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The effect of KOH pretreatment for Au/Ni Schottky contacts to GaN is investigated using I-V and x-ray photoemission spectroscopy (XPS) analysis. The molten KOH pretreatment reduces the interface trap density from 1.0×1012 to 2×1011 cm−2 eV−1, improves the on-state performance, and increases the barrier height by 10%. XPS indicates that KOH improves the GaN Schottky diode performance by eliminating an oxide layer between the metal and the semiconductor, increasing the band bending through charge transfer, and improving the GaN stoichiometry at the surface. First principle simulations indicate that the nitrogen antisite and to a minor extent the gallium antisite are also possible constituents of this interfacial layer along with gallium and nitrogen vacancies. These antisite defects can be passivated by KOH.

The effectiveness of AlN/BN and graphite annealing caps for ion implanted SiC was examined morphologically, structurally, chemically, and electrically. The AlN/BN cap more effectively blocks the out-diffusion of silicon because it is essentially inert. Relatively small amounts of silicon from the SiC diffuse out into the graphite cap and react with it. This has the effect of lowering the Si vapor pressure so that it does not create blow holes in this relatively weak structure. The graphite cap can be removed easily with an oxygen plasma, while warm KOH has to be used to remove the nitride cap, and not all of it can be removed after an 1800°C anneal. The out-diffusion of silicon through the graphite cap is most severe at 1800°C, where it roughens the SiC surface and forms reaction products when the Si reacts with the graphite at a significant rate. At lower temperatures these reaction products form small particulates on the surface that are visible only at higher magnifications. In those regions where the graphite cap crystallized, isolated morphological damage on the SiC surface was also detected, appearing to be in the vicinity of the grain boundaries. Scratches on the substrate surface were not affected even at temperatures as high as 1800°C.

In this paper we report recent advances in pulsed-laser-deposited AIN thin films for high-temperature capping of SiC, passivation of SiC-based devices, and fabrication of a piezoelectric MEMS/NEMS resonator on Pt-metallized SiO2/Si. The AlN films grown using the reactive laser ablation technique were found to be highly stoichiometric, dense with an optical band gap of 6.2 eV, and with a surface smoothness of less than 1 nm. A low-temperature buffer-layer approach was used to reduce the lattice and thermal mismatch strains. The dependence of the quality of AlN thin films and its characteristics as a function of processing parameters are discussed. Due to high crystallinity, near-perfect stoichiometry, and high packing density, pulsed-laser-deposited AlN thin films show a tendency to withstand high temperatures up to 1600°C, and which enables it to be used as an anneal capping layer for SiC wafers for removing ion-implantation damage and dopant activation. The laser-deposited AlN thin films show conformal coverage on SiC-based devices and exhibit an electrical break-down strength of 1.66 MV/cm up to 350°C when used as an insulator in Ni/AlN/SiC metal-insulator-semiconductor (MIS) devices. Pulsed laser deposition (PLD) AlN films grown on Pt/SiO2/Si (100) substrates for radio-frequency microelectrical and mechanical systems and nanoelectrical and mechanical systems (MEMS and NEMS) demonstrated resonators having high Q values ranging from 8,000 to 17,000 in the frequency range of 2.5–0.45 MHz. AlN thin films were characterized by x-ray diffraction, Rutherford backscattering spectrometry (in normal and oxygen resonance mode), atomic force microscopy, ultraviolet (UV)-visible spectroscopy, and scanning electron microscopy. Applications exploiting characteristics of high bandgap, high bond strength, excellent piezoelectric characteristics, extremely high chemical inertness, high electrical resistivity, high breakdown strength, and high thermal stability of the pulsed-laser-deposited thin films have been discussed in the context of emerging developments of SiC power devices, for high-temperature electronics, and for radio frequency (RF) MEMS.

The SiC wafers implanted with Al were capped with AlN, C, or AlN and C and were annealed at temperatures as high as 1700°C to examine their ability to act as annealing caps. As shown previously, the AlN film was effective up to 1600°C, as it protected the SiC surface, did not react with it, and could be removed selectively by a KOH etch. However, it evaporated too rapidly at the higher temperatures. Although the C did not evaporate, it was not a more effective cap because it did not prevent the out-diffusion of Si and crystallized at 1700°C. The crystalline film had to be ion milled off, as it could not be removed in a plasma asher, as the C films annealed at the lower temperatures were. A combined AlN/C cap also was not an effective cap for the 1700°C anneal as the N or Al vapor blew holes in it, and the SiC surface was rougher after the dual cap was removed than it was after annealing at the lower temperatures.

