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 F—Infrared Applications of Semiconductors II



M. Manasreh, USAF Phillips Laboratory
D. McDaniel, USAF Phillips Laboratory
Richard Miles, SDL Inc
Siva Sivananthan, Univ of Illinois-Chicago

Symposium Support 

  • Air Force Office of Scientific Research
  • Air Force Research Laboratory (PL/LIDA)
  • Air Force Research Laboratory(PL/VTMR)

Proceedings published as Volume 484 
of the Materials Research Society 
Symposium Proceedings Series.

* Invited paper

Chairs: Michael Prairie and C. H. Grein 
Monday Morning, December 1, 1997 
Salon J/K (M)

8:30 AM *F1.1 
MATERIALS FOR MID-INFRARED SEMICONDUCTOR LASERS. Alan Kost, Hughes Research Laboratories, Malibu, CA.

A variety of semiconductor materials have been used to fabricate diode lasers for the mid-infrared. Lasers using the Pb-salts (e.g. PbSnTe) have been commercially available for some time. Mid-infrared emitting III-V semiconductors (e.g. InGaAsSb) have superior thermal conductivity and diode lasers fabricated from these materials offer higher powers. Of particular interest are the III-V semiconductor lasers based on Type-II superlattices (e.g. InAs/GaInSb). Among the many unique properties attributed to the Type-II superlattices are small hole mass, reduced Auger recombination, and less intervalence band absorption - all important for better laser performance. This presentation will summarize the important semiconductor materials for mid-infrared lasers with particular emphasis on the Type-II superlattices. Studies of recombination mechanisms will covered as well as recent device results.

9:00 AM F1.2 
XSTM CHARACTERIZATION OF GASB/INAS AND GASB/ALSB INTERFACES: WHITE NOISE AND GROWTH-ORDER-DEPENDENT ROUGHNESS ASYMMETRY. J. Harper and M. Weimer, Department of Physics, Texas A&M University, College Station, TX; D. Zhang, C.-H. Lin and S.-S. Pei, Space Vacuum Epitaxy Center, University of Houston, Houston, TX.

The quality of the interfaces between the nearly-lattice-matched 6.1 Å materials (InAs, GaSb, and AlSb) is important for a number of applications, including the development of mid-IR lasers, long-wavelength photodetectors, and resonant-tunneling devices. We present new results concerning the wave-vector-dependent roughness at the GaSb/InAs and AlSb/GaSb interfaces, the latter of which is distinguished by a common anion within the adjoining layers. These interfaces are formed in GaSb/InAs/AlSb/GaSb heterostructures grown by molecular beam epitaxy (MBE) and subsequently examined with cross-sectional scanning tunneling microscopy (XSTM). Previous XSTM studies of antimonide-based heterojunctions have shown that the interface roughness spectrum is essentially Lorentzian [1,2]. We have observed a white-noise component in the filled-state roughness spectrum of the GaSb/InAs interface that is associated with the presence of point defects within the constituent materials; these defects arise from the random incorporation of As at Sb sites in the GaSb layers, and Sb at As sites in the InAs layers, during MBE growth. A comparison of the empty-state roughness amplitudes and correlation lengths at GaSb/AlSb and AlSb/GaSb interfaces reveals a cation-mediated, growth-order-dependent interface asymmetry that contrasts with the apparently anion-mediated asymmetry previously noted at GaSb/InAs interfaces [1,2]. We find that the GaSb/AlSb interface is atomically abrupt whereas the AlSb/GaSb interface is characterized by Ga penetration into the AlSb layer. This observation agrees with expectations based on Raman studies of the GaAs/AlAs system [3].

9:15 AM F1.3 
THE GROWTH OF TYPE-II INFRARED LASER STRUCTURES. M. J. Yang, W. J. Moore, B. R. Bennett, B. V. Shanabrook, and J. O. Cross, Naval Research Laboratory, Washington, DC.

We have studied the MBE growth of type-II heterostructures for mid-wavelength infrared (IR) lasers. The samples, grown on GaSb substrates, consist of a 1 m AlSb buffer layer, a 20- to 35-period superlattice, a 0.2 m AlSb and a 100 GaSb capping layer. The superlattice, comprising n-ML InAs / 10-ML InGaSb / n-ML InAs / 14-ML AlSb with 5 n 8, is grown at different temperatures and with different interfacial bonds. Layer thicknesses are calibrated by RHEED oscillations and x-ray diffraction. Based on the photoluminescence (PL) and x-ray diffraction, it is found that the quality of these heterostructures is highly sensitive to the growth temperature. The PL intensity is one order of magnitude stronger for a superlattice grown at 400oC as opposed to 450oC, where the growth temperature is estimated by assuming that the 15 to 13 RHEED phase transition of GaSb occurs at 380oC under 1.5 ML/sec Sb2 flux. In addition, the PL peak is 35 meV lower for the superlattice grown at 400oC than that at 450oC with an identical layer thickness. These findings suggest substantial layer intermixing at interfaces when the growth temperature is 450oC. A more accurate IR transmission substrate temperature monitor is being installed and results will be reported. In addition, it is possible to have different bonding between two layers when the adjacent materials have neither common cations nor anions. For instance, InSb-like or GaAs-like interfacial bonds can be formed between InAs and GaSb depending on the growth sequence. We find that samples with InSb-like interfacial bonds have sharper interfaces, narrower PL linewidths, and 10 times stronger PL intensity than samples with GaAs- and AlAs-like interfacial bonds. The effects of growth interrupts at different interfaces also will be reported.

9:30 AM F1.4 
X-RAY DIFFRACTION STUDY OF SB SEGREGATION IN MOVPE GROWN INASXSB1-X/INAS MULTI-QUANTUM WELLS. Manoj R. Pillai, Scott C. Theiring, Scott A. Barnett, Bruce W. Wessels, Department of Materials Science and Engineering, Northwestern University, Evanston, IL.

X-ray diffraction (XRD) was used to study the composition modulation in bi-axially compressed InAsxSb1-x/InAs strained layer multi-quantum wells grown by metalorganic vapor phase epitaxy (MOVPE) on InAs (111) substrates. Multi-quantum well periods were Å, with a nominal InAsxSb1-x thickness of 43 Å. The alloy compositions were varied from 0.40 < x < 0.95 to study the effect of strain on the structural quality of these multi-quantum wells. The experimental XRD patterns were simulated using a kinematical calculation with an exponential composition profile as expected for Sb segregation. The interfacial broadening due to the Sb segregation extended over 10-15 Å for most of the structures. Roughness was simulated by using random fluctuations in the thickness of each layer and defects by random fluctuations in d-spacing. The interface roughness of the highest quality InAs0.8Sb0.2/InAs structures were 1 Å. The maximum amount of surface segregated Sb was monolayer. We will also report on the effect of predeposition of Sb on the interface widths. Initial studies with 0.2 monolayer of Sb predeposited before the start of the ternary layer showed an apparent decrease in the segregation length.

9:45 AM F1.5 
RHEED STUDY OF OXIDE DESORPTION OF INSB. Jie Li, S. Iyer and S. Venkatraman, Department of Electrical Engineering, North Carolina A&T State University, Greensboro, NC.

In molecular beam epitaxy (MBE) normally oxide desorption of InSb is carried out at elevated temperatures in the presence of Sb flux for extended period of time, typically few hours. The results obtained by different workers as observed by reflection high-energy electron diffraction (RHEED) have not been consistent. Our work indicates that the sequence of introducing the Sb flux is critical in the oxide process. We report on a systematic study of the effect of Sb flux on the oxide desorption of InSb using RHEED and demonstrate a new expeditious and reproducible method. The desorption of oxide in the presence of Sb flux during the entire process always resulted in diffraction rings which could not be removed and yielded a rough surface. On introducing the Sb flux prior to the desorption, additional RHEED streaks were observed, which could be eliminated only after hours of annealing. Desorbing the oxide at 440C in the absence of Sb flux resulted in the formation of In droplets on the surface, which could however be completely removed by the Sb flux. This procedure produced a clean and smooth surface as evidenced by the observation of bright RHEED pattern and Nomarski Phase Contrast microscope, respectively. X-ray rocking curves obtained on InSb layers grown by MBE on such surfaces exhibited full width half maxima close to 13 arc sec reproducibly, attesting to the excellent quality of the epilayers. Systematic investigation of homo-epitaxial growth on growth parameters will also be reported in this paper.

10:00 AM F1.6 

An advanced metalorganic chemical vapor deposition (MOCVD) turbo disk technology employing a vertical reactor growth configuration with a high speed rotating disk has been developed and applied to the large diameter and multiple wafer growth of Infrared III-V semiconductor materials of InSb, InGaAs, InGaAsP etc. Non-destructive and whole wafer characterization are very necessary for these materials in mass production industry environment. We demonstrate our efforts in establishing a series of techniques, including high resolution X-ray diffraction (HRXRD), Nomarski micrograph (NM), sheet resistivity (SR), contactless C-V, scanning electron microscopy (SEM), optical transmission and reflectance, photoluminescence (PL), Raman scattering and Fourier transform infrared (FTIR) spectroscopy etc. to characterize thin films of InSb, InGaAs and InGaAsP in 50-100 mm (2"-4") diameter wafer scale. A series of mapping distributions of the film thickness, sheet resistivity, surface morphology, and PL features with uniformities better than or within 1-3 are illustrated. Some multiple point measurement results such as Raman features are also used to assess the film crystalline perfection in a complementary manner. Variations of above characteristic features with the growth conditions are discussed. These wafer scale material characterizations were tightly coupled with the epitaxial processes to realize the high quality and high uniformity growth.

10:45 AM *F1.7 

Mid-infrared (3-5 m) infrared lasers and LEDs are being developed for use in chemical sensor systems. As-rich, InAsSb heterostructure display unique electronic properties that are beneficial to the performance of these midwave infrared emitters. We will describe the metal-organic chemical vapor desposition (MOCVD) growth of AlAs1-xSbx cladding layers, InAsSb/InAs multiple quantum well (MQW) and InAsSb/InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. By changing the layer thickness and composition of SLSs and MQWS, we have prepared structures with low temperature (<20K) photoluminescence wavelengths ranging from 3.2 to 6.0m. We have made gain-guided, injection lasers using undoped, p-type AlAs0.16Sb0.84 for optical confinement and both strained InAsSb/lnAs MQW and InAsSb/InAsP SLS active regions. The lasers and LEDs utilize the semi-metal properties of a p-GaAsSb/n-InAs heterojunction as a source for electrons injected into active regions. Novel multiple-stage LEDs and lasers utilizing this semimetal injection scheme will be described. Gain-guided, injected lasers with a strained InAsSb/lnAs MQW active region operated up to 210 K in pulsed mode with an emission wavelength of 3.8-3.9 m and a characteristic temperature of 29-40 K. We also present results for both optically pumped and injection lasers with InAsSb/InAsP SLS active regions. The maximum operating temperature of an optically pumped 3.7 m SLS laser was 240 K. An SLS LED emitted at 4.0 m with 80 W of power at 300 K.

11:15 AM F1.8 
AN EVALUATION OF AlxGa1-xAsySb1-y GRADED-LAYER BUFFER SYSTEMS. David C. Paine and Eric B. Chen, Brown University, Division of Engineering, Providence, RI; John S. Ahearn, Kirby Nichols, and Parvez Uppal, Sanders Lockheed-Martin, Nashua, NH.

The use of GaAs substrates for the fabrication of InAs/ Ga1-xInxSb superlattices and other lattice mismatched structures for IR applications requires the use of buffer-layer systems to reduce threading defects and preserve growth surface planarity. Typical buffer-layer schemes (e.g. InGaAs) modify the Group III content to effect a gradual change in lattice parameter. In this presentation we report on a study of Sb-adjusted quaternary Al0.5Ga0.5AsySb1-y alloy buffer layer systems. A series of 10 structures were grown by MBE at 500C which utilize a multilayer grading scheme in which the Sb content of Al0.5Ga0.5AsySb1-y is successively increased in a series of 100 nm thick layers. Post growth analysis using conventional bright field and lattice resolution TEM of these Al0.5Ga0.5AsySb1-y buffer layers reveals that the growth surface remains planar throughout growth while plan view studies show that the threading dislocation density can be reduced to below the delectability limit of TEM. Spontaneously formed long range compositional modulations laying in the 001 growth direction are present in both heavily defected and defect-free quaternary layers. Satellite reflections in TEM diffraction patterns are found laying in the 001 direction and reveal that the modulations have a period that ranges with the layer composition from 1.7 to 1.9 nm. Compositional modulations in the GaAlAsSb quaternary system have not, to our knowledge, previously been reported but our defect analysis shows that, in this system, they play an important role in the development of low threading defect buffer layers. An analysis of surface kinetic and equilibrium thermodynamic miscibility effects that promote long range ordering in this system and strategies for adjusting the period of these naturally occuring compositional modulations will be presented.

11:30 AM F1.9 
SUBSTRATE MISORIENTATION EFFECTS ON EPITAXIAL GaInAsSb. C.A. Wang, H.K. Choi, and D.C. Oakley, Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA; G.W. Charache, Lockheed Martin Corporation, Schenectady, NY.

