Dr. Himani Arora

Bio

I received my Ph.D. in Physics from TU Dresden (Germany) in 2020 (specialization in ultra-thin semiconductors and their electronic applications). Prior to that, I completed my Bachelor’s in Metallurgical Engineering from IIT-BHU (India) and Master’s in Advanced Functional Materials, jointly from the University of Augsburg (Germany) and the Institut polytechnique de Grenoble (France) under Erasmus Mundus Scholarship Program.

My research is focused on the characterization and integration of nanoscale materials and their heterostructures into electronic devices (transistors, photodetectors, and sensors).

During my career, I have worked at world-renowned organizations in India, Europe, and the United States. Leading (sometimes) inter-disciplinary and multinational teams has enabled me to quickly learn new methods and smoothly integrate into new environments. I am a firm believer that learning continues throughout one’s life. Hence, I try to indulge myself in new challenges and to hone new skills to broaden my abilities and limits.

Publications

Fully Encapsulated and Stable Black Phosphorus Field-Effect Transistors

Advanced Materials Technologies 2200546 (2022)

Abstract (click to expand): Black phosphorus (BP) has quickly gained popularity in the scientific community owing to its interesting semiconducting properties, such as direct bandgap, high mobility, and intrinsic ambipolar behavior. However, its sensitivity to oxygen, moisture, and other air species has restricted its integration into active devices. Here, the lithography-free via-encapsulation scheme to fabricate fully-encapsulated BP-based field-effect transistors (FETs) is employed. The full encapsulation is achieved by sandwiching the BP layers between the top and bottom hexagonal boron nitride (hBN) layers; top hBN passivating the BP layer from the environment and bottom hBN acting as a spacer and suppressing charge transfer to the BP layer from the SiO2 substrate. The embedded via-metal electrodes allow the authors to perform reliable electrical measurements of the BP FETs. Based on these results, it is found that the electronic properties of the via-encapsulated BP FETs are significantly improved compared to unencapsulated devices. This further establishes that the via-contacting scheme leads to superior results compared to graphene-hBN heterostructures and bare hBN layers combined with evaporated metal contacts (both use top and bottom hBN to encapsulate BP) by revealing higher mobility, lower hysteresis, and long-term ambient-stability in BP FETs.

Terahertz control of photoluminescence emission in few-layer InSe

Applied Physics Letters 120, 092104 (2022)

Abstract (click to expand): A promising route for the development of opto-electronic technology is to use terahertz radiation to modulate the optical properties of semiconductors. Here, we demonstrate the dynamical control of photoluminescence (PL) emission in few-layer InSe using picosecond terahertz pulses. We observe a strong PL quenching (up to 50%) after the arrival of the terahertz pulse followed by a reversible recovery of the emission on the timescale of 50 ps at T=10 K. Microscopic calculations reveal that the origin of the photoluminescence quenching is the terahertz absorption by photo-excited carriers. this leads to a heating of the carriers and a broadening of their distribution, which reduces the probability of bimolecular electron-hole recombination and, therefore, the luminescence. By numerically evaluating the Boltzmann equation, we are able to clarify the individual roles of optical and acoustic phonons in the subsequent cooling process. The same PL quenching mechanism is expected in other van der Waals semiconductors, and the effect will be particularly strong for materials with low carrier masses and long carrier relaxation time, which is the case for InSe. This work gives a solid background for the development of opto-electronic applications based on InSe, such as THz detectors and optical modulators.

Electrical Characterization of Germanium Nanowires Using a Symmetric Hall Bar Configuration: Size and Shape Dependence

Nanomaterials 11(11), 2917 (2021)

Abstract (click to expand): The fabrication of individual nanowire-based devices and their comprehensive electrical characterization remains a major challenge. Here, we present a symmetric Hall bar configuration for highly p-type germanium nanowires (GeNWs), fabricated by a top-down approach using electron beam lithography and inductively coupled plasma reactive ion etching. The configuration allows two equivalent measurement sets to check the homogeneity of GeNWs in terms of resistivity and the Hall coefficient. The highest Hall mobility and carrier concentration of GeNWs at 5 K were in the order of 100 cm2/(Vs) and 4×1019cm−3, respectively. With a decreasing nanowire width, the resistivity increases and the carrier concentration decreases, which is attributed to carrier scattering in the region near the surface. By comparing the measured data with simulations, one can conclude the existence of a depletion region, which decreases the effective cross-section of GeNWs. Moreover, the resistivity of thin GeNWs is strongly influenced by the cross-sectional shape.

