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Optical characteristics of multiple layered InGaN quantum wells on GaN nanowalls grown on Si (111) substrate
Materials Research Bulletin 83 (2016) 563–567
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Optical characteristics of multiple layered InGaN quantum wells on GaN nanowalls grown on Si (111) substrate Yosuke Tamura* , Kazuhiro Hane Department of Nanomechanics, Graduate School of Engineering Tohoku University, 6-6-01, Aramaki Aza Aoba, Aoba-ku, Sendai, Japan
A R T I C L E I N F O
Article history: Received 3 March 2016 Received in revised form 22 May 2016 Accepted 27 June 2016 Available online xxx Keyword: Nanostructures Nitrides Semiconductors Epitaxial growth Electron microscopy X-ray diffraction
A B S T R A C T
GaN nanowall crystals and InGaN multiple quantum wells (MQWs) were grown on Si (111) substrate using molecular beam epitaxy. In high N/Ga flux ratio, dense two dimensional network GaN nanowall structures were grown with small pores. The continuity of GaN nanowall structures was decreased as decreasing the N/Ga flux ratio. In the N/Ga flux ratio of 50, nanopillars and nanowalls were mixed. The InGaN MQWs were formed on the GaN nanowall crystals which were grown in the various N/Ga flux ratios. The strong photoluminescence originated from the InGaN MQWs on GaN nanowall crystals around a wavelength of 400 nm was observed at the temperatures from 8 K to 300 K (room temperature). The ratios of the photoluminescence intensities at 8 K and the room temperature were measured for the InGaN MQWs. The higher ratio was obtained as decreasing the N/Ga flux ratio of the GaN nanowall crystal growth. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction GaN crystals have been extensively studied for optoelectronic devices such as light emitting diodes and laser diodes, which are widely commercialized in market. The GaN crystals are often grown by metalorganic vapor phase epitaxy for the commercialized applications. The flat and uniform crystals are grown on heterogeneous substrates such as sapphire and silicon with buffer layers to compensate the crystal constant mismatches. On the other hand, the nanostructures of GaN crystals are also attracting high interests due to the unique characteristics in electronic and optoelectronic properties. The nanopillars of GaN crystal have been grown by molecular beam epitaxy (MBE) and the characteristics are investigated in detail. Due to the narrow diameter of the nanopillar, the internal stress is relaxed and the excellent crystal qualities without defects are obtained [1–4]. Growing multiple quantum wells (MQWs) on the pillared GaN crystals, high efficient emissions are obtained not only in blue wavelength region, but also in yellow and red regions [5]. In the nanostructures of GaN crystals, the GaN nanowall structures are specially interesting in practical device fabrication and the characteristics are still under investigation [6–10]. The nanowalls are connected to form a honeycomb mesh structures, which are advantageous for electrical wiring
* Corresponding author. E-mail address: [email protected] (Y. Tamura). http://dx.doi.org/10.1016/j.materresbull.2016.06.042 0025-5408/ã 2016 Elsevier Ltd. All rights reserved.
since the respective nanowalls are connected for a current to flow to all the nanowalls unlike isolated nanopillars. The GaN nanowall structures were studied for the schottky diode hydrogen gas sensors by using the large surface-volume ratio and the electrical connection to a large number of nanowalls [10–12]. The light emission of InGaN MQWs formed on the nanowall was initially measured [8]. However, the detailed characteristics of InGaN MQWs on GaN nanowall crystals have not been investigated in a wide range of GaN nanowall growth conditions. Similar to the nanopillars, it is expected that InGaN MQWs on nanowalls have good properties. In this work, GaN nanowall crystals are directly grown on Si substrate by plasma assisted MBE varying the ratio of N and Ga fluxes. On the grown GaN nanowall crystals, InGaN MQWs are grown subsequently. The emission efficiency, crystal quality and surface morphology of the InGaN MQWs on the GaN nanowall crystals are investigated. 2. Experimental The MBE system (RIBER 32; RIBER) with the radio frequency plasma source (RFS-N/TH; Veeco Instruments) was used. The solid metal aluminium, gallium and indium sources with the purity of 99.9999% were used as the sources of III-elements and the nitrogen gas with the purity of 99.99995% was supplied. The samples were grown on a Si (111) substrate having the resistivity of less than 0.02 V m and a thickness of 380 mm.
