MOVPE grown low defect AlN-layer on AlN-substrates demonstrated
Aluminum nitride (AlN)-bases Ultraviolett (UV)-Technologies promise a wide range of potential market applications e.g. in the area of disinfection, agriculture, and process technologies. Integrated and highly compact UV light-emitting diode (LED) devices could, for example, be used to clean drinking water in remote areas and be applied to disinfect surfaces in hospitals and public areas.
Within the BMBF-funded consortium Advanced UV for Life, the IKZ develops processes for the production of AlN crystals and substrates as a basis for the manufacture of light-emitting diodes with ultra-short wavelengths (UVC-LED). The aim is to epitaxially grow the aluminum-rich AlxGa1-xN layers, in which the UV radiation is generated, on AlN substrates in order to realize low losses typically caused by non-radiative recombination due to a small lattice mismatch and resulting low dislocation densities. For this purpose, an AlN layer is first grown, which determines the crystalline quality of the following AlxGa1-xN layers.
A prerequisite for the growth of defect-free epitaxial layers is not only a low dislocation density in the bulk crystals but also an exact and homogeneous orientation of the substrates over the entire wafer and the preparation of a defect-free surface by suitable polishing processes. The ideal orientation (0.2° tilted to the crystal axis) determined together with the project partners Ferdinand-Braun-Institut (FBH) and the group of Prof. Kneissl at the TU Berlin could be exactly adjusted by means of a processing and measuring technology developed together with the project partner CrysTec GmbH. After the use of a standard process for chemical-mechanical polishing (CMP), there are still defects under the surface which are no longer detectable with conventional surface characterization methods, but which generate typical linear dislocation accumulations in the AlN layers grown at the FBH by metal organic gas phase epitaxy (MOVPE) [Fig. 1: left]. The damage layer can be removed before epitaxy, e.g. by plasma etching (ICP) [Fig. 1: right]. By means of transmission electron microscopy (TEM) it could be proven that the damage layer can already be completely removed with a CMP process optimized at the IKZ [Fig. 2].
In any case, the low-defect AlN epitaxial layers on precisely oriented AlN substrates provide an ideal basis for research work on aluminum-rich AlxGa1-xN layers or UVC LEDs with ultra-short wavelengths.
Figure 1: AlN substrates with MOVPE grown AlN layer and subsequent defect etching with molten salts. The inserts show AFM images with step flow before defect etching.
Left: No pre-treatment before MOVPE growth and resulting scratches below the surface, which become visible by defect etching and subsequent high dislocation density (DD > 108 cm-2).
Right: ICP etches pretreatment before MOVPE growth and resulting good surface quality and low dislocation density (DD < 106 cm-2).
Figure 2: TEM images of an AlN substrate surface after chemical-mechanical polishing. Top: Crystal volume near the surface (dark) in low magnification. Below: Near-surface ordered atomic structure without damage in high resolution.
Section: Crystalline Materials for Electronics