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An evaporated CdSe layer was incorporated with a very thin CdS layer (40 nm) to optimize the performance of CdTe solar cells. Se alloys with narrower bandgaps were formed in the CdTe deposition and annealing process. With the incorporation of the CdSe layer at the optimal thickness, the highest short-circuit current density was obtained because of the simultaneous high carrier collection in the short-wavelength region and extended carrier collection in the long-wavelength region. Although narrow bandgap Se alloys were introduced, the open-circuit voltage was also significantly increased. As suggested by dark current–voltage measurements, the current leakage paths induced by the thin CdS layer can be significantly suppressed by the incorporation of the CdSe layer. The redshift of PL emission confirms the formation of the Se alloys. As revealed by transient photo-capacitance measurement, the Se alloys are less defective compared with that of CdTe, which is also in accordance with longer carrier lifetimes extracted from photoluminescence decays. The formation of Se alloys prior to the CdTe layer is favorable for the collection of photogenerated carriers. We attribute the performance enhancement to the formation of Se alloys, which are helpful to suppress the leakage current and enhance carrier collection. The incorporation of CdSe with thin CdS layers is validated to improve the diode quality and promising for further device performance improvement.

For copper indium gallium selenide [Cu(In1−x,Gax)Se2, CIGS]-based solar cells, defect states or impurity phase always form due to both the multinary compositions of CIGS film and the difficulty of controlling the growth process, especially for high Ga concentration. To further improve device performance, it is important to understand such formation of impurity phase or defect states during fabrication. In the work presented herein, the formation mechanism of impurity phase Cu2−δSe and its depth profile in CIGS film with high Ga content, in particular CuGaSe2 (i.e., CGS), were investigated by applying different growth conditions (i.e., normal three-stage process and two-cycle three-stage process). The results suggest that impurity phase Cu2−δSe is distributed nonuniformly in the film because of lack of Ga diffusion. The formed Cu2−δSe can be removed by etching the as-deposited CGS film with bromine-methanol solution, resulting in improved device performance.

International audience; Atomic layer etching (ALE) of Ga-polar GaN (0001) using a standard inductively coupled plasma-reactive ion etching system is achieved in this work. The sequential process is using Cl2 to modify the surface in the adsorption step. For the activation step, the authors compare two rare gas plasmas, namely, Ar and Kr, and show a much larger and well-defined ALE window for the latter. The ALE of GaN is demonstrated by etching mesa structures masked with a photoresist. A constant etching rate per cycle of two monolayers is obtained. The experimental conditions of this self-limited process are found by changing both the adsorption and activation times, together with the source power. This provides an atomic-scale process for nanofabrication, with significant improvements to the GaN surface.

2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (C26H20N2), known as bathocuproine (BCP), is a commonly used cathode buffer layer in conventional structure organic solar cells (OSCs). We demonstrated that BCP layer can also be used as a buffer layer in inverted structure OSCs. Unfortunately, the device exhibited an anomalous kink in the current density–voltage (J–V) characteristics, namely, an S-shaped J–V curve, leading to a low fill factor and low power conversion efficiency (PCE). To improve device performance, Ag-doped BCP layer (Ag:BCP) was used to replace the BCP layer. The results showed that the Ag:BCP layer can eliminate the S-kink in the J–V curve, resulting in a large improvement of fill factor and PCE. The origin of the S-shaped J–V curve was demonstrated to originate from the charge accumulation at the fullerene (C60)/BCP interface. On the contrary, the C60/Ag:BCP interface has favorable electronic properties with beneficial gap states for the transport of free carriers. Together with the good conductivity of Ag:BCP layer and the smooth morphology properties, the device performance was greatly improved by Ag:BCP buffer layer.

GaInNAsSb based solar cell structure has been grown using atomic hydrogen assisted molecular beam epitaxy (H-MBE). Transient photocapacitance (TPC) spectroscopy was applied to investigate the optically active defects in the Si-doped GaInNAsSb layer ( n -GaInNAsSb) of the structure. In addition to the interband transitions, a sub-band-gap optical transition was also determined using TPC spectroscopy. This sub-band-gap transition corresponds to an electron trap ( OE 1) at 0.78 eV below the conduction band ( E C ) edge of n -GaInNAsSb material. Thermally active defects were probed by the electrical measurement using admittance spectroscopy. Thus, a defect profile in the entire band gap of the n -GaInNAsSb film was obtained.

