D. H. Rich

Temperature dependence of excitonic recombination in lateral epitaxially overgrown InGaN'GaN quantum wells studied with cathodoluminescence

We have examined in detail the optical properties of InGaN quantum wells (QWs) grown on pyramidal GaN mesas prepared by lateral epitaxial overgrowth (LEO) in a metalorganic chemical vapor deposition system that resulted in QWs on {1-101} facets. The effects of In migration during growth on the resulting QW thickness and composition were examined with transmission electron microscopy (TEM) and various cathodoluminescence (CL) imaging techniques, including CL wavelength imaging and activation energy imaging. Spatial variations in the luminescence efficiency, QW interband transition energy, thermal activation energy, and exciton binding energy were probed at various temperatures. Cross-sectional TEM was used to examine thickness variations of the InGaN/GaN QW grown on a pyramidal mesa. CL imaging revealed a marked improvement in the homogeneity of CL emission of the LEO sample relative to a reference sample for a conventionally grown In0.15Ga0.85N/GaN QW. The characteristic phase separation that resulted in ...

Highly Luminescent Zn(x)Cd(1-x)Se/C Core/Shell Nanocrystals: Large Scale Synthesis, Structural and Cathodoluminescence Studies.

Zn(x)Cd(1-x)Se/C core/shell nanocrystals with 31-39 nm semiconducting core and 11-25 nm carbon shell were synthesized from solid state precursors in large scale amounts. A mixture of spherical and tripod nanostructures were obtained only in the one-step reaction (ZC3), where the Zn- and Cd-precursors were reacted simultaneously, rather than in the two step reactions (ZC1 and ZC2), where largely spherical nanostructures were observed. Rietveld analysis of the X-ray diffraction patterns of the samples prepared in three different ways, all under their autogenic pressure, reveal varying compositions of the Zn(x)Cd(1-x)Se nanocrystal core, where the cubic phases with higher Zn content were dominant compared to the hexagonal phases. Carbon encapsulation offers excellent protection to the nanocrystal core and is an added advantage for biological applications. Cathodoluminescence (CL) measurements with spatially integrated and highly localized excitations show distinct peaks and sharp lines at various wavelengths, representing emissions from single nanostructures possessing different compositions, phases, and sizes. Transmission electron microscopy (TEM) showed striations in the nanocrystals that are indicative of a composition modulation, and possibly reveal a phase separation and spinodal decomposition within the nanocrystals. Thermal quenching of the luminescence for both the near band-edge and defect related emissions were observed in the range 60-300 K. The measured activation energies of ∼50-70 meV were related to the presence of shallow donors or acceptors, deep level emissions, and thermal activation and quenching of the luminescence due to the thermal release of electrons from shallow donors to the conduction band or a thermal release of holes from shallow acceptors to the valence band. Spatially integrated CL spectra revealed the existence of broadening and additional components that are consistent with the presence of a composition modulation in the nanocrystals. Spatial localization of the emission in isolated single nanocrystals was studied using monochromatic CL imaging and local CL spectroscopy. CL spectra acquired by a highly localized excitation of individual nanocrystals showed energy shifts in the excitonic luminescence that are consistent with a phase separation into Zn- and Cd-rich regions. The simultaneous appearance of both structural and compositional phase separation for the synthesis of Zn(x)Cd(1-x)Se nanocrystals reveals the complexity and uniqueness of these results.

Band gap narrowing and radiative efficiency of silicon doped GaN

Radiative efficiency, band gap narrowing, and band filling are studied in Si-doped GaN films as a function of carrier concentration (n), using room and low temperature cathodoluminescence (CL). Using the Kane model, a band gap narrowing ΔEg of −(3.6±0.6)×10−8 and −(2.6±0.6)×10−8n1/3 eVn1/3 is obtained for epitaxially strained and relaxed material, respectively. Band-edge CL time response and absolute external photon yield are measured. The internal radiation efficiency is deduced. Its monotonic increase as n increases is explained by the increase in the spontaneous radiative rate with a radiative free carrier band-to-band recombination coefficient B=(1.2±0.3)×10−11 cm3 s−1.

The structural and optical properties of supercontinuum emitting Si nanocrystals prepared by laser ablation in water

Laser ablation of Si(100) targets immersed in deionized water at room temperature was studied by applying the second harmonic of a ns pulsed Nd:YAG laser and by employing different methods for characterization of the resulting nanocrystals (NCs). Spontaneous Raman scattering showed first order Raman spectra of the Si NCs, which varied in their Raman shift and width. The Raman data were analyzed using a phonon confinement model, involving three-dimensional confinement and lognormal size distributions for the nanocrystallites constituting the samples, indicating the presence of particles greater than ∼2 nm diameter. High-resolution transmission electron microscopy and electron diffraction of the as-prepared samples showed NCs with diameters greater than ∼1.75 nm and quantum dots, corresponding to Si/SiOx. The optical properties of the Si NCs were studied with cathodoluminescence (CL) spectroscopy for sample temperatures in the 50–300 K range, which exhibited a supercontinuum emission ranging from the near u...

