Shallow Donor In ZnO Research Articles

On the nature of hydrogen-related shallow donors in ZnO

Vapor-phase grown ZnO samples treated with hydrogen plasma were studied by means of photoconductivity and IR absorption spectroscopy. Three bands at 180 , 240 , and 310 cm - 1 were observed in the photoconductivity spectra of hydrogenated ZnO. These are identified as electronic transitions of three independent hydrogen-related shallow donors. Two electronic transitions from the H–I defect previously associated with the bond-centered hydrogen were found in IR absorption spectra at 1430 and 1480 cm - 1 . Based on the energies of these transitions H–I was ruled out as a candidate for the hydrogen-related shallow donor in ZnO.

Hydrogen local modes and shallow donors in ZnO

The annealing behavior of the free-carrier absorption, O-H vibrational absorption, and photoluminescence lines previously associated with H-related donors in ZnO has been studied. One set of H-related defects gives rise to O-H local vibrational mode absorption at either 3326 or $3611\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$, with the relative intensities of these lines depending on the source of the ZnO starting material. These O-H lines anneal away together at 150 \ifmmode^\circ\else\textdegree\fi{}C along with $\ensuremath{\sim}80%$ of the free carriers introduced by H. The common annealing behavior of the O-H ir lines suggests that they are closely related. An additional defect produced by hydrogenation is thermally stable up to an annealing temperature of 500 \ifmmode^\circ\else\textdegree\fi{}C where a bound-exciton photoluminescence line often associated with H in ZnO is also annealed away. This more thermally stable donor accounts for $\ensuremath{\sim}20%$ of the free carriers introduced by H. These experiments on H in ZnO reveal a complex behavior with several defects being introduced and with properties that depend strongly on the source of the ZnO starting material.

Photoconductivity and infrared absorption study of hydrogen-related shallow donors in ZnO

Vapor phase grown ZnO samples treated with hydrogen and∕or deuterium plasma were studied by means of photoconductivity and infrared (IR) absorption spectroscopy. Three bands at 180, 240, and $310\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ were observed in the photoconductivity spectra of hydrogenated ZnO. These are identified as electronic transitions of three independent hydrogen-related shallow donors. Two electronic transitions from the $\mathrm{H}\text{\penalty1000-\hskip0pt}\mathrm{I}$ defect previously associated with the bond-centered hydrogen [E. V. Lavrov et al., Phys. Rev. B 66, 165205 (2002)] were found in IR absorption spectra at 1430 and $1480\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Based on the energies of these transitions $\mathrm{H}\text{\penalty1000-\hskip0pt}\mathrm{I}$ was ruled out as a candidate for the hydrogen-related shallow donor in ZnO.

Hydrogen Donors in ZnO

AbstractZinc oxide (ZnO) has shown great promise as a wide-bandgap semiconductor with a range of optical, electronic, and mechanical applications. The presence of compensating donors, however, is a major roadblock to achieving p-type conductivity. Recent first-principles calculations and experimental studies have shown that hydrogen acts as a shallow donor in ZnO, in contrast to hydrogen's usual role as a passivating impurity. Given the omnipresence of hydrogen during growth and processing, it is important to determine the structure and stability of hydrogen donors in ZnO.To address these issues, we performed vibrational spectroscopy on bulk, single-crystal ZnO samples annealed in hydrogen (H2) or deuterium (D2) gas. Using infrared (IR) spectroscopy, we observed O-H and O-D stretch modes at 3326.3 cm-1 and 2470.3 cm-1 respectively, at a sample temperature of 10 K. These frequencies indicate that hydrogen forms a bond with a host oxygen atom, consistent with either an antibonding or bond-centered model. In the antibonding configuration, hydrogen attaches to a host oxygen and points away from the Zn-O bond. In the bond-centered configuration, hydrogen sits between the Zn and O. To discriminate between these two models, we measured the shift of the stretch-mode frequency as a function of hydrostatic pressure. By comparing with first-principles calculations, we conclude that the antibonding model is the correct one.Surprisingly, we found that the O-H complex is unstable at room temperature. After a few weeks, the peak intensity decreases substantially. It is possible that the hydrogen forms H2 molecules, which have essentially no IR signature. Electrical measurements show a corresponding decrease in electron concentration, which is consistent with the formation of neutral H2 molecules. The correlation between the electrical and spectroscopic measurements, however, is not perfect. We therefore speculate that there may be a second “hidden” hydrogen donor. One candidate for such a donor is a hydrogen-decorated oxygen vacancy.

Infrared Spectroscopy of Hydrogen in ZnO

ABSTRACTZinc oxide (ZnO) has shown great promise as a wide band gap semiconductor with optical, electronic, and mechanical applications. Recent first-principles calculations and experimental studies have shown that hydrogen acts as a shallow donor in ZnO, in contrast to hydrogen's usual role as a passivating impurity. The structures of such hydrogen complexes, however, have not been determined. To address this question, we performed vibrational spectroscopy on bulk, single-crystal ZnO samples annealed in hydrogen (H2) or deuterium (D2) gas. Using infrared (IR) spectroscopy, we have observed O-H and O-D stretch modes at 3326.3 cm−1 and 2470.3 cm−1 respectively, at a sample temperature of 14 K. These frequencies are in good agreement with the theoretical predictions for hydrogen and deuterium in an antibonding configuration, although the bond-centered configuration cannot be ruled out. The IR-active hydrogen complexes are unstable, however, with a dissocation barrier on the order of 1 eV.

Electronic structure of the muonium center as a shallow donor in ZnO.

