1:30PM, Q1 *Invited
"Fabrication and Properties of Group III Nitride-Based Inner Stripe Geometry Structure:" H. AMANO, S. Sota, M. Ohta, M. Kawaguchi, H. Sakai, T. Tanaka, I. Akasaki, Department of Electrical and Electronic Engineering, Meijo University, 1-501 Shiogamaguchi, Tempaku-ku, Nagoya 468 Japan
Formation of low resistive contact to p-type layer is one of the critical issues for realization of practical short wavelength laser diode based on group III nitride. However, since work function of group III is large, it is very difficult to form low-resistivity p-type contact. In this paper, we propose inner stripe geometry as one of the most practical structure for nitride based laser. By using a inner stripe structure in combination with wide p-contact electrode, both the reduction of p-type contact resistivity and the current confinement can be achieved.
The sample was grown by MOVPE. At first n-GaN:Si/p-Al0.1Ga0.9N:Mg/p-GaN:Mg/Ga0.85In0.15N/n-GaN:Si/n-Al0.1Ga0.9N:Si/n-GaNSi separate confinement heterostructure was grown on sapphire substrate using AlN buffer layer. Top n-GaN:Si layer was used as current blocking. Then, 18 micron wide stripe of top n-GaN layer was etched off. AFM observation showed that flatness of the etched surface is almost the same as that of as-grown surface. Finally, p-GaN:Mg was regrown on the patterned wafer. 400 micron wide Ni-electrode was deposited just on the 18mm wide stripe.
Strong violet emission by current injection originating from Ga0.85IN0.15N active layer was observed. Emission pattern was monitored from the back of the sapphire substrate. Emission was strong especially at the side edge of the 18mm stripe. At the present time, spatial fluctuation of the electric field might be responsible for such non-uniform emission. Another characteristics such as high current injection property will be discussed.
"Microstructural Characterization of III-Nitride Epitaxial Structures by Transmission Electron Microscopy (TEM) and Cathodoluminescence (CL):" S.J. ROSNER, E. Carr, S.D. Lester, M.J. Ludowise, Hewlett-Packard Laboratories, 3500 Deer Creek Rd., Palo Alto, CA 94303; C. Chen, K.M. Doverspike, H. Liu, Hewlett-Packard Optoelectronics Division, 350 West Trimble Rd., San Jose, CA 95131
Gallium nitride has attracted a great deal of attention over the past several years for use in optoelectronic devices such as light emitting diodes (LEDs)1 and solid-state lasers2. With the addition of indium as a constituent material, this semiconductor system enables light emission from the ultraviolet through the red portion of the visible spectrum We will report here on the microstructure of GaN and GaN/InGaN heterostructures grown by metal-organic chemical vapor deposition (MOCVD) on Al2O3 substrates and on the wavelength-resolved luminescence from these layers.
TEM plan view examination shows the dislocations in these films to be arranged both randomly and in linear arrays of dislocations with identical burger's vectors. A large fraction of the dislocations are edge dislocations. When the arrays are present, they are often associated with lowered mobility and small in-plane orientational rotations in the epitaxial film; the nature and origin of these will be discussed as it relates to substrate type and other factors.
The room temperature cathodoluminescence spectra from these films generally consists of a band-edge peak at ~365 nm and a broad yellow band (of greatly varying intensity) from <500 nm to >600 nm. All of the luminescence is highly inhomogeneous. Morphology defects are often associated with unique luminescence wavelength ranges. When indium is added, with the associated strain and additional longer-wavelength luminescence peak, morphology defects become more likely to form. The inhomogeneities of the InGaN luminescence observed are not always spatially correlated to the base GaN inhomogeneities.
1. S. Nakamura, M. Senoh, N. Iwasa, S. Hagahama, T. Yamaea, T. Mukai, Jap. J.
Appl. Phys. 34 (1995) L1332.
