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article

1988

Experiment ; Demonstrationsunterricht ; Fotografie ; Unterrichtsmaterial ; Mikroskop ; Optik ; Physik ; Welle (Phys) ; Grafische Darstellung

Praxis der Naturwissenschaften. Physik, Bd. 37 (1988) H. 5, S. 37-41, 0177-8374

0342-8729

German

article

1997

Kulturtechnik ; Hypertext ; Grundschule ; Primarbereich ; Unterrichtsmaterial ; Deutschunterricht ; Schreiben

Computer + Unterricht, Bd. 7 (1997) H. 28, S. 21-23, 0941-519X

German

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1089-7550

AIP Digital Archive

Physics

The linear and nonlinear optical properties of a series of ternary alloy AlxGa1−xAs/AlAs multiple-quantum-well structures have been investigated and related to the multilayer configuration. The direct energy gap was found to scale with the AlAs mole fraction as predicted by Lee and Yuravel [Phys. Rev. B 21, 659 (1980)] and the band offset ratio to depend on the alloy composition. Exciton absorption bleaching was observed at room temperature and the nonlinear absorption cross sections were estimated for the first two confined excitonic states. Finally, the possibility of achieving optical gain for the type-II band alignment along the growth direction as well as in the layer plane is demonstrated.

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Physics

Non-lattice-matched GaxIn1−xAs/AlyIn1−yAs modulation-doped heterostructures grown on (100) InP by molecular-beam epitaxy suitable for application in field-effect transistors have been studied. A computer simulation of the x-ray diffraction pattern proves to be necessary to obtain precise information about the structural parameters of the samples. The high crystal quality of our samples is demonstrated by the excellent agreement between experimental and simulated x-ray-diffraction curves. The transport characteristics of Ga0.38In0.62As/AlyIn1−yAs heterostructures including the evolution of the mobility and of the two-dimensional electron-gas density with temperature and structural parameters are discussed in relation with the relevant scattering mechanisms. The use of a thin spacer layer makes it possible to obtain very high conductivities. Both x-ray and transport measurements show that the strained GaxIn1−xAs layers are pseudomorphic well above the critical thickness calculated with the mechanical equilibrium model. The highest mobilities (13 100 and 103 000 cm2 V−1 s−1 at 300 and 4 K, respectively), obtained with a sheet carrier density of 1.7×1012 cm−2, are measured on a Ga0.38In0.62As/Al0.51In0.49As heterostructure. They are among the best values reported so far for similar structures.

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Physics

GaAs:C and AlxGa1−xAs:C films, grown by solid-source molecular-beam epitaxy with doping levels beyond 1019 cm−3, have been studied by high-resolution double-crystal x-ray diffraction, Hall-effect measurements, and secondary-ion-mass spectroscopy (SIMS). Comparison between x-ray diffraction and Hall-effect data indicate that carbon is preferentially incorporated as acceptor on As lattice sites both in the GaAs:C and in the AlxGa1−xAs:C films. It was found that the higher the AlAs mole fraction the higher is the concentration of carbon incorporated on As sites (CAs). Moreover, SIMS results showed that the total amount of carbon in the host lattices largely exceeds CAs. Our findings are explained by supposing that carbon atoms are incorporated on As sites and on interstitial sites. Furthermore, it is shown that the carbon interstitial concentration can be reduced growing at higher arsenic flux and higher substrate temperature in GaAs:C as well as in AlxGa1−xAs:C layers.

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Physics

The structural and optical properties of molecular beam epitaxy-grown In0.52Ga0.18Al0.30As layers (E300 Kg(approximately-equal-to)1.18 eV), suitable for waveguide applications, have been studied by means of high-resolution x-ray diffraction, absorption, photoluminescence, photoreflectance, and high-excitation intensity photoluminescence spectroscopy. The combination of these techniques allowed us to study the free-exciton states, the impurity related transitions, and the formation of a dense electron-hole plasma.

