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CHAPTER 3

EPITAXIAL GROWTH AND CHARACTERIZATION OF

ALUMINUM NITRIDE GROWN BY METAL ORGANIC

CHEMICAL VAPOR DEPOSITION

3.1 INTRODUCTION

Deep ultra violet (DUV) light emitting diodes (LEDs) and laser

diodes (LDs) are revolutionizing the field of optoelectronics with applications

in high density optical data storage, biomedicine, water and air purification

(Schubert et al 2005, Taniyasu et al 2006 and Khan et al 2005). Aluminum

nitride (AlN) and aluminum gallium nitride (AlGaN) are the potential

materials to realize the DUV light emission due to their direct wide band gaps

(Khan et al 2008a). Also, AlN possesses high thermal conductivity (3.2 W

cm-1 K-1) and high breakdown voltage (12 MV cm-1) essential for high-power

and high temperature electronic devices (Mishra et al 2002a and Eastman et al

2002]. In addition, AlN has minimal lattice mismatch to high Al content

AlGaN layers and excellent UV light transparency (Kueller et al 2012) which

makes it a suitable buffer layer for DUV LED and LD device structures.

Generally, AlN has been grown heteroepitaxially on sapphire,

silicon carbide (SiC) and silicon (Si) substrates due to the lack of native

substrates resulting in high densities of threading dislocations (TDs) (Bai et al

2006). Indeed, growth of AlN has many challenges, which have been

addressed by adopting exceptional approaches in the growth conditions. For

example, (i) high sticking coefficient of aluminum (Al) restricting the

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migration of Al adatoms has been wiped out by growing AlN at higher

growth temperatures above 1200 °C (Ohba et al 2000) and by using pulsed

growth or migration-enhanced metalorganic chemical vapor deposition

(MEMOCVD) system (Zhang et al 2002). (ii) Unwanted parasitic reactions

between Al and ammonia (NH3) have been reduced by allowing high flow of

hydrogen (H2) carrier gas in to the reactor with low reactor pressure (Reentila

et al 2008). (iii) Cracking of AlN layers due to thermal expansion mismatch

between AlN and sapphire substrate has been encountered by multi-growth

mode modification (Okada et al 2007a). (iv) Three dimensional (3D) island

growth of AlN with poor surface morphology due to the polarity changes of

AlN and its dependency with the pre-treatment of sapphire substrate has been

studied by Paduano et al (2004). It has been found that AlN with Al polarity

exhibits smoother surface and AlN with mixed polarity (Al and nitrogen) has

rougher surface morphology (Paduano et al 2004a, Jasinski et al 2003 and

Kumagai et al 2007a). Nevertheless, pre-flow of precursors before the

deposition of nucleation layers [NLs] (Reentila et al 2008), growth

temperature of AlN-NLs (Balaji et al 2012a), thickness of NLs, density and

size of nucleation islands (NIs) have remarkable effect on the quality of AlN

layers (Wang et al 2006, Fan et al 2010a and Wang et al 2012).

The effect of initial growth stages such as pre-flow of trimethyl

aluminum (TMAl) and ammonia (NH3) prior to NL growth, direct deposition

of NL, influence of NL growth temperatures and the role of NIs coalescence

on the surface morphology and structural quality of AlN layers have been

discussed.

3.2 METAL ORGANIC CHEMICAL VAPOR DEPOSITION

SYSTEM

Manasevit et al has proposed a new approach to the growth of III-V

semiconductors by using metal organic precursors (1969).

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This approach began the rapid development of metal organic

chemical vapor deposition (MOCVD) technique in the 1970s. Earlier

difficulties in achieving high purity and abrupt interfaces have now been

faded out. Today, MOCVD has become the most promising technique for

heteroepitaxial growth and for its ability of mass production with uniform

quality. In this chapter, AlN layers grown using Aixtron 200/4 RF-S MOCVD

sytem has been analyzed. The MOCVD system has been housed in the class

10,000 clean room. MOCVD system consists of electrical cabinet with server

computer, gas mixing cabinet with metal organic sources, reactor cabinet,

glove box, process pump, radio frequency generator, scrubber, ammonia and

silane gas cabinets, exhaust pipes and purging and carrier gases. Figure 3.1

shows the MOCVD system.

3.2.1 Electrical Cabinet

All the components in the MOCVD system have been interfaced

with the safety control system (SCS) located in the electronic cabinet. These

components can be operated by the server computer (mounted in the cabinet)

through the AIXACT software, by loading the recipes. In addition SCS

prevents the entire system and switches the system to a safe state if any

dangerous situation occurs. The SCS monitors all signals, valve positions,

sensors and analog limit values. It operates stand-alone and is superior to the

server computer.

Eurotherm temperature controller in the cabinet controls the reactor

temperature. Reactor pressure has been controlled by the pressure control unit

(PCU). The PCU consists of ‘MKS’ controller and two ‘Eurotherm’

controllers for run and vent line respectively. The MKS controller controls the

position of throttle valve and in turn the reactor pressure.

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Figure 3.2 Important components and electrical cabinet of MOCVD

a – Monitoring and status LED panel; b – Eurotherm temperature

controller and differential pressure controllers; c – MKS reactor

pressure controller; d – Luxtron touch screen monitor for in-situ

reflectance and process temperature; e – server computer;

f – Electrical cabinet.

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The run and vent lines of the metal organic and hydrides precursors

are controlled by active pressure balancing. To avoid a decrease of pressure

when switching a gas source from the vent line to the run line, a differential

pressure measurement has been carried out between both the lines. Figure 3.2

depicts the LEDs panel, controllers and computer located in the electrical

cabinet.

In the Figure 3.2(a) set of LEDs at the front side of cabinet, display

alarms and system status. The LEDs under the monitoring portion indicates

the major alarm. To run the system, all alarm had to be removed. The color of

LEDs disclose the state of MOCVD system. The green color of LEDs

indicates ‘no active alarms’, red color indicates ‘active alarm’ and orange

color indicates ‘no active alarm’ but needs to reset the SCS. The LEDs under

the status portion of the panel reveals only the status of precursors, water and

pressure valves. If the valves are open then the corresponding LED glows in

green color and the LED glows in red color for closed valves. On the back

side of the cabinet, the main switch has been situated. It should be turned off

for any electrical component related testing or maintenance.

