Essay: Radiation effects on semiconductor devices

1. Introduction
Radiation effects on semiconductor devices are among one of the areas that are actively studied by researchers worldwide. For example, National Aeronautics and Space Administration (NASA) has established a Center for Radiation Engineering and Science for Space Exploration (CRESS) at Prairie View A & M University that focus on reducing NASA astronauts and their critical electronic equipment from the effects of harmful space radiation on long space missions such that in those working on the International Space Station (ISS). Therefore, proper measurement of the radiation does receive and the calculated risk needs to be determined. The research also enables researchers to improve radiation shielding capability of the LEDs for future long-duration Space missions and explorations, especially with the current race to Mars. NASA goal is to maximize the Technical Readiness Level (TRL) of new electronic devices for space applications.
The effects of ionizing radiation on light emitting diode (LED) is of great interest owing to the great need of LED as a lighting source in space, military, and extreme industrial environments. LEDs are versatile, durable, lightweight and low-powered, making it suitable to be used in challenging and power constraint environment, where in space for example, LEDs are currently being used as a light source that simulates daylight for astronauts and plants. Meanwhile, the usage of LEDs in nuclear reactor makes the device vulnerable towards neutron radiation. Furthermore, the advances in material and fabrication technology of LEDs that have occurred in the past 30 years prompt the need to understand how these LEDs perform under constant radiation.
In this research, we will study the effect of radiation on commercial LEDs as a result of it being low in cost and availability. In addition, LEDs types from several manufacturers could perform differently owing to the differences in the specifications, materials, and fabrication. In recent years, nitride based LED has attracted a great interest as one of the promising device that can provide environmental friendly light source that has high efficiency. In addition, nitride based LEDs are also considered excellent device due to its fast modulation speed, good robustness to shocks and atmospheric agents and for having linear behavior under continuous current reduction and pulsewidth modulation dimming. Furthermore, it is considering as one of the core component that is expected to replace the incandescent and compact fluorescent lamps (CFLs), indicating the start of the new era of solid state lighting. In fact, LED-based illuminating devices are free from the present of mercury unlike CFLs and this makes LED much safer compared with CFLs that can be consider as a hazardous material.
LED fabricated from Gallium Nitride (GaN) is one of the many choices of nitride commercial LED to be studied. GaN-based LED has attractive material properties of high operating temperature and breakdown strength due to the wide bandgap and is low in resistance. Furthermore, indium gallium nitride (InGaN) coated with yellow phosphor are is also of interest awing to its high heat capacity and low sensitivity to ionizing radiation. LED based on these materials are attractive candidates as a source of illumination for imaging and mapping process in a highly irradiated environment. Furthermore, these LEDs are utilized in the nuclear reactor and space environment, hence the requirement for it to be highly resistant to radiation. Source of radiation that will be used in this study are neutrons and electrons- they will be inducing displacement damage on the afore mentioned commercial LEDs where the extent to which the commercial LEDs can withstand the effect of radiation will be determined.
In addition to its luminescence efficiency, the existence of leakage current in this devices should be giving enough attention as well. The existence of leakage current will indicate the extent of the device quality, reliability, and electrostatic discharge [1][2]. Several research works have stated the dominant mechanism responsible for the occurrence of leakage current because the degradation of optical power is often linked to the existence of leakage current. LEDs with reduced leakage current are most desirable in the LED industry. Hence it is important to understand the underlying mechanism behind leakage current to improve the technology of LEDs [8].
2. Problem Statement and Its Significance:
It is important to expand the published research by conducting studies related to the effects of LEDs radiated with different dose level in order to have a wider and in depth knowledge of gallium nitride based materials. This is necessary especially in space missions, where the dose levels of Apollo missions range from0.18 rad to 0.55 rad from Apollo 11 and Apollo 17 respectively. Radiation properties also need to be measured and quantified. These include the amount of radiation in environment, the energy of radiation, and the magnitude of radiation absorbed. This is a result of the space environment, where several factors influence the level of radiation received: altitude over the earth –at higher altitudes the Earth magnetic field is weaker, thus radiation is stronger, and sun solar cycle – SunSpots based on Sun’s 11 year cycle can increased radiation. In this research, varying amounts of dose will be given to each LED to assess the extent to which it is able to function well. Radiation exposure limit is important to be determined, because there might be a limit where the LED is operatable.
