Defect Related Deep Level Emission Research Articles

Enhancement of UV emission and optical bandgap of ZnO nanowires via doping and post-growth annealing

Zinc oxide (ZnO) nanowires (NWs) are gaining importance in optoelectronics because of their excellent electrical and optical properties. However, defects in the NW structure leads to suppression of the near-band-edge (NBE) ultraviolet (UV) emission, limiting their full potential for applications in optoelectronic devices. In this work, we present enhancement in NBE emission and bandgap modulation in ZnO NWs hydrothermally grown on sputtered Al-ZnO (AZO) films. Al dopant incorporation and post-synthesis thermal annealing are found to increase the NBE emission. Compared to Al-doping, the post-growth annealing produces a more significant enhancement in the NBE emission and a substantial increase of 34.5 meV in the optical bandgap, along with suppression of defect-related deep level emissions caused by oxygen vacancies and interstitials. This further improves the applicability of the ZnO NWs in stable, room temperature emission devices.

Enhancement of the deep-level emission and its chemical origin in hexagonal boron nitride

Defect-related deep-level emissions at ∼4 eV from hexagonal boron nitride (h-BN) have been extensively investigated during the past decades. Although the emission has often been associated with deep level impurities such as carbon and oxygen, the structural and chemical origins of the emission center have not yet been identified. Here, we perform systematic photoluminescence measurements and quantitative trace impurity analysis of variously heat-treated h-BN samples with different deep-level emission intensities. In contrast to the common belief, no positive correlation between the impurity (carbon and oxygen) concentration and the deep-level emission intensity was found. We also demonstrate that the intensity of the deep-level emission is significantly enhanced by high-temperature heating (∼1900 °C) under an N2 environment and subsequent post-annealing at ∼900 °C. These results suggest that intrinsic defects created under N2-rich conditions are responsible for the deep-level emission. Furthermore, we found that besides the well-recognized zero-phonon line (ZPL) at ∼4.1 eV, additional ZPLs are induced in the near ultraviolet (NUV) region by appropriate annealing of the N2-treated h-BN sample. The results of temperature-dependent and time-resolved PL measurements on these NUV emission bands are given and discussed.

Photoluminescence of Atomic Layer Deposition Grown ZnO Nanostructures

Atomic Layer Deposition (ALD) technique was used to grow crystalline, nanostructured Zinc Oxide (ZnO) thin films, having very low surface roughness, on silicon (Si) and fused quartz (SiO2) substrates. The film thickness and structural properties were characterized by using X-ray reflectivity and X-ray diffraction techniques. The excitonic light emission of ZnO thin films wasstudied by means of photoluminescence (PL) measurements in the ultraviolet-visible spectral region. Characteristic near band edge emission (NBE) of the ZnO thin films corresponding to free excitonic recombination shows strong signal in the UV range at room temperature. Visible region of the PL spectra of ZnO nanostructures shows that there is no defect related deep level emission (DLE), suggesting the absence of defect states. The results confirm the growth of ZnO thin films of a high quality crystalline structure by ALD technique.

ALD Grown ZnO on Polymeric Template

ZnO thin film was grown on polystyrene template using atomic layer deposition (ALD) technique. The synthesized thin film was characterized using X‐ray reflectivity, X‐ray diffraction, and photoluminescence. X‐ray reflectivity confirms the growth of ZnO thin film with bulklike electron density, on top of polystyrene film without causing any perturbation in the underneath polymeric template. The X‐ray diffraction result reveals the crystalline structure of the ALD grown ZnO. Photoluminescence (PL) study of ZnO thin film on polymeric template shows the presence of strong characteristic near band edge emission (NBE) in the UV range along with a defect related deep level emission (DLE) in the visible region. The results confirm the growth of good quality crystalline ZnO thin film on polymeric template maintaining a sharp interface in metal‐oxide/polymer hybrid structure.

