AlGaN Buffer Research Articles

Investigation of the Dislocation Behavior of 6- and 8-Inch AlGaN/GaN HEMT Structures with a Thin AlGaN Buffer Layer Grown on Si Substrates

Developing cost-effective methods to synthesize large-size GaN films remains a challenge owing to the high dislocation density during heteroepitaxy. Herein, AlGaN/GaN HEMTs were grown on 6- and 8-inch Si(111) substrates using metal–organic chemical vapor deposition, and their basic properties and dislocation evolution characteristics were investigated thoroughly. With the insertion of a 100 nm thin AlGaN buffer layer, bow–warp analysis of the epitaxial wafers revealed excellent stress control for both the 6- and 8-inch wafers. HR-XRD and AFM analyses validated the high crystal quality and step-flow growth mode of GaN. Further, Hall measurements demonstrated the superior transport performance of AlGaN/GaN heterostructures. It is worth noting that dislocations tended to annihilate in the AlN nucleation layer, the thin AlGaN buffer layer, and the GaN buffer layer in the initial thickness range of 200–300 nm, which was indicated by ADF-STEM. To be specific, the heterointerfaces exhibited a significant effect on the annihilation of c-type (b = <0001>) dislocations, which led to the formation of dislocation loops. The thin inserted layers within the AlGaN buffer layer played a key role in promoting the annihilation of c-type dislocations, while they exerted less influence on a-type (b = 1/3<112¯0>) and (a+c)-type (b = 1/3<112¯3>) dislocations. Within an initial thickness of 200–300 nm in the GaN buffer layer, a-type and (a+c)-type dislocations underwent strong interactions, leading to considerable dislocation annihilation. In addition, the EELS results suggested that the V-shaped pits in the AlN nucleation layer were filled with the AlGaN thin layer with a low Al content.

Optimization of Epitaxial Structures on GaN-on-Si(111) HEMTs with Step-Graded AlGaN Buffer Layer and AlGaN Back Barrier

Recently, crack-free GaN-on-Si growth technology has become increasingly important due to the high demand for power semiconductor devices with high performances. In this paper, we have experimentally optimized the buffer structures such as the AlN nucleation layer and step-graded AlGaN layer for AlGaN/GaN HEMTs on Si (111) substrate by varying growth conditions and thickness, which is very crucial for achieving crack-free GaN-on-Si epitaxial growth. Moreover, an AlGaN back barrier was inserted to reduce the buffer trapping effects, resulting in the enhancement of carrier confinement and suppression of current dispersion. Firstly, the AlN nucleation layer was optimized with a thickness of 285 nm, providing the smoothest surface confirmed by SEM image. On the AlN nucleation layer, four step-graded AlGaN layers were sequentially grown by increasing the Al composition from undermost layer to uppermost layer, meaning that the undermost one was close to AlN, and the uppermost was close to GaN, to reduce the stress and strain in the epitaxial layer gradually. It was also verified that the thicker step-graded AlGaN buffer layer is suitable for better crystalline quality and surface morphology and lower buffer leakage current, as expected. On these optimized buffer structures, the AlGaN back barrier was introduced, and the effects of the back barrier were clearly observed in the device characteristics of the AlGaN/GaN HEMTs on Si (111) substrate such as the transfer characteristics, output characteristics and pulsed I-V characteristics.

Design and Growth of Low Resistivity P-Type AlGaN Superlattice Structure.

This work investigated the impact of periodic thickness and doping region on the doping efficiency of the P-type AlGaN superlattice. In this paper, the band structure of the simulated superlattice was analyzed. The superlattice structure of Al0.1Ga0.3N/Al0.4Ga0.6N, and the AlGaN buffer on the sapphire substrate, achieved a resistivity of ~3.3 Ω·cm. The results indicate that barrier doping and low periodic thickness offer significant advantages in introducing a reduction of the resistivity of P-type AlGaN superlattice structures.

