ChemComm
Published on 27 November 2013. Downloaded by McGill University on 26/10/2014 16:31:37.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 682 Received 22nd September 2013, Accepted 6th November 2013
View Article Online View Journal | View Issue
Highly ordered GaN-based nanowire arrays grown on patterned (100) silicon and their optical properties† Xingfu Wang,a Jinhui Tong,a Xin Chen,a Bijun Zhao,a Zhiwei Ren,a Danwei Li,a Xiangjing Zhuo,a Jun Zhang,a Hanxiang Yi,a Chao Liu,a Fang Fangb and Shuti Li*a
DOI: 10.1039/c3cc47239f www.rsc.org/chemcomm
Arrays of GaN-based nanowires have been synthesized on patterned silicon without a catalyst. The spatial density, length and average radius of the nanowires can be well-controlled. The GaN core contains two semipolar facets and a controllable polar facet. The nanowire heterostructures exhibit excellent laser behavior.
Nanowire heterostructures have attracted great attention for the potential application as functional elements in nano- and optoelectronics.1 Many advances have been achieved in the synthesis of semiconductor nanowires with controlled dimensions,2 chemical composition3 and crystal structure.4 However, great efforts still remain to be made to precisely and expediently control the chemical and physical properties of nanowires. In most cases, the use of a metal catalyst during growth2 inevitably increases the complexity of the process and may harm the intrinsic electrical and optical properties of the nanowires. To overcome these shortcomings, catalyst-free growth studies of semiconductor wires have been made by various methods (etching,5 physical and chemical vapor deposition6 and molecular beam epitaxy7), each exhibiting different growth mechanisms. In addition, single nanowires have been shown to function as ´rot cavities,8 however, the optically optical wave-guides and Fabry–Pe pumped characteristics of single nanowires with different structures and growth mechanisms still need to be further studied. Here we report a rational method to grow controllable GaN-based nanowire heterostructures on patterned (100) silicon by metal organic chemical vapor deposition (MOCVD). By adjusting the pattern on the silicon substrate and tuning the growth parameters, the spatial density, dimension and chemical composition of nanowires can be controlled precisely and effectively. Two semipolar crystallographic planes that formed during the growth of the GaN core can be utilized as growth templates for multiple quantum wells
a
Laboratory of Nanophotonic Functional Materials and Devices, Institute of Opto-Electronic Materials and Technology, South China Normal University, Guangzhou, 510631, People’s Republic of China. E-mail: [email protected] b Gold Medal Analytical & Testing Group, 511340, People’s Republic of China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cc47239f
682 | Chem. Commun., 2014, 50, 682--684
(MQWs) or device structure, which will reduce the piezoelectric fields in the active regions. Optical excitation of the single nanowire cavities produces stimulated emission and lasing. Fig. 1a schematically shows the prepared patterned Si substrate (ESI†). The trapezoidal grooves along the h1% 10i axis of the Si substrate are perpendicular to and periodically severed by the lateral ditches. Two opposite Si(111) facets of the trapezoidal groove are separated by a bottom Si(100) facet, which is confirmed by the cross-sectional SEM
Fig. 1 (a) Schematic illustration of the patterned silicon substrate. (b) SEM images of the nanowires in the plane of the Si substrate; scale bar is 3 mm. (c) SEM images of the nanowire array; scale bar is 2 mm. (d) EDX data of the DsNW. (e–g) Optical microscope images of isolated nanowires dispersed on the substrate, the transverse dimensions of the nanowires were 500 nm, 1.0 mm and 2.0 mm, respectively; scale bars are all 2 mm.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by McGill University on 26/10/2014 16:31:37.
