AlN Nucleation Layer Research Articles

Carrier localization and defect-insensitive optical behaviors of ultraviolet multiple quantum wells grown on patterned AlN nucleation layer

In this work, we report a significantly boosted defect-insensitive ultraviolet emission from InGaN/GaN multiple quantum wells (MQWs) grown on patterned AlN nucleation layer (NL) compared to samples grown on uniform NL. Carrier localization is clearly illustrated for MQWs grown on patterned AlN NL as evidenced by an S-shape profile from temperature-dependent photoluminescence characterization. The underlying mechanism for the carrier localization is demonstrated to correlate with partial relaxation of compressive strains inside epitaxial thin films. This work illustrates that carrier localization can be achieved in MQWs with very low indium content by the adoption of patterned NL during growth, and provides a promising route towards the realization of high-efficiency ultraviolet emitter.

Improvement of AlN Material Quality by High-Temperature Annealing Toward Power Diodes

AlN devices have the potential to outperform current GaN devices, especially in the high-voltage region, thanks to AlN’s larger critical electric field and thermal conductivity. In order for the AlN-based power diodes to be realized and performance to be demonstrated, high-quality AlN crystals would be the prerequisite. In this article, significantly improved AlN quality has been demonstrated through high-temperature treatment of AlN thin layers at high temperatures in the range of 1670–1730 °C, followed by regrowth of AlN layers on top. The produced AlN layer shows much improved crystalline quality when compared to AlN layer without thermal treatment. The thermal treatment of AlN nucleation layers recrystallizes the AlN materials and results in a significant reduction of the full-width at half-maximum (FWHM) values of X-ray rocking curves of AlN (002) and (102) planes. The annealing of AlN nucleation also produced much pronounced atomic step features and improves the transmittance for the nucleation layers. Regrowth of AlN templates on such annealed nucleation improves AlN crystalline quality further, producing high-quality templates for power diode applications.

Comparative Research of GaN Growth Mechanisms on Patterned Sapphire Substrates with Sputtered AlON Nucleation Layers.

The utilization of sputtered AlN nucleation layers (NLs) and patterned sapphire substrates (PSSs) could greatly improve GaN crystal quality. However, the growth mechanism of GaN on PSSs with sputtered AlN NLs has not been thoroughly understood. In this paper, we deposited AlON by sputtering AlN with O2, and we found that the variation of thickness of sputtered AlON NLs greatly influenced GaN growth on PSSs. (1) For 10 nm thin AlON sputtering, no AlON was detected on the cone sidewalls. Still, GaN nucleated preferably in non-(0001) orientation on these sidewalls. (2) If the thickness of the sputtered AlON NL was 25 nm, AlON formed on the cone sidewalls and flat regions, and some small GaN crystals formed near the bottom of the cones. (3) If the sputtered AlON was 40 nm, the migration ability of Ga atoms would be enhanced, and GaN nucleated at the top of the cones, which have more chances to grow and generate more dislocations. Finally, the GaN growth mechanisms on PSSs with sputtered AlON NLs of different thicknesses were proposed.

Electrical activity at the AlN/Si Interface: identifying the main origin of propagation losses in GaN-on-Si devices at microwave frequencies

AlN nucleation layers are the basement of GaN-on-Si structures grown for light-emitting diodes, high frequency telecommunication and power switching systems. In this context, our work aims to understand the origin of propagation losses in GaN-on-Si High Electron Mobility Transistors at microwaves frequencies, which are critical for efficient devices and circuits. AlN/Si structures are grown by Metalorganic Vapor Phase Epitaxy. Acceptor dopant in-diffusion (Al and Ga) into the Si substrate is studied by Secondary Ion Mass Spectroscopy and is mainly located in the first 200 nm beneath the interface. In this region, an acceptor concentration of a few 1018 cm-3 is estimated from Capacitance–Voltage (C–V) measurements while the volume hole concentration of several 1017 cm-3 is deduced from sheet resistance. Furthermore, the combination of scanning capacitance microscopy and scanning spreading resistance microscopy enables the 2D profiling of both the p-type conductive channel and the space charge region beneath the AlN/Si interface. We demonstrate that samples grown at lower temperature exhibit a p-doped conductive channel over a shallower depth which explains lower propagation losses in comparison with those synthesized at higher temperature. Our work highlights that this p-type channel can increase the propagation losses in the high-frequency devices but also that a memory effect associated with the previous sample growths with GaN can noticeably affect the physical properties in absence of proper reactor preparation. Hence, monitoring the acceptor dopant in-diffusion beneath the AlN/Si interface is crucial for achieving efficient GaN-on-Si microwave power devices.

