- Email: [email protected]
NH3-free growth of GaN nanostructure on n-Si (1 1 1) substrate using a conventional thermal evaporation technique
Journal of Crystal Growth 349 (2012) 19–23
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
NH3-free growth of GaN nanostructure on n-Si (1 1 1) substrate using a conventional thermal evaporation technique K.M.A. Saron a,n, M.R. Hashim a, M.A. Farrukh b a b
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800-Penang, Malaysia Department of Chemistry, GC University Lahore, 54000-Lahore, Pakistan
a r t i c l e i n f o
abstract
Article history: Received 4 February 2012 Received in revised form 4 March 2012 Accepted 22 March 2012 Communicated by K. Nakajima Available online 9 April 2012
We have investigated the influence of carrier gas on grown gallium nitride (GaN) epitaxial layers deposited on n-Si (1 1 1) by a physical vapour deposition (PVD) via thermal evaporation of GaN powder at 1150 1C. The GaN nanostructures were grown at a temperature of 1050 1C for 60 min under various gases (N2, H2 mixed with N2, and Ar2) with absence of NH3. The morphology, structure, and optical properties (SEM) images showed that the morphology of GaN displayed various shapes of nanostructured depending on the type of carrier gas. X-ray diffraction (XRD) pattern showed that the GaN polycrystalline reveals a wurtzite-hexagonal structure with [0 0 1] crystal orientation. Raman spectra exhibited a red shift in peaks of E2 (high) as a result of tensile stress. Photoluminescence (PL) measurements showed two band emissions aside from the UV emission. The ultraviolet band gap of GaN nanostructure displayed a red shift as compared with the bulk GaN; this might be attributed to an increase in the defect and stress present in the GaN nanostructure. In addition, the observed blue and green–yellow emissions indicated defects due to the N vacancy and C impurity of the supplied gas. These results clearly indicated that the carrier gas, similar to the growth temperature, is one of the important parameters to control the quality of thermal evaporation (TE)-GaN epilayers. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Si (111) substrate A1. Nanostructures A1. Gases carrier flows A2. Growth from vapor A3. Physical vapor deposition processes B1. Nanomaterials
1. Introduction Gallium nitride (GaN) is an important semiconductor with a wide direct-band gap for photonic and high-power devices [1]. The characteristics and properties of GaN nanostructures, including nanowires and nanorods, have been previously reported [2,3]. Often, GaN-based layers are epitaxially grown on sapphire or silicon carbide substrates [4,5]. However, growth on these substrates has a number of deep-level defects, such as dislocations due to lattice mismatch, which may influence the reliability and performance of GaN-based devices [6]. Silicon has attracted much attention as a substrate material for GaN growth due to its larger thermal expansion coefficient and lattice mismatch compared with SiC and Al2O3, furthermore, it is cheap. The growth of GaN on Si is highly desirable for the Si-based electronic industry, because it is a promising route for large-scale, low-cost mass production of GaN-based devices [6,7]. However, the growth of GaN on Si involves the use of catalysts, such as Au, Ni, and Ag [8,9]. The utilization of these catalysts disturbs the purity of a GaN nanostructure, and therefore, may negatively affect its performance in many semiconductor applications [10]. Many reports showed that
n
Corresponding author. E-mail address: [email protected] (K.M.A. Saron).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.03.046
nanostructures of GaN have been fabricated on Si substrates by evaporating Ga2O3 powder at 1100 1C in ammonia gas flow [11], although wurtzite GaN nanowires were synthesized via the direct reaction of Ga with ammonia (NH3) in a tube furnace [9]. Usually, GaN grows in an atmosphere of NH3 while H2, Ar2, and N2 are used as carrier gases [12–14]. However, among most of these methods, an atmosphere of superfluous NH3 is obligatory, which caused air pollution and waste. Many techniques have been employed to produce high quality GaN films, such as hydride vapour phase epitaxy (HVPE) [15], metal organic chemical vapour deposition (MOCVD), thermal evaporation technique (TE), and molecular-beam epitaxy (MBE) [16–18]. Among these techniques, thermal evaporation is relatively simple and suitable for its low melting point and low decomposition or low sublimation point oxides. However, GaN films grown in an environment free of ammonia gas contain a number of defects, such as Ga-rich, N and Ga vacancies, oxygen deep level, and carbon impurity [18]. The growth of GaN nanostructure on Si in absent of NH3 gas has not yet been demonstrated. The current paper reports the successful growth of polycrystalline GaN nanostructures on Si (1 1 1) by thermal evaporation of GaN powder using different gases as carrier gas. One goal of the present study is to control the formation of alloyed particles (GaN) under free ammonia gas and thermal evaporation method
20
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
to obtain optimum growth rate and low defects in GaN. A further study of the effects of carrier gases (N2, H2/N2, and Ar2) on grown GaN nanostructures has been presented. Moreover, the changes caused by different gas on morphological shape and diameters of nanostructure have been mentioned. Structural and optical properties of GaN nanostructure have also been reported.
2. Experimental GaN nanowires and nanorods were grown in a furnace with a horizontal three-zone quartz tube (90 mm in outer diameter and 750 mm of heated length), under different carrier gases (N2 99.999%, 2% H2 mixed with 98% N2 and Ar2 99.999%). An n-type silicon (1 1 1) wafer was used as the substrate to collect the products. Before the substrate was loaded into the furnace, it was cleaned via the standard cleaning method [14]. About 0.3 g GaN (99.99%) powder was placed into a quartz boat, and then the boat was set at the downstream centre zone, where the temperature was about 200 1C; the substrate was placed on the quartz holder and then loaded into the third zone of the quartz tube. The distance between materials and substrate (growth zone) was 32 cm. The gases (N2, H2 mixed N2, and Ar2) were supplied into the inlet before the furnace tube was run to remove oxygen with the flow rate of 4 L/min (L/min) during growth. Two of the heating zones were used for grown GaN. The temperature was adjusted between the centre zone (GaN powder) and third zone (Si (1 1 1) substrate) to be 100 1C. The third zone, the growth zone, was kept at 1050 1C during the process, while the temperature of the centre zone (GaN powder) was kept at 1150 1C and held for 60 min. After deposition of GaN, the samples were cooled down to room temperature with flowing gas. Morphology, characterization and simple composition analysis of the samples were carried out using scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDX) attached to the SEM (JOEL JSM-6460LV performed at 10 kV). The phase and purity of GaN nanostructures were examined by X-ray ˚ source, diffraction (XRD) using Cu Ka1 radiation (l ¼1.5406 A) step size 0.051, and scanning range of between 2y ¼201 and 701. The optical properties of the samples were recorded by highspatial resolution Raman spectroscopy using Jobin Yvon HR 800 UV system with an argon ion laser (514.5 nm) as an excitation source. Photoluminescence (PL) was measured at room temperature using a He–Cd laser (325 nm).
