Developing a Hall Bar Configuration for semiconducting Nanowires

Despite constant improvement in the performance of semiconducting nanowires (NWs) based devices, evaluating electrical properties of single NWs still remains a challenging task. So far, several techniques have been developed to this end. The field effect (FE) mobility measurement is the most commonly used technique, although it has some shortcomings [1,2]. The accuracy of this method depends largely on the precision of the estimated gate capacitance. Also, it characterizes only the depleted layer of charge carriers close to the gate, and estimates the carrier concentration of NWs by assuming a radially constant mobility. Unlike the FE measurement, the Hall Effect measurement provides a more direct characterization of carrier concentration by considering the entire cross-section of the semiconducting NWs [2,3]. However, the fabrication of NW based Hall devices is a challenging process and requires a very high accuracy regarding the alignment of the Hall contacts. The Hall bar configuration with narrow bars can increase the precision of Hall contacts fabrication and enhance the accuracy of the Hall Effect measurement by avoiding shorting out the Hall voltage. Recently, the Hall bar configuration has been used for SiNWs with a five-contact geometry [4]. To the best of our knowledge, such a Hall bar configuration has not been developed for GeNWs so far.

In this work, GeNWs were fabricated on Ge-on-insulator (GeOI) substrates with a top-down approach using electron beam lithography (EBL) and inductively coupled plasma reactive ion etching (ICP-RIE). To investigate the electrical properties of the fabricated NWs, we propose a six-contact Hall bar configuration with symmetric contact bars located opposite to each other, as shown in Figure 1. Using this configuration, the Hall Effect and four-probe measurements were performed on single GeNWs to quantify their carrier concentration (n), Hall mobility (µH), and resistivity (ρ). A nanowire with a width down to about 40 nm was characterized to show the capability of the proposed Hall bar configuration to reliably evaluate the electrical properties of even very small nanowires. Moreover, the effect of NW width on transport parameters such as resistivity, carrier concentration and mobility was investigated. With decreasing nanowires width, the resistivity increases and carrier concentration decreases, which is mainly attributed to the diffusion of carriers into the surface. Figure 2 shows the size-dependent resistivity and temperature-dependent Hall mobility of the Ge NWs.