RHEED Intensity Oscillations Research Articles

Model-Driven Development for scientific computing. Computations of RHEED intensities for a disordered surface. Part II

Modelling, simulation, and visualisation together create the third branch of human knowledge on equal footing with theory and experiment. Model-Driven Development (MDD) has been proposed as a means to support the software development process through the use of a model-centric approach. The objective of this paper is to address the design of an architecture for scientific application that may execute as multithreaded computations, as well as implementations of the related shared data structures. New version program summary Program title: Growth09 Catalogue identifier: ADVL_v3_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADVL_v3_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 30 940 No. of bytes in distributed program, including test data, etc.: 3 119 488 Distribution format: tar.gz Programming language: Embarcadero Delphi Computer: Intel Core Duo-based PC Operating system: Windows XP, Vista, 7 RAM: more than 1 GB Classification: 4.3, 7.2, 6.2, 8, 14 Catalogue identifier of previous version: ADVL_v2_1 Journal reference of previous version: Comput. Phys. Comm. 180 (2009) 1219 Subprograms used: Cat Id Title Reference ADUY_v4_0 RHEED1DProcess CPC 999 (9999) 9999 Does the new version supersede the previous version?: No Nature of problem: Molecular beam epitaxy (MBE) is a technique for epitaxial growth via the interaction of one or several molecular or atomic beams that occurs on a surface of a heated crystalline substrate. Reflection high-energy electron diffraction (RHEED) is an important in situ analysis technique, which is capable of giving quantitative information about the growth process of thin films and its control. The analysis of RHEED intensity oscillations has two purposes. One is to control the film growth, and the other is to understand the mechanism of the film growth using the MBE through the analysis of surface morphology as a function of time. Such control allows the development of structures where the electrons can be confined in space, giving quantum wells or even quantum dots. Such layers are now a critical part of 3 many modern semiconductor devices, semiconductor lasers, light-emitting diodes and new devices for the magnetic storage industry. Solution method: The present paper reports a practical and pragmatic approach for MDD technology [1] that has been used during design of the Growth09 program. Growth09 is a numerical model that uses multithreaded and partially nested transactions for simulation of epitaxial growth of thin films. Reasons for new version: Responding to user feedback the program has been upgraded to a standard that allows a slave process, carrying out computations of the RHEED intensities for a disordered surface, to be run. Also, functionality and documentation of the program have been improved. Summary of revisions: 1. The MDD technology has been used to design a computer model that allows the user to carry out numerical calculations layers coverage during the growth of thin epitaxial films, surface roughness, and the RHEED intensities for a disordered surface. This computer model can be applied to interpret the experimental data in real time [2]. 2. The logical structure of the Platform-Specific Model of the Growth06_v2 program has been modified according to the scheme shown in Fig. 1*. The class diagram in Fig. 1* is a static view of the main platform-specific elements of the Growth09 application architecture. Fig. 2* provides a dynamic view by showing the creation and destruction simplistic sequence diagram. Fig. 3* presents the Growth09 use case model. 3. As can be seen in Figs. 1–3* the Growth09 has been designed as a master program for the slave RHEED1DProcess (see A. Daniluk, Model-Driven Development for scientific computing. Computations of RHEED intensities for a disordered surface. Part I). 4. The slave RHEED1DProcess can be run as separate thread of the Growth09. Fig. 4* depicts the Platform-Specific Model for the development elements of the new distribution. *The figures mentioned can be downloaded, see “Supplementary material” below. Unusual features: The program is distributed in the form of main project Growth09.dproj, with associated files, and should be compiled using Embarcadero RAD Studio 2010 along with Together visual modelling platform. The program should be compiled with English/USA regional and language options. Additional comments: This version of the GROWTH program is designed to run in conjunction with the RHEED1DProcess (ADUY_v4_0) program. It does not replace the previous, stand alone, GROWTH06-v2 (ADVL_v2_1) version. Running time: The typical running time is machine and user-parameters dependent.

