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Self-catalyst growth of novel GaN nanowire flowers on Si (111) using thermal evaporation technique
Materials Chemistry and Physics 139 (2013) 459e464
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Self-catalyst growth of novel GaN nanowire flowers on Si (111) using thermal evaporation technique K.M.A. Saron, M.R. Hashim* Nano-Optoelectronics Research and Technology Laboratory (NOR), School of Physics, Universiti Sains Malaysia, Penang 11800, Malaysia
h i g h l i g h t s < GaN nanowired flowers were grown on free-catalysts Si (111) using PVD. < A higher temperature, higher uniformity, larger lengths and diameters of the NW flowers. < As substrate temperature increases the diameters and growth rate of NWs increases. < A lower temperature resulted in a high density and good crystal quality of GaN NWs. < The increase in substrate temperature increased the redshift in UV band emission.
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Article history: Received 29 March 2012 Received in revised form 16 October 2012 Accepted 12 January 2013
We investigated the effect of substrate temperature on nanowire (NW) flower GaN epitaxial layers grown on catalyst-free Si (111) through physical vapor deposition via the thermal evaporation of GaN powder at 1150 C in the absence of NH3 gas. The NW flowers were grown at various substrate temperatures from 1000 C to 1100 C for 60 min in N2 ambient. The surface morphology as well as the structural and optical properties of GaN NW flowers were examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, X-ray diffraction, and photoluminescence (PL). The results showed that the increase in substrate temperature resulted in a variation in crystal quality and surface morphology. SEM showed that the substrate temperature has a stronger effect on NW density and growth rate with respect to time. The average length of GaN flowers is estimated to be longer than 300 mm after 1 h at 1100 C, which corresponds to a fast growth rate of more than 200 mm h1 at all substrate temperatures. The PL measurements showed strong near-band-edge (NBE) emission with a weak deep level emission. The greenyellow emission (GYE) can be attributed to N vacancies or to the VGaeON-complexes. The NBE peak exhibited a redshift with increasing substrate temperature, which results from the increase in strain level. The growth mechanism of the polycrystalline GaN NWs was also discussed. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Nitrides Semiconductors Physical vapour deposition (PVD) Optical properties
1. Introduction Nanostructured materials have attracted great attention in recent years because of their unique physical properties and significant applications [1,2]. GaN is of interest for potential application in optoelectronic devices, such as laser diode and other devices, because of its wide band gap (3.4 eV) and large exciton binding energy of 26 meV [3,4]. Investigations on GaN nanostructures have focused on the synthesis and characterization of intrinsic GaN, especially on one-dimensional GaN nanowires (NWs) or nanorods
* Corresponding author. Fax: þ60 4 6579150. E-mail addresses: [email protected] (K.M.A. Saron), roslan@ usm.my (M.R. Hashim). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.01.024
[5]. Most of the reported GaN nanostructures are grown by metal organic chemical vapor deposition or molecular beam epitaxy (MBE) [6,7]. The controlled growth of GaN nanowires with good alignment on the substrate is of particular importance for their potential applications in the fabrication of devices, such as electron field emitters and solar cells [8]. Vertically aligned GaN NWs were grown by vapor deposition methods on substrates with metal nanoparticles as catalysts [9]. The utilization of these catalysts disturbs the purity of the GaN nanostructure and, therefore, may negatively affect its performance in semiconductor applications [9]. Vajpeyi et al. [6] recently reported the catalyst-free growth of GaN NWs on n-Si (111) at various substrate temperatures by MBE. Bertness et al. [7] have successfully grown GaN nanowires on Si (111) by plasma-assisted MBE. Moreover, the growth temperature range over which we observe
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NW growth is approximately from 800 C to 850 C. There is variation from group to group on the center of the range of substrate temperatures for GaN NW growth by MBE [10e12], such that the total range of reported temperatures extends from 720 C to 900 C. However, some very interesting nanowires can be obtained by simply thermally evaporating solid materials [13]. This technique can be carried out in a relatively simple setup composed of a dual-zone vacuum furnace. A previous study has reported successful growth over a temperature range from 1000 C to 1100 C with thermal evaporation of GaN in an atmosphere of NH3. Stach et al. [14] successfully grew GaN NWs on sapphire substrate via a self-catalytic vaporeliquidesolid (VLS) mechanism at high vacuum. However, the growth of GaN NW flowers on Si by thermal evaporation technique in the absence of NH3 gas has not yet been demonstrated in literature. Herein, we present a simple and low cost physical vapor deposition (PVD) method to grow GaN NW flowers directly on catalysis-free Si (111) via thermal evaporation of GaN powder in NH3-free environments. We also studied the effect of substrate temperature on the surface morphology, density, growth rate, and optical properties of GaN NW flowers spontaneously grown on n-type Si (111). 2. Experimental NW flowers were grown by thermal evaporation of GaN powder under controlled substrate temperatures ranging from 1000 C to 1100 C. GaN powder was used as source material and an n-type Si (111) wafer was used as the substrate to collect the products, as shown in Fig. 1. The furnace temperature was then raised to 1150 C in a rate of 15 C min1. The nitrogen gas was initially introduced into the tube with a flow rate of 4 L min1. GaN (99%99) powder (0.3 g) was positioned at the highest temperature center zone of a horizontal tube furnace once the temperature reached 500 C. The Si (111) substrate cleaned in advance was placed on the quartz holder and then aligned from the high- to the low-temperature zone along the downstream flow through the tube. The distance between material source and substrate was kept 250 mm. The furnace was then heated by a rate of 15 C min1 and maintained at a temperature of 1150 C for 1 h with a gas flow rate of 3 L min1. During evaporation, the products were deposited onto a catalystfree Si (111) substrate that was placed downstream within the tube. The evaporation process was conducted at 1150 C for 1 h. The condensation products were deposited onto an Si substrate placed in a temperature zone of 1000, 1050, and 1100 C under atmospheric furnace pressure. After deposition time, the samples were cooled down to room temperature with N2 gas flow. The morphology, structures, and optical properties of the products were characterized by scanning electron microscopy (SEM), energydispersive X-ray spectrometer (EDX), and X-ray diffraction (XRD) A). Room temperature photoluminescence with CuKa (l ¼ 1.5406 (PL) was excited using an HeeCd laser (325 nm). 3. Result and discussion The SEM images of the GaN NWs grown on Si (111) at various substrate temperatures, as shown in Fig. 2, indicated that numerous
Fig. 1. Schematic diagram of experimental setup in horizontal three-zone quartz tube (90 mm in outer diameter and heated length 750 mm).
