1997 EMC: Thursday Afternoon Sessions, Part 2

The following sessions are among those that will be held during the 39th Electronic Materials Conference (EMC) on Thursday afternoon June 26, at Colorado State University, Fort Collins, Colorado. To view the other Thursday afternoon sessions as well as other programming planned for the meeting, go to the EMC Calendar of Events.

SESSION S: BULK AND EPITAXIAL GROWTH OF SiC

1:30 pm

4H-SiC Vapor Phase Epitaxial Layer Structures for High-Power Switching Applications: L.B. Rowland, A.A. Burk, Jr., A.K. Agarwal, J.B. Casady, S. Seshadri, and C.D. Brandt, Northrop Grumman Corporation, Science and Technology Center, 1350 Beulah Rd., Pittsburgh, PA 15235

Devices with larger blocking voltages should be possible using 4H-SiC than can be obtained in silicon devices because of its high breakdown electric field. Initial device demonstrations in 4H-SiC have to date been performed using vapor phase epitaxial (VPE) layers. The VPE growth of high-voltage switching devices such as gate turn-off thyristors (GTO) and vertical MOSFETs in abrupt switching of epitaxial doping type as well as control of doping density from below 10¹⁵ to above 10¹⁹ cm^-3. The utmost control of the epitaxial process is necessary to maximize the electron (or hole) mobilities and minority carrier lifetimes in the blocking or drift layers in these structures. Both 4H-SiC GTO and MOSFET structures have been grown by VPE. The GTO structures consisted of a asymmetric blocking layer with a heavily Al-doped layer followed by a thick lightly Al-doped (5 x 10¹⁴ cm^-3) blocking layer of 6-12 mm thickness, a nitrogen-doped gate layer, and a heavily Al-doped anode layer. Vertical MOSFETs consisted of a 10¹² mm thick nitrogen-doped drift layers with 1 x 10¹⁵ cm^-3 doping, a moderately Al-doped gate and heavily nitrogen-doped collector. Electrical characterization on nitrogen-doped drift layers has yielded room temperature Hall mobilities of 950 cm² V^-1s^-1 and minority carrier lifetimes of 3.1 mm. With current control and optimization of VPE growth conditions, 700V, 1.5A SiC GTOs and vertical SiC MOSFETs with 1.1 kV blocking voltage have been demonstrated using these layers. The effects of VPE growth parameters such as Si/C source gas supply ratio on the proper growth of these structures will be discussed.

1:50 pm

CVD Growth of Semi-Insulating 4H-SiC Epitaxial Layers by Vanadium Doping: Barbara E. Landini, George R. Brandes, Mary Vollaro, ATMI, 7 Commerce Drive, Danbury, CT 06810

Vanadium (V) doping of CVD-grown 4H-silicon carbide (4H-SiC) epitaxial layers has been accomplished. The achievement of V-doped SiC is attractive for the development of semi-insulating (SI) epitaxial SiC layers, since V forms energy levels near midgap in SiC. V doping during sublimation growth has recently been used to successfully produce Si-SiC substrates. SI-SiC epitaxial layers can be used as device isolating buffers and in SOI-type device applications to reduce parasitic impedance. An organometallic V dopant precursor suitable for CVD applications was developed and used to grow V-doped epitaxial layers of 4H-SiC. SIMS analysis confirmed the incorporation of vanadium into the epitaxial layers. A strong correlation between epitaxial layers surface morphology and atomic V concentrations was observed. The reactor memory effect for vanadium was also measured. The V incorporation into the 4H-SiC epitaxial layers was successfully altered and controlled by variation of SiC growth parameters such as precursor vapor pressure and Si/C ratio. The electrical activity of the vanadium impurity was investigated using variable temperature resistivity measurements. The measured activation energies were consistent with vanadium acting as a compensating impurity.

2:10 pm

Defects in 6H-SiC and 4H-SiC Structures Grown by Liquid Phase Epitaxy: S.V. Rendakova, I.P. Nikitina, A.S. Tregubova, and V.A. Dmitriev1, A.F. Ioffe Institute, 26 Polytechnicheskaya Str., St. Petersburg, 194021 Russia; 1MSRCE, Howard University, 2300 6th St., Washington, D.C 20059

Recently, SiC devices with blocking voltages of a few kilovolts and forward current densities of 1 kA/cm² have been fabricated. High power SiC electronics require increased forward current. This implies reduction of the on-resistance of the devices. The two main factors currently limiting this on-resistance are (1) device area and (2) resistivity of the base region of the devices structure. In order to resolve these problems, the defect density in pn structures must be reduced and for this purpose, we have proposed fabrication of high power SiC devices, such as diodes and thyristors using liquid phase epitaxy (LPE). This method had been shown to reduce micropipe and dislocation density in SiC epitaxial structures and to produce material with a relatively large diffusion length of minority carriers (L_d) (which is required to modulate electrical conductivity of base region). Moreover, the growth rate of SiC LPE has been found to be higher than that for conventional SiC CVD method which is important for growth of the base region of power devices. We studied defect transformations such as micropipe closing and dislocation density reduction during SiC LPE growth. The layers and pn structures were grown from Si-based melts in a wide temperature range from 1200°C to 1700°C. We used substrates produced by either the Lely (6H-SiC) or modified Leley (6H- and 4H-SiC wafers up to 35 mm in diameter) techniques. The film thickness ranged from 0.5 to 50 µm and growth rate was controleld to be from 0.5 to 10µm/hr. The layers were characterized by x-ray diffraction, x-ray topography, and transmission electron microscopy. The density and distribution of dislocations and pinholes were studied using chemical etching. Electron mean induced current technique was employed to measure L_d values. It was found that in contrast to other epitaxial methods, SiC layers grown by LPE may contain zero micropipies and a dislocation density less that 10² cm^-2. Significant narrowing of x-ray rocking curves was oberved for epitaxial layers grown on modified Lely substrates. Polytype inclusions wre not detected in the films. Mechanisms of the defect formation including the effect of surface preparation and defect penetration from the substrate will be proposed. Relations between defect presented in epitaxial structures and pn junction characteristics will be discussed.

2:30 pm

Properties of n-GaN/p-SiC Heterojunctions Formed by GaN Hydride Vapor Phase Deposition on SiC Layers Grown by Liquid Phase Epitaxy: A.E. Nikolaev, S.V. Rendakova, I.P. Nikitina, K.V. Vassilevski, and V.A. Dmitriev1, A.F. Ioffe Institute, 26 Polytechnicheskaya Str., St. Petersburg, 194021 Russia

