Nanoptronics institute is to advance microelectronics science, technology, and education, by providing facilities which encourage interdisciplinary research and educational activities.
The microelectronics center is positioned to provide critical resources and infrastructure to support the education of quality students and renowned research faculty.
Theoretical and practical aspects of techniques utilized in the fabrication of semiconductor devices. In this center techniques of chemical vapour deposition and diffusion; advanced concepts of contamination control; defect-free processing; complete characterization including junction penetration, resistivity, and oxide thickness, Switching speed, junction characteristics, leakage and gain, ion implantation, and method of fabrication are studied.
Microelectronics is a branch of electronic science that is related to the study, design, manufacture of electronic devices which are very small (usually micrometer-scale).this science includes individual components (transistors, capacitors, inductors, resistors, diodes) and integrated circuits based on this components. These devices often fabricated onto the mono crystal wafer that its orientation is preserved during produce process. Fabrication steps include film deposition, impurity contamination, photolithography and packaging.
Film growth methods include Physical vapor transport (PVT), Chemical Vapor Deposition (CVD), Sol-gel processing technique and Liquid phase sintering technique.
In order to film doping can enter impurity during growth or it can be used diffusion or ion implantation for impurity transfer into prefabricated wafer.
Photolithography is used to separate specific section and change its properties. It includes photoresist deposition onto the film, UV irradiation by mask and then washing and elimination unwanted sections. After that properties of specified region (that has not photoresist) can be changed such as contamination. Latest stage is metal deposition that is used to electrical contacts.
In the recent years many attempts is concerned to miniaturization integrated circuits, using new materials with excellent properties and design of new devices with required quality of industries.
FACULTY & STAFF
MS Zahra Alaie
Emil: alaie_zahra - AT - iust.ac.ir
Dr Shahram Mohammad Nejad
Email: shahramm - AT - iust.ac.ir
Laboratory research projects
1. Design and fabrication SiC UV photodetector :
Compared with other wide band gap materials, SiC benefits from fabrication in relatively large surfaces (up to 3mm×3mm) and low dark current.SiC has a wide band gap of 3.26 eV, which makes 4H-SiC intrinsically visible-blind. 4H-SiC photodiodes show a high UV-to-visible rejection ratio. SiC has an ultra low intrinsic carrier density, which is 1018 times lower than that of Si, indicating the SiC based detectors have an ultra low leakage current. In addition, 4H-SiC has an ultra low thermal generation rate, which enables very low dark current at high temperature and makes very large detector area possible.Displacement energy defined as the minimum energy that must be impacted to a lattice atom to remove it from its lattice site, is widely used in radiation hardness measurements. The displacement energy of SiC is 22eV. Hence, SiC detectors could have better radiation hardness and a longer lifetime in strong radiation environments than Si detectors.One important advantage of 4H-SiC is that it has native thermal oxide, SiO2. During the device fabrication, surface damages can be passivated by native thermal oxide. Hence, the surface leakage current can be reduced and the device performance can be improved.SiC, with a wide band gap of 3.28 eV, is a promising semiconductor material for high power, high temperature, and high frequency applications, owning to its high breakdown electric field, high electron saturation drift velocity, and high thermal conductivity.
2. Design and simulation of silver nanoparticle influence on the electrical and optical properties of Titanium Dioxide thin film
The distinctive UV absorption characteristics make TiO2 very suitable for UV detection against the bachground of infrared and visible light. It has been reported that the optical properties of TiO2 can be improved by doping with metal particles such as Ag, Au or Cu. Nanoparticles of metals have recently become the focus of research because of their unique properties, which are different from those of bulk materials. These properties depond on the size, shape and differences in the environments of nanoparticles. Ag nanoparticles have high absorption coefficient, so the charges depict very high transition. That is why optical current increase, dark current, recombination electron-hole and noise to signal decrease. Some numerical methods are used to characterize influence of nanoparticle on absorption, transmission, reflection, Scattering and extinction. The optical properties help us to simulate electrical properties such as refractive index, conductivity and dielectric. In this project optical and electrical properties of TiO2 doped with Ag nanoparticle are discussed and simulated. We also studied effect of nanoparticle size on dark current, NEP and detectivity of TiO2 photodetectors doped with Ag nanoparticle.
