The annealing effects of gold contacts on SiC single crystal substrates have been investigated using SPM and XPS/AES. Ultra-thin Au films with thicknesses of 30 Å and 150 Å, respectively, were fabricated onto both 4H- and 6H-SiC substrates via rf sputtering. The Au/SiC systems were then undergone a series of annealing with temperatures ranging from 200 °C to 400 °C. This study reveals that the sputtering system can generally produce a thin film of uniformly distributed nano-meter sized granular particles. The size of the particles increases with both the thickness of the ultra-thin film and annealing temperature. The thicker Au film revealed pin-hole damages after 300 °C annealing and severe cracking and spalling after 400 °C. However, the thinner Au film only demonstrated the increase of particle sizes. The atomic ratio of gold to silicon in the surface region decreases with the increase of annealing temperature, which indicates the diffusion of gold into silicon carbide. No significant changes of the Schottky barrier heights have been found for Au contacts onto both 6H- and 4H-SiC in the annealing temperature range. After the 400 °C annealing, a noticeable SiO2/SiOx, peak was unveiled in the XPS spectra.

The ˜1540 nm 4I13/2 → 4I15/2 emissions of Er3+ in Er-implanted GaN annealed at temperatures in the 400 to 1000 °C range were investigated to gain a better understanding of the formation and dissociation processes of the various Er3+ sites and the recovery of damage caused by the implantation with increasing annealing temperature (TA). The monotonic increase in the intensity of the broad defect photoluminescence (PL) bands with increasing TA proves that these are stable radiative recombination centers introduced by the implantation and annealing process. These centers cannot be attributed to implantation-induced damage that is removed by postimplantation annealing. Selective wavelength pumping of PL spectra at 6 K reveals the existence of at least nine different Er3+ sites in this Er-implanted semiconductor. Most of these Er3+ PL centers are attributed to complexes of Er atoms with defects and impurities which are thermally activated at different TA. Only one of the nine observed Er3+ PL centers can be pumped by direct 4f absorption and this indicates that it is the highest concentration Er3+ center and it represents most of the optically active Er3+ in the implanted sample. The fact that this Er3+ center cannot be strongly pumped by above-gap light or broad band below-gap absorption indicates that it is an isolated center, i.e. not complexed with defects or impurities. This 4f-pumped PL spectrum appears at annealing temperatures as low as 400°C, and although its intensity increases monotonically with increasing TA, the wavelengths and linewidths of its characteristic peaks are unaltered. The observation of this high quality Er3+ PL spectrum at low annealing temperatures illustrates that the crystalline structure of GaN is not rendered amorphous by the ion implantation. The increase of the PL intensities of the various Er3+ sites with increasing TA is due to the removal of competing nonradiative channels with annealing.

GaN, AlGaN, AIN were grown on (0001) A1203 substrates by MBE using a RF plasma source and employing an AIN buffer layer. The films were characterized by RHEED, AFM, and x-ray diffraction, and electrical properties were measured by Hall technique. RHEED observations indicate that the polarity of the films is likely predominantly N-face, although Ga-face inversion domains can be observed in some films by AFM. Symmetric x-ray rocking curve widths as low as 39 arcseconds are achieved for some layers, while asymmetric peaks show widths of 240–300 arcsec. Control of Si doping over a wide range is demonstrated, which is important for design of high power device structures. Gold Schottky barrier m-v-n+diodes were fabricated which achieve high reverse electric fields before edge breakdown.

We investigate here for the first time GaN and AIGaN films by using x-ray photoemission spectromicroscopy. As compared to conventional x-ray photoemission spectroscopy (XPS), spectromicroscopy can provide spatially resolved information on the chemical composition of the sample surface. The experimental results where obtained by using MAXIMUM, a scanning photoemission microscope installed on 12.0 undulator beamline at the Advanced Light Source (ALS), Berkelely, allowing for a spatial resolution of 100 nm. We investigate here GaN and AlGaN thin films grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). The results clearly indicate the great potential of spectromicroscopy in investigating chemical inhomogeneity, inpurities and localization in GaN and AlGaN thin films.

