electrodes low contact resistancef for algan/gan structure with nitrogen vacancy … ·...

TiSi2 electrodes Low Contact Resistancef for AlGaN/GaN structure with Nitrogen

Vacancy by SiO2

Supervisor: Prof. Hiroshi Iwai

Tokyo Institute of Technology

Department of Electrical and Electronic Engineering

09_05126 Mari Okamoto

AlGaN/GaN attracts attention as power device material. To realize high performance of

AlGaN/GaN device, there are still matters to be solved. High contact resistance is one of

the issues. Aluminum(Al) and Titanium(Ti) are commonly used as ohmic contact metal

material. Ti can form nitrogen vacancies at AlGaN layer, which is essential for ohmic

contact. However, narrow process parameter can only realize low contact resistance.

This is why Ti causes crystal defect at AlGaN by high-temperature annealing. So we

proposed TiSi2 contact. Si has control over forming crystal defect and we get ohmic

contact resistance and area ratio resistance.

And so far, it is reported that SiO2 can also form nitrogen vacancies by high

temperature. So, we combine SiO2 forming nitrogen vacancies and TiSi2 which has low

metal function to realize low ohmic contact.

1

Contents

1 Introduction

1.1 Situation of saving energy…………………………………….…...5

1.2 Reason of GaN………………………………………………….….6

1.3 Issues of GaN………………………………………………………7

1.4 Currently GaN contact in use………………………………………8

1.5 Necessary conditions for lowering contact resistance……………...9

1.6 Approach to main subject…………………………………………11

Reference

2 Experiment

2.1 Experimental procedure

2.2 Experimental principle

2.2.1 SPM cleaning and HF treatment………………………………15

2.2.2 SiO2 deposition by TEOS……………………………………...16

2.2.3 Lithography…………………………………………………….16

2.2.4 Wet etching with BHF………………………………………….17

2.2.5 RF magnetron sputtering……………………………………….19

2.2.6 Dry etching by RIE……………………………………………..19

2.2.7 I-V measurement………………………………………………..19

Reference

2

3 Results and Discussion with Nitrogen Vacancy with

SiO2

3.1 XPS measurement…………………………………………………27

3.2 Analysis of interface between AlGaN and SiO2…………………...28

3.3 I-V measurement with SiO2 capped annealing……………………..29

4Result and Discussion with TiSi2 and Ti

4.1 Comparison with Ti and TiSi2……………………………….…….34

4.2 Analysis interface TiSi2 and AlGaN………………………….……36

4.3 Dependence on annealing temperature……………………….…….38

5 Conclusion and Future issues………………………….……40

Acknowledgement………………………………………………....40

3

Chapter 1

Introduction

1.1 Situation of saving energy

1.2 Reason of GaN

1.3 Issues of GaN

1.4 Currently GaN contact in use

1.5 Necessary conditions for lowering contact resistance

1.6 Approach to main subject

1.7 Structure of the Thesis

Reference

4

1.1 Situation of saving energy

Recently, some environmental problems, such as global warming, become more serious.

On the other hand, the consumption of energy per head and also in our country goes on

increasing. The table1.1 shows that electric energy is the most consumed. [1.1] So it is

the key to the saving energy that we use electric energy efficiently.

Now, electric power changing technology takes a big roll in electric power flow from

power plant to consumers. [1.2] So, we need to develop energy changing technology

high efficiently. And, there are some devices marked as power device material such as

SiC and GaN. This table1.1 shows the reason why SiC or GaN are suitable for power

device material. [1.3]

Table 1.1 Physical property of Si, SiC, GaN used as a power electric

semiconductor.

So, high electron mobility and saturated velocity cause high frequency switching,

and high breakdown field and thermal conductivity cause high power and high

temperature operation. [1.4]

5

1.2 Reason of selecting GaN

Epitaxial layer of GaN is commonly grown on sapphire however epitaxial layer

can be grown on Si substrate by MOCVD(Metal Organic Chemical Vapor

Deposition). The price of Si substrate (about 80yen/cm2) is much more lower

than sapphire (480yen/cn2), so price per unit of GaN substrate can be also

lower.[1.5] Furthermore, This achievement enabled AlGaN/GaN device to

construct lateral FET(Field-Effect-Transistor) and many devices can be produced

on large area substrate. Now, the size of GaN on Si is enlarged to substrate

six-inch caliber. [1.6]

However, it is not facilitated for high quality epitaxial layer of GaN to grow on

Si substrate. When epitaxial layer is grown, AlN layer is necessary as a buffer

layer for difference of mismatching thermal and lattice coefficient GaN on Si has

high thermal conductivity and conductivity control.[1.7] So a GaN device has the

performance of about ten MOS-FET, we realize miniaturization and weight

saving of AlGaN/GaN device.

