AlN crystal growth by PVT method

The third generation of wide bandgap semiconductor materials, including gallium nitride (GaN), silicon carbide (SiC), and aluminum nitride (AlN), exhibit excellent electrical, thermal, and acousto-optical properties. These materials address the limitations of the first and second generation of semiconductor materials, significantly advancing the semiconductor industry.

Currently, the preparation and application technologies for SiC and GaN are relatively well established. In contrast, research on AlN, diamond, and zinc oxide (ZnO) is still in its early stages. AlN is a direct bandgap semiconductor with a bandgap energy of 6.2 eV. It boasts high thermal conductivity, resistivity, breakdown field strength, and excellent chemical and thermal stability. Consequently, AlN is not only an important material for blue and ultraviolet light applications but also serves as an essential packaging, dielectric isolation, and insulation material for electronic devices and integrated circuits. It is particularly well-suited for high-temperature and high-power devices.

Moreover, AlN and GaN exhibit good thermal matching and chemical compatibility. AlN is often used as a GaN epitaxial substrate, which can significantly reduce defect density in GaN devices and enhance their performance. Due to its promising application potential, researchers worldwide are paying considerable attention to the preparation of high-quality, large-size AlN crystals.

Currently, the methods for preparing AlN crystals include the solution method, aluminum metal direct nitridation, hydride vapor phase epitaxy (HVPE), and physical vapor transport (PVT). Among these, the PVT method has become the mainstream technology for growing AlN crystals due to its high growth rate (up to 500-1000 μm/h) and superior crystal quality, with dislocation density less than 10^3 cm^-2.

Principle and process of AlN crystal growth by PVT method

AlN crystal growth by PVT method is completed through the steps of sublimation, gas phase transport and recrystallization of AlN raw powder. The growth environment temperature is as high as 2300℃. The basic principle of AlN crystal growth by PVT method is relatively simple, as shown in the following formula: 2AlN (s) =⥫⥬ 2Al (g) + N2 (g) (1)

The main steps of its growth process are as follows: (1) sublimation of AlN raw powder; (2) transmission of raw material gas phase components; (3) adsorption of gas phase components on the growth surface; (4) surface diffusion and nucleation; (5) desorption process [10]. Under standard atmospheric pressure, AlN crystals begin to slowly decompose into Al vapor and nitrogen at around 1700 °C. When the temperature reaches 2200 °C, the decomposition reaction of AlN rapidly intensifies. Figure 1 is a curve showing the relationship between the partial pressure of AlN gas phase products and the ambient temperature. The yellow area in the figure is the process temperature of AlN crystals prepared by PVT method. Figure 2 is a schematic diagram of the growth furnace structure of AlN crystals prepared by PVT method.

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