Deep Level Emission Research Articles

Electrodeposited ZnO morphology transformations under the influence of SeO2 additive: Rods, disks, nanosheets network

In this study various morphologies of ZnO including rods, disks, and nanosheets network were synthesised by electrodeposition method. The influence of the SeO2 additive concentration on morphological, structural, and optical properties of ZnO was studied by using scanning electron microscopy, X-ray diffractometry (XRD), energy-dispersive X-ray analysis (EDX), optical (UV–Vis) and photoluminescence (PL) spectroscopy methods. The formation of the disk-like ZnO structure with a diameter of ca. 300–500nm and thickness ca. 80–100nm was observed when 0.03mmol/L of SeO2 was added to the solution. Further increase of dopant concentration up to 0.2mmol/L in the solution yielded a highly-porous network structure composed of thin nanosheets with a wall thickness of ca. 20nm. According to the XRD results, all structures were composed of ZnO, rod-like crystals were strongly c-axis oriented, whereas disks and nanosheets network structures were a-axis oriented. EDX analysis revealed that Se was present in the disks and nanosheets structures. PL study of the rod sample showed UV emission at ~370nm, whereas the disks and nanosheets network showed intense deep-level emission band centred at ~540nm. The disks and nanosheets exhibited total transmittance of ∼80–90% in the region of 380–780nm, whereas transmittance of the rods was increasing from 5% to 50% in this region. According to haze factor calculations, samples possessed scattering ability of 20% for disks, 45% for sheets and ca. 50% for rods. The probable mechanism for the formation of ZnO structures was discussed in detail.

Improvement of the optical and photocatalytic properties of ZnSe QDs by growth of ZnS shell using a new approach

In this work ZnSe and ZnSe/ZnS QDs were synthesized via a rapid, room temperature photochemical approach. Zn(Ac)2, Na2SeO3 and TGA were used as precursors. As an UV-sensitive material, TGA acted simultaneously both as the capping agent and S2− source for ZnS shell growth. Crystal structures and optical properties of the QDs were characterized by means of X-ray diffraction (XRD), Fourier transform-infrared spectroscopy (FTIR), Field emission scanning electron microscope (FESEM), Photoluminescence spectroscopy (PL), Energy dispersive X-ray analysis (EDAX), UV-Visible (UV-Vis) spectroscopy. Band-gap of the ZnSe QDs was obtained about 3.45 eV. PL spectra deconvolution obtained three peaks which are related to the emission of band edge, surface states and deep levels. After ZnS shell growth, surface trap states emission was quenched significantly and band edge emission was increased considerably. ZnSe QDs indicated a good activity for Methylene Orange)MO( photo-degradation that it was improved after ZnS shell growth significantly.

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.

Synthesis of ZnO nanorods by one step microwave-assisted hydrothermal route for electronic device applications

In the present work, zinc oxide (ZnO) nanorods (NRs) were prepared by a simple Microwave-assisted hydrothermal synthesis route by using a 100 ml Teflon container. The structural, crystalline nature and purity of the synthesized material was studied by powder X-ray diffraction studies. The optical absorption wavelength observed at 368 nm has been shifted to 377 nm during hydrothermal synthesis of ZnO NRs. The shifting of wavelength reduces the energy band gap from bulk state of 3.52–3.32 eV during the Microwave-assisted hydrothermal process. There is no shift in peak observed in photoluminescence spectrum between ZnO NRs and ZnO nanoparticles (NPs) but increase in temperature increases the Deep Level Emission (DLE) intensity due to the loss of oxygen during the hydrothermal process. The FTIR analysis confirms the presence of various functional groups in the ZnO NRs and NPs. The elemental, shape and surface morphology of the ZnO NRs and NPs were determined by using EDAX and FE–SEM analysis. The surface properties of ZnO NPs and ZnO NRs were studied by BET analysis. Thermal stability of the material was increased from 180 to 277 °C after the hydrothermal synthesis. The zeta potential analysis reveals the stability of the synthesized ZnO nanostructures. The variation of dielectric constant, dielectric loss and ac conductivity with frequency for the various temperatures was measured and discussed the suitability of the material for the electronic device fabrication.

DLTS study of InGaAs and GaAsN structures with different indium and nitrogen compositions

In this article Deep Level Transient Fourier Spectroscopy experiments and various evaluation procedures were used to study emission and capture processes of deep energy levels in intentionally undoped InGaAs and GaAsN semiconductor structures. The examined samples, grown by Atmospheric Pressure Metal Organic Vapour Phase Epitaxy on GaAs substrates, were analyzed at various indium and nitrogen concentrations. Main attention was focused on differences in defect distributions, relations in possible composition and growth condition sensitive defect states. Valuable characteristics of particular In/N contents capable to eliminate or reduce specific impurities are discussed. A possible indium dependent dual state InGaAs complex and a nitrogen and growth condition dependent dual type GaAsN complex was introduced/confirmed. The most balanced samples for further utilizations were achieved for In = 8.9% and N = 1%.

