Synthesis and Characterization of GaN Rods Prepared by Ammono-Chemical Vapor Deposition ()
1. Introduction
Gallium nitride is an attractive semiconductor since its 3.4 eV gap band is suitable for use in optical devices and integrated circuits that operate in wavelengths of the blue-violet and ultra-violet regions [1]. Chemical vapor deposition (CVD) is one of the most investigated methods of GaN synthesis involving different types of gallium-containing reagents that form reactive gallium species in the vapor phase. However, this technique has countless factors that must be controlled such as the type of precursors, concentrations, substrate composition, deposition temperature, and the nature of the catalysts to produce a desired morphology. For instance, in the simplest CVD setup to prepare GaN from only metallic gallium and ammonia, the changes in reagent concentration lead to formation of amorphous GaN, pellets or rods [2].
By increasing the number of reagents, the system becomes more complex, although the use of more reagents may be justified if the reactivity of gallium precursors increases, thus affording a lower reaction temperature or if the reagents are less toxic. This is the case of a method to produce bulk GaN under high pressures with an ammonium halide salt (NH4X, where X = Cl, Br, or I) that sublimes and forms a gallium complex with metallic gallium, and then the complex reacts with NH3 to form GaN [3]. These reagents, which are inexpensive and easy to handle, have been used in a CVD system resulting in formation of GaN-containing columns [4]. However, the use of NH4Cl leads to a series of additional reactions that increase lateral growth with only a change of the reaction temperature [5].
On the other hand, a key point to grow GaN rods, i.e., to favor a larger L/D aspect ratio (length to diameter) to that of columns, is a metal catalyst, or foreign element catalytic agent (FECA) [6], which is used as a metal cluster or nanoparticle. The growth mechanism of GaN wires with metal catalysts has been described by Lieber [7,8] and is based on the capacity of the metal to dissolve gallium and nitrogen under the reaction conditions, as would happen with metallic iron or nickel at 900˚C [8] or 650˚C [9], respectively. This reaction is called the vapor-liquid-solid (VLS) mechanism [10], since gallium and nitrogen are transferred from the vapor (V) phase to the liquid (L) metal cluster phase and then solidify (S) as GaN, retaining the catalytic droplet on the tips of the rods as the special feature of this process.
In this particular metal cluster catalysis, if one element which is to form the rod is not soluble in the metal catalyst, as in the similar case of nitrogen in gold [8,11] for example, the nanorods are not formed, as confirmed earlier by Duan et al. [7]. Nonetheless, recent experiments have demonstrated the formation of GaN nanowires employing gold clusters as catalyst when triethyl gallium and ammonia were used as the reagents [8,11].
Our objective in this investigation is to prepare gold clusters in situ through the annealing of a thin film deposited on sapphire (0 0 0 1) substrate and amorphous quartz and determine whether this substrate influences the formation of clusters. Then, we intend to characterize the rods grown with this practical method in order to know their properties and propose suitable applications.
2. Materials and Methods
2.1. Chemical Vapor Deposition (CVD)
Reagents used in this work were metallic gallium (Sigma Aldrich, USA, 99.9995%), ammonium chloride (Fagalab, Mexico, 99.5%), and ammonia gas (Praxair, México, 99.99%). Sapphire (0 0 0 1) and amorphous quartz substrates areas of 1 - 1.5 cm2 were cleaned with acetone in an ultrasound bath, dried in air, and then transferred to a vacuum chamber (JEOL-JEE-400) where a gold film of ~10 nm in thickness was deposited by vacuum sputtering onto the surfaces of the substrates. The CVD system was assembled in a Lindberg-Blue horizontal furnace Model STF55433C with a 2-inch diameter quartz tube. All reactions were conducted at atmospheric pressure. A boat with 3.0 g of NH4Cl was placed at the entrance of the furnace. The temperature at this position was maintained at 350˚C, which is sufficient to sublime NH4Cl and to promote the formation of a gaseous gallium chloride precursor [3,4,12,13]. A quartz crucible with 0.7 g of gallium was placed 10 cm downstream from the position of the ammonium salt and the sapphire substrate with the gold film, separated by 1.0 cm from the crucible with the gallium metal source. Unlike other reports, where the gold film has been annealed prior to the GaN synthesis to assure the presence of gold nanoparticles, in the experiment described herein , the substrate with the gold film was placed in the CVD reactor without annealing.
