Electrical and Optical Properties of AlGaAs/GaAs Aperiodic Superlattices and Resonant Tunneling Diodes. Theory, Design and Applications

Abstract: The objective of the work presented in this thesis is to study quantum transport and optical properties of aperiodic superlattices and resonant tunneling diodes using III-V semiconductor heterostructures grown by molecular beam epitaxy. The study follows a procedure of theoretical modeling, computer-aided design and simulation, sample growth, and measurements.

The electronic energy subbands and exciton states in an irregular finite superlattice (SL) is investigated by variational approach incorporated with both tight-binding methods and exact solutions of Schrödinger equations. The field-induced Stark effect and the interaction between the localized electron states introduced by the irregularity and the extended states in the superlattices are shown to significantly affect the optical response of the SLs. Taking the electron-hole Coulomb interaction into account, the excitonic spectra and the quantum Stark effect is more flexible than in the conventional (double-) quantum-well (QW) structures. Following this theoretical model, aperiodic superlattices (ASL) are designed with varying barrier and QW dimensions but binary compound materials, in order to achieve quasi-miniband formation at a desired finite external field. The ASL is shown to have better quality than other irregular SLs designed for the similar functionality, e.g., the graded-bandgap SLs, since the control of changing material indices is a more demanding task. The low-temperature photoluminescence (PL) and time-resolved PL (TRPL) techniques have been employed in the measurements of the ASL diodes grown according to the design. The PL spectra demonstrate a drastic quenching of the detected signal due to the efficient transport of the photoexcited carriers along the formed quasi-miniband at a finite electric field. The PL excitation spectra demonstrate a triple-resonance behavior, which is observed for the first time in such irregular SL structures, owing to the field-induced anticrossing of the exciton states in the vicinity of the miniband formation. Upon the analysis of the TRPL spectra, a rebound feature of the exciton radiative decay time was observed indicating the strong mixing of the electron states in the miniband due to the Coulomb attraction of the localized holes in the widest QW, which is in reasonable agreement with our theoretical calculation.

Tunneling transport assisted by the QW Wannier-Mott excitons in the resonant tunneling diodes has been studied within the framework of the sequential tunneling formalism. The concept of this exciton-assisted tunneling (EAT) phenomenon was developed based on the model of transition between free carriers and confined 2D excitonic polaron. Consequently, the EAT probability and its unique tunneling conditions are derived and discussed in comparison with other tunneling effects observed previously. The significance of the EAT is that electrons can tunnel with a longitudinal energy even below the resonant tunneling energy by a mount of the exciton binding energy. Furthermore, our calculation results suggest that to observe the electron-EAT current intensity with the same order of magnitude as the resonant tunneling current requires injection of the QW hole density at >~10⁹ cm^-2 in the AlGaAs system and below the exciton unbinding limit of ~10¹¹ cm^-2. The EAT peaks appear on the opposite side of the phonon-replica with respect to the main RT peak in the current-voltage (I-V) characteristics, and its spectral line shape has a fingerprint which makes one easily identify and distinguish the EAT effect from the other tunneling phenomena.

For application, we have attempted to design and grow RT light-emitting diodes which is based on the graded-index waveguide heterostructure. In addition, injectors of ASL structures are incorporated into the p-i-n diodes in order to enhance simultaneous injection of electron and heavy-hole first excited subband at resonance. Multiple-wavelength electroluminescence (EL) is observed in response to different bias voltages accompanied by a pronounced S-shaped negative differential resistance region in the I-V characteristics up to 77 K. The bistability in the spectra of EL and laser generation has been demonstrated in close relation to the current bistability attributed to the efficiency of carrier injection.