We will discuss the growth and characterization of AlxGa1-xN/GaN on sapphire substrate by plasma-assisted molecular beam epitaxy. By performing the X-ray diffraction (XRD) of our sample, we could control the fraction of Aluminum in about 0%, 16%, and 31% with varying the equivalent pressure of Aluminum. Under the investigation of field emission scanning electron microscopy (FESEM) and reflection high energy electron diffraction (RHEED) pattern, we can determinate the samples are N-polarity. The photoluminescence (PL) spectra show the bowing coefficient of AlGaN is about 1.38 eV. Furthermore, The Hall mobility could be improved from 8.2 cm2/Vs to 453 cm2/Vs or 796 cm2/Vs at room temperature or liquid nitrogen temperature respectively, after changing the structure of AlxGa1-xN, such as fraction of Aluminum, the thickness of i-AlxGa1-xN and n-AlxGa1-xN, and cap layer.

We have fabricated the device of High Electron Mobility Transistor(HEMT) on Al0.18Ga0.82N/GaN heterostructures. The mobility of 2DEG of the AlGaN/GaN is 9328 cm2/Vs and carrier concentration is 7.917 1012 cm-2 obtained by conventional van der pauw Hall measurement at temperature of 77 K. We made the conducting channel of nanometer wires on the AlGaN/GaN heterostructures for researching low-dimensional transport of two-Dimensional Electron Gas by gate controlled. From the SdH measurement, we can clearly observe the SdH oscillations and obtain the SdH frequencies. For the sample of 0922GaN-200 nm and 0922GaN-100 nm at 0.39 K,two constituted peaks of Gauss¡¦s function were fitted by Non-linear Curve and Two SdH oscillations beat each other, probably due to spin-splitting. However, we can¡¦t discover any trend in the experiment of gate controlled. In the future, we will try to improve the quality and discover the suitable depth of SiO2.

In this thesis, we study the magnetro-transport properties of Fe-doped AlxGa1-xN/GaN heterostructure with different Al content by Shubnikov-de Haas measurements. On the samples KTH640(x=0.294)¡BKTH643(x=0.344)¡BKTH642(x=0.390)¡BKTH644 (x=0.397), we find that the electron have occupied the lowest two subbands, and the energy separation for each sample before illumination is 98 meV¡B107 meV¡B111 meV¡B119 meV. On the samples KTH641(x= 0.216)¡BKTH640(x= 0.294)¡BKTH642(x= 0.390)¡BKTH644(x= 0.394), we observe the electrons have spin splitting phenomenon, and the highest spin splitting energy separation¡¦s value in our experiments is 10.6 meV. For all six samples have persistent photoconductivity effect¡¦s behavior after illumination.

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High efficiency components are key elements of solid-state amplifiers for wireless application. We used HEMT (high-electron-mobility transistor) to obtain high mobility 2DEGs. AlGaN makes itself an attractive material for high frequency devices, and the system is particularly good for the investigation of quantum Hall effect. We studied the electronic properties of AlxGa1-xN/GaN heterostructures by using Shubnikov-de Haas (SdH) measurement. The T-dependent Hall measurement (from 4.2 K to 300K) was performed at the magnetic field 0.3T and the B-dependent Hall measurement (from 0.02T to 0.8 T) at constant temperature. From the field-dependent Hall measurements, we are able to calculate the individual mobility and carrier concentration for the two-subband-populated AlxGa1-xN/GaN heterostructures. Then, we can use T-dependent Hall effect measurement to calculate the binding energy of the deep-level trap Ed,which is a measure of the energy constant for the ionization of deep-level trap in thermal equilibrium.

Electroreflectance spectra of AlGaN/GaN heteostructure were measured for various biased voltage (Vbias). There are Franz-Keldysh oscillations (FKOs) exhibiting above band gap of AlGaN, and strength of electric field of AlGaN (FAlGaN) can be evaluated from periods of the FKOs. A positive polarization charge

We have studied the electronic properties of AlxGa1-xN/GaN heterostructures by using Shubnikov¡Vde Haas(SdH) measurement. Two SdH oscillations were detected on the sample of x=0.31, due to the population of the first two subbands with the energy separations of 94.2 meV. For the samples of x=0.22 and x=0.23, two SdH oscillations beat each other, probably due to a finite zero-field spin splitting. The spin-splitting energies are equal to 1.4~5.3 and 4.5 meV . The samples also showed a persistent photoconductivity effect after illuminating by blue light-emitting diode.

