Ge Crystals Research Articles

Surface flattening and Ge crystalline segregation of Ag/Ge structure by thermal anneal

We have studied the formation of an ultrathin Ge crystalline layer by thermal annealing of Ag/Ge(100) and Ag/Ge(111) to obtain insights into the growth kinetics of two-dimensional Ge crystal. An atomic force microscope was used to evaluate the effect of the surface orientation of the Ge substrate on the surface morphology change by annealing in N2 ambience at atmospheric pressure. Moreover, a very flat surface was obtained for both Ag/Ge(100) and the Ag/Ge(111) by controlling the anneal temperature. In addition, analysis of the Raman scattering spectroscopy indicated the formation of surface segregated Ge with high crystallinity on a flat Ag surface.

In Situ Transmission Electron Microscopy of Disorder–Order Transition in Epitaxially Stabilized FeGe2

Isothermal crystallization of amorphous Ge deposited on a cubic Fe3Si/GaAs(001) substrate is performed by in situ annealing within a transmission electron microscope. It was found that the formatio...

Fabrication, characterization and analysis of a prototype high purity germanium detector for ^{76}Ge-based neutrinoless double beta decay experiments

Experiments searching for the neutrinoless double beta decay in ^{76}Ge are currently achieving the lowest background level and, in connection with the excellent energy resolution of germanium detectors, they exhibit the best discovery potential for the decay. Expansion to a ton scale of the active target mass is presently considered – in this case on-site production of the detectors may be an option. In this paper we describe the fabrication and characterization procedures of a prototype detector with a small p+ contact, which enhances the abilities of the pulse shape discrimination – one of the most important tools for background reduction. Simulations of the shapes of pulses from the detector were carried out and tuned, taking the advantage of the fact that all the parameters of the Ge crystal, cryostat and of the spectroscopic chain were known. As a result, the pulse shape analyses performed on the simulated and measured data agree very well. The worked out method allows to optimize geometry and crystal parameters in terms of pulse shape analysis efficiency, before the actual production of the detectors.

Evaluation of Phonon Dispersion Relation for Bulk Silicon Germanium by Inelastic X-ray Scattering

1. Introduction Silicon germanium (SiGe) is one of the promising candidates as a next-generation material for thermoelectric devices as an ambient energy harvesting technique because it has low thermal conductivity. For a SiGe thermoelectric device, it is important to reveal the phonon scattering mechanism for the alloy material and the thermal transport in miniaturization to the nanoscale. However, the origin of the low thermal conductivity of the SiGe alloy has not been investigated in a sufficiently rigorous manner yet. In addition, the correlation among the thermal conductivity, the phonon dispersion curve, and the lifetime remains mostly unclear.From the above, we focused on direct phonon properties evaluation by inelastic X-ray scattering (IXS) with synchrotron radiation. IXS is a powerful technique that reveals the phonon dispersion curves in contrast to Raman spectroscopy, which can only give optical phonon modes at the Brillouin zone center (the Γ point). In this study, we evaluated the phonon dispersion curve of bulk SiGe by IXS. 2. Experimental method The single-crystalline SiGe samples for IXS were prepared by two different growth methods: the Czochralski (Cz) [1] and traveling liquidus zone (TLZ) methods [2]. The sample information is summarized in Table 1. Prior to the IXS measurements, the crystal orientation and Ge fraction were confirmed by X-ray diffraction (XRD) and electron backscattering pattern (EBSP) measurements, respectively.The IXS measurements were performed at the BL35XU beamline of the SPring-8 synchrotron facility. The measurements were conducted at room temperature using a reflection geometry. The incident X-ray energy was set to 17.795 keV, which corresponds to Si (9 9 9) reflection, and the overall energy resolution was around 3 meV. The incident X-ray beam size was approximately 60 x 75 μm2. The measurements were conducted along the Γ-X ([00q]) direction. 3. Results and Discussion As an example of phonon dispersion of bulk Si1-x Ge x , we show the IXS results of x = 0.45 in Fig. 1 obtained by peak positions of IXS spectra. As a result, all optical and acoustic phonon modes for bulk SiGe were observed as shown in Fig. 1. In addition, the energy of the optical phonon modes (Si-Si, Si-Ge, and Ge-Ge modes) obtained by IXS at the zone center (Γ point) was good agreement with results of Raman spectroscopy (λ = 532 nm). Both Raman and IXS indicate that, at around Γ point, the phonon energies of Si-Si and Ge-Ge modes tend to be lower than that of pure Si and Ge, respectively. These behaviors can be considered to be caused by random distribution of Si and Ge atoms in the alloy. On the other hand, it was revealed that the acoustic phonon (LA and TA modes) energies are located between those of pure Si [3] and Ge [4]. Based on the above, the origin of the dramatic thermal conductivity reduction of the SiGe alloy, which has been reported in previous study [5], may not be derived from the reduction of group velocity, but the low phonon lifetime reduction. Acknowledgements The IXS measurements were performed at SPring-8 with the approval of JASRI (Proposal Nos. 2016A1496, 2017B1630, 2019A1678, and 2019B1750). The authors thank Dr. Koji Usuda (KIOXIA Corp.) for his great support with analyzing the IXS results throughout the work.

