CHAPTER 1 INTRODUCTION
1.5 An Overview of the Dissertation
The primary contents of this dissertation are discussing subjects which are relating to the performance enhancement of InGaN-GaN LEDs, including the influence of the mirror location to the light output, the role of the n-bowl structure played in managing the view angle, and the roughening of the ITO window layer.
There are six chapters in this dissertation. In chapter 1, a brief history of the progress of lighting devices is presented. We also make a complete introduction about the LED, which contains history, advantages, applications, development of III-V nitride-based LEDs, and several key issues we might experience while studying it.
In chapter 2, we review the theory and background of this dissertation. The basics of the LED, the Snell’s law, the principles of the reflection loss of light, concave mirror, transmission
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line model, and wafer bonding are detailed.
In chapter 3, GaN LEDs with double roughened (p-GaN and u-GaN) surfaces and a silver mirror on the sapphire substrate were demonstrated using wafer-bonding and laser lift-off technologies. Effect of the Ag mirror location on the luminance intensity of LEDs was investigated.
In chapter 4, a periodic n-bowl mirror array was introduced into LEDs to improve the LED performance and adjust the view angle. According to L-I characteristics and radiation pattern, this chapter provides the direct evidence that the periodic n-bowl mirror array can not only redirect photons, but also focuses them to the vertical direction of the LED.
In chapter 5, a simple natural lithography process was used to roughen the indium tin oxide (ITO) window layer to improve the InGaN-GaN light-emitting diode (LED) performance. In this lithography process, a photoresist layer was used as a mask for inductively coupled plasma (ICP) dry etching. Transmission line model, SEM and AFM were utilized to understand the contact changing and morphology of the ITO surface after etching. L-I characteristics were also discussed.
Finally, we summarize the overall results of our studies and discuss future works in chapter 6.
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(a)
(c)
(b)
(d)
Figure 1.1 (a) Illustration of the nightly illumination of a gaslight with a thorium oxide–soaked mantle. (b) Original carbon-filament bulb from Thomas Edison. (c) A closeup of a 175W mercury vapor lamp. (d) An early compact fluorescent lamp.
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Figure 1.2 Bandgap energy of various materials for visible emission devices as a function of their lattice constant.[5]
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Figure 1.3 Chromaticity diagram. White light can be produced through color mixing, like red/blue/green, blue/yellow, or green/yellow-green/orange/purple.[13]
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Table 1.1 Thermal conductivity of the materials.[25]
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CHAPTER 2
BACKGROUND AND THEORY
2.1 Basics of LEDs
Conventional lamps rely on either incandescence or discharge in gases. Both phenomena are associated with large energy losses that are essentially inherent because of the high temperatures and large Stokes shifts involved. LEDs offer an alternative way of light generation. Spontaneous light emission in LEDs is due to radiative recombination of excess electrons and holes. Excess electrons and holes are produced by current injection with small energy losses. Subsequent radiative recombination of the injected carriers may attain quantum yields close to unity. This phenomenon, called electroluminescence, is the energy-saving promise of this generation. In this section we deal with the basics of LEDs which include device configuration, recombination of electrons and holes, injection in a p-n junction, electroluminescent structure, and LED performance.
2.1.1 Device Configuration
The schematic of a typical surface-emitting LED is shown in Fig. 2.1. Like a normal diode, the LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. The doped materials are known as the p- and n-confinement layer. Electrodes that usually contain Au and Al are placed on the top and the bottom of the structure. To make connection between the power and the chip, wire bonding is often used on the top electrode. The
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size of it must be precisely controlled. If it is too large or too small, hindering effect and current crowing will occur to decrease the luminance efficiency, respectively.
LEDs are usually built on an n-type substrate and the structure is grown by epitaxial techniques with an electrode attached to the p-type layer deposited on its surface. p-Type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate. Lattice matching is an important issue for epitaxy. Typically, films of different materials grown on the substrate are chosen to match the lattice constant of the substrate material to minimize film stress.
Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Therefore Light extraction in LEDs is also an important aspect of LED production, subject to much research and development.
