A. Chomette

Exciton Binding Energy in Small-Period GaAs/Ga(1-x)AlxAs Superlattices

We report the optical determination of exciton binding energies in small-period GaAs/Ga0.7Al0.3As superlattices by means of low-temperature photoluminescence excitation spectroscopy and photoluminescence spectroscopy as a function of temperature. The heavy-hole exciton binding energy decreases with decreasing superlattice period. Our experimental findings are in reasonable agreement with a variational calculation.

Optical studies of perpendicular transport in semiconductor superlattices

Optical studies of perpendicular transport of carriers in superlattices are reviewed. Steady-state optical measurements demonstrate the existence of perpendicular transport in superlattices. The nature of the transport has been recently investigated by using all-optical time-of-flight techniques. These studies have demonstrated that the transport for both electrons and holes indeed proceeds by extended Bloch states of the minibands in short-period superlattices. Furthermore, the mobility for such transport has been determined by these ultrafast luminescence techniques. The transition from Bloch transport to the localized transport with increasing period of the superlattice has been directly observed. >

Vertical transport in GaAs/Ga1-xAlxAs superlattices observed by photoluminescence

Abstract Photoluminescence experiments on GaAs/Ga1-xAlxAs small period superlattices in which enlarged wells have been purposely introduced reveal a transfer of photoexcited carriers from the superlattice to the enlarged well localized levels. The transfer efficiency characterized by the relative intensities of luminescence peaks increases when the superlattice period decreases. Within a simple model, ionized impurity scattering and well size fluctuations account for this carrier transfer.

Tunnelling and Relaxation in Coupled Quantum Wells

We study by time-resolved luminescence, with subpicosecond resolution, the tunnelling processes out of a narrow well (NW) coupled to a wider one (WW) by a thin barrier. Resonance and nonresonance between the first level of the NW and the second level of the WW is obtained by adjusting the WW width. At resonance, and for narrow enough barriers, the transfer of the electrons to the WW ground state takes about 2 ps and is analogous to LO phonon-assisted intersubband relaxation. When the barrier thickness is increased, even at resonance, the transfer time from the NW to the WW varies exponentially with the barrier thickness, as in the nonresonant case, showing that the coherence of the coupled states is destroyed.

Carrier capture in quantum wells

We have studied the mechanisms of capture in quantum wells by time resolved luminescence with femtosecond resolution. The measured decay times of the barrier luminescence are always shorter than 3 ps and the well luminescence rise time is of the order of 4 ps at 100 K. These coupled results indicate that electron and hole capture is always very fast. We carry out a quantum mechanical calculation which takes into account LO phonon scattering and impurity scattering as well as hot carrier effects. The short capture times in all cases are shown to be explained by the competing influence of these different scattering processes.

Picosecond luminescence approach to vertical transport in GaAs/GaAlAs superlattices

Abstract Picosecond luminescence of GaAs/GaAlAs superlattices has been measured at 5 K. Asymetrical structures where one larger well is introduced at 9000 A from the surface are studied. It is then possible to estimate the mean transfer time of photoexcited carriers through 9000 A of superlattice. This time is found to be about 4 nsec in a 40/40 A superlattice and 800 psec in a 30/30 A one. This evidences the rather high mobility of small period superlattices in the growth direction.

Band discontinuities and calculations of GaAs‐AlGaAs superlattice structures

The first two optical transition energies of GaAs‐Ga(Al)As superlattices and multiquantum wells with small well widths are quite sensitive to the band discontinuity. On a series of samples the parameters of which are determined by x‐ray diffraction, we compare the energies measured in photoluminescence and photoluminescence excitation spectroscopies with the results of calculations within the framework of the envelope function approach and with the simple Kronig–Penney model. The envelope function approximation models give transition energies systematically smaller than the experimental values and the discrepancy increases when the well width decreases. With the Kronig–Penney model, both excitonic transitions are satisfactorily calculated for each sample with a conduction‐band offset around 75%, but we do not find a unique value of the conduction‐band offset for all samples.

Optical detection of vertical transport in short-period GaAs/AlGaAs superlattices

Abstract In short-period superlattices carriers are able to move perpendicularly to the layers. Optical experiments are convenient tools to observe this transport, called vertical transport, and to determine its variations with the superlattice period and the structural disorder. Electron and hole transports are differentiated using appropriate excitation densities, and their diffusion coefficients are estimated. In agreement with the miniband width calculations, the transition from a Bloch conduction to a hopping conduction is observed at different values of the superlattice period for electrons and holes. Finally, vertical transport must be taken into account to understand the differences between absorption and excitation spectra in superlattices; AlGaAs cladding layers are necessary to avoid the escape of carriers into the substrate.

Deep levels as local probes for the study of superlattices

Two kinds of deep levels have been studied in GaAs/GaAlAs superlattices: electron irradiation‐induced defects and manganese. In both cases we confirm the localized character of the wave function by showing the invariance of the energy levels with respect to former band edges of the superlattice constituting materials. We obtain information on the energy levels of the electron and hole bands in the superlattice from the observed ionization energies of these deep levels and compare these results to energy level calculations.

Enlarged wells as probes to study superlattices

Abstract Within GaAs/Ga1−xAlx As MBE grown superlattices (SL), a few GaAs wells have been purposely enlarged. Calculations show that enlarged wells introduce localized states in the SL band gap: the distance between such a localized state and the bottom of the SL conduction band (or the top of the SL valence band) depends on the SL period and the enlarged well size. Luminescence and photoluminescence excitation results obtained on samples with different periods and enlarged well sizes are in good agreement with calculations. Once characterized, enlarged wells serve as probes in the study of SL optical and electrical properties. As examples, they were used to observe one monolayer size fluctuations and vertical transport by luminescence in GaAs/Ga1−xAlxAs superlattices.