Double Crystal X-ray Diffraction Measurements Research Articles

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.

A Novel Graphical Analysis Method for Double Crystal X-Ray Diffraction Measurements of Strained Layer Superlattices Grown on (100) Substrate

A novel graphical plot analysis method for double crystal X-ray diffraction measurements is applicable to cubic crystal epilayers grown on (100) substrate. In this method, we introduce a new term: the `difference of the virtual lattice constant, ΔA'. The ΔA values can be obtained by converting the angular difference between the relevant peak and a reference peak. By plotting ΔA values as a function of (k2+l2)/h2, parallel and perpendicular lattice constants can be easily analyzed for any sample with arbitrary relaxation rate. This advantage also reduces estimation error. This novel method and its advantages are experimentally demonstrated using InAsP/InGaAsP SLS samples.

Quality-enhanced GaAs layers grown on substrates by metalorganic chemical vapor deposition

High-quality GaAs layers were grown on Si substrates by low-pressure metalorganic chemical vapor deposition. Double crystal X-ray diffraction measurement shows that the structural properties of GaAs layers are so much improved by 5 μm thick Ge buffer layer and InGaAsGaAs strained-layer superlattice. The X-ray full-width at half-maximum of the (4 0 0) reflection obtained from 2 μm thick GaAs is 98 arcsec, one of the smallest value ever reported. The surface dislocation density of the GaAs layer estimated by molten KOH study is as low as 4 × 105cm−2. Photoluminescence and Raman measurements confirm the good optical quality of the heteroepitaxial GaAs epilayer.

Liquid-phase epitaxial growth of GaAsxP1 − x layers on GaP substrates

Abstract Liquid-phase epitaxy (LPE) was used to successfully prepare high As fraction ( x ≥ 0.4) GaAs x P 1 − x layers on GaP substrates. To accommodate the lattice mismatch between epilayer and substrate, both continuously and step-graded schemes were investigated. This is the first report of thin melt growth using the former approach. The surface morphology of multilayers prepared by step-cooled LPE were superior to those of the graded layers. The latter were found to be rippled and decorated with 60° glide lines owing to stress relaxation. Double crystal X-ray diffraction (DCXD) measurements showed that the multilayers grown by step cooling had a good uniform composition in each layer, while the alloy composition changed continuously in the graded layers. DCXD results also indicated that both the step graded multilayers and continuously graded layers were essentially fully relaxed. Photoluminescence response of the uppermost high x ( x ∼ 0.6) GaAs x P 1 − x layer of a multilayer stack showed very strong band-to-band emission. However, only a weak luminescence was detected in the graded layers.

Improvement of the crystallinity of a GaAs epitaxial film grown on a Si substrate using a Si/SiGe/Ge buffer layer

Double-crystal X-ray diffraction (DCXD), photoluminescence (PL), and Raman spectroscopy measurements on GaAs/Si heterostructures grown with various kinds of buffer layers by molecular beam epitaxy were performed in order to characterize the crystallinity of the samples. The strain and dislocation density of the GaAs epitaxial layer determined from the DCXD curve for the GaAs/Si/SiGe/Si heterostructure are 3.90×10 −3 and 6.99×10 6 cm] −3, respectively, and the full width at half maximum (FWHM) of the GaAs epitaxial layer for the heterostructures is 405 arcsec. The results of the PL spectrum for GaAs/Si with a Si/SiGe buffer showed that the FWHM of the band-to-band luminescence is only 5.4 meV. The ratio of the peak intensity of the longitudinal optical phonon to that of the transverse optical phonon for GaAs/Si with a Si/SiGe/Ge buffer increased dramatically in comparison with the corresponding rates for the GaAs/Si with Ge and SiGe/Ge buffers. These results indicate that the crystallinity of the GaAs epilayer was remarkably improved using a Si/SiGe/Ge buffer, and a strong binding energy of the SiSi bond in the top Si buffer layer reduced the formation of the dislocation in the GaAs active layer.

