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 proper setting of deposition parameters which affect film growth and result in morphological and structural changes ^4,5. By exploiting the method described here we demonstrated, for a number of oxides (e.g. WO₃, Ag₄O₄, Al₂O₃ and TiO₂), the capability to tune morphology, density, porosity, degree of structural order, stoichiometry and phase by modifying the material structure at the nanoscale ^6-11. This allows the design of materials for specific applications ^12-16. With reference to photovoltaic applications, we synthesized nanostructured TiO₂ hierarchically organized by assembling nanoparticles (<10 nm) in a nano- and mesostructure that resembles a 'forest of trees' ¹³ showing interesting results when employed as photoanodes in dye sensitized solar cells (DSSC) ¹⁷. Based on these previous results we describe the protocol for the deposition of Al-doped ZnO (AZO) films as a transparent conducting oxide.

Transparent conducting oxides (TCOs) are high bandgap (>3 eV) materials converted into conductors by heavy doping, displaying resistivity <10^-3 ohm-cm and more than 80% optical transmittance in the visible range. They are a key element for many applications such as touch screens and solar cells ^18-21 and they are typically grown by different techniques such as sputtering, pulsed laser deposition, chemical vapour deposition, spray pyrolysis and with solution-based chemical methods. Among TCOs, indium-tin-oxide (ITO) has been widely studied for its low resistivity but suffers from the drawback of the high cost and low availability of indium. Research is now moving towards indium-free systems such as F-doped SnO₂ (FTO), Al-doped ZnO (AZO) and F-doped ZnO (FZO).

Electrodes capable of providing an intelligent management of the incident light (light trapping) are particularly interesting for photovoltaic applications. To exploit the possibility to scatter visible light via structures and morphologies modulated at a scale comparable to the wavelength of light (e.g. 300-1,000 nm), a good control on the film morphology and on cluster assembly architectures is needed.

In particular we describe how to tune morphology and structure of AZO films. Compact AZO deposited at low pressure (2 Pa oxygen) and at room temperature is characterized by low resistivity (4.5 x 10^-4 ohm cm) and visible light transparency (> 90%) which is competitive with AZO deposited at high temperatures, while AZO hierarchical structures are obtained by ablating at O₂ pressures above 100 Pa. These structures display a strong light scattering capability with haze factor up to 80% and more ^22,23.

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 rinse in isopropanol, dry using a N₂ flow.

2. Laser Alignment and Selection of Laser Parameters

1. Warm-up the Nd:YAG laser and select IV harmonic emission (266 nm wavelength) by using a fourth harmonic generator (FHG) constituted by two second harmonic generators (SHG) in cascade.
2. Mount a 2%wt. Al₂O₃:ZnO circular target (2" diameter) on the target manipulator. Align the laser spot at the center of the target, start target rotation and translation and set the maximum vertical range. Check that the laser spot never touches the external steel ring used to fix the target. The target is moved with a roto-translational motion to have uniform ablation of the whole target surface.
3. Select repetition rate (e.g. 10 Hz) and pulse energy (e.g. 75 mJ). Adjust pulse energy and monitor laser stability by a power meter.
4. Move the focusing lens to a selected position and use a piece of sensitive paper attached to the target to measure the spot size. For any position of the focusing lens fire 1-5 laser shots on the paper. Select a lens position to have a laser fluence of about 1 J/cm².

3. Setting up PLD and Selection of Deposition Parameters

1. Alignment of substrate position
   1. Mount a circular paper sheet about 2" diameter as a substrate for alignment tests.
   2. Move the substrate holder to a target-to-substrate distance d_TS = 50 mm.
   3. Start pumping down the chamber with primary and turbomolecular pumps until the vacuum level reaches 10^-2 Pa.
   4. Select a gas type (i.e. oxygen) and adjust pumping speed and gas flow to have the proper gas pressure (see sections 4 and 5). Adjust x-y position of the substrate manipulator off-axis with respect to the plume center to obtain uniform film thickness over a circular corona.
   5. Start ablation by removing the beam stopper/power meter. If the target is new or if it was not used for long time, this pre-ablation is necessary to clean the target.
   6. Stop ablation when a deposit can be seen on the paper looking from a viewport.
2. Determination of plasma plume length
   1. Follow steps 3.1.1. to 3.1.5, during ablation take pictures with a digital camera with 0.5 - 1 sec accumulation time to average over different plasma plumes.
   2. Measure the visible plasma plume length from the pictures taking d_TS as a reference (see Figure 3).
3. Calibration of the film thickness
   1. Move the substrate far from the target (i.e. 100 mm and more) and move the Quartz Micro-Balance (QCM) at a distance equal to d_TS from the target.
   2. Deposit 1000 laser shots (i.e. 1' 40'') and measure the deposited mass value, then move the QCM away.
   3. Mount a Si substrate as in 1.1.
   4. Deposit a test sample (e.g. 18,000 laser shots, i.e. 30' ) and use cross sectional SEM images to calibrate the deposition rate (nm/pulse).

4. Deposition of Nanoengineered AZO Films

1. Mount the substrates prepared as in section 1 on the sample holder manipulator by using adhesive tape.
2. Follow steps 3.1.2 - 3.1.3.
3. Start substrate rotation.
4. Deposition of compact AZO films
   1. Switch on the ion gun and set ion energy at 100 eV, RF power at 75-100 W and Ar gas flux at 20 sccm (the pressure is in the 10^-2 Pa range). Clean substrates with Ar⁺ ion gun for 5-10 min. After the cleaning treatment close gas inlet and pump down the chamber to remove Argon.
   2. Insert oxygen gas and adjust pumping speed and gas flux to have 2 Pa oxygen.
   3. start ablation and deposit for 18,000 shots (30'). During ablation check that the plume length is the same as determined in step 3.2.
   4. stop ablation, close gas inlet, pump down the chamber.
5. Deposition of hierarchically structured AZO films
   1. Insert oxygen gas and adjust pumping speed and gas flow to have 160 Pa oxygen.
   2. start ablation and deposit for 18,000 shots (30'). During ablation check that the plume length is the same as determined in step 3.2.
   3. stop ablation, close gas inlet, pump down the chamber.
6. Vent the chamber and remove samples

5. Electrical and Optical Characterization

1. Measure the in-plane transport properties using four-probe techniques (i.e. Van der Pauw method). See Figure 4 for a scheme of the contacts. Typical values of the probe current are in the 1 μA to 10 mA range. The measurements is performed over a sample area reduced to 0.7 cm x 0.7 cm to ensure a better thickness uniformity (about 5%).
2. Measure the optical transmittance of the sample and of the bare substrate. Correct the spectra for the substrate contribution by setting to 1 the intensity at the glass/film interface. For a precise correction procedure make sure that the sample is mounted with the glass substrate facing the incident beam. Determine the visible light transparency by calculating the average transmittance in the 400-700 nm range. Use a 150 mm diameter integrating sphere to measure the scattered fraction of the light, the Haze factor can be calculated by dividing the scattered fraction by the total transmitted light (i.e. scattered and forward transmitted), see Figure 5.

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