Improvement Of Crystalline Quality Research Articles

Effect of annealing on photoluminescence and Raman scattering of Sb-doped ZnO epitaxial layers grown on a-Al2O3

ZnO thin films have been realized via monodoping antimony (Sb) acceptor by Molecular Beam Epitaxy on a-Al2O3 substrates. The influence of annealing on photoluminescence (PL) and micro-Raman spectra of ZnO:Sb layers was analyzed. PL spectra recorded at 6 K are dominated by two peaks: at around 3.31 eV and 3.35 eV. First one is assigned to free electrons to neutral acceptors (FA) transition and the second peak corresponds to donor bound exciton (D0X). In order to properly identify and analyze Raman modes, the room temperature Stokes Raman spectra were compared with the spectra of similar ZnO:As/Al2O3 structures. In addition to regular ZnO and Al2O3 modes, recorded ZnO:Sb Raman spectra contain 3 additional modes at around 509, 532 and 575 cm−1 that are associated with antimony doping. The intensity of doping-related bands decreases with increasing annealing temperature, which indicates the improvement of crystalline quality of the structure. The spectra also reveal the presence of compressive in-plane strain that decreases in value with increasing annealing temperature and is due to thermal residual stress, resulting from differences in thermal expansion coefficients of ZnO and Al2O3

Induced Photonic Response of ZnO Nanorods Grown on Oxygen Plasma-Treated Seed Crystallites.

We examined the influence of O2 plasma treatment for the ZnO seed layer (SL) crystallites on the material characteristics of ZnO nanorods (NRs) synthesized by the hydrothermal method. Diode photocurrent and photo-response transient characteristics of the p-Si/n-ZnO-NR heterojunction-based ultraviolet (UV) photodetectors were also examined according to the plasma treatment for the SLs. The superior optical properties of NRs were measured from the photoluminescence by exhibiting 4.6 times greater near-band edge emission when grown on the O2-plasma-treated SL. The degree of (002) orientation of the NR crystals was improved from 0.67 to 0.95, as revealed by X-ray diffraction analysis, and a higher NR surface density of ~80 rods/μm2 with a smaller mean diameter of 65 nm were also observed by the SL modification using plasma-treatment. It was shown by X-ray photo-electron spectroscopy that this improvement of NR crystalline quality was due to the recovery of stoichiometric oxygen with significant reduction of oxygenated impurities in the SL crystals and the subsequent low-energy growth mode for the NRs. UV PDs fabricated by the proposed SL plasma treatment technique showed significantly enhanced UV-to-dark current ratio from 2.0 to 83.7 at a forward bias of +5 V and faster photo-response characteristics showing the reduction in recovery time from 16 s to 9 s.

Characterization of single step electrodeposited Cu2ZnSnS4 thin films

Electrodeposited copper zinc tin sulfide (CZTS) thin films grown on conducting glass substrates are investigated in this report. The photoelectrochemical cell (PEC), optical, structural and morphological properties of the deposited films have been characterized. PEC measurements of the CZTS thin film showed p-type electrical conductivity. Optical measurement showed that the transmittance of CZTS films is observed to about 0.5–3.4% in the wavelength range 300–1100 nm. It is also observed that the absorbance of the CZTS thin films rapidly increases in the wavelength range 580–620 nm and then it decreases slowly. The band gap of the CZTS thin film is observed to be in the range 1.6–2.2 eV. The transmittance and band gap decreases upon annealing at different temperatures but the absorbance and the grain size increases due to the improvement of crystalline quality upon annealing. From the X-ray diffraction study, the CZTS films are found to be polycrystalline with (112), (200), (105), (204), (312), (220) orientations of the tetragonal structure. The average crystallite size is estimated to 23 nm for as-deposited film, 26 nm for annealed at 300 °C and 40 nm for annealed at 350 °C. It is found from SEM study that the precursor film shows non-uniform distribution of agglomerated particles with well-defined boundaries. As the annealing temperature increases, the crystallization of the films is observed to improve, and hence the morphology of as-deposited precursor film is observed to change into the larger flat grains upon annealing.

