Chemical beam epitaxy

Last updated

Chemical beam epitaxy (CBE) forms an important class of deposition techniques for semiconductor layer systems, especially III-V semiconductor systems. This form of epitaxial growth is performed in an ultrahigh vacuum system. The reactants are in the form of molecular beams of reactive gases, typically as the hydride or a metalorganic. The term CBE is often used interchangeably with metal-organic molecular beam epitaxy (MOMBE). The nomenclature does differentiate between the two (slightly different) processes, however. When used in the strictest sense, CBE refers to the technique in which both components are obtained from gaseous sources, while MOMBE refers to the technique in which the group III component is obtained from a gaseous source and the group V component from a solid source.

Contents

Basic principles

Chemical beam epitaxy was first demonstrated by W.T. Tsang in 1984. [1] This technique was then described as a hybrid of metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) that exploited the advantages of both the techniques. In this initial work, InP and GaAs were grown using gaseous group III and V alkyls. While group III elements were derived from the pyrolysis of the alkyls on the surface, the group V elements were obtained from the decomposition of the alkyls by bringing in contact with heated Tantalum (Ta) or Molybdenum (Mo) at 950-1200 °C. Typical pressure in the gas reactor is between 102 Torr and 1 atm for MOCVD. Here, the transport of gas occurs by viscous flow and chemicals reach the surface by diffusion. In contrast, gas pressures of less than 10−4 Torr are used in CBE. The gas transport now occurs as molecular beam due to the much longer mean-free paths, and the process evolves to a chemical beam deposition. [2] It is also worth noting here that MBE employs atomic beams (such as aluminium (Al) and Gallium (Ga)) and molecular beams (such as As4 and P4) that are evaporated at high temperatures from solid elemental sources, while the sources for CBE are in vapor phase at room temperatures. [3] A comparison of the different processes in the growth chamber for MOCVD, MBE and CBE can be seen in figure 1.

Figure 1: Basic processes inside the growth chambers of a) MOCVD, b) MBE, and c) CBE. CBE im1.png
Figure 1: Basic processes inside the growth chambers of a) MOCVD, b) MBE, and c) CBE.

Experimental setup

A combination of turbomolecular and cryo pumps are used in standard UHV growth chambers. The chamber itself is equipped with a liquid nitrogen cryoshield and a rotatable crystal holder capable of carrying more than one wafer. The crystal holder is usually heated from the backside to temperatures of 500 to 700 °C. Most setups also have RHEED equipment for the in-situ monitoring of surface superstructures on the growing surface and for measuring growth rates, and mass spectrometers for the analysis of the molecular species in the beams and the analysis of the residual gases. [4] The gas inlet system, which is one of the most important components of the system, controls the material beam flux. Pressure controlled systems are used most commonly. The material flux is controlled by the input pressure of the gas injection capillary. The pressure inside the chamber can be measured and controlled by a capacitance manometer. The molecular beams of gaseous source materials injectors or effusion jets that ensure a homogeneous beam profile. For some starting compounds, such as the hydrides that are the group V starting material, the hydrides have to be precracked into the injector. This is usually done by thermally decomposing with a heated metal or filament. [4]

Growth kinetics

In order to better understand the growth kinetics associated with CBE, it is important to look at physical and chemical processes associated with MBE and MOCVD as well. Figure 2 depicts those. The growth kinetics for these three techniques differ in many ways. In conventional MBE, the growth rate is determined by the arrival rate of the group III atomic beams. The epitaxial growth takes place as the group III atoms impinge on the heated substrate surface, migrates into the appropriate lattice sites and then deposits near excess group V dimers or tetramers. It is worth noting that no chemical reaction is involved at the surface since the atoms are generated by thermal evaporation from solid elemental sources. [2]

Figure 2: Growth kinetics involved in a) conventional MBE, b) MOCVD, and c) CBE. CBE im2.png
Figure 2: Growth kinetics involved in a) conventional MBE, b) MOCVD, and c) CBE.