Defect-induced excitonic recombination in TixZn1−xO thin films grown by DC-unbalanced magnetron sputtering

We study the effects of Ti doping on the near-band-edge emission (NBE) and defect-related deep-level emission (DLE) of ZnO thin films grown by DC unbalanced magnetron sputtering. DLE in pure ZnO is contributed by zinc and oxygen vacancies (VZn+VO), as revealed by photoluminescence (PL) spectroscopy, current–voltage (I–V) characteristic measurement, and spectroscopic ellipsometry. The reduction in the number of VZn states is clearly observed upon Ti doping, resulting in the enhancement of green emission from VO. Interestingly, the thin film with a Ti concentration of 1 at. % shows a higher excitonic emission. Furthermore, the temperature dependence of PL spectra shows that the enhanced excitonic emission originates from the donor-bound exciton promoted by the Ti dopant and native VO. This study shows an important role of the defects in controlling the optical and electronic properties of ZnO films for future optoelectronic applications.

Spin-coated Al-doped ZnO thin films for optical applications: Structural, micro-structural, optical and luminescence studies

Zn1−xAlxO (x = 0, 0.01, 0.02, 0.03) thin films were deposited on glass substrate using sol-gel spin coating method and the effect of Al doping on their structural, micro-structural, optical and photoluminescence properties were investigated. The XRD results confirm the single phase nature and the successful substitution of Al at Zn site in ZnO thin films. Further with Al doping, the crystallite size, lattice parameters and film thickness were found to decrease. Also, both AFM and SEM results confirm the formation of spherical grains with lower roughness of the films. Al doped (>2%) films shows ∼ 90% transparency in the visible wavelength region. Widening of band gap (3.18–3.34 eV) was observed with increase in Al doping concentration, which was attributed to the Burstein–Moss band-filling effect. The defect induced luminescence at room temperature was studied by photoluminescence spectroscopy. PL spectra of AZO films show sharp peaks in the visible wavelength region. A detailed analysis of photoluminescence of undoped and Al doped ZnO films indicates the inhibition of defect density of electronic states due to substitution of Al3+. The observed emission peaks were attributed to the defect related deep-level emissions. Results suggest possible use of Al doped ZnO films for transparent conducting oxide (TCO) electrodes and light emitting diodes (LED's) applications.

Wide-Range Thermometry at Micro/Nano Length Scales with In2O3 Octahedrons as Optical Probes.

We report the temperature-dependent photoluminescence and Raman spectra of In2O3 octahedrons synthesized by an evaporation-condensation process. The luminescence obtained here is due to the defect-related deep level emission, which shows highly temperature-dependent behavior in 83-573 K range. Both the position as well as the intensity varies with temperature. Similarly, Raman spectroscopy in 83-303 K range shows temperature-dependent variation in peak intensity but no change in the peak position. Interestingly, the variation of intensity for different peaks is consistent with Placzek theory which invokes the possibility of temperature sensing. We demonstrate the reversibility of peak intensity with temperature for consecutive cycles and excellent stability of the octahedrons toward cryogenic temperature sensing. Overall, both the temperature-dependent photoluminescence and Raman spectra can be explored to determine temperature in the cryogenic range at micro/nano length scales. As an example, we evaluate the temperature-dependent Raman spectra of WO3 that undergoes a phase transition around 210 K and temperature-dependent luminescence of Rhodamine 6G (Rh6G) where intensity varies with temperature.

Photoluminescence Properties of CdS/ZnO Nano-Composites Structure

Aiming to modification the surface optical properties, CdS/ZnO composites were synthesized by hydrothermal method. The ZnO/CdS heterostructures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), photoluminescence (PL) measurements at room temperature and the temperature-dependent photoluminescence treatment. In the room temperature PL spectra, the defect-related deep level emission can effectively suppress under optimal CdS deposition condition. With the increasing of CdS deposition time to 150 min, the donor bound exciton (D0X) peak showed a significantly red shift at low temperature (77K) PL spectrum. Through analysis the temperature-dependent photoluminescence spectra, we find that it was caused by acceptor bound exciton (A0X). Meanwhile, there appeared a donor-acceptor pair related emission at lower-energy side of the spectrum. The experiment demonstrated that the surface donor defects can effectively suppresse after CdS nanoparticles deposition on ZnO nanorods.

