GaN Buffer Layer Research Articles

InGaN/GaN Quantum Dot Light-Emitting Diodes on Silicon with Coalesced GaN Nanowire Buffer Layer

We demonstrate InGaN/GaN quantum dot light-emitting diodes on silicon substrates with a planar buffer layer formed by coalescing GaN nanowires. The GaN buffer layer and the light-emitting diode heterostructure were grown by plasma-assisted molecular beam epitaxy. The coalesced nanowire layer exhibits a defect density of the order of ∼109 cm2, a root-mean-square (rms) surface roughness of 0.562 nm, and excellent photoluminescence characteristics. Multiple layer In0.23Ga0.77N/GaN quantum dot light-emitting diodes grown and fabricated on the GaN/Si buffer layer exhibit strong electroluminescence at ∼477 nm with a very small blue shift of ∼2.4 nm, translating to a polarization field of ∼50 kV/cm, over the injection range applied. A droop is observed in the efficiency of the devices beyond an injection of 40A/cm2, most possibly due to defect-assisted Auger recombination and carrier leakage from the active region.

Особенности структурных и оптических свойств InGaN-слоев, полученных методом МПЭ ПА с импульсной подачей потоков металлов

This paper presents the results of studying the properties of InGaN layers with a high InN content (80-90%) obtained by molecular beam epitaxy with plasma activation of nitrogen on sapphire substrates with AlN / GaN buffer layers. The InGaN layers were formed using the metal modulated epitaxy (MME) method, as well as in nitrogen and metal rich conditions. It was found that the use of the MME method leads to a decrease in the density of threading dislocations in the InGaN layers. Nevertheless, despite the higher dislocation density, the smallest threshold of stimulated emission of ~ 20 kW / cm2 at 77 K was obtained for the In0.8Ga0.2N layer grown under nitrogen rich conditions, which is associated with the lowest background electron concentration in this sample (1.6•1019 cm-3).

Investigation of the effect of doping the GaN buffer layer with carbon on the avalanche breakdown effect of normally open HEMT AlGaN/AlN/GaN transistors

The influence of the GaN buffer layer doped with carbon on the avalanche breakdown effect of normally open HEMT AlGaN / AlN / GaN transistors was studied. The avalanche breakdown was simulated in a structure where the gate length is LG = 0.3 mkm, the distance between the source and gate is LSG = 1.5 mkm, and the distance between the gate and drain is LGD = 2.2 mkm. For modeling, consider a layer doped with carbon, the thickness of which is 0.3 mkm, and the layer is located at a distance of 20 nm from the channel. The Simulation showed that with an increase in the concentration of carbon doping of the buffer, the breakdown voltage increases in the range UB = 225 – 360 (V). When the layer thickness changes to 0.4 mkm, the breakdown voltage increases in the range UB = 230 – 446 (V). For a structure where the gate length is LG = 0.8 mkm, the distance between the source and the gate is LSG = 1.0 mkm, the distance between the gate and drain is LGD = 3.0 mkm, the breakdown voltage increases in the range UB = 300 – 622 (V).

Improving Transport Properties of GaN-Based HEMT on Si (111) by Controlling SiH4 Flow Rate of the SiNx Nano-Mask

The AlGaN/AlN/GaN high electron mobility transistor structures were grown on a Si (111) substrate by metalorganic chemical vapor deposition in combination with the insertion of a SiNx nano-mask into the low-temperature GaN buffer layer. Herein, the impact of SiH4 flow rate on two-dimensional electron gas (2DEG) properties was comprehensively investigated, where an increase in SiH4 flow rate resulted in a decrease in edge-type threading dislocation density during coalescence process and an improvement of 2DEG electronic properties. The study also reveals that controlling the SiH4 flow rate of the SiNx nano-mask grown at low temperatures in a short time is an effective strategy to overcome the surface desorption issue that causes surface roughness degradation. The highest electron mobility of 1970 cm2/V·s and sheet carrier concentration of 6.42 × 1012 cm−2 can be achieved via an optimized SiH4 flow rate of 50 sccm.

Understanding High-Energy 75-MeV Sulfur-Ion Irradiation-Induced Degradation in GaN-Based Heterostructures: The Role of the GaN Channel Layer

AlInN/GaN on Si high-electron mobility transistors (HEMTs) are irradiated with various fluences of 75-MeV sulfur ions to study the radiation-induced degradation mechanisms. Heavy-ion irradiation has been found to reduce saturation ON- and OFF-state current and, at higher fluence, to induce a large positive threshold voltage shift. Irradiated structures reveal a consistent carrier density and mobility reduction as a function of fluence. To demonstrate the buffer-induced threshold shift, a series of samples with various GaN channel layer thicknesses defined by carbon-doped GaN buffer layers are characterized. By comparison of the threshold voltage of nonirradiated devices with reduced channel layer thickness, we assign the observed effects mainly to carrier depletion. Thus, formation of acceptor-like traps upon irradiation is concluded as primary degradation mechanism in GaN-based HEMTs. AlInN/GaN HEMT structures are, therefore, more tolerant to heavy-ion irradiation than AlGaN/GaN transistors as their intrinsic channel conductivity is higher.

Influence of doping profile of GaN:Fe buffer layer on the properties of AlGaN/AlN/GaN heterostructures for high-electron mobility transistors

The effect of the Fe doping profile of the GaN buffer layer in the heterostructures for high-electron mobility transistors was studied experimentally and by computer simulation. The exponential Fe tail extending to the nominally undoped layers may greatly affect the properties of the structure. Reducing the distance between the channel and the Fe-doped buffer to less than 1 μm results in a decrease in the density and mobility of the two-dimensional electron gas. It also leads to the higher off-state avalanche breakdown voltage and reduced leakage current. A good agreement between simulation and experimental data is obtained when taking into account a Fe segregation effect, while an abrupt doping profile lead to significant discrepancies between them

Optimization of the growth of GaN epitaxial layers in an indigenously developed MOVPE system

Growth of GaN epitaxial layers is optimized in an indigenously developed nitride metal organic vapour phase epitaxy (MOVPE) system. The motivation of present work is to develop a simple MOVPE reactor which is though affordable but can deliver GaN epilayers of desired quality. The design and fabrication methodology of nitride MOVPE system is briefly discussed. MOVPE growth of GaN epilayers is carried out through a two-step growth process where influence of the growth parameters of low temperature buffer layer on the crystalline properties of high temperature GaN epilayer is discussed in detail. Optimum values of the growth temperature, annealing duration and thickness of GaN buffer layer are achieved. High resolution X-ray diffraction measurements (HRXRD) confirmed that the crystalline quality of GaN epitaxial layers is reasonable. A good surface morphology of the optimized sample is strongly corroborated by the results obtained from photoluminescence (PL) and HRXRD techniques. Usefulness of a systematic Williamson-Hall analysis in the optimization of growth of GaN epitaxial layers is demonstrated where a high value of lateral coherence length of GaN buffer layer is found to be a key parameter. Such an optimization process leads to the good crystalline quality of GaN epitaxial layers with low dislocation density. It is found that the high temperature GaN epilayers grown on the optimized GaN buffer layer are compressively stressed which is also revealed by the PL measurements.

Effects of GaN Buffer Resistance on the Device Performances of AlGaN/GaN HEMTs

We investigated the effects of GaN buffer resistance of AlGaN/GaN high-electron-mobility transistors (HEMTs) on direct current (DC), low-frequency noise (LFN), and pulsed I-V characterization performances. The devices with the highest GaN buffer resistance were grown on sapphire substrate using two-step growth temperature method without additional compensation doping. The proposed device exhibited the degraded off-state leakage current due to the improved GaN buffer quality compared to the reference devices with relative low buffer resistance, which is confirmed by high resolution X-ray diffraction (HRXRD). However, the proposed device with deep-level defects in GaN buffer layer showed the reduced hysteresis (∆Vth), increased breakdown voltage (BV), and enhanced pulse I-V characteristics. Regardless of GaN buffer resistance, all devices clearly showed 1/f behavior with carrier number fluctuations (CNF) at on-state but followed 1/f2 characteristic at off-state. From the 1/f2 noise characteristics, the extracted trap time constant (τi) of the proposed device can be obtained to be 10 ms, which is shorter than those of the reference devices because of the full compensation of deep-level defects in the GaN buffer layer.