Upconversion (UC) emissions in the hexagonal phase of NaYF 4 :10%Er 3+ /NaYF 4 core/shell nanocrystals have been observed by off-resonant excitation at 1620 nm, whose energy is lower than that corresponding to the lowest f–f transition of Er 3+ ( 4 I 15/2 → 4 I 13/2 ). The UC emission spectrum consists of five main peaks at around 1540 ( 4 I 13/2 → 4 I 15/2 ), 980 ( 4 I 11/2 → 4 I 15/2 ), 800 ( 4 I 9/2 → 4 I 15/2 ), 660 ( 4 F 9/2 → 4 I 15/2 ) and 540 nm ( 4 S 3/2 → 4 I 15/2 ). The infrared absorption spectrum shows a weak peak at around 1600 nm which is separated from the 4 I 15/2 → 4 I 13/2 absorption peak by 315 cm −1 , and phonon bands of NaYF 4 has been observed at about 300 cm −1 in the Raman spectrum. From these results, the UC emission is interpreted to involve a phonon-assisted excitation process. The UC emission intensity under off-resonant excitation at 1620 nm exhibits a much stronger temperature dependence than that under resonant excitation at 1550 nm, which is consistent with the phonon-assisted interpretation.

We have performed ultraviolet photoelectron spectroscopy measurements and density functional theory calculations to study the electronic structure at the interface between organic semiconductor (3-(4-biphenylyl)-4-phenyl-5-(4-tert-butyl phenyl)-1,2,4-triazole (TAZ)) and metals (Ca, Mg, Ag, and Au). The basic mechanism of interface states at organic–metal interfaces can be understood by controlling the injection of charge carriers at these interfaces. The position of highest occupied molecular orbital relative to the Fermi level and the magnitude of the interface dipole are measured for each organic–metal interface. For TAZ on Ca, Mg, and Ag, interface states are observed near the Fermi level. However, no interface state is observed for TAZ on Au. It is analyzed qualitatively that the interface state is formed due to interaction of TAZ lowest unoccupied molecular orbital composed of C2p and metal s levels. It is suggested that the interface state plays an important role in charge transport at the interface. The mechanism of formation of interface states and electrical properties are discussed.

The systematic variations in structural, optical and electrical properties of lead phthalocyanine (PbPc)-based solar cells with organic buffer layers were investigated. Transition of the PbPc crystal from a monoclinic phase to a triclinic one was observed when the buffer layers were changed. The structural properties of triclinic PbPc grown on the sexithiophene (6T) buffer layer was superior to those of PbPc grown on the other organic substrates. Since the crystal growth of organic layers is dominated by the anisotropic intermolecular interactions at organic hetero-interfaces, the highly oriented 6T buffer layer with the atomically flat morphology promotes the growth of well-ordered PbPc layers. The performance of the organic solar cells (OSCs) was in direct correlation with the structural and electrical properties of the PbPc single layers. The formation of the triclinic phase enhances the spectrum sensitivity at around 890 nm, suggesting the enhancement of the current density of OSCs. Thus, the control of both crystal quality and phase of PbPc layers using a proper organic buffer layer is effective in enhancing the device performance of OSCs.

Structural, electrical and optical properties of Al doped ZnO (Al:ZnO) thin film of various thicknesses, grown by radio-frequency magnetron sputtering system were studied in relation to the application as a window layer in Cu(In1−xGax)Se2 (CIGS) thin film solar cell. It was found that the electrical and structural properties of Al:ZnO film improved with increasing its thickness, however, the optical properties degraded. The short circuit current density, Jsc of the fabricated CIGS based solar cells was significantly influenced by the variation of the Al:ZnO window layer thickness. Best efficiency was obtained when CIGS solar cell was fabricated with electrically and optically optimized Al:ZnO window layer.

Interaction of bathocuproine (BCP) with low work function metals Ca and K was studied by density functional theory. New peaks known as interface states were observed near the Fermi level in BCP molecular gap due to interaction of metal (s) levels with BCP LUMO mainly composed of C (2p). It was analyzed that the charge injection becomes better at the interface between BCP and low work function electrodes. These findings indicate that the improvement in electrical characteristics is due to the improvement in charge injection with low work function metal electrodes. The effects of low work function on electrical properties were discussed.