Communication
images of the trapezoidal grooves (Fig. S1, ESI†). According to the epitaxial relationship between GaN and Si, the growth of GaN-based nanowires was performed on the sidewall of Si(111) facets selectively. The length of nanowires, corresponding to the length of trapezoidal groove trenches, can be adjusted by varying the distance between the adjacent lateral ditches. Fig. 1b and c are the representative SEM images of the synthesized nanowires arrays in the plane of patterned Si. On this patterned Si substrate, the InGaN/GaN MQW nanowire heterostructure (MqwNW) and the device structure nanowire heterostructure (DsNW) were fabricated (ESI†). The MqwNW consists of GaN/(InGaN/GaN)10 MQW (active layer) while the DsNW was designed as n-GaN/(InGaN/GaN)6 MQW/p-AlGaN/p-GaN. Fig. 1d shows the EDX result of the DsNW sample indicating the presence of Ga, N, In and Si. The nanowires were released and dispersed onto the substrate (ESI†) with diameters varying from 500 nm to 3.0 mm. Fig. 1e–g show the isolated nanowires with diameters of 500 nm, 1.0 mm and 2.0 mm, respectively. The length of the separated nanowires, ranging from a few micrometers to several thousand micrometers, depends on the interval of the adjacent lateral ditches and the release process. Fig. 2a shows the cross-sectional STEM image of the nanowire heterostructure that is grown selectively on the Si(111) sidewall along its (0001) axis. Meanwhile, the {11% 01} and {1% 101} semipolar facets are developed and serve as new growth templates for the second growth stage, which was confirmed by the electron diffraction pattern (Fig. 2a, inset). As the {11% 01} and {1% 101} planes are self-forming, their surface flatness is outstanding, which improves the thickness uniformity and surface smoothness of the MQWs or the device structure grown subsequently. The three boxed regions labeled A, B and C indicate the locations where magnified dark-field
Fig. 2 (a) Dark-field cross-sectional STEM image recorded along the % zone axis of an MqwNW sample. Scale bar is 100 nm. Inset: The [1120] % corresponding electron diffraction pattern indexed for the [1120] zone axis. (b) The dark-field STEM image of Si/AlN and AlN/GaN interfaces; scale bar is 50 nm; (c and d) Dark-field cross-sectional STEM images recorded % direction at the {11% 01} facet of the MqwNW sample (c) and along the [1120] the corner between the {11% 01} facet and the {1% 101} facet of the MqwNW sample (d); scale bars are 20 nm and 30 nm, respectively.
This journal is © The Royal Society of Chemistry 2014
ChemComm
STEM images are recorded and shown in Fig. 2b–d. Fig. 2b shows that the GaN core is grown on the Si(111) sidewall with 30 nm AlN as an interlayer. The contrast difference clearly reveals the abrupt Si/ AlN and AlN/GaN interfaces. In addition, it also reveals that the threading dislocations originate from the (0001) GaN/(111) Si interface. Most of the dislocations, however, turn to the direction perpendicular to the growth direction of h0001i GaN, which are indicated by the small black arrows. Thus, the density of threading dislocation can be decreased drastically in the regions about 200 nm above the AlN/GaN interface. Fig. 2c clearly shows that the 10-period InGaN/GaN MQWs are grown on the top {11% 01} facet. The interfaces are sharp without obvious interdiffusion, which is important for building high performance electronic and optoelectronic devices. This {11% 01} MQW shell was measured to be 57 nm thick and the average thickness of the InGaN well and the GaN barrier is 1.1 nm and 4.6 nm, respectively. Fig. 2d shows that the first five MQWs are grown along the polar h0001i growth direction. The polar facet is formed when there is no intersection between the {11% 01} facet and the {1% 101} facet during the growth of the GaN core. After five periods MQWs are coated, the polar plane vanishes and the apex appears. It was also found that the thickness of the {1% 101} semipolar MQWs was close to that of the top {11% 01} semipolar MQWs. More interestingly, for the first five polar MQWs, the average thicknesses of the InGaN well and the GaN barrier are 6.5 nm and 28 nm, respectively, which are larger than those on the other two semipolar facets. We believe that the polar Ga-face (0001) facet exhibits a faster shell deposition rate and there is a thicker layer of InGaN or GaN on the (0001) facet. Considering that the growth of the polar (0001) crystallographic plane can be adjusted by varying the depth of the trapezoidal groove or the duration of the n-GaN core growth, we have prepared two different types of DsNW samples to control the growth of the (0001) facet (Fig. S2, ESI†). Room-temperature micro-photoluminescence (micro-PL) measurements were carried out to characterize the optical properties of the fabricated MqwNWs and DsNWs. Fig. 3 shows the spectra recorded for the emission from both single nanowires. The wavelengths of GaN peaks are both at around 365 nm, which is consistent with the bandedge transitions in hexagonal phase GaN. Each of the spectra recorded for the DsNW sample and the MqwNW sample shows high intensity peaks at 421 nm and 430 nm, which correspond to the MQW emission from DsNW and MqwNW samples, respectively.
