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Room-temperature non-radiative recombination is studied at single dislocations in Ga0.5In0.5P quantum wells grown on metamorphic templates using cathodoluminescence and electron channeling contrast imaging. An analysis of the light emission intensity profiles around single dislocations reveals that the average recombination strength of a dislocation decreases by a factor of four and seven as a result of decreasing growth temperature of the GaInP quantum well from 725 to 675 and 625 °C, respectively. This reduction occurs despite little change in the diffusion length, precluding the prospect of inducing carrier localization by ordering and phase separation in GaInP at lower growth temperatures. These observations are rationalized by the premise that point defects or impurities are largely responsible for the recombination activity of dislocations, and the extent of decoration of the dislocation core decreases with temperature. Preliminary evidence for the impact of the Burgers vector is also presented. The lowest growth temperature, however, negatively impacts light emission away from dislocations. Carrier recombination in the bulk and at dislocations needs to be considered together for metamorphic devices, and this work can lead to new techniques to limit non-radiative recombination. [ABSTRACT FROM AUTHOR]

Selective enhancement and suppression of the photoluminescence arising from Si0.5Ge0.5/Si multiple quantum wells by photonic crystals (PCs) have been demonstrated. The formation of the stop band in PCs is designed to be a filter as well as a reflector. It is found that the self-assembled PCs are able to selectively enhance the luminescence of the type-II transitions at the interface between Si and Si0.5Ge0.5/Si layers and suppress the emission from Si. Our working principle shown here can be extended to many other material systems and should be very useful for creating high power solid-state emitters. [ABSTRACT FROM AUTHOR]

The results of the growth of thin (~3 nm) InGaN/GaN single quantum wells (SQWs) with emission wavelengths in the green region by plasma-assisted molecular beam epitaxy are present. An improved two-step growth method using a high growth temperature up to 650 °C is developed to increase the In content of the InGaN SQW to 30% while maintaining a strong luminescence intensity near a wavelength of 506 nm. The indium composition in InGaN/GaN SQW grown under group-III-rich condition increases with increasing growth temperature following the growth model of liquid phase epitaxy. Further increase in the growth temperature to 670 °C does not improve the photoluminescence property of the material due to rapid loss of indium from the surface and, under certain growth conditions, the onset of phase separation. [ABSTRACT FROM AUTHOR]

Ion channeling technique using MeV C2+ ions and high resolution X-ray diffraction were used to study the crystal quality of an InAs/AlSb-based quantum wells. We found that the InAs quality has a strong dependence on the type of the interface used. With the addition of the InSb-like interface, the crystal quality of the InAs channel was greatly improved. The InAs lattice was fully strained and aligned with the lattice of the buffer layer without any lattice relaxation. On the other hand, if the interface was of the AlAs type, the lattice of the InAs quantum well was relaxed and the crystal quality was poor. This explains why a superior InAs quantum well with high electron mobility and good surface morphology can be achieved with the use of the InSb interface. [ABSTRACT FROM AUTHOR]

The properties of intersubband transition in AlGaN/GaN multi-quantum wells (MQWs) grown on different AlGaN templates by metalorganic chemical vapor deposition are investigated. The strain states of GaN wells are studied by Raman spectra and reciprocal space mappings, which shows that the GaN wells are compressively strained and the compressive strain is increased when the Al mole composition is varied from 0 to 0.3. The Fourier transform infrared spectrometer results show that the intersubband transition wavelength in the AlGaN/GaN MQWs can be tuned from 5.14 μm to 4.65 μm when the Al mole composition of the AlGaN template is increased. The results can be attributed to the quantum confined Stark effect. [ABSTRACT FROM AUTHOR]

