AlN Nucleation Research Articles

Semipolar GaN(10–11) Epitaxial Layer Prepared on Nano‐Patterned SiC/Si(100) Template

A new approach is proposed to the synthesis of a semipolar GaN on a Si(100) substrate at the surface of which the V‐shaped nanostructures with the characteristic size of elements as low as 100 nm are formed. It has been demonstrated that application of buffer layers of 3C‐SiC and AlN enables formation of the GaN(10–11) layer characterized by the full width at half maximum value as low as ωθ ≈45 arcmin for the X‐ray diffraction rocking curve. The model based on anisotropic nucleation of AlN on the V‐shaped nanostructure is proposed to explain the growth of the GaN layer in a single semipolar direction.

Influence of surface step width of 4H-SiC substrates on the GaN crystal quality

In this work GaN films are grown on 4H-SiC substrates with different surface terrace step width and the influence of the step width on the crystal quality of GaN is studied using X-ray diffraction (XRD) method. The results prove that the different surface terrace step width has a big impact on the (0 0 2) and (1 0 2) full width at half maximum (FWHM) of GaN films. It is mainly because the nucleation and coalescence mechanism of AlN is closely related with the surface step width, resulting in different densities of threading dislocations in the subsequent grown GaN films.

Orientation of AlN Grains Nucleated on Different Diamond Substrates by Magnetron Sputtering

For AlN/diamond structure, the crystal quality of diamond substrates is of great importance for the epitaxial growth of AlN. In this paper, c‐axis‐oriented AlN films are deposited on diamond with different crystal quality by radio frequency (RF) reactive magnetron sputtering. The effects of the microstructure of diamond substrates on the orientation of AlN are studied. The results show that the crystal quality of the diamond films greatly affects the growth behavior of AlN. The surface defects of diamond, such as the amorphous phase, will cause the growth direction of AlN to deviate from the c‐axis, resulting in the disorder of grain orientation. Meanwhile, diamond crystal orientation also has influence on the growth of AlN. The (111) oriented grains of diamond are beneficial to the c‐axis growth of AlN. The interface of AlN nucleation and initial growth plays a vital role in final performance of the films, where there is a transition zone from randomly oriented AlN grains to c‐axis‐oriented grains. The suitable diamond substrate can eliminate the transition zone, and can lead to AlN films with high‐degree c‐axis orientation.

Low-temperature and global epitaxy of GaN on amorphous glass substrates by molecular beam epitaxy via a compound buffer layer

GaN epilayers are globally grown on amorphous glass substrates via a compound buffer layer including Ti pre-orienting layer and AlN nucleation layer (NL) grown by molecular beam epitaxy at 530 °C. It is shown that the ratio of V/III during AlN growth plays a key role in the crystallinity of AlN NL as a trade-off between the formation of Al clusters and the mobility of Al adatoms. The N2 flux has an optimal value of 2.4 sccm when the Al flux is fixed to 7.46 × 10−8 Torr at the RF power of 400 W. The obtained smooth GaN epilayer is hexagonal single-crystalline with the grain size in the order of submicron magnitude and root-mean-square roughness of 2.83 nm, which shows the great potential in the epitaxy of III-nitrides on amorphous glass substrates.

A GaN–SiC hybrid material for high-frequency and power electronics

We demonstrate that 3.5% in-plane lattice mismatch between GaN (0001) epitaxial layers and SiC (0001) substrates can be accommodated without triggering extended defects over large areas using a grain-boundary-free AlN nucleation layer (NL). Defect formation in the initial epitaxial growth phase is thus significantly alleviated, confirmed by various characterization techniques. As a result, a high-quality 0.2-μm thin GaN layer can be grown on the AlN NL and directly serve as a channel layer in power devices, like high electron mobility transistors (HEMTs). The channel electrons exhibit a state-of-the-art mobility of >2000 cm2/V-s, in the AlGaN/GaN heterostructures without a conventional thick C- or Fe-doped buffer layer. The highly scaled transistor processed on the heterostructure with a nearly perfect GaN–SiC interface shows excellent DC and microwave performances. A peak RF power density of 5.8 W/mm was obtained at VDSQ = 40 V and a fundamental frequency of 30 GHz. Moreover, an unpassivated 0.2-μm GaN/AlN/SiC stack shows lateral and vertical breakdowns at 1.5 kV. Perfecting the GaN–SiC interface enables a GaN–SiC hybrid material that combines the high-electron-velocity thin GaN with the high-breakdown bulk SiC, which promises further advances in a wide spectrum of high-frequency and power electronics.

