Author: Sergei Novikov Anthony Kent, Richard Campion University, and Tom Foxon Nottingham In epitaxial growth, the preparation of Group III nitride (III-N) devices is hindered due to the lack of lattice-matched bulk GaN materials. At the same time, the supply of materials has led to high prices, and even the hexagonal wurtzite structure is not ideal for all types of devices. Although there is a strong polarization field in the substrate and the attached epitaxial layer, which is the result of the interaction of the piezoelectric effect and the spontaneous polarization phenomenon, it is still helpful for the HEMT design, which weakens the recombination efficiency and thus weakens The light output of the optoelectronic device. Until now, the thickness of cubic GaN has been limited to about 1 μm, but our research team at the University of Nottingham in the UK has shown that MBE can produce thicker materials. This gives us the ability to make self-supporting GaN materials and hopefully become a pioneer in the fabrication of cubic substrates. We used a series of shifting techniques to study the properties of materials such as X-ray diffraction, reflective high energy electron diffraction (RHEED), transmission electron microscopy, photoluminescence (PL) and nuclear magnetic resonance spectroscopy. All of these techniques, including RHEED (Fig. 2), did not recognize any hexagonal inclusions as the material first grew to a thickness of 10 μm. The bandwidth of the film was 3.25 eV and 3.30 eV at room temperature and 4 K, respectively, as measured by PL. About the Author
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This shortcoming has raised interest in the growth of non-polar optical radiation III-N structures. In these devices, the polarization effect is eliminated due to growth in the non-polar direction of the crystal, such as the m-plane. However, these non-polar hexagonal ingots and stencils are difficult to produce, they are more expensive than comparable polar materials, and their quality is far from satisfactory.
The use of a non-polar (100) direction zinc blende structure III-N layer is one of many methods for how to eliminate the polarization effect of GaN-based devices, but there is another alternative method that is promising. Although these thermodynamic metastable cubic GaN layers have not attracted enough attention, they still have two major advantages against polar and non-polar wurtzite GaN: due to the geometric nature of the crystal structure, they are very It is easy to separate from the (110) crystal plane and used for device fabrication; furthermore, the cubic GaN layer can provide an order of magnitude higher carrier mobility due to improved crystal symmetry.
On platforms such as GaAs and SiC, we have made cubic GaN epitaxial layers using special growth conditions and MBE, HVPE and MOCVD techniques. Among them, the MBE method is most desirable because the hexagonal GaN manufactured by the MBE method has the lowest inclusion capacity. In contrast, the HVPE method requires a higher growth temperature; in particular, the MOCVD method, as the GaN is further grown, the hexagonal structure inclusions are also generated faster.
Figure 1. Nottingham's plasma-assisted MBE method produces a cubic GaN substrate with a thickness of 50 μm and a surface area of ​​more than 1 cm 2 .
We used a plasma-assisted MBE (PAMBE) method to grow an undoped thick-layer cubic GaN film on a semi-insulating GaAs (001) substrate, using arsenic as a surfactant to initialize the cubic phase growth. These films have a growth rate of 0.3 μm/h, although not particularly fast, but are comparable to the growth rate of hexagonal GaN crystals formed from liquid gallium at high pressure.
We produced a self-supporting cubic GaN layer using the PAMBE method, which included a small piece having a surface area greater than 1 cm2 (Fig. 1) and a thickness of 8 μm. This film is transparent and has a cubic microstructure. It is also very hard, and when the thickness is increased to 30 μm or more, it is easy to handle; this makes it possible to use it as a substrate for a cubic GaN-based structure and device.
Figure 2. Self-supporting GaN layer with a thickness of 30 μm. Its reflective high-energy electron diffraction imaging shows a high-quality cubic single crystal structure.
When the thickness was increased to 50 μm, the hexagonal inclusions were formed in the film to a concentration of 10%. We plan to further study the eigenstates, densities and defect types in cubic GaN. On these issues, we will work with the Sharp laboratory in Europe. The collaborative project was funded by the UK's Business, Corporate and Regulatory Reform Department with the aim of improving our technology for commercialization. The focus of our efforts will be on how to increase the size of the material and increase the growth rate.
Sergei Novikov is a senior researcher at the University of Nottingham's Physical Astronomy School, where he is responsible for the MBE growth of Group III nitride semiconductors. Anthony Kent is a professor at the University of Nottingham, whose research focuses on the basic physical properties of III-V semiconductor nanostructures and devices, including those based on GaAs and GaN. Richard Campion is the school's chief experimenter and is responsible for the MBE growth of III-V semiconductors. Tom Foxon (Member of the Royal Society) is a research professor at the University of Nottingham.