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Acoustic charge transport induced by the surface acoustic wave in chemical doped graphene
Applied Physics Letters
Abstract
A graphene/LiNbO3 hybrid device is used to investigate the acoustic induced charge transport in chemical doped graphene. The chemical doping of graphene via its physisorption of gas molecules affects the surface acoustic wave (SAW) charge carrier transport in a manner different from electric field drift. That transport induces doping dependent macroscopic acoustoelectric current. The chemical doping can manipulate majority carriers and induces unique acoustoelectric features. The observation is explained by a classical relaxation model. Eventually the device based on acoustoelectric current is proved to outperform the common chemiresistor for chemicals. Our finding provides insight into acoustic charge carrier transport during chemical doping. The doping affects interaction of carriers with SAW phonon and facilitates the understanding of nanoscale acoustoelectric effect. The exploration inspires potential acoustoelectric application for chemical detection involving emerging 2D nanomaterials.
... The AE effect has been studied in carbon nanotube quantum dots [33,34], CdS [35], two-dimensional electron systems (2DES) such as GaAs/Ga 1−x Al x As [36], and LaAlO 3 and SrTiO 3 interface [37], quasi-one-dimensional channel in a GaAs-AlGaAs heterostructure by a split-gate electrode [38], and broadly explored in graphene-based devices [39][40][41][42][43][44][45]. It has been demonstrated that direct control of the flow of charge carriers is possible by utilizing the field accompanying the SAW propagation. ...
... With graphene as the semiconducting material, in addition to studying the AE effect via two terminal electrodes [39][40][41], various graphene-based acousto-transistor studies [42,43,46,47] have also been done, confirming an increased current flow with SAW in a field-effect transistor (FET) structure as well. A similar observation, as discussed above, of the linear dependence of the AE current on SAW power was demonstrated, as shown in figures 9 and 10. ...
... Amongst literature on acoustic interactions with semiconductors, the majority of work has been done involving inorganic semiconductors such as GaAs, graphene, Si, WSe 2 , and DQWs such as those based on GaAs/AlGaAs, which require high-end cleanroom nanofabrication facilities. The generation of the AE current [20] has been demonstrated in carbon nanotube quantum dots [33], quasi-one-dimensional GaAs [38], 2DES [36,37], and has been elaborately studied in graphene-based devices [39][40][41][42][43][44][45]. Mostly, these materials have studied the implications of the piezoelectric field, established with SAW, on charge carrier transport. ...
This review article presents insights into acoustic interactions with semiconductors, exploring a continuum from electron dynamics to exciton behavior while highlighting recent developments in organic material systems. Various aspects of acoustic interactions, encompassing the manipulation of electrons and their transport mechanisms for applications in the fields of acoustoelectric and acousto-optics, explored by studying surface acoustic wave (SAW) devices integrated with inorganic and organic semiconductors, are presented here. SAWs are guided waves propagating along a piezoelectric material surface, inducing acoustic strain and piezoelectric fields within a semiconductor upon contact. These fields create a dragging force, transferring energy and momentum into the semiconductor, which manipulate and transport charge carriers, thereby generating an acoustoelectric current. Furthermore, SAW can influence exciton dynamics via type-II as well as type-I band-edge modulations, leading to alterations in their spatial distribution, causing transport of electron–hole pairs as distinct charge carrier packets and as bound pairs, respectively, along the SAW path. This paper explores advancements in these phenomena, shedding light on innovative applications and, especially, novel insights into the dynamic interplay between acoustics and organic semiconductor physics. The review concludes by outlining challenges and prospects in the field of SAW and semiconductor interactions, providing a roadmap for future research endeavors.
... The effect has been observed in different classes of materials such as semiconductors, quantum wires, two-dimensional (2D) electron gas, and heterostructures [3][4][5][6][7][8][9][10][11]. More recently, it has been recognized that the coupling between the surface acoustic wave (SAW) and electrons in 2D Dirac materials provides an exciting opportunity to investigate charge transport driven by the strain fields associated with the propagating SAW [12][13][14][15][16][17][18][19]. In particular, the AE effect of single-layer graphene has been investigated experimentally, and the AE current has been shown to be tunable by the application of a gate voltage [18]. ...
... Despite many studies on the piezoelectric mechanism of the AE effect in a 2D electron gas and in graphene [12][13][14][15][16][17][18][19] the analysis of the acoustoelectric effect in 2D hexagonal Dirac materials is not yet complete. In particular, the relevance of the gauge phonon for the AE effect in graphene has not been discussed to the best of our knowledge. ...
... The rectified acoustoelectric current in response to the selfconsistent scalar potential reads j AE a = χ (2) ann (Q, , −Q, − )V sc (Q, )V sc (−Q, − ), (17) where χ (2) ann is a current-density-density correlation function. The self-consistent potential is given as a summation of bare external potentials and the induced one owing to the long-range Coulomb interaction, i.e., the screening effect. ...
Using a diagrammatic scheme, we study the acoustoelectric effects in two-dimensional (2D) hexagonal Dirac materials due to the sound-induced pseudogauge field. We analyze both uniform and spatially dispersive currents in response to copropagating and counterpropagating sound waves, respectively. In addition to the longitudinal acoustoelectric current, we obtain an exotic transverse charge current flowing perpendicular to the sound propagation direction owing to the interplay of transverse and longitudinal gauge field components jT∝ALAT*. In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component jT∼sin(6θ) is achieved that stems from the inherited threefold symmetry of the hexagonal lattice. However, both longitudinal and transverse parts of the dispersive current are predicted to be strongly anisotropic ∼sin2(3θ) or cos2(3θ). We quantitatively estimate the pseudogauge field contribution to the acoustoelectric current that can be probed in future experiments in graphene and other 2D hexagonal Dirac materials.
... To demonstrate the special dependence of the AE current on the electric conductivity, the graphene carrier density has been modulated by several means, e.g. photoexcitation of electron-hole pairs [80,81] and chemical doping [82]. Figure 10 shows an example of the last case, where a p-doped graphene layer was exposed to different concentrations of nitric acid (NO 2 ) vapor. ...
... As a consequence, the amplitude of the conventional current reduced, while the AE current increased with the concentration of NH 3 , in agreement with σ approaching σ M (not shown here). Remarkably, for low gas concentrations, the relative response changes of the AE current outperformed that of the common drift current [82], thus demonstrating the potential application of graphenebased acousto-electric devices for chemical detection. ...
... Amplitude of the AE current (blue points) and conventional direct current (red points) measured under exposition of graphene to several concentrations of a NO 2 gas dopant. Reprinted from[82], with the permission of AIP Publishing. ...