Intraband absorption in quantum-dot-in-a-well (DWELL) mid-infrared photodetectors is investigated using photocurrent spectroscopy and computationally cost-effective modeling linked to experimental data. The DWELL systems are challenging for modeling the electronic structure, which involves both discrete levels and the continuum energy spectrum. We show that the latter can be successfully approximated by a quasi-continuum in a large three-dimensional (3D) 'quantum box' in which the electronic structure is calculated in the effective mass approximation using the finite element method. Experimental and simulated spectra show good agreement with each other, which justifies using the modeling for analysis of the experimental data. In particular, the origin of the peaks and the dot parameters, such as composition are deduced. Effects of dot composition and shape on the intraband absorption spectra are also predicted. Our model proves to be a useful tool in designing and analyzing advanced DWELL structures for any realistic 3D quantum dot geometry. [ABSTRACT FROM AUTHOR]

Vertical microcavity surface-emitting lasers employing quantum wells and small aperture buried-oxide current and field confinement are demonstrated with wider mode spacing and faster spontaneous carrier recombination (enhanced Purcell factor), lower threshold current, larger side mode suppression ratio (SMSR), and higher photon density and temperature insensitivity. The result is a microcavity laser that achieves higher microwave modulation bandwidth (f-3dB = 15.8 GHz) at ultra-low power consumption (1.5 mW) with a slope for the modulation current efficiency factor (MCEF) = 17.47 GHz/mA-1/2, as well as a better quality eye diagram in high-speed data transmission. The microwave behavior model for the microcavity laser is used to estimate the enhanced recombination and reduced lifetime. [ABSTRACT FROM AUTHOR]

We have investigated the InGaAs/GaAsSb/InAlGaAs/InAlAs type-II 'W' quantum wells (QWs) grown on InP substrates by molecular beam epitaxy. The photoluminescence (PL) emission wavelength longer than 2.56 μm at room temperature (RT) is demonstrated for the first time in this material system. The PL emission peaks of our designed samples can cover a wide range from 2 to 2.5 μm at cryogenic temperature. The samples show good optical quality that the reduction in integrated PL intensity is only around one order of magnitude from 35 K to RT. We found that the integrated PL intensity decreased as the emission wavelength increased, which is due to the reduction in the electron-hole wave function overlap. This is consistent with the calculated result. In the power dependent PL measurements, the emission peak of 'W' type QWs show blue shifts with the excitation power (Pex) but does not follow the Pex1/3 rule as predicted by type-II band bending model. The localized states filling effect gives reasonable explanations for the observed phenomena. [ABSTRACT FROM AUTHOR]

The interface properties of GaNxAs1-x/GaAs single-quantum well is investigated at 80 K by reflectance difference spectroscopy. Strong in-plane optical anisotropies (IPOA) are observed. Numerical calculations based on a 4 band K·P Hamiltonian are performed to analyze the origin of the optical anisotropy. It is found that the IPOA can be mainly attributed to anisotropic strain effect, which increases with the concentration of nitrogen. The origin of the strain component εxy is also discussed. [ABSTRACT FROM AUTHOR]

Highly strained InxGa1-xAs/GaAs double quantum well (DQW) structures grown by metal organic vapor phase epitaxy with different In compositions are investigated by surface photovoltage spectroscopy (SPS) in the temperature range 20–300 K. A lineshape fit of spectral features in the differential surface photovoltage (SPV) spectra determines the transition energies accurately. A comprehensive analysis of the anomalous phenomena appearing in lower temperature SPV spectra enable us to evaluate directly the band lineup of DQW and to remove the ambiguity in the identification of spectral features. The process of separation of carriers within the QW with possible capture by the interface defect traps plays an important role for phase change in SPV signal in the vicinity of light-hole related feature at low temperature. The results demonstrate the considerable diagnostic values of the SPS technique for characterizing these highly strained DQW structures. [ABSTRACT FROM AUTHOR]

