GaP Epilayers Research Articles

Gallium phosphide conformal film growth on in-situ tri-TBP dry-cleaned InGaP/GaAs using atomic hydrogen ALD

Nearly stoichiometric (high P/Ga ratio and low oxygen content) ∼ 20 nm GaP epi-layers were conformally deposited on the side-walls of InGaP layers using an atomic H ALD process with triethyl gallium (TEGa) and tri-tertiary butyl phosphine (tri-TBP). The key for uniform nucleation is an in-situ tri-TBP dry-clean in 40 mTorr Ar and 160 mTorr H2 without acidic wet-clean to generate a particle-free and damage-free InGaP surface which induces local epitaxy growth of homogeneous GaP thin films. The in-situ tri-TBP dry-clean removed the native oxide of InGaP, surface particles, and the residual impurities simultaneously. It is hypothesized that the key to the process is in-situ formation of PHx(C4H9)y surface adsorbates from atomic H and tri-TBP surface reactions. In-situ AES, cross-sectional STEM image, ex-situ XPS, AFM, and EDS confirmed the formation of high-quality GaP thin films on InGaP/GaAs with a high-quality interface and smooth surface morphology.

Interfacial dynamics of GaP/Si(100) heterostructure grown by molecular beam epitaxy

The atomic structure and surface chemistry of GaP/Si(100) heterostructure with different pre-layers grown by molecular beam epitaxy are studied. It is found that GaP epilayer with Ga-riched pre-layers on Si(100) substrate has regular surface morphology and stoichiometric abrupt heterointerfaces from atomic force microscopes (AFMs) and spherical aberration-corrected transmission electron microscopes (ACTEMs). The interfacial dynamics of GaP/Si(100) heterostructure is investigated by X-ray photoelectron spectroscopy (XPS) equipped with an Ar gas cluster ion beam, indicating that Ga pre-layers can lower the interface formation energy and the bond that is formed is more stable. These results suggest that Ga-riched pre-layers are more conducive to the GaP nucleation as well as the epitaxial growth of GaP material on Si(100) substrate.

Structure and Origin of Antiphase Domains and Related Defects in Thin GaP Epilayers on As-Modified Si(100)

Structure and Origin of Antiphase Domains and Related Defects in Thin GaP Epilayers on As-Modified Si(100)

Effect of germanium auto-diffusion on the bond lengths of Ga and P atoms in GaP/Ge(111) investigated by using X-ray absorption spectroscopy.

The germanium auto-diffusion effects on the inter-atomic distance between the nearest neighbors of the Ga atom in GaP epilayers are investigated using high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy. The GaP layers grown on Ge (111) are structurally coherent and relaxed but they show the presence of residual strain which is attributed to the auto-diffusion of Ge from the results of secondary ion mass spectrometry and electrochemical capacitance voltage measurements. Subsequently, the inter-atomic distances between the nearest neighbors of Ga atom in GaP are determined from X-ray absorption fine-structure spectra performed at the Ga K-edge. The estimated local bond lengths of Ga with its first and second nearest neighbors show asymmetric variation for the in-plane and out-of-plane direction of GaP/Ge(111). The magnitude and direction of in-plane and out-of-plane microscopic residual strain present in the GaP/Ge are calculated from the difference in bond lengths which explains the presence of macroscopic residual tensile strain estimated from HRXRD. Modified nearest neighbor configurations of Ga in the auto-diffused GaP epilayer are proposed for new possibilities within the GaP/Ge hetero-structure, such as the conversion from indirect to direct band structures and engineering the tensile strain quantum dot structures on (111) surfaces.

GaAsP/Si tandem solar cells: In situ study on GaP/Si:As virtual substrate preparation

Realization of high efficiency III-V-on-Si solar cells requires preparation of an oxygen free Si substrate surface, which allows for subsequent antiphase domain free III-V epitaxy. For GaAsP/Si tandem cells, the impact of As on established Si processing in metal organic vapor phase epitaxy (MOVPE) is essential. Here, we study the interaction of As with vicinal Si(100) surfaces, the formation of atomically well-ordered As-modified Si(100) surfaces, and its impact on subsequently grown GaP epilayers. We apply optical in situ spectroscopy during MOVPE and show that process routes strongly affect the formation of double-layer steps on Si(100). We demonstrate that exposure to As, together with HF pre-treatment, enables processing of suitable Si(100) 2° and 6° offcut surfaces at temperatures below 820°C. Subsequently grown GaP epilayers exhibit smooth surfaces free of antiphase disorder and distinct Laue oscillations in X-ray diffraction. Such low temperature processing, which is controllable in situ, is promising for further metamorphic GaAsP integration.

