Anthony D. Kurtz

Direct observation of porous SiC formed by anodization in HF

A process for forming porous SiC from single‐crystal SiC wafers has been demonstrated. Porous SiC can be fabricated by anodizing n‐type 6H‐SiC in HF under UV illumination. Transmission electron microscopy reveals pores of sizes 10–30 nm with interpore spacings ranging from ≊5 to 150 nm. This is the first reported direct observation of porous SiC formation.

Characterization of n-type beta -SiC as a piezoresistor

SiC is currently being investigated for device applications involving high temperatures. The properties of n-type beta -SiC relevant to piezoresistive devices, namely the gauge factor (GF) and temperature coefficient of resistivity (TCR), are characterized for several doping levels. The maximum gauge factor observed was -31.8 for unintentionally doped (10/sup 16/-10/sup 17//cm/sup 3/) material. This gauge factor decreases with temperature to approximately half its room-temperature value at 450 degrees C. Unintentionally doped SiC has a roughly constant TCR of 0.72%/ degrees C over the range 25-800 degrees C and exhibits full impurity ionization at room temperature. Degenerately doped gauges (N/sub d/=10/sup 20//cm/sup 3/) exhibited a lower gauge factor (-12.7), with a more constant temperature behavior and a lower TCR (0.04%/ degrees C). The mechanisms of the piezoresistive effect and TCR in n-SiC are discussed, as well as their application towards sensors. >

Characterization of nanocrystallites in porous p-type 6H-SiC

We report the formation of porous p‐type 6H‐SiC. The existence of uniformly dispersed pores was confirmed by transmission electron microscopy, with interpore spacings in the range of 1–10 nm. The porous film as a whole is a single crystal. Luminescence peaks above the normal band gap of 6H‐SiC have been observed in the porous layer, but were not distinguished in the bulk SiC substrate. Quantum confinement is discussed as a possible mechanism for the luminescence effects.

Photoelectrochemical Etching of 6H-SiC

A new photoelectrochemical etching process is described for n-type 6H-SiC, while dark electrochemistry has been used to pattern p-type material. In this two-step etching process, the SiC is first anodized to form a deep porous layer, and this layer is subsequently removed by thermal oxidation followed by an HF dip. Etch rates as high as 4000 A/min for n-SiC and 2.2 μm/min for p-SiC have been obtained during the anodization, resulting in near mirror-like etched surfaces

Characterization of monolithic n-type 6H-SiC piezoresistive sensing elements

Monolithic, junction isolated piezoresistors have been fabricated in commercially available 6H-SiC. The gauge factor (GF) of these elements has been measured up to 250/spl deg/C in both longitudinal and transverse configurations. The maximum GF observed was -29.3, corresponding to the piezoresistive coefficient /spl pi//sub 11/. A beam transducer with a four-arm integral piezoresistor network was fabricated and tested in a force sensor configuration. The data indicate that, n-type 6H-SiC has the potential to be useful in high temperature electromechanical sensors to measure parameters such as pressure, force, strain and acceleration. >

Characterization of highly doped n- and p-type 6H-SiC piezoresistors

Highly doped (/spl sim/2/spl times/10/sup 19/ cm/sup -3/) n- and p-type 6H-SiC strain sensing mesa resistors configured in Wheatstone bridge integrated beam transducers were investigated to characterize the piezoresistive and electrical properties. Longitudinal and transverse gauge factors, temperature dependence of resistance, gauge factor (GF), and bridge output voltage were evaluated. For the n-type net doping level of 2/spl times/10/sup 19/ cm/sup -3/ the bridge gauge factor was found to be 15 at room temperature and 8 at 250/spl deg/C. For this doping level, a TCR of -0.24%//spl deg/C and -0.74%//spl deg/C at 100/spl deg/C was obtained for the n- and p-type, respectively. At 250/spl deg/C, the TCR was -0.14%//spl deg/C and -0.34%//spl deg/C, respectively. In both types, for the given doping level, impurity scattering is implied to be the dominant scattering mechanism. The results from this investigation further strengthen the viability of 6H-SiC as a piezoresistive pressure sensor for high-temperature applications.

Photoelectrochemical conductivity selective etch stops for SiC

Recent advances in SiC technology have demonstrated that the material is a potentially useful semiconductor for high temperature and high frequency applications. However, unlike silicon and GaAs, SiC is chemically inert, thus limiting the amount of etchants that can be effectively used to pattern devices. In fact, no patterning technique has been reported to date for SiC which shows high selectivity between p‐ and n‐type material. In this letter, we will show how an n‐type SiC epilayer can be patterned using photoelectrochemical etching, while a p‐type substrate underneath acts as an etch stop. This process is useful for the fabrication of electromechanical transducers, mesa structures, and bipolar and CMOS devices in SiC.

Infrared reflectance of thick p-type porous SiC layers

Thick p‐type porous 6H SiC layers were fabricated by anodization of p‐type 6H SiC bulk crystals in dilute HF. Striking differences are observed in the reststrahl region room‐temperature reflectance of these porous layers compared to that of bulk 6H SiC crystals. Instead of the single broad band reflectance spectrum typically observed in bulk 6H SiC, a two‐band reflectance spectrum is observed. Several effective medium models, based on different morphologies of the component materials, 6H SiC and air, are used to obtain the frequency‐dependent dielectric function of porous SiC from which calculated reflectance spectra are generated. The best match between measured and calculated spectra is obtained for a Maxwell–Garnett model with SiC acting as the host material and air cavities acting as the inclusion material. The model reproduces the two reflectance band structure observed in the measured reflectance of the porous SiC layers. The differences in the reststrahl region reflectance spectra of the porous SiC...

Dopant-selective etch stops in 6H and 3C SiC

A novel photoelectrochemical etching process for 6H– and 3C–SiC is described. This method enables n-type material to be etched rapidly (up to 25 μm/min), while a buried p-type layer acts as an etch stop. Dissolution of SiC takes place through hole–catalyzed surface dissolution. The holes are supplied either from the bulk (e.g., p-SiC) or by UV photogeneration (in n- or p-SiC). The differing flatband potentials of n- and p-type SiC in HF solutions allow the selection of a potential range for which hole current injection occurs only in n-type materials, facilitating dopant-selective etching. This process can be utilized in controlled etching of deep features, as well as in precise patterning of multilayer films.

High Temperature Ohmic Contact Metallizations for n-Type 3C-SiC

Several high temperature metallization systems, based on Ti and W ohmic contacts have been examined for n-type β-SiC. Contact resistivities of ≃10 -4 Ω-cm 2 were measured on as deposited films. The Ti/TiN/Pt/Au metallizations exhibited limited deterioration in the electrical properties after 20 h at 650 o C in air. These contacts have the potential for utilization in short-lifetime devices in the temperature range of 600 to 700 o C and for long-term operation at temperatures of 400 to 500 o C