@phdthesis{oai:sucra.repo.nii.ac.jp:00019384,
 author = {HOSSAIN, MD ISMAIL},
 month = {},
 note = {xxii, 96p, Although the efficiency of AlGaN-based optoelectronic devices has improved in recent years, the realization of high external quantum efficiency （EQE） ultraviolet （UV） light emitting diodes （LEDs） with wavelengths below 360 nm is still challenging. In spite of the development of growth techniques, the lack of native lattice-matched, cost-effective, and suitable substrates produces high densities of threading dislocations and point defects which act as nonradiative recombination （NRR） centers or trap centers in the crystal. The defect mediated NRR centers reduces the carrier lifetime and are responsible for the low efficiency of the devices. To resolve these problems, it is indispensable not only to understand the basic mechanism of grown-in defects and imperfections in these materials, but also to find out the correlations of these defects with the performance and reliability limiting problems and impute them to their physical origin. Thus, the study of NRR centers is likely to remain an important and active research thrust for the realization of high efficiency AlGaN UV light emitting devices.
 In this study, different AlGaN-based multiple quantum wells （MQWs） samples have been investigated by photoluminescence （PL） and two-wavelength excited photoluminescence （TWEPL） methods. In the TWEPL, an intermittent below-gap excitation （BGE） light whose photon energy is lower than the bandgap energy of the material （hvBGE < Eg）, is superposed on a constant above-gap excitation （AGE） light （hvAGE > Eg） at the same point of the sample surface. The intensity change in photoluminescence （PL） due to the addition of a BGE light on an AGE light implies the presence of NRR levels in the energy position corresponding to the photon energy of the BGE source.
 The NRR centers in two UV-C （deep UV） AlGaN MQW samples, grown at two different growth temperatures 1140 °C and 1180 °C, on c-plane sapphire substrate by metal-organic chemical vapor deposition （MOCVD） technique, have been studied by TWEPL at about 25 K. The PL intensity decreased by the superposition of BGE light of photon energies between 0.93 eV and 1.46 eV over an AGE light of energy 4.66 eV. This is explained by a two-level recombination model based on Shockley-Read-Hall （SRH） statistics. The model indicates the presence of a pair of NRR centers in both samples, which are activated by the BGE. The degree of PL quenching for the sample grown at 1140 °C is higher than that of the sample grown at 1180 °C for BGE energies 0.93 eV, 1.17 eV, and 1.27 eV. The defect density ratio of 1.5, for the BGE energy of 1.27 eV, was obtained from a qualitative simulation. This result implies that a slight difference in growth conditions changes defect densities.
 Superlattice （SL） period （SLP） dependence on NRR centers of UV-B AlGaN MQW samples, grown on c-plane sapphire substrate at 1150°C by MOCVD technique, have been studied by TWEPL. The SLP affects the lattice relaxation of SL and n-AlGaN layer. The NRR centers in n-AlGaN and QW layers of these samples have been detected by adding BGE light of energies 0.93 eV, 1.17 eV, 1.27 eV, and 1.46 eV over an AGE light of energy 4.66 eV at 30 K. By the superposition of these BGE light on AGE, the PL intensity decreased and the degree of PL quenching from both the layers of the sample with SLP 100 is lower than those of other samples with SLP 50, 150, and 200. By a qualitative simulation with the dominant BGE of photon energy 1.27 eV, the density-ratio of NRR centers in n-AlGaN layers of 50:100:150:200 SLP samples is obtained as 1.7:1.0:6.5:3.4. This result implies that the number of SLP changes lattice relaxation and determine density of NRR centers in n-AlGaN layer and as a whole in QW layer which affects the performance of LEDs.
