Mutsumi Sugiyama

Localized exciton dynamics in strained cubic In0.1Ga0.9N/GaN multiple quantum wells

Radiative and nonradiative recombination dynamics in strained cubic (c-) In0.1Ga0.9N/c-GaN multiple quantum wells were studied using temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy. In contrast to hexagonal InGaN quantum wells, low-excitation photoluminescence peak energy increased moderately with decreasing well thickness L and the PL lifetime did not strongly depend on L. The results clearly indicated that the piezoelectric field was not acting on the transition process. The TRPL signal was well fitted as a stretched exponential decay from 10 to 300 K, showing that the spontaneous emission is due to the radiative recombination of excitons localized in disordered quantum nanostructures such as In clusters. The localized states were considered to have two-dimensional density of states at 300 K (quantum disk size), since the radiative lifetime increased with increasing temperature above 150 K.

Optical and structural studies in InGaN quantum well structure laser diodes

An InGaN multiple-quantum-well laser diode wafer that lased at around 400 nm was shown to have the InN mole fraction, x, of only 6% in the wells. Nanometer-probe compositional analysis showed that the fluctuation of x was as small as 1% or less, which is the resolution limit. However, the wells exhibited a Stokes-like shift (SS) of 49 meV at 300 K, which was approximately 65% of the luminescence linewidth, and effective localization depth, E0, was estimated to be 35 meV at 300 K. Since the effective electric field due to polarization in the wells was estimated to be as small as 300 kV/cm, SS was considered to originate from effective band-gap inhomogeneity. Because the well thickness fluctuation was insufficient to reproduce SS or E0 and bulk cubic In0.02Ga0.98N that does not suffer any polarization field or thickness fluctuation effect exhibited a SS of 140 meV at 77 K, the exciton localization is considered to be an intrinsic phenomenon in InGaN, which is due to the large band-gap bowing and In clusteri...

Evidence of localization effects in InGaN single-quantum-well ultraviolet light-emitting diodes

The importance of doping or alloying with In for obtaining high external quantum efficiency was shown for GaN-based single-quantum-well (SQW) structures in terms of localization effects due to quantum-disk (Q-disk [M. Sugawara, Phys. Rev. B 51, 10743 (1995)])-size potential minima in the QW plane. The ultraviolet light-emitting diode with lightly In-alloyed InGaN SQW exhibited an electroluminescence peak from the band-tail states. Monochromatic cathodoluminescence mapping images of In0.03Ga0.97N SQW indicated the presence of Q-disk-size effective bandgap variation. Furthermore, cubic InGaN QW which does not suffer from the piezoelectric field normal to the QW plane, also exhibited a broad band-tail.

Fabrication of Visible-Light-Transparent Solar Cells Using p-Type NiO Films by Low Oxygen Fraction Reactive RF Sputtering Deposition

Visible-light-transparent p-type NiO films were deposited by reactive RF sputtering under unintentional heating. An optical transmittance of >80% was obtained in the wavelength range of 500–800 nm when the films were deposited under a very low O2 fraction in the gas phase O2/(Ar+ O2) = 0.5%. This result may reflect a decrease in the concentration of Ni vacancies due to the increase in their formation energy under oxygen-poor deposition conditions. Heterostructure pn junctions consisting of p-type NiO and n-type ZnO layers were also deposited. We eventually observed a slight but noticeable photovoltaic effect.

Experimental determination of vacuum-level band alignments of SnS-based solar cells by photoelectron yield spectroscopy

Energy band offsets of SnS-based solar cell structure using various n-type semiconductors, such as CdS, SnS2, In2S3, ZnIn2Se4, ZnO, and Mg0.3In0.7O, are evaluated by photoelectron yield spectroscopy. The valence band discontinuities are estimated to be 1.6 eV for both SnS/CdS and SnS/SnS2, 0.9 eV for SnS/In2S3, 1.7 eV for SnS/ZnIn2Se4, and 1.8 eV for both SnS/ZnO and SnS/Mg0.3Zn0.7O. Using the valence band discontinuity values and the corresponding energy bandgaps of the layers, energy band diagrams are developed. This study implied a type-I heterostructure, appropriate for SnS-based solar cell, for the ZnIn2Se4 or MgxZn1−xO (0 ≤ x ≤ 0.3) interface and type-II for other junctions.