We have studied the pressure dependence of photoluminescence for various InGaN quantum wells of different Indium compositions. In contrast to classical quantum well systems such as GaAs/AlGaAs, the pressure coefficients of the photoluminescence peak position of wells of similar parameters can vary greatly, depending on conditions of growth. Generally, the pressure coefficient of the emission peak is much lower than the one predicted for a GaN-InN system. We show that this behavior can be related in most cases to the influence of the thick GaN layer compressibility on the strain in the InGaN quantum wells. However, in some cases (Nichia devices), the pressure coefficient of the emission peak seems to be unrelated to the shift of the energy gap of the material, suggesting the involvement of strongly localized electronic states

The nitridation process of (111)Si surface has been studied by in-situ reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES) measurements. It has been clarified that the nitridation of the Si surface is occurred even if the shutter of the discharged rf-plasma gun is closed. The growth process of AIN epilayer on the Si substrate by migration enhanced epitaxy (MEE) has been investigated by in-situ RHEED and atomic force microscopy (AFM) observations. It has been found that the AIN epilayer is directly grown on (111)Si by MEE growth.

Optical anisotropy of sapphire have been studied by both polarized Raman scattering and transmission variable angle spectroscopic ellipsometry (TVASE). The polarized Raman effect was measured on both c-plane (0001) and a-plane (2110) sapphire substrates. Two TO phonons (379 and 431 cm−l) and four LO phonons (418, 450, 577, and 750 cm−1) were observed when the optical axis <c> is perpendicular to the polarization of the incident laser beam (for both c-plane and a-plane). While only two TO phonons (379 and 431cm−1) and two LO phonons (418 and 645 cm−1) can be seen when the optical axis is parallel to the polarization for an a-plane sapphire. The optical axis of the a-plane sapphire can then be quickly determined by either maximizing or minimizing the 645 LO peak intensity with about ±10° error. TVASE measurements were carried out in the energy range of 0.75eV to 5.8eV at room temperature. Sizable off-diagonal Jones matrix elements Apst, and Aspt can be detected even with the optical axis 1° off the X-axis. This indicates that TVASE has a high sensitivity to optical anisotropy. By minimizing these off-diagonal elements and combining the Raman analysis, the optical axis orientation of an a-plane or m-plane (0110) sapphire can therefore be fully determined with an error of less than 1°.

Subrmicron heteroepitaxial GaAs and GaN films were grown by both conventional MOCVD and «capillary epitaxy» technique on (001) and (111) fianit (YSZ)substrates. A preliminary annealing of the substrates under vakuum was made in order to stabilize the surface by removing of some amount of oxygen. Conditions of single crystalline growth of GaAs submicron films (50–500nm) have been determined. The films had mirror-like surface morphology and high structural perfection. The distribution of Zr, O, Y across the film-substrate interface was sharp and doping impurities contents were uniform over the film. PL spectra of undoped GaN films on YSZ were studied.

BaPbl-xBixO3 phases with 0.6⋚ x⋚ 1.0 were synthesized by high temperature solid state reaction. Powder X-ray diffraction measurements show that all compositions are in single phase. Linear variation of lattice parameter is observed in BaPb1-xBixO3 with change in x indicating the random distribution of Pb in Bi sites. The activation energies for conduction of phases with x=0.8−0.6 obtained from ρ −T plots are same suggesting that the band gap does not change for compositions with x-0.8−0.6. The low activation energy obtained for BaBiO3 can be attributed to the structure of the compound. S versus (1000/T) data of x=1.0−0.8 exhibit a two-slope behavior. The orthorhombic to cubic phase transition could be the possible reason for the high power factor values of BaBiO3.

We investigated phase separation and long-range atomic ordering in InGaN alloys grown on (11–20) sapphire substrates by molecular beam epitaxy (MBE). Compositional inhomogeneities were found in films grown at substrate temperatures ranging from 650 – 675°C when the indium content was larger than 20%. Upon annealing to 725°C, these films underwent phase separation, similar to films grown at 725° C.These InGaN films also showed the evidence of long-range atomic ordering, with the order parameter being a function of film composition and the growth parameters. A competition between ordering and phase separation has been observed, suggesting that the driving force for both phenomena is lattice strain in the alloy.

In situ cleaning of MOCVD-grown GaN/AlN/6H-SiC substrates using NH3-seeded supersonic molecular beams was investigated. Removal of surface carbon and oxygen contaminants was achieved by heating at 730°C under a hyperthermal NH 3 beam. Oxygen is removed primarily by thermal desorption; however, carbon removal requires an NH3 flux. Atomically smooth surfaces with regular steps are obtained after NH3 beam cleaning. Homoepitaxial growth of smooth, highly textured GaN films was accomplished at 700°C by employing a 0.61-eV NH3 beam and an effusive Ga source.

