Ultrathin ZnO Films Research Articles

The Extraction of the Density of States of Atomic-Layer-Deposited ZnO Transistors by Analyzing Gate-Dependent Field-Effect Mobility

In this study, we investigated the density of states extraction method for atomic-deposited ZnO thin-film transistors (TFTs) by analyzing gate-dependent field-effect mobility. The atomic layer deposition (ALD) method offers ultra-thin and smooth ZnO films, but these films suffer from interface and semiconductor defects, which lead to disordered localized electronic structures. Hence, to investigate the unstable localized structure of ZnO TFTs, we tried to derive the electronic state relationship by assuming field-effect mobility can be expressed as a gate-dependent Arrhenius relation, and the activation energy in the relation is the required energy for hopping. Following this derived relationship, the DOS of the atomic-deposited ZnO transistor was extracted and found to be consistent with those using temperature-dependent measurements. Moreover, to ensure the proposed method is reliable, we applied methods for the extraction of DOSs of doped ZnO transistors, which show enhanced mobilities with shifted threshold voltages, and the results show that the extraction method is reliable. Thus, we can state that the mobility-based DOS extraction method offers practical benefits for estimating the density of states of disordered transistors using a single transfer characteristic of these devices.

Zinc oxide–copper model nanocatalysts for CO2 hydrogenation: morphology and interface effects

ZnO ultrathin films epitaxilly grown on Cu(111) as model nanocatalysts for the CO2 hydrogenation reaction toward methanol. Two primary reaction pathways were evidenced as a function of the ZnO thickness: slow (thick) and fast (thin).

Morphology-electronic effects in ultra-model nanocatalysts under the CO oxidation reaction: the case of ZnO ultrathin films grown on Pt(111).

The study of the surface morphology and interface of metal-oxides is crucial for understanding the behavior of these model systems as nanocatalysts. Besides, understanding the interplay between morphology, stability, and reactivity is crucial for designing efficient catalysts. Here, we investigated the stability and dewetting of ZnO ultrathin films on Pt(111) under CO oxidation conditions. For films <1 monolayer (ML), CO-induced dewetting occurs at the metal-oxide interface or defects. The morphology, dependent on thickness, influences reactivity. (6 × 6) structures show greater CO binding and structural changes compared to (4 × 4) structures, which exhibit resilience due to Zn-OH formation. ZnO electronic properties, as revealed by Auger spectroscopy and scanning tunneling spectroscopy (STS) investigations, vary with thickness. Low-thickness films [<2 monolayers (ML)] exhibit metallic-like behavior, possibly due to Zn-Pt interaction, while thicker films show n-type semiconductor behavior with a bandgap opening (EBG = 0.9 eV at 2 ML). DFT calculations of the local density of states (LDOS) as a function of ZnO thickness confirm the thickness-dependent electronic structure, with 0.3 ML films having a higher LDOS near the Fermi level than 1 ML films. These findings highlight the critical role of ZnO morphology in determining its stability and reactivity which opens up avenues for designing efficient and more stable ZnO-based nanocatalysts for a wide range of chemical reactions, including CO oxidation and CO2 hydrogenation.

Influence of buffer layer on optoelectronic properties of B-Mg co-doped ZnO ultrathin film for high-performance transparent conductor application

In order to develop transparent conductors for optoelectronic devices, B-Mg co-doped ZnO (BMZO) thin films and ZnO/BMZO bilayer films were prepared by using a magnetron sputtering system in this work. The properties of the films with varying operating pressure and buffer layer thickness were measured and investigated systematically. It is found that all films have high transparency, which are almost independent of the buffer layer thickness and the working pressure. Working pressure affects the chance of collision between sputtered particles and gas molecules, which impacts on the average free range of sputtered particles and the energy of the particles. Thus, the BMZO films with high crystallinity and low resistivity can be obtained at appropriate working pressure. The introduction of ZnO buffer layer makes the BMZO films more suitable for TCO applications compared to monolayer BMZO. The results showed that the thickness of the buffer layer with the introduction of ZnO buffer layer decreased the surface roughness and internal stress of the films, which increased the electrical and optical properties of the film. In particular, the ZnO/BMZO film prepared at a working pressure of 3 Pa, exhibits the optimized optoelectrical properties when the buffer layer thickness is 12 nm.