(Invited) Epitaxial Growth of Ge/III-V Films and Hetero-Layer Lift-off for Ultra-Thin GeOI Fabrication

Monolithic 3D (M3D) integration has been thought of as an attractive idea to realize multi-functional heterogeneous integration devices. M3D integration enables us to combine logic devices with different functional devices such as memory, sensor, or optics to create extremely compact and efficient system-on-chips. Ge and III-V compound semiconductors are expected to be implemented as high mobility channel materials and optical components which cannot be realized with Si. The layer transfer technique with low processing temperature has great potential for stacking non-lattice-matched crystalline materials for M3D ICs. Here, we discuss the layer transfer technology utilizing high quality epitaxial growth, low-temperature direct bonding and advanced epitaxial lift-off (ELO) process, which reveals the applicability of transferred layers for heterogeneous integration.The low temperature ELO process is essential for realizing reliable layer transfer with different materials because the process is almost free from the issues of thermal expansion mismatch. Process flow of GaAs or Ge layer transfer with ELO process is described in Fig. 1. First of all, a lattice-matched single crystal GaAs or Ge layer is epitaxially grown on GaAs substrate with an AlAs release layer by MOCVD. Subsequently, Al2O3 layer is deposited on the epitaxial layer by atomic layer deposition (ALD). ALD Al2O3 layer serves as the buried oxide (BOX) and the passivation layer for Ge or GaAs surface. Then, Ge or GaAs layer is patterned using photolithography. The patterned GaAs or Ge/AlAs/GaAs donor wafer is then bonded on host substrate by direct bonding technique with wet activation process. Finally, the epitaxial GaAs or Ge layer is transferred on host substrates by splitting GaAs donor wafer from AlAs layer with HCl side etching thanks to extremely high etching selectivity. Note that the fabrication of patterned transfer layer is effective in shortening the required time of ELO process. In this ELO process, the thickness of a transferred layer can be control by epitaxial layer thickness. After the layer transfer process, the released donor GaAs wafer can be reclaimed and reused for the subsequent re-epitaxial growth to reduce the process cost.Figs. 2 (a) and (b) show cross-sectional SEM and TEM images of fabricated GeOI substrate with different BOX structures. Ge pattern edge shows no peeling from SiO2(100nm)/Si substrate, indicating strong bonding strength at the pattern edge. In Fig. 2 (b), the bonding interface is clearly observed at Al2O3(5nm)/SiO2(50nm) interface formed by the wet activation process. The bonding interface exhibits very smooth and no defect. There is no observable defect in transferred Ge layer, suggesting dislocation density is below TEM detection limit. Compared to Smart-CutTM technique, ELO process has significant advantages in terms of crystal quality, thickness control and the surface flatness thanks to totally low temperature processing.To reduce wafer-scale thickness variation, etching stopper (ES) technique with high etching selectivity is quite useful. As a donor wafer, we prepare Ge active layer (20nm)/SiGe ES layer (5nm)/Ge buffer layer/AlAs/GaAs structures. Here, the SiGe layer serves as ES against the etching of Ge buffer layer to control the final GeOI thickness and attain the wafer-scale thickness uniformity. In addition, Si passivation on Ge surface was performed during initial epitaxial growth of donor wafer since Si passivation was proved to be effective in reducing interfacial trap density (D it) between Ge and oxide. After ELO process, Ge buffer layer /SiGe ES layer/Ge active layer / Al2O3 /SiO2/Si heterostructure was obtained. Then, the top Ge buffer layer was removed in HCl/H2O2 solution. We found the etching selectivity was more than 30 between Ge and SiGe layer, which is high enough to yield very smooth SiGe surface. Finally, SiGe layer was removed by TMAH preferentially.Further conformal thinning of Ge active layer was performed by the cyclic dry oxidation/wet digital etching (DE) at room temperature. In addition to thickness control with the etching rate of about 0.8 nm/cycle, it is found that dozens digital etching (DDE) process is effective for reducing Ge surface roughness. Finally, UTB GeOI substrates with high crystal quality down to 3 nm have been successfully fabricated through SiGe ES and DDE processes. Ultra-smooth GeOI surface with reduced thickness fluctuation was confirmed after DDE by TEM cross-sectional images, resulting in significant improvement of device performances. Figure 1

