Covalent amorphous semiconductors, such as amorphous silicon (a-Si) and germanium (a-Ge), are commonly believed to have localized electronic states at the top of the valence band and the bottom of the conduction band. Electrical conductivity is thought to be by the hopping mechanism through localized states. The carrier mobility of these materials is usually very low, in the order of ~10^-3 - 10^-2 cm^2/(Vs) at room temperature. In this study, we present the Hall effect characterization of a-Ge prepared by self-ion implantation of Ge ions. The a-Ge prepared by this method is highly homogenous and has a mass density within 98.5% of the crystalline Ge. The material exhibits an exceptionally high electrical conductivity and carrier mobility (~100 cm^2/(Vs)) for an amorphous semiconductor. The temperature-dependent resistivity of the material is very-well defined with two distinctive regions, extrinsic and intrinsic conductivity, as in crystalline Ge. These results are direct evidence for a largely-preserved band structure and non-localized states of the valence band in a-Ge, as proposed by Tauc et al. from optical characterization alone. This finding is not only significant for the understanding of electrical conductivity in covalent disordered semiconductors, but the exceptionally high mobility we have observed in amorphous Ge opens up device applications not previously considered for amorphous semiconductors.
Reversible non-equilibrium phase transformation in amorphous germanium
University of Science and Technology of China（中国科学技术大学）；Suzhou Institute for Advanced Research, University of Science and Technology of China（中国科学技术大学苏州高等研究院）；Institute of Physics, Slovak Academy of Sciences（斯洛伐克科学院物理研究所）；Carnegie Mellon University（卡内基梅隆大学）
Germanium is an extremely important material used for numerous functional applications in many fields of nanotechnology. In this paper, we study the crystallization of amorphous Ge using atomistic simulations of critical nano-metric nuclei at high temperatures. We find that crystallization occurs by the recurrent transfer of atoms via a diffusive process from the amorphous phase into suitably-oriented crystalline layers. We accompany our simulations with a comprehensive thermodynamic and kinetic analysis of the growth process, which explains the energy balance and the interfacial growth veloci…
The abundance of Sodium (Na), its low-cost, and low reduction potential provide a lucrative inexpensive, safe, and environmentally benign alternative to Lithium Ion Batteries (LIBs). The significant challenges in advancing Sodium Ion Battery (NIB) technologies lies in finding the better electrode materials. Experimental investigations revealed the real potency of Germanium (Ge) as suitable anode materials for NIBs. However, a systematic atomistic study is necessary to understand the fundamental aspects of capacity-voltage correlation, microstructural changes of Ge, as well as diffusion kinetic…
We present a contactless method for measuring charge in a thin film of amorphous germanium (a-Ge) with a nanoscale silicon MOSFET charge sensor. This method enables the measurement of conductance of the a-Ge film even in the presence of blocking contacts. At high bias voltage, the resistance of the contacts becomes negligible and a direct measurement of current gives a conductance that agrees with that from the measurement of charge. This charge-sensing technique is used to measure the temperature- and field-dependence of the conductance, and they both agree with a model of Mott variable-range…
The kinetics of intrinsic and dopant-enhanced solid phase epitaxy (SPE) is stud- ied in amorphous germanium (a-Ge) layers formed by ion implantation on <100> Ge substrates. The SPE rates were measured with a time-resolved reflectivity (TRR) system between 300 and 540 degC and found to have an activation energy of (2.15 +/- 0.04) eV. To interpret the TRR measurements the refractive indices of the a-Ge layers were measured at the two wavelengths used, 1.152 and 1.532 μm. For the first time, SPE rate measurements on thick a-Ge layers (>3 μm) have also been performed to distinguish between bulk an…
The amorphous iron-germanium system ($a$-Fe$_x$Ge$_{1-x}$) lacks long-range structural order and hence lacks a meaningful Brillouin zone. The magnetization of \aFeGe is well explained by the Stoner model for Fe concentrations $x$ above the onset of magnetic order around $x=0.4$, indicating that the local order of the amorphous structure preserves the spin-split density of states of the Fe-$3d$ states sufficiently to polarize the electronic structure despite $\mathbf{k}$ being a bad quantum number. Measurements reveal an enhanced anomalous Hall resistivity $ρ_{xy}^{\mathrm{AH}}$ relative to cry…
