P-type Transparent Conductive Oxide Research Articles

Effect of polyvinylpyrrolidone concentration on optical properties of CuCrO2 thin films

In this study, CuCrO2 thin films were fabricated by spin-coating technique on alkali-free glass substrates and a two-step annealing process. The copper-based delafossite CuCrO2 is a p-type transparent conductive oxide that exhibits a wide energy bandgap and excellent stability. To improve the transmittance of CuCrO2 thin films, polyvinylpyrrolidone (PVP) was used as a porogen for preparing thin films with high transmittance and high haze values. The highest transmittance of 73.75% was achieved with 0.5 g of PVP. The bandgaps of prepared CuCrO2 thin films were estimated at 2.82 eV to 3.11 eV. The prepared CuCrO2 thin films had a porous structure and high transmittance. The high potential transmittance of porous CuCrO2 thin films makes them applicable to photovoltaic systems, transistors and touch screens or displays in the future.

Investigating various metal contacts for p-type delafossite α-CuGaO2 to fabricate ultraviolet photodetector

Delafossite semiconductors have attracted substantial attention in the field of electro-optics owing to their unique properties and availability of p-type materials that are applicable for solar cells, photocatalysts, photodetectors (PDs) and p-type transparent conductive oxides (TCOs). The CuGaO2 (CGO), as one of the most promising p-type delafossite materials, has appealing electrical and optical properties. In this work, we are able to synthesize CGO with different phases by adopting solid-state reaction route using sputtering followed by heat treatment at different temperatures. By examining the structural properties of CGO thin films, we found that the pure delafossite phase appears at the annealing temperature of 900 °C. While at lower temperatures, delafossite phase can be observed, but along with spinel phase. Furthermore, their structural and physical characterizations indicate an improvement of material-quality at temperatures higher than 600 °C. Thereafter, we fabricated a CGO-based ultraviolet-PD (UV-PD) with a metal–semiconductor-metal (MSM) configuration which exhibits a remarkable performance compared to the other CGO-based UV-PDs and have also investigated the effect of metal contacts on the device performance. We demonstrate that UV-PD with the employment of Cu as the electrical contact shows a Schottky behavior with a responsivity of 29 mA/W with a short response time of 1.8 and 5.9 s for rise and decay times, respectively. In contrast, the UV-PD with Ag electrode has shown an improved responsivity of about 85 mA/W with a slower rise/decay time of 12.2/12.8 s. Our work sheds light on the development of p-type delafossite semiconductor for possible optoelectronics application of the future.

Study on the optical and electrical properties of Fe-doped CuAlO2 with first-principles

Fe-doped is an effective method to improve the conductivity of p-type transparent conductive oxide (TCO). In this paper, we performed first-principle calculations to study the effect of Fe-doped on the electrical structure and optical properties of the CuAlO2 crystal. To get accurate results, we used the finite size correction scheme (FNV method) to correct the defect formation energy for charged defects and the hybrid density functional method was used to adjust the band edge. The calculation results show that the formation energy for Fe occupying Al position (marked as FeAl) in CuAlO2 is lower than that for Fe occupying Cu position (marked as FeCu) and FeAl is the easiest one to form. It is also found that Fe-doping can increase the hole concentration of CuAlO2 and thus increasing the conductivity of the CuAlO2 crystal. Furthermore, we have investigated the optical spectrum of Fe-doped CuAlO2 and the absorption peak of CuAlO2 with FeAl∙∙ locates at 2.07 eV, which is close to the experimental data (2.10 eV). It can be inferred that the transmittance of the material decreases and the light absorption increases in the visible range.

Improving the p-Type CuCrO2 Thin Film's Electrical and Optical Properties.

In this research, we studied the functional properties of CuCrO2, which is the most promising p-type transparent conductive oxide (TCO). The thin films were fabricated using a spin coating technique. The diffraction patterns were obtained with the help of X-ray diffractions, and the optical properties of absorption characteristics were studied using UV-visible absorption. The physical properties of film formation and surface morphology were analyzed using FESEM analysis. The aging properties were also analyzed with the help of various precursors with different aging times. The CuCrO2 thin films' functional properties were determined by using chelating agent and precursor solution aging times. The CuCrO2 thin films have better transmittance, resistance, figure of merit (FOM), and electrical conductivity. Moreover, the resistivity values of the CuCrO2 thin films are 7.01, 9.90, 12.54, 4.10, 2.42, and 0.35 Ω cm. The current research article covers the preparation of copper chromium delafossite thin films. These thin films can be suitable for hole transport layers in transparent optoelectronic devices.