Improved nucleation of AlN on in situ nitrogen doped graphene for GaN quasi-van der Waals epitaxy

In the present work, improved crystal quality of GaN on a graphene-covered sapphire substrate was achieved compared to GaN grown on a bare sapphire substrate, and the growth mechanism of GaN quasi-van der Waals epitaxy with an AlN nucleation layer was clarified using metal-organic chemical vapor deposition. The in situ N-doping of graphene by an NH3 source during AlN growth was responsible for AlN nucleation. The first-principles calculation indicated that N atoms doped initially at the step edges of graphene and subsequently at its center and AlN nuclei followed the same sequence, which is consistent with experimental observations of nucleation. The lower migration barrier of Al atoms (0.07 eV) on graphene created larger AlN nuclei compared to that on bare sapphire (0.21 eV), leading to improved quality of the upper GaN epilayer with lower defect density. This work offers guidance for precisely controlling the nucleation morphology and density of GaN base materials and thus realizing high-quality epitaxial materials and related high-performance devices by quasi-van der Waals epitaxy.

Influence of Al pre-deposition time on AlGaN/GaN heterostructures grown on sapphire substrate by metal organic chemical vapor deposition

Aluminum (Al) pre-deposition technology was employed to grow high-quality AlGaN/GaN heterostructures on c-plane sapphire substrates by metal organic chemical vapor deposition. The effects of Al pre-deposition time on the structural and transport properties of the heterostructures were investigated in detail. The optimal Al pre-deposition time was found to be 3 s, and the AlGaN/GaN heterostructures grown under the optimal conditions possessed the surface root mean square roughness of 0.312 nm, threading dislocation density of 1.66 × 109 cm−2, two-dimensional electron gas (2DEG) mobility of 1808.09 cm2/V s and wafer resistance non-uniformity of 0.65%. Further, the influence mechanism of Al pre-deposition time on the dislocation density in the GaN buffer layer was revealed through the research on bare AlN nucleation layer grown on sapphire substrates. The results in this work not only demonstrate the great potential of AlGaN/GaN heterostructures with Al pre-deposition technology in electronic device applications, but also provide practical guidance for the growth of high‐quality group III nitride materials.

Impact of pits formed in the AlN nucleation layer on buffer leakage in GaN/AlGaN high electron mobility transistor structures on Si (111)

Limiting buffer layer current leakage is essential for the realization of high breakdown fields in GaN-on-Si high electron mobility transistors (HEMTs). In this report, we demonstrate the importance of controlling the surface morphology of the AlN nucleation layer (NL) in limiting this leakage. Testing on a self-consistent series of samples grown under two different AlN NL conditions revealed the presence of leakage paths within the epilayers grown using a single temperature AlN NL owing to the presence of surface pits. The introduction of a higher temperature AlN in the NL drastically reduced the pit density and led to a large reduction (>103) in the lateral and vertical buffer leakage in HEMT structures. Using conductive atomic force microscopy, secondary ion mass spectroscopy, and temperature-dependent carrier transport measurements, we confirm that these pits—which originate in the AlN NL, thread vertically, and propagate into the device structures—are associated with leakage paths, thus reducing the field that can be dropped across the epilayers. This is explained by invoking preferential oxygen segregation at their side-facets. It is shown that when a pit-free surface is maintained, a vertical field of 1.6 MV/cm can be achieved for HEMTs. This study is expected to benefit the development of high-performance GaN HEMTs in moving toward the theoretical breakdown field of III-nitrides.

