Nanocrystalline Ge Research Articles

Theoretical study of High-pressure compression of Nano Sized Germanium and Silicon

High pressures have been employed to examine the pressure-induced structural and mechanical characteristics of nanocrystalline Ge and Si with different grain sizes in the current work. Volume collapse in nano and bulk phases at low and high pressures was determined by authors using the isothermal equation of states Vineet, Birch-Murnaghan, and Suzuki based on a molecular dynamic’s simulation model. The acquired result was found that the high-pressure properties depend meaningfully on the particle sizes, the 4.1-nm Si nanoparticle skilled a higher decrease in volume value than the diameter of 10.3 nm but 13nm Ge particle shows a smaller reduction in volume than 49nm and 100nm sizes nanoparticles. The results of the various EOSs agreed with one another, suggesting that the EOSs of bulk materials may be used in the computations of nanoparticles.

Li reaction pathways in Ge and high-performance Ge nanocomposite anodes for Li-ion batteries

Ge is a highly researched anode material for Li-ion batteries (LIBs) owing to its high theoretical Li storage capacity and higher electrical conductivity compared to that of Si. However, Li reaction pathways in Ge have not been definitively established due to the formation of various and complicated Li-Ge alloy phases during lithiation/delithiation. Evaluation of the Li storage characteristics of bulk and nanocrystalline Ge demonstrated that nanocrystalline Ge showed higher Li reversibility than bulk Ge. Various cutting-edge ex situ techniques were used to analyze nanocrystalline Ge, and the Li reaction pathways in Ge were thoroughly demonstrated. To enhance the electrochemical Li storage characteristics of Ge, a Ge-based nanocomposite, Ge/Al2O3/C, was synthesized via simple one-pot mechanical solid-state reduction using GeO2, Al, and C. Ge/Al2O3/C exhibited very stable cyclic behavior at a current density of 100 mA g−1 over 300 cycles (capacity retention after 300 cycles: ∼95 %). Moreover, Ge/Al2O3/C exhibited excellent rate capability with high reversible capacity and stable cyclic behavior at a high 1C-rate. The superior electrochemical performance of Ge/Al2O3/C was attributed to the nanoscale dimensions of Li-active nanocrystalline Ge (∼10 nm) and Li-inactive Al2O3 (∼12 nm) embedded uniformly in the amorphous carbon matrices. The size of nanocrystalline Ge in nanocomposite was consistently maintained during repeated cycles owing to the anti-agglomeration effect of the uniformly distributed Li-inactive Al2O3 and amorphous carbon matrices. We anticipate that the electrochemical Li reaction pathways in Ge and the resulting high-performance nanocomposite anodes will be highly useful in investigating high-performance Ge-based anodes for LIBs.

Low-Temperature PECVD Growth of Germanium for Mode-Locking of Er-Doped Fiber Laser.

A low-temperature plasma-enhanced chemical vapor deposition grown germanium (Ge) thin-film is employed as a nonlinear saturable absorber (SA). This Ge SA can passively mode-lock the erbium-doped fiber laser (EDFL) for soliton generation at a central wavelength of 1600 nm. The lift-off and transfer of the Ge film synthesized upon the SiO2/Si substrate are performed by buffered oxide etching and direct imprinting. The Ge film with a thickness of 200 nm exhibits its Raman peak at 297 cm−1, which both the nanocrystalline and polycrystalline Ge phases contribute to. In addition, the Ge thin-film is somewhat oxidized but still provides two primary crystal phases at the (111) and (311) orientations with corresponding diffraction ring radii of 0.317 and 0.173 nm, respectively. The nanocrystalline structure at (111) orientation with a corresponding d-spacing of 0.319 nm is also observed. The linear and nonlinear transmittances of the Ge thin-film are measured to show its self-amplitude modulation coefficient of 0.016. This is better than nano-scale charcoal and carbon-black SA particles for initiating the mode-locking at the first stage. After the Ge-based saturable absorber into the L-band EDFL system without using any polarized components, the narrowest pulsewidth and broadest linewidth of the soliton pulse are determined as 654.4 fs and 4.2 nm, respectively, with a corresponding time–bandwidth product of 0.32 under high pumping conditions.

