Compensation effect of Mg-doped a- and c-plane GaN films grown by metalorganic vapor phase epitaxy

The electrical and optical properties of Mg-doped a- and c-plane GaN films grown by metalorganic vapor phase epitaxy were systematically investigated. The photoluminescence spectra of Mg-doped a- and c-plane GaN films exhibit strong emissions related to deep donors when Mg doping concentrations are above 1×10²⁰ cm⁻³ and 5×10¹⁹ cm⁻³, respectively. The electrical properties also indicate the existence of compensating donors because the hole concentration decreases at such high Mg doping concentrations. In addition, we estimated the N_D/N_A compensation ratio of a- and c-plane GaN by variable-temperature Hall effect measurement. The obtained results indicate that the compensation effect of the Mg-doped a-plane GaN films is lower than that of the Mg-doped c-plane GaN films.     

Effect of NH3 flow rate on m-plane GaN growth on m-plane SiC by metalorganic chemical vapor deposition

This paper reports a study of the effect of NH₃ flow rate on m-plane GaN growth on m-plane SiC with an AlN buffer layer. It is found that a reduced NH₃ flow rate during m-plane GaN growth can greatly improve the recovery of in situ optical reflectance and the surface morphology, and narrow down the on-axis (1 0 1¯ 0) X-ray rocking curve (XRC) measured along the in-plane a-axis. The surface striation along the in-plane a-axis, a result of GaN island coalescence along the in-plane c-axis, strongly depends on the NH₃ flow rate, an observation consistent with our recent study of kinetic Wulff plots. The pronounced broadening of the (1 0 1¯ 0) XRC measured along the c-axis is attributed to the limited lateral coherence length of GaN domains along the c-axis, due to the presence of a high density of basal-plane stacking faults, most of which are formed at the GaN/AlN interface, according to transmission electron microscopy.     

Photoluminescence study of Si-doped a-plane GaN grown by MOVPE

Si-doped a-plane GaN films with different doping concentrations were grown by metal-organic vapor phase epitaxy. A mirrorlike surface without pits or anisotropic stripes was observed by optical microscopy. Detailed optical properties of the samples were characterized by temperature- and excitation-intensity-dependent PL measurements. A series of emission peaks at 3.487, 3.440, 3.375–3.350, 3.290 and 3.197 eV were observed in the low-temperature PL spectra of all samples. The origin of these emissions is discussed in detail.     

Growth of high quality a-plane GaN epi-layer on r-plane sapphire substrates with optimization of multi-buffer layer

Nonpolar (1 1–2 0) a-plane GaN films have been grown using the multi-buffer layer technique on (1–1 0 2) r-plane sapphire substrates. In order to obtain epitaxial a-plane GaN films, optimized growth condition of the multi-buffer layer was investigated using atomic force microscopy, high resolution X-ray diffraction, and transmission electron microscopy measurements. The experimental results showed that the growth conditions of nucleation layer and three-dimensional growth layer significantly affect the crystal quality of subsequently grown a-plane GaN films. At the optimized growth conditions, omega full-width at half maximum values of (11–20) X-ray rocking curve along c- and m-axes were 430 and 530 arcsec, respectively. From the results of transmission electron microscopy, it was suggested that the high crystal quality of the a-plane GaN film can be obtained from dislocation bending and annihilation by controlling of the island growth mode.     

Assessment of defect reduction methods for nonpolar a-plane GaN grown on r-plane sapphire

This work assesses the relative effectiveness of four techniques to reduce the defect density in heteroepitaxial nonpolar a-plane GaN films grown on r-plane sapphire by metalorganic vapour phase epitaxy (MOVPE). The defect reduction techniques studied were: 3D–2D growth, SiN_x interlayers, ScN interlayers and epitaxial lateral overgrowth (ELOG). Plan-view transmission electron microscopy (TEM) showed that the GaN layer grown in a 2D fashion had a dislocation and basal-plane stacking fault (BSF) density of (1.9±0.2)×10¹¹ cm⁻² and (1.1±0.9)×10⁶ cm⁻¹, respectively. The dislocation and BSF densities were reduced by all methods compared to this 2D-grown layer (used as a seed layer for the interlayer and ELOG methods). The greatest reduction was achieved in the (0 0 0 1) wing of the ELOG sample, where the dislocation density was <1×10⁶ cm⁻² and BSF density was (2.0±0.7)×10⁴ cm⁻¹. Of the in-situ techniques, SiN_x interlayers were most effective: the interlayer with the highest surface coverage that was studied reduced the BSF density to (4.0±0.2)×10⁵ cm⁻¹ and the dislocation density was lowered by over two orders of magnitude to (3.5±0.2)×10⁸ cm⁻².     

