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The phonon densities of states of wurtzite AlN and ZrN

Description: Phonons are thought to play a crucial role in the high thermal conductivity of wide-bandgap semiconductors AlN and ZrN. Using time-of-flight neutron spectroscopy, we have measured the phonon spectra of AIN and ZrN up to 300 MeV (2400 cm{sup {minus}1}). The one-phonon density of states (DOS) of AIN exhibits relatively sharp bands at about 033, 63, 83 and 91 MeV. In addition, distinct multiple-phonon excitations were observed at {approximately}173 and 255 MeV. The phonon DOS of ZrN displays similar features with the corresponding phonon energies shifted toward lower energies. The measured DOS of AlN is compared with results of molecular-dynamics simulations.
Date: December 31, 1995
Creator: Loong, C.K.
Partner: UNT Libraries Government Documents Department

Filamentation and Fundamental-Mode Operation in InGaN Quantum Well Lasers

Description: Filamentation, and consequently output beam quality in InGaN quantum-well lasers are found to be strong functions of quantum-well width because of the interplay of quantum-confined Stark effect and many-body interactions. For an In{sub 0.2}Ga{sub 0.8}N/GaN gain medium the antiguiding factor in a thick 4nm quantum well is considerably smaller than that for a narrow 2nm one. As a result, lasers with the thicker quantum well maintain fundamental-mode operation with wider stripe widths and at significantly higher excitation levels.
Date: December 8, 1999
Creator: CHOW,WENG W.; AMANO,H. & AKASAKI,I.
Partner: UNT Libraries Government Documents Department

The Band Gap of AlGaN Alloys

Description: The band gap of AlXGal.XN is measured for the composition range 0s<0.45; the resulting bowing parameter, b=+O.69 eV, is compared to 20 previous works. A correlation is found between the measured band gaps and the methods used for epitaxial growth of the AlXGal_XN: directly nucleated or buffered growths of AlXGal-XN initiated at temperatures T>800 C on sapphire usually lead to stronger apparent bowing (b> +1.3 eV); while growths initiated using low-temperature buffers on sapphire, followed by high-temperature growth, lead to weaker bowing (b<+ 1.3 eV). Extant data suggests that the correct band-gap bowing parameter for AlXGal-XN is b=+O.62 (N.45) eV.
Date: January 29, 1999
Creator: Biefeld, R.M.; Crawford, M.H.; Han, J.; Lee, S.R.; Petersen, G.A. & Wright, A.F.
Partner: UNT Libraries Government Documents Department

High rate dry etching of GaN, AlN and InN in ECR Cl{sub 2}/CH{sub 4}/H{sub 2}/Ar plasmas

Description: Etch rates for binary nitrides in ECR Cl{sub 2}/CH{sub 4}/H{sub 2}/Ar are reported as a function of temperature, rf-bias, microwave power, pressure and relative gas proportions. GaN etch rates remain relatively constant from 30 to 125{degrees}C and then increase to a maximum of 2340 {angstrom}-min{sup {minus}1} at 170{degrees}C. The AlN etch rate decreases throughout the temperature range studied with a maximum of 960 {angstrom}-min{sup {minus}1} at 30{degrees}C. When CH{sub 4} is removed from the plasma chemistry, the GaN and InN etch rates are slightly lower, with less dramatic changes with temperature. The surface composition of the III-V nitrides remains unchanged over the temperatures studied. The GaN and InN rates increase significantly with rf power, and the fastest rates for all three binaries are obtained at 2 mTorr. Surface morphology is smooth for GaN over a wide range of conditions, whereas InN surfaces are more sensitive to plasma parameters.
Date: May 1, 1995
Creator: Vartuli, C.B.; Pearton, S.J.; Abernathy, C.R.; Shul, R.J.; Kilcoyne, S.P.; Crawford, M.H. et al.
Partner: UNT Libraries Government Documents Department

