The reactions between Au, Au/Ni and Au/C/Ni thin films on p-GaN have been studied using current-voltage (I-V) measurements, Auger electron spectroscopy (AES) and secondary ion mass spectrometry (SIMS). The metallization schemes consisted of ≈2000Ǻ sputtered Au, 1000Ǻ Au/500Ǻ Ni, and 1000Ǻ Au/100Ǻ C/500Ǻ Ni electron beam evaporated. The Au/Ni metallization scheme is of particular interest since it is the basis for the most commonly used ohmic p-type contacts for blue GaN LED’s. Au does not decompose the GaN matrix, while Ni has been shown to react with GaN above a temperature of 400° C for times longer than 5 minutes. Upon decomposition of the GaN by Ni, incorporation of C at the metal/GaN interface occurred. It is believed that a regrowth of GaN occurred, with the surface region being doped with C. Attempts at increasing this doping concentration by introducing an interfacial C layer were not successful.

The microstructural properties and interdiffusion reactions of Au/Ge/Ni, Ti/Pt/Au, WSix and AuBe contacts on GaN and In0.5Ga0.5N have been examined using Scanning Electron Microscopy and Auger Electron Spectroscopy. The WSix contacts possess excellent thermal stability and retained good structural properties at annealing temperatures as high as 800°C on GaN. The electrical characteristics of WSix contacts on In0.5Ga0.5N had a specific contact resistivity of 1.48×10−5Ωcm2 and an excellent surface morphology following annealing at 700°C. The increase in contact resistance observed at higher temperatures was attributed to intermixing of metal and semiconductor. In contrast the Ti/Pt/Au and Au/Ge/Ni contacts were stable only to ≤ 500°C. AuBe contacts had the poorest thermal stability, with substantial reaction with GaN occurring even at 400°C. The WSix contact appears to be an excellent choice for high temperature GaN electronics applications.

Schottky barrier heights of Ni, Pt, Pd, and Au on n-type GaN were measured using current-voltage and capacitance-voltage techniques. Measurements from the I-V technique yielded barrier heights of 0.95, 1.01, 0.94, and 0.87 eV for Ni, Pt, Pd, and Au, respectively. Barrier heights of 1.13,1.16, 1.07, and 0.98 eV, for Ni, Pt, Pd, and Au, respectively, were obtained using C-V measurements.

Schottky barriers were formed on p-GaN. p-GaN layers doped with Mg were grown by metalorganic chemical vapor deposition (MOCVD). 6H-SiC wafers were used as substrates. The barriers were made by vacuum thermal evaporation of Au. Capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the barriers were investigated. The concentration of the ionized acceptors in the p-layers was measured to be about ∼1017 cm−3. The barrier height was determined to be 2.48 eV by C - V measurements at room temperature. The forward current flow mechanism through the barriers is discussed.

Thermally stable contacts to GaN that also have desirable electrical characteristics are required for the further development of optoelectronic and high temperature devices based on GaN. To make improvements on existing contacts or to develop new ones, information on the metallurgy of potential contact systems is needed. In this work, the Metal-Ga-N ternary phase equilibria and the contact metallurgy are examined for Ti, Re, and Ni. Annealed contacts of these metals have been examined with x-ray diffraction and/or x-ray photoelectron spectroscopy, and the observed metallurgical reactions are discussed in light of estimated or experimentally determined Metal-Ga-N phase diagrams. Particular attention is paid to the gas phase equilibria and the role of the annealing environment on the metallurgical reactions. Finally, the consequences of this work for the design of thermally stable contacts are considered.

We present calculations of the specific contact resistance for metals to GaN. Our calculations include a correct determination of the Fermi level taking into account the effect of the degenerate doping levels, required in creating tunneling ohmic contacts. Using a recently reported improved WKB approximation suitable in representing the depletion width at the metal-semiconductor interface, and a two band k-p model for the effective masses, specific contact resistance was determined as a function of doping concentration. The specific contact resistance was calculated using the best data available for barrier heights, effective masses and dielectric coefficients for GaN. Because the barrier height at the metal-semiconductor interface has a very large effect on the contact resistance and the available data is sketchy or uncertain, the effect of varying the barrier height on the calculated specific contact resistance was investigated. Further, since the III-V nitrides are being considered for high temperature device applications, the specific contact resistance was also determined as a function of temperature.

We report on ohmic contacts to Si-implanted and un-implanted n-type GaN on sapphire. A ring shaped contact design avoids the need to isolate the contact structures by additional implantation or etching. Metal layers of Al and Ti/Al were investigated. On un-implanted GaN, post metalization annealing was performed in an RTA for 30 seconds in N2 at temperatures of 700, 800, and 900°C, A minimum specific contact resistance (rc) of 1.4×10−5 Ω-cm2 was measured for Ti/Al at an annealing temperature of 800°C. Although these values are reasonably low, variations of 95% in specific contact resistance were measured within a 500 µm distance on the wafer. These results are most likely caused by the presence of compensating hydrogen. Specific contact resistance variation was reduced from 95% to 10% by annealing at 900°C prior to metalization. On Si-implanted GaN, un-annealed ohmic contacts were formed with Ti/Al metalization. The implant activation anneal of 1120°C generates nitrogen vacancies that leave the surface heavily n-type, which makes un-annealed ohmic contacts with low contact resistivity possible.

