Novel Electronic Nanodevices Operating in the Terahertz Region

Abstract

A novel electronic nanodevice, the self-switching diode (SSD), is explored in this work. This includes exploration of its ability to operate as an ultra-high-speed detector at room temperature, its low-frequency noise properties, and its application to terahertz (THz) imaging. The SSDs have been realised using two novel nanolithography techniques, known as atomic-force microscope lithography and electron-beam lithography.The SSD is a unipolar two-terminal device. It has a nonlinear current-voltage characteristic which resembles the behaviour of a conventional diode. The planar structure of the SSD provides intrinsically low parasitic capacitance that enables signal rectification at higher speed than a standard vertical diode. It also allows the fabrication of a large number of SSDs in a single lithography step without the need for interconnection layers, which may introduce parasitic elements. Indeed, this is the key feature of the SSD that makes the whole fabrication process simpler, faster and lower cost, when compared with other conventional electronic nanodevices. By using large arrays of SSDs connected in parallel, the overall resistance of the devices can be reduced.A large SSD array, fabricated onto a two-dimensional electron gas (2DEG) in an InGaAs/InAlAs heterostructure material, has been defined within the fingers of an interdigital structure, located in the gaps of a coplanar waveguide. Despite of the large impedance mismatch between the SSD array and the measurement systems, the device successfully converted radio-frequency (RF) signals with frequencies up to 3 GHz (i.e., the highest frequency of the instruments used in the RF experiments) into usable DC power which may be employed in many RF applications. The obtained room-temperature results are matched very well with the theory.The development of the SSD-based THz detectors is a key objective of this work. The SSDs, coupled with either spiral or bow-tie antennas, have been fabricated onto a 2DEG in an AlGaAs/GaAs heterostructure material. Room-temperature detection of free-space radiation up to 1.5 THz using a free-electron laser has been achieved by the SSDs-based detector at unbiased condition. To the best knowledge of the author, this is the highest speed reported in room-temperature nanorectifiers to date.The first experimental study on low-frequency noise properties of the SSDs was also performed. The measurements were carried out at room and elevated temperatures using a two-channel cross-correlation technique. The noise performances of the SSDs, which are important in any detector, are discussed in terms of noise-equivalent power and corner frequency. Both parameters are comparable to those reported for state-of-the-art Schottky diodes. The observed noise in the SSDs is described using Hooge's mobility fluctuation theory. Other properties extracted from the results obtained at elevated temperatures, such as activation energy, are also presented. Based on the excellent noise properties measured, an active THz imaging experiment using an SSD-based detector was carried out successfully. A low-cost blackbody radiation source (rather than a laser) was used as a continuous-wave THz generator. Several THz images of hidden objects (e.g., a USB connector underneath its plastic cover) have been obtained by means of raster scanning.

Date of Award	1 Aug 2013
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Aimin Song (Supervisor)