Selectively dopedn-AlxGa1−xAs/GaAs heterostructures with high-mobility two-dimensional electron gas for field effect transistors

In selectively dopedn-Al_xGa_1–xAs/GaAs heterostructures with high-mobility two-dimensional electron gas (2 DEG) at the heterointerface a second conductive channel exists, if the Al_xGa_1–xAs layer is not totally depleted from free carries. The occurrence of parallel conductance has a deleterious effect on the performance of high-electron mobility transistors (HEMTs) fabricated from this material. Although in principle computable, parallel conductance depends on a large number of design parameters to be chosen for the heterostructure, which are additionally affected by the presence of deep electron traps inn-Al_xGa_1–xAs of composition 0.25n-Al_xGa_1–xAs/GaAs heterostructures is shown.     

Growth and studies of Hg1−xCdxTe based low dimensional structures

The band structure of HgTe quantum wells (QWs) has been determined from absorption experiments on superlattices in conjunction with calculations based on an 8×8 k·p model. The band structure combined with self-consistent Hartree calculations has enabled transport results to be quantitatively explained.Rashba spin–orbit, (SO) splitting has been investigated in n-type modulation doped HgTe QWs by means of Shubnikov–de Haas oscillations (SdH) in gated Hall bars. The heavy hole nature of the H1 conduction subband in QWs with an inverted band structure greatly enhances the Rashba SO splitting, with values up to 17 meV.By analyzing the SdH oscillations of a magnetic two-dimensional electron gas (2DEG) in modulation-doped n-type Hg_1−xMn_xTe QWs, we have been able to separate the gate voltage-dependent Rashba SO splitting from the temperature-dependent giant Zeeman splitting, which are of comparable magnitudes. In addition, hot electrons and Mn ions in a magnetic 2DEG have been investigated as a function of current.Nano-scale structures of lower dimensions are planned and experiments on sub-micrometer magneto-transport structures have resulted in the first evidence for ballistic transport in quasi-1D HgTe QW structures.     

Lithium ion conductive glass–ceramics with Li3Fe2(PO4)3 and YAG laser-induced local crystallization in lithium iron phosphate glasses

The glasses with the composition of 37.5Li₂O–(25 − x)Fe₂O₃–xNb₂O₅–37.5P₂O₅ (mol%) (x = 5,10,15) are prepared, and it is found that the addition of Nb₂O₅ is effective for the glass formation in the lithium iron phosphate system. The glass–ceramics consisting of Nasicon-type Li₃Fe₂(PO₄)₃ crystals with an orthorhombic structure are developed through conventional crystallization in an electric furnace, showing electrical conductivities of 3 × 10^− 6 Scm^− 1 at room temperature and the activation energies of 0.48 eV (x = 5) and 0.51 eV (x = 10) for Li⁺ ion conduction in the temperature range of 30–200 °C. A continuous wave Nd:YAG laser (wavelength: 1064 nm) with powers of 0.14–0.30 W and a scanning speed of 10 μm/s is irradiated onto the surface of the glasses, and the formation of Li₃Fe₂(PO₄)₃ crystals is confirmed from XRD analyses and micro-Raman scattering spectra. The crystallization of the precursor glasses is considered as new route for the fabrication of Li₃Fe₂(PO₄)₃ crystals being candidates for use as electrolyte materials in lithium ion secondary batteries.     