This article reviews the main theoretical and experimental advances regarding the interaction between surface acoustic waves (SAWs) and electronic excitations in graphene. The coupling of the graphene electron gas to the SAW piezoelectric field can modify the propagation properties of the SAW, and even amplify the intensity of SAWs traveling along the graphene layer. Conversely, the periodic electric and strain fields of the SAW can be used to modify the graphene Dirac cone and to couple light into graphene plasmons. Finally, SAWs can generate acousto-electric currents in graphene. These increase linearly with the SAW frequency and power but, in contrast to conventional currents, they depend non-monotonously on the graphene electric conductivity. Most of these functionalities have been reported in graphene transferred to the surface of strong piezoelectric insulators. The recent observation of acousto-electric currents in epitaxial graphene on SiC opens the way to the large-scale fabrication of graphene-based acousto-electric devices patterned directly on a semi-insulating wafer.
... use in graphene-based acoustoelectric (AE) devices [3,4]. In earlier experimental [5][6][7][8][9][10][11][12][13][14][15][16][17] and theoretical [18][19][20][21] works (for their review, see [22]), the AE response of large-area (micronscale) mono-and multilayer graphene sheets was studied. Experiments by Bandhu and Nash [12] demonstrated that a voltage applied to a hybrid graphene-lithium-niobate device using an ion-gel gate can change the doping of the graphene from p-type to n-type, thereby changing the direction of the AE current in the device to the opposite. ...
... In contrast to the graphene systems explored in the above cited papers [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21], in the narrow GNRs of interest to us here, the electron system is quasi-one-dimensional due to the lateral confinement of electron motion. This leads to size quantization of the electron energy spectrum near the Dirac points K and K ′ and, as a consequence, to the opening of a finite band gap at these points. ...
A theory is presented for the acoustoelectric (AE) effect in narrow graphene nanoribbons (GNRs) deposited on a piezoelectric substrate when a surface acoustic wave (SAW) is launched onto the surface of the piezoelectric. It is assumed that the electron density in such GNRs can be controlled by applying an external gate voltage, and the acoustic wavelength is much smaller than the mean free path of electrons, so that the quantum mode of interaction of those electrons with the SAW in realized. Using the kinetic theoryapproach, we calculate the AE current flowing through the GNR sample and arising as a result of momentum transfer from coherent SAW phonons to conduction electrons. It is shown that size quantization of the electron energy spectrum in narrow GNRs leads to giant periodic oscillations of the AE current with a change in the gate voltage. AE current surges occur whenever the Fermi level, rising with increasing gate voltage, crosses in turn the bottom of each of the successive size-quantized electron energy subbands. The oscillations predicted are giant in the sense that the maximum values of the current exceed its minimum values by at least an order of magnitude, and they can be observed in narrow GNRs ∼ 10 nm wide even at room temperature.
... The effect has been observed in different classes of materials such as semiconductors, quantum wires, two-dimensional (2D) electron gas, and heterostructures [3][4][5][6][7][8][9][10][11]. More recently, it has been recognized that the coupling between the surface acoustic wave (SAW) and electrons in 2D Dirac materials provides an exciting opportunity to investigate charge transport driven by the strain fields associated with propagating SAW [12][13][14][15][16][17][18][19]. In particular, the AE effect of single-layer graphene has been investigated experimentally, and the AE current has been shown to be tunable by the application of a gate voltage [18]. ...
... Despite many studies on the piezoelectric mechanism of the AE effect in the 2D electron gas and in graphene [12][13][14][15][16][17][18][19] the analysis of the acoustoelectric effect in 2D Dirac materials is not yet complete. In particular, the relevance of gauge phonon for the AE effect in graphene has not been discussed to the best of our knowledge. ...
Using a diagrammatic scheme, we study the acoustoelectric effects in 2D Dirac materials due to the sound-induced pseudo-gauge field. We analyze both uniform and {\em spatially dispersive} currents in response to co-propagating and counter-propagating sound waves, respectively. In addition to the longitudinal acoustoelectric current, we obtain an exotic {\em transverse} charge current flowing perpendicular to the sound propagation direction owing to the interplay of transverse and longitudinal gauge field components . In contrast to the almost isotropic directional profile of the longitudinal uniform current, a highly anisotropic transverse component is achieved that stems from the inherited three-fold symmetry of the hexagonal lattice. However, both longitudinal and transverse parts of the dispersive current are predicted to be strongly anisotropic or . We quantitatively estimate the pseudo-gauge field contribution to the acoustoelectric current that can be probed in future experiments in graphene and other 2D Dirac materials.
... One of the unique electronic properties of graphene, the ability to tune its conductivity over orders of magnitude, also allows the AE current to be tuned dynamically [26][27][28][29][30], where the ambipolar nature of graphene allows the sign of the AE current to be reversed [26][27][28][29]. In addition, using an ion gel gate to modulate the conductivity of graphene, Bandhu and Nash [30] not only reversed the direction of the current, but also demonstrated that graphene can be used to control the SAW amplitude and velocity. ...
... One of the unique electronic properties of graphene, the ability to tune its conductivity over orders of magnitude, also allows the AE current to be tuned dynamically [26][27][28][29][30], where the ambipolar nature of graphene allows the sign of the AE current to be reversed [26][27][28][29]. In addition, using an ion gel gate to modulate the conductivity of graphene, Bandhu and Nash [30] not only reversed the direction of the current, but also demonstrated that graphene can be used to control the SAW amplitude and velocity. ...
The acoustoelectric current in graphene nanoribbons, with widths ranging between 350 nm and 600 nm, has been investigated as a function of illumination. For all nanoribbon widths, the acoustoelectric current was observed to decrease on illumination, in contrast to the increase in acoustoelectric current measured in unpatterned graphene sheet devices. This is thought to be due to the higher initial conductivities of the nanoribbons compared to unpatterned devices.
... Owing to its unique electronic properties, graphene may turn out to be a new promising material for the designs of modern acoustoelectronic devices operating on ultrahigh-frequency surface acoustic waves (SAWs). At present, in the literature there are a number of experimental [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] and theoretical [16][17][18][19][20][21][22][23][24] works devoted to this important topic (for a review of those published until 2018, see [25]; for a review of more recent work, see [26]). Among them, there are several * Author to whom any correspondence should be addressed. ...