Impact of Carbon Doping on Vertical Leakage Current in AlGaN‐Based Buffer Layer Grown on Silicon Substrate

The buffer leakage phenomena have a large effect on high‐power and fast‐switching GaN high electron mobility transistor devices. Herein, the buffer leakage mechanism in a GaN‐on‐Si substrate with AlGaN buffer layers is investigated by using samples with various carbon concentrations. Comparing the leakage current with crystallinity and impurity concentration, it is found that the vertical leakage current is dominated by both threading dislocations and bulk impurities. Increasing the carbon concentration of buffer layers suppresses leakage current in the bulk conducting regime in high electric fields but increases it in the dislocation conducting regime in low electric fields. These results indicate that the optimal carbon concentration of buffer layers depends on the target electric field.

Enhancing sputtered GaN/Si film quality by adding AlGaN buffer layer in a continuous deposition process

In this work we demonstrate the achievement of significative improvements in the structural and morphological quality of wurtzite GaN films (approximately 250 nm thick) by introducing an initial growth step involving an AlGaN buffer layer on p-type Si (100) substrates through a continuous reactive sputtering process. We investigated the influence of using single and multiple AlxGa1-xN buffer layers with varying Al content (x ranging from 0.07 to 0.37) and different thicknesses (from 166 nm to 1 µm). The obtained samples underwent characterization through X-ray diffraction and scanning electron microscopy. The results reveal a slight increase in the c-axis preferred growth direction and grain sizes even with a single and thin (250 nm) Al0.07Ga0.93N buffer layer. Conversely, the use of a single Al0.37Ga0.63N buffer layer led to significant morphological changes and a remarkable improvement in the c-axis preferred orientation. The most favorable outcomes were observed with the implementation of a triple AlGaN buffer layer, featuring decreasing Al content from the substrate, indicating the attainment of a high-quality GaN top layer comparable to epitaxial GaN.

Impact of Graded AlGaN Buffer Layer Thickness on the DC, RF, Linearity, Intermodulation and Off-State Breakdown Characteristics of AlGaN Channel HEMT

This study examines the extraction of small-signal equivalent circuit parameters, linearity and intermodulation metrics, and capacitance–voltage characteristics for both AlGaN and GaN channel HEMTs. Compared to GaN channel HEMTs, AlGaN channel HEMTs have better linearity and intermodulation distortion. In the instance of AlGaN channel HEMTs, the device parasitics are decreased. Additionally, the effect of graded AlGaN buffer layer thickness on breakdown characteristics and linearity is examined. For breakdown characteristics, the 200 nm-graded AlGaN buffer layer thickness yields superior results with VBOff 157 V for a thickness of 200 nm (147 V in the case of 500 nm). At 200 nm thickness, the linearity and intermodulation distortion FOMs are also reasonably good, and the remaining device properties are essentially unchanged. Hence, the graded AlGaN buffer thickness can be reduced for high-voltage applications. For higher-voltage applications, the graded AlGaN buffer thickness can, therefore, be decreased.

Fully-vertical GaN-on-SiC Schottky barrier diode with ultrathin AlGaN buffer layer

In this letter, vertically conductive GaN epilayer on SiC substrate was achieved without the typically used conductive buffer layer. Here, in order to reduce the impacts of band offset of different layers on vertical conductivity and improve the vertical carrier transportation, an ultrathin AlGaN buffer layer was employed to replace the thick conductive buffer layer. Fully-vertical Schottky barrier diode (SBD) based on this vertically conductive epi-stack demonstrated a much lower specific on-resistance of 0.84 mΩ·cm2 with superior thermal stability. Moreover, the SBD also exhibited an on/off ratio of ∼5 × 109 and a nearly unity ideality factor of 1.08. This approach lays the foundation for the heterogeneous integration of GaN/SiC based devices.

