GaN Buffer Research Articles

MOVPE-grown GaN/AlGaN heterostructures on sapphire with polarization-induced two-dimensional hole gases

GaN/Al x Ga1−x N heterostructures were grown by metal-organic vapor phase epitaxy to study in detail the formation of two-dimensional hole gases (2DHG). In contrast to the common double-heterostructure approach to create 2DHG, which is based on GaN buffer layers and leads to the parallel formation of a two-dimensional electron gas, this concept is designed to create a 2DHG only. The Al mole fraction in the AlGaN buffer and the GaN channel thickness are each varied to investigate their influence on 2DHG properties. The carrier concentrations as determined by room temperature Hall measurements follow the expected trend given by the Al content dependence of the spontaneous polarization of the relaxed AlGaN buffer. A 2DHG density as high as of 1.6 × 1013 cm−2 with a negligible dependence on temperature (80–300 K) is determined for a GaN/Al0.29Ga0.71N heterostructure by temperature-dependent Hall measurements. Higher carrier concentrations can also be achieved, yet strain relaxation of the GaN channel degrades the transport properties for Al contents above 30%.

Characterization of traps in GaN-based HEMTs by drain voltage transient and capacitance deep-level transient spectroscopy

This paper presents a detailed investigation of trapping effect in AlGaN/GaN high-electron-mobility transistors based on the pulsed current–voltage characterization, drain voltage transient (DVT) measurement, and capacitance deep-level transient spectroscopy (C-DLTS). By monitoring the DVTs at various filling voltages and temperatures, the properties of three electron traps were obtained with the DVT measurements. Specifically, the energy levels of the former two traps were determined to be 0.28 and 0.48 eV, which was confirmed by the C-DLTS measurement performed on the same device. In addition, a third temperature-independent trap located in the GaN buffer was observed only with the DVT measurement, indicating the advantage of transient curves measurement in characterizing the traps insensitive to temperature. The combined measurements demonstrate the correlation of different techniques, which allows identifying the same trap levels to investigate the physical origin of traps.

Near-Ideal Subthreshold Swing in InAlN/GaN Schottky Gate High Electron Mobility Transistor Using Carbon-Doped GaN Buffer

This work presents a comparative study of the subthreshold swing (SS) in InAlN/GaN-based high electron mobility transistor (HEMT), with and without carbon doping in the buffer layer. It is observed that devices with carbon-doped (C-doped) buffer exhibit near-ideal SS of close to 60-mV/decade over a wide range of drain voltage for both up and downsweep. The C-doped buffer layer reduces the OFF-state drain leakage current and increases the drain current ON/OFF ratio. TCAD simulation of the vertical electric field under the gate shows that the sample with a C-doped buffer layer has a much lower value of the peak electric field at the drain side edge of the gate, resulting in lower reverse-biased gate leakage current and smaller SS compared to that of the undoped buffer. Moreover, the junction capacitance at the interface of the unintentionally doped GaN (UID:GAN) and the C-doped buffer layer, in series with the depletion capacitance, reduces the overall capacitance, which further improves the SS in the C-doped wafer.

The DC Performance and RF Characteristics of GaN-Based HEMTs Improvement Using Graded AlGaN Back Barrier and Fe/C Co-Doped Buffer

In this article, the impact of the graded AlGaN back barrier and Fe\C co-doping buffer structure on the AlGaN <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$/$ </tex-math></inline-formula> GaN high electron mobility transistors (HEMTs) is proposed and systematically investigated. Due to effective suppression of Fe tail in unintentionally doped GaN (uid-GaN) layer by the insertion of the thick graded AlGaN back barrier layer, a large maximum drain current density and a transconductance peak are achieved. Meanwhile, the breakdown voltage is significantly improved by the use of Fe\C co-doping GaN buffer design. More importantly, it is revealed that the graded-AlGaN design can reduce the range and intensity of the electrical potential distribution in uid-GaN layer and then effectively suppress the acceptor-induced trapping\detrapping effect under high drain voltage. The RF small-signal performance of Fe-doped GaN (GaN:Fe)\C co-doping buffer HEMT exhibits significant improvement. In addition, load-pull measurement at 8 GHz revealed that a saturation power increases from 38.04 to 41.07 dB, a power gain increases from 9.06 to 10.58 dB, and an associate power-added-efficiency (PAE) increased from 40.27% to 50.18%. Our proposed GaN-based epitaxial structure can not only suppress the gate lag by reduce AlGaN surface electric field, but it can also suppress the drain lag by reduce the amplitude and range of potential distribution. It indicates that our proposed device has great potential for future high-voltage RF power amplifier application.