Fig. 2. Low magnification SEM image of the GaN nanowires flower grown on n-Si (111) at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C.
flowers with lengths of up to hundreds of micrometers were uniformly distributed on the Si (111) surface. SEM images revealed that the flower density decreased and the average diameter of the NW flowers increased when the substrate temperature increased from 1000 C to 1100 C. The low-magnification SEM image of the GaN NWs (Fig. 2) shows that a high density of GaN flowers was uniformly deposited on the substrate and that these flowers combined as a bundle of NWs. A single flower is vertically aligned. The typical sizes and lengths of these flowers are 10 mme15 mm and 200 mme400 mm, respectively. The sample grown at 1000 C (Fig. 2(a)) showed a high density of flowers with an average size of 10 mm and a length of up to 100 mm were deposited on the substrate surface. The diameter and length of the flowers increased when the substrate temperature was increased to 1050 C, as shown in Fig. 2(b). The flowers revealed a clear symmetry at 1100 C and merged with one another to form large buds. This phenomenon indicates that a higher growth temperature results in a higher growth rate as well as a high uniformity of NWs with larger
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diameters [14]. A similar flower-like arrangement observed in the nanostructure was also reported in other studies [15,16], in which the growth of ZnO in the solution made enabled the NWs to bundle together easily to form a flower-like structure. We conducted a detailed study of the structural composition to confirm whether the flowers are composed of GaN. The chemical composition of the flowers was derived from the quantitative analysis by using EDX. Fig. 2, beside the SEM, shows that the NW flowers mainly consisted of Ga, N, Si, O, and C impurities. The EDX analysis of the grown NW flowers demonstrated that the Ga:N ratios were 35.7:31.8, 29.21:23.4, and 36.3:27.12 for the 1000, 1050 and 1100 C growth temperatures, respectively. EDX analysis suggests continuous N loss because of increasing temperature [13]. The O contamination changed slightly from 9% to 13.6% when the substrate temperature was increased from 1000 C to 1100 C. Note that the increase in the Ga:N ratio when the substrate temperature was increased may indicate the formation of N vacancies the samples, i.e. increasing the chance of defects in the NW flowers. The
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oxygen contaminant in GaN NWs could have originated from the atmosphere or from the residual oxygen in the reaction tube. In addition, carbon contamination resulting from the atmosphere or from the growth environment could also be occasionally detected from GaN NW flowers although at low contents. Fig. 3 shows the high-magnification SEM images of GaN NW in the form of microflowers grown at substrate temperatures (1000 Ce1100 C). The flower is three-faced, and each face consists of a high density of NWs. For the GaN NW-flowers grown at 1000 C, Fig. 3(a) shows the transformation process of microflowers into the high density of NWs. The average diameter and length of each wire of the flower-shaped structures were approximately 250 nm and several micrometers long, respectively. The sample grown at 1050 C showed that the flower contains a high density of NWs in one array (Fig. 3(b and d)). The obtained NWs exhibited smooth surface morphology with average diameters and lengths of 300 nme350 nm and several micrometers long, respectively. Thus, large-scale GaN NWs evidently grew from the flowers. Increasing
Fig. 3. High magnification SEM image of GaN nanowires flower grown on Si (111) at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C, (d) represents a single flower structure with a scale bar of 1 mm, and (e) GaN nanoparticles deposition on the silicon surface with a scale bar of 1 mm.
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temperature to 1100 C significantly increased the diameter and length of the NW flowers (Fig. 3(c)). The image obviously reveals thermally grown flower-shaped structures containing a bundle of NWs. The typical diameter of one wire in a flower is approximately 500 nm, and the length is always several tens of micrometers. 3.1. Effects of temperature Figs. 2 and 3 show the effect of growth temperature on NW flower morphology at different substrate temperatures (1000, 1050 and 1100 C). The SEM images show that NW flowers were formed in all samples. Fig. 3(a and e) displays the starting growth of GaN flowers on an Si substrate via a catalyst-free growth mechanism. The NWs formed a flowers-like structure, which showed an increase in diameter and length (Fig. 3(b and c)). Different sizes of GaN nanoparticles were also observed from the edge portion of the sample (1000 C), which suggests that these nanoparticles were not subjected to supersaturation because of the increased nucleation delay (i.e. insufficient time to grow) (Fig. 3(e)). It is believed that this is the first phase in which nucleation occurred during the growth of the NW flowers. The formation of GaN NW flowers through mechanism can be divided into two steps, namely, nucleation and growth, according to the obtained results [17e19]. The formation of NW flowers basically resulted from the deposition of nanoparticles on the surfaces of the substrates at different temperatures. The growth rate of the flowers increased with respect to temperature when the rest of the conditions were kept constant. The growth of GaN NWs was initiated by supplying GaN vapor at 1150 C, and was controlled by increasing the substrate temperature from 1000 C to 1100 C (growth area). GaN vapor was deposited on the substrate in the form of nanoparticles within a short time. The particles began to grow to form a flower-like structure through the vaporesolid (VS) mechanism and upon heating the substrate [17]. The fast growth rate was dependent on the increasing growth temperature and was achieved when the time was held constant (1 h). NW density was strongly dependent on the growth temperatures [6,7]. The substrate temperature had a stronger effect on NW density with respect to time. For 1 h growth, the NW density increases with increasing substrate temperature (1000 Ce1100 C). Therefore, the substrate temperature provides a simple method to control the NW density and growth rate. The conventional PVD growth mechanism remains suitable to account for the unique growth phenomena reported in this study. The growth mechanism can be explained in as follow: GaN decomposes at temperatures above 850 C via the possible reactions given in Eq. (1) [14,20].
