ZnO Buffer Layer Research Articles

Effects of Doping Concentration on the Structural and Optical Properties of Spin-Coated In-doped ZnO Thin Films Grown on Thermally Oxidized ZnO Film/ZnO Buffer Layer/Mica Substrate

ZnO buffer layers were deposited on mica substrates using a sol-gel spin coating method. Then, a thin film of metallic Zn was deposited onto the ZnO buffer layer/mica substrate using a thermal evaporator, and the deposited Zn thin films were then thermally oxidized in a furnace at 500 ℃ for 2 h in air. Finally, In-doped ZnO (IZO) thin films with different In concentrations were grown on the oxidized ZnO film/ZnO buffer layer/mica substrates using the sol-gel spin-coating method. All the IZO films showed ZnO peaks with similar intensities. The full width at half maximum values of the ZnO (002) peak for the IZO thin films decreased with an increase in the In concentration to 1 at%, because the crystallinity of the films was enhanced. However, a further increase in the In concentration caused the crystal quality to degrade. This might be attributed to the fact that the higher In doping resulted in an increase in the number of ionized impurities. The Urbach energy (EU) values of the IZO thin film decreased with an increase in the In concentration to 1 at% because of the enhanced crystal quality of the films. The EU values for the IZO thin films increased with the In concentration from 1 at% to 3 at%, reflecting the broadening of localized band tail state near the conduction band edge of the films. (Received March 4, 2016; Accepted July 22, 2016)

Buffer layer influence on light absorption of electron intersubband transition in binary energy level systems of quantum wells

Due to the restriction of the ZnO buffer layer on the left barrier in a wurtzite asymmetric ZnO/MgxZn1-xO single quantum well (QWs) structure with finite barriers, the other factors such as the size of the well and right barrier, and Mg component, etc. will influence the critical value of the left barrier width to form a binary level energy system. By adopting a finite difference method to solve the Schrdinger equation, the eigenstates and eigenenergies of electrons in a two-dimensional electron gas are obtained, and the influences of buffer layer ZnO, size and ternary mixed crystal effects on the formation of binary energy level system in QW are discussed. In our computation, the influences of energy band bending, material doping and built-in electric fields on a realistic heterostructure potential are considered. Furthermore, based on the Fermi's golden rule, the optical absorption coefficient of electronic intersubband transition in QW and the influences of buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal effects are discussed. Our results indicate that the critical width of left barrier increases with the increases of the right barrier width and buffer layer thickness for a binary energy level system of ZnO/MgxZn1-xO single quantum well with a ZnO buffer layer on the left side. However, the critical width of left barrier decreases with the increase of well width and Mg component. Besides, the buffer layer thickness, the widths of left barrier, well and right barrier and ternary mixed crystal also affect the light absorption induced by the electronic intersubband transitions. The increases of Mg component, the widths of right barrier and left barrier will increase the absorption peak and produce its blue-shift. Whereas, increasing well width will reduce the absorption peak and produce its red-shift. The above conclusions are expected to give theoretical guidance in improving the opto-electronic properties of materials and devices made of these heterostructures.

PH dependent studies of chemical bath deposition grown ZnO-SiO2 core-shell thin films

ZnO-SiO2 core-shell thin films were synthesized using chemical-bath deposition at different pH. Optical studies were done to optimize the thin films to find suitable parameters for solar cell buffer layers. These studies were done by measuring the transmission at 500 nm, which is the peak of the solar spectrum. All the parameters were seen to be highly pH dependent. The transmittance for a sample synthesized with a pH of 10.8 reached 85%. The transmittance was found not to depend on the bandgap values, but it was found possibly to depend on the fewer defect states created at a particular pH, as shown by Urbach energy and scanning electron microscopy (SEM) surface structure. An appreciable transmittance was observed in the blue region of the spectrum which had been missing until now in commercial CdS-based buffer layers. The Fourier-transform infrared and the energy dispersive X-ray spectra confirmed that the films were composed of only ZnO and silica only: no impurities were found. The urbach energy values and the SEM image of sample S3 clearly indicate the creation of fewer of defects, leading to higher crystallintiy and higher transmittance. Therefore, this shortcoming can be resolved by the substituted buffer layer of ZnO:SiO2 nano-composite thin film, which can enhance the blue response of the photovoltaic cells.

