Effect of trimethylgallium flow on the structural and optical properties of InGaN/GaN multiple quantum wells

InGaN/GaN multiple quantum wells (MQWs) are grown by metal-organic chemical vapour deposition on (0001) sapphire substrates. Triple-axis X-ray diffraction (TXRD) and photoluminescence (PL) spectra are used to assess the influence of trimethylgallium (TMGa) flow on structural defects, such as dislocations and interface roughness, and the optical properties of the MQWs. In this paper, a method, involving an ! scan of every satellite peak of TXRD, is presented to measure the mean dislocation density of InGaN/GaN MQWs. The experimental results show that under certain conditions which keep the trimethlyindium flow constant, dislocation density and interface roughness decrease with the increase of TMGa flow, which will improve the PL properties. It can be concluded that dislocations, especially edge dislocations, act as nonradiative recombination centres in InGaN/GaN MQWs. Also noticed is that changing the TMGa flow has more influence on edge dislocations than screw dislocations.

Self-starting passively mode-locking all-solid-state laser with GaAs absorber grown at low temperature

We realize a stable self-starting passively mode-locking all-solid-state laser by using novel GaAs mirrors as the absorber and output coupler. The GaAs mirror is grown by the technology of metal organic chemical vapour deposition at low temperature. With such an absorber as the output coupler in the laser resonator, laser pulses with duration of 42ps were generated at a repetition rate of 400MHz, corresponding to the average power of 590mW.

A 10-GHz bandwidth electroabsorption modulated laser by ultra-low-pressure selective area growth

A novel integration technique has been developed using band-gap energy control of InGaAsP/InGaAsP multi-quantum-well (MQW) structures during simultaneous ultra-low-pressure (22 mbar) selective-area-growth (SAG) process in metal-organic chemical vapour deposition. A fundamental study of the controllability of band gap energy by the SAG method is performed. A large band-gap photoluminescence wavelength shift of 83nm is obtained with a small mask width variation (0-30 mu m). The method is then applied to fabricate an MQW distributed-feedback laser monolithically integrated with an electroabsorption modulator. The experimental results exhibit superior device characteristics with low threshold of 19 mA, over 24 dB extinction ratio when coupled into a single mode fibre. More than 10GHz modulation bandwidth is also achieved, which demonstrates that the ultra-low-pressure SAG technique is a promising approach for high-speed transmission photonic integrated circuits.

Passively mode-locking Nd : Gd0.5Y0.5VO4 laser with an In(0.25)Ga(0.75)Aa absorber grown at low temperature

We have demonstrated stable self-starting passive mode locking in a diode-end-pumped Nd:Gd-0.8-Y0.5VO4 laser by using an In0.25Ga0.75As absorber grown at low temperature (LT In0.25Ga0.75As absorber). An In0.25Ga0.75As single-quantum-well absorber, which was grown directly on the GaAs buffer by use of the metal-organic chemical-vapor deposition technique, acts simultaneously as a passive mode-locking device and as an output coupler. Continuous-wave mode-locked pulses were obtained at 1063.5 nm. We achieved a pulse duration of 2.6 ps and an average output power of 2.15 W at a repetition rate of 96.4 MHz. (c) 2005 Optical Society of America.

Luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates

In this study, we report comparative luminescence properties of multi-layer InGaN quantum dots grown on C- and R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). High-density InGaN quantum dots (QDs) are formed on GaN templates by decreasing the growth temperature and increasing the adatom hopping-barrier through surface passivation. Atomic force microscopy (AFM) has been employed to estimate the size and height of these dots. Photoluminescence (PL) spectra recorded from (1120) InGaN QDs/(1102) sapphire show much stronger emission intensity compared to spectra recorded from (0001) InGaN QDs/(0001) sapphire. Due to the absence of strong spontaneous polarization and piezoelectric field, such (1150) InGaN QDs in the active layers would lead to high efficiency light emitting devices. (c) 2005 Elsevier B.V. All rights reserved.