Electronic structure of the interface between a bathocuproine (BCP) molecule and metal surface was studied by density functional theory. The bond energy of BCP with metal surface increased with decreasing work function of metals, that is, Au, Ag, Al, Mg, and Ca in this order. The charge transfer also increased with decreasing metal work function. It was analyzed that the Fermi level and the lowest unoccupied molecular orbital of BCP are composed of both BCP and metal orbital component, and electron transport properties across the BCP and metal is discussed based on these analyses.

The relationship between optical and structural properties of Er-doped GaN grown by molecular beam epitaxy on sapphire (0 0 0 1) substrates was studied. The Er-related luminescence with wavelength at around 558 nm originated from 4 S 3/2 to 4 I 15/2 transition was observed. The intensity of the Er-related luminescence increased abruptly with the Er concentration of about 2 at% and showed a maximum at about 4 at%. X-ray diffraction analyses and Rutherford backscattering measurements suggested that most of the Er ions are incorporated into substitutional lattice site with the Er concentration of up to 2 at%, and then begin to occupy interstitial sites. The incorporation site of Er was found to be dependent on the Er concentration. Based on these results, the optically active center of Er in GaN is discussed.

The authors report the effect of a low-temperature buffer layer of Cu 2 O (LTB-Cu 2 O) on the growth of Cu 2 O thin films. The samples were prepared by radio frequency-magnetron sputtering. It was found that with the introducing of LTB-Cu 2 O, the linewidth of the X-ray diffraction peak decreases and the Cu 2 O thin films have preferred (1 1 1) orientation. By introducing LTB-Cu 2 O, the grain size sufficiently increases and the surface flatness improves, as indicated from the atomic force microscopy measurement. Under the optimum growth conditions, hole mobility of 256 cm 2 V −1 s −1 , which is the highest value reported so far, with the hole concentration of 1×10 14 cm −3 was achieved.

A degradation mechanism in InGaP/GaAs heterojunction bipolar transistor (HBT) was investigated. The current gain of the HBT grown at a V/III ratio of 0.7 for the base layer decreased when annealed at 670 °C. The samples grown at V/III ratios >1.0 showed constant photoluminescence intensity independent of annealing temperature, indicating that high thermal stability is obtained, while the V/III ratios 2 –H complexes in carbon-doped GaAs decreased when annealed at >550 and 650 °C, respectively. We suggest that the decomposition of the C 2 –H complex creates defect centers, which lead to the current gain reduction.

Luminescence spectra of Eu-doped GaN with varying Eu concentration and V/III ratio were investigated. Eu-related luminescence originating from the 5 D 0 - 7 F 2 transition of Eu 3+ was observed at about 622 nm. The luminescence basically consisted of three peaks, and the relative intensity of the three peaks was affected by the Eu concentration and the V/III, ratio. These results indicate that the Eu incorporation site is very sensitive to the Eu concentration and the V/III ratio.

Structural control of the organic intrinsic layer (i-layer), which consists of a mixture of p- and n-type organic semiconductor materials, and the organic p–i–n triple-layer structure has been investigated. Significant modification of these structures, which used metal-free phthalocyanine (H2Pc) and 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) as the p- and n-type materials, respectively, was observed by introducing a modification layer deposited on a glass substrate. The orientation of the molecular planes of both H2Pc and PTCDA in the i-layer (H2Pc:PTCDA mixture) and p–i–n structure (H2Pc/i-layer/PTCDA) formed onto a PTCDA modification layer was parallel to the substrate surface. On the other hand, when H2Pc was used as the modification layer, the orientation of the molecular planes in the i-layer was found to be random. These results indicate that the internal structure of the organic mixed-layer is strongly affected by the organic layer-substrate interface. The enhancement in optical absorption observed for the orientation controlled p–i–n structure suggests the importance of the control of molecular orientation for improving the optical properties in organic device structures.