Growth and characterisation of GaN with reduced dislocation density

Two GaN MOVPE growth methods to reduce the threading dislocation (TD) density have been explored. The combined effects of (1) in situ SiN_x masking of the sapphire substrate and (2) starting the epitaxial growth at low V-to-III ratio on the GaN film quality were studied by atomic force microscopy, transmission electron microscopy and high-resolution X-ray diffraction. It was found that the annealing condition of the low-temperature nucleation layer after in situ SiN_x masking is critical in order to decrease the density of nucleation sites and hence increase the average grain size to about 5 μm. However, the coalescence of large grains with vertical side facets results in the formation of dense bundles of TDs at the grain boundaries combined with large numbers of basal-plane dislocation loops throughout the film. The formation of these dislocations can be prevented by starting the epilayer growth at low V-to-III ratio, resulting in the formation of grains with inclined side facets. The interaction of the TDs with the inclined side facets causes the dislocations to bend 90 as the grains grow in size and coalesce. GaN films with dislocation densities as low as 1×10⁸ cm⁻², giving full-width at half-maximum values of 180 and 220 arcsec for respectively (002) and (302) omega scans, were achieved by the combination of in situ masking and low V–III ratio epilayer growth. Hall carrier mobility values in excess of 900 cm² V ⁻¹ s⁻¹ were deduced for Si-doped layers.     

Giant Hall effect in (GaAs)Fe granular films

A series of (GaAs)_1 − xFe_x (x: volume fraction) films with Fe granules embedded in GaAs matrix were prepared by magnetron sputtering. Hall Effect of the films was characterized. The largest saturated Hall resistivity of was observed in (GaAs)₃₀Fe₇₀ film at room temperature, which is over 2 orders larger than that of pure Fe, about 1 order larger than that of (NiFe)–(Al₂O₃) and (NiCo)–(SiO₂) granular films prepared under the same preparation conditions, and 150% larger than that of Ge₃₀Fe₇₀.     

Photoluminescence studies of confined states in AlGaAs/GaAs asymmetric quantum well

In the recombination spectra of AlGaAs/GaAs heterostructures, a peculiar and asymmetric photoluminescence (PL) band F has previously been reported [Aloulou et al., Mater. Sci. Eng. B 96 (2002) 14] to be due to recombinations of confined electrons from the two-dimensional electron gas (2DEG) formed at AlGaAs/GaAs interface in asymmetric quantum well (AQW). Detailed experiments are reported here on GaAs/Al_0.31Ga_0.69As/GaAs:δSi/Al_0.31Ga_0.69As/GaAs samples with different spacer layer thicknesses. We show that the band F is the superposition of two PL bands F′ and F″ associated, respectively, to AQW and a symmetric quantum well (SQW). In the low excitation regime, the F′ band present a blue shift (4.4 meV) followed by important red shift (16.5 meV) when increasing optical excitation intensity. The blue shift in energy is interpreted in terms of optical control of the 2DEG density in the AQW while the red shift is due to the narrowing of the band gaps caused by the local heating of the sample and band bending modification for relatively high-optical excitation intensity. Calculation performed using self-consistent resolution of the coupled Schrödinger–Poisson equations are included to support the interpretation of the experimental data.     

Photoluminescence of Al x Ga1−x As/GaAs quantum well heterostructures grown by molecular beam epitaxy

Nominally undoped Al_xGa_1–xAs grown by molecular beam epitaxy from As₄ species at elevated substrate temperatures of 670°C exhibits well-resolved excitonic fine structure in the low-temperature photoluminescence spectra, if the effective As-to-(Al+Ga) flux ratio on the growth surface is kept within a rather narrow range of clearly As-stabilized conditions. In contrast to previous results on Al_xGa_1–xAs of composition 0.15not to shift in energy by changing the excitation intensity. This implies a simple freeelectron carbon-acceptor recombination mechanism for the line without any participation of a donor. In Al_xGa_1–xAs of composition close to the direct-to-indirect cross-over point, two distinct LO-phonons separated by 34 and 48 meV from the (D ⁰,C ⁰) peak position at x=0.43 were observed which were before only detectable by Raman scattering experiments. The intensity of the carbon-impurity related luminescence lines in bulk-type Al_xGa_1–xAs and GaAs layers was found to be strongly reduced, as compared to the excitonic recombination lines, if the respective active layer was covered by a very thin confinement layer of either GaAs on top of Al_xGa_1–xAs or vice versa grown in the same growth cycle.     

