Low-temperature AlN Buffer Layer Research Articles

Structural and optical properties of cubic GaN on U-grooved Si (100)

Cubic GaN epitaxy on large-area U-grooved silicon (100) dies is demonstrated by metalorganic chemical vapor deposition, and its structural and optical properties are reported. Scanning electron, atomic force, and transmission electron microscopy studies reveal that cubic GaN shows no discernible threading dislocations and a low stacking fault density of 3.27 ± 0.18 × 104 cm−1. Temperature-dependent photoluminescence studies reveal as-grown cubic GaN band edge emission internal quantum efficiency as 25.6% ± 0.9%. Selective etching of the low-temperature AlN buffer layer, SiO2 sidewalls, and hexagonal-phase GaN is demonstrated, which increases the cubic GaN band edge emission internal quantum efficiency to 31.6% ± 0.8%. This increase is attributed to the decrease in the radiative recombination lifetime via the removal of defective hexagonal-phase GaN. Overall, cubic GaN on U-grooved silicon with high structural and optical quality is reported, promising its suitability for next-generation devices.

Temperature dependent lattice expansions of epitaxial GaN-on-Si heterostructures characterized by in- and ex-situ X-ray diffraction

In-situ X-ray diffraction (XRD), with the sample temperature stepwise increased up to 900 °C, together with ex-situ high-resolution XRD (HRXRD), has been used to study the lattice expansion along the surface normal direction of GaN-on-Si heterostructures grown by metalorganic chemical vapor deposition (MOCVD). The thermal stabilities induced by step-graded (SG) AlxGa1−xN/AlN and multiple low-temperature (MLT) AlN buffer layers have been addressed for the heterostructures grown on 700 and 1500 µm thick Si (111) wafers, respectively. It reveals that the thermal expansion of the GaN epilayer is slightly larger than that of the Si substrate and the nitride growth tends to decrease the thermal expansion of the Si (111) wafer at lower temperatures. Onset of drop in the lattice expansions has been observed when the temperature is increased to a transition point, Ttr, and the Ttr of the MLT-AlN buffered GaN on the 1500 µm thick Si is ~500 °C, which is apparently lower than that of the SG-AlxGa1−xN/AlN buffered GaN on the 700 µm thick Si. These observations have been interpreted and attributed to convolutions among the residual lattice strains, their relaxations, and the thermal expansion coefficient mismatch induced wafer curvatures (opposite to that during MOCVD growth) at elevated temperatures.

Correlation of donor-acceptor pair emission on the performance of GaN-based UV photodetector

High responsivity UV photodetector has been realized by fabricating single crystalline epitaxial GaN-based devices grown on silicon substrate by using molecular beam epitaxy system. The influence of trap states existing within the forbidden gap on GaN-based optoelectronic devices has been investigated. The quality and performance of the fabricated GaN based UV photodetector with distinct AlN buffer layer grown at low and high substrate temperature are substantiated. A detailed analysis reveals that high temperature buffer can yield improved crystalline quality of GaN with similar morphological properties as compared to buffer layer grown at low temperature. However, the distinct buffer layer influence the optical properties as the photoluminescence analysis explains that both the films possess superior band-to-band edge emission where GaN grown with high temperature AlN buffer consists of dominant acceptor defect states in comparison to GaN with low temperature AlN buffer layer. The existence of acceptor states has been accredited by the presence of Ga vacancy related states in the band gap. The fabricated devices yield high photoresponsivity of 2.1 and 1.5 A/W at 1 V from GaN films bearing low and high accepter defect states which explain a clear correlation of device performance with trap states within the band-gap region.

The influence of V/III ratio on GaN grown on patterned sapphire substrate with low temperature AlN buffer layer by hydride vapor phase epitaxy

In this paper, 2–4 in. GaN wafers with high crystal quality were obtained with optimized V/III ratio of source gases on the patterned sapphire substrate (PSS) covered by a sputtered thin AlN buffer layer by using hydride vapor phase epitaxy (HVPE) growth technology. It was found that the morphology and crystal quality of the top GaN layer were significantly different with the variation of V/III ratio of source gases in initial nucleation growth step. Transparent single crystalline GaN layer could be obtained under relatively low V/III ratio of source gases, while it transferred to yellow and polycrystalline structure with increasing of V/III ratio of source gases. This is mainly attributed to the significant transition of GaN nucleation process with different crystallographic plane of AlN buffer layer, and corresponding variation of GaN growth mode at different V/III ratio of source gases, supported by surface, lateral SEM characterization, high-resolution X-ray diffraction (XRD) measurement, and detailed growth mechanism analysis. Moreover, the crystal quality of GaN grown under optimized condition can be further improved by increasing the thickness of GaN. It would offer a simple way to obtain GaN templates with high crystal quality at relatively low cost.

