Issues on the molecular-beam epitaxial growth of p-SiGe inverted-modulation-doped structures

Abstract

The influence of boron segregation and silicon cap-layer thickness on two-dimensional hole gases ͑2-DHGs͒ has been investigated in Si/Si 0.8 Ge 0.2 /Si inverted-modulation-doped heterostructures grown by solid-source molecular-beam epitaxy. Boron segregation, which is significant in structures with small spacer layers, can be suppressed by growth interruption after the boron doping. How growth interruption affected the electrical properties of the 2-DHG and the boron doping profile as measured by secondary ion mass spectroscopy are reported. We report also on the role played by the unpassivated silicon cap, and compare carrier transport at the normal and inverted interfaces.

Top-gating of p-Si/SiGe/Si inverted modulation-doped structures

“…The maximum hole sheet densities measured for these structures were higher than expected, but can be explained by a reduction of spacer layer thickness due to boron segregation effect. 5 The slopes of linear changes of the hole sheet density with gate voltage for these structures are in agreement with ⑀⑀ r /eL relationship seen in all gated structures, where, ⑀ and ⑀ r are respectively the permittivity of free space and dielectric constant of Si, and L is the distance between the metal gate and 2-DHG, and finally e is the electron charge.…”

“…5,6 Structures with a controllable hole density in the channel are useful in understanding the scattering mechanisms limiting mobility of holes confined near to normal ͑Si on SiGe͒ and inverted ͑SiGe on Si͒ interfaces of the Si/SiGe/Si quantum wells, so gating these structures allows one to change the hole sheet density systematically in a single device. MOS gating of undoped Si/SiGe/Si structures 4 and back-gating of normal MD structures 7,8 has been reported to study the transport properties of holes confined at the normal interface.…”

“…5 These structures consisted of the following layer sequence: 200 nm Si buffer, 30 nm Si boron-doped at 2ϫ10…”

2-D hole gas with two-subband occupation in a strained Ge channel: Scattering mechanisms

“…In this respect, modulation doped structures with the supply doping below the channel are very challenging, especially when low temperature mobility is to be optimized. In this case, segregation inevitably brings scatterers into the channel, impairing mobility [17]. Low temperature transport therefore is a very sensitive test for a growth method.…”

Wave function-dependent mobility and suppression of interface roughness scattering in a strained SiGe p-channel field-effect structure

“…1. The structure, intended for low temperature measurements only, was grown by solid-source molecular beam epitaxy 5 on a low-doped (n-type͒ Si͑100͒ substrate. The growth sequence consisted of a 200-nm-thick Si buffer layer, followed by a 30 nm Si:B doped layer (2 ϫ10 18 cm Ϫ3 ), a 20 nm Si spacer, a 20 nm coherently strained Si 0.8 Ge 0.2 layer, and finally a 180 nm Si capping layer.…”