Hydrogen Shallow Donors Research Articles

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.

Size Effect of Hemi-Toroidal Quantum Dot on the Electronic Properties in the Presence of an Off-Center Hydrogenic Shallow Donor Impurity

We have studied the electronic properties in presence of an off-center hydrogenic shallow donor impurity confined in GaAs semiconductor quantum dot with toroidal geometry by considering the infinite confinement potential. This study has been performed within the parabolic band and the effective mass approximations in the presence of an off-center donor impurity. Three-dimensional Schrödinger equations are discretized using the finite difference method on a mesh containing Nr*Nθ*Nφ nodes. The numerical results of the analytical calculations demonstrate that the variation of the geometrical and torus radii (Rg and Rc) has a remarkable effect on the donor energy and the average electron-impurity distance, which is quite remarkable in small hemi-Toroidal quantum dot. On the other hand, we've demonstrated that the donor atom's position has a considerable impact on their energy. Furthermore, our numerical results show that the geometrical radius and donor atom's position significantly affect the electron impurity binding energy.

Raman scattering in heavily donor doped β -Ga2O3

β-Ga2O3 crystals doped with the donor impurities Si or Sn are investigated by Raman spectroscopy. In addition to the well-known intrinsic Raman allowed phonon modes, we discover several spectral features when the doping concentration exceeds the Mott criterion for the metal-insulator transition (∼ 3 × 1018 cm−3). The most prominent extra Raman peak at 255 cm−1 is an asymmetrically broadened one. It is due to single-particle, electronic excitations involving the impurity band formed by effective-mass-like (hydrogenic) shallow donors. A similar type of excitation is attributed to a Raman line at 675 cm−1 that appears below room temperature in Si doped samples and might be due to a non-hydrogenic donor. Furthermore, we observe four longitudinal phonon-plasmon coupled modes at 215 cm−1, 280 cm−1, 400 cm−1, and 560 cm−1 associated with infrared active phonon modes. They arise from inelastic light scattering via charge-density fluctuations.

Unusual conduction mechanism of n-type β-Ga2O3: A shallow donor electron paramagnetic resonance analysis

We have investigated the conduction mechanism in n-type, Si doped β-Ga2O3 bulk samples and evidenced carrier dynamics in the GHz frequency range at room temperature by electron paramagnetic resonance (EPR) spectroscopy. The Si shallow donor EPR and conduction electron spin resonance (CESR) spectra show an unusual temperature dependence of the linewidth and line shape, which reveals a variable range hopping conduction and donor clustering. The temperature dependence of the EPR signal intensity can be fitted with two thermally activated processes with energies of 4 meV and 40 meV in the below and above 40 K temperature range. The value of 40 meV is attributed to the ionization energy of the Si shallow donor, indicating that hopping proceeds via the conduction band. Above T = 130 K and up to room temperature, the conduction electron spin resonance (CESR) can be observed with a decreasing linewidth of ΔB < 1 G, which indicates negligible spin flip scattering. To illustrate the unusual behavior of the shallow donor in Ga2O3, we have analyzed the hydrogen shallow donor in ZnO, for which we observe a different “classical” behavior, characterized by donor localization below 40 K and thermal ionization in the conduction band above T = 90 K. In ZnO, the CESR can only be observed in a small temperature range at 90 K due to excessive line broadening for higher temperatures.

Room-temperature ferromagnetism and hydrogen shallow donors in rare-earth Eu-doped ZnO nanorods

Prominent room-temperature ferromagnetism in rare-earth Eu-doped ZnO nanorods has been investigated by means of magnetization and electron paramagnetic resonance (EPR) measurements. The magnetism of the unhydrogenated Eu-doped ZnO nanorods is explained by a spin-1/2 species, attributed to the Zn vacancies. In the hydrogenated Eu-doped ZnO nanorods, a spin-1 species, attributed to the Zn vacancies coupled to the hydrogen shallow donors, would give rise to a slightly enhanced magnetization. The hydrogen shallow donors, identified by the electron paramagnetic resonance (EPR) measurements, can be taken to be responsible for the changes in the spin states and magnetization of the Eu-doped ZnO nanorods.

Electron paramagnetic resonance and 1H nuclear magnetic resonance study of Y-doping effect on the hydrogen shallow donors in ZnO nanoparticles

Hydrogen shallow donors in sol-gel-derived pristine and rare-earth Y-doped ZnO nanoparticles have been investigated by electron paramagnetic resonance (EPR) and high-resolution 1H nuclear magnetic resonance (NMR). It is shown by EPR measurements that the energy level of the hydrogen shallow donors in the Y-doped ZnO is much deeper (E ∼ 174 meV) than in the pristine ZnO (E ∼ 75 meV). The temperature-dependent 1H NMR chemical shift and linewidth measurements of the pristine and the Y-doped ZnO systems indicated that Y-doping effectively modifies the lattice environment and hinders the hydrogen motions in the ZnO nanoparticles.

