Growth and characterization of GaN and AlN films on (1 1 1) and (0 0 1) Si substrates

GaN and AlN films were grown on (1 1 1) and (0 0 1) Si substrates by separate admittances of trimethylgallium (or trimethylaluminum) and ammonia (NH₃) at 1000°C. A high temperature (HT) or low temperature (LT) grown AlN thin layer was employed as the buffer layer between HT GaN (or HT AlN) film and Si substrate. Experimental results show that HT AlN and HT GaN films grown on the HT AlN-coated Si substrates exhibit better crystalline quality than those deposited on the LT AlN-coated Si substrates. Transmission electron microscopy (TEM) of the HT GaN/HT AlN buffer layer/(1 1 1)Si samples shows a particular orientation relationship between the (0 0 0 1) planes of GaN film and the (1 1 1) planes of Si substrate. High quality HT GaN films were achieved on (1 1 1) Si substrates using a 200 Å thick HT AlN buffer layer. Room temperature photoluminescence spectra of the high quality HT GaN films show strong near band edge luminescence at 3.41 eV with an emission linewidth of ∼110 meV and weak yellow luminescence.     

CrO2 (1 0 0) and TiO2 (1 0 0) film heteroepitaxy on a BaF2 (1 1 1)/Si (1 0 0) substrate

Multi-domained heteroepitaxial rutile-phase TiO₂ (1 0 0)-oriented films were grown on Si (1 0 0) substrates by using a 30-nm-thick BaF₂ (1 1 1) buffer layer at the TiO₂–Si interface. The 50 nm TiO₂ films were grown by electron cyclotron resonance oxygen plasma-assisted electron beam evaporation of a titanium source, and the growth temperature was varied from 300 to 600 °C. At an optimal temperature of 500 °C, X-ray diffraction measurements show that rutile phase TiO₂ films are produced. Pole figure analysis indicates that the TiO₂ layer follows the symmetry of the BaF₂ surface mesh, and consists of six (1 0 0)-oriented domains separated by 30° in-plane rotations about the TiO₂ [1 0 0] axis. The in-plane alignment between the TiO₂ and BaF₂ films is oriented as [0 0 1] TiO₂ || BaF₂ or [0 0 1] TiO₂ || BaF₂ . Rocking curve and STM analyses suggest that the TiO₂ films are more finely grained than the BaF₂ film. STM imaging also reveals that the TiO₂ surface has morphological features consistent with the BaF₂ surface mesh symmetry. One of the optimally grown TiO₂ (1 0 0) films was used to template a CrO₂ (1 0 0) film which was grown via chemical vapor deposition. Point contact Andreev reflection measurements indicate that the CrO₂ film was approximately 70% spin polarized.     

Growth of highly oriented crystalline α-Fe/AlN/Fe3N trilayer structures on Si(1 1 1) substrates by molecular beam epitaxy

We have realized highly oriented nitride-based α-Fe/AlN/Fe₃N ferromagnetic hybrid structures on Si(1 1 1) substrates by molecular beam epitaxy using AlN/SiC intermediate layers. A two-step hysteresis loop, typically observed in magnetic tunneling junctions, was clearly observed in magnetization versus magnetic field measurements. This result indicates the formation of ferromagnetic α-Fe and Fe₃N layers separated by 8-nm-thick AlN layers over approximately 1 cm² area, and also shows the difference in coercive field between the two ferromagnetic layers.     

Fabrication of 2D ordered arrays of cobalt silicide nanodots on (0 0 1)Si substrates

Two-dimensional (2D) periodic arrays of Co metal and Co silicide nanodots were successfully fabricated on (0 0 1)Si substrate by using the polystyrene (PS) nanosphere lithography (NSL) technique and thermal annealing. The epitaxial CoSi₂ was found to start growing in samples after annealing at 500 °C. The sizes of the Co silicide nanodots were observed to shrink with annealing temperature. From the analysis of the selected-area electron diffraction (SAED) patterns, the crystallographic relationship between the epitaxial CoSi₂ nanodots and (0 0 1)Si substrates was identified to be [0 0 1]CoSi₂//[0 0 1]Si and (2 0 0)CoSi₂//(4 0 0)Si. By combining the planview and cross-sectional TEM examination, the epitaxial CoSi₂ nanodots formed on (0 0 1)Si were found to be heavily faceted and the shape of the faceted epitaxial CoSi₂ nanodot was identified to be inverse pyramidal. The observed results present the exciting prospect that with appropriate controls, the PS NSL technique promises to offer an effective and economical patterning method for the growth of a variety of large-area periodic arrays of uniform metal and silicide nanostructures on different types of silicon substrates.     

