Influence of Thickness and Substrate on the Properties of SnO2 Films Deposited by CVD

Abstract

A comparative study of the physical properties of undoped, nonstoichiometric SnO, films on glass and quartz substrate is presented. With increasing film thickness, the crystallinity improves, the preferred orientation of the grains changes from the [loll to the [200] direction, and the carrier concentration decreases. The highest conductivity and mobility obtained are 2.5 x 10, W' cm-l and 13 cmZ/V s, respectively, for films on glass and 6.0 x 10' 0-' cmand 44 cm2/V s, respectively, for films on quartz substrate. It is indicated that the diffusion of alkali metal ions degrades the properties of the films deposited on glass substrate. The scattering mechanism in the films is also investigated. The figure of merit values obtained on glass (3.1 x a-') are amongst the highest values reported for undoped SnO, films. Es wird eine vergleichende Untersuchung der physikalischen Eigenschaften von undotierten, nichtstochiometrischen Sn0,-Schichten auf Glas-und Quarzsubstraten durchgefuhrt. Mit wachsender Schichtdicke verbessert sich die Kristallinitat, die bevorzugte Orientierung der Korner andert sich von der [loll-zur [200]-Richtung und die Tragerkonzentration nimmt ab. Die hochste erhaltene Leitfahigkeit und Beweglichkeit betragt 2,5 x 10' !X' cm-' bzw. 13 cmZ/V s fur Schichten auf Glas und 6,O x 10' Rcm-' bzw. 44 cmZ/V s fur Schichten auf Quarzsubstraten. Es wird gezeigt, daB die Diffusion von Alkalimetallionen die Eigenschaften der auf Glassubstrate abgeschiedenen Schichten degradiert. Der Streumechanismus in diesen Schichten wird ebenfalls untersucht. Die auf Glas (3,l x n-')-Substraten erhaltenen Wirkungsgrade liegen zwischen den hochsten fur undotierte Sn0,-Schichten angegebenen Werten.

Process Development and Basic Studies of Electrochemically Deposited CdTe-Based Solar Cells; Annual Technical Report, Phase I: May 15, 1998 - May 14, 1999

“…Our instruments cover visible light range, near UV and near IR ranges, e.g., the spectral range of our spectrophotometer is of 250 to 2500 nm. For the SnO 2 samples with carrier concentration in the range of 1×10 20 to 6×10 20 cm -3 , the plasma frequency could be measured with an acceptable accuracy. As the concentration decreases, the plasma frequency shifts farther to the IR range (ω p ∝n 1/2 ) and the accuracy drops.…”

“…Modeling the reflection spectrum and ellipsometry data based on the Drude theory provided the plasma frequency and collision frequency data. Comparative studies of Hall effect, Seebeck effect and plasma frequency in the carrier concentration range of 2x10 20 to 6x10 20 cm -3 were used to determine the electron effective mass and its dependence on the electron energy. No indication of non-parabolicity was observed in the energy range of 0.25 to 0.75 eV above the conduction band edge.…”

“…The electron spectrum will be quantized if the electron de Broglie wavelength, λ e , is comparable to the film thickness. The electron gas in our heavily-doped material (carrier concentration ranged from 1×10 20 to 6×10 20 cm -3 ) is highly degenerate. Hence only those electronic states contribute to the transport phenomena whose energy is close to the Fermi level, that is whose wave vector K is close to the Fermi surface in the K-space.…”

“…One of the most prominent manifestations of the non-parabolicity is the growth of effective mass with Fermi level. For SnO 2 there were a few attempts to investigate non-parabolicity, but published data are controversial, see, e.g., [19] and [20]. We have studied experimentally the effective mass dependence on the Fermi level position in a wide range of the latter (from 0.25 to 0.75 eV) by means of comparison of the measured Hall and Seebeck coefficients and plasma frequency (see Sec.1.4.3).…”

“…Although increasing carrier concentration through extrinsic doping increases electrical conductivity, σ, it does so at the expense of decreased transparency, T, in the visible and near IR spectrum due to free carrier absorption and reflection (shift in the plasma reflection edge). In the figure of merit of TCOs that is commonly used, transparency is weighted much higher than low sheet resistance [2], hence the limit for the doping level is defined by transparency and is usually in the range of 5×10 20 to 1×10 21 cm 3 . The carrier concentration in commercially available TCO's often approaches this limit.…”