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41803-B10
Temperature Dependent Magneto-Optical Ellipsometry Studies of Magnetic Semiconductors
During the past year, our focus has been to study the temperature dependence of the optical properties of diluted magnetic semiconductors. We concentrated on GaMnAs, a material manifesting both semiconductor and magnetic properties. As a result of its unique characteristics, it is conceivable that GaMnAs can be used in applications related to spintronics. Since researchers have rectified several issues related to the growth of a GaMnAs, the molecular-beam-epitaxy grown films are of high quality.
It is widely believed the holes are responsible for mediating the ferromagnetic interaction in GaMnAs. Since the ferromagnetism is essentially turned off above the Curie temperature, the focus of our work was to monitor the optical signatures around this transition temperature. The method of choice was to use ellipsometry, a technique that has the potential to probe the band structure dynamics in semiconductors.
As mentioned in our last year's report, previous experiments performed near liquid nitrogen temperatures failed to give any conclusive results on GaMnAs samples. This was mainly because the GaMnAs samples chosen for those studies had a Curie temperature close to the liquid nitrogen temperature. In order to rectify this situation, without waiting for sample that had higher Curie temperatures, we decided to pursue measurements at liquid helium temperatures, which are far below the Curie temperatures.
The undergraduate student involved in this project performed several experiments using a variable-angle spectroscopic ellipsometer on a series of GaMnAs samples with different alloy concentration. The samples were mounted on a cold-finger, housed in a cryostat, and were held at temperatures between room and liquid helium temperatures. The cryostat windows were initially calibrated prior to these measurements in order to null the influence of the windows on the spectra.
After the ellipsometry spectra were taken at different temperatures, from 5 K to 300 K, they were analyzed to determine the dielectric functions of each of the GaMnAs films. This was done in two steps; first the transparent data (i.e., above ~500 nm) were analyzed using a Cauchy model to obtain the indices of refraction and the thicknesses of samples. Subsequently, the entire spectrum was analyzed to obtain both the index of refraction and the extinction coefficient of all of the samples from 200 nm to 1600 nm.
Even before analyzing the dielectric functions further, we immediately recognize that there is a noticeable difference in the dielectric functions as a function of the temperature. This verified our initial hypothesis that the ferromagnetic transition is accompanied by optical signatures which result as the band structure undergoes changes during ferromagnetic transition. In order to quantify the changes associated with the critical points in the band structure, the dielectric functions were further analyzed by taking their second derivatives. This allowed us to monitor the critical point transition energies, which are the fundamental as well as the higher order transitions responsible for the structure associated with the absorption of a material. It is very clear that two critical points undergo a minimum around their Curie-temperatures, an important discovery in understanding the origins of the ferromagnetism in GaMnAs. We are now in the process of writing a manuscript with these findings and submitting to the Journal of Applied Physics Letters.
As stated in our report last year, we continued to work on another project related to InGaMnAs, yet another diluted magnetic semiconductor recognized for its unique optical and magnetic properties. The culmination of this project was to present its results at the North American Molecular Beam Epitaxy conference held last October at Duke University. Subsequently, we prepared a manuscript and submitted it to the Journal of Vacuum Science and Technology. After a successful review, the paper was accepted, and has been published recently.
