Session Chairman: Professor Masakazu Kobayashi, Chiba University, Department of Electrical Engineering, 1-33 Yayoi-cho, Inage-ku, Chiba-shi 236, Japan. Co-Chairman: Professor Maria Tamargo, Department of Chemistry, City College of New York, Convent Ave. and 138St., New York, NY 10031
8:20AM, M1
"Lasing in ZnCdMgSe Structures Lattice-Matched to InP Substrates:" M. TAMARGO, Y. Guo, L. Zeng, A.Cavus, B. Yang, G. Alyzin, Y.-C. Chen, N. Dai, New York State Center for Advanced Technology (CAT) on Ultrafast Photonic Materials and Applications of CUNY, New York, NY 10031, Department of Chemistry and Center for Analysis of Structures and Interfaces (CASI), City College of CUNY, New York, NY 10031; Department of Physics and Astronomy, Hunter College of CUNY, New York, NY 10021
ZnCdMgSe is a material that has potential applications for the fabrication of visible emitters. Entirely lattice-matched structures of these new materials, grown on InP substrates, may offer distinct advantages to the ZnSe-based II-VI structures currently used for fabricating cw blue-green laser diodes. Among these potential advantages are improved reliability due to the complete absence of strain, improved electrical contact formation based on lattice-matched Te-based alloys and desirable band alignments for bandstructure engineering.
We have recently reported the growth of a family of these quaternary materials spanning in bandgaps from 3.2 eV to the 2.3 eV bandgap of the ZnCdSe ternary end point, all lattice-matched to the InP substrate. We have also reported the growth and properties of quantum well structures that can be tuned to emit throughout most of the visible range by changing only the QW layer thickness. Finally, the use of III-V buffer layers (InGaAs or InP) and attention to the initial nucleation of the II-VI layer on the III-V substrate have reduced our X-ray widths and defect densities to levels approaching those of ZnSSe on GaAs.
We have combined these materials advances to design and fabricate II-VI separate confinement QW laser structures entirely lattice-matched (including the QW active layer) to InP substrates. Optical pumping of these structures exhibit gain behavior with distinct threshold and spectral narrowing features typical of lasing. We will discuss the key materials issues that have led to the demonstration of lasing of our structures and their lasing characteristics. We will also discuss our progress with the remaining issues, mainly doping, before injection lasing can be performed. Our initial results suggest that these new materials expand the range of materials available for the design of the optimum laser structure and that they are alternative candidates for the fabrication of blue and visible semiconductor lasers.
8:40AM, M2
"Gain and Carrier Dynamics in ZnCdSe Quantum Wells:" J.HEGARTY, P.Rees, J. Heffernan, J. Donegan, F.P. Logue, C. Jordan, D. Fewer, S. Hewlett, E. McCabe, Physics Department, Trinity College, Dublin 1, Ireland; S. Taniguchi, T. Hino, F. Hiei, H. Yoshida, K. Nakano, A. Ishibashi, Sony Corporation Research Centre, 174, Fujitsuka-cho, Hodogaya-Ku, Yokohama-shi, 240 Japan
II-VI materials have now improved to the point that they have a good chance of being used in applications and to the point where the intrinsic physics of materials and devices can be studied at room temperature. Laser lifetimes of 101 hours have been recently announced by Sony.(1). In this paper we discuss our recent results on the mechanism of lasing in ZnCdSe/ZnSe quantum well structures, and the dynamics of carrier lifetime and diffusion. There has been some controversy regarding the mechanism of gain and lasing in II-VI structures because of the stronger electron-hole Coulomb interaction than in III-V's. The Coulomb interaction results in an excitonic part(spectrally sharp) and the so-called Coulomb enhancement of the free carrier transitions. We show by a combination of gain measurement under optical pumping and many-body calculations that the mechanism for gain from 100K upwards is electron-hole plasma but with a strong Coulomb enhancement. There is no evidence of exciton contribution to gain in this temperature region. At temperatures below 100K measurement and theory indicate the onset of a different mechanism, possibly excitonic.
The strong enhancement of the gain is also accompanied by an enhancement of the spontaneous emission rate. We have observed very long carrier lifetimes, up to 3 nsec, for the first time in these materials. The long lifetimes allow us to quantify the effect of the Coulomb enhancement on the spontaneous emission rate and to quantify carrier diffusion in the well. For the former we measure the lifetime as a function of carrier density. By comparison to our many-body calculations, we see that the spontaneous rate at low density is enhanced by up to 40% while at high density (1019 cm-3) the lifetime approaches 1 nsec with little enhancement. Knowledge of this enhancement is vital for the understanding of LED and laser operation.