Space environment normally has high radiation, which can cause damage to semiconductor devices in electronic machines. Semiconductor devices in electronic circuits have broad applications, but it is commonly used to control DC modules such as motor controllers, LED drivers, decoders, LED signal amplifiers and e-fuses for current limiting. Total dose effects have been the most prominent radiation effects in these devices. However, as technology advances utilizing semiconductor devices such as LED under harsh radiation environment have become a more important consideration for radiation effects research. Ionizing radiation effects are induced by the interaction of an ionizing particle with electronic components. Defects induced in the semiconductor lattice may affect the electrical and optical characteristics of LED. This is believed to be due to heavy radiation interaction to the LED.
In Malaysia, radiation study related to semiconductor is still at its infancy. Through ongoing literature review process, these are several papers that can be associates with the effects of radiation on semiconductor devices, ‘Effects of Total Ionizing Dose on Bipolar Junction Transistor’(by Chee Fuei Pien, UMS), Effects of Neutron on Reverse Bias Characteristics of Commercially Available Si and GaAs Diodes (by Nuurul IffahChe Omar, IIUM), Effects on the Forward Bias Characteristics of Neutron Irradiated Si and GaAs Diodes (by Nuurul Iffah Che Omar, IIUM) in collaboration with Malaysia Nuclear Agency and ‘The Radiation Analysis Conducted for RazakSAT I (by Annanthan Narayanasamy, UKM) in collaboration with a South Korean company.
Previous research has determined that gallium nitride based materials are relatively radiation tolerant compared to other semiconductor materials. However, studies on the optical characteristics of commercial InGaN/GaN LEDs subjected to electron and neutron radiation with different dose level are still lacking. This study will focus on the physics of the degradation and recombination mechanism, and the optical characteristics of the devices being irradiated. Furthermore, we will also use current commercial LEDs available in the market from different model and manufacturers.
3. Research Objectives:
The main objective of this research is to characterize the optical characteristics of commercial LEDs before and after radiation. All irradiation process will be conducted at the Malaysia Nuclear Agency as we have been collaborating with them before. Correlation between the degradation rates of the commercial LEDs with the parameter display in the LEDs datasheet will be acquired. In order to achieve the goals, specific objectives have been outlined as follow:
1. To characterize the optical characteristics of commercial InGaN/GaN LEDs before and after radiation.
2. To determine the degradation rate of commercial InGaN/GaN LEDs after radiation.
3. To analyze the superlinear dependence of the integrated EL intensity (IEL) on the injection current.
4. Research Questions/Hypothesis
1. 4.1 Research Question
The main question to be resolved in this study is to know the extent to which the effect of radiation can influence the optical and electrical characteristic of the respective commercial LEDs subjected to different amount of radiation dose. The additional sub-questions are shown below:
1. What is the degradation rate of LED under various doses with electron radiation?
2. What is the current degradation mechanisms for LEDs after subjected to electron radiation?
3. What is the light intensity degradation mechanism for LEDs after subjected to various dose of electron radiation?
2. 4.2 Research Hypothesis
The leakage current of the commercial LEDs in reverse bias will increase as the amount of radiation dose increase. This is due to the capture of free electrons by deep radiation traps. However, the characteristic of GaN in forward bias condition is expected not to show as much leakage current as shown in reverse bias because GaN is an element that has high resistivity against radiation in terms of I-V characteristic in forward bias condition.
Furthermore, the amount of light intensity will decrease and the value of wavelength will increase as the amount of radiation dose increase. This is because as the dose level increase, the size of bandgap energy of Gallium Nitride decrease, this result in a higher wavelength. Apart from that, the increasing amount of radiation dose will result in the presence of more defects hence, this will cause the light intensity of the LED to reduce.
5. Literature Review
Studies involving radiation effects in semiconductor devices is among one of the important areas of research in the world today in order to understand more about the basic physics of semiconductors as well as the applications of semiconductor materials and devices [1]. Today, many researchers started to realize the importance of the effects of radiation on the diode as it is among one of the electronic devices that often used for different types of purposes in various fields. Among one type of diode which are often applied in the field of technology today is nitride based device. Radiation can be divided in different categories and each has its own characteristics with different effects. For the experiment that is related to the nitride based diode, most of the studies that have been done before is by using a 2 MeV proton irradiation at room temperature (below 300K).
