In order to characterize the density, shape, diameter, and height size distribution of GaSb/InGaAs QDs on InP (100) substrate, the AFM and STEM measurements were carried out. Figure shows the AFM and STEM images of GaSb/In0.53
As QDs and the histogram of the height of GaSb/In0.53
As QDs. As shown in Figure , the statistical data indicate that the density of the QDs is approximately 7 × 109
and that the shape of GaSb QDs is rectangular-shaped which is the same with GaSb/GaAs QDs [9
]. Figure shows the height distribution of the GaSb/In0.53
As QDs. From the figure, we can see that the height of the quantum dots is mainly concentrated to approximately 6 nm. Due to the well known 'tip effect' of AFM, the results of AFM measurements cannot describe the precise lateral size of the QDs. The STEM measurements were used to image the configuration for overcoming this limitation of AFM measurements. Figure shows that the lateral size of the QDs is approximately 40 nm. The results indicate that the rectangular-shaped GaSb/InGaAs QDs are well developed in the SK growth mode, but no nanodash-like structures which are easily found in the InAs/InP QD system were formed [19
]. However, there seemed to be some smaller QDs (the lateral size was about 20 nm) in the AFM image. By measuring the height distribution of the QDs, we observed that they were lower than 2 nm. We did not observe such bimodal distribution in the STEM images. So, we thought that these mound-like structures were possibly from the non-optimized InGaAs buffer layer. Another possible explanation was that the formation of the InGaAsSb wetting layer resulted in the accumulation of individual atoms on the surface to form a mound-like structure, due to the intermixing of As and Sb during the growth of GaSb QD.
Figure 1 AFM and STEM images of GaSb/In0.53Ga0.47As QDs and histogram of the height of GaSb/In0.53Ga0.47As QDs. (a) The AFM image of GaSb/In0.53Ga0.47As QDs, (b) histogram of the height of GaSb/In0.53Ga0.47As QDs, and (c) the STEM image of GaSb/In0.53Ga0.47As (more ...)
Figure shows the PL spectra of four-ML QDs at 20 K with an excitation power of 3 mW. It is obvious that there are two peaks centered at 0.75eV and 0.76eV, respectively. For identifying these two peaks, low-temperature excitation power-dependent PL spectrum tests were carried out, and the results were shown in Figure . Figure shows the PL peak energies with various excitation powers. It is obvious that the low-energy peak blueshifts with the increasing excitation power, while the position of the high-energy peak is almost constant. The PL peak blueshifts with increasing excitation power is a special character of type-II heterostructures. The other supporting evidence of the type-II luminescence is the linear dependence of the PL peak energies over the third root of the excitation density [20
]. The inset of Figure shows the linear dependence of the PL peak energies and the third root of the excitation power. Many researchers attributed the high energy PL peak to the transition of the wetting layer [9
]. In these references, there is a common point where the wetting layer peak blueshifts also with increasing excitation power (type-II). However, the high-energy peak in our work is almost independent of the excitation power which is a typical feature of the type-I band transition. Therefore, the interband transition of the GaSb QD would be the only proper origin of the high-energy peak. In the growth process of the sample, the InGaAs cap layer was doped with Si. Because the dope concentration was relatively high, the Fermi level of the InGaAs layer may possibly be higher than the bottom of the conduction band of GaSb QDs. In such circumstance, the light emission intensity of the GaSb QD could be stronger than the type II transition due to the stronger spatial confinement of carriers in the QD and the nature of the direct transition type I transition. It may be the reason that the PL intensity of the direct interband transition (type I) was strong as observed in the experiment. So, these two peaks are identified as the indirect transition from the InGaAs conduction band to the GaSb hole level (type-II) and the GaSb QDs direct interband transition (type-I) respectively, as shown in Figure .
Low-temperature (20 K) PL spectra of GaSb/InGaAs QD sample on InP substrate. Dashed-dot and dashed lines show the PL spectra of type-II and type-I, respectively.
Figure 3 PL spectra and PL peak energies. (a) The low-temperature PL spectra of the sample measured under different pumping powers from 3 mW to 30 mW; (b) The PL peak energies obtained under different excitation powers and the fitting curve of the peak energies (more ...)
Schematic band diagrams. (a) Bulk GaSb/InGaAs heterostructures and (b) GaSb/InGaAs QD nanostructures forming approximately triangular wells.
To explain the PL mechanisms of the type-II GaSb/InGaAs QD structures, the schematic band diagrams of GaSb/InGaAs heterostructures are provided in Figure . The spatial separation of electrons and holes will produce an electric field near the type-II GaSb/InGaAs interface. This electric field could make the band bended and form approximately triangular wells adjacent to the heterojunction. With increasing excitation power, the accumulation of electrons and holes at the GaAs/InGaAs interface would steepen the wells. In this case, upraised energy levels in the approximately triangular wells of electrons and holes would cause the PL peak to blueshift.
As is well known, the GaSb QDs only confine the holes, while the electrons are confined in the InGaAs matrix in the type-II GaSb/InGaAs QD heterostructure. We can take advantage of these features to accomplish a charge-discharge process of QDs and then to modulate the electric property of a two-dimensional electron gas [2DEG] in QD-FET. In this kind of QD-FET structure, type-II GaSb/InGaAs QDs are embedded; even if the GaSb QDs directly contacts with the 2DEG, electrons will still be blocked by the GaSb barrier and will not enter the QDs. Therefore, this structure will prolong the lifetime of the holes. So, the QD-FET based on the above band structure can be used to improve the sensitivity of existing InAs/GaAs QD-FET. In addition, this material system can be fabricated on InP substrates. The higher electron mobility InGaAs light absorption layer with lattice matched to the InP substrate has a strong optical absorption in the range of 1.3 to approximately 1.55 μm which is the low-loss optical fiber window. All of these features will promote the application of QD-FET on quantum communications, night vision, and other fields. Besides, owing to the spatially separated electrons and hole characters of type-II QDs, the GaSb/InGaAs QD-based QDIP could have obviously better performance than the InAs/(In)GaAs QD-based QDIP.