Ryota Takabe

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

We have fabricated approximately 0.5-μm-thick undoped n-BaSi2 epitaxial films with various average grain areas ranging from 2.6 to 23.3 μm2 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 BaSi2 films using the decay time of the second decay mode, τSRH, caused by Shockley-Read-Hall recombination without the carrier trapping effect, as a measure of the minority-carrier properties in the BaSi2 films. The measured τSRH was grouped into two, independently of the average grain area of BaSi2. BaSi2 films with cloudy surfaces or capped intentionally with a 3 nm Ba or Si layer, showed large τSRH (ca. 8 μs), whereas those with mirror surfaces much smaller τSRH (ca. 0.4 μs). X-ray photoelectron spectroscopy measurements were performed to discuss the surface region of the BaSi2...

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

p-BaSi2/n-Si heterojunction solar cells consisting of a 20 nm thick B-doped p-BaSi2 epitaxial layer (p = 2.2 × 1018 cm−3) on n-Si(111) (ρ = 1–4 Ω cm) were formed by molecular beam epitaxy. The separation of photogenerated minority carriers is promoted at the heterointerface in this structure. Under AM1.5 illumination, the conversion efficiency η reached 9.0%, which is the highest ever reported for solar cells with semiconducting silicides. An open-circuit voltage of 0.46 V, a short-circuit current density of 31.9 mA/cm2, and a fill factor of 0.60 were obtained. These results demonstrate the high potential of BaSi2 for solar cell applications.

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

We investigated the effect of a 3-nm-thick amorphous Si (a-Si) capping layer on the hole transport properties of BaSi2 films. The contact resistance decreased with decreasing resistivity of p-BaSi2 and reached a minimum of 0.35 Ω·cm2. The effect of the a-Si layer was confirmed by higher photoresponsivities for n-BaSi2 films capped with the a-Si layer than for those without the a-Si layer, showing that the minority carriers (holes) were extracted efficiently across the a-Si/n-BaSi2 interface. Under AM1.5 illumination, the conversion efficiency reached 9.9% in a-Si(3 nm)/p-BaSi2(20 nm)/n-Si solar cells, the highest value ever reported for semiconducting silicides.

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

Undoped n-type BaSi2 films were grown on Si(111) by molecular beam epitaxy, and the valence band (VB) offset at the interface between the BaSi2 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 BaSi2 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 BaSi2 and the native oxide separately. The VB maximum was located at −1.0 eV from the Fermi energy for the BaSi2 and −4.9 eV for the native oxide. We found that the band bending did not occur near the native oxide/BaSi2 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-...

Evaluation of band offset at amorphous-Si/BaSi2 interfaces by hard x-ray photoelectron spectroscopy

The 730 nm-thick undoped BaSi2 films capped with 5 nm-thick amorphous Si (a-Si) intended for solar cell applications were grown on Si(111) by molecular beam epitaxy. The valence band (VB) offset at the interface between the BaSi2 and the a-Si was measured by hard x-ray photoelectron spectroscopy to understand the carrier transport properties by the determination of the band offset at this heterointerface. We performed the depth-analysis by varying the take-off angle of photoelectrons as 15°, 30°, and 90° with respect to the sample surface to obtain the VB spectra of the BaSi2 and the a-Si separately. It was found that the barrier height of the a-Si for holes in the BaSi2 is approximately −0.2 eV, whereas the barrier height for electrons is approximately 0.6 eV. This result means that the holes generated in the BaSi2 layer under solar radiation could be selectively extracted through the a-Si/BaSi2 interface, promoting the carrier separation in the BaSi2 layer. We therefore conclude that the a-Si/BaSi2 inte...

Influence of air exposure duration and a-Si capping layer thickness on the performance of p-BaSi2/n-Si heterojunction solar cells

Fabrication of p-BaSi2(20nm)/n-Si heterojunction solar cells was performed with different a-Si capping layer thicknesses (da-Si) and varying air exposure durations (tair) prior to the formation of a 70-nm-thick indium-tin-oxide electrode. The conversion efficiencies (η) reached approximately 4.7% regardless of tair (varying from 12–150 h) for solar cells with da-Si = 5 nm. In contrast, η increased from 5.3 to 6.6% with increasing tair for those with da-Si = 2 nm, in contrast to our prediction. For this sample, the reverse saturation current density (J0) and diode ideality factor decreased with tair, resulting in the enhancement of η. The effects of the variation of da-Si (0.7, 2, 3, and 5 nm) upon the solar cell performance were examined while keeping tair = 150 h. The η reached a maximum of 9.0% when da-Si was 3 nm, wherein the open-circuit voltage and fill factor also reached a maximum. The series resistance, shunt resistance, and J0 exhibited a tendency to decrease as da-Si increased. These results dem...

