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CHAPTER 1

GALLIUM NITRIDE (GaN) : PROPERTIES, GROWTH

AND APPLICATIONS – A REVIEW

1.1 INTRODUCTION

In recent years, III nitride semiconductors have several advantages

over other wide bandgap semiconductors such as SiC and diamond. They can

be doped either as p- or as n – type. They can be grown epitaxially over large

number of substrates and monocrystalline layers. This has arisen from the

need for electronic devices capable of operation at high-temperatures,

especially emitters, which are active in the blue and ultraviolet (UV)

wavelengths. Aluminium nitride (AlN), Gallium nitride (GaN), Indium

nitride (InN) and their alloys, have direct band gaps that cover the entire

visible range from 1.1 eV for InN, 3.39 eV for GaN and 6.28 eV for AlN

(Strite and Morkoc 1992), therefore covering a part of the electromagnetic

spectrum that is not covered by conventional semiconductor technology.

Among group III nitrides, GaN is a promising material because of

its band gap of 3.4 eV which makes it the best candidate for devices operating

in the blue or UV part of the spectrum. GaN has a high chemical stability at

elevated temperatures (up to about 1173 K) and is suitable for caustic

environments, but also presents a technological challenge in device

manufacturing due to its high stability. The achievement of p-type doping in

GaN has led to excellent p-n junction LEDs and by alloying with AlN and

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InN, light in the violet, blue, green and yellow bands of the visible spectrum

has been obtained.

Numerous review papers (Strite et al 1993; Monemar 1998; Pearton

et al 1999; Jain et al 2000) are available concerning III-nitride growth,

properties and their applications. Table 1.1 shows the properties of group III

nitrides.

Table 1.1 Properties of group III nitrides

Properties AlN GaN InN

Band gap energy

Eg (eV) at 300 K 6.2 3.4 1.89

Eg (eV) at 5 K 6.28 3.5 --------

Temperature coefficient dE/dT (eV K-1)

-----

-6.010-4

-1.810-4

Pressure coefficient dE/dP (eVK-1)

------ 4.210-3 --------

Lattice constants

a (Å) 3.112 3.189 3.548

c (Å) 4.982 5.185 5.760

Thermal

expansion

∆a/a (K-1) 4.210-6 3.17 10-6 --------

∆c/c (K-1) 5.310-6 5.59 10-6 ---------

Thermal conductivity (W cm-1K-1) 2.85 1.3 ----------

Index of refraction η 2.15 2.33–2.67 2.80 – 3.05

Dielectric constant (εr) 8.5 ~9 --------

This chapter briefly describes about crystal structure,

physical/chemical properties and growth aspects of GaN and its important

applications as well.

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1.1.1 Crystal Structure of GaN

The chemical bond of group III elements with N is predominantly

covalent, that is, the constituents develop four tetrahedral bonds for each

atom. Because of the large differences in electronegativity of the two

constituents, there is a significant ionic contribution to the bond, which

determines the stability of the respective phase. GaN is known to occur in the

wurtzite as well as the zinc blende crystal structure. Such polytypism is

common in wide bandgap semiconductors. The equilibrium structure is

thought to be the wurtzite structure based on experimental results. Both

crystal structures are shown in Figure 1.1.

Figure 1.1 Crystal structures of III-nitrides (a) wurtzite and

(b) zincblende

(i) Wurtzite structure

The III-nitrides crystallise mainly in wurtzite structure, which is the

thermodynamically stable structure at ambient conditions. The wurtzite

Ga or N N or Ga (a) (b)

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structure has a hexagonal unit cell and thus two lattice constants, a and c. It

contains six atoms of each type. The space group for the wurtzite structure is

P63mc (C46V). This structure consists of two interpenetrating hexagonal close

packed (HCP) sub lattices, each with one type of atom, offset along the c axis

by 5/8 of the cell height (5/8 c).

(ii) Zincblende Structure

The zincblende structure has a cubic unit cell, containing four

group III atoms and four nitrogen atoms. The space group for this structure is

T2d (F43M). The position of the atoms within the unit cell is identical to the

diamond crystal structure. Both structures consist of two interpenetrating

face-centered cubic sublattices, offset by one quarter of the distance along a

body diagonal. Each atom in the structure may be viewed as positioned at the

center of a tetrahedron, with its four nearest neighbours defining the four

corners of the tetrahedron.

