Solid-State Carbon-Doped GaN Schottky Diodes by Controlling Dissociation of the Graphene Interlayer with a Sputtered AlN Capping Layer.

Carbon-doped GaN (GaN:C) Schottky diodes are prepared by controlling the destruction status of the graphene interlayer (GI) on the substrate. The GI without a sputtered AlN capping layer (CL) was destroyed because of ammonia precursor etching behavior in a high-temperature epitaxy. The damaged GI, like nanographite as a solid-state carbon doping source, incorporated the epitaxial growth of the GaN layer. The secondary ion mass spectroscopy depth profile indicated that the carbon content in the GaN layer can be tuned further by optimizing the sputtering temperature of AlN CL because of the better capping ability of high crystalline quality AlN CL on GI being achieved at higher temperature. The edge-type threading dislocation density and carbon concentration of the GaN:C layer with an embedded 550 °C-grown AlN CL on a GI substrate can be significantly reduced to 2.28 × 109 cm-2 and ∼2.88 × 1018 cm-3, respectively. Thus, a Ni-based Schottky diode with an ideality factor of 1.5 and a barrier height of 0.72 eV was realized on GaN:C. The series resistance increased from 28 kΩ at 303 K to 113 kΩ at 473 K, while the positive temperature coefficient (PTC) of series resistance was ascribed to the carbon doping that induced the compensation effect and lattice scattering effect. The decrease of the donor concentration was confirmed by temperature-dependent capacitance-voltage (C-V-T) measurement. The PTC characteristic of GaN:C Schottky diodes created by dissociating the GI as a carbon doping source should allow for the future use of high-voltage Schottky diodes in parallel, especially in high-temperature environments.

InGaN-Based Light-Emitting Diodes Grown on a Micro/Nanoscale Hybrid Patterned Sapphire Substrate.

A hybrid patterned sapphire substrate (hybrid-PSS) was prepared using an anodic aluminum oxide etching mask to transfer nanopatterns onto a conventional patterned sapphire substrate with microscale patterns (bare-PSS). The threading dislocation (TD) suppression of light-emitting diodes (LEDs) grown on a hybrid-PSS (HP-LED) exhibits a smaller reverse leakage current compared with that of LEDs grown on a bare-PSS (BP-LED). The strain-free GaN buffer layer and fully strained InGaN active layer were evidenced by cross-sectional Raman spectra and reciprocal space mapping of the X-ray diffraction intensity for both samples. The calculated piezoelectric fields for both samples are close, implying that the quantum-confined Stark effect was not a dominant mechanism influencing the electroluminescence (EL) peak wavelength under a high injection current. The bandgap shrinkage effect of the InGaN well layer was considered to explain the large red-shifted EL peak wavelength under high injection currents. The estimated LED chip temperatures rise from room temperature to 150 °C and 75 °C for BP-LED and HP-LED, respectively, at a 600-mA injection current. This smaller temperature rise of the LED chip is attributed to the increased contact area between the sapphire and the LED structural layer because of the embedded nanopattern. Although the chip generates more heat at high injection currents, the accumulated heat can be removed to outside the chip effectively. The high diffuse reflection (DR) rate of hybrid-PSS increases the escape probability of photons, resulting in an increase in the viewing angle of the LEDs from 130° to 145°. The efficiency droop was reduced from 46% to 35%, effects which can be attributed to the elimination of TDs and strain relaxation by embedded nanopatterns. In addition, the light output power of HP-LED at 360-mA injection currents exhibits a ∼ 22.3% enhancement, demonstrating that hybrid-PSSs are beneficial to apply in high-power LEDs.

[VUV spectral properties of MO-Y2O3-B2O3 : Re (M = Mg, Sr; Re = Eu, Tb)].

Eu and Tb doped 17MO-7.88Y2O3-75B2O3 samples were prepared by the solid state reaction. VUV excitation properties and luminescence properties under VUV excitation were studied. Excitation spectra exhibited high absorption in VUV region (120-220 nm). There existed strong emission peaking at 591 and 613 nm corresponding to the 5D0 --> 7F(J) (J = 1, 2, 3, 4) emission transition of Eu3+ when MgO-Y2O3-BO3 : Eu was energized by the VUV radiation (147 nm). By the introduction of Sr into MgO-Y2O3-BO2O3: Eu absorption peaking at 147 nm and red emission peaking at 613 nm are enhanced strongly. Besides the host absorption band of MgO-Y2O3-B2O3 : Tb, there existed strong absorption band peaking at 170, 178, 195, 204, 225 nm corresponding to the 4f(8)-4f(7) 5d transition of Tb3+; emission spectra showed strong emission peaks at 491, 547, 590, 621 nm corresponding to the 5D4 --> 7F(J) (J = 6, 5, 4 and 3) emission transition of Tb3+.

[Luminescence properties of Ba2MgSi2O7 : RE].

In the present paper, alkaline earth silicates Ba2MgSi2O7 : RE were prepared under a reducing atmosphere by solid-state reaction. Under UV light excitation, Ce3+ doped pyrosilicates phosphors emit efficient bluish violet light. The emission spectra of Eu2+ doped pyrosilicates phosphors showed that Eu2+ ions could occupy two types of sites in Ba2MgSi2O7 host. The energy transfer from Ce2+ to Eu2+ in Ba2MgSi2O7 is efficient, as there is a large overlap between Ce3+ emission bands and Eu2+ excitation bands. Analysis of the spectra of Ce3+ and Tb3+ co-doped phosphor indicated the existence of energy transfer from Ce3+ to Tb3+ in Ba2MgSi2O7 under UV excitation.