A-plane InxGa1-xN/GaN (x=0.09, 0.14, 0.24, and 0.3) multiple-quantum-wells (MQWs) samples, with a well width of about 4.5 nm, were achieved by utilizing r-plane sapphire substrates. Optical quality was investigated by means of photoluminescence (PL), cathodoluminescence, and time resolved PL measurements (TRPL). Two distinguishable emission peaks were examined from the low temperature PL spectra, where the high- and low-energy peaks were ascribed to quantum wells and localized states, respectively. Due to an increase in the localized energy states and absence of quantum confined Stark effect, the quantum efficiency was increased with increasing indium composition up to 24%. As the indium composition reached 30%, however, pronounced deterioration in luminescence efficiency was observed. The phenomenon could be attributed to the high defect densities in the MQWs resulted from the increased accumulation of strain between the InGaN well and GaN barrier. This argument was verified from the much shorter carrier lifetime at 15 K and smaller activation energy for In0.3Ga0.7N/GaN MQWs. In addition, the polarization-dependent PL revealed that the degree of polarization decreased with increasing indium compositions because of the enhancement of zero-dimensional nature of the localizing centers. Our detailed investigations indicate that the indium content in a-plane InGaN/GaN MQWs not only has an influence on optical performance, but is also important for further application of nitride semiconductors. [ABSTRACT FROM AUTHOR]

The effect of a high N incorporation in self-assembled InAs quantum dots (QDs) is investigated by analyzing the electronic and structural properties around QD region. Capacitance-voltage profiling and admittance spectroscopy shows that N incorporation into the InAs QD layer leads to drastic carrier depletion in the QD layer and neighboring GaAs layers due to the formation of a deep defect state at 0.34–0.41 eV. The signature of this defect state is similar to those defects observed in strain relaxed QDs or InGaAs/GaAs quantum wells when the InAs deposition thickness exceeds a critical thickness. Accordingly, the N incorporation might result in strain relaxation either by increasing localized strain or by inducing composition inhomogeneities, which provide nucleation sources for strain relaxation. The argument of strain relaxation is supported by transmission electron microscopy that reveals lattice misfits at the QD layer and neighboring GaAs layers. [ABSTRACT FROM AUTHOR]

This work investigates the surfactant effect on exposed and buried InAs quantum dots (QDs) by incorporating Sb into the QD layers with various Sb beam equivalent pressures (BEPs). Secondary ion mass spectroscopy shows the presence of Sb in the exposed and buried QD layers with the Sb intensity in the exposed layer substantially exceeding that in the buried layer. Incorporating Sb can reduce the density of the exposed QDs by more than two orders of magnitude. However, a high Sb BEP yields a surface morphology with a regular periodic structure of ellipsoid terraces. A good room-temperature photoluminescence (PL) at ∼1600 nm from the exposed QDs is observed, suggesting that the Sb incorporation probably improves the emission efficiency by reducing the surface recombination velocity at the surface of the exposed QDs. Increasing Sb BEP causes a blueshift of the emission from the exposed QDs due to a reduction in the dot height as suggested by atomic force microscopy. Increasing Sb BEP can also blueshift the ∼1300 nm emission from the buried QDs by decreasing the dot height. However, a high Sb BEP yields a quantum well-like PL feature formed by the clustering of the buried QDs into an undulated planar layer. These results indicate a marked Sb surfactant effect that can be used to control the density, shape, and luminescence of the exposed and buried QDs. [ABSTRACT FROM AUTHOR]

Temperature dependence of the photoresponse in two superlattice infrared photodetectors with different single barriers is studied. It is found that the short-wavelength response decreases with temperature, but the long-wavelength one increases. The crossing point of those two response ranges is independent of temperature. The movement of its associated wavelength under different biases is investigated and analyzed. The relevant physical mechanisms are found and integrated into a simple model to explain the experimental results. Based on the model, another sample with a superlattice integrated with multiple quantum wells is designed to demonstrate whether its temperature dependence of the photoresponse is consistent with our understanding. The sample indeed shows a broadband response which enhances with temperature in all wavelengths. [ABSTRACT FROM AUTHOR]