The wurtzite and zincblende structures are similar. In both cases,

each group III atom is coordinated by four nitrogen atoms. Conversely, each

nitrogen atom is coordinated by four group III atoms. The main difference

between these two structures is in the stacking sequence of closest packed

diatomic planes. For the wurtzite structure, the stacking sequence of (0001)

planes is ABABAB in the <0001> direction. For the zincblende structure, the

stacking sequence of (111) plane is ABCABC in the <111> direction. It is

expected that the properties of cubic GaN would be very similar to those of

wurtzite GaN, except that cubic GaN would have lower phonon scattering

(due to its being isotropic) and different bandgap as well as impurity levels.

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1.2 PROPERTIES OF GaN

The majority of GaN researchers are currently interested in

semiconductor device applications. Conventional wet etching techniques used

in the semiconductor processing are not successful to a great extent for GaN

device technology development. To overcome this problem, well-established

chemical etching processes are essential. Promising possibilities are various

dry etching processes, which have recently been developed and reviewed by

Pearton et al (1999) and by Mohammad et al (2000). The basic properties of

gallium nitride are given in the Table 1.2.

Table 1.2 Basic Properties of GaN

Properties Wurtzite

GaN Zincblende

GaN

Density g cm-3 6.15

Band gap energy

Eg (eV) at 300 K 3.4 3.2-3.3

Eg (eV) at 5 K 3.5

Temperature coefficient dE/dT (eV K-1) -6.0x10-4

Pressure coefficient dE/dP (eV kbar-1) 4.2x10-3

Lattice constants

a (Å) 3.189 4.503

c (Å) 5.185

Thermal expansion

a/a (K-1) 5.59x10-6

c/c (K-1) 3.17x10-6

Thermal conductivity (W cm-1 K-1) 1.3

Index of refraction (n) 2.33 (1 eV)

2.67 (3.38 eV) 2.5 (3 eV)

Dielectric constant (r) ~ 9

Optical phonon energy (meV) 91.8 91.9

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The lattice parameters of wurtzite GaN are a = 3.189 Å. and

c = 5.185 Å. However, for zincblende, the calculated lattice constant based on

measured bond distance in wurtzite GaN is a = 4.503 Å. Regarding structural

properties, a group theory approach predicts the optical phonon modes of

WZ-GaN contain one A1 mode, one E1 mode, two E2 modes, and B1 modes

whose wave numbers have been measured by Raman Scattering.

1.3 GROWTH OF GALLIUM NITRIDE

1.3.1 Bulk growth

Several attempts were made to synthesize GaN crystals during the

period 1930-1960 but good quality crystals of reasonable size could not be

grown. It is very difficult to employ the well-known growth methods such as

Bridgman or Czochralski for nitride crystal growth on account of the extremely

high melting temperatures and nitrogen vapour pressures required. The two

techniques available so far for bulk GaN growth are high-temperature, high

pressure solution growth (Porowski and Grzegory 1997) and sublimation. Early

attempts at growing bulk GaN single crystals met with limited success and

small crystal size, typically of the order of a few millimeters. Later, Zetterstrom

(1970) was able to increase the crystal size to 1-2 mm platelets by heating pre-

synthesized GaN crystals in an ammonia ambient at temperatures between

1423–1453 K. Karpinski et al (1984a) have grown bulk GaN single crystals

using high nitrogen ambient pressures and reported that a nitrogen pressure of

20 kbar at the temperature of 1873 K are the best conditions for GaN

crystallization from the melt. Recently, novel techniques have been developed

to grow relatively large single crystals of GaN at temperatures below the

melting point of GaN (Porowski et al 1997). Given the unusually high

temperatures and pressures for bulk GaN crystal growth, many groups have

pursued the heteroepitaxial thin film approach to obtain high quality single

crystals. From then onwards, many research groups have demonstrated the

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growth of GaN powders through various synthetic methods (Balkas and Davis

1996). GaN powders were synthesized by injecting NH3 into molten Ga under

atmospheric pressure in the temperature range of 1173–1253 K (Shibata

1999). Senthil Kumar et al (2000) reported the X-ray diffraction spectra

(XRD), Raman studies and X–ray photoelectron spectroscopy studies of

synthesized GaN material. The study of bulk high-quality freestanding GaN

single crystals has been slowed due to extremely high processing

temperatures and pressures involved in fabricating such crystals. Growth of

GaN epitaxial layer using hydride VPE (HVPE) was reported and

subsequently, thick layers have been grown using this method. With the

increased study of the nitrides, researchers have been able to develop the

growth processes to produce high-quality GaN thin films for device

applications. The fabrication of large-area bulk crystals has also been refined

over the past two years that motivated a call for obtaining GaN substrate.

