第十二章、材料表征与分析

strained indivitual dots, relaxed dots in a buried layer
/douglas.paul/SiGe/split.html
Henry Radamson
6
GeSn alloy and its application
Intravalley scattering when the final position of the electron in E-K space are in the same valley otherwise intervalley scattering occurs when the electron’s final position is in different valleys. Alloy scattering: periodic potential is distorted by alloying Deformation potential scattering: when the acoustic phonons changes the position of lattice atom leading to conduction and valence band edge is varied. Pizzo electric scattering: If the atoms in the crystal can be ionized by acoustic phonons Carrier-carrier scattering: when the electron collide with each other and is significant ND > 1017 cm-3
Si (tensile) on relaxedSiGe
SiC (tensile) on Si
Electrons in conduction band of unstrained-Si
Strained-Si has electrons Δ2 at minimum conduction band edge
10
Henry Radamson
Formation of biaxial and uniaxial strain in Si
Biaxial strain
1) Growth of Si1-xGex or SiC
2) Formation of gate stack
2) Implantation B or P to form S/D
Uniaxial strain
1) Formation of gate stack
2) Etch of Si to form recess
3) Selective growth to form S/D
Henry Radamson
11
Inducing uniaxial strain
Recessed junctions (dry-etched) were filled by B-doped SiGe layers (selective epitaxy). This creates a compressive strain in Si channel layer.
Ge
Strain compensated
Strain relaxed
Ge1-xSnx or Si1-xGex layer
Si
Tensil Strain
Si Substrate C
SiyC1-y layer
Henry Radamson
4
Bandstructure of Si and Ge
Si
Henry Radamson
Always hole has lower mobility than electron (this is a problem in CMOS design)
Henry Radamson 14
Carrier mobility in strained-Si
The calculations show the highest mobility will be obtained along
Ge
5
Ge has nearly direct bandgap of 0.85 eV
Strain and Bandgap Engineering
Bandgap of alloys is determined from active component: 1. alloying 2. Strain (consists of two components : hydrostatic, uniaxial) Types: Compressive & Tensile (Ge/GeSn & Ge/GeSi) Designs; Biaxial & Uniaxial (or two- & one-dim strain)
Si
SiGe
Henry Radamson
9
Conduction band split (tensile Biaxial strain)
Conduction band of Si contains Δ4 and Δ2 electrons.
Tensile strain shifts Δ2 electrons downwards (these are light electrons). Two ways have been used to generate tensile strain:
Henry Radamson
12
Inducing stress of > 1 GPa in pMOSFETs
Tunning SiN stress from highly tensile to compressive stress in LPCVD and PECVD. The mechanical property of nitride layers is determined by controlling the gas phase dissociation of Silane, Amonia and gases in a plasma environment These nitride layers can be grown on embedded SiGe layers. The induced stress is additive to the induced SiGe stress.
Bandgap and mobility in group IV material systems
by Henry H. Radamson
Department of Microelectronics and Information Technology KTH, Royal Institute of Technology Henry Radamson 1
Mismatch of Si and SiGe, SiC, SiGeC, SiSn, GeSn, SnGeSi
Sn
aSn = 0.6489, aGe= 0.5646 & aSi= 0.5431 nm (Sn/Ge~15% , Sn/Si 17% and Ge/Si ~4.2% mismatch)
.
Compressive Strain
Transition of indirect-bandgap to a direct bandgap with Sn content 6-8% - Possible material system for photonic application:
- GeSn alloy system
- GeSnSi alloy system
Strain engineering in channel region is demanded
Henry Radamson 2
Strain and bandgap in group IV material
Henry Radamap Engineering
Henry Radamson
8
SiGe bandstructure (compressive Biaxial strain)
SiGe/Si systems create a compressive strain which act on the valence band (the bandgap narrowing is only from valence band shift)
Deposition of SiN layer on the top of the nMOS in order to create a tensile strain in Si channel layer. Alt. The recess is filled by As/P-doped Si1-yCy
Roadmap of the microtechnology (a transition to nanotechnology)
Increasing the number of transistors in the chip / year Down -scaling the size of the transistors (junctions, threshold voltage gate oxide thickness)
- strained Ge on GeSn or SiGeSn for direct band gap (via tensile strain)
- Unstrain Ge (or SiGe) on SiGeSn for intersubband
applications – quantum cascade lasers (QCLs) - SiSn alloy on GeSn for communication wavelengths (1.3 mm – 1.55 mm)
Gupta et al, IEDM 2011, 398
This provides the possibility for monolithic integration of photonic devices on Si
Henry Radamson 7
Strain engineering in MOSFETs
This occurs for both lateral and vertical sizes of transistors
The dopant concentrations in source/drain and in the channel has to be modified
The mobility is an important issue in the down-scaling which has to be kept high.
Four cases can be occured:
* Strain compensated (e.g. ternary systems GeSnSi & GeSiC) * Strain relaxed (Ge or GeSi virtual buffer layer) (alloying component but defect states within the bandgap) * No strained layers (e.g. ternary systems GeSnSi ) (lattice match e.g. ternary systems GeSnSi) * Locally strained (Ge, Si & Sn dots)
<110> channel direction.
Henry Radamson
15
Electron mobility in Strained-Si
Henry Radamson
16
Scattering processes in a crystal
Lattice scattering: electrons changes its momentum and energy with phonon with conservation of energy and momentum.
Arghavani et al. IEEE Electron Device Letters, V27 (2006) 114
Henry Radamson
13
Scattering and mobility
Many scattering contributions
Lower dopant concentration causes higher mobility