Origin of RHEED Intensity Oscillation during Growth Studied by Using Wave Functions

Using RHEED wave functions calculated on the basis of multiple scattering theory, one of the possible origins of RHEED intensity oscillation during growth is studied. This origin is the varying surface atomic row density which affects the interference of waves reflected from the surface. As an example, the specular beam intensity and the wave function are calculated for Si(001) surfaces with various atomic row densities.

RHEED Intensity Oscillation in Thin-film Growth of Organic and Molecular Compounds(<Special Issue>Crystal Growth of Organic Semiconductors toward Applications in Electronics)

RHEED Intensity Oscillation in Thin-film Growth of Organic and Molecular Compounds(<Special Issue>Crystal Growth of Organic Semiconductors toward Applications in Electronics)

RHEED intensity oscillation of C 60 growth on GaAs substrates

Intensity oscillation of reflection high-energy electron diffraction (RHEED) is observed during C 60 layer epitaxial growth on GaAs (1 1 1)B and (1 1 1)A substrates. The frequency of the oscillation coincides well with growth rate of C 60 layers, suggesting that C 60 layers grow with repeating nucleation and a step flow growth as with GaAs and other semiconductor materials. Unusual oscillation is observed in the initial C 60 layer growth on GaAs (1 1 1)B substrates with (2 × 2) reconstruction. The initial layer growth is completed at approximately half monolayer coverage by C 60 molecules. This phenomenon is explained by the model that C 60 absorption sites are limited due to As-trimers absorbed on (1 1 1)B surfaces. This model is strongly supported by the fact that no such effect is observed on GaAs (1 1 1)A substrates where no As-trimer is absorbed.

A study of Al1−xInxN growth by reflection high-energy electron diffraction—incorporation of cation atoms during molecular-beam epitaxy

Molecular-beam epitaxy of Al1−xInxN alloys with different indium (In) contents, x, were studied by in situ reflection high-energy electron diffraction (RHEED). Growth rates of the alloys were measured by the RHEED intensity oscillations for different source flux conditions, while the lattice parameters were derived from the diffraction patterns. It was found that under the excess nitrogen growth regime, incorporation of aluminum was complete whereas incorporation of In atoms was incomplete even at temperatures below 400°C.

Growth Mode and Surface Structure of Cr Ultrathin Film on Fe/Cu(001)

We hereby report a growth mode and surface structure of ultrathin Cr films on fcc Fe/Cu(001). We have found a clear RHEED intensity oscillation during Cr deposition up to four monolayers on 3 and 6 ML Fe/Cu(001), indicating an layer-by-layer growth. Besides, the observed (2×1) LEED pattern at low Cr thickness is similar to that of fcc Fe/Cu(001) in the 5-11 ML thickness range. We have also demonstrated the Cr/Fe multilayer growth with single monolayer of Cr inserted between fcc Fe layers. [DOI: 10.1380/ejssnt.2008.251]

Intensity beats on RHEED oscillations during MBE growth of ZnTe

During the growth of ZnTe by molecular beam epitaxy, we observed that, depending on the substrate temperature and flux ratio, the RHEED intensity oscillations can exhibit strong beats. Those beats are interpreted as a consequence of the coexistence, during growth, of c(2 × 2) and (2 × 1) surface reconstruction domains at the surface of (0 0 1)-ZnTe. The intensity beats superimposed on the regular oscillations are responsible for strong phase shift in the RHEED oscillations, which can be seriously misleading for an accurate determination of the growth rate.

Modeling RHEED intensity oscillations in multilayer epitaxy: Determination of the Ehrlich-Schwoebel barrier in Ge(001) homoepitaxy

growth that includes the diffuse scattering off surface steps and the layer interference between terraces of different heights using the kinematic approximation. The model describes the measured RHEED intensity oscillations very well for the entire range of incidence angles studied. We show that the first intensity minimum occurs well above 0.5 ML monolayer of the total deposited coverage, which contradicts the common practice of assigning the intensity minimum to 0.5 ML. By using the model to interpret the measured RHEED intensity, we find the evolution of the coverage of the first 1–2 ML. We find that second-layer nucleation takes place at low coverage, 0.3 ML, implying a substantial Ehrlich-Schwoebel ES barrier. The value inferred for the ES barrier height, 0.084± 0.019 eV, includes an analysis of the beam steering effect by step edges. Comparison is made with the value of the barrier height inferred from other measurements. The model for RHEED intensity and the method of inferring the ES barrier height can be applied to any system for which RHEED measurements can be obtained without interference from Kikuchi features.

