S. V. Ivanov

Absorption and Emission of Hexagonal InN. Evidence of Narrow Fundamental Band Gap

(a) Ioffe Physico-Technical Institute, Russian Academy of Science, Polytekhnicheskaya 26, 194021 St. Petersburg, Russia (b) Institut für Festkörpertheorie and Theoretische Optik, Friedrich-Schiller-Universität Jena, Max-Wien-Platz 1, D-07743 Jena, Germany (c) Department of Electronics and Information Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan (d) Institute of Solid State and Semiconductor Physics, Belarus Academy of Sciences, Brovki 17, 220072 Minsk, Belarus (e) LfI, University of Hannover, Schneiderberg 32, D-30167 Hannover, Germany

Radiative recombination in type‐II GaSb/GaAs quantum dots

Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self‐organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan‐view transmission electron microscopy studies reveal well developed rectangular‐shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Band Gap of Hexagonal InN and InGaN Alloys

A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 × 10 18 and 4 × 10 19 cm -3 . It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich In x Ga 1-x N alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b ∼ 2.5 eV allows one to reconcile our results and the literature data for the band gap of In x Ga 1-x N alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN-In 2 O 3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.

Experimental and theoretical studies of phonons in hexagonal InN

The first- and second-order Raman scattering and IR reflection have been studied for hexagonal InN layers grown on (0001) and (1102) sapphire substrates. All six Raman-active optical phonons were observed and assigned: E2(low) at 87 cm−1, E2(high) at 488 cm−1, A1(TO) at 447 cm−1, E1(TO) at 476 cm−1, A1(LO) at 586 cm−1, and E1(LO) at 593 cm−1. The ratio between the InN static dielectric constants for the ordinary and extraordinary directions was found to be e⊥0/e∥0=0.91. The phonon dispersion curves, phonon density-of-state function, and lattice specific heat were calculated. The Debye temperature at 0 K for hexagonal InN was estimated to be 370 K.

Mie Resonances, Infrared Emission, and the Band Gap of InN

Mie resonances due to scattering or absorption of light in InN-containing clusters of metallic In may have been erroneously interpreted as the infrared band gap absorption in tens of papers. Here we show by direct thermally detected optical absorption measurements that the true band gap of InN is markedly wider than the currently accepted 0.7 eV. Microcathodoluminescence studies complemented by the imaging of metallic In have shown that bright infrared emission at 0.7-0.8 eV arises in a close vicinity of In inclusions and is likely associated with surface states at the metal/InN interfaces.

Magnetic property investigations on Mn-doped ZnO Layers on sapphire

The need for diluted magnetic semiconductors has stimulated research on Mn-doped ZnO. However, the type of magnetic coupling (ferro/para) in ZnMnO remains an issue of debate. We have investigated the magnetic properties of Mn-doped ZnO layers grown by molecular beam epitaxy. Some samples showed a hysteresis with remnant magnetization on the order of 10−5emu, thus eventually suggesting ferromagnetism. We observed that the critical influence of the substrate substantially affects magnetic property measurements. This has to be taken into account in order to clearly confirm ferromagnetism. In our case, after subtraction of the substrate effect, there is no evidence of a ferromagnetic behavior for the ZnMnO samples.

Riemannian tori without conjugate points are flat

This statement is known as the Hopf conjecture and it has been proved by E. Hopf ([Ho]) for the case n = 2. The proof of Theorem 1 is contained in sections 1–5. The main idea of our proof is that the limit norm of such a metric (see section 1.2) is a Euclidean norm. For two unit vectors p, q in a Banach space with its unit sphere having a unique supporting linear function −Bp at p one can define something like inner product 〈p, q〉 = −Bp(q). To show that it actually is an inner product we prove that Euclidean norms possess some extremal property of integral type which makes them distinguishable among all Banach norms. Then we note that the functions Bp(q) for the limit norm of our metric can be drawn from the infinitesimal inner product by the means of integral geometry, and we check the property above for the limit norm. This proves that the limit norm is Euclidean and our inequalities for the integrals turn out to be equalities almost everywhere. Then a rather simple additional argument shows that in this case our metric is flat. Section 6 contains a brief discussion and the volume growth theorem. The history of the subject will not be touched upon in this short paper. We express our gratitude to Prof. V. Bangert for thoroughly reading the manuscript, his valuable remarks and his help in finding references. The first author also would like to use this opportunity to thank Prof. V. Bangert for his kind invitation to the University of Freiburg and actually interesting discussions.

Growth and excitonic properties of single fractional monolayer CdSe/ZnSe structures

Single fractional monolayer (FM) CdSe/ZnSe structures have been grown by molecular beam epitaxy (MBE), employing both conventional MBE and migration-enhanced epitaxy (MEE). A precise calibration of the FM mean thickness in the range of 0.15–3.0 ML has been performed for both techniques, revealing more than a 3.5 times lower Cd incorporation ability for the MEE mode at the same Cd and Se incident fluxes. Steady-state and time-resolved photoluminescence spectroscopy is used to characterize the intrinsic morphology of the CdSe FMs, with a special emphasis on the submonolayer thickness range. Both MBE and MEE grown samples exhibit inhomogeneity of the excitonic system, which can be explained by coexistence of a homogeneous alloylike layer and relatively large CdSe 2D clusters. The MEE samples display smaller fluctuations of the layer thickness and island sizes.

CdSe fractional-monolayer active region of molecular beam epitaxy grown green ZnSe-based lasers

This letter reports on the self-organized growth of nanoscale dot-like CdSe-based islands during molecular beam epitaxy of CdSe/ZnSe nanostructures with a CdSe thickness between 0.75 and 3.0 monolayers. An increase in the nominal CdSe thickness results in a higher density of islands (up to 2×1010 cm−2) and is accompanied by dramatic enhancement of the photoluminescence efficiency. The density of large relaxed islands appears to saturate at a value of (3–4)×109 cm−2. Room temperature (Zn, Mg)(S, Se)-based optically pumped lasers with an extremely low threshold (less than 4 kW/cm2), as well as (Be, Mg, Zn)Se-based injection laser diodes using a single (2.5–2.8) monolayer thick CdSe active region, both demonstrating significantly enhanced degradation stability, have been fabricated and studied.

Composition stoichiometry and growth rate control in molecular beam epitaxy of ZnSe based ternary and quaternary alloys

Abstract This paper presents an original thermodynamic description of (Mg,Zn)(S,Se) MBE growth, which is in a good quantitative agreement with experimental data. This approach provides large flexibility in choosing different growth regimes of pseudomorphic ZnSe-based heterostructures to obtain desirable alloy composition, surface stoichiometry, and growth rate. The possibility to control a nearly lattice-matched to GaAs composition of a Zn1−xMgxSySe1−y alloy from (x = 0, y = 0.09) to (x = 0.07, y = 0.13) only by variation of the Mg flux intensity has been theoretically revealed and experimentally realized in an optically pumped ZnMgSSe/ZnSSe/ZnCdSe SQW SCH laser structure with a threshold power density of 20 kW/cm2 at 300 K.