There is a lattice mismatch as well as thermal mismatch between

the III-nitrides and the substrates on which they are grown. Due to lattice

mismatch, good epilayers could not be obtained. The interest in the growth of

III-nitride epilayers was revived by Amano et al (1988) and Dam et al (2006).

Metal organic vapour phase epitaxy (MOVPE) growth of GaN epilayers on

sapphire substrate is facilitated by first depositing a thin AlN buffer layer at

low temperature (Akasaki and Amano 1994).

1.3.2 Epitaxial Growth Techniques

The growth of epitaxial layers of III nitrides and alloys using either

MOCVD, MBE and HVPE has been reported by several authors (Walker et al

1997; Minsky et al 1998; Nasser et al 2001; Lee and Harris 1996; Topf et al

(1998), Attolini et al (2000) have grown GaN by HVPE process.

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Hydride Vapour Phase Epitaxy (HVPE) (Molnar et al 1997; Fornari

et al 2001), Metal Organic Chemical Vapour Disposition (MOCVD) (Amano

et al 1988; Nakamura et al 1991; Hemmingsson et al 2007) and Molecular

Beam Epitaxy (MBE) (Markus et al 1997; Ng et al 1998; Lang et al 2005) are

some of the methods used for GaN epitaxial growth. GaN native substrates

are not available in high quality and large quantity for homoepitaxial growth

and hence, heteroepitaxial growth is widely adopted for GaN. As the melting

point of GaN is very high, the growth of GaN crystals from a liquid melt is

extremely difficult and therefore, GaN is normally grown from vapour phase

containing an equilibrium mixture of nitrogen and Ga containing gas.

1.3.2.1 Hydride Vapour Phase Epitaxy (HVPE)

HVPE is one of the techniques used for GaN growth as it offers

high growth rate and bulk GaN films. It is a very attractive technique for the

production of high quality, large diameter and thick GaN layers (> 200 m in

thickness), which are used as free standing substrates. So far free standing

GaN layers with thickness of 230 m and area up to 10 mm2 have been

achieved. In a typical HVPE system designed for deposition, the precursors

are ammonia and gallium chloride (GaCl), while the carrier gas can be either

nitrogen or hydrogen. In order to provide GaCl, pure HCl in gaseous mixture

with H2 is injected separately into the reactor and put in contact for a

sufficiently long period with liquid gallium at around with 1123 K. At this

temperature, within a few seconds, the acid reacts completely with Ga and

gives GaCl plus a negligible amount of GaCl3. At this point, ammonia and

GaCl, still separated, are transported in the deposition zone of the reactor

where the substrate is kept in the range of 1173–1373 K temperature. The

schematic diagram of the HVPE system is shown in Figure 1.2.

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Figure 1.2 Schematic representation of HVPE technique

The type of chemical reaction which occurs in the HVPE growth is

as follows:

GaCl + NH3 GaN+ HCl+ H2 (1.1)

From this reaction it is clear that, since one of the products of the

reaction is H2, the growth rate will be lower when hydrogen is used as carrier

gas instead of nitrogen. The high growth rate is indeed one of the most

important features of this technique as it may allow the preparation of very

thick layers, which can be employed as self-standing substrates for

subsequent re-growth.

1.3.2.2 Chloride Vapour Phase Epitaxy (Cl-VPE)

Figure 1.3 shows the schematic diagram of the Cl-VPE system

(Varadarajan et al 2004). The reaction chamber is made up of a quartz tube

and is kept inside a single zone resistively heated furnace. The GaCl3 cell

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NH3 gas

N2 Gas

Single Zone Furnace Reactor Tube

Substrate holder

Exhaust

SV

SV

SV

MFC

MFC NH3 Purifier

GaCl3 Cell

Assembly

assembly is made up of a quartz tube and the crystalline GaCl3 in the cell is

melted in a liquid paraffin bath. The GaCl3 vapor was transported to the

reactor by the nitrogen carrier gas. The purifier system consists of molecular

sieves to remove moisture, carbon monoxide and carbon dioxide from the

ammonia gas. The outlet of the reactor is connected to dilute H2SO4 to

neutralize the unreacted ammonia gas species. After loading the substrate, the

reactor was purged with nitrogen gas.