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At first, the Si substrate was degreased with ethanol and cleaned by the standard RCA method. The Si substrate was soaked in diluted hydrogen fluoride solution (HF: H2O = 1: 100) to remove the native oxide of Si substrate surface and the Si substrate surface was terminated by hydrogen atoms. The Si substrate was introduced in a loading chamber, immediately, and pre-heated in the loading chamber around 105 Pa to remove the hydrocarbons on Si substrate surface [12]. Finally, the substrate was transferred to the growth chamber and GaN and InGaN MQWs crystals were grown on the Si substrate. The grown GaN nanowall crystals consisted of GaN nanowall layer, AlN buffer layer and Al monolayer on Si substrate. The fiveperiods of InGaN MQWs were grown on the GaN nanowall crystal. At first, several monolayer of Al was deposited on a Si substrate at the temperatures of the substrate and the Al cell of 700 and 1160 C, respectively. The N2 plasma was then generated and several tens nanometer of AlN buffer layer was grown at the substrate temperature of 700 C and the Al cell temperature of 1160 C with a nitrogen flux of 4.2 105 Torr. Next, the GaN nanowall crystals were grown at the substrate temperature of 700 C in the different N/Ga flux ratios. The detail growth conditions of GaN nanowall crystals are described in the following paragraph. In the case of InGaN MQWs samples, five periods of InGaN MQWs with GaN barriers were grown at the substrate temperature of 600 C with nitrogen flux of 4.2 105 Torr and the temperatures of Ga and In cells of 895 and 840 C, respectively. The thickness of the InGaN QWs was designed to be 3 nm and the GaN barrier was 7 nm. During the growth of AlN, GaN and InGaM MQWs layers, the plasma source power was kept constant at 400 W. For the growth of the GaN nanowall crystals without InGaN MQWs, the N/Ga flux ratios of 350, 200, 120 and 50 were used. In the case of the GaN nanowall crystals with InGaN MQWs, the GaN nanowall crystals were grown in the N/Ga flux ratios of 50, 25 and 10. The growth conditions of GaN and InGaN MQWs samples were summarized in Tables 1 and 2. The N/Ga flux ratio was determined by the ratio of nitrogen and gallium fluxes, which were measured by beam equivalent pressures. The Nitrogen pressure was monitored during growth and the gallium pressure was measured as a function of gallium cell temperature, beforehand. The GaN nanowall samples and InGaN MQWs samples were measured by a scanning electron microscope (SEM) (SU-70, Hitachi), an X-ray diffraction (XRD) system (D8 DISCOVER, BRUKER) and a photoluminescence (PL) measurement system to evaluate surface morphology, crystal structure and quality and optical characteristics. In PL measurement, the samples were excited by a continuous He-Cd laser (325 nm). 3. Results and discussion 3.1. GaN nanowall crystal growth The SEM images of GaN nanowall crystal surfaces in the N/Ga flux ratios of 350, 200, 120 and 50 are shown in Fig. 1. The GaN nanowall structures were observed in the N/Ga flux ratios of 350 (Fig. 1(a)), 200 (Fig. 1(b)) and 120 (Fig. 1(c)). In the case of the large N/Ga flux ratio of 350 (Fig. 1(a)), the continuous GaN nanowall structure with small pores are seen. When the N/Ga flux ratio Table 1 Growth conditions of GaN nanowall crystals. GaN nanowall without InGaN MQWs N/Ga flux ratio Ga Cell temperature ( C) N2 pressure (Torr)
350 910 4.2 105
200 940 4.2 105
120 940 2.6 105
50 940 1.1 105
Table 2 Growth conditions of InGaN MQWs on GaN nanowall crystals. GaN nanowall with InGaN MQWs GaN nanowall
InGaN MQWs
N/Ga flux ratio Cell temperature ( C)
50 Ga/940
25
10
N2 pressure (Torr)
1.1 105
5.5 105
2.2 105
– Ga/895 In/840 4.2 105
