Browsing by Author "Hsieh, P. C."

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Analysis of strain relaxation in GaAs/InGaAs/GaAs structures by spectroscopy of relaxation-induced states
(2007-02-01)
Chen, J. F.
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Chiang, C. H.
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Hsieh, P. C.
;
Wang, J. S.
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電子物理學系
Strain relaxation in GaAs/In0.2Ga0.8As/GaAs structures is investigated by analyzing relaxation-induced traps. Strain relaxation is shown to cause carrier depletion by the induction of a 0.53 eV trap in the top GaAs layer, a 0.13 eV trap in the InGaAs layer, and a 0.33 eV trap in the neighboring lower GaAs layer. The 0.53 eV trap which exhibits a logarithmic function of transient capacitance is attributed to threading dislocations. The 0.33 eV trap exhibits an exponential transient capacitance, suggesting a GaAs point defect as its origin. Given its activation energy, it is assigned to the EL6 in GaAs, commonly considered to be As-i-V-Ga complexes. This trap and the 0.13 eV trap are regarded as the same, since their energy difference is comparable to the optically determined conduction-band offset. The spatial location of this trap correlates with that of misfit dislocations. Accordingly, the production of this trap is determined from the mechanism of strain relaxation. A likely mode of strain relaxation is deduced from the locations of these traps. (c) 2007 American Institute of Physics.
Deep-level emissions in GaAsN/GaAs structures grown by metal organic chemical vapor deposition
(2007-06-15)
Chen, J. F.
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Ke, C. T.
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Hsieh, P. C.
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Chiang, C. H.
;
Lee, W. I.
This work presents the deep-level photoluminescence of coherently strained GaAsN/GaAs quantum-well (QW) structures with various GaAsN thicknesses and N contents. A broad deep-level emission at similar to 1.1 eV is observed, whose wavelength is redshifted as the GaAsN thickness increases. Based on its energy separation from the QW emission, this emission is attributed to a transition between the QW electron ground state and a deep level at similar to 0.2 eV above the GaAsN valence-band (VB) edge. This level is shown to be tied to the GaAs band edge. A transition between this level and the GaAs conduction band allows the GaAsN-GaAs band alignment to be evaluated. A type II band lineup is obtained with VB offsets of 0.03 and 0.002 eV for N=0.6% and 1.8%, respectively. The decreased VB offset suggests a transition from type II to type I with increasing N content. Thermal annealing effectively removes this level and improves the QW emission. The concentration of this level is not clearly correlated with N content, suggesting that this level is induced by a low-temperature growth of the GaAsN layer to suppress the composition fluctuation. Given its energy, this level is tentatively assigned to V-Ga. (c) 2007 American Institute of Physics.
Evidence for the electron trap state associated with N-rich clusters in InGaAsN/GaAs quantum wells
(2007-06-13)
Chen, J. F.
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Hsieh, P. C.
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Hsiao, R. S.
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Wang, J. S.
;
Chi, J. Y.
An electron trap state has been found to appear in InGaAsN/GaAs quantum wells ( QWs) as a result of composition fluctuation. Evidence of composition fluctuation is shown by photoluminescence spectra which display an additional low-energy emission due to local regions of N-rich clusters. Enhancing composition fluctuation leads to undulated InGaAsN/GaAs interface with dot-like islands. The capacitance-voltage profiling shows an additional peak following the QW electron ground state peak, suggesting the presence of an electron trap state below the QW electron ground state. The emission properties of this trap state are similar to those of quantum dots immersed in a well. Enhancing composition fluctuation can cause an energy downward shift and spectral broadening of this electron trap state, suggesting an increase in island size and fluctuations of the size and composition. Thus, it is deduced that the electron trap state originated from the N-rich clusters in the well behaving like quantum dots. Upon modulation, the electrons are thermally activated from this trap state to the QW electron ground state and subsequently emitted to the GaAs bottom electrode.