Acoustic Charge Transport Research Articles

Acoustical control of charge movement in an organic semiconductor

We show experimentally the directional acoustic charge transport of optically generated charge carriers in an organic semiconductor at room temperature by surface acoustic waves (SAWs) generated on a lithium niobate substrate utilizing an easy and straightforward structure consisting of two identical interdigital transducers (IDTs) and a corresponding collector diode facing each IDT. An orthogonal arrangement of the two IDTs has been used to analyse the impact of SAW drag force on the charge carriers by interacting two SAWs. In addition, we show the frequency dependency of the acoustic dragging force on charge carriers by orthogonally interacting SAW beams of different frequencies. This relation provides the idea of using one or more SAWs to achieve an acoustic manoeuvring of charge carrier transport. The diagonal propagation of the carriers through orthogonal SAW beams of the same frequencies was also verified experimentally, thus giving rise to the possibility of an acoustic charge transfer device that may be employed as a fundamental gate for information processing based on the charge transport with the help of SAW.

Acoustic charge transport in organic semiconductor films

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 cm−2 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.

Observation of Acoustic Charge Transport in GaN-Based Quantum Well Structures Under Surface Acoustic Wave Excitations.

Though the acoustoelectric and acoustooptic interactions have been widely studied on III-V semiconductors, most studies were conducted at low temperature to avoid the influence of lattice scattering. Here, we demonstrate larger-than-1-GHz optical oscillation and acoustic charge transport at room temperature from a nitride-based LED (light-emitting diode) by acoustic wave excitation. The optical oscillation is generated by the harmonics of acoustic waves, while the acoustic transport of carriers was observed from the LED light output at different acoustic excitation frequencies. Furthermore, with the change of bias voltage, the frequency response of light output varies correspondingly due to the thermal disturbance that affects the lattice vibration and acoustic wave propagation. The results also suggest that we are able to achieve a tunable optical resonant frequency by varying bias voltages without changing the LED structure.

Acoustic charge transport induced by the surface acoustic wave in chemical doped graphene

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.

Acoustic charge transport behaviors of sulfur-doped wide gap Ga2Te3-based semiconductors

Wide gap semiconductors as the thermoelectric (TE) candidates have been increasingly interested because of their inherent high Seebeck coefficients and low thermal conductivities. Ga2Te3 is one of the typical defect compounds (Eg=1.65 eV) among the A2IIIB3VI type semiconductors, in which there are periodically self-assembled 2D vacancy planes that wrap the nanostructured domains. The vacancy planes scatter phonons highly effectively and are responsible for reducing the lattice thermal conductivity. Hence Ga2Te3 might be a good TE candidate. In the phase diagram of Ga-Te, Ga2Te3 is involved in the eutectoid and peritectic reactions at the critical temperatures (CTs) of 680 10 K and 757 10 K respectively. These reactions would lead to the generation of enthalpies of reactions, and induce the alteration of some thermo-physical properties. In the present work, we have not observed the phase transformations at CTs in the Ga2Te3-based materials with sulfur isoelectronic substitution for Te, which are prepared by powder metallurgy with the spark plasma sintering (SPS) technique, but can observe the generation of assumed enthalpies of reactions near CTs, which directly gives rise to the critical acoustic charge transport behaviors. The critical behaviors involve the remarkable increase of heat capacities and Seebeck coefficients and, at the same time, reductions of thermal diffusivities (thermal conductivities) and electrical conductivities. For example, the Seebeck coefficient () at x=0.05 increases rapidly from 376.3(VK-1) to 608.2(VK-1) when the temperature rises from 596 to 695 K, and then decreases to 213.8(VK-1) at 764 K. Similarly, all the S-doped samples, which have lowest electrical conductivities ( ) of 2.12102 (x=0.05), 0.25102 (x=0.1), 0.12102 -1m-1 (x=0.2) and 0.14102 -1m-1 (x=0.3) at 696725 K, undergo dramatic changes when the temperature rises to about 750 K, and then the electrical conductivities begin to decrease, and the changes tend to slow down. It is notable that both the Seebeck coefficients and electrical conductivities exhibit a typical zigzag temperature dependence in the temperature range from 596 to 812 K. These behaviors reveal the remarkable alterations in scattering mechanism of both phonons and carriers at temperatures near the CTs. Although the materials with these critical behaviors near CTs do not have satisfactory thermoelectric performance (ZTmax=0.17 at 793 K for x=0.3) as compared with the known binary Cu2Se, Ag2Se(S) or ternary based AgCrSe2 alloys, however, the findings of such critical transport behaviors have a great significance for future researches.

