We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed. Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

During the last few years the developments in the field of III–nitrides have been spectacular. High quality epitaxial layers can now be grown by MOVPE. Recently good quality epilayers have also been grown by MBE. Considerable work has been done on dislocations, strain, and critical thickness of GaN grown on different substrates. Splitting of valence band by crystal field and by spin-orbit interaction has been calculated and measured. The measured values agree with the calculated values. Effects of strain on the splitting of the valence band and on the optical properties have been studied in detail. Values of band offsets at the heterointerface between several pairs of different nitrides have been determined. Extensive work has been done on the optical and electrical properties. Near band-edge spectra have been measured over a wide range of temperatures. Free and bound exciton peaks have been resolved. Valence band structure has been determined using the PL spectra and compared with the theoretically calcu...

III–nitrides: Growth, characterization, and properties

The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emission spectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40 meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1 ns at 6 K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition.

Colloidal nanoplatelets with two-dimensional electronic structure

We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect enginee...

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

The fundamental relationship between radiative lifetime and spectral linewidth of free excitons is demonstrated theoretically and experimentally for quasi 2D excitons in GaAs/AlGaAs quantum wells.

Linewidth dependence of radiative exciton lifetimes in quantum wells

The optically detected magnetic resonance of electrons and excitons in type-II GaAs/AlAs quantum wells has been studied as a function of the orientation of the quantum well with respect to the direction of the magnetic field. An analysis of the anisotropy reveals that for quantum wells with AlAs layers thinner than \ensuremath{\sim}55 A\r{} the ${X}_{z}$ conduction-band valley has the lowest energy while for quantum wells with thicker AlAs layers the ${X}_{x}$ and ${X}_{y}$ valleys are lowest in energy. This is a consequence of the lattice-mismatch strain splitting of the AlAs X conduction band which dominates the confinement splitting for AlAs layers thicker than \ensuremath{\sim}55 A\r{}.

Order of the X conduction-band valleys in type-II GaAs/AlAs quantum wells.

Discrete peaks are seen in the photoluminescence excitation spectra of GaAs-(AlGa)As multiple-quantum-well samples which we identify as the excited $2s$ state of the $n=1$ heavy- and light-hole excitons. The $1s\ensuremath{-}2s$ splitting of the excitons is accurately determined and combined with calculations of the $2s$ heavy-hole exciton binding energy to give an accurate determination of the binding energy of the ground state.

Unambiguous observation of the 2s state of the light- and heavy-hole excitons in GaAs-(AlGa)As multiple-quantum-well structures.

We report the results of low-temperature photoluminescence and photoluminescence excitation studies of short-period, n=m, (GaAs${)}_{\mathrm{n}}$-(AlAs${)}_{\mathrm{m}}$ superlattices fabricated by molecular-beam epitaxy. Values of n ranged between 2 and 8. We find that the smallest energy gap does not approximate to that of an ${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$As alloy until n+m\ensuremath{\le}4 monolayers. The limits of a simple Kronig-Penney model of the electronic states are explored in relation to our observations. For the direct gap, good agreement is achieved between experiment and theory down to m=n\ensuremath{\ge}4 monolayers. The description of the ``pseudodirect'' gap breaks down at n=m\ensuremath{\sim}6 monolayers. Reasons for the failure of the simple description are discussed.

Short-period GaAs-AlAs superlattices: Optical properties and electronic structure.

We present the first direct measurements of the heavy-hole exciton binding energy in a III-V-compound quantum-well system where the well widths are 25 A\r{} or smaller. The 4-K photoluminescence excitation (PLE) spectrum from ${\mathrm{In}}_{0.15}$${\mathrm{Ga}}_{0.85}$As/GaAs multiple-well samples with 48-, 25-, and 14-A\r{} wells displays a sharp heavy-hole exciton feature well resolved from the continuum edge; the separation between these features yields exciton binding energies of 9.2, 8.7, and 6.5 meV, respectively. The decrease in binding energy with decreasing well width confirms the predicted trend from our calculations. Quantitative agreement between the model calculations and the observations is good. In addition, we have seen fine structure in the PLE spectrum of the 14-A\r{} sample in both the heavy- and light-hole excitonic peaks. The energy splitting in the fine structure equates with changes in In fraction of less than 0.5% in 15% or changes in average well width of 1\AA{}.

Karen J. Moore

Papers

Linewidth dependence of radiative exciton lifetimes in quantum wells

Order of the X conduction-band valleys in type-II GaAs/AlAs quantum wells.

Unambiguous observation of the 2s state of the light- and heavy-hole excitons in GaAs-(AlGa)As multiple-quantum-well structures.

Short-period GaAs-AlAs superlattices: Optical properties and electronic structure.

Observations and calculations of the exciton binding energy in (In,Ga)As/GaAs strained-quantum-well heterostructures