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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.

Extended x-ray absorption fine structure (EXAFS), Rutherford-backscattering ion channeling, and particle induced x-ray emission channeling (PIXE/C) measurements have been performed in order to investigate compensation centers in Cl doped ZnSeTe. The EXAFS results from Cl doped ZnSeTe suggest that almost all Cl atoms are incorporated into substitutional Se lattice sites, which seems to indicate that Cl atoms themselves are not responsible for the compensation centers. The PIXE/C angular profiles were measured across the 〈100〉, 〈110〉, and 〈111〉 axes for undoped ZnSeTe. Comparing the angular profiles for Zn Kα, Se Kα, and Te Lα x-ray yields, it was found that some portion of the Te atoms (∼1020 cm−3) are located at tetrahedral interstitial sites. From these results, the difficulty of realizing n-type ZnSeTe is considered to be due to the existence of the interstitial Te atoms which act as acceptors.

Using ion-channeling methods, the thermal stability of crystallinity for heavily Ga-doped ZnSe epitaxial layers grown on GaAs (100) substrates was investigated. Some distinctive features were found: The crystallinity of as-grown heavily Ga-doped ZnSe is similar to that of undoped ZnSe. After thermal annealing at 450 °C, the crystallinity of Ga-doped ZnSe is degraded, although that of undoped ZnSe is not significantly changed. It is expected that doped Ga atoms are displaced from the regular lattice sites after annealing. The degree of this degradation depends on the crystallinity of the as-grown Ga-doped ZnSe itself. These results indicate the degradation of the crystallinity for Ga-doped ZnSe epilayers is mainly caused by the interaction between the doped Ga atoms and grown-in point defects in the epitaxial layers.

Deep levels in Ga‐doped ZnSe films grown by molecular‐beam epitaxy have been investigated through deep‐level transient spectroscopy. Au/Ga‐doped ZnSe Schottky diodes have two well‐defined electron traps with activation energies of 0.26 eV (trap A), and 0.40–0.56 eV (trap B). The activation energy of trap B depends on the free‐carrier concentration. The trap concentration in trap A and B is linearly proportional to the free‐carrier concentration. The trap concentration, normalized by the free‐carrier concentration, increases as the VI/II (Se/Zn) ratio increases. From these observations, we conclude that traps A and B can be ascribed to the complex of either the Zn vacancy and Ga or the interstitial Se and Ga.

The properties of defect levels located 0.8 eV above the valence band in Cu(In1−x,Gax)Se2 thin films were investigated by a steady-state photocapacitance method. When illuminated by light with a photon energy of 0.8 eV at 60 K, a fast increase, followed by a slow increase, was observed in the photocapacitance transients of all samples. Upon being re-exposed, samples with a low bandgap energy showed a slow decrease in photocapacitance transients. These observations were interpreted using a configuration coordinate model assuming two states for the 0.8 eV defect: a stable state D and its metastable state D* with a large lattice relaxation. The difference in the evolution mechanisms of the photocapacitance transients was attributed to the difference in the optical transition of carriers between the two states of the 0.8 eV defect and the valence and conduction bands.

To clarify the role of buffer layer in organic solar cells (OSCs), the electronic properties of bathocuproine (BCP)/Mg interface were systematically investigated by using ultraviolet photoemissions spectroscopy, high-resolution X-ray photoemission spectroscopy, angle-resolved X-ray photoemission spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results show there are gap states at the interface, which are caused by the interaction between BCP and Mg. The formation of Mg-N bond was found at the interface. The NEXAFS measurements show that BCP molecules for 1-2 monolayers are lying-down on the substrate, whereas orient randomly for thick BCP layer. It was supposed that the gap states and the highly-ordered orientation of thin BCP layer are the reasons for improving the performance of OSC with BCP buffer layer and low work function metal cathode.

Cu(InxGa1−x)Se2 (CIGS) films, grown under various Se fluxes, have been investigated by the positron annihilation spectroscopy. The line-shape parameter (S) of the positron annihilation spectra was used to characterize the defects in the CIGS films. When Se flux was decreased, the S parameter at the surface and subsurface region of the films increased. This phenomenon was attributed to the increased concentration of the defect complex formed between Se and Cu vacancies. S parameter at the surface region was significantly higher than that in the bulk region of each film. It was explained with the model of compositional inhomogeneities along the depth of the film. Solar cell performance strongly correlated to the S parameter at the surface region of the CIGS films. Sufficient Se flux was found to be effective to reduce the S parameter, thereby, to suppress the defects in the films.