GaInAsSb alloys, which can be lattice matched to GaSb and InAs substrates, are of interest for mid-infrared emitters and detectors, and thermophotovoltaics (TPV). In this paper, we report the effect of substrate misorientation on the materials properties of GaInAsSb lattice matched to GaSb substrates. The layers were grown by low-pressure organometallic vapor phase epitaxy (OMVPE) with triethygallium, trimethlyindium, tertiarybutylarsine, and trimethylantimony, and with diethyltellurium and dimethylzinc as n- and p-type dopants, respectively. Epitaxial layers with room-temperature cutoff wavelengths 2 - 2.4 ,m were grown on (100) GaSb substrates with misorientations of 2 or 6 toward (110), (111)A, and (111)B. The growth temperature was 525 - 575C. The surface morphology is mirror-like and featureless for GaInAsSb grown on (100) substrates with a 6 toward (111)B misorientation. However, a slight haze and texture was observed for layers grown on all other misorientations. In addition, the full-width at half-maximum (FWHM) of 4K photoluminescence spectra is reduced for layers grown with a 6 toward (111)B misorientation. For example, we obtained a FWHM of 7.1 meV for Gainassb (4K PL peak energy 0.606 eV) grown on (100) 6 toward (111)B . This is the lowest value that has been previously reported for similar layers grown by OMVPE, and significantly lower than the value of FWHM = 15 meV that we measured for layers grown on (100) 2 toward (110) GaSb substrates. We also describe the effect of substrate misorientation on the n- and p-doping incorporation efficiency in Gainassb grown on semi-insulating GaAs substrates. Furthermore, we discuss the impact of substrate misorientation on the performance of GaInAsSb/GaSb TPV devices.

11:45 AM F1.10 

Thermophotovoltaic generation of electricity is attracting renewed attention due to recent advances in low bandgap (0.5-0.7 eV) III-V semiconductors. The use of mixed binary compounds allows for the tailoring of the lattice parameter and the bandgap of the matetial. Conventional deposition techniques for producing such ternary or quaternary materials are typically slow and expensive. Production of bunk single crystals of ternary materials, for example Gal-xInxSb, is expected to dramatically reduce such material costs. Bulk single crystals of Gal xInxSb have been prepared using a Bridgman technique in a two-zone furnace. These crystals are 19 mm in diameter by approximately 50 mm long and were produced using seeds of the same diameter. The effects of growth rate, seed orientation, crucible material, and other parameters on the composition and quality of these crystals will be discussed and compared with other attempts to produce single crystals of this material.

Chairs: D. L. McDaniel and Alan R. Kost 
Monday Afternoon, December 1, 1997 
Salon J/K (M)

1:30 PM *F2.1 
THEORETICAL PERFORMANCE OF MID-INFRARED BROKEN-GAP MULTILAYER SUPERLATTICE LASERS. Michael E. Flatté, J. T. Olesberg, Univ of Iowa, Dept of Physics and Astronomy, Iowa City, IA; C. H. Grein, Univ of Illinois at Chicago, Dept of Physics, Chicago, IL.

We present calculations of the laser performance of superlattices based on the InAs/InGaSb material system involving more than two layers in the repeating unit cell. Calculations of the differential gain, Auger recombination rate, and threshold current density indicate that superlattices with a single InGaSb layer in the unit cell tend to outperform those with more. The spectral structure of the intersubband absorption, which for some energies near the lasing energy can exceed 500/cm at lasing threshold, must be considered when designing an active region. Fortunately, designs which minimize Auger recombination tend to minimize intersubband absorption as well. We point out the importance of including umklapp processes in the superlattice growth direction in a calculation of the Auger rate. For a 3.7 micron optimized four-layer superlattice, including umklapp changes the calculated Auger rate by over a factor of two. The newly calculated Auger rates are in excellent agreement with measured Auger rates over a wide density range. Calculations of the Auger rate as a function of temperature will also be presented.

2:00 PM F2.2 
AUGER RECOMBINATION IN ANTIMONY-BASED, STRAIN-BALANCED, NARROW-BAND-GAP SUPERLATTICES. J. T. Olesberg, S. A. Anson, D.-J. Jang, M. E. Flatté, T. C. Hasenberg, Thomas F. Boggess, University of Iowa, Iowa City, IA; C. H. Grein, University of Illinois at Chicago, Chicago, IL.

Time-resolved all-optical techniques are used to measure the density and temperature dependence of electron-hole recombination in a four-layer InAs/GaInSb/InAs/AlGaInAsSb strain balanced superlattice grown by MBE on GaSb. This 4-micron band gap structure, which has been engineered for suppressed Auger recombination, is a candidate material for the active region for mid-infrared lasers. Similar structures have been implemented successfully in optically-pumped lasers, including a 3.7-micron device that lased at temperatures as high as 300K and a 5-micron device that lased up to 180K. While carrier lifetime measurements at room temperature show unambiguous evidence of Auger recombination, the extracted Auger rates are considerably lower than those reported for bulk materials of comparable band gap energy. We find that the Auger rate saturates at carrier densities comparable to those required for degeneracy of the valence band, illustrating the impact of Fermi statistics on the Auger process. The measured results are compared with theoretical Auger rates computed using the band structure obtained from a semi-empirical 8-band superlattice Kp model. We find excellent agreement between theoretical and experimental results when umklapp processes in the growth direction are included in the calculation. Measurements of the recombination as a function of temperature reveal that the Auger rate increases with temperature over the range of 50 to 300 K. A comparison of the measured and calculated temperature dependent Auger rates will be presented.

2:15 PM F2.3 
HIGH POWER TYPE-II QUANTUM CASCADE LASERS. B. H. Yang, D. Zhang, R. Q. Yang, C.-H. Lin, S. J. Murry, H. Wu, and S. S. Pei, University of Houston, Space Vacuum Epitaxy Center, Houston, TX.

The development of quantum cascade (QC) lasers based on intersubband transitions in III-V heterostructures represents a significant progress towards realizing high power mid-IR sources. However, the intersubband QC lasers have a low radiative efficiency due to a fast nonradiative relaxation between subbands via optical phonon scattering. Furthermore, the wavelength is limited to longer wavelength due to the small conduction band offset in InAlAs/InGaAs quantum wells. Recently, we demonstrated a new type of QC laser based on interband transitions in InAs/Ga(In)Sb/AlSb heterostructures with a type-II broken gap band alignment. It circumvents these limitations of the intersubband QC lasers, while retaining the cascade feature. We report here the high power operation of a mid-IR interband cascade laser with threshold current densities substantially lower than those reported for cascade lasers . A peak optical output power of 0.5 W/facet at 3.85 m was obtained with 1 s current pulses at 80 K. The slope efficiency was 211 mW/A per facet, corresponding to a differential external quantum efficiency of 131%.

2:30 PM F2.4 
MID-IR OPTICALLY PUMPED AND INJECTION VCSELs. I. Vurgaftman, J. R. Meyer, Code 5613, Naval Research Laboratory, Washington, DC; L. R. Ram-Mohan, Worcester Polytechnic Institute, Worcester, MA.

The development of near-IR vertical cavity surface-emitting lasers (VCSELs) has been stimulated by such attractive properties as very low threshold currents, single longitudinal mode operation, circular output beam profile, and the possibility of 2D laser arrays. Attempts have been made to demonstrate viable room-temperature cw devices at both shorter (visible) and longer (1.3-1.55 m) wavelengths. In this presentation, we will report an investigation of the feasibility and potential performance of optically and electrically pumped VCSELs emitting in the mid-IR spectral region (3-5 m). A comprehensive laser simulation program, which includes radiative and nonradiative recombination, bandstructure and optical matrix elements from the finite-element algorithm, carrier and lattice heating, electrical injection, photon propagation and diffraction, has been developed for this purpose. First, we consider optically pumped mid-IR VCSELs with a distributed Bragg reflector (DBR) semiconductor (GaSb/AlAs0.08Sb0.92) mirror, a optical cavity with a type-II W (InAs/GaSb/InAs/AlSb) active region, and a dielectric top DBR mirror through which the output beam is emitted. Simulations predict that for a 1 m pump wavelength, this laser with a 10 m diameter spot should operate up to 250 K and should be capable of producing > 2 mW of cw output power. Furthermore, by collimating the pump beam with a microlens array, gain-guided VCSEL arrays with output powers in the watt range should be attainable. Comparable powers and operating temperatures are predicted for patterned devices with electrical injection through annular contacts. An additional current-spreading region is included into the optical cavity to make current injection into the active region more uniform. By far the most attractive characteristics are expected for structures employing a type-II interband cascade laser with electron recycling, which consists of a series of W active regions separated by superlattice injectors. The simulation predicts single-element threshold currents of 150 A at 200 K and 1.1 mA at 300 K, and cw output powers of 4.7 mW and 1.2 mW, respectively.

2:45 PM F2.5 
3.2 AND 3.8 m EMISSION AND LASING IN AlGaAsSb/InGaAsSb DOUBLE HETEROSTRUCTURES WITH ASYMMETRIC BAND OFFSET CONFINEMENTS. Maya Mikhailova, Bizhigyt Zhurtanov, Konstantin Moiseev, Albert Imenkov, Oleg Ershov, Yury Yakovlev, Ioffe Physico-Technical Institute, St.Petersburg, RUSSIA.

We report first electroluminescence (EL) and lasing in laser structures with high Al-content (64%, Eg=1.5 eV) cladding layers and InGaAsSb narrow-gap active layer (Eg=0.326 eV at T=77K) LPE grown lattice-matched to GaSb substrate. Two kinds of diode lasers were fabricated on N- and P-GaSb substrates respectively: N-AlGaAsSb/p-InGaAsSb/ P-AlxGaAsSb/P-GaSb (structure A) and structure B with inverted sequence of layers, but with the same composition of quaternary alloys, P-AlGaAsSb/p-InGaAsSb/N-AlGaAsSb. Band energy diagrams of the laser structures had strongly asymmetric band offsets. Heterojuction between high Al-content layer and narrow-gap active layer have a type II broken-gap alignment at 300K. Using high Al-content barriers as stopper confined layers for electrons and holes we expected to improve some performance of our lasers. In A-laser structure spontaneous emission was obtained at 3.8 m at 77K and at 4.25m at T=300K. Full width of half maximum (FWHM) of emission band was 34 meV. Emission intensity decreased by a factor 24 from 77K to 300K. Lasing with single dominant mode was achieved at 3.774m (T=80K). Threshold current as low as 60 mA and characteristic temperature To=26K were obtained at T=80-120K in pulsed mode. Radiative recombination occurs near N-AlGaAsSb/p-InGaAsSb heteroboundary and corresponds to bulk recombination into an active layer. In turn, in the B-structure only intensive spontaneous emission was observed up to 300K. A narrow emission band (FWHM 10meV) at 3.2m was obtained at 77K. Blue shift of the emission band maximum up to 60-70 meV in comparison with the A-structure took place. Strong distinction in appearence of EL spectra in A and B structures we explaine by differ origin of radiative recombination transitions. We suppose that in the B-structure radiative recombination occurs at the type II P-AlGaAsSb/p-InGaAsSb interface and is due to tunneling-assisted optical transitions.

3:30 PM *F2.6 
HIGH-POWER LOW-THRESHOLD OPTICALLY PUMPED TYPE-II QUANTUM-WELL LASERS. Chih-Hsign Lin*, S.J. Murry, Rui Q. Yang and S.S. Pei, Space Vacuum Epitaxy Center, University of Houston, Houston, TX; H.Q. Le, MIT Lincoln Laboratory, Lexington, MA; Chi Yan and D.M. Gianardi, Jr., Rocketdyne Technical Services, Boeing Defense & Space Group, Kirtland AFB, NM; D.L. McDaniel, Jr. and M. Falcon, Semiconductor Laser Branch, Air Force Phillips Lab., Kirtland AFB, NM.

Batch X543 to 5 m mid-infrared (MIR) lasers are highly desirable for a variety of applications such as infrared countermeasures. remote chemical sensing, and MIR spectrospecopy, etc. InAs/lnGaSb/InAs/AISb typical quantum well (QW) lasers operating above room temperature have been demonstrated at wavelengths finm 2.8 to 4.6 m. However, these devices were optically pumped with short pulses at low duty cycles. For realistic applications, high average as well as high peak powers are needed. Here, we will report the recent results of type-II QW lasers under long-pulse or CW operations. For the 3.85-4.45 n lasers, lasing was observed up to room temperature, optically pumped by a pulsed 2-m Th:YAG laser. At 49 K, the CW threshold was 32 mW. When pumped with 62 mW, the CW output power was 4.2 mW/facet, indicating a differential quantum efficiency of 54%. For the 3.5-3.8 m laser, we have observed a maximum operating temperature of 226 K, pumped by InGaAs diode laser arrays with a pulse length of 6 s and a repetition rate of 2 kHz. At 83 K, the lasing peak wavelength was 3.5 m with a record-low absorbed threshold peak pump intensity of 26 W/cm2, assuming 80% of incident power absorbed in the active region. From the threshold peak pump intensity as a function of temperature, the characteristic temperature Tc is 33 K for temperatures up to 200 K. The peak output power at 73 K was about 650 mW per facet with a differential external quantum efficiency of 7%. At 120 K, the differential external quantum efficiency decreased to 3.8%, which was mainly due to the increased internal loss. We will also present the internal loss as a function of temperature and discuss the material quality based on MBE growth.

4:00 PM F2.7 
MODELING OF MID-INFRARED MULTI-QUANTUM WELL LASERS. A.D. Andreev, A.F. Ioffe Physico-Technical Institute, St. Petersburg, RUSSIA.

