Abstract Defects in solids are unavoidable and can create complex electronic states that can significantly influence the electrical and optical properties of semiconductors. With the rapid progress in the integration of 2D semiconductors in practical devices, it is imperative to understand and characterize the influence of defects in this class of materials. Here, we examine the electrical response of defect filling and emission using deep level transient spectroscopy (DLTS) and reveal defect states and their hybridization in a monolayer MOCVD-grown material deposited on CMOS-compatible substrates. Supported by aberration-corrected STEM imaging and theoretical calculations, we find that neighboring sulfur vacancy pairs introduce additional shallow trap states via hybridization of individual vacancy levels. Even though such vacancy pairs only represent ~10% of the total defect concentration, they can have a substantial influence on the off currents and switching slopes of field-effect transistors based on 2D semiconductors. Our technique, which can quantify the energy states of different defects and their interactions, allows rapid and nondestructive electrical characterization of defect states important for the defect engineering of 2D semiconductors. 

/pdf/electrical-spectroscopy-of-defect-states-and-their-1owmlljs.pdf

Electrical spectroscopy of defect states and their hybridization in monolayer MoS2

Optoelectronic devices are becoming increasingly important due to their compatibility with CMOS fabrication technology and their superior performance in all dimensions compared to currently available devices. Numerous modern applications are formulated based on various aspects of optoelectronic materials and devices, such as artificial intelligence, optical memory, optoelectronic synapses, humanoid-photodetectors, holography, solar cells, charge storage devices, bio-electronic devices, and so on. Persistent photoconductivity (PPC), an optoelectronic phenomenon that has piqued the scientific community’s interest, is a novel approach to these modern applications. In this article, we highlighted the use of PPC in a variety of emerging optoelectronic applications. PPC is a light-induced mechanism that persists after light excitation is terminated, i.e. the response does not stop immediately but remains available for a period of time. In recent years, the time duration over which the response after turning off the illumination is available has been proposed for a variety of applications. PPC has primarily been explored from a theoretical point of view, with the application component being largely ignored. Very recently, the scientific community has started exploring the possible applications pertaining to PPC such as optoelectronic synapses, holography, optical memory, bioelectronics, and artificial intelligence. Depending on the nature of the material and the type of model used in the application, a variety of mechanisms can be used to modulate the charge trapping and de-trapping methodologies for a specific application. This topical review summarizes the origins of PPC, its control mechanism, and recent advances in a variety of materials such as metal oxides, superconductors, nanofibers, 2D-semiconductors, alloys, nitrides, organic materials, topological insulators, and so on. In addition, the paper has carefully explored the development of next-generation optoelectronic applications designed for industry 4.0 leveraging the PPC phenomenon.

A review on realizing the modern optoelectronic applications through persistent photoconductivity

In this study, selective Nb doping (P-type) at the WS2 layer in a WS2-MoS2 lateral heterostructure via a chemical vapor deposition (CVD) method using a solution-phase precursor containing W, Mo, and Nb atoms is proposed. The different chemical activity reactivity (MoO3 > WO3 > Nb2O5) enable the separation of the growth temperature of intrinsic MoS2 to 700 °C (first grown inner layer) and Nb-doped WS2 to 800 °C (second grown outer layer). By controlling the Nb/(W+Nb) molar ratio in the solution precursor, the hole carrier density in the p-type WS2 layer is selectively controlled from approximately 1.87 × 107/cm2 at 1.5 at.% Nb to approximately 1.16 × 1013/cm2 at 8.1 at.% Nb, while the electron carrier density in n-type MoS2 shows negligible change with variation of the Nb molar ratio. As a result, the electrical behavior of the WS2-MoS2 heterostructure transforms from the N-N junction (0 at.% Nb) to the P-N junction (4.5 at.% Nb) and the P-N tunnel junction (8.1 at.% Nb). The band-to-band tunneling at the P-N tunnel junction (8.1 at.% Nb) is eliminated by applying negative gate bias, resulting in a maximum rectification ratio (105) and a minimum channel resistance (108 Ω). With this optimized photodiode (8.1 at.% Nb at Vg = -30 V), an Iphoto/Idark ratio of 6000 and a detectivity of 1.1 × 1014 Jones are achieved, which are approximately 20 and 3 times higher, respectively, than the previously reported highest values for CVD-grown transition-metal dichalcogenide P-N junctions.

