Researchers from India, Finland and Germany proposed to use epitaxial gadolinium oxide (Gd2 O3) as the gate insulator for gallium nitride (GaN) channel metal oxide semiconductor high electron mobility transistors (MOSHEMT) [Ritam Sarkar et al., Appl . Physics Lett., vol115, p063502, 2019].

Teams from the Bombay Institute of Technology in India, Aalto University in Finland and Leibniz University in Hanover believe that crystalline Gd2O3 should be able to withstand high-temperature post-deposition better than the more common amorphous oxide gate insulators. At high temperatures, the amorphous atomic structure tends to become polycrystalline, creating a current leakage path at the grain boundary, which has a negative impact on the performance of the transistor. Single crystal materials are more resistant to structural changes at high temperatures.

Figure 1: (a) Cross-sectional scanning electron microscope image of AlGaN/GaN heterostructure. (b) The picture of the wafer after epitaxial HEMT growth.

For low-cost mass production, the substrate is (111) silicon with a thickness of 1 mm and a diameter of 150 mm. Low-voltage metal-organic vapor phase epitaxy (MOVPE) produced a series of step-graded AlGaN layers to achieve a 1μm (0001) GaN buffer layer and channel layer (Figure 1). The top aluminum gallium nitride (AlGaN) barrier layer is composed of 1.5nm AlN, 26nm Al0.27 Ga0.73 N and a 2nm GaN cap layer.

Gd2O3 gate oxide is grown by 650°C molecular beam epitaxy (MBE). The source is Gd2O3 particles evaporated using electron beams and additional molecular oxygen to make up for the oxygen consumption during the evaporation process. Prepare the III-nitride surface for Gd2O3 by heating to 630°C for 30 minutes.

The crystalline properties of Gd2 O3 vary according to the layer thickness: according to high-resolution X-ray diffraction, the structure is hexagonal at ~2.8nm, and the structure changes to monoclinic at 15nm. A mixed state of hexagonal and monoclinic structures with a thickness of 5.5 nm was found.

X-ray analysis also shows that Gd2O3 places the underlying AlGaN under compressive strain along the c-axis of the crystal structure. The Hall measurement values ​​of the sheet carrier density and mobility of the two-dimensional electron gas (2DEG) channel near the AlGaN/GaN interface are in the range of 5-6x1012 /cm2 and 1400-1500cm2 /Vs, respectively. The researchers commented: "The small changes in mobility and electron concentration may be attributable to small fluctuations in aluminum concentration on large-diameter wafers."

Group III nitride epitaxial materials are used to manufacture circular HEMTs with annealed titanium/aluminum/nickel/gold ohmic source/drain contacts. The Schottky grid contacts are made of nickel/gold.

The maximum drain current is 175mA/mm, the drain bias is 4.5V, and the gate potential is 1V. "Compared with the results reported earlier, the relatively low drain saturation current may be due to the large circumference of the device (source-drain distance ~ 20μm)," the team explained. The threshold of the HEMT device is -2.7V; the peak transconductance is 60mS/mm. The on/off current ratio is 5x103.

Figure 2: (a) The relationship between gate leakage current and gate voltage of controlled HEMTs and MOSHEMTs with Gd2 O3 thicknesses of 2.8 nm and 5.5 nm. (b) Comparison of leakage current density with various dielectric-based MOSHEMT data reported earlier.

The epitaxial material with Gd2O3 allows the manufacture of MOSHEMT. The gate electrode is tungsten. Compared with the Schottky gate on the pure HEMT AlGaN, the insulating Gd2 O3 naturally reduces the gate leakage current by about five orders of magnitude (Figure 2). When using 5.5nm Gd2 O3, the leakage current is about 5x10-8 A/cm2 when the gate voltage is -2V.

Reducing Gd2 O3 to 2.8nm may surprisingly reduce leakage to ~4x10-9 A/cm2, which is six orders of magnitude lower than Schottky HEMT control. The researchers stated that, unlike the thicker Gd2O3 layer, the 2.8nm device benefits from "single phase (hexagon) without domain boundaries, and therefore behaves as an ideal oxide without leakage paths." According to capacitance-voltage analysis, 2.8nm Gd2 O3 also has the lowest interface trap density (Dit), which is about 2.98x1012 /cm2 -eV. The dielectric constant of 2.8nm Gd2O3 is ~15.

The carrier density of the Hall plate with 2.8nm G2 O3 is also increased by about 40%. The researchers attribute this improvement to the in-plane tensile strain of the pseudocrystalline Gd2 O3 that balances c-direction compression.

Tags: Gadolinium Oxide Gate Insulation GaN MOSHEMT AlGaN MOVPE MBE

The author Mike Cooke is a freelance technology journalist who has been working in the field of semiconductors and advanced technology since 1997.

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