Energy conversion in silicon nanostructures and devices [electronic resource]
Keywords: Electrical Engineering

We examine the physics of nanoscale energy conversion in low-dimensional silicon structures and devices. The theoretical and experimental results of this work will facilitate improved designs of the silicon nanoelectronic and nanophotonic devices vital to emerging high-density, high-bandwidth information systems. We begin by quantifying the reductions in nanotransistor performance by nonequilibrium optical phonons (OPs) generated in the drain as a result of hot electron energy relaxation. An inefficient conversion of OPs into the long-wavelength acoustic phonons (APs) responsible for heat conduction can cause an energy conversion bottleneck leading to reduced device currents and negative differential conductance instability, which poses a threat to the continued scaling of CMOS technology. We develop fully-coupled, electron-phonon, Monte Carlo Boltzmann transport simulations to assess the impact of this effect on device operation. The new approach includes electron and phonon dispersion and introduces a novel, occupation number-based stochastic rejection algorithm to efficiently couple the electron-phonon dynamics and nonequilibrium particle interaction. To address gaps in existing data, we use full-band, anharmonic perturbation theory to calculate the 2-phonon joint densities of states and lifetimes of OPs critical to hot electron energy conversion. We find that the reduction in electron mobility and the increase in leakage current due to hot OPs were greatly overestimated in previous studies and that drive current will not be impacted significantly for power densities below 200 TW/cm3. This corresponds to 20x the existing device densities and 2x what is predicted at the end of the technology roadmap. Next, we develop a new theory of the electron dynamics and nonradiative energy transfer in optically-pumped silicon nanocrystals (NCs) embedded in a dielectric host. This class of materials is promising in the development of inexpensive, CMOS-compatible, optical sources due to strong luminescence in the 600-1000 nm band and an ability to sensitize codoped Er3+ emitting near 1540 nm. We study the nonradiative recombination (NR) processes which limit the optical gain of these materials and which prevent their adoption into commercial applications using a custom built, two-color optical pump-probe system which demonstrates sub-10 picosecond temporal resolution and sensitivity within 3 dB of the shot noise limit. From an improved data set, we establish a new theory which shows the importance of long--range Coulombic dipole-dipole interaction between excited electron states at high excitation levels, an important loss mechanism neglected in all previous work. The proposed model accurately reproduces the data on bulk and quasi-2D NC films down to 10 nm over more than four decades of pump intensity and unifies existing models by showing that their solutions are limiting forms of the new model. We experimentally observe the transition from 3D to 2D behavior as a systematic reduction in NR at high excitation levels by as much as 2x which is accompanied by a characteristic change in power-law dependence on pump intensity from 1/3 to 1/4 with decreasing film thickness in exact agreement with our theory. Further reductions in NR losses and a change in power-law to 1/7 are predicted in 1-D NC laden ridges. The substantial reduction in NR losses observed in 2-D films and even greater reductions predicted in 1-D ridges and 0-D islands reduces the intrinsic lasing threshold of these materials and demonstrates a path forward to realizing silicon NC lasers.