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This article can be cited before page numbers have been issued, to do this please use: X. Yan, X. Zhang, J. Li, Y. Wu, J. Cui and X. Ren, Nanoscale, 2014, DOI: 10.1039/C4NR05486E.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C4NR05486E
Fabrication and optical properties of GaAs/InGaAs/GaAs nanowire core-multishell quantum well heterostructures Xin Yan, Xia Zhang, * Junshuai Li, Yao Wu, Jiangong Cui, and Xiaomin Ren
Abstract GaAs/InGaAs/GaAs nanowire core-multishell heterostructure with a strained radial In0.2Ga0.8As quantum well was fabricated by metal organic chemical vapor deposition. The quantum well exhibits dislocation-free phase-pure zinc-blende structure. Low-temperature photoluminescence spectra of a single nanowire exhibit distinct resonant peaks in the range from 880 to 1000 nm, corresponding to the longitudinal modes of a Fabry-Pérot cavity. This suggests a decoupling of the gain medium and resonant cavity that the quantum well provides the gain while the nanowire acts as the cavity. The resonant modes were observed at temperatures up to 240 K, exhibiting high power- and temperature-stability. The modes were blueshifted with decreasing the quantum well thickness due to enhanced quantum confinement. The results make the GaAs-based nanowire/quantum well hybrid structure promising for wavelength-tunable near-infrared nanolasers.
Corresponding Author:
[email protected]
Keywords：Nanowire, quantum well, Fabry-Pérot, heterostructure
1
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State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
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1. Introduction
DOI: 10.1039/C4NR05486E
Semiconductor nanowires (NWs) have been considered as excellent building blocks for future electronic and photonic devices due to their unique geometry and superior physical
Fabry-Pérot (F-P) microcavity which provides good optical feedback for lasing.4-6 Up to date, NW lasers have been widely reported in homogeneous materials, involving ZnO, GaN, and CdS, et al.7-9 Most recently, room-temperature lasing was observed in surface-passivated GaAs NWs.10,11 The NW lasers abovementioned have a common feature that the NW acts as both the gain medium and optical cavity. The emission can be reabsorbed during its propagation in the NW cavity, resulting in a degradation of laser performance. A structure that decouples the gain medium and optical cavity, e. g., a NW radial quantum well (QW), can avoid the reabsorption as most of the emission from the gain medium propagate in the NW core which has a larger band gap than the QW. The NW QW structure also possesses other advantages such as a stronger confinement of electrons and holes, as well as tunable emission wavelength with independent optimization of the cavity. So far, NW radial QW structures have been realized in a lot of materials, including GaN/InGaN, InP/InAs, and AlGaAs/GaAs, et al.12-14 Particularly, GaN/InGaN multi-quantum-well structure has been reported to yield lasing at room temperature with tunable wavelength, showing a bright prospect of this type of structures in nanolasers.12 GaAs/InGaAs/GaAs NW radial QW heterostructure is promising in infrared optoelectronic devices. However, the structure has been rarely reported.15 Particularly, F-P resonant modes or lasing have not been achieved in this structure up to date. In this work, we report on the 2
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properties.1-3 Single NWs are particularly promising for nanolasers as they can function as a
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DOI: 10.1039/C4NR05486E F-P resonant modes in GaAs/InGaAs/GaAs NW core-multishell QW heterostructures
fabricated by Au-catalyzed metal organic chemical vapor deposition (MOCVD) for the first time. Cross-sectional transmission electron microscopy (TEM) characterization demonstrates that the InGaAs QW exhibits dislocation-free phase-pure zinc-blende (ZB) structure.
range, corresponding to the longitudinal modes of an F-P cavity. This suggests a decoupling of the gain medium and resonant cavity that the QW provides the gain while the NW acts as the cavity. The resonant modes can be distinguished at temperatures up to 240 K, exhibiting high power- and temperature-stability. A blueshift of resonant modes with decreasing the QW thickness is observed, corresponding to an enhanced quantum confinement.
