Degenerate Semiconductors Research Articles

Scaling of resistive random access memory devices beyond 100 nm2: influence of grain boundaries studied using scanning tunneling microscopy

Resistive Random Access Memory (ReRAM) has emerged as the successor to FLASH in memory technology due to its multi-level fabrication possibilities and prospects of scaling down virtually to atomic dimensions. However, as we report here, when polycrystalline switching materials are used, the ReRAM devices scaled down to the sub-5 nm2 area show complete randomness due to inhomogeneous conductance values of grains and grain boundaries. By measuring the switching properties of grains and grain boundaries individually using a scanning tunneling microscope, we demonstrate that the doublet and triplet grain boundaries behave like degenerate semiconductors and act as conduction channels that bypass the resistive switching of the devices. Fabricating virtual devices using gold clusters deposited on top, we show that the random distribution of such highly conducting grain boundaries reduces the reliability of nano-scale ReRAM devices when scaled down to the sub-10 nm scale.

Mechanism of electron transport and bipolar resistive switching in lead oxide thin films

In this paper, we report a model that interprets the mechanism of bipolar resistive switching in thin metal oxide layers as a purely electronic process. Based on the experimental results, we find that the main transport mechanism in our compensated highly resistive semiconductor is related to space-charge-limited current traps. The S-shaped I–V characteristics of the structure layers of Pt/PbO/Pt with stable bipolar resistive switching demonstrate filamentary charge carrier injection in the bulk of the film. This leads to the formation of conductive filamentary areas in the metal-oxide film. We associate the transition from the high resistive state to the low resistive state (SET process) with the trap-filling limit being reached in the local conductive filamentary area, accompanied by the transition of this area to a state with a high degree of degeneracy. The stability of the conductive filament is provided by the potential barrier formed on the border with the main volume of the lead oxide compensated semiconductor film. The return to the initial state (RESET process) occurs at the injection of opposite charge carriers into the degenerate semiconductor in the local filamentary area, followed by the charge carrier recombination and transformation of this area into a highly resistive compensated semiconductor.

Enhanced thermoelectric performance of Mg2Si1−xSnx codoped with Bi and Cr

Magnesium silicides are favorable thermoelectric materials considering resource abundance and cost. Chromium (Cr) doping in magnesium silicides has not yet been explored. Using first-principles calculations, we have studied the stability of ${\mathrm{Mg}}_{2}\mathrm{Si}$ with chromium (1.85, 3.7, 5.55, and 6.25% Cr) and tin (12.5 and 50% Sn). Three ${\mathrm{Mg}}_{2}\mathrm{Si}$ compounds doped with Sn, (Sn + Bi), and (Sn + Bi + Cr) are used to explain doping effects on thermoelectric performance. Notably, Cr behaves nonmagnetically for $\ensuremath{\le}2%\phantom{\rule{0.28em}{0ex}}\mathrm{Cr}$, after which ferromagnetic ordering is favored ($\ensuremath{\le}12.96%$ Cr), despite its elemental antiferromagnetic state. With alloying of Sn (70.4%), ${\mathrm{Mg}}_{2}\mathrm{Si}$ remains an indirect-band-gap semiconductor, but adding small amounts of Bi (3.7%) increases the carrier concentration such that electrons occupy conduction bands, making it a degenerate semiconductor. ${\mathrm{Mg}}_{2}{\mathrm{Si}}_{0.296}{\mathrm{Sn}}_{0.666}{\mathrm{Bi}}_{0.037}$ is found to give the highest thermoelectric figure of merit (ZT) and power factor (PF) at 700 K, i.e., 1.75 and 7.04 $\text{mW}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\text{K}}^{\ensuremath{-}2}$, respectively. Adding small %Cr decreases ZT and PF to 0.78 and 4.33 $\text{mW}\phantom{\rule{4.pt}{0ex}}{\text{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\text{K}}^{\ensuremath{-}2}$, respectively. Such a degradation in thermoelectric (TE) performance is attributed to two factors: (i) uniform doping acting as an electron acceptor, decreasing conduction, and (ii) the loss of low-lying conduction band degeneracy with doping, decreasing the Seebeck coefficients. A study of configurations of Cr doping suggests that Cr has a tendency to form clusters inside the lattice, which play a crucial role in tuning the magnetic and TE performance of doped ${\mathrm{Mg}}_{2}\mathrm{Si}$ compounds.

