In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy induced ...by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72-0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of VGa and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.
Abstract
In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, blue, and green emission bands from n-type (010) Ga
2
O
3
films grown by metalorganic vapor phase ...epitaxy induced by annealing at different temperatures under O
2
ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in the formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O
2
annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find activation energy for the generation of non-radiative defects of 1.34 eV in the bulk but 2.53 eV near the surface. We also deduce activation energies for the green emission-related defects of 1.20 eV near the surface and 2.21 eV at low temperatures and 0.74 eV at high temperatures through the films, whereas the blue-related defects have activation energy in the range 0.72–0.77 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 °C. These observations are consistent with the formation and diffusion of
V
Ga
and its complexes as a dominant process during O
2
annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations.
Plastic has been used in many ways in modern life both in industry product as well as in domestic use. The waste generated by using plastic poses an enormous threat to the environment due to its ...non-degradable nature for a very long time. There has been a lot of effort to minimise plastic waste generation and reuse it. However, the plastic waste generation has been increased considerably in the past decade. This paper presents a potential use of plastic waste as fine aggregate in concrete. Experimental investigations were carried out to determine the strength characteristics of concrete by replacing fine aggregate with 10, 15, 20, and 25% of processed plastic waste. Tests were conducted on concrete specimens under tensile, compressive, and flexural loading conditions to understand its behaviour. Although there was a notable reduction in strength characteristics of the concrete specimens, the optimum strength was obtained at 15% sand replacement. The concrete mix prepared using plastic waste can be used for mass concreting and low load-bearing structures.
We report on the modeling of polarization-induced two-dimensional electron gas (2DEG) formation at -AlGaO3/ -Ga2O3 heterointerface and the effect of spontaneous polarization (Psp) reversal on 2DEG ...density in -Ga2O3/ -AlGaO3/ -Ga2O3 double heterostructures. Density-functional theory (DFT) is utilized to calculate the material properties of -Ga2O3 and -AlGaO3 alloys. Using Schrödinger-Poisson solver along with DFT calculated parameters, the 2DEG density is calculated as a function of barrier type and thickness. By optimizing the layer thicknesses of -Ga2O3/ -AlGaO3/ -Ga2O3 heterostructures, charge contrast ratios exceeding 1600 are obtained. This computational study indicates the high potential for -Ga2O3-based heterostructure devices for non-volatile memories and neuromorphic applications.
Power devices are intimately involved in generation, transmission, and consumption of electricity. Current silicon-based power devices are limited by the low breakdown field and low switching ...frequencies of silicon. Due to its large bandgap and critical breakdown field strength, β-Ga2O3 has emerged as a promising ultra-wide bandgap material for power electronic devices. DFT based transport modelling shows that by forming a two-dimensional electron gas (2DEG) at the β-(AlxGa1-x)2O3/β-Ga2O3, mobility values much higher than the β-Ga2O3 limit (~200 cm2/V. s) could be achieved.This dissertation explores doping schemes for Ga2O3 thin films and heterostructures for potential high-performance device applications. Various doping studies are undertaken to understand uniform doping, delta doping, modulation doping and polarization doping in Ga2O3. The primary focus of this work is to realize a modulation-doped 2DEG channel at β-(AlxGa1-x)2O3/β-Ga2O3 heterointerface.β-(AlxGa1-x)2O3 growth and delta doping of β-Ga2O3 are explored to understand 2DEG formation in β-(AlxGa1-x)2O3/β-Ga2O3 heterostructures. Degenerate doping up to ~8 x 1019 cm-3 is achieved in β-(Al0.26Ga0.74)2O3 by changing the silane flow. By a using a uniformly-doped β-(AlxGa1-x)2O3 barrier a sheet charge density of 2.3 x 1012 cm-2 is realized. Delta doping of β-Ga2O3 is explored to further improve the 2DEG density. The silicon incorporation and activation are studied using secondary-ion mass spectroscopy and capacitance-voltage measurements. By reducing the growth temperature to minimize surface riding of silicon dopants, sharp doping profile with a CV measured FWHM of ~3 nm is achieved. Using MOVPE based n+ regrowth process for the ohmic contacts, a completely MOVPE-based MODFET has been realized. Furthermore, a 2DEG channel with a record low sheet resistance of 5.3 kΩ/square is achieved. Further reduction in the barrier doping resulted in the formation of a pure 2DEG channel of 1 x 1012 cm-2 with a mobility of 149 cm2/V.s.This dissertation also explores 2DEG formation at ε-(AlxGa1-x)2O3/ε-Ga2O3 heterojunction with the aid of DFT calculated material properties. Finally, a new growth process was developed for achieving n-type doping in LPCVD-grown β-Ga2O3 films. The final chapter outlines ideas for future studies on improving 2DEG density in (AlxGa1-x)2O3/Ga2O3 heterojunctions and device designs for high performance devices based on (AlxGa1-x)2O3/Ga2O3 2DEG channels.
Monoclinic β-Ga2O3, an ultra-wide bandgap semiconductor, has seen enormous activity in recent years. However, the fundamental study of the plasmon–phonon coupling that dictates electron transport ...properties has not been possible due to the difficulty in achieving higher carrier density (without introducing chemical disorder). Here, we report a highly reversible, electrostatic doping of β-Ga2O3 films with tunable carrier densities using ion-gel-gated electric double-layer transistor configuration. Combining temperature-dependent Hall effect measurements, transport modeling, and comprehensive mobility calculations using ab initio based electron–phonon scattering rates, we demonstrate an increase in the room-temperature mobility to 201 cm2 V–1 s–1 followed by a surprising decrease with an increasing carrier density due to the plasmon–phonon coupling. The modeling and experimental data further reveal an important “antiscreening” (of electron–phonon interaction) effect arising from dynamic screening from the hybrid plasmon–phonon modes. Our calculations show that a significantly higher room-temperature mobility of 300 cm2 V–1 s–1 is possible if high electron densities (>1020 cm–3) with plasmon energies surpassing the highest energy LO mode can be realized. As Ga2O3 and other polar semiconductors play an important role in several device applications, the fundamental understanding of the plasmon–phonon coupling can lead to the enhancement of mobility by harnessing the dynamic screening of the electron–phonon interactions.
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