Bow-tie diodes for terahertz imaging: Comparative study

Abstract

Abstract-The concept of broadband bow-tie diodes as terahertz sensors and their possible application for imaging is presented. Different bow-tie sensors fabricated of planar GaAs, modulation doped GaAs/AlGaAs structures and InGaAs layers are compared and discussed. It is demonstrated experimentally that the sensitivity of all studied devices at room temperature is nearly independent of frequency below 1THz and amounts to 0.3V/W for GaAs-based structures and 10V/W for InGaAs sensors. Room temperature terahertz images are recorded at 0.71THz and 1.4THz. Their parameters are considered in comparison with images taken by using commercially available pyroelectric sensors.The implementation of terahertz (THz) radiation in security applications and materials inspection requires reliable and compact solutions in the development of room temperature imaging systems. In these kinds of requirements, preference goes to solid-state-based solutions. A recent major breakthrough made in all-solidstate tunable sources has opened a promising route in the progress of room temperature THz emitters [1]. As concerns room temperature solid-state THz detectors, one can mention Schottky diodes [2], nanometric field effect transistors [3,4] and microbolometers [5,6]. Very recent achievements in compact room temperature spectroscopic [7] and heterodyne THz imaging [8] allowed one to extend the family of compact THz sensors by adding a member of so-called InGaAs based bow-tie (BT) diodes. The sensors exhibit broadband operation as well as relatively simple device production and good reliability.In this paper, we present a comparative study of different BT diode sensors. An operational principle and spectral features of the device is discussed, BT diode's parameters based on InGaAs layers, modulation doped GaAs/AlGaAs structures and planar GaAs are compared.Terahertz images recorded at room temperature using InGaAs BTs are compared with images taken by using commercially available pyro-electric sensors; feasibility for real-time THz imaging is considered, too.The bow-tie diode uses an idea to combine the performance of a bow-tie antenna as a coupler of incident * E-mail: irmantak@ktl.mii.lt, † E-mail: valusis@pfi.lt radiation and the semiconducting part as radiation sensing media. The diode displays structurally broken symmetry as depicted in Fig. 1: One of the two semiconductor leaves is metalized in order to couple incident radiation into the second one, where high mobility electrons are heated nonuniformly by incident THz radiation [9]. As a result, the voltage signal across the leaves is induced without any application of a bias voltage.Three different types of bow-tie diodes having the same geometry, but fabricated of different materials, were studied. The first one was GaAs/AlGaAs modulation doped structures consisting of a 20-nm-thick GaAs cap layer, Si-doped (1×10