D. D. Coon

Hilbert spaces of analytic functions and generalized coherent states

Generalized coherent states which are associated with a generalization of the harmonic oscillator commutation relation are investigated. It is shown that these states form an overcomplete basis in a Hilbert space of analytic functions. The generalized creation and annihilation operators are bounded except in a limit in which they reduce to the usual boson creation and annihilation operators. In this limit the Hilbert space of analytic functions reduces to the Bargmann–Segal Hilbert space of entire functions and in another limit it reduces to the Hardy–Lebesgue space.

New mode of IR detection using quantum wells

A new mode of IR detection using photoemission from a single quantum well is proposed and optimization of the device performance by the proper choice of parameters is discussed. Despite the very thin device structures, theoretical calculations show large absorption at wavelengths near cutoff. The largest photoemissive response is found by adjusting the well parameters so that an excited virtual state lies just above threshold.

Exchange interactions in quantum well subbands

It is shown that exchange interactions in the two‐dimensional electron gas in quantum wells could cause observable effects on subband energies and intersubband transition energies. In the case of doped quantum wells, the intrasubband exchange interaction can produce an energy shift which is substantially larger than the direct Coulomb energy shift. Theoretical estimates of such shifts are compared with experimental measurements of the infrared photoconductivity of multiple quantum well AlGaAs/GaAs structures with wells doped at about 1018 cm−3.

Frequency limit of double barrier resonant tunneling oscillators

The frequency limit of negative differential resistance (NDR) devices employing resonant tunneling in double barrier quantum well structures is analyzed. We show that the standard theoretical approach to resonant tunneling together with a unitarity bound leads to a lower bound on NDR which is in turn related to the maximum oscillator frequency. The bound on NDR can be achieved in devices with narrow width resonances. However, too narrow a width can cause Wigner–Eisenbud resonance time delay. These considerations indicate that devices of the type studied by T. C. L. G. Sollner, P. E. Tannenwald, D. D. Peck, and W. D. Goodhue [Appl. Phys. Lett. 45, 1319 (1984)] could oscillate up to roughly 1 THz.

Heterojunction double‐barrier diodes for logic applications

The use of heterojunction double‐barrier diodes in logic circuits is examined. Switching time limitations are estimated based on circuit considerations and device architecture. Dispersion effects involving quantum mechanical resonant tunneling time delay are also included. These theoretical considerations indicate that picosecond or subpicosecond switching times might be achievable with appropriately designed AlxGa1−xAs/GaAs devices.

Narrow band infrared detection in multiquantum well structures

A theoretical analysis is performed for photoexcitation of carriers from a quantum well state into a superlattice subband. The parameters of the device structure are chosen so that (a) the first excited state of the quantum well lies within the lowest subband of the superlattice and (b) the lowest subband is narrow. The degeneracy of the first excited state with the subband provides resonant enhancement of photoexcitation into the subband while the subband extrema provide long‐wavelength and short‐wavelength cutoffs. The response band and bandwidth can be tailored and there is the possibility of high‐temperature operation associated with the narrow band response.

Resonant tunneling time delay and quantum well sheet density

Time delays, dwell times, resonant state lifetimes, and electron sheet densities associated with tunneling through quantum well structures are analyzed from a scattering theory (S‐matrix) viewpoint. Some of the results differ from intuitively motivated expressions which have appeared in the resonant tunneling literature. It is shown that the sheet density is given by a formula similar to the Tsu–Esaki formula [Appl. Phys. Lett. 22, 562 (1973)] for current density. Sheet density and dwell time are related to Hermitian matrices which are expressed in terms of the S‐matrix and in terms of partial widths associated with resonant states.

Exchange interaction effects in quantum well infrared detectors and absorbers

Infrared excitation energies between the ground‐state subband and the first excited‐state subband in quantum wells are analyzed including the effect of exchange interactions on the ground‐state subband. Analytic and numerical calculations relevant to infrared absorption and infrared detection are performed. Previous work on exchange interaction effects in narrow wells is extended to deal with well widths which can provide absorption peaks and photocurrent peaks throughout the long‐wavelength infrared and very‐long‐wavelength infrared regime. Exchange effects are shown to be substantial.

Tunneling currents and two‐body effects in quantum well and superlattice structures

Direct current (dc) tunneling through a finite superlattice or quantum well structure is analyzed in an independent particle model. A correction is made to the Tsu–Esaki [Appl. Phys. Lett. 22, 562 (1973)] one‐particle current expression. In addition, a two‐body current term is found which can, in some cases, be of the same order of magnitude as the one‐particle current terms of Tsu and Esaki.

Time‐dependent quantum‐well and finite‐superlattice tunneling

Theoretical considerations which pertain to electric currents through quantum‐well structures or finite superlattices in the presence of periodic time‐dependent applied potentials are presented. The paper includes (1) a time‐dependent generalization of the time‐independent, noninteracting electron, one‐dimensional potential model of Tsu and Esaki, (2) a derivation of generalized unitarity identities which relate all of the elastic and inelastic transitions which a particle can undergo when it interacts with a periodic time‐dependent, one‐dimensional, arbitrarily shaped potential barrier, and (3) an analysis of many‐body effects which reveals additional non‐Tsu‐Esaki current terms which disappear when the time‐dependent part of the applied potential is turned off. All of the results are expressed in terms of one‐particle scattering matrices which can be computed from the ordinary single‐particle, time‐dependent Schrodinger equation. This work may have high‐frequency device applications.