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)     

Optical properties of Ga1–xInxAs LPE-layers and p-n structures

The multiple layer structure nGaAs(n Ga_1–xIn_xAs) p Ga_1–xIn_xAs (0 ≦ × ≦ 0.18) was realized by liquid phase epitaxy from In–Ga–As-melts on (111)-oriented GaAs substrates. The InAs-content of the mixed crystal layers was found to be dominating for crystal perfection and growth rate. The cathodoluminescence spectra of p-and n-type Ga_1–xIn_xAs and spectral distribution of the electroluminescence from pn-junctions were measured at T = 77 K and 300 K. The external quantum efficiency wa found to have a maximum for diodes with x ≈ ≈ 0.006. This is caused by the decrease of the optical absorption with increasing x and increasing dislocation density on the other hand.     

Germanium Incorporation in Ga1–xInxAs LPE Layers and GaAs Bulk Crystals

Atomic absorption spectroscopy (AAS) is used to determine the Ge concentration in Ga_1–xIn_xAs and GaAs. The orientation dependence of Ge incorporation in (111)A-and (111)B oriented samples has been studied. The distribution coefficients for both the orientations were determined to be k_Ge^(111)A = 2.6 · 10⁻² and k_Ge^(111B = 1.4 · 10⁻² for Ga_1–xIn_xAs with 0 ≦ x ≦ 0.13. The differences between the two orientations are explained with the aid of the band bending model. Doping gradients in thick epitaxial layers and along crystal length of polycristalline TGS-grown GaAs ingots have been investigated too. In Ga_1–xIn_xAs layers any Ge concentration gradient couldn't be observed, but in TGS compact crystals Ge concentration increases with crystal length because the melt composition changes significantly during solidification. The results are compared with those of electrical measurements.     

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)     

LPE Growth of Ga1 – xInxSb Multi-grading Layers

Electronegativity difference approach (ENDA) has been successfully employed to obtain good prediction of the In atomic fraction, energy bandgap and lattice constant of the Ga_{1 – x}In_xSb/Ga_{1 – y}In_ySb/GaSb system. Nearly lattice-matched GalnSb epilayers with In atomic fraction of 0.42 have been obtained by the liquid phase epitaxy. The cut-off wavelength was 2.2 μm at room temperature.     

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.     

Miscibility gap calculation for Ga1−xInxNyAs1−y including strain effects

A method including strain effects is introduced for calculating the miscibility gap of the Ga_xIn_1−xN_yAs_1−y material system. The Gibbs free energy is computed using the delta lattice parameter model and the conventional solution model. The contribution caused by the strain energy due to the mismatch between substrate and epitaxial layer is added. The spinodal points can be calculated from this expression. The critical temperature above which the solid is metastable has been found to be increased due to strain effects. It is pointed out that the result of the calculation depends on the choice of substrate. The miscibility gaps for Ga_xIn_1−xAs, GaN_yAs_1−y and Ga_xIn_1−xN_yAs_1−y are evaluated by this method for a more realistic model of crystal growth.     

X-ray diffraction studies of interdiffusion in InAs—GaAs powder blends

Interdiffusion in the pseudobinary system In_xGa_1−xAs is investigated by means of X-ray diffractometry of annealed powder blends. The interpretation of the experimental results by means both of the concentric spheres and concentric cubes model yields for In_0.43Ga_0.57As an activation energy of (5.39 ± 0.11) eV. This value shows that the diffusion mechanism could be a vacancy one as in the pure compounds. According to the lower atomic radius the Ga atoms within the temperature interval of 550–625 °C diffuse more easily than the In atoms.     

Dislocation density in InxGa1–xAs films grown on GaAs substrates with intermediate buffer layer InGaAsP of variable composition

In_xGa_1–xAs films with x = 0.03 and 0.05 were grown from an In Ga As P liquid phase. Because of high value of distribution coefficient of P we have heterojunction GaAs In_yGa_1–yP_zAs_1–x–In_xGa_1–xAs. The influence of Phosphorus atom fraction (X_p) in liquid phase on dislocation density in the top In_xGa_1–xAs layer was studied. It was found that dislocation density (N_d) as a function of X_p is a curve with some minima. The minima of N_d for substrates of (111) A and (111) B orientations are observed in the different intervals of X_p axis. — The width of N_d minimum is decreased if the substrate is misoriented from the (111) plane. — It was supposed that the clusters exist in the liquid phase. On the basis of this assumption one can explain the influence of substrate position over or under the melt on the film perfection. The diameter of these clusters is estimated to be about 500 Å.     