Enhanced two-dimensional growth of MOVPE InN films on sapphire (0 0 0 1) substrates

MOVPE growth of InN on sapphire substrates is compared using two different designs of horizontal reactor. The major difference between the two designs is a variation in the reactant-gas flow-spacing between the substrate and the ceiling of the quartz chamber: 33 mm for the Type A reactor and 14 mm for Type B. Compared with the Type A reactor, the Type B reactor brings about InN films with a larger grain size. This is especially true when InN is grown at 600°C using the Type B reactor, in which case the two-dimensional (2D) growth of InN is found to be extremely enhanced. An investigation of the NH₃/TMIn molar ratio dependence of the surface morphology of grown InN films using the two reactors suggests that the enhanced 2D growth is attributed to the decrease in the effective NH₃/TMIn ratio in the growth atmosphere. Even using the Type A reactor, a film with enhanced 2D growth can be obtained when the NH₃/TMIn ratio is considerably low (1.8×10⁴). The enhanced 2D growth results in a smaller XRC-FWHM (full-width at half maximum of the X-ray rocking curve) (1500 arcsec), than that for a 3D-grown film (5000 arcsec).     

Growth of GaN based structures on Si(1 1 0) by molecular beam epitaxy

The growth of GaN based structures on Si(1 1 0) substrates by molecular beam epitaxy using ammonia as the nitrogen precursor is reported. The structural, optical and electrical properties of such structures are assessed and are quite similar to the ones obtained on Si(1 1 1) in-spite of the very different substrate surface symmetry. A threading dislocation density of 3.7×10⁹ cm⁻² is evaluated by transmission electron microscopy, which is in the low range of typical densities obtained on up to 2 μm thick GaN structures grown on Si(1 1 1). To assess the potential of such structure for device realization, AlGaN/GaN high electron mobility transistor and InGaN/GaN light emitting diode heterostructures were grown and their properties are compared with the ones obtained on Si(1 1 1).     

Heteroepitaxy of CdTe on tilting Si(2 1 1) substrates by molecular beam epitaxy

CdTe(2 1 1)B epilayers were grown on 3 in Si(2 1 1) substrates which misoriented 0–10° toward [1 1 1] by molecular beam epitaxy (MBE). The relationship of X-ray double-crystal rocking curve (XRDCRC) FWHM and deflection angle from CdTe(2 1 1) to Si(2 1 1) was studied. For 4.2–4.5 μm CdTe, the best value of FWHM 83 arcsec was achieved while deflection angle is 2.76°. A FWHM wafer mapping indicated a good crystalline uniformity of 7.4 μm CdTe on tilting Si(2 1 1), with FWHM range of 60–72 arcsec. The shear strains of these epilayers were analyzed, using reciprocal lattice points of symmetric and asymmetric reflections measured by high-resolution multi-crystal multi-reflection X-ray diffractometer (HRMCMRXD). It was found that the shear strain angle of epilayer is effectively reduced by using proper tilting Si(2 1 1) substrate. It was also proved that the lattice parameter of CdTe(2 1 1)B is affected by the shear strain and thermal strain.     

Polarities of AlN films and underlying 3C-SiC intermediate layers grown on (1 1 1) Si substrates

The hydride vapor phase epitaxy (HVPE) of {0 0 0 1} AlN films on {1 1 1} Si substrates covered with epitaxial {1 1 1} cubic SiC (3C-SiC intermediate layers) was carried out. 3C-SiC intermediate layers are essential to obtain high-quality AlN films on Si substrates, because specular AlN films are obtained with 3C-SiC intermediate layers, whereas rough AlN films are obtained without 3C-SiC intermediate layers. We determined the polarities of AlN films and the underlying 3C-SiC intermediate layers by convergent beam electron diffraction (CBED) using transmission electron microscopy. For the first time, the polarities of the AlN films and the 3C-SiC intermediate layers were determined as Al and Si polarities, respectively. The AlN films were hardly etched by aqueous KOH solution, thereby indicating Al polarity. This supports the results obtained by CBED. The result is also consistent with electrostatic arguments. An interfacial structure was proposed. The 3C-SiC intermediate layers are promising for the HVPE of AlN films on Si substrates.     

