GaAs Nanowires

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Study of Polarization Effect in InAs Quantum Dots/GaAs Nanowires Feng Cheng, Bang Li, Luying Li, Xi Wang, Shaoli Shen, Weijie Liu, He Zheng, Shuangfeng Jia, Xin Yan, Xia Zhang, Jianbo Wang, and Yihua Gao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11425 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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The Journal of Physical Chemistry

Study of Polarization Effect in InAs Quantum Dots/GaAs Nanowires Feng Cheng1, Bang Li2, Luying Li1*, Xi Wang1, Shaoli Shen1, Weijie Liu1, He Zheng3, Shuangfeng Jia3, Xin Yan2*, Xia Zhang2, Jianbo Wang3, and Yihua Gao1 1Center

for Nanoscale Characterization & Devices, Wuhan National Laboratory for

Optoelectronics and School of Physics, Huazhong University of Science and Technology, Luoyu Road 1037, Wuhan, 430074, China. 2State

Key Laboratory of Information Photonics and Optical Communications, Beijing

University of Posts and Telecommunications, Beijing 100876, China. 3School

of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of

Artificial Micro- and Nano-Structures and the Institute for Advanced Studies, Wuhan University, Wuhan, 430072, China.

ABSTRACT: The combinations of zero-dimensional quantum dots (QDs) and one-dimensional nanowires (NWs) are of great interest due to their unique performances. However, no research interpreted the performances down to the microstructure related mechanism. Here, we report a polarization effect in InAs QDs decorating GaAs NWs according to a quantitative electrostatic

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analysis at nanometer scale via off-axis electron holography on electrostatic potentials and charge densities. According to the charge distributions across the InAs QDs/GaAs NW interface, the InAs QDs tend to be n-type with a large number of free electrons at the QDs’ apexes, while the GaAs NW as substrate should have holes accumulations in the core region to allow an overall charge neutralization, which leads to a radial polarization field. This polarization effect along the radial axis of the hybrid system changes the original band structure, providing more chances for electron-hole recombinations in InAs QDs, which explains well the enhanced photoluminescence property with the introduction of InAs QDs to the GaAs NW surface.

INTRODUCTION With the development of low-dimensional semiconductors, semiconductor nanowires have drawn more and more attentions as building blocks for future optoelectronic devices such as functional nanoelectronics, batteries, singe-molecule sensors, solar cells and field effect transistors.1-4 The special geometry of one-dimensional semiconductors makes them ideal candidate for fabricating various heterostructures, e.g. core-shell NWs and quantum dots (QDs)NW heterostructures,5-8 with improved functionalities. QDs as zero-dimensional semiconductors have been successfully applied to planar optoelectronic devices, such as low threshold current QDs lasers, due to its capability of strong confinement of electrons and photons.9,10 In recent years, the hybrid structures combining QDs with NWs have attracted more attention, among which QDs-on-NW and QDs-in-NW are the general model systems showing bright prospects in many optoelectronic devices. Compared with the QDs-in-NW structure,11,12 the QDs-on-NW structure is expected to have much higher gain and broader line width.13,14 The QDs-on-NW


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structure has been realized in various materials especially III-V semiconductors for high charge carrier mobility, direct band gap and wide range of band gap engineering,15,16 including InAs QDs on InP NW,17 MnAs QDs on InAs NW,18 and InAs QDs on GaAs NW,19 to name a few. It is reported that Ge QDs embedded in planar Si substrate and Ge/Si core/shell NWs show hole accumulations in the Ge regions due to type-II band alignment at the Ge-Si heterointerfaces;20,21 the individual GaN QDs in GaN/AlN multilayer superlattices exhibit large phase shifts, which are attributed to polarization fields and the accumulation of charges at GaN/AlN interfaces.22 For QDs-on-NW scheme, the heterostructures formed by QDs base and NW surface would lead to redistribution of charges, and variable electrostatic fields across the heterostructures due to interfacial band offsets and possible stress induced band bending effect.23 The enhanced light emission efficiency of QDs-on-NW heterostructures as compared to bare nanowires,6,13,17 and the freedom of QDs-on-NWs to be integrated into nanoscale optoelectronic devices necessitate the investigation on further improvement of their electric properties at nanometer scale. In this study, the nanometer scale electrostatic potentials and charge distributions across the interfacial region in our successfully synthesized InAs QDs decorating GaAs NWs are quantitatively characterized by off-axis electron holography. A polarization effect is discovered in this hybrid structure, which changes the original band structure, makes the InAs QDs to act as gain medium, and at last enhances the photoluminescence property.

