Shallow Donor In ZnO Research Articles

Temperature dependence of the EPR signal in ZnO

Ceramics based on zinc oxide doped with lithium was fabricated. It is shown that an increase in the lithium content in an amount up to 0.1% leads to a decrease in electrical conductivity, and then in the range up to 3% to its increase. The real and imaginary parts of the permittivity decrease with increasing lithium concentration over the entire range of measured frequencies (50 kHz - 3 MHz). Similarly, the signal of the EPR line with g-factor ~1.96 decreases. An analysis of the temperature dependence of the EPR signal intensity with a g factor of ~1.96 gives an activation energy of 0.06 eV, which practically coincides with the depth of shallow donors in ZnO. This makes it possible to identify the EPR signal with g-factor ~1.96 with oxygen vacancies.

Coherent Spin Preparation of Indium Donor Qubits in Single ZnO Nanowires.

Shallow donors in ZnO are promising candidates for photon-mediated quantum technologies. Utilizing the indium donor, we show that favorable donor-bound exciton optical and electron spin properties are retained in isolated ZnO nanowires. The inhomogeneous optical line width of single nanowires (60 GHz) is within a factor of 2 of bulk single-crystalline ZnO. Spin initialization via optical pumping is demonstrated and coherent population trapping is observed. The two-photon absorption width approaches the theoretical limit expected due to the hyperfine interaction between the indium nuclear spin and the donor-bound electron.

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.

Triple group-V donors in ZnO

Triple donors have been explored in a few semiconductor materials; however, the conventional effective mass theory treatment fails at short length scales due to the high degree of localization implied by a 3+ nuclear charge. Using density functional theory, we consider the various charge states of group-V elements substituting for the Zn sublattice in ZnO under oxygen-rich conditions. For the case of Sb and Bi substitutional impurities, the (1+/0) charge state transition is shallow and has strong similarities to a (1+/0) charge transition of the more common shallow group III donors such as Ga and Al. We compare these calculations with extensive photoluminescence (PL) measurements that now exist for the Sb-related donor bound exciton in ZnO, which is known to contain substitutional Sb on Zn sites. We present new experimental data on the magneto-PL properties of the Sb-related donor bound exciton. These data confirm the strong similarity of the (+1/0) charge state transition of this center to the common group III shallow donors in ZnO. We propose that the very low binding energy (40.2 meV) of the neutral Sb donor is due to a combination of increased screening due to the two inner donor electrons, as well as the exclusion principle, resulting in a repulsive central cell potential close to the defect core.

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.

Coherence Properties of Shallow Donor Qubits in ZnO

Defects in crystals are leading candidates for photon-based quantum technologies, but progress in developing practical devices critically depends on improving defect optical and spin properties. Motivated by this need, we study a new defect qubit candidate, the shallow donor in ZnO. We demonstrate all-optical control of the electron spin state of the donor qubits and measure the spin coherence properties. We find a longitudinal relaxation time T$_1$ exceeding 100 ms, an inhomogeneous dephasing time T$_2^*$ of $17\pm2$ ns, and a Hahn spin-echo time T$_2$ of $50\pm13$ $\mu$s. The magnitude of T$_2^*$ is consistent with the inhomogeneity of the nuclear hyperfine field in natural ZnO. Possible mechanisms limiting T$_2$ include instantaneous diffusion and nuclear spin diffusion (spectral diffusion). These results are comparable to the phosphorous donor system in natural silicon, suggesting that with isotope and chemical purification long qubit coherence times can be obtained for donor spins in a direct band gap semiconductor. This work motivates further research on high-purity material growth, quantum device fabrication, and high-fidelity control of the donor:ZnO system for quantum technologies.

P-GaN/n-ZnO nanorods: the use of graphene nanosheets composites to increase charge separation in self-powered visible-blind UV photodetectors

ZnO-based heterojunctions have found applications as self-powered ultraviolet photodetectors (PDs). However, high doping levels are not compatible with high mobility for metallic doped ZnO-based PDs so further development has been inhibited. This study demonstrates a method to increase the open-circuit voltage (Voc) that allows keeping a sufficiently high level of mobility of ZnO, using a ZnO nanorod/GaN heterojunction that incorporates graphene nanosheets as the active layer. These hybrid PDs have triple the value for Voc of PDs that have only pure ZnO and better exhibit photo-response characteristics. The results of surface Kelvin probe microscopy and x-ray photoelectron spectrometer show that the complex defects that occur because Zn interstitials form a shallow donor in ZnO are mainly responsible for the increase in the value of Voc. Using this functional nanostructure as an active layer represents a new method for the manufacture of high-performance self-powered PDs.

Magnetoresistance of ZnO:Co Thin Films at Low Temperatures

Large positive magnetoresistance was observed in Co-doped thin ZnO films at low temperatures and analyzed in the frame of the model based on the exchange interaction between the conduction electrons and the electrons of a magnetic impurity for the films with temperature dependence of resistivity described by Mott’s law. Estimates of the localization length of electronic states participating in hopping conduction were obtained. The obtained values of the localization length are close to the effective Bohr radii of shallow donors in ZnO.

