Time resolved cathodoluminescence

TIME RESOLVED CATHODOLUMINESCENCE SUBBIAH SELVAKUMAR B.Eng., AU, M.Sc., NUS A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 ACKNOWLEDGEMENTS I feel honored to thank those who both directly and indirectly helped me to complete this research work successfully. First of all, I would like to thank my supervisors Dr. Daniel Chan and Dr. Jacob Phang for their inspiration and support and also spending their valuable time to discuss the findings throughout the research work. I am also grateful to my supervisors for their continued guidance, suggestions and insights during the course of the project. I am indebted to the staff and fellow students at Centre of Integrated Circuit Failure Analysis and Reliability (CICFAR), Department of Electrical and Computer Engineering, National University of Singapore. In particular, I would like to thank Mrs. Ho Chiow Mooi and Mr. Koo Chee Keong for their support and for providing necessary tools for my experimental work. I wish to thank Dr. Liu Wei, Research Scientist, Materials Growth Cluster, Institute of Materials Research and Engineering (IMRE), Singapore, for fabricating the Nitride samples for my experimental work and for discussions and insights on the results. I would also like to thank Dr. Ramam Akkipeddi, Ms. Doreen Lai Mei Ying, Ms. Hui Kim and Mr. Lim Poh Chong, SERC Nano Fabrication and Characterization cluster in IMRE, for rendering their assistance in material characterization. i I would like to express my gratitude to Dr.Paul Hosses, Stanford Computer Optics, Germany, for providing timely technical support and advice on the gated ICCD Camera I sincerely acknowledge and thank the National University of Singapore for funding the ICCD Camera purchase under Research Project R263-000-252-111, without which this research work would not have materialized. Lot of my friends helped me indirectly in last few years to complete this research work and it would not be appropriate if I not acknowledge their support and help. Since these two pages of acknowledgement are not sufficient to mention their names, I would like to thank all those people who helped me to complete this work successfully. Lastly, I am thankful to my family members for their affection and providing me moral guidance throughout the research work. I dedicate this research work to my parents to honor their guidance, support and love during all these years. ii Table of Contents _____________________________________________________________________ TABLE OF CONTENTS Acknowledgements i Table of Contents iii Summary vii List of Tables x List of Figures xi List of Symbols xvi Chapter Luminescence Techniques 1.1 Introduction 1.2 Photoluminescence 1.2.1 PL Advantages and Limitations 1.2.2 PL Applications 1.3 Cathodoluminescence 12 1.3.1 CL Advantages and Limitations 13 1.3.2 CL Applications 15 1.4 Time Resolved Cathodoluminescence 19 1.4.1 Time Resolved CL Needs and Motivation 21 1.5 Objectives and Scope of Thesis 23 Chapter Cathodoluminescence 2.1 Introduction 27 2.2 Light Emission Phenomena 28 2.2.1 Intrinsic Emissions 29 2.2.2 Extrinsic Emissions 31 2.2.3 Physics of CL 33 2.3 CL Instrumentation Techniques 39 2.3.1 CL Instrumentation and Developments 40 iii Table of Contents _____________________________________________________________________ 2.3.2 CL Detection Modes 42 2.3.3 CL Observation Considerations 45 2.4 CL Applications 47 2.4.1 Defect Contrast and Spectroscopy Study 48 2.4.2 Depth Resolved Study 54 Chapter Time Resolved Cathodoluminescence 3.1 Principle of TRCL Measurements 58 3.1.1 Carrier Relaxation Kinetics 60 3.2 TRCL Instrumentation and Techniques 62 3.2.1 Sequential Registration Technique 64 3.2.2 Simultaneous Registration Technique 65 3.2.3 Single Photon Counting 68 3.3 TRCL Applications 71 3.4 TRCL Limits and Considerations 76 Chapter Time Resolved Cathodoluminescence Microscopy and Spectroscopy System 4.1 System Design 80 4.2 System Description and Operation 83 4.2.1 Beam Blanker 86 4.2.2 Light Collector 89 4.2.3 Spectrograph 90 4.2.4 Photon Detector 91 4.3 System Alignment 96 4.4 System Calibration 99 4.4.1 Wavelength Calibration 100 4.4.2 ICCD Gate Time Verification 105 iv Table of Contents _____________________________________________________________________ Chapter Study of GaAsP Light Emitting Diode 5.1 Properties of GaAsP LEDs 109 5.2 Carrier Recombination Mechanisms 115 