High-quality GaN Epitaxial Films Research Articles

Lattice-matched GaN(0002)/LSAT(111) hetero-structures grown by PLD and analyzed by first-principles calculations

La0.3Sr1.7AlTaO6(LSAT), a desired substrate for GaN epitaxial growth with a lattice mismatch as small as 1%, has not been applied in growth of GaN films due to hetero-interface rotation in epitaxial growth. In this paper, high-quality GaN epitaxial films with a small FWHM for GaN(0002) X-ray rocking curve of 0.06° have been grown by pulsed laser deposition (PLD) with a lattice mismatch as small as 1% between GaN and LSAT, and the corresponding mechanism avoiding interface rotation has also been investigated by first-principles calculation. On the one hand, we found that GaN grown on LSAT by PLD shows an in-plane relationship of GaN[2-1-10]//LSAT[1-10], revealing a small lattice mismatch of 1%. On the other hand, the first-principles calculation based on density function theory reveals that GaN growth along GaN[2-1-10]//LSAT[1-10] shows a higher interface energy barrier compared with that along GaN[1-100]//LSAT[1-10] with a lattice mismatch of 12.5%. The high interface energy barrier can only be provided by PLD. Moreover, this work is of paramount importance for understanding the hetero-interface formation mechanism in a GaN/substrates system and will be beneficial to the development of GaN-based devices that require high-quality GaN epitaxial films.

Low-temperature direct growth of GaN epitaxial film on Cu substrate with sharp and abrupt hetero-interface

GaN grown on high thermal conductivity substrate has attracted considerable attention for the preparation of high-power GaN-based devices. In this work, high-quality ∼300 nm-thick GaN epitaxial film with an in-plane epitaxial relationship of GaN[1 1−2 0]//Cu [1−1 0] has been directly grown on Cu substrate through controlling the interfacial reactions between GaN and Cu by low-temperature pulsed laser deposition growth of 450 °C. It reveals sharp and abrupt hetero-interface, small full-width at half-maximum for X-ray rocking curves of GaN(0 0 0 2) and GaN(1 0−1 2) of 0.5° and 0.8°, and very smooth surface, respectively. These high-quality GaN epitaxial films on Cu substrates shed light on the fabrication of high-power GaN-based optoelectronic and microelectronic devices.

Stress and dislocation control of GaN epitaxial films grown on Si substrates and their application in high-performance light-emitting diodes

GaN-based light-emitting diodes (LEDs) on Si substrates are promising to replace conventional lamps due to the advantages of energy-saving and low-cost of LEDs grown on large-size Si substrates. However, high-density dislocations and cracks of GaN epitaxial films are usually formed that limit the further development and application of GaN-based LEDs. To circumvent the issues, the step-graded AlGaN buffer layers are carefully designed to grow GaN epitaxial films on Si substrates. The mechanisms of dislocations and stresses for GaN epitaxial films controlled by step-graded AlGaN buffer layers are also investigated by analyzing dislocations evolution and stresses relaxation at the hetero-interfaces. Afterwards, 3.0 μm-thick high-quality GaN epitaxial films grown on Si substrates have been obtained, and high-quality GaN-based LED wafers are obtained accordingly with small full-width at half-maximums (FWHMs) for GaN(0002) and GaN(10–12) X-ray rocking curves of 272 and 297 arcsec, respectively. The corresponding vertical-structure LED chips reveal high-performance with a high light output power of 592 mW and a small working voltage of 2.77 V @ 456 nm, at a current of 350 mA. This work provides an effective approach for the growth of high-quality crack-free GaN epitaxial films on Si substrates for the fabrication of high-performance GaN-based devices.

