Development of Aluminum Nitride Single-Crystal Substrates

High-frequency, high-power semiconductor devices are widely used in electric vehicles, medical equipment, home appliances, and communication equipment. Recently, there has been a growing demand for semiconductors that have such characteristics as high withstand voltage, high speed and high-temperature operation, low power consumption and radiation resistance. To meet this demand, silicon carbide (SiC) is increasingly being used as a substrate material instead of silicon (Si), which is reaching the limit of its material properties. For the next generation, the substrate material needs to possess a higher breakdown field and to operate under higher frequencies (Fig. 1). Meanwhile, mercury lamps have been used as ultraviolet (UV) light sources for cleaning applications in semiconductor processing and sterilization in the medical field. It would be advantageous to be able to produce a stable, high-output UV light by a small device that requires no mercury and can start instantly; these characteristics can be realized by the use of UV lightemitting diodes (LEDs). To date, UV-LEDs have been fabricated on sapphire and various other substrate materials. This has resulted in disorder in the crystalline structure inside the emitting layer (crystal defects) and also led to a low luminous efficiency because of the differences between the lattice constant and the crystal structure. Aluminum nitride (AlN) is a promising candidate for UV applications and high-power devices because of the following advantages: The widest bandgap (6.2 eV) in directtransition semiconductors, a high thermal conductivity of 3.3 W/cmK, and excellent electrical insulation properties. High-quality single-crystal substrates are in demand for use in high-efficiency and high-power devices. AlN, particularly a bulk AlN single crystal, is a promising substrate material for these devices because of the small differences in lattice constants and thermal expansion coefficients between AlN and Al1-xGaxN (0 ≤ x ≤ 1) epitaxial layers. Bulk AlN single crystals are commonly grown from the vapor phase, e.g., by sublimation(1) or by the hydride vapor-phase epitaxial (HVPE) method,(2) because their high melting point makes it difficult to grow them from the melt phase. During sublimation, the raw material is sublimated at a high temperature and then its vapor is recondensed in a lower-temperature area. Thus far two techniques have been reported to grow bulk AlN single crystals without AlN seeds: spontaneous nucleation without a seed crystal(3) and heteroepitaxial growth using a foreign substrate such as SiC(4),(5). The former can produce high-quality crystals, but has difficulties with both the growth of larger crystals and reproducibility. The latter can produce large crystals with relative ease; however, the quality of the crystals is not as high as with the former method because of lattice mismatch and SiC decomposition at high temperatures. We have already succeeded in growing free-standing AlN single crystals with high crystalline quality by using c-plane SiC as seed substrates(6)-(9). Figure 2 shows the structure and surface orientation of an AlN crystal. As the effect of a piezoelectric field can be suppressed, field-effect transistors (FETs) and other Development of Aluminum Nitride Single-Crystal Substrates