Cross-sectional observation of NaClO stain-etched Al0.5Ga0.5As/GaAs multilayer by atomic force microscopy

The cross-section of multilayered Al_0.5Ga_0.5As/GaAs epitaxial structure was investigated by atomic force microscopy (AFM). For the first time, a 5% NaClO etchant was employed to discern each layer and a clear cross-sectional image of the multilayered epitaxial structure was obtained in less than 3 s etching time. The AFM image using 0.1 m HCl was poorer than that using 5% NaClO; this is attributed to the difference in etching selectivity between HCl and NaClO solution.     

Investigation of Al/(Al,Ga)As/GaAs metal/insulator/ semiconductor-type structures and application to buried interface field effect transistors

Undoped Al_0.5Ga_0.5As is used as an insulator layer in the fabrication of metal/ insulator/semiconductor- (MIS-) like capacitors with aluminum Schottky gates. Current-voltage and capacitance-voltage characteristics were measured as a function of temperature in the range 300-77 K. At high temperatures current occurs by thermionic emission over the barrier determined by the Schottky contact and the conduction band discontinuity. As the temperature is lowered, Fowler-Nordheim tunneling is observed for sufficiently high gate biases and at 77 K conduction is ohmic. Using an Al_0.5Ga_0.5As layer 100 nm thick MIS-like buried interface field effect transistors were fabricated. When these were operated in an accumulation mode and cooled to 77 K the transconductance increased by a factor of 2 and the current by a factor of 2.5. These preliminary results represent the first observation of enhanced performance at cryogenic temperatures in this type of device as a result of increasing electron saturation velocity. At 77 K and a saturation current density of 138 mA mm^-1 the transconductance was 40 mS mm^-1, compared with a theoretically estimated value of 130 mS mm^-1.     

Features of Fermi-level pinning at the interface of Al0.3Ga0.7As with anodic oxide and stabilized zirconia

In the metal-oxide-semiconductor structures based on the Al_0.3Ga_0.7As films prepared by metal-organic epitaxy, Fermi-level pinning at the Al_0.3Ga_0.7As/oxide interface at a distance of 1.1 eV from the top of the Al_0.3Ga_0.7As conduction band was observed. In these structures, a long-term memory phenomenon was found that was caused by the inflow of electrons from Al_0.3Ga_0.7As to the anodic oxide and their capture by deep traps. The reversible change in the states of the structures due to this memory effect under the action of an electric field or light was observed.     

Low thermal drift in highly sensitive doped channel Al0.3Ga0.7As/GaAs/In0.2Ga0.8As micro-Hall element

Micro-Hall sensors with high sensitivity, low noise, and high thermal stability, 5 μm square, are fabricated using pseudomorphic Al_0.3Ga_0.7As/GaAs/In _y Ga_1-y As (0.2 ≤ y ≤ 0.3) heterostructures with Si-doped channels. The structures were optimized for thermal stability using a calculation of the self-consistent solution of Schrödinger-Poisson equations and Fermi-Dirac statistics in Hartree approximation. The optimized structure based on a Si-δ-doped 144 Å In_0.2Ga_0.8As quantum well embedded into uniformly doped GaAs channel showed thermal drifts of only 90 ppm·K^?1 in current drive mode and 192 ppm K^?1 in voltage drive mode. The measurements of the absolute magnetic sensitivity and the low frequency noise were done. The micro-Hall sensor, optimized for thermal drift, is able to resolve the magnetic field of 438 nT.     

Geometric and electronic structure of Cs adsorbed Ga0.5Al0.5As (001) and (011) surfaces: a first principles research

Using plane-wave ultrasoft pseudopotential method based on first-principles density functional theory, the adsorption of Cs atom on Ga_0.5Al_0.5As (001) and (011) surfaces is investigated. The adsorption energy, geometric structure, E-Mulliken bond population, charge-transfer index, work function, dipole moments, band structure and density of state of Cs/(001) and Cs/(011) systems are calculated, the stability, ionicity and electronic structure of Cs/(001) and Cs/(011) systems are compared. Result shows that (001) is more benefital for photoelectric activation and photoemission than (011).     

