Design, Theoretical, and Experimental Investigation of Tensile-Strained Germanium Quantum-Well Laser Structure

Abstract

Strain and bandgap engineered epitaxial germanium (ε-Ge) quantum-well (QW) laser structures were investigated on GaAs substrates theoretically and experimentally for the first time. In this design, we exploit the ability of InGaAs layer to simultaneously provide tensile strain in Ge (0.7% to 1.96%) and sufficient optical and carrier confinement. The direct band-to-band gain, threshold current density (Jth) and loss mechanisms that dominate in the ε-Ge QW laser structure, were calculated using first-principles-based 30-band k.p electronic structure theory, at injected carrier concentrations from 3x10 18 cm -3 to 9x10 19 cm -3 . The higher strain in ε-Ge QW increases the gain at higher wavelengths; however, a decreasing thickness is required by higher strain due to critical layer thickness for avoiding strain relaxation. In addition, we predict that a Jth of 300 A/cm 2 can be reduced to <10 A/cm 2 by increasing strain from 0.2% to 1.96% in ε-Ge lasing media. The measured room temperature photoluminescence spectroscopy demonstrated direct bandgap optical emission from the conduction band at valley to heavy-hole (0.6609 eV) from 1.6% tensile strained Ge/In0.24Ga0.76As heterostructure grown by molecular beam epitaxy, is in agreement with the value calculated using 30-band k.p

Multivalley Electron Conduction at the Indirect-Direct Crossover Point in Highly Tensile-Strained Germanium

“…The decoupling of carrier density and mobility from these two valleys as a function of biaxial tensile strain is still elusive to date, especially the enhancement of the mobility for L electrons. By applying biaxial tensile strain of ~ 1.6%, the indirect bandgap Ge converts to direct bandgap, demonstrated theoretically using k.p and experimentally [21][22][23] by low temperature photoluminescence and photoreflectance measurements. At this strain level, the electrons with high mobility can begin to populate the Γ valley and thus decoupling of carriers and their mobility is a significant challenge.…”

“…Moreover, the difficulty of such an investigation is compounded by Ge's pseudo-direct bandgap nature, wherein only a ~150 meV difference separates the L and Γ valley conduction band minima [11,[17][18][19]. Thus, at high strain-states (ε ≥ 1.5%) [18,19], strain-induced modification of the Ge band structure is expected to lower the Γ valley conduction band minimum below that of the L valley, thereby transitioning Ge into a direct bandgap material with the potential for competitive behavior between the two conduction band minima to arise [20,21]. However, the density of states (DOS) mass in the L and Γ valley without strain are 𝑚 0.22 𝑚 and 𝑚 0.05 𝑚 at room temperature, respectively (m e denotes the free-electron mass).…”

“…Thus, the strained enhanced mobility in ε-Ge and relative proportion of carriers in each valley as a function of strain and temperature are of importance. This transport study can find several applications; where carrier density, band offset for carrier confinement and strain level which enhance mobility, all needed for low power tunnel transistor [24,25], tunnel FET based memory and logic devices [26,27], laser [21,[28][29][30], spintronics [31,32] and qubit [33] applications. In addition, there could be an application in single electron and quantum devices as well as in cryoelectronic, in which the Γ electron participation studied here is relevant.…”

Mapping the Interfacial Electronic Structure of Strain-Engineered Epitaxial Germanium Grown on InxAl1–xAs Stressors

“…Using solid-source molecular beam epitaxy (MBE), we demonstrate the low-defect, pseudomorphic epitaxy of a highly tensile-strained Ge (ε-Ge) epilayer on an In x Al 1– x As stressor. It is anticipated that such strain-engineered group IV materials could serve as the gain medium in future QW heterostructure lasers, whereas the high-bandgap In x Al 1– x As stressor could function as the cladding . Moreover, characterization of the ε-Ge/In x Al 1– x As heterostructure material and electronic properties reveal energy band offsets (Δ E C = 1.25 ± 0.1 eV; Δ E V = 0.56 ± 0.1 eV) conducive to electro-optical confinement.…”

Monolithically Cointegrated Tensile Strained Germanium and InxGa1-xAs FinFETs for Tunable CMOS Logic

“…Thus, successful integration of Ge p-FET and high electron mobility In x Ga 1-x As n-FET would be a more viable option for monolithically co-integrated alternate channel CMOS logic [13]. In recent years, Ge is gaining popularity in photonic and optoelectronic devices [14], whereas In x Ga 1−x As has already gained popularity in the optoelectronic applications [15], [16]. Thus, integrating Ge and InGaAs based logic devices in conjunction with optical devices (lasers, photodetectors, and interconnects) would be a step closer to the long-standing dream of an integrated optoelectronic chip [17]- [19].…”