Bandgap characters in GaAs‐based ternary alloys

The existence and origins of the bowing character in the bandgap variation of GaAs‐based ternary alloys are theoretically investigated based on two different computational methods. Within the framework of the virtual crystal approximation (VCA), both the empirical sp³s * tight‐binding (TB) method with, and without, the inclusion of the spin‐orbit coupling effects, and the first‐principle full‐potential linear augmented plane wave (FP‐LAPW) technique are applied on both the common‐cation GaSb_xAs_1‐x and the common‐anion Ga_1‐xIn_xAs alloys. These methods are used to calculate the bandgap energy, the partial and total densities of states and the constituent charge ionicity versus the composition x. The results show that the bowing behavior exists in the case of common‐cation alloys (GaSb_xAs_1‐x) as a manifestation of a competition between the anion atoms (As and Sb) in trapping the made‐available‐cationic charges. The bowing parameter is found to be proportional to the electronegativity characters of the competing anions (χanion). Consistent with this in the case of common‐anion alloys (Ga_1‐xIn_xAs), as due to the lack of anion competition, the bowing is just absent and the variation of bandgap energy is found to be rather linear. The excellent agreement between our theoretical results and recent photoluminescence data has corroborated our claim. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermoelectrical properties of In1‐xGaxAs and InAs crystals irradiated with fast electrons

The thermoelectric power in In_1‐xGa_xAs (x = 0,01;0,04) solid solutions and InAs crystals irradiated with fast electrons by the energy of 6 MeV and dose of 10¹⁶‐ 2 x 10¹⁷ el/cm^‐2 on the interval 80‐400 K have been investigated. It is revealed that in the all crystals the value of the thermoelectric power is decreased under irradiation that resulted from the growth of the free electron concentration to form radiation induced defects of the donor type. It has been determined that in the initial InAs after irradiation, the charge carriers scatter on optical phonons and in In_1‐xGa_xAs solid solutions they do on optical phonons and ionized impurities. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Triple axis diffractometric investigations of the microstructure of thin AlxGa1‐xN epitaxial films

Al_xGa_1‐xN/GaN (Si doped or undoped) with the Al content in the composition range of 0.046 ≤ x ≤ 0.164 grown on the c sapphire face by atmospheric pressure MOCVD method were studied by high resolution x‐ray diffraction including symmetrical and asymmetrical ω/2θ scans and reciprocal space maps. A high sensitivity to small inhomogeneities of the layer thickness and lattice mismatch between AlGaN and GaN is reported, recognised as changes in the interference pattern of the diffraction peak observed across the sample. They are very well correlated with optical properties derived from independent photoreflectivity measurements. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality AlxGa1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al_xGa_1−x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al_xGa_1−xN was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al_0.1Ga_0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al_0.65Ga_0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al_xGa_1−x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.     

AlGaAs/InGaP interfaces in structures prepared by MOVPE

Al_0.3Ga_0.7As/In_1−xGa_xP structures were prepared by low-pressure MOVPE. Lattice matched and strained ones with top In_1−xGa_xP layers as well as reverse ones with top Al_0,3Ga_0,7As layers were examined. The structures were studied by photoluminescence, X-ray and atomic force microscope (AFM) methods. An additional photoluminescence peak from the Al_0.3Ga_0.7As/In_1−xGa_xP interface was observed in our samples and it was attributed to a type-II band offset. A conduction band offset of 0.121 eV was measured in the Al_0.3Ga_0.7As/In_0.485Ga_0.515P lattice-matched structure and a linear dependence of the conduction band offset on In_1−xGa_xP composition, with a zero offset in the Al_0.3Ga_0.7As/In_0.315Ga_0.685P structure, was determined. The valence band discontinuity had a nearly constant value of 0.152 eV.     

Phase Equilibrium of the Ga–Al–Sb system

Employing the method of liquid-phase epitaxy (LPE) solid-solutions of Ga_1–xAl_xAl_xSb (0 ≦ x ≦ 0.8) have been obtained. The dependence of Sb solubility on Al concentration in the liquid phase at 403°C, 452°C, 500°C has been established. The dependence of AlSb concentration in the solid phase on the composition of the liquid phase has been investigated at 452°C. Using the chemical constants equilibrium method, the phase equilibrium of the Ga–Al–Sb system in the region of liquid phase composition near the Ga-rich corner of the phase diagram has been calculated. The comparison of experimental and calculated data for the liquid and solid phases shows their agreement within the limits of experimental error.     