Effect of Homo-buffer Layers on the Properties of Sputtering Deposited Ga2O3 Films

β- Ga2O3 films were grown by radio-frequency magnetron sputtering method. The influence of Ga2O3 buffer layers and annealing treatment on the structural, optical, morphological and electrical properties of Ga2O3 films was studied. The results revealed an improvement of crystalline quality and transmittance of annealed β- Ga2O3 films prepared with homo-buffer layers. Ga2O3 film UV photodetectors were fabricated with a new B and Ga co-doped ZnO films (BGZO)/Au interdigitated electrode. A good ohmic contact was formed between the film and the electrode. For the detector based on Ga2O3 films with buffer layers, a higher value of photo response and faster response times was obtained.

Effects of annealing temperature and Eu concentration on the structural and photoluminescence properties of Eu-doped SnO2 thin films prepared by a metal organic decomposition method

In this study, Eu-doped SnO2 thin films were prepared by a metal organic decomposition method through a pyrolysis of organic acid salts. The effects of annealing temperature and Eu concentration on the structural and photoluminescence properties of the films were investigated. X-ray diffraction measurements showed that the crystal structure corresponded to tetragonal rutile SnO2 regardless of the annealing temperature or Eu concentration. Additionally, X-ray diffraction, Fourier transform infrared spectroscopy and atomic force microscopy measurements indicated an improvement in crystalline quality with increasing annealing temperature. Photoluminescence spectra exhibited Eu-related emissions around 588–599 nm and 620 nm. It was found that the photoluminescence properties were strongly dependent on both the annealing temperature and Eu concentration. The highest photoluminescence intensity around 588–599 nm was obtained for an Eu concentration of 7.2 at.% for samples annealed at 1000 °C.

Synthesis of dense TiO2 nanoparticle multilayers using spin coating technique

A stack of nine layers is prepared by sequential spun casting of commercially available colloidal TiO2 nanoparticles of average size of 10–15 nm. Scanning electron microscopy (SEM) is employed to investigate the surface morphology of the multilayers. SEM micrographs exhibit formation of highly uniform and dense TiO2 nanoparticle layers. The uniformity and density is found to be increasing with layer thickness. Structural characterization is carried out using X-ray diffraction (XRD) technique. XRD spectra indicate improvement in crystalline quality of all the layers with increasing layer thickness. All the layers are having mainly the anatase phase of TiO2. Optical characterization is carried out by UV–visible spectroscopy. The value of bandgap estimated on the basis of absorption coefficient is found to be 3.26 eV and approximately remains the same for the layers. The electrical characterization suggests that multilayer resistivity increases with increasing layer thickness. The good quality spin coated thin dense TiO2 layers have many applications in optoelectronics.

Optical properties of beryllium-doped GaSb epilayers grown on GaAs substrate

In this work, the effects of p-type beryllium (Be) doping on the optical properties of GaSb epilayers grown on GaAs substrate by Molecular Beam Epitaxy (MBE) have been studied. Temperature- and excitation power-dependent photoluminescence (PL) measurements were performed on both nominally undoped and intentionally Be-doped GaSb layers. Clear PL emissions are observable even at the temperature of 270 K from both layers, indicating the high material quality. In the Be-doped GaSb layer, the transition energies of main PL features exhibit red-shift up to ∼7 meV, and the peak widths characterized by Full-Width-at-Half-Maximum (FWHM) also decrease. In addition, analysis on the PL integrated intensity in the Be-doped sample reveals a gain of emission signal, as well as a larger carrier thermal activation energy. These distinctive PL behaviors identified in the Be-doped GaSb layer suggest that the residual compressive strain is effectively relaxed in the epilayer, due possibly to the reduction of dislocation density in the GaSb layer with the intentional incorporation of Be dopants. Our results confirm the role of Be as a promising dopant in the improvement of crystalline quality in GaSb, which is a crucial factor for growth and fabrication of high quality strain-free GaSb-based devices on foreign substrates.