A theory is presented for the amplification of a surface acoustic wave (SAW) due to its interaction with conduction electrons in gate-controlled epitaxial graphene (epigraphene) on a SiC substrate. It is assumed that the SAW is launched onto the substrate in the direction of an external dc electric field applied to the graphene sample and causing the conduction electrons to drift at a speed greater than the speed of the SAW. The wavelength of the SAW is assumed to be shorter than the mean free path of the electrons, so that the quantum regime of interaction of those electrons with the SAW is realized. The Green’s function method is used to calculate the SAW gain as a function of the electron concentration in epigraphene and the external dc electric field strength. It is shown that the substrate-induced band gap in the electronic spectrum of epigraphene leads to a significant (at least an order of magnitude) increase in the SAW gain as compared to the case of gapless graphene. In addition, the opening of the band gap results in a non-monotonic dependence of the SAW gain on the electron concentration, controlled by the gate voltage applied to the graphene sample. This dependence is characterized by the presence of a distinct maximum at a certain value of electron concentration (of about 2×1012 cm⁻² for typical values of the other parameters involved), which distinguishes it from the monotonic concentration dependence of the SAW gain in gapless graphene.
... There exist two primary techniques to induce changes in conductivity in a vdW material on top of a piezoelectric substrate: chemical doping and the application of a tunable gate voltage [80, 82-85, 88-90, 92, 93, 96, 109]. The investigation of chemical doping has revealed that the AE effect exhibits remarkable sensitivity in gas sensing and mass loading applications [84,92,96,109]. Conversely, gating vdW materials is a more promising avenue for applications in transistors and quantum transport. ...
Surface acoustic waves, the microcosmic cousins of seismic waves, can be generated and precisely controlled on a microscopic scale by applying a periodic electrical signal to a piezoelectric substrate. Harnessing and exploring their interactions with two-dimensional van der Waals (vdW) systems opens new frontiers in materials science and engineering. As part of a special issue on these guided elastic waves for hybrid nano- and quantum technologies, our review highlights work focusing on acoustically-induced transport phenomena at low temperatures that arise from the interaction between the surface acoustic waves in a piezoelectric substrate and a vdW material on its surface. A main focus is on technological methods to control the carrier concentration in transport and strain-related effects that can act on the carrier motion as an effective magnetic field.
... [37][38][39] In addition to their demonstration as an efficient means for the synthesis and manipulation of 2D transition metal dichalcogenides and carbides/nitrides, [40][41][42][43][44][45] SAWs have also been shown to facilitate the control and manipulation of charge carriers in nanostructures. [46][47][48] Broadly, SAWs have been used to either (i) manipulate photoexcited charge carriers in 2D semiconductors, to prevent their recombination and therefore allow their transfer over long distances (Fig. 1a), [49][50][51] or, (ii) to enhance the electrical output in non-photoexcited 2D systems (Fig. 1b). 46,[52][53][54][55][56][57][58][59][60][61] The latter effect arises as a consequence of the native SAW electromechanical coupling that facilitates interaction of the wave with the mobile carriers. ...
Two-dimensional (2D) layered metal dichalcogenides constitute a promising class of materials for photodetector applications due to their excellent optoelectronic properties. The most common photodetectors, which work on the principle of photoconductive or photovoltaic effects, however, require either the application of external voltage biases or built-in electric fields, which makes it challenging to simultaneously achieve high responsivities across broadband wavelength excitation - especially beyond the material's nominal band gap - while producing low dark currents. In this work, we report the discovery of an intricate phonon-photon-electron coupling - which we term the acoustophotoelectric effect - in SnS that facilitates efficient photodetection through the application of 100-MHz-order propagating surface acoustic waves (SAWs). This effect not only reduces the band gap of SnS, but also provides the requisite momentum for indirect band gap transition of the photoexcited charge carriers, to enable broadband photodetection beyond the visible light range, whilst maintaining pA-order dark currents - remarkably without the need for any external voltage bias. More specifically, we show in the infrared excitation range that it is possible to achieve up to eight orders of magnitude improvement in the material's photoresponsivity compared to that previously reported for SnS-based photodetectors, in addition to exhibiting superior performance compared to most other 2D materials reported to date for photodetection.
... Lee et al [17] developed a graphene acoustoelectric switch by controlling the digitised radio frequency (RF) power applied to two IDTs orthogonally placed. Zheng et al [18] manipulated the conductivity of graphene by chemical doping and explained the charge transport using a simple classical relaxation model. Theoretical studies have also been conducted by Thalmeier et al [19] and Zhang et al [20], correlating the changes in SAW oscillations to the conductivity of graphene. ...
We demonstrate the acoustic charge transport of optically induced excitons in two organic semiconductors, P3HT and MEH-PPV, up to a distance of 3 mm. The device consists of a surface acoustic wave (SAW) resonator transmitting SAW through a polymer layer where acoustic charge transport takes place and a polymer diode at the end to collect the charges. The voltage excitation is provided using an interdigital transducer (IDT) on a piezoelectric YZ lithium niobate substrate producing Rayleigh SAW at 42 MHz. Optical illumination up to 15 mW/cm2 intensity is applied to induce excitons in the polymer layer deposited on the lithium niobate substrate. The photogenerated excitons in the polymer are ionized by SAW field resulting in free carriers that are transported to the polymer diode by the travelling SAW. A surge in photovoltaic current in the diode is observed in the presence of SAW when the carriers are optically generated away from the diode. The maximum charge capacity and transfer efficiency of the acoustic transport are calculated for various SAW power and illumination intensities. A theoretical analysis of charge carrier dynamics in the presence of a moving SAW field is also performed using a semi-classical Hamiltonian of the system.
... LiNbO 3 SAW devices also have found their usage in solid state physics, for example, SAW-driven quantized charge transport [53,54], the use of SAWs to control phonon angular momentum [55], the strong optomechanical coupling of individual quantum emitters and a surface acoustic wave [56], and quantum control of surface acoustic-wave phonons [57]. In integrated photonic systems, LiNbO 3 SAW resonators can be used to confine surface phonons [58]. ...
Lithium niobate (LiNbO3) crystals are important dielectric and ferroelectric materials, which are widely used in acoustics, optic, and optoelectrical devices. The physical and chemical properties of LiNbO3 are dependent on microstructures, defects, compositions, and dimensions. In this review, we first discussed the crystal and defect structures of LiNbO3, then the crystallization of LiNbO3 single crystal, and the measuring methods of Li content were introduced to reveal reason of growing congruent LiNbO3 and variable Li/Nb ratios. Afterwards, this review provides a summary about traditional and non-traditional applications of LiNbO3 crystals. The development of rare earth doped LiNbO3 used in illumination, and fluorescence temperature sensing was reviewed. In addition to radio-frequency applications, surface acoustic wave devices applied in high temperature sensor and solid-state physics were discussed. Thanks to its properties of spontaneous ferroelectric polarization, and high chemical stability, LiNbO3 crystals showed enhanced performances in photoelectric detection, electrocatalysis, and battery. Furthermore, domain engineering, memristors, sensors, and harvesters with the use of LiNbO3 crystals were formulated. The review is concluded with an outlook of challenges and potential payoff for finding novel LiNbO3 applications.
