Effect of Femtosecond Laser Radiation on the Structure and Conductivity of Boron Doped Amorphous Hydrogenated Silicon

The comparative analysis of the influence of femtosecond laser radiation on the structure and conductivity of undoped and boron-doped amorphous hydrogenated silicon was made in this paper. It has been found that the process of nanocrystals formation in an amorphous matrix by femtosecond laser radiation for undoped amorphous hydrogenated silicon samples begins at lower laser energy densities than for doped samples. Different conductivity of undoped and doped samples of amorphous hydrogenated silicon before femtosecond radiation becomes approximately equal upon radiation with laser fluence of 150-160 mJ/ cm2.

Amorphous hydrogenated silicon (a-Si: H) has been successfully used for a long time for the production of solar cells, photodetectors, and widescreen displays. One of the main disadvantages of a-Si: H is the degradation of its parameters under illumination 1 . More stable electrical and photoelectric parameters can be achieved by the creation of nanocrystalline inclusions in a film of amorphous silicon [2][3][4][5][6][7] . This material (the material composed of amorphous silicon with crystalline silicon inclusions of nanometric size) was called nanocrystalline silicon (nc-Si:H). nc-Si: H can be obtain by femtosecond laser crystallization of a-Si:H [8][9][10][11] . This method allows crystallizing locally the selected areas of the a-Si: H film, minimizing hydrogen effusion from the film, as well as carrying out the so-called "cold" crystallization even on the fusible substrates.
Currently, femtosecond laser crystallization of a-Si:H is described in detail in the literature. In particular, there are the data on the effect of the femtosecond laser radiation on the structure, the hydrogen content, optical absorption, conductivity and photoconductivity of a-Si:H films 12-20 . However, the femtosecond laser crystallization was studied only for undoped a-Si: H. At the same time, elements of both p-and ntype are necessary for the creation of many semiconductor devices, so it is interesting to consider the effect of doping on the properties of a-Si: H, subjected to femtosecond laser crystallization.

The samples and methods of measurement
Two series of amorphous hydrogenated silicon films with thickness of 300 nm were prepared by plasma enhanced chemical vapor deposition from the gas phase of monosilane (SiH4) onto a quartz substrate coated with a layer of ITO. The temperature of the quartz substrate was 250 °C. During the preparing of second series samples diborane (B2H6) was also added to the reaction chamber, what resulted in the boron doped samples.

Volume ratio of gases in the reaction chamber was [B 2 H 6 ]/[SiH 4 ] = 10 -4 .
The obtained films were exposed to femtosecond laser pulses with a femtosecond subterawatt complex on the basis of a Cr:forsterite laser. The wavelength of emission was equal to 540 nm, and laser pulse duration was 500 fs. The radiation was carried out with laser beam scanning the film surface. The scanning speed was v = 5 mm/s, the repetition frequency of pulses incident on the sample was í = 200 kHz. The laser beam was focused onto the sample by means of an aspherical glass lens with a numerical aperture of 0.16. The focal plane was placed at a distance of 80 ìm above the sample surface. The designations of the samples in the first and second series and values of the energy density of the used laser radiation are shown in Table 1.
Measurements of Raman spectra were carried out with a spectrometer LabRam HR800 "Horiba Jobin Yvon". Excitation of the samples was conducted by light with a wavelength of 632.8 nm. To measure the conductivity of the samples the aluminum contacts with area of 1 mm 2 were deposited on their surface. The lower contact was an ITO film deposited on the substrate. To measure the conductivity of the films was used picoammeter Keithly 6487. The measurements were carried out at room temperature.

RESULTS AND DISCUSSION
Raman spectra of the films from the first and second series are shown in Figure 1, a and 1, b respectively. The volume fraction of the crystalline phase and the size of the nanocrystals were determined from the analysis of the Raman spectra in accordance to the procedure described in works [21][22][23] .
Values of volume fraction of the crystalline phase and the nanocrystals size determined from the Raman spectra for samples from two series are shown in Table 2.
As can be seen in Table 2, in undoped a-Si:H films nanocrystals are formed at laser fluence of 80 mJ/cm 2 and volume fraction of the crystalline phase may reach 22%. At the same time in boron doped a-Si:H films nanocrystals are formed at laser fluence of 110 mJ/cm 2 and higher and volume fraction of the crystalline phase doesn't exceed 10%. We can't determine the cause of this difference clearly at now. However, it should be noted that according to [24][25][26] the samples of nc-Si: H with a volume fraction of the crystalline phase less than 10% and the nanocrystal size of 3-7 nm demonstrate the best parameters for solar energy application. The appropriate samples are obtained by laser crystallization of boron doped a-Si:H films. We can conclude that femtosecond laser crystallization of boron doped a-Si:H allows to -0 obtain perspective material for solar energy application. The dependences of conductivity on volume fraction of crystalline phase for both series are demonstrated in Figure 2.
As can be seen before laser radiation the conductivity of boron doped films is larger than conductivity of undoped film by 2 orders. After laser treatment of samples the conductivity of all samples increases non-monotonically with increasing of laser fluence. The conductivity of boron doped films increases by 4 orders as conductivity of undoped films increases by 1-2 orders only. As the result after laser treatment with highest laser fluence the conductivity of undoped and boron doped samples become almost equal. It should be mentioned, for conductivity measurements the vertical configuration of electrical contacts was used and the distance between contacts was small (equal film thickness, 500 nm). The material of contacts can penetrate into the films and results to conductivity measurements error.

CONCLUSION
In summary, a-Si: H nanocrystals are formed in undoped films at laser fluence of 80 mJ / cm 2 and the volume fraction of the crystalline phase reaches more than 20%. At the same time, crystallization of boron doped films starts at laser fluence of 110 mJ / cm 2 and volume fraction does not exceed 10%. It can be assumed that the presence of boron influences the laser crystallization mechanism. So small volume fraction of the crystalline phase (less than 10%) at the femtosecond laser crystallization of boron doped amorphous silicon indicates the possibility of application in solar energy.
Before laser radiation the conductivity of boron doped a-Si: H films exceeds the conductivity of undoped a-Si: H films by 2 orders. After laser radiation the conductivity of a-Si:H(B) increases slightly with laser fluence. The conductivity of both doped and undoped films becomes alsmost equal at laser fluence of 150-160 mJ / cm 2 .