Polycrystalline Ge Thin Films Research Articles

Substrate-dependent carrier mobility in polycrystalline Ge thin films

We present the effect of substrate choice on the carrier mobility of polycrystalline Ge thin films on SiO2/Si, C-cut sapphire, and R-cut sapphire substrates. The Hall mobility of the polycrystalline Ge thin films exhibited a strong correlation with the substrates, with mobility increasing sequentially for Ge thin films on SiO2/Si, C-cut sapphire, and R-cut sapphire substrates. Hall Effect measurements further suggested minimal influence of grain boundary scattering. Electron backscatter diffraction analysis revealed similar grain sizes of ∼200 nm across different substrates, but distinct grain orientation distributions, particularly with pronounced (110) grains on R-cut sapphire substrates. This work sheds light on the importance of substrate selection in optimizing the electrical performance of polycrystalline Ge thin films and highlights avenues for further investigation in understanding the underlying mechanisms governing substrate-dependent carrier mobility.

Strain-dependent grain boundary properties of n-type germanium layers

Polycrystalline Ge thin films have attracted considerable attention as potential materials for use in various electronic and optical devices. We recently developed a low-temperature solid-phase crystallization technology for a doped Ge layer and achieved the highest electron mobility in a polycrystalline Ge thin film. In this study, we investigated the effects of strain on the crystalline and electrical properties of n-type polycrystalline Ge layers. By inserting a GeOx interlayer directly under Ge and selecting substrates with different coefficients of thermal expansion, we modulated the strain in the polycrystalline Ge layer, ranging from approximately 0.6% (tensile) to − 0.8% (compressive). Compressive strain enlarged the grain size to 12 µm, but decreased the electron mobility. The temperature dependence of the electron mobility clarified that changes in the potential barrier height of the grain boundary caused this behavior. Furthermore, we revealed that the behavior of the grain boundary barrier height with respect to strain is opposite for the n- and p-types. This result strongly suggests that this phenomenon is due to the piezoelectric effect. These discoveries will provide guidelines for improving the performance of Ge devices and useful physical knowledge of various polycrystalline semiconductor thin films.

Acceptor defects in polycrystalline Ge layers evaluated using linear regression analysis

Polycrystalline Ge thin films have recently attracted renewed attention as a material for various electronic and optical devices. However, the difficulty in the Fermi level control of polycrystalline Ge films owing to their high density of defect-induced acceptors has limited their application in the aforementioned devices. Here, we experimentally estimated the origin of acceptor defects by significantly modulating the crystallinity and electrical properties of polycrystalline Ge layers and investigating their correlation. Our proposed linear regression analysis method, which is based on deriving the acceptor levels and their densities from the temperature dependence of the hole concentration, revealed the presence of two different acceptor levels. A systematic analysis of the effects of grain size and post annealing on the hole concentration suggests that deep acceptor levels (53–103 meV) could be attributed to dangling bonds located at grain boundaries, whereas shallow acceptor levels (< 15 meV) could be attributed to vacancies in grains. Thus, this study proposed a machine learning-based simulation method that can be widely applied in the analysis of physical properties, and can provide insights into the understanding and control of acceptor defects in polycrystalline Ge thin films.

Solid‐Phase Crystallization of GeSn Thin Films on GeO2‐Coated Glass

Polycrystalline Ge thin films are promising candidates for next‐generation thin‐film transistors. However, the difficulty in Fermi‐level control due to the high density of defect‐induced acceptors has been one of the main problems for device applications. Herein, GeO2 preparation and Sn addition are combined in the advanced low‐temperature (375 °C) solid‐phase crystallization of Ge layers that have been recently developed. The GeO2 underlayer works effectively at a low Sn composition (&lt; 4%), enlarging the grain size by a magnification of ≈5. An appropriate amount of unsubstituted Sn reduces acceptor defects and achieves a minimum hole concentration of 3 × 1016 cm−3 by combining post‐annealing (500 °C). This hole concentration is the lowest level for undoped polycrystalline Ge‐based thin films, which allows control over their Fermi level widely by impurity doping. Although the lower hole concentration provides a higher energy barrier height of the grain boundary, the hole mobility is maintained at 190 cm2 V−1 s−1 owing to the large grains (15 μm diameter). This achievement paves the way for the implementation of poly‐Ge‐based thin films in various semiconductor devices, including advanced thin‐film transistors.

