Large-grained Ge Research Articles

Electrical transport and optical properties of Al doped polycrystalline SiGe alloy thin film

In this work, electrical and optical properties of large-grained Ge rich polycrystalline (poly) - SiGe alloy thin film grown via Al induced layer exchange (ALILE) crystallization process are reported. The ALILE process results in inevitable p-type doping of the poly-SiGe film. The concentration of holes in the film (~ 7 × 1019 cm−3) was large to the extent that degenerate effects became prominent in the thin film's electrical behavior. The high crystalline quality of the poly-SiGe thin film is elucidated by the large value of Hall mobility ~ 260 cm2 V−1 sec−1 in the temperature range from 4 to 50 K. The degenerate doping of the poly-SiGe thin film also impacts the film's optical absorption coefficient. There is a blue shift in the absorption edge, indicating an apparent increase in the optical band-gap due to the Burstein-Moss shift. Also, there is an increase in absorption coefficient in the NIR region due to free carrier absorption.

High photoresponsivity in a GaAs film synthesized on glass using a pseudo-single-crystal Ge seed layer

Research to synthesize a high-quality GaAs film on an inexpensive substrate has been continuing for decades in the quest to develop a solar cell that achieves both high efficiency and low-cost. Here, we applied a large-grained Ge layer on glass, formed by Al-induced layer exchange, to an epitaxial template for a GaAs film. The GaAs film, grown epitaxially from the Ge seed layer at 520 °C, became a pseudosingle crystal (grain size > 100 μm) with high (111) orientation. Reflecting the large grain size, the internal quantum efficiency reached 70% under a bias voltage of 1.0 V. This value approaches that of a simultaneously formed GaAs film on a single-crystal Ge wafer and is the highest for a GaAs film synthesized on glass at a low temperature. The application of a Ge seed layer formed by layer exchange offers excellent potential to develop high-efficiency thin-film solar cells with III–V compound semiconductors based on low-cost glass substrates.

Minority carrier lifetime of Ge film epitaxial grown on a large-grain seed layer on glass

To develop high-efficiency multijunction solar cells onto inexpensive substrates, an innovative technique for growing a large-grained Ge layer on glass is strongly desired. We investigated the epitaxial growth of a light absorbing Ge film (500-nm thickness) on a large-grained (>100 μm) Ge seed layer (50-nm thickness) formed on glass by Al-induced layer exchange. After examining molecular beam epitaxy (MBE) and solid-phase epitaxy (SPE) for both methods, Ge layers with a low Al concentration were epitaxial grown at 350 °C. Microwave photoconductivity decay revealed that the MBE-Ge layer exhibited a short minority carrier lifetime owing to the rough surface. Conversely, the SPE-Ge layer was relatively flat and exhibited a long bulk minority carrier lifetime (5.6 μs), which is close to that of a single-crystal Ge. Therefore, the seed layer concept that combines SPE with Al-induced layer exchange is a promising way for fabricating ideal bottom cells for high-efficiency multijunction solar cells based on inexpensive substrates.

