13 Molecular beam epitaxy

S.S. islam

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Introduction

 

Molecular beam epitaxy (MBE) is an epitaxial growth technique used to deposit thin-film of single crystals. J.R. Arthur, and A.Y. Cho developed the technique in late 1960s at Bell Laboratories. It is extensively used in manufacturing of semiconductor devices, e.g., transistors, and is one of the fundamental techniques used in nanotechnologies.

 

1.1 Molecular Beam Epitaxy?

 

Molecular beam epitaxy (MBE) involves taking a base material, called substrate, onto which the film will be deposited. The substrate can be a semiconductor like silicon (Si), germanium (Ge), or gallium arsenide (GaAs). The substrate is heated upto a suitable temperature (such as 500-600 °C, in case of gallium arsenide). The precursor material is heated to high temperatures such that it is present in gas state. The gaseous precursor is then bombarded onto the substrate in the form of focused beam of atoms or molecules. The source from where the beam is incident on the substrate is called gun or the effusion cell. For firing different type of atoms/molecules on the substrate, different gun is needed, i.e., same gun is not used for different materials. The atoms or molecules, on reaching the surface of the substrate condense and gradually grow into ultra-thin layers. Thus, the crystal usually grows one atomic or molecular layer at a time. This gives precise control over the thickness of the deposited films and it makes MBE an excellent technique to deposit high quality and uniform thickness thin films over a wide array of substrates. As it comprises building up the materials via atomic and molecular level manipulation, it reflects the processes carried out in nanotechnology.

 

 

Figure 1: Image showing molecular beam epitaxy (MBE) technique to deposit thin films. MBE requires ultra-high vacuum (UHV) chambers, at temperatures of ~500°C, a clean, dust-free environment. The slightest contamination can ruin the crystal required to be grown.

 

MBE is able to grow high purity crystals owing to the instruments and processes involved in the deposition process. It is carried out in highly controlled environment. Ultra high vacuum and extremely clean conditions are required to be maintained. This is needed to avoid possible contamination from any dust particle or a gas molecule, as they can interfere with the growth process of the crystal. The level of cleanliness required is much higher than the standard conditions in the semiconductor manufacturing. And the required vacuum levels implies that the pressure should be as low as possible and which can be measured.

 

MBE is analogous to inkjet printer which produces layers of colored print on the page by bombarding jets or high speed streams of ink from hot guns. Inkjet printer uses four separate guns to fire different colored inks (cyan, magenta, yellow, and black). These guns gradually create the colored image of the pattern to be printed onto the paper. Similarly, in MBE, different molecules are fired from different beams and they condense on the surface of the substrate. However, MBE is relatively slow (can even take hours to complete) than inkjet printing. Literally, epitaxial implies ‘arranged on top of’, thus MBE essentially means using beam of molecules to deposit layers on the top of the substrate.

 

1.2 Working Principle

 

MBE is performed in high or ultra high vacuum (10−8 to 10−12 Torr) conditions. Deposition rate in an MBE is a critical parameter which allows the epitaxial growth of the films. Typical deposition rates are of the order of ~3000 nm per hour. Such deposition rates demand higher vacuum conditions than other deposition techniques to achieve similar impurity levels. Owing to the presence of ultra-high vacuum environment and absence of carrier gas, this technique produces films of highest purity.

 

 

Figure 2 Typical growth chamber of an MBE equipment. A typical MBE includes multiple chambers (introduction and extraction chambers, two or more growth chambers, an analysis chamber, and a preparation chamber). All the chambers are maintained at ultra-high vacuum.

 

When solid state precursors are used, they are heated in separate quasi-Knudsen effusion cells or electron beam evaporators till the sublimation temperature is reached. In case of GaAs, elements such as gallium (Ga) and arsenic (As) in extremely pure form are used. The elements in the gas phase condense on the substrate, where they can react with each other. In case of Ga and As, single-crystal GaAs can be grown. When copper or gold are evaporated, the gas phase elements impinging on the surface of the substrate may be either adsorbed or reflected. The atoms present on the surface of the substrate may be desorbed. The rate of the material impinging at the surface of the substrate can be controlled by controlling the temperature of the source. The temperature of the substrate affects the rate of molecules’ hopping or desorption. The term ‘beam’ is used to emphasize that the evaporated atoms do not interact with each other or with the gases of the vacuum chamber until they reach the wafer, because of the long mean free paths of the atoms.

 

The growth of the crystal can be monitored in real time by using reflection high energy electron diffraction (RHEED). The thickness of each layer can be precisely controlled upto single atomic layer. This is achieved by controlling the ON and OFF of the shutters present in front of the furnace by a computer. Complex features having layers of different materials can be produced by this method. This technique has been widely used to create confined structures such as quantum wells and quantum dots. Such layers are an essential component of several modern semiconductor devices, such as semiconductor lasers and LEDs.

 

If the substrate requires cooling, cryo-pumps and cryo-panels are integrated with the growth chamber in order to create ultra-high vacuum conditions. Liquid nitrogen is used to attain temperature close to 77K. Cryogenic temperature acts as a sink for impurities in the vacuum, thus, the vacuum levels should be several orders of magnitude higher for film deposition in these conditions. Apart from this, the substrate can also be placed onto a rotating platter which can be heated to desired temperature levels during operation.

 

Organic semiconductors can also be deposited via MBE. In these cases, instead of atoms, molecules are evaporated and subsequently deposited on the substrate. Other than this, gas-source MBE (analogous to chemical vapor deposition) is also available.