Ge-Rich GST Phase-Change Alloys: Thorough Investigation of Structural Evolution in Temperature

Phase-Change Memory (PCM) is considered the most mature among the emerging Non-Volatile Memory technologies. Indeed, PCM features fast programming speed, excellent scalability down to nm scale and high endurance. It relies on the reversible and fast transition between a highly resistive amorphous phase and a low resistive crystalline phase of a chalcogenide phase-change alloy. PCM’s high maturity is demonstrated by its commercialization in Storage Class Memory (SCM) market [1] and by the recent demonstration of its manufacturability and reliability in 28 nm technology node for Automotive applications [2]. Moreover, PCM is considered the best-suited device for emerging neuromorphic systems and in-memory computing applications [3].For several years, PCM has been considered not suitable to target stability at high temperature: indeed, devices based on standard material Ge2Sb2Te5 (GST225) trigger crystallization at 150°C, limiting the target applications portfolio. With the development of Ge-rich GeSbTe (Ge-rich GST) materials [4], improved PCM stability was demonstrated, fulfilling the strict requirements in terms of data retention of embedded applications, rising the questions on the origin of such improvements.Recent works on Ge-rich GST showed the effect of Ge enrichment and N doping in such alloys, in particular highlighting the increased crystallization temperature and the phase segregation at high temperature [5, 6]. However, the fine engineering of Ge-rich alloys requires a deep investigation of their structure and the understanding of its evolution in temperature.In this work, we present a thorough investigation of GeSbTe system, comparing the alloys with and without Ge enrichment, and with and without N doping. Structural analysis is performed by a wide set of complementary techniques: Raman spectroscopy, X-Ray Diffraction (XRD) and FTIR spectroscopy, TEM/EDX analyses and electrical resistivity measurements. All the analyses were performed on as-deposited amorphous samples and on samples annealed at several temperatures ranging up to more than 400°C.Comparing the Raman spectra of Ge-rich alloys with the spectra of GST225, we highlight the importance of Sb-Te structural units and their stability at all the steps of the evolution of the layer structure in temperature. The rearrangement of Ge-Te bonds, with the following crystallization of GeSbTe phase, is accompanied by the evolution of amorphous Ge phase in the layer towards the crystalline one (e.g. Fig. 1), demonstrated also in XRD spectra and TEM images. Ge single-element layer analyses allow us to evidence the important impact of the presence of Sb-Te bonds on Ge crystallization.Correlation of the Ge-N modes in N-doped Ge and Ge-rich GST samples by FTIR spectroscopy shows the effect of N on the formation of a GeN phase in GeSbTe alloy and its evolution in temperature (e.g. Fig. 2). N doping retards the rearrangement of Ge-Te bonds and the consecutive crystallization of Ge, which is delayed at higher temperature with regard to undoped sample. The sequence of phases that appear in the system with the increasing temperature is demonstrated by XRD spectra, performed on both doped and undoped layers.In summary, in this work we present a thorough investigation of the structural evolution of Ge-rich GST system in temperature thanks to the correlation of spectra and data coming from a wide set of techniques. These results help in understanding the mechanisms behind the crystallization and layer segregation, and in designing next-generation of PCM for automotive applications.[1] Huai-Yu Cheng et al., 2019 J. Phys. D: Appl. Phys. 52 473002.[2] Paolo Cappelletti et al., 2020 J. Phys. D: Appl. Phys. 53 193002.[3] T. Kim et al., IEEE TED, vol. 67, no. 4, pp. 1394-1406.[4] P. Zuliani et al., 2019 IEEE 11th International Memory Workshop, pp. 1-4.[5] V. Sousa et al., 2015 VLSI Technology, pp. T98-T99.[6] M. Agati et al., J. Mater. Chem. C, 2019, 7, 8720. Figure 1

