Optical-gap reduction in the ordered phases of GaInAs solid solution

The measured optical band gap of the (GaAs)₁(InAs)₁ (001) superlattice is smaller than that of the corresponding Ga_0.5In_0.5As random alloy. Ab-initio pseudopotential calculations show that the effect is caused by a repulsion between the lowest conduction bands, or, equivalently, by the onset of confinement of electrons to GaAs layers, and is accompanied by the decrease of the electron effective mass. Similar reductions are obtained for the indirect band gaps, and for cubic phases of GaInAs solid solution.     

Studies on the prospects of GaInAs and GaInAsP for double-drift region heterostructure IMPATTs

Double heterojunctions having the material combinations InP/GaInAs/InP, GaAs/GaInAs/GaAs and InP/GaInAsP/InP have been studied to assess their potential for double-drift region (DDR) IMPATT diodes. An accurate and realistic computer simulation program has been framed and used for the dc and high-frequency analysis of the DDRs. The analysis is carried out both in IMPATT (IMPact Avalanche Transit Time) and MITATT (MIxed Tunnelling Avalanche Transit Time) modes. Our results indicate that the GaAs/GaInAs/GaAs DDR would provide the best mm-wave performance up to sufficiently high frequencies. Further, the performance of the DDR diodes is observed to deteriorate at high frequency of operation due to phase distortion introduced by tunnel injected current, which is found to be the least in the case of GaAs/GaInAs/GaAs DDR leading to the best performance of this DDR amongst the three.     

Selective area growth of GaInNAs/GaAs by MOVPE

Selective area growth (SAG) of GaInNAs/GaAs systems has been studied by metalorganic vapor-phase epitaxy (MOVPE) for the first time. This also includes a comparative study of SAG of the GaInAs/GaAs. The patterns consisted of various filling factors (F). The band gap changes and the growth morphology have been investigated. A red-shift observed for SAG GaInAs is 100 nm with respect to the planar GaInAs which can be attributed to both In enrichment and quantum well (QW) thickness enhancement. Selectively grown GaInNAs structures exhibit a maximum wavelength of 1.3 μm, corresponding to a red-shift of 80 nm with respect to the planar GaInNAs. Atomic force microscopy (AFM) scans reveal a three-dimensional growth behaviour for SAG GaInNAs unlike SAG GaInAs. This can be related to a certain amount of phase separation or strain that are often the signatures of N incorporation. The cathodoluminescence (CL) intensities (spectral line width) for SAG GaInNAs are larger (smaller) than those for SAG GaInAs at low F's but smaller (larger) at high F's. This indicates that at low F's, GaInAs has degraded due to very high strain but certain amount of strain compensation occurs in GaInNAs.     

Resistance in sub-μm size GaAs lines

The electrical resistance in GaAs submicron mesa lines have been studied as a function of the line width. Using molecular beam epitaxy two types of conducting layers were made: an n⁺-layer and a two-dimensional electron gas confined to an (AlGa)As/GaAs heterostructure. Processing of the lines was made by photolitography, electron beam litography and ion etching. Resistance data at 77 K and 300 K are discussed for line widths in the interval 0.2 to 5 μm. A size dependent conduction was found and interpreted in terms of geometry induced limitation of the effective conducting path.     

Cracking self-assembled InAs quantum dots

We present a cross-sectional scanning tunneling microscopy (X-STM) investigation of InAs quantum dots in a GaAs matrix. The structures were grown by molecular beam epitaxy (MBE) at a low growth rate of 0.01 ML/s and consist of five layers of uncoupled quantum dot structures. Detailed STM images with atomic resolution show that the dots consist of an InGaAs alloy and that the indium content in the dot increases towards the top. The analysis of the height versus base-length relation obtained from cross-sectional images of the dots shows that the shape of the dots resembles that of a truncated pyramid and that the square base is oriented along the [010] and [100] directions. Using scanning tunneling spectroscopy (STS) we determined the onset for electron tunneling into the conduction and out of the valence band, both in the quantum dots and in the surrounding GaAs matrix. We found equal voltages for tunneling out of the valence band in GaAs or InGaAs whereas tunneling into GaAs occurred at higher voltages than in InGaAs.     