Synthesis of a Selectively Nb-Doped WS2-MoS2 Lateral Heterostructure for a High-Detectivity PN Photodiode.

Abstract Transition metal dichalcogenides (TMDs), especially in two-dimensional (2D) form, exhibit many properties desirable for device applications. However, device performance can be hindered by the presence of defects. Here, we combine state of the art experimental and computational approaches to determine formation energies and charge transition levels of defects in bulk and 2D MX 2 (M = Mo or W; X = S, Se, or Te). We perform deep level transient spectroscopy (DLTS) measurements of bulk TMDs. Simultaneously, we calculate formation energies and defect levels of all native point defects, which enable identification of levels observed in DLTS and extend our calculations to vacancies in 2D TMDs, for which DLTS is challenging. We find that reduction of dimensionality of TMDs to 2D has a significant impact on defect properties. This finding may explain differences in optical properties of 2D TMDs synthesized with different methods and lays foundation for future developments of more efficient TMD-based devices. 

/pdf/experimental-and-theoretical-studies-of-native-deep-level-g29uiou9.pdf

Experimental and theoretical studies of native deep-level defects in transition metal dichalcogenides

Two-dimensional layered transition metal dichalcogenides (TMDCs) have demonstrated a huge potential in the broad fields of optoelectronic devices, logic electronics, electronic integration, as well as neural networks. To take full advantage of TMDC characteristics and efficiently design the device structures, one of the most key processes is to control their p-/n-type modulation. In this review, we summarize the p-/n-type modulation of TMDCs based on diverse strategies consisting of intrinsic defect tailoring, substitutional doping, surface charge transfer, chemical intercalation, electrostatic modulation, and dielectric interface engineering. The modulation mechanisms and comparisons of these strategies are analyzed together with a discussion of their corresponding device applications in electronics and optoelectronics. Finally, challenges and outlooks for p-/n-type modulation of TMDCs are presented to provide references for future studies.

p-/n-Type modulation of 2D transition metal dichalcogenides for electronic and optoelectronic devices

Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS2, WS2 and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different hosts, explaining a persistent photoconductivity above 400 K.

/pdf/chemical-trends-of-deep-levels-in-van-der-waals-hbz0ojswsz.pdf

Chemical trends of deep levels in van der Waals semiconductors

Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. We directly evaluate deep levels and their chemical trends in the bandgap of MoS2, WS2 and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different host materials, which explains the chemical trend of native electron density in the hosts as well as a persistent photoconductivity up to 400K.

/pdf/chemical-trends-of-deep-levels-in-van-der-waals-2vnw641dpg.pdf

This paper discusses the development of Electronic, Optical, and Magnetic Properties of Materials as a MOOC offered on the edX platform. In particular, we discuss the decision to include comics and comic-based themes as a component of the homework assignments in the course. We present an A/B test conducted on the edX platform that compares the performance of students given the comic-based homework assignments with students given traditional homework assignments. No difference in performance is observed between the two groups. We also present survey data that compares the reactions of MOOC learners and residential university students to the comic-based homework assignments. The university students reported a high degree of enjoyment of the comic-based homework, while the MOOC learners had a more mixed reaction.

Electronic, Optical, and Magnetic Properties of Materials: A Comic-Based MOOC

The dominant techniques for laser-based thermal measurement, time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance, generally require that samples be coated with a metal transducer layer. The properties of the transducer and its interface to the sample are then important to the interpretation of results, and heat conduction in the transducer layer can limit measurements of ultra-thin samples or in-plane conductivity. We demonstrate the feasibility of measuring semiconductor samples using standard TDTR instrumentation, without the use of a metal transducer. A mathematical model accounting for non-zero laser penetration depth as well as both thermal and charge carrier effects is presented, and the ability of the model to describe and fit to experimental data for silicon and germanium samples is demonstrated. Though at this stage measurement uncertainties are larger than for traditional TDTR, we anticipate that the use of this technique will expand the range of samples that can be measured with existing TDTR setups. Published under an exclusive license by AIP Publishing. https://doi.org/10.1063/5.0069360

Transducerless time domain reflectance measurement of semiconductor thermal properties

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Chemical trends of deep levels in van der Waals semiconductors

Chemical trends of deep levels in van der Waals semiconductors

Electronic, Optical, and Magnetic Properties of Materials: A Comic-Based MOOC

Transducerless time domain reflectance measurement of semiconductor thermal properties