1

The Physical Mechanism on the Threshold Voltage Temperature Stability Improvement for GaN HEMTs with Pre-Fluorination Argon Treatment

Yun-Hsiang Wang1,5, Yung C. Liang1,2,a) , Ganesh S. Samudra1, Chih-Fang Huang3, Wei-Hung Kuo4, Guo-Qiang Lo5

1Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119260

2National University of Singapore (Suzhou) Research Institute, Suzhou, China 215123

3Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30013

4Industrial Technology Research Institute, Chutung, Taiwan 31040

5A*Star Institute of Microelectronics, Singapore 117685.

In this paper, a normally-off AlGaN/GaN MIS-HEMT with improved threshold voltage (VTH) thermal stability is

reported with investigations on its physical mechanism. The normally-off operation of the device is achieved from

novel short argon plasma treatment (APT) prior to the fluorine plasma treatment (FPT) on Al2O3 gate dielectrics. For

the MIS-HEMT with FPT only, its VTH drops from 4.2V at room temperature to 0.5V at 200°C. Alternatively, for the

device with APT-then-FPT process, its VTH can retain at 2.5V at 200°C due to the increased amount of deep-level

traps that do not emit electrons at 200°C. This thermally-stable VTH makes this device suitable for high power

applications. The depth profile of the F atoms in Al2O3, measured by the secondary ion mass spectroscopy (SIMS),

reveals a significant increase in the F concentration when APT is conducted prior to FPT. The X-ray photoelectron

spectroscopy (XPS) analysis on the plasma-treated Al2O3 surfaces observes higher composition of Al-F bonds if APT

was applied before FPT. The enhanced breaking of Al-O bonds due to Ar bombardment assisted in the increased

incorporation of F radicals at the surface during the subsequent FPT process. The Schrödinger equation of Al2OxFy

cells, with the same Al-F compositions as obtained from XPS, was solved by Gaussian 09 molecular simulations to

extract electron state distribution as a function of energy. The simulation results show he creation of the deeper trap

states in the Al2O3 bandgap when APT is used before FPT. Finally, the trap distribution extracted from the

simulations is verified by the gate-stress experimental characterization to confirm the physical mechanism described.

Fluorine Plasma Treatment (FPT) is a promising approach applied on AlGaN/GaN heterostructure based HEMT to

achieve normally-off operations with positive threshold voltage (VTH). The technique of applying multiple FPTs by reactive

ion etching (RIE) on Al2O3 gate dielectric after partial gate recess1,2 introduces a high concentration of trapped negative

charge within the Al2O3. This will result in a high-VTH of 6.5V and a satisfactory IDMAX of 350mA/mm, while preserving two-

dimensional electron gas (2DEG) channel quality. However, the technique is unable to generate sufficient deep-level trap

states that can maintain the high-temperature (HT) VTH stability. Other works, such as adding a p-GaN cap layer on AlGaN is

another approach to achieve thermally-stable normally-off VTH of 0.9V at 200°C with ΔVTH=0.15V3 due to temperature

a) Corresponding Author Electronic mail: chii@nus.edu.sg.

2

change. However, the complexity of p-GaN growth may cause device reliability issues. Another approach uses thin AlGaN

layer of 8nm before the fluorine treatment4 and the VTH reduction can be moderately alleviated from around 0.6V at 25°C to

0.1V at 200°C. However, the low VTH achieved in both approaches is not suitable for power electronics applications. In this

reported work, the inductively-coupled plasma (ICP)-RIE, with high coil power to increase the fluorine flux, is utilized for a

single step FPT to achieve significant amount of fluorine (F) incorporation. A low cathode power to reduce the ion

bombardment energy is used to avoid severe damage to the gate dielectric. More importantly, the short argon (Ar) plasma

treatment (APT) prior to the FPT is introduced to effectively enhance the negative charge occupation at deeper trap energy

levels within the Al2O3 dielectric. With the APT-then-FPT, the VTH at 200°C maintains at the highest reported level of +2.5V

for FPT devices, leading to a strong potential in HT operations. The mechanism leading to the VTH thermal stability

improvement after the APT is attributed to the enhancement of Al-F incorporation by the additional breaking of Al-O bonds.

This is supported by the SIMS and XPS measurements. The increase in the amount of Al-F bonds will form deeper level trap

states based on the trap state distribution results from Gaussian 09 molecular simulation and gate stressing measurements on

the devices.

