Chapter 1
1.1 Aim and Motivation
Light emitting diodes (LEDs) are the ultimate light source in the lighting technology. The LED technology has flourished for the past few decades. High efficiency, reliability, rugged construction, low power consumption, and durability are among the key factors for the rapid development of the solid-state lighting based on high- brightness visible LEDs [1]. Conventional light sources, such as filament light bulbs and fluorescent lamps depend on either incandescence or discharge in gases. On the other hand, semiconductors allow an efficient way of light generation. LEDs made of semiconductor materials have the potential of converting electricity to light with near unity efficiency[1].
As the LED go through variety of developments, the packaging has improve from single package to multi package and improve the luminosity and saving the cost [2]. By examining table 1, one can see that LEDs are now efficient enough to replace incandescent lamps, although adoption is occurring slowly due to cost issues and return on investment.
Table 1.1 : Comparison of typical lighting sources with LEDs [3]
Light Source Luminous efficiency, l/w Heat lost by radiation, % Heat lost by convection,% Heat lost by conduction,%
Incandescent 10-20 >90 <5 <5
Fluorescent 75-90 40 40 20
High Intensity Discharge 100-120 >90 <5 <5
LED 30-35 <5 <5 >90
LEDs divided into three junctions which is homojunction, single heterojunction and double heterojunction.. Heterojunction LEDs made it possible to solve the considerably more general problem of controlling the fundamental parameters inside the semiconductor crystals and devices such as band gaps, refractive indices, electron energy spectrum, effective masses of the charge carriers and the mobilities [4]. It also have higher performance due to high radiative recombination.
An LED has the electrical characteristics of a diode. This means that it will pass cur- rent in one direction but block it in the reverse direction. Depending on the semiconductor material and its doping, the LED will emit light at a particular wavelength. In general, LEDs require a forward operating voltage of approximately 1.5–3 V and a forward current ranging from 10 to 30 mA, with 20 mA being the most common current they are designed to support. Both the forward operating voltage and forward current vary depending on the semiconductor material used [5].
From Table 1.0, double heterostructure is better than homostructure due to efficiency/efficacy which is the number of electrons are released by a photocell per photon of incident radiation of a given energy is higher so it will affect to the wavelength, the smaller the range of wavelength, the higher the brightness of LEDs. Then, GaN is superior to GaAs and Si due to the percentage of efficiency/efficacy which is GaN is more brightness, especially in blue LEDs.
Device structure Colour range Efficiency at 20 mA (%) Wavelength (nm) Year References
Si LED Blue 0.01 500 2013 [6]
GaAs
Homostructure Red 0.12 625 2005 [7]
GaAs double heterostructure Infrared 0.86 720 2016 [1]
GaN homostructure Blue 1.00 570 2013 [6]
GaN double heterostructure Green
Blue 2.60
17.5 517
580 2005
2009 [1]
Table 1.2: Differences between GaN, GaAs and Si based on basic properties of the semiconductor.
In indirect semiconductors, photons are absorbed much less readily, so junctions can be deep. However, heavily doping the n-type side of the junction inevitably shortens the non-radiative lifetime. The standard compromise is suboptimal doping and reduced device efficiency. Many of the tradeoffs inherent in home junctions can be resolved by using different materials for the two sides of the p-n junction which is called a single heterostructure or single heterojunction [Fig. 4, center]. The device is fabricated by growing a window layer of n-type AlGaAs on the ac- tive layer of p-type GaAs. The window layer is transparent to the photons generated in the active layer, and can therefore be made thick enough to minimize surface recombi- nation. Even though the n-type window lay- er is lightly doped, its proximity to the GaAs p-type layer, with its smaller band gap, pro- duces efficient electron injection. AIlWNcL This structu
From Table 1.1, GaN superior to GaAs and Si due to the energy bandgap which is energy bandgap of GaN larger than GaAs and Si also have great potential applications in electronics and optoelectronics. The wavelength of the light emitted, and the colour is depending on the energy bandgap of the materials to form the p-n junction light emitting light (LED).
Properties GaN GaAs Si Year References
Energy bandgap 3.4 eV 1.4 eV 1.12 eV 2013 8
Electrical breakdown field 2x 10⁶ Vcm⁻¹ 4×10⁻⁵
Vcm⁻¹ 3×10⁻⁵ Vcm⁻¹ 2013 8
Thermal conductivity 1.3 Wcm⁻¹ K⁻¹ 0.55 Wcm⁻¹ K⁻¹ 1.3 Wcm⁻¹ K⁻¹ 2013 8
Density 6.15 g 5.3176g 2.329g 2013 6
Electron mobility 440Vs⁻¹ 8500Vs⁻¹ 150Vs⁻¹ 2013 6
Colour Blue Red Blue 2013 6
Wavelength 570nm 760nm 500nm 2013 6
1.2 Problem Statement
In homojunction, p-side is thin so the light can be emitted but p-side make a problem as electron reach surface via diffusion and mix with defects present near the surface. From this problem, heterojunction or double heterojunction is used because it has different energy bandgap on both junctions which is the p-side wider from n-side so it will take the time to recombine the electron and hole. The double heterojunction is high demand in industry because it is very effective and more efficiency than homojunction. The advantage of heterojunctions is to allow the improvement of efficiency of LEDs by releasing carriers to the active region; thereby it will be avoiding diffusion of minority carriers over long distances. Other than that, the thickness of double heterojunction, molar fraction of Al and 4 voltage of p-contact also has an effect on the performance of LED. From that problem, it will be study in this investigation.