3. Results and discussion
Fig. 1. SEM images of GaN nanostructures grown on Si (1 1 1) using furnace tube under different carrier gases: (a) N2 produced nanowires and nanorods, (b) N2 mixed with H2 nanowire and nanotriangles, and (c) Ar2 nanowires like tree branches and EDX at the right side of SEM images.
Fig. 1 shows SEM images of three typical morphologies of GaN nanostructure grown on n-Si (1 1 1) obtained via different gas carriers (N2, H2 mixed with N2, and Ar2). Sample (a), which was grown under N2 gas, showed an overview of GaN nanowires (NWs) and nanorods (NRs) coverage on Si (1 1 1). The nanowires and nanorods had diameters of 100–500 nm, showing clearly that the nanowires and nanorods grown vertically on the surface of the substrate had hexagonal face. For sample (b), GaN grown under N2 mixed with H2, the nanowire was distributed randomly on the substrate like a spider web. Some nanotriangles orientations were also observed on this substrate. They had size of 50–80 nm and dimension of about 110–180 nm; their lengths spanned several micrometers of nanowires. The observed shape of the nanotriangles was due to the effect of hydrogen-mixed reaction environment, especially H2. Nanotriangles were not observed with sample (a), which was grown in N2 gas. These results are in agreement with the findings of Kuykendall et al. [12] on nanotriangles; they indicated the influence of hydrogen
on the morphology of GaN nanowire, but they did not identify the proportion of hydrogen. In the current work 2% H2 mixed with 98% N2 was used as environment carrier gas with flow rate of 4 L/min without ammonia gas. This indicated that the hydrogen flow rate of 0.1 L/min is most to obtain the nanotriangles. No catalyst and NH3 gas were used. The inset Fig. 1(b) clearly shows that the nanotriangulars grew on the c-axis of the wurtzite structure [12]. For sample (c), which was grown under Ar2, the nanowires looked similar to a tree branch with a diameter of about 200 nm. The differences in temperatures between the material source and the substrate, beside the rush of gas in the tube, changed the velocity of gas molecules, leading to the random deposition of GaN nanowires on the surface of the substrate. The individual spectrum (right side of SEM images) of Ga, N, C, and O obtained by EDX indicated the defect in GaN composition of the test samples. Thus, the differences in gases and the growth rates of these chemically and structurally distinct surfaces have a
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
21
These elements could be composed again at the substrate. During this process, some N was lost between the source and the growth area. However, N could be stabilized independently in the chemical environment (N-rich) produced from NH3. The chemical property of GaN was stable in N2 and H2 atmospheres and either gas (Ar2) at a high temperature, but it showed instability in Ar2 atmospheres at a high temperature. The single atoms of nitrogen combined with each other to be the nitrogen molecule N2, as a result of collusion between N atoms. The rate of N lost decreased when the samples were grown in N2 and H2 mixed with N2 atmospheres, as confirmed by EDX. In addition, an amount of H2 added into the carrier gas (N2) could be enhancement in reaction of Ga with incoming N. The growth rate of GaN nanostructure was observed to be faster when the N2 and N2 mixed with H2 were used as carrier gas compared with Ar2. The results showed that the carrier gas had a strong effect on the formation of GaN nanostructure. The XRD patterns of GaN nanostructures (NWs and NRs) with different carrier gases are shown in Fig. 2. Three intense peaks corresponding to (1 0 0), (0 0 2), and (1 0 1) of GaN nanostructure were located at 32.261, 34.571, and 37.31, respectively, indicating the presence of a small number of crystallites with different orientation. Further, these demonstrated that GaN nanowires possess a hexagonal wurtzite structure and have a preferred orientation h-(0 0 2) plane parallel to the (1 1 1) of the Si substrate. In sample (a), the strong peak for GaN (0 0 2) was observed at 2y ¼34.681. The XRD patterns from the GaN nanostructure indicated a typical wurtzite hexagonal structure with ˚ Fig. 2(b) showed that the peak lattice constant of (c ¼5.174 A) (0 0 2) was observed at 2y ¼34.621, with lattice constant of ˚ Fig. 2(c) showed that the weak peak (0 0 2) was (c¼5.18 2 A). ˚ The observed at 2y ¼34.81, with lattice constant of (c ¼5.16 A). shift in 2y orientation of the (0 0 2) plane with changed gas and the calculated c were found smaller than the bulk GaN (5.1855) [14]. This indicates that GaN nanostructure suffer from tensile stress, which implies that the lattice constant c decreased compared with the stress-free bulk GaN. In the literature, peak shifts
toward lower (or higher) 2y values with respect to the bulk peak profiles have been attributed to compressive (or tensile) stress in the films [19]. This effect could result in decreasing the interplanar spacing, thus leading to the observed increase in the diffraction angle. The cooling of the epitaxial nanostructure from high growth temperature to room temperature introduces stress, which is tensile, could be due to thermal mismatch and lattice mismatch between GaN and Si [20]. Furthermore, the gas carrier causes intrinsic stress resulting from the defect [21]. The variation of defects depends on carrier gases; however, low defect was achieved by N2 and N2 with a ratio of H2. In contrast, when Ar2 was used as carrier gas it weakened pre-reaction, avoided deposition of pre-reaction products and impurities, and resulted in a more proper crystal. Second, with N2 was used as carrier gas, the interface of growth became more stable and the crystal quality became purer. The intensity of the h-(0 0 2) orientation peak decreased when the Ar2 was supplied; this indicated enhanced