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Multidisciplinary University Research Initiative 2000 Phonon Enhancement of Optoelectronic and Electronic Devices INTRODUCTION: Motivation, Strategy, Approach and Goals Semiconductor heterostructure devices, which exploit the concept of “bandstructure engineering”of the electronic degrees of freedom have revolutionized modern optoelectronics and offer major new opportunities for the development of the next generation of micro/nanoelectronic devices as well. Entire generations of semiconductor lasers spanning wavelength range from the infrared to the blue are one prime example of such application of heteroepitaxy that has fundamentally changed the design of these compact, efficient optical sources. By contrast, very little attention has been given to the use of lower dimensional semiconductor and electronic material structures to exploit and control the lattice degrees of freedom in high performance devices. The importance of phonons is, of course, widely recognized, but generally in the context of performance diminishing characteristics such as increased electronic scattering due to carrier-phonon interaction. Unlike superconductivity where such interaction is fundamental to the entire phenomena and several classes of devices (SQUIDs etc), there are very few analogs in semiconductor device science, the Gunn microwave oscillator relying on phonon assisted intervalley scattering being one such rare instance. In earlier red LEDs, the use of the nitrogen isoelectronic center in GaP benefited from carrier-phonon scattering in bridging the indirect bandgap; however these LEDs have been superceded by the new generations of heterostructure light emitting devices that rely on direct electronic interband transitions. In this MURI program we begin with the premise that
extensive opportunities exist to transform the problems generally
associated with phonons presenting an overhead into a distinct benefit for
substantial enhancement of a broad range of opto The scientific strategy for the proposed program is schematically sketched in Figure 1. The left panel shows the pivotal role of the phonon enhancement processes for the optoelectronic and electronic devices which form the technological backbone of the program. The right panel identifies the core group of microscopic physical processes for the basic science component of the program, that range from quantum well dynamics to phonon assisted intersubband depopulation effects, from phonon intermediated e-e interactions in quantum transport to ballistic phonon propagation and "coherent" thermal transport. The team will pursue two different testbed outlays in applying sophisticated experimental and theoretical tools to the problem areas identified in Fig 1. First, we will pursue the growth, synthesis, nanofabrication, and study of the types of specifically tailored lower dimensional heterostructures where the phonon enhancement issues can be systematically studied. Secondly, in close collaboration with industrial partners, we will pursue the incorporation and implementation of the phonon enhancement processes to real optolectronic and electronic device structures. Advanced experimental probes and device physics modeling will be implemented for identifying the impact of specific phonon enhancement features in the high performance devices.
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Copyright © 2003, Center for Advanced Materials Research |
electronic
and electronic devices. Our explicit goal and objective is to achieve a
significant improvement and major technological impact on the performance
of a wide spectrum of advanced and novel semiconductor
optoelectronic/electronic devices, by explicitly focusing on the role of
phonon assisted and phonon dominated processes that control the
functionality and applications of such devices. The devices range from
quantum cascade and intersubband mid-IR lasers to new THz frequency laser
sources, from semiconductor lasers in the blue and near ultraviolet to
high power microwave FETs, to novel ultrahigh speed bipolar tunneling
transistors.
Additionally, new physical phenomena will be studied in which
electron-electron interaction, crucial for ballistic and coherent
electronic devices, is spectroscopically characterized via the
electron-phonon interaction. We have assembled a team of experts whose
research represents work at the leading edge of semiconductor device
science work. The interactive team has been organized specifically to
integrate a potent core of scientific expertise in phonon science, both
experiment and theory in terms of the interaction of phonons with the
electronic degrees of freedom in semiconductor nanostructures and high
speed, high power devices. Established collaborations and scientific
interactions exists within the team and coupling to leading
industrial/commercial efforts is in place for technology transfer of the
optoelectronic and electronic devices which define the technical mission
of this proposal.
The
device driven projects described in this proposal involve key classes of
semiconductor heterostructure and lower dimensional structures in which we
seek to intersect fundamental phonon physics with both material and device
design to reach new levels of phonon enhanced device performance, or to
create an opportunity for an entirely new electronic device. One example
of the former includes the intersubband and cascade IR lasers, an example
of the latter include the interband tunneling SiGe heterojunction bipolar
transistor. A summary of the principal material systems that form the core
of this proposal is shown in Figure 2. The material science expertise
within the team and its collaborators ensures that the highest quality
epitaxially grown heterostructures are available for the projects
described below. For example, the University of Houston group has
extensive molecular beam epitaxial facilities for synthesis of the
GaInAsSb system, while the collaboration with Lucent Technologies gives
the team access to state-of-the-art Si/Ge heterostructures in a closely
coupled, feedback driven mode of interactive research.
An
example of another theme within the proposed research program is the study
of ballistic phonon propagation in lower dimensional and anisotropic
heterostructures, with the goal to create directional energy flow of
lattice excitations in active electronic devices. The question of energy
dissipation and lattice heating is central to most high performance, high
power semiconductor optoelectronic devices. In submicron, mesoscale
structures, the channeling of excess energy from the electronic degrees of
freedom into the lattice forms a new paradigm, that is, the prospect of
spatially directed flow of lattice energy in the form of ballistic
acoustic phonons, for example. Our team possesses widely recognized
expertise in the field of ballistic phonon physics that will be applied
and integrated to explore the question of both thermal and nonthermal
cooling of electronic excitations by anisotropic phonon energy flow. Such
speculative prospects arise in the present advanced device circumstances
in the spirit of e.g. the ballistic phonon focusing of Figure 3, which is
computed (by Maris) as follows: One has a point source of phonons (e.g.
hot electron emitter) some distance below the surface of a cubic crystal.
This point source sends out phonons in all spatial directions. The figure
shows how these phonons arrive at the crystal surface, taken here to be a
100 surface. Each dot in the graph corresponds to a single phonon.
Longitudinal phonons are colored white, fast transverse phonons are
colored blue, and the slow transverse are in red. The high intensity areas
indicate directions where the group velocity is stationary with respect to
changes in the wave vector direction. The "piling up" of phonons at these
locations leads to the focusing effect.