Semiconductor News

A quarterly publication of the Semiconductor Society (India), www.ssindia.org.in

March 2019

From the Editor’s Desk

The crystallinity and symmetry of semiconductors is essential for their optical and electronic properties. Any change in the crystalline structure due to lattice defects disrupts the electronic band structure and the corresponding properties. Thus, it is important to study lattice defects in semiconductors for actual device applications. Defects in semiconductors have been studied for many years from the viewpoint toward improving the quality of materials. Most common defects in semiconductors are point defects, line defects and planar (extended) defects such as platelets and nanocracks. These defects influence the performance of devices made from these materials for applications in optoelectronics, high frequency, space, defence and photovoltaic applications. Considerable interest was to study the role of threading dislocations that are supposed to be the main cause of reduction in efficiency and lifetime in optoelectronic and photovoltaic devices.

Defect engineering however, also relates to controlled generation and guiding the defects in semiconductor for certain niche applications. Various methods to induce the defect in controlled manner and study the behavior of defects have been developed including co-implantation and millisecond scale annealing methods such as laser annealing and rapid thermal annealing.

Nanostructured or defect engineered materials mainly the wide band gap semiconductors (WGS) such as III-nitrides, SiC, ZnO, Ga2O3, diamond and AlN have recently been explored for varied applications. These materials have attracted intensive research attention due to prospective applications in solid state lighting, solar cells, power electronics, spintronics, thermoelectric generators and MEMS/NEMS. In the last couple of years, significant progress has been achieved in the synthesis of nanostructured WGS and promising device applications have also been demonstrated.

In this issue, we present a study on the defect engineering in wide bandgap materials for thermoelectric applications.

- Dr. Uday Dadwal, IIT Delhi

Executive Committee of SSI

Chairman: Dr. Rajesh K. Sharma, SSPL Vice-Chairman: Prof. Vinay Gupta, Delhi University Vice-Chairman: Prof. S.K. Ray, SN Bose NCBS Secretary: Prof. Rajendra Singh, IIT Delhi Treasurer: Dr. Shiv Kumar, SSPL EC members: Prof. M. Jagadesh Kumar (JNU), Dr. Ashok Kapoor (SSPL), Dr. Seema Vinayak, (SSPL), Prof. R.M. Mehra (Sharda Univ.), Prof. P.K. Bhatnagar (DU), Dr. Govind Gupta (NPL), Dr. Alok Jain (SSPL). Editorial Board: Dr. Alok Jain, SSPL (alokj_spl@yahoo.co.in) Dr. Rupesh K. Chaubey, SSPL Mr. Kamal Lohani, SSPL Dr. Uday Dadwal, IIT Delhi

Defect engineering in wide bandgap materials for thermoelectric applications

Thermoelectricity:

Figure 1: (a) Small amount of energy is produced from renewable energy sources compared to traditional sources. (b) Nuclear radiation leaked from Fukushima reactor due to strong earthquake (Reference 2).

According to a report by the U.S. Energy Information Administration, the energy generation (electricity) in developed countries still predominately depends on non-renewable energy resources (coal and gas). Stock of non-renewable energy sources is limited and hard to meet the requirements of world’s increasing population. Nuclear reactors are also efficient source for power production but the recent accident of radiation leakage from Fukushima reactor has highlighted the high risks involved with nuclear energy. Therefore, more emphasis have been focused on alternate energy technologies to lessen the dependence on fossil

fuels. This has made the research area of thermoelectricity to be a hot topic since late 90s.

Thermoelectricity (TE) is the power or voltage generated across a material when a temperature difference is applied (Figure 2a). Advantage offered by this technology is less bulky design and no moving parts involved while limitations like more expensive and less efficiency demands further refinement. To find out a suitable thermoelectric material for commercial on-chip applications which can supply high power conversion efficiency and long term stability against moisture, heat, and radiations, is the real challenge.

Figure 2: (a) Measurement principal of thermopower (Seebeck coefficient) and (b) conceptual design of thermoelectric generator (TEG).

Scope of GaN as thermoelectric material: GaN has earned the reputation of material of choice for optoelectronic and high power electronic applications e.g. Blue LED, LASER, HEMT for radars. It has also been explored for various other applications like radiation hardness, low noise devices due to its excellent semiconductor properties. An application such as thermoelectric generator (TEG) from GaN has rarely been studied. Due to its wide bandgap energy, GaN inherently has high Seebeck coefficient and offers n- and p- type conductivities. A requirement of very low thermal conductivity for TEG has been a roadblock for GaN based thermoelectric applications.