3.2.2 Safety Measures

Growth of III-nitride materials in MOCVD involves pyrophoric

metal organic (MO) sources, highly inflammable hydrogen gas (carrier gas),

toxic silane and ammonia gas. Small amount of leakage in any of these gases

will lead to an unsafe situation. Hence, all the gas lines in the MOCVD

system, such as run and vent line of metal organics and hydrides, entire gas

mixing cabinet, reactor cabinet, silane and ammonia gas cabinet have been

periodically checked by helium gas leak detector. In addition to this, leak test

has been performed while installing the new sources (MO and Hydrides) and

replacing the empty bottles.

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Figure 3.3 depicts the helium leak detector. There are two modes

for leak detection (blowing and sniffing mode). In the former mode helium

gas is blown, all over the evacuated region of the MOCVD system and in the

latter mode the helium gas is fed in to the region of interest. Then the detector

checks for the helium gas by sniffing all over the region. The blowing mode is

the suitable method for checking the cabinets and the source lines of MOCVD

system, in which the gas lines have been connected with the detector by

suitable bellows, O-ring and with K-flanges. After that, the connected regions

are evacuated by the leak detector gradually.

Figure 3.3 Helium gas leak detector for checking the MO lines, gas

mixing cabinet and reactor cabinet

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Once the region has been evacuated to the level of 10-3 bar, helium

gas is blown externally all over the evacuated region. In the case of leakage,

the detector indicates the leak by giving a continuos buzzer in both the modes.

In order to handle process gases (MO, hydrides and hydrogen), in

an emergency or accident, MOCVD system has been equipped with exhaust

pipe lines, emergency off (red color) and process stop (yellow color) buttons.

Figure 3.4 shows the emergency and process stop buttons located in the glove

box, electrical and gas mixing cabinets. Pressing the EMERGENCY-OFF

button in the dangerous situation terminates the power supply of MOCVD

system. Pressing the PROCESS-STOP button leads to the following steps

All gas lines are purged with nitrogen

Throttle valve is closed

All hydrogen and metal organic source valves are closed

Process heater is turned off

Running recipe is terminated

Figure 3.4 Emergency off and process stop buttons in MOCVD system

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Figure 3.5 (a) Drager gas sensors controlling unit, (b) Gas sensor

display and (c) Hydrogen and ammonia sensors mounted on

the wall and exhaust pipes

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In addition to these safety buttons, ammonia, silane and hydrogen

gas sensors are mounted on the walls, exhaust pipes and gas cylinder rooms.

The gas sensors are highly sensitive. It can deduct the gases in the order of

parts per million (ppm). Alarm levels of all the gases are 30 ppm. If any gas

level exceeds this value, the Drager sensor controller automatically triggers

the alarm and aborts the recipe. Figure 3.5 depicts the Drager sensors

controlling unit, display of the gas values, ammonia and hydrogen sensor

mounted on the clean room and exhaust pipes.

3.2.3 Gas Mixing Cabinet

The gas mixing cabinet (GMC) consists of a gas pipe system, gas

purifiers, metal organic sources and control components like

Valves (manual, pneumatic 2/2, 3/2, 4/2, 5/2 valves)

Mass flow controllers (MFCs) for controlling the gas flows

Pressure controllers (PCs) for controlling the source pressures

Figure 3.6 shows the baths with thermostat and MO bubbler. In the

front side of the GMC, four MO baths are located, two trimethyl gallium

(TMGa), one trimethyl aluminum (TMAl) and one bis-cyclopentadienyl

magnesium (Cp2Mg). On the rear side of the GMC, one trimethyl indium

(TMIn) and one triethyl gallium (TEGa) are situated. Hydrogen gas has been

used as the carrier gas in the front side of the MO bubblers. Cp2Mg is the

precursor of p-type dopant magnesium in III-nitrides. Silage is used as a

precursor for n-type doping in III-nitirdes.

Consequently, nitrogen gas has been used as the carrier gas for the

TMIn and TEGa MO bubblers. These precursors are used mainly for

InGaN/GaN based multi-quantum wells (MQWs) structures utilized in the

visible LED device structures.

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Figure 3.6 Metal organic bubblers (a) TMGa bath, (b) TMAl bath, and

(c) Mo bubbler kept inside a bath

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Normally, InGaN layers are grown in slightly lower growth

temperature (700 – 800 °C) for better indium incorporation than GaN layers

(~ 1000 °C). In the InGaN based MQWs, TEGa has been used as the

precursor of gallium, in order to grow the GaN layer in the same temperature

regime as that of InGaN. The temperature of precursor baths have been set

corresponding to the vapor pressure of MO bubblers. TMGa has higher vapor

pressure than all the MOs. Thus the bath temperature of TMGa was set at 0.0

°C. On the other hand, all the other precursor’s bath temperature were set at

17.0 °C due to their lower vapor pressure. To maintain the set temperature in

the bubbler, coolant liquid has been used. In the MO bubbler, the inlet line

and out let line are distinguished by the height. The inlet line height of the

bubbler is a bit smaller than the outlet line.

Figure 3.7(a) depicts the mass flow controllers (MFCs), pressure

controllers (PCs), pneumatic and manual valves, interface between the

controller components and the server computer. Figure 3.7(b) reveals the run,

vent and purge gas lines presented inside the GMC. There are three run lines.

They are used to lead the process gas from the sources to the reactor chamber.

Hydride run line leads the ammonia gas to the reactor. MO1 run-line,

comprises TMGa, TMAl and Cp2Mg precursor in the front side of the GMC.

Those precursors are fed in to the reactor cabinet by hydrogen carrier gas.