Growth and Characterizations of GeSn Films with High Sn Composition by Chemical Vapor Deposition (CVD) Using Ge2H6 and SnCl4 for Mid-IR Applications

Epitaxial GeSn films present several advantages which can provide the semiconductor devices with higher frequency operation [1-2]. GeSn films have been theoretically and experimentally shown to become direct bandgap materials with about 6-10% of Sn composition [3-7]. This opens the room for potential optoelectronic applications such as amplifiers, lasers, photodetectors [8-13]. The red shift in the absorption due to the reduction of band-gap of the GeSn film enables the optoelectronic devices to operate in the mid-infrared (MIR) region [14-16]. However, it is very challenging to incorporate Sn into Ge crystal lattice due to the extremely limited solid solubility of Sn [17]. Therefore, low temperature growth techniques were developed to create non-equilibrium growth conditions so as to incorporate higher Sn content in high quality GeSn layer. In this work, the epitaxial growth of GeSn alloys with Sn content varying from 3.4 to 11% using Ge2H6 and SnCl4 as precursors in a CVD system under reduced pressure condition is reported in detail. Characterisation results are also presented. High quality GeSn film with Sn content as high as 11% has been achieved.The growth process consists of Ge buffer layer growth followed by GeSn epitaxial growth: 1 µm of Ge buffer layer was grown on 6“ Si wafers in the CVD reactor. Prior to actual growth of Ge, the wafer was baked in H2 at 1000°C for 2 min to remove the native oxide. 10% Ge2H6 diluted in H2 was used as Ge precursor, and H2 worked as the carrier gas. The Ge growth was then carried out at 400°C for ~10 min to target a thickness of 1 µm. The wafers were annealed soon after growth at ~850°C for another 20 min in H2 environment to reduce the threading dislocation density (TDD). Subsequently, SnCl4 was introduced into the chamber. The chamber temperature was kept below 350°C. Table I summarizes the growth conditions for some of the grown GeSn films. GeSn film with Sn content as high as 11% has been achieved successfully.Atomic force microscope (AFM) images in Figure 1(a)-(d) show that the RMS value of the surface roughness of the GeSn films is 0.52, 1.09, 0.95 and 0.91 nm for sample 1 to 4, respectively. The comprehensive studies showed that higher SnCl4flow rate, lower Ge2H6flow rate or higher growth temperature resulted in higher surface roughness (Figure 1(e)-(g)). Secondary ion mass spectrometry (SIMS) results shown in Figure 2 confirm the existence of Sn element and the Sn content. For sample 1 to 4, the Sn content is 3.4, 8, 10 and 11%, respectively.The X-Ray Diffraction (XRD) results of GeSn films are shown in Figure 3(a). It revealed that the growth temperature played the most important role in varying the Sn content in the GeSn film. The Sn content value extracted from rocking curve aligned well with the SIMS data. From the RSM mapping shown in Figure 3(b), sample 3 has a film relaxation of ~49% and compressive strain of ~0.7% based on models reported in the literatures [18-20].Etching pits density (EPD) measurements were carried out to study the threading dislocation density (TDD) of the GeSn films. Figure 4(a) shows the scanning electron microscope (SEM) image of sample 3 (Ge0.9Sn0.1) after EPD. The TDD of Ge0.9Sn0.1 is ~3.33 x 107cm-2. Figure 4(b) summarised that the TDD increases as Sn content increases which is due to the increased lattice mismatch, moreover, rapid thermal annealing (RTA) does not effectively reduce the TDD of the GeSn film.Raman analysis results are shown in Figure 5. The peak position of sample 3 (Ge0.9Sn0.1) is at 293.7 cm-1. Sn content and compressive strain were calculated to be 10 and 0.7%, respectively, by using the models in literature [21]. The values are consistent with those from XRD RSM and SIMS results. Figure 6 shows the absorption coefficient of GeSn films from ellipsometry analysis. Sn-induced red shift is observed. The cut off wavelength of the absorption extends from ~1,500 nm for bulk Ge to ~2,175 nm for sample 2 (Ge0.92Sn0.08), and it enables the devices fabricated to work in the MIR region.High resolution transmission electron microscopy (HRTEM) images in Figure 7(a)-(b) confirm that Ge buffer layer has a thickness of ~916.4 nm, while the thickness of GeSn film in sample 3 is ~175.9 nm. Figure 7(c) shows the selected area electron diffraction (SAED) pattern of the GeSn film and it concludes that the film is single crystalline.We would like to acknowledge support from the National Research Foundation Singapore under the Competitive Research Program (NRF-CRP19-2017-01). Figure 1

