Appl. Phys. Lett. 102, 072105 (2013); https://doi.org/10.1063/1.4793483 102, 072105

© 2013 American Institute of Physics.

Interface charge engineering at atomic layerdeposited dielectric/III-nitride interfacesCite as: Appl. Phys. Lett. 102, 072105 (2013); https://doi.org/10.1063/1.4793483Submitted: 28 November 2012 . Accepted: 12 February 2013 . Published Online: 22 February 2013

Ting-Hsiang Hung, Sriram Krishnamoorthy, Michele Esposto, Digbijoy Neelim Nath, Pil Sung Park, andSiddharth Rajan

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Interface charge engineering at atomic layer deposited dielectric/III-nitrideinterfaces

Ting-Hsiang Hung,a) Sriram Krishnamoorthy, Michele Esposto, Digbijoy Neelim Nath,Pil Sung Park, and Siddharth RajanDepartment of Electrical and Computer Engineering The Ohio State University Columbus, Ohio 43210, USA

(Received 28 November 2012; accepted 12 February 2013; published online 22 February 2013)

Interface charges at atomic layer deposited Al2O3/III-nitride interfaces were investigated for

III-nitride layers of different polarity. A large positive sheet charge density is induced at the

Al2O3/III-nitride interface on all the orientations of GaN and Ga-polar AlGaN, and this sheet

charge can be significantly altered using post-metallization anneals. It is proposed that the charges

are caused by interfacial defects that can be passivated and neutralized through a H2 based anneal.

Tailoring of the interface charge density described here can be used to improve critical device

characteristics such as gate leakage and electron transport, and for lateral electrostatic engineering.VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4793483]

III-nitride-based high electron mobility transistors

(HEMTs) are outstanding systems for applications of high

power switching1–6 and high frequency devices7 due to the

combination of large band gap and good electronic transport

properties. Dielectric layers inserted between the gate and

the semiconductor can suppress gate leakage power switch-

ing devices and in scaled high frequency devices. Metal-in-

sulator-semiconductor HEMTs (MISHEMTs)8–19 structure

can efficiently suppress gate leakage in transistors scaled to

achieve higher frequency operation and could also be used

for applications in GaN power switching.1,3

The interface trap density and related dispersion in atomic

layer deposited (ALD) dielectric/III-nitride interfaces are sig-

nificantly lower than in III-As semiconductors,20,21 but recent

work has shown that a high density of fixed charges of the

order of 1 lC/cm2 is induced at ALD-grown Al2O3/GaN and

Al2O3/AlN structures,22–24 an effect that is unique among

previously investigated semiconductor material systems.

While this charge is not modulated by gate voltage and does

not lead to hysteresis under normal device operation, it does

significantly modify the electrostatics in the system. The high

interfacial fixed charges may degrade the mobility of the

2-dimensional electron gas (2DEG)25 and induce electric

fields in the oxide leading to increased tunneling-related

leakage currents. This fixed-charge-induced electric field also

causes leakage through gate dielectrics in N-polar III-nitride

HEMTs.26 Following previous work on post-metallization

annealing (PMA) to passivate interface states in silicon

technology,27,28 in this work, we investigated the effect of

post-metallization anneal on Al2O3/GaN interface charge

density for different polarities of GaN.

Ga-polar GaN samples were grown using a Veeco

GEN 930 plasma molecular beam epitaxy system

(PAMBE) on Fe-doped semi-insulating GaN/sapphire

templates.29 The N-polar samples were grown by PAMBE

on n-doped free standing GaN template.29 The epitaxial

layer consisted of 200 nm unintentionally doped GaN

followed by 100 nm silicon-doped GaN ([Si]� 1� 1018

cm3]. As-received bulk m-plane GaN samples ([Si]

�6� 1016 cm3) were used.30 A 29 nm Al0.3Ga0.7N/1 nm

AlN/GaN HEMT31 sample on Si substrate with 2DEG sheet

carrier density of 1.1� 1013 cm2 (as-received) was used in

the study. The Al0.3Ga0.7N layer was then recessed to 9 nm.

