Photoreflectance ͑PR͒ and contactless electroreflectance ͑CER͒ spectroscopies have been applied to study optical transitions in undoped and Si-doped AlGaN / GaN heterostructures at room temperature. Spectral features related to excitonic and band-to-band absorptions in GaN layer and band-to-band absorption in AlGaN layer have been resolved and analyzed. In addition, a broad spectral feature related to two-dimensional electron gas has been observed for the Si-doped heterostructure. It has been found that some of the mentioned optical transitions are not observed in CER spectra whereas they are very strong in PR spectra. This phenomenon is associated with different mechanisms of the modulation of built-in electric field in the investigated structure. A combination of PR and CER gives the possibility of a richer interpretation of both PR and CER spectra.

The structure, morphology and optical properties of GaN films deposited by metalorganic vapour phase epitaxy (MOVPE) on alternative substrates: ZnO, NdGaO, YSZ (yttria stabilized zirconia). Scanning electron microscopy, X‐ray diffraction and photoluminescence were used for the epitaxial layers characterisation. The obtained results have been compared to those of GaN layers grown on c‐plane sapphire substrates. It was established that the most important step towards the realisation of device quality GaN material on alternative substrates is the first stage of the growth process.

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In this work we present the influence of in situ deposited non-continous SiN layer and the chemical precursors III/V mole ratio change during GaN buffer growth for AlGaN/GaN/Si(111) heterostructures with low temperature AlN interlayer on their crystalline quality and electrical mobility of two dimensional electron gas (2DEG). We show, that application of SiN layer resulted in a decrease of (0002) full width at half maximum of diffraction peak below 400 arcsec and build-in stress in 2 µm thick heterostructures below 200 MPa without any relaxation visible on the surface. In optimized AlGaN/GaN heterostructures, by altering V/III mole ratio during growth of GaN subbuffer, the maximum 2DEG mobility of 2057 cm 2 V −1 s −1 , measured by impedance spectroscopy method, was obtained.

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Position of Fermi level on Al0.2Ga0.8N surface and distribution of electric field in Al0.2Ga0.8N/GaN transistor heterostructures without and with AlN layer were studied experimentally using contactless electroreflectance and theoretically solving Schrodinger-Poisson equation with various surface boundary conditions. It has been observed that the thin AlN layer changes very strongly the distribution of electric field in this heterostructure but the Fermi level position on Al0.2Ga0.8N surface does not change significantly. Its position is the same within experimental uncertainly (i.e., ∼0.5 eV below conduction band) for both bulk Al0.2Ga0.8N and Al0.2Ga0.8N/GaN heterostructures.

To check the influence of different process parameters, such as precursors (TMAl and TMGa) flow rates, growth interrupt time between subsequent stages of the growth for purging the reactor, on the properties of the two dimensional electron gas (2DEG) in the AlGaN/AlN/GaN heterostructure, seven sets of HEMT structures were developed and investigated. It was demonstrated that for the growth of canonically the same “sandwich” structure, several growth process schemes can be applied, and different properties of fabricated AlGaN/AlN/GaN can be obtained. It was shown, that changing Al content in the AlGaN barrier layer by modifying TMAl flow rate results in significant decrease in 2DEG mobility compared to the method in which TMGa flow rate is adjusted to keep the assumed composition. The presented results also showed, that the growth interruption between successive stages of AlGaN/AlN/GaN growth has a distinct impact on the mobility and concentration of 2DEG in HEMT’s. The transport properties of the fabricated heterostructures were studied by impedance spectroscopy technique. Specific parameters of growth process, such as growth rate and Al content, were obtained by in‐situ interferometry and photoluminescence technique. There are many publications which describes the influence of different structure parameters, e.g. composition of the AlGaN barrier layer, thickness of the AlN interlayer, however the information is rather fragmentary and it is difficult to compare quantitative results deriving from various groups. The purpose of this article was to show the key HEMT’s structure parameters in relation to MOVPE process scheme, influencing the electrical transport of 2DEG. Detailed explanation of physical phenomenas was not the main aim of this work and will be presented later. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)