As-implanted ZnO Research Articles

Au incorporation into various ZnO crystallographic cuts realised by ion implantation – ZnO damage characterization

Non-polar surfaces, such as a-plane (11–20) and m-plane (10-10), for ZnO have become more attractive as numerous efforts have recently been made to grow non-polar ZnO facets for applications in nanoscale photonic devices. Noble-metal incorporation into transparent semiconductors such as ZnO has been investigated because of the non-linear optical response of such structures. This paper presents a study of defect evolution in various ZnO crystallographic cuts caused by Au implantation. The investigation has focused on ZnO structure characterisation, Au distribution and the interior morphology of the a-, m- and c-planes ZnO single crystals implanted with 400 keV Au+ ions at the ion fluences of 5 × 1014 and 1 × 1015 cm−2 and subsequently annealed at 600 °C in O2. The structure modification was studied using Rutherford backscattering spectrometry (RBS) in the channelling mode (RBS/C) and Raman spectroscopy. After the ion-implantation process, low surface damage was observed in all ZnO orientations unlike deep structural damage. Deep structural damage grew with increased Au-ion fluence and Au did not exhibit strong out-diffusion from the depth to the surface during the post-implantation annealing. Small but noticeable differences were observed between different ZnO orientations. RBS measurements during ion implantation revealed more progressive deep-damage formation in the c- and m-planes than in the a-plane ZnO. Simultaneously, the smallest Zn sub-lattice disorder deduced from RBS/C measurements was observed in the a-plane ZnO. During post-implantation annealing, a slight structure recovery (about 4%) was observed in all orientations. Raman spectroscopy confirmed the increasing structure disorder with the enhanced ion fluence for all as-implanted ZnO orientations and a partial reconstruction of the ZnO structure during annealing, when the intensity of E2 phonons was increased and that of longitudinal optical (LO) phonons was suppressed because of the disorder recovery. E2 (high) and E1(LO) Raman phonon modes connected with oxygen sub-lattice ordering/disordering have been investigated in detail – they show a significant modification mainly in the m-plane. The cause of the different behaviour of ZnO planes as well as the differences in the incorporation and movement of Au and Er atoms in the ZnO structure are discussed in the work.

Effect of doping concentration on point defect structure in As-implanted ZnO

The effect of doping concentration on the point defect structure of As–implanted ZnO single crystal was investigated using diffuse x–ray scattering and photoluminescence spectroscopy. Based on the assumption that the low-dose ion implantation did not shift the phonon dispersion of the lattice, Huang diffuse scattering signals were obtained by subtracting thermal diffuse scattering intensities. We found that the point defects aggregate into defect clusters after annealing, and their average size decreases and concentration increases with increasing the doping concentration. The underlying mechanisms of this counter–intuition result were suggested.

Rutherford backscattering and nuclear reaction analyses of hydrogen ion-implanted ZnO bulk single crystals

The origins of low resistivity in H ion-implanted ZnO bulk single crystals are studied by Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) photoluminescence (PL), and Van der Pauw methods. The H-ion implantation (peak concentration: 1.45×1020cm−3) into ZnO is performed using a 500keV implanter. The resistivity decreases from 2.5×103Ωcm for unimplanted ZnO to 6.5Ωcm for as-implanted one. RBS measurements show that Zn interstitial as a shallow donor is not recognized in as-implanted samples. From photoluminescence measurements, the broad green band emission is observed in as-implanted samples. NRA measurements for as-implanted ZnO suggest the existence of the oxygen interstitial. The origins of the low resistivity in the as-implanted sample are attributed to both the H interstitial as a shallow donor and complex donor between H and disordered O. The activation energy of H related donors estimated from the temperature dependence of carrier concentration is 29meV.

Emission bands of nitrogen-implantation induced luminescent centers in ZnO crystals: Experiment and theory

High quality ZnO crystals with the sharp band-edge excitonic emission and very weak green emission were implanted by nitrogen ions. An additional red emission band was observed in the as-implanted ZnO crystal and investigated as a function of temperature. By employing the underdamped multimode Brownian oscillator model for the general electron-phonon coupling system, both the original green and nitrogen-implantation induced red emission bands were theoretically reproduced at different temperatures. Excellent agreement between the theory and the experiment enables us determine the energetic positions of the pure electronic levels associated with the green and red emission bands, respectively. The determined energy level of the red emission band is in good agreement with the data obtained from the deep-level transient spectroscopy measurements.

