Crystal growth and characterization of zn1-xmgxo advanced micro- and nanostructures
- Autores: Esther de Prado
- Directores de la Tesis: María Carmen Martínez-Tomás (dir. tes.)
- Lectura: En la Universitat de València ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: Vicente Muñoz-Sanjosé (presid.), Paloma Fernández Sánchez (secret.), Jesús Zúñiga Pérez (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universitat de València (Estudi General)
- Materias:
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- Resumen
Within the framework of the increasing research on semiconductor physics, the study of II-VI semiconductor compounds have become a fundamental field of material physics because their physical properties are unique to create a new generation of devices in the field of photonics and microelectronics. Between them, ZnO based semiconductors have gained substantial interest in the research community in part because of the ZnO large exciton binding energy of 60 meV, which is 2.4 times the effective thermal energy at room temperature and could lead to exciton lasing action even above room temperature.
On the other hand, the bandgap, the lattice parameters, the refractive index and many other properties of ZnO can be tuned by alloying ZnO with different materials. Among other, Zn1-xMgxO alloys have attracted considerable attention due to the similar ionic radii between Zn and Mg. Nevertheless, ZnO has a wurtzite hexagonal structure while MgO has a rocksalt cubic one and the solubility limit of Mg in ZnO at the thermodynamic equilibrium has been reported to be 4% mol. These two drawbacks make the achievement of high quality monocrystalline Zn1-xMgxO alloys a really challenging issue.
The new technological requirements demand new configurations in order to create a new generation of devices in the field of photonics and microelectronics. In this context, nanowires (NWs) and multilayer structures present a special interest because they constitute the building blocks of many advanced devices.
The growth of Zn1-xMgxO NWs has been extensive studied using different methods such as metal organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), hydrothermal, molecular beam epitaxy (MBE), vapor-liquid-solid (VLS), vapor-solid (VS) and physic vapor transport (PVT). These three latter offer some advantages which make them very interesting as growth methods, such as low cost and low temperature conditions. The benefits of NWs ordered structures is well known. The typical self-organized growth mechanism for creating nanowires is the VLS which provides free-standing crystalline nanowires with fully controlled nucleation sites and diameters from pre-formed metal catalysts. Nevertheless the contamination caused by the use of a catalyst is a drawback that must have been into account and other ways to obtain them must to be explored, as the use of patterned substrates. A very promising way to produce substrates with surface structuration consists in the irradiation of the substrate surface with ultrashort laser pulses which generates the so-called Laser Induced Periodic Surface Structures (LIPSS). Even though a complete understanding of LIPSS origin is still missing, it have been demonstrated that the use of the irradiated regions as 2D patterns enhances the growth of ordered nanostructures.
In the experiments usually reported in the literature irradiation was performed on single crystalline samples, while the use of polycrystalline substrates has not been extensively exploited.
In this sense, one of the aims of this thesis is the attainment and characterization of highly ordered Zn1-xMgxO nanowires grown by an easily, accessible and low cost method such as vapor-solid using patterned polycrystalline substrates.
Concerning the growth of Zn1-xMgxO epitaxial layers, the fast progress of crystal growth techniques over the 20th century has enabled the achievement of crystal structures that have singular and strongly interesting optical properties. Among them, semiconductor microcavities (MCs) concomitantly offer good optical confinement with a small mode volume and are promising candidates for achieve strong light-matter coupling. In this structures the cavity polaritons can undergo to a Bose-Einstein condensate and emit coherent light, giving rise to the so-called polariton laser. As they do not require an electronic population inversion, polariton lasers are expected to present lower thresholds than conventional lasers. Nevertheless, the evolution of such a laser, from a laboratory component to a useful commercial device, requires that it would be able to operate at room temperature (RT). In this sense, ZnO-based structures are an optimum choice due to their large exciton binding energy and oscillator strength. These complex heterostructures usually are fabricated by sandwiching a ZnO layer between two distributed Bragg reflectors (DBRs), which must have two key properties: they should display a small lattice mismatch with respect to ZnO and, at the same time, a large refractive index contrast is needed. This is exactly where Zn1-xMgxO layers play a major role since their lattice parameters and refraction index depend on the Mg amount and can be tuned until achieve a high reflectivity without sacrificing the MCs quality.