Fig. 3 Room-temperature Micro-PL spectra recorded for the MqwNW single nanowire (black) and the DsNW sample (red).
Chem. Commun., 2014, 50, 682--684 | 683
View Article Online
Published on 27 November 2013. Downloaded by McGill University on 26/10/2014 16:31:37.
ChemComm
The blue-shift of MQW emission in the DsNW is mainly caused due to the high temperature required for p-GaN growth and the decreased number of MQWs. Besides the main emission peak at 430 nm, the MqwNW spectrum also shows another QW peak located at 492 nm, which can be attributed to the emission of (0001) polar MQWs. An increase of the thickness of MQWs on the (0001) polar facet can be observed obviously from the dark-field STEM results. The increase of the thickness indicates a large growth rate of MQWs, which contributes to more efficient indium incorporation, thus the emission wavelength of the (0001) polar MQWs is longer. For the micro-PL spectrum of the DsNW sample, no emission peaks related to the (0001) polar MQW was observed, which confirmed that the growth time of the GaN core was long enough so that the polar plane was eliminated. The Micro-PL spectrum of this kind of sample is also exhibited (Fig. S3, ESI†). Excitation power-dependent studies of a representative MqwNW nanowire structure were carried out. At low excitation power densities, spontaneous emission was only observed from the body of a single MqwNW (Fig. 4a). With the increase of the excitation power densities, the emission intensity at nanowire ends increased (Fig. 4b) and became dominant (Fig. 4c), which indicates strong ´rot (axial) waveguiding behavior, and the cavity modes are Fabry–Pe rather than whispering gallery modes (WGMs).9 Fig. 4d shows the spectra acquired from the nanowire ends. At a low power density (160 kW cm2), a relatively broad spontaneous emission peak centered at 430 nm was observed. As the excitation power density increased, the end emission intensity increased rapidly and became dominant. Above a threshold of 500 kW cm2, this spontaneous emission peak collapsed into several well-defined narrow peaks centered at around 423 nm, thus indicating strong waveguiding properties. The spectral analysis of the emission from the nanowire
Communication
ends exhibits a blue-shift with periodic peaks at higher excitation intensity, and the full-width at half-maximum (FWHM) of these peaks is less than 1 nm. This indicates longitudinal modes in the nanowire cavity. To confirm this assumption, we calculated the mode spacing using the following formula:10 Dl ¼
l2 2ne L
(1)
where l is the emission wavelength, ne is the effective refractive index and L is the nanowire cavity length. For l = 423 nm, ne is around 2.6211 and L = 35 mm, the predicted mode spacing is around 0.98 nm, which is consistent with the measured FWHM. These results clearly indicate the lasing behavior in our MqwNW. The blue shift of lasing peaks relative to the spontaneous emission is a distinct feature of InGaN MQW lasers, which is attributed to band filling and/or photo-induced screening of internal electric fields. The optically pumped properties of DsNW samples were also investigated (Fig. S4, ESI†). Measurements made on a number of DsNW samples showed that the threshold power densities for the DsNW lasers are all above 900 kW cm2. These DsNW lasers have lasing thresholds higher than the MqwNW counterpart. Highly ordered arrays of GaN-based nanowires have been synthesized on a patterned silicon substrate. The physical and chemical properties of the nanowires can be well controlled. TEM studies demonstrate the single-crystalline hexagonal GaN nanowire cores with two semipolar planes. Optically pumped laser properties of our samples indicate excellent laser behavior. This work was supported by the National Nature Science Foundation of China (Grant No. 51172079), the Science and Technology Program of Guangdong province, China (Grant No. 2010B090400456 and 2010A081002002) and the Scientific Research Foundation of Graduate School of South China Normal University.
Note and references
Fig. 4 Images of a single nanowire excited with (a) 160 kW cm2, (b) 800 kW cm2, and (c) 1300 kW cm2. (d) PL spectra recorded at the end of a 35 mm long MqwNW nanowire sample for 200 kW cm2 and 1200 kW cm2 power densities.