The point defect structure and its transformation in As-implanted ZnO crystals

The point defect structure of As-implanted ZnO crystals was investigated using diffuse x-ray scattering and spectroscopic methods. A dynamic picture regarding the defect structure transformation was revealed. The defect type, size and combination state experienced obvious changes during annealing. Before annealing the point defects distributed isolatedly and coexisted with dislocation loops. After annealing, AsO disappeared and other point defects either aggregated into large clusters or formed defect complexes including AsZn–2VZn whose ionization energy was measured and which was discerned as the main origin of p-type conductivity.

Origins of low resistivity in Al ion-implanted ZnO bulk single crystals

The origins of low resistivity in Al ion-implanted ZnO bulk single crystals are studied by combining Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), photoluminescence (PL), and Van der Pauw methods. The Al-ion implantation (peak concentration: 2.6 × 1020cm−3) into ZnO is performed using a multiple-step energy. The resistivity decreases from ∼104 Ω cm for un-implanted ZnO to 1.4 × 10−1 Ω cm for as-implanted, and reaches 6.0 × 10−4 Ω cm for samples annealed at 1000 °C. RBS and NRA measurements for as-implanted ZnO suggest the existence of the lattice displacement of Zn (Zni) and O (Oi), respectively. After annealing at 1000 °C, the Zni related defects remain and the Oi related defects disappear. The origin of the low resistivity in the as-implanted sample is attributed to the Zni (∼30 meV [Look et al., Phys. Rev. Lett. 82, 2552 (1999)]). In contrast, the origin of the low resistivity in the sample annealed at 1000 °C is assigned to both of the Zni related defects and the electrically activated Al donor. A new PL emission appears at around 3.32 eV after annealing at 1000 °C, suggesting electrically activated Al donors.

Phase Segregation and Transformations in Arsenic-Implanted ZnO Thin Films

The conversion of ZnO into a p-type semiconductor remains a major challenge for its application in optoelectronic devices because up to now neither the suitable type of defects nor the possible role of secondary phase formation during doping has been clarified. Here, the implantation of arsenic into epitaxial ZnO thin films on α-Al2O3 (0001) and subsequent isothermal annealing in high vacuum are studied by particle-induced X-ray emission, X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The as-implanted ZnO:As films are single phase and exhibit a locally disordered ZnO lattice structure. Zn3As2 nanocrystals segregate at 700 °C. Because of annealing at 800 °C, α-As and α-As2O3 are formed. The study demonstrates the role of solid-state reactions and secondary phase formation for group V element doping of ZnO that is a promising route to convert this material into a p-type semiconductor.

Optical confinement achieved in ZnO crystal by O^+ ions implantation: analysis of waveguide formation and properties

Optical confinement in ZnO crystal was observed by O(+) implantation with different MeV energies and doses. Planar optical waveguides were formed in the as-implanted ZnO samples. The optical properties of the planar waveguide were investigated by the prism-coupling and the end-face coupling methods at the wavelength of 633 nm. The crystal lattice damage in the guiding region caused by the O(+) ions implantation was analyzed by the Rutherford backscattering/Channeling technique, results show that even high dose at 2 × 10(15) ions/cm(2) can hardly produce defect in near surface of ZnO. A theoretical model is developed to explain the principle of waveguide formation in ZnO crystal and the refractive index profile in the implanted waveguide was reconstructed accordingly. The experimental result and analysis are significant for application of ZnO crystal, especially for the design of ZnO light emitter devices.

Relationship between Aluminum and Lithium and Annealing for Reducing Lithium Contamination in Aluminum-Implanted Zinc Oxide

The relationship between Al and Li during diffusion was studied using Al-implanted ZnO. The Al donor in ZnO acts to increase the concentration of Li contamination from the atmosphere during the annealing. It is difficult to decompose the relationship formed by diffusion between Al and Li during high-temperature annealing. The most effective method to decompose the relationship is to anneal the as-implanted ZnO at a pressure of 5×10-3 torr. This annealing increases the Al solubility limit because the ZnO surface evaporates.

