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Strain modulation is crucial for heteroepitaxy such as GaN on foreign substrates. Here, the epitaxy of strain-relaxed GaN films on graphene/SiC substrates by metal-organic chemical vapor deposition is demonstrated. Graphene was directly prepared on SiC substrates by thermal decomposition. Its pre-treatment with nitrogen-plasma can introduce C–N dangling bonds, which provides nucleation sites for subsequent epitaxial growth. The scanning transmission electron microscopy measurements confirm that part of graphene surface was etched by nitrogen-plasma. We study the growth behavior on different areas of graphene surface after pre-treatment, and propose a growth model to explain the epitaxial growth mechanism of GaN films on graphene. Significantly, graphene is found to be effective to reduce the biaxial stress in GaN films and the strain relaxation improves indium-atom incorporation in InGaN/GaN multiple quantum wells (MQWs) active region, which results in the obvious red-shift of light-emitting wavelength of InGaN/GaN MQWs. This work opens up a new way for the fabrication of GaN-based long wavelength light-emitting diodes.

Effective ultraviolet light-emitting diodes (LEDs) were fabricated by clamping the n-ZnO films on the top of p-hBN/p-GaN/sapphire substrates. An ultraviolet emission originating from ZnO was measured from the diode under a forward bias, the electroluminescence (EL) spectra of which show a peak wavelength of ∼376 nm with a narrow full-width at half maximum of ∼12 nm. Compared with the reference diode fabricated by directly growing n-ZnO on the p-hBN substrates using metal-organic chemical vapor deposition, the proposed diode showed a dramatic increment of the EL intensity; meanwhile, its emission onset lowered down considerably. The improved optical property of the proposed LED is mainly ascribed to suppressing the formation of the BNO-related layer at the n-ZnO/p-hBN interface. The present work provides a simple and feasible approach for developing advanced ZnO-based optoelectronic devices.

In this work, a near-ultraviolet (380 nm) double Al0.2Ga0.8N/GaN distributed Bragg reflectors (DBRs) stack mirror was designed and fabricated. The double DBRs stack mirror consists of a 30-pair Al0.2Ga0.8N/GaN DBRs centered at 375 nm and a 20-pair Al0.2Ga0.8N/GaN DBRs centered at 385 nm. Our simulation results show that the method of double DBRs stack mirror design can broaden the stopband width greatly and increase the reflected angle efficiently, compared with the single Al0.2Ga0.8N/GaN DBRs mirror. In experiment, the double Al0.2Ga0.8N/GaN DBRs stack mirror and the reference Al0.2Ga0.8N/GaN DBRs mirror were grown on sapphire substrate by metalorganic chemical vapor deposition. The measured stopband width of the double DBRs stack mirror (~ 25 nm) is more than two times that of the reference DBRs mirror (~ 11 nm), which consists well with our simulation results. It is reasonable to believe that this work could provide a valuable information to obtain AlGaN/GaN DBRs with wide stopband width that can be used in the fabrication of GaN-based resonant cavity light-emitting diodes and vertical cavity surface emitting lasers.

In this work, GaN-based vertically conducting near ultraviolet light-emitting diodes (LEDs) were grown on n-SiC substrates by metal-organic chemical vapor deposition. 10 pairs or 20 pairs of Si-doped n-Al0.2Ga0.8N/n-GaN distributed Bragg reflectors (DBRs) were used to improve the performance of the LEDs. The central reflected wavelength of the DBRs structures agreed well with the emission wavelength of the LEDs. A 389 nm near ultraviolet electroluminescence (EL) emission peak could be clearly observed in all the LED samples in our study. The EL intensity of LEDs was obviously enhanced because of the application of DBRs. Angle-dependent EL spectra were measured as well. For all the measurement angle region, the near ultraviolet EL emission intensity of the LEDs with DBRs were higher than that of the LED without DBRs.

To obtain high-quality n-type doped β-Ga2O3 films, silane was used as an n-type dopant to grow Si-doped β-Ga2O3 films on (100) β-Ga2O3 substrates by metal-organic chemical vapor deposition (MOCVD). The electron concentrations of the Si-doped β-Ga2O3 films obtained through experiments can be stably controlled in the range of 6.5 × 1016 cm−3 to 2.6 × 1019 cm−3, and the ionization energy of Si donors is about 30 meV, as determined by analysis and calculation. The full width at half maxima of the rocking curves of the (400) crystal plane of all doped films was less than 500 arcsec, thus showing high crystal quality, while the increase of the doping concentration increased the defect density in the β-Ga2O3 films, which had an adverse effect on the crystal quality and surface morphology of the films. Compared with heteroepitaxial Si-doped β-Ga2O3 films, homoepitaxial Si-doped β-Ga2O3 films exhibited higher quality, lower defect density, and more stable electron concentration, which make them more conductive for preparing Ga2O3-based power devices.