Suppression of Rotational Twin Formation in Virtual GaP/Si(111) Substrates for III–V Nanowire Growth

Planar GaP epilayers on Si(111) are considered as virtual substrates for III–V-related optoelectronic devices such as high-efficiency nanowire-based tandem absorber structures for solar energy conversion, next generation LEDs, and fast photodetectors. Rotational twin domains in such heteroepitaxial epilayers are found to strongly impede vertical nanowire growth. We investigate the twin-induced defect density and surface morphology of B-type GaP/Si(111) virtual substrates in dependence on the GaP nucleation process by metalorganic chemical vapor deposition. By employing quantitative high-resolution X-ray diffraction (HR-XRD)), scanning electron and atomic force microscopy (SEM and AFM), we reveal the significant influence of nucleation temperature and substrate miscut direction on the formation of rotational twin domains during a two-step GaP growth approach. The epilayer defect density is drastically decreased by low temperature GaP nucleation on Si(111) misoriented 3° toward [112], where rotational twi...

Observation of anisotropic distribution of microstructure in GaP/GaAs epitaxial layers

Anisotropic distribution of microstructure in GaP/GaAs epitaxial layers is investigated by performing high resolution x-ray diffraction (HRXRD) experiments for symmetric reflections at different azimuths. The observed anisotropy leads to the different values of lattice relaxation along the [011¯] and [01¯1¯] directions as revealed by the reciprocal space maps recorded at 0° and 90° azimuths for (42¯2¯) and (422¯) reflections. The anisotropic relaxation process causes large differences in the full width at half maximum of (400) diffraction peaks of omega scans along the [011¯] and [01¯1¯] directions. It therefore provides large differences in the values of microstructural parameters of the GaP epitaxial layer along the two in-plane orthogonal directions. A systematic Williamson-Hall analysis of HRXRD data confirms the presence of large anisotropy of lattice relaxation process along the [011¯] and [01¯1¯] directions. Due to the large lattice mismatch, the GaP/GaAs epilayer presented here possesses grain boundaries and the broadening of the HRXRD pattern is strongly affected by the anisotropy of microstructure and dislocations. The kinetics of dislocations is discussed where it is observed that the misfit strain is primarily relieved by 90° partial dislocations up to a certain thickness of epilayer and thereafter the nucleation of 60° perfect dislocations dominates the strain relaxation process. The effect of the anisotropic distribution of relaxation process is also seen in the surface topography of GaP epilayer as revealed by atomic force microscopy images. The origin of the anisotropic distribution of dislocations is explained by considering the Burgers vector network, and its consequences are also observed in the cross sectional transmission electron microscopy images.

Surface induced magnetization reversal of MnP nanoclusters embedded in GaP

We investigate the quasi-static magnetic behavior of ensembles of ferromagnetic nanoparticles consisting of MnP nanoclusters embedded in GaP(001) epilayers grown at 600, 650, and 700 °C. We use a phenomenological model, in which surface effects are included, to reproduce the experimental hysteresis curves measured as a function of temperature (120–260 K) and direction of the applied field. The slope of the hysteresis curve during magnetization reversal is determined by the MnP nanoclusters size distribution, which is a function of the growth temperature. Our results show that the coercive field is very sensitive to the strength of the surface anisotropy, which reduces the energy barrier between the two states of opposite magnetization. Notably, this reduction in the energy barrier increases by a factor of 3 as the sample temperature is lowered from 260 to 120 K.

Growth of antiphase-domain-free GaP on Si substrates by metalorganic chemical vapor deposition using an in situ AsH3 surface preparation

Antiphase-domain (APD) free GaP films were grown on Si(100) substrates prepared by annealing under dilute AsH3 in situ in an MOCVD reactor. LEED and AES surface analysis of Si(100) surfaces prepared by this treatment show that AsH3 etching quickly removes O and C contaminants at a relatively low temperature (690–740 °C), and creates a single-domain “A-type” As/Si surface reconstruction. The resulting GaP epilayers grown at the same temperature are APD-free, and could thereby serve as templates for direct growth of III-V semiconductors on Si. This single chamber process has a low thermal budget, and can enable heteroepitaxial integration of III-Vs and Si at an industrial scale.

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization.

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities - high spatial resolution and richness of microstructural data - with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

Formation of GaP/Si(100) Heterointerfaces in the Presence of Inherent Reactor Residuals.

Adequate silicon preparation is a prerequisite for defect-free III-V growth on Si. We transfer the silicon processing from clean to GaP containing metalorganic vapor phase epitaxy reactors, where we monitor the entire process in situ with reflection anisotropy spectroscopy and analyze the chemical composition of the surface with X-ray photoelectron spectroscopy. Beyond a certain submonolayer threshold value of (Ga,P) residuals found on the Si(100) surface, GaP grows with an inverted majority sublattice. Analogously to III-V growth on two-domain substrates, the coexistence of Si-Ga and Si-P interfacial bonds at terraces of the same type causes antiphase disorder in GaP epilayers.

