Tsvetanka Zheleva

Lateral epitaxy of low defect density GaN layers via organometallic vapor phase epitaxy

Organometallic vapor phase lateral epitaxy and coalescence of GaN layers originating from GaN stripes deposited within 3-μm-wide windows spaced 3 μm apart and contained in SiO2 masks on GaN/AlN/6H–SiC(0001) substrates are reported. The extent and microstructural characteristics of the lateral overgrowth were a strong function of stripe orientation. A high density of threading dislocations, originating from the interface of the underlying GaN with the AlN buffer layer, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The coalesced layers had a rms surface roughness of 0.25 nm.

Dislocation density reduction via lateral epitaxy in selectively grown GaN structures

The microstructure and the lateral epitaxy mechanism of formation of homoepitaxially and selectively grown GaN structures within windows in SiO2 masks have been investigated by transmission electron microscopy (TEM) and scanning electron microscopy. The structures were produced by organometallic vapor phase epitaxy for field emission studies. A GaN layer underlying the SiO2 mask provided the crystallographic template for the initial vertical growth of the GaN hexagonal pyramids or striped pattern. The SiO2 film provided an amorphous stage on which lateral growth of the GaN occurred and possibly very limited compliancy in terms of atomic arrangement during the lateral growth and in the accommodation of the mismatch in the coefficients of thermal expansion during cooling. Observations with TEM show a substantial reduction in the dislocation density in the areas of lateral growth of the GaN deposited on the SiO2 mask. In many of these areas no dislocations were observed.

Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition

We report epitaxial growth of TiN films having low resistivity on (100) silicon substrates using pulsed laser deposition method. The TiN films were characterized using x‐ray diffraction, Rutherford backscattering, four‐point‐probe ac resistivity, high resolution transmission electron microscopy and scanning electron microscopy techniques and epitaxial relationship was found to be 〈100〉 TiN ∥ 〈100〉 Si. TiN films showed 10%–20% channeling yield. In the plane, four unit cells of TiN match with three unit cells of silicon with less than 4.0% misfit. This domain matching epitaxy provides a new mechanism of epitaxial growth in systems with large lattice misfits. Four‐point‐probe measurements show characteristic metallic behavior of these films as a function of temperature with a typical resistivity of about 15 μΩ cm at room temperature. Implications of low‐resistivity epitaxial TiN in silicon device fabrication are discussed.

The Composition Pulling Effect in MOVPE Grown InGaN on GaN and AlGaN and its TEM Characterization

InGaN films have been grown on GaN and AlGaN epitaxial layers by metalorganic vapor phase epitaxy. The “composition pulling effect” during the initial InGaN growth stages has been studied as a function of the lattice mismatch between the InGaN and the underlying epitaxial layer. The crystalline quality of the InGaN is good near the InGaN/GaN interface and the composition is close to that of GaN. However, with increasing InGaN film thickness, the crystal quality deteriorates and the indium mole fraction increases. The composition pulling effect becomes stronger with increasing lattice mismatch. It is suggested that indium atoms are excluded from the InGaN lattice during the early growth stages to reduce the deformation energy from the lattice mismatch. TEM observations of the InGaN/GaN structure reveal that the degradation of the crystalline quality of InGaN films grown on GaN is caused by pit formation which arises from edge dislocations propagating through the InGaN film from the underlying GaN.

Sublimation growth and characterization of bulk aluminum nitride single crystals

Single crystalline platelets of aluminum nitride (AlN) ⩽ 1 mm thick have been grown within the temperature range of 1950–2250°C on 10 × 10 mm2 α(6H)-silicon carbide (SiC) substrates via sublimation-recondensation in a resistively heated graphite furnace. The source material was sintered AlN. A maximum growth rate of 500 μm/h was achieved at 2150°C and a source-to-seed separation of 4 mm. Growth rates below 2000°C were approximately one order of magnitude lower. Crystals grown at high temperatures ranged in color from blue to green due to the incorporation of Si and C from the SiC substrates; those grown at lower temperatures were colorless and transparent. Secondary-ion mass spectroscopy (SIMS) results showed almost a two order of magnitude decrease in the concentrations of these two impurities in the transparent crystals. Plan view transmission electron microscopy (TEM) of these crystals showed no line or planar defects. Raman spectroscopy and X-ray diffraction (XRD) studies indicated a strain free material.

Room temperature growth of cubic boron nitride

Boron nitride thin films were deposited at room temperature with various ion energies by mass selected ion beam deposition on cubic boron nitride (c-BN) previously nucleated on Si (100) substrates at a higher temperature. Selective area diffraction, electron energy loss, and infrared spectroscopy results reveal continued growth of the cubic phase. The reported temperature threshold of about 150 °C for c-BN film formation is therefore unmistakably related to the initial nucleation of c-BN, whereas the growth of c-BN appears to be temperature independent. The latter is in accordance with predictions of the cylindrical thermal spike growth model recently proposed by our group.

