Two-dimensional electron-gas actuation and transduction for GaAs nanoelectromechanical systems

Abstract

We have fabricated doubly clamped beams from GaAs/AlGaAs quantum-well heterostructures containing a high-mobility two-dimensional electron gas ͑2DEG͒. Applying an rf drive to in-plane side gates excites the beam's mechanical resonance through a dipole-dipole mechanism. Sensitive high-frequency displacement transduction is achieved by measuring the ac emf developed across the 2DEG in the presence of a constant dc sense current. The high mobility of the incorporated 2DEG provides low-noise, low-power, and high-gain electromechanical displacement sensing through combined piezoelectric and piezoresistive mechanisms. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1516237͔ Thin, suspended two-dimensional electron gas ͑2DEG͒ heterostructures have been recently perfected, and have subsequently been employed for nanoscale conducting devices. 1,2 In this letter, we present a high-resolution displacement readout that is based upon our ability to achieve very high mobility suspended quantum wires. Molecular beam epitaxial ͑MBE͒ grown materials are directly patterned and in-plane gates are used to excite the vibration. No metallization is needed, hence high Q values can be obtained.The starting material was a specially designed, MBEgrown, 2DEG heterostructure similar to that used in Ref. 1. The structural layer stack comprises seven individual layers having a total thickness of 115 nm. The top and bottom are thin GaAs cap layers preventing oxidation of the Al 0.3 Ga 0.7 As:Si donor layers in between. The central 10-nmthick GaAs layer forms a quantum well sustaining a high mobility 2DEG located 37 nm below the top surface and surrounded by two AlGaAs spacer layers. Below the structural layer stack is a 400 nm Al 0.8 Ga 0.2 As sacrificial layer. The structure was intentionally made asymmetric to avoid neutralizing the piezoelectric effect of GaAs.After ohmic contacts were deposited, a thick layer of poly-methyl-methacrylate ͑PMMA͒ is spun on the chip, followed by a single electron-beam lithography step to expose trenches in PMMA that isolate the beam from its side gates. PMMA was then employed as a direct mask against a low voltage electron cyclotron reactor etch performed to further etch the trenches to the sacrificial layer. After stripping off the PMMA, final structure relief is achieved by removing the sacrificial layer beneath the beams with diluted HF. To minimize the damage to the 2DEG from dry etching, a Cl 2 /He plasma was chosen because of its excellent etching characteristics, such as smooth surface morphology and vertical sidewall. A stable etching speed at 35Å /s is obtained under conditions of less than 150 V self-bias ͑20 W constant rf power͒, Cl 2 and He flow rate ratio 1:9, 3 mTorr pressure, and 300 W microwave power. With the same method, we have also fabricated suspended Hall bars and extensively characterized the resulting suspended 2DEG. Before processing, the initial mobility and density after illumination are 5.1 ϫ10 5 cm 2 /Vs, 1.26ϫ10 12 cm Ϫ2 , respectively. With our improved low damage etching...

Monocrystalline AlxGa1−xAs heterostructures for high-reflectivity high-Q micromechanical resonators in the megahertz regime

“…For the vibration mode near 700 kHz (2 MHz), we calculate an approximate force sensitivity of 220 fN/ √ Hz (24 fN/ √ Hz) at 300 K, decreasing to roughly 20 fN/ √ Hz (3 fN/ √ Hz) at cryogenic temperatures. These values are on par with previous examples of GaAs-based nanomechanical resonators as presented in [34].…”

Piezoelectric Electromechanical Coupling in Nanomechanical Resonators with a Two-Dimensional Electron Gas

“…For example, papers [5,6] show that the electron transport through a quantum point contact placed on a micromechanical resonator is sensitive to mechanical vibrations. Papers [2,7,8] demonstrate that diffusive conductive channels in 2DEG can also be used as nanoelectromechanical transducers.The two fundamental key points arising in the context of NEMS are the physical mechanisms underlying actuation and transduction of the nanomechanical motion. The question about the actuation in NEMS with 2DEG is addressed elsewhere [7], while, in the present paper, we focus on the transduction mechanism.…”

“…For example, papers [5,6] show that the electron transport through a quantum point contact placed on a micromechanical resonator is sensitive to mechanical vibrations. Papers [2,7,8] demonstrate that diffusive conductive channels in 2DEG can also be used as nanoelectromechanical transducers.…”

“…The question about the actuation in NEMS with 2DEG is addressed elsewhere [7], while, in the present paper, we focus on the transduction mechanism. Most of the papers considering GaAs/AlGaAs-based suspended systems contain a proposal that a 2DEG embedded in a resonator is sensitive to its vibrations due to the change in the density of a 2DEG that screens the piezoelectrically induced bound charge [2,5,6,9]. However, there is a lack of experimental evidence for this hypothesis.…”

“…However, selective etching of a sacrificial layer (often called surface nanomachining) gives an opportunity to create also a 2DEG embedded in a thin membrane freely suspended over a substrate [1]. The nanostructures fabricated from such membranes are mechanically moveable with their movement affecting electron transport and conductivity [2]. Such electromechanical coupling gives an opportunity to probe mechanical motion at the nano-scale and it could be used to study various interesting mechanical phenomena, such as "phonon lasing" [3] and the quantum-limited motion of an artificially made object [4].…”