Amorphous Ge Research Articles

Publisher’s Note: “Nanoscale laser flash measurements of diffuson transport in amorphous Ge and Si” [APL Mater. 10, 041111 (2022)

Publisher’s Note: “Nanoscale laser flash measurements of diffuson transport in amorphous Ge and Si” [APL Mater. 10, 041111 (2022)

Temperature-dependent charge barrier height of amorphous germanium contact detector

The exploration of germanium (Ge) detectors with amorphous Ge (a-Ge) contacts has drawn attention to the searches for rare-event physics such as dark matter and neutrinoless double-beta decay. The charge barrier height (CBH) of the a-Ge contacts deposited on the detector surface is crucial to suppress the leakage current of the detector in order to achieve a low-energy detection threshold and high-energy resolution. The temperature-dependent CBH of a-Ge contacts for three Ge detectors is analyzed to study the bulk leakage current (BLC) characteristics. The detectors were fabricated at the University of South Dakota using homegrown crystals. The CBH is determined from the BLC when the detectors are operated in the reverse bias mode with a guard-ring structure, which separates the BLC from the surface leakage current (SLC). The results show that CBH is temperature dependent. The direct relation of the CBH variation to temperature is related to the barrier inhomogeneities created on the interface of a-Ge and crystalline Ge. The inhomogeneities that occur at the interface were analyzed using the Gaussian distribution model for three detectors.

Morphology of Ge thin films crystallized by Au-induced layer exchange at low temperature (220 °C)

The influence of the original amorphous Ge (a-Ge) layer thickness on the crystallization behavior by Au-induced crystallization at low temperature (220 °C) is studied. Initially, the coverage of the crystalline Ge (c-Ge) layer increases as the a-Ge layer thickness increases. A further increase in a-Ge layer thickness, however, results in the decrease of the coverage and appearance of the second Ge layer on top of the first layer, which results in the increase of surface roughness. The bottom c-Ge layer has a better crystal quality compared to the top layer. The maximum coverage of ∼97% with only a small amount of second layer is obtained by annealing an a-Ge(46 nm)/Au(29 nm) bilayer and a Hall effect hole mobility of as high as ∼85 cm2/V s is achieved.

Solid-phase crystallization of ultra-thin amorphous Ge layers on insulators

A simple method to form ultra-thin (<20 nm) semiconductor layers with a higher mobility on a 3D-structured insulating surface is required for next-generation nanoelectronics. We have investigated the solid-phase crystallization of amorphous Ge layers with thicknesses of 10−80 nm on insulators of SiO2 and Si3N4. We found that decreasing the Ge thickness reduces the grain size and increases the grain boundary barrier height, causing carrier mobility degradation. We examined two methods, known effective to enhance the grain size in the thicker Ge (>100 nm). As a result, a relatively high Hall hole mobility (59 cm2 V−1 s−1) has been achieved with a 20 nm thick polycrystalline Ge layer on Si3N4, which is the highest value among the previously reported works.

Nanoscale laser flash measurements of diffuson transport in amorphous Ge and Si

The thermal properties of amorphous materials have attracted significant attention due to their technological importance in electronic devices. In addition, the disorder-induced breakdown of the phonon gas model makes vibrational transport in amorphous materials a topic of fundamental interest. In the past few decades, theoretical concepts, such as propagons, diffusons, and locons, have emerged to describe different types of vibrational modes in disordered solids. However, experiments can struggle to accurately determine which types of vibrational states carry the majority of the heat. In this study, we use nanoscale laser flash measurements (front/back time-domain thermoreflectance) to investigate thermal transport mechanisms in amorphous Ge and amorphous Si thin-films. We observe a nearly linear relationship between the amorphous film’s thermal resistance and the film’s thickness. The slope of the film’s thermal resistance vs thickness corresponds to a thickness-independent thermal conductivity of 0.4 and 0.6 W/(m K) for a-Ge and a-Si, respectively. This result reveals that the majority of heat currents in amorphous Si and Ge thin films prepared via RF sputtering at room temperature are carried by diffusons and/or propagons with mean free paths less than a few nanometers.

