Sho Shirakata

Sharp band edge photoluminescence of high-purity CuInS2 single crystals

Temperature-dependent photoluminescence measurements were carried out between 8 and 300 K on CuInS2 single crystals grown by a traveling heater method. Ten distinct peaks were present in the near-band edge region. Four unknown peaks, observed at 8 K, were found to be due to bound exciton emission. Moreover, the luminescence remained stable up to room temperature.

Defect centers and optical absorption edge of degenerated semiconductor ZnO thin films grown by a reactive plasma deposition by means of piezoelectric photothermal spectroscopy

Undoped and Ga-doped (3 wt %) n-type ZnO thin films were grown by a reactive plasma deposition method on glass substrates at 200 °C under an oxygen flow rate from 0 to 50 SCCM. In this paper, we report on the defect and band edge related signals in the optical absorption spectra for ZnO thin film by using a piezoelectric photothermal (PPT) spectroscopy, which is effective in observing a nonradiative transition process. The PPT peak around 2.5 eV was observed only for the undoped ZnO samples grown under a low oxygen flow rate. This signal is considered to be related to the oxygen vacancies, because it disappears with the increase of the oxygen flow rates. No corresponding peak was found for the Ga-doped samples. This result indicates that Ga doping inhibits the generation of the oxygen vacancies, and it agrees with that from the first-principle electronic band structure calculations. We have also carried out the theoretical calculation for the optical absorption edge of degenerated ZnO as a function of the...

Temperature-dependent photoluminescence of high-quality GaInNAs single quantum wells

Temperature dependence of the near-bandedge photoluminescence (PL) was studied on a 10-nm Ga0.68In0.32N0.01As0.99/GaAs single quantum well (SQW) prepared by molecular-beam epitaxy for both the as-grown and the rapid-thermal-annealed (RTA) samples. Full width at half maximum (FWHM) value was 18–26 meV (for 80–280 K) and 11–19 meV (for 8–280 K) for the as-grown and the RTA samples, respectively. The FWHM of the RTA sample is the smallest among the samples prepared before and those reported so far. At low temperature (8–50 K), a PL peak due to the localized level was observed for the as-grown sample. However, no such peak was observed for the RTA sample. The PL intensity of the RTA sample was about 10–100 times larger than that of similar SQWs grown previously. These results indicate that the crystal quality of the RTA GaInNAs SQW used in this study is both outstanding and comparable with that of the corresponding GaInAs SQW.

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.

Photoluminescence and photoreflectance of GaInNAs single quantum wells

The temperature dependence of photoluminescence (PL) and photoreflectance (PR) was studied on a 10 nm GaInNAs/GaAs single quantum well prepared by molecular-beam epitaxy using a solid As source. PL was dominated by the near-band edge PL peak with a full width at half maximum (FWHM) value of 16–28 meV for 50–280 K. This indicates that the GaInNAs epilayer was of good quality. The temperature dependence of the band gap energy was studied by the PR measurements, and the dependence was comparable to that of the GaInAs. The near-band edge PL at low temperature exhibited a blueshift with an increase in excitation intensity and temperature (8–50 K). It had a large FWHM value of 24–26 meV at 8 K. These results are discussed in terms of carrier localization.

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.

Photoreflectance of Cu‐based I–III–VI2 heteroepitaxial layers grown by metalorganic chemical vapor deposition

Photoreflectance (PR) spectroscopy has been developed as a powerful technique for the characterization of the heteroepitaxial layer of the Cu–III–VI2 semiconductors having the chalcopyrite structure. PR measurements have been carried out in the energy region near the fundamental absorption edge at 77 K for the heteroepitaxial layers of CuAlSe2, CuGaSe2, CuGaS2, and CuAlS2 grown on GaAs and GaP substrates by means of the low‐pressure metalorganic chemical vapor deposition. Crystal quality, stress, and the crystallographic orientation have been well characterized, based on the analysis of the PR spectra in terms of transition energy, intensity, and broadening parameter.

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.