Minority carrier lifetime of Ge film epitaxial grown on a large-grain seed layer on glass

Abstract

To develop high-efficiency multijunction solar cells onto inexpensive substrates, an innovative technique for growing a large-grained Ge layer on glass is strongly desired. We investigated the epitaxial growth of a light absorbing Ge flim (500-nm thickness) on a largegrained (> 100 µm) Ge seed layer (50-nm thickness) formed on glass by Al-induced layer exchange. After examining molecular beam epitaxy (MBE) and solid-phase epitaxy (SPE) for both methods, Ge layers with a low Al concentration were epitaxially grown at 350 °C.Microwave photoconductivity decay revealed that the MBE-Ge layer exhibited a short minority carrier lifetime owing to the rough surface. Conversely, the SPE-Ge layer was relatively flat and exhibited a long bulk minority carrier lifetime (5.6 µs), which is close to that of a single-crystal Ge. Therefore, the seed layer concept that combines SPE with Alinduced layer exchange is a promising way for fabricating ideal bottom cells for highefficiency multijunction solar cells based on inexpensive substrates.

Metal-induced layer exchange of group IV materials

“…The motivation to replace the bulk Ge substrate, used in multijunction solar cells, with a Ge film on insulating substrates is quite high because the Ge substrate is expensive while Ge can absorb light even in a thin film. The Ge film, grown epitaxially from a large-grained ptype Ge seed layer formed by the Al-induced LE, exhibited a bulk minority carrier lifetime of 5.6 µs, which is close to that of a single-crystal Ge [188]. Ge is also useful as a seed layer for group III-V compound semiconductors because of lattice matching.…”

Optoelectrical Operation Stability of Broadband PureGaB Ge-on-Si Photodiodes with Anomalous Al-Mediated Sidewall Contacting

“…31,32 One paper on Ge/Al layerexchange at 350°C reports solid-phase-epitaxy (SPE) of Ge doped with Al to as much as 10 17 cm −3 . 33,34 To our knowledge, the literature does not report any values for the active Al dopant concentration in such Ge SPE layers. For Si, studies of layer-exchange Al-doped crystalline Si layers formed at 400°C, using secondary-ion mass spectrometry and electrical measurements, estimated an active doping level in the 10 18 cm −3 range 43 and with this p-material, p + n-type junctions were formed on n-Si.…”

“…To our knowledge, this is the first report describing this specific type of material transport system. Other systems have been studied with the purpose of achieving Al-induced crystallization of Ge or SiGe, [30][31][32][33][34] mainly for the purpose of fabricating low-cost SWIR photodiodes. This work contributes to an understanding of the situations in which Al-mediated recrystallization of Ge/Si material systems can be achieved.…”

Ultrahigh-Quality Ge-on-Glass with a 3.7% Uniaxial Tensile Strain

“…It is important to note that most Ge devices are fabricated using Ge thin films of micron-scale thickness rather than bulk Ge. Two distinct approaches are employed to create Ge thin films: one involves a bottom-up method utilizing epitaxial techniques such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) , to deposit Ge onto a substrate like Si. The other approach employs a top-down approach, which begins with bulk Ge, commonly known as Ge wafers.…”

“…Si wafers using a smart cut technology, which achieved an on and off current ratio ( I on / I off ) of Ge diode to be ∼500 at ±1 V and the surface roughness of 0.7 nm . The smart-cut method involved (1) hydrogen ion implantation into the Ge wafer, (2) wafer bonding between the Ge wafer and a handle wafer, and (3) annealing to facilitate the separation of the Ge wafer. This requires sophisticated equipment, and the high energy ion implant also introduces crystal defects in Ge that cannot be all annealed out.…”