Knowing the carrier lifetimes we can now measure carrier diffusion using spatially resolved luminescence imaging. The carrier diffusion in the well is an important parameter for small devices and for carrier migration to defect traps. We have used a confocal microscope in which an exciting laser is focused to 600 nm and the photoluminescence is imaged onto to a high resolution CCD camera. Luminescence spotsizes 3-4 larger than the laser size are measured. By combining this with the measured carrier lifetime at the densities used, a first estimate of the diffusion constant is obtained to be 10 cm2s-1. These results are important for design of materials and device structures.
This work in part is supported by Sony Sabbatical Chair Program and by Sony-Trinity College II-VI Project .
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[1] S. Taniguchi et al., submitted to Electronics Letters.
9:00AM, M3
"Spectral Ellipsometry Study of Zn0.44Cd0.56Se Lattice Matched to InP:" FRED H. POLLAK, Prakhya Ram, Todd Holden, J.L. Freeouf, B. Yang, M.C. Tamargo, Physics Department, Brooklyn College of CUNY, Brooklyn, NY 11210; Interface Studies, Inc., 27 E. Mountain, Katonah, NY 10536-0982; Chemistry Department, City College of CUNY, New York, NY 10031
ZnSe-based semiconductor alloys are important materials from both fundamental and applied perspectives, due to their potential in the fabrication of blue-green laser devices. Entirely lattice-matched structures can be grown on InP substrates, thus possibly improving the reliability of the device. However, very little work has been done on the optical properties of Zn0.53Cd0.47Se lattice-matched to InP.
Spectral ellipsometry, in the range 1.5-5.5 eV, has been used to study the bulk and surface/interface properties of a series of MBE-grown Zn0.53Cd0.47Se/InP films (~ 1 um thick). We have investigated films both as received and after a HF etch to remove the oxide including material grown (a) directly on an InP (001) substrate with no buffer layer, (b) with an InP buffer layer and (c) with an In0.53Ga0.47As buffer layer lattice matched to the InP substrate.
The direct gap, E0 (2.07 eV) exhibits a well-defined excitonic structure. The spin-orbit split component E0+[[Delta]]0 (2.55 eV) also is observed. A well resolved E1,E1+[[Delta]]1 doublet occurs in the region around 4.5 eV. The lineshape for [[epsilon]]2 in the vicinity of E0 has been fit by an excitonic (Lorentzian) and band-to band Coulomb enhanced profile. The spectral dependence of [[epsilon]]1 below E0 has been fit to the Sellmeir equation to obtain the dispersion in the index of refraction. The data in the region of E1,E1+[[Delta]]1 has been fit with a Wannier-type 2D excitonic function. The effect of oxide formation on the optical constants also has been observed. Information about the ZnCdSe/buffer (or substrate) interface has been obtained from the interference fringes observed below E0. The shape of the minima is distorted by the significant departures from the standard assumptions of flat, abrupt interfaces between layers. Specific features illustrating this effect will be presented.
9:20AM, M4+
"Refractive Index Measurement of MgZnCdSe Grown on InP Substrates:" TOSHIHIRO MORITA, Hiroyuki Shinbo, Ichirou Nomura, Akihiko Kikuchi, Katsumi Kishino, Department of Electrical & Electronics Engineering, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo 102, Japan
Refractive indices of MgZnCdSe lattice-matched to InP substrates were systematically evaluated, for various Mg compositions, for the first time. The refractive index values were fitted by the modified single effective oscillator (MSEO) method to get the fitting parameters.
Noble MgZnCdSe lattice-matching compounds on InP, having bandgap energy from 2.1 to 3.6 eV are very attractive for realizing the high-performance, yellow to green range light emitters. Recently, several groups succeeded in growing high optical quality MgZnCdSe compounds on InP substrates by MBE [1~3]. However, the material parameters of MgZnCdSe were scarcely clarified yet. Especially, refractive indices are one of important parameters for optical devices design.