FROM Si NANOWIRES TO Ge NANOCRYSTALS FOR VIS-NIR-SWIR SENSORS AND NON-VOLATILE MEMORIES: A REVIEW

"Nanocrystalline Si and Ge are ofhigh interestfor integrated Si photonics related to light emission, opticul sensors, photodetectors, solar energy harvesting and conversion devices, and also forfloating gate non-volatile memories (NVMs). In this review, we have focused on nanocrystalline porous Si (nc-PS) with extension to Si nanodots, and Ge nanocrystals (NCs)Zquantum dots (QDs)/nanoparticles (NPs) embedded in oxides (SiCh, TiCE, HfCh, AI2O3). The great asset ofnc-PS is its intense photoluminescence in VIS at room temperature (RT), while Ge NCs/NPs embedded in oxides show high photosensitivity in VISNIR-SWIR in the spectral photocurrent up to 1325 nm at RT. Ge NCs/NPs/QDsfloating gate NVMs present high memory performance, the retention characteristics corresponding to the state of the art for NCs floating gate NVMs. We prove the relevance of controlling the preparation parametersfor obtainingfilms with targetedphotoluminescence, photosensitivity and charge storage properties for applications, e.g. VIS-NIR-SWIR optical sensors and photodetectors, and electronic and photoelectric NVMs. We evidence the correlation of preparation conditions, morphology, composition and crystalline structure with optical, electrical, photoelectrical and charge storage properties and also evidence the contribution ofquantum confinement effect, localized States and trapping centers."

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.

Oxide-Free Three-Dimensional Germanium/Silicon Core-Shell Metalattice Made by High-Pressure Confined Chemical Vapor Deposition.

Metalattices are crystalline arrays of uniform particles in which the period of the crystal is close to some characteristic physical length scale of the material. Here, we explore the synthesis and properties of a germanium metalattice in which the ∼70 nm periodicity of a silica colloidal crystal template is close to the ∼24 nm Bohr exciton radius of the nanocrystalline Ge replica. The problem of Ge surface oxidation can be significant when exploring quantum confinement effects or designing electronically coupled nanostructures because of the high surface area to volume ratio at the nanoscale. To eliminate surface oxidation, we developed a core-shell synthesis in which the Ge metalattice is protected by an oxide-free Si interfacial layer, and we explore its properties by transmission electron microscopy (TEM), Raman spectroscopy, and electron energy loss spectroscopy (EELS). The interstices of a colloidal crystal film grown from 69 nm diameter spherical silica particles were filled with polycrystalline Ge by high-pressure confined chemical vapor deposition (HPcCVD) from GeH4. After the SiO2 template was etched away with aqueous HF, the Ge replica was uniformly coated with an amorphous Si shell by HPcCVD as confirmed by TEM-EDS (energy-dispersive X-ray spectroscopy) and Raman spectroscopy. Formation of the shell prevents oxidation of the Ge core within the detection limit of XPS. The electronic properties of the core-shell structure were studied by accessing the Ge 3d edge onset using STEM-EELS. A blue shift in the edge onset with decreasing size of Ge sites in the metalattices suggests quantum confinement of the Ge core. The degree of quantum confinement of the Ge core depends on the void sizes in the template, which is tunable by using silica particles of varying size. The edge onset also shows a shift to higher energy near the shell in comparison with the Ge core. This shift along with the observation of Ge-Si vibrational modes in the Raman spectrum indicate interdiffusion of Ge and Si. Both the size of the voids in the template and core-shell interdiffusion of Si and Ge can in principle be tuned to modify the electronic properties of the Ge metalattice.

On Raman scattering cross section ratio of amorphous to nanocrystalline germanium

Raman scattering spectroscopy is powerful, express and non-destructive method for control of phase composition of different materials. To determine the crystalline part, one should know the ratio of Raman cross sections of crystalline to amorphous phases. In this Letter we report on accurate comparative measurements of the Raman scattering from monocrystalline and nanocrystalline Ge, as well as from amorphous Ge films. Being accompanied by the respective optical transmittance/reflectance measurements, these data allowed us to estimate the integrated Raman scattering cross section ratios of monocrystalline and nanocrystalline Ge to amorphous Ge, for the first time. For monocrystalline Ge the obtained ratio is equal to 4, while for nanocrystalline Ge this ratio decreased monotonously with a decrease of the nanocrystal sizes. Some physical reasons of the experimentally observed dependence are proposed.

