FBH research: 21.12.2017

Impact of open-core threading dislocations on the performance of AlGaN metal-semiconductor-metal photodetectors

Design of AlGaN MSM PDs
Fig. 1: Design of AlGaN MSM PDs: (a) cross-sectional sketch of layer design, (b) light microscopic plan-view image of processed MSM PD on (dark: semiconductor, bright: metal) [2]
EQE vs. bias characteristics of several MSM PDs
Fig. 2: EQE vs. bias characteristics of several MSM PDs illuminated from the bottom side at 250 nm wavelength along a 2-inch wafer from center (black) to edge (red). All devices show increasing EQE until saturation above Usat, while values of Usat vary between 10 V and 40 V. Dark currents Idark for all devices are low (exemplarily shown for one MSM PD with high and one with low Usat) [2]
Images of the AlGaN surface by optical microscope
Fig. 3: Images of the Al0.5Ga0.5N surface by optical microscope (10x magnification) with Nomarski contrast (a) at center, (b) at edge of the 2-inch wafer. Density of hillocks (bright dots): 9 ∙ 102 cm-2 (center) and 1 ∙ 104 cm-2 (edge). (c) AFM topogram of typical hillock with hole in top facet, (d) SEM image of FIB cross section through defect shown in (c) [2]

The alloy AlxGa1-xN used as absorber material in photodetectors (PDs) offers a high sensitivity in the ultraviolet (UV) spectral range with cut-off wavelength between 200 nm (AlN) and 365 nm (GaN), adjustable by the Al content. UV PDs are of interest for applications in various fields like science, medicine, and industry. A metal-semiconductor-metal (MSM) design provides the advantage of a simple layout: No doping or ohmic contacts are needed, making MSMs relatively easy to fabricate. Under bottom illumination they show a bias-dependent saturation at high external quantum efficiencies (EQE) of up to 0.7, which was explained by two-dimensional drift diffusion modeling [1]. However, a rather high external bias is needed to get sufficiently high EQE. Recently, we found that open-core threading dislocations can have a beneficial impact on the MSM PDs by lowering the saturation voltage.

The layer design of the investigated MSM PDs consists of a 500 nm Al0.5Ga0.5N absorber on a 500 nm thick AlN buffer layer on top of the double-side polished sapphire substrate (Fig. 1a). Upon the Al0.5Ga0.5N absorber, the electrodes are processed in an interdigitated way forming a quadratic device area of 400 µm x 400 µm (Fig 1b). It was found that the EQE-bias characteristics of bottom-illuminated MSM PDs change over a 2-inch wafer (Fig. 2). MSM PDs in the center show a saturation voltage of about 35 V, which is in agreement with the calculated value from simulation [1]. In contrast to that, MSM PDs at the edge show a saturation voltage of about 10 V. Dark currents for both types of MSM PDs are in the range of pA. This difference in saturation voltages can be correlated with varying densities of open-core threading dislocations. Such dislocations terminate at the Al0.5Ga0.5N surface as hillocks in the shape of hexagonal prisms (Fig. 3a-c). MSM PDs showing the reduced (about 20 V lower) saturation voltage are located in areas with high density of open-core threading dislocations. These dislocations originate at the Al0.5Ga0.5N/AlN interface, leading to the formation of Ga-rich regions and voids, which penetrate through the Al0.5Ga0.5N absorber (Fig. 3d) [2]. Thus, they form electrically active channels connecting the electrodes with the Al0.5Ga0.5N/AlN interface, where most of the photogenerated holes accumulate. Hence, they can be swept out at a reduced bias voltage, when the absorber is not yet fully depleted.    

In conclusion, it was shown that open-core threading dislocations have to be considered as electrically active channels in AlGaN-based devices. In contrast to usually limiting effects of those dislocations, for example in LEDs, the MSM PDs benefit by enabling high EQE at lower bias voltage.

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", J. Appl. Phys., 118 (2015), 24.

[2] S. Walde, M. Brendel, U. Zeimer, F. Brunner, S. Hagedorn, M. Weyers, "Impact of open-core threading dislocations on the performance of AlGaN based metal-semiconductor-metal photodetectors", J. Appl. Phys. 123, 161551 (2018).