Effect of structural disorder on electronic states in GaAs/AlGaAs quantum wires

Perfect quantum wire structures are attractive candidates for low threshold lasers and high speed electronic devices because of the nature of the density of states and eigenfunctions. In this letter, we discuss the effect of structural disorder on the density of states as well as on the localization length of these eigenstates. We find that significant changes in the density of states and eigenfunctions occur with a small random disorder along the wire axis. Consequences for devices based on quantum wires are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70847/2/APPLAB-59-24-3142-1.pd

A new method for solving the ground‐state problem in arbitrary quantum wells: Application to electron‐hole quasi‐bound levels in quantum wells under high electric field

A method based on the Monte Carlo technique and variational principle is developed to study the ground‐state problem in arbitrary quantum wells. A technique is described to use this method to study quasi‐bound states in systems. The method is applied to AlGaAs/GaAs quantum wells subjected to high electric fields. Advantages of this approach over the conventional variational approach are identified.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71322/2/APPLAB-48-6-434-1.pd

Linear electro-optic effect due to the built-in electric field in InGaN/GaN quantum wells

A strong piezoelectric effect and large lattice mismatch allow one to incorporate high built-in electric fields in InGaN/GaN quantum wells. This letter examines the implications of these fields on the absorption spectra and refractive index changes induced by an external perpendicular electric field. We find that InGaN/GaN quantum wells show linear electro-optic effect due to quantum confined Stark effect. Our results suggest application of InGaN/GaN quantum wells in Mach–Zehnder type modulators and in electroabsorption modulators in the blue light region. © 1999 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70668/2/APPLAB-75-13-1932-1.pd

Design of high electron mobility devices with composite nitride channels

Field effect transistors based on a GaN/AlGaN system have shown remarkable performance characteristics in a wide range of device applications. However, due to the large effective mass of GaN, the mobility in the channel is small. In this work, we consider a GaN/AlGaN structure with a thin InN channel of the order of a few monolayers. We find that mobility in the channel can improve considerably while breakdown characteristics are not expected to suffer. Mobilities of ≃2500 cm2 (V s)−1 are predicted along with high sheet charges for low interface disorder. Good agreement with experimental results is observed for higher degrees of disorder within the model. At higher electric fields, we find that most electron population transfers to higher valleys or other subbands that lie in AlGaN or GaN. We also compare the low-field mobility-charge product for this structure with the conventional AlGaN/GaN structure and find that the two values are similar. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69676/2/JAPIAU-94-4-2498-1.pd

Theoretical optimization of quantum wire array lasers for low threshold current density and high modulation frequency

We show that as one decreases the cross‐sectional area of quantum wire lasers, the threshold current decreases, but the carrier relaxation time increases. Since the electron relaxation time sets the upper limit on the modulation frequency, there is a tradeoff between speed and efficiency in quantum wire lasers. We derive the optimal wire cross‐sectional area for a one‐dimensional array of quantum wire lasers based on a balance between an acceptably high maximum modulation frequency and a desirably low threshold current density. We find that for a relaxation time of 60 ps, the quantum wire of 150×150 Å cross section has the lowest threshold current density of 560 A/cm2. If high‐speed operation is not needed, the optimal choice for the quantum wire cross‐sectional area is 100×50 Å with the threshold current density of 420 A/cm2. For optimized quantum wells with the same cavity losses, the threshold current density is ≊620 A/cm2. We also present the results for the threshold current density and the relaxation time that allow one to find the optimal quantum wire structure weighing the speed and efficiency considerations in accordance with their relative importance.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70288/2/APPLAB-63-15-2024-1.pd

Tunneling and subband levels in GaAs quantum well with direct and indirect AlxGa1−xAs barriers

We present a study of coherent tunneling lifetimes for quasibound electrons confined in a GaAs quantum well by Al0.3Ga0.7As (direct band gap) and AlAs (indirect band gap) barriers, using the tight‐binding representation for the electronic states in an eight‐element (sp3) basis, and solving the time‐dependent Schrödinger equation using a unitary approximation of the evolution operator. The dependence of the lifetime on barrier thickness is found to fit a WKB‐type expression very well. Although simple effective mass theory is not applicable, the barrier thickness coefficient in the WKB exponent is determined by the Γ‐point band extrema even for indirect AlAs barriers with X‐point conduction‐band minimum. The dependence of the subband energies and their in‐plane dispersion on the mole fraction x of Al in the AlxGa1−xAs barrier is also presented, for x in the range 0.2–1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70408/2/APPLAB-59-16-1963-1.pd

