High-pressure Reflection High-energy Electron Diffraction Research Articles

Monitoring non-pseudomorphic epitaxial growth of spinel/perovskite oxide heterostructures by reflection high-energy electron diffraction

Pulsed laser deposition of spinel γ-Al2O3 thin films on bulk perovskite SrTiO3 is monitored by high-pressure reflection high-energy electron diffraction (RHEED). The heteroepitaxial combination of two materials with different crystal structures is found to be inherently accompanied by a strong intensity modulation of bulk diffraction patterns from inelastically scattered electrons, which impedes the observation of RHEED intensity oscillations. Avoiding such electron surface-wave resonance enhancement by de-tuning the RHEED geometry allows for the separate observation of the surface-diffracted specular RHEED signal and thus the real-time monitoring of sub-unit cell two-dimensional layer-by-layer growth. Since these challenges are essentially rooted in the difference between film and substrate crystal structure, our findings are of relevance for the growth of any heterostructure combining oxides with different crystal symmetry and may thus facilitate the search for novel oxide heterointerfaces.

Growth and in situ high-pressure reflection high energy electron diffraction monitoring of oxide thin films

Interface and surface physics is an important sub-discipline within condensed matter physics in recent decades. Novel concepts like oxide-electronic device are prompted, and their performance and lifetime are highly dependent on the flatness and abruptness of the layer surfaces and interfaces. Reflection high-energy electron diffraction (RHEED), which is extremely sensitive to surface morphology, has proven to be a versatile technique for the growth study of oxide thin films. A differential pumping unit enables an implementation of RHEED to pulsed laser deposition (PLD) systems, ensuring an in situ monitoring of the film growth process in a conventional PLD working oxygen pressure up to 30 Pa. By optimizing the deposition conditions and analyzing the RHEED intensity oscillations, layer-by-layer growth mode can be attained. Thus atomic control of the film surface and unit-cell control of the film thickness become reality. This may lead to an advanced miniaturization in the oxide electronics, and more importantly the discovery of a range of emergent physical properties at the interfaces. Herein we will briefly introduce the principle of high-pressure RHEED and summarize our main results relevant to the effort toward this objective, including the growth and characterization of twinned La2/3Ca1/3MnO3 thin films and ReTiO3+δ/2 (Re = La, Nd; δ = 0 ∼ 1) A n B n O3n+2 structures, on YSZ-buffered ‘Silicon on Insulator’ and LaAlO3 substrates, respectively, as well as the study of the initial structure and growth dynamics of YBa2Cu3O7−δ thin films on SrTiO3 substrate. Presently we have realized in situ monitoring and growth mode control during oxide thin film deposition process.

RHEED study of titanium dioxide with pulsed laser deposition

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the growth of thin films of titanium dioxide (TiO 2) on (1 0 0) magnesium oxide (MgO) substrates by pulsed laser deposition (PLD). The deposition is performed with a synthetic rutile TiO 2 target at low fluence. The topography and structure of the deposited layers are characterized using in situ high pressure RHEED and atomic force microscope (AFM). Based on these observations the growth mode of the films is discussed. The results will be compared to earlier results obtained for the growth of TiN films on (1 0 0) MgO.

Epitaxial growth and surface metallic nature of LaNiO3 thin films

In situ epitaxial growth control of LaNiO3 (LNO) films at high oxygen pressure has been successfully achieved using a combination of pulsed laser deposition and high-pressure reflection high-energy electron diffraction (RHEED). RHEED oscillations, indicative of epitaxial layer-by-layer growth, were clearly observed during LNO deposition under optimal conditions. The film surfaces were composed of atomically flat terraces and steps. Detailed photoelectron spectroscopy analysis of LNO grown at the optimal oxygen pressure revealed that Ni ions assume a uniform Ni3+ high-valence state and that the resultant metallic properties are preserved even at the surface and in the interface region between LNO and the LaAlO3 substrate.

