Solar energy has been sought as one of the prominent candidates among the energy harvesting methods. The energy conversion efficiency of solar cell is limited by its ability of harvesting energy from limited range in solar energy spectrum. We approach this issue by using the down-conversion effect with conventional CdSe quantum dots (QDs), increasing probability of electron-hole generation in designated solar cell. In our study, we fabricated GaAs single junction solar cells and applied QDs for down-conversion. We examine the effects of such application on the solar cell properties with various methods including TR-PL technique.
We have grown GaAs quantum dots (QDs) in Al<sub>0.3</sub>Ga<sub>0.7</sub>As matrix by droplet epitaxy for application in single photon
sources. This growth method enables the formation of QDs without strain, with emission wavelengths of around 700 nm
within the optimal detection range of cost effective silicon detector, and with reduced surface density of several tens to a
few QDs per μm<sup>2</sup> for easier isolation of single QDs. The optical properties of QDs were envisaged by exciton and
biexciton emission peaks identified from power dependent and time-resolved micro-photoluminescence (μ-PL)
measurements. The possibility of fabricating photonic crystal (PC) resonator including a single QD was shown by
obtaining precise spectral and spatial information from a few QDs in a mesa structure, utilizing cathodoluminescence
We present a digital holographic microscope that has a wide field of view. Off-axis holograms are recorded with a magnified image of microscopic objects and numerically reconstructed by calculation of scalar diffraction in the Fresnel approximation. Holograms are recorded by CCD. The distance between neighbouring pixels of a CCD is only of the order of 5 micrometer. The corresponding maximum resolvable frequency is of the order of 100 linepairs /mm. The maximum angle between the reference and object wave is therefore limited to a few degrees. The higher magnification by an objective lens with the higher power makes the wider angle object beam. Off- axis holograms with the high power objective lens has the limitation of magnification and a field of view. We present a new type of imaging system that overcomes the limitation of magnification and a field of view. It is consisted of an objective lens and additional lens array. It makes the nearly same angle between object beams and a reference beam. The overlapped angles are also less than the maximum limited angle due to CCD pixel size. It also has a maximum field of view which is decided inherently by an objective lens. It therefore overcomes the limitation of the size of CCD.
Influence of quantum dot growth on the electrical properties of Au/GaAs Schottky diode structures containing self-assembled InAs quantum dots fabricated via atomic layer molecular beam epitaxy is investigated. Current-voltage characteristics and low frequency noise measurements were performed and analyzed. Employing four different structures; containing single quantum dot layer, without quantum dot layer for a reference, thicker capping layer with single quantum dot layer, three quantum dot layers, we find the diode containing single quantum dot layer show largest leakage current and all the dots show 1/f behavior in low frequency noise characteristics. Current dependence of the noise current power spectral density shows that all the dots have linear current dependence at low bias which is explained by the mobility and diffusivity fluctuation. The Hooge parameter was determined to be in the range of 10<sup>-7</sup> to 10<sup>-8</sup>. At high bias, the diodes containing quantum dot layer(s) show I<sub>F</sub><sup>β</sup> dependence with the value of β larger than 2 (3.9, and 2.7), and the diode without quantum dot layer and thicker capping layer show the value of β smaller than 2 (1.6). The deviation of the values of β from two is explained by the random walk of electrons involving interface states at the metal-semiconductor Schottky barrier interface via barrier height modulation. It seems that the growth of quantum dots induces generation of the interface states with its density increasing towards the conduction band edge. The value of β smaller than 2 means that the interface states density is increasing towards the midgap. Typical value of the interface states density was found to be on the order of 10<sup>11</sup> to 10<sup>12</sup>cm<sup>2</sup>/Vs.
Intermixing effects of MOCVD (metal organic chemical vapor deposition) grown InGaAs SAQDs (self-assembled quantum dots) covered with SiO<sub>2</sub> and SiN<sub>x</sub>-SiO<sub>2</sub> dielectric capping layers were investigated. The intermixing of SAQDs was isothermally performed at 700°C by varying annealing time under the N<sub>2</sub>-gas ambient. It was confirmed from the PL measurement after the thermal annealing that, the emission energy of SAQDs was blue-shifted by 190 meV, the FWHM (full width at half maximum) was narrowed from 76 meV to 47 meV and the PL intensity was increased. SiNx-SiO<sub>2</sub> double capping layer have been found to induce larger PL intensity after the thermal annealing of SAQDs compared to SiO<sub>2</sub> single capping layer. The results can be implemented for increasing quantum efficiency and tuning the detection wavelength in quantum dot infrared photodetector (QDIP).
