Giovanni Attolini Technical Aspects on Crystal Growth from Vapour Phase

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3 Giovanni Attolini Technical Aspects on Crystal Growth from Vapour Phase Bulk Crystals, Epitaxy and Nanostructures

4 Copyright MMXV ARACNE editrice int.le S.r.l. via Raffaele Garofalo, 133/A B Roma (06) isbn No part of this book may be reproduced by print, photoprint, microfilm, microfiche, or any other means, without publisher s authorization. 1 st edition: June 2015

5 9 To my grandson Diego and my family

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7 Index 11 Foreword 15 Chapter I Bulk crystals Part 1: Synthesis 1. Introduction, Growth techniques, From melt, From crystallization and precipitazion, From vapour phase, Sublimation, Open tube, Closed tube, Chemical vapour transport, In open tube, In closed tube, Treatment of commercial compounds, Evaporation of the element in excess, Compensate the element in deficiency, Ampoule moving vertically method, Compounds grown with sublimation and CVT, II-VI compounds, Diluted magnetic semiconductors, Other families, Complete procedure for CVT growth, 54 Part 2: Apparatus 1. Type of the ampoule for closed tube procedure, Vacuum system, How to proceed to the evacuation, Furnaces, Two zones furnace, One zone temperature, Vertical one zone furnace, Temperature controllers, Supports, 75 References, Chapter II Vapour phase epitaxy 1. Introduction, Vapour phase techniques to grow thin film, Chemical vapour deposition (CVD), Isothermal vapour phase epitaxy (ISOVPE), Hydride vapour phase epitaxy (H-VPE), Metal-organic vapour phase epitaxy, Atomic layer deposi- 7

8 8 8 Index tion, Common components of the deposition apparatus, Gas panel for inert gases (N 2, Ar, He), Gas panel for H 2, Panel for toxic gases, Gas metering, Gas line for metal-organic precursors, Toxic gas line, Growth chambers (reactors), Heating systems, Resistance heater, Induction heater, Infrared system, Safety, Exhausts treatment, Automation, How to built simple CVD apparatus, Furnace and electronic control, Growth tube, Gas metering, How to use it, How to built a MOVPE apparatus, Gas metering, Growth chamber, Cold wall configuration, Hot wall configuration, Heating system, Exhausts treatment, Control and safety, 138 References, Chapter III Nanowires 1. Introduction, Preparation methods of nanowires, Vapour liquid solid (VLS), Vapour solid (VS), The case of silicon carbide, Core-shell SiC/SiO 2 nanowires, SiC nanowires, 151 References, Chapter IV Applications (Growth of germanium: bulk crystals, layers and nanowires) 1. Introduction, Germanium bulk crystals, Ge:I 2 system, Germanium epilayers, Germanium deposition, Growth procedure, Homoepitaxial germanium (Ge/Ge), Ge/GaAs heterostructures, Ge/Si heterostructures, Germanium nanowires, Growth method, Growth procedure, 171 References, Conclusions 181 Appendix Some aspects of chemical vapour deposition 1. Thermodynamic aspects, Thermodynamic analysis of the gaseous composition, Phase diagram od GaAs, Driving force in a phase transition associated with a heterogeneous reaction, Chemical reaction for the system GaAs/TMG/AsH 3 /H 2, Fluididynamic, Calculation of the growth rate with local equi-

9 Indice Index 9 9 librium at the interface, Diffusion in CVD and MOCVD reactors, Models for the horizontal CVD reactors, Application of the diffusive model to the MOCVD process, Kinetic aspects, Surface processes, 197 References, Acknowledgements

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11 Foreword The crystallization from vapour phase is very attractive because it can be used for the preparation of bulk crystals, layers and nanostructures of different materials from metals to semiconductors. Typically bulk crystals (size depends on the amount of starting material) are prepared by vapour phase in closed systems. If the vapour pressure of the chemical compounds is enough, may be used in physical vapour transport (PVT, physical vapour transport) sublimationcondensation. If the vapour pressure is too low with the use of a transport agent (i.e. an element that react with the components of the starting compounds), volatile compounds are generated to allow to transport the material from a source to a deposition zone to form the desired crystal (CVT, chemical vapour transport). In order to get layers, an open system (PVD, physical vapour deposition) is used almost exclusively where a substance sublimes and deposited on a substrate and/or CVD (chemical vapour deposition) where a mixture of reactants decompose and recombine on a substrate to give the desired compound with everything under the action of a carrier gas. This book is divided into four chapters, the first describes the growth of bulk crystals by chemical vapour phase transport (CVT) and sublimation of compounds of II-VI, II-III 2 -VI 4, II(M)-III 2 -VI 4 diluted magnetic semiconductors (DMS) families where M is a magnetic element; the second concerns on the growth of epitaxial layers by vapour phase epitaxy of silicon carbide. In both the cases, an initial discussion of the more general aspects such as to give the main notions, introduction to the techniques followed by a detailed description of the experimental apparatus. The third chapter describes the preparation of nanowires from general point of view by CVD technique for silicon carbide. The last chapter of the book is dedicated to demonstrate how the vapour phase technique can be applied to prepare bulk crystals, layer and nanowires of the same material with the case of germanium taken as an example. 11

