Cheng-Shih Lee

Magnetophotoluminescence properties of Co-doped ZnO nanorods

We present the detailed experimental results of the magnetic and optical properties of cobalt doped ZnO nanorods, especially the temperature and magnetic field dependence of photoluminescence up to 14 T. The Raman measurements indicate that our Co-doped ZnO nanorods have the same lattice constant as crystalline bulk ZnO. Sharp luminescence peaks centered at around 670 nm were observed at low temperature and their intensity decreased with increasing magnetic field. The luminescence peaks were attributed to d-d transitions in the Ligand field from the doped Co ions. We also observed a diamagnetic shift at a temperature of 1.5 K when the magnetic field was scanned from 0 to 14 T. The exciton radius of the Co-doped ZnO nanorods was deduced from the magnetophotoluminescence results.

Use of WN/sub X/ as the diffusion barrier for interconnect copper metallization of InGaP-GaAs HBTs

Use of WN/sub X/ as the diffusion barrier for interconnect copper metallization of InGaP-GaAs heterojunction bipolar transistors (HBTs) was studied. The WN/sub X/ (40 nm) and Cu (400 nm) films were deposited sequentially on the InGaP-GaAs HBT wafers as the diffusion barrier and interconnect metallization layer, respectively, using the sputtering method. As judged from the data of scanning electron microscopy, X-ray diffraction, Auger electron spectroscopy, and sheet resistance, the Cu--WN/sub X/--SiN and Cu--WN/sub X/--Au structures were very stable up to 550/spl deg/C and 400/spl deg/C annealing, respectively. Current accelerated stress test was conducted on the Cu--WN/sub X/ metallized HBTs with V/sub CE/=2 V, J/sub C/=140 kA/cm/sup 2/ and stressed for 55 h, the current gain (/spl beta/) of these HBTs showed no degradation and was still higher than 100 after the stress test. The Cu--WN/sub X/ metallized HBTs were also thermally annealed at 250/spl deg/C for 25 h and showed no degradation in the device characteristics after the annealing. For comparison, HBTs with Au interconnect metallization were also processed, and these two kinds of devices showed similar characteristics after the stress tests. From these results, it is demonstrated that WN/sub X/ is a good diffusion barrier for the interconnection copper metallization of GaAs HBTs.

Study of Titanium Tungsten Nitride and Tungsten Nitride Schottky Contacts on n-GaN

The Schottky diode behaviors of the TiWNx and the WNx Schottky contacts to n-GaN were investigated at different annealing temperatures. Both TiWNx and WNx films were deposited by the reactive DC sputtering method. The WNx/n-GaN contact exhibited excellent electrical characteristics even after rapid-thermal annealing up to 850°C for 10 s. The ideality factor and barrier height remained 1.09 and 0.80 eV, respectively, after 850°C annealing. However, the TiWNx/n-GaN contact was thermally stable only up to 650°C annealing; the values of the ideality factor and the barrier height were 1.14 and 0.76 eV, respectively, after 650°C annealing and started to degrade when annealed at higher temperatures. The deterioration of the TiWNx/n-GaN contact at higher temperatures was due to the inter-diffusion of the TiWN film and the GaN material.

Gold-Free Fully Cu-Metallized InGaP/GaAs Heterojunction Bipolar Transistor

A gold-free, fully Cu-metallized InGaP/GaAs heterojunction bipolar transistor using platinum as the diffusion barrier has been successfully fabricated. The HBT uses Pd/Ge and Pt/Ti/Pt/Cu for n-type and p+-type ohmic contacts, respectively, and Ti/Pt/Cu for interconnect metals with platinum as the diffusion barrier. The Ti/Pt/Cu structure was stable during annealing up to 350°C judging from the X-ray diffraction (XRD) data and sheet resistance. A current-accelerated stress test was conducted on the device with a current density JC=140 kA/cm2 for 24 h, and the current gain showed no degradation. The devices were also thermally annealed at 250°C for 24 h and showed little change. We have successfully demonstrated that an Au-free, fully Cu-metallized HBT can be realized using Pt as the diffusion barrier and Pd/Ge and Pt/Ti/Pt/Cu as the ohmic contacts.