Degradation of blue AlGaN/InGaN/GaN LEDs subjected to high current pulses

Description: Short-wavelength, visible-light emitting optoelectronic devices are needed for a wide range of commercial applications, including high-density optical data storage, full-color displays, and underwater communications. In 1994, high-brightness blue LEDs based on gallium nitride and related compounds (InGaN/AlGaN) were introduced by Nichia Chemical Industries. The Nichia diodes are 100 times brighter than the previously available SiC blue LEDs. Group-III nitrides combine a wide, direct bandgap with refractory properties and high physical strength. So far, no studies of degradation of GaN based LEDs have been reported. The authors study, reported in this paper, focuses on the performance of GaN LEDs under high electrical stress conditions. Their observations indicate that, in spite of a high defect density, which normally would have been fatal to other III-V devices, defects in group-III nitrides are not mobile even under high electrical stress. Defect tubes, however, can offer a preferential path for contact metals to electromigrate towards the p-n junction, eventually resulting in a short. The proposed mechanism of GaN diode degradation raises concern for prospects of reliable lasers in the group-III nitrides grown on sapphire.
Date: December 31, 1994
Creator: Barton, D.L.; Zeller, J.; Phillips, B.S.; Chiu, P.C.; Askar, S.; Lee, D.S. et al.
Partner: UNT Libraries Government Documents Department

Comparison of ICl- and IBr-Based Plasma Chemistries for Inductively Coupled Plasma Etching of GaN, InN and AlN

Description: A parametric study of the etch characteristics of GaN, AIN and InN has been earned out with IC1/Ar and IBr/Ar chemistries in an Inductively Coupled Plasma discharge. The etch rates of InN and AIN were relatively independent of plasma composition, while GaN showed increased etch rates with interhalogen concentration. Etch rates for all materials increased with increasing rf chuck power, indicating that higher ion bombardment energies are more efficient in enhancing sputter resorption of etch products. The etch rates increased for source powers up to 500 W and remained relatively thereafter for all materials, while GaN and InN showed maximum etch rates with increasing pressure. The etched GaN showed extremely smooth surfaces, which were somewhat better with IBr/Ar than with IC1/Ar. Maximum selectivities of- 14 for InN over GaN and >25 for InN over AIN were obtained with both chemistries.
Date: December 1, 1998
Creator: Abernathy, C.R.; Cho, H.; Donovan, S.M.; Hahn, Y.B.; Han, J.; Hays, D.C. et al.
Partner: UNT Libraries Government Documents Department

Design and performance of nitride-based ultraviolet (UV) LEDs

Description: The authors overview several of the challenges in achieving high efficiency nitride-based UV (&lt; 400 nm) LEDs. The issue of optical efficiency is presented through temperature-dependent photoluminescence studies of various UV active regions. These studies demonstrate enhanced optical efficiencies for active regions with In-containing alloys (InGaN, AlInGaN). The authors compare the performance of two distinct UV LED structures. GaN/AlGaN quantum well LEDs with {lambda} &lt; 360 nm emission have demonstrated output powers &gt; 0.1 mW, but present designs suffer from internal absorption effects. InGaN/AlInGaN quantum well LEDs with 370 nm &lt; {lambda} &lt; 390 nm emission and &gt; 1 mW output power are also presented.
Date: April 24, 2000
Creator: CRAWFORD,MARY H. & HAN,JUNG
Partner: UNT Libraries Government Documents Department

Wet Chemical Etching Survey of III-Nitrides

Description: Wet chemical etching of GaN, InN, AlN, InAlN and InGaN was investigated in various acid and base solutions at temperatures up to 75 C. Only KOH-based solutions were found to etch AlN and InAlN. No etchants were found for the other nitrides, emphasizing their extreme lack of chemical reactivity. The native oxide on most of the nitrides could be removed in potassium tetraborate at 75 C, or HCl/H{sub 2}O at 25 C.
Date: February 4, 1999
Creator: Abernathy, C. R.; Cho, H.; Hays, D. C.; MacKenzie, J. D.; Pearton, S.J.; Ren, F. et al.
Partner: UNT Libraries Government Documents Department

Chemical vapor deposition of ternary refractory nitrides for diffusion barrier applications