Effect of High Al Content AlGaN Buffer Layer on the Properties of GaN‐on‐Silicon Materials

The lattice parameter and thermal expansion coefficient mismatch between the silicon substrate and GaN lead to high tensile stress, which makes the GaN epitaxial layer prone to cracking. Effective compensation of tensile stress on GaN to prevent cracking is an important issue in GaN epitaxial growth. In this work, GaN‐on‐silicon materials with different AlGaN buffer layer structures are prepared by metal‐organic chemical vapor deposition (MOCVD). The GaN epitaxial material with a smooth and crack‐free surface is fabricated by inserting 7 AlGaN buffer layers. The crystal quality of GaN is characterized using high resolution X‐ray diffraction (HRXRD). The full‐width half maximum (FWHM) value of GaN(002) and GaN(102) crystal plane is 398 and 780 arcsec, respectively. The surface root mean square (RMS) roughness of GaN material is 0.31 nm, and the vertical breakdown voltage (BV) of the epitaxial wafer reaches 918 V. The results show that high Al content of the AlGaN buffer layer can effectively reduce the tensile stress and dislocation density of the GaN layer when the entire AlGaN layer thickness remains constant. Suppressing the generation of surface cracks and improving the crystal quality can improve the vertical breakdown voltage and two‐dimensional electron gas (2‐DEG) characteristics of GaN epitaxial wafers.

Structural, optical, and electrical characterization and performance comparison of AlGaN/GaN HEMT structures with different buffer layers

AlGaN/GaN high electron mobility transistor (HEMT) structures are promising for high power and high speed electronic applications. The selection of a good buffer structure is crucial for the achievement of high quality HEMT devices. Here, we present a comparative analysis of structural, optical, and electrical properties of AlGaN/GAN HEMT structures with three different buffer configurations: step-graded AlGaN and superlattice-based buffers on Si, and no buffer on SiC substrates. The analysis of structural properties showed that the structure on SiC has better crystal quality. The Analysis also provided an insight of the role superlattice buffer to cut-off the dislocations and enhance the GaN channel crystallinity. Optical and electrical characterizations showed the enhanced 2DEG quality in the superlattice-based structure, due to the better stress management in the top layers. The electronic and optoelectronic quality of the structures were studied by fabricating HEMT and metal-semiconductor-metal photodetector devices. The devices analysis showed that the superlattice based structure outperformed the other two structures with drain current and transconductance of 406.3 mA/mm and 81.5 mS/mm, respectively, as well as photo-to-dark current ratio that is 2–3 orders of magnitude higher than the other two structures, under 325 nm light illumination. The results provide insights into the impact of different buffer configurations on the performance of AlGaN/GaN HEMT structures, suggesting that the superlattice buffer structure is the optimal choice for fabricating high-performance AlGaN/GaN HEMT-based electronics.

Effects of physical parameters of graded AlGaN buffer on DC characteristic and short-channel effects in AlInN/GaN high-electron mobility transistors

In GaN high-electron mobility transistors (HEMTs), the graded AlGaN buffer can induce negative polarization charges over the buffer by designing the change direction of Al-content. By simulation, the effects of the physical parameters of the graded buffer [bottom Al-content (xbot) and buffer thickness (tbuf)] on the direct-current characteristics and short-channel effects (SCEs) of Al0.83In0.17N/GaN HEMTs are investigated. The results show that when tbuf is fixed, as xbot increases, threshold voltage, peak of transconductance, drain-induce barrier lower and sub-threshold swing will be approximately saturated. As tbuf decreases, the xbot becomes smaller when the saturation occurs. In the graded buffer design, the recommended xbot range is 0.15–0.2 when tbuf = 0.8–1.2 μm. Moreover, suppressing SCEs can reduce tbuf and increase xbot, but at the cost of a significant lower maximum of drain current. A tradeoff is needed in the choice of tbuf and xbot. Compared to conventional AlGaN buffer, the graded buffer can keep AlInN barrier nominally strain-free in the Al0.83In0.17N/GaN HEMTs.