The combined effect of pressure and temperature on the impurity binding energy in a cubic quantum dot using the FEM simulation

Using the finite element method, the binding energies of a hydrogenic shallow donor impurity are investigated in a cubic GaAs/Ga1−xAlxAs quantum dot structure with realistic potential barrier height under the combined effect of hydrostatic pressure and temperature in the framework of the effective mass approximation. In the calculations, we have taken into account the electronic effective mass, dielectric constant, and conduction band offset between the dot and barriers varying with pressure and temperature. The results we have obtained show that the donor binding energy varies with the dot size, the confining potential, the position of impurity, the hydrostatic pressure and temperature. It is also found that the donor binding energy increases linearly with the pressure in direct gap regime and its variation is larger for narrower dots only and drops slightly with the temperature. A good agreement is obtained with the existing literature values.

Hydrogen shallow donors in Co-doped ZnO showing room-temperature ferromagnetism

Room-temperature ferromagnetism has been identified in sol–gel-prepared Co-doped ZnO (ZnCoO), hydrogen being incorporated by a linear polymer polyvinyl pyrrolidone. The magnetic order was investigated by the X-ray photoemission and magnetization measurements in view of the interstitial hydrogen being coupled with Co2+. The electron paramagnetic resonance measurements revealed that hydrogens not contributing to the magnetic order occupy the shallow donor levels in the hydrogenated ZnCoO system.

Transition-metal acceptor complexes in zinc oxide

The incorporation of hydrogen shallow donors gives rise to ${\text{Mn}}^{2+}$ fine and hyperfine lines in the electron paramagnetic resonance of ZnO. This cannot be explained by recharging of isolated Mn atoms. First-principles density functional calculations reveal that transition metals readily form complexes with acceptors by a charge-transfer process or by bond formation. This mechanism has implications on both complex partners. Acceptors are neutralized and uncommon charge states of the transition metals are stabilized. Implications for dilute magnetic ZnO are discussed.

Modification of the optical properties of ZnO thin films by proton implantation

Optical properties of proton-implanted ZnO thin film prepared by radio-frequency (rf) magneton sputtering have been studied, the optical constants being obtained from the reflectance measurements by employing Cauchy–Urbach model. Increase in the ordinary refractive index after proton implantation was explained by that in the polarizability. Besides, a slight increase in the optical band gap by proton implantation was identified and discussed in terms of the hydrogen shallow donors introduced by the proton implantation.

Magnetic resonance spectroscopy of hydrogen shallow donors in aluminum-doped zinc oxide prepared by sol–gel processing

Hydrogen shallow donors in sol–gel-prepared pristine and Al-doped ZnO systems have been investigated by electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) measurements. It is shown that the hydrogen shallow donors in the Al-doped ZnO system mostly occupy the interstitial sites as in the case of the pristine ZnO system. Our high-resolution NMR measurements sensitively reflected the slightly distinct lattice environments in the pristine and Al-doped ZnO systems.

Hydrogen shallow donors in ZnO and rutile TiO2

A combined study of IR absorption, photoconductivity, photoluminescence and Raman measurements in ZnO samples supports the theoretical suggestions of a shallow bond-centered hydrogen donor and a shallow hydrogen donor within the oxygen vacancy. In rutile TiO2 we also identify a shallow hydrogen donor in contrast to recent theoretical predictions. A possible solution to this obvious discrepancy is proposed.

Effects of hydrogen plasma treatment on the structural and electrical properties of sputter-deposited SnO2 thin films

In this paper, we report the effects of hydrogen plasma treatment on the structural and electrical properties of SnO2 thin films prepared by the sputtering method. Whereas the hydrogen plasma treatment led to etching of SnO2 films and subsequent degradation of crystalline quality and optical transmittance, the plasma-treated films exhibited an improvement in the electrical conductivity. Hall measurements indicated an increase in the carrier concentration of SnO2 films which, following x-ray diffraction and secondary ion mass spectrometry measurements, was attributed to the generation of oxygen vacancies rather than the incorporation of hydrogen shallow donors in undoped SnO2 films.