Composition dependent properties of p- and n-type polycrystalline group-IV alloy thin films

Group-IV alloy semiconductors have garnered increasing attention as advanced thin-film materials for next-generation electronics. We have demonstrated polycrystalline Ge thin films with the highest recorded crystallinity and carrier mobility using a multistep heating process in solid-phase crystallization. In this study, we apply these recent findings in Ge to Si1-xGex (x: 0–1) and Ge1-ySny (y: 0–0.04) alloys and investigate their crystal and electrical properties. For all compositions, controlling the temperature in each stage increases the grain size to the micrometer order, improving the carrier mobility and reducing the number of defect-induced acceptors. Sb doping further enlarges the grain size (up to 10 µm) in addition to n-type conduction control, whereas the electron concentration varies with the composition. Both hole and electron mobilities significantly depend on the composition owing to the effects of carrier effective mass, grain size, and carrier concentration: the hole and electron mobilities peak at 350 and 150 cm2 V−1s−1, respectively. The relationship between the composition and various physical properties revealed in this study will contribute to the better understanding, control, and device application of polycrystalline thin films based on group-IV alloy semiconductors.

Strain effects on polycrystalline germanium thin films

Polycrystalline Ge thin films have attracted increasing attention because their hole mobilities exceed those of single-crystal Si wafers, while the process temperature is low. In this study, we investigate the strain effects on the crystal and electrical properties of polycrystalline Ge layers formed by solid-phase crystallization at 375 °C by modulating the substrate material. The strain of the Ge layers is in the range of approximately 0.5% (tensile) to -0.5% (compressive), which reflects both thermal expansion difference between Ge and substrate and phase transition of Ge from amorphous to crystalline. For both tensile and compressive strains, a large strain provides large crystal grains with sizes of approximately 10 μm owing to growth promotion. The potential barrier height of the grain boundary strongly depends on the strain and its direction. It is increased by tensile strain and decreased by compressive strain. These findings will be useful for the design of Ge-based thin-film devices on various materials for Internet-of-things technologies.

Effect of interlayer on silver-induced layer exchange crystallization of amorphous germanium thin film on insulator

Metal-induced layer exchange (MILE) is an advanced crystallization technique for fabricating high-quality semiconductor thin films on insulating substrates at low temperatures. Here, we focused on Ag as a catalytic metal for crystallizing amorphous germanium thin films on glass through MILE. The layer exchange between Ag and amorphous Ge (a-Ge) was not simple because Ag diffused into a-Ge so fast that it crystallized the top a-Ge layer before the completion of layer exchange. To suppress Ag diffusion into a-Ge, we explored the kind of interlayer material between them as well as the growth temperature. Preparing SiO2 and GeO2 interlayers allowed for a complete layer exchange, resulting in the formation of polycrystalline Ge thin films on glass at temperatures as low as 250 °C. These findings are essential for further understanding and controlling the layer exchange phenomenon.

Structural and electrical analysis of poly-Ge films fabricated by e-beam evaporation for optoelectronic applications

We have investigated the relationship between structural and electrical properties of Ge thin films deposited on single crystal silicon (100) substrates by electron beam evaporation at room temperature. Post-thermal annealing was applied to obtain poly-crystalline Ge thin films. The structural effects of the annealing temperature and annealing time on the crystallization of Ge films were analyzed using Raman and X-ray diffraction measurements. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increasing annealing temperature and annealing time. It was found that high quality poly-crystalline Ge films were obtained with crystallization ratio of 90% at an annealing temperarure of 500°C following the crystallization threshold of 450°C. Effects of structural ordering on the electrical properties were investigated through current-voltage characteristics of fabricated heterostructure devices (Ge/p-Si). Smooth cathode-anode interchange in the diode behavior has been clearly observed following the structural ordering as a function of annealing temperature in a systematic way. These outcomes could be exploited for engineering of low-cost Ge based novel electronic and opto-electronic devices.

Effects of flexible substrate thickness on Al-induced crystallization of amorphous Ge thin films

Amorphous germanium (a-Ge) thin films were directly crystallized on flexible plastic substrates at 325 °C using Al-induced crystallization. The thickness of the plastic substrate strongly influenced the crystal quality of the resulting polycrystalline Ge layers. Using a thicker substrate lowered the stress on the a-Ge layer during annealing, which increased the grain size and fraction of (111)-oriented grains within the Ge layer. Employing a 125-μm-thick substrate led to 95% (111)-oriented Ge with grains having an average size of 100 μm. Transmission electron microscopy demonstrated that the Ge grains had a low-defect density. Production of high-quality Ge films on plastic substrates allows for the possibility for developing Ge-based electronic and optical devices on inexpensive flexible substrates. • Polycrystalline Ge thin films are directly formed on flexible plastic substrates. • Al-induced crystallization allows the low-temperature growth (325 °C) of amorphous Ge. • The substrate bending during annealing strongly influences the crystal quality of poly-Ge. • A thick substrate (125 μm) leads to 95% (111)-oriented Ge with grains 100 μm in size.

