Barium
disilicide (BaSi2) has been regarded as a promising absorber
material for high-efficiency thin-film solar cells. However, it has
confronted issues related to material synthesis and quality control.
Here, we fabricate BaSi2 thin films via an industrially
applicable sputtering process and uncovered the mechanism of structure
transformation. Polycrystalline BaSi2 thin films are obtained
through the sputtering process followed by a postannealing treatment.
The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction
(XRD). A higher annealing temperature can promote crystallization
of BaSi2, but also causes an intensive surface oxidation
and BaSi2/SiO2 interfacial diffusion. As a consequence,
an inhomogeneous and layered structure of BaSi2 is revealed
by Auger electron spectroscopy (AES) and transmission electron microscopy
(TEM). The thick oxide layer in such an inhomogeneous structure hinders
further both optical and electrical characterizations of sputtered
BaSi2. The structural transformation process of sputtered
BaSi2 films then is studied by the Raman depth-profiling
method, and all of the above observations come to an oxidation-induced
structure transformation mechanism. It interprets interfacial phenomena
including surface oxidation and BaSi2/SiO2 interdiffusion,
which lead to the inhomogeneous and layered structure of sputtered
BaSi2. The mechanism can also be extended to epitaxial
and evaporated BaSi2 films. In addition, a glimpse toward
future developments in both material and device levels is presented.
Such fundamental knowledge on structural transformations and complex
interfacial activities is significant for further quality control
and interface engineering on BaSi2 films toward high-efficiency
solar cells.
We discuss possibilities of adjustment of a threshold voltage VT in normally off GaN high-electron mobility transistors (HEMTs) without compromising a maximal drain current IDSmax. Techniques of a low power plasma or thermal oxidation of 2-nm thick AlN cap over 3-nm thick AlGaN barrier are developed and calibrated for a thorough oxidation of the cap with a minimal density of surface donors at the inherent oxide-semiconductor interface. It has been shown that while a thermal oxidation technique leads to the channel and/or interface degradation, low density of surface donors and scalability of VT with additionally overgrown Al2O3 may be obtained for plasma oxidized HEMTs. With 10-nm thick Al2O3 deposited at 100 °C by atomic-layer deposition, we obtained VT of 1.6 V and IDSmax of 0.48 A/mm at a gate voltage of VGS = 8 V. Density of surface donors was estimated to be about 1.2 × 1013 cm−2, leaving most of the negative polarization charge at the semiconductor surface uncompensated. Further reduction of surface donors may be needed for even higher VT.
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