Investigation of Surface Potential Distributions of Phosphorus-doped n-BaSi 2 Thin-Films by Kelvin Probe Force Microscopy

We investigated the surface potential distributions around grain boundaries (GBs) in phosphorus (P)-doped n-BaSi2 thin-films by Kelvin probe force microscopy (KFM) and the crystal planes constituting GBs by transmission electron microscopy (TEM). By KFM measurements, it was found that the GBs in P-doped n-BaSi2 are different from those in undoped BaSi2; undoped n-BaSi2 has a downward band bending around the GBs with barrier heights of approximately 30 meV. In contrast, P-doped n-BaSi2 has an upward band bending with barrier heights of approximately 15 meV. TEM observation revealed that most of the GBs in P-doped BaSi2 are composed of BaSi2 (011)/(0-11) planes. This result is the same as that in undoped BaSi2.


Introduction
BaSi2 has an ideal band gap of approximately 1.3 eV and a large absorption coefficient of 3×10 4  cm -1 at 1.5 eV [1,2].Accordingly we consider BaSi2 as a new candidate for thin-film solar cell materials.By molecular beam epitaxy (MBE), BaSi2 can be grown on Si(111) substrates with three epitaxial variants rotated by 120° with each other around the surface normal [3,4].Thus, these films are polycrystalline and contain many grain boundaries (GBs).The grain size of BaSi2 films grown on Si(111) reached a maximum of approximately 4 µm [5].Generally, GBs are considered to be defective and thus have negative influence on solar cell performance.Therefore, we investigated the potential profiles across GB in undoped n-BaSi2 on Si(111) and found that it had a downward band bending with small potential barrier heights of approximately 30 meV for holes at GBs.Therefore, the GBs are thought not to deteriorate the minority-carrier transport in the n-BaSi2 [6].In this work, we formed phosphorus(P)-doped BaSi2 and evaluated the surface potential variations around the GBs using Kelvin probe force microscopy (KFM).We also examined the crystal planes at the GBs by transmission electron microscopy (TEM).According to our previous studies, P-doped BaSi2 showed n-type conductivity [7].

Experiment
A 200 nm-thick P-doped n-BaSi2 film was grown on Si(111) using MBE referring to our previous reports [7].An ion-pumped MBE system equipped with standard Knudsen cells for Ba, and P, and an electron-beam evaporation source for Si was used for the growth.For P-doped BaSi2, crystalline GaP was used for supplying P. The temperature of GaP was fixed at 550 °C and the substrate temperature was also set at 550 °C.The crystalline quality of the grown films were characterized by reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD).The doping of P atoms in the grown layers was confirmed by secondary ion mass spectrometry (SIMS).Reference samples with a controlled number of P atoms doped in BaSi2 have not yet been prepared.The surface potentials were characterized by KFM (Shimazu SPM-9600) and the crystal planes constituting GBs were examined by TEM.

Experimental Results and Discussions
We observed the diffraction peaks of [100]-oriented BaSi2 in the θ-2θ XRD pattern and the spotty RHEED pattern.These results indicate that a-axis-oriented P-doped BaSi2 was grown on Si(111).The electron concentration, n, was n = 4.0×10 17 cm -3 by Hall measurements.SIMS measurements revealed that the doping of P atoms in the grown layers was confirmed.Ga atoms were found to be also included.According to Ref. [8], Ga doping forms n-type BaSi2.However, the Ga concentration was smaller by 3 orders of magnitudes than P. Thus, the influence of Ga is thought to be negligibly small.
Figure 1 shows the 3×3 µm AFM topographic and KFM potential images with cross sectional profiles along white broken line AA' in the P-doped BaSi2.Both images were taken in the same area.In these profiles, the position of GBs are indicated by colored lines.In Fig. 1, we see that the electrostatic potentials at the GBs were smaller than those of grain interiors.This result means that the GBs are negatively charged compared with the grain interiors, suggesting that the upward band bending occurred around the GBs.
Next, we evaluated the potential barrier height ΔEGB at GBs by the following Eq.( 1) where VGB is the electrostatic potential at GBs, VG,ave is the average potential of inner parts of two adjoining grains, and e is the elementary charge.We applied Eq. ( 1) to 37 GBs.The histogram of barrier heights is shown in Fig. 2. The barrier height for electrons ranges from 5 to 35 meV, and we see that the GBs with barrier heights of approximately 15 meV for electrons are dominant.In undoped n-BaSi2 (n = 5.0×10 15 cm -3 [2]), the average barrier height at GBs is approximately 30 meV for holes.Therefore, the characteristics of GBs changed greatly by adding P atoms.From the results mentioned above, the band structure around GBs is determined as shown in Fig. 3.There seems to be two possibilities to explain this result; (1) the Fermi level, Ef, is almost pinned at GBs and Ef approaches the bottom of the conduction band, Ec, in the BaSi2 grains because of P doping.This may lead to the upward band bending as shown in Fig. 3 as in heavily Sb-doped n-BaSi2[9], (2) the crystal planes which form GBs changed by P doping and thereby the sign of potential barrier height at GBs with respect to the BaSi2 grains changed from undoped ones.In order to clarify the crystal planes at the GBs in P-doped BaSi2, TEM observation was performed [4].A bright-field plan-view TEM observation of the P-doped BaSi2 was first carried out.The incident electron beam direction was almost parallel to the BaSi2 [100] zone axis.From selected-area electron diffraction (SAED) pattern, we found that the GBs were composed of (011) and/or (002) plane like in undoped BaSi2.It was difficult to distinguish the diffraction of (011) from that of (002) in the SAED pattern because their lattice spacings are almost the same.Thus, we next performed dark-field (DF) plan-view TEM observations with a two-beam diffraction condition and observed only BaSi2 (00n) planes with the diffraction vectors g set to be <004>.DF plan-view TEM images under a two-beam diffraction condition are shown in Fig. 4. Bright domains in each figure corresponds to one of the three epitaxial variants.We see that the liner GBs are composed of mostly BaSi2 (011)/(0-11) planes.This result is the same as that in undoped BaSi2, meaning that the crystal planes which form GBs did not change by P doping.We therefore suppose that the upward band bending shown in Fig. 3 is caused by not the change of the planes but the movement of Ef in the inner part of BaSi2 grains [9].Further studies are mandatory to discuss further.

Conclusion
We determined the position of GBs in P-doped BaSi2 on Si(111) by AFM and investigated the surface potential distribution around the GBs by KFM.Although the downward band bending was observed in undoped n-BaSi2, the upward band bending was confirmed around GBs in P-doped BaSi2.The potential barrier height was about 15 meV for electrons.From the DF plan-view TEM observation, we found that the GBs consisted mostly of ( 011) and (0-11) planes, which is the same as in undoped BaSi2.We therefore suppose that the upward band bending occurred because EcEf became smaller in P-doped n-BaSi2 than in undoped n-BaSi2 while Ef was almost pinned at the GBs

Fig. 1 .
Fig. 1.(a) Surface topographic and (b) surface potential distribution images of P-doped BaSi2 observed in the same area.(a') and (b') are the cross sectional profiles along dotted line AA' in (a) and (b), respectively.

Fig. 4 .
Fig. 4. DF plan-view TEM images under a two-beam diffraction condition.The diffraction vector g was set to be <004> for each BaSi2 epitaxial variant.In each figure, one of the three epitaxial variants is observed bright.