Growth of GaN on 4H- or 6H-SiC (0001) Si-face substrates with various misorientation angles and directions is presented. GaN layers were directly grown on the SiC substrates by molecular-beam epitaxy using elemental Ga and rf plasma-excited active nitrogen. First, 4H-SiC (0001) 8°-off toward the [11–20] direction was investigated. Before the growth of GaN, ex-situ high-temperature gas etching was carried out, resulting in a smooth SiC initial surface. However, the surface of the subsequently grown GaN layer has wavy features with peak-to-valley height of 30 nm. Since the direction of the undulations is parallel to the misorientation direction, this feature must originate from the substrate misorientation. Step bunching and large faceting along <01–10> and <10–10> directions occurred during the growth of GaN. Lowering the growth temperature suppresses large faceting, and results in reduction of the peak-to-valley height to 3 nm. However, the surface still has the same undulating features on a smaller length scale. On the other hand such morphology was not observed for GaN grown on (0001) on-axis SiC substrates (misorientation < 0.3°). The influence of the polytype of the SiC substrate and the misorientation angle and direction are also discussed.

Praseodymium (Pr) doped aluminum nitride (AlN), gallium nitride (GaN) and boron nitride (BN) thin films deposited on Si (111) substrate are studied with cathodoluminescence. AlN:Pr and GaN:Pr films are deposited at 77 K and room temperature respectively while BN:Pr films at 750 K by reactive sputtering, using 100–200 Watts RF power, 5–10 mTorr nitrogen. Metal targets of Al and B with Pr and a liquid target of Ga with solid Pr are used. The dominant peaks observed in the visible range result from 3P0 → 3H4, 3P1→ 3H5, and 3P0 → 3F2 transitions in AlN:Pr, 3P0 → 3H4, 3P0 → 3H6, and 3P0 → 3F2 transitions in GaN:Pr and from 3P0 → 3H4, 3P1→ 3H5, 3P0 → 3H6, and 3P0 → 3F2 transitions in BN:Pr. Additional peaks are observed from AlN:Pr at 335 nm and 385 nm from 1S0 → 1D2 and 1S0 → 1I6 which are not observed in GaN:Pr and BN:Pr films.

We present micro-photoluminescence measurements on unstructured InGaN/GaN quantum well samples. Single sharp emission lines were observed and their optical properties were studied as a function of temperature and excitation density. The experimental findings such as the independence of their spectral position on the excitation density and the observation of binding and antibinding multiexcitonic states give clear evidence for the existence of strong localization centers in the InGaN quantum well, which exhibit the same characteristics as they are known for quantum dot structures.

We study the transport properties of miniband electrons in an electromagnetic field as related to the high-frequency generation in a semiconductor superlattice (SL). We study the regimes where the external ac-field is in resonance and out of resonance with the two lowest SL minibands.

In the diffusive quasi-static regime Bloch oscillations and electrical domains do not develop and high-frequency generation may occur as a result of the negative high-harmonic nonlinear conductivity. This regime is different from the frequency conversion in a stable nonlinear media. We point out that AlGaN/GaN material system has a great potential for the high-power SL terahertz (THz) source.

Electrical properties of As, Si, and [As+Si] doped GaN films grown on sapphire substrates by low temperature metalorganic chemical vapor deposition have been investigated using temperature dependent Hall-effect and deep level transient spectroscopy measurements. The Hall measurements from the GaN layers show that the concentration decreases with arsine flow (4, 40, and 400 sccm) at all temperatures. The carrier concentration of the Si-doped GaN, on the other hand, increases with the incorporation of arsine flow. This behavior is attributed to the formation of AsGa antisites which act as double donors. A deep level at around 0.82 eV below the conduction in the band gap of As doped GaN is measured by DLTS and is tentatively assigned to arsenic on gallium antisite.

Ongoing efforts of growing large AlN single crystals at NCSU using an induction-heated, high-temperature reactor are based on (1) engineered expansion of single crystalline grains with increasing boule length, as well as (2) the development of a growth process that enables seeded growth on AlN surfaces previously exposed to air. The growth process is based on physical vapor transport (PVT), where AlN powder is sublimed in a high purity nitrogen atmosphere. The growth temperature was typically in the range of 2250 to 2300 °C. In this study, tungsten crucibles were used in combination with graphite insulation and were found to be durable for AlN growth. Boule growth was interrupted several times in order to refill the AlN powder source and the growth surface was subjected to surface preparation to facilitate epitaxial re-growth. Grain expansion was studied as a function of process parameters. Crystalline quality of large single crystalline grains was correlated with their surface morphology.