Evidence of hydrogen-mediated ferromagnetic coupling in Mn-doped ZnO

Hydrogen shallow donors in undoped and doped ZnO systems have been studied by means of the electron paramagnetic resonance and nuclear magnetic resonance measurements. Experimental evidence is given in this work for coupling of hydrogen shallow donors and Mn ions in Mn-doped ZnO mediating short-range ferromagnetic spin-spin interaction.

Effects of Hydrogen Doping on the Electrical Properties of Zinc–Tin–Oxide Thin Films

Transparent conducting oxide films composed of ZnO and SnO2 were prepared on glass substrates by co-sputtering method. After surveying the electrical properties of the films according to the cationic composition and process conditions, we investigated the influences of hydrogen doping on the films' characteristic properties. With a moderate addition of H2 in sputtering gas, carrier concentration of the films increased from 3.32×1019 to 5.22×1019 cm-3, and the resistivity decreased from 7.23×10-3 to 5.29×10-3 Ω·cm. The increase in carrier concentration with H2 can be attributed to the presence of hydrogen shallow donors as well as the formation of oxygen vacancies. However, the hydrogen addition contributed to the formation of SnO local states in Zn–Sn–O films, resulting in the decreases in carrier mobility and optical transmittance. Furthermore, changes in the electrical properties of the films upon annealing in vacuum or reducing atmosphere were investigated to elucidate the state of hydrogen atoms incorporated in Zn–Sn–O films.

An analysis of the effect of nitrogen and a screened (by free carriers) Coulomb field on the binding energy of hydrogenic shallow donors in GaInAsN

The effect of nitrogen concentration on the screening with free carriers and binding energy of hydrogenic shallow donors in GaInAsN alloys is investigated. The binding energy is calculated using a novel algebraic model which was proposed recently by Gönül et al. (2006) [16], in order to find an analytical solution to the screened Coulomb potential. The results show that the nitrogen concentration is a strong factor in producing a screening field of free carriers and in affecting the binding energy of hydrogenic shallow donors.

Hydrogen as an Unstable Shallow Donor in Oxides

Hydrogen is electrochemically introduced into a monoclinic Nb2O5 in a large amount, which donates electrons to Nb5+ and decreases the resistivity of Nb2O5 by more than six orders of magnitude. Both X-ray photoelectron spectroscopy and crystal structure analyses show that ionized hydrogen, namely, proton, is not connected to an oxygen ion by a strong O–H bond, and hydrogen can be easily driven out of this oxide as molecular hydrogen by annealing at 150 °C. This finding provides a relatively direct evidence for an unstable state of a hydrogen shallow donor in oxides and should contribute much to the general understanding of hydrogen in oxides.

Hydrogen effects on the electroluminescence of n-ZnO nanorod/p-GaN film heterojunction light-emitting diodes

Through a facile low-temperature solution process, vertically n-type ZnO nanorod arrays were grown on a GaN film to form a n-ZnO nanorod/p-GaN film heterojunction. A study of the electroluminescence (EL) characteristics of the heterojunction in air and in air with 2000 ppm hydrogen revealed the sensitivity of such a device to the surrounding atmosphere. The additional hydrogen shallow donors increased the effective electron concentration in ZnO nanorods and the EL recombination zone changed from the ZnO nanorods to the GaN film, which can be identified visually from the color change.

Photoluminescence study of hydrogen donors in ZnO

Abstract A photoluminescence study of hydrogenated ZnO bulk crystals is presented. Two excitonic recombination lines at 3362.8 and 3360.1 meV are assigned to hydrogen shallow donors. Experimental evidence is presented that the corresponding donor to the line at 3362.8 meV, previously labeled I 4 , originates from hydrogen trapped within the oxygen vacancy, H O . The line at 3360.1 meV was found to be due to hydrogen located at the bond-centered lattice site, H BC . The corresponding shallow donor has an ionization energy of 53 meV.

Back Cover: phys. stat. sol. (b) 245/9

AbstractThe illustration on the back cover shows the donor wave function found using density functional theory for an interstitial Li species in diamond. The impurity, shown in green at the centre, is a candidate for production of n‐type conducting diamond. Theory shows that the outer, 2s electron of the Li atom is transferred to the diamond lattice and localised in an anti‐bonding combination of carbon sp3 orbitals, effectively breaking a neighbouring C–C bond. The tendency for Li to form localised rather than delocalised hydrogenic shallow donor states, is a common feature found for many other dopant species and complexes in diamond in computational modelling. The tendency to localise and hence form deep levels is a significant obstacle to the generation of low‐resistivity, n‐type doped diamond. The picture relates to the Review Article by J. P. Goss et al. starting on page 1679 in this issue. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)