Growth Of Multilayer Structures Research Articles

Material based on multilayer periodic structures with GeSiSn pseudomorphic layers

The growth of multilayer structures with Ge0.3Si0.7-yGey/Si heterojunction at tin content from 0 to 18% was studied. The X-ray diffractometry method shows the presence of strict periodicity of layers and a high level of tin content. It has been established that GeSiSn compounds are thermally stable in the annealing temperature range of 300-550 °C. A photoluminescence signal in the infrared range of about 3 microns is observed from a structure with pseudomorphic GeSiSn layers.

Surfactant controlled interface roughness and spin-dependent scattering in Cu/Co multilayers

In this work, we studied a correlation between spin-dependent scattering and interface roughness in Cu/Co multilayers. In Cu/Co multilayers, Cu-on-Co and Co-on-Cu interfaces are not identical because of significant difference in the surface free energy of Cu and Co. By adding Ag surfactant during the growth of multilayer structure, we found that the difference between the interface roughness of two interfaces can be minimized. By doing this we found that the strength of anti-ferromagnetic coupling increases significantly. A comparison between normalized magnetoresistance to antiferromagnetic fraction with interface roughness shows the variation of spin-dependent scattering at the interface in Cu/Co multilayers.

Electrodeposition of CuInSe2 layers using a two-electrode system for applications in multi-layer graded bandgap solar cells

In order to develop low-cost, large area multi-layer graded bandgap solar cell structures, CuInSe2 layers were grown using a simplified two-electrode system. Atomic force microscopy (AFM) shows the layers grown consist of nano- and micro-size particles. Photoelectrochernical (PEC) cell, X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) measurements confirm that it is possible to grow CuInSe2 layers with p-, i- and n-type electrical conduction, as pre-determined for applications in multi-layer device structures. XRF, XPS and PEC measurements show that Cu-richness provides p-type conduction and In-richness provides n-type conduction in electrodeposited CuInSe2 layers. It is also possible to grow materials with different bandgaps in the range 1.00-1.90eV. The combination of these two properties allows growth of multi-layer structures and preliminary work on these devices show good rectifying properties and exhibit photovoltaic activity. These new developments will be presented in this paper. (c) 2006 Elsevier B.V. All rights reserved.

Zincblende CrSb/GaAs multilayer structures with room-temperature ferromagnetism

CrSb (1 ML)/GaAs (5 nm) multilayers with period up to 4 have been grown on GaAs substrates by solid-source molecular-beam epitaxy at 250°C. Reflection high-energy electron diffraction reveals zincblende characteristics throughout the growth of multilayer structures. High-resolution cross-sectional transmission electron microscopy also indicates that the crystal structure of the multilayers is zincblende and with no dislocations at the interfaces. The presence of room-temperature ferromagnetism is confirmed by magnetization measurements.

Oxide Electrodes for Buried-Channel Field Effect Transistors

ABSTRACTIn this paper we describe the fabrication of oxide based electrodes that allow epitaxial growth of multilayer structures used to fabricate buried oxide-channel field effect transistors. The distinct characteristic of our buried electrodes is that they provide an etch stop layer which allow the opening of vias through the gate oxide using chemical etching. They can be patterned to define 1μm channel lengths and exhibit low contact resistance with channel materials such as YxPr1−xBa2Cu3O7-σ (YPBCO) or YBa2Cu3O7σ (YBCO).

Spectroscopic ellipsometry: a historical overview

The historical development of spectroscopic ellipsometry (SE) from the very early stages to the present will be briefly reviewed. It is shown that SE is a truly powerful technique of great interest and use to physicists, chemists, electrochemists, electrical and chemical engineers, etc., as will be evident from the following few of the many reasons. (a) Firstly, the full spectra of the ellipsometric parameters Δ and Ψ as a function of wavelength from UV to IR can now be determined with a high degree of precision and accuracy in a few seconds. (b) Such data can also be processed to provide (i) the most accurate values to date of the dielectric functions (i.e. the real and the imaginary parts of the optical dielectric constant as a function of wavelength) of semiconductors, metals and even wide band gap materials available only as thin films; (ii) depth-profiles of interfaces, thin films and multilayer structures with almost atomic resolution; (iii) the composition for any layers (bulk, interface, or surface) that are composites or alloys; (iv) the microroughness of the surface layer; and (v) the true near-surface temperature of the samples in their preparation chamber in the case of semiconductors. Furthermore, it will be shown that the above results obtained by SE are reliable and trustworthy, by the excellent corroboration with the results of XTEM, RBS and AFM studies on the same multilayer structures. In real-time spectroscopic ellipsometry (RTSE), which has just been developed and perfected, most of the above capabilities of SE can be achieved again through analysis of data collected in a matter of a few seconds, and hence RTSE is now ready for use in real-time monitoring and control during the growth of multilayer structures, thin films, etc. Thus the full potentialities and capabilities of this non-destructive, non-perturbing and non-invasive technique are yet to be realized.