decreases to 200 and 120 (Figs. 1(b) and (c)), the continuity of GaN nanowalls is decreased and the size of pore is increased. Further decreasing N/Ga flux ratio to 50, the GaN nanopillars are seen as shown in Fig. 1(d). Fig. 2 shows the results of PL measurement of the GaN nanowall samples at the room temperature. In the case of the N/Ga flux ratio of 350 (Fig. 2(a)), the broaden emission which has a peak position at 407 nm is observed. In general, the GaN near band edge emission is usually centered at 364 nm. According to the previous reports [13], the emission related to the Ga-vacancy is centered at the longer wavelength comparing with the GaN near band edge emission. As decreasing the N/Ga flux ratio to 200 (Fig. 2(b)) and 120 (Fig. 2(c)), the emission originated from the GaN near band edge becomes strong, while the emission originated from the GaN defects becomes weak. Furthermore, decreasing the N/Ga flux ratio to 50, the sharp emission originated from the GaN near band edge is dominant (Fig. 2(d)). The full width at half maximum (FWHM) is 9.1 nm. From these results, the GaN nanowall crystals which are grown in the low N/Ga flux ratio show the better crystal quality comparing with the GaN nanowall crystal grown in the high N/Ga flux ratio. Zhong et al. reported that GaN nanowall becomes dense and the emission originated from the GaN defects becomes larger as increasing N/Ga flux ratio [9]. Our results agree with the report. The XRD pattern was measured to investigate the crystal structure of the GaN nanowall crystal. The results of 2u-v scan are shown in Fig. 3. The peaks around 35 and 73 are clearly observed. These peaks are corresponding to the diffractions originated from GaN (0002) and GaN (0004) crystal planes, respectively. Therefore, the GaN nanowall structure consists of hexagonal GaN crystal along c-plane, which agrees with the previous reports [6,9]. In Addition, the XRD rocking curves were measured for all the samples and the spectra are shown in Fig. 4(a). The FWHM obtained from Fig. 4(a) are shows in Fig. 4(b) as a function of N/Ga flux ratio. In the case of the N/Ga flux ratios from 350 to 120, the FWHMs are around 300000 . In the case of the N/Ga flux ratio of 50, in which the GaN nanopillars and nanowalls are mixed, the FWHM is broadened to be 520000 . In general, broaden FWHM means the deterioration of crystal quality. For the sample grown in the N/Ga flux ratio of 50, the FWHM is broaden while the PL spectrum shows good crystal quality (Fig. 2(d)). We think the reason of this broaden FWHM value is not originated from poor crystal quality. The two reasons can be considered. One reason is that the nanopillars have the slight different crystal orientations even if each nanopillar has good crystal orientation. From the SEM image as shown in Fig. 1(d), the GaN nanopillars seem to have the slightly different angles to the substrate, respectively. As the result, the FWHM becomes broader. The other reason is due to the fact that the nanopillars and the nanowalls are mixed, as seen from the SEM image shown in Fig. 1(d). It is assumed the both nanopillar and nanowall may show the slight different crystal orientations to the substrate, which are superposed to generate a broaden FWHM even if both the nanopillars and nanowalls have good crystal qualities. And thus, the FWHM becomes broader comparing with the GaN nanowall crystals grown in the high N/Ga flux ratios of 350, 200 and 120.
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Fig. 1. Surface morphologies of GaN nanowall structures in the different N/Ga flux ratios, (a) 350, (b) 200, (c) 120 and (d) 50.
3.2. Optical characteristics of InGaN MQWs In previous section, the GaN nanowall crystal which is grown in the low N/Ga flux ratio shows the better crystal quality comparing with that grown in high N/Ga flux ratio. Therefore, in this experiment, the GaN nanowall crystals were grown in the low N/ Ga flux ratios of 50, 25 and 10. Subsequently, InGaN MQWs were grown on the GaN nanowall crystals. The PL characteristics of the InGaN MQWs were examined. Fig. 5 shows the top view SEM images of the InGaN MQWs grown on GaN nanowall crystals. In the N/Ga flux ratio of 50 as shown in Fig. 5(a), the surface morphology is the nanowall structure with the large pores. Decreasing N/Ga flux ratio to 25 as shown in Fig. 5(b), the nanowalls are seen with the very narrow gaps between nanowalls. Further decreasing N/Ga flux ratio to 10, the surface morphology becomes a film structure. The PL spectra of InGaN MQWs were measured by varying the temperature. The strong emission originated from the InGaN MQWs and the emission originated from the GaN near band edge were observed in all samples. The spectral width from the GaN
Fig. 2. PL spectra of GaN nanowall structures in the N/Ga flux ratios of (a) 350, (b) 200, (c) 120 and (d) 50.
