Xiaohong Tang

An accurate determination of the electronic transitions of InAs/InGaAs/InP quantum dots for midinfrared lasers using simultaneous complementary spectroscopic techniques

InAs/InGaAs/InP quantum dots (QDs) emitting at ∼2u2002μm for midinfrared laser applications are studied using the complementary spectroscopic techniques of photoluminescence (PL), photoreflectance (PR), and surface photovoltage spectroscopy (SPS). We use a procedure that ensures that the same sample spot is studied virtually simultaneously by these three different spectroscopic techniques under almost identical conditions. We are able to measure the ground and excited states transitions of the InAs QDs without any ambiguity, thus providing a complete and clear understanding of the electronic transitions. Temperature dependent PL, SPS, and PR measurements provide a systematic thermal evolution of the ground and excited states. However, the QD transitions are not all seen together at any given temperature in either the PL or SPS measurements. By contrast, the PR technique can measure the complete set of ground and two excited state QD transitions and also the InGaAs barrier energy at all studied temperatures.

Argon plasma exposure enhanced intermixing in an undoped InGaAsP∕InP quantum-well structure

Intermixing in an undoped InGaAsP∕InP quantum well structure enhanced by near-surface defects generated using inductively coupled argon plasma is temperature dependent. The group III sublattice interdiffusion can be four times as fast as that of the group V sublattice for the annealing temperature lower than 600°C, and a maximum band gap redshift of 50nm is obtained in experiment. Blueshift is obtained at 700°C when the group V sublattice interdiffusion becomes appreciable.

Arsenic incorporation into InGaAsP grown by low-pressure metalorganic vapor phase epitaxy using tertiarybutylarsine and tertiarybutylphosphine in N2 ambient

InxGa1−xAsyP1−y epilayers have been grown by low-pressure metalorganic vapor phase epitaxy (LPMOVPE) using tertiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) as group V precursors and nitrogen as the carrier gas. Arsenic incorporation into InxGa1−xAsyP1−y films grown by LPMOVPE as a function of the gas phase composition ratio and V/III ratio has been systematically studied. With optimized growth conditions, the arsenic composition of the epilayers does not change linearly with the TBA source flow. It is observed that the arsenic incorporation becomes saturated when the gas phase composition TBA/(TBA+TBP) increases to 0.4. The incorporation kinetics in MOVPE growth of InxGa1−xAsyP1−y alloy has been analyzed by using an adsorption-trapping model. The As composition (y) of the InxGa1−xAsyP1−y films varies with the TBA gas phase composition ξ=TBA/(TBA+TBP) according to the expression y=2Ns*/aN0(1−e−θβξ). It is demonstrated that with the optimized growth conditions, TBA has a higher incorporation effic...

Investigations of a New High-Performance Low-Band-Gap Photovoltaic Polymer Semiconductor

Application of our newly synthesized low-band-gap polymer, i.e., PDTBT-TT, in organic solar cells (OSCs) has been studied. We have investigated the effects of molecular weight of the PDTBT-TT on the PDTBT-TT:PC71 BM-based bulk heterojunction OSCs performance. It was found that the OSCs fabricated with higher molecular weight (Mn) PDTBT-TT polymer have the higher short-circuit current and power conversion efficiency. The better performance of the PDTBT-TT-based OSC with the higher Mn PDTBT-TT polymer is attributed to the improved charge transfer resistance and the suppressed bimolecular recombination inside the PDTBT-TT:PC71 BM blend active layer of the OSC. Moreover, the increased doping density observed in the higher Mn PDTBT-TT:PC71 BM-based OSCs increases the photocurrent and leads to the cells higher power conversion efficiency.

Free-standing GaAs nanowires growth on ITO glass by MOCVD

GaAs nanowires (NWs) are directly grown on indium tin oxide (ITO) glass substrate by metalorganic chemical vapour deposition (MOCVD), using Au nanoparticles (NPs) as catalyst. By functionalization of the ITO glass and optimization of the Au NPs deposition time, the Au NPs area density deposited on the ITO glass reaches 92 NP μm−2. Uniform and free-standing GaAs NWs without kinking or worm-shape defects have been grown at 430 °C. More than 96% of the NWs have tilt angles larger than 45° with respect of the substrate. The effects of the growth temperature and the Au NPs size on the GaAs NWs growth rate, the NW diameter, and tapering effect are investigated. These results of GaAs NWs growth are the essential step for understanding III–V NWs integration on transparent conductive oxide coated low cost substrate and developing high efficiencyhybrid solar cells.

Parameters study on the growth of GaAs nanowires on indium tin oxide by metal-organic chemical vapor deposition

After successful demonstration of GaAs nanowire (NW) epitaxial growth on indium tin oxide (ITO) by metal organic chemical vapor deposition, we systematically investigate the effect of growth parameters effect on the GaAs NW, including temperature, precursor molar flow rates, growth time, and Au catalyst size. 40u2009nm induced GaAs NWs are observed with zinc-blende structure. Based on vapor-liquid-solid mechanism, a kinetic model is used to deepen our understanding of the incorporation of growth species and the role of various growth parameters in tuning the GaAs NW growth rate. Thermally activated behavior has been investigated by variation of growth temperature. Activation energies of 40u2009nm Au catalyst induced NWs are calculated at different trimethylgallium (TMGa) molar flow rates about 65u2009kJ/mol. The GaAs NWs growth rates increase with TMGa molar flow rates whereas the growth rates are almost independent of growth time. Due to Gibbs-Thomson effect, the GaAs NW growth rates increase with Au nanoparticle s...

