Modeling the optical properties of hexagonal GaN

An adjustable broadening function instead of the conventional Lorentzian one is incorporated in the dielectric function model for hexagonal GaN. One-electron contributions at E 1 critical points and higher-state (m>1) exciton terms, which were incorrectly disregarded in the previous study, are taken into account. Model parameters were determined using the acceptance-probability-controlled simulated annealing. As a result, excellent agreement with experimental data for both real and imaginary parts in the range from 1.5 to 10 eV is obtained. Average discrepancy between experimental and calculated data for the real part of the index of refraction equals 2.75×10 -4, and for the imaginary part is 1.66×10 -3. © 1998 American Institute of Physics.published_or_final_versio

Modeling the optical constants of Hg xCd 1-xTe alloys in the 1.5-6.0 eV range

The optical constants of Hg xCd 1-xTe as a function of energy and composition x are modeled over a wide spectral range from 1.5 to 6 eV. The model employed represents an extension of Adachi's model and incorporates the adjustable broadening function rather than the conventional Lorentzian one. In this way, greater flexibility of the model is achieved, enabling us to obtain an excellent agreement with the experimental data. The relative rms errors obtained for all compositions are below 2.5% for the real part and below 6% for the imaginary part of the index of refraction. The lowest rms errors are obtained for x = 0 (0.6% for the real part and 0.7% for the imaginary part of the index of refraction), and the highest for the x = 0.91 (2.4% for the real part and 5.8% for the imaginary part). © 1999 American Institute of Physics.published_or_final_versio

Modeling the optical constants of hexagonal GaN, InN, and AlN

Optical constants of hexagonal GaN (in the range 1.5-10 eV), InN (in the range 2-10 eV), and AlN (in the range 6-20 eV) for E⊥c are modeled using a modification of Adachi's model of optical properties of semiconductors. Model parameters are determined using the acceptance-probability-controlled simulated annealing method. The employed model uses an adjustable broadening function instead of the conventional Lorentzian one. The broadening can vary over a range of functions with similar kernels but different wings. Therefore, excessive absorption inherent to Lorentzian broadening due to the large wings of a Lorentz function can be reduced, yielding better agreement with experimental data. As a result, excellent agreement with experimental data is obtained; the relative rms errors for the real part of the index of refraction are below 2% for all three materials, and, for the imaginary part, below 5% for GaN and below 3% for InN and AlN. © 1999 American Institute of Physics.published_or_final_versio

Dielectric function models for describing the optical properties of hexagonal GaN

Several different models have been employed for modeling the dielectric function of hexagonal GaN in the range from 1 to 10 eV. Models are compared in terms of number of parameters required, intricacy of model equations, and possibility of accurate estimation of important physical parameters, such as energies of critical points and exciton binding energies. Shortcomings and advantages of each model are discussed in detail. Excellent agreement with the experimental data for GaN has been achieved with three of the investigated models. It has also been shown that an assumption of adjustable broadening instead of a purely Lorentzian one improves the agreement with the experimental data and enables elimination of excessive absorption below the gap which is inherent to the models with Lorentzian broadening. © 2001 American Institute of Physics.published_or_final_versio

Calculations of the refractive index of AlGaN/GaN quantum well

We have calculated the refractive index of a AlxGa1-xN/GaN square quantum well (QW). The imaginary part of the dielectric function has been obtained by summing up the contributions of the dominant interband transitions, excitonic contributions, and the continuum contribution, obtained by weighting the well's and the barrier's continuum contributions. In the calculation of the contribution of the conduction-valence band bound-state effect without electron-hole interaction, conduction bands are assumed to be parabolic and valence bands have been calculated using Chuang's model [Phys. Rev. B 54, 2491 (1996)], but with Chan's basis expansion method [J. Phys. C 19, L125 (1986)] instead of finite-difference scheme. Excitonic contribution has been described with an expression derived by the density-matrix approach at the subband edge without the influence of band mixing. The continuum contributions have been described with the modified Adachi's model. The effects of the aluminum mole fraction x and the width of the well on the refractive index are analyzed and discussed.published_or_final_versio

Growth of SiO x nanowire bunches cocatalyzed with Ga and Ni

Si Ox nanowire bunches were fabricated on Ni (N O3) 2* 6 H2 O solution-coated Si(111) substrates in a chemical vapor deposition system in the presence of Ga and under the flow of Ar and N H3 gases. The roles of nickel nitrate hydrate, gallium, and ammonia in the formation of Si Ox nanowire bunches were investigated. It was found that Ni and Ga act as catalysts for the growth, while nickel nitrate hydrate also serves as a source of oxygen. The growth mechanisms of different nanowire structures obtained by varying the fabrication conditions are discussed. © 2005 American Institute of Physics.published_or_final_versio

