Ballistic Hole Research Articles

Improved performance of InGaN/GaN Near-UV light-emitting diodes with staircase hole injector

We report the enhanced performance of near-ultraviolet (NUV) InGaN/GaN multiple quantum-well (MQW) light-emitting diodes (LEDs) with a staircase hole injector (SHI). Simulation results indicate that the internal electrostatic field in the QW of the LED-SHI is decreased owing to the reduced sheet charge density at the interface between QW and quantum barrier (QB) caused by the smaller In content difference. Additionally, the SHI structure in the QBs suppresses the ballistic or quasi-ballistic hole transport, thus enhancing efficient hole injection into the QWs. The radiative output power of an LED-SHI is increased by 25.3% at 300 mA over that of conventional LEDs with GaN QBs. The droop of internal quantum efficiency (IQE) an LED-SHI at 300 A·cm−1 is 6.1%, while the LED with GaN QBs has an IQE droop of 17.7%. The reduced IQE droop and increased radiative output power in the LED-SHI is attributed to the reduced hole overflow, increased hole injection into the MQW and the decreased electrostatic field in the MQWs. The results show that the SHI structure in the LED is promising for improved performance in high-power GaN-based NUV LEDs.

Gaussian Thermionic Emission Model for Analysis of Au/MoS2 Schottky-Barrier Devices

Schottky-barrier inhomogeneities are expected at the metal--transition-metal-dichalcogenide (TMDC) interface and this can impact device performance. However, it is difficult to account for the distribution of interface inhomogeneity as most techniques average over the spot area of the analytical tool (e.g., few hundred micrometers squared for photoelectron-based techniques), or the entire device measured for electrical current-voltage (I-V) measurements. Commonly used models to extract Schottky-barrier heights neglect or fail to account for such inhomogeneities, which can lead to the extraction of incorrect Schottky-barrier heights and Richardson constants that are orders of magnitude away from theoretically expected values. Here, we show that a Gaussian-modified thermionic emission model gives the best fit to experimental temperature-dependent current-voltage (I-V-T) data of van der Waals $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ interfaces and allow the deconvolution of the Schottky-barrier heights of the defective regions from the pristine region. By the inclusion of a Gaussian-distributed Schottky-barrier height in the macroscopic I-V-T analysis, we demonstrate that interface inhomogeneities due to defects are deconvoluted and well correlated to the impact on the device behavior across a wide temperature range from a room temperature of 300 K down to 120 K. We verify the Gaussian thermionic model across two different types of p-${\mathrm{Mo}\mathrm{S}}_{2}$ (geological bulk crystals and synthetic flux-grown crystals), and finally compare the macroscopic Schottky-barrier heights with the results of a nanoscopic technique, ballistic hole emission microscopy (BHEM). The results obtained using BHEM are consistent with the pristine $\mathrm{Au}$/p-${\mathrm{Mo}\mathrm{S}}_{2}$ Schottky-barrier height extracted from the Gaussian-modified thermionic emission model over hundreds of nanometers. Our findings show that the inclusion of Schottky-barrier inhomogeneities in the analysis of I-V-T data is useful to elucidate the impact of defects (e.g., grain boundaries, metallic impurities, etc.) and hence their influence on device behavior. We also find that the effective Richardson constant, a material-specific constant typically treated as merely a fitting constant, is a useful parameter to check for the validity of the transport model.

Impact of random alloy fluctuations on inter-well transport in InGaN/GaN multi-quantum well systems: an atomistic non-equilibrium Green’s function study

Recent experimental studies indicate the presence of ballistic hole transport in InGaN multi quantum well (MQW) structures. Widely used drift–diffusion models cannot give insight into this question, since quantum mechanical effects, such as tunneling, are not included in such semi-classical approaches. Also atomistic effects, e.g. carrier localization effects and built-in field variations due to (random) alloy fluctuations, are often neglected in ballistic transport calculations on InGaN quantum well systems. In this work we use atomistic tight-binding theory in conjunction with a non-equilibrium Green’s function approach to study electron and hole ballistic transport in InGaN MQW systems. Our results show that for electrons the alloy microstructure is of secondary importance for their ballistic transport properties, while for hole transport the situation is different. We observe for narrow barrier widths in an InGaN MQW system that (random) alloy fluctuations give rise to extra hole transmission channels when compared to a virtual crystal description of the same system. We attribute this effect to the situation that in the random alloy case, k ∥-vector conservation is broken/relaxed and therefore the ballistic hole transport is increased. However, for wider barrier width this effect is strongly reduced, which is consistent with experimental studies. Our findings also provide a possible explanation for recent experimental results where alloying the barrier between the wells leads to enhanced ballistic (hole) transport in InGaN MQW systems.