Improved breakdown voltage mechanism in AlGaN/GaN HEMT for RF/Microwave applications: Design and physical insights of dual field plate

This work investigated and proposed the source–gate dual field plate (SG-FP) AlGaN/GaN HEMT for different gate-to-drain drift distances with a fixed gate length of 0.25 μm using ATLAS-TCAD. The increment in the GaN buffer thickness and gate-to-drain drift distance with the variation in the field plate length improves the device performance and breakdown voltages by maintaining the impact ionization mechanism. The breakdown voltage behavior, physical insights of electric field contour, impact generation rate, and potential profile intricacies of source field plate (S-FP), gate field plate (G-FP), and dual field plate (SG-FP) are illustrated. The proposed source–gate dual field plate (SG-FP) with the SiN passivation interfacial layer, SiO2, thicker GaN buffer, and extended gate-to-source drift region is best suited for optimal performance in high breakdown voltage, with optimum cut-off frequency and low gate-leakage current. The proposed dual field plate AlGaN/GaN device with the source field plate (LS-FP = 4 μm), gate field plate (LG-FP = 2 μm), the AlGaN buffer thickness of 3.697 μm, and gate-to-drain drift distance of 8 μm show the highest breakdown voltage of 562 V with the optimum frequency of 8.861 GHz. Finally, the AC/DC characteristics of the optimized dual field plate (SG-FP) are demonstrated. We observed that the source–gate dual field plate (SG-FP) device shows a 6.70% and 4.09% improvement in breakdown voltage compared to individual source field plate (S-FP) and gate field plate (G-FP) designs, respectively. This work shows that the proposed dual field plate device is used in power switching, high-power, and RF/Microwave applications due to its superior breakdown voltage performance, reduction in parasitic capacitances, and optimal cut-off frequency.

Enhancement of gallium nitride on silicon (111) using pulse atomic-layer epitaxy (PALE) AlN with composition-graded AlGaN buffer

The ability to configure the optimal buffer layer for GaN growth depends on the knowledge of relaxation processes that occurs during the cooling step while countering the tensile stresses due to the contrast of thermal expansion coefficient between GaN and Si(111) substrate. Here, we inaugurate the pulse atomic-layer epitaxy (PALE) AlN layer to reinforce the buffer layer to achieve a thick GaN epilayer which is crucial for high performance power devices. The characteristics of grown GaN on Si substrate based on PALE AlN thickness of 0 ~ 100 nm are investigated along with microstructural evolution between AlN NL and composition-graded AlGaN buffer layer. PALE AlN layer deposited with an optimum thickness of 50 nm and above was observed to exhibit a highly uniform coalesced GaN epilayer surface with root-mean square (RMS) roughness of 0.512 nm. The thickness of the PALE AlN layer substantially affected the crystallinity of the top GaN epilayer where the lowest value for symmetric (0 0 0 2) and asymmetric (1 0 -1 2) x-ray rocking curve analysis were achieved, indicating the reduction of threading dislocation density in the growth structure. Transition of the E2 (high) peak from the Raman spectrum shows that the strain compression in GaN epilayer is directly proportional to the thickness of the PALE AlN layer.

Submicrometer‐Thick Step‐Graded AlGaN Buffer on Silicon with a High‐Buffer Breakdown Field

Submicrometer‐thick AlGaN/GaN high‐electron‐mobility transistor (HEMT) epilayers grown on silicon substrate with a state‐of‐the art vertical buffer breakdown field as high as 6 MV cm−1 enabling a high transistor breakdown voltage of 250 V for short gate‐to‐drain distances despite such a thin structure are reported. HEMTs with a gate length of 100 nm exhibit good DC characteristics with a low drain‐induced barrier, going as low as 100 mV V−1 for a VDS of 30 V. Breakdown voltages of each epilayer from the decomposed heterostructure reveals that the outstanding breakdown strength is attributed to the insertion of Al‐rich AlGaN in the buffer layers combined with an optimized AlN nucleation layer. As a result, large signal measurements at 10 GHz could be reliably achieved up to VDS = 35 V despite the use of a 100 nm gate length. These results demonstrate the potential of submicrometer‐thick buffer GaN‐on‐Si heterostructures for high‐frequency applications.

