S. Dhar

Ferromagnetism and colossal magnetic moment in Gd-focused ion-beam-implanted GaN

The structural and the magnetic properties of Gd-focused ion-beam-implanted GaN layers are studied. Gd3+ ions were uniformly implanted in molecular beam epitaxy grown GaN layers at room temperature with an energy of 300keV at doses ranging from 2.4×1011to1.0×1015cm−2 which corresponds to an average Gd concentration range of 2.4×1016–1.0×1020cm−3. The implanted samples were not subjected to any annealing treatment. No secondary phase related to Gd was detected by x-ray diffraction in these layers. Magnetic characterization with superconducting quantum interference device reveals a colossal magnetic moment of Gd and ferromagnetism with an order temperature above room temperature similar to that found in epitaxially grown Gd-doped GaN layers. The effective magnetic moment per Gd atom in these samples is, however, found to be an order of magnitude larger than that found in epitaxially grown layers for a given Gd concentration which indicates that the defects play an important role in giving rise to this effect.

Element specific investigations of the structural and magnetic properties of Gd:GaN

The authors present element specific measurements of the x-ray linear dichroism and the x-ray magnetic circular dichroism (XMCD) on Gd:GaN samples. They can show that the majority of the Gd dopant atoms goes to substitutional Ga sites and that a small XMCD is detectable for Gd. There are significant deviations of the magnetic hysteresis recorded for Gd compared to superconducting quantum interference device measurements. Our measurements show that the magnetic signal from the Gd dopant atom itself is rather small highlighting the role of magnetic contributions of the GaN host crystal.

Optical metastability in undoped GaN grown on Ga-rich GaN buffer layers

Investigations on defect-related optical metastability in undoped GaN epilayers grown on GaN buffer layers are presented. The III/V ratio in the buffer layer was varied over a range such that the resistivity of the GaN epilayers traversed a semiconducting to semi-insulating transition. The high-resistive and semi-insulating GaN epilayers show photo induced metastability, which is revealed through a number of features: (i) the intensity of the blue luminescence band decreases as the intensity of the yellow luminescence band increases; (ii) quenching of photocurrent; and (iii) persistent photoconductivity, under the illumination of a low-power ultraviolet laser. It has been shown that these unwanted transient effects can be eliminated in the GaN epilayers by reducing the III/V ratio in the buffer layer. A qualitative discussion of these results suggests that the metastable defects, which are associated with both yellow and blue luminescence bands, may have important consequences for our understanding of def...

Magnetic properties of epitaxial CrN films

The authors have investigated the structural and magnetic properties of CrN films grown on MgO(001) and sapphire(0001) by rf-plasma-assisted molecular beam epitaxy. CrN∕MgO(001) exhibits a better epitaxial quality than CrN/sapphire(0001). The CrN∕MgO film shows clear paramagnetic behavior at low temperatures, whereas CrN/sapphire exhibits a ferromagneticlike response with an order temperature above room temperature which resembles the magnetic behavior found in Cr-doped dilute magnetic semiconductors. Keeping in mind that bulk CrN exhibits antiferromagnetic behavior, the dramatically different magnetic behaviors found in epitaxial CrN films grown on MgO and sapphire demonstrate the importance of epitaxial constraints in determining their magnetic properties.

Determination of energetic distribution of interface states between gate metal and semiconductor in sub-micron devices from current-voltage characteristics

Density and energetic distributions of interface states between metal-semiconductor rectifying contacts in sub-micron GaAs MESFET and AlGaAs/InGaAs pseudomorphic high electron mobility transistors (HEMTs) have been studied. Electrical properties of the interface states between gate metal and semiconductor in sub-micron devices depend on growth technique, associated processing parameters and surface states on III-V semiconductors. Correlation between nonideal current-voltage (I-V) characteristics and interface states has been established through the bias dependence of ideality factor. Ideality factor determined from I-V characteristics of MESFET and HEMT increases with bias and then decreases after reaching a maximum. A theoretical model based on nonequilibrium approach has been used to determine the density of interface states and their energetic distribution from ideality factor. Essentially, Fermi level shifts with applied bias and Schottky barrier height changes due to trapping and detrapping of electrons by the interface states, and from these changes, density of interface states and their energetic distributions have been determined.

The role of the grain boundary on persistent photoconductivity in GaN

We present an experimental investigation on temperature, excitation intensity and spectral dependence of persistent photoconductivity (PPC) in GaN. A grain boundary induced potential barrier is predicted to be responsible for PPC. The non-exponential nature of the PPC decay has been explained by a Gaussian distribution of capture barriers, arising from the trapped charges at the grain boundary interface. The spectral dependence of the PPC suggests the origin of PPC and the yellow luminescence band may arise from the same intrinsic defect.

Metastable magnetism and memory effects in dilute magnetic semiconductors

Detailed magnetic measurements on the dilute magnetic semiconductors (DMS) Cr:InN and Gd:GaN are carried out. These two materials have previously been reported to exhibit room temperature magnetic order. We show that the existence of magnetic hysteresis cannot be taken as proof for conventional ferromagnetism. Instead, we find an unparalleled metastable magnetic behavior together with memory effects in both materials, suggesting that for most DMS metastability plays a crucial role in accounting for the magnetic order even at elevated temperatures up to 400 K.