InGaN/GaN multilayer quantum dots yellow-green light-emitting diode with optimized GaN barriers

InGaN/GaN multilayer quantum dot (QD) structure is a potential type of active regions for yellow-green light-emitting diodes (LEDs). The surface morphologies and crystalline quality of GaN barriers are critical to the uniformity of InGaN QD layers. While GaN barriers were grown in multi-QD layers, we used improved growth parameters by increasing the growth temperature and switching the carrier gas from N₂ to H₂ in the metal organic vapor phase epitaxy. As a result, a 10-layer InGaN/GaN QD LED is demonstrated successfully. The transmission electron microscopy image shows the uniform multilayer InGaN QDs clearly. As the injection current increases from 5 to 50 mA, the electroluminescence peak wavelength shifts from 574 to 537 nm.     

Electrical characterization and nanoscale surface morphology of optimized Ti/Al/Ta/Au ohmic contact for AlGaN/GaN HEMT

Good ohmic contacts with low contact resistance, smooth surface morphology, and a well-defined edge profile are essential to ensure optimal device performances for the AlGaN/GaN high electron mobility transistors [HEMTs]. A tantalum [Ta] metal layer and an SiN_x thin film were used for the first time as an effective diffusion barrier and encapsulation layer in the standard Ti/Al/metal/Au ohmic metallization scheme in order to obtain high quality ohmic contacts with a focus on the thickness of Ta and SiN_x. It is found that the Ta thickness is the dominant factor affecting the contact resistance, while the SiN_x thickness affects the surface morphology significantly. An optimized Ti/Al/Ta/Au ohmic contact including a 40-nm thick Ta barrier layer and a 50-nm thick SiN_x encapsulation layer is preferred when compared with the other conventional ohmic contact stacks as it produces a low contact resistance of around 7.27 × 10^-7 Ω·cm² and an ultra-low nanoscale surface morphology with a root mean square deviation of around 10 nm. Results from the proposed study play an important role in obtaining excellent ohmic contact formation in the fabrication of AlGaN/GaN HEMTs.     

Fabrication of low-density GaN/AlN quantum dots via GaN thermal decomposition in MOCVD

With an appropriate high anneal temperature under H₂ atmosphere, GaN quantum dots (QDs) have been fabricated via GaN thermal decomposition in metal organic chemical vapor deposition (MOCVD). Based on the characterization of atomic force microscopy (AFM), the obtained GaN QDs show good size distribution and have a low density of 2.4 × 10⁸ cm^-2. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the GaN QDs were formed without Ga droplets by thermal decomposition of GaN.     

The ultrathin PEALD-GaN surface/interface layer-modulated charge dynamics in quantum dot-sensitized solar cells

In this work, gallium nitride (GaN) is employed for the first time to modulate the charge dynamics of quantum dot-sensitized solar cells (QDSCs). An ultrathin GaN layer has been coated on the surface of both mesoporous TiO₂ photoanode and quantum dots (QDs) at 240 °C by plasma-enhanced atomic layer deposition (PEALD) approach. It is revealed that there exists a stepped energy level alignment among the as-prepared TiO₂ film, GaN layer and QDs, which accelerates the extraction and collection of photogenerated electrons. Meanwhile, a type-II core-shell QD/GaN structure is formed benefiting from the self-limiting reactions of PEALD, resulting in an enhanced light absorption and a redshift of absorption edge. In addition, the dense GaN layer can also effectively inhibit the reverse transfer of photogenerated electrons from TiO₂ to QDs or electrolyte while improving the connection between TiO₂ and QDs. Ultimately, the QDSCs with a 0.68 nm-thick GaN layer achieve a 29% increase of short-circuit current density and enhanced device efficiency, even with reduced fill factor. This work has shown the multi-functions of GaN in regulating the charge dynamics of QDSCs as well as the potential advantages in replacing TiO₂ as photoanode for electronic extraction and transport.     

