The photoluminescence of ZnO thin films grown on Si (1 0 0) substrate by plasma-enhanced chemical vapor deposition

High-quality ZnO thin films have been grown on a Si(1 0 0) substrate by plasma-enhanced chemical vapor deposition (PECVD) using a zinc organic source (Zn(C₂H₅)₂) and carbon dioxide (CO₂) gas mixtures at a temperature of 180°C. A strong free exciton emission with a weak defect-band emission in the visible region is observed. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption peak in the absorption spectra, are closely related to the gas flow rate ratio of Zn(C₂H₅)₂ to CO₂. Full-widths at half-maximum of the free exciton emission as narrow as 93.4 meV have been achieved. Based on the temperature dependence of the PL spectra from 83 to 383 K, the exciton binding energy and the transition energy of free excitons at 0 K were estimated to be 59.4 meV and 3.36 eV, respectively.     

The influence of the Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.     

Epitaxial variations of Ni films grown on MgO(0 0 1)

Epitaxial Ni films were deposited on (0 0 1)MgO by DC magnetron sputtering under ultra-high vacuum conditions for studies involving magnetic-multilayer applications. The deposition temperatures of the Ni films studied in this work were 100 and 400°C. Examination by transmission electron microscopy (TEM) and electron diffraction revealed that the film deposited at the lower temperature was predominately Ni[0 0 1]MgO[0 0 1] and Ni(0 1 0)MgO(0 1 0) oriented and smooth, as expected. However, the higher temperature films were predominately of the Ni MgO[0 0 1] and Ni MgO(1 0 0) orientation and facetted. The orientation has been confirmed by X-ray diffraction, where this orientation was observed to be four-fold degenerate. For each of these four orientations there also existed a twin orientation, reflected about the MgO(1 0 0) planes, giving eight possible orientations for the Ni crystallites on MgO. This epitaxial relationship was studied by dark-field TEM and electron diffraction. Because these films were polycrystalline and hence produced many diffraction spots from both the Ni and MgO with similar lattice spacings, electron diffraction patterns of the films were indexed using an electron diffraction image processing (EDIP) technique. In this technique, the polycrystalline electron diffraction pattern was converted into a graph, with the x-axis displaying lattice spacings and the y-axis, integrated intensity.     

The growth of Pd thin films on a 6H-SiC(0 0 0 1) substrate

Pd thin films, grown on Si-rich 6H-SiC(0 0 0 1) substrates, were studied by atomic force microscopy, electron diffraction and high-resolution transmission electron microscopy. It is concluded that the growth is successful only when all the growth process takes place at room temperature. Under these conditions a very good epitaxial growth of Pd is achieved, despite the large misfit (about 8.6%) between Pd and the substrate and the existence of a semi-amorphous layer between the thin film and the substrate. A large number of twins appear in these films.     

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.     

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.     

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.     

Morphology transition of one-dimensional ZnO grown by metal organic vapour phase epitaxy on (0 0 0 1)-ZnO substrate

Metal organic vapour phase epitaxy (MOVPE) has been used to successfully grow one-dimensional (1D) ZnO deposits on (0 0 0 1)-ZnO substrate. Dimethylzinc–triethylamine and nitrous oxide were used as zinc and oxygen sources, respectively, with nitrogen as the carrier gas. Vertically aligned 1D ZnO structures were observed along the c-axis by using lower VI/II mole ratio R_VI/II<2025 and/or high growth temperatures (Tg>800 °C). The diameter, length, density and the mechanism of formation could be controlled with the growth time. Scanning electron microscopy (SEM) shows different structures, i.e., sharp-top, flat-top and open-top with slim bottom and large-top one-dimensional ZnO. A good structural quality was revealed by X-ray diffraction rocking curve with a full-width at half-maximum (FWHM) varying from 40 to 92 arcsec with increasing growth time.     

Investigations on the effect of InSb and InAsSb step-graded buffer layers in InAs0.5Sb0.5 epilayers grown on GaAs (0 0 1)

We report the structural and electrical properties of InAsSb epilayers grown on GaAs (0 0 1) substrates with mid-alloy composition of 0.5. InSb buffer layer and InAs_xSb_1−x step-graded (SG) buffer layer have been used to relax lattice mismatch between the epilayer and substrate. A decrease in the full-width at half-maximum (FWHM) of the epilayer is observed with increasing the thickness of the InSb buffer layer. The surface morphology of the epilayer is found to change from 3D island growth to 2D growth and the electron mobility of the sample is increased from 5.2×10³ to 1.1×10⁴ cm²/V s by increasing the thickness of the SG layers. These results suggest that high crystalline quality and electron mobility of the InAs_0.5Sb_0.5 alloy can be achieved by the growth of thick SG InAsSb buffer layer accompanied with a thick InSb buffer layer. We have confirmed the improvement in the structural and electrical properties of the InAs_0.5Sb_0.5 epilayer by quantitative analysis of the epilayer having a 2.09 μm thick InSb buffer layer and 0.6 μm thickness of each SG layers.     

