POLARIZATION-CHARGE INVERSION AT AL2O3/GAN INTERFACES THROUGH POST-DEPOSITION ANNEALING - MDPI

electronics
Article
Polarization-Charge Inversion at Al2O3/GaN
Interfaces through Post-Deposition Annealing
Kwangeun Kim 1, * and Jaewon Jang 2, *
 1 Department of Electronic and Electrical Convergence Engineering, Hongik University, Sejong 30016, Korea
 2 School of Electronics Engineering, Kyungpook National University, Daegu 41566, Korea
 * Correspondence: kim@hongik.ac.kr (K.K.); j1jang@knu.ac.kr (J.J.)
 



 Received: 2 June 2020; Accepted: 29 June 2020; Published: 30 June 2020 


 Abstract: The effects of post-deposition annealing (PDA) on the formation of polarization-charge
 inversion at ultrathin Al2 O3 /Ga-polar GaN interfaces are assessed by the analysis of energy band
 bending and measurement of electrical conduction. The PDA-induced positive interface charges form
 downward energy band bending at the Al2 O3 /GaN interfaces with polarization-charge inversion,
 which is analyzed using X-ray photoelectron spectroscopy. Net charge and interface charge densities at
 the Al2 O3 /GaN interfaces are estimated after PDA at 500 ◦ C, 700 ◦ C, and 900 ◦ C. The PDA temperatures
 affect the formation of charge densities. That is, the charge density increases up to 700 ◦ C and then
 decreases at 900 ◦ C. Electrical characteristics of GaN Schottky diodes with ultrathin Al2 O3 layers
 exhibit the passivation ability of the Al2 O3 surface layer and the effects of polarization-charge
 inversion through PDA. This result can be applied to improvement in GaN-based electronic devices
 where surface states and process temperature work important role in device performance.

 Keywords: post-deposition annealing; Al2 O3 /GaN; interface charge density; polarization-charge
 inversion

1. Introduction
 Chemical and physical properties at GaN surfaces and interfaces have a remarkable influence on
the electronic device performance and stability [1–5]. GaN-based Schottky contact is a prevailing basic
building block for power switching components, based on the material’s robust properties such as high
breakdown field, direct wide bandgap, radiation hardness, high electron mobility, and high saturation
velocity [6–13]. Due to the unavoidable surface and interface trap charges and defect states originated
from the dangling bonds and threading dislocations on GaN surface, drawbacks of conduction in
GaN electronic components are observed, such as leakage current, current collapse, and premature
breakdown [14–23]. Deposition of a high-k dielectric passivation layer can counteract the effects of
surface states on the conduction. Al2 O3 is a prevalent dielectric material for surface passivation owing
to large bandgap, high permittivity, and high breakdown field [24–27].
 Post-deposition annealing (PDA) is a subsequent step for atomic-layer deposition (ALD) Al2 O3 thin
film to enhance interface quality and passivation ability. Some researches represent the experimental
discovery of energy band bending at high-k dielectric/GaN interfaces with PDA [6–10,24–27]. Energy
band bending at the GaN interfaces is associated with the interface charge density and surface potential
formed by PDA. Therefore, the fundamentals of physical and chemical phenomena occurred at the
interface through the PDA should be analyzed to understand the conduction in GaN electronic
devices with Al2 O3 passivation. Up to now, the analysis on the energy band bending and surface
potential at the Al2 O3 /GaN interfaces is mainly of metal-oxide-semiconductor structure in which
the thickness of the gate oxide layer is thicker than the thickness of the oxide layer for a surface
passivation [3,6–8]. The correlation between the PDA-induced surface states and device performance

Electronics 2020, 9, 1068; doi:10.3390/electronics9071068 www.mdpi.com/journal/electronics

Electronics 2020, 9, 1068 2 of 11

remains unexplored, too, though the verification of the mechanism at the interface is prerequisite for
device design and optimization.
 In this work, the effects of PDA on the formation of polarization-charge inversion at Al2 O3 /GaN
interfaces are examined through the analysis of energy band bending and measurements of electrical
property with PDA at 500 ◦ C, 700 ◦ C and 900 ◦ C. The downward band bending is observed at
the Al2 O3 /GaN interfaces after the PDA at 700 and 900 ◦ C, attributed to the formation of positive
interface charges that lead to polarization-charge inversion. The quality of charge carriers’ flow
in GaN SDs improved with the positive charges formed by the PDA in addition to passivation
ability of Al2 O3 surface layers. However, leakage current level increases at the 900 ◦ C PDA due to
the micro-crystallization of Al2 O3 . The principles for the changes in energy band bending at the
Al2 O3 /GaN interfaces and improved conduction in the Al2 O3 /GaN SDs are discussed.

