Nitride-based epitaxial processes are performed in four MOVPE reactors at FBH. A MOVPE reactor AIX200/4-RF-S with a capacity of 1×2" is used to deposit epitaxial structures for laser diodes. The reactor is equipped with a LayTec high resolution curvature sensor (EpiCurveTT-HR). Futhermore, two planetary reactors AIX2600G3-HT with a capacity of 11×2" or 8×3"/4" are used at FBH also equipped with in-situ sensors. In this production-type reactors, high temperatures up to 1600°C can be reached in the stainless steal reactor chamber. High-quality AlN can therefore be fabricated for the use as templates for optoelectronic structures for far-UV emission. A Close Coupled Showerhead reactor (6x2") is employed for fabricating UV LEDs.
For the development of electronic devices such as AlGaN/GaN heterostructure field-effect transistors (HFET) 100 mm semi-insulating SiC substrates are used. They offer a high thermal conductivity in comparsion to the cheaper sapphire. The layer structure for HFET devices usually consists of a highly resistive GaN buffer layer, a GaN channel, and an AlxGa1-xN-barrier layer with an Al content of x=0.15 to x=0.35. Due to the spontaneous piezo-electric polarization in the AlGaN-GaN material system, carriers accumulate at the interface resulting in a two-dimensional electron gas. Typical carrier mobilities for a layer concentration of 1x1013 cm-2 are in the range of 1600 cm2/Vs. Higher mobilities of more than 2000 cm2/Vs can be reached by introducing an additional thin AlN barrier.
Optoelectronic layer structures are developed for laser diodes in the blue/violet spectral range (390 nm – 450 nm) as well as for UV light-emitting diodes (< 390 nm). The laser diodes are based on 2 - 7 nm thick InGaN quantum wells as light-emitting layers embedded in GaN and AlxGa1-xN layers for wave guiding on GaN substrates. LED structures are grown on sapphire and consist of light-emitting InyAlxGa1-x-yN layers sandwiched between AlxGa1-xN.
The polarization of GaN which is helpful for electronic devices reduces the efficiency of LEDs and laser diodes. Since polarization fields only exist along the c-axis of GaN, these field effects can be reduced by growing GaN with a different crystalline orientation. Basic research concerning the growth of GaN in different non-polar and semi-polar orientations is therefore also carried out at FBH in cooperation with the TU Berlin.