Desirable 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.

Two widely used atomic layer deposition precursors, Tetrakis (dimethylamido) titanium (TDMA-Ti) and titanium tetrachloride (TiCl₄), were investigated for use in the deposition of TiOx-based thin films as a passivating contact material for solar cells. This study revealed that both precursors are suited to similar deposition temperatures (150 °C). Post-deposition annealing plays a major role in optimising the titanium oxide (TiO_x) film passivation properties, improving minority carrier lifetime (τ_eff) by more than 200 µs. Aluminium oxide deposited together with titanium oxide (AlO_y/TiO_x) reduced the sheet resistance by 40% compared with pure TiO_x. It was also revealed that the passivation quality of the (AlO_y/TiO_x) stack depends on the precursor and ratio of AlO_y to TiO_x deposition cycles.

Germanium (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 (300–1700 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 (SRV) 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 SRV as low as 30 cm s⁻¹ 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.

The excellent field-effect passivation provided by aluminum oxide (Al2O3) on germanium surfaces relies on the high negative fixed charge present in the film. However, in many applications, a neutral or a positive charge would be preferred. Here, we investigate the surface passivation performance and the charge polarity of plasma-enhanced atomic layer deposited (PEALD) silicon oxide (SiO2) on Ge. The results show that even a 3 nm thick PEALD SiO2 provides a positive charge density (Qtot, ∼2.6 × 1011 cm−2) and a relatively good surface passivation (maximum surface recombination velocity SRVmax ∼16 cm/s). When the SiO2 thin film is capped with an ALD Al2O3 layer, the surface passivation improves further and a low midgap interface defect density (Dit) of ∼1 × 1011 eV−1 cm−2 is achieved. By varying the SiO2 thickness under the Al2O3 capping, it is possible to control the Qtot from virtually neutral (∼2.8 × 1010 cm−2) to moderately positive (∼8.5 × 1011 cm−2) values. Consequently, an excellent SRVmax as low as 1.3 cm/s is obtained using optimized SiO2/Al2O3 layer thicknesses. Finally, the origin of the positive charge as well as the interface defects related to PEALD SiO2 are discussed.

Germanium is an excellent material candidate for various applications, such as field effect transistors and radiation detectors/multijunction solar cells, due to its high carrier mobilities and narrow bandgap, respectively. However, the efficient passivation of germanium surfaces remains challenging. Recently, the most promising results have been achieved with atomic-layer-deposited (ALD) Al₂O₃, but the obtainable surface recombination velocity (SRV) has been very sensitive to the surface state prior to deposition. Based on X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), we show here that the poor SRV obtained with the combination of HF and DIW surface cleaning and ALD Al₂O₃ results from a Ge suboxide interlayer (GeO_x, x < 2) with compromised quality. Nevertheless, our results also demonstrate that both the composition and crystallinity of this oxide layer can be improved with a combination of low-temperature heating and a 300-Langmuir controlled oxidation in an ultrahigh vacuum (LT-UHV treatment). This results in a reduction in the interface defect density (D_it), allowing us to reach SRV values as low as 10 cm/s. Being compatible with most device processes due to the low thermal budget, the LT-UHV treatment could be easily integrated into many future devices and applications.

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 (SiN_x)‐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 SiN_x‐passivated Si samples, similar Ge samples contain a high amount of negative Q_tot (in the range of 10¹²cm⁻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 °C. The SiN_x‐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 × 10¹⁰ and 4 × 10¹¹ 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 (∼−1.6 × 10¹¹ cm⁻²) on Ge. The presented SiN_x‐based passivation schemes are promising for optoelectronic devices, where a low D_it and/or a low charge are favored.