Synthesis and properties of ZnO nano-ribbon and comb structures

ZnO is of great interest for photonic applications due to its wide band gap (3.37 eV) and large exciton binding energy (60 meV). A large variety of fabrication methods and nanostructure morphologies was reported up to date for this material. Obtained morphologies include nanobelts or nanoribbons, nanowires, nanorods, tetrapod nanostructures, etc. Novel nanostructures like hierarchical nanostructures, nanobridges and nanonails have also been fabricated. In this work, we report a simple method for fabrication of nanoribbon and nanocomb structures. The structures are fabricated by evaporation of a mixture of ZnO and carbon nanotubes (CNT) at 1050°C, and the deposition products have been collected on Si substrates in the temperature range 750-800°C. The growth mechanism of obtained structures is discussed. © 2005 American Institute of Physics.published_or_final_versio

Influence of the device architecture to the ITO surface treatment effects on organic solar cell performance

In this work, we investigate the influence of different indium tin oxide (ITO) surface treatments on the performance of organic solar cells with different device architectures. Two layer cells with different layer hierarchy (ITO/copper phthalocyanine (CuPc)/fullereve (C60/Al and ITO/C 60/CuPc/Cu) and three layer cells with mixed layer inserted between CuPc and C60 were fabricated. We found that in all cases the short circuit current was the parameter which was most significantly affected by ITO surface treatment. However, the performance of the cells with C60 layer in contact with ITO was markedly less sensitive to the ITO surface treatments compared to the cells with CuPc in contact with ITO. The cells with C60 layer in contact with ITO also exhibited higher efficiency compared to the cells with CuPc in contact with ITO. We also fabricated two layer cells with structures ITO/CuPc/perylene tetracarboxylic acid diimide (PTCDI)/Al and ITO/PTCDI/CuPc/Cu. In this case, we also obtain higher efficiency and lower sensitivity to ITO properties when "n type" material is in contact with ITO. The best obtained AMI power conversion efficiency was 0.4% for ITO/PTCDI/CuPc/Cu cell and ITO/C60/CuPc:C60/CuPc/Cu cells.published_or_final_versio

Phthalocyanine based Schottky solar cells

Phthalocyanine (Pc) materials are commonly used in organic solar cells. Four different phthalocyanines, nickel phthalocyanine (NiPc), copper phthalocyanine (CuPc), iron phthalocyanine (FePc), and cobalt phthalocyanine (CoPc) have been investigated for organic solar cell applications. The devices consisted of indium tin oxide (ITO) coated glass substrate, Pc layer, and aluminum (Al) electrode. It has been found that ITO/CuPc/Al Schottky cell exhibits the best performance. To investigate the influence of the active layer thickness on the cell performance, cells with several different thicknesses were fabricated and optimal value was found. Schottky cell exhibits optimal performance with one ohmic and one barrier contact. However, it is suspected that ITO/CuPc contact is not ohmic. Therefore, we have investigated various ITO surface treatments for improving the performance of CuPc based Schottky solar cell. We have found that cell on ITO treated with HCl and UV-ozone exhibits the best performance. AM1 power conversion efficiency can be improved by 30% compared to cell made with untreated ITO substrate. To improve power conversion efficiency, double or multilayer structure are required, and it is expected that suitable ITO treatments for those devices will further improve their performance by improving the contact between ITO and phthalocyanine layer.published_or_final_versio

ZnO nanorods for solar cells: Hydrothermal growth versus vapor deposition

Performance of dye-sensitized solar cells (DSSCs) based on ZnO nanorods prepared by hydrothermal and vapor-deposition methods has been investigated. In spite of their inferior optical properties, DSSCs based on hydrothermally grown rods exhibit higher power conversion efficiency, which can be attributed to the higher dye adsorption. Hydrothermally grown and vapor deposited nanorods also exhibit different dependence of photovoltaic performance on the annealing conditions of the rods, indicating significant effect of the native defects on the achievable photocurrent and power conversion efficiency. Efficiency of 0.22% is obtained for both as grown hydrothermally grown nanorods and vapor deposited nanorods annealed in oxygen at 200 °C. © 2008 American Institute of Physics.published_or_final_versio