 

To deposit oxides or other compounds of the precursor elements, reacting gases may also be introduced into the reaction chamber of MBE. Such films are of particular interest in advanced electronic, optoelectronic, as well as magnetic applications.

 

1.3 Typical uses

 

Being able to deposit high purity uniform thickness films of a diverse range of compounds, MBE is extensively used to produce: semiconductor lasers for CD players, advanced computer chips, low-temperature superconductors, solar-cells (where a thin film of photovoltaic material is deposited on a substrate). Other than industrial uses, MBE is also widely used in advanced nanotechnology research activities. In brief, precise MBE has applications in thin-film computing devices, optics, photonics, etc.

 

1.4 Advantages and disadvantages

 

The advantages of MBE include:

 

a. It can be used to prepare high quality, defect-free, and highly uniform semiconductor crystals of a wide array of compounds.

b. Precise control over film thickness is possible.

 

The disadvantages of MBE include very slow deposition rates which result in very long residence times. Additionally, it is a laborious technique. Thus, this technique is more suitable for scientific research purposes, and commercial production using this technique is expensive. The equipment is very complex and expensive which originates from the requirements of extremely clean and ultra-high vacuum conditions.

 

Who invented MBE?

 

Alfred Y. Cho and John R. Arthur, Jr are credited with the invention of molecular beam epitaxy technique. They developed the technique in 1968 while working at the Bell Laboratories. After its discovery several scientists have made significant contributions to the development of modern day MBE systems. These include, but not limited to, Leo Esaki (won the Nobel Prize in 1973 for pioneer works in semiconductor electronics) and Ray Tsu, working at IBM. First (2009) Nanotechnology International Prize, RUSNANOPRIZE was awarded to Alfred Cho for developing MBE technique.

 

Fabrication of Quantum dots by MBE method

 

The growth of epitaxial (homogeneous or heterogeneous) thin film on a single crystal surface strongly depends upon the strength of interactions between adatoms (atoms to be deposited) and the surface of the substrate. Most of the MBE processes involve growth of films epitaxially by vapour phase approach, however, it is also possible to grow epitaxial layers (often termed as epilayers) from a liquid solution.

 

The growth by MBE can be divided into three types, depending upon the lattice mismatch between the film and the substrate, these are: 3D growth, 2D growth, and Stranksi-Krastanov (or the SK) growth modes. 3D growth occurs when the interactions between adatom-adatom are stronger than the interactions between adatom and the substrate, resulting in the formation of 3D adatom clusters or islands. For this reason, 3D growth mode is often referred to as island growth. The formation of clusters makes the deposited film coarse. Thus, the films grown by 3D mode are usually rough, and multilayered at certain points. In contrast to 3D growth, the interactions of the adatoms in 2D growth are stronger with the substrate surface than their interactions with other adatoms. Therefore, adatoms prefer to attach to the surface sites, leading to the formation of atomically smooth, completely formed layers. This layer-by-layer growth is two dimensional, indicating that complete films form prior to the growth of subsequent layers. In this regard, Stranski– Krastanov mode is intermediate to both 2D and 3D growth modes, and thus both types of growth occur in this mode. Transitions from the layer-by-layer to island type growth occur at critical layer thicknesses, which strongly depends on the chemical and physical properties, such as surface energies and lattice parameters, of the substrate and the film (Figure 3).

 

 

Figure 3 Schematic representation of the lattice distortion in a 3D island.

 

Quantum dots are structures where carriers are confined in all three spatial directions and where zero-dimensional systems of carriers are realized. This can be achieved by embedding the self-assembled islands within the layers of semiconductors having larger energy gaps and suitable band discontinuities. When the size of the 3D islands approaches the de Broglie wavelength of the carriers and their mean free path lengths, the carriers are quantum confined within the levels in the quantum dots. This usually occurs in the semiconductors, because the de Broglie wavelength approaches a few tens of nanometer. Molecular beam epitaxy if a widely employed technique for producing quantum dot structures. The spontaneous formation of MBE quantum dot ensembles due to a mismatched heterostructure is attracting huge interest in both fundamental research as well as practical applications. The homogeneity in both size and shape of the QD ensemble is an important parameter that influences the optical performance of the devices fabricated from them. This factor can be improved if the quantum dots are laterally ordered or periodically aligned along some specific crystallographic directions on the surface of the substrate. Additionally, long-range ordering of quantum dots is highly significant for future micro- and optoelectronics devices that exploit the properties of a single or a well-defined number of nanostructures.

 

 

Figure 4 AFM image of InAs dot arrays on InGaAs/GaAs. Inset to the figure shows the structure of a single InAs QD.

 

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    References & Suggested Reading:

  1. Taniguchi, Norio. “On the basic concept of nano-technology.” Proc. Intl. Conf. Prod. London, 1974. British Society of Precision Engineering, 1974.
  2. http://nptel.ac.in/courses/104103019/9
  3. https://en.wikipedia.org/wiki/Molecular_beam_epitaxy
  4. https://www.aps.org/units/fiap/meetings/presentations/upload/ploog.pdf
  5. http://nptel.ac.in/courses/117106093/10
  6. https://www.ee.ucl.ac.uk/about/MBE
  7. https://warwick.ac.uk/fac/sci/physics/current/postgraduate/regs/mpagswarwick/ex5/growth/pvd/
  8. www-opto.e-technik.uni-ulm.de/forschung/jahresbericht/2002/ar2002_fr.pdf
  9. http://nptel.ac.in/courses/104103019/24