Non-Toxic Buffer Layers in Flexible Cu(In,Ga)Se2Photovoltaic Cell Applications with Optimized Absorber Thickness

Absorber layer thickness gradient in Cu(In1−xGax)Se2(CIGS) based solar cells and several substitutes for typical cadmium sulfide (CdS) buffer layers, such as ZnS, ZnO, ZnS(O,OH), Zn1−xSnxOy(ZTO), ZnSe, and In2S3, have been analyzed by a device emulation program and tool (ADEPT 2.1) to determine optimum efficiency. As a reference type, the CIGS cell with CdS buffer provides a theoretical efficiency of 23.23% when the optimum absorber layer thickness was determined as 1.6 μm. It is also observed that this highly efficient CIGS cell would have an absorber layer thickness between 1 μm and 2 μm whereas the optimum buffer layer thickness would be within the range of 0.04–0.06 μm. Among all the cells with various buffer layers, the best energy conversion efficiency of 24.62% has been achieved for the ZnO buffer layer based cell. The simulation results with ZnS and ZnO based buffer layer materials instead of using CdS indicate that the cell performance would be better than that of the CdS buffer layer based cell. Although the cells with ZnS(O,OH), ZTO, ZnSe, and In2S3buffer layers provide slightly lower efficiencies than that of the CdS buffer based cell, the use of these materials would not be deleterious for the environment because of their non-carcinogenic and non-toxic nature.

N and Al co-doping as a way to p-type ZnO without post-growth annealing

We demonstrate experimental results on p-type ZnO films grown by atomic layer deposition (ALD) and co-doped with aluminum and nitrogen (ANZO). The films were obtained at low temperature (100 °C) with different N to Al ratio and show conductivity type, which depends on the N and Al content. We applied the x-ray photoelectron spectroscopy in order to get insight into a chemical nature of dopants and we found three pronounced contributions of the N1s core level which appear at binding energies of 396.1, 397.4 and around 399 eV. Based on ANZO and undoped ZnO films, both grown by the ALD technique, the ZnO homojunction was obtained in one technological process without any post-growth high temperature processing. The rectification ratio as high as 4 × 104 at ± 2 V was achieved when an ultrathin Al2O3 layer was inserted between p- and n-type ZnO and a n-type ZnO buffer layer deposited on an insulating Si substrate was applied.

Optical optimization of high resistance transparent layers in thin film cadmium telluride solar cells

Thin film photovoltaic devices are multilayer opto-electrical structures in which light interference occurs. Light reflection at the interfaces and absorption within the window layers reduces transmission and, ultimately, the conversion efficiency of photovoltaic devices. Optical reflection losses can be reduced by adjusting the layer thicknesses to achieve destructive interference within the structure of the cell. The light transmission to the CdTe absorber of a CdS/CdTe cell on a fluorine doped tin oxide transparent conductor has been modeled using the transfer matrix method. The interference effect in the CdS layer and high resistance transparent buffer layers (SnO2 and ZnO) has been investigated. The modeling shows that due to relatively high absorption within the SnO2 layer, there are modest benefits to engineering anti-reflection interference in the stack. However, a ZnO buffer layer has limited absorption and interference can be exploited to provide useful anti-reflection effects. Optical modeling and optimization shows that for a 50 nm CdS layer, a maximum transmission of 78.5% is possible using ZnO as a buffer layer at 58 nm thickness, and 78.0% for a SnO2 buffer layer at a thickness of 48 nm.

Short-circuit current improvement in CdTe solar cells by combining a ZnO buffer layer and a solution back contact

Conventional CdTe solar cells have a CdS window layer, in which an absorption loss of photons with more than 2.4 eV occurs through the CdS layer. A thinner CdS layer was applied to enhance light transmission and a ZnO buffer layer with a band gap of 3.3 eV was introduced to suppress shunting through the thinner CdS window layer. A 100-nm thick ZnO layer sputter-deposited at 300 °C had uniform coverage on a transparent conductive oxide (TCO) after a subsequent high-temperature process. The ZnO layer was effective in preventing shunting through the CdS window layer so that the open-circuit voltage and fill factor of the CdTe solar cells were recovered and the short-circuit current was enhanced over that of the conventional CdTe solar cell. In the ZnO/CdS/CdTe configuration, the short-circuit current was further improved throughout the visible wavelength region by replacing the Cu-metal contact with a Cu solution contact. As a result the short-circuit current from 21.7 to 26.1 mA/cm2 and the conversion efficiency of the CdTe solar cell increased from 12 to 15% without antireflective coating. Our result indicates that the Cu solution back contact is a critical factor for achieving a higher cell efficiency in addition to ZnO buffer layer.