High-resolution photoluminescence spectroscopy of Sn-doped ZnO single crystals

Group IV donors in ZnO are poorly understood, despite evidence that they are effective n-type dopants. Here we present high-resolution photoluminescence (PL) spectroscopy studies of unintentionally doped and Sn-doped ZnO single crystals grown by the chemical vapor transport method. Doped samples showed greatly increased emission from the I10 bound exciton transition that was recently proven to be related to the incorporation of Sn impurities based on radio-isotope studies. The PL linewidths are exceptionally sharp for these samples, enabling a clear identification of several donor species. Temperature-dependent PL measurements of the I10 line emission energy and intensity dependence reveal a behavior that is similar to other shallow donors in ZnO. Ionized donor bound-exciton and two-electron satellite transitions of the I10 transition are unambiguously identified and yield a donor binding energy of 71meV. In contrast to recent reports of Ge-related donors in ZnO, the spectroscopic binding energy for the Sn-related donor bound exciton follows a linear relationship with donor binding energy (Haynes rule) similar to recently observed carbon related donors, and confirming the shallow nature of this defect center, which was recently attributed to a SnZn double donor compensated by an unknown single acceptor.

Electrical and optical properties of ZnO:Al films with different hydrogen contents in sputtering gas

Aluminum-doped zinc oxide (ZnO:Al) films were deposited by direct current magnetron sputtering in incorporating hydrogen in sputtering gas at room temperature. The influences of hydrogen content in sputtering gas on the structural, optical, and electrical properties of ZnO:Al films were systematically investigated. It is found that hydrogen incorporated into ZnO lattice forms shallow donors in ZnO:Al films and plays an important role in the properties of ZnO:Al films. The electrical conductivity and infrared (IR) reflectance are improved due to the increase of electron carrier concentration, and the average transmittance decreases, which is ascribed to the strong scattering from the hydrogen incorporated and oxygen vacancies in ZnO:Al films. In this study, the resistivity of 5.5 × 10−4 Ω·cm is obtained, the average transmittance of the wavelength in the range of 400–900 nm is almost 86 %, and the IR reflectance reaches 75 % at 2,500 nm, which is higher than that of reported TCO films. The band gap determined by optical absorption is a result of competition between Burstein–Moss effect and many-body perturbation effect. However, the hydrogen content in sputtering gas is above 10 %, and the optical band gap shift is independent of hydrogen content in sputtering gas.

H/Al共掺杂对ZnO基透明导电薄膜光电性质和晶体结构的影响

By incorporating suitable amount of H dopants and lowering the Al contents,the conflicts between low resistivity and high transmission in transparent conducting films have been successfully solved. The reduced resistivity of ZnO∶ Al films by hydrogen doping is attributed to the increased carrier density and carrier mobility. Hydrogen behaves as a shallow donor in ZnO and can provide plenty of electrons. Most importantly,it can increase the carrier mobility effectively by lowering the potential barrier between ZnO grains due to the passivation of O-defects around grain boundaries. The carrier mobility can also be increased due to the less impurity scattering induced by the lowering of Al dopants in ZnO films. With hydrogen doping,low resistivity(6. 3 × 10- 4Ω·cm) ZnO∶ Al samples with only 1 /10 of Al contents compared to conventional AZO films can be got. The optical transmittance in near infrared region(1 200 nm) increases from 64% to 90% by comparing samples without and with H-doping is shown. This kind of high conductivity and high transmittance ZnO thin film will be excellent transparent conducting oxide candidate for various types of thin-film solar cells to improve the efficiency of the device.

Surface‐related defects of ZnO micro and nano crystals prepared by sol‐gel technique

AbstractNano and micro crystalline ZnO were synthesized by two different sol‐gel routes. The nominally undoped ZnO crystals were investigated by X‐ray diffraction, electron paramagnetic resonance (EPR), photoluminescence (PL) and electron microscopy. These measurements are discussed together with surface and volume conductivity measured. Two intrinsic defects were observed having g factors close to that of the shallow donor in ZnO. A further defect was correlated with the conductivity of ZnO nanocrystals and is shown to be due to a surface‐related defect. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Attracting shallow donors: Hydrogen passivation in (Al,Ga,In)-doped ZnO

The hydrogen interstitial and the substitutional Al_Zn, Ga_Zn and In_Zn are all shallow donors in ZnO and lead to n-type conductivity. Although shallow donors are expected to repel each other, we show by first principles calculations that in ZnO these shallow donor impurities attract and form a complex, leading to a donor level deep in the band gap. This puts a limit on the n-type conductivity of (Al,Ga,In)-doped ZnO in the presence of hydrogen.

