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

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 Ge1-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/In0.26Al0.74As heterostructure as determined by X-ray photoemission spectroscopy (XPS). To this end, an ultrathin (∼10 nm) ε-Ge epilayer was directly integrated onto the In0.26Al0.74As 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/In0.26Al0.74As 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 Al1-x As energy band offset, demonstrating excellent agreement between experimental and theoretical results under an As0.5Ge0.5 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.

In Situ SiO2 Passivation of Epitaxial (100) and (110)InGaAs by Exploiting TaSiO x Atomic Layer Deposition Process.

In this work, an in situ SiO2 passivation technique using atomic layer deposition (ALD) during the growth of gate dielectric TaSiO x on solid-source molecular beam epitaxy grown (100)In x Ga1-x As and (110)In x Ga1-x As on InP substrates is reported. X-ray reciprocal space mapping demonstrated quasi-lattice matched In x Ga1-x As epitaxy on crystallographically oriented InP substrates. Cross-sectional transmission electron microscopy revealed sharp heterointerfaces between ALD TaSiO x and (100) and (110)In x Ga1-x As epilayers, wherein the presence of a consistent growth of an ∼0.8 nm intentionally formed SiO2 interfacial passivating layer (IPL) is also observed on each of (100) and (110)In x Ga1-x As. X-ray photoelectron spectroscopy (XPS) revealed the incorporation of SiO2 in the composite TaSiO x , and valence band offset (ΔE V) values for TaSiO x relative to (100) and (110)In x Ga1-x As orientations of 2.52 ± 0.05 and 2.65 ± 0.05 eV, respectively, were extracted. The conduction band offset (ΔE C) was calculated to be 1.3 ± 0.1 eV for (100)In x Ga1-x As and 1.43 ± 0.1 eV for (110)In x Ga1-x As, using TaSiO x band gap values of 4.60 and 4.82 eV, respectively, determined from the fitted O 1s XPS loss spectra, and the literature-reported composition-dependent In x Ga1-x As band gap. The in situ passivation of In x Ga1-x As using SiO2 IPL during ALD of TaSiO x and the relatively large ΔE V and ΔE C values reported in this work are expected to aid in the future development of thermodynamically stable high-κ gate dielectrics on In x Ga1-x As with reduced gate leakage, particularly under low-power device operation.

Transport Across Heterointerfaces of Amorphous Niobium Oxide and Crystallographically Oriented Epitaxial Germanium.

Because of the high carrier mobility of germanium (Ge) and high dielectric permittivity of amorphous niobium pentoxide (a-Nb2O5), Ge/a-Nb2O5 heterostructures offer several advantages for the rapidly developing field of oxide-semiconductor-based multifunctional devices. To this end, we investigate the growth, structural, band alignment, and metal-insulator-semiconductor (MIS) electrical properties of physical vapor-deposited Nb2O5 on crystallographically oriented (100), (110), and (111)Ge epilayers. The as-deposited Nb2O5 dielectrics were found to be in the amorphous state, demonstrating an abrupt oxide/semiconductor heterointerface with respect to Ge, when examined via low- and high-magnification cross-sectional transmission electron microscopy. Additionally, variable-angle spectroscopic ellipsometry and X-ray photoelectron spectroscopy (XPS) were used to independently determine the a-Nb2O5 band gap, yielding a direct gap value of 4.30 eV. Moreover, analysis of the heterointerfacial energy band alignment between a-Nb2O5 and epitaxial Ge revealed valance band offsets (ΔEV) greater than 2.5 eV, following the relation ΔEV(111) > ΔEV(110) > ΔEV(100). Similarly, utilizing the empirically determined a-Nb2O5 band gap, conduction band offsets (ΔEC) greater than 0.75 eV were found, likewise following the relation ΔEC(110) > ΔEC(100) > ΔEC(111). Leveraging the reduced ΔEC observed at the a-Nb2O5/Ge heterointerface, we also perform the first experimental investigation into Schottky barrier height reduction on n-Ge using a 2 nm a-Nb2O5 interlayer, resulting in a 20× increase in reverse-bias current density and improved Ohmic behavior.

Tailoring the Valence Band Offset of Al2O3 on Epitaxial GaAs(1-y)Sb(y) with Tunable Antimony Composition.

Mixed-anion, GaAs1-ySby metamorphic materials with tunable antimony (Sb) compositions extending from 0 to 100%, grown by solid source molecular beam epitaxy (MBE), were used to investigate the evolution of interfacial chemistry under different passivation conditions. X-ray photoelectron spectroscopy (XPS) was used to determine the change in chemical state progression as a function of surface preclean and passivation, as well as the valence band offsets, conduction band offsets, energy band parameters, and bandgap of atomic layer deposited Al2O3 on GaAs1-ySby for the first time, which is further corroborated by X-ray analysis and cross-sectional transmission electron microscopy. Detailed XPS analysis revealed that the near midpoint composition, GaAs0.45Sb0.55, passivation scheme exhibits a GaAs-like surface, and that precleaning by HCl and (NH4)2S passivation are mandatory to remove native oxides from the surface of GaAsSb. The valence band offsets, ΔEv, were determined from the difference in the core level to the valence band maximum binding energy of GaAs1-ySby. A valence band offset of >2 eV for all Sb compositions was found, indicating the potential of utilizing Al2O3 on GaAs1-ySby (0 ≤ y ≤ 1) for p-type metal-oxide-semiconductor (MOS) applications. Moreover, Al2O3 showed conduction band offset of ∼2 eV on GaAs1-ySby (0 ≤ y ≤ 1), suggesting Al2O3 dielectric can also be used for n-type MOS applications. The surface passivation of GaAs0.45Sb0.55 materials and the detailed band alignment analysis of Al2O3 high-κ dielectrics on tunable Sb composition, GaAs1-ySby materials, provides a pathway to utilize GaAsSb materials in future microelectronic and optoelectronic applications.

Heterointerface engineering of broken-gap InAs/GaSb multilayer structures.

Broken-gap InAs/GaSb strain balanced multilayer structures were grown by molecular beam epitaxy (MBE), and their structural, morphological, and band alignment properties were analyzed. Precise shutter sequence during the MBE growth process, enable to achieve the strain balanced structure. Cross-sectional transmission electron microscopy exhibited sharp heterointerfaces, and the lattice line extended from the top GaSb layer to the bottom InAs layer. X-ray analysis further confirmed a strain balanced InAs/GaSb multilayer structure. A smooth surface morphology with surface roughness of ∼0.5 nm was demonstrated. The effective barrier height -0.15 eV at the GaSb/InAs heterointerface was determined by X-ray photoelectron spectroscopy, and it was further corroborated by simulation. These results are important to demonstrate desirable characteristics of mixed As/Sb material systems for high-performance and low-power tunnel field-effect transistor applications.