We present a detailed and thorough study of a wide variety of quantum well infrared photodetectors (QWIPs), which were chosen to have large differences in their optical and transport properties. Both n- and p-doped QWIPs, as well as intersubband transitions based on photoexcitation from bound-to-bound, bound-to-quasi-continuum, and bound-to-continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot carrier mean free path; net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity. These results allow a better understanding of the optical and transport physics and thus a better optimization of the QWIP performance.

Photoexcited escape probability, optical gain, and noise in quantum well infrared photodetectors

We demonstrate the first long‐wavelength quantum well infrared detector using the lattice‐matched In0.53Ga0.47As/InP materials system. The responsivity has been found to be larger than that for similar GaAs/AlxGa1−xAs detectors.

InGaAs/InP long wavelength quantum well infrared photodetectors

Heart rate variability has been analyzed conventionally with time and frequency domain methods, which measure the overall magnitude of RR interval fluctuations around its mean value or the magnitude of fluctuations in some predetermined frequencies. Analysis of heart rate dynamics by methods based on chaos theory and nonlinear system theory has gained recent interest. This interest is based on observations suggesting that the mechanisms involved in cardiovascular regulation likely interact with each other in a nonlinear way. Furthermore, recent observational studies suggest that some indexes describing nonlinear heart rate dynamics, such as fractal scaling exponents, may provide more powerful prognostic information than the traditional heart rate variability indexes. In particular, the short-term fractal scaling exponent measured by the detrended fluctuation analysis method has predicted fatal cardiovascular events in various populations. Approximate entropy, a nonlinear index of heart rate dynamics, that describes the complexity of RR interval behavior, has provided information on the vulnerability to atrial fibrillation. Many other nonlinear indexes, e.g., Lyapunov exponent and correlation dimensions, also give information on the characteristics of heart rate dynamics, but their clinical utility is not well established. Although concepts of chaos theory, fractal mathematics, and complexity measures of heart rate behavior in relation to cardiovascular physiology or various cardiovascular events are still far away from clinical medicine, they are a fruitful area for future research to expand our knowledge concerning the behavior of cardiovascular oscillations in normal healthy conditions as well as in disease states.

Fractal and complexity measures of heart rate variability.

The aim of this study was to analyze the main processes that determine changes in landscape patterns and vegetation cover from 1957-1994 to develop a model for land cover dynamics Land cover and landscape patterns were assessed and compared using aerial photographs taken in 1957, 1985, and 1994 Over this period, tall grass steppe and arid garrigues increased by 6% and 4%, respectively, while crop fields decreased by 15% and tall arid brush remained the same Over the same period, tall grass steppes and arid garrigues became less fragmented Changes in land use were triggered by socioeconomic forces, which were constrained by the underlying structure of the physical landscape The best preserved vegetation (tall arid brushes) was concentrated at higher elevations, with a pronounced slope, not oriented towards the sea, and in volcanic substrate Communities tended to be better preserved further away from towns and at lower house densities Tall grass steppe was present on more gradual sea-oriented slope and in calcareous substrate, and increased at higher elevations, although not far from the town but away from high anthropogenic influence Previous studies have revealed that traditional land uses of this landscape, particularly grazing, favoured the transition from tall arid brush to tall grass steppe In this study, we analyzed to what extent the underlying structure of the physical landscape imposes limitations to the vulnerability to human activity of the main vegetation types According to the data on the probability of vegetation transition over the 37-year period, the shift from tall arid brush to tall grass steppe appeared to be favoured by gradual slopes Tall arid brush recovered from either arid garrigues or tall grass steppes at steeper slopes Thus, steep terrain had a favourable effect on the formation of brushwood and more gradual terrain favoured tall grass steppe The prevalent trends were confirmed by a projection of a transition matrix over 100 years

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Variations in landscape patterns and vegetation cover between 1957 and 1994 in a semiarid Mediterranean ecosystem

Success has been achieved in extending the spectral wavelength of GaAs/Al(x)Ga(1-x)As quantum-well infrared photodetectors to significantly longer wavelengths of 11-15 micron. High responsivity of 0.5 A/W, high quantum efficiency of 12 percent, and high detectivity as well as an excellent noise equivalent temperature difference of 4 mK have been achieved at T = 50 K. High performance of 19 mK has also been achieved at an even higher temperature of 60 K.

Extended long‐wavelength λ=11–15‐μm GaAs/AlxGa1−xAs quantum‐well infrared photodetectors

Cathodoluminescence has been investigated in diamond films, prepared by microwave plasma assisted chemical-vapor deposition. Results are presented for various diamond films differing in crystalline structure and doping. In the energy range 2.0--2.7 eV a series of 13 sharp luminescence lines has been observed at temperatures below 200 K. In a tentative interpretation they are attributed to donor-acceptor pair recombination. The Coulomb energy depends on the discrete values for the pair separation in the expected manner including a Bohr radius correction for the three closest pairs. The relative intensities of the lines depend on the growth conditions and on the film texture. Likely candidates for the pairs are the nitrogen donor and the boron acceptor. Both impurities are contained in the present samples. The sum of the binding energies is 3.57 eV, and inserting ${\mathit{E}}_{\mathit{a}}$=0.37 eV for boron yields ${\mathit{E}}_{\mathit{d}}$=3.2 eV for nitrogen.

Resolved donor-acceptor pair-recombination lines in diamond luminescence.

Intersubband transitions in GaAs/AlxGa1−xAs quantum wells have been studied as a function of the doping level by Raman scattering and infrared absorption. The n‐type dopant concentration placed in the well was varied between 1×1018 and 8×1018 cm−3. With increasing doping level Raman scattering reveals a frequency down‐shift of the single‐particle intersubband transition and a shift to higher frequencies of the collective intersubband plasmon‐phonon mode. The resonance in infrared absorption follows closely the collective mode observed in Raman scattering, demonstrating clearly that the doping‐dependent depolarization shift of the absorption peak is an important parameter that must be taken into account in device design. Possible models for the frequency down‐shift of the single‐particle transition are also discussed.

H. Biebl

Papers

Resolved donor-acceptor pair-recombination lines in diamond luminescence.

Doping density dependence of intersubband transitions in GaAs/AlxGa1−xAs quantum‐well structures

Diamond luminescence: resolved donor-acceptor pair recombination lines