H. S. Kong

Chemical vapor deposition and characterization of 6H‐SiC thin films on off‐axis 6H‐SiC substrates

High‐quality, monocrystalline 6H‐SiC thin films have been epitaxially grown on 6H‐SiC {0001} substrates which were prepared 3° off‐axis from 〈0001〉 towards 〈1120〉 at 1773 K via chemical vapor deposition (CVD). Essentially, no defects were generated from the epilayer/substrate interface as determined by cross‐sectional transmission electron microscopy (XTEM). Double positioning boundaries which were observed in β‐SiC grown on 6H‐SiC substrates were eliminated as confirmed by plan‐view TEM. A strong dependence of the surface morphology of the as‐grown thin films on the tilting orientation of the substrates was observed and reasons for this phenomenon are discussed. The unintentionally doped 6H‐SiC thin films always exhibit n‐type conduction with a carrier concentration on the order of 1016 cm−3. Au‐6H‐SiC Schottky barrier diodes were fabricated on the CVD 6H‐SiC thin films and it was found that the leakage current at a reverse bias of 55 V was only 3.2×10−5 A/cm2. This is compared to SiC films grown on oth...

Critical evaluation of the status of the areas for future research regarding the wide band gap semiconductors diamond, gallium nitride and silicon carbide

Abstract The extreme thermal and electronic properties of diamond and of silicon carbide, and the direct band gap of gallium nitride, provide multiplicative combinations of attributes which lead to the highest figures of merit for any semiconductor materials for possible use in high power, high speed, high temperature and high frequency applications. The deposition of monocrystalline diamond, at or below 1 atm total pressure and at a temperature T , has been achieved on diamond substrates; the deposited film has been polycrystalline on all other substrates but the achievement is no less significant. For electronic applications, heteroepitaxy of single-crystal films of diamond, an understanding of mechanisms of nucleation and growth, methods of impurity introduction and activation, and further device development must be achieved. Stoichiometric gallium nitride free of nitrogen vacancies has apparently not been obtained. Thus, knowledge of the defect chemistry of this material, the growth of semiconducting films on foreign substrates, and the development of insulating layers and of their low temperature deposition as well as device fabrication procedures must be achieved. By contrast, all of these problems have already been solved for silicon carbide, including the operation of a MOSFET at 923 K — the highest operating temperature ever reported for a field-effect device. However, considerable research remains to be done regarding the development of large silicon carbide substrates, of ohmic and rectifying contacts, of new types of devices, and of low temperature techniques for the deposition of insulating layers. Fugitive donor and acceptor species in unintentionally doped samples must also be identified and controlled.

Characterization of device parameters in high‐temperature metal‐oxide‐semiconductor field‐effect transistors in β‐SiC thin films

Both inversion‐ and depletion‐mode n‐channel metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) have been fabricated on β‐SiC thin films grown by chemical‐vapor deposition. The inversion‐mode devices were made on in situ doped (Al) p‐type β‐SiC(100) thin films grown on Si(100) substrates. The depletion‐mode MOSFETs were made on n‐type β‐SiC(111) thin films grown on the Si(0001) face of a 6H α‐SiC substrates. Stable saturation and low subthreshold currents were achieved at drain‐source voltages exceeding 5 and 25 V for the inversion‐mode and depletion‐mode devices, respectively. The transconductance increased with temperature up to 673 K for the short‐gate‐length devices, of either mode, and then decreased with further increases in temperature. It is proposed that the transconductances and threshold voltages for the inversion‐mode devices are greatly affected by minority‐carrier injection from the source. Stable transistor action was observed for both types of devices at temperatures up to 823 K,...

High‐temperature depletion‐mode metal‐oxide‐semiconductor field‐effect transistors in beta‐SiC thin films

Depletion‐mode n‐channel metal‐oxide‐semiconductor field‐effect transistors were fabricated on n‐type β‐SiC (111) thin films epitaxially grown by chemical vapor deposition on the Si (0001) face of 6H α‐SiC single crystals. The gate oxide was thermally grown on the SiC; the source and drain were doped n+ by N+ ion implantation at 823 K. Stable saturation and low subthreshold current were achieved at drain voltages exceeding 25 V. Transconductances as high as 11.9 mS/mm were achieved. Stable transistor action was observed at temperatures as high as 923 K, the highest temperature reported to date for a transistor in any material.

