Silvia H. Chan

OG-FET: An In-Situ

In this letter, a novel device design to achieve both low ON-resistance and enhancement mode operation in a vertical GaN FET is demonstrated. In the traditional trench MOSFET structure, a dielectric is deposited on an n-p-n trenched structure and the channel forms via p-GaN inversion at the dielectric/p-GaN interface. However, this results in a relatively high ON-resistance due to poor electron mobility in the channel. By changing the structure to include a metal-organic chemical vapor deposition (MOCVD)-regrown Un-intentionally Doped (UID)-GaN interlayer followed by an in-situ dielectric (MOCVD Al2O3) cap on the n-p-n trenched structure, a pathway (channel) for enhanced electron mobility is created, resulting in reduced ON-resistance. Preliminary results for this device design demonstrated almost 60% reduction in the ON-resistance and similar breakdown voltage compared with a traditional trench MOSFET structure while maintaining normally off operation with a threshold voltage of 2 V.

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By means of combined current-voltage and capacitance-voltage sweep and transient measurements, we present the effects of forward-bias stress and charge trapping mechanisms at oxide traps in Al2O3/GaN metal-oxide-semiconductor capacitors grown in-situ by metalorganic chemical vapor deposition. Two main current-voltage regimes have been identified: a low-field regime characterized by low gate-current and low flat-band voltage instabilities, and a high-field regime triggered for oxide field greater than 3.3 MV/cm and characterized by the onset of parasitic leakage current and positive flat-band shift. In the low-voltage regime, gate current transients convey stress/relaxation kinetics based on a power-law, suggesting that tunneling trapping mechanisms occur at near-interface traps aligned with the GaN conduction-band minimum. In the high-voltage regime, devices experience parasitic conduction mechanisms and enhanced charge-trapping at oxide-traps revealed by very slow recovery transients.

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The in situ metalorganic chemical vapor deposition (MOCVD) of Al2O3 dielectrics on N-face GaN is reported. The near-interface fixed charges are measured by using capacitance-voltage techniques on a metal-oxide-semiconductor (MOSCAP) structure, and the results are compared with those obtained on Ga-face MOSCAPs with the same in situ MOCVD Al2O3 dielectrics. The influence of GaN polarity as well as other possible mechanisms on the formation of fixed charge are identified and discussed.

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GaN is one of the best suited materials for high-power devices due to its superior material properties such as high breakdown field, wide band gap and high saturation drift velocity. Consequently, GaN power devices have gained increased attention in recent years. Numerous vertical GaN power transistors have been demonstrated in the past few years [1-4]. One of the preferred GaN vertical device designs is the trench MOSFET. In the traditional trench MOSFET structure [2-4], the channel forms via p-GaN inversion at the dielectric/p-GaN interface resulting in a relatively high on-resistance due to the poor electron mobility in the channel. In this work, we present a novel device design to lower the on-resistance in a trench MOSFET. By inserting a MOCVD regrown GaN interlayer prior to the dielectric deposition (MOCVD Al2O3) on the trenched structure, lower on-resistance is achieved due to enhancement in the electron mobility of the channel. For an optimal GaN interlayer thickness of 10 nm, a low on-resistance (active area) of 0.97 mΩ.cm2 alongside enhancement mode operation (Vth = 3 V) is demonstrated.

aN Interlayer-Based Vertical Trench MOSFET

This paper reports high two-dimensional electron gas mobility attained from the regrowth of the AlGaN gating layer on ex situ GaN surfaces. To repair etch-damaged GaN surfaces, various pretreatments were conducted via metalorganic chemical vapor deposition, followed by a regrown AlGaN/GaN mobility test structure to evaluate the extent of recovery. The developed treatment process that was shown to significantly improve the electron mobility consisted of a N2 + NH3 pre-anneal plus an insertion of a 4 nm or thicker GaN interlayer prior to deposition of the AlGaN gating layer. Using the optimized process, a high electron mobility transistor (HEMT) device was fabricated which exhibited a high mobility of 1450 cm2 V−1 s−1 (R sh = 574 ohm/sq) and low dispersion characteristics. The additional inclusion of an in situ Al2O3 dielectric into the regrowth process for MOS-HEMTs still preserved the transport properties near etch-impacted areas.

