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The carrier lifetime in CdTe is strongly limited by the nonradiative recombination via defects. Here, deep level defects in CdTe thin-film solar cells are revealed by transient photo-capacitance (TPC) measurement. A broad defect band centered at 1.07 eV above the valance band is identified at 90 K. The defect signal is reduced with the insertion of the CdSe layer between the CdS/CdTe heterojunction. The TPC signals are rapidly quenched with increased temperature, which suggests that this deep level defect is highly possible to act as an effective recombination center. Based on the thermal quenching model, the activation energy (E a) of the defect is estimated to be ∼0.2 eV. With the configuration coordinate model, the temperature-dependent TPC signal and the corresponding electronic transition process can be well interpreted. All the observations strongly indicate that the introduction of Se atoms into CdTe is promising to suppress the formation of deep defects.

To clarify the mechanism of efficiency enhancement effect by potassium (K) treatment for Cu(In1− x Ga x )(Se1− y S y )2 (CIGSSe) solar cells, we investigated the physical properties of deep and shallow defect levels of CIGSSe solar cells without or with potassium fluoride treatment via using steady state photo-capacitance and admittance spectroscopy. The results show that the defect density of deep levels (such as 0.8 and 0.7 eV above valance band) are similar, suggesting that the K treatment does not drastically change the deep defect properties. For the shallow levels, the activation energy of interface-related shallow defect decreased from 183 to 120 meV upon K treatment, suggesting the modification of interface. The modified interface may be caused by the diffusion of potassium to grain boundaries and the formation of Cu-depleted layer in the near surface region of CIGSSe, resulting in the prolonged minority carrier lifetime. A new level with activation energy of 250 meV was found after K treatment, and it contributed to the increase of carrier density. These features well explained the enhanced device performance of CIGSSe solar cells by K treatment in terms of defects.

To understand the effect of bandgap grading on carrier recombination for Cu(In,Ga)Se2 (CIGS)-based solar cells in detail, samples with different bandgaps at the CIGS surface were fabricated by changing the Ga/(Ga + In) (GGI) ratio from 0.4 to 0 at the third stage of the conventional three-stage growth process. Optoelectronic characterizations, such as photoluminescence, temperature-dependent open-circuit voltage measurement and light-intensity-dependent current–voltage measurement, indicate that the photo-generated carriers move rapidly towards the location of the bandgap minimum, and the carrier recombination occurs mainly at this location. From simulation using a one-dimensional solar cell capacitance simulator (SCAPS-1D), a single-grade sample with the smallest bandgap on the surface of CIGS showed high recombination current at the surface, while the location of the maximum recombination current moved from the surface to the bulk for double-grade samples. This study suggests that controlling the bandgap grading is one way of suppressing recombination at the interface in CIGS-based solar cells.

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.

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.

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 electronic structure at the interface between bathocuproine (BCP) and calcium (Ca) was studied by ultraviolet photoemission spectroscopy (UPS) using two kinds of thin-films: Ca and BCP codeposited film, and BCP deposited on Ca. New peaks caused by the interaction of Ca with BCP were observed near the Fermi level. The energy position of the highest occupied molecular orbital (HOMO) of BCP measured from the Fermi level did not change with the thickness of BCP on Ca and the composition ratio of BCP to Ca. The new peaks related to the interface states shifted with the thickness of BCP on Ca and the composition ratio of BCP to Ca. It is suggested that the interface states are generated by the interaction of the Fermi level of Ca with the lowest unoccupied molecular orbital rather than the HOMO.

The effect of a pentacene buffer layer on the molecular orientation of phthalocyanine (H2Pc) films was investigated by X-ray diffraction analysis, atomic force microscopy, and infrared reflection absorption spectroscopy. The results suggested that the pentacene buffer layer can control the orientation of the H2Pc molecular plane to be parallel to the substrate surface of indium tin oxide (ITO), even though the molecular plane of H2Pc films without a buffer layer is perpendicular to such ITO substrates. The optical absorbance of the H2Pc film with a pentacene buffer layer was 1.5 times larger than that without a buffer layer. Photovoltaic cells composed of H2Pc and C60 were fabricated, and the performance of the cells with and without a buffer layer was studied.