Figure 1.3 Experimental setup of Cl-VPE system for the growth of GaN

The furnace was heated to the desired temperature of 1263 K and

GaCl3 was supplied to Thu Vj3 a[[;oed the reactor using nitrogen carrier gas.

The reaction of the growth processes is given by the following equation

GaCl3 + NH3 → GaN + 3HCl (1.2)

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The typical N2 flow rate for GaCl3 transport was 100 sccm. During

the processes 3.0 slm of NH3 and 2.0 slm of N2 were introduced in the reactor.

1.3.2.3 Metal Organic Chemical Vapour Deposition (MOCVD)

MOCVD is a process for the deposition of a material that utilizes

volatile metalorganic compounds to transport metallic atoms that are

relatively nonvolatile at the convenient deposition temperature. The

organometallic compounds are usually mixed with other source materials

“Hydrides” that react to form compound semiconductor film.

Growth of GaN can be performed by introducing trimethyl gallium

(TMGa) and ammonia (NH3) simultaneously into the reaction chamber loaded

with a substrate, such as sapphire and heated to elevated temperatures (usually

1073 –1273 K)

(CH3)3Ga (g)+ NH3(g) GaN (s) + 3 (CH3)H (1.3)

Ternary compounds such as AlGaN and InGaN can be obtained by

combining TMAl or TMIn simultaneously with TMGa as described in

equation (1.3). Adjusting the gas phase composition of the TMAl and TMGa

or the TMIn and TMGa controls the solid composition.

The organometallics are transported to the heated substrate by

passing the carrier gas, usually H2 or N2, through the organometallic which is

kept in a bubbler vessel at a controlled temperature to keep the compound in

molten state. The growth temperature is the important parameter to decide the

chemical reaction between the substrate and the reaction zone. Schematic

representation of the MOCVD instrument is shown in Figure 1.4.

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Figure 1.4 Schematic representation of MOCVD technique

1.3.2.4 Molecular beam epitaxy (MBE)

Producing high quality layers with sharp abrupt interfaces and good

control of thickness, doping and different compositions are effectively

realised by the MBE technique. Conventional MBE cannot be used to grow

GaN, because N2 cannot be dissociated by using conventional effusion cells.

Plasma source (RF or electron cyclotron resonance plasma) is used to activate

N2. Ga vapour beam from an effusion cell and the activated nitrogen are

directed towards the heated substrate forming GaN film. In addition to Ga, a

MBE chamber can be equipped with Si and Mg cells for n-and p-type doping

and In and Al for InGaN and AlGaN layer growth. Ammonia or hydrazine is

another source of activated nitrogen that can be cracked into atomic nitrogen

in high temperature cracking cell. Hydrazine can be decomposed at relatively

lower temperatures than ammonia but its extreme reactivity makes it

dangerous to handle so that it cannot replace ammonia as a source for

nitrogen. The schematic picture of the MBE system is shown in Figure 1.5.

Table 1.2 compares the advantages and disadvantages of MOCVD, MBE and

HVPE techniques.

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Figure 1.5 A typical MBE system configured with ammonia injector

Table 1.3 Comparison between MOCVD, MBE and HVPE

Growth Technology Advantage

Disadvantage

MOCVD

1. Atomically sharp interfaces

2. In-situ thickness monitoring

3. Very high quality film 4. High throughput

1. Lack of in–situ characterisation

2. Large quantities of NH3 are needed

3. P-type doping associated with Mg-H complex that need post growth process to activate doping process

MBE

1. Atomically sharp interfaces

2. In-situ characterisation 3. High purity growth 4. Hydrogen free

environment

1. Need ultra-high vacuum 2. Low growth rate

(1-1.5 um/h) 3. Low throughput 4. Very expensive

HVPE

1. Simple growth technique 2. Very high growth rates 3. Reasonably good quality

film 4. Bulk GaN

1. No sharp interfaces 2. Work in Hydrogen

environment 3. Extreme temperature

condition

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1.4 SUBSTRATES

Single crystalline III nitride films are grown heteroepitaxially on a

number of closely lattice-matched substrates.

