Atypical quantized resistances in millimeter-scale epitaxial graphene p-n junctions

Abstract

We have demonstrated the millimeter-scale fabrication of monolayer epitaxial graphene p-n junction devices using simple ultraviolet photolithography, thereby significantly reducing device processing time compared to that of electron beam lithography typically used for obtaining sharp junctions. This work presents measurements yielding nonconventional, fractional multiples of the typical quantized Hall resistance at ν = 2 (R H ≈ 12906 Ω) that take the form: a b R H. Here, a and b have been observed to take on values such 1, 2, 3, and 5 to form various coefficients of R H. Additionally, we provide a framework for exploring future device configurations using the LTspice circuit simulator as a guide to understand the abundance of available fractions one may be able to measure. These results support the potential for drastically simplifying device processing time and may be used for many other two-dimensional materials.

Development of gateless quantum Hall checkerboard p–n junction devices

“…To that end, an additional set of fabrication processes was implemented using S1813 photoresist intended for protecting n-type regions and using PMMA/MMA and ZEP520A photoresists as capping materials for regions intended to be p-type [25][26]49]. The PMMA/MMA acts as a mediation layer for ZEP520A, a common electron beam resist with photoactive properties.…”

“…Though various linear Hall bar pnJ devices have been fabricated, most devices have sizes of the order 100 μm or smaller due to the typical requirement of top-gating, whose effectiveness drastically reduces with size owing to the higher probability of current leakage. This size limitation has recently been lifted, as has the requirement of using a top gate to modulate the carrier density in graphene [25][26][27][28]. Non-conventional geometries of pnJ devices have not yet been explored since fabrication methods were still quite intricate and may have posed challenges in the past.…”

“…Subsequent measurements of those devices in the quantum Hall regime were performed to compare experimental quantized resistances to those obtained with the LTspice circuit simulator [29][30]. Since all epitaxial graphene regions exhibit a resistance of R K = ℎ 2e 2 at sufficiently high magnetic flux density (where the von Klitzing constant R K is defined as containing the Planck constant h and the elementary charge e), all nonconventional quantized resistances presented herein will have resulted from the use of several contact terminals as sources or drains [25][26][27][28]. The advantage of these complex configurations is that a whole set of quantized resistances becomes accessible and can provide overwhelming evidence of device functionality in addition to being a future application for quantum-based electrical standards.…”

The quantum Hall effect in the era of the new SI

“…With the complexity of EG-based devices increasing, coupled with the capability of having effectively zero contact resistance, much larger arrays can be constructed having an abundance of new quantized resistance values. Examples of such devices are shown in Figure 4 (a) and (b), where may select any two arbitrary points between which to measure an effective quantized resistance whose exact value can be determined by various models like Kwant, a simulation program with integrated circuit emphasis (SPICE, LTspice), or other generalized parametric analyses [107][108][109].…”

“…If implementing controllable gates on EG-based devices becomes possible, then revolutionary devices like the programmable QHR may become future milestones (see Figure 4 (c)). And although pnJs for metrology need to be further developed in terms of gating, the scalability of that gating has already been demonstrated for semi-permanent gating [108]. Furthermore, the latter devices, when fed multiple sources of current at arbitrary points, have been demonstrated to exhibit electric potential redistributions resulting in atypical values of resistance [108].…”