Applied Surface Science 317 (2014) 1010–1014 Contents lists available at ScienceDirect Applied Surface Science journal h om epa ge: www.elsevier.com/locate/apsusc Low fraction of hexagonal inclusions in thick and bulk cubic GaN layers S.N. Waheeda a , N. Zainal a,∗ , Z. Hassan a , S.V. Novikov b , A.V. Akimov b , A.J. Kent b a Nano-optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800, Penang, Malaysia b School of Physics and Astronomy, University of Nottingham, Nottingham NG7 2RD, UK a r t i c l e i n f o Article history: Received 30 May 2014 Received in revised form 6 August 2014 Accepted 31 August 2014 Available online 6 September 2014 Keywords: Hexagonal inclusions Thick cubic GaN Bulk cubic GaN a b s t r a c t We report present a study of the formation of hexagonal inclusions in thick (∼5 m) and bulk (∼50 m) cubic GaN (c-GaN) layers grown on GaAs (0 0 1) substrates. Surface analysis of both samples revealed the evidence of hexagonal grains on the surface. However, larger grains were formed in the bulk sample. In the XRD measurement, the signature of the h-GaN in the bulk sample was more visible than the thick sample. This is consistent with the PL measurement, of which the h-GaN peak emission was found to be more intense in the bulk sample with respect to the thick sample. Such observations showed that the fraction of hexagonal inclusions had increased by increasing the thickness of the c-GaN layer. As presented in the Raman spectroscopy, the signals of the h-GaN were dominant, but the transverse optical (TO) and longitudinal optical (LO) modes of c-GaN were still observable at ∼559 cm -1 and ∼739 cm -1 , respectively. Finally, a micro-PL measurement showed that the average content of the hexagonal inclusions in the c- GaN layer up to ∼50 m was less than 22%, suggesting that our samples have a lower hexagonal GaN content than some reported thin c-GaN samples. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Gallium nitride (GaN) has been regarded to a preferable mate- rial for high temperature operation as well as high power and high frequency applications. Commonly, GaN is crystallized in a thermo- dynamically stable hexagonal (wurtzite) structure. However, the hexagonal GaN (h-GaN) always contains strong piezoelectric and spontaneous polarization fields [1] that would limit the perfor- mance of optoelectronic devices, such as laser and light emitting diodes (LEDs). Growing GaN in cubic structure is a possible way to eliminate the internal fields. In addition to that, cubic GaN (c- GaN) offers easy cleavage along (1 0 0) facet orientation, has high carrier mobility (especially holes due to higher crystal symmetry) and improves the reproducibility of negative differential resistance (NDR) characteristics in resonant tunneling diodes (RTDs) [2,3]. Despite of this, there still a big challenge to produce high purity c-GaN due to the metastable nature of the cubic structure. Hence, the stable h-GaN phase structure is easily incorporated within the c-GaN lattice structure. Such inclusions may affect the purity of the c-GaN and impair its potential to be adopted in device appli- cations. Note that, it is difficult to promote c-GaN growth even at ∗ Corresponding author. Tel.: +604 653 5327; fax: +604 657 9150. E-mail address: norzaini@usm.my (N. Zainal). the initial stage and even harder to maintain it for longer period of growth. For this reason, c-GaN bulk/substrate is difficult to pro- duce. In general, c-GaN layers are grown on non-lattice matched substrates like GaAs and 3H-SiC. If c-GaN bulk/substrate is avail- able, the formation of defects, strains and cracks can be minimized and therefore progress in c-GaN based research can be accelerated. Here, we demonstrate c-GaN layers grown at larger thickness (up to ∼50 m) with low hexagonal inclusions in an effort to make it suitable for further fabrications and processes. 2. Experimental procedures In this work, c-GaN layers were grown on (0 0 1)-oriented GaAs substrates at the thickness of ∼5 m (thick) and ∼50 m (bulk) using MBE Gen-II system. The growth temperature of the c-GaN lay- ers was constantly kept at 680 ◦ C. The HD-25 RF-activated plasma source was used to supply the active nitrogen (N) species for grow- ing thick layer. Details of the growth procedures can be found in [1,4]. The evidence of the hexagonal inclusions was investigated through field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), photoluminescence (PL) and Raman spec- troscopy measurements. In the FE-SEM measurement, the surface images were taken up to 50,000× of magnification. For better images of the cross-section, the samples were carefully cleaved prior to the measurement so that the evidence of stacking faults http://dx.doi.org/10.1016/j.apsusc.2014.08.186 0169-4332/© 2014 Elsevier B.V. All rights reserved.