Kajornyod Yoodee

Photoluminescence of a High Quality CuInSe2 Single Crystal

A CuInSe2 (CIS) bulk single crystal grown by the directional freezing method from the melt at 1250°C shows superior crystal quality, as evidenced by its (p-type) electrical properties. Photoluminescence at 1.5 K shows a strong and well resolved spectrum with fine structures never before observed from this compound. Free exciton emission exhibits a doublet structure which can either be described as a splitting due to the uniaxial crystal field or as a polariton. The energy gap of the CIS semiconductor as determined from the temperature dependence of the free exciton line is 1.058 eV. Two moderate free-to-bound transitions are assigned to VCu and CuIn acceptors. A weak PL peak corresponding to a deep level is interpreted as arising from the Sei acceptor. Strong phonon replicas are also observed for the first time in a CIS bulk single crystal. The phonon wavelength of 218–237 cm-1 is in good agreement with the reported Raman result of 233 cm-1 for the LO phonon.

Lattice constant values in the (Cu1−xAgx)(In1−yGay)Te2 alloys

It has been shown that single phase solid solution occurs throughout the composition range of the system (Cu1−xAgx)(In1−yGay)Te2, the structure throughout being tetragonal chalcopyrite. Values of lattice parameters a and c have been determined as functions of x and y and fitted to power series expressions. Contours of constant a and constant c are plotted, and it is indicated that these parameter values can be used to determine the composition of samples in this alloy system.

High pressure structural studies of AgInTe2

The structural phase transformations in the chalcopyrite semiconductor AgInTe2 have been studied up to 10 GPa on both pressure increase and decrease. The experiments were conducted using angle-dispersive X-ray diffraction with synchrotron radiation and an image plate. The diffraction patterns of AgInTe2 at ambient pressure reveal two coexisting phases: the first has the chalcopyrite structure while the second has a zincblende-like structure. On pressure increase both phases transformed at 3-4 GPa to a cation-disordered orthorhombic structure with spacegroup Cmcm. On pressure decrease, the chalcopyrite phase started to reappear at 0.55 GPa, and the Cmcm phase disappeared completely at ambient pressure.

The Advantages of Ga-Graded Obtained by Growth Profile Modification and Na Incorporation on Cu(In,Ga)Se2 Solar Cells

Cu (In,Ga)Se2 (CIGS) compound is a p-type semiconductor that has been used as light absorber layer in high efficiency thin film solar cell. The CIGS compound can be adjusted the band gap energy by varying the ratio of [Ga]/([In]+[Ga]) ratio (x). From theoretical and simulation, it was found that band gap grading in CIGS thin films showed the advantages to increase the efficiency of solar cells. Generally, the band gap grading can be done by the growth of non homogeneous x-ratio in depth of CIGS thin films. In this work, we develop two approaches to create band gap grading in CIGS thin films; (1) modifying the growth profile and (2) using Na incorporation in the growth process. The effects of Ga-graded would be revealed and compared with homogeneous CIGS thin films. CIGS thin films were grown on soda-lime glass and Al2O3 coated soda-lime glass substrates by molecular beam deposition method. The growth process was based on 2-stage and 3-stage growth profiles. The as grown films were characterized for their structural property, chemical composition and optical transmission as well as solar cell performance. The Auger electron spectroscopy in depth profiles revealed the variation of x-ratio increasing from the surface toward the back contact in CIGS films with our modified growth profile and Na incorporation. This result indicated Ga-graded in CIGS thin films. The structural property of Ga-graded CIGS films showed the (112) preferred orientation of the chalcopyrite structure with a broad asymmetric spectrum related to the inhomogeneous structure. The optical transmission measurements of the Ga-graded CIGS film showed the broad transition near the absorption edge indicating the effect of the band gap grading as a result of the variation in depth of the Ga-content. From I-V measurements, the solar cell efficiencies significantly increase due to the advantages of Ga-graded constitute.

Characterization of MOVPE Grown GaAs 1-x N x /GaAs Multiple Quantum Wells Emitting Around 1.3-μm-Wavelength Region

GaAs0.949N0.051/GaAs multiple quantum wells (MQWs) were grown on GaAs (001) substrates by metalorganic vapor phase epitaxy (MOVPE). Strong photoluminescence (PL) emission around the 1.3-μm-wavelength region was observed without post-growth thermal annealing, which suggests an efficient electron confinement in the GaAs0.949N0.051/GaAs MQWs. After post-growth thermal annealing, blue-shift of PL peak energy was clearly observed. This PL blue-shift, induced by thermal annealing, can be described by diffusion of N out of the quantum well and homogenization of the N concentration fluctuation. Furthermore, a reduction of PL full width at half maximum (FWHM) suggests the more homogeneous N distribution. In addition, both the as-grown and annealed GaAs0.949N0.051/GaAs MQWs exhibit fairly flat and abrupt GaAsN/GaAs interfaces, which were confirmed by high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM) measurements. Based on PL results, it is evident that the band alignment of GaAsN/GaAs hetero-structure is a type-I band lineup. Adding N to GaAs mainly affects the conduction band (CB) states leading to a large conduction band offset (ΔEC ~ 550 meV). Our results show the potential for the fabrication of 1.3 μm GaAsN QW lasers on GaAs substrates.

Valence band structure of some chalcopyrite compounds

Abstract It is shown that if the quasi-cubic model is used in the analysis of the variation of the valence to conduction band transition energies as a function of temperature, the values of spin-orbit and crystal-field splittings and their temperature variations so determined are inconsistent with those from X-ray diffraction measurements of lattice parameters and tetragonal distortion. Therefore the previously published results for five compounds have now been analyzed in terms of a recently developed theoretical model which takes into account the effects of p-d hybridization on both spin-orbit and crystal-field splittings. The analysis gives values of a dimensionless parameter M/E which is a measure of the interaction between the p and d states forming the valence bands and which determines the fractional d -like character (1 − α ) of those bands. The variations of M/E and (1 − α ) with temperature have been determined for each compound considered.