On the Mechanism of BaSi2 Thin Film Formation on Si Substrate by Vacuum Evaporation

AbstractWe report on the formation mechanism of BaSi2 thin film on Si substrate grown by vacuum evaporation using BaSi2 granules as source materials. Since the vapor flux at the initial stage of evaporation is known to be Ba-rich, Si supply from the substrate is of crucial importance to obtain homogeneous BaSi2 thin film. In fact, low substrate temperature and/or thick film deposition led to formation of rough film with voids, and the oxidation proceeded upon exposure to air. We revealed that appropriate choice of substrate temperature, film thickness, and post-growth in-situ annealing can provide enough diffusion of Si and Ba, leading to realization of homogeneous BaSi2 thin film

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

Potential variations across the grain boundaries (GBs) in a 100 nm thick undoped n-BaSi2 film on a cast-grown multicrystalline Si (mc-Si) substrate are evaluated using Kelvin probe force microscopy (KFM). The θ-2θ X-ray diffraction pattern reveals diffraction peaks, such as (201), (301), (410), and (411) of BaSi2. Local-area electron backscatter diffraction reveals that the a-axis of BaSi2 is tilted slightly from the surface normal, depending on the local crystal plane of the mc-Si. KFM measurements show that the potentials are not significantly disordered in the grown BaSi2, even around the GBs of mc-Si. The potentials are higher at GBs of BaSi2 around Si GBs that are formed by grains with a Si(111) face and those with faces that deviate slightly from Si(111). Thus, downward band bending occurs at these BaSi2 GBs. Minority carriers (holes) undergo a repelling force near the GBs, which may suppress recombination as in the case of undoped n-BaSi2 epitaxial films on a single crystal Si(111) substrate. The barrier height for hole transport across the GBs varies in the range from 10 to 55 meV. The potentials are also higher at the BaSi2 GBs grown around Si GBs composed of grains with Si(001) and Si(111) faces. The barrier height for hole transport ranges from 5 to 55 meV. These results indicate that BaSi2 GBs formed on (111)-dominant Si surfaces do not have a negative influence on the minority-carrier properties, and thus BaSi2 formed on underlayers, such as (111)-oriented Si or Ge and on (111)-oriented mc-Si, can be utilized as a solar cell active layer

Boron-doped p-BaSi 2/n-Si solar cells formed on textured n-Si(001) with a pyramid structure consisting of {111} facets

BaSi2 films were fabricated on textured Si(0 0 1) substrates that consisted of {1 1 1} facets using molecular beam epitaxy. The light-trapping effect of these films and their performance when incorporated into solar cells were measured. X-ray diffraction and reflectivity measurements showed that the BaSi2 films were grown epitaxially on the textured Si(0 0 1) substrate and confirmed the light-trapping effect. The critical thickness over which BaSi2 relaxes increased from approximately 50 to 100 nm when comparing the BaSi2 films on a flat Si(1 1 1) substrate and the textured substrate, respectively. p-BaSi2/n-Si solar cells were fabricated with varying BaSi2 layer thickness and with hole concentrations in the range between 2.0 × 1018 and 4.6 × 1018 cm−3. These cells exhibited a maximum energy conversion efficiency of 4.62% with an open-circuit voltage of 0.30 V and a short-circuit current density of 27.6 mA/cm2 when the p-BaSi2 layer was 75 nm-thick. These results indicated that the use of BaSi2 films on textured Si(0 0 1) substrates in solar cells shows great promise

Coherence of a field-gradient-driven singlet-triplet qubit coupled to many-electron spin states in 28Si/SiGe

Engineered spin-electric coupling enables spin qubits in semiconductor nanostructures to be manipulated efficiently and addressed individually. While synthetic spin-orbit coupling using a micromagnet is widely used for driving qubits based on single spins in silicon, corresponding demonstration for encoded spin qubits is so far limited to natural silicon. Here, we demonstrate fast singlet-triplet qubit oscillation (~100 MHz) in a gate-defined double quantum dot in $^{28}$Si/SiGe with an on-chip micromagnet with which we show the oscillation quality factor of an encoded spin qubit exceeding 580. The coherence time $\textit{T}_{2}$* is analyzed as a function of potential detuning and an external magnetic field. In weak magnetic fields, the coherence is limited by fast noise compared to the data acquisition time, which limits $\textit{T}_{2}$* < 1 ${\mu}$s in the ergodic limit. We present evidence of sizable and coherent coupling of the qubit with the spin states of a nearby quantum dot, demonstrating that appropriate spin-electric coupling may enable a charge-based two-qubit gate in a (1,1) charge configuration