Growth mechanisms of GaN epitaxial films grown on ex situ low-temperature AlN templates on Si substrates by the combination methods of PLD and MOCVD

Three kinds of ex situ low-temperature AlN (LT-AlN) templates grown on Si substrates by pulsed laser deposition (PLD) are employed to grow GaN epitaxial films by metal organic chemical vapor deposition (MOCVD). The single-crystalline ex situ LT-AlN template with smooth surface shows the greatest advantage to achieve high-quality GaN epitaxial films. It can effectively promote the lateral growth of high-temperature AlN layer to obtain coalesced and smooth surface, and therefore reduce the GaN nucleation energy barrier to facilitate GaN nucleation process. The as-grown truncated GaN nuclei with large sizes and low density are favorable for the two-dimensional growth of GaN. The ∼1.5 μm-thick GaN epitaxial film grown on this ex situ LT-AlN template shows a flat surface and the best crystalline quality with the minimum full-width at half maximums (FWHMs) of GaN(0002) and GaN(10–12) X-ray rocking curves (XRCs) as 504 and 566 arcsec, respectively, and the minimum FWHM of near band emission peak as 20.62 nm. According to the XRC results, compared with the GaN film grown without ex situ LT-AlN templates, the screw-type threading dislocations (TDs) density, edge- and mixed-type TDs density in as-grown GaN film are greatly reduced by 20% and 49%, respectively, proving the superiority of ex situ LT-AlN template. This work is instructional for the growth of high-quality nitride films by combination methods, and significant for achieving high-performance GaN-based devices.

Effect of growth temperature on the properties of GaN epitaxial films grown on magnesium aluminate scandium oxide substrates by pulsed laser deposition

High-quality GaN epitaxial films have been grown on magnesium aluminate scandium oxide (ScMgAlO4) substrates at low temperature of 450°C with an in-plane epitaxial relationship of GaN[1-100]//ScMgAlO4[1-100]. It is found that the ~300nm-thick GaN epitaxial films grown at 450°C show the full-width at half-maximums (FWHMs) for GaN(0002) and GaN (10-12) of 0.18° and 0.40°, respectively, and very smooth surface with a root-mean-square (RMS) surface roughness of 1.5nm. There is no interfacial layer existing between GaN and ScMgAlO4. However, as the growth temperature increases, the crystalline quality, surface morphology, and interfacial properties of as-grown GaN epitaxial films are deteriorated gradually. These GaN epitaxial films are of great importance for the fabrication of highly-efficient GaN-based devices.

A new approach to epitaxially grow high-quality GaN films on Si substrates: the combination of MBE and PLD.

High-quality GaN epitaxial films have been grown on Si substrates with Al buffer layer by the combination of molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) technologies. MBE is used to grow Al buffer layer at first, and then PLD is deployed to grow GaN epitaxial films on the Al buffer layer. The surface morphology, crystalline quality, and interfacial property of as-grown GaN epitaxial films on Si substrates are studied systematically. The as-grown ~300 nm-thick GaN epitaxial films grown at 850 °C with ~30 nm-thick Al buffer layer on Si substrates show high crystalline quality with the full-width at half-maximum (FWHM) for GaN(0002) and GaN(102) X-ray rocking curves of 0.45° and 0.61°, respectively; very flat GaN surface with the root-mean-square surface roughness of 2.5 nm; as well as the sharp and abrupt GaN/AlGaN/Al/Si hetero-interfaces. Furthermore, the corresponding growth mechanism of GaN epitaxial films grown on Si substrates with Al buffer layer by the combination of MBE and PLD is hence studied in depth. This work provides a novel and simple approach for the epitaxial growth of high-quality GaN epitaxial films on Si substrates.

A comparative study on the properties of c-plane and a-plane GaN epitaxial films grown on sapphire substrates by pulsed laser deposition