Terrace Engineering of the Buffer Layer: Laying the Foundation of Thick GaN Drift Layer on Si Substrates

AbstractVertical GaN‐on‐Si devices are promising for the next‐generation high‐voltage power electronics with low cost and high efficiency. However, their applications are impeded by the limited thickness of crack‐free GaN layers and high threading dislocation density (TDD) in the layer. Buffer layers are crucial for stress control while they usually behave with poor surface morphology, which causes early stress relaxation in GaN and limited thickness. Hereby, a terrace engineering approach for the buffer layers is proposed. Through tuning the supersaturation ratio, the terrace width can be manipulated and an atomically smooth AlGaN buffer layer can be realized, which effectively reduces the compressive stress relaxation and provides a firm foundation for thick GaN growth. As a result, a 7.5 µm thick GaN drift layer with TDD as low as 8.6 × 107 cm−2 is achieved on Si substrates. The room temperature electron mobility of the GaN drift layer can be raised up to 1210 cm2 V−1 s−1. The fabricated PiN diode shows a high breakdown voltage of 1058 V as well as a high on/off ratio of 1012. This work thus truly demonstrates the potential of high‐performance and low‐cost GaN‐based electronic as well as optoelectronic devices on Si platforms.

Epitaxial growth of AlGaN/GaN HEMTs on patterned Si substrate for high voltage power switching applications

In this work, a series of AlGaN/GaN high electron mobility transistors (HEMTs) have been grown on patterned (Al)GaN-on-Si seed layers by metalorganic chemical vapor deposition (MOCVD) to study the effect of the (Al)GaN buffer thickness and its aluminum content on the vertical breakdown voltage. The atomic force microscopy (AFM) analysis performed on the GaN seed layers shows a perfectly smooth, MOCVD-like morphology with a threading dislocation density (TDD) around 2 × 109 cm−2. We succeeded to regrow up to 10 μm and 5 μm crack-free GaN + HEMT and Al0.30G0.70N + HEMT structures, respectively and scanning electron microscopy (SEM) revealed limited lateral growth on mesas with few microscopic defects while AFM show the same surface morphology of HEMT structures as ones grown on planar substrates. NiAu Schottky contacts were deposited on the regrown (Al)GaN HEMTs as well as on (Al)GaN seed layer structures. No breakdown was noticed up to 200 V for the regrown Al0.30Ga0.70N (3 μm) HEMT and the GaN seed layer structures and the leakage current remained in the range of 1 nA. However, the leakage current in the regrown GaN (5 μm) structure reached rapidly 1 mA. Such a high leakage was not noticed for non-patterned devices. Thus, it is possibly attributed to silicon contamination in the regrown GaN layer. On the other hand, the absence of noticeable leakage in the regrown 3 μm thick Al-rich AlGaN buffer HEMT opens the way to a new competitive solution for high power electronics applications on silicon substrate.

Effect of High-Pressure GaN Nucleation Layer on the Performance of AlGaN/GaN HEMTs on Si Substrate.

A high-pressure (HP) GaN nucleation layer (NL) was inserted between AlGaN buffer and an unintentionally doped (UID) GaN layer of an AlGaN/GaN HEMT on Si. The XRD and TEM showed that when the V/III ratio was optimized during the HP-GaN NL growth, the edge dislocation density in the HP-GaN NL layer could be reduced significantly. Experimental results exhibited a lower off-state leakage current, higher maximum ID and Gm (corresponding to 22.5% and 21.7% improvement, respectively), and lower on-state resistance. These results demonstrate that the electrical properties of the AlGaN/GaN HEMT can be improved through the insertion of a HP-GaN NL.

