CRYSTAL GROWTH & DESIGN
Synthesis and Characterization of Ge-Doped GaN Crystalline Powders Deposited on Graphite and Silica Glass Substrates
2007 VOL. 7, NO. 7 1251-1255
Shiro Shimada,* Yoko Miura, and Akira Miura Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628 Japan
Takashi Sekiguchi Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba, 305-0047 Japan ReceiVed NoVember 14, 2006; ReVised Manuscript ReceiVed April 10, 2007
ABSTRACT: Gallium nitride (GaN) powders doped with various amounts of Ge were deposited at 1000 and 1180 °C on graphite and silica glass substrates by a 1-h reaction of NH3 with gaseous Ga2O and GeO, produced by the reaction of carbon with Ga2O3 and GeO2, respectively. The crystallinity of the GaN powders was determined by X-ray diffraction (XRD) and Raman spectroscopy, and their morphology was observed by scanning electron microscopy. The luminescence of the powders was measured at room temperature by cathodoluminescence (CL) to evaluate the quality of the GaN crystals. The influence of Ge-doping of the GaN powders on their crystallinity, morphology, and luminescence is discussed on the basis of the XRD, Raman spectroscopy, and CL results. It is shown that Ge-doped GaN crystals exhibit very high CL intensities accompanied by the elimination of defect-related emission at 430 nm. 1. Introduction Gallium nitride (GaN) is a most attractive III-V semiconductor material with a direct large band gap (∼3.4 eV), allowing it to be used in optoelectonic devices such as light emitting diodes and laser diodes in the blue to ultraviolet region.1,2 GaN can be also used in high power and high-temperature microelectronic devices because of its high thermal conductivity and stability without radiation damage. GaN homoepitaxial films with these useful properties have been extensively fabricated by metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or hydride vapor phase epitaxy (HVPE). GaN bulk single crystals are in great demand as substrates for the preparation of such homoepitaxial films and produced by sublimation and high-pressure solution methods.3 On the other hand, high-quality, crystalline GaN powders are used as phosphors with high luminescent intensities and efficiencies for use as multicolor vacuum fluorescent displays and field emission displays.4,5 However, synthesis of crystalline GaN of good quality is still a difficult task, and thus commercially available GaN powder is of poor quality and high cost. There have been many papers related to the synthesis of high-quality GaN powder by reaction of Ga or Ga2O3 with NH3.6-9 Balkzs and Davis have prepared single phase GaN powder of high purity by the above reactions.6 An alternative route has been developed for the synthesis of GaN powder, which can be achieved by ammonolysis of Ga2O gas formed by reduction of Ga2O3 by either metallic Ga or carbon. For example, Schwenzer et al. fabricated GaN particles by ammonolysis of Ga2O formed from a mixture of Ga and Ga2O3.9 Several authors including us have investigated the crystal growth of GaN by the reaction of NH3 with Ga2O gas formed by the carbothermal reduction of Ga2O3.10-14 These studies have been most concerned with the synthesis of nanosized GaN crystals, in contrast to the present authors, who produced millimeter-sized GaN single crystals by the above-mentioned carbothermal reduction and ammonolysis of Ga2O3.14 The rapid * To whom correspondence should be addressed. TEL/FAX: +81-11706-6576. E-mail:
[email protected].