Al Doped ZnO층 적용을 통한 ZnO 박막 트랜지스터의 전기적 특성과 안정성 개선

Recently, ZnO based oxide TFTs used in the flexible and transparent display devices are widely studied. To apply to OLED display switching devices, electrical performance and stability are important issues. In this study, to improve these electrical properties, we fabricated TFTs having Al doped Zinc Oxide (AZO) layer inserted between the gate insulator and ZnO layer. The AZO and ZnO layers are deposited by Atomic layer deposition (ALD) method. I-V transfer characteristics and stability of the suggested devices are investigated under the positive gate bias condition while the channel defects are also analyzed by the photoluminescence spectrum. The TFTs with AZO layer show lower threshold voltage () and superior sub-threshold slop. In the case of shift after positive gate bias stress, the stability is also better than that of ZnO channel TFTs. This improvement is thought to be caused by the reduced defect density in AZO/ZnO stack devices, which can be confirmed by the photoluminescence spectrum analysis results where the defect related deep level emission of AZO is lower than that of ZnO layer.

Effects of growth conditions on the microstructures and photoluminescence properties of Ga-doped ZnO films

ZnO has a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, which is recognized as one of the promising semiconductors for optoelectronic device applications. However, ZnO generally displays visible defect-related deep-level emission and/or UV near-band-edge emission, which is strongly dependent on the growth method and condition. It has been reported that doping with IIIA elements can improve the optical properties of ZnO. Among them, Ga doping is considered not to induce large lattice distortion of ZnO due to the fact that the bonding lengths of Ga-O and Zn-O are similar and ionic radii of Ga3+ and Zn2+ are also similar. The gallium related compounds such as triethylgallium, gallium nitrate and gallium oxide are used as the Ga doping sources. It has been proved that ZnO film can be grown directly by the thermal oxidation of ZnS substrate. In this research, the Ga doping is adopted in the growth of ZnO film by applying the molten gallium to the surface of ZnS substrate and performing the subsequent thermal oxidation in the air at 650 and 700 °C for 3 and 8 h, respectively. The effects of growth condition on the microstructures and photoluminescence properties of the Ga-doped ZnO film are investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and photoluminescence at room temperature. In addition, the relationship among the oxidation temperature, oxidation time, Ga doping content and photoluminescence properties is discussed. The results show that the Ga-doped ZnO films grown under different growth conditions exhibit various amounts of Ga content and the gallium is present in the ZnO matrix as Ga3+ by partially substituting Zn2+. The Ga doping affects the microstructure and photoluminescence property by changing the defect type and level, stoichiometric ratio, and crystal quality of ZnO film. As the oxidation temperature increases, the amount of Ga doping content increases. In addition, the grain size of the Ga-doped ZnO film increases and becomes uniform, and the ratio of ultraviolet emission intensity to visible emission intensity increases. However, as the oxidation time increases, the amount of Ga doping content decreases, the grain size of the Ga-doped ZnO film becomes non-uniform, and the ratio of ultraviolet emission intensity to visible emission intensity decreases.

ZnO films on transferable and low thermal resistance graphite substrate grown by ultrasonic spray pyrolysis

ZnO thin films were deposited on graphite substrates by ultrasonic spray pyrolysis method with Zn(CH3COO)2·2H2O aqueous solution as precursor. The crystalline structure, morphology, and optical properties of the as-grown ZnO films were investigated systematically as a function of deposition temperature and growth time. Near-band edge ultraviolet (UV) emission was observed in room temperature photoluminescence spectra for the optimized samples, yet the usually observed defect related deep level emissions were nearly undetectable, indicating that high optical quality ZnO thin films could be achieved via this ultrasonic spray pyrolysis method. Considering the features of transferable and low thermal resistance of the graphite substrates, the achievement will be of special interest for the development of high-power semiconductor devices with sufficient power durability.

Room temperature ferromagnetism and optical properties of Cu2+ doped ZnO nanopowder by ultrasound assisted solid state reaction technique

Cu2+ doped ZnO nanopowder was synthesized with the assistance of sonochemical method. Powder X-ray diffraction data confirms the prepared powder exhibit hexagonal wurtzite structure. From optical and EPR studies the Cu2+ ion occupied in tetragonally distorted octahedral site symmetry. The photoluminescence (PL) spectrum at room temperature shows a strong ultraviolet emission (~378) and a strong, dominated defect related deep level emission (~435) in visible regions. Vibrational bands related to metal oxide bonding are evident form FT-IR studies. The magnetic measurement of the prepared sample shows good ferromagnetic property at room temperature.

