Kian Siong Ang

Atomic layer deposition of ZrO2 as gate dielectrics for AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors on silicon

In this Letter, the device performance of AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors (MISHEMTs) on silicon substrate using 10-nm-thick atomic-layer-deposited ZrO2 as gate dielectrics is reported. The ZrO2 AlGaN/GaN MISHEMTs showed improved maximum drain current density (Idmax) with high peak transconductance (gmmax) as comparison to Schottky-barrier-gate HEMTs (SB-HEMTs). Also compared to SB-HEMTs, reverse gate leakage current was four orders of magnitude lower and forward gate bias extended to +7.4 V. At energy from −0.29 eV to −0.36 eV, low interface trap state density evaluated by AC conductance and “Hi-Lo frequency” methods indicates good quality of atomic-layer-deposited ZrO2 dielectric layer.

Direct Current and Microwave Characteristics of Sub-micron AlGaN/GaN High-Electron-Mobility Transistors on 8-Inch Si(111) Substrate

We report for the first time the DC and microwave characteristics of sub-micron gate (~0.3 µm) AlGaN/GaN high-electron-mobility transistors (HEMTs) on 8-in. diameter Si(111) substrate. The fabricated sub-micron gate devices on crack-free AlGaN/GaN HEMT structures exhibited good pin.-off characteristics with a maximum drain current density of 853 mA/mm and a maximum extrinsic transconductance of 180 mS/mm. The device exhibited unit current-gain cut-off frequency of 28 GHz, maximum oscillation frequency of 64 GHz and OFF-state breakdown voltage of 60 V. This work demonstrates the feasibility of achieving good performance AlGaN/GaN HEMTs on 8-in. diameter Si(111) for low-cost high-frequency and high-power switching applications.

Demonstration of submicron-gate AlGaN/GaN high- electron-mobility transistors on silicon with complementary metal-oxide-semiconductor-compatible non-gold metal stack

We have demonstrated 0.15-µm-gate-length AlGaN/GaN high-electron-mobility transistors (HEMTs) with direct-current (DC) and microwave performances for the first time using a complementary metal–oxide–semiconductor (CMOS)-compatible non-gold metal stack. Si/Ta-based ohmic contact exhibited low contact resistance (Rc=0.24 Ωmm) with smooth surface morphology. The fabricated GaN HEMTs exhibited gmmax=250 mS/mm, fT/fmax=39/39 GHz, B Vgd=90 V, and drain current collapse <10%. The device Johnsons figure of merit (J-FOM = fT × B Vgd) is in the range between 3.51 to 3.83 THzV which are comparable to those of other reported GaN HEMTs on Si with a conventional III–V gold-based ohmic contact process. Our results demonstrate the feasibility of realizing high-performance submicron GaN-on-silicon HEMTs using a Si CMOS-compatible metal stack.

Band alignment between GaN and ZrO2 formed by atomic layer deposition

The band alignment between Ga-face GaN and atomic-layer-deposited ZrO2 was investigated using X-ray photoelectron spectroscopy (XPS). The dependence of Ga 3d and Zr 3d core-level positions on the take-off angles indicated upward band bending at GaN surface and potential gradient in ZrO2 layer. Based on angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔEV of 1 ± 0.2 eV and conduction band discontinuity ΔEC of 1.2 ± 0.2 eV at ZrO2/GaN interface were determined by taking GaN surface band bending and potential gradient in ZrO2 layer into account.

Influence of post-deposition annealing on interfacial properties between GaN and ZrO2 grown by atomic layer deposition

Influence of post-deposition annealing on interfacial properties related to the formation/annihilation of interfacial GaOx layer of ZrO2 grown by atomic layer deposition (ALD) on GaN is studied. ZrO2 films were annealed in N2 atmospheres in temperature range of 300 °C to 700 °C and analyzed by X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. It has been found that Ga-O bond to Ga-N bond area ratio decreases in the samples annealed at temperatures lower than 500 °C, which could be attributed to the thinning of GaOx layer associated with low surface defect states due to “clean up” effect of ALD-ZrO2 on GaN. However, further increase in annealing temperature results in deterioration of interface quality, which is evidenced by increase in Ga-O bond to Ga-N bond area ratio and the reduction of Ga-N binding energy.

