Epitaxy - the Basis for our Semiconductor Devices

The basis for our devices is the epitaxial growth of III/V semiconductor structures on GaAs, InP, GaSb and InAs substrates. This is achieved by two solid source Molecular Beam Epitaxy (MBE - Fig. 2) machines and one Gas Source Molecular Beam Epitaxy (GSMBE) machine working with gaseous precursors for group V elements. The complete material system shown in Fig. 1 is available, whereby each machine focuses on a certain area in this map.

Fig. 1: Material system of our
epitaxy group

The As/P-MBE (Fig.2) is used for the AlGaAs system as well as for AlGaInAsP on InP substrate, the latter’s precise control is the basis for our VCSELs and Quantum Cascade Lasers. With GSMBE, AlGaInAsNPSb is grown on GaAs and InP-substrates. This system has the advantages of handling the phosphorous and the selective growth on especially patterned samples. The Sb-MBE is specialized for AlGaInAsSb on GaSb substrate, a system which yields in mid infrared lasers with very long wave lengths larger than two micrometers.

Fig. 2: The Molecular Beam
Epitaxy System

A forum for epitaxial growth systems can be found at  www.epitaxy.net. There are a lot of interesting links about epitaxy, as well as a map of epitaxial systems installed worldwide.

Epitaxy - Molecular Beam Epitaxy

Epitaxy system:

Our Varian Modular GEN II MBE is a three chamber system, consisting of an entry/exit -chamber for mounting wafers on a transfer trolley, the buffer chamber for heating the wafers and the main chamber. The main chamber is pumped by a ionisation pump and a cryo pump reaching a background pressure of 10-10 mbar. In this very good vacuum we evaporate Gallium (Ga at about 900°C), Indium (In, 800°C), Aluminium (Al, 1000°C), Beryllium (Be, ~1000°C), Silicon (Si, ~1300°C), Tellurium (Te, ~300°C) Arsenic (As, ~370°C) and Antimony (Sb, 600°C). All materials are filled in boron nitride crucibles, the cells are closed by a tantalum shutter. After opening the shutter a molecular beam vaporises the wafer, which is located on a substrate holder that can be rotated for better thickness and composition homogeneity. To reduce impurities all cells and the inside of the main chamber are cooled with liquid nitrogen. With our cell configuration we can grow all combinations of the three group III-elements (Ga, In, Al) with our group V-elements As and Sb. For n-doping we use Si in Arsenides and Te in Antimonides, for p-doping Be.

On standard available substrates we grow :

• AlGaAs and GaInAs on GaAs
• (Al, Ga, In)As and (Al, Ga, In)AsSb on InP
• (Al, Ga, In)AsSb on GaSb

We usually grow in a pseudomorphic growth mode, thereby the substrate lattice constant is maintained (no relaxation). The growing layer can be strained against the substrate lattice constant if a critical thickness is not exceeded (e.g. 10 nm GaInAs on GaAs with an In-content of 25%, or 5 nm InAs on InP.

Basic structures for device technology:

• Long wavelength edge emitting lasers on InP substrates up to 2.27 µm using a highly strained In(Ga)As active region
• AlInAs/AlGaInAs distributed bragg reflectors and structures for vertical cavity surface emitting lasers on InP
• AlGaInAs Schottky diodes on InP
• GaInAsSb/AlGaAsSb for long wavelength edge emitting lasers above 2 µm on GaSb

Molecular Organic Vapour Phase Epitaxy

The Metal Organic Vapor Phase Epitaxy (MOVPE) uses gaseous sources and operates at atmospheric (AP-MOVPE) or low pressure (LP-MOVPE), which means values between 50 and 200 mbar. For the incorporation of Aluminium, Gallium, Indium, Zinc and also Antimony metal organic molecules like Trimethylantimony (TMSb), Triethylgallium (TEGa) or Dimethylzinc (DMZn) are employed, which is the reason why this technique is called MOVPE. For the growth of Arsenic and Phosphorus containing alloys the hydrides Arsine (AsH3) and Phosphine are deployed and for Silicon doping Silane (SiH4) is used. Because of the high flammability and toxicity of these molecules much effort for the security of such a system has to be made. So the basic differences compared to the MBE crystal growth technique are the source materials, which are molecules instead of the pure elements, and furthermore the operating pressure, as no UHV condition is needed

Figure 1: Some source molecules and the basic MOVPE growth process are illustrated.

The fundamental MOVPE growth process is sketched in figure 1. The substrate lays on a graphite susceptor heated by IR-lamps. Due to this heat the gas molecules, which are flowing above the susceptor, are decomposed and the pure elements are incorporated into the substrate, whereas the methyl and ethyl groups desorb and are pumped away. Our AIXTRON 200/4 LP-MOVPE is equipped with TMGa, TEGa, TMIn, TMAl, DMZn, Carbontetrabromide (CBr4), Silane, TMSb, Arsine and Phosphine. For in-situ reflectance measurements we use a Laytec EpiTT. We are focused on the (AlGaIn)(AsP) material system for the fabrication of InP based VCSELs and also the growth of GaInPSb and GaInAsSb has been investigated. Besides these material related topics we use MOVPE for the overgrowth of DFB gratings and selective overgrowth, which is illustrated in figure 2.

Figure 2: MOVPE overgrowth of a
DFB grating.