Laser annealing of hydrogenated amorphous silicon–carbon films

doi:10.1016/j.tsf.2003.11.070

Thin Solid Films

Volumes 453–454, 1 April 2004, Pages 7-12

https://doi.org/10.1016/j.tsf.2003.11.070 Get rights and content

Abstract

Hydrogenated amorphous silicon–carbon films with carbon content, x=C/(C+Si), ranging from 0.08 to 0.28 have been irradiated by excimer (KrF) laser varying the incident energy density, Φ, from 64 to 242 mJ/cm². The crystallization of the silicon phase is induced in all the samples, independently of the alloy composition and the average Si crystallite size increases with the laser energy density. Cubic SiC crystallites, sized 200–450 Å, are only detectable in samples with x≥0.18 irradiated at Φ≥188 mJ/cm². The structural and electrical properties of the laser treated films have been investigated. After crystallization all films show a noticeable increase in dark conductivity.

Introduction

The wider band gap of microcrystalline silicon–carbon (μc-Si_1−xC_x) alloys together with the possibility of doping have made these materials important components of electronic devices, as solar cells, thin film transistors and light-emitting diodes [1], [2], [3].

μc-Si_1−xC_x contains Si and/or SiC crystalline phase and the dark conductivity exceeds that of amorphous silicon–carbon by several orders of magnitude.

μc-Si_1−xC_x for device applications have to be grown at low substrate temperature and the most common deposition techniques are plasma enhanced chemical vapour deposition (PECVD) [4], remote PECVD [5] and electron cyclotron resonance (ECR-PECVD) [6]. μc-Si_1−xC_x is grown from a gas mixture highly diluted in hydrogen [4], [7], [8] causing a significant damage to the device interface, moreover the incorporation of carbon up to 15 at.% suppresses completely the microcrystallinity [4], [9]. These constraints can be overcome by means of excimer laser annealing of amorphous silicon–carbon [10], [10](a), [10](b) that also induces crystallization in quasi stoichiometric alloys [11], [12] and allows to fabricate better quality interfaces.

The excimer laser annealing process is confined very close to the irradiated surface (<1 μm) due to the higher absorption coefficient in the UV region and the lower thermal conductivity of the amorphous materials as compared to its crystalline counterpart [13]. However, to avoid damaging the substrate or device interface it is necessary to know the melting threshold of the amorphous layer to be crystallized.

In this paper, the crystallization treatment of hydrogenated amorphous silicon carbon (a-Si_1−xC_x:H) films deposited by PECVD system at low substrate temperature, 200 °C, has been investigated as a function of both excimer (KrF) laser energy density and alloy composition. The process domains in which promoting the formation of the SiC crystallites in the a-Si_1−xC_x:H films with carbon content less than 0.3 have been identified.

Section snippets

Experimental details

a-Si_1−xC_x:H films were grown in a 13.56 MHz capacitively coupled PECVD, using a gas mixture of silane (SiH₄) and methane (CH₄) diluted in helium (He).

Samples were deposited by varying the methane fraction f=[CH₄]/([SiH₄]+[CH₄]) between 0 and 0.8, while keeping the helium dilution, [He]/([He]+[SiH₄]+[CH₄]), at 0.6. The other parameters were kept constant at the following values: RF power of 50 W, (RF power density of 0.03 W/cm²), plasma pressure of 0.4 Torr and substrate temperature of 200 °C.

As grown a-Si_1−xC_x:H films

The optical, electrical and structural properties of the films were determined before the laser treatment.

The absorption coefficient α, derived from the transmittance spectra is shown in Fig. 1. The curves are nearly parallel and shift towards higher energy values as the methane fraction increases. This behavior reveals a widening of the optical energy gap E₀₄ from 1.91 to 2.47 eV, due to the increase of carbon content in films [14]. The results of nuclear measurements, as C/Si and H/Si ratios,

Conclusions

a-Si_1−xC_x:H films with carbon content in the range of 0.08–0.28 have been irradiated in air by an excimer (KrF) laser varying the energy density from 64 to 242 mJ/cm².

In the range of carbon content 0.18–0.28, 3C-SiC crystallites sized 200–450 Å are laser induced in the films for Φ≥188 mJ/cm², while for x<0.18 only the segregation of the crystalline silicon phase occurs. The average crystallite size increases as a function of Φ. At Φ=242 mJ/cm² the dark conductivity of the laser treated film

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Laser annealing of hydrogenated amorphous silicon–carbon films

Abstract

Introduction

Section snippets

Experimental details

As grown a-Si1−xCx:H films

Conclusions

J. Non-Cryst. Solids

Thin Solid Films

J. Non-Cryst. Solids

Philos. Mag. B

Surf. Coat. Technol.

Phys. Status Solidi B

Phys. Rev. B

J. Appl. Phys.

Appl. Phys. Lett.

J. Appl. Phys.

As grown a-Si_1−xC_x:H films