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TDI Cracks AlN Template Trouble

GaN HEMT manufacturers and ultraviolet LED developers are using large crack-free AlN-based templates thanks to a new deposition process, say TDI's Vladimir Dmitriev and Alexander Usikov.

AlN substrates are well suited to the fabrication of ultraviolet LEDs and could boost the performance high-frequency transistors used in base-station infrastructure. The LEDs benefit from AlN s transparency at wavelengths greater than 200 nm, while the performance of RF devices is aided by AlN s very high thermal conductivity, electrical insulation and a crystal lattice that closely matches that of AlGaN.

However, despite years of development, it is still very difficult to grow single crystals of AlN with low enough defect densities and their size is insufficient for commercial applications. For example, our work at Technologies and Devices International (TDI), MD, has been restricted to the fabrication of 2 inch AlN wafers using a free-standing approach, while 2 inch substrates only became commercially available very recently through Crystal IS. Although the availability of 2 inch material represents some progress, this size is unable to satisfy the demands of electronic device manufacturers who want to use 3 and 4 inch substrates now, and 6 inch substrates in the future. This appetite for larger substrates has led to various AlGaN-based devices being developed on foreign substrates.

One way of accelerating AlGaN-based device development and commercialization is to use engineered templates, which consist of a native AlN surface for subsequent device epitaxy and a base made of a different material, like silicon, sapphire or SiC. An advantage of this is that the wafer s size is then determined by the dimensions of the base substrate (see figure 1).

Using this technique, templates are produced by depositing a single-crystal AlN epitaxial layer onto a foreign substrate at a high growth rate to form a thick, low-defect layer. Thick AlN is essential for reducing the defects that result from growth on a foreign substrate because the defect density rapidly decreases with distance from the AlN/substrate interface.

Template substrates with sufficiently thick AlN layers can also deliver excellent electrical insulation for the upper device structure because AlN s electrical resistivity is higher than 1011 Ω cm at room temperature. In addition, the AlN layer has a thermal conductivity of at least 3 W cm–1 K–1, which can boost device performance, and a native AlN surface for lattice-matched growth.

In our opinion the only suitable method for producing such thick epitaxial layers is hydride vapor phase epitaxy (HVPE). MOCVD and MBE have typical growth rates of less than 1–2 µm per hour and using these methods to deposit 10 µm or more of AlN is too expensive and time-consuming. HVPE, however, can produce low-defect GaN and AlN layers at much lower costs and at rates that can exceed 1 µm per minute.

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