The preparation of crystal for characterization consists of the following steps:
Let's start with clarifying the terms 'Crystal" and "Growth". The word "crystal" originates from the Greek κριος (coldness) or κριµος (ice). The father of crystal fabrication technology is A. Verneuil with his flame-fusion growth method he described in 1902.
What are crystals? Crystals are ordered arrangements of atoms (or molecules). Materials in crystalline form has special optical and electrical properties, in many cases improved properties over randomly arranged materials(also said to be amorphous or glassy)
What causes crystals to "grow"? The driving force for crystallization comes from the lowering of the potential energy of the atoms or molecules when they form bonds to each other.
The crystal growth process starts with the nucleation stage. Several atoms or molecules in a supersaturated vapor or liquid start forming clusters; the bulk free energy of the cluster is less than that of the vapor or liquid. The total free energy of the cluster is increased by the surface energy (surface tension), however, this is significant only when the cluster is small. A cluster of radius smaller than a critical radius, r*will evaporate (or dissolve in the solution) a cluster of radius greater than r* will become stable, will increase its size by the addition of other atoms and is thus "growing"! The critical radius r* also defines a critical energy barrier, DG, that we need to overcome in order to obtain a stable nucleus that will keep growing, eventually become a large single crystal!
Thermodynamics can help us describe the process. Assuming a spherical shape for the nucleus the free energy of its formation is:
DG = 4P r2 s + (4/3)P r3 )Gv
where DG is the total free energy; r is the radius of cluster; s is the surface tension; DGvis the free energy change per unit volume forming the stable solidification from vapor or liquid. The total free energy DG goes through a maximum DG*at a critical radius r* which can be obtained by derivation of total free energy as given above with respect to radius and solving:
(dDGo/dr) = 0
Two frequently used crystal growth methods are Physical Vapor Transport (PVT) and Bridgman methods.
PVT method is the crystal growth under vapor - solid equilibrium conditions. The temperature of the starting material (powder form) is higher than the nucleation/crystal growth region. This imposed temperature gradient leads to a mass flow resulting in a net mass transport of vapor species towards the crystal growth site. The vapor species may consist of molecules of the material itself, such as PbI2(solid) ® PbI2(vapor), or dissociated into its separate constituents, such as CdTe (solid) ® Cd(vapor) + ½ Te2(vapor), and residual gases. The reverse process occurs when vapor species nucleate and then continue to condense on the crystal growth interface at a rate of 3-5 mm/day. A typical PVT grown vanadium doped CdSSe single crystal is shown below currently being investigated for its optical properties.
The Bridgman growth method is basically a controlled freezing process taking place under liquid - solid equilibrium conditions. The growth also takes place under a temperature gradient, and the mechanism is to produce a single nucleus from which a single crystal will propagate and grow. This is achieved by allowing the solid - liquid interface to move slowly (5-50 mm/day) until the whole molten charge is solidified. A PbI2 single crystal is shown in the figure below.
Compared to other growth methods, Bridgman method is considered to be a rather simple crystal growth method, but several limitations still exist. The Bridgman method can not be applied to a material system which decomposes before it melts, systems having components with high vapor pressure, and materials exhibiting destructive solid - solid phase transformations which will compromise the crystalline quality on cooling the crystal at the end of the growth run. PbI2, lacking such a phase transition can also be grown by Bridgman method.
Thermal annealing is an important postgrowth treatment necessary to be performed before proceeding to further processing. For some crystals, whether optical or electronic, strains develop during growth and can be reduced by thermal annealing. During thermal annealing where the crystals are heated at a particular rate to a particular temperature, kept there for a particular time duration and then slowly cooled down to room temperature. Thumb rule for thermal annealing is to anneal at a temperature 2/3 of the melting point (taken in degrees Kelvin) of the material. At this temperatures the defects (dislocations) become mobile and the crystal "heals" itself Annealing can be done either in high vacuum or in the presence of inert atmosphere,i.e., in presence of argon or nitrogen atmosphere.
