High-temperature AlN Layer Research Articles

Relation of V/III ratio of AlN interlayer with the polarity of nitride

Abstract N-polar GaN film was obtained by using a high-temperature AlN buffer layer. It was found that the polarity could be inverted by a thin low-temperature AlN interlayer with the same V/III ratio as that of the high-temperature AlN layer. Continuing to increase the V/III ratio of the low-temperature AlN interlayer, the Ga-polarity of GaN film was inverted to N-polarity again but the crystal quality and surface roughness of GaN film greatly deteriorated. Finally, we analyzed the chemical environment of the AlN layer by x-ray photoelectron spectroscopy (XPS), which provides a new direction for the control of GaN polarity.

Polarity Control by Inversion Domain Suppression in N-Polar III-Nitride Heterostructures

Nitrogen-polar III-nitride heterostructures offer advantages over metal-polar structures in high frequency and high power applications. However, polarity control in III-nitrides is difficult to achieve as a result of unintentional polarity inversion domains (IDs). Herein, we present a comprehensive structural investigation with both atomic detail and thermodynamic analysis of the polarity evolution in low- and high-temperature AlN layers on on-axis and 4° off-axis carbon-face 4H-SiC (0001̅) grown by hot-wall metal organic chemical vapor deposition. A polarity control strategy has been developed by variation of thermodynamic Al supersaturation and substrate misorientation angle in order to achieve the desired growth mode and polarity. We demonstrate that IDs are completely suppressed for high-temperature AlN nucleation layers when a step-flow growth mode is achieved on the off-axis substrates. We employ this approach to demonstrate high quality N-polar epitaxial AlGaN/GaN/AlN heterostructures.

N-polar GaN Film Epitaxy on Sapphire Substrate without Intentional Nitridation.

High-temperature nitridation is commonly thought of as a necessary process to obtain N-polar GaN films on a sapphire substrate. In this work, high-quality N-polar GaN films were grown on a vicinal sapphire substrate with a 100 nm high-temperature (HT) AlN buffer layer (high V/III ratio) and without an intentional nitriding process. The smallest X-ray full width at half maximum (FWHM) values of the (002)/(102) plane were 237/337 arcsec. On the contrary, N-polar GaN film with an intentional nitriding process had a lower crystal quality. In addition, we investigated the effect of different substrate treatments 1 min before the high-temperature AlN layer’s growth on the quality of the N-polar GaN films grown on different vicinal sapphire substrates.

Enhanced crystalline quality of non-polar a-plane AlGaN epitaxial film grown with Al-composition-graded AlGaN intermediate layer

The non-polar a-AlGaN epitaxial film was successfully grown on the semi-polar r-sapphire substrate by metal-organic chemical vapor deposition technique. An Al-composition-graded AlxGa1−xN (x = 0.0 to 1.0) intermediate layer with varying film thickness from 260 to 695 nm was deposited between the high-temperature AlN layer and the non-polar a-AlGaN epitaxial film to enhance the morphological and crystalline quality. The non-polar a-AlGaN epitaxial films were investigated by using atomic force microscopy (AFM), high-resolution x-ray diffraction, photoluminescence (PL) spectroscopy and the Hall effect measurement techniques. The characterisation results indicate substantial improvements in surface morphology and crystalline quality for the non-polar a- AlGaN epitaxial film grown by adding an Al-composition-graded AlGaN intermediate layer. The surface roughness measured from AFM and the defect-related emission (yellow band) relative to the near-band-edge emission from PL spectra were decreased significantly by optimizing the layer thickness of the Al-composition-graded AlGaN layer. A relatively low background carrier concentration down to −4.4 × cm−3 was achieved from Hall effect measurement for the non-polar a-AlGaN epitaxial film.

Growth of Nitride Heteroepitaxial Transistor Structures: from Epitaxy of Buffer Layers to Surface Passivation

It is demonstrated that it is possible to use the ammonia molecular beam epitaxy for growing structurally perfect high-ohmic GaN layers which allow generating SiN/Al(Ga)N/GaN heterostructures for transistors with a high mobility of electrons. The growth conditions are determined for GaN layers with smooth surface morphology (with a mean-squared deviation of $${\sim}$$ 2 nm) appropriate for creating sharp heteroboundaries. The possibility of improving the crystalline perfection of GaN layer due to the use of buffer high-temperature AlN layer (with the growth temperature above 940 $${}^{\circ}$$ C) is demonstrated. It was shown that in situ surface passivation of Al(Ga)N/GaN heterostructures by the ultrathin SiN layer allows generating normally closed transistors with unprecedented low values of the current collapse ( $${\sim}1\%$$ ).