Direct measurement of defect and dopant abruptness at high electron mobility ZnO homojunctions

Due to a strong Fermi-level mismatch, about 10% of the electrons in a 5-nm-thick highly Ga-doped ZnO (GZO) layer grown by molecular beam epitaxy at 250 °C on an undoped ZnO buffer layer transfer to the ZnO (Debye leakage), causing the measured Hall-effect mobility (μH) of the GZO/ZnO combination to remarkably increase from 34 cm2/V s, in thick GZO, to 64 cm2/V s. From previous characterization of the GZO, it is known that ND = [Ga] = 1.04 × 1021 and NA = [VZn] = 1.03 × 1020 cm−3, where ND, NA, and [VZn] are the donor, acceptor, and Zn-vacancy concentrations, respectively. In the ZnO, ND = 3.04 × 1019 and NA = 8.10 × 1018 cm−3. Assuming the interface is abrupt, theory predicts μH = 61 cm2/V s, with no adjustable parameters. The assumption of abruptness in [Ga] and [VZn] profiles is confirmed directly with a differential form of depth-resolved cathodoluminescence spectroscopy coupled with X-ray photoelectron spectroscopy. An anneal in Ar at 500 °C for 10 min somewhat broadens the profiles but causes no appreciable degradation in μH and other electrical properties.

Fully Solution Processed Organic Light-emitting Electrochemical Cells (OLEC) with ZnO Interlayer for Lab-on-Chip Applications

Microfluidic lab-on-a-chip devices can be used for chemical and biological analyses such as DNA tests or environmental monitoring. In order to make a monolithic and cost-efficient/disposable sensing device, direct integration of excitation light source for fluorescent sensing is often required. A manufacturing process for fully solution-processed blue organic light-emitting electrochemical cells (OLECs) is presented, which consist of pre-patterned ITO, spin-coated ZnO buffer layer and blue light-emitting polymer plus dopants and an inkjet-printed PEDOT:PSS transparent top anode. Furthermore, the fully transparent blue OLEC is able to emit > 2000 cd/m2 light under pulsed driving mode, which fulfils requirements for simple fluorescent on-chip sensing applications. Furthermore, ITO electrodes can be replace by PEDOT:PSS transparent electrodes when a ZnO interlayer is solution processed on top, which enable the mask-free and fully solution processing integration on chips.

Photoactivity enhancement of zinc sulfide ceramics thin films through ultrathin buffering engineering

Zinc-sulfide (ZnS) thin films 200nm-thick with various crystal features were fabricated using RF sputtering onto patterned sapphire substrates with and without ultrathin homo-ZnS and hetero-zinc oxide (ZnO) ultrathin buffer layers (approximately 45nm in thickness). Microstructural analyses revealed that the crystalline ZnS thin films with a columnar grain feature were deposited on the various ultrathin buffer layers-coated substrates through RF sputtering. The surface morphology of the ZnS thin films became rough and the crystal defect density of the ZnS thin films increased when the ZnS thin films were grown on the buffer layers. Comparatively, the rugged and island-like ZnO buffer layer engendered the crystal growth of the ZnS thin film with a higher degree of structural disorder than that of the crystal growth on the ZnS buffer layer. An increased crystal defect number together with the highly rugged film surface of the ZnS thin film buffered with ultrathin ZnO layers efficiently enhanced the photoactivity of the 200nm-thick ZnS thin film in this study.