2. I. Akasaki, H. Amano, S. Sota, H. Sakai, T. Tanaka, M. Koike, Jap. J. Appl. Phys. 34 (1995) L1517.
"Growth of III-N Heteroepitaxial Films on Exact and Vicinal Basal-Plane Sapphire Substrates by MOCVD:" A.L. HOLMES, P.A. Grudowski, C.J. Eiting, F.A. Ponce*, R. D. Dupuis, Microelectronics Research Center, The University of Texas at Austin, MER 1.606D-R9900, Austin, TX 78712-1100;*Xerox PARC, 3333 Coyote Hill Rd., Palo Alto, CA 94304
We have studied the metalorganic chemical vapor deposition (MOCVD) growth of GaN heteroepitaxial films on exactly oriented (0001) sapphire substrates as well as on vicinal substrates oriented a few degrees off of (0001). The films are grown in an Emcore D125 reactor by low-pressure MOCVD using trimethylgallium (TMGa), trimethylgallium (TEGa), trimethylaluminum (TMAl), and ammonia (NH3) in a H2 ambient. The pressure typically employed is 75 Torr with a wafer rotation speed of 800 rpm and a total H2 carrier flow ~12 SLM. The heteroepitaxial GaN layers are grown using a low-temperature (520deg.C) GaN buffer layer ~25 nm thick and a high-temperature (1050deg.C)GaN layer ~0.3-4um thick.
X-ray diffraction studies have established that these films have excellent structural characteristics. Five-crystal (0002) X-ray diffraction rocking curves show FWHM values in the range 35-360 arc-s. Triple-axis X-ray mapping of reciprocal space for the ~35 arc-s films show the presence of Pendellosung fringes in the [[omega]]-2q scan and a FWHM of ~11 arc-s in the [[omega]] scan. Samples grown on vicinal (0001) GaN substrates exhibit broader double-crystal X-ray rocking curves with FWHM values ~12.8 arc-min. and correspondingly elongated [[omega]] and [[omega]]-2[[theta]] triple-axis X-ray peaks having FWHM values ~12.6 arc-min and ~49 arc-s, respectively.
However, both GaN films grown on (0001) and on vicinal sapphire substrates exhibit excellent 300K photoluminescence (PL) and cathodoluminescence (CL) properties. Unintentionally doped GaN films grown on both (0001) and (0001) vicinal substrates using both TEGa have doping levels n~8x1018 cm -3 and correspondingly broad PL spectra. The GaN films grown with TMGa have somewhat lower background doping. The CL spectroscopy studies of the GaN grown on vicinal sapphire substrates show a single broad emission peak near the bandage with no emission from any deep traps with the center of the peak being at 3.479 eV with a FWHM ~102 meV. The CL spatial distribution shows features with dimensions ~4 mm in diameter which coincide with the secondary electron images of the surface, however, variations in CL intensity can only be observed under extremely high contrast conditions. These variations may, in fact, be due to the surface morphological variations, instead of material variations. These CL characteristics indicate that the material is extremely uniform in quality and that the grain boundaries are essentially passivated. Furthermore, the spatial variation of the grains indicates a very low dislocation density ~108 cm -3 is confirmed by TEM analysis. We will discuss these and other results for GaN grown on (0001) and vicinal sapphire substrates.
"Deposition and Characterization of High Quality GaN-InGaN Double Heterostructure pn-Junctions with Multiple Quantum Wells in the Active Region Using Cubic (111) Spinel and Basal Plane Sapphire Substrates:" M. ASIF KHAN, J.W. Yang, C.J. Sun, Q. Chen, B. Lim, M.Z. Anwar, APA Optics, 2950 N. E 84th Lane, Blaine, MN 55449; A.V. Osinsky, H. Temkin, Colorado State University, Electrical Engineering Department, Fort Collins, CO 80523
In the past, we have reported on the deposition and characterization of high quality wurtzite GaN-InGaN single heterostructure over sapphire and (111) spinel substrates. The selection of spinel is based on  direction as the cleaved direction for both the substrate and the GaN-InGaN epilayers. This offers the possibility of cleavage laser bars with parallel facets. Realization of cleavage cavity edge emitting lasers also requires the deposition of high quality pn-junctions over the substrates. We now report the deposition and characterization of GaN-InGaN-GaN double heterostructure pn-junctions with multiple quantum wells in the active region over (111) spinel.
The structures for this study were deposited using a low pressure MOCVD system. They consisted of a 2-3 micron thick n-GaN layer followed by a GaN-InGaN multiple quantum well (MQW). This was then capped with a 0.4 micron thick p-GaN layer. Disilane was used as the dopant for the n-GaN layer and it was measured to have a carrier density of 5x1018 cm-3. The MQW region consisted of 10 repeats of GaN (50 A), InGaN (25 A) unit cell. The p-doping was achieved using bis-Mg as the dopant. The resistivity of this p-GaN layer was measured to be 1 ohm-cm. X-ray ([[theta]]-2[[theta]]) measurements clearly show the presence of satellite peaks whose separation agrees well with the superlattice period. This also agrees with values measured using cross-section TEM.