Aluminum-doped zinc oxide (AZO) ultra-thin films deposited by radio frequency sputtering for flexible Cu(In,Ga)Se2 solar cells

Zinc oxide ultra-thin films doping with aluminum (AZO) were produced through radio frequency (rf) sputtering at a fixed pressure of 10 mTorr while varying the rf power between 80 and 140 W. The crystal structure of hexagonal Wurtzite was consistent throughout, with improved crystallinity observed at higher rf powers due to optimal diffusivity of the sputtered particles during nucleation and growth. The size of the crystallite was increased from 10.37 to 16.58 nm with increasing the rf power from 80 to 140 W. The Raman spectra provided evidence of the formation of ultra-thin AZO films, with discernable changes in morphology due to the influence of rf power. The value of optical band gap fluctuated between 3.49 and 3.58 eV as a function of rf power, a basis of the Burstein–Moss effect. The resistivity of the ultra-thin AZO films declined while augmenting rf power. A bilayer structure of intrinsic ZnO (i-ZnO) and AZO was fabricated and exhibited good transmittance, well-crystalline morphology, and excellent electrical conductivity. The optimized window layer (i-ZnO and AZO) was used to produce flexible Cu(In,Ga)Se2(CIGSe) solar cells with a photo conversion efficiency of 9.53%. Therefore, ultra-thin ZnO films exhibit potential as a favorable option for a window layer in the production of high-efficient flexible solar cells in cost effective way.

Facile Synthesis of Multicolored Stacked Quantum Dot Films for Efficient White Light Emission

Quantum dot (QD)-based white light-emitting diodes (LEDs) or white QD-LEDs were successfully fabricated by stacking QD films of primary colors (i.e., red, green, and blue). An ultrathin ZnO film was deposited between each QD layer to facilitate the formation of a tricolor-stacked QD structure. Such stacked QD films effectively suppress the Förster resonance energy transfer between QDs and enable efficient blue light emission, unlike randomly mixed QD films of different colors. The photoluminescence (PL) spectra of various QD films were obtained to study the change in the corresponding color gamut with applied voltage. In addition, QD-LEDs were fabricated based on the PL behavior and electronic band diagrams of QDs of different colors. Maximum luminance and peak EQE of white QD-LED shows 5700 cd/m2 and 1.1% compared to 2200 cd/m2 and 0.3% of QD-LED with mixed QD layer. Finally, a smooth color transition and white light emission were achieved using a blue/green/red stacking sequence for the QDs, which successfully suppressed the energy transfer effect. The overall performance and brightness of the white QD-LEDs synthesized in this study can be further enhanced by developing high-performance blue QDs.

Simultaneous Measurement of Humidity and Temperature Based on ZnO-Coated Hollow Core Bragg Fiber

An optical fiber sensor for simultaneous measurement of relative humidity (RH) and temperature is proposed, which is constructed by sandwiching one section of hollow core Bragg fiber (HCBF) between two sections of single mode fiber (SMF). The ultrathin ZnO film is coated on the outer surface of HCBF as humidity-sensitive material. The dips in the transmission spectrum caused by different anti-resonant (AR) modes of HCBF have different sensitivities to the RH and temperature, and then the simultaneous measurement of above two parameters can be realized by solving a cross matrix. For the two dips monitored in the experiments, the sensitivities of the proposed sensor to RH are 0.7 pm/%RH and 16.95 pm/%RH in the range of 46–73 %RH, and the sensitivities to temperature are 25 pm/°C and 26.8 pm/°C in the range of 20-60 °C, respectively. Taking advantage of compact structure, good stability, easy fabrication and the proposed sensor exhibits great potential application in fields of chemical sensors and biochemical detection.