The cross-sectional schematic of the HEMT with the fluorinated gate dielectric is shown in Fig.1. The LG, LGS, LFP,

and LGD are 3µm, 5µm, 1.5µm, and 5µm, respectively. The AlGaN/GaN-on-Si wafer has the 2DEG carrier density and

mobility of 8.51012cm-2 and 1450cm2/Vs, respectively. After mesa isolation by BCl3-based ICP-RIE and SiO2 dielectric

deposition by PECVD, Ti/Al/Ni/Au (25/125/45/55 nm) ohmic contacts for source/drain were formed by RTA at 850C for

30s. About 10nm (50%) of AlGaN was recessed at the gate region by low power BCl3-based ICP-RIE to reduce the 2DEG

concentration to Q2DEG=4.5×1012cm-2 without damaging the AlGaN/GaN interface2. After 6.5nm of ALD-Al2O3 gate

dielectric deposition at 250°C for device B and C, only device C underwent a 30s ICP-Ar plasma treatment with cathode/coil

power of 75/100W. Afterwards, ICP-CHF3 plasma treatment was carried out with fixed cathode/coil power of 10/200W for 4

minutes for devices B and C. The incorporated F was then sealed by another 15nm of ALD-Al2O3 deposition. The selection

of fabrication recipes was based on several process short-loops to achieve optimized device performance. All devices A, B

and C received the same Al2O3 deposition process but only devices B and C received plasma treatments. Finally, Ni/Au

(15/150 nm) gate-metal deposition followed by annealing at 400°C for 5 minutes were applied.

3

Fig.1 The device cross-sectional schematic of the normally-off AlGaN/GaN MIS-HEMT with ICP-fluorinated Al2O3. trecess is

the remaining thickness of AlGaN. The inset magnifies the gate region, where the FTP-induced negative ions and the

thickness of Al2O3 gate stack (t1 and t2) are shown.

Fig.2(a) shows the ID-VG characteristics of devices A~C at room temperature (RT). It is found that ICP-FPT is able to

shift the VTH from 0.95V to as high as 4.2V (device B). This confirms that ICP-RIE is able to introduce more negative

charge into Al2O3 gate dielectric than that by conventional RIE-based single FPT whose VTH is normally about 0.6V3. For

device C, a VTH of 4.4V at room temperature is obtained. The HEMT device VTH can be modeled by Eq.(1)2, where the

barrier ϕb between Ni and Al2O3 is 3.5eV. tAlGaN and tAl2O3 are the thicknesses of AlGaN and Al2O3 respectively. ΔE1 and ΔE3

are the offsets of the conduction band (EC) between Al2O3/AlGaN and AlGaN/GaN respectively, which are added to be

2.1eV. ΔE2 is the EC offset from the Fermi level before FPT, which is 0.16eV. QN is the sheet negative trapped charge

within the dielectric layer. Permittivities are AlGaN=9.20 and Al2O3=70. QT=1.51012 cm-2 is the fixed charge density at

Al2O3/AlGaN interface2. The APT-then-FPT process (device C shown in Fig.2(b)) maintains the off-state gate leakage

current below 20nA/mm at high VG of 10V. In Fig.2(c), a good IDMAX of about 320mA/mm is found for devices B & C,

proving the preservation of 2DEG channel quality by APT-then-FPT. In Fig.2(d), both the VTH values for devices B & C

reduce at HT. The VTH for device C is retained at 2.5V at 200°C, which is much higher than VTH=0.4V for device B. Similar

RT-VTH values after numerous 200°C heating cycles indicate that the fluorine-induced charge within the Al2O3 is not

displaced at HT.

0

2N322

0

2321

32

))Q(()(

OAl

OAlTDEG

AlGaN

recessAlGaNDEGTH

tQtQqttqQ

q

EEEbV

(1)

4

0

10

20

30

40

50

60

0 1 2 3 4 5 6 7 8

Dra

in C

urr

ent

(mA

/mm

)

Gate Voltage (V)

25�C

100�C

150�C

200�C

0

50

100

150

200

250

300

350

400

450

0 1 2 3 4 5 6 7 8

Dra

in C

urr

ent

(mA

/mm

)

Drain Voltage (V)

VG-VTH=6V

VG-VTH=3V

Device A

Device B

Device C

0

10

20

30

40

50

60

70

80

90

100

-3 -2 -1 0 1 2 3 4 5 6 7 8

Dra

in C

urr

ent

(mA

/mm

)

Gate Voltage (V)

VTH

Device A -0.95VDevice B 4.2VDevice C 4.4V

VTH

@VD=1V

(b)(a)

(c) (d)

0 2 4 6 8 10

Ga

te C

urr

ent

(A/m

m)

Gate Voltage (V)

10−6

10−8

10−10

10−12

@VD=1VDevice B

Device C

Device B

Device C

Fig.2 RT characteristics of (a) ID-VG of devices A~C at VD=1V (b) IG-VG of device B and device C at VD=1V. (c) ID-VD

performance of devices A~C with VG−VTH=3V or 6V (d) ID-VG performance of device B & C for VD=1V with (a) T= 25°C,

100°C, 150°C, and 200°C.