∼500 nm-thick c-plane and a-plane GaN epitaxial films on c-plane and r-plane sapphire substrates by pulsed laser deposition have been grown and explored. The ∼500 nm-thick c-plane GaN epitaxial films grown on c-plane sapphire substrates show high crystalline quality with full-width at half-maximum (FWHM) for GaN(0002) and GaN(10–12) X-ray rocking curves (XRCs) of 0.20° and 0.37°, respectively, very smooth GaN surface with a root-mean-square (RMS) surface roughness of 1.4 nm, sharp and abrupt c-plane GaN/c-plane sapphire hetero-interfaces, a −0.116 GPa residual stress, and high optical property. These results are in striking contrast to those of ∼500 nm-thick a-plane GaN epitaxial films grown on r-plane sapphire substrates. The ∼500 nm-thick a-plane GaN epitaxial films grown on r-plane sapphire substrates show poor crystalline quality with FWHM for GaN(11–20) and GaN(10–11) XRC of 0.72° and 0.81°, respectively, rough GaN surface with an RMS surface roughness of 7.2 nm, indistinct a-plane GaN/r-plane sapphire hetero-interface with a ∼1.5 nm-thick interfacial layer, a −0.884 GPa residual stress, and poor optical property. This work brings further insight into fully understanding of the properties of GaN epitaxial films grown on c-plane and r-plane sapphire substrates for obtaining high-quality GaN epitaxial films in the application of GaN-based devices.

High-quality crack-free GaN epitaxial films grown on Si substrates by a two-step growth of AlN buffer layer

High-quality crack-free GaN epitaxial films have been grown on Si substrates by metal organic chemical vapor deposition (MOCVD) using a two-step growth of AlN buffer layer. The AlN buffer layer is first grown at a V/III ratio of 2000 to obtain a 3 dimensional (3D) growth mode, and then the V/III ratio is decreased to 800 to accelerate the coalescence of the AlN grains. In comparison to the single-step AlN buffer layer growth at a fixed V/III ratio of 800, the surface morphology of the AlN buffer layer is preferably coalesced and the crystalline quality is significantly improved via the two-step growth. Furthermore, it is demonstrated that the improvement of the AlN buffer layer driven by the two-step growth method contributes greatly to improving the crystalline quality of the subsequently grown GaN epitaxial films and suppressing the generation of cracks. The excellent quality of the GaN epitaxial films is confirmed by high resolution X-ray diffraction, with full-width at half-maximum values of 338 and 390 arcsec for GaN (0002) and GaN (10–12) rocking curves, respectively. The relationships between the properties of the AlN buffer layers, including the surface morphology and crystalline quality, and the stress condition and crystalline quality of the GaN epitaxial films are elucidated. This work offers an effective approach to achieving high-quality GaN epitaxial films on Si substrates for the application of GaN-based devices.

The impact of nanoporous SiN x interlayer growth position on high-quality GaN epitaxial films

The impact of nanoporous SiN x interlayer growth position on high-quality GaN epitaxial film was elucidated from the behavior of dislocations. The best quality GaN film was achieved when a nanoporous SiN x interlayer was grown on a rough layer, with the high-resolution X-ray diffraction rocking curve full width at half maximum for ( $1\bar 102$ ) reflection decreasing to 223 arcs, and the total dislocation density reduced to less than 1.0×108 cm−2. GaN films were grown on sapphire substrates by metal organic chemical vapor deposition. The quality of these films was investigated with high-resolution X-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. A preference for the formation of half-loops to reduce threading dislocations was observed when an SiN x interlayer was grown on a rough layer. A growth mechanism is proposed to explain this preference.

SiNx插入层的生长位置对GaN外延薄膜性质的影响

GaN films with SiNx interlayer were grown on sapphire substrates by metal organic chemical vapor deposition.The effect of nanoporous SiNx interlayer growth position on the properties of high-quality GaN epitaxial films was investigated systematically.The high-resolution X-ray diffraction spectra was achieved when a SiNx interlayer was adopted and inserted on a rough GaN layer.The screw and edge dislocation densities have been calculated.The optical and electrical properties of the GaN films were characterized by Raman scattering spectra,low temperature photoluminescence spectra and Hall measurements,etc.The position of the SiNx interlayer has no impact on the strain in GaN films,but the residual carrier concentration changes with the position of the interlayer.

