Light Effective Mass Research Articles

(Invited) Germanium-Tin P-Channel Field-Effect Transistor with Low-Temperature Si2H6 Passivation

Germanium-tin (GeSn) has high hole mobility and is a promising channel material for scaling the supply voltage towards 0.5 V for high performance p-channel field-effect transistors (pFETs) in the sub-10 nm technology nodes.In this paper, we discuss the research and development of using GeSn as the channel material of pFETs for enhancing the hole mobility and drive current. This includes GeSn planar pFETs, GeSn nanowire (NW) pFETs, as well as GeSn tunneling FETs (TFETs). Low-temperature Si2H6 passivation was developed to realize high-quality gate stack on GeSn. GeSn Planar pFETs with Various Sn Compositions and Surface Orientations GeSn (100)-oriented pFETs were demonstrated to have higher hole mobility than Ge pFETs [1]-[2]. With increasing Sn concentration in GeSn, the light hole effective mass at the G point is reduced [3], leading to higher hole mobilities. An important enabling process module for realization of high performance GeSn pFETs is the low-temperature Si2H6 passivation to achieve high-quality gate stack on GeSn [4]-[7], as shown in Fig. 1 (a) and (b).Further mobility enhancement can be achieved by a two-step Si2H6 passivation technique [8]. In addition, various channel surface orientations for the GeSn pFETs have been investigated [9], with the (111) orientation giving a higher hole mobility for a given Sn content in the channel. Fig. 1 (c) compares the effective hole mobilities of Si2H6-passivated GeSn pFETs with various Sn compositions and surface orientations at an inversion carrier density Ninv of 1 × 1013 cm-2. Uniaxially Compressive Strained GeSn Nanowire pFETs Besides planar GeSn pFETs, uniaxially compressive strained GeSn NW pFETs were realized using a CMOS compatible top-down approach [10] (Fig. 2). The uniaxial compressive strain leads to a significant reduction of the hole effective mass and an increase in the separation between light hole and heavy hole bands of GeSn compared to unstrained and biaxially compressive strained GeSn, which could enhance hole mobility. GeSn Tunneling FETs With increasing Sn content, the band gap of GeSn is reduced and GeSn becomes a direct band gap material. GeSn is therefore promising for the design of the tunneling junction in tunneling field-effect transistor (TFET) [11]-[12]. By incorporating Sn into Ge to shift the Г valley down and reduce the bandgap, GeSn TFETs with enhanced drive current and reduced subthreshold swing (SS) were demonstrated (Fig. 3).

Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics.

Graphene and transition metal dichalcogenides (TMDCs) are the two major types of layered materials under intensive investigation. However, the zero-bandgap nature of graphene and the relatively low mobility in TMDCs limit their applications. Here we reintroduce black phosphorus (BP), the most stable allotrope of phosphorus with strong intrinsic in-plane anisotropy, to the layered-material family. For 15-nm-thick BP, we measure a Hall mobility of 1,000 and 600 cm(2)V(-1)s(-1) for holes along the light (x) and heavy (y) effective mass directions at 120 K. BP thin films also exhibit large and anisotropic in-plane optical conductivity from 2 to 5 μm. Field-effect transistors using 5 nm BP along x direction exhibit an on-off current ratio exceeding 10(5), a field-effect mobility of 205 cm(2)V(-1)s(-1), and good current saturation characteristics all at room temperature. BP shows great potential for thin-film electronics, infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Assessment of Electron Mobility in Ultrathin-Body InGaAs-on-Insulator MOSFETs Using Physics-Based Modeling

We have investigated the electron mobility in ultrathin-body InGaAs-on-insulator devices using physics-based modeling that self-consistently accounts for quantum confinement and covers band-structure effects in ultrathin III-V layers. Scattering by nonpolar and polar acoustic and optical phonons, surface roughness, and thickness fluctuations, Coulomb and alloy disorder have been included in the calculations. The modeling, calibrated and verified on experimental data from the literature, has revealed a strong influence of thickness fluctuations caused by the light effective mass of Γ valley electrons. Our results indicate that InGaAs-on-insulator MOSFETs are more influenced by interface properties compared with silicon-on-insulator devices and outperform them only above certain body thickness that depends on interface quality.

Band energy effect on carrier velocity limit in graphene nanoribbon

Many studies on low-energy limit indicate that band energy of graphene nanoribbon research (GNR) is linear while it illustrates parabolic characteristic with square root approximation model near the Fermi point (k = 0) and non-parabolic characteristic when wave vector is far away from Fermi point . We report that in parabolic and non-parabolic conditions charge transport is controlled by the saturation velocity similar to the conventional one-dimensional device. Also, we claim that at narrower bandwidth the intrinsic velocity approaches saturation faster because wider widths have lighter effective mass, which leads to higher average velocity. The intrinsic velocity is approximated to be thermal velocity in nondegenerate regime and Fermi velocity in degenerate regime, which depends on carrier concentration. Even in very high electric field, because of quantum emission, the saturation velocity can be lower than the intrinsic velocity.

