Deep levels in high resistivity GaN detected by thermally stimulated luminescence and first-principles calculations

Thermally stimulated luminescence spectroscopy has been applied to study the deep centres in unintentionally doped high resistivity GaN epilayers grown by the metal organic chemical vapour deposition method on c-sapphire substrates. Two trap states with activation energies of 0.12 and 0.62 eV are evaluated from two luminescence peaks at 141.9 and 294.7 K in the luminescence curve. Our spectroscopy measurement, in combination with more accurate first-principles studies, provided insights into the microscopic origin of these levels. Our investigations suggest that the lower level at 0.12 eV might originate from C-N, which behaves as a hole trap state; the deeper level at 0.62 eV can be correlated with V-Ga that corresponds to the yellow luminescence band observed in low-temperature photoluminescence spectra.

An internally-matched GaN HEMTs device with 45.2 W at 8 GHz for X-band application

Optimized AlGaN/AlN/GaN high electron mobility transistor (HEMT) with high mobility GaN channel layer structures were grown on 2-in. diameter semi-insulating 6H-SiC substrates by MOCVD. The 2-in. diameter GaN HEMT wafer exhibited a low average sheet resistance of 261.9 Omega/square, with the resistance un-uniformity as low as 2.23%. Atomic force microscopy measurements revealed a smooth AlGaN surface whose root-mean-square roughness is 0.281 nm for a scan area of 5 x 5 mu m. For the single-cell HEMTs device of 2.5-mm gate width fabricated using the materials, a maximum drain current density of 1.31 A/mm, an extrinsic transconductance of 450 mS/mm, a current gain cutoff frequency of 24 GHz and a maximum frequency of oscillation 54 GHz were achieved. The four-cell internally-matched GaN HEMTs device with 10-mm total gate width demonstrated a very high output power of 45.2 W at 8 GHz under the condition of continuous-wave (CW), with a power added efficiency of 32.0% and power gain of 6.2 dB. To our best knowledge, the achieved output power of internally-matched devices are the state-of-the-art result ever reported for X-band GaN-based HEMTs. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.

Effect of Nitridation on Morphology, Structural Properties and Stress of AIN Films

We investigate effects of nitridation on AIN morphology, structural properties and stress. It is found that 3 min nitridation can prominently improve AIN crystal structure, and slightly smooth the surface morphology. However, 10 min nitridation degrades out-of-plane crystal structure and surface morphology instead. Additionally, 3-min nitridation introduces more tensile stress (1.5 GPa) in AIN films, which can be attributed to the weaker islands 2D coalescent. Nitridation for 10 min can introduce more defects, or even forms polycrystallinity interlayer, which relaxes the stress. Thus, the stress in AIN with 10 min nitridation decreases to -0.2 GPa compressive stress.

Simulation of light coupling enhancement and localization of transmission field via subwavelength metallic gratings

Surface plasmons(SPs) generated in nano metallic gratings on medium layer can greatly enhance the transmission field through the metallic gratings. The enhancement effect is achieved from lambda = 500 nm to near-infrared domain. The enhancement rate is about 110 % at the wavelength of about 6 10 nm and about 180 % at lambda = 700 nm and 740 nm where most kinds of thin film solar cells have a high spectral response. These structures should provide a promising way to increase the coupling efficiency of thin film solar cells and optical detectors of different wavelength response.

Carrier transport assisted by dopants in doped poly(N-vinylcarbozole) light-emitting diodes

We have investigated the transient electroluminescence (EL) onset of the double-layer light-emitting devices made from poly(N-vinylcarbozole) (PVK) doped with 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and tris(8-hydroxy-quinoline) aluminium (Alq(3)). For the double-layered device in which PVK was doped with 0.1 wt% DCJTB, the EL onset of PVK lags that of DCJTB and Alq(3), while the EL onset of DCJTB and Alq(3) is simultaneous. However, the EL emission of the double-layered device of PVK/Alq(3) originates only from Alq(3). The results show that DCJTB dopants can not only help to tunnel electrons from Alq(3) zone to PVK but can also assist electrons transfer in PVK under high electric field by hopping between DCJTB molecules or from DCJTB to PVK sites at a low doping concentration of 0.1 wt%. When the DCJTB doping concentration is 4.0 wt%, the EL onset of Alq(3) lags that of DCJTB. The difference in the EL onsets of DCJTB, Alq(3) and PVK is attributed to the slow build-up of the internal space charge in the vicinity of the interface between PVK and Alq(3). The electron potential difference of the interface between Alq(3) and PVK doped by DCJTB can be adjusted by changing the DCJTB doping concentration in double-layer devices.