Bias dependence of the admittance spectroscopy of GaInNAsSb based solar cell structure has been performed to identify and characterize the type of defects, for example interface and/or bulk type defects in a moderately Si doped GaInNAsSb (n-GaInNAsSb) layer in the structure. From the zero bias admittance spectrum, three peaks namely E1, E2, and E3 corresponding to the localized level at 0.03 eV, 0.07 eV, and 0.16 eV below the conduction band edge (EC) of n-GaInNAsSb material, respectively, were found. Constant position of E2 and E3 peak in the admittance spectra in response to the various applied DC reverse bias suggests that E2 and E3 are related to the bulk type defects being spatially homogeneous throughout the bulk of the n- GaInNAsSb film. However, bias dependence admittance of the E1 peak along with the capacitance - voltage (C-V) measurement as well as characteristic feature in the temperature dependent junction capacitance value strongly suggests that E1 peak might be originated due to the free ca...

Quantum dot solar cells (QDSCs) comprised of 10 stacked pairs of strain-compensated InGaAs/GaNAs QD structure have been fabricated by atomic hydrogen-assisted molecular beam epitaxy. A homogeneous and high-density QD array structure with improved in-plane ordering and total density of ∼1012 cm−2 has been achieved on GaAs (311)B grown at 460 °C after stacking. The external quantum efficiency (EQE) of InGaAs/GaNAs QDSC increases in the longer wavelength range due to additive contribution from QD layers inserted in the intrinsic region. The short-circuit current density measured for QDSC is 17.2 mA/cm2 compared to 14.8 mA/cm2 of GaAs reference cell. Further, an increase in EQE due to photocurrent production by 2-step photon absorption has been observed at room temperature though it is still small at around 0.1%.

We have investigated the properties of multi-stacked layers of self-organized In0.4Ga0.6As quantum dots (QDs) on GaAs (311)B grown by molecular beam epitaxy. We found that a high degree of in-plane ordering of QDs structure with a six-fold symmetry was maintained though the growth has been performed at a higher growth rate than the conventional conditions. The dependence of photoluminescence characteristics on spacer layer thickness showed an increasing degree of electronic coupling between the stacked QDs for thinner spacer layers. The external quantum efficiency for an InGaAs/GaAs quantum dot solar cell (QDSC) with a thin spacer layer thickness increased in the longer wavelength range due to additive contribution from QD layers inserted in the intrinsic region. Furthermore, a photocurrent production by 2-step photon absorption has been observed at room temperature for the InGaAs/GaAs QDSC with a spacer layer thickness of 15 nm.

The Cu-dependent phase transition in polycrystalline CuGaSe2 thin films has been studied by an electron probe micro-analyzer (EPMA) and the synchrotron x-ray diffraction method. A Cu-deficiency parameter, Z, defined as (1 − Cu/Ga) was used to study the phase transition. Upon increasing the Z-value, the composition of the films on the Cu2Se-Ga2Se3 pseudo binary tie line was found to shift from the stoichiometric CuGaSe2 (1:1:2) (Z = 0) to the Ga-rich composition through the formation of several ordered defect compounds.The structural modification in the Cu-poor CuGaSe2 film has been investigated by the synchrotron x-ray diffraction method. The existence of the Cu-poor surface phase over the near-stoichiometric bulk CuGaSe2 film was confirmed by the fitting of the accelerated voltage dependent EPMA data.The Cu-dependent phase transition in polycrystalline CuGaSe2 thin films has been studied by an electron probe micro-analyzer (EPMA) and the synchrotron x-ray diffraction method. A Cu-deficiency parameter, Z, defined as (1 − Cu/Ga) was used to study the phase transition. Upon increasing the Z-value, the composition of the films on the Cu2Se-Ga2Se3 pseudo binary tie line was found to shift from the stoichiometric CuGaSe2 (1:1:2) (Z = 0) to the Ga-rich composition through the formation of several ordered defect compounds.The structural modification in the Cu-poor CuGaSe2 film has been investigated by the synchrotron x-ray diffraction method. The existence of the Cu-poor surface phase over the near-stoichiometric bulk CuGaSe2 film was confirmed by the fitting of the accelerated voltage dependent EPMA data.

