33 Vapour and Solid state growth

Prof. P. N. Kotru

epgp books

 

TABLE OF CONTENTS

 

33.1 Growth from Vapour Phase

 

33.1.1 Introduction

 

33.1.2 Closed System Techniques

 

33.1.3 Example of Closed Systems

 

33.1.4 Different ways of Vapour Phase Growth

 

33.1.5 Basic Background of Vapour Phase Growth

 

33.1.5.1 Growth by Sublimation

 

33.1.5.2 Growth by Chemical Reaction

 

33.1.5.3 Transfer Technique

 

33.2 Solid State Growth

 

33.2.1 Annealing Techniques

 

33.2.1.1 Strain annealing

 

33.2.1.2 Zone heating

 

33.2.1.3 Sintering and hot pressing

 

33.3 Thin Layer Growth

 

LEARNING OBJECTIVES

  • In this module we learn about the basic background of crystal growth from the vapor (gas) phase. There are two categories of this method of growth, one involving close-system techniques and the other system methods.
  • The closed system techniques are explained. Un-seeded growth of chemical vapour deposition and the arrangement used in this type of growth is described. Seeded crystal growth by chemical vapour deposition method is also described
  • Deposition from the vapour phase is a technique which is performed in the fabrication of this layer of metal, insulators and semiconducting materials.
  • Different ways of achieving growth from vapour phase include by (i) sublimation, (ii) vacuum evaporation, (iii) molecular beam epitaxy, (iv) temperature oscillation method,
  • (v) gas transport processes, (vi) halide transport processes,(vii)chemical vapour deposition and (viii)vapour phase epitaxy are explained.
  • Systems in vapour phase growth under physical vapour deposition process and those under chemical vapour deposition (CVD) process along with some examples are classified and this summary is given.
  • Basic background of vapour phase growth giving description of some fundamentals regarding important techniques like growth by sublimation, growth by chemical reaction and transfer technique is described
  • Growth from solid state involving – annealing techniques viz, obtain annealing, zone heating,sintering and hot pressing is explained, layer growth from solid phase also known as “solid state epitaxy” is also described.

 

33.1 Growth from Vapour Phase

 

33.1.1 Introduction

 

 

There are methods of growth in which the materials from which the crystals have to be grown are transported to the growth zone as volatile compounds which react or decompose to give the material and a by-product.

 

The methods involved fall in two categories: i) Closed-system techniques, and ii) Open-system methods.

 

33.1.2.    Closed System Techniques

 

In closed systems the by-product is recycled. Let us consider a typical equation of the

 

T+∆T 2OSolid + Y2gas → 2OYgas →  2Osolid +  Y2 gas

 

Here, O represents the material to be grown as crystal and Y is used as an agent for transportation. The superscript specifies the state of the various species , T represents the temperature of the nutrient zone,

 

T+ ∆T is the temperature of the growth zone which may be positive or negative depending on the sign of the heat of reaction .∆T is the temperature difference between the n utrient and growth zones. If Y2 diffuses in a closed system from the growth zone to the nutrient zone, only a small amount of Y2 is required.

 

If it is an open system there would be a flow of gas through the growth region, where reactions take place such as:

 

2OYgas→ 2Osolid + Y2gas

 

Or, 2OYgas + H2gas → 2Osolid + 2HY.

The material OY that is transported may be made in a nutrient region by passing Y2 over solid O, or may be obtained from any other source. In open systems the by-products are generally eliminated as waste, Reactions that are used in closed systems are reversible reactions whereas in open systems the reactions need not be reversible. However, the use of non-reversible reactions is often advantageous. If closed systems are used, the walls of the system are required to be heated to temperatures between T and ∆T whereas in open systems it is not necessary or may not be required to be heated and the reactions used may only take place on the heated substrates. Techniques for growing materials in crystalline form in this manner are put in the category of chemical vapour deposition abbreviated as CVD. Chemical vapour deposition processes are widely used to deposit polycrystalline and amorphous materials.

 

Bulk crystal growth in closed systems has not been a priority from the commercial point of view but is widely used in research laboratories for obtaining crystal samples for research studies. The use of open systems for the growth of bulk crystals has not received much of response.