Growth and Characterisation of (CuInTe2)1‐x(2 ZnTe)x Solid Solution Single Crystals

The crystal structure as well as the optical properties in the band gap region of (CuInTe₂)_1‐x(2 ZnTe)_x solid solution single crystals grown by directional freezing have been studied. The lattice constants exhibit a linear dependence on crystal composition. The chalcopyrite‐sphalerite phase transition was observed between x = 0.3 and x = 0.4°. The variation of the band gap with respect to crystal composition can be described by a quadratic expression.     

Die mittlere quadratische Auslenkung der Atome und der Nachweis eines statischen Verzerrungsanteils in Al1–xGaxAs mit Röntgenbeugungsmethoden

The mean square atomic displacement in Al_1–xGa_xAs solid solution (x = 0.16) has been determined by integrated intensity measurements of X-ray Bragg diffraction from a polycrystalline sample at room temperature. The X-ray Debye-temperature of the solid solution was determined. The mean square displacement of the Al-atoms is smaller than those resulting for the Ga/As-atoms.     

On a Possibility of Determining Nonuniformities in AlxGa1−xAs Solid Solution Compositions

Both surface and bulk nonuniformities in Al_xGa_1−xAs solid solution compositions were determined by surface radiography of epitaxial Al_xGa_1−xAs GaAs hetero-junctions with diffused radioactive Zn⁶⁵ atoms.     

Digitally alloyed modulated precursor flow epitaxial growth of AlxGa1−xN layers with AlN and AlyGa1−yN monolayers

We propose a new growth scheme of digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG) using metalorganic and hydride precursors for the growth of Al_xGa_1−xN layers with high-Al content at relatively low temperatures. The growth of high-quality, high-Al content Al_xGa_1−xN layers (x_Al>50%) that are composed of AlN and Al_yGa_1−yN monolayers on AlN/sapphire template/substrates by DA-MPEG was demonstrated. The overall composition of the ternary Al_xGa_1−xN material by DA-MPEG can be controlled continuously by adjusting the Column III mole fraction of the atomic Al_yGa_1−yN sub-layer. X-ray diffraction and optical transmittance results show that the AlGaN materials have good crystalline quality. The surface morphology of DA-MPEG AlGaN samples measured by atomic force microscopy are comparable to high-temperature-grown AlGaN and are free from surface features such as nano-pits.     

Temperature dependence of optical absorption in In1‐xGaxAs solid solutions

Absorption spectra near the fundamental absoption edge of n‐type of In_1‐xGa_xAs are studied. The temperature coefficient of the In_1‐xGa_xAs energy gap, dE_g/dT, has been obtained and compared with calculated data. An exponential dependence of the absorption coefficient on photon energy has been found. The slope of the exponential absorption curve is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Raman scattering characterization of Mn composition and strain in Ga1–x Mnx Sb/GaSb epitaxial layers

Raman scattering (RS) experiments have been performed for simultaneous determination of Mn composition and strain in Ga_1–x Mn_x Sb thin films grown on GaSb substrate by liquid phase epitaxy technique. The Raman spectra obtained from various Ga_1–x Mn_x Sb samples show only GaSb‐like phonon modes whose frequency positions are found to have Mn compositional dependence. With the combination of epilayer strain model, RS and energy dispersive x‐ray (EDX) experiments, the compositional dependence of GaSb‐like LO phonon frequency is proposed both in strained and unstrained conditions. The proposed relationships are used to evaluate Mn composition and strain from the Ga_1–x Mn_x Sb samples. The results obtained from the RS data are found to be in good agreement with those determined independently by the EDX analysis. Furthermore, the frequency positions of MnSb‐like phonon modes are suggested by reduced‐mass model. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strahlende und nichtstrahlende Rekombination am neutralen Silizium-Akzeptor in n-Ga0,2-Al0,8As

In cathodoluminescence of Si-doped n Ga_1-xAl_xAs (x ∼0,8) the most efficient near band transition is the recombination of a free hole with a bound electron at T = 80 K. This free-to-bound luminescence is observed competing with a 300 meV lower lying luminesccence band due to donor-acceptor-pair recombination. At high excitation density the pair radiation intensity is quenched in favour of the free-to-bound emission intensity. Both peaks shift to the short wavelength range; the pair band approximately by 45 meV and the free-to-bound transition peak by 3 meV. These effects are explained on the basis of a nonradiative threecenter-Auger-process involving the trapped holes on the acceptor states.     