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).

Phase-field modeling of the non-congruent crystallization of a ternary Ge–Sb–Te alloy for phase-change memory applications

The ternary alloy of germanium, antimony, and tellurium (GST) is widely used as a material for phase-change memories. In particular, the stoichiometric compound Ge2Sb2Te5 exhibits a rapid congruent crystallization. To increase the temperature at which spontaneous crystallization erases the stored information, alloys that are enriched in germanium have been investigated. Their crystallization is accompanied by segregation and eventually the nucleation of a new, germanium-rich phase. In order to model the redistribution of alloy components and the time evolution of the microstructure during device operations, we develop a multi-phase-field model for the crystallization of GST that includes segregation and couple it with orientation fields that describe the grain structure. We demonstrate that this model is capable to capture both the emergence of a two-phase polycrystalline structure starting from an initially amorphous material, and the melting and recrystallization during the SET and RESET operations in a memory cell of the “wall” type.

Donor Complexes Formed by Cu and Zn Multi-Acceptor Impurities in Ge Crystals

Based on Hall effect measurements it is shown that quenching of complexly doped Ge〈Zn,Сu〉 crystals at 1070–1100 K leads to the formation of additional electrically active donor centers with activation energy Еd = Еc – 93 meV in them. Further annealing of crystals at 550–570 K leads to disappearance of these centers. The most likely model of additional deep donor centers is a complex consisting of a pair of substituent zinc atoms (Zns) and interstitial copper atoms (Cui). The necessity of taking into account the formation of additional donor centers in precise doping Ge〈Zn〉 crystals with copper by the method of decomposition of supersaturated solution of impurity in matrix is demonstrated.

Layer-exchange crystallization for low-temperature (∼450 °C) formation of n-type tensile-strained Ge on insulator

Layer-exchange crystallization of Ge using a group-V element has been investigated to develop a low-temperature (<500 °C) formation technique of n-type tensile-strained crystalline Ge on insulator. Here, the Sb of a group-V element is employed as a catalyst. Annealing (450 °C) of a-Ge (100 nm)/Sb (100 nm) bi-layer stacked structures generates layer-exchange crystallization. Namely, Ge and Sb layers exchange their positions, and Ge layers are crystallized on insulator substrates. However, Ge evaporation occurs during annealing, and a high concentration of Sb (∼20%) remains at the Ge/insulator interface. To solve these problems, the thickness reduction of Sb films and introduction of a-Ge thin under-layers are examined. By annealing (450 °C) a-Ge (100 nm)/Sb (50 nm)/a-Ge (5 nm) tri-layer structures, layer-exchange crystallization of Ge layers on insulator without Ge evaporation or Sb residue has been achieved. This enables formation of n-type tensile-strained (∼0.3%) Ge layers (free electron concentration: ∼5 × 1017 cm−3). Moreover, crystal orientation control of grown Ge films through the introduction of the diffusion barrier is examined. These results demonstrate the possibility of layer-exchange crystallization induced by a group-V element to realize functional thin-film devices for advanced electronics and photonics.