Description: As semiconductor device dimensions shrink, new diffusion barriers will be required. Amorphous refractory ternaries have been identified as promising barrier candidates; because sputtering may not be suitable, we have developed chemical vapor deposition processes for these materials. Acceptable deposition rates are found for each of these processes at 350 C, with all depositions performed between 300 and 450 C. The first process produces a range of Ti-Si-N compositions from Ti organometallic, SiH{sub 4}, and NH{sub 3}. Resistivity of the Ti-Si-N films changes with Si content from >1{Omega}-cm at 25 at.% Si down to that of TiN (200{mu}{Omega}-cm). Step coverage obtained is better than 80% on 0.5 {mu}m features with aspect ratios of >1.6. The second CVD process produces a range of W-Si-N film compositions from WF{sub 6}, Si{sub 2}H{sub 6}, and NH{sub 3}. Resistivities vary with composition from 350 to 20,000 {mu}{Omega}-cm. Step coverage obtained is 100% on reentrant 0.25 {mu}m features with aspect ratios of 4.0. The third process employs WF{sub 6}(reduced by SiH{sub 4}), B{sub 2}H{sub 6}, and NH{sub 3} to produce W-B-N films with a range of compositions. Resistivities range from 200 to 20,000 {mu}{Omega}-cm. Step coverage obtained is {approx}40% on 1.5 {mu}m features with aspect ratios of 5.5.
Date: June 1, 1996
Creator: Smith, P.M.; Custer, J.S.; Fleming, J.G.; Roherty-Osmun, E.; Cohn, M. & Jones, R.V.
Partner: UNT Libraries Government Documents Department

Electron scattering by native defects in III-V nitrides and their alloys

Description: We have calculated the electron mobilities in GaN and InN taking into consideration scattering by short range potentials, in addition to all standard scattering mechanisms. These potentials are produced by the native defects which are responsible for the high electron concentrations in nominally undoped nitrides. Comparison of the calculated mobilities with experimental data shows that scattering by short range potentials is the dominant mechanism limiting the electron mobilities in unintentionally doped nitrides with large electron concentrations. In the case of Al{sub x}Ga{sub 1-x}N alloys, the reduction in the electron concentration due to the upward shift of the conduction band relative to the native defect level can account for the experimentally measured mobilities. Resonant scattering is shown to be important when the defect and Fermi levels are close in energy.
Date: March 1, 1996
Creator: Hsu, L. & Walukiewicz, W.
Partner: UNT Libraries Government Documents Department

ICP dry etching of III-V nitrides

Description: Inductively coupled plasma etching of GaN, AlN, InN, InGaN and InAlN was investigated in CH{sub 4}/H{sub 2}/Ar plasmas as a function of dc bias, and ICP power. The etch rates were generally quite low, as is common for III-nitrides in CH{sub 4} based chemistries. The etch rates increased with increasing dc bias. At low rf power (150 W), the etch rates increased with increasing ICP power, while at 350 W rf power, a peak was found between 500 and 750 W ICP power. The etched surfaces were found to be smooth, while selectivities of etch were {le} 6 for InN over GaN, AlN, InGaN and InAlN under all conditions.
Date: October 1, 1997
Creator: Vartuli, C.B.; Lee, J.W. & MacKenzie, J.D.
Partner: UNT Libraries Government Documents Department

Theoretical investigation of extended defects in group-III nitrides

Description: The authors have investigated two types of extended defects commonly found in AlN, GaN and InN films using density-functional techniques. First, basal-plane stacking faults have been studied for all three compounds. Stacking-fault energies were found to be largest in AlN and smallest in GaN consistent with density-functional results for their wurtzite/zinc-blende energy differences. In addition, the 4H and 6H structures were found to have lower energies than zinc blende for all three compounds. Secondly, the authors have investigated the electronic structure and formation energy for an edge dislocation in AlN. The full-core dislocation structure was found to have a filled electronic level approximately 0.55 eV above the valence-band edge and an empty level 1.4 eV below the conduction-band edge. An open-core structure was found to have filled and empty electronic levels closer to the middle of the energy gap. Formation energies for these two geometries suggest that the full-core structure would be expected to form in p-type material whereas both are expected in n-type material.
Date: December 1, 1997
Creator: Wright, A. F.
Partner: UNT Libraries Government Documents Department