5.2.1 Direct Band-To- Band Recombination 116 5.2.2 Indirect Recombination via Deep Levels 117 5.2.3 Auger Recombination 120 5.3 GaAsP LED Sample 123 5.4 Static CL Imaging 124 5.4.1 Results and Discussion 126 5.5 Time Resolved CL Investigation 127 5.5.1 TRCL Signal Acquisition 128 5.5.2 Carrier Lifetime Measurements 129 5.5.2.1 Analysis and Discussion 133 5.5.3 Surface Recombination Effects in GaAsP LED 135 5.5.3.1 Analysis and Discussion 138 5.5.4 Minority Carrier Lifetime Mapping 144 5.5.4.1 Analysis and Discussion 145 5.5.5 Excitation Dependence on Carrier Recombination 148 5.5.5.1 Analysis and Discussion 150 5.5.6 Spectral TRCL Investigation 154 Chapter Study of GaN/InGaN Quantum Dots 6.1 Properties of InGaN Material 158 6.1.1 Structural Properties 158 6.1.2 Optical Properties 162 6.2 Details of InGaN Sample 164 6.3 Study of Threading Dislocations (TDs) in InGaN /GaN Quantum Structures 167 6.3.1 Results and Discussion 171 v Table of Contents _____________________________________________________________________ 6.4 Study of Optical and Structural Properties of InGaN /GaN QDs 185 6.4.1 Indium Surface Segregation in InGaN /GaN QDs 185 6.4.2 Indium Compositional in homogeneity in InGaN /GaN QDs 186 6.4.3 Results and Discussion 187 6.4.3.1 Photoluminescence 188 6.4.3.2 Cathodoluminescence 190 6.4.3.3 Time Resolved Cathodoluminescence 193 6.4.3.4 High Resolution TEM 199 6.4.3.5 X-Ray Diffraction 201 Chapter Conclusions and Suggestions for Future Work 7.1 Conclusions 207 7.1.1 TRCL Setup 208 7.1.2 GaAsP LED 209 7.1.3 InGaN QD 210 7.2 Suggestions for Future Work 211 7.2.1 Electron Gun 212 7.2.2 Near Field Time Resolved Cathodoluminescence 218 7.2.3 Photon Detector 220 7.2.4 Low Temperature Measurements 221 Appendices 223 References 237 vi Summary _____________________________________________________________________ SUMMARY In recent years, the semiconductor device size has been scaled down continuously according to Moore’s law and traditional semiconductor materials used for device fabrication have reached the limits of the fundamental material properties due to miniaturization. This results in the advent of nanostructured materials which attract extensive interest because of their importance in the fundamental physical research but also of their use in optoelectronics and nano technology. Naturally these materials are prone to defects due to presence of impurities intentionally or unintentionally. A systematic micro or nano scale characterization approach is essential to understand the material properties of complex hetero structures and physics behind them. Simultaneously new characterization tools capable of analyzing the material properties are being developed. The new characterization tools include scanning probe microscopy (SPM) techniques such as atomic force microscopy (AFM), high resolution transmission electron microscopy (HRTEM) and high resolution scanning electron microscopy (HRSEM), luminescence techniques such as scanning near field optical microscopy (SNOM), photoluminescence (PL) and cathodoluminescence (CL) techniques. Among these techniques, luminescence measurement technique is the most sensitive non destructive technique for characterization of defects and to study the local electronic, structural and optical properties of such materials. PL technique has a limitation on its spatial resolution and this is overcome in CL technique. The enhancement of CL vii Summary _____________________________________________________________________ technique with additional features such as temporal & spectral measurement capability enable it as a powerful tool to obtain the dynamic, spectral and spatial information that determines the optical properties of the nanostructured material. The main objective of this research work is to design and build a new time-resolved CL measurement system to overcome the limitations of other systems developed previously by other researchers and to prove its usefulness in the characterization of nano structured materials. In this work, a time-resolved cathodoluminescence (TRCL) system with spectral capability is developed with a spatial resolution of 100 nm and temporal resolution of 750 ps. The usefulness of the system demonstrated that the TRCL is a unique characterization technique to study the carrier dynamics and luminescence properties of the semiconductors. TRCL measurements are performed