Dilute Magnetic Semiconductors (DMS's) posses a strong potential to make use of the spin of carriers in spintronic devices. Experimental results and theoretical calculations predict that GaN:Mn is a potential semiconductor material for spintronic device applications. The dependence of the room temperature ferromagnetic properties of GaN:Mn/GaN:Mg double heterostructures (DHS) on the Fermi level position in the crystal is demonstrated. Several GaN:Mn/GaN:Mg DHS are grown by metal organic chemical vapor deposition on sapphire. It is shown that initially paramagnetic films can be rendered ferromagnetic by facilitating carrier transfer through the GaN:Mn/GaN:Mg interface. Additionally, it is demonstrated that ferromagnetism depends on the thickness of the GaN:Mn and GaN:Mg layers. The carrier transfer process essentially changes the Fermi level position in the crystal. By choosing the right thicknesses for GaN:Mn and GaN:Mg an optimum DHS that exhibits room temperature ferromagnetism is grown. An identical structure, with the exception of insertion of an AlGaN barrier in order to obstruct the carrier transfer at the interface, results in paramagnetic films for AlGaN barriers thicker than 25nm. These results are explained based on the change in the occupancy of the 3d-Mn impurity band, and indicate that carrier mediation is the possible mechanism for the ferromagnetism observed in the MOCVD grown GaN:Mn material system. This is the first evidence that this material system responds to electronic perturbations, hence ferromagnetism observed is not due to secondary phases or spin glass behavior.

Highly doped p-type GaN was grown by MOCVD with a Mg source for use in tunnel junctions for dual wavelength light emitting diodes. Highly doped p-type material (p+) is necessary for creating efficient tunnel junctions that act as buried current spreading layers and provide high quality contacts to p-type GaN. Molar flow ratios and growth temperature were varied to achieve maximum doping concentration. A range of post-growth annealing conditions to activate incorporated Mg was investigated. Secondary Ion Mass Spectrometry (SIMS) and Hall effect were used to measure Mg incorporation and carrier concentration, respectively. The maximum carrier concentration achieved employed a hybrid growth scheme that achieved high carrier concentration and high quality material by alternating layers of high and low growth temperature material. Atmospheric annealing was compared to standard RTA under N2 ambient. Successful GaN tunnel junctions were grown using the hybrid p-type doping scheme developed in this work. I-V measurements were taken on these devices to evaluate the effectiveness of the devices.

Optimization of GaN channel conductivity in AlGaN/GaN Heterojunction Field Effect Transistor (HFET) structures was performed using High Resistivity (HR) GaN templates grown by Metal-organic Vapor Phase Epitaxy (MOVPE). The GaN sheet resistance was tuned using final nucleation layer (NL) annealing temperature. Using an annealing temperature of 1033°C, GaN with sheet resistance of 10 Ω/sq was achieved, comparable to that of Fe-doped GaN. X-Ray Diffraction (XRD) and Photoluminescence (PL) analysis show that the high resistance GaN is achieved due to compensating acceptor levels introduced through edge-type threading dislocations. XRD analysis also shows optimization of annealing temperature provided a means to maximize GaN sheet resistance without significantly degrading material quality. AlGaN/GaN HFET layers grown using HR GaN templates gave surface and interface roughness of 14 and 7 Å, respectively. The 2DEG Hall mobility and sheet charge of HFETs grown using HR GaN templates was comparable to similar layers grown using unintentionally doped (UID) GaN templates.

Low-frequency noise measurements were performed on a number of AlGaN/GaN HEMTs with different gate recess depths, which were formed by dry etching. Detailed characterizations of the low-frequency noise properties were performed on the devices as a function of as a function of hot-electron stressing conducted at VD = 10 V and VG = -1.5 V. The room temperature voltage noise power spectral density, SV(ƒ), of the devices were found to show 1/ƒ dependence. A comparison of SV(ƒ) measured from different devices clearly indicate increase in the noise levels for the devices with large recess depths, reflecting the degradation caused by ion-impact induced damage during recess formation. Furthermore, the results of low-frequency noise measurements showed fast degradations for the devices with larger gate recess depths. Our experimental data clearly show that the dry etching process has induced damages in gates.