Microwave Performance of ‘Buffer-Free’ GaN-on-SiC High Electron Mobility Transistors

High performance microwave GaN-on-SiC HEMTs are demonstrated on a heterostructure without a conventional thick doped buffer. The HEMT is fabricated on a high-quality $0.25~\boldsymbol {\mu }\text{m}$ unintentional doped GaN layer grown directly on a transmorphic epitaxially grown AlN nucleation layer. This approach allows the AlN-nucleation layer to act as a back-barrier, limiting short channel effects and removing buffer leakage. The devices with the ‘buffer-free’ heterostructure show competitive DC and RF characteristics, as benchmarked against the devices made on a commercial Fe-doped epi-wafer. Peak transconductances of 500 mS/mm and a maximum saturated drain current of ~1 A/mm are obtained. An extrinsic $\text{f}_{\sf T}$ of 70 GHz and $\text{f}_{\sf max}$ of 130 GHz are achieved for transistors with a gate length of 100 nm. Pulsed-IV measurements reveal a lower current slump and a smaller knee walkout. The dynamic IV performance translates to an output power of 4.1 W/mm, as measured with active load-pull at 3 GHz. These devices suggest that the ‘buffer-free’ concept may offer an alternative route for high frequency GaN HEMTs with less electron trapping effects.

Growth of thin AlN nucleation layer and its impact on GaN-on-SiC heteroepitaxy

Abstract AlN nucleation layer (NL) with thickness of 35 nm was grown on semi-insulating 4H–SiC substrate by metal organic chemical vapor deposition (MOCVD). Focused on the evolutions of the surface microstructural morphology and crystalline quality, the growth mechanism of the thin NL was studied by series of growth procedures. As interval supply of ammonia (NH3) and alternative supply of Trimethylaluminum (TMA)/NH3 are applied to NL growth, complete coalescence of AlN nucleate islands with a smooth surface and low density of defects is obtained. In addition, the impact of the thin NL on the subsequent GaN buffer layer (BL) was also discussed. Overgrown on the well-coalesced NL, surface roughening and three-dimensional mode get predominant at the initial GaN growth, which facilitates high crystalline quality of GaN BL with low wafer bow.

Crystalline quality improvement of face-to-face annealed MOVPE-grown AlN on vicinal sapphire substrate with sputtered nucleation layer

Face-to-face annealed metalorganic vapor phase epitaxy (MOVPE)- grown AlN templates with sputtered AlN nucleation layers (FFA MOSp-AlN templates) were fabricated on vicinal sapphire substrates. The sputtered thin AlN layer supplies AlN seeds that are well aligned to the c-axis. The MOVPE growth with a relatively thick AlN enlarges crystal grains with a low concentration of impurities. Face-to-face annealing at high temperatures reduces the twist component of AlN films due to recrystallization, and AlN films with low threading dislocation densities are formed. The full width at half maximum (FWHM) of the (10−12) X-ray rocking curves (XRCs) of AlN templates decreased as the thickness of the MOVPE-grown AlN layer increased. The FWHMs for the (0002) and (10−12) XRCs of the 300-nm-thick AlN templates with an off-cut angle of 0.6° were 9 and 337 arcsec, respectively. The concentrations of O, Si, and C impurities in the FFA MOSp-AlN templates were lower than those in the face-to-face annealed sputtered AlN templates. Si doped Al0.7Ga0.3N films, which are typically used as n-type AlGaN layers for UVC LEDs, were grown on the FFA MOSp-AlN templates with various off-cut angles. The AlGaN films with smooth flat surfaces were obtained using the FFA MOSp-AlN template with larger off-cut angles due to the suppression of the large hillock formation.