on compound semiconductor materials such as GaAsP and InGaN quantum structures. A commercial LED is used as a specimen to study the optical properties of GaAsP material. The enhancement on the spectral capability over the current static CL system is demonstrated by performing a simultaneous acquisition of the spectral information. The minority carrier lifetime of the GaAsP material is determined and also the recombination mechanisms are studied. The application of TRCL technique is demonstrated in studying the surface recombination mechanisms in a semiconductor. The usefulness of the TRCL technique in defect finger printing is also demonstrated by studying the decay behavior and the underlying recombination mechanisms at the defect site. The spectral TRCL measurements are viii Summary _____________________________________________________________________ performed to study the multiple recombination events taking place at different time intervals. InGaN/GaN quantum dots (QDs) are grown at different growth conditions to study the impact of the indium pre deposition prior to QD growth, TMIn flow rate and growth temperature on the luminescence efficiency of the QDs. The presence of threading dislocations (TDs) is studied and its characteristics due to varying growth conditions are also discussed. TRCL measurements are performed at the “V” defect location and confirmed that TDs act as nonradiative recombination centers. The impact on optical and structural properties of the InGaN/GaN QDs due to indium compositional homogeneity is studied. The indium surface segregation at the well/barrier interface is studied and the effect of interfacial abruptness on the luminescence efficiency was also measured. The optimal growth conditions to grow InGaN QDs with broad spectral emissions with high luminescence efficiency are determined using TRCL as a one of the material characterization techniques. Lastly, modifications required to improve the performance of the TRCL system in terms of spatial, spectral and temporal resolution are proposed since these enhancements are mandatory to study the optical and structural properties of the ultrafast optoelectronic materials. Therefore this research works confirms that a high performance TRCL system is complementary to the existing characterization techniques and provides useful information for failure analysis in semiconductor research and development. ix Appendices _____________________________________________________________________ APPENDIX C DATA ACQUISITION SYSTEM The template of 4Spec software is shown in Fig C.1. The “Decs” script is run from the Windows command prompt and supplied with an input file consists of delay and gate times in a two dimensional array separated by a comma. The “Decs” script loads the delay and gate time after every exposure acquired in the 4Spec software. The sample input file is a text file and a sample is shown in Fig C.2. Fig C.1 4Spec software for data acquisition [134] 231 Appendices _____________________________________________________________________ Fig C.2 Sample input file containing delay and gate times for automatic data acquisition The automation of the TRCL data acquisition is explained with the help of the flow chart in Fig C.3. There are various processes (1-7) involved in acquiring a TRCL data automatically as shown in Fig C.3. The process starts with setting up the SEM to acquire a high resolution SE image. This process also includes setting up the CL collector and alignment which is further discussed in the next section. Lastly, the electron beam is positioned in point mode on the location of the sample, which is of interest. A line spectrum is obtained at this location to determine the peak wavelengths. The spectrograph centre wavelength is set according the spectral range to be studied. After setting up the SEM, process (2) explains the procedure to setup the ICCD camera with the help of the configuration software 4Picos.exe. This software helps to set all the hardware related parameters such as trigger source, gate control, trigger 232 Appendices _____________________________________________________________________ mode, gain, units for time scale (this unit will be used for the data from the input file for automatic data acquisition) etc. Process (3) explains the procedure for setting up the frame grabber for data acquisition with the help 4Spec.exe