Copper-Airbridged Low-Noise GaAs PHEMT With

A GaAs pseudomorphic HEMT (PHEMT) with Cu-metallized interconnects was successfully developed. Sputtered WN_x was used as the diffusion barrier and Ti was used as the adhesion layer to improve the adhesion between WN_x/Cu interface in the thin-metal structure. After copper metallization, the PHEMTs were passivated with silicon nitride to avoid copper oxidation. The Cu-airbridged PHEMT showed the saturation I_DS was 250 mA/mm and the g_m was 456 mS/mm. The Ti adhesion layer plays a significant role on the g _m and V_p uniformity of the Cu-metallized PHEMTs. The GaAs PHEMTs with Ti/WN_x/Ti/Cu multilayer have better noise figure and associated gain than those of the devices without the Ti adhesion layer. The fabricated Cu-metallized GaAs PHEMT with Ti/WN _x/Ti/Cu multilayer has a noise figure of 0.76 dB and an associated gain of 8.8 dB at 16 GHz. The cutoff frequency (f_T) is 70 GHz when biased at V_DS=1.5V. These results show that the Ti/WN_x/Ti multilayer can serve as a good diffusion barrier for Cu metallization process of airbridge interconnects on GaAs lownoise PHEMTs

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An alloyed Pd∕Ge∕Cu Ohmic contact to n-type GaAs is reported for the first time. The Pd∕Ge∕Cu Ohmic contact exhibited a very low specific contact resistance of 5.73×10−7Ωcm2 at a low annealing temperature of 250°C. This result is comparable to the reported Pd∕Ge and Au∕Ge∕Ni Ohmic contact systems to n-type GaAs with doping concentrations about 1×1018cm−3. The Ohmic contact formation mechanisms and microstructure evolution were investigated using secondary ion mass spectrometry, x-ray diffraction, transmission electron microscopy, and energy dispersive spectrometer. The Ohmic contact behavior was related to the formation of Cu3Ge and PdGaxAsy compounds after annealing.

Diffusion Barrier for High-Frequency Applications

A gold-free, fully Cu-metallized InP heterojunction bipolar transistor using non-alloyed Ti/Pt/Cu and Pt/Ti/Pt/Cu ohmic contacts and platinum diffusion barrier has been successfully fabricated. The InGaAs/Ti/Pt/Cu ohmic structure was stable after annealing up to 350°C as judged from the Auger depth profiles. A current-accelerated stress test was conducted on the device with a current density JC=80 kA/cm2 for 24 h, and the current gain showed no degradation after the current stress. The devices were also thermally annealed at 200°C for 3 h and showed almost no change in the electrical parameters after the heat treatment. The results show that the Au-free, fully Cu-metallized InP heterojunction bipolar transistor (HBT) can be realized using non-alloyed ohmic contacts and Pt diffusion barrier.

A Cu-based alloyed Ohmic contact system on n-type GaAs

A low contact resistance Pd/Ge/Cu ohmic contact to n-type GaAs is successfully developed. The Pd/Ge/Cu ohmic contact exhibits a very low contact resistivity of 5.73 x 10 Ω-cm at a low annealing temperature (250 °C). This result is comparable to the reported Pd/Ge and Au/Ge/Ni ohmic contact systems to n-type GaAs with doping concentrations about 1x10 cm. The ohmic contact formation mechanisms and microstructure reactions were investigated using x-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM).

A Gold-Free Fully Copper-Metallized InP Heterojunction Bipolar Transistor Using Non-Alloyed Ohmic Contact and Platinum Diffusion Barrier

A metamorphic high electron-mobility transistor (HEMT) manufactured with reflowed submicron T-gate using e-beam lithography for high-speed optoelectronics applications is developed. The In/sub 0.53/Al/sub 0.47/As/InGaAs HEMT uses In/sub x/Al/sub 1-x/As as the buffer layer between GaAs substrate and the InP lattice-matched HEMT structure. The T-gate developed has a gate length of 160 /spl mu/m. The fabricated metamorphic HEMT has a saturation drain current of 280 mA/mm and a transconductance of 840 mS/mm at V/sub DS/ = 1.2 V. Noise figure for 160 /spl mu/m gate-width device is less than 1 dB and the associated gain is up to 14 dB at 18 GHz. The device demonstrates a cut-off frequency f/sub T/ of 150 GHz and a maximum frequency f/sub MAX/ up to 350 GHz. The metamorphic HEMT developed has the potential for high-speed optoelectronics applications.