HRXRD Measurements Research Articles

Doping induced lattice misfit in 4H–SiC homoepitaxy

The impact of doping on the lattice constants of 4H–silicon carbide (4H–SiC) is an important material aspect influencing several steps of material and device production. Dopant incorporation in 4H–SiC causes misfit between the highly N-doped substrate and differently doped epilayers and hence, wafer bowing and the existence of a critical epilayer thickness. In this paper, the wafer bow is determined by geometrical measurements of the substrate prior to and after the epitaxial growth of single epilayers with different epilayer thicknesses and doping states, i.e. with nitrogen (N) or aluminum (Al) doping and different dopant concentrations. The misfit between substrate and epilayer is deduced from these bow measurements based on a model by Stoney. For highly Al-doped epilayers grown on highly N-doped substrates, the misfit is determined additionally by HRXRD measurements. The doping-dependent misfit, obtained from bow and HRXRD measurements, is compared to theoretical values calculated based on a model by Jacobson. For Al-doped epilayers, the experimental and theoretical values of misfit agree well. For N-doped epilayers, an effective covalent radius of N in 4H–SiC of 66 pm has to be introduced to match the theoretical misfit to the experimental one. The critical epilayer thickness is calculated based on the models by Matthews and Blakeslee and by People and Bean. The comparison of calculated values and experimental findings with respect to dislocation behavior at the substrate–epilayer interface and the dislocation densities of epilayers proves that the model by Matthews and Blakeslee underestimates the critical epilayer thickness for 4H–SiC homoepitaxy.

Microstructural characterization of GaAs-AlxGa1-xAs core-shell nanowires grown by Au-catalyst assisted MOVPE

ABSTRACTIn this work, we report on the microstructural and morphological characterization of III-V semiconductor nanowires (NWs) epitaxially grown on (111)B-GaAs substrates by Au-catalyst assisted metalorganic vapor phase epitaxy. As-grown dense (108-109 cm-2) arrays of few-micron long vertically-aligned (i.e. parallel to the <111> crystallographic axis) GaAs, AlxGa1-xAs and core-shell GaAs-AlxGa1-xAs NWs were investigated, carrying out HRXRD measurements on different (hkl) reflections and by recording reciprocal space maps (RSMs) around the materials (111) reciprocal lattice points (relps). We show that NW diffraction peaks are visible in the RSM by means of characteristic halos. In the case of GaAs NWs, the halo is located at the (111) relp indicating that the NWs are grown along the <111> direction and parallel to the <111> axis of the GaAs substrate. On the contrary, for AlxGa1-xAs NWs or intentional core-shell GaAs-AlxGa1-xAs NWs the halo is displaced (along the momentum transfer normal to the surface, Qz) with respect to the GaAs (111) relp due to the elastic lattice strain associated with the compositional variation, e.g. the Al molar fraction in the AlxGa1-xAs alloy, within the nanostructures.

Effect of growth pressure on coalescence thickness and crystal quality of GaN deposited on 4H–SiC

The influence of growth pressure on the coalescence thickness and the crystal quality of GaN deposited on 4H–SiC by low pressure metalorganic vapor phase epitaxy was studied. It was shown that growth pressure has an impact on the surface roughness of epilayers and their crystal quality. GaN coalescence thicknesses were determined for the investigated growth pressures. The GaN layers were characterized by AFM and HRXRD measurements. HEMT structures were also fabricated and characterized. Among the growth pressures studied, 50, 125 and 200 mbar, 200 mbar was found to be most suitable for GaN/SiC epitaxy.

“Step‐graded interlayers” for the improvement of MOVPE InxGa1‐xN (x ∼ 0.4) epi‐layer quality

AbstractThis paper reports the effect of step‐graded interlayers on MOVPE InGaN epi‐layer quality. Multilayer epitaxial structures consisting of Inx Ga1–xN layers with various compositions have been successfully grown. HRXRD measurements reveal that the insertion of step‐graded interlayers significantly reduces the twist while little effects are observed on tilt. Decreasing of twist with increasing the number of interlayers is found to be remarkable up to a certain value and then maintain nearly the same even with increasing the In composition. Different types of dislocation density have also been studied using the X‐ray diffraction analysis. Dislocation densities, particularly the edge dislocation density, decrease considerably with the insertion of interlayers for higher In composition. An edge dislocation of 3 × 1010 cm–2 is obtained for a smple with 0.25 In composition, that seems to come from the GaN layer beneath the step‐graded interlayers (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of thermal annealing on the properties of narrow-bandgap ZnSnAs2 epitaxial films on InP(001) substrates