... Liang et al. 48 , Zheng et al. 49 and Okuda et al. 50 reported similar behavior, where acoustic charge transportation was induced by SAW propagation in the graphene. ...
We perform self-consistent analysis of the Boltzmann transport equation for momentum and energy in the hypersound regime i.e., q l ≫ 1 ( q q is the acoustic wavenumber and l is the mean free path). We investigate the Landau damping of acoustic phonons ( LDOAP LDOAP ) in graphene nanoribbons, which leads to acoustoelectric current generation. Under a non-quantized field with drift velocity, we observed an acoustic phonon energy quantization that depends on the energy gap, the width, and the sub-index of the material. An effect similar to Cerenkov emission was observed, where the electron absorbed the confined acoustic phonon energy, causing the generation of acoustoelectric current in the graphene nanoribbon. A qualitative analysis of the dependence of the absorption coefficient and the acoustoelectric current on the phonon frequency is in agreement with experimental reports. We observed a shift in the peaks when the energy gap and the drift velocity were varied. Most importantly, a transparency window appears when the absorption coefficient is zero, making graphene nanoribbons a potential candidate for use as an acoustic wave filter with applications in tunable gate-controlled quantum information devices and phonon spectrometers.
... 2bshows the dependence of Γ q maximum amplification is obtained at Γ q = −0.16 at q = 2 . It is interesting to note that, our results is in good agreed with the work in ref.[27] where acousticphonon frequencies above 10 were attained. The field in the material can be 11.5 / . ...
We perform self-consistent analysis of the Boltzmann transport equation for momentum and energy in the hypersound regime i.e., ql >> 1 ( q is the acoustic wavenumber and l is the mean free path). Here, we investigate Landau damping of acoustic phonons (LDOAP) in graphene nanoribbon that leads to acoustoelectric current generation. Under a non-quantized field with drift velocity, we observed an acoustic phonon energy quantization which depends on the energy gap, the width and the sub-index of the material. An effect similar to Cerenkov emission was observed where the electron absorbs the confined acoustic phonons energy, causing the generation of acoustoelectric current in Graphene Nanoribbon. A qualitative analysis of the absorption and versus phonon frequency is in agreement with experimental reports. We observed a shift in the peaks when the energy gap and the drift velocity were varied. Most importantly, a transparency window appears when making graphene nanoribbon a potential candidate as an acoustic wave filter with applications in phonon spectrometers and also as tunable gate-controlled quantum information device.
... The SAW (more precisely Rayleigh wave) is an elastic wave traveling along the surface of piezoelectric substrate and is generated by applying an ac voltage to comb-shaped electrodes, as illustrated in Fig. 1A. The excited SAW can be coupled with the lattice system of thin films on the substrate, resulting in the modulation of electronic properties of the thin films (16)(17)(18)(19)(20)(21)(22). ...
We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we propose that soliton-antisoliton pairs in the charge density wave of NbSe modulated by the SAW serve as a time-dependent capacitance in the superconducting state, leading to the dc negative resistance. The present experimental result would provide a previously unexplored way to examine nonequilibrium manipulation of the superconductivity.
... The SAW (more precisely Rayleigh wave) is an elastic wave traveling along the surface of piezoelectric substrate and is generated by applying an ac voltage to comb-shaped electrodes, as illustrated in Fig. 1A. The excited SAW can be coupled with the lattice system of thin films on the substrate, resulting in the modulation of electronic properties of the thin films (16)(17)(18)(19)(20)(21)(22). ...
We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe 2 thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we propose that soliton-antisoliton pairs in the charge density wave of NbSe 2 modulated by the SAW serve as a time-dependent capacitance in the superconducting state, leading to the dc negative resistance. The present experimental result would provide a previously unexplored way to examine nonequilibrium manipulation of the superconductivity.
... The AE effect is at its strongest when a 2D semiconduct ing layer with high mobility and low carrier density is placed in close proximity with a high K 2 acoustic wave [14][15][16][17]. Fundamental investigations on AE have focused on surface acoustic waves (SAW, or Rayleigh waves) in the GaAs/AlGaAs heterostructure due to the intrinsic piezoelectricity of GaAs and the high quality of the modulation doping induced 2DEG [18,19]. ...
We report on the acoustoelectric (AE) interaction in heterogeneously integrated thin-film lithium niobate on standard resistivity and high resistivity silicon substrates (LNOSs). The monolithic LNOS platform delivers acoustic waves with a large electromechanical coupling coefficient (K ²) and draws on standard metal-oxide semiconductor field effect techniques to maximise AE interaction by impedance matching the acoustic wave with the semiconductor carriers. Preliminary results are obtained on AE attenuation (4 dB cm⁻¹) and AE gain (6 dB cm⁻¹). With further improvement of the LN/Si interface, the LNOS platform can be expected to give rise to an era of non-reciprocal silicon acoustoelectronics.
... A graphene/LiNbO 3 hybrid device was used to investigate the acoustic induced charge transport in chemically doped graphene (Zheng et al. 2016). The chemical doping of graphene via physisorption of gas molecules affected the piezoelectric surface acoustic wave charge carrier transport in a manner different from that of electric field drift. ...
... As σ BP increases with a negative gate bias from −3 V to −9 V, j or gradually decreases. The inverse dependence of j or on σ BP is attributed to the partial screening of the acousto-electric field when σ BP σ M [20]. σ M is a characteristic conductivity as the few-layer BP is placed on the BN in our system which is approximately given by σ M = υε 0 · (ε γ + 1) = 1.8 × 10 −7 S according to Weinreich relations where ε 0 = 8.85 × 10 −12 F/ m is the permittivity of free space, ε γ is relative dielectric constant of BN, υ is the speed of the acoustic wave and approximately 3990 m/s [22]. ...
... The possibility of charge transport by SAWs in graphene and the possibility of monitoring a SAW under an electric potential applied to graphene were reported in a number of works. [7][8][9][10][11][12][13][14] The SEM method can be used to study the SAW propagation in piezoelectric crystals. This method enables the visualization of acoustic wave propagation through registration of low-energy secondary electrons sensitive to electric fields accompanying the SAW propagation in piezoelectric crystals (the electric fields between SAW minima and maxima). ...
Acoustically stimulated charge transport in solids was studied using the scanning electron microscopy method (SEM). The surface acoustic wave on the surface of the YZ-cut of a LiNbO3 crystal was visualized by registration of low-energy secondary electrons in SEM, and the charge distribution on the crystal surface was visualized using the electron beam induced current method. To register the induced current, an interdigital transducer structure was formed from graphene on the crystal surface. It was shown that the charge distribution on the crystal surface corresponds to the distribution of the acoustic wave field on the crystal surface.