GaNðsÞ ¼ Ga ðlÞ þ 0:5N ðgÞ þ 0:25N2 ðgÞ
(1)
Through the thermal evaporation of GaN powder in a furnace tube at 1150 C, the occurrence of this decomposition can be observed to cause nanostructure growth in real time and at high spatial resolution [21]. We propose that during this process, Ga droplets become self-catalytic growth elements of GaN NW flowers [17]. The decomposition of the GaN nanoparticles generates N atoms and isolated nanoscale Ga droplets [21]. The evaporation of GaN powder at 1150 C also possibly produces diatomic or polymeric GaN vapor species [14]. The Ga vapor is condensed into liquid Ga droplets on the Si substrate surface [22]. The Ga droplets may react with the GaN vapor to establish a vapor-Ga/solid-GaN interface [20,23]. Given that Ga droplets were formed during the reaction, the NW flowers grow because of supersaturated Ga droplets with GaN vapor [10]. Ga metal was not observed on the tip of the NWs because of the consumption and desorption of Ga (high growth temperature) [24]. Therefore, Ga droplets are expected to
be converted into GaN nanostructures because of a high growth temperature which forbids accumulation of Ga droplets on GaN [23]. This result indicates that the GaN NW flowers grow on Si substrate via the VS mechanism. Stach et al. [14] have grown GaN NWs on sapphire via the self-catalytic VLS mechanism, in which the catalysts are nanoscale Ga liquid droplets generated by the thermal decomposition of GaN. However, they did not exclude the possibility of GaN NW growth via the VS mechanism. According to the VS mechanism, GaN vapor is condensed, and then nucleated on the Si substrate. The GaN nanoparticles participate in nucleation for the subsequent growth of GaN NW flowers. The continuous supply of GaN vapor through the heating of GaN at (1150 C) with a constant N flow rate facilitates the further growth of the GaN nucleation to form the NW flower through the VS reaction [25]. As the temperature increases, the rate of recomposition of GaN nanoparticles is significantly increased. Consequently, the reaction of the precursors Ga and N on the Si substrate is increased, thus, resulting in an increase in particle size and the facilitation of growth of NW flowers GaN structures. By increasing substrate temperatures, the growth rate is enhanced significantly because of a high supersaturation of GaN nuclei, which results in an increase in the partial pressure of the vapors component [23e25]. In other words, the growth morphology of GaN nanostructures is strongly dependent on the furnace temperature because the density of the vapor component that generated by the thermal decomposition varies with different furnace temperatures. Therefore, a high source temperature (1000 Ce1100 C) increases the partial pressure of the vapors’ component, thus yielding a high supersaturation of GaN nuclei which favors the growth of GaN NW flowers with large diameters. By contrast, when the substrate temperature is lower (1000 C) and time is insufficient, the growth of the NW flowers exhibited a nanoparticle morphology effect because of the low supersaturation of GaN nuclei, as shown in Fig. 3(e). At high temperatures, GaN nucleation is free from catalysts, and growth is expected to occur via the VS route. We conclude that a model can be proposed for the growth of novel GaN NW flowers through two possible processes. First, at high temperatures (1150 C), GaN vapor is deposited on substrates surfaces to form the nucleus of the GaN flower. As the deposition concentration increases, GaN nuclei individually grow in the form of NW flowers [23e25]. Second, thermal decomposition of GaN occurs, producing Ga vapor that condenses into Ga droplets on the Si surface [21]. The Ga-vapor reacts with N (i.e. recomposition) to establish the GaN nucleus and grow [17,19]. The newly formed NWs begin to detach the flowers from the surface as the thermal growth of GaN NW flowers continues, thus pushing the particle upward. Based on the average length of the flowers, the growth rate was estimated to be more than 500 nm min1, faster than those (w300 nm min1) of NWs grown by using another technique [25,26]. This result suggests that the substrate temperature has strong effect on the formation and growth rate of NW-flowers. Fig. 4 shows the XRD patterns of GaN NW flowers grown on Si (111) in the absence of NH3 gas with varying substrate temperatures 1000 Ce1100 C at a constant growth time of 1 h. A complete conversion of GaN powder into GaN NW flowers has been observed for the growths conducted at 1000, 1050, and 1100 C. The peaks in the XRD patterns can be indexed to a wurtzite hexagonal structure of GaN. The reflection peaks of (100), (002), and (101), correspond to the hexagonal wurtzite GaN phase with lattice parameters of approximately a ¼ 3.186 A, c ¼ 5.178 A, consistent with the reported values for bulk GaN [27]. The reflection peak of (111) corresponds to the cubic Si. An additional peak with a weak intensity of b-Ga2O3 could be observed at 2q ¼ 44.81, corresponding to reflection plane (112). The intensity of the (002) peak of GaN NW flowers gradually decreased with increasing substrate temperature. This result
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Fig. 4. The X-ray diffraction spectrum (XRD) patterns of GaN nanowires grown at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C (*) and (^) are the GaN and impurity phase of b-Ga2O3, respectively.
