the design of quantum dot lasers and amplifiers the emission wavelength and width of the gain spectrum are important parameters. We demonstrate three techniques to tailor the luminescence spectrum of Si nanocrystals in SiO2 waveguides on Si. Si nanocrystals were first synthesized by precipitation from a supersaturated solid solution of Si in SiO2 produced by ion implantation. By means of a subsequent step at 1000 we can tune the peak emission wavelength over a broad range from 670 to 920 nm. Bright field TEM and Rutherford backscattering measurements show that the nanocrystals close to the surface oxidize first, followed by nanocrystals further away from the surface. The 1/e luminescence lifetime at 700 nm increases as a function of oxidation time from 12s to 43s after 25 min. oxidation. We also show that by samples in diluted HF the luminescence spectrum can be modified. The shifts in the peak wavelength are similar to those obtained by oxidation. Finally, we show that the spectrum can be red-shifted by increasing the . The results for all three techniques are in agreement with quantum confinement theory. Prospects for waveguide light sources and lasers based on Si nanocrystals will be discussed.

11:15 AM H2.3 
TEMPERATURE-DEPENDENT PHOTOLUMINESCENCE OF SILICON NANOCRYSTALLITES PREPARED BY INERT GAS AMBIENT PULSED LASER ABLATION. I. Umezu, S. Yamaguchi, K. Shibata, A. Suginura, Dept. of Applied Physics, Konan Univ., Kobe, JAPAN; Y. Yamada, T. Yoshida, Matsushita Research Inst., Tokyo, Inc., Kawasaki, JAPAN.

Inert gas ambient pulsed laser ablation technique is one of the promising methods to prepare Si nanocrystallites which show photoluminescence (PL). However, there have been few studies about temperature-dependent PL of the Si nanocrystallites prepared by the pulsed laser ablation. We measured temperature dependence of PL spectra to elucidate radiative and non-radiative recombination process in the Si nanocrystallites prepared by this method. An ArF excimer laser beam : 193 nm, energy density: 1.0 J/cm2pulse) was focused onto the p type Si wafer as a target. During the laser ablation of Si, He gas was introduced into a vacuum chamber. The mean diameter can be controlled by varying the He gas pressure as a main process parameter. Subsequent thermal annealing was carried out at 800C for 10 min in O2 gas. The PL spectra were measured using Ar ion laser (hv: 2.54 eV) as excitation source. The Si nanocrystallites show visible PL bands in the red (1.6 eV) and green (2.2 eV) spectral regions at room temperature after the oxidation process. Intensity of the green PL band decreased during irradiation of the excitation light in the air, and then recovered in subsequent vacuum evacuation. Although the peak intensities of both the red and green PL bands increased with decreasing temperature until it reached about 100 K, they were almost constant below 100 K. The peak position of the red PL band shifted to high photon energy region with decreasing temperature, in contrast, the green band hardly showed the peak shift. Origins of the PL bands will be discussed m terms of surface effect and quantum confinement effect.

11:30 AM H2.4 
CUBIC NONLINEARITY OF ION-IMPLANTED SI AND Ge NANOCRYSTALS MEASURED BY Z-SCAN METHOD: IMPLICATIONS FOR Si-BASED PLANAR WAVEGUIDE PHASE MODULATORS. K.V. Shcheglov and Harry A.Atwater, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA.

We report cubic nonlinearities derived from Z-scan measurements performed at 1.06 m for Si and Ge nanocrystals in SiO2 and Al2O3 produced by MeV ion implantation. Care was taken in the Z-scan measurements to avoid artifacts due to measurement-induced thermal effects in the sample. 3 MeV Ge implantation in SiO2 at a dose of 2 x 1017/cm2 and annealed at 800 C for 15 min yields a 300 nm thick band of 5 nm diameter Ge nanocrystals, for which a esu is measured. Similar implantation and anneal conditions for Ge implants in Al2O3 yielded a esu. 2 MeV Si implantation in SiO2 at a dose of 4 x 1017/cm2 and annealed at 1100 C for 15 min yields a 150 nm thick band of 2-3 nm diameter Si nanocrystals, for which a esu is measured; similar implantation and anneal conditions for Si implants in Al2O3 yielded a esu. If one assumes a correlation exists between the ac and dc values of , simple calculations suggest that these cubic nonlinearities may yield planar waveguide electrooptic modulation by a /2 phase shift over lengths of 100-500 m.

11:45 AM H2.5 
STRUCTURE AND CHEMISTRY OF SILICON OXIDE NANOPARTICLES. G. Duscher, A. A. Puretzky, D. B. Geohegan and S. J. Pennycook, Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN.

We have performed the first analysis of individual nanoparticles unambiguously formed in the gas phase by laser ablation of silicon into inert background gas. Z-contrast imaging allows accurate determination of particle size, even with subnanometer, non-crystalline particles, and sizes from 1 to 5 nm were found in all samples investigated. A strong tendency for agglomeration was seen, which is believed to occur in the gas phase. Analysis of the energy near-edge structure using the silicon L2,3-edge can distinguish between pure Si, SiO2, silicon rich oxide and combinations of them. Thus a correlation between the strength of blue and red luminescence and the chemical composition can be drawn. Linescans over single particles showed that the particles do not have a shell like structure as previously assumed. Detailed structural and chemical analysis of this nature in combination with the controlled growth, should enable the mechanism of nanoparticle luminescence to be determined.

Chair: Martin D.B. Charlton 
Monday Afternoon, December 1, 1997 
Simmons (M)

1:30 PM *H3.1 
INTEGRATED OPTIC DEVICES IN SILICON AND SILICON-ON-INSULATOR MATERIALS. Bernd Schueppert and Klaus Petermann, Technische Universitaet Berlin, Fachgebiet Hochfrequenztechnik, Berlin, GERMANY.

Various techniques and ideas in the field of integrated optics in silicon and Silicon-On-Insulator will be reviewed. Different waveguide concepts, such as rib-waveguides in epitaxial silicon, rib-waveguides in Silicon-On-Insulator (SOI), rib-waveguides in SiGe-heterostructures, and SiGe-indiffused waveguides will be considered. Basic waveguide characteristics such as losses and spot-sizes for both polarizations will be given. Due to the low waveguide losses of approx. 0.1 dB/cm, and spot-sizes which are well matched to the optical field of standard single-mode fibers, silicon is well suited for integrated optical applications and devices. In comparison to straight waveguides, we will consider the excess-losses of bent waveguides dependent on the radius of curvature. For switching purposes, two different principles of couplers have been fabricated and investigated experimentally: directional couplers and multimode-interference couplers, results are presented. Electro-optical modulation and switching in silicon can be carried out either by carrier-injection or thermo-optical effects. For switching devices based on thermo-optical effects, SOI is an attractive candidate due to its good thermal isolation by the buried silicon-dioxide, yielding low electrical power requirements for switching, which will be verified by experiments. For more complex integrated optical devices, a mass production compatible integration of sources and detectors is a fundamental necessity, which, however, is not necessarily a monolithic integration. In the silicon material system, monolithic integration of photodetectors and waveguides with reasonable performance in efficiency and frequency response will be demonstrated. Numerical simulations show that such devices are useful even for higher-bitrate communication systems up to 10Gbit/s at 1.3m and 2.5Gbit/s at 1.55n.

2:00 PM H3.2 
PHOTOELASTIC WAVEGUIDES IN BULK Si AND SiGe/Si HETEROSTRUCTURES. E. Lea and B. L. Weiss, School of Electronic Engineering, Information Technology and Mathematics, University of Surrey, Guildford, Surrey, UNITED KINGDOM; J. Kim and H. E. Jackson, Department of Physics, University of Cincinnati, Cincinnati, OH.

Photoelastic waveguides are a simple low cost approach to passive waveguides in Si based structures. In this paper we present results of our modelling of strain profiles in photoelastic waveguides in bulk Si and SiGe/Si heterostructures. An analysis of the as-grown structures, which have been studied using both Raman spectroscopy and TEM micrographs, is presented here. The waveguide loss and mode profile characteristics of photoelastic waveguides in bulk Si and SiGe/Si heterojunction planar waveguide structures have been measured. In addition, annealing of the photoelastic waveguides shows that the losses can be reduced by 5ÝdB/cm. Waveguide losses down to 2.7ÝdB/cm were obtained at a wavelength of 1.523 microns. These results suggest the suitability of these waveguides for a wide range of applications.

2:15 PM H3.3 
INDEX OF REFRACTION AND STRAIN INDUCED BIREFRINGENCE OF PSEUDOMORPHIC SiGe/Si WAVEGUIDES. M. Robillard, D.M. Bruce, and P.E. Jessop, Centre for Electrophotonic Materials and Devices, McMaster University, Hamilton, Ontario, CANADA; S. Mailhot, J.M. Baribeau, A. Delage, S. Janz, H. Lafontaine, R.L. Williams, and D.X. Xu, National Research Council of Canada, Ottawa, Ontario, CANADA.

Silicon is a common substrate material for hybrid integrated optoelectronic circuits (e.g. the silicon optical bench), as well as for discrete components such as waveguide array WDM demultiplexers and splitters. Optical waveguides compatible with Si substrates can be fabricated using deposited SiO2, Si on insulator (SOI), or SiGe epitaxy. Epitaxial Si1-xGex waveguides are particularly attractive since the waveguide epilayer and substrate belong to the same material system and are compatible with standard Si processing techniques. Single mode Si1-xGex waveguides can be fabricated for both the 1300 nm and 1550 nm wavelength ranges. However, the critical optical properties of pseudomorphic SiGe layers grown on Si are not well established, and considerable discrepancies exist in the literature. 
In this paper we report new measurements of n, the refractive index step between Si and Si1-xGex, for Ge concentrations between x=0.01 and x=0.1. Values of n were determined from waveguide mode profile measurements on a series of CVD and MBE grown pseudomorphic Si1-xGex/Si waveguides for wavelengths near 1300 nm and 1550 nm. We have found that n is significantly smaller than some previously reported results, and also that pseudomorphic Si1-xGex epilayers exhibit strain induced birefringence. At a wavelength of 1300 nm, we obtain n=0.3x for TE modes, while for TM modes the index varies as n=0.5x. These results are consistent with the expected lattice distortion of the pseudomorphic SiGe layers, and measured values of the photoelastic tensor. The implications of these revised refractive index values and the strain birefringence on device design will be discussed. Current progress towards the development of Si1-xGex splitters and WDM demultiplexer chips will be presented.

2:30 PM H3.4 

In this communication we present a novel silicon optical amplitude-phase modulator integrated into a SOI waveguide and based on a three terminal electronic structure, which gives rise to definite advantages in comparison with classical p-i-n diode based modulator. The proposed device utilizes the free carrier dispersion effect to produce the desired complex refractive index variations. The MEDICI device simulator has been employed to analyze the electrical operation, with reference to the injected free carriers concentration into the optical channel, its uniformity and the required current density and electrical power. The optical investigation was carried out by means of FDM, EIM and BPM tools, giving rise to a complete evaluation of the properties of our device. We report the results for both the amplitude and phase modulators, paying attention to the static and the dynamic behavior. In particular, a modulation depth of 20 %, with a injection power expense of about 126 mW, and a switching time of 5.6 ns is achieved. Furthermore, as a phase modulator, the device exhibits a very high figure of merit, predicting an induced phase shift per volt per millimeter of about 215, for a injection power of about 43 mW, and a switching time shorter than 3.5 ns. The most attractive characteristic of the proposed device is the new bias operation mode which is based on the drift of the plasma injected into the optical channel. Respect to the p-i-n based modulator, based on the free carriers injection and depletion, the switching speed is almost one order of magnitude smaller. The described optical modulator has been realized by means of standard microelectronic techniques, and preliminary experimental results are reported.

2:45 PM H3.5 
PERFORMANCES OF POROUS SILICON OPTICAL WAVEGUIDES. Minoru Araki, Hideki Koyama, Nobuyoshi Koshida, Tokyo Univ. of Agri. & Tech., Dept. of Electron. & Info. Eng., Tokyo, JAPAN.

Quantitative evaluations of the fundamental properties and functions of luminescent porous (PS) as a component of silicon-based photonics should be sought from various viewpoints. As one of the most important aspects, we have demonstrated the usefulness of PS for optical waveguides [1]. The applicability of PS to this device is owing to that the refractive index of PS is precisely controllable in an extremely wide range. Functional waveguiding properties of PS are reported here by making use of more efficient device configurations. Based on the previous results obtained from an edge-emitting PS device with a simple step-index waveguide structure, the active PS waveguides with three-dimensionally buried structures are fabricated on p-type Si (100) wafers by monolithic planar processing. Advantageous features of this PS waveguide are shown through some optical and optoelectronic measurements. The technological potential of PS waveguides is also supported by theoretical analyses using Maxwell-equation treatment. Important results to be noted are as follows: (a) The visible-light wave propagating through the waveguide is strongly confined into the core region. This is due to a definitely different refractive indices between the core and cladding PS layers with controlled structural and optical properties. (b) The experimental and theoretical results suggest that bending losses in this system are extremely low, and that there may be no limitations in the minimum curvature radius for waveguiding. (c) Besides detailed analyses of edge-emissive and polarization-mode-selective functions, it has become possible to accurately evaluate internal losses in PS waveguides under both passive and active operations. To obtain further progress, other related functions of this waveguide will be discussed in terms of possible nonlinear optical and electro-optical effects in PS, including their combination with the EL-emission.

Chairs: Salvatore Coffa, John E. Cunningham, Albert Polman and Richard Soref 
Monday Afternoon, December 1, 1997 
3:00 P.M. 
Tufts (M)

PHOTOLUMINESCENCE FROM SURFACE MODIFIED SILICON NANOCRYSTALS. Keisaku Kimura, Himeji Inst of Technology, Dept of Material Sci, Hyogo, JAPAN; Shingo Iwasaki, Hikari Densi Kogyo Co., Kyoto, JAPAN.