Effects of V/III ratio on photoluminescence spectra of Eu-doped GaN grown by molecular beam epitaxy were investigated. The Eu-related luminescence originating from 5 D 0 – 7 F 2 transition of Eu 3+ was observed in the wavelength range of 620–624 nm under above-gap and below-gap excitations. Two luminescence peaks at 620.3 and 622.2 nm were observed in Eu-doped GaN with the Eu concentration of 1 at% under above-gap excitation, and a single peak at 620.3 nm was observed under below-gap excitation, suggesting multiple incorporation sites of Eu in GaN. The intensity ratio of the two peaks was found to vary with the V/III ratio. It was suggested that the incorporation site of Eu in Eu-doped GaN is sensitive to the V/III ratio.

We fabricated a thin film of Sm3+ doped ZnS by molecular-beam epitaxy (MBE) and investigated its photoluminescence properties. The excitation spectrum of the Sm3+ luminescence at 10 K is dominated by broad bands peaked at the free exciton energies, in contrast to many earlier reports on rare-earth-doped semiconductors. This result shows that the free exciton contributes significantly to the excitation of Sm3+ in our MBE sample, probably because the concentration of the trap centers which capture free excitons but do not transfer energy to Sm3+ is sufficiently low in our sample. The thermal quenching characteristic of the Sm3+ luminescence under interband excitation of the ZnS host is fitted well to a formula expressed by taking into account two deactivation channels. The origins of these deactivation channels are argued on the basis of previously proposed excitation mechanisms of the rare-earth ions in semiconductors.

A key prerequisite for fabricating opto-electronic devices with organic molecules is to control the molecular growth mode. In this work we present an ellipsometric study of the templating effect of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on the growth mode of metal free phthalocyanine (H 2 Pc). The difference in the anisotropic dielectric function between the H 2 Pc films grown on bare glass or silicon substrates compared to the anisotropic dielectric function of H 2 Pc films grown on PTCDA indicates a drastic change in molecular orientation. From the strong in-plane/out-of-plane anisotropy the average molecular tilt angle of H 2 Pc was found to be around 52° for the films grown on bare substrates and around 25° for H 2 Pc films grown on PTCDA. A splitting in the H 2 Pc Q-band was observed for the H 2 Pc films grown on PTCDA which indicates a deviation from the perfect H 2 Pc α-phase crystal.

The electrical properties of Al/Cu(In,Ga)Se2 (Al/CIGSe) Schottky junction and ZnO/CdS/CIGSe heterojunction were studied by admittance spectroscopy. Three distinct peaks (peaks α, ζ, and ϵ) were detected from all the CIGSe samples. The activation energies for the traps corresponding to peaks α and ζ were estimated to be approximately 10 meV and 300 meV, respectively. The peak α may be due to the shallow acceptor, and peaks ζ and ϵ may be due to defects in the CIGSe layer. The characteristics of the peak ζ have close correlation with the surface potential of the CIGSe layer. Therefore, the peak ζ may be caused by traps such as grain boundary defects near the surface of the CIGSe layer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and magnetic properties of Co doped GaN films grown by molecular beam epitaxy on sapphire(0001) substrates were studied. In the X-ray diffraction measurements and the extended X-ray absorption fine structure analysis, the existence of secondary phase that can be assigned as CoGa was suggested. Magnetic measurements were carried out over the temperature range from 5 to 300 K with the external magnetic field up to 5 Tesla. Clear hysteresis and small step in the magnetization curve suggesting ferromagnetic components were observed in the samples with Co concentration between 0.3 at% and 4.7 at%. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied the relationship between lattice-matching and crystalline quality of GaInNAs films grown on GaAs(001) substrates. The lattice mismatch between GaInNAs and GaAs was varied from –0.35% to 0.29%, and the coherent growth of GaInNAs for all of sample in this study was confirmed by (224) X-ray reciprocal space mapping. The full-width at half-maximum (FWHM) of (004) rocking curves showed minimum at the lattice-matched condition, and the FWHM did not depend on N composition. In Raman scattering measurements with the spectrum range of nitrogen-related local-vibrational mode (Ga-N like LO2 mode), a broad peak at 470 cm–1 was observed for all of GaInNAs samples. In addition to the peak at 470 cm–1, a clear shoulder peak at 490 cm–1 which has been understood as stress-relaxation mode was observed only at the lattice-matched sample. It was found that the lattice-matching has a significant effects not only on the crystal axis orientation but also local stress relaxation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The properties of the defect level at 0.8 eV above the valence band in Cu(In1−x,Gax)Se2 were studied by the two-wavelength excited photoluminescence (PL) method. The two-wavelength excited PL involves two different pumping light sources, that is, a 635 nm diode laser used as an above-gap excitation and a 1550 nm diode laser whose energy corresponds to the defect level resulting in having a role of charge saturation at the defect level. The intensity of the two-wavelength excited PL was stronger than that without 1550 nm laser irradiation for all specimens at room temperature regardless of Ga concentration. The results suggest the 0.8 eV defect level acts as recombination center at room temperature.