synthesis of high-purity GaN powder using a catalyst has recently been reported.15-17 Catalysts such as Bi, Ni, or Ti are noted as enhancing the formation of GaN by the reaction of Ga2O3 with NH3. Much attention continues to be paid to the fabrication of GaN epitaxial films doped with foreign elements such as Mg, Zn, or Si by MOCVD, MBE, or HVPE.18-20 Only one report deals with the incorporation of Ge as an n-type dopant in a GaN epitaxial layer prepared by a plasma-assisted molecular beam.21 There has been no previous report of the synthesis of GaN powders doped with Ge by any method. It is therefore of great interest to synthesize Ge-doped GaN powders by carbothermal reduction of Ga2O3 and to investigate their crystallinity and quality. The present study describes a novel method for the synthesis of Ge-doped GaN powders on graphite and silica glass substrates. The method involves the simultaneous carbothermal reduction of Ga2O3 and GeO2 with carbon and subsequent reaction of Ga2O and GeO with NH3. This procedure has several advantages in that it is easy, simple, and uses inexpensive starting materials. The Ge-doped GaN powders thus obtained were characterized by XRD, scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and cathodoluminescence measurements (CL). The influence of Ge doping on the properties of GaN is discussed on the basis of the XRD, SEM, XPS, and CL results. 2. Experimental Procedures The apparatus for synthesis of Ge-doped GaN powders is shown in Figure 1. Two silica glass tubes (φ18 × 500 mm and φ8 × 500 mm) facing each other were set up in three tube furnaces with long zones of constant temperature (40 cm) at 1000-1200 °C. The larger glass tube was used for the vaporization of Ga2O and GeO, and the smaller tube was used for the introduction of NH3 gas (purity >99.99%). The ends of the two tubes were curved downward to efficiently blend the Ga2O, GeO, and NH3 gas streams. The starting materials were Ga2O3 (Kojundo Chemical Laboratory Co. Ltd. Japan; purity >99.9%), GeO2 (Kojundo Chemical Laboratory Co. Ltd. Japan; purity >99.99%), and graphite powder (Nakarai Chemicals Co. Ltd, Japan; purity >99%). Ga2O3 powder was mixed
10.1021/cg068014p CCC: $37.00 © 2007 American Chemical Society Published on Web 05/22/2007
1252 Crystal Growth & Design, Vol. 7, No. 7, 2007
Shimada et al.
Figure 1. Apparatus for deposition of Ge-doped GaN powders. with graphite in a molar ratio of Ga2O3/C ) 1:20. Mixtures of GeO2 and graphite in a 1:10 molar ratio were prepared, and 3-300 mg of these was used to control the amount of evolved GeO gas. The two mixtures of Ga2O3/C and GeO2/C were placed separately in an alumina boat and heated at a constant temperature of 1000 °C. The Ga2O and GeO gases formed by the carbothermal reduction were transported to the outlet of the larger tube by a stream of Ar gas (100 mL min-1). NH3 gas was introduced at a rate of 100 mL min-1 from the smaller tube outlet. A graphite or silica glass plate (50 × 20 × 0.5 mm) was used as the substrate. The deposition of GaN crystals on the substrate was performed at 1000 and 1180 °C for 1 h. The vertical distance between the gas outlet and the substrate was maintained at about 20 mm. The shape, size, and number of GaN crystals were determined by scanning electron microscopy (SEM, JEOL JSM-6500F). GaN crystals were identified by X-ray powder diffraction, using Cu KR radiation (RINT 2200), and their crystallinity was determined by Raman spectroscopy with a 532 nm laser line. The quality of the GaN crystals was determined by CL at room temperature using an accelerating voltage of 5 kV. The chemical states of the Ge in the GaN crystals were determined by X-ray photoelectron spectroscopy (XPS) (Shimazu Corporation, ESCA-3200) using Mg KR radiation. The binding energies of Ge were corrected by reference to free carbon (284.6 eV).
Figure 2. XRD patterns of undoped and Ge-doped GaN powder formed on graphite substrates at 1000 and 1180 °C. (A, B) undoped and Ge-doped GaN at 1000 °C; (C, D) undoped and Ge-doped GaN at 1180 °C; the mixture of GeO2/C was 100 mg.
3. Results and Discussion Carbothermal reduction of Ga2O3 and GeO2 by graphite occurs with the formation of Ga2O and GeO gases, respectively (eqs 1 and 2).
Ga2O3(s) + 2C(s) f Ga2O(g) + 2CO(g)
(1)
GeO2(s) + C(s) f GeO(g) + CO(g)
(2)
The Ga2O and GeO gases are transported by a stream of Ar gas to the outlet of the larger glass tube and combine on the substrate with NH3 flowing from the other glass outlet. The reaction of Ga2O and GeO with NH3 results in the formation of Ge-doped GaN (eq 3)
Ga2O(g) + GeO (g) + NH3(g) f Ge-doped GaN(s) + H2(g) + H2O(g) (3)
Figure 3. (A-D) Raman spectra of undoped and Ge-doped GaN powder formed on graphite substrates at 1000 and 1180 °C. The conditions of A-D are the same as those in Figure 2.