The figure of merit (Zt) for thermoelectric material is defined as Zt = S2σ/κ, where S, σ and κ are Seebeck coefficient, electrical and thermal conductivities, respectively. This equation suggests that a good TE material must have high Seebeck coefficient, high electrical conductivity, and low thermal conductivity. Currently, commercial devices for power generation use PbTe alloys (Zt ≈ 1 at 500°C) and SiGe (Zt ≈ 0.6 at 700°C). One limitation associated with conventional thermoelectric materials is the low operational temperatures (< 800 K). These materials have low bandgap. At high temperature, intrinsic minority carrier jumps across the bandgap causing the loss of the n- or p- type character of the material and the shift in Fermi level toward midgap. This results in decreased Seebeck coefficient. Therefore, it is imperative that to maintain high Zt at higher temperature, materials need to have a wide bandgap. Bi2Te3 based materials have maximum operating temperature of 150°C apart from being toxic and scarce. III-nitrides, in particular GaN, have great potential for use in high temperature TE applications. This is due to their (III-nitrides) unique material properties, including wide bandgap, mechanical robustness and chemical stability at high temperature.

Ion beams as a tool for defect engineering: It is estimated theoretically that the maximum Zt value of In0.65Ga0.35N and Al0.4Ga0.6N alloy would be 0.15 and 0.07 at 1000 K, respectively. The low values of Zt in GaN alloys can be attributed to the relatively high bulk thermal conductivity ~200 Wm–1K–1 at room temperature (RT), which is two orders of magnitude higher than SiGe. It is crucial to reduce the thermal conductivity to get useful efficiency. Generally, the thermal conductivity of a material can be ascribed to the sum of the electronic thermal conductivity (charge carriers transport) and the

lattice thermal conductivity (due to phonon transport).

Figure 3: Seebeck coefficient (S) and power factor (P) measurements in irradiated GaN (see Reference 3 for details).

According to the Wiedemann-Franz law, the electronic thermal conductivity depends linearly on the electrical conductivity (σ), whereas the lattice thermal conductivity is independent of the electronic properties but is significantly higher in magnitude than former. Therefore, the strategies to increase Zt value may include: (1) reduction in lattice thermal conductivity by increasing the phonon scattering, such as through alloying, nanostructure engineering, defect engineering by selective implantation; and (2) spatially separate the donors (or acceptors) from electrons using bandgap engineering and modulation-doping technology. The methods of alloying and

nanostructure engineering need a very fine control on growth techniques. However, ion irradiation/implantation can be used precisely to fabricate on chip thermoelectric device along with other devices. It has been found that ion implantation/irradiation can significantly modify the thermoelectric properties in different materials (Figure 3 and Reference 3). Lack of experimental thermoelectric studies in GaN using irradiation makes it hard to pinpoint the exact transport mechanism involved. But certainly, optimization and control of thermoelectric parameters by ion beam engineering presents a promising area.

More information about the article can be found from the following references:

[1] Snyder et al., Nature Materials, 7, 105 (2008). [2] Madigan et al., Proc. Natl. Acad. Sci. U.S.A (PNAS), doi/10.1073/pnas.1204859109 (2012). [3] Kumar et al., Appl. Phys. Lett., 111, 222102 (2017).

-Dr. Ashish Kumar

(DST-INSPIRE Faculty) Inter University Accelerator Centre, New Delhi

Recent News/Events in Semiconductors

1. Researchers at the National Institute of Information and Communications Technology (NICT) and Tokyo University of Agriculture and Technology (TUAT) had demonstrated a first vertical Ga2O3 MOSFET. n++ source regions, lateral n-channel and p-current blocking layers were fabricated using three ion implantation steps. Silicon and nitrogen were taken as donor and deep acceptor dopant, respectively. Drain current density of the transistor was 0.42 kA/cm2. Transistor showed an on-resistance of 31.5 mΩcm2 with an output current on/off ratio larger than 108. Fabricated

vertical Ga2O3 transistor through ion implantation has promising application in power devices (Wong et al., IEEE Electron Device Letters, 40, 431, 2019).

2. Symposium on Wide Bandgap Semiconductors has been recently jointly organized on 15-16 March, 2019 by Semiconductor Society (India), Society for Semiconductor Devices (SSD) and National Institute of Technology Kurukshetra. The venue of the symposium was NIT Kurukshetra. The conference was inaugurated by Dr. Satish Kumar, Director, NIT Kurukshetra. During his inaugural talk Prof. Rajendra Singh (IIT Delhi), Chairman of the symposium mentioned about the importance of wide band gap semiconductors in electronic and optoelectronic devices such as LEDs, photodetector and HEMTs. The objective of the symposium was to exchange ideas, showcase respective group activities (from National laboratories and academic institutions), discuss about possible collaborative activities, and plan some joint research proposals in the area of wide bandgap semiconductors (GaN, Ga2O3, SiC, etc.). Idea of the symposium is to deliberate on the applications of wide bandgap semiconductors in defense, aerospace and communication. Symposium was attended by the prominent experts working in the field of wide bandgap semiconductors.

