Defects In ZnO Films Research Articles

Efficient Quantum Dot Infrared Photovoltaic with Enhanced Charge Extraction via Applying Gradient Electron Transport Layers

AbstractPbS quantum dot (QD) infrared (IR) solar cells that can absorb low‐energy photons are promising photovoltaic devices to improve utilization of sunlight energy by broadening absorption range of the sunlight spectrum to short‐wave infrared region. For PbS QD photovoltaics, depleted heterojunction established between photoactive layer and ZnO electron transport layer (ETL) is critical to determine the performance of devices. However, undesired defects in ZnO films and mismatched energy levels have limited the improvement of device properties. Herein, a novel and simple gradient ZnO ETL is developed for achieving high‐performance QD IR solar cells by depositing a Cs‐doped ZnO thin layer on the pristine ZnO film. The resulting gradient ETL exhibits significantly suppressive trap states and a much smoother surface, efficiently reducing the trap‐assisted nonradiative recombination at the interfaces. Meanwhile, realigned energy levels of the gradient ZnO ETL facilitate the transport and extraction of photogenerated carriers. As a result, the champion device shows a high IR open circuit voltage (VOC) of 0.446 V and IR power conversion efficiency (PCE) of 1.27% under 1100 nm filtered illumination. The VOC and PCE are 0.507 V and 11.04% under AM1.5 illumination, respectively. These results demonstrate a promising strategy for exploiting high‐performance infrared optoelectronic devices.

Tuning intrinsic defects in ZnO films by controlling the vacuum annealing temperature: an experimental and theoretical approach

The control of native defects in the ZnO material is strongly important for a wide range of technological applications. In this paper, native defects are tuned via the post-thermal treatment of ZnO films in a high vacuum atmosphere. The microstructure of the as-grown ZnO film shows columnar growth and strongly polar-oriented grains along the c-plane (002). Also, the obtained results indicate that the as-grown film contains a high amount of intrinsic defects and strong lattice distortions. After the thermal annealing, the ZnO films display significant structural changes, which are reflected in their electrical, vibrational, and optical properties. Our findings suggest that these changes were attributed to the selective cleanup effect of the native defects and the partial deoxidation process mainly on the exposed particle surface (at high temperatures) tuned up by the thermal annealing temperature. According to DFT calculations, oxygen vacancies (V O ) show lower energy, followed by zinc vacancies (V Zn ) and oxygen interstitials (O i ) indicating that V O defect is the most stable in ZnO. That sequence of stability could suggest the sequence of the annihilation of those defects, which is in line with our experimental findings and also in agreement with literature results.

Suppressed oxygen vacancy in pristine/N doped ZnO and improved ZnO homogenous p-n junction performance by H2O2 oxidant

Intrinsic oxygen vacancy n-type defects in as-prepared ZnO films have significantly restricted p-type ZnO fabrication. This paper utilized oxygen precursor H2O2 to reduce oxygen deficiency defects in ZnO films prepared using thermal atomic layer deposition. Results confirm that H2O2 can suppress oxygen deficiency for ZnO and N-doped ZnO films, with consequential reduced electron concentration. Electron concentration for N doped ZnO reduced from 2.27 × 1016 to 1.02 × 1012 cm−3, enhancing suitability for p-type ZnO fabrication. Nitrogen (H2O2 dosing directly after NH3·H2O dosing) and F co-doped ZnO exhibited good p-type quality with hole concentration = 9.28 × 1017 cm−3, and ZnO homogenous p-n junction prepared using this H2O2 dosed N and F co-doped ZnO as p-type and pristine ZnO as n-type exhibited improved single direction rectification compared with using N and F co-doped ZnO as p-type without H2O2 doping.

The low temperature growth of stable p-type ZnO films in HiPIMS

In this study, the influence of substrate temperature on properties of Al-N co-doped p-type ZnO films is explored. Benefitting from the high ionization rate in high-power impulsed magnetron sputtering, the concentration of ionized nitrogen N+ and ionized zinc Zn+ were increased, which promoted the formation of ZnO films and lowered the necessary substrate temperature. After optimization, a co-doped p-type ZnO thin film with a resistivity lower than 0.35 Ω cm and a hole concentration higher than 5.34 × 1018 cm−3 is grown at 280 °C. X-ray diffraction results confirm that Al-N co-doping does not destruct the ZnO wurtzite structure. X-ray photoelectron spectroscopy demonstrates that the presence of Al promotes the formation of acceptor (No) defects in ZnO films, and ensures the role of Al in stabilizing p-type ZnO.