In this work, GaN films were grown on SiC substrates by metal-organic chemical vapor deposition. The influence of the growth temperature, thickness, and growth rate of graded AlGaN buffer on the stress state in the GaN films were investigated. We find that the in-plane tensile stress in the GaN film can be reduced from ∼0.58 to ∼0.16 GPa by the optimization of the growth parameters of the graded AlGaN buffer. Under the optimized growth parameters, the GaN film also possess the best crystalline quality, for which the full width at half maximum (FWHM) of (0002) and ( 10 1 ¯ 2 ) X-ray diffraction (XRD) rocking curves are 240 and 288 arcsec, respectively. Based on the optimized growth parameters of the graded AlGaN buffer, the SiNx interlayer with a deposition time of 180 s was employed in the growth processes of the GaN film. As a result, the in-plane tensile stress in the GaN film is reduced to a much smaller value, which is about 0.04 GPa, and the crystalline quality of the GaN are further improved, for which the FWHM of (0002) and ( 10 1 ¯ 2 ) XRD rocking curves are 98 and 128 arcsec, respectively.

In this paper, near-ultraviolet AlGaN/GaN distributed Bragg reflectors (DBRs) were prepared on GaN/sapphire templates by metal organic chemical vapor deposition. A single low-temperature AlN interlayer was adopted to prevent the formation of cracks in DBRs. Moreover, different pairs of DBRs with AlN interlayer were grown to investigate the stress distribution through the epilayer, the surface morphology evolution and the variation of reflectivity spectra. The in-situ stress monitoring shows a compressive-to-tensile stress transition with the increase of DBRs pairs. The optical microscope and atomic force microscope images show that the AlN interlayer induces the appearance of trenches on the surface of DBRs and the trenches gradually coalesce when DBRs grow. Meanwhile, the experimental reflectivity spectra get closer to the simulated results. Finally, we obtain a smooth-surface 25-pair Al.32Ga.68N/GaN DBRs with a reflectance of 94% at 390 nm and a 16 nm stopband bandwidth. The preparation of high-quality DBRs lays the foundation for the future development of high efficiency resonant cavity UV LEDs.

N-polar GaN-based blue-green light-emitting diodes (LEDs) with p-AlGaN electron blocking layer (EBL) were numerically investigated by simulation and experimentally grown on vicinal C-face SiC substrates by metal–organic chemical vapor deposition. By numerical simulation, we can find that p-AlGaN EBL in N-polar LEDs is able to play a more important role in blocking electron overflow than that in Ga-polar LEDs due to the reversed polarization, which leads to a high output power and internal quantum efficiency. Besides, the holes injection efficiency is enhanced in N-polar LED, resulting in a lower turn-on voltage. In experimental studies, N-polar LEDs based on different numbers of quantum wells were grown on vicinal C-face n-SiC substrates. When a forward bias is applied to the epitaxial N-polar LED with two quantum wells, a strong blue-green emission located at 480 nm can be observed. This work indicates that N-polar group-III nitrides have great potential in the application of optoelectronic devices.

Al0.2Ga0.8N/Al0.45Ga0.55N multiple quantum wells (MQWs) were grown on SiC substrates by metal–organic chemical vapor deposition. We studied the influence of well width and barrier width on the structural and optical properties of AlGaN MQWs in details. The MQWs structures prepared in this work all exhibited good periodicity and abrupt interfaces. When the barrier width was fixed, the thinner well was benefit to increase the integrated photoluminescence (PL) intensity of the MQWs and obtain a shorter ultraviolet wavelength. And the thicker barrier was also contributed to improve the optical properties while the well width remained the same. Compared with the effect of the well width, the emission wavelength of MQWs was less dependent on the barrier width. A 318-nm room-temperature PL emission was achieved when the well width and barrier width were 2.5 and 11 nm, respectively.

Heteroepitaxial growth of conductive Si-doped β-Ga 2 O 3 films on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD) was successfully performed. The effect of Si content on the structural, morphological, electrical and optical properties of Si-doped β-Ga 2 O 3 films was investigated in detail. Distinctive surface morphology evolution of films depending on Si content was observed and presented. The Si-doped β-Ga 2 O 3 films exhibited high transmittance in the ultraviolet-visible regions. The temperature-dependent PL was carried out especially to discuss the photoluminescence properties of Si-doped β-Ga 2 O 3 films. More importantly, the results suggested that the conductivity of heteroepitaxial Si-doped β-Ga 2 O 3 films by MOCVD could be realized and controlled by adjusting the Si content. The minimum resistivity of 1.79×10 −1 Ω·cm was obtained for the films grown under the SiH 4 flow rate of 0.08 sccm.

The optical properties of thick InGaN epilayers grown by metal-organic chemical vapor deposition with indium (In) content range from 10% to 42% were investigated. With the increase of In content, the crystal quality of InGaN epilayers significantly degraded, particularly at increasing the edge-type dislocations density for high In content InGaN. Combining high-resolution X-ray diffraction reciprocal space mapping, the splitting of the photoluminescence (PL) peak into several PL bands was found to be caused by compositional fluctuations rather than by the strain effects. Based on the PL and optical absorption spectra, the composition-dependent Stokes shift was observed and a linear relationship between Stokes shift and Urbach tailing energy was established. Additionally, a drop of In incorporation along the growth direction was observed in InGaN epilayers with high In content. Such drop behavior of In incorporation was attributed to the formation of In droplets, which offers a competing pathway for In incorporation by gathering the incoming In atoms hence causing a decrease in the surrounding In incorporation. This will present a broader view about the nature of InGaN alloys with high In content.

