In heterostructures consisting of different transition-metal dichalcogenide monolayers, a staggered band alignment can occur, leading to rapid charge separation of optically generated electron–hole pairs into opposite monolayers. These spatially separated electron–hole pairs are Coulomb-coupled and form interlayer excitons. Here, we study these interlayer excitons in a heterostructure consisting of MoSe2 and WSe2 monolayers using photoluminescence spectroscopy. We observe a non-trivial temperature dependence of the linewidth and the peak energy of the interlayer exciton, including an unusually strong initial redshift of the transition with temperature, as well as a pronounced blueshift of the emission energy with increasing excitation power. By combining these observations with time-resolved photoluminescence measurements, we are able to explain the observed behavior as a combination of interlayer exciton diffusion and dipolar, repulsive exciton-exciton interaction.

https://iopscience.iop.org/article/10.1088/2053-1583/aa7352/pdf

Interlayer exciton dynamics in a dichalcogenide monolayer heterostructure

Highly mismatched semiconductor alloys such as GaNxAs1 − x and GaBixAs1 − x have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spin-orbit-splitting energy with increasing Bi composition. We review here the electronic structure of such alloys and their consequences for ideal lasers. We then describe the substantial progress made in the demonstration of actual GaInNAs telecommunication (telecom) lasers. These have characteristics comparable to conventional InP-based devices. This includes a strong Auger contribution to the threshold current. We show, however, that the large spin-orbit-splitting energy in GaBiAs and GaBiNAs could lead to the suppression of the dominant Auger recombination loss mechanism, finally opening the route to efficient temperature-stable telecomm and longer wavelength lasers with significantly reduced power consumption.

Band engineering in dilute nitride and bismide semiconductor lasers

Highly mismatched semiconductor alloys such as GaNAs and GaBiAs have several novel electronic properties, including a rapid reduction in energy gap with increasing x and also, for GaBiAs, a strong increase in spin orbit- splitting energy with increasing Bi composition. We review here the electronic structure of such alloys and their consequences for ideal lasers. We then describe the substantial progress made in the demonstration of actual GaInNAs telecomm lasers. These have characteristics comparable to conventional InP-based devices. This includes a strong Auger contribution to the threshold current. We show, however, that the large spin-orbit-splitting energy in GaBiAs and GaBiNAs could lead to the suppression of the dominant Auger recombination loss mechanism, finally opening the route to efficient temperature-stable telecomm and longer wavelength lasers with significantly reduced power consumption.

/pdf/band-engineering-in-dilute-nitride-and-bismide-semiconductor-4zgdqfbmvz.pdf

Traps limit the photovoltaic efficiency and affect the charge transport of optoelectronic devices based on hybrid lead halide perovskites. Understanding the nature and energy scale of these trap states is therefore crucial for the development and optimization of solar cell and laser technology based on these materials. Here, the low-temperature photoluminescence of formamidinium lead triiodide (HC(NH2)2PbI3) is investigated. A power-law time dependence in the emission intensity and an additional low-energy emission peak that exhibits an anomalous relative Stokes shift are observed. Using a rate-equation model and a Monte Carlo simulation, it is revealed that both phenomena arise from an exponential trap-density tail with characteristic energy scale of ≈3 meV. Charge-carrier recombination from sites deep within the tail is found to cause emission with energy downshifted by up to several tens of meV. Hence, such phenomena may in part be responsible for open-circuit voltage losses commonly observed in these materials. In this high-quality hybrid perovskite, trap states thus predominantly comprise a continuum of energetic levels (associated with disorder) rather than discrete trap energy levels (associated, e.g., with elemental vacancies). Hybrid perovskites may therefore be viewed as classic semiconductors whose band-structure picture is moderated by a modest degree of energetic disorder.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.201700860

Band‐Tail Recombination in Hybrid Lead Iodide Perovskite

The photoluminescence from a Ga(AsBi) sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.

Clustering effects in Ga(AsBi)

Photoluminescence in (GaIn)(NAs) quantum wells designed for laser emission was studied experimentally and theoretically. The observed temperature dependences of the luminescence Stokes shift and of the spectral linewidth evidence the essential role of disorder in the dynamics of the recombining excitations. The spatial and energy disorders can cause a localization of photocreated excitations supposedly in the form of excitons. Theoretical study of the exciton dynamics is performed via kinetic Monte Carlo simulations of exciton hopping and recombination in the manifold of localized states. Direct comparison between experimental spectra and theoretical calculations provides quantitative information on the energy scale of the potential fluctuations in (GaIn)(NAs) quantum wells. The results enable one to quantify the impact of annealing on the concentration of localized states and/or on the localization length of excitons in (GaIn)(NAs) quantum wells.