Effects of In<sub>x</sub>Ga<sub>1-x</sub>As strain relaxation layers on the optical and structural properties of InAs quantum dots (QDs) were studied systemically. 300 K-photoluminescence (PL) shows that PL peak energy of the QDs is blue-shifted in GaAs/InAs QDs/5 nm-thick In<sub>0.1</sub>Ga<sub>0.9</sub>As structure compared to GaAs/InAs QDs/GaAs structure. This is attributed to the intermixing of materials between the QDs and the InGaAs layer below the QDs, whereas capping of a 5 nm-thick In<sub>0.1</sub>Ga<sub>0.9</sub>As layer leads to red shift due to strain relaxation effect. As thickness of In<sub>x</sub>Ga<sub>1-x</sub>As capping layer (<i>TI</i>) increases, 300 K-PL peaks experience red shift below <i>TI</i> < ~7 nm. Unlikely, <i>TI</i> above 7 nm results in blue shift. Considering average height of the QDs is ~ 7 nm, this is attributed to intermixing of material between the QDs and InGaAs capping layers. The blue shift in <i>x</i> = 0.2 over <i>TI</i> > ~7 nm is relatively smaller compared to that in <i>x</i> = 0.1. It is noteworthy that strain difference between the InAs QDs and the In<sub>x</sub>Ga<sub>1-x</sub>As is smaller in <i>x</i> = 0.2 rather than in <i>x</i> = 0.1. Finally, InAs QDs are sandwiched by asymmetric thickness (7.5 nm-thick capping InGaAs, 0, 1.2, and 2.5 nm-thick bottom InGaAs) of In<sub>0.2</sub>Ga<sub>0.8</sub>As layers. 300 K-PL spectrum shows that 1.2 nm-thick bottom InGaAs leads to the longest wavelength (1306 nm) among this sample set. This is attributed to reduced barrier height and ignorable accumulated strain effect in thin bottom InGaAs layers. In this report, we justify merit of dots in an asymmetric well structure over conventional dots in a symmetric well structure and strain relaxation structure for the control of PL peak energy.
Silicon nanoparticles in the range from 2 nm to 5 nm was prepared from Zintl salt, soldium silicide (NaSi) by sonochemical method. This synthesis permits the reaction completed as fast as in a few hours and the easy alkyl-modification of nanocrystals surface at room temperature and ambient pressure. The average size of nanoparticles measured by the dynamic light scattering analysis was 2.7 nm. The high-resolution transmission electron micrograph cofirmed the material identity of nanoparticles as crystalline silicon. FT-IR spectra are consistent with the surface states of nanocrystals that is chlorine- or butyl-capped. The emission peak center moved to longer wavelength (up to 430 nm) with the reaction time, under a 325 nm excitation. The luminescence of silicon colloids looks bright bluish-white under excitation using a commercial low-intensity UV lamp.
1.55 μm multi-quantum well (MQW) broad area laser diodes with different linewidth enhancement factor (a-factor) of 2 and 4 were fabricated. The far-fields of the laser diodes were measured. It was observed that the full width half maximum (FWHM) of the far-fields and the filamentations were reduced in the laser diodes whose α-factor is 2 rather than 4. As injection current increased, the FWHM of the far-fields also increased regardless of α-factor. This phenomenon was explained by reduction of filament spacing as injection current increased
Optical response of both the gate current and the drain current in p-channel InGaP/GaAs/InGaAs double heterojunction pseudomorphic MODFET is reported and analytic models are presented. Based on quantum nature of the two-dimensional carrier statistics in the channel and a new model for the gate current, the overall current variation under optical illumination is explained. The results show power law relation between the current variation and the optical intensity. Near-threshold region in saturation region is found to be most sensitive to the optical intensity variation
Three types of thin layer were inserted between 1st and 2nd separate confinement heterostructure (SCH) layer of 1.55 μm InGaAaP/InGaAs multi-quantum well (MQW) laser diodes. The three types were Type A (p-InGaAsP, 1x10<sup>17</sup>/cm<sup>3</sup>), Type B (p-InGaAsP, 2x10<sup>18</sup>/cm<sup>3</sup>), and Type C (p-InP, 2x10<sup>18</sup>/cm<sup>3</sup>), respectively. It was shown that the light-current (L-I) characteristics for those three types were similar, while the characteristic temperature (T<sub>0</sub>) was higher for type B than others.
In this paper, we propose a new technique to suppress the non- linearity of multiple quantum well (MQW) electro-absorption (EA) modulator, mainly due to an exponential-like transmission characteristics of EA modulator and non-linearity of quantum confined stark effect (QCSE), by intermixing MQW absorption region. Optical properties and its dependence on applied bias voltages of intermixed InGaAs/InGaAsP MQW absorption region, such as transition energy and gain (or absorption) spectrum have been calculated by solving Luttinger-Kohn Hamiltonian. It has been shown that the transfer function of a MQW-EA modulator can be tailored by introducing differently intermixed regions along the waveguide direction. It has been also shown that proposed technique can suppress IMD2 (2nd order intermodulation distortion) by 39.6 dB and enhance spurious free dynamic range (SFDR) by a 3.6 dB by choosing proper combination of interdiffusion lengths and waveguide lengths.
A new model for electrical low frequency noise in semiconductor heterostructure laser diodes is developed based on number fluctuation theory. The model includes carrier number fluctuation mechanisms such as thermal activation, tunneling and random walk involving bulk traps and interface traps at the heterojunction interface. Noise sources in heterostructure semiconductor laser diodes can be divided into three parts, namely, series resistance including ohmic contacts, p-n junction and the heterojunction. The traps located at the interface and or at the bulk of the barrier layer can induce the modulation of barrier height which in turn results in the current fluctuation. Noise generation mechanisms for p-n junction is reviewed. Correlation between electrical and optical noise is also discussed.