Peter A. Barnes

Calculations of tungsten silicide and carbide formation on SiC using the Gibbs free energy

Abstract Fabricating electronic devices capable of operation at elevated temperatures requires understanding the chemical reactions at the metal–semiconductor interface. A Gibbs ternary diagram approach is used to understand the temperature sequence of silicide and carbide formation, and stability in WSiC ternary systems. Limitations of the thermodynamic approach are discussed, and comparisons with experimental results are made.

Temperature dependent electron cyclotron resonance etching of InP, GaP, and GaAs

Electron cyclotron resonance etching of InP, GaP, and GaAs in Ar, Ar/Cl 2, Ar/Cl2/H2, and Ar/Cl2/H2/CH4 plasmas is reported for substrate temperatures from 10 to 170 °C. Etch rates increased as a function of temperature for GaP and GaAs in an Ar/Cl2 plasma. With the addition of H2 or H2/CH4 to the plasma, the GaP and GaAs etch rates decreased and were essentially temperature independent. In comparison, InP etch rates showed a strong temperature dependence regardless of plasma chemistry. At 170 °C, InP etch rates were greater than GaP and GaAs in the Ar/Cl2/H2 and Ar/Cl2/H 2/CH4 plasmas. Atomic force microscopy was used to determine the root‐mean‐square roughness of the etched surfaces. The etched surface morphology for InP was strongly dependent on temperature and plasma chemistry while smooth pattern transfer was obtained for a wide range of plasma conditions for GaAs and GaP.

Plasma Chemistry Dependent ECR Etching of GaN

Electron cyclotron resonance (ECR) etching of GaN in Cl{sub 2}/H{sub 2}/Ar, C1{sub 2}/SF{sub 6}/Ar, BCl{sub 3}/H{sub 2}/Ar and BCl{sub 3}/SF{sub 6}/Ar plasmas is reported as a function of percent H{sub 2} and SF{sub 6}. GaN etch rates were found to be 2 to 3 times greater in Cl{sub 2}/H{sub 2}/Ar discharges than in BCl{sub 3}/H{sub 2}/Ar discharges independent of the H{sub 2} concentration. In both discharges, the etch rates decreased as the H{sub 2} concentration increased above 10%. When SF{sub 6} was substituted for H{sub 2}, the GaN etch rates in BCl{sub 3}-based plasmas were greater than those for the Cl{sub 2}-based discharges as the SF{sub 6} concentration increased. GaN etch rates were greater in Cl{sub 2}/H{sub 2}/Ar discharges as compared to Cl{sub 2}SF{sub 6}/Ar discharges whereas the opposite trend was observed for BCl{sub 3}-based discharges. Variations in surface morphology and near-surface stoichiometry due to plasma chemistries were also investigated using atomic force microscopy and Auger spectroscopy, respectively.

Calculations of the Specific Resistance of Contacts to III-V Nitride Compounds

We present calculations of the specific contact resistance for metals to GaN. Our calculations include a correct determination of the Fermi level taking into account the effect of the degenerate doping levels, required in creating tunneling ohmic contacts. Using a recently reported improved WKB approximation suitable in representing the depletion width at the metal-semiconductor interface, and a two band k-p model for the effective masses, specific contact resistance was determined as a function of doping concentration. The specific contact resistance was calculated using the best data available for barrier heights, effective masses and dielectric coefficients for GaN. Because the barrier height at the metal-semiconductor interface has a very large effect on the contact resistance and the available data is sketchy or uncertain, the effect of varying the barrier height on the calculated specific contact resistance was investigated. Further, since the III-V nitrides are being considered for high temperature device applications, the specific contact resistance was also determined as a function of temperature.

Low resistivity ohmic contacts to moderately doped n-GaAs with low temperature processing

A low‐temperature process for forming ohmic contacts to moderately doped GaAs has been optimized using a PdGe metallization scheme. Minimum specific contact resistivity of 1.5×10−6 Ω cm2 has been obtained with a low anneal temperature of 250 °C. Results for optimizing both time and temperature are reported and compared to GeAu n‐GaAs contacts. Material compositions were analyzed by x‐ray photoelectron spectroscopy and circuit metal interconnect contact resistivity to the low‐temperature processed PdGe contacts is reported. For the lowest‐temperature anneals considered, excess Ge on the ohmic contact layer is suspected of degrading interconnect metal contacts, while higher‐temperature anneals permitted interconnect metal formation with negligible contact resistivity. Atomic force microscopy measurements showed that the PdGe surface morphology is much more uniform than standard GeAu contacts.

Review of ohmic contacts to compound semiconductors

A solid state switch is required to pass very high current densities, J, upon demand. Electrical access to the switch is through ohmic contacts which interface the device to the external circuit. Thus, accurate characterization of these contacts is essential in the design of the switch. Most work to date has centered around the electrical characteristics of switches, in particular the resistance R, and a reduction in joule (J2R) heating. In this paper we consider both the electrical characteristics and the thermal stability of the ohmic contacts on the operation of solid state switches. Thermal stability is important due to the inherent heating of the switch during its on time. Further, some applications will place switches in environments requiring operation at high temperature. The paper describes the present status of ohmic contacts to GaAs and InP.

High Temperature Thermal Conductivity Measurements of Quasicrystalline Al70.8Pd20.9Mn8.3

We report measurements of the thermal conductivity on a potential high temperature thermoelectric material, the quasicrystal Al70.8Pd20.9Mn8.3. Thermal conductivity is determined over a temperature range from 30 K to 600 K, using both the steady state gradient method and the 3ω method. Measurements of high temperature thermal conductivity are extremely difficult using standard heat conduction techniques. These difficulties arise from the fact that heat is lost due to radiative effects. The radiative effects are proportional to the temperature of the sample to the fourth power and therefore can lead to large errors in the measured thermal conductivity of the sample, becoming more serious as the temperature increases. For thermoelectric applications in the high temperature regime, the thermal conductivity is an extremely important parameter to determine. The 3ω technique minimizes radiative heat loss terms, which will allow for more accurate determination of the thermal conductivity of Al70.8Pd20.9Mn8.3 at high temperatures. The results obtained using the 3ω method are compared to results from a standard bulk-thermalconductivity-technique on the same samples over the temperature range, 30 K to 300 K.

Upper Limitation to the Performance of Single-Barrier Thermionic Emission Cooling

We have re-examined solid-state thermionic emission cooling from first principles and report two key results. First, electrical and heat currents over a semiconductor – semiconductor thermionic barrier are determined by the chemical potential measured from the conduction band edge, not the energy band offset between the two materials as is sometimes assumed. Second, we show the upper limit to the performance of thermionic emission cooling is equivalent to the performance of an optimized thermoelectric device made from the same material. An overview of this theory will be presented and instrumentation being developed to experimentally verify the theory will be discussed.

High rate ECR etching of III-V nitride materials

The III-V nitride compound semiconductors are attracting considerable attention for blue and ultraviolet light emitting diodes (LEDs) and lasers as well as high temperature electronics due to their wide band gaps and high dielectric constants. The recent progress observed in the growth of these materials has not been matched by progress in processing techniques to fabricate more highly sophisticated devices. Patterning these materials has been especially difficult due to the relatively inert chemical nature of the group-III nitrides. The authors review dry etch techniques which have been used to pattern these materials including electron cyclotron resonance (ECR), reactive ion etch (RIE), and chemically assisted ion beam etching (CAIBE). ECR etch rates greater than 3,800 {angstrom}/min for InN, 3,500 {angstrom}/min for GaN, and 1,170 A/min for AlN are reported. Etch anisotropy, surface morphology, and near-surface stoichiometry will be discussed.