Electrical characterization of N+-implanted n-type ZnO single crystal: ZnO p-n homojunction and deep level defects

Abstract

Unintentionally doped n-type ZnO single crystals were implanted by nitrogen ions with different fluences of 1013, 1014 and 1015 cm-2 respectively under accelerating voltage of 150 keV. The oxygen face was chosen for the implantation at a substrate temperature of 300°C . Electrical characterization techniques including current-voltage (I-V), capacitance-voltage (C-V), Deep Level Transient Spectroscopy (DLTS) and double-correlation DLTS (DDLTS) were used for investigating all the as-implanted samples and control sample through Au/n-ZnO Schottky diodes. DLTS results showed that N+ implantation with the fluence of 1013 cm-2 introduced one prominent electron trap EN1 at 0.95eV below conduction band with trap concentration of 4×1015 cm-3 and capture cross section of 6×10-12 cm2 which was not observed in non-implanted control sample. This implantation induced deep level was also observed in as-implanted ZnO samples with fluences of 1014 and 1015 cm-2. ZnO p-n homojunction was successfully fabricated due to the formation of p-type layer after 650°C post-implantation annealing in air for 30 minutes. DLTS measurements showed that the deep states in as-implanted samples still persisted after 650°C annealing. However, it was found that EN1 finally disappeared after 750°C thermal annealing through DLTS spectra. On the other hand, another electron trap with activation energy of 0.174 eV and capture cross section of 4.7×10-17 cm2 was detected in 750°C-annealed p-n junction. Detailed properties of implantation induced defects were discussed based on plentiful DLTS spectra. Moreover, a broad red luminescence peak centered at 620nm was observed in low-temperature photoluminescence spectra of all the as-implanted samples, whereas absent in the non-implanted control sample. It was also found that the red luminescence peak existed in 650°C-annealed sample. As for 750°C-annealed sample, there was no obvious red emission peak and a strong green luminescence peak dominated the PL spectrum. Using multimode Brownian oscillator (MBO) model, the induced broadband had a zero phonon wavelength coincided with the energy scheme of the transition between the detected 0.95eV deep level and the valance band. Combined with first-principle calculation results, this EN1 deep level can be assigned to be oxygen vacancy (VO) which was predicted to be located ~1 eV below conduction band.