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 Å)/Ti(1500 Å) 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 Å thick and with an Al fraction of at least 22%.

No abstract

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 inter-facial 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 r-f 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 fabricated low-loss (-1 dB/cm at 1.52 pm wavelength) photoelastic waveguides in GaAs/AlGaAs heterostructures, and explored the interrelationship between the photoelastic waveguide characteristics and the stress. 0 1995 American Institute of Physics.

The technique of transferring patterned ion-cut layers from one Si wafer to another was demonstrated. The starting silicon wafer was masked with checkerboard and line patterns with a 3 m thick polymethylmethacrylate/photoresist and was implanted with 5ϫ10 16 H ϩ ions/cm 2 at 150 keV. After stripping off the mask, the wafer was bonded to an oxide-coated receptor wafer through low-temperature direct wafer bonding. Heat treatment of this bonded pair showed that the hydrogen-induced silicon surface layer cleavage ͑ion cut͒ could propagate throughout about 16 mϫ16 m of nonimplanted material with implanted regions only 4 m wide. Mask width, spacing, and implantation profiles through the mask shape were shown to have effects on the internal microfracturing mechanisms. © 1998 American Institute of Physics. ͓S0003-6951͑98͒00645-7͔Three-dimensional electronic device integration offers significant opportunities for future system improvement in microprocessors and memories. 1,2 This prospect might be implemented with the hydrogen-induced silicon layer cleavage process, which has already been reported for capacitor patterns ͑passive devices͒. 3 To cleave the implanted layer, a minimum dose of a few times 10 16 /cm 2 of implanted hydrogen is needed. 4 This large dose of hydrogen most likely will damage the devices fabricated on the silicon prior to the ion-cut process. In this study, we introduce a patterned ioncut process in which active regions of the wafer are protected from the hydrogen implantation.In this study, Czochralski-grown, ͑100͒, n-type ( ϭ5 -50 ⍀ cm), 100 mm silicon wafers were used. The Si donor wafer was coated with a layer of KTI 950K 9% polymethylmethacrylate ͑PMMA͒ and a layer of Shipley 1400-30 photoresist with a total thickness of 3 m, followed by patterning of various sizes of squares and lines for the implantation mask with different openings for the hydrogen implantation ͑see Fig. 1͒. This patterned wafer was then implanted with H ϩ ions at 150 keV with a dose of 5ϫ10 16 cm Ϫ2 . Dur-ing implantation, the wafer was kept at ambient temperature. A 3 m thick ion mask layer ͑including PMMA and the photoresist͒ was applied to prevent the hydrogen ions from reaching the silicon wafer surface, resulting in hydrogen-ion implantation only in the openings. After the implantation, the ion mask was removed by oxygen plasma ashing. To determine the cracking temperature, the patterns were annealed at temperatures from 400 to 600°C in forming gas for 5 min after the removal of the ion mask. It was found that blistering occurred between 500 and 550°C. It was clear under the microscope that all the blisters were confined to the implanted regions. This observation confirms the effectiveness of the implant mask for the protected regions.On the receptor wafer, a layer of thermal oxide 200 nm thick was grown. The two wafers were bonded directly faceto-face at room temperature or at slightly elevated temperature after standard RCA cleaning of the implanted wafer. The bonded pair was then heated in a rapid thermal annealer un...

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 photoelastic optical devices such as polarizers, splitters, and couplers are also demonstrated.