Self-assembled GaInNAs/GaAsN quantum dot lasers: solid source molecular beam epitaxy growth and high-temperature operation

Self-assembled GaInNAs quantum dots (QDs) were grown on GaAs (001) substrate using solid-source molecular-beam epitaxy (SSMBE) equipped with a radio-frequency nitrogen plasma source. The GaInNAs QD growth characteristics were extensively investigated using atomic-force microscopy (AFM), photoluminescence (PL), and transmission electron microscopy (TEM) measurements. Self-assembled GaInNAs/GaAsN single layer QD lasers grown using SSMBE have been fabricated and characterized. The laser worked under continuous wave (CW) operation at room temperature (RT) with emission wavelength of 1175.86 nm. Temperature-dependent measurements have been carried out on the GaInNAs QD lasers. The lowest obtained threshold current density in this work is ∼1.05 kA/cm² from a GaInNAs QD laser (50 × 1,700 μm²) at 10 °C. High-temperature operation up to 65 °C was demonstrated from an unbonded GaInNAs QD laser (50 × 1,060 μm²), with high characteristic temperature of 79.4 K in the temperature range of 10–60 °C.     

Effect of H2 addition during PECVD on the moisture barrier property and environmental stability of H:SiNx film

A hydrogenated silicon nitride (H:SiN_x) film with enhanced moisture barrier property and environmental stability was developed using plasma-enhanced chemical vapor deposition (PECVD) with the addition of H₂ gas at 100°C. The moisture barrier property and film density of the 100-nm-thick H:SiN_x film were ameliorated by increasing the H₂ gas flow rate during PECVD. X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy studies demonstrated that the improved performance was a result of an increase in the amount of Si–N bonds compared to hydrogen-terminated bonds with an increase in the H₂ gas flow rate. It is believed that H₂ gas assisted the formation of aminosilane, which contributed to the condensation of silicon nitride by lowering the activation energy for radicalization reactions of silane and ammonia. After the 85°C/85% RH test, the optimized H:SiN_x film maintained a water vapor transmission rate lower than 5 × 10⁻⁵ g/m²/day owing to the suppression of oxidation. The optimized H:SiN_x film was rarely oxidized owing to the decrease in hydrogen-terminated bonds and increase in the film density. The results indicated that the introduction of H₂ gas during the PECVD process strengthened the environmental stability of the H:SiN_x film.     

SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/SiN<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> multilayers for photovoltaic and photonic applications

Microstructural, electrical, and optical properties of undoped and Nd³⁺-doped SiO _x /SiN _y multilayers fabricated by reactive radio frequency magnetron co-sputtering have been investigated with regard to thermal treatment. This letter demonstrates the advantages of using SiN _y as the alternating sublayer instead of SiO₂. A high density of silicon nanoclusters of the order 10¹⁹ nc/cm³ is achieved in the SiO _x sublayers. Enhanced conductivity, emission, and absorption are attained at low thermal budget, which are promising for photovoltaic applications. Furthermore, the enhancement of Nd³⁺ emission in these multilayers in comparison with the SiO _x /SiO₂ counterparts offers promising future photonic applications.     

2D photonic crystal layer assisted thiosilicate ceramic plate with red‐emitting film for high quality w‐LEDs

In this work, we have succeeded in obtaining high quality warm w‐light‐emitting‐diodes (LEDs) by adopting hybrid two‐dimensional (2D) structure of SiN_x photonic crystal layer (PCL) assisted cyan‐emitting ceramic‐plate thiosilicate SrLa₂Si₂S₈:Ce³⁺ with red‐emitting film SrLiAl₃N₄:Eu²⁺ phosphor on a 430 nm blue LED chip at 350 mA. 2D SiN_x PCL was capped with thiosilicate is because it can enhance the luminous efficacy and maintain the low correlated color temperature (CCT) and high color‐rendering index (CRI). High luminous efficacy (82.3 lm/W), high special CRI (R₉=75) as well as the low CCT (5431 K) of the optimal w‐LED was obtained due to the assistances of 2D SiN_x PCL and narrow‐band red‐emitting phosphor with the doping percentage at 10 wt%. The synthesis processes, structural analysis, optical properties and LED device performances were detailed investigated to find out the relationship between the optimum composition and good optical properties. Based on intriguing luminescence properties by the 2D SiN_x PCL and red‐emitting film phosphor introducing, we proclaim this method could also have high potential application in other phosphor‐converted w‐LEDs.     

