【International Papers】Atomic layer deposition of gallium oxide using gallium triazenide and water

      Researchers from the Linköping University have published a dissertation titled " Atomic layer deposition of gallium oxide using gallium triazenide and water " in Materials Advances.

Background

      Wide-bandgap (WBG) semiconductors provide orders-of-magnitude higher breakdown fields and lower carrier leakage than Si. Breakthroughs in the 1990s–2000s, notably with silicon carbide (SiC) and gallium nitride (GaN), enabled high-voltage Schottky diodes, metal-oxide-semiconductor field-effect transistors (MOSFETs) and high-electron-mobility transistors (HEMTs), all of which are now widely used in electric-vehicle inverters, server power supplies, and radio-frequency amplifiers. Over the past decade, attention has shifted to ultra-wide-bandgap (UWBG) materials (bandgaps >4 eV), because they promise greater performance gains. This subclass includes β-gallium oxide (β-Ga₂O₃), diamond, and aluminium nitride. The wider bandgaps of WBG and UWBG semiconductors, relative to Si, yield exponentially lower intrinsic carrier concentrations, and enable high-temperature operation. Their high critical breakdown fields also allow devices to block high voltages with thinner, more heavily doped drift layers, dramatically reducing on-state losses. Direct-bandgap UWBG materials are strongly transparent in the deep UV-region, making them suitable for solar-blind photodetectors and other related optical devices. β-Ga₂O₃ has an optical bandgap of 4.8 eV (absorption edge ≈255 nm), rendering it solar-blind (insensitive above ∼280 nm) and attractive for UV photodetectors and optical coatings. Stoichiometric, highly pure β-Ga₂O₃ films are typically clear and highly transparent, however, O-vacancies and impurities can impart yellowish or bluish tints. The ability to tune these optical features by controlling film composition is advantageous for UV-sensor and filter applications.

Abstract

      Gallium oxide (Ga₂O₃) is an ultrawide bandgap semiconductor with promising applications in power electronics and UV-photodetectors. Herein, we present thermal atomic layer deposition (ALD) of Ga₂O₃ thin films using tris(1,3-diisopropyltriazenide)gallium(III) and water. The deposition process shows saturation in the growth per cycle of ∼1.5 Å at precursor pulses ≥2 s with a narrow ALD temperature interval between 400 and 415 °C, and a nucleation delay of ∼15 cycles. Time-of-flight elastic recoil detection analysis revealed near-stoichiometric Ga₂O₃ with <3.5 at%, of C, H, N, and Cl, all of which decreases after annealing. Grazing Incidence X-ray diffraction reveals that annealing at 700 °C converts as-deposited amorphous films into phase-pure β-Ga₂O₃. The as-deposited films were highly transparent (>96%) with an optical bandgap of ∼3.74 eV, which increased to ∼4.0 eV upon annealing. Electrical conductivity also increased from ∼3 mS cm⁻¹ in the as deposited films to ∼30 mS cm⁻¹ after annealing. This work extends the ALD chemistry of triazenide precursors, previously validated for GaN, InN, InGaN and In₂O₃, to Ga₂O₃.

Conclusion

      This work demonstrates ALD of Ga₂O₃ using Ga(triaz)₃ with water. Saturation behaviour was observed with precursor pulse durations ≥2 s, yielding a GPC of ∼1.5 Å. The process exhibited a narrow ALD temperature interval between 400 and 415 °C, and film thickness scaled linearly with cycle count after a ∼15 cycles nucleation delay. Ga(triaz)₃ showed reactivity comparable to Ga trialkyl, halide, and alkoxide precursors used in conventional thermal ALD. As-deposited films were near-stoichiometric but contained minor C, H, N, and Cl (precursor derived), and Cu from hardware. Annealing reduced these impurities and increased the O : Ga ratio to ∼1.6–1.7. Cl levels remained minimal throughout, highlighting the effectiveness of precursor purification and ligand design. As a result, the films produced here rank among the purest reported for thermal ALD of Ga₂O₃ using water as the O-source. GI-XRD confirmed amorphous as-deposited films crystallized to monoclinic β-Ga₂O₃ after annealing at 700 °C. Optically, transmittance exceeded 96% in the as-deposited state and decreased by ∼10% after annealing, and Tauc plots showed the bandgap increasing from ∼3.74 to 4.00 eV. Electrically, all films exhibit higher conductivity after annealing, consistent with β-phase formation and improved structural order. Taken together, XRD and ToF-ERDA corroborated that the property changes arised from crystallization, reduced defect/ligand residues, and a modest shift toward O-richer composition.