High-resolution Double-crystal X-ray Research Articles

Optical and structural properties of epitaxially lifted-off GaAs films

The influence of the layer thickness on the optical and structural properties of the epitaxially lifted-off (ELO) thin films has been studied. The ELO films bonded to Si, InP, and GaAs substrates have also been compared. The structure was characterized by high-resolution double-crystal x-ray diffraction and the optical properties were measured by the temperature-dependent photoluminescence spectroscopy. A biaxial compressive strain was observed for the samples bonded to Si with a buffer layer thinner than 1000 nm. Due to different thermal expansion coefficient between the grafted thin film and the host substrate, the emission spectra of the quantum wells of the lifted-off thin films are redshifted compared to the as-grown sample. The amount of the redshift is larger for thinner films.

Structures of an asymmetrically coupled double-well superlattice by double-crystal X-ray diffraction

An asymmetrically coupled (GaAs/AlAs/GaAs/AlAs)/GaAs (001) double-well supperlattice is studied by HRDCD (high resolution double-crystal X-ray diffractometry). The intensity of satellite peaks is modulated by wave packet of different sublayers. In the course of simulation, the satellite peaks in the vicinity of the node points of wave packet are very informative for precise determination of sublayer thickness and for improving accuracy.

Interfacial Abruptness in Si/SiGe Heteroepitaxy Grown by Ultrahigh Vacuum Chemical Vapor Deposition

High-quality Si/Si1-x Gex strained-layer superlattices have been grown in the temperature range 525° C–550° C by ultrahigh vacuum chemical vapor deposition. We employed X-ray reflection and high-resolution double-crystal X-ray diffraction measurements to study the dimensional and compositional variations in a Si/Si1-x Gex superlattice. From the X-ray reflection results, the interfacial roughness of Si layers on SiGe, and SiGe layers on Si, is 0.1 nm for growth at 525° C and 0.2 nm for growth at 550° C. A simple model for estimating the interfacial abruptness of Si/Si1-x Gex heterojunctions is proposed. In this model, a transition region with a linearly graded Ge composition is assumed to exist at both Si/Si1-x Gex interfaces. The Ge composition x of a Si/Si1-x Gex superlattice is found to increase with the growth time at a constant gas phase composition. This phenomenon can be explained by this model, and the thickness of the transition region and the transition time can be extracted from these fitting results. The transition thicknesses are found to be about 0.3–0.7 nm for growth at 525° C and 1–1.5 nm for growth at 550° C.

Modelling intermixing of short period strained layer superlattices by means of X-ray diffraction analysis

Intermixing of an (InAs) n /(GaAs) m short period strained layer superlattice (SPSSL) is analyzed. Lattice matched ( n ≈ mm) and indium rich ( n > m) superlattices have been prepared by molecular beam epitaxy in order to provide a ternary In xGa 1 − x As channel in InP-HFET with a reduced alloy scattering as theoretically predicted. For the optimization the interface intermixing of highly strained heterostructures was investigated using high-resolution double-crystal X-ray diffractometry combined with computer simulations. A method of modelling imperfections in semiconductor heterostructures was introduced to perform the fit between the simulated and measured data in an objective manner. The key steps of this method are (i) the sensitivity analysis of experimental diffraction curve details with respect to imperfections, and (ii) the development of objective error criteria. The result of the modelling process enables a quantitative and objective determination of the intermixing at InAs/GaAs interfaces in relation to the molecular beam epitaxy (MBE) growth parameters.

Modelling imperfections of epitaxial heterostructures by means of X-ray diffraction analysis

Interface intermixing of highly strained heterostructures was investigated using high-resolution double-crystal X-ray diffractometry combined with computer simulations. A method of modelling imperfections in semiconductor heterostructures was introduced to extract information from simulated and measured data in a fast and objective manner. The key steps of this method are i) the sensitivity analysis of experimental diffraction curves with respect to imperfections, and ii) the development of objective error criteria. The applicability of this method is demonstrated using highly strained [(InAs/GaAs)12/InGaAs]9 superlattices grown by molecular beam epitaxy (MBE) on InP substrates. The result of the modelling process enables a quantitative and objective determination of the intermixing at InAs/GaAs interfaces in relation to the MBE growth parameters which can be transferred to any other laboratory.