β gallium oxide (β-Ga2O3) is a promising material for the detection of solar-blind ultraviolet (SBUV). The quality of β-Ga2O3 crystal has a crucial influence on the performance of the photodetectors (PDs). In this paper, SBUV PDs were fabricated on high quality homoepitaxial β-Ga2O3 films grown by metal organic chemical vapor deposition (MOCVD). Both photoconductor PD and Schottky barrier diode (SBD) PD were prepared. The PDs showed excellent performance. For the photoconductor PD, the responsivity and EQE under the illumination of 254 nm light at 20 V bias were respectively 1.08 A/W and 5.32 × 102%. For the SBD PD, the I254/Idark at −20 V was 320, and the corresponding rejection ratio I254/I365 was 42.

Nitrogen-polar (N-polar) III-nitride has natural advantages in the fabrication of electronic devices. In this work, unintentionally doped N-polar GaN films were grown on sapphire substrates by metalorganic chemical vapor deposition. By optimizing the growth parameters including growth temperature, pressure, and V/III ratio, N-polar GaN with a relatively high sheet resistivity of 3.6 × 105 Ω/sq was achieved. The secondary ion mass spectroscopy and x-ray diffraction measurement results demonstrate that the increased carbon impurity concentration is primarily responsible for the high resistivity. Moreover, N-polar GaN/Al0.25Ga0.75N heterojunctions were deposited on the high-resistivity N-polar GaN template. An extremely high-density two-dimensional electron gas of up to 1.12 × 1013 cm−2 was realized at the interface between the Al0.25Ga0.75N and the GaN. It is reasonably believed that the experimental results obtained here are beneficial for the development of high-performance N-polar GaN-based electronic devices.

In this work, AlGaN/GaN superlattices (SLs) structures were grown on SiC substrates by metal-organic chemical vapor deposition (MOCVD). The influence of AlGaN/GaN period numbers, the ratio of AlGaN thickness to GaN thickness ( t A l G a N / t G a N ), and the Al content on the strain state of AlGaN/GaN SLs are investigated. Moreover, the strain relaxation mechanism of the SLs is analysed. Our results show that the surface roughening, the formation of misfit dislocations and the generation of cracks are all contribute to the strain relaxation of the SLs. Moreover, the main reason cause the formation of cracks in the SLs is the large lattice mismatch between the SLs and the template rather than the large lattice mismatch between the GaN and AlGaN in the SLs. It is worth noting that understanding the strain relaxation mechanism is crucial for the design and fabrication of high-quality optoelectronic devices based on AlGaN/GaN SLs by MOCVD.

The indium (In) incorporation induced morphological evolution and strain relaxation of high In content InGaN epilayers grown by metal–organic chemical vapor deposition (MOCVD) were investigated. With the decrease of growth temperature from 753 to 627 °C, In incorporation increases from 0.10 to 0.42, and the surface morphology evolves from initially mound-like three-dimensional surface roughness to progressive smoothness. Such morphology evolution mechanism can be well accounted for by a self-regulating model of islands’ proportions considering both strain relaxation and In surfactant effect comprehensively. Additionally, the strain relaxation and microstructural defects of such InGaN epilayers were investigated by X-ray diffraction reciprocal space mapping and cross-sectional transmission electron microscopy, respectively. It is found that, with the increase of In content, plastic relaxation via generating random stacking faults along with the surface sawtooth roughening becomes a more important strain re...

β-Ga2O3 films were grown on c-plane sapphire substrates at different substrate temperatures by metal-organic chemical vapor deposition. The effects of substrate temperature on transparence, optical band gap and photoluminescence of the films were studied systematically. With increasing the substrate temperature, the films structure was changed from amorphous to single orientation β-Ga2O3. The β-Ga2O3 films exhibited excellent optical transparency in the ultraviolet and visible regions. The ultraviolet-blue photoluminescence was observed at room temperature. The temperature-dependent photoluminescence was also carried out and the relative intensity of the blue emission with respect to the ultraviolet emission decreased with increasing the measured temperature. The origin of luminescence was discussed.

We utilized in-situ grown SiNx insertion mask to improve the quality of N-polar GaN films on sapphire substrates by metal-organic chemical vapor deposition. The influences of deposition time and position of SiNx insertion mask were studied. Under the optimal SiNx mask growth conditions, the full width at half maximum values of (0002) and (10 1 ¯ 2) XRD rocking curves of N-polar GaN films are decreased to 88″ and 172″, respectively. Simultaneously, Raman spectroscopy measurements reveal that SiNx mask can also reduce the tensile residual stress of N-polar GaN films. In addition, the electrical and optical properties of N-polar GaN films with and without SiNx insertion mask were investigated by temperature dependent Hall and photoluminescence measurements. It is found that N-polar GaN film with SiNx insertion mask has lower background carrier concentration, higher mobility and lower nonradiative recombination rate.

