The ultrafast intersubband relaxation in GaN quantum wells has been verified. Al0.65Ga0.35N/GaN multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.

Ultrafast intersubband relaxation (⩽150 fs) in AlGaN/GaN multiple quantum wells

The properties of hot electrons in systems where electrons and phonons experience quantum confinement are reviewed. The modifications to the behaviour of electrons and phonons brought about by confinement are described, particularly with reference to the principal scattering mechanism. The latter include the interaction with longitudinal optical phonons and plasmons, along with carrier-carrier effects. Some conflict in the literature concerning Fuchs-Kliewer polaritons is discussed. Low-temperature interactions with acoustic phonons are described. A central topic is that of energy relaxation, and the experimental and theoretical data relating to this form a large part of the review. Energy relaxation mechanisms in the femtosecond to nanosecond regimes, including intersubband and well-capture processes, are eventually summarized. An equally large section deals with hot-electron transport; in which negative differential resistance and other instabilities associated with parallel transport are discussed before turning to ballistic transport and impact ionization.

https://iopscience.iop.org/article/10.1088/0034-4885/54/2/001/pdf

Hot electrons in low-dimensional structures

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Quantitative characterization of micro-topography a bibliography of industrial surface metrology

Based on the dielectric-continuum model and Loudon's uniaxial crystal model, the equation of motion for the p-polarization field in an arbitrary wurtzite multilayer heterostructure is solved exactly for the interface optical-phonon modes The polarization eigenvector, the dispersion relation, and the electron\char21{}interface-phonon interaction Fr\"ohlich-like Hamiltonian are derived using the transfer-matrix method The analytical formulas can be directly applied to single heterojunctions, single and multiple quantum wells (QW's), and superlattices Considering the strains of QW structures and the anisotropy effects of wurtzite crystals, the dispersion relations of the interface phonons and the electron\char21{}interface-phonon coupling strengths are investigated for GaN/AlN single and coupled QW's We find that there are four (eight) interface optical-phonon branches with definite symmetry with respect to the symmetric center of a single (coupled) QW Typical features in the dispersion curves are evidenced which are due to the anisotropy effects of wurtzite crystals The lower-frequency modes are much more important for the electron\char21{}interface-phonon interactions than the higher-frequency modes For the lower-frequency interface phonons, the intensity of the electron-phonon interactions is reduced due to the strain effects of the QW structures For the higher-frequency interface modes, the influence of the strains on the electron-phonon interactions can be ignored

Interface optical-phonon modes and electron–interface-phonon interactions in wurtzite GaN/AlN quantum wells

The collective excitations of a double heterostructure are examined in detail. Including the important effects of the interfaces of the system the uncoupled modes are analysed, which are the long-wave optical interface phonon modes, LO phonon modes, and intra- and intersubband plasmon modes. The electron-phonon interaction is discussed and the coupling functions are calculated and discussed in graphical form. The resulting coupled modes, intra- and intersubband plasmon-phonon modes, are calculated for different approximations of the polarization function, the random-phase approximation, the plasmon-pol approximation, and the long-wave approximation. The dispersion curves of these modes are presented in graphical form for various cases. It is shown that the Landau damping of the modes strongly depends on the layer thickness and the concentration of the quasi-two-dimensional electron gas.



Die kollektiven Anregungen einer Doppelheterostruktur werden ausfuhrlich untersucht. Es werden die ungekoppelten Moden, d. h. die langwellig-optischen Grenzflachen-Phononen, die LO-Phononen sowie die Intra- und Intersubband-Plasmonen unter Einbeziehung der Grenzflacheneffekte diskutiert. Im Detail werden die Elektron-Phonon-Wechselwirkung analysiert, die Kopplungs-funktionen berechnet und graphisch dargestellt. Die resultierenden gekoppelten Moden, die Intra- und Intersubband-Plasmon-Phononen werden fur mehrere Naherung der Polarisationsfunktion, der Random-Phase-Approximation, der Plasmon-Pol-Naherung und der langwelligen Naherung berechnet. Die Dispersionskurven sind fur verschiedene Falle graphisch dargestellt. Es wird ge-zeigt, das die Landau-Dampfung der Moden stark von der Schichtdicke und der Konzentration des quasi-zweidimensionalen Elektronengases abhangt.