Ohmic contact strategies for n- and p-type GaN have been investigated electrically, chemically, and microstructurally using transmission line measurements, high-resolution EELS and cross-sectional TEM, respectively. The contributions to contact performance from work function differences, carrier concentrations, annealing treatments, and interface metallurgy have been examined. The contact materials of Ti, TiN, Au, and Au/Mg were deposited via electron beam evaporation; Al was deposited via thermal evaporation. As-deposited Al and TiN contacts to highly doped n-GaN were ohmic, with room-temperature specific contact resistivities of 8.6×10−5 Ω cm2 and 2.5×10−5 Ωcm2 respectively. The Ti contacts developed low-resistivity ohmic behavior as a result of annealing; TiN contacts also improved with further heat treatment. For p-GaN, Au became ohmic with annealing, while Au/Mg contacts were ohmic in the as-deposited condition. The performance, structure, and composition of different contact schemes varied widely from system to system. An integrated analysis of the results of this study is presented below and coupled with a discussion of the most appropriate contact systems for both n- and p-type GaN.

AlGaN/GalnN double heterostructures (DH) were fabricated by metalorganic vapor phase epitaxy on the (0001)Si 6H-SiC substrate. A cleaved edge shows a very flat surface with roughness on the order of one monolayer. Stimulated emission and laser action from the UV to blue region was observed by optical pumping at room temperature (RT). The threshold power density was 27KW/cm2 which is smaller than that of the same structure grown on a sapphire (0001) substrate by a factor of four. A AlGaN/GalnN DH UV light emitting diode, using undoped GalnN is fabricated. The power efficiency and spectra width of this LED is comparable or superior to that of an LED having the same structure but grown on sapphire.

InGaN single-quantum-well-structure (SQW) light-emitting diodes (LEDs) with an emission wavelength between violet and orange were fabricated. The maximum on-axis luminous intensity of green LEDs was 12 cd and the external quantum efficiency was as high as 6.3 % at a forward current of 20 mA, while those of blue LEDs were 2 cd and 9.1 %, respectively. The peak wavelength and the full width at half-maximum of the green LEDs were 520 nm and 30 nm, respectively, while those of blue LEDs were 450 nm and 20 nm, respectively. The color of green InGaN SQW LEDs was greener than those of conventional GaP and AlInGaP LEDs.

Asymmetric double heterostructures (ADH) of AlGaN/GaInN/GaN blue light emitting diodes (LEDs) and GaInN/GaN multiple quantum well (MQW) LEDs were fabricated by metalorganic vapor phase epitaxy (MOVPE). The ADH LEDs had spectral emissions peaking at 450 nm and the luminous intensify was 2.5 cd at 20 mA The output power was 3.6 mW at 20mA and the external quantum efficiency was as high as 5.1 % at 20 mA. The GaInN/GaN MQW structure was grown successfully by MOVPE. Fine multi-layer structures 7 – 9 nm thick were detected by secondary ion mass spectroscopy and transmission electron microscopy (TEM). The dislocation density in the MQW was as high as 0.5–2×109 cm−2 by TEM The optical efficiency of the MQW layer was higher than that of a bulk GalnN layer. The intensity of UV emission from MQW LEDs was greater than that of blue light from ADH blue LEDs. The UV emission increased as a super-linear function of injection current at -100°C.

We have shown the ability to grow thin, high mobility, GaN channel layers on high quality, highly resistive GaN. The growth, characteristics, and device results of two types of MESFET structures were discussed. The first device structure consists of a 2000Ǻ, 2 × 1017cm−3 Si-doped channel layer, grown on 3µm of highly resistive GaN, while die second structure (recessed-gate MESFET) had a 1000Ǻ, 1 × 1018cm−3 Si-doped, n+ capping layer deposited on a 2000Ǻ, 2 × 1017cm−3 Si-doped channel layer. The first MESFET structure was operational at 500°C which is the highest reported operating temperature for a GaN device while the recessed-gate MESFET had a gmas high as 41mS/mm, which is the highest reported value for a GaN MESFET.

Blue LEDs with double-heterojunction (DH) have been produced in AIN-GaN system grown on n-type 6H-SiC substrates via metalorganic chemical vapor deposition (MOCVD). These devices employ a GaN active layer bordered by Mg doped p-type Al0.1Ga0.9N and Si doped n-type Al0.1Ga0.9N. A vertical device design was utilized with a p-type Au contact centered on the chip top side for wire bonding. The backside contact to the SiC is Ni. The peak emission from these devices is 430 nm with a FWHM of 65 nm, producing a deep blue color. The output power is 1.7 mW at 20 mA which corresponds to an external quantum efficiency of 3%.