The VTH degradation at HT is caused by electron emission from the FPT-induced trap states that reduces the 2DEG

depletion field at gate region. Little VTH reduction of Device A at 200°C5 implies that electron emission from Al2O3/AlGaN

interfacial traps is not responsible for the temperature instability in FPT devices. The thermal emission of the electrons from

fluorinated traps located at an energy level within bandgap can be identified by Eq.(2),6 where ET_MAX is the deepest-bound

energy level of the trap sites from which electrons are emitted, kb is the Boltzmann’s constant and T is the device

temperature. Also n=(th/T0.5)(Nc/T

1.5)=3.251021(mn/m0), where mn/m0=0.16 is the relative electron effective mass8 within

Al2O3, th=(3kbT/ (mnm0))0.5 is the thermal velocity of electrons8 in Al2O3 and NC=2(2πmnm0kbT/h2)1.5 is the effective density

of states in the conduction band (DOS)8 in Al2O3. σAl2O3=10−16 cm2 is the capture cross-section9 of the electrons in Al2O3 and

t=50s is the device heating time to allow for sufficient device surface temperature stabilization to reach quasi-steady state10.

The ET_MAX obtained from Eq.(2) is insensitive to the typical time range in tens of seconds required for reaching quasi-steady

state. It is reasonable to assume that all of the charge trapped in shallower levels than ET_MAX will be emitted due to thermal

energy, and those at deeper levels than ET_MAX will remain trapped. The calculated ET_MAX at 100°C, 150°C, and 200°C are

0.86eV, 0.98eV, and 1.1eV respectively.

)(ln 232_ tTTkE OAlnbMAXT (2)

The depth profiles of F atoms within the Al2O3 with and without the APT prior to the FPT were measured by SIMS

and shown in Fig.3. The APT and FPT recipes are the same as those used for devices B and C, and were applied on separate

5

dies of the same wafer to make samples for SIMS measurement. 30nm of Al2O3 was deposited prior to the plasma treatment.

An additional 10nm of Al2O3 was deposited afterwards to avoid the inaccuracy issues of SIMS characterization on the

surface. In Fig.3, a clear increase in F peak concentration is observed on APT-then-FPT sample, indicating that higher F

incorporation occurs in the Al2O3 due to the additional dangling bonds created after Ar bombardment. Additionally, the F

depth profile is shifted towards the surface after APT, showing the defects created by APT are close to dielectric surface. The

O concentration drops at where F is present for both samples, indicating the replacement of Al-O bonds by Al-F bonds after

FPT. By integrating the F depth profiles and converting them to equivalent sheet charge QN located at their peaks, a similar

QNt2 product of about 1.37×107cm-1 is obtained. This explains the similar VTH between devices B & C at RT according to

Eq.(1).

0

5

10

15

20

25

30

35

40

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30 35 40 45 50

O i

nte

nsi

ty (

arb

. u

nit

s)

F c

on

cen

tra

tion

(1

02

2cm

-3)

Depth (nm)

FPT only

APT+FPT

AlGaNAl2O3

Fig. 3 The SIMS depth profiles of F and O atoms for the samples with and without APT before FPT.

To identify the underlying mechanism of APT to the increase of deeper traps and hence the better VTH thermal

stability, XPS scans of O, Al and F atoms on ALD-Al2O3 grown samples with no treatments, FPT, APT and APT-then-FPT

were carried out. The stronger signals of these atoms detected by XPS are the O 1s, Al 2p and F 1s. They can be used to

observe the changes in the chemical compositions of the atoms after APT and FPT. The 30nm-thick Al2O3 for all four

samples were grown with the same ALD configurations as before. Both APT and FPT were done with the same recipe as

those used for device C. The XPS wide-scan has found no Ar signal for the APT samples, indicating no accountable Ar

remains in the sample after APT. Possibly, the Ar bombarded the surface to break Al-O bonds and re-bounced to the ambient

during the Ar plasma treatment. Observing the O 1s spectra shown in Fig.4(a), both APT and FPT have reduced O peak

intensity as compared with the as-grown Al2O3. The APT-then-FPT sample has the lowest O 1s peak and only 28.1% of O

remained at the surface. In Fig.4(b), the F 1s spectrum of the FPT-only and the APT-then-FPT samples are compared. A

