Symposium Organizers
S. Ashok The Pennsylvania State University
Peter Kiesel Palo Alto Research Center
Jacques Chevallier CNRS
Toshio Ogino Yokohama National University
F1: Dopant and Defect Issues in Oxide and Nitride Semiconductors
Session Chairs
Peter Kiesel
Antonio Polimeni
Tuesday PM, April 10, 2007
Room 3004 (Moscone West)
9:30 AM - F1.1
Dopability, Intrinsic Conductivity, and Non-stoichiometry of the Transparent Conducting Oxides In2O3 and ZnO.
Stephan Lany 1 , Alex Zunger 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractDefect models for semiconductors and insulators usually rely on theoretical calculations, since defect formation energies ΔH are experimentally hardly accessible. For the prototype transparent conductive oxide In
2O
3, it is assumed that n-type conductivity is related to O-deficient non-stoichiometry due to oxygen vacancies. Since barely any theoretical defect calculations exist for In
2O
3, so far, such models remain speculative. For ZnO, on the other hand, a wealth of theoretical data exists, but the reported defect formation energies are highly controversial.
Applying systematic corrections to first-principles calculated formation energies ΔH, we here develop comprehensive defect models for In2O3 and ZnO, which we validate by calculating experimentally accessible quantities, i.e. defect and carrier densities [1].
We find: (i) Intrinsic acceptors in both materials have rather high formation energies, which explains high n-type dopability. (ii) The O-vacancy has a low ΔH, which explains the O-deficient non-stoichiometry. (iii) Neither the O vacancy donors nor the donor-like cation interstitials create stable n-type conductivity, as the cation interstitials have a high formation energies and O vacancies the have deep equilibrium transition levels. (iv) The O vacancies do have, however, a metastable shallow state, and explain the paradoxical coexistence of coloration (deep optical level in the visible range) and conductivity (shallow, free-electron producing state), after Zn-rich growth or annealing.
[1] S. Lany and A. Zunger, submitted.
9:45 AM - F1.2
Microscopic Origin of Amphoteric Phosphorus Doping for Stable p-type ZnO.
Xiaoqing Pan 1 , Arnold Allenic 1 , Yanbin Chen 1 , Wei Guo 1 , Guangyuang Zhao 1 , Yong Che 2 , Zhendong Hu 2 , Bin Liu 2 , Shengbai Zhang 3
1 , University of Michigan, Ann Arbor, Michigan, United States, 2 , IMRA Inc., Ann Arbor, Michigan, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe greatest challenge for ZnO optoelectronics remains to fabricate reliable and stable p-type ZnO thin films. Though nitrogen is theoretically the most promising acceptor for ZnO, its low solubility and compensation by donors such as hydrogen have been major obstacles. As alternatives to nitrogen, large size group V elements such as P, As, Sb and Bi have been widely investigated. The first principles calculations predicted a shallow acceptor level involving a group V antisite and two zinc vacancies (complex acceptor). In this work we have contacted a systematic study of microstructure, crystal defects, electrical and optical properties of epitaxial phosphorus-doped ZnO thin films grown by pulsed laser ablation. The conductivity of our P-doped ZnO (PZO) films can be tuned either n- or p-type by controlling the growth and annealing temperature, and the p-type films are stable under ambient conditions for 14 months without apparent degradation. Transmission electron microscopy reveals that the p-type films consist of a high density of dislocations, which enhances both the solubility of P and the generation of Zn vacancies to facilitate the formation of zinc vacancies for PZn-2VZn complex acceptors which make it possible to convert conductivity type from n-to-p. Photoluminescence (PL) measurements reveal the corresponding acceptor level at 151 meV. Our physical insights further allow for the fabrication of ZnO homojunctions by adjusting only growth and annealing temperatures during processing with impressive rectifying characteristics. These studies provide experimental proofs to the amphoteric doping nature of phosphorus in ZnO.
10:00 AM - **F1.3
Arsenic in ZnO and GaN: Substitutional Cation or Anion Sites?
