GaN/AlxGa1-xN heterostructures were grown by metal-organic vapor phase epitaxy (MOVPE) to study in detail the formation of two-dimensional hole gases (2DHG). In contrast to the common double-heterostructure approach to create 2DHG, which is based on GaN buffer layers and leads to the parallel formation of a two-dimensional electron gas (2DEG), this concept is designed to create a 2DHG only. The Al mole fraction in the AlGaN buffer and the GaN channel thickness are each varied to investigate their influence on 2DHG properties. The carrier concentrations as determined by room temperature Hall measurements follow the expected trend given by the Al content dependence of the spontaneous polarization of the relaxed AlGaN buffer. A 2DHG density as high as of 1.6·1013 cm-2 with a negligible dependence on temperature (80 – 300 K) is determined for a GaN/Al0.29Ga0.71N heterostructure by temperature-dependent Hall measurements. Higher carrier concentrations can also be achieved, yet strain relaxation of the GaN channel degrades the transport properties for Al contents above 30 %.

We present first experimental results of a novel electro-optical modulator for UV–vis based on III-nitrides. This device consists of a modulation layer which changes its complex dielectric constant influenced by the electric field and a field-dependent carrier distribution. Here, we fabricated and investigated different configurations for the modulation layer structure. The core of the structures was a p–i–n structure in which the i-layer was either GaN or AlGaN. Furthermore, a conventional undoped AlGaN/GaN heterostructure containing a two-dimensional electron gas was grown. Samples were characterized by high-resulution X-ray diffraction (2theta-omega scans and reciprocal space mappings), atomic force microscopy and capacitance–voltage measurements; the reflectivity modulation was determined using a UV–vis white-light reflectometer and a DC voltage source. The results show that reflectivity modulation is readily achieved, caused by the control of the charge carrier density and the electric field. We obtained a relative change in reflectivity of ±1.5% from −8 V to +2 V.

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We report on p-channel metal-insulatorsemiconductor heterostructure field-effect transistors (MISHFET) based on p-GaN/uid-GaN/Al0.29Ga0.71N single heterostructures on sapphire substrates, grown by metalorganic vapor phase epitaxy (MOVPE). The impact of p-GaN layer removal and channel layer thickness adjustment by dry-etching on the characteristics of the MISHFET are investigated. Depending on the remaining GaN thickness (tGaN), the fabricated MISHFET show either depletion-mode (d-mode) operation with a threshold voltage Vth of 3.8 V and an on-current |ID,on| of 9.5 mA/mm (tGaN = 21 nm) or enhancement-mode (e-mode) operation with Vth of -2.3 V and |ID,on| of 1.5 mA/mm (tGaN = 12 nm). Independent of the etching depth, all devices exhibit a very low off-state drain current |ID,off|~10 -8 mA/mm and a steep subthreshold swing (SS) between 80 and 89 mV/dec. Similar to n-channel devices, a Vth instability caused by charge trapping at the dielectric/semiconductor interface is found, emphasizing that careful interface engineering is required for good device performance.