2. Materials and Methods
 Wurtzite Ga-polar GaN was grown by metal organic chemical vapor deposition with a Si doping
concentration of 5 × 1017 cm−3 from the top surface to 400 nm depth for a Schottky contact region, and
5 × 1018 cm−3 from 400 to 800 nm depth for an ohmic contact region. As-grown GaN substrates were
treated with sulfuric acid peroxide mixture (SPM) and RCA cleaning procedures. The GaN go through
a sonicating in acetone and isopropyl alcohol (as-prepared GaN), followed by dipping into a SPM
solution (Piranha, H2 SO4 :H2 O2 = 3:1) and an ammonium hydroxide solution (NH4 OH:H2 O2 :DI H2 O
= 1:1:5) for organic debris removal, then a hydrofluoric acid solution (HF:DI H2 O = 1:50) for surface
oxide removal, and a hydrochloric acid solution (HCl:H2 O2 :DI H2 O = 1:1:5) for ionic debris removal
(as-cleaned GaN). Al2 O3 was deposited on the as-cleaned GaN by ALD. The GaN samples were loaded
into an ex-situ ALD chamber, where an H2 O source was applied as an oxidant at a stage temperature
of 250 ◦ C, followed by ten- and thirty-cycles deposition of Al(CH3 )3 and H2 O precursors in alternative
pulses. The growth rate of ALD layer is 1.0 Å/cycle. The Al2 O3 -deposited GaN then went through PDA
in a N2 ambient at 500 ◦ C, 700 ◦ C and 900 ◦ C for 3 min, respectively. For a circular-shaped Schottky
diode (SD) fabrication, the ground mesa was first formed by inductively coupled plasma-reactive ion
etching (ICP-RIE). After the etching, the samples were cleaned to remove PR residue and induced
charges during ICP-RIE process. After an additional pattering for ground metal area which is few
micrometers smaller in dimension than that of the pattering for Schottky gate mesa. Then, Ti/Al/Ti/Au
(20/180/20/80 nm) stack was deposited, followed by ohmic annealing at 650 ◦ C for 30 s. The gate contact
area was defined and then dipped in a diluted HF solution to etch away the Al2 O3 layer above the
contact area. Ni/Au (20/180 nm) was deposited for a Schottky contact (Figure 1a).
 Effects of PDA on the energy band bending at the ultrathin Al2 O3 /GaN interfaces were assessed
by an X-ray photoelectron spectroscopy (XPS) measurement. A monochromatic 1486.60 eV Al Kα X-ray
source was applied to scanning with 0.04 eV scan step, 100 µm spot size, 50 eV pass energy, and 50 ms
dwell time. The measurement energy scale was calibrated using the Cu 2p3/2 , Ag 3d5/2 , and Au 4f7/2
standard binding energy peak positions. The C 1s peak was referenced to 284.80 eV to offset charge
effect. The survey scans were repeated 10 times. The electrical measurements were performed by
semiconductor parameter analyzer system. All measurements were conducted at room temperature.

Electronics 2020, 9, 1068 3 of 11
Electronics 2020, 9, 1068 3 of 10

 (a)

