Search

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

We have reported electrodeposition of silicon films on metal substrates obtained through electrochemical reduction of silicon dioxide nanoparticles (SiO2-NP) at high temperature of 855 °C in the calcium chloride (CaCl2) melt. Electrodeposition was conducted using a three-electrode based electrochemical cell. Reduction of the SiO2 was found possible with applying a negative potential of 0.9 V or more negative on the metal substrate with respect to the graphite reference electrode. Mechanism of the reduction and electrodeposition process was discussed using cyclic voltammetry (CV), and chronoamperograms (CA) techniques in relation to the applied reduction potentials during experiments. Raman spectroscopy, and X-ray diffraction method confirmed formation of the Si-film on the silver (Ag) substrate using the electrodeposition technique. Effect of the various reduction potentials on the properties of the formed Si layer was studied using Raman spectroscopy, photoluminescence (PL), and Scanning electron microscopy (SEM). Crystallinity of the electrodeposited Si-films were found to be correlated with the reduction potential. Nanocrystalline Si (nc-Si) film was obtained through electrodeposition with lower reduction potentials, while higher reduction potential was found to be effective to get Si-films with uniform crystalline quality, and better morphology. Effect of the substrate materials on the electrochemical reduction of SiO2 was also investigated.

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

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

In this article, we look, from the carrier perspective, at the effect of the double grading in a CIGS absorber, in order to understand its benefit compared to a single grading. We focus on the double grading effects on the conduction band edge, generation rate and carrier collection. We first show that, when the minimum bandgap position (notch position) is outside of the absorber depletion region, a local maximum of the conduction band edge appears at the back of the depletion region, creating a hump, which hinders the collection of electrons. Interpreting the effect of the grading as an effective electric field, we show that the hump is caused by the competition between the absorber internal electric field and the grading created effective electric field. From this, we deduce that the notch should be positioned in the depletion region. As expected, the generation rate increases significantly at the notch position. However, studying carrier collection and the recombination rate, we show that, even if in the ideal case we should observe a total collection of carriers generated in the depletion region, the recombination rate in the SCR increases significantly when the notch is in it. We propose that this is caused by the increased carrier generation close to recombination centres. The ideal notch position is, therefore, a compromise between being close enough to the front to have a maximal carrier collection and avoiding the creation of a hump, while minimizing the carrier recombination.

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.

The deep red phosphor Sr4(PO4)2O:Eu2+, which has an excitation peak around 450 nm for blue LED applications, is reported. This behavior is unusual for most phosphate phosphors. The crystal structure of Sr4(PO4)2O:Eu2+ is found to be monoclinic P21 and isotypic with Ca4(PO4)2O:Eu2+, which also shows deep red emission. Sr4(PO4)2O:Eu2+ has a larger lattice volume than Ca4(PO4)2O:Eu2+, but their emission and excitation spectra at room temperature are very similar. The key factors are discussed for achieving a large redshift of the 5d levels of Eu2+ ion in order to emit red light. In particular, the importance of the anion polarizability and the distortions of the metal coordination polyhedra are discussed, including the effective coordination number. Importantly, Sr4(PO4)2O:Eu2+ lacks the yellow emission at 77 K, which is found in Ca4(PO4)2O:Eu2+. The differences in thermal quenching behavior for Eu2+ dopants in Sr4(PO4)2O:Eu2+ and Ca4(PO4)2O:Eu2+ are attributed to the degree of auto/photo-ionization due to differences in the band gaps of these compounds. The importance of the large band gap of the host lattice in avoiding non-radiative processes of energy relaxation was confirmed.

In order to characterize solar cell modules, patterning a sub-module into small-sized devices by photolithography are required. For CIGSSe sub-modules fabricated by sulfurization-after-selenization (SAS), the small-size device performance was considerably deteriorated (from 19.3 % to 11.3 % for power conversion efficiency (PCE)) as compared to the sub-module after traditional photolithography process. We found that, undesirable Cu 2-x Se phase was introduced into the film by the traditional photolithography process. By utilizing an optimized photolithograph recipe, the PCE as high as 19.5 % (nearly the same as the sub-module) for the patterned small-size unit cells were achieved. This allows us to perform specific characterizations to the small-size device in order to evaluate the properties of the sub-modules.