Ulrich Wahl 1 2 , Joao Guilherme Correia 1 2 3 , Elisabete Rita 2 , Ana Claudia Marques 2 3 , Eduardo Alves 1 2 , Jose Carvalho Soares 2
1 Fisica, Instituto Tecnologico e Nuclear, Sacavem Portugal, 2 , Centro de Física Nuclear da Universidade de Lisboa, Lisbon Portugal, 3 PH, CERN, Geneva Switzerland
Show AbstractModifying the properties of ZnO and GaN by means of incorporating arsenic impurities is of interest in both of these semiconductors, although for different reasons. In the case of ZnO, the group V element As has been reported in the literature as one of the few p-type dopants in this technologically promising II-VI compound. However, there is an ongoing debate whether the p-type character is due to As simply replacing O atoms or to the formation of more complicated defect complexes, possibly involving As on Zn sites [1]. In the case of GaN, the incorporation of high concentrations of As has been studied with respect to the formation of GaAs(x)N(1-x) alloys and the related modification of the GaN band gap and its luminescence behaviour. It has been suggested that As in GaN is amphoteric, with its lattice site preference depending on the doping character of the material, i.e. mostly substitutional Ga in p-type but also substitutional N in n-type [2].We have determined the lattice location of implanted As in ZnO and GaN by means of conversion electron emission channeling from radioactive 73As. In contrast to what one might expect from its nature as a group V element, we find that As does not occupy substitutional O sites in ZnO but in its large majority substitutional Zn sites [3]. Arsenic in ZnO is thus an interesting example for an impurity in a semiconductor where the major impurity lattice site is determined by atomic size and electronegativity rather than its position in the periodic system. The results are different in the case of As implanted into GaN, where we found roughly half of the implanted As atoms occupying Ga and the other half N sites. The amphoteric character of As therefore certainly plays a role in explaining the extreme difficulties in growing high quality GaAs(x)N(1-x) alloys with values of x above a few percent.A preliminary report will also be given on ongoing emission channeling lattice location experiments using radioactive 124Sb in ZnO and GaN.[1] S. Limpijumnong, S.B. Zhang, S.H. Wei, and C.H. Park, Phys. Rev. Lett. 92 (2004) 155504.[2] C.G. Van De Walle and J. Neugebauer, Appl. Phys. Lett. 76 (2000) 1009.[3] U. Wahl, E. Rita, J.G. Correia, A.C. Marques, E. Alves, J.C. Soares, and the ISOLDE collaboration, Phys. Rev. Lett. 95 (2005) 215503.
10:30 AM - F1.4
Design of Shallow Acceptors in ZnO
Su-Huai Wei 1 , Jinbo Li 1 , Yanfa Yan 1
1 , National Renewable Energy Lab, Golden, Colorado, United States
Show AbstractZnO has recently attracted much attention because it has interesting physical properties suitable for short-wavelength optoelectronic device applications. However, similar to most oxide materials, ZnO is difficult to be doped p-type because ZnO has a lower valence band maximum (VBM). Consequently, acceptor ionization energy in ZnO are too high. This p-type doping bottleneck has so far hindered the full utilization of ZnO as a novel optoelectronic material. In this work, by analyzing the defect wavefunction characters, we propose several approaches to lower the acceptor ionization energy by codoping acceptors with donors or isovalent atoms. Using the first-principles band-structure method, we show that the acceptor transition energies of V$_{Zn}$-O$_O$ can be reduced by introducing F$_O$ next to V$_{Zn}$ to reduce electronic potential, whereas the acceptor transition energy of N$_O$-nZn$_{Zn}$ (n=1-4) can be reduced if we replace Zn by isovalent Mg or Be to reduce the anion and cation kinetic p-d repulsion, as well as the electronic potential. Other approaches to reduced the acceptor ionization energy will also be discussed.
10:45 AM - F1.5
Formation Of Impurity Complexes During The Growth Of Undoped And Nitrogen Doped Zinc Oxide.
N. Nickel 1 , F. Friedrich 1 , J. Rommeluere 2 , P. Galtier 2
1 , Hahn-Meitner-Institut Berlin, Berlin Germany, 2 , CNRS-LPSC, Meudon France
Show Abstract11:30 AM - **F1.6
N Incorporation, Defects and Electronic Structure in Epitaxial N-doped TiO2 Rutile.
