Growth of Space Ordered on GaAs（100） Vicinal 1.3μm InAs Quantum Dots Substrates by MOCVD

Space ordered 1.3μm self-assembled InAs QDs are grown on GaAs（100） vicinal substrates by MOCVD. Photoluminescence measurements show that the dots on vicinal substrates have a much higher PL intensity and a narrower FWHM than those of dots on exact substrates, which indicates better material quality. To obtain 1.3μm emissions of InAs QDs, the role of the so called InGaAs strain cap layer （SCL） and the strain buffer layer （SBL） in the strain relaxation process in quantum dots is studied. While the use of SBL results only in a small change of emission wavelength,SCL can extend the QD＇s emission over 1.3μm due to the effective strain reducing effect of SCL.

Effect of carrier relaxation and emission on photoluminescence of InAs quantum dots

　

New way to enhance the uniformity of self-organized InAs quantum dots

Growth interruption was introduced after the deposition of GaAs cap layer, which is thinner than the mean height of Quantum dots. Uniformity of quantum dots has been enhanced because the full width of half maximum of photoluminescence decrease from 80meV to 27meV in these samples as the interruption time increasing from 0 to 120 second. Meanwhile, we have observed that the peak position of photoluminescence is a function of interruption time. This effect can be used to control the energy level of quantum dots. The phenomena mentioned above can be attributed to the diffusion of In atoms from the top of InAs islands to the top of GaAs cap layer caused by the difference of surface energies between InAs and GaAs.

Electronic investigation of self-organized InAs quantum dots

Deep Level Transient Spectroscopy (DLTS) has been applied to investigate the electronic properties of self-organized InAs quantum dots. The energies of electronic ground states of 2.5ML and 1.7ML InAs quantum dots (QDs) with respect to the conduction band of bulk GaAs are about 0.21 eV and 0.09 eV, respectively. We have found that QDs capture electrons by lattice relaxation through a multi-phonon emission process. The samples are QDs embedded in superlattices with or without a 500 Angstrom GaAs spacing layer between every ten periods of a couple of GaAs and InAs layers. The result shows that the density of dislocations in the samples with spacer layers is much lower than in the samples without the spacer layers.

The enhancement of diffusion by strain of InAs quantum dots in a GaAs matrix

We investigate the annealing behavior of Photoluminescence (PL) from self-assembled InAs quantum dots (QDs) with different thicknesses GaAs cap layers. The diffusion introduced by annealing treatment results in a blue-shift of the QD PL peak, and a decrease in the integrated intensity. The strain present in QDs enhances the diffusion, and the QDs with the cap layers of different thicknesses will experience a strain of different strength. This can lend to a, better understanding of the larger blue-shift of the PL peak of the deeper buried QDs, and the different variance of the full width at half maximum of the luminescence from QDs with the cap layers of different thicknesses.

Observation of N-Shaped Negative Differential Resistance in GaAs-Based Modulation-Doped Field Effect Transistor with InAs Quantum Dots

N-shaped negative differential resistance (NDR) with a high peak-to-valley ratio (PVR) is observed in a GaAs-based modulation-doped field effect transistor (MODFET) with InAs quantum dots (QDs) in the barrier layer (QDFET) compared with a GaAs MODFET. The NDR is explained as the real-space transfer (RST) of high-mobility electrons in a channel into nearby barrier layers with low mobility, and the PVR is enhanced dramatically upon inserting the QD layer. It is also revealed that the QD layer traps holes and acts as a positively charged nano-floating gate after a brief optical illumination, while it acts as a negatively charged nano-floating gate and depletes the adjacent channel when charged by the electrons. The NDR suggests a promising application in memory or high-speed logic devices for the QDFET structure.

The refractive nonlinearities of InAs/GaAs quantum dots above-bandgap energy

The third-order optical nonlinear refractive properties of InAs/GaAs quantum dots grown by molecular beam epitaxy have been measured using the reflection Z-scan technique at above-bandgap energy. The nonlinear refractive index and nonlinear absorption index of the InAs/GaAs quantum dots were determined for wavelengths from 740 to 777 nm. The measured results are compared with the nonlinear refractive response of several typical III-V group semiconductor materials. The corresponding mechanisms responsible for the large nonlinear response are discussed.

The effect of an electric field on the nonlinear response of InAs/GaAs quantum dots

The refractive nonlinearities of InAs/GaAs quantum dots under a dc electric field at photon energies above its band gap energy have been studied using the reflection Z-scan technique. The effect of the dc electric field on the nonlinear response of InAs/GaAs quantum dots showed similar linear and quadratic electro-optic effects as in the linear response regime at low fields. This implies that the electro-optic effect in the nonlinear regime is analogous to the response in the linear regime for semiconductor quantum dots. Our experimental results show the potential for voltage tunability in InAs quantum dot-based nonlinear electro-optic devices.

Photoluminescence investigation of InAs bimodal self-assembled quantum dots state filling

Self-assembled InAs quantum dots were prepared on GaAS(100)) substrate in a solid source molecular beam epitaxy system The distribution and topographic images of uncapped dots were studied by atomic force microscope. The statistical result shows that the quantum dots are bimodal distribution. The photoluminescence spectrum results shows that the intensity of small size quantum dots dominated, which may be due to: (1) the state density of large quantum dots lower than that of small quantum dots; (2) the carriers capture rate of large size quantum dots is small relative to that of small ones; (3) there is a large strain barrier between large quantum dots and capping layer, and the large strain is likely to produce the defect and dislocation, resulting in a probability carriers transferring from large quantum dots to small dots that is very small with temperature increasing.

