Weifeng Zhao

Direct measurement of nanoscale sidewall roughness of optical waveguides using an atomic force microscope

An atomic force microscope (AFM) with an ultrasharp tip was used to directly measure the sidewall profile of InP/InGaAsP waveguide structures etched using an inductively coupled plasma reactive ion etching (ICP-RIE) in Cl2-based plasma. A special staircase pattern was devised to allow AFM tip to access the etched sidewall of the waveguides in the normal direction. Statistical information such as correlation length and rms roughness of the sidewall profile obtained through three-dimensional imaging by AFM has been presented. rms roughness as low as 3.45 nm was measured on the sidewall of 4-μm-deep etched InP/InGaAsP heterostructures.

Characterization of sidewall roughness of InP/InGaAsP etched using inductively coupled plasma for low loss optical waveguide applications

The effects of etch depth on the sidewall roughness (SWR) of InGaAsP/InP waveguides fabricated utilizing two types of masks, NiCr/SiO2 and SiO2/NiCr/SiO2, were investigated with an atomic force microscopy. All the waveguides were etched in an inductively coupled plasma–reactive ion etching to depths ranging from 4 to 8 μm. The root-mean-square (rms) sidewall roughness values of the waveguides etched to depths of 4, 6, and 8 μm with SiO2 remasking layer were measured to be 2.97, 3.45, and 3.64 nm, respectively. Also the rms SWR values of the waveguides etched without the remasking layer were 3.2, 3.65, and 3.89 nm, respectively. The SiO2 thin remasking layer deposited on NiCr/SiO2 mask structure reduced the SWR of the waveguides. Measurements indicated that SWR increased with etch time, which is ascribed to an increase in mask erosion during etching.

Thermally stable Ge/Ag/Ni Ohmic contact for InAlAs/InGaAs/InP HEMTs

Excellent annealed ohmic contacts based on Ge/Ag/Ni metallization have been realized in a temperature range between 385 and 500/spl deg/C, with a minimum contact resistance of 0.06 /spl Omega//spl middot/mm and a specific contact resistivity of 2.62 /spl times/10/sup -7/ /spl Omega//spl middot/cm/sup 2/ obtained at an annealing temperature of 425/spl deg/C for 60 s in a rapid thermal annealing (RTA) system. Thermal storage tests at temperatures of 215 and 250/spl deg/C in a nitrogen ambient showed that the Ge/Ag/Ni based ohmic contacts with an overlay of Ti/Pt/Au had far superior thermal stabilities than the conventional annealed AuGe/Ni ohmic contacts for InAlAs/InGaAs high electron mobility transistors (HEMTs). During the storage test at 215/spl deg/C, the ohmic contacts showed no degradation after 200 h. At 250/spl deg/C, the contact resistance value of the Ge/Ag/Ni ohmic contact increased only to a value of 0.1 /spl Omega//spl middot/mm over a 250-h period. Depletion-mode HEMTs (D-HEMTs) with a gate length of 0.2 /spl mu/m fabricated using Ge/Ag/Ni ohmic contacts with an overlay of Ti/Pt/Au demonstrated excellent dc and RF characteristics.

Correlating the Schottky barrier height with the interfacial reactions of Ir gates for InAlAs∕InGaAs high electron mobility transistors

The characteristics of Ir on InAlAs and on InAlAs∕InGaAs∕InP high electron mobility transistor (HEMT) heterostructures were characterized. A maximum Schottky barrier height (φB) of 825meV was achieved for Ir∕InAlAs after annealing at 400°C. Transmission electron microscopy investigations confirmed that an amorphous layer (a layer) exists at the Ir∕InAlAs interface at that temperature. Results indicate that enhancement of φB is associated with the a layer, while beyond 400°C, the decrease of φB is due to the crystallization of the a layer and the formation of IrAs2. The enhancement of φB for Ir∕InAlAs and the slow diffusion of Ir in IrAlAs make it a superior thermally stable gate metal for InAlAs∕InGaAs HEMTs.

Electrical and structural investigations of Ag-based Ohmic contacts for InAlAs∕InGaAs∕InP high electron mobility transistors

Ge∕Ag∕Ni and AuGe∕Ni∕Au Ohmic contacts on InAlAs∕InGaAs∕InP high electron mobility transistors with excellent contact resistance of 0.07Ωmm were obtained after annealing at 425 and 265°C, respectively. The Ag-based contacts have a large processing window of >130°C. Structural analyses confirm that Ag and Au protrusions created during annealing effectively linked the two-dimensional electron gas layer with the metal contacts to produce excellent Ohmic characteristics. The formation of liquid AuGe eutectic phase in AuGe∕Ni∕Au at 300°C is believed to cause overannealing. The eutectic temperature of Ag–Ge is ∼300°C higher leading to a higher optimum annealing temperature and a wider processing window for the Ge∕Ag∕Ni contacts.

