The measurement of the surface photovoltage (SPV) is a versatile technique for the characterization of semiconductor surface conditions before and after the
deposi-Passivating a-Si1-xCx films on crystalline silicon and germanium substrates 87
tion/growth of thin films such as a-Si:H (chapter 3.7). A major field of application for SPV is the characterization and optimization of c-Si/a-Si heterostructure solar cells since the interface conditions constitute a crucial issue for device performance. In principle, SPV allows for the measurement of the surface/interface band bending ψs, the determination of the surface state density Dit and the minority carrier lifetime τ in the semiconductor.
A set of samples was prepared comprising different heterostructures formed by p- and n-type silicon as well as p-type germanium substrate and a surface passivation consisting of 30-40 nm of Si-rich intrinsic and n-doped (PH3=15 sccm) a-Si1-xCx. The standard deposition conditions and surface treatment were chosen as outlined in sec-tion 4.2.3 and the methane gas flow amounted to 30 sccm for the c-Si and 15 sccm for c-Ge samples. The different systems under investigation and the results of the lifetime, SPV and PES (photoelectron spectroscopy) measurements are summarized in Table 5-4. The lifetime measurements were performed by the QSS-PC technique (chapter 3.2) and the photo-electron spectroscopy (PES) technique was used for the determina-tion of the posidetermina-tion of the Fermi level in the a-Si1-xCx film (EF -EV refers to the position of the Fermi level with respect to the valence band edge). A description of this charac-terization method and its practical layout is e.g. given in [111]. The information given in Table 5-4 on the various heterostructures allows for the sketching of the discontinu-ous band structures at the respective interfaces, following the model proposed by Anderson [161]. The central assumption of the model is that the discontinuity of the electrostatic field F (and hence the valence and conduction band offsets ΔEV and ΔEC) is due to the difference in dielectric constants of the two materials. As a consequence, the discontinuities are invariant with doping in non-degenerate material. The condition to be fulfilled at the interface is the continuity of the electric displacement D (D=ε×F, with ε the dielectric permitivity). Sketches of the band diagrams for the heterostruc-tures based on Si-substrate are shown in Fig. 5-24. Note that the voltage profile in the
Table 5-4: Overview of the heterostructures prepared and the results of corresponding life-time, SPV and PES measurements were performed at Helmholtz-Zentrum Berlin.
substrate a-Si1-xCx
88 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
interface regions on either side of the interface was not explicitly calculated.
Discussion for passivated silicon surfaces – The measured effective lifetimes af-firm an excellent passivation quality of the deposited intrinsic and phosphorous doped a-Si1-xCx films. The only sample with comparatively high surface recombination (still yielding a τeff of 0.5 ms) is the one exhibiting the c-Si(p)/a-Si1-xCx(n) system. The measured band bendings at the p- and n-Si surfaces in combination with the n-doped a-Si1-xCx film (Fig. 5-24 b and d) reveal inversion and accumulation conditions at the interface, respectively. This finding is inherently linked to the formation of heterojunc-tions, the c-Si(p)/a-Si1-xCx(n) system forming a pn-junction and the c-Si(n)/a-Si1-xCx(n) system forming an effective high-low junction. In the dark (absence of excess carriers), the position of the Fermi level at the interface (EF,int) is almost the same. The signifi-cant discrepancy in measured lifetime however indicates a strongly increased impact of the defects in the a-Si1-xCx matrix and at the c-Si/ a-Si1-xCx interface under illumination for the pn-junction. The predominant recombination path is probably the injection of
Fig. 5-24: Sketches of band diagrams for the various hetero structures under investigation plotted on the basis of SPV and PES measurements and by consideration of the Anderson model [161]. Note that the exact course of the band bending was not explicitly calculated.
a) c-Si(p)/a-Si1-xCx(i) b) c-Si(p)/a-Si1-xCx(n) c) c-Si(n)/a-Si1-xCx(i) d) c-Si(n)/a-Si1-xCx(n).
The quantities shown in red were measured, the quantities in blue were calcu-lated/estimated.
Passivating a-Si1-xCx films on crystalline silicon and germanium substrates 89
minority carriers from the silicon (electrons) into the defect-rich amorphous film.
