Zhiyuan Dong

Analysis of solar cells fabricated from UMG-Si purified by a novel metallurgical method

In this work, solar cells fabricated from 100% upgraded metallurgical grade (UMG) silicon refined by a novel metallurgical route have been investigated. Compared with Siemens mc-Si solar cells, the UMG mc-Si solar cells fabricated in the same process present higher open voltages and fill factors, together with low initial light-induced degradation due to the material properties. By secondary ion mass spectroscopy measurements, the UMG mc-Si solar cells show shallower and better single-side abrupt junctions, resulting in higher photoelectric conversion in the wave band from 400 nm to 750 nm as external quantum efficiency measurements demonstrated. Although higher impurity content limits the minority carrier diffusion length, leading to lower shot-circuit current density, the average efficiency of the UMG-Si solar cells fabricated in standard production line is up to 16.55%, close to 16.69% of the reference solar cells. Furthermore, by an additional phosphorous gettering process, the mean efficiency of the UMG-Si solar cells increases to 16.68%, and the reverse characteristics of the corresponding cells have been significantly improved. (Some figures may appear in colour only in the online journal)

Performance improvement of c-Si solar cell by a combination of SiNx/SiOx passivation and double P-diffusion gettering treatment

SiN x /SiO x passivation and double side P-diffusion gettering treatment have been used for the fabrication of c-Si solar cells. The solar cells fabricated have high open circuit voltage and short circuit current after the double P-diffusion treatment. In addition to better surface passivation effect, SiN x /SiO x layer has lower reflectivity in long wavelength range than conventional SiN x film. As a consequence, such solar cells exhibit higher conversion efficiency and better internal quantum efficiency, compared with conventional c-Si solar cells.

Carbon agent chemical vapor transport growth of Ga2O3 crystal

Beta-type gallium oxide (β-Ga2O3) is a new attractive material for optoelectronic devices. Different methods had been tried to grow high quality β-Ga2O3 crystals. In this work, crystal growth of Ga2O3 has been carried out by chemical vapor transport (CVT) method in a closed quartz tube using C as transport agent and sapphire wafer as seed. The CVT mass flux has been analyzed by theoretical calculations based on equilibrium thermodynamics and 1D diffusional mass transport. The crystal growth experimental results are in agreement with the theoretical predictions. Influence factors of Ga2O3 crystal growth, such as temperature distribution, amount of C as transport agent used, have also been discussed. Structural (XRD) and optical (Raman spectroscopy, photoluminescence spectrum) properties of the CVT-Ga2O3 crystal are presented.

Improvement of the surface quality of semi-insulating InP substrates through a novel etching and cleaning method

Residual impurities and contamination on semi-insulating (SI) InP wafers are detrimental for epitaxial growth and device performance, especially because residual silicon on an SI-InP wafer surface is electrically active and generates an n-type conduction layer at the interface between the epilayer and the InP substrate. In order to reduce the concentration of Si and improve surface quality, the authors investigate a wet-chemical cleaning process for ready-to-use InP substrates. A novel and practical cleaning process was developed by adding an alkaline solution to the conventional acidic cleaning process. Time-of-flight secondary mass spectrometry, a very powerful analysis technique to characterize surfaces and investigate any organic and inorganic contamination present on the InP surface, was used after the samples were etched under different cleaning processes. The results show that the novel etching process effectively reduces the Si contamination.

Characterization of bulk ZnO single crystal grown by a CVT method

Hall effect, photoluminescence spectroscopy (PL), mass spectroscopy and X-ray diffraction have been used to study bulk ZnO single crystal grown by a closed seeded chemical vapor transport method. Enhancement of n-type electrical conduction and increase of nitrogen concentration are observed of the ZnO samples after high temperature annealing. The results suggest that vacancy is dominant native defect in the ZnO material. These phenomena are explained by a generation of shallow donor defect and suppression of deep level defects in ZnO after the annealing.

Native deep level defects in ZnO single crystal grown by CVT method

Hall effect, photoluminescence (PL), infrared absorption, deep level transient spectroscopy (DLTS), and Raman scattering have been used to study property and defects of ZnO single crystal grown by a chemical vapor transport method (CVT). As-grown ZnO is N type with free electron density of 1016-1017cm-3. It has a slight increase after 900°C annealing in oxygen ambient. The DLTS measurement revealed four deep level defects with energy at 0.30eV, 0.50eV, 0.68eV and 0.90eV in the as-grown ZnO sample, respectively. After the high temperature annealing, only the 0.5eV defect survive and has a concentration increase. PL results of the as-grown and annealed ZnO indicate that the well-known green emission disappear after the annealing. The result suggests a correlation between the 0.68eV defect and the green PL peak. Results of P-doped ZnO were also compared with the undoped ZnO sample. The nature of the defects and their influence on the material property have been discussed.

Electrical and optical property of annealed Te-doped GaSb

GaSb is the most suitable substrate in the epitaxial growth of mixed semiconductors of GaSb system. In this work, Te-doped GaSb bulk crystals with different doping concentration have been annealed at 550 ℃ for 100 h in ambient antimony. The annealed samples have been studied by Hall effect measurement, infrared (IR) optical transmission, Glow discharge mass spectroscopy (GDMS) and photoluminescence (PL) spectroscopy. After annealing, Te-doped GaSb samples exhibit a decrease of carrier concentration and increase of mobility, along with an improvement of below gap IR transmission. Native acceptor related electrical compensation analysis suggests a formation of donor defect with deeper energy level. The mechanism of the variation of the defect and its influence on the material properties are discussed.

Thermally induced native defect transform in annealed GaSb

Undoped p-type GaSb single crystals were annealed at 550–600 °C for 100 h in ambient antimony. The annealed GaSb samples were investigated by Hall effect measurement, glow discharge mass spectroscopy (GDMS), infrared (IR) optical transmission and photoluminescence (PL) spectroscopy. Compared with the as-grown GaSb single crystal, the annealed GaSb samples have lower hole concentrations and weak native acceptor related PL peaks, indicating the reduction of the concentration of gallium antisite related native acceptor defects. Consequently, the below gap infrared transmission of the GaSb samples is enhanced after the thermal treatment. The mechanism about the reduction of the native defect concentration and its influence on the material property were discussed.

Donor defect in P-diffused bulk ZnO single crystal

A high concentration of shallow donor defect is formed in P‐diffused ZnO single crystals. X‐ray photoelectron spectroscopy analysis indicates that P atom occupy different lattice site at different diffusion temperature. Nature of the donor defect has been discussed.

Shallow donor defect formation and its influence on semi-insulating indium phosphide after high temperature annealing with long duration

Fe-doped semi-insulating (SI) InP has become semi-conducting (SC) material completely after annealing at 900/spl deg/C for 10 hours. Defects in the SC and SI InP materials have been studied by deep level transient spectroscopy (DLTS) and thermally stimulated current spectroscopy (TSC) respectively. The DLTS only detected Fe acceptor related deep level defect with significant concentration, suggesting the formation of a high concentration of shallow donor in the SC-InP. TSC results confirmed the nonexistence of deep level defects in the annealed SI-InP. The results demonstrate a significant influence of the thermally induced defects on the electrical properties of SI-InP. The formation mechanism and the nature of the shallow donor defect have been discussed based on the results.