Double Crystal X-ray Diffraction Measurements Research Articles

Metastable SiGeC formation by solid phase epitaxy

We report the synthesis and detailed structural characterization of SiGeC metastable alloys formed by solid phase epitaxial regrowth. Epitaxial layers with 0.7 and 1.4 at. % C are formed by 700 °C regrowth of multiple energy carbon implants into preamorphized Si0.86Ge0.14 layers on Si substrates. Transmission electron microscopy and Rutherford backscattering spectrometry show heteroepitaxial regrowth of Si1−x−yGexCy layers into the metastable diamond cubic phase. Fourier transform infrared spectroscopy verifies that the carbon occupies substitutional lattice sites. Double crystal x-ray diffraction measurements of Si1−x−yGexCy and Si1−yCy reference layers quantify the C-induced tensile strain component. This strain compensates for the compressive strain in the SiGe layers, and indicates a change in lattice constant per atomic fraction C in agreement with Vegard’s law.

Observation of staggered band lineup in In0.5Ga0.5P/Al0.43Ga0.57As heterojunction grown by liquid phase epitaxy

The conduction band discontinuity for n-N isotype In0.5Ga0.5P/Al0.43Ga0.57As heterostructure grown on (100) GaAs substrate by liquid phase epitaxy was measured by the capacitance-voltage profiling method. The composition of each ternary was determined by photoluminescence and double-crystal x-ray diffraction measurement. The measurement of conduction band discontinuity shows staggered band lineup with both bands of In0.5Ga0.5P above those of Al0.43Ga0.57As, and the calculated conduction-band discontinuity ΔEc and the fixed interface charge density σi are 157 meV and −3×1010 cm−2, respectively. The nonoptimized fabrication of the light emitting devices with AlGaAs/InGaP/AlGaAs double heterostructure can be explained by the staggered band lineup of In0.5Ga0.5P/AlxGa1−xAs heterointerface for x(AlAs)≳0.43.

Optimization of interfaces in arsenide–phosphide compounds grown by gas source molecular-beam epitaxy

A novel source switching procedure in gas source molecular-beam epitaxy to achieve abrupt interfaces in lattice-matched InP/GaInAs has been demonstrated. In this process, a two-step gas source switching procedure was used at the InP-to-GaInAs interface and a single growth pause was employed at the GaInAs-to-InP interface. Gas source switching procedures were optimized through characterizing multiple quantum well samples using photoluminescence and double crystal x-ray diffraction measurements. Single quantum wells grown using the optimized switching procedure displayed the narrowest luminescence linewidth values compared to other documented samples grown by gas source molecular-beam epitaxy.

Photoluminescence study of the 2D electron gas formed at the interface of strained InGaAs/InP single heterostructures

Photoluminescence measurements have been performed in two series of strained In x Ga 1- x As/InP single heterostructures, grown by molecular beam epitaxy with different epilayer thicknesses. Tensile strained samples ( x In = 0.508) and compressive strained samples ( x In = 0.543) have been investigated. An emission 30 meV below the excitonic recombination has been observed in these structures and we ascribe it to a recombination between photocreated holes and electrons from the two-dimensional electron gas formed at the InGaAs/InP interface. The role of the epilayer thickness on the photoluminescence recombination mechanism of the two-dimensional electron gas is discussed. Measurements of the photoluminescence temperature dependence have revealed the heavy-hole and light-hole splitting in this emission. The value of this splitting is in very good agreement with the expected values obtained by double crystal X-ray diffraction measurements.

Structural and optical characterization of alternately strained GaAs/GaP/GaAs/InP superlattices grown by atomic layer molecular beam epitaxy

A new type of highly perfect strained layer superlattices matched to GaAs substrate is presented. (GaAs) N (GaP) M (GaAs) N (InP) M superlattices with N = 6, 10 and 90 and M = 2 were grown by atomic layer molecular beam epitaxy. In these superlattices, strain in Gap and InP layers has opposite signs enabling us to obtain a mean lattice parameter perfectly matched to the GaAs substrate. Thicknesses of GaP and InP layers are restricted by the critical thicknesses of these materials on the GaAs substrates. The thickness of GaAs layers in the superlattice is not limited and plays the role of tailoring the band gap of the structure, a feature that widens the applications of similar GaP/InP strained layer superlattices. Double crystal X-ray diffraction measurements show that all samples studied have a high crystalline quality, thicknesses of the different layers are very close to the designed growth values, and the superlattices as a whole are coherent with the substrate, indicating the effectiveness of compensating strains in opposite directions in the GaP and InP layers. High resolution electron microscopy analysis shows sharp interfaces with good lateral uniformity. Neither dislocations nor other crystalline defects are observed.

