Hirofumi Higuchi

HETEROEPITAXY AND CHARACTERIZATION OF CUGASE2, LAYERS GROWN BY LOW-PRESSURE METALORGANIC CHEMICAL-VAPOR DEPOSITION

CuGaSe2 chalcopyrite compounds were grown heteroepitaxially on both GaAs and GaP substrates by means of the low‐pressure metalorganic chemical‐vapor deposition method. Optical and structural properties were characterized comprehensively by photoreflectance (PR), photoluminescence (PL), x‐ray diffraction, transmission electron microscopy, transmission electron diffraction, and electron‐probe microanalysis. The CuGaSe2 epilayers had c(001) surface on GaAs(001) substrates and a(100) surface on GaP(001) substrates, respectively, the results being similar to the case of CuAlSe2. Energies of A, B, and C excitons associated with uppermost valence bands were determined from analysis of PR spectra, and the energies of good‐quality epilayers are close to those of the bulk crystal. The slight increase of the crystal‐field splitting in the valence bands were discussed in terms of the lattice strain in the epilayer caused by the lattice mismatch. Low‐temperature PL spectra exhibited an intense peak at 1.71 eV, the ene...

LP-MOCVD growth of CuAlSe2 epitaxial layers

Abstract Epitaxial growth of CuAlSe 2 has been performed on GaAs and GaP substrates by LP-MOCVD technique using cyclopentadienylcoppertriethylphosphine, trimethylaluminum, and dimethylselenium as respective Cu, Al, and Se precursors. Growth orientation of CuAlSe 2 depends on the substrate orientation, i.e., the epitaxial layers are oriented toward the appropriate direction which has the smallest lattice mismatch. The carbon incorporation from trimethylaluminium has been reduced by increasing the growth temperature and by increasing the dimethylselenium molar flow rate. The carbon incorporation has also been reduced about one order of magnitude by using the new Al precursor, ethyldimethylaminealane. The epitaxial layers have shown red, broad photoluminescence at 77K.

Photoluminescence studies in CuAlSe2 epilayers grown by low‐pressure metalorganic chemical‐vapor deposition

Low‐temperature photoluminescence (PL) spectra were investigated for CuAlSe2 epilayers grown on GaAs(001) substrates by means of low‐pressure metalorganic chemical‐vapor deposition. PL properties were studied with relation to metalorganic precursors used for the growth. High‐quality undoped epilayers exhibited PL peaks related to a free exciton (2.739 eV) and a bound exciton (2.677 eV). The other undoped epilayers exhibited PL bands at 2.3, 2.4, and 2.5 eV originating from donor‐acceptor (D‐A) pair recombinations. Some of them were found to have a common activation energy for the thermal quenching of 50±10 meV. The PL spectrum changed drastically by impurity doping. Intense green emissions at 2.51 and 2.43 eV were observed in Zn and Mg‐doped epilayers, respectively, which were interpreted as D‐A pair recombinations based on the dependencies of the PL spectra on excitation intensity, decay time, and temperature. The donor and the acceptor activation energies (ED and EA) were estimated to be 110 and 230 meV...

Excitonic photoluminescence in a CuAlSe2 chalcopyrite semiconductor grown by low-pressure metalorganic chemical-vapor deposition

Excitonic photoluminescence (PL) in a CuAlSe2 chalcopyrite semiconductor was observed. High‐quality CuAlSe2 epilayers were grown by the low‐pressure metalorganic chemical‐vapor deposition technique. Based on photoreflectance measurements, the PL peak at 2.739 eV was assigned to a free exciton emission. The PL peak at 2.677 eV was tentatively assigned to a bound exciton emission.

Heteroepitaxial Growth of CuGaS 2 Layers by Low-Pressure Metalorganic Chemical Vapor Deposition

Heteroepitaxial growth of CuGaS2 was studied by low-pressure metalorganic chemical vapor deposition using normal-tripropylgallium as a new Ga precursor, combined with cyclopentadienylcoppertriethylphosphine and ditertiarybutylsulfide. Structural and optical properties were characterized in detail. The epilayer showed the c[001]-oriented growth on both GaAs(001) and GaP(001) substrates. The lattice parameter c of the epilayers was smaller than that of the bulk single crystal. Magnitude of the residual lattice strain for CuGaS2/GaP(001) was found to be larger than that for CuGaS2/GaAs(001), even though the lattice mismatch for the former was smaller than that for the latter. The strain is considered to be introduced during cooling after the growth. A photoluminescence peak at 2.493 eV (8 K) was assigned to a free exciton emission, because the peak energy agreed with A-exciton energy obtained from photoreflectance spectra.