Structural characterization of polycrystalline Ge thin films on insulators formed by diffusion-enhanced Al-induced layer exchange

The low-temperature formation of the polycrystalline Ge thin film on an insulating substrate is investigated to develop advanced Ge-based devices onto plastic substrates. We propose a growth promotion technique in Al-induced crystallization (AIC) of amorphous Ge: the modulation of the interlayer between the Ge and Al layers for enhancing the diffusion rate of Ge atoms during annealing. By substituting a conventional AlOx interlayer with a GeOx interlayer, the growth temperature is significantly reduced from 325 to 200 °C, probably due to the difference of the diffusion coefficient of Ge in the interlayers. The electron backscatter diffraction measurement reveals that the grain size and the crystal orientation strongly depend on the annealing temperature. The 200 °C annealed sample yields a preferentially (111)-oriented Ge layer with large grains (average diameter: 57 µm). Therefore, a large-grained, orientation-controlled Ge layer is simultaneously achieved on an insulating substrate at a low temperature of 200 °C using the diffusion-enhanced AIC technique.

Nucleation and growth kinetics during metal-induced layer exchange crystallization of Ge thin films at low temperatures

The kinetics of Al-catalyzed layer exchange crystallization of amorphous germanium (Ge) thin films at low temperatures is reported. Observation of Ge mass transport from an underlying amorphous Ge layer to the Al film surface through an interposed sub-nanometer GeOx interfacial layer allows independent measurement of the areal density and average area of crystalline Ge islands formed on the film surface. We show that bias-voltage stressing of the interfacial layer can be used to control the areal density of nucleated Ge islands. Based on experimental observations, the Johnson-Mehl-Avrami-Kolmogorov phase transformation theory is used to model nanoscale nucleation and growth of Ge islands in two dimensions. Ge island nucleation kinetics follows an exponentially decaying nucleation rate with time. Ge island growth kinetics switches from linear growth at a constant growth velocity to diffusion-limited growth as the growth front advances. The transition point between these two regimes depends on the Ge nucleation site density and the annealing temperature. Knowledge of the kinetics of low-temperature crystallization is important in achieving textured polycrystalline Ge thin films with large grains for applications in large-area electronics and solar energy conversion.

Influence of hydrogen on structural and optical properties of low temperature polycrystalline Ge films deposited by RF magnetron sputtering

To improve the properties of polycrystalline Ge thin films, which are a candidate material for the bottom cells of low cost monolithic tandem solar cells, ∼300 nm in situ hydrogenated Ge (Ge:H) thin films were deposited on silicon nitride coated glass by radio-frequency magnetron sputtering. The films were sputtered in a mixture of 15 sccm argon and 10 sccm hydrogen at a variety of low substrate temperatures ( T s )≤450 °C. Structural and optical properties of the Ge:H thin films were measured and compared to those of non-hydrogenated Ge thin films deduced in our previous work. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increase in T s . It is found that the introduction of hydrogen gas benefits the structural properties of the polycrystalline Ge film, sputtered at 450 °C, although the onset crystallization temperature is ∼90 °C higher than in those sputtered without hydrogen. Compared with non-hydrogenated Ge thin films, hydrogen incorporated in the films leads to broadened band gaps of the films sputtered at different T s .

Heavily Boron-Doped Hydrogenated Polycrystalline Ge Thin Films Prepared by Cosputtering

Boron is an excellent dopant for forming germanium (Ge) shallow junctions because of its low diffusivity. This work investigates fabricating heavily boron-doped hydrogenated polycrystalline Ge thin films with an aim to develop a thin p + emitter of the bottom cell of a monolithic tandem solar cell. The films were deposited on glass by cosputtering in an argon and hydrogen mixture at 500°C. Rapid thermal annealing at 600°C for 60 s enhances the boron activation level from 6.24 × 10 19 to 1.21 × 10 20 atoms/cm 3 . The high activation level obtained with a low thermal budget indicates that the simple fabrication approach is promising.

