Research in the Petrou group during the last ten years has concentrated on the emerging field of spintronics. In particular, the group studies injection of spin-polarized carriers into semiconductor heterostructures. The devices for this work are provided by Dr. H. Luo (University at Buffalo), Dr. B.T. Jonker (Naval Research Lab), and Dr. S. Holmes (Toshiba-UK). Various techniques such as photoluminescence, electroluminescence, optical pumping, and Hanle spectroscopies are employed. The experiments are carried out in the 5-300K temperature range in the presence of magnetic fields.
Dilute Magnetic Semiconductors (DMS) quantum dots (QD), which have a discrete energy spectrum and a wider range of tunability, are useful for applications in opto-electronics, spintronics and quantum information processing. While the modern semiconductor industry is more reliant on the charge of an electrons for its operation, DMS devices give us the opportunity to harness both the charge and spin for memory applications. Although bulk DMS has been extensively studied, not much has been done on the nanoscale, particularly in case of QDs where the confinement greatly enhances the exchange interaction between the spins of charge carriers (electrons/holes) and that of the magnetic ions like Manganese. In fact there are reports of optical control of spins in II-VI QD systems. Dr. Petrou's group uses steady state and time resolved optical spectroscopic techniques to study such interactions, in both MBE grown and colloidal QD systems.
Two-dimensional layered transition metal dichalcogenides (TMDs) with the general formula MX2 (such as MoS2, WS2, WSe2, etc.) have attracted considerable attention due to their interesting properties as well as their potential for applications such as energy storage and manufacturing of electronic devices. While various bulk TMD materials were studied in the 1960s, only recently single layer crystals were prepared by exfoliation and other means. Dr. Petrou's group uses a variable temperature micro-PL/7 Tesla magnet system for Photoluminescence (PL) and Reflectivity studies. The samples are placed on the cold finger of a continuous flow optical cryostat that is operated in the 5 - 300 K temperature range. The cryostat is mounted on a three axis translator with a spatial resolution of 10μm in the x and y directions and 5μm along the z direction. The combination of a microscope objective and the other lenses gives an overall magnification ranging from x10 to x200. Dr. Petrou's group has studied different TMDs, but has focused mainly on WS2. Using the micro-PL/7 Tesla magnet system we have studied the effects of magnetic field, laser power density, laser excitation, and temperature on WS2. In the future Dr. Petrou's group plans to study different TMD materials, as well as other nano-structures like single quantum dots.
In our lab, we have carried out Hanle measurements to extract spin life time of the carriers in GaInNAs epilayers, a solar-cell material. For Hanle measurements, the circularly polarized ( σ+ ) exciting laser beam is incident at right angles to the sample surface. The emitted photoluminescence (PL) is collected along the sample normal. The magnetic field B, generated by a 7 tesla optical cryogenic-magnet, is applied in the sample plane. The samples are placed in a variable temperature optical cryostat at the center of the magnet. The emitted light is focused onto the entrance slit of a single monochromator equipped with a cooled multichannel detector. A combination of quarter wave plate-linear analyzer just before the spectrometer entrance slit is used to separate the σ+ and σ- circularly polarized components of the emitted light. By analyzing the circular polarization as a function of magnetic field, we can then extract spin life time time of the carriers in GaInNAs epilayers.