near band edge in the N/Ga flux ratios of 50, 25 and 10 were less than 10 nm and as narrow as that shown in Fig. 2(d) although the emissions from MQWs were overlapped. As an example, the PL spectra of the InGaN MQWs grown in the N/Ga flux ratio of 50 are shown in Fig. 6(a). The peaks originated from the InGaN MQWs are clearly seen around the wavelength of 400 nm. The PL peak intensities of the InGaN MQWs were plotted as a function of temperature as shown in Figs. 6(b) in the N/Ga flux ratios of 50, 25 and 10, respectively. The peak intensities of the InGaN MQWs are gradually increased as decreasing the temperature. The PL intensity ratio at the room temperature and 8 K is defined as IR here. The values of IR are plotted in Fig. 7 as a function of the N/Ga flux ratio of the GaN nanowall crystal. The values of IR are 0.16, 0.58 and 1.32% in the N/Ga flux ratios of 50, 25 and 10, respectively. It is clearly seen that the value of IR is decreased as increasing N/Ga flux ratio. It is considered that the quality of InGaN MQWs for the optical emission is improved as decreasing N/Ga flux ratio. In general, the crystal quality of grown material depends on the
Fig. 3. XRD pattern of 2u-v scan for the GaN nanowall grown in the N/Ga flux ratio of (a) 350, (b) 200, (c) 120, (d) 50.
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Fig. 4. (a) XRD rocking curve and (b) the value of FWHM as a function of N/Ga flux ratio.
Fig. 5. surface morphologies of InGaN MQWs crystals on GaN nanowalls in different N/Ga flux ratios, (a) 50, (b) 25, and (c) 10.
Fig. 6. (a) PL spectra of InGaN MQWs grown in the N/Ga flux ratio of 50, (b) PL peak intensity dependence on temperature in the N/Ga flux ratios.
quality of under-layer. As mentioned before, the GaN crystal quality was improved as decreasing N/Ga flux ratio. Therefore, in this experiment, the higher value of IR is obtained as decreasing N/ Ga flux ratio. 4. Conclusion The GaN nanowall crystals and the InGaN MQWs grown on the GaN nanowall crystals were directly deposited on Si (111) substrate using MBE and the characteristics were investigated. The GaN nanowall crystals were grown in the various N/Ga flux ratios. In the
high N/Ga flux ratio of 350, the continuous and dense GaN nanowall network structure was grown. Decreasing N/Ga flux ratio, the continuity of GaN nanowall was decreased. In the low N/ Ga flux ratio of 50, the GaN nanopillars and the nanowalls were mixed. The GaN near band edge emission became dominant as decreasing N/Ga flux ratio. On the other hand, the FWHM was larger in the low N/Ga flux ratios, which was considered from the fact that the nanopillars were slightly tilted and mixed with nanowalls. The InGaN MQWs were formed on the GaN nanowall crystals which were grown in the various N/Ga flux ratios. The strong PL
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References
Fig. 7. PL intensity ratio (IR) at the room temperature and 8 K as a function of N/Ga flux ratio.
intensities were obtained around the wavelength of 400 nm. The PL intensity ratio at room temperature and 8 K increased as deceasing N/Ga flux ratio. The quality of the InGaN MQWs was considered to be somewhat degraded with the increase of N/Ga flux ratio since the GaN nanowall network crystals seem to grow at an expense of the crystal quality. And thus, it is important to compromise the nanowall formation and the emission efficiency of MQWs. Acknowledgement The authors would like to thank Y. Kanamori and T. Sasaki for discussion. Y. Tamura appreciates the Research Fellow of Japan Society for the Promotion of Science for the financial support.
[1] M. Niebelschütz, V. Cimalla, O. Ambacher, T. Machleidt, J. Ristic, E. Calleja, Electrical performance of gallium nitride nanocolumns, Phys. E 37 (2007) 200. [2] H. Sekiguchi, T. Nakazato, A. Kikuchi, K. Kishino, Structural and optical properties of GaN nanocolumns grown on (0001) sapphire substrates by rfplasma-assisted molecular-beam epitaxy, J. Cryst. Growth 300 (2007) 259. [3] L. Cerutti, J.S. Fernandez-Garrido, E. Calleja, A. Trampert, K.H. Ploog, S. Lazic, J. M. Calleja, Wurtzite GaN nanocolumns grown on Si (111) by molecular beam epitaxy, Appl. Phys. Lett. 88 (2006) 213114. [4] L.W. Tu, C.L. Hsiao, T.W. Chi, I. Lo, K.Y. Hsieh, Self-assembled vertical GaN nanorods grown by molecular-beam epitaxy, Appl. Phys. Lett. 82 (1601) 2003. [5] A. Kikuchi, M. Kawai, M. Tada, K. Kishino, InGaN/GaN multiple