Several nanometer-sized silicon particles were prepared as a colloidal state by a conventional gas evaporation technique combined with a matrix isolation method developed in our laboratory. We have observed a clear size dependence on the emission intensity but not on its wavelength that was always located at about 500 nm. When the silicon crystals were reacted with alkaline solid such as KOH, a remarkable blue shift was observed the peak wavelength of which was reaching almost at 350 nm. The molecular-like vibronic progression was also observed superimposed on the emission spectra before and after the alkaline treatment. This structure is analyzed with reference to the IR data suggesting the emission origin being oxygen-related surface trapped sites. We have proposed a possible surface chemical reaction which is responsible to induce the emission trapped site. Moderate coupling between crystalline electron energy state and the surface trapped state is crucial to the findings. Our model can also successfully explain the results of the size dependence of the emission intensity mentioned above. Excitation spectra observed both at 480 and 400 nm revealed a clear dependence on size and a kind of alkaline species. The extrapolation of an excitation peak as a function of size to a bulk value gave 3.5 eV just corresponding to the vertical band gap energy at a gamma point. Based upon above observations, we propose an energy diagram of silicon nanocrystals.

PHOTOLUMINESCENCE IN SILICON SINGLE QUANTUM WELLS FORMED ON XIMOX WAFERS. Shinji Okamoto*, Department of Electrical and Electronic Engineering, Tottori University, Tottori, JAPAN; Yoshihiko Kanemitsu, Graduate School of Materials Science, Nara Institute of Science and Technology, Nara, JAPAN. *Presently working at Tokyo Engineering University.

Recently, reports of photoluminescence (PL) in silicon quantum wells have been increased. In low-dimensional Si structures, quantum confinement effects play essential roles in optical absorption processes. However, the PL mechanisms of Si quantum wells is still unclear: The PL peak energy does not show the dependence on size expected for quantum confinement. In this work, we have studied luminescence properties of Si single quantum wells under resonant excitation in order to clarify the PL mechanisms. Si quantum wells were formed on SIMOX (separation by implanted oxygen) wafers. A single quantum well was sandwiched between SiO2 layers. The PL spectrum of the Si quantum well exhibit a sharp peak around 1.65 eV at 2 K and is asymmetric. The asymmetric luminescence spectrum consists of two Gaussian bands in the red and infrared spectral region. From the size-dependence of the PL spectrum, resonantly excited PL spectra and PL decay dynamics, it is concluded that the strong PL band is caused by the radiative recombination in the Si-SiO2 interface states, while the weak PL band in the two-dimensional Si quantum well.


Based on first-principles calculation of a Si(100) quantum slab covered with silicon oxide, we show that Si-OH bonds at the silicon/silicon-oxide interface are a theoretically convincing candidate for reported interfacial luminescence from porous silicon and silicon nano structures. For a quantum slab with perfect silicon/silicon-oxide interfaces, wave functions of valence band top states and of some states near the conduction band bottom have a relatively large amplitude near the interface (pseudo-interface-states). When Si-OH bonds were introduced at the interface, we found that these wave functions have remarkably large amplitude near the interface. Furthermore, they are also found to localize laterally near or around the Si-OH bonds. Such strong localization comes from coupling of these states with 2p lone pair orbitals on O atoms at the interface and is significant in quantum silicon structures. Because of the localization, these pseudo-interface-states can be the source of creating localized excitons, which enhance the intensity of photoluminescence. It has been reported that photoluminescence from porous silicon and silicon nano structures originates in the transition between quantized states at confined silicon regions. On the other hand, it has also been reported that a part of the photoluminescence originates in transition at the silicon/silicon-oxide interface region. Our results suggest that interfacial luminescence can occur on silicon quantum structures if they have oxidized surfaces with Si-OH bonds.

MICROSTRUCTURE AND PHOTOLUMINESCENCE STUDIES OF NANOCRYSTALLINE Si THIN FILM. Wei Wu, Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, HONG KONG; XinFan Huang, KunJi Chen, Department of Physics, Nanjing University, Nanjing, CHINA; JianBin Xu, Dihu Chen, Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, HONG KONG.

We report the observation of visible light emission from crystallized a-Si:H/a-SiNx:H multi-quantum wells (MQWs) structures, which were deposited on SiO2/Si substrate by plasma enhanced chemical vapor deposition (PECVD) system and subsequently crystallized by KrF excimer laser. The crystallization process was performed inside a stainless-steel vacuum chamber with a 20 ns pulse length KrF (wavelength=248 nm) excimer laser at room temperature. Microstructures of the crystallized samples were analyzed by X-ray diffraction (XRD) and Raman scattering spectra, strong spectrum peaks corresponded to crystalline Si (c-Si) <111> face were shown. Transmission electron microscopy (TEM) technique revealed that in crystallized MQWs structures, nanometer Si crystallites were formed in the well layers due to the constrained crystallization between a-SiNx:H barrier layers. The crystallized MQWs samples were detected by 488 nm excitation light from an Ar+ laser to conduct the photoluminescence spectrum measurement. Room temperature visible PL emission was observed from the crystallized samples with well layers thickness 4.0 nm, dominant peak emission energy was around 2.0 eV. To further identify the origin of the visible PL, we also prepared as-deposited samples, no visible PL was found. Since the absorption coefficient of a-Si thin films is quite high in the UV-light region of the excimer laser, it will be possible to crystallize MQWs samples without thermal damage to the silicon substrate. The ability to manufacture luminescent Si films by the method compatible with the mature Si microelectronics technology will provide promising means in optoelectronics devices and allow for monolithic integration of Si technology with optical signal process.

VISIBLE ELECTROLUMINESCENCE OF CRYSTALLIZED SILICON QUANTUM WELL STRUCTURES. Wei Wu, Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, HONG KONG; Xin Fan Huang, Kunji Chen, Department of Physics, Nanjing University, Nanjing, CHINA; Jianbin Xu, Dihu Chen, Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, HONG KONG.

We had ever studied the room temperature visible photoluminescence(PL) in crystallized a-Si:H/a-SiNx:H multiquantum wells (MQWs) structures prepared by plasma enhanced chemical vapor deposition system. In this paper, we report the visible electroluminescence(EL) from nanometer Si crystallites by excimer laser annealing the EL cell of Au electrode/ a-Si:H/a-SiNx:H MQWs/ crystallized n+-a-Si:H/ SiO2/Si substrate. With a-Si:H well layer thickness less than 4 nm, when the annealing laser energy density was increased to 750 mJ/cm2, a stable light emission with orange-red color could be seen from the crystallized samples by the naked eye. The current-voltage(I-V) and light emitting characteristics of EL cells were reproducible after several months, and these characteristics were measured under both forward and reverse bias conditions, EL could all be observed with little difference. It is clear shown that irradiation laser energy density play a key role to the character of I-V, and EL properties have a close relationship with the sample conductivity. EL spectra showed multipeaks at the wavelength around 600 nm and 700 nm. Considering the size of nano-crystallites and the luminescent properties of crystallized MQWs structures, we speculate the EL phenomena as the result of carriers injection into the nano-size Si crystallites in the MQWs, and radiation recombination between quantized states.

COMPARATIVE STUDY OF IMPLANTED AND SPUTTERED SYSTEMS OF Si NANOGRAINS EMBEDDED IN SiO2. S. Charvet, R. Madelon, R. Rizk, LERMAT-ISMRA, Unite CNRS 6004, 14050 Caen cedex, FRANCE; B. Garrido, A. Perez-Rodriguez, M. Lopez, J.R. Morante, EME, Departament de Fisica Aplicada i Electronica, Universitat de Barcelona, Barcelona, SPAIN.

Si nanograins embedded in silicon oxide matrix have been grown by thermal annealing at 1100C during one hour, either after implantation of Si into thermal SiO2 layers or after magnetron co-sputtering of both Si and SiO2. The spectral distribution of the photoluminescence (PL) emission in both systems is similar and peak near the red visible spectrum (1.5-1.6eV). However, emission efficiency of implanted samples is shown higher when spectra are normalized to the total amount of crystallites. The sputtered system shows a substrate temperature (Ts) dependence with a maximum for Ts=500C. In both systems, PL intensity appears to closely correlate with the recording of infrared bands of the silica matrix approaching those for the thermal stoichiometric SiO2. Furthermore, PL emission was always observed only for samples in which precipitation of Si nanograins was indeed observed by TEM and Raman. All these experimental data are consistent with the model involving radiative centers at the grain surface when is passivated by stoichiometric SiO2. SiOx interface state make PL efficiency to drop by more than one order of magnitude. Thus, segregation of Si must be accomplished by the annealing and red light is obtained. Green-blue emission is also observed for samples annealed at lower temperatures, likely related to oxide defects. Modelling of recombination by surface states and band to band absorption will be presented.

FORMATION OF SILICON NANOCRYSTALLITES BY ELECTRON CYCLOTRON RESONANCE CHEMICAL VAPOR DEPOSITION AND ION BEAM ASSISTED ELECTRON BEAM DEPOSITION. Eun Kyu Kim, Won Chel Choi*, Suk-Ki Min, and Chong-Yun Park*, Semiconductor Materials Research Laboratory, Korea Institute of Science and Technology, Seoul, KOREA; *Department of Physics, Sung Kyun Kwan University, Suwon, KOREA.

We have observed a formation of nanocrystalline silicon(nc-Si) without any post-treatments during ion beam assisted electron beam deposition(IBAED) and electron cyclotron resonance chemical vapor deposition(ECRCVD) processes. The IBAED was performed at room temperature in 6x10-6 torr pressure with Ar ion, and then the growth rate of silicon film was 5 /s. During the ECRCVD process, the power was 300 W and the substrate temperature was changed from 50 to 200 C. The prepared samples were studied by PL, HRTEM, and AES measurements. In room-temperature PL measurements for ECRCVD samples, the peak with full width at half maximum of about 100 nm varied from 630 nm to 720 nm. It was confirmed by HRTEM that the diameter of Si nanocrystallites was 3-5 nm and distributed randomly in amorphous silicon layer. In the case of IBEAD samples, two PL bands with responsible to nc-Si appeared at 550 nm and 780 nm. With increasing the ion beam power, the 550 nm peak increased while the 780 nm peak decreased. By comparing with the result of ECRCVD, it implies that the ion beam play a role of reducing a size of Si crystallites. Also, the size of crystallites could be controlled by the change of the substrate temperature and ion beam power during the deposition process.

ON THE BLUE YELLOW-GREEN AND RED SIMULTANEOUS EMISSION FROM SiO2 MATRICES CO-IMPLANTED WITH C AND Si. M. López, B. Garrido, O. González-Varona, A. Pérez-Rodriguez, J.R. Morante, EME, Departament de Fisica Aplicada i Electrònica, Universitat de Barcelona, Barcelona, SPAIN; P. Ruterana, LERMAT-ISMRA, Caen Cedex, FRANCE.

Si and/or C implantations were performed in thermal SiO2 layers at different doses in the range 0.5-6.0x117cm-2 and annealed in N2 at temperatures up to 1100C. We found four broad photoluminescence (PL) bands, peaking at 1.5-1.7 eV (red), 1.95 eV (orange), 2.20 eV (yellow-green) and 2.75 eV (blue). The presence and relative intensity of these bands depended strongly upon implantation and annealing conditions. The 1.9 eV band disappeared after annealing and was related with ion beam induced damage in the oxide layer. Besides, the formation of Si nanocrystals (measured by TEM and Raman) with the annealing at high temperature leads to the appearance of the red band (1.5-1.7 eV). The yellow-green band was correlated with C dose and its origin related to the precipitation of graphitic carbon aggregates. Finally, high efficient red, yellow-green and blue bands were observed simultaneously in samples implanted with the same dose of Si and C and annealed at 900C. The origin of the strong blue band is still not clear although it could be related to the precipitation of SiC and/or radiative centers in the oxide. These data will be discussed in the light of a detailed TEM, Raman, IR and SIMS structural characterization to clasify the origin of the different luminescent bands, as well as to deaper in the ability to control spectral distribution and relative intensities of bands.

PHOTOEXCITATION OF RADIATIVE Si-Si SURFACE STATES IN NANOCRYSTALLITES. Munir H. Nayfeh, Nikolaos Rigakis, and Zain Yamani Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL.

Recently, intrinsic radiative localized surface states belonging to Si-Si dimers on the surface of silicon nanocrystallites have been predicted to account for the optical effect in porous silicon. We examine the various photoexcitaion pathways involved in populating these molecular states. We include both direct excitation from the groud state and indirect excitation from the photoexcited delocalized excitonic states via quantum tunneling and thermal activation. We determine the absroption and emmsion spectra and the quantum efficiency of the photoluminescence for crystallite sizes less than 2 nm. Our calculation gives a dramatic enhancement in the effeciency for sizes below about 1.4 nm. It is as if the meterial has changed from an indirect gap to a somewhat direct gap meterial. Most experiments dealing with luminescence effects in crystallites have so far used sizes in the range of 2 to 4 nm, outside the range considered here.

TIME-RESOLVED PHOTOLUMINESCENCE FROM nm-SIZED SILICON CRYSTALLITES IN SiO2. Jan Linnros, Augustinas Galeckas, Nenad Lalic and Vytautas Grivickas, Department of Electronics, Royal Institute of Technology, Kista-Stockholm, SWEDEN.

