High responsivity UV-C AlGaN photodetectors
Fig. 1: AlGaN absorber layer on ELO template: cross-section backscattered electron microscopy image (a) and monochromatic cathodoluminescence images (b, c) from cleaved edge at 100 K.
Fig. 2: AlGaN MSM photodetectors on sapphire in different interdigitated designs and several test structures.
Fig. 3: Photo (dark lines) and dark current (light lines) / voltage characteristics of AlGaN MSM PDs on ELO and planar AlN templates with different dF and orientation for Popt=5.5 W/m² and λ=270 nm .
High-power UV lamps, UV LEDs and excimer lasers are widely used for UV lithography, medical applications and disinfection. For most applications it is necessary to monitor the emitted power in a certain wavelength range. Our AlGaN UV-C photodetectors (PD) on standard sapphire substrates feature about 60 % external quantum efficiency (EQE). Reducing the dislocation density, which is very high in these largely mismatched structures, appears attractive since this is supposed to lead to longer carrier lifetimes and thus higher EQE. As a means of reduction, epitaxial lateral overgrowth (ELO) of patterned AlN/sapphire templates has successfully been developed for UV LEDs. Adopting this approach to photodetectors yields interesting and promising result.
AlN/sapphire templates with etched stripes were first overgrown with a several micrometer thick AlN layer until complete coalescence of the surface was achieved. On these ELO templates an about 1.5 µm thick Al0.4Ga0.6N layer is grown acting as absorber layer of the photodetector. Fig. 1 shows an electron microscopy image of the full layer stack and monochromatic cathodoluminescence images of the absorber layer. Due to facetted growth over bunched steps on the ELO templates there are compositional modulations indicating inhomogeneous gallium incorporation with higher GaN fraction along the triangle edges.
MSM (metal-semiconductor-metal) detectors with interdigitated Pt/Ti/Au-Schottky contacts with different finger spacings dF were fabricated on these layers with two finger orientations (Fig. 2). Devices of type ELO-P have contact fingers parallel to the etched stripes of the ELO templates; devices of type ELO-O have fingers orthogonal to the stripes. Current/voltage characteristics under illumination show big differences for the different types of PD (Fig. 3). For detectors on planar templates the dark current remains in the low pA range and the photocurrent exceeds the dark current by a factor of 1000. ELO-P PDs with the carrier flow perpendicular to the stripes and across the compositionally modulated region behave similarly. ELO-O PDs with the carriers flowing parallel to the stripes show a steep increase in photocurrent for bias UB > 3 V due to internal gain. The responsivity of such ELO-O PDs strongly increases with ascending bias over the whole wavelength range. At 30 V the EQE is about 77 - and thus an improvement of two orders of magnitude. This data clearly indicates the presence of gain mechanisms. Despite this high gain, such detectors do not show signs of persistent photoconductivity (PPC). The devices are linear in the region of gain saturation (marked in Fig. 3) above Vbias > 15 V (range C), and with sufficiently large finger spacings dF = 15 µm the dark current stays in the 200 pA range.
In conclusion, the UV-C MSM photodetectors on ELO templates with proper contact finger orientation and spacing are very promising for applications where high responsivity, a linear response and dark currents in the pA range are required.
The AlGaN photodetector activities are funded by the BMBF within the Berlin WideBaSe initiative (03WKBT02C).
Publications:
V. Kueller, A. Knauer, C. Reich, A. Mogilatenko, M. Weyers, J. Stellmach, T. Wernicke, M. Kneissl, Z. Yang, C. L. Chua, N. M. Johnson, "Modulated Epitaxial Lateral Overgrowth of AlN for Efficient UV LEDs", IEEE Photonics Technol. Lett. 24 (2012) 1603.
A. Knigge, M. Brendel, F. Brunner, S. Einfeldt, A. Knauer, V. Kueller, M. Weyers, "AlGaN photodetectors for the UV-C spectral region on planar and epitaxial laterally overgrown AlN/sapphire templates", Phys. Status Solidi C, 1–4 (2012) / DOI 10.1002/pssc.201200636.
A. Knigge, M. Brendel, F. Brunner, S. Einfeldt, A. Knauer, V. Kueller, U. Zeimer, M. Weyers, "AlGaN MSM Photodetectors on Planar and Epitaxial Laterally Overgrown AlN/Sapphire Templates for the UV-C Spectral Region", accepted for publication in Jpn. J. Appl. Phys.
FBH research: 22.01.2013