Passive Q-switching modelocked Yb3+-doped fibre laser with GaAs absorber grown at low temperature

GaAs absorber was grown at low temperature (550degreesC) by metal organic chemical vapour deposition (MOCVD) and was used as an output coupler with which we realized Q-switching modelocked Yb3+-doped fibre laser. The shortest period of the envelope of the Q-switched modelocking is about 3mus. The modelocking threshold is 4.27W and the highest average output pulse power is 290 mW. The modelocking frequency is 12 MHz.

Rapid thermal annealing properties of ZnO films grown using methanol as oxidant

ZnO thin films were grown by metal-organic chemical vapour deposition using methanol as oxidant. Rapid thermal annealing (RTA) was performed in an ambient of one atmosphere oxygen at 900 degrees C for 60 s. The RTA properties of the films have been characterized using scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, photoluminescence spectra and Hall measurement. The grains of the film were well coalesced and the surface became denser after RTA. The full-width at half maximum of rocking curves was only 496 arcsec. The ZnO films were also proved to have good optical quality. The Hall mobility increased to 43.2 cm(2) V-1 s(-1) while the electron concentration decreased to 6.6 x 10(16) cm(-3). It is found that methanol is a potential oxidant for ZnO growth and the quality of ZnO film can be improved substantially through RTA.

Temperature dependence of absorption edge in MOCVD grown GaN

We have studied the temperature dependence of absorption edge of GaN thin films grown on sapphire substrate by metal-organic chemical vapor deposition using optical absorption spectroscopy. A shift in absorption edge of about 55 meV has been observed in temperature range 273-343 K. We have proposed a theoretical model to find the energy gap from absorption coefficient using alpha = alpha(max) + (alpha(min) - alpha(max))/[1 + exp 2(E - E-g + KT)/KT]. Temperature dependence of band gap has also been studied by finding an appropriate theoretical fit to our data using E-g(T) = E-g(273 K) - (8.8 x 10(-4)T(2))/(483 + T) + 0.088 (Varshni empirical formula) and E-g(T) = E-g(273 K)-0.231447/[exp(362/T)-1] + 0.082 relations. It has been found that data can be fitted accurately after adding a factor similar to 0.08 in above equations. Debye temperature (483 K) and Einstein temperature (362 K) in the respective equations are found mutually in good agreement.

Combined structure of ZnO vertical well-aligned nanorods and net-like structures on AIN/sapphire

ZnO vertical well-aligned nanorods were grown on A1N/sapphire by using metal-organic chemical vapor deposition. We first observed the ZnO net-like structures under the nanorods. The different strain was determined in these two layers by using double crystal X-ray diffraction, Raman spectra, which revealed that the nanorods were relaxed and the net-like structures were strained. The optical properties of two layers were measured by using the cathodoluminescence and photo luminescence and the shift of UV peaks was observed. Moreover, the growth mechanism of the ZnO nanorods and the net-like structures is discussed. (c) 2007 Elsevier B.V. All rights reserved.

Enhanced Pockels effect in GaN/AlxGa1-xN superlattice measured by polarization-maintaining fiber Mach-Zehnder interferometer

Six-period 4 nm GaN/10 nm AlxGa1-xN superlattices with different Al mole fractions x were prepared on (0001) sapphire substrates by low-temperature metal-organic chemical vapor deposition. The linear electro-optic (Pockels) effect was studied by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55 mu m. The measured electro-optic coefficients, gamma(13)=5.60 +/- 0.18 pm/V, gamma(33)=19.24 +/- 1.21 pm/V (for sample 1, x=0.3), and gamma(13)=3.09 +/- 0.48 pm/V, gamma(33)=8.94 +/- 0.36 pm/V (for sample 2, x=0.1), respectively, are about ten times larger than those of GaN bulk material. The enhancement effect in GaN/AlxGa1-xN superlattice can be attributed to the large built-in field at the interfaces, depending on the mole fraction of Al. (C) 2007 American Institute of Physics.