 

33.1.3 Example of Closed System.

 

We may take up the example of producing material samples for research where use is made of a halide transport process in a sealed silica tube. In this case the material, of which the growth as crystal is desired, is placed in a tube along with enough of halide so as to produce one atmosphere pressure at the growth temperature. The tube is sealed and then put in the multi-zone furnace. One end of the sealed tube contains nutrient which is heated to a higher temperature than the other end at which growth of the material as crystal is desired. In the case of materials having higher melting points, the nutrient zone is maintained at about a temperature which is 50 to 150°C more than the temperature at the growth zone which may be around 800 to 1200°C. It may take around 6 to 40 hours before one succeeds in the growth of hardly few small crystals. This is known as unseeded growth of chemical vapour deposition and the arrangement is as shown along with temperature profile in figure 33.1

Figure 33.1: Example of closed system using CVD

 

A better production method is to make use of seeded chemical vapour deposition technique of growth as is shown in a schematic diagram of of figure 33.2.In this arrangement the seed is used in a furnace with temperature gradient as shown. Here, the nutrient is placed in the lower  end which is kept hotter and the seed is placed at the upper end which is a cooler region.

 

33.1.4 Different Ways Of Vapour Phase Growth.

 

Deposition from  the  vapour  phase   is  a  technique  which  is  preferred  in  the  fabrication of thin layers of metal , insulators and semiconducting materials. There are different ways of achieving growth from vapour phase which may be put as under:

 

Growth by sublimation

 

Growth by vacuum evaporation

 

Growth by molecular beam epitaxy

 

Growth by temperature oscillation method

 

Growth by gas transport processes

 

Growth by halide transport processes

 

Growth by chemical vapour deposition

 

Growth by vapour phase epitaxy

 

Figure 33.2: Schematic diagram showing arrangement for seeded crystal growth by chem ical vapour deposition method alongwith the temperature gradient maintained in the system. Cool region is above the hot region

Physical vapour depos ition (abbreviated as PVD) and chemical vapour deposition ( abbreviated as CVD ) are both used widely for the deposition of polycrystalline oxide and metal films on parts of any assembly for improvement of mechanical and structural properties, corrosion and abrasion resistance. It becomes possible on account of the fact that vapour phase deposition can be done on irregular-shaped substrates, inside surfaces and besides offering maximum control of materials properties such as thickness and composition.

 

The vapour phase process has a slow growth rate averaging less than 10 ─4 cm per minute for single crystal growth whereas it is about 10─2 cm per minute for growth in case of growth from molten phase (melt growth). However, better perfection and control on thickness, composition, stoichiometry and doping in vapour phase epitaxial growth (VPE growth) makes it superior for the growth of “active” layers which actually perform electronic functions. Vapour phase techniques are also very useful in the deposition of metal films required for solid state device and integrated circuit fabrication

 

There are a variety of vapour phase systems which have been developed. However, two major categories are PVD and CVD. While PVD category involves both vacuum evaporation and sputtering . In case of vacuum evaporation, the required thermal energy is supplied by resistance or electron beam heating of the source. In the case of sputtering, the source material is transformed into the vapour phase by ion bombardment of a source electrode (i.e., the target).

 

There are so many variants of growth from vapour phase that it may be beyond the scope of this text to provide details of all of them. However,it is better to be knowledgeable about the title of various types of growth from vapour phase which may be summarised here alongwith some of the several crystals that have been grown.

 

Systems under vapour phase growth are indicated in figures 33.3(a & b). The relevant details are summarised in the form of charts in these figures. The figures provide examples of most common transport agents and deposition of materials. While this cannot be claimed to be an exhaustive list, it does serve the purpose of providing some initial information regarding usefulness of the techniques.

 

Figure 33.3(a): Number of systems in vapour phase growth under PVD process

 

Figure 33.3(b): Number of systems in vapour phase growth under CVD process

 

33.1.5. Basic Background of Vapour Phase Growth.

 

The basic background of growth from the vapour phase (gas phase) may be summarily described here. Materials which decompose or sublimate before melting at atmospheric pressure and for which there is no suitable solvent, can often be grown from the vapour phase. In comparison with other growth techniques this method is much complicated for the growth of large crystals. Supersaturation can be relatively controlled more easily as compared to growth from melt. The growth process can be defined in three stages:

 

(i)      Transport of material .

(ii)    Surface reaction.

(iii)   Dissipation of heat

 

 

A very brief description of some fundamentals regarding important techniques of crystal growth from vapour phase may be given here.