Quantitative description of disorder parameters in (GaIn)(NAs) quantum wells from the temperature-dependent photoluminescence spectroscopy

We present an extensive study leading to a general understanding about the optical transitions involved in the surface photovoltage (SPV) spectra of type-I quantum well (QW) structures. SPV measurements were carried out on ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}∕\mathrm{GaAs}$ QW samples with varying QW width and on ${\mathrm{In}}_{0.15}{\mathrm{GaAs}}_{0.85}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ QW samples with varying aluminum content. The results are compared with experimental electroreflectance spectra, absorption spectra simulated with multiband $\mathbf{k}∙\mathbf{p}$ computations, and theoretical calculations of the electronic states in the QWs. Thus, the transitions which induce steps in the SPV spectra are unambiguously identified. Most remarkably, the bound electron-to-free hole transition is detected by SPV measurements. This distinguishes the SPV method from many other optical techniques like photoluminescence and photoreflectance spectroscopy that allow only the observation of bound-to-bound and free-to-free transitions. The analogous free electron-to-bound hole transition does not give rise to a feature in the SPV spectra. In addition, the bound-to-bound transitions ${e}_{1}\text{\ensuremath{-}}h{h}_{1}$, ${e}_{1}\text{\ensuremath{-}}l{h}_{1}$, and ${e}_{2}\text{\ensuremath{-}}h{h}_{2}$ are observed, if the respective states are confined. Also, the free-to-free transition $ce\text{\ensuremath{-}}ch$ is measured. We demonstrate how these transitions can typically be identified in the spectra without any further measurements or calculations. With this knowledge, the practical band offsets of the QW structure, i.e., the energy difference between the lowest confined states in the QW and the extended states in the barrier, can be extracted directly from the spectra. An advantage over conventional techniques for the determination of band offsets is that neither any additional knowledge of other QW parameters nor simulations are necessary. As an example for the application of the SPV technique, the electronic states and band offsets of a series of ${\mathrm{In}}_{0.3}{\mathrm{Ga}}_{0.7}{\mathrm{As}}_{0.98}{\mathrm{N}}_{0.02}∕\mathrm{GaAs}$ QWs with varying QW width are characterized. These experiments directly show that nitrogen affects only the conduction band states.

Bound-to-bound and bound-to-free transitions in surface photovoltage spectra: Determination of the band offsets for In x Ga 1 − x As and In x Ga 1 − x As 1 − y N y quantum wells

The band offsets of InGaAsN single quantum wells with varying nitrogen and indium content were quantitatively determined by surface photovoltage measurements. The experimental data directly show the different effect of nitrogen on the valence and on the conduction band states. While the conduction band offset strongly increases with increasing nitrogen concentration, the valence band offset is only weakly affected. In contrast, indium influences the valence and the conduction band states in the same way: both the valence and conduction band offsets increase with increasing indium content. In particular, the conduction band offset varies with In content as in N-free InGaAs quantum wells.

Nitrogen and indium dependence of the band offsets in InGaAsN quantum wells

We compare the luminescence efficiency (i.e., room-temperature photoluminescence intensity), fluctuations in composition and thickness, degree of localization, and luminescence decay times of In0.37Ga0.63As0.983N0.017 quantum wells grown by molecular-beam epitaxy at different temperatures and annealed under a comprehensive variety of conditions. Luminescence efficiency is not directly coupled to structural nonuniformity or localization, and even three-dimensional growth is not detrimental by itself. In contrast, there is always a correlation between luminescence efficiency and nonradiative decay time. Therefore, the luminescence efficiency of InGaAsN quantum wells depends almost exclusively on the density of nonradiative recombination centers, while the influence of structural nonuniformity is negligible.

Influence of structural nonuniformity and nonradiative processes on the luminescence efficiency of InGaAsN quantum wells

The band offsets of InGaAsN single quantum well samples with different indium and nitrogen concentrations have been determined by surface photovoltage measurements. With varying indium content in the quantum well, both the conduction and the valence band states are modified. On the other hand, nitrogen content variations affect only the conduction band states and leave the valence band states basically unchanged. In particular, the value of the conduction band offset ratio increases with increasing nitrogen content and decreases with increasing indium concentration. This effect shows the possibility to design structures with a fixed band gap but varied confinement of electrons and holes by changing the In/N ratio.

M. Galluppi

Papers

Quantitative description of disorder parameters in (GaIn)(NAs) quantum wells from the temperature-dependent photoluminescence spectroscopy

Bound-to-bound and bound-to-free transitions in surface photovoltage spectra: Determination of the band offsets for In x Ga 1 − x As and In x Ga 1 − x As 1 − y N y quantum wells

Nitrogen and indium dependence of the band offsets in InGaAsN quantum wells

Influence of structural nonuniformity and nonradiative processes on the luminescence efficiency of InGaAsN quantum wells

Band offsets analysis of dilute nitride single quantum well structures employing surface photo voltage measurements