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`Solid-State Lighting Based
`on Light Emitting Diode
`Technology
`
`Dandan Zhu and Colin J. Humphreys
`
`5.1
`
`5.2
`
`5.3
`
`5.4
`5.4.1
`
`5.5
`5.5.1
`5.5.2
`5.5.3
`5.5.4
`5.5.5
`5.5.6
`5.5.7
`
`5.6
`5.6.1
`5.6.2
`5.6.3
`5.6.4
`
`5.7
`5.7.1
`5.7.2
`
`5.8
`
`Historical Development of LEDs – 88
`The Importance of Nitride Materials – 89
`LED Basics – 90
`Fabrication of an LED Luminaire – 92
`Efficiency and Efficacy – 93
`
`Research Challenges – 94
`Crystal Growth – 95
`Internal Electric Field – 97
`p-Type Doping – 99
`Green Gap and Efficiency Droop – 100
`Chip Design – 102
`Generation of White Light with LEDs – 103
`LED Packaging – 105
`
`LEDs for Lighting – 106
`Quality of LED Lighting – 106
`Efficacy – 107
`Lifetime – 108
`Cost – 109
`
`LED Lighting Applications: The Present and Future – 110
`General Illumination and Energy Saving – 112
`Circadian Rhythm Lighting – 113
`
`Chapter Summary – 114
`References – 114
`
`D. Zhu  C.J. Humphreys (*)
`Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road,
`Cambridge CB30FS, UK
`NICHIA EXHIBIT 2003
`e-mail: colin.humphreys@msm.cam.ac.uk
`Lowe's Companies, Inc. et al. v. Nichia Corp.
`Case IPR2018-00601
`
`© The Author(s) 2016
`M.D. Al-Amri et al. (eds.), Optics in Our Time, DOI 10.1007/978-3-319-31903-2_5
`
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`5.1 Historical Development of LEDs
`
`More than 100 years ago in 1907, an Englishman named Henry Joseph Round
`discovered that inorganic materials could light up when an electric current flowed
`through. In the next decades, Russian physicist Oleg Lossew and French physicist
`Georges Destriau studied this phenomenon in great detail and the term ‘electro-
`luminescence’ was invented to describe this. In 1962, inorganic materials (GaAsP)
`emitting red light were first demonstrated by Holonyak and Bevacqua [1] at
`General Electric’s Solid-State Device Research Laboratory in Syracuse,
`New York, although the light emitted was so weak that it could only be seen in a
`darkened room (by comparison, the efficacy of Thomas Edison’s first incandes-
`cent light bulb was 10 times greater). Since then, the efficiency of GaP and GaAsP
`advanced significantly in the 1960s and 1970s. The AlInGaP system was developed
`later, in the 1980s, and is now the basis of most high-efficiency LEDs emitting in
`the red-to-yellow visible region. The development of the nitride material system
`(GaN, InN, AlN and their alloys) in the last two decades has enabled efficient light
`emission to expand into the blue and green spectral region, and most importantly,
`allowing the production of white light (blue is the high-energy end of the visible
`spectrum and therefore enables the production of white light using blue light plus
`phosphors). Blue LEDs were made possible by a series of key breakthroughs in
`materials science summarised in . Table 5.1, which will be discussed in greater
`detail later. In particular, the first bright blue LED was announced at a press
`conference on November 12, 1993 by Nakamura [2]. The invention of efficient
`blue LEDs has enabled white light source for illumination. In 1997, white light was
`demonstrated for the first time by combining a blue gallium nitride (GaN) LED
`with a yellow-emitting phosphor [3]. Such LEDs are called ‘white LEDs’.
