Profiling electric fields around dislocations in GaN
- PDF / 1,124,514 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 19 Downloads / 238 Views
Profiling electric fields around dislocations in GaN D. Cherns, C.G.Jiao and H. Mokhtari H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, U.K.
Abstract Electron holography has been used to measure the electric potential around edge and screw dislocations in n-GaN viewed in a near end-on geometry. It is shown that the potential at edge dislocations is 2V below that in the bulk consistent with a negative charge of 2 electrons/c (c = 0.52nm). Preliminary results, which suggest that screw dislocations are also negatively charged, are discussed. Introduction The detection of charge on dislocations in GaN is of great interest for understanding the role of dislocations in both electronic and light emitting devices. Indeed, a range of observations, including transport data [1], scanning capacitance microscopy from GaN surfaces [2] and photoelectrochemical etching [3], have suggested that dislocations in n-GaN can be highly negatively charged. In all cases, however, the conclusion that dislocations are charged is indirect. In this paper we show that electron holography carried out in the transmission electron microscope (TEM) provides a direct means of probing the charge on individual dislocations. Measurements of the charge on edge and screw dislocations are presented and discussed. Experimental method The experiments were carried out on a GaN film, highly n-doped with Si (around 1018 cm-3), which was grown by metal-organic chemical vapour deposition on (0001)sapphire. Samples were prepared by for TEM in plan view orientation by backthinning using standard methods of mechanical polishing and ion-thinning. To minimize ion beam damage, and to achieve a relatively flat surface, the final stages of ion thinning were carried out at 4kV and at an angle of incidence less than 12°. Samples were then examined at 200kV in a Hitachi HF2000 field emission gun TEM equipped with an Moellenstedt biprism for electron holography. The principle of the electron holographic method is shown schematically in fig. 1. The purpose of the electron biprism is to deflect and thereby overlap the electron beam which has passed through the specimen with a reference beam which, ideally, passes round the sample edge. Provided the beams are coherently related, an interference pattern is formed in which the fringes are displaced owing to phase changes in the sample. Under kinematical conditions, the phase change due to the specimen, ϕ is simply given by
ϕ = C E V0 t
(1)
where CE is a constant, V0 is the average inner potential and t is the film thickness. The phase change is then most sensitive to electric fields in the horizontal plane. For example, for a I10.2.1
horizontal field Ex in direction x, the change in inner potential over distance δx is δV0 = -Exδx. Given that the field associated with a charged dislocation should be in the radial direction around the dislocation line, we should examine dislocations in the end-on geometry.
Z
Specimen
Objective Lens
Biprism
+
Image Hologram Fig. 1. The principle of
Data Loading...
