Low-temperature growth of narrow optical gap highly conducting nc-Ge thin films with superior crystallinity involving dominant orientation

The nc-Ge thin films are grown at a moderately low temperature (∼220 °C) in rf PECVD via optimizing the GeH4 plasma with H2-dilution, within 97.5 ≤ D(H2)(%) ≤ 99.5. At elevated D(H2), while the bonded H-content in the matrix reduces, the film's crystallinity and dc conductivity increase. However, the optical band gap widens for 97.5 ≤ D(H2)(%) ≤ 98.5, via the elevated quantum confinement effect from the increased number of Ge-ncs of dimension below Bohr radius. Conversely, for D(H2) ≥ 98.5%, switching in the quantum effect in larger-sized Ge-ncs induces the band gap narrowing. Significantly reduced ultra-nanocrystalline-Ge (unc-Ge) component occupying the grain boundary zone, minimizes the contribution of grain-boundary defects. However, the persistent amorphous Ge matrix becomes highly defective and less dense, and those induce enhanced O-absorption from the ambient in the dominant Ge-Ox (x 98.5%. A narrow band gap (∼0.90 eV), conducting (σd ∼3.0 ×10−2 S cm−1), and photosensitive nc-Ge network, involving dominant -oriented Ge-ncs (∼35.2 nm) of enriched volume fraction (XC ∼90.2%), obtained at D(H2) ∼99.5%, seems extremely useful for need-based applications as a narrow band gap absorber layer in the bottom sub-cell of flexible high-efficiency cost-effective multijunction thin film solar cells. [Display omitted] •The nc-Ge thin films are grown at ∼220 °C in rf PECVD via optimization of H2-dilution, D(H2), to the GeH4 plasma.•At higher D(H2), crystallinity (XC), grain size, and dc σd increase with reduced ultra-nc fraction and bonded H-content.•For 97.5 ≤ D(H2)% ≤ 98.5, the optical gap (Eg) widens to ∼(0.99–1.06) eV, via QC-effect from Ge-ncs of size ∼(14.6–17.4) nm.•At D(H2) ∼99.5%, XC of ∼90.2% attains σd ∼3.0 ×10−2 S cm−1, Eg ∼0.90 eV via reversion of QC-effect in larger (∼35.2 nm) Ge-ncs.•Transition to a superior nc-Ge network involves lowering in activation energy of dipole relaxation (ΔEτ) from the a-Ge matrix.