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4.2.1 Close-space sublimation
Close-space sublimation (CSS) is a form of physical vapour deposition (PVD) and was used in this work to deposit CdTe NWs and CdTe thin films. Whilst evaporation of CdTe can prove challenging and costly due to the material’s high melting point (1092°C), direct
sublimation can be achieved at temperatures as low as 500 – 600°C, owing to CdTe having a sufficiently high vapour pressure (0.1 Torr at 600°C). However, to achieve suitable deposition rates for thin-film growth (> 1μm/min) using sublimation, it is desirable to place the target substrate in close proximity (typically a few mm) to the source material so as to ensure
efficient transfer of material from the source to the substrate. In the sublimation process, upon heating, CdTe dissociates into elemental vapours which then recombine at the substrate surface, as follows;
( ) ( ) ( ) ( ) Other materials that have high melting points, such as CdS, may be deposited using CSS,
provided they have a sufficiently high vapour pressure at the temperatures used. A schematic diagram of the system used, custom built by Electro-Gas Systems Ltd., is shown in Fig 4.1.
In this system, powdered CdTe source material (bought from Alfa Aesar, 5N pure) is placed in a source tray, above which the substrate is held - face down, 5 mm from the source. The source tray, substrate holder and chamber are constructed from high purity quartz, so to
55 limit the introduction of impurities. Two external heating elements are used, one situated below the source, and one situated above the substrate. These heaters are on rollers and are removable. They are separately controlled, although their heating cannot be completely decoupled due to their close proximity - the diameter of the chamber is ~ 110 mm.
Fig 4.1: Schematic diagram of the close space sublimation chamber used in this work
Nevertheless, a sufficient temperature gradient from source to substrate can be achieved to promote sublimation transfer. Thermocouples are used to monitor the source and substrate temperatures, Tsource and Tsub. A removable shutter could be used to start and stop growth. Since for Tsource > 500°C the growth rate could be as high as 5 μm/min. The use of the shutter was sometimes necessary for the accurate control of layer thickness.
The growth tube may be pumped to ~ 10-1 Torr using a scroll pump although most growth runs in this work were conducted under either N2, O2, H2, or N2/O2 or N2/H2 mixtures. These
gases could be introduced to the growth chamber as a static fill in the pressure range 1 – 700 Torr, or as a flow of up to 100 sccm (standard cm3 min-1).
This equipment provides a wide parameter space for growth via control of Tsource, Tsub, gas
pressure and mixture and deposition time. These variables are known to have a significant effect on the deposition rate and grain structure of generated thin-films. For instance, Major et al.1demonstrated that grain size in CSS-grown CdTe increased linearly with N2 pressure, the
grain size having a profound effect on the performance of CdTe solar cells. The exact growth conditions used in this work are included in the relevant results chapters.
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4.2.2 Radio frequency magnetron sputtering
Sputtering can be used to deposit a wide range of dielectrics, metals and semiconductors. Sputter deposition involves the ejection of material upon bombardment of a source ‘target’ with an ionised gas (i.e. a plasma), and the subsequent transfer of the ejected material onto a substrate. For the plasma, typically a non-reactive gas such as Ar is used, which is ionised and
56 accelerated by a DC or RF potential, the former for conducting and the latter for resistive materials. The ejection process occurs as a result of the momentum transfer between the impinging ions and the atoms of the target being bombarded. The transfer of material from source to substrate is a ballistic process as the pressures within the sputtering chamber are typically just a few mTorr. For magnetron sputtering, an arrangement of permanent magnets is situated behind the target to confine the plasma at the target surface, so as to enhance sputter rates. For more in-depth explanations of the principles of sputter deposition, the reader is referred to Refs 2-4.
Fig 4.2: a) Photograph of the AJA Orion Phase II-J dual chamber system. b) Schematic diagram of the sputtering set-up inside each chamber.
In this work, RF magnetron sputtering was used to deposit thin films and NW coatings of CdS, ZnO and In2O3:Sn (ITO). Fig 4.2a shows a photo of the sputtering system used here, an
57 magnetron/substrate configuration within each chamber, although each chamber may
accommodate multiple magnetrons. A brief description of the deposition process in this system from a single target is now given. For further details, the reader is referred to the PhD thesis by Treharne5.