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Physics

The structural and vibrational properties of superlattices composed of many periods of highly mismatched InAs and GaAs layers have been studied by means of x-ray diffraction and Raman scattering as a function of the sample geometry. X-ray diffraction measures the average lattice mismatch between the superlattice and the substrate. The long-range order influences the propagative acoustic phonons whereas strain and confinement effects compete in determining the optic vibration frequencies of the InAs layers. The linewidth of the main superlattice peak in the diffraction patterns and the scattering intensities of the acoustic phonons are related to the actual shape of the interfaces. We find that the stability of the structures depends on the total number of periods, in agreement with the predictions of equilibrium elasticity theory. However, the competition between the different relaxation processes is governed by the individual layer thicknesses.

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Physics

Liquid-phase epitaxy allows SiGe alloys of good quality to be grown on Si substrates. We deposit single crystalline, n-type Si1−xGex films with 0.7〈x〈1 from Bi solutions on (111)-oriented Si. The films are up to several μm thick and are uniform in thickness and in composition. The analysis by x-ray diffraction indicates good crystallinity and a dislocation density below 5×107 cm−2. Photoluminescence measurements show well-resolved peaks with the smallest linewidths reported so far for epitaxial SiGe. Hall-effect measurements yield electron concentrations around 1×1016 cm−3 and room-temperature electron mobilities of up to 340 cm2/V s.

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Physics

We report on accurate structural investigations of sputtered Nb/Pd multilayers by means of high-resolution secondary ion mass spectrometry and x-ray reflectivity. The combined use of secondary ion mass spectrometry and x-ray specular reflectivity techniques allows us to study the chemical configuration of the interfaces and to relate it to the observed superconducting properties. Secondary ion mass spectrometry analyses reveal a distinct Nb and Pd modulation and very sharp profiles with abrupt interfaces indicating a negligible interdiffusion of Nb and Pd at the interfaces. Moreover, analyzing the features in the Nb and Pd profiles and correlating them to the oxygen distribution in the multilayers and to the low-angle x-ray patterns, thin layers (3–4 nm thick) of niobium oxide were noticed at the Nb/Pd interfaces, while no oxide layers at the Pd/Nb interfaces could be detected. The role of this oxide layer in the determination of the crossover between three- and two-dimensional superconducting behavior in parallel external magnetic field, is discussed. © 2000 American Institute of Physics.

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Physics

We report on a structural study of Si/Ge multilayers grown by molecular-beam epitaxy on (100)-Si substrates. The analyses have been performed by using transmission electron microscopy, high-resolution x-ray diffraction, and secondary-ion-mass spectrometry. The investigated specimens differ in number of periods, period thickness, and in the Si/Ge periods thickness ratio. In particular, we investigate the interdiffusion of the Ge atoms in each superlattice period of the epilayer and in the epilayer as whole. The interdiffusion causes a broadening of the nominal thickness of the Ge layer producing a SixGe1−x alloy. Furthermore, the Ge content in the multilayer periods increases as a function of the growth time, i.e., the superlattice periods close to the sample surface contain more Ge atoms if compared to the periods close to the substrate/superlattice interface. We find two steps in the strain relaxation: (i) In each period the strain energy density is partially reduced by the formation of coherent islands; (ii) at a certain value of the strain energy density the shape of the islands changes and the structures relax partially or completely the accumulated strain energy by nucleation of extended defects. © 1996 American Institute of Physics.