Modification of reflection high-energy electron diffraction system for in situ monitoring of oxide epitaxy at high oxygen pressure

We have developed a modified single-stage differentially pumped RHEED system for in situ monitoring of oxide epitaxial growth at high oxygen pressure. This modification enables us to operate the RHEED system up to oxygen pressure of 100 mTorr. RHEED intensity oscillations indicative of the layer-by-layer growth were clearly observed during LaNiO 3 epitaxial growth at 30 mTorr of pure oxygen ambient. LaNiO 3 epitaxial thin films showed both atomically flat surfaces and low resistivity, suggesting that the operative oxygen pressure was high enough to epitaxially grow oxide films under optimum conditions.

Molecular beam epitaxy of Sc xEr 1−xSb on InAs(1 0 0)

The successful molecular beam epitaxial growth of single crystalline Sc x Er 1− x Sb compounds on InAs(1 0 0) substrates has been demonstrated. Ex situ high-resolution X-ray diffraction and Rutherford backscattering spectrometry with ion channeling studies indicate high crystalline quality. The surface reconstruction was monitored in situ by reflection high-energy electron diffraction (RHEED) during the growth and low-energy electron diffraction post growth. The Sc x Er 1− x Sb(1 0 0) surface exhibited a mixed (1×4)/(4×1) reconstruction. The RHEED patterns and the RHEED intensity oscillations during the growth are consistent with an embedded growth mechanism for the initial Sc x Er 1− x Sb growth on InAs.

RHEED Intensity Oscillation during Oxide Thin Film Growth

RHEED Intensity Oscillation during Oxide Thin Film Growth

Growth kinetics and properties of ZnO/ZnMgO hetero‐ structures grown by radical‐source molecular beam epitaxy

A phenomenological approach to quantitative description of Zn(Mg)O growth by radical-source molecular beam epitaxy, based on the experimental studies of RHEED intensity oscillations, has been developed. It allows a precise control of growth rate, composition and stoichiometry at any growth temperature, Along with optimization of a growth initiation procedure on a c-sapphire, it is necessary condition for fabrication of high quality ZnO epilayers and ZnO/ZnMgO heterostructures in a wide Mg composition range. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum Size Effect of 2DEG Confined Within BaTiO3/SrTiO3:Nb Superlattices

AbstractVery recently, we have found that the high density 2DEG (ne ∼1021 cm−3), which is confined within a unit cell layer thickness of SrTiO3, exhibits unusually large Seebeck coefficient (S2DEG/Sbulk ∼5)[1]. In the optimum, extremely high ZT2DEG of ∼2.4 can be obtained at room temperature, while the effective ZTeff. was only ∼0.24 because 9 unit cells of electrically insulating SrTiO3 layers were used to fabricate the 2DEG structure. Thus, high ZTeff can be realized if the insulating layer thickness is reduced significantly. We selected BaTiO3∼SrTiO3:Nb superlattice to reduce insulating layer thickness because dielectric constant of BaTiO3 is one order of magnitude large (∼3,000) as compared to that of SrTiO3 (∼300). We expected that the conduction electrons can be confined much strongly in the SrTiO3:Nb layer by sandwiching between highly dielectric BaTiO3 layers. As a result, we clarified that the critical BaTiO3 layer thickness is 1.2 nm, significantly small as compared to SrTiO3 layer (4 nm). The BaTiO3/SrTiO3:Nb superlattice films were fabricated by a pulsed laser deposition (PLD) method on (001)-face of LaAlO3 single crystal substrate at 900°C. During the film growth, we monitored RHEED intensity oscillation to control layer thickness precisely. Out-of-plane high-resolution X-ray diffraction measurements and cross sectional HAADF-STEM observations revealed that the resultant films were high quality BaTiO3/SrTiO3:Nb superlattice. Hall mobility of the SrTiO3:Nb layer was 0.4 cm2·V−1·s−1, while that of superlattice decreased gradually with increasing BaTiO3 layer thickness most likely due to that intra layer diffusion of Ba2+ ion occurred between BaTiO3 and SrTiO3:Nb layers[2], which was clearly observed by the EELS mapping. Seebeck coefficient |S|300K of SrTiO3:Nb layer was 57 μV·K−1, which corresponds carrier concentration ne of 5×1021 cm−3. The |S|300K value became large with decreasing the SrTiO3:Nb layer thickness (dSrTiO3:Nb) and it reached 305 μV·K−1, which is approximately 5 times larger than that of SrTiO3:Nb bulk. The slope of log |S|- log dSrTiO3:Nb plots was 1/2, suggesting that quantum size effect occurred. Critical BaTiO3 layer thickness for the quantum confinement of the electrons was 1.2 nm (3 unit cells of BaTiO3), which is significantly small as compared to SrTiO3 (4 nm). Thus, BaTiO3/SrTiO3:Nb superlattice would be a promising candidate to realize high ZTeff.

