Double-crystal X-ray Rocking Curve Research Articles

X-ray and DLTS characterizations of In x Ga1?x As(x<0.03)/GaAs layers grown by VPE using an In/Ga alloy source

InxGa1−xAs(x}<0.03)/GaAs lasers grown by vapor phase epitaxy using an In/Ga alloy source were characterized by double crystal X-ray (DCX) diffraction and deep level transient spectroscopy (DLTS) measurements. Based on the results obtained from (400), (511), and (¯511) DCX rocking curves, the obvious effect of In incorporation is to give an increase in the full width at half maximum of the rocking curves that correlates with a coherency of the epitaxial layers. From DLTS spectra according to the In content, the most prominent electron deep traps areE 4 (Ec-0.58eV) andE5 (Ec-0.84eV). TheE 4 trap density increases with In content while the change ofE 5 trap density is not monotonic. The trend ofE 5 trap densities versus In content is very similar to that of etch pit densities (EPDs), that is, a minimum in EPD andE 5 trap density is observed at an In content ofx∼0.003 but beyond this value the densities increase again with In content.

Stress distribution and lattice curvature determinations in multilayer structures by simulation of X-ray rocking curves

A mechanical model is presented for the calculation of the stress depth distribution and the curvature radius in multilayer crystalline structures. The model is compatible with the perpendicular strain and thickness determined for each lamella by simulation of a double-crystal X-ray rocking curve, and is suitable for coherence at all the interfaces as well as for coherence loss at one interface. An example of an application to a hypothetical practical case shows that the comparison between experimental and calculated curvatures and asymmetric rocking curves allows one to determine if loss of coherence occurred at an interface as a consequence of sample processing, and to determine the amount of incoherence and the depth position of the incoherent interface.

Improvement in the crystalline quality of heteroepitaxial GaAs on Si films grown by modulated molecular beam epitaxy

GaAs films were grown on Si substrates by a two-step method in which a thin buffer layer was first deposited using a modulated molecular beam epitaxy (MBE) technique followed by a thick layer grown by conventional molecular beam epitaxy. The film quality was evaluated using 77 K photoluminescence (PL) and double-crystal x-ray rocking curves. It was found that GaAs films grown in this way have a superior crystalline quality compared to the films prepared by normal two-step MBE. To investigate the nucleation of this buffer layer, thin GaAs layers (150 Å) were grown on Si substrates and examined by plan-view and cross-sectional transmission electron microscope. A thin, two-dimensional nucleation pattern was found in this sample, in clear contrast to the three-dimensional nucleation islands found in samples grown by conventional MBE. This showed conclusively that modulated beam molecular beam epitaxy resulted in a more uniform and two-dimensional nucleation during the initial stage of the growth as speculated earlier and is believed to be the reason for the improvement of the crystalline quality of the overgrown layer.

Structural characterization of Zn1−xCoxSe epilayers on GaAs(001)

We have grown a Co-based diluted magnetic semiconductor, the wide-gap compound Zn1−xCoxSe, by molecular-beam epitaxy on GaAs(001) substrates. We report here a detailed study of the structure and crystalline quality of these epilayers as obtained from a variety of x-ray diffraction and transmission electron microscopy (TEM) techniques. The films grow as (001) single-crystal zinc-blende epilayers which exhibit some degree of tetragonal distortion for the full range of Co concentrations (x≲0.1) and thicknesses (0.1–2.6 μm) studied. X-ray double-crystal rocking curve and topography measurements show that the crystalline quality of the thinner films (0.3 μm) approaches that of the GaAs substrates. Cross-sectional and plane-view TEM results indicate a low dislocation density and the absence of twin formation within the epilayers. High-resolution lattice images show that the interface between the epilayer and substrate is coherent and atomically abrupt.

Study of CdTe(111)B epilayers grown by molecular-beam epitaxy

The growth of high-quality Hg1−xCdxTe(111)B epitaxial layers involves the growth of a CdTe(111)B buffer layer, whether the growth is performed onto related or foreign substrates. CdTe(111)B layers have been investigated using the x-ray double-crystal rocking curve and the cathodoluminescence imaging techniques. The optimization of the growth conditions for CdTe(111)B layers is presented for a high-quality homoepitaxy. The heteroepitaxy of CdTe(111)B on GaAs(100) substrates is then discussed, in the case of GaAs substrates oriented (100), misoriented by 2° towards [110], and misoriented by 5° towards [111]. The results indicate that a 2° misorientation leads to CdTe(333) peaks with full width at half-maximum up to four times narrower then either of the other orientations tested. Furthermore, only threading dislocations were visible on this orientation by cathodoluminescence imaging.

