1:30PM, T1 *Invited
"Hydrogen In Semiconductors-Materials and Device Issues:" N.M. JOHNSON, Xerox Palo Alto Research Center, 3333 Coyote Hill Rd., Palo Alto, CA 94304
Hydrogen strongly affects the electronic properties of crystalline semiconductors. It can be present either as an uncontrolled or as a deliberately-incorporated impurity and can be introduced during either crystal growth or device processing. A variety of hydrogen-related phenomena has been identified in elemental and compound semiconductors. These include passivation of deep-level defects, neutralization of acceptor or donor dopants, minority-carrier enhanced dissociation of dopant-hydrogen complexes, charge state-dependent migration of hydrogen, formation of diatomic hydrogen complexes, and the generation of point and extended defects. Recent fundamental studies have yielded the diffusivities of charged hydrogen in silicon and gallium arsenide and the energy levels of isolated hydrogen in silicon.
The controlled alteration of electronic properties by hydrogenation suggests possible technological applications. Hydrogen passivation of defects at grain boundaries in polycrystalline silicon and of interface defects in MOS devices are well-known technological applications of hydrogenation, the adoption of which predates our understanding of the underlying microscopic processes. Examples in compound semiconductors include the use of hydrogen neutralization of dopant impurities in GaAs as a deliberate processing step in the fabrication of exploratory laser diodes and field-effect transistors.
In wide-band-gap III-V and II-VI semiconductors, which are under intense development for visible optoelectronic devices, a central electronic materials problem is the attainment of high p-type doping efficiencies. There is now evidence that this is due to inadvertent hydrogen neutralization of p-type dopants during epitaxial growth and that the empirically-established post-growth activation of dopants arises from thermal dissociation of such complexes.
In summary, in virtually all semiconductor materials that have been or are being developed for electronic and optoelectronic device applications, the presence of hydrogen either confronts our ability to control critical materials properties or vitally contributes to the realization of desired device properties.
2:10PM, T2+
"Dissociation Kinetics and Properties of Hydrogen-Extended Defect Complexes in Lattice-Mismatched InP Heteroepitaxial Layers:" B. CHATTERJEE, S.A. Ringel, Electronic Materials and Devices Laboratory, Department of Electrical Engineering, The Ohio State University, Columbus, OH 43210-1272
Hydrogen is well known to passivate dopants and deep level defects in many semiconductors. We have recently reported on the strong passivation effects of post-growth plasma hydrogenation treatments on electrically active dislocations and related defects in InP epitaxial layers grown on lattice-mismatched substrates such as GaAs and Ge, using a process being developed for solar cell applications. This was evidenced by orders of magnitude reduction in deep level concentration and reverse leakage currents, and a thermal stability of extended defect passivation far exceeding that of dopant passivation. Passivation was further found to dramatically alter the fundamental charge trapping behavior of these defects, revealing a more "point-defect-like" character after hydrogenation, and increasing the DLTS (Deep Level Transient Spectroscopy)-measured activation energy due to passivation effects that was explained on the basis of decreasing electronic defect-defect interactions.
This paper presents results on the dissociation kinetics and properties of H-extended defect complexes in InP (and to our knowledge for any III-V) for the first time. We analyze the evolution of a band of deep levels (hole traps) near Ev + 0.67 eV previously attributed to extended defects in heteroepitaxial p-InP using DLTS as a function of post-hydrogenation reverse bias annealing (RBA) conditions to account for hydrogen re-trapping effects. We show that a 2 hour hydrogen exposure reduces the trap concentration from 9x1014 cm-3 down to 8x1012 cm-3, with a corresponding increase in DLTS-determined activation energy to Ev + 0.78 eV. Subsequent RBA processing in excess of 5 volts and at temperatures exceeding 270oC for greater than 4 hours is found necessary to achieve measurable extended defect reactivation, as compared with Zn dopants which are substantially reactivated by a 100oC RBA treatment for 30 minutes. The initial stage (< 1 hour) of reactivation follows simple first order kinetics from which an H-extended defect dissociation energy of 1.71 eV was found. Longer RBA times (> 1 hr) result in a time-dependent, nonlinear increase in dissociation energy that eventually (after > 3 hours) causes the concentration of reactivated deep levels to saturate for a given temperature, unlike the case for point defects. The saturation value is found to increase toward the initial, unpassivated trap concentration as RBA temperatures are increased beyond 300oC. DLTS analysis shows the identical behavior for activation energy as a function of RBA conditions where the energy decreases from the fully passivated 0.78 eV value, toward the initial, unpassivated value of 0.67 eV as a function of RBA time and temperature, indicating that RBA causes the precise reverse process of what we have earlier reported to occur during the initial hydrogenation step. These results will be discussed as the basis of a model for hydrogen-extended defect complexes, which will include higher temperature RBA measurements. We will also present RBA results indicating that this strong stability of H-extended defect complexes actually enhances the dissociation of H from Zn dopants, resulting in a decrease in H-Zn dissociation energy for heteroepitaxial InP as compared with the values for homoepitaxial InP.
