Crystalline Ge Films Research Articles

Dopant Redistribution in High-Temperature-Grown Sb-Doped Ge Epitaxial Films

Heavily doped n-type Ge crystals are essential for Ge-based electronics and optical applications. In this report, we describe the mechanism of dopant redistribution via out-diffusion and re-evaporation processes in Sb-doped Ge epitaxial films grown by molecular beam epitaxy (MBE). The temperature-modulated depositions yielded a uniformly distributed and high Sb concentration (1019 to 1021 cm–3) on highly ordered crystalline Ge films. Here, the deposition temperatures (Td) control the transition of amorphous to epitaxial growth, as well as the Sb concentration profiles. A significant difference in the dopant levels is attributed to Sb out-diffusion that correlated with the diffusion parameters, followed by the re-evaporation from the Ge crystal in high-temperature conditions. Meanwhile, the substituted Sb atoms act as donors that form the n-type Ge film. This understanding of dopant behavior in the epitaxially grown Ge film opens a path to achieve well-defined n+-Ge thin films for Ge-based devices with improved electrical properties.

Phase Transformation in Germanium-on-Insulator (GeOI) Films Using Raman Spectroscopy and Nanoindentation

Germanium-on-insulator (GeOI) films fabricated using the SMART CUT wafer bonding and film exfoliation technology were investigated for the mechanical properties and induced phase transformations by using nanoindentation and Raman spectroscopy experiments. The hardness and modulus results of the GeOI films are significantly different from the published SOI literature and bulk germanium results obtained by Liu et al., 2020 [1] and SOI films results obtained by Miller et al., 2007 [2]. From our measured hardness results of 8 to 10 GPa for very thin single crystal Ge(100) films compared to 13.1 GPa for Bulk Ge (100), one can conclude that the thinning of crystalline Ge films leads to a considerable softening of the crystalline Ge resulting in less brittle robust films. Similarly, our measured modulus of 130 to 140 GPa for thin Ge (100) films compared to the reported bulk Ge (100) value of 157 GPa corroborates these research findings. The GeOI films are softer and more flexible as compared to bulk Ge hardness and stiffness properties. The Raman spectroscopy of the spherical indents indicate bands of metastable Ge phases @ 220 cm-1, 195 cm-1 and 184 cm-1 wavenumbers. Our results demonstrate that a spherical indenter impacted a wider area of contact and produced GeOI indented surfaces free of cracks and fracture. The spherical indenter tip kept the Ge top layer intact when compared to the Berkovich indenter tip during penetration. In contrast, the Berkovich indenter tip developed excessive fracture that resulted in displacing the Ge top layer sideways and exposed the Si substrate underneath revealing Raman spectra bands of metastable Si phases @ 350 cm-1, 399 cm-1 and 430 cm-1.Apart from the well documented electrical advantages of GeOI, the reported research on the nanomechanical properties reveals considerable softening and flexibility of the very thin exfoliated single crystal Ge films that promise added benefits and further implications for enhancing potential device application of GeOI wafers for process integration.1.] N. Liu, X-J Yang, Z. Yu, and L. Zhao, Trans. Nonferrous Met. Soc. China, 30, 181 (2020).2.] N. Miller, K. Tapily, H. Baumgart, G.K. Celler, F. Brunier, and A.A. Elmustafa, Mater. Res. Soc. Symp. Proc. Vol. 1021 © 2007 Materials Research Society, 1021-HH05-24. Boston, Massachusetts. (2007).

Temperature-dependent anomalous dispersion of polycrystalline germanium film near absorption threshold

Temperature-dependent refractive index of semiconductor germanium (Ge) near the absorption threshold has not been well studied theoretically and experimentally up to now. This paper presents a study of the temperature-dependent refractive index of Ge film near its absorption threshold (about 0.8 eV or 1550 nm), whose values deduced from the reflectivity and transmissivity of high crystalline Ge film fabricated on a fiber tip. Different from previous reports, which show a nearly linear relationship between refractive index and photon energy in the temperature range of −20–120 °C, an abnormal drop has been observed in the refractive index spectrum at the wavelengths near the absorption threshold. To theoretically understand this anomalous dispersion, we first bring the HigherCPs model and M0CP model together to provide a good description of inter-band transition and rewrite the dielectric function of Ge near its absorption edge. Then, the abnormal drop is well interpreted by introducing the Wannier exciton model to the combined model. Furthermore, we evaluate the band gap E0 and Lorentzian broadening of the exciton line as the function of temperature with this new model. The experimental and theoretical results allow for the optimized design of the Ge-based optical applications near the C-band of optical fiber communication.