Demonstration of a Broadband Photodetector Based on a Two‐Dimensional Metal–Organic Framework

Advanced Materials 32 (9), 1907063 (2020)

Abstract (click to expand): Metal–organic frameworks (MOFs) are emerging as an appealing class of highly tailorable electrically conducting materials with potential applications in optoelectronics. Yet, the realization of their proof‐of‐concept devices remains a daunting challenge, attributed to their poor electrical properties. Following recent work on a semiconducting Fe3(THT)2(NH4)3 (THT: 2,3,6,7,10,11‐triphenylenehexathiol) 2D MOF with record‐high mobility and band‐like charge transport, here, an Fe3(THT)2(NH4)3 MOF‐based photodetector operating in photoconductive mode capable of detecting a broad wavelength range from UV to NIR (400–1575 nm) is demonstrated. The narrow IR bandgap of the active layer (≈0.45 eV) constrains the performance of the photodetector at room temperature by band‐to‐band thermal excitation of charge carriers. At 77 K, the device performance is significantly improved; two orders of magnitude higher voltage responsivity, lower noise equivalent power, and higher specific detectivity of 7 × 108 cm Hz1/2 W−1 are achieved under 785 nm excitation. These figures of merit are retained over the analyzed spectral region (400–1575 nm) and are commensurate to those obtained with the first demonstrations of graphene‐ and black‐phosphorus‐based photodetectors. This work demonstrates the feasibility of integrating conjugated MOFs as an active element into broadband photodetectors, thus bridging the gap between materials' synthesis and technological applications.

Recent Progress in Contact, Mobility, and Encapsulation Engineering of InSe and GaSe

Infomat (2020)

Abstract (click to expand): The field of two‐dimensional (2D) materials has stimulated considerable interest in the scientific community. Owing to quantum confinement in one direction, intriguing properties have been reported in 2D materials that cannot be observed in their bulk form. The advent of semiconducting 2D materials with a broad range of electronic properties has provided fascinating opportunities to design and configure next‐generation electronics. One such emerging class is the family of III‐VI monochalcogenides, the two prominent members of which are indium selenide (InSe) and gallium selenide (GaSe). In contrast to transition metal dichalcogenides, their high intrinsic mobility and the availability of a direct bandgap at small thicknesses have attracted researchers to investigate the underlying physical phenomena as well as their technological applications. However, the sensitivity of InSe and GaSe to environmental influences has limited their exploitation in functional devices. The lack of methods for their scalable synthesis further hinders the realization of their devices. This review article outlines recent advancements in the synthesis and understanding of the charge transport properties of InSe and GaSe for their integration into technological applications. A detailed summary of the improvements in the device structure by optimizing extrinsic factors such as bottom substrates, metal contacts, and device fabrication schemes is provided. Furthermore, various encapsulation techniques that have been proven effective in preventing the degradation of InSe and GaSe layers under ambient conditions are thoroughly discussed. Finally, this article presents an outlook on future research ventures with respect to ongoing developments and practical viability of these materials.

2D MOFs: A New Platform for Optics?

Optics and Photonics News 31, 36-43 (2020)

Abstract (click to expand): An emerging class of 2D semiconductors, so-called metal-organic frameworks, offers intriguing potential as an active element in optical applications.