Nanostructures of silicon has recently attracted much attention due to the emission of visible light which could form the basis of optoelectronic devices. Si nanocrystals embedded in thermal oxides, is then an interesting system combining the strong PL of porous Si with the stability of an oxide passivated surface. For ultra thin layers, even electrical injection of carriers would be possible to realize basic LED structures. 
In this work time resolved photoluminescence (PL) decays have been measured for Si nanocrystallites embedded in silicon dioxide layers. The nanocrystallites were formed by implanting 40 or 160 keV Si ions to different doses into thermal oxide films of various thickness down to 10 nm, followed by thermal annealing at 900 - 1200 C. For thinner oxides, cap layers of amorphous Si were deposited to position the peak of the implantation profile in the middle of the oxide layer. The measured PL decays, peaking in the wavelength range 700 - 850 nm, exhibit a stretched exponential lineshape with shorter lifetimes at decreasing observation wavelength. Also, a decreasing lifetime was observed for reduced oxide thickness. At high excitation densities, the PL intensity saturated where the threshold for saturation was dependent on the emission wavelength. We attribute the PL saturation to the onset of Auger recombination which should have a distinct threshold when more than one electron-hole pair (i.e. one exciton) per crystallite is excited. The stretched exponential PL decays and the reduction in lifetime for very thin oxides could tentatively be attributed to the escaping of excitons from crystallites of higher bandgap.

Chair: Bernd Schueppert 
Tuesday Morning, December 2, 1997 
Simmons (M)

8:30 AM *H5.1 
MATERIALS FOR MONOLITHIC SILICON MICROPHOTONICS, J.S. Foresi, L.M. Giovane, M.T. Morse, L. Liao, A. Agarwal, X. Duan, J. Michel, A. Thilderkvist and E.A. Fitzgerald and L.C. Kimerling, Massachusetts Institute of Technology, Cambridge, MA.

Batch X36Silicon's high index of refraction and low absorption in the near infrared yield an ideal material for integrated optoelectronic circuits. We have constructed device prototypes that employ silicon materials for light emitters (Si:Er), optical waveguides and filters (Si/SiO2) and photodetectors (SiGe) Light emitting diodes based on the erbium-doped silicon exhibit sharp line luminescence at = 1.54m. Optically active erbium in silicon must be surrounded by electronegative ligands (O, N, T). The heat treatment (800-900 required to remove implantation damage limits optical activity by dissociation of the ligand field. Low temperature UHV-MOCVD has produced our brightest emission with some increase in linewidth. The Si/SiO2 system has a refractive index difference of n = 2. We have fabricated strip guide structures with dimensions of 0.2x0.5m. Taking advantage of the high optical confinement of these materials systems, we have designed and constructed bends with radii of less than 2m and losses less than 0.2dB. Y-couplers with splitting angles of greater than 10 show losses of less than 2dB. Photonic Band Gap reflectors have been fabricated using a periodic array of air holes imbedded in a planar Si/SiO2 waveguide. A gap of 400nms or 27% the midgap energy (0.8eV), was observed near = 1.54m. By omitting an air hole section, a resonant microcavity was created that has a modal volume of 0.058m3 and a Q of 265. Photodetectors for the =1.3-1.55 range have been designed based on strain-balanced superlattices of Si/SiGe. Photodetectors consisting of a relaxed Ge0.5Si p-i-n junction show charge collection efficiencies of greater than 50% with a responsivity of 3mA/W at =1.3m. Responsivities of 150mA/W are expected for the superlattice structures. The materials selection, optical design and fabrication issues of these devices will be discussed.

9:00 AM H5.2 
IMPACT EXCITATION AND AUGER PROCESSES IN Er-DOPED LIGHT EMITTING SILICON DEVICES. G. Franzo, S. Coffa, CNR-IMETEM, Catania, ITALY; F. Priolo, INFM and Physics Department, University of Catania, Catania, ITALY.

Erbium doping of crystalline Si has been recently recognized as a very powerful approach towards a VLSI compatible Si-based optoelectronics. In this work the excitation and de-excitation processes of Er3+ in crystalline Si will be discussed and correlated to the performances of light emitting diodes. In particular we have found that Auger de-excitation with energy transfer to free electrons is one of the main non-radiative quenching processes of the Er luminescence and is characterized by an Auger coefficient of 2x10-12cm3/s. In contrast Auger quenching with free holes is much weaker exhibiting a coefficient below 1.5x10-15cm2. We have applied this knowledge to the fabrication of room temperature operating Er-doped light emitting Si diodes. These devices emit light at 1.54 m under both forward and reverse bias conditions. We have studied the lineshape, efficiency and excitation mechanisms in both cases. In particular under reverse bias Er is excited by impact with hot electrons within the depletion region. Detailed investigations allowed us to obtain information on the impact excitation cross section (6x10-17cm2), excited Er lifetime (100 s) and efficiency (2x10-4). By taking full advantage of the Auger quenching, modulation at 20 MHz will be also demonstrated. These data will be reported and the future trends of this approach discussed.

9:15 AM H5.3 
MBE GROWTH AND CHARACTERIZATION OF Er/O AND Er/F DOPED Si LIGHT EMITTING STRUCTURES. W.-X. Ni, C.-X. Du, K.B Joelsson, G. Pozina, I.A. Buyanova, W.M. Chen, and G.V. Hansson, Dept of Physics, Linköping University, Linköping, SWEDEN.

Er doping in Si using Er2O3 or ErF3 as dopant sources during MBE growth has been studied. Structural and chemical analyses showed that by using a low temperature growth process the Er doping concentration can reach a level of 5x1019 cm-3 without precipitation and generation of other extended defects. Luminescence properties of these Er-doped MBE Si structures have been extensively studied using both photon and hot electron impact excitation with different excitation powers at a wide range of temperatures (2-300 K). Very similar line shape at low temperatures, and also the same activation energy value (160 meV) for the major intensity thermal quenching are observed for both PL and EL, indicating that the light emission of PL and EL can be originating from the same Er-related complexes, and there is the same de-excitation path. The onset of the thermal quenching is, however, quite different between PL and EL measurements. No PL can be observed for T>200 K, but rather intense EL can persist at room temperature from reverse biased Schottky Er-doped Si LEDs. The results indicate that a hot carrier impact excitation can be an efficient process for Er-related light emission. At the same time, large injection of electrons during EL can also passivate some non-radiative point defects to enhance the Er-excitation rate. The co-existence of efficient excitation by injecting hot carriers and a strong energy back transfer process in Er-doped Si LEDs can bring advantages to the high frequency performance of Si based opto-electronic devices. Our time-resolved EL measurements at 298 K have shown that a decay of about 60 of the EL intensity occurred with a time constant of 1 s followed by a slow decay corresponding to the Er ion spontaneous decay. As a demonstration, 1 s pulsed electric/optical signal transitions (limited by the pulse generator) have been observed from these Schottky type LEDs. Finally, the studies of waveguided Er-related emissions using SiGe layers are under way.

9:30 AM H5.4 
LIGHT EMITTING Si:Er:O DIODES OPERATING IN THE AVALANCHE REGIME. Nick Sobolev, Yuri Nikolaev, Aleksey Emel`yanov, Ioffe Physical-Technical Inst, Dept of Solid State Electronics, St. Petersburg, RUSSIA.

An appreciable decrease in the electroluminescence (EL) intensity by several orders of magnitude is observed in forward biased Si:Er:O diodes with temperature increasing from 77 to 300 K. At the same time, the temperature-related quenching of the EL in reverse biased tunnel diodes is strongly reduced. Our recent results concerning the fabrication and characterization of avalanching diodes are given. 
Erbium and oxygen ion co-implantation into n-Si and subsequent annealing were used to remove the implantation damage and activate the electrically and optically active erbium-related centers. The p-n junctions were fabricated by boron implantation. Mesa avalanching diodes were formed by a conventional technique. SIMS, RBS, TEM, X-ray diffraction, luminescence, current-voltage and capacitance-voltage measurements were used to characterize the properties of Si:Er:O. 
Si:Er:O diodes demonstrating EL at room temperature have been fabricated. The EL intensity in the avalanche breakdown regime decreases less than a factor of two with a temperature increase from 80 to 300 K. A comparative analysis of the tunnel and avalanching diodes is given.

9:45 AM H5.5 
Er-DOPED POROUS SILICON LED FOR INTEGRATED OPTOELECTRONICS. L. Tsybeskov, K.D. Hirschman, and P.M. Fauchet, Department of Electrical Engineering, University of Rochester, Rochester, NY; V. Bondarenko, Belarusian State University of Infomatics and Radioelectronics, Minsk, REPUBLIC OF BELARUS.

Porous silicon (PSi) was doped by Er using electromigration or diffusion from a solution and converted to Er-doped silicon-rich silicon oxide (SRSO:Er) by partial thermal oxidation at 900-950C. The room temperature photoluminescence (PL) at 1.5 m is intense and narrow (15 meV) and decreases by less than 50% from 12 K to 300 K. The PL spectrum reveals no luminescence bands related to Si-bandedge-recombination, point defects or dislocations, and shows that the Er3+ centers are the most efficient radiative recombination centers. A light emitting diode (LED) with an active layer made of SRSO:Er was manufactured using a preoxidation cleaning step to increase the quality of the interface between SRSO: Er and the heavily doped polycrystalline Si top electrode. Room temperature electroluminescence at 1.5 m was demonstrated.

Chair: Lionel C. Kimmerling 
Tuesday Morning, December 2, 1997 
Simmons (M)

10:30 AM H6.1 
NEAR INFRARED LIGHT DETECTORS BASED ON UHV-CVD EPITAXIAL Ge ON Si (100). L. Colace, G. Masini, F. Galluzzi, and G. Assanto, Department of Electronic Engineering, Terza University of Rome, Rome, ITALY; G. Capellini, L. Di Gaspare, E. Palange, and F. Evangelisti, Department of Physics, E. Amaldi & INFM Terza University of Rome, Rome, ITALY.

Due to the increasing use of optical fiber in the telecommunications, both for long distances and for Local Area Networks (LAN), there is an increasing effort in the material science community toward the realization of high speed and high efficiency optical detectors, operating in the low-absorption range (1.3- 1.55 m) of the silica fibers. Although III-V semiconductors provide a high detection efficiency in the range of interest, their integration in the well-established Si-based ULSI technology is very sophisticated and expensive. It is therefore desirable to grow optically efficient materials epitaxially on Si. Due to its narrow band gap, Ge is the best candidate. However, due to the high lattice mismatch with Si, it is not easy to obtain films with the characteristics suitable for the integrated electronic: thick (efficiency of the device), flat (realization of sub-micron lithography), defect-free (high-speed). In the present work we investigate detectors realized with thick relaxed Ge layers, epitaxially grown on silicon after insertion of a low-temperature-grown Ge buffer layer. By using this procedure it was possible to grow films with thicknesses comparable with light penetration depth in the 1.3-1.6m range. The films exhibited flatness on the atomic scale. Two kinds of detectors were investigated: vertical Schottky diodes and a planar Metal-Semiconductor-Metal structure. The detectors show a good responsivity at normal incidence both at 1.3 and 1.55m. The photocurrent increases with the voltage applied between the metal contact and silicon bulk, reaching a maximum responsivity of 0.12 A/W at 1.3m under a reverse bias of 4V. The leakage current density at the saturation voltage is 1nA/mm2. A complete optoelectronic characterization of the fabricated devices, including current voltage, photocurrent spectra and speed response was performed. The results show that the proposed approach is a promising one for the fabrication of 1.3 - 1.55m near infrared photodetectors integrated on silicon chips.

10:45 AM H6.2 

Electron transport characteristics and photosensitivity were measured for Al/porous silicon/c-Si (p-type) structures with porous silicon (PS) layers of 60-80 porosity. Photosensitivity spectra of Al/PS/c-Si structures with thin PS layer of high porosity are similar to Al/c-Si structures, while the structures with thick PS layer of low porosity show low photosensitivity at short-wavelength side of spectrum. It was shown that the impedance dependencies of the structures on frequency, reverse bias and thickness of PS layer are completely determined by the PS layer of high resistivity and by the space charge region in c-Si substrate on PS/c-Si heterojunction. Moreover, PS/c-Si heterojunction virtually determines photoelectric properties of Al/PS/c-Si structures. The annealing at above 100C decreases the forward current at a small bias and strongly increases the photosensitivity up to 10 A/W at 5 V reverse bias. The analysis of the experimental results leads to inevitable conclusion about the opposite band bending from two sides of the heterojunction that is caused by high concentration of charged defects at PS/c-Si heterointerface. The band diagram of PS/c-Si heterojunction is proposed. Device parameters of such a PS photodetector were measured.

11:00 AM H6.3 
GROWTH AND OPTICAL PROPERTIES OF SnxGe1-x ALLOYS FOR MONOLITHIC INTEGRATION OF INFRARED DETECTORS WITH Si (001). Regina Ragan, Gang He, Harry A. Atwater, California Institute of Technology, Thomas J. Watson Laboratory of Applied Physics, Pasadena, CA.

The ability to fabricate SnxGe1-x alloys at low temperatures (180C ) by molecular beam epitaxy and the existence of a tunable direct energy bandgap in the near to mid-infrared may enable monolithic integration of SnxGe1-x photoconductive or photovoltaic infrared detectors on fully-processed Si integrated circuits. Absorption measurements for strain-relieved SnxGe1-x alloys on Si (001) were derived from thickness-dependent transmission data and the alloys exhibited a decrease in direct energy bandgap from 0.8 eV to 0.25 eV as Sn concentration was varied from 0 < x < 0.15. More recently, high-quality, pseudomorphic single-layer SnxGe1-x alloy films and superlattices have been grown on Ge (001) for 0 < x < 0.05 as judged by X-ray reflectivity measurements, Rutherford backscattering, and transmission electron microscopy. Films were found to be fully pseudomorphic, compositionally uniform with the absence of Sn segregation, and dislocation free for single layers up to 100 nm thick. Superlattices with periods as short as 25 nm and alloy layers as thin as 5 nm exhibited abrupt interfaces. Ongoing work to be presented includes (i) characterization of the effect of pseudomorphic misfit strain on the bandgap via the determination of the optical constants from absorption measurements and (ii) photoconductivity measurements for p-type strain-relieved alloys on Si (001) and pseudomorphic alloys on Ge (001).