For the sake of simplicity, the above equation is not balanced. The gas species evolved by carbothermal reduction of GeO2 are expected to be either GeO or Ge but are taken here as GeO. 3.1. Growth of GaN Crystals at 1000 and 1180 °C on Graphite Substrates. When 0.6 g of Ga2O3/C and 100 mg of GeO2/C were used, the generation rates of Ga2O and GeO vapors were almost constant at 13 and 2 µmol min-1, respectively. Figure 2 shows the XRD patterns of undoped and Ge-doped GaN powder produced on the graphite substrates by carbothermal reduction of Ga2O3 and GeO2 at 1000 °C, followed by subsequent nitridation with NH3 at 1000 and 1180 °C. The undoped crystals formed at 1000 °C show the strong 002
reflection of GaN, the small 101, 102, 103, and 112 peaks and peaks from graphite (closed circles). The Ge-doped GaN crystals show more intense 100 and 110 peaks. At 1180 °C, undoped and Ge-doped samples give XRD patterns similar to those of the undoped sample obtained at 1000 °C, except for the increased intensities of the 002 and 103 peaks. The Raman spectra of the GaN crystals used for XRD analysis are shown in Figure 3 A-D. All the peaks except one at 300 cm-1 were assigned as wurtzite-type GaN. Undoped and Ge-doped GaN synthesized at 1000 °C possess broad peaks corresponding to A1(TO), E1(TO), and E2, consistent with the poor crystallinity
Synthesis of Ge-Doped GaN Crystalline Powders
Crystal Growth & Design, Vol. 7, No. 7, 2007 1253
Figure 4. (A-D) SEM images of undoped and Ge-doped GaN powder formed on graphite substrates at 1000 and 1180 °C. The conditions of A-D are the same as those in Figure 2.
Figure 6. XRD patterns of Ge-doped GaN powder formed on silica glass substrates at 1180 °C. (A) Undoped, (B) 0.08 mol %, (C) 0.4 mol %, (D) 0.8 mol %, (E) 4 mol %.
Figure 5. CL spectra of undoped and Ge-doped GaN powder formed on graphite substrates at 1000 and 1180 °C. 1000 °C: (A) undoped, (B) Ge-doped 1180 °C, (C) undoped, (D) Ge-doped.
of GaN. When the synthesis temperature is increased to 1180 °C, the Raman peaks, particularly the E2 peak, become sharp. Since the E2 peak is taken as a measure of GaN crystallinity, this indicates that the crystallinity of the GaN is improved at 1180 °C, consistent with the XRD results. As shown by the SEM images, the undoped GaN sample formed at 1000 °C contains agglomerated particles 2-5 µm in size, consisting of fine, triangle-shaped particles 20-50 µm in size, while the Ge-doped sample contains 10 µm hexagonal-shaped prisms (Figure 4D). Some porous powders and fibers, seen among the hexagonal crystals, are identified by energy dispersive X-ray analysis to be carbon and GaN, respectively. It is found that the higher synthesis temperature produces hexagonally shaped GaN crystals. CL spectra of the undoped and Ge-doped GaN are shown in Figure 5. The undoped crystals formed at 1000 °C show a very broad peak centered at a wavelength of 690 nm without the band-edge emission, while the spectra of Ge-doped sample are
virtually free of peaks. The undoped crystals synthesized at 1180 °C have a strong band-edge emission at 362 nm with a shoulder at 420 nm (blue luminescence). The Ge-doped crystals also have a strong peak at 362 nm with a broad peak at 570 nm (yellow luminescence). It is thus found that high-temperature synthesis at 1180 °C improves the crystallinity and quality of the GaN crystals. It is worth noting that the blue luminescence is diminished when Ge is doped into the GaN at 1180 °C. 3.2 Growth of GaN Powder Crystals at 1180 °C on Silica Glass Substrates. Figure 6 A-E shows the XRD patterns of GaN crystals produced with various amounts of Ge on silica glasses at 1180 °C. All the peaks, identified as GaN, are much stronger than those obtained on the graphite substrate. The concentration of Ge dopant is in the range 0.08-4 mol %, determined as the molar ratio of GeO to (GeO + Ga2O) calculated on the assumption that Ga2O3 and GeO2 are completely converted to Ga2O and GeO according to eqs 1 and 2; it is not clear, however, whether all the evolved GeO is doped into the GaN. The XRD pattern of the undoped GaN crystals is very similar to those formed on graphite substrates at 1180 °C. No significant difference is seen in the intensities with the Gedoping concentration, nor is any peak shift. The former observation suggests that no specific crystal orientation occurs in the Ge-doped crystals with variation in the amount of doping. Undoped GaN crystals synthesized at 1180 °C are white and become auburn with increasing Ge content. As observed by SEM (Figure 7), the undoped GaN sample consists of regularly shaped hexagonal crystals about 10 µm in size (Figure 7A); on the surface small crystals of 3-5 µm or fine particles