 (b) (c)
 Figure 1.
 Figure 1. (a)
 (a) Process
 Process steps
 steps for
 for GaN
 GaN Schottky
 Schottky diode
 diode (SD):
 (SD): (i)
 (i) preparation,
 preparation, (ii)
 (ii) atomic-layer
 atomic-layer deposition
 deposition
 (ALD) Al O , (iii) post-deposition annealing
 (ALD) Al2 3 , (iii) post-deposition annealing (PDA),
 2 3 (PDA), (iv) mesa etching using inductively coupled
 inductively coupled
 plasma-reactive ion
 plasma-reactive ion etching
 etching (ICP-RIE),
 (ICP-RIE), (v)
 (v) ground
 ground metal
 metal deposition,
 deposition, (vi) metal area formation through
 wet etch,
 wet etch, (vii)
 (vii) Schottky
 Schottky metal
 metal deposition,
 deposition, and (viii)
 (viii) optical
 optical image of circular-shaped
 circular-shaped GaN SD (scale
 (scale
 bar =
 bar = 50 µm). (b) (b) Characterization
 Characterization of of surface
 surface polarity
 polarity ofof GaN
 GaN using
 using X-ray
 X-ray photoelectron
 photoelectron spectroscopy
 (XPS). (c)
 (XPS). (c) XPS
 XPS spectra
 spectra ofof Ga
 Ga 3d
 3d core
 core levels
 levels in
 in as-prepared
 as-preparedand andas-cleaned
 as-cleanedGaN.
 GaN.

3.
3. Results and Discussion
 Results and Discussion
 XPS
 XPS is is utilized
 utilized to to identify
 identify the the surface
 surface polarity
 polarity of of GaN
 GaN by by scanning
 scanning aa near-valence
 near-valence band band (VB)(VB)
region (Figure 1b). There are two well-defined peaks (P and P
region (Figure 1b). There are two well-defined peaks (P1 and P2) emerged from the atomic p- and
 1 2 ) emerged from the atomic p- and
 s-
s-orbital states of GaN [28,29].
orbital states of GaN [28,29]. The P1 and The P 1 and P peaks
 P2 peaks
 2 at the binding energy (BE) of 4.00
 at the binding energy (BE) of 4.00 eV and 8.66 eV, eV and 8.66 eV,
individually,
individually, are are associated
 associated with with the
 the Ga
 Ga 4p-N
 4p-N 2p 2p hybridization
 hybridization and and Ga Ga 4s-N
 4s-N 2p hybridization states,
 2p hybridization states,
respectively. Here, the intensity of P peak is stronger than that of the P
respectively. Here, the intensity of P11 peak is stronger than that of the P22 peak, with the 1.14 intensity peak, with the 1.14 intensity
ratio
ratio of
 of PP11 to
 to P peaks, which
 P22 peaks, which verifies
 verifies Ga-face
 Ga-face polarity
 polarity of of GaN
 GaN [28].
 [28]. The The BEBE difference
 difference obtained
 obtained fromfrom
the
the peak shift in Ga 3d core levels can represent the surface band bending of GaN (Figure 1c). The Ga
 peak shift in Ga 3d core levels can represent the surface band bending of GaN (Figure 1c). The Ga
3d
3d peak
 peak BEsBEs ofof as-prepared
 as-preparedand andas-cleaned
 as-cleanedGaN GaNare are19.82
 19.82and and19.68
 19.68eV, eV,respectively,
 respectively,which whichindicates
 indicates a
demonstration
a demonstration ofof
 0.14
 0.14eVeVupward
 upwardband bandbending
 bendingon onthetheGaNGaNsurface
 surfaceafter aftercleaning
 cleaningsteps.
 steps. An
 An internal
 internal
spontaneous polarization (P = −0.033 C·m −2 ) of Ga-faced GaN heading from surface to bulk forms
spontaneous polarization (Psp = −0.033 C·m ) of Ga-faced GaN heading from surface to bulk forms
 sp −2
 13 −2 nearby the surface band therefore induces upward
negative
negative bound
 bound sheetsheet charges
 charges (1.81 (1.81×× 101013 cmcm−2)) nearby the surface band therefore induces upward
band bending [8,30,31].
band bending [8,30,31].
 Figure
 Figure 22 exhibits
 exhibits XPSXPS spectra
 spectra of of Ga
 Ga 3d,3d, AlAl 2p,
 2p, O O 1s,
 1s, and
 and N N 1s 1s core
 core levels
 levels atat 11nmnmAl O33/GaN
 Al22O /GaN
interfaces ◦ ◦
interfaces withwith no no PDA
 PDA (sample
 (sample A1) A1) andand with
 with PDAPDA at at 500
 500 °C C (sample
 (sample B1), B1), 700
 700 °CC (sample
 (sample C1),C1), and
 and
900 ◦ C (sample D1) for 3 min. The peak BE of Ga 3d core level of 1 nm Al O as-deposited GaN (A1) is
900 °C (sample D1) for 3 min. The peak BE of Ga 3d core level of 1 nm Al2O3 as-deposited GaN (A1) 2 3
located
is located at at
 19.55
 19.55eVeV and
 and then
 then the
 thepeak
 peakBE BEshifts
 shiftstoto19.70,
 19.70,20.16
 20.16andand20.01 20.01eVeVwith
 with thethe 33 min
 min PDA
 PDA at at
500 ◦ C, 700 ◦ C and ◦
500 °C, 700 °C and 900900 °C,C, individually
 individually (Figure
 (Figure 2a). The increase
 2a). The increase in in the
 the peak
 peak BE BE of
 of Ga
 Ga 3d3d core
 core levels
 levels
upon
upon PDAPDA indicates
 indicates thethe increase
 increase in in Ga-O
 Ga-O bonding
 bonding at at the
 the Al
 Al22O /GaN interfaces
 O33/GaN interfaces in in which
 which O O atoms
 atoms
diffuse
diffuse from Al22O33 to GaN through PDA [32,33]. The behavior of the peak shift in Al 2p core level
 from Al O to GaN through PDA [32,33]. The behavior of the peak shift in Al 2p core level
according
according to the PDA conditions is similar with that of Ga 3d core level. The peak BE of Al 2p core
 to the PDA conditions is similar with that of Ga 3d core level. The peak BE of Al 2p core
level
level at
 at the
 the 11 nm
 nm Al Al22O /GaN interface
 O33/GaN interface is is located
 locatedat at 75.60
 75.60eV, eV, then
 then shifts
 shifts toto 75.74,
 75.74, 76.22
 76.22 and
 and 76.11
 76.11 eV,eV,
with the 3 min PDA at 500 °C, 700 °C and 900 °C, respectively (Figure 2b), in which the peak position
shifts to higher BE through PDA supports the diffusion of O atoms into GaN.