Effects of Temperature on the Morphology and Optical Properties of Spark Discharge Germanium Nanoparticles.

We report the spark discharge synthesis of aerosol germanium nanoparticles followed by sintering in a tube furnace at different temperatures varying from 25 to 800 °C. The size, structure, chemical composition and optical properties were studied. We have demonstrated a melting mechanism of nanoparticles agglomerates, the growth of the mean primary particle size from 7 to 51 nm and the reduction of the size of agglomerates with a temperature increase. According to transmission electron microscopy (TEM) and Fourier transform infrared (FTIR) data, primary nanoparticles sintered at temperatures from 25 to 475 °C basically have a structure of Ge crystals embedded in a GeOx amorphous matrix, as well as visible photoluminescence (PL) with the maximum at 550 nm. Pure germanium nanoparticles are prepared at temperatures above 625 °C and distinguished by their absence of visible PL. The shape of the experimental UV-vis-NIR extinction spectra significantly depends on the size distribution of the germanium crystals. This fact was confirmed by simulations according to Mie theory for obtained ensembles of germanium nanoparticles.

Characterization of high-purity germanium (Ge) crystals for developing novel Ge detectors

High-purity germanium (HPGe) crystals are required to be well-characterized before being fabricated into Ge detectors. The characterization of HPGe crystals is often performed with the Hall Effect system, which measures the net carrier concentration, the Hall mobility, and the electrical resistivity. The reported values have a strong dependence on the size of the ohmic contacts and the geometry of the samples used in conducting the Hall Effect measurements. We conduct a systematic study using four samples cut from the same location in a HPGe crystal made into different sized ohmic contacts or different geometries to study the variation of the measured parameters from the Hall Effect system. The results are compared to the C-V measurements provided by the Ge detector made from the same crystal. We report the systematic errors involved with the Hall Effect system and find a reliable technique that minimizes the systematic error to be only a few percent from the Hall Effect measurements.

Effect of recombination process in femtosecond laser-induced modification on Ge crystal* *Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 11804227).

The dynamics of produced excited carriers under the irradiation of Ge crystal is investigated theoretically by using femtosecond laser pulse. A two-temperature model combined with the Drude model is also used to study the nonequilibrium carrier density, carrier and lattice temperatures, and optical properties of the crystal. The properties of the surface plasmon wave when excited are also studied. The influences of non-radiation and radiative recombination process on the photoexcitation of the semiconductor during pulse and the relaxation after the pulse are described in detail. The results show that the effects of Auger recombination on the nonequilibrium carrier density and optical properties of the crystal and the properties of the surface plasmon polariton are great, whereas the effect of radiative recombination is extremely small.

Four-step heating process for solid-phase crystallization of Ge leading to high carrier mobility

Solid-phase crystallization is rapidly developing as a method for obtaining high-quality polycrystalline Ge layers. We propose a four-step heating process, focusing on post-deposition annealing (PDA), which is performed between the film deposition and crystallization. PDA at an appropriate temperature (500 °C) reduces the grain size while improving the crystallinity of Ge (150 nm thickness), which leads to the reduction of the grain boundary barrier height (5 meV). The resulting hole mobility (530 cm2 V−1 s−1) is higher than that of any other semiconductor thin films (<300 nm) and even single-crystal Si wafers. These findings will further improve Ge thin-film transistors for advanced devices.