We report a detailed comparison on the role of a self-assembled monolayer (SAM) of dipolar molecules on the threshold voltage and charge carrier mobility of organic field-effect transistor (OFET) made of both amorphous and polycrystalline organic semiconductors. We show that the same relationship between the threshold voltage and the dipole-induced charges in the SAM holds when both types of devices are fabricated on strictly identical base substrates. Charge carrier mobilities, almost constant for amorphous OFET, are not affected by the dipole in the SAMs, while for polycrystalline OFET (pent…
Germanium telluride (GeTe), a phase-change material, is known to exhibit four different structural phases: three at room temperature (one amorphous and two crystalline, $α$ and $γ$) and one at high temperature (crystalline $β$). Because transitions between the amorphous and crystalline phases lead to significant changes in material properties (e.g., refractive index and resistivity), GeTe has been investigated as a phase-change material for photonics, thermoelectrics, ferroelectrics, and spintronics. Consequently, the temperature-dependent phase transitions in GeTe have been studied for bulk a…
Capacitance-voltage characteristics of individual germanium nanowire field effect transistors were directly measured and used to assess carrier mobility in nanowires for the first time; thereby removing uncertainties in calculated mobility due to device geometries, surface and interface states and gate dielectric constants and thicknesses. Direct experimental evidence showed that surround-gated nanowire transistors exhibit higher capacitance and better electrostatic gate control than top-gated devices, and are the most promising structure for future high performance nanoelectronics.
The nanometer-scale size-dependent electronic transport properties of crystalline (c-) and amorphous (a-) germanium telluride (GeTe) ultrathin films sandwiched by titanium nitride (TiN) electrodes are investigated using ab initio molecular dynamics (AIMD), density functional theory (DFT), and Green's function calculations. We find that a-GeTe ultrathin films scaled down to about 38 Angstrom (12 atomic layers) still shows a band gap and the electrical conductance is mainly due to electron transport via intra-gap states. If the ultrathin films are further scaled, the a-GeTe band gap disappears d…
Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material,[1] a light emitting medium in silicon-integrated lasers,[2,3] and a plasmonic conductor for bio-sensing.[4,5] Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repea…
Oxide interfaces provide an opportunity for electronics. However, patterning of electron gases at complex oxide interfaces is challenging. In particular, patterning of complex oxides while preserving a high electron mobility remains underexplored and inhibits the study of quantum mechanical effects where extended electron mean free paths are paramount. This letter presents an effective patterning strategy of both the amorphous-LaAlO$_3$/SrTiO$_3$ (a-LAO/STO) and modulation-doped amorphous- LaAlO$_3$/La$_{7/8}$Sr$_{1/8}$MnO$_3$/SrTiO$_3$ (a-LAO/LSM/STO) oxide interfaces. Our patterning is based…
Recently amorphous oxide semiconductors (AOS) have gained commercial interest due to their low-temperature processability, high mobility and areal uniformity for display backplanes and other large area applications. A multi-cation amorphous oxide (a-IGZO) has been researched extensively and is now being used in commercial applications. It is proposed in the literature that overlapping In-5s orbitals form the conduction path and the carrier mobility is limited due to the presence of multiple cations which create a potential barrier for the electronic transport in a-IGZO semiconductors. A multi-…
Formation of high spatial frequency laser induced periodic surface structures (HSFL) in germanium by femtosecond mid-IR pulses with wavelengths between $λ=2.0$ and $3.6 \; \mathrm{μm}$ was studied with varying angle of incidence and polarization. The period of these structures varied from $λ/3$ to $λ/8$. A modified surface-scattering model including Drude excitation and the optical Kerr effect explains spatial period scaling of HSFL across the mid-IR wavelengths. Transmission electron microscopy (TEM) shows the presence of a $30 \; \mathrm{n m}$ amorphous layer above the structure of crystalli…