Low effective mass leading to high thermoelectric performance

High Seebeck coefficient by creating large density of state (DOS) around the Fermi level through either electronic structure modification or manipulating nanostructures, is commonly considered as a route to advanced thermoelectrics. However, large density of state due to flat bands leads to large effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of high effective mass is a lower thermoelectric figure of merit when the carriers are predominantly scattered by acoustic phonons according to the deformation potential theory of Bardeen-Shockley. We demonstrate the beneficial effect of light effective mass leading to high power factor in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction.

Doping Variation of Optical Properties in ZrNCl Superconductors

Systematic optical reflectivity measurements were performed on a series of electron-doped layered nitride Li x ZrNCl superconductors with a variable carrier density. A Drude band grew up with increasing Li doping level x , providing optical evidence of a transition from a band insulator to a metal. The doping variation of the plasma edge exhibited excellent agreement with the results of first-principles calculations. Furthermore, the good agreement among the optical, thermal, and theoretical effective masses highlights the uniqueness of the present materials, indicating that relatively high T c superconductivity with T c ∼15 K can emerge from a light effective mass m * ∼0.9 m 0 with minimal renormalization.

Terahertz electromagnetic waves emitted from semiconductor investigated using terahertz time domain spectroscopy (Invited Paper)

Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy (THz-TDS). The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material, thus, THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale. We report on the observed THz emission waveforms emitted from GaAs by using a novel technology, the time domain THz electro-optic (EO) sampling, which has a bipolar feature, i.e., an initial positive peak and a subsequent negative dip that arises from its velocity overshoot. The initial positive peak has been interpreted as electron acceleration in the bottom of ?? valley in GaAs, where electrons have a light effective mass. The subsequent negative dip has been attributed to intervalley transfer from ?? to X and L valleys. Furthermore, the power dissipation spectra of the bulk GaAs in THz range are also investigated by using the Fourier transformation of the time domain THz traces. From the power dissipation spectra, the cutoff frequency for negative power dissipation (i.e., gain) under step electric field in the bulk GaAs can also be obtained. The cutoff frequency for the gain gradually increases with increasing electric fields up to 50 kV/cm and achieves saturation at approximately 1 THz at 300 K. Furthermore, based on the temperature dependence of the cutoff frequency, we find that this cutoff frequency is governed by the energy relaxation process of electrons from L to ?? valley via successive optical phonon emission.

Angular and temperature‐related specific features of averaging of hole effective masses in p‐Ge at low temperatures

AbstractMicrowave magnetoresistance of lightly doped (nondegenerate) p‐Ge has been studied by the electron spin resonance method, which can record the derivative of the microwave absorption with respect to the magnetic field. The change in the absorption is proportional to that in the conductivity of the semiconductor in the magnetic field (magnetoresistance). It was found that the averaging time of the light and heavy holes effective masses depends on temperature and on the magnetic field direction in a sample. An analysis of the derivative made it possible to determine regions of the fastest effective mass averaging.

Scaling law and critical exponent for α0at the 3D Anderson transition

Microwave magnetoresistance of lightly doped (nondegenerate) p-Ge has been studied by the electron spin resonance method, which can record the derivative of the microwave absorption with respect to the magnetic field. The change in the absorption is proportional to that in the conductivity of the semiconductor in the magnetic field (magnetoresistance). It was found that the averaging time of the light and heavy holes effective masses depends on temperature and on the magnetic field direction in a sample. An analysis of the derivative made it possible to determine regions of the fastest effective mass averaging.

Perturbation theory analysis for electronic response of DNA base pairs

The electronic responses of duplex B-DNA sequences are investigated using perturbation theory analyses. The electronic polarizability and effective mass are computed for base pair doublets and triplets and the electronic structures are calculated according to density functional theory. High polarizability and a light effective mass are obtained in a sequence such as 5′-GGG-3′. The results indicate that the concentration of GC base pairs causes a high response of the electronic states, and molecular orbitals that are very responsive to the electric field tend to be reactive with other orbitals or conducting hole carriers. Furthermore, the interaction between the electronic states and a positively charged wavepacket is discussed, assuming hole injection in the DNA sequences. The sequence of 5′-GGG-3′ responds sensitively to the external hole approach, which is presumably injected to a guanine site.

