Measurement of valence-band offset at native oxide/BaSi2 interfaces by hard x-ray photoelectron spectroscopy

Abstract

Undoped n-type BaSi 2 films were grown on Si(111) by molecular beam epitaxy, and the valence band (VB) offset at the interface between the BaSi 2 and its native oxide was measured by hard x-ray photoelectron spectroscopy (HAXPES) at room temperature. HAXPES enabled us to investigate the electronic states of the buried BaSi 2 layer non-destructively thanks to its large analysis depth. We performed the depth-analysis by varying the take-off angle (TOA) of photoelectrons as 15 , 30 , and 90 with respect to the sample surface and succeeded to obtain the VB spectra of the BaSi 2 and the native oxide separately. The VB maximum was located at À1.0 eV from the Fermi energy for the BaSi 2 and À4.9 eV for the native oxide. We found that the band bending did not occur near the native oxide/BaSi 2 interface. This result was clarified by the fact that the core-level emission peaks did not shift regardless of TOA (i.e., analysis depth). Thus, the barrier height of the native oxide for the minority-carriers in the undoped n-BaSi 2 (holes) was determined to be 3.9 eV. No band bending in the BaSi 2 close to the interface also suggests that the large minority-carrier lifetime in undoped n-BaSi 2 films capped with native oxide is attributed not to the band bending in the BaSi 2 , which pushes away photogenerated minority carriers from the defective surface region, but to the decrease of defective states by the native oxide. V C 2016 AIP Publishing LLC.

Exploring the potential of semiconducting BaSi2for thin-film solar cell applications

“…17(c'), the peak intensities of the oxides became stronger for the native-oxide/BaSi2 sample as TOA decreased; the peak located around 151 eV almost disappeared at TOA = 15°. Considering the Si 2s corelevel spectra as well as the Ba 3d5/2 core-level spectra, it is therefore reasonable to assume that a-Si suppresses surface oxidation by capping the surface with a-Si [82,83]. samples [84].…”

Effect of amorphous Si capping layer on the hole transport properties of BaSi2 and improved conversion efficiency approaching 10% in p-BaSi2/n-Si solar cells

“…For the a-Si/n-BaSi 2 interface, we found from hard X-ray photoelectron spectroscopy (HAXPES) that the barrier height of the a-Si for the minority-carriers, holes, in the n-BaSi 2 is À0.2 eV, 15 meaning that the transport of holes across the interface is not blocked, whereas this value is approximately 3.9 eV at the native oxide/n-BaSi 2 interface. 16 It was also found that the oxidation of BaSi 2 is suppressed significantly by the a-Si capping layer. The alignment of the valence-band maximums of BaSi 2 (E BaSi 2 V ), its native oxide (E oxide V ), and a-Si (E a-Si V ) is schematically shown in Fig.…”

“…25 We next investigated its effect on , and a-Si (E aÀSi V ). 15,16 Sample structures used for HAXPES measurements are schematically shown. 0003-6951/2016/109(7)/072103/4/$30.00…”

“…15 It was found from our previous study that the native oxide layer thickness is approximately 8 nm. 16 This native oxide layer disturbs the current flow and leads to an increase in R C .…”

p-BaSi2/n-Si heterojunction solar cells with conversion efficiency reaching 9.0%

“…35 However, the barrier height of the native oxide/undoped n-BaSi 2 interface for the minority-carriers (holes) in n-BaSi 2 is 3.9 eV. 36 Therefore, the transport of holes generated under solar radiation is blocked at the interface, which results in a limited conversion efficiency of g % 0.1% for a solar cell with the native oxide/n-BaSi 2 interface. 24 For this reason, a-Si was selected as a capping layer in this work.…”