A High-Resistivity ZnO Film-Based Photoconductive X-Ray Detector

High-resistivity ZnO film-based photoconductive detectors have been fabricated on glass substrate by the magnetron sputtering growth method, and tested as X-ray detectors for the first time in this letter. The devices exhibited a low dark current $\sim ~100$ pA at 40 V bias and a fast transient reproducible response to X-ray illumination (generated by bremsstrahlung with a tungsten target with tube voltage 30 kV) $\sim ~200$ ms, which was mainly attributed to the high resistivity $\sim ~10^{9} \Omega $ cm of the ZnO film deposited in the oxygen atmosphere. The high resistivity was due to the zinc vacancy defects’ compensation with the n-type defects in ZnO film as revealed by photoluminescence spectra. Furthermore, the detector achieved an X-ray pulse detection, and the rise time and full width at half maximum of the outputs signal were $139~\mu \text{s}$ and 0.9 ms at 200 V bias, respectively. Associated with material characterizations, it was demonstrated that the high-resistivity ZnO film detector had the potential to be promising for fast X-ray detection application.

Intrinsic defects in ZnO films deposited by RF sputtering

ABSTRACTZinc oxide thin films were deposited on Si (100) substrates using RF magnetron sputtering by adjusting sputtering power. The film properties were characterized by scanning electron microscopy with energy dispersive X-Ray spectroscopy, X-ray diffraction spectrometer and photoluminescence spectroscopy. Intrinsic defects could be adjusted just by modifying the sputtering power. Variation of Zn to O ratio, defects and their relative concentration of the films with the sputtering power were investigated in details.

Influence of defects of nanostructured ZnO films on the photovoltaic characteristics of perovskite solar cells

The influence of the defects in ZnO films on the electrical and photovoltaic properties of perovskite solar cells was investigated in the work. According to the results of the research it was established that the defects in ZnO films affects the concentration of defects of perovskite films synthesized on the ZnO surface. However, the difference in the defect concentration in perovskite films is about 30%, while the concentration of defects in ZnO differs by 1000 times. A less significant influence is the concentration of ZnO defects on the electrical and photovoltaic properties of perovskite solar cells. The magnitude of the short-circuit photocurrent and the open voltage of the cells are affected by the concentration of perovskite defects and the quality of the perovskite-ZnO interface.

Enhanced photoluminescence properties of Al doped ZnO films

Al doped ZnO films are fabricated by radio frequency magnetron sputtering. In general, visible emission is related to various defects in ZnO films. However, too much defects will cause light emission quench. So it is still a controversial issue to control appropriate defect concentrations. In this paper, based on our previous results, appropriate Al doping concentration is chosen to introduce more both interstitial Zn and O vacancy defects, which is responsible for main visible emission of ZnO films. A strong emission band located at 405 nm and a long tail peak is observed in the samples. As Al is doped in ZnO films, the intensity of emission peaks increases. Zn interstitial might increase with the increasing Al3+ substitute because ZnO was a self-assembled oxide compound. So Zn interstitial defect concentration in Al doped ZnO films will increase greatly, which results in the intensity of emission peaks increases.

Fullerene-Free Organic Solar Cells with Efficiency Over 12% Based on EDTA–ZnO Hybrid Cathode Interlayer

Interlayer materials play an important role in the rapid development of organic solar cells (OSCs). State-of-the-art interlayers in fullerene OSCs may be not the best choice for fullerene-free OSCs, thus new interlayers need to be developed. Here, we report high performance fullerene-free OSCs with efficiency over 12% by using low temperature (150 oC) solution processed EDTA-ZnO hybrid interlayer. The performance improvement can be attributed to the dual nature of EDTA. First, EDTA itself shows good ability in interface modification as an interlayer, leading to 10% device efficiency. Second, when combined with ZnO, EDTA can passivize defects in ZnO film due to its chelation function, leading to balanced hole and electron mobility in OSCs. As a result, high internal quantum efficiency (IQE) approaching 100% is achieved in the range from 480 nm to 720 nm. The remarkable IQE means that in OSCs with EDTA-ZnO, every absorbed photon can generate a separated pair of charged carriers and that almost all photo-gen...

Tunable zinc interstitial related defects in ZnMgO and ZnCdO films

We report tunable band gap of ZnO thin films grown on quartz substrates by radio frequency magnetron sputtering. The zinc interstitial (Zni) defects in ZnO films were investigated by X-ray diffraction, Raman scattering, Auger spectra, first-principle calculations, and Hall measurement. Undoped ZnO film exhibits an anomalous Raman mode at 275 cm−1. We first report that 275 cm−1 mode also can be observed in ZnO films alloyed with Mg and Cd, whose Raman intensities, interestingly, decrease and increase with increasing Mg and Cd alloying content, respectively. Combined with the previous investigations, it is deduced that 275 cm−1 mode is attributed to Zni related defects, which is demonstrated by our further experiment and theoretical calculation. Consequently, the concentration of Zni related defects in ZnO can be tuned by alloying Mg and Cd impurity, which gives rise to different conductivity in ZnO films. These investigations help to further understand the controversial origin of the additional Raman mode at 275 cm−1 and also the natural n-type conductivity in ZnO.