Tailoring nucleation process to gain highly oriented ZnO thin film with excellent UV luminescent and electrical performances

We present a novel approach for tuning orientation in ALD deposited ZnO nanofilm by inserting interfacial buffer layer (IBL) at interface between film and substrate, which is found to induce strong orientation and significant improvement in crystalline quality by largely decreasing grain boundary (GB) area through tailoring nucleation process. As a result, the UV emission in ZnO film with IBL is enhanced by over an order of magnitude due to the reduction of surface quenching at GB. Meanwhile, the decrease of electron scattering at GB also helps improve the electrical mobility of the film, leading to great reduction in resistivity.

Evolution of Structural and Optical Properties of ZnO Nanorods Grown on Vacuum Annealed Seed Crystallites.

In this study, the ambient condition for the as-coated seed layer (SL) annealing at 350 °C is varied from air or nitrogen to vacuum to examine the evolution of structural and optical properties of ZnO nanorods (NRs). The NR crystals of high surface density (~240 rods/μm2) and aspect ratio (~20.3) show greatly enhanced (002) degree of orientation and crystalline quality, when grown on the SLs annealed in vacuum, compared to those annealed in air or nitrogen ambient. This is due to the vacuum-annealed SL crystals of a highly preferred orientation toward (002) and large grain sizes. X-ray photoelectron spectroscopy also reveals that the highest O/Zn atomic ratio of 0.89 is obtained in the case of vacuum-annealed SL crystals, which is due to the effective desorption of hydroxyl groups and other contaminants adsorbed on the surface formed during aqueous solution-based growth process. Near band edge emission (ultra violet range of 360–400 nm) of the vacuum-annealed SLs is also enhanced by 44% and 33% as compared to those annealed in air and nitrogen ambient, respectively, in photoluminescence with significant suppression of visible light emission associated with deep level transition. Due to this improvement of SL optical crystalline quality, the NR crystals grown on the vacuum-annealed SLs produce ~3 times higher ultra violet emission intensity than the other samples. In summary, it is shown that the ZnO NRs preferentially grow along the wurtzite c-axis direction, thereby producing the high crystalline quality of nanostructures when they grow on the vacuum-annealed SLs of high crystalline quality with minimized impurities and excellent preferred orientation. The ZnO nanostructures of high crystalline quality achieved in this study can be utilized for a wide range of potential device applications such as laser diodes, light-emitting diodes, piezoelectric transducers and generators, gas sensors, and ultraviolet detectors.

Highly dispersible diamond nanoparticles for pretreatment of diamond films on Si substrate

High quality diamond film on Si substrate was synthesized by coating diamond nanoparticles prepared by polyglycerol grafting (ND-PG) dispersion as pre-treatment method. Transmission electron microscope indicates that ND-PG is much more dispersible than untreated nanoparticles in organic solvents. The surface morphology was characterized by scanning electron microscope while atomic force microscope was conducted to measure the surface roughness. Microstructure properties were carried out by Raman spectroscopy and X-ray diffraction. The results revealed an increase in nucleation density, an acceleration of growth rate and an improvement of film crystalline quality by using spin-coating ND-PG pretreatment.

Effect of NH3 plasma treatment on the transient characteristics of ZnO nanorod-gated AlGaN/GaN high electron mobility transistor-based UV sensors

In this work, we examine the effect of NH3 plasma post-treatment on the ultraviolet (UV) photo-response transient characteristics of zinc oxide (ZnO) nanorod (NR)-gated AlGaN/GaN high electron mobility transistor (HEMT) photodetectors. The recovery time of the detectors (180ms) was significantly reduced to 80ms by the NH3 plasma-treatment for 180s, while no significant reduction in response time upon UV illumination is measured from the plasma-treated devices. The photoluminescence and X-ray diffraction reveals that continuous improvement in crystalline quality of NRs is observed with the plasma duration up to 180s. The X-ray photoelectron spectroscopy measurement shows that the surface band bending of ZnO crystals is lowered with the corresponding plasma treatment.