Aseries of single energy Al, B, and Ga ion implants were performed in the energy range 50 keV to 4 MeV into 6H-SiC to characterize the implant depth profiles using secondary ion mass spectrometry (SIMS). From the implant depth profiles empirical formulae were developed to model the range statistics as functions of ion energy. Multiple energy implants were performed into 6H- and 4H-SiC and annealed with both AlN and graphite encapsulants to determine the ability of the encapsulants to protect the implants from out-diffusion and redistribution. Al and Ga were thermally stable, but B out-diffused even with AlN or graphite encapsulation. Electrical activation was determined by Hall and capacitance-voltage measurements. An acceptor substitutional concentration of 7×1016 cm−3 was achieved for 1×1017 cm−3 Al implantation.

AlN films deposited on SiC or sapphire substrates by pulsed laser deposition were annealed at 1200°C, 1400°C, and 1600°C for 30 min in an inert atmosphere to examine how their structure, surface morphology, and substrate-film interface are altered during high temperature thermal processing. Shifts in the x-ray rocking curve peaks suggest that annealing increases the film density or relaxes the films and reduces the c-axis Poisson compression. Scanning electron micrographs show that the AlN begins to noticeably evaporate at 1600°C, and the evaporation rate is higher for the films grown on sapphire because the as-deposited film contained more pinholes. Rutherford backscattering spectroscopy shows that the interface between the film and substrate improves with annealing temperature for SiC substrates, but the interface quality for the 1600°C anneal is poorer than it is for the 1400°C anneal when the substrate is sapphire. Transmission electron micrographs show that the as-deposited films on SiC contain many stacking faults, while those annealed at 1600°C have a columnar structure with slightly misoriented grains. The as-deposited films on sapphire have an incoherent interface, and voids are formed at the interface when the samples are annealed at 1600°C. Auger electron spectroscopy shows that virtually no intermixing occurs across the interface, and that the annealed films contain less oxygen than the as-grown films.

The contact resistance, ρ C , was measured for the traditional Ti/Al/Ni/Au Ohmic contact for samples implanted with Si to >10 20 cm −3 and annealed at 1100, 1150, 1200, or 1250 °C for 2, 5 or 10 min using an AlN annealing cap. These results are compared with those for samples annealed in the same way, but were not implanted. The as-grown samples were doped to 3.56 × 10 17 or 6.67 × 10 16 cm −3 or were unintentionally (UI) doped. In almost all cases, ρ C for the implanted sample was lower, and a record low ρ C = 2.66 × 10 −8 Ω cm 2 was achieved for the more heavily doped implanted sample annealed at 1200 °C for 10 min. ρ C decreased with the doping concentration, and for the UI samples, Ohmic contacts could be made only if the samples were implanted. The surface roughness was also measured, and it was found for an as-grown with an RMS roughness of 0.303 nm, the roughness increased from 0.623 after an 1100 °C anneal to 3.197 nm after a 1250 °C anneal for the implanted samples annealed for 10 min, and it increased from 1.280 nm to 5.357 nm under the same conditions for the samples that were not implanted.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Advancing technology continues to place greater and greater demands on semiconductor devices. It is clear that Si technology alone will not be able to meet all of these demands. Silicon Carbide (SiC) is a promising material for highpower and high-temperature applications, such as SiC devices for controlling power in a more electric vehicle in which the SiC device is cooled by the engine oil (200 C.) SiC is well suited for high-power/temperature applications due to its large bandgap of 3.03 eV (for 6H), high breakdown electric field of 2.4 x 106 V/cm (again for 6H), thermal stability, and chemical inertness. These properties hold the promise of reliable and robust performance, but the latter two also present challenges to fabricating such devices. For instance, a key part of making devices involves selected area doping. This is typically accomplished with ion implantation, because the rate of diffusion is so low, followed with an anneal to remove the implant damage and electrically activate the dopant.