Chapter 2 Experimental techniques
2.3 Device fabrication techniques
2.3.1
Photolithography
Photolithography, also termed as optical lithography or UV lithography, is a common process used in microfabrication to pattern parts of a thin film or a substrate. It uses UV light to transfer a pre-defined geometric defined pattern to a light-sensitive organic photoresist on the substrate. Generally, photoresist can be classified as either a positive or a negative resist, depending on the changes that occur under UV exposure. For the positive resist, exposure results in destruction of polymer bonds, increasing the resist solubility so these areas will be dissolved when placed in a developer. Hence resist is left only where the mask was opaque and the pattern is transferred onto the substrate [11, 12]. Negative resists behave in the opposite way, those unexposed areas will be developed away.
In this work Karl Suss MA6 Mask Aligner was used to fabricate solar cell devices. To fabricate the solar cell devices investigated in this thesis, samples were thoroughly cleaned by acetone, isopropanol and dried in oven at 85 °C for 3 minutes. Then the AZ 5214 E positive photoresist was spun on it at 4000 rpm for 30 s. After a 15 minutes soft-bake at 85 °C, the samples were individually placed into the MA6 mask aligner. They were aligned and then exposed to UV light from a mercury lamp for 15 s. UV exposed sample is immersed in the developer for 30-50 s (constant visual inspection is required), then rinsed with deionised (DI) water. The resist was developed in AZ 726 MIF photoresist developer, which is the solution of tetramethylammonium hydroxide in water. After thoroughly rinsing in DI water and examination under an optical microscope, the samples were hard-baked for two minutes at 110 °C. The photoresist pattern will be used as either etching mask that allows separation of top and bottom contact or define of contact pattern through subsequent metal deposition and lift-off.
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2.3.2
Wet chemical etching
Wet chemical etching is widely used for fabrication of semiconductor devices. When used in conjunction with a photolithographic process, it enables patterns to be transferred from a designed mask to the semiconductor wafer. Wet etching is a low-cost and convenient technique due to its simplicity and scalability, where the substrate is just immersed in a solution for certain time, such as an acid, a base and/or an oxidizing agent. Etching rate depends on many parameters, including solution composition and concentration, semiconductor wafer orientation and material composition, etching temperature etc. With certain concentration of a etching solution, etching temperature can also greatly affect the etching rate with a higher etching rate achieved at higher temperature. Normally, when more precise etch depths are needed the solution can be cooled down or diluted to lower the reaction speed, vice versa, heating and higher concentration can be used to increase the etching rate. Most of the etching works were performed at room temperature in this thesis. A variety of etching solutions were used for different fabrication processes in this thesis, and the main etchants are listed in Table 2.1:
Table 2.1: Etchants solution used for wet etching of different materials
Materials Etchant rate (by volume) Temperature
(°C) Comments
(Al)(In)GaAs Citric acid: 30% H2O2: H2O= 3: 43: 150
Room
temperature Selective etching GaAs 85% H3PO4: 30% H2O2:
H2O= 1: 1: 3
18 Most GaAs based III-V materials GaOx+ AsOy 36% HCl: H2O= 1: 9
Room
temperature GaAs native oxide etching SiOx+ SiNy 48% HF: H2O= 1: 9 4 Si based dielectric films
The chemical mechanism of wet etching of GaAs-based materials is the oxidation of the surface to form Ga and As oxides, and followed by the dissolving of these oxides by chemical reaction with acids or bases. Hydrogen peroxide is normally served as an oxidising agent to promote the formation of the surface oxides in most cases. A simplified representation of the chemistry typically involved in GaAs etching is given by the following equations [13]. To initiate the etching process, an oxide is formed on the surface of the GaAs by reaction with an oxidising agent, typically H2O2:
GaAs + (oxidiser) → GaOx/AsOy (2.3) The surface oxides are then attacked by either an acid or a base and will be dissolved to release Ga3+ and AsO42- ions in solution:
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GaOx/AsOy + (acidorbase) → Ga3++ AsO 4
2− (2.4) The wet etching rate for GaAs is about 1.61 μm per minute. Selective etching was used to remove the surface heavily doped GaAs capping layer while preserve the AlGaAs window layer. The etching rate for GaAs is ~6 nm per second, and ~0.4 nm per second for Al0.45Ga0.55As, the rate difference is high enough to etch two layers selectively. More operation details are shown in the section 2.4.