Films of AIN were grown on Si under vacuum pressure at 900°C and examined ex situ with infrared reflectance spectroscopy and microscopy, scanning electron microscopy, x-ray diffraction and rutherford backscattering spectroscopy. Collection of IR emission spectra for in situ characterization was successful and used to identify growth parameters during film growth.

The lack of GaN substrates is a limiting factor for the development of III-V nitride devices.Recently we proposed to use GaN/SiC epitaxial wafers, consisting of thin GaN layer depositedby hydride vapor phase epitaxy (HVPE) on SiC wafer, as substrates for subsequent growth of IIIVnitrides and devices development. These wafers are attractive to be used as substrates for GaNdevice fabrication because the GaN-based device structures can be grown on these wafers byhomoepitaxy without any buffer layer. Due to high SiC thermoconductivity and cleavagepossibility, these wafers are especially attractive for high-power electronic and optoelectronicapplications. In this paper, we focus on crystal structure, optical and electrical properties of GaNhomoepitaxial layers and p-n structures grown by HVPE on GaN/SiC epitaxial wafers. Newtypes of III-V nitride epitaxial wafers are described, insulating GaN/SiC epitaxial wafers andAIN/SiC epitaxial wafers.

The interaction of nitrogen with 6H-SiC surfaces with different orientation and composition ranging from Si-rich to graphitized was investigated by use of X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). The samples were cleaned and their surface composition was tuned by heating them in a Si flux at different temperatures or by annealing. Nitrogen exposures were performed using a RF plasma source operated at 13.56 MHz. Two different nitridation mechanisms can be distinguished by XPS. While anion exchange occurs at bulk-truncated SiC surfaces diffusion will control the nitrogen uptake of Si-rich samples. After nitridation, UPS spectra of bulk–truncated surfaces display characteristic valence-band structures of Si3N4. The thickness of the nitride layer amounts to approximately 9–12 Å. Thicker silicon nitride films on SiC are obtained by rising the sample temperature during nitrogen exposures or by deposition and subsequent nitridation of thin Si overlayers on SiC surfaces. The nitrogen uptake of graphitized samples is comparable to that of bulk-truncated surfaces. Our photoemission spectroscopy results prove that surface graphite is partly removed by N and the UPS spectra point to the formation of silicon nitride. In addition, C-N bonds exist but the exact bonding configuration cannot be resolved.

Si+ implant activation efficiencies above 90%, even at doses of 5×1015 cm−2, have been achieved in GaN by RTP at 1400–1500°C for 10 secs. The annealing system utilizes with MoSi2 heating elements capable of operation up to 1900 °C, producing high heating and cooling rates (up to 100 °C · s−1). Unencapsulated GaN show severe surface pitting at 1300 °C, and complete loss of the film by evaporation at 1400 °C. Dissociation of nitrogen from the surface is found to occur with an approximate activation energy of 3.8 eV for GaN (compared to 4.4 eV for AIN and 3.4 eV for InN). Encapsulation with either rf-magnetron reactively sputtered or MOMBE-grown AIN thin films provide protection against GaN surface degradation up to 1400 °C, where peak electron concentrations of ∼5×1020 cm-3 can be achieved in Si-implanted GaN. SIMS profiling showed little measurable redistribution of Si, suggesting Dsi ≤ 10-13 cm2 · s−1 at 1400 °C. The implant activation efficiency decreases at higher temperatures, which may result from SiGa to SiN site switching and resultant self-compensation.

The high energy implantation of boron into n-type Silicon Carbide epitaxial layers of the polytype 4H was investigated for various sample temperatures during implantation and different doping concentrations. Depth profiling using secondary ion mass spectrometry was applied to determine the boron distribution in the as-implanted epilayers and the re-distribution of boron atoms during a half hour heat treatment at a temperature of 1700 °C. The SIMS boron profiles are compared to numerical Monte Carlo calculations using the program TRIM96 and to electrical profiling by scanning capacitance microscopy.

