xAs Structures Research Articles

Band structure engineering in strain-free GaAs mesoscopic systems

We investigate the optical properties of strain-free mesoscopic GaAs/AlxGa1 − xAs structures (MGS) coupled to thin GaAs/AlxGa1 − xAs quantum wells (QWs) with varying Al content (x). We demonstrate that quenching the QW emission by controlling the band crossover between AlGaAs (X-point) and GaAs (Γ-point) gives rise to long carrier lifetimes and enhanced optical emission from the MGS. For x = 0.33, QW and MGS show typical type-I band alignment with strong QW photoluminescence emission and much weaker sharp recombination lines from the MGS localized exciton states. For x ≥ 0.50, the QW emission is considerably quenched due to the change from type-I to type-II structure while the MGS emission is enhanced due to carrier injection from the QW. For x ≥ 0.70, we observe PL quenching from the MGS higher energy states also due to the crossover of X and Γ bands, demonstrating spectral filtering of the MGS emission. Time-resolved measurements reveal two recombination processes in the MGS emission dynamics. The fast component depends mainly on the X − Γ mixing of the MGS states and can be increased from 0.3 to 2.5 ns by changing the Al content. The slower component, however, depends on the X − Γ mixing of the QW states and is associated to the carrier injection rate from the QW reservoir into the MGS structure. In this way, the independent tuning of X − Γ mixing in QW and MGS states allows us to manipulate recombination rates in the MGS as well as to make carrier injection and light extraction more efficient.

First principles study on the structural stability and optoelectronic properties of InxGa1−xAs materials with different Indium component

The III–V ternary semiconductors InxGa1−xAs (0 ≤ x ≤ 1) have attracted a great deal of interests in electronics and optoelectronics due to their superior transport properties, high-efficiency thermoelectric applications and so on. In this work, the optoelectronic properties of bulk InxGa1−xAs materials and the phase transition of InxGa1−xAs nanowires (NWs) at different Indium (In) component have been investigated by first-principles calculation using density functional theory. Different calculation models have been chosen to simulate the (2 × 2 × 1) supercells of pure bulk InxGa1−xAs structures and the most stable one has been adopted for further calculation. NWs models of In0.25Ga0.75As and In0.75Ga0.25As have been employed to represent the In-rich and Ga-rich structures, respectively. Our calculation results have clearly shown that the In-rich nanostructure show more zinc-blende characteristic while the wurtzite phase exhibit dominating role in the Ga-rich NWs. The band gap of bulk models decreases with increasing the In component, which is well consistent with the theoretical formula. With the In component increasing, the lower valence band shifts toward high energy region and the peak value increases, while the absorption edge and peak value move to lower energy side and the static dielectric constant increases. The metal reflective properties emerge in certain photon energy range. Our results offer the guidance to make use of the different properties of InxGa1−xAs materials at different In component.

Equilibrium Lattice Relaxation and Misfit Dislocations in Step-Graded In x Ga1−x As/GaAs (001) and In x Al1−x As/GaAs (001) Metamorphic Buffer Layers

The inclusion of metamorphic buffer layers (MBLs) in the design of lattice-mismatched semiconductor heterostructures is important in enhancing reliability and performance of optoelectronic and electronic devices through proper control of threading dislocations; threading dislocation can be reduced by allowing the distribution of the misfit dislocations throughout the MBL, rather than concentrating them at the interface where substrate defects and tangling can pin dislocations or otherwise reduce their mobility. Compositionally graded layers have been particularly used for this purpose and in this work we considered heterostructures involving a step-graded InxGa1−xAs or InxAl1−xAs epitaxial layer on a GaAs (001) substrate. For each structure type, we present minimum energy calculations including (i) the surface and (ii) average in-plane strain and (iii) the misfit dislocation density profile with various grading coefficients (thickness and indium composition variation). In both types of structures, the average in-plane strain and misfit dislocation density profile scale with the average grading coefficient, but InxAl1−xAs structures with a greater average elastic stiffness constants exhibit slightly higher average compressive in-plane strain (absolute valued) which is associated with higher misfit dislocation densities. However, the rate of change in the normalized relaxation percentage per unit thickness of each step with respect to the lattice mismatch of the step is lower in the InxAl1−xAs material system. The difference of the in-plane strain is small (<3%), however, so that these material systems are virtually interchangeable in terms of their mechanical behavior (<5.1% change in elastic constants).

