A comparison of grain nucleation and grain growth during crystallization of HWCVD and PECVD a-Si:H films

TL;DR: In this article, the authors compare the crystallization kinetics of HWCVD and PECVD a-Si:H films, containing different initial film hydrogen contents (CH), by annealing at 600 °C.

About: This article is published in Thin Solid Films.The article was published on 2008-01-15 and is currently open access. It has received 25 citations till now. The article focuses on the topics: Crystallization & Nucleation.

Summary

1. Introduction

• The crystallization of as deposited a-Si:H thin films is becoming increasingly important because of its potential use to produce higher mobility polycrystalline materials for use in solar cells and high performance thin film transistors.
• This process is believed, at relatively low anneal temperatures (b 1000 °C), to follow a classical model of nucleation and grain growth [1], where an amorphous incubation time, a steady state nucleation rate, grain growth of these nuclei, and a characteristic time of crystallization can be identified.
• Further, recent results have shown that the crystallization time, for a given film CH, can depend upon the a-Si:H deposition method [4].

2. Experimental

• H films were deposited by HWCVD and PECVD, using deposition conditions described previously [6,7], also known as A-Si.
• Regarding the latter, the width of the XRD diffraction peaks may result from a combination of grain size, defect density, or strain effects.
• This thickness was chosen because no sample thinning was required.
• TEM analysis was performed on a CM200 Scanning TEM using a Phillips single-slit holder and a Gatan Model 652 double-slit heating holder for in-situ annealing.

3. Results and discussion

• Fig. 1 shows TEM images of partially crystallized HWCVD and PECVD a-Si:H films annealed at 600 °C, with the annealing times indicated in the figures.
• From this figure, clear differences are seen not only in the anneal time, but also in the grain density and grain morphology between the different films.
• These differences become even more pronounced when a low CH HWCVD film is included in the comparison compared to the PECVD film seen in Fig. 1(b) [8,9]; in these comparisons, the PECVD film showed much lower grain densities and larger grains overall.
• In Table 3 the authors first present a review of the NMR data, from which the densities of the isolated and clustered hydrogen distributions can be obtained.
• H calculation, the authors use an averaged value of the clustered/isolated hydrogen ratio, obtained for ‘standard’ (low deposition rate) films deposited using 100% silane and a moderate (∼ 200–250 C) substrate temperature, also known as For the PECVD a-Si.

4. Summary and conclusions

• The authors have presented the crystallization kinetics when HWCVD films of different film CH and ‘standard’ PECVD aSi:H films have been annealed at a temperature of 600 °C to induce film crystallization.
• The authors find that the low CH HWCVD film nucleates first, and that the incubation time increases with increasing film CH.
• Not surprisingly, the films which nucleate the fastest contain the smallest grains when crystallization is complete.
• The increase in short range disorder upon film hydrogen evolution does not seem to play a primary role in the crystallization process.
• A tentative model relating the crystallization kinetics to the initial hydrogen spatial distribution in the film is presented.

Figures

Table 3 H distributions in as grown a-Si:H

Table 1 Measured crystallization parameters for HWCVD and PECVD films annealed at 600 °C

Fig. 1. TEM images taken in the early crystalline regimes of high CH HWCVD film (a) and (similar) high CH PECVD film (b). The crystalline volume fractions are roughly similar. The 600 °C anneal times are included in the figures.

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "A comparison of grain nucleation and grain growth during crystallization of hwcvd and pecvd a-si:h films" ?

Even though the bonded hydrogen evolves very early from the film during annealing, the authors suggest that the initial spatial distribution of hydrogen plays a critical role in the crystallization kinetics, and they propose a preliminary model to describe this process.