In this study, lattice-matched Mgx(ZnyCd1-y)1-xSe (y~0.48) compounds with various Mg composition (x=0-0.63) were grown on (100) InP substrates by MBE. Here, the layer thicknesses were from 0.9 to 1.5 um. The bandgap energy (Eg) values were estimated from the photoluminescence measurements at 15K (getting Eg=2.19+1.44x). Absolute power reflectances from MgZnCdSe surfaces were measured at room temperature by Hitachi U4000. Oscillations in the reflectance spectra of MgZnCdSe layers caused by cavity effects were clearly observed. Because the crest point values of reflectance spectra agreed with the reflectance values evaluated from the well-established wavelength dispersion of InP refractive index, it was considered that this measurement were carried out precisely. Refractive indices of MgZnCdSe were evaluated from the trough point values of reflectance spectra. The refractive indices decreased with increasing Mg composition. The experimental refractive index curves were fitted by MSEO method. From these result, refractive indices of Mgx(ZnyCd1-y)1-xSe were given as
n2 - 1 = + + ln
where the oscillator energy E0=5.13-1.03x, the dispersion energy Ed=24.5-15.2x and the direct bandgap energy EG=2.03+1.45x, respectively. The waveguide design of MgZnCdSe laser diodes structure using experimentally obtained refractive indices will be discussed.
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Reference: [1] N. Dai et al., Appl. Phys. Lett., 66, 2742 (1995). [2] T. Morita et al., J. Electron Mater., to be published.
9:40AM, M5 LATE NEWS 10:20AM, M6+
"Defect Identification of Etch Pits in ZnSe Based Layers:" G.D. U'REN, M.S. Goorsky, G. Meis-Haugen, K.K. Law, T.J. Miller, B. Jenichen, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, CA 90095-1595; Corporate Research Laboratories, 3M Center, St. Paul, MN 55144; Paul Drude Institute für Festkorperelektronik, Hausvogteiplatz 5-7, Berlin, Germany D 10117
The driving force in II-VI diode laser research is the reduction of as-grown defects. Refined crystal growth techniques have reduced defect densities to well below 105 cm-2 making transmission electron microscopy a less effective characterization technique for studying these defects. Etch pit and x-ray topography measurements are better suited for characterization of low defect levels. To date, etch pit techniques have not been able to distinguish among specific crystallographic defects. In this work, we have identified three distinct etch pit features in ZnSe based epitaxial layers grown by molecular beam epitaxy on GaAs (001) substrates. These features were observed with dark field optical microscopy and investigated through the use of scanning electron microscopy. Using transmission electron microscopy, we associated distinct etch pits with characteristic defects originating at or near the ZnSe/GaAs interface: i) paired, joined at the origin, extrinsic pyramidal stacking faults defined by Shockley type partial dislocations; ii) paired narrowly bound stacking faults also defined by Shockley type dislocations; and iii) individual, rather than paired, type ii stacking faults. High resolution x-ray topography shows these defects align along a <110> direction over a length of at least 2 mm. From this study, we determined that the etch pit technique is able to independently provide an accurate overall density as well as identify specific defects.
10:40AM, M7
"Surface Preparation of ZnSe Substrates for MBE Growth of II-VI Light Emitters:" W.C. HUGHES, C. Boney, M.A.L. Johnson, J.W. Cook, Jr., J.F. Schetzina, Department of Physics, North Carolina State University, Raleigh, NC 27695-8202
The quality of ZnSe-based heterostructures grown on GaAs substrates is presently limited by the density of defects, particularly twins, that originate at the substrate-epilayer heterointerface. To avoid these unwanted defects, we have focused on developing II-VI light emitting diode and laser diode structures using high-quality, bulk ZnSe substrates supplied by Eagle-Picher Industries. While the use of ZnSe substrates eliminates many of the problems associated with lattice mismatch, defects still form during nucleation of an epitaxial layer because of substrate surface roughness, surface contamination, and surface defects. At NCSU, we have employed a variety of wet chemical etches, vacuum anneals, plasma treatments, and other procedures in an attempt to improve the ZnSe substrate surface prior to MBE film growth. Processed ZnSe surfaces were evaluated by RHEED, Auger electron spectroscopy, SEM/TEM studies, and etch-pit-density counts. These studies revealed that nearly all of the wet etches employed left large amounts of carbon, and in some cases oxygen and chlorine, on the ZnSe surface. In addition, some of the etchants such as bromine/methanol left the ZnSe surface roughened and unusable, even when hydroplane-polishing techniques developed originally for preparing very high quality CdTe and CdZnTe substrate surfaces, were employed. Vacuum annealing of ZnSe produced a noticeable improvement in the RHEED patterns observed, but films grown on substrates treated only with this anneal showed evidence of poor film nucleation and large densities of defects, principally twins, in the epilayer. We have also systematically subjected the surface of selected ZnSe substrates to a hydrogen plasma, which has been shown to be an effective process for removing oxygen and carbon from the surfaces of Si and GaAs wafers. Auger spectra of plasma-processed ZnSe show that carbon and chlorine contamination can be eliminated and oxygen minimized by proper use of this method. In addition, RHEED patterns from these wafers are streaky, implying a 2-dimensional surface. However, ZnSe films and devices grown on hydrogen-cleaned ZnSe substrates exhibited larger twin densities and poorer device performance than those grown on ZnSe substrates which were only subjected to thermal annealing. We speculate that a hydrogen-terminated ZnSe surface is very reactive to a flux of Se, Te, or S such that hydrogen compounds may be formed. Issues related to MBE growth of quaternary II-VI materials will also be discussed, and alternative surface preparation techniques for ZnSe substrates will be proposed.