Low temperature rapid formation of Au-induced crystalline Ge films using sputtering deposition

We report low temperature (100–170°C) and rapid (10min) formation of crystalline Ge films between Au catalyst film and quartz glass substrate using a radio frequency magnetron sputtering deposition. The formation rate of crystalline Ge films between Au catalyst film and quartz glass substrate is proportional to the deposition rate of Ge film, namely the flux of Ge atoms. To obtain insights on the formation mechanism of crystalline Ge films, we studied dependence of grain size of Au films on annealing temperature and Au film thickness. Crystalline Ge films formed below Au films have random crystalline orientation with in-plane grain size from below 1μm. Small crystalline grain size of Au films is needed to form rapidly Au induced crystalline Ge films.

Growth of single crystalline germanium thin film on (100) silicon substrate

Epitaxial growth of germanium thin films (GeTFs) on Si (111) and Si (100) substrates was investigated, and the prepared films were compared with the films grown on SiO2 substrates. Ge films were prepared in three steps. Initially, a Ge interlayer film with thickness of ∼ 10 nm was deposited on the substrate followed by annealing and recrystallization of the film. A Ge film with a thickness of 500 nm was then deposited. A single crystalline Ge film was grown on Si (100) whereas polycrystalline films were grown on the other substrates. The growth rate of the films depends on the type of the substrates used, which in turn determines the crystallinity of the films. Highly crystalline films were obtained with slow growth rates. The single crystalline epitaxial layer of GeTFs formed on Si (100) exhibited a lower threading dislocation density as compared with those grown on Si (111) and SiO2.

Low-Temperature Formation of Sn-Doped Ge on Insulating Substrates by Metal-Induced Crystallization

Low-temperature formation of Sn-doped Ge on insulator is desired to realize next generation flexible electronics. To achieve this, metal-induced crystallization (MIC) of a-GeSn is investigated. By MIC of a-GeSn with initial Sn concentration of 5%, Sn-doped Ge is obtained at low temperatures (<250℃). The Sn concentration (0.5-2.0%) in the grown layers can be controlled by modulating the annealing temperatures in the range of 150-250℃. Introduction A technique for low-temperature (≤250°C) formation of high-quality crystalline Ge films on insulator is expected to realize advanced high performance thin film transistors (TFT) on flexible plastic substrates (softening temperature: ~300oC). To improve quality of Ge crystals, Sn-doping (<4%) is very effective, because it enables passivation of point defects in Ge [1]. This triggered the recent research of solid-phase crystallization of Sn-doped Ge (Sn concentration: 2%) on SiO2layers [2]. However, the growth temperature (425°C) reported to date is higher than the softening temperatures of plastic substrates. To decrease the growth temperature of Sn-doped Ge, in the present study, we investigate metal-induced crystallization (MIC) of a-GeSn. This successfully decreases the growth temperature below 250°C. Experimental Procedure Fig.1 schematically shows the procedure of sample preparation. Amorphous Ge1-xSnx layers (x: 0-0.2, thickness: 100 nm) were deposited on quartz substrates at room temperature. Then, Au layer patterns were formed on the amorphous GeSn layers. The samples were annealed at 150-250oC for 10-60 min to induce lateral growth in a N2ambient. The growth characteristics were analyzed by Nomarski optical microscopy and microprobe Raman spectroscopy. Results and Discussion Figs. 2(a) and 2(b) show Nomarski optical micrographs of Ge1-xSnx (x: 0.05 and 0.2, respectively) samples after annealing at 250oC for 10 min. Au pattern regions are located in the left side areas of the micrographs. In the case of the low Sn concentration (5%), different contrast regions are observed around the Au patterns, as indicated by the broken lines. On the other hand, the sample with the high Sn concentration (20%) shows no change around the Au pattern after annealing. The Raman spectra obtained from these samples are summarized in Fig. 2(c). In Raman spectra #1 for the low Sn concentration (5%), a sharp peak due to Ge-Ge bonding in c-Ge is clearly observed. On the other hand, no peaks are detected in spectra #3 for the sample with the high Sn concentration (20%). These results indicate that lateral growth is generated from the Au patterns for samples with the low Sn concentration (5%), while it is not generated for the high Sn concentration (20%).The lateral growth lengths at 150 and 250oC for samples with various Sn concentrations were estimated using theNomarski optical microscopy combined with line-scanning of microprobe Raman measurements. The results are summarized as a function of the annealing time in Fig. 3. In the case of low Sn concentrations (<5%), lateral growth lengths increase in a short annealing time (10 min). In addition, the growth lengths increase with increasing annealing temperature and exceed 20 μm at 250°C, which is sufficient for device fabrication.The substitutional Sn concentrations in grown layers for samples with the initial Sn concentration of 5% are evaluated by peak shift of Raman spectra. Substitutional Sn concentrations of 0.5−2.0% are obtained in grown layers, which is useful to passivate point defects in Ge [1]. The substitutional Sn concentration depended on the annealing temperature. This indicates that the substitutional Sn concentration can be controlled by modulating annealing temperature. This technique will be useful to realize next generation flexible electronics.

