This article is a review of the epitaxial growth of GaN-based materials on Si substrates and their application in the RF field. How to grow high-quality Si-based GaN is a question of great concern in the industry.summary : Si substrate has the advantages of large size, low cost and compatibility with existing CMOS process compatibility and other advantages make On Si substrate GaN-based RF RF) electronic materialsHarmony Components have become the next focus of attention in this field after power electronic devices. Due to its mechanical properties and low resistance And Different substrates,High resistance And On the substrate GaN BasicGrowth of plastic materialsStress controlandDislocation suppressionThe problem remains difficult and seriousRF lossProblems limit its application in the field of radio frequency electronics.Main text To introduce And On the substrate GaN The research status and challenges of radio frequency electronic materials are introduced, with a focus on the research team of Peking University.High resistance And On the substrate GaN The generation mechanism of RF loss of base materials, as well as low dislocation density and low RF loss GaN The main research progress in epitaxial growth of AndOn the substrate GaN The future development of radio frequency electronic materials and devices is prospected.0 lead Word
GaN possesses excellent properties such as a wide bandgap, high electron mobility, and a high saturated electron drift velocity. Compared to GaAs-based electronic devices, GaN-based electronic devices can operate at higher voltages and current densities. Compared to Si-based RF electronic devices, they have higher operating frequencies, making GaN particularly suitable for high-power, high-frequency RF electronic applications. Consequently, GaN-based devices are gaining increasing attention in academia and industry within the RF electronics field.Depending on the material epitaxial substrate, the main technical routes for GaN-based RF devices include GaN on Si substrate and GaN on SiC substrate.Due to the high thermal conductivity of SiC substrates and the small lattice mismatch with GaN, GaN on SiC substrates is currently relatively mature and widely used in high-power RF applications such as phased array radar and communications. However, the high price of semi-insulating SiC substrates limits its potential for civilian applications. In contrast, Si substrates offer advantages such as large size, low cost, and ease of integration with Si-based integrated circuit processes. These advantages make GaN-based RF electronic devices on Si substrates highly promising for industrial applications and have become a new research hotspot in recent years. In recent years, a research team at Peking University has systematically studied key scientific and technological issues in the epitaxial growth of GaN-based RF electronic materials on Si substrates, focusing on the mechanisms of RF losses in GaN-based materials on high-resistance Si substrates and the epitaxial growth of GaN with low dislocation density and low RF loss, achieving a series of breakthroughs.1 Research status of GaN-based RF electronic materials on Si substrates
GaN-based RF electronic devices on Si substrates have excellent characteristics such as high breakdown voltage, high current density, and high operating frequency. They also have the advantages of large size, low cost, and easy integration with Si-based CMOS processes on Si substrates. They are expected to promote the industrial application of GaN-based devices in the field of RF electronics and are currently a hot topic of concern in the international academic and industrial fields of nitride semiconductors.High-quality epitaxial materials are essential for achieving The foundation of GaN-based RF electronics.However, there is a large lattice mismatch (16.9%) and thermal expansion coefficient mismatch (56%) between GaN and Si crystals. Therefore, the GaN epitaxial layer on the Si substrate not only has a high dislocation density, but also the tensile stress caused by the thermal mismatch will cause it to have a large residual strain, and even eventually lead to film cracking. This will seriously affect the performance and reliability of GaN-based electronic materials and devices. In addition, in order to suppress theSubstrate loss, the substrate usually uses high-resistance Si,Si wafers are typically grown using the float zone (FZ) method. Compared to Si wafers grown using the conventional Czochralski (CZ) method, FZ-Si typically contains lower impurities and exhibits semi-insulating properties at room temperature, thus reducing some substrate losses. However, its mechanical strength is inferior to that of CZ-Si, placing higher demands on stress and warpage control during GaN epitaxial growth. While a series of growth methods and techniques for GaN and its heterostructures on Si substrates have been developed in recent years, including low-temperature AlN insertion layers, AlN/GaN superlattice buffer layers, and AlGaN