Low-temperature carrier transport across InGaN multiple quantum wells: Evidence of ballistic hole transport

Carrier transport across polar n-type InGaN/GaN multiple quantum wells (MQWs) has been studied by time-resolved photoluminescence (PL) using an optical marker technique. Efficiency of the hole tran ...

Visualizing metal/HfO2/SiO2/Si(001) interface electrostatic barrier heights with ballistic hole emission microscopy

The electrostatic barrier of a Au/1.4 nm HfO2/0.8nm SiO2/Si(001) structure was mapped with ballistic hole emission microscopy on p-type silicon substrates to nanoscale dimensions over a square micrometer. The 1.4 nm HfO2 layer showed three concentrations of barrier heights localized in different regions of the sample. These concentrations are consistent with the barrier heights of HfO2/Si-p, native SiO2/Si-p, and one centered at −0.45eV. The latter barrier height is attributed to an ultrathin HfO2 (1–3 monolayers). This study demonstrates the power of electrostatic barrier mapping to visualize complex and nonuniform interfaces.

Quantum Transport Study of Si Ultrathin-Body Double-Gate pMOSFETs: <inline-formula> <tex-math notation="LaTeX">${I}$ </tex-math> </inline-formula>–<inline-formula> <tex-math notation="LaTeX">${V}$ </tex-math> </inline-formula>, <inline-formula> <tex-math notation="LaTeX">${C}$ </tex-math> </inline-formula>–<inline-formula> <tex-math notation="LaTeX">${V}$ </tex-math> </inline-formula>, Energy Delay, and Parasitic Effects

A comprehensive quantum transport study of ultrathin-body silicon double-gate pMOSFETs with gate length ${L}_{\textsf {g}}= \textsf {14}$ , 10, and 7 nm is performed by in-house developed nonequilibrium Green’s function solver employing the six-band ${k} \cdot {p}$ Hamiltonian. The effects of channel length, body thickness, as well as confinement and transport crystal orientations are studied systematically. Both ${I}$ – ${V}$ and ${C}$ – ${V}$ characteristics are analyzed in this paper. Furthermore, the CMOS performance benchmarking results, such as switching delay, energy consumption, and their product, are calculated with the influence of parasitic resistances and capacitances treated in an appropriate way. It is found that the crystal orientation effect on drive current is significant at both long and short channel lengths, while its effect on effective capacitance is more observable at ${L}_{\textsf {g}} = \textsf {7}$ nm. The drive current at ${L}_{\textsf {g}} = \textsf {14}$ nm and ${L}_{\textsf {g}} = \textsf {7}$ nm can be well explained by average ballistic hole velocity and source-to-drain tunneling ratio, respectively. Meanwhile, the effective capacitance is not only determined by the density of states but also affected by the distribution of holes in the confined direction. Both intrinsic and extrinsic performance assessments suggest that (001)/[100] and (110)/[001] are the optimal confinement/transport crystal orientation configurations at ${L}_{\textsf {g}}= \textsf {7}$ nm, while ${(}\textsf {110}{)}/[\overline {1}\textsf {10}]$ and ${(}\textsf {110})/[\overline {1}\textsf {11}]$ are the best choices at longer gate lengths. As ${L}_{\textsf {g}}$ shrinks, both intrinsic and extrinsic performances will be better at a scaled body thickness.

Ballistic One-Dimensional Holes with Strong g-Factor Anisotropy in Germanium.

We report experimental evidence of ballistic hole transport in one-dimensional quantum wires gate-defined in a strained SiGe/Ge/SiGe quantum well. At zero magnetic field, we observe conductance plateaus at integer multiples of 2 e2/ h. At finite magnetic field, the splitting of these plateaus by Zeeman effect reveals largely anisotropic g-factors with absolute values below 1 in the quantum-well plane, and exceeding 10 out-of-plane. This g-factor anisotropy is consistent with a heavy-hole character of the propagating valence-band states, which is in line with a predominant confinement in the growth direction. Remarkably, we observe quantized ballistic conductance in device channels up to 600 nm long. These findings mark an important step toward the realization of novel devices for applications in quantum spintronics.

Ballistic Hole Emission Spectroscopy of Self-Assembled GeSi/Si(001) Nanoislands

Ballistic hole emission microscopy/spectroscopy has been applied to study the electronic structure of the hole states in the self-assembled GeSi/Si(001) nanoislands. The ballistic hole emission microscopic images demonstrated the spots of increased collector current related to the ballistic electron tunnelling via the confined valence band states in the GeSi/Si(001) nanoislands. In the ballistic hole emission spectra of the Ge hut clusters the stepwise features attributed to the quantum confined hole states have been observed. The results of present study demonstrate the capabilities of the ballistic hole emission microscopy/spectroscopy in the characterization of the electronic structures of the valence band states in the GeSi/Si nanostructures.