Minority-carrier lifetime and photoresponse properties of B-doped p-BaSi2, a potential light absorber for solar cells

600-nm-thick B-doped p-BaSi2 layers were grown on (111)-oriented n-Si substrates by molecular beam epitaxy, and the dependences of the minority carrier lifetime τ and photoresponsivity on the hole concentration p were investigated. p was varied from 1.4 × 1016 to 3.9 × 1018 cm−3. The highest τ of 2 µs was obtained for the sample with the lowest p of 1.4 × 1016 cm−3, reaching two orders of magnitude higher than that of the sample with the highest p of 3.9 × 1018 cm−3. The low-concentration-doped sample also exhibited an excellent external quantum efficiency (EQE) as large as 80% at a wavelength of approximately 800 nm at a reverse bias voltage of 0.2 V. This value is higher than any other EQEs we have ever achieved for BaSi2, showing the great potential of p-BaSi2 as a light absorber in solar cells

Improving the photoresponse spectra of BaSi2 layers by capping with hydrogenated amorphous Si layers prepared by radio-frequency hydrogen plasma

We studied the surface passivation effect of hydrogenated amorphous silicon (a-Si:H) layers on BaSi2 films. a-Si:H was formed by an electron-beam evaporation of Si, and a supply of atomic hydrogen using radio-frequency plasma. Surface passivation effect was first investigated on a conventional n-Si(111) substrate by capping with 20 nm-thick a-Si:H layers, and next on a 0.5 μm-thick BaSi2 film on Si(111) by molecular beam epitaxy. The internal quantum efficiency distinctly increased by 4 times in a wide wavelength range for sample capped in situ with a 3 nm-thick a-Si:H layer compared to those capped with a pure a-Si layer

Marked enhancement of the photoresponsivity and minority-carrier lifetime of BaSi2 passivated with atomic hydrogen

Passivation of barium disilicide (BaSi2) films is very important for their use in solar cell applications. In this paper, we demonstrated the effect of hydrogen (H) passivation on both the photoresponsivity and minority-carrier lifetime of BaSi2 epitaxial films grown by molecular beam epitaxy. First, we examined the growth conditions of a 3-nm-thick hydrogenated amorphous silicon (a-Si) capping layer formed on a 500-nm-thick BaSi2 film and found that an H supply duration (ta−Si:H) of 15 min at a substrate temperature of 180 °C sizably enhanced the photoresponsivity of the BaSi2 film. We next supplied atomic H to BaSi2 epitaxial films at 580 °C and changed supply duration (tBaSi;H) in the range of 1–30 min, followed by capping with an a-Si layer. The photoresponsivity of the films changed considerably depending on tBaSi;H and reached a maximum of 2.5 A/W at a wavelength of 800 nm for the sample passivated for tBaSi;H=15 min under a bias voltage of 0.3 V applied to the front-surface indium-tin-oxide electrode with respect to the back-surface aluminum electrode. This photoresponsivity is approximately one order of magnitude higher than the highest value previously reported for BaSi2. Microwave photoconductivity decay measurements revealed that the minority-carrier lifetime of the BaSi2 film with the highest photoresponsivity was 14 μs, equivalent to its bulk carrier lifetime ever reported. We performed theoretical analyses based on a rate equation including several recombination mechanisms and reproduced the experimentally obtained decay curves. We also calculated the total density of states of BaSi2 by ab initio studies when one Si vacancy existed in a unit cell and one, two, and three H atoms occupied Si vacancy or interstitial sites. A Si vacancy caused a localized state with two energy bands to appear close to the middle of the band gap. In certain cases, H passivation of the Si dangling bonds can markedly decrease trap concentration. From both experimental and theoretical viewpoints, we conclude that an atomic H supply is beneficial for BaSi2 solar cells

Spectroscopic study of Si-based semiconductor heterostructures grown by molecular beam epitaxy

報告番号: 乙13671 ; 学位授与年月日: 1998-01-29 ; 学位の種別: 論文博士 ; 学位の種類: 博士(工学) ; 学位記番号: 第13671号 ; 研究科・専攻: 工学系研究科物理工学専