Hole mobility enhancement in strained nanocrystalline architecture of group IV semiconductors

Effective hole mobility enhancements of 300% at room temperature, compared to fully relaxed all-Ge layers, are demonstrated by employing an innovative design of Boron-doped Si0.25Ge0.75 strained nanocrystalline architecture (SNA), which are prepared by sputtering deposition incorporating with rapid thermal annealing (RTA) processes. The resultant SNA with a nanocrystalline Ge buffer layer enables us not only to fully exert the global strain but also to maintain a low-density defects and high hole concentration, thus breaking a trade-off between the hole mobility and concentration. Ultimately, a record hole mobility of 1911 cm2 V−1s−1 at a high hole concentration of 1.2 × 1018 cm−3 under room temperature is obtained in group IV semiconductors. The present SNA provides an excellent platform for high-speed applications based on group IV semiconductor.

80 °C synthesis of thermoelectric nanocrystalline Ge film on flexible plastic substrate by Zn-induced layer exchange

Zn-induced layer exchange allowed us to fabricate nanocrystalline Ge on insulators at 80 °C. The Ge layer had a high hole concentration (>1020 cm−3) and was degenerate, leading to a high electrical conductivity (>200 S cm−1). The power factor reached 240 and 160 μW mK−2 for Ge on glass and even flexible plastic substrates, respectively. For the sample with glass, grain boundary phonon scattering reduced the thermal conductivity to 3 W mK−1, which is an order of magnitude smaller than that of bulk Ge. These findings will accelerate the design of flexible thermoelectric devices based on reliable group IV semiconductors.

Native oxide removal from Ge surfaces by hydrogen plasma

The mechanisms to remove the native oxide layer on Ge(001) surfaces by an in situ hydrogen plasma inside an atomic layer deposition (ALD) reactor has been studied. A strong dependence of the reaction mechanism in the temperature range commonly employed by ALD has been identified through the combined analysis of atomic force microscopy, x-ray photoelectron and Raman spectroscopy. At low temperatures (e.g., 110 °C), the hydrogen plasma removed both Ge and O species from the native GeO2 layer, but also induced surface damage to Ge substrate. At high temperatures (e.g., 330 °C), only O species were removed from the native oxide leaving a nanocrystalline Ge overlayer behind. The thermodynamically unstable nature of hydrogen passivation on Ge resulted in a Ge surface with a high density of dangling bonds. The transition temperature between the two reaction mechanisms was determined to be about 270 °C, allowing to compromise between removing a native oxide layer entirely and hydrogenating the underlying Ge surface without surface damage.

Synthesis and photoluminescence of ultra-pure α-Ge3N4 nanowires

It is a challenge to synthesize ultra-pure one-dimensional Ge3N4 nanomaterials because the current synthetic approaches are difficult to avoid oxygen to form GeOx. In this paper, we provide a novel approach to synthesize ultra-pure Ge3N4 nanowires by directly nitriding nanocrystalline Ge powder at a relatively lower temperature of 600 °C in NH3 atmosphere. The nanocrystalline Ge powder is prepared using a liquid nitrogen cryomilling method, and an amorphous GeNx layer is formed on the surface of Ge powder to significantly block oxidation. The obtained single-crystal α-Ge3N4 nanowires are ~ 80 nm in width and several tens of micrometers in length. The photoluminescence property of α-Ge3N4 nanowires has also been investigated. The result exhibits a blue-green luminescence property with the main peak at 440 nm, which is originated from the electronic transition from the conduction band to valence band.

Investigation of the some physical properties of Ge-doped ZnO thin films deposited by thermionic vacuum arc technique

We exhibit the first nano-crystalline Ge–ZnO thin films deposited on glass and PET substrates by a thermionic vacuum arc technique. The effect of Ge doping on the structural, morphological and optical properties of ZnO:Ge films were investigated. An X-ray diffraction (XRD), atomic force microscopy, field emission scanning electron microscopy (FESEM) and UV–Vis spectrophotometer were used for the analysis. XRD patterns show the polycrystalline structure of the films in the range of 20°–80°. The roughness value for the ZnO:Ge on PET substrate was increased due to agglomeration of the grains. The results are in a good agreement with the FESEM images. Using Filmetrics F20 tool, the thickness values of the deposited thin films were obtained as 60 and 80 nm on glass and PET substrates, respectively. The optical properties of the films such as transmittance, absorbance, refractive index, and reflectance were determined. The band gap values were obtained as to be 3.43 and 3.38 eV glass and PET substrates, respectively. It was found that band gap variation of ZnO is very small with Ge doping.

