I. Adesida

Gallium nitride whiskers formed by selective photoenhanced wet etching of dislocations

Gallium nitride is used to fabricate high brightness blue and green light-emitting diodes in spite of high densities of extended structural defects. We describe a photoelectrochemical etching process that reveals the dislocation microstructure of n-type GaN films by selectively removing material between dislocations. The GaN whiskers formed by the etching have diameters between 10 and 50 nm and lengths of up to 1 μm. A correlation between the etched features and threading dislocations in the unetched film is confirmed through transmission electron microscopy studies. The whisker formation is believed to be indicative of electrical activity at dislocations in GaN.

AlGaN/GaN HEMTs on SiC with f/sub T/ of over 120 GHz

AlGaN/GaN high electron mobility transistors (HEMTs) grown on semi-insulating SiC substrates with a 0.12 /spl mu/m gate length have been fabricated. These 0.12-/spl mu/m gate-length devices exhibited maximum drain current density as high as 1.23 A/mm and peak extrinsic transconductance of 314 mS/mm. The threshold voltage was -5.2 V. A unity current gain cutoff frequency (f/sub T/) of 121 GHz and maximum frequency of oscillation (f/sub max/) of 162 GHz were measured on these devices. These f/sub T/ and f/sub max/ values are the highest ever reported values for GaN-based HEMTs.

DC and microwave performance of high-current AlGaN/GaN heterostructure field effect transistors grown on p-type SiC substrates

The fabrication and characterization of high performance AlGaN/GaN heterostructure field effect transistors (HFETs) grown on p-type SiC substrates are reported for the first time. The HFETs were fabricated with gate lengths of 0.25, 0.5, and 1 /spl mu/m. These devices exhibited simultaneously high drain currents, high extrinsic transconductances, and excellent frequency response. The 0.25-/spl mu/m gate-length devices produced a peak drain current of 1.43 A/mm, a transconductance of 229 mS/mm, a unity current-gain cutoff frequency of 53 GHz, and a maximum frequency of oscillation of 58 GHz. The unity current-gain cutoff frequency also exhibited little degradation as the drain-source bias was swept up to 20 V. These results represent a significant improvement over similar HFETs grown on sapphire substrates and are attributed to the higher thermal conductivity and reduced lattice mismatch associated with SiC substrates.

Schottky barrier properties of various metals on n-type GaN

Schottky barriers of Ti, Cr, Au, Pd, Ni and Pt on n-type GaN epitaxial layers grown by low-pressure metal-organic chemical vapour deposition on sapphire have been fabricated and characterized. Measurements were carried out using current - voltage (I - V), current - voltage - temperature (I -V - T) and capacitance - voltage (C - V) techniques. A modified Norde plot was used as one of the analysis tools for the I - V - T measurements. The barrier heights, ideality factors and effective Richardson constants are presented. Barrier heights of 0.88, 0.92, 0.99 and 1.08 eV for Au, Pd, Ni and Pt respectively were obtained from the modified Norde plot. Contacts of Ti and Cr exhibited only slightly rectifying characteristics. These results show that the barrier height on n-GaN increases monotonically, but does not scale proportionately, with increasing metal workfunction.

Reactive ion etching of gallium nitride in silicon tetrachloride plasmasa)

The reactive ion etching characteristics of gallium nitride (GaN) in silicon tetrachloride plasmas (SiCl4, 1:1/SiCl4:Ar, and 1:1/SiCl4:SiF4) in the pressure range between 20 and 80 mTorr have been investigated. For the pressure range investigated, etch rates are found to be essentially identical for the different gas mixtures and also invariant with pressure. However for all gas mixtures, etch rates increased monotonically with increasing plasma self‐bias voltage exceeding 50 nm/min at 400 V. This is one of the highest etch rate ever reported for GaN. Smooth and anisotropic etch profiles are demonstrated for structures of submicrometer dimensions. The slight overcut observed in the etch profiles is attributed to the significant role of physical ion bombardment in the etching mechanism. Auger electron spectroscopy show that a wet etch in dilute HF is needed to clear the Si (in the form of SiOx) embedded in the near surface of GaN during etching thereby restoring etched surfaces to their virgin state.

Highly anisotropic photoenhanced wet etching of n-type GaN

A room-temperature photoelectrochemical etching process for n-type GaN films using a 0.04 M KOH solution and Hg arc lamp illumination is described. The process provides highly anisotropic etch profiles and high etch rates (>300 nm/min) at moderate light intensities (50 mW/cm2 @365 nm). The etch rate and photocurrent are characterized as a function of light intensity for stirred and unstirred solutions, and the etch process is found to be diffusion limited for light intensities greater than 20 mW/cm2 @365 nm. A reaction mechanism for the etch process is proposed.

Thermally-stable low-resistance Ti/Al/Mo/Au multilayer ohmic contacts on n–GaN

A metallization scheme consisting of Ti/Al/Mo/Au with excellent edge acuity has been developed for obtaining low-resistance ohmic contacts to n–GaN. Excellent ohmic characteristics with a specific contact resistivity as low as 4.7×10−7 Ω-cm2 were obtained by rapid thermal annealing of evaporated Ti/Al/Mo/Au at 850 °C for 30 sec in a N2 ambient. Additionally, no degradation in specific contact resistivity was observed for these contacts subjected to long-term annealing at 500 °C for 360 h.

Rapid evaluation of dislocation densities in n-type GaN films using photoenhanced wet etching

We describe a technique based on photoelectrochemical wet etching that enables efficient and accurate evaluation of dislocation densities in n-type GaN films. The etching process utilizes dilute aqueous KOH solutions and Hg arc lamp illumination to produce etched GaN “whiskers” by selectively etching away material around threading dislocations. The etched whiskers, each corresponding to a single threading dislocation, can be effectively imaged by plan-view scanning electron microscopy. The distribution and density of dislocations are then readily observed over very large sample areas. Transmission electron microscope and atomic force microscope studies of the GaN samples confirm the accuracy of the dislocation density obtained by the wet etching.

SMOOTH N-TYPE GAN SURFACES BY PHOTOENHANCED WET ETCHING

A room-temperature photoelectrochemical wet etching process is described that produces smoothly etched GaN surfaces using KOH solution and Hg arc lamp illumination. Atomic force microscope measurements indicate a root-mean-square etched surface roughness of 1.5 nm, which compares favorably to the unetched surface roughness of approximately 0.3 nm. Etch rates of 50 nm/min were obtained using a KOH solution concentration of 0.02 M and an illumination intensity of 40 mW/cm2. It is shown that the smooth etching occurs under conditions of low KOH solution concentration and high light intensities, which result in a diffusion-limited etch process.

Bendable GaAs metal-semiconductor field-effect transistors formed with printed GaAs wire arrays on plastic substrates

Micro/nanowires of GaAs with integrated ohmic contacts have been prepared from bulk wafers by metal deposition and patterning, high-temperature annealing, and anisotropic chemical etching. These wires provide a unique type of material for high-performance devices that can be built directly on a wide range of unusual device substrates, such as plastic or paper. In particular, transfer printing organized arrays of these wires at low temperatures onto plastic substrates yield high-quality bendable metal-semiconductor field-effect transistors. Electrical and mechanical characterization of devices on poly(ethylene terephthalate) illustrates the level of performance that can be achieved. These results indicate promise for this approach to high-speed flexible circuits for emerging applications in consumer and military electronic systems.