MO2 run-line consists of TMIn and TEGa. Nitrogen carrier gas leads these

precursors to the reactor cabinet. Figure 3.8 shows the block diagram of MO1

and MO2 lines. For each run-line, there is a vent-line. They bypass the reactor

chamber and lead the process gases into the vacuum system. Switching the

process gas between the run and vent line has been done by a 5/2-way valve,

in which two states are possible

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Figure 3.7 Gas lines in MOCVD system (a) mass flow controllers

(MFCs), pressure controllers (PCs) and valves. (b) Run,

vent and purge lines

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If the 5/2-way valve is open, the process gas flows through the

run-line into the reactor. No process gas flows in the vent-line.

If the 5/2-way valve is closed, the process gas flows through

the vent-line and skips the reactor. Only the carrier gas flows

in the run-line.

Figure 3.8 Block diagram of MO1, MO2 and hydride sources lines to

the reactor cabinet

To avoid pressure compensation reactions during the switching

processes between run and vent lines, the pressure in the run/vent lines and in

the reactor chamber will be held at the same level. The mass flow controllers

control the process gas flow. The effective functioning of flow controllers

needs three bar pressure of nitrogen/hydrogen gas. In addition, GMC gas has

several purging lines for the reactor components.

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Figure 3.9 Hydrogen, nitrogen and ammonia gas purifiers present in

the rear side of gas mixing cabinet

Figure 3.9 shows the gas purifiers for hydrogen, nitrogen and

ammonia. Gas contaminants cause process variability and poor quality of

material. The purity of the gases are inevitable for the quality of epitaxial

layers. These gas purifiers use high surface area adsorbents to remove oxygen

and carbon impurities to less than one part per billion (ppb). Impurities are

bonded to the surface of the reactive media through chemisorption

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processes. The high surface area structure of the media helps provide a large

lifetime, dependent on the inlet gas impurity levels and average flow rate. The

purifying material is a nickel mixture for hydrogen and nitrogen gases.

Manganese resinate for ammonia gas. These materials are regenerable. The

dew point of hydrogen and nitrogen are -110 and -125 °C respectively. This

value indicates the gases are of high purity.

The entire MOCVD system is equipped with liquid sensors, shown

in Figure 3.10. These liquid sensors are connected to the SCS. If a sensor

detects liquid (cooling agent from a thermostat bath), the SCS automatically

triggers the alarm. When a liquid spilt on the liquid sensor, it leads to the

connectivity between two parallel contacts that activate the ‘waterleak’ alarm

in the monitoring panel. To remove the alarm, water leak has to be arrested

and the liquid sensor needs to be free from liquid.

Figure 3.10 Liquid sensors placed all over the MOCVD system

3.2.4 Reactor Cabinet

The reactor cabinet consists of a horizontal reactor in the stainless

steel housing, wafer transfer system, susceptor, quartz liner, RF inductor,

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Double O-Ring (DOR) system and in-situ reflectometer. Figure 3.11 depicts

the reactor cabinet and the important components. On the back side of the

reactor, cooling water system has been located.

Figure 3.11 Reactor housing and components in the reactor cabinet

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There are two gas lines for purging and rotation of susceptor. Light

pipe, liner and view ports are connected with purge lines. Both nitrogen and

hydrogen are used as purge gases. The susceptor rotation has been done by

gases using a gas foil rotation technique. Speed of the susceptor rotation can

be controlled by controlling the gas flow. The reactor has three view ports, the

rotation of susceptor can be seen through the front view port, In-situ laser

reflectometer was mounted on the top view port exactly above the susceptor

and the third viewport was located at the back side of the reactor. Figure 3.12

shows the reactor components present inside the stainless steel housing.

Separation plate, ceiling plate and the liner are made up of quartz.

The separation plate (see Figure 3.12 a) placed inside the reactor makes sure

that the incoming precursors from the MO run line and the hydride run line

are not mixed until they reach the susceptor region. The liner (see Figure 3.12

b) encases the susceptor holder and forms the real process chamber. The

ceiling plate is placed on the liner. During the process the deposition of AlN

and its related alloys occur on substrate and on the liner walls and ceiling

plate. Ceiling plate can be easily removed and it has to be cleaned often to

avoid the falling of particles on the substrate during the growth. On the other

hand, this set-up provides seldom cleaning of liner.

Substrate is loaded into the reactor and unloaded from the reactor

using the transfer sytem (see Figure 3.12 c) through the reactor gate. Once

the substrate is loaded on the susceptor, the transfer system is moved inside

the reactor. Then the reactor gate is locked by three pneumatic clamps.The

reactor gate has DOR system. It consists of inner and the outer (two) O-rings.

The intermediate volume is pumped continuously to ensure reliable and

controllable sealing. Figure 3.12 (d) shows the DOR sealing surface between

the transfer system and the reactor gate.

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Figure 3.12 (a) Separation plate and RF inductor (b) ceiling plate and

liner (c) wafer transfer sytem from glove box in to reactor

(d) Double O-Ring assembly at the reactor gate

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Growth of AlN and its related alloys require higher temperature

above 1200 °C. The susceptor is coated with tantalum carbide (TaC) material

to withstand higher temperature (~ 1400 °C) . Figure 3.13 depicts the picture

of the liner (taken out from the reactor), and the susceptor disks. Susceptor

comprises of two disks. It has rotation disk at the top and a static disk at the

bottom. The bottom disk has three holes for the gases to pass through. The top

disk is mounted on the bottom disk through a centre axle pin. During the gas

foil rotation, top disk slightly floats and revolves around the axle pin and in

turn substrate loaded on the top disk of the susceptor rotates. The rotation of

the susbtrate can be seen through the front view port, provided the liner walls

are free from coating.

3.2.4.1 Radio frequency generator

The Radio Frequency (RF) generator delivers RF power to a work

piece (susceptor) via an inductor or coil at its output. There are two modes of

operations available in the RF generator. They are the internal and remote

mode. In the internal mode the generator functions independently. All the

commands necessary for the operation of the generator are produced

internally. In the remote mode, the generator is integrated into and controlled

by a higher order control system. All necessary commands for the operation

of the generator are passed through the digital interfaces and shall be operated

through recipe. The RF Generator is constructed of two parts: The power

supply unit (19" sheet metal housing) and the oscillator (Aluminium diecast

housing and aluminium sheet metal housing). The power supply unit and the

oscillator are water cooled. In the MOCVD system, the RF oscillator and

inductor altogether called as exciter is located in the reactor cabinet. The

power supply unit known as RF generator is located separately from the

MOCVD system. The power supply unit and oscillator are connected to each

other through a supply cable and a control cable.