6

1.3 Issues of AlGaN and GaN device

There are still some issues to realize to go on the market. For example, one of

the issues is normally-on transistor. Figure 1.3.1 shows normally-on state, minus

threshold voltage causes that transistor

is on state unless gate voltage is plus

value. [1.8] This result from 2DEG(2

Dimensional Electron Gas) of

AlGaN/GaN structure which shows

Figure1.3.2. Normally-on state is

necessary for safety in a high current or

power electricity used in industry. On

the other hand, one of the most important issues of AlGaN/GaN device is high

contact resistance. However contact resistance is in general 10-8 cm2 on silicon

substrate, contact resistance on GaN/AlGaN substrate is 10-5cm2.[1.9] High

contact resistance causes power loss and less efficiency. In this report we pay

attention with contact resistance.

I[A]

[V]0

Normally-on state

Figure 0.1.3.1 Normally on state

Metal

ALGaN

GaN

2DEG

Fig 2.1.3 band figure of AlGaN/GaN with metal

7

1.4 Currently contact of AlGaN and GaN

Now generally used as a contact material, Aluminum and Titanium are used as a

metal material. Ti has a roll of forming nitrogen vacancies which is essential for

ohmic contact and Al has a roll of controlling forming nitrogen vacancies by Ti.

However narrow process parameter can only realize low contact resistance. Fig

1.4.1 shows that Ti reacts locally with AlGaN layer and changes to TiN, which is

called TiN island by high temperature annealing.[1.10] The formation of

interfacial TiN was found to be critical in achieving Schottky-to-Ohmic transition.

As annealing, TiN island which have nonuniformly formed along threading

dislocations and had penetrated through the AlGaN into the underlying GaN layer.

These conductive TiN islands established intimate contact between metal and

2DEG and allowed direct transport of carriers across the AlGaN. Furthermore

keeping on annealing led to degrade contact resistance, which results from

abnormal growth of TiN islands. This causes narrow process parameter for

Ohmic contacts.

AlGaN

GaN

TiN

Ti

e-

e-

←2DEG

AlGaN

GaN

TiN

Ti

e-

e-

←2DEG

Fig1.4.1 Mechanism of ohmic contact by reaction with Ti and AlGaN

8

1.5 Necessary conditions for lowering contact resistance

To form ohmic contact, there are some necessary conditions. First, it is critical

for choosing low metal function. Fig.1.4.1 shows typical metal and its function

used as a material of metal electrode on AlGaN/GaN. [1.11]

4.9NiSi

4.7TiSi2

4.7TiN

4.52W

4.45Mo

4.14Ti

4.13Al

Work Function[eV]Material

4.9NiSi

4.7TiSi2

4.7TiN

4.52W

4.45Mo

4.14Ti

4.13Al

Work Function[eV]Material

Table.1.4.1 material and its work function

Fig.1.4.2 shows that low work function of metal leads to ohmic contact.

MetalALGaN

GaN

Metal function m

MetalALGaN

GaN

Metal function m

MetalALGaN

GaN

✖

MetalALGaN

GaN

✖

MetalALGaN

GaN

Fig 1.4.2 Mechanism of current between metal and GaN

Electrons in metal can easily flow into GaN layer for selecting low metal function.

Second, band bending is also necessary for ohmic contact.Fig.1.4.3 shows that

tunneling current occurred by band bending.

9

Metal AlGaN GaN

✖ ✖

Metal AlGaN GaN

✖ ✖

MetalAlGaN

GaNMetalAlGaN

GaN

Fig1.4.3 Mechanism of tunneling current between metal and GaN

Wide depletion region cannot flow tunneling current however band bending

causes to narrow depletion region and tunneling current applied. Band bending is

occurred by heavy doping on interface of semiconductor and applying H2-plasma

cleaning. This report proves that AlGaN and SiO2 reacts with each other and n+

layer was formed in AlGaN layer, which causes band bending and realize ohmic

contact. [1.12]

Finally, narrow AlGaN layer can also realize tunneling current by etching AlGaN

layer physically with H2-plasma etching and CH4,H2,Ar plasma.[1.13] So plasma

etching enables to cause tunneling current.

10

1.6 Approach to main subject

By measuring I-V characteristic and analyzing XPS result, we observed and

considered reaction with SiO2 and AlGaN or TiSi2 and AlGaN. So, this

observation leads to find mechanism of the reason that TiSi2 can get ohmic

contact.