In this paper, N-polar GaN films were grown on vicinal C-face SiC substrates by metal–organic chemical vapor deposition. In situ porous SiNx interlayers with different deposition times of 0 s to 120 s were adopted in the growth processes of N-polar GaN films. We find that the crystalline quality, such as surface morphology and structural characteristics, of N-polar GaN films can be controlled by changing the deposition time of the SiNx interlayers. The N-polar GaN film with a SiNx deposition time of 60 s exhibits the best crystallinity. Based on the observed porous feature of the SiNx interlayer from the phase image obtained using an atomic force microscope, we propose a model to illustrate the epitaxial growth mechanism of N-polar GaN films grown on the top of the porous SiNx interlayer. In this model, we reveal that the SiNx deposition time determines the SiNx coverage and the nucleated island density of overgrown GaN, which markedly affects the structural characteristics of the N-polar GaN films. This model can also be used to analyze the influence of SiNx deposition times on the stress states in N-polar GaN films. Additionally, the threading dislocation propagation behaviors were observed experimentally from the cross-sectional transmission electron microscopy image of N-polar GaN with a SiNx interlayer, which demonstrates the reasonability of this model. It is reasonably believed that this work provide a valuable information to obtain high-quality GaN films that can be used to the fabrication of N-polar GaN-based optoelectronic devices.

&beta, Ga2O3 films were grown on sapphire (0001) substrates with various O/Ga (VI/III) ratios by metal organic chemical vapor deposition. The effects of VI/III ratio on growth rate, structural, morphological, and Raman properties of the films were systematically studied. By varying the VI/III ratio, the crystalline quality obviously changed. By decreasing the VI/III ratio from 66.9 &times, 103 to 11.2 &times, 103, the crystalline quality improved gradually, which was attributed to low nuclei density in the initial stage. However, crystalline quality degraded with further decrease of the VI/III ratio, which was attributed to excessive nucleation rate.

In this work, blue-green InGaN/GaN multiple quantum wells (MQWs) were grown on SiC substrates by metal-organic chemical vapor deposition. The influence of growth pressure on the structural and optical properties and surface morphology of the MQWs was studied. The photoluminescence measurement results indicate that the indium content in the MQWs nearly doubles when the growth pressure increases from 100 to 400 mbar, and the MQWs grown at different pressures exhibit distinct temperature-dependent optical behaviors. Meanwhile, the growth pressure also influences the structural characteristics of the MQWs significantly. Besides, with changing the growth pressure, the surface root-mean-square roughness, V-shaped pits density, V-shaped pits size, and localization state density of the MQWs were changed accordingly.

High quality N-polar n-type GaN films were deposited on vicinal C-face n-SiC substrates by metal organic chemical vapor deposition for vertical conducting devices. We employed the SiNx interlayer to N-polar GaN growth and studied the effect of SiNx interlayer on the properties of N-polar GaN. We found that the properties of N-polar films were greatly improved by using SiNx interlayer as characterized by X-ray diffraction, photoluminescence, and I-V measurements. N-polar n-type GaN films with SiNx interlayer exhibited a full width at half maximum of 422 arcsec for (0002) omega scan and 335 arcsec for ( 10 1 ¯ 2 ) omega scan, respectively. Integrated intensity ratio of near-band-edge emission and yellow-luminescence was increased by a factor of 4 by using SiNx interlayer. Besides, the resistance of the vertical structure with SiNx interlayer was reduced by 25% in comparison with that without SiNx interlayer.

In this study, 4.5 μm thick GaN films with graded AlxGa1−xN/AlN buffer and SiNx interlayer were prepared on 6H–SiC substrates by metal–organic chemical vapor deposition. To determine the effects of SiNx interlayer on epitaxial quality and stress state of GaN films, a series of comparative experiments were carried out by changing the deposition time and the insert location of SiNx interlayer. By optimizing growth conditions of SiNx interlayer, the full width at half maximum values of $$ (0002) $$ and $$ (10\bar{1}2) $$ rocking curves of GaN films were improved to 142 and 170 arcsec, respectively. A crack-free GaN film with a small root-mean-squared roughness of 0.21 ± 0.02 nm was achieved. Simultaneously, the reduction in threading dislocation density of GaN films was confirmed by using wet etching method. In addition, stress values in GaN films were investigated by Raman and low-temperature photoluminescence spectra, which indicated that the lower tensile stress in GaN film, the higher the film’s crystallinity.

The crystal quality and stress state of Al0.5Ga0.5N epitaxial layers on 6H-SiC wafers by introducing an in-situ deposited SiNx nanomask layer grown by metal-organic chemical vapor deposition (MOCVD) were investigated. A SiNx interlayer with various growth times was inserted to the Al0.5Ga0.5N epilayers. The full width at half maximum (FWHM) of X-ray diffraction peaks and the density of etch pits decreased dramatically by the SiNx interlayer, indicating an improved crystalline quality. Also, it was found that the crack density and biaxial tensile stress in the Al0.5Ga0.5N film was significantly reduced by in situ SiNx interlayer from optical microscopy, photoluminescence spectra and Raman spectra. Finally, a crack-free 1.8 μm thick Al0.5Ga0.5N epilayer grown on 6H-SiC substrate using the optimized SiNx interlayer growth time was obtained.