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Physics

In this work we theoretically investigate the effect of the strain on the conduction and valence bands of quantum wells of two-dimensional (2D) strained semiconductor heterostructures with curved and parallel heterointerface profiles. InxGa1−xAs/GaAs heterostructures with hyperbolic heterointerface profiles are considered. The discussed structures are geometrically similar to V-shaped quantum wires, with the only difference that the well thickness is constant (absence of the crescent shape). The variation of the crystallographic orientation along the curved heterointerface profiles leads to relevant nonuniform strain fields. The nonuniform lattice deformations induce: (i) large band-gap modulations up to several tens of meV; (ii) piezoelectric effects; and (iii) lateral electron and hole potential modulations of several tens of meV. These potential modulations may allow 1D electron and hole confinement in thin InxGa1−xAs curve quantum wells (few nm thick), and should be taken into account in the calculation of the lateral confinement potentials obtained by geometrical constraints in strained V-groove-shaped quantum wires. © 1996 American Institute of Physics.

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Physics

In this work we report on transmission electron microscopy and high-resolution x-ray diffractometry studies of lattice matched AlxIn1−xAs/InP and GayIn1−yAs/InP epilayers grown by molecular beam epitaxy on InP(100) substrates. High-resolution and diffraction contrast electron microscopy measurements show the presence of different contrast zones in the epilayers. The analysis of high-resolution x-ray diffraction measurements and computer simulations ascribe these zones to the presence of a compositional gradient in the epilayers. A comparison among investigated samples grown under slightly different growth conditions combined with an analysis of the crystal defects is presented. Growth-induced small variations in the chemical composition of the epilayer can produce differences in the structural quality of the epitaxial layer. Finally, a few monolayers thick and highly strained film of InAsP, is observed in all investigated samples at the substrate/epilayer interface. The formation of this interface layer is explained by the exchange of As and P during exposure of the InP surface to As4 before the growth. © 1995 American Institute of Physics.

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Physics

In order to determine the strain field of highly mismatched semiconductor heterostructures by high-resolution x-ray diffraction with high accuracy, we derived a new second-order approximation of the incidence parameter considering an arbitrary lattice deformation. Our calculations show that, almost independently from the substrate orientation and the considered material system, for low Miller-index reflections a lattice mismatch greater than 0.004 is the value for which quadratic corrections must be considered. The quadratic approximation increases the range of validity by one order of magnitude, i.e., to a lattice mismatch up to 4%. In addition, the analytical expression which relates the strain components measured by x-ray diffraction to the lattice mismatch is derived for semiconductor epitaxial layers grown on arbitrarily oriented substrate crystals. Using Vegard's rule, our formula allows us to determine the chemical composition of ternary compounds even for low-symmetry substrate orientations. We show that in this case shear strain components have a non-negligible weight in the determination of the chemical composition of ternary compounds. Several examples considering III–V, II–VI, and IV–IV semiconductor material systems are reported and discussed. © 1996 American Institute of Physics.

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Physics

The structural characterization of ZnS epilayers grown by hydrogen transport vapor-phase epitaxy on (100)-oriented GaAs substrates is reported. X-ray-diffraction measurements in both double-axis (DA) and single-axis modes were performed to determine the residual strain tensor and the strain temperature dependence of the epilayer in the range between 25 and 650 °C. From the analysis of the data obtained by DA measurements of several symmetric Bragg reflections at different azimuth angles and several asymmetric reflections recorded in different geometries, an orthorhombic distortion of the ZnS lattice was found. The crystallographic symmetry could be explained by an asymmetric distribution of the misfit dislocation density in the interface plane along the [011] and [01¯1] directions. The temperature dependence measurements of the strain tensor components between room temperature and the growth temperature (650 °C) allowed determination of the thermal misfit between ZnS and GaAs and the linear thermal-expansion coefficient of ZnS. Finally, triple-crystal diffractometry and secondary-ion-mass spectrometry were used to investigate the chemical nature of the ZnS-GaAs interface. The results indicate the presence of an interdiffused ZnS-GaAs interface region, whose occurrence turns out to be associated with the initial defect structure of the substrate surface before the growth. © 1995 American Institute of Physics.

