Acoustic Charge Transport Research Articles

A precise angular spectrum of plane-waves diffraction theory for leaky wave materials

A computationally efficient diffraction theory technique is presented which very accurately describes leaky wave propagation in {100}-cut GaAs. This technique combines the angular spectrum of the plane-waves diffraction theory with the slowness surface measurement method of Murray and Ash [1977 Ultrasonics Symposium Proceedings, IEEE Cat. No. 77CH1264-1SU (IEEE, New York, 1977), p. 823] and extends them to deal with the anisotropic loss exhibited by the leaky wave. Both the magnitude and phase of beam profiles are predicted with great precision for beams propagating over long distances and through metallic grating structures. The implications of these results for acoustic charge transport devices on GaAs are discussed.

Reduction of surface acoustic wave grating impedance and velocity perturbations via buried bimetallic electrode geometries

The impedance and velocity perturbations presented to the leaky wave by the nondestructive sense (NDS) electrodes in GaAs acoustic charge transport (ACT) devices are of particular concern in the design of high quality ACT devices. An example is shown where large perturbations and improperly designed NDS arrays resulted in substantial leaky wave propagation loss. The full form of the Datta and Hunsinger velocity and impedance perturbation equations is presented for the case of buried bimetallic grating arrays and it is shown that, with the proper choice of groove depths and metal thicknesses, it is theoretically possible to simultaneously reduce the impedance and velocity perturbations to negligible values. It is by such a procedure that a ‘‘perturbationless’’ array can be made. Laser probe measurements of the reflection coefficient and velocity perturbation for three buried chrome-gold grating devices demonstrate the feasibility of achieving this theoretical ideal.

Optical charge injection into a gallium arsenide acoustic charge transport device

Transport of photoelectrons by the 〈110〉 propagating surface acoustic wave (SAW) on (100)-cut gallium arsenide was observed using the acoustic charge transport (ACT) principle reported by Hoskins, Morkoϑ, and Hunsinger [Appl. Phys. Lett. 41, 332 (1982)]. By directly modulating an AlGaAs light-emitting diode, pulses of near-infrared radiation (λp=730 nm) were applied to semitransparent (25 nm thick) chromium windows on the device surface. Four such windows separated by 250 μm were positioned along the otherwise opaque Schottky channel plate of the ACT device. Electron-hole pairs generated by incident radiation were separated by the electrostatic field in the depleted n-type channel region. Calculations suggested that the separation time would be small enough to reduce radiative recombination and allow enough electrons to collect in each acoustically defined well passing beneath the injection window for detection at the output. Electrons, separated from the holes, were subsequently bunched and carried along by the electric field coupled to the SAW. Output pulse delays for injection over each window were in agreement with theory taking into account the window positions and the SAW velocity (2864 m/s). Quantum efficiency, defined as the number of electrons collected at the output per incident photon on the chromium window, was approximately 0.09. Operated at 360 MHz, the experimental device had a charge transfer efficiency in excess of 0.992. It is believed that the injection technique described may be of use in future optical signal processing or imaging applications using ACT.

Acoustic charge transport in an (Al,Ga)As/GaAs heterojunction structure grown by molecular-beam epitaxy

