Smooth GaN Layers Research Articles

Active Matrix Monolithic LED Micro-Display Using GaN-on-Si Epilayers

An active matrix light emitting diode (LED) micro-display system was demonstrated with GaN-on-Si epilayers and a custom-designed CMOS backplane using an Au-free Cu/Sn-based metal bonding method. The blue micro-LED array consists of $64\times36$ pixels with a pitch size of $40\,\,\mu \text{m}\,\,\times 40\,\,\mu \text{m}$ and a pixel density of 635 pixels/in (ppi). The Si substrate for the LED growth was removed by reactive ion etching (RIE) using SF6-based gas after flip-chip bonding. Crack-free and smooth GaN layers in the display area were exposed. Images and videos with 4-bit grayscale could be clearly rendered, and light crosstalk was significantly suppressed compared to its counterpart using the GaN-on-sapphire epilayers. The demonstration suggests the tremendous potential of the low-cost and large-scale GaN-on-Si epilayers and cost-effective Au-free Cu/Sn-based bonding scheme for micro-display applications.

Unusual step meandering due to Ehrlich-Schwoebel barrier in GaN epitaxy on the N-polar surface

The stability of the Nitrogen-polar (000-1) surface of single-crystal bulk GaN substrates is studied for layers grown by plasma-assisted molecular beam epitaxy (PAMBE) in Nitrogen-rich conditions at 730 °C. It is shown that smooth GaN layers with parallel atomic steps are obtained for substrates when the surface crystal miscut angle is larger than 2o, revealing a highly stable epitaxial growth regime on single crystals. A step meandering pattern is observed on layers grown on lower miscut angle substrates. The meandering periodicity is found to have an inverse dependence on growth rate and miscut angle. This is opposite to what is observed for epitaxy on the Ga-polar surface. Combining analytic modeling and kinetic Monte Carlo simulations, it is shown that the existence of an Ehrlich-Schwoebel Barrier (ESB) in the PAMBE growth of GaN in nitrogen-rich conditions on (000-1) GaN reproduces the experimentally observed periodicity of step meandering. Assuming that ESB height depends on interactions between diffusing adatoms, all experimental phenomena are reproduced.

A combined growth process for state-of-the-art GaN on silicon

This study aims to simplify the heteroepitaxy of GaN on Si while keeping state-of-the-art III-nitride materials. The originality of this work is to combine the advantages of both NH3-MBE and metal organic chemical vapor deposition (MOCVD) growth techniques. First, the structural quality of AlN is assessed by AFM, XRD measurements, and TEM. Terraces and atomic step edges are observed by AFM showing a good structural quality of the AlN epilayer. TEM investigation reveals a sharp and well-controlled AlN/Si using NH3-MBE. Then, a 2-μm thick smooth GaN layers are obtained by MOCVD on top of these AlN templates with the use of a SiNx treatment allowing a 3D growth mode able to induce an efficient dislocation filtering to be obtained. GaN structural properties, measured by XRD, AFM, and SEM, are discussed and compared to GaN-on sapphire (Al2O3). In addition, the stress in the GaN layers grown on these AlN templates with or without a SiNx treatment is assessed. GaN epilayers with a dislocation density in the range of few 108 dislocations per cm2 have been achieved using a single AlN buffer layer (no interlayer) and only 2 μm of GaN.

Preparation of free-standing GaN substrates from GaN layers crystallized by hydride vapor phase epitaxy on ammonothermal GaN seeds

Crystallization of GaN by hydride vapor phase epitaxy (HVPE) on ammonothermally grown GaN seed crystals is overviewed. Morphology of the crystal growing surface at the beginning of the crystallization process and at the end of it is presented. Based on these results a rough growth model is proposed. Smooth GaN layers up to 1 mm thick and of a high purity, excellent crystalline quality, without any cracks, and with a low dislocation density are grown. Preparation of the free-standing HVPE-GaN crystals by slicing as well as structural, electrical and optical qualities of the resulting wafers are reported and discussed.

MOVPE growth of semi-polar GaN light-emitting diode structures on planar Si(112) and Si(113) substrates

We present semi-polar GaN light-emitting diode (LED) structures grown on non-patterned Si(112) and Si(113) substrates by metal organic vapor phase epitaxy. Cathodoluminescence and field emission scanning electron microscopy are used for sample characterization. A correlation between the structural and optical properties of the semi-polar GaN LED structures is observed. In samples, which were simultaneously grown on Si(112) and Si(113) under growth conditions optimized for Si(112), we observed that structures on Si(112) consist of a relatively smooth surface but those on Si(113) have a large number of surface pits with a three dimensional growth mode of the GaN layers resulting in a rough GaN surface. The growth conditions were further optimized to obtain smooth GaN layers on Si(113) and compared to the sample on Si(112).

