"In-situ Characterization of GaN MOCVD Growth Using Mass Spectroscopy:" Y. PARK, D. Pavlidis, Department of Electrical Engineering and Computer Science, The University of Michigan, 1301 Beal Ave., Ann Arbor, MI 48109-2122
In-situ characterization of GaN MOCVD growth was performed by means of mass spectroscopy and GaN films were characterized electrically to analyze the observed trends such as impact of nitrogen evaporation during growth. Based on the identification of gas products from the reactor, possible chemical reactions and processes occurring during GaN growth are discussed. Trimethylgallium (TMGa) and ammonia (NH3) were used as material sources for GaN growth. Gas phases were sampled just above the substrate through a capillary quartz tube and analyzed by a differentially pumped mass spectrometer.
As the substrate temperature increases, the decomposition of TMGa was observed to increase exponentially with a characteristic energy of 1.3 eV, which is somewhat lower than that reported for the homogeneous TMGa decomposition in H2 (~2.1 eV). This lower activation energy could be explained by the presence of GaN surface and/or NH3; in this study, the presence of NH3 was found to significantly promote the decomposition of TMGa. Methane was identified as the main gas product generated during the growth. This suggested that CH3 of TMGa and the hydrogen atom of NH3 react and form CH4. Also, significant suppression of the formation of CH3 and C2H6 in the presence of NH3 was observed compared to a hydrogen only environment. This implies that NH3 is a more efficient source of hydrogen atoms than the H2 carrier gas for generation of CH4.
Mass spectroscopy studies of nitrogen evaporation from the growth surface when TMGa flow was off led to the conclusion that the use of high growth rate could result in decreased background electron concentration due to nitrogen vacancy. GaN, 0.5 um thick films were grown at rates varying from 0.3 to 1.0 um/hr to confirm this observation. Film characterization by Hall effect measurements showed a threefold decrease of electron concentration with increasing growth rate, which agrees with the trends speculated from mass spectroscopy studies. Finally, the increased ratio of the peak intensity of Ga (m/e=69) to that of DMGa (Ga(CH3)2, m/e=99) with decreasing NH3 flow rate suggested the desorption of excess Ga atom from the growth surface at low NH3 flow rates.
"A X-ray Photoelectron Spectroscopy Study on Surface Chemical States of GaN, InGaN and AlGaN Epilayers Grown by MOCVD:" K. LI, A.T.S. Wee, J. Lin, K.L. Tan, Department of Physics, National University of Singapore, Singapore, 119260, Singapore; M. Schurman, Z.C. Feng, R.A. Stall, EMCORE Research Laboratory, 394 Elizabeth Avenue, NJ 08873
III-V nitrides semiconductor materials are very attractive in the applications of short wavelength optoelectronic devices, such as light emitting diodes and semiconductor lasers. In this study, we have prepared a series of GaN, AlGaN and InGaN epitaxial thin films of 1-2 mm thick grown by metalorganic chemical vapor deposition (MOCVD) with the advanced Turbo Disk technique developed at EMCORE. The surface chemical states of these GaN-based binary and ternary compounds, and, especially, the influence of different dopants have been studied with x-ray photoelectron spectroscopy. The results show that for most of the samples N 1s peak can be deconvoluted into a dominant GaN peak at the binding energy of 397.2+/-0.2 eV and a small NH3 peak at the binding energy of 398.5+/-0.2 eV, while Ga 3d can be deconvoluted into theee peaks, i.e., elemental Ga at 18.4+/-0.1 eV, GaN at 19.8+/-0.1 eV, and Ga2O3 at 20.4+/-0.1 eV. For magnesium doped GaN, both the N 1s and Ga 3d peaks become broader and the peaks shift towards a slightly lower binding energy. The percentage of elemental Ga becomes larger, while the fraction of Ga2O3 smaller. In addition, Mg doping causes the Ga:N ratio to drop from 1.2 without doping to 0.78. Compared with Mg doping, the influence of Si doping is much smaller. There is neither peak broadening nor peak shift. But Si doping still results in obvious gallium deficiency at the surface. XPS study on a ternary AlGaN sample shows that the atomic content of aluminum at the surface is 7.4%, significantly higher than the bulk value of 2.5%, indicating the surface segregation of aluminum. For the undoped InGaN (12% In), surface indium deficiency is also observed, as the content of indium is only about 2.2% at the surface. Due possibly to the unpyrolyzed ammonia (NH3) and trimethylgallium (Ga(CH3)3) left at the epilayer surface, the Zn doped InGaN sample shows a much larger N 1s peak at 398.78 eV from NH3, and an additional Ga 3d peak at 21.4 eV from Ga(CH3)3. Except for this irregularity, other features, such as peak positions and full widths at half maximum remain unchanged, suggesting that the influence of Zn doping on the surface chemical states of InGaN is small.