Time-resolved Photoluminescence Characterisation of GaAs/AlxGa1–xAs Structures Designed for Microwave and Terahertz Detectors

The time-resolved photoluminescence of donor Si-doped GaAs/AlxGa1-xAs (x = 0.3 0.25, 0.2, 0.1) structures designed for microwave and terahertz detectors have been investigated at T = 3.6 K temperature. The excitonic, impurity and defect related emission lifetimes are revealed for these structures. Possible mechanisms of carrier recombination are discussed. Concentration of acceptor, carbon and silicon, are evaluated from measured lifetimes in the structures. DOI: http://dx.doi.org/10.5755/j01.ms.20.2.6317

Photovoltaic infrared detection with p-type graded barrier heterostructures

Photovoltaic infrared detectors have significant advantages over photoconductive detectors due to zero bias operation, requiring low power and having reduced low frequency noise. They also exhibit no thermally assisted tunneling currents, leading to higher operating temperatures. p-type emitter/graded barrier GaAs/AlxGa1−xAs structures were tested as photovoltaic detectors in the infrared region, operating under uncooled conditions and without an applied bias voltage. A photovoltaic responsivity of 450 mV/W was obtained with a detectivity (D*) of 1.2 × 106 Jones at a peak wavelength 1.8 μm at 300 K. Responsivity and D* increased to ∼1.2 V/W and 2.8 × 106 Jones, respectively, at 280 K. A non-linear improvement in responsivity was observed with increased emitter thickness.

X-ray luminescence spectra of graded-gap AlxGa1−xAs structures irradiated by alpha particle

The influence of 241 Am alpha particle irradiation on X-ray luminescence spectra of the graded-gap Al x Ga 1− x As structures of different thicknesses is investigated. It is observed that the integral X-ray luminescence intensity of nonirradiated thin (15 μm) structure is 1.4 times less than that in the thick (32 μm) structure, and this difference increases to 3 times after 3×10 10 cm −2 dose of irradiation by alpha particle. The X-ray luminescence intensity of the energy hν <1.5 eV of thin nonirradiated structure is about 7 times less than that in thick one. The internal graded-gap electric field F gg is responsible of that large difference, because it shifts the X-ray generated carriers to the narrow-gap surface with great nonradiative surface recombination rate. The alpha particle irradiation increases nonradiative recombination rate and causes a decrease of the X-ray luminescence intensity of all spectra lines in the thin (15 μm) detector. The most significant drop in X-ray luminescence efficiency is observed from the region at narrow-gap surface after the initial stage (10 9 cm −2 dose) of alpha particle irradiation. In the 32 μm thick detector, the luminescence intensity of the energy hν =1.8 eV does not change up to 2×10 10 cm −2 of alpha particle irradiation dose. That means the high irradiation hardness of the thick graded-gap X-ray detector with optical response.

Reverse engineering of AlxGa1−xAs/GaAs structures composition by reflectance spectroscopy

AbstractThe paper presents the application of non-modulation reflectance method for composition profiling of epitaxial AlxGa−xAs/GaAs structures. This non-destructive method is based on spectral measurements and theoretical reflectance spectrum matching. This is a very accurate and sensitive method of determining the Al composition in AlxGa1−xAs layers and structures with resolution down to 1 nm. In this work, the authors describe theoretic principles of this method and present experimental results of characterization of different AlGaAs structures to prove the potential of the worked out method.

Intermodulation of transverse magnetoresistance oscillations in structures with strong intersubband interaction

mp F and mp F ± were derived for transverse magnetoresistance oscillations. These frequencies were experimentally found in [3] for AlxGa1–xAs structures. The oscillation intermodulation and peaks at combination frequencies were related to the elastic electron-electron (e–e) and inelastic electron-phonon interactions. In [4] it has been demonstrated that the peaks at combination frequencies are caused by resonant intersubband interaction of electrons. In the present work, we study a two-dimensional electron gas in the doped InAs/AlSb structure. The strong (up to 50%) Shubnikov–de Haas amplitude-frequency modulation (intermodulation) of oscillations of the transverse magnetoresistance xx ρ is established. As a continuation of [4], we suggest a qualitative interpretation of a dependence of peak amplitudes at combination frequencies on the intersubband interaction intensity. Reasons for different contrasts of intermodulation oscillations in InAs/AlSb and AlxGa1–xAs structures are discussed.

Conductance of meandering wires

In the past years a strong attention has been focused on the optical and transport properties of semiconductor hetero-structures owing to the possibility of tailoring material parameters such as the electron effective mass and the band gap, by changing the structure parameters. Recent experimental results on InxGa1−xAs structures cannot be explained in the framework of standard theoretical and simulative approaches for the description of carrier transport in quantum wells. In order to get a better insight on the effect of the surface roughness on the conductance, we have considered meandering quantum wires and have computed their coherent transport characteristics. We find that, at low energies, the transmission coefficient is strongly dependent on the SR parameters. The dependence of conductance upon length of the device is also analyzed.