2.3.3
Thermal and Electron-beam evaporators
For relatively large amount and quick metal deposition, thermal and electron- beam (E-beam) evaporators are the two most common equipment used for the device fabrication.
An Edward Austo 306 thermal evaporator enables deposition of a wide range of metals for various research projects. In this deposition technique, thermal energy is applied to the source material, which then evaporates after melting. These are line-of-sight depositions that take place in ultra-high vacuum conditions, where the material to be evaporated is placed inside a tungsten boat and the material is then heated resistively. Although the process is simple, the deposited materials using this method are easy to be contaminated, and cannot be used to deposit materials with high melting points [15]. This method was used to fabricate solar cell devices for growth optimization.
Alternatively, another method of depositing films on the surface of samples is by E-beam evaporation. The materials to be deposited are heated by a focused beam of electrons to a high temperature until it evaporates. The material then recondenses on all the surfaces of the evaporation chamber, including on exposed samples. In this thesis a Temescal BJD-2000 E-beam Evaporator system was used for the deposition of metal contacts (Ti, Pt, Ge, and Au) during solar cells processing to achieve Ohmic contact [15]. The system also includes an extended chamber for a larger distance between sources and samples to avoid excessive heating during thick metallization for consequent lift-off process.
2.3.4
Magnetron sputter coating technique
Magnetron sputter coating technique is a physical vapour deposition (PVD) process. During sputter deposition, the source material is a solid target and it is sputtered away by an ionised working gas (in the form of glow-discharge
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plasmas), such as Ar. A DC or radio frequency (RF) power is applied between the substrate and a metal target, which acts as the cathode, to create the discharge. As the target is bombarded with ions, the surface is eroded away and material is deposited onto the anode (substrate).
In this thesis, the AJA sputtering system was used to deposit TiO2 film on the QD solar cells as the base to form Ag nanoparticles, which will be discussed in Chapter 6. The AJA system is equipped with 6 magnetron guns (3 x DC, 3 x RF) with up to 800 ℃ substrate heating and the capability for deposition of a wide range of metal and oxide materials. The sputter has a load-lock that allowing a maximum wafer size of 100mm. A large range of metals and dielectrics are available [16].
2.3.5
Thermal annealing
2.3.5.1 Rapid thermal annealing
Rapid thermal annealing (RTA) processes are commonly used to alloy metals to form Ohmic contacts on III-V semiconductors. Also, RTA at high temperatures is carried out to produce interdiffusion effects in the devices. In this thesis, metal contact alloying and post-growth thermal annealing for QD solar cells and GaAs reference cells were accomplished by rapid thermal annealing [17]. Annealings were carried out in an AET thermal RX rapid thermal annealer or a Qualiflow JetFirst 100 annealing equipment. Cross- contamination is minimised through the use of material-specific RTA liners or graphite holders for these two machines respectively. All the anneals were carried out under high-purity Ar ambient and with fresh GaAs wafer as proximity capping to prevent As desorption which occurs at above 400 °C. The temperature was ramped up at 100 °C/s and after annealing the temperature was allowed to fall naturally under the Ar ambient. For contact metallization, devices were heated up to 400 °C for 1 minute. For post-growth thermal inter-diffusion study the QD samples were annealed at temperatures from 700 to 850 °C with the step of 50 °C for 50 s respectively. The exhaust gases and particulates are safely extracted from the laboratory.
2.3.5.2 Furnace annealing
In Chapter 6, conventional furnace annealing was used for Ag nanoparticles formation. The low temperature annealing was performed in a quartz tube at 200 °C for 50 minutes under the high-purity Ar ambient.
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