Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Summary

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Abstract

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O₂ pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (>80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO₂, Al₂O₃, WO₃ and Ag₄O₄.

Introduction

Pulsed Laser Deposition (PLD) employs laser ablation of a solid target which results in the formation of a plasma of ablated species which can be deposited on a substrate to grow a film (see Figure 1) ¹. Interaction with a background atmosphere (inert or reactive) can be used to induce homogeneous cluster nucleation in the gas phase (see Figure 2) ^2,3. Our strategy for material synthesis by PLD is based on the tuning of material properties in a bottom-up approach by carefully controlling the plasma dynamics generated in the PLD process. Cluster size, kinetic energy and composition can be varied by a prope....

Protocol

1. Substrate Preparation

1. Cut 1 cm x 1 cm silicon substrates from a Si wafer, Silicon is good for SEM characterization (plane view and cross section).
2. Cut 1 cm x 1 cm glass (soda-lime, 1 mm thick), glass is optimal for optical and electrical characterization.
3. If contacts are needed on glass substrates, Au contacts can be evaporated in vacuum by using a mask. Deposit 10 nm of Cr as an interlayer to improve adhesion of Au, deposit 50 nm of Au.
4. Cut 1 cm x 1 cm polymer sample (e.g. Ethylene Tetrafluoroethylene, ETFE).
5. Clean the substrates by sonicating in isopropanol for 5-10 min and rins....

Results

The deposition of AZO by PLD in oxygen atmosphere produces compact transparent conducting films at low background gas pressure (i.e. 2 Pa) and mesoporous forest-like structures constituted by hierarchically assembled clusters at high pressures (i.e. 160 Pa). The material is constituted by nanocrystalline domains whose size is maximum (30 nm) at 2 Pa ²².

Due to collisions between the ablated species and the background gas, the shape and length of the plasma p.......

Discussion

The plasma plume shape is closely related to the ablation process, especially in the presence of a gas; monitoring the plasma plume by visual inspection is important to control the deposition. When depositing a metal oxide by ablating an oxide target, oxygen is needed to support oxygen losses during the ablation process. At lower oxygen background gas pressure, the deposited material may have oxygen vacancies. This effect is reduced by increasing the gas pressure. To separate stoichiometry from morphology gas mixt.......

Materials

Name	Company	Catalog Number	Comments
Name of Reagent/Material	Company	Catalog Number
Pulsed Laser	Continuum-Quantronix	Powerlite 8010
Power meter	Coherent	FieldMaxII-TO
Ion Gun	Mantis Dep	RFMax60
Mass flow controller	Mks	2179 °
Quartz Crystal Microbalance	Infcon	XTC/2
Background gas	Rivoira-Praxair	5.0 oxygen
Target	Kurt Lesker	(made on request)
Isopropanol	Sigma Aldrich	190764-2L
Source meter	Keithley	K2400
Magnet Kit	Ecopia	0.55T-Kit
Spectrophotometer	PerkinElmer	Lambda 1050