Stochastic resonance in quantum-well semiconductor lasers

The quantum well (QW) semiconductor lasers have become main optical sources for optical fibre communication systems because of their higher modulation speed, broader modulation bandwidth and better temperature characteristics. In order to improve the quality of direct-modulation by means of the stochastic resonance (SR) mechanism in QW semiconductor lasers, we investigate the behaviour of the SR in direct-modulated QW semiconductor laser systems. Considering the cross-correlated carrier noise and photon noise, we calculate the power spectrum of the photon density and the signal-to-noise ratio (SNR) of the direct-modulated laser system by using the linear approximation method. The results indicate that the SR always appears in the dependence of the SNR on the bias current density, and is strongly affected by the cross-correlation coefficient of the carrier and photon noises, the frequency of modulation signal, and the photon lifetime in the laser cavity.

A 1.5 mu m n-type InGaAsP/InGaAsP modulation-doped multiple quantum well DFB laser by MOCVD

1.5 mu m n-type InGaAsP/InGaAsP modulation-doped multiple quantum well (MD-MQW) DFB lasers have been fabricated successfully by low pressure metal organic chemical vapour deposition (LP-MOCVD) technology. The experimental results indicate that n-type MD-MQWs can effectively reduce the threshold Current compared with conventional multiple quantum well DFB lasers. Theoretical analysis indicates that such an effect is due to the much smaller absorption loss and lower Auger recombination, compared with that in an undoped MQW structure. Moreover, the introduction of n-type dopant of suitable levels of concentration in the barrier layers enhances the dynamic characteristics of DFB lasers, due to a coupling between the adjacent quantum well layers and tunnelling-assisted injection, which can reduce the relatively long capture time and increase the effective differential gain 1/X dG/dn .

Realization of GaAs/AlGaAs quantum-cascade lasers with high average optical power

Quasi-continuous-wave operation of GaAs/AlGaAs quantum-cascade lasers with high average optical power is demonstrated. Double X-ray diffraction has been used to investigate the quality of the epitaxial material. The compositional gradients and the interface quality are controlled effectively. The corrected average power of per facet about 17 mW and temperature tuning coefficient of the gain peak about 0.91 nm/K from 83 K to 140 K is achieved in pulse operation. Best value of threshold current density is less than 3.0 kA/cm(2) at 83 K. (C) 2005 Elsevier Ltd. All rights reserved.

Numerical simulation of nc-Si : H/c-Si heterojunction solar cells

AMPS simulator, which was developed by Pennsylvania State University, has been used to simulate photovoltaic performances of nc-Si:H/c-Si solar cells. It is shown that interface states are essential factors prominently influencing open circuit voltages (V-OC) and fill factors (FF) of these structured solar cells. Short circuit current density (J(SC)) or spectral response seems more sensitive to the thickness of intrinsic a-Si:H buffer layers inserted into n(+)-nc-Si:H layer and p-c-Si substrates. Impacts of bandgap offset on solar cell performances have also been analyzed. As DeltaE(C) increases, degradation of VOC and FF owing to interface states are dramatically recovered. This implies that the interface state cannot merely be regarded as carrier recombination centres, and impacts of interfacial layer on devices need further investigation. Theoretical maximum efficiency of up to 31.17% (AM1.5,100mW/cm(2), 0.40-1.1mum) has been obtained with BSF structure, idealized light-trapping effect(R-F=0, R-B=1) and no interface states.

Polarity determination for GaN thin films by electron energy-loss spectroscopy

The intensity of the N K edge in electron energy-loss spectra from a GaN thin film shows a pronounced difference when the orientation of the film approaches the (0002) and (000-2) Bragg reflections, along the polar direction. This experimental result can be interpreted by the effect associated with interference between the Bloch waves of the incident electron in the GaN crystal. The theoretical calculations indicate that, at the Bragg condition of g=0002 along the Ga-N bond direction, the thickness-averaged electron current density on the N atom plane is much higher than that at g=000 (2) over bar, with a maximum as the specimen thickness is about 0.4xi(0002) (the two-beam extinction distance). The delocalization effect on the experimental spectra is also discussed. (C) 2002 American Institute of Physics.