Modeling of mid-infrared (2-5 um) III-V semiconductor multi-quantum well (MQW) lasers becomes more and more actual during last years. These lasers have a wide range of applications including laser radar systems, molecular spectroscopy and remote sensing. However, practical application of mid-infrared lasers is strongly limited by low operating temperature and low quantum efficiency of these lasers. Nevertheless it is possible to considerably improve the parameters of mid-infrared lasers by choosing the optimal structure parameters (QW width, barrier heights, strain and the number of QWs). The goal of this paper is model the performance of mid-infrared lasers with the aim of structures optimization. To model the threshold characteristics of these lasers we study the following elementary processes: Auger and radiative recombination of the excess carriers, interband absorption and the processes which lead to the carrier leakage out of the active region. The rates of the Auger processes have been calculated in the framework of multiband Kane model [1], taking account of light-heavy hole mixing and non-parabolicity of electron and hole spectrum. The threshold current is shown to be very sensitive to the variation of structure parameters. At definite values of these parameters the threshold current decreases and the internal quantum efficiency increases which is due to the suppression of the Auger process. The carrier distribution between quantum wells was calculated using drift- diffusion model. It is shown that threshold current has a minimum as a function of the number of QWs. The position of this minimum depends on QW and barrier widths, strain and temperature. It is demonstrated that the limiting operation temperature of mid-infrared MQW laser can be increased by choosing the optimal structure parameters.

4:15 PM F2.8 
OPTICAL AND ELECTRICAL PROPERTIES OF INAS/GA1-xINxSB SUPERLATTICE INFRARED PHOTODIODES. U. Weimar, F. Fuchs, W. Pletschen, J. Schmitz, J. Wagner and P. Koidl, Fraunhofer-Institute für Angewandte Festkörperphysik(IAF), Freiburg, GERMANY.

Infrared photodiodes employing InAs/Ga1-xInxSb superlattices as their active region grown by molecular beam epitaxy on <100> GaSb substrates were analyzed with respect to their optical and electrical properties. A series of n-on-p photodiodes was fabricated with varying acceptor concentration NA. The devices show electroluminescence at 10 m wavelength up to temperatures of 240 K, indicating excellent materials quality. The broad photoresponse covers both the 3-to-5 m and the 8-to-12 m atmospheric window with a peak responsivity about 2 A/W. Temperature dependent current-voltage (I-V) and capacitance-voltage analysis has been performed. The forward branch of the I-V curves is characterized by an ideality factor of 1.2. The I-V curve at higher reverse bias voltage shows leakage currents, which can be described by band-to-band tunneling. Below a critical depletion width of about 70 nm, corresponding to NA = 5 x 1016 cm-3, the devices strongly degrade because of tunneling currents. The analysis of the tunneling contributions to the reverse current leads to an effective electron mass for transport perpendicular to the SL plane of 0.03 x m0. Analyzing the I-V curves close to zero voltage the diodes with low doping NA are found to be diffusion limited at temperatures above 77 K. The dynamical impedance reaches values above 1 kcm2 (cut-off at 8 m wavelength) leading to a Johnson-noise limited detectivity at 77 K in excess of 1 x 1012 cmHz 1/2/W.

4:30 PM F2.9 
MID-IR PHOTODETECTORS BASED ON InAs/InGaSb TYPE-II QUANTUM WELLS. Gail J. Brown, William C. Mitchel, Frank Szmulowicz, Air Force Research Laboratory, Materials Directorate, Wright Patterson AFB, OH; Chih-Hsiang Lin, Space Vacuum Epitaxy Center, University of Houston, Houston, TX.

We report on the growth and characterization of InAs/Ga1-xInxSb strained layer superlattices (SLS) designed with a photoresponse cut-off wavelength of 10m. The structural parameters, layer thicknesses and compositions, were chosen to optimize the infrared absorption for a superlattice with an energy band gap of 120 meV. The energy band structure and optimized absorption coefficient were determined with an 8x8 envelope function approximation model. The superlattices were grown by a Riber 32 molecular beam epitaxy system and were comprised of 100 periods of 43.6 InAs and 17.2 Ga.77In.23Sb lattice-matched to the GaSb substrates. In order to reduce the background carrier concentrations in this material, superlattices grown with different substrate temperatures were compared before and after annealing. This set of superlattice materials was characterized using x-ray diffraction, photoresponse and Hall measurements. The measured photoresponse cut-off energy of 114 meV is in good agreement with the predicted energy band gap for the superlattice as designed. There was excellent repeatability of the measured photoresponse cut-off wavelength in this series of samples. The intensity of the measured mid-infrared photoresponse was found to improve by an order of magnitude for the superlattice grown at the lower substrate temperature and then annealed at 520C for 10 minutes. However, the x-ray diffraction spectra were very similar before and after annealing. The temperature dependent Hall measurements at low temperatures (<25K) were dominated by holes with quasi two-dimensional behavior. An admirably low background carrier concentration of lx1012 cm-2 was measured at low temperature.

4:45 PM F2.10 
PROGRESS ON GAINASSB AND INASSBP PHOTODETECTORS FOR MID-INFRARED WAVELENGTHS. Zane Shellenbarger, Michael Mauk, Paul Sims, Jeff Cox, Joseph Lesko, Jeff Bower, Joe South, Lou DiNetta, AstroPower Inc., Newark, DE.

Progress on mid-infrared photodetectors fabricated by the liquid phase epitaxial growth of GaInAsSb and InAsSbP on GaSb and InAs substrates is reported. Both p/n junction detector and separate absorption and multiplication (SAM) avalanche photodiode (APD) structures were fabricated. Measurements of detectivity, dark current, and noise current are presented as well as data characterizing detector performance versus temperature. Preliminary results indicate that these detectors will have higher detectivity with lower cooling requirements than commercially available detectors in the same wavelength range. With proper composition these quaternary alloy systems can be prepared with band gaps adjustable in wavelength from 1.7 to 4.5 microns. The major advantage of using a quaternary system over binary and ternary compounds is the ability to vary the band gap while still providing lattice matched growth to the substrate material. With proper device design both the cut-on and cut-off wavelengths can be varied over the range of the quaternary system. These photodetectors will be useful in space and terrestrial infrared imaging systems. Photodetectors operating in the mid-IR range have potential use in gas sensors since the basic absorption bands of water and many industrial gases belong in this range. These new photodetectors will have a variety of commercial applications in air pollution monitoring, industrial process control, and automobile emission monitoring. The 2-4 micron wavelength range is also of potential importance to future lightwave communication systems since novel fiber materials offer extremely low transmission losses in this region.

Chairs: Richard H. Miles and Robert M. Biefeld 
Tuesday Morning, December 2, 1997 
Salon J/K (M)

8:15 AM *F3.1 
NEW HIGH PERFORAMNCE QUANTUM CASCADE LASERS. F. Capasso, A. Tredicucci, C.G. Gmachl, J.N. Baillargeon, D.L. Sivco, and A.Y. Cho, Bell Labs, Lucent Technologies, Murray Hill, NJ; J. Faist, Dept. of Physics, University of Neuchatel, SWITZERLAND; C. Sirtori, Laboratoire Central de Recherches, THOMPSON-CSF, Orsay, FRANCE; G. Scamarcio, Dept of Physics, University of Bari, Bari, ITALY.

Recent advances in QC lasers are discussed. This includes the high power (200 mW at 80K) cw operation of 5 micron lasers, extension of the wavelength range from 3.3 to 13 microns, single mode broadly tunable (100 nanometer) single mode distributed feedback lasers at 5 and 7 microns and their chemical sensing applications, Stark-tuned photon assisted tunneling lasers and high power (up to 1 W) interminiband 8-11 micron lasers. Results on dual wavelength emitters and their potential for new mid-ir laser will be presented.

8:45 AM *F3.2 
HOT-ELECTRON EFFECTS IN LONG WAVELENGTH LASERS. Vera Gorfinkel and Serge Luryi, Department of Electrical Engineering, State University of New York at Stony Brook, Stony Brook, NY.

Hot electron effects enter the description of semiconductor laser operation owing to the well known dependence of optical gain on the carrier temperature. In near infrared lasers hot electron effects are relatively small and arise mainly from heterostructure barrier injection and free-carrier absorption of cavity radiation. Nevertheless, even these small effects are not entirely benign: they are responsible for a substantial intermodulation distortion limiting the number of channels in optical communication systems [1]. In mid-infrared region, hot-electron effects are much more dramatic. This happens because of the substantial release of power that accompanies non-radiative recombination in long wavelength materials. An important component of the non-radiative current in these materials is due to Auger recombination. In an Auger process, the potential energy of an electron-hole pair (which exceeds the semiconductor energy gap) is transferred to a free electron or hole. At high injection current I, this may lead to a substantial carrier heating. The increasing carrier temperature Te suppresses the optical gain g and leads to the appearance of a maximum gmax in the dependence g(I). If the total losses in the laser cavity exceed gmax then the structure will not lase at any current. Note that for a constant Te the dependence g(I) is monotonic. If the losses do not exceed gmax, then the laser generation regime can be reached, but the negative slope of g(I) characteristic results in peculiar instabilities for currents exceeding Icr = I( gmax ). For the same value of gain one can have two regimes that differ in carrier concentration and temperature and, most importantly, in the output radiation power. The high-Te regime corresponds to higher concentration and lower power. This regime is metastable. Illumination of the laser by a sufficiently powerful pulse of external light temporarily suppresses the Auger recombination and switches the laser into the stable regime with a high output power. Analogous phenomena can be expected in the operation of unipolar quantum cascade lasers (QCL). Instead of Auger processes, carrier heating in QCL results from non-radiative intersubband transitions. Again, an energy of the order of the lasing photon energy is transferred to the electronic system in every transition. The dependence of gain on Te arises due to the dephasing of intersubband transition by scattering processes whose rate depends on the electron energy and also from the non-parabolicity. The resultant non-monotonic g(I) is responsible [2] for the strong temperature dependence of threshold in the QCL with an abrupt disappearance of lasing above a critical temperature. Hot-electron effects in the conventional mid-IR lasers and the unipolar QCL have both similar features and fundamental differences. The most important difference lies in the type of hot-electron ensembles achievable in these structures. Unlike the bipolar laser, the QCL can naturally operate in the regime of low carrier concentrations, where the electron distribution function does not maxwellize. Under such conditions, hot-electron effects can be rendered harmless and even beneficial [2]. Hot-electron effects in the conventional mid-IR lasers and the unipolar QCL have both similar features and fundamental differences. The most important difference lies in the type of hot-electron ensembles achievable in these structures. Unlike the bipolar laser, the QCL can naturally operate in the regime of low carrier concentrations, where the electron distribution function does not maxwellize. Under such conditions, hot-electron effects can be rendered harmless and even beneficial [2].

9:15 AM F3.3 
INTERSUBBAND TERAHERTZ EMISSION FROM ELECTRICALLY PUMPED MULTIPLE QUANTUM WELLS, Bin Xu, Ben Williams, and Qing Hu, MIT, Dept. of Electrical Engineering and Computer Science, Cambridge, MA; Michael r. Melloch, Purdue Univ., School of Electrical and Computer Engineering, West Lafayette, IN.

By properly engineering intersubband scatterings and subban levels, terahertz (THz) unipolar lasers can be made from multiple quantum well (MQW) structures under appropriate bias. A three-level system was designed using coupled-triple-well structures in GaAs/AlGaAs, with the intension to achieve population inversion between the two upper subbands. THz spontaneous emission was observed in such structures, and the emission spectra were resolved using a Fourier transform infrared spectrometer (FTIR). The emission peaks were voltage tunable from 5-8 THz, and the resolved emission linewidth was less than 5 meV (1.25 THz). Evidence of population inversion was found from measuring the heights of the emission peaks corresponding to different intersubband radiative transitions. Hot electron effect was crucial in determining intersubband scattering rates and therefore the device performance, for THz emission below the longitudinal optical (LO) phonon energy. A numerical simulation, combined with experimental characterizations, was employed to address this issue. Modifications to MQW structures were made to reduce the hot electron effect. By using a metallic waveguide or plasma confinement to reduce the cavity loss, our calculation shows that the lasing threshold can be achieved in such structures, promising the operations of THz quantum cascade lasers.

9:30 AM F3.4 
TERAHERTZ INTERSUBBAND EMISSION IN OPTICALLY PUMPED QUANTUM WELLS. Ilya Lyubomirsky and Qing Hu, MIT, Dept of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Cambridge, MA; Chad Hansing and Ben G. Streetman, UT Austin, Dept of Electrical and Computer Engineering, Austin, TX.

The terahertz frequency range (30-300 m) is one of the most underdeveloped frequency regimes, mainly due to a lack of compact, coherent solid state sources. Conventional interband lasers are limited by the band gap to operate above 10 THz. Quantum wells can be used to overcome this limitation by engineering the subband separations. We used band gap and phonon engineering to design a quantum well THz laser that can be optically pumped. A multiple quantum well structure was fabricated using AlGaAs/GaAs to investigate terahertz intersubband emission. The three level structure emits THz radiation under optical pumping with a CO2 laser as a result of intersubband transition between levels E3 and E2. The emitted THz power exhibits a peak as a function of pump photon energy close to the %E3-E1 transition in accordance with the design. The emission spectrum was resolved using a Fourier transform infrared spectrometer. The center frequency was found to be 6 THz in good agreement with the absorption measurements. The width of the emission was 2.8 THz.

10:15 AM *F3.5 
NON-EQUILIBRIUM InSb/InAlSb DIODES GROWN BY MBE. Andrew D. Johnson, DERA Malvern, St Andrews Road, Great Malvern, UNITED KINGDOM.

The application of non-equilibrium transport techniques to InSb / InAlSb diodes grown by Molecular Beam Epitaxy (MBE) has produced a number of practical devices that operate at room temperature. These devices include negative luminescent diodes, which appear to contravene Kirchhoff*s Law because they will absorb infrared radiation without emitting it. Such sources have many important applications in areas such as gas sensing, radiometric referencing, dynamic infrared scene projection and cold shielding of infrared detectors, which will be described at this meeting in an associated paper by Ashley. Improved device performance has been achieved through lowering of the MBE growth temperature of the epilayers. This was previously thought not to be possible for such MBE grown InSb based structures due to the resulting poor layer morphology associated with Sb condensation. We have produced epilayers at growth temperatures as low as 275*C with excellent layer morphology as seen with Nomarski and AFM, with corresponding improvements in dopant activation and diode performance. In addition, lower growth temperatures are vitally important in the doping of the ternary material InAlSb and results will be presented indicating the benefit to device performance.

