14 Metal Organic Vapor Phase Epitaxy

S.S. islam

1. Introduction

Metal organic vapor phase epitaxy (MOVPE), also termed as organometallic vapour phase epitaxy (OMVPE) or metalorganic chemical vapor deposition (MOCVD), is a chemical vapor deposition technique which can be used to prepare single or polycrystalline thin films. It is a very complex technique to grow crystalline layers for creating complicated semiconductor multilayered structures. It is different from MBE (or molecular beam epitaxy) in the fact that the crystals are grown via chemical reactions and not physically deposited. The process does not occur in vacuum, rather a gas phase is introduced inside the reaction chamber at moderate pressure (10-to 760 Torr). MOVPE is a preferred technique in the manufacturing of devices which require incorporation of thermodynamically metastable alloys. For this reason, it has extensive applications in manufacturing of optoelectronic devices.

Figure 1 Schematic illustration of the process.

1.1 Working Principle

In MOVPE/MOCVD/OMVPE technique, precursors are used in their gaseous form. Ultra pure gas is introduced in the reaction chamber. This gas is carefully dosed with the precursor material in over to deposit an extremely fine layer of atoms on the surface of the wafer/substrate. Figure 1 shows the schematics of the growth process involved in this technique. Crystal growth is favoured by the surface reactions taking place on the substrate surface. These reactions involve organic compounds or metalorganics and hydrides (which contain the required chemical elements for producing final end product) as the reactants. The thin films of the required material or compound semiconductor is epitaxially grown on the substrate surface. This technique can be successfully used to deposit layers of compound semiconductor containing elemental combinations of Group III and Group V, Group II and Group VI, etc.

As an example, indium phosphide (InP) can be deposited onto a heated substrate in the reaction chamber. The process can be summarized as follows:

a. The precursors, trimethylindium ((CH3)3In) and phosphine (PH3), are injected into the reaction chamber.

b. The heated organic precursor molecules are decomposed in the absence of oxygen, by the process called pyrolysis. Due to pyrolysis, the atoms, to be deposited, reach the surface of the substrate.

c. These atoms attach/bound to the surface leading to the formation of a new crystalline layer at the surface of the substrate.

    The temperature at which pyrolysis occurs increases with increase in the strength of chemical bonds of the precursor molecules, as a higher energy is required to break those bonds. With increase in the number of carbon atoms attached to the central metal atom weakens the bonds. The diffusion of the atoms no the surface of the substrate depends on the atomic steps at the surface.

The vapor pressure of the metalorganic precursor is a crucial parameter in the MOCVD process. It influences the concentration of the precursor (gas state) in the growth reaction as well as the deposition rate.

1.2 Reactor Components

Figure 2 Schematic of the typical apparatus used in MOCVD.

The MOCVD process involves incorporation of reactant gases at high temperatures in the reaction chamber. The chemical reactions occurring between these gases owing to the interactions at high temperature leads to the deposition of the required compounds onto the surface of the substrate.

The reaction chamber is made from a material which should not react with the chemical species present in the chamber even at elevated temperatures. Also, the material must be able to sustain elevated temperatures. The reactor consists of reactor walls, lining, susceptor, gas injection arrangements, and temperature control unit. Most often the chamber walls are produced from stainless steel or quartz. The chamber walls and susceptor are usually separated by liners made up of ceramics or special glasses (e.g., quartz). Water may be flown through dedicated channels integrated with the chamber walls in order to cool the system and avoid overheating. The substrate is kept on the susceptor whose temperature can be controlled. The materials used to make susceptor must not react with the metal-organic compounds being used. Graphite is a commonly used susceptor material, it can also be coated with special coating layers to avoid reactions with the precursor materials (or gases). For example, during the growth of nitride- and related compounds, the graphite susceptor may corrode owing to the presence of ammonia (NH3) gas. To avoid this, the graphite is usually covered with protective coatings.

A variation of MOCVD also uses cold-wall reactors. In such reactors, a pedestal is often used to provide support to the substrate. This pedestal also acts as susceptor, and is the source of heat in the reactor. As only susceptor is heated, the gases do not react before reaching the substrate surface. Typical materials used to make susceptor are radiation absorbing materials, e.g., carbon. Contrary to this, walls of the reactor are made from quartz which is transparent towards electromagnetic radiations. The heat radiated from the hot susceptor may heat the reactor walls to a certain extent, nonetheless, the walls of reactor in a cold-wall reactor are usually cooler than the susceptor or the substrate. On the other hand, the entire reaction chamber is heated in a hot-wall CVD. In some cases, this is desirable, as the decomposition of the gases before reaching the substrate may be necessary for deposition of the desired material.

Gas Inlet and Switching System

Specialised devices, called ‘bubblers’ are used to introduce gas into the reaction chamber. In the bubbler, a carrier gas (hydrogen for arsenide and phosphide, and nitrogen for nitride based compounds), is purged though the metalorganic liquid. While passing though this liquid, the gas picks up metalorganic vapors and carries it to the reaction chamber. The concentration of metalorganic vapors transported to the reactor depends upon the flow rate of carrier gas and the temperature inside the bubbler. The amount of these vapors is generally controlled with automated controllers fitted with the system (most commonly by an ultrasonic concentration measurement feedback control system).

