TECHNICAL FEATURE Terahertz Detectors An electromagnetic wave that penetrates a US university project is looking to develop clothing could be very useful in looking a range of semiconductor detectors with for concealed weapons and other objects. spectral resolution to support such work. Dr Mike Cooke Terahertz waves offer such a possibility and Pushing semiconductor detectors into the terahertz gap The desire to see hidden threats is one of the concealed weapons and the US military is leading thrusts of the US Homeland Security keen to extend the reach of such devices, effort. Terahertz (1 THz = 1012 Hz) waves along with developing all-weather aircraft penetrate most non-metallic materials, except landing technology (Mike Cooke, “Focusing on water – hence, dry clothing should be trans- high performance and power”, III-Vs Review, parent, while metal and living matter won’t. August 2006, pp. 20–23). Terahertz detectors could also assist in detect- Signals in the terahertz range fall between ing illegal or dangerous materials hidden in conventional radio/microwave frequency elec- baggage or parcels. Spectroscopic informa- tronics up to 100 GHz and photonics above tion from THz waves can also be used to 10 THz (Figure 1). A frequency of 1 THz also detect the binding of inhibitors with protein lies in the far infrared (FIR) range of frequen- targets, allowing for rapid drug screening in cies (~300 GHz–15 THz) with wavelengths pharmaceutical research, and for distinguish- of the order 1 mm–20 µm. Some researchers ing between different tissue types for disease extend the far infrared to ‘extreme infrared’ diagnosis. Secure transmission of signals using with wavelengths of the order 2 mm–100 µm terahertz waves are also under consideration (150 GHz–3 THz). These extreme infrared in some circles. Terahertz airport security photon energies range from around 0.6 systems have already been developed to trap to 12 meV with equivalent temperatures Figure 1. The terahertz gap between electronic and photonic technologies. [From http://electron.physics.buffalo.edu/spec- tre/, the website for Dr Markelz’s group.] 36 III-Vs REVIEW THE ADVANCED SEMICONDUCTOR MAGAZINE VOL 19 - NO 8 - NOVEMBER 2006 iiivs198p36_38.indd 36 31/10/2006 12:24:39 Terahertz Detectors TECHNICAL FEATURE 5–150 K. Transistor technology is only engineering in the University of Buffalo, School now beginning to encroach on the lowest of Engineering and Applied Sciences, focuses terahertz frequencies (Mike Cooke, “Silicon on fabrication and characterization of semicon- transistor hits 500 GHz performance”, III-Vs ductor nanodevices. Review, June/July 2006, pp. 30–31). “We will gain a detailed understanding of how Researchers led by Andrea Markelz, Ph.D., of the electrical properties of semiconductor nan- the University at Buffalo (State University of odevices are modified in the presence of tera- New York) have gained a four-year $1.2m US hertz radiation,” Bird says. National Science Foundation grant to develop There are three types of FIR detector: bolo- semiconductor-based terahertz detectors that metric, photonic and plasmonic. The most can be integrated seamlessly with conventional common are bolometric photoconductors, electronics. where the heating due to absorbed FIR causes The focus will firstly be on the implementa- a net change in conductivity. These detec- tion of frequency-tuneable terahertz detec- tors usually have no frequency resolution. tors based on nanostructured semiconductor Other attempts at photonic detectors (most systems. Among the physical effects to be often resonant tunnelling diodes) are usually explored are plasmons in confined geometries overwhelmed by bolometric effects from free such as quantum dots, as well as photoexcita- carrier absorption. This type of absorption tion of electrons in quantum point contacts normally dominates because one is coupling (QPCs). Both quantum dots and QPCs are built with the electromagnetic field in the plane of in compound semiconductor environments. a two-dimensional electron gas (2DEG) – the The resulting devices are expected to be suita- coupling to the confined transitions is weak ble for integration into large-scale arrays, allow- since the confinement is perpendicular to ing sophisticated temporal and spatial signal the plane. Quantum point contacts (Figures 2 processing functions. A QPC terahertz detector and 3) add further confinement through nega- is likely to be furthest along in development at tively biased metal contacts that raise barriers the end of the grant, according to Markelz. in the 2DEG and deplete the electron popula- tion in certain regions. Markelz’s expertise lies in