10:45 AM *F3.6 

There is increasing interest in mid- and long-wavelength infrared semiconductor positive and negative luminescent sources for a number of applications. In all cases it is desirable that the infrared diodes should operate with little or no cooling in order to minimise system cost, size and power consumption. We describe uncooled mid-infrared light emitting and negative luminescent diodes made from indium antimonide based III-V compounds, and long wavelength devices made from mercury cadmium telluride. The III-V structures comprise a single thin strained heterostructure of InAlSb on the p-type side of the diode and a degenerately doped n-type side to provide effective carrier confinement, the growth of which is described by Johnson in an associated paper at this meeting. The devices have positive and negative luminescent peaks at about 6mm when the active region in the diodes comprises InSb. The use of InAlSb for the active region has been demonstrated to shift the peaks back as far as 4mm. The near constant lattice parameter across the MCT system permits a full double heterostructure to confine the electrons and holes. Positive and negative emission have been observed from these MCT devices with a peak wavelength as long as 10mm. Techniques to fabricate large area devices will be described. These include the use of degenerately doped substrates to provide transparency and the integration of optical concentrators in the substrate material to improve optical efficiency. Results will be presented from InSb sources up to 1cm2. The application of positive and negative luminescent devices to gas sensing, improved thermal imagers and imager testing will be discussed.

11:15 AM F3.7 
GaAs/AlGaAs INTERSUBBAND MID-INFRARED EMITTER. Gottfried Strasser, Lubos Hvozdara, Reinhard Zobl, Karl Unterrainer, Erich Gornik,TU Wien, Solid State Electronics, Vienna, AUSTRIA; Peter Kruck, Manfred Helm, Univ. Linz, Semiconductor Physics, Linz, AUSTRIA; James Heyman, Macalester College, St. Paul, MN

We report here the design, growth and characterization of an intersubband quantum well structure based on the GaAs/AlGaAs material system, which is designed to emit radiation at approximately 5 micrometers. We present transport behavior, infrared absorption, photocurrent spectra, and electroluminescence data. Temperature dependent photocurrent spectra show the working range of these LEDs. First attempts to fabricate a laser structure from this device encountered difficulties with the electrical properties of the AlGaAs waveguide cladding layers. Thus, we present measurements with different waveguide concepts as doped GaAlAs cladding layers and doped superlattice cladding structures.

11:30 AM F3.8 
NOVEL PIEZOELECTRIC HETEROSTRUCTURE FOR ALL-OPTICAL INFRARED LIGHT MODULATION. V. Ortiz, N.T. Pelekanos, Guido Mula, CEA/Grenoble, Dept. de Recherche Fondamentale sur la Matiére Condensée, Grenoble, FRANCE.

We demonstrate a novel all-optical quantum well (2W) modulator device, exploiting the large piezoelectric field Ep present in strained semiconductor layers grown along a polar axis. The device is all-optical in the sense that one ``control'' laser beam modulates a ``read'' beam without an external bias. The new concept here is the use Of piezoelectric barriers as opposed to piezoelectric wells, This enables efficient spatial separation of photocarriers, at the origin of the all-optical modulation.
Each period of the heterostructure, grown by MBE on (211) Cd0.96Zn0.04Te substrates, is composed of three QWs . The central (active) QW consists of 200 of Cd1-xHgxTe. Here, x=0.4, for modulation around l.5m, but modulation in the mid- and far-infrared is possible by simply increasing x. Before and after the active QW, separated by two 400 thick Cd0.68Mg0.20Zn0.12Te piezoelectric barriers (E100kV/cm), are two 400 thick Cd0.75Hg0.25Te QWa (collection QWs). 
The device works as follows: the ``control'' beam photogenerates carriers in the piezoelectric barriers. There, Ep separates electrons and holes on either side of each barrier. The collected carriers in the active QW deplete radiatively and the end result of the optical excitation is a spatially separated accumulation of electrons and holes in the collection QWs. The resulting electric field shifts the ground state of the active QW through the quantum confined Stark effect.
We monitored the photogenerated electric field by measuring the shift of the active QW photoluminescence (PL) peak as a function of the ``control'' beam power. We were able to measure at room temperature PL blueshifts of 40meV for a ``control'' power of 70W/cm2. In the low power region (<10W/cm2), the blueshift rate was 2meV per W/cm2, showing the high sensitivity of the device.

11:45 AM F3.9 
MULTIVALENT ACCEPTOR DOPED GERMANIUM LASERS: A SOLID-STATE TUNABLE SOURCE FROM 75 TO 300 m. D.R. Chamberlin, O.D. Dubon, E.E. Haller, Lawrence Berkeley National Laboratory and UC Berkeley, Berkeley, CA; L.A. Reichertz, G. Sirmain, E. Bründermann, A.M. Linhart, H.P. Röser, DLR, Institute for Space Sensor Technology, Berlin, GERMANY.

We have investigated the performance of far-infrared hole inversion lasers made from germanium doped with the multivalent acceptors Be and Cu. These lasers operate in crossed magnetic and electric fields at liquid helium temperatures. Under these conditions a population inversion occurs between the heavy- and light-hole subbands of the valence band. This leads to tunable stimulated emission which is broad-band in nature. Power as high as a few watts can be achieved under pulsed operation with pulse lengths of several s. Previously these lasers have been fabricated from Czochralski-grown Ga-doped Ge single crystals and have shown emission from 75 to 125 and 170 to 300 m. The emission gap between 125 and 170 m originates from absorption of the far-infrared light due to internal hole transitions in the neutral Ga acceptor. In this study we have used Ge crystals doped with the multivalent acceptors Be and Cu, which have internal hole transition energies well outside the emission range. These lasers demonstrate tunable emission over the entire spectral range of 75 to 300 m. By using crystals that contain these novel dopants, we have increased the duty cycle up to 10-2 range which is one order of magnitude higher than the maximum duty cycle reported for Ga-doped Ge lasers. These new lasers may offer an opportunity for achieving continuous-wave operation.

Chairs: M. O. Manasreh and Richard Singer 
Tuesday Afternoon, December 2, 1997 
Salon J/K (M)

1:30 PM *F4.1 
NEW CHALLENGES FOR INFRARED SENSOR MATERIAL. Raymond Balcerak, Defense Advaned Research Projects Agency, Arlington, VA.

As Infrared focal plane arrays (IRFPAs) move from the research laboratory to the production line, the focus of the development effort shifts toward system integration issues. The material research may no longer the paramount concern, and can be left behind in the struggle to integrate the new technology into systems. However, even as infrared focal plane array systems become available, there are still persistent, unanswered questions concerning material properties and their relationship to device performance. Infrared material research is important to understand these underlying mechanisms, but what is more important, the material research should lead the way to exciting new possibilities in sensor and system performance. This paper explores directions for research in infrared materials and the potential for new capability offered by innovations in materials research. The first consideration is simultaneous detection in multiple spectral bands, which is an important capability that relies upon sharply defined material interfaces, doping and composition control, and multiple signal processor inputs. Cooling has always been a issue with high performance infrared sensors. Material improvements may relieve the cooling requirements for infrared sensors, leading the way to low power, high performance devices for a wide range of applications. Finally, the thrust toward large high density arrays with integral signal processing depends strongly upon innovations in infrared material. The integration of signal processing with the high density arrays provides significant new systems capability. The efforts in infrared materials research will pave the way for these, and many other, new possibilities in infrared sensor systems.

2:00 PM *F4.2 
HIGH TEMPERATURE INFRARED PHOTON DETECTOR PERFORMANCE. C.H. Grein, University of Illinois at Chicago, Chicago, IL; H. Ehrenreich, Harvard University, Cambridge, MA.

We examine theoretically three combined effects designed to improve the high temperature performance of mid- to long-wavelength infrared photon detectors. They are: (i) the suppression of radiative recombination using photon recycling, (ii) the suppression of both radiative and Auger recombination with carrier depletion, and (iii) the suppression of Auger recombination using band structure engineering in strained layer superlattices. The importance of effects (i) and (ii) on bulk HgCdTe-based detectors is compared with that of effects (i), (ii), and (iii) on InAs/InGaSb strained layer superlattice-based detectors. All three effects provide significant enhancements in detectivities. However, in the long wavelength infrared the required carrier density are too low (about 1014 cm-3) and the practical gains are insufficient to raise the detectivities of photon detectors to those of thermal detectors at 300K.

2:30 PM F4.3 
FAR-INFRARED FREE HOLE ABSORPTION IN EPITAXIAL SILICON FILMS FOR HOMOJUNCTION DETECTORS. A.G.U. Perera and W.Z. Shen, Department of Physics and Astronomy, Georgia State University, Atlanta, GA; M.O. Tanner and K.L. Wang, Department of Electrical Engineering, University of California at Los Angeles, Los Angeles, CA.

Recently, homojunction interfacial work-function internal photoemission (HIWIP) far-infrared (FIR) detectors have been shown to be suitable for space astronomy applications at wavelengths greater than 50 m. The modeling studies have shown that Si HIWIP FIR detectors could have a performance comparable to that of conventional Ge FIR photoconductors or Ge blocked-impurity-band (BIB) FIR detectors, with unique material advantages. The detection mechanism of HIWIP detectors involves FIR free carrier absorption in the highly-doped thin emitter layers, followed by the internal photoemission of barrier penetration and collection. Previous studies of optical absorption in silicon were limited to relatively short wavelengths ( 40 m). Here, we report the investigation of free-carrier absorption characteristics for epitaxially grown p-type silicon thin films in the far-infrared region (50 200 m), where Si HIWIP detectors are employed. Five Si thin films were grown by MBE on different silicon substrates over a range of carrier concentrations, and the experimental absorption data were compared with calculated results. The free-hole absorption is found to be almost independent of the measured wavelength. A linear regression relationship between the absorption coefficient and the carrier concentration, in agreement with theory, has been obtained and employed to calculate the photon absorption probability in HIWIP detectors. The result shows that the quantum efficiency increases rapidly with the carrier concentration, which has been demonstrated in our recent Si HIWIP experiments.

2:45 PM F4.4 
PHOTOLUMINESCENCE AND DEVICE CHARACTERIZATION OF N-DOPED In0.15Ga0.85As/Al0.20Ga0.80As STRAINED MULTIPLE QUANTUM WELL. D.K. Sengupta+, S. Kim, A.P. Curtis, P.J. Mares, J.I. Malin, Q.Yang, T.U. Horton, H.C. Kuo, K.C. Hseih, S.L. Chuang, S.G. Bishop, M. Feng, and G.E. Stillman Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL; (+ Presently with CSMT, NASA's Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA; L. Li and H.C. Liu, Institute for Microstructural sciences, National Research Council, CANADA; W.J. Wang, Department of Electrical Engineering, Columbia University, New York, NY.

We report the temperature and excitation power dependence of the photoluminescence (PL) spectrum in conjunction with the device characteristics of n-doped In0.15Ga0.85As/Al0.20Ga0.80As quantum well infrared photodetector(QWIP). The QWIP structure consists of a strained InGaAs quantum well of 48Åand AlGaAs barriers of 300Å grown by molecular beam epitaxy. PL spectra of the QWIP show two broad emission peaks at 1470 and 1625 meV. The separation of these peaks is 155 meV which is extremely close to the 172 meV transition energy between the subbands as determined by photocurrent measurement. The lineshape of the 1625 meV emission is independent of the excitation power, unlike the 1470 meV emission, which is indicative of the luminescence originating from a single transition. The maximum responsivity of the QWIP at 77K was 0.085A/W, obtained at a peak wavelength of 7.2 m under an operating bias of 2.5V.

3:00 PM F4.5 
GERMANIUM FAR INFRARED BLOCKED IMPURITY BAND DETECTORS. C.S. Olsena,b, J.W. Beemana, W.L. Hansena and E.E. Hallera,b. aLawrence Berkeley National Laboratory and bUniversity of California, Berkeley, CA.

We are investigating Germanium Blocked Impurity Band (BIB) photoconductors for long wavelength infrared detection in the 100 to 250 m region. We use Liquid Phase Epitaxy (LPE) to grow the high purity blocking layer, and in some cases, the heavily doped infrared absorbing layer that comprise theses detectors. In order for a Ge BIB detector to function properly both epilayers require very low structural and electrical defect concentrations. To achieve these stringent demands we have developed a high purity LPE process which can be used for the growth of high purity as well as purely doped Ge epilayers. We choose a low melting point, high purity metal, Pb, as a solvent that has a negligible solubility < 1017 cm-3 in Ge at 650C and is isoelectronic with Ge. We have identified the residual impurities Bi, P, and Sb in the Ge epilayers and have determined that the Pb solvent is the source. Experiments are in progress to purify the Pb. The first tests of BIB structures with the purely doped absorbing layer grown on high purity substrates look very promising. The detectors exhibit extended wavelength cutoff when compared to standard Ge:Ga photoconductors (165 m vs. 120 m) and show the expected asymmetric current and voltage dependencies. We are currently optimizing doping and layer thickness to achieve the optimum responsivity, Noise Equivalent Power (NEP), and dark current in our devices.

3:45 PM *F4.6 
HgCdTe-AN UNEXPECTEDLY GOOD CHOICE FOR (NEAR) ROOM TEMPERATURE FOCAL PLANE ARRAYS. W.E. Tennant, Rockwell International Corporation, Thousand Oaks, CA.

