Magnetic force microscopy (MFM) has become a widely used tool for the characterization of magnetic properties. However, the magnetic signal can be overlapped by additional forces acting on the tip such as electrostatic forces. In this work the possibility to reduce capacitive coupling effects between tip and substrate is discussed in relation to the thickness of a dielectric layer introduced in the system. Single superparamagnetic iron oxide nanoparticles (SPIONs) are used as a model system, because their magnetic signal is contrariwise to the signal due to capacitive coupling so that it is possible to distinguish between magnetic and electric force contributions. Introducing a dielectric layer between substrate and nanoparticle the capacitive coupling can be tuned and minimized for thick layers. Using the theory of capacitive coupling and the magnetic point dipole–dipole model we could theoretically explain and experimentally prove the phase signal for single superparamagnetic nanoparticles as a function of the layer thickness of the dielectric layer. Tuning the capacitive coupling by variation of the dielectric layer thickness between nanoparticle and substrate allows the distinction between the electric and the magnetic contributions to the MFM signal. The theory also predicts decreasing topographic effects in MFM signals due to surface roughness of dielectric films with increasing film thickness.

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 %.

No abstract

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.