Transistors based on III-V semiconductor materials have been used for a variety of analog and high frequency applications driven by the high electron mobilities in III-V materials. On the other hand, the hole mobility in III-V materials has always lagged compared to group-IV semiconductors such as silicon and germanium. In this paper we explore the used of strain and heterostructure design guided by bandstructure modeling to enhance the hole mobility in III-V materials. Parameters such as strain, valence band offset, effective masses and splitting between the light and heavy hole bands that ar…
Germanium antimony telluride has been the most used and studied phase-change material for electronic memory due to its suitable crystallization temperature, amorphous to crystalline resistance contrast, and stability of the amorphous phase. In this work, the segregation of Ge in a Ge2Sb2Te5 film of 30 nm thickness during heating inside the transmission electron microscope was observed and characterized. The Ge2Sb2Te5 film was deposited using sputtering on a Protochips Fusion holder and left uncapped in atmosphere for about four months. Oxygen incorporated within the film played a significant r…
Germanium is the base element in many phase-change materials, i.e. systems that can undergo reversible transformations between their crystalline and amorphous phases. They are widely used in current digital electronics and hold great promise for the next generation of non-volatile memory devices. However, the ultra fast phase transformations required for these applications can be exceedingly complex even for single component systems, and a full physical understanding of these phenomena is still lacking. In this paper we study nucleation and growth of crystalline Ge from amorphous thin films at…
High-resolution neutron backscattering techniques are exploited to study the elastic and quasi-elastic response of the high-density amorphous (HDA), the low-density amorphous (LDA) and the crystalline ice Ic upon temperature changes. Within the temperature ranges of their structural stability (HDA at T > 80 K, LDA at T > 135 K, ice Ic at T < 200 K) the Debye-Waller factors and mean-square displacements characterise all states as harmonic solids. During the transformations HDA->LDA (T ~ 100 K), LDA->Ic (T ~ 150K) and the supposed glass transition with Tg ~ 135 K no relaxation processes can be d…
Herein we demonstrate the ability to optically tune the mobility of electrically powered Janus particles (JP) that are half coated with various Zinc Oxide (ZnO) semiconducting layers, i.e. polycrystalline, amorphous and amorphous with a SiO2 passivation layer. The ZnO semiconductor photo-response enables increase in its electrical conductivity with light having wavelengths of sufficient photon energy with respect to the semiconductor bandgap. This effect, termed optically modulated electrokinetic propulsion (OMEP), can be harnessed to increase the contrast in polarizability between the dielect…
Amorphous oxide semiconductors, especially indium oxide-based (InOx) thin-films, have been major candidates for high mobility with easy-to-use device processability. As one of the dopants in InOx semiconductors, we proposed Si to design a thin-film transistor (TFT) channel. Because the suppression of unstable oxygen vacancies in InOx is crucial to maintaining the semiconducting behavior, Si was selected as a strong oxygen binder that is reasonably available for large production. In this review, we focus on the overall properties observed in Si-incorporated amorphous InOx TFTs in terms of bond-…
Alloying of In/Zn oxides with various X atoms stabilizes the IXZO structures but generates electron traps in the compounds, degrading the electron mobility. To assess whether the latter is linked to the oxygen affinity or the ionic radius, of the X element, several IXZO samples are synthesized by the sol-gel process, with a large number (14) of X elements. The IXZOs are characterized by XPS, SIMS, DRX, and UV-spectroscopy and used for fabricating thin film transistors. Channel mobility and the interface defect density NST, extracted from the TFT electrical characteristics and low frequency noi…
We study the structural, dynamical and electronic properties of amorphous Si\sub{1-x}Ge\sub{x}:H alloys using first principles local basis molecular dynamics simulation. The network topology and defects in the amorphous network have been analyzed. Structural changes and an increase in number of defects have been found as the Ge atomic percentage increases from x=0.1 to x=0.5. The electronic density of states exhibits a decreasing band-gap and increased mid-gap and band-tail defect states as Ge concentration increases. Investigation of the band tails of the density of states show an exponential…