"Gas-Source Molecular Beam Epitaxial Growth and Characterization of InNxP1-x on InP:" W.G. BI, C.W. Tu, Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407
Recently there is an increasing interest in the study of N incorporation in GaP and GaAs because of the potential application in optoelectronic devices. N incorporation in InP, an important member of the III-V family, and its properties, however, have not been investigated. In this work we focused on investigating the N incorporation behavior into InP as a function of growth conditions. Very streaky reflection high-energy electron reflection patterns were observed for InNxP1-x grown on InP, indicating layer-by-layer growth of the film. The sharp X-ray diffraction peak and the clear Pendelloesung fringes in the high-resolution X-ray rocking curves indicate the high crystalline quality and uniformity of the film. They also suggest the smoothness of the interface between InP and InNxP1-x and of the surface of the InNxP1-x layer. This is further confirmed by SEM measurement on these samples, where featureless surface is obtained
The N composition was determined from X-ray (511) asymmetric reflections to account for strain-induced lattice constant change of the InNxP1-x films. Like in other mixed group-V compounds (e.g., GaAsxP1-x and InAsxP1-x1 etc.), the N composition in InNxP1-x is different from that in the gas phase, but the overall behavior is somewhat different. With increasing the N2 flow-rate fraction (N2 flow rate over total group-V gas flow rate), the solid composition increases up to a point and then seems to saturate, while for GaAsxP1-x,1 no saturation was observed. This might be due to the small solubility of N in InNxP1-x at a given temperature or incorporation of N as an interstitial. At a fixed N2 flow-rate fraction, the higher the growth temperature, the lower the N solubility seems to be; e.g., with a N2 flow rate fraction being fixed at ~ 0.37, the N concentration is decreased from 0.93% to 0.44% as the temperature is increased from 310deg.C to 420deg.C. This decreasing N incorporation with increasing growth temperature might be due to the increasing nitrogen desorption from the surface during growth. Similar behavior was also observed in GaNxAs1-x2 and GaNxP1-x3 systems.
The optical properties of these films were studied by optical absorption measurement. The results show that the absorption coefficient obeys a square law, indicating the optical absorption is from a direct bandgap material. As the N composition is increased, the band-edge of the InNxP1-x films shifts to longer wavelength, revealing a reduction in the bandgap energy from 1.34 to 1.2 eV (0.93% N). The bandgap energy of InNxP1-x as a function of the N composition was calculated using the Van Vechten model and agrees well with our experimental data.
_____________________________________ 1H. Q. Hou and C. W. Tu, J. Electron. Mat. 21, 137 (1992).
2M. Weyers and M. Sato, Appl. Phys. Lett. 62, 1396 (1993). 3S. Miyoshi, H. Yaguchi, K. Onabe, Y. Shiraki, and R. Ito, Inst. Phys. Conf. Ser. No. 141, 97 (1995).
"Lower Temperature Synthesis of Aluminum Nitride Thin Films by Atomic Layer Growth:" J. N. KIDDER, JR., J.-W. Chung, H.K. Yun, F.S. Ohuchi, J.W. Rogers, Jr., T.P. Pearsall, Department of Materials Science and Engineering, Roberts Hall, 352120, University of Washington, Seattle, WA 98195
Conventional chemical vapor deposition techniques for depositing crystalline aluminum nitride (AlN) thin films require high temperatures (>1400 K). In order to integrate AlN with other materials lower temperature processes are needed to meet the constraints present in the fabrication of electronic circuits and devices. We are studying the synthesis of AlN at lower temperatures by employing an atomic layer growth technique to promote surface reactions between a specially chosen aluminum source, dimethylethylamine:alane, and ammonia. The process is used to facilitate growth of an ordered structure through site-selective and self-limiting reactions.
Using this technique we have successfully deposited preferentially-oriented crystalline AlN thin films at temperatures as low as 573 K. X-ray diffraction studies show an increase in the crystallite size with increasing growth temperatures. We also observe a strong dependency of the AlN film orientation on substrate orientation. Films deposited on Si(100) and Al2O3(0001) substrates have the AlN(0001) planes preferentially oriented parallel to the substrate surface. The orientation of AlN films deposited on Al2O3(0112) substrates is dependent on the growth conditions where AlN(110)// Al2O3(0112) and AlN(0001)// Al2O3(0112) orientations have been observed. Growth studies show that the orientation can be controlled through variations in the atomic layer deposition process parameters. Analysis of the films by X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy shows that the films are stoichiometric AlN with some residual oxygen and hydrogen. Analysis of the XPS spectra is used to determine the film composition and the basic reaction mechanisms governing the film formation. Ellipsometry and optical absorption measurements have been done to analyze the optical properties and energy band-gaps of these films.