2.1.2 Recombination of Electrons and Holes
Carrier recombination result from interaction between electrons and other carriers, either with the lattice of the material, or with optical photons. As the electron moves from one energy band to another, its gained or lost energy must take some other form, like heat and photon. Excess carriers can recombine both radiatively and nonradiatively. Competition between radiative and nonradiative recombination processes determines the internal quantum efficiency of an LED. As shown in Fig. 2.2,[1] an intrinsic mechanism of radiative recombination is band-to-band transitions, in which an electron-hole pair recombines, emitting a photon. Radiative annihilation of excitons is the second intrinsic mechanism of light emission. In some alloys used for the fabrication of LEDs (like InGaN), the nonuniformity of the spatial distribution of constituents
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may cause considerable fluctuations of the band potential. Carriers that are localized at such fluctuations recombine radiatively with large probability. This is the third intrinsic mechanism of radiative recombination.
Other mechanisms of radiative recombination are link to impurities caused by defects and/or by intentional or unintentional doping. The impurity levels in the bandgap trap free carriers that may contribute to photon emission. For instance, a radiative transition between the conduction band and an acceptor state or between a donor state and the valence band might occur. Also, electrons trapped at donor states can recombine radiatively with holes trapped at acceptor states.
Finally, a trapped carrier can form an excitonic complex with a carrier of different type. In many semiconductors, radiative annihilation of bound excitons is the main emission mechanism at low temperatures and at low densities of excess carriers.
2.1.3 Injection in a p-n Junction
The basic element of an LED is a semiconductor electroluminescent structure that comprises, at least, a region of radiative recombination and regions of different conductivity type (p and n) that supply the recombining carriers. In the simplest design, the structure relies on a junction between a p-type semiconductor and an n-type semiconductor of the same kind with one or both conductivity regions employed as the radiative-recombination region or regions. Figure 2.3 depicts a band diagram of a p-n homojunction. Under zero bias, the majority electrons from the n-region diffuse into the p-region and the majority holes diffuse in the opposite direction. This
process creates depleted regions on both sides of the interface. The space charge of the depleted regions creates an internal electric field that counteracts the diffusion. In equilibrium, when the potential barrier is somewhat smaller than the bandgap energy, the diffusion current is
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counterbalanced by the reverse current of minority carriers that drift in this internal electric field.
When a voltage V is applied in the forward direction, the reverse currents of the minority carriers change negligibly. Meanwhile, the barrier for majority carriers decreases by qV.
Consequently, the majority-carrier diffusion current increases. Enhancement of the diffusion due to the electric field results in an excess density of minority carriers on both sides of the junction.
The injected carriers recombine both radiatively and nonradiatively.
2.1.4 Electroluminescent Structure
Conventional p-n diodes utilize doping profiles to control carrier injection. The potential barriers for electrons and holes are created by the charges of ionized donors and acceptors in the depletion region near the boundary between the n- and p-type semiconductors. LEDs based on p-n homojunctions have important shortcoming that limits their application in solid-state lighting.
The light generated in the active region is reabsorbed, to a considerable extent, in the conductive regions. This reduces the light-extraction efficiency.
Thinning the active layer is the way to further increase the rate of radiative recombination and reduce the reabsorption. In addition, using very thin active layers enables one to overcome some lattice-matching problems, since such layers are able to conform to the thick confining layers without defect generation. Such double heterostructures are called quantum well (QW) structures.[2] Single quantum wells (SQWs) and multiple quantum wells (MQWs) offer the most versatile structures for high-brightness LEDs.
Fig. 2.4 depicts a band diagram of QW structure composed of a thin layer of a semiconductor with the bandgap energy Eg2 sandwiched between thick cladding layers of a semiconductor with bandgap energy Eg1. A quantum well is a potential well that confines
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particles, which were originally free to move in three dimensions, to two dimensions, forcing them to occupy a planar region. The effects of quantum confinement take place when the quantum well thickness becomes comparable at the de Broglie wavelength of the carriers, leading to energy levels called “energy subbands”, i.e., the carriers can only have discrete energy values.
