Strain Engineered Band Structure and Optical Properties of Confined

Feb 28, 2017 - Hunan Key Laboratory for Super-micro structure and Ultrafast Process, School of Physics and Electronics, Central South University, 932 ...
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Strain Engineered Band Structure and Optical Properties of Confined GaAs Quantum Dots Cailei Yuan,*,† Yaxing Mei,† Aijun Hong,† Ting Yu,† Yong Yang,† Fanyan Zeng,† Keng Xu,† Qinliang Li,† Xingfang Luo,*,† Jun He,‡ and Wen Lei§ †

Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Communication and Electronics, Jiangxi Normal University, 99 Ziyang Avenue, Nanchang 330022, Jiangxi, China ‡ Hunan Key Laboratory for Super-micro structure and Ultrafast Process, School of Physics and Electronics, Central South University, 932 South Lushan Road, Changsha 410083, Hunan, China § School of Electrical, Electronic and Computer Engineering, The University of Western Australia, 35 Stirling Highway, Crawley 6009, Australia ABSTRACT: Understanding the physics that correlate strain and physical properties of quantum dots (QDs) is crucial for technology applications. In this paper, GaAs QDs confined in Al2O3 matrix are synthesized using the pulsed laser deposition method and rapid thermal annealing technique. It is revealed that the confined GaAs QDs experience compressive strain during the growth process. The strain can be used to improve and tailor the optical properties of confined GaAs QDs by engineering the bandgap and thus the photoluminescence emission band to a distinct wavelength. These findings presented here can engineer the properties of GaAs QDs for potential application in optoelectronic and photonic devices.

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systems through tuning external strain fields is not only interesting for fundamental physics but also crucial for device applications. Embedding QDs in solid state matrix is one way for practical device applications, which provides an ideal platform for engineering the strain in QDs.15 In this work, GaAs QDs confined in Al2O3 matrix are chosen as a test vehicle to investigate the impact of strain on the optical properties of QDs. In comparison to other common QD material systems studied, such as InAs/GaAs QDs in which the strain is mainly determined by the lattice mismatch,16,17 GaAs QDs confined in amorphous Al2O3 matrix allow more flexible strain control on QDs by changing the rapid thermal annealing (RTA) parameters, thus providing a useful approach to implement strain engineering on QDs.18 Here, GaAs QDs confined in Al2O3 matrix show a large blue-shift (∼800 meV) of their PL peak, which is far more than that (∼50 meV) caused by the size confinement. This demonstrates that QDs confined in amorphous matrix provide a valid and prominent approach to engineer the optical properties of QDs, which might lead to new device applications.

INTRODUCTION In the past decade or two, low dimensional semiconductor structures such as quantum dots (QDs) have attracted significant attention due to their interesting fundamental physical properties as well as potential applications in electronic and optoelectronic devices.1−3 These QD structures have a size around a few or a few tens of nanometers, and thus exhibit strong size confinement, leading to delta-function-like density of states, and thus optical and optoelectronic devices with enhanced performance.4,5 For example, QD lasers present lower threshold voltage, higher differential gain, and higher operating temperature compared with lasers made from bulk and quantum well structures.6,7 For the ultimate device applications of these QDs, their physical properties must be well controlled and engineered, including their electrical and optical properties. A common approach to engineer the physical properties of QDs is to engineer the size and composition of QDs and thus their physical properties.1 However, apart from size and composition, strain in the QDs plays a significant role in determining their physical properties.8 Recently, researchers already demonstrated the modulated bandgap, electrical, and optical properties by external strain in many semiconductor nanostructures such as (In, Ga) As/GaP QDs,9 InAs/GaAs QDs,10 InGaAs QDs,11 InAsP QDs in InP nanowire,12 InP nanocrystals,13 GaAs/GaP core−shell nanowires,14 etc. Therefore, it will be a great opportunity to tune GaAs QDs to exhibit desired properties using external strain. Understanding how bandgaps are modified in GaAs QDs © XXXX American Chemical Society

Received: December 7, 2016 Revised: February 23, 2017 Published: February 28, 2017 A