2. Experimental Details The structure was grown by using a Thomas Swan CCS-MOCVD system at a pressure of 100 Torr. Trimethylgallium (TMGa), trimethylindium (TMIn) and arsine (AsH3) were used as precursors. The carrier was hydrogen. An Au film with a thickness of 4 nm was deposited on GaAs (111) B substrate by magnetron sputtering. The substrate was then loaded into the MOCVD reactor and annealed at 650 oC under AsH3 ambient for 300 s to form Au-Ga alloyed particles as catalyst. The metal-catalyst nanoparticle can enhance the optical feedback in the cavity by increasing the facet reflectivity.10 GaAs NWs were grown at 440 oC for 600 s. After raising the temperature to 500 oC, a thin InGaAs shell was radially grown on the GaAs NW for 100 s. The flow rates of TMGa and TMIn are 5.40 and 1.35 µmol min-1, respectively, resulting in an In content of 20%. Then a GaAs shell was radially grown on the InGaAs shell 3
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Photoluminescence spectra of a single NW exhibit distinct resonant peaks in the infrared
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10.1039/C4NR05486E at 500 oC for 300 s. In the core-multishell structure, the InGaAs shell acts as theDOI: QW while
the GaAs core and shell function as the barrier. The morphological and structural characteristics of the NWs were characterized by TEM equipped with X-ray energy dispersive spectroscopy (EDS). Individual NWs for TEM
spreading drops from the suspension onto a holey carbon/Cu grid. For cross-section TEM studies, NW array was first embedded into an epoxy resin. Embedded NWs were cut perpendicular to the NW axis into 50-200-nm-thick slices using a diamond ultramicrotome knife and then transferred onto TEM grids. For optical characterization, as-grown NWs were mechanically
cut
down
and
dispersed
onto
a
SiO2-coated
Si
substrate.
The
microphotoluminescence (µPL) measurements were carried out using a 532 nm continuous-wave diode-pumped solid-state (DPSS) laser for excitation. The excitation beam was focused onto ~2 µm in diameter with a ×50 microscope objective on the sample placed in a cryostat. The emission through the same microscope objective was detected by the combination of a grating spectrometer and a silicon charge-coupled device (CCD).
3. Results and discussions 3.1. Morphological and structural characterization
A schematic of the structure is shown in Fig. 1a. Fig. 1b shows the TEM image of a single GaAs/InGaAs/GaAs NW core-multishell QW heterostructure. The NW has a length of 7 µm and a uniform diameter of 300 nm along the NW axis. Fig. 1c shows the cross-sectional TEM image as well as the EDS line scan result of a slice of the NW. The QW can be clearly 4
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observations were prepared by ultrasonicating the samples in ethanol for 5 min, followed by
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10.1039/C4NR05486E distinguished between the core and shell due to different brightness intensity. TheDOI: GaAs core
has a hexagonal cross-section and {112} side facets, which are commonly observed in Au-catalyzed MOCVD-grown NWs.16 The GaAs shell exhibits an inequilateral enneagon cross-section. Six main sides of the shell have a 30o rotation compared with the core,
also observed in GaAs/AlInP and GaAs/GaP NW core-shell heterostructures, indicating that {110} facets are energetically favorable at high growth temperature.17,18 From the intensity profiles of In and Ga elements we can determine that the QW has an In composition of about 20%. The intensity profile of In element exhibits a sharp increase in the QW area, suggesting an abrupt interface between the QW and neighboring GaAs barriers. This is an obvious advantage over the Au-catalyzed GaAs/In(Ga)As/GaAs NW axial heterostructures in which the heterojunction is usually not sharp due to the memory effect of group III elements in the Au droplet.19,20 Fig. 1d shows the high-resolution TEM (HRTEM) image of the QW. The QW has a uniform thickness of about 10 nm. No dislocations are observed in the QW or the GaAs barriers, indicating that the QW is fully strained. The corresponding fast Fourier transform (FFT) clearly shows two sets of diffraction spots, corresponding to the InGaAs QW and GaAs NW, respectively. The HRTEM and FFT also demonstrate that both the QW and NW exhibit pure ZB structure.
