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In clean tunneling junctions (TJs), the momentum parallel to the barrier is conserved in the tunneling process. We predict that this phenomenon can be observed in a TJ between a ferromagnet (FM) and a topological insulator (TI). The FM possesses spin-polarized electron states, while the TI has spin-momentum locked electron states. The tunneling probability is determined by the spin-matrix element between these electron states and leads to the injection of electrons into the TI with finite momentum parallel to the topological surface state. Due to the spin polarization of the electrons in the FM, the resulting current in the TI is not isotropic. The magnitude of the current is calculated quantitatively in terms of the ferromagnetic polarization and should be experimentally detectable using only an electrical setup. [ABSTRACT FROM AUTHOR]

X-ray photoelectron spectroscopy (XPS) was utilized to measure the valence band offset (ΔEV) of the TiZnSnO (TZTO)/Si heterojunction. TZTO films were deposited on Si (100) substrates using magnetron sputtering at room temperature. By using the Zn 2p3/2 and Sn 3d5/2 energy levels as references, the value of ΔEV was calculated to be 2.69 ± 0.1 eV. Combining with the experimental optical energy band gap of 3.98 eV for TZTO extracted from the UV-vis transmittance spectrum, the conduction band offset (ΔEC) was deduced to be 0.17 ± 0.1 eV at the interface. Hence, the energy band alignment of the heterojunction was determined accurately, showing a type-I form. This will be beneficial for the design and application of TZTO/Si hybrid devices. [ABSTRACT FROM AUTHOR]

We have investigated the influence of ZnO orientation on the valence band offset (ΔEV) of Mg0.2Ni0.8O(111)/ZnO (1120) and Mg0.2Ni0.8O(111)/ZnO(0002) heterostructures by x-ray photoelectron spectroscopy. Combining with the experimental optical energy band gap extracted from UV-vis transmittance spectra, the energy band alignment at the interfaces has been determined. The ΔEV has been found to be 1.8 ± 0.1 eV and 1.4 ± 0.1 eV for Mg0.2Ni0.8O(111)/ZnO (1120) and Mg0.2Ni0.8O(111)/ZnO(0002), respectively. Both the heterostructures show type-II band configurations with conduction band offsets (ΔEC) of 2.4 ± 0.1 eV and 2.0 ± 0.1 eV, respectively. The ZnO orientation correlated electronic structure in p-Mg0.2Ni0.8O/n-ZnO interfaces can be explained well by the effect of spontaneous polarization in ZnO. Furthermore, the results offer opportunity that desired band offsets in MgxNi1-xO/ZnO heterostructures can be obtained by appropriately adjusting Mg composition. [ABSTRACT FROM AUTHOR]

The microstructure and crystal orientations of polycrystalline films crucially affect the properties and performance of the films. Controlling preferred orientations (PO) and related film morphology are necessary to obtain the desirable properties. In this paper, we demonstrate a rational and effective route toward the realization of non-polar p-type ZnO thin film with surface texture on quartz substrate through Mn-Na codoping. It is uncovered experimentally and theoretically that mono-doping of Mn creates opportunity to realize PO from polar (c-axis) to non-polar ([formula], [formula], and [formula]) changing. With Mn-Na codoping, an acute modulation of the growth behavior and electrical conductivity of the film have been revealed, leading to weak p-type non-polar Zn0.94Mn0.05Na0.01O (ZMNO) texture. The dominant mechanism for the non-polar self-texture in the current paper is deliberately elucidated as resulting from the interplane surface diffusion with the cooperative effect of impurity dopants. The ZMNO films exhibit p-type conduction with hole concentration of 9.51 × 1015-1.86 × 1017 cm-3 and enhanced room temperature (RT) ferromagnetism possessing a saturation magnetization (Ms) of 1.52 μB/Mn. The results have potential applications in development of non-polar optoelectronic devices such as lighting emitting diodes (LEDs). [ABSTRACT FROM AUTHOR]

