Dong-Hyun Jang

Measurement of Internal Electric Field in GaN-Based Light-Emitting Diodes

The internal electric field due to the piezoelectric effect in an InGaN/GaN multiple quantum-well structure is measured via photoluminescence (PL), electroreflectance (ER), and photocurrent (PC) spectroscopies, and the measurement results are compared with each other and with theoretically calculated values. The flat-band voltage is estimated by measuring the applied bias voltage that induces the maximum in PL peak energy by the quantum-confined Stark effect, 180° phase shift in the ER spectrum, and the smallest band tail state in the PC spectrum. The internal electric fields estimated by the PL, PC, and ER spectra are -1.81, -2.12±0.14, and -2.04 MV/cm, respectively. The measured piezoelectric fields are in good agreement with theoretically calculated values. Possible factors affecting piezoelectric field measurements are discussed from various perspectives.

Analysis of the stress distribution in the nonuniformly bent GaN thin film grown on a sapphire substrate

We analyze the stress distribution in the nonuniformly bent GaN epilayers grown on a sapphire substrate. By using theoretical analysis combined with an analytical formula describing the realistic shape for the wafer bending of GaN epiwafers, we examine the effect of nonuniformity in the wafer bending on the stress-value variation over the entire wafer. We show that the stress on the GaN thin film can deviate by ∼1 MPa from the value obtained by the simple Stoney’s formula that is typically used for the uniformly bent wafer. We also show that the maximum value of the stress linearly increases with the bow difference along the horizontal and vertical directions.

Enhancement of Light Extraction Efficiency Using Lozenge-Shaped GaN-Based Light-Emitting Diodes

We have demonstrated both theoretically and experimentally that a lozenge-shaped light-emitting diode (LED) enhances light extraction efficiency compared with a conventional rectangular LED. The total light output power of the lozenge-shaped LED on a transmitter optical can (TO-can)-type package shows an increase of 12% at an injection current of 20 mA when compared with that of a rectangular LED. Moreover, the series resistance and the forward voltage of the lozenge-shaped LED slightly decrease compared with those of the rectangular LED. The far-field emission pattern shows that the light escaping from the lozenge-shaped LED along the horizontal direction is larger than that from the rectangular LED.

Measurement of nonuniform bowing in GaN/sapphire epi-wafers and subsequent stress analysis by using a theoretical model

We present our approach to measure the profile of nonuniformly bent GaN epi-wafers grown on sapphire substrates. By using a laser displacement sensor, the position of the epi-wafer is accurately measured and mapped. From the measured profile data, analysis of stress distributions over the nonuniformly bent wafer is performed by using a theoretical model. We show the result of theoretical analysis of how the stress tensors distribute over a wafer. The estimated stress tensors are related with optical properties such as photoluminescence of the wafer.

Strain Analysis in Pseudomorphically Grown GaN-based Multiple Epitaxial Layers

A strain analysis model in the pseudomorphically grown epitaxial multilayer system is investigated. Analytical formulas of strain parameters in each epitaxial layer are derived as the following assumptions: (1) the substrate thickness is finite, (2) the in-plane lattice constant is the same for all epitaxial layers for dislocation free growth, (3) the stress along the crystal growth direction is constant, but not necessary zero, (4) the in-plane lattice constant is determined such that the total strain energy. We find the residual stress affect the electronic properties of epitaxially grown multilayer system even though in-plane lattice constant is unchangeable compare with no stress along the crystal growth direction.