Fluorine-plasma treated AlGaN/GaN high electronic mobility transistors under off-state overdrive stress

Influences of off-state overdrive stress on the fluorine-plasma treated AlGaN/GaN high-electronic mobility transistors (HEMTs) are experimentally investigated. It is observed that the reverse leakage current between the gate and source decreases after the off-state stress, whereas the current between the gate and drain increases. By analyzing those changes of the reverse currents based on the Frenkel–Poole model, we realize that the ionization of fluorine ions occurs during the off-state stress. Furthermore, threshold voltage degradation is also observed after the off-state stress, but the degradations of AlGaN/GaN HEMTs treated with different F-plasma RF powers are different. By comparing the differences between those devices, we find that the F-ions incorporated in the GaN buffer layer play an important role in averting degradation. Lastly, suggestions to obtain a more stable fluorine-plasma treated AlGaN/GaN HEMT are put forwarded.

Preferential sublimation along threading dislocations in InGaN/GaN single quantum well for improved photoluminescence

InGaN/GaN single quantum wells were grown by molecular beam epitaxy on the silicon substrate onto thin AlN and GaN buffer layers. The InGaN/GaN structure is porosified using a combination of SixNy nanomasking and sublimation and compared with a non-porous reference. The photoluminescence efficiency at room temperature of the porosified sample is improved by a factor reaching 40 compared with the reference sample. Plan-view and cross-sectional transmission electron microscopy images reveal that the remaining material is free of dislocation cores. The regions around dislocations are, thus, preferentially sublimated. This explains the strong photoluminescence improvement of nanoporous InGaN/GaN samples.

Structural characterization of ELO-GaN film on mask-stripe patterned GaAs (0 0 1) substrate grown by metalorganic vapor phase epitaxy

GaN films were epitaxial laterally overgrown on the [110]-stripe-patterned GaAs (001) substrate by MOVPE. AlGaN intermediate layers and low-temperature GaN buffer layer were inserted in order to prevent structural defects. The epitaxial lateral overgrowth of GaN results in a trapezoidal shape with flat facets. Structural phases and defects distribution were studied by TEM. Selective-area diffraction pattern indicates the mixed phase of GaN and a high density of stacking faults at nucleation. However, with the deposition of dielectric SiNx masks, a lateral overgrowth resulted in nearly free of defects, and the cubic phase transformed into hexagonal despite a cubic template. Moreover, dark-field TEM images were used to demonstrate the structural phase transformation.

Extraction of Bias-dependent Source and Drain Resistances in AlGaN/GaN MIS-HEMTs Using Pulsed Measurement Method

A new method for extraction of bias-dependent source and drain resistances in AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) is reported. The total source/drain access resistances are extracted by measuring the pulsed transfer characteristics of test devices with various quiescent biases. With short pulse width, the charge trapping and thermal effects will not be affected by the magnitude of pulse voltages, and thus the extracted resistances correspond to the quiescent bias condition. As a result, the dependences of total access resistances on the gate and drain voltages are obtained. While employing the dual-sweep combinational transconductance technique, the source and drain resistances are separated further. To investigate the variation of access resistances with applied voltages, AlGaN/GaN HEMTs are also illuminated with a 365-nm ultraviolet (UV) source during pulsed measurements. Under UV illumination, the resistance will be less dependent on the drain voltage, indicating the charge trapping in GaN buffer layer has a large impact on the access resistance.