Molecular beam epitaxy growth and characterization of InGaAlAs-collector heterojunction bipolar transistors with 140 GHz fmax and 20 V breakdown

The molecular beam epitaxy (MBE) growth, material characterization, and device performance of InGaAlAs-collector heterojunction bipolar transistors that dramatically increase device breakdown without compromising high-frequency performance is described. This performance is achieved by the use of large band gap InGaAlAs collectors with a pulse-doped layer at the beginning of the graded collector-to-base junction to suppress current-blocking effects. Double-crystal x-ray diffraction measurements indicate these complex structure can be grown using MBE with very close lattice matching to the underlying InP substrate. From photoreflectance spectra the collector composition, field present at the collector-to-base junction, and various transitions associated with the graded emitter can be measured. The use of an In0.53(Ga0.50Al0.50)0.47As collector was found to increase breakdown characteristics (BVcbo=20 V, VA=250 V) substantially compared to an In0.53Ga0.47As collector (BVcbo=11 V, VA=7 V). For the In0.53(Ga0.50Al0.50)0.47As collector current blocking effects were only found at very high current levels (∼100 kA/cm2) allowing this device to achieve very good rf performance (ft=58 GHz and fmax=140 GHz). For higher Al compositions in the collector the breakdown voltage was found to increase further but current-blocking effects became more severe and rf performance began to suffer.

Characterization of Praseodymium-doped InP Epilayers Grown by Liquid-Phase Epitaxy

The various degrees of incorporation of praseodymium (Pr) into InP during liquid-phase epitaxial growth from an In-rich solution were investigated by double-crystal X-ray diffraction, Hall effect, and low-temperature photoluminescence (PL) measurements to investigate the effect of Pr concentration on structural, electrical, and optical properties. The lattice mismatch varies slightly with increasing Pr concentration in the growth melts. This variation in lattice mismatch with increasing Pr concentration is observed together with the shift of the PL emission energy. Examinations of the electrical property reveal that all the Pr-doped samples exhibit n-type conduction. Depending on the extent of Pr doping in the growth melts, InP epilayers have carrier concentrations ranging from 1017 cm-3 to 1016 cm-3 and mobilities varying from 1330 to 3800 cm2/V·s. It is also revealed that the impurities are gettered by Pr ions during liquid-phase epitaxial growth but the Pr-related defects formed in high Pr-doped melts may act as scattering centers and cause carries mobilities to decrease. It is found that epitaxial growth from the melt with 0.25–0.29 wt% Pr produces of the epitaxial layers highest quality. No intra-4f-shell transition line is observed from InP layers even at very high Pr concentrations.

Heavily carbon-doped GaAs grown by movpe using carbon tetrabromide for HBTs

In this paper the growth and characterization of heavily carbon-doped GaAs for heterojunction bipolar transistors is reported. The GaAs:C layers were grown by MOVPE at 650 °C using carbon tetrabromide as dopant source. The lattice mismatch of carbon-doped GaAs epilayers was analyzed using symmetric planes of double crystal X-ray diffraction. Hall effect measurements were used to study the hole mobilities and hole concentrations. Double crystal X-ray diffraction measurements show that the lattice contraction of carbon-doped GaAs epilayers is less than 0.03% for the carbon doping levels used in the base region of HBTs. Hall effect measurements show that the hole mobilities of heavily carbon-doped GaAs are higher than those observed when zinc and beryllium are used as dopant, indicating less compensation in our samples.

STUDY ON THE QUANTUM CONFINED STARK EFFECT OF InGaAs/InAlAs MULTIPLE QUANTUM WELL STRUCTURES

The quantum confined stark effect of InGaAs/InAlAs MQW heterostructures lattice-matched to InP substrate, grown by Chinese-built molecular beam epitaxy (MBE) system, is observed by absorption photocurrent spectra measurements, as well as the anisotropic elec-troabsorption of MQWs. The structure parameters of the epitaxied materials, which can be used to fabricate waveguide MQW electroabsorption modulators, were determined from double crystal X-ray diffraction measurements and computer simulations. The theoretical calculations of absorption-edge red shift compared with experimental results shows that the built-in-potential of the p-i-n junction can not be neglected.

