Melezhik E. Numerical simulation of the band structure, transport properties and noises in semi-metallic Hg1-xCdxTe quantum wells used as a channel of THz detector

The work is dedicated to numerical simulation of energy spectra, carrier wave-functions, electron mobilities and noises in semi-metallic Hg1-xCdxTe quantum well with finite band gap. This quantum well can be used as a channel of THz hot-electron bolometer. Calculations are provided for T = 77 K. Energy spectra and carrier wave-functions are calculated in the framework of 8x8 k.p Hamiltonian, which allows one to account bands mixing in the wave-function. Electron mobilities are calculated by direct iterative solution of Boltzmann transport equation, which allows one to consider the existence of inelastic scattering mechanisms, degeneracy of electron subsystem and nonparabolicity of the energy dispersion law. Graphene-like screening function is used to describe the screening of Coulomb-potential driven scattering mechanisms by two-dimensional electron and hole gases. It is shown that at moderate charged impurities concentration (about of 1e15 cm-3 or greater) charged centers scattering is dominant scattering mechanism for electrons, while optical phonons scattering is sufficiently suppressed due to strong dynamical screening which affects phonon scattering. It is found that for many sets of quantum well parameters (such as well width, molar composition x and electron concentration), holes concentration exceeds the value of 1e15 cm-3. Consequently purification of residual charged impurities concentration in the well to the values less than 1e15 cm-3 is senseless, as it would not lead to the improvement of electron mobility. This result should allow technologists to decrease sample growth costs due to the simplification of sample purification procedure. In the work there is shown that in samples with finite band gap, electron mobility at intrinsic carrier concentrations is low, compared to the mobility in the n-type samples. Growth of the electron concentration in times leads to the increase of electron mobility by orders of magnitude, approaching the value of 1e6 cm2/Vs at T=77 K. This effect is caused by simultaneous growth of screening function and decrease of holes concentration during electron concentration growth. Our estimate of the resistance in semi-metal HgCdTe quantum wells used as a channel of the THz hot-electron bolometer shows that the channel resistance varies by more than two orders of magnitude depending on the electron concentration. Also it is shown that Johnson noise is dominant for the systems with high electron concentration. Generation-recombination and photon noises are found to be much smaller. Optimal parameters of semi-metallic Hg1-xCdxTe quantum wells are obtained by numerical simulation. To produce low noise and high mobility channel of THz hot-electron bolometer, which impedance is about of the antenna impedance, one should follow several recommendations. At first, quantum well width should be greater than 10 nm to avoid interface scattering. At second, electron concentration in the well should be much greater than the intrinsic one. At third, quantum well molar composition x should be near the bands inversion point (but not too close to it to avoid of activation of additional scattering mechanisms). Finally, charged impurities concentration in such quantum well should not be greater than 1e15 cm-3. We have also assessed the advantages of the HgCdTe THz hot-electron bolometer compared to the graphene HEB. We conclude that HgCdTe semi-metallic QWs can demonstrate higher mobility, lower thermal noise, and higher operational speed and can provide much more efficient coupling to planar antennas in THz range detector applications.