Ti/Pt/Au metallization on p-type GaN/AlxGa1−xN (x=0.10 and 0.20) superlattices (SL) were investigated as ohmic contacts. Current-voltage and specific contact resistance measurements indicate enhanced p-type doping in the superlattice structures compared to that in GaN. Ti/Pt/Au is shown to be an effective ohmic metallization scheme on p-type GaN/AlxGa1−xN superlattices. A specific contact resistance of Rc = 4.6×10−4 ω-cm2 is achieved for unalloyed Ti/Pt/Au on GaN/Al0.2Ga0.8N SL. This is reduced to 1.3×10−4 ω-cm2 after annealing for 5 minutes at 300 °C.

Dry etch damage on n-GaN has been investigated using Pd Schottky diodes fabricated on surfaces etched by conventional reactive ion etching with SiCl4 plasma. The Schottky barrier height and ideality factor were investigated as a function of the plasma self-bias voltage. Current-voltage measurements revealed severe degradation of both the forward and reverse characteristics for plasma self-bias voltages in excess of -150 V.

Electrical properties of Ti (15 nm)/Pt (50 nm)/Au (80 nm) contacts on moderately doped p-GaN (N-A = 3.0x10(17)cm(-3)) are reported. Linear current-voltage characteristics were observed after annealing the contacts for 1 min at temperatures above 700 degrees C. The best ohmic contacts were obtained after annealing in a Na ambient at 800 degrees C for 2 min. These contacts exhibited a specific contact resistance R-c of 4.2x10(-5) Omega cm(2) and contact resistivity rho(c) of 21 Omega mm. Possible mechanisms for the lower contact resistivity of Ti/Pt/Au contacts are discussed. The processing for the Ti/Pt/Au ohmic contacts is compatible with routine fabrication steps for GaN devices. (C) 2000 American Institute of Physics. [S0003-6951(00)03723-2].

The authors have characterised the microwave noise performance of AlGaN/GaN HEMTs epitaxially grown on insulating BC substrates. The minimum noise figure for 0.25 mu m gate-length devices was measured to be 0.77dB at 5 GHz and 1.06dB at 10GHz. The measured minimum noise figures are comparable to those exhibited by GaAs-based FETs, which demonstrates the viability of AlGaN/GaN HEMTs for low-noise applications.

A brief overview of materials. Processing, technologies, and performance of AlGaN/GaN heterostructure field-effect transistors (HFETs) are presented. Sate-of-the-art results on the dc, microwave, power, and noise characteristics of these devices on sapphire and SiC substrates are discussed. It is evident that AlGaN/GaN HFETs will be used for high power applications at microwave frequencies in the future. It is also possible that these devices will find applications for low noise amplifiers.

One thrust in the recent AlGaN/GaN based HFET development hinges on the use of SiC substrates for the growth of the AlGaN/GaN heterostructures. We have achieved Gm and maximum drain current (Imax) as high as 222 mS mm−1 and 1.71 A mm−1 for HFETs grown on n-SiC. The HFETs on p-SiC have also shown Gm and Imax of 230 mS mm−1 and 1.43 A mm−1. These devices exhibited cut-off frequency (ft) and frequency of oscillation (fmax) of 55 and 56 GHz for HFETs on p-SiC, further demonstrating the applicability of AlGaN/GaN-based HFETs in high power microwave frequency range. The availability of high quality AlGaN/GaN heterostructure has also permitted the implementation of such new device concept as metal–insulator–semiconductor FETs (MISFETs). Our MISFETs have shown low gate leakage in ±6 V gate bias range with Gm as high as 86 mS mm−1.

The etching characteristics of AlxGa1−xN grown by metal–organic chemical-vapor deposition were investigated in an inductively coupled plasma (ICP) reactive ion etching system using Cl2/Ar gas mixtures. Etch rate variations with substrate bias voltage, ICP coil power, chamber pressure, Cl2/Ar gas mixture ratios, and gas flow rates were investigated. The optimum chamber pressure for etching was found to be dependent on both the substrate bias voltage and ICP coil power. Auger electron spectroscopy analysis showed that the stoichiometries of the etched Al0.22Ga0.78N surfaces were identical, independent of the etching conditions. Etching results were successfully applied to form highly anisotropic and smooth facets in GaN/InGaN/AlGaN heterostructure laser materials.

Experimental results of the low-frequency noise measurements on a large number of different AlGaN/GaN High Electron Mobility Transistors (HEMTs) grown on sapphire and SIC substrates have been presented. In the HEMTs grown on sapphire, the 1/f noise is an order of magnitude (or more) higher than for AlGaN/GaN HEMTs grown on SiC substrates. The devices on SIC substrates also have higher electron mobility compared to the devices grown on sapphire substrates. The temperature dependence of noise reveals a contribution to the noise from a local level with activation energy of approximately 0.42 eV for the structures grown on sapphire. A very weak temperature dependence of the low-frequency 1/f noise found for the wafers grown on SiC is very important for high temperature applications of these devices.