KOH based wet etchings were performed on both undoped and Mg-doped N-polar GaN films grown by metal-organic chemical vapor deposition. It is found that the etching rate for Mg-doped N-polar GaN gets slow obviously compared with undoped N-polar GaN. X-ray photoelectron spectroscopy analysis proved that Mg oxide formed on N-polar GaN surface is insoluble in KOH solution so that kinetic-limited etching occurs as the etching process goes on. The etching process model of Mg-doped N-polar GaN in KOH solution is tentatively purposed using a simplified ideal atomic configuration. Raman spectroscopy analysis reveals that Mg doping can induce tensile strain in N-polar GaN films. Meanwhile, p-type N-polar GaN film with a hole concentration of 2.4 y 10 17 cm ⿿3 was obtained by optimizing bis-cyclopentadienyl magnesium flow rates.

β gallium oxide (β-Ga2O3) homoepitaxy films were grown on ( 2 ‾ 01) β-Ga2O3 substrates by metal organic chemical vapor deposition (MOCVD). The effect of growth temperature on the crystalline quality was measured and systematically analyzed. The results showed the growth temperature played an important role in crystalline quality of β-Ga2O3. The optimized single crystalline β-Ga2O3 film was grown at 750 °C, and the full width at half maximum (FWHM) of X-ray diffraction (XRD) rocking curve was 21.6 arcsec, smaller than that of the β-Ga2O3 substrate (26.3 arcsec). The film exhibited a greatly smooth surface and the root-mean-square (RMS) roughness was 0.68 nm. And the arrangement of atoms inside the film was in good accordance with lattice constants of β-Ga2O3. In addition, the effect of the band structure on the luminescent properties of β-Ga2O3 was discussed.

The Si-doped β-Ga2O3 films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). Then the films were annealed in oxygen to reduce the intrinsic defects effectively. The effect of the Si doping concentration and the oxygen annealing time on the properties of β-Ga2O3 films was studied in detail. It was found that the crystal structure, surface morphology and electrical characteristics of the films can be changed regularly by adjusting the Si content and annealing time. β-Ga2O3 films with high quality and stable electron concentration can be obtained by this method.

Yellow luminescence (YL) of unintentionally doped GaN (u-GaN) peaking at about 2.2 eV has been investigated for decades, but its origin still remains controversial. In this study, ten u-GaN samples grown via metalorganic chemical vapor deposition (MOCVD) are investigated. It is observed from the room temperature (RT) photoluminescence (PL) measurements that the YL band is enhanced in the PL spectra of those samples if their MOCVD growth is carried out with a decrease of pressure, temperature, or flow rate of NH3. Furthermore, a strong dependence of YL band intensity on the carbon concentration is found by secondary ion mass spectroscopy (SIMS) measurements, demonstrating that the increased carbon-related defects in these samples are responsible for the enhancement of the YL band.

In this study, 1.5-μm-thick GaN films with AlN buffer were prepared on 6H-SiC substrates by metal-organic chemical vapor deposition. To determine the effects of growth conditions of AlN buffer on crystalline quality and stress state of GaN films, two series of experiments were carried out. By optimizing growth conditions of AlN buffer, the full width at half maximum values of (0002) and $$(10\bar{1}2)$$ rocking curves of GaN films were improved to 136 and 225 arcsec, respectively. A smooth surface was obtained with a small root-mean-squared roughness of 0.332 nm and the excellent optical property was observed. Simultaneously, threading dislocation density and tensile stress in GaN films were reduced by increasing AlN buffer growth temperature and its thickness in some extent. Besides, stress values in GaN films were confirmed by Raman and low-temperature photoluminescence spectra, which indicated that the lower tensile stress in GaN film, the higher the film crystallinity.

We report the growth of undoped N-polar GaN with a high resistivity on c-plane Al2O3 substrates by metal-organic chemical vapor deposition. The polarity of N-polar GaN has been examined and verified using a wet etching method. It is found that the density of threading dislocations has a great influence on the resistivity of N-polar GaN. By optimizing the V/III ratio, N-polar GaN with a relatively high sheet resistivity of 1.7 × 104 ohm/sq has been obtained. More importantly, the N-polar GaN films in our case are high-crystalline, with a rather low full width at half maximum (FWHM, 30–40 arc sec) of the (0002) ω-rocking curve. Simultaneously, the sheet resistivities of N-polar GaN films share the same changing trend with the FWHM of ( 10 1 ¯ 2 ) ω-rocking curve, reaching a maximum value at a V/III ratio of 1000. Through Raman spectroscopy analysis, the N-polar GaN with a sheet resistivity of 1.7 × 104 Ohm/sq is nearly stress-free. Besides, the optical properties of GaN films have been investigated by photoluminescence measurements to further access the crystallinity of N-polar GaN.

In this paper, the growth mechanism and the morphologies of GaSb islands grown on Si (1 0 0) by low-pressure metal–organic chemical vapor deposition have been studied. It was observed the GaSb growth mode transited from SK to VW mode with time, while the islands migrated in VW mode on the surface. As growth time prolonging, the islands were coarsening consistent with the considerations of Ostwald ripening substituting for migration. And it was the similar coalescence process in the various interruption time. The formation of giant islands reduced the surface energy with the island-induced strain fields which drive the islands distribution evenly.

