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Title of Thesis

Umar Saeed Qurashi
Institute/University/Department Details
Department of Physics/ Quaid-i-Azam University, Islamabad
Number of Pages
Keywords (Extracted from title, table of contents and abstract of thesis)
compound semiconductors, gap, gaas, znse, deep level transient spectroscopy, dlts, photoluminescence, pl, pl spectra, a1 doping

The work described in this thesis consists of characterization of three different compound semiconductor materials, namely GaP, GaAs and ZnSe. In addition to the deep level transient spectroscopy measurements (DLTS), photoluminescence (PL) technique (in case of GaAs) has also been used. DL TS measurements on GaP have revealed that the thermal emission of electrons from the deep level 01 due to oxygen defect is strongly dependent on the electric field. Emission rate versus field data have been obtained over a wide range of temperature (320 - 480 K) and field values. This data have been successfully fitted with the phonon-assisted tunneling model of Makram-Ebeid and Lannoo. New data on some important parameters like the Franck-Condon shift has been obtained. A1-doped GaAs grown by Molecular Beam Epitaxy (MBE) has been studied using DLTS. Samples with three different concentration of A1 (0.1%, 1% and 3%) were grown. DL TS measurements reveal the presence of nine deep levels. These deep levels have been identified with the well known M levels found in MBE-GaAs. The emission rates have been found to decrease with increase in AI concentration. This has been attributed to lattice-strain or random alloy effects. There is no decrease in the deep level concentration with 0.1 % A1 doping of GaAs as has .been reported for In and Sb. A further increase in AI concentration to 1 % increases the overall concentration of deep levels. The PL spectra on the same GaAs samples as used for the DLTS measurements show very similar overall characters for the three compositions studied. The prominent features of the spectra has been identified as the zero phonon bound exaction transition, the carbon acceptor-related tree-to-bound transition and a tree-to-bound transition at the Si acceptor with a contribution from a deeper defect. A deep-level defect-related transition with accompanied phonon replicas has also been observed. The PL intensity Has been to go down by a factor of €“ 175 when Al concentration in GaAs was increased from 0.1% to 35. These results in the overall optical emission is consistent with the strong competition from non-radiative recombination at the deep levels whose concentration is seen to increase with Al content in DLTS. We have also investigated MBE-grown ZnSe, another Technologically important semiconductor using DLTS. Four majority carrier emitting levels have bee3n observed in DLTS measurements on nitrogen doped p-type ZnSe. Detailed data on the various deep level characteristics including emission rate signatures, activation energies, hole capture cross-sections and depth profiles of the prominent levels have been reported for the first time in literature to our knowledge. These results have been compared with the limited information available from the two earlier reports. This comparison shows that the levels observed by us are most probably some new levels.

Download Full Thesis
1539.54 KB
S. No. Chapter Title of the Chapters Page Size (KB)
1 0 Contents 0
95.68 KB
2 1 Introduction 1
105.61 KB
  1.1 III- V Compound Semiconductors 1
  1.2 II- VI Compound Semiconductors 3
  1.3 Some Basic Properties of GaP , GaAs and ZnSe 4
  1.4 Brief Introduction and Motivation of Our Work 7
  1.5 Scheme of the Thesis 9
  1.6 References 10
3 2 Deep Levels - Basic Concepts and Kinetics, and Their Properties 11
105.89 KB
  2.1 Shallow Levels 11
  2.2 Kinetic Processes - S.R.H. Theory 13
  2.3 Detailed Balance Principle 15
  2.4 Electron and Hole Emission Rates 16
  2.5 Parameters Characterizing Deep Levels 18
  2.6 Trapping and Recombination 19
  2.7 Radiative and Non- Radiative Transitions 21
  2.8 References 22
4 3 Characterization Techniques 23
212.58 KB
  3.1 Deep Level Transient Spectroscopy 23
  3.2 Photolunrinescence 36
  3.3 References 47
5 4 Field Enhanced Emission - Theoretical Background 48
99.33 KB
  4.1 Introduction 48
  4.2 Potential Associated with a Defect Centre 48
  4.3 Mechanism of Field-Enhanced Emission 51
  4.4 References 59
6 5 Experimental Details 60
81.96 KB
  5.1 Samples 60
  5.2 Experimental Setup 62
7 6 Electric-Field Enhanced Thermal Emission of Electron From the Oxygen Defect in GaP 68
210.12 KB
  6.1 Introduction 68
  6.2 Results 70
  6.3 Discussion 78
  6.4 Conclusions 80
  6.5 References 81
8 7 Effects of A1 Doping on Deep Levels in MBE-Grown GaAs 89
203.38 KB
  7.1 Introduction 89
  7.2 Results 90
  7.3 Discussion 96
  7.4 107
  7.5 References 109
9 8 Photoluminescence Characterization of AI-Doped GaAs Grown by Molecular Beam Epitaxy 110
366.9 KB
  8.1 Introduction 110
  8.2 Results 111
  8.3 Discussion 128
  8.4 Conclusions 135
  8.5 References 138
10 9 Deep Levels in Nitrogen Doped MBE-Grown p-ZnSe 139
225.9 KB
  9.1 Introduction 139
  9.2 Results 140
  9.3 Discussion 154
  9.4 Conclusions 161
  9.5 References 163
11 10 Summary and Conclusion 164
56.04 KB
  10.1 Oxygen in GaP 164
  10.2 GaAs :Al 165
  10.3 ZnSe 167
  10.4 References 169