Epitaxy by molecular jet

The epitaxy by molecular jets (or MBE for Molecular Beam Epitaxy ) is a technique consisting in sending one or more molecular jets towards a beforehand selected substrate to carry out a epitaxial growth . It makes it possible to make grow nanostructurés samples of several cm² at a speed of approximately full-course atomic a second.

Conditions of Knudsen

In major the part of the cases, one wishes to carry out the growth of solid materials to room temperature. One places these materials in crucibles located within a Knudsen cell. These crucibles are produced in GNP (boron nitride pyrolitic, stable and little reagent until 2000K). The temperature of evaporation is a variable essential to control because it will determine molecular flow arriving on the sample. The growth of materials is relatively slow. Indeed, it is not necessary that evaporated molecules react with others before to have reached the substrate. One will arrange oneself so that the free course (

\ lambda

) average is higher than the distance separating the Knudsen cell to the substrate. In practice it is made so that

\ lambda

is higher than 1meter. If these conditions are observed one can then speak about molecular jets.

One can show that $\ lambda = \ frac {1} {\ sqrt {2} \ pi \ sigma^2 N}$ where

$\ sigma$ is the distance from which one considers that the molecules are in collision (one supposes the spherical molecules of ray $\ sigma$ , typically of some Ångström)
$n$ is the density of atoms (atomes/m³)

However the voluminal density of atoms is directly proportional to the pressure (

p

) and the temperature (

T

) according to

p=nkT

where

k

is the Boltzmann constant.

Advantages and disadvantages of a slow growth

It should be known that the Knudsen cells have also obturators making it possible to control the growth. Times of closing of these obturators are generally lower than the second, and thus generally lower than the time necessary with the creation of full-course. The advantage of this slow growth is thus to be able to obtain clear heterojunctions within the multi-layer material framework. A slow growth also makes it possible to control a doping homogeneous of material. Another large advantage they is measurements in real-time which one can carry out during the growths (to see not according to). However, a low speed of growth implies a very pure atmosphere without what the impurities would come to contaminate the considerable sample of manner. For that it is advisable to use several systems of pumping in order to be under a residual pressure lower than 10^-8 Pa.

Measurements in real-time

Measurements in real-time the most met in MBE are

the measurement of residual pressure using a probe of the type Bayard-Alpert
a similar probe who can be placed above the cell of evaporation (this measurement must obviously be done only before the growth)
One can determine the composition of residual gases thanks to the mass spectrometry per quadrupole (QMS)
Détermination of the temperature of the substrate thanks to an optical pyrometer detecting the infra-red radiations
But especially one can analyze the dimensional and crystalline evolution of growth thanks to the RHEED (Reflection High Energy Electron Diffraction: electron diffraction of high energy)

Surface: a dynamic balance

It should be known that the growth of surface is a dynamic process and not statics. Indeed, when a molecule reaches surface, it does not remain there simply not stuck. Typically the molecules will diffuse thanks to their thermal energy. From there, there will be nucleation: atoms will meet, to be assembled and their mobility will decrease. Other molecules will be able to associate, one speaks then about aggregation. Broadly these aggregates move little. However, they them edges are very mobile, it is what one indicates by the " diffusion of bord". Concurrently to that, the thermal energy of the molecules can be such that they leave the sample: it is what one names desorption. Lastly, certain aggregates can separate, it acts then of a dissociation.

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