Kosuke O. Hara

Determination of Bulk Minority-Carrier Lifetime in BaSi2 Earth-Abundant Absorber Films by Utilizing a Drastic Enhancement of Carrier Lifetime by Post-Growth Annealing

We have successfully determined the bulk minority-carrier lifetime in BaSi2 epitaxial films by utilizing a drastic enhancement of lifetime by post-growth annealing at 800 °C, which is attributed to strain relaxation. From the film-thickness dependence of lifetime, we reveal that the bulk lifetime is 14 µs, which is long enough for thin-film solar cell applications. In addition, the sum of surface and interface recombination velocities is found to be as low as 8.3 cm/s presumably due to the ionic nature of BaSi2. This confirms that BaSi2 is promising as an absorption-layer material for earth-abundant thin-film solar cells.

In-situ heavily p-type doping of over 1020 cm−3 in semiconducting BaSi2 thin films for solar cells applications

B-doped p-BaSi2 layer growth by molecular beam epitaxy and the influence of rapid thermal annealing (RTA) on hole concentrations were presented. The hole concentration was controlled in the range between 1017 and 1020 cm−3 at room temperature by changing the temperature of the B Knudsen cell crucible. The acceptor level of the B atoms was estimated to be approximately 23 meV. High hole concentrations exceeding 1 × 1020 cm−3 were achieved via dopant activation using RTA at 800 °C in Ar. The activation efficiency was increased up to 10%.

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...

Analysis of the electrical properties of Cr/n-BaSi2 Schottky junction and n-BaSi2/p-Si heterojunction diodes for solar cell applications

Current status and future prospects towards BaSi2 pn junction solar cells are presented. As a preliminary step toward the formation of BaSi2 homojunction diodes, diodes with a Cr/n-BaSi2 Schottky junction and an n-BaSi2/p-Si hetero-junction have been fabricated to investigate the electrical properties of the n-BaSi2. Clear rectifying properties were observed in the current density versus voltage characteristics in both diodes. From the capacitance-voltage measurements, the build-in potential, VD, was 0.53 V in the Cr/n-BaSi2 Schottky junction diode, and the Schottky barrier height was 0.73 eV calculated from the thermoionic emission theory; the VD was about 1.5 V in the n-BaSi2/p-Si hetero-junction diode, which was consistent with the difference in the Fermi level between the n-BaSi2 and the p-Si.

Realization of single-phase BaSi2 films by vacuum evaporation with suitable optical properties and carrier lifetime for solar cell applications

We have realized BaSi2 films by a simple vacuum evaporation technique for solar cell applications. X-ray diffraction analysis shows that single-phase BaSi2 films are formed on alkali-free glass substrates at 500 and 600 °C while impurity phases coexist on quartz or soda-lime glass substrates or at a substrate temperature of 400 °C. The mechanism of film growth is discussed by analyzing the residue on the evaporation boat. An issue on the fabricated films is cracking due to thermal mismatch, as observed by secondary electron microscopy. Optical characterizations by transmittance and reflectance spectroscopy show that the evaporated films have high absorption coefficients, reaching 2 × 104 cm−1 for a photon energy of 1.5 eV, and have indirect absorption edges of 1.14–1.21 eV, which are suitable for solar cells. The microwave-detected photoconductivity decay measurement reveals that the carrier lifetime is approximately 0.027 µs, corresponding to the diffusion length of 0.84 µm, which suggests the potential effective usage of photoexcited carriers.

Lattice and grain-boundary diffusions of boron atoms in BaSi2 epitaxial films on Si(111)

A 180-nm-thick boron (B) layer was deposited on a 300-nm-thick a-axis-oriented BaSi2 epitaxial film grown by molecular beam epitaxy on Si(111) and was annealed at different temperatures in ultrahigh vacuum. The depth profiles of B were investigated using secondary ion mass spectrometry (SIMS) with O2+, and the diffusion coefficients of B were evaluated. The B profiles were reproduced well by taking both the lattice and the grain boundary (GB) diffusions into consideration. The cross-sectional transmission electron microscopy (TEM) image revealed that the GBs of the BaSi2 film were very sharp and normal to the sample surface. The plan-view TEM image exhibited that the grain size of the BaSi2 film was approximately 0.6 μm. The temperature dependence of lattice and GB diffusion coefficients was derived from the SIMS profiles, and their activation energies were found to be 4.6 eV and 4.4 eV, respectively.

Fabrication and characterization of BaSi2 epitaxial films over 1 µm in thickness on Si(111)

We attempted to fabricate a-axis-oriented BaSi2 epitaxial films up to 2180 nm in thickness. First, we investigated the influence of growth temperature and growth rate on the crystalline quality of approximately 400-nm-thick BaSi2 layers, and then optimized the above two growth conditions based on X-ray diffraction measurements. We next grew BaSi2 films with various layer thicknesses at 580 °C in the range between 100 and 2180 nm, and characterized their properties. The a-axis-oriented BaSi2 thick epitaxial films had three epitaxial variants rotating 120° with each other around the surface normal. The microwave photoconductive decay measurements for the 1640-nm-thick BaSi2 epitaxial film showed that the minority-carrier lifetime was approximately 8 µs at room temperature. These achievements open up the possibilities of thin-film solar cell applications of BaSi2.

Evaluation of minority carrier diffusion length of undoped n-BaSi2 epitaxial thin films on Si(001) substrates by electron-beam-induced-current technique

We have grown a 400-nm-thick undoped n-BaSi2 epitaxial film on an n-Si(001) substrate by molecular beam epitaxy, and evaluated the diffusion length of minority carriers (holes) by an electron-beam-induced-current (EBIC) technique in the edge-scan configuration. The EBIC line-scan profile showed an exponential dependence on the distance from the tungsten probe. The diffusion length of minority carriers in the n-BaSi2 film was found to be approximately 1.5 µm. This value is much smaller than that in undoped n-BaSi2 on Si(111).

Structural Study of BF2 Ion Implantation and Post Annealing of BaSi2 Epitaxial Films

We have investigated the effects of BF2 ion implantation and subsequent annealing on the structure of epitaxial BaSi2 thin films with the aim of the fabrication of a p-type B-doped BaSi2 film. After 10 min of annealing at 600 °C and above, BaSi2 is lost at least partly accompanied by appearance of Si as evidenced by X-ray diffraction and Raman spectroscopy. Element mapping by energy dispersive X-ray spectroscopy revealed that a barium oxide is formed on the surface, which indicates that BaSi2 is oxidized into a barium oxide and Si during annealing. Such oxidation was found to be suppressed by employing rapid thermal annealing for 30 s even when the annealing temperatures of 700 and 800 °C were chosen. Analysis of the full width at half maximum of the Raman peak showed that the inhomogeneous stress in the film produced by ion implantation can be decreased to the as-grown level by rapid thermal annealing at 700 and 800 °C for 30 s. At the same time, the red shift of the Raman peak is shown, based on which the possibility of B substitution for Si is discussed.

Fabrication of single-phase polycrystalline BaSi2 thin films on silicon substrates by vacuum evaporation for solar cell applications

We report on the successful fabrication of single-phase polycrystalline BaSi2 films on Si(111) substrates by a simple vacuum evaporation method using BaSi2 granules. Substrate heating was found to be the key, and high substrate temperatures are required to realize single-phase BaSi2. The underlying mechanism is discussed considering the composition of vapor flux, and we confirmed that the vapor flux is Ba-rich at the initial stage of evaporation. The reevaporation of excess Ba atoms or their reaction with Si substrates would be responsible for the realization of stoichiometric single-phase BaSi2, which explains why high substrate temperatures are necessary.