Regrowth and strain recovery of Sb implanted Si 1− xGe x strained layers

Solid phase epitaxial growth (SPEG) and strain recovery of Sb implanted strained Si 1− x Ge x alloy layers are reported. Three sets of Si 1− x Ge x epilayers with compositions of x = 0.08, 0.10 and 0.18, MBE grown on (100)Si substrates, were implanted at room temperature with Sb + ions at energies of 200 and 100 keV, and doses of 10 12, 10 13 and 10 15 cm −2. These alloys were heat-treated in a rapid thermal annealing system at temperatures between 500 and 1000°C for durations between 5 and 600 s. Ion backscattering (channeling) measurements show a decrease in the regrowth rate compared to Sb implanted (100)Si. The growth rate for x = 0.08 was characterized by an activation energy of 2.9±0.2 eV. Double crystal X-ray diffraction measurements of lattice distortion show that the radiation damage changes the strain levels of the epilayers depending on the Ge content of the SiGe and on implantation parameters. However, the post-irradiation damage does not reduce the strain stability of SiGe layers with respect to dislocations formation.

The effect of growth temperature on plastic relaxation of In 0.2Ga 0.8As surface layers on GaAs

Single surface layers of In 0.2Ga 0.8As ranging from 10 nm to 3 μm in thickness have been grown by molecular beam epitaxy (MBE) on GaAs at two growth temperatures, 400°C and 519°C. The residual strain of each layer has been obtained from double crystal X-ray diffraction (DCXD) measurements, and the surface morphology observed by differential interference contrast microscopy. The critical thickness is found to be independent of growth temperature. The effects of the change in growth temperature on the surface morphology and the extent of relaxation with varying layer thickness are discussed.

Molecular-beam epitaxially grown Si/GaAs interfaces: Delta-doping, Si on GaAs, and GaAs on Si

Silicon layers with thicknesses covering the range from delta-doping densities to relaxed Si have been embedded in GaAs using molecular-beam epitaxy. The Si thickness for which the onset of relaxation of the Si layer occurred was found to be about 12 Å by analysis of double-crystal x-ray diffraction and Hall effect measurements. Reflection high-energy electron diffraction observations indicate the presence of GaAs at the surface of Si-on-GaAs layers. The orientation of the (2×4) reconstruction for the GaAs grown on top of the Si-on-GaAs layers was always the same as that of the substrate GaAs for strained Si layers and rotated 90° from that of the substrate for relaxed Si layers. A transition Si thickness region between 13 and 15 Å was discovered for which the GaAs cap layer had a double-domain [(2×4)+(4×2)] reconstruction.

Precise determination of aluminum content in AlGaAs

The Al composition of AlGaAs has been determined by four methods: high-resolution transmission electron microscopy (HRTEM), reflection high-energy electron diffraction (RHEED), photoluminescence (PL), and double-crystal x-ray diffraction (DCXRD). HRTEM is direct and the most accurate method because it does not involve any formula or extrapolation. Using the result obtained from HRTEM as a standard, we have calibrated the results from other methods. RHEED intensity oscillation is found to be accurate and reliable, if the growth conditions are correctly chosen. Comparing the PL results with those determined from HRTEM and RHEED, we suggest three formulas to determine the Al contents at different temperatures. We also proposed a polynomial to determine the Al concentration using the DCXRD measurement.

Determination of epitaxial AlxGa1−xAs composition from x-ray diffraction measurements

The correlation between the aluminum composition in epitaxial AlxGa1−xAs and double crystal x-ray diffraction measurements was quantitatively determined. The angular separation ΔΘ, between the diffraction peaks from the AlxGa1−xAs layer grown by metalorganic vapor phase epitaxy and the GaAs substrate increased nonlinearly with the Al content, which was independently determined using photoluminescence and electron microprobe measurements. The calibration curve was used to determine AlAs materials parameters. The AlAs lattice constant and Poisson ratio were determined to be 5.6622 Å and 0.275, respectively, assuming that the GaAs parameters are 5.65325 Å and 0.311.

Implantation damage in GaAs-AlAs superlattices of different layer thickness

We report that two GaAs-AlAs superlattices of different layer thickness show dramatically different crystal damage when ion irradiated under identical conditions. The samples, held at 77 K, were implanted with 100 keV 28Si at doses of 3×1013 cm−2 to 1×1015 cm−2. Ion channeling results show amorphization threshold doses of 1×1015 cm−2 for the 7.0 nm GaAs-8.5 nm AlAs superlattice and 4×1014 cm−2 for the 3.5 nm GaAs-5.0 nm AlAs superlattice. At low doses, the shorter period superlattice was more robust, with no damage peak observed in ion channeling spectra for doses as high as 1×1014 cm−2. For a dose of 7×1013 cm−2, double crystal x-ray diffraction measurements show a 6 arcsec broadening of the (004) peak, relative to that of the unimplanted sample, for both superlattices. However, only the finer period superlattice exhibits a broadening (10 arcsec) of the (224) diffracted peak indicating a distortion in an additional direction. A mechanism involving the formation of slightly misaligned crystal domains is suggested to describe the behavior of the finer period superlattice.

Liquid Phase Epitaxy Growth and Properties of GaInAsSb/AlGaAsSb/GaSb Heterostructures

The GaInAsSb/AlGaAsSb/GaSb heterostructures were grown by the liquid phase epitaxy (LPE) technique. The materials were characterized by means of optical microscopy, electroprobe microanalysis (EPMA), double-crystal X-ray diffraction, capacitance-voltage (C-V) and Van der Pauw measurements, infrared absorption spectra, photoluminescence and laser Raman scattering. The results show that the materials have fine surface morphology, low lattice mismatch and good homogeneity. Room-temperature light-emitting diodes with an emission wavelength of 2.2 µm were obtained by using the GaInAsSb/AlGaAsSb DH structures.

Growth and characterization of Fe-doped semi-insulating InP prepared by low-pressure organometallic vapor phase epitaxy with tertiarybutylphosphine

Fe-doped semi-insulating InP epitaxial layers were grown by low-pressure organometallic vapor phase epitaxy with tertiarybutylphosphine (TBP), triethylindium (TEI) and iron pentacarbonyl [Fe(CO)5] as the reactant gases. The growth was performed by varying the growth rate, growth pressure and V/III ratio. The epitaxial layers were characterized by optical microscopy, secondary ion mass spectrometry, double crystal x-ray diffraction and current-voltage measurements. Semi-insulating InP epitaxial layers with specular surface morphology and low defect density were obtained at TBP partial pressure higher than 0.38 torr. A premature reaction between TEI and TBP was observed which presumably formed TEI:TBP adducts and/or polymers. As a result, the growth rate of Fe-doped semi-insulating InP layers grown at low pressure with TBP in our reactor decreased by 35% as the V/III ratio was increased from 15 to 46. Electrical measurements on these layers showed that the resistivity varied from 1.7×107 to 4×108 Ω cm as the V/III ratio was increased from 15 to 46. The resistivity of TBP-grown materials is comparable to that of PH3-grown materials over a measurement temperature range of 25–110 °C. Selective growth and surface planarization of Fe-doped InP grown with TBP and trimethylindium on patterned etched mesas were achieved.

Physical characterization of OMVPE-grown Al xGa 1- xAs multi-layer films by means of non-destructive optical reflectometry

A novel Fourier transform technique which was developed by us previously, is applied in this work for non-destructive determination of thickness and refractive index steps of experimental multilayer heteroepitaxial structures. No time consuming least-squares curve-fits are required and theoretically there is no limit on the number of layers which can be analyzed. In order to demonstrate the above method experimentally, multilayer structures of Al x Ga 1- x As containing up to five layers on GaAs substrates, were grown by organometallic vapour phase epitaxy (OMVPE). For calibration purposes, initial experiments involved the growth of Al x Ga 1- x As single epilayers on GaAs substrates by the same technique. Layers were grown at five different compositions from x = 0.1 to x = 0.5. Both photoluminescence (at 12 K) and double-crystal X-ray diffraction measurements were undertaken on the single layers in order to establish the composition of the material. The results show excellent correspondence with each other. These results were also compared to secondary ion mass spectrometry data. The agreement between the optical technique developed by us and the other analytical methods was excellent.

Characterization of alloy composition in Ga 1- xAl xAs/GaAs structures: comparison of photovoltage, X-ray, SIMS and RHEED techniques

A comparative study of techniques for the characterization of alloy composition in Ga 1- x Al x As/GaAs single and multilayer structures has been carried out. Good agreement ( x ± 0.015) is demonstrated between double-crystal X-ray diffraction and photovoltage spectroscopy measurements using the generally accepted calibration of Casey and Panish. A slightly modified form of the band gap versus composition curve is presented which further improves the agreement to x±0.005. Profiling conditions achieving linear SIMS Al profiles are reported, yielding compositions in agreement with PVS and X-ray within x+-0.01 (when calibrated at a single composition). A systematic discrepancy between results from the above techniques and in-situ RHEED oscillation measurements is reported.

Quantum-well structures of InAlP/InGaP grown by gas-source molecular-beam epitaxy

Both single quantum-well (SQW) and multiple quantum-well (MQW) structures have been produced using the technique of gas-source molecular-beam epitaxy to grow the two wide band-gap ternary alloys, InAlP and InGaP. SQWs as narrow as two monolayers observed by bright field Transmission Electron Microscopy were found to be laterally uniform with abrupt InAlP/InGaP interfaces. Photoluminescence of SQWs of differing thickness produced a larger quantum confinement energy shift than expected, with emission at 570 nm for an InGaP well of 3.0 nm in thickness. The number and amplitude of peaks detected in double-crystal x-ray diffraction (DCXR) measurements of the MQW samples matched, to within the limit of the dynamic range of the DCXR system, the peaks calculated in a periodic two-layer dynamical simulation of the x-ray rocking curve.

Strained-layer InGaAs/InGaAsP MQW structures and their application to 1.67 μm lasers grown by metalorganic vapor phase epitaxy

Strained multiple quantum well (MQW) structures are grown by low pressure metalorganic vapor phase epitaxy. The strained MQWs are characterized by double-crystal X-ray diffraction and photoluminescence measurements. Close agreement with theoretical calculations clearly indicates the perfection of the strained MQWs. Buried-heterostructure laser diodes are fabricated to evaluate the strained MQW structures in a practical application. High-power, high-efficiency operation up to 61 mW is achieved at a lasing wavelength of 1.67 μm.

The crystal geometry of Al xGa 1- xAs grown by MOCVD on offcut GaAs (100) substrates

Double crystal X-ray diffraction measurements show that the cubic film unit cell defined by Vegard's law is triclinicly distorted and titled with respect to the substrate unit cell. The distortion and tilt angles oppose each other defining a crystal geometry where the substrate and film (100) vector normals remain approximately coplanar with the surface normal. Such film/substrate crystal geometry leads to formulation of a growth model. Buffer layer growth or homoepitaxy which conventionally precedes heteroepitaxial film deposition has been shown to form ordered substrate surfaces. During heteroepitaxy on offcut and ordered substrates, rapid lateral growth induces lateral strain relaxation which is interrupted periodicly by finite terrace dimensions. As it is the case for strained heteroepitaxy on substrates with zero offcut angle, strain relaxation occurs additionally in direction of the surface normal. The crystal lattice geometry produced across an average terrace length, defines the interface, which determined the observed film/substrate crystal geometry.

Strain in epitaxial GaAs on CaF2/Si(111)

Epitaxial layers of (111) GaAs of approximately 1 μm thickness were grown on epitaxial CaF2 buffer layers that were either 140 or 380 nm thick on Si(111) substrates. The best nucleation temperature for the GaAs on CaF2/Si(111) we have observed was 620 °C. This resulted in high quality GaAs films, which exhibited channeling minimum yields of 4%. The density of threading dislocations in the GaAs layers was observed by transmission electron microscopy (TEM) to be ∼108 cm−2. Double-crystal x-ray diffraction measurements showed that the strain (ε⊥) was less than 2.2×10−4 in both sets of GaAs samples. However, ion channeling revealed a large tetragonal strain of 3.5×10−3 (ε⊥=1.7×10−3) in the thinner (140 nm) CaF2 buffer layers. By doing ion channeling with high energy (2.5 MeV) protons, it was possible to determine strain more accurately. Using this technique, we were able to set an upper limit for the tetragonal strain of 2.5×10−4 in both the GaAs (which implies ε⊥<8×10−5) and CaF2 (ε⊥<1.5×10−4) layers for the thicker (380 nm) CaF2 buffer layer structure. These results are in good agreement with the strain predicted from previous strain measurements of CaF2 epitaxial layers on Si.

High precision structural measurements on thin epitaxial layers by means of ion-channeling

The possibility of obtaining a detailed description of the crystalline structure of single-crystal samples is intrinsic to several applications of the ion-channeling technique. This paper reports a new approach to the use of channeling measurements which allows a precise characterization of the crystallography of very thin crystalline layers. The use of a precise mathematical description of the sample rotations which are involved in a typical channeling experiment gives the possibility of having a direct correlation between the sample lattice structure and the angular coordinates where the axial and planar channeling minima are located. The model is fully tested and the precision of the measurements obtained by this technique is compared to the results of double crystal X-ray diffraction measurements on the same systems. This technique is particularly well adapted to the measurement of lattice strain in heteroepitaxial structures.