3.2 Optical Characterization
Fig. 2a shows the PL spectra of a single GaAs/InGaAs/GaAs NW core-multishell QW heterostructure at 80 K. A strong emission peak is observed at 985 nm, corresponding to the 5
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suggesting {110} type facets. The transformation from {112} to {110} at high temperature is
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10.1039/C4NR05486E InGaAs QW. The QW emission intensity is nearly 40 times higher than the DOI: GaAs NW,
indicating that most of the carriers are confined to the QW region. A series of periodic peaks are observed in the spectra ranging from 884 to 986 nm, which are indicative of the longitudinal modes of F-P cavity formed in the NW. For a NW F-P cavity, the mode spacing
the group refractive index.13 From inset 1, the length of the NW can be determined to be 7 µm. The group refractive index of GaAs is calculated to be 4.7-4.2 in the mode wavelength range of 884-985 nm.13 Thus the mode spacing can be calculated to be 11.9-16.7 µm, which is in agreement with the measured mode spacing 12.6-16.3 nm. In addition, as the QW has a rather small thickness, it is difficult for the QW to confine the light and support F-P modes. A reasonable explanation is that most of the emission from the QW is coupled into the GaAs core which acts as an F-P microcavity and generates resonant modes, as shown in Fig. 2b. Modal characteristics of the structure were also investigated by 3D FDTD simulations. Fig. 3a shows the schematic of the simulated structure. The radius of GaAs core was set to 100 nm, and the thickness of QW and GaAs shell was set to 10 and 50 nm, respectively. The NW was surrounded by a perfectly matched layer (PML). Several point dipole sources with the emission wavelength of 985 nm were placed at the QW position to model the emission from the QW. The simulations present three dominated modes in the structure, determined as HE11x, HE11y, and TE01 mode, respectively, as shown in Fig. 3b, c and d. For HE11x and HE11y modes, the intensity is mainly concentrated in the GaAs core. For TE01 mode, there is a substantial intensity overlap between the GaAs core and shell. The simulation results demonstrate a decoupling of gain medium and F-P cavity that the InGaAs QW provides the gain while the 6
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is given by 2 /{2L[n (dn / d )]} , where L is the NW length and n (dn / d ) is
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DOI: 10.1039/C4NR05486E GaAs NW acts as the cavity. This is an obvious advantage compared with homogenous NWs,
e. g. GaAs NWs, as the emission from the InGaAs QW wouldn’t be reabsorbed by the GaAs NW cavity, and the laser wavelength can also be tuned by adjusting the In composition in the QW with independent optimization of the cavity.
densities. We can see that the mode peak position doesn’t change at different excitation power density. For a NW F-P cavity, the mode characteristics follow the formula of m 2nL / , where m is an integer, L is the length of NW cavity, and n is the refractive index. For the NW with a certain length, the mode wavelength is only affected by the refractive index. 4 Thus the high power stability of the modes indicates that the refractive index of GaAs NW almost doesn’t change with the excitation power density. At higher excitation power densities, we can observe six dominated modes, which are centered at 910.0, 924.0, 938.2, 953.2, 968.8, and 985.0 nm, respectively. This suggests that the GaAs NW cavity has a relatively higher gain in the range of 910-985 nm. The mode with the highest intensity shifts from 985 nm to 910 nm as the excitation power density increases from 16 to 160 kW/cm2, indicating an increase of cavity loss and decrease of gain for long-wavelength modes. The mode Q factors are also calculated by fitting the spectra with Lorentzian curves. Under the excitation power density of 1.6, 16, 80 and 160 kW/cm2, the corresponding highest mode Q factor is 68.6, 70.4, 82.8, and 100.4, respectively. The low Q values can be attributed to several reasons. First, the GaAs NW cavity may have a relatively large loss and low gain in the InGaAs QW emission range, which has been theoretically confirmed by other groups.10 Second, the diameter of the NW heterostructure is relatively small for achieving lasing.10 In 7
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Fig. 4 shows the 80 K PL spectra of the InGaAs QW with different excitation power
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DOI: 10.1039/C4NR05486E addition, the InGaAs shell, which acts as the gain medium, may be not able to offer sufficient
gain due to the small thickness. For achieving lasing, the structure should be further optimized by controlling the parameters of the NW heterostructure to reduce the loss and increase the gain.
can be observed at temperatures up to 240 K. We can see that with increasing the temperature, the modes exhibit a slight red shift. From 80 to 240 K, the red shift of the modes is about 20 nm, which is much smaller than that of the band edge for InGaAs (about 40 nm for In0.15Ga0.85As).21 The high temperature-stability of the modes is probably due to a small change of refractive index of NW cavity with the variation of temperature. GaAs/InGaAs/GaAs NW core-multishell QW heterostructures with different QW thickness were also fabricated and studied. Fig. 6 shows the 80 K PL spectra of several samples with QW thickness of 2.5, 5, and 7.5 nm, respectively. The resonant modes can be observed in all the samples. However, when the QW thickness decreases, the QW emission becomes weaker, resulting in an attenuation of mode intensity and decrease of mode number. The wavelength of the main mode decreases from 880 nm to 835 nm when the QW thickness decreases from 7.5 to 2.5 nm, which is an obvious evidence of quantum confinement effect. The quantum confinement effect provides a simple way to controlling the mode wavelength and finally achieving wavelength-tunable nanolasers.