`Nowadays, solid-state lighting based on LEDs is already commercialised and
`widely used, for example, as traffic signals, large outdoor displays, interior and
`exterior lighting in aircraft, cars and buses, as bulbs in flash lights and as
`backlighting for cell phones and liquid-crystal displays. With the continuous
`improvement in performance and cost reduction in the last decades, solid-state
`
`. Table 5.1 A summary of the key steps in GaN-based LED development history
`
`1938
`
`Juza and Hahn [84]
`
`The earliest polycrystalline GaN powder was synthesised by reacting ammonia with liquid Ga metal
`
`1969 Maruska and Tietjen
`[92]
`
`First single crystal GaN film was grown by chemical vapour deposition directly on a sapphire substrate
`
`1972
`
`Pankove et al. [102]
`
`First blue GaN metal-insulator-semicondutor LED was reported
`
`1986
`
`Amano et al. [79]
`
`1989
`
`Amano et al. [43]
`
`1991
`
`Nakamura et al. [38, 94]
`
`Crack-fee GaN films with good surface morphology and crystallinity were achieved by growing a thin
`AlN buffer deposited on sapphire at low temperature before GaN growth
`
`Amano, Akasaki and co-workers demonstrated that a low-energy electron beam irradiation treatment
`in a scanning electron microscope could cause a previously highly resistive Mg-doped GaN layer to show
`distinct p-type conductivity, enabling the first GaN p–n junction LED
`
`Nakamura and co-workers showed that a ~20 nm thick GaN buffer layer deposited at low temperature
`
`
`(~500
`C) before the main GaN growth at ~1000
`C could also be used to grow smooth films on sapphire,
`including p-type material with good electrical properties
`
`1992
`
`Nakamura et al. [42]
`
`Thermal activation of Mg-doped GaN to achieve p-type conductivity
`
`1993
`
`Nakamura et al. [97]
`
`Blue and violet emitting double-heterostructure (DH) LEDs were successfully fabricated
`
`1993
`
`Nakamura et al. [2]
`
`Nakamura announced the first bright blue LED at a press conference on November 12, 1993
`
`1995
`
`Nakamura et al. [95]
`
`InGaN quantum well LEDs were fabricated
`
`1997
`
`Nakamura et al. [3]
`
`White light was demonstrated for the first time by combining a blue gallium nitride (GaN) LED with a
`yellow-emitting phosphor
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`lighting has emerged to be a realistic replacement of incandescent and fluorescent
`lamps for our homes and offices.
`Compared with any other existing lighting technology, solid-state lighting
`possesses two highly desirable features: (1) it is highly energy efficient with
`tremendous potential for energy saving and reduction in carbon emissions; (2) it
`is an extremely versatile light source with many controllable properties including
`the
`emission spectrum, direction,
`colour
`temperature, modulation and
`polarisation. The beneficial impact of LEDs on the economy, environment and
`our quality of life is so evident and well recognised that the 2014 Nobel Prize in
`Physics was awarded to the inventors of efficient blue LEDs: Isamu Akasaki,
`Hiroshi Amano and Shuji Nakamura.
`
`5.2
`
`The Importance of Nitride Materials
`
`The main compound semiconductor materials used in LEDs and their bandgap
`energies are summarised in . Fig. 5.1. For most optoelectronic devices such as
`light emitting diodes (LEDs), laser diodes, and photodetectors, a direct bandgap is
`essential for efficient device operation. This is because the optical emission pro-
`cesses in a semiconductor with an indirect bandgap require phonons for momen-
`tum conservation. The involvement of the phonon makes this radiative process
`much less likely to occur in a given timespan, which allows non-radiative processes
`to effectively compete, generating heat rather than light. Therefore semiconductors
`with an indirect bandgap are not suitable for efficient LEDs.
`Conventional cubic III–V compound semiconductors, such as the arsenides
`and phosphides, show a direct-to-indirect bandgap transition towards higher
`energies. Therefore high-efficiency devices can be achieved in the infrared and
`red-to-yellow visible spectral regions, but the efficiency decreases drastically for
`
`. Fig. 5.1 Bandgap energies at 300 K of III–V compound semiconductors, plotted from data given in Vurgaftman et al. [4] and Vurgaftman and
`Meyer [5]. For the nitrides, the hexagonal a lattice constant has been used. The energy range corresponding to the visible spectrum is also indicated
`
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`conventional III–V semiconductors as the bandgap becomes indirect. In contrast,
`the nitrides have the hexagonal wurtzite structure, and the bandgap remains direct
`across the entire composition range from AlN to InN, with the bandgap energy
`covering a wide range from the deep ultraviolet to the infrared region of the
`electromagnetic spectrum. This makes the group-III nitrides system (consisting
`of GaN and its alloys with Al and In) particularly suitable for LEDs.
`The blue/green and near-UV spectral regions can be accessed using the InGaN
`alloy, and today, the main application of the nitrides is in blue, green and white
`emitting LEDs, as well as violet laser diodes used for high-density optical storage in
`Blu-ray DVDs [6]. Since the InGaN bandgap energy spans the visible spectrum,
`extending into the infrared to ~0.7 eV for InN, this alloy covers almost the entire
`solar spectrum, and is thus a potential system for high-efficiency multi-junction
`solar cells [7].
`The wide bandgap of the AlGaN alloy system will enable the fabrication of UV
`emitters and photodetectors. Possible applications of UV optoelectronics include
`water purification, pollution monitoring, UV astronomy, chemical/biological
`reagent detection and flame detection [8, 103].
`AlGaN/GaN heterostructures are also suitable for electronic devices such as
`high electron mobility transistors (HEMTs), which have applications in micro-
`wave and radio frequency power amplifiers used for communications technology
`[9]. Such a wide bandgap materials system also allows device operation at higher
`voltages and temperatures compared to conventional Si, GaAs or InP-based
`electronics [10].
`Although this chapter will be mainly focused on nitride-based LEDs for
`lighting applications, it is worth bearing in mind the great potential of nitride
`materials in other exciting applications mentioned above. And because of their
`unique materials properties and wide range of applications, group-III nitrides are
`widely considered to be the most important semiconductor materials since Si.
`
`5.3
`
`LED Basics
`
`The simplest LED structure is a p–n junction, consisting of a layer of p-type doped
`semiconductor material connected to an n-type doped layer to form a diode with a
`thin active region at the junction. The principle for light emission in a p–n junction
`is illustrated in . Fig. 5.2. The n-type region is rich in negatively charged electrons,
`while the p-type region is rich in positively charged holes. When a voltage is
`applied to the junction (called forward bias), the electrons are injected from the
`n-type region and holes injected from the p-type region across the junction. When
`the electrons and holes subsequently meet and recombine radiatively, the energy
`released is given out as light with an emission wavelength close to the bandgap of
`the material incorporated in the active region around the junction. For high
`efficiency, a heterojunction (consisting of two semiconductor materials with dif-
`ferent bandgap) is usually preferred to a homojunction (consisting of a single
`semiconductor material) due to better carrier confinement, as shown in
`. Fig. 5.2c, i.e. the electrons and holes are spatially confined together in the active
`region with lower bandgap energy, which increase the chance of radiative recom-
`bination to produce light.
`For most high-efficiency LEDs, quantum wells (QWs) are routinely used in the
`active region, which provide additional carrier confinement in one direction,
`improving the radiative efficiency, i.e. the internal quantum efficiency (IQE).
`Quantum wells consist of a very thin (few nm thick) layer of a lower bandgap
`material, such as InGaN, between higher bandgap barriers, such as GaN (see
`. Fig. 5.3). The QW active region is sandwiched between two thicker layers of
`n-type doped and p-type doped GaN for electron and hole injection, respectively.
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`91
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`
`. Fig. 5.2 A p–n homojunction under (a) zero and (b) forward bias. A p–n heterojunction
`under (c) forward bias. EC, EF and En are the conduction band, Fermi and valence band energy.
`Filled circle and open circle represent electrons and holes, respectively. In homojunctions, carriers
`diffuse, on average, over the diffusion lengths Ln and Lp before recombination. In
`heterojunctions, carriers are confined by the heterojunction barriers (after [11])
`
`. Fig. 5.3 A schematic InGaN/GaN quantum well LED structure together with a high-
`resolution transmission electron microscope lattice fringe image of three InGaN quantum wells
`separated by GaN barriers
`
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`The recombination of electron and holes across the InGaN quantum well region
`results in the emission of light of a single colour, such as green or blue. We can
`change this colour by varying the composition and/or changing the thickness of
`the InGaN quantum well.
`
`5.4
`
`Fabrication of an LED Luminaire
`
`The LED structure described above is the essential source of light, but it often
`makes up only a tiny volume fraction of the final application, such as an LED light
`bulb or luminaire. . Figure 5.4 illustrates the fabrication procedures involved in
`making an LED luminaire. The first step is the deposition of the nitride LED
`structure on a suitable substrate wafer such as sapphire, SiC, Si or GaN. This is
`performed by crystal growth usually via a process called metal organic vapour
`phase epitaxy (MOVPE) in a heated chamber or reactor. After deposition, these
`epiwafers will be processed into LED devices according to the LED chip design,
`which usually involves several steps including wafer bonding, n and p-type contact
`patterning, etching, metallisation and surface roughening. The processed LED
`devices are then separated via cleaving, sawing or laser cutting into individual
`dies. Depending on the target applications, these individual LED dies are mounted
`on an appropriate package in a form compatible with other electronic components
`such as drivers. For white LEDs, phosphors will also be incorporated into the
`package, together with blue-emitting LED dies in most cases. These packaged LED
`devices are then ready to be used as the light source in a luminaire.
