High-resistivity GaN Research Articles

Crystallization of high-resistivity Zn-doped GaN monocrystal via hydride vapor phase epitaxy

Recently, high-resistive GaN has been studied for lateral power devices. Fe-doped GaN has high resistivity at room temperature. However, the resistivity becomes lower at high temperatures than at room temperature. Therefore, alternative dopants for GaN have been investigated to achieve resistivity higher than Fe-doped GaN at high temperatures. Several studies on the resistivity of Zn-doped GaN substrates have been conducted; however, single-layered Zn-doped GaN prepared via hydride phase epitaxy (HVPE) has not been investigated. In this study, the monocrystals of Zn-doped GaN were obtained via HVPE using Zn vapor as the dopant precursor and single-layered Zn-doped GaN substrates were fabricated by removing the starting GaN substrates. The concentration of Zn was controlled by changing the temperature of Zn metal, and two Zn-doped GaN substrates with Zn concentrations of 4 × 1017 cm−3 and 1 × 1018 cm−3 were obtained. The dependence of resistivities of the Zn-doped GaN on temperature was measured by the Hall effect method. From 500 K to 930 K, the resistivity of the Zn-doped GaN with the Zn concentration of 4 × 1017 cm−3 varied from 3 × 109 Ωcm to 5 × 102 Ωcm and that with the Zn concentration of 1 × 1018 cm−3 varied from 1 × 1010 to 1 × 105 Ωcm. These values were higher than Fe-doped GaN with almost the same concentration as the Zn-doped GaN. Therefore, Zn-doped GaN could be more preferable for power devices than Fe-doped GaN.

High-performance gallium nitride high-electron-mobility transistors with a thin channel and an AlN back barrier

In this work, high-performance high-electron-mobility transistors (HEMTs) with a thin GaN channel and an AlN back barrier were fabricated and investigated in detail. The AlN back barrier HEMTs possess a higher current density and a better linearity than traditional devices. In addition, the off-state leakage current of the AlN back barrier HEMTs is more than one order of magnitude lower than that of traditional devices with a high-resistance Fe-doped GaN buffer, even though they do not involve any intentional doping technique. Additionally, the high-temperature performance of the AlN back barrier HEMTs is excellent, with less attenuation in the drain current density and less increase in the off-state leakage current. Moreover, the breakdown voltage of the AlN back barrier HEMTs is as high as 309 V with an LGD of 2.5 μm, resulting in a high Baliga figure of merit of 0.354 GW/cm2. The superior performance of the AlN back barrier devices is further demonstrated by the calculation and simulation results. The results in this work not only show the great potential of AlN back barrier HEMTs but also provide a useful direction for overcoming the limiting issues of nitride devices.

Determination of the electron trap level in Fe-doped GaN by phonon-assisted conduction phenomenon

We acoustically measured the energy level for thermally activated conduction (TAC) in high-resistivity Fe-doped GaN using the non-contacting antenna-transmission acoustic-resonance method. The acoustic attenuation takes a maximum at a specific temperature, where the TAC is accelerated with the help of phonon energy. The Debye-type relaxation is thus observed for acoustic attenuation, and its activation energy (0.54 0.04 eV) was determined with attenuation measurements at various frequencies and temperatures. This value agrees with the E3 level in GaN, indicating that thermally associated conduction originates from the E3 trap level. We also measured the five independent elastic constants at high temperatures.

High-resistivity nitrogen-polar GaN for GaN/AlGaN high electron mobility transistors by metalorganic chemical vapor deposition

Nitrogen-polar (N-polar) III-nitride has natural advantages in the fabrication of electronic devices. In this work, unintentionally doped N-polar GaN films were grown on sapphire substrates by metalorganic chemical vapor deposition. By optimizing the growth parameters including growth temperature, pressure, and V/III ratio, N-polar GaN with a relatively high sheet resistivity of 3.6 × 105 Ω/sq was achieved. The secondary ion mass spectroscopy and x-ray diffraction measurement results demonstrate that the increased carbon impurity concentration is primarily responsible for the high resistivity. Moreover, N-polar GaN/Al0.25Ga0.75N heterojunctions were deposited on the high-resistivity N-polar GaN template. An extremely high-density two-dimensional electron gas of up to 1.12 × 1013 cm−2 was realized at the interface between the Al0.25Ga0.75N and the GaN. It is reasonably believed that the experimental results obtained here are beneficial for the development of high-performance N-polar GaN-based electronic devices.

