Passivation of trap states in polycrystalline Si by cyanide treatments

SSC 4994

PERGAMON

Solid State Communications 113 (2000) 195–199 www.elsevier.com/locate/ssc

Passivation of trap states in polycrystalline Si by cyanide treatments E. Kanazaki a, K. Yoneda b, Y. Todokoro c, M. Nishitani d, H. Kobayashi a,e,* a

The Institute of Scientific and Industrial Research, Osaka University, 8-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan ULSI Process Technology Development Center, Matsushita Electronics Corporation, 19, Nishikujo-Kasugacho, Minami-ku, Kyoto 601-8413, Japan c Corporate Planning Department, Semiconductor Group, Matsushita Electronics Corporation, 1, Koutari-yakemachi, Nagaokakyo, Kyoto 617-8520, Japan d Display Device Development Center, Matsushita Electric Industrial Co., Ltd. 1-1-3, Yagumonakamachi, Moriguchi, Osaka 570-0005, Japan e Research Center for Photoenergetics of Organic Materials, Osaka University, Osaka, Japan b

Received 15 July 1999; accepted 1 October 1999 by S. Ushioda

Abstract The cyanide treatment in which polycrystalline Si is immersed in a KCN solution followed by rinsing in boiling water increases the energy conversion efficiency of kITO/silicon oxide/polycrystalline Sil junction solar cells to 12.5%. The XPS measurements under bias show that the trap density in polycrystalline Si is markedly decreased by the cyanide treatment, especially the decrease near the Fermi level being remarkable. The dark current density for the cells without the cyanide treatment depends only weakly on the temperature, indicating that tunneling is a dominant mechanism for the current flow through the Si depletion layer. The cyanide treatment increases the temperature-dependence markedly, showing that thermal excitation of majority carriers becomes necessary for the current flow due to the elimination of the trap states in the Si depletion layer. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; D. Electronic states (localized); D. Photoconductivity and photovoltaics; D. Recombination and trapping; E. Photoelectron spectroscopies

1. Introduction Polycrystalline Si (poly-Si)-based solar cells are one of the most important candidates for low-cost and high-efficiency solar cells used in large scale for electric power generation. For the achievement of a high efficiency, the passivation of trap states in poly-Si is of importance. For this purpose, hydrogen plasma treatments are often performed [1,2]. However, Si–H bonds formed from Si dangling bonds are ruptured by irradiation [3,4] or by heat treatment above 5508C [5,6]. Other species such as phosphorus [7], flourine [8], and oxygen [8] are also found to

* Corresponding author. Tel.: 181-6-6879-8450; fax: 181-66879-8450. E-mail address: [email protected] (H. Kobayashi)

have effects to passivate trap states. However, these methods are time-consuming and expensive. We have recently developed a new method to eliminate defect states in the semiconductor band-gap called “cyanide method” [9–13]. This method is very simple which requires no expensive apparatus, i.e. just the immersion of semiconductors in a KCN solution followed by the rinse in water. Moreover, the cyanide method can be performed at relatively low temperatures below 1008C. The cyanide treatment is found to decrease the interface states at single crystalline Si/SiO2 interfaces [9,10], trap states in poly-Si [11], dangling bond states in amorphous Si [12] and interface states at GaAs/oxide interfaces [13]. In the present study, the cyanide treatment is applied to poly-Si-based solar cells with the kindium–tin oxide (ITO)/silicon oxide/ poly-Sil structure. It is found that the cyanide treatment greatly improves the solar cell characteristics due to the elimination of trap states in poly-Si.

0038-1098/00/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00457-3

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by 50 mV. For this increase in VOC, the dark current density 0 (Jdark ) is calculated to be reduced to , 1=7 using the following equation [17]: ! Jph nkT VOC ˆ …1† ln 0 1 1 ; e Jdark

Fig. 1. Photocurrent–photovoltage curves for the solar cells with the kITO/silicon oxide/poly-Sil structure measured under AM 1.5 100 mW cm 22 irradiation: (a) without the cyanide treatment; and (b) with the cyanide treatment.