CuAlSe2 chalcopyrite epitaxial layers grown by low-pressure metalorganic chemical vapor deposition

Epitaxial layers of CuAlSe2 chalcopyrite crystals were successfully grown on GaAs and GaP substrates by low-pressure metalorganic chemical vapor deposition. The epilayers were comprehensively characterized by X-ray diffraction, transmission electron microscope, transmission electron diffraction, electron-probe microanalysis, secondary-ion mass spectrometry, photoreflectance (PR), photoluminescence (PL), and Hall measurements. Dependence of growth rate and film properties on growth parameters, such as growth temperature and input molar flow ratio of the metalorganic precursors were studied. It was found that the good-quality epilayers were grown in the narrow range of the growth conditions. The valence to conduction band transition energies of such good-quality epilayers determined by the PR spectra are close to those for the bulk crystals. Stoichiometric CuAlSe2 epilayers exhibited a yellow-green PL peak at 2.27 eV, which is the shortest-wavelength PL ever reported for CuAlSe2 single crystals.

Preparation and characterization of CuAlxGa1−xSe2 alloy layers grown by low‐pressure metalorganic vapor phase epitaxy

CuAlxGa1−xSe2 alloy layers were successfully grown on GaAs(001) by low‐pressure metalorganic vapor phase epitaxy. The distribution coefficient of Al was unity. All alloy layers had their c‐axis normal to the substrate plane. Exciton resonance energies were determined as a function of x by means of photoreflectance measurements. A quadratic dependence of exciton energies on x was confirmed. The spin‐orbit splittings of the epilayers were approximately the same as that of bulk crystals. The magnitudes of crystal‐field splittings were larger than that of bulk crystals, and this was explained in terms of residual tensile biaxial strain in the epilayers. The color of the low‐temperature photoluminescence (PL) changed from red to crimson, orange, yellow, green, and bluish‐purple with increasing x. A peak due to a free‐to‐acceptor transition was dominant in the PL spectra of the alloy layers. The acceptor ionization energy increased with increasing x, and the result may reflect an increase of the hole effective ...

Low-pressure metalorganic chemical vapor deposition of a CuGaSe2/CuAlSe2 heterostructure

A single-heterostructure of CuGaSe2/CuAlSe2 chalcopyrite semiconductors was successfully grown epitaxially on a GaAs(001) substrate using a low-pressure metal organic chemical vapor deposition technique. The valence-band discontinuity for the CuGaSe2/CuAlSe2 heterointerface was estimated to be 0.8±0.1 eV by means of X-ray photoelectron spectroscopy. The CuGaSe2/CuAlSe2 structure was found to have the straddling-type heterointerface, and the band discontinuity was found to be distributed mainly in the valence band.

Low-pressure metalorganic chemical vapor deposition of cualse2 epitaxial films

A low-pressure metalorganic chemical vapor deposition of CuAlSe2 epitaxial films was performed using several kinds of metalorganic precursors. The film quality varied remarkably depending on the source precursors. Free exciton emission at 2.739 eV was observed in the CuAlSe2 films grown using triisobutylaluminium and diethylselenium, both of which have lower decomposition temperatures.

2.51 eV Donor-Acceptor Pair Photoluminescence from Zn-Doped CuAlSe2 Epilayer Grown by Low-Pressure Metalorganic Chemical Vapor Deposition

A donor to acceptor pair photoluminescence (PL) peak at 2.51 eV was observed in Zn-doped CuAlSe2 epilayers grown by low-pressure metalorganic chemical vapor deposition. The summation of ionization energies of the donor and the acceptor was obtained to be 330 meV from the excitation intensity dependence of the PL peak energy. The Zn impurity is shown to act as a defect-killer because of the apperance of exciton emission at 2.716 eV.