Investigation of improving organic light-emitting diodes efficiency using an ultra-thin ultraviolet-ozone-treated Nb-doped ZnO film as anode buffer layer

In this study, ultraviolet (UV)-ozone treated ultrathin Nb-doped ZnO (NZO) films are developed as an efficient anode buffer layer to improve the overall performance of organic light-emitting diodes (OLEDs). The results show that the UV-ozone treated NZO buffer layer containing 1 mol% of Nb2O5 possesses a higher oxygen content and a greater work function (5.22 eV) as compared to that of an indium tin oxide (ITO) film (~4.7 eV) signifying a reduction in hole injection barrier and thus an improvement in the injection efficiency. In addition, UV-ozone treatment can increase the surface energy of NZO films while reducing their surface roughness. Importantly, the UV-ozone treated 1 nm-thick NZO film with a Nb2O5 doping concentration of 1 mol% can help to lower the turn-on voltage from 3.2 V to 2.8 V, increase the luminance from 10,450 cd/m2 to 25370 cd/m2, and improve the current efficiency from 3.46 cd/A to 5.26 cd/A, (~52 % enhancement as compared to the standard OLED device). Moreover, OLEDs with the developed buffer layer reveal a significant improvement in the roll-off phenomenon under high current densities, indicating a key role of the optimized NZO film in enhancing the carrier balance of the devices. When applied to the p-i-n structure, the NZO film also lead to a superior device performance as compared to the conventional p-i-n structure using NPB:MoO3 as a hole injection layer, suggesting a widespread use of the developed thin film. These findings therefore show a promising use of the UV-ozone treated NZO ultrathin film as an effective anode buffer material for enhancing OLED overall performance.

Nanoscale Heating of an Ultrathin Oxide Film Studied by Tip-Enhanced Raman Spectroscopy

We report on the nanoscale heating mechanism of an ultrathin ZnO film using low-temperature tip-enhanced Raman spectroscopy. Under the resonance condition, intense Stokes and anti-Stokes Raman scattering can be observed for the phonon modes of a two-monolayer (ML) ZnO on an Ag(111) surface, enabling us to monitor local heating at the nanoscale. It is revealed that the local heating originates mainly from inelastic electron tunneling through the electronic resonance when the bias voltage exceeds the conduction band edge of the 2-ML ZnO. When the bias voltage is lower than the conduction band edge, the local heating arises from two different contributions, namely direct optical excitation between the interface state and the conduction band of 2-ML ZnO or injection of photoexcited electrons from an Ag tip into the conduction band. These optical heating processes are promoted by localized surface plasmon excitation. Simultaneous mapping of tip-enhanced Raman spectroscopy and scanning tunneling spectroscopy for 2-ML ZnO including an atomic-scale defect demonstrates visualizing a correlation between the heating efficiency and the local density of states, which further allows us to analyze the local electron-phonon coupling strength with ∼2 nm spatial resolution.

Atomic Layer Deposition of Ultrathin ZnO Films for Hybrid Window Layers for Cu(Inx,Ga1-x)Se2 Solar Cells.

The efficiency of thin-film chalcogenide solar cells is dependent on their window layer thickness. However, the application of an ultrathin window layer is difficult because of the limited capability of the deposition process. This paper reports the use of atomic layer deposition (ALD) processes for fabrication of thin window layers for Cu(Inx,Ga1−x)Se2 (CIGS) thin-film solar cells, replacing conventional sputtering techniques. We fabricated a viable ultrathin 12 nm window layer on a CdS buffer layer from the uniform conformal coating provided by ALD. CIGS solar cells with an ALD ZnO window layer exhibited superior photovoltaic performances to those of cells with a sputtered intrinsic ZnO (i-ZnO) window layer. The short-circuit current of the former solar cells improved with the reduction in light loss caused by using a thinner ZnO window layer with a wider band gap. Ultrathin uniform A-ZnO window layers also proved more effective than sputtered i-ZnO layers at improving the open-circuit voltage of the CIGS solar cells, because of the additional buffering effect caused by their semiconducting nature. In addition, because of the precise control of the material structure provided by ALD, CIGS solar cells with A-ZnO window layers exhibited a narrow deviation of photovoltaic properties, advantageous for large-scale mass production purposes.