(Invited) Unseeded Growth of Poly-Crystalline Ge with (111) Surface Orientation on Insulator by Pulsed Green Laser Annealing

Thin-film transistors (TFTs) with a high channel mobility for both electrons and holes are required for the three-dimension large scale integrations or system-on-panels. Lower temperature process is more favorable for TFT fabrication because the application of plastic substrate to system-on-panels is expected to add the flexibility function. Germanium (Ge) has higher hole and electron mobilities than Si, suggesting that large-grain poly-crystalline or single-crystalline Ge thin films lead to high channel mobility TFTs. This paper describes unseeded growth of poly-crystalline Ge with well-oriented large-size grains on insulator from amorphous Ge by scanning pulsed green laser annealing. In experimental, a 50-nm-thick amorphous Ge layer was deposited on a Si substrate with a 50-nm-thick SiO2 film. The Ge layer was patterned to narrow stripes of 2-µm-width. Then a capping layer of 650-nm-thick SiO2 was deposited. For this processed template, a scanning pulse green laser annealing was performed. Q-switch Nd:YAG laser beam with the following properties was used for annealing; a Gaussian line beam of 70 x 1400 µm2 at a laser wavelength of 532 nm, a pulse frequency of 7.5 kHz, a scanning velocity of 150 µm/s, and energy densities from 0.5 to 0.7 J/cm2. The Ge stripes were crystallized by a single scanning, where the beam was scanned perpendicular to the longitudinal direction of the beam. The Ge stripes crystallized at the optimum energy of 0.56 J/cm2 by the scanning pulsed laser annealing was characterized by electron back scattering diffraction patterns (EBSD). The EBSD mapping of normal direction to the sample surface indicates that the crystallized Ge film has (111) surface orientation in the 600 µm-long stripe. The (111) Ge crystal begins within only 2 µm after the starting point of the laser scanning. It was observed that the crystal orientation was recovered to (111) even if the prior part of crystalized Ge film was rotated from (111) surface. In the mapping of rolling direction, which corresponds to the laser scanning direction, two types of grains are dominant. One shows (101) and the other shows (112) for the laser scanning direction. From the normal direction and rolling direction images of EBSD, the Ge stripes were crystallized with the grains of 2 µm wide and ~10 µm long. The maximum grain length of 20 µm was also observed. Raman scattering was measured to evaluate the crystalline quality of the (111) surface-oriented Ge stripes. The full-width at half-maximum of 2.35 cm-1 shows good crystal quality of the film, which is smaller than the values of 3.2-3.3 cm-1 for the single crystalline Ge film obtained by lateral liquid-phase epitaxy from a Si seed. For laser fluence dependence, Ge stripes were not crystallized when the fluence was smaller than 0.56 J/cm2, and Ge stripes show non-oriented crystal with a long grain boundary in the center of stripe width when the fluence was larger than 0.67 J/cm2. Simulation analyses of thermal diffusion reveal the crystallization process. The pulse laser irradiation heats the Ge stripe and melts it. Simultaneously the underlying Si in the outside region of the Ge stripe patterns absorbs the laser energy and is also heated up. During the cooling period after each pulse a temperature gradient between the edge and the center is produced in the molten Ge stripe, because of suppression of cooling at the edge. Then the temperature at the center of the Ge stripe drops below the solid point first rather than the center. Crystal growth occurs from the center to the edges, resulting that random nucleation at the edge is suppressed and crystal growth proceeds with controlled orientation in the Ge stripe. This crystallization method is promising to realize high-mobility (111) Ge TFTs and easily extended to the glass or plastic substrate by making amorphous Si layer, which works as a laser absorbing layer, and SiO2, which works as a heat insulating layer, on the substrate.