A study of the structure and electrical properties of CdxHg1−xTe grown by metalorganic vapor phase epitaxy (interdiffused multilayer process)

Epitaxial layers of CdxHg1−xTe (CMT) have been grown by the interdiffused multilayer process (IMP) at 350 °C. The electrical properties are fixed by the equilibrium conditions of the interdiffused structure and subsequent cooling of the layer in a Hg ambient. Indiffusion of Hg during cooldown can be prevented by the growth of a 1000-Å-thick cap layer of CdTe. X-ray double-crystal rocking curves for the 400 reflections have been measured to find whether IMP introduces any residual structure or strain into the CMT. For CMT grown onto CdTe/GaAs, a postgrowth anneal of 30 min or longer reduces rocking curve widths to below 100 arc s, with a best value of 55 arc s. The best value on CdTe substrates was 72 arc s. Depth profiling of x-ray peak widths for CMT/CdTe/GaAs structures has shown that the CMT/CdTe interface improves the crystalline quality of the epilayer. One contribution to peak width broadening has been identified as pyramidlike morphological defects. Epitaxial layers have been grown onto GaAs substrates with regions &amp;gt;1 cm2 free of pyramids.

Metalorganic chemical vapor deposition growth of Cd1−yZnyTe epitaxial layers on GaAs and GaAs/Si substrates

As a step toward growth of HgCdTe on GaAs and GaAs/Si substrates, Cd1–yZnyTe buffer layers were grown on these substrates. A conventional pyrolytic process at relatively high temperature (441 °C) was used. Epitaxial growth on GaAs was achieved for all alloy compositions, 0≤y≤1. The solid–vapor distribution curve for alloy deposition was determined experimentally for one particular set of deposition parameters, and compared with a thermodynamic model. Deposit uniformity was also studied experimentally and compared with a model incorporating coupled fluid dynamics and mass transport (but not chemical kinetics) with realistic reactor boundary conditions and thermal effects. Epitaxial growth on the GaAs(123) substrates orientation was studied by x-ray techniques. Cd1–yZnyTe epitaxial layers grown on these substrates were tilted with respect to the substrate toward 〈111〉, about a 〈12̄1〉 axis. The degree of tilt depended on the alloy composition y, ranging from 1.7° for y=1 to 3.2° for y=0. These results are consistent with a model of the GaAs(123) surface as a terraced structure consisting of (111) oriented ledges and steps parallel to [12̄1]. The crystal lattice perfection of the epitaxial layers was assessed by x-ray double-crystal rocking curve analysis. The best full width at half-maximum (FWHM) obtained for CdTe layers was 80 arc s, and for ZnTe layers it was 150 arc s. Cd0.96Zn0.04Te layers also grown on GaAs/Si(001) tilted 3.5° toward 〈111〉. Some layers grown on these substrates also had rocking curve FWHM≤100 arc s. Hg0.7Cd0.3Te was grown by liquid phase epitaxy on a CdZnTe/GaAs/Si substrate, and the resulting layer had a rocking curve FWHM of 165 arc s.

Long and middle wavelength infrared photodiodes fabricated with Hg1−x CdxTe grown by molecular-beam epitaxy

Long and middle wavelength infrared (LWIR, MWIR) p+-n photodiodes have been fabricated with Hg1−xCdxTe (0.20&amp;lt;x&amp;lt;0.30) grown by molecular-beam epitaxy (MBE). The epilayers were grown on (211)B lattice-matched ZnCdTe substrates. The surface morphology was smooth and free of in-plane twins. The Cd concentration (x) was uniform across the wafer, with standard deviations (Δx) as low as 0.0017. Structural properties were measured by double-crystal x-ray rocking curve and dislocation etching; FWHM values as low as 34 arcsec and etch pit density values as low as 1×105 cm−2 were measured. p+ -n homojunctions were formed by arsenic diffusion; unpassivated mesa photodiodes were fabricated by standard photolithographic techniques. MWIR and LWIR photodiodes fabricated with MBE material exhibited good diode performance, comparable to that obtained on photodiodes fabricated with the more matured technique of liquid-phase epitaxy. 77-K R0A products of the diodes measured were 6.35×107, 22.3, and 1.76 Ω cm2 with cutoff wavelengths of 4.66, 9.96, and 12.90 μm, respectively. The R0A product for a VLWIR photodiode was 1.36×102 Ω cm2 with a cutoff wavelength of 16.23 μm at 35 K. LWIR diodes with no antireflection coating had a quantum efficiency of 48.6%. The present results represent a significant step toward the demonstration of MBE as a viable growth technique for the fabrication of large infrared focal plane arrays.