High Breakdown Voltage and Low-Current Dispersion in AlGaN/GaN HEMTs With High-Quality AlN Buffer Layer

We have successfully grown AlGaN/GaN high electron mobility transistor (HEMT) structure on the high-quality undoped thick AlN buffer layer with large band offset to replace the conventional high-resistivity GaN buffer layer. The AlGaN/GaN HEMT fabricated on this AlN buffer layer exhibits low OFF-state leakage current with high ${I}_{\mathrm{\scriptscriptstyle ON}}/{I}_{\mathrm{\scriptscriptstyle OFF}}$ of $\sim 10^{{6}}$ due to enhanced confinement of the electrons in the 2-D electron gas (2-DEG) channel. The undoped AlN buffer layer is responsible for suppressing the trapping effects to greatly reduce the current dispersion in pulsed ${I}_{\text {D}}$ – ${V}_{\text {D}}$ characteristics, which is hardly avoided in conventional deep acceptor-doped GaN buffer layer. The device also demonstrates high breakdown voltage of 2154 V with very high figure of merit (FOM) of ~1.8 GV $^{2-1}$ cm $^{-{2}}$ , one of the highest ever reported, suggesting that the AlGaN/GaN-based Hemts WITH AlN buffer layer are promising for high-performance RF and power applications.

Normally-off p-GaN Gated AlGaN/GaN HEMTs Using Plasma Oxidation Technique in Access Region

Normally-off p-GaN gated AlGaN/GaN high electron mobility transistors (HEMTs) were developed. Oxygen plasma treatment converted a low-resistive p-GaN layer in the access region to a high-resistive GaN (HR-GaN); that oxygen plasma treatment used an AlN layer as an oxygen diffusion barrier layer to prevent further oxidizing of the underlying AlGaN barrier layer, and to ensure that the low-resistive p-GaN layer in the access region was fully oxidized. Relative to conventional p-GaN gated AlGaN/GaN HEMTs, these AlGaN/GaN HEMTs with HR-GaN layers achieved a lower drain leakage current of 4.4 x 10 -7 mA/mm, a higher drain current on/off ratio of 3.9 x 10 9 , a lower on-state resistance of 17.1 Ω·mm, and less current collapse.

Undoped High-Resistance GaN Buffer Layer for AlGaN/GaN High-Electron-Mobility Transistors

It is shown that intentionally undoped high-resistance GaN buffer layers in AlGaN/GaN heterostructures with high electron mobility for transistors can be formed by ammonia molecular beam epitaxy. The GaN growth conditions have been optimized using calculations of the background impurity and point defect concentrations at different ratios of the gallium and ammonia fluxes.

Normally-off p-GaN/AlGaN/GaN high-electron-mobility transistors using oxygen plasma treatment

We propose a method of oxygen plasma treatment to realize normally-off p-GaN/AlGaN/GaN high-electron-mobility transistors. The fabricated device features an oxide surface passivation layer and a high-resistivity GaN cap layer at the access region, both transformed from p-GaN by the oxygen plasma treatment technique. With optimized treatment conditions, a low sheet resistance of 682 Ω/□ has been successfully obtained and the fabricated device shows high performance with a positive threshold voltage of +1.02 V, a maximum drain current of 301 mA mm−1, a high on/off ratio of ∼108, a breakdown voltage of 660 V, and low current collapse.