The influence of confining wall profile on quantum interference effects in etched Ga0.25In0.75As/InP billiards

We present measurements of the potential profile of etched GaInAs/InP billiards and show that their energy gradients are an order of magnitude steeper than those of surface-gated GaAs/AlGaAs billiards. Previously observed in GaAs/AlGaAs billiards, fractal conductance fluctuations are predicted to be critically sensitive to the billiard profile. Here we show that, despite the increase in energy gradient, the fractal conductance fluctuations persist in the harder GaInAs/InP billiards.     

InGaP/GaAs heterojunction bipolar transistor grown by solid-source molecular beam epitaxy with a GaP decomposition source

Lattice-matched InGaP epilayers on GaAs (001) and InGaP/GaAs heterojunction bipolar transistors (HBTs) were successfully grown by solid-source molecular beam epitaxy (SSMBE) with a GaP decomposition source. A 3 μm thick InGaP epilayer shows that low temperature photoluminescence (PL) peak energy is as large as 1.998 eV, full width at half maximum (FWHM) is 5.26 meV, which is the smallest ever reported, and X-ray diffraction (XRD) rocking curve linewidth is as narrow as that of GaAs substrate. The electron mobilities at room temperature of nominally undoped InGaP layers obtained by Hall measurements are comparable to similar InGaP epilayer grown by solid-source molecular beam epitaxy (SSMBE) with other sources or other growth techniques. The large area InGaP/GaAs HBTs show very good Dc characteristics.     

Magnetotransport characterization of THz detectors based on plasma oscillations in submicron field-effect transistors

Magnetotransport characterization of field-effect transistors in view of their application as resonant detectors of THz radiation is presented. Three groups of different transistors based on GaAs/GaAlAs or GaInAs/AlGaAs heterostructures are investigated at liquid-helium temperatures and for magnetic fields of up to 14 T. The magnetic-field dependence of the transistor resistance is used for evaluation of the electron density and mobility in the transistor channel. The electron mobility and concentration determined from magnetotransport measurements are used for the interpretation of recently observed resonant detection of terahertz radiation in 0.15 μm gate length GaAs transistors and for the determination of the parameters of other field-effect transistors processed for resonant and voltage tunable detection of THz radiation. From Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 138–145. Original English Text Copyright ? 2004 by Lusakowski, Knap, Dyakonova, Kaminska, Piotrowska, Golaszewska, Shur, Smirnov, Gavrilenko, Antonov, Morozov. This article was submitted by the authors in English.     

Evidence for Fermi level shift in GaInAs/GaAs quantum wells upon nitrogen incorporation

GaInAs/GaAs and GaInNAs/GaAs quantum well (QW) structures grown by metalorganic vapor phase epitaxy have been studied by contactless electroreflectance spectroscopy. In addition to the N-related redshift of QW transitions, an increase in the electric field in the GaAs cap layer has been observed after the incorporation of nitrogen atoms into the GaInAs QW. This observation is associated with the tendency of the Fermi level shift to a given energy in the GaInNAs QW region due to N-related defects.     

In0.2Ga0.8As/GaAs interface revealed by an AlAs marker layer

A detailed analysis of the microstructure of layered semiconductor heterostructures is only possible if the interface between the various layers can be accurately located. Microstructural features whose characterisation is dependent on the location of the interface include the planarity and width of the layers, the composition profile and the geometry of misfit dislocations (in the particular case of lattice mismatched heterostructures). We report on an investigation to image, at high resolution in the transmission electron microscope, the interface in an In_0.2Ga_0.8As/GaAs structure grown by molecular beam epitaxy. The difficulty in locating the interface, due to similarity in electron optical behaviour of GaAs and In_0.2Ga_0.8As when imaged in a 110 direction, was overcome by incorporating into the structure a marker layer of AlAs two unit cells thick (11.4 Å in total) between the GaAs and the In_0.2Ga_0.8As layers. Lattice fringe images of an In_0.2Ga_0.8As/AlAs/GaAs structure are presented which show, at near atomic resolution, the location of the misfit dislocations relative to the In_0.2Ga_0.8As/GaAs interface.     