1.4.1 Sapphire (Al2O3) and Silicon carbide (6H- SiC)

Sapphire is the most extensively used substrate material for the

growth of GaN. It is transparent, stable at high temperature and the

technology of growth of the nitrides on sapphire is quite mature. The lattice

mismatch between sapphire and GaN is ~13%. Though sapphire possesses a

substantially different lattice constant and thermal expansion coefficient from

the nitrides, it is most commonly used because of its good thermal stability

and hexagonal crystal structure. Good optical quality, availability of large size

and low cost also makes sapphire as a suitable substrate material. Nitridation

of sapphire substrate prior to AlN buffer layer growth provides the best

quality GaN layer.

Currently, Silicon Carbide (SiC) materials system is challenging the

GaN/sapphire system in both the optoelectronic and electronic arena.

(Matsushita et al 1995 and Sriram et al 1996). SiC offers a higher electrical

and thermal conductivity compared to sapphire and is available in the

hexagonal crystal structure. Its lattice mismatch with GaN is only 3.5 %, and

with AlN, the mismatch is very small. Despite these advantages, SiC suffers

from being substantially more expensive compared to sapphire. The

prohibitive cost of using SiC has limited its usefulness and availability to only

a small number of research groups.

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1.4.2 Other Substrate Materials

In spite of rapid improvements in the growth and materials quality

of GaN on sapphire, a variety of other substrate materials have also been

investigated. In the early stages, many researchers explored the growth of

GaN on GaAs substrates in order to obtain the metastable zincblende phase of

GaN (Okumura et al 1991; Kikuchi et al 1994). Other substrates, which were

used in order to achieve the zincblende GaN phase include MgAl2O3 (Powel

et al 1990) and Si (Lei et al 1991).

1.4.3 Buffer Layers

The heteroepitaxial growth of GaN thin films typically uses various

techniques to minimize the generated defects during deposition. These

approaches focus primarily on optimizing a buffer layer between the main

GaN epilayer and sapphire substrate. AlN buffer layers, (Amano et al 1986)

and later low-temperature GaN buffer layers (Nakamura 1991) of thickness

ranging between 50-100 nm, were recognized as a way to relieve the stress

and associated defects in GaN thin films grown on large lattice-mismatched

substrates. The buffer layer acted as a template to supply nucleation sites for

growth of the GaN epilayer.

This buffer relieves some of the large lattice mismatch that is

detrimental to achieving high-quality films. The layer also effectively absorbs

the stress created by the lattice and thermal expansion coefficient mismatch

through the generation of extended defects such as dislocations and stacking

faults. The creation of these defects helps in eliminating the significant stress

build up within the main GaN epilayer.

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1.5 ALLOYS AND HETEROSTRUCTURES OF GaN

By alloying AlN, GaN and InN, which is critical for making high

efficiency visible light sources and detectors. InGaAlN semiconductor alloy is

the promising material for the active layer of double heterostructure LED’s

and LDs. AlGaN can have bandgap ranging from 3.4 eV to 6.2 eV. The

compositional dependence of the lattice constant, the direct energy gap,

electrical and luminescence properties of the AlGaN alloys have been studied

(Yoshida et al 1982). Most semiconductor devices are optimised by

heterojunctions, which are commonly achieved through the use of alloys.

Many of the properties such as band gap energy, effective mass of electrons

and holes and dielectric constant are dependent on alloy compositions.

Of the lattice constant, the direct energy gap, electrical and

luminescence properties of the AlGaN alloys have been studied. The lattice

mismatch between AlN and GaN is 2.4%. InGaN can have a direct bandgap

ranging between 0.8 eV and 3.43 eV. The ternary alloy InxGa1-xN grown on

GaN is an important semiconductor heterostructure having wide applications

in the fabrication of lasers and optoelectronic devices. InxGa1-xN epitaxially

grown films with x < 0.1 have been well studied due to their potential

applications as blue emitters. The quaternary InGaAlN system is a wide direct

banad gap semiconductor and having wurtzite structure. By varying the In,

Ga, and Al mole fractions, this material allows the construction of a double

heterostructure (DH structure) even when each layer is lattice-matched

(Matsuoka et al 1992). This structure is also needed for semiconductor laser

diodes in shorter than blue wavelength regions, so this material system is

promising for high performance devices.

