Improvement of electron wavefunction symmetry in InAs/GaAs quantum dots embedded in an InGaAs strain-reducing layer

Abstract

Summary form only given . Semiconductor quantum dot (QD) devices for the generation of single photon and entangled photon pairs have been eagerly studied for their applications in quantum information technology. Although the single-photon generation has been progressed with vertical micro-cavity, the generation of entangled photon pairs has not yet been achieved with the devices. The well-known Stranski-Krastanov(SK)-type self-assembled QD has an in-plain asymmetric shape, and the symmetry of the electron wavefunction in the QD state is critical for generating entangled photon pairs using biexiton vertical emission. In this paper, we report that the InGaAs strain-reducing layer improves the in-plane symmetry of electron wavefunction in InAs/GaAs self-assembled QDs. The SK-type QD samples were prepared using molecular beam epitaxy. InAs/GaAs QDs were covered by a d-nm (d = 0 -10) thick In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> As (x = 0 -0.17) strain-reducing layer and capped by a 100-nm GaAs layer. Photoluminescence (PL) perpendicular to the sample surface was measured at room temperature and its polarization dependence was investigated to evaluate structural anisotropy. We compared photoluminescence intensity in the most intense polarization direction with that in its 90deg-rotated direction. The authors show that the polarization dependence was reduced by increasing indium composition and thickness of strain reducing layer. The characteristics of the pyramidal and the domical self-assembled InAs QD structures were investigated theoretically with the three-dimensional finite element method (3D FEM). To evaluate the symmetry, they defined the major and minor axes of the QD structure and that of the electron wavefunction to be L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">major</sub> , L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">minor</sub> , L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Phi</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">major</sub> , and L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Phi</sup> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">minor</sub> . Throughout the calculation, it is assumed that L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">major</sub> has a value of 20 nm and that the QD height is 10 nm. The symmetry of the wavefunction is improved by the strain-reducing layer. The symmetry is superior to that of the QD structure, even without the strain-reducing layer (i.e., x = 0). The higher the indium composition and the greater the thickness of strain-reducing layer, the better the resulting symmetry of the wavefunction. They also see in there analysis that the lower the original QD symmetry, the greater the improvement, and that the wavefunction symmetry is better in a domical dot than in a pyramidal dot. The improvement of symmetry in the calculation owes to the spread of the wavefunction into the strain-reducing layer. The higher the indium composition of strain-reducing layer, and the lower the original QD symmetry, the larger the probability amplitude out of QD. To summarize, they report that the InGaAs strain-reducing layer improves the in-plane symmetry of electron wavefunction in InAs/GaAs self-assembled QDs. This is suggested experimentally by the polarization dependence of photoluminescence intensity of QD. The calculation based on the 3D FEM also supports the experiments. The results will aid in the design of an entangled photon generator that uses self-assembled QDs.