Development of High-Efficiency GaAs Solar Cells Grown on Nanopatterned GaAs Substrates

Abstract

One approach to reducing the cost of high-efficiency III–V devices involves adding patterned layers to heteroepitaxial or homoepitaxial substrates to facilitate substrate removal and reuse. However, few studies have focused explicitly on high-quality devices grown over patterned substrates, which is required for any cost saving to be beneficial. In this work, we demonstrate the growth of high-efficiency GaAs solar cells on GaAs substrates patterned with an array of nanoscale SiO X mask stripes. We show that reducing the pattern dimensions to submicron length scales with nanoimprint lithography enables defect-free coalescence. By varying the growth conditions, faceting of the epilayer material during overgrowth of the patterned mask was also controlled. A V/III ratio of 200 during MOVPE overgrowth produced smooth coalesced epilayers, which is desirable for the growth of subsequent device layers. Inverted GaAs front homojunction devices grown on patterned GaAs(001) substrates achieved threading dislocation densities below 5 × 105 cm–2 and maintained >23% solar cell efficiencies at one sun illumination, equivalent to control devices grown on unpatterned epi-ready substrates.

Design of Planarizing Growth Conditions on Unpolished and Faceted (100)-Oriented GaAs Substrates Using Hydride Vapor Phase Epitaxy

“…10 Mangum et al also showed high-efficiency GaAs solar-cell device growth on surfaces that had an approximately 20 nm RMS roughness after coalescence over nanoimprint lithography patterns. 28 These studies imply that the features and roughness remaining after planarization overgrowth are unlikely to negatively affect device performance.…”

“…10 showed high-efficiency GaAs solar-cell device growth on surfaces that had an approximately 20 nm RMS roughness after coalescence over nanoimprint lithography patterns. 28 These studies imply that the features and roughness remaining after planarization overgrowth are unlikely to negatively affect device performance. These results demonstrated that planarization overgrowth by HVPE is a promising technique that may enable device growth on spalled surfaces without the need for ex situ CMP steps.…”

Coalescence of GaP on V-Groove Si Substrates

“…Regardless of the reason, this delay in relaxation is likely beneficial to the final material quality of the thin film. When relaxed islands of growth coalesce, grown-in sessile dislocations can form due to alignment errors between existing dislocations in neighboring islands; 43 however, here, the lack of relaxation in the Vgrooves likely prevents such issues with grown-in dislocations, 21 so the population of dislocations present after the film relaxes will be largely glissile.…”

“…As opposed to conventional thin-film growth on planar substrates, conformal growth on the V-groove substrate is not always desired. For films grown by MOVPE, lateral growth is typically encouraged with a high V/III ratio to promote coalescence when growing over nanopatterns. ,, Additionally, the dislocation dynamics differs between a V-groove system and planar substrates. The coalescence of relaxed, lattice-mismatched material can drive the formation of grown-in, sessile threading dislocations .…”

“…The direct growth of high-quality III-V semiconductors on Si substrates for optoelectronic devices has been a key research goal for decades. Recently, advances in the control of crystalline defects in the epitaxial III-V layers − has enabled rapid performance increases for III-V-on-Si solar cells − and lasers. − V-groove Si, a type of selective area growth (a strategy used in many epitaxial systems − ) where a substrate uses selective etching of nanopatterns to produce {1 1 1}-faceted trenches on a (0 0 1)-oriented Si wafer, is one of the technologies that has enabled this progress. , The {1 1 1}-faceted surface of these templates stops the formation of antiphase domains (APDs), which historically have been a persistent problem for III-V-on-Si growth. V-grooves have been used for the direct growth of GaAs, − InP, − GaSb, and GaP − on Si for laser and solar cell applications via metalorganic vapor phase epitaxy (MOVPE).…”