Effect of InGaN Channel on Radio‐Frequency Performance in High‐Electron‐Mobility Transistors with an InAlGaN Barrier

The effect of the InGaN channel on the performance of high‐electron‐mobility transistors (HEMTs) through the Silvaco Atlas simulator is investigated in detail. It is shown that the 2D electron gas (2DEG) density and carrier confinement of the device can be enhanced by replacing the GaN channel with the InGaN channel. The higher 2DEG density significantly increases the saturation drain current density, thus improving the current drive capability of the device. Meanwhile, better carrier confinement alleviates the short channel effects (SCEs) and traps state‐related current collapse in the GaN buffer, effectively improving the reliability of the device. Better carrier confinement also increases the current gain cutoff frequency (fT) and maximum oscillation frequency (fmax) to 133 and 226 GHz, while the fT and fmax of conventional GaN channel HEMTs are 104 and 167 GHz, respectively. These results indicate that AlInGaN/InGaN HEMTs achieve better performance in direct current (DC) and radio frequency (RF).

Investigation on Capacitance Collapse Induced by Secondary Capture of Acceptor Traps in AlGaN/GaN Lateral Schottky Barrier Diode.

In this study, a dedicated dynamic measurement system was used to investigate the transient capacitance and recovery process of AlGaN/GaN lateral Schottky barrier diodes (SBDs). With the consideration of acceptor traps in the C-doped buffer, the C-V characteristics and transient capacitance were measured and analyzed, and the results were simulated and explained by Silvaco TCAD (technology computer aided design). The ionization of acceptor traps and the change of electric potential were monitored in transient simulation to investigate the origin of the capacitance collapse in the SBD. The results suggest the significant impact of traps in the GaN buffer layer on the capacitance collapse of the device, and the secondary capture effect on the variation of acceptor ionization. Based on the study of transient capacitance of SBD, this work could be extended to the Miller capacitance in high electron mobility transistor (HEMT) devices. Moreover, the report on the stability of capacitance is essential for GaN devices, and could be further extended to other aspects of device research.

Charge Control in Schottky-Type p-GaN Gate HEMTs With Partially and Fully Depleted p-GaN Conditions

The Schotty-type p-GaN gate high-electron-mobility transistors (HEMT) feature a unique gate structure. A comprehensive understanding of the charge control mechanism in the p-GaN gate region is a fundamental step for the optimization of this technology. In this work, a physics-based analytical model is presented which takes into consideration all the capacitive effects from gate metal deep into the GaN buffer. According to our analysis, the p-GaN layer can be either partially depleted by the metal/p-GaN Schottky junction or fully depleted, depending on the doping concentration and thickness of the p-GaN layer. Our model accurately captures the charge control properties under both conditions and is validated against TCAD numerical simulations. For a certain p-GaN thickness, a lightly doped p-GaN leads to a full-depletion condition, such that the acceptor concentration directly affects the band diagram at AlGaN/GaN interface. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> of the HEMT increases quickly with acceptor concentration in p-GaN. With sufficiently high acceptor concentration in p-GaN, the device reaches the partial-depletion condition, the acceptor concentration loses its influence over the band diagram at the location of the AlGaN/GaN interface, since the Fermi-level at the AlGaN surface is pinned near the valence band of p-GaN. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> starts to decrease with acceptor concentration, but at a relatively slow rate. The maximum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> is obtained near the boundary between partial-and full-depletion conditions. In consideration of the process margin, the device designed with a partially depleted p-GaN is preferable, since it ameliorated the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> sensibility against acceptor concentration.

A Π-shaped p-GaN HEMT for reliable enhancement mode operation

This paper presents a TCAD based investigation of virtually fabricated Π-shaped Gate p-GaN HEMT for reliable enhancement mode operation. A detailed comparison with the Conventional T-Gate counterpart, with a focus towards the trap related dispersive effect reveals the superiority of Π-shaped Gates P-GaN HEMT for suppressing both the vertical substrate and gate leakage currents, thereby enhancing the breakdown characteristics of the device. A modification of the electric fields, into a stepped profile, demonstrates effective suppression of the leakage through the surface defects, and spillover into GaN buffer, thereby mitigating the trap related dispersive effects. Dispersive effect of the two devices has also been compared using current Slump Ratio (SR) which is lower for Π-shaped Gate P-GaN HEMT. Further analysis into laterally scaled drain-access regions, demonstrates superiority of Π-shaped Gates in achieving a reliable thermal operation, and applicability of Π-shaped Gate P-GaN HEMT for future RF and High Power applications.