In MOCVD, group III alkyls are already partially dissociated in the gas stream. These diffuse through a stagnant boundary layer that exists over the heated substrate, after which they dissociate into the atomic group III elements. These atoms then migrate to the appropriate lattice site and deposit epitaxially by associating with a group V atom that was derived from the thermal decomposition of the hydrides. The growth rate here is usually limited by the diffusion rate of the group III alkyls through the boundary layer. Gas phase reactions between the reactants have also been observed in this process. [2]

In CBE processes, the hydrides are cracked in a high temperature injector before they reach the substrate. The temperatures are typically 100-150 °C lower than they are in a similar MOCVD or MOVPE. [5] There is also no boundary layer (such as the one in MOCVD) and molecular collisions are minimal due to the low pressure. The group V alkyls are usually supplied in excess, and the group III alkyl molecules impinge directly onto the heated substrate as in conventional MBE. The group III alkyl molecule has two options when this happens. The first option is to dissociate its three alkyl radicals by acquiring thermal energy from the surface, and leaving behind the elemental group III atoms on the surface. The second option is to re-evaporate partially or completely undissociated. Thus, the growth rate is determined by the arrival rate of the group III alkyls at a higher substrate temperature, and by the surface pyrolysis rate at lower temperatures. [2]

Compatibility with device fabrication

Selective growth at low temperatures

Selective growth through dielectric masking is readily achieved using CBE as compared to its parent techniques of MBE and MOCVD. Selective growth is hard to achieve using elemental source MBE because group III atoms do not desorb readily after they are adsorbed. With chemical sources, the reactions associated with the growth rate are faster on the semiconductor surface than on the dielectric layer. No group III element can, however, arrive at the dielectric surface in CBE due to the absence of any gas phase reactions. Also, it is easier for the impinging group III metalorganic molecules to desorb in the absence of the boundary layer. This makes it easier to perform selective epitaxy using CBE and at lower temperatures, compared to MOCVD or MOVPE. [5] In recent developments patented by ABCD Technology, substrate rotation is no longer required, leading to new possibilities such as in-situ patterning with particle beams. [6] This possibility opens very interesting perspectives to achieve patterned thin films in a single step, in particular for materials that are difficult to etch such as oxides.

p-type doping

It was observed that using TMGa for the CBE of GaAs led to high p-type background doping (1020 cm−3) due to incorporated carbon. However, it was found that using TEGa instead of TMGa led to very clean GaAs with room temperature hole concentrations between 1014 and 1016 cm−3. It has been demonstrated that the hole concentrations can be adjusted between 1014 and 1021 cm−3 by just adjusting the alkyl beam pressure and the TMGa/TEGa ratio, providing means for achieving high and controllable p-type doping of GaAs. This has been exploited for fabricating high quality heterojunction bipolar transistors. [4]

Advantages and disadvantages

CBE offers many other advantages over its parent techniques of MOCVD and MBE, some of which are listed below:

Advantages over MBE

  1. Easier multiwafer scaleup: Substrate rotation is required for uniformity in thickness and conformity since MBE has individual effusion cells for each element. Large effusion cells and efficient heat dissipation make multiwafer scaleup more difficult.
  2. Better for production environment: Instant flux response due to precision electronic control flow.
  3. Absence of oval defects: These oval defects generally arise from micro-droplets of Ga or In spit out from high temperature effusion cells. These defects vary in size and density system-to-system and time-to-time. [3] [7]
  4. Lower drifts in effusion conditions that do not depend on effusive source filling.
  5. In recent developments patented by ABCD Technology, substrate rotation is no longer required. [6]

Advantages over MOCVD

  1. Easy implementation of in-situ diagnostic instruments such as RHEED.
  2. Compatibility with other high vacuum thin-film processing methods, such as metal evaporation and ion implantation. [3] [7]

Shortcomings of CBE

  1. More pumping required compared to MOCVD.
  2. Composition control can be difficult when growing GaInAs. At high temperature, we have a better incorporation of Ga, but we face the problem related desorption of In.

So, a compromise should be found between high and low temperature for a good composition control.

  1. High carbon incorporation for GaAlAs. [3] [7]

See also

Related Research Articles

<span class="mw-page-title-main">Gallium arsenide</span> Chemical compound

Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.

<span class="mw-page-title-main">Epitaxy</span> Crystal growth process relative to the substrate

Epitaxy refers to a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited crystalline film is called an epitaxial film or epitaxial layer. The relative orientation(s) of the epitaxial layer to the seed layer is defined in terms of the orientation of the crystal lattice of each material. For most epitaxial growths, the new layer is usually crystalline and each crystallographic domain of the overlayer must have a well-defined orientation relative to the substrate crystal structure. Epitaxy can involve single-crystal structures, although grain-to-grain epitaxy has been observed in granular films. For most technological applications, single-domain epitaxy, which is the growth of an overlayer crystal with one well-defined orientation with respect to the substrate crystal, is preferred. Epitaxy can also play an important role while growing superlattice structures.