... Details of the preparation can be found in our previous work. 19 Typically, surface acoustic waves penetrate into the material to a depth comparable to the acoustic wavelength. Therefore, to keep energy from leaking into the silicon substrate, thicker AlN or smaller wavelength is preferred. ...
We fabricated a hybrid on-chip acousto-electric (AE) and field-effect device to investigate the modulation of acoustic carrier transportation by gate voltage. The device fabrication exploited a surface micromachining aluminum nitride process on a silicon wafer, facilitating an integration of a surface acoustic wave(SAW) delay line and a graphene field-effect transistor. The SAW device induced an AEcurrent in graphene, which scales linearly with the input power and remains essentially constant when subtracting the offset current at different DC biases. At a constant DC bias, the AEcurrent can be modulated by the gate voltage, due to the change of the carrier mobility in graphene. A four-fold enhancement in the AEcurrent was realized when ∼35 V voltage was applied to the gate electrode. The highly integrated device proves to be a powerful tool to understand the AEcurrent in graphene, and since it supports integration for versatile functionality, it opens an avenue to explore the properties of diverse nanomaterials.
... These have been measured in micron-scale graphene monolayers 7,8 , and we have previously studied the acoustoelectric response of large-area graphene sheets produced by chemical vapour deposition (CVD) transferred to LiNbO 3 SAW devices at room temperature 9 , low temperature 10 , and under illumination 11 . Theoretical and experimental reports have examined DC-biased mono-and multi-layer graphene sheets for SAW amplification 12,13 , while others have studied the use of electrolytic solutions 14 , and gases 15 to control the carrier concentration in graphene films, opening up possibilities for low-power chemical sensors. In contrast, by using an ion gel gate to modulate the conductivity of graphene, Bandhu and Nash 16 demonstrated that graphene can be used to control the SAW amplitude and velocity shift. ...
Surface acoustic waves (SAWs) propagating on piezoelectric substrates offer a convenient, contactless approach to probing the electronic properties of low-dimensional charge carrier systems such as graphene nanoribbons (GNRs). SAWs can also be used to transport and manipulate charge for applications such as metrology and quantum information. In this work, we investigate the acoustoelectric effect in GNRs, and show that an acoustoelectric current can be generated in GNRs with physical widths as small as 200 nm at room temperature. The positive current in the direction of the SAWs, which corresponds to the transportation of holes, exhibits a linear dependence on SAW intensity and frequency. This is consistent with the description of the interaction between the charge carriers in the GNRs and the piezoelectric fields associated with the SAWs being described by a relatively simple classical relaxation model. Somewhat counter-intuitively, as the GNR width is decreased, the measured acoustoelectric current increases. This is thought to be caused by an increase of the carrier mobility due to increased doping arising from damage to the GNR edges.
While electronics sits at the forefront of logic processing and data communication in microprocessors, the ever-increasing trend of on-chip power density has rendered the sustainability of device scaling questionable. This has also predominantly inspired the exploration of alternative pathways for energy-efficient data communication and information processing. Excitonics, not only offers a viable alternative for low-power logic processors but also, low-footprint communication. At room temperature, excitons only exist in a handful of semiconducting media - organic materials and recently discovered transition metal dichalcogenides (TMD) monolayers. While the room temperature existence makes it technologically relevant, controlling the exciton dynamics remains a challenge. The thesis addresses the challenge by exploring strain-based approaches. We first investigate the uniaxial strain modulated photophysics in an organic excitonic guest: host blend Alq3: DCM, where the excited state properties are governed by a phenomenon called solid-state solvation. We utilize buckled SiO2 microbeams to apply axial strain on an overlying host: guest thin film and tune the excited state photoluminescence from the guest molecules. We further verify that the observed spectral shift indeed originates from modulation in local dielectric polarization triggered by the molecular density under axial strain. We also demonstrate dynamic tuning of the molecular fluorescence by electrostatic actuation of a Si3N4 microbeam fabricated on a Si3N4/Si substrate. Later, we study piezoelectric modulation of excitonic photoluminescence in a monolayer semiconductor at room temperature under strong dielectric screening. As an archetype system, we study the photoluminescence spectral modulation in monolayer WSe2 transferred on piezoelectric LiNbO3 substrate. Strong screening from the substrate leads to enhanced exciton dissociation and electric field-dependent shift of exciton emission energy – both can be further tuned by optically generated free carrier density. Finally, we investigate exciton transport under traveling strain in monolayer WSe2 encapsulated by bulk h-BN (h-BN/WSe2/h-BN). We generate high-frequency Rayleigh type SAW on piezoelectric Y-cut LiNbO3 substrate by patterning interdigitated electrodes (IDTs). Traveling strain generated by the SAW wave creates out-of-phase modulation of the monolayer energy bands - also known as the Type – I modulation. We also demonstrate acoustic steering of the photogenerated exciton density by precision control over the instantaneous SAW phase using phase synchronized time-correlated single photon counting scheme (TCSPC). We find that the acoustic transport of excitons in monolayer TMDs at room temperature is limited by the intrinsic exciton mobility and the spatial extent of the monolayer. Lower intrinsic mobility results in the ‘weak coupling’ of the photo-generated excitons to the traveling strain wave.
This work reports for the first time acoustoelectric charge transport in an organic semiconductor, regioregular poly(3-hexylthiophene), (rrP3HT) deposited as thin films on surface acoustic wave (SAW) devices generating shear horizontal and Rayleigh SAW. With the propagation of SAW, an enhancement in current was observed, especially when the applied bias field was in the direction of wave propagation. The momentum and energy transfer from SAW helped to reduce charge trapping at trap states and increase the free charge density, assisting the drift current. The mechanical and electrical effects of the acoustic wave were independently studied by using a non-piezoelectric substrate and by screening the mechanical deformations of SAW, respectively, in order to support the observations. The devices displayed high stability on repeated exposure of the SAW while they were monitored for 1 week under ambient conditions (temperature at around 24 °C and humidity at around 44% RH).
Recent studies strongly indicate that graphene can be used as a channel material for converting surface acoustic waves to acoustoelectric current, which is a resource for various exciting technological applications. On the theoretical side, studies on phonon amplification/attenuation and acoustoelectric current at low temperatures in graphene have reported approximate analytical results under exceedingly simplifying conditions using the Boltzmann transport equation. Overcoming the earlier simplifying assumptions, we investigate both numerically and analytically the governing kinetic equations for amplification/attenuation and acoustoelectric current, taking into account the piezoelectric and deformation potential electron phonon coupling mechanism in the semi classical Boltzmann transport formalism approach, and obtain analytical results that are in reasonable agreement with the reported experimental results.
Electric fields in a surface acoustic wave in a piezoelectric substrate can pattern charge in an adjacent graphene film via the acousto-electric effect and thus reconfigure the optical transmission in an unpatterned graphene metasurface.