indicates that the worst crystal quality is obtained at the highest growth temperature. The reduction of intensity peaks is attributed to the decrease in the conversion of Ga to GaN. Some N could have been lost between the vapor source and the growth area during GaN growth (NH3-free) [13,28]. The XRD measurement exhibited variations (002) in the diffraction peaks with increasing substrate temperature, which indicates a decreased, lattice constant (c) of GaN NW flowers compared with that of stress-free GaN (co ¼ 5.1855) [27]. The (002) diffraction peaks are shifted to higher 2q values (34.85 , 34.67, and 34.75 ) compared with the (002) peak positions of bulk GaN (34.5 ), implying that all the GaN NW flowers are under tensile stress along the c-axis [29]. With increase in temperature, the intensity peak became lower and less asymmetric. However, tensile stress (i.e. intrinsic stress) is typically introduced by impurities, defects, and lattice distortions in the crystal [30]. With an increase in substrate temperature, more defects are formed during the growth process, which might result in a crystal defect that facilitates the reduction of the lattice constant and increases the tensile stress. The thermal expansion, lattice mismatch, and growth defect thus account for most of the stress present in the GaN produced in this study. The NW flowers grown at 1000 C (Fig. 4(a)) revealed a high-intensity peak with a larger shift in the (002) diffraction peak, while the intensity peak of the b-Ga2O3 phase reduced. As shown in Fig. 3(b), the intensity of XRD peaks of this sample decreased as the growth temperature increased to 1050 C, whereas the intensity peak of the b-Ga2O3 phase increased slightly, which resulted from oxygen diffusion [28]. The XRD peaks of GaN NW flowers were significantly lower when the temperature increased to 1100 C, but the traces of b-Ga2O3 were found to increase in the reaction product, as shown in Fig. 4(c). The NW flowers grown at 1050 C and 1100 C showed significantly lower XRD intensity peaks than the NW flowers grown at 1000 C, which can be attributed to the greater number of defects formed during high-temperature growth. The increase in growth temperature results in the degradation of the crystalline quality of GaN NW flowers [31]. The formation of b-Ga2O3 might be attributed to the presence of oxygen traces in the reaction tube furnace. The kinetic barriers of the N atoms might be reduced at high growth temperature, and the diffusion length of O in Ga may be increased, thus resulting in a sufficient reaction of O atoms with Ga to produce the Ga2O3 phase [27]. These XRD measurement results indicate that high growth
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temperature can increase the number of crystal defects and degrade the crystalline quality of NW flowers. The room temperature PL spectra of the GaN NW flower-like structure were measured using an HeeCd 325 nm wavelength laser as the excitation source (Fig. 5). The spectra of the NW flower grown at 1000 C (Fig. 5(a)), displayed two band emissions, namely, a strong UV emission at approximately 369 nm (3.36 eV) and a broad deep-level emission (DLE) in the green-yellow emission (GYE) at 536 nm (2.25 eV). The PL spectra of NW flower grown at 1050 C exhibited a strong UV near band edge emission (NBE) at approximately 387 nm (3.32 eV) and a broad GYE centered at 540 nm (Fig. 5(b)). The NW flower grown at 1100 C (Fig. 5(c)) shows a strong UV NBE at approximately 393 nm (3.31 eV) and a broad GYE at 537 nm. Generally, the strong UV emission ranging from 369 nm to 394 nm is the NBE emission resulting from the recombination of free excitons [32]. The GYE centered at approximately 540 nm is attributed to the singly ionized nitrogen vacancy and to the emission resulting from the radiative recombination of a photo generated hole with an electron occupying the nitrogen vacancy [33]. The GaN NW flowers grown at 1000, 1050 and 1100 C showed UV emission peaks that were red shifted to lower energy. The redshift of UV emission peaks increases slightly as growth temperature increases, because of the narrowing of the energy band gap, which probably resulted from an increase in crystal intrinsic defects (tensile stress) [30]. Thus, the redshift of the UV emission of different GaN nanostructures from 369 nm to 394 nm indicated different contents of N vacancies in each NW flower, which resulted from different growth conditions. This phenomenon might also be attributed to a kind of lattice defect in the crystal, depending on the content of the reactive Ga vapor through the particle formation process. Therefore, the redshift of PL peak position could be correlated with the band gap change resulting from the in-plane tensile stress of GaN NWs along the axial direction [34]. As the temperature increases the PL is dominated mainly by the UV emission with redshift. We note that as temperature increases the wire diameter were increased, thus, quantum confinement effect is less (i.e. no blueshift), therefore, the UV peak shifted to red [35]. However, the NBE peaks are more intense than the GYE peaks for all measured samples, thus, indicating the optical quality of the NW flowers. The DL emission in PL can be attributed to several factors such as; incorporation of impurity-induced disorder, strain-induced structural defects, and surface defects during
Fig. 5. PL spectra of the NWs obtained at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C.
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the growth process [36]. The sample grown at 1000 C showed the highest GYE intensity, which can be associated with the presence concentrated N vacancies (VN) in thinner NWs. These vacancies form the defect levels in the forbidden band gap and give rise to this GYE [36]. The sample grown at 1050 C had a weakest GYE, whereas, for growth temperature at 1100 C, the UV peak shifts to lower energy, and its GYE also reduced. The higher concentration of surface states in thinner wires can also be explained by the larger surface-to-volume ratio in thinner wires compared with their thicker counterparts. Therefore, N vacancy-related complexes or extended defects can be correlated with an increase in the intensity of the GYE at approximately 540 nm, the degradation of the crystal quality, and an increase in the concentration of electrons [36]. This broad DL emission is associated with vacancy-related structural defects, including N, Ga, and VGaeON complexes, which are energetically located within the band gap [36]. The presented GYE might be attributed to the defect related to the O impurity [37]. The correlation between the intensity of the YE band in GaN and the concentration of O has been reported by Reshchikov et al. [36]. Our study yields insights into the behavior and nature of UV NBE emission and defect-related visible emission in GaN NW flowers. The results of our experiment indicated that substrate temperatures strongly affect the optical properties of GaN NW flowers. We thus suggest a simple method for producing electronically pure polycrystalline semiconductor NW flowers. 4. Conclusions The growth of GaN flower-like NW arrays on catalyst-free Si substrates has been demonstrated at 1000, 1050, and 1100 C in the absence of NH3. At increased substrate temperatures, the diameters of both NWs and flowers increase which indicates increased growth rate. At higher substrate temperatures, growth rate accelerates. A lower substrate temperature resulted in good crystal quality of GaN (NWs). Increasing substrate temperature resulted in further oxygen contamination and in the formation of the b-Ga2O3 phase. Oxygen traces might be responsible for the increase in growth rate at higher temperatures in the absence of NH3. The PL measurements showed strong NBE emission with a weak GYE, which confirms the high optical quality of the fabricated GaN NW flowers. The NBE consequently shifted to the narrower energy, because of tensile stress. These results suggest that GaN powder is a promising self-catalyst for growing GaN NW flowers via thermal evaporation with controlled substrate temperature.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Self-catalyst growth of novel GaN nanowire flowers on Si (111) using thermal evaporation technique K.M.A. Saron, M.R. Hashim* Nano-Optoelectronics Research and Technology Laboratory (NOR), School of Physics, Universiti Sains Malaysia, Penang 11800, Malaysia