Charge transport in two-dimensional materials and their electronic applications

Doctoral Dissertation (2020)

Photoluminescence dynamics in few-layer InSe

Phys. Rev. Materials 4, 044001 (2020)

Abstract (click to expand): We study the optical properties of thin flakes of InSe encapsulated in hexagonal boron nitride. More specifically, we investigate the photoluminescence (PL) emission and its dependence on sample thickness and temperature. Through the analysis of the PL line shape, we discuss the relative weights of the exciton and electron-hole contributions. Thereafter we investigate the PL dynamics. Two contributions are distinguishable at low temperature: direct band-gap electron-hole and defect-assisted recombination. The two recombination processes have lifetimes of τ 1 ∼ 8 ns and τ 2 ∼ 100 ns , respectively. The relative weights of the direct band-gap and defect-assisted contributions show a strong layer dependence due to the direct-to-indirect band-gap crossover. Electron-hole PL lifetime is limited by population transfer to lower-energy states and no dependence on the number of layers was observed. The lifetime of the defect-assisted recombination gets longer for thinner samples. Finally, we show that the PL lifetime decreases at high temperatures as a consequence of more efficient nonradiative recombinations.

Autocorrected Off-Axis Holography of Two-Dimensional Materials

Physical Review Research 2, 043360 (2020)

Abstract (click to expand): The reduced dimensionality in two-dimensional materials leads to a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high-resolution microscopy techniques are indispensable. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the two-dimensional material. We demonstrate these features with the example of h-BN, for which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.

Effective Hexagonal Boron Nitride Passivation of Few-Layered InSe and GaSe to Enhance Their Electronic and Optical Properties

ACS Applied Materials and Interfaces 11 (46), 43480–43487 (2019)

Abstract (click to expand): Indium selenide (InSe) and gallium selenide (GaSe), members of the III–VI chalcogenide family, are emerging two-dimensional (2D) semiconductors with appealing electronic properties. However, their devices are still lagging behind because of their sensitivity to air and device fabrication processes which induce structural damage and hamper their intrinsic properties. Thus, in order to obtain high-performance and stable devices, effective passivation of these air-sensitive materials is strongly required. Here, we demonstrate a hexagonal boron nitride (hBN)-based encapsulation technique, where 2D layers of InSe and GaSe are covered entirely between two layers of hBN. To fabricate devices out of fully encapsulated 2D layers, we employ the lithography-free via-contacting scheme. We find that hBN acts as an excellent encapsulant and a near-ideal substrate for InSe and GaSe by passivating them from the environment and isolating them from the charge disorder at the SiO2 surface. As a result, the encapsulated InSe devices are of high quality and ambient-stable for a long time and show an improved two-terminal mobility of 30–120 cm2 V–1 s–1 as compared to mere ∼1 cm2 V–1 s–1 for unencapsulated devices. On employing this technique to GaSe, we obtain a strong and reproducible photoresponse. In contrast to previous studies, where either good performance or long-term stability was achieved, we demonstrate a combination of both in our devices. This work thus provides a systematic study of fully encapsulated devices based on InSe and GaSe, which has not been reported until now. We believe that this technique can open ways for fundamental studies as well as toward the integration of these materials in technological applications.

Exciton Localization in MoSe2 Monolayers Induced by Adsorbed Gas Molecules

Applied Physics Letters 114, 172106, (2019)

Abstract (click to expand): Lattice defects and dielectric environment play a crucial role for 2D materials. Gas molecules can get physisorbed easily on the surface through van der Waals forces and can modify dramatically their electronic and optical properties. In this work, we investigate the impact of the physisorbed gas molecules on the optical properties of MoSe2 monolayers by means of low-temperature photoluminescence (PL). More specifically, we focus on the physics of excitons localized by gas molecules. The associated PL peak is observed to show a systematic and large red-shift with temperature and a blue-shift with laser irradiation. Both energy shifts are explained in terms of thermal instability of the localization in combination with hopping effects. Finally, a model is presented, which can reproduce the experimental data with excellent agreement.