Growth of high-quality nitrogen-polar GaN film by two-step high-temperature method

Normally, nitrogen-polar (N-polar) GaN films with better crystalline quality tend to have poor surface morphology, whereas those with good surface morphology suffer from degraded crystalline quality. In this work, we use a two-step high-temperature (HT) growth method to grow N-polar GaN films on vicinal c-plane sapphire substrates by metalorganic chemical vapor deposition, which can effectively solve the irreconcilability between surface morphology and crystalline quality. The GaN film consists of two GaN layers (HT-GaN 1 and HT-GaN 2), which were grown at different growth parameters, respectively. The 300-nm-thick HT-GaN 1 layer was directly grown on the low-temperature GaN buffer layer under the higher V/III ratio and pressure at 1040 °C to reduce dislocation density. The 1.7-μm-thick HT-GaN 2 layer was grown under the lower V/III ratio and pressure to improve surface morphology. Besides, the growth temperature of HT-GaN 2 layer was ramped up to 1070 °C to suppress the yellow-band luminescence. Finally, high-quality N-polar GaN films with smooth surface morphology and high crystalline quality were obtained, for which the full-width at half-maximum values of (0002) and (101¯2) planes X-ray diffraction rocking curves are 131 and 390 arcsec, respectively, the root mean square roughness over an area of 20 × 20 μm2 is 1.88 nm, and the residual compressive stress is as low as 0.025 GPa. It is reasonably believed that this work provides an effective approach to preparing high-quality N-polar GaN films for the application of N-polar nitride devices.

GaN HEMTs on low resistivity Si substrates with thick buffer layers for RF signal amplification and power conversion

We report GaN high-electron-mobility transistors (HEMTs) with a thick (7.7 µm) GaN buffer on a Czochralski low resistivity Si (LRS) substrate. The GaN HEMTs exhibit high performance for both radio-frequency (RF) amplification and power conversion. The thick GaN buffer was grown by means of vacancy engineering, delivering a low dislocation density of ∼1.6 × 108 cm−2, contributing to suppressed RF signal coupling to the lossy Si substrate and a high vertical voltage blocking capability. For RF performance, GaN HEMTs with a 650 nm gate exhibit an fT/fMAX value of 25.1/32.3 GHz and a maximum output power POUT of 2.2 W/mm at 4 GHz with a drain voltage VDS of 20 V, which is comparable with the performance of RF GaN HEMTs on a high-resistivity silicon substrate without the existence of the field plate. For power performance, the vertical breakdown voltage of the wafer is 1160 V, and the three-terminal lateral breakdown voltage is 885 V in a GaN HEMT with a gate-to-drain distance of 8 µm. The thick GaN layer on the LRS substrate scheme thus provides a compelling platform for monolithic integration of high-performance RF devices and high-voltage power devices.

Robustness of Cascode GaN HEMTs in Unclamped Inductive Switching

Surge-energy robustness is essential for power devices in many applications such as automotive powertrains and electricity grids. While Si and SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s can dissipate surge energy via avalanche, the GaN high-electron-mobility transistor (HEMT) has no avalanche capability and withstands surge energy by its overvoltage capability. However, a comprehensive study into the surge-energy robustness of the cascode GaN HEMT, a composite device made of a GaN HEMT and an Si <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> , is still lacking. This article fills this gap by investigating the failure and degradation of 650-V-rated cascode GaN HEMTs in single-event and repetitive unclamped inductive switching (UIS) tests. The cascode was found to withstand surge energy by the overvoltage capability of the GaN HEMT, accompanied by avalanche in the Si <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> . In single-event UIS tests, the cascode failed in the GaN HEMT at a peak overvoltage of 1.4–1.7 kV, which is statistically lower than the device's static breakdown voltage (1.8–2.2 kV). In repetitive UIS tests, the device failure boundary was found to be frequency dependent. At 100 kHz, the failure boundary (∼1.3 kV) is even lower than the single-event UIS boundary. After 1 million cycles of 1.25-kV UIS stresses, devices show large but recoverable parametric shifts. All these failure and degradation behaviors can be explained by the buffer trapping in the GaN HEMT and the resulted change in its dynamic breakdown voltage. Moreover, the GaN buffer trapping is believed to be assisted by the Si <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> avalanche. An analytical model was also developed to extract the charges and losses produced in the Si avalanche in a UIS cycle. These results provide new insights into the surge-energy and overvoltage robustness of cascode GaN HEMTs.

