研究目的
To serve as an electron transport layer (ETL) or a buffer layer for the third-generation solar cells, a compact and uniform gallium nitride (GaN) thin layer with suitable energy level is needed. Meanwhile, it is also meaningful to explore its low-temperature deposition especially on transparent electrodes.
研究成果
280-300°C is identified as the optimized deposition temperature for forming a compact and uniform n-type GaN layer on FTO substrate. The 50-200 PEALD cycles of GaN layers show an amorphous structure, and their bandgap values ranging from 3.95 to 3.58 eV have been displayed. From the UPS results, as the GaN thickness increases, Fermi level moves upward obviously along with a reduction of CBM value and an increase of VBM value. This thickness-dependent band structure can be reasonably explained as the relaxation of compressive stress and increased carrier concentration for a thicker GaN layer, and thus shows the possibility to regulate its energy level via controlling thickness.
研究不足
The high roughness of FTO substrate is adverse to purge process and thus leads to carbon residue. Besides, the oxygen impurities in the GaN films are mainly from the plasma source and residual oxygen in the reaction chamber.
1：Experimental Design and Method Selection:
GaN thin films were prepared using the Angstrom-dep III PEALD reactor. Triethylgallium (TEGa) precursor as Ga source was used. A gas mixture of N2/H2/Ar (3:6:1) plasma and high-purity Ar were used as nitrogen source and purging gas, respectively.
2：Sample Selection and Data Sources:
FTO conductive glass was used as the substrate for the GaN deposition.
3：List of Experimental Equipment and Materials:
Angstrom-dep III PEALD reactor, TEGa precursor, N2/H2/Ar plasma, high-purity Ar, FTO glass.
4：Experimental Procedures and Operational Workflow:
The cleaned FTO substrates were fixed in the reaction chamber and then pumped vacuum to
5：15 Torr. Prior to GaN deposition, the substrates should keep at the reaction temperature at least 30 min to reach temperature balance. Especially, 5 cycles of plasma pretreatment were firstly performed to eliminate the residual oxygen from chamber and FTO surfaces. For each PEALD cycle of GaN deposition, included 30 s plasma, 30 s Ar purge, 5 s TEGa release, and 30 s Ar purge. Data Analysis Methods:
The surface morphology and roughness of GaN were measured by SEM, AFM, and TEM. Crystalline characteristic of the samples was analyzed by a GIXRD. Chemical features were measured using an XPS. The valence band spectrum and work function were obtained by UPS. UV-visible absorption and transmission were carried out by UV-vis spectrophotometer.
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