A Breakthrough in Semiconductor Physics

Semiconductors find use in many electronic applications. A semiconductor uses the properties of a conductor, as well as an insulator to derive electrical conductivity. Semiconductor examples include silicon (Si) and germanium (Ge).

Semiconductor devices display a wide variety of properties such as conductivity, resistance, and sensitivity to light or heat. Doping, which is the process by which impurities are added to a semiconductor device, and passing electrical current, and light, are ways by which the electrical conductivity of a semiconductor device can be increased.

Recently, materials known as low-dimensional transition metal dichalcogenides (TMDC) have been attracting a lot of attention from researchers. These are atomically thin semiconductor materials with the formula MX2, where M is a transition metal (Molybdenum (Mo), Tungsten (W)), and X is a chalcogen atom (Sulphur (S), Selenium (Se), Tellurium (Te)). A chalcogen is an element from Group 16 of the periodic table, also known as the oxygen family.

Transition metal dichalcogenides have many useful properties and find use in optoelectronics and valleytronics. Photo-induced dynamics in these systems have been studied from the point-of-view of individual quasi-particles such as excitons, bi-excitons, and trions. Quasi-particles refer to a physical concept which treats elementary excitations in solids, like spin waves, as particles.

The role of exciton dynamics, the associated collective behaviour, condensation, and inter-excitonic interactions remain intriguing and seek attention, especially in room-temperature scenarios which are relevant for device applications.

Under suitable conditions, the condensation of these excitonic particles into quantum clusters such as electron-hole liquid (EHL) phase can occur. The distinguishing behaviour of the quantum nature of EHL in contrast to a typical classical liquid has been explored in coupled semiconductor quantum well recently.

These quantum phases play a crucial role in realizing room-temperature applications exploiting the unique properties of low-dimensional transition metal dichalcogenides such as molybdenum disulphide (MoS2) and realizing devices out of them.

There is therefore a lot of research going into the dynamics of these condensates to realize such devices. However, condensed excitonic phases such as EHL remain scarcely available especially at room-temperature as low-critical temperature is required for their formation in bulk or even most nanoscale systems. However, EHL phase can be stable at room- temperature with the help of low-dimensional transition metal dichalcogenides.

In this study, conducted by Ms. Pritha Dey, Mr. Anubhab Sahoo, Prof. Cheriyanath Vijayan, and Prof. Sivarama Krishnan from the Department of Physics, Indian Institute of Technology Madras, Chennai, India (Prof. Sivarama Krishnan is also from the Quantum Center for Diamond and Emerging Materials, Indian Institute of Technology Madras, Chennai, India), Dr. Tejendra Dixit from the Optoelectronics and Quantum Devices Group, Department of Electronics and Communication Engineering, Indian Institute of Information Technology Design and Manufacturing, Kancheepuram, Chennai, India, and Dr. Vikash Mishra from the Department of Physics, Nano Functional Materials Technology Center and Materials Science Research Center, Indian Institute of Technology Madras, Chennai, India, for the first time to the best of the authors’ knowledge, using femtosecond (a quadrillionth of a  second (10-15)) transient absorption spectroscopy (fs-TAS), time-resolved dynamics of bangap renormalization (BGR) arising from the highly correlated EHL phase has been measured and revealed. EHL phase was formed on picosecond (a trillionth of a second (10-12)) timescales in the ultrafast relaxation of photoexcited multilayered molybdenum disulphide nanoparticles.

Bandgap renormalization (BGR) happens when Coulomb interaction (Coulomb interaction is the primary force determining the behaviour of colliding atoms or molecules) between charges of the same sign results in a decrease in the quasi-particle energy.

To the best of the authors’ knowledge, this is the first time experimental evidence for EHL-induced BGR has been reported.

The dynamics of electron-hole liquid (EHL) and electron-hole plasma (EHP) phases were analyzed from the temporal evolution of excitonic resonances. Insights into the individual contributions of the droplet, EHP, excitonic and phononic states (phonons are collective excitations in a periodic, elastic arrangement of atoms and molecules in condensed matter) into the total transient absorbance as well as the BGR were obtained.

The dynamics was dominated by the EHP phase at initial timescales of 2 to 4 picoseconds, after which the quantum EHL phase emerged and started to dominate the transient absorption approximately till 10 picoseconds. Within this period, the trapping of carriers resulted in phonon emission, which subsequently took over the dynamics until these carriers decayed via radiative or non-radiative processes.

The quantum nature of EHLs further emphasizes its applicability in quantum circuits. The results of the numerical modelling and experiments, on the formation and evolution of the excitonic condensates in this study, lead to identification of this new paradigm, opening up ample scope for harnessing the electron-hole liquid at room-temperature, in likely quantum devices.           

Prof. Sai Santosh Kumar Raavi, Associate Professor from the Department of Physics, Indian Institute of Technology Hyderabad, Telangana, India, confirmed the importance of this study with the following comments: “Low-dimensional materials have opened new doors for physics and applications in electronics, photonics, optoelectronics and even quantum devices. The arrival of graphene made waves a decade ago, but its lack of bandgap led to a search for other candidates. Transition metal dichalcogenides (TMDCs) emerged as favourites, with the molybdenum sulphide system as an early success for realizing semiconducting devices. This work on time-resolved spectroscopy by the IIT Madras group has revealed an unforeseen state of electrons and holes following the excitation of MoS2 quantum dots by ultrashort pulses. The results indicate that even at room temperature, electrons and holes can form a quantum liquid-like condensed aggregate which lives for several picoseconds after the excitation. In terms of an analogy from day-to-day life, this is like mist formation on leaves or grass. The revelation of this intermediate state is an essential piece of information and knowledge which impacts the realization of optoelectronic and quantum electronic devices using low-dimensional TMDCs as a generic platform for technology.”

Article by Akshay Anantharaman
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