JOURNAL OF ELECTRONIC MATERIALS
	ABSTRACTS Volume 24, Number 3, March 1995 This Month Featuring: Proceedings of the symposium on Short Wavelength Optical Sources Based on Wide Bandgap II-VI Heterostructures and Regular Issue Papers from the 36th Electronic Materials Conference in Boulder, Colorado, June 22-24, 1994. View March 1995 Table of Contents.

SPECIAL ISSUE PAPERS

• the molecular beam epitaxy of the (Zn,Mg)(S,Se) quaternary alloys,
• the p-type doping using the nitrogen plasma source,
• the bandgap engineering of very sophisticated quantum well heterostructures necessary to provide optical and electronic confinement,
• the use of a graded Zn(Se,Te) multilayered structure to form an ohmic contact to the p-type ZnSe layers, and
• advances in the proper formation of the II-VI/III-V heterovalent interface consisting primarily of ZnSe/GaAs.

At the 36th Electronic Materials Conference (EMC) held June 22-24, 1994 in Boulder, Colorado, researchers from around the world joined together to address many of the key issues still facing the II-VI community in their quest for commercial short wavelength optical sources. Twenty papers were presented from scientists originating from the United States, France, Japan, United Kingdom, Korea, and Italy. Issues related to the p-type doping employing solid source molecular beam epitaxy, as well as gaseous source epitaxy technologies, were highlighted. Of course, electronic properties of ZnSe-based structures demonstrating p-type conductivity could only be investigated by utilizing various methods to form ohmic contacts. Of particular relevance was the first observation of persistent photoconductivity, in conjunction with an anomalously large acceptor activation energy in (Zn,Mg)(Se,S) alloys, which suggested the presence of an acceptor-like "AX" center. New quantum well structures were proposed that were composed of a variety of binary pseudo-alloys, and also contained new layered II-VI configurations to explore alternate II-VI materials having various lattice constants. In addition, techniques for the homoepitaxy of ZnSe and the heteroepitaxy on InP were described.

Building on the previous successes of the 34th and 35th EMC meetings, the 36th EMC was equally successful in highlighting the numerous advances in the field of materials science and characterization of II-VI materials, quantum well physics and heterostructure device fabrication. The key issues to be addressed at the present time include identification and elimination of the mechanisms contributing to defect generation and propagation within the laser and light emitting device structures. Once the II-VI community solves these issues of defects, perhaps commercial II-VI light emitters will finally be a reality. The upcoming 37th Electronic Materials Conference promises to provide the forum for pioneering breakthroughs in wide bandgap II-VI semiconductor optical devices.

Professor Leslie A. Kolodziejski
Massachusetts Institute of Technology

Guest Editor

Design of Pseudo-Ternary and Quaternary Alloys by Superlattices Consisting of (Zn,Cd)(S,Se) Binary II-VI Compounds
A.W. JIA, M. KOBAYASHI, and A. YOSHIKAWA
Department of Electrical and Electronics Engineering, Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba-shi 263, Japan.

KEY WORDS Effective bandgap, expression of in-plane lattice constant, lattice-matched superlattice, ZnCdSSe,

Theoretical design of pseudo-ternary and quaternary alloys by superlattice structures consisting of (Zn,Cd)(S,Se) binary II-VI compounds has been studied. For pseudo-ternary ZnCdS and ZnCdSe alloys, the superlattices with two layers in a cycle, i.e., ZnS/CdS and ZnSe/CdSe are considered, and for pseudo-quaternary ZnCdSSe alloy, the two superlattice structures with more than two layers in a cycle are considered. In order to design and evaluate these superlattices, the expression for the equilibrium in-plane lattice constant of these superlattices has been derived by minimizing the total elastic strain energy in the cycle. The combinations of layer thicknesses in a cycle and the effective bandgap of these superlattices have been calculated while the elastic strain effect was included. The usefulness of these superlattice structures has been evaluated.

Optical Properties, Electronic Structure, and Exciton Binding Energies in Short Period ZnS-ZnSe Superlattices
T. CLOITRE, L. AIGOUY, M. DI BLASIO, B. GIL, P. BIGENWALD, N. BRIOT, O. BRIOT, D. BOUCHARA, M. AVEROUS, and R.L. AULOMBARD
Universití Montpellier II-Groupe d'Etudes des Semiconducteurs, Case Courrier 074-34095 Montpellier Cedex 5 - France.