The effect of thin gap insertion layer on InP nanostructure grown by metal–organic vapour phase epitaxys

The effect of thin GaP insertion layers on the structural and optical properties of InP/In_0.49Ga_0.51P self‐assembled quantum dots (SAQDs) on GaAs (001) substrate grown by metal–organic vapour phase epitaxy has been reported. The properties of InP/In_0.49Ga_0.51P SAQDs are modified when a thin (1–4 ML) GaP layer is inserted underneath the InP quantum dots (QDs). Deposition of the GaP insertion layer affects the dot dimension and improves the size uniformity. The density, dimension and uniformity of InP QDs strongly depend on the GaP insertion layer thickness. This variation in QD size is a result of a material nucleation effect caused by atomic intermixing between the InP QDs and underlying GaP insertion layer and surface energy. The insertion of GaP layer led to tuning the emission wavelength and narrowing of full width at half maximum (FWHM) when they are characterised by PL measurements at room temperature. © 2012 Canadian Society for Chemical Engineering     

Growth,doping and characterization of epitaxial thin films and patterned structures of AlN,GaN, and AlxGa1−xN

Advancements in the doping of GaN and Al_xGa_1−xN thin films, and the growth of GaN and Al_xGa_1−xN structures on patterned heterostructure substrates via metalorganic vapor phase epitaxy are reported. The acceptor-type behavior of Mg-doped GaN films grown in N₂ diluents is presented. Net ionized impurity concentrations up to 8×10¹⁸ cm⁻³ and Hall mobilities up to ≈14 cm² V⁻¹ s⁻¹ were measured for Mg-doped films grown in N₂ in the as-grown condition. Donor and acceptor doping of Al_xGa_1−xN alloys was performed. Acceptor doping of Al_xGa_1−xN for x≤0.13 and donor doping for x≤0.58 were achieved for films deposited at 1100 °C. Lateral epitaxial overgrowth of GaN and Al_xGa_1−xN layers was investigated. The growth and coalescence of GaN and Al_xGa_1−xN stripes patterned in SiO₂ and/or SiN_x masks deposited on GaN, including aligned second lateral epitaxial overgrowth on initial laterally overgrown GaN layers, are discussed.     

Mechanical and field emission properties of CGed Si(C,N) films synthesized by PECVD from HMDS precursor

Compositionally graded (CGed) Si(C,N) films were prepared by Ar/H₂/N₂ plasma enhanced chemical vapor deposition from liquid injected hexamethyldisiloxane precursor. The films were characterized by scanning/transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Monolithic crystalline SiC and amorphous SiN_x films were produced from Ar/H₂ and Ar/H₂/N₂ thermal plasma, respectively. The CGed SiC–SiN_x film was obtained by changing N₂ flow rate from 2 L/min to zero in Ar/H₂/N₂ during the deposition process, and it was composed of an uppermost crystalline SiC layer, a thin intermediate layer containing nanocomposite c-SiC/a-SiN_x and an innermost layer of amorphous SiN_x. The CGed SiN_x–SiC film, in which SiN_x acts as a top layer with a SiC layer underneath, was fabricated by an inverse change of the plasma gas supply from initial Ar/H₂ to Ar/H₂/N₂. Microhardness increase and promising field emission properties were obtained from these CGed films in comparison with monolithic SiC and SiN_x films.     

