The InGaAs surface quantum dots grown on GaAs surface without a capping layer (surface quantum dots, SQDs) are expected to play an important role for sensor applications due to their special surface sensitive properties. In this research, we investigated the photoluminescence (PL) characteristics of such In0.35Ga0.65As/GaAs SQDs with a layer of buried InGaAs QDs (BQDs) as reference. The uncapped InGaAs SQDs are integrated into a hybrid nanostructure with SQDs and buried quantum dots (BQDs) spaced by a 70 nm GaAs layer. Due to this thick GaAs spacer, we assert there is no quantum coupling between the SQDs and BQDs so that each layer of QDs has independent emission. The PL spectra show that the SQD PL intensity is far less than BQDs at low temperature but exceeds BQDs at high temperature, indicating a possible carrier transfer between the SQDs and surface states. With increasing excitation intensity, the PL spectra show clearly broaden on the high energy side and a blueshift for both the SQDs and BQDs. Therefore, there is lateral carrier transfer among each layer of QDs due to their high areal density. The intra-layer carrier transfer among SQDs as well as the inter-layer carrier transfer between SQDs and surface states attribute to carriers dynamics that make the SQDs having optical performance very different from the BQDs.
The GaSb quantum dots (QDs) with type II band alignment have attracted great attention recently. They are predicted to be optimizing active region materials for achieving high efficient intermediate-band solar cells and for obtaining ultra-long storage time for memory cells. In this research, GaSb QDs sandwiched inside InAlAs matrix lattice-matched to InP (001) substrate have been obtained via droplet epitaxy. The droplet epitaxy enable us to achieve low density (~2.6 x 10^9/cm^2) and large size (average height ~6.5nm) for the QDs while the lattice mismatch between the GaSb and InAlAs matrix is only ~4%. PL measurements reveal a type-II band alignment for these GaSb/InAlAs/InP QDs. The PL peak energy of QDs shows a blue-shift of >100 meV when the laser intensity increases by six orders of magnitude. Time-resolved PL measurements further confirm the type-II band alignment for the QDs by showing a maximum carrier lifetime of ~4.5 ns. The abnormal dependence of peak energy of QD PL band on the temperature in together with the special PL decay curve indicate that these GaSb/InAlAs QDs likely have different physics mechanism from common GaSb/GaAs type-II QDs. This study provide useful information for understanding the band structure and carrier dynamics of the GaSb/InAlAs QDs grown on InP surface.
Recently, to sandwich InAs QDs with GaAs1-xSbx layers have attracted enormous attention. It is expected to achieve a transition from type-I to type-II band alignment at ~ x=12%. The type-II InAs/GaAs1-xSbx QDs are predicted to be one of the optimizing active region materials for achieving high efficient intermediate-band solar cells.
We first investigate PL properties of InAs/GaAs1-xSbx QD structures of different Sb compositions (x=0, 0.15, and 0.25) in the GaAs1-xSbx capping layer. By capping the InAs QDs with GaAs0.85Sb0.15 and GaAs0.75Sb0.25 layer, we are able to gain type II QDs. These type II QDs exhibit a clear multi-peaks characteristic under increasing laser excitation intensity, which stems from different carriers recombination routes due to the combination of InAs QDs with GaAs1-xSbx capping layer. Time-resolved PL measurements further confirm our assignment for the mnulti-peaks in the PL spectra. We then study carrier coupling inside vertically aligned InAs/GaAs and InAs/GaAs1-xSbx QD pairs. The features of InAs/GaAs1-xSbx QD pairs are very different from the traditional InAs/GaAs QD-pairs. Again, clear multi-peaks characteristic are observed under stronng laser excitation intensity, which stems from different carriers recombination routes from the top InAs/GaAs1-xSbx QDs.
Our investigations indicate that the optical behavior and carrier dynamics in type-II InAs/GaAs1-xSbx QDs and QD-pairs are much more complicated than the InAs/GaAs QDs counterparts. This study provide useful information for understanding the band structure and carrier dynamics of the InAs/GaAs1-xSbx QDs for high efficiency solar cell applications.
Layered cobalt oxide thin films with tilted structures exhibit large light-induced transverse voltage signals due to the
transverse thermoelectric effect and have great potential applications in uncooled broad-band light detectors. In this
paper, we investigated the photoresponse in c-axis tilted Bi2Sr2Co2Oy thin films coated with a layer of nano-structured silver light absorber by using a 532 nm continuous wave laser as the incident light. The incidence direction of the laser beam was directly perpendicular to the sample surface. The laser spot was located at the centre position between the two electrodes and its diameter was about 2 mm. The induced lateral voltage signals were recorded using a 2400 Keithley source meter. Open-circuit voltage signals were observed when the sample surface was illuminated by the 532 nmradiation. Appropriate lateral size and thickness of the nano-structured silver layer can increase the photo-thermal-electric conversion efficiency in this photoresponse process due to the effective absorption of the light at the absorption layer, leading to the improvement in voltage sensitivity. The result offers important guidance of designing the light absorption layer for high performance broad-band light detectors based on the light-induced transverse voltage effect.
this paper, we have prepared c-axis inclined NaxCoO2 thin films on 10° and 20° tilted c-Al2O3 substrates and studied its light-induced thermoelectric voltage effect by using an ultraviolet pulsed laser as light source. A giant open-circuit voltage signal with the peak voltage Vp of tens of voltage was observed when the film surface was illuminated by the 308 nm pulsed radiation, and the Vp increased linearly with the inclination angle as well as the laser energy on the film. In addition, we found that Ag doping in NaxCoO2 films can improve the sensitivity of the thermoelectric voltage signal. The results demonstrate that c-axis inclined NaxCoO2 thin film has a great potential application in the detection of weak ultraviolet pulsed radiation.
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