Time-Resolved In Situ Spectroscopy During Formation of the GaP/Si(100) Heterointerface.

Though III-V/Si(100) heterointerfaces are essential for future epitaxial high-performance devices, their atomic structure is an open historical question. Benchmarking of transient optical in situ spectroscopy during chemical vapor deposition to chemical analysis by X-ray photoelectron spectroscopy enables us to distinguish between formation of surfaces and of the heterointerface. A terrace-related optical anisotropy signal evolves during pulsed GaP nucleation on single-domain Si(100) surfaces. This dielectric anisotropy agrees well with the one calculated for buried GaP/Si(100) interfaces from differently thick GaP epilayers. X-ray photoelectron spectroscopy reveals a chemically shifted contribution of the P and Si emission lines, which quantitatively corresponds to one monolayer and establishes simultaneously with the nucleation-related optical in situ signal. We attribute that contribution to the existence of Si-P bonds at the buried heterointerface. During further pulsing and annealing in phosphorus ambient, dielectric anisotropies known from atomically well-ordered GaP(100) surfaces superimpose the nucleation-related optical in situ spectra.

Defects limitation in epitaxial GaP on bistepped Si surface using UHVCVD–MBE growth cluster

We report on the association of Ultra High Vaccum Chemical Vapor Deposition (UHVCVD) and Molecular Beam Epitaxy (MBE) to achieve III–V (GaP) integration on Si/Si(100) substrates. We first demonstrate that a very good flatness (0.3nm) can be obtained when growing directly GaP on a chemically prepared Si substrate. X-ray diffraction pole figure however demonstrates that a residual amount of micro-twins originating from the hetero-interface still remains. Silicon homoepitaxial buffer layer is then optimized in the UHVCVD chamber on different Si substrates misorientation (+/-0.15–6°-off) . A flat, clean and bistepped Si surface is achieved during the homoepitaxial growth on 6°-off silicon substrates. Samples are then transferred under UHV conditions to the MBE chamber to perform GaP overgrowth. Keeping the same III–V overgrowth conditions, influence of silicon homoepitaxial buffer layer on micro-twins generation is determined quantitatively using Synchrotron X-Ray Diffraction. We finally demonstrate that growing a flat, clean and bistepped silicon buffer layer on a 6°-off substrate, and transferring it under UHV to the MBE chamber for GaP overgrowth reduces significantly the amount of anisotropic defects generated in the GaP epilayers.

Ferromagnetic resonance measurements of GaP epilayers with embedded MnP nanoclusters grown on GaP(001)

The relation between crystallographic texture and magnetic properties of GaP epilayers with embedded MnP nanoclusters grown by metal-organic vapor phase epitaxy is investigated with angle-dependent ferromagnetic resonance (FMR) measurements. A systematic methodology entirely based on magnetic characterization identifies all the crystallographic orientations of the MnP nanoclusters relative to the GaP matrix as well as the volume fraction of MnP in each orientation. For all 18 crystallographic orientations of the MnP nanoclusters that were determined for samples grown at 650 ◦C, the MnP c axis is oriented along GaP 〈110〉. Our approach also reveals that approximately 90% of the clusters have their b axis oriented along GaP 〈111〉, while the remaining clusters have their b axis oriented along GaP 〈100〉. In addition, FMR indicates that the ratio of the anisotropy field along the intermediate magnetic axis over that of the hard magnetic axis of the embedded MnP clusters is increased by more than 20% compared to bulk MnP. Calculations suggest that a magnetoelastic and/or a surface contribution to the magnetic anisotropy of the nanoclusters is required to explain the observed difference. The methodology presented in this work is particularly suited for semiconducting epilayers with embedded nanomagnets but could be easily applied to other similar systems.

Plasmonic enhancement of the magneto-optical response of MnP nanoclusters embedded in GaP epilayers

We report on the magneto-optical activity of MnP nanoclusters embedded in GaP epilayers and MnP thin film as a function of temperature, magnetic field, and wavelength in the near infrared and visible. The measured Faraday rotation originates from the ferromagnetic magnetization of the metallic MnP phase and exhibits a hysteretic behavior as a function of an externally applied magnetic field closely matching that of the magnetization. The Faraday rotation spectrum of MnP shows a magnetoplasmonic resonance whose energy depends on the MnP filling factor and surrounding matrix permittivity. At resonance, the measured rotary power for the epilayer systems increases by a factor of 2 compared to that of the MnP film in terms of degrees of rotation per MnP thickness for an applied magnetic field of 410 mT. We propose an effective medium model, which qualitatively reproduces the Faraday rotation and the magnetocircular dichroism spectra, quantitatively determines the spectral shift induced by variations in the MnP volume fraction, and demonstrates the influence of the shape and orientation distributions of ellipsoidal MnP nanoclusters on the magneto-optical activity and absorption spectra.