For the first time GaN-based MESFETs which have been recessed using a wet etching process are presented. Photoelectrochemical etching was used to recess openings through the heavily-doped n-GaN cap and into the n-GaN channel. The DC and RF characteristics of recessed-gate GaN MESFETs are presented.

The electrical characteristics of Re Schottky contacts on AlxGa1-xN (x = 0, 0.15, 0.22 and 0.26) grown by MOCVD on sapphire substrates have been investigated. The effective barrier heights were obtained from current-voltage and capacitance-voltage measurements and were found to increase with aluminium concentration.

The effects of reactive ion etching n-GaN surfaces with both SiCl4 and Ar plasmas have been investigated using transmission line measurements. The measurements were made from ohmic contacts consisting of Al (as-deposited) and Ti/Al (as-deposited and rapid thermal annealed). The contact resistance, specific contact resistance, and sheet resistance were investigated as functions of the dc plasma self-bias voltage and etch time. The contact resistance extracted from contacts fabricated on surfaces etched with SiCl4 was found to be improved over the unetched samples for all conditions investigated. Dry etching the surface with Ar severely degraded the contact resistance over the unetched sample except at the lower self-bias voltages. Rapid thermal annealing of etched samples prior to Al deposition was found to be effective in removing some of the reactive ion etching/SiCl4-induced damage.

The ability to remove or etch surface materials is a fundamental device processing step, and as an example, this is required to obtain mesa stripes in laser diode fabrication. In addition, etching can be utilized to realize laser facets as an alternative to cleaving. This is particularly important for GaN-based laser diodes. We will describe and present results on the wide range of dry and wet etching techniques that have been developed for GaN-based materials over the last few years

The fabrication and characterization of highperformance AIGaN/GaN heterostructure field effect transistors (HFET's) grown on p-type SiC substrates are reported for the first time, The HFET's were fabricated with gate lengths of 0.25, 0.5, and 1 mu m. These devices exhibited simultaneously high drain currents, high extrinsic transconductances, and excellent frequency response, The 0.25-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 HFET's grown on sapphire substrates and are attributed to the higher thermal conductivity and reduced lattice mismatch associated with SiC substrates.

The group-III nitrides have become versatile semiconductors for short wavelength emitters, high temperature microwave transistors, photodetectors, and field emission tips. The processing of these materials is significant due to the unusually high bond energies that they possess. The dry and wet etching methods developed for these materials over the last few years are reviewed. High etch rates and highly anisotropic profiles obtained by inductively-coupled-plasma reactive ion etching are presented. Photoenhanced wet etching provides an alternative path to obtaining high etch rates without ion-induced damage. This method is shown to be suitable for device fabrication as well as for the estimation of dislocation densities in n-GaN. This has the potential of developing into a method for rapid evaluation of materials.

We report on the high-power performance of the 0.25-mu m gate Doped-Channel GaN/AIGaN Heterostructure Field Effect Transistors (DC-HFET's), At a drain bias voltage of 18 V and drain bias current of 46 mA, these 100-mu m wide devices exhibit high gain at 8.4 GHz with a power density reaching 1.73 W/mm. The de,ices also display high gain at moderate power over a wide range of frequencies, This high gain at high frequency is a result of an optimal doping level in the AIGaN layer that gives rise to a high sheet charge density while maintaining a high-channel electron mobility. These results demonstrate the excellent microwave power capability of the GaN/AIGaN based heterostructure field effect transistors.

A summary of the performance of AlGaN/GaN heterostructure field effect transistors on sapphire and SiC substrates are presented. High total power have been demonstrated by these devices at microwave frequencies. The prospects of utilizing the devices for high power integrated amplifiers are excellent although the issue of thermal management will need to be addressed especially for devices and circuits on sapphire.

Contacts consisting of various single layer metals to n-type GaN have been formed and characterized. The current-voltage characteristics were measured for 17 different metals (Sc, Hf, Zr, Ag, Al, V, Nb, Ti, Cr, W, Mo, Cu, Co, Au, Pd, Ni, and Pt) deposited on the same epitaxial growth layer. The barrier height, ideality factor, breakdown voltage, and effective Richardson coefficients were measured from those metals which exhibited strong rectifying behavior. The barrier heights for these metal contacts were measured using current-voltage-temperature and capacitance-voltage techniques. It was found that an increase in metal work function correlated with an increase in the barrier height. The surface state density of GaN was approximated to be very similar to CdS and almost a factor of ten less than GaAs.