Reduction of threading dislocation density beyond the saturation limit by optimized reverse grading

Abstract

The threading dislocation density (TDD) in plastically relaxed Ge/Si(001) heteroepitaxial films is commonly observed to progressively decrease with their thickness, owing to mutual annihilation. However, there exists a saturation limit, known as the geometrical limit, beyond which a further decrease of the TDD in the Ge film is hindered. Here, we show that such limit can be overcome in SiGe/Ge/Si heterostructures thanks to the beneficial role of the second interface.Indeed, we show that Si0.06Ge0.94/Ge/Si(001) films display a TDD remarkably lower than the saturation limit of Ge/Si(001). Such result is interpreted with the help of Dislocation Dynamics simulations. The reduction of TDD is attributed to the enhanced mobility acquired by pre-existing threading dislocations after bending at the new interface to release the strain in the upper layer.Importantly, we demonstrate that the low TDD achieved in Si0.06Ge0.94/Ge/Si layers is preserved also when a second, relaxed Ge layer is subsequently deposited. This makes the present reversegrading technique of direct interest also for achieving a low TDD in pure-Ge films.

Tensile Strained Germanium Microstructures: A Comprehensive Analysis of Thermo‐Opto‐Mechanical Properties

“…Silicon–germanium–tin (GeSn) alloy can be a promising solution [ 4 ] because its band structure can be controlled through its composition toward high emission efficiency in a broad spectral range, but these ternary alloys present several technological challenges for the material growth. [ 5 ] The alternative route for band structure control is the introduction of tensile strain in Ge and GeSn alloys. Here, the target is to exploit tensile strain to reduce the energy barrier between the L and Γ minima of the conduction band, realizing a quasi‐direct bandgap material and thus enhancing the radiative efficiency.…”

Current leakage mechanisms related to threading dislocations in Ge-rich SiGe heterostructures grown on Si(001)

“…Here we present a comprehensive analysis of the influence of the grown-in TDD on the vertical transport mechanisms in as-grown intrinsic Si 0.06 Ge 0.94 /Ge/Si heterostructures featuring the same thickness and degree of plastic relaxation without introducing implantation-induced defects [15]. We conveniently use these Gerich SiGe RGVS because of the recent demonstrated capability to tune their TDD down to the low 10 6 cm -2 range, thanks to the presence of the Si 0.06 Ge 0.94 /Ge heterointerface [16]. Furthermore, this composition range is of special interest for applications using superlattice structures due to the requirement of strainsymmetrization between quantum-wells and tunnel barriers [17].…”

“…More details on the deposition process can be found in Ref. [16] . The three Si 0.06 Ge 0.94 epilayers feature the same thickness and degree of relaxation (R= 106% [16]) but different TDD values of 3×10 6 (sample SA), 9×10 6 (SB) and 2×10 7 cm -2 (SC) as measured by the Secco etch pit count over a surface area of 55 µm 2 .…”

Strain Modulation of Selectively and/or Globally Grown Ge Layers

“…However, the growth of high-quality Ge on Si faces the difficulty of large lattice mismatch and large thermal expansion coefficients between Ge and Si. To decrease the threading dislocation densities (TDDs) and surface roughness of the Ge layers, several methods have been proposed, such as introducing a Ge buffer of low-temperature (LT) and high-temperature (HT) growth [21][22][23][24], As-doped LT-Ge buffer [25], ultra-thin SiGe/Si superlattice buffer layer [26], reversed graded SiGe buffer [27], high-temperature H 2 annealing [28,29], cyclic thermal annealing [30], and selective epitaxial growth (SEG) of Ge buffer [31]. By using the above-mentioned methods, material quality for Ge epilayers has significantly improved.…”