11

Reference

[1.1]Agency for Natural Resources and Energy: ‘The white paper on energy

usage’ the second chapter (2011)

[1.2]L.M.tollbert, T.J.King, B.Ozepinec :’Power Electronics For Distributed

Energy Systems and Transmission and Distribution Applications’ The University

of Tennessee-Knoxville. (2005)

[1.3]VE Chelnokov, AL Syrkin:’Overview of SiC power electronics’, Elecron

Device, IEEE (1989)

[1.4]M.Hikita, M.managahara: ’GaN-Based Devices’, Panasonic Technical

Journal Vol.55(2009)

[1.5]Pierre Gibart, ’Metal organic vapour phase epitaxy of GaN and lateral

overgrowth’, Institute of Physics Pubilishing. (2004)

[1.6]D.Zhu, C.Mcaleese: ’GaN-LEDs grown on 6-inch diameter Si(111)

substarates by MOVPE’ Proc. Of SPIE Vol.7231

[1.7]M.Sakai, K.Asano: ‘Grownth of AlGaN/GaN Heterostructure Grown on

Epitaxial AlN/Sappire Templates by MOVPE’ (2002)

[1.8]N.Ikeda, Li Jiang :’Novel GaN Device for High-Power Application with

Thin AlGaN/GaN Heterostructure Layer.’(2008)

[1.9]Bin Lu, D.Piedra, T.palacios: ‘GaN Power Electronics’Advanced

Semiconductor Devices (2010)

[1.10]Liang Wang, Titih M.Mohammed, ’Formation mechanism of Ohmic

contacts on AlGaN/GaN heterostructure Electrical and microstructure

characterizations’ Journal of Applied physics (2008)

12

[1.11]M,Kanamura, T.Oki, T.Kikkawa: ‘GaN-HEMT Technology for Future

Applications’Fujitsu. 60,5 (2009)

[1.12] Y. Taur, T. H. Ning: “Fundamentals of MODERN VLSI DEVICES”,,

Cambridge University Press (1998)

[1.13]T.Hasizume, H.Hasegawa: ‘Effects of nitrogen deficiency on electronic properties

of AlGaN surfaces subjected to thermal and plasma process.’ RCIQE(2004)

13

Chapter 2

Experiment



2.2.1 SPM cleaning and HF treatment

2.2.2 SiO2 deposition by TEOS

2.2.3 Lithography

2.2.4 Wet etching with BHF

2.2.5 RF magnetron sputtering

2.2.6 Dry etching by RIE

2.2.7 I-V measurement

Reference

14


Fig. 2,1 shows experimental procedure of contact. Contact was fabricated on

AlGaN(26nm) on GaN(1.3um). The silicon-die-oxide(100nm) was deposited

by plasma-TEOS sputtering. And SiO2 and AlGaN layer were cut down by

RIE(Reactive Ion Etching). The silicon-nitride(50nm) was deposited by

magnetron sputtering and successively SiO2(100nm) was deposited after

photolithography. The samples were annealed at 1000 for 10 min. SiO2. The

SiO2 was removed by BHF and metal layer such as TiSi2, TiN and Ti for

electrode was deposited by magnetron sputtering. Finally, metal layers were

removed by RIE and the samples were I-V measured.

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.6]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.3]

SPM,HF last treatment ・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.1]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.2]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.4]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.5]

Photolithography

Wet etching by BHF

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.7]

Lithography, Dry etching by RIE(Cl2)

TiN/TiSi2 deposition by TEOS

TiN/Ti deposition by TEOS

IV mesurement

Annealing

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.8]

SiO2 deposition by TEOS

Dry etching by RIE(Cl2)


Photolithography, Wet etching by BHF

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.6]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.3]


・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.2]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.4]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.5]

Photolithography

Wet etching by BHF

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.7]




IV mesurement

Annealing

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.8]





・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.3]


・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.2]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.4]

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.5]

Photolithography

Wet etching by BHF

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.7]




IV mesurement

Annealing

・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・Chapter [2.2.8]





Fig.2.1 Experimental procedure of contact

15


2.2.1 SPM cleaning and HF treatment

Particles and organic substance at the surface of Si substrate become a cause of false

operation. Therefore, it is important to clean the surface of Si substrate. SPM cleaning is

one of the effective cleaning methods. The cleaning liquid is made from H2O2 and

H2SO4 (H2O2:H2SO4 = 1:4). Because of its oxidizability, particles and organic

substance are oxidized and separated from the surface of Si substrate. However, the

surface of Si substrate is oxidized and SiO2 is formed during SPM cleaning. 1% HF is

used to eliminate the SiO2.