The recombination properties of exciton in fullerene (C60) deposited onto Ag-doped BCP layer with different doping concentrations were studied by photoluminescence (PL). The results show that low doping does not affect the exciton quenching of C60. On the contrary, the exciton quenching of C60 can be observed when the molar ratio of Ag to BCP is more than 1:1. The exciton quenching may be caused by the increase of metal-induced nonradiation centers at the interface since the existence of Ag cluster was confirmed in this case.

The structural and optical properties of europium-doped GaN were studied. The Eu-related luminescence was observed at 622 nm and originated from the intra-4f transition of the Eu 3+ ion. The intensity of the luminescence increased as the Eu concentration increased up to about 3 at%, and then abruptly decreased. In-situ reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) were carried out to study this luminescence quenching, and it was discovered that there is a close relationship between the luminescence intensity at 622 nm and the structural properties. The cause of the concentration quenching is likely related to the polycrystalline growth as well as to the formation of EuN.

Cyanide treatment which simply involves immersion of semiconductors in cyanide solutions can passivate interface states as well as surface states. When Si surfaces are treated with KCN solutions, a surface photovoltage greatly increases, and the surface recombination velocity is calculated to be decreased from ∼3000 cm/s to less than 200 cm/s. When the cyanide treatment is applied to ultrathin SiO2/single-crystalline Si structure, interface states are passivated. The passivation of the SiO2/Si interface states increases the energy conversion efficiency of 〈indium tin oxide (ITO)/SiO2/Si〉 MOS solar cells to 16.2% and decreases the leakage current density for 〈aluminum (Al)/SiO2/Si〉 MOS diodes to 1/3–1/8. When the cyanide treatment is performed on polycrystalline (poly-) Si, defect states in Si up to at least 0.5 μm depth from the surface are passivated, resulting in a vast increase in the energy conversion efficiency of 〈ITO/SiO2/poly-Si〉 solar cells and a decrease in the dark current density of 〈Al/SiO2/poly-Si〉 MOS diodes to 1/100–1/15 that without cyanide treatment. The defect passivation is attributed to the formation of SiCN bonds from defect states. SiCN bonds are found not to be ruptured by heat treatment at 800 °C and AM 1.5 100 mW/cm2 irradiation for more than 1000 h. Density functional calculations show that the thermal and irradiation stability results from strong SiCN bonds with the bond energy of 4.5 eV. When the cyanide treatment is performed on oxide/GaAs(1 0 0) structure, the interface state density decreases to ∼50%. The cyanide treatment can also passivate defect states in Cu2O films, resulting in increases in the carrier density and the band-to-band photoluminescence intensity.

Magnetic properties of the rare-earth-doped III-nitride semiconductor Ga1−xEuxN (x = 0.02) were studied together with X-ray absorption fine structure (XAFS) measurements. Experimental results show that the GaEuN layer is not only paramagnetic originating from the non-magnetic 7F0 ground level of trivalent Eu ions, but exhibits also ferromagnetic-like behaviour with easy axis perpendicular to the sample plane. This ferromagnetic component seen also at 300 K seems to be related to divalent Eu ions. The observed magnetic behaviour was also studied theoretically, by introducing a c-axis oriented molecular-field term that acts via the spin-polarized valence band of GaN. This result suggests that Eu ions interact with other Eu ions through the RKKY-type interaction. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetic properties of Er-doped GaN and Tb-doped GaN films grown by molecular beam epitaxy (MBE) on sapphire substrates (0001) were studied. Magnetization measurements were carried out under magnetic fields ranging from −5 to 5 T, and both samples were reasonably interpreted to exhibit a predominant paramagnetic character. However, for Er-doped GaN, clear finite steps around zero fields were observed throughout the temperature range of 5 K to 300 K, suggesting the coexistence of ferromagnetic order.