3. Design and simulation of ZnO based UV photodetectors with improving of quantum efficiency & dark current:
In this project, the current transport mechanism of ZnO-based metal-semiconductor-metal ultraviolet photodetectors with various contact electrodes is discussed and simulated. The simulation is based on the thermionic emission theory and tunneling effects. It was found that the lowest dark current, 6.04×10-10A at 3V biased, is obtained when the RU contact electrode is used. Moreover, it is shown that in order to achieve a large schottky barrier height on ZnO and more reduction of dark current, one can insert a thin oxide layer between contacts and ZnO layer. Also, the influence of the thickness of the insulator layer on the dark current of the MIS photodetector has been analyzed. Based on the simulations results, the dark current at 3V photodetector biased, with various thicknesses of interfacial insulator layer 3, 5, 7 and 10nm is 2.87×10-11, 8.23×10-13, 2.36×10-14 and 1.15×10-16A, respectively. Furthermore, quantum efficiency in our simulations with an antireflection coating has been improved. It is found that in these devices, the quantum efficiency with TiO2 thin oxide with thickness of 1nm is 46.7%.
4. Simulation and optimization of the behave of quantum well infrared photodetectors based on GaAs/AlGaAs :
In this research, making use of quantum mechanics rules and relations, the operation of quantum well infrared photodetectors, based on super lattice Ga/As/AlGaAs/GaAs in fitting physical and calculatory models, has been simulated.
The first stage of performed simulations contains the formation of profile potential, conduction band of detector and the place of sub band confinement energy in wells making use of the envelope function, the form of similar wave with any confinement energy level making use of the Bloch wave and assignment of effective mass of carriers in well and barrier, making use of Kroing and Penny model.
In the next stage, the preparation of producing of noise current and photocurrent in these detectors, making use of the effects of carrier capturing, the escaping of carrier, injection of carrier through the contact layers in the sides of devices, tunneling carrier through the barriers and the established perturbation in conduction band by means of electrical field has been simulated. In the last stage, the frequency behave of these detectors is simulated, too.
5. Dual band UV-IR detection in reticle seekers :
Target position detection in presence of flares is one of the most important aspects of electronic wars. Among the various methods of target position detection, studying and improving the reticle seekers seems to be important. Reticles are fundamentally unable to distinguish the target from flares. But being simple and cheap are their main advantages. In this research, we have used dual band UV-IR detection to distinguish the target from flares in reticle seekers. Because of sensible difference of emitted power of flare and target in UV band, we can deduce flare position from UV detector and target position from both IR and UV detectors by the algorithm we have designed. We have converted the processing complexity to hardware complexity. So, we will have faster position detection in comparison with methods like Rosette or ICA and unlike traditional reticles, we can detect target position in presence of flares.
6. Growth process and calculating special characteristics such as accommodation coefficient, strain & stress and … in PbCdS system :
The objectives of this research are to study the growth process and special characteristics of semiconductors Pb1-xCdxS. Meanwhile, different subjects have been discussed. First, characteristics and properties of PbCdS films are described. Research on crystal structure and electrical properties of PbCdS films, have been investigated. Then, the growth of these films and besides study the different stages of growth such as etching and alloy preparation, a scientific discussion was done on the reasons acceptance of Cd with different percentages in these films and also investigated on PbCdS/PbS super lattices. Then, diffusion into PbCdS films was studied. Various diffusion mechanisms were discussed and a computerized simulation for calculating diffusion coefficient in these films was done. After it deals with mechanical effects on films (stress & strain) and accommodation coefficient. Different methods of measuring stress was studied and accommodation coefficient as an important parameter in thin films specially PbCdS , were discussed. For calculating these parameters, computerized simulations were done on each one and quantities were observed for each parameter in current lab temperatures and thicknesses for PbCdS. Finally, films and super lattices had made in lab and their X-ray effects were studied. In addition, by drawing simulated X-ray curves, a comparison was done between lab and simulated figures.