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Figure 3.13 (a) Quartz liner, (b) static bottom disk of susceptor with axle

pin, (c) top surface of rotation disk and (d) bottom surface

of rotation disk

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The exciter generates the RF signal. It produces a sinusoidal,

alternating voltage from the DC source voltage of the intermediate circuit.

The frequency of this alternating voltage is determined by the oscillator

circuit and it is the working frequency. The oscillator circuit consists of an

inductor and one or more capacitors. The transfer of RF power onto the work

piece is carried out via an inductor. The inductor is dependent on the work

piece and is matched to the frequency. The current consumption of the RF

generator is determined by the loss in the oscillator circuit and the load. The

monitoring electronics monitor the exciter for surge voltage, water

malfunctions, auxiliary voltage and the working frequency. The monitoring

electronics also transfers the work data of the exciter to the control electronics

in the power supply unit. Figure 3.14 discloses the RF generator and the

oscillator.

Figure 3.14 RF generator and RF oscillator of the MOCVD system

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3.2.4.2 In-situ reflectometer

One of the important components in the reactor cabinet is Epitune

III In-situ reflectometer placed above the top view port of the reactor (see

Figure 3.11). Epitune III is an optical system for temperature and reflection

measurements on wafers. The key features of Epitune III are

In-situ measurement of the wafer temperature

In-situ measurement of the light reflection on a wafer

Graphical analysis of the temperature and reflection of

measurement

Control of the reactor temperature during the process

Measurement of growth rate and thickness

Epitune III consists of temperature and reflection measuring unit,

lightpipe and detector for temperature measurements at the bottom of the

susceptor and Epitune III controller (see Figure 3.2 d). The optical measuring

unit measures the temperature and reflection values using the laser

wavelength source of 635 nm and also by an LED source of 880 nm. Laser

source at 635 nm is mostly used to obtain the reflectance and wafer

temperature information. At the same time, the temperature is measured at the

bottom of the susceptor through a light pipe. It is connected with a detector

which measure the radiation from the susceptor. Both the optical measuring

unit and light pipe are controlled by Epitune III controller. The controller

converts the measurement signals of the measuring units (raw data) to the

temperature and the reflection values. These values are displayed on the

touchscreen and made available to the AIXACT server for further graphical

analysis.

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Optical temperature measurement

The optical temperature measurement is based on the thermal

radiation that every body emits at a temperature above absolute zero. If the

emissivity of the measured object is known, the measured radiation energy

may be used to calculate the body’s temperature. The calculation is based on

Planck’s law of black body radiation. If the measurement is performed at one

specific wavelength, the radiated energy depends only on the temperatue of

the body

( , ) = x a1

-5

ea2T-1

(3.1)

where

a1 = 2hc2 and a2 = hc/k, E – Radiated energy, – Wavelength,

T – Temperature, – Emissivity, h – Planck’s constant and c – Speed of light

Reflection measurement

The measuring unit emits a light ray normal to the wafer surface

and measures the reflected radiation. This provides information about the

properties of the grown layer. Figure 3.15 shows the behaviour of incident

and reflected radiation from the gas to layer and susbtrate

The measurement of the growth rate and the layer thickness is

based on the principle of interference of monochromatic light. At the interface

of materials with different refractive indices, light rays are transmitted as well

as reflected. The resulting reflected intensity as a function of time is a

characteristic fingerprint of the layer. If the refractive indices of substrate.

layer and the gas in the reactor are known, the growth rate and the layer

thickness can be calculated from the reflection measurements.

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Figure 3.15 Incident and reflected radiation with respect to the

refractive indices

n1 – refractive index of the susbstrate; n2 – refractive index of

the layer; nG – refractive index of the gas;

Figure 3.16 shows the typical curve of the reflected intensity as a

function of time. The crest and the trough in the curve are caused by the

interference of the incident and the reflected radiation. T is the oscillation

time. The following formula can be used to determine the growth rate the

layer

= (3.2)

= (3.3)

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where

d – Thickness of the layer which has grown during the time T

T – Oscillation time

– Wavelength of the light source (635 nm)

n – Refractive index of the layer

r – Growth rate of the layer

Figure 3.16 Reflectance intensity versus time

In addition to these components, throttle valve and water cooling

system are situated at the rear side of the reactor cabinet. The pressure gauge

mounted near the reactor measures the pressure in the reactor and forwards

the measured pressure to the MKS pressure control unit (see Figure 3.2 (c)).

MKS controller controls the position of the throttle/butterfly valve in order to

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adjust pressure in the reactor according to the set point. Figure 3.17 shows the

throttle valve.

Figure 3.17 Throttle valve in the reactor cabinet

There are two separate water lines for the RF generator and the

reactor cabinet. Figure 3.18 depicts the cooling water system. RF generator

requires water flow rate of 21 liters/minute (+ or – 2). The reactor cabinet in

the MOCVD system needs 6 bar of the inlet water pressure for cooling system

and the maximum tolerable outlet water pressure is 2.5 bar. Drop and surge in

the water pressures triggers alarms and in turn aborts the process. Routine

checking of water lines are mandatory before the start of growth. There are

separate flow meters with flow limiter for reactor, flanges, RF oscillator, coil

or inductor and glove box water lines. If necessary the water flow can be

changed using manual valves for the standard flow rate. The inlet pressure

can be adjustable with the hand wheel and a water filter is located in the inlet

line to remove the dust particles. There is a provision to clean the filter by

means of back washing through the drain hose. This is a quick cleaning

process and there is no need for removing the filters.

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Figure 3.18 Cooling water system at the rear side of reactor cabinet

3.2.5 Glove Box

The glove box contains pure nitrogen atmosphere. It protects the

reactor from contaminations during loading/unloading the wafer through

transfer system and during maintenance. Two gloves are integrated with the

glove box for both the right and left hand to access the components such as

load lock chamber, wafer transfer system, vacuum tweezers and wand inside

the glove box. Substrates are transported into the glove box via the load lock.