(Invited) Gold-Induced Low-Temperature (

I NTRODUCTION Low-temperature (≤300oC) formation of quasi-single crystal SiGe, i.e., orientation-controlled large-grain crystal (≥10 μm) SiGe, on insulator is desired for realization of advanced flexible electronics, where high-speed transistors and high-efficiency optical devices are integrated on flexible plastics substrates (softening temperature: ~350°C). This is because SiGe provides higher carrier mobility and superior optical properties compared with Si. Recently, we developed gold-induced layer-exchange crystallization (GIC) using a-Ge/Au stacked structures.[1-3] This enabled selective formation of (100)- or (111)-oriented large-grain Ge (≥20 μm) through controlling nucleation during GIC by using anisotropic surface energy on substrates.[1] We have developed this technique onto plastic and realized orientation-controlled large-grain crystals on flexible plastic substrates with low concentration holes (~2×1017 cm-3), which enables high carrier mobility (~160 cm2/Vs).[2,3] However, reports of orientation-controlled growth by GIC are limited to pure Ge crystals so far. In the present study the GIC technique is developed to SiGe. Consequently, quasi-single crystal SiGe is achieved on insulating substrates at low temperatures (~300oC). EXPERIMENTAL The sample structure and experimental procedure are schematically shown in Fig. 1. In the experiment, fused-quartz chips were employed as substrates. On the substrates, a-SiGe (Ge concentration: 0-100%, thickness: 100 nm) /Au (thickness: 100 nm) stacked structures were formed. Here, amorphous Al2O3 interface-layers (thickness: 4-7 nm) were introduced between a-SiGe and Au layers as diffusion barrier to make interface-nucleation dominate over random bulk-nucleation in Au layers.[1] These samples were annealed at 250-300oC to induce crystallization. The grown layers were analyzed by Nomarski optical microscopy, electron backscattering diffraction (EBSD), and micro-probe Raman scattering spectroscopy (excitation-laser spot-diameter: ~1 μm). The Nomarski optical microscopy observations were carried out from the back sides of the samples through the transparent quartz substrates to characterize the bottom layers. RESULTS AND DISCUSSION The growth features of samples with various Ge fractions and Al2O3 interface layer thicknesses were investigated by Nomarski optical microscopy observations after annealing at 250°C for 50 h. Typical Nomarski micrographs of samples with Ge concentrations of 50% and 100% are shown in Figs. 2(a)-2(b) and 2(c)-2(d), respectively. In these micrographs, dark and bright contrast regions are observed. Micro-probe Raman measurements revealed that the dark regions dominantly observed in Figs. 2(a), 2(c), and 2(d) are crystal, while bright regions dominant in Fig. 2(b) are not crystal. These results indicate that layer exchange crystallization occurs for the samples shown in Figs. 2(a), 2(c), and 2(d), while it does not proceed for the sample shown in Fig. 2(b). EBSD measurements revealed that the grown layers for Fig. 2(d) consisted of (111)-oriented large-grains (≥10 μm). On the other hand, grown layers for Figs. 2(a) and 2(c) consisted of randomly-oriented small-grains (1-3 μm). Here, we examined the Nomarski micrographs of a no-grown Si0.5Ge0.5 sample (diffusion barrier thickness: 5 nm), shown in Fig. 2(b). It is found that nuclei are formed for the Si0.5Ge0.5 sample, though lateral growth does not proceed from the nuclei. From this finding, it is speculated that for the Si0.5Ge0.5 sample (diffusion barrier thickness: 5 nm), (111)-oriented nucleation occurs at Au/SiO2 interface. However, lateral growth does not proceed even after long time annealing. Thus, layer-exchange is not observed. It is suggested that lateral crystallization in GIC is retarded by Si introduction. This consideration triggers an idea that (111)-oriented large-grains can be obtained even for the Si0.5Ge0.5 sample by generating lateral growth from nuclei. Thus, we examine annealing at a higher temperature of 300°C. A Nomarski micrograph of Si0.5Ge0.5 sample (diffusion barrier thickness: 5 nm) after annealing at 300°C for 50 h is shown in Fig. 3(a). Micro-probe Raman measurements reveled layer-exchange crystallization in the entire area. The crystal structures of the grown layer are examined by EBSD measurements. The result is shown in Fig. 3(b), which reveals (111)-oriented large-grains (≥10 μm). As a result, quasi-single crystal SiGe is realized on insulator. This technique will facilitate realization of advanced flexible electronics. ACKNOWLEDGEMENT Part of this work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks.

High carrier mobility in orientation-controlled large-grain (≥50 μm) Ge directly formed on flexible plastic by nucleation-controlled gold-induced-crystallization

High-carrier-mobility semiconductors on flexible-plastic are essential to realize flexible electronics. For this purpose, electrical properties of orientation-controlled large-grain Ge crystals on flexible-plastic directly formed by nucleation-controlled gold-induced-crystallization (GIC) are examined, and compared with those obtained by aluminum-induced-crystallization (AIC). The Ge crystals show p-type conductions. Here, hole concentrations are 2.2 × 1017 and 5.8 × 1020 cm−3 for GIC-Ge and AIC-Ge, respectively, which are explained on the basis of the solubility of Au and Al in Ge. Thanks to the low hole concentration, GIC-Ge shows high hole mobility (160 cm2 V−1 s−1) compared with AIC-Ge (37 cm2 V−1 s−1). These demonstrate significant advantage of GIC to realize high-performance flexible-electronics.