Ballistic hole emission spectroscopy of self-assembled GeSi/Si(001) nanoislands

AbstractBallistic hole emission microscopy/spectroscopy has been applied to study the electronic structure of the hole states in the self-assembled GeSi/Si(001) nanoislands. The ballistic hole emission microscopic images demonstrated the spots of increased collector current related to the ballistic electron tunnelling via the confined valence band states in the GeSi/Si(001) nanoislands. In the ballistic hole emission spectra of the Ge hut clusters the stepwise features attributed to the quantum confined hole states have been observed. The results of present study demonstrate the capabilities of the ballistic hole emission microscopy/spectroscopy in the characterization of the electronic structures of the valence band states in the GeSi/Si nanostructures.

Analysis of ballistic and quasi-ballistic hole transport properties in germanium nanowires based on an extended “Top of the Barrier” model

The ballistic hole transport properties in rectangular cross-sectional germanium nanowire transistors with various geometries were studied based on the “Top of the Barrier” model. Then, by an extension of this model, the quasi-ballistic hole transport was discussed taking into account phonon and surface roughness scattering in the channel and source-to-drain direct tunneling. Among several nanowire geometries targeted in this study, the [110]-oriented nanowire with large height along [11¯0] ([110]/(11¯0) NW) exhibited the largest ballistic current. This was understood from its large density of states and resulting high hole density. Large density of states, however, enhances backscattering in the channel. An approximation analysis of quasi-ballistic transport suggested that the [110]/(001) NW with higher mobility can outperform [110]/(11¯0) NW when scattering and tunneling are considered.

Nanoscale mapping of the W/Si(001) Schottky barrier

The W/Si(001) Schottky barrier was spatially mapped with nanoscale resolution using ballistic electron emission microscopy (BEEM) and ballistic hole emission microscopy (BHEM) using n-type and p-type silicon substrates. The formation of an interfacial tungsten silicide is observed utilizing transmission electron microscopy and Rutherford backscattering spectrometry. The BEEM and BHEM spectra are fit utilizing a linearization method based on the power law BEEM model using the Prietsch Ludeke fitting exponent. The aggregate of the Schottky barrier heights from n-type (0.71 eV) and p-type (0.47 eV) silicon agrees with the silicon band gap at 80 K. Spatially resolved maps of the Schottky barrier are generated from grids of 7225 spectra taken over a 1 μm × 1 μm area and provide insight into its homogeneity. Histograms of the barrier heights have a Gaussian component consistent with an interface dipole model and show deviations that are localized in the spatial maps and are attributed to compositional fluctuations, nanoscale defects, and foreign materials.

Schottky barrier height measurements of Cu/Si(001), Ag/Si(001), and Au/Si(001) interfaces utilizing ballistic electron emission microscopy and ballistic hole emission microscopy

The Schottky barrier heights of both n and p doped Cu/Si(001), Ag/Si(001), and Au/Si(001) diodes were measured using ballistic electron emission microscopy and ballistic hole emission microscopy (BHEM), respectively. Measurements using both forward and reverse ballistic electron emission microscopy (BEEM) and (BHEM) injection conditions were performed. The Schottky barrier heights were found by fitting to a linearization of the power law form of the Bell-Kaiser BEEM model. The sum of the n-type and p-type barrier heights are in good agreement with the band gap of silicon and independent of the metal utilized. The Schottky barrier heights are found to be below the region of best fit for the power law form of the BK model, demonstrating its region of validity.

Quantum interference and control of the dynamic Franz-Keldysh effect: Generation and detection of terahertz space-charge fields

The Dynamic Franz Keldysh Effect (DFKE) is produced and controlled in bulk gallium arsenide by quantum interference without the aid of externally applied fields and is spatially and temporally resolved using ellipsometric pump-probe techniques. The ∼3 THz internal driving field for the DFKE is a transient space-charge field that is associated with a critically damped coherent plasma oscillation produced by oppositely traveling ballistic electron and hole currents that are injected by two-color quantum interference techniques. The relative phase and polarization of the two pump pulses can be used to control the DFKE.

Size- and Orientation-Dependent Strain Effects on Ballistic Si p-Type Nanowire Field-Effect Transistors

The size- and orientation-dependent uniaxial strain effects on ballistic hole transport in nanowire field-effect transistors are investigated using an sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">*</sup> -based tight-binding formalism coupled with a compact electrostatics model and a semiclassical transport model. It is found that the strain-induced reduction of the valence band density of states leads to an increased ballistic hole current. This is explained by the product of a small reduction in hole density and a significant increase in the average ballistic hole velocity under uniaxial compression. While uniaxial compressive strain is beneficial for both 〈110〉 and 〈100〉 devices, the strain response of 〈110〉 nanowires is much larger than their 〈100〉 counterparts. Ultrascaled 〈110〉 nanowires have the highest hole drive current under both strained and unstrained conditions, despite the reduction of strain-induced ballistic hole current enhancement for narrower devices.