Microstructure of annealed low-temperature-grown GaAs layers

The crystal structure and orientation of As precipitates in annealed low-temperature GaAs (LT-GaAs) layers have been investigated by transmission electron microscopy. Three types of As precipitates were identified in layers grown by molecular beam epitaxy at substrate temperatures from 180° to 210° C. In the monocrystalline LT-GaAs layers small pseudocubic As precipitates (2–3 nm diameter) coherent with the GaAs lattice were observed. These precipitates lose their coherency when a certain critical size is exceeded. Precipitates of similar sizes are occasionally found for which a TEM lattice image cannot be obtained. These precipitates are believed to be amorphous. Larger As precipitates with a hexagonal structure (>4 nm diameter) were also found in the layers. These hexagonal As precipitates were observed to be largest near structural defects. The effect of these precipitates on the structure and on the electronic properties of the host GaAs is discussed.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Dependence of the microstructural properties on the substrate temperature in strained CdTe (1 0 0)/GaAs (1 0 0) heterostructures

CdTe thin films were grown on GaAs (1 0 0) substrates by using molecular beam epitaxy at various temperatures. The bright-field transmission electron microscopy (TEM) images and the high-resolution TEM (HRTEM) images showed that the crystallinity of CdTe epilayers grown on GaAs substrates was improved by increasing the substrate temperature. The result of selected-area electron diffraction pattern (SADP) showed that the orientation of the grown CdTe thin films was the (1 0 0) orientation. The lattice constant the strain, and the stress of the CdTe thin film grown on the GaAs substrate were determined from the SADP result. Based on the SADP and HRTEM results, a possible atomic arrangement for the CdTe/GaAs heterostructure is presented.     

N,Sb和单分子层数对GaAs/GaInNAsSb超晶格性能的影响

用Keating的价力场 (valenceforcefield)模型和蒙特卡罗方法计算了GaAs GaInNAsSb超晶格中键的分布、原子的精确位置以及应变 .用折叠谱法 (foldedspectrummethod)结合Williamson经验赝势法计算了GaAs GaInNAsSb超晶格应变条件下的电子结构 .讨论了N和Sb原子以及超晶格单分子层数对电子结构的影响 .发现导带底电子态在N原子周围的局域化减小了光跃迁矩阵元 ,从而影响了该超晶格的发光性能 .计算并讨论了超晶格的电子和空穴的有效质量 .     

Quantitative measurement of image intensity in transmission electron microscope images

We have made a thorough comparison of the ability of image simulations to predict the contrast in high-resolution electron microscope lattice images of GaAs. Simulations of the diffracted beam intensities from thickness fringes generally agreed with observations to within ∼20% over a range of GaAs thicknesses up to 150 nm. Likewise, simulations of lattice images agreed qualitatively with experimental lattice images over a range of defocus and sample thicknesses up to 20 nm. However, using the same parameters as for the diffracted beam intensities, lattice fringe amplitudes were calculated to be typically two to three times higher than observed experimentally.     

Characterization by X-ray diffraction and electron microscopy of GaInAs and GaAsN single layers and quantum wells grown on GaAs

Experiments were performed on thick GaInAs and GaAsN layers and on GaInAs quantum wells grown on (1 1 1)B and (0 0 1) GaAs substrates. The aim of this work is to develop an experimental procedure in order to evaluate the chemical compositions and relaxation state of the samples at global as well as nanometre scale. Chemical analyses (EDS, RBS, etc.), X-ray diffraction (reciprocal space map, sin²ψ, etc.) and XTEM were carried out.The validity of the sin²ψ method on the above mentioned thin layers has been tested. Good accuracy is obtained for In and N composition but more work has to be done in order to optimize the determination of the relaxation state. Coupling TEM observations to these calculations gives valuable information on the relaxation mechanisms (misfit dislocations, stacking faults, microtwins, etc.).     

Spatially resolved STM light emission spectra of cleaved (1 1 0) AlGaAs/GaAs heterostructures

We have measured the spectra of light emitted from individual single GaAs quantum wells of cleaved (1 1 0) AlGaAs/GaAs heterostructures using the STM (scanning tunneling microscope) tip as a local electron injection source. The cross-sectional STM images of the quantum wells were obtained, and the light emission spectra were measured by locating the STM tip over individual quantum wells of interest. Single emission peaks were observed to shift to the high-energy side with decreasing well width. The peak positions agree with the calculated transition energies for the corresponding well width. The thermalization length of the injected electron was estimated by observing the change in the emission intensity as the tip is moved at different distances from a given well.     