Plasma versus thermal annealing for the Au-catalyst growth of ZnO nanocones and nanowires on Al-doped ZnO buffer layers

We successfully synthesized ZnO nanocones and nanowires over polycrystalline Al-doped ZnO (AZO) buffer layers on fused silica substrates by a vapor-transport process using Au-catalyst thin films. Different Au film thicknesses were thermal or plasma annealed in order to analyze their influence on the ZnO nanostructure growth morphology. Striking differences have been observed. Thermal annealing generates a distribution of Au nanoclusters and plasma annealing induces a fragmentation of the Au thin films. While ZnO nanowires are found in the thermal-annealed samples, ZnO nanocones and nanowires have been obtained on the plasma-annealed samples. Enhancement of the preferred c-axis (0001) growth orientation was demonstrated by x-ray diffraction when the ZnO nanocones and nanowires have been grown over the AZO buffer layer. The transmittance spectra of the ZnO nanocones and nanowires show a gradual increase from 375 to 900 nm, and photoluminescence characterization pointed out high concentration of defects leading to observation of a broad emission band in the visible range from 420 to 800 nm. The maximum emission intensity peak position of the broad visible band is related to the thickness of the Au-catalyst for the thermal-annealed samples and to the plasma power for the plasma-annealed samples. Finally, we proposed a model for the plasma versus thermal annealing of the Au-catalyst for the growth of the ZnO nanocones and nanowires. These results are promising for renewable energy applications, in particular for its potential application in solar cells.

Influence of a ZnO Buffer Layer on the Structural, Optical, and Electrical Properties of ITO/ZnO Bi-Layered Films

Influence of a ZnO Buffer Layer on the Structural, Optical, and Electrical Properties of ITO/ZnO Bi-Layered Films

Characteristics of SnO2:Sb Films as Transparent Conductive Electrodes of Flexible Inverted Organic Solar Cells.

Antimony-doped tin oxide (ATO) films were deposited on polyethersulfone (PES) substrates by means of a radio frequency (RF) magnetron sputtering method, using a SnO2 target mixed with 6 wt% Sb at room temperature and using various RF powers; these films were used as transparent electrodes in inverted organic solar cells (IOSC). We investigated the structural, optical, and electrical properties of the resulting films by means of various analyses, including X-ray diffraction (XRD), UV-visible spectroscopy, and Hall effect measurements. The crystallinity and conductivity of the ATO films were increased by increasing the RF power used. Based on the experimental data acquired, we fabricated IOSCs based on ATO electrodes deposited by using various conditions. Each IOSC device was composed of an ATO electrode, a ZnO buffer layer, a photoactive layer (P3HT:PCBM), and an Al cathode. The IOSC based on an ATO electrode fabricated at the RF power of 160 W exhibited good device performance due to the electrode's high conductivity and crystallinity.

Improving source/drain contact resistance of amorphous indium–gallium–zinc-oxide thin-film transistors using an n+-ZnO buffer layer

To avoid high temperature annealing in improving the source/drain (S/D) resistance (RDS) of amorphous indium–gallium–zinc-oxide (α-IGZO) thin-film transistors (TFTs) for flexible electronics, a simple and efficient technique using a sputtering-deposited n+-ZnO buffer layer (BL) sandwiched between the S/D electrode and the α-IGZO channel is proposed and demonstrated. It shows that the RDS of α-IGZO TFTs with the proposed n+-ZnO BL is reduced to 8.1 × 103 Ω as compared with 6.1 × 104 Ω of the conventional one. The facilitation of carrier tunneling between the S/D electrode and the α-IGZO channel through the use of the n+-ZnO BL to lower the effective barrier height therein is responsible for the RDS reduction. Effects of the chamber pressure on the carrier concentration of the sputtering-deposited n+-ZnO BL and the thickness of the BL on the degree of improvement in the performance of α-IGZO TFTs are analyzed and discussed.

Improved UV photoresponse properties of high-quality ZnO thin films through the use of a ZnO buffer layer on flexible polyimide substrates

An oxidized ZnO buffer layer was prepared by using thermal oxidation of a Zn buffer layer on a polyimide (PI) substrate; then, ZnO thin films with (sample 1) and without (sample 2) an oxidized ZnO buffer layer were grown by using the sol-gel spin-coating method. The intensities of the ZnO (002) diffraction peaks observed in sample 1 were stronger than those observed in sample 2, implying that the crystal quality was enhanced by the oxidized ZnO buffer layer. Moreover, the residual stress of sample 1 was reduced compared to that of sample 2 due to the decreased number of defects. Sample 2 exhibited defect-related deep-level orange-yellow emissions, which almost disappeared with the introduction of the ZnO buffer layer (sample 1). The values of the responsivity were 0.733 (sample 1) and 0.066 (sample 2) mA/W; therefore, the proposed method could provide a pathway to the easy fabrication of fast-response UV sensors.