Extraction of ZnO thin film parameters for modeling a ZnO/Si solar cell

Zinc oxide (ZnO) is a semiconductor with promising electrical and optical properties. Pure ZnO is an n-type degenerate semiconductor. It is almost entirely transparent in the optical region of the electromagnetic spectrum. In this work sol–gel spin-coating method was used to grow a ZnO thin film from Zinc acetate decomposition. The optical properties of the thin film were extracted and investigated. A ZnO/p-Si heterostructure solar cell was realized using this film and its performance (efficiency) was found to be quite poor. A numerical procedure is used to elucidate this poor performance. A first step is to use the extracted optical parameters in the simulation. However, these parameters did not lead to the reproduction of measured current-voltage and figures of merit of the solar cell by simulation when other parameters of ZnO are taken from literature. It was therefore considered that ZnO is similar to an amorphous semiconductor with a continuous distribution of states in its band gap. The density of states model used is composed of tail bands and Gaussian distribution deep level bands. Eventually, and by adjusting the constituents of the band gap states and by increasing the surface recombination velocity in ZnO/p-Si interface, it was possible to obtain a good agreement between simulated and measured J-V characteristics of this solar cell.

The conduction band non-parabolicity of degenerate AZO semiconductors: k.p method

We present a discussion regarding the conduction band non-parabolicity and the Fermi energy of Al doped ZnO (AZO) degenerate semiconductors using the higher orders of Fermi–Dirac (F-D) integrals. We find an analytical expression for Fermi energy, based on two-band k.p theory and modified Boltzmann's classical equation. We examine the accuracy of resulted expression using absolute error value.

Electric field effect in boron and nitrogen doped graphene bilayers

Unlike single layer graphene, in the case of AB-stacked bilayer graphene (BLG) one can induce a non-zero energy gap by breaking the inversion symmetry between the two layers using a perpendicular electric field. This is an essential requirement in field-effect applications, particularly since the induced gap in BLG systems can be further tuned by the magnitude of the external electric field. Doping is another way to modify the electronic properties of graphene based systems. We investigate here BLG systems doped with boron and nitrogen in the presence of external electric field, in the framework of density functional theory (DFT) calculations. Highly doped BLG systems are known to behave as degenerate semiconductors, where the Fermi energy depends on the doping concentration but, in addition, we show that the electronic properties drastically depend also on the applied electric field. By changing the magnitude and the orientation of the electric field, the gap size and position relative to the Fermi level may be tuned, essentially controlling the effect of the extrinsic doping. In this context, we discuss in how far the external electric field may suitably adjust the effective doping and, implicitly, the conduction properties of doped BLG systems.

Backward-Diode Heterostructure Based on a Zinc-Oxide Nanoarray Formed by Pulsed Electrodeposition and a Cooper-Iodide Film Grown by the SILAR Method

A heterostructure promising for designing a backward diode is formed from a zinc-oxide nanorod array and a nanostructured copper-iodide film. The effect of modes of successive ionic layer adsorption and reaction (SILAR) deposition and the subsequent iodization of CuI films on smooth glass, mica, and fluorine-doped tin oxide (FTO) substrates and on the surface of electrodeposited nanostructured zinc-oxide arrays on the film structure and electrical and optical properties is investigated. A connection between the observed variations in the structure and properties of this material and intrinsic and iodination-induced point defects is established. It is found that the cause and condition for creating a backward-diode heterostructure based on a zinc-oxide nanoarray formed by pulsed electrodeposition and a copper-iodide film grown by the SILAR method is the formation of a p+-CuI degenerate semiconductor by the excessive iodination of layers of this nanostructured material through its developed surface. The n-ZnO/p+-CuI barrier heterostructure, which is fabricated for the first time, has the I–V characteristic of a backward diode, the curvature factor of which (γ = 12 V–1) confirms its high Q factor.

The synthesis of CuO and NiO nanoparticles by facile thermal decomposition of metal-Schiff base complexes and an examination of their electric, thermoelectric and magnetic Properties

In this study, CuO and NiO NPs have been synthesized by facile thermal decomposition of Schiff base complexes and their physical properties have been examined. The synthesized CuO and NiO NPs are crystallized in polycrystalline monoclinic and cubic structures with average particle size 17 ± 0.4 and 62 ± 1 nm, respectively. The electrical conductivity measurements confirm that the conduction in the CuO NPs is a band-to-band type while the conduction in the NiO NPs is governed by the large polaron conduction mechanism. The thermoelectric measurements confirm that the CuO and NiO NPs are P-type degenerate semiconductors with room temperature Seebeck coefficients equal to 89.9 and 12.1 mV/K, respectively. The magnetic measurements exhibit that the NPs have weak ferromagnetic ordering and antiferromagnetic to paramagnetic transition with high dipolar interaction.