Magnetic properties of molecular-beam epitaxy grown Eu-doped GaN are studied. At high temperatures, clear hysteresis and clear saturation indicating the existence of a ferromagnetic-like phase are observed in the magnetization versus magnetic curves, while at low temperatures gradually saturating nonlinear field dependence of the magnetization is also observed. The easy-axis for the ferromagnetization is perpendicular to the sample plane. A theoretical model is proposed to account for the experimental results, assuming that a molecular field is acting on Eu ions via the spin-polarized valence-band of GaN. The experimental results are accounted for by assuming the coexistence of Eu2+ ions and Eu3+ ions interacting through the Ruderman-Kittel-Kasuya-Yoshida-type interaction.

The effects of the passivation of defects in polycrystalline nitrogen-doped cuprous oxide (Cu 2 O) thin films with hydrogen or cyanide treatment were studied. In the photoluminescence (PL) measurements, although the emission was not observed before treatment, luminescence of Cu 2 O at around 680 nm was observed after each treatment. This improvement in the luminescence property may be due to the passivation of non-radiative recombination centers by H or CN. The hole carrier concentration increased from the order of 10 16 to 10 17 cm -3 with hydrogen or cyanide treatment. From these results, both the hydrogen and cyanide treatments were found to be very effective to passivate defects and improve the optical and electrical properties of polycrystalline Cu 2 O thin films. The thermal stability of the passivation effects by the cyanide treatment is, however, superior to that by the hydrogen treatment.

Eu-doped GaN with various Eu concentrations were grown by gas source molecular beam epitaxy, and their structural and optical properties were investigated. With increasing Eu concentration from 0.1 to 2.2 at%, deterioration of the structural quality was observed by reflection high-energy electron diffraction, atomic force microscopy and X-ray diffraction. Such a deterioration may be caused by an enhancement of island growth and formation of dislocations. On the other hand, room temperature photoluminescence spectra showed red emission at 622 nm due to an intra-atomic f–f transition of Eu 3+ ion and Fourier transform infrared spectra indicated an absorption peak at about 0.37 eV, which may be due to a deep defect level. The intensity of the red luminescence and the defect-related absorption peak increased with increasing Eu concentration, and a close correlation in the increasing behavior was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu 3+ ion in GaN and the optical process for the luminescence at 622 nm was discussed with relation to the defect.

The valence states of Eu ions doped in GaN were investigated by the core-level X-ray and resonant photoemission spectroscopy. Although the photoluminescence spectra show the emissions from only trivalent Eu ions, the existence of both divalent and trivalent Eu ions is suggested by the Eu 3d core-level spectra. Taking into account the results of the 4d→4f resonant photoemission measurements, it can be concluded that valence transition from Eu 3+ to Eu 2+ occurs near the surface of GaN.

The effects of hydrogen treatment for polycrystalline nitrogen-doped cuprous oxide (Cu 2 O) thin films on their electrical and optical properties were studied. Although the emission was not observed before the treatment by hydrogen plasma, near band edge emission of Cu 2 O at around 680 nm was observed after the treatment. This result indicates that the non-radiative recombination centers possibly existing on and in polycrystalline grains are passivated by hydrogen. The hole density was controlled from the order of 10 16 –10 18 cm −3 by hydrogen treatment. The control of the hole density in such a range was achieved for the first time. From these results, hydrogen treatment was found to be very effective to passivate defects and improve electrical and optical properties of polycrystalline Cu 2 O.

Single crystalline Eu- or Tb-doped GaN was grown with the rare earth content up to 2% by molecular beam epitaxy (MBE) using ammonia. Sharp luminescence peaks originating from intra-atomic f–f-transition of the rare earth were observed, however, the intensity of Eu-doped GaN is two orders of magnitude stronger than that of Tb. The cause of the remarkable difference in the luminescence intensity between Eu- and Tb-doped GaN was discussed based on the coordination symmetry around the rare earth elements.