Effects of TiN Buffer Layer Thickness on GaN Growth

Smooth GaN layers were successfully grown on metallic TiN buffer layers by metalorganic chemical vapor deposition (MOCVD). One important factor in controlling GaN layer smoothness was the TiN layer thickness. We investigated systematically the effects of this thickness, and found an optimal thickness of 5 nm, at which the smallest average grain size (20 nm) and smoothest surface were obtained. The TiN layers increased surface coverage with GaN hexagons at an early stage of GaN growth, indicating that enhancing the GaN nucleation is essential for smooth GaN layer growth, and small grain size and smooth surface are needed to enhance GaN nucleation. Further reduction in TiN layer thickness to 2 nm decreased the surface coverage with GaN hexagons, and a high density of grooves and holes were observed in the surface of the 2-μm-thick GaN layers. Defect structures in the GaN layers grown on the TiN layers were remarkably changed on reduction of TiN layer thickness from 5 nm to 2 nm. GaN growth was found to be sensitive to the TiN layer thickness between 2 nm and 5 nm.

Influence of the substrate misorientation on the properties of N-polar GaN films grown by metal organic chemical vapor deposition

Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the a and the m plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar GaN films grown in the same reactor.

Effect of Reducing Thickness of TiN Buffer Layers on Epitaxial Growth of GaN Layes

We investigated effect of reducing thickness of TiN buffer layers on growth of the smooth GaN layers. The sputtered TiN layers with thicknesses in the range of 2 to 100 nm were deposited on sapphire substrates. The sputtered TiN layers were exposed NH3 + H2 mixed gas atmosphere at about 1000°C to enrich nitrogen concentration of the layers. GaN layers were deposited on the nitrogen-enriched TiN layer using a MOCVD method. Average grain size of the nitrogen-enriched TiN layer was minimized at the thickness of 5 nm. In the initial stages of GaN growth, density of GaN hexagons grown on the 5nm-thick TiN layers was the highest. The 2μm-thick GaN layers grown on the 5nm-thick TiN layers exhibited the smoothest surface. Thus, the 5nm thickness is believed to be the best thickness for the smooth GaN growth on the sapphire/TiN substrates.

Growth of GaN on Nitriding TiN Buffer Layers

The breakthrough of GaN epitaxial layer growth with mirror-like surface on the sapphire substrate using a MOCVD (metal organic-chemical vapor deposition) technique was made by discovery of an AlN buffer layer prior to GaNdeposition about 20 years ago. Since then. extensive efforts have been made to develop a conductive substrate for MOCVD grown GaN layers. In the present study, we explored a possibility of growing continuous, flat GaN layers on a metallic TiN buffer layer, and focused our experiments to investigate the effect of nitrogen contents of the TiN buffer layers on lateral growth of the GaN layers. It was concluded that nitrogen composition in the TiN buffer layers should be higher than that of stoichiomeflric TiN to grow smooth GaN layers. It was found that nitriding the TiN buffer layers after deposition enriched significantly the nitrogen content compared with increasing the ratio of N 2 to N 2 + Ar (N 2 /(N 2 + Ar) ratio) during TiN deposition. Choice of the moderate N 2 /(N 2 + Ar) ratio during TiN deposition and nitriding the TiN buffer layers after TiN deposition were essential to grow continuous, flat GaN layers, since the reduction of the N 2 /(N 2 + Ar) ratio promoted few opening areas in the GaN layers.

Chemistry and kinetics of the GaN formation by plasma nitridation of GaAs: Anin situreal-time ellipsometric study

The chemistry and kinetics of the nitridation of GaAs (100) surfaces by ${\mathrm{N}}_{2},$ ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{H}}_{2},$ and ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{N}\mathrm{H}}_{3}$ radio-frequency plasmas, in a remote configuration, are investigated in situ and in real time using spectroscopic ellipsometry. The effects of the surface temperature in the range 70--700 \ifmmode^\circ\else\textdegree\fi{}C and of the gas-phase chemistry on both the nitridation kinetics and the composition of the resulting GaN layer are highlighted. Pure ${\mathrm{N}}_{2}$ plasmas yield stoichiometric and smooth GaN layers with As segregation at the GaN/GaAs interface. The As segregation inhibits GaAs nitridation, because the N atoms scavenge the free As, and thereby limits the GaN thickness to a few angstroms. Thicker GaN layers (g100 \AA{}) are obtained by ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{H}}_{2}$ and ${\mathrm{N}}_{2}{\ensuremath{-}\mathrm{N}\mathrm{H}}_{3}$ plasmas, since hydrogen reduces the As segregation by the formation and desorption of ${\mathrm{AsH}}_{x}$ species. For the three plasma mixtures, the self-limiting nature of the GaAs nitridation process is revealed and explained using simple kinetic and chemical models based on the fact that the GaAs nitridation can be considered to be a topochemical reaction. Also demonstrated is the ineffectiveness of the nitridation at $Tg~600\ifmmode^\circ\else\textdegree\fi{}\mathrm{C},$ which is accompanied by the GaAs substrate decomposition and yields both a rough and Ga-rich GaN layer.