The mobility−lifetime products (μτ) for electrons and holes in CdS nanowires were quantitatively determined by scanning photocurrent microscopy
of devices with ohmic contacts. Ohmic contacts were fabricated by ion bombardment of the contact regions. By analyzing the spatial profiles
of the local photoconductivity maps, we determined that electron transport (μeτe ≈ 5 × 10-7 cm2/V) was more efficient than hole transport
(μhτh ≈ 10-7 cm2/V). The results demonstrate that photocurrent mapping can provide quantitative insight into intrinsic carrier transport properties
of semiconductor nanostructures.
Three types of two-terminal CdS nanowire devices with distinct current versus voltage characteristics were fabricated by forming Schottky and/or Ohmic contacts in a controlled manner. Argon ion bombardment of CdS nanowires increased the carrier concentration allowing the formation of Ohmic Ti-CdS contacts. Scanning photocurrent microscopy ͑SPCM͒ was used to explore the influence of the contacts on the spatially resolved photoresponse in two-terminal devices and to analyze charge carrier transport processes. Modeling of the spatial profiles of the local photocurrent images enabled the quantitative extraction of electron and hole mobility-lifetime products in Ohmic devices and the hole mobility-lifetime product in Schottky devices. Analysis of the evolution of SPCM images with bias suggests that the electric field is localized to the optical generation region in the Ohmic devices and localized beneath the contacts in the Schottky devices.
Particle-based ensemble semi-classical Monte Carlo (MC) methods employ quantum corrections (QCs) to address quantum confinement and degenerate carrier populations to model tomorrow's ultra-scaled metal-oxide-semiconductor-field-effect-transistors. Here, we present the most complete treatment of quantum confinement and carrier degeneracy effects in a three-dimensional (3D) MC device simulator to date, and illustrate their significance through simulation of n-channel Si and III-V FinFETs. Original contributions include our treatment of far-from-equilibrium degenerate statistics and QC-based modeling of surface-roughness scattering, as well as considering quantum-confined phonon and ionized-impurity scattering in 3D. Typical MC simulations approximate degenerate carrier populations as Fermi distributions to model the Pauli-blocking (PB) of scattering to occupied final states. To allow for increasingly far-from-equilibrium non-Fermi carrier distributions in ultra-scaled and III-V devices, we instead generate the final-state occupation probabilities used for PB by sampling the local carrier populations as function of energy and energy valley. This process is aided by the use of fractional carriers or sub-carriers, which minimizes classical carrier-carrier scattering intrinsically incompatible with degenerate statistics. Quantum-confinement effects are addressed through quantum-correction potentials (QCPs) generated from coupled Schrödinger-Poisson solvers, as commonly done. However, we use these valley- and orientation-dependent QCPs not just to redistribute carriers in real space, or even among energy valleys, but also to calculate confinement-dependent phonon, ionized-impurity, and surface-roughness scattering rates. FinFET simulations are used to illustrate the contributions of each of these QCs. Collectively, these quantum effects can substantially reduce and even eliminate otherwise expected benefits of considered In0.53Ga0.47As FinFETs over otherwise identical Si FinFETs despite higher thermal velocities in In0.53Ga0.47As. It also may be possible to extend these basic uses of QCPs, however calculated, to still more computationally efficient drift-diffusion and hydrodynamic simulations, and the basic concepts even to compact device modeling.
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