B. P. Gila

Magnetic properties of n-GaMnN thin films

GaMnN thin films were synthesized using gas-source molecular-beam epitaxy. Mn concentrations between 3 and 12 at. % were investigated. No evidence of second-phase formation was observed by powder x-ray diffraction or high-resolution cross section transmission electron microscopy in films with 9% or less Mn. The films were n type as determined by capacitance–voltage or Hall analysis. Magnetic characterization performed using a squid magnetometer showed evidence of ferromagnetic ordering at room temperature for all samples. In agreement with theoretical predictions, material with 3% Mn showed the highest degree of ordering per Mn atom. At 320 K, the samples show a nonzero magnetization indicating a TC above room temperature.

GaN-based diodes and transistors for chemical, gas, biological and pressure sensing

An apparatus for testing of integrated circuits (ICs) has parallel plates arranged at a fixed distance from each other, each having a plurality of aligning bores, and a plurality of equally oriented contact elements made of resilient sheet material. The elements have a first straight end portion sitting in a bore of one of said plates and a second straight end portion sitting in an aligning bore of the other of said plates. A pressure member is used to move an IC toward the outer side of one of the plates and the first end portions of the contact elements, which first ends extend slightly beyond the outer side of one of the plates and form a pattern which corresponds to the pattern of contact points of the IC to be tested. The second end portions of the contact elements are adapted to be connected to a testing device, and the portion of the contact elements between said first and second end portions lie between the plates and are offset relative to the end portions and experience multiple deflections as axial pressure is exerted on the first end portions. The contact elements are formed out of a flat strip, and the center portion of each element is bent out of the plane defined by the strip.

Influence of MgO and Sc2O3 passivation on AlGaN/GaN high-electron-mobility transistors

Unpassivated AlGaN/GaN high-electron-mobility transistors show significant gate lag effects due to the presence of surface states in the region between the gate and drain contact. Low-temperature (100 °C) layers of MgO or Sc2O3 deposited by plasma-assisted molecular-beam epitaxy are shown to effectively mitigate the collapse in drain current through passivation of the surface traps. These dielectrics may have advantages over the more conventional SiNX passivation in terms of long-term device stability.

Electrical transport properties of single ZnO nanorods

Single ZnO nanorods with diameters of ∼130nm were grown on Au-coated Al2O3 substrates by catalyst-driven molecular beam epitaxy. Individual nanorods were removed from the substrate and placed between Ohmic contact pads and the current–voltage characteristics measured as a function of temperature and gas ambient. In the temperature range from 25to150°C, the resistivity of nanorods treated in H2 at 400°C prior to measurement showed an activation energy of 0.089±0.02eV and was insensitive to the ambient used (C2H4,N2O,O2 or 10% H2 in N2). By sharp contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors.

Rectification at graphene-semiconductor interfaces: Zero-gap semiconductor-based diodes

Diodes based on metal-semiconductor interfaces are common place in semiconductor electronics. What happens when the normal metal is replaced by monolayer graphene? A group of physicists at University of Florida experimentally demonstrate that graphene-semiconductor interfaces make interesting diodes for a surprisingly wide variety of semiconductors.

Pressure-induced changes in the conductivity of AlGaN∕GaN high-electron mobility-transistor membranes

AlGaN∕GaN high-electron-mobility transistors (HEMTs) show a strong dependence of source∕drain current on the piezoelectric-polarization-induced two-dimensional electron gas. The spontaneous and piezoelectric-polarization-induced surface and interface charges can be used to develop very sensitive but robust sensors for the detection of pressure changes. The changes in the conductance of the channel of a AlGaN∕GaN high electron mobility transistor (HEMT) membrane structure fabricated on a Si substrate were measured during the application of both tensile and compressive strain through changes in the ambient pressure. The conductivity of the channel shows a linear change of −(+)6.4×10−2mS∕bar for application of compressive (tensile) strain. The AlGaN∕GaN HEMT membrane-based sensors appear to be promising for pressure sensing applications.

AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors using Sc2O3 as the gate oxide and surface passivation

We demonstrated that Sc2O3 thin films deposited by plasma-assisted molecular-beam epitaxy can be used simultaneously as a gate oxide and as a surface passivation layer on AlGaN/GaN high electron mobility transistors (HEMTs). The maximum drain source current, IDS, reaches a value of over 0.8 A/mm and is ∼40% higher on Sc2O3/AlGaN/GaN transistors relative to conventional HEMTs fabricated on the same wafer. The metal–oxide–semiconductor HEMTs (MOS–HEMTs) threshold voltage is in good agreement with the theoretical value, indicating that Sc2O3 retains a low surface state density on the AlGaN/GaN structures and effectively eliminates the collapse in drain current seen in unpassivated devices. The MOS-HEMTs can be modulated to +6 V of gate voltage. In particular, Sc2O3 is a very promising candidate as a gate dielectric and surface passivant because it is more stable on GaN than is MgO.

MgO/p-GaN enhancement mode metal-oxide semiconductor field-effect transistors

We report the initial demonstration of an enhancement mode MgO/p-GaN metal-oxide-semiconductor field-effect transistor (MOSFET) utilizing Si+ ion-implanted regions under the source and drain to provide a source of minority carriers for inversion. The breakdown voltage for an 80-nm-thick MgO gate dielectric was ∼14 V, corresponding to a breakdown field strength of 1.75 MV cm−1 and the p-n junction formed between the p-epi and the source had a reverse breakdown voltage >15 V. Inversion of the channel was achieved for gate voltages above 6 V. The maximum transconductance was 5.4 μS mm−1 at a drain-source voltage of 5 V, comparable to the initial values reported for GaAs MOSFETs.

Characteristics of MgO/GaN gate-controlled metal–oxide– semiconductor diodes

Gate-controlled n+p metal–oxide–semiconductor diodes were fabricated in p-GaN using MgO as a gate dielectric and Si+ implantation to create the n+ regions. This structure overcomes the low minority carrier generation rate in GaN and allowed observation of clear inversion behavior in the dark at room temperature. By contrast, diodes without the n+ regions to act as an external source of minority carriers did not show inversion even at measurement temperatures of 300 °C. The gated diodes showed the expected shape of the current–voltage characteristics, with clear regions corresponding to depletion and inversion under the gate. The MgO was deposited prior to the Si implantation and was stable during the activation annealing for the Si-implanted n+ regions.

Gd2O3/GaN metal-oxide-semiconductor field-effect transistor

Gd2O3 has been deposited epitaxially on GaN using elemental Gd and an electron cyclotron resonance oxygen plasma in a gas-source molecular beam epitaxy system. Cross-sectional transmission electron microscopy shows a high concentration of dislocations which arise from the large lattice mismatch between the two materials. GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated using a dielectric stack of single crystal Gd2O3 and amorphous SiO2 show modulation at gate voltages up to 7 V and are operational at source drain voltages up to 80 V. This work represents demonstrations of single crystal growth of Gd2O3 on GaN and of a GaN MOSFET using Gd2O3 in the gate dielectric.