Characterization of polycrystalline Ge thin films fabricated by short-pulse XeF excimer laser crystallization

XeF excimer laser-induced melting and recrystallization dynamics of amorphous germanium are investigated using time-resolved optical reflection and transmission measurements with a nanosecond time resolution, field-emission scanning electron microscopy, and micro-Raman spectroscopy. It is found that the disc-shaped grain with a diameter of approximately 0.8 µm is located in the complete melting regime with a melt phase duration of approximately 141–200 ns. The significant change of transmissivity is a key phenomenon revealing the excessive excimer laser fluence during excimer laser crystallization by in-situ optical measurements. Differences between the melting and recrystallization phenomenon for Si and Ge thin films are also discussed.

Poly-crystalline Ge thin films prepared by RF sputtering method for thermo photo voltaic application

AbstractThe poly crystalline Ge films were prepared by the RF sputtering method with Ar and Ar-H2 mixture gases. The crystallization temperature increases from 400 °C to 500 °C due to the H2 introduction into the sputtering gases. On the other hand, the H2 introduction decreases the absorption coefficients in the long wavelength region corresponding to lower energy values than the energy gap of Ge, although much higher absorption coefficients are observed in the case of the Ar sputtering. Probably, the gap state density decreases due to the hydrogen termination of dangling-bonds in the grain-boundaries. The (220) preferential orientation is stressed and the highest Hall mobility is obtained with 75% of the gas flow ratio [Ar/ (Ar+H2)]. The preferential growth probably causes higher quality Ge films.

Thermal decomposition mechanisms of tetraethylgermane in metal-organic chemical vapor deposition

Polycrystalline Ge thin films have been grown by MOCVD in an atmospheric laminar flow reactor using GeEt 4 as precursor. Hydrogen is required to remove the carbon contamination of the films which is observed under inert atmosphere. The decomposition mechanism of GeEt 4 in the CVD reactor has been investigated from analyses of the gaseous by-products in a variety of chemical environments. The overall reaction is the growth of Ge thin film with formation of H 2 and C 2H 4 as gaseous by-products, likely by the β-hydrogen elimination mechanism rather than radical pathways. The decomposition process under inert atmosphere is predominantly homogeneous with likely formation of intermediates as nutrient species for the film. Secondary heterogeneous processes including polymerization reactions or incomplete removal of the ligands and subsequent dehydrogenation lead to carbon contamination of the layers. In ambient H 2, the formation of C 2H 6, likely by the hydrogenation of C 2H 4, prevents the polymerization of the olefin and accounts for the beneficial influence of H 2 in this low temperature deposition process of pure Ge thin films.

Chemical vapor deposition of ge thin films using geet 4: study of the reaction mechanisms

Summary Polycrystalline Ge thin films were grown by MOCVD using GeEt 4 as a precursor. H 2 is required to remove the carbon contamination of the films which is observed under inert atmosphere. The decomposition mechanism of GeEt 4 has been investigated from analyses of the gaseous by- products in a variety of chemical environments. The process is predominantly homogeneous and the primary reaction is the formation of Ge thin film, H 2 and C 2 H 4 , possibly by β-hydrogen elimination mechanism rather than a radical process.

Electrical properties of evaporated polycrystalline Ge thin-films

Electrical properties of Ge thin films evaporated on Si 3N 4 CVD-coated Si substrate were improved by introducing a heat treatment after the deposition of Ge films. Evaporation conditions were optimized by changing the substrate temperature and deposition rate, and then, heat treatment was performed. At substrate temperatures during the evaporation lower than 300 °C and higher than 400 °C, deposited films were amorphous and polycrystalline, respectively. At substrate temperatures lower than 400 °C, Ge films were evaporated without degrading the surface roughness. The Hall mobility of films evaporated at room temperature increased with increasing the substrate and heating temperature and showed about 400 cm 2 V −1 s −1 for the hole concentration of 4 × 10 17 cm −3 at the heating temperature of 900 °C. This value was almost comparable to that of p-type Ge single crystal.

Improvement of Responsivity of Antenna-Coupled Warm Carrier Device Using Polycrystalline Ge Thin Films

Electrical properties of Ge thin films evaporated on Si3N4 substrates were improved by introducing a heat treatment after the deposition. The Hall mobility of the deposited films increased with increasing heat-treatment temperature and was 280 cm2/V·s for a hole concentration of 6×1017 cm-3 at a heat-treatment temperature of 900° C. This value of the Hall mobility was about three times larger than that obtained for Ge films without heat treatment. A point-contact warm carrier device was also fabricated using evaporated Ge film with heat treatment, and the detected voltage induced by CO2 laser radiation was measured. The fabricated device operated as an antenna-coupled device and the detected voltage was about ten times higher than that obtained using a device without heat treatment.