An extension of the computer program for dynamical calculations of RHEED intensity oscillations. Heterostructures

A practical computing algorithm working in real time has been developed for calculations of the reflection high-energy electron diffraction from the molecular beam epitaxy growing surface. The calculations are based on a dynamical diffraction theory in which the electrons are scattered on a potential, which is periodic in the direction perpendicular to the surface. New version program summary Title of program:RHEED_v2 Catalogue identifier:ADUY_v1_1 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADUY_v1_1 Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Catalogue identifier of previous version:ADUY Authors of the original program:A. Daniluk Does the new version supersede the original program:Yes Computer for which the new version is designed and others on which it has been tested: Pentium-based PC Operating systems or monitors under which the new version has been tested: Windows 9x, XP, NT, Linux Programming language used:C++ Memory required to execute with typical data:more than 1 MB Number of bits in a word:64 bits Number of processors used:1 Number of bytes in distributed program, including test data, etc.:1 074 131 No. of lines in distributed program, including test data, etc.:3408 Distribution format:tar.gz Nature of physical problem: Reflection high-energy electron diffraction (RHEED) is a very useful technique for studying the growth and the surface analysis of thin epitaxial structures prepared by the molecular beam epitaxy (MBE). RHEED rocking curves recorded from heteroepitaxial layers are used for the non-destructive evaluation of epilayer thickness and composition with a high degree of accuracy. Rocking curves from such heterostructures are often very complex because the thickness fringes from every layer beat together. Simulations based on dynamical diffraction theory are generally used to interpret the rocking curves of such structures from which very small changes in thickness and composition can be obtained. Rocking curves are also used to determine the level of strain and its relaxation mechanism in a lattice-mismatched system. Method of solution: The new version of the program retains the design and structure of the previous one [A. Daniluk, Comput. Phys. Comm. 166 (2005) 123. [1]]. Reasons for the new version: Responding to the user feedback we presented an extension of the RHEED program that enables computing the crystalline potentials for epitaxial heterostructures and corresponding values of the amplitude of the RHEED intensity oscillations. Summary of revisions: (1) In this paper we show how the dynamical approach may be applied to creation of a practical computing algorithm to calculate of the intensity of the specularly reflected RHEED beam during MBE growth of Pb on Si(111). The structural properties of the Pb Si interface have been [Display omitted] [b] [Display omitted] [Display omitted] meticulously studied by Howes and co-workers [P.B. Howes, K.A. Edwards, D.J. Hughes, J.E. Macdonald, T. Hibma, T. Bootsma, M.A. James, Surf. Sci. Lett. 331 (1995) 646; K.A. Edwards, P.B. Howes, J.E. Macdonald, T. Hibma, T. Bootsma, M.A. James, Surf. Sci. 424 (1999) 169. [2,3]], and Lucas and Loretto [C.A. Lucas, D. Loretto, Surf. Sci. Lett. 344 (1995) 1219. [4]] (X-ray diffraction). The new version of the RHEED program has the same design as the previous one [A. Daniluk, Comput. Phys. Comm. 166 (2005) 123. [1]]. To simulate the structural variations of whole crystalline heterostructure along the surface normal direction the substrate and layers are divided into an assembly of n atomic layers. Each of these layers is further divided into an assembly of i thin slices parallel to the surface and each slice is assumed to have a constant potential normal to the surface as shown in Fig. 1. The Fourier component of the scattering potential of the whole crystalline heterostructure can be determined as a sum of contributions coming from all thin slices of n individual atomic layers. To carry out one-dimensional calculations we used the self-consistent thicknessZi_Substrate(), thicknessZi_Layers(), thicknessZn_Substrate(), thicknessZn_Layers(), crystPotUgSubstrate() and crystPotUgLayers() functions. Fig. 2 presents the crystalline potentials (real part) calculated for some Pb layers on a Si(111) substrate at 70 K. Fig. 3 shows a dynamically calculated one-beam rocking curve for Pb/Si(111). [Display omitted] [Display omitted] (2) The presented algorithm is a generalization of the previous one. By attributing 0 to the numberOfLayers and NLayers constant parameters (Fig. 4) and removing appropriate functions from the main program (Fig. 5), we obtain the same results as in the case of monocrystal [A. Daniluk, Comput. Phys. Comm. 166 (2005) 123. [1]]. Typical running time: The typical running time is machine and user-parameters dependent. Unusual features of the program: The program is presented in the form of a basic unit RHEED_v2.cpp. It is not tied to any specific hardware and systems software platform, and could be compiled using C++ compilers, including C++ Builder, VC++ and g++.