Artificial synapses based on Josephson junctions with Fe nanoclusters in the amorphous Ge barrier

We have measured the synaptic properties of a magnetic Josephson junction (MJJ) consisting of a barrier of amorphous Ge containing size-selected Fe nanoclusters between two Nb electrodes. We show that the critical current of the device varies with the magnetic order of the clusters and that the magnetic order can be tuned in a quasi-analog way with short electrical pulses of 16 pJ applied through the device. This is the first demonstration of critical current tuning via magnetic order in a nano-clustered MJJ in a material system other than SiMn. This result eliminates the need for a postdeposition annealing step for synaptic MJJs in neuromorphic applications and opens new options to optimize materials, which has the potential to improve the performance of the MJJs and provide a better understanding of the physics of the device.

Nanostructured Ge and GeSn films by high-pressure He plasma sputtering for high-capacity Li ion battery anodes

We fabricated nanostructured Ge and GeSn films using He radio-frequency magnetron plasma sputtering deposition. Monodisperse amorphous Ge and GeSn nanoparticles of 30–40 nm size were arranged without aggregation by off-axis sputtering deposition in the high He-gas-pressure range of 0.1 Torr. The Ge film porosity was over 30%. We tested the charge/discharge cycle performance of Li-ion batteries with nanostructured Ge and GeSn anodes. The Ge anode with a dispersed arrangement of nanoparticles showed a Li-storage capacity of 565 mAh/g after the 60th cycle. The capacity retention was markedly improved by the addition of 3 at% Sn in Ge anode. The GeSn anode (3 at% Sn) achieved a higher capacity of 1128 mAh/g after 60 cycles with 92% capacity retention. Precise control of the nano-morphology and electrical characteristics by a single step procedure using low temperature plasma is effective for stable cycling of high-capacity Ge anodes.

Low-Temperature and High-Speed Fabrication of Nanocrystalline Ge Films on Cu Substrates Using Sub-Torr-Pressure Plasma Sputtering

We fabricated nanocrystalline Ge films using radio-frequency (RF) magnetron plasma sputtering deposition under a high Ar-gas pressure. The Ge nanograins changed from amorphous to crystalline when the distance between the Ge sputtering target and the substrate was decreased to 5 mm and the RF input power was 11.8 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (60 W), where the deposition rate was as high as 660 nm/min. In addition, the size of the nanocrystalline grains increased from 100 to 307 nm when the RF input power for plasma production was increased from 11.8 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (60 W) to 17.7 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (90 W). In the developed narrow-gap plasma process at sub-Torr pressures, nanocrystalline Ge films were successfully fabricated on Cu substrates at low temperatures, without the substrate being heated. However, when annealing was conducted under an N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> atmosphere, which is the conventional method to induce solid-phase crystallization, the amorphous Ge layer on a Cu substrate changed to a Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ge crystal layer through interdiffusion of Ge and Cu atoms at 400–500 °C.

Effect of off-stoichiometry on the thermal conductivity of amorphous GeTe

The reversible phase change of Germanium Telluride (GeTe) is essential for developing advanced non-volatile devices. We investigate off-stoichiometric effect on the thermal and structural properties of amorphous Ge1−δ Te (0 ≤ δ ≤ 0.12) via molecular dynamics. The structural optimization due to off-stoichiometry was taken into account with an empirical potential. Our simulated thermal conductivity is in the range of experimental observations. With increasing δ, the thermal conductivity tends to be slightly reduced. Analysis on the coordination number and the bond angle distribution indicates that the off-stoichiometric Ge1−δ Te still retain its ability of rapid phase transition. These results are helpful in reliable device design and modeling.

