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Growth methods
In an effort to improve the quality of GaN crystals and reduce dislocation density, several growth methods were used. The growth methods that have been used are vapour phase epitaxy (VPE), which includes both hydride (HVPE) [7] and metal organic vapour phase epitaxy (MOVPE) [8], and molecular beam epitaxy (MBE) [9]. MOVPE is a chemical vapour deposition method, also termed metal organic chemical vapour deposition (MOCVD), organometallic chemical vapour deposition (OMCVD) or organometallic vapour phase epitaxy (OMVPE). This method uses ammonia (NH3) and trimethyllgallium (TMG) as precursors for nitrogen and gallium, respectively. In the case of other nitrides, trimethyllaluminium (TMAl) and trimethylinduim (TMIn) are used as sources for Al in AlN and In in InN, respectively. The chemical equation for the growth of GaN is given by: G a CH NH GaN CH. The MOCVD method requires a high partial pressure for NH3 and a high growth temperature, ranging from 1000 to 1100°C. The first step is growing the material at low temperature producing GaN crystallites that covers the substrate. The final film is grown at a higher temperature to control and reduce contamination in the material. GaN is doped with Si or Mg for n-type and p-type doping. In n-type doping, Si is sourced from methyl silane, while in p-type doping Mg is sourced from biscyclopentadithyl. Figure 2.2 shows the growth process of the IIINitrides. The carrier gas introduces the required element for the growth of GaN and the by products are expelled from the system, with the semiconductor growing onto a substrate.
MBE is an ultra high vacuum technique for growing semiconductor crystals. High purity Ga is heated in an effusion cell until it evaporates and deposits slowly onto a substrate. Nitrogen atoms are supplied from a plasma source. The growth of GaN is controlled by conditions that allow atoms of Ga and N to be deposited layer by layer onto a heated substrate. The MBE method operates in an ultra high vacuum chamber to minimize crystal contamination during growth. This method is capable of producing heterostructures with sharp interfaces and of growing zinc-blende structure GaN (normally, GaN is grown in a wurtzite structure). The chamber is also equipped with Si, Mg, In and Al effusion cells for alloying purposes. The disadvantage of the MBE method is the low growth temperature, 700 to 800°C as compared to MOCVD where temperatures are 1000 to 1100°C. GaN is a thermodynamically unstable material in a vacuum and the thin film may decompose into Ga and N in the MBE, when the deposition rate becomes lower than the decomposition rate due to a temperature difference between the chamber and the substrate. The low substrate temperature reduces surface atom mobility, resulting in increased densities of defects [10]. Figure 2.3 shows the schematics of the MBE growth method.
Substrates for GaN
In addition to the growth method, the crystal quality of semiconductors depends on the suitability of the substrate. In growing GaN, sapphire (α-Al2O3), GaAs, Si, SiC, LiGaO2, LiAlO2 and ZnO have been used as a substrate [3]. The criterion for choosing a suitable substrate is lattices match. In addition, practical properties such as crystal structure, surface finish and composition, chemical, thermal and electrical properties are also considered. Currently, it has become important to study the effects of treatments of a substrate (e.g. heating or chemical processing) prior to the deposition of GaN. For example, it has been shown that wet etching of sapphire prior to the deposition of GaN crystals reduces threading dislocations [12]. α-Al2O3 has been extensively used as a substrate for GaN. It has a crystal orientation parallel to GaN c-plane, and the lattice mismatch is about 15 %, leading to a dislocation density of about 1010 cm-2. α-Al2O3 has a rhombohedral structure and is highly anisotropic. Like GaN, it exhibits extremely high chemical and thermal stability with a melting point of 2040 °C. Its bandgap of 9.1 eV permits excellent optical transmission. Furthermore, the coefficient of the thermal expansion of α-Al2O3 is greater than that of GaN, resulting in comprehensive stress in the grown film during cooling. Such stress causes cracks in both GaN and α-Al2O3.
A continually improving technique to produce GaN with less threading dislocations is the epitaxial lateral overgrowth (ELOG), considered to be an alternative substrate [13]. Figure 2.4 depicts the schematics of the ELOG substrate. The ELOG technique takes advantage of the large anisotropy of the GaN growth rate in the [0001] direction, controlling dislocations through the patterned substrate such that they do not reach the surface of the final layer as shown in figure 2.4 (D). GaN thin buffer layer is grown on sapphire as a usual practice for GaN growth as shown in figure 2.4 (A). A dielectric material such as SiO2 or SiN mask is then patterned onto the GaN buffer layer {figure 2.4 (B)}. The thin film is then grown onto the patterned GaN buffer layer {figure 2.4 (C)}. Using MOCVD, ELOG and several of its variations has been shown to significantly reduce the dislocation density of GaN crystals to as low as 106 cm–3 [14].