Sr-doped LaMoN3 and LaWN3: New degenerate p-type nitrides

Structural and optoelectronic behaviours of Lanthnum molybdate and Lanthnum tungstate based semiconducting stable nitrides perovskite, LaMoN3 and LaWN3, are investigated using density functional theory based first-principles calculations. The equation of state curve fitting confirms that LaMoN3 and LaWN3 exhibit the ground state structural stability in C2/c and R3C phases, respectively. Electronic structure calculations using HSE 06 functional reveal that the energy band gap of these materials lies in the visible region along with high dispersive nature of top of the valance band result in smaller hole effective masses. Doping of divalent cation Sr2+ to replace La3+ in these materials with the stoichiometry La1−xSrxMoN3(x = 0.0625, 0.125) and La1−xSrxWN3(x = 0.0825, 0.165) significantly changes the electronic structure of the compositions and shifts the Fermi level into valance band indicates the degenerate p-type nature of these semiconducting compositions. The shifting of the Fermi level into the valence band reproduces significant intraband transitions in the optical spectra of both the compositions. The transmittance of lightly doped La1−xSrxMoN3(x = 0.0625) and La1−xSrxWN3(x = 0.0825) increases significantly in the optical region due to lower values of the reflectivity. The lower electron and hole effective masses with considerable transmittance in the visible region indicate the suitability of these compositions as degenerate p-type semiconductors for a diverse range of applications such as in light emitting diodes and in photovoltaics.

Degenerate and non-degenerate In2O3 thin films by pulsed electron beam deposition

Pulsed electron beam deposition (PED) was used to grow indium oxide thin films on c-cut sapphire single crystalline substrates between room temperature and 500 °C under oxygen gas. A slight difference in oxygen pressure during the PED growth (from 2 × 10−2 to 1.3 × 10−2 mbar) has strong effects on the electrical and optical film properties. The indium oxide thin films grown in these conditions changed from a non-degenerate semiconducting behaviour (at 2 × 10−2 mbar) to a degenerate semiconductor one (at 1.3 × 10−2 mbar), with a metal-insulator transition at 149 K. This crossover from strong to weak localization, evidenced in the temperature dependent resistivity curves, may be due to the effects of a structural disorder in such films. The direct optical band gap was estimated from transmission spectra taking into account non-degenerate/degenerate behaviour.

Coherent transmission of superconducting carriers through a ∼2 μm polar semiconductor

Coherent transmission of Cooper pairs through a non-superconducting medium is a key challenge for hybrid electronics with superconductors, normal metals and semiconductors. While superconductor–normal metal–superconductor (SNS) junctions have been known for quite sometime, including a low carrier density region or a two-dimensional electron gas (2DEG) in the path of superconducting electrons is relatively less explored. Indeed, this is due to the limited choice of materials that would make ohmic contacts to such systems, while simultaneously supporting a superconducting phase. In this paper we show a coherent transmission of supercurrent through a degenerate semiconductor over a length ≈2 μm with a critical magnetic field Bc ≈ 8 T at 1.6 K and Tc ≈ 5 K at zero magnetic field. This length scale is much larger than the typical thickness of a Josephson junction. Our system is a fragment of a GaN nanowall network that has been shown to support a high mobility 2DEG (μn > 104 cm2 V−1 s−1 ). The current and voltage probes were superconducting tungsten–gallium composite electrodes and the measurements could be done in four-probe geometry. We demonstrate ballistic type carrier transport with a near ideal transparency of 1 and a critical current (Ic) large enough such that the Josephson coupling parameter . Some features in magneto-transport data suggest that there is possibly a small magnetic moment forming in the semiconductor fragment. In addition the combination of a Tc typical of elemental metallic superconductors, but a critical field that appears to be higher than the Clogston–Chandrasekhar limit, may be indicative of the emergence of a triplet pairing mechanism in these structures.