Multithreaded transactions in scientific computing: New versions of a computer program for kinematical calculations of RHEED intensity oscillations

Writing a concurrent program can be more difficult than writing a sequential program. Programmer needs to think about synchronisation, race conditions and shared variables. Transactions help reduce the inconvenience of using threads. A transaction is an abstraction, which allows programmers to group a sequence of actions on the program into a logical, higher-level computation unit. This paper presents multithreaded versions of the GROWTH program, which allow to calculate the layer coverages during the growth of thin epitaxial films and the corresponding RHEED intensities according to the kinematical approximation. The presented programs also contain graphical user interfaces, which enable displaying program data at run-time.

In situ growth regime characterization of AlN using reflection high energy electron diffraction

Methods for characterizing the growth regime of AlN are presented to deal with the unresolved critical issue of in situ growth regime determination during molecular beam epitaxy. Previously, there were no known reports of in situ measurements to determine the growth regime of AlN, making reproducibility difficult. Reflection high energy electron diffraction (RHEED) intensity transitions were observed upon opening and closing Al effusion cells, and three key signatures were identified. First, the time constant of the falling RHEED specular spot intensity upon Al shutter opening was found to vary inversely with the Al flux, providing a means of in situ calibration of Al flux. Second, a RHEED intensity spike or oscillation feature was observed at the onset of Al flux. Third, the behavior of the RHEED intensity rise upon Al shutter closing can be used to identify the Al-rich intermediate and Al-rich droplet regimes. These techniques and observations provide in situ methods of determining the growth regime of AlN, improving reproducibility and control.

Electrical conductance at initial stage in epitaxial growth of Pb on modified Si(1 1 1) surface

The electrical conductance and RHEED intensities as a function of the coverage have been measured during Pb depositions at 105 K on Si(1 1 1)–(6 × 6)Au with up to 4.2 ML of annealed Pb. The experiments show the strong influence of used substrates on the behavior of the conductance during the epitaxy of Pb atoms, especially for very initial stage of growth. Oscillations of the conductance during the layer-by-layer growth are correlated with RHEED intensity oscillations. The analysis of the conductance behavior is made according to the theory described by Trivedi and Aschcroft [N. Trivedi, N. Aschcroft, Phys. Rev. B 38 (1988) 12298].

MBE-grown high-efficiency GaInAsSb mid-infrared detectors operating under back illumination

This paper describes molecular beam epitaxial growth, processing and room temperature characterization of lattice-matched GaInAsSb mid-infrared detectors on GaSb substrates for room temperature operation. For the first time, we demonstrate GaInAsSb detectors operating under back-illumination, a critically important geometry for flip-chip-mounted focal plane arrays, and achieve performance equal or superior to front-illuminated detectors. Very high quantum efficiency and flat spectral response are achieved for the back-illuminated detectors due to improved carrier collection efficiency, photon recycling and reduced carrier recombination. In situ RHEED intensity oscillations and post-growth XRD are used for coarse and fine tuning of GaInAsSb lattice matching, respectively.