1.3 Objective and Scope
The objective of this project is to study in detail on the factor and material affecting the performance of LED. Specific objectives of this studies are lists as below:-
i. To study working operation of Gallium Nitride (GaN) based heterostructure LED.
ii. To investigate how bandgap and doping energies affecting the performance of LED using a simulation software, Comsol Multiphysics to achieve desired improvement.
iii. To simulate design and do analysis based on how semiconductor interface can be used to evaluate optical emission.
iv. To calculate the emission spectrum, intensity and efficiency as a function of the driving current in order to evaluate the optimum operating conditions.
The main scope of this project is focusing on the characteristic and ideal operating range of GaN heterostructure LED. Furthermore this study will validate which process need to be included and focusses on dopant thickness variation in the n and p layer. Thus a current-voltage relationship and internal quantum efficiency with variation dopant thickness can be studies. Simulation can be important tool in enhancing product development activities. Details are listed below:-
1.4 Report Structure
This report is separated into several chapters covering different aspects of the research. An analysis of different techniques and procedures directly used also mentioned in this report structure.
Chapter 1 as the introduction. This section describe the introduction of the project title. Besides, this section also consists of some problem statement which will be studied and evaluated. One of the objectives is study in detail on the factor and material affecting the performance of LED. Differences between Si,GaN and GaAs based on basic properties of the semiconductor also reported.
Chapter 2 as the literature review of the project. In this section, relevant projects conducted by other researchers are being studied and identified. Some investigation of various techniques and procedures currently used by other researchers included in this chapter. In this chapter includes basics operation of LED, double heterostructure and previous research that have been done regarding LED. The study then continued by investigate the method of this project using Comsol software in Chapter 3.
Chapter 3 as the methodology which comprises the theoretical analysis of the body of methods and principles associated with the projects. The steps on completing the simulation is discussed one by one. A flow chart is done to arrange and explain all the main activities that have been carried out throughout the research including the steps in setting the doping and all the testing. Results of each steps are provided thus the working process will be more clear and easy learning.
In Chapter 4, in this section, all the studies regarding the project are being summarize in order to explain the activities and working process during the 14 weeks of the first part of this project.
Chapter 2
Literature review
2.1 Introduction.
Chapter 2 is the literature review of the project. In this part the previous research and studies handles by other researcher are being examined and set side by side. This chapter also consist of analysis based on various techniques that had been conducted. Furthermore, the basic operation structure and investigation that related to topic also reported in this section. More sonsideration will be taken regarding the topic of this project.
2.2 History
For decades, semiconductor device researchers have dreamed of obtaining blue LED. The stunning technological breakthroughs by Nakamura in producing GaN-based blue and green LEDs in early 1990s have a profound impact on the LED technology. Since the development of the high brightness blue LEDs, the LED market has grown significantly. By combining red, green and blue,the three primary colors, full-color display and white light source could be realized.
The light-emitting diodes (LEDs) have been known for many years. LEDs based on the electroluminescence which is the production of light by the flow of electrons and discovered in 1907 by H.J Round of Marconi Laboratories, who invented a crystal of Silicon Carbide (SiC) and a cat’s whisker detector to prove it [9]. In 1955, the Radio Corporation of America (RCA), Rubin Braunstein investigated the infrared emission from Gallium Arsenide (GaAs) and semiconductor alloys [10] and further demonstrated that such devices could be used for non-radio communication across a short distance. In 1962, Texas Instruments began a project to make an infrared diode and announced the first LED commercial product which employed a pure GaAs crystal to emit a 900nm output [11]. Finally, in 1994, Nakamura produced the first high-brightness blue LED based on Indium Gallium Nitride (InGaN) [12].
The three most popular and well-established approaches to using GaN LEDs to generate white light are shown in Fig 1. They are a blue LED with yellow phosphors, a UV LED with red, green and blue phosphors, and a device that combines red, green and blue LEDs. Currently, the blue GaN LED plus yellow phosphor dominates the white LED industry.