The chamber is pumped to high vacuum (~ 10-6 – 10-7 Torr) and Ar is introduced at pressures of 1 – 20 mTorr. The AJA system uses the ‘sputter up’ configuration and the substrate is heated by halogen lamps to up to 800°C. The target is rotated during growth to improve uniformity; this is necessary as the targets are not placed centrally. Reactive gases may also be introduced during growth: In Chapter 7, the use of CdS:O films, generated by sputtering CdS in the presence of O2, is presented (CdS:O has a wider band-gap than CdS -
see Chapter 7 - and therefore may be more suitable as a window layer). Deposition rates are typically of the order ~ 10 nm/min, and sputter generated films commonly consist of grains < 50 nm in diameter.
For sputter deposition variation of the growth conditions is well-known to influence both the growth rate and properties of the films. While there are some general trends – i.e.
increasing the RF power and decreasing the gas pressure both have the effect of increasing the growth rate – the details may be system dependent. The reader is referred to Ref. 5 for a specific report of the electrical and optical properties of ITO, ZnO, SnO2, CdS and CdTe
prepared in this particular sputtering system.
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4.2.3 Chemical bath deposition
Chemical bath deposition (CBD) is achieved via the precipitation of a solid phase from a solution onto a substrate following chemical reactions. The substrate is immersed in the aqueous solution that contains the precursors. CBD is a convenient and low cost technique conducted at relatively low temperatures (< 100°C), and is often used to grow the window layers for thin-film solar cells6. Grain sizes are typically small (< 50 nm) therefore the films can be grown relatively thin (< 100 nm) whilst maintaining high levels of surface coverage. For the case of CdS, CBD is achieved by the thermal decomposition of aqueous thiourea (SC(NH2)2) in the presence of Cd2+ (aq).7
In this work, CBD was used to coat CdTe NWs with a CdS shell layer. A solution of 318.2g H2O + 31.8g NH4OH was prepared, which was put into a reaction vessel and heated to
67°C by a water bath. 0.1456g of CdSO4 was then added to the vessel. NW samples on
substrates were then suspended in the vessel and 0.333g of thiourea was added to initiate the reaction. A nitrogen ‘bubbler’ was put in place in the vessel and used intermittently during the reaction (once a minute) to displace any gas bubbles that may accumulate on the substrate
58 surface. Following deposition, samples were removed and rinsed in DI water. Using this method, layers of ~ 100 nm were deposited after 20 minutes of reaction time. In this work, CBD was carried out at Northumbria University.
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4.2.4 Metal-organic chemical vapour deposition
In metal organic vapour deposition (MOCVD), organometallic precursors are thermally decomposed over a heated substrate, normally in the presence of hydrogen, often at
atmospheric pressure. A polycrystalline deposit forms on the substrate (MOCVD) or in the case of a single crystal substrate, an epilayer may form (metal organic vapour phase epitaxy, or MOVPE). Dopants may be introduced in organometallic or gaseous form and the method is capable of high levels of materials control. High performance CdTe solar cells have been generated by MOCVD at the University of Glyndwr8, where MOCVD was carried out in this work.
A schematic diagram of the system used here is shown in Fig 4.3. In this work, CdZnS and CdS shell layers were deposited onto CdTe NWs. The precursors used were dimethylcadmium (DMCd), ditertiarybutylsulphide (DtBS) and diethylzinc (DEZn). H2 was used as the carrier
gas, controlled using mass flow controllers. Reactions took place with the substrate held at 360°C for 15 – 20 mins; heating being provided by an RF heater. Film thickness was monitored and controlled in-situ by a triple wavelength laser reflectometer.
Fig 4.3: Schematic diagram of the metal-organic chemical vapour deposition system.
4.2.5 Thermal evaporation
Thermal evaporation is a common PVD technique to deposit thin films and involves the vaporisation of a source from liquid phase upon heating under vacuum. In this work, Au and CdCl2 were deposited using thermal evaporation, in separate systems. For Au, a UNIVEX 300
59 evaporator, having a base pressure of 10-4 Torr and a quartz crystal thickness monitor was used. For fine control of layer thickness, the growth could be lowered to 5 nm/min. Patterned growth could be achieved by placing substrates in contact with a shadow mask. Au of 99.99% purity was obtained from Advent.
CdCl2 was evaporated in a custom built diffusion pumped system operating at 10-5 Torr.
The CdCl2 was of 99.99% purity and was from Aldrich Ltd. In CdTe device fabrication,
doping with CdCl2 is achieved by evaporation of a layer onto the CdTe surface followed by
annealing 9 - in this work the annealing step was carried out in a tube furnace.