C. Zhu

Deep level defects in a nitrogen-implanted ZnO homogeneous p-n junction

Nitrogen ions were implanted into undoped melt grown ZnO single crystals. A light-emitting p-n junction was subsequently formed by postimplantation annealing in air. Deep level transient spectroscopy was used to investigate deep level defects induced by N+ implantation and the effect of air annealing. The N+ implantation enhanced the electron trap at EC−(0.31±0.01)eV (E3) and introduced another one at EC−(0.95±0.02)eV (D1), which were removed after annealing at 900 and 750°C, respectively. Another trap D2 (Ea=0.17±0.01eV) was formed after the 750°C annealing and persisted at 1200°C.

Arsenic doped p-type zinc oxide films grown by radio frequency magnetron sputtering

As-doped ZnO films were grown by the radio frequency magnetron sputtering method. As the substrate temperature during growth was raised above ∼400 °C, the films changed from n type to p type. Hole concentration and mobility of ∼6×1017 cm−3 and ∼6 cm2 V−1 s−1 were achieved. The ZnO films were studied by secondary ion mass spectroscopy, x-ray photoelectron spectroscopy (XPS), low temperature photoluminescence (PL), and positron annihilation spectroscopy (PAS). The results were consistent with the AsZn–2VZn shallow acceptor model proposed by Limpijumnong et al. [Phys. Rev. Lett. 92, 155504 (2004)]. The results of the XPS, PL, PAS, and thermal studies lead us to suggest a comprehensive picture of the As-related shallow acceptor formation.

Characterization of tumor suppressive function of P300/CBP-associated factor at frequently deleted region 3p24 in esophageal squamous cell carcinoma

Deletion of 3p is one of the most frequent genetic alterations in many tumors, including esophageal squamous cell carcinoma (ESCC). In our recent study, deletion of 3p24 was frequently detected in ESCC and one candidate tumor suppressor gene (TSG), p300/CBP-associated factor (PCAF), was identified within the region. In this study, downregulation of PCAF was detected in 23/40 (57.5%) of primary ESCCs and 4/9 (44.4%) of the ESCC cell lines. A further study found that downregulation of PCAF was also associated with hypermethylation of the promoter region of PCAF gene. Methylation-specific PCR found that promoter methylation was detected in 28/40 (70%) of primary ESCCs and 5/9 (55.6%) of ESCC cell lines. In addition, the expression of PCAF could be reactivated in ESCC cell line KYSE510 after demethylation treatment with 5-aza-dC. Functional studies showed that PCAF was able to suppress tumorigenicity of ESCC cells both in vitro and in vivo, including foci formation, colony formation in soft agar and tumor formation in nude mice. Molecular study found that the tumor suppressive mechanism of PCAF was associated with its role in cell cycle arrest at the G1/S checkpoint by the downregulation of CDK2 and upregulation of p21waf1/Cip1, Smad4, Rb and p27Kip1. In conclusion, PCAF might be the target TSG responsible for the 3p24 deletion event, which has an important role in the development and progression of ESCC.

Comprehensive study of the p-type conductivity formation in radio frequency magnetron sputtered arsenic-doped ZnO film

Arsenic doped ZnO and ZnMgO films were deposited on SiO2 using radio frequency magnetron sputtering and ZnO–Zn3As2 and ZnO–Zn3As2–MgO targets, respectively. It was found that thermal activation is required to activate the formation of p-type conductivity. Hall measurements showed that p-type films with a hole concentration of ∼1017 cm−3 and mobility of ∼8 cm2 V−1 s−1 were obtained at substrate temperatures of 400–500 °C. The shallow acceptor formation mechanism was investigated using x-ray photoelectron spectroscopy, positron annihilation, low temperature photoluminescence, and nuclear reaction analysis. The authors suggest that the thermal annealing activates the formation of the AsZn-2VZn shallow acceptor complex and removes the compensating hydrogen center.

Influence of electron irradiation on hydrothermally grown zinc oxide single crystals

The resistivity of hydrothermally grown ZnO single crystals increased from similar to 10(3) Omega cm to similar to 10(6) Omega cm after 1.8 MeV electron irradiation with a fluence of similar to 10(16) cm(-2), and to similar to 10(9) Omega cm as the fluence increased to similar to 10(18) cm(-2). Defects in samples were studied by thermally stimulated current (TSC) spectroscopy and positron lifetime spectroscopy (PLS). After the electron irradiation with a fluence of 10(18) cm(-2), the normalized TSC signal increased by a factor of similar to 100. A Zn vacancy was also introduced by the electron irradiation, though with a concentration lower than expected. After annealing in air at 400 degrees C, the resistivity and the deep traps concentrations recovered to the levels of the as-grown sample, and the Zn vacancy was removed.

Vacancy-type defects in 6H–silicon carbide induced by He-implantation: a positron annihilation spectroscopy approach

Six-fold helium ion implantation was carried out on nitrogen doped n-type 6H–SiC epi samples. A box-shaped He-implantation profile and damage region was thus introduced. Vacancy-type defects in the implanted region were studied by positron annihilation spectroscopy using a monoenergetic positron beam. The average size of the vacancy-type defect detected in the as-He-implanted sample was the divacancy (V2). Thermal annealing had the effect of shrinking the defective region. Annealing at temperatures lower than 900 °C had the effect of removing vacancy-type defects in the defective region. While the annealing temperature is above 900 °C, the size of the vacancy-type defects in the defective region increased with annealing temperature. At the annealing temperature of 1600 °C, the defective region reduced to ~100 nm and the vacancy-type defects within the region agglomerated to clusters having an average size of 14 V2.