Properties of (Ga0.47In0.53)As epitaxial layers grown by metalorganic vapor phase epitaxy (MOVPE) using alternative arsenic precursors

In this study, the use of novel, liquid, organic arsenic precursors as substitutes for the highly toxic hydride gas arsine (AsH 3 ) in low pressure metalorganic vapor phase epitaxy (LP-MOVPE) of (GaIn)As lattice matched on InP has been investigated. The model precursors out of the classes of (alkyl)3- n AsH n ( n = 0,1,2) are tertiarybutyl arsine (TBAs), ditertiarybutyl arsine (DitBAsH) and diethyltertiarybutyl arsine (DEtBAs). The MOVPE growth has been investigated in the temperature range of 570–650°C using V III ratios from 2 to 20. The obtained epitaxial layer quality as examined by means of optical and scanning electron microscopy (SEM), high resolution double crystal X-ray diffraction, temperature-dependent van der Pauw-Hall, as well as photoluminescence (PL) measurements, will be compared for the different source molecules. Under optimized conditions almost uncompensated n-type (GaIn)As layers with carrier concentrations below 1 × 10 15 cm −3 and corresponding mobilities above 80 000 cm 2 /V · s have been realized. For TBAs and DitBAsH in combination with the corresponding P sources TBP and DitBuPH, respectively, we have worked out a process parameter area for the growth of layers with device quality, as proven by the realization of a pin-detector structure.

Photoluminescence from ordered and disordered Si-SiGe superlattices

Ordered (Ord-SL) and disordered (Dis-SL) Si-SiGe superlattices are grown using ultrahigh vacuum chemical vapour deposition (UHVCVD). The results of cross-sectional transmission electron microscopy (XTEM) and high-resolution double crystal x-ray diffraction (HRXRD) indicate that high quality Si-SiGe superlattices can be achieved. Well-defined band-edge excitonic luminescence is observed for the Si0.86Ge0.14-Si superlattice. Stronger phosoluminescence (PL) is observed for the Si-SiGe disordered superlattice compared with the corresponding Si-SiGe ordered superlattice. Furthermore, PL peak energy of the Dis-SL shifts to lower value with respect to the peak position of the corresponding Ord-SL. The stronger intensity of the no-phonon (NP) peak and the red shift of the PL peak are possibly a result of two probable mechanisms: (i) the tunnelling effect and (ii) the formation of localized states.

P 2-induced [formula omitted] exchange on GaAs during gas-source molecular beam epitaxy growth interruptions

P As exchange behavior on GaAs under a P 2 flux in a gas-source molecular beam epitaxy (GSMBE) system has been studied by reflection high-energy electron diffraction (RHEED), high-resolution double-crystal X-ray rocking curves (DCXRC) and cross-sectional scanning tunneling microscopy (STM). We found that the exposure of GaAs to a P 2 flux results in severe substitution of As atoms by P atoms at the normal temperature for growing GaAs ( T s = 570°C). When T s < 500°C, a streaky RHEED pattern can be maintained even during P 2 flux exposure of over 10 min. DCXRC of a 25-period P 2-exposed GaAs structure grown at 550°C show a set of satellite peaks, indicating formation of a periodic structure. Approximately an average of 0.45 monolayer of phosphorus is incorporated at each interface via As P exchange. High-resolution cross-sectional STM images reveal non-uniform (both laterally and vertically) interfacial layers of width as large as 10 monolayers. For the sample grown at 450°C, no clear satellite peaks are present. STM images of the sample exhibit much thinner interlayers, consistent with a lower level of phosphorus substitution.

Characterization of low temperature GaAs grown by molecular beam epitaxy

Low temperature GaAs has been grown by molecular beam epitaxy at substrate temperatures between 200°C–400°C, and subsequently annealed in the growth chamber at 600°C. These samples have been characterized by X-ray and Raman spectroscopy. From the rocking curve of a high resolution double crystal X-ray diffraction system we observed that GaAs layers grown at low temperatures were highly strained and contained about 1% excess arsenic. These results have been compared with an analysis of LO and TO frequency splitting in the Raman spectra.

Disordered [formula omitted] superlattices grown by ultrahigh vacuum chemical vapor deposition

A series of Si Si 1−x Ge x disordered superlattices with various degrees of disorder were grown in an ultrahigh vacuum chemical vapor deposition system (UHV/CVD). High-resolution double-crystal X-ray diffraction (HRXRD), and conventional cross-sectional transmission electron microscopy were used to evaluate the crystalline quality of these superlattices. A dynamical X-ray simulation program was employed to analyze the experimental rocking curves. Excellent matches between experimental rocking curves and simulated ones were obtained for all superlattices with various degrees of disorder.