2.2.2 SiO2 deposition by using TEOS

TEOS(tetraethoxysilane) is usually used for depositing SiO2 and as a gate insulator

material in semiconductor.[2.1] Here, we deposited SiO2 on samples for covering

AlGaN layer in isolation. By CVD(chemical vapor deposition) method using TEOS gas,

SiO2 layer with which O2 gas reacts can be formed rapidly and at low temperature.

The plasma CVD method is described below. First, oxygen gas and TEOS gas used as

material gases are injected into reactor. Applying high frequency electric field generates

plasma which makes gases excited. Excited atoms are reacted chemically and deposited.

TEOS is commonly reacted with SiH4, O2 and Ar, however, we use CF4, O2 and Ar in

this report. Deposition temperature is 200°C and deposition rate is 5nm/min. We

deposited 100nm for 20min.

16

Process Gases O2RF power

By-products to the pump

Heater

Plasma

Process Gases O2Process Gases O2RF power

By-products to the pump

Heater

Plasma

Fig.2.2 2.1 Schematic illustration of RIE

CH

H H

H

O

H

C CO

H H

H

H

H

C

C H

H

H

O

HC

H

Si

C H

O

H

H

HC

H

CH

H H

H

O

H

CCH

H H

H

O

H

C CO

H H

H

H

H

CCO

H H

H

H

H

C

C H

H

H

O

HC

H C H

H

H

O

HCC

H

SiSi

C H

O

H

H

HC

H C H

O

H

H

HCC

H

CH

H H

H

O

H

C CO

H H

H

H

H

CSi

C H

O

H

H

HC

H

Sisub

CH

H H

H

O

H

CCH

H H

H

O

H

C CO

H H

H

H

H

CCO

H H

H

H

H

CSi

C H

O

H

H

HC

H C H

O

H

H

HCC

H

Sisub

CH

HH

H

O

H

C CO

HH

HHH

C

Si

C H

O

H

H

HC

H

Si

sub

CH

HH

H

O

H

C CO

HH

HHH

C

Si

C H

O

H

H

HC

HC

H

HH

H

O

H

CC

H

HH

H

O

H

C CO

HH

HHH

CC

OH

H

HHH

C

Si

C H

O

H

H

HC

H C H

O

H

H

HCC

H

Si

sub

CH

H H

H

O

H

C CO

H H

H

H

H

C

C H

H

H

O

HC

H

Si

C H

O

H

H

HC

H

CH

H H

H

O

H

CCH

H H

H

O

H

C CO

H H

H

H

H

CCO

H H

H

H

H

C

C H

H

H

O

HC

H C H

H

H

O

HCC

H

SiSi

C H

O

H

H

HC

H C H

O

H

H

HCC

H

CH

H H

H

O

H

C CO

H H

H

H

H

CSi

C H

O

H

H

HC

H

Sisub

CH

H H

H

O

H

CCH

H H

H

O

H

C CO

H H

H

H

H

CCO

H H

H

H

H

CSi

C H

O

H

H

HC

H C H

O

H

H

HCC

H

Sisub

CH

HH

H

O

H

C CO

HH

HHH

C

Si

C H

O

H

H

HC

H

Si

sub

CH

HH

H

O

H

C CO

HH

HHH

C

Si

C H

O

H

H

HC

HC

H

HH

H

O

H

CC

H

HH

H

O

H

C CO

HH

HHH

CC

OH

H

HHH

C

Si

C H

O

H

H

HC

H C H

O

H

H

HCC

H

Si

sub

Fig 2.2.2.2 Transformation of TEOS by CVD method

2.2.3 Rapid thermal annealing

Rapid thermal annealing (RTA) was used for produce of nitrogen vacancies. The heat

chamber was vacuum and filled in nitrogen gas, so that the effect of prevention

oxidation of the sample. The samples were annealed by infrared ray for 1 minute.

17

2.2.4 Lithography

Photolithography is a process used in microfabrication to pattern the substrate. [2.2] On

the substrate, the liquid called “resist” which has photosensitivity was put on. The resist

has two types, “positive type” and “negative type”. The former type is that the part of

substrate irradiated with ultraviolet rays is removed by liquid developer. We use this

type. After exposure , the substrates are soaked in liquid developer and baked on the

heater at 150℃ for 10min to stabilize resist.