4. Summary In conclusion, we have fabricated GaAs/InGaAs/GaAs NW core-multishell heterostructures 8
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Fig. 5 illustrates the temperature-dependent PL spectra of the InGaAs QW. The F-P modes
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DOI: 10.1039/C4NR05486E with a strained radial In0.2Ga0.8As QW by MOCVD. The structure exhibits dislocation-free
phase-pure ZB structure. F-P cavity modes are clearly observed from the PL spectra of QW, suggesting a decoupling of the gain medium and resonant cavity that the QW provides the gain while the NW acts as the cavity. The modes can be observed at temperatures up to 240 K,
quantum confinement. The results make the GaAs/InGaAs NW/radial QW heterostructure a promising candidate for wavelength-tunable near-infrared nanolasers.
Acknowledgments This work was supported by the National Basic Research Program of China (2010CB327600), the National Natural Science Foundation of China (61020106007 and 61376019), the Natural Science Foundation of Beijing (4142038), the Specialized Research Fund for the Doctoral Program of Higher Education (20120005110011), and the 111 Program of China (B07005).
References 1 W. Lu and C. M. Lieber, Nat. Mater., 2007, 6, 841-50. 2 R. Yan, D. Gargas and P. Yang, Nat. Photonics, 2009, 3, 569. 3 Y. Li, F. Qian, J. Xiang and C. M. Lieber, Mater. Today, 2006, 9, 18. 4 X. Duan, Y. Huang, R .Agarwal and C. M. Lieber, Nature, 2003, 421, 241. 5 B. Hua, J. Motohisa, Y. Ding, S. Hara and T. Fukui, Appl. Phys. Lett., 2007, 91, 131112. 6 Y. Ding, J. Motohisa, B. Hua, S. Hara and T. Fukui, Nano Lett., 2007, 7, 3598-602. 9
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and the mode wavelength decreases with decreasing the QW thickness due to enhanced
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DOI: 10.1039/C4NR05486E 7 M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo and P. Yang, Science,
2001, 292, 1897. 8 Q. Li, J. B. Wright, W. W. Chow, T. S. Luk, I. Brener, L. F. Lester and G. T. Wang, Opt. Express, 2012, 20, 17873.
10 D. Saxena, S. Mokkapati, P. Parkinson, N. Jiang, Q. Gao, H. H. Tan and C. Jagadish, Nat. Photonics, 2013, 7, 963. 11 B. Mayer, D. Rudolph, J. Schnell, S. Morkötter, J. Winner, J. Treu, K. Müller, G. Bracher, G. Abstreiter, G. Koblmüller and J. J. Finley, Nat. Commun., 2013, 4, 2931. 12 F. Qian, Y. Li, S. Gradečak, H. Park, Y. Dong, Y. Ding, Z. L. Wang and C. M. Lieber, Nat. Mater., 2008, 7, 701. 13 P. Mohan, J. Motohisa and T. Fukui, Appl. Phys. Lett., 2006, 88, 133105. 14 M. Fickenscher, T. Shi, H. E. Jackson, L. M. Smith, J. M. Yarrison-Rice, C. Zheng, P. Miller, J. Etheridge, B. M. Wong, Q. Gao, S. Deshpande, H. H. Tan and C. Jagadish, Nano Lett., 2013, 13, 1016. 15 M. Moewe, L. C. Chuang, S. Crankshaw, K. W. Ng and C. Chang-Hasnain, Opt. Express, 2009, 17, 7831. 16 B. A. Wacaser, K. Deppert, L. S. Karlsson, L. Samuelson and W. Serfert, J. Cryst. Growth, 2006, 287, 504. 17 N. Sköld, J. B. Wagner, G. Karlsson, T. Hernán, W. Seifert, M. Pistol and L. Samuelson, Nano Lett., 2006, 6, 2743. 18 G. Zhang, K. Tateno, T. Sogawa and H. Nakano, Appl. Phys. Express, 2008, 1, 064003.
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9 R. Agarwal, C. J. Barrelet and C. M. Lieber, Nano Lett., 2005, 5, 917.
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10.1039/C4NR05486E 19 I. Regolin, D. Sudfeld, S. Lüttjohann, V. Khorenko, W. Prost, J. Kästner, G. Dumpich, C. DOI: Meier, A.
Lorke and F. –J. Tegude, J. Cryst. Growth, 2007, 298, 607. 20 M. Paladugu, J. Zou, Y. Guo, X. Zhang, Y. Kim, H. J. Joyce, Q. Gao, H. H. Tan and C. Jagadish, Appl. Phys. Lett., 2008, 93, 101911.
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21 Z. Hang, D. Yan and F. H. Pollak, Phys. Rev. B, 1991, 44, 10546.
