Interplay Between Strain and Thickness on the Effective Carrier Lifetime of Buffer-Mediated Epitaxial Germanium Probed by the Photoconductance Decay Technique

Abstract

We report contactless effective minority carrier lifetime of epitaxially grown unstrained and in-plane <110> biaxially tensile-strained (001) germanium (ε-Ge) epilayers measured using microwave-reflectance photoconductance decay measurements. Strained Ge epilayers were grown using In x Ga 1−x As linearly graded buffers on (001) GaAs substrates. Using homogeneous excitation of unstrained Ge epilayers, thickness-dependent separation of minority carrier lifetime components under low injection conditions yielded a bulk lifetime of 114 ± 2 ns and low surface recombination velocity of 21.3 ± 0.04 cm/s. More notably, an effective minority carrier lifetime of >100 ns obtained from sub-50 nm 1.6% tensilestrained Ge epilayers showed no degradation relative to the unstrained counterpart. Detailed material characterization using X-ray diffractometry revealed successful strain transfer of 0.61 and 0.89% to the Ge epilayers via In x Ga 1−x As metamorphic buffers and confirms pseudomorphic growth. Lattice coherence observed at the ε-Ge epilayer and In x Ga 1−x As buffer heterointerfaces via transmission electron microscopy substantiates the prime material quality achieved. The relatively high carrier lifetimes achieved are an indicator of excellent material quality and provide a path forward to realize lowthreshold Ge laser sources.

Elucidating the Role of InGaAs and InAlAs Buffers on Carrier Dynamics of Tensile-Strained Ge Double Heterostructures

“…For this work, we used the μ-PCD technique to characterize the effect of the underlying buffer and top overgrowth on the effective carrier lifetime in the ε-Ge epilayers. Details of the measurement technique have been reported in our previous work . Under low-injection conditions, it can be shown that the reciprocal of effective carrier lifetime, τ eff , is expressed as the cumulative response from two reciprocal components, τ surface and τ bulk , as follows: , 1 τ eff = 1 τ surface + 1 τ bulk where τ surface and τ bulk are effective lifetime components from the surface and bulk, respectively.…”

“…The Ge epilayers in heterostructures A and B in this work are grown well within the critical layer thickness, adhering to the strain balance model by People and Bean, and hence, the Ge epilayers should be fully strained. This is indeed observed in our previous work where high-magnification transmission electron micrographs obtained from the constant composition InGaAs virtual substrate, the ε-Ge epilayer, and the ε-Ge/InGaAs heterointerface show a high degree of coherence and absence of misfit dislocations, alluding to the pseudomorphic epitaxy of Ge. The exact cause for this observed disparity between XRD- and Raman-estimated strain levels, especially on InAlAs buffer, currently eludes the authors and needs additional research.…”

“…Typical symmetric (004) and asymmetric (115) reciprocal space maps (RSMs) were recorded from each heterostructure for quantification of the effective In incorporation in the virtual substrates and relaxation achieved in the buffer layers and final strain state and crystalline quality of the ε-Ge epilayers. We have reported XRD measurements from heterostructure A in our previous work, 37 and they are reported here to aid in direct comparison with heterostructure B.…”

High-κ Gate Dielectric on Tunable Tensile Strained Germanium Heterogeneously Integrated on Silicon: Role of Strain, Process, and Interface States

“…The lower Al 2 O 3 thickness was selected for lower EOT. As high-quality GeO 2 ,,, IPL layer is necessary for passivating surface defects of Ge, thermal oxidation of 0.2% ε-Ge and 1.2% ε-Ge stack was carried out at 550 °C (which is the highest oxidation temperature one can select in order to prevent the strain relaxation in this work) for different oxidation duration. Figure shows the C – V characteristics of the Ge MOS-Cs of (a) 0.2% ε-Ge, where GeO 2 oxidation was performed only for 0.5 min, and (b–d) 1.2% ε-Ge where oxidation duration was varied from 0.5 to 2.5 min, respectively.…”

“…Group IV based materials, germanium (Ge), silicon–germanium (SiGe), and germanium–tin (GeSn), are under consideration for nanoelectronics and photonics. − Due to their high carrier mobilities, supplementing these materials along with tunable compositional In x Ga 1– x As (0.1 ≤ x ≤ 0.4) as channel materials will boost the on-current and ultimately device/circuit performance in an alternate channel CMOS, , tensile-strained Ge/In x Ga 1– x As based tunnel transistors, , energy-efficient SRAM cell architecture for ultralow voltage applications, and photonic devices. ,,, For high-performance ultralow voltage CMOS logic, RF circuits, and mixed signal low-noise amplifier circuits, it is widely accepted that InGaAs and Ge will serve as n -channel and p -channel transistor materials, respectively. ,− However, implementing these two different materials (i.e., Ge and InGaAs) on a Si wafer requires defect-controlled buffer engineering for the monolithic heterogeneous integration process rather than direct growth of Ge or SiGe on Si. Furthermore, Ge is an excellent choice for CMOS logic due to its 2.8× and 4.2× higher electron and hole mobilities, respectively, compared to Si.…”