quantum disk nanocolumn light-emitting diodes grown on (111) Si substrate, Japn. J. Appl. Phys. 43 (2004) L1524. [6] M. Kesaria, S.M. Shivaprasad, Nitrogen flux induced GaN nanostructure nucleation at misfit dislocations on Al2O3 (0001), Appl. Phys. Lett. 99 (2011) 143105. [7] M. Kesaria, S. Shetty, S.M. Shivaprasad, Evidence for dislocation induced spontaneous formation of GaN nanowalls and nanocolumns on bare C-plane sapphire, Cryst. Growth Des. 11 (2011) 4900. [8] A. Zhong, K. Hane, Characterization of GaN nanowall network and optical property of InGaN/GaN quantum wells by molecular beam epitaxy, Jpn. J. Appl. Phys. 3 (2013). [9] A. Zhong, K. Hane, Growth of GaN nanowall network on Si (111) substrate by molecular beam epitaxy, Nanoscale Res. Lett. 7 (2012) 686. [10] A. Zhong, T. Sasaki, K. Hane, Platinum/porous GaN nanonetwork metalsemiconductor Schottkydiode for room temperature hydrogen sensor, Sens. Actuators A 209 (2014) 52. [11] A. Zhong, T. Sasaki, K. Hane, Comparative study of Schottky diode type hydrogen sensors based on a honeycomb GaN nanonetwork and on a planar GaN film, Int. J. Hydrogen Energy 36 (2014) 8564. [12] D.J. Eaglesham, M. Cerullo, Dislocation-free stranski-Krastanow growth of Ge on Si, Phys. Revi. Lett. 64 (1943) 1990. [13] M.A. Reshchikov, H. Morkoç, Luminescence properties of defects in GaN, J. Appl. Phys. 97 (2005) 061301.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Optical characteristics of multiple layered InGaN quantum wells on GaN nanowalls grown on Si (111) substrate Yosuke Tamura* , Kazuhiro Hane Department of Nanomechanics, Graduate School of Engineering Tohoku University, 6-6-01, Aramaki Aza Aoba, Aoba-ku, Sendai, Japan
A R T I C L E I N F O
Article history: Received 3 March 2016 Received in revised form 22 May 2016 Accepted 27 June 2016 Available online xxx Keyword: Nanostructures Nitrides Semiconductors Epitaxial growth Electron microscopy X-ray diffraction
A B S T R A C T
GaN nanowall crystals and InGaN multiple quantum wells (MQWs) were grown on Si (111) substrate using molecular beam epitaxy. In high N/Ga flux ratio, dense two dimensional network GaN nanowall structures were grown with small pores. The continuity of GaN nanowall structures was decreased as decreasing the N/Ga flux ratio. In the N/Ga flux ratio of 50, nanopillars and nanowalls were mixed. The InGaN MQWs were formed on the GaN nanowall crystals which were grown in the various N/Ga flux ratios. The strong photoluminescence originated from the InGaN MQWs on GaN nanowall crystals around a wavelength of 400 nm was observed at the temperatures from 8 K to 300 K (room temperature). The ratios of the photoluminescence intensities at 8 K and the room temperature were measured for the InGaN MQWs. The higher ratio was obtained as decreasing the N/Ga flux ratio of the GaN nanowall crystal growth. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction GaN crystals have been extensively studied for optoelectronic devices such as light emitting diodes and laser diodes, which are widely commercialized in market. The GaN crystals are often grown by metalorganic vapor phase epitaxy for the commercialized applications. The flat and uniform crystals are grown on heterogeneous substrates such as sapphire and silicon with buffer layers to compensate the crystal constant mismatches. On the other hand, the nanostructures of GaN crystals are also attracting high interests due to the unique characteristics in electronic and optoelectronic properties. The nanopillars of GaN crystal have been grown by molecular beam epitaxy (MBE) and the characteristics are investigated in detail. Due to the narrow diameter of the nanopillar, the internal stress is relaxed and the excellent crystal qualities without defects are obtained [1–4]. Growing multiple quantum wells (MQWs) on the pillared GaN crystals, high efficient emissions are obtained not only in blue wavelength region, but also in yellow and red regions [5]. In the nanostructures of GaN crystals, the GaN nanowall structures are specially interesting in practical device fabrication and the characteristics are still under investigation [6–10]. The nanowalls are connected to form a honeycomb mesh structures, which are advantageous for electrical wiring
* Corresponding author. E-mail address: [email protected] (Y. Tamura). http://dx.doi.org/10.1016/j.materresbull.2016.06.042 0025-5408/ã 2016 Elsevier Ltd. All rights reserved.