We demonstrate in situ gate-tunable acoustoelectric transport in exfoliated monolayer graphene by measuring the voltage created as high-frequency surface acoustic waves dynamically drive charge carriers in the graphene. We employ a flip-chip device configuration to conduct acoustoelectric measurements while simultaneously controlling the graphene carrier density with a metal-oxide back-gate. At high carrier density we observe dependence of the acoustoelectric signal on the sign of the graphene charge carriers, while at low densities we observe anomalous sign reversals of the acoustoelectrically generated voltage. We attribute these anomalous sign reversals to spatially heterogeneous conduction in the vicinity of charge neutrality.
We have combined a graphene field-effect transistor (GFET) and a surface acoustic wave (SAW) sensor on a LiTaO3 substrate to create a graphene surface acoustic wave (GSAW) sensor. When a SAW propagates in graphene, an acoustoelectric current (IA) flows between two attached electrodes. This current has unique electrical characteristics, having both positive and negative peak values with respect to the electrolyte-gate voltage (VEg) in solution. We found that IA is controlled by VEg and the amplitude of the SAW. It was also confirmed that the GSAW sensor detects changes of electrical charge in solution like conventional GFET sensors. Furthermore, the detection of amino-group-modified microbeads was performed by employing a GSAW sensor in a phthalate buffer solution at pH 4.1. The hole current peak shifted to the lower left in the IA–VEg characteristics. The left shift was caused by charge detection by the GFET and can be explained by an increase of amino-groups that have positive charges at pH 4.1. In contrast, the downward shift is thought to be due to a reduction in the amplitude of the propagating SAW because of an increase in the mass loading of microbeads. This mass loading was detected by the SAW sensor. Thus we have demonstrated that the GSAW sensor is a transducer capable of the simultaneous detection of charge and mass, which indicates that it is an attractive platform for highly sensitive and multifunctional solution sensing.
This chapter reports on the state of the art of piezoelectric micro-/nano-mechanical devices in frequency control and sensing applications. Recent studies on bulk acoustic wave (BAW) devices are introduced, including investigation of high-coupling materials and filter and oscillator designs. A novel class of frequency devices based on Lamb waves is also reviewed. Micro- and nano-mechanical sensors for various sensing applications and integrated module are outlined.
We investigate the acoustoelectric properties of graphene and extract its acoustoelectricattenuation Γ as a function of the carrier densityn, tuned via ionic liquid gating. Acoustoelectric effects in graphene are induced by launching surface acoustic waves(SAWs) on a piezoelectric LiNbO3 substrate. We measure the acoustoelectric current Iae through graphene and extract the SAWattenuation factor Γ as a function of n. The magnitude of Iae increases with decreasing n when the n is far from the charge neutral point (CNP). When n is tuned across the CNP, Iae first exhibits a local maximum, vanishes at the CNP, and then changes sign in accordance with the associated change in the carrier polarity. By contrast, Γ monotonically increases with decreasing n and reaches a maximum at the CNP. The extracted values of Γ, calibrated at the central frequency of 189 MHz, vary from ∼0.4 m⁻¹ to 6.8 m⁻¹, much smaller than the values for known two-dimensional systems. Data analysis suggests that the evolution of Iae and Γ with n manifests the electronic states of graphene. Our experimental findings provide insightful information for developing innovative graphene-based devices.
We demonstrate charge pumping in semiconducting carbon nanotubes by a traveling potential wave. From the observation of pumping in the nanotube insulating state we deduce that transport occurs by packets of charge being carried along by the wave. By tuning the potential of a side gate, transport of either electron or hole packets can be realized. Prospects for the realization of nanotube based single-electron pumps are discussed.
The effects of humidity on the electronic properties of mono- and bi-layer chemical vapour deposition graphene transferred on SiO2 are investigated via simultaneous global transport and local work function measurements using the van der Pauw method and Kelvin probe force microscopy, respectively. It is found that mono-layer graphene on SiO2 is extremely sensitive to water vapour, with water molecules acting as physisorbed loosely bound p-dopants. In the case of a bi-layer stack, produced by double transfer of two graphene layers, the layers are randomly oriented and decoupled with respect to each other. As a consequence, the bottom layer of the bi-layer stack is mostly affected by substrate charges, while the top graphene layer behaves as a decoupled layer which is externally doped by the water vapour. Moreover, we provide evidence that ambient humidity is only partly responsible for the p-doping of graphene. These findings will assist in the development of reliable graphene-based electronics such as sensors working in ambient air.
Lithium niobate is the archetypical ferroelectric material and the substrate of choice for numerous applications including surface acoustic wave radio frequencies devices and integrated optics. It offers a unique combination of substantial piezoelectric and birefringent properties, yet its lack of optical activity and semiconducting transport hamper application in optoelectronics. Here we fabricate and characterize a hybrid MoS2/LiNbO3 acousto-electric device via a scalable route that uses millimetre-scale direct chemical vapour deposition of MoS2 followed by lithographic definition of a field-effect transistor structure on top. The prototypical device exhibits electrical characteristics competitive with MoS2 devices on silicon. Surface acoustic waves excited on the substrate can manipulate and probe the electrical transport in the monolayer device in a contact-free manner. We realize both a sound-driven battery and an acoustic photodetector. Our findings open directions to non-invasive investigation of electrical properties of monolayer films.
Using a high-resolution X-Ray diffraction measurement method, the surface acoustic wave
(SAW) propagation in a graphene film on the surface of a Ca3TaGa3Si2O14 (CTGS) piezoelectric crystal was investigated, where an external current was driven across the graphene film. Here, we show that the application of the DC field leads to a significant enhancement of the SAW magnitude and, as a result, to amplification of the diffraction satellites. Amplification of 33.2 dB/cm for the satellite +1, and of 13.8 dB/cm for the satellite +2, at 471 MHz has been observed where the external DC voltage of +10 V was applied. Amplification of SAW occurs above a DC field much smaller than that of a system using bulk semiconductor. Theoretical estimates are in reasonable agreement with our measurements and analysis of experimental data for other materials.
The contactless Surface Acoustic Wave (SAW) technique was implemented to probe the high-frequency (ac) conductivity in a high-mobility p-SiGe/Ge/SiGe structure in the integer quantum Hall (IQHE) regime. The structure was grown by low-energy plasma-enhanced chemical vapor deposition and comprised a two-dimensional channel formed in a compressively strained Ge layer. It was investigated at temperatures of 0.3–5.8 K and magnetic fields up to 18 T at various SAW intensities. In the IQHE regime, in minima of the conductivity oscillations with small filling factors, holes are localized. The ac conductivity is of the hopping nature and can be described within the “two-site” model. Furthermore, the dependence of the ac conductivity on the electric field of the SAW was determined. The manifestation of non-linear effects is interpreted in terms of nonlinear percolation-based conductivity.
