Nonuniform Carrier Heating Induced Nonlinear Electron Transport Properties in Asymmetrically Necked InAs Mesa Structures
Authors: Ohashi Ryota , Shimada Daichi , Koyama Masatoshi , Maemoto Toshihiko , Sasa Shigehiko 
Published: December 2020

Introduction
Terahertz (THz) wave has been very attractive light for scientific and practical applications such as light source for characterizing fundamental physics on ultra-fast carrier dynamics in semiconductors and imaging applications in the submillimeter wavelength regime. Therefore, there have been studied on various kind of emitting and detecting methods on THz waves in this quarter century. So far, we also have proposed simple and low-cost THz wave source with InAs thin films grown on GaAs substrates combining with a time-domain spectroscopy system [1, 2]. In case of THz detectors, there are not so many techniques which enable to detect THz wave at room temperature with high responsivity without biasing. For example, square-law field rectifier detectors, such as Schottky diodes and InGaAs-based bow-tie diodes, respond to THz electric field at room temperature with no external power supply [3]. Nonlinear current-voltage characteristics of the latter technique is based on unique operation principle which is generated by nonuniform carrier heating in asymmetric bow-tie shaped planar structures. Since InAs possesses one of the highest electron mobility at room temperature among semiconductors, this material based thin films will be hopeful candidate for detecting THz waves by fabricating this unique nonlinear device structures on them. In this paper, we devise utilizing InAs THz wave emitter thin films to base material for asymmetrically necked bow-tie mesa structures for the THz detector. As a first step of this idea, nonlinear current-voltage characteristics of these devices are presented. We especially discuss on the curvature of current-voltage characteristic at zero-bias point which is closely related to the sensitivity on square-law detection for THz wave.
Experimental
Nominally undoped InAs thin films were epitaxially grown on semi-insulating GaAs (100) substrate by molecular beam epitaxy. For improving the crystal quality of InAs channel layer, 150 nm thick InAs buffer layer was directly grown with gradually changing growth temperature from 300 to 500°C on GaAs substrate. An 1 µm think InAs channel layer was grown on this buffer layer at 450°C. This growth condition is same as that of THz emitter InAs films we have reported [2]. The patterning of asymmetric bow-tie mesa structure was carried out by photo lithography and phosphoric acid based wet chemical etching. After defining mesa structures, in order to investigate the film thickness dependence on nonlinear current properties, some samples were performed recess etching for InAs channel, which were made each thickness to be 265 nm, 435 nm, 625 nm and 835 nm. Finally, non-alloyed ohmic contact metals of Ti/Pt/Au (50/30/170 nm) were deposited by electron-beam evaporation technique and defined by lift-off. The structure of asymmetric bow-tie diode and the measurement configuration are shown in Fig. 1. Current-voltage characteristics were measured by using Keithley 4200A-SCS system at room temperature.
Results and Conclusion
Typical current-voltage characteristic of the investigated InAs asymmetric bow-tie diode is represented in Fig. 2, which was the result measured with the sample of 265 nm thick InAs channel. It showed clear asymmetric current-voltage characteristic depended on the polarity of applied voltage, which was considered originated from nonlinear carrier transport related to non-uniform carrier heating in asymmetric mesa structure. In order to evaluate the nonlinearity of current-voltage curve at zero-biased point, the zero-biased curvature coefficient γ was calculated, which is given by the formula written in Fig. 3. A fifth order polynomial fit was utilized to current-voltage curves in determining the curvature. As shown in Fig. 3, the zero-bias curvature clearly depended on thickness of InAs thin film. The highest zero-bias curvature of 0.12 V-1 was obtained in the thinnest sample. In addition, this value was about 2.2 times larger than that of the GaAs/AlGaAs heterostructure-based asymmetric bow-tie diode which had the same shape and size (not shown here). These results indicate that the InAs thin film for THz radiation has possibility to be applied to square-law detection device for THz wave. The details of nonlinearity which depends on mesa shape and size and comparison between InAs based structures and that of GaAs/AlGaAs will be discussed at the conference.
References
[1] S. Sasa et al., Journal of Physics: Conf. Series 906, 012015 (2017).
[2] S. Sasa et al. Int. Conf. on Infrared, Millimeter, and Terahertz Waves. IRMMW-THz 2018.
[3] V. Palenskis et al. Sensors 18, 3760 (2018).

Figure 1