Structural and optical properties of sol–gel synthesised Zn1−xMgxO nanocrystals

Zn1−xMgxO nanocrystals with various compositions (x=0–0.1) were synthesised by sol–gel process using tri ethanol amine (TEA) as a capping agent. The structural properties of the prepared materials were studied by X-ray diffraction analysis. Un-agglomerated nanocrystals with spherical morphology were observed by scanning electron microscope (SEM). The presence of Mg in the Zn1−xMgxO was confirmed by X-ray photoelectron spectroscopy (XPS). Moreover, the Mg composition relatively increases in the synthesised nanocrystals as it increases in the precursor solution. The optical absorption studies of the Zn1−xMgxO samples show the blue shift from the pure ZnO due to the incorporation of Mg in ZnO lattice. The photoluminescence studies demonstrated that the intensity of defect related deep level emission increases drastically with increasing the x value from 0.02 to 0.08 and decreases sharply on further increase of Mg from 0.08 to 0.1. The mechanism for the enhanced green emission was explained based on the structural properties of the Zn1−xMgxO samples.

Intrinsic photoluminescence from low temperature deposited zinc oxide thin films as a function of laser and thermal annealing

An investigation into the modification of low temperature deposited ZnO thin films by different annealing processes has been undertaken using laser, thermal and rapid thermal annealing of 60 nm ZnO films deposited by high-target-utilization sputtering. Single-pulse laser annealing using a KrF excimer laser (λ = 248 nm) over a range of fluences up to 318 mJ cm−2 demonstrates controlled in-depth modification of internal film microstructure and luminescence properties without the film degradation produced by high temperature thermal and RTA processes. Photoluminescence (PL) properties show that the ratio of defect related deep level emission (DLE, 450–750 nm, 2.76–1.65 eV) to excitonic near band-edge emission (NBE at 381 nm, 3.26 eV) is directly correlated to processing parameters. Thermal and rapid thermal processing results in the evolution of a strong visible orange/red DLE PL (with peaks at 590 nm, 2.10 eV and 670 nm, 1.85 eV) dominated by defects related to excess oxygen. At higher temperatures, the appearance of a green/yellow emission (530 nm, 2.34 eV) indicates a transition of the dominant radiative transfer mechanism. In contrast, laser processing removes defect related DLE and produces films with intense NBE luminescence, correlated to the observed formation of large grains (25–40 nm).

High optical quality ZnO films grown on graphite substrate for transferable optoelectronics devices by ultrasonic spray pyrolysis

ZnO thin films were deposited on graphite substrates by ultrasonic spray pyrolysis method. The effects of substrate temperature and film thickness on the crystalline structure, morphology, and optical properties of the as-grown ZnO films were investigated systemically. Results illustrated that dense ZnO films with hexagonal wurtzite structure were uniformly distributed on the substrate. Strong near-band edge ultraviolet (UV) emission peaks were observed in room temperature photoluminescence (PL) spectra for the samples prepared under optimized parameters, yet the usually observed defect related deep level emissions were nearly undetectable, indicating high optical quality ZnO films could be achieved via this easy process under optimal conditions. The successful growth of polycrystalline ZnO films on graphite offers the significant opportunity to be readily transferred onto any rigid or flexible foreign substrates, since the graphite substrates consist of weakly bonded layer structure.

Study of ZnO:Zn Phosphors Prepared by Sol-gel and Ionimplantation Techniques

ZnO thin films were firstly deposited on glass substrates by the sol-gel process, and then Zn ions with energy of 56 keV were implanted into the ZnO films to a dose of 1×1017 cm-2. The effects of ion implantation on the structural and optical properties of the ZnO films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), photoluminescence (PL) and optical transmittance measurements. The results showed that the as-deposited films had hexagonal wurtzite structure with high c-axis preferred orientation and (002) peak disappeared after ion implantation. Both the near band edge (NBE) excitonic UV emission at 390 nm and the defect related deep level emission (DLE) centered at 470 nm in the visible region were greatly decreased after Zn ion implantation. The optical band gap redshifted from 3.10 to 3.04 eV after ion implantation.

