Molecular beam epitaxy: Difference between revisions

Template:WikipediaMolecular Beam Epitaxy (MBE) is a material fabrication process used to produce high-purity nano-scale materials. A material is grown through interactions between a substrate and one or more beams of atoms or molecules incident upon the substrate's surface. MBE exhibits many advantages over similar thin film deposition processes: significantly improved purity, arbitrarily sharp deposition resolution, and operation at low temperatures. Template:MECH370

System Description

• Insert Image of MBE layout.

The main MBE apparatus is composed of two major sections: the substrate chamber and the effusion cells. All sections must be evacuated to low pressures to ensure that the molecular beam condition is achieved. This condition requires that the free path of a particle be greater than the dimensions of the substrate chamber. For a typical MBE set up, a pressure of 10E-4 Torr is sufficient, but pressures on the order of 10E-11 Torr are commonly used.

Effusion Cells

Within the effusion cells, the system can be considered contained when the shutters are closed. Thus, the state of the system can be approximated using thermodynamics. The phase behaviour of the source material is important. Gibb's phase rules states that the number of control variables for a system are is related to the system properties as:

 F = N +2 – P

Where F is the number of degrees of freedom, N is the number of chemical components, and P is the number of phases present. Within the effusion cell, two phases are present, and there is only one chemical component. The number of degrees of freedom of the system is one, indicating that the system pressure is dependent upon the temperature. This relationship yields an Arrhenius-type relationship, with:

 P(T) ~ exp(dH / kT)

Where dH is the latent heat of vaporisation of the material, T is the material temperature, and k is Boltzmann's constant. The pressure of the vapour can be controlled by varying the temperature of the solid phase, which in turn is controlled by variations in crucible heating. Because the beam intensity within the substrate chamber depends on the effusion cell pressures, the amount of material reaching the surface can be controlled by varying the source temperature.

The shutters at the end of the collimation tunnel enable an operator to terminate the material flux from each cell separately. The shutter closing time is significantly smaller than a single layer deposition time. This means that the composition of the final material can be controlled at the monolayer level.

Substrate Stage

A schematic of a basic substrate chamber is shown >. The effusion cells are attached to the chamber and oriented so as to direct the beam upon the substrate holder. The substrate holder is heated with a resistive element, as in the effusion cells. Upon the substrate holder is placed a material upon which the beams are to interact and the source material is to be deposited. Common substrate materials are silicon, GaAs, and sapphire, but other substrates can be used. The choice of substrate is very important, and has been shown to effect the thin film microstructure. The substrate stage can be rotated during deposition to increase the surface homogeneity.

A RHEED (reflection high energy electron diffraction) monitoring system is implemented in modern MBE equipment. The RHEED system is used to monitor the growth surface during deposition. High energy electrons are incident upon the surface, producing a diffraction pattern upon the RHEED screen. The diffraction pattern can be analyzed to determine surface characteristics, like the material structure, as well as composition. The intensity of the diffraction pattern has been found to vary predictably with the surface layer formation, and can be used to determine when a monolayer has been fully formed. This information can be used to control the effusion cell shutters to produce an atomically flat surface.

Each component of the MBE apparatus must have a number of special properties. Due to the high vacuum, all components must be de-gassed before system operation. The materials chosen must have very low rates of gas evolution to prevent their vapour from being deposited on the substrate and contaminating the sample. This property is of particular importance for materials chosen for use in the resistive heating elements, as the increased surface temperature can lead to significant increases in gas evolution. Boron nitride is used for the crucibles and substrate stage, and graphite is commonly used for the heating elements. Similarly, all moving parts of the apparatus must operate oil-free, to avoid evaporation and sample contamination.

Growth Processes

During deposition, one or more beams is directed toward the substrate. When a particle reaches the substrate, it will be absorbed on the surface. Depending on the substrate and surface conditions, the particle will then either join the surface, or be desorbed and return to the atmosphere. The conditions for absorption are highly dependent upon the material being grown and the substrate material. Study of growth kinetics is very difficult in the field of MBE due to the interconnectivity of growth parameters (e.g. altering beam flux also changes the atmosphere at the substrate). Another problem in MBE is that the deposition process does not take place at or close to thermodynamic equilibrium. The temperatures of the molecular beams are usually very different from that of the substrate surface.

Work in MBE has been focused on production of either doped substances or binary compounds, often for applications in semiconductors. The two most common types of MBE are IV and III-V. These names correspond with the group in which the source materials are found in the periodic table.

IV Systems

Silicon is the most common IV material for applications in MBE, and the GaAs system is particularly common example of a III-V system. Other systems (notably II-VI) are possible and have been investigated, but the amount of work on these systems is minute compared to IV and III-V systems. One of the major focuses of IV MBE is in the doping of silicon for semiconductor applications. In contrast with traditional diffusion based doping mechanisms, MBE allows the creation of arbitrarily sharp doping profiles. To produce, for example, an n-type silicon film, beams of silicon and phosphorus should be used. The silicon flux should be greater that of phosphorus, which can be achieved by adjusting the amount of heating in the phosphorus effusion cell. P-type silicon can be produced in a similar fashion, and a pn- or pin-junction can be produced by alternating the beams.

III-V Systems

During early work with the production of GaAs films, an interesting effect was observed. The sticking coefficient of a chemical is the probability of a particle that reaches the substrate remaining on the substrate. It was found that the sticking coefficient of III particles was unity. In the absence of III particles, no absorption of VI particles occurred. This led to the conclusion that the material growth rate depends solely on the flux of the III beam. In the case of GaAs, the As molecules were absorbed on the surface, and either reacted with Ga on the surface, or desorbed. The Ga particles would collect as droplets on the substrate surface in the absence of sufficient As flux to maintain growth. These results can be related to the differences in bonding behaviour of III and V chemicals, and the resulting energies associated with surface formation. For Ga, the formation of solid-liquid and liquid-vapour interfaces is energetically favourable on a GaAs substrate, and droplets are formed. This process is unfavourable for As, so As particles would instead form gaseous molecules (As2, As4). This phenomenon is important in the development of III-V films, and highlights one method used to control growth rates in MBE processes. No similar phenomena occurs for IV particles, which is one major difference between the two growth processes.

Process Considerations

Upon completion of any growth process, it is recommended that a flux of low-temperature particles be directed toward the growth surface. The particles should be non-reactive with the deposited material. This additional flux will serve to both cool the surface, reducing surface diffusion, and flush the atmosphere above the substrate, restricting further deposition. The non-reactive flux will displace any reactive particles, ensuring surface cleanliness.

Efficiency Advances

MBE has been researched for several decades, and many advances have been made to the basic apparatus. A large number of patents have been issued for improvements to the apparatus. Some of the improvements include:

• enhancements to the shutter system to enable arbitrary flux variations^[1]
• heat shielding of the substrate chamber heating element to drastically increase energy efficiency^[2]
• readily removable effusion cell crucibles to avoid system shut-down during refilling^[1]

References