3. The flagship event of Semiconductor Society of India (SSI), International Symposium on Semiconductor Materials and Devices (ISSMD), this year held in collaboration with Visvesvaraya National Institute of Technology (VNIT) at the orange city of India, Nagpur between 30th Nov.–02nd Dec., 2018. The event marked the successful completion of 5th biennial edition of ISSMD, last one was held at Jadavpur University in the year 2016. The symposium

was held under the convenorship of Prof. Rajendra M. Patrikar and Prof. Raghvendra B. Deshmukh and supported by Center for VLSI and Nanotechnology at VNIT Nagpur. The highly fruitful three days event brought together the academicians, researchers and students working in the area of semiconductor materials, devices and process technologies. The 2018 edition of ISSMD featured invited talks presented by scientists and academic leaders, as well as witnessed the outstanding contributed papers not only by students but also from scientists and academicians. The inaugural session was held in the morning of 30th Nov., 2018 preceded by Prof. Vikram Kumar, Professor, IIT Delhi together with the Director, VNIT Nagpur. In the inaugural talk itself Prof. Vikram Kumar set the high tone for the event by highlighting the major roles of semiconductors now a days playing in bringing sophistication in human life. He also highlighted the journey of semiconductor research and development in Indian institutions, both academics as well as research, which was a treat to know and learn. Another major highlight of the event was the plenary talk delivered by Prof. B. M. Arora, Prof., IIT Bombay. He addressed the very vital issue of degradation and failure detection of solar devices and modules, which is critical for successful commercialization of the solar cell technologies. The other eminent speakers, including Prof. Rajendra Singh (IIT Delhi), Dr. Govind Gupta (CSIR NPL, Delhi), Prof. Rajiv Dusane (IIT Bombay) and Dr. Mukesh Kumar (IIT Ropar) also highlighted some of the specialized processing technologies for the design of next generation devices including group III-nitride based devices such as hot wire CVD, MBE and MOCVD. Prof. MM Nayak (IISc Bangalore) and Dr. SRK Vanjari (IIT Hyderabad) discussed the use of semiconductor technology for sensing and actuation during the

event, as the subject becomes important because of evolution of MEMS devices and continuous deployment in the systems. Another notable highlight of the event was that it brought together both the semiconductor and photonics community on the same platform. There were very intense and interesting discussions on the photonics and optoelectronic devices applications with various semiconductor materials like silicon photonics, photonic band gap structures and 2D van der Waals materials through eminent speakers that included Prof R. Vijaya (IIT Kanpur), Dr. SK Varshney (IIT Kharagpur), Venu Gopal Achanta (TIFR, Mumbai), Dr. Manish Mathew (CSIR CEERI, Pilani) and Dr. Saurabh Lodha (IIT Bombay). Many of the explored interesting topics in the symposium also included novel modelling and simulation approaches for designing nano-scale semiconductor devices by Prof. S. Karmalkar (IIT Madras) and Prof. Abhinav Kranti (IIT Indore). The symposium also covered topics related of semiconductor materials for energy applications especially by Prof. BR Sankapal (VNIT Nagpur), Prof. Sidhharta Duttagupta (IIT-Bombay) and Prof. CJ Panchal (CSUB, Vadodara). Rest many topics related to semiconductor were also reflected in contributory talks and posters. A special feature of this event was the demonstration of the QuDSim-3D as a TCAD simulator by Dr. Ashutosh Mahajan, developed at Centre for VLSI and Nanotechnology, VNIT, Nagpur to simulate electrical characteristics of nanoscale devices. It is the Finite-Element-Method (FEM) based tool that solves self-consistent Poisson-Schrodinger equation on 2D/3D device geometry. This software will be available to the students on the web soon. The event also included the talk by IEEE distinguished lecturer, Prof. Anil Kottantharayil from IIT Bombay on Graphene based devices. All the talks were of

very high standard and helped the target audience to understand the basic concept of the subject. The event particularly gave opportunity to the research scholars and students to learn, interact and discuss their ideas to the experts of the fields. The banquet dinner on the penultimate day gave the opportunity to the participants to discuss and share their ideas with the masters of the field. The five best poster paper awards were also distributed during the session. The posters were judged by the six members committee. The presence of many other distinguished contributors and participants from all around the country, enlightened the event through sharing their expertise and expand the knowledge horizons of ISSMD-2018. This edition of the event, due to its high standard, got the financial support from various government agencies including DRDO, ISRO, CSIR, DST-SERB and TEQIP-III. With the hope that the biennial event will continue to strive similar further successes, the organizers officially marked adieu to all the eminent invited speakers and participants. The next edition of the symposium will held in the year 2020.

4. Forthcoming Conference: 20th International Workshop on the Physics of Semiconductor Devices (IWPSD), will be organized by SN Bose National Center for Basic Sciences during December 16-20, 2019, at Kolkata. Prof. S. K. Ray is the Chairman for this edition of the workshop. The details about the workshop can be seen at: http://newweb.bose.res.in/Conferences/IWPSD2019/