Three Al2O3 layers of nominal thickness 6 nm, 12 nm, and

18 nm were deposited by ALD on each Ga-polar, N-polar

GaN, and m-plane GaN sample at 300 �C, using trimethyla-

luminum (TMA) and H2O as precursors. For the case of

Al2O3 on AlGaN, 17 nm Al2O3 was deposited by ALD. Six

H2O pulses (0.1 s for each) were used at the beginning of

deposition and then followed by TMA precursor. The dif-

ference between estimated and measured Al2O3 thickness

was less than 0.3 nm as confirmed by ellipsometry. Post-

deposition anneal (PDA) consisted of 700 �C in forming gas

for 1 min. Gate patterns were defined by optical contact

aligner, and a Ni/Au/Ni (30/200/30 nm) stack was deposited

using an e-beam evaporator. Large contacts for ohmic con-

tact were patterned using contact lithography, and buffered

oxide etch (BOE) 10:1 was used to locally remove the

oxide layer for large features in order to get ohmic contacts.

After gate metallization, PMA was carried out in the form-

ing gas (5% H2, 95% N2) in a rapid thermal annealing

system at temperatures varying from 400 �C to 550 �C.

A quantitative analysis of the Ni/Al2O3/GaN capacitors

was done to determine conduction band discontinuity, elec-

tric field in the dielectric layer, and interface fixed charge

for each polarity. C-V measurements were performed using

an Agilent B1500 semiconductor device analyzer equipped

with medium power source/monitor units (MPSMUs) and

multi frequency capacitance measurement unit (MFCMU).

Figure 1(a) shows the C-V measurement result for three

different thicknesses of Al2O3 on Ga-polar GaN before

PMA treatment. The flat band voltage (VFB) was derived

from capacitance voltage profiles and was found to be

�2.8 V,�1.7 V, and �0.5 V for 18 nm, 12 nm, and 6 nm

Al2O3/GaN, respectively. The conduction band diagram of

Ni/Al2O3/GaN MIS structure is shown in the inset of

Figure 1(a).

a)Author to whom correspondence should be addressed. Electronic mail:

hung.110@osu.edu. Tel.: þ1-614-688-8458.

0003-6951/2013/102(7)/072105/4/$30.00 VC 2013 American Institute of Physics102, 072105-1

APPLIED PHYSICS LETTERS 102, 072105 (2013)

A simple analytical expression relating the applied flat

band gate voltage to the interfacial parameters can be derived

as

qVgi ¼ ðub � DEc � usÞ � qFoxtoxi; (1)

where ub is the Ni/Al2O3 conduction band barrier height, Fox

is the electric field in the oxide, us is the energy difference

between conduction band and Fermi level in GaN, DEc is the

conduction band offset between Al2O3 and GaN, and Vgi is

the flat band gate bias for Al2O3 thickness toxi.

From C-V measurements, the VFB extracted was found

to vary linearly with oxide thickness, confirming the validity

of Eq. (1) (Figure 1(b)) and indicating that the charge was

distributed as a sheet, rather than throughout the oxide. Since

the flat band voltage dependence on Al2O3 thickness is lin-

ear, the electric field in the oxide can be assumed to be inde-

pendent of the oxide thickness, indicating the absence of a

significant density of bulk oxide charges. From Eq. (1), the

slope of this curve gives the oxide field, while the intercept

can be used to determine the conduction band offset. us is

estimated as 18 meV using doping density in GaN, and the

Ni/Al2O3 conduction band barrier height ub was estimated to

be 3 eV from internal photoemission measurements.32 Using

the slope and intercept of the fitted line, Fox was determined

to be 1.92 MV/cm before PMA. Using Fox at the Al2O3/GaN

interface, we estimate a net sheet charge density of

þ9.5� 1012 cm�2. The total interface fixed charge (rfix),

derived by adding the interface net charge (rnet) and GaN

spontaneous polarization charge (rsp_GaN¼ 1.81� 1013

cm�2) in the Ga-polar case, is 2.7� 1013cm�2. Figure 1(b)