To determine the minority carrier lifetime, room temperature time-resolved photoluminescence (TR-PL) measurements have been performed on a set of Cu2ZnSn(S,Se)4 (CZTSSe) samples with different Cu/Sn ratios of 1.65, 1.75, and 1.85. TR-PL measurements were carried out on the bare CZTSSe thin films, CdS covered CZTSSe films and on solar cell structure using a femtosecond laser. The sample containing high Cu/Sn ratio of 1.85 shows the lowest lifetime, while films with Cu/Sn ratios of 1.65 and 1.75 show almost equal lifetime. The difference in lifetime between the CdS covered and solar structure samples is not remarkable. This demonstrates domination of recombination than charge separation by electric field. The bare films show extremely small lifetime. To examine surface quality, TR-PL emission spectra of uncovered CZTSSe and Cu(In,Ga)Se2 (CIGS) films were measured with two different excitation wavelengths of 420 and 750 nm, which generate excess carriers at different depths in absorbers. This comparison confirms the dominant surface recombination by CZTSSe than CIGS.

Inverted organic solar cells (OSCs) based on boron subphthalocynine chloride (SubPc) and fullerene (C60) were fabricated and the device structure was optimized by inserting a bathocuproine (C26H20N2) buffer layer. The power conversion efficiency was greatly improved from 0.8 to 1.6%. The roles of bathocuproine in this inverted device were investigated by photoluminescence and transient photovoltage/photocurrent measurements. The results show that the bathocuproine in the device not only blocks the exciton quenching, but also prohibits the build-up of charge trapping and suppresses the trap-assisted recombination.

The quantitative characterization of the deep-level defect located at around 0.8 eV above the valence band in Cu(In1−x,Gax)Se2 (CIGS) thin films was performed by steady-state photocapacitance (SSPC) and time-resolved photoluminescence (TRPL) measurements. The defect concentration, photoionization cross section and electron capture cross section were evaluated to be on the order of 1013–1014 cm−3, 10−17–10−16 cm2, and 10−14 cm2, respectively, in the Ga content x range from 0.30 to 0.80; the first two values increased with increasing Ga content while the last one remained almost constant. By comparing the obtained values with the actual device performance, the cause of the degradation of the conversion efficiency of CIGS solar cells with higher Ga content was discussed in relation to the 0.8 eV defect.

The density of deep defects at 0.8 eV above the valence band in Cu(In,Ga)Se2 (CIGS) thin films was investigated by a transient photocapacitance (TPC) method. A relatively quantitative comparison of deep defect densities showed that the defect density seems to increase with increasing Ga/III ratio. This could be one reason for the poor device performance of CIGS samples with high Ga content.

The energy level alignment of C60/bathocuproine (BCP)/Ca interfaces as a function of BCP layer thickness has been studied by ultraviolet photoelectron spectroscopy. The results show that the energy level alignment is very sensitive to the BCP layer thickness. The energy levels of the lowest unoccupied molecular orbital of C60 and BCP were found to be almost the same when the thickness of BCP interlayer is less than 1.6 nm. Such energy level alignment, which is favorable to the carrier transport, may be caused by an interaction between C60 and Ca, where Ca passed through the BCP interlayer and diffused to the C60 layer. The role and optimum design for BCP interlayer were discussed.

The carrier recombination processes in Cu(In1-x ,Ga x )Se2 (CIGS) thin films were investigated by time-resolved microscopic-photoluminescence (µ-PL) measurement at room temperature. For films with x = 0.45, the spatial distribution of the donor–acceptor pair luminescence is much larger than the grain size. The PL decay lifetime is directly correlated with the µ-PL intensity, but the spectral shape is identical regardless of the sample position. These results suggest that the nonuniform distribution of nonradiative recombination centers mainly affects the carrier recombination in CIGS thin films. At relatively high Ga concentrations (x ≥0.7), the spatial inhomogeneity in µ-PL intensity is enhanced and decay accelerated, suggesting an increase in the density of nonradiative recombination centers. Thus, suppression of nonradiative recombination is critical to enhancing the performance of CIGS-based solar cells.

The transformation of SiC etching shapes by high-temperature annealing has been investigated. Without silicon atoms on the surface, transformation of the etching shapes hardly occurred even after annealing in pure Ar at 1700 °C, where transformation should occur without the loss of silicon atoms. When SiH4 was added to Ar, the surface tended to revert to SiC, and the transformation was enhanced with increasing SiH4 addition. Therefore, the presence of silicon atoms is necessary to transform the etching shapes on SiC surfaces.

The structural properties of a bulk heterojunction of poly(3-hexylthiophene) (P3HT) and 6,6-phenyl-C61 butyric acid methyl ester (PCBM) was investigated using synchrotron X-ray diffraction and atomic force microscope techniques. The growth of anisotropic crystals of P3HT components in the bulk heterojunction was observed. The π–π stacking structure between the facing thiophene rings of P3HT was disturbed by the introduction of PCBM, whereas the side chain crystallization along the a-axis of P3HT occurred smoothly. We discuss a possible mechanism of the anisotropic crystallization on the basis of the configuration of P3HT. The importance of controlling the π–π stacking and phase separation of the composed materials on the performance of the bulk heterojunction cell is suggested.