Effect of surface morphology of metallic zinc films deposited by ion beam sputter deposition on the formation of ZnO nanowires

Metallic zinc film with various surface roughnesses was deposited on Si (100) substrates by ion beam sputter deposition utilizing beam energies at 8, 12 and 16 keV. The surface roughness of the metallic zinc film increased as ion beam energy increased and was found to act as a crucial factor for the formation of ZnO nanowires by subsequent thermal oxidation. ZnO nanowires with diameters of ∼30 nm and average length of ∼1 μm were obtained from 12 to 16 keV ion beam deposited samples while no ZnO nanowires were found on 8 keV ion beam deposited samples. Photoluminescence study of ZnO nanowires exhibits a strong UV emission at 377.2 nm (3.287 eV) with a full-width at half maximum of 95.0 meV and negligible defect related deep level emission. The ZnO nanowires are grown along the [110] direction and the growth mechanism is likely due to a solid state based-up diffusion process. Field-emission measurement shows a turn-on field of 7.9 MV/m and a field enhancement factor β of 691 is achieved.

Effect of annealing on structural and optical properties of zinc oxide films

Nanocrystalline ZnO thin films were prepared using modified liquid flow deposition method (LF method). The effects of annealing in temperature range of 350–550 °C on the structural and optical properties of the ZnO films have been studied. The structure, surface morphology, chemical composition and optical properties of the films were determined using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis using X-rays (EDAX), UV–vis–NIR and photoluminescence (PL) spectroscopy. X-ray diffraction measurements showed that the annealed ZnO films were polycrystalline with (1 0 0), (0 0 2) and (1 0 1) oriented crystallites of hexagonal wurtzite structure and full width half maximum (FWHM) of (0 0 2) XRD peak is observed to decrease as annealing temperature increases above 350 °C. The film annealed at 550 °C possesses c-axis oriented hexagonal wurtzite ZnO nanoparticles. The crystallinity as well as grain size of the films was found to increase on post-annealing. SEM micrographs of the films reveal the presence of hexagonal shaped nanoparticles and EDAX measurements confirm the removal of impurities after annealing treatment. The optical band gap of ZnO films initially blue shifted (3.39–3.41 eV) when annealed at 350 °C and a red shift (3.41–3.32 eV) was observed above the annealing temperature 350–550 °C. Room temperature photoluminescence (PL) measurement of the films showed UV and defect related deep level emissions including blue, green and yellow emissions in the visible region. The possible mechanisms of these emissions were studied in detail.

The structure，optical and electrical properties of Li-N dual-acceptor doped p-type ZnO thin films prepared by sol-gel method

采用溶胶-凝胶法在n型Si(100)衬底上沉积Li-N双掺杂ZnO薄膜，经X射线衍射和扫描电镜图片分析，所制备薄膜具有多晶纤锌矿结构和高的c轴择优取向.室温下霍尔效应测试结果显示Li-N双掺杂ZnO薄膜具有p型导电特性.在Li掺杂量为15.0at%，Li/N（摩尔比）为1∶1，700℃退火等优化条件下得到的最佳电学性能结果是：电阻率为0.34 Ω·cm，霍尔迁移率为16.43 cm2/V·s，载流子浓度为2.79×1019 cm-3</s

Controllable growth of well-aligned ZnO nanorod arrays by low-temperature wet chemical bath deposition method

Well-aligned ZnO nanorod arrays were synthesized by low-temperature wet chemical bath deposition (CBD) method on Si substrate under different conditions. Results illustrated that dense ZnO nanorods with hexagonal wurtzite structure were vertically well-aligned and uniformly distributed on the substrate. The effects of precursor concentration, growth temperature and time on nanorods morphology were investigated systematically. The mechanism for the effect of preparation parameters was elucidated based on the chemical process of CBD and basic nucleation theory. It is demonstrated that the controllable growth of well-aligned ZnO nanorods can be realized by readily adjusting the preparation parameters. Strong near-band edge ultraviolet (UV) emission were observed in room temperature photoluminescence (PL) spectra for the samples prepared under optimized parameters, yet the usually observed defect related deep level emissions were nearly undetectable, indicating high optical quality ZnO nanorod arrays could be achieved via this easy process chemical approach at low temperature.