GaAs-on-Si: Improved MBE Growth Conditions, Properties of Undoped GaAs, High 2DEG Mobility, and Fabrication of High Performance AlGaAs/GaAs SDHT's and Ring Oscillators.

We report improved growth conditions by molecular beam epitaxy (MBE) and fabrication of state-of-the-art AlGaAs/GaAs selectively doped heterostructure transistors (SDHT's) and ring oscillators on Si substrates. In MBE growth, use of minimum As sub 4 Ga flux ratio during initial nucleation combined with in-situ thermal cycles gave a marked improvement in material quality. With this method, FWHM of x-ray rocking curves was measured as low as 135 arc s for a 3.5microns thick GaAs layer on Si. The method also improved the forward and reverse current voltage characteristics of the Schottky diodes making them as good as on GaAs substrates. Although 3microns thick undoped GaAs buffer layers on p-type Si substrates were highly resistive and were fully depleted under a Schottky contact, a parallel n-type conduction path confined in a thin region (0.1micron) near the GaAs/Si interface was sometimes observed whose sheet density (10 sup (12) - 10 sup (13) cm sup (-2)) and mobility (600-900 cm 2 V sup (-1) s sup (-1)) were independent of temperature between 300K and 77K. This parallel conduction did not affect the dc behavior and switching speed of SDHT's, and it was successfully prevented by doping 0.1micron GaAs with 5-10 x 10 sup (16) cm sup (-3) Be atoms near the interface. For a 2DEG sheet density of 10 sup (12) cm sup (-2), a mobility as high as 53,500 cm sup 2 V sup (-1) s sup (-1) at 77K was obtained, as against a mobility of ~ 70,000 cm sup 2 V sup (-1) s sup (-1) for a similar structure on GaAs substrates. For 1micorn-gate-length devices, maximum transconductances of 220 and 365 mS/mm were measured at 300K and 77K, respectively. A minimum propagation delay time, tau sub d, of 28 ps/stage was measured at 300K for direct coupled FET logic (DCFL) ring oscillators for 1. 1 mW/stage power dissipation. tau sub d decreased to 17.6 ps/stage at 77K. From microwave S-parameter measurements at 300K, current gain and power gain cutoff frequencies of 15 GHz and 22 GHz, respectively, were measured. These results are comparable to that of SDHT technology on GaAs substrates.