2, Stony Brook University, Stony Brook, New York, United States
Amorphous materials can be uniformly deposited over a large area at lower cost compared to crystalline semiconductors (Silicon or Germanium). This property along with its high resistivity and wide band-gap has found many applications in devices like rectifiers, xerography, xero-radiography, ultrahigh sensitivity optical cameras, digital radiography, and mammography (2D and 3D tomosynthesis). Amorphous selenium is the only amorphous material that undergoes impact ionization where only holes avalanche at high electric fields. This leads to a small Excess Noise Factor which is a very important performance comparison matrix for avalanche photodetectors.
Thus, there is a need to model high field avalanche in amorphous selenium. At high fields, the transport in amorphous selenium changes from low values of activated trap-limited drift mobility to higher values of band transport mobility, via extended states. When the transport shifts from activated mobility with high degrees of localization to extended state band transport, then the wavefunction of the amorphous material resembles that of its crystalline counterpart.
The only form of amorphous selenium used in ultra-fast radiation detectors is thin-films made by the process of Vapor-deposition. The local topology of a Vapor Deposited amorphous selenium has a predominantly ring-like symmetry, which has a distinct resemblance to the monomer ring like structure of crystalline monoclinic selenium. Thus, we model the transport of crystalline monoclinic selenium, to study the electrical properties of amorphous selenium.
We have studied the transport phenomena in crystalline monoclinic selenium by using a bulk Monte Carlo technique to solve the semi-classical Boltzmann Transport Equation. Here, we primarily consider acoustic and non-polar optical phonon scattering mechanisms. The band structure and the Density of States function for monoclinic selenium was obtained by using an atomistic simulation tool called the Atomistic Toolkit in the Virtual Nano Lab, Quantum Wise, Copenhagen, Denmark.
In this work, we have simulated the velocity and energy against time characteristics for a wide range of electric fields (1-1000 kV/cm), which is further used to find the hole drift mobility. The low field mobility is obtained from the slope of the velocity vs. electric field plot. The low field hole mobility was calculated to be 5.51 cm2/Vs at room temperature. The experimental value for low field hole mobility is 7.29 cm2/Vs. The energy vs. electric field simulation at high fields is used to match the experimental onset of avalanche (754 kV/cm) for an ionization threshold energy of 2.1eV. Finally, we present the Arrhenius Plot for Mobility against temperature and compare it with published experimental data. The experimental and simulation results show a close match, thus validating our study.