Inspired by Holstein's work on small polaron hopping, the evolution equations of localized states and extended states in presence of atomic vibrations are derived for an amorphous semiconductor. The transition probabilities are obtained for four types of transitions: from one localized state to another localized state, from a localized state to an extended state, from an extended state to a localized state, and from one extended state to another extended state. At a temperature not too low, any process involving localized state is activated. The computed mobility of the transitions between loc…
The atomic and electronic structure of the p-type transparent amorphous semiconductor CuI is calculated by ab-initio molecular dynamics. It is found to consist of a random tetrahedrally bonded network. The hole effective mass is found to be quite low, as in the crystal. The valence band maximum (VBM) state has a mixed I(p)-Cu(t2g)-I(p) character, and its energy is relatively insensitive to disorder. An iodine excess creates holes that move the Fermi level into the valence band, but it does not pin the Fermi level above the VBM mobility edge. Thus the Fermi level can easily enter the valence ba…
The performance of strained silicon as the channel material for transistors has plateaued. Motivated by increasing charge-carrier mobility within the device channel to improve transistor performance, germanium (Ge) is considered as an attractive option as a silicon replacement due to its highest p-type mobility in all of the known semiconductor materials and being compatible with today's conventional CMOS manufacturing process. However, the intrinsically high carrier mobility of Ge becomes significantly degraded because Ge's native oxide is unstable and readily decomposes into several GexOy su…
Glasses usually represent the lower limit for the thermal conductivity of solids, but a fundamental understanding of lattice heat transport in amorphous materials can provide design rules to beat such a limit. Here we investigate the role of mass disorder in glasses by studying amorphous silicon-germanium alloy (a-Si$_{1-x}$Ge$_x$) over the full range of atomic concentration from $x=0$ to $x=1$, using molecular dynamics and the quasi-harmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-Si$_{1-x}$Ge$_x$ as a function of $x$ exhibits a smoother U-shape than …
Germanium single crystal having 45 mm diameter and 100 mm length of 7N+ purity has been grown by Czochralski method. Structural quality of the crystal has been characterized by Laue diffraction. Electrical conduction and Hall measurements are carried out on samples retrieved from different parts of the crystal along the growth axis. Top part of the crystal exhibits lowest impurity concentration (~10^12/cm3) that gradually increases towards the bottom (10^13/cm3). The crystal is n-type at room temperature and the resistivity shows non-monotonic temperature dependence. There is a transition from…
Numerical studies of amorphous silicon in harmonic approximation show that the highest 3.5% of vibrational normal modes are localized. As vibrational frequency increases through the boundary separating localized from delocalized modes, near omega_c=70meV, (the "mobility edge") there is a localization-delocalization (LD) transition, similar to a second-order thermodynamic phase transition. By a numerical study on a system with 4096 atoms, we are able to see exponential decay lengths of exact vibrational eigenstates, and test whether or not these diverge at omega_c. Results are consistent with a…
Advanced Power Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST)（先进电力电子研究中心，产业技术综合研究所（AIST））；Global Zero Emission Research Center, National Institute of Advanced Industrial Science and Technology (AIST)（全球零排放研究中心，产业技术综合研究所（AIST））
A rough interface seems to be one of the possible reasons for low channel mobility (conductivity) in SiC MOSFETs. To evaluate the mobility by interface roughness, we drew a boundary line between amorphous insulator and crystalline 4H-SiC in a cross-sectional image obtained by a transmission electron microscope (TEM), by using the deep learning approach of convolutional neural network (CNN). We show that the CNN model recognizes the interface very well, even when the interface is too rough to draw the boundary line manually. Power spectral density of interface roughness was calculated.
