In Situ Electrical Studies of Ozone Based Atomic Layer Deposition on Graphene

One of the major advancements in the field of nanoelectronics in the last decade was the successful isolation of graphene [1], which led to the proposal of many novel electronic and photonic devices [2-4]. Graphene, a single sheet of carbon atoms packed in a hexagonal arrangement is expected to have interesting electronic properties which arise from its Dirac cone like band structure [2-4]. Due to its two-dimensional nature and exceptional properties such as high carrier mobilities, high saturation velocity and long mean free path, it has been predicted to play a major role not only in future CMOS applications, but also in post-CMOS electronics. Although graphene has a huge potential in future electronics applications, there are a number of technological issues which need to be resolved. This includes developing a technique for depositing highquality dielectrics on graphene. Deposition of dielectrics on graphene is important from the perspective of a gate insulator for CMOS applications, as a tunneling barrier in case of spin-FETs and also for passivation purposes. Atomic layer deposition (ALD) is the technique of choice in the recent times for depositing thin and conformal highdielectrics, but, graphene has a hydrophobic surface and an attempt to deposit oxides on it with a H2Obased ALD process results in non-uniform films [4-5]. In order to overcome this problem, thin seed-layers (functionalization layer) such as evaporated metal layers, self-assembled monolayers (SAMs), polymers are introduced on graphene prior to the ALD process [4-5]. But, such techniques result in an unwanted low quality interfacial layer which limits the scalability of the dielectric thickness, which is required for greater control of the graphene channel in an FET configuration. Several other techniques for direct deposition of dielectrics [5] such as physical vapor deposition (PVD), plasmaenhanced chemical vapor deposition (PECVD) have also been explored, but, such techniques either damage the graphene or result in unintentional doping in graphene. In order to overcome these issues our group has developed a novel technique based on ozone (O3) functionalization for depositing ALD oxides such as Al2O3 [6]. Top-gate graphene field-effect transistors (GFETs) were fabricated using this technique and mobilities as high as 5000 cm/Vs were achieved [6]. In this technique, first, a seed-layer of Al2O3 is deposited using 6 cycles of TMA/O3 at room-temperature (25 °C), followed by a higher temperature (200 °C) Al2O3 deposition using TMA/H2O in order to achieve a conformal and pinhole free dielectric on graphene [6]. It has been predicted based on First-Principle calculations that O3 can functionalize (adsorb on) the graphene surface either through a physisorption or a chemisorption process depending on the temperature and O3 concentration [7]. In order to understand the mechanism of O3 functionalization more carefully, the effects of O3 on charge transport properties of graphene were investigated using back-gated GFETs by exposing them to O3 and characterizing them in-situ. It was observed that when back-gated GFETs were exposed to O3 at room-temperature (300 K), the Dirac voltage (VDirac) or the minimum conductivity point shifts to a positive voltage and by increasing the partial pressure of O3 (or O3 exposure dose) there is a further positive shift in the VDirac (see Figure 1). This is because O3 adsorbed on the graphene surface acts as an acceptor and pushes the Fermi level (EF) into the valence band resulting in p-type doping of graphene (Figure 2). But, when the samples exposed to O3 were left in vacuum for an extended period of time, the mobility and VDirac of the GFETs returned to the original values. The observed charge scattering mechanisms and the effect on mobility due to the interaction of O3 with graphene as a function of O3 exposure dose will be presented. The effect of other common oxidants used in ALD process such as H2O, O2 and metal precursor such as TMA on the transport properties of graphene will also be presented.