684 | Chem. Commun., 2014, 50, 682--684
1 S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang and Z. L. Wang, Nat. Nanotechnol., 2010, 5, 366–373; W. Guo, M. Zhang, A. Banerjee and P. Bhattacharya, Nano Lett., 2010, 10, 3355–3359; X. Wang, J. Song, J. Liu and Z. L. Wang, Science, 2007, 316, 102–105. 2 B. Mukherjee, A. Peterson and V. Subramanian, Chem. Commun., 2012, 48, 2415–2417; Y. Shan, A. K. Kalkan, C.-Y. Peng and S. J. Fonash, Nano Lett., 2004, 4, 2085–2089. 3 M. S. Gudiksen, L. J. Lauhon, J. Wang, D. C. Smith and C. M. Lieber, Nature, 2002, 415, 617–620. 4 X. Sun, Q. Li, Y. N. Lu and Y. B. Mao, Chem. Commun., 2013, 49, 4456–4458; F. M. Davidson, D. C. Lee, D. D. Fanfair and B. A. Korgel, J. Phys. Chem. C, 2007, 111, 2929–2935. 5 C. Y. Wang, L. Y. Chen, C. P. Chen, Y. W. Cheng, M. Y. Ke, M. Y. Hsieh, H. M. Wu, L. H. Peng and J. Huang, Opt. Express, 2008, 16, 10549–10556. 6 V. V. Kuryatkov, W. Feng, M. Pandikunta, J. H. Woo, D. Garcia, H. R. Harris, S. A. Nikishin and M. Holtz, Appl. Phys. Lett., 2010, 96, 073107. 7 S. D. Carnevale, J. Yang, P. J. Phillips, M. J. Mills and R. C. Myers, Nano Lett., 2011, 11, 866–871. 8 S. Gradecak, F. Qian, Y. Li, H. G. Park and C. M. Lieber, Appl. Phys. Lett., 2005, 87, 173111; F. Qian, Y. Li, S. Gradecak, H. G. Park, Y. J. Dong, Y. Ding, Z. L. Wang and C. M. Lieber, Nat. Mater., 2008, 7, 701–706. 9 J. C. Johnson, H. J. Choi, K. P. Knutsen, R. D. Schaller, P. D. Yang and R. J. Saykally, Nat. Mater., 2002, 1, 106–110. 10 Y. Cui and C. M. Lieber, Science, 2001, 291, 851–853. 11 J. A. Martin and M. Sanchez, Semicond. Sci. Technol., 2005, 20, 290–295.
This journal is © The Royal Society of Chemistry 2014
Published on 27 November 2013. Downloaded by McGill University on 26/10/2014 16:31:37.
COMMUNICATION
Cite this: Chem. Commun., 2014, 50, 682 Received 22nd September 2013, Accepted 6th November 2013
View Article Online View Journal | View Issue
Highly ordered GaN-based nanowire arrays grown on patterned (100) silicon and their optical properties† Xingfu Wang,a Jinhui Tong,a Xin Chen,a Bijun Zhao,a Zhiwei Ren,a Danwei Li,a Xiangjing Zhuo,a Jun Zhang,a Hanxiang Yi,a Chao Liu,a Fang Fangb and Shuti Li*a
DOI: 10.1039/c3cc47239f www.rsc.org/chemcomm
Arrays of GaN-based nanowires have been synthesized on patterned silicon without a catalyst. The spatial density, length and average radius of the nanowires can be well-controlled. The GaN core contains two semipolar facets and a controllable polar facet. The nanowire heterostructures exhibit excellent laser behavior.
