The surface and bulk electronic structures of In0.7Ga0.3N epilayers are investigated by angle-resolved hard X-ray photoelectron spectroscopy (HX-PES) combined with soft X-PES. The unintentionally and Mg-doped In0.7Ga0.3N (u-In0.7Ga0.3N and In0.7Ga0.3N:Mg, respectively) epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. Here three samples with different Mg concentrations ([Mg] = 0, 7 × 1019, and 4 × 1020 cm−3) are chosen for comparison. It is found that a large downward energy band bending exists in all samples due to the formation of a surface electron accumulation (SEA) layer. For u-In0.7Ga0.3N epilayer, band bending as large as 0.8 ± 0.05 eV occurs from bulk to surface. Judged from the valence band spectral edge and numerical analysis of energy band with a surface quantum well, the valence band maximum (VBM) with respect to Fermi energy (EF) level in the bulk is determined to be 1.22 ± 0.05 eV. In contrast, for In0.7Ga0.3N:Mg epilayers, the band bending increases and the VBM only in the bulk tends to shift toward the EF level owing to the Mg acceptor doping. Hence, the energy band is considered to exhibit a downward bending structure due to the coexistence of the n+ SEA layer and Mg-doped p layer formed in the bulk. When [Mg] changes from 7 × 1019 to 4 × 1020 cm−3, the peak split occurs in HX-PES spectra under the bulk sensitive condition. This result indicates that the energy band forms an anomalous downward bending structure with a singular point due to the generation of a thin depleted region at the n+ p interface. For In0.7Ga0.3N:Mg epilayers, the VBM in the bulk is assumed to be slightly lower than EF level within 0.1 eV.

To evaluate the polarity, energy band diagram, and oxygen (O) distribution of a heavily Mg-doped InN (InN:Mg+) epilayer with a Mg concentration of 1.0 ± 0.5 × 1020 cm−3, the core-level and valence band (VB) photoelectron spectra are investigated by angle-resolved soft and hard X-ray photoelectron spectroscopies. The InN:Mg+ epilayers are grown by radio-frequency plasma-assisted molecular beam epitaxy. In this doping level, the polarity inversion from In-polar to N-polar occurs with the increase in the Mg flow rate under the same growth conditions, and the VB spectrum clearly indicates the direction of polarity of InN:Mg+, which is N-polar. The energy band diagram is considered to exhibit a two-step downward bending structure due to the coexistence of the n+ surface electron accumulation layer and heavily Mg-doped p+ layer formed in the bulk. The O concentration rapidly increases until ∼4 nm with respect to the surface, which is deduced to be one of the reasons of the formation of the anomalous two-step energy band profile.

Optical properties of Ga0.82In0.18N p-n homojunction light-emitting-diode are investigated by the photovoltaic, photoluminescence-excitation, electroluminescence (EL), and photoluminescence (PL) measurements. Although the X-ray diffraction measurements indicate a uniform InN molar fraction x in the sequentially grown n- and p-type Ga0.82In0.18N layers, the EL and PL exhibited different peak energies at room temperature. The difference is explained by the emission models in the n- and p-type of Ga0.82In0.18N layers. The results demonstrate a potential use of Ga1-xInxN p-n homojunction for further development of functional device structures.

Although several magnetic resonance studies address the Mg acceptor in GaN, there are few reports on Mg doping in the alloys, where hole production depends strongly on the Al or In content. Our electron paramagnetic resonance (EPR) measurements of the p-type alloys suggest that the Mg impurity retains the axial symmetry, characteristic of a p-type dopant in both alloys; however, In and Al produce additional, different characteristics of the acceptor. In InGaN, the behavior is consistent with a lowering of the acceptor level and increasing hole density as In concentration increases. For AlGaN, the amount of neutral Mg decreases with increasing Al content, which is attributed to different kinetics of hydrogen diffusion thought to occur in samples with higher Al mole fraction.