Phase separation-suppressed and strain-modulated improvement of crystalline quality of AlGaN epitaxial layer grown by MOCVD

High crystalline quality AlGaN films were grown on GaN templates by metalorganic chemical vapor deposition (MOCVD). Inhomogeneous distributions of Al compositions in both the vertical and lateral growth direction caused by the strong gas-phase pre-reaction occurring between Al precursors and NH3 were suppressed by optimizing the growth conditions. The residual strain in AlGaN/GaN induced by lattice mismatch, which results in the degradation of crystallinity, was modulated by varying thickness of a high temperature (HT) AlN interlayer (IL) inserted between AlGaN and GaN layers. Both the crystalline quality and Al-incorporation into the AlGaN are influenced by the residual strain related to the AlN IL thickness. Lastly, the understanding of the AlGaN growth and of the strain modification was employed to grow an AlGaN/GaN HEMT structure showing good electrical characteristics and uniformity.

Study on the effect of annealing under pressure on the material properties of Cu2ZnSn(S,Se)4 thin films

In this work, we report the processing of Cu2ZnSn(S,Se)4 (CZTSSe) thin films by doctor blade technique, a vacuum-free and low-cost method. For this purpose, nanoparticles of the precursor materials ZnS, SnS and CuS were synthesized by wet chemical reaction. A paste of these three binary compounds mixed in appropriate proportions were coated on glass substrates, and submitted to thermal treatment under Se atmosphere at different temperatures and pressures to obtain the CZTSSe films. The impact of temperature and pressure on structural, compositional, optical and electrical properties was analyzed and the information was used to optimize the CZTSSe physical properties. By increasing temperature from 450°C to 600°C, improvements in CZTSSe crystalline quality as well as better Se incorporation were found for all chamber pressures. As an interesting result, it was observed that the band-gap of kesterite film can be tailored as a function of annealing temperature and pressure. In particular, band-gap energy values in the range of 1.16 −1.25eV were obtained. Finally, it was found that sample annealed at 600°C under 375Torr for 30min showed the best physical properties suitable for solar cell applications.

Epitaxial growth and characterization of non-polar a-plane AlGaN films with MgN/AlGaN insertion layers

The MgN/AlGaN insertion layers were applied for the first time in the growth of non-polar a-plane AlGaN epi-layers by metal organic chemical vapor deposition technology. The full-width-at-half-maximum value of X-ray rocking curve for the a-plane AlGaN epi-layer was decreased by approximately 50.6% and the root-mean-square value of the surface was reduced by 74% by inserting the MgN/AlGaN insertion layers with optimized number of insertion pairs, which revealed that the compressive strain within the a-plane AlGaN epi-layers was effectively reduced, leading to significant improvements in crystalline quality and surface morphology, which is very helpful to fabricate high quality AlGaN-based ultraviolet light-emitting-diodes.

Epitaxial growth and characterization of nonpolar a-plane AlGaN films with MgN/AlGaN insertion layers

MgN/AlGaN insertion layers were applied for the first time to the growth of nonpolar a-plane AlGaN epilayers by metal organic chemical vapor deposition technology. The full width at half maximum of the X-ray rocking curve for the a-plane AlGaN epilayers was reduced by approximately 50.6% and the root-mean-square roughness value of the surface was reduced by 74% by applying the MgN/AlGaN insertion layers with an optimized number of insertion pairs. These results reveal that the compressive strain within the a-plane AlGaN epilayers was effectively reduced, leading to significant improvements in crystalline quality and surface morphology. These improvements are very helpful in fabricating high-quality AlGaN-based ultraviolet light-emitting diodes.

High-quality AlN epitaxy on sapphire substrates with sputtered buffer layers

In this work, a novel strategy for high-quality AlN templates epitaxy on sapphire substrates with sputtered buffer layers combined with a low- and high-temperature alteration technique is proposed. The best full width at half maximum values for (0002) and (11¯02) reflections are 207 and 377 arcsec, respectively. Investigations indicate the joint effect of growth mode control and sputtered buffer layer results in the improvement of AlN crystalline quality. Firstly, threading dislocations density can be significantly decreased due to the alteration from three-dimensional to two-dimensional growth mode. Moreover, the graded composition of AlON layer in the sputtered buffer layer is believed to alleviate lattice mismatch between sapphire substrates and AlN, which also contributes to low dislocations density in AlN templates.