III–V super-lattice SPST/SPMT pin switches for THz communication: theoretical reliability and experimental feasibility studies

A physics-based Quantum-Modified CLassical Drift–Diffusion (QMCLDD) non-linear mathematical model has been developed for design and characterisation of GaN/AlGaN asymmetrical superlattice pin based single-pole single-throw (SPST) and single-pole multi-throw (SPMT) switches for sub-MM wave communication systems. The simulator has incorporated several important physical phenomena that arise in superlattice structure, including quantum confinement effects, generation-recombination and tunnelling generation of carriers as well as scattering limited carrier mobility and velocity in GaN/AlGaN structure. In order to reduce the dislocation density and in turn the series resistance in GaN/AlGaN heterostructure, thin AlN nucleation layer and a buffer layer are considered in the simulation. It is observed that the RF series resistance of the pin device, operating around 0.2 THz frequency regime, reduces in case of proposed superlattice structure. The advantages of super-lattice pin diode over the conventional Si devices are the faster reverse recovery time (~ 9 vs 35 ns) and lowering of forward RF series resistances (0.39 vs 1.20 Ω). This study also reveals that SPST and SPMT switches made up with super-lattice devices are characterized by low insertion loss (~ 0.13 and 0.15 dB, respectively) and high isolation (~ 65.4 and 82.5 dB, respectively). A good agreement between theory and experiment establishes the superiority of the present model over the others. A comparative analysis of Si and GaN/AlGaN super-lattice pin SPST and SPMT switches establishes the potential of the later for its application in high-frequency THz-communication. In addition to this, a detailed thermal modelling of the device has also been done to make the analysis more realistic. The junction temperature of the designed GaN/AlGaN superlattice pin switch will be as high as 377 K, which is quite moderate compared to its flat profile counterpart. To the best of authors’ knowledge, this is the first ever report on QMCLDD non-linear modelling of pin SPST and SPMT switches (series-shunt combination) at THz-arena.

Study of dual nitridation processes in growth of non-polar a-plane AlGaN epi-layers

The effect of dual nitridation processes for both r-plane sapphire and low temperature-grown AlN (LT-AlN) nucleation layer on non-polar a-plane AlGaN epi-layer was studied intensively. A root-mean-square value as small as 1.54 nm for a-plane Al0.53Ga0.47N epi-layer was achieved. It was revealed that the generation of AlN grains as well as the coalescence and recrystallization of LT-AlN islands were the key factors for growing a-plane AlGaN epi-layers with smooth surface morphology. Meanwhile, the evolution of surface morphology with varied nitridation processes and the mechanisms for improving surface morphology of a-plane AlGaN epi-layers were also investigated.

Investigation on Carrier Transport Through AlN Nucleation Layer From Differently Doped Si(111) Substrates

To get a better insight into the vertical leakage mechanism of GaN-on-Si, the carrier transport from n+, n, p+, and p-Si(111) substrates through the AlN nucleation layer was investigated. A plateau in the current-voltage curve was found only for the AlN/p-Si heterojunction due to depletion of the p-Si substrate. Detailed study illustrated that it was the leaky AlN that cannot effectively block the increasing amount of electrons in the inversion layer at the interface and triggered the depletion. Temperature-dependent characterization suggested that the forward vertical leakage mechanism of AlN/Si could be explained sequentially by Ohm’s law, space-charge-limited conduction, variable-range hopping, and trap-assisted tunneling. A model involving shallow donor traps, interface traps, and deep level traps was proposed to explain the leakage characteristics. This paper shows that the carrier concentration of the Si substrates strongly impacts the vertical leakage characteristics, and also that the carrier transport from the Si substrate through the AlN nucleation layer is heavily influenced by traps.