Pressure dependence of refractive index of Ge near the absorption edge

Pressure-dependent refractive index of semiconductor germanium (Ge) near the absorption edge has not been well studied theoretically and experimentally to date. In this paper, we present a pressure-dependent refractive index of Ge film near its absorption threshold (about 1550 nm), deduced from the reflectivity of high crystalline Ge film coated on a fiber end. The thin Ge layer is deposited on one end of an optical fiber by using an E-beam evaporation machine equipped with a substrate heater of a quartz halogen lamp. In order to obtain high crystalline film, the quartz halogen lamp heater provides a constant substrate temperature of 450 ℃ during film deposition. After the film forming, the sample is transferred into a muffle furnace with a nitrogen atmosphere and annealed at 600 ℃ for 20 h to guarantee the formation of higher crystalline film. The process of light propagating through the optical fiber and reflecting from the Ge thin-film involves multi-beam interference. An abnormal dispersion is observed in the refractive index spectra of the polycrystalline Ge near the absorption edge. A comparison shows that the refractive index spectrum of the amorphous Ge is normal dispersion. Unlike previously reported results that the pressure-dependent refractive index had a negative value, in our experiment it is observed to be a positive value near the absorption edge. To better understand this phenomenon, we use a critical point model including the pressure effect to successfully fit the experimental data. We obtain an abnormal dispersion range of 1505-1585 nm and a range of negative value of pressure-dependent refractive index of 1500-1580 nm from the critical point model. In this paper, we adopt the method of high crystalline Ge film coated on a fiber end. This method has the advantages of small volume, high precision and strong stability, which can be used to measure the optical properties of many thin film materials under the different conditions (temperature, electric field or magnetic field, etc.). In this work, we obtain the refractive index of the Ge film and its pressure dependence near the C-band of the optical fiber communication, and these results are conducive to optimized design of the Ge-based optical systems near the C-band of the optical fiber communication.

Thermal boundary resistance at Au/Ge/Ge and Au/Si/Ge interfaces

Amorphous Ge (a-Ge), crystalline Ge (c-Ge), and amorphous Si (a-Si) thin films were deposited on a Ge substrate at different temperatures by magnetron sputtering.

Enhancement of the crystalline Ge film growth by inductively coupled plasma-assisted pulsed DC sputtering.

The effect of pulsed DC sputtering on the crystalline growth of Ge thin film was investigated. Ge thin films were deposited on the glass substrates using ICP-assisted pulsed DC sputtering. The Ge target was sputtered using asymmetric bipolar pulsed DC sputtering system with and without assistance of ICP source. The pulse frequency of 200 Hz and the pulse on time of 500 μsec (duty cycle = 10%) were kept during sputtering process. Crystal structures were studied from X-ray diffraction. The X-ray diffraction patterns clearly showed crystalline film structures. The Ge thin films with randomly oriented crystalline were obtained using pulsed DC sputtering without ICP, whereas they had well aligned (220) orientation crystalline using ICP source. Moreover, the combination of ICP assistance and pulsed DC sputtering enhanced the growth of crystalline Ge thin films without hydrogen and metal by in situ deposition. The structure and lattice of the films were studied from TEM images. The cross-sectional TEM images revealed the deposited Ge films with columnar structure.

Improvement of Ge-on-Si photodiodes by black silicon light trapping

A light-trapping scheme for normal incidence Ge-on-Si photodiodes, utilizing needle-like black silicon nanostructures is presented. Simulations reveal that light absorption in thin rear Ge films can be enhanced several times due to both the antireflection and the scattering effect of the nanostructure. It is shown that especially films with thicknesses ≤100 nm benefit from the black silicon nanostructure, e.g., resulting in a 5 to 10 times higher absorptance at 1500 nm for a 100 nm thick film. Theoretical predictions are experimentally proved by reflectance-transmittance measurements of crystalline Ge films sputtered on black silicon substrates.

