We have calculated the change of interband absorption spectra of a quantum well based on hexagonal group-III nitride semiconductors under photo-injection of high densities of electron-hole pairs. The screening of internal electric fields by such optical excitation is known to blue-shift and reinforce the ground-state optical transition. Due to the large values of densities of states and of internal fields, we predict novel properties that rather concern optical absorption via transitions between excited states. The absorption coefficient can be strongly enhanced by the optical excitation itself, in this particular spectral region, yielding the possibility for self-induced absorption properties. In other words, if sufficiently intense, an excitation laser can increase the absorption coefficient of the system at its own wavelength, thus providing a strong nonlinear optical response. Finally, we briefly discuss the potential application of these optical phenomena.

We have systematically studied the effects of wet chemical etching and high temperature nitridation on the resulting sapphire surface morphology and chemical transformation. The etching of c-plane sapphire substrates using H2SO4, H3PO4, and a 3:1 H2SO4 :H3PO4 mixture as a function of temperature and etching time was studied and compared with H2 etching at 1100°C and air-annealing at 1400°C. The surface nitridation using NH3 and N2 at 1100°C was studied as a function of NH3 concentration, nitridation time, and surface pretreatment. Atomic force microscopy and x-ray photoelectron spectroscopy were used to study the surface morphology and chemical composition. The detailed surface morphology after chemical etching was a function of the chemical composition and the specific time and temperatures. The smoothest, pit-free sapphire surface was obtained by etching in pure H2SO4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al2(SO4)3 and Al2(SO4)3·17H2O crystalline deposits on the surface. Phosphoric acid and the 3:1 H2SO4:H3PO4 mixture etched the sapphire preferentially at defect sites and resulted in pits formation on the surface. The high temperature sapphire nitridation resulted in nitrogen incorporation into the surface. The nitrogen content of nitridation layer depends on NH3 concentration, nitridation time, and surface pretreatment. The nitrogen contents of sapphire treated with H2SO4 and 3:1 H2SO4:H3PO4 are about the same as the as-received sapphire. While the nitrogen content of the air-annealed sapphire is ∼1.6 times higher then the nitrogen content of the as-received sapphire.

Sharp intense peaks are sometimes detected in the low-temperature photoluminescence (PL) spectrum of undoped GaN samples in the photon energy range of 3.0 – 3.46 eV. Some of these peaks can be attributed to excitons bound to dislocations and inversion domains, whereas some others originate from the GaN surface because they can be affected essentially by surface treatment. In our samples, grown by molecular beam epitaxy on sapphire substrate, the 3.42 eV peak always disappeared after removing the surface layer by etching for a few seconds in hot phosphoric acid. Atomic force microscopy images confirmed that such light etching modifies the surface morphology, although the etched depth is negligibly small. Moreover, intensities of two other peaks (at 3.32 and 3.35 eV) were observed to depend on sample etching, as well as on the length of subsequent exposure to air. The 3.32 and 3.35 eV peaks evolved with time of UV illumination, increasing by several times and demonstrating memory effect at low temperature. We attribute the 3.42 and 3.35 eV peaks to bound excitons, whereas the 3.32 eV peak is tentatively attributed to a surface donor-acceptor pair transition.

The magnitudes and evolutions of two photoinduced absorption nonlinearities, absorption bleaching and field screening, were compared and investigated by employing electroabsorption and femtosecond pump-probe spectroscopy in a biased InGaN/GaN p-i-n double heterostructure. Steady state electroabsorption measurements indicate the field induced absorption coefficient changes in this structure are caused by the Franz-Keldysh effect. The temporal resolution of the absorption bleaching spectra suggests that the photoinduced carriers rapidly relaxed to the InGaN band edge within several picoseconds. As the applied reverse bias field was increased, the transition of the differential absorption spectral signature from the signature for absorption bleaching to the signature for field screening was observed. The magnitude of the change in absorption due to photoinduced carrier screening of the applied field is quantified and compared to absorption bleaching.

The result of first-principles density functional calculations of the bulk modulus and related structural and electronic properties of the total 25 group III-V binary phases with zinc-blende and wurtzite structures are presented. The behavior of energy band structure variation under high pressures is also studied. It is found that the bulk modulus is more sensitive to the local atom configuration than the lattice structure. The crystallographic geometry plays an important role in the electronic property of these phases.

A contacted electroreflectance technique was used to characterize the electronic properties of AlGaN/GaN heterostructure field-effect transistors (HFETs). By studying variations in the electroreflectance with applied electric field, spectral features associated with the AlGaN barrier, the 2-dimensional electron gas at the interface, and bulk GaN were observed. Barrier-layer composition and electric field were determined from the AlGaN Franz-Keldysh oscillations. Comparing HFETs grown on SiC and sapphire substrates, the measured AlGaN polarization electric field (0.25±0.05 MV/cm) approached that predicted by a standard model (0.33 MV/cm) for the higher mobility HFET grown on SiC.