Electron magnetotransport in coupled quantum wells with double-sided doping

Magnetoresistance in a weak magnetic field was measured in AlxGa1−xAs/GaAs/AlxGa1−xAs structures with coupled quantum wells separated by a thin AlAs central barrier. The experimental data on negative magnetoresistance were analyzed in terms of the weak-localization model and in the kinetic approach using the density matrix. In some cases, the kinetics approach allows a more accurate description of the experimental data.

Hydrogenation of strain engineered InAs/In x Ga 1− x As quantum dots

InxGa1−xAs/InAs/InxGa1−xAs structures have been grown by atomic layer molecular beam epitaxy on top of a GaAs buffer and substrate. In these structures, the thickness d and/or the In composition x of the lower InxGa1−xAs confining layer control the strain in self-assembled quantum dots. This strain engineering has allowed achieving emission energies as low as 1.5 μm at low T, with a rapid quenching of the photoluminescence (PL) signal at high T. Hydrogen irradiation of these structures leads to an increase in the PL efficiency, higher in samples with higher x, with a blue shift in the peak energy. A higher concentration of non radiative defects in confining layers richer in indium is responsible for the observed PL quenching, more than an increased thermal escape of carriers toward the InxGa1−xAs barriers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ga0.51In0.49P/InxGa1−xAs/GaAs doped‐channel FETs (DCFETs) and their applications on monolithic microwave integrated circuits (MMICs)

AbstractIn this paper, lattice‐matched Ga0.51In0.49P/GaAs and strained Ga0.51In0.49P/In0.2Ga0.8As doped‐channel FETs (DCFETs) were investigated in terms of DC and microwave performances, including frequency response, noise figure, power‐added efficiency (PAE), and output power. In addition, small‐signal and large‐signal models were created for designing monolithic microwave integrated circuits (MMICs). The heterostructures were both grown by gas‐source molecular beam epitaxy (GSMBE) on semi‐insulating (100) GaAs substrates. In situ reflection high‐energy electron diffraction (RHEED) was used to calibrate the growth rate of InP and GaP. Because of the high etching selectivity between Ga0.51In0.49P and In0.2Ga0.8As/GaAs, the uniformity of the measured electrical properties of our fabricated devices is quite satisfying, which indicates that these Ga0.51In0.49P/InxGa1−xAs structures are very suitable for mass production. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 39: 56–62, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11126

Graded-gap AlxGa1−x As X-ray detector with collected charge multiplication

An increase in sensitivity of the graded-gap AlxGa1−xAs/GaAs X-ray detectors is achieved using multiplication of charge generated in a detector body. A thin n-GaAs layer was grown on a narrow-gap side of the AlxGa1−xAs structure. This layer is used as a photoconductor. The multiplication of the charge injected from the graded-gap structure to the photoconductive n-GaAs layer exceeds a few hundred times. This sensitive structure was tested as a detector of single alpha particles (from an 241Am source) .

X-ray luminescence spectra of graded-gap Al xGa 1− xAs structures

X-ray luminescence from graded-gap Al x Ga 1− x As structures is investigated. The presented measurements show that only part of the Al x Ga 1− x As layer with the Al fraction x<0.1 is active as a light emitter.

Quantitative Secondary Ion Mass Spectrometry (SIMS) of III-V Materials

AbstractSecondary ion mass spectrometry (SIMS) provides direct methods to characterize the chemical composition of III-V materials at major, minor and trace level concentrations as a function of layer depth. SIMS employs keV primary ions to sputter the surface and sensitive mass spectrometry techniques to mass analyze and detect sputtered secondary ions which are characteristic of the sample composition. In-depth compositional analysis of these materials by SIMS relies on a number of its unique features including: (1) keV primary ion sputtering yielding nanometer depth resolutions, (2) the use of MCs+ detection techniques for quantifying major and minor constituents, and (3) ion implant standards for quantifying trace constituents like dopants and impurities. Nanometer depth resolution in SIMS sputtering provides accurate detection of diffusion of dopants, impurities and major constituents. MCs+ refers to the detection of “molecular” ions of an element (M) and the Cs+ primary beam. MCs+ minimizes SIMS matrix effects in analysis for major and minor constituents, thus providing good quantification. This paper presents a SIMS study of AlxGa1−xAs structures with three different x values. MCs+ (M=Al or Ga) data are presented for the accurate determination of major and minor components. Rutherford backscattering spectrometry (RBS) and x-ray diffraction (XRD) data were crosscorrelated with the MCs+ results. Three specimens with different x values were ion implanted with H, C, O, Mg, Si, Zn and Se to study quantification of trace levels. SIMS data acquired on a double focusing instrument (CAMECA IMS-4f) and a quadrupole instrument (PHI ADEPT 1010) are also compared. Lastly, we discuss our efforts to improve the analysis precision for pand n-type dopants in AlGaAs which currently is ± 3% (1 sigma).