The pn-junction mesa structures were then fabricated using Ni/Au as the p-contact and Ni/Al as the n-contact metallization. The mesa diameters range from 200 to 500 microns. The IV measurements indicated the fabrication of high quality pn-junctions both over sapphire and spinel substrates. A forward turn on of 3-4 Volts with a total series resistance of 10-20 ohms was measured. These values represent the some of the lowest reported values to date. Details of material deposition procedures and the characterization results will be presented. We will elucidate growth factors controlling the electrical and optical properties.
3:30 PM, Q5
"Deposition and Characterization of High Quality GaN-InGaN Heterostructures over (111) Silicon Substrates:" J.W. YANG, M. Asif Khan, C.J. Sun, Q. Chen, B. Lim, M.Z. Anwar, APA Optics, Inc., 2950 NE 84th Lane, Blaine, MN 55449; G.A. Seryogin, S.A. Nikishin, A.V. Osinsky, H. Temkin, Colorado State University, Electrical Engineering Department, Colorado State University, Ft. Collins, CO 80523; C. Hu, S. Mahajan, Carnegie Mellon University, Department of Materials Science and Engineering, Pittsburgh, PA 15213-3890
Growth of high quality GaN-InGaN heterostructures on Si substrates should be interesting for a variety of electronic and optoelectronic devices. In this work, we describe preparation of hexagonal GaN-InGaN on (111) oriented Si substrates.
The (111) silicon substrates used here were cleaned using a two step chemical etch process. A ~2 nm thick layer of SiO2 is formed first and then removed in a dilute HF:water solution. The second step produces a thin, 0.3-0.5 nm, oxide layer which is removed with a HF:methanol solution. This results in a H-passivated surface. When placed in the MBE chamber a (1x1) surface reconstruction can be observed at room temperature. Hydrogen is removed when the sample is heated to 600deg.C. The initial layer of GaAs, 20 nm thick, is grown at 300deg.C and the final high quality layer, 10-20 nm thick, is grown at 550deg.C. The GaAs layer is then nitrided and a GaN layer, ~1 um thick, deposited by low pressure MOCVD, at 76 torr and 850deg.C, using ammonia and triethylgalium as precursors. This was followed by the deposition of a 0.3um thick layer of InGaN at 750deg.C using trimethylindium as the In-source. Layers of GaN and heterostructures of InGaN-GaN show excellent surface morphology.
The single layers of GaN and GaN-InGaN heterostructures were characterized for the crystalline perfection and electrical properties. X-ray diffraction measurements show that the layers are of good single crystal quality. We also use atomic force microscopy (AFM) and photoluminescence (PL) to show that the GaN layers are hexagonal. Low temperature PL shows a strong bandedge emission at 364 nm, characteristic of hexagonal GaN, and a weak deep level at 564 nm. These features, similar to those observed in the material grown on sapphire, are indicative of good optical quality. AFM measurements reveal GaN islands with clear hexagonal facets oriented along the (110) direction of the (111) Si plane. Results of the cross-sectional TEM describing crystalline perfection of these layers and the GaN/Si interface will be presented.
"Effect of Growth Parameters and Local Gas Phase Concentrations on the Uniformity and Materials Properties of GaN/Sapphire Grown by HVPE:" SYED A. SAFVI1, , M.N. Horton2, N.R. Perkins2, D. Zhi2, R. Matyi2, T. F. Kuech1,2 1University of Wisconsin, Department of Chemical Engineering, 2University of Wisconsin, Materials Science Program, 1509 University Ave., Madison, WI 53706-1595
We are investigating the HVPE growth of thick layers of GaN on (0001) sapphire for use in subsequent MOVPE homoepitaxial GaN growth. The materials properties of HVPE GaN/sapphire are investigated using x-ray diffraction (XRD) and photoluminescence (PL) while the growth uniformity are determined using cross sectional optical microscopy. GaN films are produced at growth rates between 60 and 120 microns/hour, yielding a total film thickness of up to 300 microns. XRD rocking curves typically have FWHM between 200-500 arcseconds, with broadening mostly due to mosaic spread. PL results show strong excitonic emission with surprisingly little deep level yellow luminescence.