Interfacial studies and electrical characterisation of heteroepitaxial InSb on GaAs (100) grown by MBE

The growth by molecular beam epitaxy of InSb layers (0.5 < t < 10 μm) on undoped GaAs (100) substrates has been undertaken. In situ reflection high energy electron diffraction and Auger electron spectroscopy have been used to investigate the initial stages of InSb layer nucleation. The GaAs/InSb interface has also been investigated ex vacuo using transmission electron microscopy and secondary ion mass spectrometry. The data reveal a maximum interface width of 25 nm and indicate that the large mismatch (14.6% at room temperature) is accommodated by a network of misfit dislocations at the epilayer substrate interface. Nevertheless, transmission electron microscopy studies and data obtained from double crystal X-ray rocking curves show that the structural quality of the heteroepitaxial InSb improves considerably with increasing layer thickness. The electrical and morphological properties of the InSb heteroepitaxial layers have also been assessed using 77 K Hall, 2 K photoluminescence and Nomarski optical interference microscopy. All layers grown have exhibited p-type carrier concentrations (p ∽ 2 x 10 15 cm -3), a value somewhat higher than observed for the corresponding homoepitaxial material.

Growth and Characterization of GaAs on Si by Mbe

AbstractHigh quality of Si-doped n-type MBE GaAs grown directly on 4° off (001) towards [0111 Si substrates has been achieved by a multiple step MBE growth procedure. Epilayers have been characterized for the doping range 2x1016 to 3x1018cm-3 with thickness up to lOm by double crystal x-ray rocking curves, Hall mobilities, photoluminescence spectra, dislocation etching pit densities and I-V characteristics of MESFET of GaAs/Si. The results show that the electron mobilities are 2900 to 1430cm2 /Vs while free electron concentration of 3.5x1017 to 2.7x1018cm-3. The FWHM of the DCRC are 175 to 350 arc sec, the EPD are 1x106 to 2x107 cm-2 for most of samples and both of them appear variation with thickness of epilayer. The main peak of PL appears a few meV shift to long wavelength in comparison with GaAs on GaAs. The FWHM of the PL are 21 meV for n-type GaAs/Si in a doping level of 3.5x1017cm-3. MESFET show a acceptable I-V characteristics. All of epilayers with thickness less than 5pm appear good morphology. Some slightly orthogonal cracking was observed in the 5 to 10µm thick films of GaAs/Si.

Ultraviolet photon-induced heteroepitaxy of CdTe on GaAs

Ultraviolet photon-induced metalorganic vapor phase epitaxy of CdTe films on GaAs substrates has been investigated using diethyltelluride and dimethylcadmium as the precursor gases. The relationship between the deposition parameters and the properties of the epilayers have been examined using transmission electron microscopy and x-ray rocking curves. Epilayers grown at 6μm/h show an x-ray double-crystal rocking curve full width at half-maximum (FWHM) of 250 arcsec.