software. In this step, the image readout specifications such as pixel details, scan sequence such as number of frames integration, axes definition, curve definition etc are set. All the parameters can be configured by loading a preconfigured file to minimize the configuration time. A background image is acquired with entrance to the camera closed to get the black noise of the ICCD. The background image is saved in the 4Spec software and “Automatic background subtraction” feature in 4Spec.exe software is used to offset the black noise during TRCL measurement. The calibration data is also loaded in this step according to the centre wavelength set in the spectrograph. Once the ICCD camera and frame grabber is configured as per the process (2) and (3), the next step is power up the beam blanker to modulate the electron beam excitation. The optical light guide from the CL collector is also connected to the inlet of the spectrograph. An input file for automatic data acquisition consisting of various gate delays and gate time is also prepared. Upon running the “Decs” script, the input file is supplied to start the automatic data acquisition. The first data set of gate delay and gate time is loaded in 4Picos.exe and an image is produced in the 4Spec.exe after integration of as many frames as defined. Upon completion of the image construction, a curve is generated based on the pixels defined. 233 Appendices _____________________________________________________________________ This curve is related to the first time data set in the input file. Then the second data set of gate delay and gate time is loaded in 4Picos.exe and process continues. This will continue until the last data set in the input file and as many curves as the number of data set defined in the input file are generated. The Intensity vs. Time plot can be generated by an integrals function in 4Spec.exe software by selecting the curves from the data set which are of interest. It is also possible to select the spectral range of a curve for which the time integral has to be created. In this way, the software is capable of producing a 3D CL data set. The raw data and the time integral data can also be exported to a spreadsheet for further analysis. Time-resolved data acquisition is typically a time consuming process due to integration of multiplier exposures and frames in studying weak emissions. Manual data acquisition process is prone to errors and repetitive experiments. Automation of the data acquisition in such iterative experiments really helps to eliminate experimental errors and minimize the data acquisition time. 234 Appendices _____________________________________________________________________ Start 1. 2. 3. 4. Load sample into SEM specimen chamber Obtain high resolution SE image Move in the CL light collector Align the CL light collector with the electron beam 5. Swtich the electron beam scan mode to point mode 6. Position the electron beam on the sample 7. Set the spectrograph centre wavelength 1. Connect the 4Picos ICCD camera according to the mode of operation 2. Powerup the ICCD camera 3. Run the 4Picos.exe software and set all the camera parameters for optimal detection 1. Run the 4Spec.exe software 2. Load the image readout, scan sequence defaults into 4Spec.exe software 3. Close the inlet to the camera and acquire a background image 4. Enable autobackground subtraction 5. Load the calibration data according to the spectrograph centre wavelength 1. Connect the optical light guide to the spectrograph 2. Powerup the beam blanker 3. Create a text file for delay time and gate time in this format ( t d , t s1 , .t dn , t sn ) ) 4. Run the decs.exe script and point the above text file for automatic data capture Fig C.3 TRCL data acquisition Flowchart (contd.) 235 Appendices _____________________________________________________________________ 1. Load 1st set of delay and gate times into 4Picos.exe software 2. Obtain an integrated image and respective spectrum in 4Spec.exe software 3. Load next set of delay and gate times into 4Picos.exe software No Is current set of data is equal to nth set? Yes 1. 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Phys. 30 (1991) 411 S.Tanaka, H.Omiya and 246 [...]