Effects of Microwave and Furnace Annealing for P-Type SnO Thin Film Material in Oxygen Ambient

Transparent conductive oxide (TCO) semiconductors are attracted considerable attention due to a wide range of applications, such as flat panel display (FPD), touch panels, solar cells, and other optoelectronic devices. Owing to the different carrier conduction paths between n-type and P-type TCOs, the n-type TCO used in TFTs usually have high Ion/Ioff current ratio (>107) and high electron mobility (>10 cm²/V·s), P-type TCO TFTs are both lower than that of n-type one. For complementary circuits design and applications, however, both P-type and n-type semiconductor materials are equally important. For SnO thin films, it is important to adjust the ratio of Sn2+ (SnO P-type) and Sn4+ (SnO₂ n-type) in order to modulate the electrical characteristics. In this investigation of post treatment for SnO thin films, both microwave annealing (MWA) and furnace annealing process with 0₂ ambient are studied. The results show that SnO thin films are optimized at 300 °C, 30 minutes furnace annealing, the P-type SnO/SnO₂ thin film shows surface mean roughness 0.168 nm, [Sn2+]/[Sn4+] ratio as 0.838, at least 80% transmittance between 380 nm-700 nm visible light. Withthe results, SnO can be even used to fabricate high performance P-type thin film transistors (TFTs) device for future applications.

Optimized Stoichiometry for CuCrO2 Thin Films as Hole Transparent Layer in PBDD4T-2F:PC70BM Organic Solar Cells.

The performance and stability in atmospheric conditions of organic photovoltaic devices can be improved by the integration of stable and efficient photoactive materials as substituent of the chemically unstable poly (3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS), generally used as organic hole transport layer. Promising candidates are p-type transparent conductive oxides, which combine good optoelectronic and a higher mechanical and chemical stability than the organic counterpart. In this work, we synthesize Cu-rich CuCrO2 thin films by aerosol-assisted chemical vapour deposition as an efficient alternative to PEDOT:PSS. The effect of stoichiometry on the structural, electrical, and optical properties was analysed to find a good compromise between transparency, resistivity, and energy bands alignment, to maximize the photovoltaic performances., Average transmittance and bandgap are reduced when increasing the Cu content in these out of stoichiometry CuCrO2 films. The lowest electrical resistivity is found for samples synthesized from a solution composition in the 60–70% range. The optimal starting solution composition was found at 65% of Cu cationic ratio corresponding to a singular point in Hackee’s figure of merit of 1 × 10−7 Ω−1. PBDD4T-2F:PC70BM organic solar cells were fabricated by integrating CuCrO2 films grown from a solution composition ranging between 40% to 100% of Cu as hole transport layers. The solar cells integrating a film grown with a Cu solution composition of 65% achieved a power conversion efficiency as high as 3.1%, representing the best trade-off of the optoelectronic properties among the studied candidates. Additionally, despite the efficiencies achieved from CuCrO2-based organic solar cells are still inferior to the PEDOT:PSS counterpart, we demonstrated a significant enhancement of the lifetime in atmospheric conditions of optimal oxides-based organic photovoltaic devices.

Investigation of crystallinity, electrical conductivity, and optical transmittance of Mg-doped CuCrO2 deposited on buffer layer

Copper-based delafossite materials such as CuCrO2 are promising candidates for application as p-type transparent conductive oxide because they have high optical transparency and high electrical conductivity. Here, we investigate how the crystallinity of the Mg-doped CuCrO2 and the buffer layer affects both the transparency and conductivity of Mg-doped CuCrO2. Our results show that the properties that are the transparency and the conductivity, correlate with the crystallinity of the materials. Furthermore, we find that the addition of the buffer layer enhances the formation of highly oriented CuCrO2, making optimization of transparency and conductivity possible.

Efficient Resistive Switching and Spike Rate Dependent Plasticity in a New CuCrO2 Memristor for Plausible Neuromorphic Systems

