Shigeru Niki

Band gap energies of bulk, thin-film, and epitaxial layers of CuInSe2 and CuGaSe2

Band gap and excitonic resonance energies of high-quality bulk single crystals, polycrystalline thin films, and epitaxial layers of CuInSe2 and CuGaSe2 were determined as a function of temperature by means of photoreflectance, optical absorption (OA), and photoluminescence measurements. OA spectra were fit including excitonic absorption from low temperature up to room temperature (RT). The band gap energy of 1.032 eV and free exciton (FE) resonance energy of 1.024 eV were obtained at RT for strain-free CuInSe2 giving an exciton binding energy of 7.5 meV. The band gap energy of both CuInSe2 and CuGaSe2 was found to be essentially independent of the molar ratio of Cu to group-III atom (Cu/III) for near-stoichiometric and Cu-rich samples. The disappearance of the FE absorption in the In-rich (Cu/In<0.88) CuInSe2 thin films was explained by plasma screening of Coulomb interactions. A slight decrease in the band gap energy of the In-rich films was attributed to a degradation of film quality such as high-densit...

Heteroepitaxy and characterization of CuInSe2 on GaAs(001)

Abstract CuInSe 2 (CIS) films with Cu/In ratios of γ = 0.81–1.81 have been grown on (001)-oriented GaAs substrates by molecular beam epitaxy at substrate temperatures of T s = 350–550° C . Film properties were found to be substantially different for Cu- and In-rich regions. Cu-rich films were p-type, and streaky reflection high-energy electron diffraction (RHEED) patterns and sharp photoluminescence (PL) emissions were observed, suggesting high quality epitaxial films. In-rich films were highly resistive, and contained a large number of twins formed on {112} planes. A broad and strong PL emission, a donor-acceptor pair emission, was observed, which blue-shifted with increasing excitation power, indicating heavy compensation.

Excitonic emissions from CuInSe2 on GaAs(001) grown by molecular beam epitaxy

CuInSe2 epitaxial films grown on (001)‐oriented GaAs substrates by molecular beam epitaxy have been characterized by means of low temperature photoluminescence spectroscopy at T=2–102 K. Distinct emission lines were observed near the band gap, and have been investigated further with varying sample temperature. An emission at 1.0386 eV (EX1) became broader with increasing sample temperature, and still remained up to T=102 K. A distinct, sharp emission at 1.0311 eV (IX1) which disappeared at a significantly lower temperature than the other peaks was observed only in films with weak donor‐related emissions. We attribute such emissions to the ground‐state free exciton [FEn=1] and exciton bound to neutral acceptor [A0,X], respectively. The band gap of CuInSe2 epitaxial films was also determined to be Eg=1.046 eV at 2 K using the reported exciton binding energy of Eex=7 meV.

High quality CuInSe2 films grown on pseudo‐lattice‐matched substrates by molecular beam epitaxy

CuInSe2 films have been grown by molecular beam epitaxy on pseudo‐lattice‐matched substrates that consist of a 1‐μm‐thick In0.29Ga0.71As layer grown on a linearly composition‐graded InxGa1−xAs buffer (0≤x≤0.29) grown in turn on GaAs (001). The properties of these films have been compared with those of the films grown directly on GaAs (001). High resolution x‐ray diffraction analysis on CuInSe2 grown on pseudo‐lattice‐matched substrates indicated substantial reduction on residual strain in the CuInSe2 films. A photoluminescence spectrum dominated by sharp free exciton emissions has been observed for the first time from CuInSe2 films indicative of significant improvement in crystalline quality and substantial reduction in the point defect density.

OPTICAL CHARACTERIZATIONS OF CUINSE2 EPITAXIAL LAYERS GROWN BY MOLECULAR BEAM EPITAXY

CuInSe2 (CIS) films with Cu/In ratios of γ=0.82–1.79 have been grown on a GaAs (001) substrate by molecular beam epitaxy. Piezoelectric photoacoustic (PPA) measurements were carried out from liquid helium to room temperature to investigate nonradiative carrier recombination processes in comparison with photoluminescence (PL) measurements which directly detected radiative carrier recombination processes. Three PPA signal peaks which corresponded to band gap energies of the CIS (AB and C bands) and the GaAs substrate, were clearly obtained between liquid helium and room temperatures. A free-exciton emission line was observed up to 200 K in the PL spectra. Two additional peaks on intrinsic defects which are Cu vacancy (VCu) and interstitial In (Ini) were observed in the In-rich CIS samples. The PPA measurements were useful in investigating the defect levels and the band gap energy in the CIS/GaAs thin films.

