PL Study on the Effect of Cu on the Front Side Luminescence of CdTe/CdS Solar Cells

The effect of Cu on highly efficient CdTe thin solid film cells with a glass/TCO/CdS/CdTe structure subjected to CdCl2 treatment was investigated by low-temperature photoluminescence (PL). The PL of the CdS/CdTe junction in samples without Cu deposition revealed a large shift in the bound exciton position due to the formation of CdSxTe1−x alloys with Eg (alloy) ≅ 1.557 eV at the interface region. After Cu deposition on the CdTe layer and subsequent heat treatment, a neutral acceptor-bound exciton (A0Cu,X) line at 1.59 eV and two additional band-edge peaks at 1.54 and 1.56 eV were observed, indicating an increase in the energy gap value in the vicinity of the CdTe/CdS interface to that characteristic of bulk CdTe. These results may suggest the disappearance of the intermixing phase at the CdTe/CdS interface due to the presence of Cu atoms in the junction area and the interaction of the Cu with sulfur atoms. Furthermore, an increase in the intensity of CdS-related peaks in Cu-doped samples was observed, implying that Cu atoms were incorporated into CdS after heat treatment.

The Properties of Dual Acceptor Delta-Doped ZnO Thin Films

ABSTRACTWe report the investigation of ZnO thin films delta-doped with lithium and phosphorus introduced simultaneously. The films were deposited from high purity ceramic targets of ZnO and Li3PO4 on c-plane sapphire substrates by RF magnetron sputtering. An undoped ZnO film with a low background electron concentration was used as the buffer layer on the sapphire substrate. The doped films were prepared by carrying simultaneous sputtering from the ZnO and Li3PO4 ceramic targets. For uniform doped films, the simultaneous deposition from the ZnO and Li3PO4 was uninterrupted. For the delta-doped films on the other hand, deposition from the ZnO target was uninterrupted while that from the Li3PO4 was interrupted periodically using a shutter. Post-deposition annealing was carried using a rapid thermal processor in O2 at 900 oC for 3 min. Results obtained from photoluminescence spectroscopy measurements at 12 K revealed acceptor-related luminescence peaks at 3.35 eV, possibly due to the transition from exciton bound to a neutral acceptor. The x-ray diffraction 2θ-scans showed a single peak at about 34.4o. Hall effect measurements revealed p-type conductivities with an average Hall concentrations of 3.8 x 1013 cm-3 in uniform doped samples and 1.5 x 1016 cm-3 in delta doped samples. However, in some cases the Hall coefficients had both positive and negative values, making the determination of the carrier type inconclusive. The fluctuation in the carrier type could be due to the lateral inhomogeneity in the hole concentration caused by signal noise impacting the small Hall voltages in the measurements.

Photoluminescence and acceptor level state of p-type nitrogen-doped MgZnO films

A wurtzite nitrogen-doped MgZnO (MgZnO:N) film was grown by plasma-assisted molecular-beam epitaxy (PAMBE) on c-plane sapphire using radical NO as oxygen source and nitrogen dopant. The as-grown film shows n-type conduction at room temperature, but transforms into p-type conduction after annealed. Photoluminescence (PL) spectrum measured at 80 K is dominated by neutral donor-bound exciton emission (D0X) located at 3.522 eV for the n-type MgZnO:N film, but by neutral acceptor-bound exciton emission (A0X) located at 3.515 eV for the p-type MgZnO:N film. By fitting exciton emission intensity of temperature-dependent PL spectra, the binding energies of the D0X and A0X were estimated to be 32 and 43 meV, respectively. Based on the energy shift of exciton emission, the band gap of the MgZnO:N film is estimated to be 3.613 eV, which is 179 meV larger than that of ZnO. Using the Haynes rule, the acceptor energy level of the MgZnO:N film was evaluated to be about 176 meV above the valence band.

Optical Properties for ZnSe Epilayer Obtained From Photoluminescience Measurement

The ZnSe epilayers were grown on the GaAs substrate by hot wall epitaxy. After the ZnSe epilayers treated in the vacuum-, Zn-, and Se-atmosphere, respectively. The defects of the epilayer were investigated by means of the low-temperature photoluminescence measurement. The dominant peaks at 2.7988 eV and 2.7937 eV obtained from the PL spectrum of the as-grown ZnSe epilayer were found to be consistent with the upper and the lower polariton peak of the exciton, I2 (Do, X), bounded to the neutral donor associated with the Se-vacancy. This donorimpurity binding energy was calculated to be 25.3 meV. The exciton peak, I1 d, at 2.7812 eV was confirmed to be bound to the neutral acceptor corresponded with the Zn-vacancy.

Photoluminescence investigation of CdZnTe:In single crystals annealed in CdZn vapors

CdZnTe:In single crystals were annealed in CdZn vapors through a method involving a high-temperature step and a low-temperature step in sequence. The effects of annealing on the properties of CdZnTe:In were characterized with photoluminescence (PL) spectra. The neutral acceptor bound exciton (A0, X) peak, which was on the right shoulder of the neutral donor bound exciton (D0, X) peak, disappeared after annealing. A fine donor-acceptor pair structure and its longitudinal optical phonon replicas were clear before annealing. However, both of them became undistinguishable in the PL spectrum of annealed CdZnTe:In. The two phenomena imply that the annealing treatment can remove the impurities from CdZnTe:In wafers effectively. In addition, the intensity of Dcomplex band fell remarkably after annealing, which confirmed that Cd vacancies were well-compensated in the annealing treatment.

PHOTOLUMINESCENCE STUDIES OF GaInArSb HIGHLY DOPED WITH TELLURIUM GROWN BY LIQUID PHASE EPITAXY ON (100) GaSb

Tellurium-doped GaInArSb epitaxial layers with electron concentration in the range of 3 × 1017 – 2 × 1020 cm -3 are grown at 530°C on (100) GaSb substares by liquid phase epitaxy (LPE). To dope the layers we used pellets of Sb 3 Te 2 in preparing growth melts. The low temperature photoluminescence (PL) spectra (20 K) showed a dominant peak composed of three transitions associated to excitons bound to residual acceptor impurities. For highly Te-doped layers the excitonic transitions related to exciton bound to neutral acceptor, BE 2, disappears.