Doping behavior and electrical activation of carbon in GaAs

Heavily C-doped GaAs films, grown by metalorganic chemical vapor deposition with CCl4 as external carbon source, have been studied by Hall-effect measurements, high-resolution double-crystal x-ray diffraction, and secondary-ion-mass spectroscopy (SIMS). Comparison among x-ray diffraction and Hall-effect measurements and SIMS results indicates that carbon is preferentially incorporated as acceptor on As lattice sites and electrical activation rate achieves 100%. There is no evidence of carbon incorporated on Ga sites as donors and incorporated on interstitial lattice sites.

Residual and thermal strain of ZnS epitaxial layers grown on (100)-GaAs by vapour phase epitaxy

In this work, ZnS epitaxial layers grown by vapour phase epitaxy on (100)-oriented GaAs substrates are investigated by X-ray diffraction. The residual strain status of the as-grown samples was determined by high-resolution double-crystal X-ray diffraction measurements. Eleven diffraction curves were recorded in the vicinity of the (400), (422) and (531) Bragg reflections in different diffraction geometries and for several azimuth angles. The analysis of the experimental data was performed by using a general model, which relates the angular distances between diffraction peaks and strain tensor components in the second-order approximation. This model considers the lowest crystallographic symmetry (triclinic) for the lattice distortion of a cubic unit cell. Our results indicate that the crystallographic symmetry of the distorted ZnS unit cell is orthorhombic. In order to determine the strain contribution due to the different thermal expansion coefficients of ZnS and GaAs (thermal strain) the temperature variation of the residual strain was measured between 25 degrees C and the growth temperature (650 degrees C) by using a single-crystal X-ray diffractometer. From our temperature-dependent measurements we determined the thermal misfit between ZnS and GaAs and the linear thermal expansion coefficient of ZnS.

Selective epitaxial growth of GaInP by LP-MOCVD using ethyldimethylindium, trimethylindium, trimethylgallium and triethylgallium as group III sources

The present experiments demonstrate the epitaxial growth (EG) and selective epitaxial growth (SEG) of GaInP by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) using ethyldimethylindium (EDMI), trimethylindium (TMI), trimethylgallium (TMG) and triethylgallium (TEG) as group III sources. In epitaxial growth of GaInP, the Ga incorporation efficiency using TEG + EDMI is found to be lower than those using other combinations. Completely selective epitaxy using TEGa + EDMIn can be achieved at a growth temperature of 675°C and at a growth pressure of 40 Torr, while other combinations (TEG + TMI, TMG + TMI, and TMG + EDMI) can achieve SEG of GaInP at 650°C. High resolution double-crystal X-ray measurements are used to investigate the compositional variation in the selectively epitaxial-grown area.

Quantum confinement effects of Si/SiGe strained-layer superlattices grown by an ultrahigh vacuum/chemical vapor deposition technique

Well-resolved band-edge luminescence is observed for Si0.86Ge0.14/Si strained-layer superlattices grown by an ultrahigh vacuum/chemical vapour deposition technique at 550 ‡C. High-resolution double-crystal X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM) were used to determine the strain and other parameters for these strained-layer superlattices. Quantum confinement is observed for a SiGe well as thin as 1.3 nm. The blue shift of the emission peaks with decreasing well width is found to be in good agreement with theoretical calculation.

Observation of a triclinic lattice distortion of InxGa1− xAs (100)-oriented epitaxial layers by high-resolution double-crystal X-ray diffraction

In this work the lattice distortion of In x Ga 11− x As epitaxial layers with different InAs mole fractions (0.08 < x < 0.28), grown on (1 0 0)-oriented GaAs substrates by molecular beam epitaxy, is studied in detail by high-resolution X-ray diffractometry. The strain field and the chemical composition are determined by recording rocking curves in different diffraction geometries in the vicinity of the (4 0 0), (4 2 2) and (5 3 1) GaAs reflections. In order to analyze the experimental data we used a 2nd-order approximation of the strain function in the Takagi-Taupin equation. Our results reveal that a triclinic lattice distortion occurs in all of the investigated heterostructures despite the misorientation of the (1 0 0) substrate surface was less than 0.2°.

Characterization of Si/SiGe Strained-Layer Superlattices Grown by an Ultrahigh Vacuum/Chemical Vapor Deposition Technique

High-resolution double-crystal X-ray diffraction and cross-sectional transmission electron microscopy were used to characterize the Si/SiGe strained-layer superlattices grown by the ultrahigh vacuum/chemical vapor deposition (UHV/CVD) system. A dynamical X-ray simulation was employed to analyze the experimental rocking curves. Good matches between the experimental rocking curves and the simulated ones demonstrate that the high quality Si/SiGe strained-layer superlattices with abrupt interface and excellent thickness and composition uniformity have been achieved. Thickness uniformity was further confirmed by the cross-sectional transmission electron microscopy. In addition, high-resolution double-crystal X-ray diffraction was proven to be a powerful technique for determining the doping level and the thickness of heavily doped contact layer in Si/SiGe device structures prior to further processing. Good wafer-to-wafer uniformity can be simultaneously achieved by the UHV/CVD technique; as a result, the UHV/CVD technique is readily applicable to manufacturing.

