Advanced Experimental Methods for Low-temperature Magnetotransport Measurement of Novel Materials

Summary

We describe the methodology of mechanical exfoliation and deposition of flakes of novel materials with micron-sized dimensions onto substrate, fabrication of experimental device structures for transport experimentation, and the magnetotransport measurement in a dry helium close-cycle cryostat at temperatures down to 0.300 K and magnetic fields up to 12 T.

Abstract

Novel electronic materials are often produced for the first time by synthesis processes that yield bulk crystals (in contrast to single crystal thin film synthesis) for the purpose of exploratory materials research. Certain materials pose a challenge wherein the traditional bulk Hall bar device fabrication method is insufficient to produce a measureable device for sample transport measurement, principally because the single crystal size is too small to attach wire leads to the sample in a Hall bar configuration. This can be, for example, because the first batch of a new material synthesized yields very small single crystals or because flakes of samples of one to very few monolayers are desired. In order to enable rapid characterization of materials that may be carried out in parallel with improvements to their growth methodology, a method of device fabrication for very small samples has been devised to permit the characterization of novel materials as soon as a preliminary batch has been produced. A slight variation of this methodology is applicable to producing devices using exfoliated samples of two-dimensional materials such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (TMDs), as well as multilayer heterostructures of such materials. Here we present detailed protocols for the experimental device fabrication of fragments and flakes of novel materials with micron-sized dimensions onto substrate and subsequent measurement in a commercial superconducting magnet, dry helium close-cycle cryostat magnetotransport system at temperatures down to 0.300 K and magnetic fields up to 12 T.

Introduction

The pursuit of materials platforms for advanced electronics technology demands methods for high-throughput materials synthesis and subsequent characterization. Novel materials of interest in this pursuit may be produced in bulk by direct reaction synthesis^1,2, electrochemical growth^3,4, and other methods⁵ in a more rapid fashion than more involved single crystal thin film deposition techniques such as molecular beam epitaxy or chemical vapor deposition. The conventional method to measure the transport properties of bulk crystal samples is to cleave a rectangular prism-shaped fragment with dimensions of approximately 1 mm x 1 mm x 6 mm ....

Protocol

1. Preparation of Substrate

1. Obtain 4 inch silicon (Si) wafer composed of heavily-doped p-doped Si covered by approximately 300 nm of SiO₂. This substrate structure will allow the substrate to serve as a back gate.
2. Using drafting/design software, design a 1 cm × 1 cm pattern with evenly spaced features, such as enumerated crosses, in the x and y direction to use as positional locators on the substrate for transferred sample flakes and alignment marks for electron beam lithography (Figure 1).
   1. Open a new file in a drafting program such as AutoCAD.
   2. Use polylines to draw the following marks: i) c....

Results

Figure 3 shows a typical Hall bar device patterned for the purpose of a low temperature magnetotransport experiment. The optical image in the upper figure shows a successfully-fabricated Graphene/hBN Hall bar; the lower image shows the device schematic with the Landauer-Büttiker edge states that arise from the Landau levels (LLs), a transport mechanism that can be used to calculate the values of the quantized Hall resistances, the experimental investigation of which will .......

Discussion

After acquisition of high quality bulk samples, characterized to ensure appropriate composition and structure, samples are patterned into the geometry depicted by exfoliating flakes of sample onto 1 cm × 1 cm pieces of substrate. Substrates composed of heavily p-doped Si covered by approximately 300 nm of SiO₂ are preferred as they increase the experimental parameter space by allowing the application of a back gate. Samples must be sufficiently thin — fewer than 10 nm — to produce a suffi.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Cryogenic Limited 12 T CFMS	Cryogen Limited	CFM-12T-H3- IVTI-25	Magnetotransport system customized with modulated field magnet (step 4)
7270 DSP Lock-in amplifier	Signal Recovery	7270	lock-in amplifier for source/drain and voltage measurements (step 4)
GS200 DC Voltage/Current Source	Yokogawa	GS200	Voltage source for gate voltage application (step 4)
2636B System Sourcemeter	Keithley	2636B	Sourcemeter for source/drain and voltage measurements
DWL 2000 Laser Pattern Generator	Heidelberg Instruments	DWL 2000	Generate chrome mask for lithography of substrate location/alignment feature pattern (step 1.3)
Suss MicroTec MA6/BA6 Contact Aligner	Suss	MA6	Used for the lithography of substrate location/alignment feature pattern (step 1.4.12)
JEOL Direct Write Electron Beam Lithography System	JEOL	JBX 6300-FS	Perform high-resolution lithography of devices
Discovery 550 Sputtering System	Denton Vacuum	Discovery 550	Perform SiO2 sputtering (step 2.5)
Infinity 22 Electron Beam Evaporator	Denton Vacuum	Infinty 22	Perform Cr/Au deposition (steps 1.5 and 3.7)
Unaxis 790 Reactive Ion Etcher	Unaxis	Unaxis 790	Etch sample into Hall bar structure (step 3.4)
PMMA 495 A4	MicroChem	PMMA 495 A4	Polymer coating/electron beam mask for lithography (step 3.5.1)
PMMA 950 A4	MicroChem	PMMA 950 A4	Polymer coating/electron beam mask for sample dicing and lithography (steps 1.7.3, 3.3.1, and 3.5.2)
S1813 positive photoresist	MicroChem	S1813 G2	Positive photoresist (step 1.4.8)
LOR resist	MicroChem	LOR 3A	Lift off resist (step 1.4.3)
1:3 MIBK:IPA PMMA developer	MicroChem	1:3 MIBK:IPA	PMMA developer
MF-321 Developer	MicroChem	MF-321	Novolac positive photoresist-compatible developer solution (step 1.4.15)
Diglycidiyl ether-terminated polydimethylsiloxane	Sigma Aldrich	SA 480282	For layered material stacking (step 2.6.1)
Polypropylene carbonate	Sigma Aldrich	SA 389021	For layered material stacking (step 2.6.2)