Evaluation of zinc interstitial in Si-ion implanted ZnO bulk single crystals by a Rutherford backscattering study: An origin of low resistivity

The relation between low resistivity and Zn interstitial in ZnO single crystals implanted with the peak Si atomic concentration of 2.62 × 1020 cm−3 is studied by combining Rutherford backscattering spectroscopy/channeling, photoluminescence, and Van der Pauw methods. The variation in resistivity from ∼104 Ω cm for un-implanted ZnO to ∼10−3 Ω cm for as-implanted ones is observed. The lattice displacement of Zn atoms of ∼0.10 Å from the 〈0 0 0 1〉 row is estimated from the normalized angular yield profiles, preserving the single crystallinity in as-implanted ZnO with a minimum yield (χmin) of ∼11%. The low resistance is maintained by the 1000 °C annealing, accompanied with the lattice displacement of Zn atoms of ∼0.07 Å. These results suggest the existence of the shallow donor consisting of Zn interstitial and/or Zn interstitial-related complex corresponding to a newly appeared 3.33 eV-emission located at ∼40 meV from the bottom of the conduction band.

Recovery of Structural and Luminescent Properties in Zinc-Implanted ZnO Films

This paper reports that Zn ion implantation to a dose of 1 × 1017 ions/cm2 was performed on highly c-axis-orientated ZnO thin films deposited on Si (100) substrates by the sol-gel technique. After ion implantation, the as-implanted ZnO films were annealed in argon ambient at different temperatures from 300−500 oC. The effects of ion implantation and post-implantation annealing on the structural and luminescent properties of the ZnO films were investigated by x-ray diffraction, photoluminescence (PL). Results show that the intensities of (002) peak and photoluminescence (PL) were evidently decreased by Zn ion implantation. The recovery of (002) peak and photoluminescence occurs at ~300oC.

Effects of high-dose Ge ion implantation and post-implantation annealing on ZnO thin films

This paper reports that ion implantation to a dose of 1×1017 ions/cm2 was performed on c-axis-orientated ZnO thin films deposited on (0001) sapphire substrates by the sol-gel technique. After ion implantation, the as-implanted ZnO films were annealed in argon ambient at different temperatures from 600-900 °C. The effects of ion implantation and post-implantation annealing on the structural and optical properties of the ZnO films were investigated by x-ray diffraction (XRD), photoluminescence (PL). It was found that the intensities of (002) peak and near band edge (NBE) exitonic ultraviolet emission increased with increasing annealing temperature from 600-900 °C. The defect related deep level emission (DLE) firstly increased with increasing annealing temperature from 600−750 °C, and then decreased quickly with increasing annealing temperature. The recovery of the intensities of NBE and DLE occurs at ~ 850 °C and ~ 750 °C respectively. The relative PL intensity ratio of NBE to DLE showed that the quality of ZnO films increased continuously with increasing annealing temperature from 600 − 900 °C.

Dependence of photoluminescence and electrical properties with rapid thermal annealing in nitrogen-implanted ZnO films

Undoped (as-grown) ZnO films grown by pulsed laser deposition on Al 2O 3 (0001) substrates were doped with nitrogen by means of an ion implantation process. Post-implantation annealing behavior in the temperature range between 500 and 700 °C has been studied by photoluminescence and Hall effect measurements. The implanted films show no peak other than the excitonic recombination emission in the as-implanted state, however, after rapid thermal annealing at 700 °C they reveal a nitrogen acceptor related emission at 3.273 eV. The as-implanted ZnO films show more electron concentrations than the as-grown, unimplanted ZnO film. In contrast, after annealing, the electron concentration in the implanted films is significantly reduced, indicating that the incorporated nitrogen becomes activated after the thermal annealing, then produces holes and eventually compensates for certain amount of electrons. The results imply that a proper nitrogen implantation and subsequent annealing may be a way to produce p-type ZnO films.

Crystallinity-damage recovery and optical property of As-implanted Zno crystals by post-implantation annealing

Since As doping in ZnO using ion implantation must always follow a damage caused by the energetic As ion penetration, there is a growing demand for crystallinity recovery from the surface damage through the post-implantation annealing. The results obtained from our experiments using double crystal X-ray diffraction and the atomic force microscopy indicate that the crystallinity recovery of the As-implanted sample presents the optimum condition when it is annealed at 800 °C for 1 h. From the Raman and photoluminescence measurements through the post-implantation annealing, the As-related optical properties were observed in As-implanted ZnO crystals. Thereby, we confirmed that the surface of the unimplanted ZnO was clearly converted into As-doped p-type ZnO layer by As ions.