Search

Defect levels in Cu(In 1 − x ,Ga x )Se 2 (CIGS) were studied by using admittance spectroscopy with varying Ga concentration of x from 0.38 to 1.0. Two distinct peaks ( α and ζ ) were detected from all of CIGS samples. Peak α could be attributed to the shallow acceptors. Peak ζ was closely correlated with the surface potential of CIGS; therefore, it may have been caused by traps near the surface of CIGS such as grain boundary defects. The intensity of peak ζ increases with increasing Ga concentration. Further, this intensity was correlated with the ideality factor of the Al/CIGS Schottky junction. We suggest that the degradation of the performance of CIGS solar cell with increasing Ga concentration may be concerned with defects corresponding to peak ζ ˙

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.

To understand the degradation mechanism of organic solar cells (OSCs), the charge dynamics of conventional and inverted planar heterojunction OSCs based on boron subthalocyanine chloride (SubPc) and fullerene (C60) with identical buffers during the air exposure were investigated. The results of light intensity dependent open circuit voltage show that the bimolecular recombination is dominated in the fresh devices, regardless of the device structure. The appearance of transient peak in photocurrent after turn-on and the light intensity independent turn-off traces in transient photocurrent suggest that the rapid degradation of conventional device is due to the energy loss originated from the aggravated trap mediated recombination. In contrast, the half-lifetime of inverted device is ∼25 times longer than the conventional one. The improvement of stability is ascribed to the decrease of the trap generation possibility and the suppression of trap mediated recombination in the case of inverted structure, where the penetration of oxygen and water through buffer layer is avoided.

Temperature dependent time resolved photoluminescence (TR-PL) measurements have been performed in similar structured kesterite Cu2ZnSn(S,Se)4 and chalcopyrite Cu(In,Ga)Se2 thin film absorbers. It is investigated that at 26 K the measured lifetime values for a set of CZTSSe samples are more than one order magnitude higher than at room temperature whereas in CIGS this difference is not significant. At room temperature the significantly lower photoluminescence emission and minority carrier lifetime for CZTSSe comparing to CIGS suggest that the dominant non-radiative recombination processes are associated with CZTSSe than CIGS and it can limit the efficiency of CZTSSe based solar cells.

CuGaSe2 thin films with different Cu/Ga ratio were studied using photoluminescence (PL) method. Measurement at 1.4 K shows asymmetrically broadened PL spectra for all the samples where PL-peaks shift to lower energy with a decrease of the Cu/Ga ratio. Excitation power and temperature dependent PL spectra were explained using donor to acceptor pair transition under the influence of potential fluctuation. Magnitude of the potential fluctuation was calculated from lower energy tail of the emission spectra. Effect of Cu-deficiency on the properties of the CuGaSe2 films and fabricated solar cells were discussed in relation to the potential fluctuation at the band-edge.

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)

Deposition conditions of cuprous oxide (Cu2O) thin films on glass substrates and nitrogen doping into Cu2O were studied by using reactive radio-frequency magnetron sputtering method. The effects of defect passivation by crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, were also studied. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu2O at around 680 nm was enhanced, and the hole density was increased from 1016 to 1017 cm−3. Finally, polycrystalline p-Cu2O/n-ZnO heterojunctions were grown for use in solar cells. Two deposition sequences were studied, ZnO deposited on Cu2O and Cu2O deposited on ZnO. It was found that the crystallographic orientation and current–voltage characteristics of the heterojunction were significantly influenced by the deposition sequence, both being far superior for the heterojunction with structure Cu2O on ZnO than for the inverse structure. We successfully obtained a photoresponse for the first time in the deposited thin film of Cu2O/ZnO.

Structural properties of Eu-doped GaN with the Eu concentration of about 2 at% were studied by cross sectional transmission electron microscope (TEM) observation, and electrical and optical properties of n-GaN/Eu-doped GaN/p-GaN structure were discussed. Selected area diffraction pattern from Eu-doped GaN showed hexagonal structure and no other anomalous pattern was observed. These results suggest that the segregation of Eu and EuN were not formed. From the high resolution TEM observation of Eu-doped GaN, in addition to a small portion of cubic phase of GaN, high density of stacking irregularity which was hardly observed in undoped GaN was detected. Heterostructure of n-GaN/Eu:GaN/p-GaN was grown by molecular beam epitaxy on the p-type GaN template which was prepared by metalorganic chemical vapor deposition. Although the rectification behavior was observed, the electroluminescence was obtained not from the Eu-doped layer but from p-GaN layer. These results suggest that the hole mobility in Eu-doped GaN is extremely low probably due to the high density of stacking irregularity.

