Mapping the Interfacial Electronic Structure of Strain-Engineered Epitaxial Germanium Grown on In x Al1- x As Stressors

Abstract

The indirect nature of silicon (Si) emission currently limits the monolithic integration of photonic circuitry with Si electronics. Approaches to circumvent the optical shortcomings of Si include band structure engineering via alloying (e.g., Si _x Ge_1-x-y Sn _y ) and/or strain engineering of group IV materials (e.g., Ge). Although these methods enhance emission, many are incapable of realizing practical lasing structures because of poor optical and electrical confinement. Here, we report on strong optoelectronic confinement in a highly tensile-strained (ε) Ge/In_0.26Al_0.74As heterostructure as determined by X-ray photoemission spectroscopy (XPS). To this end, an ultrathin (∼10 nm) ε-Ge epilayer was directly integrated onto the In_0.26Al_0.74As stressor using an in situ, dual-chamber molecular beam epitaxy approach. Combining high-resolution X-ray diffraction and Raman spectroscopy, a strain state as high as ε ∼ 1.75% was demonstrated. Moreover, high-resolution transmission electron microscopy confirmed the highly ordered, pseudomorphic nature of the as-grown ε-Ge/In_0.26Al_0.74As heterostructure. The heterointerfacial electronic structure was likewise probed via XPS, revealing conduction- and valence band offsets (ΔE _C and ΔE _V) of 1.25 ± 0.1 and 0.56 ± 0.1 eV, respectively. Finally, we compare our empirical results with previously published first-principles calculations investigating the impact of heterointerfacial stoichiometry on the ε-Ge/In _x Al_1-x As energy band offset, demonstrating excellent agreement between experimental and theoretical results under an As_0.5Ge_0.5 interface stoichiometry exhibiting up to two monolayers of heterointerfacial As-Ge diffusion. Taken together, these findings reveal a new route toward the realization of on-Si photonics.

Figure 1

(a) Cross-sectional schematic diagram of the ε-Ge/In_0.26Al_0.74As heterostructure grown on (001)GaAs. (b) Graphic representation of the influence of biaxial tensile stress on the in-plane (a_∥) and out-of-plane (a_⊥) lattice constants of a pseudomorphic thin film (red) grown onto a lattice-mismatched stressor (blue).

Figure 2

High-resolution reciprocal space maps (RSMs) taken along (a) symmetric (004) and (b) asymmetric (115) crystallographic orientations. The separation between the Ge reciprocal lattice point and that of the substrate (GaAs) in the Q_z coordinate is indicative of compressive out-of-plane stress and thus tensile in-plane stress.

Figure 3

Atomic force micrograph of a representative 20 μm × 20 μm region of the as-grown ε-Ge/In_0.26Al_0.74As surface and related line height profiles recorded along the two orthogonal ⟨110⟩ symmetric directions.

Figure 4

Raman spectra collected from a (001)Ge substrate and the ε-Ge epilayer grown on In_0.26Al_0.74As. The shift (Δω = −7.27 cm^–1) in the unstrained Ge LO-related mode (ω₀ ∼ 300 cm^–1) corresponds to an in-plane strain of 1.75%.

Figure 5

(a) Low-magnification cross-sectional transmission electron micrograph (X-TEM) of the entire ε-Ge/In_xAl_1–xAs/GaAs heterostructure, highlighting the confinement of lattice mismatch-induced defects below the region of interest. (b) and (c–e) High-magnification X-TEM of the ε-Ge/In_0.26Al_0.74As heterointerface and associated FFT patterns, respectively, revealing coherent strained-layer epitaxy with no observable relaxation-induced interface defects.

Figure 6

X-ray photoelectron spectroscopy (XPS) spectra of (a) Ge 3d CL (E_Ge 3d^ε – Ge) and valence band maximum (E_VBM^ε – Ge) from the ε-Ge thin-film, (b) As 3d (E_As 3d^{In_0.26Al_0.74As}) and VBM (E_VBM^{In_0.26Al_0.74As}) from the In_0.26Al_0.74As stressor, and (c) As 3d and Ge 3d CLs measured at the ε-Ge/In_0.26Al_0.74As heterointerface. (d) Schematic flat-band diagram for the ε-Ge/In_0.26Al_0.74As heterostructure illustrating the relatively large valence (ΔE_V = 0.56 ± 0.1 eV) and conduction (ΔE_C = 1.25 ± 0.1 eV) band offsets found in this work.

Figure 7

Calculated valence band offset (ΔE_V, left, blue) and conduction band offset (ΔE_C, right, red) as a function of arsenic (As) diffusion length into a ε-Ge epilayer overlying an As-terminated In_0.25Al_0.75As stressor. Solid lines have been adapted from ref (35), whereas dashed lines represent ΔE_C when recalculated using the In_xAl_1–xAs bandgap provided in ref (45). Symbols (and associated error) correspond to the experimental energy band offsets as determined via XPS and reported in this work. The experimental data (symbols) were overlaid with the modeled “trend” (lines) to approximate the extent of As diffusion in the as-grown (empirical) ε-Ge/In_0.26Al_0.74As heterostructure studied herein.