Direct Correction of Residual Symmetric Aberrations in Electron Holograms of Weak Phase Objects

Microscopy and Microanalysis 25 (Suppl 2), 98–99, (2019)

High-mobility band-like charge transport in a semiconducting two-dimensional metal–organic framework

Nature Materials 17, pp. 1027–1032 (2018)

Abstract (click to expand): Metal–organic frameworks (MOFs) are hybrid materials based on crystalline coordination polymers that consist of metal ions connected by organic ligands. In addition to the traditional applications in gas storage and separation or catalysis, the long-range crystalline order in MOFs, as well as the tunable coupling between the organic and inorganic constituents, has led to the recent development of electrically conductive MOFs as a new generation of electronic materials. However, to date, the nature of charge transport in the MOFs has remained elusive. Here we demonstrate, using high-frequency terahertz photoconductivity and Hall effect measurements, Drude-type band-like transport in a semiconducting, π–d conjugated porous Fe3(THT)2(NH4)3 (THT, 2,3,6,7,10,11-triphenylenehexathiol) two-dimensional MOF, with a room-temperature mobility up to ~ 220 cm2 V–1 s–1. The temperature-dependent conductivity reveals that this mobility represents a lower limit for the material, as mobility is limited by impurity scattering. These results illustrate the potential for high-mobility semiconducting MOFs as active materials in thin-film optoelectronic devices.

Electrical Characterization of Two-Dimensional Materials and Their Heterostructures

IOP Conference Series: Materials Science and Engineering 198, 012002, (2017)

Abstract (click to expand): Two-dimensional (2D) materials have gained enormous attention in recent years owing to their huge potential in future electronics and optics. On the one hand, conventional 2D materials like graphene, MoS2, h-BN are being intensively studied, on the other hand, search for novel 2D materials is at a rapid pace. In this study, we have investigated electrical properties of 2D nanosheets of ultrathin Indium Selenide (InSe), a member of the III-VI chalcogenides' family. The InSe layers were prepared via micromechanical cleavage of its bulk crystal and were integrated into a field-effect transistor (FET) device as the transport channel. On characterizing the InSe-based FETs, InSe showed n-type conductance with the mobility of 2.1×10−4 cm2V−1s−1.

Amorphous indium-gallium-zinc-oxide as electron transport layer in organic photodetectors

Applied Physics Letters 106, 143301 (2015)

Abstract (click to expand): Amorphous indium-gallium-zinc-oxide (a-IGZO) is demonstrated as an electron transport layer (ETL) in a high-performance organic photodetector (OPD). Dark current in the range of 10 nA/cm2 at a bias voltage of −2 V and a high photoresponse in the visible spectrum were obtained in inverted OPDs with poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester active layer. The best results were obtained for the optimum a-IGZO thickness of 7.5 nm with specific detectivity of 3 × 1012 Jones at the wavelength of 550 nm. The performance of the best OPD devices using a-IGZO was shown to be comparable to state-of-the-art devices based on TiOx as ETL, with higher rectification achieved in reverse bias. Yield and reproducibility were also enhanced with a-IGZO, facilitating fabrication of large area OPDs. Furthermore, easier integration with IGZO-based readout backplanes can be envisioned, where the channel material can be used as photodiode buffer layer after additional treatment.

Study of Sliver Defects on Cold Rolled Coils: Effect of Casting Process Parameters

TATA Search 2, 209-214 (2012)

Read my Ph.D. thesis/dissertation here.

Completed Projects

• Investigation of Temperature Dependent Electronic Properties of Encapsulated Black Phosphorus.

• Development of Highly Stable and Sensitive Black Phosphorus Based Gas Sensors as well as Black Phosphorus Based Reconfigurable 2D Circuits.

• Investigation of (opto)electronic properties of π–d conjugated 2D MOFs.

• Characterization of Germanium Nanowires (sub-40 nm) Using a Reliable Hall-bar Configuration.

• Fabrication and Characterization of p–n Junctions Based on Implanted Germanium Nanowires for Photodetection Applications.

Awards and Honors

News and Media Coverage

Personal

Language fluency in Hindi, English, and German.

Check out the amazing amazing Peace Slam Project, of which I am a proud member.

Former player of Belgium National Women’s Cricket Team

Mountaineering enthusiast. Expedition to Surya Top in Himalayas (4,200 meters above sea level) with Ms. Bachendri Pal (first Indian woman to climb Mt. Everest).