Eu-doped GaN has been grown by gas source molecular beam epitaxy on two kinds of sapphire (0 0 0 1) substrates; one has regular atomic step (AS) surface and the other one has random roughness (RR) surface. The effect of the substrate on the structural and optical qualities of Eu doped GaN was investigated. The atomic force microscope showed the epilayer on the AS substrate owns better surface morphology at each growth step such as nitridation, buffer layer, undoped GaN and Eu doped GaN layer. In the asymmetric X-ray diffraction of (1 1 2 4) , the full-width at half-maximum of the Eu-doped GaN on the AS substrate was almost a half value compared with that on the RR substrate. The room temperature photoluminescence spectra showed red emission due to f–f transition of Eu 3+ ion from both epilayers, however, the intensity of the red emission from the epilayer grown on the AS substrate is five times stronger than that on the RR substrate. The cause of the improvement in the structural and optical properties of Eu-doped GaN grown on the AS substrate is discussed.

A sharp luminescence peak from Sm doped ZnS at 650 nm which can be assigned as 4 G 5/2 - 6 H 9/2 transition of Sm 3+ ion was observed; however, no luminescence peak related to Eu 3+ , which can have luminescence in the similar spectral region, was observed from Eu doped ZnS. A defect level situated at about 0.36 eV above the valence band was detected only in Sm doped ZnS by infrared absorption spectroscopy. The cause of the difference in the luminescence properties between Sm doped ZnS and Eu doped ZnS was reasonably explained by the model of defect related energy transfer associated with the 0.36 eV level.

The dynamics of free carriers in polycrystalline ${\mathrm{CuIn}}_{1\ensuremath{-}x}$${\mathrm{Ga}}_{x}$${\mathrm{Se}}_{2}$ (CIGS) thin films were studied using picosecond time-resolved photoluminescence (PL) and femtosecond transient-absorption (TA) measurements. The PL spectrum and the TA decay component due to the band-to-band recombination of free carriers were observed in the picosecond time region. From excitation-photon-energy-dependent TA measurements, we identified a slow intraband relaxation of free carriers in the CIGS thin films. Collectively, the combination of PL and TA experiments reveal a global feature of energy relaxation and recombination processes of free carriers in the femtosecond to nanosecond time regions.

We report single-crystalline growth of Tb-doped GaN grown by gas source molecular beam epitaxy using NH3 as a nitrogen source, and the incorporation site of Tb in GaN. It was found that most of the Tb atoms are substitutionally incorporated into the Ga lattice site by Rutherford-backscattering ion channelling and extended x-ray absorption fine-structure analysis. Photoluminescence spectra, which show sharp peaks originating from 4f intra-atomic transitions of Tb3+ ions, with near-band-edge emission of GaN, were observed from the film whose Tb content was roughly estimated to be 2%, and their properties are discussed.

Sharp luminescence originating from intra-atomic 4f-4f transition from both Eu and Tb doped GaN was observed at 622 nm which can be assigned to 5 D 0 - 7 F 2 transition of Eu 3+ and at 545 nm which can be assigned to 5 D 4 - 7 F 5 transitions of Tb 3+ . However, the luminescence intensity of Eu 3+ is two orders of magnitude stronger than that of Tb 3+ though the content of the rare earth elements is almost the same. The cause of the difference in the luminescence properties was studied based on the defect-related energy transfer model, and the role of the defects observed by Fourier transform infrared (FTIR) spectra was discussed.

The electronic structure and chemical bonding at the interface between 1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) film and metal, Au and In were investigated by a photoemission spectroscopy. It was found that the NTCDA film on Au does not react with Au, on the other hand, the diffusion of In atoms into NTCDA film was observed for NTCDA on In, following the reaction with the carboxylic anhydride which is the end groups of NTCDA molecules. This reaction produces in-gap states above the highest occupied molecular orbital of NTCDA. The reactivity between NTCDA and metal seems to strongly depend on the ionization energy of metal atom.

The effects of annealing on the interface properties between p-GaN and Ni were investigated by ultra-violet photoemission spectroscopy. Photoemission spectra of p-GaN showed the existence of oxidized Ga, however, the oxidized Ga disappeared by Ni deposition. In addition, photoemission spectra of Ni/p-GaN after annealing showed the generation of metal-like Ga and the formation of Ni islands. These results indicate that the Ni deposition on GaN and subsequent annealing have an effect of making clean surface of GaN. Such cleaning effect and the island formation may play an important role in ohmic contact formation.