Properties of AlN layers grown on c-sapphire substrate using ammonia assisted MBE

AlN epilayer properties (120 nm thick) grown by ammonia assisted molecular beam epitaxy on c-sapphire substrates with different low temperature AlN buffer layers (LT-BL) have been studied. The role of the LT-BL on the AlN structural and optical properties was investigated as a function of the LT-BL thickness and growth temperature. Optimum growth conditions were identified with LT-BL thickness of 3 nm and growth temperature between 480 °C and 520 °C. It was shown that by optimizing these conditions, a reduction of both mixed and edge threading dislocation densities up to 75% is achieved. The impact of the growth temperature of the AlN epilayer was also studied showing an additional improvement of the AlN crystal and morphological properties while growing at higher temperature. A correlation between the epilayer strain and the PL emission was also investigated. Finally, an Al0.7Ga0.3N:Si doped layer was grown on the top of the optimized AlN template showing a smooth surface with monoatomic steps and a roughness ∼0.2 nm, confirming the potential of such templates for the fabrication of AlGaN based heterostructures.

Fabrication of high-crystallinity a-plane AlN films grown on r-plane sapphire substrates by modulating buffer-layer growth temperature and thermal annealing conditions

High-crystallinity a-plane AlN(112̅0) films containing a low-temperature AlN (LT-AlN) buffer layer and a high-temperature AlN (HT-AlN) film were grown on r-plane sapphire(11̅02) substrates. We investigated the effect of the growth temperature and thermal annealing conditions for the LT-AlN buffer layers on the crystallinity and surface morphology. The surface roughness of the buffer layers became smooth with the decrease in growth temperature to 900°C, and the crystallinity of the buffer layers was improved by thermal annealing at temperatures over 1600°C. HT-AlN films were then grown on the annealed LT-AlN buffer layers at 1500°C. The optimum crystallinity of HT-AlN films without any facet formation at the surfaces was obtained with full width at half maximum values of the X-ray rocking curves for AlN(112̅0)//[11̅00]AlN at 770 and (0002) at 640″.

Effects of AlN buffer layer thickness on the crystallinity and surface morphology of 10-µm-thick a-plane AlN films grown on r-plane sapphire substrates

10-µm-thick a-plane AlN films containing a low-temperature AlN (LT-AlN) buffer layer and a high-temperature AlN (HT-AlN) film were prepared on r-plane sapphire substrates. The crystallinity of all the samples with different LT-AlN buffer layer thicknesses was improved after thermal annealing and HT-AlN growth, mainly owing to the elimination of domain boundaries and the concurrent suppression of facet formation. The optimum crystallinity of HT-AlN films was obtained with full widths at half maximum of the X-ray rocking curves of 660 arcsec for AlNAlN and 840 arcsec for (0002) using a 200-nm-thick LT-AlN buffer layer.

Anisotropic structural and optical properties of semi-polar (11-22) GaN grown on m-plane sapphire using double AlN buffer layers.

We report the anisotropic structural and optical properties of semi-polar (11–22) GaN grown on m-plane sapphire using a three-step growth method which consisted of a low temperature AlN buffer layer, followed by a high temperature AlN buffer layer and GaN growth. By introducing double AlN buffer layers, we substantially improve the crystal and optical qualities of semi-polar (11–22) GaN, and significantly reduce the density of stacking faults and dislocations. The high resolution x-ray diffraction measurement revealed that the in-plane anisotropic structural characteristics of GaN layer are azimuthal dependent. Transmission electron microscopy analysis showed that the majority of dislocations in the GaN epitaxial layer grown on m-sapphire are the mixed-type and the orientation of GaN layer was rotated 58.4° against the substrate. The room temperature photoluminescence (PL) spectra showed the PL intensity and wavelength have polarization dependence along parallel and perpendicular to the [1–100] axis (polarization degrees ~ 0.63). The realization of a high polarization semi-polar GaN would be useful to achieve III-nitride based lighting emission device for displays and backlighting.

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

The Blue LED Nobel Prize: Historical context, current scientific understanding, human benefit

Reflectance analysis on the MOCVD growth of AlN on Si(111) by the virtual interface model