Computational Study of Tunneling Transistor Based on Graphene Nanoribbon

Tunneling field-effect transistors (FETs) have been intensely explored recently due to its potential to address power concerns in nanoelectronics. The recently discovered graphene nanoribbon (GNR) is ideal for tunneling FETs due to its symmetric bandstructure, light effective mass, and monolayer-thin body. In this work, we examine the device physics of p-i-n GNR tunneling FETs using atomistic quantum transport simulations. The important role of the edge bond relaxation in the device characteristics is identified. However, the device has ambipolar I-V characteristics, which are not preferred for digital electronics applications. We suggest that using either an asymmetric source-drain doping or a properly designed gate underlap can effectively suppress the ambipolar I-V. A subthreshold slope of 14mV/dec and a significantly improved on-off ratio can be obtained by the p-i-n GNR tunneling FETs.

Light Effective Mass in the Widely-Dispersed Valence Band of Single Crystalline Rubrene Observed by High-Resolution Angle-Resolved Ultraviolet Photoelectron Spectroscopy

AbstractHigh-resolution angle resolved ultraviolet photoelectron spectroscopy measurements were conducted on rubrene single crystals successfully through relief of the sample charging assisted by a laser illumination. Significant dispersion of the valence band was clearly resolved. The band width W and the hole effective mass mh* were estimated to be 0.4 eV and 0.7m0, respectively, along the most conductive direction. The present results strongly suggest that the transport nature in rubrene single crystals should be described in the band transport framework of a delocalized charge carrier.

Electron tunneling in a strained n-type Si1−xGex/Si/Si1−xGex double-barrier structure

We report electrical measurements on an n-type Si1−xGex/Si/Si1−xGex double-barrier structure grown on a partially relaxed Si1−yGey buffer layer. Resonance tunneling of Δ4 band electrons is demonstrated. This is attributed to the strain splitting in the SiGe buffer layer where the Δ4 band is lowest in energy at the electrode. Since the Δ4 band electrons have a much lighter effective mass along the direction of tunneling current in comparison with that of the Δ2 band electrons, this work presents an advantage over those SiGe resonant-tunneling diodes in which tunneling of Δ2 band electrons is employed.

Study of valence intersubband absorption in tensile strained Si/SiGe quantum wells

The hole subband structures and effective masses of tensile strained Si/Si1—ZyGey quantum wells are calculated by using the 6×6 k · p method. The results show that when the tensile strain is induced in the quantum well, the light-hole state becomes the ground state, and the light hole effective masses in the growth direction are strongly reduced while the in-plane effective masses are considerable. Quantitative calculation of the valence intersubband transition between two light hole states in a 7nm tensile strained Si/Si0.55Ge0.45 quantum well grown on a relaxed Si0.5Ge0.5 (100) substrates shows a large absorption coefficient of 8400 cm-1., Si/SiGe

Excited-state relaxation in PbSe quantum dots

In solids the phonon-assisted, nonradiative decay from high-energy electronic excited states to low-energy electronic excited states is picosecond fast. It was hoped that electron and hole relaxation could be slowed down in quantum dots, due to the unavailability of phonons energy matched to the large energy-level spacings ("phonon-bottleneck"). However, excited-state relaxation was observed to be rather fast (< or =1 ps) in InP, CdSe, and ZnO dots, and explained by an efficient Auger mechanism, whereby the excess energy of electrons is nonradiatively transferred to holes, which can then rapidly decay by phonon emission, by virtue of the densely spaced valence-band levels. The recent emergence of PbSe as a novel quantum-dot material has rekindled the hope for a slow down of excited-state relaxation because hole relaxation was deemed to be ineffective on account of the widely spaced hole levels. The assumption of sparse hole energy levels in PbSe was based on an effective-mass argument based on the light effective mass of the hole. Surprisingly, fast intraband relaxation times of 1-7 ps were observed in PbSe quantum dots and have been considered contradictory with the Auger cooling mechanism because of the assumed sparsity of the hole energy levels. Our pseudopotential calculations, however, do not support the scenario of sparse hole levels in PbSe: Because of the existence of three valence-band maxima in the bulk PbSe band structure, hole energy levels are densely spaced, in contradiction with simple effective-mass models. The remaining question is whether the Auger decay channel is sufficiently fast to account for the fast intraband relaxation. Using the atomistic pseudopotential wave functions of Pb(2046)Se(2117) and Pb(260)Se(249) quantum dots, we explicitly calculated the electron-hole Coulomb integrals and the P-->S electron Auger relaxation rate. We find that the Auger mechanism can explain the experimentally observed P-->S intraband decay time scale without the need to invoke any exotic relaxation mechanisms.

A surface-gated InSb quantum well single electron transistor

Single electron charging effects in a surface-gated InSb/AlInSb QW structure are reported. This material, due to its large g-factor and light effective mass, offers considerable advantages over more commonly used materials, such as GaAs, for quantum information processing devices. However, differences in material and device technology result in significant processing challenges. Simple Coulomb blockade and quantized confinement models are considered to explain the observation of conductance oscillations in these structures. The charging energy (e2/C) is found to be comparable with the energy spectrum for single particle states (ΔE).