Many of the useful crystals consist of more than one component, ie. compound crystals, for example, CdS, CdSe, GaAs, InP etc. In the cases where the vapor pressure of one of the component is higher compared to the other, a deficit of the more volatile component occurs. To prevent such deviations from the stoichiometric composition we anneal such crystals in an atmosphere containing an excess of the component having the higher vapor pressure. For example CdS can be annealed in sulfur atmosphere, GaAs can be annealed in As atmosphere.
The growing interest in electronic and optical materials, has made this field one of the fastest growing areas of research. The preparation of semiconductors starts with the purification of the material down to impurity levels below 1 ppm (parts per million) and in some cases only ppt (parts per trillion) levels of impurity are allowed.
Typically, starting materials which may purchased from commercial vendors with nominal purity of 99.9999% or synthesized from pure elements. Zone-refining and vacuum sublimation are two standard processes to achieve this goal. Zone-refining was first introduced in 1952by Pfann, and has been successfully implemented to purify elements and compounds. The main principle is difference in solubility of an impurity in the liquid and solid phases of a material. The characteristic effect of zone-refining is to accumulate impurities at the ends of an ingot, thus leaving pure material in the central section. Vacuum sublimation is a routine method to purify starting materials from impurities having higher (under dynamic vacuum) or lower (closed tube) vapor pressures.
Principle of zone refining: A moving heater melts a zone of the material that can accumulate impurities. The impurities are "swept" to the right end of the ingot. The heater is next moved fast to the left (without melting the material) and the process is repeated several times.
As cut crystal surfaces are very rough. For any application whether it is electronic or optical, the crystal surface should be optically flat. When the surface roughness is about l/10, the surface is called optically flat (l is in the wavelength of the light for the particular application). To achieve an optically flat surface, the crystal needs to be polished in a special mounting device. There are few steps to polish the crystals. Crystals are first lapped in coarse carborundum (SiC) powder followed by lapping in fine carborundum powder. Next comes polishing in coarse alumina emulsion (3 micron), and gradually polished in fine (0.05 micron) alumina (Al2O3) emulsion. This procedure results a mirror finish optically flat surface.
All as-polished surfaces contain mechanically induced defects ("scratches") caused by the polishing particles. These defects affect the electronic properties of the device to be fabricated on this crystal. Prior to any device fabrication process, the polished surface is sometimes etched in order to remove the damaged/ defective layer produced during polishing process. Chemical etchant are commonly used for this purpose and different crystals have different etchants. Other than removing the damaged layer, etchants are also useful for revealing bulk defects present in the crystals. Some etchants attack preferentially defect sites on the crystal, revealing dislocations and other atomic arrangement faults. Some etchants are also used for determining the orientation of the crystal surface. Thus etching is a very powerful technique for removing the damaged layer, determining defect density of the crystal and also the orientation of the crystal surface.
Sputtering is a cold evaporation technique which employs pure metals, alloys and compounds to deposit a layer of material onto a suitable substrate. We can use RF sputtering (instead of the more straight forward DC sputtering), because some of the materials are insulating materials.
Sputtering as a process to deposit thin films dates back to 1852 and has found limited use until 1928 when it was used for such applications as coating gold onto early phonograph cylinder masters.
Sputtering utilizes a gas plasma (argon, neon, krypton or xenon) to remove material from a negatively charged source called the target and to deposit it as a thin film coating onto asubstrate.
Magnets (magnetron) are placed under the target to constrain the secondary electrons from leaving the plasma volume before they have had a chance to ionize a gas atom. In this way the ionization rate in the plasma volume is increased. There are many process parameters which can influence the physical and chemical properties of the coating. For instance, the film composition depends on the target composition, the ionized gas pressure, the substrate temperature, and the energetic particle radiation (photons as well as other particles). For instance, in an RF discharge configuration, not only the electrons, but also positive ions, produced in the bulk plasma, are accelerated across the plasma and may arrive at the substrate with significant energies.