Available Equipment List
Microelectronic device fabrication needs equipments for thin film growth, n &p impurity contamination (diffusion or ion implantation), photolithography, metal contact deposition and optical and electrical characterization tests. Available equipments in microelectronic lab are shown:
| (Spin Coater)|| (Spin Coater)|| (laminar air flow bench)|
|(UV-Vis Spectrometer) || (Oven-Furnace)|
1. Z. Alaie, S. Mohammad Nejad, “ Investigation and Simulation of properties of SiC ultraviolet Detectors ,” 2nd Iranian Conference on Optics & Laser Engineering, Isfahan-Malke-e-ashtar University of Thechnology, 2011
Abstract: SiC devices have high break down voltage and great band gap Which is suitable Particularly in applications featuring high temperatures or hostile environments. In this article some of the electro-optical properties of SiC detector such as change of band gap with work temperature of this semiconductor, temperature changes of density of intrinsic carriers, electron and hole mobilities in 6H-SiC and 4H-SiC , drift velocities base on electrical field at different temperatures, impurity ionization degree and current density-voltage curves is simulated to obtain different properties of this detectors at different conditions.
2 . S. Mohammad Nejad, S. G. Samani, E. Rahimi, “Optical characteristics of ZnO - Based photodetectors doped with Au nanoparticles,” 2nd International Conference on Mechanical and Electronics Engineering (ICMEE), Aug 2010, pp. V2-408 - V2-412
Abstract: Zinc oxide as a primary material for ultraviolet detectors has shown to have a very high ratio of photo current to dark current which makes it suitable for photodetectors. Au Nanoparticles with specific diameters are exposed to improve optical characteristics of ZnO-based photodetectors. They have a high absorption coefficient, so charge transition is increased and photo current will be amplified as a result. Optical properties of photodetectors depend on nanoparticles characteristic. For photodetectore application we need to identify which of optical properties (scattering, absorption) are beneficial and then design suitable nanoparticle to maximize these effect while minimizing unwanted optical properties.Effect of Nanoparticles on optical parameters of photodetectors based on ZnO is discussed in this paper.
3 . S. Mohammadnejad, S. G. Samani, E.Rahimi, “Characterization of Resposivity and Quantum Efficiency of TiO2-Based Photodetectors Doped with Ag Nanoparticles,” International Conference on Mechanical and(ICMEE 2010), 1-3 Aug 2010, pp v2-394
Abstract: There is a motivation to reduce the thickness of photodetectore to improve the efficiency of extracting executed carrier. However, reducing the thickness of thin fdm also reduces the amount of light absorbed. To alleviate this problem we can use light trapping. Ag Nanoparticles with specific diameters are exposed to improve optical characteristics of Ti02-based photodetectors. Optical properties of photodetectors depend on optical properties of nanoparticles. There for photodetector application we need to identify which of these properties(scattering, absorption) are beneficial and then design suitable nanoparticle to maximize these effect while minimizing unwanted optical properties. Nanoparticles diameter is usually between S-100nm. If the diameter of nanoparticles is about more than 100nm, light will not be absorbed efficiently, and when it is smaller than Snm, charge transition through channels fades and scattering is increased. Effect of nanoparticles on absorption coefficient, quantum efficiency, and responsively of photodetectors based on Ti02 are discussed in this paper.
4 . S. Mohammad Nejad and M. Gharavi, "Design and simulation of a quantum well detector at 9 micrometer wave length and low noise under 77 Kelvin" , loth Photonic conference. The International Science & Advanced Technology and Environmental center Kerman, Feb 2003.
5. Sh. Mohammad Nejad, Sh. Enayati Maklavani, and Ehsan Rahimi "Dark Current Reduction in Zno-Based MSM Photodetectors With Interfacial Thin Oxide Layer" , Proceeding on Honet 2008, Malayzia. 2008.
Abstract: In this paper the current transport mechanism of ZnO-based metal-semiconductor-metal ultraviolet photodetectors with various contact electrodes is discussed and simulated. The simulation is based on the thermionic emission theory and tunneling effects. It was found that the lowest dark current attributes to the Ru contact electrode. Moreover, it is shown that in order to achieve a large Schottky barrier height on ZnO and more reduction of dark current, one can insert a thin oxide layer between contacts and ZnO layer. The influence of the thickness of the insulator layer on the dark current of the MIS photodetector has also analyzed.
6. S.Mohammad Nejad, M. Listani, S.Olyaee, “Target and Flares Seperation in the Reticle Seekers Based on the Dual UV-IR Detection ”, 1st National Conference on Optics and Laser Engineering, Isfahan, Iran, pp. 693-698, May 20-21, 2009.