Enabling n-type polycrystalline Ge junctionless FinFET of low thermal budget by in situ doping of channel and visible pulsed laser annealing

A low-thermal-budget n-type polycrystalline Ge (poly-Ge) channel that was prepared by plasma in-situ-doped nanocrystalline Ge (nc-Ge) and visible pulsed laser annealing exhibits a high electrically active concentration of 2 × 1019 cm−3 and a narrow Raman FWHM of 3.9 cm−1. Furthermore, the fabricated n-type poly-Ge junctionless FinFET (JL-FinFET) shows an Ion/Ioff ratio of 6 × 104, Vth of −0.3 V, and a subthreshold swing of 237 mV/dec at Vd of 1 V and DIBL of 101 mV/V. The poly-Ge JL-FinFET with a high-aspect-ratio fin channel is less sensitive to Vth roll-off and subthreshold-swing degradation as the gate length is scaled down to 50 nm. This low-thermal-budget JL-FinFET can be integrated into three-dimensional sequential-layer integration and flexible electronics.

Influence of crystallinity of as-deposited Ge film on formation of quantum dot in carbon-mediated solid-phase epitaxy

The influence of crystallinity of as-deposited Ge films on Ge quantum dot (QD) formation via carbon (C)-mediated solid-phase epitaxy (SPE) was investigated. The samples were fabricated by solid-source molecular beam epitaxy (MBE). Ge/C/Si structure was formed by sequential deposition of C and Ge at deposition temperature (TD) of 150–400°C, and it was heat-treated in the MBE chamber at 650°C. In the case of amorphous or a mixture of amorphous and nano-crystalline Ge film grown for TD ≤250°C, density of QDs increased with increasing TD due to the increase of C-Ge bonds in Ge layer. Ge QDs with diameter of 9.2±2.1nm were formed in the highest density of 8.3×1011cm−2 for TD =250°C. On the contrary, in the case of polycrystalline Ge film for TD ≥300°C, density of QDs decreased slightly. This is because C incorporation into Ge layer during SPE was suppressed due to the as-crystallized columnar grains. These results suggest that as-deposited Ge film in a mixture of amorphous and nano-crystalline state is suitable to form small and dense Ge QDs via C-mediated SPE.

Facile Synthesis of Ge/N-Doped Carbon Spheres with Varying Nitrogen Content for Lithium Ion Battery Anodes.

The simple fabrication of composites of germanium nanoparticles dispersed on nitrogen-doped carbon nanospheres (Ge/NC) of varying nitrogen content and their performance in lithium ion battery anodes are reported. A heavily nitrogen-doped carbon gel was formed by condensing m-phenylenediamine with formaldehyde (PF-gel); a less heavily N-doped gel was formed by condensing resorcinol and m-phenylenediamine with formaldehyde (RPF-gel); and an undoped gel was formed by condensing resorcinol with formaldehyde (RF-gel). Pyrolises of the gels with GeCl4 at 750 °C produced nanocrystalline Ge composites with 7.5 atom % N-doped carbon, termed Ge/NC (PF), with 3.9% N-doped carbon, termed Ge/NC (RPF) and undoped carbon, termed Ge/C (RF). The heavily N-doped Ge/NC (PF) anode retained a reversible capacity of 684 mAhg-1 at a specific current of 0.2 Ag-1 after 200 cycles, versus 337 mAhg-1 retained by anode made with Ge/NC (RPF) and 278 mAhg-1 retained by anode made with undoped Ge/C (RF). At a specific current 2.0 Ag-1, the capacity of the Ge/NC (PF) anode was 472 mAhg-1, versus the 210 mAhg-1 of the Ge/NC (RPF) anode and 83 mAhg-1 of the Ge/C (RF) anode. The enhanced performance of the Ge/NC (PF) anode is attributed to the better electrical conductivity of Ge/NC (PF) and to the higher density of Li+ binding defects in its N-doped carbon.