Room-temperature, ground-state lasing for red-emitting vertically aligned InAlAs/AlGaAs quantum dots grown on a GaAs(100) substrate

The effect of the growth temperature on the properties of InAlAs/AlGaAs quantum dots grown on GaAs(100) substrates is investigated. The optical efficiency and structural uniformity are improved by increasing the growth temperature from 530 to 560 degreesC. The improvements of InAlAs/AlGaAs quantum-dot characteristics could be explained by suppressing the incorporation of oxygen and the formation of group-III vacancies. Furthermore, edge-emitting laser diodes with six quantum-dot layers grown at 560 degreesC have been fabricated. Lasing occurs via the ground state at 725 nm, with a room-temperature threshold current density of 3.9 kA/cm(2), significantly better than previously reported values for this quantum-dot systems. (C) 2002 American Institute of Physics.

Growth and characterization of InGaAs/InAlAs quantum cascade lasers

The growth and characterization of quantum cascade (QC) lasers based on InGaAs/InAlAs material system are investigated. Pronounced intersubband absorption from stacked active region of QC structure is used to monitor the wavelength of QC laser and disclose the material quality. The precise control of the epilayer thickness and the good quality of interfaces are demonstrated by the abundant narrow satellite peaks of X-ray diffraction. Laser action in quasi-continuous wave operation is achieved at lambda approximate to 5.1-5.2 mum up to 300 K. For 10 x 800 mum(2) laser device, peak output power of similar to7.2 mW and threshold current density of 3 kA/cm(2) at room temperature are obtained. For some devices, if keep the peak output powers at the similar to2 mW level, quasi-continuous wave operation at room temperature persists more than 1 h are recorded. (Q) (C) 2001 Elsevier Science Ltd. All rights reserved.

Self-assembled quantum dots, wires and quantum-dot lasers

Molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs and InAs/InAlAs/InP quantum dots (QDs) and quantum wires (QWRs) have been studied. By adjusting growth conditions, surprising alignment. preferential elongation, and pronounced sequential coalescence of dots and wires under specific condition are realized. The lateral ordering of QDs and the vertical anti-correlation of QWRs are theoretically discussed. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 3.6 W from both uncoated facets is achieved fi-om vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). A RT CW output power of 0.6 W/facet ensures at least 3570 h lasing (only drops 0.83 dB). (C) 2001 Elsevier Science B.V, All rights reserved.

Investigation on InGaAs/InAlAs quantum cascade lasers

The preparation and main characteristics of the InGaAs/InAlAs quantum cascade laser were given. The device has a reinforced ridge waveguide structure. The threshold current obtained at 80K is about 0. 5A, and the corresponding threshold current density is about 5kA/cm(2).

Strain-compensated quantum cascade lasers operating at room temperature

Quantum cascade (QC) lasers based on strain-compensated InxGa(1-x)As/InyAl(1-y)As grown on InP substrate using molecular beam epitaxy is reported. The epitaxial quality is demonstrated by the abundant narrow satellite peaks of double-crystal X-ray diffraction and cross-section transmission electron microscopy of the QC laser wafer. Laser action in quasi-continuous wave operation is achieved at lambda approximate to 3.6-3.7 mum at room temperature (34 degreesC) for 20 mum x 1.6 mm devices, with peak output powers of similar to 10.6mW and threshold current density of 2.7kA/cm(2) at this temperature. (C) 2000 Published by Elsevier Science B.V.