Apart from proton radiation, there are also researchers that conducted studies regarding the effects of neutron radiation on GaN diode. However, more research focused on the effects of proton radiation and neutron radiation on GaN diode rather than the effects of electron radiation. Therefore, in this paper, the main focus is on studying the effects of neutron and electron radiation on the electrical and optical characteristic of GaN and InGaN diode.
GaN is believe to become one of the promising material in the field of electronics and technology due to its hardness against radiation. The hardness of GaN is due to the fact that it has a stronger bonding of atom compared with Gallium Arsenide (GaAs). There are a number of previous studies have shown that GaN diode capable of withstanding the effects of radiation more effectively than GaAs. The first study involved the comparison between the electroluminescence of GaAs and gallium nitride diode [2]. In second study, is about the photoluminescence of GaN diode [3] and third study is while the third study involves photoluminescence measurements of GaN after being bombarded with proton radiation [4]. While the study was conducted, energy level being bombarded from proton radiation is placed at the same level and kept constant for both diodes, GaN and GaAs. Through this we saw that GaN diode only begins to show degradation when it is two orders of magnitude higher compared with GaAs technology. These differences contribute to the stronger bonding of GaN and this bond strength is representing the energy required to change the position of an atom from its lattice position which is also known as the atomic displacement energy, Ed.
According to Carlone and Houdayer in 2002, the higher value of Ed indicates that the semiconductor is more radiation hard. Through several analyzing that has been made regarding the transport properties of electron-irradiated GaN films, it has been conclude that the Ed(Ga) is 20.5 eV ,Ed(N) is 10.8 eV [4] and the Ed of GaAs is 9.8 eV [5]. This proof that GaN is definitely more radiation hard compared with others. In this research various materials of LED including InGaN and GaN will be chosen in order to analyze the radiation effects on different materials and structures.
Other researchers in this field have identified few degradation mechanisms on the semiconductor materials under heavy proton and neutron radiation. Literature review reported that great efforts are being put into designing, growing and fabricating LED but at the same time it is very important to develop and investigate radiation sustainable LED which can operate under harsh radiation environment. This is because of its performance in lightning sensor, optocouplers, LED drivers, and LED signal amplifiers which is used in the space robot and military equipment makes the LED vulnerable towards radiation. Its operation varies depends on the various energy impact.
Some of the higher level energy impacts that can be harmful to semiconductor devices such as LED are electron radiation. Electron radiation consists of negatively charged particle. The particles are released as a result of collisions between electrons with high kinetic energy, with an atom. The collusion caused tightly bond electrons to be freed causing changes in the atom’s crystalline system. The Van Allen radiation belt around the Earth is an example where majority of charged particles are fast moving electrons. According to NASA, this fast moving electron gave rise to the occurrence of damage to the electronic devices of orbiting satellites and hence leads to their premature deaths [6]. The radiation induced defects leads to degradation of semiconductor. In the past years, commercial radiation hardened LEDs have been studied for the radiation harsh environment by many researchers. Studies reported that heavy radiation have high impact on optical characteristics on lightly doped samples [7]. Furthermore, Parameters.Shasidhara et al reported that 8MeV of electron beam could cause the leakage current in the commercial LED to increase [5]. As the leakage current increase, the optical performance of the LED will reduce and hence this will cause the LED to malfunction. Hence, improvement in the lens structure, lens material, and radiation hardened against the particle radiation is essential to extend the operating life. Several research has been done regarding the effects of radiation on semiconductor device. However, there are still open issues that need to be tackle. Previous research mainly focus on the effects of proton radiation and minimal attention is being given to the effect of electron radiation despite electron radiation being the dominant radiation inside the Van Allen Radiation Belt. Furthermore, previous research mainly focuses on the current-voltage characteristics of the semiconductor device after radiated and lack of attention is being given to the changes experienced by capacitance-voltage characteristics. In this research, we will focus on the effects caused by electron and neutron radiation in terms of current-voltage, capacitance-voltage, light intensity and shifting of wavelength.