This work presents the deep-level photoluminescence of coherently strained GaAsN/GaAs quantum-well (QW) structures with various GaAsN thicknesses and N contents. A broad deep-level emission at ∼1.1 eV is observed, whose wavelength is redshifted as the GaAsN thickness increases. Based on its energy separation from the QW emission, this emission is attributed to a transition between the QW electron ground state and a deep level at ∼0.2 eV above the GaAsN valence-band (VB) edge. This level is shown to be tied to the GaAs band edge. A transition between this level and the GaAs conduction band allows the GaAsN–GaAs band alignment to be evaluated. A type II band lineup is obtained with VB offsets of 0.03 and 0.002 eV for N=0.6% and 1.8%, respectively. The decreased VB offset suggests a transition from type II to type I with increasing N content. Thermal annealing effectively removes this level and improves the QW emission. The concentration of this level is not clearly correlated with N content, suggesting that this level is induced by a low-temperature growth of the GaAsN layer to suppress the composition fluctuation. Given its energy, this level is tentatively assigned to VGa. [ABSTRACT FROM AUTHOR]

The broadband absorption of metal-oxide-semiconductor SiGe/Si quantum dot infrared photodetectors is demonstrated using boron δ doping in the Si spacer. The peak at 3.7–6 μm results from the intersubband transition in the SiGe quantum dot layers. The other peak at 6–16 μm mainly comes from the intraband transition in the boron δ-doping wells in the Si spacers. Since the atmospheric transmission windows are located at 3–5.3 and 7.5–14 μm, broadband detection is feasible using this device. The δ doping in SiGe quantum dots and Si0.9Ge0.1 quantum wells is also investigated to identify the origin of the absorption. [ABSTRACT FROM AUTHOR]

Several approaches have been employed to grow high-quality ZnSe epilayers on Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si virtual substrates. The ZnSe epilayers were characterized by photoluminescence spectroscopy. Migration enhanced epitaxy and inserting an in situ thermal annealing ZnSe buffer layer effectively reduced the intensity of deep level emissions from the ZnSe epilayer grown on a 6°-tilted Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si virtual substrate. Optimized conditions for growing high-quality ZnSe were used to deposit ZnCdSe/ZnSe multiple quantum wells on Ge/Ge0.95Si0.05/Ge0.9Si0.1/Si virtual substrates. Photoluminescence spectroscopy revealed quantum-confinement effect in the ZnCdSe multiple quantum wells. The evolution of the exciton emission peak energy and the linewidth as a function of temperature indicate a low density of localized sites in the sample with a well width of 1 nm. In the high-temperature regime, the thermal quenching of the excitonic emission intensity from ZnCdSe quantum well structures was governed by the thermal activation of carriers from quantum-well-confined states into barrier states. [ABSTRACT FROM AUTHOR]

With the miniaturization of a solid device, quantum and interface effects become dominant not only in the static properties but also in the transport dynamics in the solid. Here we examine the dc conductance of a nanosolid by introducing the depression of the intra-atomic trapping potential in the surface skin [Sun, Phys. Rev. B 69, 045105 (2004)] as a perturbation to the conventional “quantum well” for tunneling electrons. It is derived that downshifts of the dc conductance peaks (that is, the incident energies for resonant tunneling) happens in an oscillatory way, depending on the strength of the perturbation. Besides, the downshifts of the peak positions due to the trapping potential well depression decrease as the size of the nanosolid is increased. [ABSTRACT FROM AUTHOR]

Room temperature photocurrent measurements were carried out on two InAs[sub x]P[sub 1-x]/In[sub 0.53]Ga[sub 0.47] As asymmetric quantum wells with InP barriers, grown by molecular beam epitaxy. The lowest energy exciton transition in these samples was close to 1.5 μm. A significant blueshift of this transition was observed on application of an electric field. We attribute this blueshift to the type II band lineup that exists between InAsP and InGaAs for the samples studied. The resulting stepped well causes separation of the electron and hole wave functions at zero bias, which is then reduced by the applied field. X-ray diffraction spectra indicated layer thicknesses and compositions to be close to the intended values. However, to fit the observed transition energies, spectral shifts, and x-ray data simultaneously, it was necessary to assume low levels of arsenic contamination in the InP barriers and of gallium in the InAsP layers. From these results, we infer the maximum value of the conduction band offset (ΔE[sub c]) for InAsP/InP to be approximately 65% of the band-gap difference (ΔE[sub g]). © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report a detailed investigation on the optical transitions of strained type II GaAs[SUB0.7]Sb[SUB0.3]/GaAs (100) multiple quantum wells. For the theoretical calculations, both of the elastic deformational potential of intrinsic compressive biaxial strain, and quantum confinement effects are included. The asymmetric photoluminescence spectra reveal the features of excited state transition and quantum confinement Stark effect at high and low temperatures, respectively. The asymmetry features have also been investigated and confirmed by low-temperature photoluminescence experiments under different excitation power. From polarized photoluminescence excitation and photoconductivity spectra, both of the type I and type II optical transitions can also be clearly identified. [ABSTRACT FROM AUTHOR]

