Degradation mechanism of AlInGaP light
emitting diodes during PMMA encapsulation
and operation
Dem Department Physik
der Universität Paderborn
zur Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigte
Dissertation
von
Stephan Preuß
Paderborn, November 2007
Contents
1 Introduction 1
2 Fundamentals of semiconductor devices 5
2.1 Semiconductors 5
2.2 Band structure of p-n junctions 8
2.2.1 Doping of semiconductors 8
2.2.2 p-n junction 9
2.2.3 Double heterostructures and quantum wells 11
2.3 Radiative recombination 14
2.4 Modelling of the emission spectra 15
2.5 Light extraction from a LED 18
2.6 Non-radiative recombination 21
3 Micro electroluminescence set-up 23
3.1 Principle of the set-up 23
3.2 Optics 25
3.2.1 Step size at the chip surface 26
3.2.2 Space resolution 26
3.3 The positioning system 27
3.4 The Ulbricht sphere 27
4 Micro electroluminescence - results and discussion 29
4.1 Description of the samples 29
4.1.1 The HWFR-B 410 LED: basic properties 30
4.2 Sample preparation 34
4.3 Experimental details for micro electroluminescence
measurement 34
4.4 Temperature analysis 35
4.5 Experiment 36
4.5.1 Glue hardening 36
4.5.2 Spectral measurements along the z-axis 37
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4.6 Discussion of the results 40
4.6.1 Glue hardening 40
4.6.2 Micro electroluminescence along the z-axis 41
4.6.3 Modelling A1 and A2 versus z 41
4.6.4 Current dependence 45
4.6.5 Measurements along the x-axis 47
5 Aging of LEDs 49
5.1 Degradation under high forward current-chip only 49
5.2 Positive aging 49
5.3 Long-term aging 55
6 Injection molding of LED clusters 61
6.1 General set-up and temperature measurements 61
6.1.1 The module (molded part) 63
6.1.2 Optics 65
6.1.3 Temperature measurement 66
6.1.4 Temperature simulation 71
6.2 Influence of injection molding on the LED performance 74
6.2.1 Results and discussion 74
6.3 Radiation pattern of the PMMA optic 80
7 Diffusion model for LED degradation 83
7.1 The dependence of electroluminescence on trap
concentration 83
7.2 Identifying magnesium as the p-type doping material 86
7.3 Magnesium diffusion into the active layer 89
7.4 Data analysis 92
7.4.1 The activation and diffusivity of magnesium
in AlInGaP 95
8 Summary 99
9 Appendix 101
Bibliography 107
ii
List of Figures
2.1 Bandstructure of direct semiconductor InP. 6
2.2 Bandstructure of indirect semiconductor GaP. 7
2.3 Band gap and wavelength of alloy AlInGaP versus lattice constant.
For large In-content the direct bandgap dominates. The red line indicates
the spectral range of AlInGaP LEDs lattice matched on GaAs with a direct
bandgap. 8
2.4 Band structure of the p-n junction under zero bias 10
2.5 Band diagram of a forward-biased double heterostructure. The p-type
confinement layer consists of a lightly doped layer close to the active
region and a higher doped layer further away from the active layer. 12
2.6 Square-well potential of a quantum well structure. The well is formed by
decreasing the mole fraction x 13
2.7 Density of state for a quantum well structure (2D) and for bulk material (3D). 13
2.8 The emission spectrum of a QW LED, red curve, is the product of the density
of state and the distribution of carriers. 16
2.9 Emission spectrum of an AlInGaP LED. The black line indicates the
measurement, the blue line the model function 17
2.10 A captured photon in an LED structure. The active layer is dark red, the
escape cones are hatched. Only photons inside the escape cones can escape
the device. A captured photon with an angle >βcrit is shown as light red rays. 19
2.11 Auger-recombination. The energy released by a electron hole recombination
is absorbed by another electron. 21
3.1 Electroluminescence set-up, the emission from the LED chip is coupled
into a monochromator. A diode array detects the diffracted light and the
data are sent to a PC. 23
3.2 Sketch of the micro-electroluminescence set-up. The light from the LED is
magnified by a factor of 500 and projected onto a screen. One point of the
image with a diameter of 1mm is in coupled to an optical fibre and guided
to the monochromator. Data is stored in a PC. 24
3.3 Optical set-up. The LED is outside the focal length of lens 1. Lens 1 generates
a magnified real intermediate image. Lens 2 magnifies this intermediate image. 25
3.4 Schematic set-up of an Ulbricht sphere. The coating has a high reflectivity and
the reflection is diffuse. Thus the light flux is uniform in the sphere. 28
4.1 Layer structure of the LED chips. “EP” denotes the edge planes (red hatched). 30
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