AbstractThe reflectance‐time profile of AlN‐on‐Si(111) growth by MOCVD as acquired through an in‐situ optical reflectance monitor is analyzed. It is found that, similar to the case of GaN‐on‐sapphire nucleation and growth through the low temperature GaN or AlN buffer layer techniques, the reflectance profile for AlN‐on‐Si also contains information that relate to the growth mechanism, which can then be correlated to the as‐grown material quality. Based on the equations for the Virtual Interface model, a set of 6 parameters can be used to curve‐fit the reflectance traces of AlN growth as acquired by the in‐situ optical monitor. By analyzing a range of 405 nm AlN reflectance‐time data from single layer AlN growth to multilayer AlN/AlGaN/GaN stack grown on Si(111), the “goodness of fit” statistics are found to correlate with material quality metrics such as the XRD (002) rocking curve FWHM for AlN, microscopic morphological roughness of the AlN surface, as well as the resultant wafer bow and curvature for GaN/AlGaN grown on the AlN/Si(111) substrate. The correlations provide useful insight into the desirable mechanism and growth mode for the high temperature MOCVD growth of AlN and GaN on Si(111). When fully established, the technique has the potential of being used as an in‐situ pass/fail screen for the MOCVD growth of GaN‐on‐Si. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of AlN based lateral polarity structures

AbstractGrowth and characterization of AlN based lateral polarity structures (LPS) are presented. The LPS were grown by MOCVD using patterned low temperature AlN/sapphire substrates: the Al‐polar and N‐polar AlN grew on AlN buffer layer and nitrided sapphire, respectively. AFM images showed a height difference between the two adjacent domains of different polarity on the order of 30 nm, which was equivalent to the low temperature AlN buffer layer thickness. SEM images provided an insight in the growth mode of the two polarities. It was shown that Al‐polar AlN grew two‐dimensionally, leading to a smooth well coalesced layer, while N‐polar AlN grew primarily three‐dimensionally, leading to a columnar structure. This difference in the growth mode led to different properties of domains with opposite polarity, including strain and defect incorporation, as observed by X‐ray diffraction, photoluminescence, and Raman spectroscopy measurements. Finally, an outlook on the applicability and future development of AlN‐based LPS is given. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth and characteristic of high orientation indium nitride films grown on (100) silicon substrate

In this study, high orientation InN films were prepared on Si (100) substrates using plasma-assist metal-organic molecular beam epitaxy (RF-MOMBE) system. These buffers include the nitrided treatment of the Si substrate, a low-temperature AlN buffer layer and oxide layer. The crystalline structure, surface morphology and optical properties of the InN films were characterised by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscope (SEM), atomic force microscopy (AFM) and photoluminescence (PL) measurements. XRD results revealed that the wurtzite InN grows on oxide layer with preferential (0002) orientation. Bright-field XTEM images of InN films exhibit a hexagonal structure with highly preferred orientation along direction. InN film direct growth on Si substrate exhibited the formation of metallic indium droplets on the surface. AFM images revealed that InN films with oxide layer had a root-mean-square (RMS) roughness of 16.1 nm. The optical properties of InN/oxide layer were determined according to the photoluminescence, revealing a near band-edge of 0.719 eV.

GaN epitaxy on Cu(110) by metal organic chemical vapor deposition

We demonstrate that Cu can be a suitable substrate material for c-plane GaN epitaxy using metal organic chemical vapor deposition. By using a low temperature AlN buffer layer, Ga and Cu alloying can be prevented so that GaN layer can be grown on Cu at a temperature of 1000 °C. An epitaxial relation of GaN (0001)//Cu(110) is observed using cross-section transmission electron microscopy and electron back scatter diffraction studies. The single crystalline GaN epilayer shows a threading dislocation density of 3 × 109 cm−2 and strong band edge emission at room temperature. The site alignment between GaN (0001) and Cu(110) shows a mesh ratio of 4/3 and 5/3 in GaN [10-10] and GaN [-1100] directions, which is attributed to the epitaxial relation observed.

TEM study of defects in Al xGa 1− xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al x Ga 1− x N layers with various Al mole fractions ( x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al x Ga 1− x N layer ( x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed. The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al x Ga 1− x N layer with both polarities. The lattice relaxation of metal-polar Al 0.2Ga 0.8N was observed, while N-polar Al 0.2Ga 0.8N did not relax. The dislocation densities in the N-polar Al 0.2Ga 0.8N and Al 0.4Ga 0.6N layers were 5.5×10 9 cm −2 and 9×10 9 cm −2, respectively, and in metal-polar Al 0.2Ga 0.8N and Al 0.4Ga 0.6N layers these were 1×10 10 cm −2 and 6×10 9 cm −2, respectively. Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.

Effect of AlN buffer layer properties on the morphology and polarity of GaN nanowires grown by molecular beam epitaxy

Low-temperature AlN buffer layers grown via plasma-assisted molecular beam epitaxy on Si (111) were found to significantly affect the subsequent growth morphology of GaN nanowires. The AlN buffer layers exhibited nanowire-like columnar protrusions, with their size, shape, and tilt determined by the AlN V/III flux ratio. GaN nanowires were frequently observed to adopt the structural characteristics of the underlying AlN columns, including the size and the degree of tilt. Piezoresponse force microscopy and polarity-sensitive etching indicate that the AlN films and the protruding columns have a mixed crystallographic polarity. Convergent beam electron diffraction indicates that GaN nanowires are Ga-polar, suggesting that Al-polar columns are nanowire nucleation sites for Ga-polar nanowires. GaN nanowires of low density could be grown on AlN buffers that were predominantly N-polar with isolated Al-polar columns, indicating a high growth rate for Ga-polar nanowires and suppressed growth of N-polar nanowires under typical growth conditions. AlN buffer layers grown under slightly N-rich conditions (V/III flux ratio = 1.0 to 1.3) were found to provide a favorable growth surface for low-density, coalescence-free nanowires.