ZnSnAs2 epitaxial film has been grown on epi-ready semi-insulating InP(001) substrates by low-temperature molecular beam epitaxy (LT-MBE) technique. The MBE-grown sample was then cleaved into pieces, three of which were subjected to lowtemperature annealing at different temperatures of 300 ∘C, 320 ∘C and 340 ∘C which are equal or slightly higher than growth temperature using face-to-face proximity capping by GaAs wafers to simulate arsenic atmosphere. HR-XRD measurements showed that increasing annealing temperature decreases the lattice constant towards the bulk value. This suggests that indeed the relatively higher lattice constant of ZnSnAs2 epitaxial films is partly, if not wholly, due to defects consequence of low temperature growth. For the as-grown control sample, resistivity of 4.31×10−2 Ω cm, mobility of 17.7 cm2/V −s and hole concentration of 8.18×1018 cm−3 were obtained at room temperature. After annealing at 340 ∘C, the resistivity was increased to 21.0×10−2 Ω cm, the mobility increased to 60.9 cm2/V −s, and the hole concentration was decreased to 4.88×1017 cm−3.

Structural Characterization of GaN Epitaxial Layers Grown on 4H-SiC Substrates with Different Off-Cut

The influence of surface preparation of 4H-SiC substrates on structural properties of GaN grown by low pressure metalorganic vapour phase epitaxy was studied. Substrate etching has an impact on the crystallographic structure of epilayers and improves its crystal quality. The GaN layers were characterized by RBS/channelling and HRXRD measurements. It was observed that on-axis 4H-SiC is most suitable for GaN epitaxy and that substrate etching improves the crystal quality of epilayer.

Modifications of Defects Concentration Induced by Ammonia Flow Rate and its Effects on Gallium Nitride Grown by MOCVD

AbstractOptical and schottky diode characteristics of unintentionally doped GaN films grown by MOCVD were reported. GaN epilayers were grown with different V/III ratio by varying the source ammonia (NH3) flowrate. It exhibit changes in the density of threading dislocations (TDs) and reduced carbon and oxygen impurity incorporation. The density of dislocations determined from hot-wet chemical etching and atomic force microscopy show that on decreasing the ammonia flowrate, threading dislocations decreases. Low energy positron beam was employed to study the Ga vacancies in the epilayers. S-parameter vs. positron beam energy curves clearly shows increase in SL on increasing the V/III ratio indicating that the point defects trapping positron increases. Corroborative HRXRD, Photoluminescence and Hall measurements confirm the reduction in trapping defects and threading edge dislocations with reducing V/III molar ratio. The effects of such variation of compensating centres and radiative centres as a function of MOCVD growth conditions on optical properties and schottky device characteristics like radiative decay lifetime, barrier height and reverse leakage current respectively were discussed.

Growth and properties of Al-rich InxAl1−xN ternary alloy grown on GaN template by metalorganic chemical vapour deposition

An Al-rich InxAl1−xN ternary alloy was grown on a GaN template by metal–organic chemical vapour deposition (MOCVD). The GaN template was fabricated on a c-plane sapphire with a low temperature GaN nucleation layer. The growth of the 300 nm thick InxAl1−xN layer was carried out under various growth temperatures and pressures. The surface morphology and the InN molar fraction of the InxAl1−xN layer were assessed by using atomic force microscopy (AFM) and high resolution x-ray diffraction, respectively. The AFM surface images of the InxAl1−xN ternary alloy exhibited quantum dot-like grains caused by the 3D island growth mode. The grains, however, disappeared rapidly by increasing diffusion length and mobility of the Al adatoms with increasing growth temperature and the full width at half maximum value of ternary peaks in HR-XRD decreased with decreasing growth pressure. The MOCVD growth condition with the increased growth temperature and decreased growth pressure would be effective to grow the InxAl1−xN ternary alloy with a smooth surface and improved quality. The optical band edge of InxAl1−xN ternary alloys was estimated by optical absorbance and, based on the results of HR-XRD and optical absorbance measurements, we obtained the bowing parameter of the InxAl1−xN ternary alloy at b = 5.3 eV, which was slightly larger than that of previous reports.