KEY WORDS II-VI wide bandgap semiconductors, built-in strain, exciton-related phenomena, photoluminescence characterization, superlattices

We present a detailed examination of the optical properties and electronic structure taken from photoreflectance and photoluminescence data collected on a series of short-period ZnS-ZnSe superlattices grown by low pressure metalorganic vapor phase epitaxy. We studied the band offset problem and calculated the excitor binding energy using several variational models. The temperature dependence of the photoluminescence properties of these superlattices was analyzed in the context of a model which includes the influence of the interfacial disorder.

The Study of Nitrogen Doping in ZnSe and ZnSe:Te
Y. FAN,¹ J. HAN,¹ R.L. GUNSHOR,¹ J. WALKER,² N.M. JOHNSON,² and A.V. NURMIKKO³
1--School of Electrical Engineering, Purdue University, West Lafayette, IN 47907-1285. 2--Xerox PARC, Palo Alto, CA 94304. 3--Division of Engineering, Brown University, Providence, RI 02912.

KEY WORDS Hall-effect measurement, isoelectronic impurity, nitrogen doping, ZnSe, ZnTe

A comparative study of nitrogen doping in ZnSe and ZnTe has been performed and the results suggest that dopant solubility seems to be the limiting factor, at least under our growth conditions, in obstructing degenerate p-type doping of ZnSe. In an effort to increase the nitrogen acceptor concentration in ZnSe, we have investigated the effects of Te isoelectronic impurity on the nitrogen doping concentration in ZnSe. It was found that the total nitrogen concentration and the nitrogen acceptor concentration are indeed increased, but the room temperature free hole concentration actually drops slightly. Temperature dependent transport measurements were performed and the results show a large increase in compensation ratio as well as a dramatical reduction in hole mobility. The latter is attributed to the tendency for hole localization at the isoelectronic impurity.

Thermal Annealing Effects on p-type Conductivity of Nitrogen-Doped ZnSe Grown by Metalorganic Vapor Phase Epitaxy
SHIZUO FUJITA, TSUYOSHI TOJYO, TETSU YOSHIZAWA, and SHIGEO FUJITA
Department of Electrical Engineering, Kyoto University, Kyoto 606-01, Japan.

KEY WORDS Metalorganic vapor phase epitaxy (MOVPE), nitrogen doping, p-type ZnSe, thermal annealing

Post-growth thermal annealing (e.g., 500°C, 30 min), is proposed as one of the promising techniques to realize and to improve the quality of p-type ZnSe layers grown by metalorganic vapor phase epitaxy (MOVPE). The layers were grown at low temperature (350°C) by photo-assisted MOVPE with doping nitrogen from tertiarybutylamine (t-BuNH₂). The flow rate of t-BuNH₂ was limited to be relatively low, in order to avoid heavy doping, with which as-grown layers exhibited electrically high-resistivity; but the thermal annealing converted the layers to p-type. As the as-grown layers exhibited the stronger donor-to-acceptor pair recombination lines or the weaker donor-bound excitonic emission (I_x) lines in photoluminescence, the annealed layers resulted in higher net acceptor concentration, which was 1 x 10¹⁷ cm^-3 at the optimum conditions at present.

Au and Ag Electrical Contacts to p-ZnSe
J.J. FIJOL, L.C. CALHOUN, R.M. PARK, and P.H. HOLLOWAY
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611.

KEY WORDS Ag, Au, electrical contacts, Schottky contacts, ZnSe

The reverse bias break-down voltage and resistance of Au and Ag contacts sputter deposited on nitrogen doped p-type ZnSe, having free hole concentrations in the low to mid 10¹⁷ cm^-3 range, have been studied. Samples were heat treated over the range of 160-400°C for times of 15-45 min. The minimum break-voltage for the Au contacts (3.0 V) was found to occur following heat at 350-400°C for 30 min and for Ag contacts (2.3 V) following heat treatments at 150°C for 45 min. Secondary ion mass spectrometry and Auger electron spectroscopy were used to identify changes in the contacts induced by heat treatments. No evidence was found for the formation of new interfacial compounds, but Au diffused into the ZnSe at T >350°C. The data suggest that conduction through the Au/ZnSe contacts was dominated by avalanche breakdown assisted by deep acceptor levels formed by the diffusion of Au into the ZnSe. The results from the Ag contacts suggest that interfacial O forms a highly doped region in the ZnSe leading to conduction dominated by field emission currents.