Photoluminescence of AlxGa1−xAs/GaAs quantum well heterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence measurements on nominally undoped Al_xGa_1–xAs/GaAs quantum well heterostructures (QWHs) grown by molecular beam epitaxy (MBE) exemplified the exclusivelyintrinsic free-exciton nature of the luminescence under moderate excitation conditions. Neither any spectroscopic evidence for alloy clustering in the Al_xGa_1–xAs barriers nor any extrinsic luminescence due to recombination with residual acceptors has been detected in single and double QWHs when grown at 670 °C under optimized MBE growth conditions. Carrier confinement in Al_xGa_1–xAs/GaAs QWHs starts at a well width ofL _z30 nm when x0.25. The minor average well thickness fluctuation ofL _z=4×10^–2nm as determined from the excitonic halfwidth allowed the realization of well widths as low asL _z=1 nm and thus a shift of the free-exciton line as high as 2.01 eV which is close to the conduction band edge of the employed Al_0.43Ga_0.57As confinement layer. The measurements further revealed a strongly enhanced luminescence efficiency of the quantum wells as compared to bulk material which is caused by the modified exciton transition probabilities due to carrier localization.     

Thermal oxidation of n-type ZnN films made by -sputtering from a zinc nitride target, and their conversion into p-type films

Transparent p-type thin films, containing zinc oxide phases, have been fabricated from the oxidation of n-type zinc nitride films. The zinc nitride thin films were deposited by rf-magnetron sputtering from a zinc nitride target in pure N₂ and pure Ar plasma. Films deposited in Ar plasma were conductive (resistivity 4.7×10⁻² Ω cm and carrier concentrations around 10²⁰ cm⁻³) Zn-rich Zn_xN_y films of low transmittance, whereas Zn_xN_y films deposited in N₂ plasma showed high transmittance (>80%), but five orders of magnitude lower conductivity. Thermal oxidation up to 550 C converted all films into p-type materials, exhibiting high resistivity, 10²–10³ Ω cm, and carrier concentration around 10¹³ cm⁻³. However, upon oxidation, the Zn_xN_y films did not show the zinc oxide phase, whereas Zn-rich Zn_xN_y films were converted into films containing ZnO and ZnO₂ phases. All films exhibited transmittance >85% with a characteristic excitonic dip in the transmittance curve at 365 nm. Low temperature photoluminescence revealed the existence of exciton emissions at 3.36 and 3.305 eV for the p-type zinc oxide film.     

Classical in-plane negative magnetoresistance and quantum positive magnetoresistance in undoped InSb thin films on GaAs (1 0 0) substrates

Magnetoresistance (MR) effects have been investigated in perpendicular and parallel magnetic fields at 300, 80 K and liquid He temperatures for undoped InSb thin films 0.1–2.3 μm thick grown on GaAs(1 0 0) substrates by MBE. At high temperatures, the intrinsic carriers show the parabolic negative MR observable only in magnetic fields parallel to the film. The skipping-orbit effect due to surface boundary scattering in the classical orbits in the plane vertical to the film has been argued to be responsible for the negative MR. At low temperatures (T=80 K), the transport is dominated by the two-dimensional (2D) electrons in the accumulation layers at the InSb/GaAs(1 0 0) hetero interface; MR is positive and shows a logarithmic increase with anisotropy between parallel and perpendicular field orientation, arising from the 2D weak anti-localization (WAL) that reflects the interplay between the spin-Zeeman effect and strong spin–orbit interaction caused by the asymmetric potential at the interface (Rashba term). The zero-field spin splitting energy of Δ₀13 meV, the electron effective mass of m^*0.10m₀ seven times of the band edge mass in bulk InSb and the effective g-factor of |g^*|15 in the accumulation layer have been inferred from fits of MR for the 0.1 μm thick film to the 2D WL theory.     

