Z. F. Guan

Low resistance ohmic contacts on AlGaN/GaN structures using implantation and the “advancing” Al/Ti metallization

The ohmic contact formation of Al/Ti on AlGaN/GaN heterostructure field effect transistors (HFETs) with and without Si implantation was investigated. Direct implantation and implantation through an AlN capping layer were studied. Compared to implantation through AlN, direct implantation is more effective in reducing the contact resistance. An Al(200 A)/Ti(1500 A) bilayer structure, called the “advancing” metallization, was used in this investigation to take advantage of consuming nearly all the top AlGaN layers for easy carrier access to the GaN layer underneath. Combining the direct implantation and the advancing metallization, low contact resistance of the order of 0.25 Ω mm (∼5.6×10−6 Ω cm2) can be readily obtained on HFET structures with an AlGaN layer about 340 A thick and with an Al fraction of at least 22%.

DIRECT MEASUREMENT OF THE REFRACTIVE INDEX CHANGE OF SILICON WITH OPTICALLY INJECTED CARRIERS

Silicon phase modulators are important silicon optoelectronic devices. The relationship of the refractive index change with injected carrier concentration is the basis for the operation of silicon phase modulators. No direct experimental data of this relationship have been reported. We have developed a new method for the direct measurement of this relationship using a Fabry–Perot interference and optical injection of carriers. The experimental results of the refractive index change are reported for the first time in the range of injected carrier concentration between 1013 and 1015 cm−3. It should be noted that our experiment results are about 5 to 10 times larger than those predicted by theory. The reliability of our experiments is also discussed.

Photoelastic waveguides and the controlled introduction of strain in III‐V semiconductors by means of thin film technology

We have investigated the use of thin film technology to introduce controllable and thermally stable stress into semiconductor heterostructures. Two simple schemes are used. The first scheme is to use interfacial reactions between a metal and the substrate, such as Ni, Co, Pd, and Pt on GaAs/AlGaAs. The induced stress in the structure is reproducible and controllable because the volumetric change for a given reaction is fixed, as long as the deposited film is fully reacted to form a compound. The stability of the stress depends on the stability of the compound. In the case of Ni and Co on GaAs/AlGaAs, the induced stress is thermally stable up to 600 °C. Evaporated films and reacted films are usually under tension. The second scheme is to use rf sputtered W or WNi alloy films where W or WNi is sputtered onto a negative dc biased substrate. This scheme effectively provides highly compressed films. The thermal stability depends on the concentration of Ni in the WNi alloy. Using the two schemes above, we have ...

Structural investigation of self-aligned silicidation on separation by implantation oxygen

Self-aligned silicidation is a well-known process to reduce the source, drain, and gate parasitic resistances of submicron metal-oxide-semiconductor devices. This process is particularly useful for devices built on very thin Si layer (∼1000 A or less) on insulators. Since the amount of Si available for silicidation is limited by the thickness of the Si layer, once the Si in the source and drain region is fully consumed during silicidation, excessive silicide formation could lead to void formation near the silicide/silicon interface beneath the oxide edge. In this article, we study the effects of different metals (Ti, Ni, Co, and Co/Ti bilayer) with varying thickness on the formation of voids. A change in the moving species during lateral silicide formation was found to be the likely cause for the voids, even if the metals are the moving species during silicidation in the thin film case.

Silicon on insulator photoelastic optical waveguide and polarizer

Single‐mode silicon on insulator photoelastic waveguides and polarizers have been investigated. It was found that waveguides induced by the photoelastic effect can be low loss (1.5 dB/cm at 1.53 μm) and can serve as polarizers as well. A polarization distinction ratio of 14 dB for both TE and TM mode has been obtained.

