Molecular beam epitaxy (MBE)

MBE technology is a technique in which an epitaxial thin layer is grown by spraying a molecular (or atomic) beam of its constituents onto a substrate under ultra-high vacuum conditions. The background vacuum of the modern MBE growth system can reach 1.33×10-10Pa, and there is no collision between the molecular beam and the molecular beam and between the molecular beam and the background molecule.

First, the key and main features of MBE technology:

III, the Group V elements are heated to the temperature Ti, respectively, and the beam formed by Tj is introduced onto the substrate with a temperature of Ts. The Ts is carefully selected, and the excess Group V elements are evaporated from the surface of the substrate to grow the chemical proportioning material; A suitable growth temperature allows the adsorbed atoms to have sufficient energy to migrate to a suitable equilibrium position for epitaxial growth. Too low a temperature may grow polycrystalline or amorphous; if the temperature is too high, the adsorbed atoms will evaporate again and desorb.

1. Growing under ultra-high vacuum, with little pollution, can grow high-purity epitaxial materials

2. The growth rate is generally 0.1-10 monoatomic layers/s. The fast switching of the baffles enables fast switching of the beam to achieve precise control of epitaxial layer thickness, composition and doping.

3. The substrate temperature is low, which can reduce the interdiffusion of the heterojunction interface and the easy growth of the abrupt junction.

4. MBE growth is not carried out under thermal equilibrium conditions, and it is possible to grow thin film materials which are difficult to grow by ordinary heat balance methods. Easy to grow a variety of new materials

5. MBE grows into a two-dimensional growth model, so that the surface interface of the epitaxial layer has atomic level flatness (RHEED intensity periodically corresponds to the thickness of the monolayer)

6. High vacuum, a variety of surface analysis instruments can be used for real-time in-situ monitoring of the epitaxial growth process and provide information on growth rate, surface morphology and composition of the epitaxial layer at any time, facilitating the study of growth process and growth mechanism.

7. MBE equipment can be vacuum-connected with other semiconductor process equipment to enable epitaxial material growth, evaporation, ion implantation and etching under vacuum conditions to improve device performance and yield.

Second, MBE equipment

Vacuum system

2. Growth system: injection chamber, pretreatment chamber (substrate storage chamber), growth chamber

3. Monitoring system: quadrupole mass spectrometer - vacuum detection, monitoring the composition of residual gas and molecular beam; ionization meter - measuring molecular beam flow; electron diffractometer - observing crystal surface structure and growth surface smoothness; Auger spectrum Instrument - detects surface composition, stoichiometry and surface contamination.

Third, the growth process and growth principle

1. The source evaporates to form a molecular beam having a certain beam current density and is directed toward the substrate under high vacuum;

2. The molecular beam is epitaxially grown on the substrate. From the growth process, MBE has three basic regions: the molecular beam generation region, the cross-mixing region of each molecular beam, the reaction and the crystallization process region.

3. The molecular beam emitted from the source is adsorbed on the surface of the substrate and adsorbed

4. The adsorbed molecules (atoms) migrate and decompose on the surface

5. The atom enters the lattice position and epitaxially grows

6. Molecules that do not enter the crystal lattice leave the surface due to thermal desorption

Adhesion coefficient

Adhesion coefficient = number of molecules grown on the substrate / number of incident molecules

Ga is chemically adsorbed on the surface of the GaAs substrate, and the general adsorption coefficient is 1.

The group V atom (As) is first physically adsorbed, and after a series of physical and chemical processes, it is converted into chemisorption, and its adhesion coefficient is related to the surface state of the substrate and the substrate temperature.

Different As sources correspond to different physicochemical processes

1. Ga source: Ga

2. As source: As or GaAs

As is formed by As to form a molecular beam of As4. Disadvantages: large beam current, difficult to control, especially for As+P solid solution; advantages: Ga and As can be controlled separately. When there is no Ga beam incident, the adhesion coefficient of As4 is 0, and when Ga beam is incident, the adhesion coefficient of As4 increases. When it is 450K or less, As4 does not decompose; when it is 450k or more, As4 can be decomposed to generate As.

The GaAs is used to form the As2 molecular beam. The advantage is that the beam size is suitable and easy to control. Disadvantages: Ga and As cannot be controlled separately.

3. GaAs source: When the substrate temperature is between 775k and 800k, Ga:As=1:10, Ga, As can obtain GaAs with Ga:AS 1:1, and the adhesion coefficient of As2 is 0.1~0.15.

Comparison with other extension methods

1. Chemical beam epitaxy growth CBE

Background: MBE uses solid-state source, when growing related solid solution, it is not easy to heat the As, P solid source accurately and repeatedly control its composition ratio;

CBE: gaseous source MBE, MO-MBE;

Substituting a solid source of Al, Ga, In, etc. with an MO source of a group III element;

Replace the source of the V group element with AsH3 and PH3;

Epitaxial growth in an MBE growth system;

It basically combines the advantages of MBE and MOVPE technology, and is especially suitable for phosphide materials with high vapor pressure which are difficult to grow MBE.

2. Compare the growth mechanism of MBE, MOVPE and CBE

MBE: Group III elements are emitted to the substrate in the form of atoms or molecular beams, adsorption → crystallization (desorption);

MOVPE: MO thermally decomposes in the gas stream and on the surface of the substrate, and the group III atoms decomposed in the gas stream diffuse through the boundary layer to reach the surface of the substrate;

CBE: MO is only thermally decomposed on the surface of the substrate, and there is no boundary layer;

3. Atomic layer epitaxy, molecular layer epitaxy MLE

Two elemental sources (gas or beam) of the constituent compounds are introduced into the growth chamber, respectively, alternately deposited on the substrate. A single molecular layer is epitaxially grown on the substrate every time (introduced), and the speed of epitaxial growth depends on the time required for the components to alternately adsorb on the substrate, and the pulse growth source may be used in actual growth;

ALE is a growth "mode", it does not have its own "dedicated" equipment, VPE, MBE, CBE equipment can perform ALE growth

1. Growth mechanism of ALE

2. ALE growth stop mechanism (SLM)

When Ga, In, As, and P elements are used as the source, the higher vapor pressures of As and P act as SLM;

In the case of TMG as a source, TMG is an acceptor of electrons, As is an electron donor, and alkyl R acts as an SLM.

   Characteristics of ALE epitaxial technology

1. Precise thickness control and good repeatability;

2. Due to the monolayer growth, it is easy to grow with an atomic-level abrupt junction interface;

3. The thickness of the epitaxial layer is uniform, basically independent of the structure of the pedestal, the shape of the airflow, the flow velocity and other parameters. The total thickness depends only on the number of cycles of alternating growth, and is called “digital extension”;

4. The surface quality is good, and the surface of the specular epitaxial layer can be produced.

(Author: jiangwei68 Source: Photo News blog)

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