12 12 Foreword In this publication, the author write about the growth from vapour phase of semiconductor materials both in bulk, layer and nanowire forms. The classes of bulk materials are of the II-VI, II-III 2 -VI 4 and DMS families prepared by CVT or sublimation; while for epitaxial layers the VPE technique is described that includes the metal-organic vapour phase epitaxy (MOVPE) of SiC, germanium and III-V s compounds. In addition, a comprehensive description of the experimental apparatus and many construction details make the reader able to apply these techniques to grow materials in different forms. The choice of the class of materials is from the direct experience of the author in many years of his work; generality with which they will be described to make it applicable in many other materials. For the above reasons, the book contains figures taken from the laboratory notebook and inserted in the text without graphical changes but translated from Italian to English. The apparatus described are realized at IMEM Institute (former MASPEC) along with the grown materials. It describes the growth of bulk crystals by chemical vapour phase transport (CVT) and sublimation of compounds of the II-VI, II-III 2 -VI 4 and II(M)-III 2 -VI 4 diluted magnetic semiconductors (DMS) families where M is a magnetic element. The growth techniques considered can be summarized as follow: Sublimation Open tube Closed tube Chemical transport Open tube (Bulk crystals, Epitaxy) Closed tube (Bulk crystals)

13 Foreword 13 Epitaxy (layers) Direct synthesis Chemical vapour deposition (layers, nanostructures, coatings) The purpose of this book is to present the vapour phase growth technique in all its aspects from general point of view to more technical. The reader should be able to apply completely this technique by the choice of material with appropriate procedure.

14 13 Preface

15 Chapter I Bulk Crystals Part 1 1.Introduction The growth of crystals is of interest in semiconductor technology; silicon, germanium, III-V and II-VI's compounds represent the most studied materials, however, also other materials are interesting for their physical properties. The technique used for growth bulk crystal is from melt for those compounds that have a relatively low melting point. While for the compounds that have melting points much higher (and often not easily obtainable under normal pressures ) is used the vapor phase method. The vapor phase methods are used for materials that sublime at low temperatures and for materials which decompose before melt. Here I will describe the sublimation and chemical transport methods. The compounds considered are those belonging to the families II-VI, II III 2 - VI 4 and diluted magnetic materials. In this chapter I will describe the two methods from the fundamental point of view with examples and descriptive of the basics to growth from the vapor phase. The section on experimental setups gives you all the details of construction of the necessary elements for growth from the vapour phase. A crystal can be defined as a solid having the atoms arranged with specific geometry repeated in the space; however, structural defects can be present. Single crystal is a material that has the crystal lattice repeated in all the dimensions without any grain boundaries or twins that can modify their properties; polycrystal is an aggregation of many crystals separated by grain boundaries

16 16 22 Forme Technical canoniche aspects e on funzioni crystal growth di matrici from vapour phase Bulk crystals are defined as a crystal with large volume with a final mass proportional in a reproducible way to the mass of the starting charge [1]. The most interesting bulk crystals are semiconductor compounds, metals, oxides, organic compounds and ceramics. The most used techniques to grow the above materials are from melt and vapour phase methods, few materials are grown from crystallization. 2. Growth techniques 2.1 From melt Many techniques have been developed for growing crystal from melt. These techniques are easy to use when the melting point of the material (elements or compounds) is not high and the compounds do not decompose before reach the melting point. When the melting point is high and there is decomposition, high pressure is necessary to avoid it and also the growth furnace can become complicated. In practice, the developed melt techniques take into account of the physical and chemical properties of the materials (semiconductors, oxides, metals, organic compounds, ceramics etc ); for example the pulling method is applicable for semiconductors, oxide and ceramics; vertical Bridgman for oxides and ceramics; zone melting and horizontal Bridgman for semiconductors and metals. The suitability of the above mentioned techniques is influenced by chemical reactivity, volatility (or instability) and the melting point of the materials [2, 3]. 2.2 From crystallization and precipitation [4] Crystallization and precipitation are widely used in chemical laboratories as a mean of preparing different class of materials. Many compounds, including organic, inorganics and polimers are obtained by reaction which occur in liquid phase contained the required components.