In this article, we introduce a new class of p-type transparent conductive oxide (TCO) CuCrO2 (150 nm) heterogeneously integrated onto fluorine doped tin oxide (FTO)/glass for forming-free memristor-based neuromorphic applications. The fabricated Al/CuCrO2/FTO memristors demonstrate a reliable bipolar resistive switching with an ON/ OFF ratio of 1000. The retention of the device was found to be steady even beyond 106 s, which demonstrates its nonvolatility. The current–voltage ( ${I}$ – ${V}$ ) characteristics were fit to evaluate its transport properties and a band diagram was projected to have a better insight of the device operational principles. To validate the experimental observations, a new model has been developed, and the simulated ${I}$ – ${V}$ behavior was analogous to the experimental one. Efforts were then devoted to observe long-term potentiation (LTP) and long-term depression (LTD) utilizing identical but opposite pulses to evaluate the device’s efficacy for synaptic applications. The synaptic behavior was well controlled by the pulse (pulse amplitude and width) variations. The conductance change was found to be symmetric and then saturated, which reflects the popular biological Hebbian rules. Finally, a long-term synaptic modulation has been implemented by establishing the spike rate dependent plasticity (SRDP) rule, which is a part of spiking neural networks and advantageous to mimic the brain’s capability at low power. All the obtained experimental results were systematically corroborated by neural network simulation. Overall, our approach provides a new road map toward the development of TCO-based alternative memristors, which can be employed to mimic the synaptic plasticity for energy-efficient bioinspired neuromorphic systems and non-von Neumann computer architectures.

The Optoelectronic Properties of p-Type Cr-Deficient Cu[Cr0.95-xMg0.05]O2 Films Deposited by Reactive Magnetron Sputtering.

CuCrO2 is one of the most promising p-type transparent conductive oxide (TCO) materials. Its electrical properties can be considerably improved by Mg doping. In this work, Cr-deficient CuCrO2 thin films were deposited by reactive magnetron sputtering based on 5 at.% Mg doping. The influence of Cr deficiency on the film’s optoelectronic properties was investigated. As the film’s composition varied, CuO impurity phases appeared in the film. The mixed valency of Cu+/Cu2+ led to an enhancement of the hybridization between the Cu3d and O2p orbitals, which further reduced the localization of the holes by oxygen. As a result, the carrier concentration significantly improved. However, since the impurity phase of CuO introduced more grain boundaries in Cu[Cr0.95−xMg0.05]O2, impeding the transport of the carrier and incident light in the film, the carrier mobility and the film’s transmittance reduced accordingly. In this work, the optimal optoelectronic performance is realized where the film’s composition is Cu[Cr0.78Mg0.05]O2. Its Haacke’s figure of merit is about 1.23 × 10−7 Ω−1.

Double perovskite Ba2BiTaO6 as a promising p-type transparent conductive oxide: A first-principles defect study

The realization of p-type transparent conductive oxides (TCOs) is a long-standing difficulty because of their generally large hole effective mass and low doping limit ascribed to the strong electronegativity of oxygen. By introducing a Bi atom with a lone-pair s orbital on the B site, double perovskite Ba2BiTaO6 could exhibit a dispersive upper valence band and a small hole effective mass due to the strong sp antibonding coupling. In contrast to n-type TCOs, i.e., ZnO and In2O3, first-principles studies on the intrinsic defects of Ba2BiTaO6 show that it is able to exhibit a good p-type property or stay intrinsic instead of being n-type by tuning the chemical potentials and its dominated defects (VBa, VBi, BaBi, and BiTa) are shallow acceptors, indicating that Ba2BiTaO6 is able to be a promising p-type TCO. This work shows that perovskites could be a class of potential p-type TCOs by introducing strong sp coupling with lone-pair-s-orbital elements.

Effects of substrate temperature on characteristics of the p-type Ag-doped SnOx thin films prepared by reactive DC magnetron sputtering

P-type transparent conductive oxides are one of long-persued research topics in scientific community. Substitutes for unstable p-type zinc oxide are in urgent need than ever and doped tin monoxide can be a material-of-choice. In this report, we present Ag-doped SnOx thin film produced by reactive DC magnetron sputtering from Sn + Ag alloy targets. We investigated and proved the strong influence of substrate temperatures on crystal structure, optical and electrical properties. Tin monoxide thin films showed tetragonal crystal structure and preferred diffraction peaks of (001), (101), (110), (002). As substrate temperature increased, grain size also increased, leading to reduced grain boudaries, higher hole mobility and lower resistivity. Films deposited at low (150 °C) and high (300 °C) substrate temperatures had the presence of exotic β-Sn and SnO2 phases, respectively.They acted as undesired scattering centers or induce harmful carrier recombination which decreased mobility or hole concentration. The Ag-doped SnO films fabricated at 250 °C had the best electrical properties, hole concentration 5.82 × 1018 cm−3, mobility 4.1 cm2/Vs, resistivity 2.262 × 10-1Ωcm. Current-voltage characteristics of thin film diode structures of glass/FTO/n-nc-Si:H /p-Ag:SnO/Cu and glass /FTO/n-nc-Si:H /undoped SnO/Cu were studied. They showed remarkably improved electric properties of SnO: Ag films compared to the pristine SnO counterpart.