Fueled by a broad range of Government needs and funding, HgCdTe materials and device technology have matured significantly over the last two decades. Also in this same time period, we have come to understand better the phenomenology which limits image performance. As a result of these developments, it appears that HgCdTe arrays may be tailored in wavelength to outperform GaAs-based image intensifier devices in sensitivity and to compete with bolometric and pyroelectric imaging arrays in NEDT at temperatures at or near room temperature (250K-295K). These benefits can be fully realized, however, only if HgCdTe can be brought to a level of maturity where the material and detectors made from it are limited by fundamental mechanisms. We will discuss the state of HgCdTe near room temperature performance and the practical and theoretical limits which constrain it.

4:15 PM F4.7 
UNCOOLED MICROBOLOMETER SENSOR PERFORMANCE. Margaret Kohin, Walter Watson, Robert Murphy, Neal R. Butler, Lou Perich, Charles A. Marshall, Lockheed Martin IR Imaging Systems, Lexington, MA.

Lockheed Martin IR Imaging Systems is developing low cost, high performance, uncooled microbolometer infrared imaging products for both military and commercial applications. The microbolometer FPAs used in our systems consist of a 320 x 240 pixel array of LWIR (8 - 14 µm) absorbing microbridges suspended above a CMOS readout integrated circuit (ROIC). The microbridges contain vanadium oxide (VOx) which exhibits a change in resistance with temperature. This change in resistance is read out at a frame rate of either 60 Hz (NTSC) or 50 Hz (PAL) by the ROIC. The pixels have a pitch of 46.25 µm and a typical detector time constant of 14 msec. The ROIC produces a single multiplexed 14 bit digital output. On chip analog-to-digital converters and 6 bits per pixel coarse correction are standard. The total power consumption of the ROIC is < 500 mW. Built-in test and calibration references are available at the unit cell level. Thirty dB of global gain control and x-y image inversion are also built into the ROIC. Measurements of the performance of our uncooled sensors have been completed by both Lockheed Martin and NV&ESD (Night Vision and Electronic Sensors Directorate) after a series of performance improvements. The MTF, linearity, spatial noise, temporal noise, Minimum Resolvable Temperature Difference and stability between calibrations have been measured and are reported in this paper. Spatial and temporal noise are well below 100 mk over a very wide range of scene temperatures because the nonlinearity is < 0.5% over a scene temperature range of 95ƒ C. The Minimum Resolvable Temperature Difference is less than 0.4ƒ C at spatial frequencies that are beyond Nyquist. These measurements confirm that uncooled sensors have reached performance levels that allow them to be useful in a wide variety of applications. A video tape showing some recent imagery provides qualitative confirmation of the performance of Lockheed Martinís uncooled camera.

4:30 PM F4.8 
BANDGAP-ENGINEERING OF HgCdTe FOR TWO-COLOR IR DETECTOR ARRAYS BY MOVPE*. Pradip Mitra, Francine C. Case and Scott L. Barnes, Lockheed Martin Vought Systems, Dallas, TX; Martha R. Krueger, Peter O'Dette, Margaret H. Weiler and Marion B. Reine, Lockheed Martin IR Imaging Systems, Lexington, MA.

MOVPE growth of Hg1-xCdxTe has progressed to the extent that multilayer structures with single or multiple p-n junctions can now be grown in situ with complete flexibility in the control of alloy composition, and donor and acceptor doping. This flexibility enables the realization of IR detector devices that incorporate the most theoretically optimal device designs through bandgap engineering. In this paper we will describe the growth of independently accessed, stacked two-color IR detector structures in the p-n-N-P configuration that operate simultaneously in the MW (3-5 m) and LW (8-12 m) spectral bands. The MW detector is an N-on-P heterojunction on which an LW p-on-n heterojunction detector is grown. The 28-30 m thick HgCdTe multilayer structures have been grown on nominally lattice-matched CdZnTe substrates in the (100) orientation. SIMS depth profile data show the interdiffused widths in alloy compositions between x=0.30 and x=0.40 layers are 1m. A composition barrier layer, with x0.025 between the MW md LW layers, was incorporated to minimize spectral crosstalk between the two detectors. The n-type layers are doped with iodine from the precursor ethyl iodide at (1-3)x 1015 cm-3. P-type doping was achieved with arsenic from the precursor tris-dimethylaminoarsenic at (2-4)x1017 cm-3. Both the n-type and p-type doping are obtained at levels precisely as targeted and near 100% activations have been achieved. The mobilities and lifetimes measured in single layers exhibit values comparable to that achieved in the highest quality epitaxial HgCdTe grown by any technique. A number of p-n-N-P structures have been grown and detailed characterization of the films demonstrate that repeatable growth has been achieved. These structures are among the most challenging devices grown in situ and amply demonstrate the effectiveness of MOVPE for bandgap engineering of HgCdTe.

4:45 PM F4.9 
MULTISPECTRAL HgCdTe INFRARED DETECTORS GROWN BY MOLECULAR BEAM EPITAXY. R.D. Rajavel, D.M. Jamba, J.E. Jensen and O.K. Wu, Hughes Research Laboratories, Malibu, CA; J.A. Wilson, J.L. Johnson, J.T. Caulfield, E.A. Patten, K. Kosai, and P. Goetz, Santa Barbara Research Center, Goleta, CA.

The flexibility over composition and doping control offered by molecular beam epitaxy was used for the growth of n-p-n HgCdTe-photodetectors having sensitivity in two IR spectral regions. Our mature in-situ n- and p-type doping capability, employing In and As as the donor and acceptor impurities, respectively, was used to dope the layers and grow a device structure with two p-n junctions integrated into each pixel. The device structures were grown on near-lattice matched CdZnTe substrates in one continuous growth process. The two n-type layers with electron concentrations of 1-3x1015 cm-3 serve as the absorber layers for the generation of electron-hole paris at the IR wavelength of interest. The simultaneous detection of two IR wavelengths was accomplished by fabricating devices with contacts to both n-type layers as well as the p-type layer and operating the diodes as two independent p-n junctions. Multispectral n-p-n devices for the detection of two different combinations of IR bands will be discussed. The first example consists of a device for the simultaneous detection of two closely spaced sub-bands at 4.1 and 4.5 um in the midwave IR spectrum. For these devices, the spectral response of the two diodes exhibit sharp turn-off, a characteristic essential for high performance in two adjacent sub-bands. RoA values in excess of 1x106-cm2 at 78K, and quantum efficiencies greater than 70% (non-AR coated) were measured in both diodes. The second multispectral device structure was designed for the simultaneous detection of 5.5 and 10.0 m radiation at 78K. Results of the evaluation of materials characteristics and electrical properties of the individual layers and device performance of both MWIR and LWIR detector elements will be presented and discussed.

Chairs: M. O. Manasreh, D. L. McDaniel, Richard H. Miles and Siva Sivananthan 
Tuesday Evening, December 2, 1997 
8:00 P.M. 
Salons E-G (M)


Metalorganic vapor phase deposition has been used for years as the most suited technology for the large area deposition of semiconductors layers, mainly for light emitting devices and solar cells. With the antimonide compounds a new class of III-V semiconductor materials has gained much attraction now. Thus it appeared to be consequent to apply the proven concept of the multiwafer Planetary ReactorsR for the growth of antimonides. Especially indium antimonide (InSb) is a highly interesting material. One of the main reasons for this is that the magneto-resistance effect at room temperature is stronger in InSb than it is in any other III-V semiconductor which makes it particularly suited for magnetic field detectors. We use the AIXTRON 2400 Planetary ReactorR to grow indium antimonide on eight 3 inch GaAs wafers per run. To meet the narrow specifications for automobile components, the standard deviation of the sheet resistivity and the thickness were reduced below 1.5%. This uniformity is the result of the optimisation of the growth process as is the specular surface of the layers despite a mismatch of 0.146 lattice constants of GaAs. The optimisation encompassed the determination of the best process temperature and the optimum flow. The gas velocity and depletion of the gas phase has a large impact on the uniformity of the layers. By rotation of the wafers, non-uniformities caused by depletion can be completely levelled out.

MINORITY CARRIER LIFETIMES IN INFRARED ALLOYS. Srinivasan Krishnamurthy, A.Sher, M. A. Berding, SRI International, Applied Physical Sciences, Menlo Park, CA; A.-B. Chen, Auburn University, Physics Dept, Auburn, AL.

The alloys InTlP, InTlAs, and InTlSb have been proposed as viable alternatives to the HgCdTe alloy for infrared (IR) applications,1 several groups are currently trying to grow these materials. The efficiency of IR devices is limited by minority carrier recombination lifetimes. The Auger recombination rates in small-gap semiconductor alloys are calculated using full band structures. In addition to the usual coulomb interactions, we consider the electron-electron interaction mediated by longitudinal optical phonons. The rates are calculated using second-order perturbation theory, and the expression for lifetimes appropriate for photoconductive decay experiments are derived. We find that the calculated lifetimes are sensitive to band structure details and the results can differ by two orders of magnitude depending on the approximations used to describe the energy bands and wave functions. The lifetimes calculated with full temperature-dependent band structure agree reasonably well with experiments. Similar calculations were carried out for hole lifetimes in In0.67Tl0.33P, In0.85Tl0.15As, and In0.92Tl0.08Sb as a function of temperature. The alloy concentrations were adjusted to have a band gap of 100 meV at 77 K. We find that the minority carrier lifetime in In0.67Tl0.33P and In0.92Tl0.08Sb are consistently shorter than that in Hg0.78Cd0.22Te at all temperatures. However, the low-temperature minority carrier lifetime in In0.85Tl0.15As is an order of magnitude longer than that in Hg0.78Cd0.22Te. These results are explained in terms of matrix elements and temperature-dependent energy gap variations. Importantly, our calculations further suggest a possibility of increasing the lifetimes of minority carriers by decreasing the density of states outside a critical energy and momentum region. This explains the increased lifetimes observed in the strained materials. Details of these calculations and a systematic evaluation of various approximations will be presented.

EFFECT OF ALLOY COMPOSITION ON THE SURFACE AND ELECTRICAL PROPERTY OF MOCVD-GROWN GAINASSB. Yongqiang Ning, Baolin Zhang, Tianming Zhou, Hong Jiang, Guang Yuan, Yuan Tian, Yixin Jin, Changchun Inst of Physics, Chinese Academy of Sciences, Jilin, CHINA.

MOCVD growth and characterization of GaSb-rich and InAs-rich GaInAsSb on GaSb substrates were investigated. The surface of InAs-rich GaInAsSb epilayers was much different from that of GaSb-rich film. The flux of TMSb in vapor phase might be the origin of the different surface features between GaSb-rich and InAs-rich epilayers. The n-type conduction of InAs-rich epilayers was the opposite of GaSb-rich film. Residual impurity carbon might occupy different sublattice sites, resulting in the two types of conductions. This site preference was caused by the different strength of chemical bonds between carbon and III-group elements or between carbon and V-group elements.

MOCVD GROWTH AND CHARACTERIZATION OF UNDOPED AND Sn-DOPED GaSb. Yongqiang Ning, Baolin Zhang, Tianming Zhou, Hong Jiang, Guang Yuan, Yuan Tian, Yixin Jin, Changchun Inst of Physics, Chinese Academy of Sciences, Changchun, CHINA.

Undoped and Sn-doped GaSb are grown on GaSb substrates through atmospheric pressure MOCVD system. TESn is used as the n-type dopant. The flux of TESn affects the surface quality of epilayers, espicially when the flux of TESn is beyond 50ml/min. Hall measurements show that the electron concentration of Sn-doped epilayers increases linearly with increasing the flux of TESn. We confirmed that TESn is a promising n-type dopant although it is usually amphoteric. Unusual compression of crystal lattice is found with increasing the flux of TESn. The reason for this compression is still unclear.

ELECTRICAL PROPERTIES AND DOPING INVESTIGATION FOR GaInAsSb SYSTEM MATERIALS GROWN BY MOCVD. Tianming Zhou, Hong Jiang, Yongqiang Ning, Baolin Zhang, Yixin Jin, Changchun Inst of Physics, Chinese Academy of Sciences, Changchun, CHINA.

GaInAsSb system materials have received much attention in recent years due to their potential applications in light sources and detectors for infrared field.Undoped GaSb,GaInSb and GaInAsSb alloy both in rich GaSb and rich InAs have been grown by AP-MOCVD. The dependence of the electrical properties on the growth parameters has been studied.It is found that the undoped alloys in rich GaSb are p-type,and n-type in rich InAs alloys.The hole concentration of alloys increases with III/V ratio of input vapor phase sources.For GaInSb,the hole concentration of epilayers increases with Ga/In ratio in vapor phase.In addition,the hole concentration of GaInAsSb epilayers decreases with increasing og Ga/In ratio ans Sb/V ratio in vapor phase as well as growth temperature.The epilayers with the background hole concentration of 1 times 10 (16)cm(-3) and 1.2 times 10(16)cm(-3) have been obtained for undoped GaSb and GaInAsSb,respectively . DESn,DETe and TESn were investigated as p-type and n-type dopant sources in GaInAsSb alloy growth.It is found that the doped efficiency of DEZn increases with III/V ratio in vapor phase and growth temperature. The hole concentration between 5 times 10 (17)cm(-3) and 1 times 10 (19) cm(-3) has been achived by using DEZn at 600 degree. and III/V=0.8. The electron concentration between 5 times 10(17)cm(-3) and 1 times 10(19)cm(-3) have been achived for using both DETe and TESn dopants.It is found that there is memory effect for Te dopant,on the contrary no memory effect for Sn dopant.The concnetration n of epilayers is proportional to the input concentration of TESn.