Investigation and optimization of AlN nucleation layers grown on 4H-SiC by MOCVD

The AlN Nucleation layers (NLs) with different thickness under very high V/III ratio (∼5000) growth conditions were grown on 4H-SiC by metal organic chemical vapor deposition (MOCD). The thickness of the NLs were varied between 10 and 300 nm. Growth mechanism was investigated by atomic force microscopy and high-resolution X-ray diffraction measurements. The crystalline, surface qualities and relaxation of the AlN were found to closely depend on the thickness of the AlN epi-layers. AlN NL with 100 nm thickness were found to have lower RMS (root mean square) roughness of ∼ 0.5 nm with smoother surface morphology and improved interface and crystalline quality. It was also observed that relaxation changed from 30 to 65 percent when the thickness was changed from 40 to 300 nm, respectively.

The influence of AlN nucleation layer on RF transmission loss of GaN buffer on high resistivity Si (111) substrate

A sufficiently low transmission loss in radio frequency (RF) is one of the critical requirements for GaN-on-Si RF devices to achieve high performance. We have systematically studied the mechanism and effect of the AlN nucleation layer on the RF loss of the GaN-on-Si device buffer stack. Our results show that the RF loss is strongly influenced by the growth parameters of the AlN nucleation layer during epitaxial process. It is observed that the AlN nucleation layer grown at a low thermal budget with a low density of deep surface pits can efficiently reduce the AlN/Si interface loss by suppressing the conductive channel at AlN/Si interface which is governed largely by the thermal diffusion of Al and Ga into the Si substrate. By optimizing the growth process of the AlN nucleation layer, the RF loss of the GaN-on-Si device buffer can be dramatically reduced by up to ∼40%.

The Influence of AlN Nucleation Layer on Radio Frequency Transmission Loss of AlN‐on‐Si Heterostructure

Reducing radio frequency (RF) transmission loss is a key requirement when fabricating GaN‐on‐Si RF devices. To get a better insight into the RF loss mechanism in the GaN‐on‐Si structure, the RF loss of an AlN/Si template is investigated by varying the growth temperature of AlN during a metalorganic chemical vapor deposition process. The results show that the RF loss of the AlN/Si template is dominated by the interface loss due to the p‐type conductive channel at the AlN/Si interface, which is induced by the thermal diffusion of Al during the high‐temperature growth. Although a low growth temperature of the AlN nucleation layer can suppress the RF loss in the AlN/Si template, it results in a low crystalline quality of AlN for practical use. Optimizing the growth temperature of the AlN nucleation layer is essential to obtain a good balance between the crystalline quality, morphological quality, and RF loss such that the AlN/Si template is suitable for epitaxial growth of the complete GaN‐on‐Si RF device structure.

Investigation of AlGaN/GaN Heterostructures Grown on Sputtered AlN Templates with Different Nucleation Layers.

In this work, a sputtered AlN template is employed to grow high-quality AlGaN/GaN heterostructures, and the effects of AlN nucleation layer growth conditions on the structural and electrical properties of heterostructures are investigated in detail. The optimal growth condition is obtained with composited AlN nucleation layers grown on a sputtered AlN template, resulting in the smooth surface morphology and superior transport properties of the heterostructures. Moreover, high crystal quality GaN material with low dislocation density has been achieved under the optimal condition. The dislocation propagation mechanism, stress relief effect in the GaN grown on sputtered AlN, and metal organic chemical vapor deposition AlN nucleation layers are revealed based on the test results. The results in this work demonstrate the great potential of AlGaN/GaN heterostructures grown on sputtered AlN and composited AlN nucleation layers for microelectronic applications.

Stress evolution in AlN growth on nano-patterned sapphire substrates

The stress evolution behavior of AlN grown on nano-patterned substrates (NPSSs) has been investigated. It is found that there are two sources of the tensile stress for AlN grown on NPSSs. One originates from the coalescence of grain islands of the AlN nucleation layer, and then the voids provide a channel for the gradual release of the tensile stress to nearly stress-free state accompanied by the lateral growth process of the AlN columns on the mesas. The other originates from the contacting of adjacent columns to complete the coalescence process, which is responsible for the residual tensile stress in the top AlN epilayer after completing the coalescence. This understanding of the stress evolution is certainly of great significance in AlN-based material and devices.