"Room Temperature Growth of GaN Epilayers on GaAs(100) Substrates by Hot Plasma Chemical Vapor Deposition:" JIE WANG, Ziqiang Zhu, Takafumi Yao, Institute for Materials Research, Tohoku University, Aoba-Ku, Sendai 980, Japan
The III-V nitrides and their alloys with direct wide bandgap open a very broad field of optoelectronic applications in visible and near-UV wavelength ranges. To date, many techniques have been developed to grow GaN films in which metalorganic chemical vapor deposition (MOCVD) is the dominant one. In the normal MOCVD, Triethylgallium (TMG) and NH3 are used as sources and the substrates have to be heated to roughly 1000oC to thermally dissociate NH3. Thus, a disadvantage of MOCVD is due to thermal mismatch with all of the available substrate materials. Postgrowth cooling introduces significant of strain and defects into nitride films. In addition, the high growth temperatures may encourage other undesirable effects such as dopant and group-III metal desorption, diffusion and segregation, and it become more difficult to choice proper substrates due to high growth temperature.
In order to overcome the disadvantage of conventional MOCVD of GaN, we have developed a novel growth technique, namely, hot plasma CVD, for producing GaN films at room temperature for the first time. A hot plasma nitrogen source in a growth chamber is generated in an inductively coupled quartz tube by supplying high rf power up to 5kW at a nitrogen background pressure of 3x10-2 torr. In this way, abundant active nitrogen atoms are generated, which results in growth rate as high as 4um/hr at room temperature. Additionally, strong light emissions from the hot plasma irradiate onto the substrate and promote the dissociation of TMG during the growth, which results in the realization of the room temperature growth of GaN films.
Prior to the growth of GaN, GaAs(100) substrates were heated to 600oC with or without the irradiation of nitrogen plasma beam, and then cooled down to room temperature. The partial pressures of N2 and TMG were set at 3x10-2 and 1x10-2 torr, respectively. The GaN films were characterized by X-ray diffraction (XRD), reflection high-energy electron diffraction (RHEED) and photoluminescence spectroscopy. The RHEED and XRD results indicated that the structures of GaN films depends on the surface treatment prior to growth: a hexagonal structure obtained without the irradiation of nitrogen plasma beam and a cubic structure with the irradiation. However, all the samples showed a mirror-like surface, and strong and broad emissions centered at 2.997 eV at room temperature.
"Temperature Effects on Schottky Diodes on n-type GaN:" A.C. SCHMITZ, A.T. Ping, M. Asif Khan*, I. Adesida, Center for Compound Semiconductor Microelectronics and Department of Electrical and Computer Engineering, University of Illinois-Champaign, Urbana, IL 61801, *APA Optics, Inc. 2950 NE 84th Lane, Blaine, MN 55449
The III-V nitrides have recently attracted considerable attention for their application in short wavelength optoelectronic devices. In addition, the nitrides are also appealing because their chemical/thermal stability and wide energy gaps make these materials suitable for high power and high temperature electronic devices. Electronic devices such as heterojunction field effect transistors (HFET)  and heterojunction bipolar transistors (HBT)  have been demonstrated to operate at temperatures of up to 300oC. However, in order to realize the potential of the III-V nitrides for high temperature applications, high quality Schottky and ohmic contacts are required. These contacts must not deteriorate significantly under high temperature operation in order to useful. To date, no work has been reported on the effect of high temperatures on the characteristics of Schottky contacts. This work is thus necessary to understand which metals are appropriate for gates in high temperature field effect transistors.
In this work, Pd, Au, Ni, Pt, W, and Mo contacts on n-GaN have been formed and characterized. Schottky barrier heights have been determined using current-voltage (I-V), current-voltage-temperature (I-V-T), and capacitance-voltage (C-V) measurement techniques. The ideality factor and effective Richardson constant were also determined for each metal. Barrier height on n-GaN shows dependence upon the metal workfunction, probably due to the ionic nature of GaN. The high temperature effects on contacts of various metals have also been investigated. Measurements of barrier height for the different metals are taken by I-V measurements over the temperature range 25oC-500oC. Also, the diodes were subjected to temperature treatments up to 800oC. A steady degradation for each Pd and Ni barrier heights is observed with increasing temperature.