Crystal growth and investigation of CuAlxGa1−xTe2 solid solutions

Alloys of the CuGaTe₂‐CuAlTe₂ system have been synthesized by the one‐temperature method and homogenized by annealing. It was established for the first time by X‐ray and DTA methods that a continuous row of solid solutions is formed in the system. Large block crystals of CuAl_xGa_1−xTe₂ solid solutions have been grown by the Bridgman method (horizontal version). Measurements of density, microhardness and thermal expansion have been carried out on the grown crystals. It has been established, that the density and thermal expansion coefficients are changed linearly with composition, microhardness has a maximum at x = 0.6. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

TEM study of defects in AlxGa1−xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al_xGa_1?xN layers with various Al mole fractions (x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al_xGa_1?xN layer (x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed.The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al_xGa_1?xN layer with both polarities.The lattice relaxation of metal-polar Al_0.2Ga_0.8N was observed, while N-polar Al_0.2Ga_0.8N did not relax. The dislocation densities in the N-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers were 5.5×10⁹ cm^?2 and 9×10⁹ cm^?2, respectively, and in metal-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers these were 1×10¹⁰ cm^?2 and 6×10⁹ cm^?2, respectively.Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.     

Preparation and investigation of (CuInSe2)x(2ZnSe)1‐x and (CuInTe2)x(2ZnTe)1‐x solid solution crystals

The (CuInSe₂)_x(2ZnSe)_1‐x and (CuInTe₂)_x(2ZnTe)_1‐x solid solution crystals prepared by Bridgman method and chemical vapor transport have been studied. The nature of the crystalline phases, the local structure homogeneity and composition of these materials have been investigated by X‐ray diffraction (XRD) and Electron Probe Microanalysis (EPMA) methods. The analysis revealed the presence of chalcopyrite‐sphalerite phase transition between 0.6 ≤ X ≤ 0.7. Lattice constants, value of σ position parameter and bond length between atoms were also calculated. It was found that the lattice parameters exhibit a linear dependence versus composition. The transmission spectra of solid solution crystals in the region of the main absorption edge were studied. It was established that the optical band gap of these materials changes non‐linearly with the X composition. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and magnetic studies of Mn‐doped ZnO

The bulk samples of Mn‐doped ZnO were synthesized with the nominal compositions Zn_1‐xMn_xO (x = 0.02, 0.05, 0.10, 0.15) by the solid‐state reaction and sol‐gel methods. In both the methods the samples were finally sintered at ∼700 °C in air. The X‐ray diffraction (XRD) studies of the samples synthesized by the solid‐state reaction method exhibit the presence of wurtzite (hexagonal) crystal structure similar to the parent compound (ZnO) in all the samples, suggesting that doped Mn ions sit at the regular Zn sites. However, same studies spread over the samples with Mn content ≥5% and synthesized by the sol‐gel method reveal the occurrence of some secondary phase in addition to the majority wurtzite phase. The magnetic measurements by vibrating sample magnetometer (VSM) clearly indicate ferromagnetic interaction at room temperature in all the samples. The Curie temperatures (T_c) and magnetization vary with concentration of Mn ions in the samples. However, the samples synthesized by sol‐gel method were found to have lower Tc values and also lower magnetization as compared to the corresponding samples synthesized by solid‐state reaction method. It could possibly be due to the presence of antiferromagnetic islands and smaller crystallite sizes in the samples prepared by sol‐gel method. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Calculations of homogeneous region in Ga1−xAlxAs and GaAs1−xPx ternary solid solutions