The interaction of Na impurities and O vacancies (VO) in ZnO was studied in theory and experiment. The VO facilitated substitutional Na (NaZn/NaZn-) acceptors with inhibiting interstitial Na (Nai/Nai+) donors, which may benefit the p-type conductivity. The formed (NaZn-VO)+/0 complexes induced the change of band structures of ZnO, with the broadening of gap energies and disappearance of the VO2+-related impurity level, which confirmed by the blue shift of the near-band-edge (NBE) UV emission and the decay of the visible emission in PL spectra. The strong and dominant NBE UV emission suggested acceptable luminescence behaviors of Na-doped ZnO. [ABSTRACT FROM AUTHOR]

We propose a specifically designed structure to fabricate thin-film transistors using amorphous indium-gallium-zinc-oxide (a-IGZO) films as the active channel layers. The I-shaped gate electrode is employed to define the channel width, reducing overlaps between the gate and source/drain electrodes. The devices with such a structure exhibit acceptable electrical performance and stability after annealing treatment. The XPS data show that the as-deposited a-IGZO film has not a very dense structure that may induce shallow traps. A shallow trap model is proposed to explain the large threshold voltage shifts of the as-deposited device. Annealing treatment can eliminate these shallow traps and improve the device stability. [ABSTRACT FROM AUTHOR]

Dislocations in the ZnMgO pleated nanosheets were observed from high-resolution transmission electron microscopy (TEM). The Burgers vector was identified as [formula] from the Fourier filtered TEM images. Raman spectra revealed that there were microstructure defects in the ZnMgO nanosheets with the increasing Mg content. The incorporation of the Mg impurities in the ZnMgO nanosheets may promote the congregation of oxygen vacancies and thus facilitate the generation of dislocations. The oxygen deficiency as well as surface dislocations played an important role in determining the optical properties of ZnMgO pleated nanosheets. [ABSTRACT FROM AUTHOR]

Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2/3 for ne≤4.2×1019 cm-3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4-8.4×1019 cm-3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widening and the latter an offsetting narrowing. The band gap narrowing (ΔEBGN) derived from the band gap renormalization effect did not show a good ne1/3 dependence predicated by a weakly interacting electron-gas model, but it was in excellent agreement with a perturbation theory considering different many-body effects. Based on this theory a simple expression, ΔEBGN=Ane1/3+Bne1/4+Cne1/2, was deduced for n-type ZnO, as well as p-type ZnO, with detailed values of A, B, and C coefficients. An empirical relation once proposed for heavily doped Si could also be used to describe well this gap narrowing in AZO. [ABSTRACT FROM AUTHOR]

(Zn,Al)O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0–50 at. %. The structural, optical, and electrical properties of (Zn,Al)O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region (<10 at. %), Al-doped ZnO (AZO) thin films could be obtained at 400 °C in an Ar–O2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4 at. %, with low resistivity ∼10-4 Ω cm, high transmittance ∼90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52 eV at 4 at. % Al due to the Burstein-Moss shift [E. Burstein, Phys. Rev. 93, 632 (1954)] modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the residual, compressive stress in film, which may be the reason for the improvement in film stability and the increment in grain size as well. In a medium Al content region (10–30 at. %), however, the film quality was degraded, which was presumably due to the formation of clusters or precipitates in the grains and boundaries. Besides the (002) plane, other diffraction peaks such as (100) and (101) planes of ZnO were observed, but the (Zn,Al)O films still exhibited a single-phase wurtzite ZnO structure. An intragrain cluster scattering mechanism was proposed to interpret the reduction of carrier mobility in films with the Al contents in the 7–20 at. % region. The solubility limit of Al in ZnO film was identified to be in the 20–30 at. % range, much higher than the thermodynamic solubility limit of 2–3 at. % in ZnO. In a high Al content region (>=30 at. %), there were distinct observations for (Zn,Al)O films. As the Al content was 30 at. %, the film appeared in a two-phase nature with ZnO hexagonal and Al2O3 rhombohedral structures. At the 50 at. % Al content, the matrix of the (Zn,Al)O film was Al2O3, and no evident trace of wurtzite ZnO was observed. The electrical and optical properties for both cases were also very different from those at the Al contents below 30 at. %. [ABSTRACT FROM AUTHOR]