CBE growth of high‐quality ZnO epitaxial layers

AbstractFurther improvements on the recently reported novel approach to zinc oxide Chemical Beam Epitaxy (CBE) are presented. Hydrogen peroxide is employed as a very efficient novel oxidant. ZnO layers with a thickness from 100 nm to 600 nm were grown on c‐sapphire using a MgO buffer. PL‐mapping as well as conductivity mapping shows a good uniformity across the 2 inch ZnO‐on‐sapphire epiwafers. The measured surface roughness for the best layers is as low as 0.26 nm. HRXRD measurements of the obtained ZnO layers show excellent quality of the single crystalline ZnO. The FWHM of the HRXRD (0002) rocking curves measured for the 2 inch ZnO‐on‐sapphire wafers is as low as 27 arcsec with a very high lateral homogeneity across the whole wafer. Plane view HRTEM observations reveal the very good quality of the ZnO films. The results indicate that CBE is a suitable technique to fabricate ZnO of very high structural quality, which can eventually be used as an alternative to bulk ZnO substrates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characterization of InAlP/InGaAs double barrier RTDs

The material system InAlP/InGaAs is of interest especially due to the high conduction band discontinuity at the heterojunction. The composition of In 0.67Al 0.33P results in a bandgap of E G,InAlP=2.01 eV. In this work we present a double barrier In 0.67Al 0.33P/In 0.55Ga 0.45As/In 0.67Al 0.33P resonant tunnelling diode. The device function is very sensitive to interface effects like roughness, abruptness and homogeneity. The growth of this material system is challenging because of the large lattice mismatch of the Al containing material on an InP substrate. A fully non-gaseous source LP-MOVPE configuration is used for the growth. The resonant tunnelling diode structures are characterized by HRXRD measurements, which are compared to simulation result, and dc measurement. A high peak current density of S p=3×10 5 A/cm 2 at V Peak=1.3 V with a peak-to-valley current ratio PVR=1.2 has been achieved. Both, the symmetric behaviour of the dc-measurement and the X-ray results confirm a high quality of the highly strained InAlP barrier layers and abrupt interfaces to InGaAs.

Chemical beam epitaxial growth of AlInAs and investigations of electrolytes for ECV profiling

ALInAs has been grown by CBE technique for ultra-fast switching device applications. The materials were grown in the temperature range of 693–803 K and a low growth rate of 0.3 μm/h was employed in order to enhance the surface ad-atom residence time. The grown materials have excellent crystallographic and optical properties as studied by HRXRD and low temperature photoluminescence (LT PL) measurements. There is no suitable electrolyte available to successfully profile the dopants in AlInAs by ECV method. An effort has been made to develop an appropriate electrolyte for profiling AlInAs material. A number of electrolytic solutions were investigated and among the probed solutions, two different electrolytes namely HNO 3:H 2O 2::HF:H 2O and NH 4F have been realised. Both have very low electrolytic resistance and a low free chemical etch rate values with good I– V and C– V characteristics. Using these electrolytes, the estimated carrier concentration value were in very good agreement with independently measured HALL effect measurement values. But the ECV measured thickness value does not agree well with that of stylus profiler measurement thickness value. This discrepancy has been explained on the basis of etching properties of aluminium containing compounds during electrochemical etching process.

MOCVD Growth of ZnO for Optoelectronic Applications

In this paper we report on the metal-organic chemical vapour deposition of ZnO layers. We focus on heteroepitaxial growth on c-plane Al2O3 and the influence of the VI/II ratio during growth on the properties of the layers. The layer quality has been investigated by HRXRD, PL and reflectivity measurements. Under optimized growth conditions the photoluminescence is dominated by strong near band edge emission lines with half widths below 4 meV and the excitonic signals are clearly visible in reflectivity measurements. Hall effect measurements indicate an n-type background doping in the 1017 cm—3 range, and mobilities of more than 100 cm2/Vs can be reached. Moreover, a post-growth annealing step is found to improve the quality of the layers grown under suboptimal conditions. The problem of strain will be addressed and a tensile strain can be confirmed in the heteroepitaxial ZnO layers.

