What Is Amorphous Silicon?

Amorphous silicon is a non-crystalline allotrope of silicon in which atoms are arranged without long-range periodic order. Unlike single-crystal silicon, where each silicon atom occupies a precise lattice site bonded to four neighbors in a diamond cubic structure, amorphous silicon has a disordered network with varying bond angles and bond lengths. This structural disorder introduces a high density of broken bonds, called dangling bonds, and localized electronic states within the energy gap that would otherwise be forbidden. The form that dominates practical applications is hydrogenated amorphous silicon (a-Si:H), in which hydrogen atoms bond to dangling silicon sites and dramatically improve the material's electronic quality.

Amorphous silicon emerged as a research topic in the 1960s, but its practical importance was established in the late 1970s when PECVD (plasma-enhanced chemical vapor deposition) techniques enabled the deposition of device-quality a-Si:H films over large substrates at temperatures below 300 °C. This low-temperature compatibility with glass and later with plastic substrates made it the material of choice for thin-film electronics at a scale and cost inaccessible to silicon wafer technology.

Atomic Structure and Hydrogenation

In unhydrogenated amorphous silicon, the density of dangling bond defects reaches approximately 10^20 per cm^3, sufficient to pin the Fermi level and make the material unsuitable for transistor or photovoltaic operation. Hydrogen incorporation during PECVD reduces this density by several orders of magnitude, to values as low as 10^15 to 10^16 per cm^3 in optimized films. The hydrogen content in a-Si:H films typically falls in the range of 5 to 20 atomic percent, with the hydrogen primarily residing at silicon monohydride (Si-H) and dihydride (Si-H2) configurations. The resulting material retains amorphous disorder but shows band-tail slopes and defect densities compatible with field-effect transistor and p-i-n photovoltaic operation. An overview of amorphous silicon structure and properties is available through ScienceDirect reference entries on amorphous silicon.

Optical and Electrical Properties

The optical bandgap of a-Si:H lies between 1.7 and 1.8 eV, compared with 1.12 eV for crystalline silicon. This wider gap produces stronger optical absorption in the blue and green portions of the visible spectrum, allowing photovoltaic absorbers as thin as 300 nm to capture most of the incident solar radiation, versus the 100 to 200 micrometers required for crystalline silicon cells. The electron field-effect mobility in a-Si:H TFTs is typically 0.5 to 1.0 cm^2/V·s, substantially lower than crystalline silicon but adequate for pixel-switching applications. A persistent limitation is the Staebler-Wronski effect, in which light soaking creates additional dangling bond defects and degrades photoconductivity by 15 to 30% before the material reaches a stabilized state. This degradation, described in a review on thin-film amorphous silicon solar cells published in the journal Silicon, is partially reversible by thermal annealing above 150 °C.

Thin-Film Deposition

PECVD using silane (SiH4) gas is the standard deposition method for a-Si:H. An RF or VHF plasma dissociates silane molecules, and film-forming species deposit on the heated substrate. Substrate temperatures in the range of 150 to 250 °C favor good hydrogen passivation without excessive thermal budget. The process scales readily to substrates exceeding 1 m^2, which enabled the thin-film display industry to manufacture active-matrix LCD panels. Doping of a-Si:H is achieved by adding phosphine (n-type) or diborane (p-type) to the process gas, though diffusion-based doping used in crystalline silicon is impractical given the absence of long-range order. Details of PECVD process optimization and device integration are covered in MIT OpenCourseWare materials on amorphous materials and glass.

Applications

Amorphous silicon has applications in a range of electronic and photonic devices, including:

• Thin-film photovoltaic modules on glass or flexible substrates
• Active-matrix liquid crystal display backplanes using a-Si:H TFTs
• Flat-panel X-ray detectors for medical imaging and baggage inspection
• Photoreceptor drums in laser printers and photocopiers
• Large-area photodetector arrays for document scanners