The molecular orientation and optical properties of phthalocyanine (H2Pc) films grown on glass with a 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) intermediate layer were investigated by infrared reflection absorption spectroscopy and absorption spectrum analysis. The orientation of the H2Pc molecular plane became almost parallel to the substrate surface with the introduction of a thin PTCDA layer on a glass substrate during initial growth. However, the orientation of the H2Pc molecular plane turned from parallel to oblique with increasing layer thickness, and finally the angle approached 26.5°, which indicates the formation of a crystalline phase of α-H2Pc with the b-axis perpendicular to the substrate surface. The optical absorption of the film was significantly affected by the molecular arrangement, and the importance of molecular orientation control was suggested.

The thickness dependences of the molecular orientation and crystal structure of phthalocyanine (H2Pc) films deposited on 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) template layers were investigated by X-ray diffraction (XRD) analysis and infrared reflection absorption spectroscopy (IR-RAS). It was found that the H2Pc molecular plane is almost parallel to the substrate surface when the H2Pc layer thickness is less than 10 nm, however, it tilts with increasing H2Pc thickness. In a H2Pc thickness of more than 300 nm, the tilt angle of the H2Pc molecular plane approached 26.5°, suggesting the growth of α-H2Pc with the b-axis perpendicular to the substrate surface. The images obtained by scanning electron microscopy showed that the thin and thick H2Pc layers have a columnar structure and a tangled fiber structure, respectively, consistent with the results of XRD analysis and IR-RAS.

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)

A single crystalline Eu-doped GaN was grown by gas-source molecular beam epitaxy and photoluminescence (PL) properties were studied. The PL spectra show red-emission at 622 nm originating from intra 4f–4f transition of Eu3+ 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 less than 1.6 meV, and thermal quenching of the luminescence was found to be small compared with the band-to-band transition. Fourier transform infrared spectra showed 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 intensity was observed between them. These results suggest that the deep defect level plays an important role in the radiative transition of Eu3+ ion in GaN and the optical process for the luminescence at 622 nm was discussed with the relation to the defect.

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.

Polycrystalline n-ZnO/p-Cu2O heterojunctions were fabricated by reactive sputtering for photovoltaic applications. Although electrical rectification was not reproducibly obtained in the as-grown samples, rectification was observed in the samples fabricated with cyanide treated Cu2O. We have previously shown that cyanide treatment has an effective role in passivation of defects in Cu2O, in analogy to hydrogen passivation. It is, therefore, believed that the observed improvement in the electrical rectification of the ZnO/Cu2O heterojunctions is attributable to the defect passivation near the hetero-interface. The effectiveness of the cyanide treatment and the importance of defect passivation in ZnO/Cu2O heterojunctions are presented. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We argue that the charge-transfer state of Eu3+ due to an electron transfer from N to Eu plays important roles in the luminescence properties of GaN:Eu3+ grown by gas-source molecular beam epitaxy. On the basis of the excitation spectrum of the Eu3+ luminescence, the excitation mechanism of Eu3+ under the GaN interband excitation is interpreted in terms of the energy transfer from the above charge-transfer state. The intra-4f shell 5D0–7F2 transition strength of Eu3+ in GaN, which is responsible for red luminescence, is large compared with those of most Eu3+-doped oxide materials. This fact can be attributed to the low-energy position of the charge-transfer state of Eu3+ in GaN. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

We probed defects in Eu- and Tb-doped GaN films grown on sapphire substrates by gas-source molecular-beam epitaxy with a monoenergetic positron beam. In both Eu- and Tb-doped samples, we observed vacancy clusters consisting of two or more vacancies. These defects were introduced by replacing Ga with rare-earth elements, and resulting in distortion of the host matrix. We studied the correlation between luminescence originating from the intra-4f-transitions of Eu3+ and the crystal quality of the GaN film. In film doped at 2-at. % Eu, the mean open volume of the vacancies near the interface between the GaN film and the sapphire substrate was found to be larger than that in the subsurface region. The increase in the open volume of the defects correlated with the lowering coordination symmetry of Eu3+ and the increase in the transition rate of its 4f-electrons.