MnP nanoclusters embedded in GaP epitaxial films grown by organometallic vapor-phase epitaxy: A reciprocal space mapping and transmission electron microscopy study

Full three dimensional x-ray diffraction reciprocal space maps combined with transmission electron microscopy measurements provide a systematic determination of the texture of GaP epilayers containing embedded MnP nanoclusters grown on GaP(001) by organometallic vapor phase epitaxy. This approach reveals that the texture of the MnP clusters depends on the growth surface morphology and bonding configuration and on the lattice mismatch at the cluster/matrix interfaces during growth. It demonstrates that the orthorhombic MnP nanoclusters are oriented along specific GaP crystallographic directions, forming six well defined families, whose population is influenced by the growth temperature and the film thickness. The clusters principally grow on GaP(001) and GaP{111} facets with a small fraction of clusters nucleating on higher-index GaP{hhl} facets. Most epitaxial alignments share a similar component: the MnP(001) plane (c-axis plane) is parallel to the GaP{110} plane family. Axiotaxial ordering between the MnP clusters and the GaP matrix is also observed. Furthermore, with this systematic approach, all phases present in these heterogeneous films can be identified. In particular, traces of hexagonal Mn2P precipitates have been observed while their formation can be avoided by lowering the growth temperature. Comparing the structural results presented here with magnetic measurement carried out on similar samples confirms that the effective magnetic properties of the heterogeneous layer can be tuned by controlling the texture of the ferromagnetic nanoclusters.

Micromechanical properties of GaP〈Dy〉 epilayers

The physicomechanical properties of GaP epilayers grown by liquid phase epitaxy from indium-based high-temperature solutions have been studied using microindentation. The results demonstrate that the growth of GaP epilayers from indium-based high-temperature solutions leads to a reduction in their microhardness, microbrittleness, and fracture toughness. The addition of a rare-earth dopant to a high-temperature solution has an ambiguous effect on the strength of the epilayers. Low rare-earth concentrations may both reduce and increase the microhardness of the epilayers, depending on epitaxy conditions. The random microhardness distribution of samples containing rare-earth inclusions has two peaks: one due to the rareearth inclusions, and the other due to the region with a relatively low lanthanide concentration.

Non-Destructive, Large-Scale Imaging of Anti-Phase Disorder in GaP Epilayers on Si(001) Using Low-Energy Electron Microscopy

We introduce low-energy electron microcopy (LEEM) as a surface-sensitive technique for the investigation of anti-phase disorder and other defects in III-V semiconductor layers on Si(100) and present first results for 40 nm GaP films on Si(100). Using an actively pumped ultra-high vacuum transfer chamber, we were able to preserve the surface structure and chemical composition of the MOVPE-grown GaP films on Si(100) during transfer from the MOVPE reaction chamber to the remote electron microscope. Applying dark- and bright-field imaging modes we demonstrate the unique potential of LEEM for non-invasive, fast and large-scale inspection of anti-phase disorder and surface defects in heteroepitaxial III-V systems on Si.

Non-Destructive, Large-Scale Imaging of Anti-Phase Disorder in GaP Epilayers on Si(001) Using Low-Energy Electron Microscopy

not Available.

MnP films and MnP nanocrystals embedded in GaP epilayers grown on GaP(001): Magnetic properties and local bonding structure

MnP nanostructures embedded in GaP epilayers, and MnP polycrystalline films, grown from the vapor phase on GaP(001) substrates using metalorganic precursors are compared with bulk MnP. We observe a large increase of the low transition temperature from the ferromagnetic to the antiferromagnetic screw phase, from TN = 47 K for bulk to 82 K for nanocrystals in MnP:GaP films, while the Curie temperature TC, close to room temperature, varies only slightly. A net magnetic moment is measured in the nanocrystals and films at 5 K, as well as large coercive fields, contrary to bulk MnP. X-ray absorption spectroscopy and diffraction show that epilayers and films contain MnP grains in the nanometric range with average Mn–P bond lengths very close to those of bulk MnP. The MnP film lattice parameters are almost identical to bulk values (within 0.5%) and the main crystallographic preferential orientations are those also present in the epilayers but with different relative populations. Overall the local structures of all MnP forms are very similar, except for indications of more disorder in the nanocrystals. Such combined changes of TN and TC are in apparent contradiction with the known response of bulk MnP to strains induced by hydrostatic, uniaxial or chemical pressure. We conclude that the differences in the low temperature magnetic behavior are most probably originated by local structural disorder at the surface of the nanostructures and by finite size effects.