Ga-doped ZnO (GZO)/graphene multilayer film was fabricated on the polyethylene terephthalate substrate at room temperature. The obtained GZO/graphene multilayer showed a transparence of 75% at 550nm and a sheet resistance of 721Ω/sq. Our findings indicate that the graphene intermediate layer plays a critical role in improving the conductivities of GZO. Under a tensile strain, the sheet resistance of the GZO electrode without graphene exhibited a sharp increase owing to the plenty of cracks formed in the GZO layer. In contrast, the GZO/graphene multilayer displayed a relatively stable and low resistance during the tensile deformation due to the excellent mechanical and electrical stabilities of the graphene. [ABSTRACT FROM AUTHOR]

High-resistive Zn0.95Mn0.05O and weak p-type nonpolar-oriented Zn0.94Mn0.05Na0.01O thins films were grown on quartz by pulsed laser deposition. Both samples exhibit room temperature ferromagnetism while with Mn-Na codoping, the saturation magnetic moment is greatly enhanced. It is revealed that the doped Mn impurities are substitutionally incorporated into the ZnO host. Magnetic anisotropy was also observed in the Zn0.94Mn0.05Na0.01O film, which is the indication for intrinsic ferromagnetism. The first-principles calculations reveal that codoping of Na in Zn0.94Mn0.05Na0.01O changes the antiferromagnetic interaction to ferromagnetic due to the hybridization between spin-split delocalized Mn 3d and shallow acceptor states of Na 2p, thereby enhancing the ferromagnetism. [ABSTRACT FROM AUTHOR]

Bilayer films consisting of Ga-doped ZnO (GZO) and Cu layers were grown at room temperature by magnetron sputtering. The structural, electrical, and optical properties of GZO/Cu bilayer films were investigated in detail. The crystallinity and transparent-conductive properties of the films were correlated with the Cu layer thickness. The GZO/Cu bilayer film with the Cu layer thickness of 7.8 nm exhibited a low resistivity of 7.6×10-5 Ω cm and an average visible transmittance of 74%. The reflectance was up to 65% in the near-infrared region for this film. The transparent conductive and near-infrared reflective GZO/Cu bilayer films could be readily deposited at room temperature. The GZO/Cu bilayer films were thermally stable when annealed at temperatures as high as 500 °C. [ABSTRACT FROM AUTHOR]

N-doped, Li-doped, and Li–N doped Zn0.89Mg0.11O thin films have been prepared by pulsed laser deposition. Hall-effect measurements indicate that the doping technique plays an important role in the p-type behavior of ZnMgO. Li doping produces high-resistivity p-type ZnMgO films, whereas N doping produces ZnMgO films with indeterminate carrier type. In contrast, Li–N codoping results in low resistivity and stable p-type ZnMgO films. The enhancement by Li and N coincorporation is investigated by secondary ion mass spectroscopy and is possibly responsible for the good p-type conduction in Li–N dual-acceptor doped Zn0.89Mg0.11O. [ABSTRACT FROM AUTHOR]

Nonpolor a-plane and polar c-plane ZnO thin films were prepared on r-plane sapphire and quartz substrates, respectively. The electronic structure of the interface between subsequently fabricated NiO/ZnO heterojunctions has been investigated by x-ray photoelectron spectroscopy measurements and the band offsets are determined together with information yielded from UV-vis transition spectra. It is found that a type-II band alignment forms at the interface for both the samples. The revealed ZnO-orientation dependent band offsets are analyzed and are attributed mainly due to the variations in internal electric field arose from spontaneous polarization effect. The accurate determination of the band alignment is important for the design and application of NiO/ZnO based hybrid devices. [ABSTRACT FROM AUTHOR]

