SiC substrates

Tongay S, Schumann T, Hebard AP (2009) Graphite based Schottky diodes formed on Si, GaAs, and 4H-SiC substrates. Appl Phys Lett 95 222103... 

LED due to the direct bandgap of the Ill-nitrides. However, due to the lack of a native substrate for GaN, sapphire or SiC substrates were and are still used. The biggest use of semiconductor-grade SiC is still for LEDs, but now it serves the role as the substrate for the active GaN layer rather than both the substrate and the active layer. Today there are high-freqnency metal-semiconductor-field effect transistors (MES-EETs) offered commercially, as well as an emerging market for Schottky diodes made from SiC. We are still at the beginning of the SiC revolution, however, and the material s full potential has yet to be realized. 

However, such comparison is a meaningless exercise. A comparison of silicon, sapphire, GaAs, or other established materials, rather than other SiC growth techniques, should occur instead. The blue and white LEDs can, for instance, be made on either sapphire or SiC substrates. From a performance point of view, there appear to be advantages with the SiC substrate but the substrate cost is lower on sapphire, therefore making it the most-used substrate. Granted, once larger substrates of SiC are made available, it is doubtful that the LED cost will be lower on sapphire. 

Work is ongoing to reduce defects in SiC material. One of the more interesting concepts is the reduction of defects through epitaxial growth on porous SiC substrates [64]. This approach has clearly demonstrated a reduction in intrinsic defects, as evidenced by photoluminescence measurements. It is too early to tell whether this technique can provide a path forward for the bipolar devices but it will clearly find its applicability in several areas where SiC will have a market. 

A depth profile analysis of trace and matrix elements (B, Na, Ni, Fe, Mg, V, A1 and C) in a 26p.m Si layer on a SiC substrate measured by GDMS, yielded impurity profiles, for example, with constant Ni contamination in the Si layer and enrichment at the interface layer.45 However, with respect to depth profiling of thin layers using dc GDMS with a depth resolution between 50 and 500 nm, this technique plays a subordinate role compared to the commercially available and cheaper GD-OES (glow discharge optical emission spectrometry). 

The surface morphology of ZnO films on sapphire, 3C-SiC/Si, and 6H-SiC substrates was also investigated by scanning electron microscopy (SEM) as... 

For epitaxial layers on sapphire, their polarity depends on the buffer structure [9], Most of such layers contain defects where the polarity is reversed [10], For SiC substrates, the nitride layers reproduce the substrate polarity the Si-face is overgrown with a Ga-terminated layer and the C-face is overgrown with an N-terminated layer. 

Hossain et al [15] measured die cathodoluminescence in undoped aluminium nitride (AIN) thin films at 300, 77 and 4.2 K. These films were grown on sapphire and SiC substrates by LP-MOCVD at 1473 K. As shown in FIGURE 1, two distinct peaks were observed at about 6.1 and 5.9 eV, respectively. The energy position of these peaks increases and the linewidth becomes narrower, as the temperature is decreased. They believed that these two peaks are due to exciton recombination. Recently, MacMillan et al measured cathodoluminescence in AlN-GaN superlattices [16],... 

FIGURE 4 1.7 K reflectance spectra for GaN epilayers grown on various C-plane AI2O3 and 6H-SiC substrates. The strains are those measured from the radius of curvature at 293 K and corrected to 1.7 K. The data are taken from Skromme [1], who can also fit the evolution of the oscillator strengths with strain using his zero strain value for the A]s exciton line and a crystal field splitting parameter of 3.7 1.4 meV. 

As well as the lattice mismatch between the layer and the substrate, other factors influence the FWHM of the RC, including thickness of the buffer layer, miscut of the substrate, substrate quality, growth conditions etc. For example, Warren Weeks et al [16] reported 00.2 FWHMs of 58 and 151 arc sec for 1.4 pm GaN layers grown on on-axis and off-axis (3 - 4° toward the <11.0>) SiC substrates, respectively. The corresponding FWHMs for the 0.1 pm AIN buffer layers were approximately 200 and 400 arc sec. For a thicker GaN layer of 2.7 pm, the off-axis sample also exhibited an FWHM of 66 arc sec. 

FIGURE 1 Near-infrared transition metal PL in GaN samples grown (a) on p-SiC substrate by the sublimation sandwich technique, and (b) on sapphire substrate bv HVPE. 

B2.10 Lateral epitaxy and microstructure in selectively grown GaN on SiC substrates... 

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