Low-optical-loss, low-resistance Ag/Ge based ohmic contacts to n-type InP for membrane based waveguide devices

L Shen,G Roelkens,V Dolores-Calzadilla,J.J.G.M Van Der Tol,D Heiss,Y Jiao,C.W.H.A Wullems,H.P.M.M Ambrosius,A Higuera-Rodriguez,A Millan-Mejia,M.K Smit

doi:10.1364/ome.5.000393

Abstract

We present the development of Ag/Ge based ohmic contacts to n-type InP with both low contact resistances and relatively low optical losses. A specific contact resistance as low as 1.5×10−6 Ω cm2 is achieved by optimizing the Ge layer thickness and annealing conditions. The use of Ge instead of metal as the first deposited layer results in a low optical absorption loss in the telecommunication wavelength range. Compared to Au based contacts, the Ag based metallization also shows considerably reduced spiking effects after annealing. Contacts with different lengths are deposited on top of InP membrane waveguides to characterize the optical loss before and after annealing. A factor of 5 reduction of the propagation loss compared to the conventional Au/Ge/Ni contact is demonstrated. This allows for much more optimized designs for membrane photonic devices.

Highlights

The ever-growing demand in data transport networks has promoted the development of highdensity high-speed photonic integrated circuits [1]
The thermal stability is improved compared to Au/Ge systems due to a much higher eutectic phase temperature of the Ag/Ge alloy [10]. We investigate these potential advantages of Ag/Ge as a new ohmic contact solution for InP-membrane based photonic devices
In order to check the contact-InP interface and the spiking effect due to annealing, scanning electron microscope (SEM) cross-sectional images are taken on focused ion beam (FIB) cut facets

Summary

Introduction

The ever-growing demand in data transport networks has promoted the development of highdensity high-speed photonic integrated circuits [1]. A more advanced solution involves Ni as the first deposited layer for its ability to consume native oxides and the possibility of forming compounds at the InP surface with a lower barrier height [5] To our knowledge, this optimized contact holds the lowest resistance level and is still being used in many InP based devices. Ag is a widely used metal in plasmonic devices due to its very low optical loss It can be deposited with an adhesion layer, like Ge, to different substrates [8, 9]. The significantly lower optical absorption of Ge at 1550 nm compared to that of conventionally used metal adhesion layers (like Ni, Ti or Cr) promises low loss contacts. The effects of Ge on the contact resistance and the optical loss are discussed

Specific contact resistance

Annealed contact interface

Waveguide loss measurements

Conclusion