h i g h l i g h t s < GaN nanowired flowers were grown on free-catalysts Si (111) using PVD. < A higher temperature, higher uniformity, larger lengths and diameters of the NW flowers. < As substrate temperature increases the diameters and growth rate of NWs increases. < A lower temperature resulted in a high density and good crystal quality of GaN NWs. < The increase in substrate temperature increased the redshift in UV band emission.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2012 Received in revised form 16 October 2012 Accepted 12 January 2013
We investigated the effect of substrate temperature on nanowire (NW) flower GaN epitaxial layers grown on catalyst-free Si (111) through physical vapor deposition via the thermal evaporation of GaN powder at 1150 C in the absence of NH3 gas. The NW flowers were grown at various substrate temperatures from 1000 C to 1100 C for 60 min in N2 ambient. The surface morphology as well as the structural and optical properties of GaN NW flowers were examined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy, X-ray diffraction, and photoluminescence (PL). The results showed that the increase in substrate temperature resulted in a variation in crystal quality and surface morphology. SEM showed that the substrate temperature has a stronger effect on NW density and growth rate with respect to time. The average length of GaN flowers is estimated to be longer than 300 mm after 1 h at 1100 C, which corresponds to a fast growth rate of more than 200 mm h1 at all substrate temperatures. The PL measurements showed strong near-band-edge (NBE) emission with a weak deep level emission. The greenyellow emission (GYE) can be attributed to N vacancies or to the VGaeON-complexes. The NBE peak exhibited a redshift with increasing substrate temperature, which results from the increase in strain level. The growth mechanism of the polycrystalline GaN NWs was also discussed. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Nitrides Semiconductors Physical vapour deposition (PVD) Optical properties
1. Introduction Nanostructured materials have attracted great attention in recent years because of their unique physical properties and significant applications [1,2]. GaN is of interest for potential application in optoelectronic devices, such as laser diode and other devices, because of its wide band gap (3.4 eV) and large exciton binding energy of 26 meV [3,4]. Investigations on GaN nanostructures have focused on the synthesis and characterization of intrinsic GaN, especially on one-dimensional GaN nanowires (NWs) or nanorods
* Corresponding author. Fax: þ60 4 6579150. E-mail addresses: [email protected] (K.M.A. Saron), roslan@ usm.my (M.R. Hashim). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.01.024
[5]. Most of the reported GaN nanostructures are grown by metal organic chemical vapor deposition or molecular beam epitaxy (MBE) [6,7]. The controlled growth of GaN nanowires with good alignment on the substrate is of particular importance for their potential applications in the fabrication of devices, such as electron field emitters and solar cells [8]. Vertically aligned GaN NWs were grown by vapor deposition methods on substrates with metal nanoparticles as catalysts [9]. The utilization of these catalysts disturbs the purity of the GaN nanostructure and, therefore, may negatively affect its performance in semiconductor applications [9]. Vajpeyi et al. [6] recently reported the catalyst-free growth of GaN NWs on n-Si (111) at various substrate temperatures by MBE. Bertness et al. [7] have successfully grown GaN nanowires on Si (111) by plasma-assisted MBE. Moreover, the growth temperature range over which we observe
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NW growth is approximately from 800 C to 850 C. There is variation from group to group on the center of the range of substrate temperatures for GaN NW growth by MBE [10e12], such that the total range of reported temperatures extends from 720 C to 900 C. However, some very interesting nanowires can be obtained by simply thermally evaporating solid materials [13]. This technique can be carried out in a relatively simple setup composed of a dual-zone vacuum furnace. A previous study has reported successful growth over a temperature range from 1000 C to 1100 C with thermal evaporation of GaN in an atmosphere of NH3. Stach et al. [14] successfully grew GaN NWs on sapphire substrate via a self-catalytic vaporeliquidesolid (VLS) mechanism at high vacuum. However, the growth of GaN NW flowers on Si by thermal evaporation technique in the absence of NH3 gas has not yet been demonstrated in literature. Herein, we present a simple and low cost physical vapor deposition (PVD) method to grow GaN NW flowers directly on catalysis-free Si (111) via thermal evaporation of GaN powder in NH3-free environments. We also studied the effect of substrate temperature on the surface morphology, density, growth rate, and optical properties of GaN NW flowers spontaneously grown on n-type Si (111). 2. Experimental NW flowers were grown by thermal evaporation of GaN powder under controlled substrate temperatures ranging from 1000 C to 1100 C. GaN powder was used as source material and an n-type Si (111) wafer was used as the substrate to collect the products, as shown in Fig. 1. The furnace temperature was then raised to 1150 C in a rate of 15 C min1. The nitrogen gas was initially introduced into the tube with a flow rate of 4 L min1. GaN (99%99) powder (0.3 g) was positioned at the highest temperature center zone of a horizontal tube furnace once the temperature reached 500 C. The Si (111) substrate cleaned in advance was placed on the quartz holder and then aligned from the high- to the low-temperature zone along the downstream flow through the tube. The distance between material source and substrate was kept 250 mm. The furnace was then heated by a rate of 15 C min1 and maintained at a temperature of 1150 C for 1 h with a gas flow rate of 3 L min1. During evaporation, the products were deposited onto a catalystfree Si (111) substrate that was placed downstream within the tube. The evaporation process was conducted at 1150 C for 1 h. The condensation products were deposited onto an Si substrate placed in a temperature zone of 1000, 1050, and 1100 C under atmospheric furnace pressure. After deposition time, the samples were cooled down to room temperature with N2 gas flow. The morphology, structures, and optical properties of the products were characterized by scanning electron microscopy (SEM), energydispersive X-ray spectrometer (EDX), and X-ray diffraction (XRD) A). Room temperature photoluminescence with CuKa (l ¼ 1.5406 (PL) was excited using an HeeCd laser (325 nm). 3. Result and discussion The SEM images of the GaN NWs grown on Si (111) at various substrate temperatures, as shown in Fig. 2, indicated that numerous
Fig. 1. Schematic diagram of experimental setup in horizontal three-zone quartz tube (90 mm in outer diameter and heated length 750 mm).