11:15 AM H6.4 
ERBIUM DOPED Si INFRA-RED DETECTOR. N. Hamelin, P.G. Kik and A. Polman, FOM-Institute for Atomic and Molecular Physics, Amsterdam, NETHERLANDS.

Because of band-gap limitation there is no efficient Si based infra-red detector. Impurity doping is a potential route to improve sub-bandgap response through the impurity photovoltaic effect. In this paper Er ions at 3 MeV were implanted to a dose of 1x1013 ions/cm2 and 3x1013 ions/cm2 in shallow p-n junctions defined by P implantation at 10 keV in p-type CZ-Si(100). Photoluminescence and lifetime measurements showed that a two step thermal anneal at 600C and 1000C is required to optimized crystal re-growth and Er3+ luminescence after implantation. The Er+3 doped p-n diode showed a photo-current at 1.54 m due a back-transfer process. In this process the energy of an Er+3 ion, acquired through sub-bandgap photon, is transferred to an impurity induced level within the Si bandgap which in turn, via a phonon-assisted process leads to exciton dissociation and free carrier generation. Temperature dependence of the photo-current was measured in the range of 15 to 300 K for the visible and infra-red regions. In the infra-red region the photo-current increases with temperature proving the phonon assisted nature of the process. Optimum doping concentration and quantum efficiency will also be discussed.

11:30 AM H6.5 
GAIN LIMITATIONS OF Er CO-SPUTTERED Al2O3 RIDGE WAVEGUIDE AMPLIFIERS ON SILICON OPERATING AT 1.53 m. P.G. Kik and A. Polman, FOM Institute for Atomic and Molecular Physics, Amsterdam, NETHERLANDS; J.W.M. van Uffelen and M.K. Smit, Delft University of Technology, Delft, NETHERLANDS.

Planar waveguide amplifiers form a crucial component in the realization of optical integrated circuits on silicon. In order to achieve maximum gain, both amplification medium and device structure have to be optimized. Previous measurements done in our group have shown a net gain in 1.53 m of 0.5 dB/cm in Er implanted Al2O3 ridge waveguides, using a pump power of 9mW at 1.48%. This talk focuses on a novel, improved amplifier design, and addresses the fundamental gain limitations of this structure. The erbium doped waveguide amplifiers were fabricated on an oxidized silicon substrate by RF sputtering from an Al_2O_3, a 3 cm interaction length could be fitted on 1 mm^2. Gain measurements applying either 1.48or 980 nm pump light will be presented. The results are analyzed using numerical calculations that take into account the highly confined signal and pump mode profiles. The relative contributions of excited state absorption, signal absorption in low pump power regions, and cooperative upconversion throughout the waveguide are addressed.

11:45 AM H6.6 
GUIDED WAVE FLUORESCENCE IN THIN FILM Er-DOPED BARIUM TITANATE. D. M. Gill, B. A. Block, B. W. Wessels, S. T. Ho, Northwestern University, Materials Research Center, Evanston, IL.

Ferroelectric oxide thin film waveguides are being developed for utilization in Si-based optoelectronic integrated circuits. We have been fabricating epitaxial barium titanate waveguides for amplifiers and modulators. Here we report on guided wave absorption and fluorescence of 1.54 micron light in channel waveguides fabricated using Er-doped BaTiO3 on MgO. The epitaxial layers are prepared by metalorganic chemical vapor deposition. Er concentrations as high as 0.5 atomic percent have been incorporated into the thin film waveguides. Strong guided wave fluorescence has been observed at 1.54 microns. Heavily doped films show an up-conversion of the 1.5 micron pump to 1.35 microns. Issues of integration of epitaxial BaTiO3 with Si using MgO buffer layers will also be discussed.

Chair: Mark L. Brongersma 
Tuesday Afternoon, December 2, 1997 
Simmons (M)

1:30 PM H7.1 
ELECTROLUMINESCENT DEVICES BASED ON POLYCRYSTALLINE SILICON FILMS FOR LARGE-AREA APPLICATIONS. Nobuyoshi Koshida, Satoshi Arai, Hideki Koyama, Eiichiro Takizawa, Toshiyuki Sameshima, Tokyo Univ. of Agri. & Tech., Dept. of Electron. & Info. Eng., Tokyo, JAPAN.

From technological viewpoints, it is desirable to establish fabrication technology of silicon optoelectronic devices in the form of thin films. We report here that polycrystalline silicon (poly-Si) films are available for applications to electroluminescence (EL) diodes and possibly to integrated large-area devices. The experimental diodes are composed of top contact (semitransparent thin Au film), luminescent porous poly-Si (PPS) layers (grain size: about 60 nm, thickness: 1 m), p-type silicon substrates, and ohmic back contacts. The PPS layers were formed by conventionally anodizing boron-doped LP-CVD poly-Si films (1.5 cm in resistivity) in ethanoic HF solutions. The EL intensity and spectra of the diodes, including their temperature dependences, were measured in relation to the photoluminescence (PL) characteristics. The experimental results are compared with those obtained from the porous silicon (PS) diodes fabricated on single crystalline silicon substrates. The major experimental results are summarized as follows: (a) PS diodes exhibit a definite rectifying behavior. Uniform visible EL emission was observed at a forward and reverse bias voltages of about 5 V and 40 V, respectively. The EL intensity at sufficient injection level increases in proportion to the diode current. (b) The relationship between the EL efficiency and the diode current remained unchanged even at low temperatures. The EL spectra of PPS diodes are similar to those of our conventional PS diodes. (c) The fundamental PL characteristics of PS (the decay dynamics, polarization memory, and electric-field PL quenching) were also obersved in PPS as well. These results indicate that the injection-EL scheme holds in PPS diodes through the common luminescence mechanism with PS-LEDs. The design and device physics of integrated PPS-LED devices with switching thin film transistors will be also discussed.

1:45 PM H7.2 
STRONG BLUE AND VIOLET LIGHT EMISSION FROM SILICON AND GERMANIUM IMPLANTED SILICON DIOXIDE. L. Rebohle, I.E. Tyschenko, J. von Borany, B. Schmidt, R. Grötzschel, A. Markwitz, R.A. Yankov, W. Skorupa, FZ Rossendorf, Dresden, GERMANY; H. Fröb, Institut für Angewandte Photophysik, TU Dresden, GERMANY.

We have obtained strong blue and violet photo- and electroluminescence at room temperature from Silicon and Germanium nanostructures using a dry technology compatible to microelectronic manufacturing: Thermally grown, 500 nm thick SiO2-fllms are doubly implanted with 1016... 1017 cm-2 Si+- or Ge+-ions in the energy range of 100 to 450 keV, followed by different annealing procedures. Photoluminescence (PL), Electroluminescence (EL), High-Resolution Transmission Electron Microscopy (HRIEM) and Rutherford Backscattering Spectrometry (RBS) are used for sample characterization. The PL spectra of Ge-rich layers reach a maximum after annealing at 500C both PL and EL intensities of Ge-rich layers are distinctly higher than these of Si-implanted layers and easily visible by the naked eye at ambient light. HRTEM investigations show that Si or (Ge nanocrystals are formed only in samples annealed at temperatures higher than 900C, proving that the blue PL is not due to quantization in nanocrystals. Pure radiation damage as a source of distinct PL can be excluded by comparison to Ar+-implanted SiO2 which does not show notable PL. Based on excitation spectra, we tentatively interpret the blue PL as due to the oxygen vacancy in silicon dioxide. The RBS results for the Ge depth profiles show complicated redistribution and agglomeration effects for annealing temperatures higher than 900C. EL measurements are carried out at MOS capacitors using an ITO front contact and an electric field of about 6 to 8 MVcm-l. The EL spectrum of the Ge-implanted oxide correlates very well with the PL one and shows a linear dependence on the injected current over three orders of magnitude. First results without any optimization give EL efficiencies in the range of 10-5 to 10-4 for Ge+-implanted silicon dioxide.

2:00 PM H7.3 
BLUE THIN FILM ELECTROLUMINESCENT DEVICES FROM QUANTUM CONFINED Si AND Ge NANOCRYSTALS. Howard W.H. Lee, Lawrence Livermore National Laboratory, Livermore, CA; Gildardo R. Delgado, University of California at Davis, Davis, CA; Chrysostomus H.M. Maree, Vanderbilt University, Nashville, TN.

We report on a novel nanocrystalline semiconductor system for fabricating low-cost, high brightness, blue, thin film light emitting devices (LEDs). Blue electroluminescent (EL) devices were fabricated with quantum confined Si and Ge nanocrystals. These nanocrystals were synthesized through a controlled synthetic chemical reaction. Ultrasonic fracturing of porous silicon (PSi) was also used to produce Si nanocrystals. The active EL material consists of fully surface-passivated Si and Ge nanocrystals embedded in a variety of wide bandgap, thin film host matrices such as polyvinylcarbazole (PVK), polystyrene, silica sol-gels and other organic materials. Major advantages of this composite material system are the ease and low-cost of producing high quality, thin, conformal EL films. Several device configurations were used to induce EL processes that rely on radiative electron-hole recombination within the nanocrystals. We report on the optical and electrical properties of these devices including the PL and EL spectra, quantum efficiency, and current-voltage (I-V) behavior. Comparison of the PL and EL spectra indicates that the blue EL originates from quantum confined nanocrystals. Mechanisms for the carrier injection are discussed and include space charge limited and tunneling injection in various operating regimes. These EL devices perform as highly efficient light emitting devices. The low-cost and ease of processing these materials make them ideal for many light emitting applications such as flat panel displays and optical memories. Work at LLNL was perfomred under the auspices of the U.S. DOE by LLNL under contract No. W-7405-ENG-48.

2:15 PM H7.4 

Light emitting nanocrystalline -SiC films on c-Si wafers can be fabricated by C+-implantation following by annealing and oxidation. X-ray photo-electron spectrum and high resolution transmission electron microscope analysis shows that -SiC nanocrystallites with size of 3 nm have been formed during annealing. After oxidizing for 8 hours, SiO2 layers were formed around the embedded -SiC nanocrystallites. A stable photoluminescence peak at 420 nm was observed under the excitation of 250 nm at room temperature. The dependence of the PL peak position and intensity on implantation dose and the explanation of the light emission have also been studied. We also have studied the electroluminescence from porous -SiC formed on C+-implanted silicon followed by annealing and anodization. A stable (blue-green) light emission was observed at a forward bias. The EL spectra have a main peak at 560 nm and a blue shoulder at 450 nm. The EL is caused by the recombination of the carriers which are injected from Au contact and p-Si into porous layer. The high energy light emission comes from porous SiC and the low energy light emission comes from porous Si.

2:30 PM H7.5 
PHOTOLUMINESCENCE STABILITY IN SILICON SUBOXIDE THIN FILMS. F. Wang, B.J. Hinds, D. M. Wolfe, and G. Lucovsky, Department of Physics, Materials Sciences and Engineering, Electrical and Computer Engineering, North Carolina State University, Raleigh, NC.

Silicon suboxide (SiOx, x<2) has long been known to have efficient visible photoluminescence (PL) over a broad range of temperatures, however its thermal and lightsoaking stability require further study. The films were deposited by remote plasma enhanced CVD using silane and oxygen gas mixtures, with film oxygen content (x) varied from 0.7 to 1.5. The films were characterized by PL, FTIR, optical transmission and electrical conductivity. The experimental results are: (1) As oxygen content (x) increases, the PL spectra shifts towards visible, however an infrared component below 1.5 eV exists in all the films. (2) The PL intensity decreases as the temperature increases while temperature stability increases with x, consistent with higher band gap and broader tail state distribution in the oxygen rich films. (3) The PL intensity decreases under strong light soaking, which can be attributed to the photogeneration of defect states similar to a-Si:H. Oxygen rich films is relatively stable compared to oxygen poor one. (4) The PL intensity increases as a power law function of the excitation intensity (exponent decreases from 0.95 to 0.55 with laser intensity), which indicates a change from geminate to non-geminate radiative recombination mechanism. 5) With high temperature annealing (>650 C) all PL is quenched by Si dangling bond defects. H2 plasma treatment restores PL in the IR region (<1.4eV) indicative of silicon suboxide decomposition into amorphous Si and SiO2. All these results will be discussed in correlation with structure and optoelectronic properties. This work is supported by ONR, NSF and SRC.

2:45 PM H7.6 
ELECTROLUMINESCENCE STUDIES OF Si BULK MATERIALS. C.-X. Du, W.-X. Ni, K.B Joelsson, and G.V. Hansson, Dept. of Physics, Linkoeping University, Linkoeping, SWEDEN.

Electroluminescence (EL) of single crystal Si has been studied by injecting carriers through Al/Si contacts to an FZ Si substrate (200 W-cm) under a forward biased condition. Rather sharp near infrared light emissions associated with TO- (1.1 m, FWHM=13 meV), TA-, and two-TO-phonon assisted processes have been observed from these Schottky light emitting structures at low temperatures (37K), and the spectrum shapes are identical to the PL spectra of bulk Si. The EL intensity is, however, much higher and can persist up to room temperature with an FWHM value of 65 meV and an integrated intensity only 60 lower than that observed at 37 K. Compared the measured EL peak shift with the calculated energy values of the Si bandgap, a transition between the exciton and free carrier interband recombination likely occurred at 200 K. The fact that luminescence can be more efficient by using electric rather than optical excitation is probably due to interactions between exceeded electrons and some point defects, and eventually a high density of injected electrons can passivate some non-radiative competing channels. The time resolved EL measurements showed that the transient behavior of the EL from these structures is complex, and there are over shootings of the luminescence intensity at both rising and fall edges of the input electric pulse, which may be associated with the junction capacitance and the applied electric field. The major luminescence decay time is 0.7 s at 37 K, which is about two orders of magnitude faster than that observed in PL measurements. However, an even lager time constant (4 s) for EL decay is observed at 300 K. Further studies of these luminescence properties are under way, and expected to provide some useful knowledge for a realistic Si-based opto-electronics.

