FBH research: 05.07.2017

Degradation of AlGaN-based metal-semiconductor-metal UV photodetectors

Degradation experiment of an unpassivated AlGaN/AlN MSM PD
Fig. 1: Plan-view light microscopic images of degradation experiment of an unpassivated AlGaN/AlN MSM PD biased at 20 V and illuminated locally by 250 nm radiation (top). After 25 hours dark dendritic features decorate the cathodes (C), but not the anodes (A), in the illuminated region only (bottom).
SEM image of degraded cathode & representative EDX spectrum
Fig. 2. Plan-view SEM image taken at 10 kV and 318 pA showing the tip of a degraded cathode on the unpassivated AlGaN absorber layer (top). Representative EDX spectrum of the (Al,Ga)O dendrites (bottom).
Illustration of the anodic oxidation process at the Pt/ambient/AlGaN electrochemical cell
Fig. 3: Illustration of the anodic oxidation process at the Pt/ambient/AlGaN electrochemical cell.
1 -> 2: Electrons (e-) from cathode and adsorbed water (H2O) from ambient produce hydroxyl (OH-).
2 -> 3: Decomposition of AlGaN to (Al,Ga)O due to presence of OH- and photoexcited holes (h+) at the AlGaN surface

AlxGa1-xN-based metal-semiconductor-metal photodetectors (MSM PDs) are very attractive for the detection of ultraviolet (UV) radiation in a variety of medical, scientific, industrial, and military applications. Although high external quantum efficiencies can be achieved when the PD is illuminated from the substrate side [1-2], to our knowledge no data on the long-term operation of AlGaN-based MSM PD is published, yet.

We investigated the degradation behavior of bottom-illuminated Al0.5Ga0.5N/AlN MSM PDs grown by metal-organic vapor phase epitaxy (MOVPE) on double-side polished c-sapphire substrates. For devices without any passivation only in the illuminated area dark dendritic features can be observed near the biased electrodes (Fig. 1) after the first 25 h of operation at 20 V and 0.4 mW/cm² intensity at 250 nm wavelength. These features are identified by EDX analysis as amorphous (Al,Ga)O growing on the surface of the AlGaN absorber (Fig. 2), most likely due to field-induced electrochemical reactions such as the anodic oxidation of the AlGaN layer, as reported by Gao et al. for AlGaN/GaN transistors [3]. Fig. 3 illustrates the corresponding processes at the Pt-electrode/air/AlGaN-system (electrochemical cell): The formation of (AlGa)O is driven by the presence of hydroxyl ions (OH-)—which are provided by adsorbed H2O from the ambient and electrons from the biased electrode—and photoexcited holes reaching the AlGaN surface. Obviously, the operation at higher UV intensities enhances this process, and as a consequence, the (Al,Ga)O finally lifts-off the biased electrodes from the AlGaN layer leaving the device useless after a couple of days of operation (< 60 h). Applying a SiN passivation layer on the AlGaN absorber (and the electrodes) suppresses the oxidation process, and therefore, significantly enhances the lifetime of the AlGaN MSM detectors. A passivated device with 340 nm SiN was operated for about 500 h at 20 V and 20 mW/cm² at 254 nm without showing any of the abovementioned signs of degradation (Fig. 2).

In conclusion, the anodic oxidation of the AlGaN absorber was identified as the dominant degradation mechanism limiting the lifetime of AlGaN-based MSM PDs. Applying a SiN-passivation layer suppressed the formation of (Al,Ga)O and enhanced the operation time from several 10 hours up to at least 500 hours. Further studies considering different SiN-thicknesses are still ongoing.

This study was partially supported by the German “Federal Ministry of Education and Research” (BMBF) within the “Advanced UV for Life” consortium and the “German Research Foundation” as part of the Collaborative Research Center ‘Semiconductor Nanophotonics’ CRC787.

Publications

[1] M. Brendel, M. Helbling, A. Knigge, F. Brunner, M. Weyers, "Measurement and simulation of top- and bottom-illuminated solar-blind AlGaN metal-semiconductor-metal photodetectors with high external quantum efficiencies", Journal of Applied Physics, 118(24), 2015.

[2] M. Brendel, F. Brunner, A. Knigge, M. Weyers, "AlGaN-based metal-semiconductor-metal photodetectors with high external quantum efficiency at low operating voltage", Proc. SPIE 10104, 2017.

[3] F. Gao, S.C. Tan, J.A. del Alamo, C.V. Thompson, T. Palacios, "Impact of water-assisted electrochemical reactions on the OFF-state degradation of AlGaN/GaN HEMTs", IEEE Trans. Electron Device, 61–2, pp. 437–444, 2014.