InAlAs/InGaAs/InP heterostructures for RTD and HBT device applications grown by LP-MOVPE using non-gaseous sources

We report on the applicability of a fully non-gaseous source (ngs) configuration for MOVPE growth of InAlAs/InGaAs/InP heterostructures. The ngs configuration is based on TBAs/TBP/ TMAs as group-V precursors and DitBuSi/CBr 4 as dopant sources. Conventional group-III metalorganics and nitrogen-carrier gas are used. InAlAs/InGaAs RTD layer structures combined with carbon-doped InP/InGaAs HBT layers are characterized in detail by HRXRD measurements and simulations. The InAlAs layers exhibit n-type background concentration of n=1.5×10 16 cm −3, N=1.7×10 17 cm −3 determined by Hall and CV-measurements, respectively. Abrupt InAlAs/InGaAs heterojunction interfaces are deduced from the X-ray characterization. Fabricated RTDs exhibit a high peak current density of S p>1×10 5 A/cm 2 and a highly symmetric I– V characteristics. The RTD/HBT combination devices exhibit state-of-the-art device performance like a DC-current gain of B>1 0 0 and a transit frequency of f T=39 GHz.

Evaluation of strain distribution in freestanding and buried lateral nanostructures

A free-standing lateral nanostructure based on GaAs[001] containing a ${\mathrm{Ga}}_{0.97}{\mathrm{In}}_{0.03}\mathrm{As}$ single quantum well and similar structures after the overgrowth with GaAs and AlAs, respectively, have been investigated by high-resolution x-ray grazing incidence diffraction (GID) and conventional x-ray diffraction (HRXRD). The wire shape of the freestanding structure and the lateral density variation in the overgrown samples, were determined by running scans with constant length of the scattering vector (transverse scans) across the grating truncation rods (GTR's) close to the $(2\ifmmode\bar\else\textasciimacron\fi{}20)$ reflection. The in-plane strain distribution became available crossing the $(220)$ GTR's by a scan in the longitudinal direction. Exploiting the capability of GID for depth resolution, the in-plane strain distribution was analyzed for different values of depth below the sample surface. The strain analysis was completed by HRXRD measurements close to the (004) reflection. The x-ray measurements were interpreted in terms of the distorted wave Born approximation applied for GID geometry. The strain distribution is determined by comparing the measured GTR intensities with the corresponding simulations containing the displacement fields obtained from finite-element calculations. At the freestanding wire structure we find laterally compressive strain of about $\ensuremath{\Delta}{a/a}_{\ensuremath{\Vert}}=\ensuremath{-}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at the single quantum well (SQW) with a steep strain gradient close to the wire side walls. Both overgrown samples show pronounced lateral strain variation within the overgrown layer, which still appears up to the completely planar surface. Within the SQW the in-plane strain is still compressive after GaAs overgrowth and of similar amount compared to the freestanding grating. The strain is increased by about 30% after overgrowth with AlAs. For both overgrown samples the strain gradient near the wire side walls is reduced, but reaches a maximum close to the SQW. Accompanied by the defect passivation, these findings explain the difference in the energy shift of the photoluminescence line between freestanding and overgrown lateral nanostructures.

Carbon delta doping in chemical beam epitaxy using CBr4

Abstract We describe the growth of carbon δ-doped GaAs by chemical beam epitaxy (CBE) using CBr 4 as the dopant during growth interrupts. Characterisation of the δ-doped samples using secondary ion mass spectrometry (SIMS) at a series of impact energies showed that the C was confined to a planar sheet less than 1 nm thick. This is in agreement with HRXRD measurements on C δ-doping superlattices. The results of SIMS profiling of layers with a range of interrupt times indicate that there is an initial surface coverage of C from CBr 4 of approximately 9 × 10 12 cm −2 which increases relatively slowly during an extended interrupt.

Effect of Oxygen Dose Variation on the Simox Microstructure

AbstractThe effect of oxygen dose variation on the SIMOX SOI structure has been investigated extensively in this work. Keeping other implantation parameters constant, the dose has been varied from 0.2x1018cm–2 to 1.4xl018cm–2. Our studies show that a critical dose exists for a particular implant energy, below which a continuous buried oxide layer does not form. Further, the SiO2 precipitates enhance the pinning of defects in the as-implanted state which subsequently grow into threading dislocations. A dose window exists, where the dislocation density is a minimum.The oxygen depth distribution in the as-implanted and annealed state has been studied using SIMS. HRXRD measurements have been performed to reveal the strain state as a function of dose variation. The defect microstructure has been investigated using PTEM, XTEM and Secco Etching. AFM studies show the quality of the Si surface and Si/BOX (Buried OXide) interface roughness. This work gives us a fundamental understanding of the evolution of the defect microstructure and strain from the as-implanted to the annealed state using independent implantation parameter control.