Transmorphic epitaxial growth of AlN nucleation layers on SiC substrates for high-breakdown thin GaN transistors

Interfaces containing misfit dislocations deteriorate electronic properties of heteroepitaxial wide bandgap III-nitride semiconductors grown on foreign substrates, as a result of lattice and thermal expansion mismatches and incompatible chemical bonding. We report grain-boundary-free AlN nucleation layers (NLs) grown by metalorganic chemical vapor deposition on SiC (0001) substrates mediated by an interface extending over two atomic layers L1 and L2 with composition (Al1/3Si2/3)2/3N and (Al2/3Si1/3)N, respectively. It is remarkable that the interfaces have ordered vacancies on one-third of the Al/Si position in L1, as shown here by analytical scanning transmission electron microscopy and ab initio calculations. This unique interface is coined the out-of-plane compositional-gradient with in-plane vacancy-ordering and can perfectly transform the in-plane lattice atomic configuration from the SiC substrate to the AlN NL within 1 nm thick transition. This transmorphic epitaxial scheme enables a critical breakdown field of ∼2 MV/cm achieved in thin GaN-based transistor heterostructures grown on top. Lateral breakdown voltages of 900 V and 1800 V are demonstrated at contact distances of 5 and 20 μm, respectively, and the vertical breakdown voltage is ≥3 kV. These results suggest that the transmorphic epitaxially grown AlN layer on SiC may become the next paradigm for GaN power electronics.

Design of AlGaN/AlN Dot‐in‐a‐Wire Heterostructures for Electron‐Pumped UV Emitters

This article describes the fabrication of nitrogen‐polar AlxGa1−xN/AlN (x = 0, 0.1) quantum dot (QD) superlattices (SLs) integrated along GaN nanowires (NWs) for application in electron‐pumped UV sources. The NWs are grown using plasma‐assisted molecular beam epitaxy on n‐type Si(111) wafers using a low‐temperature AlN nucleation layer. Growth conditions are tuned to obtain a high density of noncoalesced NWs. To improve the uniformity of the height along the substrate, the growth begins with a long (≈900 nm) NW base, with a diameter of 30–50 nm. The AlxGa1−xN/AlN active region is 400 nm long (88 periods of QDs), long enough to collect the electron–hole pairs generated by an electron beam with an acceleration voltage ≤5 kV. The spectral response is tuned in the 340–258 nm range by varying the dot/barrier thickness ratio and the Al content in the dots. Internal quantum efficiencies as high as 63% are demonstrated.

Stress evolution during growth of AlN templates on c-Al2O3 substrates by plasma-assisted molecular beam epitaxy

We describe stress evolution during plasma-assisted molecular beam epitaxy (PAMBE) of AlN nucleation and buffer layers on c-Al2O3 substrates at varying growth temperatures of 780 and 850°C. Moreover, different mechanisms of stress generation in the growing AlN films, related to the processes of grain coalescence and impact of Me-excess during PA MBE by migration enhanced epitaxy and metal-modulated epitaxy, are considered.

N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality

Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and crystalline quality of AlN NLs on on-axis C-face SiC (0001̄), C-face SiC (0001̄) off-cut towards the [112̄0] by 4°, and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.

Suppressing the Initial Growth of Sidewall GaN by Modifying AlN-Coated Patterned Sapphire with KOH-Based Etchant

Micron-sized patterned sapphire substrates (PSSs) with an ex situ sputtered AlN nucleation layer (NL) have been used to improve the performance of GaN-based light-emitting diodes (LEDs). The growth of GaN was enhanced not only from bottom c-plane, but also from the sidewall of the micron-sized patterns. In this study, KOH solution was used to etch AlN (especially sidewall AlN) for the first time. The additional etching process is very simple. It was found that KOH etching 1 min did enhance the light output power (LOP) of LED. However, with the increase of etching time to 4 min, the LOP decreased. Besides, the effect of remained AlN on GaN growth mechanism was investigated in detail.