Characteristics of oxide-free InGaAlAs layers grown by narrow stripe selective MOVPE

Narrow stripe selective MOVPE has been used to grow high quality oxide-free InGaAlAs layers on an InP substrate patterned with SiO2 masks at optimized growth conditions. Mirror-like surface morphologies and abrupt cross sections are obtained in all samples without spike growth at the mask edge. For the narrow stripe selectively grown InGaAlAs layers with a mesa width of about 1.2 mu m, a bandgap wavelength shift of 70 nm, a photoluminescence (PL) intensity of more than 80% and a PL full width at half maximum (FWHM) of less than 60 meV are obtained simultaneously with a small mask width variation from 0 to 40 mu m. The characteristics of the thickness enhancement ratio and the PL spectrum dependence on the mask width are presented and explained by considering both the migration effect from a masked region and the lateral vapour diffusion effect.

1.55-mu m ridge DFB laser integrated with a buried-ridge-stripe dual-core spot-size converter by quantum-well intermixing

A ridge distributed feedback laser monolithically integrated with a buried-ridge-stripe spot-size converter operating at 1.55 mu m was successfully fabricated by means of low-energy ion implantation quantum-well intermixing and dual-core technologies. The passive waveguide was optically combined with a laterally exponentially tapered active core to control the mode size. The devices emit in a single transverse and single longitudinal mode with a sidemode suppression ratio of 38.0 dB. The threshold current was 25 mA. The beam divergence angles in the horizontal and vertical directions were as small as 8.0 degrees x 12.6 degrees, respectively, resulting in 3.0-dB coupling loss with a cleaved single-mode optical fiber.

Laser diode monolithically integrated with an electroabsorption modulator and dual-waveguide spot-size converter

A 1.60-mu m laser diode and electroabsorption modulator monolithically integrated with a dual-waveguide spot-size converter output for low-loss coupling to cleaved single-mode optical fiber is demonstrated. The devices emit in a single transverse and quasi-single longitudinal mode with a side mode suppression ratio of 25.6 dB. These devices exhibit a 3-dB modulation bandwidth of 16.0 GHz, and modulator extinction ratios of 16.2 dB dc. The beam divergence angle is about 7.3x10.6 deg, resulting in 3.0-dB coupling loss with cleaved single-mode optical fiber. (c) 2005 Society of Photo-optical Instrumentation Engineers.

Monolithically integrated semiconductor optical amplifier and electroabsorption modulator with dual-waveguide spot-size converter input and output

We have demonstrated an electroabsorption modulator and semiconductor optical amplifier monolithically integrated with novel dual-waveguide spot-size converters (SSC) at the input and output ports for low-loss coupling to a planar light-guide circuit silica waveguide or cleaved single-mode optical fibre. The device was fabricated by means of selective-area MOVPE growth, quantum well intermixing and asymmetric twin waveguide technologies with only a three-step low-pressure MOVPE growth. For the device structure, in the SOA/EAM section, a double ridge structure was employed to reduce the EAM capacitances and enable high bit-rate operation. In the SSC sections, buried ridge structure (BRS) was incorporated. Such a combination of ridge, ATG and BRS structure is reported for the first time in which it can take advantage of easy processing of the ridge structure and the excellent mode characteristic of BRS. At the wavelength range of 1550-1600 nm, lossless operation with extinction ratios of 25 dB dc and more than 10 GHz 3 dB bandwidth is successfully achieved, The beam divergence angles of the input and output ports of the device are as small as 8.0 degrees x 12.6 degrees, resulting in 3.0 dB coupling loss with a cleaved single-mode optical fibre.

Monolithically integrated laser diode and electroabsorption modulator with dual-waveguide spot-size converter input and output

We have demonstrated a 1.60 mu m ridge-structure laser diode and electroabsorption modulator monolithically integrated with buried-ridge-structure dual-waveguide spot-size converters at the input and output ports for low-loss coupling to a cleaved single-mode optical fibre by means of selective area growth and asymmetric twin waveguide technologies. The devices emit in single transverse and quasi-single longitudinal modes with a side mode suppression ratio of 25.6 dB. These devices exhibit 3 dB modulation bandwidth of 15.0 GHz and modulator extinction ratios of 14.0 dB dc. The output beam divergence angles of the spot-size converter in the horizontal and vertical directions are as small as 7.3 degrees x 10.6 degrees, respectively, resulting in 3.0 dB coupling loss with a cleaved single-mode optical fibre.