Light-emitting structures based on QWs require optimization of the injection efficiency. The drawback is that in materials used for LED fabrication, electron mobility is very high and electrons can leak into the p-type cladding layer without being captured by the QW states. The leakage is less probable for holes, which have much less mobility. Usually, an electron blocking layer (EBL) made of a wider-gap p-material is introduced between the QW and the p-conductive layer to prevent leakage of electrons into the p-type conductive layer and thus to improve injection efficiency.[3]
2.1.5 Efficiency
An injection-electroluminescence device is characterized by its radiant efficiency (also called wall-plug efficiency)
e ext f
η =η ×η (2.1)
where ηext is the external quantum efficiency and ηf is the feeding efficiency. Feeding efficiency is the ratio of the mean energy of the photons emitted and the total energy that an electron-hole pair acquires from the power source when passing through the LED:
f
h qV
η = ν (2.2)
where h is the Planck’s constant (4.14× 10-15 eV·sec), ν is the mean frequency of photons, V is the forward voltage drop across the LED and q is the elementary charge (1.60×10-19 C).
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External quantum efficiency (EQE) is the ratio of the number of photons emitted and the number of electrons passed through the LED. Explicitly, it is the product of the injection efficiency, ηinj; radiative efficiency, ηrad; and optical efficiency, ηopt:
ext inj rad opt
P hI
q
η = ν =η ×η ×η (2.3)
where P is the total power emitted out of the LED, ν is the frequency of the photon, and I is the injection current. Injection efficiency is the fraction of the electrons passed through the LED that are injected into the active region, where radiative recombination takes place:
n
where In is the injection current introduced in the p-region, In0 is the reverse current for minority electrons, kB is the Boltzmann constant, and T is the temperature of the crystal. Radiative efficiency (also called internal quantum efficiency, IQE) is the ratio of the number of electron-hole pairs that recombined radiatively to the total number of pairs that recombined in the active region: recombine radiatively and nonradiatively, respectively. Finally, optical efficiency (also called light-extraction efficiency) is the fraction of the photons generated that escape from the device.
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2.2 Snell’s Law
Snell’s law is used to determine the direction of light rays through refractive media with varying indices of refraction.[4] The indices of refraction of the media, labeled n1, n2 and so on, are used to represent the factor by which a light ray’s speed decreases when traveling through a refractive medium, such as glass or water, as opposed to its velocity in a vacuum.
As light passes the border between media, depending upon the relative refractive indices of the two media, the light will either be refracted to a lesser angle, or a greater one. These angles are measured with respect to the normal line, represented perpendicular to the boundary. In the case of light traveling from air into water, light would be refracted towards the normal line, because the light is slowed down in water; light traveling from water to air would refract away from the normal line. Refraction between two surfaces is also referred to as reversible because if all conditions were identical, the angles would be the same for light propagating in the opposite direction.
Snell’s law is generally true only for isotropic or specular media (such as glass). In anisotropic media such as some crystals, birefringence may split the refracted ray into two rays, the ordinary or o-ray which follows Snell’s law, and the other extraordinary or e-ray which may not be co-planar with the incident ray.
Named after Dutch mathematician Willebrord Snellius, one of its discoverers, Snell’s law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of velocities in the two media, or equivalent to the opposite ratio of the indices of refraction:
1 1 2
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Snell’s law follows from Fermat’s principle of least time, which in turn follows from the propagation of light as waves.
2.3 Reflection Losses of Light
When light travels from one medium to another, two types of reflection losses will occur because of the indices difference between two mediums. One is total internal reflection and the other is Fresnel loss.
2.3.1 Total Internal Reflection
When light travels from a medium with a higher refractive index to one with a lower refractive index, Snell’s law seems to require in some cases (whenever the angle of incidence is large enough) that the sine of the angle of refraction be greater than one. This of course is impossible, and the light in such cases is completely reflected by the boundary, a phenomenon known as total internal reflection.[5]
Total internal reflection is an optical phenomenon that occurs when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. The largest possible angle of incidence which still results in a refracted ray is called the critical angle; in this case the refracted ray travels along the boundary between the two media.