Scott Chambers 1 , Irene Cheung 1 , Pannusami Nachimuthu 1 , Alan Joly 1 , Mark Engelhard 1 , Michael Bowman 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractWe have investigated the growth and properties of well-defined epitaxial TiO2-xNx rutile for the first time. This material is of interest because of its potential for photochemical water splitting to make hydrogen. It has been known for years that TiO2 (Eg = ~3 eV) can be used to photochemically split water to make hydrogen via UV irradiation. It is of significant current interest to find ways to lower the bandgap so that water splitting can be achieved with visible light absorption. Cation doping extends absorbance into the visible. However, deep-level traps act as recombination centers and render the material ineffective. In contrast, anion-doped TiO2 appears to be better suited for bandgap reduction. Numerous recent studies of N-doped TiO2 powders show an enhancement of visible-light photocatalytic activity, but the underlying causes are not apparent. Several groups have used film growth in an attempt to make model materials and gain deeper fundamental understanding. Critically important questions include N speciation, the mechanism by which N is incorporated into the lattice, the maximum achievable dopant concentration, and the effect of dopant concentration on photocatalytic activity. Despite considerable recent effort to answer these questions using films as model materials, what is lacking is an investigation of well-defined TiO2-xNx epitaxial films. As a result, little is known about the properties of TiO2-xNx prepared under highly controlled conditions, and without high defect concentrations. Doing so is the focus of our work. Our growth method of choice is plasma assisted molecular beam epitaxy. Mixed beams of N and O radicals were prepared in an electron cyclotron resonance plasma source and impinged on various substrates, along with an atomic Ti beam. The associated materials properties were investigated using RHEED, XPS, UPS, XANES, XRD, EPR and UV-visible light absorption. We have found that the structural, compositional and electronic properties depend sensitively on the three atomic fluxes, as well as the substrate. In the absence of extensive defect creation, N incorporation is limited to ~1 at. %. Interstitial Ti resulting from Ti indiffusion during growth generates shallow donors that fully compensate N acceptors, precluding p-type character. Filled Ti-N hybridized states fall deep in the gap and give rise to enhanced optical absorption in the visible above ~2.5 eV. However, it is not yet known whether this new state results in itinerant electrons and holes at and above 2.5 eV. At the time of abstract preparation, we are building apparatus to carry out photoconductivity measurements in order to answer this important question.
12:00 PM - F1.7
Chlorine Doped ZnO grown by MOCVD.
Ekaterine Chikoidze 1 2 , Vincent Sallet 1 , Julien Barjon 1 , Ouri Gorochov 1 , Pierre Galtier 1
1 GEMAC, CNRS, MEUDON France, 2 Material Science Department, Tbilisi State University, Tbilisi Georgia
Show AbstractZnO is a semiconductor oxide material with low resistivity, high transmittance up to UV with a good chemical stability under strong reducing environments. It is thus a promising Transparent Conductive Oxyde (TCO) and a possible alternative to tin oxide and indium oxide to be used as transparent electrode for photovoltaic solar cell for example [1]. Although the achievement of p-type doping ZnO is still currently an issue, high carrier levels have been demonstrated on the n-type side. Up to now, metal elements like Al, Ag, In, Si, Sn, substituting to Zn, have been widely used for this purpose whereas except for a few works [1-3], doping of ZnO with anion impurity in substitution to oxygen, has not been widely studied. However, the use of non-metal dopants in substitution to O was suggested as a better way to achieve high carrier concentration and mobility while keeping good transparency, thanks to the weaker perturbation of the ZnO conduction band expected in this configuration [4].We present a study of chlorine doping of ZnO. ZnO:Cl layers have been grown by MOCVD technique, in a vertical geometry reactor. The optimal growth temperature for the quality of the layers was found T=425°C , also respectively law temperature is a pre-requisite for further device processing compatibility. In order to achieve acceptable structural quality, the growth was performed on (0001) oriented sapphire. Hydrogen or helium was used as vector gas. Thickness of the layers was ranging between 1µm to 4µm, depending on the growth conditions. The Theta-2Theta X-ray diffraction scans present the ZnO wurtzite symmetry structure without any additional phase. Transport properties were studied for samples with different content of chlorine. Hall effect measurements show the increase of electron carrier concentration and decreases of electron mobility while increasing the amount of chlorine incorporated in ZnO. Carrier concentration as high as 6.51020cm-3 has been achieved with resistivity of ρ=1.4 x10-3 Ohm cm. Low temperature cathodoluminescence spectrometry show strong UV excitonic emission for all ZnO:Cl reflecting the conservation of the optical properties of the layers. These results demonstrate that the use of Chlorine is an interesting route to achieve high level of n-type doping by MOCVD. [1]A.Guillen-Santiago, M.Olivera, A.Maldonado, et al. Phys.Stat.Sol.(a) 201, (2004), 952[2]H.Y.Xu, Y.C.Liu, R.Mu, Appl.Phys.lett. 86, (2005),123107 [3]A El Hichou, A.Bourgine, J.Bubendorff, Semic.Sc.Technol.(2002), 607[4] R.Gordon, MRS Bulettin, (2002), 52
12:15 PM - F1.8
Defect States in Carbon Co-Doped n- and p-type GaN Grown by Molecular Beam Epitaxy.