Investigation of electrical and photoluminescent properties of MBE-grown Al x Ga1−x As layers

The electrical and photoluminescent (PL) properties of Al _x Ga{1–x}As layers grown by MBE have been investigated. Some experimental factors, e.g. vacuum conditions, substrate growth temperature and As/group-III flux ratio, have been considered. Undoped AlGaAs layers exhibit slight p-type characteristics due to C accepters and its concentration is lower than 10¹⁵cm^–3. Both n- and p-type AlGaAs layers achieve semi-insulated high resistance when the substrate temperature is lower than 580° C. The experimental results are comparable to other reports. In summary, the excellent vacuum environment, higher substrate temperature (T _s>580° C) and lower As/group-III flux ratio are the necessary conditions for growing high quality Al _x Ga_1–x As layers.     

Crystal Structure and Optical Absorption Spectra of Ga0.5Fe0.5InS3 and Ga0.5Fe0.25In1.25S3 Crystals

Ga_0.5Fe_0.5InS₃ and Ga_0.5Fe_0.25In_1.25S₃ crystals are grown by chemical vapor transport and the Bridgman method, respectively. As determined by x-ray diffraction analysis, they have trigonal (sp. gr. P3m1, a= 3.796 × 2 Å, c = 12.210 Å) and rhombohedral (sp. gr. R3m, a= 3.786 × 2 Å, c = 36.606 Å) structures, respectively. From edge absorption data, the band gap is determined to be E _g = 1.885 eV in Ga_0.5Fe_0.5InS₃ and 1.843 eV in Ga_0.5Fe_0.25In_1.25S₃.     

Photoluminescence of heterostructures with highly strained GaAs quantum wells in Al0.48In0.52As and Ga0.47In0.53As layers

Heterostructures comprising highly strained GaAs quantum wells in Al_0.48In_0.52As and Ga_0.47In_0.53As layers have been grown by the metalorganic chemical vapor deposition method on InP(100) substrates. The photoluminescence spectra of these structures have been studied, and it is established that GaAs quantum wells form type-I heterojunctions with Al_0.48In_0.52As layers and type-II heterojunctions with Ga_0.47In_0.53As layers.     

High field transport characteristics of minority electrons in p-In0.53Ga0.47As

Measurements of the static current-voltage characteristics of photoexcited minority electrons in p-In_0.53Ga_0.47As are reported. Photoluminescence measurements are also presented. These results are discussed in terms of the theory for high field photoconductivity and the recently reported velocity-field characteristics for minority electrons in In_0.53Ga_0.47As.     

Effects of gate sidewall recess on Al<Subscript>0.2</Subscript>Ga<Subscript>0.8</Subscript>As/ In<Subscript>0.15</Subscript>Ga<Subscript>0.85</Subscript>As PHEMTs by citric-based selective etchant

In this report, an effective and simple method of selective gate sidewall recess is proposed to expose the low barrier channel at mesa sidewalls during device isolation for Al_0.2Ga_0.8As/ In_0.15Ga_0.85As PHEMTs (pseudomorphic high electron mobility transistors) by using a newly developed citric-acid-based etchant with high selectivity (> 250) for GaAs/Al_0.2Ga_0.8As or In_0.15Ga_0.85As/Al_0.2G_0.8 As interfaces. After sidewall recess, a revealed cavity will exist between the In_0.15Ga_0.85As layers and gate metals. Devices with 1 × 100μm² exhibit a very low gate leakage current of 2.4μA/mm even at V_GD = −10 V and high gate breakdown voltage over 25 V. As compared to that of no sidewall recess, nearly two orders of reduction in magnitude of gate leakage current and 100% improvement in gate breakdown voltage can be achieved.     

Observation of e2/h Conductance Steps in a Side-Gate Point Contact on In0.75Ga0.25As/In0.75Al0.25As Heterostructure

In 1996, it was suggested by K. J. Thomas et al. (Phys. Rev. Lett. 77, 135 (1996)) that 0.7(2e²/h) conductance structure in Quasi-1DEG (Q1DEG) at GaAs/AlGaAs heterostructure might be related to spin polarization at zero magnetic field. We have recently studied spontaneous spin-splitting in the 2DEGs formed at normal metamorphic In_0.75Ga_0.25As/In_0.75Al_0.25As heterojunctions grown on GaAs substrates and obtained the value of ?10 meV as the zero-field splitting at Fermi level (Y. Sato et al., J. Appl. Phys. 89, 8017 (2001)). In this work, we attempted to observe spin-related phenomena in this heterojunction Q1DEGs at zero magnetic field. We observed e²/h conductance steps in low electron concentration side-gate point contact.     

