Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

When group-III nitrides go infrared: New properties and perspectives

Molecular beam epitaxy growth of GaN, AlN and InN

Prior experimental work had found that the Fermi level at InN growth surfaces is pinned well above the conduction band edge, leading to strong surface band bending and electron accumulation. Using cross-sectional scanning photoelectron microscopy and spectroscopy, we show definitive evidence of unpinned Fermi level for in situ cleaved a-plane InN surfaces. To confirm the presence or absence of band bending, the surface Fermi level relative to the valence band edge was precisely measured by using both the Fermi edge of Au reference sample and the core level of ultrathin Au overlayer. It is confirmed that flat surface bands only occur at cleaved nonpolar surfaces, consistent with the recent theoretical predictions.

Absence of Fermi-Level Pinning at Cleaved Nonpolar InN Surfaces

We demonstrate that vertically aligned InN nanorods can be grown on Si(111) by plasma-assisted molecular-beam epitaxy. Detailed structural characterization indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c axis. Near-infrared photoluminescence (PL) from InN nanorods can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder (especially for the low-temperature grown InN nanorods) and strong surface electron accumulation effects.

Near-infrared photoluminescence from vertical InN nanorod arrays grown on silicon: Effects of surface electron accumulation layer

The influence of dislocations on electron transport properties of undoped InN thin films grown by molecular-beam epitaxy on AlN(0001) pseudosubstrates is reported. The microstructure and the electron transport in InN(0001) films of varying thickness were analyzed by transmission electron microscopy and variable temperature Hall-effect measurements. It was found that crystal defects have strong effects on the electron concentration and mobility of the carriers in the films. In particular, the combined analysis of microscopy and Hall data showed a direct dependence between free carrier and dislocation densities in InN. It was demonstrated that threading dislocations are active suppliers of the electrons and an exponential decay of their density with the thickness implies the corresponding decay in the carrier density. The analysis of the electron transport yields also a temperature-independent carrier concentration, which indicates degenerate donor levels in the narrow band-gap InN material. The relative in...

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Effect of dislocations on electrical and electron transport properties of InN thin films. II. Density and mobility of the carriers

High-quality InN epitaxial films have been grown by nitrogen-plasma-assisted molecular-beam epitaxy on Si(111) substrates using a double-buffer technique. Growth of a (0001)-oriented single crystalline wurtzite–InN layer was confirmed by reflection high-energy electron diffraction, x-ray diffraction, and Raman scattering. At room temperature, these films exhibited strong near-infrared (0.6–0.9 eV) photoluminescence (PL). In addition to the optical absorption measurement of absorption edge and direct band nature, the PL signal was found to depend linearly on the excitation laser intensity over a wide intensity range. These results indicate that the observed PL is due to the emission of direct band-to-band recombination rather than the band-to-defect (or impurity) deep emission.

J. T. Hsu

Papers

Heteroepitaxial growth of wurtzite InN films on Si(111) exhibiting strong near-infrared photoluminescence at room temperature