Optical properties of MOVPE-grown a-plane GaN and AlGaN

a-Plane GaN and AlGaN were grown on r-plane sapphire by low-pressure metal-organic vapor epitaxy (LP-MOVPE), and the effects of reactor pressure (from 40 to 500 Torr) and growth temperature (from 1020 to 1100 °C) on the crystalline quality and surface morphology of a-plane GaN were studied. The a-plane GaN grown under 40 Torr had a smooth-surface morphology but a poor crystalline quality; however, the a-plane GaN grown under 500 Torr had higher crystalline quality and optical properties, whose full-width at half-maximum of the X-ray rocking curve (XRC-FWHM) and intensity of yellow luminescence (YL) were smaller. Furthermore, the optical properties of a-plane GaN were investigated by photoluminescence (PL) in detail. We also studied the emission properties of a-plane Al_0.35Ga_0.65N grown at room temperature.     

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.     

Fabrication of freestanding m-plane GaN wafer by using the HVPE technique on an aluminum carbide buffer layer on an m-plane sapphire substrate

A freestanding m-plane GaN wafer is fabricated by using the hydride vapor-phase epitaxy (HVPE) technique on an aluminum carbide buffer layer on an m-plane sapphire substrate. X-ray pole-figure measurements show a clear m-plane orientation of the GaN surface. The full-width at half-maximum (FWHM) of GaN (1 1¯ 0 0) X-ray rocking curve (XRC) with the scattering vector along the [1 1 2¯ 0] direction is approximately 800 arcsec; this indicates good crystallinity. On the other hand, the FWHM for the case in which the scattering vector is oriented along the [0 0 0 1] direction is broad; this suggests the influence of structural defects along this direction. In fact, basal plane stacking faults (BSF) with a density of approximately 3×10⁵ cm⁻¹ is observed by transmission electron microscopy (TEM). The preparation of a 45-mm-diameter m-plane GaN wafer due to spontaneous separation of the GaN layer from the sapphire substrate is demonstrated.     

Hydrothermal growth of ZnO nanorods on a-plane GaN/sapphire template

ZnO nanorod arrays are grown on a-plane GaN template/r-plane sapphire substrates by hydrothermal technique. Aqueous solutions of zinc nitrate hexahydrate and hexamethylenetetramine were employed as growth precursors. Electron microscopy and X-ray diffraction measurements were carried out for morphology, phase and growth orientation analysis. Single crystalline nanorods were found to have off-normal growth and showed well-defined in-plane epitaxial relationship with the GaN template. The 〈0 0 0 1〉 axis of the ZnO nanorods were observed to be parallel to the 〈1 0 1¯ 0〉 of the a-plane GaN layer. Optical property of the as-grown ZnO nanorods was analyzed by room temperature photoluminescence measurements.     

One-sidewall-seeded epitaxial lateral overgrowth of a-plane GaN by metalorganic vapor-phase epitaxy

The selective regrowth of GaN during sidewall-seeded epitaxial lateral overgrowth was performed. In addition to adjusting the V/III ratio, control of offset angle of the sidewall was found to be effective for realizing one-sidewall-seeded a-plane (1 1 2¯ 0) GaN on r-plane (1 1¯ 0 2) sapphire. The number of coalescence regions on the grooves was reduced, and threading-dislocation and stacking-fault densities as low as 10⁶–10⁷ cm⁻² and 10³–10⁴ cm⁻¹, respectively, were successfully realized.     

Preparation of single-crystalline ZnO films on ZnO-buffered a-plane sapphire by chemical bath deposition

High-quality zinc oxide (ZnO) films were successfully grown on ZnO-buffered a-plane sapphire (Al₂O₃ (1 1 2¯ 0)) substrates by controlling temperature for lateral growth using chemical bath deposition (CBD) at a low temperature of 60 °C. X-ray diffraction analysis and transmission electron microscopy micrographs showed that the ZnO films had a single-crystalline wurtzite structure with c-axis orientation. Rocking curves (ω-scans) of the (0 0 0 2) reflections showed a narrow peak with full width at half maximum value of 0.50° for the ZnO film. A reciprocal space map indicated that the lattice parameters of the ZnO film (a=0.3250 nm and c=0.5207 nm) were very close to those of the wurtzite-type ZnO. The ZnO film on the ZnO-buffered Al₂O₃ (1 1 2¯ 0) substrate exhibited n-type conduction, with a carrier concentration of 1.9×10¹⁹ cm⁻³ and high carrier mobility of 22.6 cm² V⁻¹ s⁻¹.     

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.     

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.     

Mg doping behavior of MOVPE InxGa1−xN (x∼0.4)

The Mg doping behavior of MOVPE indium gallium nitride (InGaN), such as secondary ion mass spectrometry (SIMS) Mg profile, crystalline quality and n–p conversion of the films are described and discussed in this paper. The SIMS analysis reveals that the memory effect of Cp₂Mg as a doping source deteriorates the controllability of Mg doping level and profile, especially for thin (−0.4 μm) InGaN. The high residual donors (10¹⁹–10²⁰ cm⁻³) in InGaN with In content from 0.05 to 0.37 can be compensated by Mg doping and p-type conduction is obtained for those with In content up to 0.2. It is found that a higher Cp₂Mg flow rate is needed to get p-type conduction in InGaN with a higher In content x; for example, Cp₂Mg/(TEG+TMI)≈0.5% for x=0 (GaN), ≈2% for x=0.05 and ≈4% for x=0.2. Such a high Cp₂Mg flow rate is needed due to the high residual donor concentration (10¹⁹–10²⁰ cm⁻³) of InGaN films and the low activation efficiency of Mg. The crystalline quality of InGaN is deteriorated with increasing In content as well as Mg doping level. To achieve a p-type InGaN with a lower Mg doping, it is essential to improve the crystalline quality of non-doped InGaN. For this purpose, the use of a thicker GaN interlayer is effective.     