Effect of mask thickness on the nanoscale sidewall roughness and optical scattering losses of deep-etched InP∕InGaAsP high mesa waveguides

Deep-etched mesa waveguide in InP∕InGaAsP heterostructures were fabricated using different thicknesses of NiCr etching mask in an inductively coupled plasma reactive ion etching system with Cl2-based plasmas. An atomic force microscope (AFM) loaded with a carbon nanotube tip was utilized to measure the sidewall roughness of the etched structures. The root-mean-square (rms) roughness values were calculated separately for the horizontal and vertical directions of the waveguide sidewall. As the NiCr mask thickness increased from 10 nm to 150 nm, the rms roughness value in the vertical direction of the waveguide sidewall decreased from 19.5 nm to 2.8 nm. On the other hand, the rms roughness value in the horizontal waveguiding direction increased from 2.5 nm to 4.0 nm. The correlation lengths along the horizontal direction for the etched waveguide structures were also extracted from the AFM roughness data. The rms roughness values and correlation lengths were used as parameters in three-dimensional beam propag...

Study of the evolution of nanoscale roughness from the line edge of exposed resist to the sidewall of deep-etched InP∕InGaAsP heterostructures

The evolution of line edge roughness and sidewall roughness was monitored during the fabrication of deep-etched optical waveguides in InP∕InGaAsP heterostructures. Scanning electron microscopy was used to extract line edge profiles of the electron beam exposed resist and the lifted-off NiCr metal mask. Atomic force microscopy with an ultrasharp tip was utilized to extract the sidewall profiles of InP∕InGaAsP mesa waveguides that were etched using inductively coupled plasma reactive ion etching. The processing step that critically influences the roughness of the etched waveguides was determined by studying the evolution of the roughness.

Micro-racetrack notch filters based on InGaAsP/InP high mesa optical waveguides

Experimental and theoretical investigations were carried out on the spectral responses of micro-racetrack notch filters based on InP/InGaAsP high mesa optical waveguides that were fabricated by utilizing electron beam lithography and inductively-coupled-plasma reactive ion etching (ICP-RIE) technique. The critical factors determining the performance of micro-racetrack resonator-coupled devices were identified to be the optical power coupling efficiency between the bus waveguide and the racetrack resonator and the round-trip loss of the racetrack resonator. These parameters were extracted from the measured spectral responses of three single resonator-coupled waveguide devices with different gap spacings between the bus waveguides and the racetrack resonators, which were 0.2, 0.35, and 0.5 µm. These extracted parameters can be used to uniquely determine the pole and zero locations of the unit racetrack-coupled waveguide filter. The phase responses as well as the magnitude responses of optical filters can be calculated using the uniquely determined pole and zeros. The extracted parameters were used to calculate the spectral responses of a high-order racetrack-coupled device that was designed by cascading the three single racetrack-coupled devices having gap spacings of 0.2, 0.35, and 0.5 µm. The calculated and measured spectral responses of the high-order filters were compared to verify the parameter extraction process. The measured spectral response of the filters matched very well with the theoretically calculated response using the extracted parameters from the first-order racetrack resonator-coupled devices.

Application of the Ag-Based Ohmic Contact to the Realization of Thermally-Stable InAlAs/InGaAs/InP High Electron Mobility Transistors

Auger electron spectroscopy analysis of Ag-based Ge/Ag/Ni ohmic contacts on InAlAs/InGaAs/InP revealed that the in-diffusion of the Ag down to the semiconductor layers was necessary for the formation of excellent ohmic contact. Atomic force microscopy characterization of the surface morphology showed that the root mean square roughness of the ohmic contact after annealing was as small as 3.0 nm. High electron mobility transistors with a gate length of 0.2 µm fabricated utilizing Ag-based ohmic contact showed excellent DC and RF characteristics including: gm,max of 835 mS/mm, ID,max of 813 mA/mm, fT of 156 GHz, and fmax of 245 GHz. Due to the stable property of the Ag-based source and drain ohmic contacts, these devices were shown to be thermally stable through preliminary thermal storage tests at 215 °C.

Monolithic integration of thermally stable enhancement-mode and depletion-mode InAlAs/InGaAs/InP HEMTs utilizing Ir-gate and Ag-ohmic contact technologies

This paper reports a newly developed fabrication process for monolithic integration of thermally stable InAlAs/InGaAs/InP E/D-HEMTs based on Ir-gate and Ag-ohmic contact technologies. The Ir-gate and Ag-ohmic contacts were annealed simultaneously after passivation using a SiNx layer. Both integrated E/D-HEMTs with gate-length of 0.2 mum demonstrated excellent DC and RF characteristics