Investigation of void formation beneath thin AlN layers by decomposition of sapphire substrates for self-separation of thick AlN layers grown by HVPE

Void formation at the interface between thick AlN layers and (0 0 0 1) sapphire substrates was investigated to form a predefined separation point of the thick AlN layers for the preparation of freestanding AlN substrates by hydride vapor phase epitaxy (HVPE). By heating 50–200 nm thick intermediate AlN layers above 1400 °C in a gas flow containing H₂ and NH₃, voids were formed beneath the AlN layers by the decomposition reaction of sapphire with hydrogen diffusing to the interface. The volume of the sapphire decomposed at the interface increased as the temperature and time of the heat treatment was increased and as the thickness of the AlN layer decreased. Thick AlN layers subsequently grown at 1450 °C after the formation of voids beneath the intermediate AlN layer with a thickness of 100 nm or above self-separated from the sapphire substrates during post-growth cooling with the aid of voids. The 79 μm thick freestanding AlN substrate obtained using a 200 nm thick intermediate AlN layer had a flat surface with no pits, high optical transparency at wavelengths above 208.1 nm, and a dislocation density of 1.5×10⁸ cm⁻².     

MOVPE growth of single-crystal hexagonal AlN on cubic diamond

We have obtained single-crystal aluminum nitride (AlN) layers on diamond (1 1 1) substrates by metalorganic vapor-phase epitaxy (MOVPE). When the thermal cleaning temperature of the substrate and growth temperature of the AlN layer were below 1100 °C, the AlN layer had multi-domain structures mainly consisting of rotated domains. An interface layer, consisting of amorphous carbon and poly-crystal AlN, was formed between the AlN layer and the diamond substrate. On the other hand, when the thermal cleaning temperature and growth temperature were above 1200 °C, a single-crystal AlN layer was grown and no interface layer was formed. Therefore, we attribute the multi-domain structures to the interface layer. Even at the growth temperature of 1100 °C, by performing the thermal cleaning at 1200 °C, the single-crystal AlN layer was obtained, indicating that the thermal cleaning temperature of the substrate is a critical factor for the formation of the interface layer. The epitaxial relationship between the single-crystal AlN layer and the diamond (1 1 1) substrate was determined to be [0 0 0 1]_AlN∥[1 1 1]_diamond and [1 0 1¯ 0]_AlN∥[1 1¯ 0]_diamond. The AlN surface had Al polarity and no inversion domains were observed in the AlN layer.     

Low-pressure HVPE growth of crack-free thick AlN on a trench-patterned AlN template

A thick AlN layer was grown on a trench-patterned AlN/sapphire template by low-presssure hydride vapor phase epitaxy (LP-HVPE). Compared with the AlN layer grown on a flat AlN/sapphire template, the AlN layer grown on the trench-patterned AlN/sapphire template had a crack-free and smooth surface. The typical full-widths at half-maximum (FWHMs) of X-ray rocking curves (XRC) for the (0 0 0 2), (1 0 1¯ 2), and (1 0 1¯ 0) diffractions of the AlN layer on the trench-patterned AlN/sapphire template were 132, 489, and 594 arcsec, respectively. In addition, atomic steps were observed on the AlN layer on the trench-patterned AlN/sapphire template, and the root-mean-square (RMS) roughness of the AlN layer was determined to be 0.602 nm by atomic force microscopy (AFM).     

AlGaN epitaxial lateral overgrowth on Ti-evaporated GaN/sapphire substrate

AlGaN growth using epitaxial lateral overgrowth (ELO) by metalorganic chemical vapor deposition on striped Ti, evaporated GaN on sapphire, has been investigated. AlGaN/AlN films growth on GaN/AlGaN superlattices (SLs) structure on the Ti masks, with various SLs growth temperature (1030, 1060 and 1090 °C) were grown. With increasing the growth temperature, AlGaN surface became flat. The AlGaN film had a cathodoluminescence peak around 345 nm. However, in secondary ion mass spectrometry (SIMS) measurement, Ti signal was detected on the top of AlGaN surface when GaN/AlGaN SLs was grown on Ti striped masks. By inserting the AlN blocking layer on SLs, Ti diffusion was stopped at the AlN layer, and the AlGaN crystalline quality was improved.     