Semiconductor heterojunctions are widely used to improve characteristics of solid state devices, particularly for bipolar junction transistors (BJT) and thyristors. It was shown that the current gain in SiC BJT is limited by a carrier transport in the base region. This problem may be solved by employing a wide band gap emitter structure. BJT with a GaN/SiC heterojunction has been proposed. Recently, we have reported on GaN/SiC structures made on p-SiC bulk material. In this paper, we report on material properties and electrical characeristics of GaN/SiC pn heterojunctions formed on p-SiC epitaxial material. The SiC/GaN pn heterojunctions were fabricated by GaN hybride vapor phase epitaxy (HVPE) on 6H-SiC epitaxial layers. The 6H-SiC layers were initially grown by liquid phase epitaxy on 6H-SiC 30 mm diameter wafers. Layers were grown on the (00001)Si face of the wafers. The silicon carbide layers were doped with Al. Aluminum concentration in SiC varied from 10¹⁷ to 10²⁰ at/cm³. The thickness of the SiC layers ranged from 0.5 to 10 µm. SiC structures were charcterized by x-ray diffraction and secondary ion mass spectrometry. GaN layers were grown by HVPE in the temperature range from 950 to 1050°C. The GaN growth rate was controlled from 0.02 to 0.6 µm/min. Thickness of GaN layers ranged from 0.2 to 2 µm. GaN layers were unintentionally doped. Background N_d - N_a concentration in GaN ranged from 10¹⁸ to 10¹⁹ cm^-3. The crystal quality of GaN was examined by reflectance high energy electron diffraction and x-ray diffraction. In order to study heterojunction properties, mesa structures were formed by reactive ion etching. Ti/Ni and Al were used to form ohmic contacts to n-GaN and p-SiC, respectively. The heterojunctions were characterized by electron beam induced current. Current - voltage and capacitance - voltage characteristics of the heterojunctions were measured. The characteristics of the heterojunctions will be discussed for applications in GaN/SiC heterojunction bipolar transistors and thyristors for high temperature and high power electronics.

2:50 pm

Model of the Epitaxial Growth of SiC-Polytypes Under Surface-Stabilized Conditions: J. Furthmüller, P. Käckell, F. Bechstedt, Institut für Festkörpertheorie und Theoretische Optik and A. Fissel, K. Pfennighaus, B. Schröter, W. Richter, Institut für Festkörperphysik, FSU Jena, Max-Wien-Platz 1, D 07743 Jena, Germany

Heterosturctures or superlattices of different SiC-polytypes are expected to be interesting materials with adjustable bandgap and other promising properties. However, the growth of SiC-polytypes in a definite way is difficult, expecially under conditions far from the thermodynamical equilibrium (as e.g. during molecular beam epitaxy). Recently, by means of MBE experiments, we have demonstrated that even under such conditions a definite growth of a certain SiC polytype is achievable. In this growth process the stabiliztion of certain surface superstructures is of improtance. It is one aim of the paper to give a consistent physical picture for these findings. Investigations have been performed concerning Si-rich superstructures becasue an accurate layer-by-layer growth was only achieved under Si-stabilized conditions at low tempertures (lower 1000°C). Surface-stabilized conditions have been realized in such a way that Si and C were supplied alternately to the 6H-SiC(0001) surface. During this supply the surface superstructures between 3 X 3 "1 x 1", representing a phase of higher Si-coverage, results in a 6H-SiC growth whearas switching between the same "1 x 1" phase and 3 x 3, with the highest Si-coverage, results in 3C-SiC growth. Supplementary theoretical investigations have been performed concerning the atomic and electronic structure of various possible models for the relevant surface superstructures. The calculations have been realized within the framework of density functional theory in the local density approximation using ultrasoft Vanderbilt pseudopotentials in a plane-wave basis. From this phase diagram we conclude that in general Si-rich surfaces are energetically clearly favoured over C-rich surfaces. Therefore, we suggest a two-step-growth process. Firstly a Si-rich surface is formed. Switching, secondly to C-rich conditions these Si-rich structures become now energetically less favourable compared to the less-Si-rich structures, i.e., C-adsorption becomes very easy on these Si-rich surface (SiC is formed, whereas C-adsorption is very difficult on less Si-rich or clean surfaces. This is consistent with experimental findings of carbonization of Si-rich surfaces. Moreover, we find that a T₄-site Si- adatom structure represents the most stable model for the structure over a wide range of Si chemical potentials. However, in Si-rich environemtns (chemical potential of Si close to Si bulk), we find that other more Si-rich structures are energetically even more favourable. However, under more Si-rich conditions various energetically close structures might compete leading to less stable surface conditions. This might be a explanation why one always observes growth of 3C-SiC if working under more Si-rich conditions (such as on the 3 x 3 surface).

3:10 pm, Break

3:30 pm

Dislocation Structures in High Quality 4H-SiC Single Crystal Wafers: W. Si and M. Dudley, Dept. of Materials Science and Engineering, SUNY at Stony Brook, NY 11794-2275; R. Glass, V. Tsetkov, and C. Carter, Jr., Cree Research, Inc., Durham, NC 27713

In physical vapor transport (PVT) grown 4H-SiC single crystals, there are mainly two types of dislocation structures: supercrew dislocations (including micropipes) with their Burger vectors running along the [0001] axis and basal plane dislocatiosn which have line directions and Burgers vectors confined to the {0001} planes. In the present study, synchrotron white beam x-ray topography (SWBXT) and Normarski Optical Microscopy (NOM) have been used to investigate the characters of these two types of dislocations and their interactions. The density and distribution of the superscrew dislocations can be measured accurately from back-reflection x-ray topographs. Basal plane dislocations are visible via kinematic diffraction contrast and display bimodal, double-line contrast under certain diffraction conditions. Basal plane dislocations often emanate from the superscrew dislocations and bow out to form the beginnings of Frank-Read sources. The Burgers vectors of basal plane dislocations are a/3 <> {0001} (a = 3.073Å), and most of them are mainly screw type. The magnitude of the Burgers vectors of the basal plane dislocations was confirmed from the calculations of their bimodal image widths. The formation mechanisms of both superscrew dislocations and basal plane dislocations are briefly discussed. Research supported by AFWL/ARPA and DOE.

3:50 pm

Defect Formation During SiC Crystal Growth: I. Khlebnikov, Y. Khlebnikov, E. Solodovnik, V. Madangarli, and T.S. Sudarshan, Department of Electrical and Computer Engineering, University of South Carolina, Columbia, SC 29208

Defect formation during SiC crystal growth has been studied. A mathematical model of this process is developed, and experimental evidence that supports the validity of the developed model is presented. To grow high quality SiC crystals it is necessary to satisfy certain minimum conditions during the growth process. First of all, the initial nucleation process has to be critically controlled by suppressing the growth at random nucleation spots, as well as any metastable phase. Second, a strict control over the variation axial and radial temperature gradient must be maintained. Third, a constant flow of the evaporated (sublimated) species must also be maintained. It is difficult to satisfy these conditions due to high temperatures of crystal growth and the reactive nature of the Si vapor. The rate of growth and the super-saturation inside the furnace during crystal growth critically influence the of the SiC grown. A low super-saturation leads to a slow growth rate which is generally suited for obtaining high quality crystals, with smaller density of defects such as micropipes. Due to this reason, presently, most crystal growth techniques adopted are very slow and takes a very long time to grow large diameter wafers of high quality. In this paper, we study the primary causes for the formation of defects during the crystal growth process. We have developed a geneal structural-mathematical-model of SiC growth, involving a system of fourth order nonlinear differential equations which describes a functional relationship between the variation in super-saturation and temperature with time, resulting in the formation of defects such as micropipes. Computer simulation of the model which has been developed on the basis of BCF (Burton, Cabrera, and Frank) theory, clearly demonstrates that the formation of micropipes is initiated from a spiral growth. Experiments of crystal along of the c-axis on SiC substrated performed in our laboratory clearly demonstrates the validity of our mathematical model. Research performed in our laboratory shows that screw dislocation only initiate micropipe formation. Further propagation of micropipe with enlargement of its diameter cannot be explained by dislocation theory of crystal growth. Observed micropipes having a diameter greater than 1 µm are probably formed as result of a capillary mechanism involving predominantly a liquid phase of Si. It is a clear that micropipe formation is usually correlated with the localization of a metastable phase. These results are useful for getting a better understanding of defect formation, specifically that of micropipes, during SiC crystal growth.