Optical properties of GeO[SiO] and GeO[SiO2] solid alloy layers grown at low temperature

The optical properties of GeO[SiO] and GeO[SiO2] solid alloy films grown on Si(001) substrates were studied using Raman, Fourier transform infrared absorption (FTIR) and photoluminescence (PL) spectroscopies. A PL signal was observed in the infrared region both for as-deposited and annealed germanium silicate suboxide films. Furnace annealing led to an increase of the PL signal and to a redshift of the PL maximum. The PL at ∼1100 nm is most probably due to defect-induced radiative transitions while the PL at ∼1500 nm may be rather caused by amorphous Ge nanoclusters and Ge nanocrystals. Finally, the temperature dependence of the PL was studied. Anomalous quenching of the PL signal is observed which does not follow the classical Arrhenius law. The temperature dependence of the PL intensity is well explained by using a Berthelot model.

Magnetic Ge:Mn nanocrystals grown by MBE on insulator substrate for solar cell and photodetector applications

In this work, we successfully grew novel dilute magnetic Ge:Mn nanocrystals (Ge:Mn NCs) on SiO2 thin film for metal-oxidesemiconductor (MOS) solar cell and photodetector applications. The innovative Ge:Mn NCs have been grown via solid-state dewetting of ultra-pure thin amorphous Ge:Mn nanolayer deposited by high precision molecular beam epitaxy (MBE) on thin SiO2/Si film. The morphological and structural properties of the Ge:Mn NCs have been determined by atomic force microscopy (AFM) and high-resolution transmission electron microscopy (HR-TEM). The results indicate the formation of homogeneous Ge:Mn NCs on SiO2 with a well-defined spherical shape and small size (∼8 nm). The incorporation of Mn element into Ge matrix has been confirmed by scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX). The magnetic properties of the Ge:Mn NCs have been determined by the superconducting quantum interference device (SQUID). The results show that the Ge:Mn NCs are ferromagnetic and having a Curie temperature above room temperature (∼310 K). The electrical transport and the photocurrent generation in the novel Ge:Mn NCs embedded in the SiO2 film of MOS structure are studied by I-V and photocurrent spectroscopies. The results show that when the Ge:Mn NCs are inserted as an active layer in a SiO2 film of MOS structure, the photocurrent of the MOS structure increase 10 times higher than the dark current.The excellent structural, magnetic and optoelectric properties results indicate that the novel Ge:Mn NCs grown on insulator (SiO2) thin-film via solid-state dewetting can be integrated with spintronic and optoelectronic technologies to produce novel high-performance devices fully compatible with complementary oxide metal (CMOS) technology.

Germanium nanoparticles film as a room-temperature electron transport layer for organic solar cells

The performances of the organic solar cells (OSCs) largely depends on the selection and preparation of the electron transport layer (ETL). In this paper, the uniform and compact Ge nanoparticles (NPs) film was fabricated by using the plasma enhanced chemical vapor deposition (PECVD) method at room temperature. The Ge NPs film is formed by close packing of amorphous Ge particles with an average diameter of 6.05 nm, and the film shows a fairly smooth surface with a root-mean-square (RMS) roughness of 0.6419 nm. The results shows that Ge NPs film exhibits n-type characteristics, and the longitudinal conductivity of the film is 1.90 × 10−5 S/cm, which is better than that of the ZnO NPs film. The excellent longitudinal charge transport characteristics is attributed to the existence of the columnar structures in the Ge NPs. In addition, the Ge NPs film shows a visible light transmittance comparable to that of the ZnO NPs film. More importantly, the energy levels of the Ge NPs film meets the requirements of the PTB7:PC70BM OSCs for the ETL, and the devices with the Ge NPs ETL shows the power conversion efficiency (PCE) of 7.38%. The characteristic of room temperature preparation make the Ge NPs ETL may become a reliable choice for the flexible devices in the near future.

