Search

Initial stages of molecular beam epitaxial (MBE) growth of highly mismatched InxGa1-xAs/GaAs(100) have been studied by planar and cross-sectional transmission electron microscopy. For In0.5Ga0.5As growth, we find drastic differences in morphology obtained by reducing the growth temperature form 475 to 420°C. We also observe differences in morphology between alloy growth and short period superlattice ((InAs)n/(GaAs)m (m = 1 monolayer, n = 2 monolayers) growth of equivalent effective composition. In the case of growth by formation of large islands, we present direct evidence of strain relief at the island edges and discuss defect formation in these islands.

A transmission electron microscope study of the twin structure in thin GaAs films grown by molecular‐beam epitaxy on (111)B GaAs reveals the twinned region to be composed of three distinct crystallites which are single‐ and double‐twin variations of the bulk crystal. A three‐dimensional structure of the twin region involving variants of these crystallites, as required by the threefold symmetry of the (111)B plane, is proposed.

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.

Mesas defined on GaAs(111)B substrates by photolithography followed by wet chemical etching are found to be triangular pyramids with truncated tops. The areal dimensions of the unetched tops of the pyramidal mesas diminish with etch time and can be reduced to an apex on continued etching. Transmission electron microscopy is used to examine a post‐growth etched GaAs/Al0.3Ga0.7As multilayer structure grown on GaAs(111)B. A three‐dimensionally confined GaAs volume of the order of 105 atoms is observed.

Direct evidence for interplanar relaxation in islands at the initial stages of strained epitaxy is presented using molecular beam epitaxically deposited In0.5Ga0.5As on GaAs(100). Concomitant existence of atomic displacements in the substrates to unexpectedly large depths of ∼150 A is found. In incoherent islands, defects are found to be introduced symmetrically near the island edges.

A transmission electron microscope study of GaAs grown on Si tilted 4° off (100) has for the first time revealed that of the two twin variants propagating to the GaAs film surface, a substrate tilt about a 〈011〉‐type direction favors one over the other.

In a thick strained GaAs/InGaAs MQW we recently reported the unusual observation of the exciton linewidth initially narrowing upon application of a reverse bias, before the usually observed broadening set in with further increase in the bias. The phenomena suggested the existence of a spatially varying electric field in the MQW region arising from a depletion of the net charge density with increasing reverse bias. Here we provide an explanation for the unusual observation arrived at through a systematic examination of the sample behavior using electro-transmission, electro-photoluminescence, capacitancevoltage profiling and transmission electron microscopy. We conclude that shallow levels cannot account for the observation and that the presence of strain induced point defect related deep levels (either n of p type) offers a consistent explanation. This is the first clear manifestation of the influence of deep levels on the free exciton electroabsorption behavior and has practical implications for MQW based electroabsorptive / electrorefractive light modulators.

We report the realization of high quality strained InGaAs/GaAs multiple quantum wells (MQW) grown on planar GaAs (100) substrates through optimization of molecular beam epitaxical (MBE) growth conditions and structure. Such MQWs containing ∼ 11% In have lead to the realization of an asymmetric Fabry-Perot (ASFP) reflection modulator with a room temperature contrast ratio of 66:1 and an on-state reflectivity of 30%. For In composition ≥ 0.2, the improved optical quality for very thick (gt;2μm) InGaAs/GaAs MQWs grown on pre-patterned substrates is demonstrated via transmission electron microscopy (TEM) and micro-absorption measurements.

We have studied the initial stages of highly mismatched InxGa1-x As growth on GaAs (100) by transmission electron microscopy (TEM). We find direct evidence of (i) deformation in the substrate below each island to depths of about 150 A and (ii) strain relief in the islands at their lateral edges. Growth morphology as a function of growth temperature is studied and it is found that the tendency for islanding for x as high as 0.5 can be supressed to a large extent by a reduction in the growth temperature. We also present results of correlation between TEM determined different growth morphologies (brought about by different growth temperatures ) and the electrical characteristics of AlAs/InxGa1-xAs resonant tunneling diode structures grown on GaAs (100).

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.

The spatial distribution of the long‐wavelength luminescence in thick In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) grown on patterned and unpatterned substrates has been investigated using cathodoluminescence (CL) imaging and spectroscopy. By spatially correlating the luminescence arising from the MQW exciton recombination (λ≊950 nm) with the longer wavelength (1000≲λ≲1200 nm) luminescence arising from the defect‐induced recombination, we demonstrate that it is possible to determine the regions of highest film quality in both the mesa and valley regions for growth on patterned GaAs substrates. The present approach enables a judicious determination of the optimal regions to be used for active pixels in InGaAs/GaAs devices. For growth on unpatterned substrates, the CL spectra show defect‐induced broad bands between 1000≲λ≲1600 nm. These bands exhibit spatial variations which correlate with the dark line defects (DLDs) observed in the λ=950 nm exciton luminescence imaging. Transmission electron microscopy...