The combination of semiconductor quantum well structures and strongly piezoelectric crystals leads to a system in which surface acoustic waves with very large amplitudes can interact with charge carriers in the well. The surface acoustic wave induces a dynamic lateral superlattice potential in the plane of the quantum well which is strong enough to spatially break up a two-dimensional electron system into moving wires of trapped charge. This transition is manifested in an increase of the electron transport velocity with sound amplitude, eventually reaching the sound velocity. The sound absorption by the electron system then becomes governed by nonlinearities and is strongly reduced. We study the transition from the linear towards the strongly nonlinear regime of interaction and present a theoretical description of such phenomena in a 2D system.
Motivated by the recent realization of graphene sensors to detect individual gas molecules, we investigate the adsorption of H2O, NH3, CO, NO2, and NO on a graphene substrate using first-principles calculations. The optimal adsorption position and orientation of these molecules on the graphene surface is determined and the adsorption energies are calculated. Molecular doping, i.e., charge transfer between the molecules and the graphene surface, is discussed in light of the density of states and the molecular orbitals of the adsorbates. The efficiency of doping of the different molecules is determined and the influence of their magnetic moment is discussed.
The acoustoelectric effect in a hybrid of a strong piezoelectric material and a semiconductor layer containing a two-dimensional electron system is investigated. Caused by the very strong interaction between a surface acoustic wave and the mobile carriers in the semiconductor, the acoustoelectric effect is very large as compared to other materials, which might be interesting for device applications. Moreover, the tunability of the sheet conductivity of the electron system enables us to tune the magnitude of the acoustoelectric effect over a wide range. We present experimental results for a GaAs/LiNbO3 layered hybrid system at room temperature and describe our experimental findings quantitatively using a recently developed model calculation. © 1998 American Institute of Physics.
The transport properties of electronic materials have been long interpreted independently from both the underlying bulk-like behavior of the substrate or the influence of ambient gases. This is no longer the case for ultra-thin graphene whose properties are dominated by the interfaces between the active material and its surroundings. Here, we show that the graphene interactions with its environments are critical for the electrostatic and electrochemical equilibrium of the active device layers and their transport properties. Based on the prototypical case of epitaxial graphene on (000) 6 H-SiC and using a combination of in-situ thermoelectric power and resistance measurements and simulations from first principles, we demonstrate that the cooperative occurrence of an electrochemically mediated charge transfer from the graphene to air, combined with the peculiar electronic structure of the graphene/SiC interface, explains the wide variation of measured conductivity and charge carrier type found in prior reports.
We demonstrate the directed control of charge carriers in graphene using the electric field that accompanies the propagation of surface acoustic waves (SAWs) on a piezoelectric surface. Graphene grown by chemical vapor deposition was transferred to the surface of lithium niobate, allowing its direct integration with interdigital transducers used for SAW generation and detection. Radio frequency (RF) signal applied to the transducers at their resonant frequency was found to generate a direct current flow by the transport of p-type charge carriers. The acoustically induced current scales linearly with the applied RF power and can be observed even in presence of a counter-flow current induced by an applied bias. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697403]
Electrons in a metal are indistinguishable particles that interact strongly with other electrons and their environment. Isolating and detecting a single flying electron after propagation, in a similar manner to quantum optics experiments with single photons, is therefore a challenging task. So far only a few experiments have been performed in a high-mobility two-dimensional electron gas in which the electron propagates almost ballistically. In these previous works, flying electrons were detected by means of the current generated by an ensemble of electrons, and electron correlations were encrypted in the current noise. Here we demonstrate the experimental realization of high-efficiency single-electron source and detector for a single electron propagating isolated from the other electrons through a one-dimensional channel. The moving potential is excited by a surface acoustic wave, which carries the single electron along the one-dimensional channel at a speed of 3 μm ns(-1). When this quantum channel is placed between two quantum dots several micrometres apart, a single electron can be transported from one quantum dot to the other with quantum efficiencies of emission and detection of 96% and 92%, respectively. Furthermore, the transfer of the electron can be triggered on a timescale shorter than the coherence time T(2)* of GaAs spin qubits. Our work opens new avenues with which to study the teleportation of a single electron spin and the distant interaction between spatially separated qubits in a condensed-matter system.
The ultimate aim of any detection method is to achieve such a level of sensitivity that individual quanta of a measured entity can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects, which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here, we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene's surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.
Thin semiconductor quantum well structures fused onto LiNbO/sub 3/ substrates using the epitaxial lift-off (ELO) technology offer the possibility of controlling the surface acoustic wave (SAW) velocity via field effect. The tunability of the conductivity in the InGaAs quantum well results in a great change in SAW velocity, in general, accompanied by an attenuation. We show that an additional lateral modulation of the sheet conductivity reduces the SAW attenuation significantly, enhancing device performance. At high SAW intensity the bunching of electrons in the SAW potential also leads to a strong reduction of attenuation. These effects open new possibilities for voltage-controlled SAW devices. We demonstrate a novel, wireless, passive voltage sensor, which can be read out from a remote location.
The strong piezoelectric fields accompanying a surface acoustic wave on a semiconductor quantum well structure are employed to dissociate optically generated excitons and efficiently trap the created electron hole pairs in the moving lateral potential superlattice of the sound wave. The resulting spatial separation of the photogenerated ambipolar charges leads to an increase of the radiative lifetime by orders of magnitude as compared to the unperturbed excitons. External and deliberate screening of the lateral piezoelectric fields triggers radiative recombination after very long storage times at a remote location on the sample. Comment: 4 PostScript figures included, Physical Review Letters, in press
We report on the excitation of acousto-eletric (AE) charge transport in monolayer graphene by acoustic transducers based on aluminum nitride thin films. The acoustic waves induced macroscopic current flow that linearly scaled with input power. The AE current exhibited unique frequency dependence due to special configuration and piezoelectric properties of the transducer, which led to transitions between traveling and standing acoustic waves across a characteristic frequency. A Finite Element model was built to investigate and understand the phenomena and the underlying mechanisms.
The acoustoelectric current in graphene has been investigated as a function of illumination, using blue (450 nm) and red (735 nm) light-emitting diodes
(LEDs), and surface acoustic wave
(SAW) intensity and frequency. The measured
acoustoelectric current increases with illumination, more than the measured change in the conductivity of the graphene, whilst retaining a linear dependence on the SAW intensity. The latter is consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model suggesting that the change in the acoustoelectric current is caused by the effect of the illumination on the electronic properties of the graphene. The increase in the acoustoelectric current is greatest under illumination with the blue LED, consistent with the creation of a hot electron distribution.