Investigation of Schottky barrier height using area as parameter: Effect of hydrogen peroxide treatment on electrical optical properties of Schottky diode

RF sputtered 25 nm ZnO thin film surface treated with H2O2 has produced the Schottky diodes of improved electrical and optical properties. The enhancement is associated with adsorbed and introduced oxygen at the film's surface and in bulk while H2O2 treatment as revealed by the XPS analysis. Further, H2O2 treatment also has improved the ZnO thin film's surface morphology, crystal structure, and optical properties studied by XRD, SEM, and PL measurement. The experimentally measured energy band gap is compared with density functional theory computation-based result to find the possible cause behind the change in edge excitation energy band after treatment. Besides, an alternative method by considering the Schottky diode area as a parameter is introduced to calculate barrier height. The five diodes average barrier height, calculated by the conventional Schottky model, was lower than the barrier height obtained by this proposed method. This result is obtained for the diodes fabricated on the nontreated and treated sample. The conventional and proposed methods showed Schottky diodes potential barrier lowering under UV illumination, and it was associated with the change in carrier density and desorption of adsorbed oxygen on the ZnO surface. The treated surface's low conductivity and high oxygen concentration governed the superior UV detection capability of Schottky diodes fabricated on it. The literature demonstrates many similar studies for large thicknesses of ZnO 150–1000 nm but not on ultra-thin (25 nm) ZnO film, even though the UV light can penetrate ZnO approximately to this depth.

Plasma-induced oxygen vacancies enabled ultrathin ZnO films for highly sensitive detection of triethylamine

Metal oxide semiconductor (MOS) thin films hold great promise for electronic devices such as gas sensors. However, the low surface activity of pristine MOS often leads to inferior sensitivity and the sensitization mechanism of ultrathin MOS films has received rare attention. Herein, we report a high performance gas sensor based on plasma-etched ZnO thin films. The ultrathin ZnO films (20 nm) were deposited on SiO2 wafers by atomic layer deposition (ALD), which enables high-throughput production of sensor devices. The ZnO sensor shows typical n-type conductivity, which is highly variable to the exposure of triethylamine (TEA). Annealing temperature of the films is found to impact the sensor response, revealing calcination at a moderate temperature, i.e. 700 °C, leads to the best response. Further treatment by Ar plasma results in a remarkable decrease of sensor working temperature from 300 °C of untreated films to 250 °C and nearly 4-fold enhancement in the sensor response to 10 ppm TEA. Notably, the plasma-treated ZnO sensor also shows decent response even at room temperature (RT), which has been seldom reported for ZnO-based sensors. Structure and mechanism investigations reveal that the superior sensor properties are derived from the abundant oxygen vacancies generated by Ar plasma etching.

Atomic Layer Deposition of ZnO for Modulation of Electrical Properties in n-GaN Schottky Contacts

ZnO films (5 nm and 20 nm) have been grown on GaN single-crystal substrates by thermal atomic layer deposition (ALD) and the electrical properties of n-GaN Schottky contacts modified by such ultrathin ZnO films have been characterized. Compared with 5-nm-thick ZnO, 20-nm-thick ZnO exhibited a better rectifying nature. The average barrier height and ideality factor at room temperature were extracted to be 0.64 eV and 2.33 eV, and 1.01 eV and 1.16 eV, for 5-nm- and 20-nm-thick ZnO, respectively. These results indicate that both the barrier height and ideality factor were altered effectively by changing the ZnO thickness. The temperature-dependent reverse current–voltage (I–V) characteristics revealed that tunneling was dominant for the 5-nm-thick ZnO. A laterally inhomogeneous barrier was appropriate to explain the forward I–V characteristics for both samples. Based on the parallel conductance method and forward I–V data, a lower interface state density was observed for 20-nm-thick ZnO, implying improved interface quality. These results suggest that the electrical properties of n-GaN Schottky contacts could be easily modulated by changing the ZnO thickness.

