A rate equation model is developed to investigate the plasma assisted MBE growth of GaN in the presence of a fractional monolayer of Mg. Four distinct cases were identified and modeled - (i) Ga-limited regime (ii) Low N-limited regime (iii) Medium N-limited regime and (iv) High N-limited regime. In the model, it is assumed that Ga arriving on a Mg site undergoes faster incorporation into the epilayer through an exchange reaction compared to Ga arriving directly on a N surface. Additionally the incorporation rate of Ga was assumed to depend on the size of the Ga cluster. The results of the model are in good agreement with that of experiments. The non-monotonic behavior of growth rate with Ga flux for moderate Mg coverage is explained based on the incorporation rate dependence of Ga on the cluster size.

Gallium nitride films are grown by plasma-assisted molecular beam epitaxy (MBE) on 6H-SiC(0001) substrates with no miscut and with 3.5° miscuts in both the [1 0 0] and [1 1 0] directions. The hydrogen-etched substrates display straight or chevron shaped steps, respectively, and the same morphology is observed on the GaN films. X-ray rocking curves display substantially reduced width for films on the vicinal substrates compared to singular substrates, for the same Ga/N flux ratio used during growth.

A review of surface structures of bare and adsorbate-covered GaN (0001) and (000) surfaces is presented, including results for In, Mg, Si, and H adsorbates. Emphasis is given to direct determination of surface structure employing experimental techniques such as scanning tunneling microscopy, electron diffraction, and Auger electron spectroscopy, and utilizing first principles computations of the total energy of various structural models. Different surface stoichiometries are studied experimentally by varying the surface preparation conditions (e.g. Ga-rich compared to N-rich), and the stoichiometry is included in the theory by performing calculations for various chemical potentials of the constituent atoms. Based on the work reviewed here, surface reconstructions for plasma-assisted molecular beam epitaxy growth of GaN (0001) and (000) surfaces are fairly well understood, but reconstructions for reactive molecular beam epitaxy or for metal-organic vapor phase epitaxy (both involving H, at moderate and high temperatures, respectively) are less well understood at present.

Anisotropic chemical etching is an important means for characterizing the polarity and defect density of single crystals. In this letter, we present the results of our studies on the etching of bulk AlN crystals in aqueous potassium hydroxide solution. The nitrogen polarity (0001) basal plane initially etched rapidly, while the aluminum polarity basal plane, and prismatic (100) planes were not etched. The etch rate of the nitrogen polarity basal plane eventually decreased to zero, as the surface became completely covered with hexagonal hillocks which were bounded by {101} planes. The hillock density for the self-seeded AlN crystals studied was typically in the range of 5×107cm −2 to 109cm −2. From our analysis of etched AlN crystals, we infer that freely nucleated crystals predominately have the nitrogen to aluminum direction pointing out from the nucleation surface, that is the ends of the AlN crystals facing the source are aluminum polarity.

High quality epitaxial films of AlxGa1−xN, grown on SiC substrates, were investigated using spatially resolved cathodoluminescence (CL), scanning electron microscopy, and atomic force microscopy. A variation in the observed peak energy position of the CL was related to alloy fluctuations. CL was used to reveal relative alloy fluctuations of approximately 1% on a sub-micrometer scale, with a precision difficult to surpass with other available techniques. By correlating data from the different techniques, a model was derived. The main feature of it is an alloy fluctuation on the micrometer scale, seeded during the initial growth and extending through the epitaxial film. These alloy fluctuations seems to be related to terrace steps (≈5 nm in height), formed preferentially at scratches on the SiC surface. This investigation indicates that the initial growth of epitaxial films is critical and structures formed at the beginning of the growth tend to persist throughout the growth. Further, a strain gradient from the SiC interface extending towards the surface, was observed.

The hardness of single-crystal GaN and AlN of 0.5-mm-thickness was measured by the Vickers indentation method in the temperature range 20 - 1400°C. The hardness of GaN and AlN is 10.2 and 17.7 GPa, respectively, at room temperature. The nano-indentation hardness of single-crystal AlN was measured at room temperature as 18 GPa, harder than GaN and InN. Up to about 1100°C, GaN and AlN maintain its hardness similar to that of SiC and thus, a high mechanical stability for GaN and AlN at elevated temperatures is deduced. Yield strength of nitrides is discussed.

In0.1Ga0.9N/In0.01Ga0.99N multiple quantum wells (MQWs) with heavily Si-doped barriers, grown with Metal Organic Vapor Phase Epitaxy (MOVPE) at about 8000C, have been studied in detail with optical spectroscopy. Such structures are shown to be very sensitive to a near surface depletion field, and if no additional layer is grown on top of the MQW structure the optical spectra from the individual QWs are expected to be drastically different. For a sample with 3 near surface QWs and Si-doped barriers, only the QW most distant from the surface is observed in photoluminescence (PL). The strong surface depletion field is suggested to explain these results, so that the QWs closer to the surface cannot hold the photo-excited carriers. A similar effect of the strong depletion field is found in an LED structure where the MQW is positioned at the highly doped n-side of the pn-junction. The internal polarization induced electric field in the QWs is also rather strong, and incompletely screened by carriers transferred from the doped barriers. The observed PL emission for this QW is of localized exciton character, consistent with the temperature dependence of peak position and PL decay time. The excitonic lineshape of 35-40 meV in the QW PL is explained as caused by a combination of random alloy fluctuations and interface roughness; the corresponding localization potentials are also responsible for the localization of the excitons in the low temperature range (<150 K). These samples show no evidence of localization due to nanoscale In fluctuations, these commonly observed problems are concluded to be not present in our samples. A second PL feature at lower energy, observed at low temperatures, is shown to be related to an electron pocket at the interface to the underlying n-GaN buffer layer in these samples.

GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random opening sizes. This produces a significant decrease of the threading dislocation (TD) density compared to the best GaN/sapphire templates. Ultra Low Dislocation density (ULD) GaN layers were obtained with TD density as low as 7×107cm−2 as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). Time-resolved photoluminescence experiments show that the lifetime of the A free exciton is principally limited by capture onto residual donors, similar to the situation for nearly dislocation-free homoepitaxial layers.

A systematic investigation of damage accumulation in GaN films induced by Ar+ as a function of implantation temperature and dose rate has been conducted. Depth distribution of disorder was measured by Rutherford Backscattering/Channeling spectrometry. Two disordered regions were identified in the damage depth distribution: a surface peak and a bulk damage peak. These regions exhibited different behavior as a function of implantation temperature. The displaced atomic density in the bulk damage peak displayed a “reverse annealing” behavior in temperature range from 500 °C to 700 °C, which we attributed to formation of characteristic secondary defects. The influence of implantation temperature and dose rate on the radiation damage accumulation is discussed.