Anisotropic strain relaxation and high quality AlGaN/GaN heterostructures on Si (110) substrates

We have investigated the growth and relaxation mechanisms of anisotropic lattice misfit strain in AlN and GaN layers on Si (110) substrates. A qualitative model is proposed to explain the relaxation process. It is revealed that the anisotropic misfit strain is quickly relaxed in the low temperature AlN layer by the formation of interface misfit dislocations, small misoriented grains, and lattice distortion. As a result, isotropic properties and atomically smooth surface are observed in the high temperature AlN layer. Based on this isotropic AlN layer, a high quality GaN layer and AlGaN/GaN heterostructures with a high electron mobility of 2160 cm2/(V · s) have been obtained. This work will have important impacts on the understanding of the epitaxy of isotropic semiconductor films on anisotropic substrates.

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.

Defect Reduction in AlN Epilayers Grown by MOCVD via Intermediate-Temperature Interlayers

In this work, significant reduction of the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates via metalorganic chemical vapor deposition has been obtained by insertion of thin intermediate-temperature interlayers (IT-ILs). The growth temperature of the IT-ILs ranged from 750°C to 950°C after the initial growth at high temperature of 1200°C. Detailed characterizations were performed to understand the mechanisms of the reduction in dislocation density. It is found that the relatively low growth temperature of the IT-ILs can modify the originally two-dimensional (2D) growth mode to a three-dimensional (3D) growth process and creates a high density of small islands at the interface. During the subsequent growth of the high-temperature AlN layer, the AlN islands initially coalesce to form larger grains as the growth proceeds, and some TDs present in the previous AlN epilayers are found to bend near the interlayer, leading to dislocation merging and annihilation.

Deposition of GaN Layers with a lowered dislocation density by molecular-beam epitaxy

The deposition of a multilayer buffer layer that includes a high-temperature AlN layer grown at a temperature above 1100°C has made it possible to reduce the dislocation density in a GaN layer by 1.5–2 orders of magnitude to values in the range from 9 × 108 to 1 × 109 cm−2, compared with the case of growth on a thin low-temperature AlN nucleation layer. The decrease in the dislocation density causes a substantial increase in the electron mobility in the GaN layers to 600–650 cm2 V−1 s−1, which is in agreement with the results of calculations and is indicative of the high crystalline perfection of the layers.

Effect of TMGa preflow on the properties of high temperature AlN layers grown on sapphire

AbstractThe effect of a trimethylgallium (TMGa) preflow on the structural and optical properties of MOCVD grown AlN layers on sapphire substrates is investigated. Secondary ion mass spectroscopy measurements were performed to investigate the incorporation of Ga in the layers. It is shown that for AlN layers grown with a TMGa preflow Ga atoms incorporate mainly near the substrate/epilayer interface. Photoluminescence spectra exhibit a free exciton and a donor bound exciton. By analyzing the full width at half maximum of the free exciton and the defect luminescence an increased optical quality for the sample grown with TMGa preflow is demonstrated. Additionally, Raman spectroscopy reveals a higher crystal quality for this sample. Comparing the results from Raman spectroscopy and luminescence measurements a shift rate of 60 meV GPa−1 is determined for the free A‐exciton. Finally, cross‐sectional Raman spectroscopy is used to compare the strain at the AlN/sapphire interface for a sample grown with and without TMGa preflow.

