PowerPoint Presentation

High-Brightness InGaN Blue, Green and Yellow Light-Emitting Diodes with Quantum Well Structures

KW universityCollege of Electronic materials engineering2011734036 Woo chang hee

1

() - () -

Index

Abstract

Background

Conclusion

Introduction

Discussion

References

Introductionpercentage of photons hitting the device's photoreactive surface that produce charge carriers.Quantum efficiencywidth of a spectrum curve measured between those points on the y-axis which are half the maximum amplitudeFull width at half maximum (FWHM)measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angleluminous intensity(cd)Quantum wellpotential well with only discrete energy values

Background

InGaN/AlGaN LEDProblem about the crystal quality of the InGaN active layer

AlInGaP LEDLow quantum efficiency(band structure of AlInGaP close to an indirect transition)

The lifetime of the led is short.II-VI based LED

ingan/algan led .4

Discussion

Wide active layerNarrow active layerIn the DH structure, the thickness of active layer has an influence on LED`s inner quantum efficiency. If the width of active layer is too thick in the DH structure, there is no advantage in using DH structure. The thickness must thinner than distance of diffusion. Unless, there is no differences between hetero-junction and homo-junction.

SQW. LED LED . , . . SQW LED LED SQW . . , .5

Discussion

C-face sapphireGaN buffer layer (300)N-type GaN:si (4m)

N-type AlGaN:Si (1000)

N-type InGaN:Si (500)Undoped InGaN(20A) well layerP-type AlGaN:Mg (1000)

P-type AlGaN:Mg (1000)P-type GaN:Mg (0.5m)

- SQW structure- Reduce interface trap

Hi, Im the second presenter Chang hee, woo. Ji-hyun told you about basic keywords and single quantum well. Continually, I would like to explain the structure of improved LED. This LED is made of well structure above mentioned. Sapphire with 0001 orientation we called this C face, of two-inch diameter, was used as a substrate. GaN buffer layer is grown at a low temperature. The active region forms a single-quantum-well structure(SQW) consisting of a 20A InGaN well layer sandwiched by 500A n-type InGaN and 1000A p-type AlGaN barrier layers. The carriers injected by each electrode, moves into active layer, and other doped layers exist in order to enhance injection and quantum efficiency. The buffer layer is being use for reducing interface trap between sapphire substrate and n-type GaN. 6

() - . . () - 3-5 two flow cvd . () - .Discussion

Under no stress condition, peak wavelength of green is about 490nmIn experimental, peak wavelength of green LED is 525nm.

525nm, 45nm

590nm, 90nm

450nm, 20nm

Different thermal expansion coefficientLarge lattice mismatchIf indium mole fraction increases, the energy gap is getting smaller. It causes the reduction of wavelength.

590nm, 90nm

Band Gap Narrowing

This page deals with results related on electroluminescence. It shows longest emission wavelength and its fwhm of each color. As you can see this figure, it is divided into blue, green and yellow color LED. It is because of different indium mole fractions. If you see green color LED, you can notice that the longest emission wavelength is about 525nm. This data is higher than 490nm. For reference, we can get 490nm peak wavelength under no stress. Its because of band gap narrowing. We can explain this band gap narrowing through quantum size effects, mismatch of the lattice and different thermal expansion coefficients.

7

() - in led 2.ingan sqw led el . () - 525nm . () - () - 490nm . () - 2 . () - , tensile stress . () - fwhm () - 2Discussion

When the peak wavelength becomes longer, the value of the FWHM of the EL spectra increases.Increase indium mole fractionIt is caused by the mismatch of the lattice, andthe thermal expansion coefficients between well and barrier layers.

Reducing of energy band gapIncrease differences and strain

FWHM . In SQW (starin) .This page shows the FWHM of the EL spectra as a function of the peck wavelength. If the peak wavelength becomes longer, the value of FWHM of the EL spectra increases. We have to control the indium mole fraction to reduce energy band gap. Its due to the strain between well and barrier layers of quantum well. It is caused by the mismatch of the lattice and the thermal expansion coefficients between well and barrier layers.

8

Discussion

4mW, 7.3%

1mW, 2.1%

0.5mW, 1.2%

Saturation generation of heatDifferences between the well and the walls thermal expansion coefficientInGaN has large lattice mismatch.

Blue SQW LED > Green, Yellow

This page shows output power of SQW LEDs of each colors and external quantum efficiency. Above 60mA, the output power almost saturates, probably due to the generation of heat. The blue led has much higher output power and external quantum efficiency. We can explain the reason by 2 ways. First, Its because of different thermal expansion coefficient. Second, InGaN has large lattice mismatch.

9

() - () - 1. () - . () - . () - () - ingan .

Peak wavelength Output power

The large strain between well and barrier layers.

Discussion

This figure is the last of this article. It shows output power as a function of the peak wavelength. If the peak wavelength becomes longer, output power decreases. It is because of lattice mismatch between well and barrier layers, and thermal expansion effects.10

LEDOutput powerPeak wavelengthInGaN SQW0.5mW 1mW590nm525nmGaP0.04mW555nmAlInGaP0.4mW570nm

InGaN SQW LED GaP/AllnGaP LED

Discussion

More bright + More green

Let's compare with newly developed InGaN SQW LED and former LED which is made of GaP, AlInGaP. SQW LED has longest peak wavelength. Commonly, the output power has to weak, when peak wavelength is high. But it shows strong output power. Also, the former LED has similar peak wavelength with yellowish green. But InGaN SQW LED has similar peak wavelength with pure green color. In its final analysis, we can make pure green color LED through InGaN single quantum well. 11

Conclusion

1. Highbrightness InGaN green SQW LEDs were grown by MOCVD on sapphire substrates2. The peak wavelength and the FWHM of the green LEDs were 525nm and 45nm3. The color of green InGaN SQW LEDs was greener than those of conventional GaP and AlInGaP LEDs4. Fabrication of practical visible LEDs in the range from blue to yellow is possible using III-V nitride materials.

12

Reference

Q&A