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The novel use of a GaAsyP1-y/GaP compressively-strained superlattice (CSS) to provide enhanced control over misfit dislocation (MD) evolution and threading dislocation density (TDD) during GaP/Si m...

Metamorphic III-V/Si materials with low threading dislocation density (TDD) are critical to realizing high-efficiency III-V/Si multijunction photovoltaics. In pursuit of a dual junction III-V/Si design with a GaAs 0.75 P 0.25 top junction epitaxially integrated on a Si bottom junction, we report on progress made in the development of GaP/Si and GaAsyP l-y /Si materials with significantly reduced TDD. Using a tightly integrated study of fundamental dislocation dynamics, rapid electron microscopy based feedback on dislocation populations, and MOCVD process development, we have fully re-engineered the GaP on Si growth process. Our new approach results in a TDD of 7x104 cm−2 for 50 nm thick films. Implementation of a novel dislocation glide enhancing heterostructure then enabled subsequent growth of fully-relaxed, 500 nm total thickness n-GaP with a TDD of 2.4×106 cm−2. When applied to the production of full GaAs 0.75 P 0.25 /Si tandem solar cell structures, but without any significant optimization thus far, this low TDD is effectively maintained, yielding a terminal TDD of only 3.0x106 cm−2, sufficient to support high photovoltaic performance.

To identify the complex relationships between early-stage growth processes and the resultant defect microstructure in GaP/Si heteroepitaxy, a holistic study of several key metal–organic chemical vapor deposition (MOCVD) parameters was conducted, focusing on Si surface preparation and GaP atomic layer epitaxy (ALE) based nucleation processes. Crystalline defects related to the lattice mismatch and/or interfacial heterovalency, namely misfit dislocations (MD), threading dislocations (TD), and stacking fault pyramids (SFP), were quantitatively characterized via electron channeling contrast imaging (ECCI) and correlated against the different process variations. Choice of Si surface preparation method between the two examined (dilute SiH4 annealing versus Si2H6 based homoepitaxy) had little impact on resultant GaP film morphology and defect content, whereas differing GaP ALE nucleation conditions produced much more substantial changes. In particular, the initial precursor species (tert-butylphosphine versus triethygallium) and ALE cycle purge times both yielded significant influence over threading dislocation densities (TDD) in thin (100 nm), post-critical thickness GaP/Si films, with TDD spanning two orders of magnitude, from 6.7 × 107 cm−2 to 7.1 × 105 cm−2, depending on the specific process conditions employed. SFP densities were also found to follow a similar trend, ranging from 2.0 × 107 cm−2 to 1.8 × 105 cm−2, but with no apparent causal relationship between SFP density and TDD. To help explain the dramatic differences observed, detailed, large-area MD network characterization was used to provide statistically-relevant quantitative analyses of the critical dislocation dynamics (introduction rates and glide velocities) associated with the different process variants. These extracted values are then correlated against the ALE process variants to provide insight into the potential mechanistic roles of the different growth processes.

The growth of BxGa1−xP alloys by hybrid solid/gas-source molecular beam epitaxy, with B supplied via the BCl3 gas precursor, is demonstrated. Compositional control ranging from pure GaP to B0.045Ga0.955P has thus far been achieved. Slightly tensile-strained B0.031Ga0.969P grown on nearly pseudomorphic, compressively strained GaP/Si was used to produce an effectively strain-free (0.06% tensile misfit at growth temperature) 160 nm total III–V thickness BxGa1−xP/Si virtual substrate with a threading dislocation density of

Compositionally-graded InAs1−ySby metamorphic buffers, grown on GaSb wafers via metal organic chemical vapor deposition and designed to serve as virtual substrates for InAs0.6Sb0.4 long wave infrared detectors, were investigated for their potential to provide low residual threading dislocation densities. Structural properties of a range of step-graded buffer designs, including surface morphology, strain relaxation, and residual threading dislocation density were investigated as a function of substrate orientation, initial layer composition (lattice mismatch), grading rate, and compositional step size and thickness, and used to guide optimization of the metamorphic buffer structure. A minimum threading dislocation density of 2 × 106 cm−2 within the InAs0.6Sb0.4 terminal layer was achieved through use of a 7-layer buffer design with a strain grading rate of −0.78% μm−1, with potential for further reduction by continued optimization and interfacial control.