shows the flat band voltage (VFB) vs. oxide thickness curves

for 5-min post-metallization anneals at 400 �C, 450 �C, and

500 �C. The slope of the curve in Figure 1(b) decreases as

the PMA temperature is increased, indicating a reduction in

the electric field and the net interface charge density, as

shown in Table I. After 500 �C PMA, the interface net posi-

tive charges were reduced to 1.1� 1012 cm�2, almost 95%

TABLE I. Electric field in the oxide and interface charge density for different polarities of GaN after PMA.

PMA conditionGa-polar No PMA 400 �C 5 min 450 �C 5 min 500 �C 5 min

Fox (MV/cm) 1.92 0.83 0.45 0.22

rnet (cm�2) þ9.5� 1012 þ4.1� 1012 þ2.2� 1012 þ1.1� 1012

PMA conditionN-polar 400 �C 5 min 450 �C 5 min 500 �C 5 min 550 �C 5 min

Fox (MV/cm) 1.67 1.08 0.51 0.16

rnet (cm�2) þ9.2� 1012 þ5.9� 1012 þ2.8� 1012 þ9.0� 1011

PMA conditionnon-polar No PMA 400 �C 5 min 450 �C 5 min

Fox (MV/cm) 1.42 0.46 �0

rnet (cm�2) þ7.8� 1012 þ2.5� 1012 �0

FIG. 1. (a) C-V characteristics for Al2O3/

Ga-polar GaN before PMA. (b) Flat-band

voltage versus oxide thickness for the

Ga-polar GaN for different PMA

temperatures.

FIG. 2. (a) C-V profile for Al2O3/N-polar

GaN after 400 �C PMA. (b) Flat-band

voltage versus oxide thickness for Al2O3/

N-polar GaN at different temperature

anneals.

072105-2 Hung et al. Appl. Phys. Lett. 102, 072105 (2013)

lower than the pre-anneal charge density, and the total rfix

was reduced to 1.9� 1013 cm�2. The intercept of the curves

was used to determine the conduction band offset, which

was found to be 2.1 eV before PMA. After PMA, DEc was

found to approximately the same (2.3 eV), showing that the

annealing does not change the band line-ups significantly.

While the N-polar samples have opposite surface

termination and spontaneous polarization to the Ga-polar

samples, our results indicated that the electrostatics of the

ALD/GaN interface is similar to the Ga-polar surface. After

ALD deposition, a net positive charge density was found at

the N-polar GaN surface. Figure 2(a) shows the C-V

measurement results of the Al2O3/N-polar GaN after differ-

ent PMA treatments. We extracted flat band voltage from

C-V results, and the relation between VFB and oxide thick-

ness for N-polar case is shown in Figure 2(b). The conduc-

tion band offset for the N-polar case was found to be

2.1 eV. The electric field in the oxide was found to reduce

from 1.7 MV/cm to 0.2 MV/cm, indicating that the inter-

face net charges were reduced to 9.0� 1011 cm�2 as the

PMA temperature was increased from 400 �C to 550 �C.

This indicates that, in N-polar case, positive GaN spontane-

ous polarization charges were neutralized after higher tem-

perature of PMA. The oxide field Fox and net interface

charge density rnet are listed in Table I.

To determine the effects of polarization on the interfa-

cial properties, a series of PMA experiments were done on

non-polar surfaces using Al2O3/m-plane GaN structures.

Figures 3(a) and 3(b) show the C-V measurement and VFB

verses oxide thickness plot for this structure. Before PMA,

positive interface fixed charges (7.8� 1012 cm�2) were

found to exist at the Al2O3/m-plane GaN interface, causing

an electric field in the oxide under flat band conditions in the

GaN. The conduction band offset as determined from the

measurements was 2.1 eV. PMA was found to remove the

interface charges completely, similar to the Ga- and N-polar

samples.

