Thomas Boggess Research

Electronic and Optical Properties of Novel Semiconductor Heterostructures:

 

Modern semiconductor growth techniques, such as molecular beam epitaxy (MBE) can be used to create synthetic semiconductors with unique optical and electronic properties that can be vastly different from those of the bulk parent compounds from which they are comprised. Such structures are typically composed of alternating layers of differing semiconductors with each layer perhaps only a handful of atoms thick. The difference in band gap energies for the different layers causes electrons in the structure to see alternating potential wells and potential barriers. If electrons readily tunnel through the barriers into an adjacent well, the electron "sees" a periodicity in the direction perpendicular to the layers (growth direction) that is in addition to the periodicity of the crystal lattice. This leads to a band structure (i.e., electron energy dispersion curves) that is different in the growth direction compared to that within the layers. That is, an entirely new highly-anisotropic semiconductor, referred to as a superlattice (SL), is created. If the potential barriers are sufficiently high and/or thick, the electron cannot tunnel into an adjacent potential well. For sufficiently thin wells, such a structure is described by a formalism quite similar to that used for the familiar particle-in-a-box problem of elementary quantum mechanics. Just as for this simple quantum mechanics problem, discrete energy levels associated with electron motion in the growth direction arise. Such a structure is referred to as a multiple quantum well (MQW), or for a single potential well sandwiched between two barriers, simply a quantum well. Again, an entirely new semiconductor is created.

Superlattices and quantum wells form the basis for many state-of-the-art electronic, optoelectronic, and optical devices. This, and the unique nature of these quantum-confined structures, provide fruitful avenues for both basic and applied research. Structures of particular current interest to us are superlattices with small band gap energies, such as those based on thin layers of InAs/InGaSb (and other Sb-containing ternary, quaternary, and quinternary compounds) grown on GaSb substrates. These materials are of interest for lasers, detectors, and modulators operating at mid-infrared (2-5 micron) wavelengths. Our research involves the application of all-optical techniques to the determination of electronic and optical properties of these interesting and unique semiconductor structures. These properties include carrier energy relaxation (hot-carrier cooling), carrier recombination, carrier-density-dependent absorption and refraction, and carrier transport.

Much of this work is performed in collaboration with Professor Thomas C. Hasenberg and Professor Michael E. Flatté of the Department of Physics and Astronomy at the University of Iowa. Professor Hasenberg's expertise is MBE semiconductor growth and device physics, while Professor Flatté is a condensed matter theorist with expertise in the area of optical and electronic processes in semiconductors (as well as in many other areas of condensed matter physics).

More recently, we have begun to use ultrafast optical techniques to explore carrier dynamics in In(Ga)As/GaAs self-assembled quantum dots. These remarkable structures exhibit quantum confinement effects in all three dimensions and form the basis for a new generation of high-performance 1.3 mm lasers. This work, which is done in collaboration with professor Dennis Deppe's group at the University of Texas at Austin, is currently focused on measurements of the size dependence of the carrier dynamics. The overall goal is to use the basic understanding of the physics of these novel structures to develop improved, ultra-low-threshold lasers for telecommunication and interconnect applications. An atomic force microscope image of one of the samples that we are studying is shown below, along with data illustrating the carrier relaxation and radiative lifetimes as a function of temperature in small (250x30Å) InAs QDs and large (350x110Å) InGaAs QDs. These data demonstrate a size dependence to the dynamics and the potential for engineering the dynamics for optimal laser performance.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Another recent research program is directed at understanding the dynamics of spin-polarized electronic distributions in semiconductors. Up to now, the foundation of modern electronics has been based on manipulation of electronic charge. Recently, however, researchers have begun to focus on the another fundamental electronic property, namely spin, as a potential property on which to base a new generation of electronic devices, including high-speed magnetically-addressed transistors and diodes, and even quantum computers. Our work in this area currently focuses on electronic spin relaxation in InAs and the "no-common-anion" family of heterostructures, InAs/GaSb/AlSb. Measurements are based on the optical injection of spin-polarized carriers by excitation with circularly-polarized ultrafast pulses and the probing of the transmission of the excited system with pulses of either the same or opposite circular polarization. Distinctly different signatures are observed in the two cases; for the same circular polarization (SCP) the probe "sees" the decay of the spin polarized distribution into a random spin distribution, while for the opposite circular polarization (OCP), as the initially spin-polarized distribution randomizes, the probe "sees" an increase in population with spin opposite that initially created. This is illustrated below for bulk InAs. The measurement has allowed us to for the first time measure the longitudinal electronic spin relaxation time of 19 ps at 300K in InAs. In the case of bulk semiconductors, only a partially-spin-polarized carrier distribution can be generated due to the nature of the valence band states. However, in quantum structures, such as superlattices, quantum, wells, and QDs, it is possible to create a fully spin-polarized population.