Session Chairman: Ilesanmi Adesida, University of Illinois, Urbana-Champaign Micro Lab, 208 N. Wright St., Urbana, IL 61801 Co-Chairman: Russ Dupuis, University of Texas-Austin, Microelectronics Research Center, MER 1.606D/R9900, Austin, TX 78712-1100
1:30PM, F1+
"Improvement of Luminescence Properties of InGaP/InAlP Heterostructures with Oxidized InAIP Cladding Layers Grown by MOCVD:" M.R. ISLAM, R.D. Dupuis, A.P. Curtis, N.F. Gardner, G.E. Stillman, Microelectronics Research Center, The University of Texas at Austin, MER 1.606D/R9900, Austin, TX 78712-1100; Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 208 North Wright St., Urbana, IL 61801-2991
Data are presented on the luminescence performance of InGaP/InAlP heterostructures with oxidized InAlP cladding layers grown by MOCVD. The epitaxial layers are grown in a modified Emcore Model GS3200 UTM reactor at a pressure of 60 Torr using adduct-purified trimethylindium (TMIn), triethylgallium (TEGa), trimethylaluminum (TMAl), and arsine (AsH3) as sources and H2 as carrier gas. The QW test structures consist of 200 Å In0.5Ga0.5P QW or an In0.5Ga0.5P "bulk" layer sandwiched between two In0.5Al0.5P bulk barriers or between two 10-period In0.5Ga0.5P/InxGa1-xP strain-modulated aperiodic superlattice heterobarriers (SMASHs); where x varies from 0.5 to 0.45 and the period of the superlattice is ~ 30 Å. The structure has an InAlP top (0.25-0.43 um) and bottom (0.25-0.73 um) cladding layer and a 450 Å GaAs capping layer. For oxidation of the top InAlP cladding layer of the InAlP/InGaP heterostructures, the GaAs cap is selectively removed, and the exposed InAlP epitaxial layer is oxidized for 2-5.5 hours at 500deg.C in an ambient of H2O vapor saturated in a N2 carrier gas (~1.0 slm).
Extensive photoluminescence (PL) and time-resolved photoluminescence (TRPL) studies at room temperature show that as a result of the oxidation of the top InAlP cladding, the emission intensity and luminescence lifetime from InGaP QWs increase significantly. The room-temperature PL peak intensity for an InGaP QW heterostructure sample with top InAlP cladding thickness of 0.25 um increases by a factor of ~ 1.5 when about 0.2 um of the InAlP is converted to oxide. For the sample with ~ .43 um top InAlP barrier, the 300 K PL peak intensities are about 1.7 and 3.25 times higher than that form the as-grown sample when ~ 0.2 um and ~ 0.35 um of the InAlP is oxidized, respectively. This improvement in photoluminescence intensity with oxidation of the top InAlP window is also observed from thick (0.75 um) InGaP layers having both bulk InAlP barriers and InAlP/InGaP SMASH barriers.
TRPL experiments at room temperature show that a QW sample with a 0.25 um-thick top InAlP barrier layer has a decay time constant of about 62 ns for all excitation powers when ~ 0.2 um of the InAlP cladding layer is oxidized. The as-grown piece of the same structure exhibits a decay time constant of about 53 ns for all the excitation intensities measured (ranging over an order of magnitude). The luminescence decay is non-exponential for all of the measurements, displaying characteristics both of high-excitation and Hall-Shockley-Read (HSR) recombination. We have also observed improvement in the luminescence decay times from thick InGaP layers having both InAlP bulk barriers and InAlP/InGaP SMASH barriers with top InAlP window layer partially converted to oxide. These thick InAlP/InGaP DH samples exhibit longer carrier lifetimes ~ 120-200 ns. More extensive oxidation, PL, and TRPL experiments involving the InAlP/InGaP QWH samples grown under a variety of conditions are presently underway. These results will be discussed in detail.