Integration of 650 V GaN Power ICs on 200 mm Engineered Substrates

GaN power ICs on engineered substrates of Qromis substrate technology (QST) are promising for future power applications, thanks to the reduced parasitics, thermally matched substrate of poly-AlN, high thermal conductivity, and high mechanical yield in combination with thick GaN buffer layers. In this article, we will elaborate in detail on epitaxy, integration, and trench isolation. Electrical characterizations show that the GaN buffer bears a breakdown voltage of > 650 V under the leakage criterion of $10~{\mu }\text{A}$ /mm2 at 150 °C. The fabricated 36 mm power HEMTs with ${L} _{\mathrm{ GD}}$ of $16~\mu \text{m}$ show a high threshold voltage of 3.1 V and a low OFF-state drain leakage of $ /mm until 650 V. The horizontal trench isolation breakdown voltage exceeds 850 V. The device dispersion is well controlled within 20% over full temperature and bias range. Finally, GaN power ICs on this platform are demonstrated.

Influence of Fe in the buffer layer on the laser lift-off of AlGaN/GaN HEMT film: phenomena and mechanism

In this paper, a study is presented of the effect of Fe in the buffer layer on the laser lift-off (LLO) of GaN high electron mobility transistors (HEMTs). AlGaN/GaN HEMTs grown on Fe-doped and unintentionally doped buffer layers were separated from sapphire substrates using a 248 nm KrF excimer laser lift-off system. We analyzed the variations of the crystal characteristics, two-dimensional electron gas (2DEG) characteristics and the strain state of AlGaN/GaN HEMT films before and after LLO by the x-ray diffraction, Hall and Raman methods. The measurement of the distribution of elemental Fe in the GaN buffer layer was performed by secondary ion mass spectroscopy. The results show that a peak Fe concentration of 9.56 × 1018 cm−3 appears at the interface. Moreover, the crystal quality and 2DEG characteristics of the Fe-doped GaN film obviously degenerated, with the formation of micro-cracks under the threshold laser separation energy density (Es). In this case, the threshold laser damage energy density (ED) was used to measure the laser damage tolerance of GaN in LLO. The analysis considers that additional laser absorption centers, induced by the Fe impurity energy level, coupled with a large residual stress of up to 1.0763 GPa in Fe-doped GaN, reduced the threshold laser damage energy density (ED).

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.

Short Circuit Capability Characterization and Analysis of p-GaN Gate High-Electron-Mobility Transistors Under Single and Repetitive Tests

Short circuit (SC) capability of 650-V Schottky-type p-GaN gate high-electron-mobility transistors (HEMTs) under single and repetitive tests is characterized in this article. The investigated devices exhibit strong capability under a single SC test, but weak capability under repetitive SC tests with a bus voltage of 400 V and a gate drive voltage of 6 V. The failure mechanism under repetitive SC tests is revealed through electrothermal simulation and microscale failure spot analysis. Thermal fatigue cracks are formed due to the high temperature spike and local temperature fluctuations in the narrow GaN channel and buffer layers, leading to weak repetitive SC capability. The unique heat confinement effect in the GaN layer plays an important role in the formation of high temperature spike and fatigue cracks. The withstand time in a single SC test is several hundred microseconds due to fast drain current drop that results from high temperature and, to a lesser degree, the dynamic threshold voltage shift during the SC transient. The device failure in a single-event SC test is related to heat diffusion to a wider region. Some guidelines are proposed for handling and improving the repetitive SC capability.

