Poly-Si films with low aluminum dopant containing by aluminum-induced crystallization

Abstract

Typically, highly p-doped (2×10 18 cm −3 ) poly-Si films fabricated by the aluminum induced layer exchange (ALILE) process are not suitable for solar cell absorber layers. In this paper, the fabrication of high-quality, continuous polycrystalline silicon (poly-Si) films with lower doping concentrations (2×10 16 cm −3 ) using aluminum-induced crystallization (AIC) is reported. Secondary-ion-mass spectroscopy (SIMS) results showed that annealing at different temperature profiles leads to a variety of Al concentrations. Hall Effect measurements revealed that Al dopant concentration depends on the annealing temperature and temperature profile. Raman spectral analysis indicated that samples prepared via AIC contain some regions with small grains. polycrystalline silicon, low doping concentration, AIC, crystallizationThe crystallization of amorphous silicon (a-Si) deposited on glass substrates is gaining increasing interest for its importance in polycrystalline silicon (poly-Si) thin film solar cells [1]. One of the most important challenges for the development of polycrystalline silicon (poly-Si) thin film solar cells on glass substrates is the growth of crystalline silicon at low-temperature. A promising means to overcome this problem is the deposition of a-Si followed by subsequent low-temperature crystallization. The crystallization techniques that attracting most attention so far have been the solid phase crystallization (SPC) [2], laser crystallization (LC) [3], and rapid photo-thermal crystallization (RTP) [4]. As an alternative approach, metal-induced crystallization (MIC) has caught considerable interest, because the crystallization temperature of a-Si are much lower than reported temperature for SPC of a-Si of about 600°C. When a-Si is in contact with certain metal, remarkably, the crystallization temperature is well below the eutectic temperature of the corresponding metal/Si system [5]. In MIC of silicon, the silicide forming and nonsilicide forming crystallization is distinguishable. In contrast to compound forming systems (e.g. Ni/Si, Cu/Si, and Ag/Si, etc.), simple eutectic systems, such as Al/Si (AIC), do not form stable silicides [6]. The Al/Si system does not form a stable silicide; a lower crystallization temperature can be achieved [7]. For Al/Si system, the crystallization temperatures as low as 100°C has been reported [8]. The mechanism for AIC is the aluminum-induced layer exchange (ALILE) process which was first introduced by Nast et al. [9]. Annealing of a glass/Al/a-Si composite layer at temperature below the eutectic temperature of the Al/Si system (577°C) leads to the formation of a glass/poly-Si/Al(+Si) composite layer. During the annealing, silicon atoms diffuse into the aluminum layer and then nucleate in the aluminum matrix. The nuclei grow in all directions until confined within the aluminum layer between the glass and the a-Si layer. Crystal growth then continues in a lateral direction only until adjacent crystallites form a continuous poly-Si layer. The

Crystallization of amorphous silicon thin films deposited by PECVD on nickel-metalized porous silicon

“…The as-deposited a-Si layer presents a broad band that can be deconvoluted into two peaks (Figure 5b). The broad peak centered at about 476 cm −1 corresponds to the amorphous silicon [7,8], and the narrowest one located at 511 cm −1 is generally ascribed to transverse optic mode of the phonon in nc-Si [8-12]. This result suggests that the a-Si layer contains a crystalline phase with crystallites in the nanometer scale.…”

Metal-induced layer exchange of group IV materials

“…This behavior is considered to more likely occur as the LE is closer to the equilibrium state. Some papers reported that the hole concentration in the Si layer formed by Al-induced LE, where Al works as an acceptor, can be controlled, that is, not completely restricted with the solid solubility limit [168][169][170]. These behaviors possibly occur by bringing LE closer to a non-equilibrium state.…”

Thin film poly-crystalline silicon fabrication based on Rapid Thermal Annealing (RTA) process

“…Crystallization methods, including excimer laser crystallization (ELC) [1] , solid-phase crystallization (SPC) [2] , rapid thermal process(RTP) [3] and medal induced crystallization (MIC) [4] . SPC demands a relatively high temperature and long period (over 700℃) annealing process， calling for a specific kind of glass substrate and rising the production cost, which cannot meet the large scale manufacturing requirements.…”

“…One of the most studied Si-metal systems is the Si-Al system [6] .In this work, the a-Si films were sputtered on low cost glass substrate followed by sputtering Al thin films on top. The whole structure was followed by different annealing process.…”