Hetero- and homo-epitaxial growth of III-nitride based junctions and devices by molecular beam epitaxy

The family of III-Nitrides semiconductors (InN, GaN, AlN and their alloys) is an area of intense interest for the development of power electronic, optoelectronic and mico-electromechanical systems (MEMS) based devices due to the unique properties of these materials. Presently, light emitting diodes (LEDs) and lasers emitting in the blue part or the electromagnetic spectrum have become available based on these wide band gap materials. Further interest exists in the development of p-n and Schottky based junctions for application as solar-blind ultraviolet (UV) detector arrays and power rectifiers. Solar blind UV detectors require the growth of high quality Al_xGa_1−x N alloys containing up to 60% AlN mole fraction. Both devices require the development of high-quality p-type doped material. This dissertation addresses the growth of GaN and Al_xGa_1−xN alloys by molecular beam epitaxy (MBE) as well as the processing of these materials for the fabrication of optoelectronic and power devices.; The growth of III-Nitrides materials by MBE requires the development of an appropriate source of active nitrogen due to the large binding energy of molecular nitrogen (9.5 eV). Two methods for producing active N are the cracking of molecular nitrogen using a plasma source and the catalytic decomposition of ammonia on a heated substrate. The first method is explored using a compact electron cyclotron resonance (ECR) plasma source, and it is found that smooth films are grown by this technique under group-III rich conditions. The second method is explored using ammonia gas that enters the MBE system through an ammonia injector. In contrast to the plasma-assisted technique, smooth films can be grown under N-rich conditions. Due to the lack of native substrates for these materials, these films are generally deposited heteroepitaxially on c-plane sapphire or (0001) Si-face 6H-silicon carbide (SiC) substrates. The polarity of films grown on the former was observed to be dependent on the nucleation conditions, while Ga-polar films are deposited directly on the latter without the use of buffer layers. GaN films of both polarities were doped p-type using Mg, but Ga-polar material is observed to dope more efficiently than N-polar material under Ga-rich conditions. The n-type doping of Al_xGa_1−xN alloys containing up to 50% AlN mole fraction has been explored and it is found that up to 50% AlGaN alloys can be doped degenerately to ∼1019 cm −3.; A number photodetectors and power electronic devices have been fabricated based on these materials. Schottky diodes have been fabricated based on homoepitaxial growth of GaN by MBE on GaN templates grown by hydride vapor phase epitaxy (HVPE) on c-plane sapphire substrates. Photodetectors based on these materials have high quantum efficiency, 70%. GaN pn junctions fabricated by depositing a p-GaN layer on similar GaN templates show reduced leakage current in comparison to heteroepitaxially grown devices. A number of high power rectifiers have been fabricated by depositing n-GaN films on p-type epitaxial SiC. These heterojunctions have a very large breakdown (>830 V) and good ideality. A number of pnpn thyristor structures have been fabricated for application as optically gated switches. These devices show forward breakover voltage of 14V without requiring any rapid thermal annealing. The holding current and voltage for these devices was found to be 11 mA and 2 V.