Coherent tunneling of mixed state hole wave packets in coupled quantum well structures

The time‐dependent Schrödinger equation is solved numerically to study the coherent tunneling of hole wave packets in asymmetric coupled quantum wells. The importance of selection rules and band mixing is evident in the extremely low rates of the wave‐packet leakage from heavy‐hole state to a resonant light‐hole state at zero in‐plane wave vector (k∥). But these rates increase dramatically away from k∥=0, when the hole states acquire mixed character, and rapidly become comparable to the heavy‐hole to heavy‐hole resonant tunneling rates. The effect of inhomogeneous level broadening arising from well size fluctuations in multicoupled quantum well systems is shown to greatly reduce the effective tunneling rates near resonance.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69802/2/APPLAB-58-14-1509-1.pd

A self‐consistent approach to spectral hole burning in quantum wire lasers

In a semiconductor laser above threshold, carriers are extracted at the lasing energy at a high rate due to stimulated emission and are injected at higher energies. This creates a ‘‘hole burning’’ phenomenon resulting in gain compression. This effect is studied in a quantum wire laser by solving the Boltzmann equation with sink and source terms by a novel Monte Carlo technique. The results for various values of the characteristic injection times are given. A formalism is also proposed for the fully self‐consistent determination of the laser operating parameters from the rate equations with the inclusion of nonlinear gain effects by substituting the correct form of the distribution function in presence of hole burning into the standard expressions for the laser material gain. The nonlinear gain effect is then described completely starting from the wire band structure and scattering rates. The generality of the proposed technique and its possible extensions and applications to the problem of nonlinear gain in quantum‐well lasers are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70724/2/JAPIAU-74-10-6451-1.pd

Channel effective mass and interfacial effects in Si and SiGe metal-oxide-semiconductor field effect transistor: A charge control model study

We present results of a numerical formalism developed to address the band structure and charge control problem in nn- and pp-type silicon and silicon-germanium metal-oxide-semiconductor field effect transistors. We focus on the following issues: (i) the dependence of the in-plane carrier effective mass on sheet charge density and germanium content; (ii) the fraction of charge near the interface and the evaluation of the interface roughness matrix element. Results are compared to existing models. For nn-type structure, the effective mass approximation and deformation potential theory is used to describe the electron states. However, for pp-type structure, a six-band k⋅p Kohn–Luttinger formulation is used to describe the hole states due to the strong coupling of heavy-hole, light-hole, and split-off bands. This allows us to examine the influence of the coupling of the heavy-hole, light-hole, and the split-off bands. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70471/2/JAPIAU-83-8-4264-1.pd

Probabilistic Framework for Behavior Characterization of Traffic Participants Enabling Long Term Prediction

This research aims at developing new methods that predict the behaviors of the human driven traffic participants to enable safe operation of autonomous vehicles in complex traffic environments. Autonomous vehicles are expected to operate amongst human driven conventional vehicles in the traffic at least for the next few decades. For safe navigation they will need to infer the intents as well as the behaviors of the human traffic participants using extrinsically observable information, so that their trajectories can be predicted for a time horizon long enough to do a predictive risk analysis and gracefully avert any risky situation. This research approaches this challenge by recognizing that any maneuver performed by a human driver can be divided into four stages that depend on the surrounding context: intent determination, maneuver preparation, gap acceptance and maneuver execution. It builds on the hypothesis that for a given driver, the behavior not only spans across these four maneuver stages, but across multiple maneuvers. As a result, identifying the driver behavior in any of these stages can help characterize the nature of all the subsequent maneuvers that the driver is likely to perform, thus resulting in a more accurate prediction for a longer time horizon. To enable this, a novel probabilistic framework is proposed that couples the different maneuver stages of the observed traffic participant together and associates them to a driving style. To realize this framework two candidate Multiple Model Adaptive Estimation approaches were compared: Autonomous Multiple Model (AMM) and Interacting Multiple Model(IMM) filtering approach. The IMM approach proved superior to the AMM approach and was eventually validated using a trajectory extracted from a real world dataset for efficacy. The proposed framework was then implemented by extending the validated IMM approach with contextual information of the observed traffic participant. The classification of the driving style of the traffic participant (behavior characterization) was then demonstrated for two use case scenarios. The proposed contextual IMM (CIMM) framework also showed improvements in the performance of the behavior classification of the traffic participants compared to the IMM for the identified use case scenarios. This outcome warrants further exploration of this framework for different traffic scenarios. Further, it contributes towards the ongoing endeavors for safe deployment of autonomous vehicles on public roads