The heteroepitaxial growth of a GaN single crystal by metal-organic vapor phase epitaxy on a 4H-SiC (3038) substrate was demonstrated. The crystallographic orientation of GaN was found to be dependent on growth pressure. When the growth pressure was 1000 hPa, the orientation of the GaN single crystal was consistent with that of the SiC substrate, where the c-plane of the GaN was single crystal tilted 54.7° from the surface plane. Then, we fabricated a violet-light-emitting diode (LED) with a GaInN multiple-quantum-well (QW) active layer grown on the GaN layer, which coherently grew on the 4H-SiC (3038 ) substrate. The blue shift of the peak wavelength with increasing injection current of up to 100 mA was confirmed to be two times smaller than that of a conventional LED on a c-plane sapphire substrate due to a low internal polarization.

The aim of this contribution is to determine theoretically the maximum mobility that can be expected in AlGaN/GaN quantum wells as soon as the free carriers are only submitted to intrinsic scattering mechanisms associated with phonons and the carrier-carrier interaction. In our model, we consider that the carrier-carrier two bodies collisions do not constitute by themselves a relaxation mechanism since they conserve the momentum and the energy of the electron system. Thus, we assume that the free carriers act only through their contribution to the dynamical dielectric response of the material and, at least, through their collective behavior resulting into plasmons which, when damped, constitute now a real relaxation mechanism. The full scattering strength is connected with the imaginary part of the total reversed dielectric function including the lattice and the free carrier contributions. This approach automatically includes the scattering mechanisms associated with hybrid phonon/plasmon particles.

This paper reports the impact of the Mn incorporation on the structural and magnetic properties of Ga1−xMnxN on the metal-organic vapor phase deposition (MOCVD). Crystalline quality and phase purity were determined by high-resolution X-ray diffraction and indicated that no macroscopic second phases are formed during growth. Atomic force microscopy revealed a 2-dimensional MOCVD step-flow growth pattern in the Mn-incorporated samples. Various annealing steps were applied to some of the samples to reduce compensating defects and to investigate the effects of post processing on the growth. SQUID measurements showed an apparent ferromagnetic hysteresis behavior. However, none of the requirements for room temperature ferromagnetism in the prevailing mean field DMS theories were found. Therefore, different origins of the ferromagnetic signal are discussed.

The Raman modes of ZnO crystallites of the wurtzite structure were investigated via micro-Raman scattering at resonant and out-of-resonant conditions. The E2 mode was the predominant mode in the spectra for out-of-resonant conditions. For the resonant conditions one mode at the spectral range of the LO's of ZnO single crystal was the predominant mode: its frequency was found to be at ∼ 580 cm−1. The A1(LO) and the E1(LO) modes of a reference ZnO single crystal were found to be 568 cm−1 and 586 cm−1 respectively. Two possible mechanisms were considered that may explain the mode frequency of the crystallites: it can be regarded as a confined E1(LO) mode or as a quasi-LO mode. The UV-photoluminescence of the crystallites was found to have the same energy as that of the single crystal ∼ 3.3 eV, indicating the lack of size-confinement of the electronic states in the crystallites, and inter alia that of the phonon states. Our analysis indicated that the observed frequency can be explained in terms of Loudon's model of a quasi-mode behavior which is due to the crystallites tilt.

The junction temperature of AlGaN/GaN ultraviolet (UV) Light-Emitting Diodes (LEDs) emitting at 295 nm is measured by using the temperature coefficients of the diode forward voltage and emission peak energy. The high-energy slope of the spectrum is explored to measure the carrier temperature. A linear relation between junction temperature and current is found. Analysis of the experimental methods reveals that the diode-forward voltage is the most accurate method (± 3 °C). A theoretical model for the dependence of the diode junction voltage (Vj) on junction temperature (T) is developed that takes into account the temperature dependence of the energy gap. A thermal resistance of 87.6 K/W is obtained with the AlGaN/GaN LED sample mounted with thermal paste on a heat sink.

Single crystals of AlN have been grown by seeded (on SiC substrates) and unseeded (spontaneous nucleation) sublimation techniques. Tantalum carbide coated graphite crucibles were used. Temperature gradient and source-substrate distance have been found to be the most influencing parameters of crystal growth. AlN crystals of maximum dimension 12 mm (length) × 10 mm (width) × 300 μm (thickness) were grown on 6H-SiC substrates and the best (0002) oriented crystal showed an XRD omega rocking curve FWHM of 4.81 arcmin. AlN nucleated as independent hexagonal islands and coalesced as growth progressed on. Growth rate of AlN grown on C-face SiC has been found to be higher than that on Si-face SiC. Tantalum carbide coated crucibles have been found to be better suited for AlN growth as the impurity incorporation in to the crystals due to crucible was very less. Spontaneously nucleated crystals exhibited an incompleted pyramid-like structure with (1010) and (1100) as their prominent faces.

