Effect of exciton-carrier thermodynamics on the GaAs quantum well photoluminescence

Precise radiative lifetimes in bulk crystals from first principles: the case of wurtzite gallium nitride

Gallium nitride (GaN) is a key semiconductor for solid-state lighting, but its radiative processes are not fully understood. Here we show a first-principles approach to accurately compute the radiative lifetimes in bulk uniaxial crystals, focusing on wurtzite GaN. Our computed radiative lifetimes are in very good agreement with experiment up to 100 K. We show that taking into account excitons (through the Bethe-Salpeter equation) and spin-orbit coupling is essential for computing accurate radiative lifetimes. A model for exciton dissociation into free carriers allows us to compute the radiative lifetimes up to room temperature. Our work enables precise radiative lifetime calculations in III-nitrides and other anisotropic solid-state emitters.

Photon-induced carrier recombination in the nonlayered-structured hybrid organic-inorganic perovskite nano-sheets

The hybrid organic-inorganic perovskites (HOIPs) have attracted much attention recently due to their preeminent efficiency in solar cells. According to the difference on the crystalline structure, the HOIPs could be classified into layered and non-layered perovskites. Very recently, it has been realized that the non-layered HOIPs with common-vertex structure possess even better opto-electrical performance. Yet the carrier recombination mechanism in perovskite remains not very clear, and a clear understanding of this mechanism is essential to pinpoint the working mechanism of photovoltaic and electroluminescent materials. Here we report the optical studies on the hybrid perovskite crystalline nano-sheet of CH₃NH₃PbBr₃ with common-vertex structure. It is shown that the non-layered perovskite crystalline nanosheets possess the exciton binding energy about two orders of magnitude smaller than that of the layered perovskite and the colloidal nanoplates, which is beneficial for the designing of the high-efficiency photovoltaic devices. By measuring the temperature-dependent photoluminescence (PL) spectra, the excitation-power-variant PL spectra, and the time-resolved PL spectra, we identify that both the free-carrier and the localized exciton recombination channels may coexist in the crystallites. Further, for the thin crystallite (∼60 nm), the free-carrier recombination channel dominates; whereas when the thickness increases beyond 200 nm, the localized exciton recombination channel plays the major role. We suggest these results are helpful to improve further the photovoltaic and electroluminescent performances of perovskite devices.

Experimental Evidence for Dark Excitons in Monolayer WSe_{2}

Transition metal dichalcogenides in the class MX_{2} (M=Mo, W; X=S, Se) have been identified as direct-gap semiconductors in the monolayer limit. Here, we examine light emission of monolayer WSe_{2} using temperature-dependent photoluminescence and time-resolved photoluminescence spectroscopy. We present experimental evidence for the existence of an optically forbidden dark state of the band-gap exciton that lies tens of meV below the optically bright state. The presence of the dark state is manifest in the strong quenching of light emission observed at reduced temperatures. The experimental findings are consistent with theoretical predictions of spin-polarized conduction and valence bands at the K point of the Brillouin zone, with the minimum gap occurring between bands of opposite electron spin.

Direct observation of charge-carrier heating at WZ-ZB InP nanowire heterojunctions

We have investigated the dynamics of hot charge carriers in InP nanowire ensembles containing a range of densities of zinc-blende inclusions along the otherwise wurtzite nanowires. From time-dependent photoluminescence spectra, we extract the temperature of the charge carriers as a function of time after nonresonant excitation. We find that charge-carrier temperature initially decreases rapidly with time in accordance with efficient heat transfer to lattice vibrations. However, cooling rates are subsequently slowed and are significantly lower for nanowires containing a higher density of stacking faults. We conclude that the transfer of charges across the type II interface is followed by release of additional energy to the lattice, which raises the phonon bath temperature above equilibrium and impedes the carrier cooling occurring through interaction with such phonons. These results demonstrate that type II heterointerfaces in semiconductor nanowires can sustain a hot charge-carrier distribution over an extended time period. In photovoltaic applications, such heterointerfaces may hence both reduce recombination rates and limit energy losses by allowing hot-carrier harvesting.

Determination of the exciton formation in quantum wells from time-resolved interband luminescence

We present the results of a detailed time-resolved luminescence study carried out on a very high quality InGaAs quantum well sample where the contributions at the energy of the exciton and at the band edge can be clearly separated. We perform this experiment with a spectral resolution and a sensitivity of the setup, allowing us to keep the observation of these two separate contributions over a broad range of times and densities. This allows us to directly evidence the exciton formation time, which depends on the density as expected from theory. We also denote the dominant contribution of excitons to the luminescence signal, and the lack of thermodynamical equilibrium at low densities.

Excitonic photoluminescence in semiconductor quantum wells: plasma versus excitons

Time-resolved photoluminescence spectra after nonresonant excitation show a distinct 1s resonance, independent of the existence of bound excitons. A microscopic analysis identifies exciton and electron-hole plasma contributions. For low temperatures and low densities, the excitonic emission is extremely sensitive to details of the electron-hole-pair population making it possible to identify even minute fractions of optically active excitons.