Work supported by grants from ARO, ONR, ARPA, and by Eagle-Picher Industries.
11:00AM, M8
"Surface Treatment of ZnSe Substrates and Homoepitaxy of ZnSe:" T. YAO, M.W. Cho, K.W. Koh, K. Arai, H.D. Jung, K. Morikawa, Z. Zhu, Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980, Japan; Y. Okada, Electrotechnical Laboratory, Umezono, Tsukuba 305, Japan
The unstability of the hetero-interface between ZnSe-based alloys and GaAs(100) substrates is crucial to effect a cw lifetime of ZnSe-based blue-green diode lasers. For an instance, the dislocation growth during the lasing performance originated from the hetero-interface. Thermal stress in the epilayers grown on the GaAs is larger than 108disl./cm2 which is responsible for the dislocation growth. An approach for the improvement of the device performance would be the use of a ZnSe substrate instead of the GaAs, which is free of the thermal stress and dislocation due to lattice mismatch. A key technique for the growth of high crystal quality ZnSe epilayers is the surface treatment of the ZnSe substrate which consists of the chemical etching prior to loading into a growth chamber and the surface deoxygenation process prior to growth. In the present work, we have investigated the effects of versatile chemical treatments and atomic hydrogen beam cleaning on the surfaces of ZnSe(100) substrates by means of RHEED and X-ray photoemission spectroscopy (XPS). The crystallinity of ZnSe epilayers on the treated surface has been characterized by X-ray diffraction (XRD) and photoluminescence spectroscopy (PL).
For the chemical pre-treatment, etchants, of HF, KMnO4, Br and their combination were used to remove contaminated surface layers. XPS signals related to ZnOx and Zn were detected and the intensity ratio of ZnOx and Zn signals were used to evaluate surface oxidation degree by the etchants. A surface chemically etched by the KMnO4 solution followed by HF gives a smallest ZnOx/Zn ratio of 0.15 and a ZnSe epilayer grown on the surface showed a smallest FWHM value of XRD (311) rocking curve among samples investigated.
The surface deoxygenation process prior to growth strongly influences the quality of ZnSe epilayers. A radical atomic H-beam was used to remove oxygen from the surface. When the substrate was impinged simultaneously by a radical atomic H-beam and a Zn flux at a temperature of 500deg.C for 30 minutes, the RHEED showed a mixture streaky reconstruction pattern of (2x1) and c(2x2), indicating the formation of a clean surface. FWHM of (400) and (311) rocking curves obtained from a 1.5um-thick ZnSe epilayer grown on the surface was 74 and 72 arcsec, where the FWHM of the substrate was 20 and 40 arcsec, respectively.
11:20AM, M9
"Anisotropic Behavior and Equivalent Rates of ZnSe and ZnMgSSe in Electron Cyclotron Resonance Plasma Etching Using a Cl2/H2 Mixture:" T. YOKOGAWA, S. Kamiyama, S. Yoshii, K. Ohkawa, A. Tsujimura, Y. Sasai, Matsushita Electric Ind. Co., Ltd., Semiconductor Research Center, 3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan
Currently, there is much interest in utilizing electron cyclotron resonance (ECR) plasma etching for III-V devices. However, there are only few reports on dry etching of wide bandgap II-VIs. In this paper, we report on anisotropic behavior and equivalent rates of ZnSe and ZnMgSSe in ECR plasma etching with C12/H2 mixture and discuss the reaction mechanism. The anisotropic and low damage etching is also demonstrated by realizing the ridge waveguide II-VI laser with a stable fundamental transverse mode.
