High-resolution imaging of 1:1 [0 0 0 1] ordered a-plane Al0.3Ga0.7N

We report the observation of ordering in Al_0.3Ga_0.7N as part of an epitaxial lateral overgrowth (ELO) of GaN carried out using (1 1 2¯ 2) GaN templates grown by metal-organic chemical vapor deposition on m-plane sapphire. Transmission electron microscopy showed that the crystalline quality of the ELO GaN was greatly improved when the ELO SiO₂ mask was patterned along the [1 1 2¯ 0]_sapphire direction. The ELO GaN wings had an inclined columnar shape with smooth (0 0 0 1) and (1 1 2¯ 0) facets. Layers of 1:1 [0 0 0 1] ordered a-plane Al_0.3Ga_0.7N were observed on the a-plane GaN facets by high-resolution transmission electron microscopy and high-angle annular-dark-field scanning transmission electron microscopy. However, no ordering was observed for c-plane Al_0.3Ga_0.7N layers grown at the same time on the c-plane GaN facets.     

N-type doping of GaN/Si(1 1 1) using Al0.2Ga0.8N/ALN composite buffer layer and Al0.2Ga0.8N/GaN superlattice

The characteristics of Si-doped and undoped GaN/Si(1 1 1) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(1 1 1) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200 nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(1 1 1), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al_0.2Ga_0.8N/AlN and superlattice of Al_0.2Ga_0.8N/GaN on 2-in. Si(1 1 1) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823 arcsec and 745 arcsec for n-GaN and undoped GaN/Si(1 1 1) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×10⁹ and 1.32×10⁹ cm⁻² for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(1 1 1) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0 nm related with 3.377 and 3.378 eV, respectively, using 325 nm He-Cd laser as an excitation source under room temperature.     

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.     

Synchrotron X-ray diffraction studies of heteroepitaxial ZnO films grown by pulsed laser deposition

Heteroepitaxial ZnO films were grown by pulsed laser deposition on various substrates such as GaN-buffered C-Al₂O₃, C-Al₂O₃, A-Al₂O₃, and R-Al₂O₃. The epitaxy nature of the films was investigated mainly by synchrotron X-ray diffraction. The results showed that the GaN interlayer plays a positive role in growing an unstrained, well-aligned epitaxial ZnO film on the basal plane of Al₂O₃. Importantly, the ZnO film grown on R-Al₂O₃ has two differently aligned domains. The dominant (1 1 0) oriented domain has much better alignment in the in-plane direction than the minor portion of (0 0 1) oriented domain, while in the out-of-plane direction the two domains have almost the same mosaic distribution.     

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.     

Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer

Nonpolar (1 1–2 0) a-plane GaN films have been grown using the multi-buffer layer technique on (1–1 0 2) r-plane sapphire substrates. In order to obtain epitaxial a-plane GaN films, optimized growth condition of the multi-buffer layer was investigated using atomic force microscopy, high resolution X-ray diffraction, and transmission electron microscopy measurements. The experimental results showed that the growth conditions of nucleation layer and three-dimensional growth layer significantly affect the crystal quality of subsequently grown a-plane GaN films. At the optimized growth conditions, omega full-width at half maximum values of (11–20) X-ray rocking curve along c- and m-axes were 430 and 530 arcsec, respectively. From the results of transmission electron microscopy, it was suggested that the high crystal quality of the a-plane GaN film can be obtained from dislocation bending and annihilation by controlling of the island growth mode.     

Heteroepitaxial growth of Cu2O thin film on ZnO by metal organic chemical vapor deposition

Cuprous oxide (Cu₂O) thin films were grown epitaxially on c-axis-oriented polycrystalline zinc oxide (ZnO) thin films by low-pressure metal organic chemical vapor deposition (MOCVD) from Copper(II) hexafluoroacetylacetonate [Cu(C₅HF₆O₂)₂] at various substrate temperatures, between 250 and 400 °C, and pressures, between 0.6 and 2.1 Torr. Polycrystalline thin films of Cu₂O grow as single phase with [1 1 0] axis aligned perpendicular to the ZnO surface and with in-plane rotational alignment due to (2 2 0)_Cu2O(0 0 0 2)_ZnO; [0 0 1]_Cu2O[1 2¯ 1 0]_ZnO epitaxy. The resulting interface is rectifying and may be suitable for oxide-based p–n junction solar cells or diodes.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

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.     

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.     