Almost all the studies performed on this topic until the beginning of this thesis were based on (0001) oriented ZnO, that is, without exploiting the benefits of other orientations. In nonpolar and semipolar Zn1-xMgxO/ZnO heterostructures it is possible to reduce, eventually nullify, the internal electric fields arisen due to the spontaneous and piezoelectric polarization mismatches between the different materials. Thus it is desirable the growth of nonpolar or semipolar ZnO based MCs.
Thus, given the interest of Zn1-xMgxO for constituting MCs based on nonpolar and semipolar ZnO, a detailed study is needed, both of Zn1-xMgxO/ZnO single layers, multilayers and complete MCs. In this respect the second aim of this thesis is to deep in the study of nonpolar and semipolar Zn1-xMgxO/ZnO systems grown by MBE in order to obtain high quality MCs, making special attention to the structural characterization.
For all the explained above, the objective of this thesis is to deep into some aspects related to such an interesting material as the ZnO-MgO alloy thus covering some of the lacks revealed by the bibliography. First, the growth of high ordered Zn1-xMgxO nanowires by the vapor solid method without the use catalyst on polycrystalline substrates patterned by LIPPS formation. Next, the growth and characterization of high quality m- and r- oriented ZnO microcavities with Zn1-xMgxO/ZnO distributed Bragg reflectors.
For the obtainment of the required quality structures not only is necessary to control the growth mechanism but also to be able to enhance the growth conditions. In this way growth and characterization constitute a self-reinforcing cycle, so that, chapter 2 and 3 aims to give an overview of the main properties of the ZnO and MgO materials and some concepts about the crystalline growth methods used in this thesis to obtain its alloy as well as of the techniques that have been employed for characterizing the obtained samples.
Chapters 4,5 and 6 illustrates the growth and the morphological, optical and structural characterization of ordered Zn1-xMgxO NWs, m-oriented MCs and Zn1-xMgxO r-oriented layers as a first step towards future semipolar ZnO-based MCs. From these results, arise the following conclusions: In order to found the best conditions for the VS growth, a previous study of the randomly growth of Zn1-xMgxO NWs was made, obtaining that the best conditions for this type of growth were a low time milling 5 h of the precursor mixture and a content of 5 wt. Mg %. Out of these conditions low covered substrates and non-homogeneous distribution of NWs were found, joined to the presence of the binary compound MgO.
The analysis of the optical properties of the randomly grown Zn1-xMgxO NWs has shown a narrowing of the Near Band Edge (NBE) band and a decreasing of the Deep Level (DL) band for increasing values of the Mg content.
For obtaining patterned substrates, self-organized LIPSS structures have been fabricated by irradiation with a high repetition rate femtosecond laser on polycrystalline ZnO based samples. All samples were prepared in the previously determined best conditions for the growth process. The capability of the structured surfaces to serve as substrate template for the growth of low dimensional structures has been investigated, showing that the best regions to be used as growth templates are those irradiated in the highest energy deposition regime. On these patterned substrates, wires with large aspect ratios, arranged in arrays with the scanning line spatial period have been obtained after the VS growth. The luminescent characteristics of the so obtained Zn1-xMgxO nanostructures consisted mainly on an increasing of the DL band intensity in respect pure ZnO. This aspect is of great importance for the performance of ZnO-based sensing devices, since gas sensitivity in NWs was found to be strongly related to the luminescence intensity of the DL band.
Given the high interest on Zn1-xMgxO /ZnO microcavities (MCs) constituted by Distributed Bragg Reflectors (DBR) and based on nonpolar and semipolar ZnO, a careful study, mainly about their structural characteristics, has been developed. Thus, Zn1-xMgxO thin layers grown by Molecular Beam Epitaxy (MBE) have been analyzed in two different configurations, m- Zn1-xMgxO /ZnO systems (both single layers and multilayers) and r-oriented Zn1-xMgxO /ZnO single layers, as building blocks for the achievement of MCs.
