In addition to the introduction, this dissertation is organized into five chapters and two appendices. Chapter 2 presents a small-signal method to characterize the III-nitride LEDs. The RF measurement setup is described, and the results of the method are validated. Different components of modulation bandwidth of LEDs are also investigated, and bandwidth limitations are discussed.
Chapter 3 studies the high-speed InGaN/GaN LEDs. Based on the knowledge from the previous chapter, DLT as the limiting element of modulation bandwidth is targeted for design of high-speed LEDs. Thus, the dependency of DLT and modulation bandwidth on crystal orientation is investigated, and then GHz-bandwidth LEDs operating at low current densities are demonstrated on nonpolar GaN materials. Next, different active region designs are studied for shorter DLT and higher modulation bandwidths. At the end of this chapter, co-optimization of efficiency and bandwidth is discussed for two different active region designs in semipolar LEDs.
Chapter 4 presents the determination of injection efficiency under electrical injection and studies carrier and current distributions in the LED structure. Behavior of injection efficiency is explained by studying the differential carrier lifetimes. Injection efficiency is excluded as the primary cause of efficiency droop.
In chapter 5 a new method known as pulsed-RF measurement is developed to characterize the LEDs under isothermal conditions. This technique was used to measure the RF characteristics of the LEDs as a function of current density for elevated temperatures. The differential analysis of the extracted carrier lifetimes resulted in
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calculation of carrier density and radiative and non-radiative recombination rates in the QW. As a result, origins of efficiency and thermal droop are identified.
Chapter 6 consists of a conclusion of the dissertation and proposed future work. The future work focuses on the development of efficient, high-speed LEDs for emerging applications, digging more into physics of the carrier mechanisms, study of green gap, and exploring how to expand/tailor this method to characterize other photonic devices such as laser diodes (LDs), superluminescent diodes (SLDs), photodetectors (PDs), and vertical-cavity surface-emitting lasers (VCSELs).
Appendix A gives the technical information on the RF setup. It gives useful tips for future users of the RF setup that was developed for this dissertation. Tips are on operating the network analyzer, calibration of the setup, noise cancelation techniques, and pulsed- RF measurement. The components of the setup are also discussed.
Appendix B is focused on the growth, fabrication, and basic characterization of the LEDs. Details of growth, surface roughness, and XRD results are discussed. Mask design and fabrication processes are presented, and finally basic electrical characterization of the LEDs are realized.
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Chapter 2 Small-signal RF method to study carrier dynamics in III-
nitride LEDs
In this chapter, the framework of a unique method to study carrier dynamics of the LEDs under electrical injection is laid out. This method will be used in the following chapters to design high-speed LEDs for VLC applications, determine the injection efficiency, and study the origins of efficiency and thermal droop. The focus of this chapter is only on developing this characterization method. However, fabricated micro- LEDs are used as the target device to realize the technique. The details of growth and fabrication of the LEDs will be discussed in chapter 3 and appendix B.
Using small-signal rate equations, a small-signal equivalent electrical circuit for III- nitride LEDs considering both recombination and transport effects is derived. The input impedance and modulation response of the circuit are simultaneously fit to the measured input impedance and modulation response of semipolar SQW InGaN/GaN micro-LEDs on free-standing GaN. Nonlinear regression is used to achieve excellent fittings to the RF data without pre-defining or assuming any parameters based on DC measurements. The complete modulation response and impedance characteristics are reconstructed from the model. The DLT and transport related parameters are obtained as a function of current density for four different micro-LED sizes. Steady-state form of the rate equations is also used to derive expressions for the circuit parameters. The fittings are validated by checking the consistency of different parameters vs. current density and the LED diameter. Then, approximations for the modulation response under low and high injection levels are derived and showed that the leakage of carriers affects the modulation response
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of the device at low current densities. Only considering the recombination process for predicting the modulation response introduces deviations from the full model, especially at low injections. Additionally, at high current densities the DLT is separated and an RC time constant related to transport effects is defined. Decoupling the effects of transport and recombination on the modulation response is important to establish device designs with low RC time constant and short recombination lifetime, which are both needed for high-speed VLC applications.