7. S.Mohammad Nejad and M.A. Mazlumin, "PbCdS Accommodation Coefficient Calculation"; ICM’2000, Tehran University, Oct.31-Nov 2, 2000
Abstract: In this paper, the accommodation coefficient of PbCdS is calculated for the first time. The key elements describing accommodation coefficient, are presented. Based on the theoretical adaptation of the existing formulas, and the data obtained from the variety of PbCdS films made by Hot Wall Epitaxy, the accommodation coefficient of the system was calculated. The calculation depicts the decrease of accommodation coefficient with increasing temperature. The results would agree with experimental data.
Information of UV Photodetectors
1. UV Radiation
Ultraviolet (UV) radiation covers a wide wavelength span from 400 nm to 10 nm including:
Near ultraviolet (NUV) 400-300 nm
Mid ultraviolet (MUV) 300-200 nm
Far ultraviolet (FUV) 200-100 nm
Extreme ultraviolet (EUV) 100-10 nm
Some time, the following subdivisions may be also encountered:
Ultraviolet-A (UVA) 400-320 nm
Ultraviolet-B (UVB) 320-280 nm
Deep ultraviolet (DUV) 350-190 nm
Vacuum ultraviolet (VUV) 200-10 nm
Ultraviolet research began in the latter half of the 19th century, when the invisible radiation beyond the blue end of visible spectrum began to attract attention. Significant attention has been paid for a long time to the detection of UV radiation, owing to its wide applications in astronomy, biotechnology, material science, ecology and UV location, etc.
2. Photodetector parameters
The main parameters of semiconductor photodetectors are the following:
(1) Responsivity (Ri), gain (g) and quantum efficiency (η).
Responsivity is defined as the photocurrent per unit of incident optical power. It is determined by the quantum efficiency number of electron–hole pairs generated per incident photon) and gain (number of carriers detected per photogenerated electron–hole pair), using the following expression
where λ is the radiation wavelength, h is the Planck constant, c is the speed of light and q is the electron charge.
(2) Response time and bandwidth (BW).
The photodetector response time is characterized by the decay time, τ d (or rise time, τ r), defined as the time in which the photocurrent drops from 90% to 10% (or increases from 10% to 90%) of its maximum value, when the device is excited with rectangular light pulses. Bandwidth is defined as the frequency at which the photocurrent is 3 dB lower than the low-frequency response. In the special case of exponential transient response, the decay time and the rise time are related to the exponential time constant, τ, and to the bandwidth, by the expression:
It is very important to notice that reliable information about the device bandwidth can only be obtained from the
Photocurrent rise time if the excitation pulse is rectangular, with a rise time much shorter than τr, and a pulse width much larger than τr, so that photocurrent increases from zero to a steady-state value.
(3) Noise equivalent power (NEP) and detectivity (D).
The NEP is the optical input power for which the signal-to-noise ratio is equal to one. In the case of white noise, the NEP increases with the square root of the detector bandwidth. Thus, to evaluate the noise performance of a detector, it is more convenient to give the NEP normalized for frequency bandwidth,
Detectivity is the reciprocal of NEP value, and it is usually normalized for bandwidth and detector active area, Aopt:
3. Photodetector Classifications :
There are different types of semiconductor photodetectors: photoconductors, Schottky barrier photodiodes, metal–semiconductor–metal (MSM) photodiodes, metal–insulator–semiconductor (MIS) structures, p–n and p–i–n photodiodes, and field-effect and bipolar phototransistors. The schematic structure of these devices is depicted in figure 2.
Figure 2. Different types of UV photodetectors.
4. Methods of SiC synthesis :
1) Physical vapor transport (PVT)
Physical vapor transport (PVT), also known as the seeded sublimation growth, has been the most popular and successful method to grow large sized SiC single crystals. The first method of sublimation technique, known as the Lely method (Lely, Keram, 1955) was carried out in argon ambient at about 2500°C in a graphite container, leading to a limited SiC crystal size. Nevertheless, although the Lely platelets presented good quality, this technique has presented major drawbacks. The control of SiC growth by the PVT method is difficult and the adjustment of the gas phase composition between C and Si complements and/or dopant species concentration is also limited. For this reason, (Tairov, Tsvetkov, 1978) have developed a modified- Lelly method also called physical vapor transport method or seed sublimation method. In fact, this latter was perfected by placing the source and the seed of SiC in close proximity to each other, where a gradient of temperature was established making possible the transport of the material vapor in the seed at a low argon pressure.
