Doping of a-Si

An undoped a-Si H film is thought to have n-type characteristics, while holes travel a longer distance than electrons in a photoconductive target. Therefore, a p-type a-Si H is preferable for the present purpose. 

However, when the incident light is 600 nm, photocarriers are generated 

Electric Field ( V crft1) Electric Field (Vent1) 

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A completely different approach for the doping of a-Si H has been followed by Beyer and Fischer (1977). They showed that n-type doping is also obtained by interstitially incorporated lithium brought into the films by in-diffusion as well as by ion implantation. The possibility of introducing substitutional dopants in a-Si H by ion implantation has been proved by Muller et al. (1977). Subsequently, these authors demonstrated the doping effect of most elements of the groups I, III, and V of the periodic table by ion implantation experiments (Kalbitzer et al., 1980). 

Fig. 6. Conductivity a (open symbols) and quantity Q (closed symbols) at 300°K as a function of dopant concentration for gas-phase doping of a-Si H O, P-, B-doped films. [Data from Beyer ei a/. (1977a) and Beyer and Mell (1977).] Dashed curve, P-, B-doped films. [From Spear and LeComber (1976).] V, As-, B-doped films. [From Jan et al. (1979,1980).] , B-doped films. [Data from Tsai et al. (1977).]...

The discharge activated deposition of amorphous and microcrystalline silicon (a-Si and pc-Si, respecti>%ly) is presently among the most important applications of plasma chemistry This field has been opened up by Spear and Le Comber who where the first to demonstrate in 1975-76 the possibility of substitutional doping of a-Si but only recently some, although still incomplete, understanding of the chemical processes during the deposition has been achieved The... 

Fig. 3. Logarithm of room temperature electrical conductivity of a-Si H as a function of doping with diborane, B2H, and phosphine [7803-51-2] where is the ratio of the number of diborane to silane molecules Nppp /Ng pp is the ratio of phosphine to silane molecules. Both ratios refer to...

In 1977 it was observed that extended illumination with visible light of a-Si H produced a decrease in photoconductivity and dark conductivity (the Staebler-Wronski effect), which is reversible upon annealing, as shown in Fig. 7 (Staebler and Wronski, 1977,1980). The effect can be quite dramatic, producing a decrease in dark conductivity of over four orders of magnitude, though the extent of the decrease depends on the initial defect density and doping level of the sample. The degraded conductivity state is... 

If VEB is increased, IEB increases and the current density at the electrode eventually becomes equal to JPS. It has been speculated that this first anodic current peak is associated with flat-band condition of the emitter-base junction. However, data of flat-band potential of a silicon electrode determined from Mott-Schottky plots show significant scatter, as shown in Fig. 10.3. However, from C-V measurement it can be concluded that all PS formation occurs under depletion conditions independent of type and density of doping of the Si electrode [Otl]. 

There are two mechanisms that limit the built- in potential in a-Si H solar cells. One is the existence of band-tail states as mentioned earlier, and the other is the low doping efficiency of a-Si H. Spear and LeComber (1976)... 

The net result is that doped a-Si H films are not very conductive (typically 10-3-10-2 Q-1 cm-1)- The Fermi level is — 0.2 eV below the conduction band in phosphorus-doped a-Si H (Spear, 1977) and is—0.5 eV above the valance band in boron-doped a-Si H (Jan et al., 1980). Since the optical gap of undoped a-Si H is typically about 1.7 eV, the built-in potential of a-Si Hp-i-n solar cells is about 1.0 eV (Williams et al., 1979). Improving the conductivity of the doped layers should lead to larger built-in potentials and consequently higher conversion efficiencies. The conductivity can be increased significantly by forming microcrystalline-doped Si H films (Mat-suda et al., 1980), but since these films contain both amorphous and crystalline phases, there is no significant increase in the built-in potential (Carlson and Smith, 1982). 

The built-in potential of a-Si H p-i-n cells has been increased by alloying the p layer with carbon (Tawada et al., 1982). However, as shown in Fig. 5, the resistivity of the p layer increases as the optical gap (or carbon content) increases. Thus, the carbon alloying is decreasing the doping efficiency in this case. The increase in with increasing carbon content of the p layer is apparently associated with a suppression of the dark current by the wide-band-gap p layer. 

As shown in Fig. 19, the conversion efficiency of a-Si H solar cells has improved dramatically in recent years, and we project that efficiencies of — 12-14% will be obtained in the next few years for single-junction cells. Most of the future improvement will probably come from the development of better alloys for doped layers. 

Fig. 3. Doping feasibility of a-Si H containing oxygen (0.01 wt. %). The conductivity in the dark ( ) and under illumination (O) (6328.4 nm, 200 /tW cm-2) are plotted as a function of gaseous ratio of dopants to silane. For comparison, the dashed line shows the dark conductivity for a-Si H without oxygen. [From Wakita et al. (1982).]...

The photoconductivity increases when the a-Si H is lightly doped with phosphorus (Anderson and Spear, 1977). However, phosphorus doping causes very slow decay of photoresponse. The photoresponse characteristic for the phototconductive sensor using undoped a-Si H is shown in Fig. 3. The illumination is the modulated light from a GaP LED. The modulation ratio is defined as M = (it — i2)/i2, where is the peak photocurrent and i2 is the bottom current just prior to the next pulse. Figure 4 shows the modulation ratio of a-Si H versus the pulse width T, compared to that of the CdS-CdSe photoconductive sensor. The CdS-CdSe sensor modulation ratio decreases as the repetition time becomes shorter. On the other hand, in the a-Si H photoconductive sensor, the modulation ratio does not decrease... 

Electronic defects reduce the photosensitivity, suppress doping and impair the device performance of a-Si H. Their high density in pure amorphous silicon makes this material of lesser interest and is the reason for the attention on the hydrogenated material, in which the defect density is greatly reduced. The remaining defects in a-Si H control many electronic properties and are centrally involved in the substitutional doping process. The phenomena of metastability, which are described in Chapter 6, are caused by the defect reactions. 

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