Exciton localization and diffusion in low-dimensional nanostructures formed on non-planar substrates

Quantum wells (QWs), quantum wires (QWRs) and quantum dots (QDs) are semiconductor heterostructures with nanoscopic dimensions. At this length scale, their properties are governed by quantum mechanics. The interest in these nanostructures is motivated by applications in the domain of electronics and opto-electronics. QWs are widely used in diodes and lasers. QDs now attract much attention because of the ability to precisely tailor their interaction with light. With carrier dimensionality in between, QWRs have been less considered, partly because of their higher sensitivity to disorder and the difficulties in fabricating homogeneous structures approaching ideal electronic and photonic 1D systems. The common technique for fabricating semiconductor quantum nanostructures is epitaxial growth. Metalorganic vapor phase epitaxy (MOVPE) is one of the major implementations of the epitaxial process. One of the exciting aspects of this technique is that one can make use of self-organization mechanisms to control the growth front of a crystalline surface. In this thesis, we make use of these phenomena to improve the homogeneity of GaAs/AlxGa1-xAs QWs and QWRs. In the case of QWs, we investigated the growth of GaAs/AlxGa1-xAs structures on vicinal substrates. Discontinuities of the atomic planes are known to occur on the surface of such structures. We found that the growth mode is very sensitive to the initial miscut angle. It allowed us to create very different configurations of the disorder at the interfaces of QWs. In particular, we determined conditions for which the optical characteristics of QWs grown by MOVPE reached unprecedented quality. Moreover, by using a selective etching technique, we were able to correlate the narrow optical linewidth observed to a step-flow morphology of the hetero-interfaces. In that case, terraces are smooth over the length scale determined by the exciton radius. The GaAs/AlxGa1-xAs QWRs that we studied are formed at the center of V-grooves patterned onto the substrate. A lateral self-ordering mechanism creates a quasi one-dimensional (1D) region with a crescent-shaped cross-section. These structures suffer from strong fluctuations of the interfaces along the QWR axis. We investigated the possibility of reducing the effects of this disorder by decreasing the potential height of the confining barriers. By properly adjusting the growth conditions to grow AlxGa1-xAs barriers with low Al concentration, we obtained a strongly reduced spectral linewidth of the emission. When the probed area is of the order of one micron, the spectral line of the QWR decomposes into two main components, the origin of which are discussed. To improve the homogeneity of V-groove QWRs while retaining a strong confinement, we also investigated the effect of growing these structures on vicinal patterned substrates. We evidenced a strong modification of the relative growth rates of the facets forming the QWRs and a narrower spectral linewidth for QWRs grown on substrates with a large miscut angle. This narrower linewidth offers new possibilities for the study of 1D physics. Finally, we also addressed the question of exciton diffusion in QWRs. This subject has been at the center of important theoretical efforts, but experimental data have been scarce so far. We present a systematic study of the exciton diffusion as a function of the lattice temperature. Using a time-of-flight technique, we have evidenced an activation of the diffusion at intermediate temperatures (~50 K). We present data suggesting that excitons are localized below this temperature and that the diffusion is determined by interface roughness. In summary, this thesis presents important improvements of the properties of QWs and QWRs grown by MOVPE. Record low spectral linewidths of the emission are obtained for both types of structures. Yet, diffusion measurements in QWRs indicate that the interface roughness is still the dominant factor in limiting the exciton mobility at low temperatures. The growth mechanisms that we evidenced offer new routes for further improvement of the homogeneity of V-groove QWRs.