Selective growth of N-polar InN by exploiting the similar to 100 degrees gap between the upper limits of the growth temperatures of In- and N- polarities has been introduced. An InN epilayer grown at a temperature in this gap on a sapphire substrate covered with an ultrathin AlN layer has demonstrated N-polarity. The dislocation density of such-grown InN layer has been significantly reduced (one order of magnitude lower than the conventional InN epilayers). The results have demonstrated great potential for improving the crystalline quality of hetero-epitaxial InN films. This concept can be easily adopted for other substrates. (C) 2014 AIP Publishing LLC.

We proposed recently a new RF-MBE method called droplet elimination by radical-beam irradiation (DERI) for growing high-quality InN-based III-nitride films. DERI consists of two growth processes: a metal-rich growth process (MRGP) and a droplet elimination process (DEP). In InGaN growth, Ga was preferentially and selectively captured from the Ga/In wetting layer and droplets during MRGP. Then, excess In was swept out from the growing InGaN surface. The swept In was transformed to InN, epitaxially grown on top of InGaN during DEP using nitrogen radical beam irradiation. By repeating this process, an InN/InGaN MQW structure was successfully fabricated. Thick and uniform InGaN films were also successfully obtained by additionally irradiating the same Ga beam flux as that in MRGP even during DEP. When the irradiated Ga/N* beam flux ratio in DEP was changed from that in MRGP, an InxGa1-xN/InyGa1-yN MQW structure was successfully fabricated.

We have studied the temperature dependence and anharmonic coupling of the local vibrational modes (LVMs) associated with Mg-H complexes in heavily doped InN:Mg. Two main LVM peaks are observed which are probably related to two different H-impurity bond lengths. The temperature dependence of the higher-frequency mode, which exhibits a monotonic frequency downshift and broadening with increasing temperature, can be explained by LVM dephasing due to acoustic phonon scattering. The lower-frequency mode displays an anomalous behavior as its frequency decreases initially and then starts to increase linearly above room temperature. The anharmonic coupling of the lower-frequency mode to a molecular mode of the impurity complex is suggested as a possible cause for this behavior.

A new growth method has been proposed and verified to be effective for improving the crystalline quality of GaN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) at low temperature. In this new method, an indium (In) layer is predeposited before the main growth of low-temperature (LT) GaN. The improved quality for LT GaN probably comes from a selective growth process between In and gallium (Ga) in predeposited In coverage, which is similar to the process of several-monolayer-level liquid-phase epitaxy. This method may enable the extension of the application field of nitride semiconductors, for example, to solar cells.

A real-time investigation of In-polar InN growth by radio-frequency plasma-assisted molecular beam epitaxy has been conducted using laser reflection and reflected high-energy electron diffraction (RHEED). Laser reflection intensity is very sensitive to surface changes in the growth front. Clear reflection intensity changes have been observed for In-stabilized surface, In adlayer, and In droplet formation. Simultaneously, the envelope of reflection intensity variation shows interference oscillation due to the increasing InN thickness. Laser reflection intensity increases during In deposition and decreases during the following N irradiation, in contrast to RHEED intensity. In situ monitoring using both techniques has demonstrated complementary information on the growth front. On the basis of the observation from both techniques, the growth mechanisms and roles of the In adlayer and droplets in the InN growth are discussed.

We use cathodoluminescence (CL) mapping and Kelvin probe force microscopy (KFM) to investigate fluctuations of the InN mole fraction in InGaN films grown by radio-frequency plasma-assisted molecular beam epitaxy. Both techniques reveal that InGaN films on GaN templates with scratches exhibit clear composition fluctuations. CL spectra of InGaN obtained in scanning mode have two peaks. Monochromatic CL maps at these two peaks reveal a clear contrast between flat regions and scratches. The shorter-wavelength CL peak is from scratched regions and the longer-wavelength peak is from flat regions. These results reveal that less In is incorporated in scratched regions than in flat regions. KFM measurements reveal that scratched and flat regions have different surface potentials. Experiments on a series of InGaN films with InN mole fractions that span the entire composition range confirm that the surface potential of InGaN depends on the In content. This demonstrates that KFM is useful for studying microscale fluctuations in the InN mole fraction in InGaN. A possible mechanism for this fluctuation is discussed. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim