X-ray Rocking Curve Width Research Articles

Plasma-assisted molecular beam epitaxy of ZnO thin films on sapphire substrates with an MgO buffer

We report on plasma-assisted molecular beam epitaxy of ZnO on Al 2O 3 (0 0 0 1) substrates with an MgO buffer layer. We found a thin MgO buffer greatly influence the initial growth of ZnO by facilitating the adhesion and promoting the lateral epitaxy of ZnO. Consequently, a flat surface with a (3×3) reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Both in situ investigations and structural analysis revealed that the MgO buffer has a rocksalt structure and consists of a 2D wetting layer and 3D islands above. The change in surface energy by covering the substrate with an MgO wetting layer results in a drastic suppression of columnar growth and layer-by-layer epitaxy of ZnO is achieved. The resulting improvement in crystal quality of the ZnO epilayers is confirmed by a large decrease in the X-ray rocking curve widths of both symmetrical and asymmetrical diffraction peaks, as well as the presence of free exciton recombinations in low temperature photoluminescence and the low n-type background carrier concentration.

Properties of lattice matched ZnMgSeTe quaternary alloys grown on ZnTe substrates

Lattice-matched high crystal quality Zn 1− x Mg x Se y Te 1− y quaternary alloys are grown on ZnTe substrates. For high crystalline and homogeneous alloy growth, the molecular beam pressure ratio is controlled as P Se< P Zn+ P Mg< P Te and ( P Se+ P Te)/( P Zn+ P Mg)⪢1. The lattice-matching condition to ZnTe substrates is obtained as y=0.63 x by both the correlated-function-expansion method and experiment. The growth temperature is optimized in terms of the X-ray rocking curve and photoluminescence line width. The observed narrow temperature window (300–320°C) for high-quality crystal growth is explained in terms of the bond energy difference of the binary alloys, which compose ZnMgSeTe. Mg composition is determined by RHEED intensity oscillations of ZnTe and ZnMgTe, which shows good agreement with the values estimated from X-ray analysis and RHEED intensity oscillations of MgTe. The photoluminescence properties of ZnMgSeTe are characterized by strong excitonic luminescence intensity with a narrow line width (5.95 meV) and very weak deep level luminescence. The luminescence intensity of typical ZnMgSeTe is 3.5 times stronger than that of bulk ZnSe under the same experimental condition. The narrow luminescence line width and small Stokes shift of ZnTe/ZnMgSeTe quantum wells are indicative of the homogeneity of the quaternary alloy.

Heteroepitaxy of CuInxSey: a review of the material and interface properties

Molecular beam epitaxy has been used to grow heteroepitaxial CuInxSey layers on Si and GaAs substrates. Layers of different Cu/In ratios have been grown to study the formation of different phases such as CuInSe2 chalcopyrite (α-phase) and defect-chalcopyrite/stannite phases (β-phase) in this material system. A variety of characterization methods are used to identify if the layer is a single phase compound or a mixture of two compounds. Structural and optoelectronic properties strongly depend on the Cu/In ratio. Epitaxial layers with an ion channeling yield of 7% and an X-ray rocking curve width of about 770 arcsec have been grown. Linear and planar defects as well as interdiffusion of elements are expected in the interfacial region due to layer-substrate mismatch and growth conditions. These defects have been identified with high resolution transmission electron microscopy. The chemical composition of the interfacial layer has been investigated by Rutherford backscattering spectroscopy, microprobe energy dispersive X-ray analysis and X-ray diffraction. An interfacial CuSexSiy layer is formed during the growth of CuInxSey on Si. In the case of GaAs substrates, Ga from the substrate diffuses into the CuInxSey and a quaternary compound Cu(In,Ga)xSey is formed at the interface.

Improved Heteroepitaxial MBE GaN Growth with a Ga Metal Buffer Layer

ABSTRACTWe demonstrate that the use of pure gallium (Ga) as a buffer layer results in improved crystal quality of GaN epilayers grown by plasma-assisted molecular beam epitaxy on c-plane sapphire. The resulting epilayers show electron Hall mobilities as high as 400 cm2/Vs at a background carrier concentration of 4 × 1017 cm−3, an outstanding value for an MBE-grown GaN layer on sapphire. Structural properties are also improved; the asymmetric (101) X-ray rocking curve width is drastically reduced with respect to that of the reference GaN epilayer grown on a low-temperature GaN buffer layer. Nitrided Ga metal layers were investigated for different Ga deposition time. These layers can be regarded as templates for the subsequent Ga main layer growth. It was found that there is an optimum Ga metal layer deposition time for improving the electron mobility in the epilayer. Heating of the Ga metal layer to the epilayer growth temperature under nitrogen plasma is found to be sufficient to produce highly oriented GaN crystals. However, nonuniform surface morphology and incomplete surface coverage were observed after nitridation of comparatively thick Ga metal layers. This is shown to be the reason for the decreasing electron mobility of the epilayers as the Ga metal layer thickness exceeds the optimum value.

Evolution of mosaic structure in Si0.7Ge0.3 epilayers grown on Si(001) substrates

In this article, we report a study of mosaic structures in partially relaxed Si0.7Ge0.3 epilayers grown on Si(001) substrates by x-ray double- and triple-axis diffractometry. The samples have different layer thicknesses and hence different degrees of strain relaxation. Our results show that, at early stages of strain relaxation, the films contain mosaic regions laterally separated by perfect regions. This is because the mosaic structure caused by a misfit dislocation is effectively localized in a lateral range of the layer thickness. Therefore, far from the dislocations, the film is virtually a perfect crystal. With the increase in the degree of strain relaxation, and consequently in the dislocation density, the mosaic regions of the layer expand while the perfect regions shrink and finally vanish completely. Moreover, our results indicate that the conventional method of estimating dislocation density from the x-ray rocking curve width fails in our case.

Strain and Mosaic Structure in Si0.7Ge0.3 Epilayers Grown on Si (001) Substrates Characterized by High Resolution X-Ray Diffraction

AbstractX-ray double- and triple-axis diffractometries were employed to characterize the strain status and structural deformation in Si0.7Ge0.3 alloy layers grown on Si (001) substrates. We show that, at low levels of strain relaxation, x-ray peaks of the Si0.7Ge0.3 alloy layers contain two components, a narrow one superimposed on a broad one. Such a peak profile corresponds to a layer structure consisting of mosaic regions laterally separated by more perfect regions. With the increase in the degree of strain relaxation and consequently in the dislocation density, the narrow component of the x-ray peak gradually disappears as a result of expansion of the mosaic regions and shrinkage or even vanishing of the perfect regions in the layer. Moreover, our results indicate that the conventional method of estimating dislocation density from the x-ray rocking curve width fails in our case.

High Quality Hydrothermal ZnO Crystals

Zinc Oxide crystals have historically been grown in hydrothermal autoclaves with a basic mineralizer; however, doubts have been raised about the quality of such crystals because they have often exhibited large x-ray rocking curve widths and low photoluminescence (PL) yield with large linewidths. Several ZnO crystals were grown hydrothermally and sliced parallel to the c-plane. This resulted in opposite surfaces (the C+ and C−) exhibiting pronounced chemical and mechanical differences. Different surface treatments were investigated and compared by PL both at room temperature and liquid helium temperatures, and by double axis X-ray rocking curve measurements. The high quality of hydrothermally-grown ZnO is substantiated by the narrow rocking curve widths and sharp PL peaks obtained. A critical factor in obtaining these results was found to be surface preparation.

Relationship between physical properties and gas purification in GaN grown by metalorganic vapor phase epitaxy

This paper reports on the structural, optical and electrical properties of LP-MOCVD GaN grown on miscut and exactly oriented sapphire substrates as a function of gas purification. SIMS and C–V experiments achieved on GaN layers indicate that high purity NH 3 is a significant source of oxygen contamination. Transport studies performed on oxygen-contaminated GaN samples have shown that oxygen acts as a very shallow donor. An activation energy ranging from 4 to 10 meV is extracted as a function of oxygen concentration. Hall mobilities of unintentionally oxygen-doped GaN samples are comparable to the data already published in the literature for silicon doped samples over carrier density range: 5×10 17–3×10 19 cm −3 [S. Nakamura et al., Jpn. J. Appl. Phys. 31 (1992) 2283]. Structural and optical GaN properties are not degraded by oxygen-contamination. [0 0 2] X-ray rocking curve (ω-scans) widths around 26–80 arcsec are obtained and a strong correlation is observed between low broad band yellow emission and highly oxygen-contaminated GaN samples.

MOVPE growth of ZnS xSe 1− x/GaAs(1 0 0) using ditertiarybutylselenium, tertiarybutylmercaptan and dimethylzinc triethylamine as precursors

Metal-organic vapor-phase epitaxy (MOVPE) at atmospheric pressure has been used to grow ZnS x Se 1− x epitaxial layers on (1 0 0)GaAs substrates. Ditertiarybutylselenium (DTBSe), tertiarybutylmercaptan (t-BuSH) and dimethylzinc triethylamine (DMZn : Et3N) were used as the selenium, sulfur, and zinc sources, respectively. The dependence of the composition of the ZnS x Se 1− x films on the partial pressures ratios R VI = p S/( p S + p Se) and R IV/II = ( p S + p Se)/ p Zn was investigated. The conditions for growing ZnS x Se 1− x alloy with x = 0.07 (lattice matched on GaAs) were therefore determined. In addition, the growth of ZnS x Se 1− x /ZnSe quantum wells was investigated. The structural characterization data show a strong dependence on the growth conditions. Excellent layer quality with sharp PL peaks and low values of the X-ray rocking curve width for x around 0.07 was obtained. A very intense and sharp (FWHM = 2 meV) PL emission was obtained for a ZnS 0.08Se 0.92/ZnSe quantum well with a 10 nm thick well.

High Quality Hydrothermal ZnO Crystals

AbstractZinc Oxide crystals have historically been grown in hydrothermal autoclaves with a basic mineralizer; however, doubts have been raised about the quality of such crystals because they have often exhibited large x-ray rocking curve widths and low photoluminescence (PL) yield with large linewidths. Several ZnO crystals were grown hydrothermally and sliced parallel to the c-plane. This resulted in opposite surfaces (the C+ and C-) exhibiting pronounced chemical and mechanical differences. Different surface treatments were investigated and compared by PL both at room temperature and liquid helium temperatures, and by double axis X-ray rocking curve measurements. The high quality of hydrothermally-grown ZnO is substantiated by the narrow rocking curve widths and sharp PL peaks obtained. A critical factor in obtaining these results was found to be surface preparation.

Growth of Iii-Nitrides by Rf-Assisted Molecular Beam Epitaxy

ABSTRACTGaN, AlGaN, AIN were grown on (0001) A1203 substrates by MBE using a RF plasma source and employing an AIN buffer layer. The films were characterized by RHEED, AFM, and x-ray diffraction, and electrical properties were measured by Hall technique. RHEED observations indicate that the polarity of the films is likely predominantly N-face, although Ga-face inversion domains can be observed in some films by AFM. Symmetric x-ray rocking curve widths as low as 39 arcseconds are achieved for some layers, while asymmetric peaks show widths of 240–300 arcsec. Control of Si doping over a wide range is demonstrated, which is important for design of high power device structures. Gold Schottky barrier m-v-n+diodes were fabricated which achieve high reverse electric fields before edge breakdown.

Growth of Ti thin films on sapphire substrates

Titanium thin films have been grown by electron-beam physical vapor deposition on the (0001) surface of sapphire (single crystal α−Al2O3) substrates at growth temperatures ranging from 295 to 1223 K. Single phase α−Ti films grew at all growth temperatures, even at 1223 K which is above the α-β transition temperature of Ti. Crystal quality, as measured by the width of x-ray rocking curves, was found to improve, and the elastic strain to increase, as the growth temperature increased from 295 K to 1023 K. The epitaxial relationship between the Ti and sapphire was (0002)Ti ‖ (0006)Al2O3 and [1120]Ti ‖ [1010]Al2O3. The extent of interdiffusion across the Ti/Al2O3 interface was observed to be small (&lt; 20 nm) at all growth temperatures.

Optimization of the structural properties of Hg1−x CdxTe (x = 0.18−0.30) alloys: Growth and modeling

The conditions for the metalorganic molecular beam epitaxial growth of Hg1−xCdxTe (x = 0.18−0.32) alloys at very low growth temperatures (T ≤150°C) have been optimized by correlating the surface properties and crystalline perfection with the incident Hg flux. A window for growth has been defined for x = 0.18, 0.23, and 0.32. A thermodynamic model has been developed to account for void formation. A neural net model has been used for the first time to model the dependence of void density on the Hg flux and the x-ray rocking curve widths on growth parameters. The combination of these two complementary modeling techniques allows for a flexible process optimization to be carried out with a minimum effort spent in calibration runs.

Orthoferrite Single Crystals Growing by Modified Czochralski Method and their Properties

To obtain bulb large-sized orthoferrite single crystals with different rare-earth elements a new crystal growth technology is proposed including modified Czochralski method and using no metal crucibles for melt suspension. For this purpose two unusual Czochralski method modifications are used. In the first one the melt of crystallized material is obtained at heating the upper surface of polycrystalline block with intensive light radiation focused by 3 powerful single ellipsoidal mirror light concentrators, and in the second one the combination of surface heating with 3 intensive light concentrators and induction of high-frequency heating of the melt placed in cold crucible is used to produce it. In the last case the melt volume is considerably larger and this circumstance allows to obtain large-sized single crystals. For optimum thermal field formation needed for crystal pulling the relation between cold crucible diameter, frequency of inductive coil and power of the focused light inputted in the melt plays the main role. Using this technology rather perfect single crystals of yttrium gadolinium, thulium etc. orthoferrites 25-30 mm in diameter and up to 50 mm long were grown. The maximum crystal diameter depends on the diameter of the cold crucible used and can be really increased up to 50 mm Ynrium orthoferrite single crystals obtained by this technology have the following perfection characteristics: X-ray rocking curve width is 10 in the central part of the crystal. the dislocation density is 1-10 /cm -2 in the central part and up to 10 4 on the periphery. blocks and twins are absent.

Mapping of CdZnTe substrates and CdHgTe epitaxial layers by X-ray diffraction

A simple procedure is described for mapping of lattice parameter and X-ray rocking curve width across Cd 1− y Zn y Te (CZT) substrates and Cd x Hg 1− x Te (CMT) epitaxial layers. It is shown how screening of the crystallinity and composition of CZT substrates can be utilized to make subsequent X-ray characterizations of epitaxial layers more reliable. This also allows selection of substrates with a close lattice matching to a given CMT composition, to reduce or avoid layer relaxation. Substrates with laterally varying Zn content give a varying lattice mismatch on a single epitaxial layer. Correlation of the lattice parameter maps of substrate and layer shows the resulting variations in layer strain, and the onset of relaxation can be clearly identified. Mapping of rocking curve peak widths on good quality CZT substrates and closely lattice matched CMT layers shows that the peak width can be below 6 arcsec, limited essentially by the intrinsic peak width of the materials. Additional broadening of the layer peaks is introduced primarily by substrate defects and misfit dislocations, unless epitaxial growth conditions deviate significantly from their optimum.

Hardness, elastic modulus and structure of indium nitride thin films on AlN-nucleated sapphire substrates

The structural properties of epitaxial indium nitride thin films were characterized and related to the mechanical properties as measured by nanoindentation. The seven epitaxial InN films examined were deposited over the temperature range 200–400°C by reactive magnetron sputtering on (0 0.1) sapphire substrates buffered with a thin AlN layer. The hardness, surface texture and crystal quality of the InN films were functions of the deposition temperature, with the maximum hardness (11.2 GPa), the smoothest surface, the minimum c-lattice constant (0.5708 nm), and the minimum (0 0.4) X-ray rocking curve width (0.6°) all occurring at the deposition temperature of 350°C. The crystal quality and the hardness of the InN films degraded at both higher and lower temperatures.

