Atsuhiko Fukuyama

Effect of the carbon acceptor concentration on the photoquenching and following enhancement of the photoacoustic signals of semi-insulating GaAs

The spectral and the time dependent piezoelectric photoacoustic (PPA) measurements under the continuous light illumination were carried out at 85 K to investigate nonradiative recombination processes involving EL2 defect levels in carbon concentration controlled and not intentionally doped semi-insulating (SI) GaAs. The decrease of the PPA signal due to the photoquenching effect of EL2 is observed for a short period of illumination in the photon energy region from 1.0 to 1.3 eV. Since almost all of the carbon acceptors are compensated by deep donor EL2 in SI GaAs, electron occupancy of EL2 level can be controlled by changing the carbon acceptor concentration. It is found that the photoquenching becomes drastic with increasing the carbon concentration. After fully photoquenching, the PPA signal increases again through a local minimum by the continuous light illumination and finally exceeds the initial value before illumination until the saturation level is reached. The deep donor level EL6 and its metastab...

Piezoelectric detection of the photoacoustic signals of n‐type GaAs single crystals

Piezoelectric photoacoustic (PA) measurements on liquid‐encapsulated‐Czochralski‐grown n‐GaAs were carried out at room temperature. A continuous broad band below 1.35 eV and a peak at 1.383 eV were observed in the PA amplitude spectra. By comparing with the optical‐absorption spectra, it is concluded that the broad band is due to the electron transition involving the EL2 deep‐lying defect levels. For the observed peak at 1.383 eV, the origin is considered to be dislocation related. The possibility that this peak is an apparent one expected from the proposed models for the PA signal generation is denied.

OPTICAL CHARACTERIZATIONS OF CUINSE2 EPITAXIAL LAYERS GROWN BY MOLECULAR BEAM EPITAXY

CuInSe2 (CIS) films with Cu/In ratios of γ=0.82–1.79 have been grown on a GaAs (001) substrate by molecular beam epitaxy. Piezoelectric photoacoustic (PPA) measurements were carried out from liquid helium to room temperature to investigate nonradiative carrier recombination processes in comparison with photoluminescence (PL) measurements which directly detected radiative carrier recombination processes. Three PPA signal peaks which corresponded to band gap energies of the CIS (AB and C bands) and the GaAs substrate, were clearly obtained between liquid helium and room temperatures. A free-exciton emission line was observed up to 200 K in the PL spectra. Two additional peaks on intrinsic defects which are Cu vacancy (VCu) and interstitial In (Ini) were observed in the In-rich CIS samples. The PPA measurements were useful in investigating the defect levels and the band gap energy in the CIS/GaAs thin films.

Annealing time and temperature dependence for photo-induced crystallization in amorphous GeSe2

Abstract Annealing time and temperature dependence for thermal- and photo-induced crystallization in amorphous GeSe2 was studied by Raman measurement. Amorphous GeSe2 crystallizes to a low temperature form, LT-GeSe2 or a high temperature form, HT-GeSe2. We investigated the growth conditions of crystallization with and without laser irradiation for LT-GeSe2 and HT-GeSe2 from the relation of a time–temperature–transformation (T–T–T) diagram. The activation energy for thermal crystallization from amorphous GeSe2 to LT-GeSe2 is about 4.6 eV, whereas that for photo-induced crystallization is about 2.6 eV.

Low-Temperature Photoluminescence of Nanostructured ZnO Crystal Synthesized by Pulsed-Laser Ablation

The optical properties of ZnO nanorods and nanowires grown by our newly developed nanoparticle-assisted pulsed-laser deposition (NAPLD) technique were studied by photoluminescence (PL) spectroscopy. This NAPLD technique, which does not require a catalyst for crystal growth, is expected to synthesize high-quality nanostructured ZnO crystals. The green luminescence (GL) band and ultraviolet luminescence (UVL) observed at low temperature were investigated for these crystals. The UVL band consists of several sharp emission lines due to free excitons, donor-bound excitons, donor-acceptor pair (DAP) transitions, and their longitudinal-optical (LO) phonon replicas. The free-exciton binding energy was estimated to be 59 meV and the band gap of these ZnO crystals was determined to be 3.435 eV at 10 K. The temperature variation for the energy positions of free-exciton emission was fitted by the Manoogian–Woolley equation and the Debye temperature was estimated to be 505 K. The low-temperature PL investigation of our nanostructured ZnO crystals that were easily synthesized by our newly developed NAPLD technique demonstrated their excellent crystallinity.