Fig. 2. Low magnification SEM image of the GaN nanowires flower grown on n-Si (111) at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C.
flowers with lengths of up to hundreds of micrometers were uniformly distributed on the Si (111) surface. SEM images revealed that the flower density decreased and the average diameter of the NW flowers increased when the substrate temperature increased from 1000 C to 1100 C. The low-magnification SEM image of the GaN NWs (Fig. 2) shows that a high density of GaN flowers was uniformly deposited on the substrate and that these flowers combined as a bundle of NWs. A single flower is vertically aligned. The typical sizes and lengths of these flowers are 10 mme15 mm and 200 mme400 mm, respectively. The sample grown at 1000 C (Fig. 2(a)) showed a high density of flowers with an average size of 10 mm and a length of up to 100 mm were deposited on the substrate surface. The diameter and length of the flowers increased when the substrate temperature was increased to 1050 C, as shown in Fig. 2(b). The flowers revealed a clear symmetry at 1100 C and merged with one another to form large buds. This phenomenon indicates that a higher growth temperature results in a higher growth rate as well as a high uniformity of NWs with larger
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diameters [14]. A similar flower-like arrangement observed in the nanostructure was also reported in other studies [15,16], in which the growth of ZnO in the solution made enabled the NWs to bundle together easily to form a flower-like structure. We conducted a detailed study of the structural composition to confirm whether the flowers are composed of GaN. The chemical composition of the flowers was derived from the quantitative analysis by using EDX. Fig. 2, beside the SEM, shows that the NW flowers mainly consisted of Ga, N, Si, O, and C impurities. The EDX analysis of the grown NW flowers demonstrated that the Ga:N ratios were 35.7:31.8, 29.21:23.4, and 36.3:27.12 for the 1000, 1050 and 1100 C growth temperatures, respectively. EDX analysis suggests continuous N loss because of increasing temperature [13]. The O contamination changed slightly from 9% to 13.6% when the substrate temperature was increased from 1000 C to 1100 C. Note that the increase in the Ga:N ratio when the substrate temperature was increased may indicate the formation of N vacancies the samples, i.e. increasing the chance of defects in the NW flowers. The
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oxygen contaminant in GaN NWs could have originated from the atmosphere or from the residual oxygen in the reaction tube. In addition, carbon contamination resulting from the atmosphere or from the growth environment could also be occasionally detected from GaN NW flowers although at low contents. Fig. 3 shows the high-magnification SEM images of GaN NW in the form of microflowers grown at substrate temperatures (1000 Ce1100 C). The flower is three-faced, and each face consists of a high density of NWs. For the GaN NW-flowers grown at 1000 C, Fig. 3(a) shows the transformation process of microflowers into the high density of NWs. The average diameter and length of each wire of the flower-shaped structures were approximately 250 nm and several micrometers long, respectively. The sample grown at 1050 C showed that the flower contains a high density of NWs in one array (Fig. 3(b and d)). The obtained NWs exhibited smooth surface morphology with average diameters and lengths of 300 nme350 nm and several micrometers long, respectively. Thus, large-scale GaN NWs evidently grew from the flowers. Increasing
Fig. 3. High magnification SEM image of GaN nanowires flower grown on Si (111) at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C, (d) represents a single flower structure with a scale bar of 1 mm, and (e) GaN nanoparticles deposition on the silicon surface with a scale bar of 1 mm.
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temperature to 1100 C significantly increased the diameter and length of the NW flowers (Fig. 3(c)). The image obviously reveals thermally grown flower-shaped structures containing a bundle of NWs. The typical diameter of one wire in a flower is approximately 500 nm, and the length is always several tens of micrometers. 3.1. Effects of temperature Figs. 2 and 3 show the effect of growth temperature on NW flower morphology at different substrate temperatures (1000, 1050 and 1100 C). The SEM images show that NW flowers were formed in all samples. Fig. 3(a and e) displays the starting growth of GaN flowers on an Si substrate via a catalyst-free growth mechanism. The NWs formed a flowers-like structure, which showed an increase in diameter and length (Fig. 3(b and c)). Different sizes of GaN nanoparticles were also observed from the edge portion of the sample (1000 C), which suggests that these nanoparticles were not subjected to supersaturation because of the increased nucleation delay (i.e. insufficient time to grow) (Fig. 3(e)). It is believed that this is the first phase in which nucleation occurred during the growth of the NW flowers. The formation of GaN NW flowers through mechanism can be divided into two steps, namely, nucleation and growth, according to the obtained results [17e19]. The formation of NW flowers basically resulted from the deposition of nanoparticles on the surfaces of the substrates at different temperatures. The growth rate of the flowers increased with respect to temperature when the rest of the conditions were kept constant. The growth of GaN NWs was initiated by supplying GaN vapor at 1150 C, and was controlled by increasing the substrate temperature from 1000 C to 1100 C (growth area). GaN vapor was deposited on the substrate in the form of nanoparticles within a short time. The particles began to grow to form a flower-like structure through the vaporesolid (VS) mechanism and upon heating the substrate [17]. The fast growth rate was dependent on the increasing growth temperature and was achieved when the time was held constant (1 h). NW density was strongly dependent on the growth temperatures [6,7]. The substrate temperature had a stronger effect on NW density with respect to time. For 1 h growth, the NW density increases with increasing substrate temperature (1000 Ce1100 C). Therefore, the substrate temperature provides a simple method to control the NW density and growth rate. The conventional PVD growth mechanism remains suitable to account for the unique growth phenomena reported in this study. The growth mechanism can be explained in as follow: GaN decomposes at temperatures above 850 C via the possible reactions given in Eq. (1) [14,20].