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Figure Captions
DOI: 10.1039/C4NR05486E
Fig. 1 (a) Schematic of the GaAs/InGaAs/GaAs NW core-multishell QW heterostructure on GaAs substrate. (b) TEM image of a single NW. (c) Cross-sectional TEM image as well as
shows the corresponding FFT of the structure. The scale bars in (b), (c), and (d) are 1 µm, 100 nm, and 10 nm, respectively. Fig. 2 (a) PL spectra of a single GaAs/InGaAs/GaAs NW core-multishell QW heterostructure at 80 K. The arrows indicate the F-P cavity modes. Inset 1 shows the microscopy image of a single NW used for optical measurement. The scale bar is 10 µm. Inset 2 shows the peak of GaAs NW. (b) Schematic of the decoupling of gain medium and optical cavity in the structure. Fig. 3 (a) Schematic of the simulation structure. (b)-(d) Electric field of the structure computed by a 3D-FDTD simulation. (b), (c) and (d) show the HE11x , HE11y and TE01 mode, respectively. Fig. 4 80 K PL spectra of the InGaAs QW with different excitation power densities. Fig. 5 (a) Temperature-dependent PL spectra of the InGaAs QW. (b) Mode wavelengths of of the QW at different temperatures. Fig. 6 80 K PL spectra of the GaAs/InGaAs/GaAs core-multishell QW heterostructure with different QW thickness. The arrows indicate the main mode.
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the EDS of a slice of NW. (d) Cross-sectional HRTEM image of the radial QW. The inset
Figure 1.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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This article can be cited before page numbers have been issued, to do this please use: X. Yan, X. Zhang, J. Li, Y. Wu, J. Cui and X. Ren, Nanoscale, 2014, DOI: 10.1039/C4NR05486E.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
www.rsc.org/nanoscale
Page 1 of 18
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DOI: 10.1039/C4NR05486E
Fabrication and optical properties of GaAs/InGaAs/GaAs nanowire core-multishell quantum well heterostructures Xin Yan, Xia Zhang, * Junshuai Li, Yao Wu, Jiangong Cui, and Xiaomin Ren
Abstract GaAs/InGaAs/GaAs nanowire core-multishell heterostructure with a strained radial In0.2Ga0.8As quantum well was fabricated by metal organic chemical vapor deposition. The quantum well exhibits dislocation-free phase-pure zinc-blende structure. Low-temperature photoluminescence spectra of a single nanowire exhibit distinct resonant peaks in the range from 880 to 1000 nm, corresponding to the longitudinal modes of a Fabry-Pérot cavity. This suggests a decoupling of the gain medium and resonant cavity that the quantum well provides the gain while the nanowire acts as the cavity. The resonant modes were observed at temperatures up to 240 K, exhibiting high power- and temperature-stability. The modes were blueshifted with decreasing the quantum well thickness due to enhanced quantum confinement. The results make the GaAs-based nanowire/quantum well hybrid structure promising for wavelength-tunable near-infrared nanolasers.
Corresponding Author:
[email protected]
Keywords：Nanowire, quantum well, Fabry-Pérot, heterostructure
1
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State Key Laboratory of Information Photonics and Optical Communications, Beijing University of Posts and Telecommunications, Beijing 100876, China
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1. Introduction
DOI: 10.1039/C4NR05486E
Semiconductor nanowires (NWs) have been considered as excellent building blocks for future electronic and photonic devices due to their unique geometry and superior physical
Fabry-Pérot (F-P) microcavity which provides good optical feedback for lasing.4-6 Up to date, NW lasers have been widely reported in homogeneous materials, involving ZnO, GaN, and CdS, et al.7-9 Most recently, room-temperature lasing was observed in surface-passivated GaAs NWs.10,11 The NW lasers abovementioned have a common feature that the NW acts as both the gain medium and optical cavity. The emission can be reabsorbed during its propagation in the NW cavity, resulting in a degradation of laser performance. A structure that decouples the gain medium and optical cavity, e. g., a NW radial quantum well (QW), can avoid the reabsorption as most of the emission from the gain medium propagate in the NW core which has a larger band gap than the QW. The NW QW structure also possesses other advantages such as a stronger confinement of electrons and holes, as well as tunable emission wavelength with independent optimization of the cavity. So far, NW radial QW structures have been realized in a lot of materials, including GaN/InGaN, InP/InAs, and AlGaAs/GaAs, et al.12-14 Particularly, GaN/InGaN multi-quantum-well structure has been reported to yield lasing at room temperature with tunable wavelength, showing a bright prospect of this type of structures in nanolasers.12 GaAs/InGaAs/GaAs NW radial QW heterostructure is promising in infrared optoelectronic devices. However, the structure has been rarely reported.15 Particularly, F-P resonant modes or lasing have not been achieved in this structure up to date. In this work, we report on the 2
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properties.1-3 Single NWs are particularly promising for nanolasers as they can function as a
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DOI: 10.1039/C4NR05486E F-P resonant modes in GaAs/InGaAs/GaAs NW core-multishell QW heterostructures
fabricated by Au-catalyzed metal organic chemical vapor deposition (MOCVD) for the first time. Cross-sectional transmission electron microscopy (TEM) characterization demonstrates that the InGaAs QW exhibits dislocation-free phase-pure zinc-blende (ZB) structure.