Quasialigned, single-crystal Zn1-xMgxO (x=0–0.32) nanorods were synthesized on Si substrates by thermal evaporation. Zn1-xMgxO nanorods grew along the [0001] crystal direction and had uniform hexagonal planes with diameters of 420–120 nm. The predominant ultraviolet luminescence could be tuned from 379 (x=0) to 305 nm (x=0.32) at room temperature. This blueshift indicated the band gap engineering in Zn1-xMgxO nanorods. Temperature-dependent photoluminescence was used to illustrate the free-exciton emission from Zn1-xMgxO nanorods. The exciton binding energy decreased from 59 (x=0) to 49 meV (x=0.18) and then increased to 54 meV (x=0.32). [ABSTRACT FROM AUTHOR]

ZnO homojunction light-emitting diodes (LEDs), comprised of N–Al codoped p-type ZnO and Al-doped n-type ZnO layers, were fabricated on Si substrates with homobuffer layers. The current-voltage measurements showed typical diode characteristic with a threshold voltage of about 3.3 V. The electroluminescence (EL) bands at 110 K consisted of a near-band-edge emission at 3.18 eV and a deep level emission at 2.58 eV. The EL emissions were assigned as radiative recombinations, presumably of donor-acceptor pairs, in the p-type layer of the LED. The quenching of EL with temperature was attributed to the degradation of p-type conducting of the ZnO:(N,Al) layer. [ABSTRACT FROM AUTHOR]

The authors report on reproducible growth of ZnMgO quantum dots (QDs) by a metal organic chemical vapor deposition method. Mg is introduced into ZnO QDs, as confirmed by x-ray photoelectron spectroscopy and photoluminescence measurements. The size, density, and optical band gap of ZnMgO QDs can be well controlled by simply modulating the growth parameters. [ABSTRACT FROM AUTHOR]

Transparent conducting Al-doped Zn1-xMgxO thin films were grown on glass substrates by chemical vapor deposition. The resistivity could be lowered to 10-3 Ω cm with optical transmittance above 85% in visible regions. The influence of carrier concentration on band-gap shift in Zn1-xMgxO alloys was systematically studied. The shift of energy gap could be fully explained by the Fermi-level band filling and band-gap renormalization effects. As the Mg content increased, the electron effective masses in Zn1-xMgxO (x=0–0.21) alloys increased from 0.30m0 to 0.49m0. The Al-doping efficiency was reduced with the increase in alloy composition. [ABSTRACT FROM AUTHOR]

p-type ZnO thin films have been fabricated by In and N codoping. The carrier type in the In–N codoped ZnO can be controlled by adjusting the growth conditions and good p-type conductivity is obtained at temperatures between 490 and 580 °C. The p-type behavior is improved when a buffer layer is used. The lowest reliable room temperature resistivity is found to be 7.85 Ω cm in the presence of a buffer layer. The ZnO-based homostructural p-n junctions comprising a n-ZnO:In layer on a p-ZnO:(In,N) layer on a buffer layer display apparent electrical rectification in the authors’ repeated measurements. [ABSTRACT FROM AUTHOR]

Li-doped ZnO films were prepared by pulsed laser deposition. The carrier type could be controlled by adjusting the growth conditions. In an ionized oxygen atmosphere, p-type ZnO was achieved, with the hole concentration of 6.04 × 1017 cm-3 at an optimal Li content of 0.6 at. %, whereas ZnO exhibited n-type conductivity in a conventional O2 growth atmosphere. At a Li content of more than 1.2 at. % only high-resistivity ZnO was obtained. The amount of Li introduced into ZnO and the relative concentrations of such defects as Li substitutions and interstitials could play an important role in determining the conductivity of films. [ABSTRACT FROM AUTHOR]

Electrical characteristics have been studied for ZnO p-n and p-i-n homojunctions, with optimization of device structures for improved performance. Capacitance-voltage measurements confirm the formation of abrupt junctions. The current-voltage characteristics exhibit their inherent electrical rectification behavior. The p-ZnO:(N,Al)/n-ZnO:Al homojunctions fabricated on sapphire substrates combining with the intrinsic ZnO buffer layer have acceptable p-n diode characteristics, with the forward turn-on voltage of 1.4 V and the reverse breakdown voltage of 5.3 V. By introduction of an intrinsic (Zn,Cd)O layer, the resultant p-ZnO:(N,Al)/i-(Zn,Cd)O/n-ZnO:Al homojunction exhibits a high reverse breakdown voltage of ∼18 V. [ABSTRACT FROM AUTHOR]