Plasma etching of the Group-III nitrides

Description: In reactive ion etching (RIE) of GaN, the ion bombardment can damage the material, so it is necessary to develop plasma etch processes. This paper reports etching of GaN in an ECR (electron cyclotron resonance) etch system using both the ECR/RIE mode and the RIE-only mode. Group III (Ga, In, Al) nitride ECR etching is reviewed as a function of plasma chemistry, power, temperature, and pressure; as the ECR microwave power increased, the ion density and etch rates increased, with the etch rate increasing the most for InN. GaN etch rates > 6500 {angstrom}/min have been observed in the ECR/RIE mode. 2 figs, 6 refs.
Date: January 1, 1996
Creator: Shul, R.; Pearton, S.J. & Abernathy, C.R.
Partner: UNT Libraries Government Documents Department

Dry etching of III-V nitrides

Description: The chemical inertness and high bond strengths of the III-V nitrides lead to slower plasma etching rates than for more conventional III-V semiconductors under the same conditions. High ion density conditions (>3{times}l0{sup 9}cm{sup {minus}3}) such as those obtained in ECR or magnetron reactors produce etch rates up to an order of magnitude higher than for RIE, where the ion densities are in the 10{sup 9}cm{sup {minus}3} range. We have developed smooth anisotropic dry etches for GaN, InN, AlN and their alloys based on Cl{sub 2}/CH{sub 4}/H{sub 2}/Ar, BCl{sub 3}/Ar, Cl{sub 2}/H{sub 2}, Cl{sub 2}/SF{sub 6}, HBr/H{sub 2} and HI/H{sub 2} plasma chemistries achieving etch rates up to {approximately}4,000{angstrom}/min at moderate dc bias voltages ({le}-150V). Ion-induced damage in the nitrides appears to be less apparent than in other III-V`s. One of the key remaining issues is the achievement of high selectivities for removal of one layer from another.
Date: December 1, 1995
Creator: Pearton, S.J.; Shul, R.J.; McLane, G.F. & Constantine, C.
Partner: UNT Libraries Government Documents Department

Role of C, O and H in III-V nitrides

Description: The light ion impurities C, 0 and H have been implanted or diffused into GaN and related compounds and their effect on the electrical properties of these materials measured by Hall, C-V and SIMS as a function of annealing temperatures from 300--11OO{degree}C. While C in as-grown GaN appears to create an acceptor under MOMBE conditions, implanted C shows no measurable activity. Similarly, implanted 0 does not show any shallow donor activity after annealing at {le}700{degree}C, but can create high resistivity regions (10{sup 6} {Omega}/{open_square}) in GaN, AlInN and InGaN for device isolation when annealed at 500--70O{degree}C. Finally, hydrogen is found to passivate shallow donor and acceptor states in GaN, InN. InAlN and InGaN, with dissociation of the neutral complexes at >450{degree}C. The liberated hydrogen does not leave the nitride films until much higher annealing temperatures (>800{degree}C). Typical reactivation energies are {approximately}2.0 eV for impurity-hydrogen complexes.
Date: December 1, 1995
Creator: Abernathy, C.R.; Pearton, S.J.; MacKenzie, J.D.; Lee, J.W.; Vartuli, C.B.; Wilson, R.G. et al.
Partner: UNT Libraries Government Documents Department

Design and performance of nitride-based UV LEDs

Description: In this paper, the authors overview several of the critical materials growth, design and performance issues for nitride-based UV (&lt; 400 nm) LEDs. The critical issue of optical efficiency is presented through temperature-dependent photoluminescence studies of various UV active regions. These studies demonstrate enhanced optical efficiencies for active regions with In-containing alloys (InGaN, AlInGaN). The authors discuss the trade-off between the challenging growth of high Al containing alloys (AlGaN, AlGaInN), and the need for sufficient carrier confinement in UV heterostructures. Carrier leakage for various composition AlGaN barriers is examined through a calculation of the total unconfined carrier density in the quantum well system. They compare the performance of two distinct UV LED structures: GaN/AlGaN quantum well LEDs for {lambda}&lt; 360 nm emission, and InGaN/AlGaInN quantum well LEDs for 370 nm &lt;{lambda}&lt; 390 nm emission.
Date: February 16, 2000
Creator: CRAWFORD,MARY H.; HAN,JUNG; CHOW,WENG W.; BANAS,MICHAEL ANTHONY; FIGIEL,JEFFREY J.; ZHANG,LEI et al.
Partner: UNT Libraries Government Documents Department

Electrolytic In-process Dressing (ELID) for high-efficiency, precision grinding of ceramic parts: An experiment study