Chairs: Salvatore Coffa, John E. Cunningham, Albert Polman and Richard Soref 
Tuesday Afternoon, December 2, 1997 
3:00 P.M. 
Tufts (M)

ION-BEAM SYNTHESIZED SEMICONDUCTION -FeSi2 CONTROLLED BY ANNEALING PROCEDURES AND PHASE-TRANSITIONS. Yoshihito Maeda, Takumi Fujita, Tomaki Akita, Kenji Umezawa, Dept of Materials Sciences, Osaka Prefecture University, Sakai, Okaka, JAPAN; Kiyoshi Miyake, Power & Industrial Systems R&D Division, Hitachi, Ltd., Hitachi, Ibaraki, JAPAN.

Semiconducting -FeSi2 has been expected to be a promising material for near IR optoelectronics devices because of its direct band gap of 0.85 eV (1.46 m). Ion-beam synthesis (IBS) has been employed to make -FeSi2/Si heterojunctions. -FeSi2 has an orthorhombic structure (D2h18) and its semiconducting property comes from lattice distortion of a higher symmetric lattice due to a Jahn-Teller effect. So the optoelectronic properties are very sensitive to lattice imperfections. Therefore, it is important to control the complicated lattice structure of FeSi2. In this study, we examine which process controls the growth of -FeSi2 in IBS by RBS, XRD, Raman spectroscopy and SEM. Samples were prepared by some kinds of ion implantation methods at room temperature. The doses of 56Fe+ into n type-FZ grown Si(100) were 5x1016 - 1x10^17)%% ions/cm2. The lamp-annealing of the samples was carried out by some procedures at 500 - 800C for 0.5 - 8 h. The RBS data showed that the Fe atoms implanted near the surface (200 nm in depth) hold the distribution even after annealing. The XRD and Raman spectra showed that the -FeSi2 growth is controlled by the following processes: (1) precipitation of metastable metallic -FeSi2 of a fluorite structure with a= 0.5428 nm at 500 - 600C, (2) precipitation of -FeSi2 above 600C and (3) phase-transition from -FeSi2 to -FeSi2 induced by the Jahn-Teller effect above 800C. From the SEM observations, we found that morphology and crystallinity of -FeSi2 grown on Si(100) strongly depend on the processes(1)-(3), and especially that the -FeSi2 - > -FeSi2 transition is the most important to form the smooth -FeSi2 surfaces with good crystallinity.

SILICON/POROUS SILICON HETEROJUNCTION LED: PERFORMANCE AND PHYSICAL CHARACTERIZATION. L. Pavesi, F. Piazza, INFM and University of Trento, Dept Physics, Povo, ITALY; P. Bellutti, Microelectronics and sensors Division, IRST-ITC, Povo, ITALY.

We develop a process CMOS compatible to form porous silicon (p-Si) based LED. This process exploits the selectivity of the p-Si formation with respect to the doping-type of the substrate. We identify via implantation n-type doped stripes in p-type doped wafers. After the whole micro-electronics processing occurred, the structure was covered and p-type windows were formed to allow locally the electrochemical attack. As a result the n-type doped stripes float on the p-Si layer. In this way, the electrical injection into the p-Si is provided by silicon/p-Si heterojunction in the present LED structure. At the conference the performance and the physical mechanism of injection into these heteojuntion LED will be presented and discussed also in comparison with other LED structure reported in the litterature.

A LED BASED ON POROUS POLYCRYSTALLINE SILICON. W.N.Huang, K.Y.Tong, Hong Kong Polytechnic University, Dept of Electronic Engineering, HONG KONG; P.W.Chan, Hong Kong Polytechnic University, Dept of Applied Physics, HONG KONG.

Previous studies on electroluminescence in porous silicon were based on crystalline wafers. In this paper, we shall report the characteristics of a LED based on porous effects in a cast polycrystalline silicon substrate. A layer of porous region was first formed on a cast polycrystalline silicon substrate by anodization, followed by the deposition of a semi-transparent Au layer. Under forward bias, the LED emits stable yellowish white light (with the presence of bright spots) for currents above 20 mA/sq. cm. The electroluminescence is very different from the photoluminescence which appears reddish orange. From the electroluminescence spectra measured, we suggest that the emission is due to the recombination of electron-hole pairs in a microplasma region. We propose a model where the microplasma is present in the depletion region of the heterojunction formed between the bulk polysilicon and the surface porous polysilicon. The defects and grain boundaries in a polycrystalline material enhance the formation of such microplasma. The heterojunction model will also be used to explain the current characteristics of the LED. The effect on the LED characteristics due to indium coating on the porous substrate prior to Au deposition was studied, and the results agree with the heterojunction model. Our work shows that cast polycrystalline silicon substrates have potential for LED fabrication in cheap and large area applications.

OPTICAL ACTIVATION OF ERBIUM-DOPED POROUS SILICON BY HYDROGEN PLASMA TREATMENT. Tohru Dejima, Riichiro Saito, Shigemi Yugou, Hideo Isshiki, and Tadamasa Kimura, Univ. of Electro-Communications, Dept. of Electronics Engineering, Tokyo, JAPAN.

Er3+-doped porous silicon (Er:PS) shows a strong room temperature emissions at 1.54m. However,its spectra is usually as broad as that of Er-doped polycrystalline silicon or amorphous silicon (full-width at half maximum - FWHM - is 10 nm). It is probably because Er ions are located in amorphous phases. We report in this paper that strong and very sharp Er3+ 1.54m emissions are obtained, when Er:PS samples are treated in hydrogen plasma after Er-doping. Porous silicon layers are formed by anodic etching and then doped with Er3+ ions in an ErCl3/ethanol solution by an electrochemical method, and then treated in hydrogen plasma at 1000C from 0.5min to 90min for the optical activation. A negative bias is applied to the samples during the hydrogen plasma treatment, which is found to enhance the peak intensity of the Er3+-related 1.54m luminescence. Several sharp peaks are observed at 20K, of which the strongest peak is located at 1.538m with an FWHM less than 1 nm. This value is comparable to that obtained from Er3+-doped crystalline silicon formed by means of MBE or ion implantation. Comparison is made among hydrogen plasma, argon plasma, H2 flow and vacuum for the post-dope annealing atmosphere. FTIR and SIMS measurements are also carried out. We conclude that a preferential etching of amorphous surface layers including silicon oxides by hydrogen plasma, termination of dangling bonds of porous silicon with hydrogen atoms and the formation of Er-H complexes may be responsible for the strong and sharp Er3+-related luminescence.

TIME DECAY CHARACTERISTICS OF THE Yb3+-RELATED 0.98 m EMISSIONS IN POROUS SILICON. Tadamasa Kimura, yasuhiro Nishida, Tohru Dejima, Riichiro Saito, Hideo Isshiki, Univ. Electro-Communications, Dept. Electronics Engineering, Tokyo, JAPAN.

Porous silicon is a good host for rare-earth ions to realize a strong room-temperature emissions. Rare-earth ions can easily be incorporated, for example, by an electrochemical method into porous silicon layers, and the widened bandgap of the host porous silicon makes possible the emission of rare earth ions at energies higher than the bandgap of crystalline Si. We have shown that incorporation of Yb3+ ions into porous silicon layers and post-dope annealing in Ar at high temperatures result in the Yb3+-related emissions at 0.98 m with a small temperature quenching.(1) The peak intensity is decreased from 20K to 300K by about a factor of 10. This paper studies the time decay characteristics as functions of temperature between 20K and 300K to understand the cause of the small temperature quenching of the Yb3+-related emissions in porous silicon. Porous silicon layers are formed by anodic etching and then pre-annealed in an Ar/O2 atmosphere at 900C for 30 min prior to Yb3+ doping. Thereafter, porous silicon is doped with Yb3+ ions in YbCl3/alcohol solution, and then annealed in H2 (typically at 1100C for 30sec) for the optical activation. The pre-annealing process is found not only to stabilize the luminescence spectra of the host porous silicon against the post-dope annealing but also to enhance the Yb3+-related emissions by 10 times. Time decay measurements show that there are two major Yb-centers. One shows a short decay time (40 s at 20K ) which decreases rapidly with increasing temperature. The other has a long decay time (400 s) which is almost independent of temperature, and is responsible for the strong room-temperature luminescence.

LATTICE SITES AND STABILITY OF IMPLANTED Er IN FZ AND CZ Si. Ulrich Wahl, Andre Vantomme, Guido Langouche, Univ of Leuven, Instituut voor Kern- en Stralingsfysica, Leuven, BELGIUM; Joao Guilherme Correia, The ISOLDE Collaboration, CERN, Dept of PPE-IS, Geneva, SWITZERLAND.

We report on direct lattice location of Er in Si using the emission channeling technique. This experimental method makes use of the fact that charged particles emitted from radioactive isotopes in a single crystal experience channeling or blocking effects along crystallographic axes and planes. This leads to an anisotropic emission yield from the cystal surface, which depends in a characteristic way on the occupied lattice sites of the probe atoms. We have applied this method to study the lattice sites of Er-167m (2.3 s half life) in oxygen-rich CZ Si and oxygen-lean FZ Si. In this contribution, we compare the lattice sites and thermal stability of Er in both types of Si and for different Er implantation doses. In both types of Si, already a high fraction of Er (>80%) occupies near-tetrahedral interstitial (T) sites directly following 60 keV room temperature implantation of the mother isotope Tm-167m (9 days half life) at doses of 5E12/cm2. For higher implantation doses (5E13/cm2), the as-implanted near-T fractions of Er visible by emission channeling are smaller, due to beginning amorphization. Following the removal of implantation damage at 600 deg C, more than 90% of Er is found on near-T sites in both FZ and CZ Si. In FZ Si, Er exhibits a remarkable thermal stability. In-situ channeling measurements at 900 deg C yielded still ca. 90% on near-T sites, and even prolonged annealing for several hours did only slightly reduce this fraction. On the other hand, annealing of CZ Si at 900 deg C for several minutes resulted in an isotropic electron emission yield for the majority of Er probes. This is characteristic for lattice sites in amorphous surroundings or of very low crystal symmetry. We therefore suggest that segregation of Er and O into disordered rare earth oxide precipitates is responsible for the increase in luminescence yield which is usually observed for Er-doped CZ Si annealed at 900 deg C.


We report on a novel method of depositing high quality erbium doped silicon:oxygen thin films by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) of SiH4 and O2 with concurrent sputtering of erbium, and on the composition dependence of room temperature 1.54 m Er3+ photoluminescence from the deposited films. The Si:O ratio was varied from 3:1 to 1:2, thus ranging from oxygen-doped hydrogenated amorphous silicon (a-Si:H:O) to SiO2. Some samples were also annealed in the temperature range of 500-900 C. A-Si:H:O-like samples luminesce most brightly as deposited, demonstrating the advantage of direct deposition. SiO2-like samples require high temperature annealing for luminescence. The most intense luminescence is observed from sample with Si:O ratio of 1:1.2 after 900 C anneal and formation of silicon nanoclusters, or from silicon-rich silicon oxide (SRSO). Based on pholuminescence excitation spectroscopy and visible photoluminescence results, we identify efficient excitation via carriers and complete passivation of defects by oxygen as key factors in producing the intense Er3+ luminescence from SRSO.

ERBIUM {[(TRIMETHYLSILYL)]AMIDE} AS A DOPANT SOURCE FOR INCORPORATION OF ERBIUM INTO SILICON. Oliver Just and William S. Rees, Jr, Georgia Institute of Technology, School of Chemistry and Biochemistry, School of Materials Science and Engineering and Molecular Design Institute, Atlanta, GA; Lionel C. Kimmerling and Michael T. Morse, Massachusetts Institute of Technology, Dept of Materials Science and Engineering, Cambidge, MA.

The development of silicon-based optoelectronic technology has increasingly attracted attention in recent years this market area has been dominated up to date by III/V materials such as binary (GaAs), ternary (AlGaAs) and quaternary (InGaAIAs) compositions. Due to their small wafer size, high cost as well as poor mechanical and thermal properties III/V-based optoelectronic devices have rapidly reached their commercial limits. Silicon, an alternative semiconducting candidate for fabrication of electroluminescent materials with potentially superior properties, exhibits an intrinsic indirect band-gap preventing it from becoming a light source. This fundamental obstacle can be surmounted by introducing optically-active rare earth ions with inherent energy overlap and proper crystal field environment into the silicon host lattice. Due to its unique electronic properties, (characteristic intra-4f shell emission at 1.54 coincides with the minimum absorption wavelength of silica based optical fibers) erbium emerged among the lanthanides as a suitable dopant precursor for growth of Si:Er films. Singe the originally utilized erbium tris(cyclopentadienyl) and (tetramethylheptanedionate) CVD derivatives deposit carbon and/or oxygen during the thermal decomposition process, erbium tris(amide) examples have been selected as a class of compounds displaying no direct metal-carbon or metal-oxygen interactions. Earlier evaluations of erbium {tris[bis(trimethylsilyl)]amide} as a source precursor demonstrated the high potential for doping of GaAs and AlGaAs laser diodes. Ensuing investigations into doping of silicon have confirmed its incorporation efficiency (1021 by SIMS) along with high levels of optically active erbiunt centers. Glancing angle X-ray diffraction techniques demonstrated unparalleled crystalline quality of deposited Si:Er films exhibiting a strong photoluminescence signal at 1.54 . The synthesis, purification, characterization and evaluation of these compounds will be presented.