The critical angle is the angle of incidence above which total internal reflection occurs. The angle of incidence is measured with respect to the normal at the refractive boundary. The critical
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where n2 is the refractive index of the less optically dense medium, and n1 is the refractive index of the more optically dense medium. Because of the critical angle, there is a cone-like region formed near the surface when the light travels between these two mediums. Only the light within the cone can be extracted, the light outside the cone will experience total internal reflection until it is absorbed.
2.3.2 Fresnel Loss
When light moves from a medium of a given refractive index n1 into a second medium with refractive index n2, both reflection and refraction of the light may occur.[6] In Fig. 2.5, an incident light ray PO strikes at point O the interface between two media of refractive indexes n1 and n2. Part of the ray is reflected as ray OQ and part refracted as ray OS. The angles that the incident, reflected and refracted rays make to the normal of the interface are given as θi, θr and θt, respectively. The relationship between these angles is given by the law of reflection and Snell's law.
The fraction of the incident power that is reflected from the interface is given by the reflectance R, which is also called the Fresnel losses, and the fraction that is refracted is given by the transmittance T.
The calculations of R and T depend on polarisation of the incident ray. If the light is polarised with the electric field of the light perpendicular to the plane of the diagram above (s-polarised), the reflection coefficient is given by:
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where θt can be derived from θi by Snell's law and is simplified using trigonometric identities.
If the incident light is polarised in the plane of the diagram (p-polarised), the R is given by:
( )
A concave mirror, or converging mirror, has a reflecting surface that bulges inward (away from the incident light). Concave mirrors reflect light inward to one focal point, therefore they are used to focus light. Unlike convex mirrors, concave mirrors show different image types depending on the distance between the object and the mirror.
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These mirrors are called “converging” because they tend to collect light that falls on them, refocusing parallel incoming rays toward a focus. This is because the light is reflected at different angles, since the normal to the surface differs with each spot on the mirror. In this study, we used concave mirror to redirect photons to the vertical direction of the LED which generated from the MQW and originally emitted downward.
As shown in Fig. 2.6, if light source is placed at the left side of the curvature center of the concave mirror, there will be an image formed at the right side of the curvature center. From the
ΔPAC and ΔPAP', we can easily have the relation as follow From (2.13) and (2.14), we can receive
1 1 2
'
S+S = R (2.15)
In order to collect photons from the vertical direction of the LED (S'= ∞ ), MQW must be placed at the focal plane (focal length = f) of the concave mirror. From (2.15), we can derive
2
The transmission line model (TLM) originally proposed by Shockley offered a convenient
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method for determining the specific contact resistance (ρC) for planar ohmic contacts.[7]
Shockley proposed an experiment in which the total resistance RT between any two contacts (of length d and width W) separated by a distance L could be measured and plotted as a function of L.
As shown in Fig. 2.7, the total resistance (RT) between any two contacts is given by:
2 SH
T C
R R R L
= + W (2.17) where RSH is the sheet resistance of the semiconductor layer outside the contact region. The contact resistance RC can be shown as:
coth
where RSK is the modified sheet resistance under the contact and LT is the transfer length. When d
>> LT, coth ~ 1
Using ordinary least squares (OLS), the linear relationship between RT and L can be exactly performed and the variables can be defined by the intercepts and the slope as follow:
Intercept of the axis RT =2RC (2.21)
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2.6 Wafer Bonding
To join two different materials, wafer bonding technique shows a benefit when compares with the ordinary growth technique because defects can be confined at the interface. Wafer bonding has two stages: first is wafer contacting and second is wafer binding. Surfaces of these two materials must be smooth while contact with each other. Weak forces form between them which appear in the following three types: (1) van der Waals force, (2) Capillary action, and (3) electrostatic force, as shown in Fig. 2.8.[8]
Although weak forces will form at the contact process, it still needs a high temperature annealing and a uniaxial pressure to construct high strength bonds between the two materials in binding process. By using a special treatment to the wafer surface, wafer binding can also be proceeded at low temperature. The weak bonds can finally be transferred to covalent bonds.
Although weak forces will form at the contact process, it still needs a high temperature annealing and a uniaxial pressure to construct high strength bonds between the two materials in binding process. By using a special treatment to the wafer surface, wafer binding can also be proceeded at low temperature. The weak bonds can finally be transferred to covalent bonds.