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1.6 GALLIUM NITRIDE NANOSTRUCTURES

Nanostructured semiconducting materials have had a monumental

impact on our society. They are the foundation of modern technology and the

basis for the entire information revolution. In recent years, group III–V

compound semiconductors are the choice for many optoelectronic and high

speed electronic applications mainly due to their direct band gap, high

saturation velocity and high electron mobility. GaN nanostructures have

attracted much attention because of their potential for near visible and UV

optoelectronic applications. Synthesis of GaN nanowires has been achieved

by various techniques such as laser ablation (Duan and Lieber 2000), carbon–

nanotube – confined reaction (Han et al 1997) etc. In particular, a large

number of crystalline GaN nanowires have been obtained by a catalytic

synthesis based on a vapour-liquid-solid (VLS) growth mechanism.

Srivastava et al (2005) have reported on the XRD and AFM results of GaN

nanowires grown by chemical vapour transport technique.

1.7 DOPING ISSUES OF GaN

Unintentionally doped GaN is the foremost problem in growing

device quality material. Most material grown so far has been n-type with

carrier concentrations typically in the range from 1016 to 1019 cm-3. Since no

impurities have so far been observed to account for such a large background

carrier concentrations it is thought to be due to nitrogen vacancies. To

overcome this problem, many groups tried to compensate with shallow p-type

dopants. Most potential dopants that were tried produced a highly resistive

material, but none seemed to generate p-type material. Nevertheless, doping

GaN p-type was finally accomplished by Amano et al (1989) using Mg to

grow compensated GaN and converting it to p-type material with low energy

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electron beam irradiation (LEEBI). Later, Nakamura et al (1992) improved

those results and were the first to report on the doping mechanism.

1.7.1 Transition metals doping in GaN

The possibility of using electrons' spins in addition to their charge

in information technology has created much enthusiasm for a new field of

electronics popularly known as "spintronics." An intensely studied approach

to obtaining spin-polarized carriers for data-storage devices is the use of

diluted magnetic semiconductors created by doping ions like Mn, Fe, or Co

having a net spin into a semiconducting host such as GaAs, ZnO, or GaN. The

interaction among these spins leads to ferromagnetic order at low

temperatures, which is necessary to create spin-polarized carriers (Figure 1.7).

The formation of magnetic semiconductors is of wide spread

interest due to the possibility of making highly efficient spin injectors. Many

research groups have demonstrated RT ferromagnetism in a variety of

wide band gap doped semiconductors, such as TiO2, ZnO and AlN

(Matsumoto et al 2001). Reed et al (2001) and Sato et al (2001) reported various

magnetic properties ranging from spin-glass-like to ferromagnetic for GaN

incorporating various concentrations of Cr, Co, Fe, Mn and Ni based on a

local spin-density approximation which assumed that Ga atoms were

randomly substituted with the magnetic atoms and did not take into account

any additional carrier doping effects.

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Figure 1.6 Types of semiconductors: (a) nonmagnetic, (b) Diluted

magnetic semiconductor and (c) DMS with ferromagnetic

order mediated by charge carriers

Thaler et al (2002) and Kane et al (2005) reported the magnetic and

optical properties of Ga1-xMnxN grown by metal organic chemical vapour

deposition (MOCVD) technique. Manganese doped GaN nanowires have

been of great interest recently because it was predicted that GaMnN solid

solutions might be magnetic semiconductors with a curie temperature above

RT. Recently Choi et al (2005) reported the single crystalline magnetic Mn

doped GaN nanowires have been grown without the extrinsic phase formation

in a controlled manner via a unique chloride – based chemical vapour

transport method.

1.7.2 Rare earth doping in GaN

The doping of nitrides with rare earth ion may serve as an

alternative to transition metal doping with the possibility of room temperature

ferromagnetism mediated either by carriers or by conduction through bands

induced by the rare earth ions. Rare-earth (RE) element doping of the wide

band gap semiconductors has recently attracted great attention, as light

emission extending from the infrared to the blue emission. Recently

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ferromagnetism with Tc above 300 K has been observed in very dilute

Gadolinium (Gd) doped GaN (Dhar et al 2006).

1.8 APPLICATIONS OF GaN

The successful development of a method for the production of

p-type GaN has resulted in the fabrication of blue light emitting diodes and

various microelectronics devices. With the fabrication of efficient blue LED's

it is now feasible to work on full color LED flat panel displays as well. The

main industries profiting from such a display are TV and computer

manufacturers. Flat panel displays could replace bulky CRT tubes and permit

large light-weight displays with superior resolution. This becomes especially

important with the current development efforts of high resolution television.

