Black Germanium Photodetector Exceeds External Quantum Efficiency of 160%

Abstract

Inspired by the formation of black Si, [14,15] black Ge with nanospiked structures was achieved by femtosecond laser irradiation in SF 6 environment. [16] However, optical properties of the structure were not characterized. There have been intensive research efforts to fabricate black Ge by using the standard dry etching technique based on sulfur hexafluoride (SF 6 ) and chlorine (Cl 2 ) gases. [17,18] Although Pasanen et al. fabricated black Ge surface by SF 6 -based inductively coupled plasma reactive ion etching (ICP-RIE), [19] the etching process was carried out at −120 °C, resulting in substantial challenges for processing. Steglich et al. realized black Ge by using Cl 2 -based RIE at room temperature, but the process requires a longer etching time of 45 min. [18] The extremely high absorbance of black Ge reaching 99% was experimentally obtained in the 300-1600 nm wavelength range. However, more importantly, the excellent light absorption property of black Ge has not translated into optoelectronic performance of any functional devices.In this paper, we report the demonstration and comprehensive performance characterization of black Ge-based metal-semiconductor-metal (MSM) PDs. Spike-like Ge nanostructures with optimized dimension and uniformity are formed by Cl 2 -based room-temperature standard RIE process in 2 min. Black Ge surface exhibits an optical reflection as low as 1% at a wavelength range of 1000-2000 nm. The surface is covered with aluminum oxide (Al 2 O 3 ) to reduce dark current via an atomic layer deposition (ALD). The optoelectrical properties of black Ge PDs are compared to those of the Ge PDs made on planar surface to evaluate the applicability of black Ge toward light trapping at a device level. Responsivity of black Ge MSM PDs is enhanced significantly in the NIR range of 1.5-2 µm with a slight degradation of dark current. In addition, EQE of black Ge PDs not covered with Al 2 O 3 layer is as high as 161% at 1550 nm. Therefore, our black Ge fabricated by the simple RIE process exhibits outstanding potential for efficient NIR photodetection. Results and DiscussionFigure 1 illustrates the schematic fabrication process of black Ge PDs. Detailed process is provided in Method. In short, undoped Ge substrate (n-type, resistivity > 50 Ω cm) was thoroughly cleaned by acetone, isopropyl alcohol (IPA), and DI water. The dry etching was performed in Cl 2 environment for 2 min to In this work, a viable method is demonstrated to realize high-performance germanium (Ge) photodetectors (PDs) on the nanostructured Ge surface, namely black Ge, formed by chlorine (Cl 2 ) gas-based reactive ion etching at room temperature. Black Ge surface has spike-like pyramidal structures with a width and height up to 150 and 570 nm, respectively. Average reflection of black Ge is reduced to 2% at a wavelength range from 1 to 2 µm, while that of planar Ge is ≈37%. Light absorption is strongly enhanced by the significantly reduced reflection, thereby leading to an increase in responsivity of black Ge PDs. Moreover, external...

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