Electronics 2020, 9, 1068 4 of 11

with the 3 min PDA at 500 ◦ C, 700 ◦ C and 900 ◦ C, respectively (Figure 2b), in which the peak position
shifts to higher
Electronics
Electronics 2020, 9,
 2020, BE through PDA supports the diffusion of O atoms into GaN.
 9, 1068
 1068 44 of
 of 10
 10

 (a) (b) (c) (d)
 Figure 2.
 Figure 2. XPS
 2. XPS spectra
 spectra of
 of (a)
 (a) Ga
 Ga 3d,
 3d, (b)
 (b) Al
 Al 2p,
 2p, (c)
 2p, (c) O
 (c) O 1s,
 O 1s, and
 1s, and (d)
 and (d) N
 (d) N 1s
 N 1score
 1s corelevels
 core levelsat
 levels at1nm
 at 1nmAl
 1nm Al222O
 Al O333/GaN
 O /GaN
 interfaces with
 interfaces with post-deposition
 with post-deposition annealing
 post-depositionannealing (PDA)
 annealing(PDA) conditions:
 (PDA)conditions: nono
 no
 conditions: anneal
 anneal (A1),
 (A1),
 anneal 500500
 500
 (A1), °C for
 °C for
 ◦ 33 min
 C formin
 3 min(B1),(B1),
 (B1), 700
 700
 °C
 700for
 ◦ 3 min
 °C forC3for
 min (C1), and
 (C1),(C1),
 3 min 900
 and and °C
 900 900for
 ◦ 3 min
 °C forC3for
 min (D1).
 (D1).(D1).
 3 min

 displays XPS spectra of Ga
 Figure 3 displays Ga 3d,
 3d, Al
 Al 2p,
 2p, OO 1s,
 1s, and
 and N N 1s
 1s core
 core levels
 levels at
 at33nm
 nmAl O333/GaN
 Al222O /GaN
interfaces with no PDA (sample A3) and with PDA at at 500 ◦ C (sample B3), 700 ◦
 700 °C (sample C3), and
 500 °C (sample B3), C (sample
900 ◦°C
 C (sample D3) for 3 min. The peak BEs of Ga 3d are 19.50, 19.64, 20.22 and 20.05 eV (Figure 3a),
and theBEs
and the BEsofof
 Al Al 2p 75.56,
 2p are are 75.56,
 75.68,75.68, 76.3076.16
 76.30 and andeV 76.16 eVsamples
 for the for the A3,
 samples
 B3, C3,A3,
 andB3,D3,C3, and D3,
 respectively
(Figure 3b). The
respectively increase
 (Figure in the
 3b). The peak BEs
 increase of peak
 in the Ga 3d and
 BEs ofAl
 Ga2p 3dcore
 andlevels
 Al 2p through
 core levelsPDA agree PDA
 through with
agree
the with theofoutcomes
 outcomes of O
 O diffusion at diffusion
 the 1 nm at O3 /GaN
 Al2the 1 nm interfaces.
 Al22O33/GaN Theinterfaces. The XPS measurement
 XPS measurement results are
summarized in Table 1. in Table 1.
results are summarized