The practicability of today's laptop computers is mainly limited by their

operating time on a battery charge. Tremendous efforts are made by laptop

manufacturers to minimize the power consumption. Current full color

displays are the main user of the precious energy and with the introduction of

efficient LED screens the battery lifetime can be extended dramatically.

The great achievement as result of high brightness blue and green

LEDs is the fabrication of white LEDs. Traffic lights may prove to be a fertile

application for the blue/green GaN LEDs. Traffic lights using InGaN/AlGaN

blue-green LEDs promise to save vast amounts of energy. By combining high

power and high brightness blue,green and red LDs many kinds of applications

such as LED full coulour displays and LED white lamp are now possible with

the benefits of high reliability, high durability and low energy consumption.

Blue/violet LDs with lifetime of over 10,000 h were successfully

demonstrated (Nakamura et al 1997) and consequently commercialized.

Implementation of blue LDs into CD/DVD read/write systems in computers

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had increased the data storage capacity by five times. This enhancement will

become ever larger when using UV LDs instead.

Several high performance electronic devices such as Schottky

diodes, JFETS, HFETS, MESFETS and MOSFETS with promising results

have been demonstrated based on GaN and its alloys (Asif Khan et al 1993;

Carrano et al 1998). UV devices could also take advantage of the properties of

the earth's ozone layer, which nearly completely absorbs radiation in

the 240-280 nm (~4.75 eV) band. This would permit space to space

communication undetectable from earth and surveillance of objects leaving

the atmosphere on the dark side of the earth. Like most wide band gap

semiconductors, the nitrides are expected to exhibit superior radiation

hardness compared to GaAs and Si, which also makes them attractive for

space applications.

1.9 ION IRRADIATION: A REVIEW

Most of the interesting properties of matter in the solid state are

related to the presence of defects and impurities. The imperfections in solids

differentiate the real solids from an ideal crystalline structure. In

semiconductors the defects are introduced due to either thermodynamic

considerations or the presence of impurities during the crystal growth process.

The defects can also be introduced in the crystalline semiconductors by ion

irradiation. Ion beam processing of semiconductors is an integral part of

device fabrication process in the semiconductor industry. The defects in

perfect solids may be considered as a perturbation in the local symmetry

around the atoms. In general, defects in crystalline semiconductors can be

characterized as point defects or extended defects. They can be due to

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chemical impurities, interstitials, vacancies, vacany-impurity complex or due

to internal surfaces, glide planes, dislocations and grain boundaries.

1.9.1 Energy loss and damage

After penetrating inside materials, ions lose their kinetic energy

through collisions with (1) nuclei, (2) bound electrons of the target atoms and

(3) free electrons inside the target material. Ions penetrate some distance

inside the material until they stop. In case of high energy ions, the ions slow

down mainly by electronic energy loss in the beginning of the slowing down

process and they move atoms in a straight path. When the ions are slowed

down sufficiently, the collision with nuclei (the nuclear stopping) become

more and more probable, and eventually ions are stopped by nuclear

scattering. When atoms of the solid receive significant recoil energies they are

removed from their lattice positions and produce a cascade of further

collisions in the material.

The calculation of range requires the knowledge of the rate of

energy loss of ions. According to the classical scattering theory, the

interaction of the moving ions with the target atoms is described assuming

two separate processes, collisions with nuclei and collision with electrons.

The former is due to the coulomb repulsion between the ion and the target

nuclei. The nuclear stopping component is usually considered separately

because the heavy recoiling target nucleus can be assumed to be unconnected

from its lattice during the passage of the ion. The elastic recoil energy

transferred to it can be treated simply as the elastic scattering to two heavily

screened particles. Excitations or ionization of electrons are only a source of

energy loss and do not influence the collision geometry. This is justified if the

energy transferred to the electrons is small compared to the exchange of

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kinetic energy between the atoms, a condition usually fulfilled in ion

implantation. The ion is thus deflected by nuclear encounters and

continuously loses energy to the electrons.

1.9.2 Ion beam and Semiconductors

Ion beam irradiation can play an important role in the study of

defects related to semiconductors, as controlled amount of defects can be

introduced by selecting suitable irradiation parameters. Defects like point

defects or extended defects can also be selectively introduced by proper

choice of ion mass and energy of the irradiation ions.

Ion beams can be classified into three broad groups on the basis of

energy and mass of the irradiated ions.

