Showing papers by "Masao Nishioka published in 2003"

Strain-induced modifications of the electronic states of InGaAs quantum dots embedded in bowed airbridge structures

Abstract: We have fabricated bowed airbridges in which self-assembled InGaAs quantum dots are embedded. Strong strain distribution induced in the bowed airbridge and the effect on the electronic states of the quantum dots are investigated through the measurement of the photoluminescence from the individual dots and the theoretical calculation. A finite element calculation shows the strain in the bowed airbridge to distribute from tensile to compressive along the growth direction. The strain effect on the electronic states of the dots is probed through the photoluminescence peak shift following the deformation of the GaAs matrix of the dots from a wall-shaped structure to the bowed airbridge. The magnitude of the peak shift varies systematically with the position of the quantum dot along the growth direction, clearly reflecting the strain distribution in the bridge. The energy level shift following the deformation is calculated by solving the three-dimensional Schrodinger equation taking into account the strain distribution around the dots embedded in the bridge. The calculation, which agrees well with the experiment, demonstrates that the characteristic strain distribution around the dot embedded in the bowed airbridge modifies not only the energy levels, but also the wave functions. The electron and hole wave functions are modified differently, mainly due to the opposite contribution of the biaxial strain to the hydrostatic ones.

Nondegenerate pump and probe spectroscopy in InGaN thin films

Abstract: Carrier dynamics in In0.10Ga0.90N/In0.03Ga0.97N multiple quantum wells and epilayers were studied by nondegenerate femtosecond pump and probe spectroscopy and by using monochromatic continuous wave light at room temperature. Structurally dependent transient absorption spectra were observed after pump pulse excitation. These spectral changes are caused by excitonic absorption quenching and screening of internal piezoelectric fields by photoinduced carriers. Both rapid (∼400 ps) and slow (∼90 μs) temporal behavior was observed in the transient absorption spectra. The results of time-resolved photoluminescence measurements and the existence of saturation pump fluences indicate that trapped carriers in potential minima originating from In-rich regions and/or midgap carrier traps are responsible for the lasting screening of the internal electric field.

Effect of strain variation on photoluminescence from InGaAs quantum dots in air‐bridge structures

Abstract: We have fabricated freestanding wire (air-bridge) structures with a bowed shape by introducing a strain layer to vary the strain around quantum dots. The photoluminescence peak energy shift following the shape change of the bridge can be observed for individual InGaAs quantum dots. We find systematic dependence of the peak shift on the dot position along the growth direction. The dependence of the peak shift is explained by strain distribution in the bridge. The strain distribution in the bridge as well as in the dot is calculated using a finite element method. Using the strain data, the electronic structures of the dots embedded in the bridge structures are calculated within the effective mass approximation. The calculated energy shifts agree well with the experimental ones.

Thickness dependence of transient absorption spectrum for InGaN thin films

Abstract: Nondegenerate femtosecond pump and probe spectroscopy was performed in In0.10Ga0.90N/In0.03Ga0.97N multiple quantum wells and epilayers at room temperature. The structural dependence of the photoinduced differential absorption (Δα) spectrum was studied on four In0.10Ga0.90N layers of different thickness (5.5, 17, 34, and 66 nm). A single bulge was clearly observed in Δα spectra except for the 5.5 nm thick sample. The bulges are sharper in thicker In0.10Ga0.90N layer samples. These spectrum changes are caused by excitonic absorption quenching and screening of internal piezoelectric fields by photoinduced carriers. Both rapid and slow temporal behaviors were observed in transient absorption spectra. The results of time-resolved photoluminescence measurements and existence of saturation pump fluences indicate that trapped carriers in potential minima originate from In-rich regions and/or midgap carrier traps are responsible for the observed lasting screening of the internal electric field.