First and second order Raman scattering spectroscopy of nonpolar a-plane GaN

Nonpolar a-plane [(1120)] GaN samples have been grown on r-plane [(1102)] sapphire substrates by low-pressure metal-organic chemical-vapor deposition. The room-temperature first and second order Raman scattering spectra of nonpolar a-plane GaN have been measured in surface and edge backscattering geometries. All of the phonon modes that the selection rules allow have been observed in the first order Raman spectra. The frequencies and linewidths of the active modes have been analyzed. The second order phonon modes are composed of acoustic overtones, acoustic-optical and optical-optical combination bands, and optical overtones. The corresponding assignments of second order phonon modes have been made. (c) 2007 American Institute of Physics.

ZnO nanostructures grown on AlN/sapphire substrates by MOCVD

ZnO nanorods and nanotubes are successful synthesized on AlN/sapphire substrates by metal-organic chemical vapour deposition (MOCVD). The different morphology and structure properties of ZnO nanorods and nanotubes are found to be allected by the AlN under-layer. The photoluminescence spectra show the optical properties of the ZnO nanorods and nanotubes, in which a blueshift of UV emission is observed and is attributed to the surface effect.

Effects of rapid thermal annealing on the emission properties of highly uniform self-assembled InAs/GaAs quantum dots emitting at 1.3 mu m

The authors report the effects of rapid thermal annealing (RTA) on the emission properties of highly uniform self-assembled InAs quantum dots (QDs) emitting at 1.3 mu m grown on GaAs substrate by metal organic chemical vapor deposition. Postgrowth RTA experiments were performed under N-2 flow at temperatures ranging from 600 to 900 degrees C for 30 s using GaAs proximity capping. Surprisingly, in spite of the capping, large blueshifts in the emission peak (up to about 380 meV at 850 degrees C) were observed (even at low annealing temperatures) along with enhanced integrated photoluminescence (PL) intensities. Moreover, pronounced peak broadenings occurred at low annealing temperatures (< 700 degrees C), indicating that RTA does not always cause peak narrowing, as is typically observed with traditional QDs with large inhomogeneous PL linewidths. The mechanism behind the large peak blueshift was studied and found to be attributed to the as-grown QDs with large size, which cause a larger dot-barrier interface and greater strain in and near the QD regions, thereby greatly promoting Ga-In intermixing across the interface during RTA. The results reported here demonstrate that it is possible to significantly shift the emission peak of the QDs by RTA without any additional procedures, even at lower annealing temperatures. (c) 2007 American Institute of Physics.

Temperature dependence of the Raman-active modes in the nonpolar a-plane GaN film

Temperature dependences of the polarized Raman scattering spectra in the backscattering configuration of the nonpolar a-plane (or [11 (2) over bar0]-oriented) GaN thin film are analyzed in the range from 100 to 570 K. The nonpolar a-plane GaN film is grown on an r-plane [or (1 (1) over bar 02)-oriented] sapphire substrate by metal organic chemical vapor deposition. The spectral features of the Raman shifts, intensities, and linewidths of the active phonons modes A(1)(TO), E-1(TO), and E-2(high) are significantly revealed, and corresponding temperature coefficients are well deduced by the empirical relationships. With increasing the measurement temperature the Raman frequencies are substantially redshifted and the linewidths gradually broaden. The compressive-strain-free temperature for the nonpolar a-plane GaN film is found to be at about 400 K. Our studies will lead to a better understanding of the fundamental physical characteristics of the nonpolar GaN film. (c) 2007 American Institute of Physics.

Effect of an indium-doped barrier on enhanced near-ultraviolet emission from InGaN/AlGaN: In multiple quantum wells grown on Si(111)

Low indium content InGaN/AlGaN multiple quantum wells (MQWs) have been grown on Si(111) substrate by metal-organic chemical vapour deposition (MOCVD). A new method of using an isoelectronic indium-doped AlGaN barrier has been found to be very effective in improving the crystalline quality and interfacial abruptness of InGaN quantum well layers. We grew five periods of In0.06Ga0.94N/Al0.20Ga0.80N:In MQWs with In-doped barrier layers and obtained strong near-ultraviolet (UV) emission (similar to 400 nm) at room temperature. An In-doped AlGaN barrier improves the room-temperature PL intensity of InGaN/AlGaN MQWs, making it a candidate barrier for a near-UV source on Si substrate.