Abstract
This article presents a complete plasma etching process to etch high aspect ratio patterns on III-V/Ge solar cell heterostructure with low damage for the fabrication of multijunction solar cells with a through cell via contact architecture. A SiCl 4 /H 2 chemistry was studied with different hydrogen dilutions within the plasma (0%, up to 67%) and with different cathode temperatures (20 ∘ C, up to 200 ∘ C). This chemistry choice creates a SiCl x -based inhibiting layer on the sidewalls that promotes anisotropic etching through the epitaxial heterostructure. The study suggests that a high hydrogen flow and a low temperature reduce the chemical reactions that create sidewall erosion. A high hydrogen flow appears to provide a hydrogen passivation of the non-radiative defects on the III-V heterostructure sidewall during the etching process. III-V/Ge triple junction solar cells with standard grid line and busbar front and back contacts have been fabricated and via-holes were plasma-etched through the active layers in order to investigate the impact of hydrogen passivation on the photovoltaic performance. The results demonstrate that the hydrogen passivation enables an open-circuit voltage increase that persists even after 5 months. This plasma process can also be used for the mesa etching step on multijunction solar cells with standard contacts. Thus, it could provide an appealing pathway to increase the conversion efficiency of state-of-the-art multijunction solar cells with standard contacts. To complete the etching process, a liner is used to protect the sidewall properties and a time-multiplexed Ge etching process is proposed to finalize the patterning and even open a pathway towards III-V/Ge plasma dicing. • A complete etching process is proposed to etch deep and anisotropic vias through a III-V/Ge heterostructure. • A low temperature SiCl 4 /H 2 plasma process is well suited to etch III-V/Ge heterostructures with limited sidewall erosion. • High H 2 flow enables a defect passivation on the etched sidewalls and it persists over several months.
Highlights
IntroductionPlasma etching anisotropic via-holes with low sidewall damage and high aspect ratio (AR) still represents a challenge in fabricating through cell via contacts on III-V/Ge triple junction heterostructures
Ciency increase of 3 % could be expected by using through cell via contact instead of standard contacts. [Richard et al (2016)] The busbar suppression could increase the power yield per wafer by 20%. [Richard et al (2016)] Another study has shown that this architecture could reduce the losses from the light non-uniformities. [Richard et al (2018)] this new architecture has been successfully fabricated on InGaP/AlGaAs dual junction heterostructure. [Salvetat et al (2016); Oliva et al (2016)] To the best of our knowledge, through cell via contacts were never fabricated on III-V/Ge triple junction heterostructures
Isotropic etching results from the chemical action of chlorine radicals, which is known to be activated by temperature, explaining the lateral etch rate increase. [Ohori et al (2019); Pearton et al (1994)] As for the In-rich InGaP, InClx are supposed to be the etch byproducts, which volatility limits InGaP etching at low temperature and those compounds become volatile above 150◦C. [Chen et al (2000); Asakawa et al (1998)] This explains why the InGaP lateral etching is considerably enhanced above 150◦C
Summary
Plasma etching anisotropic via-holes with low sidewall damage and high aspect ratio (AR) still represents a challenge in fabricating through cell via contacts on III-V/Ge triple junction heterostructures. [Zhao et al (2013); de Lafontaine et al (2019)] hydrogen addition to the plasma chemistry further reduces the sidewall erosion. A complete plasma etching process is proposed and characterized to pattern high aspect ratio via-holes on III-V/Ge multijunction heterostructures. Multijunction solar cells with standard contacts and via-holes etched with this process have been fabricated in order to evaluate the possible photovoltaic performance loss induced by the plasma process
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