Andrew Armstrong 1 , Christiane Poblenz 2 , Umesh Mishra 2 , James Speck 2 , Steven Ringel 1
1 Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio, United States, 2 Materials and Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractCarbon doping in GaN produces semi-insulating (SI) behavior, which finds important application for SI GaN:C buffer layers to provide sharp current-pinch off in AlGaN/GaN high electron mobility transistors (HEMT) grown by molecular beam epitaxy (MBE). To emphasize SI behavior via carbon doping while minimizing undesired effects arising from trapping states or deep levels, the overall impact of carbon-related defects, and in particular, the mechanism for SI behavior in MBE-grown GaN:C must be understood. Therefore, to identify electrically active carbon-related defect states and determine how their site selectivity depends on the Fermi level
Ef position, we have employed deep level transient spectroscopy (DLTS), deep level optical spectroscopy (DLOS) and lighted capacitance-voltage (LCV) measurements to track
quantitatively the evolution of the defect spectrum of MBE-grown GaN with systematically varied electrical conductivity ranging from n-type to SI to p-type that is achieved using carbon co-doping.
A series of co-doped GaN:C:Si films were investigated as a function of systematically increasing [C] that produced electrical conductivity ranging from n-type to SI. Thus, the preferential incorporation site of carbon was studied as Ef receded from Ec toward mid-gap with increasing [C], thereby enabling the identification of carbon-related bandgap states that render GaN semi-insulating. The compensating role of a carbon-related shallow acceptor at Ec – 3.28 eV attributed to CN has was established from investigation of n-type GaN:C:Si, and this bandgap state dominated the deep level spectrum of SI GaN:C. The lack of p-type activity despite the pre-eminence of this near-Ev carbon acceptor in SI GaN:C films strongly suggests the presence of an additional carbon-related donor state, resulting in carbon auto-compensation that fixes Ef near mid-gap, as suggested for GaN:C grown by metalorganic chemical vapor deposition [1]. Such a carbon donor level arising from the substitutional CGa defect has been predicted to form near Ec[1] but has yet to be observed. Consistent with this model, we report a deep donor particular to SI GaN:C:Si via DLTS at Ec - 0.11 eV. Since CGa is predicted to be the preferred site for carbon in SI and p-type GaN [1], the DLOS spectrum of p-GaN:Mg was also studied with the expectation that residual carbon should incorporate primarily as CGa. Indeed, a bandgap state at Ev + 3.26 eV (Ec – 0.14 eV) was found for p-GaN:Mg. In light of the good agreement between the DLTS and DLOS results, we attribute these levels to the putative CGa donor. To further bear out the actuality of the carbon donor and its role in the auto-compensation model for SI GaN:C, progress of DLOS study of co-doped GaN:C:Mg as a function of increasing [C] will be discussed where now Ef will be intentionally receded from Ev toward mid-gap, and [Ev + 3.26 eV] is expected to track increasing acceptor compensation.
[1] Seager et al. J. Appl. Phys. 92 6553, (2002).