17 1. Richiami sull agebra 1. Nanowires Bulk delle crystals matrici Crystallization from these reactions allows the separation of products in convenient solid forms (crystals). The important properties of these products can be dependent in composition, crystal habit, sizes and purity. A solution (solute and solvent) containing product is fed to the container (named crystalizer) and is supersaturated by means cooling, evaporation of the solvent or a combination of the two, depending on the properties of the chemical system [5]. 2.3 From Vapour Phase When the melt method is not practicable because the high melting point (it can not be reached at atmospheric pressure) of the compound, we need to use the vapour phase approach, it permits to growth bulk crystal more easily. The vapour phase methods generally are used to growth material with high melting point, but sublimate at lower temperature and if decomposes at the melting point. To obtain a crystal from vapour phase (crystalline solid phase) starting from a source (polycrystalline solid phase) it must through a mobile phase (vapour phase) constituted by the elements of the starting polycrystalline solid phase. From the mobile phase you get the formation of the crystal only in presence of a chemical potential difference between the components of the mobile phase and the crystal. The transition from the equilibrium to the metastable phase is defined as oversaturation, obtained by varying the state parameters. When the oversaturation is to high many nuclei are formed which give a polycrystalline phase (many crystals); while to have few nuclei it is necessary to control the oversaturation with small value to have monocrystals. With defined experimental growth conditions there is a little range where it is possible to control the growth regime, this regions is called Ostwald-Miers region [5/A,6]. The crystal growth from vapour phase offers the possibility to obtain large crystals taking advantage of the change from the solid phase to vapour simply by sublimation (PVD = Physical Vapour Deposition) or by reaction of the solid phase with a chemical

18 18 24 Forme Technical canoniche aspects e on funzioni crystal growth di matrici from vapour phase compound or an element to react and form volatile species to give a solid phase deposition (CVD = Chemical Vapour Deposition). It will be considered below the following cases: - Sublimation (in open and closed tube) - Chemical vapour transport (in open and closed tube) - The two cases above but moving the ampoule To utilize the above methods it is necessary to arrange the experimental apparatus that will be explained in the second part of this chapter Sublimation The sublimation is possible when a compound evaporates as molecule or it thermally decomposes congruently (elements are in the vapour phase in the same ratio as in the compound) to give in the deposition zone the starting compound in the crystal form. Starting from a polycrystalline materials (source) contained in a closed ampoule or in an open tube under a flow of inert gas to the effect of temperature gradient from the source (T s ) and the deposition zone (T d ), the vapours crystalize to give the crystals. The mass transfer occurs by diffusion and/or thermal convection of the gaseous species formed by sublimation. In the closed tube we have diffusion contribution and thermal convection in the open tube Open tube The open tube configuration consists in a long quartz tube where we have sublimation under a gas flow inside. In figure 1 we can see a quartz tube with connections to introduce the carrier gas (inlet) and the outlet of the exhaust gases prior to release into the atmosphere. The connections can be made simply in rubber stopper inserted with a quartz tube connected with the gas source by means plastic tube.

19 1. Richiami sull agebra 1. Nanowires Bulk delle crystals matrici Figure 1. Simple example of the open tube set-up. In a more sophisticated set-up the connections can be made in stainless steel suitably carried out in the workshop on design to fit the tube as shows in figure 2. Figure 2. Stainless steel connections for quartz tube. A one zone tubular furnace is used in this experiment, the temperature profile must permit the right temperature for the source, the crystals will growth at lower temperature on the tube wall, figure 3.

20 20 26 Forme Technical canoniche aspects e on funzioni crystal growth di matrici from vapour phase Figure 3. Schematic view of an open tube system with related temperature profile Closed tube When sublimation has done in closed tube it is very important to use as starting material a compound with very well controlled and precise stoichiometry. If commercial compounds are used it is necessary a pre-treatment to adjust the stoichiometry, see specific section in this chapter. The treated powder is placed in a quartz ampoule prepared with selected diameter ranging from 10 to 20 mm, 180 mm in length. A quartz tube is worked by melt using oxy-propane flame: first, the tube is closed in one side then it is restricted at a distance of 180 mm; this permit to seal it easily using the flame. The ampoule is filled with the starting material, evacuated using a special vacuum equipment and sealed, figure 4a. The ampoule is placed in a two zones furnace for having the source at the temperature T s and deposition zone at T d with T s >T d, see figure 5.