2. Experimental Phosphorus-doped n-type poly-Si wafers with the resistivity of ,1 V cm produced using a cast method were cut into 1 × 1 cm2 pieces and etched with a CP4-A solution (i.e. hydrofluoric acid: nitric acid: acetic acid ˆ 3 : 5 : 3† for 2 min, followed by the immersion in a 1% hydrofluoric acid solution for 2 min. Then, the wafers were immersed in a 0.1 M KCN aqueous solution for 2 min and then washed with boiling water for 10 min. A thin silicon oxide layer on poly-Si was formed by the heat treatment at 4508C in an oxygen atmosphere. For electrical measurements, a ,600 nm-thick indium–tin oxide (ITO) layer was deposited by a splay pyrolysis method [10,11,14]. For X-ray photoelectron spectroscopy measurements under bias to obtain trap state spectra [9,15,16], a ,3 nm-thick platinum (Pt) layer was deposited by an electron beam evaporation method instead of an ITO film. The thickness of the silicon oxide layers for the electrical measurements and XPS measurements was ,1.5 and ,2.5 nm, respectively. XPS measurements were performed using a VG SCIENTIFIC ESCALAB 220i-XL spectrometer with a monochromatic AlKa radiation source. During the XPS measurements, the front Pt layer was grounded and a bias voltage was applied to the rear Si surface.

3. Results and discussion Fig. 1 shows the photocurrent–photovoltage (Iph –Vph) curves for the kITO/SiO2/poly-Sil solar cells under Air Mass (AM) 1.5 100 mW cm 22 irradiation. Without the cyanide treatment, the cell characteristics were poor, especially the fill factor being low. In this case, the conversion efficiency was 6.6%. When the cyanide treatment was performed on poly-Si, the cell characteristics were greatly improved, and the conversion efficiency was increased to 12.5%. The open circuit photovoltage (VOC) was increased

where Jph is the photocurrent density and n is the ideality factor which is assumed to be unity. Such a vast decrease in 0 Jdark results from a change in the mechanism of the dark current flow caused by the elimination of trap states by the cyanide treatment, as explained later. Fig. 2 shows the energy distribution of trap states obtained from the analysis of the XPS spectra measured under bias. The Si 2p peak of the substrate was shifted by the application of a bias voltage to the Si with respect to the Pt overlayer, due to the accumulation of charges in the trap states. The analysis of the shift vs. the bias voltage gives the energy distribution of trap states in the Si band-gap [9,15,16]. It is seen that high density trap states were present in the diodes without the cyanide treatment (spectrum a). The cyanide treatment decreased the density of the trap states to ,1=5 (spectrum b). Fig. 3 shows the dark current density vs. voltage (I–V) curves for the kITO/SiO2/poly-Sil diodes measured at various temperatures. For the diodes without the cyanide treatment, the dark current density was only slightly increased with the temperature, (Fig.3(a)) indicating that a tunneling mechanism is important for the current flow. For the diodes with the cyanide treatment, on the other hand, the current density depended strongly on the temperature, and it was considerably reduced (Fig.3(b)). For the diodes without the cyanide treatment, the semilog plot of the dark saturation 0 current density, Jdark ; (determined from the extrapolation of the semilog plot in the bias region between 0.3 and 0.45 V to the voltage axis vs. the temperature) is linear, showing that the trap-assisted multistep tunneling is a dominant current 0 flow mechanism because Jdark for this mechanism is given by [18] 0 Jdark ˆ PJt exp…BT†;

…2†

where Jt is a constant proportional to the trap density, P is the tunneling probability of majority carriers through the oxide layer, and B is given by B ˆ Ak ln

NC ; ne

…3†

where NC is the effective density of conduction bands, ne is the carrier density, and A is a temperature-independent constant. The change in B in Eqs. (2) and (3) by varying the temperature from 20 to 808C is calculated to be only ,3% of B, and thus negligible. For the diodes with the cyanide treatment, the semilog 0 plots of Jdark =T 2 vs. 1/T is linear, showing that thermionic emission becomes a dominant current flow mechanism since

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Fig. 2. Energy distribution of trap states for the diodes with the kITO/silicon oxide/poly-Sil structure obtained from XPS measurements under bias: (a) without the cyanide treatment; (b) with the cyanide treatment.