Properties of Si-doped GaN and AlGaN/GaN heterostructures grown by RF-MBE on high resistivity Fe-doped GaN

Silicon-doped GaN epilayers and AlGaN/GaN heterostructures were developed by nitrogen plasma-assisted molecular beam epitaxy on high resistivity iron-doped GaN (0001) templates and their properties were investigated by atomic force microscopy, x-ray diffraction and Hall effect measurements. In the case of high electron mobility transistors heterostructures, the AlN mole fraction and the thickness of the AlGaN barrier employed were in the range of from 0.17 to 0.36 and from 7.5 to 30 nm, respectively. All structures were capped with a 2 nm GaN layer.Despite the absence of Ga droplets formation on the surface, growth of both GaN and AlGaN by RF-MBE on the GaN (0001) surfaces followed a step-flow growth mode resulting in low surface roughness and very abrupt heterointerfaces, as revealed by XRD. Reciprocal space maps around the reciprocal space point reveal that the AlGaN barriers are fully coherent with the GaN layer.GaN layers, n-doped with silicon in the range from 10¹⁵ to 10¹⁹ cm⁻³ exhibited state of the art electrical properties, consistent with a low unintentional background doping level and low compensation ratio. The carrier concentration versus silicon cell temperatures followed an Arhenius behaviour in the whole investigated doping range. The degenerate 2DEG, at the AlGaN/GaN heteroiterfaces, exhibited high Hall mobilities reaching 1860 cm²/V s at 300 K and 10 220 cm²/V s at 77 K for a sheet carrier density of 9.6E12 cm⁻².The two dimensional degenerate electron gas concentration in the GaN capped AlGaN/GaN structures was also calculated by self-consistent solving the Schrödinger–Poisson equations. Comparison with the experimental measured values reveals a Fermi level pinning of the GaN (0001) surface at about 0.8 eV below the GaN conduction band.     

Tunneling measurements on (La1−x Ce x )Al2 single crystals with wax barriers

In the investigation of the Kondo superconductor (La_1–x Ce _x )Al₂ a new technique is developed to fabricate tunnel junctions of the type: bulk sample — wax barrier — evaporated counterelectrode. Pb, Al, Ag, Au and Mg are used as counterelectrodes. Measurements employing Pb and Al yield the order parameter of nominally pure LaAl₂: ₀=(0.47±0.03) meV=1.63kT _c. The large influence of localized states within the gap is confirmed.     

Extremely narrow luminescence linewidth in GaAs single quantum wells by insertion of thin AlAs smoothing layers

We propose a new method to considerably reduce the overall growth interruption for high-quality GaAs single quantum wells during molecular beam epitaxy. The insertion of ultrathin AlAs smoothing layers at the constituent GaAs/Al _x Ga_1–x As heterointerfaces and growth interruptions of not more than 15 s yields an improvement of the luminescence linewidth (FWHM) to 0.56 meV for a 13 nm wide GaAs well and to a value as low as 0.195 meV for a 27 nm wide GaAs well. In addition, no Stokes shift between absorption and emission and no line splitting due to monolayer fluctuations in the well width is observed.     

Materials and device properties of pseudomorphic In x Ga1−x As/Al0.3Ga0.7As/GaAs high electron mobility transistors (0<x<0.5)

Modulation doped Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures for device application have been grown using molecular beam epitaxy. Initially the critical layer thickness for InAs mole fractions up to 0.5 was investigated. For InAs mole fractions up to 0.35 good agreement with theoretical considerations was observed. For higher InAs mole fractions disagreement occurred due to a strong decrease of the critical layer thickness. The carrier concentration for Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with a constant In _x Ga_1–x As quantum well width was investigated as a function of InAs mole fraction. If the In _x Ga_1–x As quantum well width is grown at the critical layer thickness the maximum carrier concentration is obtained for an InAs mole fraction of 0.37. A considerable higher carrier concentration in comparison to single-sided -doped structures was obtained for the structures with -doping on both sides of the In _x Ga_1–x As quantum well. Al_0.3Ga_0.7As/In _x Ga_1–x As/GaAs high electron mobility transistor structures with InAs mole fractions in the range 0–0.35 were fabricated for device application. For the presented field effect transistors best device performance was obtained for InAs mole fractions in the range 0.25–0.3. For the field effect transistors with an InAs mole fraction of 0.25 and a gate length of 0.15 m a f _T of 115 GHz was measured.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Correlation between morphological, electrical and optical properties of GaN at all stages of MOVPE Si/N treatment growth

