Rytis Dargis

Strain relaxation in single crystal SrTiO3 grown on Si (001) by molecular beam epitaxy

An epitaxial layer of SrTiO3 grown directly on Si may be used as a pseudo-substrate for the integration of perovskite oxides onto silicon. When SrTiO3 is initially grown on Si (001), it is nominally compressively strained. However, by subsequent annealing in oxygen at elevated temperature, an SiOx interlayer can be formed which alters the strain state of SrTiO3. We report a study of strain relaxation in SrTiO3 films grown on Si by molecular beam epitaxy as a function of annealing time and oxygen partial pressure. Using a combination of x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy, we describe the process of interfacial oxidation and strain relaxation of SrTiO3 on Si (001). Understanding the process of strain relaxation of SrTiO3 on silicon will be useful for controlling the SrTiO3 lattice constant for lattice matching with functional oxide overlayers.

Monolithic integration of rare-earth oxides and semiconductors for on-silicon technology

Several concepts of integration of the epitaxial rare-earth oxides into the emerging advanced semiconductor on silicon technology are presented. Germanium grows epitaxially on gadolinium oxide despite lattice mismatch of more than 4%. Additionally, polymorphism of some of the rare-earth oxides allows engineering of their crystal structure from hexagonal to cubic and formation of buffer layers that can be used for growth of germanium on a lattice matched oxide layer. Molecular beam epitaxy and metal organic chemical vapor deposition of gallium nitride on the rare-earth oxide buffer layers on silicon is discussed.

Facet analysis of truncated pyramid semi-polar GaN grown on Si(100) with rare-earth oxide interlayer

After epitaxial growth of GaN on Si(100) substrates using an Er2O3 interlayer, two dominant growth orientations can be observed: semi-polar (101¯3) as well as non-polar (112¯0). Epilayers with the (101¯3) orientation lead to the formation of truncated pyramids, which were studied in detail by high-resolution X-ray diffraction, photoluminescence, and scanning electron microscopy (SEM). Depending on the GaN growth orientation and in-plane relation to the Er2O3 interlayer, lattice mismatches in the growth plane were calculated. In order to understand the formation of truncated pyramids, a method for facet identification from SEM images under different tilt angles was developed. This method was used to reconstruct truncated pyramids from our experiments. These were then compared with calculations of the corresponding kinetic Wulff construction, to explain the preferential growth of (101¯3) GaN.

Study of thermal stability of distributed Bragg reflectors based on epitaxial rare-earth oxide and silicon heterostructures

Thermal stability of rare-earth oxide/silicon multilayer heterostructure-based distributed Bragg reflectors under typical metal organic chemical vapor deposition process temperatures with an ambience of typical process gases was studied. Gadolinium oxide or erbium oxide based two period (¼λ oxide, ¾λSi) blue light (λ = 450 nm) mirrors with Si layer on the top were annealed in H2, N2, and H2 + NH3 gases for 20 min at 1015 °C in a metal organic chemical vapor deposition chamber. Changes in the structure were analyzed using x-ray diffraction and transmission electron microscopy. Reflectivity measurements were used for evaluation of optical properties of the reflectors before and after annealing. Chemical reaction between gadolinium oxide and silicon are revealed by reduction of reflectivity of Gd2O3/Si based distributed Braggs reflector and GdSi2-x attributed peaks detected by x-ray diffraction. No major changes were detected in the Er2O3/Si heterostructure-based reflector, indicating good chemical and crys...

Rare-earth-metal oxide buffer for epitaxial growth of single crystal GeSi and Ge on Si(111)

Ternary and binary rare-earth oxides that are used as a template buffer, which accommodates the crystal lattice mismatch between substrate and a semiconductor layer, are discussed here. The oxides were grown on Si(111) substrates and exhibit the cubic bixbyite crystal structure. Stabilization of the cubic bixbyite structure of ternary erbium-neodymium oxide and lanthanum oxide was analyzed using structural investigation of the epitaxially grown oxides and ab initio density functional theory calculations. The authors demonstrate that despite the more energetically favorable hexagonal structure of bulk lanthanum oxide a pseudomorphic single crystal cubic lanthanum oxide layer grows under nonequilibrium conditions of a molecular beam epitaxy process on gadolinium oxide. Growth of hexagonal lanthanum oxide begins when the critical thickness of the layer is reached. Germanium was epitaxially grown on the cubic bixbyite lanthanum sesquioxide. Due to a higher surface energy, germanium starts to grow in the form ...