EXPERIMENTS AND METHODS



Preparation of InAs QDs/GaAs NW heterostructure. The -orientated GaAs NWs were grown epitaxially on GaAs (111) B substrate following the vapor-liquid-solid (VLS) growth mode using a Thomas Swan CCS- metal organic chemical vapor deposition (MOCVD) system at a pressure of 100 Torr, and the growth of InAs QDs followed the Stranski-Krastanov (S-K) growth mode. The carrier gas was hydrogen and the precursors were trimethylgallium (TMGa), trimethylindium (TMIn), and arsine. The steps of growth are listed below: (1) the Au/Ga alloy catalyst was formed by depositing an Au film with thickness of 4 nm on GaAs (111) B substrate, which was then loaded into the MOCVD reactor at 650 °C for 300 s; (2) the GaAs NWs were grown at 470 °C for 500 s, and TMGa was switched off after growth while the flow rate of AsH3 was kept constant at 2.87×10-3 mol min-1. (3) After increasing the temperature to 505 °C, the growth of InAs QDs was initiated for 90 s with a TMIn flow rate of 11.3 μmol min-1 and an AsH3 flow rate of 71.8 μmol min-1. Transmission electron microscopy (TEM) sample preparation. The NWs were removed from the substrate through sonication in alcohol, and then deposited on copper grid with holey carbon film for TEM observations. The cross-sectional specimen was prepared by focused ion beam (FIB) using FEI-Quanta-3D-FEG. Characterizations and Measurements. The high-resolution transmission electron microscopy (HRTEM) images, electron energy-dispersive spectroscopy (EDS) data, and off-axis electron holograms were obtained using JEM-ARM200CF with double Cs correctors operating at 200 kV. The photoluminescence (PL) measurements were performed using a 632.8 nm continuous-wave He-Ne laser for excitation.


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RESULTS AND DISCUSSION

Figure 1 Schematic diagrams of fabrication, experimental setup, charge distribution and band structure of GaAs NW and InAs QDs/GaAs NW heterostructure. (a) The growth model of InAs QDs/GaAs NW heterostructure. (b) The setup of off-axis electron holography: the object wave that passes through the sample and the reference wave that transmits in vacuum are overlapped by biprism forming interference fringes. Via off-axis electron holography, we can confirm the polarization effect (c) with the introduction of InAs QDs on the GaAs NW surface, and the band structures (d) of GaAs NW and InAs QDs/GaAs NW heterostructure.

Figure 1a shows the formation of NW and QDs following the VLS and S-K growth modes, respectively. The QDs/NW heterostructure would lead to a polarization effect, which can



be quantitatively characterized by off-axis electron holography, a powerful electron-interference technique for quantitative characterization of phase shifts due to electrostatic or magnetic fields, which has been successfully applied to characterization of a wide range of semiconductor heterostructures.24-28 As shown in the setup image of off-axis electron holography (Figure 1b), the object wave passing through the sample contains both amplitude and phase information. By analyzing the phase information, quantification of electrostatic properties, including electrostatic potentials, charge distributions, polarization fields from NW to QDs (Figure 1c) and band structures (Figure 1d), can be realized. Figures 2a-b show bright-field TEM images of InAs QDs/GaAs NW heterostructure, and the GaAs NW as substrate is ~ 5.5 μm in length and ~ 200 nm in width. The HRTEM image of the InAs QDs/GaAs NW heterostructure (Figure 2c) shows that the height of the QD is about 10 nm, and the QD and NW are both of zinc blende structure with good crystal quality. The darker contrasts at the heterostructural interface could be attributed to the lattice mismatch of ~ 7% between InAs QD and GaAs NW,13 and the observable stacking faults in the InAs QDs are signatures of strain relaxation.29,30 The corresponding selected area electron diffraction (SAED) pattern projected along [0 1 1] of the interfacial region (Figure 2d) confirms the NW growth direction of [111]. Two sets of diffraction spots of the same symmetry appear, with the bright ones attributing to GaAs NW and the faint ones to InAs QD. As measured in the SAED pattern, the corresponding lattice mismatch ratio is evaluated to be 6.68% for InAs (InAs = 0.6182 nm) and GaAs (GaAs = 0.5795 nm), leading to the splitting of diffraction spots with the same index. This lattice mismatch ratio is relatively smaller than that of film heterostructure composed of the same materials (lattice mismatch of ~ 7.2%),30 which reflects the better strain relaxation capability of NW and QD morphologies.