A systematic study on the energy level alignment, chemical interaction, and electron doping at interfaces between bathocuproine (BCP) and various types of metals (Au, Cu, Ag, Mg, and Ca) was carried out by performing ultraviolet photoelectron spectroscopy and electronic conductivity measurements. The energy level alignment at BCP/metal interfaces was found to depend on the metal work function (Φm). For BCP on Au and Cu, whose Φm exceeds 4.3 eV, the energy shift in the highest occupied molecular orbital (HOMO) level with respect to the metal Fermi level (EF) almost accords with the variation in Φm. For BCP on Ag, Mg, and Ca, whose Φm is below 4.3 eV, the HOMO energy level is fixed at 3.7 eV with respect to EF regardless of Φm and new electronic states, called gap states, appeared at BCP/metal interfaces. Since the appearance of gap states is correlated with the energy of the lowest unoccupied molecular orbital (LUMO) level with respect to EF, these states appear to have formed mainly through the interactio...

application/pdf, Electron emission spectra resulting from thermal collision of He*(23S) atoms with 2,9-demethyl-4,7-diphenyl-1,10-phenanthroline (BCP) films deposited on metal substrates were measured to characterize gap states emerged at the organic-metal interface. For BCP on Au, the gap state is originated from weak chemisorption and serves as a mediator of metal wave functions to the first layer. For BCP on K, organic-metal complex is formed by spontaneous diffusion, yielding the gap states delocalized over the film. In the interfacial region, all the gap state reveals an incommensurate shift with the valence band top of the film, indicating the breakdown of the Schottky–Mott model as evaluating the transport characteristics in organic-metal system.

application/pdf, The systematic variations in the structural, optical, and electrical properties of polycrystalline Cu(In,Ga)Se2 (CIGS) thin films with Na doping level were investigated. Precise control of the Na concentration in CIGS films was demonstrated using alkali-silicate glass thin layers of various thicknesses deposited on substrates prior to CIGS growth. The CIGS grain size was observed to decrease with increasing Na concentration, although the surface morphology became smoother and exhibited a stronger (112) texture, which has been demonstrated consequence of larger grain size. The Ga composition gradient in the CIGS films was found to become large due to the presence of Na during growth, which in turn led to a decrease in the nominal band gap energy. Variations in the photoluminescence spectra and electrical properties suggested that the formation of an acceptor energy state, which may originate from OSe point defects, was enhanced in the presence of Na. This result suggests that not only Na, but also the presence of O in combination with Na contributes to the compensation of point defects and enhances p-type conductivity in CIGS films.

application/pdf, A relationship between intra-4f transitions of Er and vacancy-type defects in Er-doped GaN was studied by using a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured for Er-doped GaN grown by molecular beam epitaxy. A clear correlation between the defect concentration and the photoluminescence (PL) intensity was observed. The major defect species detected by positrons was identified as a Ga vacancy VGa, and its concentration increased with increasing Er concentration [Er]. For the sample with [Er]=3.3 at. %, the maximum integrated intensity of PL was observed. The VGa concentration was above 1018 cm−3 and additional vacancies such as divacancies started to be introduced at this Er concentration. For the sample with higher [Er], the PL intensity decreased, and the mean size of vacancies decreased due to an introduction of precipitates and/or metastable phases.

The interface of a GaAs/AlxGa1−xAs heterostructure grown by metalorganic chemical vapor deposition has been studied by photoluminescence spectroscopy by using a step‐etching technique. Luminescence peaks associated with vacancy complexes emitted from both GaAs and AlxGa1−xAs layers were observed, and these peaks were remarkably intense on both sides of the GaAs/AlxGa1−xAs interface region. This result can be explained by the accumulation of vacancies in the interface region. The cause of this vacancy accumulation at the interface is discussed.

Photoluminescence at various temperatures of highly doped (5×1017 cm−3) Al0.3Ga0.7As grown by molecular‐beam epitaxy has been studied. A luminescence peak at about 100 meV below the edge emission was observed. The intensity of this peak decreases as the As4/(Ga+Al) flux ratio increases for p‐type samples, but decreases for n‐type samples. An analysis based on the configurational coordinate model suggests that the origin of the luminescence may be a vacancy‐dopant complex which acts as a nonradiative center at room temperature, as the intensity of the peak is inversely proportional to the intensity of the edge emission at room temperature.