Nanowire heterostructures have attracted great attention for the potential application as functional elements in nano- and optoelectronics.1 Many advances have been achieved in the synthesis of semiconductor nanowires with controlled dimensions,2 chemical composition3 and crystal structure.4 However, great efforts still remain to be made to precisely and expediently control the chemical and physical properties of nanowires. In most cases, the use of a metal catalyst during growth2 inevitably increases the complexity of the process and may harm the intrinsic electrical and optical properties of the nanowires. To overcome these shortcomings, catalyst-free growth studies of semiconductor wires have been made by various methods (etching,5 physical and chemical vapor deposition6 and molecular beam epitaxy7), each exhibiting different growth mechanisms. In addition, single nanowires have been shown to function as ´rot cavities,8 however, the optically optical wave-guides and Fabry–Pe pumped characteristics of single nanowires with different structures and growth mechanisms still need to be further studied. Here we report a rational method to grow controllable GaN-based nanowire heterostructures on patterned (100) silicon by metal organic chemical vapor deposition (MOCVD). By adjusting the pattern on the silicon substrate and tuning the growth parameters, the spatial density, dimension and chemical composition of nanowires can be controlled precisely and effectively. Two semipolar crystallographic planes that formed during the growth of the GaN core can be utilized as growth templates for multiple quantum wells
a
Laboratory of Nanophotonic Functional Materials and Devices, Institute of Opto-Electronic Materials and Technology, South China Normal University, Guangzhou, 510631, People’s Republic of China. E-mail: [email protected] b Gold Medal Analytical & Testing Group, 511340, People’s Republic of China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cc47239f
682 | Chem. Commun., 2014, 50, 682--684
(MQWs) or device structure, which will reduce the piezoelectric fields in the active regions. Optical excitation of the single nanowire cavities produces stimulated emission and lasing. Fig. 1a schematically shows the prepared patterned Si substrate (ESI†). The trapezoidal grooves along the h1% 10i axis of the Si substrate are perpendicular to and periodically severed by the lateral ditches. Two opposite Si(111) facets of the trapezoidal groove are separated by a bottom Si(100) facet, which is confirmed by the cross-sectional SEM
Fig. 1 (a) Schematic illustration of the patterned silicon substrate. (b) SEM images of the nanowires in the plane of the Si substrate; scale bar is 3 mm. (c) SEM images of the nanowire array; scale bar is 2 mm. (d) EDX data of the DsNW. (e–g) Optical microscope images of isolated nanowires dispersed on the substrate, the transverse dimensions of the nanowires were 500 nm, 1.0 mm and 2.0 mm, respectively; scale bars are all 2 mm.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by McGill University on 26/10/2014 16:31:37.
Communication
images of the trapezoidal grooves (Fig. S1, ESI†). According to the epitaxial relationship between GaN and Si, the growth of GaN-based nanowires was performed on the sidewall of Si(111) facets selectively. The length of nanowires, corresponding to the length of trapezoidal groove trenches, can be adjusted by varying the distance between the adjacent lateral ditches. Fig. 1b and c are the representative SEM images of the synthesized nanowires arrays in the plane of patterned Si. On this patterned Si substrate, the InGaN/GaN MQW nanowire heterostructure (MqwNW) and the device structure nanowire heterostructure (DsNW) were fabricated (ESI†). The MqwNW consists of GaN/(InGaN/GaN)10 MQW (active layer) while the DsNW was designed as n-GaN/(InGaN/GaN)6 MQW/p-AlGaN/p-GaN. Fig. 1d shows the EDX result of the DsNW sample indicating the presence of Ga, N, In and Si. The nanowires were released and dispersed onto the substrate (ESI†) with diameters varying from 500 nm to 3.0 mm. Fig. 1e–g show the isolated nanowires with diameters of 500 nm, 1.0 mm and 2.0 mm, respectively. The length of the separated nanowires, ranging from a few micrometers to several thousand micrometers, depends on the interval of the adjacent lateral ditches and the release process. Fig. 2a shows the cross-sectional STEM image of the nanowire heterostructure that is grown selectively on the Si(111) sidewall along its (0001) axis. Meanwhile, the {11% 01} and {1% 101} semipolar facets are developed and serve as new growth templates for the second growth stage, which was confirmed by the electron diffraction pattern (Fig. 2a, inset). As the {11% 01} and {1% 101} planes are self-forming, their surface flatness is outstanding, which improves the thickness uniformity and surface smoothness of the MQWs or the device structure grown subsequently. The three boxed regions labeled A, B and C indicate the locations where magnified dark-field
Fig. 2 (a) Dark-field cross-sectional STEM image recorded along the % zone axis of an MqwNW sample. Scale bar is 100 nm. Inset: The [1120] % corresponding electron diffraction pattern indexed for the [1120] zone axis. (b) The dark-field STEM image of Si/AlN and AlN/GaN interfaces; scale bar is 50 nm; (c and d) Dark-field cross-sectional STEM images recorded % direction at the {11% 01} facet of the MqwNW sample (c) and along the [1120] the corner between the {11% 01} facet and the {1% 101} facet of the MqwNW sample (d); scale bars are 20 nm and 30 nm, respectively.