defects and poor crystallinity of the nanostructure. The highest peak intensity of h-(0 0 2) orientation was observed at N2 and H2 mixed with N2. On the other hand, the absence of NH3 gas during the growth of GaN nanostructure could cause more defects. The high intense peak was presented at sample b, indicating the reduction of a defect, and thereby, impact of interaction between hydrogen with oxygen. This interaction led to the reduced rate of oxygen in GaN [9]. Results of the present study suggest that the type of introduced gas in the growth of GaN without NH3 gas would produce defective concentrations in the crystals of GaN. Fig. 3 shows the Raman spectrum of three samples (GaN) nanostructure grown on Si (1 1 1) by thermal evaporation in different gas carriers. Two dominated Raman-active phonons corresponding to E2 (high) around 567 cm 1 and A1 (LO) around 733 cm 1 were observed from GaN nanostructure and their peaks intensity were found varied. A strong band is observed at about 521 cm 1, which is the contribution from the Si (1 1 1) substrate due to (AO) and a band at 302 cm 1 due to the acoustic phonons of Si. The Raman peak of E2 high mode indicated the GaN nanostructure had a hexagonal wurtzite structure [22]. The Raman peaks of GaN nanostructure E2 high mode for the samples a, b and c, located at 565.3, 567, and 565 cm 1, respectively, are indicating large red-shifts compared with the h-GaN bulk-phase peaks’ positions at 568 cm 1 [23]. The red shift was most likely due to tensile stress in the GaN epilayer. Additional peaks as
Fig. 2. XRD pattern of GaN nanostructures grown on Si (1 1 1) substrate using furnace tube under different carrier gases: (a) N2, (b) N2 mixed with H2, and (c) Ar2.
Fig. 3. Raman spectrum of GaN nanostructures grown at different carrier gases: (a) N2, (b) N2 mixed with H2, and (c) Ar2. The intense band at 520 cm 1 arises from the Si substrate.
major impact on nanostructure morphologies. To investigate the influence of gas on growth structure and chemical at 1150 1C in the presence of N2, N2 mixed with H2, and Ar2 atmosphere, the following Eq. (1) was used [9] for the decomposition of GaN. GaNðSÞ-2Gað1Þ þ N2 ðgÞ
ð1Þ
22
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
However, the stress in GaN nanostructures was due to intrinsic defects, caused by growth condition. The blue emission present in sample (a) was due to the effect of N2 gas, which introduced rich Ga with N vacancies and caused the loss of some N and gain of some C impurity [31]. The green/yellow emission of the GaN material has been reported by Ogino et al. [24] which attributed to the presence of the CGa–ON complex. However, in the samples of the present study, the green/yellow emission was attributed to the VN or the Ga-richness and C impurity. EDX confirmed that because the sample was grown under N2, rich Ga and slight O gain are observable. This result suggests that the grown GaN under N2 flow gas is suitable for visible emission and solar cell devices owing to an enhanced defect in GaN, especially Ga richness [18]. Result suggested that the type of carrier gas has strong effect on crystal quality and photoluminescence band emission.
4. Conclusions Fig. 4. shows the photoluminescence spectra measured at room temperature of the GaN nanostructure grown under different carrier gases: (a) N2, (b) N2 mixed with H2, and (c) Ar2.
contribution of Al longitudinal optical (LO) mode were observed at 733, 732, and 733.2 cm 1 in GaN nanostructure of samples a, b and c, respectively; these had obvious red shifts as well, indicating tensile stress in the GaN epilayer. The E2 mode intensity was directly proportional to the gas supply. The thermal expansion mismatch and growth defect thus accounted for most of the stress present in the GaN produced from the present study. The tensile stress in the GaN epilayer could be caused by the defect in the GaN nanostructure during growth due to N vacancies and impurity. Fig. 4 shows the photoluminescence spectra measured at room temperature. GaN nanostructure grown on Si (1 1 1) indicated the variation of peak intensities and peak wavelengths with different carrier gases. A He–Cd laser [l ¼325 nm], used to optically excite the GaN nanostructure, showed two emissions peaks of UV, and green–yellow emissions. The spectrum for GaN (sample a) showed three main contributions: the band emission of GaN in the UV region (364 nm at 3.4 eV), blue emission (4 1 8), and broad green–yellow band (GYB) emission (centred at 519 nm at 2.4 eV). The emission became visible, which is generally assigned to crystal defects, such as N vacancies, C impurities, and O complexes [24–26]. Sample (b) had two peak emissions; the near band edge (NBE) in UV at 371 nm (3.34 eV) and green–yellow band (GYB) emission centred at 543 nm (2.28 eV). These peaks are possibly attributable to the existence of intrinsic defects or surface states [27,28]. The spectra of sample (c) showed two strong peak emissions, one at 397 nm (3.13 eV) NBE of GaN in the UV region and another at 536 nm (2.34 ev). The UV emission band presented by all samples was due to a radiative recombination from a GaN active layer, which is in good agreement with the experimental band gap value (3.4 eV [29]) of bulk GaN. The variation of the position and intensity of this emission was due to the effect of the gas carrier on GaN nanostructures during the growth process as the concentration defect in the GaN crystal. Therefore, the red shift could be considered as likely to be a severe effect, such as a strain, crystal defect, or impurity. The red shift in the position of the UV peak was presented in samples that were due to the tensile stress produced by the defect contained in GaN crystals. Knowing the impact of stress on the band gap, where the energy band increases from compressive stress and decreases from tensile stress, is therefore necessary [30].