Spectroscopic analysis of ultrathin amorphous ZnO films grown by atomic layer deposition

Ultrathin amorphous Zinc Oxide films were prepared on glass substrates by homemade atomic layer deposition system using Diethyl Zinc and Deionized water as precursors and high pure Argon gas as purging gas. The substrate temperature was set as 125℃ and cycle number was varied. Different spectroscopic techniques were employed to unveil the structural and optical properties of films. The effect of quantum confinement on band gap energy values reduces with increase in thickness. Resonant Raman scattering was observed for all the samples when excited with laser wavelength of 325 nm.

Strain and support effects on phase transition and surface reactivity of ultrathin ZnO films: DFT insights

Strain and support effects play a crucial role in heterogeneous catalysis, which has been intensively studied over metal-based catalysts. In contrast, there is little discussion about the two effects in oxide systems. In this work, using an ultrathin ZnO film as an example, we investigate strain and support effects on the structure and surface reactivity of oxide catalysts through density functional theory calculations. Our results suggest that tensile strain increases the surface reactivity of ZnO films as indicated by enhanced CO and NH3 adsorptions and compressive strain renders an early phase transition from an inert graphene-like phase to a more reactive wurtzite-like phase. The support (Au, Pt, and Ru) can promote the phase transition and surface reactivity concurrently, which exhibits a larger effect on the reactivity than the strain. The support effect can be ascribed to the increasing rumple and polarization of ZnO films through the strong ZnO–substrate interaction, which enhances the surface reactivity. The insight helps us to develop advanced oxide-based catalysts through the strain and/or substrate engineering.

Growth and Raman spectroscopy of ultrathin ZnO(0001) films on Ag(001)

The growth and vibrational properties of ultrathin ZnO(0001) films on Ag(001) were investigated by X-ray and ultraviolet photoemission spectroscopy (XPS/UPS), low-energy electron diffraction (LEED), and Raman spectroscopy. Our results indicate the formation of well-defined stoichiometry and crystalline phase ~5 ML thick ZnO(0001) films characterized by two domains rotated by 30°. Further film analysis using Raman spectroscopy showed that the ultrathin films display substantial intensity enhancement of their Raman combination modes. Our findings suggest a significant role of the finite-size film electronic and structural properties on its vibrational properties. These observations may serve as a pathway toward further investigations dependent on film thickness and orientation. More important, the modification of the vibrational properties is expected to be relevant to understanding general phenomena in ZnO nanostructures (i.e. thin films, nanoparticles, nanorods).

Refractive index of ZnO ultrathin films alternated with Al2O3 in multilayer heterostructures

The design of optoelectronic devices made with ZnO superlattices requires the knowledge of the refractive index, which currently can be done only for films thicker than 30 nm. In this work, we present an effective medium approach to determine the refractive index of ZnO layers as thin as 2 nm. The approach was implemented by determining the refractive index of ZnO layers ranging from 2 nm to 20 nm using spectroscopic ellipsometry measurements in multilayers. For a precise control of morphology and thickness, the superlattices were fabricated with atomic layer deposition (ALD) with alternating layers of 2 nm thick Al2O3 and ZnO, labeled as N ZnO-Al2O3, where N = 10, 20, 30, 50, 75 and 100. The total thickness of all superlattices was kept at 100 nm. The approach was validated by applying it to similar superlattices reported in the literature and fitting the transmittance spectra of the superlattices.

Dramatic Enhancement of Tip-Enhanced Raman Scattering Mediated by Atomic Point Contact Formation.