12:30 PM - F1.9
Unusually High Be Diffusivity in GaAs1-xNx (x<<0.01)
Wenkai Zhu 1 , Alex Freundlich 1
1 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show Abstract The unusual bandgap shrinkage associated with the introduction of small amounts of nitrogen in III-V compounds has sparked a strong interest in the development of dilute nitride heterostructures. In molecular beam epitaxy and related techniques N is introduced using active (atomic) nitrogen species often obtained using rf-plasma sources. However the difficulty in effectively blocking (using shutters) the nitrogen flux emanating from rf-plasma sources after the fabrication of the dilute nitride epilayers leads to a slight non-intentional incorporation of N(0.01-0.2%) in subsequent III-V epilayers (e.g.barriers for MQW structures). Here the effect of such low level nitrogen contamination/doping upon the Be doping and diffusion properties is investigated. GaAs:N samples with different Be doping concentrations ranging from 5x1017 cm-3 to 3x1019 cm-3 were grown at 5250C on (001) GaAs substrates by chemical beam epitaxy in a Riber 32CTM chamber, using an EPI Uni-BulbTM nitrogen RF plasma source with UHP nitrogen (7N). Triethylgallium and pre-cracked arsine were used as group III and V precursors respectively and Be doping was achieved by a solid effusion cell. The actual nitrogen concentrations in the epilayers grown here were extracted by secondary ion mass spectroscopy (SIMS) and were found to be constant throughout the epilayer (no correlation with Be profile) at about 0.01% (~5-8x1017 cm-3). In order to assess the diffusion properties of Be, samples were subjected to post-growth long and rapid thermal annealings (5500C < T < 9000C). SIMS and electrochemical capacitance-voltage profilometry (ECV) analysis were implemented to obtain Be and dopant concentration profiles in GaAs:N. The diffusivity of Be was extracted by simulating numerically experimental diffusion profiles using a model derived from Fick’s diffusion law. Surprisingly, and despite the small amount of nitrogen present in the GaAs:N epilayers, the diffusivity of Be measured here exceeds by nearly two orders of magnitude the one commonly reported for Be in GaAs. For the temperature range of this study the diffusivity of Be in GaAs:N follows an Arrhenius like behavior with an activation energy of about Ea= 1.41 eV (instead of 1.95 eV for Be in GaAs) suggesting that the presence of small amounts of nitrogen in GaAs alters significantly the diffusion mechanism of Be, which in turn may be of critical importance in the design and fabrication of dilute nitride devices.
12:45 PM - F1.10
Structural Origins of the Systematic Crystallographic Tilt in Micron-Sized InAs Islands on (100) GaAs.
Xueyan Song 1 , Ganesan Suryanarayanan 2 , Anish Khandekar 3 , Thomas Kuech 3 2 , Susan Babcock 1 2
1 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractThe continued and increasing interest in the use of the 6.1Å materials (InAs, GaSb, InSb and their alloys) for a broad spectrum of optical and electrical applications that spans lasers, detectors and thermal-photovoltaic cells, intensifies the need to understand the mechanism of strain relax between the device film and available GaAs substrate and to develop new substrates and growth processes that minimize the defects in the manufactured device. The 7% mismatch between InAs and GaAs derived strain leads to a Stranski-Krastanov growth in this system. Backscattered electron Kikuchi pattern (BEKP) studies have established clearly that islands grown under condition that favor growth over nucleation develop a distinct diamond shape and characteristic domain substructure. In islands a few micron in lateral extent, the InAs is tilted a few degrees relative to the substrate in one of six specific directions. The aim of the present study is to investigate the origin of the systematic tilt. Microstructure analysis was focused on the mismatch dislocation structure in islands with lateral dimensions of about 1 micron, somewhat smaller than the largest islands in which the tilt substructure is fully developed. The topography, morphology, and crystallographic tilt of the micron-sized island were first investigated by plan-view scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The interfacial dislocations that accommodate the lattice mismatch were subsequently characterized using cross section high-resolution TEM (HREM). Most of the islands with the lateral dimension of ~ 1 micron, posses 5 sections, including the center of the island, which is crystallographic aligned with the GaAs substrate, and 4 other sections that are tilted as much as 4 degrees relative to the substrate. Cross-section HREM images reveal that the crystallographic alignment/tilt is established at the InAs/GaAs interface. The mismatch dislocation array is comprised primarily of 60o dislocations. In the aligned volume near the center of the island, the 60° dislocations exist in pairs that produce a zero out of plane component to the net burgers vector in that region of the interface. On the contrary, the outer-edge volumes of the islands possess unevenly spaced 60° dislocations with like Burgers vector, rendering a net out of plane component to the dislocation array that is oriented in the direction needed to produce the crystal tilt observed in the same volume. The present experimental results on the segregation of one set of the 60° dislocation with like Burgers vector on one side of the islands and resultant in crystal tilt are fully consistent with the model proposed by B. Spencer and J Tersoff.1,2 These results enhance our understanding the strain relaxation mechanisms in the large misfit systems.[1].B.J. Spencer, and J. Tersoff, Appl. Phys. Lett. 77 (1997) 2533.[2].B.J. Spencer, and J. Tersoff, Phys. Rev. B63 (2001) 205424.