In summary, GaN nanostructures have been successfully grown on the n-Si (1 1 1) substrate without any catalyst and NH3 gas using three zones in a tube furnace under difference gas carriers. SEM images show that the nanostructures have been observed under different gas carriers exhibiting different shapes (nanowires, nanorods, and nanotriangles) and diameters. According to SEM images of the sample grown under H2 mixed with N2, the ratio of hydrogen in the carrier gas has a strong influence on the morphology of the structure events for nanotriangles shape. The XRD pattern indicates that the tip is poly-crystalline hexagonal GaN with the (0 0 2) crystal phase. A shift in peak position indicated stress on the GaN nanostructure due to crystal defect. The decomposition and composition of GaN leads to the dominant loss of N, resulting in enhanced defects in GaN nanostructures, such as C impurity, oxygen complex, and Ga richness. The green–yellow emission band in GaN is from a transition between a shallow donor and a deep level. The presence of peaks observed in the green yellow region is due to defects, such as VN and C. Ga/N ratio in GaN crystal is depends on the type of carrier gas in case of NH3gas absence and impact of this ratio can determine the extent of compositional imbalance of GaN, which affects the optical and electrical properties. PL spectra suggest that the GaN nanowires grown in N2 environment have a visible emission. Overall results indicate that the gas carrier had a strong effect on the properties of grown GaN nanostructures. References [1] R. Marco, D. Julien, R. Raffaele, D. Marcus, V. Christian, F. Eric, C. Antonino, C. Gatien, C.J. Franc- ois, G. Nicolas, Applied Physics Express 3 (2010) 061002. [2] S.D. Hersee, X. Sun, X. Wang, Nano Letters 6 (2006) 1808. [3] M. Narukawa, S. Koide, H. Miyake, K. Hiramatsu, Journal of Crystal Growth 311 (2009) 2970–2972. [4] O. Briot, J.P. Alexis, M. Tchounkeu, R.L. Aulombard, Materials Science and Engineering 43 (1997) 147–153. [5] Z. Sitar1, L.L. Smith, R.F. Davis, Journal of Crystal Growth 141 (1994) 11–21. [6] Y. Nakano, Y. Irokawa, M. Takeguchi, Applied Physics Express 1 (2008) 091101. [7] S. Guha, N.A. Bojarczuk, Applied Physics Letters 72 (1998) 415. [8] A. Soudi, E.H. Khan, J.T. Dickinson, Y. Gu, Nano Letters 9 (2009) 1844–1849. [9] G. Seryogin, I. Shalish, W. Moberlychan, V. Narayanamurti, Nanotechnology 16 (2005) 2342–2345. [10] G. Cheng, A. Kolmakov, Y. Zhang, M. Moskovits, R. Munden, M.A. Reed, G. Wang, D. Moses, J. Zhang, Applied Physics Letters 83 (2003) 1578. [11] Y. Wang, C. Xue, H. Zhuang, Z. Wang, D. Zhang, Y. Huang, W. Liu, Applied Surface Science 255 (2009) 7719–7722. [12] T. Kuykendall, P. Pauzauskie, S. Lee, Y. Zhang, J. Goldberger, P. Yang, Nano Letters 3 (2003) 1063–1066. [13] M. Zervos, A. Othonos, Nanoscale Research Letters 6 (2011) 262. [14] K.S.A. Butcher, Afifuddin, P.P.T. Chen, M. Godlewski, A. Szczerbakow, E.M. Goldys, T.L. Tansley, J.A. Freitas Jr, Journal of Crystal Growth 246 (2002) 237–243. [15] H.-M. Kim, T.W. Kang, K.S. Chung, Advanced Materials 15 (2003) 567–569. [16] Z. Chen, C. Cao, W.S. Li, C. Surya, Crystal Growth & Design 9 (2009) 792–796.
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
[17] T.Y. Tang, K.L. Averett, J.D. Albrecht, W.Y. Shiao, Y.S. Chen, C.C. Yang, C.W. Hsu, L.C. Chen, Nanotechnology 18 (2007) 445601. [18] F.K. Yam, Z. Hassan, Superlattice Microstructure 43 (2008) 1–23. [19] B.C. Mohanty, Y.H. Jo, D.H. Yeon, I.J. Choi, Y.S. Cho, Applied Physics Letters 95 (2009) 062103. [20] A. Chakraborty, C.K. Sarkar, Physica B 404 (2009) 1148–1153. [21] S. Tanaka, M. Takeuchi, Y. Aoyagi, Japanese Journal of Applied Physics 39 (2000) 831–834. [22] N. Marquestaut, D. Talaga, L. Servant, P. Yang, P. Pauzauskie, F.L. Labarthet, Journal of Raman Spectroscopy 40 (2009) 1441–1445. [23] D.D. Manchon, A.S. Barker, P.J. Deana, R.B. Zetterstroma, Solid State Communications 8 (1970) 1227–1231. [24] T. Ogino, M. Aoki, Japanese Journal of Applied Physics 19 (1980) 2395.