Real-time control by multiwavelength ellipsometry of plasma-deposited multilayers on glass by use of an incoherent-reflection model

Real-time control by multiwavelength phase-modulated ellipsometry (PME) of the growth of multilayer structures deposited on transparent glass is presented. The structures consist of plasma-deposited SiO(2) and SiN(x) stacks. A model that takes into account incoherent reflection in the substrate is described and tested. A generalized feedback control method that incorporates the incoherent modeling of the transparent substrate is further applied to the growth of a Fabry-Perot and a standard quarter-wave filter. The resulting optical coatings characterized by spectroscopic PME and transmission measurements show a reproducible precision, with less than 1% error between target and measured spectral responses.

An in-situ laser-light scattering study of the development of surface topography during GaAs and In xGa 1 − xAs chemical beam epitaxy

The development of surface topography during the annealing of epi-ready (001) GaAs wafers and the subsequent CBE growth of GaAs and In x Ga 1 − x As layers were investigated using laser-light scattering (LLS) from a rotating substrate. The change in scattered light intensity during the pre-growth anneal is usually attributed to oxide desorption. However, we have found that a wide range of behaviour is possible depending on the growth history of the chamber. We suggest that this is due to the desorption of previously deposited species from the cryopanel as a result of radiant heating from the substrate, leading to contamination of the wafer surface. Procedures to minimise this contamination are outlined. LLS was used to study the changes in surface topography during growth, particularly the development of elliptical islands during the growth of GaAs and the onset of the “cross-hatch” pattern during the growth of InGaAs. It has recently been established that the ridges of the cross-hatch pattern are associated with dislocations which lie parallel to the surface. The development of these ridges parallel to 〈110〉 is related to the critical thickness of the layer. Interference oscillations in scattered light intensity during the growth of multilayer structures are also shown to be observable using LLS.

Selective area bandgap modification during MBE growth of InGaAs/GaAs quantum wells for mode locked laser applications

Strained layer InGaAs/GaAs quantum wells (QWs) have been grown by molecular beam epitaxy on (100) GaAs substrates patterned into ridges and grooves by wet etching. Indium migration off (311)A ridge sidewalls during growth locally increases the indium concentration in areas adjacent to the sidewalls. Aside from the ridge dimensions (i.e. width, sidewall angle and etch depth), the amount of excess indium incorporated into QWs on the ridges is determined by the indium mobility over the growth surface. We report, for the first time, an arsenic flux dependence of the indium mobility. Photoluminescence and photoluminescence excitation have been used to investigate the QWs and low temperature cathodoluminescence has been used for spatial mapping of the indium concentration with micrometer resolution across the pattern features. QWs on ridges and in grooves were observed to luminesce with the same efficiency. The novel observation that indium migration can be enhanced or suppressed by varying the arsenic flux makes possible the growth of multilayer structures in which the number of active layers can differ from area to area.

Spectral Reflectance as an in Situ Monitor for Mocvd

AbstractNear normal-incidence spectral reflectance has been used to monitor the growth of AlAs, GaAs, and AlGaAs thin films by MOCVD in real time. The method has been used in a research rotating disk CVD reactor and in a modified commercial horizontal channel reactor. Reflectance is simple, robust, and is insensitive to perturbations such as mechanical strains, wafer rotation, and imperfect windows commonly encountered in an actual MOCVD system. Data may be collected over a wide spectral range, 390 to 950 nm, in less than one second using a multichannel detector and broad-band light source. This wide bandwidth provides detailed compositional discrimination and greater thickness sensitivity than single-wavelength measurements. The technique may be used to extract wavelength-dependent optical constant data under growth temperature conditions. Examples from the growth of multi-layered structures used in the fabrication of Vertical Cavity Surface Emitting Lasers is presented. The method shows promise as a valuable tool in process modeling, optimization, and control.

