tant, a, and effective mass, m* are taken as a linear interpolation between CdTe and ZnTe.
Fig. 40. Calculated electron mobility of Cd1-xZnxTe at T=300 K as a function of x using Ue=0.3 eV(solid) and Ue=0.7 eV(dashed). Here Ue is the alloy scattering potential
Ideally carrier mobilities can be calculated from the scattering rates for acoustic phonon, polar optical phonon, piezoelectric, impurity and allo -y scattering, with polar optical scattering being dominant for nonalloyed II-VI and III-V materials at room temperature .Suzuki et al. found that for Cd0.8Zn0.2Te:Cl the hole mobility was limited by trappindetrapping involving the Acenter complex (VCdClTe)-.
Transport properties 、 Electrical contacts and
current versus voltage measurements of CZT
content
Transport properties Electrical contacts and current versus voltage measurements
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Fig. 39. Calculated electron mobilities for CdTe (solid) and ZnTe (dashed) as a function of temperature
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In the case of Cd1-xZnxTe
Ue =0.3 eV Ue =0.7 eV, Uh=0.1 eV
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On the other hand Szeles et al.have argued that the composition dependence of hole trap ionization energies implies that either the inert atomic-like character of localized levels or the small valence band offset is invalid. This may be a moot point since Singh has pointed out that that the alloy scattering potential is a fairly ill-defined quantity. It is not clear theoretically whether it corresponds to the difference in electron affinities, the difference in bandgaps, the band offsets, or some other quantity. Furthermore, clustering effects also affect the alloy scattering rate. Hence, alloy scattering potentials are determined by fitting of experimental data.
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Transport properties
Carrier mobility Experimental determination of mobility
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The effect of electron and hole transport properties on detector performance.
Transport is characterized by two parameters for each carrier type: mobility,μ,and mean trapping lifetime,. The quantity which is directly related to detector performance is the product μE, where E is the electric field. This quantity is the mean drift length, sometimes called the schubweg. Despite the underlying complexity of trapping phenomena, simple macroscopic models based on the use of the mean drift length predict detector behavior remarkably.
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3.3.1 Carrier mobility
One of the great strengths of CdTe compared to other high-Z materi -als is its relatively high electron mobility (1100 cm2/V s at 300K). The hole mobility, on the other hand, is about a factor of 10 lower. It is typically found that the electron mobility of Cd1-xZnxTe is comparable to that of CdTe, while the hole mobility is somewhat lower than in CdTe.
where nhh/nlh is the ratio of occupancies of the two bands, which is approximately equal to
Scattering between the heavy-hole and light-hole bands must be taken into account.
After Wiley the mobilities are calculated separately for light-hole and heavy-hole bands, and the overall mobility is then estimated as:
The latter result is more intuitively pleasing, since U is often identified with the band offset between the constituents, and we expect the conduction band offset to be much greater than the valence band offset for CdTe/ZnTe, based on the common-anion rule.
Eq. (23) is solved by iteration, with the f（E±kT)terms (o2n4) the right hand side being set to zero in the first pass. After each iteration the mobility is calculated as
The dielectric constant is assumed to be a linear interpolation between CdTe (=10.9) and ZnTe (= 9.7) and using a zinc fraction of 0.13, as determined from the PL spectrum, a Bohr radius of 62.6 Å is predicted. From this one can obtain an exciton reduced mass of 0.090m0, which implies a hole effective mass of 0.40m0, which is a little below the conductivity effective mass (0.47m0 for ZnTe, 0.53m0 for CdTe).
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Two factors are relevant to the dependence of the carrier mobilities on alloy composition : first, the alloy scattering rate varies as x(1-x) and therefore is greatest in the middle of the composition range; second, all scattering rates increase with the effective mass of the carrier, which in turn increases with bandgap.
for electrons.
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Valence band wave functions have p-like symmetry rather than s-like, which makes the matrix elements that appears in the scattering probability more difficult to evaluate.