23
[25] W. You, X.D. Zhang, L.M. Zhang, Z. Yang, H. Bian, Q. Ge, W.X. Guo, W.X. Wang, Z.M. Liu, Physica B 403 (2008) 2666. [26] J.L. Lyons, A. Janotti, C.G.V. Walle, Applied Physics Letters 97 (2010) 152108. [27] L.H. Robins, K.A. Bertness, J.M. Barker, N.A. Sanford, J.B. Schlager, Applied Physics Letters 101 (2007) 113506. [28] M.A. Reshchikov, H. Morkoc, Journal of Applied Physics 97 (2005) 061301. [29] A. Denis, G. Goglio, G. Demazeaua, Materials Science and Engineering R 50 (2006) 167–194. [30] J. Xie, S. Mita, A. Rice, J. Tweedie, L. Hussey, R. Collazo, Z. Sitar, Applied Physics Letters 98 (2011) 202101. [31] D.C. Reynolds, D.C. Look, B. Jogai, J.E. Van Nostrand, R. Jones, J. Jenny, Solid State Communications 106 (1998) 701–704.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
NH3-free growth of GaN nanostructure on n-Si (1 1 1) substrate using a conventional thermal evaporation technique K.M.A. Saron a,n, M.R. Hashim a, M.A. Farrukh b a b
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, 11800-Penang, Malaysia Department of Chemistry, GC University Lahore, 54000-Lahore, Pakistan
a r t i c l e i n f o
abstract
Article history: Received 4 February 2012 Received in revised form 4 March 2012 Accepted 22 March 2012 Communicated by K. Nakajima Available online 9 April 2012
We have investigated the influence of carrier gas on grown gallium nitride (GaN) epitaxial layers deposited on n-Si (1 1 1) by a physical vapour deposition (PVD) via thermal evaporation of GaN powder at 1150 1C. The GaN nanostructures were grown at a temperature of 1050 1C for 60 min under various gases (N2, H2 mixed with N2, and Ar2) with absence of NH3. The morphology, structure, and optical properties (SEM) images showed that the morphology of GaN displayed various shapes of nanostructured depending on the type of carrier gas. X-ray diffraction (XRD) pattern showed that the GaN polycrystalline reveals a wurtzite-hexagonal structure with [0 0 1] crystal orientation. Raman spectra exhibited a red shift in peaks of E2 (high) as a result of tensile stress. Photoluminescence (PL) measurements showed two band emissions aside from the UV emission. The ultraviolet band gap of GaN nanostructure displayed a red shift as compared with the bulk GaN; this might be attributed to an increase in the defect and stress present in the GaN nanostructure. In addition, the observed blue and green–yellow emissions indicated defects due to the N vacancy and C impurity of the supplied gas. These results clearly indicated that the carrier gas, similar to the growth temperature, is one of the important parameters to control the quality of thermal evaporation (TE)-GaN epilayers. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Si (111) substrate A1. Nanostructures A1. Gases carrier flows A2. Growth from vapor A3. Physical vapor deposition processes B1. Nanomaterials
1. Introduction Gallium nitride (GaN) is an important semiconductor with a wide direct-band gap for photonic and high-power devices [1]. The characteristics and properties of GaN nanostructures, including nanowires and nanorods, have been previously reported [2,3]. Often, GaN-based layers are epitaxially grown on sapphire or silicon carbide substrates [4,5]. However, growth on these substrates has a number of deep-level defects, such as dislocations due to lattice mismatch, which may influence the reliability and performance of GaN-based devices [6]. Silicon has attracted much attention as a substrate material for GaN growth due to its larger thermal expansion coefficient and lattice mismatch compared with SiC and Al2O3, furthermore, it is cheap. The growth of GaN on Si is highly desirable for the Si-based electronic industry, because it is a promising route for large-scale, low-cost mass production of GaN-based devices [6,7]. However, the growth of GaN on Si involves the use of catalysts, such as Au, Ni, and Ag [8,9]. The utilization of these catalysts disturbs the purity of a GaN nanostructure, and therefore, may negatively affect its performance in many semiconductor applications [10]. Many reports showed that
n
Corresponding author. E-mail address: [email protected] (K.M.A. Saron).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.03.046
nanostructures of GaN have been fabricated on Si substrates by evaporating Ga2O3 powder at 1100 1C in ammonia gas flow [11], although wurtzite GaN nanowires were synthesized via the direct reaction of Ga with ammonia (NH3) in a tube furnace [9]. Usually, GaN grows in an atmosphere of NH3 while H2, Ar2, and N2 are used as carrier gases [12–14]. However, among most of these methods, an atmosphere of superfluous NH3 is obligatory, which caused air pollution and waste. Many techniques have been employed to produce high quality GaN films, such as hydride vapour phase epitaxy (HVPE) [15], metal organic chemical vapour deposition (MOCVD), thermal evaporation technique (TE), and molecular-beam epitaxy (MBE) [16–18]. Among these techniques, thermal evaporation is relatively simple and suitable for its low melting point and low decomposition or low sublimation point oxides. However, GaN films grown in an environment free of ammonia gas contain a number of defects, such as Ga-rich, N and Ga vacancies, oxygen deep level, and carbon impurity [18]. The growth of GaN nanostructure on Si in absent of NH3 gas has not yet been demonstrated. The current paper reports the successful growth of polycrystalline GaN nanostructures on Si (1 1 1) by thermal evaporation of GaN powder using different gases as carrier gas. One goal of the present study is to control the formation of alloyed particles (GaN) under free ammonia gas and thermal evaporation method
20
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
to obtain optimum growth rate and low defects in GaN. A further study of the effects of carrier gases (N2, H2/N2, and Ar2) on grown GaN nanostructures has been presented. Moreover, the changes caused by different gas on morphological shape and diameters of nanostructure have been mentioned. Structural and optical properties of GaN nanostructure have also been reported.
2. Experimental GaN nanowires and nanorods were grown in a furnace with a horizontal three-zone quartz tube (90 mm in outer diameter and 750 mm of heated length), under different carrier gases (N2 99.999%, 2% H2 mixed with 98% N2 and Ar2 99.999%). An n-type silicon (1 1 1) wafer was used as the substrate to collect the products. Before the substrate was loaded into the furnace, it was cleaned via the standard cleaning method [14]. About 0.3 g GaN (99.99%) powder was placed into a quartz boat, and then the boat was set at the downstream centre zone, where the temperature was about 200 1C; the substrate was placed on the quartz holder and then loaded into the third zone of the quartz tube. The distance between materials and substrate (growth zone) was 32 cm. The gases (N2, H2 mixed N2, and Ar2) were supplied into the inlet before the furnace tube was run to remove oxygen with the flow rate of 4 L/min (L/min) during growth. Two of the heating zones were used for grown GaN. The temperature was adjusted between the centre zone (GaN powder) and third zone (Si (1 1 1) substrate) to be 100 1C. The third zone, the growth zone, was kept at 1050 1C during the process, while the temperature of the centre zone (GaN powder) was kept at 1150 1C and held for 60 min. After deposition of GaN, the samples were cooled down to room temperature with flowing gas. Morphology, characterization and simple composition analysis of the samples were carried out using scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDX) attached to the SEM (JOEL JSM-6460LV performed at 10 kV). The phase and purity of GaN nanostructures were examined by X-ray ˚ source, diffraction (XRD) using Cu Ka1 radiation (l ¼1.5406 A) step size 0.051, and scanning range of between 2y ¼201 and 701. The optical properties of the samples were recorded by highspatial resolution Raman spectroscopy using Jobin Yvon HR 800 UV system with an argon ion laser (514.5 nm) as an excitation source. Photoluminescence (PL) was measured at room temperature using a He–Cd laser (325 nm).
