Chapter 5 Plasma Nitridation

Chapter 5 Plasma Nitridation This chapter deals with the nitridation of GaAs and porous GaAs for the formation of GaN using N2+H2 gas mixture as the plasma forming gas. High Speed M2 steel has also been nitrided to form Iron Nitride Chapter 5:Plasma Nitr. 108 Index Introduction 109 5A Section A: Gallium Nitride (GaN) 109 5A.1. Background Literature 109 5A. 1.1 Importance of GaN 109 5A. 1.2 Surface chemistry 110 5A.1.3 Electrical properties 110 5A. 1.4 The applications of GaN 111 5A. 1.5 Different techniques of synthesizing GaN 112 i) Chemical process 112 ii) Physical process 112 iii) Nitridation of porous GaAs 112 5A.2 Present experimental techniques of synthesis 114 5A.2.1 Nitridation of GaAs 114 5A.2.2 Nitridation of Porous GaAs 114 5A.3 Results and discussions 115 5A.3.1 Nitridation of GaAs 116 5A.3.2 Nitridation of porous GaAs 119 5A.4 Conclusions 122 5B: Section B: Iron Nitride 122 5B.1 Background 122 5B. 1.1 Surface Hardening 122 5B. 1.2 Nitridation 123 5B.2 Experimental details 125 5B.3 Results and discussions 126 5B.4 Conclusions 132 References Chapter 5:PIasma Nitr. 109 CHAPTER 5 Plasma Nitridation Introduction Present chapter deals with the process of nitridation and presents some of results obtained by plasma nitridation over two important classical materials, namely GaAs and Steel. The main intention of the work is to demonstrate the potential of ECR plasma in reducing the time of nitridation usually required in the conventional chemical methods. However, there are different techniques of producing surface nitrides which involve deposition of a nitride layer on the required surface. Few attempts have been made in which GaN is deposited on GaAs by CVD. The chapter is split into two sections. The first section refers to the nitridation of GaAs whereas the second section refers to the nitridation of steel. The purpose of obtaining the nitrides on these two different classes of materials is completely different. GaN is a semi conducting optical material whereas nitrided steel is a mechanically important material for tool steel industry. Importance of these two kinds of materials is described in details in the following section. 5A Section A: Gallium Nitride (GaN) 5A.1. Background literature 5A.1.1 Importance of GaN GaN is an important III-V group semiconductor well known for its stability and as a blue light emitter '. Some people 2'3 have developed a process for obtaining p-type GaN to demonstrate first P-N junction light emitting diode. The nitrides of III-V semiconductors have gained much importance recently because of their tremendous applications in the field of opto electronics 4. GaN is one of the most important candidates of these nitrides because of its potential applications for a solid-state lasers and optical data storage devices ' . Modern researchers have used improved crystal growth and processing technology to overcome many of the difficulties reported by the earlier workers. It is a highly stable material against temperature variation & its wide band gap has made it attractive material for device operation in high Chapter 5:Plasma Nitr. 110 temperature and caustic environment. It is an excellent candidate for protective coatings due to its hardness. Blue and UV wavelengths are technologically important regions of the electromagnetic spectrum in which efforts to develop semiconductor device technology are being carried out7 . Current semiconductor components emit wavelength from IR to green wavelength. With the advancement in the blue wavelength semiconductor components, emission and detection of the three primary colors of the visible spectrum is possible, which would have a major impact on imaging and graphics applications. Another technologically significant band occurs in the 240- 280 nm range (~4.75eV) where absorption by ozone makes the earth's atmosphere nearly opaque. Space to space communication in this band would be secure from the earth, although vulnerable to satellite surveillance. On the dark side of the earth, shielded by the sun's radiation imaging detectors operating in this band would provide extremely sensitive surveillance of objects coming out of the atmosphere. Like most wide band gap semiconductors, the nitrides are expected to exhibit superior radiation hardness compared to GaAs and Si, which makes them attractive for space applications. GaN is by far the most studied of the III-V nitrides, yet compared to the more commonly studied Si and GaAs semiconductors, relatively little is known about GaN. Large background n-type carrier concentrations, the lack of a suitable substrate material, difficulties with GaN p-type doping, and processing difficulties have discouraged many workers in the past. Films of GaN have been prepared by various methods. There are many reports 8"" of synthesizing thin films of GaN by exposing the surface of GaAs to nitrogen ions generated by different resources. This chapter deals with the nitridation of bulk GaAs which yields a layer of GaN on its surface. 