The electronic structure of the interface between a bathocuproine (BCP) molecule and metal surface was studied by density functional theory. Ca(111) and Au(111) surfaces composed of 13 atoms were considered as the metal surface and one BCP molecule was put on the metal surface. In case of BCP on Au, the interaction was found to be small; however, for BCP on Ca, electron density of states near the Fermi level was observed. The results were compared with those of the experiments. It was analyzed that the electron density of states near the Fermi level were formed by an interaction between Ca 4s and C 2p. The effects of the interaction on the electrical properties were discussed.

The influences of a 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) layer on the molecular orientation and crystalline structure of phthalocyanine (H2Pc) films grown on PTCDA/glass structures were studied by X-ray diffraction analysis, grazing incidence X-ray diffraction analysis, atomic force microscopy and infrared reflection absorption spectroscopy. The orientation of the H2Pc molecular plane became parallel to the substrate surface with the introduction of a thin PTCDA layer on a glass substrate, although the molecular plane of a H2Pc film without a PTCDA layer was almost perpendicular to the substrate surface. Such a parallel orientation of the H2Pc molecular plane was observed even when the PTCDA layer thickness was only 5 Å. It was found that the intensity of the optical absorption of a H2Pc film is significantly influenced by the molecular orientation of H2Pc, and the importance of controlling this molecular orientation is suggested.

The effects of nitrogen doping into Cu2O thin films deposited by reactive radio-frequency magnetron sputtering were studied. It was found that nitrogen is an effective p-type dopant for Cu2O and the hole density can be controlled from 1×1015 cm-3 to approximately 1017 cm-3. The acceptor level of nitrogen was estimated to be about 0.14 eV by temperature-dependent Hall effect measurements and this value roughly agrees with that obtained by the effective mass theory. No significant degradation of structural and optical properties induced by nitrogen doping were observed. The resistivity of 15.2 Ωcm was obtained for a relatively high nitrogen flow rate, which is the lowest value reported to date for Cu2O thin films.

Deposition conditions of cuprous oxide (Cu2O) thin films on glass substrates by reactive radio-frequency (rf) magnetron sputtering method were studied. The substrate temperature was found to be important for obtaining high-quality films, and the optimum substrate temperature was about 500°C. The Cu2O deposited at 500°C shows a band-gap energy of about 2.0 eV and a typical hole concentration of the order of 1015 cm-3 with a Hall mobility of 60 cm2/V·s, which is the highest mobility reported thus far.

Bright and sharp photoluminescence was obtained from Eu-doped GaN films grown by gas-source molecular beam epitaxy using ammonia. It was found that the decrease in luminescence intensity is small between 80 K and 300 K, compared with the red emission from InGaN. The red emission from Eu-doped GaN also showed only a small peak shift within 1.6 meV in the same temperature range. From these results, the advantage of using Eu-doped GaN as a stable optoelectronic material against temperature variation is shown.

Effects of nitrogen ion irradiation on photoluminescence (PL) properties at 77 K of undoped GaN grown by plasma enhanced molecular beam epitaxy were investigated. It was found that for epitaxial layers grown under ion irradiation conditions, the PL spectra were dominated by near band-edge emission at 357 nm (3.47 eV). However, deep-level emission at around 567 nm (2.2 eV) was also observed and the intensity of which was considerably strong. On the other hand, for GaN films grown with no ion irradiation, the intensity of the deep-level emission was two orders of magnitude less than that of the near band-edge emission. This result suggests that the nitrogen ions degrade the PL properties, i.e., create defect levels relating to emission at around 567 nm (2.2 eV).

We report results on the 77 K photoluminescence (PL) of undoped GaN epilayers grown on c-plane sapphire substrate by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). It was found that the PL properties of undoped GaN epilayers are strongly affected by the growth conditions such as nitrogen pressure, growth temperature and ECR plasma power. The intensities of the donor-to-acceptor (D-A) pair emission peak at about 3.2 eV and of the deep-level emission peak at around 2.3 eV change with the variation of these three growth conditions. Assuming that the D-A peak and deep-level emission peak are related to native defects and carbon impurities, respectively, the systematic change of the PL spectra with growth conditions can be reasonably interpreted. At the optimum growth conditions, the PL spectra at 77 K are dominated by excitonic peak at 3.47 eV and the intensity of deep-level emission peak is one order of magnitude less than that of the excitonic peak. However, since the region regarded as optimum is relatively narrow, it is important to control the growth conditions precisely.