The demonstration of charge transport in GaAs epitaxial layers by surface acoustic waves in 1982 has led to a great deal of interest in using acoustic charge transport (ACT) technology for high-speed analog signal processors. Successful implementation of ACT effects requires confining the moving charge packets to a buried layer. This was originally accomplished by biasing a thick, lightly doped n-GaAs layer (about 4 μm low-1015/cm3) with electrodes at the epilayer surface and at the back of the semi-insulating substrate, leading to problems in device fabrication and reproducibility, as well as making monolithic integration with field effect transistors (FET’s) difficult. The logical solution to these problems seemed to be using the charge confining capability inherent in heterojunctions. We report here the first observation of acoustic charge transport in a heterojunction structure. The heterojunction acoustic charge transport (HACT) epitaxial structures were grown by molecular-beam epitaxy and are comprised of a GaAs quantum well with (Al,Ga)As barriers. The top (Al,Ga)As layer is doped to satisfy surface states and to facilitate formation of Ohmic contacts. These structures are similar to quantum well or double heterojunction high electron mobility transistor (HEMT) [modulation-doped FET (MODFET)] epilayers, but with somewhat different material requirements for good device operation. As examples, acoustic charge transport devices operate at low current levels (about 100 μA) and thus require Schottky contacts with very low leakage. But since they operate at relatively low frequencies (hundreds of MHz) they do not need extremely high mobility two-dimensional electron gases. Initial results indicate that these devices function best with a low sheet carrier concentration, also in contrast to HEMT’s. Further details of materials and device fabrication will be presented.

Heterojunction acoustic charge transport devices on GaAs

We have demonstrated a new type of buried-channel acoustic charge transport device in which charge is transported in an (Al,Ga)As/GaAs/(Al,Ga)As heterojunction channel. Traveling-wave potential wells, associated with a surface acoustic wave (SAW) propagating on the 〈100〉 surface of a GaAs crystal, transport electrons at the SAW velocity by means of a large-signal acoustoelectric interaction. Heterojunction acoustic charge transport (HACT) delay lines have been fabricated, and the transport of charge demonstrated. Charge packets with up to 16×106 electrons/cm were measured in a delay 1.4 μs long. The HACT device is much simpler, the transport channel is more reliably produced (by molecular beam epitaxy or metalorganic chemical vapor deposition), and the device has potential for higher dynamic range when compared to the previously developed acoustic charge transport technology. This new device type is useful for the implementation of high-speed monolithic signal processors.

Performance of Acoustic Charge Transport Chirp Filters

The measured performance of acoustic charge transport (ACT) based linear FM dispersive delay line filters is presented and compared to modelled performance. The excellent agreement between the theoretically predicted device performance and the measured results shows clearly that anodization of the nondestructive sensing array is an effective means for achieving weighted ACT filter responses.

Acoustic charge transport device and method

Method and device for delaying an electrical signal, the method and device being useable in various signal processing applications. A buried channel (41) is formed by disposing a portion of a layer of piezoelectric semiconductor material (40) between confining layers (50, 30). A surface acoustic wave is established in the piezoelectric semiconductor material via an input transducer (55). Majority carriers are depleted from the channel, and an electrical signal is injected into the channel via an interdigital transducer (55). A delayed version of the electrical signal, which had been carried along by the surface acoustic wave, can then be extracted from the channel via an output diode (71).

Simple theory of buried channel acoustic charge transport in GaAs

A theory is developed which describes the fundamental charge transfer characteristics of basic buried channel GaAs structures which are illuminated by large amplitude surface acoustic waves. Several approximations which simplify the analysis are shown to be valid in this channel structure. The theoretical approach treats carrier diffusion as a perturbation on the diffusionless two-dimensional electrostatic problem. Simple closed form expressions are obtained for packet charge density and boundary shape in terms of the channel parameters. It is shown that four sets of numerically generated and normalized curves are sufficient for the determination of the transport characteristics of a wide range of channel potential geometries. The dependence of diffusion induced transfer inefficiency on charge load and wave potential is investigated. The results indicate that high speed acoustic charge transport is capable of supporting typical buried channel charge loads at very high transfer efficiency.

Experimental performance of the buried-channel acoustic charge transport device

High-speed electron transport is accomplished in a new type of buried-channel charge transfer device which is based on the nonlinear acoustoelectric interaction between large amplitude surface acoustic waves (SAW) and electrons in GaAs. The performance of the acoustic charge transport (ACT) device is enhanced by design improvements which provide precise channel definition and charge injection structures. A transfer efficiency in excess of 0.996 has been measured in an experimental device operating at the SAW clock frequency of 358 MHz.