MOCVD growth of ZnO nanowires on Ni–W metallic substrates

AbstractHere we investigate the possibility to grow ZnO nanowires on (100) textured Ni‐W substrates by MOCVD at 850 °C. Due to the oxidation of Ni, direct growth of ZnO nanowires on those metallic substrates was not achieved whereas it was easily obtained on sapphire in the same conditions. Therefore, a study of a ZnO buffer layer grown at low temperature was performed in order to prevent the NiO formation. The morphology and the crystallographic orientation of the as‐grown interfacial layers were characterized by scanning emission microscopy (SEM) and XRD diffraction, and are shown to depend on the growth temperature. Subsequently, the growth of ZnO nanowires was carried out on ZnO buffer/Ni‐W pseudo‐substrates. The SEM observations reveal the growth is not homogeneous and depends on the grain orientation. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Effect of Buffer Layer on ZnO Nanorods on PES Substrate.

In general, growing ZnO nanorods with the hydrothermal method requires the deposition of ZnO buffer layers. This is the most essential step, because the morphology of ZnO nanorods is strongly influenced by the buffer layer on a flexible substrate. In this work, we study the effect of the buffer layer when ZnO nanorods are grown. ZnO nanorods were grown on ZnO-buffered PES substrates with the hydrothermal method. As a result, it was found that the ZnO buffer layer using PLD had excellent characteristics compared with the ZnO buffer layer using sputtering.

Electronic Properties of CdTe/CdS Solar Cells as Influenced by a Buffer Layer

ABSTRACTWe considered modification of the defect density of states in CdTe as influenced by a buffer layer in ZnO(ZnS, SnSe)/CdS/CdTe solar cells. Compared to the solar cells employing ZnO buffer layers, implementation of ZnSe and ZnS resulted in the lower net ionized acceptor concentration and the energy shift of the dominant deep trap levels to the midgap of CdTe. The results clearly indicated that the same defect was responsible for the inefficient doping and the formation of recombination centers in CdTe. This observation can be explained taking into account the effect of strain on the electronic properties of the grain boundary interface states in polycrystalline CdTe. In the conditions of strain, interaction of chlorine with the grain boundary point defects can be altered.

Metal versus dielectric back reflector for thin‐film Si solar cells with impact of front electrode surface texture

AbstractThin‐film Si solar cells employ a back reflector (BR) for a more efficient use of the long wavelength light. Here, we have carried out a cross evaluation of metal (Ag‐based) and dielectric (white paint‐based) BR designs. Conclusive results have been reached regarding the most suitable BR type depending on the front electrode morphology, both with crater‐like and pyramidal texture. The ZnO/Ag BR is found to be optically more efficient because of improved light trapping, although the gain tends to vanish for rougher front electrodes. Thanks to non‐conventional Raman intensity measurements, this dependence on the front texture has been linked to the different weight of front and back interfaces in the light trapping process for the different morphologies. With rougher substrates, because the minor optical gain is accompanied by sputter‐induced electronic deterioration of the solar cell during the ZnO buffer layer deposition, the white paint‐based BR design is preferred. Copyright © 2016 John Wiley & Sons, Ltd.

Enhanced Light Scattering by Preferred Orientation Control of Ga Doped ZnO Films Prepared through MOCVD

We have explored the effective approach to fabricate GZO/ZnO films that can make the pyramidal surface structures of GZO films for effective light scattering by employing a low temperature ZnO buffer layer prior to high temperature GZO film growth. The GZO thin films exhibit the typical preferred growth orientations along the (002) crystallographic direction at deposition temperature of 400°C and SEM showed that column-like granule structure with planar surface was formed. In contrast, GZO films with a pyramidal texture surface were successfully developed by the control of (110) preferred orientation. We found that the light diffuse transmittance of the film with a GZO (800 nm)/ZnO (766 nm) exhibited 13% increase at 420 nm wavelength due to the formed large grain size of the pyramidal texture surface. Thus, the obtained GZO films deposited over ZnO buffer layer have high potential for use as front TCO layers in Si-based thin film solar cells. These results could develop the potential way to fabricate TCO based ZnO thin film using MOCVD or sputtering techniques by depositing a low temperature ZnO layer to serve as a template for high temperature GZO film growth. The GZO films exhibited satisfactory optoelectric properties.