Cathodoluminescence imaging and spectroscopy of excited states in InAs self-assembled quantum dots

We have examined state filling and thermal activation of carriers in buried InAs self-assembled quantum dots (SAQDs) with excitation-dependent cathodoluminescence (CL) imaging and spectroscopy. The InAs SAQDs were formed during molecular-beam epitaxial growth of InAs on undoped planar GaAs (001). The intensities of the ground- and excited-state transitions were analyzed as a function of temperature and excitation density to study the thermal activation and reemission of carriers. The thermal activation energies associated with the thermal quenching of the luminescence were measured for ground- and excited-state transitions of the SAQDs, as a function of excitation density. By comparing these activation energies with the ground- and excited-state transition energies, we have considered various processes that describe the reemission of carriers. Thermal quenching of the intensity of the QD ground- and first excited-state transitions at low excitations in the ∼230–300-K temperature range is attributed to dis...

Influence of Metal Deposition on Exciton–Surface Plasmon Polariton Coupling in GaAs/AlAs/GaAs Core–Shell Nanowires Studied with Time-Resolved Cathodoluminescence

The coupling of excitons to surface plasmon polaritons (SPPs) in Au- and Al-coated GaAs/AlAs/GaAs core-shell nanowires, possessing diameters of ~100 nm, was probed using time-resolved cathodoluminescence (CL). Excitons were generated in the metal coated nanowires by injecting a pulsed high-energy electron beam through the thin metal films. The Purcell enhancement factor (FP) was obtained by direct measurement of changes in the temperature-dependent radiative lifetime caused by the nanowire exciton-SPP coupling and compared with a model that takes into account the dependence of FP on the distance from the metal film and the thickness of the film covering the GaAs nanowires.

Correlating exciton localization with compositional fluctuations in InGaN∕GaN quantum wells grown on GaN planar surfaces and facets of GaN triangular prisms

We have used spatially and temporally resolved cathodoluminescence (CL) to study the carrier recombination dynamics of InGaN quantum wells (QWs) grown on (0001)-oriented planar GaN and {11¯01}-oriented facets of GaN triangular prisms prepared by lateral epitaxial overgrowth in a metal-organic chemical vapor deposition system. The effects of In migration during growth on the resulting QW thickness and composition were examined. We employed a variable temperature time-resolved CL imaging approach that enables a spatial correlation between regions of enhanced exciton localization, luminescence efficiency, and radiative lifetime with the aim of distinguishing between excitons localized in In-rich quantum dots and those in the surrounding Ga-rich QW regions.

Temperature dependence of exciton-surface plasmon polariton coupling in Ag, Au, and Al films on InxGa1−xN/GaN quantum wells studied with time-resolved cathodoluminescence

The optical properties and coupling of excitons to surface plasmon polaritons (SPPs) in Ag, Au, and Al-coated InxGa1−xN/GaN multiple and single quantum wells (SQWs) were probed with time-resolved cathodoluminescence. Excitons were generated in the metal coated SQWs by injecting a pulsed high-energy electron beam through the thin metal films. The Purcell enhancement factor (Fp) was obtained by direct measurement of changes in the temperature-dependent radiative lifetime caused by the SQW exciton-SPP coupling. Three chosen plasmonic metals of Al, Ag, and Au facilitate an interesting comparison of the exciton-SPP coupling for energy ranges in which the SP energy is greater than, approximately equal to, and less than the excitonic transition energy for the InGaN/GaN QW emitter. A modeling of the temperature dependence of the Purcell enhancement factor, Fp, included the effects of ohmic losses of the metals and changes in the dielectric properties due to the temperature dependence of (i) the intraband behavior...

Carrier relaxation dynamics and steady-state charge distributions in coupled InGaN∕GaN multiple and single quantum wells

We have examined the carrier capture dynamics and excitation dependent charge distributions of coupled InGaN∕GaN multiple quantum well samples. We measured the temporal evolution of time-delayed cathodoluminescence (CL) spectra to study the temperature- and excitation-dependent transfer of carriers from a surrounding confinement region into a coupled single quantum well. Samples possessing two different structures for the confinement region [i.e., number of quantum wells (QWs) and varying widths] were examined with CL. In order to study state filling of the SQW and QWs in the confinement region, we calculated the quasi-Fermi levels and carrier densities by utilizing a model that involves self-consistent solutions of the nonlinear Poisson-Schrodinger equation for wurtzite QWs including strain, deformation potentials, and polarization fields. Band-edge and effective mass parameters were first obtained from a strain- and In composition-dependent k⋅p calculation for wurtzite InxGa1−xN, using a 6×6 k⋅p Hamilto...

Polarized light from excitonic recombination in selectively etched GaN/AlN quantum dot ensembles on Si(111)

Multiple layers of GaN/AlN quantum dot (QD) ensembles were grown by the Stranski–Krastanov method on Si(111) using molecular beam epitaxy. During the subsequent cooling from growth temperature, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing the vertically stacked QDs leads to an additional biaxial tensile stress of 20–30 kbar in the III-nitride film. We have selectively modified the thermal stress in the QD layers by etching a cross-hatched pattern into the as-grown sample using inductively coupled Cl2/Ar plasma reactive ion etching. The results show that a suitable choice of stripe width from ∼ 2t o 10µm and orientation along [11−20] and [1−100] can create regions of in-plane uniaxial stress that enable a selective and local control of the polarized luminescence from ensembles of QDs which were probed with cathodoluminescence. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing e–h pair excitation for the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼46 K). A theoretical modelling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schr¨ odinger and Poisson equations using the 6×6 k·p and effective-mass methods for calculations of the e–h wavefunctions and electron and hole quasi-Fermi levels for varying levels of state filling. We attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during e–h pair excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing e–h pair excitation density at T = 300 K. (Some figures may appear in colour only in the online journal)