4:10 pm, Student Paper

Growth of High Quality Bulk -SiC Single Crystals on Silicon Removed (100) Epitaxial -SiC Film: H.N. Jayatirtha, M.G. Spencer, Materials Science Research Center of Excellence, School of Engineering, Howard University, 2300 6th Street, NW, Washington, DC 20059

Silicon carbide (SiC) is a valuable wide bandgap semiconductor material for high power and high temperature applications. Beta-SiC () is particularly attractive because of high low field mobility and the zincblende structure. In this study, we report on the growth of high quality bulk -SiC single crystals with growth rates as high as 180 microns per hour using a modified Lely technique, on silicon removed (100)-SiC epitaxial films. We are successful in growing a thick transparent yellow material at 1900°C seed temperature without polytype transformation in the range of 0.1 - 0.04 torr growth pressure. Optical micrographs indicate smooth growth pressure. Optical micrographs indicate smooth growth layers. The x-ray doubel rockign curve measurements indicate a high degree of crystalllinity with full with at half maximum (FWHM) corresponding to 26 arcsec. Optical measurements indicate reflectance values within 5% compared to good quality Lely crystal in the range of 4000-400 wave numbers. Cathodoluminescence results indicate the presence of aluminum impurity in the crystal lattice. Etch results indicate 3 orders of magnitude (3 - 8) x 10⁴ #/cm²) reduction in stalking fault density as compared to the growth substrate. Further study is in progress to control source powder graphitiztion necessary for prolonged growth. Currently, epitaxial 3C layers are being grown on these substrates and results of these studies will also be discussed.

4:30 pm

Mechanisms For Micropipe Formation in 4H-SiC Grown By PVT: V. Balakrishna, G. Augustine, G.T. Dunne, H. McD. Hobgood, and R.H. Hopkins, Northrop Grumman Science and Technology Center, 1350 Beulah Rd., Pittsburgh, PA 15235-5080

SiC is gaining importance as a wide bandgap semiconductor material for future, high-temperature and high power electronic device applications. However, the presence of micropipes in bulk grown SiC substrates is of concern in the performance of future large area devices. Significant reduction in micropipe defect density (<100 cm^-2 averaged over whole areas; best wafers having <25 cm^-2) has been achieved in PVT-grown 4H-SiC by careful control of the growth conditions. However, more stringent device requirements mandate further reduction in the defect density. In-depth understanding of the mechanisms of micropipe formation is essential in order to devise approaches to eliminate them. Experiments have been performed to elucidate the origins of micropipes; the role of crystal growth orientation, source composition and purity, and growth conditions in the formation of micropipes have been examined. Results indicate that source stoichiometry and purity play a significant role in the nucleation of micropipes. Experiments performed to test the validity of this model will be discussed, and results for the wafer growth of [0001}-oriented, 1.5 inch diameter, low defect 4H-SiC crystals will be presented. Details of microstructural characterazition on as grown crystals wil also be presented. This work was partially supported by the Advanced Research Projects Agency under Contract No.: F33615-95-5427.

4:50 am, Late News

1:30 pm, Student Paper

Growth and Characterization of InN_xAsyP_1-x-y/InP Strained Quantum Well Structures: W.G. Bi and C.W. Tu, Department of Electrical and Computer Engineering, University of California, San Diego, CA 92093-0407

Nitrogen containing mixed group-V compounds have received much attention in recent years. When nitrogen, which has a large electronegativity, is added into InP, GaAs, or related compounds, the conduction and valence band edges are lowered. Therefore, we expect a larger conduction band offset, which has many advantages for electronic and optoelectronic device applications, for example, a better electron confinement, which will improve the high-temperature performance of lasers. In this work, we propose a novel material InN_xAs_yP_1-x-y(InNAsP for short) on InP for improving high-temperature performance of long-wavelength lasers. InNAsP/InP and InNAsP/InGaAsP quantum well (QW) samples were grown by gas-source molecular beam epitaxy, with a RF nitrogen plasma source. Very streaky reflection high-energy electron diffraction patterns were routinely observed for these samples and Nomarski observations indicate featureless surface morphology. InNAsP/InP single QWs with different well width have strong photoluminescence (PL) in the range from 1.1 ~ 1.5 µm, demonstrating their suitability for long-wavelength applications. The high-quality of these samples is evidenced by a PL linewidth of 25 meV at room temperature for a 10-period InNAsP/InGaAsP multiple QWs. High-resolution X-ray rocking curve (XRC) measurement shows that adding N into InAsP reduces the net strain of the system, as is confirmed by the observation that the zeroth peak of an InNAsP/InP MQW shifts closer to the InP substrate peak compared with that of an InAsP/InP MQW. Besides, although very distinctive X-ray satellite peaks are clearly seen for both samples, the overall peak intensity is much stronger for the InNAsP/InP MQW than the InAsP/InP MQW, suggesting better structural quality as a result of the smaller net strain of the former. Very sharp and distinct satellite peaks as well as Pendellösung fringes are also observed in XRCs for InNAsP/InGaAsP multiple QWs, indicating good crystalline quality, lateral uniformity, and vertical periodicity. The excellent agreement between simulations based on a dynamical theory and the experimental data suggests the sharpness of the interfaces of the QWs. In summary, we have demonstrated the successful growth of a novel material InNAsP, which can be combined with InP and/or InGaAsP barriers to form QWs for improved high-temperature performance of lasers.

1:50 pm

Growth and Properties of Low-bandgap GaAsN Layers on GaAs: Y. Le Bellego, A. Ougazzaden, E.V.K. Rao, M. Juhel, France Télécom, CNET, BP 107m 92225 Bagneux Cedex, France

GaAsN layers with good structural quality and surface morphology are grown on GaAs substrate using metalorganic chemical vapor deposition (MOCVD). A new combination of precursors namely, dimethylhydrazine (DMHy) for nitrogen and tertiarybutylarsine (TBAs) instead of arsine (AsH3) for arsenic are used here for the first time. This association permitted growths at temperatures as low as 500°C, where the nitrogen incorporation rate is found to be large. The pseudomorphic GaAsN layers are grown sandwiched between two nominally undoped GaAs layers on semi-insulated GaAs substrate. Their nitrogen content is independently determined from SIMS and X-ray double diffraction measurements. The good structural quality is also confirmed by transmission electron microscopy measurements. The mechanism of nitrogen incorporation in GaAs as revealed from this study can be described as follows. The nitrogen incorporation significantly increases with a decrease of growth temperature in the 530 to 640°C range. On the other hand, for temperature below 530°C, the incorporation of nitrogen is found to depend only on the fractional flow of DMHy. Layers with nitrogen content as high as 3% are obtained, corresponding to a room temperature photoluminescence wavelength of 1.17 µm, the largest red-shift reported to date. Conventional optical transmission measurements further confirmed the band to band nature of this transition. Using the same growth condition, with the addition of trimethylindium (TMIn), a further red-shift of the bandgap and a simultaneous reduction of the lattice mismatch to GaAs is obtained by growing InGaAsN quaternary alloys. However, we noticed a significant decrease in the nitrogen incorporation rate with In introduction. The subsequent limitation in the maximum red-shift achievable with this alloy will be discussed.