CHARACTERIZATION OF QUATERNARY GaInAsSb LAYERS GROWN BY MOCVD. Hong Jiang, Yixin Jin, Tianming Zhou, Baolin Zhang, Yongqiang Ning, Yuan Tian, Changchun Institute of Physics, Chinese Academy of Sciences, Changchun, CHINA.

Carrier concentration and mobility were investigated by Hall measurements deduced from Van der Pauw method on heterostructures (GaInAsSb/SI-GaAs) for temperature range of 77K - 300K. Hall measurement samples were unintentionally doped GaInAsSb epilayers grown on semi-insulating GaAs substrates in an atmospheric metalorganic chemical vapor deposition system with trimethylgallium(TMGa), trimethylindium(TMIn), trimethylantimony(TMSb) and AsH3 as the source materials. The growth temperature was varied from 580oC to 620oC for III/V ratio of 0.5-0.7. The results indicated that the undoped MOCVD-GaInAsSb epilayers have room temperature carrier concentration range from 5.0x1015 to 6.5x1016 cm-3 and Hall mobilities in the range of 40 cm2/V.s. to 260 cm2/V.s.. The function of the carrier concentration for 1/kT shows that four shallow energy levels exist in the band gap of undoped GaInAsSb epilayers. The hole carrier scattering mechanism has been discussed. It was found that the Hall mobility increased with increasing the measurement bath's temperature. Moreover, the compensation coefficient was calculated.

PHOTOREFLECTANCE STUDY OF GaSb AT THE E0 AND E0+0 TRANSITION ENERGIES AND THEIR TEMPERATURE DEPENDENCE. S. Iyer, S. Mulugeta, J. Li, and B. Mangalam, Department of Electrical Engineering, N.C. A&T State University, Greensboro, NC.

We report on low temperature photoreflectance (PR) studies of Te doped GaSb from 0.72 eV to 1.8eV. PR signal corresponding to the fundamental gap E0 and its spin orbit split component E0+ are obtained in this energy range. A variety of samples have been examined, which include bulk samples, liquid phase electroepitaxially grown epilayers and molecular beam epitaxially grow epilayers. No PR signals were obtained from undoped GaSb. The PR spectra line shapes for the E0 and E+ transition energies are similar. The PR spectra are best represented by the theoretical low field first derivative of an exciton. Our results are compared and discussed in relation to experimental data reported in literature by electroreflectance. The temperature dependence of E0 and E0+ transition energies has been investigated in the temperature range of 4K to 300K. The temperature dependence of these two transitions is found to be different. To our knowledge this is the first report of PR of n-GaSb and its temperature dependence.

MODIFICATION OF PHOTOELECTRIC PROPERTIES OF InSb CRYSTALS SUBJECTED TO PULSE LASER IRRADIATION. Volodymyr Gnatyuk, Olena Gorodnychenko, Petro Mozol, Oleksandr Vlasenko, Institute of Semiconductor Physics, National Academy of Sciences, Kiev, UKRAINE

The goal of this work is to study the possibilities for purposeful change of the photoelectric characteristics of InSb crystals using nanosecond ruby laser pulses. Used as the objects of study were n-type InSb crystals with mechanically polished faces and chemically etched ((110) orientation) as well as with a nature cleaved surface. The photoconductivity spectra were measured at T=77 K. The photosensitivity region of the original samples was in wavelength range =1.2-5.8 m with maximum at 5.4 m. The photoconductivity spectrum had a typical shape, indicating an increase in the rate of recombination in the strong absorption region. Irradiation of InSb crystals with laser pulses of subthreshold intensity resulted in the following changes in the photoconductivity spectrum: 1) the photoconductivity signal increased (by a factor of 20 at the maximum of the spectrum); 2) the short wavelength wing rose tending to form a plateau that was evidence a reduction in the surface recombination rate; 3) the maximum and the long-wavelength edge toward shorter wavelengths. Irradiation with .... resulted in deterioration of the surface region and the photosensitivity fell somewhat compared with the samples with the maximum sensitzation but it will remained higher than the initial value. The reasons for the photosensization and the mechanics of laser affect were determined by investigating the temperature dependences of conductivity and nonequilibrium carrier lifetime of InSb crystals subjected to laser irradiation with different intensity. The modification of the photoconductivity spectrum was attributed to laser-stimulated change of the point defect structure in the surface layer. Laser irradiation with nanosecond pulse of subthreshold intensity is considered as effective method of treatment of surface region in order to improve the photoelectric properties of InSb crystals used in optoelectronic devices.

SURFACE AND INTERFACE PROPERTIES OF InSB EPITAXIAL THIN FILMS GROWN ON GaAs BY LOW PRESSURE METALORGANIC CHEMICAL VAPOR DEPOSITION. K. Li, *Z.C. Feng, + A.T.S. Wee, * C. Beckham, + J.Y. Lin, * M. Pelczynski, + K.L. Tan, * I. Ferguson, + R.A. Stall. + * Department of Physics, National University of Singapore, SINGAPORE; + EMCORE Corporation, Somerset, NJ.

There is increasing interest in the epitaxial growth of high quality InSb thin films on GaAs substrates, which have many device applications in the infrared optoelectronics and automobile industry. Because of the large lattice mismatch of 14.6 between InSb and GaAs, different growth techniques and conditions may affect the interface property and film quality greatly. We present a surface science study, by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), on the surface and interface properties of InSb/GaAs grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD). It is found from XPS data that the surface InSb, In2O3 and Sb2O3 relative components are varied with different III/V ratios and In- or Sb rich surfaces. The interdiffusion phenomena are studied by AES depth profiles and the width of interdiffusion region is determined for samples grown under different conditions. In order to examine if there is any new chemical compound formation at the interface during fabrication process, the Ga LMM Auger peaks at different depth, using different sputtering times, from the surface, are traced. These data are in comparison with the results obtained from high resolution X-ray diffraction, Raman scattering and Fourier transform infrared (FTIR) spectroscopy measurements to enhance our understanding on the surface and interface properties of MOCVD-grown InSb/GaAs.

ON THE PHOTOEMISSION FROM SEMICONDUCTOR SUPERLATTICES WITH GRADED STRUCTURES IN THE PRESENCE OF QUANTINZING MAGNETIC FIELD. Kamakhya P. Ghatak, Dept. of Electronic Sciences, Calcutta Univ., Calcutta, INDIA; P.K. Bose and G. Majumder, Dept. of Mech. Engg., Jadavpur University, Calcutta, INDIA.

In recent years, there has been considerable interest in studying the influence of high magnetic field (B) on the various electronic properties of different heavily doped semiconductor superlattices for their potential in uncovering new phenomena in material science. In this paper, we have investigated photoemission from such heterojunctions with graded structures under quantizing magnetic filed, considering broadening and all types of anisotropies of the energy band spectrum within the frame work of k.p. formalism on the basis of a new electron dispersion law. It is found, taking InAs/GaSb superlattices as an example, that the photoemission increases with increasing photon energy in a ladder like manner and also oscillates with inverse quantizing magnetic field and electron concentration respectitively. The well known results for wide gap undoped materials for B = O have also been obtained as special cases from our analysis. In addition, the theoretical formulation is in quantitative agreement with the experimental datas as given elsewhere.

1.95 m COMPRESSIVELY STRAINED InGaAs/InGaAsP QUANTUM WELL DFB LASER WITH LOW THRESHOLD. Jie Dong, Akinori Ubukata, Koh Matsumoto, Tsukuba Lab, Nippon Sanso Corp., Ibaraki, JAPAN.

In 1.2m2m wavelength region, there are many potential applications such as remote sensing, pollutant detection and molecular spectroscopy. We have been developing distributed feedback (DFB) lasers for molecular spectroscopy [1]. Usually, several milliwatts output power with continuous wavelength tuning is needed in molecular spectroscopy. It is not reliable for the previous DFB lasers with 0.1mW output power [2] and mode hopping [3]. In this work, we demonstrate the fabrication and characteristics of 1.95m highly compressively strained InGaAs /InGaAsP quantum well DFB laser gown by LP MOCVD. The laser wafer was grown on (100) p-InP substrate at 650C. Active layer was double In0.81Ga0.19As quantum wells (+1.9%, 10 nm) with a lattice-matched InGaAsP (g=1.36m, 20nm) barrier. Capped mesa burried heterostructure grown also by LP-MOCVD was used for cw operation. Thirty-two cleaved laser chips were measured. Typical threshold current was 5mA15mA (84% sarnples). Lowest threshold current of 3.7mA was obtained in a 400m-long device. Single mode yield with SMSR larger than 30dB was 44%. Maximum power and differential quantum efficiency were 7.2mW and 15.9%, respectively. Total wavelength tuning range was 3.5nm from 2040C with a tuning rate of 0.012nm/mA.

STRAIN COMPENSATED InGaAs SURFACE EMMITING LASER. N.K. Dutta, M.B. Tayahi, Photonics Research Center and Department of Physics, University of Connecticut, Storrs, CT; W.S. Hobson, D. Vakhshoori, J. Lopata, J. Wynn, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.

High power surface emitting lasers are important as sources for high power cladding pumped fiber lasers due to ease of coupling to the large area fibers. The fabrication and performance characteristics of a vertical cavity surface emitting laser design which utilizes lateral current injection for low resistance and lateral oxidation for carrier confinement is described. The lasers have InGaAs active quantum wells with GaAsP barrier layers and InGaP cladding layers. The 4 m diameter devices have threshold current of 1 .5 mA and emit few mW. The 50 m diameter devices have threshold current of 200 mA and emit 250 mW in a single wavelength. The lasers emit near 1 m.

EFFECT OF GEOMETRIC FACTORS ON POLARIZATION PROPERTIES OF VERTICAL-CAVITY SURFACE-EMITTING LASERS WITH TILTED PILLAR STRUCTURES. M. S. Park and B. T. Ahn, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, KOREA; H. Y. Chu, B.-S. Yoo, and H.-H. Park, Electronics and Telecommunications Research Institute, Taejon, KOREA.

We report on the polarization properties of vertical-cavity surface-emitting lasers (VCSELs) with tilted pillar structure. The VCSEL structure is composed of InGaAs/GaAs quantum well and AlAs/GaAs distributed Bragg reflector designed for 990nm wavelength. Circular and square shape laser pillars were etched by reactive ion beam etching with Cl2 gas. Tilted pillar structures were obtained by inclining the substrate toward the beam direction. The tilt angle was varied in a range of 1540. The light emission was dominantly polarized along the perpendicular direction to the tilt direction. This result is attributed to the difference of the optical losses between the parallel and perpendicular direction to the tilted direction. We found that the polarization selectivity roughly increased with increasing the tilted angle and decreasing device size between 20m7m diameters. From the tilted laser structures, we recorded the best polarization suppression ratio of 320:1. We will discuss the polarization controllability considering the optical losses in the tilted waveguide geometries.

1.55m SURFACE EMITTING LASER STRUCTURE WITH In0.53Al0.14Ga0.33As/In0.52Al0.48As DISTRIBUTED BRAGG REFLECTOR AND SINGLE CAVITY ACTIVE LAYER GROWN BY METALORGANIC CHEMICAL VAPOR DEPOSITION METHOD. Jong-Hyeob Baek, Bun Lee, Jason M. Smith, and El-Hang Lee, ETRI, Taejon, SOUTH KOREA; Won Seok Han, Chunam National Univ, Dept of Physics, Taejon, SOUTH KOREA; Boo Sung Jung, Yonsei Univ, Dept of Physics, Seoul, SOUTH KOREA.

Surface emitting laser (SEL) structure designed at 1.55 m was grown by low pressure metalorganic chemical vapor deposition method. The SEL structure contains top and bottom distributed Bragg reflector (DBR) and single cavity active layer. The DBR was grown with In0.53Al0.14Ga0.33As and In0.52Al0.48As quarter lamda wavelength layer, alternatively. The growth temperature was as high as 750C to prevent ordering and phase separation of the In0.52Al0.48As layer. The In0.52Al-ray detector and imaging devices at present is hampered by material issues that limit the yield of large-size and high-quality crystals. These include ingot cracking, formation of pipes, material homogeneity and the reproducibility of the material from growth to growth.

8:30 AM *F9.1 
ATOMIC DIFFUSION IN SEMICONDUCTORS: BASIC PRINCIPLES AND ITS IMPACT ON OPTOELECTRONIC DEVICES. Louis J. Guido, Center for Microelectronic Materials and Structures, Yale University, New Haven, CT.

Even though atomic diffusion is one of the oldest and most extensively investigated subjects the reduced length scales, high doping concentrations, and abrupt interfaces associated with modern semiconductor devices present an extraordinarily complex problem the outcome of which plays a crucial role in device performance. Diffusion phenomena can be classified into three basic categories: self-diffusion, interdiffusion and impurity diffusion. The focus of this seminar is on interdiffusion. The most widely accepted description of interdiffusion assumes equilibrium is achieved instantaneously between vapor and solid phases and treats all solid phases in an identical manner. This viewpoint ignores the inherent coupling between point defect and host atom diffusion and neglects to consider the film/substrate interaction. The experiments described below address these issues. The AlxGal-x As material system was used as a standard platform to investigate diffusion related phenomena. A pair of 8 m thick AlGaAs-GaAs heterostructure crystals consisting of five, equally spaced GaAs quantum wells were grown simultaneously on n-type and semi-insulating GaAs substrates by metalorganic chemical vapor deposition. These samples were susequently annealed under an arsenic overpressure at temperatures ranging from 800 to 900C for times between 1 and 12 h. Annealing-induced changes in the optical transition energies for all five quantum wells were tracked by performing spatially resolved photoluminescence measurements across the freshly cleaved edges of the samples. The coupled diffusion equations were solved under general boundary conditions and fitted to the experimental data to obtain interdiffusion parameters. Monte Carlo sampling was used to determine the margins of error for these parameters with the variations across the samples as inputs. This work will be discussed within the context of ongoing efforts to understand and control interdiffusion in a variety of other compound semiconductor systems.