Optimizing the GaAs capping layer growth of 1.3 mu m InAs/GaAs quantum dots by a combined two-temperature and annealing process at low temperatures

We report on optimizing the GaAs capping layer growth of 1.3 mu m InAs quantum dots (QDs) by a combined two-temperature and annealing process at low temperatures using metalorganic chemical vapor deposition. The initial part (tnm) of the capping layer is deposited at a low temperature of 500 degrees C, which is the same for the growth of both the QDs and a 5-nm-thick In0.15Ga0.85As strain-reducing capping layer on the QDs, while the remaining part is grown at a higher temperature of 560 degrees C after a rapid temperature rise and subsequent annealing period at this temperature. The capping layer is deposited at the low temperatures (<= 560 degrees C) to avoid postgrowth annealing effect that can blueshift the emission wavelength of the QDs. We demonstrate the existence of an optimum t (=5 nm) and a critical annealing time (>= 450s) during the capping, resulting in significantly enhanced photoluminescence from the QDs. This significant enhancement in photoluminescence is attributed to a dramatic reduction of defects due to the optimized capping growth. The technique reported here has important implications for realizing stacked 1.3 mu m InAs/GaAs QD lasers. (C) 2008 Elsevier B.V. All rights reserved.

Photoluminescence of Charged Low-Density InAs/GaAs Quantum Dots

We obtain low-density charged InAs quantum dots with an emission wavelength below 1 mu m using a low InAs growth rate. The quantum dots have a bimodal size distribution with an emission wavelength of around 1340 nm and 1000 nm, respectively. We observe the photoluminescence of the singly charged exciton in the modulation doped quantum dots in 77 K.

Thermal annealing effect on InAs/InGaAs quantum dots grown by atomic layer molecular beam epitaxy

The effect of rapid thermal annealing on the InAs quantum dots (QDs) grown by atomic layer molecular beam epitaxy and capped with InGaAs layer has been investigated using transmission electron microscopy and photoluminescence (PL). Different from the previously reported results, no obvious blueshift of the PL emission of QDs is observed until the annealing temperature increases up to 800 degreesC. The size and shape of the QDs annealed at 750 degreesC have hardly changed indicating the relatively weak Ga/In interdiffusion, which is characterized by little blueshift of the PL peak of QDs. The QD size increases largely and a few large clusters can be observed after 800 degreesC RTA, implying the fast interdiffusion and the formation of InGaAs QDs. These results indicate that the delay of the blueshift of the PL peak of QDs is correlated with the abnormal interdiffusion process, which can be explained by two possible reasons: the reduction of excess-As-induced defects and the redistribution of In, Ga atoms around the InAs QDs resulted from the sub-monolayer deposition of InGaAs capping layer. (C) 2004 Elsevier B.V. All rights reserved.

Effect of the InAlAs and InGaAs combination strain-reducing layer on 1.3 mu m emission self-assembled InAs/GaAs quantum dots

Self-assembled InAs quantum dots (QDs) with differing deposition thicknesses covered by InxAl1-xAs (x = 0.2, 0.3) and In0.2Ga0.8As combination strain-reducing layers (CSRLs) were grown by molecular beam epitaxy. Their structural and optical properties were investigated by atomic force microscopy and photoluminescence spectroscopy, respectively. The emission peak position of InAs QDs capped by CSRL can reach 1.34 mum at room temperature with a relatively larger energy splitting of 93 meV between the ground and first excited states.

Rapid thermal annealing effects on structural and optical properties of self-assembled InAs/GaAs quantum dots capped by InAlAs/InGaAs layers

Effects of rapid thermal annealing on the optical and structural properties of self-assembled InAs/GaAs quantum dots capped by the InAlAs/InGaAs combination layers are studied by photoluminescence and transmission electron microscopy. The photoluminescence measurement shows that the photoluminescence peak of the sample after 850 degrees C rapid thermal annealing is blue shifted with 370meV and the excitation peak intensity increases by a factor of about 2.7 after the rapid thermal annealing, which indicates that the InAs quantum dots have experienced an abnormal transformation during the annealing. The transmission electron microscopy shows that the quantum dots disappear and a new InAlGaAs single quantum well structure forms after the rapid thermal annealing treatment. The transformation mechanism is discussed. These abnormal optical properties are attributed to the structural transformation of these quantum dots into a single quantum well.

Structural and optical properties of InAs/GaAs quantum dots emitting at 1.5 mu m

We have demonstrated 1.5 mum light emission from InAs quantum dots (QDs) capped with a thin GaAs layer. The extension of the emission wavelength can be assigned to the large QD height. We also investigate the effect of growth interruption on the PL properties and the shape of InAs QDs fabricated by migration-enhanced growth (MEG). Contrary to expectation, we observed a remarkable blueshift of the emission energy with the growth interruption in MEG mode. Detailed investigations reveal that the blueshift is related to the reduced island height with the growth interruption, which is confirmed by reflection high-energy electron diffraction (RHEED) patterns and atomic force microscopy (AFM) measurement results. Accordingly, the structure changes of the islands are interpreted in terms of thermodynamic and kinetic theories. (C) 2004 Elsevier B.V. All rights reserved.