In situ spectroscopic ellipsometry in molecular beam epitaxy

Noninvasive, real-time material growth monitoring is becoming increasingly important as epitaxial layer structures become more complex and the thickness and alloy composition tolerances are reduced. The technique of spectroscopic ellipsometry has been adapted to a commercial III–V semiconductor molecular beam epitaxy (MBE) system to monitor layer thickness, ternary alloy composition, and substrate temperature in real time during the growth of multilayer structures. Practical system considerations for reliably implementing the rotating analyzer ellipsometer in a hydride source MBE environment that contains a high pressure of dimeric group V species will be discussed. Measurement of substrate temperature from room temperature to typical growth temperatures will then be presented as well as in situ determination of alloy composition and thickness for efficient growth calibration. A growth run of GaAs/AlGaAs epitaxial layers will be examined to observe surface smoothing upon group III pulse deposition and growth interruption at heterointerfaces. The growth of quantum well structures is then monitored and analyzed in attempts to extract heterojunction interface abruptness. Finally, a summary of epitaxial reactor design issues that need to be addressed to implement the spectroscopic ellipsometry (SE) technique in commercial systems is provided.

Growth and properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on non-lattice-matched and polycrystalline substrates

The authors have investigated the nature of the in-situ growth of c-axis normal YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on non-lattice-matched, vicinal, and polycrystalline MgO substrates and on buffer layers of MgO. It is found that the preparation of the MgO surface determines the structural and transport properties of the films. In particular, the authors were able to reproducibly grow films exhibiting either weak link behavior or very high critical current densities. The variation of the thin-film microstructure, oxygen content, and superconductive properties with the changes in the thin-film crystal structure that result from the different growth situations is discussed. The structural and transport measurements of YBCO films grown on various MgO surfaces clearly indicate the importance of obtaining a nucleation stage which will result in high-quality YBCO on non-lattice-matched substrates. Such considerations are of prime importance when the low-temperature growth of the cuprate superconductors is required in order to obtain a morphology which facilitates the use of lithographic processing and the growth of multilayer structures.

Vapor Levitation Epitaxy—A New Concept in Crystal Growth

Bell Communications Research (Bellcore) scientists Herbert M. Cox, Steve G. Hummel and Vassilis G. Keramidas recently unveiled a revolutionary crystal growth process used in growing semiconductor materials of importance to many areas of fiber optic communications. The technique, called Vapor Levitation Epitaxy (VLE), is based on the concept of floating wafers while growing thin film crystalline layers on them. The method has produced exceptionally pure materials such as GaAs, InP, InGaAs, InGaAsP and GalnP. The capability of growing ultra high purity materials combined with the ability to grow ultra thin layers-a few atoms thick-makes the new technique uniquely powerful. In addition, exceptional uniformity of thickness and materials composition can be achieved across the whole area of two-inch diameter wafers. These capabilities are essential to the growth of multilayer structures needed for new applications in photonic and electronic devices and integrated circuits, and integrated opto-electronics for fiber optic communications. The VLE apparatus has two adjacent growth chambers capped with porous disks and a horizontal levitation track with a series of small gas jet holes along its center (see Figure 1). The gas jets suspend the wafer on a cushion of gas as it is moved between the wafer loading area and the growth region of the apparatus. Each growth chamber is supplied independently with vapors for the particular material to be grown. During growth, the wafer floats a few tenths of a millimeter above the chamber suspended stably by the growth vapors as

LPE GaAs for microwave applications

The liquid phase epitaxial (LPE) growth method has been successfully used to fabricate GaAs microwave device structures. General material requirements for the GaAs microwave devices currently of interest are reviewed in this paper. An LPE technology based on a multiple-well graphite slider boat, utilized for growth of multilayer structures, is described. Selection of dopants and control of unwanted background impurities are discussed in detail. Doping profiles and rf performance data on Gunn and Avalanche devices are presented to define the state-of-the-art in GaAs LPE capability.