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Figure 2 Morphologies, crystal structures and chemical information of InAs QDs/GaAs NW heterostructures. (a) TEM image of GaAs NW with an Au/Ga alloy particle as catalyst on the top. (b) Magnified TEM image of the same NW showing InAs QDs on the NW surface. (c) HRTEM image of InAs QDs/GaAs NW heterostructure. (d) SAED pattern of the interface region projected along [0 1 1] direction. The lower inset is magnified pattern of the region framed in the white box in (d). The brighter spot is from GaAs NW, and the darker spot is from InAs QD. (e) HAADF image of InAs QDs/GaAs NW. The corresponding 2D elemental maps and 1D elemental line profiles of Ga, As, C, In are shown in respective colors. The elemental maps of Ga, As, C and In are presented in respective colors in Figure 2e. While the Ga signals are confined in the NW region, the In signals are mostly distributed within the QDs. The corresponding EDS line profiles of the region labeled by a red line in the HAADF image also show distribution of In signals in the QD, and the content ratio of Ga and As in the GaAs NW region is close to 1:1 as expected. It should be noted that there is also small amount of 7 Environment ACS Paragon Plus


carbon, which appears to be more in InAs QDs than in the GaAs NW. In order to eliminate the influence of the supporting carbon film, the part of the NW far away from the carbon film is selected for EDS test. We speculate that the carbon is originated from the graphite boat as supporter of the substrate. It is reported that carbon is an amphoteric dopant for III-V NWs since it can result in either p- or n-type doping depending on how it is incorporated into the material. However, in the case of InAs, the In-carbon bond is relatively weak, and carbon tends to act as donor in InAs,31,32 which leads to the formation of n-type InAs QDs. To further confirm the locations of carbon within the InAs lattices, the formation energies of carbon replacing In and carbon replacing As within InAs lattices are calculated by density functional theory (DFT) calculations (S1), and the result shows that carbon atoms are more inclined to replace In atoms which leads to a relatively lower formation energy.


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Figure 3 The strain analysis and electron holographic characterization of InAs QDs/GaAs NW. (a) GPA of the InAs QDs/GaAs NW heterostructure. (b) Electron hologram of the heterostructure. (c) Reconstructed phase image of the same region in (b). (d) Phase profiles across QDs/NW region and the pure NW region as indicated by the red dashed arrow and black dashed arrow in (c), respectively.

The geometric phase analysis (GPA) procedure33 compatible with Digital Micrograph is utilized to analyze the normal strain level at specific InAs QD/GaAs NW interface, as shown in Figure 3a. The analysis is based on the HRTEM image in Figure 2c, which is tilted to allow a vertical interface, and reveals compressive strain in the InAs QD and tensile strain in the GaAs NW. It is reported that the tensile reduces the band gap, while the influence of the compressive strain on the band gap may depend on local crystal deformation with nearly parabolic behavior, different levels of compressive strain may lead to either increase or decrease of the band gap.34,35 Thus, the interfacial strain field may influence the band structures of both InAs QD and GaAs NW. Since the volume of the InAs QD is much smaller than the GaAs NW, the band structure of InAs QD is expected to be affected more. Figures 3b-c are electron hologram and corresponding reconstructed phase shift image of the QDs/NW heterostructure, the sample is tilted ~ 3° away from the [0 1 1] zone axis while the interface is kept edge on to minimize dynamical diffraction contrasts. The phase image in Figure 3c is presented in pseudo colors with the color bar calibrated in radians shown at the bottom. The regions used for phase-shift line profiles across QDs/NW and NW only are indicated by reddashed and black-dashed arrows, respectively, and the phase-shift profiles are shown in Figure



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3d. Compared with the profile of pure GaAs NW region, the profile of InAs QD/GaAs NW region shows excess phase shift at the dot base as well as the surface of the NW. Thus, the introduction of InAs QDs to the surface of GaAs NW indeed changes the interfacial electrostatic potential distributions and charge accumulations. On the other hand, the phase shift of the GaAs NW region away from the QDs/NW interface is barely affected by the introduction of InAs QDs. For nonmagnetic materials in the absence of fringing fields and phase shift due to

v diffraction, the electrostatic potential ΔV (r ) could be extracted from the phase shift and projected thickness information using:

v

v

v

v

 (r)= CE (VMIP (r)+ ΔV(r))t(r)

(1)

v where  (r) is the phase shift, CE is an energy-related constant (0.00728 rad ·V−1·nm−1 for an

v accelerating voltage of 200 kV), VMIP (r) is mean inner potential (MIP) of the material36 (VMIP-GaAs =14.18V, VMIP-InAs =14.50V)37,38 and t is projected thickness along the incident electron beam.