range, corresponding to the longitudinal modes of an F-P cavity. This suggests a decoupling of the gain medium and resonant cavity that the QW provides the gain while the NW acts as the cavity. The resonant modes can be distinguished at temperatures up to 240 K, exhibiting high power- and temperature-stability. A blueshift of resonant modes with decreasing the QW thickness is observed, corresponding to an enhanced quantum confinement.
2. Experimental Details The structure was grown by using a Thomas Swan CCS-MOCVD system at a pressure of 100 Torr. Trimethylgallium (TMGa), trimethylindium (TMIn) and arsine (AsH3) were used as precursors. The carrier was hydrogen. An Au film with a thickness of 4 nm was deposited on GaAs (111) B substrate by magnetron sputtering. The substrate was then loaded into the MOCVD reactor and annealed at 650 oC under AsH3 ambient for 300 s to form Au-Ga alloyed particles as catalyst. The metal-catalyst nanoparticle can enhance the optical feedback in the cavity by increasing the facet reflectivity.10 GaAs NWs were grown at 440 oC for 600 s. After raising the temperature to 500 oC, a thin InGaAs shell was radially grown on the GaAs NW for 100 s. The flow rates of TMGa and TMIn are 5.40 and 1.35 µmol min-1, respectively, resulting in an In content of 20%. Then a GaAs shell was radially grown on the InGaAs shell 3
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Photoluminescence spectra of a single NW exhibit distinct resonant peaks in the infrared
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10.1039/C4NR05486E at 500 oC for 300 s. In the core-multishell structure, the InGaAs shell acts as theDOI: QW while
the GaAs core and shell function as the barrier. The morphological and structural characteristics of the NWs were characterized by TEM equipped with X-ray energy dispersive spectroscopy (EDS). Individual NWs for TEM
spreading drops from the suspension onto a holey carbon/Cu grid. For cross-section TEM studies, NW array was first embedded into an epoxy resin. Embedded NWs were cut perpendicular to the NW axis into 50-200-nm-thick slices using a diamond ultramicrotome knife and then transferred onto TEM grids. For optical characterization, as-grown NWs were mechanically
cut
down
and
dispersed
onto
a
SiO2-coated
Si
substrate.
The
microphotoluminescence (µPL) measurements were carried out using a 532 nm continuous-wave diode-pumped solid-state (DPSS) laser for excitation. The excitation beam was focused onto ~2 µm in diameter with a ×50 microscope objective on the sample placed in a cryostat. The emission through the same microscope objective was detected by the combination of a grating spectrometer and a silicon charge-coupled device (CCD).
3. Results and discussions 3.1. Morphological and structural characterization
A schematic of the structure is shown in Fig. 1a. Fig. 1b shows the TEM image of a single GaAs/InGaAs/GaAs NW core-multishell QW heterostructure. The NW has a length of 7 µm and a uniform diameter of 300 nm along the NW axis. Fig. 1c shows the cross-sectional TEM image as well as the EDS line scan result of a slice of the NW. The QW can be clearly 4
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observations were prepared by ultrasonicating the samples in ethanol for 5 min, followed by
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10.1039/C4NR05486E distinguished between the core and shell due to different brightness intensity. TheDOI: GaAs core
has a hexagonal cross-section and {112} side facets, which are commonly observed in Au-catalyzed MOCVD-grown NWs.16 The GaAs shell exhibits an inequilateral enneagon cross-section. Six main sides of the shell have a 30o rotation compared with the core,
also observed in GaAs/AlInP and GaAs/GaP NW core-shell heterostructures, indicating that {110} facets are energetically favorable at high growth temperature.17,18 From the intensity profiles of In and Ga elements we can determine that the QW has an In composition of about 20%. The intensity profile of In element exhibits a sharp increase in the QW area, suggesting an abrupt interface between the QW and neighboring GaAs barriers. This is an obvious advantage over the Au-catalyzed GaAs/In(Ga)As/GaAs NW axial heterostructures in which the heterojunction is usually not sharp due to the memory effect of group III elements in the Au droplet.19,20 Fig. 1d shows the high-resolution TEM (HRTEM) image of the QW. The QW has a uniform thickness of about 10 nm. No dislocations are observed in the QW or the GaAs barriers, indicating that the QW is fully strained. The corresponding fast Fourier transform (FFT) clearly shows two sets of diffraction spots, corresponding to the InGaAs QW and GaAs NW, respectively. The HRTEM and FFT also demonstrate that both the QW and NW exhibit pure ZB structure.