(a) Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai,Tsukuba 305-8573, Japan(b) Photon Factory, Institute of Materials Structure Science,High Energy Accelerator Research Organization, Oho, Tsukuba 305-0801, Japan(Received July 22, 2002; accepted October 1, 2002)PACS: 61.10.Ht; 68.55.Ln; 71.20.Eh; 78.55.CrSingle crystalline Er-doped GaN was grown by ammonia source molecular beam epitaxy, and lumi-nescence properties, structural properties were studied. It was found that the intensity of the lumi-nescence in green spectral region from Er-doped GaN is two orders of magnitude stronger thanthat of Tb-doped GaN, which shows luminescence in green spectral region as well, and that thecoordination symmetry of N around Er in GaN was lower than that of Tb. The lower coordinationsymmetry may be one of the reasons for the improved luminescence of Er-doped GaN.

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.

Interface properties between p-GaN and Ni were investigated by vacuum ultra violet photoemission spectroscopy and atomic force microscopy. It was found that residual oxide on the GaN surface was removed by Ni deposition and subsequent annealing, and the layered Ni became an island-like shape with annealing in O2 ambient. Such annealing effects result in the direct contact of Au with atomically clean GaN in a Au/Ni/p-GaN structure. On the basis of these results, it can be concluded that the cleaning effect of Ni and island formation of oxidized Ni may play an important role in forming an ohmic contact to p-GaN.

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.

Properties of deep-level defects in CuGaSe2 thin-film solar cells were investigated using transient photo-capacitance (TPC) spectroscopy. Two Gaussian-shaped deep-level defects centered at around 0.8 eV and 1.54 eV above the valence band were identified. The electronic structure of the two defects was illustrated by a configuration coordinate model to explain the thermal quenching effect in the two defects, which considered a large lattice distortion for the 0.8 eV defect while no distortion for the 1.54 eV defect.

SrGa2S4:Eu is promising as a phosphor for blue-light excitation, for example in a blue LED. We have found that changing the gallium ratio of SrGa2S4:Eu in preparation influences the optical properties and crystal structure of SrGa2S4:Eu. As a result, the addition of excess gallium significantly affected the characteristics of SrGa2S4:Eu.

The effect of doping on position of interface states for metal doped bathocuproine (BCP) was studied with density functional theory (DFT). The doping of Ca atoms with BCP induces the formation of interface states with shift in their relative positions from Fermi level and approximately no shift in HOMO position of BCP molecule. The shift in the position of interface states towards higher binding energy was believed to be due to the presence of doping excess electrons from Ca at the interface. The analysis of modification in intensity of LUMO or E F or interface states, suggests the formation of multiply charged anions in heavily doped film. It clearly gives the direct evidence for the origin of the doping interface states in organic molecules. The effects of Ca doping on electrical properties were discussed.

Defect properties of Cu 2 ZnSn(S x ,Se 1−x ) 4 (CZTSSe) were investigated by admittance spectroscopy (AS). Two defect states (labeled E A1 and E A2 ) were observed in CZTSSe (x=0.15) with different Cu/Sn ratio. When the Cu/Sn ratio increased from 1.75 to 1.95, the activation energy of E A1 and E A2 decreased and the defect densities increased. The capture cross sections of E A1 and E A2 defects are in the order from 10−16 cm2 to 10−18 cm2, indicating that these two defects possibly do not impact on device performance.

Room-temperature two-wavelength excited photoluminescence (PL) measurements have been performed in the kesterite Cu 2 ZnSnS 4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) thin film absorbers. A defect level at 0.8 eV from the valence band and its properties are investigated. Two light sources of 635 nm and 1550 nm diode lasers, respectively, were used for above bandgap and 0.8 eV defect level excitation. The two-wavelength excited PL intensity was stronger than that only above-gap laser irradiation for the CZTS specimen. This phenomenon strongly suggests that the 0.8eV defect level acts as recombination center at room temperature. On the other hand, this defect may act as a trap in lower gap CZTSSe.

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.