Band gap engineering, enhanced morphology and photoluminescence of un-doped, Ga and/or Al-doped ZnO nanoparticles by reflux precipitation method

In this study, un-doped ZnO, Ga/Al-doped and co-doped ZnO nanoparticles (NPs) at 2mol% Ga doping (GZO), 2mol% Al doping and combined Ga and Al doping concentrations summing up to 2mol% (ranging from 1:1 to 1.9:0.1mol% Ga/Al) were synthesized using the reflux precipitation method. The effects of increasing the mole ratios of Ga in the ZnO host material or reducing Al mol%, on the structural, luminescence and optical properties of impurity-doped ZnO were investigated along with the properties of ZnO NPs. The X-ray diffraction patterns for all samples exhibit highly crystalline NPs with a hexagonal wurtzite structure. The crystallite size and average relative intensity of the diffraction peaks of the samples increased with increasing Ga doping indicating improved crystallinity with an increase in the content of gallium in the ratio. Photoluminescence spectra also displayed an increase in exciton peak emission with increasing Ga doping levels and reached a maximum at a doping concentration of 2.0mol% Ga, as well as a corresponding decrease in the deep level emission intensity, indicative that the best optical quality was obtained at the 2mol% Ga level. It was observed a blue shift in the PL exciton emission peak position and the band edge emission of the reflectance spectra with increasing concentration of Ga in the sample. The optical band gap calculated from the reflectance data also increased with increasing Ga concentration. These GZO NPs with improved crystallinity, minimum lattice stress, and enhanced luminescence intensity and optical properties would be suitable for deposition on a glass substrate to form the seed layer to produce a transparent conducting oxide for a dye- sensitized solar cell based thin films.

Ultra-fast epitaxial growth of ZnO nano/microrods on a GaN substrate, using the microwave-assisted hydrothermal method

Well-aligned ZnO nano/micro-rods with identical crystallographic orientation were synthesized on a c-plane GaN template, using a microwave-assisted hydrothermal method at 50 °C for duration of 2 min. The ZnO nanorods exhibited true epitaxial growth, in contrast to most of the previously reported methods, which involve nucleation on a ZnO buffer layer pre-deposited on the substrate. Homogeneous in-plane alignment as well as the c-axis orientation were confirmed by X-ray diffraction measurements and TEM analysis. More importantly, in the photoluminescence spectra of the nanorods a strong, narrow excitonic emission and an extremely weak deep level emission were observed, indicating high optical quality. The diameter was controlled by adjusting pH of the solution used in the growth process. The main achievement of the research was the opportunity to obtain oriented, high quality ZnO nano/microrods, using a surprisingly quick, cheap, and safe growth process, without the use of toxic substances or ultra-high purity compounds.

Fabrication of n-ZnO/p-Si (100) and n-ZnO:Al/p-Si (100) Heterostructures and Study of Current–Voltage, Capacitance–Voltage and Room-Temperature Photoluminescence

Heterojunction diodes of n-ZnO/p-Si (100) and n-ZnO:Al/p-Si (100) were fabricated by spray pyrolysis technique. X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDX), and field emission scanning electron microscopy (FESEM) were used to characterize the as-prepared samples. The XRD pattern indicates the hexagonal wurzite structure of zinc oxide (ZnO) and Al-doped ZnO (AZO) thin films grown on Si (100) substrate. The compositional analysis by EDX indicates the presence of Al in the AZO structure. The FESEM image indicates the smooth and compact surface of the heterostructures. The current–voltage characteristics of the heterojunction confirm the rectifying diode behavior at different temperatures and illumination intensities. For low forward bias voltage, the ideality factors were determined to be 1.24 and 1.38 for un-doped and Al-doped heterostructures at room temperature (RT), respectively, which indicates the good diode characteristics. The capacitance–voltage response of the heterojunctions was studied for different oscillation frequencies. From the 1/C 2–V plot, the junction built-in potentials were found 0.30 V and 0.40 V for un-doped and Al-doped junctions at RT, respectively. The differences in built-in potential for different heterojunctions indicate the different interface state densities of the junctions. From the RT photoluminescence (PL) spectrum of the n-ZnO/p-Si (100) heterostructure, an intense main peak at near band edge (NBE) 378 nm (3.28 eV) and weak deep-level emissions (DLE) centered at 436 nm (2.84 eV) and 412 nm (3.00 eV) were observed. The NBE emission is attributed to the radiative recombination of the free and bound excitons and the DLE results from the radiative recombination through deep level defects.

