Paper VT-TuP4
Formation and Characterization of Hydrogenated Amorphous Silicon (a-Si:H) Thin Films Deposited by ECR-CVD with Different RF Powers

Tuesday, November 8, 2016, 6:30 pm, Room Hall D

Hydrogenated amorphous silicon (a-Si:H) films have been deposited on Si substrates using electron cyclotron resonance (ECR) plasmas. ECR systems are downstream plasma reactors, which allow a separate control of ion energy and ion flux, and can operate at low pressures (1-50mTorr) and can allow reducing sharply ion surface sputtering. A 2.45GHz microwave ECR source generates the plasma at high power (up to 1000W). A 13.56MHz RF power source biases separately the sample chuck. The 2.45GHz ECR source and RF chuck power control the ion flux and ion energy, respectively, allowing low temperature (20⁰C), low pressure (4mTorr) and low damage chemical vapor deposition (CVD). Due to high discharge power conditions in ECR plasmas, high dissociation degree of silane (SiH4), gas molecules can be obtained. SiH4 diluted in 98% of Ar allows low silane concentration in the ECR plasma that, with high degree of dissociation, can reduce the Si-H bond incorporation in the films deposited. Moreover, to optimize the composition and the microstructure of the deposited a-Si:H films, the H incorporation should be kept at less than 20%, depending on RF power source bias. In this work, to study the effects of RF chuck power in the H incorporation into films, different RF powers of 1, 3 and 5W were deposited on silicon substrates, with fixed parameters: ECR power of 500W, pressure of 4mTorr, substrate temperature of 20⁰C, gas flows of SiH4 and Ar, 200 and 20 sccm and 20 minutes. The hydrogen incorporation in the films was determined by FTIR (Fourier Transformed Infrared). These films were annealed at 1000⁰C, during 60s (Rapid Thermal Annealing (RTA) process). Thus, before and after the RTA annealing, for films deposited with RF powers between 1 and 5W: i) The crystalline level of each film, obtained by Raman spectroscopy, changed from totally amorphous film to amorphous and polycrystalline (19.4% - 32.4%) structures; ii) The images from optical microscopy were used to identify the presence of pin holes on the film surfaces. Using scan profiler system, it was extracted the depth (100µm - 2.2µm) and diameter (1800Å - 90 Å) of them. For that, it can be concluded that if the RF power values increases, the crystalline level increases and pin holes dimensions and densities decreases. Considering that the pin holes are generated due to H incorporation into the films, if RF chuck power is higher, the intensities due to Si-H bonds are reduced (extracted from FTIR analyses), indicating the low H concentration into the films. Thus, the a-Si:H films deposited by ECR-CVD that are part of the hybrid solar cells with a-Si:H-p+/c-Si-n++ structures are going to be presented at the conference.