High-performance strain-compensated InGaAs/InAlAs quantum cascade lasers

We report on the realization of quantum cascade (QC) lasers based on strain-compensated InxGa(1-x)As/In(y)A((1-y))As grown on InP substrates using molecular beam epitaxy. X-ray diffraction and cross section transmission electron microscopy have been used to ascertain the quality of the QC laser materials. Quasi-continuous wave lasing at lambda approximate to 3.54-3.7 mum at room temperature was achieved. For a laser with 1.6 mm cavity length and 20 mum ridge-waveguide width,quasi-continuous wave lasing at 34 degreesC persists for more than 30 min, with a maximum power of 11.4 mW and threshold current density of 1.2 kA cm(-2), both record values for QC lasers of comparable wavelength.

Design consideration and performance of high-power and high-brightness InGaAs-InGaAsP-AlGaAs quantum-well diode lasers (lambda=0.98 mu m)

In this paper, we conduct a theoretical analysis of the design, fabrication, and performance measurement of high-power and high-brightness strained quantum-well lasers emitting at 0.98 mum, The material system of interest consists of an Al-free InGaAs-InGaAsP active region and AlGaAs cladding layers. Some key parameters of the laser structure are theoretically analyzed, and their effects on the laser performance are discussed. The laser material is grown by metal-organic chemical vapor deposition and demonstrates high quality with low-threshold current density, high internal quantum efficiency, and extremely low internal loss. High-performance broad-area multimode and ridge-waveguide single-mode laser devices are fabricated. For 100-mum-wide stripe lasers having a cavity length of 800 mum, a high slope efficiency of 1.08 W-A, a low vertical beam divergence of 34 degrees, a high output power of over 4.45 W, and a very high characteristic temperature coefficient of 250 K were achieved. Lifetime tests performed at 1.2-1.3 W (12-13 mW/mum) demonstrates reliable performance. For 4-mum-wide ridge waveguide single-mode laser devices, a maximum output power of 394 mW and fundamental mode power up to 200 mW with slope efficiency of 0.91 mW/mum are obtained.

Room temperature (34 degrees C) operation of strain-compensated quantum cascade lasers

A short wavelength (lambda similar or equal to 3.5 mu m) strain-compensated InxGa(1-x)As/InyAl(1-y)As quantum cascade laser is reported. Quasi-continuous wave operation of this device at 34 degrees C with an output power of 11.4mW persisted for more than 30 minutes without obvious degradation. A very low threshold current density of 1.2KA/cm(2) at this temperature was observed.

Self-assembled InAs/GaAs quantum dots and quantum dot laser

Systematic study of molecular beam epitaxy-grown self-assembled In(Ga)As/GaAs, In-AlAs/AlGaAs/GaAs, and InAs/InAlAs/InP quantum dots (QDs) is demonstrated. By adjusting growth conditions, surprising alignment, preferential elongation, and pronounced sequential coalescence of dots under the specific condition are realized. Room-temperature (RT) continuous-wave (CW) lasing at the wavelength of 960 nm with output power of 1 W is achieved from vertical coupled InAs/GaAs QDs ensemble. The RT threshold current density is 218 A/cm(2). An RT CW output power of 0.53 W ensures at least 3 000 h lasing (only drops 0.83 db). This is one of the best results ever reported.

Si1-xGex/Si resonant-cavity-enhanced photodetectors with a silicon-on-oxide reflector operating near 1.3 mu m

We report on a Si1-xGex/Si multiple quantum-well resonant-cavity-enhanced (RCE) photodetector with a silicon-on-oxide reflector as the bottom mirror operating near 1.3 mu m. The breakdown voltage of the photodetector is above 18 V and the dark current density at 5 V reverse bias is 12 pA/mu m(2). The RCE photodetector shows enhanced responsivity with a clear peak at 1.285 mu m and the peak responsivity is measured around 10.2 mA/W at a reverse bias of 5 V. The external quantum efficiency at 1.3 mu m is measured to be 3.5% under reverse bias of 16 V, which is enhanced three- to fourfold compared with that of a conventional p-i-n photodetector with a Ge content of 0.5 reported in 1995 by Huang [Appl. Phys. Lett. 67, 566 (1995)]. (C) 2000 American Institute of Physics. [S0003-6951(00)00628-8].