Interpretation of the photomagnetic EMF spectra in a Ge crystal obtained by modulation of linearly polarized radiation

Alternating filling of the conduction band valleys by nonequilibrium charge carriers occurs in a semiconductor germanium crystal as a result of modulation of linearly polarized wave's field azimuth. Optical anisotropy in the crystal appeared when it was localized in a magnetic field. The intervalley component of the photomagnetic electromotive force (PEF) arose at the crystal contacts. The intervalley component of the PEF is associated with the intervalley filling of electrons under the effect of light polarization. The spectral dependence of the PEF consists of four resonant extremes. The origin of these extremes is associated with vertical resonance transitions from two subbands of the valence band Еv into the conduction band Еc at point Г of the zone center and point Δ with transitions E1 and E1 + Δ1. The following conclusion was made based on modulation polarimetry properties, Kramers-Kronig relations, and also taking into account the results of other authors. The nature of the extremes is the result of physical differentiation of the spectral dependence of the crystal absorption coefficient on energy (frequency).

Crystallization of Ge Thin Films on Sapphire(0001) by Thermal Annealing

Introduction>A formation of single crystalline semiconductor such as Silicon on Sapphire substrate has been intensively studied for applying the high frequency devices with low power consumption and high-speed operation. Recently, two-dimensional honeycomb crystals consisted with Si and Ge atoms so called sillicene and germanene have been attracted much attention as a post-graphene material [1,2]. One of the key issues to exhibit their unique electronic properties for device application is the formation of silicone and germanene on the insulating substrate [3]. Therefore, the purpose of this work is to get an insight into the formation of ultra-thin Ge crystal on Sapphire substrate. We have investigated an influence of thermal annealing on the surface morphology and the solid-phase-crystallization of amorphous Ge thin films formed on Sapphire(0001) substrate. Experimental procedure > Surface treatment of Sapphire(0001) substrate was performed by the ultrasonic cleaning with acetone and subsequent annealing at 1000 °C for 5 hours in N 2 ambience to obtain the cleaned flat surface. Then, a Ge thin film with a thickness of 8 nm was deposited by RF magnetron sputtering with a power density of 254 W/cm2 at Ar pressure of ~0.9 Pa. In some samples, a SiO2 capping layer with a thickness of ~ 15 nm was formed by ALD using BEDAS gas and O2 plasma at a substrate temperature of 300ºC. After that, annealing in N2 ambience at temperatures of 600 °C and 650 °C for 30 minutes were carried out to crystalize the Ge thin films. During the annealing, ramp rate was set at 20 °C/min. The surface morphology and crystallinity of the Ge thin layer for the samples were evaluated by atomic force microscope (AFM) and Raman scattering spectroscopy. Results and discussion > Figure 1 shows the AFM topographic images of the Ge thin film formed on the Sapphire(0001) substrate after the annealing at 650 °C. It was also confirmed that atomically flat surface of Sapphire(0001) with step and terrace structure was obtained by the surface treatment, and the flatness was almost unchanged even after the deposition of Ge thin film and the SiO2 capping layer (data not shown). For the samples after the annealing at 650 °C, the SiO2 capping layer was effectively suppressed an increase in the surface roughness due to the Ge (or GeO) desorption and migration, and line shapes originated from the crystalline structure near the surface were partially detected in the AFM image (Fig. 1(a)).An impact of the annealing on the crystallinity of the Ge thin film for the samples with SiO2 capping layer was evaluated from Raman scattering spectra excited by green laser with a wavelength of 532 nm as shown in Fig. 2. A broad Raman feature around 275 cm-1 due to the amorphous Ge (a-Ge) was dominant for the samples before the annealing and after the annealing at 600 °C. After the annealing at 650 ºC, a sharp signal of the TO phonon mode in the crystalline phase (c-Ge : ~300 cm-1) was clearly observed, which peak width was slightly larger than that of a reference spectrum of Ge substrate. Also, the peak tailing toward the lower Raman shift side from c-Ge signals originated from poly-crystalline were detected. From these results, it was found that the crystal nucleation in the 8 nm-thick amorphous Ge film on Sapphire(0001) substrate occurs at around 650 °C. In addition, the peak shift of c-Ge signals as compared to the reference Ge wafer was detected, which was likely to be responsible for the tensile stress caused by the lattice mismatch between Ge and Sapphire.In summary, the Ge thin films with a thickness of ~8nm on the cleaned Sapphire(0001) substrate was found to be crystallized with keeping the surface flatness using SiO2 capping layer by the annealing at 650 ºC. References > [1] K. Ito et al., Jpn. J. Appl. Phys. 57, 06HD08 (2018). [2] J. Yuhara et al., ACS Nano 12, 11632 (2018). [3] Z. Guo, et al., J. Phys. Soc. of Jpn., 82, 063714 (2013). Acknowledgement > This work was supported in part by JSPS Grant-in Aids for Scientific Research Nos. 19H02169. Figure 1