37.9% increase in the XPS spectrum area is observed for the latter, demonstrating the increase in the amount of F

incorporation at the surface. According to the Al 2p spectrum for FPT-only and APT-then-FPT samples shown in Fig.4(c) &

6

(d), the relative proportion of Al-O bonds (with binding energy11 of 75.81 eV) and Al-F bonds (binding energy12 of 77.17eV)

can be obtained by fitting the measured spectra. The Al-F bonds increases from 20.7% to 36.13% for the APT-then-FPT

sample comparing with the FPT-only sample.

0

500

1000

1500

2000

2500

3000

3500

525 530 535 540In

ten

sity

(a

rb. u

nit

)

Binding Energy (eV)

No

treatmentAr

treatmentF

treatmentAr & F

treatments

O 1s

78.6%

42.1%

28.1%

80

130

180

230

280

330

380

70 72 74 76 78 80 82

Inte

nsi

ty (

arb

. u

nit

)

Binding Energy (eV)

F treatment

Al-F

Al-O

combined

Al-F: 20.7%

Al-O: 79.3%

Al 2p

0

1000

2000

3000

4000

5000

6000

680 682 684 686 688 690

Inte

nsi

ty (

arb

. u

nit

)

Binding Energy (eV)

F

treatmentAr & F

treatments

F 1s

80

130

180

230

280

330

380

70 72 74 76 78 80 82

Inte

nsi

ty (

arb

. u

nit

)

Binding Energy (eV)

Ar & F treatments

Al-F

Al-O

combined

Al-F: 36.13%

Al-O: 63.87%

Al 2p

(a) (b)

(c) (d)

Fig.4 The XPS spectrum at the Al2O3 surfaces for (a) O 1s peak with the remaining percentage of oxygen of the samples

compared with the non-treated sample; (b) F 1s peak; and Al 2p peak for samples with (c) FPT-only and (d) APT-then-FPT.

To investigate the link between the Al-F bonds and the formation of trap states at deeper energy levels, the solution of

the Schrödinger equation incorporating the defect potentials is used. A numerical solution was obtained for a primitive cell

with 2 Al atoms bonded to a mix of O and F atoms. It yields relative density of states (DOS) data with energy bandgap (Eg)

of 8.6eV, similar to the Eg of 8.7eV for crystalline α-Al2O311. The bonding schematics of Al2OxFy primitive cells are shown in

Fig. 5(a), where each of the O atoms at the cell edge are counted as 0.5O in the cell formula as they are shared with adjacent

primitive cells. The same Eg is obtained regardless of the simulated cluster size. The Schrödinger equation in presence of the

traps13 for such a system is expressed as (H0+V)Φ=EΦ, where H0 is the Hamiltonian of the perfect crystal, V is the defect

potential, Φ is the wave function and E is its energy eigenvalue. When any impurities are introduced to the perfect lattice to

substitute the host atoms, an additional potential V arises near the impurities to manifest as states with finite DOS forming

the trap centres14,15. The defect potential is related to the difference in the atomic electronegativity between impurity and host

atoms, thus deeper traps can be formed when more F atoms (electronegativity=3.98)16 with larger electronegativity are

substituting the O atoms (electronegativity=3.44)17 to form more Al-F bonds by APT-then-FPT. To quantitatively investigate

the relationship between the number of Al-F bonds to the trap state distribution within the bandgap, the numerical atomistic

first-principle simulations were carried out by the Gaussian 09 software.18 The orbital energies of Al2OxFy lattice cell with

different amount of Al-F bonds were calculated with the Hartree-Fock self-consistent approach with the basis set of 6-31G

for a good accuracy.19 The amount of Al-F bonds used in the simulation were similar to the Al-F bond composition of 20%

7

and 36% of the samples underwent FPT-only and APT-then-FPT shown in Fig. 4(c) & (d) respectively. The simulated total

DOS of the cells are plotted by the Multiwfn wavefunction analyzer20 in Fig.5(b). For Al2O2.5F simulation (Al-F bond=17%),

a trap state symmetric distribution with peak DOS at about 0.96eV below EC is found. For Al2O2F2 simulation (Al-F

bond=33%), the two F atoms can either be bonded with different (denoted as Al2O2F2(I)) or the same Al atoms (denoted as

Al2O2F2(II)). Both of the calculated ET levels are deeper than that of Al2O2.5F. Deepest ET of 1.44eV is obtained for

Al2O2F2(II) as the proximity of the two F atoms provides the strongest local defect potential, which hinders electron de-

trapping.