We investigate photoluminescence from reproducible Li-doped p-type ZnO thin films prepared by dc reactive magnetron sputtering. The LiZn acceptor state, with an energy level located at 150 meV above the valence band maximum, is identified from free-to-neutral-acceptor transitions. Another deeper acceptor state located at 250 meV emerges with the increased Li concentration. A broad emission centered at 2.96 eV is attributed to a donor-acceptor pair recombination involving zinc vacancy. In addition, two chemical bonding states of Li, evident in x-ray photoelectron spectroscopy, are probably associated with the two acceptor states observed. [ABSTRACT FROM AUTHOR]

Self-assembled ZnO quantum dots (QDs) were achieved by a vapor phase transport process. ZnO nanodots were naturally formed on solid substrates in the Volmer-Weber growth mode. Size control of nanodots could be readily realized by varying the growth time. The as-prepared ZnO QDs are of high quality and very stable after formation. The blueshift of band gap energies derived from quantum confinement effects was confirmed by optical absorption spectra. Photoluminescence spectra revealed the tunable behavior of ultraviolet luminescence due to exciton localization. The realization of size-tuned color from ZnO QDs makes them more promising for practical applications. [ABSTRACT FROM AUTHOR]

We report on intrinsic p-type ZnO thin films by plasma-assisted metal-organic chemical vapor deposition. The optimal results give a resistivity of 12.7 Ω cm, a Hall mobility of 2.6 cm2/V s, and a hole concentration of 1.88×1017 cm-3. The oxygen concentration is increased in the intrinsic p-type ZnO, compared with the n-type layer. Two acceptor states, with the energy levels located at 160 and 270 meV above the valence band maximum, are identified by temperature-dependent photoluminescence. The origin of intrinsic p-type behavior has been ascribed to the formation of zinc vacancy and some complex acceptor center. [ABSTRACT FROM AUTHOR]

A Li–N dual-acceptor doping method has been developed to prepare p-type ZnO thin films by pulsed laser deposition. The lowest room-temperature resistivity is found to be ∼0.93 Ω cm, much lower than that of Li or N monodoped ZnO films. The p-type conductivity of ZnO:(Li,N) films is very reproducible and stable, with acceptable crystal quality. The acceptor activation energy in ZnO:(Li,N) is about 95 meV. ZnO-based homostructural p-n junctions were fabricated by depositing an n-type ZnO:Al layer on a p-type ZnO:(Li,N) layer, confirmed by secondary ion mass spectroscopy. The current-voltage characteristics exhibit their inherent rectifying behaviors. [ABSTRACT FROM AUTHOR]

We report a breakthrough in fabricating ZnO homojunction light-emitting diode by metal organic chemical vapor deposition. Using NO plasma, we are able to grow p-type ZnO thin films on n-type bulk ZnO substrates. The as-grown films on glass substrates show hole concentration of 1016–1017 cm-3 and mobility of 1–10 cm2 V-1 s-1. Room-temperature photoluminescence spectra reveal nitrogen-related emissions. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 2.3 V. Electroluminescence at room temperature has been demonstrated with band-to-band emission at I=40 mA and defect-related emissions in the blue-yellow spectrum range. [ABSTRACT FROM AUTHOR]

A vapor phase transport growth process has been developed to synthesize ZnO quantum dots (QDs) on Si substrates. The characteristics were investigated for as-prepared ZnO QDs without any additional treatment. The formation of ZnO QDs with 6 nm in height and 15 nm in diameter is confirmed by scanning electron microscope and atomic force microscopy. Room-temperature photoluminescence reveals that the as-prepared ZnO QDs exhibit a predominant ultraviolet emission at 3.32 eV while the low energy defect-related blue-green emission is significantly quenched. The band gap of ZnO QDs is determined to be 3.41 eV, which evidently indicates the quantum confinement effects. [ABSTRACT FROM AUTHOR]

