CdTe is a group V compound semiconductor whose structure is similar to that of Si and Ge. That is, its crystals are mainly covalently bonded, but they have certain ionic properties. Compared with W series semiconductors of the same cycle, the bonding strength of CdTe is similar. Great. Therefore, at room temperature, the conductivity of CdTe semiconductors is mainly determined by doping. At the same time, the composition, structure and heat treatment process of the thin film also have great influence on the resistivity and conductivity type of the CdTe thin film.
CdTe has a direct band gap structure. For light with a wavelength less than the absorption limit, the CdTe film has a very high light absorption coefficient, as long as the film thickness reaches about m, enough to absorb most of the sunlight of hu>1.45 eV, thereby reducing the requirement for the material diffusion length. In the film deposition process, the deposition parameters affect the light absorption characteristics of CdTe films obtained by thermal evaporation. For different thicknesses of CdTe films, the absorption coefficient varies with the absorption limit and the incident photon energy near the absorption limit. Experiment 13 shows that the thinner the film, The higher the absorption coefficient.
This work uses a vacuum vapor deposition method, heated in a vacuum chamber with a molybdenum boat, while evaporating Cd + Te material to form a CdTe film. To improve its performance, In was doped in the material and the film was heat treated in nitrogen. The films before and after heat treatment were tested and analyzed to study their electrical and optical properties. Experiments show that the electrical and optical properties of CdTe films are mainly affected by the elemental composition ratio of the material, the uniformity of the components, the degree of crystallization, the lattice structure, and the grain boundary. Therefore, it is very important to study the electrical and optical properties of CdTe thin films for the preparation of thin films with good photovoltaic properties.
1 Experiment 1 Deposition of CdTe thin film In a vacuum chamber, a vacuum vapor deposition method was used. The raw material was a high-purity Cd+Te material, heated with a molybdenum boat, and a CdTe thin film was deposited on a glass substrate. Doping is performed during the deposition process, In is added to the material in proportion and the film thickness is measured and controlled.
1.2 Heat Treatment of CdTe Thin Films When heat treating CdTe thin films, nitrogen was used as a protective gas, and the gas flow rate was about 900 mL/min. The results were obtained for undoped CdTe thin films. 2 Results and Discussion 1 Electrical Characteristics of CdTe Thin Films Four-probe methods were used to test the electrical resistance of thin films. rate. The resistivity of the undoped Indium CdTe thin film is high regardless of the blending ratio, confirming that the undoped Indium-doped CdTe material is a typical high-resistivity semiconductor. The effect of different heat treatment conditions on the resistivity of CdTe thin films. From a can be found in the 50C heat treatment 7min, the thin film resistivity, greater than or less than 7min, the resistivity is high. From b in the same heat treatment time (7min), after the heat treatment temperature exceeds 50C, with the increase of temperature, the resistivity of the fund project: National Natural Science Foundation of China (6956001) temperature conditions of different heat treatment time on the resistivity of CdTe thin film The effect of the heat treatment temperature T = 5QC on the CdTe thin film heat treatment 7min, before and after heat treatment thin film resistivity changes. It can be seen that when the 1716 and 15 samples with Cd:Te=0.6:1 are heat treated, the resistivity drops faster. 16 samples prepared with a deposition time of 3 min, the resistivity decreased after heat treatment. The light absorption response curves of different CdTe films before heat treatment. When the heat treatment time is too long, the Te content in the film decreases due to evaporation of Te, and the composition of the film and the stoichiometric ratio are shifted. Large, so the resistivity of the thin film increases again. In addition, when the heat treatment temperature is high, due to the difference in thermal expansion coefficient between the substrate and CdTe, a crystal defect occurs in the film, which also has a significant influence on the structure and characteristics of the film, which increases the resistivity of the CdTe film.
3 (1, 351, 401 heat treatment, the test results show that after the heat treatment of the film, the resistivity is still very large. It can be seen that pure CdTe is a high-resistivity semiconductor material.
The conductivity of the sample was measured by the hot and cold probe method. The results showed that the samples were all P type, but the P type was stronger after heat treatment.
The optical properties of 2.2CdTe thin film 4 grains gradually Jinn grain boundary effect reduced free current-carrying fees lishings-34QQ spectrophotometer measurement 1 transmission spectrum of the film solar energy 23 after the management of the price of 4 thin film forbidden width For â–²Yeci.cXblishing reflectivity spectra and transmission spectra of glass substrates, the absorption spectrum of CdTe films was calculated using a computer. The optical absorption spectra of unheat treated CdTe films are given. It can be seen that different film samples have different absorption curves. This is due to the influence of deposition parameters on the light absorption of CdTe films obtained by the thermal evaporation method. The absorption of the film near 4580A is the strongest, and the long-wave absorption is higher. weak. For the heat treatment time t = 7 min, different heat treatment temperature and film absorption rate (sample Cd 6:1, no incorporation of In). It can be seen that the high heat treatment temperature has good absorption, but when the temperature rises to 350C, the absorption rapidly decreases. This is because the heat treatment makes the grain of the thin film grow and the structure tends to be complete, which is beneficial to the light absorption of the film. When the heat treatment temperature is higher than 350C, evaporation of Te occurs, the deviation of the stoichiometric ratio of the film-rich Cd increases, the defects increase, and the light absorption characteristics are seriously affected. It can be seen that at the different heat treatment temperatures, the absorption of the film still occurs at around X=458A, and then decreases. The heat absorption time of the CdTe thin film of the heat treatment time T=5C with different heat treatment time (60% of sample Cd 50%) is increased by the visible heat treatment time, and the absorption is good. This is because the heat treatment makes the grain in the thin film grow and the structure tends to Complete, thus facilitating the light absorption of the film.
Since CdTe is a direct bandgap semiconductor and satisfies the direct interband transitions, the reports that the absorption coefficient a and the photon energy hu satisfy the following relationships are basically consistent. It can be seen that the doping In improves not only the conductivity of the CdTe semiconductor, but also improves the optical properties.
3 Conclusion Pure CdTe thin film has a high resistivity and is a high-resistivity semiconductor compound. Doping with In reduces the resistivity of the film, and the resistivity of the film decreases further after heat treatment.
Appropriate heat treatment conditions can improve the electrical and optical properties of the film.
The In doped CdTe film has a higher light absorption coefficient. The film prepared in this experiment is about 41×14 cm, and the optical properties are better than those of the undoped In film.
The forbidden band width of the CdTe film before heat treatment at room temperature is about 1.46 eV, and the forbidden band width of the heat treated CdTe film is about 1.40 to 1.44 eV. Ji Binghou et al. Advances in Research and Application of Rural Energy and New Energy in the Cold Regions of Northern China 丨 M. Hohhot: Inner Mongolia University Press, 1997.298-310. Liu Enke, Zhu Bingkai, Luo Jinsheng, et al. Semiconductor physics. Fourth edition.
Beijing: National Defense Industry Press, 1994! C. Chiev. Semiconductor Physics (Chinese translation). Jinan: Shandong Institute of Electronics Publishing.
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