Low‐Temperature Operation of High‐Efficiency Germanium Quantum Dot Photodetectors in the Visible and Near Infrared

Abstract

The temperature-dependent operation of high efficiency Ge quantum dot (QD) photodetectors (PDs) is reported, that shows spectral responsivity of 1.2 A W À1 , internal quantum efficiency (IQE) of 228% and signal-to-noise ratio (SNR) equal to 7 Â 10 6 at a wavelength of 640 nm for 12 mW of incident power. The performance of these photodetectors can be improved by reducing the operating temperature, especially at low incident power. For instance, at 10 nW of 640 nm illumination power, lowering temperature from 300 to 100 K improves SNR from 2 Â 10 4 to 2 Â 10 5 and specific detectivity D Ã from 1.2 Â 10 11 to 2 Â 10 13 cm Hz 1/2 W À1 . This enhanced performance is attributed to saturation of the charging process within the QD layer, that leads to longer hole lifetimes and IQE exceeding 22 000%. Also, the nearinfrared performance of these PDs is reported, finding that below 200 K there is a significant near-IR photocurrent (three orders of magnitude larger than the dark current at 1100 nm and two orders of magnitude larger than the dark current at 1300-1550 nm, where only the Ge QDs contribute to optical absorption), leading to operational PDs, albeit at lower D Ã .

Ge nanoparticles in SiO2 for near infrared photodetectors with high performance

“…In the last decade, many research groups have paid attention on amorphous germanium nanoparticles (Ge-NPs) embedded in different oxide matrices because of their attractive electrical and optical properties which are suitable for different applications like photo-detectors 1–3 , solar cells 4 , light-emitting diodes 5 , memory devices 6 , MOSFET transistors 7,8 and lithium-ion batteries with high charge-discharge rate 9,10 . This effort has the main aim to extend the sensitivity domain of photodetectors toward near infrared (NIR) and therefore to develop the optoelectronics in this wavelength range as Si based photodetectors are usually limited to 1.1–1.2 μ m. Ge is the main candidate that has attracted attention for Si replacement because Ge has a higher carrier mobility than Si 11 .…”

Broadband visible-to-telecom wavelength germanium quantum dot photodetectors

“…For T ¼ 100 K, I ph shows a sublinear behavior across the entire incident power range for both illumination wavelengths, which is attributed to the saturation of the QD charging process and the reduction of available tunneling paths through the QD network as temperature is lowered, due to suppressed phonon-assisted tunneling. 16 The sublinear behavior of the photocurrent for T ¼ 100 K at both k results in an increasing IQE as P in is reduced, reaching a maximum of 1000% for k ¼ 1550 nm at P in ¼ 10 nW.…”

“…Previously, we reported PDs based on Ge QDs embedded in a SiO 2 matrix, fabricated on Si substrates, with spectral responsivities up to 4 A/W and high internal gain up to 700% in the k ¼ 400-1100 nm range. [13][14][15][16] However, those PDs were incapable of producing a significant photoresponse beyond k ¼ 1100 nm, corresponding to the Si substrate bandgap. 17 In this work, Ge QD PDs were fabricated on Ge substrates, extending the working spectral range up to the k ¼ 1550 nm telecom wavelength.…”

“…The fabrication process followed was the same as described in previous work, 15,16 but here Ge and SiO 2 targets were co-sputtered on n-Ge substrates so as to extend the photoresponse up to k ¼ 1550 nm, based on the fact that photon absorption takes place predominantly in the substrate, which in turn determines the PD's working spectral range. 13 The n-Ge substrates used were lightly doped (q ¼ 2.6 X cm) with the aim of minimizing recombination of photogenerated carriers and were kept at 400 C during deposition.…”

“…Finally, the structure was completed by attaching the device to a sapphire substrate with a back indium contact. 15,16 The schematic geometry of the Ge QD PD structure is depicted in Fig. 1(a), whereas the energy band diagram including the corresponding interband optical transitions responsible for the photoresponse is provided in Fig.…”