A fabrication method for type-II Ge clathrate film by annealing of Ge film covered with Na layer

A simplistic and improved technique was employed for type-II Ge clathrate film fabrication on the sapphire substrate. This method involves the deposition of metal Na layer onto amorphous Ge film and subsequent annealing for several hours by an infrared lamp heater under high vacuum in the same chamber. X-ray diffraction and Raman measurements confirmed the synthesis of type II Ge clathrate film and the crystallographic parameters were refined by Rietveld analysis. Scanning electron micrographs revealed the sample to be 1.2 μm thick and granular film with a lot of cracks. Nevertheless, the sample appeared homogeneous and uniform with the naked eye and the optical transmission spectrum indicated periodic fluctuation due to the light interference.

Low temperature formation of high-quality crystalline Ge (1 1 1) by improving Au catalyst crystallinity

We have investigated the Au catalyst crystallinity modulation effect of solid phase crystallization for amorphous Ge / Au / SiO2 stacked structure. Without using Au crystallinity modulation, the preferentially (111) oriented crystalline Ge was formed after annealing at 200 °C, however, Au atoms were highly included (~50 %) in the crystallized Ge. On the other hand, when Au crystallinity modulation by post Au-evaporation annealing utilized, the Au atom diffusion into Ge was significantly suppressed (<1%). We have confirmed that low temperature formation of the preferentially (111) oriented crystalline Ge without Au contamination by improving Au catalyst crystallinity.

Effect of the thermal stress on the defect evolution at GaAs/Si wafer bonding with a-Ge intermediate layer

In this paper, the bonding of GaAs wafer and Si wafer is achieved by introducing an amorphous Ge as the intermediate layer. Dislocations are observed on the GaAs surface when the GaAs/Si bonded wafers are annealed at 150 °C. The dislocation density is found to increase with the increase of the annealing temperature (from 150 °C to 350 °C). This feature can be explained by the increase of the thermal stress in the GaAs wafer due to the thermal mismatch between GaAs and Si wafers. With higher annealing temperature, such as 300 °C and 350 °C, some pits which originate from the partial cracking of GaAs surface are observed at the boundary of the bonded and unbonded regions. According to the stress simulation, the thermal stress at the bonding interface increases rapidly and reaches its maximum near the boundary of the bonded and unbonded regions, leading to the cracking of GaAs surface (formation of pits). In addition, large numbers of dislocations are generated near the pits. This may be attributed to the reduction of the nucleation energy of dislocations.

Controlling solid–liquid–solid GeSn nanowire growth modes by changing deposition sequences of a-Ge:H layer and SnO2 nanoparticles

Alloying Ge with Sn is one of the promising ways for achieving Si compatible optoelectronics. Here, GeSn nanowires (NWs) are realized via nano-crystallization of a hydrogenated amorphous Ge (a-Ge:H) layer with the help of metal Sn droplets. The full process consists of three steps: (1) SnO2 nanoparticle (NP) reduction in a hydrogen plasma to produce Sn catalyst; (2) a-Ge:H deposition at 120 °C and (3) annealing. GeSn alloys with rich morphologies such as discrete nanocrystals (NCs), random, and straight NWs were successfully synthesized by changing process conditions. We show that annealing under Ar plasma favors the elaboration of straight GeSn NWs in contrast to the conventional random GeSn NWs obtained when annealing is performed under a H2 atmosphere. Interestingly, GeSn in the form of discrete NCs can be fabricated during the deposition of a-Ge:H at 180 °C. Even more, the synthesis of out-of-plane GeSn NWs has been demonstrated by reversing the deposition sequence of SnO2 NPs and a-Ge:H layer.

