Advanced atmospheric pressure CVD of a-Si:H using pure and cyclooctane-diluted trisilane as precursors†

Abstract

Liquid silanes can be used for low-cost, fast deposition of hydrogenated amorphous silicon (a-Si:H) as an alternative to state-of-the-art deposition processes such as plasma enhanced chemical vapor deposition or electron beam evaporation. However, liquid silane deposition techniques are still in their infancy. In this paper, we present a new version of the atmospheric pressure chemical vapor deposition technique designed to improve the reproducibility of a-Si:H deposition. With this new tool, we explore ways to improve the quality of the material. The films can be prepared using pure trisilane as a precursor; frequently, however, trisilane is diluted with cyclooctane for better handling and process control. Currently, the influence of this dilution on the film quality is not well understood. In our work, we investigate and compare both precursor strategies. This paper presents a comprehensive analysis of the effects of cyclooctane dilution, deposition temperature, process duration, and precursor amount on the structure stoichiometry and electronic properties of the resulting films. The analysis was performed using a range of techniques, including Fourier transform infrared spectroscopy, electronic spin resonance spectroscopy, Raman spectroscopy, ellipsometry, secondary ion mass spectrometry, and conductivity measurements. For films deposited with pure silane, we found a low oxygen (O) and carbon (C) impurity incorporation and an adjustable H content up to 10%, resulting in a photosensitivity of up to 10⁴. Dependent on the dilution and deposition temperature, the films deposited with cyclooctane dilution showed various amounts of C incorporation, culminating in an a-Si:H/a-SiC:H structure for high temperatures and dilutions. High purity a-Si:H films as a-Si:C:H films are promising for application in solar cells and transistors either as an amorphous functional layer or as a precursor for recrystallization processes, e.g., in TOPCon solar cell technology.