 NRR processes through defect states and their temperature dependence in UV-B AlGaN MQW sample on sapphire substrate grown by MOCVD technique have been studied by photoluminescence （PL） spectroscopy. We detected NRR centers by adding a below-gap excitation light with photon energies from 0.93 eV to 1.46 eV on an above-gap excitation light of 4.66 eV. All the BGE energies decreased PL intensity at 25 K, and the most-distinct quenching is observed by 1.27 eV BGE at the same BGE photon number density. The temperature-dependent PL intensity for the BGE energy of 1.27 eV is interpreted by three NRR centers. The one-level model dominates over that of two-level model in the temperature range 58 K < T < 88 K. The two-level model prevails in other region of temperature. The combination of one-level and two-level models is consistent with the spectral peak-energy shift as a function of temperature., ABSTRACT ............................................................................................................ i
ACKNOWLEDGEMENTS .............................................................................................. iii
LIST OF PUBLICTIONS ................................................................................................. v
VITA .............................................................................................................. vii
CONTENTS ........................................................................................................... ix
LIST OF TABLES ....................................................................................................xiii
LIST OF FIGURES .................................................................................................... xv
LIST OF ABBREVIATIONS ......................................................................................... xxi
CHAPTER 01 .......................................................................................................... 2
INTRODUCTION ........................................................................................................ 2
1.1 Motivation ...................................................................................................... 2
1.2 Objectives ...................................................................................................... 3
1.3 Organization of the thesis ...................................................................................... 4
CHAPTER 02 .......................................................................................................... 6
BACKGROUND .......................................................................................................... 6
2.1 Basic properties of AlGaN materials......................................................................... 6
2.2 Defects in AlxGa1-xN materials .............................................................................. 8
2.3 Quantum well and multi-quantum wells .................................................................. 10
2.4 Shockley-Read-Hall (SRH) recombination theory ...................................................... 11
2.5 Two-wavelength Excitation PL (TWEPL): a powerful tool for defect characterization ...... 14
CHAPTER 03 ......................................................................................................... 16
EXPERIMENTAL DETAILS ............................................................................................ 16
3.1 Photoluminescence .............................................................................................. 16
3.2 Two-wavelength Excited Photoluminescence (TWEPL) .............................................. 17
CHAPTER 04 ......................................................................................................... 22
EFFECT OF GROWTH TEMPERATURE ON NONRADIATIVE RECOMBINATION CENTERS IN UV-C AlGaN QW ................................ 22
4.1 Introduction ................................................................................................... 22
4.2 Experimental methods ........................................................................................ 23
4.2.1 Sample structure ............................................................................................. 23
4.2.2 Measurement .................................................................................................. 24
4.3 Results and discussion ......................................................................................... 24
4.3.1 PL intensity comparison .................................................................................... 24
4.3.2 TWEPL measurement ....................................................................................... 27
4.3.3 Rate equation analysis ....................................................................................... 30
4.4 Conclusions .................................................................................................... 34
CHAPTER 05 ......................................................................................................... 36
SUPERLATTICE PERIOD DEPENDENCE ON NONRADIATIVE RECOMBINATION CENTERS OF UV-B AlGaN QW STRUCTURE .................... 36
5.1 Introduction ................................................................................................... 36
5.2 Experimental methods ........................................................................................ 37
5.2.1 Sample structure ............................................................................................. 37
5.2.2 Measurement .................................................................................................. 39
5.3 Results and discussion ......................................................................................... 40
5.3.1 Conventional PL measurement .................................................................................. 40
5.3.2 TWEPL Measurement ......................................................................................... 44
5.3.3 Rate Equation Analysis ....................................................................................... 51
5.4. Conclusions ................................................................................................... 54
CHAPTER 06 ......................................................................................................... 56
TEMPERATURE DEPENDENCE OF NONRADIATIVE RECOMBINATION PROCESSES IN UV-B AlGaN QW .................................... 56
6.1 Introduction ................................................................................................... 56
6.2 Experimental methods ........................................................................................ 57
6.2.1 Sample structure ............................................................................................. 57
6.2.2 Measurement .................................................................................................. 58
6.3 Results and discussion ......................................................................................... 59
6.3.1 PL measurement ............................................................................................... 59
6.3.2 TWEPL measurement ......................................................................................... 62
6.3.3 Rate equation analysis ....................................................................................... 69
6.4 Conclusion ..................................................................................................... 73
CHAPTER 07 ......................................................................................................... 74
SUMMARY AND FUTURE DIRECTIONS ....................................................................... 74
7.1 Summary ........................................................................................................ 74
7.2 Future directions .............................................................................................. 76
APPENDIX ........................................................................................................... 78
A.1 Calibration of output power of different BGE sources ................................................ 78
A.2 Beam parameters of different laser sources ............................................................. 80
REFERENCES.......................................................................................................... 82, 指導教員 : 鎌田憲彦, text, application/pdf},
 school = {埼玉大学},
 title = {Optical Characterization of Defect levels in AlGaN-based Light Emitting Materials},
 year = {2020},
 yomi = {ホセイン, エムディ イスメイル}
}