Satoru Seto

Defect-induced emission band in CdTe

We report on a distinct correlation between the 1.47 eV emission band and the dislocation density in bulk CdTe. The 1.47 eV band intensifies around the high-dislocation area (lineage structure) and at the position just on dislocation bundle. On the other hand, the 1.47 eV band was hardly observed in the low-dislocation area (etch pit density less than 2 × 105 cm-2) or at the position away from the dislocation bundle. Furthermore, the 1.47 eV band was intensified by γ-ray irradiation of 1.7 × 107 Gy, which produced a great number of Frenkel defects. It was shown that the 1.47 eV band is related not only to an extended defect such as a dislocation, but also to a point defect such as a Frenkel defect. These results suggest that the strain field induced in the vicinity of the defects is responsible for the recombination center of the 1.47 eV band.

Chlorine-related photoluminescence lines in high-resistivity Cl-doped CdTe

Abstract High-resistivity Cl-doped CdTe crystals grown by the gradient-freeze method were studied by high-resolution photoluminescence (PL) and electrical measurements. A sharp PL line ascribed to a Cl-related defect was observed at 1.5903 eV in all of the high-resistivity crystals doped with different amounts of chlorine. A broad line around 1.586 eV was clearly observed in highly doped crystals. The intensities of these lines increased with the amount of chlorine. Annealing under Cd-saturated atmosphere eliminated the 1.5903 eV line and intensified the neutral donor-bound exciton line together with its two-electron transition. The high-resistivity crystals were also converted to n-type with low resistivity by the Cd annealing. These results show that the 1.5903 eV line is a key emission line for the high-resistivity, and suggest that the line is associated with Cd-vacancy/chlorine complex, probably in the form of V CD -Cl.

Compensating related defects in In-doped bulk CdTe

We report on annealing behaviors of electrical and optical properties of In-doped CdTe to investigate the self-compensation in high-resistivity CdTe. A characteristic luminescence line at 1.5842 eV in high-resistivity In-doped CdTe is found to be due to emissions from bound excitons trapped at compensating related defects, which is ascribed to a Cd-vacancy/indium complex. These defect complexes act as acceptors and are responsible for the self-compensation through the balance between the doped indium donors and the defect complex acceptors. Magneto-optical measurements also support this self-compensation mechanism.

Carrier drift mobilities and PL spectra of high resistivity Cadmium Telluride

Abstract Time-of-flight (TOF) measurements and photoluminescence (PL) properties of Cl-doped semi-insulating CdTe, grown by the gradient freeze (GF) method are described. By choosing appropriate growth conditions, drift mobilities as high as 1100 and 80 cm 2 /V·s for electrons and holes respectively were obtained, which are comparable to those grown by the traveling heater method (THM). From a comparison of PL and TOF results, correlations between the intensity of PL line W (at 1.587 eV) and the drift mobilities of both electrons and holes have been found. The relations of the PL lines W and G (at 1.591 eV) are analyzed by a compensating defect model composed of (V Cd Cl Te ) and (V Cd 2Cl Te ).

Photoluminescence and annealing behavior of Ga-doped CdTe crystals

Ga-doped CdTe crystals grown by the vertical gradient freeze method were studied by measuring high-resolution photoluminescence (PL) and electrical properties. Comparisons were made for as-grown and annealed samples under Cd saturation. The as-grown samples grown from slightly Te rich melt were of high resistivity (108 Ω cm) although gallium on the Te site alone in CdTe acts usually as a shallow donor. The Cd-annealed samples, on the other hand, were n-type with low resistivity. A new prominent PL line at 1.5841 eV was found in the high-resistivity as-grown crystals. Furthermore, the 1.5841 eV line completely disappeared after Cd annealing. These results suggest that the luminescence center related to the 1.5841 eV line is a Cd-vacancy/gallium associated and is responsible for high resistivity. This is direct evidence to explain the high-resistivity mechanism due to so-called self-compensation in CdTe(Ga). The two-electron transitions in Ga donors measured by PL are also reported.