3.2 Optical Characterization
Fig. 2a shows the PL spectra of a single GaAs/InGaAs/GaAs NW core-multishell QW heterostructure at 80 K. A strong emission peak is observed at 985 nm, corresponding to the 5
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suggesting {110} type facets. The transformation from {112} to {110} at high temperature is
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10.1039/C4NR05486E InGaAs QW. The QW emission intensity is nearly 40 times higher than the DOI: GaAs NW,
indicating that most of the carriers are confined to the QW region. A series of periodic peaks are observed in the spectra ranging from 884 to 986 nm, which are indicative of the longitudinal modes of F-P cavity formed in the NW. For a NW F-P cavity, the mode spacing
the group refractive index.13 From inset 1, the length of the NW can be determined to be 7 µm. The group refractive index of GaAs is calculated to be 4.7-4.2 in the mode wavelength range of 884-985 nm.13 Thus the mode spacing can be calculated to be 11.9-16.7 µm, which is in agreement with the measured mode spacing 12.6-16.3 nm. In addition, as the QW has a rather small thickness, it is difficult for the QW to confine the light and support F-P modes. A reasonable explanation is that most of the emission from the QW is coupled into the GaAs core which acts as an F-P microcavity and generates resonant modes, as shown in Fig. 2b. Modal characteristics of the structure were also investigated by 3D FDTD simulations. Fig. 3a shows the schematic of the simulated structure. The radius of GaAs core was set to 100 nm, and the thickness of QW and GaAs shell was set to 10 and 50 nm, respectively. The NW was surrounded by a perfectly matched layer (PML). Several point dipole sources with the emission wavelength of 985 nm were placed at the QW position to model the emission from the QW. The simulations present three dominated modes in the structure, determined as HE11x, HE11y, and TE01 mode, respectively, as shown in Fig. 3b, c and d. For HE11x and HE11y modes, the intensity is mainly concentrated in the GaAs core. For TE01 mode, there is a substantial intensity overlap between the GaAs core and shell. The simulation results demonstrate a decoupling of gain medium and F-P cavity that the InGaAs QW provides the gain while the 6
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is given by 2 /{2L[n (dn / d )]} , where L is the NW length and n (dn / d ) is
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DOI: 10.1039/C4NR05486E GaAs NW acts as the cavity. This is an obvious advantage compared with homogenous NWs,
e. g. GaAs NWs, as the emission from the InGaAs QW wouldn’t be reabsorbed by the GaAs NW cavity, and the laser wavelength can also be tuned by adjusting the In composition in the QW with independent optimization of the cavity.
densities. We can see that the mode peak position doesn’t change at different excitation power density. For a NW F-P cavity, the mode characteristics follow the formula of m 2nL / , where m is an integer, L is the length of NW cavity, and n is the refractive index. For the NW with a certain length, the mode wavelength is only affected by the refractive index. 4 Thus the high power stability of the modes indicates that the refractive index of GaAs NW almost doesn’t change with the excitation power density. At higher excitation power densities, we can observe six dominated modes, which are centered at 910.0, 924.0, 938.2, 953.2, 968.8, and 985.0 nm, respectively. This suggests that the GaAs NW cavity has a relatively higher gain in the range of 910-985 nm. The mode with the highest intensity shifts from 985 nm to 910 nm as the excitation power density increases from 16 to 160 kW/cm2, indicating an increase of cavity loss and decrease of gain for long-wavelength modes. The mode Q factors are also calculated by fitting the spectra with Lorentzian curves. Under the excitation power density of 1.6, 16, 80 and 160 kW/cm2, the corresponding highest mode Q factor is 68.6, 70.4, 82.8, and 100.4, respectively. The low Q values can be attributed to several reasons. First, the GaAs NW cavity may have a relatively large loss and low gain in the InGaAs QW emission range, which has been theoretically confirmed by other groups.10 Second, the diameter of the NW heterostructure is relatively small for achieving lasing.10 In 7
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Fig. 4 shows the 80 K PL spectra of the InGaAs QW with different excitation power
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DOI: 10.1039/C4NR05486E addition, the InGaAs shell, which acts as the gain medium, may be not able to offer sufficient
gain due to the small thickness. For achieving lasing, the structure should be further optimized by controlling the parameters of the NW heterostructure to reduce the loss and increase the gain.
