Effect of Heat Treatment of Al Substrate on GaN Film Electrodeposited in Aqueous Solution

Most reports on the fabrication of high-quality Gallium nitride (GaN) are typically based on physical techniques that require very expensive equipment. Therefore, the electrodeposition was adopted and examined to develop a simple and economical method for GaN synthesis. GaN films are synthesized on aluminum substrates that are heat-treated at various temperatures using a low-cost and low-temperature electrochemical deposition technique. The electrochemical behavior of source ions in aqueous solutions is examined by cyclic voltammetry (CV). In the solution at pH 1.5 containing 0.1M Ga(NO 3 ) 3 , 2.5 M NH 4 NO 3 and 0.6 M H 3 BO 3 , reduction of gallium and nitrate ions are observed in CV. The presence of hexagonal GaN and gallium oxide (Ga 2 O 3 ) phases is detected for the films deposited on Al substrates at −3.5 mA∙cm -2 for 3 h. The energy dispersive X-ray and mapping results reveal that Ga, O, and N coexist in these films. Raman analysis shows hexagonal GaN formation on Al substrates. The changes in the morphology and preferred orientation of GaN were found, which was caused by the reactivity of aluminum surface and the aluminum oxide layer formed by the heat treatment.


Introduction
Gallium nitride (GaN) is one of the most promising III-V semiconductor materials that use nitrogen as a group V element [1] [2] [3] [4] [5]. In particular, GaN has been recognized to be the most important material for blue and ultraviolet optoelectronic devices and has recently attracted considerable attention after the How to cite this paper: Kang successful fabrication of high-efficiency blue light-emitting diodes [6] [7] [8] [9].
During the last two decades, numerous studies have been conducted on GaN semiconductors, and several reports have been published on the synthesis technology of these semiconductors [10]- [19]. Various dry plating methods, such as metal-organic chemical vapor deposition (MOCVD) [20] [21], molecular beam epitaxy (MBE) [22] [23] [24], thermal ammoniation [25], physical vapor deposition [26], and chemical vapor deposition [27] [28] [29], have been adopted and used in many researches. However, the high cost of these equipments is one of the major challenges to be overcome for commercialization and widespread application. Moreover, the use of high-temperature systems and ultra-pure gases for GaN fabrication entails considerable complexity and additional manufacturing cost [30].
Therefore, the electrodeposition has recently been proposed as one of low temperature and low-cost synthesis methods of semiconductor materials, and has been studied by some researchers as an alternative for GaN synthesis. Several reports have also confirmed the possibility of GaN synthesis by electrodeposition [31]- [37]. Most low-temperature GaN syntheses have been conducted on silicon (Si) substrate because of their high quality, large size, and low cost. However, Si and deposited metal usually exhibit a weak interaction that leads to three-dimensional island-like growth or Volmer-Weber mechanism [38] [39]. This is because the rate of electrochemical reaction on Si is slower than on metals [40].
The substrate effect of various aluminum compounds, such as aluminum nitride (AlN) [41] [42], aluminum gallium nitride (AlGaN) [43] [44] and aluminum oxide (γ-Al 2 O 3 ) [45], which can improve the crystallinity of GaN, has been studied and reported by many researchers [46] [47]. High-quality GaN have been grown on Al-based intermediate layers. It can be considered that Al plays an important role in GaN growth. In particular, it can be expected to be more suitable as substrate material for fabrication of GaN film in wet processes since Al has better conductivity than Si. Therefore, the synthesis of GaN film was performed on Al substrate in this study. In addition, the effect of heat treatment of Al substrate on the GaN formation was investigated.

Electrode Preparation
The working electrodes are 1.0 × 1.0 cm 2 Al plates with a 99.9% purity, and a Pt coil is used as counter electrode. An Ag|AgCl|sat'd-KCl electrode is employed as reference electrode. The Al substrates are prepared by polishing using emery paper to a grade of 2000. They are ultrasonically cleaned with ethanol for 10 min, followed by deionized water for another 10 min. The Pt coil is immersed in hydrochloric acid (HCl, 35%, Nacalai Tesque) to clean the surface and is thereafter washed with distilled water. Finally, all electrodes are washed with distilled water and dried under atmospheric conditions. Prior to electrodeposition, the Al plate is covered with a Nitoflon adhesive tape (Nitto Denko) to yield an electrode area of 0.5 × 0.5 cm 2 .

Heat Treatment of Al Substrates
The influence of electrode conditions, such as the composition and crystallinity of various electrodes, on material synthesis has been widely investigated [48].
Here, the Al substrates are prepared to examine the influence of Al oxide layer

Electrodeposition Process
All the electrochemical depositions are performed using a potentiostat (Hokuto Denko, HA3003A) in a three-electrode cell configuration at atmospheric pressure and temperature of 25˚C. As shown by the E-pH diagram of Ga in Figure   1(a) [49], gallium ion (Ga 3+ ) exists in the form of passivated gallium oxide were obtained at this current density [50]. The working and counter electrodes are separated by a distance of approximately 1.0 cm, and the electrolyte is vigorously stirred during the electrodeposition process. After deposition, the specimens are immediately rinsed thoroughly with distilled water.

Characterization and Tests
The Horiba Jobin Yvon HR system, and an argon ion laser (514.5 nm) is utilized as an excitation source. The Raman scattering experiment is implemented in the z (x, unpolarized) z scattering configuration. The resolution of this system is 1 cm −1 , and the integral time is 3000 ms.