Tip-enhanced Raman scattering (TERS) in ångström-scale plasmonic cavities has drawn increasing attention. However, Raman scattering at vanishing cavity distances remains unexplored. Here, we show the evolution of TERS in transition from the tunneling regime to atomic point contact (APC). A stable APC is reversibly formed in the junction between an Ag tip and ultrathin ZnO or NaCl films on the Ag(111) surface at 10 K. An abrupt increase of the TERS intensity occurs upon APC formation for ZnO, but not for NaCl. This remarkable observation is rationalized by a difference in hybridization between the Ag tip and these films, which determines the contribution of charge transfer enhancement in the fused plasmonic junction. The strong hybridization between the Ag tip and ZnO is corroborated by the appearance of a new vibrational mode upon APC formation, whereas it is not observed for the chemically inert NaCl.

Synthesis of ZnO Ultra-Thin Film-Based Bottom-Gate Phototransistors for UV Detection

The present study illustrates the fabrication of ZnO ultra-thin film (25 nm)-based bottom gate phototransistors using RF sputtering and thermal evaporation on SiO2/Si substrate for UV detection. According to the literature, phototransistors have the ability to solve persistent photoconductivity (PPC). PPC increases the response time of metal oxide semiconductor-based conventional two-terminal photodetectors. Prior to transistor fabrication, the surface of the deposited ZnO thin film was treated with hydrogen peroxide (H2O2) in order to improve its crystal structure, surface morphology, energy bandgap, and electrical conductivity. The characteristics of ZnO thin film were investigated by atomic force microscope (AFM), field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), photoluminescence (PL), and x-ray photoelectron spectroscopy (XPS). The electrical and optical performance of phototransistors were investigated by measuring their output and transfer characteristics in dark and UV light. H2O2 treatment was found to be effective in producing efficient optical detection phototransistor. Optoelectronics properties (for UV detection) of the fabricated phototransistors were studied by using low-intensity and low power commercial LEDs of 365 nm wavelength.

Ultrathin Al-Doped ZnO Films Coated Li4Ti5O12 As an Anode Material with Excellent Cycling Stability and Rate Capability for Lithium-Ion Batteries

Lithium titanate oxide (Li4Ti5O12, LTO) is a lithium ion intercalated compound, which was identified as a promising anode material for LIBs with a theoretical capacity of 175 mAh g-1 within a typical potential range of 1.0 V–3.0 V. Extending the end voltage from 1.0 V to 0.01 V is an effective way to improve the electrochemical performance of LTO; the tetrahedral (8a) sites of LTO would be available for lithium ion storage to have an “extra” reversible capacity, when the end potential is lower than 0.6 V. However, this approach also leads to some other problems: (1) a lower end potential will cause a more intensive decomposition of electrolyte to form the solid electrolyte interface (SEI), which means that the initial irreversible capacity loss will increase, and (2) the change in lattice dimension during the lithium ion insertion/extraction will be increased and thus the structure stability of LTO will become worse. In this study, an Al-doped ZnO (AZO) film with various thicknesses and compositions were deposited on LTO particles by atomic layer deposition (ALD) method with different deposition temperatures and different ratios of the number of Al2O3 ALD to that of ZnO ALD. The electrochemical characterization results indicated that the coating of AZO significantly improved the electrochemical performance of LTO between 0.1 V and 3.0 V with a proper coating thickness. After 250 cycles of charge/discharge at a 1 C rate, the capacity of the optimum AZO-coated LTO sample decreased from 203 to 190 mAh g-1 with a capacity retention of ~94% at room temperature and from 224 to 216 mAh g-1 with a capacity retention of ~96% at 55 ℃. The AZO coating layer with an appropriate thickness not only increased the capacity of LTO by enhancing conductivity, but also assisted the LTO electrodes to form a beneficial interface layer to protect the LTO from the continuous attack by harmful components in the electrolyte, especially under extreme conditions, such as high temperature and high current rates.