GaN has been grown using Si/N treatment growth by MOVPE on sapphire (0001) in a home-made vertical reactor. The growth was monitored by in situ laser reflectometry. The morphological, electrical and optical properties of GaN are investigated at all the growth stages. To this aim, the growth was interrupted at different stages. The obtained samples are ex situ characterized by scanning electron microscopy (SEM), room temperature Van der Pauw–Hall electrical transport and low temperature (13 K) photoluminescence (PL) measurements. The SEM images show clearly the coalescence process. A smooth surface is obtained for a fully coalesced layer. During the coalescence process, the electron concentration (n) and mobility (μ) vary from 2×10¹⁹ cm⁻³ to 2×10¹⁷ cm⁻³ and 12 cm²/V s–440 cm²/V s, respectively. The PL maxima shift to higher energy and the FWHM decreases to about 4 meV. A correlation between PL spectra and Hall effect measurements is made. We show that the FWHM follows a n^2/3 power law for n above 10¹⁸ cm⁻³.     

Effects of S incorporation on Ag substitutional acceptors in ZnO:(Ag, S) thin films

Ag–S codoped ZnO thin films have been fabricated on Si substrates by radio frequency (RF) magnetron sputtering using a thermal oxidation method. XRD and SEM measurements showed that the sample has hexagonal wurtzite structure with a preferential (002) orientation and the surface is composed of compact and uniform grains. Ag_Zn–nS_O defect complexes were observed in the Ag–S codoped ZnO films by XPS analysis. Low temperature PL spectra showed neutral acceptor bound exciton emission related to Ag_Zn–nS_O. The corresponding acceptor ionization energy of 150 meV is much lower than that of monodoped Ag (246 meV), which is favorable for p-type doping of ZnO.     

Far-infrared dielectric functions and phonon spectra of alloys determined by spectroscopic ellipsometry

We have used spectroscopic ellipsometry to determine the complex dielectric function of a series of ternary Be_xZn_1−xTe thin films grown by molecular beam epitaxy. The II–VI semiconductor alloys were grown on InP substrates that had an InGaAs buffer layer. After the growth, X-ray diffraction experiments were performed in order to determine the alloy concentration. A standard inversion technique was used to obtain the dielectric functions from the measured ellipsometric spectra, obtained between 2000 nm (5000 cm⁻¹) and 40,000 nm (250 cm⁻¹). By modelling the dielectric function as a collection of oscillators, representing longitudinal and transverse optical phonons of the Be_xZn_1−xTe lattice, we were able to recover the phonon spectra for this alloy system. It is argued that the additional phonon modes that are obtained from ellipsometry are best understood from the recently-proposed percolation model.     

In-plane photovoltage measurements on MBE grown InN

Temperature dependent measurements of the in-plane photovoltage (IPV) arising from Fermi level fluctuations constitute a powerful tool for determining surface and bulk states and trap activation energies in nominally undoped semiconductors. In this work we report the first IPV measurements on high quality InN grown using the MBE technique. The temperature dependent IPV results coupled with Hall mobility and carrier density measurements provide ample evidence for the existence of a high density of states within the range 54 to 68 meV below the conduction band. Temperature dependent PL measurements indicate a band gap of 0.77 eV where a very weak temperature dependence is observed between 77 and 300 K. These results are shown to be in accord with the IPV measurements.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

Effect of structural defects on InSb/AlxIn1−x Sb quantum wells grown on GaAs (0 0 1) substrates

We investigate oriented abrupt steps (OASs), a type of surface defect in InSb/Al_xIn_1−xSb quantum-well (QW) samples grown on GaAs (0 0 1) substrates. Previous atomic force microscopy studies have reported that the OASs are oriented along the [1 1 0] and directions and have an inclination angle of 5°–15° with respect to the sample surface. Our plan-view and cross-sectional transmission electron microscopy analyses reveal that the OASs are the terminal edges of threading micro-twins at the sample surface. Hall effect measurements indicate that the density of OASs correlates with the electron mobility in the InSb QWs.