K. Dovidenko

Defects and interfaces in epitaxial ZnO/α-Al2O3 and AlN/ZnO/α-Al2O3 heterostructures

We have investigated the nature of epitaxy, defects (dislocations, stacking faults, and inversion domains), and heterointerfaces in zinc oxide films grown on (0001) sapphire and explored the possibility of using it as a buffer layer for growing group III nitrides. High quality epitaxial ZnO films were grown on sapphire using pulsed laser deposition in the temperature range 750–800 °C. The epitaxial relationship of the film with respect to (0001) sapphire was found to be (0001)ZnO∥(0001)sap, with in-plane orientation relationship of [0110]ZnO∥[1210]sap. This in-plane orientation relationship corresponds to a 30° rotation of ZnO basal planes with respect to the sapphire substrate, which is similar to the epitaxial growth characteristics of AlN and GaN on sapphire. The threading dislocations in ZnO were found to have mostly 1/3〈1120〉 Burgers vectors. The planar defects (mostly I1 stacking faults) were found to lie in the basal plane with density of about 105 cm−1. We have grown epitaxial AlN films at tem...

Aluminum nitride films on different orientations of sapphire and silicon

The details of epitaxial growth and microstrictural characteristics of AlN films grown on sapphire (0001), (1012) and Si (100), (111) substrates were investigated using plan‐view and cross‐sectional high‐resolution transmission electron microscopy and x‐ray diffraction techniques. The films were grown by metalorganic chemical vapor deposition using TMA1+NH3+N2 gas mixtures. Different degrees of epitaxy were observed for the films grown on α‐Al2O3 and Si substrates in different orientations. The epitaxial relationship for (0001) sapphire was found to be (0001)AlN∥(0001)sap with in‐plane orientation relationship of [0110]AlN∥[1210]sap. This is equivalent to a 30° rotation in the basal (0001) plane. For (1012) sapphire substrates, the epitaxial relationship was determined to be (1120)AlN∥(1012)sap with the in‐plane alignment of [0001]AlN∥[1011]sap. The AlN films on (0001) α‐Al2O3 were found to contain inverted domain boundaries and a/3〈1120〉 threading dislocations with the estimated density of 1010 ...

Heteroepitaxial structures of SrTiO3/TiN on Si(100) by in situ pulsed laser deposition

High‐quality ceramics based heteroepitaxial structures of oxide‐nitride‐semiconductors, i.e., SrTiO3/TiN/Si(100) have been fabricated by in situ pulsed laser deposition. The dependence of substrate temperature and oxygen partial pressure on the crystalline quality of the SrTiO3 films on Si with epitaxial TiN template has been examined. We found that epitaxial growth occurs on TiN/Si(100) above 500 °C, initially at a reduced O2 pressure (10−6 Torr), and followed by a deposition in the range of 5–10×10−4 Torr. X‐ray diffraction (Θ, ω, and Φ scans) and transmission electron microscope (TEM) results revealed an excellent alignment of SrTiO3 and TiN films on Si(100) with a cube‐on‐cube epitaxy. Rutherford backscattering and ion channeling results show a channeling minimum yield (χmin) of ∼13% for the SrTiO3 films. High‐resolution TEM results on the SrTiO3/TiN interface show that the epitaxial SrTiO3 film is separated from the TiN by an uniform 80–90 A crystalline interposing layer presumably of TiNxO1−x (oxy‐n...

Characteristics of stacking faults in AlN thin films

We have investigated growth characteristics and atomic structure of defects in AlN thin films grown by the metalorganic chemical vapor deposition technique on the (1012) r plane of α-Al2O3. The AlN films were single crystal and exhibited the following epitaxial relationship: (1120)AlN∥(1012)sap with the in-plane alignment of [0001]AlN∥[1011]sap. Using high-resolution electron microscopy and multislice image simulation, the predominant defects in AlN thin films grown on the r plane of α-Al2O3 were found to be low-energy intrinsic stacking faults of type I1 lying in the basal plane. This fault, with a 1/6[2023] resultant displacement vector, can be formed by removing one (0002) plane and then shearing the remaining half-crystal by displacement of 1/3[1010]. The faults appear as a single face-centered-cubic stack ABC inserted into the normal …ABAB… hexagonal sequence. We discuss the possible role of these defects in optical properties of semiconductor devices.

Influence of oxygen background pressure on crystalline quality of SrTiO3 films grown on MgO by pulsed laser deposition

We have systematically investigated the effect of oxygen partial pressure (PO2) on the crystalline quality of SrTiO3 films grown on MgO (001) substrates using pulsed laser deposition and established optimized conditions for the growth of high-quality epitaxial films. The crystalline quality is found to improve significantly in the O2 pressure range of 0.5–1 mTorr, compared to the films deposited at higher pressures of 10–100 mTorr. The x-ray diffraction rocking curves for the films grown at PO2 of 1 mTorr and 100 mTorr yielded full width at half-maximum (FWHM) of 0.7° and 1.4°, respectively. The in-plane x-ray φ scans showed epitaxial cube-on-cube alignment of the films. Channeling yields χmin were found to be <5% for the 1 mTorr films and ∼14% for 100 mTorr films. Thermal annealing of the SrTiO3 films in oxygen further improves the quality, and the 1 mTorr films give FWHM of 0.13° and χmin of 1.7%. In-plane misorientations of the annealed SrTiO3 films calculated using results of transmission electron mic...

