Effects of Uniaxial and Biaxial Strain on Semiconductors

Strain in semiconductors has been shown to push the limits of electronic device performance.  Common methods include strain-graded buffer layers, uniaxial, and biaxial strain.  The last two can provide dislocation-free materials.  This is important to minimize strain relaxation and loss in the carrier mobility.


Uniaxial Strain

Uniaxial strain is the current method for improving carrier mobility in microelectronics.  It can offer benefits not obtained by biaxial strain such as creating a direct band gap in materials of certain orientation [1] or a superlattice of strain[2].  As size effects become more important, it is of interest to probe how strain is distributed throughout uniaxially strained semiconductors and how it affects their band structure. For this, photoemission electron microscopy (PEEM) is used to observe the changes in the conduction band minimum with high spatial and energy resolution. We expect our results to compliment the theoretical work [1], [3] and have applications in the fields of thermoelectrics and nanoelectronics.


SiNx stripes on Si substrate
Figure 1. SiNx stripes on Si substrate.


Biaxial Strain

Biaxial strain can create sheets of uniformly strained material, most commonly in the form of nanomembranes.  Strained (001) silicon nanomembranes have recently been produced through epitaxial growth of SiGe stressor layers, with strain up to 1% in the Si layer [4].  These nanomembranes provide flexible, transferable Si with increased electron mobility.  Biaxial strain in Si breaks the 6-fold degeneracy of the delta valley in the conduction band into a 4-fold degenerate level in-the-plane of strain and a 2-fold degenerate level out-of-plane of strain.  This reduces the intervalley scattering which increases the electron mobility[5].

The use of PECVD silicon nitride enables virtually any substrate to be strained in either a compressive or tensile direction for either uniaxial or biaxial strain.  My work focuses on applying this versatile SiNx to fully discover the potential of strain in semiconducting materials. 

Schematic of SiNx/Si membrane liftoff


Figure 2. Schematic of SiNx/Si membrane liftoff.


Author: Anna Clausen

[1] Zhang et al.  Prediction that uniaxial tension along <111> produces a direct band gap in germanium.  PRL 102, 156401 (2009).
[2] Huang et al.  Mechano-electronic superlattices in silicon nanoribbons.  ACSNano 3, 721 (2009).
[3] Hoshina et al.  First-principles analysis of indirect-to-direct band gap transition of Ge under tensile strain.  JJAP 48, 04C125 (2009).
[4] Roberts, M. et al., Nature Mat. 5, 388-393 (2006).
[5] Euaruksakul et al.  Influence of strain on the conduction band structure of strained silicon nanomembranes.  PRL 101, 147403 (2008).

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