<span class="mw-page-title-main">Molecular-beam epitaxy</span> Crystal growth process

Molecular-beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. MBE is widely used in the manufacture of semiconductor devices, including transistors, and it is considered one of the fundamental tools for the development of nanotechnologies. MBE is used to fabricate diodes and MOSFETs at microwave frequencies, and to manufacture the lasers used to read optical discs.

<span class="mw-page-title-main">Pulsed laser deposition</span>

Pulsed laser deposition (PLD) is a physical vapor deposition (PVD) technique where a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. This material is vaporized from the target which deposits it as a thin film on a substrate. This process can occur in ultra high vacuum or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films.

A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, integrated passive devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.

An epitaxial wafer is a wafer of semiconducting material made by epitaxial growth (epitaxy) for use in photonics, microelectronics, spintronics, or photovoltaics. The epi layer may be the same material as the substrate, typically monocrystaline silicon, or it may be a silicon dioxide (SoI) or a more exotic material with specific desirable qualities. The purpose of epitaxy is to perfect the crystal structure over the bare substrate below and improve the wafer surface's electrical characteristics, making it suitable for highly complex microprocessors and memory devices.

<span class="mw-page-title-main">Metalorganic vapour-phase epitaxy</span> Method of producing thin films (polycrystalline and single crystal)

Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films. It is a process for growing crystalline layers to create complex semiconductor multilayer structures. In contrast to molecular-beam epitaxy (MBE), the growth of crystals is by chemical reaction and not physical deposition. This takes place not in vacuum, but from the gas phase at moderate pressures. As such, this technique is preferred for the formation of devices incorporating thermodynamically metastable alloys, and it has become a major process in the manufacture of optoelectronics, such as Light-emitting diodes. It was first demonstrated in 1967 at North American Aviation Autonetics Division in Anaheim CA by Harold M. Manasevit.

Hydride vapour-phase epitaxy (HVPE) is an epitaxial growth technique often employed to produce semiconductors such as GaN, GaAs, InP and their related compounds, in which hydrogen chloride is reacted at elevated temperature with the group-III metals to produce gaseous metal chlorides, which then react with ammonia to produce the group-III nitrides. Carrier gasses commonly used include ammonia, hydrogen and various chlorides.

<span class="mw-page-title-main">Isobutylgermane</span> Chemical compound

Isobutylgermane (IBGe, Chemical formula: (CH3)2CHCH2GeH3, is an organogermanium compound. It is a colourless, volatile liquid that is used in MOVPE (Metalorganic Vapor Phase Epitaxy) as an alternative to germane. IBGe is used in the deposition of Ge films and Ge-containing thin semiconductor films such as SiGe in strained silicon application, and GeSbTe in NAND Flash applications.

Indium gallium aluminium nitride is a GaN-based compound semiconductor. It is usually prepared by epitaxial growth, such as metalorganic chemical vapour deposition (MOCVD), molecular-beam epitaxy (MBE), pulsed laser deposition (PLD), etc. This material is used for specialist opto-electronics applications, often in blue laser diodes and LEDs.

Selective area epitaxy is the local growth of epitaxial layer through a patterned amorphous dielectric mask (typically SiO2 or Si3N4) deposited on a semiconductor substrate. Semiconductor growth conditions are selected to ensure epitaxial growth on the exposed substrate, but not on the dielectric mask. SAE can be executed in various epitaxial growth methods such as molecular beam epitaxy (MBE), metalorganic vapour phase epitaxy (MOVPE) and chemical beam epitaxy (CBE). By SAE, semiconductor nanostructures such as quantum dots and nanowires can be grown to their designed places.

<span class="mw-page-title-main">Vapor–liquid–solid method</span> Mechanism to grow nano wires

The vapor–liquid–solid method (VLS) is a mechanism for the growth of one-dimensional structures, such as nanowires, from chemical vapor deposition. The growth of a crystal through direct adsorption of a gas phase on to a solid surface is generally very slow. The VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can subsequently occur from nucleated seeds at the liquid–solid interface. The physical characteristics of nanowires grown in this manner depend, in a controllable way, upon the size and physical properties of the liquid alloy.

<span class="mw-page-title-main">Veeco</span> American manufacturing company

Veeco is a global capital equipment supplier, headquartered in the U.S., that designs and builds processing systems used in semiconductor and compound semiconductor manufacturing, data storage and scientific markets for applications such as advanced packaging, photonics, power electronics and display technologies.