Structural and electrical contact properties of LPE grown GaAs doped with indium

We have studied the effects of In doping on the structural and electrical properties of a liquid phase epitaxially (LPE) grown GaAs. The results of surface morphology studies show that macroscopically, a terrace-free area in certain regions can be seen on the surface of a GaAs layer doped with In of 2.4 × 1019 cm3. The full widths at half-maximum (FWHM) of x-ray double crystal rocking curves show that a GaAs epi-layer of good crystalline quality can be obtained by doping In to a concentration up to 4.3 × 1019 cm-3, beyond which a sharp increase in the FWHM is observed. Etch pit density (EPD) study also shows that the dislocation density is reduced by doping the epi-layer with In. At an optimum In concentration, 2.4 × 1019 cm-3, the EPD was reduced by a factor of 20 when measured at the surface of a 9μm thick epi-layer. The I-V characteristics of Au-GaAs Schottky diodes show, for the layer with an optimal In concentration, an ideality factor close to 1.04 over more than seven decades of current. For the same layer, the reverse I-V characteristics are close to an ideal Schottky diode and could be fitted by a theoretical curve, combining the thermionic field emission and thermionic emission. For doping levels higher than 6 × 1019 cm-3, the epitaxial layer quality deteriorated. We report the results obtained from the Nomarski optical microscope, double crystal x-ray rocking curves, etch pit density, forward and reverse I-V characteristics, and the theoretical current transport models.

Interference structures in double-crystal X-ray rocking curves from very thin multiple epitaxial layers

Interference fringes have been observed in X-ray double-crystal rocking curves from a two layer epitaxial structure consisting of 920 AA GaxIn1-xAs1-yPy grown on InP and capped by 0.38 mu m of InP. It is shown that the fringes can be used to measure the InP cap thickness to a precision of about 100 AA. The effects on the fringe spacing of variations in layer thickness and lattice mismatch are investigated by simulation. The fringe spacing is independent of the lattice parameter difference between cap and substrate. Simulations reveal that the fringes are modulated with a periodicity equal to the Bragg case Pendellosung period for a layer of thickness equal to that of the confined quaternary layer. This modulation is shown to be independent of cap thickness and layer mismatch. The fringe spacing and the modulation period thus provide an easy and non-destructive method for the determination of layer thicknesses to a precision of 100 AA for two-layer hetero-epitaxial structures.

Characterisation of InGaAs/GaAs superlattice structures by X-ray double crystal diffraction

Experimental X-ray double crystal diffraction rocking curves for different InGaAs/GaAs superlattices grown by molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD) are used to obtain theoretically calculated rocking curves, based on Takagi's theory of dynamic X-ray diffraction of distorted crystal. A detailed assessment and accurate information about these superlattice structures are possible by a comparison of the experimental and theoretical curves. A comparison is also made between theoretical rocking curves obtained by dynamic and kinematical approaches.

A study of layer composition of InGaAs/InP multiquantum wells grown by metalorganic chemical vapor deposition using double-crystal x-ray diffraction theory and experiment

InGaAs/InP multiquantum wells grown by metalorganic chemical vapor deposition have been studied by fitting computer simulated theoretical double-crystal x-ray rocking curves to experimental data. It is shown that the technique can identify the carry over of As into the InP barriers and the concentration and distribution of As are determined. The highest concentration of As observed was 21%, which was confined to the first 50 Å of the barrier and fell to 2% towards the following interface. Both the maximum and minimum concentration of As was found to decrease as a function of the pause time between growth of the InGaAs quantum well and the InP barrier layers. These observations were confirmed by secondary ion mass spectrometry (SIMS) and other experimental data on the same material is shown to support the results.

Se passivation and regrowth of ZnSe(001) epilayers on GaAs(001)

We report the use of a Se overlayer to passivate and protect the surface of ZnSe epilayers during prolonged exposure to atmosphere. The samples were grown by molecular-beam epitaxy (MBE) on GaAs (001) substrates and passivated by the deposition of a Se coating near room temperature before removal from the MBE system. The Se coating is readily desorbed in ultrahigh vacuum at substrate temperatures of 115–150 °C. The resultant single-crystal surfaces are clean, stoichiometric, and well ordered, as demonstrated by Auger electron spectroscopy and reflection high-energy electron diffraction. Regrowth of ZnSe on these surfaces yields epilayers which exhibit excellent crystalline order comparable to that of continuously grown reference samples, as shown by x-ray double-crystal rocking curves. This Se passivation technique offers a simple means of transferring such II-VI epilayers through atmosphere from the growth system to other facilities while preserving surface single-crystal order and cleanliness.