The measured surface photovoltages for silicon surfaces in combination with in-trinsic a-Si1-xCx reveal a change of sign of the surface potential depending on the dop-ing type of the substrate (Fig. 5-24 a and c). The surface conditions in the dark inferred from the band bendings are an inversion and a depletion of minority carriers for the c-Si(p)/a-Si1-xCx(i) and the c-Si(n)/a-Si1-xCx(i) systems, respectively. As already out-lined in the former section (5.4.4), this finding is in qualitative accordance with the modeled surface parameters and points to the existence of amphoteric defects related to dangling bonds at the c-Si/a-Si1-xCx interface. For a classification of the surface potentials with respect to other (dielectric) passivation layers, it is somewhat instruc-tive to relate the measured band bendings to a net fixed charge density situated at the very interface. Assuming the a-Si1-xCx film furthermore to be an adequate insulator, a fixed charge density Qf can be directly associated with the measured surface potential following the basic calculations for MIS (metal insulator semiconductor) structures given in any semiconductor textbook (e.g. [14]). The diagrams of the variation of the fixed charge density Qf at the c-Si surface as a function of the surface potential ψs and the position of passivating a-Si1-xCx, SiNx and (annealed) Al2O3 are given in Fig. 5-25.
The fixed charge densities of +2×1012 cm-3 and -1.3×1012 cm-3 for the latter dielectrics were taken from [162]. These charges can be considered as independent of the sub-strate doping and the injection conditions [45, 52]. Note that the latter is not a priori true for a-Si1-xCx since the charges refer to interface defect states which may be charged and uncharged under varying excess carrier density conditions.
Fig. 5-25: Variation of the fixed charge density Qf at the c-Si surface as a function of the surface potential ψs for an ideal MIS structure. The calculations were carried out for room temperature and dark conditions (Δn=0). Left: 1 Ωcm p-type silicon.
Right: 1 Ωcm n-type silicon.
90 Passivating a-Si1-xCx films on crystalline silicon and germanium substrates
Taking into account the invariance of the band offsets regarding (non-degenerate) doping conditions, the band diagrams of the various investigated systems furthermore allow for the estimation of the valence band offset for the c-Si(100)/a-Si1-xCx system to 0.35 eV< ΔEV <0.38 eV. For comparison, the value derived by Korte via PES meas-urements for the c-Si(111)/a-Si system is ΔEV = 0.46 eV [111]. The same technique applied on c-Si(100)/a-Si1-xCx systems with x ranging from 0 to 50 % resulted in val-ues of ΔEV = 0.44 to 1.00 eV for increasing C-content [163].
Discussion for passivated germanium surfaces – The surface photovoltage meas-ured at the c-Ge/a-Si1-xCx interface is rather small (0.04 eV), resulting in a minor ac-cumulation of minority carriers (holes) at the interface (Fig. 5-26). The increase of the lifetime of the germanium sample from 80 µs in the as-deposited state to 290 µs after annealing (425°C, 30 min), without any detectable difference in SPV before and after the temperature treatment, is instructive regarding the mechanism of Ge surface passi-vation: the increase in passivation quality cannot be related to a possible increase in charges at the c-Ge/a-Si1-xCx interface since this would result in an additional band bending. Hence, it is necessarily the interface state density that is reduced during an-nealing. Since the saturation of interface defects at the interface by hydrogen atoms can be ruled out due to the considerations and experimental findings outlined in the sections above, a direct saturation of Ge dangling bonds by silicon (and carbon) atoms is likely to occur. The increased temperature stability of this passivation scheme might then be related to the higher probability of Si-H bond rupture due to molecular hydro-gen formation as compared to the probability of the mere rupture of Ge-Si (or Ge-C) bonds without additional energy gaining mechanism. The role of carbon in the a-Si1-xCx network remains unclear. A possible issue might be the lattice mismatch between c-Ge and (epitaxially grown) silicon resulting in stress and lattice misfits
Fig. 5-26: Band diagram at the c-Ge/a-Si1-xCx interface. Fermi level position and band bend-ing were measured by the PES and SPV techniques, respectively.
Passivating a-Si1-xCx films on crystalline silicon and germanium substrates 91
critically impacting the recombination kinetics at the interface (chapter 5.1). The intro-duction of carbon (CHn groups) into the matrix might contribute to a relaxation of the surface near layers.