We categorized threading dislocations in GaN and GaInN multiple quantum wells and epitaxially lateral overgrown GaN into three types of line defects (edge, screw and mixed dislocations), and investigated the optical properties. It was confirmed by cathodoluminescence measurements that not only screw and mixed dislocations but also edge dislocations act as non-radiative centers in GaN. Epitaxial lateral overgrowth (ELO) technique can reduce the densities of all line-defects in a several μm wide wing region. Growth steps in the wing region were disturbed by the defects which were left in a seed region, and a complicated structure was formed at the surface of GaN and GaInN layers grown on ELO-GaN at low temperature. We believe that this surface structure formed by high supersaturation is a cause of In compositional spatial fluctuation and phase separation of GaInN alloy.

Photoluminescence (PL) properties of Sm-doped ZnSSe/ZnS multi-quantum wells were studied. The PL spectra were dominated by red-emission around 651 nm due to intra-atomic 4f–4f ( 4 G 2/5 – 6 H 9/2 ) transition of Sm 3+ . It was found that the PL intensity significantly increased and the peak position shifted to longer wavelength with decreasing well width. A significant enhancement of the PL intensity can be interpreted as an increase of the energy transfer probability from quantum wells to the Sm 3+ ions. A novel material which has a high efficiency of radiative recombination is suggested.

The quenching mechanism of luminescence by heavy Sm doping in ZnS was investigated by the Rutherford backscattering ion channeling (RBS/C) method. Red luminescence around 650 nm due to the 4f intra transition of Sm 3+ ion was observed in the Sm-doped ZnS, and the emission intensity becomes maximum at an Sm concentration of about 2×10 19 cm −3 . The RBS/C measurements showed a much shallower dip in the angular profile of Sm yields and a shallower and broader Zn-dip in the heavily Sm-doped ZnS than those in the moderately Sm-doped sample. These results indicate that the quenching of the luminescence by heavy doping of Sm is caused by the deterioration of the structural quality of ZnS and the large displacement of Sm atoms from the lattice sites.

A single crystalline Eu-doped GaN was grown by gas-source molecular beam epitaxy and photoluminescence properties were studied. The PL spectra show red-emission at 622 nm originating from intra 4f–4f transition of Eu 3+ ion without band-edge emission of GaN. The peak shift of the red-emission with the temperature variation from 77 K to room temperature is 1.6 meV, and thermal quenching of the luminescence was found to be very small for the same temperature range. A novel optical material whose emission wavelength and intensity are extremely stable with temperature was suggested.

The density of the vacancy-type defect in Er doped GaN was measured by positron annihilation spectrometry (PAS) and the correlation between the intensity of the Er-related luminescence was studied. A luminescence peak at 558 nm originating from 4 S 3/2 to 4 I 15/2 transition of Er 3+ was observed in Er-doped GaN. The intensity of the luminescence increased with increasing Er concentration and showed the maximum with the Er concentration of around 4.0 at%. The PAS measurements showed that the vacancy-type defect density increased with increasing Er concentration up to 4 at%, and around 4 at% of Er, the formation of defect complex such as V Ga – V N was suggested. The contribution of the defect to the radiative recombination of intra-4 f transition of Er is discussed.

The effect of Se/(Ga+In) ratio, expressed as Se/III, on the electrical, optical and structural properties of Cu(In,Ga)Se 2 , abbreviated as CIGS, thin film and on the solar cell performance was studied. CIGS films with various Se/III ratios were grown by the molecular beam epitaxy (MBE) process. With decreasing Se/III ratio, the resistivity of CIGS film was found to be increased and majority carrier hole concentration was decreased. This is probably due to the formation of Se-vacancy and/or vacancy complex. Again, the band-gap energy of CIGS film increased and the Cu content in the bulk of the film decreased with decreasing Se/III ratio despite the Ga/III ratio in the film remained almost unchanged. These results may suggest the formation of anti-site defects such as In Cu in CIGS. Solar cell performance degraded with decreasing Se/III ratio in CIGS absorber layer and the cause of the degradation was discussed.