2:30PM, T3+
"Oxygen Related Defects in Low Phosphorus Content GaAs1-yPy Ternary Alloys Grown by MOVPE:" J.G. CEDERBERG, T.F. Kuech, Department of Chemical Engineering, University of Wisconsin-Madison, 1415 Engineering Dr., Madison, WI 53706
Oxygen is a persistent impurity found in both elemental and compound semiconductors. A great deal is known of the properties of oxygen in the binary semiconductors such as GaAs. Less is known about the evolution of these properties within alloy systems. Previous studies of GaAs1-y Py have focused on the P-rich compositions of this alloy due to the importance of this defect in the fabrication of LED structures. We have developed an oxygen doping technique which allows for the controlled introduction of oxygen into a wide variety of semiconductor host materials. This molecular doping precursor, diethyl aluminum ethoxide, incorporates oxygen into the growing crystal through the presence of a strong Al-O bond. We present results from the study of oxygen doped GaAs1-y Py for y<0.2. The deep structures through the alloy region was determined and compared to previous results on high P-content material as well as GaAs.
The metal organic vapor phase growth system used trimethylgallium(TMG), arsine (AsH3), and phosphine (PH3) as precursors. Diethyaluminum ethoxide (DEAlO) was used as the oxygen precursor. Disilane was used as the n-type dopant for the formation of electrical test structures. A low pressure (78 Torr) reactor was used and the alloy was grown at 600 oC. To reduce the density of misfit dislocations, the samples were graded using 0. 05 um layers and increasing the phosphorous composition by 2 percent each layer. Samples with compositions of y = 0, 0.10, and 0.17 were grown co-doped with oxygen and silicon, as well as non-oxygen doped control samples. SIMS and electrochemical capacitance voltage (ECV) were performed to determine the quantity of oxygen incorporated and the fraction which is electrically active. Deep Level Transient Spectroscopy (DLTS) was used to determine the effect of alloy composition on the oxygen level. DLTS spectra show three peaks; two shallower levels and a large DLTS peak corresponding to a deeper level. The two shallower levels have energies of 0.37 and 0.20 eV below the conduction band in y = 0. 17. The deeper level has an energy of 0.82 eV. The defect at 0.37 eV is due to misfit defects introduced during growth since it is present in both the oxygen-doped and control samples. The defects at 0.20 eV and 0.82 eV are shifted relative to the same oxygen levels in GaAs. This data now allows for a comprehensive picture of the oxygen level across this entire important alloy system.
2:50, T4
"Strain and Temperature Effects on Interdiffusion in InAsP/InP Heterostructures," D.J. TWEET, H. Matsuhata, P. Fons and H. Oyanagi, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, lbaraki 305 Japan; H. Kamei, Sumitomo Electric Industries, 1, Taya-cho, Sakae-ku, Yokohama 244, Japan
Previously, we have presented evidence for large, strain-dependent interdiffusion in InAsl-xPx layers (x = 0.0 and 0.4) grown at 620oC on InP(001) substrates by low-pressure organometallic vapor phase epitaxy (OMVPE). Here we show that the P-As intermixing is drastically reduced in layers grown with slightly less strain (x = 0.6) and also in x = 0.4 layers grown at a somewhat lower temperature (580oC).
The samples range in thickness from 40 to 1000 A with an intended value of x of 0.0, 0.4, or 0.6, corresponding to strains of 3.2%, 1.9%, and 1.3%, respectively. These were all grown at a substrate temperature of 620oC. In addition, another set of films with x = 0.4 was grown at 580oC. The structure of the layers was examined using X-ray diffraction, AFM, TEM, and energy dispersive X-ray analysis.