1:50PM, F2
"Thermal Oxidation of AlAs in Water Vapor:" M. OCHIAI, G.E. Guidice, H. Temkin, Electrical Engineering Department, Colorado State University, Ft. Collins, CO 80523; J.W. Scott, T.M. Cockerill, Vixel Corporation, 325 Interlocken Parkway, Broomfield, CO 80021
The thermal oxidation of AlxGa1-xAs layers with high Al content in water vapor forms a stable oxide suitable for current and optical confinement in lasers. Despite record device characteristics of vertical cavity surface emitting lasers with oxide defining apertures, little is understood about the growth parameters which determine the oxidation process. For example, there are discrepancies in the literature whether the oxidation follows parabolic or linear growth laws [1,2,3]. We have investigated the lateral thermal oxidation of AlAs layers in water vapor in vertical cavity surface emitting laser structures as a function of time, temperature, and layer thickness.
At low temperatures and short oxidation times oxidation was found to be reaction rate limited. Conversely, diffusion across the oxide was the rate controlling mechanism at higher temperatures and longer times. Lasers are typically processed at intermediate values of temperatures and time. The observed growth can be modeled by rate equations by which the two component growth mechanism can be separated. Activation energies of 1.6eV and 0.8eV were determined for the reaction rate and diffusion limited mechanisms, respectively. In general, thicker AlAs layers oxidize at higher rates, with a increasing thickness dependence observed for longer oxidation times, where the growth is the diffusive regime.
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[1] H. Nickel, J. Appl. Phys, 78, 5201 (1995).
[2] K. D. Choquette, K.L. Lear, R. P. Schneider, K.M. Geib, J.J. Figiel, R.
Hull, IEEE Photon. Technol. Lett., 7, 1237 (1995).
[3] F.A. Kish, S. J. Caracci, N. Holonyak, K. C. Hsied, J. E. Baker, S.A.
Maranowski, A. R. Sugg, J. M. Dallesasse, R. M. Fletcher, C. P. Kuo, T.D.
Osentowski, M. G. Crawford, J. Electron. Mat., 21, 1133
(1992).
2:10PM, F3
"Design of Graded AlxGa1-xAs Layers for Tapered Al-Oxide Apertures in Vertical Cavity Lasers:" R.L. NAONE, E.R. Hegblom, B.J. Thibeault, J.C. Ko, L.A. Coldren, Materials Department, Electrical and Computer Engineering Department, University of California at Santa Barbara, Santa Barbara, CA 93106
Improved vertical-cavity lasers (VCLs) have been constructed in several laboratories [1-4] by using current and optical mode confinement apertures formed by lateral oxidation of high-aluminum-content AlGaAs layers [5,6]. We have recently shown that it is desirable to use thin or tapered apertures to further improve the properties of such devices [3,7]. Such apertures better approximate ideal lenses, which in principle, should provide a stable lateral cavity mode without any optical scattering loss. All other prior work has used apertures with abrupt (or blunt) oxidation fronts that give unwanted optical scattering loss which can become large at small diameters. In this paper, we shall review the theoretical and experimental properties of the tapered-aperture VCLs as well as detail their design and construction.
To form the tapered apertures we have made use of the large change in oxidation rate that results from small changes in aluminum content [8]. Oxidation will proceed furthest where the aluminum content is the highest. By using a step-graded composition we can form a tapered oxidation front due to the combination of rapid lateral oxidation in the high Al content layer and slow transverse oxidation into the adjacent lower Al content layers. Structures with varying degrees of compositional grading have been grown by solid-source molecular beam epitaxy. These structures have been oxidized under various conditions and then characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microscopy shows the desired tapers as well as additional information about the quality of the materials involved. Two-beam TEM conditions which provide strain-contrast have revealed no lattice distortion in the layers adjacent to the oxidized material, indicating a fully relaxed structure. Microvoids may be responsible for the relaxation as well as the reaction rate limited oxidation by expediting the transport of reactants and products.