This journal is © The Royal Society of Chemistry 2014
ChemComm
STEM images are recorded and shown in Fig. 2b–d. Fig. 2b shows that the GaN core is grown on the Si(111) sidewall with 30 nm AlN as an interlayer. The contrast difference clearly reveals the abrupt Si/ AlN and AlN/GaN interfaces. In addition, it also reveals that the threading dislocations originate from the (0001) GaN/(111) Si interface. Most of the dislocations, however, turn to the direction perpendicular to the growth direction of h0001i GaN, which are indicated by the small black arrows. Thus, the density of threading dislocation can be decreased drastically in the regions about 200 nm above the AlN/GaN interface. Fig. 2c clearly shows that the 10-period InGaN/GaN MQWs are grown on the top {11% 01} facet. The interfaces are sharp without obvious interdiffusion, which is important for building high performance electronic and optoelectronic devices. This {11% 01} MQW shell was measured to be 57 nm thick and the average thickness of the InGaN well and the GaN barrier is 1.1 nm and 4.6 nm, respectively. Fig. 2d shows that the first five MQWs are grown along the polar h0001i growth direction. The polar facet is formed when there is no intersection between the {11% 01} facet and the {1% 101} facet during the growth of the GaN core. After five periods MQWs are coated, the polar plane vanishes and the apex appears. It was also found that the thickness of the {1% 101} semipolar MQWs was close to that of the top {11% 01} semipolar MQWs. More interestingly, for the first five polar MQWs, the average thicknesses of the InGaN well and the GaN barrier are 6.5 nm and 28 nm, respectively, which are larger than those on the other two semipolar facets. We believe that the polar Ga-face (0001) facet exhibits a faster shell deposition rate and there is a thicker layer of InGaN or GaN on the (0001) facet. Considering that the growth of the polar (0001) crystallographic plane can be adjusted by varying the depth of the trapezoidal groove or the duration of the n-GaN core growth, we have prepared two different types of DsNW samples to control the growth of the (0001) facet (Fig. S2, ESI†). Room-temperature micro-photoluminescence (micro-PL) measurements were carried out to characterize the optical properties of the fabricated MqwNWs and DsNWs. Fig. 3 shows the spectra recorded for the emission from both single nanowires. The wavelengths of GaN peaks are both at around 365 nm, which is consistent with the bandedge transitions in hexagonal phase GaN. Each of the spectra recorded for the DsNW sample and the MqwNW sample shows high intensity peaks at 421 nm and 430 nm, which correspond to the MQW emission from DsNW and MqwNW samples, respectively.
Fig. 3 Room-temperature Micro-PL spectra recorded for the MqwNW single nanowire (black) and the DsNW sample (red).
Chem. Commun., 2014, 50, 682--684 | 683
View Article Online
Published on 27 November 2013. Downloaded by McGill University on 26/10/2014 16:31:37.
ChemComm
The blue-shift of MQW emission in the DsNW is mainly caused due to the high temperature required for p-GaN growth and the decreased number of MQWs. Besides the main emission peak at 430 nm, the MqwNW spectrum also shows another QW peak located at 492 nm, which can be attributed to the emission of (0001) polar MQWs. An increase of the thickness of MQWs on the (0001) polar facet can be observed obviously from the dark-field STEM results. The increase of the thickness indicates a large growth rate of MQWs, which contributes to more efficient indium incorporation, thus the emission wavelength of the (0001) polar MQWs is longer. For the micro-PL spectrum of the DsNW sample, no emission peaks related to the (0001) polar MQW was observed, which confirmed that the growth time of the GaN core was long enough so that the polar plane was eliminated. The Micro-PL spectrum of this kind of sample is also exhibited (Fig. S3, ESI†). Excitation power-dependent studies of a representative MqwNW nanowire structure were carried out. At low excitation power densities, spontaneous emission was only observed from the body of a single MqwNW (Fig. 4a). With the increase of the excitation power densities, the emission intensity at nanowire ends increased (Fig. 4b) and became dominant (Fig. 4c), which indicates strong ´rot (axial) waveguiding behavior, and the cavity modes are Fabry–Pe rather than whispering gallery modes (WGMs).9 Fig. 4d shows the spectra acquired from the nanowire ends. At a low power density (160 kW cm2), a relatively broad spontaneous emission peak centered at 430 nm was observed. As the excitation power density increased, the end emission intensity increased rapidly and became dominant. Above a threshold of 500 kW cm2, this spontaneous emission peak collapsed into several well-defined narrow peaks centered at around 423 nm, thus indicating strong waveguiding properties. The spectral analysis of the emission from the nanowire
