Band Gap and Carrier Mobility in GeSiSn Alloys

Some of the Group IV semiconductors have antibonding character in their conduction band at Γ-point, while the valance bands have bonding character. The inverse of these structures are found in Sn which has bonding and antibonding character in conduction and valance band at Γ-point respectively. This is reason why Sn has negative band gap

Large lattice mismatch between Sn (6.489 Aº) and Ge (5.646 Aº) or Si (5.431 Aº), approximately 15% and 17% respectively.

By adding Sn to Ge, direct () and indirect (L), band gab of Ge decreases. However, the direct band gab decreases faster by reducing the band gap and larger lattice constant.

Achieving a cross over from indirect to direct gap material at ≈ 0.5 eV for 6-8% Sn.

Predicted properties of high carrier mobility, direct band-gap crossover (around x = 0.15). The Ge1-x Snx lattice constant of Ge ranges from 5.66 Å to more than 5.90 Å,

Since Si has smaller lattice constant than Ge and a higher band gap, the incorporation of Si will generally widen the band gap and decrease the alloy’s lattice constant. Furthermore, the incorporation of Si into GeSn provides additional flexibility for tuning the band gaps and lattice constants

Adding Sn to GeSi to create Ge1-x-ySix Sny, adjusts the behaviour of the band gap from indirect band gap to direct band gap for y ≤ 0.12

Note; there is special case for Ge1-x (Si 0.79Sn 0.21) x, the materials exhibit lattice constants identical to Ge

I-V characterization curves shows that three samples exhibit schottky behavior. In addition, the three samples have low current approximately less than 0.01 mA at reverse baise - 2 Volt which indicate that to the three samples are suitable for DLTS measurements

Figures above show that barriers in height increase slightly with increasing temperature. However, the ideality factors decrease with temperature.

High ideal factor and low barrier height at low temperature due to the frozen the carrier. Where, the carriers at low temperature overcome through interfaces state rather than thermionic emission

Notably, the barriers in height in the sample have high Ge content, at high temperature sample, are larger or equal the band gap of GeSiSn (0.90 eV). This behaviour reported in NiGe/n-(100) Ge Schottky contacts by Chi et al., (2005). They interpreted by inversion layer formation at the interface.

Two peaks opposite to each other at high temperature were observed in cIn. It is not acceptable to refers these peaks to electron or hole trap since the nature of concentration for n-type and p-type are the same

Increasing the activation energy for traps with increasing Ge content were observed between samples contain Ge 30 and 40%. The activation energy 0.62 and 0.64 in 40 and 50 % Ge content respectively are close to each other. Therefore, these traps are similar to the trap observed in n-type Ge0.30Si0.70 in by Grillot, & Ringel, (1996). The scholars explained that this trap with activation energy 0.60 is related to Kink site defect

Increasing the activation energy of traps with increasing Ge content was observed. Also the activation energy in Ge0.3 were reported in SiGe/Si heterostructures by Jinggang Lu, Park, and Rozgonyi (2008)

The activation of this trape in Vr=1 and Vr=2 increase with increasing Ge content. This observation was reported in p-type SiGe/Si hetero-structures by Jinggang Lu, Park, and Rozgonyi (2008) who proposed that the increase is due to the band gap narrowing as a result of Ge alloying.

There is no an observation of Sn traps with activation energy between (0.11 and 0.27eV) as reported by Markevich et al., (2011).