Piezoelectric photothermal study of AlxGa1−xAs epitaxial layer (x=0.22, 0.28, and 0.5) grown on semi-insulating GaAs substrate

Piezoelectric photothermal measurements of an AlxGa1−xAs (x=0.22, 0.28, and 0.5) epitaxial layer grown on a GaAs substrate were carried out in the temperature range of 297 to 80 K. In addition to the band gap signal of the GaAs substrate, the direct transition gaps of AlGaAs were clearly observed in the higher photon energy region. It was experimentally confirmed that the temperature coefficient of the direct transition gap of AlxGa1−xAs alloy decreases with increasing Al mole fraction. By conducting the quenching light illumination measurements at 80 K we concluded that the photoexcited electrons in the AlGaAs epitaxial layer drifted under the influence of an electric field present at the AlGaAs/GaAs interface. The drifted electrons eventually recombined with the ionized EL2 centers in the SI GaAs substrate.

Piezoelectric Photothermal and Photoreflectance Spectra of InxGa1-xN Grown by Radio-Frequency Molecular Beam Epitaxy

Piezoelectric photothermal spectroscopy (PPTS) measurements were carried out on InxGa1-xN (x=0.01?0.32) thin films grown by radio-frequency molecular beam epitaxy. We found that the band energy shifts to the lower energy side of the spectrum (red shift) with an increase in the indium composition from 0.01 to 0.32. For samples with a lower indium composition, we were able to observe the exciton contribution, and the binding energy was estimated to be 27 meV (x=0.01). Since conventional photoreflectance (PR) spectroscopy was unable to observe signals for the samples with a higher indium content (x=0.13, 0.2, and 0.32), the usefulness of this PPTS method for samples with phase fluctuation is demonstrated.

Investigation of deep levels in semi-insulating GaAs by means of the temperature change piezoelectric photo-thermal measurements

The temperature variation of the piezoelectric photo-thermal (PPT) signal intensity of semi-insulating (SI) GaAs from 20 to 150 K was measured. Four peaks at 50, 70, 110, and 125 K were observed in the PPT signal. From the theoretical analysis based on the rate equations of electrons in the conduction band and deep levels, we concluded that the observed four peaks were due to the nonradiative electron transitions through EL6, EL7, EL15, and an unspecified deep level, respectively. Deep levels with extremely low concentration (1012–1015 cm−3) were clearly identified in SI GaAs by using the PPT method.

Temperature dependence of photoacoustic spectra in CuInSe2 thin films grown by molecular beam epitaxy

CuInSe2 (CIS) thin films, the thickness of about 1.0 μm and composition of CuIn ratio (γ = 1.79), were grown on (0 0 1)-oriented GaAs substrate by molecular beam epitaxy (MBE) at substrate temperature of Ts, = 450°C. The samples have been characterized by means of piezoelectric photoacoustic (PPA) measurements between liquid helium (4.2 K) and room temperature (300 K). Two distinct PPA signals due to band gap of CIS and GaAs are observed in the whole temperature range from 4.2 to 300 K and the PPA signals of CIS decrease curvilinearly at the temperature range. Since the PPA signals of CIS thin films can be obtained up to room temperature, the PPA measurements are quite effective to obtain the optical characterizations, especially for the nonradiative recombination processes.

Piezoelectric Photothermal and Surface Photo-Voltage Studies of Carrier Recombination Mechanism at Interface of Si

Room-temperature piezoelectric photothermal spectroscopy (PPTS) and surface photo-voltage spectroscopy (SPVS) were carried out on two types of Si p–n junction sample to investigate carrier recombination processes at the interface. It was found that the SPVS peak positions of each spectrum depend on the direction of illumination: a 1.18 eV peak for substrate-side illumination and a 1.25 eV peak for epitaxial-layer-side illumination. We concluded that peak position corresponds to the specific photon energy at which optical penetration length is equal to the distance from the irradiated surface to the p–n junction. On the contrary, the PPTS peak positions depended on the structure of the samples: a 1.18 eV peak for the p/N sample and a 1.25 eV peak for the n/P sample. The PPTS amplitude spectrum of the n/P sample split into two distinctive peaks as a result of frequency being increased. One was a 1.25 eV peak, whose origin is not clear yet. The other was a newly appearing 1.18 eV peak, whose position corresponds to that shown by SPVS.