buffer layers with gradient Al composition, these methods have initially met the material requirements for GaN-based power electronics devices on low-resistance Si substrates. However, GaN and its heterostructures on high-resistance Si substrates still suffer from high dislocation density, making stress and warpage control difficult, severely impacting device performance and reliability.In addition, despite the use of high-resistance Si substrate,RF loss remains a key bottleneck hindering the development of GaN-based RF electronic materials and devices on Si substrates.The implication is that GaN-based RF devices on Si substrates experience transmission losses during signal transmission, which can lead to a decrease in signal accuracy and device operating efficiency.The RF loss of GaN-based RF devices on Si substrates mainly includes four parts:1) Conductor loss, also known as transmission line loss. This part of the loss is mainly related to the selected transmission line structure, geometric dimensions, metal materials and processes, and can be optimized through the corresponding process. To solve.2) Substrate loss.This part of the loss can be solved by using a high-resistance Si substrate.3) Due to the conductive layer at the interface between the Si substrate and the semiconductorInterface loss.4) Unavoidable radiation lossThis loss is usually negligible and only needs to be considered in severe mismatches or special circuit designs. Therefore, the first three factors are still the main factors. Among them, the interface loss caused by the conductive layer at the interface between the Si substrate and the semiconductor is currently an unresolved issue and is the most important source of RF loss. Therefore, the formation mechanism of parasitic conductance at the interface between the Si substrate and nitride and its suppression methods have become the focus of current research.。In recent years, various research groups at home and abroad have proposed some solutions to the above problems and made some progress. Luong et al. used a high-temperature-low-temperature-high-temperature method to grow an AlN nucleation layer. They found that this method could suppress the tensile stress caused by the lattice mismatch between AlN and Si, thereby reducing the RF loss due to the interface polarization electric field to 0.4 dB/mm at 10 GHz. Chang et al. suppressed the interfacial p-type conductive channel caused by Al diffusion by lowering the AlN nucleation layer growth temperature, thereby reducing the RF loss to 0.2 dB/mm at 10 GHz. Mauder et al. confirmed that Al diffusion is the cause of parasitic conduction at the AlN/Si interface through spreading resistance profiling (SRP) and other testing methods. Similarly, they achieved low RF loss (0.2 dB/mm at 28 GHz) for high-resistance GaN on Si substrates by using a similar cooling growth method, while maintaining high crystal quality and low warpage (<20 μm). Zhan et al. achieved direct growth of 2.2 μm crack-free GaN on an AlN buffer layer using a quasi-two-dimensional nucleation method on a 6-inch (1 inch = 2.54 cm) high-resistance Si substrate. The (002)/(102) plane XRD full width at half maximum (FWHM) was 457/509 arcsec, and the warpage was as low as less than 8 μm. Although the above methods can alleviate the difficulties faced by GaN-based RF electronic materials on Si substrates to a certain extent, the performance of current GaN-based RF electronic devices on Si substrates is still far inferior to that of GaN-based RF electronic devices on SiC substrates. Its performance improvement depends on more in-depth research on materials and breakthroughs in related technologies.2 Large-Mismatch Heteroepitaxial GaN-Based Materials on High-Resistivity Si Substrates
2.1 Challenges of GaN epitaxial growth on high-resistance Si substratescompared to GaN on SiC substrate. Currently, GaN-based RF electronic materials and devices on Si substrates are still immature.Facing a series of challenges: 1) There is a gap between the Si substrate and GaNLarger lattice mismatch, which will lead to a higher density of threading dislocations in the GaN epitaxial layer, affecting device reliability. 2) There is a gap between the Si substrate and the GaNLarger thermal mismatch, resulting in large residual stress and even cracking of GaN on Si substrate. 