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Figure 4 The cross sectional view of InAs QDs/GaAs NW, and the extraction of potentials across the interface. (a) The cross sectional TEM image of InAs QDs/GaAs NW, where the red dashed arrows indicate the incident electron beam direction for electron holography measurement. (b) Magnified image of the yellow box in (a). (c) The schematic diagram showing the orientation relationship of the incident electron beam, InAs QDs and GaAs NW. (d) The calculated projected thickness profile across the QDs/NW interface based on the above schematic model. (e) The extracted electrostatic potential profile across the QDs/NW interface.

In order to obtain the local projected thicknesses across the QDs/NW interface, cross sectional sample of the NW is prepared by FIB. Figures 4a-b are cross sectional TEM images of the InAs QDs/GaAs NW heterostructure at various magnifications. The red dashed arrows in Figure 4a indicate the incident electron beam direction for electron holography measurement,



and the cross sectional shape of the GaAs NW is approximately hexagonal bounded by {110} crystal planes. A detailed inspection of the corner region reveals a spherical cap shaped QD closely attached to the {112} plane of the GaAs NW, based on which a schematic diagram is displayed in Figure 4c including the relative locations of the incident electron beam, the InAs QDs and the GaAs NW. The projected thicknesses across the InAs QDs/GaAs NW are calculated based on the above schematic model and presented in Figure 4d. The thicknesses increase monotonously in both QD and NW regions, with a sudden leap at the interface. Similar thickness profile is also extracted from the holographic amplitude image (Figure S1) which verifies the rationality of the model for the QD. While the former thickness profile fits well with the latter, the slight differences between them would just affect the absolute values of the local charge densities, but not the distribution of the charges which is the main focus of our study (Figures S1 and S2). The electrostatic potentials (ΔV) across the interface are calculated by subtracting those phase contributions from MIPs according to equation (1) (the phase of vacuum region is set to be zero), and the resultant potential profile is presented in Figure 4e. Since the phase contributions from MIPs have been subtracted, the observed potential variations should be solely attributed to possible charge accumulations and band structure engineering at the interface. The electrostatic potential of the InAs QD region is apparently negative, which indicates possible distribution of n-type dopants in the InAs QD, especially in the dot apex. Since higher signals of carbon normally acting as donor is detected in the InAs QD as compared to those in the GaAs NW, as shown in the EDS elemental map and line profile in Figure 2e, it is reasonable to expect excess electrons accumulated in the QD, and thus negative electrostatic potentials. The positive slope of the potential profile in the InAs QD region and the


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positive potential values on the dot base indicate positive charges distributed at the dot base, and an electric field pointing to the dot apex. On the other hand, the sudden drop of the potential at the GaAs NW surface indicates the accumulation of negative charges on the NW side close to the interface.

Figure 5 Polarization effect, band structure and performance enhancement of the InAs QDs decorating GaAs NWs. (a) The profile of charge density across the InAs QDs/GaAs NW heterointerface. (b) The scheme diagram of band structure for InAs QDs/GaAs NW. (c) The PL spectra of InAs QDs/GaAs NW. (d) The PL spectra including the GaAs peak of a pure GaAs NW (black) and InAs QDs/GaAs NW heterostructure (red).

In order to quantitatively analyze the results, charge densities across the InAs QDs/GaAs NW hetero-interface are calculated as second derivative of the electrostatic potentials according



to Poisson equation, and presented in Figure 5a. It shows that a large number of negative charges are accumulated at the apex of InAs QD. Meanwhile, positive and negative charges of similar densities are distributed on QD side and the NW side of the hetero-structural interface, and the corresponding depletion layer thicknesses are 1.22 nm (QD side) and 1.15 nm (NW side), respectively. Due to the different volumes of the depletion regions of the QD and NW, the amount of the negative charges distributed on NW side is much larger than that of the positive charges on QD side. The above electron holographic analyses have been applied to QDs/NW interfacial regions of variable QD sizes, while the corresponding electrostatic potential profiles across the interfaces show similar trend, the QDs of smaller size confine less electrons close to the dot apex. To further analyze the corresponding charge distributions, the band structures for InAs QDs/GaAs NW hetero-interface are simulated using a 1D Poisson Solver.39 During the simulation, InAs QD is set to be n-type semiconductor with a donor concentration of 1020 cm-3 (the data is obtained from Figure 5a), and the result is shown in Figure S3. Figure 5b is schematic diagram of the simulated band structures. The band gap of InAs QD is smaller than that of the GaAs NW forming type-I band alignment as reported.13 It is well known that the Fermi level of GaAs is close to the middle of the band gap, and that of InAs pins at the conduction band,40,41 which lead to local band bending and redistribution of charge carriers. As Figure 5b shows, the conduction band of GaAs is higher than that of InAs, and large amount of electrons accumulate at InAs QD conduction band. Moreover, the type-I band alignment at the hetero-interface and the slightly tilted band structures would cause accumulation of holes at the right of the InAs QD valence band, and electrons would be left on the GaAs NW side of the interface. The accumulation of electrons and holes in valence band leads to the distribution of


