Ultra-thin crystalline silicon films produced by plasma assisted epitaxial growth on silicon wafers and their transfer to foreign substrates

Abstract

We have developed a new process to produce ultra-thin crystalline silicon films with thicknesses in the range of 0.1−1 μm on flexible substrates. A crystalline silicon wafer was cleaned by SiF4 plasma exposure and without breaking vacuum, an epitaxial film was grown from SiF4, H2 and Ar gas mixtures at low substrate temperature (T sub ≈ 200• C) in a standard RF PECVD reactor. We found that H2 dilution is a key parameter for the growth of high quality epitaxial films and modification of the structural composition of the interface with the c-Si wafer, allowing one to switch from a smooth interface at low hydrogen flow rates to a fragile one, composed of hydrogen-rich micro-cavities, at high hydrogen flow rates. This feature can be advantageously used to separate the epitaxial film from the crystalline Si wafer. As a example demonstration, we show that by depositing a metal film followed by a spin-coated polyimide layer and applying a moderate thermal treatment to the stack, the fragile interface breaks down and allows one to obtain an ultrathin crystalline wafer on the flexible polyimide support.

Low temperature plasma deposition of silicon thin films: From amorphous to crystalline

“…1) shows that the film is 100% c-Si. As previously shown [12,17], epitaxial films can be described by a three layer model: i) a very thin (~1 nm) interface layer between the substrate and the film made up of a mixture of c-Si and voids, ii) a bulk layer modeled by a mixture of monosilicon [18], large and small grain polysilicon [19], and iii) a roughness layer made up of a mixture of c-Si and voids. The very thin interface layer is responsible for the interference fringes observed in the low photon energy range (< 3 eV).…”

“…While much more work is required to better understand the growth process, there is no doubt that the possibility of growing crystalline silicon films in the same equipment as that used for silicon thin films opens new perspectives. A recent example is the production of thin crystalline foils on foreign substrates [12].…”

“…In the case of heterojunction solar cells, epitaxial growth has been shown to lead to poor surface passivation; therefore efforts have been done to avoid epitaxial growth [10,11]. However, one can take benefit of this to produce ultrathin crystalline silicon films which can be transferred to foreign substrates [12] as well as solar cells featuring such epitaxial films [13]. Besides their applied interest, they also raise questions about the growth mechanism of such epitaxial films at low temperature.…”

Thin crystalline silicon solar cells based on epitaxial films grown at 165°C by RF-PECVD

“…Pursuing this approach, we have studied the growth of thick epitaxial layers and have found that the epitaxial growth can be sustained up to a thickness of 3 µm at such low-temperature conditions. Moreover, by using an SiF 4 plasma etching step on the wafer substrate before deposition, we have found that it is possible to produce a crystalline silicon layer having a porous interface with the crystalline silicon wafer, which allows to easily detach the epitaxial film from the substrate [8,9]. In this way it is possible to produce thin and flexible crystalline silicon wafers that could be used for electronic devices, in a similar way to the "smart-cut" process [10].…”

Direct Current Sputter Epitaxy of Heavily Doped p+ Layer for Monocrystalline Si Solar Cells

“…For further cost reduction of monocrystalline Si solar cells (SCs), both decreasing material usage by means of thin film and increasing power generation by increasing energy conversion efficiency are two effective methods. As for the approach by means of thin films, transferring of thin epitaxially grown Si film on Si wafer to foreign substrates is a promising method [1][2][3]. As for the latter approach, several structures of Si SC have been proposed and realized.…”