Impact of defects on photoexcited carrier relaxation dynamics in GeSn thin films.

We report the results of a study that was conducted to investigate the recombination paths of photoexcited charge carriers in GeSn thin films. The charge carrier lifetime was predicted as a function of temperature from a description of photoconductivity transients, assuming co-influence of Shockley-Read-Hall and radiative carrier recombination paths. We identify that dislocations are the source of a band of electronic states with the highest occupied state at E V + (85÷90) meV that acts as Shockley-Read-Hall centers determining the charge carrier lifetime. The photoluminescence (PL) and photoconductivity spectroscopy have been applied to distinguish between the contribution of both band-to-band and dislocation-related electron transitions. The PL band was found to demonstrate a low-energy shift of about 80 ± 20 meV relative to the edge of the photoconductivity spectra in the indirect bandgap GeSn films with dislocations. The role of a different nature deeper acceptor level at E V + (140 ÷ 160) meV in the recombination processes of the GeSn layers with better structural quality and the Sn content higher than 4% was discussed. This detailed understanding of the recombination processes is of critical importance for developing GeSn/Ge-based optoelectronic devices.

Charge carrier relaxation in InGaAs-GaAs quantum wire modulation-doped heterostructures.

The time dependencies of the carrier relaxation in modulation-doped InGaAs-GaAs low-dimensional structures with quantum wires have been studied as functions of temperature and light excitation levels. The photoconductivity (PC) relaxation follows a stretched exponent with decay constant, which depends on the morphology of InGaAs epitaxial layers, presence of deep traps, and energy disorder due to inhomogeneous distribution of size and composition. A hopping model, where electron tunnels between bands of localized states, gives appropriate interpretation for temperature-independent PC decay across the temperature range 150-290 K. At low temperatures (T < 150 K), multiple trapping-retrapping via 1D states of InGaAs quantum wires (QWRs), sub-bands of two-dimensional electron gas of modulation-doped n-GaAs spacers, as well as defect states in the GaAs environment are the dominant relaxation mechanism. The PC and photoluminescence transients for samples with different morphologies of the InGaAs nanostructures are compared. The relaxation rates are found to be largely dependent on energy disorder due to inhomogeneous distribution of strain, nanostructure size and composition, and piezoelectric fields in and around nanostructures, which have a strong impact on efficiency of carrier exchange between bands of the InGaAs QWRs, GaAs spacers, or wetting layers; presence of local electric fields; and deep traps.

Band offsets and photocurrent spectroscopy of Si/Ge heterostructures with quantum dots.

Raman and lateral photoconductivity spectra of self-assembled SiGe nanoislands were studied with a height of ∼2 nm and a base of ∼20 nm formed at a temperature of 500 °C. It was estimated that the value of elastic deformation (ε(xx)) was -0.022 (ε(zz) = 0.017), while the germanium content in the islands (x) was 0.66. The obtained values of x and ε were used to calculate band offsets at the interfaces and the energy of interband transitions of structures under study. It was shown that the minimal energy of photocurrent observation is 0.52 eV, which is below the bandgap of the QDs under study. The first photocurrent component which began to contribute at 0.52 eV and had a peak at 0.68 eV is explained by optical transitions of electrons from the QD HH localized states of the valence band to the conduction band Δ(2) valley of the surrounding silicon matrix in which tensile strains are present. The second component with limiting energy of 0.73 eV can be caused by interband electron transitions from the HH valence band of the QDs to the Δ(4) valley of the QD conduction band.