CHARACTERIZATION OF ERBIUM-DOPED SILICON DIOXIDE LAYERS ON SILICON BY SPARK PROCESSING. John St. John, Jeffery L. Coffer, Texas Christian University, Department of Chemistry, Ft. Worth, TX; Young Gyu Rho, Patrick Diehl, F. Pinizzotto, University of North Texas, Denton, TX; Thomas D. Culp, Kevin L. Bray, Department of Chemical Engineering, University of Wisconsin, Madison, WI.

Alternative routes to ion implantation of erbium in Si are of extensive interest. Previous studies have shown that processing silicon wafer surfaces with the high frequency arc from a tesla coil in air produces a layer of porous SiO-2 on the silicon surface. If an erbium salt layer is deliberately deposited on the silicon surface and the substrate is spark processed, then the resulting Si porous layer and SiO-2 cap are doped with the desired rare earth ion. We have characterized these luminescent Er-doped porous SiO-2 layers on Si by SEM, energy-dispersive x-ray measurements, as well as visible and near IR PL spectroscopies. Energy-dispersive x-ray maps indicate that the erbium concentration in the porous layer can be controlled by varying the molarity of the erbium solution deposited on the substrate prior to spark processing. Visible PL measurements reveal that the concentration of Er3+ is proportional to the resultant intensity of the visible fluorescence transitions; for the near IR fluorescence peak at 1.54 m, self-quenching due to erbium clustering occurs at higher concentrations. PL intensity measurements of the 1.54 m Er3+ transition as a function of excitation power and temperature show a sub-linear increase in intensity with increasing power. Low temperature, high resolution PL spectra show that the erbium atoms occupy multiple site symmetries and do not form a separate erbium oxide phase.

A STUDY OF PHOTOLUMINESCENCE FROM COPPER AND CERIUM IMPURITIES IN SILICON FILMS. Won Chel Choi*, Eun Kyu Kim, Suk-Ki Min, and Chong-Yun Park*, Semiconductor Materials Research Laboratory, Korea Institute of Science and Technology, Seoul, KOREA; *Department of Physics, Sung Kyun Kwan University, Suwon, KOREA.

Photoluminescence(PL) originated from the impurities in copper diffused silicon dioxide film and cerium doped silicon film has been studied. The structural effects of impurities in silicon and silicon dioxide were measured also by AES, SEM, and TEM. In Cu diffused sample, two PL signals peaked at 723 nm and 862 nm appeared. These results implies that the optical recombination centers induced from a chemical bonding such as Cu-O-Si or Cu-O were generated. The 723 nm signal disappears at the temperature above 120 K, while the 862 nm shows at room temperature. In the case of the Ce-doped silicon, strong PL peaks at 574 nm, 615 nm and 621 nm are obtained at room temperature. These peaks have the full width at half maximum(HWHM) of 3-4 nm range. These signals might be originated from the atomic levels of Ce ion in silicon. However, after thermal annealing at 1000 C for 10 min, a new signal appears at 450 nm range with 70 nm HWHM, which caused from nanocrystalline silicon confirmed by TEM measurement. The optical properties of Cu and Ce impurities in silicon materials have enormous potential applications in Si-based optical devices.

Chair: Yoshihiko Kanemitsu 
Wednesday Morning, December 3, 1997 
Simmons (M)

8:30 AM *H9.1 
BEYOND-THE-ROADMAP TECHNOLOGY: SILICON HETEROJUNCTIONS AND QUANTUM DEVICES. Alan Seabaugh, Texas Instruments Inc., Corporate Research Laboratories, Dallas, TX.

The roadmap for silicon device technology has been drawn, extending to the year 2010, and featuring a CMOS transistor whose gate length will shrink to 0.07m at the roadmap's end [1]. Beyond this point, the availability of a silicon heterojunction technology could provide a significant boost to device performance and functionality. Today SiGe/Si and SiGeC/Si heterojunctions are receiving the greatest attention, but heterojunctions now being developed to realize silicon resonant tunneling diodes are greatly increasing the heterojunction options for silicon quantum and optical devices. In this talk we will outline progress on silicon quantum-well heterojunction development which now includes II-VI materials, crystalline oxides, nitrides, and fluorides, and new process techniques for forming crystalline materials over amorphous barriers.

9:00 AM H9.2 
TYPE II Si/Si1-yCy/Si1-xGex QUANTUM WELL STRUCTURES PREPARED BY MBE. Kenneth B. Joelsson, Wei-Xin Ni, Chun-Xia Du, Galia Pozina, Lars Hultman, and Goran V. Hansson, Department of Physics and Measurement Technology, Linkoeping University, Linkoeping, SWEDEN.

C doping of Si have been obtained from sublimation of SiC in a home-built high-temperature cell during solid source molecular beam epitaxy (MBE). With this technique Si/Si1-yCy andSi/Si1-yCy/Si1-xGex quantum well (QW) structures have been prepared. Different characterization techniques have been used including high-resolution X-ray diffraction (HRXRD), transmission electron microscopy (TEM), photoluminescence (PL), and electroluminescence (EL) to study the structure properties. Multiple Si1-yCy QWs with the C content up to 8 have been prepared without extended defect formation. Near-band gap PL spectra consisting of both NP and TO peaks have been obtained both from single- and multi-QW structures. In general the intensity of the NP emission is higher than that of the TO phonon replica. As Brunner et al.1 has reported, even strongly enhanced NP transition should be obtained from neighboring Si1-yCy/Si1-xGex QWs. We have grown similar neighboring QWs for both PL and EL studies. Electroluminescence peaked at 1.05 m has been observed from a p-i-n diode with neighboring QWs embedded in the middle undoped region between the temperatures 37 and 80 K. Further studies of the luminescence properties are under way.

9:15 AM H9.3 
CONTROL OF SIZES AND OPTICAL EMISSION OF SiGe QUANTUM DOTS PREPARED ON ORDERED MESOPOROUS SILICA COATED Si WAFER. Y.S.Tang, S.Cai, D.Wang, G.Jin, J.Duan, K.L.Wang, UCLA, Electrical Engineering Dept, Los Angeles, CA; H.M.Soyez, B.S.Dunn, UCLA, Materials Science Dept, Los Angeles, CA.

In this paper, we present our recent results on a new way of preparing wafer sized SiGe and Ge quantum dots at extremely low cost, and their optical properties as measured by low temperature photoluminescence. It was found that two different controlled sizes of any quantum dots such as SiGe, III-V, II-VI or even magnetic dots can be formed simultaneously into two layers on the same wafer with very good dot size uniformity distributions. The dot sizes can be controlled between 2nm and several hundred nanometers in diameter. Our initial experiments on SiGe and Ge system suggest that it is possible to squeeze the SiGe dots for much improved optical emission, as what occurs in dry etched small Si-SiGe dots which showed two orders of magnitude enhanced light emission. Unlike the emission from ordinary Si-SiGe structures, the emission from our dots has only a single intense photoluminescence peak, no phonon replica can be detected, which suggests that a crystal lattice symmetry change and reduction of the lattice constant in the dots may be realized. The great advantages of this dot preparation method are its fully compatibility with the Si-technology, its simplicity in dot preparation and extremely low cost.

9:30 AM H9.4 
LUMINESCENT AMORPHOUS SILICON LAYERS. G. Allan, C. Delerue, M. Lannoo, IEMN, Dept ISEN, Villeneuve d'Ascq Cedex, FRANCE.

Silicon amorphous layers have been recently grown by molecular beam epitaxy, magnetron sputtering or plasma enhanced chemical vapor deposition [1]. They show a blue shift of the luminescence which seems to be due to quantum confinement. We have calculated the electronic structure of amorphous silicon layers within the empirical tight binding approximation using the Wooten-Winer-Weaire atomic structure model [2]. We predict an important blueshift due to the confinement for layer thickness below 3 nm and we compare with crystalline silicon layers. The radiative recombination rate is enhanced by the disorder and the confinement but remains quite small. The comparison of our results with experimental results shows that the density of defects and localized states in the studied samples must be quite small.

9:45 AM H9.5 
STRUCTURAL BAND GAP ENGINEERING. Alexander A. Demkov1,2, Otto F. Sankey1, and M. Fuentes1, 1Department of Physics, Arizona State University, Tempe AZ; 2Semiconductor Products Sector, Motorola Inc, Mesa, AZ.

A relatively small indirect band gap of Si is the main obstacle of Si-based optoelectronics. So far, the best way to overcome this problem has been the hybrid integration of III-V based devices on Si. Alloying of Si with other group IV materials such as C or Ge did not produce a band gap increase but rather resulted in the opposite. The quantum confinement effects in microcrystalline and 'porous' (anodically etched) Si generally produce visible photoluminescence (1.6-1.9 eV). Structural band gap engineering is an alternative approach in which the band gap is modified via crystal structure tailoring. Recently we have proposed a whole new class of pure Si materials, which we called silisils, with crystal structures derived from those of zeolites. These Si phases a very low energy (only 0.07-0.1 eV/atom above the ground state diamond Si), and have band gaps varying from about 0.5 eV to about 1.9 eV (which makes them comparable to porous Si). If made experimentally, silisils will open an important avenue in Si-based electronics. Two silisils, also known as Si clathrates Si(46) and Si(136) have been successfully synthesized in our lab at ASU and also elsewhere, and are currently under intense investigation. In this talk we will discuss ordering of impurity atoms endohedrally incorporated in the cages of clathrates, and effects of endohedral doping on the electronic properties of silisils. Also, we will discuss point defects such as vacancies and substitutional isoelectronic imputities in these exciting materials, and properties of SiGe clathrate alloys. In addition we will discuss Ge analogs of silisils, which theoretically promise to be direct band gap group IV semiconductors.

Chair: Christoph Buchal 
Wednesday Morning, December 3, 1997 
Simmons (M)

10:30 AM *H10.1 
VAPOR PHASE EPITAXIAL LIFTOFF OF GaAs. Wei Chang, Guido A. Pike, Chung-Pao Kao, Eli Yablonovitch, University of California, Electrical Engineering Dept., Los Angeles, CA.

A modified Epitaxial Liftoff (ELO) technique which deposits GaAs thin film devices onto Si circuitry is reported. The separation of GaAs thin films from their mother substrate and subsequently transfer and bonding onto new host substrates has drawn great attention in recent years due to potential applications in the field of semi-monolithic opto-electronic integrated systems (OEIS). One of the major applications being investigated is the integration of GaAs and Si. Such an integration will enable the design of high performance systems that utilize the best attributes of both Si and GaAs technologies. In recent years, research has focused on ELO technique to integrate GaAs thin films with Si substrates while maintaining the ultimate performance of both. The essence of the ELO process is the manipulation of thin films and their transfer back-and-forth between different substrates and support carriers. This is done by using a variety of thin films supporting materials (e.g. Apiezon black wax, polyimide), selective etching acids (e.g. hydrofluoric acid, HF), and selective organic solvents, which release one layer but leave another layer unaffected. We report the development of a new manufacturable ELO and bonding technique. Arrays of discrete GaAs thin film light emitting diodes (LED's) are separated from their substrates by HF vapor exposure of etching channels defined in a thick photoresist spacer layer. The sapphire disk is used as a thin film carrier during the liftoff and transfer processes. The thin films are then "flipped" and bonded onto a Si wafer coated with a thin layer of polyurethane. This integration process presents several advantages; first, black wax is eliminated and replaced by a more robust and transparent sapphire carrier. In addition, it can be reused for further ELO process; second, it improves not only the robustness of ELO process but its efficiency; third, this process has great potential for full wafer-scale integration.

11:00 AM *H10.2 

Active Silicon integrated Optical Circuits (ASOCTM) is a technology based on single-mode rib waveguides formed on silicon-on-insulator that is being used to manufacture commercial integrated optics components. Silicon waveguides have excellent properties for many applications in the 1.3 and 1.55 micron telecommunications bands that include, for example, low loss (<0.2dB/cm) and relatively small birefringence (<10-3). An important aspect of ASOCTM technology is the development of a set of waveguide-based elements that can be assembled into practical integrated optics devices. The fundamental waveguide elements include bends, couplers and fiber-waveguide interfaces. More sophisticated waveguide elements that have been developed include free-carrier modulators and waveguide gratings. Another important aspect of ASOCTM is the incorporation of discrete lasers and photodetectors to form hybrid devices. ASOCTM elements are being used to manufacture devices for applications in telecomunications and optical sensing. A major product is a bi-directional optical transceiver, where the waveguide connects the laser, detector and fiber to form a compact device that is used in optical access networks. The most recent version of this device includes a wavelength dependent element to allow the device to transmit on 1550nm and receive at 1310nm and vica versa.

11:30 AM H10.3 
AMORPHOUS SILICON BASED WAVEGUIDES AND LIGHT MODULATORS FOR SILICON LOW-COST PHOTONIC INTEGRATED CIRCUITS. Giuseppe Cocorullo, Francesco G. Della Corte, Ivo Rendina, Istituto Ricerca Elettromagnetismo e Componenti Elettronici - CNR, Naples, ITALY; Alfredo Rubino, Ezio Terzini, ENEA-Centro Ricerche Portici, Naples, ITALY.

The integration of optics and electronics on the same substrate is a challenge that must be won for lowering the cost of optical interconnection systems. Silicon is considered an ideal candidate for integrated optoelctronics at the infra-red communication wavelenghts of 1300 and 1550 nm. However is strong the call for passive and active optical devices readily integrable inside standard microelectronic circuits. We propose a-Si:H for the realization of waveguides which can serve also as the base for the fabrication of light modulators. A rib waveguide was defined out of an a-SiC:H(500nm)/a-Si:H(3000nm) stack. This heterostructure was grown on a Silicon wafer by low-temperature PECVD. The step refractive index change between Silicon carbide (undercladding)and a-Si produces a strong confinement of the radiation. Structure ranging between 4 and 20 m in width were defined by CF4 plasma etching. The absorption losses have been measured to be less than 2 db/cm. This interesting result was obtained thanks to the advances made in the deposition technology of a-Si:H, allowing today to maintain a low density of states in the gap. Fabry-Perot interferometers were realized by cleavage of these structures. Light amplitude modulation was obtained by thermo-optic effect, i.e. through a shift of the cavity modes induced by temperature. The result from the complete characterization of the devices will be presented at the Symposium. The analyzed structure is fully compatible with the processes of the microelectronic industry. For this reason it could represent a rapid and inexpensive mean for an immediate integration of passive and active optoelectronic devices on almost any silicon standard microelectronic chip.