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[1] K. L. Lear et al., Electron. Lett. 31, 208 (1995)
[2] G. M. Yang et al. , IEEE Photon. Tech. Lett., 7, (11), 1228
(1995)
[3] B. J. Thibeault et al., IEEE Photon. Tech. Lett., 8, (5) (1996)
[4] D. L. Huffaker et al., Appl. Phys. Lett. 65 (1) 97 (1994)
[5] J. M. Dallesasse et al., Appl. Phys. Let. 56 (24) 2436
(1990)
[6] J. M. Dallesasse et al., J. Appl. Phys. 68 (5) 2235 (1990)
[7] E. R. Hegblom et al., to be published in Appl. Phys. Lett.
(1996)
[8] K. D. Choquette et al., Electronics Lett., 30 (24) 2043 (1994)
2:30PM, F4
"Lateral, Selective Steam Oxidation of AlAsSb Lattice Matched to InP:" O. BLUM, K. Geib, M.J. Hafich, J.F. Klem, K.L. Lear, C.I.H. Ashby, Sandia National Laboratories, MS 0603, PO Box 5800, Albuquerque, NM 87185
We demonstrate lateral, selective steam oxidation of an AlAsSb layer lattice matched to an InP. Upon oxidation, AlAsSb is converted into an aluminum oxide with an elemental antimony layer at the top oxide-InGaAs interface.
Recent advances in wet thermal oxidation of AlGaAs compounds have led to dramatic improvements in the performance of InGaAs/AlGaAs vertical cavity surface emitting lasers (VCSELs) in the [[lambda]]<1 um wavelength regime [1]. Structures grown on InP cannot benefit from this technology since AlGaAs is mismatched to InP. While InAlAs provides an Al-bearing material which can be lattice matched to InP, its low Al mole fraction results in very slow oxidation requiring high temperatures [2]. An alternative Al-bearing material which can be lattice matched to InP is AlAsSb, which has an Al mole fraction of 1.0. It is thus extremely reactive and, similarly to AlAs, oxidizes quire readily.
Several AlAsSb samples were grown by solid-source molecular beam epitaxy. Samples consisted of 1088 Å of In0.53Ga0.47As, followed by 2484 Å of AlAs0.56Sb0.44 capped by another 1088 Å layer of In0.53Ga0.47As, grown lattice matched to InP substrates heated to 520deg.C. Several mesa stripes of widths varying from 2 um to 200 um were lithographically defined and dry etched prior to the oxidation. Oxidation was accomplished in an open tube furnace held at 350deg.C for 1 hour or at 325deg.C for 15.5 hours by bubbling N2 through water heated to 85deg.C.
Cross-sectional scanning electron microscope images of an oxidized mesa indicate that upon oxidation an interfacial Sb layer emerges out of the AlAsSb layer leaving behind Al2O3. The oxidized regions of the stripe appear swollen compared to the unoxidized regions due to formation of the interfacial Sb layer. An Auger depth profile before oxidation indicates there is no O present and the Sb signal is observed concurrently with Al. After oxidation a Sb spike is visible between the top InGaAs cap and the oxidized AlAsSb layer, with no corresponding O spike indicating that Sb layer is not oxidixed. Quantitative analysis of the Auger data, calibrated on a known Al2O3 standard, indicates that the oxidized layer is Al2O3 with no Sb present. Raman spectroscopic measurements confirm the elemental nature of the Sb film which segregated from the oxidized areas. Following oxidation, the Raman peaks (LO = 373 cm-1) associated with the buried AlAsSb layer are completely attenuated by conversion to Al2O3 and/or absorption by interfacial Sb layer. This Sb layer is evidenced by the appearance of a strong peak at 155 cm-1, which correlates with A1g peak of crystalline Sb at 150 cm-1 [3].
