Abstract:The strong influences of temperature and vacuum on the optical properties of In 0.3 Ga 0.7 As surface quantum dots (SQDs) are systematically investigated by photoluminescence (PL) measurements. For comparison, optical properties of buried quantum dots (BQDs) are also measured. The line-width, peak wavelength, and lifetime of SQDs are significantly different from the BQDs with the temperature and vacuum varied. The differences in PL response when temperature varies are attributed to carrier transfer from the SQDs to the surface trap states. The obvious distinctions in PL response when vacuum varies are attributed to the SQDs intrinsic surface trap states inhibited by the water molecules. This research provides necessary information for device application of SQDs as surface-sensitivity sensors.

Abstract:The relative coupling efficiency of two-dimensional (2D) grating based on surface plasmon for very long wavelength quantum well infrared detector is analyzed by using the three-dimensional finite-difference time domain (3D-FDTD) method algorithm. The relative coupling efficiency with respect to the grating parameters, such as grating pitch, duty ratio, and grating thickness, is analyzed. The calculated results show that the relative coupling efficiency would reach the largest value for the 14.5 m incident infrared light when taking the grating pitch as 4.4 m, the duty ratio as 0.325, and the grating thickness as 0.07 m, respectively.

InGaAs/GaP QD self-assembled quantum dot (QD) systems are of tremendous interest, for GaP can be directly grown on Si, enabling true monolithic integration of optoelectronics with Si technology [1]. Promising optical properties such as light emitting diodes (LEDs) at room temperature [2] and lasing at 80 K [3] have been demonstrated in these systems. It is critical that we develop a detailed microscopic understanding of the QD structures and elemental distributions of InGaAs/GaP QD systems because the band structure and band alignment, and thus the optical properties, depend on the detailed atomic structure of the QDs and spatial distributions of each element (In, Ga, As) [4][5][6], particularly In [7].So far, two-beam TEM imaging has been the popular choice to image these QDs ( Figure 1). In addition, the In distribution in these QDs has been inferred from lattice constant changes or diffraction contrast in TEM [8] or STM [9]. These indirect measurements can be potentially inaccurate due to factors such as strain relaxations in thin TEM samples and other elements (As, Ga, or P) affecting the lattice constant. Low-loss electron energy loss spectroscopy (EELS) was used to report the elemental distribution [10], but careful post subtraction of the zero loss peak is necessary. Therefore, direct measurements of In, Ga, and As in InGaAs/GaAs and InGaAs/GaP QD systems are highly desired.We directly measure two-dimensional (2D) inhomogeneous distributions of In and As in InGaAs QDs grown in the GaAs and GaP matrix, using energy-dispersive X-ray spectral imaging in a scanning transmission electron microscope (EDX-STEM). The 2D chemical maps were acquired within several minutes, made possible by the combination of the high brightness electron source and high-solid-angle area, quadrant EDX detector, dramatically increasing the signal to noise ratio of the EDX maps.Our main finding is that, in InGaAs/GaP QD systems, the In elemental distribution does not closely follow the QD structures while the As profile does (Figure 2). In contrast, for InGaAs/GaAs QD systems, In is concentrated within the QDs and along the interfacial wetting layer (Figure 2). Comparing the InGaAs/GaAs (Figure 2j-m) and InGaAs/GaP (Figure 2c-g) QD systems, In is more broadly distributed in the GaP matrix. Our findings show that the In elemental profile is much more complicated than previously assumed and that replacing the GaAs matrix with GaP for monolithic integration with Si results in different elemental profiles of InGaAs QDs. These findings challenge our current understanding of the band structure model of InGaAs/GaP QD systems [4,6]. The detailed analysis of elemental distributions of InGaAs QDs on GaAs and GaP gives us a better understanding of the atomic structure of the InGaAs QDs, which can lead to improved QD structures and better optical properties by optimizing the growth parameters.