Low-temperature properties of Ga0.5Mo2S4

Specific heat measurements from 2 to 27 K and crystal structure and magnetic properties for 4.2–300 K are reported for the spinel Ga_0.5Mo₂S₄. AtT _p =42 K this ternary chalcogenide undergoes a phase transition. At liquid helium temperatures it has a rhombohedral crystal structure with lattice parametera=9.275 Å and rhombohedral angle =90.45°. It follows from the magnetization measurements that ferromagnetic ordering takes place at about 20 K, and close to the Curie point an anomaly in the temperature dependence ofC _p has been found. The magnetic behavior of Ga_0.5Mo₂S₄ at low temperatures can be explained by a small concentration of magnetic ions and far-distant-neighbor superexchange interactions partly weakened by vacancies located in 4(c) positions. Magnetocaloric measurements indicate the dominance of ferromagnetic interactions.     

Influence of Codoping on the Magnetoresistance of Paramagnetic (Ga,Mn)As

We studied the correlation between magnetoresistance (MR) effects and the type of conducti- vity in paramagnetic Ga_1–x Mn _x As with x < 0.5%. Series of samples of (Ga,Mn)As with different codoping levels of Te have been prepared by metal-organic vapor-phase epitaxy (MOVPE). With increasing Mn-content from doping levels to x = 0.5% in paramagnetic Ga_1–x Mn _x As, the MR at 1.6 K changes continuously from positive to strongly negative. This manifests the complex interplay between p–d exchange induced valence band splitting and the metal–insulator transition. Codoping with Te leads eventually to a dominant conduction band transport. It is characterized by a small negative contribution at low magnetic fields to the parabolic MR similar to that found in highly n-doped diamagnetic semiconductors. It shows that the Mn-induced conduction band splitting is much smaller than the valence band splitting, i.e., |N₀| |N₀|.     

Investigation on in situ Al0.5FeSi0.5/Al composites prepared by transient liquid phase sintering

In situ Al_0.5FeSi_0.5/Al composites were prepared by transient liquid-phase sintering. The hardness and wear resistance of the composites were investigated with an XHV-1000 microhardness tester and an M-2000 wear tester. Results show that with increased sintering temperature and holding time, the in situ needle-like reinforcement is transformed into short, bar-like, massive particles. At a sintering temperature of 510 °C and holding time of 4 h, the reinforcement consists of short, bar-like Al_0.5FeSi_0.5; moreover, the hardness of the in situ Al_0.5FeSi_0.5/Al composites peaks to a value eight times that of pure aluminum and 2.5 times that of Al–Si alloy. Accordingly, the wear resistance of the composites is the highest, i.e., 6.6 that of pure Al and 4.5 times that of Al–Si alloy.     

In(Ga)As quantum dots on Ge substrate

The effects of growth temperature and deposition thickness on the formation, size, density, and uniformity of InAs and In_0.5Ga_0.5As islands grown by metalorganic vapor phase epitaxy on a germanium substrate are investigated. Atomic force microscopy images show InAs islands when 0.8–2.0 monolayers are deposited. At the nominal deposition thickness of 2.0 monolayers an island density of 2.5×10¹⁰ cm^–2 is achieved. InAs islands covered with a GaAs layer show low-temperature luminescence at around 1.15 eV. The In_0.5Ga_0.5As islands grown at 550 °C show a maximum density of 3.5×10¹⁰ cm^–2 at a nominal three monolayers deposition thickness.     

Electronic and optical properties of (Al x Ga1−x )1−y Mn y As single crystal: a new candidate for integrated optical isolators and spintronics

We have explored the electronic and optical properties of cubic (Al _x Ga_1?x )_1?y Mn _y As system using the FP-LAPW method. The unit cell has 64 atoms, so that one manganese (Mn) atom is placed in the position of gallium site, which corresponds to 3.125 % doping concentration with x = 12.5 %. Our calculations, using local density approximation + U (Hubbard parameter) scheme, predict that the ferromagnetic state for AlGaMnAs, with a magnetic moment of about 4.014 μ_B per Mn dopant is more favorable. Despite its electronic properties being strongly affected by inducing small amounts of Mn substitutional atoms in the cationic sublattice of AlGaAs, (Al _x Ga_1?x )_1?y Mn _y As possesses optical properties strictly less than those of Al _x Ga_1?x As, especially its optical conductivity at the peak 1.256 eV. The results indicate that AlGaMnAs may be a good candidate for optoelectronics when exploited in optical fiber networks, and it can still be of great interest because of its promising potential when used for spintronics.     

