Morphology Control in As-Grown GaN Nanoporous Particles Joan J. Carvajal* ,†,‡ and J. Carlos Rojo †,§ Department of Materials Science and Engineering, State UniVersity of New York, Stony Brook, New York 11794, and Fı ´sica i Cristal · lografia de Materials (FiCMA), UniVersitat RoVira i Virgili (URV), Campus Sescelades, c/ Marcel · lı ´ Domingo s/n, 43007, Tarragona, Spain ReceiVed May 14, 2008; ReVised Manuscript ReceiVed September 18, 2008 ABSTRACT: The influence of different parameters, such as temperature, flow rate of NH 3 , and pressure, in the crystal growth process of GaN micron-size nanoporous particles through the direct reaction of Ga and NH 3 has been studied. Temperature influences porosity and the coalescence of the individual pores. Flow rate of NH 3 influences the degree of nanoporosity of the particles. Pressure is the main parameter controlling the external shape of the particles by modulation of the crystal growth rate along or perpendicular to the c crystallographic direction. It also seems to play a role in controlling the size of the pores that can be obtained. Programmed and controlled changes in pressure during the growth experiment resulted in interesting nanoporous structures with benefits for extraction of light and fabrication of electrical contacts on these particles. 1. Introduction Recent advances in the technologies to produce porous semiconductor materials has propelled the use of these materials in the fabrication of enhanced devices for advanced optoelec- tronics, 1,2 sensors, 3,4 interfacial structures 5 and catalysis, 6 as porous semiconductors exhibit unique properties when compared to their bulk counterparts. 7 Among these materials, wide bandgap semiconductors (SiC, GaN, AlN, ZnO, BN, etc.) are expected to enable novel technologies in optoelectronics, magnetism, catalysis, and biotechnology due to the band gap shift, efficient luminescence, high surface area, and size selective adsorption that porous semiconductors show. 8 The actual application of these materials does, however, critically hinge on the development of processing methods able to precisely control the optical and electrical properties of the resulting porous materials. Furthermore, the use of these porous semi- conductors is partially restrained by an incomplete understanding of pore-forming mechanisms. Particular interest in the production of porous GaN arises from the potential to shift the absorption band edge of this material further into the ultraviolet due to quantum confine- ment effects. Porous GaN thin films have also found application as buffer layers or templates for heteroepitaxial growth of lattice-mismatched materials with low density of defects. 9,10 These porous templates could be potentially important for rich-Al AlGaN and rich-In InGaN epitaxial growth for deep UV and green applications, respectively, where the lattice and thermal expansion mismatch between a foreign substrate or even GaN and the active layer results in defects generation that adversely affect the lifetime, reliability, and performance of opto-electronic devices. Another area of technological interest can arise from the strong photoresponse that porous GaN exhibit when il- luminated by a broad spectra light source with photon energies from 2.5 to 4.0 eV. 11 This particular effect makes porous GaN structures even more attractive than bulk GaN for the development of sensors and detectors operating in the visible and the UV wavelengths of the electromagnetic spectrum due to the increase in surface area. It has been shown that GaN and some of its alloys exhibit a photocata- lytic response with application in the production of hydrogen by water splitting. 12 Here, the use of porous GaN structures is clearly attractive due to the enhanced effect resulting from the higher surface-volume ratio when compared with its bulk counterparts. Finally, nanoporous GaN particles represent optically homogeneous media through which the electro- magnetic radiation can propagate without internal scattering but where the presence of pores induces an optical anisotropy that may be of particular importance for nonlinear optical and photonic applications when a certain order exists in the position of the pores. 13 Recently, we reported a novel and simple technique for the synthesis of GaN porous structures by the direct reaction of Ga and NH 3 in a chemical vapor deposition (CVD) system. 14 This novel technique presents several advantages over other tech- niques used to produce porous GaN films that are based in corrosion and etching methods, as it allow us to produce porous GaN without any postgrowth treatment. Corrosion techniques may induce surface damage; 15 pores tend to coalesce with etching time 16 and the induced porosity is highly dependent on the uniformity of the substrate used. 17 This makes control of pore morphology and arrangement in the GaN film difficult. As these parameters are generally regarded as fundamental during the fabrication of nanoporous materials, exploiting the properties of porous GaN has been hampered, because it requires good control over its morphology, surface chemistry, optoelec- tronic properties, and in situ in-plane patterning on substrate. Our novel method for the production of as-grown GaN micron- sized nanoporous particles represents a new opportunity to overcome these difficulties. In this paper, we report the influence of different reaction parameters, such as temperature, flow rate of ammonia, and pressure, on the external morphology and nanopore density of the GaN micron-sized nanoporous particles synthesized by the direct reaction of Ga and NH 3 . A careful control of these parameters will allow us to establish different strategies to produce GaN micron-sized nanoporous particles with the desired morphology and density of nanopores, as we discuss in the last section of the paper. * Corresponding author. E-mail: joanjosep.carvajal@urv.cat. † State University of New York. ‡ Universitat Rovira i Virgili. § Present address: GE Global Research. CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 320–326 10.1021/cg800498y CCC: $40.75  2009 American Chemical Society Published on Web 11/20/2008