Carrier mobilities in multicrystalline silicon wafers made from UMG‐Si

Abstract

We investigate majority‐ and minority‐carrier mobilities in multicrystalline silicon (mc‐Si) made from upgraded metallurgical‐grade (UMG) feedstock. Since UMG‐Si contains high amounts of both boron and phosphorus, a decrease of the carrier mobility due to increased scattering at ionized impurities is expected. Minority‐carrier mobilities are determined by measuring effective carrier lifetimes τeff on as‐cut wafers, where τeff is limited by carrier diffusion to the unpassivated surfaces. By examining a wafer cut vertically from the mc‐Si ingot, we indeed find a reduction in minority‐carrier mobility μmin with increasing dopant density. In addition, we find a further strong reduction of μmin in the transition region from p ‐ to n ‐type silicon. Similar results are obtained regarding the majority‐carrier mobility μmaj, which is investigated by combining measurements of the resistivity ρ and the equilibrium hole concentration p0 (equilibrium electron concentration n0 in n ‐type material) obtained from electrochemical capacitance‐voltage measurements. Apart from an overall reduction in μmaj compared to values measured in non‐compensated p ‐type mc‐Si, an additional pronounced decrease of the mobility with increasing compensation level is observed. This additional reduction can be explained by reduced screening of the ionized scattering centers. We propose a parameterization of the experimental data based on the Brooks‐Herring equation and find excellent agreement between the two (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electronic properties of highly-doped and compensated solar-grade silicon wafers and solar cells

“…This extra reduction in l Hall , observed here for C l >6, has been recently reported by different research groups, using the Hall effect 3,4 or a combination of electrochemical capacitance-voltage (ECV) and q measurements. 9,11 The origin of this drop is still under investigation and is variously attributed to an additional or enhanced carrier scattering mechanism introduced by compensation (scattering by electrostatic disorder, 3 unscreening of dopants, 11 altered electron-hole scattering 6 …) or by some deviation in the measurement associated to a high inhomogeneity in transport properties at high C l , 29,30 all of these mechanisms possibly working in parallel.…”

“…In other words, SoG M -Si features high and similar concentrations of mainly B and P [B and P], resulting in modified electrical properties that need to be investigated. In particular there is no consensus yet on the validity of standard mobility (l) models such as Klaassen's 2 in compensated Si, although many studies have been carried out in this direction [3][4][5][6][7][8][9][10][11] using various techniques including the Hall effect, 3-7,9,10 electrochemical 6,7,9,11 or standard 3 capacitance-voltage (CV/ECV), fourier transform infraRed spectroscopy (FTIR) for free carrier absorption (FCA) measurements, 8 all together with resistivity (q) measurements, or other independent techniques involving carrier lifetime measurements. [6][7][8]11 To complement previous work, we investigate in the first part of this study the effects of compensation on majority carrier mobilities (l maj ) in two highly doped SoG M -Si ingots.…”

Models for numerical device simulations of crystalline silicon solar cells—a review

“…The mobility differs between minority and majority carriers because the screening and scattering is different. Recently, heavily compensated material such as upgraded metallurgical grade (UMG) material has become available, and the evaluation of the carrier mobility is in progress [110][111][112][113]. To be precise, mobility would need to be parameterized as a tensor where the relative current-directions of electrons and holes are taken into account, especially in highly-injected solar cells [114][115][116].…”

Temperature‐dependent Hall‐effect measurements of p‐type multicrystalline compensated solar grade silicon

“…. No agreement has been reached yet to explain the observed reduction in room temperature majority carrier mobility in compensated samples, but various mechanisms have been suggested, for example, reduced screening of the ionized scattering centers caused by the decrease of free carriers , or carrier trapping caused by potential fluctuations and introduction of deep level (recombination) centers within the bandgap . However, the contribution of these various mechanisms is temperature dependent, and their balance at low temperature is still unknown, with no measurements reported in the literature for multicrystalline compensated silicon.…”

“…In particular, carrier mobility (m) is of interest owing to its effect on carrier lifetime and, hence, on the performance of the solar cells. A number of studies have been recently published on majority carrier mobility in both multicrystalline and monocrystalline compensated materials measured by different techniques, such as Hall effect [1][2][3][4][5][6][7], free carrier absorption [8,9], electrochemical capacitance voltage [3,5,8,[10][11][12] or Fe-acceptor pairing [13]. All these investigations show a reduction of majority carrier mobility at room temperature in compensated samples compared with uncompensated ones.…”