Midgap density of states in hydrogenated polymorphous silicon

Abstract

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n+-i-n+ structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3–4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material.

Light induced electrical and macroscopic changes in hydrogenated polymorphous silicon solar cells

“…Through spectroscopic ellipsometry, Rutherford Backscattering and ERDA measurements, it has been shown that pm-Si:H films are denser than a-Si:H films, in spite of their high hydrogen content, in the range of 15−20% [7]. The peculiar structure of pm-Si:H results in a low defect density (of the order of 10 14 cm −3 eV −1 at Fermi level as measured by SCLC and modulated photocurrent) and higher resistance to light-soaking than aSi:H [8][9][10]. In particular improved hole transport appears to be a key point for the application of this material in solar cells [11].…”

Geometrical optimization and electrical performance comparison of thin-film tandem structures based on pm-Si:H andμc-Si:H using computer simulation

“…Also used as the top cell in such structures is polymorphous silicon, a nanostructured material deposited by PECVD at high pressure and RF power, in a regime where silicon clusters and nanocrystals synthesized in the plasma contribute to the growth along with silicon radicals [9]. It has been reported in previous studies that pm-Si:H has better electronic properties and stability than conventional a-Si:H [10][11][12][13]. Moreover, the pm-Si:H optical gap being slightly larger than the a-Si:H one, the use of pm-Si:H in a tandem structure in place of a-Si:H allows one the possibility to increase the opencircuit voltage of the entire device and therefore to increase the electric output of the photovoltaic modules.…”

The Influence of the Hot Wire Temperature on the Crystallization of μc-Si:H Films Prepared by Hot Wire-Assisted-ECR-CVD

“…The stability of a-Si:H films is related to the amorphous network structure, whereas the improvement of amorphous network may finally lead to the formation of micro or nanocrystalline silicon (μc-or nc-Si), which shows no noticeable light-induced degradation, but a poor photosensitivity. It has been confirmed in recent years that hydrogenated silicon films deposited in the regime just above the phase transition from amorphous to crystalline state could gain both the fine photoelectronic properties like a-Si:H and high stability like μc-Si:H [1][2][3][4][5][6]. The demand of these applications is not only a high quality but also a high deposition rate on the preparation.…”