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[1] K. L. Lear, K. D. Choquette, R. P. Schneider, Jr., S. P. Kilcoyne, K. M. Geib, Electron Lett., 31, 208, 1995
[2] S. J. Caracci, M. R. Krames, N. Holonyak, Jr., M. J. Ludowise and A. Fischer-Colbrie, J. Appl. Phys., 75, 2706, 1994
[3] J. B. Renucci, W. Richeter, M. Cardona, E. Schönherr, Phys. Stat. Sol. B, 60, 299, 1973
2:50PM, F5
"Photochemical Deposition of Multiple Dielectric Layers for Optical Interconnects on InP OEICs:" Y.J. NISSIM, A. Sayah, France Telecom/CNET/PAB Laboratoire de Bagneux, 196 Avenue H. Ravera, 92225 Bagneux Cedex, France
Multiple dielectric layers have been largely studied and developed for optical passive devices fabricated on Silicon. The aim of this research effort is to use silicon as a platform for hybrid optoelectronic integration. In the field of telecommunication, most of the optoelectronic devices are made on InP substrates. Monolithic integration requires the fabrication of the passive optical devices, such as waveguides, optical couplers, directly on InP. Most of the time compound semiconductors or polymers are utilized for this application. The use of dielectric materials is attractive since they offer a large range of refractive index and their technology and stability are fully compatible with InP. However no reported work has been made on this subject.
In this work, we have investigated the use of photo-CVD to deposit multi-Silicon based dielectric layers for guiding purpose at 1.5 um . Two types of lamps have been mounted in a CVD reactor. IR lamps ( halogen lamps) allow fast temperature cycling and thus allows high temperature deposition on materials as fragile as InP. This deposition techniques (RTCVD) makes high quality thin dielectric films such as SiO2, Si3N4 or SiOxNy with very low residuals of H and OH bonds. These layers will be used as guiding layers. UV lamps (low pressure mercury lamps) allows cold deposition. The resulting layers are still very stochiometric but less dense. They will serve as stress release in the multilayer structures.
A conventional structure made of InP/SiO2/SiOxNy/SiO2 layers has been deposited on InP. The tension developed on the structure due to the large thickness of the films (4 um, 1 um, 2 um), made the dielectric to peal off and slip lines to appear on the substrate. Stress measurements were carried out using the divergence of two parallel beams after ten meters. This very sensitive measurement technics allowed us to design a guiding structure that is stable on InP. It consists of a Si3N4 film for sticking and tension compensation followed by a RTCVD SiO2 layer, followed by a RTCVD SiOxNy guiding layer, finally covered by a UVCVD SiO2 layer. The measured attentuation of this guide is 10dB/cm which is high due to the residual stresses. Finally we have developed an anti-resonant structure composed of the following layers: InP/SiO2/Si/SiO2 (guiding layer)/air. The Si layer act as a complete stress release layer so that the structure is fully compatible with InP. All layers were deposited by RTCVD resulting in the best material quality. An atenuation of 1dB/cm was obtained on the as-deposited structure which is among the best as deposited result ever reported. Using this structure, directional coupler were fabricated and the result will be reported at the conference.
3:30PM, F6+
"DLTS Study of GaAs MOS Capacitors With Al2O3 As the Gate Insulator:" PRIMIT PARIKH, Sanjay Jain, Lee McArthy, Prashant Chavarkar, James Champlain, James Ibbetson, Song Stone Shi, Evelyn Hu, Umesh Mishra, University of California at Santa Barbara, ECE Department, Santa Barbara, CA 93106
With the proliferation of wireless communications, there is an increasing need for low power dissipation, single power supply devices. Enhancement mode FETs, are a natural choice for such a technology. To this end we are developing an enhancement-mode MOSFET and CMOS technology in GaAs with Al2O3 formed by the steam oxidation of AlAs as the gate insulator. Our initial experiments have focused on the gate capacitor which is an ideal structure for studying the MOS system.
In this study, we report the first (to the best of our knowledge) DLTS measurements on a GaAs based MOS system with Al2O3 (formed by wet oxidation of AlAs) as the gate insulator. 500 Å AlAs was grown on p-GaAs doped a 3x1017 cm-3, on a p+ substrate and was capped with 200 Å of GaAs. The steam oxidation was done at 450deg.C with nitrogen bubbling through the water at 90deg.C. The oxide was subsequently subjected to hydrogenation treatment. Metallization for the bottom (Ti/Au) and the top (W) contacts completed the MOS capacitor.