5A. 1.2 Surface chemistry Though it is said that GaN is highly stable, it is noticed that the stability depends on its environment. It is less stable in HC1 & H2 environment but stable in N2 environment. It is the chemical stability at elevated temperatures combined with its wide band gap that has made GaN an attractive material for device operation in high temperature and caustic environments. GaN is also an excellent candidate for protective coatings due to its hardness . 5A. 1.3 Electrical properties Unintentionally doped GaN has been observed to be n-type with the best samples with the electron concentration of ~4xl016/cm-3.The mobility was also found likewise & it was Chapter 5:Plasma Nitr. Ill 2 observed that it depends on temperature .At room temperature it is 600 cm /Vs while at 2 13 temperature of liquid N2 it is 1500 cm /Vs . Table 5.1 Structural Properties of GaN Crystal Structure Wurtzite Band Gap 3.39 eV at 300 K 3.50 eV at 1.6 K Lattice Constant a = 3.189Au c = 5.185 Au Coefficient of thermal Aa/a=5.59* 10"b /K expansion Ac/c = 3.17 *10"6/Ka= 3.189 A0 Thermal conductivity K=1.3w/cmK a + Electron effective mass - m e ( 0.20 . 0.02 )m0 Refractive index at 300 K 2.29 n-type doping Substitutional O on N site and N vacancy P type doping C,Be,Mg,Zn,Cd 5A. 1.4 The applications of GaN As stated previously that GaN is very much important for its ability of emitting in the blue region of the visible spectrum hence has been used for the optoelectronic and electronic devices. GaN is also used in high pressure annealing up to 1600°C for modification of the material microstructure & chemical composition. It is used largely for high mobility transistors, which have been used widely for display l4, data storage and power amplifications , and high power microwave devices . The explosive increase in the AlGalnN family of materials, in recent years has been fueled by the application of blue / green / UV light emitting diodes in the full-colour displays traffic lights, automatic lighting & general room lighting using the so called white light16'17. In addition blue / green laser diodes are currently being used in high storage 1 o capacity DVDs. GaN based photo detectors are also useful for solar blind UV detection & have applications as flame sensors 19. The high band gap & small lattice constant of GaN is fairly well- lattice matched to SiC substrates, which have the concomitant advantages of durability & high thermal conductivity relative to the more commonly used AI2O3 substrate . There is, currently, a lot of interest in the science & potential technical applications of spin transport electronics (or spintronics) in which the spin of charge carriers (electrons / holes) is exploited to provide new functionality for micro electronic devices 2J"23. The phenomenon of giant magneto resistance & tunneling magneto resistance have been exploited in all metal - insulator-metal magnetic sensors, magnetic systems for read & write heads in computer hard devices, magnetic sensors, and magnetic random access memories (MRAM) etc. Chapter 5:Plasma Nitr. 112 5A.1.5 Different Techniques of synthesizing GaN i) Chemical process There are many techniques to grow GaN thin films but the oldest reported technique to grow GaN was by Halide Vapor Phase Epitaxy (HVPE) and the nitrogen precursor was ammonia. The metallic Ga was converted into GaN keeping Ga in an NH3 stream at elevated temperatures. The reaction is as follows 2Ga + 2NH3 -» 2GaN + 3H2 ...(5.1) According to the other reports, it is possible to make GaN by the reaction given below GaCl + NH3 •* GaN + HC1 + H2. ... (5.2) Using this method, the first single crystal GaN thin films were realized. The growth rate was quite high which allows making extremely thick films, whose properties were less influenced by thermal & lattice mismatches with the substrate 16. ii) Physical process In this case, a GaAs film which has been cleaned with a proper sequential etching process is used for nitridation process. When this GaAs faces the nitrogen ions made within a plasma chamber, GaN is formed on the surface of the GaAs. The process can be represented as given below GaxAsx+N2+H2 -» GaxAsx.y+yN+H+H -» GaN + GaAs ... (5.3) The nitridation process occurs when the nitrogen ions are accelerated by the electric field towards the GaAs surface. iii) Nitridation of porous GaAs Porous semiconductors (Si, Ge, InP, GaP, GaAs, GaN, etc.) have attracted much attention recently since they allow one to engineer optical properties in a relatively simple way ' . Such materials are usually formed by electrochemical etching of the nonporous semiconductors in some special electrolytes.

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