Comparative study of InAs quantum dots grown on different GaAs substrates by MOCVD

We present a comparative study of InAs quantum dots grown on Si-doped GaAs (10 0) substrates, Si-doped GaAs (10 0) vicinal substrates, and semi-insulating GaAs (10 0) substrates. The density and size distribution of quantum dots varied greatly with the different substrates used. While dots on exact substrates showed only one dominant size, a clear bimodal size distribution of the InAs quantum dots was observed on GaAs vicinal substrates, which is attributed to the reduced surface diffusion due to the presence of multiatomic steps. The emission wavelength is blueshifted during the growth of GaAs cap layer with a significant narrowing of FWHM. We found that the blueshift is smaller for QDs grown on GaAs (10 0) vicinal substrates than that for dots on exact GaAs (100) substrates. This is attributed to the energy barrier formed at the multiatomic step kinks which prohibits the migration of In adatoms during the early stage of cap layer growth. (c) 2005 Elsevier B.V. All rights reserved.

Lossless electroabsorption modulator monolithically rntegrated with a semiconductor optical amplifier and dual-wavegulde spot-size converters

Semiconductor optical amplifier and electroabsorption modulator monolithically integrated with dual-waveguide spot-size converters at the input and output ports is demonstrated by means of selective area growth, quantum-well intermixing, and asymmetric twin waveguide technologies. At the wavelength range of 1550 similar to 1600 nm, lossless operation with extinction ratios of 25-dB dc and 11.8-dB radio frequency and more than 10-GHz 3-dB modulation bandwidth is successfully achieved. The output beam divergence angles of the device in the horizontal and vertical directions are as small as 7.3 degrees x 10.6 degrees, respectively, resulting in 3.0-dB coupling loss with cleaved single-mode optical fiber.

Frequency dependence of junction capacitance of GaN p-i-n UV detectors

In this paper frequency dependence of small-signal capacitance of p-i-n UV detectors, which were fabricated on GaN grown on sapphire substrate by metalorganic chemical vapor deposition, has been studied. The Schibli-Milnes model was used to analyze the capacitance-frequency characteristics. According to high frequency C-V measurements, the deep level mean concentration is about 2.98 x 10(20) cm(-3). The deep level is caused by the un-ionised Mg dopant. The calculated Mg activation energy is 260 meV and the hole thermal capture cross section of the deep level is about 2.73 x 10(-22) cm(2). The applicability of the Schibli-Milnes model is also discussed when the concentration of deep levels exceeds that of the heavily doped n-side. It is concluded that the analytic expression of the Schibli-Milnes model can still be used to describe the capacitance-frequency characteristics of GaN p-i-n UV detectors in good agreement with experiment. (c) 2005 Elsevier Ltd. All rights reserved.

Laser diode integrated with a dual-waveguide spot-size converter by low-energy ion implantation quantum well intermixing

A ridge laser diode monolithically integrated with a buried-ridge-structure dual-waveguide spot-size converter operating at 1.58 mu m is successfully fabricated by means of low-energy ion implantation quantum well intermixing and asymmetric twin waveguide technology. The passive waveguide is optically combined with a laterally tapered active core to control the mode size. The devices emit in a single transverse and quasi single longitudinal mode with a side mode suppression ratio of 40.0dB although no grating is fabricated in the LD region. The threshold current is 50 mA. The beam divergence angles in the horizontal and vertical directions are as small as 7.3 degrees x 18.0 degrees, respectively, resulting in 3.0dB coupling loss With a cleaved single-mode optical fibre.

Diode-end-pumped passively mode-locked ceramic Nd : YAG Laser with a semiconductor saturable mirror

We report on a diode- pumped CW passively mode locked ceramic Nd: YAG laser with SESAM ( semiconductor saturable absorber mirror), wavelength 1064nm. At a pump power of 7.6w, the pulse width was estimated to be similar to 8.3ps with repetition rate similar to 130MHz and the average output power was 1.59w. To our knowledge, this was the first demonstration that ceramic Nd: YAG was used for diode pumped CW passively mode locking. (C) 2005 Optical Society of America.