 (a) (b) (c) (d)
 Figure 3.
 Figure 3. XPS
 3. XPS spectra
 spectra of
 of (a)
 (a) Ga
 Ga 3d,
 3d, (b)
 (b) Al
 Al 2p,
 2p, (c)
 2p, (c) O
 (c) O 1s,
 O 1s, and
 1s, and (d)
 and (d) N
 (d) N 1s
 N 1s core
 1s corelevels
 core levelsat
 levels at333nm
 at nmAl
 nm Al222O
 Al O333/GaN
 O /GaN
 interfaces with
 withPDA
 PDAconditions:
 conditions:no
 noanneal
 anneal(A3),
 (A3),500 ◦
 500C°C
 °Cfor
 for ◦ ◦C
 interfaces
 interfaces with PDA conditions: no anneal (A3), 500 for 33 min
 3 minmin (B3),
 (B3),
 (B3), 700C°C
 700700 °C for
 forfor 33 min
 3 min min (C3),
 (C3), and
 andand
 (C3), 900
 900900
 °C for
 for
 °C for 33 min
 3 min min (D3).
 (D3).
 (D3).

A3 3 nm N. A. 19.50 75.56 531.48 397.23
 B3 3 nm 500 °C 19.64 75.68 531.58 397.38
 C3 3 nm 700 °C 20.22 76.30 532.26 397.97
 D3 3 nm 900 °C 20.05 76.16 532.14 397.81
Electronics 2020, 9, 1068 5 of 11
 The energy band bending at the 1 nm Al2O3/GaN interfaces with the PDA conditions are shown
in Figure 4a‒d. (‒) 0.27 eV surface potential (Ψs) with an upward band bending is formed on the GaN
 Table 1. Binding energies of core elements at Al2 O3 /GaN interfaces.
surface after the deposition of 1 nm Al2O3. The Ψs decreases to (‒) 0.10 eV, 0.34 eV, and increases back
toSample
 0.19 eVNo.
 for the
 Al2samples
 O3 (nm) A1, B1,(3C1,
 PDA and D1.
 min) Ga 3dThe
 (eV)valence band
 Al 2p (eV)offset O
 (VBO)
 1s (eV)at the N
 Al1s
 2O(eV)
 3/GaN