IQE PLC is a British semiconductor company founded 1988 in Cardiff, Wales, which manufactures advanced epitaxial wafers for a wide range of technology applications for wireless, optoelectronic, electronic and solar devices. IQE specialises in advanced silicon and compound semiconductor materials based on gallium arsenide (GaAs), indium phosphide (InP), gallium nitride (GaN) and silicon. The company is the largest independent outsource producer of epiwafers manufactured by metalorganic vapour phase epitaxy (MOCVD), molecular beam epitaxy (MBE) and chemical vapor deposition (CVD).

Indium aluminium nitride (InAlN) is a direct bandgap semiconductor material used in the manufacture of electronic and photonic devices. It is part of the III-V group of semiconductors, being an alloy of indium nitride and aluminium nitride, and is closely related to the more widely used gallium nitride. It is of special interest in applications requiring good stability and reliability, owing to its large direct bandgap and ability to maintain operation at temperatures of up to 1000 °C., making it of particular interest to areas such as the space industry. InAlN high-electron-mobility transistors (HEMTs) are attractive candidates for such applications owing to the ability of InAlN to lattice-match to gallium nitride, eliminating a reported failure route in the closely related aluminium gallium nitride HEMTs.

Manijeh Razeghi is an Iranian-American scientist in the fields of semiconductors and optoelectronic devices. She is a pioneer in modern epitaxial techniques for semiconductors such as low pressure metalorganic chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE), molecular beam epitaxy (MBE), GasMBE, and MOMBE. These techniques have enabled the development of semiconductor devices and quantum structures with higher composition consistency and reliability, leading to major advancement in InP and GaAs based quantum photonics and electronic devices, which were at the core of the late 20th century optical fiber telecommunications and early information technology.

<span class="mw-page-title-main">Low-energy plasma-enhanced chemical vapor deposition</span>

Low-energy plasma-enhanced chemical vapor deposition (LEPECVD) is a plasma-enhanced chemical vapor deposition technique used for the epitaxial deposition of thin semiconductor films. A remote low energy, high density DC argon plasma is employed to efficiently decompose the gas phase precursors while leaving the epitaxial layer undamaged, resulting in high quality epilayers and high deposition rates.

Gallium arsenide antimonide, also known as gallium antimonide arsenide or GaAsSb, is a ternary III-V semiconductor compound; x indicates the fractions of arsenic and antimony in the alloy. GaAsSb refers generally to any composition of the alloy. It is an alloy of gallium arsenide (GaAs) and gallium antimonide (GaSb).

Aluminium arsenide antimonide, or AlAsSb (AlAs1-xSbx), is a ternary III-V semiconductor compound. It can be considered as an alloy between aluminium arsenide and aluminium antimonide. The alloy can contain any ratio between arsenic and antimony. AlAsSb refers generally to any composition of the alloy.

References

  1. ↑ Tsang, W. T. (1984). "Chemical beam epitaxy of InP and GaAs". Applied Physics Letters. AIP Publishing. 45 (11): 1234–1236. Bibcode:1984ApPhL..45.1234T. doi:10.1063/1.95075. ISSN   0003-6951.
  2. 1 2 3 4 5 Tsang, W.T. (1987). "Chemical beam epitaxy of Ga0.47In0.53As/InP quantum wells and heterostructure devices". Journal of Crystal Growth. Elsevier BV. 81 (1–4): 261–269. Bibcode:1987JCrGr..81..261T. doi:10.1016/0022-0248(87)90402-7. ISSN   0022-0248.
  3. 1 2 3 4 5 Tsang, W.T. (1989). "From chemical vapor epitaxy to chemical beam epitaxy". Journal of Crystal Growth. Elsevier BV. 95 (1–4): 121–131. Bibcode:1989JCrGr..95..121T. doi:10.1016/0022-0248(89)90364-3. ISSN   0022-0248.
  4. 1 2 3 Lüth, Hans (1994). "Chemical beam epitaxy — a child of surface science". Surface Science. 299–300: 867–877. Bibcode:1994SurSc.299..867L. doi:10.1016/0039-6028(94)90703-X. ISSN   0039-6028.
  5. 1 2 Benchimol, Jean-Louis; Alexandre, F.; Lamare, Bruno; Legay, Philippe (1996). "Benefits of chemical beam epitaxy for micro and optoelectronic applications". Progress in Crystal Growth and Characterization of Materials. Elsevier BV. 33 (4): 473–495. doi:10.1016/s0960-8974(96)00091-5. ISSN   0960-8974.
  6. 1 2 G. Benvenuti, Large area deposition in high vacuum with high thickness uniformity. WO_2003093529_A2[1].
  7. 1 2 3 M.A. Herman and H. Sitter. Molecular Beam Epitaxy. Heidelberg: Springer, 1996.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.