Electronic structure tuning of stanene monolayers from DFT calculations: Effects of substitutional elemental doping

Stanene is one of the most important quantum spin hall insulators and can be considered as an efficient material for the fabrication electronic and optoelectronic devices and next-generation integrated circuits. Density functional theory calculations were carried out to investigate the band gap variations in stanene, and the effects of elemental doping were taken into account. For the pristine stanene sheet, the valence and conduction bands of stanene consist of Sn 5p orbitals. The pristine stanene system is a zero band gap material with a Dirac cone located at the K point. The Al-doped, B-doped, N-doped and P-doped systems show metallic characteristics. In the Al-doped and B-doped stanene, the Fermi level is shifted towards the valence band edge, while in the N-doped and P-doped ones, the Fermi level shifts towards the conduction band edge. These doped systems behave as degenerate semiconductors, due to the significant shifts in the Fermi levels. Other doping elements such as Si and Ge were also considered in this study. Both Si and Ge-doped stanene show the properties similar to the pristine stanene, which is a consequence of the similar electronic structure of Si, Ge and Sn atoms. In the N-doped stanene, the Dirac cone vanishes, whereas in the other doping patterns, the Dirac cone exists around the Fermi level. The electronic band structure of the Si and Ge-doped systems is principally the same as that of pristine one. Our results thus suggest a theoretical basis for the potential application of such doped systems in electronic and optoelectronic devices.

Thermoelectric properties and intrinsic conduction processes in DBSA and NaSIPA doped polyanilines

Seeking to gain fundamental understanding of the thermoelectric (TE) behavior of polyanilines (PANIs), structure- property relationships of PANI nanorods, doped with dodecylbenzenesulfonic acid (DBSA) and 5-sulfoisophtalic acid sodium salt (NaSIPA), and prepared by an indirect synthetic route, are discussed in terms of the contribution of the acid concentrations on the thermoelectric properties.The synergistic combination of high doping level and layer structure, accounts for the moderately high electrical conductivities (σ) and low constant Seebeck coefficients (α) of PANI-DBSA. Conversely, the poor doping ability of NaSIPA and low crystallinity degree explain the low electrical conductivities along with significant increases in Seebeck coefficient values. In relation to conduction mechanisms, PANI-DBSA shows a hopping behavior with a carrier concentration of c≈0.49 (hole type), while PANI-NaSIPA displays a diffusive regime, characteristic of degenerate metallic semiconductors, with an estimated charge carrier density of n ≈3 × 1021 e/cm3.

Single-Crystal Growth of Cl-Doped n-Type SnS Using SnCl2 Self-Flux.

SnS is a promising photovoltaic semiconductor owing to its suitable band gap energy and high optical absorption coefficient for highly efficient thin film solar cells. The most significant carnage is demonstration of n-type SnS. In this study, Cl-doped n-type single crystals were grown using SnCl2 self-flux method. The obtained crystal was lamellar, with length and width of a few millimeters and thickness ranging between 28 and 39 μm. X-ray diffraction measurements revealed the single crystals had an orthorhombic unit cell. Since the ionic radii of S2- and Cl- are similar, Cl doping did not result in substantial change in lattice parameter. All the elements were homogeneously distributed on a cleaved surface; the Sn/(S + Cl) ratio was 1.00. The crystal was an n-type degenerate semiconductor with a carrier concentration of ∼3 × 1017 cm-3. Hall mobility at 300 K was 252 cm2 V-1 s-1 and reached 363 cm2 V-1 s-1 at 142 K.

Experimental determination of lattice thermal conductivity and Lorenz number as functions of temperature for n-type PbTe

The thermal conductivity of materials, κ, consists of two components, i.e. the lattice contribution from phonon transport, κL, and the electrical contribution from electron/hole transport, κE, and κ = κL + κE. The Lorenz number (L) relates κE to electrical conductivity, σ, and temperature, T, as L = κE/σT. In the analyses of thermoelectric properties on non-degenerate semiconductors, L has commonly been adopted as 2.44 × 10−8 W/S-K2 to derive the lattice thermal conductivity from the measured thermal and electrical conductivity. However, this value was obtained using a model of free electron gas and, thus, is only valid for metals or degenerate semiconductors. In this study, the thermal conductivity and electrical conductivity have been measured at 13 different temperatures between 37 and 615°C on about 100 n-type doped PbTe samples processed by melt growth of directional solidification, which resulted in the wide ranges of data values. By analyzing the plot of thermal conductivity vs. electrical conductivity with linear regression, the lattice thermal conductivity and Lorenz number have been experimentally determined from the intercept and slope at each temperature for n-type PbTe. The former values represent the intrinsic case for the lattice thermal conductivity, i.e. the heat conduction of an ideal PbTe lattice without any extrinsic electrical contribution, and the latter values give the relationship between electrical conductivity and its contribution to thermal conductivity for the n-type PbTe material.

