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 Ω·mm [specific contact resistivity, ρc=(0.78±0.22)×10-6 Ω·cm2] 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.

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We have demonstrated 0.17-µm gate-length In 0.17 Al 0.83 N/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 (R c ) of 0.36 Ω-mm. This contact resistance is comparable to the conventional gold-based (Ti/Al/Ni/Au) ohmic contact resistance (R c = 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 µm 2 ). 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 (V gs0 = -5 V, V ds0 = 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 R c ohmic contact for the realization of high-frequency operating InAlN/AlN/GaN HEMTs on Si substrates without using recess etching and regrowth processes.

This work investigates the conduction mechanism of non-gold Ta/Si/Ti/Al/Ni/Ta ohmic contact in un-doped AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on Si. The temperature dependent I-V measurement reveals that the conduction mechanism is primarily via Thermionic Emission(TE). The extracted mean barrier height(Φ B ) values are 0.113 and 0.121 eV and the mean contact resistance(R c ) values are 0.24 and 0.28 Ω•mm respectively for annealing temperature at 850 ºC and 900 ºC. The low R c is attributed to the formation of low work function Ti x Si y at the metal-semiconductor interface. The HR-TEM and EDX analysis also provide structural evidence to support the TE mechanism.

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The far ultraviolet C (UVC) light sources based on carbon nanotube (CNT) field emitters as excitation sources have become promising light sources for sterilization, disinfection, and water purification. However, the low light extraction efficiency of UVC–CNT light sources still hinders the practical application of these structures. Herein, we report an optimized aluminum (Al) reflector to enhance the light extraction efficiency of UVC–CNT light sources. Optical analysis of UVC-CNT light sources covered by the Al reflectors with various thicknesses ranging from 30 to 150 nm was performed to realize the optimized reflector. The UVC-CNT light sources exhibit the highest light extraction efficiency when the Al reflector layer has an optimized thickness of 100 nm. For comparison, the cathodoluminescence (CL) spectra were recorded for UVC–CNT light sources with and without the optimized Al reflector. The measured light output power and the estimated power efficiency of the UVC–CNT light-source-tube with Al reflector were enhanced by about 27 times over the reference. This enhancement is mainly attributed to the outstanding reflection effect of the Al reflector.

We report charge-compensated modified uni-traveling-carrier photodiodes (MUTC-PDs) with high photocurrent and fast response, grown using liquid group-V precursor, in an AIXTRON MOCVD system. The liquid group-V precursors involve less toxicity with better decomposition characteristics. Device fabrication is completed with standard processing techniques with BCB passivation. DC and RF measurements are carried out using a single mode fiber at 1.55 μm. For a 24-μm-diameter device (with diode ideality factor of 1.34), the dark current is 32.5 nA and the 3-dB bandwidth is ≫20 GHz at a reverse bias of 5 V, which are comparable to the theoretical values. High photocurrent of over 150.0 mA from larger diameter (>60 μm) devices is obtained. The maximum DC responsivity at 1.55 μm wavelength is 0.51 A/W, without antireflection coating. These photodiodes play a key role in the progress of the future THz communication systems.