Ohmic Contact and Transport Properties of II-VI Green/Blue Laser Diodes
JUNG HAN,¹ R.L. GUNSHOR,¹ and A.V. NURMIKKO²
1--School of Electrical Engineering, Purdue University, West Lafayette, IN 47907. 2--Division of Engineering, Brown University, Providence, RI 02912.

KEY WORDS AX center, DX center, persistent photoconductivity, ZnSe, ZnMgSSe, Zn (Se,Te) graded contact

The employment of the Zn(Se,Te) pseudo-graded contacting scheme to p-type ZnSe-based alloys contributes directly to the recent demonstration of room temperature continuous-wave operation of II-VI green-blue laser diodes. Contact ohmicity is maintained down to cryogenic temperatures which enabled the investigation of electrical transport properties associated with the p-type nitrogen-doped ZnSe, Zn(S,Se), and (Zn,Mg)(S,Se). The observation of both persistent photoconductivity and a metastable population of holes which are in thermodynamic equilibrium with hydrogenic acceptors having reduced activation energy suggests the presence of a DX-like behavior for holes in p-type (Zn,Mg)(S,Se).

Misfit Strain Induced Tweed-Twin Transformation on Composition Modulation Zn_1-xMg_xS_ySe_1-y Layers and the Quality Control of the ZnSe Buffer/GaAs Interface
L.H. KUO,¹ L. SALAMANCA-RIBA,¹ B.J. WU,² and J.M. DE PUYDT²
1--Department of Materials and Nuclear Engineering, University of Maryland, College Park, MD 20742-2115. 2--3M Company, St. Paul, MN 55144-1000.

KEY WORDS Composition modulation, misfit accommodation, Zn1-xMgxSySe1-y films, twinning

[100] composition modulation as well as [101] and [] tweed strain contrast were observed in 0.72 µm thick Zn_1-xMg_xS_ySe_1-y epitaxial films grown on ZnSe buffer layers. The lattice distortion induced tweed strain contrast disappears in relaxed Zn_1-xMg_xS_ySe_1-y layers of thicknesses above ~ 0.8-1 µm even though the [100] composition modulation remains. Instead, the formation of microtwins takes place to relieve the strain in the distorted lattice of the quaternary films. The Zn_1-xMg_xS_ySe_1-y layers were obtained by growing a ZnSe buffer layer on As stabilized GaAs substrates with Zn treatment of the substrate prior to the growth of the film. The samples with film thickness of ~0.72 µm were of very high quality with a defect density of less than 5 x 10⁴/cm². Some samples showed rough ZnSe/GaAs interfaces and a high density of Frank partial dislocations originating at the ZnSe/GaAs interface. The interface roughness is believed to result from an As-rich GaAs surface after the oxide desorption.

Deep Level Electronic Structure of ZnSe/GaAs Heterostructures
A. RAISANEN,¹ L.J. BRILLSON,¹ A. FRANCIOSI,² R. NICOLINI,² L. VANZETTI,² and L. SORBA²
1--Xerox Webster Research Center, 800 Phillips Road 0114-41D, Webster, NY 14580. 2--Laboratorio Tecnologie Avanzate Superfici e Catalisi dell' Istituto Nazionale di Fisica della Materia, Area di Ricerca, Padriciano 99, 1-34012 Trieste, Italy, and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455.

KEY WORDS Cathodoluminescence (CL), deep levels, heterojunction interface, molecular beam epitaxy (MBE), photoluminescence (PL), ZnSe/GaAs

We performed 1-2 keV cathodoluminescence measurements and He-Ne and He-Cd excited photoluminescence studies of ZnSe/GaAs(100) heterostructures grown by molecular beam epitaxy. Our goal was to investigate the deep level electronic structure and its connection with the heterojunction band offsets. We observed novel deep level emission features at 0.8, 0.98, 1.14, and 1.3 eV which are characteristic of the ZnSe overlayer and independent in energy of overlayer thickness. The corresponding deep levels lie far below those of the near-band-edge features commonly used to characterize the ZnSe crystal quality. The relative intensity and spatial distribution of the deep level emission was found to be strongly affected by the Zn/Se atomic flux ratio employed during ZnSe growth. The same flux ratio has been shown to influence both the quality of the ZnSe overlayer and the band offset in ZnSe/GaAs heterojunctions. In heterostructures fabricated in Se-rich growth conditions, that minimize the valence band offset and the concentration of Se vacancies, the dominant deep level emission is at 1.3 eV. For heterostructures fabricated in Zn-rich growth conditions, emission by multiple levels at 0.88, 0.98, and 1.14 eV dominates. The spectral energies and intensities of deep level transitions reported here provide a characteristic indicator of ZnSe epilayer stoichiometry and near-interface defect densities.