The results of RHEED, FTIR and Raman spectroscopy study of silicon and germanium films with the thickness up to 200 nm grown from molecular beams on dielectric Si$_3$N$_4$/SiO$_2$/Si(001) substrates are presented. Noticeable changes of the intensity of the N$-$H and Si$-$N absorption bands have been observed in the IR absorbance spectra as a result of the deposition of the silicon and germanium films. The thicker was the deposited film, the more considerable were the decrease of N$-$H absorption band intensity and the increase in that of the Si$-$N band. This tendency has been observed during …
Structure, Dynamics and Hydrogen Transport in Amorphous Polymers: An Analysis of the Interplay Between Free Volume Element Distribution and Local Segmental Dynamics from Molecular Dynamics Simulations
Rutile germanium dioxide (r-GeO$_2$) is a recently predicted ultrawide-band-gap semiconductor with potential applications in high-power electronic devices, for which the carrier mobility is an important material parameter that controls the device efficiency. We apply first-principles calculations based on density functional and density functional perturbation theory to investigate carrier-phonon coupling in r-GeO$_2$ and predict its phonon-limited electron and hole mobilities as a function of temperature and crystallographic orientation. The calculated carrier mobilities at 300 K are $μ_{\text…
First-principles molecular dynamics is employed to describe the atomic structure of amorphous SiN, a non-stoichiometric compound belonging to the Si$_x$N$_{y}$ family. To produce the amorphous state via the cooling of the liquid, both the Car-Parrinello and the Born-Oppenheimer approaches are exploited to obtain a system featuring sizeable atomic mobility. At high temperatures, due to the peculiar electronic structure of SiN, exhibiting gap closing effects, the Car-Parrinello methodology could not be followed since non-adiabatic effects involving the ionic and electronic degrees of freedom do …
It is argued that topological disorder in amorphous solids can be described by local strains related to local reference crystals and local rotations. An intuitive localization criterion is formulated from this point of view. The Inverse Participation Ratio and the location of mobility edges in band tails is directly related to the character of the disorder potential in amorphous solid, the coordination number, the transition integral and the nodes of wave functions of the corresponding reference crystal. The dependence of the decay rate of band tails on temperature and static disorder are deri…
Thin film transistors, with channels composed of In-X-Zn oxides, IXZO, with X dopants: Ga, Sb, Be, Mg, Ag, Ca, Al, Ni, and Cu, were fabricated and their I-V characteristics were taken at selected temperatures in the 77K<T<300K range. The low field mobility, mu, and the interface defect density, Nst were extracted from the characteristics for each of the studied IXZOs. At higher T the mobility follows the Arrhenius law with an upward distortion, increasing as T was lowered, gradually transforming into the exp [-(T0/T)1/4] variation. We showed that mu(T, Nst) follows mu0exp[-Eaeff(T,Nst)/kT], wi…
The thermal conductivity of polycrystalline semiconductors with type-I clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr and/or Eu) exhibit lattice thermal conductivities typical of amorphous materials. Remarkably, this behavior occurs in spite of the well-defined crystalline structure and relatively high electron mobility ($\sim 100 cm^2/Vs$). The dynamics of dopant ions and their interaction with the polyhedral cages of the structure are a likely source of the strong phonon scattering.
Germanium is a widely used material for electronic and optoelectronic devices and recently it has become an important material for spintronics and quantum computing applications. Donor spins in silicon have been shown to support very long coherence times ($T_{2}$) when the host material is isotopically enriched to remove any magnetic nuclei. Germanium also has non-magnetic isotopes so it is expected to support long $T_{2}$s while offering some new properties. Compared to Si, Ge has a strong spin-orbit coupling, large electron wavefunction, high mobility, and highly anisotropic conduction band …
Atomistic simulations are employed to study structural evolution of pore ensembles in binary glasses under periodic shear deformation with varied amplitude. The consideration is given to porous systems in the limit of low porosity. The initial ensembles of pores are comprised of multiple pores with small sizes, which are approximately normally distributed. As periodic loading proceeds, the ensembles evolve into configurations with a few large-scale pores and significantly reduced number of small pores. These structural changes are reflected in the skewed shapes of the pore-size distribution fu…
Diluted magnetic semiconductors (DMSs) based on group-IV materials are desirable for spintronic devices compatible with current silicon technology. In this work, amorphous Mn-doped SiGe thin films were first fabricated on Ge substrates by radio frequency magnetron sputtering and then crystallized by rapid thermal annealing (RTA). After the RTA, the samples became ferromagnetic (FM) semiconductors, in which the Curie temperature increased with increasing Mn doping concentration and reached 280 K with 5% Mn concentration. The data suggest that the ferromagnetism came from the hole-mediated proce…
