| Electron tunneling spectroscopy was applied toward Si/SiGe resonant interband tunnel diodes (RITDs) to ascertain the phonon spectra of the composite tunneling barrier which consists of Si1- xGex and Si layering. Two issues were identified: the error in the second-order derivative which would reduce the measurement sensitivity, and the contact resistance which would affect phonon energy measurement. A harmonic detection system was used to measure the first-order derivative with an accuracy of 10-4, followed by first-order numerical differentiation to obtain high quality second-order derivative. Contacts to silicon using nickel silicide (NiSi) and delta doping technique were studied, and preliminary results indicate that the contact resistance was very low. With typical Si/SiGe RITD configurations, the phonon spectra were found to be dominated by the phonon peaks of bulk Si. Of the Si/SiGe RITD structures that were measured, only those with a relatively thick Si1-xGex layer in the composite tunneling barrier and a high Ge content x = 0:6 produced phonon spectra in which the transverse acoustic (TA) phonon of the Si0.4Ge0.6 were observable, but the longitudinal acoustic (LA), longitudinal optical (LO) and transverse optical (TO) phonons of Si1-xGex were not observed. The energy of the TA phonon of Si0.4Ge 0.6 in Si/SiGe RITDs grown on Si substrates was determined to be about 4+/-1 meV higher than that measured using Esaki tunnel diodes. This increase is attributed to the compressive strain in the Si0.4Ge0.6 layer in the Si/SiGe RITDs. |