Growth and Characterization of Zn1-xMnxSe1-ySy Epilayers and Related Heterostructures

AbstractWe describe the growth of (Zn,Mn)(S,Se) epitaxial layers of high structural quality on (100) GaAs substrates. Double crystal x-ray diffraction (DCXRD) measurements indicate that quaternary epilayers nearly lattice-matched with GaAs are characterized by DCXRD curves with a foil width at half maximum in the range 30 – 60 arc seconds. Photoluminescence (PL) spectroscopy is employed to map the variation of the energy gap of the quartenary alloys over a wide range of alloy compositions. Finally, temperature dependent PL is used to examine the viability of (Zn,Mn)(S,Se) alloys as confining layers for ZnSe and (Zn,Cd)Se quantum wells. While efficient exciton confinement is demonstrated through the observation of robust PL from such quantum wells up to high temperatures, the renormalization of the quartenary band gap by spin fluctuations leads to a rapid decrease in confinement with increase in temperature.

The growth and characterization of (211) and (133) Oriented (Hg,Cd)Te epilayers (211)B GaAs by organometallic vapor phase epitaxy

The growth of CdTe buffer layers on (211)B GaAs substrates by organometallic vapor phase epitaxy (OMVPE) was studied, and it was found that, depending on the growth conditions, either the (211) or (133) epitaxial orientation could be formed. In some cases, an epilayer showing a mixed (211) and (133) orientation was also observed. The influence of several growth parameters on the orientation of the CdTe layer was investigated, and it was found that the Te/Cd ratio, together with the growth temperature, have the most significant effect in determining the epilayer orientation. From these results, it was then possible to select nominally optimized growth conditions for CdTe buffer layers of both orientations. (Hg,Cd)Te layers of the same orientations could then be grown and characterized. Although double crystal x-ray diffraction measurements indicated a somewhat better crystalline perfection in the (133) (Hg,Cd)Te layers, these layers showed a poor surface morphology compared to the (211) orientation. Measurement of etch pit densities also indicated defect densities to be typically half an order of magnitude higher in the (133) orientation. Diodes were formed by ion implantation in both orientations and significantly better results were obtained on the (211) (Hg,Cd)Te layers.

Growth of high-quality InGaP and application for modulation-doped structure by molecular beam epitaxy with a GaP source

High-quality In 0.48Ga 0.52P lattice-matched to GaAs has been successfully grown by solid-source molecular beam epitaxy (MBE). To avoid the problem associated with white phosphorus, a dimer phosphorus molecular beam was produced by the sublimation of GaP. The In 0.48Ga 0.52P layers have been characterized by double-crystal X-ray diffraction (DCXR), photoluminescence (PL) spectroscopy and Hall measurements. The full width at half maximum of the (004) DCXR peak from 1.7 μm thick layer is 13 arc sec, which is comparable to the smallest value ever reported. The PL linewidth at 10 K is 16 meV and the electron mobilities are comparable to similar InGaP/GaAs layers grown by gas-source MBE. To confirm the layer quality for device application, InGaP layers were used for pseudomorphic AlGaAs/InGaAs/GaAs modulation-doped field-effect transister (MODFET) structures. Since wet chemical etchants with pronounced selectivity between InGaP and GaAs are well established, InGaP layers are attractive for the device fabrication as etch-stopper. In addition, Si-doped InGaP can replace the Si-doped AlGaAs layer, which causes the problem of persistent photoconductivity (PPC) at low temperatures. We have grown three different samples to compare the properties of the MODFET structures. The insertion of the etch-stopper did not deteriorate the properties of the MODFET structures and the PPC effect was significantly improved, although the carrier concentration was reduced.

Growth of Ga xIn 1− xAs/InP thin layer structures by chemical beam epitaxy

Multiple layer structures of InP and Ga x In 1− x As have been grown on InP substrates by chemical beam epitaxy (CBE). The samples were analyzed by photoluminescence (PL) spectroscopy at 5 K, double-crystal X-ray diffraction (DCXRD) and transmission electron microscopy (TEM). From 2000 Å thick layers, the PL measurements revealed the variation of the composition as a function of growth temperature. The influence of the growth interruption sequence (GIS) on the interfacial layers of 10-period superlattices with a 20 Å Ga 0.47In 0.53As well and a 40 Å InP barrier is investigated. The time responsible for the exchange of P by As atoms at the “lower” ( InP → Ga 0.47 In 0.53 As) interface is varied from 0 to 10 s at a growth temperature of 525°C. A minimal PL linewidth was observed for 1 s. TEM analysis showed abrupt transitions and a period thickness of 61 ± 3 Å. Both simulation of DCXRD measurements and calculation of PL transition energies revealed the formation of minimally 1 ML InAs y P 1− y during the substitution at the “lower” interface.