The temperature-dependent behavior of excitonic photoluminescence observed in ZnO/MgZnO multiple quantum wells (MQWs) in the temperature range of 5-300 K is described. In a ZnO/Mg[SUB0.27]Zn[SUB0.73]O MQW grown by laser molecular-beam epitaxy, the luminescence was dominated by localized exciton (LE) emission throughout the whole temperature range studied. Luminescence of free excitons (FEs) was not observed. A simple rate equation is used to describe the quenching of LE emission. The activation energy for LE luminescence quenching is of the order of the localization energy of excitons, suggesting that the thermionic emission of the LEs out of the localization potentials leads to nonradiative recombination. In a ZnO/Mg[SUB0.12]Zn[SUB0.88]O MQW having lower barriers, the luminescence was dominated by LE emissions at low temperatures, while the FE transition was dominating emissions at temperatures above 175 K. A rate equation assuming one nonradiative recombination channel is used to describe the quenching of the transitions observed. The activation energy for LE luminescence quenching deduced in this sample is also of the order of the localization energy of excitons. The temperature dependences of FE emission intensities are also discussed by using a simple rate equation in which a thermal release effect of LEs toward FEs is taken into account. [ABSTRACT FROM AUTHOR]

Effects of growth interruption on the optical and the structural properties of InGaN/GaN quantum wells were investigated by using photoluminescence, transmission electron microscopy, optical microscopy, and high resolution x-ray diffraction. The In[sub x]Ga[sub 1-x]N/GaN (x>0.2) quantum wells used in this study were grown on c-plane sapphire by using metalorganic chemical vapor deposition. The interruption was carried out by closing the group-III metalorganic sources before and after the growths of the InGaN quantum well layers. The transmission electron microscopy images show that with increasing interruption time, the quantum-dot-like regions and well thickness decreased due to indium reevaporation or the thermal etching effect. As a result the photoluminescence peak position was blueshifted and the intensity was reduced. Temperature- and excitation-power-dependent photoluminescence spectra support the results of transmission electron microscopy measurements. The sizes and the number of V defects did not differ with the interruption time. The interruption time is not directly related to the formation of defects. The V defect originates at threading dislocations and inversion domain boundaries due to higher misfit strain. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Provides information on a study on the effect of interdiffusion on the subbands and optical properties of InGaAs/InAIAs double quantum well. Modeling of InGaAs/InAIAs quantum well structure; Effects of interdiffusion; Effects of injected carriers; Effects of applied electric fields; Conclusions.

Presents a study which calculated the ideal threshold current densities of IR lasers for active layers composed of indium arsenide/InGaSb superlattices, InGaAsSb quantum well quaternaries, InAsSb bulk ternaries, and HGCdTe superlattices. Formalism employed to calculate the superlattice band structure; Classes of laser structures considered in the study; Usefulness of band-structure engineering to obtain optimum laser performance.

Presents a study which examined the broadening factor due to electron-phonon collisions in semiconductor quantum wells. Theoretical background on electron-phonon scattering process; Description of the theoretical model; Results and discussion.

Presents a study that investigated photoluminescence in strained gallium antimony/indium gallium antimony quantum wells by metalorganic chemical vapor deposition. Details of the experiment; Results and discussion; Conclusion.