Mask

Resist

Ultraviolet

Positive type Negative type

Fig.2.2.4 Different process with negative type and positive type

18

2.2.5 Wet etching by BHF

Buffered HF (BHF) are used for wet etching process. When the RE-oxides uncovered

with resist are etched, HCl is used as etching liquid which is called etchant. When the

SiO2 is etched, BHF is used as etchant.

2.2.6 Dry etching by RIE

Reactive ion etching (RIE) is one of the patterning methods. Etching gas becomes the

plasma in a similar way with RF sputtering. However, RIE is not only physical but also

chemical reaction. For etching of aluminum-gallium-nitride and silicon-die-oxide,

chemistry is used as etching gas in this study. The aluminum-gallium-nitride and

silicon-die-oxide which are uncovered with resist reacts with Cl- and becomes SiCl4

andAlCl3 which are gas at room temperature.[2.3] When the resist is eliminated, O2 is

used as etching gas and this process is called ashing.

2.2.7 RF magnetron sputtering

Titanium which is used as contact electrode in this study is deposited by radio

frequency (RF) magnetron sputtering with Ar gas.[2.4] An RF with 13.56 MHz at a

power of 150 W is applied between substrate side and target (Ti) side. Because of the

difference of mass, Ar ions and electrons are separated. A magnet is set underneath the

target, so that the plasma damage is minimized. Electrons run through the circuit from

substrate side to target side, because substrate side is subjected to be conductive and

target side is subjected to be insulated. Then, target side is biased minus and Ar ions hit

the target.

19

In this study, silicon is also used as a target and titanium and Fig .2.3 shows that silicon

are laminated by changing targets for appropriate time and at low power of 80W

Fig.2.2 Schematic illustration of RF magnetron sputtering

Fig.2.3 lamination layer of TiSi2

20

2.2.8 I-V measurement method

TLM is short for Transmission Line Model. It is usually used as a method for

evaluating I-V measurement and Contact resistance. [2.5]

The contact resistance measurement technique was shown in Fig.2.4.

ZW

d

I I

Fig .2.4 Contact resistance structure. The contact width and length

are Z and L and the diffusion width is W.

To measure I-V characteristic is needed for determining contact resistance. I-V

characteristic is measured by applying voltage on electrodes with variable electrode

distances. Fig.2.5 shows relation between total resistance and the distance between

electrodes.

2

Fig .2.5 Total resistance and distances between electrodes

21

Rt is total resistance, Rc is contact resistance, and Lt is transmission length. Total

resistance, which consists of sheet resistance and contact resistance, is written as

(2.1)

Rsh is sheet resistance. Intercept of this graph is the contact resistance. That is, contact

resistance is written as

(2.2)

can be thought of as that distance over which most of the current transfers from the

semiconductor into the metal of from the metal into the semiconductor. The transfer

length is on the order of 1m or less for such contacts.

Lt

Lt

V

Fig.2.6 shows electric potential difference between contacts. Basically, the relation

between voltage and contact distance is linear. However, at directly under contact

I

Fig.2.7 current transfer from semiconductor to metal represented by the arrows.

22

340m

250m

290m

300m80m

290m60m

40m

320m

150m340m

250m

290m

300m80m

290m60m

40m

320m

150m

Fig.2.8 mask pattern used in the process

23

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

GaN

AlGaN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaN

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaN

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaN

GaN

AlGaNSiO2

I-V measurement

I II I

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

SiO2

GaN

AlGaN

GaN

AlGaN

GaN

AlGaN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaN

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaN

GaN

AlGaNSiO2

TiSi2/TiN

GaN

AlGaNSiO2

GaN

AlGaNSiO2

GaN

AlGaN

GaN

AlGaNSiO2

I-V measurement

I II I

Fig 2.8 Schematic illustration of contact process.

24

Reference

[2.1] Alex Lubnin: ‘SiO2 TEOS-OZONE CVD’ DUMIC symposium(1998)

[2.2]H.Levinson: ‘Principles Of Lithography’p402-405(2005)

[2.3]N.Hayasaka: ‘Method for removing composite attached to material by dry

etching’(1994)

[2.4] Hari Singh Nalwa: ‘Handbook of Thin Films’Five-VoloumeSet, p410-p421.(2001)

25

Chapter 3

Results and Discussion

with Nitrogen vacancy

with SiO2

3.1 XPS measurement

3.2 Analysis of interface between AlGaN and SiO2

3.3 I-V measurement with SiO2 capped annealing

26

3.1 XPS measurement

XPS(X-ray photoelectron Spectroscopy) is one of the spectroscopic technique which

measures elemental composition .Because it uses high energy X-ray as exciting light, it

is possible to excite a core electron which strongly bound to an atom and to emit

photoelectron. Each material has their own core electron binding energy. It is quite

different between materials each other. Therefore, the information which atom is caused

can be known after core electron binding energy is analyzed and the peak of that is

measured. We can perform quantitative analysis by comparing with peak intensity. To

determine exactly binding energy, that of Si in the sample is set to the standard, and that

of other material is measured relate to a standard.