2) Chemical vapor deposition (CVD)
Chemical vapor deposition (CVD) techniques have the largest variability of deposition parameters. The chemical reactions implicated in the exchange of precursor-to-film can include thermolysis, hydrolysis, oxidation, reduction, nitration and carbonation, depending on the precursor species used. During this process, when the gaseous species are in proximity to the substrate or to the surface itself, they can either adsorb directly on the catalyst particle or on the surface. Thus the diffusion processes as well as the concentration of the adsorbents (super saturation) leads to a solid phase growth at the catalyst–surface interface.
CVD is one of the suitable used methods to produce SiC in various shapes of thin films powders and nanorods.
Figure 3. Reactions occur in the surface of film during CVD processing.
3) Sol-gel technique for synthesizing SiC
Sol-gel processing received extensive attention of researchers because of it is novel and low temperature method. The basic advantages of using the sol-gel synthesis approach have been the production of high pure product with extremely uniform and disperse microstructures not achievable using conventional processing techniques because of problems associated with high melting temperatures or crystallization. In addition, the production by the sol-gel method permits preparation of material at far lower temperatures than is possible by using conventional method. Moreover, the sol-gel process has proved to be an effective way for synthesis of nanopowders, films and fibers as well as bulk pieces. The sol-gel process comprises two main steps that are hydrolysis and polycondensation. The first one starts by the preparation of a Silica-glass by mixing an appropriate alkoxide as precursor, with water and a mutual solvent to form a solution. Hydrolysis leads to the formation of silanol groups (SiOH) subsequently condensed to produce siloxane bonds (SiOSi). The silica gel formed by this process leads to a rigid network consisting of submicrometer pores and polymeric chains. After drying process, solvent is removed and microstructure is obtained.
5. Optoelectronic properties of SiC:
Silicon carbide has been known as a wide band gap semiconductor and as a material well-suited for high temperature operation, high-power, and/or high-radiation conditions in which conventional semiconductors like silicon (Si) cannot perform adequately or reliably. Additionally, SiC exhibits a high thermal conductivity (about 3.3 times that of Si at 300 K for SiC). Moreover it possesses high breakdown electric-field strength about 10 times that of Si for the polytype 6H-SiC.
SiC possesses a much higher thermal conductivity than the semiconductor GaAs as well as a band gap of approximately twice the band gap of GaAs. Moreover, it has a saturation velocity (νsat) at high electric fields which is superior to that of GaAs.
In the following table a comparison of several important semiconductor material properties is given. Silicon carbide also has a good match of chemical, mechanical and thermal properties that making it more suitable for use in sensor applications where the operating environments are chemically harsh.
Table 2. Comparison of several important semiconductor material properties .
6. SiC photodetectors:
Schottky photodiodes :
SiC Schottky photodiodes are of interest due to their surface junction, large detection area and relatively easier fabrication process. The Schottky barrier height on SiC depends on the metal work function. When high speed detection is not required, 4H-SiC Schottky photodiode with large detection area is an excellent candidate for low lever UV and EUV detection from 10 nm to 400 nm, with high quantum efficiency, low leakage current, and high spectral detectivity.
Figure 4. Sample of SiC Schottky photodiodes.
SiC avalanche photodiodes:
4H-SiC avalanche photodetectors when working at the voltage higher than the avalanche breakdown, benefit from a high gain and Ultra low dark current can be achieved. With proper design, 4H-SiC avalanche photodetectors present a high quantum efficiency in the UV range from 270 -280 nm. They can be used for single photon counting detection applications. They have High frequency, high speed UV detection and they are visible blind with a high UV-to-visible rejection ratio.
Fiqure 5. . Sample of SiC avalanche photodiodes.
