Hisayuki Nakanishi

Preparation and some properties of CuInSe2 single crystals

Abstract A brief review of our previous research on the electrical properties and p-n junction characteristics of CuInSe2 single crystals prepared by the normal freezing method is presented. Recently the optical absorption spectra have been measured as functions of temperature and pressure. The temperature and pressure coefficients of the absorption edge are found to be −1.1 × 10 −4 eV K −1 and +2.8 × 10 −11 eV Pa −1 respectively. Some trends may be seen in the distribution of the absorption edge energies measured for various n- and p-type samples.

Band‐edge photoluminescence of CuGaSe2 films grown by molecular beam epitaxy

Slightly Cu‐rich CuGaSe2 films were grown on [001] oriented GaAs substrates by molecular beam epitaxy. Photoluminescence of the films showed a remarkable emission peaked at 1.71 eV at low temperature, which is attributed to recombination of free excitons and bound excitons. The dissociation energy of free excitons and their localization energy to a center are found to be 16.2 and 3.3 meV, respectively. The band‐gap energy Eg is estimated to be 1.7310 eV at low temperature. It is suggested that the temperature variation of Eg is dominated by interaction with phonons of 26 meV which corresponds to the mean energy of the optical phonons in CuGaSe2.Slightly Cu‐rich CuGaSe2 films were grown on [001] oriented GaAs substrates by molecular beam epitaxy. Photoluminescence of the films showed a remarkable emission peaked at 1.71 eV at low temperature, which is attributed to recombination of free excitons and bound excitons. The dissociation energy of free excitons and their localization energy to a center are found to be 16.2 and 3.3 meV, respectively. The band‐gap energy Eg is estimated to be 1.7310 eV at low temperature. It is suggested that the temperature variation of Eg is dominated by interaction with phonons of 26 meV which corresponds to the mean energy of the optical phonons in CuGaSe2.

Raman Spectra of CuGaxIn1-xSe2

We have measured Raman spectra of CuGaxIn1-xSe2 (0≤x≤1) single crystals at low temperature. We have examined all Raman active modes. Every phonon shows the linear dependence of the frequency vs the Ga composition x, respectively. The results are in good agreement with the infrared results and our Raman data of CuInSe2, but are inconsistent with the previous report of the Raman spectra of CuInSe2 [J. N. Gan et al.: Phys. Rev. B 13 (1976) 13610].

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.

Comparison of Optical Properties of GaN/AlGaN and InGaN/AlGaN Single Quantum Wells

Static, field-modulated and time-resolved spectroscopies were carried out to compare the electronic states between GaN/AlGaN binary and InGaN/AlGaN ternary single quantum wells (SQWs). The internal field that exists across the quantum well (QW) naturally induces a quantum-confined Stark effect (QCSE), namely the redshift of the QW resonance energy and decrease of the electron-hole wavefunction overlap. The GaN/AlGaN SQW exhibited a weak emission due to QCSE. However, optical absorption and degenerate pump-probe measurements revealed that excitonic character was maintained for thin QWs with the well width nearly the same as the bulk free exciton Bohr radius even under an electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW exhibited a bright luminescence peak in spite of an effective bandgap inhomogeneity in the QW, which was confirmed by the point excitation and monochromatic cathodoluminescence (CL) mapping method. The lateral interval of each light-emitting area was estimated from the spatial resolution of the CL mapping to be smaller than 60 nm. Such local potential minima is considered to be formed due to the presence of a structure similar to quantum-disks [M. Sugawara: Phys. Rev. B 51 (1995) 10743]. Carriers generated in the InGaN QWs are effectively localized in these regions to form localized QW excitons exhibiting highly efficient spontaneous emissions.

Ultraviolet photoluminescence from CuAlS2 heteroepitaxial layers grown by low‐pressure metalorganic chemical vapor deposition

Photoluminescence (PL) studies were carried out on CuAlS2 heteroepitaxial layers grown by low‐pressure metalorganic chemical vapor deposition. The epilayers exhibited an intense near‐band‐edge PL peak at 3.525 eV (77 K) in the ultraviolet (UV) wavelength region together with PL bands at 2.76 and 2.1 eV. This near‐band‐edge PL peak showed the following properties: (i) the PL was strongly polarized parallel to the c axis, (ii) the peak energy varied from 3.497 to 3.525 eV at 77 K with increasing epilayer thickness from 0.49 to 0.67 μm, and (iii) the emission was observed up to room temperature. This UV‐PL was tentatively assigned to an exciton‐related emission, energy shifted due to the residual lattice strain.

Electroreflectance of CuInSe 2 Single Crystals

Electrolyte electroreflectance (ER) measurements have been carried out at room temperature (297 K) on a series of bulk single crystals of CuInSe2 prepared by the normal freezing method with various Cu/In ratios. The Cu-rich crystals exhibited sharp ER peaks and the AB-transition energy is 1.026 eV, while In-rich crystals exhibited broader ER peaks and smaller AB-transition energies (1.00–1.02 eV) than those of the Cu-rich ones. The spin-orbit splitting of the uppermost valence band is 0.236 eV. The effect of the Cu/In ratio on the ER spectra is discussed in terms of the Urbach energy determined by optical absorption measurement, and the bandgap energy of the stoichiometric CuInSe2 crystal is discussed.

Growth and doping characteristics of ZnSeTe epilayers by MOCVD

Abstract Compositional control, band gap evaluation and acceptor doping characterization for ZnSeTe have been studied. Zinc selenotelluride layers were grown on GaAs (100) substrates by atmospheric metalorganic chemical vapor deposition. The composition of the ZnSexTe(1−x) solid is proportional to the gas phase composition when the VI II ratio is close to unity. Accurate band gap energy values for ZnSeTe epilayers were obtained at room temperature by photoreflectance spectroscopy. The bowing parameter is 1.647 eV. The p-type carrier concentration of ZnSeTe:As is drastically quenched with increasing Se incorporation. The quenching is possibly attributable to the lattice distortion.

Improved quality of CuGaSe2 and CuAlSe2 epilayers grown on CuGa0.96In0.04Se2 substrates

Nearly strain free CuGaSe2 and CuAlSe2 thin films were successfully grown by metalorganic vapor phase epitaxy on CuGa0.96In0.04Se2 substrates prepared by the traveling heater method (THM). CuAlSe2 (112) epilayers exhibited fine surface structures which reflected the crystal geometry. The CuGaSe2 epilayers exhibited intense near-band-edge emission even at room temperature. THM-grown single crystals were demonstrated to be a suitable substrates for epitaxy of chalcopyrite compounds.

Structural studies of Cu–III–VI2 chalcopyrite semiconductor heteroepitaxial films grown by low-pressure metalorganic vapor phase epitaxy

Growth orientation, domain structures, and residual strain of Cu(Al,Ga)(S,Se)2 epitaxial films grown by low-pressure metalorganic vapor phase epitaxy on various substrates were systematically investigated. All epilayers grew in such a manner that the lattice mismatch between the epilayer and the substrate became minimum. Domain structure of the epilayer changed depending on the type of substrate. Most residual strain in the epilayer was assigned as being due to the pseudomorphic stress for the epilayers having small lattice mismatch ( 1%) ones.