11:45 AM H10.4 
CONTROLLABILITY AND HOMOGENEITY OF OPTICAL PROPERTIES OF THIN POROUS SILICON FILM FOR SUPERLATTICE OPTICAL DEVICES. Shingo Uehara, Takakazu Sato and Tadashi Matsubara, Seikei Univ, Dept of Electrical Engineering and Electronics, Tokyo, JAPAN.

Reflection spectroscopy was applied to study optical thickness and homogeneity of thin porous silicon (PS) layer. Å@Optical thickness of thin PS mono-layer was measured as a function of anodizing current density and time.Å@In the measurement, the difficulty arising from the ambiguous interference fringe order was avoided by applying a criterion taking the PS index dispersion into account. Successive formation of PS layers with different porosity is essential in fabricating superlattice and other optical devices, we therefore measured the optical thickness for the case of multi-layer PS film. A 5-10 % increase in the optical thickness relative to the mono-layer case was observed for a small porosity layer formed under a large porosity layer, while little change was observed for a large porosity layer formed under a small porosity layer. To evaluate homogeneity of the PS layer, reflectance map was obtained by scanning the PS surface by a focused He-Ne laser. The reflectance variation was then converted into optical thickness variation by using the reflection spectrum averaged over the porous area. For the total anodization area of 15 mm diameter, the measured variation was less than 5 % in the area of 12 mm diameter. The anodization electrode configuration was found to have little influence on the homogeneity. Bragg reflectors were designed and fabricated by using the data above. The realized characteristics (center frequency, bandwidth etc.) were in good agreement with the designed values.

Chair: Alan Seabaugh 
Wednesday Afternoon, December 3, 1997 
Simmons (M)

1:30 PM *H11.1 

Photonic band gaps (PBG's) are essentially multidimensional dielectric superlattices which forbid the propagation of photons with certain energies. PBG's may allow the control of spontaneous emission in active opto-electronic devices such as lasers and LED's. Since the first demonstration of a PBG at microwave frequencies, it has proved technologically difficult to create a PBG in the optical region of the spectrum because the lattice pitch is of the order of half the stop band wavelength. Following the demonstration of a waveguide device with a polarisation selective band flap at 633nm in the visible red region of the spectrum, we now report some extremely unusual and unpredicted optical effects which introduce a host of new applications for PBG's as passive devices in optical computing and WDM communication systems. Our devices consist of a triangular array of air holes plasma etched through the layers of a planar semiconductor waveguide structure. In the design of the devices we use a three-dimensional plane wave analysis in combination with conventional waveguide theory to ensure the existence of guided Bloch modes at wavelengths outside the bandgap. Unlike previous waveguide devices they are fabricated from a relatively low index material (silicon nitride) which cannot support a polarisation insensitive PBG. Our devices conclusively demonstrate that the well known 'in plane' polarization dependance of a nominally two-dimensional lattice, extends to three dimensions. In addition, we have extended nanofabrication technology to fabricate unusual waveguide designs which overcome restrictions on PBG bandwidth imposed by unfavourable waveguide boundary conditions. Although device dimensions are below the Rayleigh resolution limit, (which precludes normal diffraction) we observe effects such as wavelength selective beam splitting and broadband polarisation selective reflection. We suggest applications for PBGs as monolithically integrated WDM demutiplexers, bi-directional wavelength selective cross connects and polarisation selective routers.

2:00 PM H11.2 
THE PROPERTIES OF SILICON CLUSTERS IN ZEOLITE. Masahiko Hirao, Shinya Muramatsu, Univ. of Tokyo, Dept. of Chemical System Engineering, Tokyo, JAPAN.

Nanometer-size silicon clusters show photoluminescence, but some technique to embed them in bulk materials are needed for practical device applications. Thus we calculated the properties of silicon clusters in zeolite, whose framework composes such large cages as the clusters. We chose hydrogenated silicon cluster Si10H16 embedded in dehydrated all-silica zeolite-A. We first performed molecular dynamics simulations using simulated annealing technique to obtain a stable structure. A thermodynamically stable structure was yielded with a negative stabilization energy. The cluster was located near the center of the cage. Then we calculated the electronic structure and optical properties of this stable structure by density functional method. Absorption spectrum of this material was nearly equal to the superposition of those of Si10H16 and zeolite-A, showing that the properties of silicon clusters are retained even in zeolite cages. These results suggest that silicon clusters in zeolite may have an potential to be used as optoelectronic materials.

2:15 PM H11.3 
STUDY OF THE Si/ZnS MULTILAYER SYSTEM FOR OPTOELECTRONIC APPLICATIONS. L. C. Lew Yan Voon, Physics Dept, Worcester Polytechnic Institute, Worcester, MA.

One of the current major thrusts in Si-based technology is to integrate Si with a high barrier material. An example system which is approaching maturity is Si/ZnS. In this work, we focus on the physical properties which are relevant to the design of optical and tunneling devioes. We compare the advantages and disadvantages of the Si/ZnS system with respect to the Si/Ge system. Results to be presented include: accessible wavelengths for interband and intersubband devices, directness of the band gap, optical strengths of the transitions, and effective masses. Preliminary results indicate that very thin Si layers (< 20 Åare required for interband light-emitters (with a small gain for interband lasing) and thin ZnS layers are required for tunneling devices.

2:30 PM H11.4 
POROUS SILICON AS A SACRIFICAL MATERIAL FOR MICROMACHINING OF SILICON OPTICAL PLATFORMS. Mohammed Guendouz, Pierre Joubert, Groupe de Microélectronique et Visualisation, Université de Rennes, Lannion, FRANCE; Nicole Pédrono, Jean Le Rouzic, France-Télécom, CNET, Lannion, FRANCE.

Silicon substrates are available in large area and can be machining down to several microns. Owing to its excellent mechanical properties and its low cost, silicon may be a good candidate for optical devices coupling applications and to provide platforms for hybrid optoelectronic integration. Several processes were previously investigated in order to put and to line optical fibers on single-crystalline silicon substrate. In this work, we used an electrochemical etching method which consists in a first step of porous silicon formation in located areas delineated by using photolithography. Then, the porous-silicon sacrificial layer is selectively removed in a KOH chemical solution, leading to well defined grooves (125 m wide and 62.5 m deep). This unexpensive and accurate technique gives advantages as isotropic formation which does not depend on the crystallographic orientations contrary to any others chemical etching techniques. In this work we used photoresist and/or metallic masking layers with several apertures. We present and discuss some results on porous silicon formation through the openings of the masking layer, i.e., etching rate, groove shape, undercutting and dimensions accuracy as a function of anodization parameters and doping of the Si substrate. We show that the shape of the grooves obtained by the sacrificial Si-porous method is convenient to position single-mode optical fibers on a silicon optical platform with a good accuracy.

Chairs: Salvatore Coffa, John E. Cunningham, Albert Polman and Richard Soref 
Wednesday Afternoon, December 3, 1997 
3:00 P.M. 
Tufts (M)

THE ORIGIN OF LIGHT EMISSION FROM POROUS SILICON. D. Kovalev, H. Heckler, B. Averboukh, M. Ben-Chorin*, M. Schwartzkopff, F. Koch, Tech. Univ. Munich, Physics Dept., Garching, GERMANY; *Weizmann Institute of Science, Dept. of Chemical Physics, Rehovot, ISRAEL.

The origin of the light emission from porous silicon is a subject of a long debate. We report on luminescence hole burning experiments that provide a clue for the mechanism of the photoluminescence of porous silicon. A HeCd laser excites the red luminescence band. An intense resonant pump beam is used to suppress the luminescence, by introducing an Auger non-radiative recombination. We find that about 80% - 90% of the total light emission is suppressed by the presence of the pump beam. Furthermore, the spectrum of this hole, burnt in the luminescence band, has two well defined onsets related to the TO momentum conserving phonons of Si. At low temperatures the hole persists for hours. An increase of the temperature heals the spectral hole and restores the initial photoluminescence spectrum. During the heating process a thermoluminescence signal is observed, which also exhibits the TO phonon steps in its spectrum. All these results allow us to conclude that most of the luminescence of porous Si arises from radiative recombination between states confined inside the nanocrystals.

OPTICAL PROPERTIES OF AS-ANODIZED AND PASSIVATED 6H AND 4H POROUS SILICON CARBIDE. Jonathan E. Spanier* and Irving P. Herman, Columbia Radiation Laboratory and Department of Applied Physics, Columbia University, New York, NY.

We present the results of recent studies of the optical properties of porous silicon carbide (PSC) formed by the electrochemical dissolution of silicon carbide substrates of both 6H and 4H polytypes. The passivation of PSC has been accomplished using a short thermal oxidation, shown previously to markedly enhance the photoluminescence (PL) efficiency and shorten the PL wavelength as-anodized PSC and bulk SiC[1]. In this investigation, Fourier Transform Infrared (FTIR) reflectance spectroscopy and PL (at different temperatures) are employed at different stages of the thermal oxidation process. In particular, the characteristics of the reststrahlen band normally seen in bulk SiC are altered and are found to depend on the porosity of the PSC film. Changes in the FTIR and PL spectra with porosity and oxidation time are examined as a possible means of determining the origin of the enhanced PL, as well as the oxidation rate and the nature of the oxide film formed in PSC.

PHOTO- AND ELECTRO-LUMINESCENCE FROM DEUTERIUM TERMINATED POROUS SILICON. Takahiro Matsumoto, Takahiro Matsumoto, Yasuaki Masumoto, Single Quantum Dot Project, ERATO, JST, Tsukuba, JAPAN; Nobuyoshi Koshida, Tokyo University of Agriculture and Technology, Koganei, JAPAN.

We will report the optical and the electrical properties of deuterium terminated porous Si (D-PS). The structural characterization by Raman spectroscopy shows that the nanometer crystalline structure of D-PS is almost the same as that of H-PS except surface termination. Although both PS show the same optical absorption spectrum, the photoluminescence spectrum of D-PS was different from that of H-PS. These results indicate that the surface vibration of terminated atoms couples to the excited energy states of quantum-confined carriers. In addition to these basic results, we have found that the replacement of hydrogen with deuterium greatly reduces the degradation of photoluminescence [1]. We will also introduce the electrical properties of light emitting diodes based on D-PS to stabilize the electroluminescence. The luminescence degradation-mechanism will be discussed from the point of a photoinduced oxidation model.

THE OXIDATION BEHAVIOR OF SILICON NANOCRYSTALS IN THE SUBMONOLAYER REGION. J. Diener*, M. Ben-Chorin, D. Kovalev, G. Polisski and F. Koch, *Department of Physics, University of California, Berkley, CA; Technische Universitat Munchen, Physik-Department, Garching, GERMANY.

Luminescing porous silicon (Si), produced from bulk Si by electrochemical etching in ethanoic HF solution, is a highly disordered material with a hugh surface to volume ratio (300-1000 m2/cm3). Usually, with porosities in the region of 70%, a major part of remaining Si atoms are surface atoms ( 30%). Directly after the etching procedure the main part of the surface is hydrogen (H) passivated, covered by Si-Hx species (x 1,2,3). Stored in ambient conditions oxygen is gradually incorporated into the specimen. The importance of this process for the photoluminescence properties of porous silicon manifests itself in a significant increase of the quantum efficiency during this process. We will focus hereby on the formation of the first monolayer of oxygen compounds on the surface of the Si nanocrystals. Based on FTIR measurements we will construct a microscopic picture for this process. We will show that the dynamic of this oxidation is, even in the submonolayer region, well described by that of a diffusion limited process, similar to bulk Si or mesoporous Si. Based on this analysis we will show that annealing at elevate temperatures (200C) allows to control the amount of incorporated oxygen.

DISPERSIVE FOURIER TRANSFORM SPECTROSCOPY OF FREE-STANDING POROUS SILICON FILMS. J. Salonen*, K. Saarinen, J. Peura, J. Viinikanoja, I. Salomaa, E. Laine*, J. Kauppinen, Univ of Turku, *Dept of Physics and Dept of Applied Physics, Turku, FINLAND.

We have investigated optical constans of free-standing porous silicon films by dispersive Fourier transform spectroscopy (DFTS)[1-3] in NIR-VIS range. This allows the spectral variation of both the absorption coefficient and the refractive index of a material to be determined from the measurements of the attenuation and phase shift imposed on an electromagnetic wave by its interaction with a specimen. Using these optical constants, i.e. complex refractive index, we have studied the complex dielectric function and the complex conductivity. The corresponding surface resistance and penetration depth spectra were also calculated. To avoid the additive error in the absorption spectrum arising from pseudocoherence [4],we measured the transmission spectrum by conventional Fourier transform spectroscopy (FTS). Using the refraction spectrum derived from the DFTS measurements, we have corrected for reflection losses in calculation of the absorption spectrum from the FTS transmission spectrum. The dielectric constant can be expressed in terms of the polarizability of the medium and while the free carrier concentration (cm-3)is known, one can estimate an average effective mass [5]. The knowledge of that coupled to conductivity permits a calculation of the carrier relaxation time. The changes in the absorption coefficient and the refractive index due to oxidation, which is the most common aging phenomenon in porous silicon [6], have been studied using samples with different types of oxidation.[2]

INFLUENCE OF PROLONGED ETCHING IN HF AND STORAGE IN THE AIR ON THE POROUS SILICON PHOTOLUMINESCENCE. V.A. Makara, M.S. Boltovets, O.I. Datsenko, T.D. Kotikova, S.M. Naumenko, T.V. Ostapchuk, O.V. Rudenko, and O.V. Vakulenko, Taras Shevchenko Kyiv University, Dept of Physics, Kyiv, UKRAINE.