The recovery of light-induced degradation of the tandem micromorph solar cell by applying reverse bias is compared with the single-junction amorphous silicon solar cell. The illuminated current density-voltage characteristics and external quantum efficiency show that the degradation of both the micromorph and the amorphous silicon cells can be recovered by applying sufficient reverse bias. The micromorph cell was recovered at smaller reverse bias than amorphous silicon cell. The abundant H in the microcrystalline silicon bottom cell of the micromorph cell can act as a reservoir to repair the defects in the amorphous silicon top cell at the reverse bias. This is responsible for small recovery bias of tandem cells. [ABSTRACT FROM AUTHOR]

A set of ZnO/Mg[sub 0.12]Zn[sub 0.88]O multiple quantum wells (MQWs) with well widths, L[sub w], varying from 6.91 to 46.5 Å has been grown by laser molecular-beam epitaxy. We estimated the L[sub w] dependence of the radiative and nonradiative recombination times of localized excitons at 5 K. Suppression of quantum efficiency can be avoided even in the MQWs having small L[sub w]s studied in this work. Effects of excitonic localization are discussed in order to explain the L[sub w] dependence of radiative recombination time at 5 K. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Focuses on the use of magnetophotoluminescence excitation spectroscopy to study GaAs-AlGasAs coupled double quantum wells. Technique used to measure the hole Landau levels; Photoluminescence excitation spectrum at zero magnetic field; Effects of the magnetic field.

We have theoretically studied the InP based InGaAs/GaAsSb /InAlAs type-II 'W' quantum wells (QWs) using the eight band k.p theory. The trade-off between the emission wavelength and the magnitude of the transition matrix element was investigated with various structural parameters of the 'W' QWs. For the same emission wavelength, the devices with thinner InGaAs/GaAsSb layers and a higher Sb content in GaAsSb could provide higher transition strength. The gain spectra and their peak values at various carrier densities were calculated. We have also found that a more balanced electron and hole masses in the type-II 'W' QWs can benefit the material gain. In our designed cases, we have seen that the reduced hole effective mass due to a higher Sb content can partially compensate the gain loss caused by the reduced transition matrix element. Based on the optimized design, a material gain above 103 cm-1 is readily achievable for a single properly designed 'W' quantum well. [ABSTRACT FROM AUTHOR]

The effects of different scattering on two-dimensional electron gases (2DEGs) in lattice matched In0.53Ga0.47As/In0.52Al0.48As quantum wells with silicon δ-doped in one barrier layer have been investigated by means of magneto-transport measurements. For the studied samples, the 2DEGs have occupied two subbands. It is found that the dominant scattering mechanism is ionized impurities scattering for the two subbands' electrons. Besides the dominant scattering mechanism of ionized impurities scattering, Coulomb scattering also plays a role in scattering mechanism for both subbands. Both the transport scattering time and the quantum scattering time of the second subband are larger than those of the first subband. It is because that the electrons of the first subband are, on average, closer to the ionized impurities in the doped layer, they will be scattered more strongly than those in the second subband. Due to the electron wave functions for the second subband spread more widely in the quantum wells, the alloy disorder scattering is more important for the carriers in the second subband than that in the first subband. [ABSTRACT FROM AUTHOR]

Optoelectronic properties of undoped ZnSe/ZnMgSSe multiple quantum wells (MQWs) grown by molecular beam epitaxy on (001) GaAs substrates have been investigated by photoluminescence, photoconductivity, and persistent photoconductivity (PPC) measurements. The features related to the band-to-band excitonic and impurity-related transitions of ZnSe/ZnMgSSe MQWs are observed and discussed. In addition, the parameters that describe the temperature dependence of the interband transition energy and broadening function of the excitonic feature are evaluated. PPC has been observed in undoped ZnSe/ZnMgSSe MQWs. The decay kinetics of the PPC effect can be described by a stretched-exponential function, I[sub ppc](t)=I[sub ppc](0)exp[-(t/τ)[sup β]], (0<β<1). Through the study of the PPC effect under various conditions, such as different temperature, different photon energy of photoexcitation, and different ZnSe well width, we identify that the carrier excitation from the defect level in ZnMgSSe barrier layer into the ZnSe well layer is the origin of the PPC effect in ZnSe/ZnMgSSe MQWs. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]