... : Minority carrier lifetime ε : Strain τ A : Auger recombination lifetime ϕCL : Steady state CL intensity τ bulk : Minority carrier lifetime at the bulk xviii List of Symbols _ τ eff : Effective lifetime τ nr : Nonradiative minority carrier lifetime τ rr : Radiative minority carrier lifetime τ SRH : SRH lifetime τ surf : Minority carrier lifetime at the surface ν :... studied on a time scale and referred to as time- resolved PL and CL, respectively Sometimes these techniques are also referred to as dynamic or transient PL and CL respectively Time- resolved CL technique is an emerging and useful characterization technique to study the electronic and optical properties of the group III-V semiconductors This research work is focused on the development of a novel timeresolved... the average recombination time The photo excited carriers then recombine at a rate that is characteristic of the recombination path they follow This technique is referred to as time- resolved PL (TRPL) measurement and is used to determine carrier lifetimes, and to identify and characterize various recombination mechanisms in the material The experimental setup required for timeresolved PL depends on the... continuous-wave PL intensity and spectrum is quick and straightforward compared to the time- resolved PL which is more challenging, especially if recombination process are fast Instrumentation for time- resolved PL detection can be expensive and complex Alternative luminescence probing technique is sought to address the limitations of the PL Cathodoluminescence (CL) is one such technique with a spatial resolution... measurement includes CL imaging, both panchromatic [39,40] and monochromatic modes [41,42] and CL spectroscopy includes depth -resolved CL and time- resolved CL A spectrum corresponding to a selected area of the sample is used to determine the carrier recombination kinetics [43] Time- resolved CL is further discussed in the next section 15 Chapter 1 Luminescence Techniques ...List of Tables _ LIST OF TABLES Table 4.1 Features of other time- resolved CL system Table 4.2 Lamps used for wavelength calibration Table 4.3 Spectral dispersion chart Table 5.1 Test conditions for panchromatic TRCL measurements Table 5.2 Test conditions for carrier lifetime determination Table 5.3 Test conditions for surface recombination study Table 5.4 Test conditions... timeresolved PL depends on the desired resolution but the most common detection scheme is time- correlated single photon counting technique [30] Pulsed laser systems yield very high time resolution in order of femtoseconds Such high resolution is required to study the ultra-fast recombination kinetics in a heterostructure [31] 1.3 CATHODOLUMINESCENCE In recent years there has been continuous scaling down of device... yields sharp, well resolved peaks in the PL [29] Under the continuous wave excitation in a PL experiment, the system quickly reaches a steady state The rate of excitation equals the rate of recombination and the photo excited carrier density is constant in time However if the material is excited by a series of short laser pulses, the photo-excited carrier density depends strongly on time, because the... velocity N D : Donor concentration T : Absolute temperature ne : Excess electron concentration xvii List of Symbols _ t d : ICCD gate delay time ϕ CLD : CL intensity at the defect t s : ICCD gate open time ϕ ( z ) : Distribution of electron-hole pairs t sr : Streak range γ : Backscattered coefficient U : Net recombination rate η ext : External quantum efficiency Vacc :... for testing beam blanker operation Fig 4.8 Intensity vs Position plot for semi-ellipsoidal mirror Fig 4.9 Wavelength calibration setup Fig 4.10 ICCD gate time verification setup Fig 4.11 Laser pulse characteristics Fig 4.12 Timing diagram for ICCD gate time verification Fig 5.1 Optical transitions in GaAsP material doped with an optically active nitrogen impurity Fig 5.2 Band structure of GaAs1-XPX material . Limitations 6 1.2.2 PL Applications 9 1.3 Cathodoluminescence 12 1.3.1 CL Advantages and Limitations 13 1.3.2 CL Applications 15 1.4 Time Resolved Cathodoluminescence 19 1.4.1 Time Resolved. 6.4.3 Results and Discussion 187 6.4.3.1 Photoluminescence 188 6.4.3.2 Cathodoluminescence 190 6.4.3.3 Time Resolved Cathodoluminescence 193 6.4.3.4 High Resolution TEM 199 6.4.3.5 X-Ray. prove its usefulness in the characterization of nano structured materials. In this work, a time-resolved cathodoluminescence (TRCL) system with spectral capability is developed with a spatial resolution