In general we found that boron is moving towards the implantation damage already during the implantation of a profile with four different energies. The defect enhanced diffusion of boron is more pronounced during the high temperature anneal, where a higher concentration of boron diffusing towards the sample surface was observed. Into the bulk of the epilayer the boron diffusion is on a lower level and shows an exponential decay with a length of up to 5 Pm. An out-diffusion of boron during the post implantation anneal was not observed at implantation temperatures up to 500 °C.

Electrical profiling using scanning capacitance microscopy revealed that complicated layer structures with several pn-junctions can occur due to the re-distribution of boron during post implantation anneal.

Epitaxially grown GaN by metal organic chemical vapor deposition (MOCVD) on SiC were implanted with 100 keV Si+ (for n-type) and 80 keV Mg+ (for p-type) with various fluences from 1×1012 to 7×1015 ions/cm2 at liquid nitrogen temperature (LT), room temperature (RT), and 700 °C (HT). High temperature (1200 °C and 1500 °C) annealing was carried out after capping the GaN with epitaxial AIN by MOCVD to study damage recovery. Samples were capped by a layer of AIN in order to protect the GaN surface during annealing. Effects of implant temperature, damage and dopant activation are critically studied to evaluate a role of ion implantation in doping of GaN. The damage was studied by Rutherford Backscattering/Channeling, spectroscopic ellipsometry and photoluminescence. Results show dependence of radiation damage level on temperature of the substrate during implantation: implantations at elevated temperatures up to 550 °C decrease the lattice disorder; “hot implants” above 550 °C can not be useful in doping of GaN due to nitrogen loss from the surface. SE measurements have indicated very high sensitivity to the implantation damage. PL measurements at LT of 80 keV Mg+ (5×1014 cm-2) implanted and annealed GaN showed two peaks : one ∼100 meV and another ∼140 meV away from the band edge.

6H silicon carbide wafers were implanted with 40–50 keV aluminum ions to a dose of 1.5 × 1014 – 1.5 × 1016 cm−2 at high temperatures (1100°C–1700°C). The substrate temperature and the implantation dose were varied to investigate the influence of the implantation parameters on the formation of structural defects. Conventional transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) techniques were applied to study the defects. We found that for low dose implants {0001} interstitial dislocation loops are formed but for high dose implants aluminum precipitates associated with {0001} half-loops are formed.

GaN etching can be affected by a wide variety of parameters including plasma chemistry and plasma density. Chlorine-based plasmas have been the most widely used plasma chemistries to etch GaN due to the high volatility of the GaClx and NClx etch products. The source of Cl and the addition of secondary gases can dramatically influence the etch characteristics primarily due to their effect on the concentration of reactive Cl generated in the plasma. In addition, high-density plasma etch systems have yielded high quality etching of GaN due to plasma densities which are 2 to 4 orders of magnitude higher than reactive ion etch (RIE) plasma systems. The high plasma densities enhance the bond breaking efficiency of the GaN, the formation of volatile etch products, and the sputter desorption of the etch products from the surface. In this study, we report GaN etch results for a high-density inductively coupled plasma (ICP) as a function of BCl3:Cl2 flow ratio, dc-bias, chamber pressure, and ICP source power. GaN etch rates ranging from ∼100 Å/min to > 8000 Å/min were obtained with smooth etch morphology and anisotropic profiles.

We report on the dry etching damage effects on the Ohmic contact characteristics of Sidoped n-type GaN. The 2-µm thick n-type GaN samples whose carrier concentrations are ∼7 × 1017/cm3 are grown on c-plane sapphire substrates by metalorganic chemical vapor deposition(MOCVD). The samples are processed to measure the transmission line method(TLM). We evaporated Ti/Al/Ni/Au=150/2700/400/4000 Å on the samples using an ebeam evaporator under 5 × 10−7 torr and annealed at 700 °C for lminute using a rapid thermal annealer(RTA). AES and XPS data show that the surface stoichiometry of the sample etched with H2 gas is gallium- richer than that of the one etched with Cl2 gas. According to the previous report[l], N-vacancy acts as shallow donor in GaN. However, specific contact resistivity of the sample etched with H2 gas is 3.03 × 10−6 Ω cm 2, which is much higher than that of the sample etched with C12 gas, 7.9 × 10 −7 Ω cm2. The difference of the N- vacancy formed during the etching process cannot completely explain the result. We suggest that formation of GaxOy during etching process, and the weakening of the bonding by ion bombardments might be responsible for the difference in the ohmic characteristics.