The case of Al2O3 on AlGaN is of special technologi-

cal interest for AlGaN/GaN HEMTs. Figure 4 shows C-V

profile before and after PMA treatment at different temper-

atures for Al2O3/AlGaN structures. We find that there is a

higher positive charge at the Al2O3/AlGaN interface than at

the Al2O3/GaN interface, as predicted from previous

FIG. 3. (a) C-V profile for Al2O3/m-plane

GaN without PMA. (b) Flat-band voltage

versus oxide thickness for the m-plane GaN

for different PMA temperatures.

FIG. 4. C-V profile for Al2O3/Al0.3Ga0.7N/AlN/ GaN before and after

400 �C, 450 �C, 500 �C PMA.

FIG. 5. Gate leakage current in Al2O3/Ga-polar GaN after different PMA temperatures showing suppression of leakage with increased PMA.

072105-3 Hung et al. Appl. Phys. Lett. 102, 072105 (2013)

work.24 The flat band voltage shifts in the positive direction

after higher temperature PMA, similar to the case of Al2O3

on GaN, showing that the positive charge density has been

reduced.

Based on the energy band diagrams, the bare n-type

GaN surface and the AlGaN surface without ALD dielectric

have net negative surface charges (equal to the positive

depletion layer charge). Our results show that after ALD

deposition, net positive charges are induced at the ALD/GaN

interface for all polarities, showing that rnet changes from

negative to positive after depositing ALD Al2O3. We also

find that PMA treatment can efficiently reduce the interface

positive net charges in Al2O3 on Ga-polar, N-polar, m-plane

GaN and AlGaN. The origin of this net positive charge is not

known—it could be attributed to either the increase of posi-

tive interface fixed charges or a reduction in the number of

previously existent negative surface states after Al2O3 depo-

sition. The polarity-independent result indicates that the

source of the Al2O3/III-nitride interface fixed charge does

not originate from intrinsic polarization-charge inversion22

but rather may be due to defects at the surface.33 We

hypothesize that the gate metal assists the absorption of

forming gas at the Al2O3/III-nitride interface and compen-

sate the spontaneous polarization charge. The absorption of

hydrogen atoms becomes more efficient when we increased

the PMA temperature, and the interface net charge is reduced

substantially.

The use of PMA to control the Al2O3/III-nitride in inter-

face net charges is important for design of AlGaN/GaN

HEMTs for various applications. The positive interface

charge also affects the dielectric leakage characteristics sig-

nificantly. In Figure 5, the gate leakage current for the

Al2O3/Ga-polar GaN structures discussed earlier in this letter

is shown. As expected, the decrease in the electric field in

oxide reduces the leakage currents by suppressing field-

assisted tunneling mechanisms (Fowler-Nordheim tunnel-

ing). The high interface fixed can act as remote scattering

centers and decrease electron mobility especially when gate

oxide thickness is scaled,25 and the channel is close (several

nm) from the fixed charges. The results described here pro-

vide a method to reduce the interface charge density, thereby

eliminating mobility degradation due to remote ionized im-

purity scattering. Perhaps unique to the III-nitride system,

the ability to tune high dielectric/semiconductor charge

densities (of the order of 1013) can provide a new way to

engineer lateral band structures and charge density in semi-

conductor devices. For example, selective patterning of

metals can be used to create different band profiles in differ-

ent regions of a device, giving unprecedented flexibility in

lateral band structure design.

In conclusion, that post metallization anneal in a hydro-

gen containing ambient can effectively reduce the Al2O3/III-

nitride interface net charges in Ga-polar, N-polar, non-polar

GaN, and AlGaN cases. The investigation gives further

insight of the origin of the interface fixed charges at oxide/

III-nitride interface and provides a method to engineer inter-

face charge density. The suppression of the interface charges

using PMA is shown to reduce the gate leakage in reverse

bias and could be critical for mitigating remote ionized im-

purity scattering in GaN-based MISFETs.

This work was funded by the ONR DEFINE MURI

(N00014-10-1-0937) program (Program manager: Dr. Daniel S.

Green).

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