DOI: 10.1021/acs.jpcc.6b12343 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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EXPERIMENT GaAs QDs confined in Al2O3 matrix were synthesized by pulsed laser deposition (PLD) and RTA methods. A KrF pulsed laser beam of 248 nm wavelength with a frequency of 10 Hz was used to ablate the target in an ultrahigh vacuum chamber. A high-purity (99.99%) Al2O3 round target (∼40 mm in diameter) and high-purity (99.99%) single crystal GaAs square target (∼8 mm in length) were used as the targets for laser ablating. The GaAs/Al2O3 film was deposited on Si substrate at room temperature in a high vacuum system with a background pressure of about 1.5 × 10−7 Torr. The thickness of GaAs/ Al2O3 film was about 300 nm. After deposition, GaAs/Al2O3 thin films were subjected to a RTA process at 600 °C for 300 s with nitrogen ambient protection. The structural properties of the GaAs/Al2O3 films deposited were investigated using transmission electron microscopy (TEM) with a JEOL 2010 microscope. The TEM electron diffraction pattern was matched against a simulated diffraction pattern generated by Java Electron Microscopy Simulation (JEMS) TEM simulating software.19 X-ray photoelectron spectroscopy (XPS) measurements were carried out with a Kratos XSAM800 spectrometer employing Al Kα radiation. The optical properties of GaAs/ Al2O3 thin films were investigated with photoluminescence (PL) and Raman spectroscopies at room temperature with a HORIBA LabRAM HR Evolution, using an excitation wavelength of 488 and 514 nm, respectively.

grown in Al2O3 matrix. Obviously, a large number of GaAs QDs confined in Al2O3 matrix are formed in the sample. Figure 1d presents the TEM image of a single GaAs QD grown in Al2O3 matrix. As observed, the Al2O3 matrix still remains an amorphous phase after annealing, while crystalline GaAs QD with spherical shape is formed and confined in Al2O3 matrix. The average size of GaAs QDs is around 10 nm. Figure 1e shows the electron diffraction pattern of GaAs QDs. The diffraction pattern is matched against a simulated diffraction pattern generated by JEMS software. Combining the experimental diffraction patterns with the simulated ones, it can be deduced that GaAs QDs have a cubic crystal structure (space group F4̅3m), which is consistent with the crystal structure of GaAs bulk. As observed in Figure 1e, for the GaAs QDs grown in Al2O3 matrix, the lattice parameters and unit cell volume are about a = b = c = 5.508 and 167.1 Å3, respectively. In contrast, for the GaAs bulk, the lattice parameters and unit cell volume are about a = b = c = 5.654 and 180.75 Å3, respectively.20 The relative volume difference (VQDs − Vbulk)/ Vbulk is about −7.6%. Obviously, the bond lengths and volume of unit cell for GaAs QDs confined in Al2O3 matrix are smaller than those of GaAs bulk, which could be attributed to the compressive strain existing on the GaAs QDs confined in Al2O3 matrix. Because of the thermal expansion mismatch between GaAs and Al2O3, the formation and growth of GaAs QDs in Al2O3 matrix through PLD and RTA processes are accompanied by the generation and accumulation of strain. During the growth process, the Al2O3 matrix exerts compressive strain on the GaAs QDs due to the volume expansion of confined GaAs QDs with respect to the Al2O3 matrix.18,19 Moreover, in the case of hydrostatic compression, the energy bandgap shift of semiconductor is ΔEg = αΔV/V, where α is the deformation potential of the semiconductor and ΔV/V is the relative change in volume of the semiconductor.21 Using the bulk GaAs deformation potential (−8.33 eV)22 and the relative volume change between GaAs QDS and bulk, it can be calculated that the energy bandgap shift of strain GaAs QDs is about 633 meV. To study the surface chemical composition and stoichiometry of the GaAs QDs confined in Al2O3 matrix, XPS analysis was employed. Figure 2 shows the XPS spectra of the Ga (3d), As (3d), Al (2p), and O (1s) peaks of the GaAs QDs and Al2O3 matrix. The positions of the Ga 3d and As 3d peaks are at 19.4 and 41.4 eV, respectively, which is in agreement with the corresponding values for Ga 3d and As 3d in GaAs.23 The binding energy of Al 2p is at 74.2 eV, and the binding energy of O 1s shows that the Al−O bond is at about 530.5 eV without any shift.24 The results indicate that GaAs QDs are formed in the insulating layer without interacting with the Al2O3 matrix, which is consistent with TEM observation. To further confirm the large strain existing in the GaAs QDs, Raman spectroscopy was performed on these GaAs QDs confined in Al2O3 matrix. Raman spectroscopy is a powerful tool for the characterization of nanostructures, which can provide valuable information on the microstrain and confinement effect in nanostructures.25,26 Recently, it has been shown in strained GaAs/GaP core−shell nanowires that it is possible to gain a consistent picture by directly measuring the strain in each layer using Raman scattering on a single nanowire.4 Figure 3a displays room temperature Raman spectra of GaAs bulk and GaAs QDs confined in the Al2O3 matrix excited at a wavelength of 518 nm, respectively. As can be seen, the Raman spectra of the GaAs bulk have two peaks, which are located at 268 and at 292 cm−1 and can be assigned to its transverse optical (TO)

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RESULTS AND DISCUSSION Figure 1a is the planar HRTEM image of the as-deposited film with its corresponding electron diffraction pattern shown in

Figure 1. (a) HRTEM image of as-deposited film; (b) electron diffraction pattern of as-deposited film; (c) TEM image of GaAs QDs grown in Al2O3 matrix; (d) TEM image of a single GaAs QD grown in Al2O3 matrix; (e) electron diffraction pattern of GaAs QDs grown in Al2O3 matrix.