since the respective nanowalls are connected for a current to flow to all the nanowalls unlike isolated nanopillars. The GaN nanowall structures were studied for the schottky diode hydrogen gas sensors by using the large surface-volume ratio and the electrical connection to a large number of nanowalls [10–12]. The light emission of InGaN MQWs formed on the nanowall was initially measured [8]. However, the detailed characteristics of InGaN MQWs on GaN nanowall crystals have not been investigated in a wide range of GaN nanowall growth conditions. Similar to the nanopillars, it is expected that InGaN MQWs on nanowalls have good properties. In this work, GaN nanowall crystals are directly grown on Si substrate by plasma assisted MBE varying the ratio of N and Ga fluxes. On the grown GaN nanowall crystals, InGaN MQWs are grown subsequently. The emission efficiency, crystal quality and surface morphology of the InGaN MQWs on the GaN nanowall crystals are investigated. 2. Experimental The MBE system (RIBER 32; RIBER) with the radio frequency plasma source (RFS-N/TH; Veeco Instruments) was used. The solid metal aluminium, gallium and indium sources with the purity of 99.9999% were used as the sources of III-elements and the nitrogen gas with the purity of 99.99995% was supplied. The samples were grown on a Si (111) substrate having the resistivity of less than 0.02 V m and a thickness of 380 mm.
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At first, the Si substrate was degreased with ethanol and cleaned by the standard RCA method. The Si substrate was soaked in diluted hydrogen fluoride solution (HF: H2O = 1: 100) to remove the native oxide of Si substrate surface and the Si substrate surface was terminated by hydrogen atoms. The Si substrate was introduced in a loading chamber, immediately, and pre-heated in the loading chamber around 105 Pa to remove the hydrocarbons on Si substrate surface [12]. Finally, the substrate was transferred to the growth chamber and GaN and InGaN MQWs crystals were grown on the Si substrate. The grown GaN nanowall crystals consisted of GaN nanowall layer, AlN buffer layer and Al monolayer on Si substrate. The fiveperiods of InGaN MQWs were grown on the GaN nanowall crystal. At first, several monolayer of Al was deposited on a Si substrate at the temperatures of the substrate and the Al cell of 700 and 1160 C, respectively. The N2 plasma was then generated and several tens nanometer of AlN buffer layer was grown at the substrate temperature of 700 C and the Al cell temperature of 1160 C with a nitrogen flux of 4.2 105 Torr. Next, the GaN nanowall crystals were grown at the substrate temperature of 700 C in the different N/Ga flux ratios. The detail growth conditions of GaN nanowall crystals are described in the following paragraph. In the case of InGaN MQWs samples, five periods of InGaN MQWs with GaN barriers were grown at the substrate temperature of 600 C with nitrogen flux of 4.2 105 Torr and the temperatures of Ga and In cells of 895 and 840 C, respectively. The thickness of the InGaN QWs was designed to be 3 nm and the GaN barrier was 7 nm. During the growth of AlN, GaN and InGaM MQWs layers, the plasma source power was kept constant at 400 W. For the growth of the GaN nanowall crystals without InGaN MQWs, the N/Ga flux ratios of 350, 200, 120 and 50 were used. In the case of the GaN nanowall crystals with InGaN MQWs, the GaN nanowall crystals were grown in the N/Ga flux ratios of 50, 25 and 10. The growth conditions of GaN and InGaN MQWs samples were summarized in Tables 1 and 2. The N/Ga flux ratio was determined by the ratio of nitrogen and gallium fluxes, which were measured by beam equivalent pressures. The Nitrogen pressure was monitored during growth and the gallium pressure was measured as a function of gallium cell temperature, beforehand. The GaN nanowall samples and InGaN MQWs samples were measured by a scanning electron microscope (SEM) (SU-70, Hitachi), an X-ray diffraction (XRD) system (D8 DISCOVER, BRUKER) and a photoluminescence (PL) measurement system to evaluate surface morphology, crystal structure and quality and optical characteristics. In PL measurement, the samples were excited by a continuous He-Cd laser (325 nm). 3. Results and discussion 3.1. GaN nanowall crystal growth The SEM images of GaN nanowall crystal surfaces in the N/Ga flux ratios of 350, 200, 120 and 50 are shown in Fig. 1. The GaN nanowall structures were observed in the N/Ga flux ratios of 350 (Fig. 1(a)), 200 (Fig. 1(b)) and 120 (Fig. 1(c)). In the case of the large N/Ga flux ratio of 350 (Fig. 1(a)), the continuous GaN nanowall structure with small pores are seen. When the N/Ga flux ratio Table 1 Growth conditions of GaN nanowall crystals. GaN nanowall without InGaN MQWs N/Ga flux ratio Ga Cell temperature ( C) N2 pressure (Torr)