Electric breakdown of mesa terminated GaN and AlGaN pn diodes was investigated. The nitride pn structures were grown on 6H-SiC (0001) wafers by metalorganic chemical vapor deposition (MOCVD). Mesa structures were fabricated by reactive ion etching (RIE). Capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the pn junctions were measured. It was found that the junctions were linearly graded. Microplasmic breakdown was observed. The breakdown voltage for GaN and AlGaN diodes ranged from 40 to 150 V and from 40 to 100 V, respectively. The electric breakdown field and temperature coefficient of the breakdown voltage were measured.

We review our recent results on GaN based optoelectronic devices, which include InGaN-AlGaN Light Emitting Diodes (LEDs), GaN photoconductive, Schottky barrier, and p-n junction ultraviolet detectors, and optoelectronic AlGaN-GaN Heterostructure Field Effect Transistors. GaN-based optoelectronic devices cover a wide spectral range and demonstrate visible blind operation. A high quality of the epitaxial layers, the recent development of high performance GaN-based heterostructure field effect transistors, and transparent substrates make this material system uniquely suited for optoelectronic integrated circuits operating in visible and ultraviolet range.

We have observed laser emission with well-defined cavity modes in optically pumped GaN-Al0.1Ga0.9N double-heterostructure (DH) lasers. The laser structures were grown using an electron-cyclotron-resonance nitrogen-discharge source and gas-source molecular beam epitaxy (ECR-GSMBE) on thick (≥10 µm) GaN buffers grown by hydride vapor-phase epitaxy (HVPE) on c-plane sapphire. Transversely pumped cavities using a 337.1 nra nitrogen laser pump source exhibit a threshold pump fluence ranging from 0.15 to 0.3 mJ/cm2 at 77 K, a linear light output above threshold, a lasing wavelength of 358 nm, and an estimated differential quantum efficiency of 1%. The room-temperature threshold is about 1.7 times higher. Longitudinal mode structure has been resolved in a shorter-cavity (23 µn) device at 77 K. The measured mode spacing of 0.56 nm corresponds to a group index of 5.0. Far-field measurements in a plane perpendicular to the plane of the heterostructure indicate a double-lobed pattern for a 1000 Ǻ thick GaN active region, and a single lobe with a FWHM of 60° for a 4000 Ǻ active region. The thick HVPE GaN buffer layer provides for a lattice-matched growth and results in improved nucleation in MBE, as indicated by a high-quality reflection-electron-diffraction pattern of the as-loaded wafers. The surface morphology of the MBE layers on the HVPE buffer shows improved optical smoothness as compared to layers grown directly on sapphire using a low-temperature, MBE-grown GaN buffer. Laser facets were formed either by saw cutting or cleaving of the GaN buffer and epilayer along crystal planes. Details of the material development and laser performance are described.

Optical gain of wurtzite GaN/AlGaN quantum wells has been studied from a first-principles calculation using the k • p method. Most of the parameters in the k • p method were determined by fitting the band structures by the first-principles calculation. Owing to the small spin-orbit splitting energies of the wurtzite GaN and AIN, the optical gain has been calculated using the 6×6 Hamiltonian for the valence band. It is found that the large hole effective masses and the small spin-orbit splitting cause the higher threshold current density of wurtzite GaN/AlGaN quantum well lasers.

We report on the current status of room-temperature life testing of Nichia NLPB-500 blue light emitting diodes. So far, two tests have been completed. During the first 1000-h test, a constant current of 20 mA was maintained in all devices. During the second 1650 h test, groups of 3 or 4 devices were driven at currents ranging from 20 mA to 70 mA. Very little degradation has been observed in devices driven at normal conditions (20-30 mA), with a noticeable increase in degradation rate above 60 mA.

Studies of electrical characteristics of nitride-based light emitting diodes (LEDs) are of interest as they can shed light on carrier transport across the p-n heterojunction. In addition, they provide a convenient way of investigating degradation processes associated with high electrical stress. We present electrical characteristics of Nichia NLPB500 blue LEDs based on AlGaN/InGaN/GaN material system in the temperature range of 9-340 K. Two components of current are identified. High-density current stress leads to diode degradation by shunt formation caused by metal electromigration. At 8 K, blue emission was observed at currents as small as 20 nA.

We have studied GaN films grown by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) on sapphire substrates using different buffer layer structures. Surface morphology was characterized by scanning electron microscopy (SEM). Optical properties were measured using photoluminescence (PL), cathodoluminescence (CL) spectroscopy and catho-luminescence wavelength imaging (CLWI) method. It is found that the hexagonal pit-like defects in morphology are associated with the D-A/e-A transition band in the PL and CL spectra. The same correlation of morphology with optical properties is observed for the GaN films grown by selective area epitaxy (SAE). In addition, the possibility of improving optical quality by SAE is investigated. The SAE depth profile is simulated for the first time, and satisfactory results are obtained.