Figure 1b. Obviously, it shows a void of QDs, which is due to the fact that GaAs QDs are dispersed in atomic states in the Al2O3 matrix and all of the GaAs QDs are not consumed or nucleated at room temperature. It confirms that the Al2O3 thin film remains amorphous. Then, the as-deposited film was subjected to postannealing at 600 °C for 300 s in N2 ambient. Figure 1c shows the TEM image of the synthesized GaAs QDs B

DOI: 10.1021/acs.jpcc.6b12343 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 2. XPS spectra of Ga (3d), As (3d), Al (2p), and O (1s) electrons from GaAs QDs and Al2O3 matrix.

0).28−31 The positive value of Δω means that the GaAs QDs suffer large compressive strain, and the compressive strain effect dominates the phonon confinement in these confined GaAs QDs. Therefore, the Raman measurements clearly demonstrate that the GaAs QDs grown in Al2O3 matrix are under significant compressive strain, which is consistent with the TEM observation and simulation results. Parts b and c of Figure 3 present the Raman intensity maps of the TO and LO phonon peaks over the whole thin film with confined GaAs QDs. These two figures show that the intensity distributions are uniform in two different phonon peaks. As the TO and LO phonon peak positions can be influenced by the strain effect and confinement effect, it indicates GaAs QDs with homogeneously distributed size and strain are formed in the Al2O3 matrix during the growth process. Figure 4a shows the room temperature PL spectra measured for the referenced amorphous Al2O3 thin film and the thin film with strained GaAs QDs confined in amorphous Al2O3 film excited at a wavelength of 488 nm. Obviously, no PL peaks can be observed in the referenced amorphous Al2O3 film. In contrast, a PL band centered at about 2.2 eV can be clearly observed. Therefore, it can be deduced that the observed PL peak is associated with the GaAs nanoparticles. However, the bandgap of unstrained GaAs bulk at room temperature is only 1.4 eV.32 This indicates that a blue shift of 800 meV occurs for the strained GaAs QDs, which has been the largest blue shift observed for GaAs reported in the literature.33−35 Although the size confinement effect also contributes to the blue shift of the PL peak, the blue shift caused by size confinement is only around 50 meV,36 which is far smaller than the blue shift observed here. Therefore, it can be deduced that the compressive strain dominates the blue-shift of the PL peak observed. Figure 4b presents the PL intensity mapping image of the GaAs/Al2O3 QD sample. It should be noticed that the highlight of the image is homogeneously distributed, indicating the growth of strained GaAs QDs confined in Al2O3 matrix with homogeneous optical properties. This indicates that strain plays a dominant role in determining the optical properties of

Figure 3. (a) Raman spectra of GaAs bulk and GaAs QDs confined in the Al2O3 matrix. Raman intensity maps of (b) TO and (c) LO phonon lines of the thin film with confined GaAs QDs.

phonon and longitudinal optical (LO) phonon,27 respectively. The Raman spectra of the GaAs QDs confined in the Al2O3 matrix also display both TO and LO phonon peaks, which are located at 275 and 299 cm−1, respectively. Compared with those of bulk GaAs, the LO phonon and TO phonon peaks of GaAs QDs confined in Al2O3 matrix present an obvious wavenumber shift (Δω = 7 cm−1). Usually, Δω is induced by the independent and combined influences of the strain effect and confinement effect. It is well-known that the confinement effect can cause a red-shift wavenumber (Δω < 0), while the compressive strain can induce a blue-shift wavenumber (Δω > C