E-mode All-GaN-Integrated cascode MISHEMT with GaN/InAlGaN/GaN backbarrier for high power switching performance: Simulation study

In this work, simulation-based research work based on a novel All-GaN-Integrated Cascode (AGC) MISHEMT with quaternary layer InyAlxGa1-x-yN as backbarrier has been investigated. The AGC-MISHEMT device consists of series combination of enhancement mode (recessed gate MISHEMT) and depletion-mode (MIS-gate) GaN devices with GaN/InAlGaN/GaN sandwich back barrier (SW-BB). TCAD simulations indicate that AGC-MISHEMTs with sandwich back barrier (SW-BB) exhibits simultaneous improvement in terms of drain current density (267 ​mA/mm), high ION/IOFF ratio (108), transconductance-peak (191 ​mS/mm) and breakdown voltage (371 ​V) as compared to AGC-MISHEMT with GaN buffer. The impact of varying mole fractions (x, y), thickness and position of InyAlxGa1-x-yN layer on breakdown voltage has also been examined. Improvement in drain current density up to 40% has been accomplished with increased gate oxide thickness under gate 2 (depletion-mode device) for AGC-MISHEMT with SW-BB. Besides, power switching performance has been compared for AGC-MISHEMT with SW-BB to that of AGC-MISHEMT with GaN buffer and Recessed Single Gate (RSG) MISHEMT for inductive load circuit. Significant reduction in power switching losses (i.e. 43% during turn-ON time and 87% during turn-OFF time) has been observed for AGC-MISHEMT with SW-BB as compared to RSG-MISHEMT. In addition, introduction of quaternary InAlGaN layer in backbarrier for AGC-MISHEMT also results in considerable degradation (∼89%) in power loss when input gate is in OFF-state due to improved ION/IOFF ratio in contrast with recessed single gate device.

The Effect of Heavy Fe-Doping on 3D Growth Mode and Fe Diffusion in GaN for High Power HEMT Application.

The high electron mobility transistor (HEMT) structures on Si (111) substrates were fabricated with heavily Fe-doped GaN buffer layers by metalorganic chemical vapor deposition (MOCVD). The heavy Fe concentrations employed for the purpose of highly insulating buffer resulted in Fe segregation and 3D island growth, which played the role of a nano-mask. The in situ reflectance measurements revealed a transition from 2D to 3D growth mode during the growth of a heavily Fe-doped GaN:Fe layer. The 3D growth mode of Fe nano-mask can effectively annihilate edge-type threading dislocations and improve transfer properties in the channel layer, and consequently decrease the vertical leakage current by one order of magnitude for the applied voltage of 1000 V. Moreover, the employment of GaN:C film on GaN:Fe buffer can further reduce the buffer leakage-current and effectively suppress Fe diffusion.

(001) and (11n)n = 1,3 GaAs substrate orientations for growth of GaN layers by AP-MOVPE: impact of GaN buffer layer thickness

(001) and (11n)n = 1,3 GaAs substrate orientations for growth of GaN layers by AP-MOVPE: impact of GaN buffer layer thickness

Highly Transparent p‐AlGaN‐Based (326–341 nm)‐Band Ultraviolet‐A Light‐Emitting Diodes on AlN Templates: Recent Advances and Perspectives

The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed.

Study of Drain Injected Breakdown Mechanisms in AlGaN/GaN-on-SiC HEMTs

Breakdown mechanism in 0.25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mu } \text{m}$ </tex-math></inline-formula> gate length AlGaN/GaN-on-SiC iron doped high electron mobility transistors (HEMTs) with background carbon is investigated through the drain current injection technique. The measurement results reveal that it can be divided into two distinct stages according to the gate voltage levels. The first stage of the measured drain injected breakdown is mainly due to the initiation of the punchthrough process under the gate, and the second stage of breakdown is associated with the potential barrier between the unintentionally doped (UID) GaN and the Fe doped p-type GaN buffer layer which also has a higher carbon density. The electroluminescence (EL) results suggest that the first stage shows uniform punchthrough current flow, but localized leakage current flow associated with a snapback breakdown mechanism replaces the uniform punchthrough current flow and dominates the second stage. A 2D-TCAD simulation has been implemented and shows the current paths under uniform flow conditions.