Crystallization Of Amorphous Ge Films Research Articles

Crystallization Of Tensile Strained n-Type Ge By Continuous Wave Laser Annealing

In recent years, germanium (Ge) has attracted a lot of attention for the development of next generation devices due to its higher carrier mobilities compared with silicon (Si) and its compatibility for complementary metal-oxide-semiconductor (CMOS) applications. It is widely known that Ge is an indirect-band semiconductor like Si. However, by introducing tensile strain, the 136 meV difference between direct and indirect gaps can be reduced. Furthermore, in the case of 0.2-0.3% tensile strained Ge, n-type doping in the order of 1019 cm-3 were expected to be resulted in quasi direct-band light emission around 1550 nm wavelength. [1] The realization of this tensile strained n-Ge is promising for the integration of light sources on next generation Ge-based devices. Here, we focused on n-type Ge deposition using molecular beam epitaxy (MBE) method and have succeeded in realizing highly Sb-doped epitaxial n-Ge films by modulating the deposition temperatures. [2] Moreover, we have recently reported the crystallization of amorphous Ge by high-speed continuous wave laser annealing (CWLA), which more importantly also resulted in the introduction of 0.55-0.62% tensile strain. [3] In this study, we applied the similar annealing technique for crystallization of Sb-doped Ge toward the realization of tensile strained n-type Ge films.Sb-doped poly-crystalline Ge films with high Sb concentration (approximately 1019 cm-3) and thickness of about 100 nm were deposited on quartz substrate by molecular beam deposition at 450ºC substrate temperature. Then, 300 nm of SiO2 capping layer was deposited by sputtering in room temperature, such that the sample structure become that shown in Fig.1(a). The samples then annealed in the CWLA system equipped with Nd:YVO4 solid state laser with wavelength of 532 nm as the light source, which scan laser light at the speed (vscan) of 800 m/min. Also, the laser is focused to 20 μm diameter and shifted at 5 μm to scan the samples. Here, the laser power (Elaser) was changed from 300 to 1000 mW. After laser annealing, the capping layer were removed then the samples were characterized by micro-Raman spectroscopy.The annealed sample surface shows brighter contrast compared to the as-deposited samples, as shown in Fig.1(b) and (c), which indicate the structural change of Ge layer. Raman spectra measured at surface of annealed Ge layers are summarized in Fig.2. Here, Ge-Ge peak around 300.2 cm-1 confirmed on the samples, indicating the crystallization of the Ge layers. This peak is not shifted in low Elaser but largely shifted in high Elaser at up to -2.4 cm-1 for undoped and -3.6 cm-1 for the Sb-doped Ge films. This difference could be explained by the liquid phase recrystallization after Ge layer melted in high Elaser (≥600 mW). During recrystallization process, tensile strain is accumulated upon cooling as the tensile strain amount (0.43-0.63%) is agree with the thermal expansion value of Ge between solidification temperature and room temperature (-0.60%). This mechanism is similar to the crystallization of amorphous Ge films on quartz substrates in our previous work. [3] These results show the successful crystallization of tensile strained n-Ge layer by the CWLA method. This finding will be useful for the growth of Ge-based films and a promising step toward the development of CMOS-integrated optoelectronics. Further investigation of the annealing conditions and its relations with the properties of Sb-doped Ge films will be discussed in the main presentation. References Liu, J., Kimerling, L. C. & Michel, J. Semicond. Sci. Technol. 27, (2012).Saputro, R.H., Matsumura, R. & Fukata, N. Cryst. Growth & Des. 21, 6523–6528 (2021).Matsumura, R. & Fukata, N. ECS J. Solid State Sci. Technol. 9, 063002 (2020). Figure 1

Effect of Au on the crystallization of germanium thin films by electron-beam evaporation

Metal induced crystallization is a widely used method to form crystalline/polycrystalline structures at low temperatures. In this work, Au was applied to enhance the crystallization of amorphous Ge films. Ge films with thicknesses of ∼1.5μm were fabricated by electron beam evaporation on c-Si substrate with and without very thin Au layer. Crystallization properties of Ge films were analyzed for different growth and post annealing temperatures varied between 270°C and 730°C. The structures of polycrystalline Ge films were investigated by employing X-ray diffraction (XRD), Raman spectra and scanning electron microscopy (SEM).The existence of thin Au layer showed significant impact on the crystallization of amorphous Ge films in terms of reducing the crystallization temperature. In post annealing processes, it was noticed that the impact of Au thin layer on crystallization slightly reduces above the temperature of ∼400°C, and almost no remarkable differences were observed between the films with and without Au layer in this temperature region. It is observed that the growth temperature has a stronger effect on the crystallization than post annealing temperatures in the presence of Au thin film. It is also shown that Au layer catalyzes the axial growth in the presence of planar Ge layer on the substrate surface.

Effect of Ge/Al thickness on Al‐induced crystallization of amorphous Ge layers on glass substrates

AbstractLow‐temperature (350 °C) crystallization of amorphous Ge films on SiO2 was investigated using Al‐induced layer exchange (ALILE) process. Thicknesses of Ge and catalytic Al layers were varied in the range of 30‐300 nm, which strongly influenced the ALILE growth morphology. Based on the study, the Ge thickness was adjusted to 40 nm while the Al thickness was adjusted 50 nm. This sample satisfied both of the surface coverage of polycrystalline‐Ge and the annihilation of randomly oriented Ge regions. Moreover, the enhancement of the heterogeneous Ge nucleation improved the (111) orientation and the grain size. As a result, the area fraction of the (111)‐orientation reached as high as 97% and the average grain size as large as 70‐μm diameters. This (111)‐oriented Ge layer with large‐grains promises to be the high‐quality epitaxial template for various functional materials to achieve next‐generation devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Large-Grained Polycrystalline (111) Ge Films on Insulators by Thickness-Controlled Al-Induced Crystallization