Photoreflectance spectroscopy of semiconductor structures at hydrostatic pressure: A comparison of GaInAs/GaAs and GaInNAs/GaAs single quantum wells

The pressure dependence of optical transitions in Ga_0.64In_0.36As/GaAs and Ga_0.64In_0.36N_0.01As_0.99/GaAs single quantum well (SQW) structures were studied in photoreflectance (PR) spectroscopy. In order to apply high hydrostatic pressure, up to ∼11 kbar, the liquid-filled clamp-pressure cell with a sapphire window for optical access has been adopted in the PR set-up with the so called ‘bright configuration’. It has been found that the linear hydrostatic pressure coefficient for the ground state transition are equal to 8.6 and 7.3 meV/kbar for the GaInAs/GaAs and GaInNAs/GaAs SQWs, respectively. This result shows that the incorporation of only 1% of N atoms into GaInAs/GaAs leads to ∼15% decrease in the pressure coefficient. In addition, a non-linearity in the pressure dependence of the ground state transition has been resolved for the GaInNAs/GaAs SQW.     

High-Q factor single mode circular photonic crystal nano-resonator

In this work, the analysis, fabrication and optical characterization of a two-dimensional circular photonic crystal (2D-CPC) nano-resonator based on an air/GaAs/air slab waveguide are presented. Four InAs/InGaAs quantum dots (QDs) stacked layers emitting around 1300 nm at room temperature were embedded in a GaAs waveguide layer grown on an Al_0.7Ga_0.3As layer and GaAs substrate. The patterning of the structure and the membrane release were achieved by using electron beam lithography, ICP plasma etching and selective wet etching of the AlGaAs sacrificial layer. The micro-luminescence spectrum recorded from the fabricated nano-cavity shows a narrow optical transition at the resonance wavelength of about 1282 nm with a FWHM and Q-factor of 6.2 Å and more than 2000, respectively.     

Raman spectroscopic studies of ZnSe/GaAs interfaces

ZnSe/semi‐insulating GaAs interfaces were studied by observing photogenerated plasmon–LO (PPL) coupled modes by nonresonant micro‐Raman spectroscopy. The effect of the carriers generated by the focused laser beam was investigated for a series of different thicknesses of ZnSe epitaxial layers. The PPL mode in GaAs was observed in the micro‐Raman spectra for all samples, but with different magnitude. The plasma is believed to be an electron gas as a result of the negative nature of the interfacial region that contains predominantly hole traps. The free carrier concentration is estimated to be > 10¹⁸ cm⁻³ and their lifetime ∼0.1 ns. This relatively long lifetime suggests that the ZnSe/GaAs interface has to be of high structural quality leading to a low recombination velocity. ZnSe/GaAs heterostructures of less crystalline quality (as determined by resonant Raman measurements) shows the effect of photogenerated carriers only to lesser extent. Copyright © 2007 John Wiley & Sons, Ltd.     

Hot electron cooling in parabolic and modulation doped quantum wells and doped superlattices

Hot electron cooling in variously structured and doped quantum wells and superlattices has been studied by low temperature steady-state photoluminescence. A parabolic quantum well realized by thickness grading of Al_0.3Ga_0.7As and GaAs epitaxial layers deposited by molecular beam epitaxy with electron level spacings of ∼25 meV did not show increased electron plasma temperatures compared to thick epitaxially deposited GaAs or square quantum wells with electron level spacings greater than the LO phonon energy of GaAs; this implies that mechanisms involving intersubband Δk ≠ 0 transitions and interfacial recombination are dominant in the parabolic structure. Investigations as a function of carrier concentration in modulation-doped quantum wells and n-type superlattices with strong miniband formation indicate that increasing the carrier concentration in either structure above ∼ 5 × 10¹⁷ cm^-3 significantly increases the electron plasma temperatures, even under low light excitation, suggesting that such structures may be suited for high efficiency hot electron photovoltaic and photoelectrochemical cells.