Communication pubs.acs.org/JACS
Confined High-Pressure Chemical Deposition of Hydrogenated Amorphous Silicon Neil F. Baril,†,¶ Rongrui He,†,¶ Todd D. Day,†,¶ Justin R. Sparks,†,¶ Banafsheh Keshavarzi,‡ Mahesh Krishnamurthi,§,¶ Ali Borhan,‡ Venkatraman Gopalan,§,¶ Anna C. Peacock,# Noel Healy,# Pier J. A. Sazio,# and John V. Badding*,†,¶ †
Department of Chemistry, ‡Department of Chemical Engineering, §Department of Materials Science and Engineering, and Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, United States # Optoelectronics Research Centre, University of Southampton, Highfield, Southampton SO17 1BJ, United Kingdom ¶
S Supporting Information *
vapor deposition (PECVD)1,2 approach include the considerable cost and complexity of the equipment needed to maintain a uniform plasma over large substrate areas and an inability to deposit conformally into deep trenches, pores, and other extreme aspect ratio templates, which is desirable for the fabrication of state-of-the-art nanomaterials, nanostructures, and nanodevices.5 High reactant concentrations that activate silane decomposition via high rates of molecular collision could allow for increased reaction rates at a given temperature. However, in conventional reactors, high SiH4 precursor concentrations lead to homogeneous nucleation and growth of undesired silicon nano-/microparticles in the gas phase rather than the desired heterogeneous deposition of films on substrates.6,7 We report that in reactors of microscale to nanoscale dimensions, such as small-diameter optical fiber capillaries, this difficulty can be overcome. Deposition in this manner (Figure 1a) allows for practical growth of uniform a-Si:H films (Figure 1b) without any particulate formation under SiH4 partial pressures that are thus far as high as several megapascals (MPa). It is widely recognized that the scaling of the surface area to volume can have a major impact on the properties of nanosize objects. This scaling also has a large impact on the micro-/nanoreactors employed here, such that particulate formation is avoided. The use of MPa SiH4 partial pressures increases reaction rates by many orders of magnitude at temperatures that are low enough to allow for hydrogen incorporation. We find that aSi:H film deposition rates of 3.7 Å s−1 (vs 1−10 Å s−1 for PECVD)1 can be realized. The high-pressure approach is also advantageous in its efficient use of chemical precursors because minimal volumes of SiH4 precursor are used in the microscale reactors and nearly all of it can be converted to silicon in the “closed” pore geometry described below. In contrast, the conversion efficiency for PECVD is typically significantly lower such that some fraction of the valuable silane precursor is wasted.2 The minimal volumes of toxic and pyrophoric SiH4 employed reduce the hazards associated with its use at high pressure.
ABSTRACT: Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH4 precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, smalldiameter optical fiber capillary templates. The semiconductor materials deposited have ∼0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.
H
ydrogenated amorphous silicon (a-Si:H) is produced on a very large scale for electronic and energy-related applications such as thin-film transistors and solar cells.1,2 The SiH4 precursor molecule typically used for a-Si:H deposition is thermodynamically unstable but does not pyrolize at an appreciable rate unless heated to temperatures >550 °C because of a significant kinetic barrier.3,4 Conventional thermal pyrolysis techniques are therefore unsuitable for production of a-Si:H semiconductor films from SiH4, as these temperatures are too high to allow for much hydrogen incorporation. Without hydrogen to passivate its dangling bonds, amorphous silicon has poor electronic and photonic properties. The central challenge in depositing a-Si:H films is thus to overcome the kinetic barrier to decomposition at low enough temperatures; nonthermal plasmas that produce very reactive chemical species are commonly employed below atmospheric pressures.1 Disadvantages of this plasma-enhanced chemical © 2011 American Chemical Society
Received: July 20, 2011 Published: December 8, 2011 19
dx.doi.org/10.1021/ja2067862 | J. Am. Chem.Soc. 2012, 134, 19−22
Journal of the American Chemical Society
Communication
facility and used as reactors to carry out the high-pressure aSi:H deposition. SiH4 at 1.7 MPa partial pressure mixed with 33.3 MPa of helium carrier gas was configured to flow in the capillaries (see Supporting Information (SI)), which were heated in a furnace to 400−420 °C to induce deposition (Figure 1a,b). Two experimental configurations were used in which the end of the capillary opposite the gas inlet was either open to allow for gas flow or sealed closed to inhibit it. In both cases, well-developed films were formed when the capillary diameter was confined to