2:10 pm

Epitaxial Growth and Characterization of Single Phase GaAs_1-xN_x with High Nitrogen Concentration: Y. Qiu, S.A. Nikishin, H. Temkin, Electrical Engineering Department, Texas Tech University, Lubbock, TX 79409; N.N. Faleev, Yu.A. Kudriavtsev, A.M. Mintairov and P.A. Blagnov, A.F. Ioffe Institute, St. Petersburg, Russia 194021

Recent progress in the growth of low band gap III-V-V' materials such as GaAs_1-xN_x promises major improvements in the performance of optoelectronic devices based on well known III-V materials and their heterostructures. However, the growth of GaAs_1-xN_x is still in its infancy and only small nitrogen fractions have been reported. The mechanism of nitrogen incorporation is also not well known. We report systematic experiments on the growth of high quality GaAs_1-xN_x by metalorganic molecular beam epitaxy (MOMBE) using solid As, triethylgallium and nitrogen. Active nitrogen flux was generated by a microwave plasma source operating at 230-250 watts. The epitaxial growth was carried out on (001) GaAs substrates at growth temperatures between 400 and 450°C. The growth rate was in the range of 0.6-1.0 µm/hr, independent of the N content of the layer. We prepared single phase layers of GaAs_1-xN_x with x up to 0.10, higher concentrations were obtained in polycrystalline samples. The layers of GaAs_1-xN_x were evaluated by double crystal and powder X-ray diffraction, secondary ion mass spectroscopy (SIMS), photoluminescence (PL), Raman spectroscopy, and atomic force microscopy. In the range of growth temperatures used in our experiements the N/As₄ flux ratio plays a crucial role in the preparation of single phase layers. The requirement of an appropriate flux ratio suggests that the N and As atoms compete with each other for the same anion sites, i.e. are bonded to Ga. This was further confirmed by SIMS measurements where identical depth profiles were obtained by monitoring the N⁺ (m/e=14) and GaN⁺ (m/e=83). By adjusting the N/As₄ flux ratio, instead of simply supplying a sufficient N flux, we were able to control the N incorporation. High resolution X-ray diffraction shows that the GaAs_1-xN_x layers are single phase and of high quality. Powder X-ray diffraction shows the absence of any other phases. Good agreement is obtained between N concentrations derived from SIMS and dynamic simulation of the X-ray data. Room temperature PL has been observed from coherent epitaxial layers of GaAs_1-xN_x for the first time. Raman measurements show the presence of strongly confined GaAs-type modes, which we interpret as evidence for local ordering.

2:30 pm

A Significant Improvement of the Optical Quality of Low-bandgap GaAsN Layers: E.V.K. Rao, A. Ougazzaden, Y. Le Bellégo and M. Juhel, France Télécom - SA, CNET/PAB, BP 107, 196 Av. Henri Ravera, 92225, Bagneux, Cedex, France

The low-bandgap nitrides with N incorporation in GaAs are an attractive alternative for the development of emission devices in the ~1.3 µm wavelength region useful for optical communication systems. In contrast to the conventional InGaAsP/InP structures, lasers made out of these low-gap nitrides on GaAs are predicted to exhibit higher characteristic temperatures (T_O). However, a careful scrutiny of all data to date suggests that neither their optical quality nor the exact emission wavelength are yet suitable to attempt the fabrication of ~1.3 µm lasers operating at 300K. Indeed, their inadequate optical quality can be witnessed from the low luminescence efficiency at room temperature (RT) and the prominence of deep level emissions in the low temperature photoluminescence (LTPL). In this context, the present work constitutes a step ahead in the advancement of these new materials. We report here on a significant improvement of the optical quality of AP-MOCVD grown GaAsN layers on GaAs substrates subsequent to post-growth treatments. The inherently tensile strained pseudomorphic GaAsN layers studied here are obtained using a new combination of precursors which facilitated their growth in the low temperature range (T_g~500 to 550°C) with a maximum of N incorporation (~4%). The PL measurements in a wide range of temperatures and incident powers and conventional transmission measurements at RT to ascertain the band to band nature of the transitions have been performed. The major findings of this study are as follows. Firstly, in agreement with earlier data, all layers exhibited low RT PL efficiency independent of their N content and growth temperature (T_g). And at 10K, a number of (deep level or) low energy emissions have been detected but none related to the GaAsN near bandedge transition. Secondly, a prolonged exposure to the probing laser beam (514 nm line of argon laser) at moderate powers (~200W.cm^-2) and at 10K, surprisingly resulted in the emergence of an intense new emission which we later on attributed to the near bandedge transition of the GaAsN layer. This irrevesible process, not frequently observed in III-V materials, is quite analogous to defect annealing occuring under intense nonradiative recombination (i.e., recombination enhanced defect annealing). Lastly and most importantly, post-growth heat treatments in the same epitaxial reactor at temperatures above T_g have successfully led to the detection of an uniform near bandedge emission with enhanced RT PL efficiency all over the samples surface irrespective of their N content. All of the above mentioned data, observed for the first time in the low-gap nitride layers, will be presented and discussed in light of microscopic incorporation behavior of N in GaAs. Besides, these results are also examined by considering the role of atomic hydrogen incorporated in the layers during growth.

2:50 pm

In-Situ Investigation of Nitridation Processes on GaAs(001) Surfaces Using Reflectance Difference Spectroscopy and Reflection High Energy Electron Diffraction: H.D. Jung, N. Kumagai, T. Hanada, Z. Zhu and T. Yasuda, Institute for Materials Research, Tohoku University, Sendai 980-77, Japan; T. Yao, Institute for Materials Research, Tohoku University and Joint Research Center for Atom Technology, National Institute for Advanced Interdisciplinary Research, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan; K. Kimura, Joint Research Center for Atom Technology, Angstrom Technology Partnership, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan

Although zincblende GaN is metastable, it possesses a number of advantages over wurtzite GaN for device applications. Several groups reported successful growth of zincblende GaN on GaAs(001) substrates by molecular beam epitaxy, and the control of initial nitridation of the GaAs surface is one of the key processes to improve the quality of the epilayers. The importance of background As for the initial stage of zincblende GaN growth was pointed out by several groups; however, the mechanism is not yet clear. In this work, nitridation processes of GaAs(001) surfaces have been systematically investigated by in-situ reflectance difference spectroscopy, reflection high energy electron diffraction, and in-line Auger electron spectroscopy. We have studied the influence of the As background pressure and various initial GaAs surface reconstructions on the nitridation. Our results show that several monolayers of wurtzite GaN can only be stably formed under sufficiently low As background pressure. High As background pressure results in an As-rich surface, which hinders nitridation of the GaAs surface and is crucial for the growth of zincblende GaN. It is concluded that in order to grow zincblende GaN on GaAs(001), nitridation time should be minimized and the background pressure of As plays an important role in suppression of nitridation. We will propose a model to interpret these observations and discuss the implication of the present results for improving the GaN epitaxy on GaAs.

