Growth of AlN layer on patterned sapphire substrate by hydride vapor phase epitaxy

Abstract

Even though a patterned sapphire substrate (PSS) has been used for the growth of a high-quality epilayer because of its many advantages, it has not been successfully used to grow an AlN epilayer for ultraviolet (UV) light-emitting diodes (LEDs) on a PSS up to now. We report the growth of a high-quality AlN epilayer on a PSS, as a substrate for the manufacture of UV LEDs, by hydride vapor phase epitaxy (HVPE). The X-ray diffraction (XRD) peaks for the AlN epilayer grown on the PSS indicate that crystalline AlN with a wurtzite structure was grown successfully on the PSS. Furthermore, HVPE combining both in situ HVPE technology and liquid-phase epitaxy (LPE) using a mixed source is proposed as a novel method for the growth of a flat AlN epilayer on a PSS.

Epitaxial Growth of AlN on (0001) Sapphire: Assessment of HVPE Process by a Design of Experiments Approach

“…These data allow the determination of mean values of residual stress. For all reactor designs and all optimized conditions by the authors [18][19][20][21][22][23][24][25][26][27][28], FWHM values were in the range of 100 to 600 arcsec and 500 to 800 arcsec for on-axis and off-axis peak, respectively. It is clear that the optimal temperature range is 1200-1400 • C to increase the Al mobility on the surface while avoiding sapphire degradation and/or etching [38].…”

“…In most recent papers [17][18][19][20][21][22][23][24][25][26][27][28] dealing with HVPE growth of AlN at high temperature from chlorinated precursors, intense efforts have been made to reduce defect densities caused by stress [29][30][31] before optimizing optical properties [18]. In situ etching to control void formation and stress release [17], multi-step methods to control island coalescence [32][33][34] or N/Al ratio [33,35], pulse injection of precursors [36], substrate position in turbulent flow [37] have been proposed to reduce strain, high dislocation densities and the appearance of cracks.…”

“…It is clear that the optimal temperature range is 1200-1400 • C to increase the Al mobility on the surface while avoiding sapphire degradation and/or etching [38]. Very recently, attempts to reduce strain by lateral epitaxial growth (LEO) have been proposed [21,26,34,39]. The common LEO developed for GaN has been difficult to adapt to AlN films.…”

“…The approach consists in growing films on substrates possessing elevated mesas separated by lower trench regions to promote dislocation bending [39]. Despite lateral growth and coalescence over the mesas or pillars [21,34], only one order of magnitude reduction in the threading dislocation density were observed (10 9 to 3 × 10 8 /cm 2 ). The observations that can be made in all these studies is the difficulty to have a good knowledge of the actual N/Al ratio at the substrate-gas interface strongly dependent on the reactor design, supersaturation just above the substrate [40,41] and many other parameters related to the evolution of stress during the growth.…”

High Luminous Efficacy Phosphor-Converted Mass-Produced White LEDs Achieved by AlN Prebuffer and Transitional-Refraction-Index Patterned Sapphire Substrate

“…For many decades, many efforts have been taken to improve the internal quantum efficiency ( η int ), light extraction efficiency ( η ext ), and conversion efficiency of phosphor ( η phos ), which improves the overall η L of PC-LEDs. For η int and η ext , the typical techniques focus on (a) improving the crystalline quality of epilayers by using low-temperature (LT) buffer layers [ 20 ], epitaxial lateral overgrowth [ 21 , 22 ], and patterned sapphire substrate (PSS) [ 23 , 24 ]; (b) optimization of the device structures including multiple quantum wells (MQWs) with stronger radiative recombination [ 25 , 26 ], electron blocking layer (EBL) suppressing current leakage [ 27 , 28 ], and p -type layer with high hole concentration [ 29 ]; and (c) high η ext design, involving PSS [ 23 , 24 ], microstructured air cavities [ 30 ], and mirror reflectors [ 31 , 32 ]. In terms of η phos , multiple types of phosphors have been introduced to improve color rendering index while manifesting high efficiency [ 33 , 34 ].…”