An examination of double positioning boundaries and interface misfit in beta‐SiC films on alpha‐SiC substrates

Beta‐SiC thin films epitaxially grown on 6H‐SiC (0001) substrates were examined via scanning‐reflection x‐ray topography (XRT) and x‐ray rocking curve analysis. The lattice misfit between the (111) plane of the β‐SiC epitaxial layer and the (0001) plane of the 6H‐SiC substrate was determined to be 7.6×10−4. Boundaries separating regions of the β‐SiC film rotated from each other by 60° were observed and identified by XRT as double positioning boundaries (DPBs), which are a special type of twin boundary. The XRT maps showed that the mosaic structure examined via optical microscopy was also caused by the DPBs. Finally, many stacking faults were generated at the boundaries as revealed by plan‐view transmission electron microscopy, indicating the high internal energy of these DPBs. The formation mechanisms of the DPBs is also discussed.

Epitaxial growth of β‐SiC thin films on 6H α‐SiC substrates via chemical vapor deposition

Epitaxial films of cubic β‐SiC(111) have been grown via chemical vapor deposition at 1683 K on (0001) substrates of hexagonal 6H α‐SiC. Optical microscopy of the surface indicated that a decrease in the ratio of the sum of the C and Si source gases to the H2 carrier gas changed the crystallization behavior from polycrystalline to monocrystalline. Cross‐sectional transmission electron microscopy showed almost no line or planar defects at the substrate/film interface and very few defects within the bulk of the film.

Hall measurements as a function of temperature on monocrystalline SiC thin films

Hall measurements were conducted at temperatures up to 1000 K on unintentionally doped n‐type β(3C)‐ and α(6H)‐SiC thin films epitaxially grown on both on‐axis and vicinal Si (100) and α(6H)‐SiC (0001) by chemical vapor deposition. The carrier concentration versus temperature data were analyzed using a compensation model. The β‐SiC films grown on Si were highly compensated (NA/ND=0.73–0.98). The compensation ratio was not as large in the SiC films grown on α‐SiC (NA/ND=0.36, for β‐SiC on α‐SiC, and 0.02, for α‐SiC on α‐SiC). The donor ionization energy for β‐SiC on Si was calculated to be 14–21 meV. Analogous values for β‐ and α‐SiC films on α‐SiC were 33 and 84 meV, respectively. These values are smaller than those for N determined from photoluminescence studies.

Temperature dependence of the current‐voltage characteristics of metal‐semiconductor field‐effect transistors in n‐type β‐SiC grown via chemical vapor deposition

Metal‐semiconductor field‐effect transistors (MESFET’s) have been fabricated in an unintentionally doped, n‐type β‐SiC thin film grown by chemical vapor deposition (CVD). This n‐type layer was deposited on a monocrystalline p‐type β‐SiC (100) CVD layer previously grown on a p‐type Si (100) substrate. The buried p layer allowed the devices to be fabricated several microns away from the SiC/Si interface region which contained numerous defects formed as a result of the poor lattice match and different coefficients of thermal expansion between SiC and Si. Thermally evaporated Au was utilized for the gate contact. Sputtered TaSi2 was employed for the source and drain contacts. The gate lengths and channel depths of these MESFET’s were 3.5 and 0.60 μm, respectively. Saturation of the drain currents was achieved at room temperature. Furthermore, the current‐voltage characteristics, measured from 298 to 623 K for the first time, indicated that these MESFET’s performed reasonably well throughout this temperature r...

Defects in neutron irradiated SiC

Deep level transient spectroscopy and resistivity measurements have been used to characterize defects in as‐grown and neutron irradiated epitaxially grown 3C‐SiC on Si(100) substrates. The thick epilayers were free of defects; neutron irradiation induced an electron trap with an activation energy of 0.49 eV. The SiC‐Si interface has a large density of defects and dislocations. Most of the irradiation defects are confined to the lower two‐thirds of the band gap. Ninety percent of these defects can be removed by annealing at 350 °C.

Amorphization And Recrystallization Processes In Monocrystaline Beta Silicon Carbide Thin Films

Individual, as well as multiple doses of /sup 27/Al/sup +/, /sup 31/P/sup +/, /sup 28/Si/sup +/, and /sup 28/Si/sup +/ and /sup 12/C/sup +/, were implanted into (100) oriented monocrystalline ..beta..-SiC films. The critical energy of approx. =16 eV/atom required for the amorphization of ..beta..-SiC via implantation of /sup 27/Al/sup +/ and /sup 31/P/sup +/ was determined using the TRIM84 computer program for calculation of the damage-energy profiles coupled with the results of RBS/ion channeling analyses. In order to recrystallize amorphized layers created by the individual implantation of all four ion species, thermal annealing at 1600, 1700, or 1800/sup 0/C was employed. Characterization of the recrystallized layers was performed using XTEM. Examples of SPE regrown layers containing precipitates and dislocation loops, highly faulted-microtwinned regions, and random crystallites were observed.