GaNðsÞ ¼ Ga ðlÞ þ 0:5N ðgÞ þ 0:25N2 ðgÞ
(1)
Through the thermal evaporation of GaN powder in a furnace tube at 1150 C, the occurrence of this decomposition can be observed to cause nanostructure growth in real time and at high spatial resolution [21]. We propose that during this process, Ga droplets become self-catalytic growth elements of GaN NW flowers [17]. The decomposition of the GaN nanoparticles generates N atoms and isolated nanoscale Ga droplets [21]. The evaporation of GaN powder at 1150 C also possibly produces diatomic or polymeric GaN vapor species [14]. The Ga vapor is condensed into liquid Ga droplets on the Si substrate surface [22]. The Ga droplets may react with the GaN vapor to establish a vapor-Ga/solid-GaN interface [20,23]. Given that Ga droplets were formed during the reaction, the NW flowers grow because of supersaturated Ga droplets with GaN vapor [10]. Ga metal was not observed on the tip of the NWs because of the consumption and desorption of Ga (high growth temperature) [24]. Therefore, Ga droplets are expected to
be converted into GaN nanostructures because of a high growth temperature which forbids accumulation of Ga droplets on GaN [23]. This result indicates that the GaN NW flowers grow on Si substrate via the VS mechanism. Stach et al. [14] have grown GaN NWs on sapphire via the self-catalytic VLS mechanism, in which the catalysts are nanoscale Ga liquid droplets generated by the thermal decomposition of GaN. However, they did not exclude the possibility of GaN NW growth via the VS mechanism. According to the VS mechanism, GaN vapor is condensed, and then nucleated on the Si substrate. The GaN nanoparticles participate in nucleation for the subsequent growth of GaN NW flowers. The continuous supply of GaN vapor through the heating of GaN at (1150 C) with a constant N flow rate facilitates the further growth of the GaN nucleation to form the NW flower through the VS reaction [25]. As the temperature increases, the rate of recomposition of GaN nanoparticles is significantly increased. Consequently, the reaction of the precursors Ga and N on the Si substrate is increased, thus, resulting in an increase in particle size and the facilitation of growth of NW flowers GaN structures. By increasing substrate temperatures, the growth rate is enhanced significantly because of a high supersaturation of GaN nuclei, which results in an increase in the partial pressure of the vapors component [23e25]. In other words, the growth morphology of GaN nanostructures is strongly dependent on the furnace temperature because the density of the vapor component that generated by the thermal decomposition varies with different furnace temperatures. Therefore, a high source temperature (1000 Ce1100 C) increases the partial pressure of the vapors’ component, thus yielding a high supersaturation of GaN nuclei which favors the growth of GaN NW flowers with large diameters. By contrast, when the substrate temperature is lower (1000 C) and time is insufficient, the growth of the NW flowers exhibited a nanoparticle morphology effect because of the low supersaturation of GaN nuclei, as shown in Fig. 3(e). At high temperatures, GaN nucleation is free from catalysts, and growth is expected to occur via the VS route. We conclude that a model can be proposed for the growth of novel GaN NW flowers through two possible processes. First, at high temperatures (1150 C), GaN vapor is deposited on substrates surfaces to form the nucleus of the GaN flower. As the deposition concentration increases, GaN nuclei individually grow in the form of NW flowers [23e25]. Second, thermal decomposition of GaN occurs, producing Ga vapor that condenses into Ga droplets on the Si surface [21]. The Ga-vapor reacts with N (i.e. recomposition) to establish the GaN nucleus and grow [17,19]. The newly formed NWs begin to detach the flowers from the surface as the thermal growth of GaN NW flowers continues, thus pushing the particle upward. Based on the average length of the flowers, the growth rate was estimated to be more than 500 nm min1, faster than those (w300 nm min1) of NWs grown by using another technique [25,26]. This result suggests that the substrate temperature has strong effect on the formation and growth rate of NW-flowers. Fig. 4 shows the XRD patterns of GaN NW flowers grown on Si (111) in the absence of NH3 gas with varying substrate temperatures 1000 Ce1100 C at a constant growth time of 1 h. A complete conversion of GaN powder into GaN NW flowers has been observed for the growths conducted at 1000, 1050, and 1100 C. The peaks in the XRD patterns can be indexed to a wurtzite hexagonal structure of GaN. The reflection peaks of (100), (002), and (101), correspond to the hexagonal wurtzite GaN phase with lattice parameters of approximately a ¼ 3.186 A, c ¼ 5.178 A, consistent with the reported values for bulk GaN [27]. The reflection peak of (111) corresponds to the cubic Si. An additional peak with a weak intensity of b-Ga2O3 could be observed at 2q ¼ 44.81, corresponding to reflection plane (112). The intensity of the (002) peak of GaN NW flowers gradually decreased with increasing substrate temperature. This result
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Fig. 4. The X-ray diffraction spectrum (XRD) patterns of GaN nanowires grown at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C (*) and (^) are the GaN and impurity phase of b-Ga2O3, respectively.