Growth and characterization of c-plane AlGaN on γ-LiAlO2

This study demonstrates a pure c-plane AlGaN epilayer grown on a γ-LiAlO₂ (1 0 0) (LAO) substrate with an AlN nucleation layer grown at a relatively low temperature (LT-AlN) by metal-organic chemical vapor deposition (MOCVD). The AlGaN film forms polycrystalline film with m- and c-plane when the nucleation layer grows at a temperature ranging from 660 to 680 °C. However, a pure c-plane AlGaN film with an Al content of approximately 20% can be obtained by increasing the LT-AlN nucleation layer growth temperature to 700 °C. This is because the nuclei density of AlN increases as the growth temperature increases, and a higher nuclei density of AlN deposited on LAO substrate helps prevent the deposition of m-plane AlGaN. Therefore, high-quality and crack-free AlGaN films can be obtained with a (0 0 0 2) ω-rocking curve FWHM of 547 arcsec and surface roughness of 0.79 nm (root-mean-square) using a 700-°C-grown LT-AlN nucleation layer.     

Growth characteristics of AlN on sapphire substrates by modified migration-enhanced epitaxy

We investigated the effect of growth parameters for obtaining high-quality AlN grown directly on sapphire substrates by a hybridized method, derived from simultaneous source supply and conventional migration-enhanced epitaxy. At an optimal growth temperature of 1200 °C, AlN was atomically smooth and pit-free, while below and above 1200 °C, AlN was rough and with pits, respectively. Surface morphologies also depended on the V/III ratio. Rough surfaces became atomically smooth but then pits appeared, as the V/III ratio increased. The crystallinity revealed by X-ray diffraction changed accordingly. The 600-nm-thick AlN grown under the optimal conditions showed X-ray line widths of as narrow as ∼43 and ∼250 arcsec for (0 0 0 2) and (1 0 1¯ 2) diffractions, respectively.     

HRTEM investigation of high-reflectance AlN/GaN distributed Bragg-reflectors by inserting AlN/GaN superlattice

A high-quality AlN/GaN distributed Bragg-reflectors (DBR) was successfully grown on sapphire substrate by low-pressure metal-organic chemical vapor deposition using ultra-thin AlN/GaN superlattice insertion layers (SLILs). The reflectivity of AlN/GaN DBR with ultra-thin AlN/GaN SLIL was measured and achieved blue peak reflectivity of 99.4% at 462 nm. The effect of ultra-thin AlN/GaN superlattice insertion layer was examined in detail by transmission electron microscopy, and indicated that the crack of AlN/GaN DBR can be suppress by inserting AlN/GaN SLIL. For electronic properties, the turn on voltage is about 4.1 V and CW laser action of vertical-cavity surface-emitting laser (VCSEL) was achieved at a threshold injection current of 1.4 mA at 77 K, with an emission wavelength of 462 nm.     

HVPE growth of semi-polar (1 1 2¯ 2)GaN on GaN template (1 1 3)Si substrate

The hydride-vapour-phase-epitaxial (HVPE) growth of semi-polar (1 1 2¯ 2)GaN is attempted on a GaN template layer grown on a patterned (1 1 3) Si substrate. It is found that the chemical reaction between the GaN grown layer and the Si substrate during the growth is suppressed substantially by lowering the growth temperatures no higher than 900 °C. And the surface morphology is improved by decreasing the V/III ratio. It is shown that a 230-μm-thick (1 1 2¯ 2)GaN with smooth surface is obtained at a growth temperature of 870 °C with V/III of 14.     

Growth of GaN films on PLD-deposited TaC substrates

GaN films were grown by metal organic chemical vapor deposition on TaC substrates that were created by pulsed laser deposition of TaC onto (0 0 0 1) SiC substrates at ∼1000 °C. This was done to determine if good quality TaC films could be grown, and if good quality GaN films could be grown on this closely lattice matched to GaN, conductive material. This was done by depositing the TaC on on-axis and 3° or 8° off-axis (0 0 0 1) SiC at temperatures ranging from 950 to 1200 °C, and examining them using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The GaN films were grown on as-deposited TaC films, and films annealed at 1200, 1400, or 1600 °C, and examined using the same techniques. The TaC films were polycrystalline with a slight (1 1 1) texture, and the grains were ∼200 nm in diameter. Films grown on-axis were found to be of higher quality than those grown on off-axis substrates, but the latter could be improved to a comparable quality by annealing them at 1200–1600 °C for 30 min. TaC films deposited at temperatures above 1000 °C were found to react with the SiC. GaN films could be deposited onto the TaC when the surface was nitrided with NH₃ for 3 min at 1100 °C and the low temperature buffer layer was AlN. However, the GaN did not nucleate easily on the TaC film, and the crystallites did not have the desired (0 0 0 1) preferred orientation. They were ∼10 times larger than those typically seen in films grown on SiC or sapphire. Also the etch pit concentration in the GaN films grown on the TaC was more than 2 orders of magnitude less than it was for growth on the SiC.     