The actual wavelength of light generated and its color that corresponds to the emitted wavelength is dependent on the band gap energy of the materials used to form the p-n junction. []. Different colour of light have different active region of semiconductor material such as Gallium Nitride (GaN) and Gallium Phospide (GaP) will produce green LEDs, meanwhile Aluminium Gallium Indium Phosphide (AlGaInP) and Aluminium Gallium Arsenide (AlGaAs) alloys for red LEDs while Silicon Carbide (SiC) and Zinc Selenide (ZnSe) produce blue LEDs [14]
2.2.1 Principle of Light Emitting Diode (LED) Operation
One way to construct an LED is to deposit three semiconductor layers on a substrate. Between p-type and n-type semiconductor layers, an active region emits light when an electron and hole recombine.
2.2 Working Operation
A LED is typically made from a direct band gap semiconductor. Electron Hole Pair (EHP) recombination results in the emission of a photon. The emitted photon energy is approximately equal to the band gap energy.
Figure 2.2: Energy band diagram of p-n junction [12].
The energy band diagram of p-n junction without any bias. Built-in potential (Vo) prevents electrons from diffusing from n to p side. The applied bias reduces Vo and thereby allows electron to diffuse, be injected, into the p-side [12]. Recombination around the junction and within the diffusion length of the electrons in the p-side leads to photon emission.
Wavelength range, efficiency coefficient and typical efficacy of LED recombination primarily occurs within the depletion region and within a volume extending over the diffusion length of the electron in the p-side. This recombination zone is frequently called the active region. The phenomenon of light emission from EHP recombination as a result of minority carrier injection is called injection electroluminescence [13]. Because of the statistical nature of the recombination process between electrons and holes, the emitted photons are in random direction. They result from spontaneous processes in contrast to stimulated emission.
The LEDs realized using two differently doped semiconductors that are the same material is called a homojunction. A junction between 2 device band gap semiconductors is called a heterojunction. When they are realized using different bandgap materials they are called a heterostructure device. A heterostructure LED is brighter than a homojunction LED [20]. The refractive index of a semiconductor materials depends on its band gap. The wider the semiconductor gap the lower refractive index. This means that by constructing LED from heterostructures, we can build a dielectric waveguide within the device. Thereby channel photons out from the recombination region.
(a)
(b)
Figure 2.5: (a) Homojunction under forward bias. (b)Heterojunction under forward bias [20]
Heterojunctions have clear advantages over homojunction devices. Heterojunction devices employ two types of semiconductors, namely a small-bandgap active region and a large-bandgap barrier region. If a structure consists of two barriers, two large-bandgap semiconductors, then the structure is called a double heterostructure [21]. The effect of heterojunctions on the carrier distribution is shown in Fig. 2.5 (b). Carriers injected into the active region of the heterojunction are confined to the active region by means of the barriers. As a result, the thickness of the region in which carriers recombine is given by the thickness of the active region rather than the diffusion length.
2.2.2 Advantages of Light Emitting Diode (LED)
High power LED provides a high luminous and high efficient for using as a lighting source. Because of the advent for LED as lighting device, LED lighting becomes one of the new trends in the lighting industry. A principal advantage of LED lighting over traditional lighting technologies is its high energy efficiency [psss].
LED lights have relatively long life spans as they possess a high luminous efficiency, lack of hazardous materials, and they are environmentally friendly.
2.3 Double Heterostructure in Light Emitting Diode
Heterostructure electronics are widely used in many areas of human civilization. It is hardly possible to imagine our recent life without double heterostructure (DHS) laser-based telecommunication systems, heterostructure-based light-emitting diodes (LED’s), heterostructure bipolar transistors, or low- noise high-electron-mobility transistors for high- frequency applications including, for example, satellite television. [nobel lecture].
The wide distribution of carriers and the correspondingly low carrier concentration can be avoided by the employment of double heterostructure. carriers in the active region of a double heterostructure have a much higher concentration than carriers in homojunctions, which are distributed over several diffusion lengths. For this reason, all high-efficiency LED designs employ double heterostructure or quantum well designs. Figure 2.3, it shows the structure of InGaN sandwiches with AlGaN to form double heterostructure light emitting diodes (LEDs).
The structure of the InGaN/AlGaN double-heterostructure blue LED.
2.4 Researchers on GaN/InGaN/AlGaN
A few studies related on GaN/InGaN/AlGaN has been conducted by other researchers from outside. Various type of study based on the GaN/InGaN/AlGaN with different techniques also implemented. A study considered the design of an efficient, high brightness polar InGaN/GaN light emitting diode (LED) structure with AlGaN capping layer for green light emission. The deposition of high In (>15%) composition within InGaN quantum well (QW) has limitations when providing intense green light.
