Tuning of Surface Properties of AlGaN/GaN Sensors for Nanodroplets and Picodroplets

Modifications of AlGaN surfaces have been carried out in order to tune their wetting properties. A hydrophilic surface is achieved by a wet and dry thermal oxidation, whereas the deposition of fluorocarbon (FC) layers leads to a passivation with a hydrophobic behavior. It was found that both surfaces still change their properties in the first days/hours after the modification. For the FC layers, differences are observed in dependence of the deposition method, which are explained by the different chemical-bond structures of the various FC films

I. Introduction

Group-III nitrides are known to be pyroelectric, i.e., they exhibit a spontaneous and piezoelectric polarization [1]. At their heterointerfaces, a polarization-induced sheet charge is formed, which attracts mobile electrons to form a dense two-dimensional electron gas [2]. Since its sheet carrier density is strongly influenced by the surface potential, AIGaN/GaN heterostructures are very promising candidates as sensors for ions, polar liquids, and gases. Due to their physical and chemical stability, they even work at high temperatures or in hostile and toxic environments. Such a sensor mainly consists of a gateless transistor structure with two ohmic contacts. The details of the sensing mechanism have been published by Neuberger et al. [3], [4]. This paper is focused on the optimization of such heterostructures by surface modifications to produce a sensing device for smallest liquid droplets with volumes less than 1 nL, which allows a fast and cheap screening of rare substances, expensive pharmaceuticals, bacteria, etc. For a good sensor performance, a hydrophilic surface is necessary to make the droplets spread over the whole active area. A hydrophobic surrounding supports a self-positioning effect and protects the ohmic metal contacts from contamination [5]. The hydrophilic surface is obtained by a dry or wet oxidation of the AIGaN surface. Fluorocarbon (FC) layers are investigated as a hydrophobic periphery. Their chemical inertness against most liquids ensures that no reaction with the measuring specimen takes place.