Нелегированный высокоомный буферный слой GaN для HEMT AlGaN/GaN

In this paper the possibility of obtaining the intentionally undoped high resistance GaN buffer layers in AlGaN/GaN heterostructures with high electron mobility for transistors by ammonia molecular beam epitaxy was demonstrated. The growth conditions based on background impurity concentrations and point defects calculations for different gallium and ammonia flows ratios were optimized.

3P1-1 Carrier dynamics of hopping conduction in high-resistance GaN studied by resonant ultrasound spectroscopy

3P1-1 Carrier dynamics of hopping conduction in high-resistance GaN studied by resonant ultrasound spectroscopy

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Thermal stability of F ion-implant isolated AlGaN/GaN heterostructures

The thermal stability of F ion-implanted isolated AlGaN/GaN heterostructures was investigated, with B ion-implanted isolation shown for comparison. The sheet resistance of as-implanted samples with F ions was lower than that with B ions due to enhanced hopping conduction. The leakage current for both implanted samples initially decreased and then increased with increases in post-annealing temperature, indicating a damage-induced isolation mechanism. Leakage reached a minimum value after 400°C annealing, which was over 108 times lower than the saturated current (Isat) of the as-grown structure, suggesting a successful isolation. After relatively high-temperature annealing, the leakage for F implantation showed a small change, whereas that for B implantation showed significant increase, suggesting that F-implanted isolation exhibited excellent thermal stability. Leakage current measured at 300°C for F ion-implantation was 103 times lower than the Isat of the as-grown structure, demonstrating that F ion isolation was applicable up to 300°C. Differences in current and activation energy (Ea) between the high-resistance GaN layer and F or B ion-implanted samples indicated that the leakage current originated from the region above the high-resistance GaN layer. Variation in Ea revealed that the optimal contributing states for conduction changed with operating temperature, annealing temperature, and ion species.

The Effect of the Method by Which a High-Resistivity GaN Buffer Layer Is Formed on Properties of InAlN/GaN and AlGaN/GaN Heterostructures with 2D Electron Gas

AlGaN/AlN/GaN and InAlN/AlN/GaN structures with 2D electron gas have been grown on sapphire substrates by metal-organic vapor-phase epitaxy. The suppression of the parasitic conductivity of buffer GaN layers was provided either by intentionally raising the density of edge dislocations or by doping with iron (GaN:Fe). It was shown that using GaN buffer layers with a better crystal perfection and more planar surface results in the electron mobility in the 2D channel for carriers becoming 1.2–1.5 times higher.

Etched Al 0.32 Ga 0.68 N/GaN HEMTs with high output current and breakdown voltage (>600 V)

By etching the AlGaN layer close to the gate edge of the drain side, the novel AlGaN/GaN high-electron-mobility transistor with the etched AlGaN epitaxial layer (etched AlGaN/GaN high-electron-mobility transistors, HEMTs) is proposed in this work with the high-resistance GaN buffer layer. The relationship between the 2D electron gas (2DEG) concentration and the thickness of the AlGaN layer has been explored. The 2DEG density under the etched barrier is reduced to optimise the surface electric field and mobility of 2DEG. The electric field is reshaped by the etched AlGaN layer, featuring an extra electric field peak far away from the gate edge. The electric field peak near the gate is decreased effectively because of the extra electric field peak, which therefore improves the breakdown voltage (BV). The mobility of 2DEG is increased because of the low 2DEG concentration, which improves the output current (I DS). The experimental BV is improved from 152 V for the conventional structure to 620 V for etched AlGaN/GaN HEMTs because of the optimised surface electric field. The maximum output current (I max) is increased from 381 to 478 mA/mm. The trade-off between the BV and R on,sphas been improved to break the silicon limit. It is concluded that the gate-edge etching technique can significantly increase both BV and I DS for AlGaN/GaN HEMTs.