Activation energy of metastable amorphous Ge2Sb2Te5 from room temperature to melt

Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level show an exponential decline with temperature matching with the steady-state thin-film resistivity measured at 858 K (melting temperature). This suggests that the free carrier activation mechanisms form a continuum in a large temperature scale (300 K – 858 K) and the metastable amorphous phase can be treated as a super-cooled liquid. The effective activation energy calculated using the resistivity versus temperature data follow a parabolic behavior, with a room temperature value of 333 meV, peaking to ∼377 meV at ∼465 K and reaching zero at ∼930 K, using a reference activation energy of 111 meV (3kBT/2) at melt. Amorphous GST is expected to behave as a p-type semiconductor at Tmelt ∼ 858 K and transitions from the semiconducting-liquid phase to the metallic-liquid phase at ∼ 930 K at equilibrium. The simultaneous Seebeck (S) and resistivity versus temperature measurements of amorphous-fcc mixed-phase GST thin-films show linear S-T trends that meet S = 0 at 0 K, consistent with degenerate semiconductors, and the dS/dT and room temperature activation energy show a linear correlation. The single-crystal fcc is calculated to have dS/dT = 0.153 μV/K2 for an activation energy of zero and a Fermi level 0.16 eV below the valance band edge.

New photoelectrical properties of InN: Interband spectra and fast kinetics of positive and negative photoconductivity of InN

Using a Fourier transform spectrometer and a pulsed laser, we study spectra and kinetics of positive and negative photoconductivity (PC) along with other types of photoexcitation spectra of n-type InN epilayers with Hall concentrations from 3 × 1017 to 5 × 1019 cm−3 at room and low temperatures. The PC, absorption, and photoluminescence spectra of the investigated InN samples correspond to a degenerate homogenous direct band semiconductor. In contrast to previous works, we observed positive and negative PC with relaxation times in the nano- and microsecond ranges. The sign of PC depends on the temperature and the equilibrium carrier concentration.

Quasi-Classical Model of the Static Electrical Conductivity of Heavily Doped Degenerate Semiconductors at Low Temperatures

Germanium, silicon, gallium arsenide, and indium antimonide n-type crystals on the metal side of the insulator–metal transition (Mott transition) are considered. In the quasi-classical approximation, the static (direct current) electrical conductivity and the drift mobility of electrons of the c band, and electrostatic fluctuations of their potential energy and the mobility edge are calculated. It is considered that a single event of the elastic Coulomb scattering of a mobile electron occurs only in a spherical region of the crystal matrix with an impurity ion at the center. The results of calculations using the proposed formulas without using fitting parameters are numerically consistent with experimental data in a wide range of concentrations of hydrogenlike donors at their weak and moderate compensation by acceptors.

Study of the electronic transport in the semiconducting Bi0.5Sb1.5Te3 and Bi1.5Sb0.5Te3 alloys

This work presents a study of the temperature dependence on the electronic transport properties for p-type (Bi0.5Sb1.5Te3) and n-type (Bi1.5Sb0.5Te3) pellets prepared by spark plasma sintering from ball-milled powders. Seebeck coefficient and electrical resistivity displayed a metallic behavior in both samples in the temperature range from 20 to 300 K, resembling to the carrier transport of a degenerate semiconductor. The magnitude of the Seebeck coefficient of the n-type sample was lower than that p-type sample, which was attributed to bipolar conduction. That is, in the n-type sample lie an acceptor level and a donor level, generating simultaneously electrons and holes. The donor level that promotes the n-type conductivity in Bi1.5Sb0.5Te3 could be attributed to the SbBi antisite defect as reveled by crystalline cell simulations.

P-type Al-doped Cr-deficient CrN thin films for thermoelectrics

Thermoelectric properties of chromium nitride (CrN)-based films grown on c-plane sapphire by dc reactive magnetron sputtering were investigated. In this work, aluminum doping was introduced in CrN (degenerate n-type semiconductor) by co-deposition. Under the present deposition conditions, over-stoichiometry in nitrogen (CrN1+δ) rock-salt structure is obtained. A p-type conduction is observed with nitrogen-rich CrN combined with aluminum doping. The Cr0.96Al0.04N1.17 film exhibited a high Seebeck coefficient and a sufficient power factor at 300 °C. These results are a starting point for designing p-type/n-type thermoelectric materials based on chromium nitride films, which are cheap and routinely grown on the industrial scale.