Growth mechanism, microstructure and transport properties of Sr1−x La x CuO2 (x=0.10−0.15) thin films

Sr1−xLaxCuO2 (x=0.10−0.15) thin films with an infinite-layer type structure were grown on BaTiO3 buffered (001) SrTiO3 substrates by pulsed laser deposition (PLD). The evolution of the growth front was monitored, in-situ, by high-pressure reflection high-energy electron diffraction (RHEED), while the surface morphology was analyzed by means of atomic force microscopy (AFM), ex-situ. X-ray diffraction (XRD) was used to determine the evolution of the film structure with deposition and cooling parameters, as well as to study the type and level of epitaxial strain in the Sr1−xLaxCuO2 films. The RHEED data showed that the Sr1−xLaxCuO2 films grow on BaTiO3/SrTiO3 following a 2D or Stranski-Krastanov mechanism, depending on the La doping level. The transition point (critical thickness dc) from layer-by-layer like (2D) to island (3D) growth depends on the film stoichiometry: decreasing the La doping concentration x from 0.15 to 0.10, the critical thickness dc increases from ∼45 nm to ∼75 nm. In order to induce superconductivity, the Sr1−xLaxCuO2 films were cooled down under reduction conditions. The as-deposited films showed semiconducting or metallic behavior, the resistivity decreasing with increasing La concentration. Post-deposition vacuum annealing resulted in a superconducting transition onset (but no zero resistance down to 4.2 K) only for some of the x=0.15 Sr1−xLaxCuO2 films.

Growth of thin films of TiN on MgO(100) monitored by high-pressure RHEED

Reflection high-energy electron diffraction (RHEED) operated at high pressure has been used to monitor the initial growth of titanium nitride (TiN) thin films on single-crystal (100) MgO substrates by pulsed laser deposition (PLD). This is the first RHEED study where the growth of TiN films is produced by PLD directly from a TiN target. At the initial stage of the growth (average thickness ∼2.4 nm) the formation of islands is observed. During the continuous growth the islands merge into a smooth surface as indicated by the RHEED, atomic force microscopy and field emission scanning electron microscopy. These observations are in good agreement with the three-dimensional Volmer–Weber growth type, by which three-dimensional crystallites are formed and later cause a continuous surface roughening. This leads to an exponential decrease in the intensity of the specular spot in the RHEED pattern as well.

Real-time monitoring of organic vapor-phase deposition of molecular thin films using high-pressure reflection high-energy electron diffraction

The crystalline thin film growth of the organic material, copper phthalocyanine (CuPc), by organic vapor-phase deposition (OVPD) is studied using high-pressure reflection high-energy electron diffraction (HP-RHEED). In situ growth of this material was monitored, in real time, on both highly oriented pyrolytic graphite and native SiO2 on Si(100) substrates. The growth of the first several monolayers on both substrates was found to be independent of the growth conditions; however, the crystalline texture of thicker films was controlled through changes in the substrate temperature and deposition rate. Higher substrate temperatures lead to an increase in crystalline ordering for growth on both substrates. This work shows that HP-RHEED is a powerful tool for real-time monitoring of growth morphology in the low-vacuum OVPD environment, ultimately leading to in situ control of thin film crystalline order.

Eucentric four-axis ultrahigh vacuum goniometer for reflection high-energy electron diffraction applications

The design and performance of a four-axis low-profile eucentric UHV goniometer for in situ reflection high-energy electron diffraction (RHEED) studies during film deposition is reported. The design provides one translational and three rotational degrees of freedom that are fully independent. Although developed to facilitate high-pressure RHEED during the growth of oxide thin films by pulsed laser deposition, this goniometer design is applicable to other UHV techniques including molecular beam epitaxy. The goniometer requires only a single DN 100 CF flange (6in. o.d., 100mm i.d.), making it suitable for small deposition systems, too. Samples, attached to a resistively heated holder, can be easily transferred on and off of the goniometer without breaking vacuum. The holder accommodates samples up to 10mm×10mm in size and allows them to be heated to 900°C in pure oxygen while being attached to the goniometer. Full eucentric motion of the hot sample is possible with a typical axis precision of <0.1°. Most of the mechanism is located in air, allowing the use of standard materials and lubricants, substantially reducing the in-vacuum mechanics, and increasing the precision, reliability, and robustness of the system.

In-situ monitoring of the growth of oxide thin films in PLD using high-pressure reflection high energy electron diffraction

Reflection high-energy electron diffraction (RHEED) is very surface sensitive and often used for the analysis and monitoring of thin film growth in ultrahigh vacuum deposition system (for instance, Molecular Beam Epitasis). In order to in-situ monitor the growth of oxide thin films at high oxygen pressure up to 50Pa, a high-pressure RHEED designed and fabricated by our group was used for first time in our pulsed laser deposition system (PLD). Using the PLD system the Nb-doped SrTiO3 (STNO) and Y1Ba2Cu3O7 (YBCO) thin films have been epitaxially grown on SrTiO3 (001) substrates. T he RHEED patterns of the STNO and YBCO films and the oscillation of the intensity of the pattern have been measured by the high-pressure RHEED during deposition. In addition,the surface morpholoyg of the films and the dynamic analysis of film growth process were discussed.

