H. Selke

Single zero-dimensional excitons in CdSe/ZnSe nanostructures

Zero-dimensional excitons (0DXs) in CdSe/ZnSe nanostructures have been studied by time- and spatially resolved photoluminescence spectroscopy. The three-dimensional confinement is confirmed by an exciton lifetime up to 550 ps, independent of temperature up to 130 K. By preparing mesa structures with diameters down to 50 nm as local probes, an extremely high spatial resolution is achieved, giving experimental access to single 0DXs. A splitting of the ground state into a linearly polarized doublet with an energy spacing up to 1.5 meV is found, varying from dot to dot in sign and magnitude. This indicates a noncircular shape with no preferential orientation of the dots. The dot density is estimated to increase from 5×1010 to 1.5×1011 cm−2, when changing the nominal CdSe layer thickness from 1 to 3 ML, i.e., close to the critical thickness.

INCORPORATION OF INDIUM DURING MOLECULAR BEAM EPITAXY OF INGAN

We report on the incorporation of In during growth of InxGa1−xN by molecular beam epitaxy under varying In/Ga flux ratios and with different film thicknesses. The incorporation efficiency studied by energy dispersive x-ray microanalysis, high-resolution x-ray diffraction and photoluminescence spectroscopy is strongly affected by the chosen fluxes of Ga and N and is limited by the excess of nitrogen compared to gallium. Furthermore, thick films exhibit a decrease of the In content in growth direction. The behavior can be explained by considering the different stabilities of the two binary compounds InN and GaN.

CdSe/ZnSe quantum structures grown by migration enhanced epitaxy: Structural and optical investigations

Migration enhanced epitaxy has been applied to induce the change from two-dimensional growth to formation of self-assembling islands in the growth of CdSe on ZnSe. Transmission electron microscopy images from samples with a ZnSe caplayer show a transition from a flat quantum well to an interrupted layer with pronounced thickness fluctuations when the CdSe exceeds its critical thickness. These results are confirmed by high resolution x-ray diffraction measurements, as only for the former sample the 004 rocking curve can be simulated assuming a flat quantum well with abrupt interfaces. Compared to bulk CdSe, the photoluminescence peak is blueshifted by about 0.5 eV. PL excitation experiments indicate that the interrupted layer consists of CdSe islands embedded in Zn1−xCdxSe with a composition gradient. Atomic force microscopy images of uncapped samples show spherical islands with a height of 20 nm and a diameter-to-height ratio of 4:1.

Formation of self-assembling II-VI semiconductor nanostructures during migration enhanced epitaxy

We have studied the transition from two- to three-dimensional (3D) growth during migration enhanced epitaxy (MEE) of CdSe on ZnSe by systematically varying the CdSe thickness. Transmission electron microscopy (TEM) images show structures that contain islands within the quantum well (QW) region when the CdSe exceeds a critical thickness. The transition to a 3D interface could be confirmed by high-resolution X-ray diffraction (HRXRD) measurements. (0 0 4) diffraction profiles could be simulated assuming flat interfaces only for CdSe below the critical thickness. Photoluminescence (PL) shows emission energies up to 2.7 eV for flat QWs while samples with CdSe layers above the critical thickness show a broad emission around 2.3 eV. Photoluminescence excitation (PLE) measurements from the latter samples support the assumption that CdSe islands within the QW region act as a final centre of recombination. Heterostructures containing a CdTe or a ZnTe layer have been investigated for comparison.

Compositional inhomogeneities in InGaN studied by transmission electron microscopy and spatially resolved cathodoluminescence

The structural and optical properties of InGaN epilayers grown by different molecular beam epitaxy (MBE) techniques were studied with high spatial resolution. Mappings of the local emission wavelength obtained by cathodoluminescence (CL) spectroscopy indicate lateral and spectral inhomogeneities in the luminescence of InGaN epilayers grown with continuous In and Ga fluxes. These results agree well with variations in the chemical composition in the lateral and in the growth direction seen in mappings of the local composition which were obtained by energy-dispersive X-ray (EDX) microanalysis in cross-sectional transmission electron microscopy (TEM). Possible origins of these variations are discussed. In comparison, epilayers grown with alternating deposition of (In, Ga)N and (Ga)N are more homogeneous.

