Visible photoluminescence and electroluminescence in wide-bandgap hydrogenated amorphous silicon

Abstract

The work describes basic photoluminescence (PL) and electroluminescence (EL) properties of wide-bandgap (2.0 eV or greater) hydrogenated amorphous silicon (a-Si: H). Thin films of wide-bandgap (high-hydrogen-content) a-Si: H were prepared by microwave electron cyclotron resonance plasma-enhanced chemical vapour deposition from SiH₄ under high dilution with He. The films exhibit spectrally broad (full width at half-maximum, 0.4 eV or greater) visible PL at room temperature, peaked at about 1.5eV. On the basis of measurements of the PL temperature dependence, PL dynamics, infrared absorption spectra, picosecond pump-and-probe experiments and hydrogen thermal desorption spectroscopy the dominant microscopic mechanism of visible PL has been revealed to be the radiative de-excitation of oligosilane (-(SiH₂)₂-) units or of a specific defect in their close vicinity. EL has been investigated in sandwich p⁺‒i‒n⁺ and p⁺‒p‒n‒n⁺ structures with (Cr‒Ni)/indium tin oxide contacts. The as-grown structures exhibit good rectifying properties, low conductivity and no EL. After being subjected to a ‘forming’ procedure (application of a sufficiently high voltage), the current flowing through the structures increases abruptly by several orders of magnitude, up to about 10 mA, and the structures begin to emit weak EL at room temperature. The EL occurs in reverse bias only and its external quantum efficiency is about 10⁻⁵%. It is concluded that the forming procedure leads to partial crystallization of a-Si: H between the contacts and impact lattice ionization can participate in the light emission.