UC Santa Barbara

The III-nitride material system, i.e., (In, Ga, Al)N, which has a direct bandgap ranging from 0.7 eV to 6 eV, is well-suited for a wide range of optoelectronic devices. One of them would be the InGaN-based light-emitting diode (LED), which has become one of the leading general illumination technologies. The success of nitride LEDs has also sparked interest in other optoelectronic devices including photodetectors. This dissertation focuses on the development of both III-nitride photodetectors and LEDs.

In recent years, increasing interest in visible light communication (VLC) has created demand for high-speed photodetectors in the visible light spectrum. InGaN-based photodetectors, which offer great wavelength-selective response due to its tunable bandgap, are promising candidates for optical receivers in VLC links. Instead of the InGaN double heterostructure devices, InGaN-based photodetectors with multiple quantum well (MQW) structure designs are favored due to their significant advantages in terms of growth. The speed limiting factors of such devices were studied to improve their speed performance. On top of the usual RC time constant, InGaN/GaN MQW photodetectors are also limited by carrier escape lifetimes due to carrier trapping in the QWs. By reducing the thickness of the quantum barriers, carrier escape lifetimes were reduced, resulting in a twofold improvement in the 3-dB bandwidth of the photodetectors.

Despite the huge success of InGaN-based LEDs in solid-state lighting, one of the most enduring challenges that still limit the LEDs is the efficiency droop phenomenon, which refers to the decrease in the quantum efficiency with increasing injection current density. In conventional c-plane LEDs, polarization-induced electric fields further exacerbate the droop problem. The large internal electric fields in the quantum wells lead to a reduction in the overlap of the electron and hole wavefunctions. This lowers the recombination coefficients and causes an increase in carrier density at a given current density, leading to an early onset of efficiency droop due to the nonlinear Auger recombination. Additionally, the large internal electric fields also prevent the use of thick QW active region designs to reduce the carrier densities. One approach to reduce the internal electric field in c-plane QWs is through the use of doped barriers. However, the heavily doped Mg(Si)-doped p(n)-type GaN barriers also lead to a higher defect density. Growth optimization was performed with the aim of maximizing the field reduction and minimizing the detrimental impact of the doped barriers. With doped barriers, we demonstrated a 9-nm-thick single QW LED with a low efficiency droop. Biased photocurrent spectroscopy was also carried out to illustrate the effect of doped barriers on the internal electric fields. Device simulations were used in tandem with experimental results to guide the interpretation of the results. Lastly, differential carrier lifetime measurements were performed to determine the impact of doped barriers on the recombination coefficients of the LEDs. The improvement in the radiative coefficients in the LEDs with doped barriers, coupled with the blueshift of the emission wavelengths, indicates an enhancement in wavefunction overlap and a reduction of quantum confined Stark effect (QSCE) as a result of the reduced internal electric field.