3:10 pm, Break

3:30 pm

Chemical, Strain, and Clustering Effects in the Band Gap and Bowing Parameters of GaAsN and GaPN Alloys: L. Bellaiche, S.-H. Wei and A. Zunger, national Renewable Energy Laboratory, Golden, CO 80401

We investigate theoretically the band gap variation of GaAs_1-xN_x and GaP_1-xN_x alloys as a function of composition x using large (512 atoms) supercells and the plane-wave empirical pseudopotential method. We consider both bulk alloys and epitaxial alloys. To understand and isolate the physical factors contributing to the band gap, we perform the calculation in three steps illustrated here for GaAsN: (1) compress GaAs and expand GaN to the volume of the GaAsN alloy, (2) bring GaN and GaAs together at the GaAsN volume to form the 512-atom random GaAsN supercell, and (3) relax the atomic positions inside the supercell. Step 1 gives the hydrostatic effect, step 2 gives the constant-volume chemical effect, and step 3 gives the relaxation effect. We find that our approach produces completely different results than other calculations that skip some steps, in particular those using the VCA, or those using unrelaxed small supercells. Steps 1-3 are repeated also for epitaxial films. Our calculated band gaps are in excellent agreement with recent experimental measurements of Bi and Tl. Finally, we have studied the effects of atomic clustering on the optical properties by repeating the 512-atoms supercell calculation, but arranging the atoms to exhibit local clustering. We find that non-randomness can lead to significant changes in the bowing coefficient of the nitride alloys, especially at the low concentration limit. This work is supported by BES/OER/DMS under contract No. DE-AC36-83CH10093.

3:50 pm

Surface Enhanced Solubility of Nitrogen in III-V Semiconductors: S.B. Zhang and A. Zunger, National Renewable Engergy Laboratory, Golden, CO 80401

The solubility of nitrogen in III-V semiconductors is central to the growth of GaNAs, GaNP and InNP alloys. Recent vapor phase growth experiments showed that the amount of N that can be incorporated into these materials, (e.g., 16% in GaP) far exceeds the calculated bulk solubility limits of N in these materials. The explanation for the unusually low bulk solubility of N is the large size mismatch between N and P or As--the atomic radius of nitrogen (0.75Å) is 39% smaller than that of P (1.23 Å). We point out that while in bulk a large site mismatch limits solubilities, on a reconstructed surface it could actually enhance it: In MBE growth of GaAs or GaP, the growing (001) surface atoms dimerize, consequently there is a subsurface selectivity for occupation by small (N) atoms under the strained dimer rows and by large atoms (P or As) underneath the opening between dimer row. The high, surface-induced N concentration can subsequently be frozen in as the surface is covered. We have studied the trend in surface strain energies for isovalent Group V impurity (N, P, As or Sb) in various III-V (001) surfaces. We find that (i) without dimerization, there is already a partial surface relief of the strain energy (of ~0.4eV for GaAs:N) due to relaxations normal to the surface; (ii) with dimerization, there is a pronounced subsurface layer selectivity of ~0.8 eV per N for GaAs:N. The calculated N solubilities are thus 3, 4 and 5 orders of magnitude larger than their bulk counterparts for InP, GaP and GaAs, respectively, at 1000K. Our theory also predicts that the solubility of nitrogen in GaP should be further enhanced if an equal amount of As or Sb can be co-incorporated during growth. This work is supported by DOE BES/OER through contract No. DE-AC36-83CH10093.

4:10 pm

Thermodynamic Considerations in Epitaxial Growth of Solid Solutions of GaAs_1-xN_x and GaP_1-xNx: V.A. Elyukhin, Y.A. Kudriavtsev, A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia 194021; Y. Qiu, S. Nikishin, H. Temkin, Electrical Engineering Department, Texas Tech University, Lubbock, TX 79409

The zinc-blende structure alloys of GaAs_1-xN_x and GaP_1-xN_x offer exciting possibilities of extending the performance range of optoelectronic devices. However, large differences in lattice constants between the constituent binaries lead to great internal stresses which are likely to result in extensive miscibility gaps. The determination of appropriate crystal growth conditions is thus important. Distortions in the GaAs(P)_1-xN_x crystal structure are investigated on a microscopic level using the valence-force-field (VFF) model. We determine the conditions under which these alloys are thermodynamically stable, metastable, and unstable with respect to decomposition or ordering. The difference between the values of strain energies in the random and completely ordered alloys is larger than the difference between the respective entropy terms, up to the fusion temperature of GaN. Therefore, the processes of spinodal decomposition and ordering in GaAs(P)_1-xN_x random alloys must be determined by kinetics rather than thermodynamic conditions. The lower bound temperatures of the region stable with respect to the ordering, for random alloys with x=0.25, 0.50 and 0.75, was found to be larger than the fusion temperature of these alloys. We compare our thermodynamic model with experimental growth results obtained for solid solutions of GaAs_1-xN_x with a wide range of nitrogen concentrations. The equilibrium constants of reactions needed for the formation of single phase solid solution are calculated and compared with our experimental pressure and growth temperature data. In order to grow alloys with high nitrogen concentration the flux of As₄, as well as the flux of active nitrogen, must be increased. This is a consequence of very large interaction parameter of the strictly regular solution approximation assumed here. Good agreement between the model and experimental growth conditions was obtained.

4:30 pm, Late News

4:50 pm, Late News

1:30 pm

Super-Flat Interfaces in In_0.53Ga_0.47As/In_0.52Al_0.48As Quantum Wells Grown on (411)A InP Substrates by Molecular Beam Epitaxy: T. Kitada, T. Saeki, M. Ohashi, S. Hiyamizu, and S. Shimomura, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan; A. Adachi, Research Development Division, Nissin Electric Co., Umezu-Takase-cho, Ukyouku, Kyoto 615, Japan; Y. Okamoto, Research and Headquarters Kubota LTD., Armagasaki, Hyogo 661, Japan; N. Sano, Faculty of Science, Kwansei-Gakuin University, Nishinomiya, Hyogo 662, Japan