Epitaxial growth of quaternary AlGalnN compounds by plasma-assisted molecular beam epitaxy has been demonstrated. Two-dimensional growth is achieved under In excess, with a monolayer of In segregating at the growth front. The maximum In incorporation is significantly affected by the substrate temperature and the Al mole fraction of the alloy. This behavior has been attributed to the enhancement of In segregation due to the high binding energy of A1N compared to InN and GaN.

Recombination dynamics in a variety of InGaN/GaN quantum systems has been studied by time resolved photoluminescence (PL). We have discovered that the time-decay of PL exhibits a scaling law: the nonexponential shape of this decay is preserved for quantum wells and quantum boxes of various sizes while their decay time varies over several orders of magnitude. To explain these results, we propose an original model for electron-hole pair recombination in these systems, combining the effects of internal electric fields and of carrier localization on a nanometer-scale. These two intricate effects imply a separate localization of electrons and holes. Such a microscopic description accounts very well for both the decays shape and the scaling law.

The effects of SiC surface treatment on the lattice relaxation of AlN buffer layers and the crystalline quality of GaN layers grown on the buffer layers were studied. AlN buffer layers and GaN main layers were grown by plasma-assisted molecular-beam epitaxy on on-axis 6H-SiC (0001)Si substrates. High-temperature HCl-gas etching resulted in an atomically flat SiC surface with (√3×√3)R30° surface reconstruction, while HCl-gas etching followed by HF chemical treatment resulted in an atomically flat surface with (1×1) structure. The AlN layer grown on the (1×1) surface showed slower lattice relaxation. GaN grown on the AlN buffer layer exhibited a (0002) X-ray rocking curve of 70 arcsec and 107 cm−2 of screw-type dislocation density, which was superior than that of GaN grown on (√3×√3)R30° surface.

In this paper, we study the surfactant capability of In for the growth of AlGaN/GaN heterostructures by plasma-assisted molecular beam epitaxy. Growth conditions were determined to have a self-regulated 1×1 In adlayer on AlxGa1-xN (0001). The presence of this In film favors two dimensional growth of AlGaN under stoichiometric conditions, and inhibits the formation of metal droplets on the surface. The quality of these layers was assessed by high resolution X-ray diffraction, atomic force microscopy and photoluminescence.

A new technique is presented that employs luminescence downconversion using an ultrashort gating pulse to enable the characterization of UV light emission from III-nitride semiconductors with subpicosecond temporal resolution. This technique also allows one to measure PL rise times and fast components of multiple decays in the subsequent time evolution of the PL intensity. Comparison of luminescence emission intensity and lifetime in GaN and AlGaN with ∼0.1 Al content grown homoepitaxially on GaN templates with the same quantities measured in heteroepitaxial layers grown on sapphire indicate significant improvement in the homoepitaxial layers due to reduction in dislocation density. Fast (<15 ps) initial decays in the AlGaN are attributed to localization associated with alloy fluctuations and subsequent recombination through gap states.

The large defect densities in heteroepitaxially grown group-III-nitride layers on sapphire or SiC cannot be tolerated in applications such as lasers. We report here on a defect reduction by overgrowth of patterned n-6H-SiC(0001)surfaces.

First, we formed mesa structures in the windows of metal masks and then after removal of the masks layers of AlxGa1–xN and GaN were grown by low-pressure MOVPE under conditions of high lateral growth rates. We demonstrate that layers and layered structures can be grown with smooth surfaces and reduced defect densities.

Hydrogen removal from Mg-doped GaN is necessary to activate p-type conductivity, but the exact chemical process is not yet clear. We have investigated this issue by monitoring the intensity of an electron paramagnetic resonance (EPR) signal attributed to Mg through a series of isochronal and isothermal anneals between 600 and 1000 °C. Measurements made on GaN:Mg epitaxial layers deposited on SiC and annealed between 700 and 850 °C indicate that the Mg-related EPR signal increases with temperature as expected for depassivation of a Mg complex by removal of hydrogen. However, data obtained outside this temperature range suggest that additional processes may occur. For example, as-deposited films contain a signal resembling the Mg acceptor that is quenched by a 650 °C N2 anneal. Also, for all samples, N2 annealing at T>850 °C irreversibly decreases the signal thought to be due to Mg. Although the presence of the signal in the as-grown films is not fully understood, the effects observed at T>850 °C may be attributed to preferential N-desorption from Mg-N-H complexes.

In this paper, threading dislocation densities in GaN and AlGaN epitaxial layers have been evaluated using two different X-ray analysis techniques; a Williamson Hall (WH) plot and reciprocal space mapping (RSM). GaN and AlGaN have crystalline growth composed of columnar structures that can be estimated by coherence length and angular misorientation measured by X-ray. A WH plot can provide information about coherence length and tilt angle from a linear fit to the linewidth of the triple axis rocking curve (000l) symmetric reflections. RSM is typically used to obtain this data, but it is more involved in technique. The two dominant components of threading dislocation densities (screw and edge types) in the GaN and AlGaN epitaxial layers were found to be similar by both techniques. The treading dislocation density correlates to the size of columnar structure as determined by coherence length, tilt angle, and twist angle. The effect of Al composition in AlGaN alloys on these dislocation densities was investigated and found to depend on strongly on the type of nucleation layer, GaN or AlN.