Trapping Effects on Leakage and Current Collapse in AlGaN/GaN HEMTs

In this paper, we showcase our investigation regarding the effect of acceptor traps in GaN buffer and AlGaN barrier layers on the leakage current and current collapse in GaN high-electron-mobility transistors. The dependence of current collapse and leakage current on the density and energy level of traps is carefully considered. With an increase in trapping density from 1015 cm−3 to 1018 cm−3, the leakage current was significantly reduced from 80.2% to 1.76% in the buffer layer and 95% to 12.6% in the barrier layer, while the current collapse increased from 6% to 89.8% in the buffer layer and 0.3% to 17.5% in the barrier layer. The effects of current collapse and leakage were more noticeable in the buffer layer than in the barrier layer. Different energy levels (0.75 eV, 1.8 eV, and 2.85 eV) of acceptor traps were likewise studied. It was demonstrated that high-energy traps induced a lower amount of leakage, while the current collapse was greater. Based on these results, a balanced trade-off between the current collapse and the leakage current is proposed.

Correlation between Kink effect and trapping mechanism through H1 hole trap in Al0.22Ga0.78N/GaN/SiC HEMTs by current DLTS: field effect enhancement

A cryogenic investigation of the Kink effect with drain-source bias sweeping process during output characteristics is suggested. An exhaustive study of the field effect dependence on the emission rate from hole traps in AlGaN/GaN HEMT transistors has been realized by means of current DLTS spectroscopy (I-DLTS). We have found that the Kink effect was induced by impact ionization of electron trapped in acceptor-like deep levels with activation energies at about 0.85 eV overhead the valence band of the GaN buffer layer. Using I-DLTS method, three holes traps, labeled A, H1, and H5, have been distinguished. The H1 deep level might correspond to the carbon substituting the N site (CN) which is supposed to be the main cause of the Kink effect. The major H5 trap seems to be gallium vacancy complex (VGa–ON). For the hole trap H1, the phonon-assisted tunneling emission is the dominant mechanism for holes to escape from the trapping centers while for the H5 trap their field dependence shows a classical pure tunneling effect.

High-Temperature Spontaneous Emission Quantum Efficiency Analysis of Different InGaN MQWs for Future Power Electronics Applications

The commercial light-emitting diode (LED) epitaxy materials-blue (i.e., InGaN/gallium nitride (GaN) multiple quantum wells (MQWs)) for lighting and display applications and green (i.e., InGaN/GaN MQWs) for display applications-are evaluated with a varied range of temperatures (10-800 K) for future applications in high-density power electronic modules. Six different InGaN/GaN MQW structures (peak wavelengths λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 448, 467, and 515 nm) are studied and compared to evaluate whether they can satisfy the light output requirements in the optocouplers at high temperatures. The spontaneous emission quantum efficiency (QE) of these LED epitaxy materials are studied using temperature-dependent and power photoluminescence (PL) spectroscopy. The LED materials, which have no pre-QWs, a higher number of QWs, and GaN buffer layer, show the highest QEs at high temperatures. All six LED materials' exhibit 65% QE at 500 K and stable operation at 800 K were without failure or flickering. At 800 K, the spontaneous emission QEs are between 56% and 60% for blue and green LEDs for displays, respectively. This work evaluates the high-temperature capability of various LED epitaxy materials for future applications in the high-temperature optocouplers as the galvanic isolations in high-density power modules.