Vertically aligned ZnO/MgO coaxial nanowire (NW) arrays were prepared on sapphire substrates by metal-organic chemical vapor deposition combined with a sputtering system. We present a comparative investigation of the morphological and optical properties of the produced heterostructures with different MgO layer thicknesses. Photoluminescence measurements showed that the optical performances of ZnO/MgO coaxial NWs were strongly dependent on the MgO layer thickness. The intensity of deep-level emission (DLE) decreased monotonously with the increase of MgO thickness, while the enhancement of ultraviolet (UV) emission showed a critical thickness of 15 nm, achieving a maximum intensity ratio (∼226) of IUV/IDLE at the same time. The significantly improved exciton emission efficiency of the coaxial NW structures allows us to study the surface passivation effect, photogenerated carrier confinement and transfer in terms of energy band theory. More importantly, we achieved an ultralow threshold (4.5 mA, 0.58 A cm(-2)) electrically driven UV lasing action based on the ZnO/MgO NW structures by constructing an Au/MgO/ZnO metal/insulator/semiconductor diode, and the continuous-current-driven diode shows a good temperature tolerance. The results obtained on the unique optical properties of ZnO/MgO coaxial NWs shed light on the design and development of ZnO-based UV laser diodes assembled with nanoscale building blocks.

We report the fabrication of vertically aligned two-dimensional (2D) ZnO nanowall networks (NNWs) by a catalyst-free metal–organic chemical vapor deposition system via a two-step growth method. The growth mechanism of the resulting NNWs was investigated detailedly by monitoring the morphology evolution at different growth stages. The formation of rough and striped grains in the initial stage was the determinant factor, which facilitated and guided the subsequent growth of NNWs. In our case, the obtained ZnO NNWs were of single-crystal wurtzite structure with a small full width at half-maximum (481 arcsec) of (0 0 0 2) ω -rocking curve. We also demonstrated the feasibility of controlling the morphology, structural and optical features of ZnO NNWs by adjusting the oxygen flow rate. A controllable evolution of micro-hole size and density of ZnO NNWs was observed. In addition, possible microcavity-related light oscillation and contaminants adsorption behaviors were proposed to explain the shift of ultraviolet peak and deep-level emission variation for the produced ZnO NNWs. Temperature-dependent photoluminescence results behaving with strong exciton effects further confirmed the relatively excellent quality of the obtained ZnO NNWs. It was reasonably believed that the novel 2D nanostructures featuring high crystallinity and remarkable optical properties can be employed in reliable optoelectronic nanodevices.

Epitaxial ZnO structures with inherent two layers of nanorods layer on film layer were fabricated on c-Al2O3 by metal-organic chemical vapor deposition (MOCVD) and studied by photoluminescence. Specially, photoluminescence spectra for the film layer were obtained by rendering the excitation from the substrate side. Different defect levels configurations between nanorods and film were revealed. Zinc vacancies tend to form in top nanorods layer, whereas abundant zinc–oxygen divacancies accumulate in bottom film layer. An acceptor state with activation energy of ~200 meV is exclusive to the film layer. The stacking fault related acceptor and Al introduced donor are present in both layers. Besides, two other defect related donors contained in the nanorods layer perhaps also exist within the film layer.

Cone-shaped patterned sapphire substrate was prepared by inductively coupled plasma etching and GaN nucleation layer was grown on it by metal-organic chemical vapor deposition. A selective growth of GaN nucleation layer was found on the slope of the cone-shaped patterned sapphire substrat, and the distribution morphology of GaN had significantly changed after it was recrystallized. GaN selective growth and redistribution were analyzed by investigating the distribution of crystallographic planes on the cone surface and the atom array of specific planes at atom level.

ZnO was grown on Si (1 1 1) by low-pressure metal–organic chemical vapor deposition. The nucleation and evolution of ZnO thin films were investigated by varying growth temperature and growth time during the nucleation process. It is found that nucleation temperature and time play a key role on the nucleation process. With the increase of nucleation temperature, the size of ZnO nuclei and the spacing between nucleation sites increases. And as the nucleation time increases, the size of ZnO nucleation islands (NIs) increases and the density decreases due to the coalescence of ZnO nuclei. The density and size of initial ZnO NIs have intensely affected the growth model and quality of ZnO epitaxial layers. In addition, the crystalline quality of ZnO thin films was improved by decreasing nucleation temperature and increasing nucleation time.

GaSb quantum dots have been widely applied in optoelectronic devices due to its unique electrical and optical properties. The effects of metal-organic chemical vapor deposition(MOCVD) parameters, such as growth temperature and vapour V/III ratio[V/III ratio means the molar ratio of trimethylgallium(TMGa) and triethylantimony( TESb)], were systematically investigated to achieve GaSb quantum dots with high quality and high density. The features of surface morphology of uncapped GaSb quantum dots were characterized by atomic force microscope( AFM) images. The results show that the surface morphologies of quantum dots are strongly dependent on growth temperature and vapour V/III ratio. GaSb quantum dots with an average height of 4.94 nm and a density of 2.45×1010 cm–2 were obtained by optimizing growth temperature and V/III ratio.

