Preferred Orientation of BaSi 2 Thin Films Fabricated by Thermal Evaporation

Thermal evaporation is a simple and high-speed method to grow a BaSi2 thin film, which is an emerging candidate for an absorber-layer material of thin-film solar cells. In this study, we have investigated the preferred orientation of BaSi2 films grown at substrate temperatures of 600–700 ◦C by thermal evaporation using X-ray diffraction. 2θ–ω scans show that peaks derived from (100) orientation grow steadily with increasing substrate temperature. By Xray pole figure analysis, the (100)-oriented crystals are proven to be epitaxially grown on Si(100) with two variants. The reason of epitaxial growth is discussed from the epitaxial temperature.


Introduction
Orthorhombic BaSi 2 is an attractive material for thin-film solar cell applications because of the abundance of constituent elements in the earth's crust as well as excellent properties such as a suitable band gap of 1.1-1.3eV [1][2][3][4], high absorption coefficients [3], and long minority-carrier lifetime (up to 11 µs [5]) and diffusion length (10 µm [6]).Control of the carrier type and conductivity is possible by impurity doping [7][8][9], which enables one to design various device structures.Recently, a high power conversion efficiency reaching 9.0 % has been reported by using a p-type BaSi 2 /n-type Si heterojunction structure [10].
Fundamental properties and device applications of BaSi 2 have been studied mainly using (100)-oriented epitaxial films grown by molecular beam epitaxy (MBE) on Si substrates [11,12].For practical applications, however, a more simple and high-speed film growth method is preferable.We are hence developing a thermal evaporation technique using a BaSi 2 source.So far, single-phase BaSi 2 films have been formed on various substrates such as Si [13][14][15][16][17], glass [4,14], and CaF 2 [18].On single-crystal Si substrates, the BaSi 2 films have been proven to be oriented polycrystals [13].The preferred orientation is, however, not decided.For further understanding of the film growth mechanism, the information on preferred orientation is important.In this study, therefore, we investigated the crystal orientation of evaporated BaSi 2 films, especially focusing on the effects of substrate temperature (T sub ).It is shown that partial epitaxial growth with (100) orientation is possible by the simple evaporation technique at high T sub 's, which suggests that the (100) orientation of BaSi 2 is preferable on Si(100) from an energetic viewpoint.

Experimental methods
Commercial BaSi 2 lumps (99 % in purity, Kojundo Chemical Lab.) were used as a source material of evaporation after grinding.They were loaded onto a tungsten boat in the vacuum chamber and were melted by resistive heating of the boat.After complete melting of BaSi 2 , the shutter was opened and the heating current was raised for film deposition.The substrate used was p-type Si(100) wafer with a resistivity of over 1000 Ω•cm.T sub was varied in a range of 600-700 • C. Base pressure of the chamber was below 1 × 10 −3 Pa.Produced films were characterized by scanning electron microscopy (SEM; JEOL JSM-6500F and Hitachi TM3030), 2θ-ω X-ray diffraction analysis (XRD; Bruker Discover D8) with Cu Kα radiation, and X-ray pole figure analysis with an area detector (Bruker Discover D8).

