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

Extensive research efforts of strained germanium (Ge) are currently underway due to its unique properties, namely, (i) possibility of band gap and strain engineering to achieve a direct band gap, thus exhibiting superior radiative properties, and (ii) higher electron and hole mobilities than Si for upcoming technology nodes. Realizing lasing structures is vital to leveraging the benefits of tensile-strained Ge (ε-Ge). Here, we use a combination of different analytical tools to elucidate the effect of the underlying InGaAs/InAlAs and InGaAs overlaying heterostructures on the material quality and strain state of ε-Ge grown by molecular beam epitaxy. Using X-ray analysis, we show the constancy of tensile strain in sub-50 nm ε-Ge in a quantum-well (QW) heterostructure. Further, effective carrier lifetime using photoconductive decay as a function of buffer type exhibited a high (low) defect-limited carrier lifetime of ∼68 ns (∼13 ns) in 0.61% (0.66%) ε-Ge grown on an InGaAs (InAlAs) buffer. These results correspond well with the measured surface roughness of 1.289 nm (6.303 nm), consistent with the surface effect of the ε-Ge/III-V heterointerface. Furthermore, a reasonably high effective lifetime of ∼78 ns is demonstrated in a QW of ∼30 nm 1.6% ε-Ge, a moderate reduction from ∼99 ns in uncapped ε-Ge, alluding to the surface effect of the overlying heterointerface. Thus, the above results highlight the prime quality of ε-Ge that can be achieved via III-V heteroepitaxy and paves a path for integrated Ge photonics.

GeSn-on-GaAs with photoconductive carrier lifetime >400 ns: role of substrate orientation and atomistic simulation.

Group IV GeSn quantum material finds application in electronics and silicon-compatible photonics. Synthesizing these materials with low defect density and high carrier lifetime is a potential challenge due to lattice mismatch induced defects and tin segregation at higher growth temperature. Recent advancements in the growth of these GeSn materials on Si, Ge, GaAs, and with substrate orientations, demonstrated different properties using epitaxial and chemical deposition methods. This article addresses the effect of GaAs substrate orientation and misorientation on the materials' properties and carrier lifetimes in epitaxial Ge0.94Sn0.06 layers. With starting GaAs substrates of (100)/2°, (100)/6°, (110) and (111)A orientations, Ge0.94Sn0.06 epitaxial layers were grown with an intermediate Ge buffer layer by molecular beam epitaxy and analyzed by several analytical tools. X-ray analysis displayed good crystalline quality, and Raman spectroscopy measurements showed blue shifts in phonon wavenumber due to biaxial compressive strain in Ge0.94Sn0.06 epilayers. Cross-sectional transmission electron microscopy analysis confirmed the defect-free heterointerface of Ge0.94Sn0.06/Ge/GaAs heterostructure. Minority carrier lifetimes of the unintentionally doped n-type Ge0.94Sn0.06 epilayers displayed photoconductive carrier lifetimes of >400 ns on (100)/6°, 319 ns on (100)/2°, and 434 ns on (110) GaAs substrate at 1500 nm excitation wavelength. On the other hand, Ge0.94Sn0.06 layer showed poor carrier lifetime on (111)A GaAs substrate. The observed differences in carrier lifetimes were correlated with the formation energy of the Ge on (100)/6° and (100)/2° GaAs heterointerface using Stillinger-Weber interatomic potential model-based atomistic simulation with different heterointerfacial bonding by Synopsys QuantumATK tool. Total energy computation of 6280-atom Ge/GaAs supercell on (100)/6° leads to lower formation energy than (100)/2°, making it more thermodynamically stable. Hence, the growth of the GeSn/III-V material system using misoriented (100) substrates that are more thermodynamically stable will enhance the performances of optoelectronic devices.

Investigation of Sub-Bandgap Emission and Unexpected n-Type Behavior in Undoped Polycrystalline CdSexTe1-x.

Se alloying has enabled significantly higher carrier lifetimes and photocurrents in CdTe solar cells, but these benefits can be highly dependent on CdSexTe1-x processing. This work evaluates the optoelectronic, chemical, and electronic properties of thick (3 µm) undoped CdSexTe1-x of uniform composition and varied processing conditions (CdSexTe1-x evaporation rate, CdCl2 anneal, Se content) chosen to reflect various standard device processing conditions. Sub-bandgap defect emission is observed, which increased as Se content increased and with "GrV-optimized CdCl2" (i.e., CdCl2 anneal conditions used for group-V-doped devices). Low carrier lifetime is found for GrV-optimized CdCl2, slow CdSexTe1-x deposition, and low-Se films. Interestingly, all films (including CdTe control) exhibited n-type behavior, where electron density increased with Se up to an estimated ≈1017 cm-3. This behavior appears to originate during the CdCl2 anneal, possibly from Se diffusion leading to anion vacancy (e.g., VSe, VTe) and ClTe generation.

Temperature Coefficients of Perovskite Photovoltaics for Energy Yield Calculations.

Temperature coefficients for maximum power (T PCE), open circuit voltage (V OC), and short circuit current (J SC) are standard specifications included in data sheets for any commercially available photovoltaic module. To date, there has been little work on determining the T PCE for perovskite photovoltaics (PV). We fabricate perovskite solar cells with a T PCE of -0.08 rel %/°C and then disentangle the temperature-dependent effects of the perovskite absorber, contact layers, and interfaces by comparing different device architectures and using drift-diffusion modeling. A main factor contributing to the small T PCE of perovskites is their low intrinsic carrier concentrations with respect to Si and GaAs, which can be explained by its wider band gap. We demonstrate that the unique increase in E g with increasing temperatures seen for perovskites results in a reduction in J SC but positively influences V OC. The current limiting factors for the T PCE in perovskite PV are identified to originate from interfacial effects.

Measuring temperature-dependent activation energy in thermally activated processes: a 2D Arrhenius plot method.

Thermally activated processes are characterized by two key quantities, activation energy (E(a)) and pre-exponential factor (nu(0)), which may be temperature dependent. The accurate measurement of E(a), nu(0), and their temperature dependence is critical for understanding the thermal activation mechanisms of non-Arrhenius processes. However, the classic 1D Arrhenius plot-based methods cannot unambiguously measure E(a), nu(0), and their temperature dependence due to the mathematical impossibility of resolving two unknown 1D arrays from one 1D experimental data array. Here, we propose a 2D Arrhenius plot method to solve this fundamental problem. Our approach measures E(a) at any temperature from matching the first and second moments of the data calculated with respect to temperature and rate in the 2D temperature-rate plane, and therefore is able to unambiguously solve E(a), nu(0), and their temperature dependence. The case study of deep level emission in a Cu(In,Ga)Se(2) solar cell using the 2D Arrhenius plot method reveals clear temperature dependent behavior of E(a) and nu(0), which has not been observable by its 1D predecessors.