MOVPE Nitrides

Nitride-based epitaxial processes are performed in two different MOVPE reactors at FBH. For optoelectronic devices a MOVPE reactor AIX200/4-RF-S with a capacity of 1×2" is used. The reactor is equipped with a LayTec high resolution curvature sensor (EpiCurveTT-HR). Futhermore a planetary reactor AIX2600G3-HT with a capacity of 8×3" or 11×2" is used at FBH also epuipped with in-situ sensors. In this production-type reactor, electronic devices and GaN templates as starting substrates for the single wafer reactor and the HVPE reactor are fabricated. High temperatures up to 1600°C can be reached due to the stainless steal reactor chamber. High-quality AlN can therefore be fabricated for the use as templates for optoelectronic structures for far-UV emission.

For the development of electronic devices such as AlGaN/GaN heterostructure field-effect transistors (HFET) 75 mm semi-insulating substrates are used. They offer a high thermal conductivity in comparsion to the cheaper saphire. The layer structure for HFET devices usually consists of a highly resistant GaN buffer layer, a GaAs channel and an Al_xGa_1-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/violett and green spectral range (390 nm – 520 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 und Al_xGa_1-xN layers for wave guiding on GaN substrates. LED structures are grown on sapphire and consist of light-emitting In_yAl_xGa_1-x-yN layers sandwiched between Al_xGa_1-xN layers.

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.