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.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

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.     

Epitaxial NiO (1 0 0) and NiO (1 1 1) films grown by atomic layer deposition

Epitaxial NiO (1 1 1) and NiO (1 0 0) films have been grown by atomic layer deposition on both MgO (1 0 0) and α-Al₂O₃ (0 0 l) substrates at temperatures as low as 200 °C by using bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II) and water as precursors. The films grown on the MgO (1 0 0) substrate show the expected cube on cube growth while the NiO (1 1 1) films grow with a twin rotated 180° on the α-Al₂O₃ (0 0 l) substrate surface. The films had columnar microstructures on both substrate types. The single grains were running throughout the whole film thickness and were significantly smaller in the direction parallel to the surface. Thin NiO (1 1 1) films can be grown with high crystal quality with a FWHM of 0.02–0.05° in the rocking curve measurements.     

Mg segregation in a (1 1¯ 0 1) GaN grown on a 7° off-axis (0 0 1) Si substrate by MOVPE

Redistribution behavior of magnesium (Mg) in the N-terminated (1 1¯ 0 1) gallium nitride (GaN) has been investigated. A nominally undoped GaN layer was grown on a heavily Mg-doped GaN template by metalorganic vapor-phase epitaxy (MOVPE). Mg dopant profiles were measured by secondary ion mass spectrometry (SIMS) analysis. A slow decay of the Mg concentration was observed in the nominally undoped GaN layer due to the surface segregation. The calculated decay lengths of the (1 1¯ 0 1) GaN are ∼75–85 nm/decade. These values are shorter than the decay length determined in the sample grown on the Ga-terminated (0 0 0 1) GaN. This result indicates that Mg exhibited weak surface segregation in the (1 1¯ 0 1) GaN as compared to the (0 0 0 1) GaN. The weak surface segregation is in agreement with the high efficiency of Mg incorporation on the (1 1¯ 0 1) face. The high density of hydrogen was obtained in the (1 1¯ 0 1) GaN, which might enhance the Mg incorporation.     

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).     

Electron microscopy analysis of dislocation behavior in HVPE-AlGaN layer grown on a stripe-patterned (0 0 0 1) sapphire substrate

Microstructures were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in order to clarify the dislocation behavior in AlGaN layers HVPE-grown on a stripe-patterned sapphire (0 0 0 1) substrate. SEM observation revealed very clearly the growth process: if AlGaN starting to grow from the side-wall of patterned substrate develops, a poly-crystalline region is formed up to the top surface of thin film. When the growth from the upper side (terrace) of patterned substrate is predominant, AlGaN becomes a single-crystalline layer with a flat surface. Threading dislocations (TDs) generated from the interface to the terrace propagate upwards, inclining to the wing regions. They are scarcely merged with one another. The AlGaN layer on the patterned substrate with a wider groove has a smaller density of dislocation to be about 1×10⁹ cm⁻². There are four types of dislocations: (1) TDs inclining toward 〈1 1¯ 0 0〉 normal to their Burgers vector B; (2) TDs inclining toward 〈2 1¯ 1¯ 0〉 on their slip-plane; (3) TDs inclining largely or horizontal dislocations (HDs) along 〈2 1¯ 1¯ 0〉 and (4) roundly curved HDs lying on (0 0 0 1) plane. Some TDs change the direction of inclination, suggesting that internal stress changed intricately during the growth.     

Thickness-dependent structural transition in GaAs nanocrystals grown on Si (1 1 1) surface

Structural stabilities in GaAs nanocrystals grown on the Si (1 1 1) substrate have been studied by transmission electron microscopy in order to see the structure and growth mechanism. The GaAs nanocrystals grown epitaxially on the Si (1 1 1) surface kept at 573 K have thin shapes consisting of a flat surface which is parallel to the Si (1 1 1) surface. The crystalline structure of the initial growth layer approximately below 5 nm in thickness is the zincblend structure, but with increasing thickness the structure changes to the wurtzite structure by formation of orderly-arranged stacking faults. The small difference in the driving force between the wurtzite structure and the zincblende structure could bring about a situation, where the kinetic rate of nucleus formation is high for the wurtzite structure than for the zincblende structure. It would highly increase the probability that the wurtzite structure is formed as a non-equilibrium state.