Germanium on Insulator (GOI) Structure Using Hetero-Epitaxial Lateral Overgrowth on Silicon

A technique to achieve germanium on insulator (GOI) structure on Si platform using hetero-epitaxial lateral overgrowth is presented. On (100) Si wafer, SiO2 is thermally grown, and patterned to locally reveal Si surface on which Ge is grown via selective epitaxy. After filling the growth window, Ge starts growing laterally, and adjacent Ge crystals coalesce with each other on SiO2. After the coalescence, <100> directional growth is made dominant, to fill the valley and planarize the surface. Thus, single crystalline Ge film sitting on SiO2 can be made for GOI applications.

The evolution of Ge nanostructures growth on silicon substrate by reduction of GeO2

Various shaped and sized Ge nanostructures were obtained by reducing GeO2 powder under a H2 atmosphere in a high-temperature tubular furnace. At a high depositing temperature region, crystalline Ge film was epitaxially grown on a silicon substrate. Jellyfish-like Ge/SiO2NWs composite structures were obtained next to germanium epitaxial film grown by Ge-catalyzed vapor–liquid–solid phase segregation mechanism. In addition, at the lower depositing temperature zones, Ge nanowires with various morphologies were formed owing to the temperature and concentration gradient by oxide-assisted vapor–solid mechanism and Ostwald ripening.

Solid phase epitaxy of amorphous Ge films deposited by PECVD

Amorphous thin Ge films were deposited by plasma enhanced chemical vapor deposition (PECVD) on Si(1 1 1) substrates and then annealed at 600 °C in N 2 atmosphere for 1 min. The amorphous Ge films are transformed into single crystalline Ge films by solid phase epitaxy. The influence of deposition temperature on crystalline quality, morphology, and twin density of annealed Ge films was studied. X-ray diffraction shows that after annealing the Ge films are mono-crystalline with [1 1 1] orientation parallel to the Si [1 1 1] orientation. Amorphous Ge layers were deposited at temperatures between 50 and 300 °C. Deposition at 100 °C shows the best crystal quality after annealing, with X-ray rocking curve FWHM of the (1 1 1) diffraction peak of 155 arc sec for a thickness of 53 nm. These mono-crystalline Ge films on Si(1 1 1) can be used as Ge substrate for epitaxial growth of III/V materials.

Thermally evaporated single-crystal Germanium on Silicon

Using conventional and polarization-dependent Raman spectroscopy we investigate the structural properties of Germanium films thermally evaporated on Silicon under various conditions. The analysis suggests that the Ge films can be crystalline, amorphous and poly-oriented, depending on the substrate temperature. We use both comparison with Raman spectra of Ge films grown on amorphous substrates and polarization-dependent Raman measurements to demonstrate that in the 250–450 °C interval, crystalline Ge films are epitaxial. This result is validated by means of large angle X-ray diffraction measurements. We employ these films to fabricate and characterize near infrared heterojunction photodiodes that exhibit high responsivities and low dark current densities.

Overlayer Surface-Enhanced Raman Spectroscopy for Studying the Electrodeposition and Interfacial Chemistry of Ultrathin Ge on a Nanostructured Support

Ultrathin films of germanium (Ge) have been electrodeposited onto surface-enhanced raman spectroscopy (SERS)-active, polycrystalline gold (Au) nanoparticle film electrodes from aqueous solutions containing dissolved GeO2. An overlayer SERS strategy was employed to use the SERS-activity of the underlying Au electrode to enhance the Raman signatures separately for the Ge phonon mode and vibrational modes of surface groups. Electrochemical and spectroscopic data are presented that demonstrate monolayer-level detection of the electrodeposited material and the preparation of crystalline Ge films exhibiting quantum-confinement effects. Potential-dependent Raman spectra are shown that identify electrodeposition conditions where Ge films can be deposited with either long- or short-range crystalline order. Raman spectra collected with electrodeposited Ge films immersed in solutions containing CN-(aq) did not indicate a significant presence of pinholes that exposed the underlying Au(s) substrate. Raman spectra were also collected that identified a potential-dependence for Ge hydride formation at the interface of these films. Separate spectra were collected for the oxidative dissolution of Ge in solution and the complete dry oxidation of Ge to GeOx in air. These data sets cumulatively represent the first demonstration of the overlayer SERS strategy to follow surface chemical processes at crystalline, nanostructured, Ge materials in situ and in real time.

