Shigehiro Isomura

Visible and ultraviolet photoluminescence from Cu-III-VI2 chalcopyrite semiconductors grown by metalorganic vapor phase epitaxy

We optimized the overall process of heteroepitaxial growth of wide-gap Cu–III–VI2 chalcopyrite semiconductors by using the low-pressure metalorganic vapor phase epitaxy technique, and obtained very high quality epilayers of them. All end-point compounds, namely CuGaSe2, CuGaS2, CuAlSe2 and CuAlS2, exhibited predominant free and bound excitonic photoluminescence (PL) peaks at low temperature. The color of the emission varied from red to ultraviolet. The room temperature (RT) PL spectra exhibited predominant near-band-edge emission except for that for CuAlSe2. A noticeable excitonic feature was found in the PL spectra for CuAlS2 and CuGaS2 even at RT. Time-resolved and excitation intensity-dependent PL measurements revealed the existence of free-to-bound (FB) and donor-acceptor pair recombination emission centers in some undoped and impurity-doped compounds and alloys. The ionization energy of the recombination centers increased with increasing band gap energy of the matrix, reflecting an increase in the hole effective mass. The potential of heteroepitaxial layers of chalcopyrite semiconductors as new light-emitting materials was demonstrated.

HETEROEPITAXY AND CHARACTERIZATION OF CUGASE2, LAYERS GROWN BY LOW-PRESSURE METALORGANIC CHEMICAL-VAPOR DEPOSITION

CuGaSe2 chalcopyrite compounds were grown heteroepitaxially on both GaAs and GaP substrates by means of the low‐pressure metalorganic chemical‐vapor deposition method. Optical and structural properties were characterized comprehensively by photoreflectance (PR), photoluminescence (PL), x‐ray diffraction, transmission electron microscopy, transmission electron diffraction, and electron‐probe microanalysis. The CuGaSe2 epilayers had c(001) surface on GaAs(001) substrates and a(100) surface on GaP(001) substrates, respectively, the results being similar to the case of CuAlSe2. Energies of A, B, and C excitons associated with uppermost valence bands were determined from analysis of PR spectra, and the energies of good‐quality epilayers are close to those of the bulk crystal. The slight increase of the crystal‐field splitting in the valence bands were discussed in terms of the lattice strain in the epilayer caused by the lattice mismatch. Low‐temperature PL spectra exhibited an intense peak at 1.71 eV, the ene...

Properties of Cu(In,Ga)Se2 Thin Films Prepared by Chemical Spray Pyrolysis

Thin polycrystalline films of Cu(In,Ga)Se2 alloy with the single-phase chalcopyrite structure were successfully grown by the chemical spray pyrolysis (CSP) method on a glass substrate at 360 and 400°C. Alloy composition in the film was well controlled by that in the spray solution. The films were characterized by X-ray diffraction, optical absorption, Raman spectroscopy, van der Pauw measurement and scanning electron microscopy. It was found that lattice constants, the optical band-gap energy and the A1 mode phonon frequency changed continuously with the alloy composition x. A grain size of about 1 µm was obtained for Cu-rich films.

Preparation of c-axis oriented ZnO films by low-pressure organometallic chemical vapor deposition

Abstract ZnO films have been deposited on sapphire (0001) substrates at 370 °C at a H 2 O partial pressure of 2.7 × 10 −2 to 2.7 × 10 −3 Pa, using zinc acetate as a precursor. All of the resultant ZnO films are insulators at room temperature. ZnO film with a smooth surface has been prepared at a H 2 O partial pressure of 6.7 × 10 −3 Pa, and its X-ray diffraction pattern is not affected by annealing at 500 °C for 10 h in air. In photoluminescence spectra at 77 K, an intense broad band is observed in near-UV region (3.3–3.4 eV) for as-deposited ZnO films, whereas the emission in the near-UV region is significantly diminished by the annealing.

