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Gallium Nitride - Molecular Beam Epitaxy

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Fig. 1
Fig. 1: Surface morphology of an AlGaN/GaN heterostructure (see text) grown under Ga-lean stoichiometry on a defect-rich template. Up to 30 nm deep pits are detected.

During the past years we developed comprehensive knowledge and skills in growing ultra-pure AlGaN/GaN heterostructures with smooth and atomically-sharp interfaces by molecular beam epitaxy (MBE). The resulting epitaxial layer stacks are a test ground for novel lateral and vertical electrical and optical device concepts.
Despite the perfect 2-dimensnional growth mode and the ultra-pure material grown, surface defects from MBE-grown layers altered the electrical properties of vertical GaN-based MOSFET devices. A systematic study figured out that the origin of these surface defects is the threading dislocation density of the underlying substrate in combination with the Ga/N stoichiometry during growth. Screw and mixed type dislocations proceed into the MBE layers and Ga-rich or –lean growth conditions lead to a different decoration of these dislocations. MBE growth on substrates with low dislocation density supports this finding and ultimately it was shown that defect-free growth of structures resulting in vertical and lateral device schemes has become reality.

Fig. 2
Fig. 2: Surface morphology of an AlGaN/GaN heterostructure (see text) grown under Ga-rich stoichiometry on a defect-rich template. Hillocks with heights of less than 1 nm are vsible.

 

In the aforementioned systematic study a nominally identical AlGaN/GaN heterostructure was grown on substrates synthezied with different vapor phase growth techniques exhibiting dislocation densities in a wide range between 5x104 cm-2 and 5x109 cm-2. The MBE layer structure consists of 1 µm GaN, 16 nm Al0.06Ga0.94N and a 3 nm thick GaN cap. At the GaN/AlGaN interface a 2-dimensional electron gas is confined featuring an electron density of ~2x1012cm-2. That type of structure is used for subsequent fabrication of lateral field-effect transistor prototype devices to compare substrate-dependent switching characteristics and optical properties. The surface morphology after the MBE step is monitored with atomic force microscopy (AFM).

Fig. 3
Fig. 3: Surface morphology of a Ga-rich grown AlGaN/GaN heterostructure (see text) on a defect-lean template. None of the above described defects are visible. Instead 260 pm high monolayer steps are observed caused by the template miscut.

 

 

 

The surface morphology of the layer stack grown under Ga-lean stoichiometry exhibits several 10 nm deep pits (Fig 1). Its Ga-rich counterpart shows the formation of hillocks with a height of less than 1 nm (Fig 2). The density of pits and hillocks for both growth regimes is comparable and in agreement with the substrate specification. Defect-free growth was demonstrated using a GaN substrate with a specified dislocation density of 5x104 cm-2. On a 5x5 µm2 AFM scan no defect is observed (Fig 3). This impressively shows that the ultimate structural surface morphology of the MBE layer is set by the dislocation density of the substrate. A solid side aspect of the finding is the retrospective possibility of the substrate dislocation density analysis, either in comparison with e.g. vendor specifications or as a determination of its previously non-accessible value.

 

Contact: Dr. Stefan Schmult

 

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