The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

In this paper, we review the metal–GaN contact technology to shed light on some of the critical issues such as GaN surface cleaning for metallization, Schottky barrier formation to GaN, thermal stability of contacts, low contact-resistance ohmic contacts to n-GaN and to GaN/AlGaN heterostructures, and ohmic contacts to p-GaN. Some general conclusions are drawn on the electrical behavior of metal contacts on n-GaN for further advances in this exciting field of GaN-based devices and materials.

A review of the metal–GaN contact technology

Synchrotron radiation-based x-ray photoemission spectroscopy was used to study the surface Fermi level position within the band gap for thin metal overlayers of Au, Al, Ni, Ti, Pt, and Pd on n–GaN and p–GaN. Nonequilibrium effects were taken into account by measuring the Fermi edge of the metal overlayer. There are two different behaviors observed for the six metals studied. For Au, Ti, and Pt, the surface Fermi level lies about 0.5-eV higher in the gap for n-type than for p-type GaN. For Ni, Al, and Pd, the surface Fermi level position is independent of doping, but varies from one metal to the other. Results for Ni, Pd, and Al fit a modified Schottky–Mott theory, while Au, Ti, and Pt demonstrate a more complex behavior. Atomic force microscopy was used along with photoemission to investigate the growth mode of each metal on the GaN surface.

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X-ray photoemission determination of the Schottky barrier height of metal contacts to n-GaN and p-GaN

Recently, ultraviolet photoelectronics emerged in response to the needs of medicine, biology, military equipment, and the problem of the hole in the ozone layer. A specific feature of this field of photoelectronics is the need to detect weak (albeit appreciably affecting vital human functions) signals against a background of intense radiation in the visible and infrared regions of the spectrum. Ultraviolet electronics relies on Si-based p-n structures and GaP-based Schottky barriers, p-n structures and Schottky barriers based on GaN and AlGaN (“solar-blind,” i.e., solar-radiation-insensitive devices), SiC structures with potential barriers (high-temperature devices), and ZnO-and ZnS-based photoresistors and Schottky diodes. In this review, the parameters of starting wide-gap semiconductors are given, physical foundations for photoelectric conversion and the principles of formation of ohmic contacts are described, characteristics of corresponding devices are given, and the envisaged lines of further studies are outlined.

Semiconductor photoelectric converters for the ultraviolet region of the spectrum

Large Schottky barriers were measured for Ni contacts formed on low Mg-doped p-GaN. In order to improve leaky Schottky characteristics, low-Mg doping was examined. This provided atomically flat surfaces and a low dislocation density of 5.5×108 cm−2. The Schottky barrier height (qφB) as high as 2.4±0.2 eV and n values of 1.84±0.06 were obtained from current–voltage measurements. These results are in good agreement with the prediction that the sum of qφB of n and p types adds up to the band gap. In the capacitance–voltage measurements, a transient response of capacitance was observed. This indicates that the evaluation of deep levels close to the valence band is possible, which could result in improvement of p-GaN growth.

Large Schottky barriers for Ni/p-GaN contacts

Schottky barriers were formed on p-GaN. p-GaN layers doped with Mg were grown by metalorganic chemical vapor deposition (MOCVD). 6H-SiC wafers were used as substrates. The barriers were made by vacuum thermal evaporation of Au. Capacitance-voltage (C-V) and current-voltage (I-V) characteristics of the barriers were investigated. The concentration of the ionized acceptors in the p-layers was measured to be about {approximately} 10{sup 17} cm{sup {minus}3}. The barrier height was determined to be 2.48 eV by C-V measurements at room temperature. The forward current flow mechanism through the barriers is discussed.

Schottky Barriers on p-GaN

4H-SiC p+n photodiodes based on ultrathin-junctions have been fabricated with distinct processes for the p+-region creation: either with Aluminium conventional ion implantation, or with Boron Plasma Ion Immersion Implantation. Spectral sensitivity measurements were performed at several temperatures from room temperature up to 340°C, with incident wavelengths ranging from 200 to 400 nm. Both responses are characterized by a stability between 200 and 270 nm, and a important increase with temperature between 270 and 380 nm. This fact has to be related to the two different kinds of optical absorption phenomena in SiC with respect to the wavelength, which are direct and indirect (phonon assisted) transitions. When decreasing the temperature, we noticed a hysteresis effect, which could be due to charge trapping by temperature activated defects. After strong proton and electron irradiations, the diodes showed a stability of the response below 270 nm, making them suitable for use in harsh environments. Simulation was performed at room temperature, with a good correlation between simulated and experimental room temperature curves.

E.V. Kalinina

Papers

Schottky Barriers on p-GaN

4H-SiC P+N UV Photodiodes: Influence of Temperature and Irradiation