Curvature evolution of 200 mm diameter GaN-on-insulator wafer fabricated through metalorganic chemical vapor deposition and bonding

Crack-free 200 mm diameter N-polar GaN-on-insulator (GaN-OI) wafers are demonstrated by the transfer of metalorganic chemical vapor deposition (MOCVD)-grown Ga-polar GaN layers from Si(111) wafers onto SiO2/Si(100) wafers. The wafer curvature of the GaN-OI wafers after the removal of the original Si(111) substrate is correlated with the wafer curvature of the starting GaN-on-Si wafers and the voids on the GaN-on-Si surface that evolve into cracks on the GaN-OI wafers. In crack-free GaN-OI wafers, the wafer curvature during the removal of the AlN nucleation layer, AlGaN strain-compensation buffer layers and GaN layers is correlated with the residual stress distribution within individual layers in the GaN-OI wafer.

Influence of the AlN nucleation layer on the properties of AlGaN/GaN heterostructure on Si (1 1 1) substrates

AlGaN/GaN heterostructures were grown on Si (1 1 1) substrates with different AlN nucleation layers (NL) by metal–organic chemical vapor deposition (MOCVD). The results indicate that the growth temperature of AlN NL has a noticeable influence on the structural, electronic and optical properties of the AlGaN/GaN heterostructures. Optimizing the growth temperature to 1040 °C led to quasi-2D smooth surface of the AlN NL with providing sufficient compressive stress to suppress cracking of the subsequent GaN layer during the cooling process, resulting in improved crystalline quality of GaN layer and superior two-dimensional electron gas (2DEG) performance of the AlGaN/GaN heterostructure.

Improved crystalline quality of AlN epitaxial layer on sapphire by introducing TMGa pulse flow into the nucleation stage

The crystalline quality of AlN epitaxial layers on sapphire substrates was improved by introducing trimethylgallium (TMGa) pulse flow into the growth of AlN nucleation layers. It was found that the density of both screw- and edge-type threading dislocations could be significantly reduced by introducing the TMGa pulse flow. With increasing TMGa pulse flow times, the lateral correlation length (i.e. the grain size) increases and the strain in the AlN epilayers changes from tensile state to compressive state. Unstrained AlN with the least dislocations and a smooth surface was obtained by introducing 2-times TMGa pulse flow. The crystalline improvement is attributed to enhanced lateral growth and improved crystalline orientation by the TMGa pulse flow.

High efficiency ultraviolet GaN-based vertical light emitting diodes on 6-inch sapphire substrate using ex-situ sputtered AlN nucleation layer.

We demonstrated the growth of crack-free high-quality GaN-based UV vertical LEDs (VLEDs) (λ = 365 nm) on 6-inch sapphire substrates by using an ex-situ sputtered AlN nucleation layer (NL) and compared their performance with that of UV VLEDs with an in situ low temperature (LT) AlGaN NL. The X-ray diffraction (XRD) results showed that the ex-situ AlN sample contained lower densities of screw-type and edge-type threading dislocations than the in situ AlGaN NL sample. The micro-Raman results revealed that the ex-situ AlN sample was under more compressive stress than the in situ AlGaN sample. As the current was increased, the electroluminescence peaks of both of the samples blue-shifted, reached a minimum wavelength at 1000 mA, and then slightly red-shifted. Packaged VLEDs with the ex-situ AlN NL yielded 6.5% higher light output power at 500 mA than that with the in situ AlGaN NL. The maximum EQEs of the VLED with the in situ AlGaN and ex-situ AlN NLs were 43.7% and 48.2%, respectively. Based on the XRD and Raman results, the improved light output power of the ex-situ AlN sample is attributed to the lower density of TDs.

Polar Dependence of Threading Dislocation Density in GaN Films Grown by Metal-Organic Chemical Vapor Deposition**Supported by the National Key Research and Development Program of China under Grant No 2016YFB0400100, and the China Postdoctoral Science Foundation under Grant No 2015M582610.