Studies in evolution of porous silicon (PS) properties in the process of etching in HF and the further prolonged storage of PS in the air at the day-time were undertaken with an effort to find out the effective methods to increase the PS photoluminescence intensivity (PL). We have employed PS formed by the electrolytic anodizing in aqueous solutions of HF and KDB-10 single crystal silicon of orientation[111]. The integral intensity of PL(IFL) excited by the pulse nitrogen laser (337 nm) was measured. In the process of etching the samples were taken out at regular intervals from HF from measuring IFL. The samples were dried using CCL. It is found that the behavior of IFL with the etching time t depends on the initial value of PLIg. At Ig=0 the dependence IFL(t) is of dome-like character. At high values of Ig(the quantum yield >10%)IFL decreases sharply in time down to small values and for the samples with the quantum yield <5% the value of IFL changes slightly on etching of samples for 3 hours. The investigations of IFL changes in the process of storing samples in the air at the day-time have shown that IFL increases in the time. the growth rate depending on the etching time in HF. It increases on etching for 6 hours and decreases on further etching. The transformation of PL spectra during investigations and quantitative chemical analysis of PS permitted arriving at the conclusion about the existence of radiative recombination centres of different origin. In this case the contribution of these centres to the PL spectrum is determined by the prehistory of PS samples. The chemical treatment in HF and again in the air influence actively the state of recombination centres. According to the above investigation it is possible to propose effective methods for an increase of the PS radiating capacity and stability.

COMPLEX STUDIES OF EXCITATION MECHANISMS OF POROUS SILICON PHOTOLUMINESCENCE. Moissei Sheinkman, Tatiana Torchinskaya, Nadezhda Korsunckaya, Berdishukur, Dzhumaev, Larisa Khomenkova, Photoelectronics Department of Institute of Semiconductor Physics, Kyiv, UKRAINE.

The investigation of aging phenomena of porous silicon (PS) layers is practically important to understand the photoluminescence (PL) origin of PS and consider the potential application of the PS material for fabrication of light emitting silicon devices integrated within the core of the silicon technology. We investigated the dependencies of the PL and photoluminescence excitation (PLE), FTIR and SlMS spectra of PS upon the aging time in different conditions (atmospheres and temperatures). It has been found that the PLE spectrum of as-prepared PS consists of the visible band (400-520 nm) and the steep rise In ultraviolet (UV) region. During aging at the room temperature the variation of PL intensity is characterized by two stages. At the first one the visible band in PLE spectrum decreases whereas the UV band changes slightly. At the second stage the UV band increases only. The dependence of aging process kinetics upon the atmosphere indicates that the first aging stage is connected with desorption of some surface species from PS layer, whereas second state is due to oxidation. The FTIR and SIMS results confirm these conclusions. As FTIR studies show desorption of CH, OH or H2O complexes take place during aging mainly. We suppose that the visible PL excitation in as-prepared PS is connected with the light absorption by the some entities (probably H2O or OH) adsorbed on the Si wire surface and following transfer the excitation to the luminescence centers in silicon oxide. The UV excitation is due to direct light absorption by silicon oxide.

SPECTROSCOPIC CHARACTERISATION OF POROUS SILICON EMBEDDED WITH CdSe SEMICONDUCTOR COMPOUND. A.I. Belogorokhov, Institute of Rare Metals, Leninsky prosp., Moscow, RUSSIA; L.I. Belogorokhova, Moscow State University, Moscow, RUSSIA.

The formation of the ohmic contacts to a porous medium such as porous silicon (PS) is, still a challenging problem. There is experimental evidence that this problem can be solved by embedding the PS with a II-VI semiconductor. Here we investigate photoluminescence (PL) and infrared (FTIR) properties of these new porous materials Despite extensive efforts, the nature of the visible PL from PS, whether due to the spatial and quantum confinement in the Si crystallites, to surface states, or to various Si compounds, is still intensely debated. In this work, an attempt is made to reveal the relationship between the PS microstructure, PL and FTIR properties and the surface chemical composition, especially the changes in hydrogen and oxygen termination of pores. We compare the PL and FTIR spectra of the pure PS area of samples with those obtained from the embedded areas on the same wafer. The results are consistent with the stabilization of the PL peak in the case of CdSe in spite of the different PL peak positions of the pure PS. PL spectra of the PS were examined as function of laser irradiation time and laser intensities.

NANOSIZED LIGHT EMITTING SILICON IN POROUS ALUMINA MEMBRANES. Andreas Heilmann, Michael Kroell, Thomas Sawitowski, Guenter Schmid, Universitaet Essen, Institut fuer Anorganische Chemie, Essen, GERMANY; Peter Jutzi, Universitaet Bielefeld, Fakultaet fuer Chemie, Bielefeld, GERMANY.

Since the discovery of the luminescence of anodized porous silicon many researchers focus on developing nanostructured silicon. This includes the formation of new type of nanosilicon compounds. These compounds are formed by means of anodizing silicon wafers as well as CVD processes on different surfaces. Experiments were also performed on generating silicon colloids in micro- and mesoporous hosts like zeolites. One important goal of those experiments is the ability to achieve different luminescence wavelengths by engineering size and band gap of those silicon compounds. In this paper we present the formation of silicon nanostructures in meso- and macroporous alumina which show luminescence in the UV/visible range. These structures are generated by means of CVD in porous alumina membranes. The membranes are produced by anodic oxidation of high purity aluminum plates in diprotic acids and provide an excellent template while the diameter of the pores is proportional to the applied voltage during anodizing. Membranes with pore diameters from 20-200nm and thickness varying from 40-80m are used in these experiments. CVD was performed by two steps: vacuum filling of the pore with an appropriate silane, thermal decomposition of the silane in the pores. By means of this process layers of silicon are formed on the pore walls which is proved by HREM, IR and photoluminescence studies. While the layer thickness of the silicon is a function of the pore size, luminescence can be varied in the range from 400nm up to 750nm.

ELECTRONIC STRUCTURES OF HALOGENATED POLYSILANES. Hideshi Motoyama, Kyozaburo Takeda, Waseda Univ, Dept of Materials Science and Engineering, Tokyo, JAPAN; Kenji Shiraishi, NTT Basic Research Laboratories, Kanagawa, JAPAN.

The electronic structures of halogenated polysilanes (PSi), whose side chains are replaced by halogen (X) atoms, have been theoretically investigated based on the first-principles calculations. It is found that non-bonding (n) electrons localizing at X atom cause an important orbital mixing with the electrons of the valence band delocalized in the direction of the PSi skeleton (-n mixing). This -n mixing splits the top of the valence bands, and creates unoccupied states in the band gap. This valence-band-splitting effectively narrows down the band gap to a visible range, and the unoccupied state evident in the band gap has the potential to produce a conducting nature. Moreover, we also investigated several replacement patterns as well as the kind of replaced halogen atoms(X=F, Cl, Br, I). The dispersion and energy position of the unoccupied state(s) can be artificially tuned by these chemical modifications. Thus, halogenation can changes PSi into optelectronic polymers with visible photoluminecence.

Chairs: Salvatore Coffa, John E. Cunningham, Albert Polman and Richard Soref 
Wednesday Afternoon, December 3, 1997 
3:00 P.M. 
Tufts (M)

OPTICAL CHARACTERIZATION OF As2Te3 FILMS FOR OPTICAL INTERCONNECTS. Tatiana Globus, Dept of Electrical Engineering, Univ of Virginia, Charlottesville, VA; D. Kurt Gaskill, Naval Research Laboratory, Washington, DC; Tom Groshens, Naval Air Warfare Center, China Lake, CA.

The dielectrics As2Te3 and related materials are potentially useful for applications in optical connect and imaging technology. However, information regarding optical characteristics of these thin film materials are sparse or not available. In this paper, we present results absorption coefficient and refractive index measurements of stoichiometric As2Te3 thin films grown on Si and KBr substrates. The innovative growth process uses organometallic sources for both the As and Te which react at reduced pressures to produce stoichiometric films in the presence of a surface at very low temperatures; no growth occurs otherwise. Films were grown at 300 and 373 K. The film thicknesses were about 0.2-1.2 µm. Optical characteristics were recorded in the spectral range 0.8 to 2.2 µm. Since the thickness of substrate was 2-3 orders of magnitude greater than film thickness, bands with small absorption coefficients in the substrate completely masked the absorption from As2Te3 film. Conventional techniques based on measurements of optical transmission did not work in this case. To circumvent this difficulty, the interference technique was utilized which permitted us to separate the effects of absorption in the film from that of the silicon substrate and extract information on As2Te3 absorption even in the range of noticeable absorption in Si. The results of these measurements have been compared with data obtained from ellipsometry taken in the visible and UV. Our results confirm that the As2Te3 formed in this study behaves as indirect bandgap semiconductor, being either amorphous or partially crystallized, with an optical gap obtained from Tauc plots in the range 0.83-0.9 eV, close to earlier results. The refractive index has strong dispersion near the edge, and rapidly changes from the value of 3.8 at the wavelength of 1.5 µm for shorter wavelengths.

COMBINED IN-BUILT ELECTRONIC AND OPTICAL INTERCONNECTS FOR INTEGRATED CIRCUITS. S. Lazarouk, P. Jaguiro, V. Borisenko, Belarusian State University Informatics and Radioelectronics, Minsk, BELARUS.

We have developed design and technology to produce in-built aluminum/alumina interconnects by electrochemical aluminum anodic oxidation. Aluminum lines appear to be in-built in its own anodic oxide. They demonstrate advantages as compared to the conventional metallization formed with etching techniques. First of all, in-built interconnects have a planar structure. Secondly, anodic alumina layers can be used as an optical waveguide between porous silicon based light emitting diode and photodetector. The developed technology provides submicron element sizes and is compatible with existing silicon technology. Thus, electronic and optoelectronic interconnects are shown to be formed on the same silicon substrate by standard technological operation.

STRAIN EVALUATION OF GaAs LAYERS GROWN ON ULTRAHIGH - PRESSURE - ANNEALED STRAIN - FREE GaAs - ON - Si STRUCTURES. Takehito Jimbo, Hiroshi Ishiwara, Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, JAPAN.

We have reported a strain-free GaAs-on-Si structure by annealing under ultrahigh pressure (UHP) around 2.0 GPa [1]. In this method, the difference of thermal expansion coefficients in a heteroepitaxial structure can be compensated by the elastic strain produced by hydrostatic pressure. We have also reported from X-ray diffraction (XRD) analysis and photoluminescence (PL) measurements that the residual strain of the GaAs film reannealed at atmospheric pressure is smaller at the surface than the average value throughout of the GaAs film [2]. In this work, additional GaAs layers were grown using molecular beam epitaxy (MBE) on strain-free GaAs-on-Si structures and the residual strain in the layers was evaluated. The strain-free GaAs-on-Si structure used in this work has a 3-m-thick GaAs layer which was grown by MBE at C on -off Si(100) substrate. The structure was first annealed at C for 20 min at around 2.0 GPa to decrease the residual strain. It was found that the initial residual strain of was reduced to less than by the UHP annealing. Then, a 500-nm-thick GaAs layer was grown on this strain-free GaAs-on-Si structure at C. It was found that the residual strain measured from XRD analysis was about , whereas the strain evaluated from PL measurements was at most . This result shows that the surface strain is smaller than the average value even in the case that an additional GaAs layer is grown on a strain-free GaAs-on-Si structure, which has been formed by annealing under ultrahigh pressure.

INTERPLAY BETWEEN PLANAR DEFECTS AND THREADING DISLOCATIONS IN GaAs-on-Si. A. Delimitis, Ch.B. Lioutas, J. Stoemenos, Aristotle University of Thessaloniki, Physics Department, Thessaloniki, GREECE; K. Michelakis, A. Georgakilas, FORTH, IESL, Microelectronics Research Group, Crete, GREECE and University of Crete, Physics Department, Crete, GREECE.

Heteroepitaxial layers of GaAs and related compounds on Si substrates may provide all kinds of optoelectronic devices required for Si-based optoelectronics. However, a necessary prerequisite is the minimization or elimination of crystal defect densities in this material and this task requires to gain further insight into crystal defects and develop new schemes to reduce their density. In this paper we will report on the interrelation between planar defects and threading dislocations in GaAs on Si layers. We have investigated by planar and cross-sectional TEM observations the structure of GaAs-on-Si layers grown on off-oriented (100) Si substrates with various surface plane tilting angles. No planar faults but a threading dislocation density in the mid 108cm-2 range appears in 2 m as-grown films on moderate angle values, i.e. 4.5. The dislocation density is minimized significantly for tilting angles below 1 and no dislocations were observed for 0.5 angle. However, planar defects such as stacking faults, antiphase domain boundaries and microtwins start appearing with increasing density as the angle reduces below 1.5. These observations indicate that planar defects are absorbing efficiently, during growth, the initially created threading dislocations. We propose that proper control of heteroepitaxial process parameters such as the substrate off-orientation tilting angle, MBE growth conditions epilayer thickness and post growth annealing may be able to eliminate defects in device active regions.

III-V/Si INTEGRATION BY COMPLIANT SUBSTRATE ENGINEERING. M.F. Stumborg, F. Santiago, T.K. Chu, and K.A. Boulais, Naval Surface Warfare Center- Dahlgren Division, Dahigren, VA.

Compliant Substrate Engineering (CSE) has been used to fabricate lattice mismatched heteroepitaxies. In our approach to CSE the substrate surface is chemically modified, resulting in a monolayer scale "template" layor able to accommodate the stress induced by the lattice mismatch. Successful examples include PbTe and CdTe epitaxial films grown on Si(100).