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opposite charges at heterojunction interface, which is well matched with the holography results. Meanwhile, the core region of GaAs NW should also have large number of holes to allow overall charge neutralization of the system, and a polarization field would form from the NW center directing to the NW surface, as shown in Figure 1c. Figure 5c shows the PL spectra of this hybrid structure recorded at 77 K, where the main peaks at 826 nm and 876 nm are well matched with the report.42 The peak at 826 nm corresponding to the GaAs NW is close to the theoretical value (822.5 nm) of bulk GaAs.43 The other peak centered at 876 nm with a line width of 30 nm is attributed to the InAs QDs.8,13,44 The peak intensity of InAs is much higher than that of GaAs, which can be explained as follows: During the irradiation in PL measurement, more electron-hole pairs would form in the QDs because of the narrower band gap of InAs QDs, and their location sites surrounding the GaAs NWs. Then the electrons of much higher density mainly accumulating at the InAs QD apex would recombine with the generated holes in InAs QDs, resulting in the strong peak of 876 nm. There is also a small peak centered at 812 nm, which might be attributed to the sparse regions of wurtzite structure inserted within the matrix of zinc blende structure of GaAs.45,46 Figure 5d is the PL spectra (800 ~ 850 nm) including the GaAs peaks in pure GaAs NW and InAs QDs/GaAs NW (enlarged from Figure 5c). As mentioned in Figure 3d, the holographic phase shifts change abruptly at the interfacial region, and converge to the values of pure NW away from the interface, which reflects the much less impact on the GaAs NW body with the introduction of InAs QD. On the other hand, the GaAs peak of InAs QDs/GaAs NW heterostructure exhibits an observable red shift, which corresponds to a decreased band gap due to the tensile strain with the introduction of InAs QDs on the NW surface. Moreover, the possible diffusion of In in the GaAs NW might contribute to the decrease of the band gap as well.47-49



CONCLUSIONS In summary, QDs-on-NW hybrid structure of InAs QDs/GaAs NW has been successfully grown by MOCVD. Both of the GaAs NWs and InAs QDs are of zinc blende structure with good crystallinity. The corresponding strain analyses reveal compressive strain in InAs QD and tensile strain in GaAs NW. Off-axis electron holography analysis across the hetero-interface shows a polarization effect in InAs QDs decorating GaAs NWs. The InAs QDs tend to be n-type with a large number of electrons at the QDs’ apexes, leading to negative electrostatic potential, while the core region of the GaAs NW should have hole accumulations to allow an overall charge neutralization. The resultant polarization effect in the hybrid system changes the original band structure, makes the InAs QDs to act as gain medium, and at last enhances the photoluminescence property. This quantitative characterization by off-axis electron holography shows great promise in revealing the mechanism of photoluminescence properties from atomic basis, which is also suitable for similar material systems.

ASSOCIATED CONTENT Supporting Information Density functional theory (DFT) calculation, Comparison of sample thickness from the hologram thickness image and model calculation, Band diagram of the InAs QDs/GaAs NW heterostructure.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Author Contributions Feng Cheng and Luying Li performed the experiments, analyzed the data and wrote the draft of the manuscript. Bang Li, Xin Yan and Xia Zhang prepared the samples. He Zheng and Shuangfeng Jia contributed to the electron holography characterization. Jianbo Wang and Yihua Gao raised the concepts, mechanism and analysis. Luying Li devised this investigation, and revised the manuscript. All authors including Xi Wang, Weijie Liu and Shaoli Shen discussed the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (51871104, 51371085, 11674113, 61504010, 61774021 and 61376019). The authors would like to thank Professor Zhong Lin Wang (BINN, CAS) and Dr. Yong Ding (GIT, USA) for their fruitful discussions.

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TOC

The InAs quantum dots decorating GaAs nanowires are successfully synthesized. The electrostatic analysis at nanometer scale reveals charge redistribution across the heterojunction, and formation of a radial polarization field, which provides more chances for electron-hole recombinations in InAs quantum dots, and explains well the enhanced photoluminescence property.


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GaAs Nanowires

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