 M. Asif Khan et al., Appl. Phys. Lett. 66, 1083 (1995).
 J. Pankove et al., IEDM 94-389 (1994).
"Facet Formation in GaN/AlGaN Heterostructures Using Chemically Assisted Ion Beam Etching with HCl Gas:" C. YOUTSEY, A.T. Ping, G. Bulman*, I. Adesida, Center for Compound Semiconductor Microelectronics and Department of Electrical and Computer Engineering, University of Illinois-Champaign, Urbana, IL 61801;*CREE Research, Inc. 2810 Meridian Parkway, Durham, NC 27713
Recent progress in the growth and processing of III-V nitrides have brought potential device applications for this material system ever closer to reality. The wide, direct bandgaps and robustness of the nitrides have attracted much interest for their use as short-wavelength light sources and detectors. The annoucement late last year by Nichia Chemical Industries of pulsed, room temperature operation of an InGaN-based multi-quantum well (MQW) laser diode is widely viewed as an important milestone in the effort to obtain commercial blue semiconductor lasers . The Nichia device was grown by MOCVD on a sapphire substrate and utilized reflective mirror cavity facets formed by reactive ion etching (RIE). At present, sapphire and silicon carbide are the substrate materials of choice for nitride growth. The difficulty of reliably cleaving these substrates underscores the importance of improving dry etching techniques for the formation of high-quality facets in nitride materials. To date, chemically assisted ion beam etching (CAIBE) and RIE using chlorinated gases have been shown to be effective methods for etching of III-V nitrides, though little has been reported on the etching of heterostructures of these materials.
In this work, we demonstrate the use of chemically assisted ion beam etching with HCl gas for fabricating smooth, vertical facets in GaN/AlGaN heterostructures. The CAIBE technique utilizes simultaneous fluxes of energetic Ar ions and reactive HCl gas to obtain high etch rates with good anisotropy. Details of the basic process have previously been reported for etching of GaN layers only . The process conditions during etching as well as the quality of the etch mask both play an important role in the resulting smoothness and verticality of the etched facets. Optimization of the etch process as well as etch mask for obtaining mirror-quality etched facets will be discussed.
 Nakamura et al., Jpn. J. Appl. Phys., Jan. 15, 1996.
 A.T. Ping et al., Appl. Phys. Lett. 67 (9), 1250 (1995).
"Reactive Ion Etching-Induced Damage on n-GaN Surfaces Evaluated by Schottky Diodes:" A.T. PING, A.C. Schmitz, M. Asif Khan*, J.W. Yang*, I. Adesida, Center for Compound Semiconductor Microelectronics and Department of Electrical and Computer Engineering, University of Illinois-Champaign, Urbana, IL 61801; *APA Optics, Inc. 2950 NE 84th Lane, Blaine, MN 55449
Recent progress in the dry etching of high quality III-V nitrides, in particular GaN, has overcome one obstacle obstructing the realization of novel optoelectronic devices with operating wavelengths ranging from blue to the ultraviolet. Dry etching techniques are a necessity for reliable pattern definition in the III-V nitrides due to the chemical stability of these materials against wet chemical solutions. Thus far, reports have concentrated on characterizing the etch rates, etch profiles, and changes in surface stoichiometry and morphology using various etching techniques. However, little work has been done to determine the extent of the damage introduced to the nitrides as a result of dry etching. Pearton et al. have investigated the change in sheet resistance of InN, InGaN, and InAlN layers exposed to Ar plasmas under both ECR-RIE and RIE conditions . It was found that InGaN was the most resistant of the three to damage. The characterization of dry etch-induced damage using Schottky diodes has been utilized extensively for GaAs and other III-V compounds. To date, only preliminary work has been reported on the use of Schottky diode for damage assessment in GaN . More work is required in this area in order to quantify the utility of dry etching for the fabrication of high-quality devices such as recessed-gate field effect transistors.
In this paper, we will present our investigations on the electrical characteristics of reactive ion etched surfaces of n-type GaN using Schottky diodes. Reactive ion etching was performed in SiCl4 and Ar plasmas. Both of these gas chemistries were used to compare damages resulting from etching with chemical + physical components versus purely physical mechanisms. The barrier heights, ideality factors, and effective Richardson coefficients were investigated as functions of plasma self-bias voltage, etch time, and chamber pressure. A modified Norde plot of the current-voltage-temperature (I-V-T) measurements was used to determine the barrier heights and effective Richardson coefficients. Measurements from capacitance-voltage (C-V) techniques indicated the surface doping is not significantly altered for etching up to plasma voltages of -200 V. The use of rapid thermal annealing after etching was investigated for the removal of induced damages. The Schottky characteristics of etched GaAs surfaces were also investigated for comparison.
 S. J. Pearton et al., Appl. Phys. Lett. 67, 2329 (1995).
 A.T. Ping et al., 1995 MRS Fall Meeting Proceedings, Symposium AAA.
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