C. J. Summers

Magnetic and optical properties of Ga1−xMnxN grown by metalorganic chemical vapour deposition

Epitaxial layers of Ga1−xMnxN with concentrations of up to x = 0.015 have been grown on c-sapphire substrates by metalorganic chemical vapour deposition. No ferromagnetic second phases were detected via high-resolution x-ray diffraction. Crystalline quality and surface structure were measured by x-ray diffraction and atomic force microscopy, respectively. No significant deterioration in crystal quality and no increase in surface roughness with the incorporation of Mn were detected. Optical measurements show a broad emission band attributed to a Mn-related transition at 3.0 eV that is not seen in the underlying GaN virtual substrate layers. Room temperature ferromagnetic hysteresis has been observed in these samples, which may be due to either Mn-clustering on the atomic scale or the Ga1−xMnxN bulk alloy.

Synthesis of silicon quantum dot buried SiOx films with controlled luminescent properties for solid-state lighting

Highly luminescent Si quantum dot embedded SiOx films were studied as down-converting emitters for solid-state lighting applications. Strong red photoluminescence was observed from these Si nanocrystal embedded films prepared by thermal evaporation of SiO in vacuum or an O2 atmosphere followed by anneal at 1100 °C. The stoichiometry (1.0<x<1.9) and refractive indices (1.5–1.75) of these films could be well controlled by varying the oxygen flow rate and the deposition rate. The emission peak shifted from 840 to 745 nm with increasing O2 flow rate due to a decrease in the size of the Si QDs. Two excitation bands, peaked at 280 and 370 nm, were observed from these samples. The 370 nm band was much stronger than the 280 nm band, which is near the UV LED emission range required for solid-state lighting applications. Blue and green emissions were also observed from samples annealed at a lower temperature.

The Fermi level dependence of the optical and magnetic properties of Ga1−xMnxN grown by metal–organic chemical vapour deposition

The suppression of the ferromagnetic behaviour of metal–organic chemical vapour deposition grown Ga1−x Mnx Ne pilayers by silicon co-doping, and the influence of the Fermi level position on and its correlation with the magnetic and optical properties of Ga1−x Mnx Na re reported. Variation in the position of the Fermi level in the GaN bandgap is achieved by using different Mn concentrations and processing conditions as well as by co-doping with silicon to control the background donor concentration. The effect on Mn incorporation on the formation of defect states and impurity induced energy states within the bandgap of GaN was monitored by means of photoluminescence absorption an de mission spectroscopy. A broad absorption detected around 1.5 eV is attributed to the presence of a subband introduced by Mn induced energy states due to temperature independent transition energies and linewidths. The intensity and the linewidth of the absorption band correlate with the Mn concentration. Similarly, the magnitude of the magnetization decreases as the Fermi level approaches the conduction band, as the Fermi energy is increased above the Mn(0/−) acceptor state. Silicon concentrations >10 19 cm −3 caused the complete loss of ferromagnetic behaviour in the epilayer. The absorption band at 1.5 eV is also not observed upon silicon co-doping. The observed spectroscopic data favour a double-exchange-like mechanism rather than an itinerant free carrier mechanism for causing the ferromagnetism. This behaviour significantly differs from the properties reported for widely studied (Ga, In)MnAs.

Enhancement of white luminescence from SiNx films by surface roughening

White photoluminescence was obtained from multi-layered silicon nitride thin films prepared by plasma-enhanced chemical vapor deposition. The emission colors from single-layered silicon nitride films could be adjusted over the range of 440–660 nm by controlling the SiH4/NH3 flow ratio during deposition. Surface roughening by anisotropic KOH etching of the Si(100) substrate significantly improved the emission extraction efficiency and changed the color-rendering properties from the silicon nitride thin films. This was attributed to the suppression of internal reflection and interference effects from the thin films.

Molecular‐beam epitaxy growth of strontium thiogallate

The molecular‐beam epitaxy growth and characterization of cerium doped strontium thiogallate (SrGa2S4:Ce) thin film phosphors are reported. The layers were grown on GaAs, and glass/indium tin oxide/dielectric stack substrates for device fabrication. Ga2S3/Sr beam equivalent pressure ratios of 20–100 and CeCl3/Sr flux (molecules cm−2 s−1) ratios of 1/20–1/10 were investigated in this study. The substrate temperature was varied between 530 and 575 °C. A typical SrGa2S4:Ce film growth rate of 0.5 μm/h was obtained with Sr, Ga2S3, and CeCl3 beam equivalent pressures of 2.0×10−7, 1.0×10−5, and 3.5×10−8 Torr, respectively. Characterization of the layers’ structural and optical properties by x‐ray diffraction, transmission electron microscopy, energy dispersive x‐ray spectroscopy, and photoluminescence spectroscopy is presented.