0 its Jdark is written as 0 Jdark

p 2

ˆ PA T exp…2f=kT†; p

…4†

where A is a modified Richardson constant and f is the barrier height in Si. The change in the dark current flow mechanism is due to the elimination of trap states in the depletion layer: Without the cyanide treatment, high density trap states are present in the Si depletion layer, and the majority carriers (electrons)

in the Si bulk flow through the depletion layer by hopping the trap states. The cyanide treatment eliminates the trap states and consequently thermal excitation becomes necessary for majority carriers to flow through the depletion layer. Theoretical calculations using a density functional method recently performed by us show that an isolated Si dangling bond has an energy level near the midgap [19]. The calculations also show that the gap states are eliminated by the formation of Si–CN bonds from Si dangling bonds. XPS

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Fig. 3. Dark current density–voltage curves at various temperatures for the diodes with the kITO/silicon oxide/poly-Sil structure: (a) without the cyanide treatment; (b) with the cyanide treatment.

measurements show that about 3% monolayer of CN species is present on the Si surface after the cyanide treatment [10]. Considering these results, it is highly probable that high density trap states (most probably Si dangling bonds) are present in the Si depletion layer before the cyanide treatment and they are eliminated by the formation of Si–CN bonds from the Si dangling bonds.

The Si depletion layer width is estimated to be ,0.5 mm from the donor density. For the elimination of the trap states in the depletion layer, CN 2 ions are necessary to penetrate into at least ,0.5 mm from the surface. We think that the penetration occurs easily due to the small size of CN 2 ions. For amorphous Si, it is found that CN 2 ions penetrate into ,0.2 mm even at room temperature [12]. In the present experiment, the poly-Si was rinsed with boiling water after the cyanide treatment and moreover the heat treatment was performed at 4508C. CN 2 ions are likely to penetrate deep into the Si bulk during these treatments and make chemical bonds selectively with the trap states. The mechanism of the penetration of CN 2 ions is not clear but we think that the penetration is enhanced by an electrical field induced by CN 2 ions adsorbed on the Si surface. The cyanide treatment increases the fill factor from 0.48 to 0.68 and the short circuit photocurrent density from 27.93 to 33.42 mA cm 22. These improvements are likely to result from a reduction in the density of the defect recombination current which has a large ideality factor [20]. At a low bias voltage, the Si band-bending is high and the Fermi level in the depletion layer crosses the midgap as shown in Fig. 4(a). Since the recombination rate is high when trap states are located near the midgap [21], the recombination current density is high at the low bias voltage. At a high bias voltage, on the other hand, the Si band-bending becomes small and the Si Fermi level does not cross the midgap (Fig.4(b)). In this case, however, majority carriers in the Si bulk transport through the depletion layer by hopping the trap states near the conduction band.

4. Summary Fig. 4. Roles of trap states: (a) under a low bias; (b) under a high bias. Trap states near the midgap act as effective recombination centers (a), while those near the conduction band play an important role in the majority carrier transport through the depletion layer (b).

Trap states present in the depletion layer of poly-Si are effectively eliminated by the cyanide treatment. The elimination of the trap states changes the majority carrier

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transport mechanism from trap-assisted multistep tunneling to thermionic emission, resulting in a vast decrease in the dark current density. Consequently, high conversion efficiency of 12.5% is achieved in spite of the simple structure of the kITO/silicon oxide/poly-Sil solar cells.

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