Intersubband transitions in quantum well mid-infrared photodetectors

A study of intersubband transitions in quantum well infrared detectors working at high temperatures has been reported. This study allows a greater tunability in the device designs, with the ability to control the peak wavelength, the absorption coefficient, the dark current, the quantum efficiency and the detectivity of the modeled structure operating around 3.3 μm wavelength. The detection energy and absorption coefficient dependences with an applied electric field are given. Then, the electro-optic performances of the modeled mid-infrared detector are estimated, the dark current dependence with the applied voltage and temperature as well as the quantum efficiency and the detectivity are investigated and discussed. High detectivities were found at high temperatures revealing the good performances of the designed photodetector, especially at 3.3 μm wavelength.     

Intersubband transitions in an asymmetric double quantum well

The intersubband optical absorption in an asymmetric double quantum well for different barrier widths and the right well widths are theoretically calculated within the framework of effective mass approximation. The results obtained show that the intersubband transitions and the energy levels in an asymmetric double quantum well can be importantly modified and controlled by the barrier width and the well width. The sensitivity to the barrier and well widths of the absorption coefficient can be used in various optical semiconductor device applications.     

Intersubband optical absorption in a quantum well under a tilted magnetic field

By using an appropriate coordinate transform we have calculated the intersubband optical absorption in the single square well under a tilted magnetic field. In this study the dependence of the intersubband transitions on the magnetic field strength and the direction of the magnetic field (tilt angle) is discussed. We show that intersubband optical absorption is sensitive to the tilt angle. This behaviour in the intersubband optical absorption gives a new degree of freedom in regions of interest device applications.     

Intersubband transitions and refractive index changes in coupled double quantum well with different well shapes

In this study, both the linear intersubband transitions and the refractive index changes in coupled double quantum well (DQW) with different well shapes for different electric fields are theoretically calculated within framework of the effective mass approximation. Results obtained show that intersubband transitions and the energy levels in coupled DQW can importantly be modified and controlled by the electric field strength and direction. By considering the variation of the energy differences, it should point out that by varying electric field we can obtain a blue or red shift in the intersubband optical transitions. The modulation of the absorption coefficients and the refractive index changes which can be suitable for good performance optical modulators and various infrared optical device applications can be easy obtained by tuning applied electric field strength and direction.     

Energy relaxation by hot carriers in wurtzite GaN epilayers

In this paper, we report hot carrier energy relaxation processes studied by acoustic phonon emission in wurtzite GaN epilayers, using the heat pulse technique. In this method, the carriers were heated up by means of short (≈ 10 ns) voltage pulses and emitted phonons were detected by Al bolometers biased at their superconducting transition. Obtained phonon signals indicate that the optical phonon emission threshold has not been reached and longitudinal acoustic and transverse acoustic modes can be clearly resolved. This paper specifically concentrates on the electron temperature dependence of the energy relaxation rates and compares the experimental results with the existing theory.     

Temperature relaxation and energy loss of hot carriers in graphene

The temperature relaxation and energy loss of hot Dirac fermions are investigated theoretically in graphene with carrier–optical phonon scattering. The time evolutions of temperature and energy loss for hot Dirac fermions in graphene are calculated self-consistently. It shows that the carrier–optical phonon coupling results in the energy relaxation of hot carriers excited by an electric field, and the relaxation time for temperature is about 0.5–1 ps and the corresponding energy loss is about 10–25 nW per carrier for typically doped graphene samples with a carrier density range of 1–5×10¹² cm^?2. Moreover, we analyze the dependence of temperature and energy relaxation on initial hot carrier temperature, lattice temperature and carrier density in detail.     

Intersubband scattering of cold electrons in a coupled quantum well with subband spacing below

We report measurements of the intersubband scattering rate between the first and second subband in a quantum-well structure with subband spacing (11 meV) smaller than the optical phonon energy. We measure the electron population in the second subband under CW excitation by a far-infrared laser tuned to the intersubband absorption frequency. This allows us to determine the intersubband relaxation rate using detailed balance. These measurements are novel because they are performed at very low excitation densities (I10 μW/cm²). In this regime the heating of the electron gas is negligible, so that the optically excited population in the upper subband greatly exceeds any thermal population induced by laser heating. Therefore, the relaxation rate we measure is controlled by intersubband scattering rather than carrier cooling. At low temperature we obtain an intersubband lifetime of which is power independent below 10⁻¹ W/cm², and approximately temperature independent for lattice temperatures between T=10 and 2.5 K.     

Extraction of current carriers by photons in a quantum well

The extraction of holes by photons in an infinitely deep semiconductor quantum well is studied. Both interzone and intersubzone optical transitions, which make separate contributions to this effect, are examined. In calculating the extraction current, it is assumed that the optical transition probability depends on the photon momentum only as a result of its appearance in the energy and momentum conservation laws. Fiz. Tverd. Tela (St. Petersburg) 40, 1710–1711 (September 1998)     

Intersubband optical absorption coefficients and refractive index changes in modulation-doped asymmetric double quantum well

The linear and the third-order nonlinear optical absorption coefficients and refractive index changes in a modulation-doped asymmetric double quantum well are studied theoretically. The electron energy levels and the envelope wave functions in this structure are calculated by the Schrödinger and Poisson equations self-consistently in the effective mass approximation. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. In this regard, the linear, nonlinear and total intersubband absorption coefficients and refractive index changes are investigated as a function of right-well width (Lw₂) of asymmetric double quantum well. Our results show that the total absorption coefficients and refractive index changes shift toward higher energies as the right-well width decreases. In addition, the total optical absorption coefficients and refractive index changes is strongly dependent on the incident optical intensity.     

Energy relaxation and thermalization of hot electrons in quantum wires

We develop a theory of energy relaxation and thermalization of hot carriers in clean quantum wires. Our theory is based on a controlled perturbative approach for large excitation energies and emphasizes the important roles of the electron spin and finite temperature. Unlike in higher dimensions, relaxation in one-dimensional electron liquids requires three-body collisions and is much faster for particles than holes which relax at nonzero temperatures only. Moreover, comoving carriers thermalize more rapidly than counterpropagating carriers. Our results are quantitatively consistent with a recent experiment.     

Scattering of excitons by free carriers in semiconducting quantum well structures

We have theoretically investigated the scattering of excitons by free electrons and holes in a two-dimensional semiconducting quantum well system. The scattering cross-sections have been calculated using the Born approximation for both the elastic and inelastic scattering of the excitons by the free carriers. The threshold for inelastic scattering is increased over the value in a bulk semiconductor because of the enhancement of the exciton binding energy by its confinement. The behavior of the scattering cross-section as a function of the energy of relative motion of the free carriers and the excitons is different than in the bulk and the cross-section is a more sensitive function of the ratio of the electron and hole masses than in the bulk. In fact, the scattering of light hole excitons is suppressed relative to that of heavy hole excitons by several orders of magnitude.     

Intersubband scattering as a tool to study the symmetry properties of a parabolic quantum well

The electron mobility in parabolic quantum wells is measured as a function of carrier density at low temperatures. The curves display pronounced maxima that indicate the onset of intersubband scattering, i.e. the population of another subband. We find that the population of a given subband only depends on the total carrier density almost independent of the position of the wavefuntion within the well which can be tuned via a front and back gate electrode. This is in agreement with predictions for a perfect parabolic potential. If a potential spike is inserted in the center of the PQW we find a complicated behavior for the mobility versus carrier density maxima which reflects the changed symmetry of the potential.