In situ accurate control of 2D-3D transition parameters for growth of low-density InAs/GaAs self-assembled quantum dots
© Li et al.; licensee Springer. 2013
Received: 26 December 2012
Accepted: 6 February 2013
Published: 18 February 2013
A method to improve the growth repeatability of low-density InAs/GaAs self-assembled quantum dots by molecular beam epitaxy is reported. A sacrificed InAs layer was deposited firstly to determine in situ the accurate parameters of two- to three-dimensional transitions by observation of reflection high-energy electron diffraction patterns, and then the InAs layer annealed immediately before the growth of the low-density InAs quantum dots (QDs). It is confirmed by micro-photoluminescence that control repeatability of low-density QD growth is improved averagely to about 80% which is much higher than that of the QD samples without using a sacrificed InAs layer.
KeywordsInAs quantum dots Sacrificed InAs layer Molecular beam epitaxy Reflection high-energy electron diffraction Micro-photoluminescence Low density 78.67.Hc 78.55.Cr 78.55.-m
Single self-assembled semiconductor quantum dots (QDs) are of increasing interest due to their applications in low-threshold lasers, single-photon and entangled photon sources[2, 3], quantum computing, and quantum information processing[4, 5]. Several techniques have been developed to obtain low-density QD structures, such as the Stranski-Krastanov self-assembled growth of QDs on a substrate patterned with mesa/holes[6, 7], stopping of the rotation of the substrate to obtain a gradient density of InAs QDs[8, 9], and a modified droplet epitaxy method to lower the QDs' density; especially one of the most effective method is to stop the InAs deposition at the onset of a two-dimensional to three-dimensional (2D-3D) growth transition by controlling the parameters of 2D-3D growth transition such as temperature, growth rate, deposition amount of indium, and interruption time. However, the narrow range of deposition in the 2D-3D growth transition determines that allowed deviations of controllable parameters are quite limited for repeatable growth of low-density QDs.
In this paper, to increase the repeatability and to obtain good single-photon characteristics, we investigated a growth technique to obtain in situ the critical deposition in 2D-3D growth transition and slightly change the critical conditions to achieve InAs QDs with good single-photon characteristics. The success ratio is improved averagely to about 80% which is much higher than that of the traditional QD samples (less than 47%).
Growth parameters of sample 1 to sample 9
Growth temperature of SQD/QD (°C)
Growth rate (ML/s)
Deposition θc + Δ (ML)
Interruption time (s)
Annealing temperature (°C)
θc + 0.15
θc + 0.075
θc + 0.025
θc + 0
θc − 0.05
θc − 0.075
θc + 0
θc + 0
Results and discussion
By growing a reference sample to obtain the critical growth parameters, then increasing growth interruption and growth temperature, and decreasing deposition of InAs, a very low density of QDs can be realized. However, the repeatability is very low if the critical conditions were obtained from samples in different batches because of the accidental error and system error, such as differences caused by different molybdenum sample holder blocks, ambience in the growth chamber, measurement of growth rate and temperature, and so on. For our samples used in this method, the repeatability is less than 47%.
Another reason for the low repeatability is that the condition of the low-density InAs QD for single-photon source devices is strict, so a small deviation of deposition may affect the micro-PL seriously. The micro-PL spectra of samples 3 and 4 at 80 K are shown in Figure 4c,d. The sharp single peak indicates that sample 4 has a good single-photon characteristic. The multiple peaks of sample 3 demonstrate that a slight change (0.025 ML) of deposition may determine the optical characteristic, so the critical growth parameters obtained from the reference sample ex situ make the repeatability low.
It is an important issue to accurately control the 2D-3D transition parameters for the growth of low-density self-assembled InAs QDs. We have proposed a method of introducing a sacrificial InAs layer to determine in situ the 2D-3D critical condition as a spotty pattern appears in RHEED. After annealing of the InAs sacrificial layer at 610°C, the expected low-density QDs can be grown with highly improved repeatability. As confirmed by micro-PL spectroscopy, high optical-quality low-density QDs were obtained under the growth temperature of 5°C higher than that of the SQD layer and the same deposition of InAs. The slight increase of the InAs deposition amount dramatically deteriorates the PL properties. Our result provides a useful way to accurately control the critical condition of the low-density InAs QDs and thus to improve the fabrication repeatability.
M-FL, YY, J-FH, L-JW, YZ, and X-jS are Ph.D. students at the Institute of Semiconductors, Chinese Academy of Sciences. H-QN is associate researcher, and Z-CN is a researcher at the State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences.
Reflection high-energy electron diffraction
Sacrificed InAs quantum dots
Transmission electron microscopy.
This work is supported by the National Natural Science Foundation of China (under grant nos. 90921015, 61176012, 61274125), the National Key Basic Research Program of China (grant nos. 2013CB933304, 2010CB327601, 2012CB932701), and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (grant no. XDB01010200).
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