Focusing Electron Gun

X‐ray

Focusing MonochromaterCrystal

Hemispherical Electron Energy Analyzer

Fast MultichanneDtector

Figure3.1.1 Schematic illustration of XPS

27

3.2 Analysis of interface between AlGaN and SiO2

Figure3.1.2 shows analysis sample A and B by XPS measurements. A is the sample

which was processed by SPM cleaning and HF cleaning. B is processed by SPM and

HF cleaning and annealed at 1000°C for 10min in N2 ambientafter deposition with SiO2

layer. An x-ray energy of 8keV was used as a source for XPS. So that photoelectrons as

deep as 40 nm can be collected.

1559 1561 15621560 1563 1564 1565

Inte

nsi

ty[a

.u.]

Binding Energy[eV]

0.2eV

Al1s without SiO2 capped annealing

(h=7940eV, TOA=80o)

Al1s with SiO2 capped annealing

1559 1561 15621560 1563 1564 1565

Inte

nsi

ty[a

.u.]

Binding Energy[eV]

0.2eV

1559 1561 15621560 1563 1564 15651559 1561 15621560 1563 1564 1565

Inte

nsi

ty[a

.u.]

Binding Energy[eV]

0.2eV

Al1s without SiO2 capped annealing

(h=7940eV, TOA=80o)

Al1s with SiO2 capped annealing

Figure 3.2.1 Al1s spectrum of the sample with and without SiO2 capped annealing

The Figure shows that Al 1s peak shift into high energy band by 0.2 keV. Fig3.2.2

shows that This result indicates that nitrogen vacancy is formed at AlGaN and Al

changes into AlN, which make n+ layer formed. That proved that SiO2 layer can form

28

nitrogen vacancy by high temperature annealing.

Metal

ALGaN

GaN

2DEG

vvMetal

ALGaN

GaN

2DEG

vv

Fig 3.2.2 Band structure with annealing

3.3 I-V measurement with annealed SiO2

Figure 3.3.1 shows I-V measurement of the sample which is processed with annealing at

1000oC for 1min after SiO2 deposited and annealed for 1min at from 0oC to 950oC.

This result shows that less current flows with the higher annealing temperature is.

950OC

asdepo

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

950OC

asdepo

-5 -2 -1-3-4 0 1 2 3 4 5-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

Fig 3.3.1 I-V measurement with SiO2 capped annealing at asdepo and 950oC

29

Figure 3.3.2 shows I-V measurement of the sample which is processed without

annealing and annealed for 1min at from 0oC to 950oC. Different from result of SiO2

capped annealing, current flow increases with high temperature annealing for the

sample without SiO2 capped annealing.

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Voltage [V]

Cur

rentm

asdepo

950OC

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Voltage [V]

Cur

rentm

asdepo

950OC

Fig 3.3.2 I-V measurement without SiO2 capped annealing at asdepo and 950oC

Figure 3.3.3 shows Relationships with Total resistance and annealing temperature. This

results also with high temperature annealing

30

Annealing temperature [OC]

Tot

al R

esis

tanc

e TiSi2 with SiO2 capped annealing

TiSi2 with SiO2 capped annealing

1.41.21.00.80.60.40.20.0

0 500 600400300 700 800 900 9500 500 600400300 700 800 900 950

1.41.21.00.80.60.40.20.0

0 500 600400300 700 800 900 9500 500 600400300 700 800 900 950

Annealing temperature [OC]

Tot

al R

esis

tanc

e TiSi2 with SiO2 capped annealing


1.41.21.00.80.60.40.20.0

0 500 600400300 700 800 900 9500 500 600400300 700 800 900 950

1.41.21.00.80.60.40.20.0

0 500 600400300 700 800 900 9500 500 600400300 700 800 900 950

Fig 3.3.3 Relationships with total resistance and annealing temperature of the

samples with SiO2-capped annealing and without annealing.

This result indicates that the samples with SiO2-capped annealing at low temperature

can get the lower contact resistance than the samples without SiO2-capped annealing.