Li-doped, p-type ZnO thin films have been realized via dc reactive magnetron sputtering. An optimized result with a resistivity of 16.4 Ω cm, Hall mobility of 2.65 cm2/V s, and hole concentration of 1.44×1017 cm-3 was achieved, and electrically stable over a month. Hall-effect measurements supported by secondary ion mass spectroscopy indicated that the substrate temperature played a key role in optimizing the p-type conduction of Li-doped ZnO thin films. Furthermore, ZnO-based p-n homojunction was fabricated by deposition of a Li-doped p-type ZnO layer on an Al-doped n-type ZnO layer. [ABSTRACT FROM AUTHOR]

The valence band structures of Al–N-codoped [ZnO:(Al, N)] and N-doped (ZnO:N) ZnO films were studied by normal and soft x-ray photoelectron spectroscopy. The valence-band maximum of ZnO:(Al, N) shifts up to Fermi energy level by about 300 meV compared with that of ZnO:N. Such a shift can be attributed to the existence of a kind of Al–N in ZnO:(Al, N), as supported by core level XPS spectra and comparison of modified Auger parameters. Al–N increased the relative quantity of Zn–N in ZnO:(Al, N), while N–N decreased that of Zn–N in ZnO:N. [ABSTRACT FROM AUTHOR]

p-type ZnO thin films have been realized by the In–N codoping method. Secondary ion mass spectroscopy revealed that the nitrogen incorporation was enhanced by the presence of indium in ZnO. The as-grown In–N codoped ZnO film shows acceptable p-type behavior at room temperature with high film quality. A conversion from p-type conduction to n type in a range of temperature was confirmed by Hall effect measurement. The lowest reliable room-temperature resistivity was found to be 3.12 Ω cm with a carrier concentration of 2.04×1018 cm-3 and a Hall mobility of 0.979 cm2 V-1 S-1. The p-type behavior is stable. [ABSTRACT FROM AUTHOR]

ZnO p-n homojunctions were fabricated on quartz substrates by depositing Al-doped n-type ZnO layer on Al–N-co-doped p-type ZnO layer through reactive magnetron sputtering. In/Zn metal contacts to as-grown ZnO show ohmic behavior, and the ohmic contacts can be improved by annealing in an Ar ambient. The optimal annealing temperatures for Al–N-co-doped ZnO and Al-doped ZnO are 550 °C and 600 °C, respectively. The p-n junction characteristic is confirmed by current-voltage measurements. The turn-on voltage is 2 V, with a low leakage current under reverse bias. Series resistances of the ZnO p-n junctions can be lowered by optimizing the annealing temperature, increasing the grain size of the ZnO, or increasing the hole concentration of the p layer. [ABSTRACT FROM AUTHOR]

p-type ZnO thin films have been realized by the N–Al co-doping method. Secondary ion mass spectroscopy demonstrated that the N incorporation was enhanced evidently by the presence of Al in ZnO. The lowest room-temperature resistivity was found to be 57.3 Ω cm with a Hall mobility of 0.43 cm2/V s and carrier concentration of 2.25×1017 cm-3 for the N–Al co-doped p-type ZnO film deposited on glass substrate. The results were much better than those for the N-doped p-type ZnO. Moreover, the co-doped film possesses a good crystallinity with c-axis orientation and a high transmittance (90%) in the visible region. [ABSTRACT FROM AUTHOR]

A correction to the article "ZnO light-emitting diode grown by plasma-assisted metal organic chemical vapor deposition," that was published online on April 20, 2009 is presented.

A correction is presented to "Rational synthesis and tunable optical properties of quasialigned Zn(sub 1-x)Mg(sub x) nanorods," which appeared in Applied Physics Letters no. 91 in 2007.

A correction to the article "Identification of acceptor states in Li-doped p-type ZnO thin films" that was published in a previous issue is presented.