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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.

application/pdf, The effects of group-IV element dopants on the structural and electrical properties of Cu2O thin films were studied. Similar dopant-induced behavior was found in the observed variations of the growth orientation and electrical properties of Si- and Ge-doped Cu2O thin films. Ge doping was found to induce electrically active acceptors with an activation energy of 0.18 eV, comparable to the 0.19 eV value of Si-doped Cu2O. These results suggest that locally formed silicate and germanate have the same effect on the structural and electrical properties of Cu2O. On the other hand, Sn and Pb likely act as donors when incorporated substitutionally onto Cu-lattice sites, although further study may be required to suppress self-compensation effects in Cu2O to achieve n-type conductivity.

application/pdf, The dependence of Europium (Eu)-related luminescence intensity on the Eu concentration in Eu-doped GaN was studied. This luminescence is observed at 622 nm and originates from the intra-4f transition of the Eu3+ ion. The intensity of the luminescence increased with increasing Eu concentration, up to about 2 at. %, and then abruptly decreased. It was found that polycrystalline growth began to be induced at Eu concentrations of more than 2 at. %. In addition, clear evidence for the formation of EuN compounds was obtained by x-ray diffraction and extended x-ray absorption fine structure analysis. The cause of the concentration quenching is likely to be related to the polycrystalline growth as well as EuN formation.

A comparative study with focusing on carrier recombination properties in Cu2ZnSn(S,Se)4 (CZTSSe) and the CuInGaSe2 (CIGS) solar cells has been carried out. For this purpose, electroluminescence (EL) and also bias-dependent time resolved photoluminescence (TRPL) using femtosecond (fs) laser source were performed. For the similar forward current density, the EL-intensity of the CZTSSe sample was obtained significantly lower than that of the CIGS sample. Primarily, it can be attributed to the existence of excess amount of non-radiative recombination center in the CZTSSe, and/or CZTSSe/CdS interface comparing to that of CIGS sample. In case of CIGS sample, TRPL decay time was found to increase with the application of forward-bias. This can be attributed to the reduced charge separation rate resulting from the reduced electric-field at the junction. However, in CZTSSe sample, TRPL decay time has been found almost independent under the forward and reverse-bias conditions. This phenomenon indicates that the charge recombination rate strongly dominates over the charge separation rate across the junction of the CZTSSe sample. Finally, temperature dependent VOC suggests that interface related recombination in the CZTSSe solar cell structure might be one of the major factors that affect EL-intensity and also, TRPL decay curves.

Crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, is studied in relation to electrical and optical properties of nitrogen-doped, polycrystalline Cu2O thin films, and its effect is compared with that of hydrogen treatment. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu2O at around 680 nm is enhanced, and the hole density is increased from the order of 1016 to 1017 cm−3, analogous to hydrogen treatment. The effects of the passivation by the hydrogen treatment completely disappear after annealing at 350 °C, while those of the crown-ether cyanide treatment stay unchanged after the same annealing treatment. From these results, the crown-ether cyanide treatment for polycrystalline Cu2O thin films can be concluded to be a more suitable method of passivating defects than the hydrogen treatment.

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.

Deep-level defects were investigated in Cu2ZnSn(S,Se)4 and Cu2ZnSnS4 thin-films using transient photocapacitance (TPC) spectroscopy. A deep-defect, OH1 centered around 1.0 eV above the valance-band (EV) of Cu2ZnSnS4 has been identified at room temperature (RT). However, OH1-defect could be identified in Cu2ZnSn(S,Se)4 at low temperature only. Absence of OH1-defect in Cu2ZnSn(S,Se)4 at RT explains its better performance comparing to Cu2ZnSnS4 solar-cell. A comparative study of the TPC spectra of the Cu(In,Ga)Se2 solar-cells was performed. Low intensity of defect-signal together with lower broadening of exponential band-tail in the TPC spectra were attributed to superior performance of Cu(In,Ga)Se2 solar-cells comparing to Cu2ZnSn(S,Se) counterpart.

The photocarrier dynamics of CuIn1−xGaxSe2 (CIGS) thin films were studied using white-light transient absorption (TA) measurements, as an understanding of this behavior is essential for improving the performance of solar cells composed of CIGS thin films. A characteristic double-peak structure due to the splitting of the valance bands in the CIGS was observed in the TA spectra under near-band-gap resonant excitation. From a comparison of the TA decay dynamics monitored at these two peaks, it was found that the slow-decay components of the electron and hole relaxation are on the nanosecond timescale. This finding is clear evidence of the long lifetimes of free photocarriers in polycrystalline CIGS thin films.

Low resistance Ohmic contacts of Au(Pt)Pd to p‐ZnTe were studied. The specific contact resistance of these contacts depends strongly on the annealing temperature and the Pd layer thickness. The specific contact resistance, measured by the transmission line model, is as low as 5×10−6 Ω cm2 when a sample is annealed at 200 °C. The optimum Pd layer thickness is 5–10 nm. This value of the specific contact resistance is two orders of magnitude lower than that of Au or Pt contacts to p‐ZnTe. The depth profiles of these contacts were investigated by Auger electron spectroscopy. The possible role of the Pd layer is discussed.