Photoresponse properties of BaSi2 film grown on Si (100) by vacuum evaporation

We have succeeded in the observation of high photoresponsivity of orthorhombic BaSi2 film grown on crystalline Si by a vacuum evaporation method, raising the prospect of its promising application in high-efficiency thin-film solar cells. Photocurrent was observed at photon energies larger than 1.28 eV, which corresponds to the band gap of evaporated BaSi2 film, indicating that the photoresponsivity originates from the BaSi2 film. The effect of the substrate temperature on the films properties was also investigated. The films grown at a substrate temperature larger than 500 °C are single-phase polycrystalline BaSi2 films, while those grown at a substrate temperature of 400 °C is a mixture of phases. We confirmed that undoped evaporated BaSi2 films are an n-type material with high carrier concentration. High carrier lifetime of 4.8 and 2.7 μs can be found for the films grown at 500 °C and 400 °C, respectively. BaSi2 film grown at a substrate temperature of 500 °C, which is crack-free and single-phase, shows the best photoresponsivity. The maximum value of photocurrent was obtained at photon energy of 1.9 eV, corresponding to an external quantum efficiency of 22% under reverse applied voltage of 2 V.

Characterization of defect levels in undoped n-BaSi2 epitaxial films on Si(111) by deep-level transient spectroscopy

Electrically active defect levels in 650-nm-thick undoped n-BaSi2 epitaxial layers grown by molecular beam epitaxy were investigated by deep-level transient spectroscopy (DLTS) using undoped n-BaSi2/p-Si heterojunction diodes. The layer structure was designed so that the depletion region extended toward the n-BaSi2 layers under reverse bias conditions. DLTS revealed the presence of majority-carrier (electron) traps located at approximately 0.1 and 0.2 eV below the bottom of the conduction band. The densities of these trap levels were approximately 1 × 1015 cm−3. Photoresponse spectra are also discussed in relation to these trap levels. Minority-carrier traps were not detected in this work.

Photoresponse properties of undoped BaSi2 epitaxial layers on n+-BaSi2/p+-Si(001) by molecular beam epitaxy

We investigated the photoresponse properties of 500-nm-thick undoped BaSi2 epitaxial layers formed on Sb-doped n+-BaSi2/p+-Si heterostructures by molecular beam epitaxy using Si(001) substrates. The external quantum efficiency (EQE) reached approximately 5% when 1 V was applied between the top and bottom electrodes. However, the EQE decreased to approximately 0.2% after inserting a solid-phase-epitaxy crystalline Si (c-Si) layer between the undoped and Sb-doped n+-BaSi2 layers to prevent Sb diffusion during the growth of the undoped BaSi2 overlayer. This result was unexpectedly different from that obtained for BaSi2 layers on Si(111), where the c-Si layer improved the EQE significantly.

Impact of Ba to Si deposition rate ratios during molecular beam epitaxy on carrier concentration and spectral response of BaSi2 epitaxial films

Undoped 0.5-μm-thick BaSi2 epitaxial films were grown on Si(111) substrates with various ratios of the Ba deposition rate to the Si deposition rate (RBa/RSi) ranging from 1.0 to 5.1, and their electrical and optical properties were characterized. The photoresponse spectra drastically changed as a function of RBa/RSi, and the quantum efficiency reached a maximum at RBa/RSi = 2.2. Hall measurements and capacitance versus voltage measurements revealed that the electron concentration drastically decreased as RBa/RSi approached 2.2, and the BaSi2 films with RBa/RSi= 2.0, 2.2, and 2.6 exhibited p-type conductivity. The lowest hole concentration of approximately 1 × 1015 cm−3 was obtained for the BaSi2 grown with RBa/RSi = 2.2, which is the lowest value ever reported. First-principles calculations suggest that Si vacancies give rise to localized states within the bandgap of BaSi2 and therefore degrade the minority-carrier properties.