The load lock chamber comprises of two doors: The outer door is used to load

and unload the substrates. The substrates for the growth is kept in the load

lock chamber through the outer door. Then the outer door of load lock

chamber is closed. Load lock chamber is purged with nitrogen gas followed

by evacuation. This cycle is repeated for three times in order to terminate

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oxygen and moisture entering into the glove box from the clean room. After

purging the load lock chamber, the inner door is opened through the gloves.

The inner door is used to transfer substrates between load lock chamber and

glove box. Figure 3.19 shows the load lock chamber, vacuum tweezers and

vacuum wand.

Figure 3.19 Load lock chamber, vacuum tweezers and vacuum wand in

the glove box

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To load and unload the substrate onto the susceptor, vacuum

tweezers is used. Vacuum tweezers handle the substrate only at the bottom. It

avoids the tweezers mark on the substrate’s surface completely. Vacuum

wand is a tool to take out the susceptor’s rotation disk from the axle. Both

vacuum tweezers and wand are operated with a pedal switch.

3.2.6 Process Pump and Scrubber

Process pump is used for evacuating the reactor up to 1 millibar.

This pump is connected with the reactor pressure control unit (see Figure 3.2

c) via the throttle valve. The process pump is an oil pump and it is purged

with nitrogen gas. The pump uses special white oil, which is normally clear

and transparent. If the oil darkens, it should be changed. The oil level can be

seen from the front side of the pump. The oil level of the pump should be

checked before the start-up. If necessary the oil should be topped up. The

process pump can pump condensable gases and vapours, provided that the gas

ballast valve is open and the pump has attained its operating temperature.

Proper operation of the pump is ensured in the ambient temperature range

between 12 to 40 °C.

A stainless steel pipe connects the process pump and the scrubber.

The scrubber has two tanks and the tanks are filled with the cleansing

solution. The cleansing solution is a mixture of sulphuric acid and deionized

water. The gaseous by products are fed into the liquid scrubber. There, the

gases are detoxified. The scrubber has a touch screen controller. The

controller alerts when the solution level is low and if there are other problems

while pumping. The pH value of the solution is maintained at 5. The

cleansing solution should be replaced, if there is any sign of crystallization.

Switching on and off shall be done in the touch screen controller. Figure 3.20

and 3.21 depict the Leybold process pump and scrubber.

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Figure 3.20 Process pump of the MOCVD system

Figure 3.21 The liquid scrubber in which the gaseous by products are

fed through the process pump

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3.2.7 Ammonia and Silane Gas Cabinet

Figure 3.22 shows the ammonia, nitrogen and silane cabinet.

Ammonia is the precursor for nitrogen and silane is the precursor for silicon

(n-type dopant of III-nitrides). Ammonia cabinet has the provision of loading

two cylinders at a time. If one cylinder is at service, another cylinder is in

standby mode. If the service cylinder is empty during the growth, the

ammonia will be delivered by the standby cylinder without any interruption,

by means of an automatic change over valve. Since the nitrogen gas generated

from liquid nitrogen is being used, the dry nitrogen gas cabinet has not been

put in use.

Figure 3.22 Ammonia, nitrogen and silane gas cabinets

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3.2.8 Process Technology of MOCVD

Chemical vapor deposition is a common method to deposit thin

films. In a CVD process, volatile compounds are transported into the reactor

chamber via the gas phase. There the precursors are decomposed on a hot

substrate by thermal energy. The following reaction takes place step by step.

Figure 3.23 displays the CVD reaction steps

1 Transport of the reagents in the gas phase to the substrate, by

carrier gas

2 Diffusion of the molecules through the phase interface

gas/boundary layer

3 Adsorption on the substrate surface

4 Migration of the molecules to the crystallization zones

5 Chemical decomposition

6 Desorption of the by-products and diffusion and convection

out from the boundary layer

7 Transport of the by-products out from the reactor

Figure 3.23 Chemical vapour deposition reaction steps A – Gas phase;

B– Boundary layer; C – Substrate;

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3.3 EXPERIMENTAL PROCEDURE

Trimethylaluminum (TMAl) and ammonia (NH3) were used as

precursors for aluminum (Al) and nitrogen (N) respectively. Hydrogen was

used as the carrier gas. 2 c-plane sapphire substrate was loaded on the

susceptor and the surface of substrate was thermally cleaned at 1150 °C for 10

minutes in H2 ambient. Initially, three AlN layers were grown on nucleation

layers (NLs) deposited at 850 °C with a V/III ratio of 1024. Thickness of NLs

was around 30 nm. Only the pre-flow conditions prior to the growth of NLs at

850 °C were modified to optimize the initial growth condition. In the first

sample, the NL was grown directly after the thermal cleaning of the sapphire.

For the second sample, TMAl was fed in to the reactor for 30 seconds prior to

the NL growth called as alumination was carried out. In the third sample, pre-

flow of NH3 (nitridation) for 2 minutes was done before the NL growth. All

the AlN samples with thickness of about 2 m, were grown at 1300 °C with a

V/III ratio of 512 and, after the recrystallization of nucleation layers at

1250 °C. The nucleation layer growth was carried out between 850 and

1250 °C as explained below. Structural quality and surface morphology of

these AlN layers were investigated by high resolution x-ray diffraction (HR-

XRD) and atomic force microscopy (AFM) respectively.

The growth temperature of direct NLs grown beneath the high

temperature AlN layers was varied as 950 °C, 1050 °C, 1150 °C and 1250 °C

without altering other growth parameters such as recrystallization temperature

(Tr) of AlN-NLs (Tr ~1250 °C), growth temperature (Tg) of AlN layers (Tg ~

1300 °C), V/III ratios of AlN-NLs (1024) and AlN layers (512). The AlN

layers deposited on direct NLs grown at different temperatures were

characterized using HR-XRD, AFM and Raman spectroscopy. Further, to

understand the effect of NLs on the quality of AlN layers, three NLs growth

temperature were selected. NLs were deposited at the selected temperatures of

117

950, 1050 and 1150 °C on sapphire substrates and subsequent

recrystallization was performed at 1250 °C for 2 minutes. The surface

morphology of recrystallized NLs and coalescence of NIs were assessed by

AFM.