Effect of quantum confinement and influence of extra charge on the electric field gradient in ZnO

By means of electron-nuclear double resonance (ENDOR), it is shown that the Al impurity, which acts as a shallow donor in ZnO, leads to a significant reduction of the electric field gradient in ZnO single crystals. In ZnO quantum dots, however, the gradient on the Al sites remains virtually unchanged. When the Zn2+ ion is substituted by Mn2+ in a ZnO single crystal, the electric field gradient slightly increases (by about 20%). Therefore, the Mn2+ ions can be used as probes to monitor the electric field gradients in ZnO crystals.

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.

Bound excitons in ZnO: Structural defect complexes versus shallow impurity centers

ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons ($Y$ lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers ($I$ lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all $Y$ lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II--VI and III--V semiconductors.

High-Frequency EPR and ENDOR Spectroscopy on Semiconductor Quantum Dots

It is shown that high-frequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy are excellent tools for the investigation of the electronic properties of semiconductor quantum dots (QDs). The great attractions of these techniques are that, in contrast to optical methods, they allow the identification of the dopants and provide information about the spatial distribution of the electronic wave function. This latter aspect is particularly attractive because it allows for a quantitative measurement of the effect of confinement on the shape and properties of the wave function. In this contribution EPR and ENDOR results are presented on doped ZnO QDs. Shallow donors (SDs), related to interstitial Li and Na and substitutional Al atoms, have been identified in this material by pulsed high-frequency EPR and ENDOR spectroscopy. The shallow character of the wave function of the donors is evidenced by the multitude of ENDOR transitions of the 67Zn nuclear spins and by the hyperfine interaction of the 7Li, 23Na and 27Al nuclear spins that are much smaller than for atomic lithium, sodium and aluminium. The EPR signal of an exchange-coupled pair consisting of a shallow donor and a deep Na-related acceptor has been identified in ZnO nanocrystals with radii smaller than 1.5 nm. From ENDOR experiments it is concluded that the deep Na-related acceptor is located at the interface of the ZnO core and the Zn(OH)2 capping layer, while the shallow donor is in the ZnO core. The spatial distribution of the electronic wave function of a shallow donor in ZnO semiconductor QDs has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine interactions as monitored by ENDOR spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. In addition, the effect of confinement on the g-factor of SDs in ZnO as well as in CdS QDs is observed. Finally, it is shown that an almost complete dynamic nuclear polarization (DNP) of the 67Zn nuclear spins in the core of ZnO QDs and of the 1H nuclear spins in the Zn(OH)2 capping layer can be obtained. This DNP is achieved by saturating the EPR transition of SDs present in the QDs with resonant high-frequency microwaves at low temperatures. This nuclear polarization manifests itself as a hole and an antihole in the EPR absorption line of the SD in the QDs and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with nuclear magnetic resonance techniques.

Role of Si and Ge as impurities in ZnO

The electronic and structural properties of Si in ZnO are studied using density functional calculations with both the generalized gradient approximation and a hybrid functional. Our calculations show substitutional Si on the Zn site SiZn to be significantly lower in energy than the Si interstitial Sii and Si on an O site SiO. SiZn is predicted to be a shallow donor in ZnO with a formation energy low enough to lead to significant incorporation of Si in the ZnO crystal. Interestingly, we find that SiZn has a lower formation energy and therefore higher solubility under Zn-rich conditions than under O-rich conditions. We also study the properties of Ge in ZnO for comparison, finding behavior similar to the Si impurity, but with an even lower formation energy. Our results suggest Si and Ge as suitable n-type dopants in ZnO.

Electrical characterization of H+ ion irradiated n-ZnO

Schottky barrier diodes (SBD’s) were fabricated with 20/80/40/80nm Ti/Al/Pt/Au ohmic contacts on the O face and circular 0.5mm in diameter, 50nm thick Ru Schottky contacts in the Zn face. Current (I) deep level transient spectroscopy (I-DLTS) was used to study the shallow level defects introduced during 1.8MeV proton irradiations with fluences ranging from 1×1013cm−2 to 2.4×1014cm−2. These measurements revealed that this irradiation introduced a defect with an energy level at 0.03–0.036eV below the conduction band and with an electron capture cross section of 9×10−16 to about 1.4×10−15cm2. This defect may be hydrogen-related as theory has predicted that hydrogen forms a shallow donor in ZnO, or it may be due to the ZnI, as it was reported that electron irradiation with an energy of >1.6MeV produces the ZnI in ZnO which is a shallow level defect at 0.03eV below the conduction band.

Electric Dipole and Current Induced Spin Resonances of Shallow Donor in ZnO

In this paper we present our observation of electric dipole spin resonance and current induced spin resonance in ZnO bulk crystals, both effects result from spin–orbit coupling. Electric dipole spin resonance originates from admixture of spin state due to spin–orbit interaction which leads to probability of spin-flip caused by electric component of microwave field. In current induced spin resonance phenomenon the electric component of microwave field induces electron motion leading to mean ac spin–orbit field that acts on electron spin. Spin–orbit interaction can be described in terms of the Rashba field, an effective magnetic field that induce spin resonance. In contrast to magnetic and electric dipole spin resonances the current induced resonance is characterized by dispersive-like line shape.