Zn1-xMgxO single layers and complete MCs have been obtained with flat surfaces and interfaces confirmed by atomic force microscopy and high resolution X ray diffraction (HRXRD) measurements. Surface roughness has been studied as a function of the number of layers in the DBR and it has been proven that the roughness of these heterostructures raised as the total thickness increased, but with a lower raising rate than single layers do. Mg composition has been found to be homogeneous, even for heterostructures with a thickness in the range of micrometers. Despite these good qualities, samples thicker than 400 nm presented cracks along to <11.0> in plane direction. Nevertheless, it has been proven that these cracks do not affect the macroscopic optical and structural properties of the system.
HRXRD has shown that m-oriented layers were grown pseudomorphically, that is totally in-plane strained, exhibiting almost the same in-plane lattice parameters than the substrate. Measurements of the lattice parameters revealed the typical behavior of ZnO-MgO alloy, an increased lattice parameter a and a decreased lattice parameter c with respect to ZnO ones. In-plane measurements indicated a relatively high (-0.32%) compressive mismatch along the [11.0] in-plane direction and a relatively small (+0.078%) tensile mismatch along the [00.1] in-plane direction, that could be the origin of cracks. Nevertheless, the high difference in the thermal expansion coefficients of the layer and the substrate could also contribute.
When optical properties have been studied, it has been shown that the cathodoluminescence (CL) spectra of microcavities based on m-oriented multilayers are dominated by the NBE of ZnO, in company with a weak DL emission. These characteristics are indicative of a high NBE radiative efficiency and excellent optical properties. Reflectivity measurements show a high and wide stop band with a cavity mode centered at 2.94-2.96 eV.
As a culmination of all of the above, the possibility of stacking a large number of bilayers, together with the high contrast of index in the DBR mirrors and their excellent structural properties, have enabled to obtain optical microcavities with quality factors in the order of 600 with a photonic disorder one order of magnitude smaller than the state-of-the-art in wide bandgap microcavities.
In the case of r-oriented microcavities, only the first step of the equivalent study made previously on m-oriented multilayers system has been carried out. Thus MBE growth conditions were first optimized for obtaining (01.2) Zn1-xMgxO /ZnO single layers and a deep study of the structural properties was carried out as a function of the Mg content. During the development of this study, satellite peaks in 2theta-omega scans were observed that gave us the opportunity of studying an unusual and barely studied case of multiple diffraction phenomenon called hybrid multiple diffraction (HMD) which is a consequence of the high crystalline quality of heterostructures. As far as we know, HMD has never been studied on systems with a symmetry as low as that of r-oriented wurtzite structures.
Related to roughness, r-oriented layers also display flat surfaces, independently of the Mg content of the studied range. In addition, the samples showed no cracks even for thicknesses higher than 400 nm.
HRXRD has shown that these layers were also pseudomorphic with respect to the substrate, independently of the Mg content of the studied range. These measurements revealed also that the lattice parameters follow the previously described as typical behavior of ZnO with Mg content. However they also bring out the fact that the basal plane had an orthorhombic distortion, with the gamma angle slightly higher than 120 deg. increasing this deviation with the increasing Mg content.
Structural analysis of layers by HRXRD confirmed the good quality of layers, although exhibiting satellite peaks in 2theta-omega scans. The value of the Bragg angle at which these satellite peaks were observed was dependent of the Mg content. These satellite peaks could be interpreted in the frame of the HMD, a special kind of multiple diffraction in which the participating planes in the phenomenon belong to different lattices, in this case substrate and layer.
Two sets of hybrid reflections were found, one exhibiting smaller 2theta with an approximate six-fold azimuthal symmetry and the other one exhibiting a larger one, with an approximate two-fold symmetry in the azimuthal angle. Interestingly, for these hybrid reflections the difference between forward and reverse beam path is not 180 deg. azimuthaly.
Planes that contribute to HMD were identified, finding five sets of planes which explain all the observed satellite peaks. This set of planes allow the calculation of the Bragg angles 2theta_H and the azimuthal angles at which HMD appears, finding an admirably good agreement with experimental values.
The analysis and the achieved angular precision leads us to propose a very interesting application of HMD, as is the accurate measurement of lattice parameters. This method provides an easy way to shorten the measurement time without sacrificing accuracy.
From a global point of view, this thesis has allowed, not only the aforementioned achievements, but also the acquisition of skills and wide knowledge on different growth and characterization techniques.