Intrinsic reduction of ballistic hole current due to quantum mechanical coupling of heavy and light holes in p-type Si nanowire field effect transistors

Rigorous quantum mechanical transport calculations based on the multi-band k · p Hamiltonian are performed in this work to show that the coupling of heavy and light holes (LHs) greatly reduces on-state hole current in ultra-scaled p-type Si nanowire FETs. If the coupling between the heavy and light holes is artificially suppressed, on-current of the p-type devices almost doubles and becomes comparable to that of n-type counterparts. It is found that the effect of the coupling on the hole transport is maximized at the channel width of around 5 nm.

Weak Localization in Mesoscopic Hole Transport: Berry Phases and Classical Correlations

We consider phase-coherent transport through ballistic and diffusive two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show that intrinsic heavy-hole-light-hole coupling gives rise to clear-cut signatures of an associated Berry phase in the weak localization which renders the magnetoconductance profile distinctly different from electron transport. Nonuniversal classical correlations determine the strength of these Berry phase effects and the effective symmetry class, leading even to antilocalization-type features for circular quantum dots and Aharonov-Bohm rings in the absence of additional spin-orbit interaction. Our semiclassical predictions are confirmed by numerical calculations.

Ballistic induced hole quantum wires fabricated on a (1 0 0)-oriented AlGaAs/GaAs heterostructure

We have fabricated and studied ballistic one-dimensional hole quantum wires using a high mobility undoped (1 0 0)-oriented AlGaAs/GaAs heterostructure. The devices exhibit clear quantized conductance plateaus with highly stable gate characteristics. These devices provide a platform for studying one-dimensional phenomena in hole systems with minimal crystallographic effects.

Defects Responsible for the Hole Gas in Ge/Si Core−Shell Nanowires

The origin of the ballistic hole gas recently observed in Ge/Si core-shell nanowires has not been clearly resolved yet, although it is thought to be the result of the band offset at the radial interface. Here we perform spin-polarized density-functional calculations to investigate the defect levels of surface dangling bonds and Au impurities in the Si shell. Without any doping strategy, we find that Si dangling bond and substitutional Au defects behave as charge traps, generating hole carriers in the Ge core, while their defect levels are very deep in one-component Si nanowires. The defect levels lie to within 10 meV from or below the valence band edge for nanowires with diameters larger than 33 A and the Ge fractions above 30%. As carriers are spatially separated from charge traps, scattering is greatly suppressed, leading to the ballistic conduction, in good agreement with experiments.

The kinetic magnetoelectric effect in laterally boundary-confined ballistic two-dimensional hole gases

A theoretical investigation is presented on the characteristics of the kinetic magnetoelectric effect in laterally boundary-confined ballistic two-dimensional hole gases. It was shown that, though the momentum-dependent effective magnetic fields felt by charge carriers due to the spin-orbit interaction are in-plane orientated in such systems, both in-plane polarized and normal polarized nonequilibrium spin polarization densities could be electrically induced by the kinetic magnetoelectric effect, and the induced nonequilibrium spin polarizations exhibit some interesting characteristics. The characteristics we found indicate that there may be some possible relation between this effect and some recent experimental findings.

Orientation Effects in Ballistic High-Strained P-type Si Nanowire FETs

In order to design and optimize high-sensitivity silicon nanowire-field-effect transistor (SiNW FET) pressure sensors, this paper investigates the effects of channel orientations and the uniaxial stress on the ballistic hole transport properties of a strongly quantized SiNW FET placed near the high stress regions of the pressure sensors. A discrete stress-dependent six-band k.p method is used for subband structure calculation, coupled to a two-dimensional Poisson solver for electrostatics. A semi-classical ballistic FET model is then used to evaluate the ballistic current-voltage characteristics of SiNW FETs with and without strain. Our results presented here indicate that [110] is the optimum orientation for the p-type SiNW FETs and sensors. For the ultra-scaled 2.2 nm square SiNW, due to the limit of strong quantum confinement, the effect of the uniaxial stress on the magnitude of ballistic drive current is too small to be considered, except for the [100] orientation. However, for larger 5 nm square SiNW transistors with various transport orientations, the uniaxial tensile stress obviously alters the ballistic performance, while the uniaxial compressive stress slightly changes the ballistic hole current. Furthermore, the competition of injection velocity and carrier density related to the effective hole masses is found to play a critical role in determining the performance of the nanotransistors.