Effects of AlN nucleation layers on the growth of AlN films using high temperature hydride vapor phase epitaxy

AlN layers were grown on c-plane sapphire substrates with AlN nucleation layers (NLs) using high temperature hydride vapor phase epitaxy (HT-HVPE). Insertion of low temperature NLs, as those typically used in MOVPE process, prior to the high temperature AlN (HT-AlN) layers has been investigated. The NLs surface morphology was studied by atomic force microscopy (AFM) and NLs thickness was measured by X-ray reflectivity. Increasing nucleation layer deposition temperature from 650 to 850°C has been found to promote the growth of c-oriented epitaxial HT-AlN layers instead of polycrystalline layers. The growth of polycrystalline layers has been related to the formation of dis-oriented crystallites. The density of such disoriented crystallites has been found to decrease while increasing NLs deposition temperature. The HT-AlN layers have been characterized by X-ray diffraction θ−2θ scan and (0002) rocking curve measurement, Raman and photoluminescence spectroscopies, AFM and field emission scanning electron microscopy. Increasing the growth temperature of HT-AlN layers from 1200 to 1400°C using a NL grown at 850°C improves the structural quality as well as the surface morphology. As a matter of fact, full-width at half-maximum (FWHM) of 0002 reflections was improved from 1900 to 864arcsec for 1200°C and 1400°C, respectively. Related RMS roughness also found to decrease from 10 to 5.6nm.

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.

Demonstration of AlGaN/GaN high-electron-mobility transistors on 100 mm diameter Si(111) by plasma-assisted molecular beam epitaxy

AlGaN/GaN high-electron-mobility transistor structures grown on 100 mm high-resistivity Si(111) substrates using plasma-assisted molecular beam epitaxy are reported. The two-dimensional electron gas (2DEG) formation in the heterostructures was realized by the growth optimization of two-step low temperature and high temperature AlN layers and GaN buffer layer. High-electron mobility of 1100 cm2/V s with a sheet carrier density of 9×1012 cm−2 was achieved. The presence of 2DEG in the AlGaN/GaN interface was confirmed by temperature dependent Hall measurements and capacitance-voltage carrier profiling. The fabricated 1.5 μm gate length high electron mobility transistor exhibited a maximum drain current density of 530 mA/mm and a peak extrinsic transconductance of 156 mS/mm.

High performance InGaN LEDs on Si (1 1 1) substrates grown by MOCVD

We report high performance InGaN multiple-quantum well (MQW) light-emitting diodes (LEDs) grown on Si (1 1 1) substrates using metalorganic chemical vapour deposition (MOCVD). A high-temperature thin AlN layer and AlN/GaN multilayers have been used for the growth of a high-quality GaN-based LED structure on Si substrates. Reduction of the high-temperature AlN layer thickness promotes the formation of a tunnel junction at the AlN/Si interface which reduces the LED operating voltage. Optical output power of the LED on Si saturates at a higher injected current density due to higher thermal conductivity of Si than that of a sapphire substrate. At a high injection current, output power of the LED on Si is higher than that of the LED on sapphire. Cross-sectional transmission electron microscopy (TEM) indicates that the active layer of these LEDs consists of a dislocation-free pyramid-shaped (quantum-dot-like) structure. Additionally, the crack-free thin-film LED epilayer region was transferred onto a copper carrier using metal-to-metal bonding and the selective lift-off technique. A LED with high output power, low operating voltage and low series resistance was realized by this technique. Furthermore, optimization of LED on Si by insertion of an Al0.06Ga0.94N/GaN strained-layer superlattice underlayer into the structure exhibits improved internal quantum efficiency (ηiqe) in the MQW, higher optical emission intensity with higher saturation current, lower operation voltage of 3.2 V at 20 mA and a series resistance of 16 Ω, as well as narrower electroluminescence spectra.

High performance of InGaN LEDs on (111) silicon substrates grown by MOCVD

We report on the high-performance of InGaN multiple-quantum well light-emitting diodes (LEDs) on Si (111) substrates using metal-organic chemical vapor deposition. A high-temperature thin AlN layer and AlN-GaN multilayers have been used for the growth of high-quality GaN-based LED structure on Si substrate. It is found that the operating voltage of the LED at 20 mA is reduced to as low as 3.8-4.1 V due to the formation of tunnel junction between the n-AlGaN layer and the n-Si substrate when the high-temperature AlN layer is reduced to 3 nm. Because Si has a better thermal conductivity than sapphire, the optical output power of the LED on Si saturates at a higher injected current density. When the injected current density is higher than 120 A/cm/sup 2/, the output power of the LED on Si is higher than that of LED on sapphire. The LED also exhibited the good reliability and the uniform emission from a large size wafer. Cross-sectional transmission electron microscopy observation indicated that the active layer of these LEDs consists of the dislocation-free pyramid-shaped (quantum-dot-like) structure.