Thickness dependent growth of Ge nanoparticles in amorphous Ge/SiO2 multilayers

Amorphous Ge/SiO2 multilayers were deposited in a high vacuum by an e-gun assisted physical vapor deposition technique with continuous Ge layers either 2 or 4 nm thick, separated by 2 nm thick SiO2 layers. The aim was to explore whether annealing of these multilayers at a rather low temperature will produce Ge nanoparticles suitable for solar cell applications. All samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (ARM), transmission electron microscopy (TEM) in cross-section, grazing incidence X-ray diffraction, Raman spectroscopy and grazing incidence small-angle X-ray scattering (GISAXS). It is found that annealing of very thin continuous amorphous Ge layers will lead to mostly spherical crystalline nanoparticles (NPs) of the desired size. On the other hand, from thicker Ge layers, a bimodal distribution is obtained consisting of smaller sphere-like and larger oblate spheroids, as confirmed by a calculated simulation.

Valence Band Structure of Chalcogenide Obtained by X‐Ray Photoelectron Spectroscopy and Etching Technique

Chalcogenide glass, such as GeSe, has been widely used in the selector device for high‐density vertically stackable memory application due to their nonlinear electrical property. The electronic structure of the valence band, from which the nature of the bonding, the short‐range structure, and the Fermi level can be obtained, is of great importance to understand their unique electrical behavior. However, the surface oxidation issue makes it difficult to obtain the accurate valence band structure by X‐ray photoelectron spectroscopy (XPS). Herein, XPS depth profiles using the combination of the monatomic and cluster ion etching are performed to determine the valence band structures of amorphous Ge–Se films capped with a thin carbon layer. The completely different etching behavior dependent on composition in the depth profiles may be closely associated with the intrinsic bonding configurations of the amorphous films. After obtaining the fresh surface, the intrinsic valence band structures of amorphous Ge–Se samples demonstrate the different bonding behavior and short‐range structure. Most importantly, the Fermi level of amorphous GexSe100−x compounds is also determined.

Polycrystalline Ge Nanosheets Embedded in Metal‐Semiconductor Heterostructures Enabling Wafer‐Scale 3D Integration of Ge Nanodevices with Self‐Aligned Al Contacts

AbstractThe ability to stack nanosheet transistors is an important prerequisite for the realization of vertically monolithic 3D integrated circuits enabling higher integration densities of functions and novel circuit topologies that relax miniaturization constraints. In this respect, a wafer‐scale platform is presented embedding high‐quality nanoscale polycrystalline Ge channels into monolithic metal‐semiconductor heterostructures. Thereto, a fabrication scheme comprising a combination of flash lamp annealing, crystallizing ultra‐thin amorphous Ge nanosheets, and a thermally induced Al‐Ge exchange reaction is demonstrated, facilitating the formation of self‐aligned Al leads enabling sharp Al‐Ge heterojunctions. The high quality of the obtained polycrystalline Al‐Ge‐Al heterostructure nanosheets is confirmed by µ‐Raman, scanning transmission electron microscopy, energy‐dispersive X‐ray spectroscopy, and electron backscatter diffraction measurements. Embedded in back‐ and top‐gate field‐effect transistor architectures, the electrical transport in polycrystalline Al‐Ge‐Al heterostructures is systematically analyzed. Enabling a complementary metal‐oxide‐semiconductor compatible wafer‐scale accessibility of high‐quality polycrystalline Ge with self‐aligned Al contacts, the proposed platform significantly contributes to the development of a broad spectrum of emerging 3D integrated Ge nanodevices.

Non-equilibrium solid-phase growth of amorphous GeSn layer on Ge-on-insulator wafer induced by flash lamp annealing

The effect of flash lamp annealing on the crystal growth of amorphous germanium-tin (GeSn) layer deposited on Ge-on-insulator wafer was investigated. It was found that the presence of Sn in the amorphous Ge significantly promoted solid-phase growth (SPG). The diffusion of Sn during the SPG of GeSn was completely suppressed owing to milli-second thermal processing, providing a high-quality GeSn layer with Sn content of 13%, which far exceeds the equilibrium solid solubility limit (∼1%). The fabricated GeSn/Ge-on-insulator wafer exhibited improved infrared absorption beyond 2000 nm, which would be a suitable platform for near-infrared image sensors based on group-IV materials.