Properties of transparent conducting tin monoxide(SnO) thin films prepared by chemical spray pyrolysis method

Transparent conducting Stannous Oxide (SnO) thin films were obtained by chemical spray pyrolysis method on glass substrates for 0.1 M and 0.25 M concentration of precursor solutions. Their structural, morphological, optical and electrical properties were investigated. X-ray diffraction (XRD) study shows polycrystalline nature of the films with orthorhombic crystal structure. The morphological analysis was carried out by Scanning electron microscopy (SEM) and elemental analysis was done by Energy dispersive X-ray spectroscopy (EDX). The band gap of 0.1 M and 0.25 M thin film samples were found to be 3.58eV with 82% transmission and 3 eV with 30% transmission respectively. The film thickness, refractive index (n) and extinction coefficient (k) of the films were obtained by ellipsometric technique. Hall effect measurements reveal p-type conduction with mobility 7.8 cm2V−1s−1 and 15 cm2V−1s−1 and conductivity of 8.5 S/cm and 17.1 S/cm respectively for the 0.1 M and 0.25 M samples. Photoluminescence (PL) spectrum of the samples show a broad emission which covers near band edge (NBE) as well as deep level emission (DLE) in the region 380 nm-620 nm.

Ultraintense UV emission from ZnO-sheathed ZnS nanorods

Short-wavelength luminescence is essential for high-performance optoelectronic device applications. There have been efforts to obtain intense ultraviolet (UV) emission by encapsulating ZnO one-dimensional (1D) nanostructures with materials such as ZnS. However, the encapsulation of ZnS 1D nanostructures with ZnO has not been reported. In this paper, we report ultraintense UV emission from ZnS nanorods coated with ZnO, i.e., ZnS-core/ZnO-shell nanorods. UV emission from the ZnS-core/ZnO-shell nanorods was much more intense than that obtained from the extensively studied ZnO-core/ZnS-shell nanorods. The highest intensity of the near-band-edge emission from the ZnS-core/ZnO-shell nanorods was obtained with a ZnO shell layer thickness of 35 nm, which is ∼16 times higher than that of pristine ZnS nanorods. Moreover, the deep level (DL) emission was suppressed completely. The substantial enhancement of the UV emission from the ZnS nanorods and the complete suppression of the DL emission by ZnO sheathing can be rationalized by combining the following four effects: the reinforcement of the UV emission by the overlap of the UV emissions from the ZnS core and ZnO shell, enhancement of the emission from the ZnO shell by the carrier transfer from the ZnS core to the ZnO shell, suppression of the capture of carriers by the surface states on the ZnS surface, and suppression of the visible emission and nonradiative recombination in ZnS.

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.

Effects of the ZnO layer on the structure and white light emission properties of a ZnS:Mn/GaN nanocomposite system.

ZnO films were inserted between the ZnS:Mn films and GaN substrates by pulsed laser deposition (PLD). The structure, morphology, and optical properties of the ZnS:Mn/ZnO/GaN nanocomposite systems have been investigated. X-ray diffraction results show that there are three diffraction peaks located at 28.4°, 34.4°, and 34.1°, which correspond to the β-ZnS(111), ZnO(002), and GaN(002) planes, respectively. Due to the insertion of ZnO films, the diffraction peak intensity of ZnS:Mn in ZnS:Mn/ZnO/GaN is stronger than that of ZnS:Mn in ZnS:Mn/GaN, and the full width at half-maximum is smaller. Though the transmittance of ZnS:Mn/ZnO films is slightly lower than that of ZnS:Mn films, the transmittance is still higher than 80%. Compared with ZnS:Mn/GaN, an ultraviolet (UV) emission at 387nm (originated from the near-band emission of ZnO) and a green light emission at about 520nm appeared in the photoluminescence (PL) spectra of ZnS:Mn/ZnO/GaN, in addition to the blue emission at 435nm and the orange-red emission at 580nm. The emission at 520nm may be related to the deep-level emission from ZnO and the interface of ZnS:Mn/ZnO.The PL spectrum of ZnS:Mn/ZnO/GaN covers the visible region from the blue light to the red light (400-700nm), and its color coordinate and color temperature are (0.3103,0.3063) and 6869K, respectively, presenting strong white light emission.