9:00 AM *F9.2 
QUANTUM WELL INTERMIXING FOR OPTOELECTRONIC APPLICATIONS. C. Jagadish, H.H. Tan, S. Yuan, Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, Australian National University, Canberra, AUSTRALIA; M. Gal, School of Physics, University of New South Wales, Sydney, AUSTRALIA.

Quantum well intermixing (QWI) has drawn considerable interest in recent years due to its potential for the fabrication of photonic integrated circuits. Interdiffusion of quantum wells lead to the modification of band gap as well as optical properties. Several techniques used for QWI include, impurity induced disordering (IID), impurity-free vacancy disordering (IFVD), photoabsorption induced disordering and implantation induced intermixing (III). In this work, we present the results on implantation induced intermixing of GaAs/AlGaAs quantum wells for H, O and As ions. Low temperature photoluminescence (PL) has been used to monitor QWI. PL energy shifts were found to increase with increase in ion dose for all the species, though saturation in energy shifts was observed at higher doses for heavier species. The PL energy shifts were also found to be dependent on the Al composition in the barriers, well thickness, implantation and annealing temperatures, indicating that QWI is influenced by various parameters. This technique was extended to GaAs/AlGaAs quantum well lasers where wavelength shifts of up to 40 nm were observed. Recently, we have developed another QWI technique based on pulsed anodic oxides of gallium arsenide. Anodic-Oxide Induced Intermixing (AOII) is a promising technique, particularly in device applications due to its simplicity. Results on AOII of GaAs/AlGaAs quantum wells and quantum wires will be presented. By using AOII of the side walls in quantum wire structures grown on non-planar substrates, enhanced PL emission from the quantum wire regions at bottom of the V-grooves was observed.

9:30 AM F9.3 
THE EFFECT OF TENSILE STRAIN ON A1GaAs/GaAsP INTERDIFFUSED QUANTUM WELL LASER. K.S. Chan, City University of Hong Kong, Dept. of Physics and Materials Science, Kowloon, HONG KONG.

Several studies have reported that tensile strained quantum well (QW) lasers show improvements in performances including lower threshold currents, higher differential gains, and wider frequency modulation band width as compared to unstrained QW lasers. In a tensile strained QW, the biaxial tensile strain shifts the light hole bands closer to the conduction band minimum than the heavy hole bands. With a suitable choice of materials and structure parameters, it is possible to optimize the performances of QW laser by tuning the separation of the heavy and light hole subband energies. In order to shift the heavy and light holes energy levels, the engineering of band-gap structures by thermal-induced composition intermixing is a useful tool. The intermixing process involves the interdiffusion of the constituent atoms of the heterostructure by controlling the annealing temperature, annealing time and defect distribution. During the process the as-grown square-QW compositionalprofile is modified to a graded profile thereby altering the confinement profile and subband structure in the interdiffused QW (DFQW). In this paper, the interdiffusion of GaAsyP1 y/AI0.33GaO.67As single QW structure with a well width of 60Å is studied. Different P concentrations in the as-grown well are chosen to induce different tensile strains. Interdiffusion causes variations in the tensile strain and potential profiles, which consequently change the valence band structure and the optical gain spectrum.

9:45 AM F9.4 
REDUCED Al-Ga INTERDIFFUSION IN GaAs/AlGaAs MULTIPLE QUANTUM WELL STRUCTURE BY INTRODUCING Si-RICH SiN CAPPING LAYER FOR DIELECTRIC CAP QUANTUM WELL DISORDERING. Won Jun Choi, Sang Min Han, Syed Ijaz Shah, Suk Geun Choi, Deok Ha Woo, Seok Lee, Hwe Jong Kim, Il Ki Han, Sun Ho Kim, Jung Il Lee, Kwang Nham Kang, Korea Institute of Science and Technology, Photonics Research Center, Seoul, KOREA; Jaewon Cho, Kwangwoon University, Dept of Physics, Seoul, KOREA.

We present the results of the dielectric cap quantum well disordering (DCQWD) of GaAs/AlGaAs multiple quantum well (MQW) structure grown by molecular beam epitaxy (MBE) with SiNx capping films. The MQW sample consists of 4 pairs of 7 nm thick GaAs well and 10 nm thick Al0.2Ga0.8As barrier. The composition of the SiN capping layers over the MQW structure was varied by changing the flow rate of NH3 during film growth by plasma enhanced chemical vapor deposition method. DCQWD was carried out by rapid thermal annealing (RTA) at 950 oC for 30 seconds without GaAs proximity cap. Photoluminescence (PL) measurement of the disordered MQW samples showed that Si-rich SiN capping film caused smaller blue shift than that of stoichiometric SiN capping film. Diffusion equation and the Schrodinger's equation were solved numerically to obtain the compositional profile and transition energy in the disordered MQW structure. We obtained interdiffusion coefficient by comparing the experimental results and calculation result. The interdiffusion coefficient, DAl-Ga of the Si-rich SiN capped sample was about an order of magnitude smaller than that of stoichiometric SiN capped sample, though stoichiometric SiN capped sample and SiO2 capped sample exhibited nearly same interdiffusion coefficients at the same RTA conditions. This results can be used for the integration of optoelectronic devices.

10:30 AM F9.5 

Heteroepitaxy in lattice-mismatched systems are becoming more and more important for achieving high-performance electronic and optoelectronic devices. In the growth of lattice-mismatched systems, a critical layer thickness (CLT) exists beyond which coherently strained pseudomorphic growth alters the growth with misfit dislocations. This restricts the design of the device structure. In the theoretical calculations, the first accepted model introduced by Matthews and Blakeslee in 1974 is based on the energy equilibrium which gave a result in agreement with the experimental data. Recently, the thermally induced composition intermixing in the heterostructure QW is becoming a popular choice. The intermixing process involves the interdiffusion of the constituent atoms of the heterostructure, the processing temperature, and time. During the process the as-grown square-QW compositional profile is modified to a graded profile thereby altering the confinement profile and subband structure in the interdiffused QW (DFQW). The CLT of the DFQW structures is not clearly defined by a conventional methods. The conventional way to determine the CLT of a strained QW is only to consider the abrupt interface while the DFQW has the graded compositional profiles of the interface. Therefore, it is difficult to define the QW width for DFQW. In this paper, the proposed theoretical model for tackling the CLT in DFQW is now considered. The theory is based on the equilibrium of two type of energies: the strain relief energy and the self energy of misfit dislocation. These two energies can be calculated with the graded and continuous profile of the DFQW. The results are useful information for applying the DFQW structure in the optoelectronic devices fabrication.

10:45 AM F9.6 
ACTIVE ANTI-GUIDE VERTICAL CAVITY SURFACE EMITTING LASERS WITH DIFFUSED QUANTUM WELLS STRUCTURE. S.F. Yu, E. Herbert Li and W.M. Man, The University of Hong Kong, Department of Electrical and Electronic Engineering, HONG KONG.

Single longitudinal mode operation, low threshold current and narrow output beam are the promising characteristics of vertical cavity surface emitting lasers (VCSEL's) for various applications such as high speed parallel optical communication and computing systems. However, VCSEL's exhibit multiple transverse modes operation at high power which deteriorate the performance of optical communication systems. The excitation of multiple transverse modes is caused by the increase in refractive index, arises from carrier spatial hole burning and thermal lensing, inside the core region of the active layer. In this paper, a simple DFQW's structure of VCSEL's is proposed to enhance single-mode operation. The QW active layer of this laser consists of a step DFQW's structure which can be defined selectively within the core region. This will create a non-uniform stepped refractive index profile such that an anti-guiding structure for the transverse modes is obtained. The purpose of the step DFQW's structure is to compensate the influence of self-focusing and stabilize the profile of transverse modes. It is found that by introducing a step diffused quantum wells structure inside the core region of quantum-well active layer, the influence of self-focusing (due to carrier spatial hole burning and thermal lensing) on the profile of transverse modes can be minimized. Therefore stable single-mode operation in vertical cavity surface emitting lasers can be improved. Details of the calculation will be discussed in the presentation.

11:00 AM F9.7 
MODELING SELECTIVE COMPOSITIONAL MIXING IN AlGaAs/GaAs SUPERLATTICES. Richard Gass, and Howard E. Jackson. Department of Physics, University of Cincinnati, Cincinnati, OH.

Compositional interdiffusion in Al.03Ga0.7As/GaAs superlattice structures with equal 3.5 nm barrier and well widths induced by Si focused ion beam implantation and subsequent rapid thermal annealing has been calculated within a new model. QuickTime movies of the time evolution of the compositional diffusion will be presented for the models of Kahen, Rajeswaran, and Lee and for the new model. Distinctive differences are observed, for instance, in the strong depth dependence of the mixing process is observed at a Si++ energy of 100 keV and at a dose of 1x1014 cm-2. The model includes the dynamics of the vacancy spatial profile, and results in good agreement with experimental results. Preliminary explorations of the effects of lateral spatial interdiffusion will also be presented.

11:15 AM F9.8 
CATION INTERDIFFUSION IN GaInP/GaAs SINGLE QUANTUM WELLS. Joseph Micallef, Andrea Brincat, Univ of Malta, Dept of Microelectronics, Msida, MALTA.

The effects of cation interdiffusion in GaInP/GaAs single quantum wells are investigated using an error function distribution to model the compositional profile after interdiffusion. Two interdiffusion conditions are considered: cation interdiffusion only, and dominant cation interdiffusion. For both conditions the fundamental absorption edge exhibits a red shift with interdiffusion, with a large strain build up taking place in the early stages of interdiffusion. In the case of cation interdiffusion only, an abrupt carrier confinement profile is maintained even after significant interdiffusion, with a well width equal to that of the as-grown quantum well. When the interdiffusion takes place on the two sublattices, but with the cation interdiffusion dominant, the red shift saturates and then decreases. The model results are correlated with reported experimental results. The effects of the interdiffusion-induced strain on the carrier confinement profile can be of interest for device applications in this material system.

11:30 AM F9.9 
INTERDIFFUSION MECHANISMS IN GAAS/ALGAAS QUANTUM WELL HETEROSTRUCTURES INDUCED BY SIO2 CAPPING AND ANNEALING. A. Pepin, C. Vieu, M. Schneider, H. Launois, Laboratoire de Microstructures et de Microelectronique, CNRS, Bagneux, FRANCE; E. V. K. Rao, Centre National d'Etudes des Telecommunications, Bagneux, FRANCE.

Post-growth tailoring of semiconductor quantum wells via intermixing has received considerable interest over the past decade. Indeed, it is well known that interdiffusion changes the shape of quantum wells and thus induces a modification of the confined electronic levels. We have investigated intermixing enhancement in GaAs/AlGaAs quantum well heterostructures obtained by SiO2 capping and subsequent annealing using low temperature photoluminescence, in conjunction with secondary ion mass spectroscopy and cross-sectional transmission electron microscopy. Various dielectric deposition techniques were compared. The highest quality interdiffusion, with respect to interdiffusion rate, homogeneity, material quality, and reproducibility, was obtained with the purest, denser SiO2 films produced by rapid thermal chemical vapor deposition. Evidence of fast Ga pumping inside the SiO2 layer during anneal and simultaneous generation of excess Ga vacancies under the SiO2/GaAs interface is presented. The interdiffusion rate is found to increase linearly with oxide thickness and reveals a sponge-like behavior of SiO2. The SiO2 film quickly absorbs Ga up to a fixed solubility limit, which increases with anneal temperature. A simple model involving the thermal stress arising from the difference in thermal expansion coefficients between SiO2 and GaAs, is proposed to account for the abnormally fast Ga vacancy diffusion inside the heterostructure and the outstanding homogeneity of the resulting enhanced intermixing process. A spatial control of the interdiffused areas can be achieved if a suitable stress field is imposed on the surface of the semiconductor by the capping layers. We show experimental evidence of this effect using a specific patterning of SiO2/Si3N4 bilayers. Potential applications for quantum nanostructures fabrication will be discussed.

11:45 AM F9.10 
A1GaAs/GaAs QUANTUM WELL ELECTRO-OPTIC PHASE MODULATOR WITH DELINEATED OPTICAL CONFINEMENT. Bernard L. Weiss and Wallace C.H. Choy, University of Surrey, Department of Electronic Electrical Engineering, UNITED KINGDOM.

Two waveguide type phase modulators with 0.5m and lm Quantum Wells active regions using masked ion-implantation to produce lateral cladding have been theoretically investigated. These devices are designed for single mode operation at 0.87m for 50kV/cm operation and 0.875m for l00kV/cm operation, in which it is found that the extension of QW interdiffusion can be used to tune the propagating modes of the device. The metal contact width of 4m is made deliberately larger than the waveguide width of 3m, to avoid the discontinuity of refractive index profile at the lateral cladding core interface. In designing a single mode phase modulator with strong optical confinement, the peak ion concentration should be at the center of the guiding layer. Moreover, the annealing time should be selected at the midpoint of the overlapping region between the two single mode cutoff ranges for two desired applied field values. Low impurity concentrations are also required to minimize lattice damage and thus retain the electro-optical properties of the device structure. By comparing the modulation properties of 0.5m and l.0m MQW device structure, the I.0m structure can provide higher optical confinement and larger total phase change per unit mm. The 0.5m structure can operate either in 0 and 50kV/cm or in 0 and l00kV/cm. The more important comparison is of the two structures and the 0.5m structure can provide more efficient modulation since its value of is twice that of the l.0m structure. of the 0.5m structure can be farther reduced by increasing the applied field from 50 to l00kV/cm since the longer can be used, although this increase results in a reduction of . It is important to note that of 13.2 that is obtained for a field of l00kV/cm operation is still large enough for a good phase modulator. The results chronologically shown here can be as a guideline to develop an intermixing optical confined phase modulator.