We find evidence for large, strain-dependent interdiffusion and for the existence of a "critical strain": if the strain is 1.9% or more a great deal of P-As mixing occurs, but for smaller strain the mixing is greatly decreased. Specifically, in the x = 0.0 and 0.4 sets, initially strong interdiffusion occurs, producing pseudomorphic islands of intermediate composition. As these grow and start to relax, the strain decreases. When it reaches 1.9% the intermixing suddenly stops and islands of the intended composition begin to appear. In support of this idea the x = 0.6 set, which has a strain of less than the critical value of 1.9%, shows virtually no intermixing, even in the thinnest films investigated. The interdiffusion is also highly sensitive to temperature. The x = 0.4 set grown at 580oC exhibits a factor of -5 decrease in P-As mixing compared to that grown at 620oC.
3:30PM, T5
"Low-Stacking Fault Density ZnSe Growth on GaAs by Molecular Beam Epitaxy:" T.J. MILLER, G.M. Haugen, M.A. Haase, K. K. Law, D.C. Grillo, P.F. Baude, Science Research Lab, 3M Company, 3M Center 201-1N-35, St. Paul, MN 55144
3:50PM, T6
"Roles of Interface Chemistry and Surface Stoichiometry on Defect Generation in ZnSe/GaAs:" L.H. KUO, K. Kimura, T. Yasuda, S. Miwa, C.G. Jin, K. Tanaka, T. Yao, Joint Research Center for Atom Technology (JRCAT) - Angstrom Technology Partnership, Higashi 1-1-4, Tsukuba, Ibaraki 305 Japan; JRCAT-National Institute for Advanced Interdisplinary Research (NAIR), Higashi 1-1-4, Tsukuba, Ibaraki 305, Japan; Tsukiba University, Tsukuba 305, Japan; Institute for Materials Research, Tohoku University, Sendai 980, Japan
We have investigated the quality of ZnSe/GaAs as a function of interfacial chemical preparation, and II/VI flux ratios during growth to realize the mechanism of defect generation. Two sets of samples were grown with different interface chemistry. One set had 1 min Zn exposure at 250oC of As-rich surface of GaAs epilayers. The other set had 1 min Se exposure at 250oC of (4x6) Ga-rich GaAs surfaces with and without 1-2 min of annealing at 520oC. A very low density of faulted defects of ~ 5x104/cm2 was obtained in samples with Zn treatment on a (2x4) As-rich GaAs surface. However, very high densities of As precipitates and extrinsic Shockley partials (~ 1x109 /cm2) were generated in samples with 1 min Zn treatment on a c(4x4) As-rich GaAs surface. Density of As precipitates increased as the increased c(4x4) feature on As-rich GaAs surface and associated with an increase on the density of extrinsic Shockley partials. On the other hand, densities of Frank and Shockley partials are of ~ 5x107/cm2 in samples with Se treatment on a (4x6) Ga-rich GaAs surface. Annealing on the Se-treated (4x6) Ga-rich GaAs surface generated a high density of vacancy loops (1x109/cm2) and an increase on the density of Shockley partials (5x108/cm2) after the growth of the films. Furthermore, we have studied the dependence of generation and structure of Shockley-type stacking faults on II/VI flux ratios in samples with Zn treatment on a (2x4) As-rich GaAs surface. The surface stoichiometry under varied flux ratios was characterized as evidence by the observation of (lxl), c(2x2), coexistence of c(2x2) and (2xl), and (2xl) reflection high energy electron diffraction patterns. The density and core structure of Shockley partials were changed in the ZnSe films under varied Se/Zn flux ratios. A diagram for the structure and density of Shockley partials versus the surface stoichiometry under a wide range of Se/Zn flux ratios will be discussed.
4:10PM, T7
"Failure Analysis of AlGaN/InGaN/GaN Blue Light Emitting Diodes Degraded During Life Testing:" MAREK OSINSKI*, Daniel L. Barton, Christopher J. Helms, Niel H. Berg, Piotr Perlin*, *Center for High Technology Materials, University of New Mexico, Albuquerque, NM 87131-6081; Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185-1081
Until very recently, efforts to develop short-wavelength visible light sources concentrated on either II-VI materials, or second harmonic frequency doubling of GaAs/AlGaAs lasers. The situation has changed dramatically following the commercial introduction by Nichia Chemical Industries of high-brightness blue LEDs based on gallium nitride and related compounds. The reliability of devices fabricated in GaN and related alloys, especially under high current densities as would be found in lasers. has yet to be fully characterized.
