The piezoelectricity and its influences on optoelectronic performances in GaN-based light-emitting diodes

Abstract

Light-emitting diodes (LEDs) have been of keen interest due to their wide range of valuable applications in the real world. This work is an effort towards understanding the characteristics of the LEDs specifically in mainly three parts. The first part discusses the piezoelectric field induced by polarization in the GaN-based light-emitting diodes (LEDs) which is one of the main hindrance in improving the performance of the LEDs. This piezoelectric behavior is strongly dependent on Indium (In) content in the quantum well region i.e. higher the In-content, stronger is the effect of piezoelectric field. However, the relation between the two has not been found linear. Reported theoretical studies show a consistent relationship between the In-content and the piezoelectric field. But experimental results show a different scenario altogether.

Also, the experimental study of the piezoelectric field in single and double quantum well green light emitting diodes has been reported for the first time. The higher In-content makes it difficult to grow good quality InGaN epilayer because of the increased mismatch of lattice and thermal expansion coefficient. This hindrance in the visible spectrum of the green LEDs is, often referred to as ‘green gap’.

The second part gives an insight on the performance of blue single quantum well (SQW) vertical LED (VLED), in terms of numerical simulations. This work has been possible by using the simulator called APSYS (Advanced Physical Models of Semiconductor Devices) by Crosslight Software Inc. APSYS is a general purpose 2D/3D modeling software program for semiconductor devices. Based on finite element analysis, it includes many advanced physical models such as hot carrier transport, heterojunction models and thermal analysis. APSYS offers a very flexible and simulation environment for modern semiconductor devices. The goal is to propose and discuss the mechanisms that could reduce the built-in field caused by polarization in the real device. Therefore, the thesis discusses the effect of compound polarity and In-grading on the performance of the device.

The third part discusses the numerical effect of polarization strength on the device and then incorporates the concept in the standard ABC-model using experimental data. The study has a useful finding in terms of why the polarization effect needs to be considered and what are the real scenarios where polarization effect must be considered.