Effects of Interface Disorder on Valley Splitting in SiGe/Si/SiGe Quantum Wells

A sharp potential barrier at the Si/SiGe interface introduces valley splitting (VS), which lifts the 2-fold valley degeneracy in strained SiGe/Si/SiGe quantum wells (QWs). This work examines in detail the effects of Si/SiGe interface disorder on the VS in an atomistic tight binding approach based on statistical sampling. VS is analyzed as a function of electric field, QW thickness, and simulation domain size. Strong electric fields push the electron wavefunctions into the SiGe buffer and introduce significant VS fluctuations from device to device. A Gedankenexperiment with ordered alloys sheds light on the importance of different bonding configurations on VS. We conclude that a single SiGe band offset and effective mass cannot comprehend the complex Si/SiGe interface interactions that dominate VS.Comment: 5 figure

Brillouin-zone Unfolding of Perfect Supercells Having Nonequivalent Primitive Cells Illustrated with a Si/Ge Tight-Binding parameterization

Numerical calculations of nanostructure electronic properties are often based on a nonprimitive rectangular unit cell, because the rectangular geometry allows for both highly efficient algorithms and ease of debugging while having no drawback in calculating quantum dot energy levels or the one-dimensional energy bands of nanowires. Since general nanostructure programs can also handle superlattices, it is natural to apply them to these structures as well, but here problems arise due to the fact that the rectangular unit cell is generally not the primitive cell of the superlattice, so that the resulting E(k) relations must be unfolded to obtain the primitive- cell E(k) curves. If all of the primitive cells in the rectangular unit cell are identical, then the unfolding is reasonably straightforward; if not, the problem becomes more difficult. Here, we provide a method for zone unfolding when the primitive cells in a rectangular cell are not all identical. The method is applied to a Si(4)Ge(4) superlattice using a set of optimized Si and Ge tight-binding strain parameters

Valley splitting in strained silicon quantum wells modeled with 2 degrees miscuts, step disorder, and alloy disorder

Valley splitting (VS) in strained SiGe/Si/SiGe quantum wells grown on (001) and 2 degrees miscut substrates is computed in a magnetic field. Calculations of flat structures significantly overestimate, while calculations of perfectly ordered structures underestimate experimentally observed VS. Step disorder and confinement alloy disorder raise the VS to the experimentally observed levels. Atomistic alloy disorder is identified as the critical physics, which cannot be modeled with analytical effective mass theory. NEMO-3D is used to simulate up to 10(6) atoms, where strain is computed in the valence-force field and electronic structure in the sp(3)d(5)s(*) model

Valley Degeneracies in (111) Silicon Quantum Wells

(111) Silicon quantum wells have been studied extensively, yet no convincing explanation exists for the experimentally observed breaking of 6 fold valley degeneracy into 2 and 4 fold degeneracies. Here, systematic sp3d5s* tight-binding and effective mass calculations are presented to show that a typical miscut modulates the energy levels which leads to breaking of 6 fold valley degeneracy into 2 lower and 4 raised valleys. An effective mass based valley-projection model is used to determine the directions of valley-minima in tight-binding calculations of large supercells. Tight-binding calculations are in better agreement with experiments compared to effective mass calculations.Comment: 4 pages, 3 figures, to appear in Applied Physics Letter

Multiscale Metrology and Optimization of Ultra-Scaled InAs Quantum Well FETs

A simulation methodology for ultra-scaled InAs quantum well field effect transistors (QWFETs) is presented and used to provide design guidelines and a path to improve device performance. A multiscale modeling approach is adopted, where strain is computed in an atomistic valence-force-field method, an atomistic sp3d5s* tight-binding model is used to compute channel effective masses, and a 2-D real-space effective mass based ballistic quantum transport model is employed to simulate three terminal current-voltage characteristics including gate leakage. The simulation methodology is first benchmarked against experimental I-V data obtained from devices with gate lengths ranging from 30 to 50 nm. A good quantitative match is obtained. The calibrated simulation methodology is subsequently applied to optimize the design of a 20 nm gate length device. Two critical parameters have been identified to control the gate leakage magnitude of the QWFETs, (i) the geometry of the gate contact (curved or square) and (ii) the gate metal work function. In addition to pushing the threshold voltage towards an enhancement mode operation, a higher gate metal work function can help suppress the gate leakage and allow for much aggressive insulator scaling

Multiscale Metrology and Optimization of Ultra-Scaled InAs Quantum Well FETs

A simulation methodology for ultra-scaled InAs quantum well field effect transistors (QWFETs) is presented and used to provide design guidelines and a path to improve device performance. A multiscale modeling approach is adopted, where strain is computed in an atomistic valence-force-field method, an atomistic sp3d5s* tight-binding model is used to compute channel effective masses, and a 2-D real-space effective mass based ballistic quantum transport model is employed to simulate three terminal current-voltage characteristics including gate leakage. The simulation methodology is first benchmarked against experimental I-V data obtained from devices with gate lengths ranging from 30 to 50 nm. A good quantitative match is obtained. The calibrated simulation methodology is subsequently applied to optimize the design of a 20 nm gate length device. Two critical parameters have been identified to control the gate leakage magnitude of the QWFETs, (i) the geometry of the gate contact (curved or square) and (ii) the gate metal work function. In addition to pushing the threshold voltage towards an enhancement mode operation, a higher gate metal work function can help suppress the gate leakage and allow for much aggressive insulator scaling

Performance Analysis of a Ge/Si Core/Shell Nanowire Field Effect Transistor

We analyze the performance of a recently reported Ge/Si core/shell nanowire transistor using a semiclassical, ballistic transport model and an sp3s*d5 tight-binding treatment of the electronic structure. Comparison of the measured performance of the device with the effects of series resistance removed to the simulated result assuming ballistic transport shows that the experimental device operates between 60 to 85% of the ballistic limit. For this ~15 nm diameter Ge nanowire, we also find that 14-18 modes are occupied at room temperature under ON-current conditions with ION/IOFF=100. To observe true one dimensional transport in a Ge nanowire transistor, the nanowire diameter would have to be much less than about 5 nm. The methodology described here should prove useful for analyzing and comparing on common basis nanowire transistors of various materials and structures

Accurate six-band nearest-neighbor tight-binding model for the pi-bands of bulk graphene and graphene nanoribbons

Accurate modeling of the pi-bands of armchair graphene nanoribbons (AGNRs) requires correctly reproducing asymmetries in the bulk graphene bands as well as providing a realistic model for hydrogen passivation of the edge atoms. The commonly used single-pz orbital approach fails on both these counts. To overcome these failures we introduce a nearest-neighbor, three orbital per atom p/d tight-binding model for graphene. The parameters of the model are fit to first-principles density-functional theory (DFT) - based calculations as well as to those based on the many-body Green's function and screened-exchange (GW) formalism, giving excellent agreement with the ab initio AGNR bands. We employ this model to calculate the current-voltage characteristics of an AGNR MOSFET and the conductance of rough-edge AGNRs, finding significant differences versus the single-pz model. These results show that an accurate bandstructure model is essential for predicting the performance of graphene-based nanodevices.Comment: 5 figure