Creating a better semiconductor in femtoseconds with ‘photo-doping’

June 29, 2015

Certain compounds can exhibit multiple quantum phases, including Mott insulator, superconductor, and spin or charge density wave (CDW) states based on subtle physical tunings, including applying heat, pressure (P), and doping (x) (credit: Tzong-Ru T. Han et al./Science Advances)

Michigan State University (MSU) researchers have developed a “photo-doping” process by shooting an ultrafast laser pulse into a semiconductor* material — rapidly changing its properties as if it had been chemically “doped.”

Changing the electrical properties of semiconductors formerly required a complex, expensive process of adding different dopants, or trace chemical impurities.

The new research could lead to development of next-generation electronic materials and even optically controlled switching devices without requiring doping of semiconductor materials.

“The material we studied is an unconventional semiconductor made of alternating atomically thin layers of metals and insulators,” said Chong-Yu Ruan, an associate professor of physics and astronomy who led the research effort at MSU.

“This combination allows many unusual properties, including highly resistive and also superconducting behaviors to emerge, especially when ‘doped.'”

By varying the wavelengths and intensities of the laser pulses, the researchers were able to observe phases with different properties that are captured on the femtosecond timescale. A femtosecond is 1 quadrillionth, or 1 millionth of 1 billionth, of a second.

“The laser pulses act like dopants that temporarily weaken the glue that binds charges and ions together in the materials at a speed that is ultrafast and allow new electronic phases to spontaneously form to engineer new properties,” Ruan said. “Capturing these processes in the act allows us to understand the physical nature of transformations at the most fundamental level.”

The research is described in a open-access paper published in the journal Science Advances.

* A semiconductor is a substance that conducts electricity under some conditions but not others, making it a good medium for the control of electrical current. Semiconductors are used in many electronic devices, including computers.


Abstract of Exploration of metastability and hidden phases in correlated electron crystals visualized by femtosecond optical doping and electron crystallography

Characterizing and understanding the emergence of multiple macroscopically ordered electronic phases through subtle tuning of temperature, pressure, and chemical doping has been a long-standing central issue for complex materials research. We report the first comprehensive studies of optical doping–induced emergence of stable phases and metastable hidden phases visualized in situ by femtosecond electron crystallography. The electronic phase transitions are triggered by femtosecond infrared pulses, and a temperature–optical density phase diagram is constructed and substantiated with the dynamics of metastable states, highlighting the cooperation and competition through which the macroscopic quantum orders emerge. These results elucidate key pathways of femtosecond electronic switching phenomena and provide an important new avenue to comprehensively investigate optical doping–induced transition states and phase diagrams of complex materials with wide-ranging applications.