MOVPE growth of InN buffer layers on sapphire

We report on the MOCVD growth of InN buffer layers on sapphire substrate for InN growth. The approach used assumes that an optimized InN buffer layer has to exhibit at least the same crystalline quality and sapphire surface coverage than the GaN buffer layers allowing to grow high crystalline quality GaN on sapphire. The buffer layers were characterized by AFM and GID measurements. Sapphire nitridation was investigated: it has a strong influence on in-plane crystalline quality. Two kinds of buffer layers were optimized according to the GaN buffer layer specifications: one of them only presented In droplets at its surface. It was shown that the small amount of In droplets increases the adatoms mobility of the main layer overgrown, leading to a 25% decrease of its in-plane mosaicity, compared to InN films directly grown on sapphire. To achieve a same improvement on InN buffer layer free of In droplets, the InN main layer growth temperature had to be increased from 550 °C. to 600 °C.     

The influence of the Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate

The influence of Al pre-deposition on the properties of AlN buffer layer and GaN layer grown on Si (1 1 1) substrate by metalorganic chemical vapor deposition (MOCVD) has been systematically studied. Compared with the sample without Al pre-deposition, optimum Al pre-deposition time could improve the AlN buffer layer crystal quality and reduce the root mean square (RMS) roughness. Whereas, overlong Al-deposition time deteriorated the AlN crystal quality and Al-deposition patterns could be found. Cracks and melt-back etching patterns appeared in the GaN layer grown without Al pre-deposition. With suitable Al-deposition time, crack-free 2.0 μm GaN was obtained and the full-width at half-maximum (FWHM) of (0 0 2) plane measured by double crystal X-ray diffraction (DCXRD) was as low as 482 arcsec. However, overlong Al-deposition time would result in a great deal of cracks, and the crystal quality of GaN layer deteriorated. The surface of GaN layer became rough in the region where the Al-deposition patterns were formed due to overlong Al-deposition time.     

Epitaxial NiO (1 0 0) and NiO (1 1 1) films grown by atomic layer deposition

Epitaxial NiO (1 1 1) and NiO (1 0 0) films have been grown by atomic layer deposition on both MgO (1 0 0) and α-Al₂O₃ (0 0 l) substrates at temperatures as low as 200 °C by using bis(2,2,6,6-tetramethyl-3,5-heptanedionato)Ni(II) and water as precursors. The films grown on the MgO (1 0 0) substrate show the expected cube on cube growth while the NiO (1 1 1) films grow with a twin rotated 180° on the α-Al₂O₃ (0 0 l) substrate surface. The films had columnar microstructures on both substrate types. The single grains were running throughout the whole film thickness and were significantly smaller in the direction parallel to the surface. Thin NiO (1 1 1) films can be grown with high crystal quality with a FWHM of 0.02–0.05° in the rocking curve measurements.     

Improvements in a-plane GaN crystal quality by AlN/AlGaN superlattices layers

Non-polar (1 1 2¯ 0) a-plane GaN films have been grown by low-pressure metal-organic vapor deposition on r-plane (1 1¯ 0 2) sapphire substrate. We report on an approach of using AlN/AlGaN superlattices (SLs) for crystal quality improvement of a-plane GaN on r-plane sapphire. Using X-ray diffraction and atomic force microscopy measurements, we show that the insertion of AlN/AlGaN SLs improves crystal quality, reduces surface roughness effectively and eliminates triangular pits on the surface completely.     

MOVPE growth and properties of GaN on (1 1 1)Si using an AlInN intermediate layer

Using an AlInN intermediate layer, GaN was grown on (1 1 1)Si substrate by selective metalorganic vapor phase epitaxy. The variation of the surface morphology was investigated as a function of the In composition and thickness of the AlInN layer. It was found that the In composition in the AlInN layer was a function of the growth temperature and thickness. Because of the small band offset at the AlInN/Si hetero-interface, we have achieved a low series resistance of the order of 9 Ω (0.0036 Ω cm²) across the GaN/AlInN/AlN/Si layer structure.     