The p–n junction:
SiC p–n junction photodiodes were the first WBG detectors to reach the market. The advantage of SiC photodiodes for UV detection is the extremely low reverse current. The spectral response of p–n junctions with a thin semiconductor front layer is quite similar to that of semitransparent SiC Schottky photodiodes. For increasing operating temperature, there is a redshift in the peak response, together with an enhancement of long-wavelength responsivity. These devices exhibit quantum efficiencies, showing excellent performance at high temperatures.
The p–i–n photodiodes:
A PIN diode is similar to p-n photodiode that have an intrinsic region between a p-type semiconductor and an n-type semiconductor region. P-i-n photodiode is used further than p-n photodiode because of generation electron-hole pairs in intrinsic layer. Thickness of intrinsic layer is optimized to improve the performance of photodetectors.
Figure 6. Sample of SiC P-i-n photodiode.
Like diodes, all transistors are light-sensitive. Phototransistors are designed specifically to take advantage of this property to detect ion incident light. The Phototransistor is similar in operation to the amplifying Transistor, but it is controlled by light rather than by the electric current of the emitter. So, a phototransistor amplifies variations in the light striking it. The Phototransistor has a high power output for a photo-electric device and gives good response to a rapidly fluctuating light source. It is particularly sensitive to the wavelengths of light and they have low impedance. They can be bipolar or field effect transistors. Structure diagram of this photodetectors can be seen in figure 7.
Figure7. Sample of SiC Phototransistors.
• History of microelectronics technology
Originally, all devices were discrete devices existing as separate units in their own package. Example: (capacitors, vacuum tubes, transistors, etc.)
The work of Kilby and then Noyce in 1958 gave rise to the of microelectronics technology which made it possible to create many devices arranged as an electrical circuit on a chip of semiconductor material.
As the of microelectronics technology matured, increasing numbers of devices could be integrated to form highly complex circuits.
• History of ultraviolet radiation
The first mention of ultraviolet (UV) radiation was made in 1801, when J W Ritter discovered that certain chemical reactions were catalyzed by exposure to non- visible radiation with a shorter wavelength than violet. Shortly after, in 1804, T. Young demonstrated that this chemically active radiation followed the interference laws. This observation, together with the work of many other researchers, made it possible to establish that both visible and UV emissions were manifestations of the same kind of electromagnetic radiation, only differing in their wavelength.
Nowadays, it is widely established that the UV region occupies the spectral interval of λ = 400–10 nm. It is a highly ionizing radiation, which activates many chemical processes. The most important natural UV source is the Sun. Approximately 9% of the energy received from the Sun at the higher layers of the atmosphere is in the UV range.
• History of photodetectors
Photodiode technology developments came out of the basic developments of the PN junction diode that started in the 1940s in earnest. Applications for the use of the PN junction diode were found outside the basic use of rectifying signals. It was found that they could be used for many photonic applications - photodiodes, solar cells and light emission.
Photodiode technology was refined in the 1950s and in the latter part of that decade the PIN photodiode was developed. Light absorption in the wide depletion area of the PIN structure was first investigated in a paper published in 1959 by Gartner. Although silicon has been the favored material for photodiodes, germanium can also be used, and its use was first demonstrated in 1962 by Riesz.
While PIN photodiode technology has been the most widely used format for diodes, other types including the avalanche diode were also demonstrated. The first step along the road was undertaken in 1953 by McAfee and McKay who first addressed the concept of avalanche multiplication and later work appeared on avalanche photodiodes in 1963 and the following years.
Another form of photodiode, named the Schottky photodiode has also been addressed. Some of the first research on point contact photodetectors appears to have been undertaken around 1962, and later diodes using evaporated metals films were also studied.
The idea of the photo transistor has been known for many years. William Shockley first proposed the idea in 1951, not long after the ordinary transistor had been discovered. It was then only two years before the photo transistor was demonstrated. Since then phototransistors have been used in a variety of applications, and their development has continued ever since.
The development of the SiC sensors is based on progress in the following technologies: 1) improved electrical and mechanical properties of SiC films produced (optimization of SiC deposition process), 2) SiC film processing (optimization of etching process and metallization appropriate for high temperature applications), 3) microfabrication technology to fabricate miniaturized sensors and 4) sensors packaging for harsh environments.
• SiC technology road map
Road map is Future perspective of technology that is based on recent improvements of that branch of science. Recent and future advances in SiC technology can be seen in figure 8.
Figure 8. Road map of SiC technology.