Anisotropic photoluminescence characteristics of Al0.08Ga929292As single quantum well laser structure

A nominal well width (20 nm) of Al_0.08Ga_0.92As quantum well structure has been fabricated by molecular beam epitaxy technique with the aim of obtaining a lasing device. The temperature evolution of quantum well photoluminescence was studied in the range 10–300 K which shows excitons being trapped at the interfacial defects below 100 K. The linear polarization effects in the photoluminescence have been studied for the incident and collected light propagating parallel to the plane of the well layer. In a very careful study, the luminescence was found to be fully polarized for the incident electric vector parallel to well layers, while it showed depolarized behaviour for the incident electric vector perpendicular to the well layers. The earlier conclusions based on photoluminescence excitation and absorption studies of heavy- and light-hole emissions are supported. The 20 nm quantum well structure has been corroborated using scanning tunnelling microscopy.     

Ga as an additive in the As2Te3 glass

Results of measurements of the mean atomic volume (V), the glass transition temperature (T _g), the activation energy for glass transition (E _t) and the d. c. electrical conductivity () are reported and discussed for ten glass compositions of the Ga–As–Te system. The glasses studied can be represented as Ga _x (As_0.4Te_0.6)_100–x glasses, with the additive Ga ranging from 0 to 12 atomic percent (at.%) in the parent As₂Te₃glass. In the Ga _x (As_0.4Te_0.6)_100–x glasses, changes in slope are observed in the V, T _g, E _t, and other electronic properties, at the composition with a Ga content of 2 at.%. The results are compared with those obtained on introduction of Ag and Cu to the As₂Te₃and the [0.5As₂Te₃–0.5As₂Se₃] glasses. Analysis of the data suggest formation of GaAs, Ga₂Te₃and excess Te structural units (s.u.) in lieu of some of the original As₂Te₃s.u., for addition of Ga up to 2 at.% to the parent As₂Te₃glass; for higher Ga contents, formation of GaAs, GaTe and excess Te s.u. are indicated.     

Alloying (In,Mn)As and (Ga,Mn)As: Ferromagnetic (In,Ga,Mn)As Lattice-Matched to InP

An effect of alloying two ferromagnetic semiconductors (In,Mn)As and (Ga,Mn)As on the ferromagnetic properties of resultant (In,Ga,Mn)As alloys is reported. For conditions close to lattice-matching to InP substrates, y = 0.53 in (In _y Ga_1–y )_1–x Mn _x As, ferromagnetism up to Curie temperatures T _C = 100–110 K could be achieved for a Mn composition x = 0.13. Trends in the Curie temperature in (In,Ga,Mn)As are compared with (Ga,Mn)As and (In,Mn)As as a function of Mn content. Hole concentrations determined from magnetotransport, taking into account the anomalous Hall contribution to Hall resistance, gives p/Mn = 0.03 ratio to Mn composition in metallic case for x = 0.13. We mention the possible role of chemical ordering (short range) of Mn impurity atoms on hole concentration and, consequently, for the ferromagnetic properties.     

Synthesis of a novel langanite-type compound La3Al5.5Nb0.5O14 by the sol-gel process

Langanite (La₃Ga_5.5Nb_0.5O₁₄, LGN) and its isomorphs are a few piezoelectric materials which have the unique temperature compensation and piezoelectric properties. But the high Ga₂O₃ content makes them very expensive and limits their applications. We reported a new langanite-type compound La₃Al_5.5Nb_0.5O₁₄ (LAN) which has the advantage of no Ga₂O₃ content. Chemically homogeneous La₃Al_5.5Nb_0.5O₁₄ sol was synthesized using La (NO₃)₃·6H₂O, Al (NO₃)₃·9H₂O and niobium citrate as starting materials. Single-phased LAN powder was prepared by decomposition of a citrate polymer precursor and subsequent reactions at high temperatures. TG-DTA, XRD and FTIR were employed to investigate the transformation process of gel to LAN powder. The results showed that, after calcination at 900 °C, pure La₃Al_5.5Nb_0.5O₁₄ polycrystalline powder with a narrow particle size distribution was obtained, which has the same structure with La₃Ga_5.5Nb_0.5O₁₄.