The Effects of the Post-Annealing Atmosphere on the Ultraviolet Responsivity of n-ZnO/p-Silicon Nanowire Heterojunction

UV photoresponse of n-ZnO/p-Silicon nanowire (SiNW) photodiodes is investigated by varying post-treatment conditions. Based on spectral responsivity measurement, the responsivity of our photodiodes decreases with increasing the post-annealing temperature in vacuum atmosphere, and the biggest responsivity reaches to as high as ~42 A/W under a reverse bias of 2V near 390 nm when the post-annealing temperature is at 100 °C. And the responsivity is higher when our devices are annealed in vacuum atmosphere than in oxygen atmosphere. We think that the higher responsitivity may be due to the smaller grain size and more oxygen-vacancy-related defects in ZnO film deposited at lower temperature. These results may benefit potential applications of n-ZnO/p-SiNW photodetectors in ultraviolet region.

Influence of annealing temperature on ZnO thin films grown by dual ion beam sputtering

We have investigated the influence of in situ annealing on the optical, electrical, structural and morphological properties of ZnO thin films prepared on p-type Si(100) substrates by dual ion beam sputtering deposition (DIBSD) system. X-ray diffraction (XRD) measurements showed that all ZnO films have (002) preferred orientation. Full-width at half-maximum (FWHM) of XRD from the (002) crystal plane was observed to reach to a minimum value of 0.139° from ZnO film, annealed at 600 °C. Photoluminescence (PL) measurements demonstrated sharp near-band-edge emission (NBE) at ~ 380 nm along with broad deep level emissions (DLEs) at room temperature. Moreover, when the annealing temperature was increased from 400 to 600 °C, the ratio of NBE peak intensity to DLE peak intensity initially increased, however, it reduced at further increase in annealing temperature. In electrical characterization as well, when annealing temperature was increased from 400 to 600 °C, room temperature electron mobility enhanced from 6.534 to 13.326 cm2/V s, and then reduced with subsequent increase in temperature. Therefore, 600 °C annealing temperature produced good-quality ZnO film, suitable for optoelectronic devices fabrication. X-ray photoelectron spectroscopy (XPS) study revealed the presence of oxygen interstitials and vacancies point defects in ZnO film annealed at 400 °C.

Analysis of Li-related defects in ZnO thin films influenced by annealing ambient

Li-doped ZnO thin films were grown on quartz substrates by radio frequency magnetron sputtering and in situ annealing under O2 or Ar ambient. Li-related defects in ZnO films strongly depend on the annealing ambient. AFM and XRD indicated that ZnO films possessed a good crystallinity with c-axis orientation, uniform thickness and dense surface. Electrical and optical properties demonstrated that, an amount of LiZn defect had existed in ZnO annealed under O2ambient and an amount of Lii(o) defect had existed in ZnO annealed under Ar ambient. First-principle calculations were performed to calculate formation energies of Li-doped ZnO in order to explain the formation mechanism of Li-related defects in ZnO.

Effect of oxygen partial pressure on the behavior of dual ion beam sputtered ZnO thin films

Undoped ZnO thin films were grown on p-type Si (1 0 0) substrates at different oxygen partial pressure by dual ion beam sputtering deposition system at a constant growth temperature of 400 °C. The crystallinity, surface morphology, optical, elemental and electrical properties of these ZnO thin films was studied. The minimum value of full-width at half-maximum of the θ-rocking curve obtained from x-ray diffraction of the ZnO (0 0 2) plane, was reported to be 0.1865° from ZnO film grown at 50% of (O2/(O2 + Ar))%. Crystalline property of ZnO films was observed to degrade with the increase in oxygen partial pressure. Photoluminescence measurements demonstrated sharp near-band-edge emission at ∼381 nm at room temperature. X-ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies as point defects in ZnO films. Electrical resistivity of ZnO was found to increase with the increase in oxygen partial pressure.