Poisson ratio and bulk lattice constant of (Sr0.25La0.75)CrO3 from strained epitaxial thin films

About 30 nm thick (001)-oriented (Sr0.25La0.75)CrO3 (SLCO) epitaxial thin films were grown by solid-source oxide molecular beam epitaxy on four different single-crystalline cubic or pseudocubic (001)-oriented oxide substrates, LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3, and DyScO3, which result in lattice mismatch ranging from –2% to +1.7%. All the films are of high-quality, flat, and strained by the substrates. By assessing the evolution of the out-of-plane lattice parameter as a function of the in-plane lattice parameter of the samples, we determine both the Poisson ratio (ν = 0.32) and the bulk lattice constant (ab = 3.876 Å) of SLCO. The Poisson ratio significantly differs from LaCrO3 (ν = 0.23), and the (SrxLa1−x)CrO3 solid solution appears to obey structural Vegard's law. Since SLCO is the only one p-type transparent conductive oxide of perovskite structure and has promising thermoelectric properties, integrating SLCO in heterostructures and devices is, therefore, of paramount importance, which confers on our results their strong interest. Besides, the method used here can be straightforwardly applied to other complex oxides.

Thermal solid-phase crystallization of amorphous CuCrO2:N thin films deposited by reactive radio-frequency magnetron sputtering

In order to fabricate highly-oriented delafossite CuCrO2 (CCO) thin film which is one of the candidate materials for a p-type transparent conductive oxide, thermal solid-phase crystallization of an amorphous N-doped CCO (CCO:N) film was investigated. The amorphous CCO:N films were deposited at room temperature by reactive radio-frequency sputtering in an Ar/N2 gas mixture atmosphere, and they were subsequently calcined in N2. A N2 partial pressure (αN2) during the film deposition and a calcination temperature (TC) were varied within the range from 0 to 90% and 500 to 900°C, respectively. By the calcination at 550°C and above, c-axis orientation was observed regardless of αN2. Furthermore, at αN2 of 70 and 90%, a six-fold symmetry of CuCrO2[110]//Al2O3[300] was confirmed at TC of 800°C and above. Average optical transmittance (λ=450–800nm) of the as-deposited CCO:N films was 45% and increased to 60% and over simultaneously with c-axis orientation. Resistivity of the calcined CCO:N film decreased with increasing TC up to 600°C but it rebounded drastically at TC of 650°C. This implies that resistivity was mainly affected by the number of intrinsic defects. From X-ray diffraction and X-ray photoelectron spectroscopy measurements, metal nitrides were formed in the as-deposited CCO:N film and it effectively acted to assist the generation of Cu+ and CuCrO2 bonds. N atoms were desorbed during the calcination but it was confirmed the initial formation of these desirable bonds effectively improved orientation of the delafossite structure.

P-type conductive NiOx: Cu thin films with high carrier mobility deposited by ion beam assisted deposition

Transparent conductive NiO thin films with 18at% Cu dopant were fabricated by ion beam assisted deposition (IBAD). Their structural and optoelectronic properties were compared with undoped NiO films and NiO films doped with 12at% Cu, and also compared with NiO:Cu (18at%) films deposited by RF sputtering as reported in our previous work. The results show that the crystallinity of NiO thin films deposited through IBAD technology is much better than that of the films deposited by RF sputtering. Thanks to this reason, the highest carrier mobility above 45cm2V−1s−1 for NiO:Cu (18at%) film can be realized here. Meanwhile, the films’ resistivity remains an acceptable value, varying from 2.05 to 0.064Ωcm with oxygen ion beam current changing from 0.2 to 0.8A. This feature is imperative for p-type transparent conductive oxides (TCOs) applied in various domains. In addition, with oxygen ion beam current increase, the increase of the Ni3+/Ni2+ ratio leads to more Ni2+ vacancies be introduced into NiO films, which is beneficial to generate holes and improve carrier concentration. In this work, the optimal carrier mobility of NiO film doped with 18at% Cu is obtained when the oxygen ion beam current is 0.2A. Its carrier concentration and electrical resistivity are 7.26 ×1016cm−3 and 2.05Ωcm, respectively.

Codoping and Interstitial Deactivation in the Control of Amphoteric Li Dopant in ZnO for the Realization of p-Type TCOs.