Figure3.3.4 shows I-V measurement of the samples which is processed with and

without SiO2-capped annealing at asdepo state

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0



Cur

rentm

Voltage [V]-5 -2 -1-3-4 0 1 2 3 4 5-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0



Cur

rentm

Voltage [V]

Fig 3.3.4 I-V measurement with and without SiO2 capped annealing at asdepo state

31

From this result that is the sample with SiO2 capped annealing has lower total resistance

than the sample without SiO2 capped annealing, it is considered that nitrogen vacancy is

formed at AlGaN layer by depositing SiO2 and annealed.

32

Chapter 4

Results and Discussion

with Contact of TiSi2

4.1 Comparison with Ti and TiSi2

4.2 Analysis interface TiSi2 and AlGaN

4.3 Dependence on annealing time

33

4.1 Comparison with Ti and TiSi2

Chapter 3 shows SiO2 can form nitrogen vacancy, on the other hand the samples

without SiO2 capped annealing gets ohmic contact of TiSi2. So we use TiSi2 electrode

and measured I-V characteristic without SiO2 capped annealing. Compared with TiSi2,

Ti(50nm) was deposited as a contact material and measured I-V characteristic as well as

TiSi2 by annealing for 1min and N2 ambient.

Figure 4.1.1 shows I-V measurement of Ti with annealing at 700oC to 950OC.

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

950℃

900℃

850℃

800℃750℃

700℃

300m

150m

80m

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]-5 -2 -1-3-4 0 1 2 3 4 5-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

950℃

900℃

850℃

800℃750℃

700℃

300m

150m

80m

300m

150m

80m

Fig 4.1.1 I-V measurement of Ti annealed from 750oC to 950oC

This result indicates that ohmic contacts were formed at 750OC annealing. The higher

temperature annealed more than 750OC, the less current flows.

I-V measurement of TiSi2 is shown in Fig 4.1.2.

34

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

850℃800℃

750℃

700℃

950℃

900℃

300m

150m

80m

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]-5 -2 -1-3-4 0 1 2 3 4 5-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

850℃800℃

750℃

700℃

950℃

900℃

300m

150m

80m

Fig 4.1.2 I-V measurement of TiSi2 annealed from 750oC to 950oC

This result indicates that ohmic contacts were formed at 950oC annealing. The higher

temperature annealing caused that current flow is increasing from 700oC to 950oC. Then,

Fig 4.1.3 shows total resistance dependency on of TiSi2 electrode with contact distance

with annealed at 950oC in N2 ambient.

4000 150 20010050 250 300 350

Distance between contacts(m)

Tot

alre

sist

ance

(m

m)

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

04000 150 20010050 250 300 350

Distance between contacts(m)

Tot

alre

sist

ance

(m

m)

0.18

0.16

0.14

0.12

0.1

0.08

0.06

0.04

0.02

0

Fig4.1.3 Relation with Total resistance of TiSi2 and distance between contacts

35

So, ohmic contact of TiSi2 is proportional of specific area. Using TLM method and

contact specific resistance is c=9.6E-4[cm]

Contact resistance is lowere, however this contact resistance needs improvement for

reducing contact resistance.

4.2 Analysis interface TiSi2 and AlGaN

Figure4.2.1 shows analysis interfaces with AlGaN and Ti, AlGaN and TiSi2 by XPS,

which are the samples processed by SPM cleaning and HF cleaning and annealed at

900°C for 1min in N2 ambient after deposition metal.

Binding energy (eV)1115111611171118111911201121

Inte

nsit

y (a

.u.)

Ti(20nm)

TiSi2(20nm)

Ti-Ga

Ga-N

0.16eV

Binding energy (eV)1115111611171118111911201121

Inte

nsit

y (a

.u.)

Ti(20nm)

Ti-Ga

Ga-N

0.16eV

TiSi2(20nm)

Fig 4.2.1 XPS result of TiSi2 and AlGaN

Intensity and binding energy suggested following band structure shown in Fig 4.2.2.

36

Fig 4.2.2 Band structure with TiSi2, AlGaN and GaN with annealing

This XPS result of Ti and TiSi2 and band structure indicate that TiSi2 can generate

nitrogen vacancy at AlGaN layer and band bending. Especially XPS result supposed

that Ti forms alloy such as TiGa, however TiSi2 doesn’t make alloy and reacts with

AlGaN layer over the entire surface.

4.3 Dependence on annealing temperature

The samples which of metal are Ti and TiSi2 were annealed at 950OC and each I-V

characteristic was measured by changing annealing time from 1min to 20 min. This

result is shown in Fig 4.3.1 and Fig 4.3.2. Fig 4.3.1 shows I-V measurement of Ti and

Fig 4.3.2 shows I-V measurement of TiSi2.