Defect reduction in GaN epilayers and HFET structures grown on (0 0 0 1)sapphire by ammonia MBE

The quality of GaN epilayers grown by molecular beam epitaxy on substrates such as sapphire and silicon carbide has improved considerably over the past few years and in fact now produces AlGaN/GaN HEMT devices with characteristics among the best reported for any growth technique. However, only recently has the bulk defect density of MBE grown GaN achieved levels comparable to that obtained by MOVPE and with a comparable level of electrical performance. In this paper, we report the ammonia-MBE growth of GaN epilayers and HFET structures on (0 0 0 1)sapphire. The effect of growth temperature on the defect density of single GaN layers and the effect of an insulating carbon doped layer on the defect density of an overgrown channel layer in the HFET structures is reported. The quality of the epilayers has been studied using Hall effect and the defect density using TEM, SEM and wet etching. The growth of an insulating carbon-doped buffer layer followed by an undoped GaN channel layer results in a defect density in the channel layer of 2×10⁸ cm⁻². Mobilities close to 490 cm²/Vs at a carrier density of 8×10¹⁶ cm⁻³ for a 0.4 μm thick channel layer has been observed. Growth temperature is one of the most critical parameters for achieving this low defect density both in the bulk layers and the FET structures. Photo-chemical wet etching has been used to reveal the defect structure in these layers.     

Controllable cubic and hexagonal GaN growth on GaAs(0 0 1) substrates by molecular beam epitaxy

Two kinds of GaN samples were grown on GaAs(0 0 1) substrates. One is grown on nitridized GaAs surface, the other is grown on nitridized AlAs buffer GaAs substrate. X-ray diffraction and photoluminescence measurements find that the GaN sample directly grown on GaAs substrate is pure cubic phase and those grown on AlAs buffer is pure hexagonal phase. The present study shows that the phase of GaN samples grown on GaAs substrates can be controlled using different buffer layers.     

Molecular beam epitaxy (MBE) growth and structural properties of GaN and AlN on 3C-SiC(0 0 1) substrates

AlN and GaN was deposited by molecular beam epitaxy (MBE) on 3C-SiC(0 0 1) substrates on low-temperature (LT) GaN and AlN buffer layers. It is shown that not only GaN but also epitaxial AlN can be stabilized in the metastable zincblende phase. The zincblende AlN is only obtained on a zincblende LT-GaN buffer layer; on the other hand, AlN crystallizes in the wurtzite phase if it is grown directly on a 3C-SiC(0 0 1) substrate or on a LT-AlN buffer layer. The structural properties of the layers and in particular the orientation relationship of the wurtzite AlN on the 3C-SiC(0 0 1) were analyzed by conventional and high-resolution transmission electron microscopy.     

Selective area growth of GaN microstructures on patterned (1 1 1) and (0 0 1) Si substrates

Selective MOVPE growth of GaN microstructure on silicon substrates has been investigated using SiO₂ mask having circular or stripe window. In case of (0 0 1)substrate, grooves with (1 1 1) facets at the sides were made by using the etching anisotropy of a KOH solution. On the (1 1 1) facets of patterned silicon substrate (or on the as opened window region of (1 1 1) substrate), growth of wurtzite GaN was performed, of which the c-axis is oriented along the 1 1 1 axis of silicon. The photoluminescence and X-ray diffraction analysis were performed to characterize the single crystal to reveal the effect of the growth conditions of the intermediated layer and the microstructure.     

Heteroepitaxy of InSb films grown on a Si(0 0 1) substrate with AlSb buffer layer

The growth of InSb films on a Si(0 0 1) substrate with AlSb buffer layer was performed in an ultra high vacuum chamber (UHV) by a co-evaporation of elemental Indium (In) and antimony (Sb) sources. The samples were characterized by Auger electron spectroscopy (AES), X-ray diffraction (XRD) and atomic force microscopy (AFM). The surface morphology and the crystal quality of the grown films strongly depend on the flux ratio of Sb/In. It is found that the optimized flux ratio for the one-step growth procedure is about 2.9 to obtain the InSb films with smooth surface and good crystal quality, for the growth temperature of 300 °C. The two-step growth procedure was also used to further improve the crystal quality of the films.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Hexagonal AlN films grown on nominal and off-axis Si(0 0 1) substrates

Nucleation and growth of wurtzite AlN layers on nominal and off-axis Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy is reported. The nucleation and the growth dynamics have been studied in situ by reflection high-energy electron diffraction. For the films grown on the nominal Si(0 0 1) surface, cross-sectional transmission electron microscopy and X-ray diffraction investigations revealed a two-domain film structure (AlN¹ and AlN²) with an epitaxial orientation relationship of [0 0 0 1]_AlN || [0 0 1]_Si and AlN¹ || AlN² || [1 1 0]_Si. The epitaxial growth of single crystalline wurtzite AlN thin films has been achieved on off-axis Si(0 0 1) substrates with an epitaxial orientation relationship of [0 0 0 1]_AlN parallel to the surface normal and 0 1 1 0_AlN || [1 1 0]_Si.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)_ZnO‖(1 1 1)_Si and [1 1 2¯ 0]_ZnO‖[0 1 1]_Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10⁹ cm⁻². Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.