Correlation of Nanoscale Structure and Magnetic Properties in Manganese Doped Germanium Dilute Magnetic Semiconductors

ABSTRACTSpintronic devices generally require the spin of carriers to be utilized in the storage or manipulation of data. One theoretical model for ferromagnetism in dilute magnetic semiconductors (DMS) results from the percolation of ferromagnetic regions around dilute dopants such as Mn atoms in III-V or group IV materials through the interaction of Mn atoms with carriers. Our work employed Mn implantation in Ge with subsequent rapid thermal annealing or TEM in-situ annealing to study the correlation between structure and magnetic properties. The magnetic properties of 300-350 ºC implanted Ge:Mn (which produced crystalline Ge films) varied significantly with implantation dose and annealing condition due to precipitation and transformation of different MnxGe1-x secondary phases. It was found that Mn substitution of Ge and MnxGe1-x secondary phases can both result in ferromagnetic properties. By combining TEM in-situ annealing and ex-situ magnetic characterization, we have demonstrated detailed correlation of magnetic properties with nanoscale structures in Mn implanted Ge DMS materials.

An interface study of crystalline Fe/Ge multilayers grown by molecular beam epitaxy

Fe/Ge multilayers were grown on single crystal Ge(001) substrates by molecular beam epitaxy. The structural, electronic and magnetic properties of Fe/Ge have been studied. The analysis shows that Fe grows in a layer-by-layer epitaxial growth mode on Ge(001) substrates at 150°C and no intermixing has been observed. Growth of a crystalline Ge film at 150°C on a single crystal Fe film has been observed. At this temperature Ge films grow by means of the island growth mode according to reflection of high energy electron diffraction patterns. Fe layers of 36nm thickness, deposited at 150°C on Ge(001) substrates, show two magnetization reversal values indicating the growth of Fe in two different crystal orientations. 36nm thick Fe and Ge layers grown at 150°C in Ge/Fe/Ge/Fe/Ge(001) sequence shows ferromagnetic behavior, however, the same structure grown at 200°C shows paramagnetic behavior.

Low Temperature Metal-Induced Lateral Crystallization of Si1-xGex Using Silicide/Germanide-Forming-Metals

Metal-induced lateral crystallization (MILC) of Si1-xGex (0≤x≤1) using Ni, Co, and Pd is carried out at 375 °C. It is found that the MILC rates increase with increasing Ge fraction when Ni and Co are used; however, the rate reaches a maximum at Ge mole fraction of 0.7 for Pd induced lateral crystallization of SiGe. The difference in these two trends is due to different contributions of the three processes involved during the metal-induced crystallization. Using Ni and Co, the MILC rates of Si1-xGex are limited by the silicon germanide formation and diffusion process for all Ge mole fractions. While the MILC rate of Si1-xGex using Pd is also controlled by these two processes when x is smaller than 0.7, the rate is dominantly limited by the lattice mismatch between silicon germanide and SiGe when the Ge mole fraction is larger than 0.7. Comparing the quality of MIC Ge films induced by the three metals, crystalline Ge film induced by Co has the largest grain size and the smoothest surface. Regarding the MILC rate, Pd has a similar rate as Ni in MILC of Ge, and a much higher rate than those of Ni and Co in MILC of Si0.3Ge0.7.

Low temperature high-rate growth of crystalline Ge films on quartz and crystalline Si substrates from VHF inductively-coupled plasma of GeH 4

We have studied the temperature dependence of crystalline Ge film growth on quartz, Si(100), Si(110) and Si(111) substrates from VHF-inductively-coupled plasma (ICP) of GeH 4+H 2. The ICP was generated by supplying 60 MHz power to an external single-turn antenna which was placed on a quartz plate window of a stainless steel reactor and parallel to the substrate. We have found that the crystallinity of Ge film grown on c-Si is markedly improved, especially on Si(110) and Si(100), and the signals originating from the disordered network becomes hardly observed at 350 °C in three different crystallographic orientations.