3.4 RESULTS AND DISCUSSION

3.4.1 Effect of Initial Growth Stages

Structural quality and surface morphology of 2 m thick AlN layers

grown on NLs (directly, with alumination and nitridation) deposited at 850 °C

have been presented in Figure 3.24. The x-ray rocking curve (XRC) full width

at half maximum (FWHM) values of (002) and (102) reflections and

corresponding AFM root mean square (RMS) roughness values were shown

in Figure 3.24 (a). The three dimensional (3D) AFM images of AlN layer

grown on direct NL, with alumination and with nitridation have been depicted

in Figure 3.24 (b), (c) and (d) respectively. The symmetric (002) and

asymmetric (102) peaks are related to the screw and edge threading

dislocations (Srikant et al 1997 and Heinke et al 2000). Screw and edge

threading dislocations have been found to originate from the tilt and twist of

the AlN-NIs with respect to the substrate (Fini et al 1998 and Chierchia et al

2003).

Figure 3.24 (a) reveals that the FWHM of (002) and (102)

reflections of the AlN layer on direct NL is as low as 450 and 2124 arcsec

compared with aluminated and nitridated AlN layers. Thus, the broadening of

(002) FWHM to 954 arcsec in the aluminated AlN layer indicates that the pre-

flow of TMAl prior to the NL increases the tilt of NIs perpendicular to the

surface normal. Higher (102) FWHM of 4248 arcsec in the nitridated AlN

layer shows the larger twist of NIs around the surface normal (Taniyasu et al

118

2007). It appears that edge dislocations and RMS roughness values increase

for aluminated and nitridated AlN layers.

Figure 3.24 (a) XRC-FWHM of (002) and (102) reflections and

corresponding AFM-RMS roughness values of AlN layers

deposited on 850 °C grown NLs directly, with alumination

and nitridation. (b) 3D-AFM images of AlN layer on 850 °C

NLs grown directly, (c) with alumination and (d) with

nitridation. All scans are 10 µm × 10 µm.

119

RMS roughness values of AlN layer grown on direct NL, with

alumination and with nitridation have been estimated from the AFM images

of scan area 10 µm × 10 µm as 5, 15 and 22 nm from Figure 3.24 (b), (c) and

(d) respectively. Both islands and flat surfaces have been observed on the AlN

layers grown with direct NL and aluminated NL. However the number of

islands has been noted to be lesser in AlN layer having direct NL. Observance

of flat surface is totally annihilated in the AlN layer grown with nitridated NL

(Figure 3.24 d), which could be the reason for higher surface roughness. On

the whole, alumination and nitridation of sapphire prior to the growth of NLs

leads to broader (002) and (102) FWHM of AlN layers respectively. Surface

morphology of nitridated AlN layer has been poor with prevailing island

growth. Direct deposition of NL on sapphire substrate revealed impact on the

quality of AlN layers. Thus, direct growth of NL has been adapted to improve

the quality of AlN layers.

3.4.2 Influence of Nucleation Layers Growth Temperature on the

Quality of AlN Layers

Figure 3.25 depicts the graph between reflectivity and growth time.

In-situ reflectance pattern of AlN layers grown with nucleation layers

deposited at 850 °C, 950 °C and 1050 °C shown in Figure 3.25 (a), (b) and (c)

respectively. In Figure 3.25 region ‘1’ represents the reflectance peak of AlN

nucleation layer and region ‘2’ represents the start of high temperature AlN

growth. Figure 3.25(a) reveals the gradual decrease in the reflectance

oscillation amplitudes. It indicates the surface of the AlN is not smooth. In

Figure 3.25 (b) the amplitudes of AlN layer reflectance oscillation is stable,

that shows the smoothness of AlN layer surface. Figure 3.25 (c) discloses AlN

layer surface is very rough, the amplitude of reflectance peak has been

drastically reduced.

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From the in-situ reflectance graph, it has been found that the AlN

layer grown on 950 °C NL has the smooth surface morphology than AlN

layers with 850 and 1050 °C NL.

Figure 3.25 In-situ reflectance pattern of AlN layer versus growth time

during the growth in MOCVD. (a) AlN layer grown on NLs

deposited at 850 °C, (b) 950 °C and (c) 1050 °C

1 – Reflectance peak of AlN-nucleation layer (NL); 2 – High

temperature AlN layer deposition starts after the

recrystallization of NL;

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Figure 3.26 reveals AlN layers XRC-FWHM of (002) and (102)

reflections as the function of NL growth temperatures used for the AlN layers.

The FWHM of (002) reflections are 202, 214, 164 and 288 arcsec. On the

other hand, FWHM of (102) reflections are 900, 864, 1368 and 1404 arcsec

for the AlN layers deposited on the NLs grown at 950, 1050, 1150 and

1250 °C respectively.

Figure 3.26 AlN layers XRC-FWHM of (002) and (102) reflections as the

function of NL growth temperatures from 950 to 1250 °C

used for the AlN layers. ((002) and (102) FWHM of AlN

layer grown on 850 °C NL can be seen in Figure 3.24 a)

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Both the (002) and (102) FWHM values are found to decrease as

the NLs growth temperature of AlN layers increase from 850 °C (Figure 3.24

a) to 950 °C. This trend continues for the AlN layer grown on 1050 °C NL

with slight increment and decrement in the (002) and (102) FWHM values.

The lowest (002) FWHM value of 164 arcsec has been obtained for the AlN

layer with 1150 °C NL. At the same time, the (102) FWHM has been found to

be as high as to 1368 arcsec. In the case of AlN layer with 1250 °C NL, both

the symmetric and asymmetric peaks FWHM are found to be broader. This

might be partly explained by the thermal decomposition of c-plane sapphire

substrate occur above 1200 °C in the H2 ambient (Akiyama et al 2007a),

resulting in the deterioration of AlN layer. HR-XRD results elucidate the

enormous effect of NLs growth temperatures on the AlN layers and edge

dislocations are dominant in the AlN layers.

