Huang-Ming Lee

New nanometer T-gate fabricated by thermally reflowed resist technique

Novel nanometer T-gate process has been developed utilizing electron beam (EB) lithography and thermally reflowed resist technique. Through well-controlled EB exposure dosage, heating time and reflow temperature, the resist structures can be efficiently reflowed to form the desired T-gate configuration with dimension ranging from 150 nm to 30 nm. After Ti/Pt/Au metal deposition by electron gun evaporation and lift-off process, the nanometer T-gates with thickness of about 500 nm were formed. With the optimized conditions, ultra-short 30 nm T-shaped gate was clearly demonstrated on the GaAs substrate. This is the smallest T-gate reported with the thermally reflowed technique in the literature so far and can practically be used in the GaAs monolithic microwave integrated circuit (MMIC) fabrications.

Electronic transport characteristics in a one-dimensional constriction defined by a triple-gate structure

We investigate the electronic transport characteristics of a one-dimensional (1D) narrow constriction defined in a GaAs∕AlxGa1−xAs heterostructure by a simple triple-gate structure consisting of a pair of split gates and an additional surface Schottky gate (center gate) between them. Comparison between devices with and without a center gate reveals that the center gate, even when zero biased (VCG=0V), significantly modifies the surface potential and facilitates the 1D confinement in a deep two-dimensional electron system. The pinch-off voltages at VCG=0V for various channel widths W (=0.4–0.8μm) and lengths L (=0.2–2μm) are well described by the analytical formula based on the pinned-surface model [J. H. Davies et al., J. Appl. Phys. 77, 4504 (1995)]. Nonlinear transport spectroscopy with an additional dc bias shows that the lowest 1D subband energy separation (ΔE1,2) changes linearly with VCG and can be enhanced by 70% for VCG=0.8V. A simple model assuming an infinitely long channel and no self-consisten...

Composite-Channel Metamorphic High Electron Mobility Transistor for Low-Noise and High-Linearity Applications

A metamorphic high-electron-mobility transistor (MHEMT) with In0.55Ga0.45As/In0.67Ga0.33As/In0.55Ga0.45As composite channel layers was developed for low-noise and high-linearity applications. The use of a composite channel results in high electron mobility and good confinement of electrons in the channel region which are the desired characteristics of low-noise and high-linearity devices. The noise figure of the 0.25 ×160 µm2 devices is 0.23 dB with 15.06 dB associated gain, and the output third-order intercept point (IP3) is 18.67 dBm at 6 GHz. The device shows great potential for high-linearity and low-noise applications at high frequencies.

InAlAs/InGaAs Metamorphic High Electron Mobility Transistor with Cu/Pt/Ti Gate and Cu Airbridges

The use of a Cu/Pt/Ti Schottky contact structure and Cu-based airbridges for high-frequency metamorphic high electron mobility transistor (MHEMT) is successfully developed. The material characteristics of the Cu/Pt/Ti Schottky contact on iInAlAs were studied. Judging from the results of the X-ray diffraction analysis, Auger electron spectroscopy, and transmission electron microscopy, the Cu/Pt/Ti Schottky contact structure on InAlAs was very stable after annealing at 350 � C. However, after 400 � C annealing, the reaction of copper with the layers underneath started to occur and formed the Cu4Ti phase. The Cumetallized MHEMT using the proposed Cu/Pt/Ti T-gate structure and Cu-based airbridges has a saturated drain current of 673 mA/mm and a maximum transconductance of 750 mS/mm. The gate to drain breakdown voltage measured was 14.5 V at a gate reverse current of � 1 mA/mm. The device also demonstrated a cutoff frequency Ft of 90 GHz and a maximum frequency of oscillation Fmax of 165 GHz. An MHEMT with a Au/Pt/Ti gate was fabricated and compared with an MHEMT fabricated with the proposed Cu/Pt/Ti gate. These two kinds of MHEMTs showed similar Ft and Fmax. These results demonstrate that the Cu/Pt/Ti T-gate and Cu-based airbridges can be used for MHEMT fabrication with excellent electrical characteristics. [DOI: 10.1143/JJAP.46.2848]

Flower-Like Distributed Self-Organized Ge Dots on Patterned Si (001) Substrates

Self-organized Ge dots were obtained utilizing ultra high vacuum chemical molecular epitaxial growth of Ge on electron beam lithographically patterned Si (001) substrates. The dimensions of these etched Si mesa are 65/23/200 nm in diameter/height/period. The sizes and arrangement of the Ge dots were characterized by scanning electron microscopy and atomic force microscopy. The Ge dots have an average base width of 10 nm and the size is quite uniform. Due to the energetically favorable sites, the Ge dots tend to form homocentrically along the Si mesa edge, and their distribution is flower-like.

Controlled placement of self-organized Ge dots on patterned Si (001) surfaces

The positioning and ordering of the self-organized Ge dots were controlled using the ultra high-vacuum chemical molecular epitaxial growth of Ge on electron beam lithographically patterned Si (001) substrates. The experimental results indicate that the edge smoothness of the etched Si holes affects the distribution and long-range ordering of the Ge dots formed. With optimized etching conditions, the Ge dots can be grown uniformly on the edges of the holes etched on the patterned Si substrates. The sizes of the Ge dots are approximately 10 nm. The proposed technique can be applied to the fabrication of regimented arrays in signal processing applications.

Mechanism of Microstructure Evolution for the Cu/Ta/GaAs Structure after Thermal Annealing

The diffusion behavior and microstructure evolution of Cu/Ta/GaAs multilayers after thermal annealing are investigated and the mechanism is proposed. A thin 30 nm tantalum layer was sputtered as a diffusion barrier to block Ga and As diffusion into the Cu layer. From the results of sheet resistance measurement, X-ray diffraction analysis, Auger electron spectroscopy and transmission electron microscopy, the Cu/Ta films on GaAs were found to be very stable up to 500 °C without Cu migration into GaAs. After annealing at 550 °C, the interfacial mixing of Ta with GaAs substrate occurred, resulting in the formation of TaAs2, and the diffusion of Ga through the Ta layer formed the Cu3Ga phase at the Cu/Ta interface. After annealing at 600 °C, the reaction of GaAs with Ta and Cu formed TaAs and Cu3Ga owing to Ga migration and interfacial instability.

A low noise composite-channel metamorphic HEMT for wireless communication application

A composite-channel metamorphic high electron mobility transistor (MHEMT) was developed for low noise high linearity application. The MHEMT was grown by molecular beam epitaxy (MBE) on GaAs substrates with InAlAs graded buffer. The composite-channel layers in the MHEMT include a top In/sup 0.55/Ga/sup 0.45/As layer, a middle In/sup 0.67/Ga/sup 0.33/As layer, and a bottom In/sup 0.55/Ga/sup 0.45/As layer. The design of this structure provides better electron confinement in the channel with less impact ionization as compared to conventional dual delta doped MHEMTs. This results in devices with higher linearity and drain to gate voltage as compared to the conventional metamorphic HEMTs. The 0.25/spl times/160/spl mu/m/sub 2/ device with the novel channel structure exhibits a maximum frequency of oscillation f/sup max/ of 290 GHz and a current gain cut-off frequency f/sup t/ of 110 GHz. The noise figure of the device at 6 GHz is 0.23db and an associated gain was 15.06dB. The IP3 of the device at 6 GHz is 18.67dBm. The composite channel metamorphic HEMT shows great potential for high linearity and low noise application at high frequencies.

A metamorphic high electron-mobility transistor with reflowed submicron T-gate for high-speed optoelectronics applications

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.