ECR plasma etching was carried out with C12 and H2 gas mixture at room temperature. Pure C12 gas or C12/He mixture were also studied for the reaction mechanism. The etching characteristics in ZnSe and ZnMgSSe were investigated. In case of only using C12 gas, ZnSe etched 2 times faster than ZnMgSSe. This is due to the fact that ZnC12 (Melting Point: 290 deg.C) is much more volatile than MgC12 (M.P.: 714deg.C). In the C12/H2 mixture, however, the etching rate for ZnMgSSe was as high as that for ZnSe, which is useful for etching ZnMgSSe based heterostructures. The anisotropic behavior in the C12/H2 ECR etching enabled us to achieve a nearly vertical mesa side wall (20deg. incline from the surface normal) and a smooth surface. As for the reaction mechanism, equivalent rate, smooth surface and anisotropic profile are attributed to the reduction of C1 radical density by diluting C12 with H2. This reduction suppresses the desorption rate of ZnC12 product so as to be comparable with that of MgC12. Similar etching characteristics were also obtained by diluting C12 with an inert gas of He.
The crystalline quality of etched p-type ZnSe was investigated by TEM and photoluminescence (PL) at 12 K. The depth of damage region was estimated to be less than 30 nm. From PL results, there was only very slight degradation in the total PL intensity after etching, which implies this ECR etching causes few defects acting as nonradiative recombination centers.
Ridge waveguide II-VI lasers were fabricated by using this ECR plasma etching with a C12/H2 mixture. The lasers with a 5-um-wide ridge consist of the ZnCdSe/ZnSSe/ZnMgSSe SCH structure. The lasing was obtained at room temperature. In lateral far-field characteristics, the output beam had constant full-angle at half-maximum for a wide range of injected currents. The single lateral mode operation was obtained at up to 18 mW. These characteristics are clear evidences of the anisotropic behavior and low damage in this ECR etching with C12/H2 mixture.
In conclusion, we have demonstrated ECR plasma etching of II-VIs using a C12/H2 gas mixture. Equivalent etching rates of ZnSe and ZnMgSSe, vertical side wall and smooth surface with low damage were obtained by this ECR etching technique, which is very useful for the fabrication of II-VI heterostructure lasers.
11:40AM, M10
"Novel Wet Chemical Etch For Nanofabrication of II-VI Materials:" A. OSINSKY, Y. Qiu, J. Mahan, H. Temkin, Electrical Engineering Department, Colorado State University, Ft. Collins, CO 80523; S.A. Gurevich, S.I. Nesterov, E.M. Tanklevskaia, V. Tretyakov, O.A. Lavrova, V.I. Skopina, A.F. Ioffe Physical-Technical Institute, 26 Politekhnicheskaia, 194021 St. Petersburg, Russia
Fabrication of nanostructures based on large bandgap II-VI compounds is difficult since the required etchants are not available. We describe here a simple and well behaved wet chemical etch which produces stable surfaces on II-VI materials. This etch is applied, in conjunction with reactive ion etching, to form high quality CdZnSe/ZnSSe quantum wires as narrow as 10 nm. The etch chemistry, etch rates, and the quality of the resulting surface are investigated.
We find that solutions of H2SO4:H2O2:H2O mixed in a 1:X:1 ratio, where X>20, reacts with the surface of ZnSe. The etchant produces an amorphous layer which grows into the semiconductor. The etch rate can be controlled by varying the H2O2 fraction from x=20 to x=200. This changes the growth rate from 150 nm/min down to 5 nm/min. The rate of growth is found to be diffusion-limited. X-ray microprobe analysis shows that the amorphous layer contains only Se, no traces of Zn or O could be detected. This layer can be subsequently removed with a solution of (NH4)2S. This produces a substitutional reaction which results in an S-passivated surface of ZnSe. The S-passivated layer is volatile and can be removed by simple heat treatment.
The etch was used to remove sidewall damage on 30-50 nm mesas formed on CdZnSe/ZnSSe structures by reactive ion etching (RIE). The RIE was done through a holographically defined mask with a 0.23 um period. The mesas were formed by low energy plasmas of BC13:Ar and CH4:H2. A follow up etch in a 1:20:1 solution resulted in controllable narrowing of the CdZnSe/ZnSSe stripes to 10-20 nm.
We also show that etched surfaces of ZnSe are stable enough to allow high
quality homoepitaxy and overgrowth by MOMBE.
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