N‐doped p‐type ZnMgSSe was grown by molecular beam epitaxy. Nitrogen was induced by electron cyclotron resonance plasma. The maximum net acceptor concentration (NA−ND) and the activation energy of the nitrogen acceptor (EN) depend on the band‐gap energy of ZnMgSSe. With increasing band‐gap energy, the maximum NA‐ND is decreased and EN is increased. The maximum NA‐ND and the EN of ZnMgSSe with a band‐gap energy of 3.05 eV at 77 K are 2.5×1016 cm−3 and 140 meV, respectively.

We have experimentally investigated refractive indices n of ZnxMg1−xSySe1−y, using the ellipsometry method and reflection‐spectrum measurement. The samples are epitaxial films of undoped ZnxMg1−xSySe1−y grown by molecular beam epitaxy on semi‐insulating GaAs substrates. The obtained dispersion relations of n in the transparent region are classified by the band‐gap energy Eg. We have found that the refractive index n of ZnxMg1−xSySe1−y decreases as Eg increases. These results will be available for the design of blue laser diodes containing ZnMgSSe.

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.

Admittance spectroscopy has been performed to investigate the effect of antimony (Sb) on GaInNAs material in relation to the deep-level defects in this material. Two electron traps, E1 and E2 at an energy level 0.12 and 0.41 eV below the conduction band (E C), respectively, were found in undoped GaInNAs. Bias-voltage dependent admittance confirmed that E1 is an interface-type defect being spatially localized at the GaInNAs/GaAs interface, while E2 is a bulk-type defect located around mid-gap of GaInNAs layer. Introduction of Sb improved the material quality which was evident from the reduction of both the interface and bulk-type defects.

We studied vacancy type defects in Ga‐doped ZnSe films grown by molecular beam epitaxy using an energy‐variable monoenergetic positron beam. We found that the concentration of negative charged vacancies, such as Zn vacancies, increases as the Ga atom concentration increases. This result indicates that the doping by Ga atoms induces the formation of Zn vacancies in a ZnSe film. We believe that these defects will cause the saturation of active carriers in n‐type ZnSe films.

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.

Polycrystalline CuGaSe2 thin films grown with various Cu/III(=Cu/Ga) ratios were investigated by positron annihilation spectroscopy (PAS). The line-shape parameter S of the spectra was used to characterize defects in CuGaSe2 films. The S-parameter in positron annihilation spectra increased with decreasing bulk Cu/III ratio in the CuGaSe2 film. Experimental results combined with theoretical calculation show the formation of multiple vacancy-type defect complexes in the near-surface region of the CuGaSe2 film when Cu-content in the film is decreased. These point defects appear to cause the higher S-parameter in PAS measurement.

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...

Erbium-doped GaN samples grown with different V/III ratios through gas source molecular beam epitaxy were prepared to investigate the influence of the V/III ratio on the defect formation and the optical activity of the Er-related luminescence center. Obvious V/III ratio dependence was observed in photoluminescence measurement. Positron annihilation spectroscopy measurements suggested that VGa-VN vacancy-complexes formed in these samples and that the VN/VGa ratio depended on the V/III ratio. The generation of Er-VN defect complexes, which act as high optical active luminescence centers, is suggested as the cause of improved optical properties of Er-doped GaN grown with a lower V/III ratio.

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.

Variation of the luminescence spectra of Eu-doped GaN with varying Eu concentration ranging from 0.6to8.0at.% was investigated. Eu-related luminescence originating from the D05‐F27 transition of Eu3+ was observed at about 622nm. The luminescence basically consisted of three peaks. The relative intensity of the three peaks changed remarkably at the Eu concentration of around 2.0–3.5at.% which corresponds to a structural phase transition from a single crystalline to a polycrystalline structure. This indicates that the incorporation site of Eu in GaN is very sensitive to the structural properties of the GaN host material.

application/pdf, We have studied photoluminescence (PL) properties of Eu3+-doped GaN (GaN:Eu3+) epitaxial films by microscopic PL imaging spectroscopy. The GaN:Eu3+ epitaxial films exhibit efficient red luminescence related to intra-4f transitions of Eu3+ ions. The intensity and the spectral shape of the Eu3+-related PL are sensitive to the Eu3+ concentration, the excitation wavelength, and the monitored position. Microscopic PL imaging spectroscopy revealed that efficient red luminescence of GaN:Eu3+ epitaxial films is due to Eu3+ ions around point defects and dislocations.

application/pdf, Hole traps in p-type Cu2O were studied by means of deep level transient spectroscopy in the heterostructure of p-Cu2O/i-ZnO/n-ZnO. In addition to the trap level at about 0.45 eV from the valance band edge, which is already reported as being due to Cu vacancy, we found a new trap level at about 0.25 eV. The new trap is tentatively assigned as Cu-di-vacancy from the trap concentration dependence on oxygen flow rate and substrate temperature.

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.

Degradation of GaAs/AlGaAs double‐heterostructure light‐emitting diodes under accelerated aging tests has been investigated through cathodoluminescence measurements. We observed an increase in intensity of the band‐edge emission and a decrease in that of defect‐related deep level emision for the cladding layers. As for the active layer, a decrease was observed in the intensity of the band‐edge emission. These results suggest that defects migrate gradually from the cladding layer into the active layer.