Depth distribution of the strain in the GaN layer with low-temperature AlN interlayer on Si(111) substrate studied by Rutherford backscattering/channeling

The depth distribution of the strain-related tetragonal distortion e(T) in the GaN epilayer with low-temperature AlN interlayer (LT-AlN IL) on Si(111) substrate is investigated by Rutherford backscattering and channeling. The samples with the LT-AlN IL of 8 and 16 nm thickness are studied, which are also compared with the sample without the LT-AlN IL. For the sample with 16-nm-thick LT-AlN IL, it is found that there exists a step-down of e(T) of about 0.1% in the strain distribution. Meanwhile, the angular scan around the normal GaN <0001> axis shows a tilt difference about 0.01degrees between the two parts of GaN separated by the LT-AlN IL, which means that these two GaN layers are partially decoupled by the AlN interlayer. However, for the sample with 8-nm-thick LT-AlN IL, neither step-down of e(T) nor the decoupling phenomenon is found. The 0.01degrees decoupled angle in the sample with 16-nm-thick LT-AlN IL confirms the relaxation of the LT-AlN IL. Thus the step-down of e(T) should result from the compressive strain compensation brought by the relaxed AlN interlayer. It is concluded that the strain compensation effect will occur only when the thickness of the LT-AlN IL is beyond a critical thickness. (C) 2004 American Institute of Physics.

Passively Q-switched mode-locking of diode-pumped Nd : YVO4 laser with GaAs intracavity absorber grown at low temperature

Using a low temperature grown GaAs wafer as an intracavity saturable absorber, a temporal envelope duration of 11 ns of Q- switched and mode- locked ( QML) 1064 nm operation was achieved in a very simple compact plane- concave cavity Nd: YVO4 laser, it was so short that the pulses can be used as Q- switching pulses. The maximal average output power is 808 mW with the repetition rate of 25 kHz, and the corresponding peak power and energy of a single Q- switched pulse was 2.94 kW and 32.3 mu J, respectively. The mode- locked pulse trains inside the Q- switched pulse envelope had a repetition rate of 800 MHz.

The influence of V/III ratio in the initial growth stage on the properties of GaN epilayer deposited on low temperature AlN buffer layer

The V/III ratio in the initial growth stage of metalorganic chemical vapor deposition has an important influence on the quality of a GaN epilayer grown on a low-temperature AIN buffer layer and c-plane sapphire substrate. A weaker yellow luminescence, a narrower half-width of the X-ray diffraction peak, and a higher electron mobility result when a lower V/III ratio is taken. The intensity of in situ optical reflectivity measurements indicates that the film surface is rougher at the beginning of GaN growth, and a longer time is needed for the islands to coalesce and for a quasi-two dimensional mode growth to start. A comparison of front- and back-illuminated photoluminescence spectra confirms that many threading dislocations are bent during the initial stage, leading to a better structural quality of the GaN layer. (C) 2007 Elsevier B.V. All rights reserved.

Low-threshold high-temperature operation of lambda similar to 7.4 mu m quantum cascade lasers

We report low-threshold high-temperature operation of 7.4 mu m strain-compensated InGaAs/InAlAs quantum cascade lasers (QCLs). For an uncoated 22-mu m-wide and 2-mm-long laser, the low-threshold current densities, i.e. 0.33 kA/cm(2) at 81 K in pulsed mode and 0.64 kA/cm(2) at 84 K in cw mode, are realized. High-temperature operation of uncoated devices, with a high value of 223 K, is achieved in cw mode.

A bidirectionall diode-pumped, passively mode-locked Nd : YVO4 ring laser with a low-temperature-grown semiconductor saturable absorber mirror

We report the operation of a bidirectional picosecond pulsed ring Nd:YVO4 laser based on a low-temperature-grown semiconductor saturable absorber mirror. Except for the laser crystal, the six-mirror ring laser cavity has no intra-cavity elements such as focusing lens or mirror. The bidirectional mode locked pluses are obtained at the repetition rate of 117.5 MHz, pulse duration of 81 ps, power of 2 x 200 mW.

Rapid photoluminescence quenching in GaInNAs quantum wells at low temperature

Temperature dependence of optical properties of GaInNAs/GaAs quantum wells (QWs) has been studied by photoluminescence (PL) and time-resolved PL. A rapid PL quenching is observed even at very low temperature and is of the excitation power dependence. These results strongly suggest that the non-radiative recombination process plays a very important role at low temperature. In the TRPL measurement the shape of the PL decay curve shows significant difference under different excitation powers. It is attributed to the different involvement of non-radiative recombination in the overall recombination process. The TRPL data are well fitted with the rate equation involving both the radiative and non-radiative recombination. (c) 2006 Elsevier B.V. All rights reserved.

Pressure dependence of the near-band-edge photoluminescence from ZnO microrods at low temperature

The temperature and pressure dependences of band-edge photo luminescence from ZnO microrods have been investigated. The energy separation between the free exciton (FX) and its first order phonon replica (FX-1LO) decreases at a rate of k(B)T with increasing temperature. The intensity ratio of the FX-1LO to the bound exciton (BX) emission is found to decrease slightly with increasing pressure. All of the exciton emission peaks show a blue shift with increasing pressure. The pressure coefficient of the FX transition, longitudinal optical (LO) phonon energy, and binding energy of BX are estimated to be 21.4, 0.5, and 0.9 meV/GPa, respectively. (c) 2006 Elsevier Ltd. All rights reserved.