In-situ stress evolution and its correlation with structural characteristics of GaN buffer grown on Si substrate using AlGaN/AlN/GaN stress mitigation layers for high electron mobility transistor applications

In-situ stress evolution as a function of thickness has been investigated and correlated with the structural properties and surface morphology of GaN buffer layer grown on AlGaN/AlN/GaN stress mitigating layers (SMLs). For comparison, GaN buffer was also grown on AlN/GaN SMLs. AlGaN/AlN/GaN SMLs exhibited efficient stress mitigation characteristics resulting in higher compressive mean stress during the growth and convex bow at the end of the growth. Horizontal screw-type misfit dislocations generated at the GaN/AlGaN and AlGaN/AlN interfaces were attributed to the stress mitigation property. The residual compressive stress in the GaN buffer was found to be lower with the AlGaN/AlN/GaN SMLs, which resulted in rough surface morphology. Increased V/III ratio used for GaN buffer growth was found to result in reduced stress relaxation at the interface leading to higher residual compressive stress and enhanced diffusion of ad-atoms. This consequently reduced the kinetic roughening and improved surface morphology. Thus, stress engineering by using AlGaN/AlN/GaN SMLs and by changing of the V/III ratio of GaN buffer, the mean stress of heterostructure was controlled and relatively smoother surface morphology was achieved, respectively. Reasonably good uniformity in electrical characteristics with a standard deviation of 7%, 1% and 8% for the sheet resistance, carrier concentration and mobility, respectively, were achieved for GaN high-electron-mobility transistor heterostructures across the 100 mm substrate.

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.

Effect of InGaN/GaN Superlattice on Lattice Curvature of GaN Layers Grown on Sapphire Substrates

The lattice curvature due to the difference in the thermal expansion coefficients between sapphire and GaN decreases the device performance and production yield. There is room to improve the lattice curvature and radius of curvature of GaN layers grown on sapphire substrates. Herein, an InGaN/GaN superlattice (SL) is grown on a low‐temperature GaN buffer layer through metalorganic vapor‐phase epitaxy to improve the lattice curvature. The lattice curvature of the GaN layer is investigated in terms of In content of the InGaN layer, the number of SL layers, cap layer, and GaN thickness. The radius of curvature increases with the increase in the number of SL layers and In content. The radius of curvature is 45 m when a 10‐period SL structure is used and the In content is 23.4%. When using a 20‐period SL structure, the GaN layer partially separates despite its very low thickness (2 μm). The improvement in the lattice curvature can be attributed to the thermal decomposition of InGaN during the annealing process from the growth temperature of InGaN to that of GaN.

Effect of V/III ratio on the surface morphologies of N-polar GaN films grown on offcut sapphire substrates

We investigated the influence of the V/III ratio on the surface morphologies of N-polar GaN epilayers during the stage of high-temperature (HT) GaN growth. The epilayers were grown on 4°-offcut sapphire substrates by metal-organic vapor phase epitaxy. We observed prominent undulations on the GaN surface when a low V/III ratio was used, and ridged hillocks on the GaN surface when a high V/III ratio was used. With the V/III ratio increased from 128 to 2237, the density of ridged hillocks increased but the surface roughness was significantly reduced from 13.8 nm to 1.7 nm in 25 × 25 μm2 AFM images. The results differ from the existing reports in which a smoother N-polar GaN surface morphology was usually obtained with a lower V/III ratio. Non-uniform island formation in the initial growth of HT-GaN and enhanced step-bunching effect under a low V/III ratio were blamed for the large surface roughness of the N-polar GaN film. The increasing density of ridged hillocks on the N-polar surface grown under a high V/III ratio was attributed to the reduced diffusion rate of Ga adatoms due to excessive N. Completely hillock-free smooth N-polar GaN surfaces were achieved by further increasing the growth temperature of the GaN buffer layer.

GaN-on-diamond technology platform: Bonding-free membrane manufacturing process

GaN-on-diamond samples were demonstrated using a membrane-based technology. This was achieved by selective area Si substrate removal of areas of up to 1 cm × 1 cm from a GaN-on-Si wafer, followed by direct growth of a polycrystalline diamond using microwave plasma chemical vapor deposition on etch exposed N-polar AlN epitaxial nucleation layers. Atomic force microscopy and transmission electron microscopy were used to confirm the formation of high quality, void-free AlN/diamond interfaces. The bond between the III-nitride layers and the diamond was validated by strain measurements of the GaN buffer layer. Demonstration of this technology platform is an important step forward for the creation of next generation high power electronic devices.