Wafer‐scale Ge epitaxial foils grown at high growth rates and released from porous substrates for triple‐junction solar cells

Abstract

Germanium is listed as a critical raw material, and for environmental and economic sustainability reasons, strategies for lower consumption must be implemented. A promising approach is Ge lift‐off concepts, which enable to re‐use the substrate multiple times. Our concept is based on the Ge‐on‐Nothing approach that is the controlled restructuring at high temperature of a macroporous Ge surface, forming a Ge foil weakly attached to its parent wafer. Its suitability as III–V epitaxy seed and support substrate has previously been demonstrated with proof‐of‐concept solar cells. This work focuses on bringing this concept to the next level, by upscaling the detachable area to a full 200‐mm wafer scale, increasing foil thickness for sufficient light absorption in the Ge bottom cell, and improving the control on the strength that is bonding the suspended foil to its parent. By introducing a new high growth‐rate epitaxy process from GeCl4, and by engineering the GeON structure to introduce pillars with ad hoc density and shape, we fabricated P‐type foils with tunable boron doping up to 15 μm in thickness and 11 cm × 11 cm in area, for which the detachment strength could be adapted to the stresses induced by the solar cell process steps. The surface roughness and the electrical and crystal qualities of these foils were inspected by AFM, SIMS, SRP, ECCI, and TEM to check the GeCl4‐based epitaxy conditions and to check that the ad hoc pillars were not introducing any damage. Small‐area triple‐junction lattice‐matched GaInP/GaInAs/Ge solar cells were fabricated on 7‐μm‐thick Ge foils with various pillar densities and on a standard reference Ge wafer. The III–V layer nucleation was virtually the same on both substrates and the solar cells on the GeON foils performed in the same way as the cells on the Ge wafer, albeit a small loss in short‐circuit current and open‐circuit voltage that can be attributed to the thickness reduction and absence of rear‐side passivation. We conclude that it is possible to gain control on the GeON detachability and upscale the concept to areas relevant for the space PV industry, proving that porous germanium is a serious candidate for replacement of bulk Ge wafers in view of a more sustainable multijunction solar cell process.

Large‐Scale Formation of Uniform Porous Ge Nanostructures with Tunable Physical Properties

“…Many applications can take advantage of this kind of versatility such as energy storage systems, [ 7 ] thermoelectric devices, [ 11 ] or nanoengineered compliant substrates for epitaxial growth. [ 21,23 ]…”

“…Many applications can take advantage of this kind of versatility such as energy storage systems, [7] thermoelectric devices, [11] or nanoengineered compliant substrates for epitaxial growth. [21,23] The demonstration of wafer-scale production and use of porous Ge substrates for various applications, comes with a crucial need to develop fast and nondestructive characterization methods easily applicable for post-production PGe wafer's quality assessment. To date, PGe layers are mainly characterized by SEM.…”

“…[17][18][19] Moreover, PGe has recently been demonstrated as an efficient virtual substrate for epitaxial growth of detachable Ge membranes [20] and III-V heterostructures with high crystalline quality [21,22] paving the way to direct application in the development of lightweight and flexible photovoltaics and optoelectronics. [23,24] Nevertheless, to bring these applications to the real world, a largescale formation of homogeneous PGe layers with on demand characteristics is necessary.…”

“…The fabrication of PGe nanostructures was demonstrated using techniques such as thermal reduction of GeO 2 nanoparticles, [25] oxidative polymerization of the deltahedral [Ge 9 ] 4− cluster, [26] spark processing, [27] reduction-alloying-dealloying approach, [28] ion implantation, [29,30] growth by Molecular Beam epitaxy, [15] coupled plasma chemical vapor deposition, [31] metal-assisted chemical etching, [32] lithography and dry etching, [23] and electrochemical etching. [33][34][35] Some of the major challenges of the aforesaid techniques are the use of expensive precursors, high investment in equipment and labor, low yields, random crystallite orientation, low purity of PGe structures, and intricate procedures making them nonviable for low-cost/ large-scale production.…”

Wafer-scale detachable monocrystalline germanium nanomembranes for the growth of III–V materials and substrate reuse

“…Although, this generates various defects in the layer, it has been shown that high efficiency devices can be still fabricated on such a substrate, with limited inuence on their performance. 21 Recently, another technique called Germanium-on-Nothing 7,22 (GoN) has been demonstrated. This process uses a sequence of photolithography, plasma etching, and annealing steps to engineer a voided weak layer at the interface between the bulk material and the Ge NM, which enables the separation of the membrane from the substrate.…”