Remarkable Improvement in Emission Efficiency of ZnCdSe/Zn(S)Se LEDs by Thermal Annealing
YOSHIKATSU ICHIMURA, KATSUMI KISHINO, MASARU KURAMOTO, MITSUNARI SATAKE, and ATSUSHI YOSHIDA
Department of Electrical and Electronics Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102, Japan.

KEY WORDS Light emitting diode (LED), MgZnSSe, thermal annealing, ZnCdSe/Zn(S)Se

Thermal annealing effects on optical and electrical characteristics for p-type and n-type II-VI compound layers (ZnSe, ZnSSe, and MgZnSSe) and on the emission efficiency of ZnCdSe/Zn(S)Se 6 quantum well (QW) light emitting diodes (LEDs) grown by molecular beam epitaxy were investigated. It was clarified that serious degradation of optical and electrical characteristics was not observed up to an annealing temperature of 400°C. In the case of p-MgZnSSe, the maximum permitted annealing temperature was lower than that of Zn(S)Se. The light output of the ZnCdSe/Zn(S)Se multi QW LEDs was enhanced by a factor of three at optimum thermal annealing conditions. The study suggests that this thermal effect for LEDs was produced by the improved crystal quality of ZnCdSe QWs by thermal annealing.

Molecular Beam Epitaxial Growth of Green Light Emitting Diodes on ZnSe Wafers
M.H. JEON, L.C. CALHOUN, and R.M. PARK
Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611-2066.

KEY WORDS Green light emitting diodes, ZnSe homoepitaxy

We have performed a preliminary investigation into the use of ZnSe bulk crystals fabricated by Sumitomo Electric Industries, Ltd. as substrates for the epitiaxial deposition of ZnSe-based materials and light emitting devices. A low temperature (<380°C) in-situ cleaning process has been developed for the (100) oriented ZnSe wafers involving the use of a remotely generated atomic hydrogen beam. The process produces a (1 x 1) atomically smooth ZnSe surface which is highly suitable for epitaxy. Hall-effect measurements performed on nitrogen-doped ptype ZnSe/S.I. ZnSe epilayers have revealed free-hole concentrations in the homoepitaxial material as high as 2.1 x 10¹⁷cm^-3, so far, with room temperature and 77K hole mobility values of 20 and 100 cm²V^-1s^-1, respectively. Finally, green light emitting diodes have been grown on the ZnSe wafers having Cd_0.2Zn_0.8Se/ZnSe multiple quantum well active regions which have exhibited electroluminescence peak linewidths around 9.9 nm at room temperature.

REGULAR ISSUE PAPERS

KEY WORDS In-situ phosphorus doping, plasma-enhanced deposition, Si and Si1-xGex epitaxy

Remote plasma-enhanced chemical vapor deposition has been applied to grow in-situ doped e-type epitaxial Si and Si_1-xGe_x with the introduction of phosphine. Growth rates and dopant incorporation have been studied as a function of process parameters (temperature, rf power, and dopant gas flow). Growth rates remain unaltered with the introduction of PH₃ during deposition, unlike in many other low temperature growth techniques. Phosphorus incorporation shows a linear dependence on PH₃ flow rate, but has little if any dependence on the other growth parameters, such as radio frequency power and substrate temperature, for the ranges of parameters that were examined. Phosphorus concentrations as high as 4 x 10¹⁹ cm^-3 at 14 W have been obtained.

