Hg1−xCdxTe or mercury cadmium telluride (also cadmium mercury telluride, MCT, MerCad Telluride, MerCadTel, MerCaT or CMT) is a chemical compound of cadmium telluride (CdTe) and mercury telluride (HgTe) with a tunable bandgap spanning the shortwave infrared to the very long wave infrared regions. The amount of cadmium (Cd) in the alloy can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. CdTe is a semiconductor with a bandgap of approximately 1.5 eV at room temperature. HgTe is a semimetal, which means that its bandgap energy is zero. Mixing these two substances allows one to obtain any bandgap between 0 and 1.5 eV.
Properties
Physical
Hg1−xCdxTe has a zincblende structure with two interpenetrating face-centered cubic lattices offset by (1/4,1/4,1/4)ao in the primitive cell. The cations Cd and Hg are statistically mixed on the yellow sublattice while the Te anions form the grey sublattice in the image.
Electronic
The electron mobility of HgCdTe with a large Hg content is very high. Among common semiconductors used for infrared detection, only InSb and InAs surpass electron mobility of HgCdTe at room temperature. At 80 K, the electron mobility of Hg0.8Cd0.2Te can be several hundred thousand cm2/(V·s). Electrons also have a long ballistic length at this temperature; their mean free path can be several micrometres. The intrinsic carrier concentration is given by
n i ( T , x ) = ( 5.585 − 3.82 x + ( 1.753 ⋅ 10 − 3 ) T − 1.364 ⋅ 10 − 3 T ⋅ x ) ⋅ ( 10 14 ⋅ E g 3 / 4 ⋅ T 3 / 2 ⋅ e − E g 2 k T ) {\displaystyle n_{i}(T,x)=(5.585-3.82x+(1.753\cdot 10^{-3})T-1.364\cdot 10^{-3}T\cdot x)\cdot (10^{14}\cdot E_{\text{g}}^{3/4}\cdot T^{3/2}\cdot e^{\frac {-E_{\text{g}}}{2kT}})}
where k is the Boltzmann constant, q is the elementary electric charge, T is the material temperature, x is the percentage of cadmium concentration, and Eg is the bandgap given by
E g ( T , x ) = − 0.302 + 1.93 ⋅ x + ( 5.35 ⋅ 10 − 4 ) ⋅ T ⋅ ( 1 − 2 ⋅ x ) − 0.81 ⋅ x 2 + 0.832 ⋅ x 3 {\displaystyle E_{\text{g}}(T,x)=-0.302+1.93\cdot x+(5.35\cdot 10^{-4})\cdot T\cdot (1-2\cdot x)-0.81\cdot x^{2}+0.832\cdot x^{3}}
Using the relationship λ p = 1.24 E g {\displaystyle \lambda _{\text{p}}={\frac {1.24}{E_{\text{g}}}}} , where λ is in μm and Eg. is in electron volts, one can also obtain the cutoff wavelength as a function of x and t:
λ p = ( − 0.244 + 1.556 ⋅ x + ( 4.31 ⋅ 10 − 4 ) ⋅ T ⋅ ( 1 − 2 ⋅ x ) − 0.65 ⋅ x 2 + 0.671 ⋅ x 3 ) − 1 {\displaystyle \lambda _{\text{p}}=(-0.244+1.556\cdot x+(4.31\cdot 10^{-4})\cdot T\cdot (1-2\cdot x)-0.65\cdot x^{2}+0.671\cdot x^{3})^{-1}}
Minority carrier lifetime
Auger recombination Two types of Auger recombination affect HgCdTe: Auger 1 and Auger 7 recombination. Auger 1 recombination involves two electrons and one hole, where an electron and a hole combine and the remaining electron receives energy equal to or greater than the band gap. Auger 7 recombination is similar to Auger 1, but involves one electron and two holes. The Auger 1 minority carrier lifetime for intrinsic (undoped) HgCdTe is given by
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