3) Despite the use of high-resistance Si substrate, the performance of GaN-based devices on Si substrate is still poor.RF lossCompared with GaN-based devices on SiC substrates, the performance is still much higher. At the same time, the high temperature of the device will further generate a large number of intrinsic carriers in the high-resistance Si substrate, causing further increase in RF loss. This will seriously affect the performance of the device.Including output power, power added efficiency, gain, etc.4) The thermal conductivity of Si substrate itself is poorer than that of SiC, which makes it difficult to use in high power applications.Insufficient heat dissipation performanceIn addition, the buffer layer between the Si substrate and GaN typically includes an AlN nucleation layer, a gradient AlGaN ternary alloy, or an AlN/AlGaN superlattice. Complex transition layer design further increases the thermal resistance between the Si substrate and GaN, affecting device heat dissipation.In general, the crystal quality of GaN epitaxial materials on high-resistance Si substrates is still far behind that of GaN on SiC substrates.Stress/dislocation control problemsStill difficult,Device reliability issues remain unresolved.In particular, severe RF losses limit its application in RF electronics. Recent research has revealed that the diffusion of Al into the Si substrate during epitaxial growth is the primary source of parasitic conductance and RF losses in GaN on high-resistance Si substrates. This requires not only excellent stress and dislocation control during epitaxial growth, but also the coordinated consideration of suppressing this parasitic conductance. This poses a significant challenge to the epitaxial growth of GaN on high-resistance Si substrates. Therefore, how to comprehensively address these challenges through improved epitaxial growth methods and structural design is a pressing issue.2.2 Mechanism of RF loss in GaN-based materials on high-resistance Si substratesHigh resistanceA key scientific and technological issue facing GaN-based RF electronic devices on Si substrates is RF loss. Although the use of high-resistance Si substrates can reduce the loss of RF devices during signal transmission to a certain extent, researchers have found that GaN-based materials on high-resistance Si substrates still have quite seriousRF loss. Its possible source mechanisms mainly include parasitic conductance at the AlN/Si interface, dielectric loss in the AlN epitaxial layer, and loss caused by thermally excited carriers generated by the self-heating effect of the device during operation in the high-resistance Si substrate.The mainstream international view is that the parasitic conductance at the AlN/Si interface is the most dominant factor. However, the formation mechanism of this parasitic conductance remains controversial. Some research teams believe that the parasitic conductance at the AlN/Si interface is caused by the diffusion of Al/Ga atoms (as acceptor impurities) into the high-resistance Si substrate during high-temperature growth, forming a p-type conductive channel.As shown in Figure 1Another view is that the parasitic conductance at the AlN/Si interface is caused by the electron channel formed by the bending of the semiconductor band at the AlN/Si interface. Therefore, a deep understanding of the formation mechanism of the parasitic conductance at the AlN/Si interface is of great research significance for suppressing the parasitic conductance at the interface and radio frequency loss.。picture1 AndOn the substrateIII-NSchematic diagram of epitaxial layer growth
It can be seen that the key to the above dispute lies inDetermination of its interface conductivity type. Wei Lai and others from Peking University directly measured the p-type carrier signal at the AlN/Si interface through the Hall experiment. They epitaxially grew 60, 100, and 200 nm AlN films on high-resistance Si substrates, respectively. The carriers of the three samples measured by the Hall experiment were positively correlated with the thickness of the AlN film. Analysis shows that the p-type carrier signal cannot come from the n-type doped Si substrate and the unintentionally doped high-resistance AlN film, so it can only come from the interface parasitic conductance. In order to confirm that the p-type parasitic conductance comes from the diffusion of Al atoms, they performed secondary ion mass spectrometry (SIMS) measurements.As shown in Figure 2(a)As shown in Figure 2, SIMS results confirm that a considerable number of Al atoms have indeed diffused into the high-resistance Si substrate, and the diffusion depth of Al atoms in the sample increases with the thickness of the AlN epitaxial film. This is because as the growth time increases, Al atoms diffuse more severely into the Si substrate in thicker AlN samples, demonstrating that Al atomic diffusion occurs during the high-temperature AlN growth process.As shown in Figure 2 (b)As shown in Figure 2, the Al impurity surface densities obtained from the Hall experiment and SIMS measurements are essentially consistent. This demonstrates that the parasitic conductance at the AlN/Si interface originates from the diffusion of Al into the high-resistance Si substrate during the high-temperature growth process.picture2 SIMS Test and fitAndIn the substrateAl Impurity concentration distribution curve(a),as well asHall andSIMS TestedAl Comparison of surface density of impurities(b)