Applications of GaN-based materials.
III-nitrides are suitable semiconductor materials for use in optoelectronic devices, as both emitters and detectors. They can also be used to fabricate high power and high temperature electronic devices [51]. The allowed energy bandgaps of these materials are suitable for band-toband light generation with colours ranging from potentially red to UV wavelengths, rendering them an advantageous addition to the already existing semiconductor systems for colour displays. It has been demonstrated that nitrides can be used as Bragg reflectors [52], UV detectors [53], UV and visible light emitting diodes (LEDs) for applications in flat panel displays, lighting and indicator lights on devices, advertisements and traffic signals [54]. As coherent sources, lasers are important for high-density optical read-write technologies [55]. The diffraction-limited optical storage density increases approximately quadratically as the probe Property laser wavelength is reduced, making the GaN-based materials suitable for coherent sources at lower wavelengths of the electromagnetic radiation. Optical storage enables the storage and retrieval of data in vast quantities. Medical applications of UV LEDs and lasers include surgery [56], phototherapy of neonatal jaundice [57], photodynamic therapy [30], photo-polymerization of dental composites [30], phototherapy of seasonal affective disorder [30], and sterilization [58]. When used in surgery, UV lasers are seen as most suitable due to the fact that UV can sterilize. In photosynthesis, the high brightness LEDs are suitable for the growing of plants and for photo bioreactors [59]. Finally, the LEDs and laser diodes (LDs) are suitable for use in optical measurements such as time domain and frequency domain spectroscopy [60]. Furthermore, exposure to UV-B radiation causes skin cancer to fair skinned people. The use of AlGaN ultraviolet detectors will help prevent such disease, where a handheld device will be able to communicate to user how much ultraviolet radiation was absorbed.
There is great concern all over the world about the contribution of uncontrolled effluents to global warming which is an unexpected change in climate. The effluents stem from aerosols, car fumes, industries and wild fires, and add to the concentration of CO2 in the atmosphere. When installed in jet engines, cars and furnaces, the UV detectors would monitor and control contaminants for a cleaner environment. In addition, UV detectors operating in the solar-blind region of the electromagnetic spectrum, when made from GaN-based materials, record a high detectivity and are useful in the detection of UV-C (280 nm to 10 nm) and UV-B (320 nm to 280 nm) [61]. UV-C and UV-B are not detectable naturally because the ozone layer is a natural UV filter for all radiation less than 280 nm [35]. It has been observed that power lines emit UV-C radiation as a result of ionization of nitrogen around them.
1 Introduction
1.1 Introduction
1.2 Aims of the study
1.3 Synopsis of thesis
2 GaN-based materials for Ultraviolet detectors
2.1 Introduction
2.2 Progressive development of GaN
2.3 Properties of GaN-based materials
2.4 Applications of GaN
2.5 AlGaN photodetectors
2.6 Ohmic contacts to AlGaN
3. Schottky Barrier Ultraviolet Photodetectors
3.1 Introduction
3.2 Ultraviolet photodetectors
3.3 Schottky-Mott theory and its modifications
3.4 Current transport mechanisms
3.5 Theory of ultraviolet photodetectors
4 Experimental Techniques
4.1 Introduction
4.2 Sample preparation
4.3 Surface characterization
4.4 Electrical and Optical Characterization
5 Analysis of GaN Cleaning procedures
5.1 Introduction
5.2 Experimental
5.3 Results and Discussion
5.4 Conclusions
6 Study of metal contacts on GaN for transmission of UV light
6.1 Introduction
6.2 Choice of metal for transparent contacts
6.3 Experimental
6.4 Results and Discussion
6.5 Conclusions
7 Chemical treatment effect on Au/GaN diodes
7.1 Introduction
7.2 Experimental
7.3 Results and Discussion
7.4 Conclusions
8 Fabrication of GaN/AlGaN Schottky barrier photodiodes
8.1 Introduction
8.2 Experimental
8.3 Results and Discussion
8.4 Conclusions
9 Conclusions