3. Results and discussion
Fig. 1. SEM images of GaN nanostructures grown on Si (1 1 1) using furnace tube under different carrier gases: (a) N2 produced nanowires and nanorods, (b) N2 mixed with H2 nanowire and nanotriangles, and (c) Ar2 nanowires like tree branches and EDX at the right side of SEM images.
Fig. 1 shows SEM images of three typical morphologies of GaN nanostructure grown on n-Si (1 1 1) obtained via different gas carriers (N2, H2 mixed with N2, and Ar2). Sample (a), which was grown under N2 gas, showed an overview of GaN nanowires (NWs) and nanorods (NRs) coverage on Si (1 1 1). The nanowires and nanorods had diameters of 100–500 nm, showing clearly that the nanowires and nanorods grown vertically on the surface of the substrate had hexagonal face. For sample (b), GaN grown under N2 mixed with H2, the nanowire was distributed randomly on the substrate like a spider web. Some nanotriangles orientations were also observed on this substrate. They had size of 50–80 nm and dimension of about 110–180 nm; their lengths spanned several micrometers of nanowires. The observed shape of the nanotriangles was due to the effect of hydrogen-mixed reaction environment, especially H2. Nanotriangles were not observed with sample (a), which was grown in N2 gas. These results are in agreement with the findings of Kuykendall et al. [12] on nanotriangles; they indicated the influence of hydrogen
on the morphology of GaN nanowire, but they did not identify the proportion of hydrogen. In the current work 2% H2 mixed with 98% N2 was used as environment carrier gas with flow rate of 4 L/min without ammonia gas. This indicated that the hydrogen flow rate of 0.1 L/min is most to obtain the nanotriangles. No catalyst and NH3 gas were used. The inset Fig. 1(b) clearly shows that the nanotriangulars grew on the c-axis of the wurtzite structure [12]. For sample (c), which was grown under Ar2, the nanowires looked similar to a tree branch with a diameter of about 200 nm. The differences in temperatures between the material source and the substrate, beside the rush of gas in the tube, changed the velocity of gas molecules, leading to the random deposition of GaN nanowires on the surface of the substrate. The individual spectrum (right side of SEM images) of Ga, N, C, and O obtained by EDX indicated the defect in GaN composition of the test samples. Thus, the differences in gases and the growth rates of these chemically and structurally distinct surfaces have a
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
21
These elements could be composed again at the substrate. During this process, some N was lost between the source and the growth area. However, N could be stabilized independently in the chemical environment (N-rich) produced from NH3. The chemical property of GaN was stable in N2 and H2 atmospheres and either gas (Ar2) at a high temperature, but it showed instability in Ar2 atmospheres at a high temperature. The single atoms of nitrogen combined with each other to be the nitrogen molecule N2, as a result of collusion between N atoms. The rate of N lost decreased when the samples were grown in N2 and H2 mixed with N2 atmospheres, as confirmed by EDX. In addition, an amount of H2 added into the carrier gas (N2) could be enhancement in reaction of Ga with incoming N. The growth rate of GaN nanostructure was observed to be faster when the N2 and N2 mixed with H2 were used as carrier gas compared with Ar2. The results showed that the carrier gas had a strong effect on the formation of GaN nanostructure. The XRD patterns of GaN nanostructures (NWs and NRs) with different carrier gases are shown in Fig. 2. Three intense peaks corresponding to (1 0 0), (0 0 2), and (1 0 1) of GaN nanostructure were located at 32.261, 34.571, and 37.31, respectively, indicating the presence of a small number of crystallites with different orientation. Further, these demonstrated that GaN nanowires possess a hexagonal wurtzite structure and have a preferred orientation h-(0 0 2) plane parallel to the (1 1 1) of the Si substrate. In sample (a), the strong peak for GaN (0 0 2) was observed at 2y ¼34.681. The XRD patterns from the GaN nanostructure indicated a typical wurtzite hexagonal structure with ˚ Fig. 2(b) showed that the peak lattice constant of (c ¼5.174 A) (0 0 2) was observed at 2y ¼34.621, with lattice constant of ˚ Fig. 2(c) showed that the weak peak (0 0 2) was (c¼5.18 2 A). ˚ The observed at 2y ¼34.81, with lattice constant of (c ¼5.16 A). shift in 2y orientation of the (0 0 2) plane with changed gas and the calculated c were found smaller than the bulk GaN (5.1855) [14]. This indicates that GaN nanostructure suffer from tensile stress, which implies that the lattice constant c decreased compared with the stress-free bulk GaN. In the literature, peak shifts
toward lower (or higher) 2y values with respect to the bulk peak profiles have been attributed to compressive (or tensile) stress in the films [19]. This effect could result in decreasing the interplanar spacing, thus leading to the observed increase in the diffraction angle. The cooling of the epitaxial nanostructure from high growth temperature to room temperature introduces stress, which is tensile, could be due to thermal mismatch and lattice mismatch between GaN and Si [20]. Furthermore, the gas carrier causes intrinsic stress resulting from the defect [21]. The variation of defects depends on carrier gases; however, low defect was achieved by N2 and N2 with a ratio of H2. In contrast, when Ar2 was used as carrier gas it weakened pre-reaction, avoided deposition of pre-reaction products and impurities, and resulted in a more proper crystal. Second, with N2 was used as carrier gas, the interface of growth became more stable and the crystal quality became purer. The intensity of the h-(0 0 2) orientation peak decreased when the Ar2 was supplied; this indicated enhanced