Results and discussion
The morphology of the produced films observed by SEM is shown in Fig. 1.At T sub = 600 • C, a dense film without cracks is formed.On the other hand, at higher T sub 's, cracks appear on the surface.The density of cracks is higher for T sub = 700 • C than 650 • C, indicating that cracking is because of thermal stress, as has been observed in previous studies [4,19].The BaSi 2 film thicknesses determined from the cross-sectional pictures are in a range of 460-510 nm.
Figure 2 shows 2θ-ω XRD patterns of evaporated BaSi 2 films.All peaks are attributed to orthorhombic BaSi 2 phase and Si substrate, assuring that the film consists of BaSi 2 .It is clearly observed that the relative peak intensity of BaSi 2 is significantly different from the simulated powder pattern, which shows that the BaSi 2 films are preferentially oriented.
At T sub = 600 • C, 301 or 004 diffraction is relatively strong.Since 002 diffraction at 2θ = 15.4 • was not observed, 301 is more plausible than 004, corresponding to the (301) preferential orientation.At T sub = 650  111) orientations are preferred at T sub = 600 and 650 • C, respectively, because any straightforward trends of diffraction intensity are not observed.On the other hand, the diffraction intensity of h00 increases steadily with increasing T sub .This result suggests that the (100) orientation of BaSi 2 is more preferable on the Si(100) surface from an energetic viewpoint.Other orientations may have been kinetically stabilized at low temperatures.
Since the (100) orientation of BaSi 2 on Si(100) surface is similar to BaSi 2 films grown on Si(100) by MBE, the evaporated BaSi 2 film at T sub = 700 • C may have also grown epitaxially on Si(100).To determine the crystal orientation of evaporated BaSi 2 films, therefore, we also performed X-ray pole figure measurement.Each two-dimensional (2D) XRD pattern was taken by scanning the φ axis with a 2θ angle of the area detector fixed at 31. the outer one is 203, the 2θ angle of which is 30.7 • .This pole figure clearly shows that the evaporated film was grown epitaxially on Si(100) with two epitaxial variants.The orientation relationships are BaSi 2 [010]//Si[011] and BaSi 2 [001]//Si[011] with BaSi 2 (100)//Si(100) for two variants.These relationships are the same as those of an epitaxial BaSi 2 film grown on Si(100) by MBE [12].
Similar two sets of spots are observed from the evaporated film grown at 650 • C. The intensity of spots are, however, weaker than 700 • C. By considering the 2θ-ω scan and pole figure results, a part of this film is epitaxially grown with the same relationship at 700 • C, while the rest consists of polycrystals with (111) preferential orientation.At a lower temperature of 600 • C, strong spots are not observed.Instead, signals are relatively strong around the normal direction, which agrees with strong 301 diffraction in the 2θ-ω pattern.The evaporated film grown at 600 • C, therefore, has preferred orientation of (301) without epitaxy.
It is known that epitaxial growth occurs with evaporation methods when the growth temperature exceeds a minimum temperature (epitaxial temperature), which has been reported for various materials including metals and semiconductors grown on metal, semiconductor, oxide, and halide substrates [20][21][22].From the present results, the epitaxial temperature of BaSi 2 on Si(100) is speculated to be above 650 • C when grown by thermal evaporation.Although the epitaxial temperature of MBE-grown BaSi 2 on Si(100) is not reported, it is known for Si(111) substrate.With a reactively-deposited template layer, epitaxial growth is reported in a wide T sub range of 450-700 • C [11].On the other hand, without a template layer, epitaxial growth occurs below 650 • C [11].This significant difference of epitaxial temperatures between thermal evaporation and MBE is presumably due to a different crystal growth mechanism.The existence of a template layer formed by the reactive deposition epitaxy [23] also significantly influences the epitaxial temperature.During thermal evaporation, Ba-rich vapor is supplied to the substrate at the initial stage.The initial BaSi 2 layer formed by the reaction between Ba-rich vapor and Si substrate may serve similarly as the template layer in MBE, which may be one of the reasons of epitaxial growth during thermal evaporation at high T sub .At low T sub , thermal diffusion may not be enough to reach thermal equilibrium due to high deposition rates of thermal evaporation, which might account for other kinetically-stabilized preferred orientations.

Conclusions
In this study, we have investigated the crystal orientation of BaSi 2 thin films grown by thermal evaporation.It has been found by 2θ-ω XRD and pole figure analyses that (100)-oriented epitaxial growth of BaSi 2 is possible on Si(100) by increasing T sub to 700 • C. The epitaxial relationship of BaSi 2 (100)/Si(100) with two epitaxial variants is the same for thermal evaporation and MBE.A significant difference of epitaxial temperatures between thermal evaporation and MBE is probably due to different crystal growth mechanisms.

Fig. 1 .
Fig. 1.SEM pictures of evaporated BaSi 2 films grown at T sub = 600-700 • C. Plan-view pictures are backscattered electron images (BEI) taken by TM3030 while cross-sectional ones are secondary electron images (SEI) by JSM6500F.
Fig. 2. 2θ-ω XRD patterns of evaporated BaSi 2 films together with theoretical powder pattern of BaSi 2 .The asterisk symbol denotes the peak from Si(100) substrate.Three digit numbers are diffraction indices for BaSi 2 .isprobably (111) and the strong diffractions are 111 and 222.By further increasing T sub to 700 • C, preferential orientation changes again.The 111 and 222 diffractions become invisible.The strongest peak is 600 diffraction at this temperature.The 200 and 400 diffractions are also strong while the other diffractions are very weak, indicating that almost all BaSi 2 crystals are (100)-oriented at T sub = 700 • C. From the present experimental results, it is difficult to find out the reason why (301) and ( • C, 301 peak becomes weak.Instead, 222 or 123 diffraction is strongest.Because 111 or 102 diffraction is also strong, preferential orientation