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.
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.
Effective mass ratios, m*, of electrons in near intrinsic and n-type Hg1-xCdxTe for 0.20 ⩽ x ⩽ 0.30 over the temperature range 77 K ⩽ T ⩽ 296 K were measured using Faraday rotation spectroscopy. Effective masses were found to be about twice as large at room temperature as band edge effective mass, m*be, calculations. Measured effective masses diverge further from the theoretical formulations as temperature increases which appears to be due to a thermal excitation effect that is not accounted for in theoretical calculations. These calculations can be corrected using a linear correction factor, m**, where the true effective mass ratio, m* = m** m*be, where m** was found empirically to be m** = 4.52 x 10-3 T + 0.78. Carrier concentrations were measured using Hall or van der Pauw tests. Soldered contacts to high mobility materials like HgCdTe using even the purest indium solder inevitably result in contamination that can add significant numbers of impurity carriers to the material and severely decrease mobility. A simple method of burnishing contacts to the material without heat using indium solder is presented. These cold contacts do not effect the material properties and are very effective in n-type HgCdTe making good physically strong contacts that remain ohmic to at least 10 K. This is a review paper.
GaN /AlGaN transistors are being developed for a variety of RF electronic and high temperature elctronics applications that will replace GaAs and Silicon devices and circuits for commercial and military applications. AlGaN/ GaN based HEMT device structure shows significant potential to meet these needs. In this paper, we present a GaN/AlGaN based HEMT design with modeling results, that includes AlN buffer layer followed by AlGaN layers on lattice matched semi-insulating SiC substrates. These devices were grown using RF Plasma Assisted MBE Technique. This approach has demonstrated very uniform epitaxial layers. Key to high quality HEMT structures is the ability to grow high quality AlN Buffer layers. Details of the electrical and optical characteristics of the HEMT layers and devices are presented and a short overview of semi-insulating SiC crystal growth is given.
An optical method for measuring carrier concentration and mobility in n-type semiconductors is presented. The method depends on an accurate knowledge of the free electron effective mass,(M), as a function of temperature and dopant concentration. These values are not generally available in the literature. Measured values for M in HgCdTe, for example, are rare and, according to several authors, significantly higher than calculated values in the literature. M for n-type HgCdTe, InSb, GaAs, and Si were measured using Faraday rotation and van der Pauw testing over the range 77 K < T < 296 K. Good agreement was obtained with available published values of M in InSb and GaAs. M in HgCdTe was found to be on the order of 90 percent higher than calculated values at 296 K. The specific results have ben sent to the open literature for publication and consequently cannot be included for republication here. Faraday rotation and absorption can be combined with a constant, C, to yield carrier mobility. C was measured in the above materials at selected wavelengths. With M and C for a material known, carrier concentration and mobility can be determined from Faraday rotation and absorption measurements alone.
A nondestructive characterization technique to determine free carrier concentration and mobility that is applicable to most n-type semiconductors, including infrared detector materials such as InSb and HgCdTe, is presented. The technique utilizes absorption and Faraday rotation in regions of the spectrum where these phenomena are due mostly to the free carriers themselves. Carrier concentration is directly proportional to the free carrier component of the rotation signal. Free carrier mobility is proportional to a simple ratio of the free carrier rotation to the free carrier absorption. Good agreement with Hall mobility data was obtained using a simpler version of this technique at room temperature. The proportionality constant is actually a complex quantum mechanical quantity that is temperature- and material-dependent. In this study, absorption data was obtained in the temperature range 77 K to 300 K in HgCdTe, n-type InSb, GaAs, and Si. Rotation data was measured at 300 K and calculated for lower temperatures using known values of electron effective mass and refractive index to determine this constant and make it available for practical mobility determinations at various infrared detector operating temperatures.
A magneto-optical system is described that allows for spatial mapping of Faraday rotation and infrared transmission of HgCdTe thin films. Composition, thickness, and absorption coefficient of HgCdTe samples are determined from analysis of transmission spectra. Carrier concentration is extracted from analysis of Faraday rotation spectra. The system provides noncontact, nondestructive rapid screening or detailed diagnostics of HgCdTe material. We also show that the results of resonant magneto-optical spectroscopy support the observation of Faraday rotation caused by optical transitions from shallow compensating acceptors as well as near-midgap defect levels in material with similar x-value. We show that these magneto- optical methods are powerful tools for the study of impurity and defect levels in HgCdTe as well as for characterizing and screening HgCdTe.
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