Heterostructures with atomically flat interfaces is necessary for obtaining high performance quantum electronics devices such as resonant tunneling diodes. In our recent work, atomically flat interfaces over a macroscopic area (> 1 cm²) (the super-flat interfaces) had been achieved in GaAs/Al_0.3Ga_0.7As quantum wells (QWs) grown on (411)A GaAs substrates by molecular beam epitaxy (MBE). It can be expected that the (411)A plane is a very stable crystal plane generally for III-V semiconductor materials during MBE growth. In this paper, we report, for the first time, extremely flat interfaces over a large area realized in In_0.53Ga_0.47As/In_0.52Al_0.48As QWs grown on the (411)A InP substrates by MBE. An In_0.53Ga_0.47As/In_0.52Al_0.48As QW was grown on the (411)A InP substrate at T_s = 570°C under V/III = 6 (in pressure). Growth rates of In_0.53Ga_0.47As and In_0.52Al_0.48As were 1.0 µm/h. An InGaAs well width (L_w) was 0.6 nm and InAlAs barriers were 20 nm-thick. Surface morphology of the QW grown on the (411)A substrate showed quite featureless in contrast with a rough surface for the (100) sample. Surface roughing of the InGaAs/InAlAs QW on the (100) substrate drastically occurred above T_s = 570°C, but it never occurred for the (411)A QW, indicating that the (411)A plane is also very stable even for the InGaAs/InAlAs system lattice-matched to InP substrates. Photoluminescence (PL) spectra at 20 K from the QWs on a (411)A and (100) InP substrate. The (100) QW for this PL measurement was grown at T_s = 560°C under V/III = 8 instead of T_s = 570°C and V/III = 6, because it showed good surface morphology. Full width at half maximum (FWHM) of a PL peak from the (411)A QW was as small as 14.5 meV, which is 20 % narrower than that of the (100) QW (18.3 meV) and is comparable to the best value (14.5 meV for L_w = 0.8 nm) reported for the InGaAs/InAlAs QWs grown on the (100) InP substrates by growth-interruption technique. This result suggests that the super-flat interfaces (atomically flat interfaces over a macroscopic area) are realized in the In_0.53Ga_0.47As/In_0.52Al_0.48As QW grown on a (411)A InP substrate by MBE.

1:50 pm

Molar Fraction and Substrate Orientation Effect on Carbon Incorporation in InGaAs Grown by Solid Source MBE Using Carbon Tetrabromide: D.I. Lubyshev, M. Micovic, W.Z. Cai, D.L. Miller, The Pennsylvania State University, Electronics and Processing Research Laboratory, University Park, PA 16802

Recently, much attention has focused on carbon gas sources for p-type doping for Ga(Al)As in conventional molecular beam epitaxy due to the very high maximum doping level (~10²⁰ cm^-3) attainable and the low diffusion coefficient of carbon. Carbon doping for InGaAs, however, suffers from amphoteric behavior and also a reduced incorporation efficiency at high indium molar fraction. Here we study the amphoteric behavior and doping efficiency of carbon incorporation from CBr₄ as function of In molar fraction for (100) In_xGa_1-xAs from x=0.53 to x=1.0. We have also studied the growth of carbon doped GaAs, InAs and In_0.53Ga_0.47As on the high index (211), (311) and (511) A and B planes of GaAs. In the first set of experiments, In_xGa_1-xAs (x=0.53-1) doped layers were grown on (100) InP. The molar fraction of indium was determined by RHEED oscillation measurements and confirmed by x-ray diffraction. The substrate temperature was 450°C. The concentration and mobility were taken from Van der Pauw measurements at 300 K. The hole concentration was constant (p=6.x10¹⁸ cm^-3) for In molar fraction from 0.53 to 0.7 and then drastically decreased to p=6x10¹⁷ cm^-3 for x=0.8. For layers with x=0.9 and 1.0 we observed n-type conductivity with electron concentration and mobility of 5x10¹⁶ cm^-3 and 2500 cm²/V s respectively. In order to clarify further the amphoteric behavior and efficiency of tetrabromide doping, GaAs, In_0.53Ga_0.47As and InAs were grown on (211), (311), (511) A/B and (100) GaAs substrates mounted side- by- side on the same holder. For In_0.53Ga_0.47As, the hole concentration had a maximum p=1.2x10¹⁹ cm^-3 on (211)A plane and monotonically decreased to 2x10¹⁷ cm^-3 as the orientation was changed from (211)A to (100) to (211)B. The mobility slightly increased from 30 cm²/V s to 50 cm²/V s with the orientation change from (211)A to (100), and then drastically decreased to ~20 cm²/V s for (311)B and (211)B oriented samples. Based on these measurements, we believe that carbon autocompencation, related to redistribution of C atoms between the cation and anion sublattices, determines the doping efficiency for In_0.53Ga_0.47As. The efficiency of carbon incorporation in InGaAs as an acceptor is related to the formation of group III metal-carbon complexes. We will discuss the effects of surface kinetics and the density of arsenic vacancies on carbon incorporation and autocompensation.

2:10 pm

Lateral Modulation of Material Properties in Strained-Modulated Epitaxial Growth of InGaAs on GaAs Bottom Patterned Compliant Substrates: C. Carter-Coman, A.S. Brown, J. Pickering, R.A. Metzger, N.M. Jokerst, L. Bottomley, Georgia Institute of Technology, Atlanta, GA 30332

Compliant substrates can be used to extend the conventional critical thickness in strained overlayers by accommodating part of the strain in a thin substrate. Strain-modulated epitaxy utilizes compliant substrates that are patterned on the bottom, bonded surface. Therefore, strain partitioning between a mismatched epitaxial layer and the compliant substrate creates lateral band structure variations. Additionally, strain-dependent growth kinetics can be used to further modulate properties such as composition and thickness in the epitaxial layer. It has been shown previously that the properties of InGaAs layers on GaAs bottom patterned compliant substrates can be laterally modulated at relatively high growth temperatures (>520°C). Recent results from strain-modulated epitaxy experiments utilizing bottom patterned compliant substrates will be reported. In these experiments, thin layers of 2500 Å In_0.07Ga_0.93As were grown on GaAs patterned thin film substrates with various pattern geometries and thick GaAs reference substrates in a Riber 2300 MBE with several substrate temperatures. The samples were characterized by atomic force microscopy (AFM) and 4K photoluminescence (PL). AFM showed that the InGaAs layers grown on the bottom patterned compliant substrates exhibited pattern-dependent modulations in thickness across the sample, as well as mounds which are spatially aligned with the pattern. On the samples with the highest mound density, islands with a spatial extent of less than ~100Å formed on the mounds. Furthermore, the behavior of the mound formation as a function of growth temperature was dramatically different for the InGaAs layers grown on the bottom patterned compliant substrates compared to layers grown on the reference substrates. The formation of these mounds and islands will be discussed. In addition to the AFM studies, low temperature PL has been used to show for the first time that the GaAs and InGaAs peaks on the compliant substrates shift compared to the reference substrates. The experimental shifts in the PL peaks compare well with what is predicted using strain partitioning.

2:30 pm

Selective Area Epitaxy of GaAs Using Tri-isopropylgallium: R. Zhang, R. Tsui, K. Shiralagi, J. Tresek, Phoenix Corporate Research Laboratories, Motorola, Inc., 2100 East Elliot Road, M/S-El308, Tempe, AZ 85284

Selective area epitaxy (SAE) is a powerful technology in the fabrication of advanced devices and circuits. In some applications, however, it is desirable to achieve SAE at a low substrate temperature (Ts). This is particularly so in a selective regrowth process, where the characteristics of the underlying layers can be degraded by a high Ts. For GaAs, SAE at low Ts can be achieved by using tri-methylgallium as the Ga source, but its use results in a high background concentration of C in the layers. Tri-isopropylgallium (TIPG) has been reported to provide high purity growth at low Ts, and appears to be a good candidate for use in SAE. We have studied the SAE of GaAs using TIPG as the Ga source and GaAs oxide as a mask. Experiments were carried out in a chemical beam epitaxy (CBE) system using pre-cracked arsine (AsH₃) as the group V source. We investigated the effect of growth rate, Ts and AsH₃ flux on the epitaxial selectivity of the GaAs. Our results show that GaAs can be selectively grown by CBE in the window areas of the oxide masking layer at a Ts as low as 380°C. The low selective growth temperature mini-mizes the degradation of the oxide mask. Selective GaAs layers as thick as ~ 0.3 µm have been obtained under optimized growth conditions. Details of the technique as well as properties of the selectively grown layers will be presented.

