We report on quantum dot ͑QD͒ lasers made of stacked InAs dots grown by metalorganic chemical vapor deposition. Successful growth of defect-free binary InAs/GaAs QDs with high lateral density (d l у4ϫ10 10 cm Ϫ2) was achieved in a narrow growth parameter window. The room-temperature photoluminescence ͑PL͒ intensity is enhanced up to a factor of 3 and the PL peak width is reduced by more than 30% when a thin layer of In 0.3 Ga 0.7 As is deposited onto the InAs QDs. A QD laser with a single sheet of such InAs/InGaAs/GaAs QDs exhibits threshold current densities as low as 12.7 and 181 A/cm 2 at 100 and 300 K, respectively. Lasers with threefold stacked QDs show ground-state lasing and allow for cw operation at room temperature.
The authors have studied the electronic structure of InN and GaN employing G 0 W 0 calculations based on exact-exchange density-functional theory. For InN their approach predicts a gap of 0.7 eV. Taking the Burnstein-Moss effect into account, the increase of the apparent quasiparticle gap with increasing electron concentration is in good agreement with the observed blueshift of the experimental optical absorption edge. Moreover, the concentration dependence of the effective mass, which results from the nonparabolicity of the conduction band, agrees well with recent experimental findings. Based on the quasiparticle band structure the parameter set for a 4 ϫ 4 k · p Hamiltonian has been derived. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2364469͔The group III-nitrides AlN, GaN, and InN and their alloys have become an important class of semiconductor materials, in particular, for use in optoelectronic devices such as green and blue light emitting diodes and lasers. Among the three materials InN is still the least explored, due to difficulties in synthesizing high quality single crystals. Only very recently these problems have been overcome, 1 but many of the key band parameters have not been conclusively determined until now.1,2 The most controversially discussed parameter is currently still the fundamental band gap of InN. For many years it was believed to be approximately 1.9 eV, but essentially any value between 0.65 and 2.3 eV has been reported in the literature over the last 30 years.1 However, more recent experiments on high quality samples grown by molecular beam epitaxy and recent ab initio calculations support a significantly lower value around 0.7 eV. [3][4][5][6][7] Different hypotheses have been proposed to explain the large variation in the measured band gaps. Defects could be responsible for inducing states in the band gap or give rise to a pronounced Burnstein-Moss effect due to a shift in the Fermi level caused by a high intrinsic electron density. Nonstoichiometry may increase the defect concentration or alter the crystaline structure. The formation of oxides and oxynitrides would increase the band gap, whereas the precipitation of In clusters leads to additional features in optical absorption spectra. 1,8 In this letter we demonstrate that first principles calculations can contribute to the solution of this fundamental question. By combining density-functional theory ͑DFT͒ with many-body perturbation theory in the G 0 W 0 approximation, 9 which is currently the method of choice for calculating quasiparticle excitations in solids, 10,11 we combine atomistic control over the material with accurate calculations for the band structure and the band gap of stoichiometric and defect-free structures.Previous ab initio studies were aggravated by the fact that DFT calculations in the local-density approximation ͑LDA͒ predict InN to be metallic in the zinc blende and wurtzite structures. Subsequent G 0 W 0 calculations only open the gap to 0.02-0.05 eV, 12,13 while adding self-interaction correcti...
We present gain measurements and calculations for InAs/GaAs quantum dot injection lasers. Measurements of the modal gain and estimation of the confinement factor by transmission electron microscopy yield an exceptionally large material gain of 6.8(±1)×104 cm−1 at 80 A cm−2. Calculations including realistic quantum dot energy levels, dot size fluctuation, nonthermal coupling of carriers in different dots, and band filling effects corroborate this result. A large maximum differential gain of 2×10−12 cm2 at 20 A cm−2 is found. The width of the gain spectrum is determined by participation of excited quantum dot states. We record a low transparency current density of 20 A cm−2. All experiments are carried out at liquid nitrogen temperature.
By inserting stacked sheets of nominally 0.7 monolayer CdSe into a ZnSe matrix we create a region with strong resonant excitonic absorption. This leads to an enhancement of the refractive index on the low-energy side of the absorption peak. Efficient waveguiding can thus be achieved without increasing the average refractive index of the active layer with respect to the cladding. Processed high-resolution transmission electron microscopy images show that the CdSe insertions form Cd-rich two-dimensional (Cd, Zn)Se islands with lateral sizes of about 5 nm. The islands act as quantum dots with a three-dimensional confinement for excitons. Zero-phonon gain is observed in the spectral range of excitonic and biexcitonic waveguiding. At high excitation densities excitonic gain is suppressed due to the population of the quantum dots with biexcitons.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.