Effects of initial stages on the crystal quality of nonpolar a-plane AlN on r-plane sapphire by low-pressure HVPE

A 4–6 μm thick a-plane (1 1 2¯ 0) AlN was grown on r-plane sapphire substrate by low-pressure hydride vapor phase epitaxy (LP-HVPE), using a direct growth without any nitridation and buffer layer, a single-step nitridation growth, a two-step nitridation growth and a two-step buffer growth method. For the two-step buffer growth procedure, smoother surface is observed with the lower full widths at half maximum (FWHM) of X-ray rocking curves (XRC) compared with the other two kinds of nitridation procedures. A smaller FWHM of in-plane XRC peak anisotropy features are reversed, which is consistent with the smaller in-plane stress anisotropic distribution in a-plane AlN, when the two-step nitridation or buffer growth method is used. In four kinds of initial growth procedures, the two-step buffer method is the suitable method for the growth of a-plane AlN by HVPE with the high crystal quality and more isotropic distribution.     

MOVPE growth of InN films using 1,1-dimethylhydrazine as a nitrogen precursor

InN films have been successfully grown on sapphire substrates by MOVPE using trimethylindium (TMIn) and 1,1-dimethylhydrazine (DMHy) with N₂ carrier. DMHy is an advantageous precursor of N as it decomposes efficiently at relatively low temperature (T₅₀=420 °C) compatible with the InN growth. The reactor is specially designed so as to avoid parasitic reaction between TMIn and DMHy occurring at room temperature. The growth feature was studied by varying growth temperature, V/III ratio, TMIn flow and reactor pressure. The InN films were obtained at 500–570 °C and 60–200 Torr with a V/III ratio optimized to 100–200. The In droplets are seen on the grown surfaces, indicating an excess supply of TMIn. It is demonstrated that the InN films grows on the sapphire substrate in a single domain with an epitaxial relationship, [1 01¯ 0]_InN//[1 1 2¯ 0]_sapphire.     

Characterizations of GaN films and AlGaN/GaN heterostructures on vicinal sapphire (0 0 0 1) substrates grown by MOCVD

GaN films and AlGaN/GaN heterostructures grown on vicinal sapphire (0 0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that surface morphologies of GaN films depend on the vicinal angle, however, they are not sensitive to the inclination directions of the substrate. The optimized vicinal angle for obtaining excellent surface morphology is around 0.5°. This conclusion is also confirmed by characterizing the electrical property of two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure.     

The effects of annealing on non-polar (1 1 2¯ 0) a-plane GaN films

Non-polar a-plane (1 1 2¯ 0) GaN films were grown on r-plane sapphire by metal–organic vapor phase epitaxy and were subsequently annealed for 90 min at 1070 °C. Most dislocations were partial dislocations, which terminated basal plane stacking faults. Prior to annealing, these dislocations were randomly distributed. After annealing, these dislocations moved into arrays oriented along the [0 0 0 1] direction and aligned perpendicular to the film–substrate interface throughout their length, although the total dislocation density remained unchanged. These changes were accompanied by broadening of the symmetric X-ray diffraction 1 1 2¯ 0 ω-scan widths. The mechanism of movement was identified as dislocation glide, occurring due to highly anisotropic stresses (confirmed by X-ray diffraction lattice parameter measurements) and evidenced by macroscopic slip bands observed on the sample surface. There was also an increase in the density of unintentionally n-type doped electrically conductive inclined features present at the film–substrate interface (as observed in cross-section using scanning capacitance microscopy), suggesting out-diffusion of impurities from the substrate along with prismatic stacking faults. These data suggest that annealing processes performed close to film growth temperatures can affect both the microstructure and the electrical properties of non-polar GaN films.     

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10⁷ cm⁻²) and a higher edge-dislocation density (8.157×10⁹ cm⁻²) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.     

Effects of temperature and carrier gas flow amount on the formation of GaN nanorods by the HVPE method

We fabricated one-dimensional GaN nanorods on AlN/Si (1 1 1) substrates at various temperatures, and carrier gas flow amount, using the hydride vapor phase epitaxy (HVPE) method. An AlN buffer layer of 50 nm thickness was deposited by RF sputtering for 25 min. Stalagmite-like GaN nanorods formed at a growth temperature of 650 °C. The diameters and lengths of GaN nanorods increase with growth time, whereas the density of nanorods decreases. And we performed the experiments by changing the carrier gas flow amount at a growth temperature of 650 °C and HCl:NH₃ flow ratio of 1:40. GaN nanorods, with an average diameter of 50 nm, were obtained at a carrier gas flow amount of 1340 sccm. The shape, structures, and optical characteristics of the nanorods were investigated by field-emission scanning electron microscopy, X-ray diffraction, and photoluminescence.