Materials studies of high Al-content (> 30%) AlGaN epilayers and the performance of AlGaN-based LEDs with emission wavelengths shorter than 300 nm are reported. N-type AlGaN films with Al compositions greater than 30% reveal a reduction in conductivity with increasing Al composition. The reduction of threading dislocation density from the 1–5 × 1010 cm-2 range to the 6–9 × 109cm-2 range results in an improvement of electrical conductivity and Al0.90Ga0.10N films with n= 1.6e17 cm-3 and μ=20 cm2/Vs have been achieved. The design, fabrication and packaging of flip-chip bonded deep UV LEDs is described. Large area (1 mm × 1 mm) LED structures with interdigitated contacts demonstrate output powers of 2.25 mW at 297 nm and 1.3 mW at 276 nm when operated under DC current. 300 μm × 300 μm LEDs emitting at 295 nm and operated at 20 mA DC have demonstrated less than 50% drop in output power after more than 2400 hours of operation. Optimization of the electron block layer in 274 nm LED structures has enabled a significant reduction in deep level emission bands, and a peak quantum well to deep level ratio of 700:1 has been achieved for 300 μm × 300 μm LEDs operated at 100 mA DC. Shorter wavelength LED designs are described, and LEDs emitting at 260 nm, 254nm and 237 nm are reported.

Passivation of AlGaN/GaN HEMT by SiN has been shown by several authors to improve device performance. In this paper we demonstrate a new method of passivating the top AlGaN layer, that leads to a drastic improvement of the device characteristics. On top of the AlGaN/GaN HEMT layer structures, a thin SiN layer is grown at high temperature in-situ, i.e. in the MOCVD reactor prior to unloading the HEMT epiwafers. The quality of the SiN is shown to be very high. As the AlGaN surface is protected against any air contamination, ohmic contacts deposited directly on the SiN layer exhibit better properties than on uncapped AlGaN layers. Transistor performance increases impressively: the source-drain dc current, for a positive gate voltage of 2 V, increases from 0.5 A/mm to 1.2 A/mm for similar AlGaN/GaN HEMT structures by depositing SiN in-situ. RF characteristics also show some improvement. The positive impact of in-situ SiN layer has different origins, some being intrinsically linked: surface protection against oxide formation, better ohmic contact formation, passivation of the surface states, reduction of AlGaN relaxation. All this leads to an increase of the 2DEG carrier concentration.

For nitride based devices such as LEDs, high power FETs and laser diodes, single crystal substrates of AlN are highly desirable. While the sublimation technique is suitable for growing bulk AlN crystals, appropriate seeds are also necessary for growing large diameter oriented boules. 4H- and 6H-SiC substrates which are readily available commercially can potentially be implemented as seeds for bulk AlN growth. However, issues regarding SiC decomposition at high temperatures, thermal expansion mismatch, single crystal growth, etc. need to be addressed. Towards this end, a series of growth experiments have been carried out in a resistively heated reactor using on and off-axis 4H- and 6H-SiC substrates as seeds for AlN growth from the vapor phase. Several hundred microns thick AlN layers have been grown under different growth conditions. Synchrotron white beam x-ray topography (SWBXT) has been used to map the defect distribution in the grown layers and high resolution triple axis x-ray diffraction (HRTXD) experiments were carried out to record reciprocal space maps from which tilt, mismatch and strain data can be obtained. These results are analyzed with respect to the growth conditions in order to gain a better understanding of this growth process.

An effective gate insulator for compound semiconductors has been a challenging goal for the materials research community for nearly 40 years. Recent developments on the epitaxial deposition of complex gate oxides as gate insulators have shown promise with the demonstration of enhancement mode high electron mobility transistors (e-mode HEMTs). In this work, gate oxide epilayers deposited on III-V semiconductors for field effect transistors (III-V MOSFETs) are examined using transmission electron microscopy (TEM) to identify the structure of the oxide/semiconductor interface. The high resolution images of the cross-sectional structures for the first time reveal a crystalline nature of the interface between the oxide and the III-V semiconductor. The composition of the oxide layers are determined by Z-contrast Electron Energy Loss Spectroscopy (EELS). The surface morphology of the FET structures is investigated by atomic force microscopy (AFM) both before and after gate oxide deposition, and the structural results are related to device DC electrical characteristics. With an underlying GaN/InGaN heterojunction grown by metal-organic chemical vapor deposition (MOCVD) on sapphire, the MOSFET devices exhibit the characteristics of a substantially unpinned interface, including the capacity for significant charge accumulation and transconductance at positive gate voltages.