0

0.2

0.4

0.6

0.8

1

1.2

-14 -12 -10 -8 -6 -4 -2 0

Tota

l D

ensi

ty o

f S

tate

s (e

V-1

)

Energy (eV)

Eg=8.9 eV

ET=0.96eV

ET=1.44eVET=1.10eV

Al2O3

Al2O2.5FAl2O2F2(I)Al2O2F2 (II)

(a) (b)

Fig.5(a) The bonding schematic of the simulated Al2O3, Al2O2.5F, Al2O2F2(I), and Al2O2F2(II) primitive cells, where the

dashed line represents the bonds connecting with adjacent cells. (b) The simulated total DOS distrubution of Al2OxFy along

the energy band.

To verify the simulated trap levels obtained in Fig.5(b) are realistic, the characterization of trap state densities (DF) of

FPT in Al2O3 bandgap is carried out by the pulsed ID-VG measurement on devices B & C. Based on Eq.(3) from the Poole-

Frenkel trap emission theory5,21, stressing the gate with a negative base voltage (VB) is able to provide an external electric

field (ξ) that enhances the emmission of negative charges trapped at deeper energy level, as indicated in the conduction band

(EC) schematic diagram shown in Fig. 6(c). In pulsed ID-VG measurement, a base voltage VB from 0V to 40V is applied for

50s before starting the measurement pulse sequence under RT to allow sufficient time to deplete the trapped charge at states

below ET_MAX. Pulsed ID-VG characteristics with VB=15V, 20V and 25V together with detailed timing sequence are

shown in Fig 6(a) as they are most distinct in this range. Based on the pulsed ID-VG experimental outcomes with various

pulse separation times, 18ms is used as it obtains similar VTH to the ones with much longer separation times. In Eq.(3), QN,0

and QN,VB, which are the QN before and after applying gate stress respectively, can be calculated from the change in VTH with

Eq.(1)2. They are used to derive ET_MAX levels associated with different gate stressing voltages, which is shown in Fig.6(b).

With the extracted QN and ET_MAX, the DF distributions for devices B & C can be calculated from the difference in QN at

adjacent energy levels and are shown in Fig.6(d). For device C, the peak of DF moves towards deeper levels than that of

8

device B. Since more trap states are deeper than the ET_MAX=1.1eV at 200°C, the VTH is much better sustained under HT.

Comparing with the simulated DOS of Al2O2.5F in Fig.5(b), it shows that device B has a similar DF peak location at around

ET=1eV. For device C, it is found that the DOS of Al2O2F2(II) provided a closer fit with the pulsed measurements than that of

Al2O2F2(I). Around 15% mismatch on DF values along the range between 0.6eV and 1.2eV is found. Therefore, Al2O2F2(II) is

considered as a more suitable model for the APT-then-FPT gate stack to explain its improvement in VTH thermal stability

attributed to the formation of F-Al-F bonds.

)1ln()( VB,0,N032_ NOAlMAXT QQqkTqE (3)

where )( 3232 OAlOAlG qtb

tV

Fig.6 (a) Pulsed ID-VG performance of devices B & C for VD=1V with VB from 15V to 25V for 50s (b) The relationship of

VB to ET_MAX. (c) The EC along the gate stack region for fluorinated gate with (solid line) and without (dotted line) gate bias.

(d) The DF distribution within the A2O3 bandgap at energy levels referenced to the EC for devices B & C

In conclusion, for the normally-off AlGaN/GaN power MIS-HEMTs, an argon plasma pre-treatment before ICP-FPT

process on gate dielectric Al2O3 is able to achieve a high VTH of 4.4V at 25°C and 2.5V at 200°C. We reported the physical

mechanism on the VTH temperature stability improvement by the short APT pre-treatment. The SIMS and XPS

characterizations reveal improved F incorporation after APT process, which resulted in the formation of F-Al-F bonds and

deeper trap states creation. Theoretical prediction by solving of Schrödinger equation within repetitive structure of Al2OxFy

molecules and the Gaussian 09 molecular orbital energy simulations were both carried out. The outcomes are further verified

by the gate-stress trap state characterization measurements on devices B & C.

9

Acknowledgement

We acknowledge the research funding support by the Grant 51477108 from the National Natural Science Foundation

of China.

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