`From the fabrication procedure, we can see that there are many components
`contributing to the overall efficiency of a packaged LED device. These can be
`broken down into:
`
`. Fig. 5.4 Illustration of the fabrication procedures involved in making LED luminaries. The corresponding efficiency and losses involved in each
`procedure are also listed
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`1. Internal quantum efficiency (ηIQE)
`2. Light extraction efficiency from the chip (ηLEC)
`3. Electrical efficiency (ηEE)
`4. Phosphor conversion efficiency (ηconv)
`5. Light extraction efficiency from the package (ηLEP)
`
`The IQE is defined as the number of photons emitted from the active region
`divided by the number of electrons injected into the active region. The IQE is
`primarily determined by the LED structure design, such as the choice of material
`compositions, layer thicknesses, doping profile; and for a given structure, the
`material quality linked to the growth conditions used during the epitaxy proce-
`dure. The IQE is also a function of the current density through the LED. At high
`current density the IQE falls, a phenomenon known as ‘efficiency droop’.
`The light generated in the quantum well region needs to be extracted from the
`semiconductor material: most III–V semiconductors have high optical refractive
`indices (GaN: n ~ 2.4; InGaP: n ~ 3.5), and only a small portion of the light
`generated in the quantum well region can escape. This is because much of the
`light is trapped inside the LED by total internal reflection. Various advanced chip
`designs have been developed and used during the wafer and die level fabrication
`procedures to increase the possibility of light extraction from LED chips (LEC) and
`to minimise the electrical
`losses caused by the electrical contact and series
`resistances. Today, an LEC value >85 % is achieved for high performance com-
`mercial LED devices with a ThinGaN chip structure, as shown in . Fig. 5.5b [12].
`Furthermore LED dies need to be packaged before they can be incorporated
`with other electronic components in a real application. LED packaging is also
`critical to achieve high luminous efficiency, dissipate heat generated from the LED
`chip,
`improve reliability and lifetime and control
`the colour for specific
`requirements, as well as to protect the LED chips from damages due to electrostatic
`discharge, moisture, high temperature and chemical oxidation. A schematic struc-
`ture of a high power LED package is shown in . Fig. 5.5a, together with a picture
`of a commercial white LED package shown in . Fig. 5.5c. The light extraction
`efficiency from a package (LEP) such as this is as high as 95 %. For white light
`generation, a yellow-emitting cerium-doped yttrium aluminium garnet (YAG)
`phosphor plate is added on top of the nGaN layer. To achieve a high phosphor
`conversion efficiency, the phosphor material is carefully chosen to match the LED
`emission for optimum excitation.
`
`5.4.1 Efficiency and Efficacy
`
`For a single colour LED such as blue, green and red LEDs, wall-plug efficiency is
`usually used as a measure of the overall efficiency. The wall-plug efficiency,
`measured by the light output power (measured in watts) divided by the electrical
`input (also in watts), is dimensionless and is usually expressed as a percentage. For
`white LEDs, a different term, efficacy, is usually used instead of efficiency. The unit
`of efficacy is lumens per watt (lm/W), corresponding to light power output
`(as perceived by the human eye and measured in lumens) relative to electrical
`power input (measured in watts). The terms efficiency and efficacy are both widely
`used in lighting, and care must be taken not to confuse them. The efficacy of a
`white light source will be explained in more detail later in this chapter. The term
`efficacy takes into account the sensitivity of the human eye to different colours: it is
`a maximum for green light at 555 nm.
`It should also be noted that the efficiency or efficacy of a luminaire would be
`lower than the packaged LED devices due to additional losses caused by other
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`

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`5
`
`. Fig. 5.5 (a) The schematic structure of a high-power LED package with good optical efficiency and thermal management, as required for high
`power LED chips. (b) Cross-section of a high power ThinGaN LED chip, illustrating the complex structure of state-of-the-art white LEDs for
`illumination. (c) A picture of a high power white LED package from Osram
`
`components such as optics, heat sinks and electrical drivers. When discussing the
`efficiency of LED lighting, it is important to be clear about the form of the light
`source: whether it is a bare die, packaged LED device or luminaire.
