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In this section the technique of magnetron sputtering and the subsequent processing steps used in device fabrication are described in detail. Common features of the deposition and processing are presented but a discussion of conditions and parameters associated with specific materials or experiments are deferred until the relevant points in subsequent chapters.

4.2.1 Radio frequency magnetron sputtering

Radio frequency magnetron sputtering (RFMS) is a thin-film deposition technique that can be used to deposit a wide range of dielectric and semiconductor materials. Figure 4.1 shows schematic of the magnetron/substrate arrangement used within this work. The magnetrons contain a symmetrical configuration of permanent magnets to which a disc of the desired target material is fixed. They are water cooled to prevent the targets from overheating. A summarised description of the sputtering process, from a single target, is as follows:

ˆ The vacuum chamber is filled with a pressure (1 - 20 mTorr) of argon.

ˆ An RF voltage (13.56 MHz) is applied accross the substrate holder and the mag-netron.

ˆ Residual electrons, within the vacuum chamber, are accelerated by the voltage and confined to spiral paths around the magnetic field lines at the target surface.

ˆ The Ar atoms within the region of the target surface are ionized by electrons, forming a plasma, and are accelerated towards the target. The motion of the Ar⁺ ions is not influenced by the magnetic field because of their large mass, relative to electrons.

Figure 4.1: Schematic of sputtering chamber.

Figure 4.2: AJA Orion Phase II-J dual chamber sputtering kit.

ˆ Ar⁺ ions that impact upon the target surface knock out neutrally charged species of target material which travel, with long mean free paths, until they encounter the substrate.

For a more detailed description of the principles behind the sputtering process and technical reviews of the current state of the art the reader is referred to references [1–5].

The use of an RF voltage is necessary for dielectric materials in order to overcome the build-up of positive charge at the target surface. Because of this, the growth rates associated with RFMS are typically half those of direct current magnetron sputtering (DCMS) as the Ar⁺ ions spend as much time travelling away from the target as travelling towards it. In general, it is only possible to use DCMS to deposit from metallic targets where the high conductivity of the materials prevent a charge build-up. In this work, RFMS was performed using an AJA Orion Phase II-J dual chamber kit (figure 4.2). Chamber 1 was used in the deposition of metal-oxides (i.e. In2O3, SnO2, ZnO, TiO2, SiO2) and chamber 2 was used to deposit semiconductor films required for PV devices (i.e. CdS and CdTe). Features of the kit include:

ˆ Substrate heating: halogen lamps located behind the substrate holder were used to heat substrates to a maximum of 800^◦C.

ˆ Growth monitoring: A quartz crystal oscillator, located in the vicinity of the sub-strate, could be used to monitor the change in film thickness during deposition. How-ever, it was necessary to calibrate this oscillator against a physical measurement, e.g.

surface profilometry (section 4.3.3).

ˆ Reactive gases: incorporation of H2, O₂, CHF₃ and CF₄ gases to the chamber during deposition permitted reactive sputtering, achieving reduction, oxidation and doping of resultant films respectively. Gas flow rates were controlled by mass flow controllers (MFCs) that permitted flows in the range 0.15 - 10 sccm.

ˆ Co-sputtering: the arrangement of multiple magnetrons within each chamber (four in chamber 1 and two in chamber 2) permitted materials from different targets to be deposited simultaneously and the growth rates from each to be controlled indepen-dently.

ˆ DC bias: The application of a DC voltage between the substrate and chamber gener-ated a plasma that was confineable to the substrate surface and used to back-sputter from the substrate surface. This feature was used exclusively prior to deposition to clean the glass substrates.

ˆ Load lock: substrates were loaded into chambers via a load lock so that the chambers did not have to be opened after each run. This helped to preserve the conditioning of the targets and dramatically reduced the pumping time required.

ˆ Chamber transfer: the substrates were transferred between chambers 1 and 2 without breaking vacuum.

ˆ Computer control: All runs could be programmed and completely automated using a LabView based program. This was particularly useful for the deposition of com-plicated multi-layer structures and eliminated the potential for human error during growth. The use of datalogging software during growth also ensured that if a run was aborted then it was possible to determine why.

The three key deposition parameters of RFMS are RF power, pressure and substrate temperature. Some general characteristics relating to the effect on the growth rate of varying these three parameters include:

ˆ A linear increase in the growth rate of a material with RF power.

ˆ An increase in growth rate with a reduction in pressure.

ˆ A relatively small decrease in growth rate with increasing substrate temperature.

4.2.2 Device fabrication and processing

The following is a brief description of how CdTe devices were prepared from start to finish.

In general all devices were deposited onto 10 cm × 10 cm × 3.2 mm low Fe soda-lime glass (SLG) substrates provided by Pilkington (OptiWhite). The thickness uniformity of layers deposited on this scale was deemed excellent with a typical variation of < 5% accross each layer. Following deposition, via RFMS, the 10 cm × 10 cm samples were cut into either four 5 cm × 5cm or sixteen 2.5 cm × 2.5 cm pieces. Doing this permitted investigations into the effect of multiple post growth treatments, the details of which are deferred to section 6.3.2.

Glass cleaning

Prior to deposition the substrates underwent the following cleaning regime:

ˆ EX SITU

– Scrub with nylon brush and de-ionized (DI) water.

– Ultrasonication in boiling DI water + 2% Decon90 detergent.

– Rinse with DI water followed by rinse with isopropanol alcohol.

– Nitrogen blow dry.

ˆ IN SITU

– 10 - 15 min DC bias plasma etch: RF power = 50 W, pressure = 5 mTorr (Ar only)

RFMS of device structures

Each target was pre-sputtered, under the conditions of the subsequent film deposition, for a minimum of 10 mins prior to the opening of the shutter (located directly above target surface). This helped to ensure that any contaminants were removed from the target surface before deposition. The TCO (ITO) and buffer (ZnO) layers were deposited firstly

in chamber 1 (see section 4.3.1) and then the sample was transferred, under vacuum, to chamber 2 where films of CdS and CdTe were deposited. The sample underwent natural cooling to room temperature (typically taking ∼ 1 hour) before being removed via the load lock.

Post growth treatment with CdCl₂

The 10 cm²samples were quartered into 5 cm²pieces and films of CdCl₂were deposited onto the CdTe surfaces of each piece via vacuum evaporation using a custom built evaporator unit. A CdCl₂ thickness of 200 nm, determined by a callibrated oscillator crystal, was maintained for all samples within this work. Each piece was then quartered again into 2.5 cm² pieces and annealed, one by one, in a tube furnace in air. A range of annealing temperatures (375 - 420^◦C) and times (5 - 35 min) were used, the specific details of which are described at the relevant points in Chapter 7. Following annealing, each piece was rinsed in warm DI water and dried with nitrogen prior to contacting. Note that no chemical etch, commonly used for such devices, was performed prior to contacting.

Contacting

Back contacts were formed by vacuum deposition of Au (99.999% purity) in a Leybold Univex 300 evaporator system. The contacts, deposited using a mask array, were square and 5 mm × 5 mm in size. It was possible to deposit nine such contacts onto each 2.5 cm² piece. A front contact was made by using a scalpel blade to scratch away part of the CdTe film at the edge of each piece. A cotton bud swab of a dilute hydrochloric acid was used to remove the CdS beneath this and reveal the ZnO layer to which a direct contact could be made during electrical characterisation of the device (see section 4.4).