Structure and optical properties of upper In(Ga)As quantum dots on a different seed layer with a thin GaAs spacer layer

The structure and optical properties of In(Ga)As grown with the introduction of InGaAlAs or InAlAs seed dots layers are investigated. The area density and size homogeneity of the upper InGaAs dots are efficiently improved with the introduction of a layer of high-density buried dots. When the GaAs spacer layer is too thin to cover the seed dots, the upper dots exhibit the characterization of a quantum well. By analyzing the growth dynamics, we refer to it as an empty-core structure dot. (C) 2002 Elsevier Science B.V. All rights reserved.

Selectively excited photoluminescence of GaAs1-xSbx/GaAs single quantum wells

GaAsSb/GaAs single quantum wells grown by molecular-beam epitaxy are studied by selectively excited photoluminescence measurements. We have simultaneously observed the photoluminescence (PL) from both type-I and type-II transitions in the samples. The two transitions exhibit different PL behavior under different excitation energies. As expected, the peak energy of the type-I emission remains constant in the entire excitation energy range we used, while the type-II transition shows a significant blueshift with increasing excitation energy. The observed blueshift can be well explained by an electron-hole charge separation model at interface. This result, along with the excitation-power-dependent PL and the measured longer carrier decay time, provides more direct information on the type-II nature of the band alignment in GaAsSb/GaAs quantum well structures. (C) 2002 American Institute of Physics.

Proof of InAs/GaAs self-organized quantum dot lasing and the experimental determination of local Strain effect on the band structures

Confirmation of quantum dot lasing have been given by photoluminescence and electro-luminescence spectra. Energy levels of QD laser are distinctively resolved due to band filling effect, and the lasing energy of quantum dot laser is much lower than quantum well laser. The energy barrier at InAs/GaAs interface due to the built-in strain in self-organized system has been determined experimentally by deep level transient spectroscopy (DLTS). Such barrier has been predicted by previous theories and can be explained by the apexes appeared in the interface between InAs and GaAs caused by strain.

Quantum-confined magneto-Stark effect in diluted magnetic semiconductor coupled quantum wells

The magneto-Stark effect in a diluted magnetic semiconductor (DMS) coupled quantum well (CQW) induced by an in-plane magnetic field is investigate theoretically. Unlike the usual electro-Stark effects, in a DMS CQW the Lorenz force leads to a spatially separated exciton. The in-plane magnetic field can shift the ground state of the magnetoexciton from a zero in-plane center of mass (CM)/momentum to a finite CM momentum, and render the ground state of magnetoexciton stable against radiative recombination due to momentum conservation. (C) 2002 American Institute of Physics.

Exciton localization in In0.15Ga0.85As/GaAs quantum wire structures grown on (553)B-oriented GaAs substrate

Using time-resolved photoluminescence (PL) measurements, we have studied the exciton localization effect in InGaAs/GaAs quantum wire (QWR) structures formed in corrugated narrow InGaAs/GaAs quantum wells (QWs) grown on (553)B GaAs substrate. The PL decay time in the QWR structure was found to be independent of the temperature for T < 70 K, showing a typical dynamical behavior of the localized excitons. This result is in striking contrast to the corresponding quantum well structures, where a linear increase of the PL decay time was observed. In addition, an increase of the exciton lifetime was observed at low temperature for the QWR structure as compared to a reference InGaAs/GaAs quantum well sample (1200 vs 400 ps). The observed longer decay time was attributed to the reduction in the spatial coherence of excitons in the QWR-like structure. In PL measurements, a significant polarization anisotropy was also found in our narrow InGaAs/GaAs QWs grown on (553)B GaAs. (C) 2001 American Institute of Physics.

Strong three-level resonant magnetopolaron effect due to the intersubband coupling in heavily modulation-doped GaAs/AlxGa1-xAs single quantum wells at high magnetic-fields

