The lattice constant (or lattice constant) refers to the side length of the cell, that is, the side length of each parallelepiped unit. It is an important basic parameter of the crystal structure.
Lattice constant, or lattice parameter, refers to the physical size of the cell in the lattice.
latticeLattice Constant is the basic structural parameter of crystal material, which is the same asatomInterbinding energyThere is a direct relationship.The change of lattice constant reflects the internal composition and stress state of the crystalchange。latticeConstant is also calledlatticeConstant.
Related Introduction
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In material science research, in order to facilitate the analysis of particle arrangement in crystals, a representativeBasic unit(Usually the smallest parallelepiped), as the constituent unit of the lattice, is called the unit cell. The unit cell is not necessarily the smallest repeating unit, but it is generally an integral multiple of the volume of the original cell (generally considered to be the smallest unit of the crystal).
The lattice in three-dimensional space generally has three lattice constants, which are represented by a, b and c respectively.However, in the special case of cubic crystal structure, these three constants are all equal, so they are only represented by a.Similarly, there is a hexagonal crystal structure, where the constants a and b are equal, so we only use a and c.A family of lattice constants can also be collectively referred to as lattice parameters.But in fact, the complete lattice parameters should be described by three lattice constants and three included angles.
For example, for a common diamond, its lattice constant is a=3.57 ∨ (300K).The crystal cell here is an equilateral structure, but the actual structure of diamond cannot be inferred only from the lattice constant.In addition, in practical applications, the average lattice constant is usually given.On the surface of the crystal, the lattice constant is the result of the deviation of the average value of the surface reconstruction.This deviation is particularly important for nanocrystals due to the large surface nanocrystal nucleus ratio.With the length of the lattice constant, its SI unit is meter.The lattice constant is usually on the order of a few angstroms (that is, a few tenths of a nanometer).The lattice constants can be determined by techniques such as X-ray diffraction and atomic force microscopy.
In epitaxial growth, the lattice constant is a measure of structural compatibility between different materials.The growth of thin layers of other materials with lattice constant matching is very important;When the constant is different, the strain is introduced into the layer to prevent the thick layer from epitaxial growth without defects.
Lattice matching
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By matching the lattice structure between two different semiconductor materials, the band gap change can form a region without introducing crystal structure change in the material.This allows the construction of advanced light-emitting diodes and laser diodes.
For example, gallium arsenide, aluminum arsenide, gallium arsenide, and aluminum arsenide have almost equal lattice constants, so that they can grow in almost any layer.
Grading of lattices
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In general, the selection of different materials for films grown on previous films or substrates matches the lattice of existing layer constants to reduce film stress.
Another method is to control the alloy ratio of changing grade from one value to another lattice constant during film growth.At the beginning of the grading layer, there will be a layer below the matching base lattice and the final lattice required for the alloy match at the end of layer growth.
The rate of change in the alloy must be determined by weighing the layer strain penalty, therefore, the defect density, the time cost of the epitaxial tool.
For example, the band gap of the InGaP layer above 1.9 eV can be graded in the GaAs wafer growth index.
Related issues
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Calculate the lattice constant of unknown matter?
For the cube structure, add a small amount of diffractive substance, such as Si powder, into the powder sample, accurately measure a sample peak close to a silicon peak, calculate the diffraction angle of the two peaks, check the correct diffraction angle of the silicon peak on the PDF card, and add the difference between the measured value and the standard value to get the accurate diffraction peak without instrument error,The lattice constant can be calculated according to the crystallographic calculation formula.
Non cubic structures are more troublesome.It is basically impossible to use this method to calculate.Moreover, this calculation still contains errors, because only when the diffraction angle is equal to 90 can there be no systematic error.
If software, such as JADE, is used to measure the full spectrum of the diffraction spectrum of a pure silicon sample and sample under the same experimental conditions, and the refined results are obtained by removing the instrument error according to the software method and then fitting the full spectrum.
The latter method can calculate the unit cell parameters of different phases in multiphase samples respectively.Due to the full spectrum, the phase with complex structure can also be calculated.But the precision is obviously inferior to that of single phase.