Studying phase change memory devices by coupling scanning precession electron diffraction and energy dispersive X-ray analysis

A new simultaneous acquisition software for Precession Electron Diffraction (PED) and Energy Dispersive X-ray Spectroscopy (EDX) mapping has been developed. This approach has been used to study the RESET operation in Ge-rich Ge-Sb-Te (Ge-rich GST) non-volatile Phase-Change Memory (PCM). Such an operation consists in the local amorphization of the device's active volume, which defines the memory state. Using PED patterns indexing, we were able to segment the amorphous area, and access its composition through EDX quantification. Moreover, both the spatial distribution and the chemical composition of the two crystalline phases overlapped in the chalcogenide layer were retrieved by using the spectral unmixing approach and PED patterns indexing. It evidences that Ge and Ge2Sb1Te2 (GST-212) crystals are separated in the studied devices. Despite this, we show that Ge-enrichment is preserved in the active volume of a functional device, which is crucial to guarantee the RESET state stability at high temperature. We also link the origin of the failure of the RESET operation observed in a dysfunctional device to the larger Ge-depleted area.

Impact of charge trapping on the energy resolution of Ge detectors for rare-event physics searches

Charge trapping degrades the energy resolution of germanium (Ge) detectors, which require increased experimental sensitivity in searching for dark matter and neutrinoless double-beta decay. We investigate the charge trapping processes utilizing nine planar detectors fabricated from USD-grown crystals with well-known net impurity levels. The charge collection efficiency as a function of charge trapping length is derived from the Shockley–Ramo theorem. Furthermore, we develop a model that correlates the energy resolution with the charge collection efficiency. This model is then applied to the experimental data. As a result, charge collection efficiency and charge trapping length are determined accordingly. Utilizing the Lax model (further developed by CDMS collaborators), the absolute impurity levels are determined for nine detectors. The knowledge of these parameters when combined with other traits such as the Fano factor serve as a reliable indicator of the intrinsic nature of charge trapping within the crystals. We demonstrate that electron trapping is more severe than hole trapping in a p-type detector and the charge collection efficiency depends on the absolute impurity level of the Ge crystal when an adequate bias voltage is applied to the detector. Negligible charge trapping is found when the absolute impurity level is less than 1.0 × 1011 cm−3 for collecting electrons and 2.0 × 1011 cm−3 for collecting holes.

Fabrication of p+ contact by thermally induced solid state regrowth of Al on p-type Ge crystal

Formation of p+ contact on germanium is important for its applications in diode detector and other electronic devices. In this work, thermally deposited Al on Ge crystal is annealed at 350 °C followed by slow cooling for solid-state regrowth of Al-Ge p+ contact on Ge. Depth profile analysis by secondary ion mass spectrometry (SIMS) is carried out to investigate the occurrence of Al and Ge along the depth of the regrown Al-Ge layer. Evidence of regrowth is observed due to inter-diffusion of both Ge and Al across the layers although Ge diffusion in Al layer is found to be more prevalent. Thickness of the evaporated Al layer is varied to understand the diffusion profile of Al, Ge and estimate the depth of Al incorporation in Ge crystal underneath. Hall measurement at different depth of Al-Ge regrown layer reveals that Al impurity induces p+ doping in p-type Ge and its concentration gradually reduces towards the Ge crystal. Top surface of the Al-Ge layer exhibits lowest sheet resistance that varies with the thickness of the as deposited Al layer.