Wael Z. Tawfik

Stress-induced piezoelectric field in GaN-based 450-nm light-emitting diodes

We investigated the influence of the built-in piezoelectric field induced by compressive stress on the characteristics of GaN-based 450-nm light-emitting diodes (LEDs) prepared on sapphire substrates of different thicknesses. As the sapphire substrate thickness was reduced, the compressive stress in the GaN layer was released, resulting in wafer bowing. The wafer bowing-induced mechanical stress altered the piezoelectric field, which in turn reduced the quantum confined Stark effect in the InGaN/GaN active region of the LED. The flat-band voltage was estimated by measuring the applied bias voltage that induced a 180° phase shift in the electro-reflectance (ER) spectrum. The piezoelectric field estimated by the ER spectra changed by ∼110 kV/cm. The electroluminescence spectral peak wavelength was blue-shifted, and the internal quantum efficiency was improved by about 22% at a high injection current of 100 mA. The LED on the 60-μm-thick sapphire substrate exhibited the highest light output power of ∼59 mW at an injection current of 100 mA, with the operating voltage unchanged.

Stress Engineering by Controlling Sapphire Substrate Thickness in 520 nm GaN-Based Light-Emitting Diodes

Controlling the compressive stress in 520 nm GaN-based light-emitting diodes (LEDs) prepared on sapphire substrates with different thicknesses was investigated. As the sapphire substrate thickness is reduced, the compressive stress in the GaN layer is released, resulting in wafer bowing. The wafer bowing-induced mechanical stress alters the piezoelectric fields, which in turn reduces the quantum-confined Stark effect in the InGaN/GaN active region of the LED. Thus, the electroluminescence spectral peak wavelength was blue-shifted, and the internal quantum efficiency was improved by about 11% at an injection current of 20 mA. The LED with an 80-µm-thick sapphire substrate exhibited the highest light output power of 11.5 mW.

Uni-axial external stress effect on green InGaN/GaN multi-quantum-well light-emitting diodes

The influence of uni-axial external stress on green InGaN/GaN multi-quantum-well (MQW) light-emitting diodes (LEDs) is evaluated. LEDs with external tensile stress show an improvement in light output power up to 35% at an injection current of 20?mA. The maximum output power can be obtained by considering the residual mechanical stress induced by the wafer bowing. In contrast, when the LEDs were exposed to an external compressive stress, the light output power was reduced by ?7% at an injection current of 20?mA. Moreover, when the compressive strain developed in InGaN/GaN MQW active region is relaxed, the peak wavelength of electroluminescence was blue-shifted. The results confirmed that applying external tensile stress effectively compensates for the compressive strain and alters the piezoelectric field in the InGaN/GaN active region, and hence increases the probability of radiative recombination.

Electrochemical removal of hydrogen atoms in Mg-doped GaN epitaxial layers

Hydrogen atoms inside of an Mg-doped GaN epitaxial layer were effectively removed by the electrochemical potentiostatic activation (EPA) method. The role of hydrogen was investigated in terms of the device performance of light-emitting diodes (LEDs). The effect of the main process parameters for EPA such as solution type, voltage, and time was studied and optimized for application to LED fabrication. In optimized conditions, the light output of 385-nm LEDs was improved by about 26% at 30 mA, which was caused by the reduction of the hydrogen concentration by ∼35%. Further removal of hydrogen seems to be involved in the breaking of Ga-H bonds that passivate the nitrogen vacancies. An EPA process with high voltage breaks not only Mg-H bonds that generate hole carriers but also Ga-H bonds that generate electron carriers, thus causing compensation that impedes the practical increase of hole concentration, regardless of the drastic removal of hydrogen atoms. A decrease in hydrogen concentration affects the current-voltage characteristics, reducing the reverse current by about one order and altering the forward current behavior in the low voltage region.

Numerical Analysis of the Temperature Impact on Performance of GaN-Based 460-nm Light-Emitting Diode

The influence of temperature on the characteristics of a GaN-based 460-nm light-emitting diode (LED) prepared on sapphire substrate was simulated using the SiLENSe and SpeCLED software programs. High temperatures impose negative effects on the performance of GaN-based LEDs. As the temperature increases, electrons acquire higher thermal energies, and therefore LEDs may suffer more from high-current loss mechanisms, which in turn causes a reduction in the radiative recombination rate in the active region. The internal quantum efficiency was reduced by about 24% at a current density of 35 A/cm2, and the electroluminescence spectral peak wavelength was redshifted. The LED operated at 260 K and exhibited its highest light output power of ~317.5 mW at a maximum injection current of 350 mA, compared to 212.2 mW for an LED operated at 400 K. However, increasing temperature does not cause a droop in efficiency under high injection conditions. The peak efficiency at 1 mA of injection current decreases more rapidly by ~15% with increasing temperature from 260 to 400 K than the efficiency at high injection current of 350 mA by ~11%.