Threading dislocations and in-plane stress of AlN layers, originates

due to large lattice and thermal expansion mismatch between AlN and

sapphire substrate (Yim et al 1973) were estimated. Threading dislocations

(TDs) with screw components NS (screw + mixed type TDs) and edge

components NE (edge + mixed type TDs) have been calculated from the

Equations (3.4) and (3.5)

=( )

( ) (3.4)

=)

( )(3.5)

where,

bS = 4.982 Å and bE = 3.112 Å are the AlN Burger vector lengths of screw

components and edge components respectively (Dunn et al 1957).

123

FWHM(002) and FWHM(102) have to be converted in radian for

calculation. In-plane stress has been calculated using the following

Equation:

)2(33

213

12110

c

ccc

a

aall (3.6)

where ‘a’ is the lattice constant of AlN layers, a0 = 3.112 Å is the lattice

constant of unstrained AlN and the elastic constants of AlN Cij are C11 = 410

GPa, C12 = 140 GPa, C13 = 100 GPa and C33 = 390 GPa (Deger et al 1998). In

which AlN lattice constant ‘a’ has been calculated using the equation:

2

2

2

22

2)(

3

41

c

l

a

khkh

dhkl (3.7)

Screw NS, edge NE components of threading dislocation densities

and in-plane stress of the AlN layers deposited on NLs grown at temperatures

that varied from 850 to 1250 °C have been tabulated in Table 3.1.

As can be seen from Table 3.1, screw dislocation densities of AlN

layer are decreased by an order from 4.3 × 108 cm-2 to 6 × 107 cm-2 as NL

growth temperature increases from 850 to 1150 °C. It is then increased to 1.7

× 108 cm-2 for 1250 °C NL. Edge dislocation densities of AlN layers also

decrease from 2.5 × 1010 cm-2 to 4.1 × 109 cm-2 when increasing NLs growth

temperature of AlN layers from 850 to 1050 °C and start to increase further

with increasing NLs growth temperature to 1150 °C and 1250 °C. Over all

threading dislocation density has been found to be minimum for the AlN

layers grown on 950 °C and 1050 °C NLs, and might be attributed to the

reduced tilt and twist of NIs at these temperatures.

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Table 3.1 Nucleation layers growth temperature, screw NS and edge

NE components of threading dislocation density and

In-plane stress of AlN layers

NL growth

temperature of

AlN layers (°C)

Threading dislocation density In-plane

stress

(GPa)

Screw (108 cm-2)

components NS

Edge (1010 cm-2)

components NE

850 4.3 2.5 -0.32

950 0.8 0.44 -0.91

1050 0.9 0.41 -0.82

1150 0.6 1.03 -1.05

1250 1.7 1.09 -1.18

Moreover, the calculated in-plane stress, ( ) shows that all the

AlN layers are in compressive strain, negative sign indicating the compressive

strain. The in-plane compressive strain has been found to increase with NL

growth temperatures, excluding a small drop in the stress value for AlN layer

grown on 1050 °C NL. XRD results enunciate that 950 °C and 1050 °C grown

NLs are the optimal NL growth temperature to reduce the structural defects in

the AlN layer.

Figure 3.27 shows three dimensional AFM images of AlN layer in

the scan area of 10 µm × 10 µm. Figure 3.27 (a-d) reveal the images of AlN

layers grown with NLs from 950 - 1250 °C. AlN surface morphology

becomes rougher on increasing the NLs growth temperature. It is worth

noticing the clear steps and smooth surface of AlN layer grown on 950 °C NL

in Figure 3.27 (a). Step flow and emerging island like structure are seen in

Figure 3.27 (b) corresponding to the AlN layer with 1050 °C NL. In Figure

3.27 (c) and (d) granular or island structures are observed for the AlN layers

125

grown on 1150 and 1250 °C NLs. Appearance of layer by layer (Frank-Van

der Merwe) and layer-island (Stranski-Krastanov) growth modes pronounces

the influence of NLs growth temperature used for AlN layers. This could be

also due to the differences in the growth rate (Kumagai et al 2007) between

AlN-NLs (5 nm/min) and AlN layers (35 nm/min). RMS roughness value

measured in Figure 3.27 (a-d) are found to be 1, 13, 42 and 32 nm for AlN

layers grown on 950, 1050, 1150 and 1250 °C NLs. The higher surface

roughness is obvious due to the absence of step growth and prevalence of

island growth on increasing the NLs growth temperature. Specular surface

morphology with RMS roughness of 1 nm indicates high crystalline quality

(Kaeding et al 2004) of AlN layer grown on 950 °C NLs. The AFM results

are well accordance with in-situ recflectance pattern (see Figure 3.24). In-situ

reflectance, structural quality and surface morphology of AlN layers are noted

to favour the 950 °C grown NL than the1050 °C NL.

Raman spectra has been recorded for AlN layers deposited on NLs

grown at 850, 950 , 1050 , 1150 and 1250 °C using laser excitation source of

532 nm at room temperature in a backscattering geometry. Raman spectral

resolution is 0.9 cm-1. In backscattering geometry, the allowed Raman phonon

modes for (002) oriented AlN are E2 modes and the A1 (LO) mode (Kuball

2001 a). Figure 3.28 shows the Raman spectra of AlN layers. A1 (TO) and E1

(TO) phonon modes are usually forbidden in the backscattering geometry.

Dotted circle and circles in the Figure 3.28 indicates the weak signals of A1

(TO) and E1 (TO) phonon modes, respectively. It may be due to the presence

of misaligned 3D-islands or grains (Deng et al 2005 and Harima 2002),

for AlN with partial and complete granular surface as can be seen in the AlN

layers deposited on NLs grown at 850 (Figure 3.24 b), 1050 (Figure 3.27 b),

1150 (Figure 3.27 c) and 1250 °C (Figure 3.27 d).

