Paper EM+OX-WeA11
High-Electron-Mobility SiGe on Sapphire Substrate for Next Generation Ultrafast Chipsets

Wednesday, October 31, 2012, 5:20 pm, Room 009

In the conventional silicon-on-sapphire (SOS) technology with the epitaxy of Si on r-plane (1-102) sapphire, typical device region do not need reverse bias between the substrate and device area for electrical separation, because SOS wafer is separated by the sapphire itself, a best insulator. The advantage is that the sapphire insulator is very thick, which engenders an ultra-small capacitance and therefore it can reduce parasitic capacitance and leakage current at a high operating frequency. However, SOS wafer has a limitation in carrier mobilities due to the silicon material. The mobilities of SiGe can be a few times higher than those of silicon due to the high carrier mobilities of germanium (p-type Si: 430 cm²/V·s, p-type Ge: 2200 cm²/V·s, n-type Si: 1300 cm²/V·s, n-type Ge: 3000 cm²/V·s at 10¹⁶ per cm³ doping density). Therefore, RF devices which are made with rhombohedral SiGe on c-plane sapphire can potentially run a few times faster than RF devices on SOS wafers.

NASA Langley’s rhombohedral epitaxy uses an atomic alignment of the [111] direction of cubic SiGe on top of the [0001] direction of the sapphire basal plane (c-plane). It shows a sample of rhombohedrally grown SiGe on c-plane sapphire with a single crystalline percentage of 95%. Twin defects exist only at the edge of the wafer. The electron mobilities of the tested samples are between those of single crystal Si and Ge. For instance, the electron mobility of 95% single crystal SiGe is 1538 cm²/V·s which is between 350 cm²/V·s (Si) and 1550 cm²/V·s (Ge) at 6x10¹⁷ /cm³ doping concentration. Typically, a rhombohedral single crystal SiGe has 2 or 3 times higher carrier mobility than monocrystalline silicon. If the defects in SiGe can be removed, transistors with higher operational frequencies can be fabricated for a new generation of ultrafast chipsets.