Double-crystal x-ray topography and rocking curve studies of epitaxially grown ZnSe

X-ray double-crystal rocking curve and topography measurements have been used to obtain a detailed evaluation of the crystalline quality of ZnSe(001) films grown by molecular-beam epitaxy on GaAs(001). Rocking curves were measured for three incident x-ray wavelengths to identify and determine the contribution of specific physical defects to the linewidth through their wavelength dependence. X-ray topography was used to obtain a very direct, nondestructive depth-resolved record of the physical structure of both epilayer and substrate through the use of various asymmetric reflections having different penetration depths for dynamical scattering. We find that the GaAs substrate is strained by the misfit to the ZnSe epilayer. The epilayers exhibit a fine microstructure, consisting of slightly misoriented mosaic blocks, which is very uniform both laterally and throughout the thickness of the film. The rocking curve linewidth for the epilayer is dominated by the lattice tilt between these blocks. The resultant dislocation density as estimated from the linewidth is on the order of 1.4×108 cm−2.

Epitaxial growth and x-ray structural characterization of Zn1−xFexSe films on GaAs(001)

We have grown single-crystal (001) epilayers of the diluted magnetic semiconductor Zn1−xFexSe on GaAs(001) substrates by molecular beam epitaxy. The films were characterized with Auger electron spectroscopy (AES) and reflection high-energy electron diffraction, followed by x-ray θ–2θ, rocking curve, and topography measurements. AES data show that the surface stoichiometry does not change significantly under electron beam exposure, indicating that (Zn,Fe)Se is a more stably bonded compound than (Zn,Mn)Se, where large variations are observed. The variation of the lattice parameters both parallel and perpendicular to the surface is obtained as a function of Fe concentration. The Zn1−xFexSe epilayers are cubic in structure for x≤0.22 and thicknesses up to at least 2 μm, although films with x&amp;gt;0.05 exhibit a slight tetragonal distortion (1≤c/a&amp;lt;1.002). The onset of this distortion is reflected in a broadening of the x-ray double-crystal rocking curve. Dislocation densities obtained from the rocking curve linewidth range from 1 to 7.5×108 cm−2. X-ray topographs reveal a degree of mosaic structure which increases with Fe content.

Annealed Czochralski grown silicon crystals: A new material for the monochromatisation of synchrotron radiation and X-rays above 60 keV

The diffraction properties of silicon single crystals of 10 cm diameter and 1 cm thickness have been studied in Laue geometry using the 316.5 keV γ-line of an iridium source ( λ = 0.0392 A ̊ ). In addition, sections of the samples have been characterized by means of high resolution X-ray topography as well as by measuring double crystal X-ray rocking curves with approximately 0.25″ resolution in Bragg geometry. The material annealed at temperatures around 1200°C allows efficient monochromatisation of high energy X-rays and presents new possibilities for achieving a better compromise between resolution and intensity in X-ray spectroscopy. After annealing, the integrated reflecting power measured with 316.5 keV γ-radiation at the 220 reflection of a 1 cm thick silicon crystal is approximately 30 times the average dynamical value calculated for an ideally perfect crystal. The increase in intensity is uniform over the whole crystal. The experimental data are interpreted consistent with Darwin's extinction theory. The full width at half maximum (fwhm) of the intrinsic diffraction pattern (mosaic spread) does not exceed 3″ even for path lengths in the crystal up to 10 cm. The fwhm of the double crystal X-ray rocking curves is of the order of 1.5″, i.e. about 10 times the dynamical value. The X-ray topographs reveal the degree of homogeneity of the internal deformation achieved for different oxygen contents and annealing cycles.

Effect of Substrate Misorientation on the Structual Strain and Defect Distribution of Mocvd Grown GaAs on Si

Astract:A systematic study of the structural properties and defect distribution of GaAs layers grown by metalorganic chemical vapor deposition on Si substrates misoriented 1°, 1.5°, 2°, 4°, and 6° from [100] toward [011] is reported. Double crystal x-ray rocking curves, cross-section and plan-view Transmission Electron Microscopy (TEM) are used to characterize the structural strain and defect distribution of as-grown and annealed GaAs layers. Both strain and defect density in the GaAs layers are found to be dependent of the degree of substrate misorientation as well as the direction in which measurements are made. Plan-view TEM shows an asymmetric distribution of microtwins in two perpendicular directions. There exists a correlation between the directionality of the strain and of the defect density. Furnace annealing at 850°C for 30 minutes in an arsine overpressure can reduce significantly the defects, the strain and the strain anisotropy. It is found that microtwins are of the highest density when the substrate is misoriented about 4 degrees for the as-grown samples. Though a reduction of defects after annealing occurs for all samples, the least misoriented one shows the most improvement.