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.     

Ammonothermal epitaxy of wurtzite GaN using an NH4I mineralizer

Purely wurtzite phase needle crystals and epitaxial layers of GaN were grown by the ammonothermal method using an NH₄I mineralizer. The inclusion of zincblende phase GaN was effectively eliminated by increasing the growth temperature higher than 500 °C. Accordingly, an approximately 20-μm-thick GaN epitaxial layer was achieved on the Ga-polar face of a c-plane GaN seed wafer at 520 °C. Although the characteristic deep state emission band dominated the room temperature photoluminescence spectrum, the near-band-edge emission of GaN was observed for both the needle crystals and the epitaxial layers. These results encourage one to grow better quality GaN crystals at a high growth rate under high-temperature growth conditions.     

Growth of (1 0 1¯ 3¯ ) semipolar GaN on m-plane sapphire by hydride vapor phase epitaxy

The crystalline, surface, and optical properties of the (1 0 1¯ 3¯) semipolar GaN directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) were investigated. It was found that the increase of V/III ratio led to high quality (1 0 1¯ 3¯) oriented GaN epilayers with a morphology that may have been produced by step-flow growth and with minor evidence of anisotropic crystalline structure. After etching in the mixed acids, the inclined pyramids dominated the GaN surface with a density of 2×10⁵ cm⁻², revealing the N-polarity characteristic. In the low-temperature PL spectra, weak BSF-related emission at 3.44 eV could be observed as a shoulder of donor-bound exciton lines for the epilayer at high V/III ratio, which was indicative of obvious reduction of BSFs density. In comparison with other defect related emissions, a different quenching behavior was found for the 3.29 eV emission, characterized by the temperature-dependent PL measurement.     

Investigation of nonpolar (1 1 2¯ 0) a-plane ZnO films grown under various Zn/O ratios by plasma-assisted molecular beam epitaxy

Structural and optical properties of nonpolar a-plane ZnO films grown with different II/VI ratios on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. Even by increasing the II/VI ratio across the stoichiometric flux condition a consistent surface morphology of striated stripes along the ZnO 〈0 0 0 1〉 direction without any pit formation was observed, which is contrary to polar c-plane ZnO films. Root mean square surface roughness, full width at half maximum values of X-ray rocking curves, defect densities, and photoluminescence were changed with the II/VI ratio. The sample grown with stoichiometric flux condition showed the lowest value of rms roughness, the smallest threading dislocation and stacking fault densities of ∼4.7×10⁸ cm⁻² and ∼9.5×10⁴ cm⁻¹, respectively, and the highest intensity of D^oX peak. These results imply that the stoichiometric flux growth condition is suitable to obtain superior structural and optical properties compared to other flux conditions.     

Epitaxial lateral overgrowth on (2 1¯ 1¯ 0) a-plane GaN with [0 1¯ 1 1]-oriented stripes

Epitaxial lateral overgrowth was applied to a-plane GaN on r-plane sapphire using SiO₂ stripe masks oriented parallel to [0 1¯ 1 1]. Coalescence and defect distribution was studied using scanning electron microscopy and cathodoluminescence. Defects, i.e., threading dislocations and basal plane stacking faults from the template propagate into the overgrown layer through the mask openings. Stacking faults spread into the whole overgrown layer, whereas threading dislocations are laterally confined in the region above the mask where a part of them is terminated at the inclined coalescence boundary. Lateral overgrowth and dislocation termination at the coalescence boundary leads to an improvement in luminescence intensity and crystal quality, in comparison to the template. The measured XRD rocking curve FWHM were 453″ with incidence along the [0 0 0 1] c-direction and 280″ with incidence along the [0 1 1¯ 0] m-direction.     

Electrical properties and deep traps spectra in undoped M-plane GaN films prepared by standard MOCVD and by selective lateral overgrowth

Electrical properties, deep traps spectra and structural performance were studied for m-GaN films grown on m-SiC substrates by standard metalorganic chemical vapor deposition (MOCVD) and by MOCVD with lateral overgrowth (ELO) or sidewall lateral overgrowth (SELO). Standard MOCVD m-GaN films with a very high dislocation density over 10⁹ cm⁻² are semi-insulating n-type with the Fermi level pinned near E_c−0.7 eV when grown at high V/III ratio. For lower V/III they become more highly conducting, with the electrical properties still dominated by a high density (∼10¹⁶ cm⁻³) of E_c−0.6 eV electron traps. Lateral overgrowth that reduces the dislocation density by several orders of magnitude results in a marked increase in the uncompensated shallow donor density (∼10¹⁵ cm⁻³) and a substantial decrease of the density of major electron traps at E_c−0.25 and E_c−0.6 eV (down to about 10¹⁴ cm⁻³). Possible explanations are briefly discussed.