We propose a true solid-state alternative to the vacuum photomultiplier tube using amorphous selenium (a-Se) as the bulk avalanche i-layer. A-Se is a unique photosensing material in which carrier transport can be shifted entirely from localized to extended states where only holes get hot and undergo impact ionization, resulting in deterministic and non-Markovian avalanche gain. To achieve reliable and repeatable impact ionization gain without irreversible breakdown, a non-insulating metal oxide ntype hole-blocking/electron-transporting layer is needed. For the first time, we have deposited a solution-processed quantum dot (QD) hole blocking layer over an a-Se film at room temperature, without any surface or bulk crystallization. We have measured the lowest dark current density ever reported (30 pA/cm 2 at the onset of avalanche) compared to any other solid-state avalanche sensor at room temperature. Our results provide new strategies for the development of advanced solid-state photomultipliers via efficient QD-based interface layers to fully exploit the deterministic avalanche properties of a-Se.

Amorphous selenium is emerging as a viable large-area imaging detector with avalanche multiplication gain for low-light and low-dose radiation detection applications. A key feature of its avalanche process is that only holes become “hot” carriers and undergo impact ionization. Thus, understanding the transport of non-equilibrium hot holes in extended states is pivotal to all the device applications. One of the interesting aspects of elemental selenium is the similar general feature of the electronic structure for various phase modifications. This stems from the strikingly similar short-range order between the crystalline and amorphous phases of selenium. At high electric fields, hole mobility in amorphous selenium loses its activated behavior and saturates with transport shifted entirely from localized to extended states. Thus, we expect the general details of the extended-state hole-phonon interaction in the amorphous phase to be described by the band-transport lattice theory of its crystalline counterparts, namely, monoclinic and trigonal selenium. To that effect and due to the intrinsic meta-stability of the monoclinic phase and high trap density in prepared specimens, we study hole transport in crystalline trigonal selenium semiconductors using a bulk Monte Carlo technique to solve the semi-classical Boltzmann transport equation. We validated our transport model by showing the excellent match between experimentally calculated hole drift mobilities with that calculated using the bulk Monte Carlo technique. Furthermore, calculations of the field-dependent carrier energy showed that holes in selenium can break the thermal equilibrium and get hot at which point the rate of energy gain from the applied electric field exceeds that of energy loss from the lattice.

Amorphous selenium lacks the structural long-range order present in crystalline solids. However, the stark similarity in the short-range order that exists across its allotropic forms, augmented with a shift to non-activated extended-state transport at high electric fields beyond the onset of impact ionization, allowed us to perform this theoretical study, which describes the high-field extended-state hole transport processes in amorphous selenium by modeling the band-transport lattice theory of its crystalline counterpart trigonal selenium. An in-house bulk Monte Carlo algorithm is employed to solve the semiclassical Boltzmann transport equation, providing microscopic insight to carrier trajectories and relaxation dynamics of these non-equilibrium “hot” holes in extended states. The extended-state hole–phonon interaction and the lack of long-range order in the amorphous phase is modeled as individual scattering processes, namely acoustic, polar and non-polar optical phonons, disorder and dipole scattering, and impact ionization gain, which is modeled using a power law Keldysh fit. We have used a non-parabolic approximation to the density functional theory calculated valence band density of states. To validate our transport model, we calculate and compare our time of flight mobility, impact ionization gain, ensemble energy and velocity, and high field hole energy distributions with experimental findings. We reached the conclusion that hot holes drift around in the direction perpendicular to the applied electric field and are subject to frequent acceleration/deceleration caused by the presence of high phonon, disorder, and impurity scattering. This leads to a certain determinism in the otherwise stochastic impact ionization phenomenon, as usually seen in elemental crystalline solids.

Amorphous selenium (a-Se) with its single-carrier and non-Markovian, hole impact ionization process can revolutionize low-light detection and emerge to be a solid-state replacement to the vacuum photomultiplier tube (PMT). Although a-Se-based solid-state avalanche detectors can ideally provide gains comparable to PMTs, their development has been severely limited by the irreversible breakdown of inefficient hole blocking layers (HBLs). Thus, understanding of the transport characteristics and ways to control electrical hot spots and, thereby, the breakdown voltage is key to improving the performance of avalanche a-Se devices. Simulations using Atlas, SILVACO, were employed to identify relevant conduction mechanisms in a-Se-based detectors: space-charge-limited current, bulk thermal generation, Schottky emission, Poole–Frenkel activated mobility, and hopping conduction. Simulation parameters were obtained from experimental data and first-principle calculations. The theoretical models were validated by comparing them with experimental steady-state dark current densities in avalanche and nonavalanche a-Se detectors. To maintain bulk thermal generation-limited dark current levels in a-Se detectors, a high-permittivity noninsulating material is required to substantially decrease the electric field at the electrode/hole blocking layer interface, thus preventing injection from the high-voltage electrode. This, in turn, prevents Joule heating from crystallizing the a-Se layer, consequently avoiding early dielectric breakdown of the device.

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