indicates that the worst crystal quality is obtained at the highest growth temperature. The reduction of intensity peaks is attributed to the decrease in the conversion of Ga to GaN. Some N could have been lost between the vapor source and the growth area during GaN growth (NH3-free) [13,28]. The XRD measurement exhibited variations (002) in the diffraction peaks with increasing substrate temperature, which indicates a decreased, lattice constant (c) of GaN NW flowers compared with that of stress-free GaN (co ¼ 5.1855) [27]. The (002) diffraction peaks are shifted to higher 2q values (34.85 , 34.67, and 34.75 ) compared with the (002) peak positions of bulk GaN (34.5 ), implying that all the GaN NW flowers are under tensile stress along the c-axis [29]. With increase in temperature, the intensity peak became lower and less asymmetric. However, tensile stress (i.e. intrinsic stress) is typically introduced by impurities, defects, and lattice distortions in the crystal [30]. With an increase in substrate temperature, more defects are formed during the growth process, which might result in a crystal defect that facilitates the reduction of the lattice constant and increases the tensile stress. The thermal expansion, lattice mismatch, and growth defect thus account for most of the stress present in the GaN produced in this study. The NW flowers grown at 1000 C (Fig. 4(a)) revealed a high-intensity peak with a larger shift in the (002) diffraction peak, while the intensity peak of the b-Ga2O3 phase reduced. As shown in Fig. 3(b), the intensity of XRD peaks of this sample decreased as the growth temperature increased to 1050 C, whereas the intensity peak of the b-Ga2O3 phase increased slightly, which resulted from oxygen diffusion [28]. The XRD peaks of GaN NW flowers were significantly lower when the temperature increased to 1100 C, but the traces of b-Ga2O3 were found to increase in the reaction product, as shown in Fig. 4(c). The NW flowers grown at 1050 C and 1100 C showed significantly lower XRD intensity peaks than the NW flowers grown at 1000 C, which can be attributed to the greater number of defects formed during high-temperature growth. The increase in growth temperature results in the degradation of the crystalline quality of GaN NW flowers [31]. The formation of b-Ga2O3 might be attributed to the presence of oxygen traces in the reaction tube furnace. The kinetic barriers of the N atoms might be reduced at high growth temperature, and the diffusion length of O in Ga may be increased, thus resulting in a sufficient reaction of O atoms with Ga to produce the Ga2O3 phase [27]. These XRD measurement results indicate that high growth
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temperature can increase the number of crystal defects and degrade the crystalline quality of NW flowers. The room temperature PL spectra of the GaN NW flower-like structure were measured using an HeeCd 325 nm wavelength laser as the excitation source (Fig. 5). The spectra of the NW flower grown at 1000 C (Fig. 5(a)), displayed two band emissions, namely, a strong UV emission at approximately 369 nm (3.36 eV) and a broad deep-level emission (DLE) in the green-yellow emission (GYE) at 536 nm (2.25 eV). The PL spectra of NW flower grown at 1050 C exhibited a strong UV near band edge emission (NBE) at approximately 387 nm (3.32 eV) and a broad GYE centered at 540 nm (Fig. 5(b)). The NW flower grown at 1100 C (Fig. 5(c)) shows a strong UV NBE at approximately 393 nm (3.31 eV) and a broad GYE at 537 nm. Generally, the strong UV emission ranging from 369 nm to 394 nm is the NBE emission resulting from the recombination of free excitons [32]. The GYE centered at approximately 540 nm is attributed to the singly ionized nitrogen vacancy and to the emission resulting from the radiative recombination of a photo generated hole with an electron occupying the nitrogen vacancy [33]. The GaN NW flowers grown at 1000, 1050 and 1100 C showed UV emission peaks that were red shifted to lower energy. The redshift of UV emission peaks increases slightly as growth temperature increases, because of the narrowing of the energy band gap, which probably resulted from an increase in crystal intrinsic defects (tensile stress) [30]. Thus, the redshift of the UV emission of different GaN nanostructures from 369 nm to 394 nm indicated different contents of N vacancies in each NW flower, which resulted from different growth conditions. This phenomenon might also be attributed to a kind of lattice defect in the crystal, depending on the content of the reactive Ga vapor through the particle formation process. Therefore, the redshift of PL peak position could be correlated with the band gap change resulting from the in-plane tensile stress of GaN NWs along the axial direction [34]. As the temperature increases the PL is dominated mainly by the UV emission with redshift. We note that as temperature increases the wire diameter were increased, thus, quantum confinement effect is less (i.e. no blueshift), therefore, the UV peak shifted to red [35]. However, the NBE peaks are more intense than the GYE peaks for all measured samples, thus, indicating the optical quality of the NW flowers. The DL emission in PL can be attributed to several factors such as; incorporation of impurity-induced disorder, strain-induced structural defects, and surface defects during
Fig. 5. PL spectra of the NWs obtained at different substrate temperatures (a) 1000 C, (b) 1050 C and (c) 1100 C.
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the growth process [36]. The sample grown at 1000 C showed the highest GYE intensity, which can be associated with the presence concentrated N vacancies (VN) in thinner NWs. These vacancies form the defect levels in the forbidden band gap and give rise to this GYE [36]. The sample grown at 1050 C had a weakest GYE, whereas, for growth temperature at 1100 C, the UV peak shifts to lower energy, and its GYE also reduced. The higher concentration of surface states in thinner wires can also be explained by the larger surface-to-volume ratio in thinner wires compared with their thicker counterparts. Therefore, N vacancy-related complexes or extended defects can be correlated with an increase in the intensity of the GYE at approximately 540 nm, the degradation of the crystal quality, and an increase in the concentration of electrons [36]. This broad DL emission is associated with vacancy-related structural defects, including N, Ga, and VGaeON complexes, which are energetically located within the band gap [36]. The presented GYE might be attributed to the defect related to the O impurity [37]. The correlation between the intensity of the YE band in GaN and the concentration of O has been reported by Reshchikov et al. [36]. Our study yields insights into the behavior and nature of UV NBE emission and defect-related visible emission in GaN NW flowers. The results of our experiment indicated that substrate temperatures strongly affect the optical properties of GaN NW flowers. We thus suggest a simple method for producing electronically pure polycrystalline semiconductor NW flowers. 4. Conclusions The growth of GaN flower-like NW arrays on catalyst-free Si substrates has been demonstrated at 1000, 1050, and 1100 C in the absence of NH3. At increased substrate temperatures, the diameters of both NWs and flowers increase which indicates increased growth rate. At higher substrate temperatures, growth rate accelerates. A lower substrate temperature resulted in good crystal quality of GaN (NWs). Increasing substrate temperature resulted in further oxygen contamination and in the formation of the b-Ga2O3 phase. Oxygen traces might be responsible for the increase in growth rate at higher temperatures in the absence of NH3. The PL measurements showed strong NBE emission with a weak GYE, which confirms the high optical quality of the fabricated GaN NW flowers. The NBE consequently shifted to the narrower energy, because of tensile stress. These results suggest that GaN powder is a promising self-catalyst for growing GaN NW flowers via thermal evaporation with controlled substrate temperature.
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