Low-temperature (180 °C) formation of large-grained Ge (111) thin film on insulator using accelerated metal-induced crystallization

The Al-induced crystallization (AIC) yields a large-grained (111)-oriented Ge thin film on an insulator at temperatures as low as 180 °C. We accelerated the AIC of an amorphous Ge layer (50-nm thickness) by initially doping Ge in Al and by facilitating Ge diffusion into Al. The electron backscatter diffraction measurement demonstrated the simultaneous achievement of large grains over 10 μm and a high (111) orientation fraction of 90% in the polycrystalline Ge layer formed at 180 °C. This result opens up the possibility for developing Ge-based electronic and optical devices fabricated on inexpensive flexible substrates.

(111)-oriented large-grain (≥50 µm) Ge crystals directly formed on flexible plastic substrate by gold-induced layer-exchange crystallization

Orientation-controlled large-grain Ge crystals grown on plastic substrates (softening temperature: ∼300 °C) are desired to realize advanced flexible electronics, where various functional devices are integrated on flexible substrates. To achieve this, gold-induced crystallization (annealing temperature: 250 °C) using a-Ge/Au stacked structures is developed on plastic substrates, where thin-Al2O3 layers (thickness: ∼7 nm) are introduced at a-Ge/Au interfaces. Interestingly, (111)-oriented nucleation at the Au/plastic interface dominates over random bulk nucleation in Au layers. As a result, the formation of (111)-oriented large-grain (≥50 µm) Ge crystals directly on flexible plastic substrates is realized. This technique will pave the way for advanced flexible electronics.

Temperature dependent Al-induced crystallization of amorphous Ge thin films on SiO2 substrates

High-quality crystalline Ge thin films on low-cost glass substrates are desired to reduce the fabrication cost of high-efficiency tandem solar cells. We applied an Al-induced crystallization technique to amorphous-Ge films (50-nm thickness) on SiO2 glass substrates. The annealing temperature of the sample strongly influenced the grain size and the crystal orientation in the grown polycrystalline Ge layers: low annealing temperatures resulted in large grains and high (111)-orientation fractions. As a result, annealing at 325°C provided 98% (111)-oriented grains with average diameters of 30μm. Moreover, the grown Ge layers could be used as an epitaxial template for chemical vapor deposition. This large-grained Ge film on a SiO2 substrate appears promising for use as a Ge light-absorbing layer, as well as an epitaxial buffer layer for group III–V compound semiconductors.

Formation of Large-Grain Ge(111) Films on Insulator by Gold-Induced Layer-Exchange Crystallization at Low Temperature

A formation technique of orientation-controlled large-grain (< 10 μm) Ge crystals on insulator at low temperature ({less than or equal to} 350oC) has been investigated to realize advanced flexible electronics. Previously, we proposed a gold-induced crystallization technique, and demonstrated low temperature crystallization (250oC) of Ge on insulator. In the present study, we intentionally inserted thin Al2O3 layers between gold and a-Ge layer, to control crystal nucleation. Consequently, (111)-oriented large-grain (< 20 μm) Ge crystals are obtained at 350oC by optimizing interfacial oxide layer thickness. It is speculated that this phenomena is attributed to suppression of random bulk nucleation of Ge in Au films and resultant dominance of interfacial nucleation at Ge/SiO2 interfaces.

Formation of Large-Grain Ge(111) Films on Insulator by Gold-Induced Layer-Exchange Crystallization at Low Temperature

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Low Temperature Deposition and Crystallization of Large-Grained Ge Films on TiN

Germanium films were deposited by low-pressure chemical vapor deposition on TiN substrates using a Si seeding technique. By depositing a self-limiting Si film on TiN via SiH 4 decomposition at 380°C, a continuous, amorphous Ge film could subsequently be deposited by GeH 4 decomposition at 320°C. Amorphous Ge films were crystallized by thermal annealing at ≥405°C for 60 m. Polycrystalline Ge films on TiN are shown to have larger grains and fewer stacking faults when deposited amorphous and subsequently crystallized, compared to films polycrystalline as-deposited.