Optimization of AlGaN UV photodetectors for high quantum efficiency at low voltage
Fig. 1: Layout of AlGaN-based MSM PD. (a) Epitaxial layer structure. (b) Planar electrode design (top) with symmetric (middle) or asymmetric (bottom) electrode pair configuration.
Fig. 2: EQE-bias characteristics of 0.5 µm thick MSM PD at a wavelength of 250 nm measured for two symmetric devices with a different number of electrode pairs under top illumination (black) and two devices consisting of 29 electrode pairs but different electrode geometry under bottom illumination (red).
Fig. 3: EQE-bias characteristics of 0.1 µm thin MSM PD at a wavelength of 250 nm measured under bottom illumination for two devices consisting of 29 electrode pairs but having different electrode geometry.
The AlxGa1-xN material system is attractive for the detection of ultraviolet (UV) radiation due to its large band gap energy (3.4 eV to 6.2 eV). This feature makes it UV-robust and enables to adjust the cut-off wavelength between 365 nm and 200 nm by varying the Al mole fraction x in the photoactive layers. The planar design (Fig. 1a) of metal-semiconductor-metal photodetectors (MSM PD) simplifies epitaxy and processing since no doping or ohmic contacts are needed. However, the achievable external quantum efficiency critically depends on layer design and electrode geometry. As we reported earlier, MSM PD with thin AlGaN absorber layers on AlN/Sapphire templates show high external quantum efficiencies (EQE), saturating above 50% when the PD is illuminated from the substrate side and a certain saturation voltage is applied to the co-planar electrodes [1].
We developed solar-blind Al0.5Ga0.5N/AlN MSM PDs with geometrically symmetric or asymmetric electrode configuration (Fig. 1b) on absorber layers with thicknesses tabs of 500 nm and 100 nm [2]. According to the EQE-bias characteristics of symmetric devices with tabs = 500 nm under top illumination (black data in Fig. 2), the EQE can be enhanced by a factor of 1.7 by increasing the number of illuminated space-charge regions from 29 to 49. However, the EQE just approaches about 17% at 50 V. The asymmetric devices with 29 electrode pairs consisting of two 8 µm and 2 µm wide electrodes show a slight asymmetry of the EQE under bottom illumination conditions with respect to bias polarity (red data in Fig. 2). Especially, the EQE below reverse saturation (-20 V) is enhanced by a factor of three, compared to the corresponding symmetric device with also 29 electrode pairs due to the larger carrier collection volume below each broadened (left) electrode. In the 500 nm thick absorber layer most of the photo-generated charges recombine far from the space-charge regions, and therefore, only an EQE of 8% is achieved below reverse saturation. Consequently, by reducing the absorber layer thickness to 100 nm the EQE below threshold was enhanced (Fig. 3), reaching about 25% already at a very low voltage of only +1 V due to the reduced recombination losses.
In conclusion, the combination of a sufficiently thin absorber layer and a geometrically asymmetric electrode scheme enables for AlGaN-based MSM photodetectors with high EQE values at very low bias voltages under bottom illumination conditions.
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 effciency at low operating voltage”, Proc. SPIE 10104, 2017.