Description: This report describes Electrolytic In-process Dressing (ELID) as applied to the efficient, high-precision grinding of structural ceramics, and describes work performed jointly by Dr. B.P. Bandyopadhyay, University of North Dakota, and Dr. R. Ohmori, of the Institute of Physical and Chemical Research (RINEN), Tokyo, Japan, from June through August, 1994. Dr. Ohmori pioneered the novel ELID grinding technology which incorporates electrolytically enhanced, in-process dressing of metal bonded superabrasive wheels. The principle of ELID grinding technology is discussed in the report as will its application for rough grinding and precision grinding. Two types of silicon nitride based ceramics (Kyocerals Si{sub 3}N{sub 4}, and Eaton`s SRBSN) were ground under various conditions with ELID methods. Mirror surface finishes were obtained with {number_sign} 4000 mesh size wheel (average grain size = 4 {mu}m). Results of these investigations are presented in this report. These include the effects of wheel bond type, type of power supply, abrasive grit friability, and cooling fluid composition. The effects of various parameters are discussed in terms of the mechanisms of ELID grinding, and in particular, the manner of boundary layer formation on the wheels and abrasive grit protrusion.
Date: August 1, 1995
Creator: Bandyopadhyay, B.P.
Partner: UNT Libraries Government Documents Department

Microstructure and thermal stability of transition metal nitrides and borides on GaN

Description: Microstructure and thermal stability of ZrN/ZrB2 bilayer deposited on GaN have been studied using transmission electron microscopy methods (TEM) and secondary ion mass spectrometry (SIMS). It has been demonstrated that annealing of the contact structure at 1100 C in N2 atmosphere does not lead to any observable metal/ semiconductor interaction. In contrast, a failure of the integrity of ZrN/ZrB2 metallization at 800 C, when the heat treatment is performed in O2 ambient has been observed.
Date: June 28, 2000
Creator: Jasinski, J.; Kaminska, E.; Piotrowska, A.; Barcz, A. & Zielinski, M.
Partner: UNT Libraries Government Documents Department

Role of Intergranular Films in Toughened Ceramics

Description: Self-reinforced silicon nitride ceramics rely the generation of elongated grains that act as reinforcing elements to gain increases in fracture toughness. However, the size and number of the reinforcing grains must be controlled, along with the matrix grain size, to optimize the fracture toughness and strength. Furthermore, the toughening processes of crack bridging are dependent upon retention of these reinforcing grains during crack extension by an interfacial debonding process. Both the debonding process and the resultant toughening effects are found to be influenced by the composition of the sintering aids which typical are incorporated into the amorphous intergranular films found in these ceramics. Specifically, it is shown that the interface between the intergranular glass and the reinforcing grains is strengthened in the presence of an epitaxial SiAlON layer. In addition, the interface strength increases with the Al and 0 content of the SiAlON layer. Micromechanics modeling indicates that stresses associated with thermal expansion mismatch are a secondary factor in interfacial debonding in these specific systems. On the other hand, first principles atomic cluster calculations reveal that the debonding behavior is consistent with the formation of strong Si-0 and Al-O bonds across the glass-crystalline interface.
Date: May 10, 1999
Creator: Becher, P.F.; Hsueh, C.H.; More, K.L.; Painter, G.S. & Sun, E.Y.
Partner: UNT Libraries Government Documents Department

Reactive plasma atomization of aluminum nitride powder

Description: Experiments were performed to synthesize AlN powders by reacting Al with N using a conventional dc arc plasma as heat source. Feeding Al powder into Ar/N plasma open to atmosphere produced mainly Al oxide. Experiments using a chamber backfilled with nitrogen suppressed the Al oxide, but little AlN was formed. A furnace and crucible assembly was designed to feed molten Al directly into a DeLaval nozzle attached to the face of the dc arc plasma gun. Resulting submicron powders show a significant increase in AlN formation. This was dependent on chamber pressure, plasma velocity, and molten liquid feed rate. Experimental parameters, equipment design, effects of atomization/vaporization/condensation are discussed.
Date: February 1, 1997
Creator: Prichard, P.; Besser, M.; Sordelet, D. & Anderson, I.
Partner: UNT Libraries Government Documents Department