Shallow Donor Research Articles

Hydrogen-like impurities in Si and Ge quantum dots in connection with 5 nm and beyond metal-oxide-semiconductor technologies

The industrial drive towards smaller semiconductor devices is at less than 10 nano-meters. In the next few years a three-dimensional quantum confined device structure is expected. We have studied the electronic structure of hydrogen-like donor impurities in Si and Ge quantum dots with varied shape of the confinement potential. These calculations are carried out within the Density-Functional-Effective-Mass Theory (DF-EMT). We have paid particular attention to the electron-electron interaction energy in a negative charge state of the donor impurity. The effect of electron correlation on the binding energy has been explored. In addition to these, we show that the second order correction to the ground state energy obtained through perturbative approach agrees well with the DF-EMT based numerical results, down to very small sizes of the quantum dots when the depth of the confinement is moderate or small. A non-monotonic shallow-deep transition of the binding energy of neutral and charged donors of hydrogen-like impurities is expected to occur with the reduction in the size of the quantum dot. A similar effect is expected for hydrogen-like acceptor impurities as well. Furthermore, the optical gap also shows non-monotonic shallow-deep behaviour with the quantum dot size reduction. Deepening of shallow donor or acceptor level implies that the carriers will freeze out at small sizes of the dot. We believe this is quite important in the context of metal-oxide-semiconductor (MOS) devices beyond 5 nm technologies, based on Si, Ge or a combination (SiGe). We also point out possible variabilities in the binding energy of donors and acceptors if there are variations in the size of host quantum dots in a large assembly of devices in future integrated circuits. This will lead to the variabilities in the characteristics from device to device. These results should be incorporated in the Technology Computer Aided Design (TCAD) simulation of less than or equal to 5 nm MOS devices.

Oscillator strengths, intracenter absorption and photoionization cross sections of optical transitions of shallow donors in silicon

Oscillator strengths, intracenter absorption and photoionization cross sections of optical transitions of shallow donors in silicon

Tailoring the electric, dielectric and optical properties of template-free hydrothermally synthesized CuO nanostructures via Co doping

Facile template-free hydrothermal method was utilized for preparing CuO nanostructures (NSs) doped with Co element (Cu1-xCoxO (x = 0.0, 0.02, 0.04, 0.06 and 0.08)). A single-phase monoclinic crystal with changing in morphology from rod-to sphere-like nanoparticles was observed. The temperature dependency of DC electrical conductivity revealed that the shallow and deep donor levels are responsible for the DC conduction in low and high temperature ranges, respectively. The Ac electrical conductivity, dielectric constant and dielectric loss were also measured as functions in frequency and temperature. The frequency dependence of the AC conductivity was well represented by the Jonscher's universal power law. The AC conduction was governed by the Correlated Barrier Hopping model where the inter-well and intra-well hopping mechanisms contribute to the electrical conduction in the low and high frequency ranges, respectively. The activation energies and the barrier height decreased while the dielectric constant and dielectric loss increased with increasing the Co content. The relaxation and deformational polarizations predominate and the overall behavior of dielectric constant fits well with the Koops' model. The Cole-Cole plots indicated a decrease in the sample resistance with increasing temperatures and Co concentrations. UV–vis absorption spectra exhibited two peaks at wavelengths 252 and 323 nm. The optical energy gap was calculated using Tauc's equation and decrease from 2.24 to 2.1 eV with increasing the Co concentrations. The effect of Co-doping on CuO was explored for oxygen evolution reaction (OER) and found a decrease in the overvoltage (μ) about 50 mV at 0.08 ratios, which declares that the doping of Co element enhances the electrical conductivity and feasibility for further electrocatalytic improvement.

Zr dopant acting shallow donor and active site in CdS lattice for co-catalyst free photocatalytic H2 evolution

Hydrogen evolution by photocatalysis shows great potential in solar energy conversion process and thus has become an active field of research. Separation efficiency of photogenerated carriers, light absorption and surface reaction are the dominant aspects that affect the photocatalytic hydrogen evolution. Creating traps to prolong the lifetime of photogenerated charge carriers has been widely used but may be considered detrimental if they lead to recombination process of the photogenerated charge carriers. Herein, we design a kind of efficient atomic substitution strategy to create shallow donor for highly efficient hydrogen evolution reaction. The photocatalytic hydrogen evolution reaction rate reaches 3766.9 μmol h−1 g−1, which is about 2.98 times higher of pure CdS. The injected foreign atoms (Zr) not only reduce bandgap to extend light absorption but also generate shallow trap level to reduce recombination of photogenerated carriers. Moreover, the introduced atoms could downshift Fermi level to supply more carriers and act the role as active center to reduce Gibbs free energy, leading to the enhanced hydrogen generation rate and superior stability. This report offers new enlightenment into designing noble-metal free photocatalysts with extraordinary catalytic performance and excellent stability.