X-ray diagnostics of P-HEMT AlGaAs/InGaAs/GaAs structures

The structural characteristics of the P-HEMT AlGaAs/InGaAs/GaAs heterostructure have been studied by high-resolution X-ray diffractometry. The parameters of the heterostructure layers were determined by simultaneous analysis of the X-ray reflection curves for the (004) and (113) crystallographic planes. Interface diffusion has been established for the In_yGa_1?yAs quantum well and the Al_xGa_1?x As spacer layer, which are characterized by reconstructed profiles of the lattice parameter distribution and anisotropic distribution of random displacements in the layer plane and in the perpendicular direction.     

Study of InxGa1–xP/GaAs Films Formation on the Basis of In–Ga–P Liquidus Precised Investigation

It is shown that the In–Ga–P melt which is prepared by contacting with the GaP seed becomes supersaturated in reality. Some peculiarities of In_xGa_1–xP/GaAs films formation at quasi-equilibrium conditions are described. It is observed that the saturated In–Ga–P melt dissolves the GaAs substrate when isothermally contacting if the In_xGa_1–xP equilibrium solid has the lattice parameter less than that of GaAs. As a result some InGaAsp deposit arises on the substrate. This phenomenon takes place if the In–Ga–P melt is just supercooled. This instability of the liquid-solid interface is explained on the basis of the relaxation theory of non-equilibrium liquid-solid contact which has been created by the author in previous papers.     

Investigation of GaAs—InyGa1−yPzAs1−z–InxGa1−x-As grading heterojunctions formation

The initial stages of growth of GaAs–InGaAsP_var–In_xGa_1−xAs heterostructures (x = 0.1 and 0.17) were investigated for the equilibrium-cooling method of LPE growth. Similar investigations were carried out for GaAs–InGaAsP_var–In_0.05Ga_0.95As heterocompositions, but for the step-cooling technique. The scheme of growing of In_0.17Ga_0.83As films of GaAs substrates with several intermediate buffler InGaAsP_var layers is represented. These heterostructures were shown to have less than 10⁶ cm⁻² dislocation density on the overall area of the film (> 2 cm²).     

Characterization of the optical properties of LPE InxGa1−xAsyP1−y thin layers grown on InP

A series of In_xGa_1?xAs_yP_1?y single-crystal thin layers have been grown on an InP substrate in a vertical liquid phase epitaxy furnace with a rotating slide boat system. The optical properties of these LPE quaternary alloys lattice-matched to InP have been investigated mainly by photoluminescence and electroreflectance measurements. Photoluminescence spectra of In_xGa_1?xAs_y P_1?y epitaxial layers are dominated by a strong luminescence line due to band-edge emission. At low temperatures, around 4.2 K, we have observed complicated luminescence bands with many fine structures. Electroreflectance spectra for the LPE In_xGa_1?xAs_yP_1?y layers are sufficiently broad to fulfil the low-field condition, and the analysis enabled us to determine precisely the band gap energy.     

Structure of the interfaces of the InxGa1 − x As quantum well from X-ray diffraction data

The structure of the interfaces of a 10-nm-thick In_xGa_{1 ? x} As quantum well buried in the semiconductor GaAs matrix has been studied by the method of double-crystal X-ray diffractometry. It has been shown that, in comparison with the well-known photoluminescence method, the X-ray diffraction method has considerable advantages in characterization of multilayer systems. The detailed analysis of the rocking curves provided the reconstruction of the profiles of indium distributions in quantum wells for specimens with different indium concentrations.     

The electrical and structural properties of InyGa1 ? yAs/InxAl1 ? xAs/InP quantum wells with different InAs content

In _x Al_{1 − x} As/In _y Ga_{1 − y} As/In _x Al_{1 − x} As/InP HEMT structures has been investigated with a change in the InAs molar fraction both in the quantum well and the buffer layer. The electrical parameters of the samples are measured at different temperatures. The structural parameters of the layers and the characteristics of the interfaces between them are determined by double-crystal X-ray diffraction. An increase in the Hall mobility and electron concentration, as well as in the structural quality of the samples, is observed alongside an increase in the InAs molar fraction in the quantum well. It is established that high electron mobility is retained at small (to 5%) mismatches between the buffer layer and substrate.     