350 910 4.2 105
200 940 4.2 105
120 940 2.6 105
50 940 1.1 105
Table 2 Growth conditions of InGaN MQWs on GaN nanowall crystals. GaN nanowall with InGaN MQWs GaN nanowall
InGaN MQWs
N/Ga flux ratio Cell temperature ( C)
50 Ga/940
25
10
N2 pressure (Torr)
1.1 105
5.5 105
2.2 105
– Ga/895 In/840 4.2 105
decreases to 200 and 120 (Figs. 1(b) and (c)), the continuity of GaN nanowalls is decreased and the size of pore is increased. Further decreasing N/Ga flux ratio to 50, the GaN nanopillars are seen as shown in Fig. 1(d). Fig. 2 shows the results of PL measurement of the GaN nanowall samples at the room temperature. In the case of the N/Ga flux ratio of 350 (Fig. 2(a)), the broaden emission which has a peak position at 407 nm is observed. In general, the GaN near band edge emission is usually centered at 364 nm. According to the previous reports [13], the emission related to the Ga-vacancy is centered at the longer wavelength comparing with the GaN near band edge emission. As decreasing the N/Ga flux ratio to 200 (Fig. 2(b)) and 120 (Fig. 2(c)), the emission originated from the GaN near band edge becomes strong, while the emission originated from the GaN defects becomes weak. Furthermore, decreasing the N/Ga flux ratio to 50, the sharp emission originated from the GaN near band edge is dominant (Fig. 2(d)). The full width at half maximum (FWHM) is 9.1 nm. From these results, the GaN nanowall crystals which are grown in the low N/Ga flux ratio show the better crystal quality comparing with the GaN nanowall crystal grown in the high N/Ga flux ratio. Zhong et al. reported that GaN nanowall becomes dense and the emission originated from the GaN defects becomes larger as increasing N/Ga flux ratio [9]. Our results agree with the report. The XRD pattern was measured to investigate the crystal structure of the GaN nanowall crystal. The results of 2u-v scan are shown in Fig. 3. The peaks around 35 and 73 are clearly observed. These peaks are corresponding to the diffractions originated from GaN (0002) and GaN (0004) crystal planes, respectively. Therefore, the GaN nanowall structure consists of hexagonal GaN crystal along c-plane, which agrees with the previous reports [6,9]. In Addition, the XRD rocking curves were measured for all the samples and the spectra are shown in Fig. 4(a). The FWHM obtained from Fig. 4(a) are shows in Fig. 4(b) as a function of N/Ga flux ratio. In the case of the N/Ga flux ratios from 350 to 120, the FWHMs are around 300000 . In the case of the N/Ga flux ratio of 50, in which the GaN nanopillars and nanowalls are mixed, the FWHM is broadened to be 520000 . In general, broaden FWHM means the deterioration of crystal quality. For the sample grown in the N/Ga flux ratio of 50, the FWHM is broaden while the PL spectrum shows good crystal quality (Fig. 2(d)). We think the reason of this broaden FWHM value is not originated from poor crystal quality. The two reasons can be considered. One reason is that the nanopillars have the slight different crystal orientations even if each nanopillar has good crystal orientation. From the SEM image as shown in Fig. 1(d), the GaN nanopillars seem to have the slightly different angles to the substrate, respectively. As the result, the FWHM becomes broader. The other reason is due to the fact that the nanopillars and the nanowalls are mixed, as seen from the SEM image shown in Fig. 1(d). It is assumed the both nanopillar and nanowall may show the slight different crystal orientations to the substrate, which are superposed to generate a broaden FWHM even if both the nanopillars and nanowalls have good crystal qualities. And thus, the FWHM becomes broader comparing with the GaN nanowall crystals grown in the high N/Ga flux ratios of 350, 200 and 120.
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Fig. 1. Surface morphologies of GaN nanowall structures in the different N/Ga flux ratios, (a) 350, (b) 200, (c) 120 and (d) 50.
