Analysis of germanium epiready wafers for III–V heteroepitaxy

Frequently, when growing III–V semiconductors on germanium substrates, unexpected differences between nominally identical substrates are encountered. Using atomic force microscopy (AFM), we have analysed a set of germanium substrates sharing the same specifications. The substrates come from the same vendor but different results come about in terms of the morphology of the epilayers produced by the same epitaxial routine (i.e. substrate W1 produced epilayers with good morphology while substrate WX produced epilayers with defects). The morphological analysis has been carried out on (a) epiready substrates; (b) samples after a high-temperature bake at 700 °C; and (c) on the samples after a hydride (PH3) annealing at 640 °C. In the two first stages all substrates (both W1 and WX) show the same good morphology with RMS roughness below 3 Å in all cases. It is in the third stage (annealing in PH₃) that the morphology degrades and the differences between the samples become apparent. After phosphine exposure at 640 °C, the RMS roughness of both substrates approximately doubles, and their surface appears as full of peaks and valleys on the nanometre scale. Despite the general appearance of the samples being similar, a careful analysis of their surface reveals that the substrates that produce bad morphologies (WX) show higher peaks, and some of their roughness parameters, namely, surface kurtosis and the surface skewness, are considerably degraded.

Source:Journal of Crystal Growth

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Radiotracer study of cobalt diffusion and solubility in electronic-grade germanium wafers

The diffusion rate of Co in Ge is found to be as fast as 2×10^-6cm²s^-1 at 900 °C, whereas the Co solubility comes close to 1×10¹⁶cm^-3 at the same temperature. Based on these properties and its acceptor activity, Co may cause serious contamination problems during the fabrication of Ge-based electronic devices. In contrast to an early radiotracer study, we observe common diffusion profiles of complementary error function type, which are indicative of a constant diffusivity depending only on temperature. A preliminary analysis of the data points to the dissociative diffusion mechanism involving interstitial–substitutional exchange via vacancies. However, in contrast to Cu, which migrates via the vacancy-controlled mode of the dissociative mechanism, the diffusion of Co may be rate-limited by the transport properties of its interstitial modification (Coi).

Source: Physica B: Condensed Matter

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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.

Source: Journal of Materials Processing Technology

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High quality Germanium-On-Insulator wafers with excellent hole mobility

We present the fabrication and characterization of ultra thin and relatively thick SiGe-On-Insulator wafers with different Ge contents prepared by Ge condensation technique. The fabrication procedures as well as the structural analysis are detailed. The electrical properties of advanced strained SiGe-On-Insulator (SGOI) and relaxed Germanium-On-Insulator (GeOI)wafers were investigated using the Pseudo-MOSFET method and then compared with Silicon-On-Insulator (SOI) and strained Silicon-On-Insulator (sSOI) structures. GeOI wafers with 10-nm and 100-nm film thickness show exceptionally high hole mobility as compared to both SOI and sSOI structures. The hole mobility can reach 400 cm²/V s. It is found that the mobilities for holes and electrons vary in opposite directions as the Ge fraction is increased. The Ge content also impacts the threshold and flat-band voltages.

Source: Solid-State Electronics

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Assessment of the overall resource consumption of germanium wafer production for high concentration photovoltaics

The overall resource requirements for the production of germanium wafers for III–V multi-junction solar cells applied in concentrator photovoltaics have been assessed based on up to date process information. By employing the cumulative energy demand (CED) method and the cumulative exergy extraction from the natural environment (CEENE) method the following resources have been included in the assessment: fossil resources, nuclear resources, renewable resources, land resources, atmospheric resources, metal resources, mineral resources and water resources. The CED has been determined as 216 MJ and the CEENE has been determined as 258 MJ_ex.In addition partial energy and exergy payback times have been calculated for the base case, which entails the installation of the high concentration photovoltaics (HCPVs) in the Southwestern USA, resulting in payback times of around 4 days for the germanium wafer production. Due to applying concentration technology the germanium wafer accounts for only 3% of the overall resource consumption of an HCPV system. A scenario analysis on the electricity input to the wafer production and on the country of installation of the HCPV has been performed, showing the importance of these factors on the cumulative resource consumption of the wafer production and the partial payback times.

Highlights

• The Ge-wafer production for concentrator solar cells was inventoried and assessed. • The cumulative energy demand was determined as 216 MJ wafer⁻¹.

• The cumulative exergy extraction from the natural environment was 258 MJ_ex wafer⁻¹.

• System installation in the SW USA results in Ge-wafer payback times of ca. 4 days.

• The Ge-wafer represents only 3% of the concentrator PV system resource requirements.

Source: Thin Solid Films

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