Silver nanoparticles enhanced luminescence and stability of CsPbBr3 perovskite quantum dots in borosilicate glass

Series of silver nanoparticles (NPs) embedded CsPbBr₃ quantum dots (QDs) glass was synthesized via the melt-quench method. Ag NPs and CsPbBr₃ QDs coexist in the TEM image of the Ag-doped glass sample. Photoluminescence (PL) spectra show that the 0.1 molar ratio Ag₂O-doped sample had a PL intensity 2.37 times than the undoped sample. This increase is generated by localized surface plasmon resonance coupling between the Ag NPs and CsPbBr₃ QDs. Excessive Ag doping weakens the PL intensity due to spectral self-absorption of the Ag NP surface plasmon resonance (SPR). Self-adsorption of SPR is detrimental to luminescence properties because it increases the amount of photogenerated charge carriers, which proceed through nonradiative relaxation pathways. In addition, stability results of Ag NP-doped-CsPbBr₃ QD glass show that they have excellent stability. This study on Ag NP-doped-CsPbBr₃ QD glass provides a new idea for the future development of perovskite QD optoelectronic devices.     

Preparation strategy for low-stress and uniform SiC-on-diamond wafer: A silicon nitride dielectric layer

Reducing the self-heating of SiC- and GaN/SiC-based high-powered devices by integrating diamond films offers promising performance enhancement of these devices. However, such a reduction strategy faces serious problems, such as diamond nucleation on SiC and stress accumulation greater than 10 GPa. In this work, a SiN_x dielectric layer (～50 nm) was coated onto the C polar face of a 4H–SiC wafer using microwave plasma chemical vapor deposition (MPCVD) to improve direct dense diamond nucleation and growth, significantly reduce the stress, and build Si–C(SiC)?Si?C(diamond) bond bridges. This SiN_x thin layer, prepared by activating Si ions under Ar/N plasma during magnetron sputtering, gave rise to local Si₃N₄ crystal features and a low effective work function (EWF) for promoting surface dipoles with electronegative carbon-containing groups. In the H plasma environment during diamond growth, the local Si₃N₄ crystal was amorphized, and the N atoms escaped as a result of atomic H and the high temperature. At the same time, C atoms diffused into the SiN_x and formed C–Si bonds (49.7% of the total C bonds) by replacing N–Si and Si–Si, thus increasing the direct nucleation density of the diamond grains. The diamond thin film grew rapidly and uniformly, with a grain size of approximately 2 μm in mixed orientation, and the stress of the 2-inch SiC-on-diamond wafer was extremely low (to ～0.1–0.2 GPa). In comparison, partially connected diamond grains (>10 μm) on the bare SiC in the preferential (110) orientation resulted in a film with twin-grain features and significant stress, which was associated with the hexagonal lattice interface of 4H–SiC. These results are considered the material and surface/interface bases for actively controlling wafer fabrication and enhancing the heat dissipation of SiC and GaN/SiC electronics.     

Diamond based field-effect transistors with SiNx and ZrO2 double dielectric layers

A diamond-based field-effect transistor (FET) with SiN_x and ZrO₂ double dielectric layer has been demonstrated. The SiN_x and ZrO₂ gate dielectric are deposited by plasma-enhanced chemical vapor deposition (PECVD) and radio frequency (RF) sputter methods, respectively. SiN_x layer is found to have the ability to preserve the conduction channel at the surface of hydrogen-terminated diamond film. The leakage current density (J) of SiN_x/ZrO₂ diamond metal-insulator-semiconductor FET (MISFET) keeps lower than 3.88 × 10^− 5 A·cm^− 2 when the gate bias was changed from 2 V to − 8 V. The double dielectric layer FET operates in a p-type depletion mode, whose maximum drain-source current, threshold voltage, maximum transconductance, effective mobility and sheet hole density are determined to be − 28.5 mA·mm^− 1, 2.2 V, 4.53 mS·mm^− 1, 38.9 cm²·V^− 1·s^− 1, and 2.14 × 10¹³ cm^− 2, respectively.     