3.2. Optical characteristics of InGaN MQWs In previous section, the GaN nanowall crystal which is grown in the low N/Ga flux ratio shows the better crystal quality comparing with that grown in high N/Ga flux ratio. Therefore, in this experiment, the GaN nanowall crystals were grown in the low N/ Ga flux ratios of 50, 25 and 10. Subsequently, InGaN MQWs were grown on the GaN nanowall crystals. The PL characteristics of the InGaN MQWs were examined. Fig. 5 shows the top view SEM images of the InGaN MQWs grown on GaN nanowall crystals. In the N/Ga flux ratio of 50 as shown in Fig. 5(a), the surface morphology is the nanowall structure with the large pores. Decreasing N/Ga flux ratio to 25 as shown in Fig. 5(b), the nanowalls are seen with the very narrow gaps between nanowalls. Further decreasing N/Ga flux ratio to 10, the surface morphology becomes a film structure. The PL spectra of InGaN MQWs were measured by varying the temperature. The strong emission originated from the InGaN MQWs and the emission originated from the GaN near band edge were observed in all samples. The spectral width from the GaN
Fig. 2. PL spectra of GaN nanowall structures in the N/Ga flux ratios of (a) 350, (b) 200, (c) 120 and (d) 50.
near band edge in the N/Ga flux ratios of 50, 25 and 10 were less than 10 nm and as narrow as that shown in Fig. 2(d) although the emissions from MQWs were overlapped. As an example, the PL spectra of the InGaN MQWs grown in the N/Ga flux ratio of 50 are shown in Fig. 6(a). The peaks originated from the InGaN MQWs are clearly seen around the wavelength of 400 nm. The PL peak intensities of the InGaN MQWs were plotted as a function of temperature as shown in Figs. 6(b) in the N/Ga flux ratios of 50, 25 and 10, respectively. The peak intensities of the InGaN MQWs are gradually increased as decreasing the temperature. The PL intensity ratio at the room temperature and 8 K is defined as IR here. The values of IR are plotted in Fig. 7 as a function of the N/Ga flux ratio of the GaN nanowall crystal. The values of IR are 0.16, 0.58 and 1.32% in the N/Ga flux ratios of 50, 25 and 10, respectively. It is clearly seen that the value of IR is decreased as increasing N/Ga flux ratio. It is considered that the quality of InGaN MQWs for the optical emission is improved as decreasing N/Ga flux ratio. In general, the crystal quality of grown material depends on the
Fig. 3. XRD pattern of 2u-v scan for the GaN nanowall grown in the N/Ga flux ratio of (a) 350, (b) 200, (c) 120, (d) 50.
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Fig. 4. (a) XRD rocking curve and (b) the value of FWHM as a function of N/Ga flux ratio.
Fig. 5. surface morphologies of InGaN MQWs crystals on GaN nanowalls in different N/Ga flux ratios, (a) 50, (b) 25, and (c) 10.
Fig. 6. (a) PL spectra of InGaN MQWs grown in the N/Ga flux ratio of 50, (b) PL peak intensity dependence on temperature in the N/Ga flux ratios.
quality of under-layer. As mentioned before, the GaN crystal quality was improved as decreasing N/Ga flux ratio. Therefore, in this experiment, the higher value of IR is obtained as decreasing N/ Ga flux ratio. 4. Conclusion The GaN nanowall crystals and the InGaN MQWs grown on the GaN nanowall crystals were directly deposited on Si (111) substrate using MBE and the characteristics were investigated. The GaN nanowall crystals were grown in the various N/Ga flux ratios. In the
high N/Ga flux ratio of 350, the continuous and dense GaN nanowall network structure was grown. Decreasing N/Ga flux ratio, the continuity of GaN nanowall was decreased. In the low N/ Ga flux ratio of 50, the GaN nanopillars and the nanowalls were mixed. The GaN near band edge emission became dominant as decreasing N/Ga flux ratio. On the other hand, the FWHM was larger in the low N/Ga flux ratios, which was considered from the fact that the nanopillars were slightly tilted and mixed with nanowalls. The InGaN MQWs were formed on the GaN nanowall crystals which were grown in the various N/Ga flux ratios. The strong PL
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References
Fig. 7. PL intensity ratio (IR) at the room temperature and 8 K as a function of N/Ga flux ratio.
intensities were obtained around the wavelength of 400 nm. The PL intensity ratio at room temperature and 8 K increased as deceasing N/Ga flux ratio. The quality of the InGaN MQWs was considered to be somewhat degraded with the increase of N/Ga flux ratio since the GaN nanowall network crystals seem to grow at an expense of the crystal quality. And thus, it is important to compromise the nanowall formation and the emission efficiency of MQWs. Acknowledgement The authors would like to thank Y. Kanamori and T. Sasaki for discussion. Y. Tamura appreciates the Research Fellow of Japan Society for the Promotion of Science for the financial support.
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