Effect of Substrate Smoothness on the Microstructure of
YBa₂Cu₃O_7-x/Y₂O₃/YBa₂Cu ₃O_7-x Trilayers
G.L. WAYTENA,¹ H.A. HOFF,¹ R.R. WOLCOTT, JR.,² P.R. BROUSSARD,¹ C.L. VOLD,¹ and CLINTON B. LEE³
1--Naval Research Laboratory, Washington, DC 20375. 2--North Carolina State University, Raleigh, NC 27695-8202. 3--North Carolina A. and T. State University, Electrical Engineering Department, Greensboro, NC 27411.

KEY WORDS Josephson junctions, strain, substrate smoothness, superconductors, thin films, yttria particles

Transmission electron microscopy was used to analyze the microstructure of YBa₂Cu₃O_7-x/Y₂O₃/YBa₂Cu ₃O_7-x trilayer structures deposited by off-axis sputtering on MgO substrates with varying degrees of roughness. Substrate surface hillocks with a peak-to-valley height of about 4.5 nm were found to contribute to strain that extended through the film and disrupted the smoothness of the Y₂O₃ layer. In some cases, these hillocks served as nucleation sites for yttria precipitates. Such defects may seriously jeopardize the realization of weak-link Josephson junctions.

Uniform Intermixing of Quantum Wells in p-i-n Modulator Structures by Impurity Free Vacancy Diffusion
S.J. LYCETT,¹ A.J. DEWDNEY,¹ M. GHISONI,¹ C.E. NORMAN,¹ R. MURRAY,¹ D. SANSOM,² J.S. ROBERTS³
1--Interdisciplinary Research Centre for Semiconductor Materials, Imperial College, London, United Kingdom SW7 2AX. 2--Materials Department, Imperial College, London, United Kingdom SW7 2AX. 3--SERC III-V Facility, University of Sheffield, Sheffield, United Kingdom.

KEY WORDS Photoluminescence, quantum wells, Si-doped GaAs

We present the results of an investigation of impurity free vacancy diffusion (IFVD) post-growth treatments of p-i-n modulator structures. The investigation is in two parts. We first establish that gallium vacancies (V_Ga) are produced during IFVD (by measuring the intensity of the low temperature 1.2 eV signal from Si-V_Ga complexes) in a thick Si-doped GaAs sample. The second part of this work investigates the degree of intermixing of three 80Å GaAs quantum wells embedded in the intrinsic region of a p-i-n modulator at depths between 1-2 µm from the surface. Photoluminescence studies on etched samples and cathodo-luminescence showed that no significant depth dependence occurs as a result of IFVD.

Internal Strain and Dislocations in In_xGa_1-xAs Crystals Grown by Liquid Phase Epitaxy/Electroepitaxy
B. BRYSKIEWICZ,^1,3 T. BRYSKIEWICZ,² and E. JIRAN²
1--National Research Council, Montreal Rd., M-50, Ottawa, Ontario K1A OR6, Canada. 2--MPB Technologies, Inc., NRC, Montreal Rd., M-50, Ottawa, Ontario K1A OR6, Canada. 3--Present address: Bell-Northern Research, P.O. Box 3511, Station C, Ottawa, Ontario, Canada K1Y 4H7.

KEY WORDS Dislocations, internal strain, liquid phase electroepitaxy, selective lateral overgrowth, ternary crystals

Strain relaxed, low dislocation density In_xGa_1-xAs crystals, 0 < x < 0.2, have been successfully grown by liquid phase electroepitaxy on the GaAs substrate, despite the crystal/substrate lattice mismatch. Residual strain in these novel substrates is below 10^-4, at least an order of magnitude lower than in the molecular beam epitaxially (MBE) or metalorganic chemical vapor deposition-grown ternary buffer layers of similar composition. Threading dislocation density induced by both the crystal/substrate lattice mismatch and unavoidable composition variations has been reduced from the low 10⁶ cm^-2 range, while growing directly on GaAs, to the mid 10⁴ cm^-2 by employing both the MBE grown ternary buffer layer and selective lateral overgrowth of an SiO₂ mask which, prior to the crystal growth, was deposited on the buffer layer and patterned by photolithography with 10 µm wide, oxide free seeding windows. The full width at half maximum of the rocking curves measured for In_xGa_1-xAs crystals grown by liquid phase epitaxy/electroepitaxy on patterned, closely lattice matched buffer layers was in the 20-23 arc s range. Further reduction of the dislocation density and a more uniform dislocation distribution is expected by modifying the initial growth conditions, improving substrate preparation, and optimizing the seeding win dow geometry.