In situ cleaning of GaAs and AlxGa1−xAs surfaces and production of ohmic contacts using an atomic hydrogen source based on a reflected arc discharge

A method for the production of ohmic contacts to n-type GaAs and to n-type and p-type AlxGa1−xAs has been proposed where the surface cleaning in atomic hydrogen and the metal film deposition are performed in situ. A feature of the method is that it is realized in a system for vacuum deposition of metal films with the residual pressure kept equal to ∼5×10−4 or ∼(4–10)×10−5Pa when GaAs or AlxGa1−xAs structures, respectively, are cleaned. The atomic hydrogen flow was formed by a source whose operation is based on a reflected arc discharge with a hollow cathode and a self-heating electrode. In the process of cleaning the hydrogen pressure was 10−2 Pa and the temperature of the specimens and the time of their treatment were varied in the ranges from 300 to 400 °C and from 1 to 90 min, respectively. AuGe/GaAs interfaces with the contaminant content below the sensitivity threshold of the method of Auger electron spectroscopy (AES) have been produced. With some technological expedients, an AuGe/Al0.6Ga0.4As interface with the oxygen content &amp;lt;1% and the contents of other impurities below the sensitivity threshold of the AES method have been produced. A comparative investigation of the formation of an ohmic contact by the proposed method and by a conventional technology using “wet” chemical cleaning has shown that the contacts produced with the use of atomic hydrogen cleaning show a better morphology of the surface and a more even edge of the contact pad, high adhesion of the metal film to the semiconductor, and a low contact resistance. The technological process for the production of ohmic contacts is characterized by a high reproducibility. The application of the proposed method together with hydrogenation of the near-surface region of semiconductor structures used in the production of light diodes have raised the output power of the diodes by 30%–40%.

Characterization of semiconductor heterostructures and quantum dots by friction force microscopy

The capability of friction force microscopy to obtain compositional maps of semiconductor structures grown by molecular beam epitaxy was studied. Experiments on InP/InGaAs multiquantum wells determined a compositional spatial resolution of 3 nm. Additionally, variations in indium concentration smaller than 10% were detected in InxGa1 − xAs structures. Friction maps of InAs and InSb quantum dots on InP(001) are also presented. The experimental results show that the frictional force is sensitive to the presence or absence of a wetting monolayer. Based on these experiments, the potential of friction force microscopy to develop a spatially resolved spectroscopy is discussed.

High Quality in.Ga1−xas Heterostructures Grown on GaAs With Movpe

AbstractInxGa1−xAs structures with compositionally graded buffers were grown by metal-organic vapor phase epitaxy (MOVPE) on GaAs substrates and characterized with plan-view and cross-sectional transmission electron microscopy (PV-TEM and X-TEM), atomic force microscopy (AFM), and x-ray diffraction (XRD). The results show that surface roughness experiences a maximum at growth temperatures where phase separation occurs in InxGa1−xAs. The strain energy due misfit dislocations in the graded buffer indirectly influences phase separation. At growth temperatures above and below this temperature, the surface roughness is decreased significantly; however, only growth temperatures above this regime ensure nearly complete relaxed graded buffers with the most uniform composition caps. With the optimum growth temperature for grading InxGa1−xAs determined to be 700°C, it was possible to produce In0.33Ga0.67As diode structures on GaAs with threading dislocation densities &lt; 8.5 × 106/cm2

The study of ripple formation and degradation of depth resolution in GaAs and Al xGa 1−xAs structures

The present work investigates the characteristics of ripple formation in Al xGa 1−xAs samples under oxygen irradiation. The wavelength and the amplitude of the ripples have been determined under different conditions and are examined within the scope of two existing theoretical models: the model of Bradley and Harper [1] and the local incorporation model [12]. It is shown that oxygen bombardment of Al xGa 1−xAs (with 0 ≤ × ≤ 0.56) structures with an impact energy of 8 keV at 37° always produces ripples. Both the wavelength as well as the growth rate of the ripples are affected by the stoichiometric composition of the sample analyzed, the temperature, and the oxygen pressure near the sample. The work shows that the local incorporation mechanism contributes to the ripple growth and explains correctly the increase in ripple amplitude when introducing oxygen gas near the sample. Moreover, it also accounts for the material dependence and the strong reduction in ripple growth rate at the highest pressures. Neither of the two models can explain the dependence of the ripple formation as a function of the temperature.

Origin of XAS Resonances for 3D Metals in Rutile Compounds

In this work, we want to clear up the origin of the XAS structures for rutile compounds. To determine the connections between the atomic arrangement and XAS structures, we have calculated the Ti K edge in TiO 2 and compared results when few atoms are removed from the cluster. Multiple Scattering approach from FEFF6 code is used.