1.9.2.1 Irradiation with low energy ions

Low energy ions in the range of 50 keV to 500 keV are extensively

used for doping in semiconductors. Low energy light ions produce point

defects and heavy ions produce extended defects in materials but range of

both light and heavy ions is very low as compared to that in case of high

energy ions.

For the last several decades, low energy ion beams have been used

extensively for the purpose of ion implantation. Ion implantation is an

essential process for the production of modern devices and integrated circuits

(IC) based on Si and compound semiconductors. In the case of III-V

compound semiconductors, there are two important applications. The first is

the implantation of dopants to create n-or p-type III-V semiconductors. The

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second is the implantation of ions like H, He, B, O, etc to convert a doped

layer to a highly resistive one through defect engineering (Ahmed et al 2001).

Ion beam interaction in semiconductors with ions having energy, such that

during slowing down, the energy of the ion is dominantly lost through nuclear

energy loss process is well understood. During slowing down, an ion interacts

inelastically with electrons and elastically with other target atoms. If the

kinetic energy, E1, transferred to the host atom is higher than the displacement

threshold energy Ed (~8eV for Ga and ~6eV for As), the knock-on atom

leaves its lattice site and depending on the residual kinetic energy, E1-Ed, they

can move for a certain path length. These atoms recoil and collide with other

atoms giving rise to further generation of collisions which produce many low-

energy recoils and induce small displacements in nearly random directions.

This sequence of collisions and displaced atom multiplication is often called

collision cascade and it lasts for ~10-13 seconds, that is the ion range divided

by the average ion velocity.

This fast process is followed by a redistribution of the energy into

surrounding material by both lattice and electron conduction, and lasts for an

additional time interval of 10-11 seconds, resulting in local thermal

equilibrium. In the following 10-9 seconds, the unstable disorder relaxes and

some ordering occurs by a local diffusion process and the system attains total

thermal equilibrium.

1.9.2.2 Irradiation with swift heavy ions (SHI)

1.9.2.2.1 Irradiation with high energy light ions (HELI)

These ions are most suitable for defect engineering, because the

point defects produced by these ions are almost uniformly distributed within

deep inside the sample and the ions get implanted at a depth of more than

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100 µm. This excludes the possibility of any interferences from implanted

ions in modifying the material properties. In some cases, the samples can be

made thinner than the range of ions where the ions pass through the samples

or samples may be grown on substrates such that the range of the ions is more

than the film thickness. In contrary to interaction of swift heavy ion (SHI)

where electronic energy loss above threshold causes track formation, the

damage accumulation by high energy light ions (HELI) is largely due to the

nuclear energy loss (Kamarou et al 2005). The HELI irradiation produces

point defects due to nuclear energy loss in the samples and can be estimated

using Stopping Range of Ions in Matter (SRIM) calculation. Moreover, the

electronic energy loss of HELI is very high compared to the nuclear energy

loss but much less than the threshold energy for track formation, which can be

uniquely utilized for defect engineering and material modification through

ionization of native defects.

1.9.2.2.2 Irradiation with high energy heavy ions (HEHI)

During slowing down process, low energy ions lose energy through

nuclear energy loss process. But in the case of high energy ions the electronic

energy loss dominates over nuclear energy loss. Heavy ions lead to extremely

strong electronic excitations inside a narrow cylinder around each ion path.

The initial interaction processes of the energy transfer from a high energy

heavy ion to electrons bound to inner shells take only 10-19 to 10-17 s and

slightly longer for collective electronic excitations like formation of plasmons

(Schiwietz 2004). Hence, just after the passage of the SHI, the narrow

cylindrical target zone coaxial with the ion path consists of a two component

plasma of cold lattice atom and hot electrons. Such a narrow region is often

called ionization spike.

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1.10 SCOPE OF THE PRESENT INVESTIGATIONS

In the last two (2006-2007) years GaN nanostructures and related

compounds have emerged as the key materials for applications in LEDs, LDs,

UV detectors and high temperature - high power electronics. There has been a

substantial worldwide interest in the development of GaN based low noise

circuit elements, particularly transistors with superior characteristics. While

the research on the current issue moves towards the realisation of better

quality layers, the fundamental material studies are still in progress with many

issues not fully resolved or understood. The aim of the present investigations

is to study GaN nanostructures and Mn doped GaN nanostructures and

irradiation effects on MOCVD grown GaN epilayers.