Study of the Structural and Thermal Properties of Single Crystalline Epitaxial Rare-Earth-Metal Oxide Layers Grown on Si(111)

Rare-earth-metal oxides (REO) grown epitaxially on silicon have attracted much attention because of their applications as gate dielectrics for MOS devices or as a buffer layer for epitaxial high carrier mobility semiconductor growth on silicon. Some of the oxides have stable cubic crystal structures and are suitable for growth on silicon. However, their crystal lattice is smaller if compared to that of twice that of silicon (from 0.5 % for Gd2O3 to 4.2% for Lu2O3). Stress and dislocations appearing at the interface and in the oxide layer influence the electronic band structure (it is of importance for REOs application as gate dielectric) and structure of a high mobility semiconductor layer if grown on the top of the oxide. Additionally, the thermal expansion coefficient of the bulk oxides is higher than that of silicon. For this reason, control of thermal stress in the REO-silicon structure is of high importance. In this work, results of thermal stress studies of REO layers epitaxial grown by molecular beam epitaxy (MBE) on Si(111) substrates are presented. In contrast to the similar works published earlier [1],[2] that investigated thermal stress in nanometer thick (up to 20 nm) oxide layers grown on silicon substrates, the thickness of the oxide layers in this study is in range from 100 nm to 500 nm. This is a typical thickness used for buried dielectric layer for semiconductor on isolator (SOI) structures. The stress behaviour in such “thick” layers is expected to be different from that in the thin layers. Additionally, the poor thermal conductivity of thermal silicon dioxide as a buried dielectric layer in SOI structure and the self-heating effects in CMOS devices is one of the most important factors that motivate looking for alternative dielectric materials. There is very little data in the literature of the thermal conductivity of bulk rareearth-metal oxides and no data about the thermal conductivity of single crystalline thin REO layers. The gadolinium oxide and erbium oxide layers were grown in an MBE system by the evaporation of the metals from effusion cells and molecular oxygen from a gas manifold. The crystal structure and thermal stress of the layers were investigated using high resolution X-ray diffractometer with in-situ heating of a sample up to 1000 C temperature in nitrogen ambience. The time-domain thermoreflectance method [3] was used for measurement of thermal conductivity of the REO layers. Results of the structure analysis of rare-earth metal – oxide layers grown on silicon show that at room temperature, the gadolinium oxide (and erbium oxide) inplane and out of plane crystal lattice parameters are similar whithin the error limits indicating an almost fully relaxed cubic lattice (Fig. 1). On the other hand, an increase of the out of -plane lattice constant with oxide thickness is clearly seen (Fig.2). This could be explained by minor tetragonal distortion of the oxide lattice [4]. However in our case, the oxide lattice is under tension in the in-plane direction which should result in a decrease of the out of-plane lattice constant. This distortion is more notable for thinner layers.

Formation of single-orientation epitaxial islands of TiSi2 on Si(001) using Sr passivation

Epitaxial islands of C49-phase TiSi2 of up to 100 nm in size, and with a single crystallographic orientation, have been fabricated on Si(001) substrates. The growth process involves passivation of the Si surface using Sr, followed by deposition of Ti in the form of SrTiO3, which prevents the reaction between Ti and Si. Decomposition of SrTiO3 at temperatures above 800 °C drives off Sr and O completely, leaving epitaxial islands of TiSi2 dispersed on the Si surface. The TiSi2 islands have (010) orientation and an in-plane epitaxial relationship of Si[110]∥TiSi2[100]. Density functional calculations of the surface and interface energies show that the island sizes and contact angles are consistent with surface energy minimization.