Influence of grain size and surface condition on minority-carrier lifetime in undoped n-BaSi2 on Si(111)

Abstract

We have fabricated approximately 0.5-lm-thick undoped n-BaSi 2 epitaxial films with various average grain areas ranging from 2.6 to 23.3 lm 2 on Si(111) by molecular beam epitaxy, and investigated their minority-carrier lifetime properties by the microwave-detected photoconductivity decay method at room temperature. The measured excess-carrier decay curves were divided into three parts in terms of decay rate. We characterized the BaSi 2 films using the decay time of the second decay mode, s SRH , caused by Shockley-Read-Hall recombination without the carrier trapping effect, as a measure of the minority-carrier properties in the BaSi 2 films. The measured s SRH was grouped into two, independently of the average grain area of BaSi 2. BaSi 2 films with cloudy surfaces or capped intentionally with a 3 nm Ba or Si layer, showed large s SRH (ca. 8 ls), whereas those with mirror surfaces much smaller s SRH (ca. 0.4 ls). X-ray photoelectron spectroscopy measurements were performed to discuss the surface region of the BaSi 2 films.

Potential variation around grain boundaries in BaSi2 films grown on multicrystalline silicon evaluated using Kelvin probe force microscopy

“…[3][4][5] In addition, both the minority-carrier lifetime (ca. 10 ls) [6][7][8] and the minoritycarrier diffusion length (ca. 10 lm) 9,10 in undoped n-type BaSi 2 epitaxial films on Si(111) are sufficiently large for thin-film solar cell applications.…”

Potential variations around grain boundaries in impurity-doped BaSi2 epitaxial films evaluated by Kelvin probe force microscopy

“…[1][2][3][4] Large absorption coefficients come from the band structure of BaSi 2 , where the localized Ba d-like states form flat energy bands in the conduction band. [5][6][7] Minority-carrier properties, such as a long minority-carrier diffusion length (ca.10 lm) 8,9 and a long minority-carrier lifetime (ca.10 ls), [10][11][12] in a-axis-oriented undoped n-type BaSi 2 epitaxial films spurred interest in this material. One of the striking features of this material is the fact that both large minority-carrier diffusion length and large absorption coefficient are available.…”

Formation of BaSi2 heterojunction solar cells using transparent MoOx hole transport layers

“…During air exposure, oxides such as SiO x and BaO x are likely to form, which function like a passivation layer on the surface of BaSi 2 . 31 It was thus speculated that the J-V characteristics would therefore be improved in sample B. SiO x is more likely to form because measurements using coaxial impact-collision ion scattering spectroscopy and atomic force microscopy suggest that the surface of a-axis-oriented BaSi 2 film grown on Si(111) is terminated by Si. 32 However, the formation of BaSi 2 surface oxides seems fast in air.…”

“…30 Second, an approximately 600 nm thick a-axis-oriented undoped BaSi 2 layer was grown epitaxially by MBE at 600 C for 8 h. The sample was then removed from the chamber and exposed to air to form a native oxide layer on the BaSi 2 (except for sample A) because long minority-carrier lifetime is ensured by the formation of oxides on the surface of BaSi 2 . 31 The duration of exposure was 1 h, 5 min, and 1 min for samples B, C, and D, respectively. A 15 nm thick MoO x (x < 3) layer was then formed on the BaSi 2 surface at room temperature (RT).…”