ICMCTF2003 Session H2-1: Thin Films and Processes for Next Generation Devices

Future high-speed receiver and logic applications that require lightweight power supplies, long battery lifetimes, improved efficiency, or high component density will require transistors that consume less power. Military and commercial applications that would significantly benefit from reduced power consumption include space-based communications, imaging, sensing, high data rate transmission, microair-vehicles, wireless applications and other portable systems. The use of Sb-based transistors in lownoise high-frequency amplifiers, digital circuits, and mixed-signal circuits will provide the enabling technology needed to address these rapidly expanding needs.

AlSb/InAs HEMTs have intrinsic advantages for some of these high-speed, low-power-consumption applications due to the attractive material properties of this heterojunction material system that include high values of mobility, channel conductivity, and peak electron velocity at low electric field. Intrinsic fT values of 250 GHz have been obtained at V_DS = 600 mV and an fT of 90 GHz has been measured at VDS = 100 mV¹. HEMTs with a TiW/Au gate metallization and a reduced 50Å spacer layer above the channel exhibit a maximum transconductance at VDS = 550 mV of 1.4 S/mm. HEMTs with the addition of a stable AlGaSb buffer layer were also recently fabricated and characterized². The AlGaSb insert enables a shallow mesa isolation etch and provides a method of device formation that is completely compatible with existing space-flight qualified production MMIC processes.

Heterojunction bipolar transistors using In_zGa_1-zSb for the base and In_xAl_1-xA_sySb_1-y alloys for the collector and emitter are also being explored. The InGaSb base is attractive due to its narrow bandgap, its good hole transport characteristics, and its ease in forming low-resistance ohmic contacts. Equally important is the fact that the InGaSb base can be used with a collector and an emitter made from a variety of InAlAsSb alloys enabling a wide range of heterojunction design flexibility. Modeling of the DC current-voltage characteristics indicate that current gain in excess of 500 may be obtained. Good diode characteristics with an ideality factor of 1.1 have recently been obtained for InAlAsSb p-n junctions grown at a lattice constant of 6.2Å using Te for the n-type dopant and Be for the p-type dopant³. Lowresistance Pd/W/Au ohmic contacts to the p-InGaSb base material which are shallow and thermally stable have also recently been demonstrated⁴. In this talk, the current status of the design, fabrication, and characterization of Sb-based HEMTs (6.05 Å) and HBTs (6.2 Å) in our group will be presented including emphasis on the Schottky and ohmic contact methodologies employed to achieve shallow contacts and improved thermal stability.

¹ J. B. Boos et al., J. Vac. Sci. Technol. B, 17 (3), 1022-1027, (1999).

² R. Tsai et al., Proc. of the 2002 Lester Eastman Conference on High Performance Devices

³ R. Magno et al., Proc. of the 2002 Lester Eastman Conference on High Performance Devices

⁴ S. H. Wang et al., submitted for publication in J. Vac. Sci. Technol. This work was supported by the Defense Advanced Research Projects Agency and the Office of Naval Research.

As devices become scaled to ever smaller dimensions, parasitic components of larger versions of devices start to not only become important to performance but can actually dominate performance especially at high current densities. Of the many parasitic components, none is more intriguing and perplexing than excessive ohmic contact resistance. A major reason for this is there is no agreed-on theory that adequately explains the electronic behavior at metal/semiconductor interfaces.

Therefore, the technical community has had to rely on "empirical engineering" to develop suitable enabling contact technologies. This talk will review the current status of ohmic contact technology for commercially important compound semiconductors and present new enablers that yield adequate contact resistances.

Thin films, from novel B-site substituted barium strontium titanate (BSTO) bulk targets, have been deposited using the pulsed laser deposition (PLD) technique. The measured electrical properties of these thin films will be compared with the electrical properties of the bulk materials. In these materials, an applied electric-field can be used to change the dielectric constant of the material, hence, the capacitance or phase velocity in RF/microwave devices can be tuned in real time for a particular application. The microstructure of the film influences the electronic properties which in turn influence the performance of the film.

Selected charge-balanced binary substitutions, using A³⁺ = Y³⁺ and Sc³⁺ and A⁵⁺ = Ta⁵⁺ were made for Ti⁺⁴ to obtain Ba_{0.60√sub 0.40}Sc_0.05Ta_0.05Ti_0.90O₃ and Ba_{0.60√sub 0.40}Y_0.05Ta_0.05Ti_0.90O₃ targets. Thin films were synthesized on MgO (100) substrates, at substrate temperatures ranging from 400⁰C to 600⁰C and oxygen partial pressure ranging from 30 to 50 mTorr, at 500 mJ laser energy and 10 pulses per second using the pulsed laser deposition technique. The thickness of the thin films varied from 1.5 to 3.0 µm. Shallow glazing angle x-ray diffraction (GAXRD) studies of the synthesized thin films have revealed a preferred orientation. The effect of the ionic substitution and substrate temperature in conjunction with the effect of the oxygen partial pressure on the microstructure, and mechanical and electrical properties of the thin films were studied using x-ray diffraction, SEM, AFM, Rutherford backscattering, nanoindentation and capacitance measurements and will be discussed in detail.