A theoretical study of the effects of uniaxial stress and spatial dielectric functions on the density of states of shallow donor impurities in a GaAs quantum well dot of circular geometry

In the present work, we have carried out a comparative study of the effects of uniaxial stress and spatial dielectric functions on the density of impurity states (DOIS) of shallow donor impurities in a GaAs quantum well dot of circular cross-section. Using a trial wave function in the effective mass approximation, we carried out calculations for a range of binding energies of hydrogenic (dielectric constant) and non-hydrogenic (spatial dielectric functions) donors for various applied uniaxial stress and for different uniaxial lengths of the quantum dot. Our results show that, for a constant axial length of the quantum dot and constant uniaxial stress, the DOIS for the donor impurity is markedly enhanced for the non-hydrogenic donor impurity over that for purely hydrogenic donor impurity. At constant axial length, the applied uniaxial stress enhances the DOIS in both cases. The density of impurity states has also been studied for a constant applied uniaxial stress for different axial lengths of the quantum dot. Here, again, the DOIS increases with increasing axial length of the quantum dot. In fact, the enhanced DOIS is observed throughout the range of binding energies considered. These results show that not only does the DOIS vary with the applied uniaxial stress and spatial dielectric functions they are also different for various axial lengths of the quantum dot. These findings indicate that is important to take into account the effect of applied uniaxial stress and spatial dielectric functions when performing experimental studies of electronic, optical and transport properties of such nanostructures as quantum dots.

Halide Perovskites Breathe Too: The Iodide-Iodine Equilibrium and Self-Doping in Cs2SnI6.

The response of an oxide crystal to the atmosphere can be personified as breathing-a dynamic equilibrium between O2 gas and O2- anions in the solid. We characterize the analogous defect reaction in an iodide double-perovskite semiconductor, Cs2SnI6. Here, I2 gas is released from the crystal at room temperature, forming iodine vacancies. The iodine vacancy defect is a shallow electron donor and is therefore ionized at room temperature; thus, the loss of I2 is accompanied by spontaneous n-type self-doping. Conversely, at high I2 pressures, I2 gas is resorbed by the perovskite, consuming excess electrons as I2 is converted to 2I-. Halide mobility and irreversible halide loss or exchange reactions have been studied extensively in halide perovskites. However, the reversible exchange equilibrium between iodide and iodine [2I-(s) ↔ I2(g) + 2e-] described here has often been overlooked in prior studies, though it is likely general to halide perovskites and operative near room temperature, even in the dark. An analysis of the 2I-(s)/I2(g) equilibrium thermodynamics and related transport kinetics in single crystals of Cs2SnI6 therefore provides insight toward achieving stable composition and electronic properties in the large family of iodide perovskite semiconductors.

Conduction mechanism and shallow donor defects in Nb-doped β-Ga2O3 single crystals

Electrical properties, electronic defects, and photoluminescence (PL) of Nb-doped β-Ga2O3 crystals grown by the floating zone method have been studied in the temperature range from 10 to 350 K. The activation energies of shallow and deep traps were obtained and compared with their counterparts. The 0.25-Nb-doped Ga2O3 crystal has a metallic behavior at room temperature and a semiconducting behavior at low temperatures. This behavior was interpreted by the quantum corrections to the electrical resistivity resulting from the weak localization and Coulomb interaction. In addition, the 0.25-Nb-doped Ga2O3 crystal has a shallow donor ionization energy of 0.03 eV, indicating that it requires less energy to enable n-type conductivity. Hence, Nb is a good choice for improving the electrical conductivity of the β-Ga2O3 crystal. Three defect trap states were found at EC-0.81 eV, EC-0.49 eV, and EC-0.12 eV below the conduction band edge. The PL spectra of the undoped sample show two characteristic bands in the UV and blue regions under the excitation wavelength of 256 nm. On the other hand, in the Nb-doped β-Ga2O3 sample, a novel UV emission band with a shorter wavelength was observed.

