Sep 26, 2016

Characteristics of Germanium-on-Insulators Fabricated by Wafer Bonding and Hydrogen-Induced Layer Splitting

There is considerable interest in germanium-on-insulator (GeOI) because of its advantages in terms of device performance and compatibility with silicon processing. In this paper, fabricating GeOI by hydrogen-induced layer splitting and wafer bonding is discussed. Hydrogen in germanium exists in molecular form and is prone to outdiffusion, resulting in a storage-time dependence of blistering. In contrast to the case of silicon, little effect of substrate doping on blistering is observed in germanium. Hydrogen implantation in germanium creates both {100}- and {111}-type microcracks. These two types of platelets are located in the same region for (111)-oriented wafers, but in different zones for (100) samples. This variation in distribution explains the smoother splitting of (111) surfaces than that of (100) surfaces. Hydrogen implantation also introduces a significant concentration of charged vacancies, which affect dopant diffusion in the transferred germanium film. Boron, with a negligible Fermi-level dependence, shows an identical diffusion profile to that of bulk germanium. In contrast, phosphorus diffusion is enhanced in the fabricated GeOI layers. These results also shed light on the understanding of dopant diffusion mechanisms in germanium.

Keywords:  germanium-on-insulator (GeOI);  wafer bonding;  silicon;  hydrogen-induced layer splitting ;  

Source:  iopscience

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Sep 20, 2016

Yttrium–scandium oxide as high-k gate dielectric for germanium metal–oxide–semiconductor devices

The feasibility of employing yttrium–scandium oxide (YScOx) as high-k gate dielectrics for germanium-metal–oxide–semiconductor (MOS) devices has been investigated. The composition and chemical structures of the film were studied using x-ray photoelectron spectroscopy (XPS). Conduction and valence band discontinuities at YScOx/Ge (1 0 0) interfaces were also determined by measuring the O 1s photoelectron energy loss and valence band spectra. Indeed, high-k(YScOx)/Ge MOS characteristics, exhibiting fairly good electrical characteristics, especially low leakage current density and low density of interface states, have been achieved due to the formation of stable Y-Sc-germanate at the interface. The effects of various scandium oxide (ScO) concentrations in YScOx on the electrical characteristics are also reported. It has been demonstrated that higher ScO concentration in YScOx may cause Vfb to shift to its even lower positive value even if considering hysteresis while it causes degradation in interfacial properties. Besides, the effects of several annealing treatments have been investigated in order to optimize the process conditions. This work suggests that ScO concentration up to 50% in YScOx along with post-metallization annealing treatment at 500 °C will be the key to ensure reasonable electrical performance of Ge MOS devices.

Keywords:  employing yttrium–scandium oxide (YScOx);  germanium-metal–oxide–semiconductor (MOS) ;  x-ray photoelectron spectroscopy (XPS);   Y-Sc-germanate; various scandium oxide (ScO)

Source:  iopscience

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Sep 6, 2016

Slicing, cleaning and kerf analysis of germanium wafers machined by wire electrical discharge machining

This paper investigates the slicing of germanium wafers from single crystal, gallium-doped ingots using wire electrical discharge machining. Wafers with a thickness of 350 μm and a diameter of 66 mm were cut using 75 and 100 μm molybdenum wire. Wafer characteristics resulting from the process such as the surface profile and texture are analyzed using a surface profiler and scanning electron microscopy. Detailed experimental investigation of the kerf measurement was performed to demonstrate minimization of material wastage during the slicing process using WEDM in combination with thin wire diameters. A series of timed etches using two different chemical etchants were performed on the machined surfaces to measure the thickness of the recast layer. Cleaning of germanium wafers along with its quality after slicing is demonstrated by using Raman spectroscopy.

Keywords:  Wire electrical discharge machining;  Wafer slicing;  Single crystal germanium;  Raman spectroscopy;  Kerf;  Wafer cleaning;  Recast layer thickness

Source: Sciencedirect

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Debris-free rear-side picosecond laser ablation of thin germanium wafers in water with ethanol


•Picosecond laser cutting of fragile 150 μm thin germanium wafers (typically used for solar cell applications) in liquid results in debris-free surfaces.

•Liquid-assisted laser cutting is much better than air-assisted laser cutting in terms of recast, debris and cleanness of the resultant grooves.

•Laser cutting in ethanol–water mixtures result in better cut quality than those performed in pure water but lead to less cutting efficiency.

•Low repetition rate (10 kHz), mixed solution (1 wt% ethanol in water) and moderate scanning speed (100 μm/s) are preferable for ultrafine high-quality debris-free cutting.

In this paper, we perform liquid-assisted picosecond laser cutting of 150 μm thin germanium wafers from the rear side. By investigating the cutting efficiency (the ability to allow an one-line cut-through) and quality (characterized by groove morphologies on both sides), the pros and cons of this technique under different conditions are clarified. Specifically, with laser fluence fixed, repetition rate and scanning speed are varied to show quality and efficiency control by means of laser parameter modulation. It is found that low repetition rate ablation in liquid gives rise to a better cut quality on the front side than high repetition rate ablation since it avoids dispersed nanoparticles redeposition resulting from a bubble collapse, unlike the case of 100 kHz which leads to large nanorings near the grooves resulting from a strong interaction of bubbles and the case of 50 kHz which leads to random cutting due to the interaction of the former pulse induced cavitation bubble and the subsequent laser pulse. Furthermore, ethanol is mixed with pure distilled water to assess the liquid's impact on the cutting efficiency and cutting quality. The results show that increasing the ethanol fraction decreases the ablation efficiency but simultaneously, greatly improves the cutting quality. The improvement of cut quality as ethanol ratio increases may be attributed to less laser beam interference by a lower density of bubbles which adhere near the cut kerf during ablation. A higher density of bubbles generated from ethanol vaporization during laser ablation in liquid will cause stronger bubble shielding effect toward the laser beam propagation and therefore result in less laser energy available for the cut, which is the main reason for the decrease of cut efficiency in water–ethanol mixtures. Our findings give an insight into under which condition the rear-side laser cutting of thin solar cells should be performed: high repetition, pure distilled water and high laser power are favorable for high-speed rough cutting but the cut kerf suffers from strong side effects of ripples, nanoredeposition occurrence, while low laser power at low repetition rate (10 kHz), mixed solution (1 wt% ethanol in water) and moderate scanning speed (100 μm/s) are preferable for ultrafine high-quality debris-free cutting. The feasibility of high-quality cut is a good indication of using rear laser ablation in liquid to cut thinner wafers. More importantly, this technique spares any post cleaning steps to reduce the risk to the contamination or crack of the thin wafers.

Graphical abstract

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Keywords:  Laser cutting;  Liquid cutting;  Germanium cutting


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