APCVD法生长的硅纳米线的化学表面钝化

1. Introduction

In recent years, nanowires including nanorods based solar cells have attractive interest due to their characteristics and processing benefifits. The nanowires-enabled solar cells allow for decoupling the light absorption from the direction of carrier transport, such that in materials where the diffusion length of minority carriers is much shorter than the thickness of material required for optimal light absorption, current densities can be improved. Nanostructure solar cells, such as organic–inorganic materials, compound semiconductor and many researchers [1–3] studied hetero or homojunction silicon structures. These results indicate that the nanowires are attractive to enhance charge transport in nanostructures solar cells compared with conventional solar cells or other nanostructured solar cells.

In particular, silicon nanowires (SiNWs) are allows effificient charge transport. SiNWs can easily dope with various impurities to fabricate n- or p-type Si semiconductor [4] and [5]. The SiNWs have been synthesized by using various methods and metal catalysts via well-known vapor–liquid–solid (VLS) mechanism [6,7]. Moreover, various materials, such as Au, Al, Ga, In, Pb, Sn and Zn [6–13], have been used for synthesis of silicon nanowires. Recently, hetero-junction solar cell using SiNWs have been demonstrated by Tsakalakos et al. [2] and Thony et al. [3]. They fabricated an allinorganic SiNWs solar cell using Au thin fifilm and colloidal nanoparticles. As mentioned above, Au nanoparticles are well-used metal catalyst and it easily synthesizes the SiNWs at low eutectic temperature. However, it is well-known that Au creates deep leveldefect when it incorporate with silicon. In contrast with Au, tin (Sn) appears to be the favorable catalyst because the Sn–Si alloy has relatively low eutectic temperature of 262  C [2]. Moreover, Sn-catalyzed SiNWs were easily controlled by introducing hydrogen flflow ratios [14]. In this work, we synthesized SiNWs using Au nanoparticles on single crystalline silicon (p-type-Si) wafer for base layer for solar cell application and their characteristics are explored. In this present work, SiNWs were deposited on p-type c-Si by an atmospheric pressure chemical vapor deposition (APCVD). The structural and vibration properties of SiNWs were characterized by different SiH4 flflow rates. Diluted HF and HCl have been used for SiNWs passivation. Finally, we evaluated minority carrier lifetime of SiNWs/p-type c-Si with different chemical treatments.

2. Experimental details

Boron-doped p-type silicon (1 0 0) (1–3 X-cm) wafers were used for SiNWs deposition. The wafers were dipped in diluted hydroflfluoric (2% HF) solution to remove native oxide layer and dried in N2 atmosphere. The Au metal fifilm was deposited on the Si wafer by spluttering technique at a rate of less than 1 nm/s. After deposition of Au fifilm of approximately 5 nm thickness onto the Si wafer, the wafer was transferred into the experimental chamber and ageing at 700  C. For synthesis of SiNWs, SiH4, H2 and N2 were introduced into the CVD chamber. SiNWs were synthesized for 120 min at 800  C. Details of other experiment conditions are summarized in Table 1. The SiNWs are characterized by Field emission scanning electron microscopy (FESEM-JEOL JSM-6700F), Fourier transform infrared (FTIR-Nicolet 2000) and micro-Raman Spectroscopy (HORIABA Jobin Yvon, FRANCE). The lifetime measurementswere performed by a microwave photoconductive decay (l-PCD) method using a WT-85 lifetime scanner (SEMILAB). A pulsed laser diode with a wavelength of 904 nm was used, and the injection photon density was 1  1013 cm 2 . The light from the laser diode was illuminated from the front of the wafer. A 10.3 GHz microwave was used to obtain the conductivity decay curve.

Fig1

Raman spectroscopy is an important tool to determine crystalline fraction, confifined photonics properties in SiNWs. Su et al. described the Raman spectra of SiNWs and observed amorphous signal is more prominent at high laser powers [17]. Fig. 3a–c shows the Raman spectra of SiNWs at different SiH4 flflow rates. The variation of Raman intensity only depends on the density of SiNWs. As can be seen from the fifigure, the Raman spectra of SiNWs centered at 499 cm 1 and 491 cm–1 for (b) and (c), indicates the core of SiNWs are nanocrystalline silicon. For the better clarity we have deconvoluted Raman peak into three main Gaussian distribution represented two longitudinal phonon and one transverse phonon. Fukataa et al. reported that increase in power of the laser beam or increase in irradiation time cause downshifts and broadens in the fifirst-order TO (transverse optic) mode of the Raman spectra of SiNWs as shown in SiNWs mixed nanosilicon [18]. Intense Raman line at 520 cm 1 with the full width at half maximum (FWHM) of 4.7 cm 1 was witnessed in the Raman spectrum of crystal Si. This peak corresponds to the degenerate zone-center optical phonon mode of crystal Si. All SiNWs samples exhibit similar Raman spectral peaks red shifted from 520 cm 1 and a small shoulder at 495 cm 1 . The main peak near 520 cm 1 corresponds to the fifirst-order optical phonon of crystalline Si. The small broad peak at 495 cm 1 was attributed to the amorphous silicon that covers SiNWs or distributed on the silica substrate, which has a Raman spectrum between 400 and 550 cm 1 peaked at 480 cm 1 . Two optical phonons and two acoustic phonons are simultaneous coexist in between  175 cm 1 and 550 cm 1 . The fifirst order longitudinal phonon and optical phonon are set to be 400 cm 1 and 500 cm 1 indicates nanocrystalline nature of SiNWs [18–20]. The longitudinal phonon is dominant over the optical phonon in SiNWs intensity ratios of LO/TO of SiNWs. Generally, the ratio between LO and TO is a parameter signifified to the atomic network in the SiNWs due to phonon confifinement. The vibrational frequencies at 250 cm 1 and  300 cm 1 corresponding to transverse acoustic (TA) and longitudinal acoustic (LA) of SiNWs. The Raman shifts of following vibrations have corresponding higher frequencies than a-Si:H and its alloys observed by different groups [27]. The vibrational differences of longitudinal modes (LO-LA) are related to ring structure geometry in amorphous network, closely associated with diameter of SiNWs. The (LO-LA) vibration increases from 99.9 cm 1 to 110.1 cm 1 with increase of nanowires concentrations