defects and poor crystallinity of the nanostructure. The highest peak intensity of h-(0 0 2) orientation was observed at N2 and H2 mixed with N2. On the other hand, the absence of NH3 gas during the growth of GaN nanostructure could cause more defects. The high intense peak was presented at sample b, indicating the reduction of a defect, and thereby, impact of interaction between hydrogen with oxygen. This interaction led to the reduced rate of oxygen in GaN [9]. Results of the present study suggest that the type of introduced gas in the growth of GaN without NH3 gas would produce defective concentrations in the crystals of GaN. Fig. 3 shows the Raman spectrum of three samples (GaN) nanostructure grown on Si (1 1 1) by thermal evaporation in different gas carriers. Two dominated Raman-active phonons corresponding to E2 (high) around 567 cm 1 and A1 (LO) around 733 cm 1 were observed from GaN nanostructure and their peaks intensity were found varied. A strong band is observed at about 521 cm 1, which is the contribution from the Si (1 1 1) substrate due to (AO) and a band at 302 cm 1 due to the acoustic phonons of Si. The Raman peak of E2 high mode indicated the GaN nanostructure had a hexagonal wurtzite structure [22]. The Raman peaks of GaN nanostructure E2 high mode for the samples a, b and c, located at 565.3, 567, and 565 cm 1, respectively, are indicating large red-shifts compared with the h-GaN bulk-phase peaks’ positions at 568 cm 1 [23]. The red shift was most likely due to tensile stress in the GaN epilayer. Additional peaks as
Fig. 2. XRD pattern of GaN nanostructures grown on Si (1 1 1) substrate using furnace tube under different carrier gases: (a) N2, (b) N2 mixed with H2, and (c) Ar2.
Fig. 3. Raman spectrum of GaN nanostructures grown at different carrier gases: (a) N2, (b) N2 mixed with H2, and (c) Ar2. The intense band at 520 cm 1 arises from the Si substrate.
major impact on nanostructure morphologies. To investigate the influence of gas on growth structure and chemical at 1150 1C in the presence of N2, N2 mixed with H2, and Ar2 atmosphere, the following Eq. (1) was used [9] for the decomposition of GaN. GaNðSÞ-2Gað1Þ þ N2 ðgÞ
ð1Þ
22
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
However, the stress in GaN nanostructures was due to intrinsic defects, caused by growth condition. The blue emission present in sample (a) was due to the effect of N2 gas, which introduced rich Ga with N vacancies and caused the loss of some N and gain of some C impurity [31]. The green/yellow emission of the GaN material has been reported by Ogino et al. [24] which attributed to the presence of the CGa–ON complex. However, in the samples of the present study, the green/yellow emission was attributed to the VN or the Ga-richness and C impurity. EDX confirmed that because the sample was grown under N2, rich Ga and slight O gain are observable. This result suggests that the grown GaN under N2 flow gas is suitable for visible emission and solar cell devices owing to an enhanced defect in GaN, especially Ga richness [18]. Result suggested that the type of carrier gas has strong effect on crystal quality and photoluminescence band emission.
4. Conclusions Fig. 4. shows the photoluminescence spectra measured at room temperature of the GaN nanostructure grown under different carrier gases: (a) N2, (b) N2 mixed with H2, and (c) Ar2.
contribution of Al longitudinal optical (LO) mode were observed at 733, 732, and 733.2 cm 1 in GaN nanostructure of samples a, b and c, respectively; these had obvious red shifts as well, indicating tensile stress in the GaN epilayer. The E2 mode intensity was directly proportional to the gas supply. The thermal expansion mismatch and growth defect thus accounted for most of the stress present in the GaN produced from the present study. The tensile stress in the GaN epilayer could be caused by the defect in the GaN nanostructure during growth due to N vacancies and impurity. Fig. 4 shows the photoluminescence spectra measured at room temperature. GaN nanostructure grown on Si (1 1 1) indicated the variation of peak intensities and peak wavelengths with different carrier gases. A He–Cd laser [l ¼325 nm], used to optically excite the GaN nanostructure, showed two emissions peaks of UV, and green–yellow emissions. The spectrum for GaN (sample a) showed three main contributions: the band emission of GaN in the UV region (364 nm at 3.4 eV), blue emission (4 1 8), and broad green–yellow band (GYB) emission (centred at 519 nm at 2.4 eV). The emission became visible, which is generally assigned to crystal defects, such as N vacancies, C impurities, and O complexes [24–26]. Sample (b) had two peak emissions; the near band edge (NBE) in UV at 371 nm (3.34 eV) and green–yellow band (GYB) emission centred at 543 nm (2.28 eV). These peaks are possibly attributable to the existence of intrinsic defects or surface states [27,28]. The spectra of sample (c) showed two strong peak emissions, one at 397 nm (3.13 eV) NBE of GaN in the UV region and another at 536 nm (2.34 ev). The UV emission band presented by all samples was due to a radiative recombination from a GaN active layer, which is in good agreement with the experimental band gap value (3.4 eV [29]) of bulk GaN. The variation of the position and intensity of this emission was due to the effect of the gas carrier on GaN nanostructures during the growth process as the concentration defect in the GaN crystal. Therefore, the red shift could be considered as likely to be a severe effect, such as a strain, crystal defect, or impurity. The red shift in the position of the UV peak was presented in samples that were due to the tensile stress produced by the defect contained in GaN crystals. Knowing the impact of stress on the band gap, where the energy band increases from compressive stress and decreases from tensile stress, is therefore necessary [30].