Preparation of SnS Films by Sulfurization of Sn Sheet

Tin sulfide (SnS) films were grown by sulfurization using an inexpensive Sn sheet and S powder. The Sn sheet was sulfurized in a closed ampoule at a low temperature between 150 and 300 °C. The resulting sulfurized films had a flat surface and large grains, and exhibited adhered very well to the substrate. These samples exhibited X-ray diffraction peaks corresponding to SnS and contained no extra phases. These results represent the first step toward realizing an optical device such as a solar cell using a SnS film grown by a sulfurization method.

Experimental Determination of Valence Band Discontinuities at Cu(Al,Ga)(S,Se)2/GaAs(001) Heterointerfaces Using Ultraviolet Photoemission Spectroscopy

Ultraviolet photoemission spectroscopy measurement was carried out for c(001) plane Cu(Al,Ga)(S,Se)2 chalcopyrite structure epilayers grown on GaAs(001) substrates to determine valence band discontinuities, ΔEv, at the heterointerfaces. The values of ΔEv were estimated to be about 1.2 eV for CuAlS2/GaAs, 1.0 eV for CuAlSe2/GaAs, 1.1 eV for CuGaS2/GaAs and 0.3 eV for CuGaSe2/GaAs. From these values and bandgap energies of the corresponding compounds, Cu(Al,Ga)(S,Se)2 system is considered to offer the TYPE-I heterostructures between the corresponding narrow bandgap materials and wide bandgap ones.

Sulfurization Growth of SnS Films and Fabrication of CdS/SnS Heterojunction for Solar Cells

Polycrystalline tin sulfide (SnS) films were grown by sulfurization of a Sn precursor at low temperatures of 120–220 °C. The SnS film grown at 170 °C comprises densely packed 3–5-µm-diameter columnar grains, which is appropriate for use in the photoabsorption layers of solar cells. The SnS film had an optical bandgap of approximately 1.3 eV and p-type conductivity. Using an appropriate SnS film, an n-CdS/p-SnS heterojunction was fabricated on Mo-coated soda-lime glass substrates. These results are the first step toward realizing an optical device as a solar cell using a SnS film grown by sulfurization.

Metalorganic vapor phase epitaxy of Cu(AlxGa1−x)(SySe1−y)2 chalcopyrite semiconductors and their band offsets

Abstract Cu-chalcopyrite (Ch) Cu(Al,Ga,In)(S,Se)2 compounds and alloys were grown epitaxialy on various substrates by low-pressure metalorganic vapor phase epitaxy. They grew in such a manner that the lattice mismatch between the epilayer and the substrate became minimum. Growth of anion and cation alloys revealed that the vapor pressure of Al–S reactants is much higher than that of Ga–S or Al–Se ones. Most of residual strain in the epilayer was assigned as being due to the pseudomorphic stress for those having small lattice mismatch ( 1%) ones, and the strain problem was solved by the use of Ch structure substrate. All single-domain compound epilayers, namely CuAlS2, CuAlSe2, CuGaS2, CuGaSe2, CuInSe2, and their alloys exhibited predominant excitonic photoluminescence peaks. A noticeable excitonic feature was found in the PL spectra of CuAlS2, CuGaS2 and CuGaSe2 even at 300 K. Band diagram of Cu(Al,Ga)(S,Se)2 system was discussed, and they are considered to offer the TYPE-I heterostructures between the corresponding narrow bandgap materials and wide bandgap ones.

Band gap bowing and exciton localization in strained cubic InxGa1-xN films grown on 3C-SiC (001) by rf molecular-beam epitaxy

Spontaneous emission mechanisms in InGaN alloys were studied by determining the effective band gap energies using photoluminescence excitation spectroscopy and static and time-resolved photoluminescence (PL) measurements on fully strained cubic (c-) InxGa1−xN films on c-GaN templates, which were grown by rf molecular-beam epitaxy on smaller lattice-mismatched 3C-SiC (001) substrates prepared on Si (001). The c-InxGa1−xN alloys exhibited large band gap bowing. The PL decay dynamics showed that the emission is due to recombination of localized excitons, the same as in the case of hexagonal InGaN. The c-InxGa1−xN exhibited a larger Stokes-like shift and a larger localization depth, showing that the material’s inhomogeneity is much enhanced compared to that of the hexagonal polytype.