We investigate the threading dislocation (TD) density in N-polar and Ga-polar GaN films grown on sapphire substrates by metal-organic chemical vapor deposition. X-ray diffraction results reveal that the proportion of screw type TDs in N-polar GaN is much larger and the proportion of edge type TDs is much smaller than that in Ga-polar. Transmission electron microscope results show that the interface between the AlN nucleation layer and the GaN layer in N-polar films is smoother than that in Ga-polar films, which suggests different growth modes of GaN. This observation explains the encountered difference in screw and edge TD density. A model is proposed to explain this phenomenon.

Impact of AlN/Si Nucleation Layers Grown Either by NH3‐MBE or MOCVD on the Properties of AlGaN/GaN HFETs

Epitaxial growth of group III nitrides on silicon substrates is challenging as large lattice and thermal expansion coefficient mismatches have to be accounted for. It is generally accepted that AlN is the best nucleation layer for GaN‐on‐Si growth. However, some open questions remain about this AlN nucleation layer. In this study, it is investigated how the choice of deposition method of the AlN nucleation layer affects the material and electrical properties of a full HFET structure. First, electrical characterization is performed on thin AlN layers deposited on two inch p‐type silicon substrates grown by NH3‐MBE. CV characteristics indicate that electron injection from an inversion‐like layer at the AlN/Si‐interface is suppressed. Based on previous studies, this leads to the expectation that choosing MBE‐AlN may actually result in superior properties at the device level. Finally, the full HFET stack is grown by MOCVD on both types of AlN nucleation layers. Hall measurements on active devices yield comparable mobilities for both nucleation methods. Preliminary data suggest that MBE‐AlN does not have any detrimental impact the dynamic properties of the buffer. The vertical breakdown voltage measured at 1 μA mm−2 is increased by 30% and the leakage current measured at 600 V is reduced by two orders of magnitude.

Nucleation layer design for growth of a high-quality AlN epitaxial film on a Si(111) substrate

A high-quality AlN epitaxial film has been grown on a Si(111) substrate by metal–organic chemical vapor deposition through designing the AlN nucleation layer.

Effect of thermal annealing on AlN films grown on sputtered AlN templates by metalorganic vapor phase epitaxy

To obtain low-dislocation-density c-plane AlN on sapphire, we have studied the annealing of AlN films at 1650 °C. We prepared an AlN film with a thickness of 20 nm by radio-frequency (RF) sputtering, on which an AlN epilayer with a thickness of 280 nm was grown by metalorganic vapor phase epitaxy (MOVPE) at different growth temperatures. Finally, the AlN film on sapphire was annealed at 1650 °C. The full width at half maximum (FWHM) of the X-ray rocking curve (XRC) for AlN(0002) was approximately 50 arcsec for all samples, whereas that for AlN() significantly decreased from 6500 to 350 arcsec after thermal annealing. A surface morphology with atomic steps and a terrace structure was clearly observed after thermal annealing. From these results, we concluded that a high-quality AlN film can be obtained by annealing a stacking structure of MOVPE-grown AlN (buffer layer)/sputtered AlN (nucleation layer)/sapphire substrate.

Stress in (Al, Ga)N heterostructures grown on 6H-SiC and Si substrates byplasma-assisted molecular beam epitaxy

The paper describes experimental results on low temperature plasma-assisted molecular beam epitaxy of GaN/AlN heterostructures on both 6H-SiC and Si(111) substrates. We demonstrate that application of migration enhanced epitaxy and metal-modulated epitaxy for growth of AlN nucleation and buffer layers lowers the screw and edge(total)threading dislocation (TD) densities down to 1.7·108 and 2·109 cm-2, respectively, in a 2.8-μm-thick GaN buffer layer grown atop of AlN/6H-SiC. The screw and total TD densities of 1.2·109 and 7.4·109 cm-2, respectively, were achieved in a 1-μm-thickGaN/AlNheterostructure on Si(111). Stress generation and relaxation in GaN/AlN heterostructures were investigated by using multi-beam optical stress sensor (MOSS) to achieve zero substrate curvature at room temperature. It is demonstrated that a 1-μm-thick GaN/AlN buffer layer grown by PA MBE provides planar substrate morphology in the case of growth on Si substrates whereas 5-μm-thick GaN buffer layers have to be used to achieve the same when growing on 6H-SiC substrates.