In summary, GaN nanostructures have been successfully grown on the n-Si (1 1 1) substrate without any catalyst and NH3 gas using three zones in a tube furnace under difference gas carriers. SEM images show that the nanostructures have been observed under different gas carriers exhibiting different shapes (nanowires, nanorods, and nanotriangles) and diameters. According to SEM images of the sample grown under H2 mixed with N2, the ratio of hydrogen in the carrier gas has a strong influence on the morphology of the structure events for nanotriangles shape. The XRD pattern indicates that the tip is poly-crystalline hexagonal GaN with the (0 0 2) crystal phase. A shift in peak position indicated stress on the GaN nanostructure due to crystal defect. The decomposition and composition of GaN leads to the dominant loss of N, resulting in enhanced defects in GaN nanostructures, such as C impurity, oxygen complex, and Ga richness. The green–yellow emission band in GaN is from a transition between a shallow donor and a deep level. The presence of peaks observed in the green yellow region is due to defects, such as VN and C. Ga/N ratio in GaN crystal is depends on the type of carrier gas in case of NH3gas absence and impact of this ratio can determine the extent of compositional imbalance of GaN, which affects the optical and electrical properties. PL spectra suggest that the GaN nanowires grown in N2 environment have a visible emission. Overall results indicate that the gas carrier had a strong effect on the properties of grown GaN nanostructures. References [1] R. Marco, D. Julien, R. Raffaele, D. Marcus, V. Christian, F. Eric, C. Antonino, C. Gatien, C.J. Franc- ois, G. Nicolas, Applied Physics Express 3 (2010) 061002. [2] S.D. Hersee, X. Sun, X. Wang, Nano Letters 6 (2006) 1808. [3] M. Narukawa, S. Koide, H. Miyake, K. Hiramatsu, Journal of Crystal Growth 311 (2009) 2970–2972. [4] O. Briot, J.P. Alexis, M. Tchounkeu, R.L. Aulombard, Materials Science and Engineering 43 (1997) 147–153. [5] Z. Sitar1, L.L. Smith, R.F. Davis, Journal of Crystal Growth 141 (1994) 11–21. [6] Y. Nakano, Y. Irokawa, M. Takeguchi, Applied Physics Express 1 (2008) 091101. [7] S. Guha, N.A. Bojarczuk, Applied Physics Letters 72 (1998) 415. [8] A. Soudi, E.H. Khan, J.T. Dickinson, Y. Gu, Nano Letters 9 (2009) 1844–1849. [9] G. Seryogin, I. Shalish, W. Moberlychan, V. Narayanamurti, Nanotechnology 16 (2005) 2342–2345. [10] G. Cheng, A. Kolmakov, Y. Zhang, M. Moskovits, R. Munden, M.A. Reed, G. Wang, D. Moses, J. Zhang, Applied Physics Letters 83 (2003) 1578. [11] Y. Wang, C. Xue, H. Zhuang, Z. Wang, D. Zhang, Y. Huang, W. Liu, Applied Surface Science 255 (2009) 7719–7722. [12] T. Kuykendall, P. Pauzauskie, S. Lee, Y. Zhang, J. Goldberger, P. Yang, Nano Letters 3 (2003) 1063–1066. [13] M. Zervos, A. Othonos, Nanoscale Research Letters 6 (2011) 262. [14] K.S.A. Butcher, Afifuddin, P.P.T. Chen, M. Godlewski, A. Szczerbakow, E.M. Goldys, T.L. Tansley, J.A. Freitas Jr, Journal of Crystal Growth 246 (2002) 237–243. [15] H.-M. Kim, T.W. Kang, K.S. Chung, Advanced Materials 15 (2003) 567–569. [16] Z. Chen, C. Cao, W.S. Li, C. Surya, Crystal Growth & Design 9 (2009) 792–796.
K.M.A. Saron et al. / Journal of Crystal Growth 349 (2012) 19–23
[17] T.Y. Tang, K.L. Averett, J.D. Albrecht, W.Y. Shiao, Y.S. Chen, C.C. Yang, C.W. Hsu, L.C. Chen, Nanotechnology 18 (2007) 445601. [18] F.K. Yam, Z. Hassan, Superlattice Microstructure 43 (2008) 1–23. [19] B.C. Mohanty, Y.H. Jo, D.H. Yeon, I.J. Choi, Y.S. Cho, Applied Physics Letters 95 (2009) 062103. [20] A. Chakraborty, C.K. Sarkar, Physica B 404 (2009) 1148–1153. [21] S. Tanaka, M. Takeuchi, Y. Aoyagi, Japanese Journal of Applied Physics 39 (2000) 831–834. [22] N. Marquestaut, D. Talaga, L. Servant, P. Yang, P. Pauzauskie, F.L. Labarthet, Journal of Raman Spectroscopy 40 (2009) 1441–1445. [23] D.D. Manchon, A.S. Barker, P.J. Deana, R.B. Zetterstroma, Solid State Communications 8 (1970) 1227–1231. [24] T. Ogino, M. Aoki, Japanese Journal of Applied Physics 19 (1980) 2395.
23
[25] W. You, X.D. Zhang, L.M. Zhang, Z. Yang, H. Bian, Q. Ge, W.X. Guo, W.X. Wang, Z.M. Liu, Physica B 403 (2008) 2666. [26] J.L. Lyons, A. Janotti, C.G.V. Walle, Applied Physics Letters 97 (2010) 152108. [27] L.H. Robins, K.A. Bertness, J.M. Barker, N.A. Sanford, J.B. Schlager, Applied Physics Letters 101 (2007) 113506. [28] M.A. Reshchikov, H. Morkoc, Journal of Applied Physics 97 (2005) 061301. [29] A. Denis, G. Goglio, G. Demazeaua, Materials Science and Engineering R 50 (2006) 167–194. [30] J. Xie, S. Mita, A. Rice, J. Tweedie, L. Hussey, R. Collazo, Z. Sitar, Applied Physics Letters 98 (2011) 202101. [31] D.C. Reynolds, D.C. Look, B. Jogai, J.E. Van Nostrand, R. Jones, J. Jenny, Solid State Communications 106 (1998) 701–704.