`The performance of LEDs has improved dramatically over the last decade with
`sustained improvements in the material quality, LED structure, chip design and
`packaging. Before moving to the discussions on LED performance and
`applications, it is worthwhile to first review the historical development of nitride
`LEDs, in particular the research challenges involved.
`
`5.5
`
`Research Challenges
`
`The research in nitride materials and LED devices is a very broad and interdisci-
`plinary field,
`spanning
`crystal
`growth, physics, materials
`science
`and
`characterisation, device processing, device physics, luminaire design and others.
`From a materials science point of view, nitride materials are highly defective
`compared with conventional semiconductor materials such as Si and GaAs, and
`the remarkable success of nitride-based LEDs is based on a series of wonderful
`achievements in science and engineering.
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`5.5.1 Crystal Growth
`
`As with many other semiconductor materials, III-nitrides do not exist naturally, so
`the crystals need to be grown by some chemical reaction. The predominant growth
`method for the group-III nitrides is metalorganic vapour phase epitaxy (MOVPE,
`also called metalorganic chemical vapour deposition, MOCVD), both for research
`and mass-production of devices such as LEDs and lasers.
`It should be noted that one key difference between the nitrides and the other
`III–V compound semiconductors mentioned earlier in this chapter is the lack of a
`suitable substrate for heteroepitaxial growth (namely, crystal growth on a different
`substrate material) of GaN. Bulk substrates of GaAs, GaP and InP can be used for
`epitaxy of most of the III–Vs and even II–VI compounds. Unfortunately, the
`nitrides have very high melting temperatures and dissociation pressures at melting,
`~2800 K and ~40 kbar, respectively, for GaN, which means that bulk crystals
`cannot be grown from stoichiometric melts using the usual Czochralski or
`Bridgman methods [13,14]. Not only have bulk substrates of GaN been unavailable
`in a sufficient size and at reasonable cost, there is also no other suitable substrate
`material with a close lattice match to GaN. The properties of the GaN epitaxial
`layer such as crystal orientation, defect density, strain and surface morphology are
`to a large extent determined by the substrates used. Most commercial GaN-based
`LEDs are grown on sapphire or silicon carbide (SiC) substrates. Recently, the use
`of large area Si substrates has attracted great interest because high quality Si wafers
`are readily available in large diameters at low cost [106]. In addition, such wafers
`are compatible with existing sophisticated automated processing lines for 6 inch
`and larger wafers commonly used in the electronics industry.
`Sapphire was the original substrate material, and remains the most commonly
`used to this day, but it has a lattice mismatch of 16 % with GaN. This is so large
`that attempts at direct epitaxial growth inevitably result
`in rough surface
`morphologies and a very high density of defects called dislocations that thread
`up through the growing layer: a typical density of such dislocations passing
`through the active InGaN quantum well region is five billion per square centimetre
`(5  109 cm
`2), as shown in . Fig. 5.6.
`The development of growth techniques for the reduction of the threading
`dislocation (TD) density in GaN on sapphire has resulted in considerable
`improvements. There are numerous methods in the literature, mostly related to
`the annealing of a low temperature nucleation layer [15], island formation and
`subsequent
`coalescence, as detailed in Figge
`et al.
`[16] and Kappers
`et al. [17,18]. An example of TD reduction using an SiNx interlayer is shown in
`. Fig. 5.7. The mechanism by which TD density can be reduced is as follows: the
`thin SiNx interlayer constitutes a mask containing random holes through which
`small facetted GaN islands form on regrowth; aided by the inclined facets of the
`islands, the TDs bend laterally and react with other dislocations to annihilate and
`form half
`loops, hence halting their upward propagation, as illustrated in
`. Fig. 5.7a. It was also found that the growth conditions of the GaN regrowth
`on top of the SiNx interlayer have a pronounced effect on the degree of the TD
`reduction. By using a special ‘slow’ coalescence method, the TD density of the seed
`layer (5  109 cm
`2) was reduced to 5  108 cm
`2 and successively deployed
`SiNx interlayers reduce the TD density further to 1  108 cm
`2, as shown in
`. Fig. 5.7b.