Imposed layer-by-layer growth with pulsed laser interval deposition

In order to create crystal structures by depositing consecutive atomic or unit-cell layers of different materials, a layer-by-layer growth mode is a prerequisite: nucleation of each next layer may only occur after the previous layer is completed. We introduced a growth method based on a periodic sequence: very fast deposition of the amount of material needed to complete one monolayer followed by an interval in which no deposition takes place and the film can reorganize. This makes it possible to grow in a layer-by-layer fashion in a growth regime (temperature, pressure) where otherwise island formation would dominate the growth. Here, we present imposed layer-by-layer growth of homo-epitaxial SrTiO3 as monitored by high-pressure reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM).

Laser ablation of manganite thin films monitored by in situ RHEED

Reflection high-energy electron diffraction (RHEED) during crystal growth has become a standard analytical tool in the fabrication of semiconducting materials by molecular-beam epitaxy in an ultrahigh vacuum environment. For the fabrication of high-quality epitaxial and heteroepitaxial thin film structures of oxide materials such as perovskite manganites or high-temperature superconductors, it is also highly desirable to use RHEED technique during epitaxial growth. However, the complex oxide materials are commonly fabricated by pulsed laser deposition (PLD) at high oxygen pressure above 10 Pa preventing the use of standard RHEED systems. A two-stage differential pumping system can serve to circumvent this problem [1]. In this way, the electron path in the high-pressure oxygen atmosphere within the deposition chamber can be reduced considerably. Then the specular beam intensity of the RHEED system is sufficiently high to allow the observation of intensity oscillations during the growth of oxide materials. We have used high-pressure RHEED and in situ atomic force microscopy (AFM) for the investigation of the surface morphology of oxide thin films prepared by PLD.

Imposed layer-by-layer growth by pulsed laser interval deposition

Pulsed Laser Deposition (PLD) has become a significant technique to study the thin film growth of novel materials. Here, one has benefited from the main advantages of PLD, the relative easy stoichiometric transfer of material from target to substrate and the almost free choice of (relatively high) background pressures. However, the applicability of PLD is still hampered, because real time in situ growth monitoring was almost not available. For example, Reflection High-Energy Electron Diffraction (RHEED) was limited to low background pressures only until recently. High pressure RHEED, which makes it possible to in situ monitor during deposition of oxides at the higher pressures, opened new possibilities [1]. Besides observed intensity oscillations due to layer by layer growth, enabling accurate growth rate control, it became clear that intensity relaxation observed due to the typical pulsed way of deposition leads to a wealth of information about growth parameters [2]. Here, PLD in combination with high pressure RHEED is used to study the influence of the different parameters on the growth mode, resulting in a new approach to impose layer by layer growth by interval deposition.

In Situ Initial Growth Studies of SrTiO3 on SrTiO3 by Time Resolved High Pressure RHEED

AbstractThe initial growth of pulsed laser deposited SrTiO3 on SrTiO3 has been studied using high pressure Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). For this, we developed a Pulsed Laser Deposition (PLD)-RHEED system, with the possibility to study the growth and to monitor the growth rates, in situ, at typical PLD pressures (10-50 Pa). Using perfect single crystal SrTiO3 substrate surfaces, we observe true 2D intensity oscillations at different temperatures. Simultaneously, information on the diffusion of the deposited material on the surface could be extracted from the relaxation of the intensity after each laser pulse. The characteristic times depend on pressure and temperature as well as the 2D coverage during growth.

In-Situ Monitoring during PLD of Complex Oxides using Rheed at High Oxygen Pressure

ABSTRACTSeveral groups have monitored the growth of complex oxides in-situ using Reflection High Energy Electron Diffraction (RHEED). In order to utilize RHEED during growth, films are deposited at low background oxygen pressures. Because of the low oxidation power at low pressures, low substrate temperatures have to be used. This hampers, in general, the film crystallinity. Furthermore, the background pressure in Pulsed Laser Deposition (PLD) is an important parameter, because it influences the shape and size of the plasma and, therefore, the deposition rate and homogeneity of the film.We have developed a high-pressure RHEED system, which can be used for growth monitoring during the deposition of complex oxides at standard PLD conditions. Clear diffraction patterns are observable up to 50 Pa, due to the minimized scattering losses.SrTi03 substrate treatments as well as growth studies of Yba2Cu3O7−δ, using atomic layers of SrO or BaO as a buffer-layer, will be discussed in this contribution. It will be shown that monitoring and control of thin film growth by PLD on an atomic level is feasible, even in quite high background pressures.