Low-Contact-Resistance Non-Gold Ta/Si/Ti/Al/Ni/Ta Ohmic Contacts on Undoped AlGaN/GaN High-Electron-Mobility Transistors Grown on Silicon

Low-contact-resistance (Rc) non-gold Ta/Si/Ti/Al/Ni/Ta ohmic contacts were realized on an undoped AlGaN/GaN high-electron-mobility transistor (HEMT) grown on a silicon substrate. Optimization of the rapid thermal process reveals that Rc decreases drastically from the annealing temperature of 700 to 850 °C and slightly increases from 875 to 900 °C. The sample annealed at 850 °C exhibited the lowest Rc of 0.22±0.03 Ωmm [specific contact resistivity, ρc=(0.78±0.22)×10-6 Ωcm2] with a smooth surface morphology (RMS roughness ~5.5 nm). The low Rc is due to the formation of TixSiy and the intermixing of TixSiy with the bottom Ta layer at the metal/semiconductor interface.

Impact of post-deposition annealing on interfacial chemical bonding states between AlGaN and ZrO2 grown by atomic layer deposition

The effect of post-deposition annealing on chemical bonding states at interface between Al0.5Ga0.5N and ZrO2 grown by atomic layer deposition (ALD) is studied by angle-resolved x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy. It has been found that both of Al-O/Al 2p and Ga-O/Ga 3d area ratio decrease at annealing temperatures lower than 500 °C, which could be attributed to “clean up” effect of ALD-ZrO2 on AlGaN. Compared to Ga spectra, a much larger decrease in Al-O/Al 2p ratio at a smaller take-off angle θ is observed, which indicates higher effectiveness of the passivation of Al-O bond than Ga-O bond through “clean up” effect near the interface. However, degradation of ZrO2/AlGaN interface quality due to re-oxidation at higher annealing temperature (>500 °C) is also found. The XPS spectra clearly reveal that Al atoms at ZrO2/AlGaN interface are easier to get oxidized as compared with Ga atoms.

Modal Gain and Photoluminescence Investigation of Two-State Lasing in GaAs-Based 1.3 µm InAs/InGaAs Quantum Dot Lasers

Two-state lasing was investigated on the fabricated high-performance InAs/InGaAs quantum dot (QD) lasers. Both excitation-dependent photoluminescence (PL) measurements and modal gain measurements based on the Hakki Paoli method, for the first time, on the two-state transitions were performed. The two lasing states are attributed to ground state (GS) and excited state (ES) transitions. The significant carrier recombination competition has been observed at the GS and ES transitions from both PL and modal gain analyses. Unlike that in the quantum well lasers, the gain was not pinned at the GS state after GS lasing in QD lasers.

Record-low contact resistance for InAlN/AlN/GaN high electron mobility transistors on Si with non-gold metal

We have demonstrated 0.17-µm gate-length In0.17Al0.83N/GaN high-electron-mobility transistors (HEMTs) on Si(111) substrates using a non-gold metal stack (Ta/Si/Ti/Al/Ni/Ta) with a record-low ohmic contact resistance (Rc) of 0.36 Ω mm. This contact resistance is comparable to the conventional gold-based (Ti/Al/Ni/Au) ohmic contact resistance (Rc = 0.33 Ω mm). A non-gold ohmic contact exhibited a smooth surface morphology with a root mean square surface roughness of ~2.1 nm (scan area of 5 × 5 µm2). The HEMTs exhibited a maximum drain current density of 1110 mA/mm, a maximum extrinsic transconductance of 353 mS/mm, a unity current gain cutoff frequency of 48 GHz, and a maximum oscillation frequency of 66 GHz. These devices exhibited a very small (<8%) drain current collapse for the quiescent biases (Vgs0 = −5 V, Vds0 = 10 V) with a pulse width/period of 200 ns/1 ms. These results demonstrate the feasibility of using a non-gold metal stack as a low Rc ohmic contact for the realization of high-frequency operating InAlN/AlN/GaN HEMTs on Si substrates without using recess etching and regrowth processes.

0.25-μm gate In0.48Ga0.52P/In0.20Ga0.80As/GaAs pseudomorphic high electron mobility transistors grown by solid-source molecular beam epitaxy

Abstract In0.48Ga0.52P/In0.20Ga0.80As/GaAs pseudomorphic high electron mobility transistor (p-HEMT) structures were grown by solid-source molecular beam epitaxy (SSMBE) using a valved phosphorus cracker cell. Device with a mushroom gate of 0.25 μm gate length and 80 μm gate width achieved a peak transconductance (Gm) of 420 mS/mm and drain current density of nearly 500 mA/mm. A high cut-off frequency (fT) of 58 GHz and maximum oscillation frequency (fmax) of 120 GHz were obtained. The results showed that the In0.48Ga0.52P/In0.20Ga0.80As/GaAs material system grown by SSMBE using the valved phosphorus cracker cell for the In0.48Ga0.52P Schottky and spacer layers is clearly a viable technology for high frequency p-HEMT device applications.