F2: Defect Properties, Activation, Passivation
Session Chairs
Tuesday PM, April 10, 2007
Room 3004 (Moscone West)
2:30 PM - **F2.1
Are the Materials Properties of Indiumnitride Dominated by Defects?
Petra Specht 1 , Johnny Ho 1 , Joanne Yim 1 , Eicke Weber 1 , Til Bartel 2 , Christian Kisielowski 2
1 Mat. Sci. & Eng., UC Berkeley, Berkeley, California, United States, 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratories, Berkeley, California, United States
Show AbstractIndiumnitride (InN) is a promising, yet technologically challenging material with a high defect density and unusual material properties. Its high electron mobility may be utilized in high power electronic devices. However, to date it is still debated how much the evident electron accumulation at the materials surface contributes to the measured electrical transport properties. The results for optical response, absorption and photo-luminescence, of epitaxial InN resulted in a large correction of the fundamental bandgap from originally 1.9 eV to now around 0.7 eV. Yet, it is still debated if the commonly measured optical transitions below the original high bandgap values may be caused by a large concentration of defects, in the order of 1E20/ccm, instead of reflecting a low fundamental bandgap. The materials application in high efficient solar cell technology, however, is primarily dependent on the successful production of a contacted p-n junction which was not yet achieved. This contribution addresses the controversy in the bandgap discussion of InN. Standard optical and electrical characterization will be compared with results from a transmission electron microscopy characterization. Valence electron energy loss spectroscopy (VEELS) of InN is applied and discussed. Specifically, characterization of InN epilayers from three different suppliers, all deposited by molecular beam epitaxy, will be evaluated and differences in local versus standard optical characterization will be pointed out. VEELS analysis consistently shows high energy transitions around 1.8 eV in all three materials. The presence of In clusters in some epilayers will be demonstrated and surface effects in standard optical characterization will be presented. The presently available results in literature and from own experiments show that it is possible that InN is a high bandgap material and its materials properties may be dominated by its large concentration of defects.
3:00 PM - F2.2
Nature of Stacking Faults in Quaternary InxAlyGa1-x-yN Layers.
Fanyu Meng 1 , Nathan Newman 1 , Subhash Mahajan 1
1 School of Materials, Arizona State University, Tempe, Arizona, United States
Show AbstractQuaternary InxAlyGa1-x-yN layers on top of GaN buffer layers were grown on (0001) sapphire substrates using metal organic chemical vapor deposition. One In.12Al.29Ga.59N layer was comprehensively studied using various transmission electron microscopy (TEM) techniques including weak beam dark field (WBDF) imaging, selected area diffraction pattern (SADP), high resolution electron microscopy (HREM) and annular dark filed (ADF) imaging in STEM mode. High-density stacking faults were found in this layer. Complete WBDF analysis using g vectors of (0002), (11(-2)0) and (1(-1)00) and HREM images revealed stacking faults are bounded by Shockley partials. Z-contrast ADF images revealed stacking faults are Al rich. It is thus rationalized low surface mobility of Al atoms at low growth temperature leads to stacking faults formation. Al is generally known to have high reactivity. In result, Al atoms form the strongest bonds among Al, Ga and In atoms, thus mobility of Al atoms must be lowest among Al, Ga and In atoms in the same growth environment. Low surface mobility of Al atoms prohibits them to move around surface and find the correct bonding sites. A perfect wurtzitic structure would have ABABAB stacking sequence of (0001) cation planes. Instead of finding A or B positions for a perfect wurtzitic structure, Al atoms might reside on C positions where they firstly arrive on the surface, stacking error would thus form. Our study on other quaternary layers including In.10Al.02Ga.88N, In.05Al.06Ga.89N, and In.06Al.18Ga.76N showed they all contain stacking faults with same structures. Stacking faults are locally zincblende structures within wurtzitic matrix. Though theoretical work predicted zincblende structure would have smaller band gap than wurtzitic structure at same composition, the fact these stacking faults are Al rich makes the situation complicated. It is mostly possible Al rich stacking faults are larger in band gap than the surrounding matrix. Instead of forming self-assembled quantum well structures to enhance emission efficiency, these Al rich faults may act as non-effective emission centers thus reduce emission efficiency. Devices built on quaternary InxAlyGa1-x-yN layers should take this fact into consideration.
3:15 PM - F2.3
Conductivity Characteristic in Heavily Boron-Doped Diamond Films.