New types of surface acoustoelectric waves and acoustic charge transport in GaAs crystals

It is shown for the first time that longitudinal and transverse surface acoustoelectric waves can propagate in the basal plane of GaAs crystals. In contrast to well-known Gulyaev-Bleustein transverse waves, these waves are localized in a surface layer with a thickness on the order of the wavelength. The field of mechanical displacements and the electric-potential field of the wave are shifted in phase by π/2. The electric-field vector is circularly polarized in the sagittal plane. The effect of acoustoelectric charge transport by wave packets of new-type acoustoelectric waves is considered.

High-frequency acoustic charge transport in GaAs nanowires

The oscillating piezoelectric fields accompanying surface acoustic waves are able to transport charge carriers in semiconductor heterostructures. Here, we demonstrate high-frequency (above 1 GHz) acoustic charge transport in GaAs-based nanowires deposited on a piezoelectric substrate. The short wavelength of the acoustic modulation, smaller than the length of the nanowire, allows the trapping of photo-generated electrons and holes at the spatially separated energy minima and maxima of conduction and valence bands, respectively, and their transport along the nanowire with a well defined acoustic velocity towards indium-doped recombination centers.

Acousto-electric transport in epitaxial monolayer graphene on SiC

We report on the piezoelectric excitation and acoustic charge transport by gigahertz surface acoustic waves (SAWs) in epitaxial monolayer graphene (EG) on SiC. The GHz SAWs frequencies were generated by interdigital transducers fabricated on a piezoelectric island on the SiC substrate. Acoustic transport studies in a Hall bar geometry show that the propagating SAW field transports carriers in EG, the transport direction being determined by the direction of the acoustic beam. Carrier transport is driven by drift in the piezoelectric field induced by the SAW in EG.

Ultrahigh-frequency surface acoustic wave generation for acoustic charge transport in silicon

We demonstrate piezo-electrical generation of ultrahigh-frequency surface acoustic waves on silicon substrates, using high-resolution UV-based nanoimprint lithography, hydrogen silsequioxane planarization, and metal lift-off. Interdigital transducers were fabricated on a ZnO layer sandwiched between two SiO2 layers on top of a Si substrate. Excited modes up to 23.5 GHz were observed. Depth profile calculations of the piezoelectric field show this multilayer structure to be suitable for acoustic charge transport in silicon at extremely high frequencies with moderate carrier mobility requirements.

Experimental methods to suppress random telegraph signal noise in acoustic charge transport nanostructure devices

AbstractWe present an experimental investigation of how the different operating parameters affect the random telegraph signal (RTS) noise in surface acoustic wave (SAW) devices. The altered parameters include: SAW power and frequency, phase difference between two counterpropagating SAW beams, and asymmetrical voltage applied to the two side gates of the quantum point contact. The results indicate that proper tuning of the relevant parameters can reduce the RTS noise, which would be beneficial for metrological applications. Building on the experimental results, physical models have been proposed to explain the reason why the effective reduction of the noise can be achieved.

The effects of two counterpropagating surface acoustic wave beams on single electron acoustic charge transport

We present a comprehensive study of the effects of two counterpropagating surface acoustic waves on the acoustoelectric current of single electron transport devices. A significant improvement in the accuracy of current quantization is achieved as a result of an additional surface acoustic wave beam. The experiments reveal the sinusoidally periodical modulation in the acoustoelectric current characteristic as a function of the relative phase of the two surface acoustic wave beams. Besides, by using standing surface acoustic waves, the acoustoelectric current is detected which we consider as the so-called anomalous acoustoelectric current produced by acoustic wave mechanical deformations. This kind current is contributed to one component of the acoustoelectric current in surface acoustic wave device, which could enable us to establish a more adequate description of acoustoelectric effects on single-electron acoustic charge transport.

Fidelity of the charge transport by surface acoustic waves in a GaAs quantum well

Charge transport in one- and two-dimensional GaAs-based electronic systems by surface acoustic waves (SAWs) is expected to find application in quantum information processing and metrology. In the charge transport by a SAW (referred to as the acoustic charge transport (ACT)), electrons are transported in packets residing in the SAW potential minima along channels formed in a GaAs quantum well or heterojunction, which play a role similar to that of wires in standard electronic circuits. The potential of ACT circuits for quantum information processing stems from their ability to precisely control the position and the spin of single electrons. Traps in the ACT channel may capture an electron from one SAW minimum and release it later into another, thereby reducing the fidelity of the ACT transport and compromising device performance. In this paper, we describe a novel experimental technique for the characterization of the ACT fidelity using an acoustoelectric Y-branch switch. The results are used to assess the performance of an acoustoelectric discriminating photon detector for quantum information processing.