Crystalline dependence of optical and interfacial properties of InGaN/GaN quantum wells on nonpolar a-plane GaN/ r-sapphire

We investigated the optical and the crystal qualities of InGaN/GaN quantum wells (QWs) grown on nonpolar a-plane (1 1 –2 0) GaN/ r-sapphire by introducing the novel 2-step growth method without low temperature GaN or AlN buffer layer. In spite of achievement of macroscopic specular surface structure of a-plane GaN epilayer, the surface defects were developed after growing InGaN/GaN QWs. The surface defect density of InGaN was decreased by enhancing the crystallinity of a-plane GaN template. From high-resolution X-ray ω/2θ scan, the interfacial qualities of InGaN/GaN QWs would deteriorate with increasing surface defect density. In addition, high PL emission intensity and uniformity of InGaN/GaN QWs were obtained by improving the crystal quality of a-plane GaN/ r-sapphire due to the abrupt interfacial quality and the reduction of the surface defect in InGaN active regions.

Structural and optical characterization of (11‐22) semipolar GaN on m ‐plane sapphire without low temperature buffer layer

AbstractWe reported the high quality semipolar (11‐22) GaN grown on m‐sapphire by using the novel two‐step growth method without low temperature GaN or AlN buffer layer. It is found that macroscopic surface morphology of semipolar GaN epilayer was very smooth, while microscopic surface structure was arrowhead‐like surface structure toward the direction of [1‐21‐1]. Anisotropic crystal properties of semipolar GaN/m‐sapphire were also observed by two incident directions of X‐ray beam. Therefore, we suggested that the anisotropic crystal properties and arrow‐head like surface structure would be caused by heteroepitaxial crystallograhpic difference between semipolar GaN and m‐sapphire. Additionally, photoluminescence (PL) measurements showed the strong bandedge emission of n‐type semipolar GaN without yellow luminescence (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of the MgO substrate on the growth of GaN

We investigate the effect of the MgO substrate on the growth of GaN by molecular beam epitaxy with radio frequency plasma source. MgO substrate is advantageous for its closer lattice constant to GaN. However, epitaxial GaN layer grown on MgO (1 1 1) substrate is often inclined about 2 ∘ toward a particular a-axis of the GaN. This inclination is found to be caused by the domain structure of the MgO substrates. On the tilted domains of the substrate, the strain is effectively relaxed through the inclination. Another difficulty of MgO substrate is the diffusion of Mg atoms into the GaN layer. This Mg diffusion from the MgO substrate has successfully been suppressed by introducing the thin low-temperature grown AlN buffer layer. The AlN buffer also improves the crystalline quality of the GaN layer.

Epitaxial growth of GaN films grown on single crystal Fe substrates

GaN films have been grown on single crystalline Fe(110), Fe(100), and Fe(111) substrates with low-temperature AlN buffer layers by the use of pulsed laser deposition. AlN films with 30° rotated domains and polycrystalline AlN films grow on Fe(100) and Fe(111), respectively. On the other hand, AlN(0001) films grow epitaxially on Fe(110) substrates. X-ray reflectivity measurements have revealed that the heterointerface of AlN∕Fe(110) is quite abrupt and that its abruptness remains unchanged, even after annealing at 700°C. GaN epitaxial films have been grown on AlN∕Fe(110) structures with an in-plane relationship of GaN[11−20]∥AlN[11−20]∥Fe[001]. The first-principles calculations have revealed that this in-plane epitaxial relationship is determined by the adsorption process of nitrogen atoms on the Fe surfaces at the initial stage of the growth.

Dislocation reduction in GaN with double Mg xN y/AlN buffer layer by metal organic chemical vapor deposition

Unintentionally doped GaN with conventional single low-temperature (LT) AlN buffer layer and with double Mg x N y /AlN buffer layers both were prepared. It was found that we could reduce defect density and thus improve crystal quality of the GaN by using double Mg x N y /AlN buffer layers. GaN with double Mg x N y /AlN buffer layers reveals an asymmetrical reflection (1 0 2) and (0 0 2) with a smaller full-width at half-maximum (FWHM), and a higher mobility, lower background concentration and lower etching pit density (EPD) than the GaN with the LT-AlN buffer layer.