Zainal, Norzaini Binti
(2010)
Study of cubic III-V nitrides for device applications.
PhD thesis, University of Nottingham.
Abstract
This thesis describes the optimisation of the growth of bulk cubic GaN with low hexagonal content and with the intention of making it a commercial substrate for device applications. The optimised material was then applied for fabrication of cubic AlxGa1-xN/GaN based double barrier resonant tunnelling diode (DBRTD) devices. The devices with a clear negative differential resistance (NDR) and high reproducibility are demonstrated.
In the early part of this project, we reported a study on cubic GaN material with the variation of III/V ratio, growth rate and wafer position. Using PL and XRD measurements, we found that all these factors influence the increase of hexagonal inclusions in cubic GaN, leading to poor quality in cubic nitride growth. This problem however is more significant when the thickness of cubic GaN is increased. From the calibration work, a ~50μm cubic GaN layer has been grown for the first time with a low average proportion of hexagonal inclusions of around 10% and just few percent at the interface of cubic GaN and GaAs substrate. Thus, the interface would be the most suitable surface for further growth.
Next, we investigated the fundamental properties of cubic GaN using picosecond acoustic measurements. In this material, the sound velocity is found to be 6.9±0.1 kms-1, elastic constant = 285±8GPa and the refractive index at 400nm = 2.63±0.04. Comparison with hexagonal GaN films indicated that these parameter values differ considerably in different symmetry of GaN. These show the usefulness of our layers for determination of the basic properties of cubic GaN using a wide range of techniques.
From the Hall transport measurement, the electrical properties of undoped cubic GaN samples depend on growth conditions and thickness. In this work, we successfully demonstrated p-type cubic GaN:Mn using C-doping and n-type behaviour from Si-doped cubic GaN. However, these samples have high electron density but low mobility as the residual impurities and intrinsic defects were found to be higher inside the samples.
We extended the technology of growing cubic GaN to cubic AlxGa1-xN. A number of cubic AlxGa1-xN samples with different Al content, x were grown and characterised by PL measurement. We found that the hexagonal PL starts to dominate when x is increased, even for thin samples. This could be due to the problems of maintaining cubic AlxGa1-xN growth. It could also be due to the miscibility gap between AlN and GaN. More results and data are required to explain this behaviour.
In this thesis, we demonstrated potential cubic GaN substrates for device applications for the first time. The study on bulk cubic GaN showed that the interface between the cubic GaN and the GaAs substrate has only few percent of hexagonal content. Thus, the surface that was in contact with GaAs is the most suitable surface for further processing and growth. Due to the effect of strain, As inclusions and defects were already formed on the surface. By polishing the surface for ~2 hours, these problems were minimised and the surface still had low hexagonal content. In this work, the first working InGaN LED device grown on a polished free-standing cubic GaN substrate has been demonstrated. Our polished cubic GaN substrates also improve the quality of the grown device as been measured by luminescence and I-V characteristics.
In the last part of this thesis, we investigated the potential of cubic GaN for developing cubic AlxGa1-xN/GaN DBRTD devices. In the first stage of this work, the I-V characteristics of the cubic tunnel devices were calculated for various band offset, well width, barrier composition and barrier thickness parameters. From this work, optimal designs of cubic AlxGa1-xN/GaN tunnel diodes that could be fabricated and characterised were proposed. The result was then used as a starting point for the growth of cubic AlxGa1-xN/GaN DBRTD. A number of cubic AlxGa1-xN/GaN tunnel devices with different structural parameters were grown. Some devices showed a clear NDR effect but not all of them are reproducible due to breakdown of the device. This factor may also contribute to the irreproducibility of wurtzite (hexagonal) nitride based tunnel diodes in addition to the problem related to charge trapping.
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