Strain relaxation induced misfit defects and their effects on the electron emission of InAs quantum dots
| Autor: | Yan -Zung Wang, 汪炎宗 |
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| Rok vydání: | 2007 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 95 The electrical and optical properties of relaxation-induced lattice misfits and their effects on the electron emission of InAs QD are investigated by photoluminescence (PL), current-voltage (CV), admittance spectroscopy, deep-level transient spectroscopy (DLTS), and cross-sectional transmission electron microscopy (TEM). Three samples with different InAs deposition thicknesses of 2.0, 2.3, 3.3 ML are grown by molecular beam epitaxy (MBE). From previous experiments, when the InAs thickness exceeds the critical thickness (between 2.7 and 3.0 ML), strain is relaxed by introducing misfits in the QD and neighboring GaAs bottom layer. In a relaxed 3.3 ML sample, capacitance-voltage profiling shows an electron accumulation peak at the QD with a long emission time, followed by additional carrier depletion caused by the misfits in the GaAs bottom layer. By comparison with anther 2.8 ML relaxed InAsSb QD which displays no emission-time increase because the relaxation-induced misfits are not located in the GaAs bottom layer but are located in the QD, the emission-time lengthening in the relaxed sample is explained by the suppression of tunneling for the QD excited states due to the additional carrier depletion caused by misfits in the bottom GaAs. As a result, electrons are thermally activated from the QD states to the GaAs conduction band, consistent with the CV and conductance-frequency (G-F) measurements which show emission energies of 0.160 and 0.068 eV, close to the confinement energies of the QD electron ground and first-excited states, respectively, relative to the GaAs conduction band. This is in contrast to non-relaxed samples in which emission energy of 60 meV is observed, corresponding to the emission from the QD electron ground state to the first-excited state. Thus, placing the misfit dislocations in the GaAs bottom layer can have the effect of lengthening the emission time for electrons escaping from the QD. DLTS reveals a trap at 0.41 eV in the bottom GaAs layer, which is attributed to the misfits. Its intensity is about two orders of magnitude less than the misfit intensity, suggesting that most of the misfits are not active traps. The effect of post-growth annealing on the 3.3 ML relaxed sample is also studied. After annealing at 700 ℃ for 1min, a blueshift of both the ground-state and first-excited-state emissions is observed, suggesting an interdiffusion between In and Ga after annealing. Annealing is found to increase the emission time foe the electrons trapped on the misfit-related traps. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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