Charakterisierung und Optimierung von (Al, Ga)N-basierten UV-Photodetektoren

In this work, the influence of dislocation density, heterostructure design and detector geometry on the electro-optic properties of AlGaN-based metal-semiconductor-metal photodetectors (MSM PD) was investigated. The focus was on the realization of Schottky-type MSM PD suitable for photodetection below wavelengths of 300 nm with a high external quantum efficiency (EQE). First, experimental and simulated results were combined to give a detailed study of the performance of top- and bottom-illuminated Al0.5Ga0.5N/AlN MSM PD on planar c-sapphire templates. Of particular note here is the threshold and saturation behavior of the EQE as a function of the operating voltage observed for all detectors with an AlN buffer layer under bottom illumination, since an EQE of 50 % and 68 % was achieved at 50 V for absorber layer thicknesses of 0.1 μm and 0.5 μm, respectively. The analysis of 2-dimensional simulation data showed that this is essentially caused by the polarization charge located at the AlGaN/AlN heterojunction, leading to an accumulation of photogenerated minority carriers along this interface. The transport of these charge carriers to the Schottky contact correlates to a certain threshold voltage which decreases from 22 V to 5 V with decreasing absorber layer thickness. For thin detectors therefore an EQE of about about 30 % was obtained at 7 V or 1 V for geometrically symmetric or asymmetric electrode configuration, respectively. The use of an alloyed and a non-alloyed electrode comb in the MSM design (a-MSM) finally allowed an EQE of 24 % with no external bias. In addition, top-illuminated Al0.4Ga0.6N MSM PD fabricated on two different template types were examined, which by means of epitaxial lateral overgrowth (ELO) of AlN were produced on patterned AlN/sapphire to investigate the device behavior at reduced threading dislocation density. Using SEM and CL it was found that the Al0.4Ga0.6N absorber layers on ELO-AlN showed a periodically modulated distribution of the threading dislocation density as well as an inhomogeneous material composition. With thin absorber layers (ELO) high dark currents due to internal interfaces as well as photoconductive gain due to hole accumulation at these interfaces were observed. For thick absorber layers (ELOB), in which the material inhomogeneity is buried far from the electrodes, an increase in the EQE over that of the planar PD of same geometry was observed, that was explained by the threading dislocation density being reduced from 9×10^9 /cm² in the planar PD to 1.4×10^9 /cm² in the ELOB PD. Finally, the anisotropy of the EQE of MSM PD with respect to substrate miscut and electrode orientation was explained by approximating the modulated dislocation density as a periodically altering arrangement of AlGaN regions of different specific resistance promoting the flow of photo-induced current between two electrodes for the parallel case. These results have also been reproduced based on 1-dimensional calculations. Overall understanding of transport processes in AlGaN MSM PD has been improved and approaches for the realization of detectors with high EQE but without gain as well as very sensitive, yet linear detectors with gain have been demonstrated.

Author dc:creator

• Brendel, Moritz

Advisor dc:contributor.advisor

• Weyers, Markus

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Language dc:language.iso

de

Identifier URI

http://dx.doi.org/10.14279/depositonce-5645

OAI identifier oai:identifier

oai:depositonce.tu-berlin.de:11303/6064