Organometallic vapor phase epitaxy growth of upright metamorphic multijunction solar cells

We demonstrate an integrated metamorphic AlGaInP/AlGaInAs/GaInAs/Ge 4 J solar cell on Ge substrate using organometallic vapor phase epitaxy (OMVPE). A step graded GaInAs buffer was grown right after the Ge subcell was formed to change the lattice constant from that of Ge to that of Ga_0.8In_0.2As lattice constant followed by a 1.14 eV Ga_0.8In_0.2As subcell, a 1.5 eV (AlGa)_0.8In_0.2As subcell, and a 1.85 eV Al_xGa_0.32?xIn_0.68P subcell. Transmission electron microscope (TEM) study shows the threading dislocation density (TDD) is about 6×10⁶ cm^?2. The X-ray diffraction reciprocal space map (RSM) shows that the structure is 100% relaxed. Bandgap dependent (Al_xGa_1?x)_0.32In_0.68P subcell performance is systematically investigated. As the Al_xGa_0.32?xIn_0.68P cell bandgap goes up to 1.9 eV, the external quantum efficiency (EQE) goes down significantly. Theoretical simulation shows that the decrease of diffusion length causes the lower EQE, which indicates the material quality degrades with the increasing Al content. Integrated 4 J solar cells are fabricated and characterized with spectral response and tested under the AM1.5D terrestrial spectrum at both 1 sun and 2000 suns.     

Ga and In incorporation rates in Ga1−xInxAs growth by chemical beam epitaxy

GaAs, InAs and Ga_1?xIn_xAs layers were grown by chemical beam epitaxy (CBE) using triethylgallium, trimethylindium and tertiarybutylarsine as precursors for Ga, In and As, respectively. The growth rate during the homoepitaxial growth of GaAs and InAs, deduced from the frequency of reflection high-energy electron diffraction intensity oscillations, was used to calibrate the incorporation rates for the III elements. The In content of the Ga_1?xIn_xAs layers was measured by Rutherford backscattering spectrometry and compared with the value predicted from the above calibration data; while the measured In mole fraction is close to the predicted value for the samples grown for low In to Ga flux ratios (x<0.2), the In incorporation is enhanced for larger values of this ratio. The results obtained on layers grown at different substrate temperatures show that In mole fraction is almost constant at growth temperatures in the range 400–500 °C, but a strong dependence on the substrate temperature has been found outside this range. The above results, not observed for samples grown by solid source molecular beam epitaxy, indicate that some interaction between Ga and In precursors at the sample surface could take place during the growth by CBE.     

Composition latching phenomenon and lattice mismatch effects in LPE-grown In1−xGaxAs on InP substrate

We describe the possible composition range of In_1?xGa_xAs grown directly on InP substrates by LPE, the expansion of the range by use of In_0.53Ga_0.47As as a buffer layer, and the composition latching phenomenon in which only an x = 0.445 ± 0.005 layer is grown from the melt of any solidus composition between 0.40 and 0.45. This phenomenon does not occur on the larger composition side relative to the lattice matching compostition of x = 0.47, but only on the smaller side. The effects of the growth temperature, cooling rate, and step-cooling interval on this phenomenon, and the effects of the lattice mismatch on the crystal perfection and surface morphology are also described.     

Electrical and structural characteristics of metamorphic In0.38Al0.62As/In0.37Ga0.63As/In0.38Al0.62As HEMT nanoheterostructures

The influence of the metamorphic buffer design and epitaxial growth conditions on the electrical and structural characteristics of metamorphic In_0.38Al_0.62As/In_0.37Ga_0.63As/In_0.38Al_0.62As high electron mobility transistor (MHEMT) nanoheterostructures has been investigated. The samples were grown on GaAs(100) substrates by molecular beam epitaxy. The active regions of the nanoheterostructures are identical, while the metamorphic buffer In _x Al_{1 ? x} As is formed with a linear or stepwise (by Δ _x = 0.05) increase in the indium content over depth. It is found that MHEMT nanoheterostructures with a step metamorphic buffer have fewer defects and possess higher values of two-dimensional electron gas mobility at T = 77 K. The structures of the active region and metamorphic buffer have been thoroughly studied by transmission electron microscopy. It is shown that the relaxation of metamorphic buffer in the heterostructures under consideration is accompanied by the formation of structural defects of the following types: dislocations, microtwins, stacking faults, and wurtzite phase inclusions several nanometers in size.     

Studies on the lattice match and strain in the epitaxial growth of III – V alloys on inas and GaSb substrates

The compositional conditions to deposit the epitaxial films of III – V materials lattice matched to InAs and GaSb substrates have been investigated and the results are presented in the form of charts. The validity of our theoretical predictions has been tested by comparing with the available experimental results. Strain in the Ga_xIn_{1 − x}As_ySb_{1 − y} quaternary alloy has been visualised as three-dimensional perspective plots.