Buffer-layer-free growth of high-quality epitaxial GaN films on 4H-SiC substrate by metal-organic chemical vapor deposition

High-quality GaN epitaxial films were successfully grown directly on 4H-SiC substrates. The difficulty in the nucleation as well as the overgrowth of GaN islands, which is often observed during metal-organic chemical vapor deposition, was solved using the concept of an epitaxial lateral overgrowth technique. The continuous mirror-flat GaN epitaxial film could be obtained at a reduced reactor pressure (76 Torr) and a lower N/Ga supply ratio. The full-width half-maximums (FWHMs) of the (0 0 0 2) and (1 0 1 ¯ 3) rocking curves of the GaN epitaxial films on the 4H-SiC substrate at the optimized condition were 105 and 118 arcsec, respectively, under a skew symmetric diffraction geometry. The intense bound exciton lines with a FWHM of 10 meV and a free exciton peak appeared in the low-temperature photoluminescence spectrum, illustrating the very high quality of the GaN film on 4H-SiC.

Iodine vapor phase growth of GaN: dependence of epitaxial growth rate on process parameters

High quality GaN epitaxial films have been grown on sapphire using iodine vapor phase growth (IVPG). The growth mechanism has been investigated to determine the effect of pressure, flow rate, and V–III ratio on the crystalline quality of GaN epi-layers. A major distinction between this process and conventional hydride vapor phase epitaxy (HVPE) is the replacement of HCl gas by iodine as a reactant. Ammonia is the source of nitrogen, and forming gas or hydrogen is used as carrier gas. C-plane sapphire wafers with pre-deposited 1 μm MOCVD films were used as substrates. Growth conditions were investigated in the range of pressure between 1 and 0.1 atmosphere, while maintaining the same temperature gradient and mass flow rates for the reactant gases. Within these parameters, it was found that the fastest growth rate for GaN layers is obtained at vapor pressures close to equilibrium. The data indicates crystal quality is also optimized under these conditions. Crystal quality was determined from the FWHM of X-ray rocking curves and room temperature photoluminescence (PL). The crystal quality was found to improve under conditions where the ambient pressure and supersaturation approach equilibrium.

Improvement of the crystallinity of GaN epitaxial layers grown on porous Si (100) layers by using a two-step method

A new approach was used for combining GaN and porous Si with the goal of producing high-quality GaN epitaxial layers for optoelectronic integrated circuit devices based on Si substrates. Reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), photoluminescence (PL), and Van der Pauw–Hall effect measurements were performed to investigate the structural, optical, and electrical properties of the GaN epitaxial films grown on porous Si(100) by plasma-assisted molecular-beam epitaxy with a two-step method. The RHEED patterns were streaky with clear Kikuchi lines, which was direct evidence for layer-by-layer two-dimensional growth of GaN epitaxial layers on porous Si layers. The XRD curves showed that the grown layers were GaN(0001) epitaxial films. The results of the XRD and the PL measurements showed that the crystallinities of the GaN epilayers grown on porous Si by using a two-step growth were remarkably improved because the porous Si layer reduced the strains in the GaN epilayers by sharing them with the Si substrates. Hall-effect measurements showed that the mobility of the GaN active layer was higher than that of the GaN initial layer. These results indicate that high-quality GaN epitaxial films grown on porous Si(100) by using two-step growth hold promise for potential applications in new kinds of optoelectronic monolithic and ultralarge integrated circuits.

Effects of multiple buffer layers on structural electronic properties of GaN growth by atmospheric pressure Organometallic Vapor Phase Epitaxy

High-quality GaN epitaxial films have been grown on sapphire by organometallic vapor phase epitaxy using multiple-pair buffer layers. Each pair of buffer layers consists of a thin GaN nucleation layer grown at a low temperature around 500°C and a thick GaN epitaxial layer around 4 μm thick grown at a high temperature around 1000°C. The sample with four-pair buffer layers showed much improved GaN epitaxial films, as compared to the sample with only one-pair of buffer layers. The better qualities include narrower full width at half maximum of 150 arc-s and stronger intensity in double-crystal X-ray diffraction, higher electron mobility of 420 cm 2 V-s −1, lower background concentration of 3×10 17 cm −3, and lower etch-pit density of mid-10 5 cm −2.