Gas Source Molecular Beam Epitaxy of ZnSe on (In,Ga)P

ABSTRACTThe wide bandgap semiconductor ZnSe has been nucleated on epitaxial (In,Ga)P buffer layers (on GaAs substrates) having various In compositions, and hence various lattice constants. The III-V ternary alloy offers a wide range of lattice constants for the heteroepitaxy of a multitude of potential II-VI light emitting devices, such as blue pn injection lasers composed of the (Zn,Mg)(S,Se) material system. Since the II-VI and III-V layers are grown using gas source molecular beam epitaxy in separate dedicated reactors, the technique of amorphous As deposition is employed to passivate the (In,Ga)P surface prior to the ex situ transfer. High resolution double crystal x-ray diffraction measurements on the ZnSe/(In,Ga)P/GaAs heterostructures indicate that for In compositions of 50-52%, the buffer layers with a thickness of 4 μm were only partially relaxed on the GaAs substrates, with the residual mismatch remaining at the ZnSe/III-V heterointerface. The critical thickness of (In,Ga)P, with In concentrations near 52-56%, on GaAs greatly exceeds the predicted critical thickness from either the energy balancing or force balancing model. For an In composition of 56% (and a film thickness of 4 μm), the buffer layers contain an in-plane lattice constant equal to that of ZnSe, and therefore represent the lattice-matched condition, even though the film is not fully relaxed. For (In,Ga)P buffer layers lattice-matched to ZnSe, but mismatched to GaAs, the surface exhibits the expected cross-hatched surface morphology. The occurrence of the cross-hatched surface is significantly alleviated by the addition of a pseudomorphic layer of GaAs positioned between the ZnSe and (In,Ga)P layer.

Heterostructural Characterization of Pseudomorphic, Partially Strained, and Highly Mismatched Semiconductors Using Double Crystal X-Ray Diffraction, TEM, and SEM

ABSTRACTHeterostructural properties of pseudomorphic (AlGaAs/GaAs), partially strained (GaInAs/GaAs), and highly strained (GaAs/Si) semiconductor systems have been studied using High Resolution Double-Crystal X-ray Diffraction (DXRD), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM). Using the high resolution DXRD with CuKα1 and two-reflection Si (220) monochromator, we obtained (004) symmetric and (115) or (224) asymmetric reflection rocking curves for samples grown by molecular beam epitaxy. With 0.5 μm thick samples, perpendicular and in-plane lattice mismatches were calculated using elastic theory and compared with each other. The different degree of relaxation for these samples was observed and correlated with the lattice mismatch, X-ray layer peak broadening (i.e., full width at half maximum), and SEM surface morphology. For a GaInAs/GaAs sample, the strain relaxation along one of the &lt;110&gt; directions was more than the other direction, that is, the strain relaxation is not isotrophic. Also we observed that the lines were mainly parallel in one direction, i.e., they did not form a cross-hatch pattern. TEM images from both cross-sectional and planar views of the samples will be presented.

Carbon incorporation in GaAs and AlxGa1−xAs layers grown by molecular-beam epitaxy

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.

Growth and structure of CdTe/Cd 1− xMn xTe multiple quantum wells showing excitonic 2S states

Multiple quantum well structures of CdTe and the dilute magnetic semiconductor Cd 1−x Mn x Te(x⪅0.08) have been grown by molecular beam epitaxy on InSb substrates. High resolution double crystal X-ray diffraction analysis using rocking curve simulations based on the dynamical theory of diffraction shows that these layers are of excellent structural quality and uniformity. The diffraction data indicate the presence of a highly mismatched layer a few monolayers thick at the interface between the InSb substrate and the CdTe buffer layer, which is consistent with previous Raman spectroscopy studies identifying the phase as In 2Te 3. Photoluminescence and photoluminescence excitation spectroscopy measurements indicate that well width fluctuations occur on a monolayer scale. Observation of the 2S state of both light and heavy hole excitons allows the binding energy of both excitons to be deduced—17.1 meV and 19.7 meV respectively—considerably enhanced from their three-dimensional values.