To conclude, this research aims to study the effects of high energy particle irradiation such as ionization damage and displacement damage. This research will further investigate the optical characteristics and electrical characteristics of InGaN and GaN LED under electron and neutron radiation.
6. Research Scope
The scope of this study is to analyze the characteristics of electrical and optical GaN diodes before and after it being irradiated with an electron radiation. Electrical characteristic referred to is current-voltage measurements and capacitance-voltage acquired while the intended proceedings of the optical characteristic is a measure of the level of photoluminescence GaN diodes before and after it being irradiated with an electron and neutron radiation.
7. Research Methodology
To achieve the objectives that have been targeted, the research will be done by following the steps below:
A) Experimental Setup
-Identifying and exposure to the required instrumentation for electrical and optical characteristics experiments such as the Keithley source measurement unit and HORIBA photoluminescence setup.
B) Selection of commercial LEDs
– Commercial LEDs will be chosen based on required criteria such as the its operating voltage, the wavelength of the light emitted, the active material and the cost. The LEDs will be purchased from Element14 which is a trusted online shop. Datasheet of the LEDs will be compared to select a common set.
C) Exposure to electron and neutron radiation
– Irradiation of the LED will be implemented in Malaysian Nuclear Agency at various energy levels. In Malaysian Nuclear Agency, they have the electron and neutron beam radiation facility which can covers up to 3MeV of energy. The LEDs will be radiated with incrementing electron energy to study the degradation characteristics.
D) Electrical measurements
-Room temperature I-V (current-voltage) measurements, and room temperature C-V (conductance -voltage) measurements, will be conducted before and after radiation. These will be done using Keithley source measuring unit.
E) Optical measurement
-Room temperature optical measurement will be conducted before and after radiation. These measurements will be conducted by using HORIBA photoluminescence setup.
F) Result analysis
-Based on the I-V and C-V measurements characteristics, the current degradation mechanisms of the commercial LED will be analyzed before and after radiation. This will be done by modelling the dark current with available theoretical equations.
– Based on the optical measurement characteristics, the light intensity degradation and shifting of wavelength will be analyzed before and after radiation. This will be done by modelling the peak wavelength and light intensity with available theoretical equations.
G) Final report.
– Writing comprehensive reports on studies that have been done and send the report within the time specified
8. Expected Outcome/Contributions
1) In depth understanding regarding the effects of electron and neutron radiation on LED
2) Able to determine the dominant mechanism responsible for the presence of leakage current in the forward and reverse bias region.
3) Publishing journal related to this field in an international journals indexed in ISI
4) The knowledge can be used to develop radiation hard electronic circuits for safety equipment and robots.
5) The knowledge gain through this research will help Malaysia to develop more radiation hard satellite’s electronics where it can increase reliability of the satellite thus reducing cost in long time. The result will not only benefit to space industry but also can be applied to any radiation related field such as nuclear and medical.
9. Research Schedule and Milestones
Work schedule for the first year
2015 2016
Task Time June July August Sept Oct Nov Dec Jan Feb Mac April May
Literature review and background study
Identify the samples and design the experimental set-up
Pre-radiation optical and electrical measurement
Samples being send to ANM to be radiated with electron and neutron radiation
Work schedule for the second year
2016 2017
Task Time June July August Sept Oct Nov Dec Jan Feb Mac April May June
Literature review and background study
Samples being send to ANM to be radiated with electron and neutron radiation
Post -radiation optical and electrical measurement
Analysis of results
Publication process
10. Conclusion
We proposed to study the optical and electrical characterization of commercial LED subjected to electron and neutron radiation. The proposed research will focus on the impacts of radiation on the current-voltage and capacitance-voltage characteristics of the device in terms of electrical characterization. Meanwhile, in terms of optical characterization, the value of light intensity before and after radiation, and the shifting of wavelength will be the main focus. It is expected that the value of leakage current in reverse bias will increase as the dose of radiation increase while the value of capacitance will decrease. Furthermore, it is also expected that the light intensity will reduce as the dose of radiation increase and the shifting of wavelength will occur after the LED being radiate with electron and neutron radiation. All these changes occur due to the presence of defects. Through the findings obtain by this research, the invention of radiation hard semiconductor device can be improved and we will able to gain new knowledge regarding the effects of radiation on semiconductor device.
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