`Dislocations are known to be non-radiative recombination centres [19] that
`should strongly quench light emission. Indeed, if the dislocation density in other
`semiconductors, for example, GaAs, exceeds around 1000 per square centimetre
`2), the operation of light emitting devices is effectively killed. However,
`(103 cm
`commercial InGaN blue and white LEDs show high performance despite the fact
`
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`. Fig. 5.6 Transmission electron microscopy (TEM) images showing the high density of threading dislocations resulting from the growth of GaN
`on sapphire substrate. The lattice mismatch between GaN and (0001) sapphire is 16 %, which gives rise to a dislocation density in the GaN of
`typically 5  109 cm
`2, unless dislocation reduction methods are used
`
`. Fig. 5.7 (a) Cross-sectional TEM image of an SiNx interlayer (arrowed) deposited on a GaN seed layer followed by the regrowth of GaN islands.
`Threading dislocations can be observed as bright lines in the image. (b) Weak beam dark field TEM image, g ¼ (11–20), showing the reduction of
`edge and mixed TDs with successive SiNx interlayers and a ‘slow’ coalescence of GaN between the layers
`
`2. The reason
`that the TD density of such devices is usually in the range of 108 cm
`that InGaN LEDs are much more tolerant of TDs than other conventional III–V
`materials is probably due to carrier localisation effects [20–26]. The first
`contributing factor is the monolayer height interface steps on the InGaN quantum
`wells. Since the QWs are strained and because of the high piezoelectric effect in
`GaN, a monolayer interface step produces an additional carrier-confinement
`energy of about 2kBT at room temperature, where kB is the Boltzmann constant
`and T is the temperature. This is sufficient to localise the electrons. Recent three-
`dimensional atom-probe studies also confirmed that InGaN is a random alloy.
`Calculations show that random alloy fluctuations on a nanometer scale strongly
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`localise the holes at room temperature. Thus, the above two mechanisms can
`localise both the electrons and the holes, reducing diffusion to non-radiative
`defects like TDs. It is interesting to note that the electrons and holes are localised
`by different mechanisms in InGaN quantum wells.
`Although high threading dislocation densities seem to be not very detrimental
`for InGaN LEDs, laser diodes and AlGaN-based UV-emitters do show a strong
`dependence of lifetime on dislocation density. Moreover, the growth conditions
`will also affect many microstructural properties of nitride materials as well as
`impurity levels and thus the final device properties. Therefore, the research in
`crystal growth remains highly relevant and important for high performance
`devices.
`
`5.5.2 Internal Electric Field
`
`The nitrides normally crystallise in the hexagonal wurtzite structure, which is
`non-centrosymmetric and has a unique or polar axis along a certain direction (the
`c-axis). Since the bonding is partially ionic due to the difference in electronegativ-
`ity of the group III and V atoms, a spontaneous polarisation will exist in the crystal
`because of the lack of symmetry. In addition, most nitride devices involve the use
`of strained heterojunctions, such as InGaN/GaN. Because the in-plane lattice
`constant of InGaN is larger than for GaN, the InGaN layer will be under compres-
`sive strain perpendicular to the c-axis and under tensile strain along the c-axis
`when grown epitaxially on GaN. An applied strain along or perpendicular to the c-
`axis will cause an internal displacement of the metal sublattice with respect to that
`of the nitrogen, effectively changing the polarisation of the material. This strain
`effect provides an additional contribution to the polarisation of the material,
`referred to as the piezoelectric component, and is particularly relevant to strained
`heterostructures.
`Virtually all commercial GaN-based LEDs are grown along the c-axis of the
`crystal. Since this is a polar direction, there exists an electric field across the InGaN
`quantum well due to a difference in polarisation for the well and barrier material.
`The electric field will cause a tilting of the conduction and valence bands in the
`well, separating the electrons and holes and shifting the quantum well emission
`wavelength to lower energy, as illustrated in . Fig. 5.8. This is known as the
`quantum confined stark effect (QCSE).
`There are some general observations about the QCSE relevant to nitride QWs:
`with the presence of an electric field, the transition energy is shifted to a lower
`value (from ΔEg,QW to ΔEg1) and this shift is roughly equal to the sum of the shifts
`of the first electron (ΔEe1) and hole (ΔEh1) levels;
`it is the hole state that
`contributes most due to the larger effective mass; electrons and holes are separated
`from each other spatially by the electric field across the quantum well, resulting in
`a reduced overlap of electron and hole wave functions and thus a longer radiative
`lifetime; wider wells (QW2) show more obvious effects of the QCSE and a larger
`potential drop (ΔEE2) across the well. For a sufficiently wide well, the emission can
`be lower energy than the bandgap of the quantum well material itself.