can be observed at temperatures up to 240 K. We can see that with increasing the temperature, the modes exhibit a slight red shift. From 80 to 240 K, the red shift of the modes is about 20 nm, which is much smaller than that of the band edge for InGaAs (about 40 nm for In0.15Ga0.85As).21 The high temperature-stability of the modes is probably due to a small change of refractive index of NW cavity with the variation of temperature. GaAs/InGaAs/GaAs NW core-multishell QW heterostructures with different QW thickness were also fabricated and studied. Fig. 6 shows the 80 K PL spectra of several samples with QW thickness of 2.5, 5, and 7.5 nm, respectively. The resonant modes can be observed in all the samples. However, when the QW thickness decreases, the QW emission becomes weaker, resulting in an attenuation of mode intensity and decrease of mode number. The wavelength of the main mode decreases from 880 nm to 835 nm when the QW thickness decreases from 7.5 to 2.5 nm, which is an obvious evidence of quantum confinement effect. The quantum confinement effect provides a simple way to controlling the mode wavelength and finally achieving wavelength-tunable nanolasers.
4. Summary In conclusion, we have fabricated GaAs/InGaAs/GaAs NW core-multishell heterostructures 8
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Fig. 5 illustrates the temperature-dependent PL spectra of the InGaAs QW. The F-P modes
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DOI: 10.1039/C4NR05486E with a strained radial In0.2Ga0.8As QW by MOCVD. The structure exhibits dislocation-free
phase-pure ZB structure. F-P cavity modes are clearly observed from the PL spectra of QW, suggesting a decoupling of the gain medium and resonant cavity that the QW provides the gain while the NW acts as the cavity. The modes can be observed at temperatures up to 240 K,
quantum confinement. The results make the GaAs/InGaAs NW/radial QW heterostructure a promising candidate for wavelength-tunable near-infrared nanolasers.
Acknowledgments This work was supported by the National Basic Research Program of China (2010CB327600), the National Natural Science Foundation of China (61020106007 and 61376019), the Natural Science Foundation of Beijing (4142038), the Specialized Research Fund for the Doctoral Program of Higher Education (20120005110011), and the 111 Program of China (B07005).
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and the mode wavelength decreases with decreasing the QW thickness due to enhanced
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9 R. Agarwal, C. J. Barrelet and C. M. Lieber, Nano Lett., 2005, 5, 917.
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21 Z. Hang, D. Yan and F. H. Pollak, Phys. Rev. B, 1991, 44, 10546.
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Figure Captions
DOI: 10.1039/C4NR05486E
Fig. 1 (a) Schematic of the GaAs/InGaAs/GaAs NW core-multishell QW heterostructure on GaAs substrate. (b) TEM image of a single NW. (c) Cross-sectional TEM image as well as
shows the corresponding FFT of the structure. The scale bars in (b), (c), and (d) are 1 µm, 100 nm, and 10 nm, respectively. Fig. 2 (a) PL spectra of a single GaAs/InGaAs/GaAs NW core-multishell QW heterostructure at 80 K. The arrows indicate the F-P cavity modes. Inset 1 shows the microscopy image of a single NW used for optical measurement. The scale bar is 10 µm. Inset 2 shows the peak of GaAs NW. (b) Schematic of the decoupling of gain medium and optical cavity in the structure. Fig. 3 (a) Schematic of the simulation structure. (b)-(d) Electric field of the structure computed by a 3D-FDTD simulation. (b), (c) and (d) show the HE11x , HE11y and TE01 mode, respectively. Fig. 4 80 K PL spectra of the InGaAs QW with different excitation power densities. Fig. 5 (a) Temperature-dependent PL spectra of the InGaAs QW. (b) Mode wavelengths of of the QW at different temperatures. Fig. 6 80 K PL spectra of the GaAs/InGaAs/GaAs core-multishell QW heterostructure with different QW thickness. The arrows indicate the main mode.
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the EDS of a slice of NW. (d) Cross-sectional HRTEM image of the radial QW. The inset
Figure 1.
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Figure 3.
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Figure 4.
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Figure 5.
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Figure 6.
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