Hydrogen-included plasma-assisted reactive sputtering for conductivity control of ultra-wide bandgap amorphous gallium oxide

Conductivity control of a-Ga2Ox films by cation/anion off-stoichiometry such as oxygen vacancy formation and hydrogen doping have been achieved by hydrogen-included plasma-assisted reactive sputter deposition system and physical and electrical properties of a-Ga2Ox films formed by this system have been investigated. The change in resistivity of a-Ga2Ox thin films deposited by the hydrogen-included plasma-assisted reactive sputtering was then investigated by changing the H2 flow rate ratio H2/(Ar + H2). The a-Ga2Ox thin films with semiconducting properties with a resistivity as low as 102 Ωcm was demonstrated using the plasma-assisted reactive sputtering system with addition to H2. Along with the low resistivity, the a-GaOx thin films with high film density and band gap energy of 5.2 g cm−3 and 4.8 eV were realized. The electrical resistivity of the a-Ga2Ox thin films can be controlled from 102 Ωcm to 105 Ωcm by appropriately controlling the amount of hydrogen introduced from the plasma. The results indicate that the hydrogen acts as a shallow donor, which increases the carrier concentration, can be efficiently introduced by using the plasma-assisted reactive sputtering system with addition to H2.

Hybrid density functional theory simulation of sodium impurity and impurity–vacancy defect complexes in germanium: perspectives of defect engineering for activation of shallow donors

At present, germanium (Ge) as a high-mobility material is considered as a possible replacement for Si in microelectronics. The main obstacle in the method is the large vacancy concentration obtained after the implantation of shallow donors. This prevents the formation of n+ regions and has a negative impact on device performance. One way to eliminate the detrimental effects of such defects is through the codoping approach. In this study, the behavior of Na impurities in germanium together with the formation of defect complexes with vacancies, divacancies and shallow donors (P and As) were investigated using hybrid density functional theory calculations. The site preference, diffusion barriers, charge states and binding energies of various complexes, as well as the activation of donor–vacancy complexes with both F and Na codoping were investigated. For this purpose, two extreme cases were considered. Firstly, we calculate the concentrations of substitutional and interstitial Na together with monovacancies in the case of quasi-equilibrium conditions realized for doping from melt. Another extreme case is Na codoping during ion implantation. Our calculations show that Na passivates vacancies, divacancies and donor–vacancy complexes (E-centers) with a large energy gain, which prevents rapid donor diffusion similar to the case of doping with fluorine. We have shown that the passivation of E-centers with both F and Na impurities leads to the back transformation of such complexes into shallow donors. This means that codoping with Na may neutralize the harmful effects of vacancies and is a perspective for defect engineering.

Reliability Engineering of High‐Mobility IGZO Transistors via Gate Insulator Heterostructures Grown by Atomic Layer Deposition

AbstractThe reliability of oxide‐semiconductor (OS) thin‐film transistors (TFTs) is significantly influenced by the gate insulator (GI). During electrical bias stress, the defect sites near the semiconductor/GI interface and/or within the GI may trap electrons, which makes the threshold voltage (Vth) shift toward positive values. On the other hand, carbon (C) or hydrogen (H) atoms may diffuse from the GI into the active layer, and act as shallow donors, which induce negative Vth shifts (ΔVth). In this paper, an in situ atomic layer deposition (ALD)‐based GI heterostructure is introduced, which consists of a stack of two complementary materials, namely Al2O3 and SiO2. Here, a competition occurs between electron trapping in Al2O3 (positive ΔVth) and carrier generation from H atoms in SiO2 (negative ΔVth) which allows the achievement of nearly zero ΔVth under positive bias temperature stress (PBTS). This strategy is successfully applied to a high‐mobility (>50 cm2 Vs−1) ALD‐based indium‐gallium‐zinc oxide (IGZO) device, resulting in a net ∆Vth of −0.02 V under PBTS and drain current variation (∆ID) of +0.49% under constant current stress (CCS). The application of an in situ ALD process thus offers valuable insights to resolve the mobility versus reliability trade‐off in high‐performance oxide TFTs.

First-principles study of hydrogen- and oxygen-related complexes in ScN

Scandium nitride (ScN) is an attractive material for electronic applications due to its high n-type conductivity. Native defects and unintentional impurities may limit its electron concentration and reduce its mobility; therefore, it is important to control their formation and incorporation. Hydrogen and oxygen are unintentional impurities that are commonly present during growth and processing. They act as shallow donors in ScN and hence may be regarded as harmless or even favorable to achieving n-type conductivity. Here we show, using state-of-the-art first-principles calculations, that these impurities can be detrimental because they readily form complexes with scandium vacancies (VSc). Isolated VSc have relatively high formation energies and thus have low concentrations and little impact on electronic properties. However, complexes between VSc and either hydrogen or oxygen form more readily than the pristine vacancy and will act as both compensating and scattering centers. Our results point to the importance of controlling the incorporation of hydrogen and oxygen in ScN (and AlScN alloys) to avoid degradation of the electronic properties.