Multi-angle ZnO microstructures grown on Ag nanorods array for plasmon-enhanced near-UV-blue light emitter

Metal enhanced ultraviolet light emission has been explored in ZnO/Ag hybrid structures prepared by hydrothermal growth of multi-angled ZnO nanorods on slanted Ag nanorods array fabricated by the thermal evaporation technique. Slanted Ag nanorods are realized to be the stacking of non-spherical Ag nanoparticles, resulting in asymmetric surface plasmon resonance spectra. The surface roughness of Ag nanorod array films significantly influences the growth mechanism of ZnO nanorods, leading to the formation of multi-angled ZnO microflowers. ZnO/Ag hybrid structures facilitate the interfacial charge transfer from Ag to ZnO with the realization of negative shift in binding energy of Ag 3d orbitals by ∼0.8 eV. These high quality ZnO nanorods in ZnO/Ag hybrid nanostructures exhibit strong ultraviolet emission in the 383–396 nm region without broad deep level emission, which can be explained by a suitable band diagram. The metal enhanced photoluminescence is witnessed mainly due to interfacial charge transfer with its dependence on surface roughness of bottom layer Ag nanorods, number density of ZnO nanorods and diversity in the interfacial area between Ag and ZnO nanorods. The existence of strong ultraviolet light with minor blue light emission and appearance of CIE shade in strong violet-blue region by ZnO/Ag hybrid structures depict exciting possibilities towards near UV-blue light emitting devices.

Strong Deep-Level-Emission Photoluminescence in NiO Nanoparticles.

Nickel oxide is one of the highly promising semiconducting materials, but its large band gap (3.7 to 4 eV) limits its use in practical applications. Here we report the effect of nickel/oxygen vacancies and interstitial defects on the near-band-edge (NBE) and deep-level-emission (DLE) in various sizes of nickel oxide (NiO) nanoparticles. The ultraviolet (UV) emission originated from excitonic recombination corresponding near-band-edge (NBE) transition of NiO, while deep-level-emission (DLE) in the visible region due to various structural defects such as oxygen vacancies and interstitial defects. We found that the NiO nanoparticles exhibit a strong green band emission around ~2.37 eV in all samples, covering 80% integrated intensity of PL spectra. This apparently anomalous phenomenon is attributed to photogenerated holes trapped in the deep level oxygen vacancy recombining with the electrons trapped in a shallow level located just below the conducting band.

Surface morphology evolution with fabrication parameters of ZnO nanowires toward emission properties enhancement

ZnO nanowire (NW) arrays were successfully grown on pre-seeded indium Tin oxide (ITO)-coated glass substrate using hydrothermal synthesis. Herein, the effects of ZnO seed layer density on the performance of ZnO NWs were investigated in details. The orientation and the dimension of ZnO NWs were found to depend on seeded substrate density as shown by scanning electron microscopy (SEM) micrographs which revealed that the typical morphology with the most uniform size can be obtained. From the X-ray diffraction (XRD) measurement, it can be seen that hexagonal c -axis oriented NWs were grown. The resonant Raman scattering was also investigated in details. It confirmed the wurtzite structure of the NW arrays and expected for good optical properties. The optical band gaps of synthesized ZnO NWs were found to decrease comparing to bulk ZnO and as function of seed layer. The photoluminescence (PL) spectra at room temperature have shown strong UV excitonic emission and weak deep-level emissions which reveal that the as-grown NW arrays have good optical properties with limited deep-level defects.

A study of the red-shift of a neutral donor bound exciton in GaN nanorods by hydrogenation

In this paper we account for the physics behind the exciton peak shift in GaN nanorods (NRs) due to hydrogenation. GaN NRs were selectively grown on a patterned Ti/Si(111) substrate using plasma-assisted molecular beam epitaxy, and the effect of hydrogenation on their optical properties was investigated in detail using low-temperature photoluminescence measurements. Due to hydrogenation, the emissions corresponding to the donor–acceptor pair and yellow luminescence in GaN NRs were strongly suppressed, while the emission corresponding to the neutral to donor bound exciton (D0X) exhibited red-shift. Thermal annealing of hydrogenated GaN NRs demonstrated the recovery of the D0X and deep level emission. To determine the nature of the D0X peak shift due to hydrogenation, comparative studies were carried out on various diameters of GaN NRs, which can be controlled by different growth conditions and wet-etching times. Our experimental results reveal that the D0X shift depends on the diameter of the GaN NRs after hydrogenation. The results clearly demonstrate that the hydrogenation leads to band bending of GaN NRs as compensated by hydrogen ions, which causes a red-shift in the D0X emission.

