AlP/GaP quantum wells for implementing intersubband devices in the 30-60 µm wavelength region.

International audienc

Excitation correlation photoluminescence in the presence of Shockley Read Hall recombination

Excitation correlation photoluminescence ECPL measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non radiative recombination path, but not for a non linear recombination rate as in the general Shockley Read Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley Read Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs InAlAs that were additionally cross checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreemen

Band offsets of InGaP/GaAs heterojunctions by scanning tunneling spectroscopy

Scanning tunneling microscopy and spectroscopy are used to study InGaP/GaAs heterojunctions with InGaAs-like interfaces. Band offsets are probed using conductance spectra, with tip-induced band bending accounted for using three-dimensional electrostatic potential simulations together with a planar computation of the tunnel current. Curve fitting of theory to experiment is performed. Using an InGaP band gap of 1.90 eV, which is appropriate to the disordered InGaP alloy, a valence band offset of 0.38±0.01 eV is deduced along with the corresponding conduction band offset of 0.10±0.01 eV (type I band alignment)

Concept and demonstration of an intermediate band tandem device for solar energy conversion

We have realized a tandem solar cell design that combines a pin junction with a photovoltaic intersubband absorber. This concept allows harvesting light in the visible range and the near and mid infrared at the same time, and theoretically, energy conversion efficiencies beyond the Shockley Queisser limit could be achieved. A test structure was grown, and the operation of this concept could be confirmed, in principal with an optical two beam experiment. The basic characteristics of the device can be explained with an equivalent circuit design that consists of three individual cells, and we find an obvious analogy to the concept of the intermediate band solar cell with noteworthy advantages at some points. Our results show, that for a working device it is crucial to adjust the properties of the photovoltaic intersubband absorber for optimal charge separating performance at the working point of the solar cel

Magnetospectroscopy of AIP quantum wells

International audienc

Light emission despite doubly-forbidden radiative transitions in AlP/GaP quantum wells: Role of localized states

International audienceThe GaP/AlP/GaP heterostructure has an indirect gap both in real as well as momentum space, making the first order radiative recombination doubly forbidden. Nevertheless, we have observed relatively efficient emission from these structures. This paper comprehensively studies the origin of this improved light emission through a detailed analysis of the photoluminescence (PL) spectra. Our observations suggest that localized excitons within the acceptor states in GaP close to the heterostructure interface are enough for efficient light emission in these structures, doing away with the need for more complicated structures (superlattices or neighboring confinement structures). This real space localization of holes, close to the interface, apart from increasing the wave function overlap, also relaxes the delta-function momentum selection rule. Independent experimental evidence for this assertion comes from (i) the PL spectrum at high excitation power where transitions from both the localized as well as extended states are independently observed, (ii) the observation that extended states emission has the expected band-bending-induced blue-shift with increase in excitation power, whereas the localized states do not, (iii) observation of phonon replicas for PL from localized states, and (iv) observation of persistent photoconductivity at low temperature. Finally, we propose a simple analytical model that accounts for both the type-II nature as well as the indirect bandgap to explain the improvement of radiative recombination efficiency with increased localization. The experimental observations are reproduced within an order of magnitude. The model is very general and it also provides a framework to study the optical properties of other such (type-II and/or indirect gap) heterostructures