Doping, Activation of Impurities, and Defect Annihilation in Gan by High Pressure Annealing

AbstractGaN semiconductor is characterized by strong bonding and high vapor pressure of nitrogen. The strong bonds limit an efficiency of annealing procedures required for variety of semiconductor technologies, for example, post-implantation annealing, or doping by diffussion. Maximum temperatures employed up to now (at ambient pressure) in GaN annealing have not exceeded about 1100°C. A desired increase of annealing temperatures would cause a decomposition of GaN unless an elevated pressure of N2 is supplied. In this work, we report onapplication of high pressure annealing procedures (temperatures up to 1550°C and pressures up to 16 kbar) which enabled us to study variation of the properties of epitaxial films and bulk crystal of GaN. In particular, we discuss the following results obtained using high pressure annealing: i) structural quality improvement and increase of the thermal strain in as grown epitaxial layers of GaN/A12O3, The annealing at above 1300°C resulted in the decrease of the X-ray rocking curve width from about 700 arc sec. down to 470 arc sec., ii) drastic increase of bandedge (bound exciton) photoluminescence intensity iii) enhancement in removal of implant damage, iv) increase of diffusivity of Zn and Mg atoms (introduced by implantation and/or diffusion from external source). For Zn in epitaxial layers of GaN/A12O3 a diffusion starts at 1200–1250°C, v) enhancement of the blue-photoluminescence intensity in Zn and Mg implanted GaN. The performed experiments give an evidence of the importance of the defect (dislocations) in diffusion of Zn and Mg in the GaN semiconductor.

Properties of expitaxial Pb 1− xSn xSe on CaF 2 covered Si(111) substrates

Epitaxial narrow gap Pb 1− x Sn x Se layers on Si-substrates are used for infrared focal plane arrays for thermal imaging in the 8–12 μm wavelength range. The density of threading dislocations in the 3–4 μm thick layers due to the lattice mismatch is about 3 × 10 7 cm −2, in accordance with the width of X-ray rocking curves (100 arc s range). These narrowest widths are obtained only in layers which exhibit high carrier mobilities. Due to the thermal expansion mismatch, misfit dislocations formed during growth glide on each temperature change along the {100} main glide planes. The interaction probabilities of such crossing dislocations leading to strain hardening is extremely low, a rough estimate leads to a value of 10 −5. This is because the cumulative plastic deformation (applied by repeated temperature cycling) is as high as 500%.

Role of threading dislocation structure on the x-ray diffraction peak widths in epitaxial GaN films

In this letter we demonstrate that the anomalously low (002) x-ray rocking curve widths for epitaxial hexagonal GaN films on (001) sapphire are a result of a specific threading dislocation (TD) geometry. Epitaxial GaN films were grown on c-plane sapphire by atmospheric pressure metalorganic chemical vapor deposition (MOCVD) in a horizontal flow reactor. Films were grown with (002) rocking curves (ω-scans) widths as low as 40 arcsec and threading dislocation densities of ∼2×1010 cm−2. The threading dislocations in this film lie parallel to the [001] direction and within the limit of imaging statistics, all are pure edge with Burgers vectors parallel to the film/substrate interface. These TDs will not distort the (002) planes. However, distortion of asymmetric planes, such as (102), is predicted and confirmed in (102) rocking curve widths of 740 arcsec. These results are compared with films with (002) rocking curves of ∼270 arcsec and threading dislocation densities of ∼7×108 cm−2.

Optical Characterization of Europium-Doped Calcium Fluoride Thin Films Grown on Silicon by Molecular Beam Epitaxy

AbstractPhotoluminescence measurements have been made on Eu-doped CaF2 thin films grown on Si(100) substrates. The dependence of the integrated intensity of both the zero-phonon line and the vibronic sideband on temperature and Eu concentration has been studied in the range of 10 – 300 K and 0.14 – 7.48 at. %, respectively. An anti-Stokes feature is visible in the emission spectra at 75 K, indicating a significant occupation of the excited vibrational states of Eu2+ by phonons before the transition at temperatures above 75 K. The integrated intensity of the vibronic sideband increases slightly as temperature increases. The solubility of Eu in CaF2 thin films grown on Si(100) is found to be somewhere around 4.05 at. % as confirmed by the increase of the line width of x-ray rocking curves and the decrease in the integrated intensity of the zero-phonon line with Eu concentration.