126

Figure 3.27 3D-AFM images of AlN layers deposited on NLs grown at

(a) 950 °C, (b) 1050 °C, (c) 1150 °C and (d) 1250 °C. AlN

layer grown on 850 °C NL was shown in Figure 3.24(b).

All scans are 10 µm × 10 µm.

127

Figure 3.28 Raman spectra of AlN layers deposited on NLs grown at

850-1250 °C. Dotted circle represents A1(TO) mode and

open circles show E1(TO) mode. The inset depicts observed

phonon modes of AlN layers from 600 to 900 cm-1

In the AlN layer grown on 950 °C NL, there are no distinct feature

of A1 (TO) and E1 (TO) phonon modes. Inset shows the Raman phonon modes

of AlN layers from 600 to 900 cm-1. The E2 (high) phonon mode is found to

be sensitive to in-plane strain. The E2 (high) phonon mode position of

128

unstrained AlN layer is at 657 cm-1. Shift towards higher frequency side in the

E2 (high) phonon modes as 658.22, 657.5, 658.24, 658.26 and 659 cm-1 for

AlN layers with NLs grown at 850 – 1250 °C has been observed. It exhibits

that all the AlN layers are in compressive strain. The calculated in-plane

stress values using the equation (3.6) and the position of E2 (high) phonon

peaks ascertain the in-plane compressive strain of AlN layers. Raman spectra

show AlN layer with 950 °C NL is free from misaligned AlN grains.

Figure 3.29 UV-Visible transmission spectra of AlN layers

Figure 3.29 depicts UV transmission spectra of AlN layers as the

function of wavelength. AlN layer with 950 °C NL has the higher optical

transparency throughout the visible region of the spectrum and well into the

UV region. The transmission curve also has a sharp drop off at 200 nm

wavelength, which corresponds to the bandgap of AlN. Decrease in the

transparency for other AlN layers might be partially due to the surface

129

morphology. The rough surfaces of AlN layer decrease the transmission due

to the reflection and scattering of light.

3.4.3 Investigation of Nucleation Layers

In this section, NLs have been examined to understand their role in

determining the quality of AlN layers. Figure 3.30 (a), (b) and (c) disclose

three dimensional AFM images of recrystallized (at 1200 °C for 2 minutes)

NLs grown at 950, 1050 and 1150 °C respectively.

Firstly, 3D-AlN islands nucleate on the sapphire surface and start to

coalesce during the recrystallization of NLs. The 3D-islands tend to tilt and

twist around each other in order to undergo coalescence. In this process,

screw dislocations are found to originate from the tilt of NIs, edge

dislocations arise at the island or grain boundaries due to the NIs twist. Most

of these dislocations propagate in the subsequent AlN growth. Uniform

coalescence of nucleation islands have pronounced effects in suppressing the

intrusion of threading dislocations and formation of specular surface with step

and terrace growth pattern in the AlN layers. The NL grown at 950 °C shown

in Figure 3.30 (a) is found to exhibit the uniformly coalesced NIs, which

could be attributed to the step and terrace growth of AlN layer (Figure 3.27 a).

Whereas uneven coalescence of NIs is observed in Figure 3.30 (b), 1050 °C

grown NL leads to the step flow and emerging island like structure of AlN

layers (Figure 3.27 b). In Figur 3.30 (c) harsh undulations of coalesced NIs

and inadequate surface coverage of NIs are seen in NL grown at 1150 °C, it

could be attributed to the very rough surface of AlN layer (Figure 3.27 c).

130

Figure 3.30 3D-AFM images of recrystallized NLs grown at (a) 950 °C,

(b) 1050 °C and (c) 1150 °C. All scans are 1µm × 1µm

131

NIs density, diameter, height and RMS roughness values are

presented in Table 3.2. The NIs densities have been in the order of 1010 cm-2

for all three recrystallized NLs. The variation in the NIs diameter and height

has been found to increase with NL growth temperatures. RMS roughness

also increases with NLs growth temperatures. Smaller NIs and very low

variations in the diameter and height of NIs aid to uniform coalescence in the

NL grown at 950 °C. Relatively higher NIs density, diameter, height and

variation in the size could be attributed to uneven coalescence in the NL

grown at 1050 °C. Low density and big size of NIs have been found essential

to obtain good surface morphology and structural quality (Datta et al 2004).

Table 3.2 NL growth temperature, nucleation island density,

diameter, height and AFM-RMS roughness values for the

nucleation layers grown at various temperature.

NL growth

Temperature

(°C)

NI

density

(1010

cm-2

)

NI

diameter

(nm)

NI

height

(nm)

AFM-RMS

roughness

(nm)

950 7 80-100 7-10 1.5

1050 9.6 40-300 4-13 3

1150 3 100-360 3-16 3.9

Despite this fact, lower NIs density and large size of NIs obtained

for the NLs grown at 1150 °C have failed to fulfill the anticipation. It might

have been due to insufficient coverage of NIs on the surface and high

variation in NIs height, which favours the non-uniform coalescence of NIs

with highly undulated surface. Consequently, surface roughness of the

recrystallized NLs has been found to increase with increasing NL growth

temperatures. It is worth noticing that coalescence of NIs play a vital role in

determining the structural quality and surface morphology of the AlN layer.

132

Optical microscope images (not shown) confirmed that AlN layers

are free from cracks. Thermal expansion co-efficient differences (extrinsic

stress) between AlN and sapphire are found to cause cracks in the AlN layers.

However, in-plane compressive strain of AlN layers found from XRD and

Raman spectroscopy can also be attributed to the absence of cracks.

3.5 CONCLUSION

Direct deposition of AlN NL, effect of nucleation layer growth

temperatures, role of nucleation islands coalescence with respect to the

density, size and height have been investigated. The influence of NL

temperature in the aspect of producing evenly sized NIs and the uniformity in

the coalescence of NIs have been found as important growth parameters to

achieve the device quality AlN layers. Increasing the NL growth temperature

from 850 to 950 °C has been found to improve the quality of AlN layers.

Further increase in the NL growth temperature destructs the structural quality

and surface morphology due to the uneven coalescence of nucleation islands.