Preparation of CuInSe2 Thin Films by Chemical Spray Pyrolysis

CuInSe2 thin films have been prepared by chemical spray pyrolysis (CSP) on glass substrate from the ethanol aqueous solution containing CuCl2, InCl3 and N,N-dimethylselenourea. Properlies of the CuInSe2 films (electrical, structural, optical absorption and morphological properties) have been systematically studied in terms of substrate temperature (T s), pH and the ion ratio (Cu/In) of the spray solution. Good chalcopyrite CuInSe2 films with large grains have been grown using the neutralized spray solution (pH=4) at the growth temperature of 360° C. On the other hand, low values of T s, pH and Cu/In led to the production of sphalerite films.

Photoluminescence studies in CuAlSe2 epilayers grown by low‐pressure metalorganic chemical‐vapor deposition

Low‐temperature photoluminescence (PL) spectra were investigated for CuAlSe2 epilayers grown on GaAs(001) substrates by means of low‐pressure metalorganic chemical‐vapor deposition. PL properties were studied with relation to metalorganic precursors used for the growth. High‐quality undoped epilayers exhibited PL peaks related to a free exciton (2.739 eV) and a bound exciton (2.677 eV). The other undoped epilayers exhibited PL bands at 2.3, 2.4, and 2.5 eV originating from donor‐acceptor (D‐A) pair recombinations. Some of them were found to have a common activation energy for the thermal quenching of 50±10 meV. The PL spectrum changed drastically by impurity doping. Intense green emissions at 2.51 and 2.43 eV were observed in Zn and Mg‐doped epilayers, respectively, which were interpreted as D‐A pair recombinations based on the dependencies of the PL spectra on excitation intensity, decay time, and temperature. The donor and the acceptor activation energies (ED and EA) were estimated to be 110 and 230 meV...

Excitonic photoluminescence in a CuAlSe2 chalcopyrite semiconductor grown by low-pressure metalorganic chemical-vapor deposition

Excitonic photoluminescence (PL) in a CuAlSe2 chalcopyrite semiconductor was observed. High‐quality CuAlSe2 epilayers were grown by the low‐pressure metalorganic chemical‐vapor deposition technique. Based on photoreflectance measurements, the PL peak at 2.739 eV was assigned to a free exciton emission. The PL peak at 2.677 eV was tentatively assigned to a bound exciton emission.

Electroreflectance studies in CuGaS2

Room temperature electroreflectance (ER) measurements have been performed near the fundamental absorption edge of CuGaS2 single crystals grown by both iodine-transport and melt-growth methods. Transition energies and broadness of the ER spectra are examined and the results are discussed in terms of photoluminescence properties. The band gap energy obtained from the ER spectrum is 2.492 eV at 290 K, and the value is larger than the widely used band gap energy of 2.43 eV determined by the previous ER studies.

Heteroepitaxial Growth of CuGaS 2 Layers by Low-Pressure Metalorganic Chemical Vapor Deposition

Heteroepitaxial growth of CuGaS2 was studied by low-pressure metalorganic chemical vapor deposition using normal-tripropylgallium as a new Ga precursor, combined with cyclopentadienylcoppertriethylphosphine and ditertiarybutylsulfide. Structural and optical properties were characterized in detail. The epilayer showed the c[001]-oriented growth on both GaAs(001) and GaP(001) substrates. The lattice parameter c of the epilayers was smaller than that of the bulk single crystal. Magnitude of the residual lattice strain for CuGaS2/GaP(001) was found to be larger than that for CuGaS2/GaAs(001), even though the lattice mismatch for the former was smaller than that for the latter. The strain is considered to be introduced during cooling after the growth. A photoluminescence peak at 2.493 eV (8 K) was assigned to a free exciton emission, because the peak energy agreed with A-exciton energy obtained from photoreflectance spectra.

Studies on CuIn precursor for the preparation of CuInSe2 thin films by the selenization technique

Abstract The effect of temperature on the degree of alloy formation between Cu and In layers has been studied for the preparation of CuInSe 2 films by the selenization technique. The mechanism of alloy formation is different in bilayers annealed at temperatures lower and higher than the melting point of In, or prepared by deposition of Cu and In at 200°C and 150°C, respectively. While the annealing of the precursor up to 200°C produces an alloy in the interface region of Cu and In layers, annealing at 500°C completely changes its morphology. The effect of alloy formation in the precursor on the selenized CuInSe 2 films is studied using structural, morphological, optical and compositional properties.