Photoluminescence measurements on undoped CdZnTe grown by the high-pressure bridgman method

The low temperature photoluminescence of Cd0.91Zn0.09Te grown by the high-pressure Bridgman (HPB) method exhibits a neutral donor bound exciton emission (D0X) at 1.65603 eV with its excited state (D0X*) at 1.65798 eV and neutral acceptor bound exciton emissions (A0X) at 1.64566 eV and 1.65201 eV. Assuming a direct generation and subsequent relaxation of excitons at the D0X* state, we demonstrate that the temporal evolution of the above emission bands is well reproduced by a set of rate equations. The resultant radiative-lifetime of 1.4 ns for the D0X and 1.5 and 2.0 ns for the A0Xs are compared with various CdZnTes (CZTs) grown by the other methods to demonstrate the particular nature of the HPB CZT.

Drift mobility and photoluminescence measurements on high resistivity Cd 1−x Zn x Te crystals grown from Te-rich solution

The drift mobilities of chlorine doped high resistivity Cd0.8Zn0.2 Te have been investigated by using the time-of-flight technique. Electron as well as hole mobility in the as-grown crystals is limited by trap-controlled carrier transport. The energy locations of the defects responsible for carrier trapping are determined to be Ec- 0.03 and Ev+ 0.14 eV for electrons and holes, respectively. After annealing at 400°C for 80 h, no evidence of trap-controlled mobility was recognized for electrons. On the other hand, no significant change before and after the annealing was observed for hole transport. Those results and the change in the photoluminescence spectra before and after the annealing are explained by the complex defect model composed of the Cd vacancy and chlorine donor. Further, the alloy scattering potentials of ΔUe and ΔUh were estimated by employing the theoretical calculation method recently reported by D. Chattopadhyay [Solid State Commun. 91, 149 (1994)].

Activation of Nitrogen Acceptor in ZnSe Homo‐Epilayer Grown by MOCVD

A ZnSe epitaxial layer doped with nitrogen (N) has been grown on ZnSe substrate at optimized conditions by a low-pressure metalorganic chemical vapor deposition (MOCVD) system using hydrogen as a carrier gas and ammonia as a dopant source. In order to enhance the activation ratio of nitrogen in ZnSe epitaxial layer, ZnSe:N/ZnSe was annealed in atmosphere with zinc-saturated vapor pressure. Photoluminescence spectra measured at 4.2K from annealed ZnSe : N epilayers showed the stronger I 1 N emission. C-V measurement showed that the ZnSe:N epitaxial layer is of p-type conductivity and the highest net acceptor concentration reaches as high as 6.7 x 10 1 7 cm - 3 . This is the highest value for the ZnSe homosystem epitaxial layers grown by the MOCVD method at present. Furthermore, the ionization energy of nitrogen in ZnSe was estimated to be 109 meV by examining the dependence of DAP emission on the measuring temperature.

Acceptor defects and annealing behavior in indium doped Cd1 − xZnxTe (x > 0.7)

The doping and annealing behavior of indium doped Cd 1-x Zn x Te is investigated by using admittance spectroscopy and photoluminescence measurements. Two acceptor traps are induced by indium doping: one is the well-known A-center at E v + 0.15 eV and the other at E v + 0.27 eV. The latter one is suggested to be a key-defect for the abrupt change of conductivity observed in this ternary system at x>0.7.

Epitaxial Growth of Hexagonal CdS Films on Hydrogen-Terminated Si(111) Substrates

Hexagonal CdS films were epitaxially grown directly on hydrogen-terminated Si(111) substrates with nominally 0° (just-orientated) and 3°-off-orientated surfaces by the hot-wall epitaxy technique. X-ray diffraction θ-2θ scans and pole figures confirmed the epitaxial relationship between the grown CdS film and Si substrate to be CdS(0001) ∥ Si(111) and CdS[1120] ∥ Si[110]. Strong excitonic emissions as well as donor-acceptor pair emissions were observed for the CdS film grown on the 3°-off-orientated Si(111) substrate. In the CdS film on the just-orientated Si(111) substrate, on the other hand, no excitonic emissions could be observed.