Communication
ends exhibits a blue-shift with periodic peaks at higher excitation intensity, and the full-width at half-maximum (FWHM) of these peaks is less than 1 nm. This indicates longitudinal modes in the nanowire cavity. To confirm this assumption, we calculated the mode spacing using the following formula:10 Dl ¼
l2 2ne L
(1)
where l is the emission wavelength, ne is the effective refractive index and L is the nanowire cavity length. For l = 423 nm, ne is around 2.6211 and L = 35 mm, the predicted mode spacing is around 0.98 nm, which is consistent with the measured FWHM. These results clearly indicate the lasing behavior in our MqwNW. The blue shift of lasing peaks relative to the spontaneous emission is a distinct feature of InGaN MQW lasers, which is attributed to band filling and/or photo-induced screening of internal electric fields. The optically pumped properties of DsNW samples were also investigated (Fig. S4, ESI†). Measurements made on a number of DsNW samples showed that the threshold power densities for the DsNW lasers are all above 900 kW cm2. These DsNW lasers have lasing thresholds higher than the MqwNW counterpart. Highly ordered arrays of GaN-based nanowires have been synthesized on a patterned silicon substrate. The physical and chemical properties of the nanowires can be well controlled. TEM studies demonstrate the single-crystalline hexagonal GaN nanowire cores with two semipolar planes. Optically pumped laser properties of our samples indicate excellent laser behavior. This work was supported by the National Nature Science Foundation of China (Grant No. 51172079), the Science and Technology Program of Guangdong province, China (Grant No. 2010B090400456 and 2010A081002002) and the Scientific Research Foundation of Graduate School of South China Normal University.
Note and references
Fig. 4 Images of a single nanowire excited with (a) 160 kW cm2, (b) 800 kW cm2, and (c) 1300 kW cm2. (d) PL spectra recorded at the end of a 35 mm long MqwNW nanowire sample for 200 kW cm2 and 1200 kW cm2 power densities.
684 | Chem. Commun., 2014, 50, 682--684
1 S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang and Z. L. Wang, Nat. Nanotechnol., 2010, 5, 366–373; W. Guo, M. Zhang, A. Banerjee and P. Bhattacharya, Nano Lett., 2010, 10, 3355–3359; X. Wang, J. Song, J. Liu and Z. L. Wang, Science, 2007, 316, 102–105. 2 B. Mukherjee, A. Peterson and V. Subramanian, Chem. Commun., 2012, 48, 2415–2417; Y. Shan, A. K. Kalkan, C.-Y. Peng and S. J. Fonash, Nano Lett., 2004, 4, 2085–2089. 3 M. S. Gudiksen, L. J. Lauhon, J. Wang, D. C. Smith and C. M. Lieber, Nature, 2002, 415, 617–620. 4 X. Sun, Q. Li, Y. N. Lu and Y. B. Mao, Chem. Commun., 2013, 49, 4456–4458; F. M. Davidson, D. C. Lee, D. D. Fanfair and B. A. Korgel, J. Phys. Chem. C, 2007, 111, 2929–2935. 5 C. Y. Wang, L. Y. Chen, C. P. Chen, Y. W. Cheng, M. Y. Ke, M. Y. Hsieh, H. M. Wu, L. H. Peng and J. Huang, Opt. Express, 2008, 16, 10549–10556. 6 V. V. Kuryatkov, W. Feng, M. Pandikunta, J. H. Woo, D. Garcia, H. R. Harris, S. A. Nikishin and M. Holtz, Appl. Phys. Lett., 2010, 96, 073107. 7 S. D. Carnevale, J. Yang, P. J. Phillips, M. J. Mills and R. C. Myers, Nano Lett., 2011, 11, 866–871. 8 S. Gradecak, F. Qian, Y. Li, H. G. Park and C. M. Lieber, Appl. Phys. Lett., 2005, 87, 173111; F. Qian, Y. Li, S. Gradecak, H. G. Park, Y. J. Dong, Y. Ding, Z. L. Wang and C. M. Lieber, Nat. Mater., 2008, 7, 701–706. 9 J. C. Johnson, H. J. Choi, K. P. Knutsen, R. D. Schaller, P. D. Yang and R. J. Saykally, Nat. Mater., 2002, 1, 106–110. 10 Y. Cui and C. M. Lieber, Science, 2001, 291, 851–853. 11 J. A. Martin and M. Sanchez, Semicond. Sci. Technol., 2005, 20, 290–295.
This journal is © The Royal Society of Chemistry 2014
