The growth and optical properties of III-V nanostructures grown by Molecular Beam Epitaxy

This thesis is dedicated to the growth and characterization of the optoelectronic properties of III-V semiconductor nanostructures namely nanowires and nanoscale membranes. III-V semiconductors possess promising intrinsic properties like direct band gap, high electron/- hole mobility and spin-orbit interaction which makes them interesting for a wide range of applications such as high speed electronics, optoelectronics and photovoltaics. Nanostruc- tures enable the exploitation and further functionalization of the inherent semiconductor properties. The nanostructures we study in the scope of this thesis are grown with molecular beam epitaxy which enables us to obtain ultra-pure nanostructures with high crystalline quality. In the first part of this thesis we investigate and optimize the growth of GaAs nanowires both on silicon and GaAs substrates. We employ the self-catalyzed growth technique in order to avoid the risk of foreign metal contamination. Growth on Si substrates is interesting in views of enabling the integration of existing Si microtechnology and III-V technology. GaAs nanowire growth on (111) Si substrates is achieved with self-assisted and position controlled nanowire growth techniques. In both techniques, the effects of the silicon oxide thickness and composition along with the nature of the openings are investigated. GaAs nanowires grown on (111)B GaAs substrates are employed in optomechanical and optoelectronics applications. Pristine GaAs nanowires grown on (111)B GaAs substrates are employed in scanning force microscopy thanks to asymmetric orthogonal modes they exhibit. Furthermore, GaAs/AlGaAs heterostructure nanowires demonstrate lasing when their dimensions and AlGaAs capping are optimized. The rest of this thesis is dedicated to the growth of defect-free structures. Two methods are presented to create defect-free pure zinc-blende GaAs nanostructures. The first one is to modify the polarity of GaAs nanowires. We optimize the growth parameters to obtain a high yield of (111)A nanowires on (100) GaAs. GaAs nanowires grown in (111)A direction exhibited a defect-free structure in contrast to the nanowires grown in (111)B direction. Our second approach is to grow elongated nanostructures and control their orientation to âlock outâ the defects. When these nanostructures, GaAs nanoscale membranes, are oriented in direction on a (111)B GaAs substrate they exhibit pure zinc-blende crystalline structure. Their superior crystalline quality is confirmed with transmission electron microscopy and optical characterization techniques, i.e. photoluminescence and cathodoluminescence. Their growth mechanism and parameter window is investigated in detail. Next, GaAs nanoscale membranes are used as templates for quantum heterostructures. GaAs quantum wells are embedded in an AlGaAs shell around the nanoscale membranes. They are found to exhibit bright luminescence with narrow linewidth. Additionally, local alloy fluctuations in the AlGaAs shell are investigated. They exhibited sharp and localized luminescence characteristics like self-assembled quantum dots.

Ordered nanostructures on silicon substrates

Jelena Vukajlovic Plestina

Silicon is today the main material used in electronics. It is a very advanced and mature technology. It is therefore clear that new technological concepts and materials should be introduced through the integration on the silicon platform. III-V semiconductors, such as GaAs and InAs, are high performance semiconductors, with direct bandgap and high carrier mobility, what makes them very prominent candidates for future electronics and optoelectronics. Lattice, thermal and polarity mismatch have been for decades limiting their integration on Si in the form of thin film technology. The nanowire geometry brings new prospects in this direction by reducing the contact area between mismatched materials,. Now, while growth of defect-free III-V structures on silicon is important, use in applications requires the nanostructures to be placed deterministically following a certain design/order. In this thesis we have studied different mechanisms to obtain ordered nanostructures on a silicon substrate, with the goal of increasing its functionality. The core part of this work was dedicated to the integration of III-V nanowires on Si in the formof ordered arrays. We have considered both GaAs and InAs nanowires. This process has several requirements: substrate fabrication and the growth process should be CMOS compatible, nanowires within the arrays should be grown vertically with a yield close to 100% and nanowires should be clones-looking and performing the same. An additional point to consider is that the fabrication process should be compatible with large scale techniques. The substrate requirements for obtaining ordered arrays of Ga-assisted GaAs nanowires on silicon were identified. In particular, we focused on the initial stages of growth, that turned to be key for achieving a high yield in the arrays. We found that the positioning of the Ga droplet within the predefined holes on the substrate determined whether the nanowire would grow perpendicularly to the substrate. Our HRTEMstudies on the titled nanowires show that the initial nanowire seeds seem to nucleate at the corner of the nanoscale holes. Instead, the crystal seeds of vertical nanowires occupy homogeneously the nanoscale holes. Achieving high yield in the growth of GaAs arrays on silicon also allowed us to study their evolution in time. We demonstrated that there is an incubation time for nanowires to start growing. This incubation time is different from NW to NWand leads to a relatively broad length distribution. We proposed a solution and showed how an increase in the supersaturation in the Ga droplet leads to the formation of homogeneous arrays. Growth of InAs nanowires in ordered arrays on silicon was based on the concept of guided growth. We used a SiO2 nanotube templates to promote vertical growth. Due to the directionality of MBE, achieving growth inside a nanotube was especially challenging. Our results provided new insights on the role of different pathways of the In and As4 adatoms during growth. In this part, large scale patterning by phase shift photolithography was demonstrated as an alternative to the conventionally used electron beam lithography. Stain etching method for producing porous silicon was applied on top-bottom fabricated Si micropillars. This led to geometrically driven electrochemical dissolution of silicon trough the center of the microstructure forming microtubes. The optical properties were also studied and used to produce functional 3D LED.