Experimentally, it has been found that unusually high flow rates (10 slpm), along with a 30:1 V/III ratio, are required to obtain smooth, typically clear, single crystal material. In these samples, epilayer uniformity varies by approximately 50% across the 2 inch wafer, with thicker areas corresponding to the center of the multiannular gas inlet. The growth thickness exhibits a steep drop-off near the wafer edge with a more uniform central region. When the substrate is mounted normal to gas flow and kept close to the inlet during growth, a dark circular region of rough morphology appears in the center of the epilayer, surrounded by areas of smooth transparent film. This rough surface region decreases in diameter as the substrate is moved sequentially away from the inlet until the entire epilayer is optically clear and smooth. Rough growth areas typically display slightly elevated growth rates, but thickness uniformity remains approximately the same. A finite element based model of the system has been developed to study the correlation between gas phase reactant concentrations and the trends in morphology, growth rate, and materials properties. The numerical model revealed that the ammonia mole fraction in the center of the film during growth is very low due to the high total flowrate and the reactor geometry in which the ammonia is injected from an outer annulus of the reactor. Correlated with improvements in film morphology, the mole fraction of ammonia across the film became more uniform as the substrate was moved further away from the point of gas mixing, allowing more time for radial gas phase diffusion. As an alternative to this flow regime, lower flow rates were tested with the substrate nearer to the gas inlet. Although films are more uniform, as predicted from our model, film morphology was very rough. These results suggest that our model may be incomplete and reactant mixing and possible gas phase prereaction, which partially determine the local reactant concentration at the growth front, can be primary factors affecting growth morphology and materials properties. It is believed that optimal design in this system requires attention to the detailed mass transport and gas phase chemistry within the reactor. Additionally, materials properties as a function of radial position and film morphology will be presented and compared to the local gas phase concentrations at the growth front obtained from the infinite element model.
"Effect of Thin Sputtered AlN Buffer Layer on HVPE Grown GaN Films:" HEON LEE, James S. Harris, Jr., Solid State Electronics Laboratory, Stanford University, Stanford, CA 94305; Kyusik Sin, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305
Because of the absence of substrate that has a good lattice and thermal expansion match to GaN, it is very difficult to grow GaN with low defect density on any substrate and thus homoepitaxial growth of GaN is very attractive. Due to its high growth rate of up to a few tens of um/hr, HVPE (Hydride Vapor Phase Epitaxy) technique can be used to grow thick GaN films for the substrate of homoepitaxial growth. The surfaces of GaN films grown by HVPE are not very smooth, because it is difficult to obtain uniform distribution of GaN nucleations on sapphire surface. In this work, sputtered AlN films with thickness of 100 to 700Å were used as a buffer layer to improve surface morphology and crystal quality.
Up to 10um thick GaN films were grown on sapphire substrates by a new HVPE technique using GaCl3N2 and NH3/N2. Unlike conventional HVPE growth of GaN, sublimed GaCl3 in N2 is used as a Gallium precursor, thus eliminating H2 and HC1 gases in the process. The reactor is an open flow horizontal quartz tube Solid GaCl3 (Tmelt = 76deg.C) was separately heated to 110deg.C in a stainless steel cell and its vapor was transported by flowing N2. The supply of GaCl3 was controlled by both N2 carrier gas flow rate and Tcell. The growth temperature was from 950deg.C to 1075deg.C and V/III ratio ([NH3]/[GaCl3]) was in the range of 500 to 700.
Prior to growth, sapphire substrates were degreased and chemically etched with a hot HCl + H2PO4 (3:1) solution. Thin AIN films (100 ~ 500Å) were then deposited by rf sputtering. The AIN layers were polycrystal from cross-sectional TEM and the surfaces were very smooth. GaN films grown under optimized conditions without AIN buffer layer were single crystal with photoluminescence spectrum showing only band edge emission related peaks at both room temperature and 77deg.K. For GaN films grown with AIN buffer layer, AIN buffer layer was recrystallized to a single crystal during the growth and GaN overlayers were also single crystal. From RBS analysis, only 3 to 5% yield was obtained when GaN crystal was aligned. From the X-ray [[phi]]-scan of GaN ( ) plane, the crystallographic relationship between sapphire substrate and GaN film was the same as that of GaN films grown without AIN buffer layer. From SEM observations, the surface morphologies of the GaN films grown on an AIN buffer layer were much smoother and featureless compared to those grown directly on sapphire. Also, improvement of crystalline quality was observed from the analysis of X-ray rocking curve and [[phi]]-scan. Optimization of the AIN buffer thickness and GaN growth conditions will be reported. GaN films grown on a 500Å thick AIN buffer layer at 1050deg.C showed the best crystalline quality and the smoothest surface morphology.
"Electrical and Optical Properties of Se-Doped GaN:" GYU-CHUL YI, Bruce
W. Wessels, Department of Materials Science and Engineering, Materials Research
Center, Northwestern University, Evanston, IL 60208
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