In this thesis, we will discuss that GaN template which was grown on the sapphire by metal organic vapour phase epitaxy (MOVPE). Then GaN epi-layer, intrinsic AlGaN (as spacer) and N-type AlGaN(doping Si) which offers carrier grow by molecular-beam epitaxy. We changed the content of aluminium to find out any difference on AlGaN/GaN heterostructure . For our experimental, we tried our best to keep all the parameters in steady besides the vapor of aluminum. If the vapor of aluminum every 4.65*10-9 torr is set to be one unit, then ratio of the pressure from sample A to E is 0.5, 1.0, 1.5, 2 and 3. We can get the best growth parameters by hall measurement, reflection high-energy electron diffraction, scanning electron microscope, atomic force microscopic and X-ray diffraction analysis to improve the quality of the sample. From scanning electron microscope and reflecting high energy electronic diffraction picture, the roughness of all samples is good which confirms that the samples should be in two-dimensional (2D) growth mode. We can find the same result by atom force microscope. After comparing 3D picture, we find out the surface of Sample B is the smoothest, meanwhile the roughness is 1.404 (nm) has been calculated through the functions. Due to Hall measurement in 77 K, the electron mobility of this series of samples are very high, especially Sample C is 10995(cm2/Vs) and sample D is 10697 (cm2/Vs) respectively. Quite narrow Full Width at Half Maximum(FWHM) of AlGaN which is about only 300 (arcsec) has been showed under the analysis of X-ray in rocking curve mode and these results indicate these samples have extraordinary qualities better than previous one.

Photoreflectance (PR) spectra of a GaN thin film and an AlGaN/GaN heterostructure were measured by using a HeCd laser or a mercury lamp as a pump beam. The wavelengths (£f) of the HeCd laser and the mercury lamp are 325 nm and 253.7 nm, respectively. The energy of the HeCd laser is lower than band-gap energy of AlxGa1-xN (x > 0.2) so that electron-hole pairs cannot be generated in the AlGaN layer. Hence, the PR of the AlGaN was measured by using Argon ion laser (£f=300 nm) or quadrupled Nd:YAG (£f=266 nm) rather than HeCd laser in the previous works. In this work, the mercury lamp (£f=254 nm)was used as the pump beam. The problem with using the mercury lamp as the pump beam is because it is a diffused source so that it cannot be focused to a small spot. Nevertheless, defocused pump and probe beams were used in the PR measurement to improve signal to noise ratio. Hence, the diffused property of the mercury lamp is not a hindrance to the PR measurements.

Recent advances in growth techniques have lead to the production of high quality GaN and this has played a vital role in the improvement of GaN based devices. A number of device types can be produced from GaN. Spectrally selective devices can be produced by creating ternary or quaternary material systems by partially substituting either Al or In for Ga in GaN. This allows a wide spectral range that can be achieved ranging from the visible to the ultraviolet. The applications of detectors based on these material systems are vast and include areas such as biological, military, environmental, industrial and scientific spheres. In front illuminated Schottky barrier photodetectors, two major factors influencing the sensitivity of the device are the reverse leakage current and the transparency of the Schottky contact. In order to reduce the reverse current of semiconductor based devices, increase the barrier height, and enhance the adhesion of a metal on a semiconductor it is important to subject the contact to annealing. Annealing studies have been performed on AlGaN based photodiodes to investigate the evolution of the optical and electrical properties. In this study, the electrical and optical characteristics of AlGaN based Ni/Au and Ni/Ir/Au Schottky photodiodes were investigated. The electrical properties of the photodiodes were optimised by annealing in an Ar ambient. An increase in the Schottky barrier height and a decrease in the reverse leakage current were observed with increasing annealing temperature up to 500 oC. This effect was observed for both the Ni/Au and Ni/Ir/Au photodiodes. The optical characteristics of the photodiodes, which include the responsivity and the quantum efficiency, were also investigated. UV/visible rejection ratios of as high as 103 were obtained. The transmittance of Ni/Au and Ni/Ir/Au metal layers deposited on a quartz substrate were optimised by annealing. This was under the same ambient conditions as the Schottky photodiode. The transmittance increased with annealing temperature for the Ni/Au metal layer whereas it decreased at higher temperatures for the Ni/Ir/Au layer. The transmittance of the Ni/Au metal layer reached as high as 85 % after 500 oC annealing. The transmittance of the Ni/Ir/Au only reached a high of 41 % after 400 oC annealing. / Dissertation (MSc)--University of Pretoria, 2013. / Physics / unrestricted