Electron cyclotron resonance CR) measurements have been carried out in magnetic fields up to 32 T to study electron-phonon interaction in two heavily modulation-delta -doped GaAs/Al0.3Ga0.7As single-quantum-well samples. No measurable resonant magnetopolaron effects were observed in either sample in the region of the GaAs longitudinal optical (LO) phonons. However, when the CR frequency is above LO phonon frequency, omega (LO)=E-LO/(h) over bar, at high magnetic fields (B>27 T), electron CR exhibits a strong avoided-level-crossing splitting for both samples at frequencies close to (omega (LO)+ (E-2-E-1)1 (h) over bar, where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large with the minimum separation of 40 cm(-1) occurring at around 30.5 T. A detailed theoretical analysis, which includes a self-consistent calculation of the band structure and the effects of electron-phonon interaction on the CR, shows that this type of splitting is due to a three-level resonance between the second Landau level of the first electron subband and the lowest Landau level of the second subband plus one GaAs LO phonon. The absence of occupation effects in the final states and weak screening or this three-level process yields large energy separation even in the presence of high electron densities. Excellent agreement between the theory and the experimental results is obtained.

Capacitance-voltage characteristic as a trace of the exciton evolvement from spatially direct to indirect in quantum wells

We have investigated the photo-excited capacitance-voltage (C-V) characteristics as well as the photoluminescence spectra under different biases of a wide quantum well (QW) embedded in an n(+)-i-n(+) double-barrier structure. The pronounced peak feature at zero bias in the C-V spectrum observed upon illumination is regarded as a kind of quantum capacitance related to the quantum confined Stark effect, originating from the spatial separation of the photo-generated electron and hole gas in the QW. This fact is further demonstrated through the comparison between the C-V curve with the PL intensity versus applied voltage relationship under the same excitation. The results may provide us with a more direct and sensitive means in the detection of the separation and accumulation of both types of free carriers-electrons and holes-in low-dimensional semiconductor structures, especially in a new type of optical memory cell.

Influence of dual incorporation of In and N on the luminescence of GaInNAs/GaAs single quantum wells

The optical properties of above- and below-band-edge transitions have been investigated by incorporating In atoms into GaNAs/GaAs single quantum wells. The experimental results show that with increasing In concentration the interband luminescence is improved and the luminescence intensity below the band edge in GaInNAs/GaAs decreases significantly. An interpretation is given that N atoms are preferable to form a covalent bond with In than with Ga atoms in a GaInNAs alloy, due to the compensation of the atomic-size difference between In and N atoms on the GaAs substrate. The photoreflectance spectra of the GaInNAs/GaAs single quantum well support the assignment of an intrinsic mechanism to the high-energy luminescence peak. (C) 2000 American Institute of Physics. [S0003- 6951(00)01752-6].

Photoluminescence studies on pseudomorphic delta-doped AlGaAs/InGaAs/GaAs quantum wells

Photoluminescence (PL) measurements were performed on several series of single-side Si-doped pseudomorphic high electron mobility transistors (p-HEMTs) quantum well (QW) samples, with different spacer layer widths, well widths and Si delta -doped concentrations , under different temperatures and excitation power densities. The dynamic competitive luminescence mechanism between the radiations of e2-hh1 and e1-hh1 was discussed in detail. The confining potential, subband energies, corresponding envelope functions, subband occupations and transferring efficiency etc., were calculated by self-consistent finite differential method at different temperatures in comparison with the present experiment results. The relative variation of the integrated luminescence intensity of the two transitions (e1-hh1 and e2-hh1) was found to be dependent on the temperature and the structure's properties, e. g. spacer layer width, dopant concentration and well width.

Evolution from point defects to arsenic clusters in low-temperature grown GaAs/AlGaAs multiple quantum wells

Optical transient current spectroscopy (OTCS), photoluminescence (PL) spectroscopy and excitonic electroabsorption spectroscopy have been used to investigate the evolution of defects in the low-temperature grown GaAs/AlGaAs multiple quantum well structures during the postgrowth rapid thermal annealing. The sample was grown at 350 degrees C by molecular beam epitaxy on miscut (3.4 degrees off (001) towards (111)A) (001) GaAs substrate. After growth, the sample was subjected to 30s rapid thermal annealing in the range of 500-800 degrees C. It is found that the integrated PL intensity first decreases with the annealing temperature, then gets a minimum at 600 degrees C and finally recovers at higher temperatures. OTCS measurement shows that besides As,, antisites and arsenic clusters, there are several relatively shallower deep levels with excitation energies less than 0.3 eV in the as-grown and 500 degrees C-annealed samples. Above 600 degrees C, OTCS signals from As,, antisites and shallower deep levels become weaker, indicating the decrease of these defects. It is argued that the excess arsenic atoms group together to form arsenic clusters during annealing. (C) 2000 Elsevier Science B.V. All rights reserved.