EPFL2017

Formation mechanisms of low-dimensional semiconductor nanostructures grown by OMCVD on nonplanar substrates

Semiconductor quantum wires (QWRs) are promising structures for optoelectronics applications, since they can provide quantum confinement for charge carriers in two dimensions. The advantage that they offer over conventional quantum wells (QWs) is due to the sharper density of states characteristic of these structures, yielding narrower spectral lines and higher optical gain. However, to exhibit clear confinement characteristics, QWRs must meet stringent requirements in terms of size, uniformity and interfacial quality. Different methods have been explored for QWR fabrication. Techniques based on etching and regrowth suffer from defect incorporation into the lateral interfaces, since they are not formed in situ, and are limited in size by the lithographic features. On the other hand, growth of fractions of monolayers on vicinal substrates, although overcoming the above limitations, gives rise to size nonuniformities and graded interfaces. In this project, (In)GaAs/AlxGa1-xAs QWRs are obtained by organometallic chemical vapor deposition (OMCVD) growth of quantum wells on patterned, V-grooved substrates. In this way, the lithographically defined pattern serves as a seed for QWR formation. The self-ordering properties of OMCVD on nonplanar surfaces ensure the creation of a self-limiting profile at the bottom of the grooves, on which the wires are grown. This method overcomes the size limitations imposed by lithography, allows the in situ formation of interfaces and, thanks to the self-ordering mechanism, yields structures with high uniformity, whose characteristics are determined solely by the growth conditions. Although nonplanar growth has been employed for more than ten years for QWR fabrication, the understanding of the self-ordering mechanisms originating the profiles at the bottom of the grooves has been until now only phenomenological. The attainment of self-limiting profiles takes place via transients of the growth rates at the bottom of the groove. Current models of nonplanar growth can predict the formation, evolution or disappearance of facets at the 100nm-μm size. However, they cannot describe the transient behaviors at the nm scale that lead to self-limiting growth. This thesis project has been aimed at elucidating the physical mechanisms of this self-organized growth. A fundamental part of the project has been the creation of a wide experimental database to understand the dependence of the self-limiting profiles on the materials and growth conditions. The profiles at the bottom of the groove exhibit a faceted structure, consisting of a central (100) plane, surrounded by two {311}A ones. Cross-sectional transmission electron microscopy (TEM) shows that the bottom facets become wider as the growth temperature increases and as the Al mole fraction x of AlxGa1-xAs layers decreases. It appears therefore that surface diffusion is a key element in determining self-limiting growth. TEM cross sections show also that the establishment of self-limiting profiles takes place via self-adjusting growth rates on these facets. In addition to this geometrical self-ordering, AlxGa1-xAs alloys exhibit also a compositional self-ordering at the bottom of the groove. Due to the higher mobility of Ga species, with respect to the Al ones, the facets at the bottom of the groove are Ga rich, with respect to the sidewall planes, giving rise to so-called vertical quantum wells (VQWs). To determine the composition of the VQWs, we have developed a technique employing cross-sectional atomic force microscopy (AFM) in air. This method is based on the dependence of the AlxGa1-xAs oxidation rates on the Al content x. Through a calibration on a reference sample, we were able to measure compositions with an accuracy of ±0.02. The enhanced Ga content of the VQWs follows classical models of segregation, and reaches a maximum of Δx ≅ 0.15 for x ≅ 0.55 at a growth temperature of 700°C. We also studied the three dimensional structure of the self-limiting surface profiles by top-surface AFM in air of the nonplanar samples after cool-down and removal from the OMCVD reactor. Each of the planes composing the groove presents a monolayer step structure that reflects directly the morphology of surfaces of the same orientation found in planar epitaxy. However, on the facets forming on corrugated substrates the step structure exhibits a higher degree of ordering, with respect to planar epitaxy. This is due to a modification of surface diffusion, when the trench width becomes comparable to or lower than the adatom surface diffusion length. In the last part of the project, we have developed a model ascribing the self-ordering phenomena observed above to local variations of the surface chemical potential μ. Since μ becomes lower as the concavity of the surface increases, it induces a curvature-dependent capillarity flux towards the bottom of the groove. In the absence of capillarity, if the growth rate is higher on the sidewall planes than on the bottom facets, the capillarity-enhanced growth rate at the bottom can balance exactly the sidewall growth rate, thus leading to self-limiting growth. The different behavior of nonplanar OMCVD (where self-ordering is usually observed at the bottom of the grooves) and molecular beam epitaxy (where self-ordering rather takes place at the top of the corrugations) can be explained by the different growth rate anisotropies of the two techniques. In a ternary alloy, the composition is locally different at the bottom of the groove, due to the different diffusion lengths of Ga and Al. The resulting entropy of mixing, which is lower than the one for a uniform composition, tends however to oppose this segregation, thus affecting the alloy self-limiting profiles. The predictions of the model have been successfully verified on our OMCVD-grown profiles. They can be used to design and optimize a variety of nanostructures, including VQWs, QWRs and QWR superlattices in the GaAs/A1GaAs system, and can be further extended to the strained InGaAs/AlGaAs system.

EPFL1998

Formation mechanisms of low-dimensional semiconductor nanostructures grown by OMCVD on nonplanar substrates

EPFL1998

Ordered nanostructures on silicon substrates

Jelena Vukajlovic Plestina

EPFL2017