IEEE ELECTRON DEVICE LETTERS, VOL. 35, NO. 12, DECEMBER 2014 1275 Gold-Free Fully Cu-Metallized InGaP/InGaAs/Ge Triple-Junction Solar Cells Ching-Hsiang Hsu, Member, IEEE, Edward Yi Chang, Fellow, IEEE , Hsun-Jui Chang, Hung-Wei Yu, Hong Quan Nguyen, Chen-Chen Chung, Jer-Shen Maa, Member, IEEE, Krishna Pande, Fellow, IEEE Abstract— Copper contacts and interconnects were developed for GaAs and Ge for low-cost solar cell application. In addition, thermally annealed Pd/Ge and Pt/Ti/Pt metallizations were created for ohmic contacts to n-GaAs and p-Ge with contact resistance of 4.4 × 10 -6 and 6.9 × 10 -6 cm 2 , respectively. Utilizing such metallization structure for InGaP/InGaAs/Ge triple-junction device structure solar cells were fabricated that delivered conversion efﬁciency of 23.11%, which is average efﬁciency for the above device structure. Index Terms— III-V concentrator solar cell, copper metalliza- tion, ohmic contact, low cost. I. I NTRODUCTION C OST effective proliferation of solar cell requires continuous enhancement of its efﬁciency which in turn leads to panel size reduction. Therefore novel device structures and metallization schemes must be developed for efﬁciency improvements. Solar cells built on wide band-gap multi-junction structures using III-V semiconductors have yielded highest efﬁciency as a result of better (∼97%) absorp- tion of solar spectrum [1]. Such cells are suitable for thin ﬁlm format, resistant to radiation damage and lend well with concentrators. In view of these advantages, we have developed InGaP/InGaAs/Ge triple junction solar cells with potential for commercialization. In order to improve the efﬁciency and its commercialization, we developed several technologies including a new metallization scheme, anti-reﬂection layer (sub-wavelength structure) [2], new epitaxy layer design (lattice match, current match) [3], utilization of Ge/Si substrate as a replacement for costly p-Ge [4] and cells amenable to Manuscript received August 19, 2014; revised September 16, 2014, September 17, 2014, and October 1, 2014; accepted October 3, 2014. Date of publication November 5, 2014; date of current version November 20, 2014. This work was supported in part by the NCTU-UCB I-RiCE Program, in part by the Ministry of Science and Technology, Taiwan, and in part by Taiwan Semiconductor Manufacturing Company, Ltd., Hsinchu, Taiwan, under Grant NSC-103-2911-I-009-302. The review of this letter was arranged by Editor J.-M. Liu. C.-H. Hsu, H.-J. Chang, and J.-S. Maa are with the Institute of Lighting and Energy Photonics, National Chiao Tung University, Hsinchu 300, Taiwan. E. Y. Chang is with the Department of Materials Science and Engineering and the Department of Electronic Engineering, National Chiao Tung Univer- sity, Hsinchu 300, Taiwan (e-mail: edc@mail.nctu.edu.tw). H.-W. Yu, H. Q. Nguyen, and C.-C. Chung are with the Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan. K. Pande is with the Department of Electronic Engineering, National Chiao Tung University, Hsinchu 300, Taiwan. Color versions of one or more of the ﬁgures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/LED.2014.2361923 concentrator technology. Our metallization scheme involves Cu based contacts, instead of conventional Au based contacts, both as front side interconnects and backside metallization. Further, as reported earlier [5], [6], Pd/Ge/Cu was chosen as contact for n-type GaAs and a novel Pt/Ti/Pt/Cu/Cr metal stack for p-type Ge for this letter. Utilizing fully copper based metallization InGaP/InGaAs/Ge triple junction solar cells were fabricated and their performance is reported here. For the Pd/Ge/Cu contact, the Pd layer can react with GaAs layer to form Pd 12 Ga 5 As 2 [7]. In addition, Ge can alloy with Cu to form Cu 3 Ge which is of low resistance and can prevent the Cu diffusion. Furthermore, these two compounds can create the Ga vacancies on the surface of GaAs which allows Ge to diffuse into these vacancies and form a high doping layer without depletion region. This provides easy carrier transport with low contact resistance [8], [9]. On the other hand, for the Pt/Ti/Pt/Cu/Cr contact, the ﬁrst Pt layer can effectively reduce the barrier height at the interface between the ohmic material and p-Ge substrate since Pt has high work function [10]. In addition, the second Ti layer and third Pt layer can effectively prevent Cu diffusion since Ti and Pt have high melting point. For the interconnect and back- side metals, Pt/Ti/Pt was used as the adhesion and diffusion barrier layers and Cu was evaporated as the seed layer after Pt/Ti/Pt deposition. Finally, the Cu layer was electroplated to 5-μm thick. For performance comparison, III-V solar cells with the conventional Au-based interconnects and the n-type Au/Ge/Ni/Au and p-type Ti/Pt/Au ohmic structures were also fabricated on the same GaInP/GaAs/Ge wafer. II. DEVICE FABRICATION For solar cell fabrication, n-GaAs epitaxial layer and p-Ge substrate will be the semiconductor layers for contacts on front side and backside of the GaInP/InGaAs/Ge wafer. The samples were ﬁrst cleaned in a the HCl solution to remove the native oxides and loaded into the evaporator. For Cu-metallization, Pd (15-nm)/Ge (150-nm) layers were deposited on the n-GaAs contact layer and Pt (5-nm)/ Ti (50-nm)/Pt (60-nm) layers were deposited on the p-Ge substrates by E-gun evaporator. Then Cu (150-nm) and Cr (10-nm) layers were deposited on top of the two types of interconnects by sputtering. Finally, the front side metal patterns were formed using the lift-off method and the rapid thermal annealed from 100 °C ∼390 °C. The samples were annealed for 30 sec in 0741-3106 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.