A new version of a computer program for dynamical calculations of RHEED intensity oscillations

We present a new version of the RHEED program which contains a graphical user interface enabling the use of the program in the graphical environment. The presented program also contains a graphical component which enables displaying program data at run-time through an easy-to-use graphical interface. New version program summary Title of program: RHEEDGr Catalogue identifier: ADWV Program summary URL: http://cpc.cs.qub.ac.uk/summaries/ADWV Program obtainable from: CPC Program Library, Queen's University of Belfast, N. Ireland Catalogue identifier of previous version: ADUY Authors of the original program: A. Daniluk Does the new version supersede the original program: no Computer for which the new version is designed and others on which it has been tested: Pentium-based PC Operating systems or monitors under which the new version has been tested: Windows 9x, XP, NT Programming language used: Borland C++ Builder Memory required to execute with typical data: more than 1 MB Number of bits in a word: 64 bits Number of processors used: 1 Number of lines in distributed program, including test data, etc.: 5797 Number of bytes in distributed program, including test data, etc.: 588 121 Distribution format: tar.gz Nature of physical problem: Reflection high-energy electron diffraction (RHEED) is a very useful technique for studying growth and surface analysis of thin epitaxial structures prepared by the molecular beam epitaxy (MBE). The RHEED technique can reveal, almost instantaneously, changes either in the coverage of the sample surface by adsorbates or in the surface structure of a thin film. Method of solution: RHEED intensities are calculated within the framework of the general matrix formulation of Peng and Whelan [1] under the one-beam condition. Reasons for the new version: Responding to the user feedback we designed a graphical package that enables displaying program data at run-time through an easy-to-use graphical interface. Summary of revisions: (1) In the present form the code is an object-oriented extension of previous version [2]. Fig. 1 shows the static structure of classes and their possible relationships (i.e. inheritance, association, aggregation and dependency) in the code. (2) The code has been modified and optimized to compile under the C++ Builder integrated development environment (IDE). (3) A graphical user interface (GUI) for the program has been created. The application is a standard multiple document interface (MDI) project from Builder's object repository. The MDI application spawns child window that reside within the client window; the main form contains child object. (4) We have added an original graphical component [3] which has been tested successfully in the C++ Builder programming environment under Microsoft Windows platform. Fig. 2 shows internal structure of the component. This diagram is a graphic presentation of the static view which shows a collection of declarative model elements, such as classes, types, and their relationships. Each of the model elements shown in Fig. 2 is manifested by one header file Graph2D.h, and one code file Graph2D.cpp. Fig. 3 sets the stage by showing the package which supplies the C++ Builder elements used in the component. Installation instructions of the TGraph2D.bpk package can be found in the new distribution. (5) The program has been constructed according to the systems development live cycle (SDLC) methodology [4]. Typical running time: The typical running time is machine and user-parameters dependent. Unusual features of the program: The program is distributed in the form of a main project RHEEDGr.bpr with associated files, and should be compiled using Borland C++ Builder compilers version 5 or later.

Design and development of an ultra-compact drum-shaped chamber for combinatorial pulsed laser deposition

We have designed a compact combinatorial pulsed laser deposition (PLD) chamber as a building block of a desktop laboratory for advanced materials research. Development of small-size systems for the growth and characterization of films would greatly help in interconnecting a variety of analytical tools for rapid screening of advanced materials. This PLD chamber has four special features: (1) a drum-shaped growth chamber, (2) a waterwheel-like combinatorial masking system, (3) a multi-target system having one feedthrough, and (4) a small reflection high-energy electron diffraction (RHEED) system. The performance of this system is demonstrated by the RHEED intensity oscillation during homoepitaxial growth of SrTiO 3 as well as by simultaneous fabrication of a ternary phase diagram of rare earth-doped Y 2O 3 phosphors.