GaN epilayers were grown on Al2O3(0001) substrate by electron cyclotron resonance molecular beam epitaxy. The effects of growth parameters such as growth temperature, nitrogen pressure, Ga cell temperature and substrate-surface nitridation on crystal quality were investigated by scanning electron microscope and X-ray diffraction. It was found that the formation of Ga droplets at the growth surface depends strongy on growth temperature and Ga cell temperature, and flat and smooth surfaces were obtained at the growth temperature of 750° C with the growth rate of 0.5 µ m/h. The III/V ratio has a large effect on the full width at half maximum (FWHM) of the X-ray diffraction (XRD), i.e., on the relaxation of misfit stress. However, substrate-surface nitridation has little effect on the relaxation of misfit stress.

We observed the lasing operation of ZnSSe-ZnMgSSe double heterostructure (DH) photopumped blue lasers up to 500 K. The wavelength at room temperature (RT) and threshold intensity at 300 K were 464.5 nm and 105 kW/cm2, respectively. Characteristic temperature (T 0) was estimated to be 170 K. These results indicate that this structure has good thermal stability. Both n-type and p-type doping in ZnMgSSe were successfully performed. Both n-type and p-type ZnMgSSe show good properties for fabricating laser diodes.

We report that a Ti/Pt/Au multilayered metal structure can make a nonalloy ohmic contact to n-type ZnSe with an electron concentration greater than 1019 cm-3. A specific contact resistance as low as 3.4×10-4 Ω-cm2 was achieved for Cl-doped ZnSe with an electron concentration of 2×1019 cm-3. We think that the lower contact resistance can be ascribed to the higher reactivity and adhesion of Ti metal with the surface of ZnSe at low temperature, and to the lower work function of Ti metal. The dominant current flow may be due to the quantum mechanical tunneling of electrons through the potential barrier at the ZnSe surface, because the specific contact resistance decreases with an increase in the electron concentration. A small improvement in the specific contact resistance was made by thermal annealing at a temperature of 250°C for 5 min, which yielded the lowest value of 1.1×10-4 Ω-cm2. With annealing at 300°C and below, the contact resistance was not drastically increased. The electrical properties of the contact are therefore thermally stable up to 300°C.

Nitrogen-doped p-type ZnSe films have been grown by molecular beam epitaxy using electron cyclotron resonance (ECR) plasma of N2. The nitrogen concentration in the film from 1×1017 to 6×1018 cm-3 has been controlled by adjusting both the aperture area of the ECR cell and the input microwave power. A net acceptor concentration as high as 4.5×1017 cm-3 was obtained by C-V measurements, and up to this level, the ratio of the net acceptor concentration to the incorporated nitrogen impurity concentration was almost unity. Donor-to-acceptor (DA) emission dominated the photoluminescence spectra at 4.2 K, and the emission intensity ratio of deep to DA emissions was less than 1/1000 for the ZnSe films with nitrogen concentrations less than 2×1018 cm-3.

We report the first operation of ZnSe-ZnMgSSe double-heterostructure photopumped blue lasers with a threshold excitation intensity at 150 kW/cm2 at room temperature. The lasing was confirmed by measuring the polarization. This result indicates that ZnMgSSe is a promising material for the cladding layer of a blue laser diode. Until now, no material had been lattice-matched to a GaAs substrate and had sufficient band-gap energy. If p-type and n-type doping is possible, this ZnSe-ZnMgSSe double heterostructure can be applied to a blue laser diode.

Blue electroluminescence has been obtained from ZnSe p-n junctions. ZnSe films were grown on n-type GaAs(100) substrates by molecular beam epitaxy. The dopants used for n-and p-type ZnSe were Ga and O, respectively. The electron-beam-induced current strongly suggests the formation of a p-n junction. The built-in Potential of the p-n junction and carrier concentration of p-type ZnSe layer estimated from the capacitance-voltage relation were about 2.3 V and 1.2×1016cm-3, respectively. The electroluminescence spectra from the p-n junction were dominated by band-edge emissions of 466 nm at room temperature and 446 nm at 77 K.

Electroluminescence has been obtained from ZnSe p-n junctions. ZnSe films were grown on n-type GaAs substrates by molecular beam epitaxy. The dopant used for n-type ZnSe was Ga, and p-type ZnSe was formed by nitrogen ion implantation into undoped ZnSe which was grown on the Ga-doped ZnSe layer. Rapid thermal annealing was performed using an infrared lamp in N2 ambient. The formation of a p-n junction was confirmed by measuring electron beam-induced current. The electroluminescence spectra were dominated by two peaks of band-edge emission at 446 and 459 nm at 77 K.