Based on the above experimental results, they proposed thatNitriding pretreatment methodexist AndA layer of about 2 nm Thick and indefiniteForm a silicon nitride layer to block AlTowards Diffusion of Si substrate.picture 3 As shown, the sample after nitriding treatment AlN/AndAt the interface AlAtomic concentrationThe density is significantly reduced, and the diffusion depth is also suppressed to a certain extent, especially for thicker AlNFor thin film samples, the inhibitory effect is more obvious.TestedTest, using nitridation pretreatment to grow high resistance AndOn the substrate AlNThe RF loss of the sample is lower than that of the conventionally grown sample.44%。However, this layerThe presence of amorphous silicon nitride will AlN It will have a negative impact on the nucleation and subsequent growth of GaNCrystal quality and stress control。
picture3 SIMS in AlN samples based on different substrate processing methods
2.3 Epitaxial Growth of GaN-Based Materials with Low Dislocation Density and Low RF Loss on High-Resistivity Si SubstratesExperiments show thatAlN/And The main mechanism of parasitic conductance at the interface is the high temperature growth processAlTo high resistance AndDiffusion of the substrate.againstThere are some solutions to this problem proposed internationally, including reducing AlN Growth temperature，Nitriding pretreatment to form silicon nitride barrierlayer，use 3C-SiC Composite structurewaitHowever, these methodsThis usually comes at the expense of epitaxial layer crystal quality or increased device cost. For example, the most mainstream solution isreduce AlN Growth temperaturedegree, thereby suppressing Al Diffusion during growth,In the experiment, we also obtainedHowever, at low temperaturesAlThe limited mobility of atoms AlN Growth deviates significantly from its optimal window，Ultimately leads to AlN The dislocation density increases and the surface deteriorates. And On the substrate AlN As GaN The cornerstone of stress and defect control in epitaxial growth, its quality degradation will lead to To GaN The stress control is more difficult and the dislocation density increases，Finally reflected in GaN Film cracking or deviceReliability decreases. therefore,How to reduce RF loss and effectively control stress and dislocation during epitaxial growth?Is high resistanceAndOn the substrateEpitaxial growth GaN Key to radio frequency electronic materials.To address this problem, Cai Zidong et al. from Peking University proposed akindUltra-low saturation Al Pretreatment technology to grow high quality at low temperature AlN buffer layer, Through this low temperature AlN Buffer layer technology can achieve4Inch high resistance And On the substrate GaN Stress/dislocationsComprehensive control of RF loss。So-called ultra-low saturation AlPretreatment technology, its connotation is growth AlN slowBefore flushing, introduce extremely low flow TMAl They compared Two with pre-processing traffic growth And Low temperature on substrate AlN Buffer layer sample, through X X-ray diffraction (XRD) The experiment obtained ultra-low flow pretreatment Sample handling AlN(002) and( 102) The full width at half maximum of the rocking curve decreased significantly ( See Figure 4) , which indicates that its dislocation densitySignificantly reduced; at this pointAlso by cross-sectional transmission electron microscopy (HE HAS) observations were confirmed ( See picture 5) .also,STEM The test found that the high flow pretreatment sample Goods AlN/Si interface exists 2Amorphous around nm Al-Si-N layer, while the interface of the sample pretreated by ultra-low flow rate is very clear ( See picture 6) ， This fully demonstrates the ultra-low saturation Al Pretreatment technology for low temperature AlN Suppression of dislocations in the buffer layer.picture4 Ultra-low saturationAl Preprocessing(sampleAND)With conventional pretreatment(sampleA)ofAlN SampleXRD Swing Curve
picture5 sample A(a)、(b)and samples AND(c) 、(d) Under double-beam conditionsHE HAS Brightfield image
picture6 sampleA( a) and samplesAND( b) cross sectionHAADF-VESTpicture
To explore AlPretreatment for low temperature AlNThe mechanism of dislocation suppression in the high-resistance AndSubstrate samplessurface.Scanning electron microscope (AS) The experiment found that high flow pretreatment AndThe substrate surface has complex etching morphology and is accompanied by smallof AlNNucleation island ; Ultra-low flow pretreatment AndThere is no etching on the substrate surface, instead there is a large hexagonal symmetric AlNnucleationisland( See picture 7) 。The analysis shows that high traffic TMAl Due to its high supersaturation, it is easy toAccumulation Aldrop,and AndsubstrateReaction Al-Si alloy, and Al-Si alloys During the growth phase, AlN preferentially reacts with ammonia to form amorphous Al-Si-N layer. In contrast, ultra-lowTraffic preprocessing AndThe substrate surface will not be etched, and due to its ultra-low supersaturation, AlNNucleation IslandThe boundary size increased significantly, Reduces the boundaries of grain closure during subsequent growth, thereby inhibiting the generation of dislocations。
picture7 sampleA( a) and samplesAND( b) ofASphoto