Tuning the electronic structure of the transparent conducting oxide Cu2O

The electronic structure of Cu2O is important for its application as a p-type transparent conducting oxide (TCO). To be useful as a TCO, a material needs to show enhanced transparency in the visible range (band gap >3 eV) as well as good conduction properties. While Cu2O has too small a band gap, alloys of Cu2O and Al2O3 or Cu2O and alkaline earth oxides are known to display enhanced transparency, with little degradation of electrical properties. It is of interest to consider how to dope Cu2O p-type, e.g. Cu vacancies (oxidation) or cationic dopants. We present a study of the electronic structure and effective hole masses of stoichiometric and oxidised Cu2O and study metal cation doping, using density functional theory (DFT), to analyse p-type doping scenarios. We show that formation of a Cu vacancy is relatively facile, introducing delocalised hole states, with a light hole present. Substitutional cation doping with Al and Au/Ag is found to decrease the band gap but maintains a light hole effective mass necessary for p-type conduction.

Numerical Analysis of Thermoelectric Properties on Bi88Sb12

The large maxima of thermopower as functions of the temperature observed for Bismuth Antimony alloy is studied theoretically based on the Boltzmann transport theory of single carrier. The Fermi statistics is fully taken into account, with the chemical potential being calculated self-consistently at each temperature from experimental Hall coefficients. It stays constant at lower temperatures, but starts to rise at T ¼ 70 K, where the maximum of the thermopower is observed. The temperature dependence of the calculated thermopower agrees well with the measurements. The occurrence of the maxima is thus shown to be associated with the extrinsic-to-intrinsic transition, thus providing theoretical confirmation to the known empirical rule. tentative evaluation of the chemical potential � ðTÞ in the semi-classical treatment, and found that it increases rapidly with increasing temperature for T > 70 K, whereas it increases only slowly for T < 70 K. Then it is applied the Boltzmann transport theory of single carrier using the calculated chemical potentials, assuming only one scattering parameter for the whole temperature range. This simple treatment was able to reproduce qualitatively the observed temperature dependences of all the measured quantities, and it is concluded the change in the electronic configuration and not the change in the scattering mechanism, which is responsible for the characteristic temperature dependence of the transport quantities. The Bi100� xSbx alloys are characterized by the narrow band gap and the light effective mass of the conduction electrons. The increase of the chemical potential from relatively low temperatures is therefore plausible. However it implies that the system must be recognized as a strongly degenerate semiconductor. The above preliminary calcula- tion has practically revealed that the chemical potential rise into the conduction band, so that this classical treatment is inadequate. It is seemed that the extrinsic-to-intrinsic transition is related to high thermoelectricity of the materials. In this paper, it is shown only the results of the polycrystal- line Bi88Sb12. The results will be basically the same for other antimony concentrations of x ¼ 8{17.

Effective masses and complex dielectric function of cubic HfO2

The electronic band structure of cubic HfO2 is calculated using an ab initio all-electron self-consistent linear augmented plane-wave method, within the framework of the local-density approximation and taking into account full-relativistic contributions. From the band structure, the carrier effective masses and the complex dielectric function are obtained. The Γ-isotropic heavy and light electron effective masses are shown to be several times heavier than the electron tunneling effective mass measured recently. The imaginary part of the complex dielectric function ϵ2(ω) is in good agreement with experimental data from ultraviolet spectroscopic ellipsometry measurements in bulk yttria-stabilized HfO2 as well as with those performed in films deposited with the tetrakis diethylamido hafnium precursor for energies smaller than 9.5eV.

Polariton laser and polariton superfluidity in microcavities

Excitons coupled to light in microcavities form exciton–polaritons (polaritons) having very light effective masses. These 2-dimensional quasiparticles exhibit local condensation or Kosterlitz–Thouless phase transition towards superfluidity, allowing for polariton lasing at temperatures that could be higher than 300 K. Polariton lasers are devices producing monochromatic and coherent light spontaneously emitted by Bose-condensed exciton–polaritons. They do not require inversion of population and, theoretically, have no threshold. We show great advantages of cavity polaritons with respect to excitons to achieve bosonic phase transition. We review experimental and theoretical activity that has been developed in this research field since the discovery of strong exciton-light coupling in microcavities 10 years ago. We present basic thermodynamic properties of polariton systems modeled as a weakly interacting Bose gas at equilibrium. We present an analysis of the relaxation kinetics of cavity polaritons in the framework of a semiclassical Boltzmann equation. The main obstacle that prevents polariton condensation is their finite lifetime: polaritons disappear before they have time to condense. We discuss the ways to improve their relaxation kinetics in real structures.