Ternary solid solutions of A^IIIB^V compounds are considered as pseudobinary A(x)^IIIB(x)^v compounds, where the behaviour of A(x)^III and B(x)^v pseudoatoms is quite similar to A^III and B^v atoms in a binary A^IIIB^v crystal. Weak dependence of point defect contribution into Gibb's energy of A^IIIB^v crystal on its defect nature, random character of ternary solid solutions of A^IIIB^v compounds allow to use already for binary compounds developed formalism in the determination of component thermodynamic potentials of solid solution. Basing on literature data for the equilibrium solidus of AlAs the approximation for the temperature dependence of thermodynamic potential of an AB quasimolecule in A^IIIB^v crystal is revised. This result together with the well-known parameters for the equilibrium liquidus in Ga–P, Ga–As, and Al–As systems were used for calculations of the nonstoichiometric factor at the boundary of a homogeneous region in Ga_1−xAl_xAs and GaAs_1−xP_x ternary solid solutions. The results are compared with the known literature data.     

X‐ray studies on quaternary system RbXK1‐XBrYCl1‐Y

Single crystals of the quaternary crystalline solid solution system Rb_xK_1‐xBr_yCl_1‐y have been grown by melt technique for three different compositions y = 0.25, 0.50 and 0.75, keeping x = 0.5 and X‐ray diffraction studies have been carried out for each case independently, using spherically ground single crystals. The results indicate an enhancement of Debye‐Waller B(total) factors for the compositions y = 0.25 and 0.75 whereas for the intermediate composition, y = 0.5, the factor B(total) is close to the end member value. This experimental investigation show that for this composition, the crystalline solid solution behaves like an end member crystal.     

Molecular beam epitaxy as a growth technique for achieving free-standing zinc-blende GaN and wurtzite AlxGa1-xN

Currently there is a high level of interest in the development of ultraviolet (UV) light sources for solid-state lighting, optical sensors, surface decontamination and water purification. III-V semiconductor UV LEDs are now successfully manufactured using the AlGaN material system; however, their efficiency is still low. The majority of UV LEDs require Al_xGa_1-xN layers with compositions in the mid-range between AlN and GaN. Because there is a significant difference in the lattice parameters of GaN and AlN, Al_xGa_1-xN substrates would be preferable to those of either GaN or AlN for many ultraviolet device applications. However, the growth of Al_xGa_1-xN bulk crystals by any standard bulk growth techniques has not been developed so far.There are very strong electric polarization fields inside the wurtzite (hexagonal) group III-nitride structures. The charge separation within quantum wells leads to a significant reduction in the efficiency of optoelectronic device structures. Therefore, the growth of non-polar and semi-polar group III-nitride structures has been the subject of considerable interest recently. A direct way to eliminate polarization effects is to use non-polar (001) zinc-blende (cubic) III-nitride layers. However, attempts to grow zinc-blende GaN bulk crystals by any standard bulk growth techniques were not successful.Molecular beam epitaxy (MBE) is normally regarded as an epitaxial technique for the growth of very thin layers with monolayer control of their thickness. In this study we have used plasma-assisted molecular beam epitaxy (PA-MBE) and have produced for the first time free-standing layers of zinc-blende GaN up to 100 μm in thickness and up to 3-inch in diameter. We have shown that our newly developed PA-MBE process for the growth of zinc-blende GaN layers can also be used to achieve free-standing wurtzite Al_xGa_1-xN wafers. Zinc-blende and wurtzite Al_xGa_1-xN polytypes can be grown on different orientations of GaAs substrates - (001) and (111)B respectively. We have subsequently removed the GaAs using a chemical etch in order to produce free-standing GaN and Al_xGa_1-xN wafers. At a thickness of ～30 µm, free-standing GaN and Al_xGa_1-xN wafers can easily be handled without cracking. Therefore, free-standing GaN and Al_xGa_1-xN wafers with thicknesses in the 30–100 μm range may be used as substrates for further growth of GaN and Al_xGa_1-xN-based structures and devices.We have compared different RF nitrogen plasma sources for the growth of thick nitride Al_xGa_1-xN films including a standard HD25 source from Oxford Applied Research and a novel high efficiency source from Riber. We have investigated a wide range of the growth rates from 0.2 to 3 µm/h. The use of highly efficient nitrogen RF plasma sources makes PA-MBE a potentially viable commercial process, since free-standing films can be achieved in a single day.Our results have demonstrated that MBE may be competitive with the other group III-nitrides bulk growth techniques in several important areas including production of free-standing zinc-blende (cubic) (Al)GaN and of free-standing wurtzite (hexagonal) AlGaN.