Acoustic charge and spin transport in GaAs quantum wires

AbstractOptical investigation of the transport of electrons and holes in GaAs quantum wires by surface acoustic waves shows that carriers can be efficiently transported over several tens of mm. Despite the efficient charge transport, the transport distances for photoexcited spins were found to be very short.These short spin transport lengths indicate that spin dephasing is not limited by spin motion (the DP mechanism) but rather by spin relaxation during momentum scattering. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Acoustic charge transport in GaN nanowires

We present acoustic charge transport in GaN nanowires (GaN NWs). The GaN NWs were grownby molecular beam epitaxy (MBE) on silicon(111) substrates. The nanowires were removed fromthe silicon substrate, aligned using surface acoustic waves (SAWs) on the piezoelectric substrateLiNbO3 and finally contacted by electron beam lithography. Then, a SAW was used to create anacoustoelectric current in the GaN NWs which was detected as a function ofradio-frequency (RF) wave frequency and its power. The presented method and ourexperimental findings open up a route towards new acoustic charge transport nanostructuredevices in a wide bandgap material such as GaN.

An acoustoelectric single photon detector

We propose a novel single photon detector for quantum information processing and for general applications in areas where the detection of ultra weak photon fluxes is required. The detector is also capable of discriminating between different numbers of photons in a light pulse. It uses acoustic charge transport in order to transfer photogenerated electrons and holes to areas where their charge can be detected by single electron transistors. Preliminary experimental results on the fidelity of the acoustic charge transport are presented, which allows us to make estimates of the detector's performance.

Quantum-dot thermometry of electron heating by surface acoustic waves

We investigate the effect of applied microwave signals on the electron temperature in a surface-acoustic-wave (SAW) device, using a quantum dot as a thermometer. We find that with the continuous application of the radio-frequency signal at ∼3GHz, the electron temperature rises from a base temperature of 0.5K to over 1.3K, even with a power two orders of magnitude smaller than normally used for acoustic charge transport. As a solution to this problem, we show that this heating can be considerably reduced by pulse modulating the SAW. The methods we have developed to allow this type of electron thermometry also allow them to measure the SAW amplitude from the splitting of conductance peaks.

Experimental investigation of the surface acoustic wave electron capture mechanism

We have investigated surface acoustic wave charge transport through a one-dimensional channel, where the form of the potential is defined by two split gates placed in series. It is found that, when the second gate is set to shut off the acoustoelectric current, sweeping the first gate to negative voltages recovers the current and a set of current-quantization plateaux are observed. At a certain negative gate voltage, a sudden transition occurs and the current starts to decrease to zero, again showing plateaux. Calculation of the channel potential shows that this counterintuitive behavior is caused by the switching of maximum potential slope between two different locations that give different current dependencies. This result indicates that in the high transducer-power regime, the charge transport by surface acoustic wave is determined by the potential gradients.

Acoustic charge transport in a n-i-n three terminal device

We present an unconventional approach to realize acoustic charge transport devices that takes advantage from an original input region geometry in place of standard Ohmic input contacts. Our scheme is based on a n-i-n lateral junction as electron injector, an etched intrinsic channel, a standard Ohmic output contact and a pair of in-plane gates. We show that surface acoustic waves are able to pick up electrons from a current flowing through the n-i-n junction and steer them toward the output contact. Acoustic charge transport was studied as a function of the injector current and bias, the SAW power and at various temperatures. The possibility to modulate the acoustoelectric current by means of lateral in-plane gates is also discussed. The main advantage of our approach relies on the possibility to drive the n-i-n injector by means of both voltage or current sources, thus allowing to sample and process voltage and current signals as well.

Lateral n–p junction for acoustoelectric nanocircuits

We report the experimental realization of a device comprising a lateral n–p junction grown by focused-ion molecular-beam epitaxy and a transducer to generate a surface acoustic wave. Acoustic charge transport across the junction and the accompanying photon emission are demonstrated. This type of light-emitting diode is suitable for integration into acoustoelectric nanocircuits in which quasione-dimensional semiconductor channels serve as “wires” through which packets of charge are transported by surface acoustic waves. The diode provides a means by which to extend the functionality of acoustoelectric nanocircuits into the optical domain.

Injection-Current Dependence of the Acoustoelectric Signal in AlGaAs/GaAs Heterostructures

Acoustic charge transport is studied experimentally and theoretically. To this end, an AlGaAs/GaAs heterostructure is designed and fabricated with set geometrical and electrical parameters of the layers. Its transfer characteristic is measured, i.e., the variation of output voltage with injection current. The behavior of the curve is explained within an analytical model. The conclusions are supported by numerical simulation.