Strain compensation effect on stacked InAs self-assembled quantum dots embedded in GaNAs layers
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0891-EE03-01.1
Strain compensation effect on stacked InAs self-assembled quantum dots embedded in GaNAs layers Ryuji Oshima, Takayuki Hashimoto, Hidemi Shigekawa, and Yoshitaka Okada Institute of Applied Physics, University of Tsukuba 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ABSTRACT We have studied the effect of strain compensation in multiple stacking of InAs self-assembled quantum dots on GaAs (001) substrates grown by atomic hydrogen assisted RF-molecular beam epitaxy. The GaNxAs1-x material was used as a strain compensating spacer layer. We confirmed by high resolution x-ray diffraction measurements that a 40 nm GaN0.005As0.995 strain compensating layer provides compressive strain to compensate for tensile strain induced by 2.0 ML InAs quantum dots. Consequently, we achieved a multiple stack of InAs QDs up to 30 layers without formation of coalesced QDs, and the density of QDs exceeded 3×1012cm-2.
INTRODUCTION Recently, studies on the self-assembled quantum dots (QDs) have attracted a strong attention because QDs have the potential to improve the properties of optoelectronic devices such as QD lasers [1], semiconductor optical amplifiers [2], and next generation photovoltaic device [3]. In order to achieve these improvements, the fabrication technique to obtain sufficient densities in active region is required. It is well known that stacking is a powerful way of increasing the density. However, too many layers of stacking lead to degradation of quality because of accumulation of internal strain with an increase number of QDs layers. In InAs QDs/GaAs systems, to overcome these problems, several novel techniques such as In-flush methods [4] and columnar QD structures [5] have been reported. On the other, the strain compensation technique has been proposed by using InGaP [6] or GaNAs [7] as strain compensating spacer layers (SCL). However, this approach has been carried out to stack only less than 10 QD layers and a further increase of QD stacking is required to realize a more useful QD superlattice structure. In this letter, we propose a novel fabrication procedure in order to multiply the number of InAs QDs on GaAs (001) substrates without degradation of size uniformity, by strain compensation scheme using GaNAs as an embedding layer.
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EXPERIMENTAL DETAILS For fabrication of multiple stacked self-assembled InAs QDs on GaAs (001) substrates, we employed atomic hydrogen-assisted molecular beam epitaxy (H-MBE) [7,8] with a radio frequency (RF) nitrogen plasma source. After native oxide desorption at 500°C by irradiating atomic hydrogen, a 250 nm-thick GaAs buffer layer was grown at a growth rate of 1µm/h at 580°C. Then, 2.0 monolayers (MLs) of InAs QD layer and 40 nm-thick GaNxAs1-x SCL were consecutively grown in stack from 10 up to 30 multiple cycles at 480°C. The growth rate of QD and spacer layers were 0.1 and 1.2 µm/h, respectively. In a separate experiment, we determined that for the first QD layer, the average QD size was 23.8 nm in diameter, 3.1 nm in height, 13.4 % in diameter di
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