Effects of annealing on CuInSe2 films grown by molecular beam epitaxy

CuInSe2 (CIS) thin films with a range of CuIn ratios were grown by molecular beam epitaxy on GaAs (0 0 1) at substrate temperatures of Ts = 450–500°C and the effects of annealing under various atmospheres have been investigated. Photoluminescence spectra obtained from an ex-situ vacuum annealed CIS film at a temperature of TA = 350°C showed a red-shift and a broadening of an emission peak (peak c) which originally appeared at 0.970 eV before annealing and the red-shifted peak c was found to consist of two overlapping peaks. The excitation power dependence of these overlapping peaks indicated the radiative recombination processes associated with the emissions to be a conduction band to acceptor transition (peak at 0.970 eV) and a transition due to donor-acceptor pairs (peak at 0.959 eV), indicating the formation of a shallow donor-type defect during the vacuum annealing process. The origin of this defect has tentatively been attributed to Se vacancies. On the other hand, the molar fraction of oxygen increased with increasing annealing temperature in dry-air. An epitaxially grown In2O3 phase was found both in Cu-rich and In-rich films annealed at TA ⩾ 350°C, which was not observed in the films annealed in Ar atmosphere. Thermodynamic calculations based on the CuInSeON system showed In2O3 to be the most stable phase in good agreement with the experimental results.

Temperature dependence of photoacoustic spectra in CuInSe2 thin films grown by molecular beam epitaxy

CuInSe2 (CIS) thin films, the thickness of about 1.0 μm and composition of CuIn ratio (γ = 1.79), were grown on (0 0 1)-oriented GaAs substrate by molecular beam epitaxy (MBE) at substrate temperature of Ts, = 450°C. The samples have been characterized by means of piezoelectric photoacoustic (PPA) measurements between liquid helium (4.2 K) and room temperature (300 K). Two distinct PPA signals due to band gap of CIS and GaAs are observed in the whole temperature range from 4.2 to 300 K and the PPA signals of CIS decrease curvilinearly at the temperature range. Since the PPA signals of CIS thin films can be obtained up to room temperature, the PPA measurements are quite effective to obtain the optical characterizations, especially for the nonradiative recombination processes.

Temperature variation of nonradiative carrier recombination processes in high-quality CuGaSe2 thin films grown by molecular beam epitaxy

The piezoelectric photoacoustic (PPA) measurements for Cu-rich CuGaSe2(CGS)/GaAs(001) epitaxial layer were carried out between liquid helium and room temperatures. The band gap energies of CGS (A, B, and C bands) were measured to be 1.73, 1.83, and 2.04 eV at liquid nitrogen temperature, respectively. The A band was clearly obtained from 5 to 300 K, and the temperature dependence of the peak energy was fitted with the modified Manoogian–Woolley equation. PPA signals for CGS/GaAs (001) epitaxial layers were obtained between liquid helium and room temperature.

Photoluminescence properties of CuInSe2 grown by molecular beam epitaxy

Abstract Photoluminescence (PL) properties of CuInSe 2 thin films with Cu/In ratio ranging from 0.81 to 1.81 grown by molecular beam epitaxy have been investigated for photovoltaic applications. The PL spectra of Cu-rich CuInSe 2 epitaxial films unlike those of polycrystalline CuInSe 2 showed well-defined emission lines, suggesting that high quality CuInSe 2 epitaxial films have been grown. Such fine PL spectra made the identification possible of phono replicas, in other words, the separation of such phonon replicas from substantive emmissions. PL properties are found to be very sensitive to the growth parameters such as Cu/In ratio and substrate temperature. A broad peak at λ ∼ 1.45 μm becomes dominant in In-rich films, and excitation power dependence of such a broad emission indicated a pair-type radiative recombination, most likely the emission due to donor-acceptor pair or their complex.

Piezoelectric photoacoustic spectra of CuInSe2 thin film grown by molecular beam epitaxy

Abstract A CuInSe2 (CIS) film with Cu/In ratio of γ = 1.79 has been grown on (001) oriented GaAs substrate by molecular beam epitaxy (MBE) at substrate temperature of Ts = 450 °C, Piezoelectric photoacoustic (PPA) spectra were measured at liquid nitrogen and room temperatures. Two signals due to the non-radiative carrier recombination that correspond to bandgap energies of CIS and GaAs substrate were obtained. After illuminating with secondary light (λ = 1100 nm), two additional PPA peaks were observed for the CIS film at liquid nitrogen temperature. These signals are due to intrinsic defects in the Cu-rich CIS film.