|JOURNAL OF ELECTRONIC MATERIALS
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
This issue of the Journal of Electronic Materials is the third Special Issue highlighting the significant advances made by the worldwide II-VI community toward achievement of long lived, blue/green light emitters based on the Zn-chalcogenide material family. As of November 1994, the Sony Corporation reported the longest lived blue/green pn junction injection laser operating continuously at room temperature for one hour. Prior to this substantial advance, various other groups, including the university team of Purdue/Brown researchers and a group at North American Philips Corporation, had also achieved CW room temperature lasers exhibiting lifetimes of one minute and several seconds, respectively. The demonstration of commercially viable optical devices based on wide bandgap II-VI compounds represents a series of pioneering breakthroughs in many different areas of technology. Technological advances have been made in
However, although there is "light at the end of the tunnel," many issues continue to require immediate attention. This Special Issue of the Journal of Electronic Materials addresses many of these needs.
- 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
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.
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.
Effective bandgap, expression of in-plane lattice constant, lattice-matched superlattice, ZnCdSSe,
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.
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.
II-VI wide bandgap semiconductors, built-in strain, exciton-related phenomena, photoluminescence characterization, superlattices
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.
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.
Hall-effect measurement, isoelectronic impurity, nitrogen doping, ZnSe, ZnTe
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.
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 (Ix) 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.
Metalorganic vapor phase epitaxy (MOVPE), nitrogen doping, p-type ZnSe, thermal annealing
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.
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.
Ag, Au, electrical contacts, Schottky contacts, ZnSe
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.
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).
AX center, DX center, persistent photoconductivity, ZnSe, ZnMgSSe, Zn (Se,Te) graded contact
Misfit Strain Induced Tweed-Twin Transformation on Composition Modulation Zn1-xMgxSySe1-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.
 composition modulation as well as  and  tweed strain contrast were observed in 0.72 µm thick Zn1-xMgxSySe1-y epitaxial films grown on ZnSe buffer layers. The lattice distortion induced tweed strain contrast disappears in relaxed Zn1-xMgxSySe1-y layers of thicknesses above ~ 0.8-1 µm even though the  composition modulation remains. Instead, the formation of microtwins takes place to relieve the strain in the distorted lattice of the quaternary films. The Zn1-xMgxSySe1-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.
Composition modulation, misfit accommodation, Zn1-xMgxSySe1-y films, twinning
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.
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.
Cathodoluminescence (CL), deep levels, heterojunction interface, molecular beam epitaxy (MBE), photoluminescence (PL), ZnSe/GaAs
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.
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.
Light emitting diode (LED), MgZnSSe, thermal annealing, ZnCdSe/Zn(S)Se
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.
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 Cd0.2Zn0.8Se/ZnSe multiple quantum well active regions which have exhibited electroluminescence peak linewidths around 9.9 nm at room temperature.
Green light emitting diodes, ZnSe homoepitaxy
REGULAR ISSUE PAPERS
In Situ P-Doped Si and Si1-xGex Epitaxial Films Grown by Remote Plasma Enhanced Chemical Vapor Deposition
S. THOMAS, J. FRETWELL, D. KINOSKY, R. QIAN, A. MAHAJAN, P. MUNGUÍA, S. BANERJEE, A. TASCH, and C. MAGEE*
Microelectronics Research Center, University of Texas, Austin 78712. *Evans East Inc., Plainsboro, NJ 08536.
Remote plasma-enhanced chemical vapor deposition has been applied to grow in-situ doped e-type epitaxial Si and Si1-xGex 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.
In-situ phosphorus doping, plasma-enhanced deposition, Si and Si1-xGex epitaxy
Effect of Substrate Smoothness on the Microstructure of
YBa2Cu3O7-x/Y2O3/YBa2Cu 3O7-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.
Transmission electron microscopy was used to analyze the microstructure of YBa2Cu3O7-x/Y2O3/YBa2Cu 3O7-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.
Josephson junctions, strain, substrate smoothness, superconductors, thin films, yttria particles
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.
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 (VGa) are produced during IFVD (by measuring the intensity of the low temperature 1.2 eV signal from Si-VGa 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.
Photoluminescence, quantum wells, Si-doped GaAs
Internal Strain and Dislocations in InxGa1-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.
Strain relaxed, low dislocation density InxGa1-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 InxGa1-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.
Dislocations, internal strain, liquid phase electroepitaxy, selective lateral overgrowth, ternary crystals
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