A schematic representation of this setup is shown elsewhere [4,14]. The furnace was heated at a rate of 30˚C·min−1 and maintained at 800˚C. The tube was purged with ammonia when the temperature in the furnace was 300˚C and the flow rate was constant at 180 sccm until the end of the reaction. Then, the system was cooled to room temperature by switching off the heater. The exhaust gases were collected in an aqueous HCl solution trap.
2.2. Characterization
Electron scanning microscopy images were collected with a JEOL JSM5300 system and a cathode luminescence (CL) system using an electron beam with an energy of 15 keV. The structures were analyzed with a Phillips X’pert-MDP diffractometer with Cu kα radiation (0.15404 nm). Nanoparticle dimensions were estimated by analyzing SEM images with Image Tool v 3.0 [15]. TEM images were collected with a JEOL 2010 microscope and a FEI model TECNAI F-30 operated at 300 keV adapted with an EDS detector. Atomic Force Microscopy (AFM) images were acquired with a Nanoscope III system.
3. Results and Discussion
3.1. SEM
The first step in the synthesis of GaN rods is the deposition of gold clusters onto the substrate surface. Instead of separately annealing the gold film prior to CVD synthesis [4], we prepared the gold clusters in situ by introducing the substrate with the gold film directly into the CVD reactor along with the reagents required for GaN deposition. This is the actual environment in which gold clusters promote the growth of GaN. This in situ annealing seemed feasible since the minimal temperature to form the clusters is 200˚C, independent of time, which can range from 1 to 30 minutes [16]. Thus, by applying a heating rate of 30˚C·min−1, there is sufficient temperature and time (ca. 26 min) to form the clusters before reaching the operational temperature of 800˚C for this reaction. A similar in situ annealing was successful when gold was deposited onto a silicon substrate [17]. Sapphire and quartz were used to verify influences of substrate crystallinity on either the formation of gold clusters or the growth of GaN.
GaN deposition only starts when the temperature in the furnace is 800˚C, since this corresponds to 350˚C at the entrance of the furnace where the NH4Cl sublimes and transfer the metal gallium to the vapor phase [4,18].
In two experiments, the substrates were removed at this step to analyze the clusters by XRD and SEM (Figures 1(a), (b)). The depositions carried out with quartz and sapphire substrates were identified by XRD as GaN with a hexagonal wurtzite-type structure (patterns not shown herein), matching with the JCPDS card 74-0243 [19]. The GaN deposited onto quartz exhibited relative intensities similar to that of the powder pattern of the card, whereas the sample grown onto sapphire showed greater intensity for the [002] reflection, as a consequence of the preferential orientation of the rods in rela-
tion to the substrate.
Other substrates with gold films were treated with the same procedure and allowed to react 30 minutes at 800˚C to obtain GaN rods (Figures 1(c) and (d)). The images were analyzed with an image program to determine particle dimensions [15].
It should be noted that due to the NH3 atmosphere (applied when the furnace reached 300˚C) and the reagents in the CVD reactor, traces of gallium, nitrogen or chloride could be present in the cluster. Henceforth, the clusters formed before the start of NH4Cl sublimation will be called “clusters”, while the particles which remained on the tips of GaN rods will be named “droplets” because they form liquid droplets during the growth of the GaN rods [6-8,10].
Figure 2 shows the AFM image of gold clusters formed by annealing the gold film on sapphire. The sizes are ~250 nm, close to the results obtained from the analysis of SEM images.