Hydride-assisted growth of GaN nanowires on Au/Si(0 0 1) via the reaction of Ga with NH3 and H2

High quality, straight GaN nanowires (NWs) with diameters of 50 nm and lengths up to 3 μm have been grown on Si(0 0 1) using Au as a catalyst and the direct reaction of Ga with NH₃ and N₂:H₂ at 900 °C. These exhibited intense, near band edge photoluminescence at 3.42 eV in comparison to GaN NWs with non-uniform diameters obtained under a flow of Ar:NH₃, which showed much weaker band edge emission due to strong non-radiative recombination. A significantly higher yield of β-Ga₂O₃ NWs with diameters of ≤50 nm and lengths up to 10 μm were obtained, however, via the reaction of Ga with residual O₂ under a flow of Ar alone. The growth of GaN NWs depends critically on the temperature, pressure and flows in decreasing order of importance but also the availability of reactive species of Ga and N. A growth mechanism is proposed whereby H₂ dissociates on the Au nanoparticles and reacts with Ga giving Ga_xH_y thereby promoting one-dimensional (1D) growth via its reaction with dissociated NH₃ near or at the top of the GaN NWs while suppressing at the same time the formation of an underlying amorphous layer. The higher yield and longer β-Ga₂O₃ NWs grow by the vapor liquid solid mechanism that occurs much more efficiently than nitridation.     

Effect of Mn concentration and growth temperature on nanostructures and magnetic properties of Ge1−xMnx grown on Si

The nanostructures and magnetic properties of Ge_1−xMn_x thin films grown on Si substrates by molecular beam epitaxy, with different nominal Mn concentrations (1−4%) and different growth temperatures, have been systematically investigated by transmission electron microscopy and superconducting quantum interference device. It was discovered that when Ge_1−xMn_x thin films were grown at 70 °C, with increase in Mn concentration, Mn-rich tadpole shaped clusters started to nucleate at 1% Mn and become dominate in the entire film at 4% Mn. While for the thin films grown at 150 °C, tadpoles was firstly seen in the film with 1% Mn and subsequently Mn-rich secondary precipitates became dominant. The magnetic properties show specific features, which are mainly related to the nature and amount of Mn-rich clusters/precipitates within these thin films.     

Band alignment at the interfaces of Al2O3 and ZrO2-based insulators with metals and Si

Internal photoemission of electrons was used to determine the band alignment in metal (Mg, Al, Ni, Cu, Au)-oxide-silicon structures with Al₂O₃- and ZrO₂-based insulators. For Al₂O₃- and ZrO₂ layers grown on Si by atomic layer deposition the barrier height between the Si valence band and the oxide conduction band was found to be 3.25 and 3.1 eV, respectively. Thermal oxidation of the Si/oxide stacks results in a barrier height increase to ≈4 eV for both cases due to formation of a silicate interlayer. However, there is a significant sub-threshold electron emission both from silicon and metals, indicating a high density of states in the band gap of the insulators. These states largely determine the electron transport across metal oxides and may also account for a significant dipole component of the potential barrier at the metal/ZrO₂ and metal/Al₂O₃ interfaces.     

Reconstruction of the -GaO (1 0 0) cleavage surface to hexagonal GaN after NH nitridation

The potential use of β-Ga₂O₃ wafers as transparent conductive substrates for compounds with hexagonal structure, in particular the GaN-system, is shown. Nitridation of the main cleavage plane under high temperature and NH₃ gas results in the substitution of O by N on the surface, and a simultaneous surface reconstruction. The 2-fold symmetry of the initial (1 0 0) plane changes to a 6-fold symmetry, keeping the [0 1 0] azimuth as a symmetry direction. The RHEED pattern suggests the formation of GaN on the β-Ga₂O₃ surface, with the b-axis of β-Ga₂O₃ parallel to the direction of GaN, so that β-Ga₂O₃ can be used as a substrate with no lattice mismatch to GaN.     

High quality MOCVD GaN film grown on sapphire substrates using HT-AlN buffer layer

In this work we report on the growth and characterization of high quality MOCVD GaN film grown on Al₂O₃ substrates by using a HT (>1150 °C)-AlN buffer layer. We have investigated the most favorable growth conditions in terms of temperature, thickness and growth rate of AlN buffer layer in order to optimize the high temperature GaN layer. The improved morphological and structural properties of GaN layer were verified by AFM and XRD measurements. The optimized GaN layer presents a smooth surface with a rms value of 1.4 Å. The full width at half maximum (FWHM) for 800 nm thick GaN films is 144″. Furthermore PL measurements and C–V analysis confirm that in GaN layer grown on HT-AlN buffer layer defect density is drastically reduced.     