Epitaxial growth of GaNAs/GaAs heterostructure materials

A series of systematic experiments on the growth of high quality GaNAs strained layers on GaAs (001) substrate have been carried out by using DC active Nz plasma, assisted molecular beam epitaxy. The samples of GaNAs between 3 and 200 nm thick were evaluated by double crystal X-ray diffraction (XRD) and photoluminescence (PL) measurements. PL and XRD measurements for these samples are in good agreement. Some material growth and structure parameters affecting the properties of GaNAs/GaAs heterostructure were studied; they were: (1) growth temperature of GaNAs epilayer; (2) electrical current of active N-2 plasma; (3) Nz flow rate; (4) GaNAs growth rate; (5) the thickness of GaNAs strained layer. XRD and PL measurements showed that superlattice with distinct satellite peaks up to two orders and quantum well structure with intensity at 22 meV Fourier transform infrared spectroscopy (FWHM) can be achieved in molecular beam epitaxy (MBE) system. (C) 2000 Published by Elsevier Science S.A. All rights reserved.

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-delta-doped GaAs/Al0.3Ga0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E-LO at B > 27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E-LO + (E-2 - E-1), where E-2, and E-1 are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm(-1) around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LI, of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence: of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

Quantum-dot superluminescent diode: A proposal for an ultra-wide output spectrum

We propose a novel superluminescent diode (SLD) with a quantum dot (QD) active layer, which should give a wider output spectrum than a conventional quantum well SLD. The device makes use of inhomogeneous broadness of gain spectrum resulting from size inhomogeneity of self-assembled quantum dots grown by Stranski-Krastanow mode. Taking a design made out in the InxGa1-xAs/GaAs system for example, the spectrum characteristics of the device are simulated realistically, 100-200 nm full width of half maximum of output spectrum can be obtained. The dependence of the output spectrum on In composition, size distribution and injection current of the dots active region is also elaborated.

Ultrafast low-temperature grown AlGaAs/GaAs photorefractive quantum wells using point defects as capture centers

At a medium substrate temperature of 400 degrees C and a lower As flux, we have grown an ultrafast AlGaAs/GaAs photorefractive multiple quantum well (MQW) structure by molecular beam epitaxy. The as-grown sample exhibits strong photorefractive effect under the transverse Frantz-Keldysh geometry. A peak electroabsorption of 2100 cm(-1) is measured in the as-grown sample in an 11 kV/cm dc electric field, and the peak photorefractive diffraction efficiency can be 1.2%. After postgrowth annealing, the photorefractive effect becomes weak and disappears in samples annealed above 700 degrees C. Using optical transient current spectroscopy, deep levels are measured in these samples. It is found that deep levels are stable against annealing until 700 degrees C. Using a pump-probe technique, carrier lifetimes are measured at room temperature. We find that the as-grown sample has a lifetime of 20 ps, while the 700 degrees C annealed sample has a lifetime of more than 200 ps. The ultrafast lifetime in the as-grown sample is caused by point defects, not by As clusters. Our result show that AlGaAs/GaAs MQW structure grown around 400 degrees C has better performance of the photorefractive effect. (C) 1999 American Institute of Physics. [S0003-6951(99)04036-X].

Influence of (001) vicinal GaAs substrates on the optical properties of defects in low-temperature grown GaAs/AlGaAs multiple quantum wells

Photoluminescence (PL) spectroscopy and carrier lifetime measurement has been used to characterize optical properties of defects in the low-temperature (LT) grown GaAs/AlGaAs multiple quantum well structures. Two sets of samples were grown at 400 degrees C by molecular beam epitaxy on nominal (001) and miscut [4 degrees off (001) towards (111) A] GaAs substrates, respectively. After growth, samples were subjected to 30 s rapid thermal annealing at 600-800 degrees C. It is found that after annealing, two defect-related PL features appear in the samples grown on nominal (001) GaAs substrates, but not in those grown on miscut (001) GaAs substrates. The carrier lifetimes are about 31 and 5 ps in as-grown samples grown on nominal and miscut (001) GaAs substrates, respectively. The different PL spectra and carrier lifetimes in two sets of samples are attributed to different structures of the As-Ga-like defects formed during LT growth. (C) 1999 American Institute of Physics. [S0003-6951(99)00230-2].