High-performance 1.55 mu m low-temperature-grown GaAs resonant-cavity-enhanced photodetector

A 1.55 mu m low-temperature-grown GaAs (LT-GaAs) photodetector with a resonant-cavityenhanced structure was designed and fabricated. A LT-GaAs layer grown at 200 degrees C was used as the absorption layer. Twenty- and fifteen-pair GaAs/AlAs-distributed Bragg reflectors were grown as the bottom and top mirrors. A responsivity of 7.1 mA/W with a full width at half maximum of 4 nm was obtained at 1.61 mu m. The dark current densities are 1.28x10(-7) A/cm(2) at the bias of 0 V and 3.5x10(-5) A/cm(2) at the reverse bias of 4.0 V. The transient response measurement showed that the photocarrier lifetime in LT-GaAs is 220 fs. (c) 2006 American Institute of Physics.

Experimental investigation of intracavity absorber low temperature GaAs in diode-pumped Nd : GdVO4 laser

A passively Q-switched and mode-locked diode-pumped Nd:GdVO4 laser was demonstrated using a low-temperature-grown GaAs wafer (LT-GaAs) as an intracavity saturable absorber. The maximal Q-switched mode-locked average output power was 750 mW with the Q-switched envelop having a repetition rate of 167 kHz. The mode-locked pulse trains inside the Q-switched pulse envelope had a repetition rate of similar to 790 MHz.

GaAs absorber grown at low temperature used in passively Q-switched diode pumped solid state laser

We report, for the first time to the best of our knowledge, on a passively Q-switched Nd:YVO4 laser with a GaAs absorber grown at low temperature (LT) by metal organic vapor phase expitaxy. Using the LT GaAs absorber as well as an output coupler, a passively Q-switched laser whose pulse duration is as short as 90 ns, was obtained.

Crack control in GaN grown on silicon (111) using In doped low-temperature AlGaN interlayer by metalorganic chemical vapor deposition

The effects of In doped low-temperature (LT) AlGaN interlayer on the properties of GaN/Si(111) by MOCVD have been investigated. Using In doping LT-interlayer can decrease the stress sufficiently for avoiding crack formation in a thick (2.0 mu m) GaN layer. Significant improvement in the crystal and optical properties of GaN layer is also achieved. In doping is observed to reduce the stress in AlGaN interlayer measured by high-resolution X-ray diffraction (HRXRD). It can provide more compressive stress to counteract tensile stress and reduce crack density in subsequent GaN layer. Moreover, as a surfactant, indium is observed to cause an enhanced PL intensity and the narrowed linewidths of PL and XRD spectra for the LT-interlayer. Additionally, the crystal quality of GaN layer is found to be dependent on the growth parameters of underneath In-doped LT-AlGaN interlayer. The optimal parameters, such as TMIn flow rate, TMAl flow rates and thickness, are achieved to obtain nearly 2.0 mu m thick crack free GaN film with advanced optical and crystal properties. (c) 2005 Elsevier B.V. All rights reserved.

Q-switched and mode-locked diode-pumped Nd : GdVO4 laser with low temperature GaAs saturable absorber

Low temperature GaAs (LT-GaAs) was successfully grown at the temperature of 550 degrees C by metal organic vapor phase epitaxy on a semi-insular GaAs substrate. With such an absorber as well as an output coupler we obtain Q-switched mode-locked (QML) 1064 nm Nd:GdVO4 laser pumped by diode laser with high repetition rate, formed with a simple flat-flat cavity. The repetition rate of the Q-switched envelope increased from 100 to 660 kHz as the pump power increased from 2.28 to 7.29 W. The mode-locked pulses inside the Q-switched pulse envelope had a repetition rate of similar to 1.36 GHz. A maximum average output power of 953 mW was obtained. The dependence of the operational parameters on the pump power was also investigated experimentally. (C) 2005 Elsevier B.V. All rights reserved.

Influence of the cavity on the low-temperature photoluminescence of SiGe/Si multiquantum wells grown on a silicon-on-insulator substrate

The influences of the cavity on the low-temperature photoluminescence of Si0.59Ge0.41/Si multiquantum wells grown on silicon-on-insulator substrates are discussed. The positions of the modulated photoluminescence (PL) peaks not only relate to the nature of SiGe/Si multiquantum wells, but also relate to the characteristic of the cavity. With increasing temperature, a redshift of the modulated PL peak originating from the thermo-optical effect of the cavity is observed.

Growth of low-density InAs/GaAs quantum dots on a substrate with an intentional temperature gradient by molecular beam epitaxy

InAs was deposited by molecular beam epitaxy (MBE) on a GaAs substrate with an intentional temperature gradient from centre to edge. Two-dimensional (2D) to three-dimensional (3D) morphology evolution was found along the direction in which the substrate temperature was decreasing. Quantum dots (QDs) with density as low as similar to 8 x 10(6) cm(-2) were formed in some regions. We attribute the morphological evolution to the temperature-dependent desorption of deposited indium and the intermixing between deposited indium and gallium from the buffer.

Effect of a low-temperature thin buffer layer on the strain accommodation of In0.25Ga0.75As grown on a GaAs(001) substrate

Various low-temperature (LT) ultra-thin buffer layers have been fabricated on the GaAs (001) substrate. The buffer layer is decoupled from the host substrate by introducing low-temperature defects. The 400 nm In0.25Ga0.75As films were grown on these substrates to test the 'compliant' effects of the buffer layers. Atomic force microscopy, photoluminescence, double crystal x-ray diffraction and transmission electron microscopy were used to estimate the quality of the ln(0.25)Ga(0.75)As layer. The measurements indicated that the misfit strains in the epilayer can be accommodated by the LT ultra-thin buffer layer. The strain accommodation effects of the LT defects have been discussed in detail.