DOI: 10.1021/acs.jpcc.6b12343 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) PL spectra of the referenced amorphous Al2O3 thin film and the thin film with strained GaAs QDs confined in Al2O3 matrix. (b) PL intensity map of thin film with strained GaAs QDs confined in Al2O3 matrix. (2) Kroutvar, M.; Ducommun, Y.; Heiss, D.; Bichler, M.; Schuh, D.; Abstreiter, G.; Finley, J. J. Optically programmable electron spin memory using semiconductor quantum dots. Nature 2004, 432, 81− 84. (3) Dai, X. L.; Zhang, Z. X.; Jin, Y. Z.; Niu, Y.; Cao, H. J.; Liang, X. Y.; Chen, L. W.; Wang, J. P.; Peng, X. G. Solution-processed, highperformance light-emitting diodes based on quantum dots. Nature 2014, 515, 96−99. (4) Nakamura, H.; Nishikawa, S.; Kohmoto, S.; Kanamoto, K.; Asakawa, K. Optical nonlinear properties of InAs quantum dots by means of transient absorption measurements. J. Appl. Phys. 2003, 94, 1184−1189. (5) Ho, J. F.; Tatebayashi, J.; Sergent, S.; Fong, C. F.; Ota, Y.; Iwamoto, S.; Arakawa, Y. A Nanowire-based plasmonic quantum dot laser. Nano Lett. 2016, 16, 2845−2850. (6) Liu, H. Y.; Wang, T.; Jiang, Q.; Hogg, R.; Tutu, F.; Pozzi, F.; Seeds, A. Long-wavelength InAs/GaAs quantum-dot laser diode monolithically grown on Ge substrate. Nat. Photonics 2011, 5, 416− 419. (7) Chen, S. M.; Li, W.; Wu, J.; Jiang, Q.; Tang, M. C.; Shutts, S.; Elliott, S. N.; Sobiesierski, A.; Seeds, A. J.; Ross, I.; et al. Electrically pumped continuous-wave III-V quantum dot lasers on silicon. Nat. Photonics 2016, 10, 307−311. (8) Stepanov, P.; Elzo-Aizarna, M.; Bleuse, J.; Malik, N. S.; Cure, Y.; Gautier, E.; Favre-Nicolin, V.; Gerard, J. M.; Claudon, J. Large and uniform optical emission shifts in quantum dots strained along their growth axis. Nano Lett. 2016, 16, 3215−3220. (9) Robert, C.; Nestoklon, M. O.; da Silva, K. P.; Pedesseau, L.; Cornet, C.; Alonso, M. I.; Goni, A. R.; Turban, P.; Jancu, J. M.; Even, J.; et al. Strain-induced fundamental optical transition in (In,Ga) As/ GaP quantum dots. Appl. Phys. Lett. 2014, 104, 011908. (10) Laghumavarapu, R. B.; El-Emawy, M.; Nuntawong, N.; Moscho, A.; Lester, L. F.; Huffaker, D. L. Improved device performance of InAs/GaAs quantum dot solar cells with GaP strain compensation layers. Appl. Phys. Lett. 2007, 91, 243115. (11) Zhang, J. X.; Wildmann, J. S.; Ding, F.; Trotta, R.; Huo, Y. H.; Zallo, E.; Huber, D.; Rastelli, A.; Schmidt, O. G. High yield and ultrafast sources of electrically triggered entangled-photon pairs based on strain-tunable quantum dots. Nat. Commun. 2015, 6, 10067. (12) Bouwes Bavinck, M. B.; Zieliński, M.; Witek, B. J.; Zehender, T.; Bakkers, E. P. A. M.; Zwiller, V. Controlling a nanowire quantum dot band gap using a straining dielectric envelope. Nano Lett. 2012, 12, 6206−6211. (13) Díaz, J. G.; Bryant, G. W. Theory of InP nanocrystals under pressure. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 245433. (14) Montazeri, M.; Fickenscher, M.; Smith, L. M.; Jackson, H. E.; Yarrison-Rice, J.; Kang, J. H.; Gao, Q.; Tan, H. H.; Jagadish, C.; Guo,

GaAs/Al2O3 QDs, and thus provides an effective approach to engineer the physical properties of QDs.

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CONCLUSIONS In summary, GaAs QDs confined in Al2O3 matrix are synthesized using the PLD method and RTA technique. The HRTEM analyses clearly reveal that GaAs QDs confined in Al2O3 matrix experience compressive strain during the growth process, which can modify the atomic structure of GaAs QDs and lead to the bandgap shift of the strained GaAs QDs. XPS analyses also indicate the formation of GaAs QDs without interacting with Al2O3 matrix. It is demonstrated that the strain can be used to improve and tailor the optical properties of confined GaAs QDs such as bandgap and other optical properties. The 800 meV blue-shift of the PL peak observed indicates strain provides effective guidelines to tune the bandgap of GaAs QDs and thereby engineer their properties for potential application in optoelectronic and photonic devices.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: xfl[email protected]. ORCID

Cailei Yuan: 0000-0002-8088-0313 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant Nos. 51461019, 51661012, 51361013, 51561012, and 61664005), the Project for Young Scientist Training of Jiangxi Province (Grant No. 20153BCB23016), the Natural Science Foundation of Jiangxi Province (Grant Nos. 20151BAB202004 and 20161BAB201022) and Australian Research Council (Grant Nos. FT130101708, DP170104562, and LE170100233).

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DOI: 10.1021/acs.jpcc.6b12343 J. Phys. Chem. C XXXX, XXX, XXX−XXX