interfaces
 A1 are determined
 1 nm by the equation
 N. A. [6,24,25]:
 19.55 75.60 531.52 397.27
 B1 1 nm 500 ◦ C 19.70 75.74 531.65 397.41
 VBO = [ − ] − [ − ] − [ − ] (1)
 C1 1 nm 700 ◦ C 20.16 76.22 532.13 397.85
whereD1 1 nm
 ECL are the BEs 900 ◦levels,
 of atomic core C 20.01
 EV are 76.11 and subscripts
 the BEs of VBMs, 532.02 b and i397.68
 stand for
 A3 3 nm N. A. 19.50 75.56 531.48 397.23
the bulk GaN and Al2O3/GaN interface, respectively. The second term for bulk Al2O3 is estimated to
 B3 3 nm 500 ◦ C 19.64 75.68 531.58 397.38
be 71.60
 C3 eV. The VBO values
 3 nm at the
 700 C Al2O3/GaN
 1 ◦nm 20.22interfaces 76.30
 with the different PDA conditions
 532.26 397.97 are
determined
 D3 to be 1.94,
 3 nm 1.93, 1.95 and ◦
 900 C 1.99 eV for the
 20.05 samples A1, B1,
 76.16 C1, and D1,
 532.14 individually.
 397.81 The
bandgap (Eg) of Al2O3 is 7.00 eV and conduction band offsets (CBOs) are obtained by mathematical
operations.
 The energy band bending at the 1 nm Al2 O3 /GaN interfaces with the PDA conditions are shown
 For the cases of 3 nm Al2O3-deposited GaN, same methodology with the 1 nm Al2O3 cases is
in Figure 4a-d. (-) 0.27 eV surface potential (Ψs ) with an upward band bending is formed on the GaN
applied to the determination of Ψs, VBO, and CBO at the 3 nm Al2O3/GaN interfaces and the
surface after the deposition of 1 nm Al2 O3 . The Ψs decreases to (-) 0.10 eV, 0.34 eV, and increases
verification of the effect of oxide layer thickness on the surface band bending. The energy band
back to 0.19 eV for the samples A1, B1, C1, and D1. The valence band offset (VBO) at the Al2 O3 /GaN
bending at the 3 nm Al2O3/GaN interfaces with the PDA conditions are exhibited in Figure 5a‒d.
interfaces are determined by the equation [6,24,25]:
 For both cases of 1 and 3 nm Al2O3 interfaces, the directions of band bending change from
upward without andVBO with=500 °C PDA
 [EGaN GaN (A1, B1,AlA3,
 2 O3 and
 − EV 2B3)
 Al O3 to downward
 ]b − [EGaN Al2with
 O3
 ]i 700 °C and 900 °C
 CL − EV ]b − [ECL CL − ECL (1)
PDA (C1, D1, C3 and D3). The surface polarity inversion possibly occurs due to the compensation of
internal
where ECL polarization
 are the BEs charges
 of atomicbycore
 thelevels,
 ionized EV donors in space
 are the BEs of VBMs,charge
 andregion of n-GaN
 subscripts b and iand
 standpositive
 for the
bulk GaN and Al2 O3 /GaN interface, respectively. The second term for bulk Al2 O3 is estimated to be 71.60
interface charges formed at the Al 2 O 3 /GaN interface through PDA. The polarity inversion implies
that the VBO
eV. The negative bound
 values at thesheet Al2 O3 /GaN
 1 nmcharges are fully compensated.
 interfaces Since thePDA
 with the different ionized donorsare
 conditions only partially
 determined
compensate for1.95
to be 1.94, 1.93, the and
 effect of eV
 1.99 negative
 for the polarization
 samples A1, charges [24,25],
 B1, C1, and the main reason
 D1, individually. for the built-in
 The bandgap (Eg ) of
positive
Al2 O3 is polarity can be
 7.00 eV and the generation
 conduction of positive
 band offsets (CBOs)charges at the Alby
 are obtained 2Omathematical
 3/GaN interface. operations.

 (a) (b) (c) (d)
 Figure
 Figure 4. Energy
 Energy band
 band bending
 bending at
 at 1 nm Al22O /GaNinterfaces
 O33/GaN interfaceswith
 withestimated
 estimatedsurface
 surfacepotential
 potential(Ψs),
 (Ψs),
 valence
 valence band
 band offset
 offset (VBO),
 (VBO), and
 and conduction
 conduction band
 band offset
 offset (CBO)
 (CBO) of
 of samples
 samples (a)
 (a) A1,
 A1, (b)
 (b) B1,
 B1, (c)
 (c) C1,
 C1, and
 and
 (d) D1.

 For the cases of 3 nm Al2 O3 -deposited GaN, same methodology with the 1 nm Al2 O3 cases
is applied to the determination of Ψs , VBO, and CBO at the 3 nm Al2 O3 /GaN interfaces and the
verification of the effect of oxide layer thickness on the surface band bending. The energy band bending
at the 3 nm Al2 O3 /GaN interfaces with the PDA conditions are exhibited in Figure 5a-d.
 For both cases of 1 and 3 nm Al2 O3 interfaces, the directions of band bending change from
upward without and with 500 ◦ C PDA (A1, B1, A3, and B3) to downward with 700 ◦ C and 900 ◦ C
PDA (C1, D1, C3 and D3). The surface polarity inversion possibly occurs due to the compensation
of internal polarization charges by the ionized donors in space charge region of n-GaN and positive
interface charges formed at the Al2 O3 /GaN interface through PDA. The polarity inversion implies
that the negative bound sheet charges are fully compensated. Since the ionized donors only partially
compensate for the effect of negative polarization charges [24,25], the main reason for the built-in
positive polarity can be the generation of positive charges at the Al2 O3 /GaN interface.