Electrochemical Behavior of Gallium and Nitrogen Ions
The CV experiments are performed for aqueous solutions containing several chemicals to examine the electrochemical behavior of ions. The pH level of all solutions is adjusted by HNO 3 . Table 1 Although the reduction potential of 3 NO − ion is very positive than HER, it can be seen that the 3 NO − ion does not reduce easily before HER. And if the reaction continues, N species ion may change to most stable ammonium ( 4 NH + ) ion.
In the graph of Figure 2 It is generally known that hydrogen discharge is preferred for Ga deposition, thus the reduction reaction of Ga 3+ can therefore occur after the HER on the cathode surface [54]. The curve appearing at −1.7 V thereafter corresponds to the reduction reaction of 3 NO − of Equation (1) and Equation (2). That is, even if Ga is adsorbed on the Al substrate, the reduction reaction of 3 NO − ion can occur, and it can be expected that the reaction of 3 NO − ion occurs at a more negative potential under the influence of the adsorbed gallium (Ga ads ).
In the graph (d) obtained from a solution containing Ga 3+ , 3 NO − and 4 NH + ions, three clearly separated cathodic peaks appear. After the large current peak corresponding to the precipitation of Ga, Ga c i , a distinct reduction peak is found at the potential region corresponding to the formation of N ads , ads N c i , confirmed from graph (b). In addition, the increased and separated current peak with the addition of Ga 3+ ion indicates the formation of new phase precipitates on the Al substrate. During the reactions of Equations (3) and (4)   Ga NH 4e GaN 2H The third current may be considered to correspond to the reduction of H + ion caused by the consumption of 4 NH + ion, H c i + . When H 3 BO 3 is added (graph (e)), the current density decreases, and the amount of hydrogen gas bubbles generated on the Al surface is reduced, and also the bubble size becomes smaller. Boric acid thus acts as a surface agent and may be expected to function as a buffer to retard the generation rate of hydrogen ion (H + ) as well as prevent hydrogen adsorption, which can interfere with the permeation of N source ion on the Al surface [54].  (Figure 3(a)). In the XRD results of heat-treated Al samples (Figure 3

Electrodeposition and Characterization
The electrodeposition of GaN film in an aqueous solution is attempted on Al substrates, and their morphology and composition are characterized by SEM and EDX. Figure 5 shows the SEM images of GaN films deposited with a current density of −3.5 mA cm −2 for 3 h in the solution corresponds to    As shown in Figure 5, the film formed on the Al plate exhibit an island-like morphology. And the island-like particles are connected and grow in the form of a thick plate for the film deposited on Al200HT (Figure 5(b)). In the film deposited on Al500HT, a cauliflower-like structure, which was found in the γ-Al 2 O 3 or β-Ga 2 O 3 samples, is further formed [55] [56].
The EDX results summarized in Table 2 clearly reveal that more than 35 mol% of N and Ga are present with O in the GaN film grown on the Al substrate. This indicates that the GaN compound film is formed according to Equation (6). The mapping micrographs of Figure 6 clearly reveal the presence of Ga and N on the Al substrate. These elements are uniformly dispersed in the deposited films. Similarly, the N content is more than 28 mol% at any given position in the films that form on Al200HT and Al500HT. On the other hand, oxygen increases by more than 20 mol%, which can be attributed to γ-Al 2 O 3 . The mapping results in Figure 6 also show that the films containing a large amount of N are formed on the Al substrates, and Ga, N and O coexist. Figure 7 shows the XRD spectra of GaN films that are prepared by electrodeposition. Two peaks that correspond to the reflection of h-GaN are observed at 2θ = 32.7˚ and 36.8˚; these peaks are caused by reflections from the ( 1 100 ) and ( 10 11) planes for the film deposited on the Al plate, respectively [57] [58]. In contrast, a strong peak corresponding to the reflection of (0002) plane is detected at 2θ = 34.4˚ in the diffraction result for the film deposited on Al200HT (Figure 7(b)). In the film formed on Al500HT (Figure 7    Three Raman active optical phonon modes corresponding to h-GaN have been observed from GaN film deposited on the Al plate. One mode is at 144 cm −1 because of E 2 (LO), and two bands are at 518 and 718 cm −1 of modes A 1 (TO) and E 1 (LO), respectively [63]. In the film formed on Al200HT, the band that corresponds to A 1 (TO) is intensified, and the signal at 570 cm −1 that corresponds to E 1 (TO) is also detected. In contrast, weak bands are detected from the film deposited on Al500HT because of the influence of a thick oxide layer. As can be observed from Figure 9(b) and Figure 9(c), strong bands corresponding to

Effect of Heat Treatment of Al on GaN Formation
The effects of the Al substrate on GaN formation by electrodeposition are considered. It is confirmed that Al(OH) 3

Conclusions
The GaN films are synthesized on Al substrates heat-treated different temperatures by electrodeposition under ambient temperatures. The CV analysis reveals that starting from a potential value that approaches the Ga reduction potential, 3 NO − ion could be combined by adsorbing Ga and O during the deposition process, resulting in the formation of GaN films.
The GaN films are prepared by applying a current density of -3.5 mA•cm − The GaN films produced in this work contain a considerable amount of O and exhibit insufficient crystallinity compared to single-crystal GaN. After the formation of GaN, additional processing, such as annealing at a high temperature in the presence of ammonia, may be required, because the addition of O to the GaN film may cause lattice deformation or diminish material properties. Nevertheless, the possibility of synthesizing GaN on Al substrate using a low-cost temperature method that can replace the current expensive process has been confirmed. The fact that GaN films can be formed on metals other than semiconductors will have many advantages for various potential applications.