Defect study of the CuGaSe2 thin-films deposited with three-stage evaporation process under different Se-flux (PSe) conditions were performed using admittance spectroscopy (AS) technique. A dominant defect, A2 has been identified around 230~350 meV above the valance band (Ev) of the CuGaSe2. In general, density of defects was found to decrease with an increase in the PSe condition during deposition of the CuGaSe2 layer. Effect of the Se-flux on the performance of the fabricated solar-cells were discussed in relation to the defect-properties of the corresponding CuGaSe2 absorber-layer identified by admittance spectroscopy.

Silicon (Si) layer was formed on the silver (Ag) substrates using electrodeposition technique through electrochemical-reduction of the purified (99%) diatomaceous earth (SiO2) at high temperature of 855° C using calcium chloride (CaCl2) molten-salt. Electrodeposition was conducted using a three-electrode electrochemical -cell. Reduction of the SiO2was possible with applying negative potential of more than 0.7 volt on the Ag-electrode with respect to graphite reference-electrode. Raman spectroscopy and X-ray diffraction method confirm formation of the Si-layer on the Ag-substrate from SiO2using electrodeposition technique. Scanning electron microscopy indicates that deposited Si has morphology of rough surfaces.

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

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

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

A novel synthesis method for hexagonal (β)-phase NaYF 4 :Er nano-crystals (NCs) which showed up-conversion (UC) from infrared to visible spectral region was developed. The NaYF 4 :Er NCs were synthesized in oleic acid (OA) and 1-octadecene (ODE) with Y 2 (CO 3 ) 3 · x H 2 O, Er 2 (CO 3 ) 3 · x H 2 O, Na 2 CO 3 and NH 4 F as precursors. This proposed method was simple and less toxic compared with generally used method so far. The XRD results showed that the molar ratio of OA/ODE and the temperature were key factors for phase control of NaYF 4 :Er NCs. The UC emission spectra were obtained with the emission wavelength at about 980 nm ( 4 I 11/2 → 4 I 15/2 ), 800 nm ( 4 I 9/2 → 4 I 15/2 ), 660 nm ( 4 F 9/2 → 4 I 15/2 ) and 540 nm ( 4 S 3/2 → 4 I 15/2 ) from Er 3+ ions, by excitation wavelength of 1550 nm. The slope values, n , in the pump-power dependence, showed that the emission at 980 and 800 nm were generated by 2-step UC and at 660 nm and 540 nm were 3-step UC. The optical process for the UC excitation was discussed.

Structural and electrical properties of polycrystalline CuGaSe2 thin films have been studied by changing the Ga/Cu ratio in the films. CuGaSe2 thin films with various Ga/Cu ratio were grown over Mo-coated soda-lime glass substrates. With the increase of Ga content in CuGaSe2, morphology of the films was found to deteriorate which is associated with the smaller grain size and the appearance of impurity phases presumably due to the phase transition from the chalcopyrite structure to the defect-related phase on the surface of the films. Properties of the Ga poor films were affected by the Cu rich secondary phases. Electrical properties of the films were strongly influenced by the structural properties and degraded with increasing the Ga/Cu ratio in the film. Device performances, fabricated with the corresponding CuGaSe2 films, were found to be correlated with the Ga/Cu ratio in the films and consistent with the observed structural and electrical properties.

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

Quantitative phase analysis of Cu(In 1− x Ga x )Se 2 (CIGS) thin film grown over Mo coated soda lime glass substrates was studied by Rietveld refinement process using room temperature X-ray data at θ –2 θ mode. Films were found to contain both stoichiometric Cu(In 1− x Ga x )Se 2 and defect related Cu(In 1− x Ga x ) 3 Se 5 phases. Best fitting was obtained using crystal structure with space group I-42d for Cu(In 1− x Ga x )Se 2 and I-42m for Cu(In 1− x Ga x ) 3 Se 5 phase. The effects of Ga/III (=Ga/In+Ga= x ) ratio and Se flux during growth over the formation of Cu(In 1− x Ga x ) 3 Se 5 defect phase in CIGS was studied and the correlation between quantity of Cu(In 1− x Ga x ) 3 Se 5 phase and solar cell performance is discussed.

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

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

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

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.

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.

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)

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.

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.

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.

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.