Double- and triple-crystal X-ray diffraction analysis of semiconductor quantum wires

In this work we show the potential of high-resolution X-ray diffractometry for the non-destructive investigation of nanostructures and low-dimensional semiconductor systems. A characterization of the geometrical and structural properties of corrugated semiconductor surfaces (crystalline surface gratings) and semiconductor quantum wires is presented. Double-crystal and triple-crystal X-ray diffraction measurements and reciprocal-space mapping are used in order to determine the geometrical parameters such as the wire periodicity, width and height and the corrugation profile as well as the strain status of quantum wires.

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.

Photoluminescence, intersubband absorption, and double crystal x-ray diffraction in p-doped InGaAs/AlGaAs strained multiple quantum wells

We report the correlation of photoluminescence (PL), infrared intersubband absorption, and double crystal x-ray diffraction (DCXRD) data for a p-doped InGaAs/AlGaAs strained multi- ple-quantum-well structure grown by molecular beam epitaxy. A PL doublet at 1.476 and 1.563 eV involves two confined holes states and their 87 meV separation is in good agreement with the measured intersubband absorption of about 14.5 μm (85 meV). Furthermore, when the well width obtained from DCXRD measurement is included excellent agreement with an envelope function calculation is found for the energy levels determined by PL and intersubband absorption energy.

High-quality InGaP and InGaP/InAlP multiple quantum well grown by gas-source molecular beam epitaxy

Using gas-source molecular beam epitaxy, we have obtained high-quality GaInP and (AlGa)InP epilayers lattice-matched to (100) GaAs substrates. All grown layers exhibited mirror-like surfaces. For a 1.7 μm thick Ga 0.5In 0.5P film, the Hall electron mobility was 3400 and 30,000 cm 2 /V·s at 300 and 77 K, respectively. The luminescence wavelength of (Al x Ga 1− x )InP samples ranged from 680 nm (for GaInP) to 590 nm (for AlInP) at room temperature, and from 644 to 513 nm at 77 K. The multiple quantum well (MQW) structure with well width of 40 Å showed strong luminescence intensity with wavelength of 647 nm (300 K) or 622 nm (80 K). The satellite peaks can be detected in double-crystal X-ray (DCXR) diffraction measurements of the MQW samples, which indicates the perfect structural periodicity.

Strain relaxation studied by photoluminescence and by double crystal X-ray diffraction measurements in strained InGaAs

Photoluminescence (PL) and double crystal X-ray diffraction (DCXD) experiments have been carried out on strained In x Ga 1 − x As grown on InP substrate by molecular beam epitaxy with different epilayer thicknesses, in order to study lattice relaxation. Samples with tensile strain ( x In = 0.508 ) and compressive strain ( x In = 0.543 ) have been investigated. Strain was measured by DCXD in conjunction with detailed rocking curve analysis within a dynamical framework. From PL results, we have evaluated the strain values in two different ways. First, we used the PL transition shift induced by strain and second, from PL temperature dependence we measured the heavy-hole-light-hole splitting, which is a direct measurement of strain. Our results have shown good agreement between PL and DCXD measurements and also the existence of residual strain even in samples with a thickness considerably greater than the critical layer thickness.

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.

In situ metalorganic vapor phase epitaxy control of GaAs/AlAs Bragg reflectors by laser reflectometry at 514 nm

In situ reflectometry with a 514-nm laser beam was used to monitor AlAs and GaAs layer thicknesses grown by metalorganic vapor phase epitaxy. The effective optical indices of these materials have been calibrated at the growth temperature by using an original method based on ex situ double crystal x-ray diffraction measurement. According to these measured indices, the in situ laser reflectometry at 514 nm appears to be well suited for a real-time thickness control of the GaAs/AlAs based Bragg reflectors. Finally, Bragg reflectors centered at 980 nm have been grown using the reflectometry at 514 nm. X-ray diffraction and reflectivity measurements performed on these reflectors confirm a 1% reproducibility and accuracy of the wavelength stop band center.