Low-temperature (350°C) crystallization of amorphous Ge films on SiO2 was investigated using Al-induced layer exchange (ALILE) process. Thicknesses of Ge and catalytic Al layers were varied in the range of 30–300 nm, which strongly influenced the ALILE growth morphology. Based on the study, the Ge thickness was adjusted to 40 nm while the Al thickness was adjusted 50 nm. This sample satisfied both of the surface coverage of polycrystalline-Ge and the annihilation of randomly oriented Ge regions. Moreover, the enhancement of the heterogeneous Ge nucleation improved the (111) orientation and the grain size. As a result, the area fraction of the (111)-orientation reached as high as 97% and the average grain size as large as 70-μm diameters. This (111)-oriented Ge layer with large-grains promises to be the high-quality epitaxial template for various functional materials to achieve next-generation devices.

In situ transmission electron microscopy observations of the crystallization of amorphous Ge films

The crystallization of amorphous Ge films has been studied as a function of annealing temperature between 400 and 700°C by in situ transmission electron microscopy (TEM). It is found that crystallization does not occur until the annealing temperature reaches 650°C, which is nearly 250°C higher than the crystallization temperature in previous reports. The high crystallization temperature and average crystal size obtained by in situ TEM are in agreement with those from Raman spectroscopy and X-ray diffraction measurement. The kinetics analysis indicates that homogeneous nucleation is the dominant crystallization mode and the activation energy is up to about 3.1 eV.

Crystallization of amorphous Ge films induced by semiconductor diode laser annealing

Crystallization of amorphous Ge (a-Ge) films induced by semiconductor diode laser (SDL) irradiation has been investigated. 500-nm-thick a-Ge films are crystallized by scanning 807 nm CW SDL light focused to 4.0 × 0.1 mm 2 with the scanning speed ( v) ranging from 100 to 350 mm/s. From X-ray diffraction and Raman scattering spectra, it is confirmed that the films crystallized at v = 100 mm/s show [220] preferential orientation with no residual amorphous component. On the basis of in-situ monitoring, it has been confirmed that a-Ge films annealed at a v higher than 200 mm/s are transformed to crystalline phase via solid phase crystallization, while films annealed with v lower than 170 mm/s are melted and recrystallized.

Influences of ion species (Ge +, Sb +, Au 2+) on crystallization of amorphous Ge films induced by high energy ion irradiation

Amorphous Ge films deposited on CaF 2 substrates were irradiated with Ge, Sb and Au ions and their structural changes were examined. Samples during irradiations were kept at a constant temperature about 380°C since a formation of randomly oriented crystals in amorphous matrix was closely related to temperature. It was found that the crystal quality of the epitaxial Ge films became good and the formation of randomly oriented crystals was retarded with increasing ion mass. These results presumably suggested that the crystal quality was correlated with the amount of randomly oriented crystals. Furthermore, it was assumed that the formation of randomly oriented crystals were affected by the single collision cascade whose size was increased with ion mass. From these results, it was concluded that heavy ion irradiation was effective for the enhancement of the epitaxial crystallization and the suppression of the formation of randomly oriented crystals.

Triggering of diffusion and crystallization of amorphous germanium in contact with antimony

Abstract The diffusion and crystallization of amorphous Ge films in contact with Sb has been studied in bilayer Ge–Sb films. Under prolonged annealing in vacuum, the Sb layer sublimes and, similarly to what happens under etching, the reactions which had occurred at the interface are “recorded” at the surface of the residual Ge film. The results show that the bulk crystallization of the Ge layer occurs at 75% of the eutectic temperature (in degrees Kelvin) of the Ge-Sb system although the trigger of preferential diffusion and crystallization at the defects of the Sb grains is observed at lower temperatures. A comparison of the results obtained in a-Ge films grown onto a Sb layer and films covered by an Sb layer show that the interface in the former film configuration is more active than in the latter and leads to a different crystallization mechanism.

Crystallization-front velocity during scanned laser crystallization of amorphous Ge films

An optical transmission technique has been used to measure the propagation velocity of the crystallization front during scanned laser crystallization of amorphous Ge films on fused-silica substrates. The measurements confirm our theoretical model for the solid-phase amorphous-to-crystalline transformation. According to this model, the periodic structural features of the crystallized films are formed because the crystallization front moves in a series of periodic jumps between rest positions, with the velocity of the front during each jump much higher than the laser scanning velocity. The measured values of the crystallization-front velocity range from 140 to 260 cm/sec, compared with the laser scanning velocity of only 0.5 cm/sec.

Structural relaxation and crystallization of amorphous Ge films

The irreversible changes in conductivity occurring in evaporated amorphous Ge films just after deposition and during annealing are investigated in detail. Their analysis leads to a model for the structural relaxation of real amorphous networks. Electron diffraction and electron microscope experiments provide additional information on annealing effects and crystallization processes.

Solid-phase growth of large aligned grains during scanned laser crystallization of amorphous Ge films on fused silica

Well-aligned grains of dimensions up to 2–3×100 μm have been obtained by laser crystallization of amorphous Ge films on fused-silica substrates. A theoretical model has been developed for the dynamics of the crystallization process, a solid-phase transformation that is greatly accelerated by liberation of the latent heat of transformation, resulting in either periodic or runaway motion of the crystallization front. We believe that scanning of amorphous films with an energy beam of large aspect ratio may be developed into an effective process for preparing large-grained or even single-crystal sheets of Ge and other semiconductors, including Si and GaAs.