Thermal mismatch stress relaxation via lateral epitaxy in selectively grown GaN structures

A reduction in the dislocation density of 104–105 cm−2 has been achieved via lateral epitaxial overgrowth (LEO) of GaN films selectively grown from stripes etched in SiO2 masks deposited on GaN/AlN/6H–SiC(0001) heterostructures. The magnitudes and distribution of stresses generated in the LEO GaN layer and the SiO2, due primarily to differences in the coefficients of thermal expansion, were modeled using finite element (FE) analysis. These calculations showed that localized compressive stress fields of ≈3 GPa occurred at the edges of the LEO GaN in the vicinity of the GaN/SiO2 interface. Localized compression along the GaN substrate/SiO2 interface and tension along the 〈0001〉 direction were responsible for the change in shape of the SiO2 stripes from rectangular with flat sides to an airfoil shape with curved sides. The FE calculations also revealed that an increase in the width of the LEO GaN regions over the SiO2 or the reduction in the separation between the GaN stripes (all other dimensions being fixe...

The formation of crystalline defects and crystal growth mechanism in InxGa1 - xN/GaN heterostructure grown by metalorganic vapor phase epitaxy

Abstract The composition pulling effect at the initial growth stage of In x Ga 1− x N grown on a GaN epitaxial layer is studied in relation to the lattice mismatch between In x Ga 1− x N and the GaN epitaxial layer. TEM observation of the In x Ga 1− x N/GaN heterostructure reveals that the degradation of the In x Ga 1− x N layer is caused by pit formation, which is converted from the edge dislocations penetrating to the In x Ga 1− x N layer from the GaN layer. By increasing the layer thickness, the crystalline quality becomes worse, and In x Ga 1− x N consists of two types of regions: a homogeneous, good crystalline quality layer and a bad crystalline quality layer. Crystalline quality of In x Ga 1− x N is good near the interface of In x Ga 1− x N/GaN, and EDX composition analysis shows that the composition of In x Ga 1− x N near the interface is close to that of GaN.

Preparation of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttria‐stabilized zirconia buffer layers by laser physical vapor deposition technique

We have fabricated high‐quality 〈00l〉 textured Pb(Zr0.54Ti0.46)O3 (PZT) thin films on (001)Si by interposing 〈00l〉 textured YBa2Cu3O7−δ (YBCO) and yttria‐stabilized zirconia (YSZ) buffer layers using pulsed laser deposition (KrF excimer laser, λ=248 nm, τ=20 ns). The YBCO layer provides a seed for PZT growth and can also act as an electrode for the PZT films, whereas YSZ provides a diffusion barrier as well as a seed for the growth of YBCO films on (001)Si. These heterostructures were characterized using x‐ray diffraction, high‐resolution transmission electron microscopy, and Rutherford backscattering techniques. The YSZ films were deposited in oxygen ambient (∼9×10−4 Torr) at 775 °C on (001)Si substrate having 〈001〉 YSZ//〈001〉Si texture. The YBCO thin films were deposited in situ in oxygen ambient (200 mTorr) at 650 °C. Temperature and oxygen ambient for the PZT deposition were optimized to be 530 °C and 0.4–0.6 Torr, respectively. The laser fluence to deposit this multistructure was 2.5–5 J/cm2. The 〈00...

Pendeo-Epitaxy - A New Approach for Lateral Growth of Gallium Nitride Structures

A new process route for lateral growth of nearly defect free GaN structures via Pendeoepitaxy is discussed. Lateral growth of GaN films suspended from {11 2 0} side walls of [0001] oriented GaN columns into and over adjacent etched wells has been achieved via MOVPE technique without the use of, or contact with, a supporting mask or substrate. Pendeo-epitaxy is proposed as the descriptive term for this growth technique. Selective growth was achieved using process parameters that promote lateral growth of the { 11 2 0) planes of GaN and disallow nucleation of this phase on the exposed SiC substrate. Thus, the selectivity is provided by tailoring the shape of the underlying GaN layer itself consisting of a sequence of alternating trenches and columns, instead of selective growth through openings in SiO 2 or SiN x mask, as in the conventional lateral epitaxial overgrowth (LEO). Two modes of initiation of the pendeo-epitaxial GaN growth via MOVPE were observed: Mode A - promoting the lateral growth of the {11 2 0} side facets into the wells faster than the vertical growth of the (0001) top facets; and Mode B - enabling the top (0001) faces to grow initially faster followed by the pendeo-epitaxial growth over the wells from the newly formed {11 2 0} side facets. Four-to-five order decrease in the dislocation density was observed via transmission electron microscopy (TEM) in the pendeo-epitaxial GaN relative to that in the GaN columns. TEM observations revealed that in pendeo-epitaxial GaN films the dislocations do not propagate laterally from the GaN columns when the structure grows laterally from the sidewalls into and over the trenches. Scanning electron microscopy (SEM) studies revealed that the coalesced regions are either defect-free or sometimes exhibit voids. Above these voids the PEGaN layer is usually defect free.