White photoluminescence from SiNx films prepared by plasma enhanced chemical vapor deposition

Intense visible blue to red emissions were obtained from SiNx thin films prepared by plasma enhanced chemical vapor deposition (PECVD) using SiH4 and NH3 as the source gases. A continuous blue shift of the photoluminescence (PL) peak from 660nm to 440nm was observed by increasing the NH3 flow rate from 20 to 150sccm, while the flow rate of N2 diluted 2% SiH4 was fixed at 650sccm. This controllable PL was attributed to the quantum confinement effect of Si quantum dots (QDs) which were formed during the deposition process and embedded in the SiNx films. White photoluminescence with multiple emission peaks was achieved for potential solid state lighting applications from multi-layered SiNx films by changing the SiH4/NH3 ratio during the deposition process. This was attributed to a combination of Si quantum dots with different sizes within the different layers. Surface texturing of the thin film samples was conducted by potassium hydroxide (0.56%) etching the (100) Si substrate for 3~40 min at 80°C before deposition. The reflectivity of the etched samples decreased with increasing etch time due to increased surface roughness. The extraction efficiency of light emission from the textured SiNx thin films was significantly improved, owing to a depression of the internal reflection and interference effects. In addition, the elimination of the multiple emission peaks by surface texturing significantly affected the color coordinates of the output spectrum.

Development of new substrate technologies for GaN LEDs: atomic layer deposition transition layers on silicon and ZnO

Al2O3 layers have been deposited by atomic layer deposition (ALD) on both silicon and zinc oxide (ZnO) substrates as a transition layer for MOCVD growth of GaN. These Al2O3 layers have been shown to reduce tensile strain and cracking in GaN thin films on Si, and they have also been shown to help suppress impurity diffusion from the ZnO substrate into the GaN layers. Surface morphology of the ALD-grown layers was investigated using scanning electron microscopy (SEM), and structural properties were studied using high resolution x-ray diffraction (HR-XRD). GaN thin films were then grown on these layers to determine the effects of the Al2O3 layer on subsequent GaN quality. The optical and structural properties of these films were studied, as well as surface morphology. GaN layers grown using the Al2O3 layers on Si in particular exhibit structural and optical properties approaching those of typical GaN thin films on sapphire, which shows significant promise for high performance GaN-based devices on Si substrates.

Growth of InGaN with high indium content on ZnO based sacrificial substrates

ZnO has been considered as a substrate for epitaxial growth of III-Nitrides due to its close lattice and stacking order match. This paper will cover growth of InxGa1-xN epitaxial layer on lattice-matched ZnO substrates by metal-organic chemical vapor deposition (MOCVD). InGaN of various indium compositions from different growth temperatures were well controlled in the InGaN films on ZnO substrates. High-resolution X-ray diffraction (HRXRD) confirmed the epitaxial growth of InGaN film on ZnO. The optical and structural characterization of InGaN epilayer on ZnO substrates was measured by room temperature photoluminescence, temperature-dependent photoluminescence, and field-emission secondary electron microscope. In addition, a transition layer of Al2O3 on ZnO substrates have been employed for InGaN growth to help prevent Zn and O diffusion into the epilayers as well as assist nitride epilayer growth. HRXRD results show a single crystal InGaN film has been successfully grown on annealed Al2O3 coated ZnO substrates.

Challenges in the fabrication of an optical frequency ground plane cloak consisting of silicon nanorod arrays

The application of transformation optical techniques to photonic crystal-like dielectric structures has facilitated the creation of invisibility cloaks that operate at optical wavelengths. In this article, the authors present the fabrication processes for an all-dielectric ground plane cloak structure that consists of multiple silicon nanorod arrays connected by input and output waveguides. An advantage of this particular design is that it does not use metals to obtain metamaterial-like device behavior. The structure consists only of dielectrics that can be processed by the use of electron beam nanofabrication technologies and is designed to operate in the 1400–1600 nm wavelength range.

Multilayer stacked electroluminescent devices

A device concept, the multilayer stacked electroluminescent device is presented. This device consists of a series of double insulating-layer electroluminescent units stacked up on top of one another, separated by transparent electrodes and alternately biased in opposite directions. This unique design allows independent control of the drive voltage and the total phosphor thickness. The drive voltage depends only on the individual phosphor layer thickness while the total phosphor thickness, and thus the total brightness, can be increased by increasing the number of layers. The anticipated enhancement in brightness was predicted by equivalent circuit analysis and demonstrated by prototype devices fabricated by atomic layer epitaxy.