Recombination balance parameters for GaN/InGaN/AlGaN single-quantum-well green-lightemitting diodes are extracted from optical power and carrier lifetime measurements. The radiative recombination coefficient B is found to depend on two-dimensional carrier density N, with a low-carrier-density limit of Bo=1.2×10^4 cm^2/s. Based on blue emitting diodes as primary light sources, white luminescence conversion LEDs (LUCOLEDs) have been fabricated. Using commercially available perylene dyes or YAG:Ce phosphors as the luminescent material, the LED radiation is converted into light of longer wavelengths by luminescence down-conversion (Stokes shift).
2.5 LED Efficiency
The efficiency of LEDs is affected by many physical phenomena which are external quantum efficiency (EQE) for describing the efficiency of LEDs. The EQE are the ratio for describing the numbers of extracted photons are divided by the number of electrons flowing through the device. The equations that use to calculate the percentage of IQE as follow [24]:
Ƞ𝑬𝑸𝑬 = 𝑷𝒐𝒑𝒕/𝒉𝝎
𝑰/𝒒 (2. 0)
where
Popt = optical power
ω = the photon angular frequency
I = is the total current
2.3 LED Material Selection (GaN)
Figure 2.3: Schematic drawing of GaN based heterostructure LED [14].
Silicon is the most widely used substrate for GaN-based LEDs with excellent mechanical property and chemical stability. Consequently, the original substrate for GaN is hard to be removed during device fabrication process. The conventional GaN structure, namely the lateral structure is commonly used in LED chip fabrication process, with relatively low difficulty [14]. Typically, the epitaxial structure of GaN-LED consists of an n-GaN layer grown on silicon substrate, a p-GaN layer on the top surface and active region lay between n-GaN and p-GaN.
2.3.1 Effect of GaN thickness in GaN LED
The GaN thickness on the light output power and internal quantum efficiency of the GaN-based LEDs investigated by experiments and simulations. For example, the light output power of the GaN based LED improves as the p-GaN thickness decreases [15]. The emission energy of photoluminescence and electroluminescence increases with the barrier thickness decreased and also with the Si doping level increased. The forward voltage measured at 20 mA raises with reducing barrier doping level and thinning barrier thickness [16].
2.3.2 Energy Band Structure
Figure 2.4: Bandgap energy versus bond length for various semiconductors [17].
The bandgap in the (Al, Ga, In)N based materials system ranges from 1.9eV (InN), 3.4 (GaN) to 6.2 eV (AIN). The band structure is currently thought to be a direct bandgap across the entire alloy range [17]. Therefore as Figure 2.4 almost the entire visible range and deep UV wavelengths are spanned in the group III nitride alloy system. This direct bandgaps especially fortuitous as it allows for high quantum efficiency light emitters to be fabricated in the group-III nitride system.
2.3.3 GaN Physical Properties
The Group-III nitrides possess several remarkable physical properties which make them particularly attractive for reliable solid state device applications. The wide bandgap materials possess low dielectric constants with high thermal conductivity pathways [18]. As shown in Table 2.2 Group III nitrides have high critical breakdown field and very high saturated electron mobility or velocity. These lead to inhibit dislocation motion and improve reliability in comparison to other Group VI and V materials.
Table 2.1: Semiconductor material parameter [19].
Semiconductor Silicon Gallium Arsenide Indium Phosphide Silicon Carbide Gallium Nitride
Characteristic Unit
Bandgap eV 1.1 1.42 1.35 3.25 3.49
Electron Mobility at 300 °K Saturated cm2/Vs 1.5K 8.5K 5.4K 700 1K-2K
Saturated Electron Velocity Critical cm/s 1 1.3 1 2 2.5
Critical Breakdown Field MV/cm 0.3 0.4 0.5 3 3.3
Thermal Conductivity W/cm °K 1.5 0.5 0.7 4.5 1.5
Relative Dielectric Constant εr 11.8 12.8 12.5 10 9
2.4.3 LED I-V Characteristics
Before a light emitting diode can “emit” any form of light it needs a current to flow through it, as it is a current dependant device with their light output intensity being directly proportional to the forward current flowing through the LED.
Figure 2.6: Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves showing the different colours available [23].
For a Light Emitting Diode to emit any form of light it needs a current to flow through it, as it is a current dependant device. As the LED is to be connected in a forward bias condition across a power supply it should be current limited using a series resistor to protect it from excessive current flow. From the graph above we can see that each LED has its own forward voltage drop across the PN-junction and this parameter, which is determined by the semiconductor material used, is the forward voltage drop for a given amount of forward conduction current [23].
Overall of this Chapter 2 is about the researches that have been done on the Gallium Nitride (GaN), double heterostructure and their effect on LEDs. GaN double heterojunction/heterostructure is been choose due to a wide range of application of light emitting diode (LED) which is its use in blue LED due to high efficiency and brightness also wide bandgap so it easily to make a better recombination of electron and hole. Other than that, the double heterostructure has many effects on the performance of LED such a brightness of LEDs.