K. Rammohan

Optically active three-dimensionally confined structures realized via molecular beam epitaxical growth on nonplanar GaAs (111)B

We report the first realization on nonplanar patterned substrates of optically active three‐dimensionally confined semiconductor volumes created in situ via a one‐step molecular beam epitaxial growth. Growth is carried out on pyramidal mesas on (111)B substrates and in a regime that results in the emergence of three equivalent {110} side facets which overtake the as‐patterned {100} side facets and lead to mesa pinch‐off. Transmission electron microscopy along with spatially and spectrally resolved cathodoluminescence provide evidence for emission from laterally confined regions with lateral linear dimensions ≲100 nm.

Study of μm‐scale spatial variations in strain of a compositionally step‐graded InxGa1−xAs/GaAs(001) heterostructure

The relaxation of strain in compositionally step‐graded InxGa1−xAs layers grown on GaAs(001) has been examined with cathodoluminescence (CL) wavelength and linearly polarized imaging approaches. A polarization anisotropy in CL is found, and this correlates with spectral shifts in the peak positions of excitonic luminescence. Varying asymmetries in misfit dislocation densities from transmission electron microscopy are found to be consistent with the μm‐scale spatial variations in strain that is deduced from the CL.

Realization of three‐dimensionally confined structures via one‐step in situ molecular beam epitaxy on appropriately patterned GaAs(111)B and GaAs(001)

The realization of three‐dimensionally confined GaAs/AlGaAs structures on GaAs (111)B and GaAs (001) substrates via one step in situ molecular beam epitaxy is reported. Growth is carried out on nonplanar patterned substrates with crystallographically equivalent mesa top edges. Equivalent side facets evolve during growth and completely surround the mesa top. Adatom migration from these facets to the mesa top result in shrinkage of the mesa top area leading to mesa pinch‐off. Scanning and transmission electron microscopy provide evidence for the realization of structures with lateral linear dimensions ≲500 A. Cathodoluminescence images from the (111)B structures attest to their high optical quality.

Anisotropic structural, electronic, and optical properties of InGaAs grown by molecular beam epitaxy on misoriented substrates

We have investigated the structural, electronic, and optical properties of partially strain‐relaxed InxGa1−xAs layers, grown by molecular beam epitaxy on both misoriented and nominally flat (001) GaAs substrates. We find large anisotropies in bulk strain relaxation, interfacial misfit dislocation density, dark‐line defect density, and electron mobility, as well as a polarization anisotropy in cathodoluminescence for epilayers grown on misoriented substrates, in comparison with those grown on flat substrates.

Influence of misfit dislocations on thermal quenching of luminescence in InxGa1−xAs/GaAs multiple quantum wells

The temperature dependence of the cathodoluminescence (CL) originating from In0.21Ga0.79As/GaAs multiple quantum wells has been studied between 86 and 250 K. The CL intensity exhibits an Arrenhius‐type dependence on temperature (T), characterized by two different activation energies. The influence of misfit dislocations and point defects associated with strain relaxation on the thermal quenching of luminescence has been investigated, and the spatial variation in the activation energies has been examined. The CL intensity dependence on temperature for T≲150 K is controlled by thermally activated nonradiative recombination. For T≳150 K the decrease in CL intensity is largely influenced by thermal re‐emission of carriers out of the quantum wells.

Ambipolar diffusion anisotropy induced by defects in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells

The influence of strain‐induced defects on the ambipolar diffusive transport of excess electrons and holes in the δ‐doped InGaAs/GaAs multiple quantum well system has been examined with a new technique called electron‐beam‐induced absorption modulation (EBIA). The excess carrier lifetime and diffusion coefficient are obtained by a one‐dimensional diffusion experiment that utilizes EBIA. An anisotropy in the ambipolar diffusion along both high‐symmetry 〈110〉 directions is found, and this is seen to correlate with the distribution of dark line defects observed in cathodoluminescence.

Electron beam-induced absorption modulation imaging of strained In0.2Ga0.8As/GaAs multiple quantum wells

We have examined the effects of electron‐hole plasma generation on excitonic absorption phenomena in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) using a novel technique called electron beam‐induced absorption modulation imaging. The electron‐hole plasma is generated by a high‐energy electron beam in a scanning electron microscope and is used as a probe to study the MQW absorption modulation. The influence of structural defects on the diffusive transport of carriers is imaged with a μm‐scale resolution.

Thermal processing of strained InGaAs/GaAs quantum well heterostructures bonded to Si via an epitaxial lift-off technique

We have investigated the effects of thermal cycling on the optical properties of In0.2Ga0.8As/GaAs single quantum well films bonded to Si(001) via the epitaxial lift-off technique. The optical and structural quality of the bonded films were monitored using cathodoluminescence (CL) imaging and spectroscopy. The films were stable through the temperature range (500–700 °C) used in normal InxGa1−xAs device processing. However, annealing at temperatures greater than ∼700 °C resulted in layer intermixing accompanied by a blue-shift in the CL peak energy. The shifts in the CL peak energy were modeled by considering In–Ga interdiffusion at the interface and solving the Schrodinger equation using appropriate band profiles for this region.

Absorption modulation induced by electron beam excitation of strained In0.2Ga0.8As/GaAs multiple quantum wells

The effects of excess carrier generation on excitonic absorption phenomena in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) was examined using a novel technique called electron beam‐induced absorption modulation imaging. The nipi‐doping induced barrier height is determined by measuring the frequency response of the absorption modulation as a function of temperature and employing a model which is based on thermal excitation of carriers in the limit of Boltzmann statistics. Spatial steps in the absorption modulation which correlate with the positions and orientation of dark line defects imaged in cathodoluminescence are observed. These results indicate the existence of defects in the MQWs which impede the ambipolar diffusive transport of the spatially separated electron‐hole plasma.

Influence of GaAs(001) substrate misorientation towards {111} on the optical properties of InxGa1-xAs/GaAs

Local variations in the optical properties of thick In0.13Ga0.87As films grown on GaAs(001) substrates misoriented toward {111} planes have been studied with polarized and spectrally‐resolved cathodoluminescence (CL) imaging. The degree of anisotropic relaxation and density of dark line defects (DLDs) in CL was found to depend on the choice of the substrate miscut orientation. An enhanced anisotropy in DLD density and strain relaxation was found for a misorientation towards (111)A relative to that for a misorientation towards (111)B. Local variations and spatial correlations in polarization anisotropy, band‐gap energy shifts, luminescence efficiency, and defect‐induced long‐wavelength luminescence were examined.