37

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

1min

5min

20min

10min

40min 60min

300m

150m

80m

300m

150m

80m

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

1min

5min

20min

10min

40min 60min

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]-5 -2 -1-3-4 0 1 2 3 4 5-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

1min

5min

20min

10min

40min 60min

300m

150m

80m

300m

150m

80m

Fig 4.3.1 I-V measurement of Ti with annealing for 1min to 60min

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

1min

5min

20min

10min

40min 60min

300m

150m

80m

300m

150m

80m

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

1min

5min

20min

10min

40min 60min

-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]-5 -2 -1-3-4 0 1 2 3 4 5-5 -2 -1-3-4 0 1 2 3 4 5

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

2.0

0.5

1.0

1.5

0.0

-0.5

-1.0

-1.5

-2.0

Cur

rentm

Voltage [V]

1min

5min

20min

10min

40min 60min

300m

150m

80m

300m

150m

80m

Fig 4.3.2 I-V measurement of TiSi2 with annealing for 1min to 60min

For Ti electrode, contact resistance increases with long high-temperature

annealing, however for TiSi2 electrode, it is efficient that longer high

38

temperature can make less contact resistance. Furthermore, relation total

resistance of Ti electrode annealed at 750oC and TiSi2 annealed at 900oC and

current at 2 V was shown in Fig 4.3.3.

0.0

0.4

0.8

1.2

1.6

Cur

rent

@2V

(m

A)

1 10

Annealing time (min)2 5 20 50

Ti

100

TiSi2

non -linear IV

Ohmic

Ohmic in all range

(750 oC)

(950 oC)

0.0

0.4

0.8

1.2

1.6

Cur

rent

@2V

(m

A)

1 10 100

Annealing time (min)2 5 20 50

Ti

TiSi2 Ohmic in all range(950 oC)

non -linear IV

Ohmic

(750 oC)

Fig 4.3.3 Relation to current and annealing time

TiSi2 can get ohmic contact for all range from 1min to 60min at 950 oC, however Ti

can get ohmic contact only for 5min to 20 min at750oC. So, TiSi2 is able to have wide

process time of ohmic contact.

39

Chapter 5

Conclusion

and Future Issues

40

Nitrogen vacancy is essential for ohmic contact of AlGaN and GaN. As mentioned in

Chapter 2, Ti made TiN and TiGa into crystal dislocation at AlGaN layer and current

flows through these alloy and connect 2DEG. On the other hand, TiSi2 forms nitrogen

vacancy at AlGaN layer and this reaction didn’t make alloy, that is, TiSi2 doesn’t

depend on crystal dislocation and reaction with area surface. With high annealing

temperature and long annealing time, TiSi2 can make much nitrogen vacancies and

contact resistance can be reduced.

Furthermore, result of XPS objected interface of AlGaN and SiO2 with annealing at

1000oC for 1min. SiO2. This result proves that SiO2 can form nitrogen vacancy at

AlGaN layer, so generates band bending which causes tunneling current. However, we

cannot get ohmic contact by SiO2 capped annealing.

Future issue is that SiO2 which can form nitrogen vacancy to bend GaN energy

combines TiSi2 which has low metal function and form nitrogen vacancy. To establish

process that SiO2 and TiSi2 are combined would make low ohmic contact.

Fig5.1 Ideal process of Ohmic contact of AlGaN and GaN

41

42

Acknowledgement

First of all, I would like to express my gratitude to my supervisor Prof. Hiroshi Iwai for

his continuous encouragement and advices for my study. He also gave me many

chances to attend conferences. The experiences are precious for my present and future

life.

I deeply thank to Prof. Takeo Hattori, Prof. Kenji Natori, Prof. Nobuyuki Sugii, Prof,

Akira Nishiyama, Prof. Kazuo Tsutsui, Associate Prof. Parhat Ahmet, and Assistant

Prof. Kuniyuki Kakushima for useful advice and great help whenever I met difficult

problem.

I also thank research colleagues of Iwai Lab. for their friendship, active many

discussions and many of encouraging words.

I would like to appreciate the support of secretaries, Ms. Nishizawa and Ms.

Matsumoto.

I thank to my elder of my laboratory, Kana Tsuneishi, Chen Jiang Ning who touched

me how to use

Finally, I would like to thank my parents Yoshifumi and Hiroko and my elder sister

Nozomi for their endless support and encouragement.

electrodes low contact resistancef for algan/gan structure with nitrogen vacancy … ·...

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