Ca3N2 as a flux for crystallization of GaN

In this work Ca₃N₂ was investigated as a potential flux for crystallization of GaN. Melting temperature of the potential flux at high N₂ pressure evaluated by thermal analysis as 1380 °C is in good agreement with the theoretical prediction. It is shown that Ca₃N₂ present in the liquid gallium in small amount (1 at%) dramatically accelerates synthesis of GaN from its constituents. On the other hand, it does not influence significantly the rate of GaN crystallization from solution in gallium in temperature gradient for both unseeded and seeded configurations. However the habit and color of the spontaneously grown GaN crystals change drastically. For 10 mol% Ca₃N₂ content in the liquid Ga it was found that the GaN thick layer and GaN crystals (identified by micro-Raman scattering measurements) were grown on the substrate. For growth from molten Ca₃N₂ (100%) with GaN source, the most important observations were (i) GaN source material was completely dissolved in the molten Ca₃N₂ flux and (ii) after experiment, GaN crystals were found on the sapphire substrate.     

High-quality silicon/insulator heteroepitaxial structures formed by molecular beam epitaxy using Al2O3 and Si

Heteroepitaxial growth of γ-Al₂O₃ films on a Si substrate and the growth of Si films on the γ-Al₂O₃/Si structures by molecular beam epitaxy have been investigated. It has been found from AFM and RHEED observations that, γ-Al₂O₃ films with an atomically smooth surface with an RMS values of ∼3 Å and high crystalline quality can be grown on Si (1 1 1) substrates at substrate temperatures of 650–750°C. Al₂O₃ films grown at higher temperatures above 800°C, did not show good surface morphology due to etching of a Si surface by N₂O gas in the initial growth stage. It has also been found that it is possible to grow high-quality Si layers by the predeposition of Al layer followed by thermal treatment prior to the Si molecular beam epitaxy. Cross-sectional TEM observations have shown that the epitaxial Si had significantly improved crystalline quality and surface morphology when the Al predeposition layer thickness was 10 Å and the thermal treatment temperature was 900°C. The resulting improved crystalline quality of Si films grown on Al₂O₃ is believed to be due to the Al₂O₃ surface modification.     

Calculation of phase diagrams in AlxIn1−xAs/InP,AsxSb1−xAl/InP and AlxIn1−xSb/InSb nano-film systems

The models for calculation of phase diagrams of semiconductor thin films with different substrates were proposed by considering the contributions of strain energy, the self-energy of misfit dislocations and surface energy to Gibbs free energy. The phase diagrams of the Al_xIn_1−xAs and As_xSb_1−xAl thin films grown on the InP (1 0 0) substrate, and the Al_xIn_1−xSb thin films grown on the InSb (1 0 0) substrate at various thicknesses were calculated. The calculated results indicate that when the thickness of film is less than 1 μm, the strain-induced zinc-blende phase appears, the region of this phase extends with decreasing of the layer thickness, and there is small effect of surface energies of liquid and solid phases on the phase diagrams.     

Method for modulating the wafer bow of free-standing GaN substrates via inductively coupled plasma etching

The bowing curvature of the free-standing GaN substrate significantly decreased almost linearly from 0.67 to 0.056 m⁻¹ (i.e. the bowing radius increased from 1.5 to 17.8 m) with increase in inductively coupled plasma (ICP) etching time at the N-polar face, and eventually changed the bowing direction from convex to concave. Furthermore, the influences of the bowing curvature on the measured full width at half maximum (FWHM) of high-resolution X-ray diffraction (HRXRD) in (0 0 2) reflection were also deduced, which reduced from 176.8 to 88.8 arcsec with increase in ICP etching time. Decrease in the nonhomogeneous distribution of threading dislocations and point defects as well as V_Ga–O_N complex defects on removing the GaN layer from N-polar face, which removed large amount of defects, was one of the reasons that improved the bowing of the free-standing GaN substrate. Another reason was the high aspect ratio of needle-like GaN that appeared at the N-polar face after ICP etching, which released the compressive strain of the free-standing GaN substrate. By doing so, crack-free and extremely flat free-standing GaN substrates with a bowing radius of 17.8 m could be obtained.     

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.