Chairs: Ravindra Pandey and Gregory J. Salamo 
Thursday Afternoon, December 4, 1997 
Salon J/K (M)

1:30 PM *F10.1 
SECOND-HARMONIC AND SUM-FREQUENCY GENERATION in CdGeAs2. Eiko Tanaka, Keio University, Yokohama, Japan; Kiyoshi Kato, Japan Defense Agency, Tokyo, Japan.

CdGeAs2 has been found to be phase matchable for type-2(b) SFG between the fundamental and second Harmonic of a CO2 laser down to 10.4406 m at 20 C. The Sellmeier's equations and the thermo-optic dispersion formula which reproduce well the phase matching properties of SHG and direct FHG of the CO2 lasers at 77K are presented together with the absolute value of the nonlinear optical constant.

2:00 PM F10.2 
LOW OPTICAL LOSS WAFER BONDING GaAs STRUCTURES FOR QUASI-PHASE MATCHED SECOND HARMONIC GENERATION. YewChung Sermon Wu, Robert S. Feigelson, Roger K. Route, Dong Zheng, Leslie A. Gordon, Martin M. Fejer, Robert L. Byer Center for Nonlinear Optical Materials, Stanford University.

A periodic GaAs wafer-bonded structure has been proposed for quasi-phase-matched (QPM) second harmonic generation (SHG). However, current bonding processes often lead to unacceptable optical losses and poor device performance. In this study, three sources of optical losses in wafer-bonded structures were investigated, (1) interfacial defects between the wafers, (2) bulk defects within the wafers, and (3) decomposition at the exposed outer surfaces. Surface losses due to incongruent evaporation were easily eliminated by repolishing the outer surfaces. However, to minimize the losses from interfacial and bulk defects, it was necessary to investigate the relationship between these defects and the bonding parameters. It was found that an increase in bonding temperature and/or time led to a decrease in interfacial defects, but an increase in bulk and surface defects. Through a trade-off process, optimized processing conditions were developed which permitted the preparation of bonded stacks containing over 50 layers of (100) GaAs wafers, and about 40 layers of (110) GaAs wafers. Optical losses as low as 0.1-0.3% interface (at 5.3 and 10.6 ) were observed for the (110) oriented multi-layer structures.

2:15 PM F10.3 
PHASE-MATCHED 2ND HARMONIC GENERATION IN ASYMMETRIC DOUBLE QUANTUM WELLS. Konstantin Vodopyanov, Kevin O'Neill, Chris Phillips, Imperial College, London, UNITED KINGDOM; Igor Vurgaftman, Jerry Meyer, Naval Research Laboratory, Washington DC.

Intersubband-based second harmonic generation (SHG) processes in semiconductor multiple quantum wells (MQWís) are particularly attractive, since asymmetric QW structures yield giant resonant second-order nonlinear susceptibilities. Nonetheless, SHG conversion efficiencies reported so far have been rather small and to improve on this, longer interaction lengths are needed, necessitating the matching of the phase-velocities of the fundamental and second harmonic beams to maximise the energy transfer between them. A special phase-matched waveguide structure, lattice matched to InP, was used (8m thick and 200m long) consisting of an active SHG region incorporating 178 periods of repeated asymmetric InGaAs/InAlAs double quantum wells (30‰ InGaAs-18‰ InAlAs-54‰ InGaAs-100‰ InAlAs) with subband resonances close to =8.5 m and its harmonic, placed in-between two phase-matching InGaAs/InAlAs regions containing 139 periods of QW (58‰ InGaAs-100‰ InAlAs). The frequencies of the intersubband transitions in the phase-matching QWís were chosen midway between the fundamental and SHG frequencies, so that the additional dispersion they produced (calculated from the Kramers-Kronig transform of the intersubband absorption line) cancelled the normal material dispersion. Single 90 picosecond pulses from a novel widely tunable (3.3-19m)IR optical parametric generator were focussed onto the sample using a f=50mm BaF2 lens. The pump intensity, in a 100mm focal spot, was varied in the range 0.3-50 MW/cm2. Only about 8% of the fundamental harmonic (FH) energy entered the SHG waveguide due to its small thickness. The resulting SHG signal was monitored using an InSb detector ( 5.6 m longwave cut-off) and SHG dependence evidences the strongly resonant spectral behaviour of the SHG signal, peaking at 8.5 m, near the design wavelength. To resume, we demonstrate intersubband-based second harmonic generation in an asymmetric InGaAs/InAlAs double quantum well structure with an additional phase-matching region. A resonance enhancement near =8.5 m and 0.2% external conversion efficiency were obtained.

2:30 PM F10.4 
DARK SOLITON FORMATION FOR LIGHT-INDUCED WAVEGUIDING IN PHOTOREFRACTIVE InP. M.E. Chauvet, S. A. Hawkins, and G.J. Salamo, Dept of Physics, University of Arkansas, Fayetteville, AR; M. Segev, Dept of Electrical Engineering and Advanced Center for Photonics and Optoelectric Materials, Princeton University, Princeton, NJ; and D.F. Bliss, G. Bryant, U.S. Air Force, Rome Laboratory, Hanscom Air Force Base, MA.

Optical spatial solitons1 in photorefractive crystals offer potential applications in the field of alloptical switching and beam steering. A photorefractive soliton is created when a photo-induced index change exactly compensates for the diffraction of the beam. In this sense the beam is able to create its own waveguide. These effects have been extensively studied in ferroelectric oxide and sillenite oxide crystals for visible wavelengths. For the near infra-red wavelengths used in telecommunications, InP:Fe crystals have already demonstrated interesting photorefractive properties. For example, self-trapping of a laser beam has been reported in InP:Fe2. In this paper we report the first observation of a dark soliton formed at a wavelength of lm in InP. 
For the expenment a dark notch is produced in the center of a laser beam from a Ti-sapphire laser by reflecting the incident Gaussian beam off a phase plate which introduces a 180 phase shift between two halves of the beam. The beam profile at the phase mirror is then imaged onto the crystal entrance face An electric field is applied along <100> the laser beam propagates along <110>, and is polarized along <110>. The beam notch size at the entrance face of the crystal has a diameter of about 12m and diverges to a 36m diameter at the exit face when no field is applied to the crystal. The InP:Fe crystal length is 0 6cm in the direction of the propagation. When a 12 kV/cm field is applied to the crystal, the beam notch is trapped and the notch diameter at the exit face is reduced to about 12m.

3:00 PM *F10.5 
ZnGeP2 AND ITS RELATION TO OTHER DEFECT SEMICONDUCTORS. A.W. Vere, L.L. Taylor, P.C. Smith, C.J. Flynn, M.K. Saker and J.A.C. Terry, Defence Evaluation and Research Agency, Malvern, UNITED KINGDOM.

ZnGeP2 is a semiconductor currently showing great promise as a material for 3-5m waveband coherent optical sources based on optical parametric oscillators (OPOs). Power outputs of >3 watt, with quantum efficiencies of >50% have been quoted. Significant reductions in extrinsic optical absorption in the 1-3m OPO pump waveband have been achieved. We have studied the growth of this material by horizontal and vertical Bridgman and gradient freeze techniques and will present a comparison of the results obtained. The best results were obtained from vertical Bridgman which showed 2.05m absorbance values as low as 0.45cm-1 in the as-grown state. Early problems related to mosaic cracking in the material will be discussed and solutions based on seed orientation and the reduction of thermal stresses during cooling will be described. Attention in the research community is now increasingly directed to understanding the interactions between native defects and between native defects and impurities. These interactions control the pump-band absorption. In our work we have noted hitherto unreported comparisons with similar defect structures in other semiconductor groups, in particular, the II-VI materials, CdTe, CdHgTe and ZnSe. Some of these will be presented as a basis for discussion of possible models for the absorption process.

3:30 PM *F10.6 
GROWTH OF NLO CHALCOPYRIT MATERIALS BY OMVPE. Michael Timmons, Research Triangle Institute, Research Trangle Park, NC; Klaus Bachmann, North Carolina State University, Material Science and Engineering, Raleigh, NC.

In this presentation we describe the growth of chalcopyrite materials that may be suitable for NLO applications. The focus of the paper is ZnGeAs2, although growth details for ZnGeP2 are also included. The growth technology is organometallic vapor phase epitaxy (OMVPE). The growth of the materials by OMVPE is difficult. Growth "windows" are small. The differing vapor pressures of the reactants are largely responsibility for this difficulty. Nonetheless, very good quality epitaxy has been achieved for both ZnGeAs2 and ZnGeP2. ZnGeAs2 has been grown on large-area Ge and GaAs substrates. X-ray rocking curves show the same FWHM for the epilayers and substrates. Strong photoluminescence has also been measured. The main challenge for devices made from these materials is control of the p-type conductivity that results from native defects, mainly Zn vacancies and Zn on Ge sites. The presentation includes doping studies using Group VI and Group III elements. We have not been able to produce n-type material with these dopants, but indium does seem to reduce the heavy p-type behavior. For ZnGeAs2 carrier concentrations in the 1016-cm-3 have been measured. The presence of interstitial Zn in the material, which produces n-type behavior, is also examined.

4:00 PM F10.7 
IMPROVED CHALCOPYRITES FOR INFRARED LASER APPLICATIONS. P.G. Schunemann, T.M. Pollak, and I. Zwieback, Sanders-A Lockheed Martin Company, Nashua, NH.

Substantial improvements in the properties of I-III-VI2 and II-IV-V2 chalcopyrite crystals in the last year have resulted in the demonstration of high-average-power mid-infrared lasers with 3-5 m output powers exceeding l0 Watts. ZnGeP2 and AgGaSe2 are the most mature materials in the class. Five-fold reductions in the ZnGeP2 near-band-edge absorption coefficient boosted the performance of 2 m-pumped optical parantric oscillators, while phase- matched growth of AgGaSe2 crystals increased the interaction length for more efficient frequency doubling of CO2 lasers. Even as these crystals continue to be refined, new materials are needed which offer higher nonlinear coefficients, additional wavelength coverage, and ideally ``noncritical phase-matching'' (NCPM) for optimal laser frequency conversion. CdGeAs2 exhibits the highest nonlinear coefficient of all chalcopyrites, but continued reductions in loss are required for multi-watt operation at non-cryogenic temperatures. Mixed-crystal chalcopyrites such as AgGa1-x, lnx,Se2 and CdGe(As1-xPx)2 can offer improved device performance by engineering the refractive indices to eliminate birefringence walk-off. Finally, crystal growth in a transparent furnace has facilitated rapid development and evaluation of unexplored compounds in the chalcopyrite family. Progress on all of these fronts will be reported, along with the latest device performance milestones.

4:15 PM *F10.8 
GaSe NONLINEAR DIFFERENCE FREQUENCY GENERATION OF TUNABLE MID-IR USING NEARVISIBLE DIODE LASERS. Roger S. Putnam, Aerodyne Research Inc., Billerica, MA; David Lancaster, Rice Quantum Institute, Rice University, Houston, TX.

We report the generation of tunable 10-15 micron infrared using convenient, room temperature 800-900 nm diode lasers by difference frequency generation in 5-8 mm thick GaSe crystals. Absorption spectra of ammonia and acetylene and detection of chlorofluorcarbon 12 have been demonstrated using smooth temperature and current tuning of the diode lasers. Pure GaSe crystals cleave across the z-axis (1) and are phase matched by rotating in the y-z plane to an internal angle of approximately 15 degrees as calculated (2). The collinear input beams are combined using a polling beamsplitter and are removed from the generated mid-IR by a germanium window in front of the detector(3). We also report a modification of the standard figure of merit for angle tuned nonlinear materials used for DFG which includes the effect of beam walkoff in the case of optimum elliptical beams: figure of merit=(d2/n3)*(1/(nIR*rho)) where rho is the walkoff angle in radians, n is the optical index, and d is the nonlinear coefficient. We also report computer controlled beam alignment of the difference frequency generator for automated mid-IR molecular spectroscopy using compact CCD cameras with motorized mirrors for 4 micron beam positioning, 0.01 degree accurate crystal angle positioning, automated diode laser beam collimation, and diode laser temperature and current tuning through spreadsheet lookup tables.

4:45 PM F10.9 
LASER DAMAGE STUDIES OF SILVER GALLIUM SULFIDE SINGLE CRYSTALS. Warren Ruderman and John Maffetone, INRAD Corp., Northvale, NJ; and David E. Zelmon, Materials Directorate, Air Force Research Laboratory, Wright Patterson AFB OH.

Silver gallium sulfide single crystals have exceptional optical properties including a high nonlinear optical susceptibility and a transmission from 0.45 to 13 microns. No other nonlinear crystal that is transparent in this region can be phase matched in an optical parametric oscillator (OPO) pumped by a 1.064 micron laser. Such an OPO can produce tunable laser radiation from 1.5 to 11 microns. Silver gallium sulfide crystals have laser damage thresholds far below the values that are needed for useful OPO systems. Laser damage occurs at the crystal surface, rather than in the bulk. We have studied the nature of surface laser damage in silver gallium sulfide crystals, and we will describe our findings. Polished, cleaved, and ion beam sputtered surfaces of silver gallium sulfide single crystals, characterized by electron spectroscopy for chemical analysis (ESCA) and by Auger electron spectroscopy, showed anomalous values of the stoichiometry at the surface compared with the bulk. Surface laser damage sites were also analyzed by ESCA.