Effect of growth temperature on structural, electrical and optical properties of dual ion beam sputtered ZnO thin films

ZnO epitaxial thin films were grown on p-type Si(100) substrates by dual ion beam sputtering deposition system. The crystalline quality, surface morphology, optical and electrical properties of as-deposited ZnO thin films at different growth temperatures were studied. Substrate temperature was varied from 100 to 600 °C at constant oxygen percentage O2/(O2 + Ar) % of 66.67 % in a mixed gas of Ar and O2 with constant chamber pressure of 2.75 × 10−4 mBar. X-Ray diffraction analyses revealed that all the films had (002) preferred orientation. The minimum value of stress was reported to be −0.32 × 1010 dyne/cm2 from ZnO film grown at 200 °C. Photoluminescence measurements demonstrated sharp near-band-edge emission (NBE) was observed at ~375 nm along with deep level emission (DLE) in the visible spectral range at room temperature. The DLE Peak was found to have decrement as ZnO growth temperature was increased from 200 to 600 °C. The minimum FWHM of the NBE peak of 16.76 nm was achieved at 600 °C growth temperature. X-Ray photoelectron spectroscopy study revealed presence of oxygen interstitials and vacancies point defects in ZnO film grown at 400 °C. The ZnO thin film was found to be highly resistive when grown at 100 °C. The ZnO films were found to be n-type conducting with decreasing resistivity on increasing substrate temperature from 200 to 500 °C and again increased for film grown at 600 °C. Based on these studies a correlation between native point defects, optical and electrical properties has been established.

Study on oxygen source and its effect on film properties of ZnO deposited by radio frequency magnetron sputtering

The structural and optical properties of ZnO thin films deposited at various oxygen partial pressure and rf-power of rf magnetron sputtering were investigated. The sputtered ZnO films are mainly formed with the oxygen which was supplied from a sputtering gas; therefore the film stoichiometry can be controlled by the oxygen partial pressure and rf-power. From photoluminescence study, it was found that the wide emission band above 550nm was observed due to an increase of oxygen vacancies when the ZnO film changed from O-rich to Zn-rich. The chemical stoichiometry of the film will help us to understand the formation mechanism of intrinsic defects in ZnO films.

High Field-Effect Mobility Bottom-Gated Metallorganic Chemical Vapor Deposition ZnO Thin-Film Transistors with SiO2 ∕ Si3N4 Bilayer Gate Dielectric

Bottom-gated thin-film transistors (TFTs) on glass substrates were fabricated using ZnO channel layer grown by metal organic chemical vapor deposition with two different gate dielectrics. The gate dielectrics employed were plasma enhanced chemical vapor deposition prepared and bilayer stack. The TFTs employing bilayer gate dielectric exhibit a field-effect mobility of and an on/off current ratio of . The TFTs using only gate dielectric, on the other hand, show a lower field-effect mobility of and a lower on/off current ratio of . The enhancement of device properties is attributed to a better -axis crystal orientation, a larger grain size, and a lower density of point defects in ZnO films grown on the bilayer stack.

Changing vacancy balance in ZnO by tuning synthesis between zinc/oxygen lean conditions

The nature of intrinsic defects in ZnO films grown by metal organic vapor phase epitaxy was studied by positron annihilation and photoluminescence spectroscopy techniques. The supply of Zn and O during the film synthesis was varied by applying different growth temperatures (325–485 °C), affecting decomposition of the metal organic precursors. The microscopic identification of vacancy complexes was derived from a systematic variation in the defect balance in accordance with Zn/O supply trends.

Structural defects in an epitaxial ZnO∕(011¯2) r-plane sapphire studied by high-resolution electron microscopy and computer simulation

The structure of planar defects in ZnO films on (011¯2) r-plane sapphire was studied by the combination of high-resolution electron microscopy and computer simulations. The dominant defects present in these films were identified as a type I1 intrinsic-stacking fault with a displacement vector of (a∕6)[202¯3] and a density of ∼6×104cm−1, lying in the (0001) basal plane. We found the formation of these stacking faults to be growth related, corresponding to the face-centered-cubic stacking sequence in wurtzite ZnO.

Dependence of Intrinsic Defects in ZnO Films on OxygenFraction Studied by Positron Annihilation

Defects in ZnO films grown by radio-frequency reactivemagnetron sputtering under variable ratios between oxygen andargon gas have been investigated by using the monoenergetic positronbeam technique. The dominate intrinsic defects in these ZnOsamples are O vacancies (VO) and Zn interstitials (Zni)when the oxygen fraction in the O2/Ar feed gas does not exceed70% in the processing chamber. On the other hand, zinc vacanciesare preponderant in the ZnO films fabricated in richer oxygenenvironment. The concentration of zinc vacancies increases withthe increasing O2 fraction. For the oxygen fraction 85%, thenumber of zinc vacancies that could trap positrons will be smaller. Itis speculated that some unknown defects could shield zinc vacancies. Theconcentration of zinc vacancies in the ZnO films varies with the oxygenfraction in the growth chamber, which is in agreement with the resultsof photoluminescence spectra.