The HF C-V measurements on the MOS capacitors indicate that it is possible to go from accumulation to inversion and vice-versa in this system. From the observed Cox the values of [[epsilon]] for the oxide is estimated to be around 5. The gate leakage was under 10 pA (600deg.um2 area) at 5 volts. The DLTS peak signal gives a dominant interface trap level at around 0.74 eV above the valence band, with a capture cross section of 2x10-14 cm2. DLTS measurements were also done at different quiescent and pulse bias voltage values to ensure that the signal is not due to bulk traps in the GaAs or due to inversion charge. From the magnitude of the DLTS signal, the density of interface states is calculated to be around 5.7x1011/cm-2.
These results indicate the Al2O3 is a possible gate insulator for GaAs based MOS technology. Future efforts will concentrate on reducing the interface state density in the Al2O3/GaAs system to form an enabling technology for a low gate leakage dielectric to be used for enhancement mode MOSFETs and GaAs CMOS technology.
3:50PM, F7
"Wet Oxidation of AlxGa1-xAs for MIS and Dielectric Applications: Sensitivity to Residual Arsenic Levels:" CAROL I.H. ASHBY, John P. Sullivan, Nancy A. Missert, Hong Hou, B.E. Hammons, Paula P. Newcomer, Sandia National Laboratories, Mail Stop 0603, Albuquerque, NM 87185-0603
The thermal oxidation of AlAs has shown promise for the formation of low-interface-state-density insulator/GaAs interfaces for potential applications in metal-insulator-semiconductor field effect transistors (MISFETs). Time- and temperature-dependent studies of the wet thermal oxidation of AlxGa1-xAs (1>=x>=0.90) on GaAs have been performed to elucidate the oxidation kinetics and the microstructures and electrical properties of the resulting oxide layers. Two critical oxidation regimes have been identified. The first regime involves the oxidation of Al and Ga in the AlxGa1-xAs alloy to form an amorphous oxide layer, as determined by thin-film X-ray and electron diffraction and by Raman spectroscopy. The second regime involves the oxidation and elimination of residual As, which has been observed to remain in the oxide layer after the formation of the amorphous oxide appears complete.
DC transport and capacitance measurements performed on MIS diode structures have been found to be critically sensitive to the amount of residual As present in the amorphous oxide layer. After oxidation at 450deg.C until no residual As was observed by Raman spectroscopy, the oxide layers were highly insulating with bulk resistivities approaching 4 x 1013 [[Omega]]-cm and electrical permittivities of 5.1[[epsilon]]0, indicative of a high quality dielectric. Raman spectroscopy on samples processed at lower temperatures or for shorter times revealed the presence of unreacted amorphous and/or crystalline As; the relative amounts of amorphous and crystalline material depended on the starting composition and oxidation conditions. Residual As can result in up to a two order of magnitude increase in leakage current and up to a 30% increase in the dielectric constant. Residual As also strongly affected the interface-state density of the oxidized AlxGa1-xAs/GaAs interface, as manifested by the onset of strong Fermi-level pinning and high leakage currents. The dependence of the interface-state density on residual As in the oxidized layers and on the starting composition of the AlxGa1-xAs alloy will be discussed.
_____________________________________
This work was performed at Sandia National Laboratories and supported by the U.S. Department of Energy under Contract No. DE-AC04-94AL85000.
4:10PM, F8
"Photoluminescence Study of Hydrogenated Aluminum Oxide-Semiconductor Interfaces:" SONG STONE SHI, Evelyn L. Hu, Center for Quantized Electronic Structures (QUEST), University of California at Santa Barbara, Santa Barbara, CA 93106
Oxidation of AlAs layers to form insulating aluminum oxide has recently found applicability to electrical isolation in semiconductor lasers, and to gate oxides in GaAs-based metal-oxide semiconductor filed effect transistor. The quality of the oxide and oxide-semiconductor interface is naturally a critical determinant of the device performance. This study uses a quantum well (QW) situated in close proximity to the semiconductor-oxide interface; the photoluminescence of that QW is used to assess the quality of the interface. We further report on improvements in the oxide quality brought about through hydrogen ion irradiation of the oxide.