Photoelastic waveguides formed by interfacial reactions

The fabrication of low‐loss photoelastic waveguides in GaAs/AlGaAs layered structures by thin film reactions is investigated. The waveguides are formed by opening a narrow window stripe, a few microns wide, in an otherwise continuous Ni layer under tension deposited on a semiconductor structure. The local tensile stress induced by the Ni layer in the semiconductor causes the local refractive index to increase, thus providing the guiding mechanism. Annealing the sample at 250 °C for 1 h induced an interfacial reaction between the Ni film and the substrate to form Ni3GaAs. The formation of an interfacial compound stabilizes the stresses, making the stress state independent of the deposition system and technique. Single‐mode waveguide propagation losses as low as 1.4 dB/cm at 1.53 μm wavelength have been obtained on annealed waveguides. Further annealing up to 600 °C did not cause degradation in the optical confinement, thus indicating a thermally stable planar waveguide fabricated by this process. Other pho...

Simultaneous disordering and isolation induced by ion mixing in InGaAs/InP superlattice structures

The phenomenon of simultaneous compositional disordering and the formation of electrical resistive layers induced by oxygen implantation in InGaAs/InP superlattices has been investigated. The disordering characteristics have been studied as a function of implantation temperature and ion dose. It was found that implantation at elevated temperatures (referred to as the IM or ion mixing process) usually leads to much more efficient disordering compared to implantation at room temperature followed by annealing at the same elevated temperature (referred to as the implantation plus annealing process). Of particular interest is the observation that ion mixing at 550 °C with 1×1013 O+/cm2 leads to significantly more disordering than implantation with the same dose at room temperature followed by annealing at 550 °C for the same period of ion mixing time. In addition, the electrical resistance of the ion‐mixed layer at 550 °C increases 2600 times for the p‐type InGaAs/InP superlattice structure, whereas the sample...

Planar, low‐loss optical waveguides fabricated by solid‐phase regrowth

Planar, low‐loss AlGaAs/GaAs waveguides have been fabricated using the solid‐phase regrowth (SPR) process. Single‐mode waveguide with a propagation loss as low as 1.6 dB/cm have been obtained. This process requires only thin‐film deposition and low‐temperature short‐duration annealing (i.e., 650 °C for 30 s), thus making the SPR method a much simplified technique to induce compositional disordering. Simultaneous electrical isolation and compositional disordering are also demonstrated with the SPR process.

Compositional disordering by solid phase regrowth

The principle of solid phase regrowth (SPR)has been used to induce compositional disordering in AlGaAs/GaAs superlattice structures in the temperature range of 400 °C (30 min)–650 °C (30 s) as compared to the conventional diffusion method in the temperature range of 600–850 °C for hours. The SPR process is simple to implement, requiring only thin‐film deposition and annealing. The crystal quality as well as the photoluminescence signals emerging from the disordered region generally improve with increasing processing temperature. The simplicity, the low process temperature, and the short process duration of the SPR technique are distinct advantages for optoelectronic applications, especially for self‐aligned devices.

Planar 1.3 and 1.55 μm InGaAs(P)/InP electroabsorption waveguide modulators using oxygen ion mixing and the photoelastic effect

Efficient 1.3 and 1.55 μm InP‐based electroabsorption waveguide modulators with planar device structures have been demonstrated. Elevated temperature oxygen ion implantation and/or the photoelastic effect induced by W metal stressor stripes deposited on the semiconductor surface have been used to produce these self‐aligned planar guided‐wave devices. The oxygen ion mixing process has been used to simultaneously achieve compositional disordering and electrical isolation of superlattice material while the photoelastic effect has been used to improve the lateral mode confinement. A 1.3 μm Franz–Keldysh modulator with a ≳10 dB extinction ratio at 2 V and a 1.55 μm device with a ≳10 dB extinction ratio at 7 V are reported. These single growth step planar processing techniques have also been used to fabricate relatively low‐loss (<4 dB/cm) double heterostructure InGaAs(P)/InP single‐mode optical waveguides which demonstrate their usefulness in developing InP‐based photonic integrated circuits.