Dynamical Screening, Collective Excitations, and Electron-Phonon Interaction in Heterostructures and Semiconductor Quantum Wells. Application to Double Heterostructures

A theory of dynamical screening in heterostructures and semiconductor quantum wells is developed which contains electron–electron interaction and electron–phonon interaction. In this theory both the types of long-wave optical phonons, LO phonons and interface phonons, which occur in those layered structures are included. An expression is derived for the longitudinal screened interaction of subband electrons in heterostructures and quantum-wells. The obtained screened interaction potential allows the derivation of the total longitudinal dielectric function and the dispersion relation of the coupled intra- and intersubband plasmon-phonon modes. This screened interaction potential is used to develope a dynamically screened and finite-temperature theory of the electron–phonon interaction in heterostructures and quantum wells. The leading-order electron self-energy is disscussed. An analytic expression of the polarization function of the quasi-two-dimensional electron gas is calculated within the random-phase approximation. Further approximations, the plasmon pole approximation and the long-wave approximation are discussed and the resulting analytic expressions of the polarization function are given.



Es wird eine Theorie der dynamischen Abschirmung in Heterostrukturen und Quanten-Wells entwickelt. Die dynamische Abschirmung beinhaltet sowohl die Elektron–Elektron- als auch die Elektron–Phonon-Wechselwirkung, wobei in letzterer die zwei Typen von langwellig-optischen Phononen (LO Phononen und Grenzflachen-Phononen), die in solchen schichtformigen Strukturen auftreten, explizit berucksichtigt werden. Das longitudinal abgeschirmte Wechselwirkungs-potential der Subband-Elektronen in Heterostrukturen und Quanten-Wells wird berechnet. Ausgehend von diesem Potential wird ein Ausdruck fur die totale longitudinale dielektrische Funktion und fur die Dispersionsrelation der gekoppelten Intra- und Intersubband-Plasmon–Phonon-Moden abgeleitet. Weiterhin wird aus diesem Potential eine Theorie der dynamisch abgeschirmten Elektron–Phonon-Wechselwirkung bei eindlichen Temperaturen in Heterostrukturen und Quanten-Wells abgeleitet. Die aus dieser Wechselwirkung resultierende Selbstenergie der Elektronen wird diskutiert. Es wird ein analytischer Ausdruck fur die Polarisationsfunktion des quasi-zweidimensionalen Elektronengases in der Random-Phase Approximation abgeleitet. Weitere Naherungen, die Plasmon-Pol-Naherung werden diskutiert und analytische Ausdrucke fur die Polarisationsfunktion abgeleitet.

Dynamical Screening, Collective Excitations, and Electron–Phonon Interaction in Heterostructures and Semiconductor Quantum Wells. General Theory

The dispersion relation of an unretarded surface polariton propagating on a rough surface covered with a thin transition layer is derived. The interfaces are assumed to be completely correlated Numerical calculations of the dispersion relation are performed for an Al substrake with an Al2O3 layer. It is found that the magnitude of the frequency shift in comparison with an analogous smooth system depends on the layer thickness and the correlation length of the roughness. The position of the maximum shift depends on the correlation length only.



Die Dispersionsrelation eines sich an einer rauhen, mit einer dunnen Ubergangsschicht bedeckten Oberflache ausbreitenden unretardierten Oberflachen-Polaritons wird abgeleitet. Die Grenzflachen werden als vollstandig korreliert angenommen. Numerische Berechnungen werden fur ein Al-Substrat mit einer Al2O3-Schicht durchgefuhrt. Es wird gefunden, das die Grose der Frequenzverschiebung im Vergleich zu einem analogen glatten System von der Schichtdicke und der Korrelationslange der Rauhigkeit abhangt. Die Lage der maximalen Frequenzverschiebung hangt nur von der Korrelationslange ab.

R. Pechstedt

Papers

Dynamical Screening, Collective Excitations, and Electron-Phonon Interaction in Heterostructures and Semiconductor Quantum Wells. Application to Double Heterostructures

Dynamical Screening, Collective Excitations, and Electron–Phonon Interaction in Heterostructures and Semiconductor Quantum Wells. General Theory

Effects of a thin transition layer on the dispersion relation of surface polaritons on a randomly rough surface