Examining the influence of magnetic field on a donor dopant’s photoionization cross-section in a double quantum box

This study systematically explored the binding energy and the photoionization cross-section (PCS) of a hydrogenic shallow donor impurity within a GaAs double quantum box structure incorporated into a Ga(1−x)Alx As matrix under the influence of an external intense magnetic field. The computations are performed using the effective-mass approximation (EMA) and the finite element method (FEM), taking into account a finite potential across all surfaces of the nanosystem. The results emphasize the significant role of the magnetic field, primarily in enhancing the donor binding energy within the double quantum box system. We demonstrate that increasing the magnetic field results in an augmentation of binding energy across various aluminum concentrations. The PCS of a shallow donor impurity undergoes a blue shift when shifted from the barrier center to the box center, attributed to the increased binding energy. When the double quantum box is subjected to a magnetic field effect, the amplitude peaks of PCS show an elevation coupled with a blue shift. By setting magnetic field values at 0 T, 35 T, or 70 T and varying aluminum concentrations, we observe a shift in the PCS towards higher energy levels along with an increase in peak amplitude. We also study the PCS of the donor impurity as a function of photon energy, mainly examining the effects of box width and barrier width modifications in the double quantum box.

Adjusting photocatalytic hydrogen production of TiO2 hollow nanospheres through nitrogen-induced shallow trapping levels

N-doped titanium dioxide (TiO[Formula: see text] hollow nanospheres with abundant oxygen vacancies were successfully synthesized by coupling urea treatment and annealing in an N2 atmosphere. The pristine TiO2 hollow nanospheres exhibit a shallow donor level for electron trapping, while the urea treatment generates a N 2p acceptor level for hole trapping. After annealing in N2, the sufficient N atoms generate abundant oxygen vacancies for trapping electrons, resulting in further improved charge separation efficiency. The N-doped TiO2 exhibits the highest H2 evolution rate, reaching 2867[Formula: see text][Formula: see text]mol[Formula: see text]g[Formula: see text][Formula: see text]h[Formula: see text], which is six times higher than that of pristine TiO2 hollow nanospheres. The introduction of oxygen vacancies by interstitial N provides a promising way to improve the photocatalysis activity of photocatalysts.

Origin of Black Color in Heavily Doped n‐Type GaN Crystal

In semiconductor materials, doping is used mainly for controlling the electrical properties. There have been attempts to grow low‐resistivity n‐type gallium nitride (GaN) crystals by doping oxygen, germanium, and silicon, because a low‐resistivity GaN substrate is required to reduce the power losses of optical and electrical devices. However, in those efforts, the crystal color turns black with the increase in the concentration of the n‐type additives, even though they are shallow donors. Herein, it is explained why heavily doped n‐type GaN crystals exhibit low transparency. From optical absorption profiles, the appearance of a band tail from the band edge to 1.5 eV is observed. Considering the band tail theory and our observations, it is concluded that Ga vacancy or Ga vacancy complexes behaving as acceptors induce the band tail and the black color. It is proposed that neutralizing the high charge of defects ensures that low‐colored GaN crystals with low resistivity can be obtained. Moreover, the fabrication of low‐resistivity wafers sliced from a large crystal with a laser produces inexpensive wafers and allows the spread of high‐efficiency GaN devices fabricated on low‐resistivity substrates for saving electric power.

Epitaxial Tantalum‐Doped β‐Ga2O3 Thin Films Grown on Mgo (001) Substrate by Pulsed Laser Deposition

Epitaxial thin films of tantalum‐doped β‐Ga2O3 (Ta‐Ga2O3) are grown on MgO (001) substrates to study the effect of Ta doping on the electrical properties of β‐Ga2O3 films. X‐Ray diffraction (XRD) measurements show that the films with different Ta doping concentrations are (00l)‐oriented single‐crystalline β‐Ga2O3 without impurity phases. The incorporation of the Ta element modifies the electrical properties of Ta‐Ga2O3 films significantly. At a very low doping ratio of 0.05 mol%, the Ta‐Ga2O3 film showed a minimum resistivity of 2.32 Ω cm and a carrier concentration of 2.48 × 1017 cm−3. The corresponding activation energy of Ta element in the film is 16.8 meV, suggesting that the Ta element is a promising shallow donor dopant. The X‐ray photoelectron spectroscopy (XPS) analysis confirms that the Fermi level of the Ga2O3 films shifts toward the conduction band minimum after the introduction of Ta ions. These results indicate that the transition metal element Ta could be an effective n‐type dopant for modulating the carrier transport behavior of β‐Ga2O3 films.

