We report on AlInGaN/GaN heterostructure field effect transistors (HFETs) and the effect of different barrier material compositions. The analytical model for the interface charge in quaternary nitride heterostructures is described in detail and is applied in the calculation of the expected sheet carrier density. Experimental results from different lattice-matched AlInGaN/GaN heterostructures are presented and compared with the analytical predictions. Three heterostructures with AlInGaN barriers grown on sapphire substrates were processed and have been investigated. Each barrier layer was lattice-matched to GaN and the gallium content was 0.1, 0.15 and 0.2 at a barrier thickness of 13.5, 12.8 and 11.3 nm, respectively. Additionally, from these experiments, the basic trends for quaternary nitride Schottky barrier contacts are discussed. Finally, comprehensive dc characterizations have been performed. All devices had a gate length of 1 μm and exhibited a good transconductance of around 260 mS mm −1 at nearly the same current density level. An increase in threshold voltage as well as a decrease in gate leakage current for increasing GaN content has been observed. The nearly constant electron mobility in the range of 1700 cm 2 V −1 s −1 at room temperature is within the best reported so far for HFETs with InN-containing barriers.

The optical properties of quaternary AlxInyGa1-x-yN alloy films with 0.16<x<0.64 and 0.02<y<0.13 are presented. The (0001)-oriented AlInGaN layers were grown by metal-organic vapor phase epitaxy on thick GaN/sapphire templates. High-resolution x-ray diffraction measurements revealed the pseudomorphic growth of the AlInGaN films on the GaN buffer. Rutherford backscattering and wavelength-dispersive x-ray spectroscopy analysis were used in order to determine the composition of the alloys. The ordinary dielectric function (DF) of the AlInGaN samples was determined in the range of 1–10 eV by spectroscopic ellipsometry (SE) at room temperature (synchrotron radiation: BESSY II). The sharp onset of the imaginary part of the DF defines the direct absorption edge of the alloys. At higher photon energies, pronounced peaks are observed in the DF indicating a promising optical quality of the material. These features are correlated to the critical points of the band structure (van Hove singularities). An analytical model, which permits us to accurately describe the dielectric function (or optical constants) in the range of 1–10 eV, is also presented. The band-gap and high-energy interband transition values are obtained by fitting the experimental DF with the analytical model. The strain influence on the bandgap is evaluated by using the k×p formalism. Furthermore, an empirical expression is proposed which allows us to calculate the AlInGaN band-gap and high-energy inter-band transitions in the whole compositional range (x, y). The band-gap values obtained from the empirical expression are in good agreement with both the calculated ab initio and the experimental values determined by SE.

The anisotropic film properties of m-plane GaN deposited by metal organic vapour phase epitaxy (MOVPE) on LiAlO 2 substrates are investigated. To study the development of layer properties during epitaxy, the total film thickness is varied between 0.2 and 1.7 µm. A surface roughening is observed caused by the increased size of hillock-like features. Additionally, small steps which are perfectly aligned in (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) planes appear for samples with a thickness of ~0.5 µm and above. Simultaneously, the X-ray rocking curve (XRC) full width at half maximum (FWHM) values become strongly dependent on incident X-ray beam direction beyond this critical thickness. Anisotropic in-plane compressive strain is initially present and gradually relaxes mainly in the [11][12][13][14][15][16][17][18][19][20] direction when growing thicker films. Low-temperature photoluminescence (PL) spectra are dominated by the GaN near-band-edge peak and show only weak signal related to basal plane stacking faults (BSF). The measured background electron concentration is reduced from ~10 20 cm -3 to ~10 19 cm -3 for film thicknesses of 0.2 µm and ~1 µm while the electron mobilities rise from ~20 to ~130 cm 2 /Vs. The mobilities are significantly higher in [0001] direction which we explain by the presence of extended planar defects in the prismatic plane. Such defects are assumed to be also the cause for the observed surface steps and anisotropic XRC broadening. . This is especially helpful for the design of thick quantum wells or double heterostructure devices to mitigate the effects of the efficiency droop [2]. Limited availability of large-scale freestanding nonpolar GaN wafers as well as their high price favours the heteroepitaxial growth on foreign substrates. (100) LiAlO 2 is an interesting option for this approach because of the very low lattice mismatch with m-plane (1-100) GaN. It amounts to -1.7 % and -0.3 % in [11][12][13][14][15][16][17][18][19][20] and [0001] direction of GaN, respectively, giving rise to anisotropic strain in the layer [3]. In addition to that, the material is produced by the comparably cheap Czochralski pulling method. A major disadvantage of this substrate is the thermal and chemical instability which demands special care on the epitaxial process and leads to highly n-type doped films, most likely related to oxygen incorporation [4]. The growth of high-quality m-plane GaN on LiAlO 2 was already reported using various techniques as molecular beam epitaxy (MBE) [5], hydride vapour phase epitaxy (HVPE) [6] and metal organic vapour phase epitaxy (MOVPE) [7]. However, only few reports contain detailed data on the anisotropic film properties [8]. In this contribution, we present an elaborate investigation on the development of the anisotropic crystal properties during MOVPE of m-plane GaN on LiAlO 2 .

The line shape of X‐ray diffraction (XRD) rocking curves of GaN layers grown epitaxially on (0001) oriented sapphire substrates is analyzed. Measurements performed with double‐ and triple‐crystal setup show a q‐3 and q‐4 intensity decay, respectively, as expected for peak broadening dominated by randomly distributed dislocations. A model developed in [2], based on a restricted random dislocation distribution is fitted to the entire peak shape and used to extract dislocation densities and correlation lengths for edge and screw type threading dislocations. Parameters extracted by double‐ and triple‐crystal x‐ray diffraction measurements agree well with each other but still must be verified by systematic cross‐sectional TEM measurements. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

In this work, we report on the thermal oxidation of AlInN/AlN/GaN heterostructures. A “nearly native” Al2O3 oxide was formed during this oxidation procedure, which can be used as a gate oxide and thus enables the fabrication of metal insulator semiconductor hetero field effect transistors. A constant barrier height of ΦB ≈ 2.34 eV was obtained for all oxidized samples, independent of the oxidation time and temperature, indicating a stable AlInN-oxide interface. The interface state density was approximated to be as low as Nint = 2.5 × 1012 cm-2. Oxide thicknesses were estimated to be in the range of 0.6 nm and 3.2 nm, resulting in a suppression of reverse leakage currents oflarge area metal insulator semiconductor diodes by up to three orders of magnitude. Two-dimensional electron gas density and, in particular, carrier mobility are strongly affected by the thermal oxidation in the O2 atmosphere. A narrow processing window for successful thermal oxidation was identified, covering temperatures between 700 °C and 800 °C and durations of few minutes. The resulting oxide thickness scales well with the square root of oxidation time, indicating diffusion of oxygen atoms into the barrier.