2:50 pm

Selective Area MOVPE of GaAs Using a Novel Ga₂O₃ Mask Layer: S. Hirose, A. Yoshida, M. Yamaura, K. Hara, and H. Munekata, Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226, Japan

We have been studying selective area growth of GaAs for semiconductor microstructures using patterned Ga₂O₃/GaAs(100) substrates on the basis of the following three expectations: (1) Ga₂O₃ would be stable enough to survive during the growth by metal organic vapor phase epitaxy (MOVPE), (2) it may be eliminated in the growth reactor immediately after the growth by reduction reaction with a H₂ atmosphere at high temperature, and (3) electronic quality of the reexposed GaAs surface would still remain high to allow overlayer growth. In this paper, we demonstrate the selective area growth of GaAs by horizontal low-pressure MOVPE (LP-MOVPE), and the elimination of Ga₂O₃ mask layers by H₂ gas and H₂⁺ plasma treatments. Ga₂O₃ layers were deposited by RF sputtering on GaAs(100) substrates using a 5N-Ga₂O₃ powder target. Annealing experiments have showed that Ga₂O₃ layers could be eliminated under the 100-Torr H₂ atmosphere with substrate temperature (T_s) of 480°C or higher, whereas the layers remained unchanged under the reduced H₂ atmosphere of 10 Torr. This indicates that the elimination of Ga₂O₃ is controllable though the reduction as expected. Selective area MOVPE was studied with source gases of trimethylgallium (TMGa) and 20 % AsH₂-H₂ mixture at Ts = 640-700°C, and a reactor pressure of 10 Torr. For the growth with high V/III ratio (= 800), GaAs layers are deposited on both masked and unmasked areas. On the contrary, when V/III ratio is reduced to be 20, epitaxial growth occurs only on unmasked area, resulting in perfect selective growth. The sensitive dependence of the V/III ratio on growth selectively can probably be explained in terms of the As-dependent migration rate of Ga species. Elimination of a Ga₂O₃ mask layer was examined by various H₂ treatments for samples on which selective area epitaxy was successively achieved. We have found that the mask layer can be eliminated with smooth GaAs surface finish by the mild hydrogen-plasma treatment (40 W, T_s = 480°C) followed by H₂ annealing at T_s = 640°C with a reactor pressure of 500 Torr.

3:10 pm, Break

3:30 pm

High-Purity AlGaAs Grown with Dimethylaluminumhybride by Metalorganic Vapor Phase Epitaxy: H.Q. Hou, B.E. Hammons, M.H. Crawford, R.J. Hickman, Sandia National Laboratories, MS 0603, Albuquerque, NM 87185; R.A. Stall and E.A. Armour, EMCORE Corp., Somerset, NJ 08873

We present results on metalorganic vapor phase epitaxy (MOVPE) growth and characterization of low oxygen and carbon background AlGaAs using an alternative aluminum precursor, dimethylaluminumhydride (DMAH), to the traditional trimethylaluminum (TMA). The AlGaAs growth was carried out using DMAH (or TMA), trimethylgallium (TMG), and 100% arsine in an EMCORE rotating disk reactor. The growth rate, measured by an in situ reflectometer, was examined as a function of the growth temperature, growth pressure, substrate rotation speed, and V/III ratio. No measurable pre-reaction of DMAH by itself or between DMAH and TMG precursors was observed. Excellent surface morphology of AlGaAs was obtained over a wide range of growth conditions. A comparative study on background C and O concentrations was performed for AlGaAs grown with both TMA and DMAH by secondary-ion mass spectroscopy (SIMS) measurements. It is found that both O and C backgrounds in GaAs are below 5x10¹⁵ cm^-3, slightly above the SIMS detection limit. Thus the background impurities in AlGaAs are predominantly from AlAs. It is also found that the respective concentration of O and C in AlAs can be as high as 1x10¹⁹ and 2x10¹⁷ cm^-3. However, these concentrations in AlAs grown with DMAH can be significantly reduced to 1x10¹⁷ and 5x10¹⁶ cm^-3, respectively, at a higher V/III ratio and higher growth pressure (e.g., 110 torr). These levels are much lower than those in AlAs grown with TMA. Room-temperature photoluminescence measurements on high Al-content Al_0.24Ga_0.76As (80 Å)/Al_0.42Ga_0.58As quantum wells show that the efficiency from DMAH material is 40% more than that from a separately optimized TMA structure, indicative of lower concentration of O traps in DMAH materials. We will also report the results of all-AlGaAs 700 nm vertical-cavity surface emitting lasers using active layers grown with DMAH and TMA. This work is supported by the US DOE under contract No. DE-AC04-94AL85000.

3:50 pm

Initial Stages of Alloy Ordering in Single Variant GaInP Grown on GaAs: J.F. Geisz, J.M. Olson, S.P. Ahrenkiel, W.E. McMahon, and Y. Zhang, National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401

The extent of alloy ordering in Ga_0.5In_0.5P grown on singular (001) GaAs by metal-organic vapor phase epitaxy (MOVPE) has previously been shown to develop slowly with thickness from 1 to 10 microns corresponding to the development of 4°B facets. Well ordered, single variant GaInP layers of thicknesses less than 1 micron can easily be grown on (001) GaAs substrates miscut 4-6° toward the []B direction under proper growth conditions. We have observed the development of ordering in GaInP grown on 6°B miscut substrates for thicknesses 10 - 600Å using in-situ reflectance difference spectroscopy (RDS). The amplitude of the E0 peak in RD spectra drops off dramatically for layer thicknesses less then approximately 200Å. This RDS peak amplitude has been shown to correlate directly to the order parameter. This low degree of ordering in GaInP layers less than 200Å has also been observed by polarization dependent photoluminescence. Dark field transmission electron microscopy (DF-TEM) images also show very little ordering within the first few hundred angstroms of the GaInP layers. V-shaped ordered domains appear to originate near the interface approximately 1300Å apart amidst mostly disordered GaInP and widen significantly within the first 1000Å of growth. Popular theories for the ordering mechanism require the presence of step edges and/or a particular terrace reconstructions. In an attempt to explain the gradual development of ordering we have observed the evolution of these surface features. The surface reconstruction of GaInP observed by RDS during growth of highly ordered GaInP has been confirmed by low energy electron diffraction (LEED) to be 2x1. The RDS peaks at 1.9 and 3.0 eV corresponding to this 2x1 reconstruction (at growth temperature) become fully developed during growth within the first 10 seconds (or 30Å at this growth rate). We have also observed from splitting of LEED spots the presence of evenly spaced monolayer steps on the surface of GaInP layers as thin as 50Å. Because both the step structure and the terrace reconstruction appear to be well developed at thicknesses much less than the "critical thickness" for ordered material, we conclude that these surface features are not sufficient criterion for GaInP ordering. These results strongly suggest that some other surface perturbation or structure not yet detected by the probes used in this study are required to initiate ordering. The results of this study has relevance to potential quantum structures that would rely on ordering in GaInP.

