Vladimir Volodin

Silicon Thin Film

Novosibirsk State University, A.V. Rzhanov Institute of Semiconductor Physics SB RAS

Lavrentiev Ave. 13, 630090, Novosibirsk, Russia

Email: volodin@isp.nsc.ru

Biography

Abstract for Presentation

Femto- and picosecond infrared laser crystallization of amorphous Si:H films with included amorphous Ge layers

   Pulsed laser annealing is efficiently used for crystallization and nano-structuring of amorphous semiconductor films. In the case of narrow-band semiconductor (for example Ge) layers included in a-Si, a pulsed infrared laser annealing (PIRLA) technique allows to selectively crystallize regions that absorb IR light. Such structures can be used in p-i-n photodiodes or solar cells.

In the present study, a-Si:H/a-Ge:H multilayer structures (MLS) consisted of 4 silicon (40 nm) and 3 germanium (15 nm) alternating amorphous layers were produced by plasma enhanced chemical vapor deposition on glass and Si(100) substrates. The MLS samples were annealed by a picosecond laser (HiLASE PERLA-B, λ=1030 nm, pulse duration 1.4 ps) and a femtosecond laser in combination with optical parametric amplifier (Coherent Astrella/TOPAS, λ=1500 nm, 70 fs) in a wide range of the pulse energy density (laser fluence) from 20 to 200 mJ/cm2 under single-shot and multishot irradiation conditions. The structural analysis was performed by Raman spectroscopy using a T64000 spectrometer with micro-Raman setup and an Ar laser (514.5 nm) as an excitation source. The Raman spectrum of the as-deposited MLSs contains broad bands at ~480 cm–1 and ~275 cm–1 related to maximum density of vibrational states of a-Si and a-Ge, respectively. The vibration frequencies of Ge–Si bonds, which are present at the Ge/Si heteroboundaries, are ~400 cm–1 but the spectrum of as-deposited MLS reveals almost no features at this frequency indicating that the concentration of these bonds is low.

Several regimes of PIRLA with increasing laser fluence are found for both ps and fs pulses. (1) At low fluences, the Ge layers are crystallized while the Si layers remain amorphous without noticeable intermixing of Ge and Si. (2) The Ge layers are almost fully crystallized without observable Si crystallization but some Ge-Si intermixing occurs. (3) Complete crystallization of the Ge layers with partial crystallization of Si layers and Ge-Si intermixing. (4) Almost complete intermixing of the Ge and Si layers in the MLS. (5) Partial ablation of the MLS.

A theoretical analysis of ps and fs PIRLA of the MLS, taking into account one- and two-photon excitation, shows that, in a wide range of laser fluence, germanium can be laser-heated to the melting point while heating of silicon is negligible.