Junction-less poly-Ge FinFET and charge-trap NVM fabricated by laser-enabled low thermal budget processes

A doping-free poly-Ge film as channel material was implemented by CVD-deposited nano-crystalline Ge and visible-light laser crystallization, which behaves as a p-type semiconductor, exhibiting holes concentration of 1.8 × 1018 cm−3 and high crystallinity (Raman FWHM ∼ 4.54 cm−1). The fabricated junctionless 7 nm-poly-Ge FinFET performs at an Ion/Ioff ratio over 105 and drain-induced barrier lowering of 168 mV/V. Moreover, the fast programming speed of 100 μs–1 ms and reliable retention can be obtained from the junctionless poly-Ge nonvolatile-memory. Such junctionless poly-Ge devices with low thermal budget are compatible with the conventional CMOS technology and are favorable for 3D sequential-layer integration and flexible electronics.

Fabrication of Nanostructured Objects by Thermal Vacuum Deposition of Ge Films onto (100)GaAs Substrates

The technique of thermal vacuum deposition of Ge onto GaAs substrates has been used for obtaining nanocrystalline Ge films. Nanocrystalline character of the films is confirmed by atomic force microscopy of their surface and by the data of Raman light scattering. The most probable size of the nanocrystallites forming the films decreases monotonically with decreasing their thickness. And raise of the deposition temperature results in their enlargement. Electro conductivity of such films proves to be high enough (resistivity of 1-10 Ohm cm at room temperature) and has character of variable range hopping conduction of the Mott's type. The hops, presumably, take place through the localized states connected with the grain boundaries.

Nanocrystalline Ge films created by thermal vacuum deposition on GaAs substrates: structural and electric properties

The technique of thermal vacuum deposition of Ge onto GaAs substrates has been used for obtaining nanocrystalline Ge films. Nanocrystalline character of the films is confirmed by atomic force microscopy of their surface and by the data of Raman light scattering. The most probable size of the nanocrystallites forming the films decreases monotonically with decreasing their thickness. Electro conductivity of such the films proves to be high enough (1-10 Ohm·cm at room temperature) and has a character of variable range hopping conduction of Mott's type. The hops, presumably, take place through the localized states connected with the grain boundaries.

Structural and electrical properties of laser-crystallized nanocrystalline Ge films and nanocrystalline Ge/SiNx multilayers

Nanocrystalline Ge (nc-Ge) single layers and nc-Ge/SiNx multilayers are prepared by laser annealing amorphous Ge (a-Ge) films and a-Ge/SiNx multilayers. The microstructures as well as the electrical properties of laser-crystallized samples are systematically studied by using various techniques. It is found that the optical band gap of nc-Ge film is reduced compared with its amorphous counterpart. The formed nc-Ge film is of p-type, and the dark conductivity is enhanced by 6 orders for an nc-Ge single layer and 4 orders for a multilayer. It is suggested that the carrier transport mechanism is dominant by the thermally activation process via the nanocrystal, which is different from the thermally annealed nc-Ge sample at an intermediate temperature. The carrier mobility of nc-Ge film can reach as high as about 39.4 cm2·V−1·s−1, which indicates their potential applications in future nano-devices.

Interface traps and quantum size effects on the retention time in nanoscale memory devices

Based on the analysis of Poisson equation, an analytical surface potential model including interface charge density for nanocrystalline (NC) germanium (Ge) memory devices with p-type silicon substrate has been proposed. Thus, the effects of Pb defects at Si(110)/SiO2, Si(111)/SiO2, and Si(100)/SiO2 interfaces on the retention time have been calculated after quantum size effects have been considered. The results show that the interface trap density has a large effect on the electric field across the tunneling oxide layer and leakage current. This letter demonstrates that the retention time firstly increases with the decrease in diameter of NC Ge and then rapidly decreases with the diameter when it is a few nanometers. This implies that the interface defects, its energy distribution, and the NC size should be seriously considered in the aim to improve the retention time from different technological processes. The experimental data reported in the literature support the theoretical expectation.