Effect of 3C-SiC intermediate layer in GaN—based light emitting diodes grown on Si(111) substrate

GaN-based light emitting diodes (LEDs) have been grown by metalorganic chemical vapor deposition on Si(111) substrate with and without 3C-SiC intermediate layer (IL). Structural property has been characterized by means of atomic force microscope, X-ray diffraction, and transmission electron microscope measurements. It has been revealed that a significant improvement in crystalline quality of GaN and superlattice epitaxial layers can be achieved by using 3C-SiC as IL. Regarding of electrical and optical characteristics, it is clearly observed that the LEDs with its IL have a smaller leakage current and higher light output power comparing with the LEDs without IL. The better performance of LEDs using 3C-SiC IL can be contributed to both of the improvements in epitaxial layers quality and light extraction efficiency. As a consequence, in terms of optical property, a double enhancement of the light output power and external quantum efficiency has been realized.

Lateral epitaxial overgrowth of ZnO layers on hexagonally patterned buffer layers in low-temperature aqueous solution

Abstract In conjunction with lift-off patterning and thermal annealing, we have fabricated hexagonal arrays of epitaxial ZnAl2O4 buffer layers on sapphire substrates. Taking advantage of the small lattice mismatch between ZnO (0001) and ZnAl2O4 (111), site-controlled epitaxial growth of ZnO prisms occurred on the pre-patterned, lattice-matched buffer layers. Subsequently, using a continuous flow reactor, long-duration maskless lateral epitaxial overgrowth (LEO) of ZnO prisms led to coalescence into continuous, thick layers within aqueous solutions at low temperature. X-ray diffraction (XRD) confirmed that LEO on the hexagonally patterned buffer layers facilitated the fabrication of strain-relieved, wing-tilt-free, thick ZnO layers. The dislocation density at the coalesced LEO-grown ZnO layer was approximately 108 cm−2. The spatial variation in the micro-photoluminescence characteristics of the LEO-grown layer suggested that the improvement in crystalline quality occurred in the overgrown wings, originating from the decrease in dislocation in the ZnO layer during the LEO process.

InGaN/GaN quantum well improved byin situSiNxpretreatment of GaN template

In situ SiNx pretreatment was employed to modify the growth behavior and optical properties of InGaN/GaN quantum wells (QWs). With moderate SiNx pretreatment surface smoothness of InGaN/GaN QWs was improved and attributed to enhanced layer growth by Ga surfactant effect. Significant increase of photoluminescence peak intensity and relatively uniform and bright cathodoluminescence images were observed, which were attributed to the improvement in crystalline quality and strain reduction for the InGaN/GaN QWs with moderate SiNx pretreatment.

Investigation on the Effect of Crystal Orientation Dependence of Pulse Porous Silicon for White Light Emission

Porous silicon (PS) was formed by using an electrochemical pulse etching (PC) and conventional direct current (DC) etching techniques. The study aims to compare the effect of crystal orientations (n-type (100) and n-type (111)) on the formation of the PS under various conditions. For DC etching technique, the silicon wafers were etched in Hydrofluoric (HF) based solution with a current density of J=10 mA/cm2 for 30 minutes. While for the PC process, an electroless chemical etching with a different delay time (Td) of 0 min and 2 min were imposed before PC process starts. After that, the pulse current of J=10 mA/cm2 with the cycle time (T) of 10 ms and pause time (Toff) of 4ms were supplied in 30 min etching time in HF based electrolyte. Three samples from n-type (100) are DC1, PC1 and PC2 while the three samples from n-type (111) are DC2, PC3, and PC4 respectively. Field Emission Scanning Electron Microscopy (FESEM) images showed that the samples from n-type (100) produce more uniform circular structures and dense compared to n-type (111). The introduction of 2 minutes delay during PC process resulted in better structural of PS formation and also the optical properties shown by the Raman and Photoluminescence (PL) spectroscopies. For PL observation, the as grown Si shows no emission at the visible spectrum while all the PS samples (DC and PC techniques) exhibited significant broad spectrum between 500 nm to 900 nm respectively. It can be seen that the uniform circular pore of n-type (100) enhanced the PL emission indicated by the higher PL intensity (PC1 and PC2) compared to PC3 and PC4 from n-type (111). Raman spectroscopy showed that an improvement in the crystalline quality of PS in PC technique compared to DC indicated by the reduction of full width at half maximum (FWHM).