Gas Exhaust and Cleaning System

Toxic wastes (gaseous state) generated during and after the growth process should be converted into liquid or solid forms to be recycled (preferred) or disposed. Additionally, tuning the processes so as to generate least amount of waste is an important research topic.

Environment, Health and Safety

MOCVD has gained much popularity as nanomaterials fabrication technique. There is an important concern about its potential impact on the safety of persons and environment. This technique involves handling of highly toxic and hazardous materials not only during particles synthesis, but also during the development of the equipment itself. Presently, the safety and responsible environmental care are the significant parameters having extreme importance in the MOCVD based crystal growth of compound semiconductors.

Quantum Dot Fabrication

MOCVD is a chemical vapor deposition technique used to grow materials epitaxially. It is widely used to produce compound semiconductors as well. Usually one of the reaction species containing the required chemical elements is an organic compound or metalorganics and metal hydrides, which has a low boiling temperature. Pyrolysis of the precursor materials at the surface of the substrate leads to the formation of the epitaxial layer. The semiconductor’s growth does not occur in the vacuum, rather with gas in the reactor at a moderate pressure (2 – 100 kPa). The III-V semiconductors (InN, GaN, GaAs etc), II-VI semiconductors (HgCdTe, ZnO), IV semiconductors (SiC, SiGe) are usually grown by MOCVD. Today it is predominantly used to fabricate LDs, solar cells, and LEDs in the industrial world. Today’s MOCVD GaN growth reactor was developed from the approach of Maruska and Tietjen, which is capable of depositing films with AlN and InN. The reactors can be of two types: horizontal (Figure 3(a)), and vertical (Figure 3(b)). In the case of horizontal reactor, the precursor (in gas form) is injected into the reaction chamber from left side, and the material gets deposited at the surface of the substrate in the middle of the reactor. For growing uniform films with desirable crystallinity, the substrate is generally inclined with respect to the horizontal flow direction of precursor gases. In case of vertical reactors, the precursor is injected into the reaction chamber from the upside inlet and the desired materials deposit on the surface of the substrate. The substrate is usually rotated to ensure the uniformity of the deposited films. The gaseous wastes, in both reactors, are removed from the reaction chamber by the carrier gas from the exhaust. Usually TMG, trimethylaluminum (TMA) and trimethylindium (TMI) are used as group III precursors. These precursors, usually diluted with H2, react with NH3 at a substrate (usually made up of SiC or sapphire) which is heated to around 1000 °C. MOCVD reactors for group-III nitride film growth incorporate laminar flow at high operating pressures and have separate inlets for the nitride precursors and the ammonia to minimize predeposition reactions. Figure 4 shows complex physical and chemical processes in the reactor. Initially TMG and NH3 are transported into the reaction chamber to undergo gas-phase reactions (TMG and NH3) in the chamber. The reactants are then transported onto the wafer surface. The diffusion and desorption of adatoms and radicals occurs on the substrate surface. Finally after surface reaction GaN gets deposited onto the surface.

Figure 3 Two types of MOCVD reactor

Figure 4 Major physical and chemical processes involved in MOCVD

Volmer and Weber predicted that, if the adhesion between the substrate and the depositing crystals is weak, only 3D nuclei can be formed on the substrate. The internal strain in these crystals depends upon the lattice mismatch, and can be high or low. This approach of epitaxial crystal growth is termed as Volmer Weber (VM) mechanism.

In the case of strong adhesion between the substrate and the depositing clusters and relatively small crystallographic lattice mismatch, 2D adsorption or phase formation may take place already at equilibrium and even at under saturations. 2D nuclei are formed also at super saturations and the growth follows a layer-by-layer mode as predicted by Frank and van der Merwe. This mode of epitaxial crystal growth is known as Frank-van der Merwe (FM) mechanism.

In the case of a strong adhesion between the substrate and the depositing clusters but significant crystallographic lattice mismatch the layer-by-layer growth takes place only during the deposition of the first few MLs. After that, the accumulated internal strain energy due to the strong lattice mismatch compensates the attractive forces with the substrate and internally strained 3D nuclei form on the top of the 2D MLs. This mode of the epitaxial crystal growth is known as a Stranski-Krastanov (SK) mechanism named after I. N. Stranski and L. Krastanov who considered such type of nucleation and crystal growth phenomenon.

For the growth of QDs using MOCVD all three growth modes are used, the most common technique exploits SK growth mode. Self-assembled QDs are formed when growing a semiconductor layer on top of a substrate material of smaller lattice constant. Above the critical thickness and under certain conditions, the strain relaxes by forming small islands, where at the surface the QD lattice constant relaxes to its bulk value Generally, a thin layer, which is known as the wetting layer, will remain, completely covering the substrate. The wetting layer forms a quantum well, which usually shows photoluminescence below the quantum dot emission wavelength. QDs are finally capped with (usually) the substrate semiconductor material to obtain a high quantum yield, i.e., avoiding non-radiative recombination via surface states.

References

1. MOCVD Epitaxy Johnson Matthey GPT
2. How MOCVD works. Deposition Technology for Beginners, Aixtron, May 2011
3. MOCVD Basics and Applications, Samsung Advanced Institute of Technology, 2004
4. Metalorganic chemical vapor deposition (MOCVD) Archived September 27, 2010, at the Wayback Machine.
5. http://www.anu.edu.au/CSEM/machines/MOCVD.htm
6. https://en.wikipedia.org/wiki/Metalorganic_vapour_phase_epitaxy