The Strain Status of the Buried Self-assembled InAs quantum dots using MeV technique
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0916-DD02-04
The Strain Status of the Buried Self-assembled InAs quantum dots using MeV technique H.Y. Wang1, C.H. Chen2, H. Niu3, S.C. Wu2, and C.P. Lee1 1
Electronics Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu, 30043, Taiwan 2
Physics, National Tsing Hua University, Hsinchu, 30043, Taiwan
3
Nuclear Science and Technology Development Center, National Tsing Hua University, Hsinchu, 30043, Taiwan
ABSTACT The strain status of the buried self-assembled InAs quantum dots was comprehended by measurement first time. Results show the in-plane strain is compressive and the lattice in the growth direction is lager than the lattice of GaAs. The strain of the sample annealed at 650°C relaxes in the growth direction. The growth and the lateral direction become relaxed in the sample annealed at 750°C. INTRODUCTION InAs self-assembled quantum dots (QDs) have been extensively used in optoelectronic devices, such as semiconductor lasers [1-3], and detectors [4-5]. The formation of self-assembled QDs is driven by the strain originated from the lattice mismatch between InAs and GaAs [6]. The physical properties of the dots are strongly dependent on the amount of strain in and around the dots [7]. However, the strain of these buried dots is difficult to measure and so far only a very few experimental studies have yielded quantitative information on the strain [8-10]. MeV ion channeling has long been used in analyzing the atomic ordering in crystal structures and has been successfully applied for the study of strain in quantum wells. By using angle scan, one is able to calculate the amount of strain in quantum wells by measuring the shift of the scan curve from the quantum wells and that of the substrate. In this work, the strain distribution of self-assembled InAs QDs in GaAs was studied using channeling of MeV C++ ions. Because of the use heavy ions, we were able to measure the angular shift of the channeled signal due to the lattice displacement in the strained QDs. The strain relaxation of the QDs after thermal annealing was also studied using this technique.
EXPERIMENT The samples of InAs QDs used in this study were grown by a Varian GenII molecular beam epitaxy (MBE) system. A buffer layer of 500 nm GaAs was grown first on a semi-insulating GaAs (100) substrate. Then InAs QD layers (each with 2.6 monolayers) was grown at a temperature of 520˚C. All samples were capped with a 50 nm thick GaAs layer. Arsenic pressure was 4–5×10-6 Torr. Growth rates were 1 µm/h for GaAs and 0.056 µm/h for InAs. From the atomic force microscope (AFM) image, the density of QDs was determined to be approximately 1×1010 cm-2. RBS/w channeling measurement was performed by the 9SDH-2 Tandem accelerator at National Tsing Hua University. The beam divergence was less than 0.02°, defined by two sets of slits with 2.3 m apart. The ion beam current density was 200 nA/cm2 on the target. The sample was mounted on a three-axis goniometer with an angular resolution of less than 0.01°. Backscattered particles were collected by a
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