Ultrafast carrier dynamics and the role of grain boundaries in polycrystalline silicon thin films grown by molecular beam epitaxy

Abstract

We have used time-resolved terahertz spectroscopy to study microscopic photoconductivity and ultrafast photoexcited carrier dynamics in thin, pure, non-hydrogenated silicon films grown by molecular beam epitaxy on quartz substrates at temperatures ranging from 335 °C to 572 °C. By controlling the growth temperature, thin silicon films ranging from completely amorphous to polycrystalline with minimal amorphous phase can be achieved. Film morphology, in turn, determines its photoconductive properties: in the amorphous phase, carriers are trapped in bandtail states on sub-picosecond time scales, while the carriers excited in crystalline grains remain free for tens of picoseconds. We also find that in polycrystalline silicon the photoexcited carrier mobility is carrier-density-dependent, with higher carrier densities mitigating the effects of grain boundaries on inter-grain transport. In a film grown at the highest temperature of 572 °C, the morphology changes along the growth direction from polycrystalline with needles of single crystals in the bulk of the film to small crystallites interspersed with amorphous silicon at the top of the film. Depth profiling using different excitation wavelengths shows corresponding differences in the photoconductivity: the photoexcited carrier lifetime and mobility are higher in the first 100-150 nm from the substrate, suggesting that thinner, low-temperature grown polycrystalline silicon films are preferable for photovoltaic applications.

Microscopic origin of the Drude-Smith model

“…Such a system will exhibit persistent carrier confinement, resulting in a suppression of the DC conductivity, 18,26 whereas Bruggeman EMT would predict a Drude-like effective conductivity if the microscopic metallic conductivity were assumed to be Drude.…”

“…23,29,[34][35][36]39,71,73,76 Alternatively, the Drude-Smith model has also been used as an EMT in its own right and fit directly to the measured THz conductivity. [9][10][11][12][13][14][15][16][17][18][19]21,22,[25][26][27]30,31,33,38,[41][42][43][44][45]48 Each approach begins with a different physical process and leads to its own difficulties when one interprets the subsequent fits to the data. In the former case, depolarization fields are included explicitly and treated as independent from weak confinement.…”

Terahertz Spectroscopy of Nanomaterials: a Close Look at Charge‐Carrier Transport

“…[110] In even smaller (nanometersized) nanocrystals, the first excitonic transition appears far above the THz range. [114] Regardless of the nanoparticle shape, size and spatial distribution and orientation, many works still employ the Drude-Smith model as a base for fitting the THz conductivity spectra of a variety of nanostructures, including silicon nanowires, [115] nanocrystals [116][117][118] and polycrystalline films, [119] SnO 2 nanowires, [120][121][122] nanowhiskers [123] and mesoporous films, [124] CdSe nanobelts, [125] CdS x Se 1−x nanobelts [126,127] and nanowires, [128] ZnSe nanocrystals, [129] Au nanostructures, [130] granular kesterite films, [131] organic perovskite CsPbBr 3 nanocrystals, [132] VO 2 nanogranular films, [133,134] or graphene nanoribbons. [111,112] The excitonic polarizability can be calculated on a microscopic level, e.g., using a multiband effectivemass model.…”

“…[112,113] A transition from an extended electron state into a localized exciton state was observed in CdSe nanorods. [114] Regardless of the nanoparticle shape, size and spatial distribution and orientation, many works still employ the Drude-Smith model as a base for fitting the THz conductivity spectra of a variety of nanostructures, including silicon nanowires, [115] nanocrystals [116][117][118] and polycrystalline films, [119] SnO 2 nanowires, [120][121][122] nanowhiskers [123] and mesoporous films, [124] CdSe nanobelts, [125] CdS x Se 1−x nanobelts [126,127] and nanowires, [128] ZnSe nanocrystals, [129] Au nanostructures, [130] granular kesterite films, [131] organic perovskite CsPbBr 3 nanocrystals, [132] VO 2 nanogranular films, [133,134] or graphene nanoribbons. [135] Some of these analyses further combine the Drude-Smith model with an effective medium approximation; this approach was used for example in order to describe the response of silicon nanowires and nanocrystals, [136] ZnO nanowires, [137] ZnO/In 2 S 3 core/shell nanorod heterojunctions, [138] or silver nanowires.…”

Ultrafast Photoconductivity and Terahertz Vibrational Dynamics in Double‐Helix SnIP Nanowires

“…[6,10] Alternatively, modified forms of Drude conductivity, such as the Drude-Smith model, [38][39][40][41] are often applied to describe nanomaterial conductivity when carrier localization arises from nanoscale morphology. [23,33,34,37,[42][43][44][45][46][47][48][49] The Drude-Smith model has been shown to fit photoconductivity spectra over a broad frequency range, [42] yield comparable conductivity to standard transport measurements, [44] and provide qualitative information about carrier localization. [34] The functional form is given by,…”