 

33.1.5.1 Growth by Sublimation

 

In this method, the substance is converted directly from solid to the gas (via liquid) phase. The substance from the gas phase is now converted back to the solid phase without intermediate liquid state. This process is frequently carried in high vacuum and heating or cooling of the said crystal or area where the growth initiation is desired, is often required. Formation of ice crystals in nature occurs in this way.

 

 

33.1.5.2. Growth by Chemical Reaction

 

In the growth from gas phase, using chemical reaction, the material to be grown is prepared right in the growth region. For example, in the formation of silicon carbide (SiC), a volatile silicon containing substance  such as silicon tetrachloride ( SiCl4 ) and a carbon containing substance such as toluene (C7H8 ) are each vaporised in a separate inert gas stream and brought together in a heated zone , where the two react giving silicon carbide (SiC) and other products. An important application of this technique is the epitaxial growth of silicon.This is obtained from silicon tetrachloride upon reaction with hydrogen gas as per this equation:

 

SiCl4 + 2 H2 →   Si + 4 HCl

 

The silicon so obtained is made to deposit an additional layer of silicon on a heated silicon crystal, giving rise to the epitaxial growth. Such silicon crystals are used in the construction of transistor devices.

 

 

33.1.5.3.   Transfer Technique

 

This is also an important process of crystal growth from gas phase. The technique is performed in a closed and sealed quartz tube with silicon feed material at one end maintained at 1100⁰ C and silicon seed crystal at the other end maintained at 900⁰ C. A small amount of iodine is added before the tube is evacuated and sealed. Iodine reacts with seed silicon as:

 

Si + I2  → SiI2

 

At the lower temperature of the seed region SiI2 decomposes to form SiI4 with the growth of crystal as:

 

2 SiI2    → SiI4  + Si

 

The product SiI4 moves back to the feed region to produce more SiI4  by reverse reaction:

 

SiI4 + Si → 2 SiI2

 

Growth by this process is very slow and also difficult to control.

 

Greenockite (CdS), SiC and Wurtzite (ZnS) have been grown by this technique. These techniques have been frequently used for the growth of organic crystals, metal crystals and semiconductor materials such as silicon.

 

Chemical transport methods are widely used for the growth of semiconducting, insulating and magnetic crystals, using closed tube arrangements. As for example, crystals of most admantine compounds could be grown in sealed quartz ampoules. About 5 mg. per c.c. of I2 are added to a powdered charge of the compound in vacuum. Crystals grow at the cold end of the tube as shown in figure 33.4

 

 

Figure 33.4: Chemical transport method using Iodine vapour to transport compounds containing non-volatile metals.

 

In this method iodine transports the non-volatile metal atoms by formation of an iodide which is volatile at low temperature end which then decomposes in the region at higher temperature. The whole process is carried out in a directional or a two-zone furnace. The tube is tilted by around 10⁰ C to promote convection. Though the closed tube methods are very useful in research but they are very costly for production. Open tube methods are preferred for production purposes, using gas phase reactions.

 

33.2 Solid State Growth

 

Growth from solid state is because of atomic diffusion which is very slow in many cases. However, in certain exceptional cases involving superionic materials small cations are quite mobile. On account of its limitations solid state growth techniques are not very commonly used, especially when there is an option for growth by other techniques. The metallurgical processes including annealing, heat treatment, sintering and quenching assume great importance in tailoring the properties of materials.

 

The growth of single crystals either from polycrystalline or amorphous solids has certain advantages and some disadvantages.

 

The advantages are:

i)  Its simplicity.

ii) It is necessary to anneal the sample

iii) Possibility of avoiding contamination as no solvents are used and the sample has very limited areas of

contact with foreign materials.

 

However, it has certain disadvantages which may be summarised as follows: i) It lacks reproducibility, ii) There are difficulties in attaining high perfection materials. iii) Some crystallographic defects are unavoidable inspite of all care in processing.

 

Prior to development of techniques like float zone refining and cold crucible techniques, solid phase growth was being extensively used in the production of pure single crystals of metals .

 

33.2.1 Annealing Techniques

 

There are two main techniques:

 

(i) Strain annealing and

(ii)  Zone heating

 

33.2.1.1 Strain annealing

 

Cold-worked metals show more grain growth than unstrained samples. It is observed that a critical strain induces nucleation and growth of new grains -a process known as secondary re-crystallization. Below the critical strain only primary crystallization takes place. Above it several new grains nucleate and grow. Temperature gradient strain annealing, using a two-zone furnace, has been successfully used in causing one grain to grow down specimens of copper, aluminium, lead and tin. This technique is named as “grain boundary migration technique”.