Hitoshi Ishiwata 1 , Tomohiro Takenouchi 1 , Ryusuke Okada 1 , Shingo Iriyama 1 , Yoshihiko Takano 2 , Hiroshi Kawarada 1
1 Nano Science Engineering, Waseda University, Setagaya-ku, Tokyo, Japan, 2 Nano-frontier Material, National Institute for Materials Science, Tsukuba, Ibaragi, Japan
Show Abstract3:30 PM - F2.4
Theoretical Study of the Nature of Defect States in PbTe Thin Films.
Subhendra Mahanti 1 , Khang Hoang 1 , Puru Jena 2
1 Physics and Astronomy, Michigan State University, East Lansing, Michigan, United States, 2 Physics , Virginia Commonwealth University, Richmond, Virginia, United States
Show AbstractLead chalcogenides (PbTe, PbSe, and PbS) are IV-VI narrow band gap semiconductors whose studies over several decades have been motivated by their importance in infrared detectors, light-emitting devices, infrared lasers, photovoltaics, and high temperature thermoelectrics. It is well known that defects and impurities control the electronic properties of semiconductors and therefore play a crucial role in emerging semiconductor technologies. Furthermore, the nature of the defect states depend on the geometry of the host material, whether it is a bulk material, or a 2D film, a nanowire, a nanodot. Recently we have studied the nature of deep defect states in bulk PbTe using ab initio density functional theory and a supercell model when Pb and Te atoms are substituted by different types of defects [1,2]. In particular, we found that substitution of Pb by the trivalent impurities Ga, In, and Tl gave rise to hyper deep defect states (HDS) below the valence band (VB) and deep defect states (DDS) near the band gap region. In this talk we will discuss how these states are affected by going to a PbTe film. A PbTe (001) thin film was modeled by 9-layer (2x2) centrosymmetric slab [3] (separated by a vacuum) without or with the impurities either in the first, the second, or the third layer on each side of the slab. We find that the undoped slabs exhibit an oscillatory geometric relaxation. There is a surface state near the top of the Te s band, and surface resonance states near the top of the VB and the bottom of the conduction band (CB).There are no surface states in the fundamental gap. The HDS and DDS of Ga, In, and Tl are preserved in the film geometry. As one goes from the bulk-like layers to sub-surface and surface layers, the former tends to move closer to the bottom of the VB and its width gets narrower, and the latter also gets modified. The calculated formation energy of the impurities as a function of the distance from the surface shows interesting features: all three impurities have lowest formation energy in the first layer but it increases monotonically after that in case of Ga, whereas there is a potential barrier in the second layer and a shallow potential “valley” between the second and the bulk-like layers in the case of In and Tl. This should have a significant impact on doping. We expect that Ga atoms will be easily annealed out to the surface whereas In and Tl atoms can be trapped in the subsurface layers.* Work at MSU partially supported by ONR-MURI Grant No. N00014-03-10789 and at VCU partially supported by DOE. 1. S. Ahmad, S. D. Mahanti, K. Hoang, and M. G. Kanatzidis, Phys. Rev. B 74, 155205 (2006).2. S. Ahmad, K. Hoang, and S. D. Mahanti, Phys. Rev. Lett. 96, 056403 (2006); 96, 169907(E) (2006)3. F. Bechstedt, Principles of Surface Physics (Springer-Verlag, Berlin, 2003).
3:45 PM - F2.5
Predominance of Alternate Diffusion Mechanisms for Interstitial-Substitutional Impurities in Si.
Hui Li 1 , Na Li 1 , Subhash Joshi 1 , Teh Tan 1
1 Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina, United States
Show AbstractWe propose a general model to describe the diffusion of interstitial-substitutional (i-s) impurities in Si. In this model the kick-out (KO) and the Frank-Turnbull (FT) mechanisms take effect simultaneously and independently. A novel factor is introduced to evaluate the relative contribution of each mechanism during the diffusion process. Satisfactory fits are obtained for a group of experimental data for both Au diffusion into Si and out of Si. The latter is facilitated by Al gettering at the same temperature of the Au indiffusion process. Our simulation showed that KO dominates for Au indiffusion, which has been well established, and FT dominates for Au outdiffusion, which did not receive the deserved attention previously.
4:30 PM - **F2.6
Defect Engineering in Oxide Semiconductors.
Chris Van de Walle 1
1 , University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractIn spite of rapid progress in materials quality, oxide semiconductors still suffer from serious problems in controlling their conductivity. We are addressing these