X-ray scattering from GaN epitaxial layers - an example of highly anisotropic coherence

GaN(0001) epitaxial layers were grown by molecular beam epitaxy on a few-nanometres thick low-temperature GaN nucleation layers on c-plane sapphire. Despite extremely high densities of extended defects, the layers show a narrow (002) x-ray diffraction peak, superimposed by broad diffuse scattering. Triple-axis transverse and radial scans were measured for (00l) reflections of different orders and for various GaN layer thicknesses. The results can be described by an interfacial displacement-difference correlation function. Its microscopic origin is assigned to either inversion domain boundaries or edge-type threading dislocations in the GaN layers, in agreement with findings of transmission electron microscopy. These defects are associated with an only weak rotational disorder perpendicular to the growth plane as proven by the x-ray scattering characteristics.

High-resolution x-ray diffraction investigations of highly mismatched II-VI quantum wells

High-resolution x-ray diffraction (HRXRD) was used to systematically investigate CdSe and ZnTe quantum wells one to three monolayers thick sandwiched between a ZnSe buffer and cap layer grown at different substrate temperatures. For comparison high-resolution transmission electron microscopy (HRTEM) measurements were performed which were evaluated by digital analysis of lattice images. The x-ray diffraction profiles show typically two main layer peaks. Their intensity ratio depends critically on the quantum well thickness and varies only weakly with the thickness of the ZnSe layers. The total Cd or Te content determined from comparisons of experimental and simulated (004) -2 scans is well confirmed by the results from digital analysis of HRTEM lattice images. For quantum well thicknesses larger than 1.5 (ZnTe) or 2.0 (CdSe) monolayers, no simulation parameters could be found to achieve good agreement between theoretical and measured diffraction profiles. This transition is more clearly visible in diffraction profiles of asymmetrical reflections. By HRTEM measurements, this could be correlated to the occurrence of stacking faults at these thicknesses. The formation of quantum islands detected by HRTEM was not reflected in the HRXRD -2 scans.

Buffer layers for the growth of GaN on sapphire by molecular beam epitaxy

Abstract The change of structural, optical and electrical properties of GaN grown by plasma assisted molecular beam epitaxy on sapphire due to the introduction of GaN and AlN buffer layers is investigated. Whereas the layer surface becomes smoother its structural features remain very small. Photoluminescence and resistivity measurements confirm an increased density of extended defects produced by the buffer layer. A predominant lateral growth mode as known from metalorganic vapour-phase epitaxy is not observed.

Influence of composition and well-width fluctuations on optical gain in (In, Ga)N multiple quantum wells

The influence of composition and well-width fluctuations on the optical gain in (In, Ga)N multiple quantum wells for different barrier dopings and compositions is studied under quasistationary conditions. Systematic temperature- and excitation-density-dependent gain measurements were performed by means of the variable-stripe-length method. The correlation between optical properties such as gain or temperature behaviour of the laser threshold and the crystal quality of the samples is demonstrated. In addition, by means of photoluminescence measurements, the temperature dependence of the quantum efficiency is determined, which gives information on the activation processes to nonradiative recombination channels. Thermal activation energies obtained from these measurements can be correlated with the depth of potential minima caused by composition fluctuations in the quantum well. The experimental temperature-dependent threshold density and gain can be successfully explained by use of a simple model of band-to-band transitions considering localization effects.

Influence of buffer layers on the structural properties of molecular beam epitaxy grown GaN layers

Abstract The influence of low temperature buffer layers on the structural characteristics of GaN grown by molecular beam epitaxy on sapphire (0001) substrates was investigated. Layers grown on GaN and AlN buffers were studied by high-resolution X-ray diffraction and transmission electron microscopy (TEM). For both buffer materials, the variation of the buffer parameters, like their thickness and growth temperature, is reflected in a clear change of the GaN (0002) rocking curve width. For strongly decreased as well as for increased Bragg reflection width a deterioration of optical and electrical properties of GaN layers grown on buffers with respect to reference samples without buffer layers was observed. Moreover, layers grown on thin GaN buffer layers show extremely narrow ω scans and layer thickness interferences in 2θ/ω direction, while TEM reveals a high defect density throughout the entire layer. Therefore, not only the width of the rocking curves but also their shape has to be considered for the estimation of the defect densities by X-ray diffraction.