Preparation and Characterization of Silica-Coated Magnetic–Fluorescent Bifunctional Microspheres

Bifunctional magnetic–fluorescent composite nanoparticles (MPQDs) with Fe₃O₄ MPs and Mn:ZnS/ZnS core–shell quantum dots (QDs) encapsulated in silica spheres were synthesized through reverse microemulsion method and characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, vibration sample magnetometer, and photoluminescence (PL) spectra. Our strategy could offer the following features: (1) the formation of Mn:ZnS/ZnS core/shell QDs resulted in enhancement of the PL intensity with respect to that of bare Mn:ZnS nanocrystals due to the effective elimination of the surface defects; (2) the magnetic nanoparticles were coated with silica, in order to reduce any detrimental effects on the QD PL by the magnetic cores; and (3) both Fe₃O₄ MPs and Mn:ZnS/ZnS core–shell QDs were encapsulated in silica spheres, and the obtained MPQDs became water soluble. The experimental conditions for the silica coating on the surface of Fe₃O₄ nanoparticles, such as the ratio of water to surfactant (R), the amount of ammonia, and the amount of tetraethoxysilane, on the photoluminescence properties of MPQDs were studied. It was found that the silica coating on the surface of Fe₃O₄ could effectively suppress the interaction between the Fe₃O₄ and the QDs under the most optimal parameters, and the emission intensity of MPQDs showed a maximum. The bifunctional MPQDs prepared under the most optimal parameters have a typical diameter of 35 nm and a saturation magnetization of 4.35 emu/g at room temperature and exhibit strong photoluminescence intensity.     

Improvement of performance of InAs quantum dot solar cell by inserting thin AlAs layers

A new measure to enhance the performance of InAs quantum dot solar cell is proposed and measured. One monolayer AlAs is deposited on top of InAs quantum dots (QDs) in multistack solar cells. The devices were fabricated by molecular beam epitaxy. In situ annealing was intended to tune the QD density. A set of four samples were compared: InAs QDs without in situ annealing with and without AlAs cap layer and InAs QDs in situ annealed with and without AlAs cap layer. Atomic force microscopy measurements show that when in situ annealing of QDs without AlAs capping layers is investigated, holes and dashes are present on the device surface, while capping with one monolayer AlAs improves the device surface. On unannealed samples, capping the QDs with one monolayer of AlAs improves the spectral response, the open-circuit voltage and the fill factor. On annealed samples, capping has little effect on the spectral response but reduces the short-circuit current, while increasing the open-circuit voltage, the fill factor and power conversion efficiency.     

Effect of the post-heating temperatures on the microstructure,mechanical and electrical properties of silicon nitride thin films

Silicon nitride (SiN_x) thin film is a potential candidate for the fabrication of the insulating layer of thin-film thermocouples, which can be utilised to measure cutting temperatures, owing to its excellent insulation properties and hardness and a thermal expansion coefficient similar to that of carbide tool. Thus, it is necessary to investigate the stability of the SiN_x microstructure and its mechanical and electrical properties at high temperatures. In this study, SiN_x thin films were deposited using reactive magnetron sputtering, followed by post-heating in an air atmosphere at 200–600 °C. The microstructure, adhesion, and sheet resistance were investigated using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scratch tests, and four-probe resistance tests. The results showed that the SiN_x film had a typical amorphous structure. During the heating process, the grain size increased, as did the content of columnar structures. When the temperature was increased from room temperature to 200 °C, the SiN_x film was oxidised to SiN_xO_y. The oxidative product (SiO₂) and escaping nitrogen gas were not observed until the film was heated above 400 °C, revealing the different oxidation reactions and products induced by the elevated temperature. The adhesive strength of the SiN_x film increased monotonically with increasing temperature but was severely weakened when the film was heated to temperatures above 400 °C. Oxygen could not completely invade the deeper layers of the film until the temperature reached 600 °C. The sheet resistance of the SiN_x film improved at 200 °C, but reduced severely when the temperature exceeded 400 °C.     