Raman spectroscopy tests show that ultra-low saturation The AlN buffer layer prepared by Al pretreatment method canEffectively reduce residual tensile stress in GaN epitaxial layers(See Figure 8)Based on the above work, the Peking University research team successfully grew a 1.5 μm GaN epitaxial layer directly on the high-quality low-temperature AlN buffer layer, and the full width at half maximum of the GaN (002)/(102) surface rocking curve was as low as 390 /440 arcsec(See Figure 9) 。Figure 8differentAlN On the buffer layerGaNRaman spectrum
picture9 1. 5 mm GaNofXＲD Swing Curve
RF loss is 0.29 dB/mm at 10 GHz(See Figure 10) 。Comparison of GaN dislocation density and RF loss values with other international research workFigure 11shown.picture10 RF loss of the sample
Figure 11InternationallyAndOn the substrateGaNDislocation density and RF lossesData comparison chart
2.4 PVD-AlN/Si composite substrate suppresses GaN RF lossThe above experiments show that low temperature AlN buffer layers can effectively reduce RF losses in GaN grown on high-resistance Si substrates. Improvements in epitaxial growth methods can also, to some extent, compensate for the degradation in crystal quality caused by lowering the growth temperature. However, because nitride growth cannot significantly deviate from its optimal growth window, the adjustment range for lowering the temperature is relatively limited. Further reducing RF losses and meeting the material requirements of RF electronic devices requires an innovative approach. To address this issue, Liu Danshuo and others at Peking University have developed a PVD-AlN/Si composite substrate to reduce RF losses in GaN grown on high-resistance Si substrates. This method involves depositing a PVD-AlN layer on a 2-inch high-resistance Si substrate to replace the AlN nucleation layer grown by traditional metal-organic chemical vapor deposition (MOCVD). The GaN epitaxial layer is then grown on top. The advantage is that the PVD-AlN layer blocks the diffusion of Al/Ga atoms into the Si substrate during MOCVD growth, thereby suppressing the formation of a p-type parasitic conduction channel. They studied the epitaxial growth of GaN films on PVD-AlN/Si composite substrates and designed samples with different structures, which differed in the selection of AlN nucleation layer and the growth mode of GaN.The use of low-density, large-grain-size PVD-AlN and low V/III ratio epitaxial growth conditions can effectively delay the transition of GaN growth from three-dimensional to two-dimensional growth.Thus successfully obtained1.5 μmThick crack-free GaN epitaxial film, the full width at half maximum of the XRD (002) and (102) rocking curves are525 and 527 arcsec,The root mean square roughness of the atomic steps measured by AFM is 0.16 nm. SIMS testing confirms that PVD-AlN effectively prevents surface Ga atoms from diffusing into the Si substrate compared to MO-AlN. Furthermore, the diffusion length of Al atoms is significantly reduced due to the lower growth temperature and time of PVD-AlN, as shown in Figure 12. This reduces the RF loss of GaN epitaxial films on Si substrates to 0.20 dB/mm at 10 GHz.picture12 PVD-AlN/SiandMOCVD-AlN/SiComposite substrateAl、HereImpurity concentration distribution curve
3. Conclusion and Outlook
GaN-based RF electronic devices, with their superior properties such as high efficiency, high power, wide bandwidth, and miniaturization, are core components supporting innovative development in next-generation mobile communications, defense, and military applications. Currently, GaN-based RF devices are primarily fabricated on expensive semi-insulating SiC substrates. With the development of next-generation mobile communications, the demand for the number and efficiency of RF devices is rapidly increasing. Existing RF chip solutions are unable to meet the urgent need for low cost and high efficiency for large-scale applications. Si substrates offer advantages such as large size, low cost, and compatibility with existing CMOS processes. These advantages have made GaN-based RF electronic devices on Si substrates the next focus of attention in this field after power electronics, and have garnered significant attention from both academic and industrial communities. Significant progress has been made in epitaxial growth of GaN on high-resistance Si substrates, with wafer sizes continuously expanding. However, low dislocation density, low RF loss, and stress and defect control in GaN films still lag behind those of GaN-based materials on SiC substrates, and device performance, influenced by epitaxial growth quality, requires further improvement. From the perspective of device development trends, GaN-based RF electronic devices for millimeter wave applications and low-voltage GaN-based RF devices for mobile phones and other terminals are the current research and production of GaN-based RF electronic materials and devices on Si substrates.The mainIn this direction, high linearity, high efficiency and high reliability devices are still the focus of research.
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