The material structure studied consists of a 70 Å GaAs quantum well, 80 Å AlGaAs tunneling barrier, and a 500 Å AlAs layer, capped by a 500 Å GaAs. The AlAs was oxidized in two ways: a) the 500 Å GaAs cap was selectively removed from the material, which was exposed to air, and (b) the AlAs was laterally oxidized at 425deg.C under N2 gas bubbled through water at 95deg.C for different time durations. The oxidation proceeded laterally from the edges of lithographically defined stripes (100 um in width on 150 um center-to-center spacing). Photoluminescence measurements were made of the GaAs QW, in close proximity to the semiconductor-oxide interface. For the air-exposed structure, the QW luminescence disappeared. However, a partial recovery of the luminescence was achieved through hydrogen ion irradiation at 80eV energy and 5x1016 cm-2 dose. For the wet-oxidized samples, the normalized PL intensity was found to decrease roughly in proportion to the ratio of oxidized/non-oxided interface, with full disappearance of the QW peak for the fully oxidized AlAs layer. Previous studies on oxides of both GaAs and AlGaAs have suggested the presence of elemental arsenic at the semiconductor-oxide interface. Such excess arsenic may promote the formation of AsGa antisite defects, with mid band-gap energy. These form non-radiative recombination sites that reduce the luminescence of the nearby QW. Our previous studies have shown that hydrogen ion irradiation can react with the near-surface As, forming arsine which will desorb from the substrate. The improvement in the luminescence of the hydrogenated air-oxidized samples is consistent with this. Optimal hydrogenation parameters for the wet thermally oxidized sample have not yet been found and, but are being actively pursued. The difference of hydrogenation results in these two cases may be related to the difference in the oxide structures.
4:30PM, F9+
"Characterization of Transparent Electrodes on InAlAs and AlGaAs for Metal Semiconductor-Metal Photodetector Applications:" W.A. WOHLMUTH, C. Caneau*, I. Adesida, Center for Compound Semiconductor Microelectronics and Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, IL 61801; *Bell Communications Research Laboratory, Red Bank, NJ 07701
Heterojunction III-V metal-semiconductor-metal photodetectors (MSMPDs) are becoming popular elements in optical communication systems [1]. Low resistivity, transparent electrodes are required to obtain good signal-to-noise ratio while maintaining high frequency performance in MSMPDs. The transparent metals, indium-tin-oxide (ITO) and cadmium-tin-oxide (CTO), possess low resistivity and are highly transparent to optical radiation in the 0.85 to 1.55 um wavelength region [2]. Lattice-matched Schottky barrier enhancement layers such as InAlAs and AlGaAs are used in InGaAs- and GaAs-based MSMPDs to increase the Schottky barrier height and reduce interface states [1,3]. Thus far, reports of Schottky barrier studies of ITO and CTO on InAlAs have found that the Schottky barrier height is almost independent of the electrode material leading to the belief that the Fermi level at the semiconductor surface is pinned [4]. To the authors' knowledge there have been no reports of the electrical properties of ITO and CTO on AlGaAs.
In this paper we investigate the electrical characteristics of Schottky diodes on InAlAs and AlGaAs using ITO and CTO. The transparent conductors were deposited using RF magnetron sputtering at a substrate temperature of 300deg.C. An annealing treatment was performed on the Schottky diodes at a temperature of 400deg.C in a nitrogen ambient after the Schottky diodes were delineated. It was found that the as-deposited CTO/AlGaAs diodes displayed ohmic characteristics. The Schottky barrier heights and ideality were determined from I-V measurements and from Norde plots. The barrier heights of ITO and CTO Schottky diodes on InAlAs were found to be very dissimilar, indicating that the Fermi level is not pinned. The composition of ITO and CTO as determined using x-ray photoelectron spectroscopy and x-ray diffraction will also be presented.
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[1] H.T. Griem et al., Appl. Phys. Lett., 56 (11), p. 1067, (1990).
[2] R. Lewin et al., Vacuum, 36, p. 95, (1986).
[3] M. Klingenstein et al., Solid State Electron., 37 (2), p. 333, (1994).
[4] W. Gao et al., Appl. Phys. Lett., 66 (25), p. 3471, (1995).
4:50PM, F10
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