Compositional Mapping of the AlGaN Alloy Composition in Graded Buffer Structures Using Cathodoluminescence

Herein, the use of cathodoluminescence (CL) hyperspectral mapping in the quantification of the AlGaN alloy composition in graded buffer structures is explored. The quantification takes advantage of the known parabolic dependence of the AlGaN bandgap on the alloy composition allowing the AlGaN near‐band‐edge (NBE) emission energy recorded from CL to be converted to a composition. The proposed quantification method is first applied to cleaved cross‐sections of two nominally step‐graded AlGaN buffer structures each containing five AlGaN layers with different compositions. By comparing the compositions obtained from CL to those calculated using X‐ray diffraction, a close agreement between values from both techniques is observed. However, due to a change in the bowing parameter, some deviation is observed for layers with compositions near 75%. The method is then applied to cleaved cross‐sections of an AlGaN buffer whose group III precursor flow molar ratio is varied linearly throughout the growth. Herein, the hyperspectral nature of the CL datasets is exploited such as to produce composition maps by converting the relevant AlGaN‐NBE emission energy at each pixel of the CL data to a composition.

Low stress AlGaN/GaN heterojuction with AlGaN buffer grown on 6-inch semi-insulating SiC substrate

AlGaN/GaN heterojuction with AlGaN buffer was grown by MOCVD on 6-inch semi-insulating SiC substrate. Compared to GaN buffer, the structures of GaN/AlGaN compound buffer and pure AlGaN buffer could reduce the tensile stress caused by mismatch within epilayer and SiC substrate effectively. The relationship between stress and of AlGaN buffer thickness was analyzed. As AlGaN buffer thickness decreased from 2000 nm to 500 nm, the stress status of GaN channel reversed from tensile to compressive. The crystal quality of AlGaN buffer was improved by increasing AlN nucleation growth temperature to 1150℃. High propertiescrystal quality and low stress AlGaN/GaN heterojuction with AlGaN buffer grown on 6-inch semi-insulating SiC substrate was obtained.

Effects on RF Performance for AlGaN/GaN HEMT on Si Substrate with AlGaN Buffer Engineering

In this paper, we demonstrate the AlGaN/GaN high electron mobility transistors (HEMTs) on Si substrate using an AlGaN back-barrier (BB) and super-lattice (SL) buffer to achieve high breakdown and low current collapse (CC) properties for Radio-frequency (RF) applications. The HEMTs also demonstrated low initial vertical leakage current, high thermal stability, and smaller drain-lag with reduced leakage currents as compared to the devices using step-graded (SG) AlGaN buffer. Also, the device showed improved RF performances with higher f T and f max as compared to the devices with SG buffer.

Design of step-graded AlGaN buffers for GaN-on-Si heterostructures grown by MOCVD

For the growth of low-defect crack-free GaN heterostructures on large-area silicon substrates, compositional grading of AlGaN is a widely adapted buffer technique to restrict the propagation of lattice-mismatch induced defects and balance the thermal expansion mismatch-induced tensile stress. So far, a consolidation of the design strategy of such step-graded buffers has been impaired by the incomplete understanding of the effect of individual buffer design parameters on the mechanical and microstructural properties of the epilayers. Herein, we have analyzed a series of metal-organic chemical vapor deposition grown GaN/graded-AlGaN/AlN/Si heterostructures through in situ curvature measurements and post-growth x-ray diffraction (XRD). Our results reveal that in such epi structures, the GaN layer itself induces more compressive stress than the AlGaN buffer, but the underlying AlGaN layers dictate the magnitude of this stress. Furthermore, for a fixed AlGaN buffer thickness, the mean-stress accumulated during the GaN growth is found to be correlated with its structural properties. Specifically, one µm thick GaN layers that acquire 1.50 GPa or higher compressive mean-stress are seen to possess XRD ω-FWHM values less than 650 arc-sec. Also, the evolution of instantaneous stresses during the growth of the AlGaN layers is found to be a valuable indicator for buffer optimization, and composition difference between successive layers is established as a crucial criterion. The results also show that increasing the total buffer thickness (for a fixed number of steps) or increasing the number of steps (for a fixed total buffer thickness) may not always be beneficial. Irrespective of the buffer thickness, optimized high electron mobility transistor structures show similarly low sheet-resistance (∼350 Ω □)−1 and high mobility (∼2000 cm2 V−1 s −1) at room temperature.