Semi-insulating wurtzite GaN:C of high optical quality is obtained with CCl4 or CS2 doping sources in plasma-assisted molecular-beam epitaxy in Ga-rich growth conditions. The highest resistivity (107 Ω-cm) is found for [C] in the low 1018 cm−3 range. An increasing fraction of carbon appears to form electrically inactive pair defects for higher doping levels causing the concentration of uncompensated residual donors to be higher in films with [C] in the 1019 cm−3 range compared with [C] in the 1018 cm−3 range. Blue (2.9 eV) and yellow (2.2 eV) luminescence bands are associated with carbon-related defects, and additional support is provided for the association of the blue luminescence with the carbon-acceptor deactivating pair defect. Finally, the temperature dependence of the resistivity is described within the grain-boundary controlled transport model of Salzman et al., Appl. Phys. Lett. 76, 1431 (2000).

Utilizing plasma source molecular beam epitaxy (PSMBE), we have grown epitaxial InxAl1-xN films on (0001) sapphire substrates; the indium concentration, x, varied from 0 to 1. The atomic force microscopy of the films reveals characteristic surface patterns of nanometer scale. The feature size distribution is determined by the film composition and thickness. Both absorption and reflection spectra of the films have additional peaks below the fundamental absorption threshold. These peaks cannot be associated with N vacancies or any other known crystal defects and impurities. We attribute the peaks to electron confinement in the hillocks of the lateral structure by the strong electric field of piezoelectric and spontaneous polarization that is characteristic to nitride semiconductor compounds. The calculated values of the electron energy levels are in good agreement with the spectroscopic data; moreover, the electron confinement model explains the observed temperature dependence of the additional peaks. The hillock size control will allow one to control the optical and transport properties of the films.

New ultraviolet (UV) light emitting device structures address the problems of small carrier concentrations and large band-offsets in wide bandgap Aluminum Gallium Nitride (AlGaN) heterostructures through the use of graded epilayers for electron and hole injection. For light emission at 280–290 nm, a multiple-quantum-well separate confinement heterostructure (MQWSCH) employs a graded AlGaN structure for the injection of majority carriers from the metal-semiconductor contact layers into the spacecharge region of the pn-junction with a higher bandgap energy. Sample LED mesa devices were fabricated and have shown light emission of 289 nm under a forward bias of 12V (20mA). These results provide a ‘proof-of-concept’ for this new graded device structure which can be employed for the development of both UV-LEDs and laser diodes.

Waveguide prism-coupling methods were used to measure the ordinary and extraordinary refractive indices of AlxGa1-xN films grown on sapphire substrates by HVPE and MOCVD. Several discrete wavelengths ranging from 442 nm to 1064 nm were used and the results were fit to one-term Sellmeier equations. The maximum standard uncertainty in the refractive index measurements was ± 0.005 and the maximum standard uncertainty in the self-consistent calculation for film thickness was ± 15 nm. Analysis of normal-incidence spectroscopic transmittance and reflectance measurements, correlated with the prism-coupling results, was used to determine the ordinary refractive index as a continuous function of wavelength from the band gap wavelength of each sample (between 252 nm and 364 nm) to 2500 nm. The Al compositions of the samples were determined using energy-dispersive X-ray spectroscopy analysis (EDS). HVPE grown samples had compositions x = 0.279, 0.363, 0.593, and 0.657. MOCVD samples had x = 0.00, 0.419, 0.507, 0.618, 0.660, and 0.666. The maximum standard uncertainty in the absolute EDS-determined value for x was ± 0.02.

The surface acoustic wave velocities, electromechanical coupling coefficients, and the spatial distributions of both elastic displacement and electric potential have been calculated for various configurations of gallium nitride and aluminum nitride. The electromechanical coupling coefficient values of 0.13 % in GaN and 0.29 % in AlN have been predicted. The maximum electromechanical coupling coefficient values of 0.24 % at Euler angles (0, 54°, 90°) in GaN and 1.08 % at (0, 53°, 90°) in AlN have been found. For GaN layer-on- sapphire substrate structures, the SAW velocity and electromechanical coupling coefficient have been calculated as functions of layer thickness and acoustic wavelength. The experimentally measured values of the surface acoustic wave velocity and electromechanical coupling coefficient are in satisfactory agreement with the calculation results.

We have performed electroreflectance and photoreflectance studies of Pt/GaN Schottky diodes with Ga- and N-face polarity as well as AlGaN/GaN based transistor heterostructures. The experimental data were analyzed using electric field-dependent dielectric functions of GaN and AlGaN. Inhomogeneities in the electric fields were taken into account by application of a multi-layer formalism. We observed an increase of the electric field strength underneath the Schottky contact and in the AlGaN barrier with increasing temperature. The results are explained in terms of temperature dependent densities of ionized impurities and surface charges.