High-resistivity unintentionally carbon-doped GaN layers with nitrogen as nucleation layer carrier gas grown by metal-organic chemical vapor deposition

In this letter, high-resistivity unintentionally carbon-doped GaN layers with sheet resistivity greater than 106 Ω/□ have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). We have observed that the growth of GaN nucleation layers (NLs) under N2 ambient leads to a large full width at half maximum (FWHM) of (102) X-ray diffraction (XRD) line in the rocking curve about 1576 arc sec. Unintentional carbon incorporation can be observed in the secondary ion mass spectroscopy (SIMS) measurements. The results demonstrate the self-compensation mechanism is attributed to the increased density of edge-type threading dislocations and carbon impurities. The AlGaN/GaN HEMT grown on the high-resistivity GaN template has also been fabricated, exhibiting a maximum drain current of 478 mA/mm, a peak transconductance of 60.0 mS/mm, an ON/OFF ratio of 0.96×108 and a breakdown voltage of 621 V.

Breakdown Enhancement and Current Collapse Suppression by High-Resistivity GaN Cap Layer in Normally-Off AlGaN/GaN HEMTs

In this letter, a device structure of high-resistivity-cap-layer HEMT (HRCL-HEMT) is developed for normally-off p-GaN gate HEMT toward high breakdown voltage and low current collapse. It demonstrates that the breakdown capability and current collapse of the device were effectively improved due to the introduction of a thick HR-GaN cap layer. The fabricated HRCL-HEMT exhibits a high breakdown voltage of 1020 V at ${I}_{\text {DS}} = 10~\mu \text{A}$ /mm with the substrate grounded. Meanwhile, the dynamic ${R}_{\text {on}}$ is only 2.4 times the static ${R}_{\text {on}}$ after off-state ${V}_{\text {DS}}$ stress of 1000 V with the substrate grounded (the OFF to ON switching time interval is set to $200~\mu \text{s}$ ).

Investigation of high density two-dimensional electron gas in Zn-polar BeMgZnO/ZnO heterostructures

Zn-polar BeMgZnO/ZnO heterostructures grown by molecular beam epitaxy on high resistivity GaN templates producing high-density two-dimensional electron gas (2DEG) are investigated. This is motivated by the need to reach plasmon-longitudinal optical (LO) phonon resonance for attaining minimum LO phonon lifetime. Achievement of high 2DEG concentration in MgZnO/ZnO heterostructures requires growth of the MgZnO barrier at relatively low temperatures, which compromises the ternary quality that in turn hinders potential field effect transistor performance. When this ternary is alloyed further with BeO, the sign of strain in the BeMgZnO barrier on ZnO switches from compressive to tensile, making the piezoelectric and spontaneous polarizations to be additive in the BeMgZnO/ZnO heterostructures much like the Ga-polar AlGaN/GaN heterostructures. As a result, a 2DEG concentration of 1.2 × 1013 cm−2 is achieved in the Be0.03Mg0.41Zn0.56O/ZnO heterostructure. For comparison, a 2DEG concentration of 7.7 × 1012 cm−2 requires 2% Be and 26% Mg in the barrier, whereas the same in the MgZnO/ZnO system would require incorporation of more than 40% Mg into the barrier, which necessitates very low growth temperatures. Our results are consistent with the demands on achieving short LO phonon lifetimes through plasmon-LO phonon resonance for high carrier velocity.

Ultraviolet photoresponse of surface acoustic wave device based on Fe-doped high-resistivity GaN

The ultraviolet (UV) photoresponse of a surface acoustic wave (SAW) device fabricated on an Fe-doped high-resistivity GaN epitaxial film grown by hydride vapor phase epitaxy is investigated. The SAW device exhibits large variations in transmission characteristics under UV illumination, but shows a very low recovery rate after ceasing the illumination. Through the characterization of photocurrent and the simulation of the displacement field of Rayleigh wave, it is suggested that the persistent photoconductivity of the Fe-doped GaN is the origin of the retarded photoresponse of the SAW device, and that the wide surface depletion layer contributes to the large photoresponse of the SAW device.

Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD

High-resistive GaN (>108 Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1−xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.