The desire for UV-emitting photonic devices has produced much activity in the growth of AlxGa1-xN-based devices. A typical growth process by the metal-organic chemical vapor deposition (MOCVD) method for aluminum nitride or aluminum gallium nitride consists of the growth of a thin nucleation layer (typically less than 10 nm) at a temperature typically between 400° C. and 700° C. Common precursors when the nucleation layer is AlN are ammonia and trimethyl aluminum (TMA) mixed together in the deposition chamber or separately pulsed into the deposition chamber; typical pressures are between 40 and 150 Torr. Following the formation of this very thin nucleation layer, a thicker AlN or high-Al-content AlxGa1-xN layer is grown at temperatures in excess of 1050° C. and at pressures typically below 150 Torr. Growth rates of the high-temperature AlN or AlxGa1-xN layer under these conditions are typically near or less than about 1 micrometer per hour. For AlN films on sapphire substrates, threading dislocation densities with an edge or mixed edge+screw character are typically very high, greater than 1×1010/cm2. Much lower densities (≦108/cm2) are highly desirable for devices. The growth of structures with epitaxial layer thicknesses in excess of 1 micrometer without cracking can be problematic as well with conventional growth techniques.
Wang et al. (T. Wang, J. Bai, P. J. Parbrook, and A. G. Cullis, “Air-bridged lateral growth of an Al0.98Ga0.02N layer by introduction of porosity in an AlN buffer,” Appl. Phys. Lett. vol. 87 (2005) pp. 151906-1-151906-3) reports significant dislocation reduction by introduction of a porous AlN buffer via metalorganic chemical vapor deposition. An approximately 500-nm AlN layer with a high density of pores was directly grown on a sapphire substrate at 1150° C. An important feature is pointed out that, although the column-like islands do not coalesce, the top of each island is very flat. Decreasing temperature and increasing V/III ratio can increase the area of the pores, resulting in a rough surface for the high-Al-composition AlGaN layer subsequently grown on the porous layer.
Kida et al. (Y. Kida, T. Shibata, H. Miyake, and K. Hiramatsu, “Metalorganic Vapor Phase Epitaxy Growth and Study of Stress in AlGaN Using Epitaxial AlN as Underlying Layer,’Jpn. J. Appl. Phys. Vol. 42, (3002) pp. L572-L574) report the growth of AlGaN on an AlN underlying layer. An epitaxial AlN film of 1 micrometer thickness grown on sapphire (0001) by low-pressure metalorganic vapor phase epitaxy (LO-MOVPE) was used as a substrate; it had an atomically flat surface.
Hiramatsu et al. (K. Hiramatsu, H. Miyake, H. Yoshida, T. Urushido, and Y. Terada, “Group III nitride semiconductor film and its production method,” U.S. Patent Application No. 20040157358) reports a group III nitride semiconductor film involving few lattice defects and having a large thickness, and a process for making the film. Dry-etching is conducted while a quartz substrate and a group III nitride semiconductor are placed on the top of a lower electrode. Nano-pillars are formed on the top of the group III nitride semiconductor. Another group III nitride semiconductor film is deposited on the nano-pillars.