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Physics

Silver, copper, and mixed silver–copper nanocluster-doped silica thin layers were prepared by the sol-gel process. Samples were heat treated in different annealing atmospheres (air, argon, or 5%H2–95%N2) in the temperature range 500–1100 °C. Specimens were characterized by optical absorption spectroscopy, Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy. Cluster growth and dissolution, as well as migration of metal atoms towards the sample surface, with a subsequent evaporation, were observed to occur at temperatures that depend on the annealing atmosphere. In the mixed silver–copper system, the formation of Ag–Cu phase-separated clusters was observed. © 1996 American Institute of Physics.

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Physics

In this work we investigate the structural properties of SrCuO2/CaCuO2 infinite layer superlattices by high-resolution x-ray diffraction and x-ray specular reflectivity measurements. The infinite layer superlattices are grown by pulsed laser deposition on slightly misoriented (001) SrTiO3 substrates. We demonstrate that good quality superlattices with few monolayers thick constituent SrCuO2 and CaCuO2 layers can be grown having an interface roughness of less than 3–4 A(ring). A strain analysis of the epitaxial film shows that the SrCuO2 layers are completely relaxed with respect to the substrate. However, the CaCuO2 layers are elastically strained with respect to the SrCuO2 layer. The Poisson ratio of the CaCuO2 is estimated to be 0.40±0.08. © 1996 American Institute of Physics.

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Physics

Strained multiple quantum wells of InxGa1−xAs/GaAs were grown by low pressure metalorganic chemical vapor deposition (LP-MOCVD) and characterized by secondary ion mass spectrometry, x-ray diffraction, and optical spectroscopy. The structural analysis demonstrates the excellent control of the interface morphology and composition achieved by MOCVD growth. Temperature dependent optical absorption, photoluminescence, and magnetotransmission were used to evaluate the well-width dependence of the major excitonic properties. The samples show sharp excitonic resonances with distinct excited states evolving into Landau-type excited states in high magnetic field. The well-width dependence of the excitonic eigenstates and of the exciton binding energy as well reproduced by envelope function and variational calculations, also in the presence of external electric field. Finally, nonlinear electro-optic modulation induced by the quantum confined Stark effect is demonstrated in a Schottky diode with extremely low switching threshold. © 1996 American Institute of Physics.

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1077-3118

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Physics

Ag and Cu (pure and/or mixture) nanoclusters doped silica films were prepared by the sol-gel process. In the case of Ag and Cu codoped silica films, Cu/Ag molar ratio was 1, 2, and 3 at constant (Ag+Cu)/SiO2 molar ratio of 0.175. Separated Ag and Cu nanoclusters are formed in the silica matrix. The size of the clusters and their distribution are dependent on the film composition. Optical absorption was measured from 185 to 800 nm. Intensity-dependent nonlinear refractive index was measured for pure Cu and Ag–Cu (1:1 molar ratio) samples using z-scan technique in the wavelength range from 570 to 596 nm. The measured nonlinear refractive index is of the order of 10−13 m2/W at a pulse repetition rate of 15.2 MHz. © 1996 American Institute of Physics.

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1077-3118

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Physics

The short-period (GaAs)n1(AlAs)n2 superlattices with parameters (n1,n2)=(21,25), (6,42), (14,16), and (23,8) have been grown by molecular beam epitaxy on GaAs substrates along the [012] direction and characterized by x-ray and Raman scattering spectroscopy. The appearance of distinct satellite peaks around the Bragg reflections demonstrates the formation of high quality superlattices. The observed TO and LO confined modes have frequencies which map closely those of the optical phonons of bulk GaAs and AlAs in the Γ-W-X direction.

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1077-3118

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Physics

We report on spectroscopic experimental evidence of type II band alignment in a (GaAs)6/(AlAs)6 superlattice grown by molecular beam epitaxy along the (111) direction. This result is in contrast to recent theoretical calculations predicting that the (GaAs)n/(AlAs)n superlattices grown along the (111) direction should be direct for all n's.

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