 

33.2.1.2  Zone heating

 

By this technique metals like tungsten and molybdenum in the form of single crystal wires are produced. Figure 33.5 is a zone heating arrangement. The wire is electrically heated in vacuum and a small travelling heater passes a hot zone up and down the wire. In this way, whole length of the metal can be grown into a single crystal.

 

33.2.1.3 Sintering and hot pressing

 

It involves the process of annealing of a pre-compressed powder known as sintering. The process of annealing of a powder under pressure known as hot pressing is also used here. Both these processes are used for homogenization of alloys and grain growth. Both these techniques have been used in metallurgy for production of ingots and shaped components of difficult alloys. However, these are complicated processes but do not offer any scope for the growth of good crystals.

 

33.3 Thin Layer Growth

 

The growth of thin layers from solid phases, popularly known as solid-state epitaxy is a technique which has recently generated a lot of interest. Both amorphous and polycrystalline materials can be crystallised at high rates. Ion implantation is used to make semiconductors amorphous which results into formation of layers having high resistivity. These layers are selectively recrystallized by using energy beams, resulting into conducting patterns.

 

One is able to achieve growth of nearly isolated crystalline layers by using standard lithographic and etching process to create a hole in oxide on a particular substrate followed by depositing amorphous or polycrystalline material from plasma. A stripe of crystal is grown by starting at the hole site and traversing a laser beam. The process is shown in figure 33.6. Here, the figure shows an oxide layer on a crystalline substrate. Amorphous or polycrystalline film deposition is then made. A stripe of crystal is then grown by using a laser beam. The process is sequentially shown in the figure.

Figure 33.5: Schematic diagram showing zone-heating arrangement for the production of single crystal wires.

 

Figure 33.6: Schematic illustration of the growth of thin layers from solid phase; the process shownin sequential order

 

SUMMARY

  • In this module we learn about the basic background of crystal growth from the vapor (gas) phase. There are two categories of this method of growth, one involving close -system techniques and the othe r system methods.
  • The closed system techniques are explained. Un-seeded growth of chemical vapour deposition and the arrangement used in this type of growth is described. Seeded crystal growth by chemical vapour deposition method is also described.
  • Deposition from the vapour phase is a technique which is performed in the fabrication of this layer of metal, ins ulators and semiconducting materials.
  • Different ways of achieving growth from vapour phase include by (i) sublimation, (ii) vacuum evaporation, (iii) molecular beam epitaxy, (iv) temperature oscillation method,(v) gas trans port processes, (vi) halide transport processes,(vii)chemical vapour deposition and (viii)vapour phase epitaxy are explained.
  • Systems in vapour phase growth unde r physical vapour deposition process and those under che mical vapour deposition (CVD) process along with some examples are classified and this summary is given.
  • Basic background of vapour phase growth giving description of some fundame ntals regarding important techniques like growth by sublimation, growth by che mical reaction and transfer technique is described.
  • Growth from solid state involving – annealing techniques viz, obtain annealing, zone heating,sintering and hot pressing is explained, layer growth from solid phase also known as “solid state epitaxy” is explained.
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References.

 

1.Pamplin, B.R.: “ Crystal Growth”,Pergamon press,Oxford, 1980.

2.Brice,J.C.: “ Crystal Growth Processes”, Blackie Halsted Press,London, 1986.

3.Stringfellow, G.B.: “ Vapour Phase Growth” in “ Crystal Growth”, Ed.Pamplin ,B.R Pergamon Press, Oxford,

1980.

4.Gilman, J.J.: “ The Art & Science of Growing Crystals”, John Wiley,N.Y.,1963.

 

Suggested Reading.

  1. Austerman,S.B.: “ Single Crystal Growth by Vapour Transport Processes” in “ Crystal Growth & Characterization”ed, Ueda,R&Mullin,J.B.,North-Holland Pub.Co.Amsterdam, 1975.
  2. Schafer,H. : “ Chemical Transport Reactions”, Academic Press,N.Y.,1964.
  3. Powll,C.F.;Oxley,J.H. &Blocker,J.M.:” Vapour Deposition”,Wiley,N.Y.,1967.