Substrate Temperature Dependent Surface Morphology and Photoluminescence of Germanium Quantum Dots Grown by Radio Frequency Magnetron Sputtering

The visible luminescence from Ge nanoparticles and nanocrystallites has generated interest due to the feasibility of tuning band gap by controlling the sizes. Germanium (Ge) quantum dots (QDs) with average diameter ~16 to 8 nm are synthesized by radio frequency magnetron sputtering under different growth conditions. These QDs with narrow size distribution and high density, characterized using atomic force microscopy (AFM) and field emission scanning electron microscopy (FESEM) are obtained under the optimal growth conditions of 400 °C substrate temperature, 100 W radio frequency powers and 10 Sccm Argon flow. The possibility of surface passivation and configuration of these dots are confirmed by elemental energy dispersive X-ray (EDX) analysis. The room temperature strong visible photoluminescence (PL) from such QDs suggests their potential application in optoelectronics. The sample grown at 400 °C in particular, shows three PL peaks at around ~2.95 eV, 3.34 eV and 4.36 eV attributed to the interaction between Ge, GeO_x manifesting the possibility of the formation of core-shell structures. A red shift of ~0.11 eV in the PL peak is observed with decreasing substrate temperature. We assert that our easy and economic method is suitable for the large-scale production of Ge QDs useful in optoelectronic devices.     

Synthesis of Multifunctional Silica Composites Encapsulating a Mixture Layer of Quantum Dots and Magnetic Nanoparticles

Multifunctional silica colloidal composites with enhanced photoluminescence (PL) and superparamagnetism are reported. Enhanced PL and superparamagnetism were achieved by encapsulating a mixture layer of quantum dots (QDs) and superparamagnetic iron oxide nanoparticles (SPIONs) within a silica sphere, wherein QDs and SPIONs were capped by 3-mercaptopropionic acid (MPA) and 2-carboxy ethyl phosphonic acid (CEPA), respectively. The silica composites encapsulating a mixture layer of QDs and SPIONs, i.e., S(Q,M)S core(layer)shell architectures with various diameters (80, 360, and 900 nm) were successfully prepared by utilizing electrostatic interaction between positively charged amine-functionalized silica (S) and negatively charged mixture of QD–MPA (Q) and SPION–CEPA (M) and then, by forming a silica shell of 10–20 nm. The S(Q,M)S showed more than twice higher PL intensity than MPA-capped QD with the same QD concentration. Increasing the molar ratio of M/Q from 0.02 to 0.05 in the S(Q,M)S increased the saturation magnetization value from 0.15 to 0.62 emu/g. The S(Q,M)S composites with enhanced PL intensity and superparamagnetism are expected to be a plausible probe material for bioimaing and sensing application. Also, the current synthetic strategy for S(Q,M)S composites is expected to be extendible to include other functional nanoparticles.     

Structural and optical characterization of pure Si-rich nitride thin films

The specific dependence of the Si content on the structural and optical properties of O- and H-free Si-rich nitride (SiN_x>1.33) thin films deposited by magnetron sputtering is investigated. A semiempirical relation between the composition and the refractive index was found. In the absence of Si-H, N-H, and Si-O vibration modes in the FTIR spectra, the transverse and longitudinal optical (TO-LO) Si-N stretching pair modes could be unambiguously identified using the Berreman effect. With increasing Si content, the LO and the TO bands shifted to lower wavenumbers, and the LO band intensity dropped suggesting that the films became more disordered. Besides, the LO and the TO bands shifted to higher wavenumbers with increasing annealing temperature which may result from the phase separation between Si nanoparticles (Si-np) and the host medium. Indeed, XRD and Raman measurements showed that crystalline Si-np formed upon 1100°C annealing but only for SiN_x<0.8. Besides, quantum confinement effects on the Raman peaks of crystalline Si-np, which were observed by HRTEM, were evidenced for Si-np average sizes between 3 and 6 nm. A contrario, visible photoluminescence (PL) was only observed for SiN_x>0.9, demonstrating that this PL is not originating from confined states in crystalline Si-np. As an additional proof, the PL was quenched while crystalline Si-np could be formed by laser annealing. Besides, the PL cannot be explained neither by defect states in the bandgap nor by tail to tail recombination. The PL properties of SiN_x>0.9 could be then due to a size effect of Si-np but having an amorphous phase.