FZ silicon

However, since liquid silicon reacts markedly with any crucible material including fused silica (Si02), FZ silicon can be grown about 100 times purer than CZ silicon mainly concerning, oxygen from the crucible, carbon from the graphite heater and, last but not least, transition metals (Cu, Fe, Ni,... 

The reasons why FZ silicon, up to now, could not be commercially launched in the terrestrial PV market are the smaller maximum crystal sizes (diameter, lengths) and the need of expensive high-quality feed rods being of limited sizes, too. Today, the biggest FZ crystal diameter on the market is 200 mm. This would fit a quadratic wafer format of 150 x 150 mm2, but growing such crystals is still extremely sophisticated and expensive, too. However, future progress in FZ growth development, as mentioned before, could make FZ solar cells competitive or even superior to CZ cells. 

With FZ solar cells, the amount of silicon per Watt peak (Wp) would probably be the smallest of the crystalline silicon solar cells. Low-cost silicon feedstock development suitable for FZ is crucial for the future of FZ silicon based photovoltaic. 

A combination of proton implantation and thermal annealing of CZ or FZ silicon has also been shown to produce shallow donors [85]. Hydrogen can also be introduced in a region near the semiconductor surface by hydrogen plasma treatments as long as the semiconductor surface does not suffer excessive plasma etching. 

Table 6.1. Measured FWHMs of donor lines in different FZ silicon samples. The spectral resolution is 5vs...

Measurement of the intensity differences of the 2p line at LHeT in intrinsic FZ silicon samples and the same samples after NTD with different doses have been made by Pajot and Debarre [202]. The peak absorption coefficient K2P i of the 2p i line was measured with a spectral resolution <)vs of 0.45 cm-1 (53 peV) and for the P concentrations introduced, the observed FWHM of 2p i was equal to 5i>s. For P concentrations up to 1 x 1015cm-3, the valid relationship for 5vs > 0.45 cm 1 is ... 

Fig. 6.26. Part of the absorption spectrum of a silicon sample showing low-energy TDDi+ lines and unidentified lines denoted by an asterisk. A high-purity FZ silicon sample used as a reference allows the subtraction of the silicon two-phonon absorption. Line Cs is the vibrational mode of residual carbon at 607.4 cm 1...

Table 6.32. LHeT positions (meV (cm-1 in parentheses)) of lines of HDi spectra in as-irradiated NTD FZ silicon after annealing in the 240-400° C range in a 1H2 plasma...

The results of piezospectroscopic measurements on the Al-X spectrum are consistent with a trigonal <111> oriented centre [33]. Early measurements of the X-centre concentration showed no dependence on the concentration of interstitial oxygen, but a linear dependence on [Cs] which led to their attribution to (C, acceptor) pairs [85], However, subsequent measurements on NTD Ga-doped FZ silicon with undetectable [Cs] showed the presence of Ga-X centres after annealing at 450-550°C [145] so that the atomic structure of these centres is still an open problem. Calibration factors for lines l(B-X), 2(AZ-X), 2(Ga-X), and 2(In-X) given by Jones et al. [85] are 1.1, 2.5, 2.5, and 28 x 1013 cm-1, respectively. 

In B- and AZ-doped FZ silicon samples subjected to NTD, two new EM acceptor spectra are observed after annealing at 500-600° C, besides the above-discussed B-X and Al-X spectra [179]. These spectra, called Bntd and AZntd, are similar to those of the isolated acceptors, but their ionization energies are 28.24 and 43.25meV for Bntd and AZntd, respectively. The ionization energy of Bntd is smaller than the EM energy calculated for acceptors in silicon (31.6 meV) by Baldereschi and Lipari [13]. Such centres have also been... 

Information on the broadening of donor and acceptor lines in FZ silicon between 5 and 60 K is given in a concise report by Agladze et al. [3], where the spectra were obtained when necessary under high resolution. Representative examples are given in Fig. 8.40. In the original figure, the lines considered were not indicated and tentative attributions have been made here. 

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