Indium nitride (InN), like other group-III nitrides, is a polar semiconductor in wurtzite structure, which exhibits strong orientation-dependent electronic and optical properties. It is now well recognized that the surface and interface properties of III-nitride films grown in the polar directions are drastically different from their counterparts grown in the nonpolar directions. In this talk, a cross-sectional view of wurtzite III-nitrides will be presented for studying the electronic properties of nonpolar InN surface and interface. For these studies, a wurtzite N-polar (–c-axis), undoped InN/GaN/AlN heterostructure, consisting of a InN top layer, a GaN intermediate layer, and a AlN bottom layer, was grown by plasma-assisted molecular beam epitaxy (PA-MBE) on a Si (111) substrate with relaxed lattices. Using this III-nitride heterostructure sample grown on Si substrates, in situ cleavage becomes a viable approach to expose the clean, nonpolar {-1-120} surface (i.e., the a-plane) under ultra-high vacuum conditions. To investigate the cleaved surfaces which present a small cross section on the order of a few mm, a special experimental technique, cross-sectional scanning photoelectron microscopy and spectroscopy (XSPEM/S), was applied to achieve the required spatial resolution. The unique features of XSPEM/S for studying InN surface and interface are: (1) Elimination of the polarization effects associated with the interface charges/dipoles normal to the cleaved cross-sectional surfaces of III-nitride heterojunctions, and (2) Formation of nonpolar InN surface without the influence of indium metal adlayers.

    At the III-nitride heterojunctions, the spontaneous and piezoelectric polarizations [1] manifest themselves with bound interface charges and built-in electrostatic fields. It has been found that the discontinuities of spontaneous and piezoelectric polarizations play an important role in the "apparent" band lineups at the III-nitride heterojunctions. Martin et al. reported that the strong asymmetry of measured valence band offset (VBO) vs. growth sequence is due to the built-in piezoelectric fields with opposite directions [2]. Very recently, we also reported that, under nearly strain-free conditions, large variation in VBO can still be found for InN/GaN heterojunctions grown with different crystal polarities (cation-polar vs. anion-polar) [3]. This phenomenon was explained by the presence of discontinuity of spontaneous polarizations across the III-nitride heterojunction. Therefore, the "intrinsic" VBOs of III-nitride heterojunctions can be obtained only after eliminating the effects of polarization fields. The XSPEM/S measurement of "intrinsic" VBO of InN/GaN heterojunction by using photoelectron emissions from the in-situ cleaved a-plane surface [4] will be presented here as an example to demonstrate its unique feature.

    In addition, indium nitride (InN), due to its narrow direct bandgap and superior electron transport properties, has recently emerged as a technologically important semiconductor for use in near-infrared optoelectronics, high-efficiency solar cells, and high-speed electronics. The current obstacle for extensive fundamental studies and widespread applications of InN is mostly related to its unique surface electronic properties [5], resulting from the exceptionally large electron affinity. In particular, it has been found that the growth surfaces of InN exhibit a striking phenomenon that an intrinsic electron accumulation layer can form in the near-surface region of as-grown, unintentional n-type InN. This phenomenon has been experimentally confirmed by a variety of different techniques [6,7] and all results indicate that the Fermi level at InN growth surfaces is pinned well above the conduction band edge. Based on the first-principles calculations, Segev and Van de Walle suggested that the microscopic origin of donor-type surface states is mostly associated with In-In bonding states on the InN surfaces with In adlayers. Furthermore, they have predicted that the phenomenon of surface electron accumulation will be absent on reconstructed nonpolar InN surfaces without In adlayers [8]. Very recently, using the technique of XSPEM/S, we have provided definitive evidence of unpinned Fermi level for in situ cleaved a-plane InN surfaces [9]. To confirm the presence or absence of band bending, the surface Fermi level relative to the valence band edge was precisely measured by using both the Fermi edge of Au reference sample and the core level of ultrathin Au overlayer. Experimental data unambiguously show the absence of band bending for the in situ cleaved a-plane InN surface. In contrast, all the growth surfaces of InN (polar or nonpolar) with In adlayers exhibit a large downward surface band bending.

Acknowledgment:

This work was supported by grants from the National Science Council, Taiwan.

References:

1. F. Bernardini, V. Fiorentini, and D. Vanderbilt, Phys. Rev. B 56, R10024 (1997).
2. G. Martin, A. Botchkarev, A. Rockett, and H. Morkoç, Appl. Phys. Lett. 68, 2541 (1996).
3. C.-L. Wu, H.-M. Lee, C.-T. Kuo, S. Gwo, and C.-H. Hsu, Appl. Phys. Lett. 91, 042112 (2007).
4. C.-L. Wu, H.-M. Lee, C.-T. Kuo, C.-H. Chen, and S. Gwo, Appl. Phys. Lett. 92, 162106 (2008).
5. R. E. Jones, K. M. Yu, S. X. Li, W. Walukiewicz, J. W. Ager, E. E. Haller, H. Lu, and W. J. Schaff, Phys. Rev. Lett. 96, 125505 (2006).
6. H. Lu, W. J. Schaff, L. F. Eastman, and C. E. Stutz, Appl. Phys. Lett. 82, 1736 (2003).
7. I. Mahboob, T. D. Veal, C. F. McConville, H. Lu, and W. J. Schaff, Phys. Rev. Lett. 92, 036804 (2004).
8. D. Segev and C. G. Van de Walle, Europhys. Lett. 76, 305 (2006)
9. C.-L. Wu, H.-M. Lee, C.-T. Kuo, C.-H. Chen, and S. Gwo, Phys. Rev. Lett. 101, 106803 (2008).

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/18306 must be provided.