Alignment of InAs Quantum Dots on GaAs Using the Manipulation of Strain Fields
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Mat. Res. Soc. Symp. Proc. Vol. 618 © 2000 Materials Research Society
EXPERIMENT The epitaxial layers were grown by molecular beam epitaxy (MBE) technique. We deposited 0.51im thick GaAs buffer layer at a growth rate of IML/s on GaAs (001) substrate. During the buffer layer growth, substrate temperature was 580°C. After the buffer layer, substrate temperature was lowered to 500'C. And then strained layers were formed which consisted of IML thick lnAs/4MLs GaAs superlattice followed by 20MLs thick GaAs mediated layer. In order to enhance the uniformity of strain fields and the surface migration of adatoms, InAs layers were grown at a slow growth rate of 0.14ML/s. The number of cycles of superlattice was 10, 15, 20, 25, and 30. Finally, we deposited 2MLs thick InAs on strained layer to form the aligned QDs by the repetitive delivery for the indium of 1 second followed by the interruption of 5 seconds. We also prepared samples for the measurement of photoluminescence with the same structures as described above except the deposition of 180MLs thick GaAs cap layer. The 514.5nm line of Ar* laser was used for the excitation source with the power density of 20 W/cm2 in PL measurement. RESULTS AND DISCUSSION According to Hull et al. [10], strained superlattice system has commensurate mole-fraction in alloy composition. Considering that our strained layer is composed of InAs/GaAs superlattice (# of cycles: n) and 20 MLs thick GaAs, the equivalent commensurate mole-fraction (x) is calculated to be x =
n in In.Ga 1.,As. Here, '5n+20' means the total thickness of strained 5n + 20 layer and 'n' the total thickness of InAs in MLs, respectively. i~-o n0110
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Cycles of superiattice (n)
Fig. 1 The number of cycles of superlattice versus critical thickness and the thickness of strained layer Because of 20MILs thick GaAs as a mediated layer in the strained layer, equivalent commensurate mole-fraction (x) is not a constant value. The thickness of strained layer, d can be calculated from the relation d = 1.435n + 5.65 nm. And then, the critical thickness of strained layer h, can be calculated by the equivalent commensurate mole-fraction from Mattew's model [11]. Thus, we can choose h, from the critical thickness of 60' misfit dislocation h,6. When d is thicker than h, of strained layer, misfit dislocation will be generated. Figure 1 shows the dependence of critical thickness and strained layer thickness as a function of the number of cycles of superlattice. We can obtain the result of 'n > 11' from 148
solving the equation of' d(n) Ž_hk(n)' because d and t, of strained layer are a function of the cycles of superlattice n. Thus, the sample of n=-10 is expected to be under the critical thickness whereas those of n=15, 20, 25, 30 to be over the critical thickness. We can also expect that InAs QDs will be started to align along the misfit dislocation lines in the sample of n=15. The strained layer, in spite of having 20MLs thick GaAs mediated layer which i
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