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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - PTMCnano (Post-transition metal chalcogenides: 2D nanoelectronics and photonics)

Teaser

Summary

The family of two-dimensional (2D) crystals encompasses materials from insulators and semiconductors to superconductors, and is rapidly expanding. Such crystals can be visualized as a single molecular plane peeled off of a layered bulk (3D) crystal. Stacking layers of different 2D materials on top of each is similar in effect to the molecular beam epitaxy used to produce the highest quality semiconductor devices, but much more versatile: such multilayer heterostructures are easier to fabricate, and there is a huge number of potential combinations. Since 2D heterostructures may have exciting new properties that are completely different from those of the parent materials, they will likely revolutionalize the semiconductor industry in the following decades.
Recent results on a subgroup of this family, post-transition metal chalcogenides (PTMCs), like indium or gallium selenide (InSe, GaSe), suggest that transistors and more advanced nanodevices of exceptional electronic quality can be fabricated of these materials. Moreover, they are excellent candidates for photodetectors, LEDs, and other photonic applications.
The main objectives of the project were the following:
1. Fabricate and electrically characterize PTMC transistors, in order to find the best methods for fabrication, including combinations with other 2D crystals.
2. By using so-called gate electrodes over the surface of a PTMC layer, electrostatically trap electrons to quasi isolated islands called quantum dots (QDs). These can potentially be exploited to create quantum bits for quantum computers. Since no QDs have been made of PTMCs before, it is important to understand their electronic transport characteristics, to determine their viability as quantum bits.
3. Fabricate PTMC devices for optical applications.

Conclusions:
During the action, several PTMC transistors have been fabricated and electrically characterized. Of these, indium selenide (InSe)-based devices showed the greatest promise. Many devices included top gates: with these, quasi one dimensional conducting channels have been successfully defined, and conductance quantization has been observed, indicating their outstanding quality. Single quantum dots have also been created in InSe for the first time. Working light emitting diodes (LEDs) have also been fabricated. However, sample quality was sometimes inadequate, likely due to surface contamination. Therefore, a technique has been developed to stack 2D layers on each other in total vacuum: measurements on the first transistors indicate high quality, showing that this is a promising way forward.

Work performed

\"The main results related to the objectives above, described in more detail:
1. The Researcher performed electric measurements on several PTMC based devices. They were all contacted with graphene, and encapsulated in hexagonal boron nitride (hBN) for protection against environmental contamination or oxidation. GaSe devices showed insulating behaviour, probably due to low quality contacts. Therefore, the InSe devices had top gates over the graphene-InSe overlap (see the attached image for illustration), to locally tune the areal density of electrons and thus improve contact resistances. These samples were conductive, but contact resistances were inconsistent, scattered over several orders of magnitude. Applying a top gate voltage at the contacts produced varying responses.
2. In spite of the moderate yield of working contacts and high-quality samples, it was possible to fabricate a few devices where electrons were electrostatically trapped into narrow conducting channels, or into islands: quantum dots (QD). This was accomplished by applying a voltage to local top gates, illustrated on the attached image. These results were evaluated by the Researcher and have been published in the journal Nano Letters, in 2018, with the title \"\"Gate-Defined Quantum Confinement in InSe-Based van der Waals Heterostructures\"\". Measurements on the channels demonstrated conductance quantization. Single quantum dot measurements showed that, though the confinement was weak, electrons were indeed isolated to the small area of the dot. These are the first demonstration of one and zero dimensional confinement, as well as conductance quantization, in InSe based nanodevices.
3. Several InSe heterostructures were fabricated, consisting of graphene, hBN, InSe, hBN and graphene stacked on each other. When voltage was applied between the outermost (graphene) layers, these devices produced electroluminescence, i.e. they worked as light emitting diodes (LEDs). Their current-voltage characteristics were compared with theoretical simulations developed by the Researcher. These results have been accepted to Nature Communications for publication. The characterization methods described in the manuscript could be exploited in the future to probe the finer features in a 2D deviceâ€™s band structure. Furthermore, the appearance of electroluminescence demonstrated that InSe can be used in the fabrication of infrared LEDs, where the output wavelength varies with the number of layers.
4. Due to the moderate yield of working contacts and high quality devices, and the limited effectiveness of top gates to induce confinement, the fabrication procedure had to be further improved. A promising solution to these issues is to prepare each layer and stack them in an ultra-high vacuum (UHV) chamber, to avoid contamination and corrosion. The Researcher successfully adapted a layer stacking technique used in atmosphere, and fabricated the first 2D samples in UHV. In general, these devices showed less contamination, while their electronic qualities were comparable to the best samples prepared in a glovebox, the cleanest environment before this approach.

Besides the publications mentioned above, these results have been widely disseminated in the form of lectures or poster presentations at several seminars, workshops, and international conferences, including Graphene Week 2018, or the Basel Workshop on 2D Materials 2018.\"

Final results

InSe-based one-dimensional channels were shown to exhibit conductance quantization, which so far has only been observed in the highest quality 2D samples, mainly made of graphene. Moreover, measurements on InSe quantum dots, fabricated successfully for the first time, showed that electrons can be confined by electrostatic gating.
The Researcher developed a semi-automated control program for the motorized sample stages of a UHV system (used to align and stack layers on top of each other), and adapted a fabrication recipe for heterostructures to UHV conditions.
The former results are significant milestones in the research of PTMCs. The latter developments have the potential to produce devices of exceptional quality with a high yield, and especially to prepare materials that are sensitive to air and water vapor, such as GaSe. The UHV fabrication technique will be exploited to make complex nanoelectronic samples like quantum dots or photonic devices, as well as other layered structures.