`The impact of the internal field, especially the piezoelectric field caused by
`strain, on quantum well recombination behaviour has been confirmed experimen-
`tally and reported in various III-nitride-based heterostructures [27–32]. Redshifts
`of emission energy and lower emission intensity were found in strained quantum
`wells based on III-nitrides, confirming the strong influence of the strain-induced
`piezoelectric field. However, with increasing carrier injection, a blue shift of the
`emission peak was observed by several researchers [33,34] and attributed to the
`reduction of the QCSE due to the in-well field screening by carriers. Therefore, in
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`D. Zhu and C.J. Humphreys
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`5
`
`. Fig. 5.8 Schematic plot showing the effects of the QCSE on InGaN/GaN quantum wells: black: QW1 without electric field; blue: QW1 with
`electric field; red: QW2 (thicker quantum well) with electric field
`
`. Fig. 5.9 Schematic of the principle polar, non-polar and semi-polar planes of GaN. The QCSE effect should be eliminated by growing along a
`non-polar direction such as [1–100] and [11–20] or minimised along a semi-polar direction such as [11–22]
`
`an LED structure, the electric field across the quantum wells is not only deter-
`mined by the polarisation field but also affected by the carrier density and
`distribution in the quantum well region. The carriers may be from carrier injection
`(optical or electrical), as well as from doping, either intentional dopants or
`non-intentional impurities.
`From the discussion above, it is obvious that the QCSE is not desirable for
`LEDs of high efficiency and good colour consistency. . Figure 5.9 shows the main
`polar, non-polar and semi-polar planes of GaN. In principle, the QCSE should be
`eliminated by growing along a non-polar direction such as [1–100] and [11–20] or
`minimised along a semi-polar direction such as [11–22]. The efficiency of
`non-polar and semi-polar light emitting structures is therefore expected to be
`enhanced over that of polar.
`However, it was found that the defect density is currently much higher in GaN
`structures grown in such directions [35], unless expensive freestanding non-polar
`or
`semi-polar GaN substrates are used [36]. Furthermore,
`the indium
`incorporation in the InGaN MQWs grown along non-polar direction is 2–3
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`99
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`5
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`Chapter 5 · Solid-State Lighting Based on Light Emitting Diode Technology
`
`times lower than along the c-plane for similar growth conditions [37]. The output
`power of the non-polar LEDs also reduced dramatically when the emission
`wavelength was longer than 400 nm. Therefore, a non-polar plane is considered
`not suitable for LEDs with emission wavelengths longer than blue and semi-polar
`planes are preferred for blue, yellow and red LEDs with reduced internal field, but
`again high defect densities are a problem. Despite the potential advantages of
`reduced internal field, non-polar and semi-polar LEDs are currently not commer-
`cially viable due to their lower overall performance and the requirement of
`expensive freestanding GaN substrates.
`
`5.5.3 p-Type Doping
`
`For III-nitrides, p-type doping is problematic and the realisation of p-type con-
`ductivity was another major breakthrough in the historical development of nitride-
`based LEDs. Non-intentionally doped GaN usually shows n-type conductivity;
`however, the improvement in crystal growth methods has managed to reduce this
`sufficiently to allow controllable p-type doping
`background doping level
`[38]. Many potential p-type dopants have been tried and so far magnesium is
`the most successful p-type dopant for GaN, AlGaN and InGaN with low Al and In
`mole fractions.
`There are two main issues involved in Mg doping: (1) the presence of hydrogen
`in MOVPE and HVPE growth environments results in the passivation of Mg by
`forming Mg–H complexes that are electrically inactive; (2) Mg forms relatively
`deep acceptor states ~160–200 meV above the valence band [39], resulting in only
`a small fraction activated at room temperature and therefore low conductivity of
`p-type GaN. This means the hole concentration will always be more than an order
`of magnitude lower than the Mg concentration. Furthermore, heavily Mg-doped
`GaN is subject to self-compensation due to the formation of donor-like structural
`defects [40].
`The first issue can be solved by thermal annealing under an N2 ambient at a
`
`temperature higher than 700
`C [