On trapping mechanisms at oxide-traps in Al2O3/GaN metal-oxide-semiconductor capacitors

In this paper, we report on the growth and electrical characterization of (Al,Si)O dielectrics grown by metalorganic chemical vapor deposition (MOCVD) using trimethylaluminum, oxygen, and silane as precursors. The growth rates, refractive indices, and composition of (Al,Si)O films grown on Si(001) were determined from ellipsometry and XPS measurements. Crystallinity and electrical properties of (Al,Si)O films grown in situ on c-plane GaN were characterized using grazing incidence X-ray diffraction and capacitance–voltage with current–voltage measurements, respectively. Si concentration in the films was found to be tunable by varying the trimethylaluminum and/or oxygen precursor flows. The Si incorporation suppressed the formation of crystalline domains, leading to amorphous films that resulted in reduced interfacial trap density, low gate leakage and ultra-low hysteresis in (Al,Si)O/n-GaN MOS-capacitors.

In situ metalorganic chemical vapor deposition of Al2O3 on N-face GaN and evidence of polarity induced fixed charge

GaN trench-gate MOSFETs with m- and a-plane-oriented sidewall channels were fabricated and characterized. The trench-gate MOSFET performance depended strongly on the sidewall-MOS-channel plane orientation. The m-plane-oriented MOS channel devices demonstrated higher channel mobility, higher current density, lower sub-threshold slope, and lower hysteresis with similar threshold voltage and on–off ratio compared to a-plane MOS channel devices. These results indicate that orienting trench-gate MOSFET toward the m-plane would allow for better on-state characteristics while maintaining similar off-state characteristics.

A novel device design to lower the on-resistance in GaN trench MOSFETs

In recent years, GaN trench MOSFETs have been actively investigated to achieve low on-resistance and high breakdown voltage [1-8]. The absence of a JFET region makes the trench MOSFET a favorable device structure to reduce the on-resistance. However, poor (electron) channel mobility in GaN trench MOSFETs lead to increased channel resistance. This could potentially result in reliability issues and/or high on-resistance as a large gate bias is needed to reduce the channel resistance. In our previous works, we demonstrated a novel device design (OG-FET), where enhanced channel mobility was obtained by inserting a MOCVD-regrown GaN interlayer between the trenched structure and the in-situ gate dielectric [7, 8]. The breakdown performance of OG-FETs reported in previous work was limited due to the absence of edge termination [8]. In this work, OG-FETs were fabricated with field plate based edge termination which resulted in an enhanced breakdown from 600 V (EBR ∼ 1.5 MV/cm) to 1000 V (EBR ∼ 2 MV/cm).

High electron mobility recovery in AlGaN/GaN 2DEG channels regrown on etched surfaces

This work reports on the electrical characterization of Al₂O₃/GaN MOS capacitors grown by means of metal-organic chemical vapor deposition. Novel results for 25-nm-thick Al₂O₃ show (i) an interface-state density measured by means of UV-assisted C-V technique lower than 7x10¹¹ cm^-2; (ii) flat-band voltage shift lower than 100 mV up to a gate voltage of 4 V (1.6 MV/cm); (iii) a breakdown strength of 8.8 MV/cm; (iv) timeto-breakdown of 20 years for electric field smaller than 3.7 MV/cm at room temperature, and charge to breakdown of 13.6 C/cm² if measured at 10 mA/cm² with an electric field of ~5.6 MV/cm.

Metalorganic chemical vapor deposition and characterization of (Al,Si)O dielectrics for GaN-based devices

Thin AlN interlayers are widely used in (In,Al,Ga)N based high-electron-mobility transistors to improve the mobility of the two-dimensional electron gas forming at the GaN/(In,Al,Ga)N interface. AlN layers grown by metal-organic chemical vapor deposition, however, were recently shown to contain high amounts of gallium caused by carry over reactions, resulting in AlxGa1−xN layers with x ~ 0.5 under typical deposition conditions. By modifying the AlN growth conditions in this study, layers with an Al mole fraction up to 0.78 were obtained. The unintentional Ga incorporation had a negligible effect on the electronic properties of GaN/AlN/AlGaN structures with nominally 0.7 nm thick AlN interlayer and sheet carrier densities in the order of 1 × 1013 cm−3. It resulted, however, in low electron mobility values for samples with thicker nominal AlN layers and/or higher sheet carrier densities.