The electronic structure and the location of muonium centers (Mu) in single-crystalline ZnO were determined for the first time. Two species of Mu centers with extremely small hyperfine parameters have been observed below 40 K. Both Mu centers have an axial-symmetric hyperfine structure along with a <0001> axis, indicating that they are located at the antibonding (AB(O, parallel )) and bond-center (BC( parallel )) sites. It is inferred from their small ionization energy ( approximately 6 and 50 meV) and hyperfine parameters ( approximately 10(-4) times the vacuum value) that these centers behave as shallow donors, strongly suggesting that hydrogen is one of the primary origins of n type conductivity in as-grown ZnO.

A FP–LMTO study on the native shallow donor in ZnO

First-principle calculations are performed on ZnO:V O , ZnO:V Zn and ZnO:Zn I using a FP–LMTO (full potential linear muffin-tin orbital) method. The results prove that the Zn I is the dominant donor in ZnO, and V Zn , a shallow acceptor, while V O is a deep donor.

Powder Pattern Hyperfine Spectroscopy of Shallow- Donor Muonium Centres

The hyperfine spectroscopy of muonium in II–VI semiconductors is reviewed, suggesting that whereas hydrogen is a deep-level defect in ZnS, ZnSe and ZnTe, it constitutes a shallow donor in ZnO, CdS, CdSe and CdTe. Shallow and deep states coexist in CdTe. Using new data for ZnO, it is shown that the principal values of the muonium hyperfine tensor may be obtained with equal facility from measurements in longitudinal or in transverse magnetic field, and from samples that are polycrystalline powders or single crystals. Spin density on the central muon in the shallow states correlates with the electron binding energy or donor depth.

High Temperature Characterization of Electrical Barriers in ZnO Varistors

Two main models have been proposed to describe the potential barriers in ZnO varistors: the surface oxidation and the surface states. It has been difficult to decide which of them better corresponds to the experimental observations. High temperature electrical characterization of these materials is an important tool to understand the formation of the electrical barriers. In this work, using literature data describing ZnO varistor characteristics at high temperature, up to 1153°C, we calculate the energy position of the equilibrium Fermi level at the grain boundary interface, and found that this parameter decreases with the increase of temperature, and for temperatures higher than ∼700°C it stays close to the ZnO band gap without crossing it. This behavior shows that the interface never presents a p-type character, a starting point to develop the surface states model. On the other hand, 700°C is a temperature too low for the surface oxidation mechanism to be operative. It is then proposed that, during cooling down to ∼700°C, the interface Fermi level stays close to the middle of the band gap due to the adsorption and subsequent reaction of oxygen with ZnO surfaces/grain boundaries. For lower temperatures, when the interface Fermi level separates from the middle of the band gap, it is proposed that it follows the variation of the bulk Fermi level, which in turn is caused by shallow donors in ZnO. A calculation assuming a reduced electroneutrality condition, gave a donor density of ∼ 3 × 1017 cm-3, which corresponds approximately to the density of carriers in the material for temperatures down to room temperature. This value is in a good agreement with those available in the literature. Knowing both the bulk and the interface Fermi levels, it is then possible to calculate the barrier height at any temperature, and it is observed that it is almost constant from room temperature up to ∼400°C, with a value of 0.8 eV, and than decreases monotonously up to 1153°C. Taking these values, it is possible to calculate the variation of the low voltage conductivity with temperature, and it is found that, apart from the variation between room temperature and 400°C, with no special significance, the decrease of the barrier height from 400°–1153°C induces an extra change of the conductivity from which a fictitious activation energy of 1.5 eV is obtained. Therefore, these two energies are not related to shallow and deep donors in ZnO grains.

Residual Native Shallow Donor in ZnO

High-energy electron irradiation in ZnO produces shallow donors at about ${E}_{C}\ensuremath{-}30\mathrm{meV}$. Because the production rate is much higher for Zn-face (0001) than O-face $(000\overline{1})$ irradiation, the donor is identified as a Zn-sublattice defect, most likely the interstitial ${\mathrm{Zn}}_{I}$ or a ${\mathrm{Zn}}_{I}$-related complex. The donor energy is quite close to that of the unirradiated sample, and of other samples discussed in the literature, strongly suggesting that ${\mathrm{Zn}}_{I}$ (and not ${V}_{\mathrm{O}}$) is the dominant native shallow donor in ZnO. An exceptionally high displacement threshold energy $(\ensuremath{\sim}1.6\mathrm{MeV})$ is quantitatively explained in terms of a multiple-displacement model.

A magnetic double-resonance investigation of the quadrupole interaction in ZnO single crystals

Shallow donors in ZnO are used to investigate the quadrupole interaction in single crystals. From the resolved quadrupole splitting, we determine the electric field gradients at the 67Zn and the 17O site. A magnetic double-resonance technique both increases the sensitivity and the accuracy of the field gradient determination. The data for ZnO are compared to previous results on GaN.

Magnetic resonance studies of shallow donors in zinc oxide

Shallow donors in ZnO were studied by electron spin resonance (ESR) and optically detected magnetic resonance (ODMR). Indium and gallium donors were identified by their hyperfine structure. They have g‖ = 1.957 and g⊥ = 1.956 and the hyperfine constants are A( 69Ga)=4.2 and A( 115In)=36.6 G. The ESR and ODMR linewidths resulting from 67Zn interactions are analysed in the effective mass model. The hyperfine data indicate that Ga is quite accurately hydrogenic whereas there is a marked central-cell effect for In. At high donor concentration, electron transport and/or donor-donor exchange interactions induce motional narrowing of the ESR lines. The ESR of Zn-treated ZnO and the origin of the n-type conductivity are discussed.