Low-temperature photoluminescence characteristic of Tm-doped Ga2O3 films for light emitting diodes application

Tm-doped Ga2O3 films hold significant promise for application in light-emitting diodes (LEDs). A comprehensive understanding of their photoluminescence properties is pivotal for unlocking their full potential. In this work, we present findings on the photoluminescence (PL) spectra of Tm-doped Ga2O3 films, elucidating variations at different measurement temperatures and excitation energies utilizing synchrotron radiation light. The intensity of ultraviolet light (UVL1 and UVL2) exhibits a temperature dependence and its activation energy illustrates an ionization energy of shallow donor and migration through tunneling of self-trapped holes to triply ionized Ga vacancy (VGa) centers forming doubly ionized VGa acceptors. Additionally, the excitation energy-dependent PL spectra provide insights into the excitation mechanism of Tm ions, indicating a most likely through defects in the Ga2O3 host.

Transition Metal Impurities as Shallow Donors in β‐Ga2O3

An in‐depth investigation of transition metal impurities (Hf, Zr, Nb, and W) is presented as shallow donors in monoclinic Ga2O3 using first‐principles calculations within the framework of density functional theory. A combination of semilocal and hybrid functionals is used to predict their binding energies and hyperfine parameters. The generalized gradient approximation (GGA) allows performing calculations for supercells of up to 2500 atoms, enabling an extrapolation to the dilute limit. The shortcoming of GGA in correctly describing the electron localization is then overcome by the use of the hybrid functional. Results are presented and discussed in light of the application of these transition‐metal elements as shallow donors in Ga2O3 and their identification in the experiment. The methodology applied here can be used in calculations for shallow donors in other systems.

Interstitial Doping of SnO2 Film with Li for Indium-Free Transparent Conductor

SnO2 films exhibit significant potential as cost-effective and high electron mobility substitutes for In2O3 films. In this study, Li is incorporated into the interstitial site of the SnO2 lattice resulting in an exceptionally low resistivity of 2.028 × 10−3 Ω⋅cm along with a high carrier concentration of 1.398 × 1020 cm−3 and carrier mobility of 22.02 cm2/V⋅s. Intriguingly, Li i readily forms in amorphous structures but faces challenges in crystalline formations. Furthermore, it has been experimentally confirmed that Li i acts as a shallow donor in SnO2 with an ionization energy ΔE D1 of −0.4 eV, indicating spontaneous occurrence of Li i ionization.

Spinel LiGa5O8 prospects as ultra-wideband-gap semiconductor: Band structure, optical properties, and doping

LiGa5O8 in the spinel type structure is investigated as a potential ultra-wideband-gap semiconductor. The band structure is determined using the quasiparticle self-consistent GW method, and the optical properties are calculated at the Bethe Salpeter Equation level including electron-hole interaction effects. The optical gap including exciton effects and an estimate of the zero-point motion electron phonon coupling renormalizations is estimated to be about 5.2±0.1 eV with an exciton binding energy of about 0.4 eV. Si doping as potential n-type dopant is investigated and found to be a promising shallow donor.

PVP assisted sol-electrospraying, unlike sol electrospinning, yields highly pure rhombohedral zinc titanate particles that reduce 4-nitrophenol under visible light

This work demonstrates for the first time that switching from sol-electrospinning to sol-electrospraying using PVP as a carrier polymer leads to enhanced purity of rhombohedral zinc titanate (Rh-ZTO) after calcination. Unlike in sol-electrospinning, the purity systematically increases and crystallite size decreases as PVP concentration is increased in sol-electrospraying. 100% pure Rh-ZTO powders were prepared with PVP fraction of 31.25 wt%. After careful characterization by XRD, rheology, zeta potential measurements, and SEM, we conclude that the interaction of multipole moments of the incipient sol-clusters with the electrostatic field influences the purity. We propose that in electrospinning the sol clusters spatially separate to yield multiple crystalline phases whereas in electrospraying they intermix due to polymeric cages in droplets and yield a single crystalline phase. The highest purity samples were found to enhance photo-reduction of 4-nitrophenol in presence of visible light, with rate constant increasing from 0.000415 to 0.00104 min−1. This could result from complex formed between oxygen vacancy and the shallow donor states of Ti3+ as confirmed by DRS, XPS, and PL.