Producing deep UV-LEDs in high-yield MOVPE by improving AlN crystal quality with sputtered AlN nucleation layer * * Project supported by the National Natural Sciences Foundation of China (Nos. 61334009, 61474109, 61306050).

High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes (UV-LEDs). In this work, a high-temperature AlN epitaxial layer was grown on sputtered AlN layer (used as nucleation layer, SNL) by a high-yield industrial metalorganic vapor phase epitaxy (MOVPE). The full width half maximum (FWHM) of the rocking curve shows that the AlN epitaxial layer with SNL has good crystal quality. Furthermore, the relationships between the thickness of SNL and the FWHM values of (002) and (102) peaks were also studied. Finally, utilizing an SNL to enhance the quality of the epitaxial layer, deep UV-LEDs at 282 nm were successfully realized on sapphire substrate by the high-yield industrial MOVPE. The light-output power (LOP) of a deep UV-LED reaches 1.65 mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31 mW at an injection current of 60 mA. Hence, our studies supply a possible process to grow commercial deep UV-LEDs in high throughput industrial MOVPE, which can increase yield, at lower cost.

Influence of AlN Growth Temperature on the Electrical Properties of Buffer Layers for GaN HEMTs on Silicon

Despite the fact that a lower growth temperature is generally considered as a drawback for achieving high crystal quality, the necessity to reduce the nucleation temperature of AlN on Silicon has permitted Molecular Beam Epitaxy (MBE) to demonstrate high performance for GaN transistors operating at high frequency. Compared to Metal‐Organic Vapor Phase Epitaxy (MOVPE), the control of the interface between the AlN nucleation layer and the substrate is easier and the reduced growth temperature allows to obtain a more electrically resistive interface while keeping good crystal quality. Moreover, it is shown that further reducing the growth temperature within the nucleation and stress mitigating layers has a noticeable impact on the lateral and vertical buffer leakage currents. At the same time the buffer of MBE grown HEMT structures exhibits low RF propagation losses (below 0.5 dB mm−1 up to 70 GHz). Also, results obtained with structures regrown by MOVPE on MBE AlN‐on‐Si templates confirm that the thermal budget is critical for the resistivity of AlN/Si. On the other hand, the insertion of a 1.5 μm thick Al0.05Ga0.95N layer within a 2 μm HEMT structure significantly improves the vertical breakdown voltage up to 740 V permitting to compare favorably with MOVPE epilayers with similar total thickness.

Comparative study of GaN-based ultraviolet LEDs grown on different-sized patterned sapphire substrates with sputtered AlN nucleation layer

GaN-based ultraviolet-light-emitting diodes (UV LEDs) with 375 nm emission were grown on different-sized patterned sapphire substrates (PSSs) with ex situ 15-nm-thick sputtered AlN nucleation layers by metal–organic chemical vapor deposition (MOCVD). It was observed through in situ optical reflectance monitoring that the transition time from a three-dimensional (3D) island to a two-dimensional (2D) coalescence was prolonged when GaN was grown on a larger PSS, owing to a much longer lateral growth time of GaN. The full widths at half-maximum (FWHMs) of symmetric GaN(002) and asymmetric GaN(102) X-ray diffraction (XRD) rocking curves decreased as the PSS size increased. By cross-sectional transmission electron microscopy (TEM) analysis, it was found that the threading dislocation (TD) density in UV LEDs decreased with increasing pattern size and fill factor of the PSS, thereby resulting in a marked improvement in internal quantum efficiency (IQE). Finite-difference time-domain (FDTD) simulations quantitatively demonstrated a progressive decrease in light extraction efficiency (LEE) as the PSS size increased. However, owing to the significantly reduced TD density in InGaN/AlInGaN multiple quantum wells (MQWs) and thus improved IQE, the light output power of the UV LED grown on a large PSS with a fill factor of 0.71 was 131.8% higher than that of the UV LED grown on a small PSS with a fill factor of 0.4, albeit the UV LED grown on a large PSS exhibited a much lower LEE.