Thermal reactions and micro-structure of TiN–AlN layered nano-composites

Abstract Bilayer and multilayer structures of TiN and AlN thin films were synthesized using pulsed laser deposition technique in a substrate temperature range 300–700°C. The chemical reactions at TiN–AlN interfaces and the formation of different alloy phases were studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was observed that TiN–AlN interface remains sharp and stable for deposition temperatures up to ∼650°C. At higher deposition temperatures, however, substantial chemical reactions were found to occur. The ternary alloy phases such as Ti 3 Al 2 N 2 and Ti 3 AlN have been observed, for the first time. The composites synthesized at temperatures lower than 650°C and subsequently annealed at higher temperature were found to exhibit very limited or no interfacial chemical reactions. The effect of layer thickness on the microstructure is also studied. The studies revealed that by controlling the thickness of individual layers and substrate temperatures, it was possible to control microstructure and obtain composite coatings consisting of ternary Ti–Al–N alloy phases. The results are discussed in terms of characteristic features of pulsed laser ablation process in which evaporated flux contains energetic ions, electrons and neutral particles. Preliminary nano-indentation measurements and oxidation measurements reveal that these composites possess desirable mechanical properties at high temperatures.

CUBIC GAN FORMATION UNDER NITROGEN-DEFICIENT CONDITIONS

We have studied crystal structure and associated defects in GaN/α-Al2O3 (0001) films grown under nitrogen-deficient conditions by metalorganic chemical vapor deposition and pulsed laser deposition. N-deficient films exhibit polycrystalline structure with a mixture of cubic zinc-blende and wurtzite hexagonal GaN grains retaining tetragonal bonding across the boundaries and hence the epitaxial orientations and polarity. Renucleation of the wurtzite phase at different {111} planes of cubic GaN results in a rough and faceted surface of the film. We elucidate that the cubic phase is more stable under the nitrogen deficiency.

Comparative study of typical defects in III-nitride thin films and their alloys

The microstructure and typical defects in GaN and GaN/GaAlN thin film heterostructures grown on (0001)α-A1 2 O 3 were investigated using different transmission electron microscopy (TEM) techniques including diffraction contrast analysis, multiple dark field imaging, and highresolution TEM. The films were grown by metal-organic chemical vapor deposition (MOCVD) technique. All of the films exhibited good electrical/optical properties. Yet, films were found to be of two distinctive types in terms of the microstructure. Films of the first type (A) were found to contain high, up to the 10 9 cm 2 , density of inversion domains (IDs) as well as pure edge (b=l/3[11 20]), screw and mixed type dislocations with the average density of 10 9 - 10 10 cm −2 . Smoother surface of the film, absence of IDs, and low (down to 10 7 cm −2 in the device quality layers) density of screw and mixed type dislocations were found to be characteristic for the second type (B) microstructure. The majority of defects present in these B-type GaN and GaN/GaA1N thin films were found to be threading pure edge dislocations associated with low angle tilt sub-grain boundaries. Despite the 10 10 cm −2 density of the edge dislocations, the films displayed the devicequality electrical characteristics. Type A and type B microstructure can be obtained by the variation of growth conditions. The correlation between the optical and structural properties are discussed.

Defects and Interfaces in III-Nitride/Sapphire Thin Film Heterostructures

High quality III-nitrides and zinc oxide thin film heterostructures have been fabricated using pulsed laser deposition (PLD) on highly lattice mismatched sapphire substrates. The structural quality of PLD films grown at 700-800°C is comparable with those grown by MOCVD in the temperature range 950-1050°C. The PLD films were found to be single crystalline with columnar structure having the dislocation density of ~10 10 cm 2 at a film thickness 150-200 nm. The typical structure of defects and interfaces in GaN and AIN films is analyzed and compared with those obtained from specimens grown by MOCVD at much higher temperatures. Low temperature PLD ( 2 O 4 intermediate layer.

Crystal Structure and Defects in Nitrogen-Deficient GaN

We have studied crystal structure and associated defects in GaN films grown on sapphire under nitrogen-deficient conditions by metalorganic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD). The structural quality of the PLD films grown at 750 °C was comparable with those grown by MOCVD at 1050 °C having threading dislocations density of about 1010 cm -2 at a film thickness 150-200 nm. Microstructure of the PLD films grown at temperatures above 780°C was found to be similar to that of nitrogen-deficient MOCVD films indicating the loss of nitrogen due to thermal decomposition of the nitride layers. Nitrogen-deficient MOCVD and PLD films exhibit polycrystalline structure with a mixture of cubic zinc-blende and wurtzite hexagonal GaN grains retaining tetragonal bonding across the boundaries and hence the epitaxial orientations and polarity. Renucleation of the wurtzite phase at different {111} planes of cubic GaN results in a rough and faceted surface of the film. Most of the stoichiometric films displayed (0001) Ga-face polarity, but the renucleated inclined wurtzite grains grew in the opposite N-face polarity. The major defects related to the cubic structural metastability are stacking faults and microtwins which being nuclei of the metastable cubic phase have an extremely low energy. We elucidate that the cubic phase is more stable under the nitrogen deficiency and, therefore, can exist without decomposition at higher nitrogen vacancy concentrations in the material.