Efficient photon capture on germanium surfaces using industrially feasible nanostructure formation

Nanostructured surfaces are known to provide excellent optical properties for various photonics devices. Fabrication of such nanoscale structures to germanium (Ge) surfaces by metal assisted chemical etching (MACE) is, however, challenging as Ge surface is highly reactive resulting often in micron-level rather than nanoscale structures. Here we show that by properly controlling the process, it is possible to confine the chemical reaction only to the vicinity of the metal nanoparticles and obtain nanostructures also in Ge. Furthermore, it is shown that controlling the density of the nanoparticles, concentration of oxidizing and dissolving agents as well as the etching time plays a crucial role in successful nanostructure formation. We also discuss the impact of high mobility of charge carriers on the chemical reactions taking place on Ge surfaces. As a result we propose a simple one-step MACE process that results in nanoscale structures with less than 10% surface reflectance in the wavelength region between 400 nm and 1600 nm. The method consumes only a small amount of Ge and is thus industrially viable and also applicable to thin Ge layers.Comment: 8 pages, 4 figures. Full citation details and link to manuscript published in Nanotechnology were adde

(oral talk) Nanostructured germanium with >99 % absorption for near-infrared sensor applications

| openaire: EC/H2020/777222/EU//ATTRACTPeer reviewe

Nanostructured germanium with >99% absorption at 300-1600 nm wavelengths

Tarkista embargo, kun artikkeli julkaistu. | openaire: EC/H2020/777222/EU//ATTRACTNear-infrared (NIR) sensors find numerous applications within various industry fields, including optical communications and medical diagnostics. However, the state-of-the-art NIR sensors made of germanium (Ge) suffer from rather poor response, largely due to high reflection from the illuminated device surface. This work demonstrates a method to increase the sensitivity of Ge sensors by implementing nanostructures to the wafer surfaces. The absorbance of nanostructured Ge wafers is measured to be >99% in the whole UV–vis–NIR spectrum up to 1600 nm wavelength, which is a significant improvement to bare Ge wafers that reach absorption of only 63% in maximum. The process is shown to be capable of producing uniform nanostructures covering full 100 mm diameter substrates as well as wafers with etch mask openings of different sizes and shapes, which demonstrates its applicability to complementary metal oxide semiconductor (CMOS) sensor manufacturing. The results imply that nanostructured Ge has potential to revolutionize the sensitivity of Ge-based sensors.Peer reviewe

Comparison of SiNx-based Surface Passivation Between Germanium and Silicon

Germanium (Ge) has attracted much attention as a promising channel material in nanoscale metal-oxide-semiconductor devices and near-infrared sensing because of its high carrier mobilities and narrow bandgap, respectively. However, efficient passivation of Ge surfaces has remained challenging. Herein, silicon nitride (SiNx)-based passivation schemes on Ge surfaces are studied and the observations are compared to Si counterparts. These results show that instead of a high positive charge density (Q(tot)) that is found in SiNx-passivated Si samples, similar Ge samples contain a high amount of negative Q(tot) (in the range of 10(12 )cm(-2)). The maximum surface recombination velocity of the samples is shown to reduce by a factor of three in both Si and Ge samples by a post-deposition anneal at 400 degrees C. The SiNx-coated samples are capped with an atomic-layer-deposited aluminum oxide (Al2O3) layer, which reduces the midgap interface defect density (D-it) after annealing to 7 x 10(10) and 4 x 10(11) cm(-2) eV(-1) in Si and Ge, respectively. Interestingly, while the Al2O3 capping seems to have no impact on Q(tot) of the Si samples, it turns the stack virtually neutral (similar to-1.6 x 10(11) cm(-2)) on Ge. The presented SiNx-based passivation schemes are promising for optoelectronic devices, where a low D-it and/or a low charge are favored.Peer reviewe

(oral talk) Controlling charge polarity and defect density at Ge/Al2O3 interface using SiNx interlayer

| openaire: EC/H2020/777222/EU//ATTRACTPeer reviewe

Achieving surface recombination velocity below 10 cm/s in n-type Germanium using ALD Al2O3

| openaire: EC/H2020/777222/EU//ATTRACTDesirable intrinsic properties, namely, narrow bandgap and high carrier mobility, make germanium (Ge) an excellent candidate for various applications, such as radiation detectors, multi-junction solar cells, and field effect transistors. Nevertheless, efficient surface passivation of Ge has been an everlasting challenge. In this work, we tackle this problem by applying thermal atomic layer deposited (ALD) aluminum oxide (Al2O3), with special focus on the process steps carried out prior to and after dielectric film deposition. Our results show that instead of conventional hydrofluoric acid (HF) dip, hydrochloric acid (HCI) pre-treatment is an essential process step needed to reach surface recombination velocities (SRVs) below 10 cm/s. The main reason for efficient surface passivation is found to be a high dielectric charge that promotes the so-called field-effect passivation. Furthermore, the results demonstrate that the post-deposition anneal temperature, time, and ambient play a role in passivating Ge-dangling bonds, but surprisingly, good surface passivation (SRV below 26 cm/s) is obtained even without any post-deposition annealing. The results pave the way for high-performance n-type Ge optoelectronic devices that could use induced junctions via negatively charged Al2O3 layers.Peer reviewe

Superior gamma-detection and IR imaging via ALD-passivated germanium nanostructures

| openaire: EC/H2020/777222/EU//ATTRACTNon peer reviewe

Efficient surface passivation of germanium nanostructures with 1% reflectance

| openaire: EC/H2020/777222/EU//ATTRACT | openaire: EC/H2020/101004462/EU//ATTRACT2Germanium (Ge) is a vital element for applications that operate in near-infrared wavelengths. Recent progress in developing nanostructured Ge surfaces has resulted in > 99 % absorption in a wide wavelength range (300 nm- 1700 nm), promising unprecedented performance for optoelectronic devices. However, excellent optics alone is not enough for most of the devices (e.g. PIN photodiodes and solar cells) but efficient surface passivation is also essential. In this work, we tackle this challenge by applying extensive surface and interface characterization including transmission electron microscopy and X-ray photoelectron spectroscopy, which reveals the limiting factors for surface recombination velocity of the nanostructures. With the help of the obtained results, we develop a surface passivation scheme consisting of atomic-layer-deposited aluminum oxide and sequential chemical treatment. We achieve surface recombination velocity as low as 30 cm/s combined with ~1 % reflectance all the way from ultraviolet to NIR. Finally, we discuss the impact of the achieved results on the performance of Ge-based optoelectronic applications, such as photodetectors and thermophotovoltaic cells.Peer reviewe