(Invited) Characterization of Metal/GaN Schottky Contacts - Review from the Early Days

GaN-based materials have been intensively developed for blue light-emitting-diodes, blue laser diodes, power amplifiers in wireless base-stations, and high-power switching devices. Schottky contacts are one of the most important components in electron device fabrication. The electrical characteristics of GaN Schottky barrier contacts have been continuously studied for more than twenty years as improvement of GaN crystal quality and understanding physics of metal/semiconductor interfaces. In the early days, for the achievement of the high-power and high-temperature operation, thermally stable contacts were explored. Interfacial reactions and electrical characteristics were intensively examined by annealing studies for various kinds of metal contacts such as Ni, Pd, Pt, and Re. As a result of those, Ni has been widely used as a stable Schottky contact, because less thermal reaction with N atoms, high Schottky barrier height (qφ B), and good adhesion. As for the correlation with GaN crystal quality, in the early days, because a GaN epitaxial layer contained large-structure killer defects, rectifying characteristics were hardly observed in the Schottky contacts. Since after the n-GaN crystal quality improved with electron mobility over 100 cm2V−1 s−1, rectifying characteristics have been reproducibly obtained.[1] Even though the dislocation density was as high as 109–1010 cm−2 in epitaxial GaN layers, good current–voltage (I–V) characteristics and qφ B values below 1.1 eV were reported in Ni/n-GaN contacts. Moreover, in order to clarify the effect of dislocations on I–V characteristics, a 0.5-µm-diameter Schottky-dot array was formed on n-GaN grown by MOCVD, and I –V measurements were conducted using AFM with a conductive probe.[2] Neither mixed nor pure edge dislocations affected the I –V characteristics, but a large structural defect with a diameter of a few hundred nanometers shorted the contact. In addition, further improvement of the GaN layer grown on low dislocation (∼106 cm−2) free-standing GaN substrates provided ideal Schottky contacts with high qφ B values around 1.1 eV. It has been reported that the forward and reverse I–V characteristics of Ni contacts formed on hydride-vapor-phase-epitaxy grown n-GaN obeyed the thermionic emission and the thermionic field emission (TFE) models, respectively.[3] The same trend was also reported in nine different metal contacts on both c- and cleaved m-plane n-GaN crystals.[4] We also reported a post-metallization annealing at 400°C for 10 min was effective to reduce the reverse biased current significantly to the prediction by the TFE model.[5] Basic understanding of p-GaN Schottky contacts was slower than that of n-GaN, because p-GaN contacts were intensively studied for ohmic contacts of p-n-junction-base optical devices. In the early days, very leaky characteristics were reported and a reasonable qφ B value was not obtained. We improved leaky characteristics of Ni/p-GaN Schottky contacts by low Mg doping. The qφ B as high as 2.4 eV and the n-values of 1.8 were obtained from I –V.[6] We found a memory effect that carrier capture and emission from acceptor-like deep level defects caused depletion layer width to vary significantly.[7] Finally, as our original technique, scanning internal photoemission spectroscopy was developed to map the electrical characteristics of metal/semiconductor interfaces nondestructively.[8] We have demonstrated the mapping of characteristics for interfacial reactions of Ni/n-GaN at 400°C; degradation under applied voltage stress up to 45 V for vertical n-GaN contacts; and process induced surface damages by ion-implantation, dry etching, and photo-electrochemical etching.[9] This technique was confirmed to be useful for the development of the wide-bandgap-semiconductor high-power devices. K. Shiojima, J. Woodall, P. Grudowski, C. Eiting and R. Dupuis, J. Vac. Sci. Tech., B17(5), p. 2030 (1999). K. Shiojima, and T. Suemitsu, J. Vac. Scie. Tech., B21(2) p. 698(2003). J. Suda, K. Yamaji, Y. Hayashi, T. Kimoto, K. Shimoyama, H. Namita, and S. Nagao, Appl. Phys. Express, 3, p. 101003 (2010). H. Imadate, T. Mishima, and K. Shiojima, Jpn. J. Appl. Phys., 57, p. 04FG13 (2018). K. Shiojima, R. Tanaka, S. Takashima, K. Ueno, and M. Edo, Jpn. J. Appl. Phys., 60, 056503 (2021). K. Shiojima, T. Sugahara and S. Sakai, Appl. Phys. Lett., 74, p. 1936 (1999). K. Shiojima, T. Sugahara and S. Sakai, Appl. Phys. Lett. 77, p. 4353 (2000). T. Okumura and K. Shiojima, Jpn. J. Appl. Phys., vol. 28, p. L1108 (1989). K. Shiojima, ECS Transactions, 104, p. 69 (2021).