The acoustoelectric current in graphene has been investigated as a function of temperature, surface acoustic wave
(SAW) intensity, and frequency. At high SAW frequencies, the measured
acoustoelectric current decreases with decreasing temperature, but remains positive, which corresponds to the transport of holes, over the whole temperature range studied. The current also exhibits a linear dependence on the SAW intensity, consistent with the interaction between the carriers and SAWs being described by a relatively simple classical relaxation model. At low temperatures and SAW frequencies, the measured
acoustoelectric current no longer exhibits a simple linear dependence on the SAW intensity, and the direction of the acoustoelectric current is also observed to reverse under certain experimental conditions.
We demonstrate macroscopic acoustoelectric transport in graphene,
transferred onto piezoelectric lithium niobate substrates, between
electrodes up to 500 μm apart. Using double finger interdigital
transducers we have characterised the acoustoelectric current as a
function of both surface acoustic wave intensity and frequency. The
results are consistent with a relatively simple classical relaxation
model, in which the acoustoelectric current is proportional to both the
surface acoustic wave intensity and the attenuation of the wave caused
by the charge transport.
We report the first observation of the direct current induced by a surface acoustic wave through a quantum point contact defined in a GaAs - AlGaAs two-dimensional electron gas by means of a split gate. We have observed giant oscillations in the acoustoelectric current as a function of gate voltage, with minima corresponding to the plateaux in quantum point contact conductivity. A theoretical consideration is presented which explains the observed peaks in terms of the matching of sound velocity with electron velocity in the upper one-dimensional subband of the quantum point contact.
The acoustoelectric interaction of surface phonons with conduction electrons in piezoelectric semiconductors is investigated in the frequency region where the piezoelectric coupling is the predominant mode of interaction. We derive the interaction explicitly taking the elastic anisotropy of the semiconductors into account. The electronic state near the semiconductor surface, due to the presence of the surface space-charge layer, is also properly considered to specify the interaction. Applying the interaction to the study of the amplification characteristics of surface acoustic waves of GHz frequencies (propagated on the basal plane of a cubic crystal), we find that the elastic anisotropy reduces the interaction significantly compared with that of the isotropic approximation. Furthermore, when the wavelengths of the surface phonons become comparable to the width of the surface-depletion layer, the frequency dependence of the amplification rates suffers modifications also qualitatively.
The interaction between surface acoustic waves and quasi-two-dimensional inversion electron systems on GaAs/AlxGa1-xAs heterojunctions is investigated in high magnetic fields and at low temperatures. The interaction of the surface acoustic wave with high-mobility inversion electrons leads to strong quantum oscillations in both the transmitted surface wave intensity as well as in the sound velocity, reflecting the quantum oscillations of the magnetoconductivity as a function of an applied magnetic field. We study the dependence of this interaction on the magnetic field and on the surface-acoustic-wave power and frequency, and discuss the results using simple models. The influence of slight spatial inhomogeneities in the carrier density on the line shape of the quantum oscillations is analyzed in detail and related to their influence on the quantum Hall effect. First experimental results on the interaction of surface acoustic waves with two-dimensional electron systems in gated heterojunctions providing an adjustable carrier density are presented.
The presence in a semiconductor of both signs of current carriers makes
it possible to disturb the spatial distribution of these carriers
appreciably without giving rise to electrical space charge. As a result,
the interaction of particles with acoustic waves leads to certain
effects which, at least at low frequencies, are peculiar to
semiconductors; these include a (complex) addition to the elastic
modulus and the "acoustoelectric effect."
This paper reviews the interaction between coherently stimulated acoustic phonons in the form of surface acoustic waves with light beams in semiconductor based photonic structures. We address the generation of surface acoustic wave modes in these structures as well as the technological aspects related to control of the propagation and spatial distribution of the acoustic fields. The microscopic mechanisms responsible for the interaction between light and surface acoustic modes in different structures are then reviewed. Particular emphasis is given to the acousto-optical interaction in semiconductor microcavities and its application in photon control. These structures exhibit high optical modulation levels under acoustic excitation and are compatible with integrated light sources and detectors.
Acoustoelectric amplification, in the linear approximation, is considered for a system containing several types of carriers in the two limiting cases of fast and slow carrier equilibration. The pertinent expressions are first derived for a piezoelectric semiconductor and then expanded to the more practical case of a layered system containing one or more semiconducting layers coupled to a piezoelectric element. An experimental example is given in the form of a guided elastic plate wave in lead zirconate titanate coupled to a plate of near‐intrinsic Ge.
The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited carriers, as well as to spatially control exciton recombination in GaAs-based nanowires (NWs) on a subns time scale. The experiments are carried out in core-shell NWs transferred to a SAW delay line on a LiNbO(3) crystal. Carriers generated in the NW by a focused laser spot are acoustically transferred to a second location, leading to the remote emission of subns light pulses synchronized with the SAW phase. The dynamics of the carrier transport, investigated using spatially and time-resolved photoluminescence, is well-reproduced by computer simulations. The high-frequency contactless manipulation of carriers by SAWs opens new perspectives for applications of NWs in opto-electronic devices operating at gigahertz frequencies. The potential of this approach is demonstrated by the realization of a high-frequency source of antibunched photons based on the acoustic transport of electrons and holes in (In,Ga)As NWs.
We report on optical experiments performed on individual GaAs nanowires and the manipulation of their temporal emission characteristics using a surface acoustic wave. We find a pronounced, characteristic suppression of the emission intensity for the surface acoustic wave propagation aligned with the axis of the nanowire. Furthermore, we demonstrate that this quenching is dynamical as it shows a pronounced modulation as the local phase of the surface acoustic wave is tuned. These effects are strongly reduced for a surface acoustic wave applied in the direction perpendicular to the axis of the nanowire due to their inherent one-dimensional geometry. We resolve a fully dynamic modulation of the nanowire emission up to 678 MHz not limited by the physical properties of the nanowires.
The dynamic nature of the first water adlayers on solid surfaces at room temperature has made the direct detection of their
microscopic structure challenging. We used graphene as an atomically flat coating for atomic force microscopy to determine
the structure of the water adlayers on mica at room temperature as a function of relative humidity. Water adlayers grew epitaxially
on the mica substrate in a layer-by-layer fashion. Submonolayers form atomically flat, faceted islands of height 0.37 ± 0.02
nanometers, in agreement with the height of a monolayer of ice. The second adlayers, observed at higher relative humidity,
also appear icelike, and thicker layers appear liquidlike. Our results also indicate nanometer-scale surface defects serve
as nucleation centers for the formation of both the first and the second adlayers.
- Lv R.