Germanium CMOS potential from material and process perspectives: Be more positive about germanium

Abstract

CMOS miniaturization is now approaching the sub-10 nm level, and further downscaling is expected. This size scaling will end sooner or later, however, because the typical size is approaching the atomic distance level in crystalline Si. In addition, it is said that electron transport in FETs is ballistic or nearly ballistic, which means that the injection velocity at the virtual source is a physical parameter relevant for estimating the driving current. Channel-materials with higher carrier mobility than Si are nonetheless needed, and the carrier mobility in the channels is a parameter important with regard to increasing the injection velocity. Although the density of states in the channel has not been discussed often, it too is relevant for estimating the channel current. Both the mobility and the density of states are in principle related to the effective mass of the carrier. From this device physics viewpoint, we expect germanium (Ge) CMOS to be promising for scaling beyond the Si CMOS limit because the bulk mobility values of electrons and holes in Ge are much higher than those of electrons and holes in Si, and the electron effective mass in Ge is not much less than that in III-V compounds. There is a debate that Ge should be used for p-MOSFETs and III-V compounds for n-MOSFETs, but considering that the variability or nonuniformity of the FET performance in today's CMOS LSIs is a big challenge, it seems that much more attention should be paid to the simplicity of the material design and of the processing steps. Nevertheless, Ge faces a number of challenges even in case that only the FET level is concerned. One of the big problems with Ge CMOS technology has been its poor performance in n-MOSFETs. While the hole mobility in p-FETs has been improved, the electron mobility in the inversion layer of Ge FETs remains a serious concern. If this is due to the inherent properties of Ge, only p-MOSFETs might be used for device applications. To make Ge CMOS devices practically viable, we need to understand why electron mobility is severely degraded in the inversion layer in Ge n-channel MOSFETs and to find out how it can be increased. In the Si CMOS technology, the SiO 2 /Si interface has long been investigated and cannot be ignored even after the introduction of high-k gate stack technology. In that sense, the GeO 2 /Ge interface should be intensively studied to make the best of Ge's advantages. Therefore we first discuss the GeO 2 /Ge interface with regard to its physical and electrical characteristics. When we regard Ge as a channel material beyond Si for high performance ULSIs, we also have to seriously consider the gate stack scalability and reliability. The source/ drain engineering, as well as the gate stack formation, is another challenge in Ge MOSFET design. Both the higher metal/Ge contact resistance and the larger p/n junction leakage current may be the consequences of Ge's intrinsic properties because they are derived from the strong Fermilevel pinning and the narrow energy band gap, respectively. Even if the...

Improving carrier mobility of polycrystalline Ge by Sn doping

“…Ge has attracted attention as the most promising candidate for next-generation material because it has a higher carrier mobility than Si for both electrons and holes and is compatible with conventional Si processing 1 – 3 . Effective mobilities in Ge metal-oxide-semiconductor field-effect transistors (MOSFETs) have exceeded those in Si-MOSFETs because of the development of device technologies including gate stacks 4 – 8 . In addition, Ge has a lower crystallization temperature and grain-boundary potential than Si 9 – 12 .…”

“…By using these techniques, Ge-TFTs have been fabricated on thermally oxidized Si 21 , 26 , 27 , glass 28 – 31 , and even flexible substrates 22 , 32 . Since gate stack technology for Ge has developed sufficiently 8 , recent Ge-TFTs performance is limited by the properties of the poly-Ge thin film itself 21 , 22 , 26 – 32 . Some of these TFTs exhibited effective hole mobilities greater than 100 cm 2 /V s 22 , 26 , 28 .…”

Sb-doped crystallization of densified precursor for n-type polycrystalline Ge on an insulator with high carrier mobility

“…Ge complementary metal-oxide-semiconductors (CMOSs) are expected to be promising for scaling beyond the Si-CMOS limit because Ge has a higher carrier mobility than Si for both electrons and holes and is compatible with conventional Si processing. [1][2][3][4] For both p and n channels, effective mobilities in Ge MOS field-effect transistors (MOSFETs) have exceeded those in Si-MOSFETs because of the development of device technologies including gate stacks. [4][5][6][7][8] The most promising usage of such high-performance Ge-CMOS is to integrate it into Si large-scale integrated circuits (LSIs) or flat-panel displays.…”

“…[1][2][3][4] For both p and n channels, effective mobilities in Ge MOS field-effect transistors (MOSFETs) have exceeded those in Si-MOSFETs because of the development of device technologies including gate stacks. [4][5][6][7][8] The most promising usage of such high-performance Ge-CMOS is to integrate it into Si large-scale integrated circuits (LSIs) or flat-panel displays. To achieve this, low-temperature Ge-on-insulator (GOI) technology has been developed, including solid-phase crystallization (SPC), [9][10][11][12][13] laser annealing, [14][15][16][17][18] chemical vapor deposition, 19,20 flash-lamp annealing, 21 the seed layer technique, 22 and metal-induced crystallization.…”

Germanium photovoltaic cells with MoOx hole-selective contacts

“…To our knowledge, this is the first demonstration of a hole-selective contact made with a TMO on an n-type semiconductor different than c-Si. Thus, it might open the door to new device architectures, not only for PV applications, but also in photonics and CMOS electronics, where the integration of TMOs is being investigated (Sanchez et al 2016), along with the use of different semiconductors having higher carrier mobilities and extended spectral response than c-Si, such as Ge (Reboud et al 2017;Toriumi and Nishimura 2017).…”