Using e-beam lithography on a single layer of polymethylmethacrylate (PMMA) we designed a relatively thick
subwavelength aluminum mesh on top of sapphire. The 100 nm thick mesh consisted of two perpendicularly oriented
sets of 100 nm wide parallel metal lines with a center to center distance as low as 260 nm. Due to the large proximity
effect during e-beam exposure and the small spacing between metallic lines the use of an adhesion promoting layer
appeared necessary to avoid premature peeling of the photoresist. Using a monoatomic layer of hexamethyldisilazane
(HMDS) as an adhesion promoter between the sapphire and the PMMA, a 500 nm thick photoresist layer could be
exposed and developed with excellent control over the features sizes. Line spacing distances from 500 nm down to 160
nm were achieved. An oxide plasma etch was found to be necessary for metal adhesion during the lift-off process. Due to
the small spacing between the aluminum lines, use of a bi-layer photoresist technique to achieve undercut was not
possible. Thermal evaporation of aluminum was performed and e-beam evaporation didn't help smoothing the metal
surface. An additional ultrasonic bath in acetone was found necessary to ease the lift-off process.
GaN / Al1-xGaxN-based hetero-structures have demonstrated a versatility in RF electronic applications which
is practically unmatched by any other material system. There are many device structures under consideration
for use in RF and Power amplifiers, suitable for both commercial and military applications.
In this paper, we will discuss HEMT device design and growth of GaN/AlGaN layers on semi-insulating SiC
substrates by MBE and MOCVD. Both of the growth techniques have shown high quality GaN /AlGaN
epitaxial layers and have demonstrated very uniform epitaxial layers with high mobility. The MBE growth
was carried out using RF Plasma Assisted MBE. The MOCVD growth was performed in a close-coupled
showerhead reactor operating at low pressure. All HEMT structures were grown on 2-inch semi-insulating
SiC substrates.
Several of the HEMT wafers grown by these two growth techniques were characterized in detail using AFM
measurements of the surface roughness, and non-destructive characterization via contact-less sheet resistance
mapping, optical reflectance, and high-resolution X-ray diffraction.
Several of the wafers were fabricated into HEMT devices, and the results on these devices are also
presented.
This paper reports temperature-dependent DC and small-signal RF characteristics of a 0.4-mm-radius sapphire-based GaN p-i-n diode between -60°C and 175°C. Deep levels approximately 1 eV below the conduction band were observed in both persistent
photo-conductance and photo-capacitance measurements. Self-heating effects were also observed and modeled with the measured thermal resistance and time constant. Based on these characteristics, an equivalent-circuit model was constructed, which accurately predicted the temperature-dependent DC and RF characteristics of the diode.
GaN /AlGaN transistors are being developed for a variety of RF electronic devices that will
eventually replace GaAs- and silicon-based devices for commercial and military applications. In
this paper, we present results from the optimization of the growth conditions for GaN/AlGaN
HEMT structures. The HEMT epitaxial layers are grown via MOCVD. We demonstrate that the
key to high quality HEMT structures is the ability to grow uniform AlGaN layers. Details of the
structural, electrical and optical characteristics of the HEMT epitaxial layers are presented. In
addition, we present results on an innovative ICP etching used for HEMT fabrication. This
technique allows for low damage device processing and improved reliability.
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