Our previous work investigated the degradation of GaN-based blue light emitting diodes (LEDs) under high pulsed current stress. This work indicated a possible correlation between the high crystal defect density and failures caused by metal migration along these defect tubes. To assess the impact of this data on devices under more normal conditions, several LEDs from both older and more recent production lots were placed in a controlled temperature and current environment for several thousand hours. The test started with a constant 20 mA current for the first 1000 hours and continued at a range of currents up to 70 mA for the next 1650 hours, all at a temperature of 23oC. One of the older generation LED's output degraded by more than 50% during the test. The I-V characteristics of this device indicated that there was an ohmic leakage path across the junction which was similar to, but of much higher resistance, than was observed on failures from the previous high current tests. The similarity indicated that the device may have failed at room temperature and at moderate currents in the same manner as was observed at high currents. Subsequent failure analysis proved that this was not the case, since a crack caused by latent mechanical damage was found on the degraded LED which isolated part of the active region from the p-contact. The tests were continued for 500 hours at 30oC and for another 500 hours at 35oC. During this last 500 hour test, one of the newer generation LEDs which had been in the group held at a constant 70 mA degraded to 57% of its original intensity. The failure analysis of this LED and any others which fail during subsequent testing at gradually increasing temperatures will be presented in detail at the conference.
4:30PM, T8
"Defect Structures Induced During Degradation of Proton-Implanted GaAs Quantum Well Vertical-Cavity Surface-Emitting Lasers:" Y. MICHAEL CHENG, Robert W. Herrick, Pierre M. Petroff, Department of Materials Engineering, University of California, Santa Barbara, CA 93106; M. Hibbs-Brenner, R.A. Morgan, Honeywell Technology Center, 12001 State Hwy. 55, Plymouth, MN 55441-4799
The degradation of gain-guided 0.85 um GaAs quantum well vertical-cavity surface emitting lasers (VCSELS) has been studied. The planar, top emitting devices are grown by MOCVD, and use proton-implantation to provide current confinement. These devices have record performances and excellent reliability of over 5x106 hours MTTF at 40oC with a 1 eV activation energy. The aging of devices has been performed under the extreme high currents and high temperatures, as well as at the normal operation current (10 mA) with an elevated temperature for over thousands of hours. Dark line defects (DLDs) oriented along <110> direction have been observed under the high stress aging conditions using electron-beam induced-current (EBIC). By using cross-sectional cathodoluminescence and spectrally-filtered electroluminescence, the DLDs and dark area defects (DADs) have been also observed in the p-DBR and the active regions for the devices which are both aged at high current and at normal operating current. The transmission electron microscopy (TEM) measurements have shown these DLDs and DADs are related to the small dislocation loops and precipitates at the interfaces between the AlAs and AlGaAs layers of p-DBR mirror stacks. Our results show that degradation of this type of VCSELs is due to DLDs and DADs which are formed in the p-DBR mirror. We will discuss the processes of possible recombination enhanced defect reactions that are involved in the p-DBR mirror regions of these devices.
_____________________________________
1) J.K. Gunter, et al., "Reliability of Proton Implanted Vertical
Cavity Surface Emitting Lasers", Proc. Of SPIE, Vol. 2683, paper 16 (1996)
2) Y.M. Cheng, et al., "Degradation Studies of Proton-Implanted
Vertical Cavityt Surface Emitting Lasers", Appl Phys. Lett. Vol. 67, 1648
(1995)
3) R.W. Herrick, et al., "Spectrally-Filtered Electroluminescence of
Vertical Cavity Surface Emitting Lasers", Photon. Technol. Lett., Vol. 7, 1107
(1995)
4:50PM, T9
"Dislocation Generation by Process-Induced Stress in GaAs Microwave Amplifier Circuits:" ALLAN WARD III, Robert W. Hendricks, Materials Science and Engineering Department, 213 Holden Hall, Blacksburg, VA 24061-0237
GaAs metal-semiconductor field effect transistors (MESFETS) configured as microwave power amplifiers have been observed to degrade under normal device operations at high gate-to-drain fields. The nature of this degradation is an increase in the gate current, with a subsequent decrease in the gain. We present evidence that crystallographic defects in the active region are responsible for this "power slump" and that these defects originate during device operation due to the high strain fields which exist as the result of passivation layer processing.
Strain data and X-ray topographic images support our assertion that
passivation layer processing induces high strain in and around the
gate-to-drain region of the device. The process-induced strain is a
superposition of film edge effects and macrostrain resulting in shear stresses
which are near the yield stress of the material. Topographic images show that
an increase in dislocation density occurs in the highly stressed regions after
power slump. By varying certain processing parameters, we can produce
passivation films which induce less stress in the active region, resulting in
less dislocation generation and a less severe power slump.
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