An efficient and effective design of InP nanowires for maximal solar energy harvesting

Solar cells based on subwavelength-dimensions semiconductor nanowire (NW) arrays promise a comparable or better performance than their planar counterparts by taking the advantages of strong light coupling and light trapping. In this paper, we present an accurate and time-saving analytical design for optimal geometrical parameters of vertically aligned InP NWs for maximal solar energy absorption. Short-circuit current densities are calculated for each NW array with different geometrical dimensions under solar illumination. Optimal geometrical dimensions are quantitatively presented for single, double, and multiple diameters of the NW arrays arranged both squarely and hexagonal achieving the maximal short-circuit current density of 33.13xa0mA/cm2. At the same time, intensive finite-difference time-domain numerical simulations are performed to investigate the same NW arrays for the highest light absorption. Compared with time-consuming simulations and experimental results, the predicted maximal short-circuit current densities have tolerances of below 2.2% for all cases. These results unambiguously demonstrate that this analytical method provides a fast and accurate route to guide high performance InP NW-based solar cell design.

MOCVD Growth of High-Quality and Density- Tunable GaAs Nanowires on ITO Catalyzed by Au Nanoparticles Deposited by Centrifugation

High-quality and density-tunable GaAs nanowires (NWs) are directly grown on indium tin oxide (ITO) using Au nanoparticles (NPs) as catalysts by metal organic chemical vapor deposition (MOCVD). Au catalysts were deposited on ITO glass substrate using a centrifugal method. Compared with the droplet-only method, high-area density Au NPs were uniformly distributed on ITO. Tunable area density was realized through variation of the centrifugation time, and the highest area densities were obtained as high as 490 and 120 NP/μm2 for 10- and 20-nm diameters of Au NPs, respectively. Based on the vapor–liquid–solid growth mechanism, the growth rates of GaAs NWs at 430xa0°C were 18.2 and 21.5xa0nm/s for the highest area density obtained of 10- and 20-nm Au NP-catalyzed NWs. The growth rate of the GaAs NWs was reduced with the increase of the NW density due to the competition of precursor materials. High crystal quality of the NWs was also obtained with no observable planar defects. 10-nm Au NP-induced NWs exhibit wurtzite structure whereas zinc-blende is observed for 20-nm NW samples. Controllable density and high crystal quality of the GaAs NWs on ITO demonstrate their potential application in hybrid a solar cell.

An Analytic Approach for Optimal Geometrical Design of GaAs Nanowires for Maximal Light Harvesting in Photovoltaic Cells

Semiconductor nanowires(NWs) with subwavelength scale diameters have demonstrated superior light trapping features, which unravel a new pathway for low cost and high efficiency future generation solar cells. Unlike other published work, a fully analytic design is for the first time proposed for optimal geometrical parameters of vertically-aligned GaAs NW arrays for maximal energy harvesting. Using photocurrent density as the light absorbing evaluation standard, 2u2009μm length NW arrays whose multiple diameters and periodicity are quantitatively identified achieving the maximal value of 29.88u2009mA/cm2 under solar illumination. It also turns out that our method has wide suitability for single, double and four different diameters of NW arrays for highest photon energy harvesting. To validate this analytical method, intensive numerical three-dimensional finite-difference time-domain simulations of the NWs’ light harvesting are also carried out. Compared with the simulation results, the predicted maximal photocurrent densities lie within 1.5% tolerance for all cases. Along with the high accuracy, through directly disclosing the exact geometrical dimensions of NW arrays, this method provides an effective and efficient route for high performance photovoltaic design.

Effective coupled optoelectrical design method for fully infiltrated semiconductor nanowires based hybrid solar cells

We present a novel coupled design method that both optimizes light absorption and predicts electrical performance of fully infiltrated inorganic semiconductor nanowires (NWs) based hybrid solar cells (HSC). This method provides a thorough insight of hybrid photovoltaic process as a function of geometrical parameters of NWs. An active layer consisting of GaAs NWs as acceptor and poly(3-hexylthiophene-2,5-diyl) (P3HT) as donor were used as a design example. Absorption spectra features were studied by the evolution of the leaky modes and Fabry-Perot resonance with wavelength focusing firstly on the GaAs/air layer before extending to GaAs/P3HT hybrid active layer. The highest absorption efficiency reached 39% for the hybrid active layer of 2 μm thickness under AM 1.5G illumination. Combined with the optical absorption analysis, our method further codesigns the energy harvesting to predict electrical performance of HSC considering exciton dissociation efficiencies within both inorganic NWs and a polymeric shell of 20 nm thickness. The validity of the simulation model was also proved by the well agreement of the simulation results with the published experimental work indicating an effective guidance for future high performance HSC design.