In this work we demonstrated the feasibility of controlling the morphology and crystallinity features of ZnO nanostructures by adjusting the growth temperature and reaction pressure in the appropriate windows. The microstructure of the films was made to change from a three-dimensional nanorods form to a two-dimensional nanowall networks form, and then to a two-dimensional dense film form. The controllable morphology evolution of the ZnO nanostructures was studied by analyzing the growth mechanisms and growth kinetics within different temperature and pressure windows. We also investigated the differences on the crystallinity and optical characteristics of three typical ZnO nanostructures. By optimizing the growth conditions, single-crystal ZnO films with a low full width at half-maximum (221 arcsecond) of the (0002) ω-rocking curve was obtained. It is believed that the promising results will provide valuable information for the fabrication of dimension controllable ZnO nanostructures that can be employed in reliable ZnO-based nanodevices.

NiZnO thin films with different Ni and O contents were grown by photo-assist metal–organic chemical vapor deposition. Electrical characteristics of films including resistances and charge carrier concentrations were analyzed. The intrinsic donor defects in NiZnO films could be effectively compensated by the increasing the Ni and O contents. A p-type NiZnO film with a high hole concentration could be obtained by controlling the Ni and O content of the film. This was further confirmed by analyzing defect levels from photoluminescence measurements.

Direct fabrication of semiconductor light emitting devices on metal foils is beneficial, because it brings flexibility and good heat sink in the devices. In this work, we have grown ZnO on the commercially available stainless steel foils by metal–organic chemical vapor deposition for the first time. With the increase of growth temperature, the morphology changes from a thin film structure to closely stacked columns, and eventually to nanorods. The change in the migration ability of adatoms due to the increase of growth temperature plays an important role in the evolution of morphology. The samples with nanorod morphology exhibit relatively better crystallinity and optical quality. A PEDOT:PSS/PMMA/ZnO device was fabricated based on the grown ZnO nanorods. The metal–insulator–semiconductor type device shows an uncommon symmetric I–V curve. Under reverse bias, the device emits fairly pure UV light, which comes from the near band edge emission of ZnO. The working mechanism of the devices has been discussed, and a model mainly based on the Poole–Frenkel effect is proposed to describe the charge transportation of the devices.

High-quality ZnO thin films were deposited on sapphire substrates using a photoassisted metal–organic chemical vapor deposition (PA-MOCVD) system. A controllable morphology evolution with varying d...

Polarization-induced doping has been shown to be effective for wide-bandgap III-nitrides. In this work, we demonstrated a significantly enhanced hole concentration via linearly grading an N-polar AlxGa1-xN (x = 0–0.3) layer grown by metal-organic chemical vapor deposition. The hole concentration increased by ∼17 times compared to that of N-polar p-GaN at 300 K. The fitting results of temperature-dependent hole concentration indicated that the holes in the graded p-AlGaN layer comprised both polarization-induced and thermally activated ones. By optimizing the growth conditions, the hole concentration was further increased to 9.0 × 1017 cm−3 in the graded AlGaN layer. The N-polar blue-violet light-emitting device with the graded p-AlGaN shows stronger electroluminescence than the one with the conventional p-GaN. The study indicates the potential of the polarization doping technique in high-performance N-polar light-emitting devices.

In this paper, N-polar GaN films with different V/III ratios were grown on vicinal C-face SiC substrates by metalorganic chemical vapor deposition. During the growth of N-polar GaN film, the V/III ratio was controlled by adjusting the molar flow rate of ammonia while keeping the trimethylgallium flow rate unchanged. The influence of the V/III ratio on the surface morphology of N-polar GaN film has been studied. We find that the surface root mean square roughness of N-polar GaN film over an area of 20 × 20 μm2 can be reduced from 8.13 to 2.78 nm by optimization of the V/III ratio. Then, using the same growth conditions, N-polar InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) were grown on the rough and the smooth N-polar GaN templates, respectively. Compared with the LED grown on the rough N-polar GaN template, dramatically improved interface sharpness and luminescence uniformity of the InGaN/GaN MQWs are achieved for the LED grown on the smooth N-polar GaN template.

Two InGaN/GaN multiple-quantum-well (MQW) samples with identical epitaxial structures are grown at different growth rates via metal-organic chemical vapor deposition system. The room temperature photoluminescence intensity of the fast-grown sample is much stronger than that of the slow-grown one. In addition, the fast-grown sample has two luminescence peaks at low temperatures, and the height of main peak anomalously increases with increasing temperature below 100 K. Such improved emission efficiency and the untypical temperature-induced increase of peak height can be attributed to the carrier's transferring between two kinds of localized traps with different potential depth in the fast-grown sample, where the distribution of indium is seriously inhomogeneous. The enhanced fluctuation of indium is caused by the reduced migration time of adsorbed atoms due to the increased growth rate during the epitaxial growth of MQW region.

In this paper ZnO films are grown on GaAs/Al 2 O 3 substrates at different temperature by metal-organic chemical vapor deposition (MOCVD). The GaAs/Al 2 O 3 substrates are formed by depositing GaAs layer (∼35 nm) on the Al 2 O 3 substrate. The results of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) demonstrate that most of the Ga and As atoms form Ga–As bond and the GaAs layer does not present any orientation. The characters of the ZnO films grown on GaAs/Al 2 O 3 substrates are investigated by XRD, photoluminescence (PL), atomic force microscopy (AFM) and Raman scattering. Compared with ZnO film grown on Al 2 O 3 substrate, ZnO film prepared by our fabrication scheme has good crystal and optical quality. Meanwhile its grain size becomes bigger according to the AFM image. Raman analysis indicates that the intrinsic defects and the in-plane tensile stress are obviously reduced in ZnO/GaAs/Al 2 O 3 samples.