characterizations of terahertz optical systems and materials at “In our device, we combine the high sensitiv- these frequencies while co-principal investiga- ity of a quantum point contact, with the pho- tor Jonathan Bird, Ph.D., professor of electrical tonic transitions of confinement in the plane Figure 2. In research being conducted by Andrea Markelz and Jonathan Bird at the University at Buffalo, the grey and yellow regions create a quantum point contact nanowire device that detects terahertz radiation emitted by a targeted substance. www.three-fives.com 37 iiivs198p36_38.indd 37 31/10/2006 12:24:47 TECHNICAL FEATURE Terahertz Detectors carried out on an initial prototype for a quan- tum dot plasmonic detector. The electron momentum relaxation time was derived using a Drude model and the researchers found an unexpectedly smaller relaxation time compared with that found from DC measure- ments. The normal Drude like response seen in highly doped semiconductors at room tempera- ture evolves to a non-Drude response at low temperatures. The effect begins to be noticeable at 100 K and increases down to 10 K. The differ- ence is also more pronounced at higher frequen- cies. The suggestion is that small-angle scattering processes lead to weaker heating of the 2DEG compared with the results from DC mobility measurements. Figure 3. Close-up of the to allow for maximal coupling and sensitivity,” The money for the research program comes quantum point contact. says Markelz. “We are aiming to operate the QPC from the NSF Nanoscale Interdisciplinary detector at more than 40 K, but higher tempera- Research Teams (NIRT) initiative. The grant tures are likely to suffer reduced sensitivity due is one of only 10 that the NSF has funded to dark currents arising from the thermal from more than 400 applications received. background. The biggest advantage to the quan- In addition to the University at Buffalo, tum point contact terahertz detector that we researchers will come from the University of are developing is that it will provide spectral California at Santa Barbara, and the Queens information, revealing many wavelengths at once, and Kingsborough Community colleges of allowing for far more precise distinctions among the City University of New York (CUNY). This similar objects.” collection of scientists has wide expertise Markelz adds: “We are also continuing to work in semiconductor nanodevice fabrication, on plasmonic devices using both two-dimen- DC and THz characterization, and theoretical sional and three-dimensional confinement. modelling. Some collaboration is also expect- In this case we will have tuneable frequency ed with the Institute of Physical and Chemical resolution through gate defined length Research (RIKEN, Japan) and the US Sandia scales thus changing the confined plasmon National Laboratory. resonances.” Beyond research, the project has educational ‘Hybrid’ devices combining bolometric and and social aims with the inclusion of commu- plasmonic techniques are also being considered. nity-college students and even ‘engagement’ One idea is to have a metal grating rather than with high school teachers. Indeed, the theo- the simple gate structure of the QPC that, when rists on the project are based at the CUNY colleges that do not have graduate programs. negatively biased, creates a grating for the elec- The collaboration aims to provide state-of- tron waves that is tuned to a particular terahertz the-art research opportunities to students at frequency. Improvements over existing technol- these institutions and encourage them to go ogy are expected to include widely-tuneable on to higher degrees in science and engineer- response frequency, low power consumption, ing. Markelz is also keen to use the project to and enhanced sensitivity. attract more women and other under-repre- A recent Applied Physics Letters paper (Kabir sented groups to the sciences, through such et al., Appl. Phys. Lett., 89, 132109, 2006) organisations as the GGems (Girls and Guys involving a number of University of Buffalo Exploring Math and Science) educational pro- researchers (and researchers from the US gram for high school students that she set up Sandia National Laboratory and Arizona State some years ago with NSF backing. University) studies complex conductivity of high mobility GaAs and InAs two-dimensional Mike Cooke is a freelance technology journalist electron gas (2DEG) systems using terahertz who has been reporting on the semiconductor time domain spectroscopy. The tests were industry since 1997. 38 III-Vs REVIEW THE ADVANCED SEMICONDUCTOR MAGAZINE VOL 19 - NO 8 - NOVEMBER 2006 iiivs198p36_38.indd 38 31/10/2006 12:24:47.