Dilute-Nitride Low-Dimensional Nanostructures Formed on Non-Planar Substrates

Romain Carron

The properties of semiconductors heterostructures of nanoscopic dimensions change from that of bulk material according to the rules of quantum mechanics. The planar quantum wells (QWs) are widely used in various diode and laser devices thanks to the relative ease of fabrication and to their improved electronic and optical performance compared to bulk materials. Quantum effects become more apparent when the charge carriers are confined in more than 1 spatial dimension. Much scientific interest was initially dedicated to the quantum wire (QWR) structures, which confine carriers into a quasi-1D space. But their sensitivity to disorder and the development of efficient fabrication methods of quantum dots (QD) shifted the attention to this latter system. The carrier confinement in the three directions of space confers to these structures a discrete spectrum of energy states, with the state occupancy ruled by the Pauli exclusion principle. The prospective applications are numerous in domains such as ultra-low threshold lasers, quantum cryptography, true random numbers generation, quantum electrodynamics experiments, etc. One of the prominent semiconductor growth techniques is the metalorganic vapour phase epitaxy (MOVPE). We used this method to produce ordered QWRs and QDs at the bottom of V-shaped and tetrahedral recesses, respectively. These nanostructures form by the complimentary actions of nano-capillarity and growth rate anisotropy in these recesses etched in GaAs substrates. This fabrication process offers some key advantages over other methods. The emission energy is very well controlled, with a narrow inhomogeneous broadening and a high uniformity across the wafer. Combined with the highly precise control on the formation site, this offers the possibility of high-yield integration of one or even several nanostructures, e.g., into photonic crystal devices. Other advantages of this approach are the impressive tunability of the electronic potential within the nanostructures and the possible use of well-defined intraband transitions. However, the emission energy of V-groove QWRs and pyramidal QDs studied so far is quite limited due to the formation mechanism imposing a low degree of strain. Incorporation of nitrogen has dramatic effects on the band structure of GaAs-based materials. Dilute concentrations (< 5 %) lead to a huge shrinkage of the bandgap, as well as to large changes in various other electronic properties. Addition of nitrogen into QWRs and QDs nanostructures may thus allow emission wavelengths above 1 μm, possibly up to the 1.3 μm telecommunication window of silica optical fiber. In view of these opportunities, the first part of this work is dedicated to the study of N incorporation into QWs grown on vicinal-(100) GaAs substrates. The substrate miscut angle is often neglected in growth studies, and is of great importance in the cases of V-grooves and pyramidal recesses because of the large misorientations of the recesses facets. In our studied dilute-nitride QWs grown by MOVPE, large emission redshifts are achieved (> 250 meV). The substrate miscut and the surface corrugations are shown to play an important role in the N incorporation efficiency: QWs grown on large substrate misorientations emit at longer wavelength than those grown on usual (100)-"exact" substrates, while exhibiting a comparable luminescence efficiency. The importance of a uniform N distribution within the QW is stressed, which appears difficult to achieve when the effect of surface corrugation is combined with that of In segregation. The second part of the work focuses on the N incorporation into V-groove QWRs. Important emission redshifts are achieved, in the ∼ 250 meV range. We first detail the emission spectrum and assert the 1D-character of the carrier wavefunctions. The influence of various growth and structural parameters is explored, leading to the fabrication of QWRs emitting at 1.3 μm at room temperature. The evolution of the polarization properties with temperature is also characterized. The third main topic and primary goal of this thesis is the nitrogen incorporation into QDs formed in inverted pyramids etched on (111)B GaAs substrates. A study is first conducted to understand the effects of several growth and structural parameters on the emission properties of InGaAs QDs. Nitrogen incorporation into QDs is then successfully demonstrated. The monitoring of the lateral QWR emission energy suggests a peculiar N incorporation pattern, or a significant perturbation of the formation of these lateral nanostructures. By contrast to what achieved with QWs and QWR structures, only limited emission redshifts were achieved (∼ 75 meV). The QD linewidths, degree of linear polarization and fine structure splitting are significantly deteriorated when compared to the InGaAs counterparts. These results cast serious doubts on the perspective of high-quality GaAs-based QDs in pyramids emitting at long wavelength. Our results demonstrate that nitrogen does not have the potential to shift the emission wave-length of InGaAs pyramidal QDs up to 1.3 μm, while simultaneously satisfying strict quality requirements. But other material systems may offer such opportunities. We briefly explore the possibilities of growing InGaAs/InAlAs nanostructures on patterned InP wafers. This ongoing project may open new possibilities for exploiting the pyramidal QD system. A kinetic Monte-Carlo numerical algorithm was implemented, reproducing by deposition and diffusion processes the evolution of the pyramidal template during growth. The numerical experiments were compared with post-growth AFM measurements of real samples. The recesses are observed to strongly affect the monatomic step flow on the neighboring (111)B surfaces. The simulations especially evidence the strong attraction of the pyramid apex on the atoms of the surrounding area, tending to elevate the QD formation site from the nominal C3V symmetry toward a hexagonal one.

EPFL2013

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

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

Submonolayer epitaxial growth is obtained by the deposition of less than a complete layer of atoms on a single crystal surface. It is of fundamental interest for industrial applications (e.g. in the semiconductor industry) as well as from the point of view of basic research. For example, it is known that nanometer-sized atomic structures (nanostructures) exhibit remarkable physical and chemical properties which can differ greatly from those of bulk matter. In order to investigate these properties, it is often necessary to create large quantities of well defined and possibly spatially ordered nanostructures. One way to achieve this result is self-organized growth where one deposits atoms on a clean crystalline surface and lets the growth process evolve freely. Here, nanostructures result from the atomic diffusion and aggregation processes taking place at the surface. Understanding the exact nature of these processes is of ongoing interest in the field of nanostructure growth. In this thesis we report about submonolayer growth experiments of the ferromagnetic transition metal cobalt. Cobalt was chosen because it exhibits remarkable magnetic properties. These experiments were performed in an ultra-high vacuum chamber and molecular beam epitaxy was used to grow the nanostructures. Observations of the surface were made using a variable-temperature scanning tunneling microscope (STM). In order to get a better understanding of the atomic processes happening at the surface, we developed two adapted computational simulation methods. Kinetic Monte-Carlo (KMC) simulations were used to get an atomistic picture of the surface while mean field rate equations were integrated numerically to yield cluster densities. We study the growth of cobalt on three different surfaces. By depositing Co on a clean Pt(111) crystal surface, we observe that the platinum surface reconstructs by forming star-shaped partial dislocations for sample temperatures above 180 K (-93°C). We also observe that island densities deviate from predictions of all known models towards higher values for these same temperatures. By simulation we are able to show that insertion of Co into the topmost platinum layer creates a repulsive network of dislocations. We show that these dislocations act as diffusion inhibiting barriers and thus influence the island density by constraining the free movement of atoms at the surface. We also show that the Co dimer and trimer diffuse on Pt(111) before dissociating and are able to extract corresponding activation barriers. We also study the deposition of Co on a Ru(0001) surface which shows a more conventional temperature dependence. By comparing simulation and experiment we extract all relevant diffusion activation barriers and show that at high temperatures the Co dimer and trimer dissociate. Finally, we investigate the growth of Co on the hexagonal boron nitride superlattice which forms on a Rh(111) surface. On this surface Co clusters form three-dimensional hemispherical dots. We show how the substrate corrugation influences diffusion of Co atoms and that clusters up to the size of the pentamer can diffuse. By simulation, we also extract the relevant migrations and desorption barriers.

EPFL2008