We report on first principle investigations about the electrical character of Li-X codoped ZnO transparent conductive oxides (TCOs). We studied a set of possible X codopants including either unintentional dopants typically present in the system (e.g., H, O) or monovalent acceptor groups, based on nitrogen and halogens (F, Cl, I). The interplay between dopants and structural point defects in the host (such as vacancies) is also taken explicitly into account, demonstrating the crucial effect that zinc and oxygen vacancies have on the final properties of TCOs. Our results show that Li-ZnO has a p-type character, when Li is included as Zn substitutional dopant, but it turns into an n-type when Li is in interstitial sites. The inclusion of X-codopants is considered to deactivate the n-type character of interstitial Li atoms: the total Li-X compensation effect and the corresponding electrical character of the doped compounds selectively depend on the presence of vacancies in the host. We prove that LiF-doped ZnO is the only codoped system that exhibits a p-type character in the presence of Zn vacancies.

On the growth of transparent conductive oxide ternary alloys Zn–Ir–O (ZIRO) by the means of rf magnetron co-sputtering

We report here on an alternative approach for forming p-type transparent conductive oxide ternary alloys Zn–Ir–O (ZIRO) by the means of rf magnetron co-sputtering of Ir and zinc nitride targets in a mixed argon and oxygen plasma. The structural and optical properties of ZIRO thin films formed on both glass and Si (100) substrates have been studied by Energy Dispersive X-ray spectrometry (EDX), X-Ray Diffraction (XRD), Raman and optical spectrophotometry. The EDX results outlined that the Ir incorporation in ZIRO alloys saturates at 3.3% when the rf co-sputtering is done in a plasma containing above 2% O2. XRD and Raman spectroscopy analysis revealed that all films were polycrystalline containing only the ZnO and IrO2 phases. The presence of Ir in ZIRO thin films does affect their optical transparency below 500nm while it remains comparable to that of ZnO above 500nm down to the infrared spectral range. Moreover the physical properties of the ZIRO films have been studied after sequentially annealing in air or forming gas (95% N2–5% H2) at elevated temperatures up to 700°C. All ZIRO films were found to remain highly resistive (MΩ range) regardless of annealing treatment conditions. The optical transparency of these films was found to be further reduced upon annealing at temperatures above 500°C.

Identification of Novel Cu, Ag, and Au Ternary Oxides from Global Structural Prediction

We use ab initio global structural prediction, and specifically the minima hopping method combined with high-throughput calculations, to explore the periodic table in search of novel oxide phases. In total, we study 183 different compositions of the form MXO2, where M = (Cu, Ag, Au) and X is an element of the periodic table. The choice of this specific set is motivated by the fact that it includes Cu delafossite compounds, which are the best-known p-type transparent conductive oxides. Our calculations identify 81 stable compositions, out of which only 36 are already included in materials databases. We then prescreen these new phases for applications as p-type transparent conductors by calculating their electronic band gap and hole effective masses, finding a few potentially good candidates. These results allow us to conclude that structural prediction combined with high-throughput calculations is a viable approach to contribute to the discovery of new materials for specific applications.

CuBO2: a new highly transparent p-type wide band gap electron field emitter

Nanocrystalline copper boron oxide, the latest addition to the p-type transparent conductive oxide family, was synthesized via the cost effective sol–gel technique and here we report the first-time observation of its excellent electron field emission property. Proper phase formation of the as-prepared samples was confirmed from x-ray diffraction studies whereas energy dispersive x-ray analysis showed the composition of the samples to be near stoichiometric. High resolution transmission electron microscopic studies were carried out to obtain morphological information and the exact particle size. The information regarding the work function was theoretically obtained using density functional theory. The optical transmittance and band gaps of the samples were obtained from ultraviolet-visible spectroscopic studies. The electron field emission property of the sample was studied with a standard field emission measurement set-up and the samples exhibited very good field emission performance compared to other Cu-delafossites, resulting in a turn-on field as low as 2.2 V μm−1 with the maximum emission current density up to 872 μA cm2, which gradually decreased with increasing particle size.

Preparation and Properties of the P-Type Transparent Conductive Oxide CuAlO<sub>2</sub>

The P-type transparent conductive oxide CuAlO2 was prepared by solid state reaction in this paper. The chemical state, phase structure, microstructure and optical properties were characterized by Differential Thermal Analysis ( DTA),X-ray diffraction ( XRD),Scanning Electron Microscope ( SEM) and Infrared Spectroscopy ( IR) respectively. Testing results show that the product is really p-type transparent conductive oxide CuAlO2. CuAlO2 phase began to form at about 1047.7 °C.It can be formed with high density and a high infrared light transmittance of the conductive oxide at 1150 °C.

The crystal structure of p-type transparent conductive oxide CuBO2

Abstract