Heterojunction light-emitting diodes with ZnO/Si structure were fabricated on both high-resistivity (p) and low-resistivity (p+) Si substrates by metal–organic chemical vapor deposition technology. Fairly good rectifications were observed from the current–voltage curves of both heterojunctions. Ultraviolet (UV) and blue-white electroluminescence (EL) from ZnO layer were observed only from ZnO/p+-Si heterojunction under forward bias at room temperature (RT), while strong infrared (IR) EL emissions from Si substrates were detected from both ZnO/p-Si and ZnO/p+-Si heterojunctions. The UV and IR EL mechanisms have been explained by energy band structures. The realization of RT EL in UV–visible and IR region on Si substrate has great applicable potential for Si-based optoelectronic integrated circuits.

ZnO thin films were deposited on semi-insulating (0 0 1) GaAs substrates at different growth conditions by metal-organic chemical vapour deposition. The effect of growth temperature and ambient oxygen partial pressure on the properties of the ZnO films was studied. It was found that these two important parameters affected the structure, crystallinity, surface morphology, and optical and electrical characteristics of the ZnO films. At the present growth conditions, the full width at half maximum of a ZnO (0 0 2) diffraction peak decreased to 0.185° at 610 °C under an oxygen partial pressure of 45 Pa. By increasing the oxygen partial pressure from 45 to 68 Pa, the surface of the ZnO films became smoother and the grain size smaller and more homogeneous. In addition, the near-band-edge emission and the deep-level emission in the photoluminescence spectra and the electrical properties of the ZnO thin films strongly depended on the ambient oxygen content.

Undoped n-ZnO thin films were successfully grown on p-Si (1 0 0) substrates by low-pressure metalorganic chemical vapour deposition (MOCVD). The c-axis oriented ZnO films were grown on Si at different temperatures using diethylzinc (DEZn) and oxygen (O2). The structural and optical properties of ZnO films were investigated using x-ray diffraction (XRD) and photoluminescence (PL) spectra, respectively. It is found that the ZnO film grown at 610 °C shows the best crystallinity and optical quality. Current–voltage (I–V) characteristics of all n-ZnO/p-Si heterojunctions exhibit nonlinear and rectifying characteristics with a small current leakage in the reverse direction. Junction leakage of the heterojunction deposited at 620 °C is higher than those of the other heterojunctions.

ZnO films were grown on GaAs(0 0 1)substrates at different surface treatment conditions at different growth temperatures by metal-organic chemical vapor deposition. ZnO(1 0 0) and ZnO(0 0 2) films that had different preferred orientations were grown according to different chemical etching condition. The crystallinity and optical properties of ZnO films were investigated with X-ray diffraction and the room-temperature photoluminescence spectra. Compared with ZnO(0 0 2) films, ZnO(1 0 0) films that have undergone variation at the preferred orientation have better crystal structure. Moreover, all samples have similar optical characteristics at the same growth temperature. For GaAs substrates etched under different conditions, the analysis results of X-ray photoelectroscopy show that the preferred orientation variations of ZnO films take place because rich As layer are formed in process of chemical etching.

Zinc oxide (ZnO) thin film was grown on CaF2 (11–21) substrate by metalorganic vapor phase epitaxy (MOVPE) and the structure, surface morphology and the opto-electrical properties of the film were investigated. It was found that preferential c-axis oriented ZnO film was highly transparent and quite smooth. In photoluminescence (PL) spectrum at 10 K, the neutral donor bound exciton (D0X) emission was identified at 3.362 eV (I4), the neutral acceptor bound exciton (A0X) emission at 3.346 eV (I9). Free A- and B-exciton emissions at 3.374 eV and 3.386 eV, respectively, as well as the phonon replicas of free A-exciton and D0X were also observed, indicating high optical quality of the ZnO film. Temperature-dependent PL spectra demonstrated that the free exciton emission was dominant at a temperature larger than 215 K. Hall measurement showed that the ZnO/CaF2 film exhibited n-type conduction, with the resistivity of 114 Ω cm and the Hall mobility of 15.7 cm2/Vs.

ZnO films were grown on (0 0 0 1) orientation (C-plane) and (0 1 ¯ 1 2) orientation (R-plane) sapphire substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). The crystallinity, surface morphology and optical properties of ZnO films on C and R sapphires were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL) spectrum and transmission spectrum, respectively. The XRD patterns of the films indicated that ZnO grew on C sapphire with a (0 0 0 2) orientation, while (1 1 2 ¯ 0) ZnO grew on R sapphire. The epitaxial relationships of the two films were determined by X-ray θ −2 θ and ϕ scans. The SEM images exhibited that the ZnO film grown on C sapphire had a columnar morphology, and that the ZnO film on R sapphire possessed a smooth and dense surface. The room temperature PL spectra showed that both of the films had a good optical quality, but the intensity of the near band-edge emission (NBE) peak of the ZnO film on C sapphire was more intense than that of the film on R sapphire. Besides, the transmission spectra showed that the two ZnO films were highly transparent in the visible region.