Influence of the sol gel synthesis parameters on the photoluminescence properties of ZnO nanoparticles

In this work, ZnO NPs were successfully synthesized by the sol–gel method without any organic additives or post annealing. The effect of the preparation process on the structural and optical properties of the resulting NPs was investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. The structural characterization demonstrated clearly that the NPs crystallize in pure ZnO würtzite structure without any other secondary phases. Furthermore, we show that it is possible to perform the control of the crystalline growth orientation of ZnO NPs, which is a key parameter when seeking to develop ZnO NPs with piezoelectric properties for nano-transducer applications. In fact, TEM observations show that the reduction of the NaOH flow changes the NPs shape from hexagonal NPS to short nanorods grown along the c-axis. The PL spectra of the obtained NPs excited at 280nm, present an UV emission centered at approximately 380nm with a slight shift when varying the synthesis temperature and/or the NaOH flow. Moreover, as the visible region (from 400 to 650nm) is concerned, it was shown that the increasing of the synthesis temperature affects strongly the kind of interstitial defects (Oi, Zni and VoZni) formed in ZnO nanostructures. However, the excitation at 320nm revealed a broad deep-level emission for all the samples that can be deconvoluted into two Gaussian peaks centered at 514nm (P1) and 581nm (P2). These last results have been discussed in the light of a physical mechanism based on the Schottky barrier.

Role of silver doping on the defects related photoluminescence and antibacterial behaviour of zinc oxide nanoparticles

The Ag doped ZnO (ZnO:Ag) NPs with a hexagonal wurtzite structure were synthesized by a solution combustion method. X-ray absorption near edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) were used to study the defects, local electronic and atomic structures before and after Ag doping. XPS and XANES studies confirmed the deficiency of concentration of defects in ZnO after Ag doping. The photoluminescence study showed the deep level emission in the orange-red region in addition to the band to band emission. It was also found that the defect related emission of ZnO was decreased with an increasing in Ag concentration. The antibacterial behaviour of ZnO and ZnO:Ag NPs was studied against the gram positive and gram negative bacteria. The role of Ag doping and defects in the ZnO NPs were discussed for the observed antibacterial and photoluminescence behaviour.

Structural and optical characterization of (Sn/Li) co-doped ZnO thin films deposited by spray pyrolysis technique

(Sn/Li) co-doped ZnO (LTZO) thin films have been deposited by spray pyrolysis technique and their structural, morphological and optical properties have been investigated. The films were characterized by X-ray diffractometer (XRD), field-emission scanning electron micrograph (FESEM), (UV–Vis) spectroscopy and photoluminescence spectroscopy. XRD results revealed that all thin films (LTZO) are polycrystalline with a hexagonal wurtzite structure, moreover when the Li reach (5 at.%) content, the surface thin films of (LTZO5) become covered with bigger and clear nano-sized crystallites and the average value of grain size is 147 nm with mainly hexagonal grains. The (UV–Vis) spectroscopy showed high transmittance in the visible region, which varied between 84% and 93% and the doping of 5 at.% for all the films of Li content thin films (LTZO5) red shifted. The doping ZnO with lithium and tin were eliminates the deep-level emission in orange/red bands (around to 600 nm).

Effects of precursor concentrations on the optical and morphological properties of ZnO nanorods on glass substrate for UV photodetector

In this study, zinc oxide (ZnO) nanorod arrays with various precursor concentrations (0.05, 0.10 and 0.15 M) were synthesized using a simple hydrothermal reaction on ZnO seeds/glass substrate and tested for UV-photodetector. The structural and optical properties of the ZnO nanorods have been investigated. The results demonstrate that morphology and crystallinity of ZnO nanorods influenced by the concentration of the precursor and had a high impact on the UV-photosensing. As the precursor concentration increased by 0.1 M, the density and the surface-to-volume ratio of ZnO nanorods decreased by 49 nanorods per μm2 and 22.9 respectively. At the lowest concentration of 0.05 M, the ZnO nanorods grew with high density of 85 nanorod per μm2 and high surface-to-volume ratio of 28.5. Moreover, a strong ultraviolet (UV) emission peak (λ = 375 nm) was observed with a low deep-level emission peak (λ = 580 nm), which indicated high optical property and crystallinity of the nanorods. The synthesized ZnO nanorods exhibited different sensing characteristics that correlated with the morphological and structure of ZnO nanorods formed at different concentrations. The sample grew with concentration of 0.05 M showed the highest photocurrent of 1.949 × 10−4 A and current gain of 562.1 under low power intensity UV illumination at λ = 375 nm with 5 V bias.