4:10 pm

Anti-Phase Domain-Free GaAs Grown on Ge Substrates by Molecular Beam Epitaxy Using Either As or Ga Terminated Ge Surfaces: R.M. Sieg, S.A. Ringel, The Ohio State University, Department of Electrical Engineering, 2015 Neil Avenue, Columbus, OH 43210-1272; S.M. Ting and E.A. Fitzgerald, Massachusetts Institute of Technology, Room 13-4053, Cambridge, MA 02139

GaAs growth on (001) Ge substrates is an ideal system for the study of III-V/IV heteroepitaxy due to the close lattice matching, and is of great interest as the material of choice for high-efficiency solar cells for space satellite applications. However, the GaAs material quality is often degraded by the presence of anti-phase domains (APDís), and control of these APDís during epitaxy is vital for the successful application of this material system. We present a study of growth factors which contribute to APD suppression in GaAs grown on offcut (001) Ge substrates by MBE. Regardless of the Ge wafer cleaning method used, we find that a Ge epitaxial layer must be grown to achieve a smooth, clean surface prior to GaAs nucleation. A key step in suppressing APD formation is annealing the epitaxial Ge above 640°C, which produces a double-stepped surface as indicated by RHEED. The next critical step is to initiate growth using a complete monolayer of a single atomic species (either As or Ga). In order to carefully define the GaAs nucleation conditions, we initiated growth using migration enhanced epitaxy (MEE), starting with either a pure As or a pure Ga initial monolayer. This MEE step was done at low substrate temperature (~350°C) to prevent adatom desorption. After 10 GaAs monolayers were deposited by MEE, the substrate was raised to 500°C under dimeric As flux and GaAs coevaporation was initiated, using a slow 0.1 ml/sec growth rate for the first 100 nm. We find that terminating the annealed, epitaxial Ge surface with As results in APD-free GaAs/Ge. However, we are also able to obtain nearly APD-free GaAs/Ge by using a Ga terminated surface but only if the background As pressure is kept to <10-10 Torr. Any significant As-Ga mixture during the initial layer deposition results in a high APD density. Thus, a requisite condition for APD-free growth is achieving a single species initial monolayer prior to epitaxy. For either an As or Ga prelayer, however, RHEED indicates the same GaAs domain is obtained, and the 2x4 reconstruction of this dominant domain is observed after only a few monolayers have been deposited. This is a strong indication of large-scale monolayer re-arrangement during the initial GaAs nucleation. The high material quality is confirmed cross-sectional and plan-view TEM, and by electrical measurements which show controllable n-type doping at n=2x10¹⁵ cm^-3 can be achieved, indicating no significant outdiffusion of Ge, and also by DLTS measurements which do not reveal any detectable traps, indicating the very high material quality of our GaAs grown on Ge substrates.

4:30 pm

Epitaxial Growth of Si/CdF₂/CaF₂ Heterostructures on Si Substrate: W. Saitoh, Y. Aoki, J. Nishiyama, M. Watanabe and M. Asada, Research Center for Quantum Effect Electronics, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152, Japan

Si/CdF₂/CaF₂ heterostructures are possible new heterostructure system for quantum-effect devices on Si substrate. As a basic device which can be extended to field effect quantum devices, we proposed a very short channel tunneling FET with these heterostructures. To realize this transistor, growth of CoSi₂/Si/CdF₂/CaF₂/CoSi₂ heterostructures on Si substrate is necessary. CoSi₂/Si, CdF₂/CaF₂ and CaF₂/CoSi₂ in these heterostructures have already been reported. In this paper, we demonstrate epitaxial growth of Si/CdF2, which has not been reported so far. It is shown that growth with two-temperature step is essential to obtain flat epitaxial Si layer on CdF² for Si thicker than about 0.9 nm. Si (1.8 nm)/ CdF₂ (5.0 nm)/ CaF₂ (3.1 nm) heterostructures were grown on Si(111) substrate by MBE. CaF₂ was grown on Si pseudomorphically at substrate temperature 650°C for the 1st 0.6 nm and 200 °C for the 2nd 2.5 nm. CdF₂ was grown on CaF₂ at 50°C. In the growth of 1.8 nm-thick Si on CdF₂, growth at a constant temperature between 50 and 200 °C showed halo RHEED pattern, while growth at 300°C showed ring pattern and islandish AFM image. At low temperature in these results, migration energy was not enough for Si layer thicker than 0.9 nm, although a few monolayer single crystalline Si can grow. At high temperature, CdF₂ was damaged thermally or by reaction with Si. Thus, we employed the growth technique with two-temperature step for Si on CdF₂: the 1st 0.9 nm-thick Si was grown at 50°C, and then, the 2nd 0.9 nm-thick Si at 300°C. A streaky RHEED pattern and flat surface image of Si layer were observed using this technique. As a result, epitaxial growth of Si thicker than about 0.9 nm was achieved on CdF₂ by growth with two-temperature step.

4:50 pm, Student Paper

Epitaxial Growth and Characterization of DyP/GaAs and GaAs/DyP/GaAs Heterostructures and Devices: P.P. Lee, R.J. Hwu and L.P. Sadwick, H. Balasubramaniam, Engineering, University of Utah, Salt Lake City, UT 84112; R. Alvis, Advance Micro Device, 3625 Peterson Avenue, Santa Clara, CA 95054

There is a significant interest in the area of increasing the high temperature stability of III-V device processing, fabrication, and operation. An attractive materials system that offers strong promise in this area is dysprosium phosphide/gallium arsenide (DyP)/GaAs. Details of the epitaxial growth of DyP/GaAs by MBE and the fabrication and device performance of DyP/GaAs MESFETs will be presented. DyP is very stable at high temperature in air and is highly lattice matched to GaAs with a room temperature mismatch of less than 0.01%. High quality DyP epilayers, as determined by XRD, AFM, and TEM measurements, were obtained for MBE growth temperatures ranging from 590 to 640°C at a DyP growth rate of approximately 0.5 micron/hour. The surface morphology, as determined by AFM, of DyP grown at different substrate, cracker, and e-cell temperatures, As/P interruption times, and III/V ratios will be presented. The DyP epilayers are n-type with measured electron concentrations on the order of 3 to 4 x10²⁰ cm^-3 with room temperature mobilities of 250 to 300 cm^2/Vs. FTIR and XPS results indicate that DyP is a semimetal. The Schottky barrier height of DyP on GaAs is approximately 0.75 eV with an ideality factor near unity as determined by I-V measurements. The specific contact resistance of DyP/GaAs measured by the transmission-line-method (TLM) as a function of growth parameters will also be presented. A detailed study of the surface morphology, including RMS roughness, as a function of substrate temperature has been performed and will be presented. Typical surface RMS roughness was between 0.4 to 0.6 nm, at a growth temperature of 600°C. DyP/GaAs heterointerfaces and contacts are thermodynamically stable in air ambient with no changes in the electrical or material properties to temperatures well in excess of 400°C. Details of the material and surface science properties of DyP/GaAs and GaAs/DyP/GaAs to be reported include Hall measurements, XRD, TEM, AFM, and SIMS results. We will also report on GaAs grown on DyP/GaAs epilayers. Details of the surface morphology, orientation, defects, and roughness as determined by AFM, TEM, and XRD will also be presented.