Plasma-Gas-Condensation Deposition of Nb Clusters to Obtain Giant Permittivity 9 Jan 2013 Jennifer DeCerbo Materials Engineer AFRL/RQQE

Program Goals Overall: Use the giant permittivity (ε ~1010) of Nb12 clusters to enhance the permittivity of a dielectric film in order to produce ultrahigh energy storage devices Year 1: Demonstrate preliminary formation of Nb clusters using procured cluster deposition source Year 2: Optimize process inputs (sputtering power, gas flow rates, condensation length) for desired cluster size and distribution Year 3: Investigate the approach of incorporating small size Nb clusters into a thermally stable, high resistance dielectric (ex. carbon nitride or anodized alumina) to develop high energy storage materials

Why Nb Clusters? Unique Properties Inert nature of specific formations (Nb12) Ferroelectric effect (dipole moments) Potential for giant permittivity when embedded in dielectrics (Gor’kov-Eliashberg effect) Potential material for ultrahigh energy storage devices Novel high-k dielectric for energy storage, filtering, and signaling applications

First Phase Goal Study the effects of process conditions on cluster formation to develop a better understanding of: Mechanisms of the nanoparticle formation How to control nanoparticle diameter and size dispersion Parameters: Argon Flow Rate Source Current Aggregation Length

The Deposition System In-house developed software Controls gas flow rates and source voltage Monitors temperature and pressure Mantis Nanogen50 Deposition System 2” sputter source 4 mm/ 5 mm/ 6 mm apertures 2 - 106 amu mass filter 30-130 mm aggregation length 1-15 nm cluster size 5-175 sccm flow rate (Ar) 2 gas inlets Liquid cooled cluster aggregation zone

Cluster Formation Mechanism Cluster formation is a two step process Initial cluster nucleation Cluster growth Metallic Nanocluster growth theory Quesnel, et. al., J. Appl. Phys. 107, 054309, 2010 Metallic vapor temperature must drop below a critical value for stable clusters to nucleate Quantified as intensity of peak in mass spectrum Cluster size depends the length traversed in the growth/aggregation zone after nucleation Quantified as position of peak in mass spectrum

A Unique Feature of Nb Clusters A dual cluster distribution has been observed under certain conditions, which is a unique feature of Nb clusters – not reported by many studies in literature on metals other than Nb Nb mass spectra profiles varying the Ar flow rate Source current = 200 mA Aggregation length = 35 mm Some lines have been offset upward for display clarity The broader peak has a varying location, typically ranging from 5–10 nm diameter The second peak at ~1 nm diameter appears at flow rates > 120 sccm

A Unique Feature of Nb Clusters (cont.) We consider the peak at ~1 nm diameter to be stable and structured clusters, rather than simple condensation products, for two reasons: The cluster size remains constant at various experimental conditions. In a particular experiment with a moderate source current and a high Ar flow rate, the peak intensity was observed to decrease as the aggregation length changes from 0 to 50 mm, which is different from the larger clusters. It appears to suggest a different formation mechanism than the simple condensation process, i.e., one that requires a minimum temperature to overcome the reaction barrier.

Other Results Atomic gas temperature is key to cluster nucleation and growth There is a critical temperature above which clusters are unstable and will not nucleate We can alter cluster size and intensity by manipulating the impact of processes inputs on the temperature There is a range for Ar flow rate within which we can control the cluster size as expected by the mean of varying the aggregation length

Argon Flow Rate Dependence (source current=200mA, aggregation length=35mm) Nucleation Increased Ar density from increased flow enhances frequency of 3-body collisions assisting in the nucleation of clusters Above a threshold, further increasing Ar flow rate begins to collimate the plasma, reducing heat transfer with the chamber walls, and therefore reducing nucleation Cluster Growth Below a critical level, the Ar concentration is too low to effectively conduct heat to the chamber walls Above a flow threshold, the plasma begins to collimate, reducing heat transfer with the chamber walls, reducing nucleation and creating a shorter growth region

Source Current Dependence (Ar flow rate=80sccm, aggregation length=100mm) Nucleation Increased current increases the atomic metal species, facilitating nucleation Above a threshold, further increasing current raises the temperature too high for efficient nucleation Cluster Growth At lower currents, the temperature profile facilitates nucleation to occur earlier, allowing longer time for cluster growth At higher currents, the temperature profile reaches a steady state, resulting in a constant cluster size distribution

Aggregation Length Dependence (Ar flow rate=40sccm, source current=200mA) Nucleation As the length increases, the nucleation zone is enlarged and more clusters are formed Above a threshold, the effect of diffusion-and-quench of clusters dominates Cluster Growth As the length increases, the enlarged nucleation zone results in both increase of the cluster size and the size dispersion

For larger clusters (red) For smaller clusters (blue) Aggregation Length Dependence (Ar flow rate=150sccm, source current=200mA) For larger clusters (red) At short lengths, there is a narrow region in which nucleation can occur, resulting in a smaller number of clusters As the length increases, the nucleation zone is enlarged Above a threshold, the effect of diffusion-and-quench of clusters dominates For smaller clusters (blue) At longer lengths, the intensity follows the same trend as the larger clusters At lengths below 50 mm, the intensity decreases as the length increases, suggesting a different formation mechanism than the larger clusters

Future Work Having gained knowledge of the cluster source and the nucleation mechanisms, we will move on to the study of the formation of a special cluster, Nb12. It requires a hardware modification, in particular of that related to the mass filter, for the purpose of accurately detecting the mass of the clusters in the mass range of 100-2000 amu Perform surface deposition and analysis to study the effects of the cluster size and size dispersion on the properties of the modified surface

Personnel Name Degree Discipline Air Force Personnel Jennifer DeCerbo M.S. Chemistry Biswa Ganguly Ph.D. Physics Contractors Kevin Bray Chemical Engineering William Lanter B.S. Charles Jiao Physical Chemist