Nanoindentation cracking in gallium arsenide: Part II. TEM investigation

The nanoindentation fracture behavior of gallium arsenide (GaAs) is examined from two perspectives in two parent papers. In the first paper (part I), we address the morphology of the crack field induced by different types of indenters by means of in situ nanoindentation inside a scanning electron microscope (SEM) and of cleavage cross-sectioning techniques. In the present paper (part II), we investigate the early stage of crack nucleation under wedge nanoindentation through cathodoluminescence and transmission electron microscopy. We find that the apex angle of the wedge indenter influences the dislocation microstructure and, as a consequence, the mechanism of crack nucleation under nanoindentation. The formation of microtwins depends on both the orientation of the indenter with respect to the orientation of the GaAs crystal and on the apex angle of the indenter. For dicing applications of GaAs wafers, it is desirable to have an opening angle of the indenter smaller than 70° to facilitate the formation of precursor crack

Exciton hopping probed by picosecond time-resolved cathodoluminescence

The exciton transport is studied in high quality ZnO microwires using time resolved cathodoluminescence. Owing to the available picosecond temporal and nanometer spatial resolution, a direct estimation of the exciton average speed has been measured. When raising the temperature, a strong decrease of the effective exciton mobility (hopping speed of donor-bound excitons) has been observed in the absence of any remarkable change in the effective lifetime of excitons. Additionally, the exciton hopping speed was observed to be independent of the strain gradient value, revealing the hopping nature of exciton movement. These experimental results are in good agreement with the behavior predicted for impurity-bound excitons in our previously published theoretical model based on Monte-Carlo simulations, suggesting the hopping process as the main transport mechanism of impurity-bound excitons at low temperatures

Electron localization by a donor in the vicinity of a basal stacking fault in GaN

We study the effects of the vicinity between a shallow donor nucleus and an I1-type basal stacking fault (BSF) in GaN. We propose a numerical calculation, in the “effective potential” formalism, of energies and envelope functions of electrons submitted to the conjunction of attractive potentials caused by the BSF and the donor. We show that the donor localizes the electron along the plane of the BSF, even when the donor lies as far as 10 nm from the BSF. Conversely, the presence of the BSF enhances the donor binding energy by up to a factor of 1.8, when the donor is placed exactly on the BSF. We briefly discuss the probability of occurrence of such a situation in, e.g., a-plane GaN, as well as its consequences on transport and optical properties of this material

Impact of biexcitons on the relaxation mechanisms of polaritons in III-nitride based multiple quantum well microcavities

We report on the direct observation of biexcitons in a III nitride based multiple quantum well microcavity operating in the strong light-matter coupling regime by means of nonresonant continuous wave and time-resolved photoluminescence at low temperature. First, the biexciton dynamics is investigated for the bare active medium (multiple quantum wells alone) evidencing localization on potential fluctuations due to alloy disorder and thermalization between both localized and free excitonic and biexcitonic populations. Then, the role of biexcitons is considered for the full microcavity: in particular, we observe that for specific detunings the bottom of the lower polariton branch is directly fed by the radiative dissociation of either cavity biexcitons or excitons mediated by one LO-phonon. Accordingly, minimum polariton lasing thresholds are observed, when the bottom of the lower polariton branch corresponds in energy to the exciton or cavity biexciton first LO-phonon replica. This singular observation highlights the role of excitonic molecules in the polariton condensate formation process as being a more efficient relaxation channel when compared to the usually assumed acoustical phonon emission one.This work was supported by the NCCR Quantum Photonics, research instrument of the Swiss National Science Foundation, through Grant No. 129715 and Grant No. 200020-113542, and by the EU-project Clermont4 (Grant No. FP7-235114)

Implementation of spatio-time-resolved cathodoluminescence spectroscopy for studying local carrier dynamics in a low dislocation density m-plane InGaN epilayer grown on a freestanding GaN substrate

Spatio-time-resolvedcathodoluminescence (STRCL) spectroscopy isimplemented to assess the local carrier dynamics in a 70 nm-thick, very low threading dislocation (TD) density, pseudomorphic m-plane In GaN epilayer grown on a freestanding GaN substrate by metalorganic vapor phase epitaxy. Although TDs or stacking faults are absent, sub-micrometer-wide zonary patterns parallel to the c-axis and 2 um-long-axis figure-of-8 patterns parallel to the a-axis are clearly visualized in the monochromatic cathodoluminescence intensity images. Because the STRCL measurement reveals very little spatial variation of low-temperature radiative lifetime, the considerable peak energy variation is interpreted to originate from nonidentical In-incorporation efficiency for the growing surfaces exhibiting various miscut angles. The figure-of-8 patterns are ascribed to originate from the anisotropic,severe m-plane tilt mosaic along the a-axis of the GaNsubstrate,and the zonary patterns may originate from the m-plane tilt mosaic along the c-axi

Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence.

International audienceWe present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs

Thermal carrier emission and nonradiative recombinations in nonpolar(Al,Ga)N/GaN quantum wells grown on bulk GaN.

International audienceWe investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320K range. We first show that the effective lifetime of QW excitons tau increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing tau depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs

High-temperature Mott transition in wide-band-gap semiconductor quantum wells

The crossover from an exciton gas to an electron-hole plasma is studied in a GaN/(Al,Ga)N single quantum well by means of combined time-resolved and continuous-wave photoluminescence measurements. The two-dimensional Mott transition is found to be of continuous type and to be accompanied by a characteristic modification of the quantum well emission spectrum. Beyond the critical density, the latter is strongly influenced by band-gap renormalization and Fermi filling of continuum states. Owing to the large binding energy of excitons in III-nitride heterostructures, their injection-induced dissociation could be tracked over a wide range of temperatures, i.e., from 4 to 150K. Various criteria defining the Mott transition are examined, which, however, do not lead to any clear trend with rising temperature: the critical carrier density remains invariant around 1012cm−2

Thermal carrier emission and nonradiative recombinations in nonpolar (Al,Ga)N/GaN quantum wells grown on bulk GaN

We investigate, via time-resolved photoluminescence, the temperature-dependence of charge carrier recombination mechanisms in nonpolar (Al,Ga)N/GaN single quantum wells (QWs) grown via molecular beam epitaxy on the a-facet of bulk GaN crystals. We study the influence of both QW width and barrier Al content on the dynamics of excitons in the 10-320 K range. We first show that the effective lifetime of QW excitons s increases with temperature, which is evidence that nonradiative mechanisms do not play any significant role in the low-temperature range. The temperature range for increasing s depends on the QW width and Al content in the (Al,Ga)N barriers. For higher temperatures, we observe a reduction in the QW emission lifetime combined with an increase in the decay time for excitons in the barriers, until both exciton populations get fully thermalized. Based on analysis of the ratio between barrier and QW emission intensities, we demonstrate that the main mechanism limiting the radiative efficiency in our set of samples is related to nonradiative recombination in the (Al,Ga)N barriers of charge carriers that have been thermally emitted from the QWs