Electronics 2020, 9, 1068 6 of 11
Electronics 2020, 9, 1068 6 of 10

 (a) (b) (c) (d)
 Figure 5. Energy
 Energy band
 bandbending
 bendingatat33nm
 nmAl
 Al2O
 2O 3 /GaN
 3/GaN interfaces with
 interfaces Ψs,Ψs,
 with VBO, and
 VBO, CBO
 and of samples
 CBO (a)
 of samples
 A3,A3,
 (a) (b)(b)
 B3,B3,
 (c) (c)
 C3,C3,
 andand
 (d)(d)
 D3.D3.

 Some reports show that the density of positive charges at GaN surface increased owing to the
formation of of Ga-O
 Ga-O bonds
 bonds [8,30].
 [8,30]. From
 From the the comparison
 comparison of of ΨΨss values
 values at at the
 the Al /GaN interfaces with
 Al22O33/GaN
different thickness of Al Al22O O33butbutsame
 samePDA PDAtemperature
 temperature(A1 (A1 and A3, B1 and B3,B3,
 and A3, B1 and C1 C1 andandC3,C3,
 andand D1
D1
andandD3),D3), it isitnoticed
 is noticed thatthatthe the direction
 direction of band
 of band bending
 bending at the
 at the 1 nm 1 nm andand 3 nm 3 nm Al2Al O3 interfaces
 O32interfaces are
are
same same
 andand the magnitude
 the magnitude of Ψsof Ψs at
 at the 3 nmthe Al3 nm Al2 O3 interface
 2O3 interface is largeristhan largerthatthan that
 of the of the
 1 nm Al2O1 3nm Al2 O3
 interface.
interface.
Considering Considering the measurement
 the measurement principle principle
 of XPSofwhere XPS where X-ray X-ray penetrates
 penetrates toptop surfaceup
 surface uptoto few
nanometers, the values measured from from the the GaN
 GaN interfaced
 interfaced with with 33 nm nm Al Al22O
 O33 top layer can represent
the ΨΨs relatively close to outer surface side, compared compared to to the
 the values
 values from from the the GaN
 GaN withwith11nm nmAl Al22O
 O33
layer,
layer, which
 whichshows shows thethe Ψs relatively
 Ψs relatively at inner bulk side.
 at inner bulkTherefore, the physical
 side. Therefore, the and chemical
 physical andproperties
 chemical
obtained
properties from the 3 nm
 obtained from Al2theO3 /GaN
 3 nm structure
 Al2O3/GaN canstructure
 be interpreted
 can befor the surfacefor
 interpreted property of GaN
 the surface at the
 property
Al O
of 2GaN /GaN interface.
 3 at the Al2O3/GaN interface.
 To
 To investigate
 investigate the the effects
 effects of of PDA
 PDA on on the
 the formation
 formation of of Ga-O bonds and associated positive charge
densities
densities atatthe theAlAl O
 2 3
 2 O/GaN
 3 /GaN interfaces,
 interfaces, the Ga-O/Ga-N
 the Ga-O/Ga-N ratio (%) are
 ratio (%)obtained
 are obtained from the from XPS GaXPS
 the 3d spectra
 Ga 3d
of samples
spectra A3, B3, C3,
 of samples A3,and B3,D3.C3,TheandGa-OD3. area
 The ratio
 Ga-Oincreases
 area ratio from 19.45% from
 increases to 21.01%19.45%andto 22.24%
 21.01% byand
 the
application
22.24% by the of PDA at 500 ◦ C
 application ofand
 PDA 700at◦500
 C, respectively,
 °C and 700 then decreases to 21.79%
 °C, respectively, with theto
 then decreases PDA at 900
 21.79% ◦ C.
 with
Here,
the PDA the trend
 at 900of°C. changes
 Here, in thethe Ga-O/Ga-N
 trend of changes ratioinandtheΨGa-O/Ga-N
 s at the interface ratio areandidentical,
 Ψs at thewhich reveals
 interface are
the strong which
identical, correlation
 revealsbetween the PDA
 the strong and surface
 correlation polarity
 between the formation.
 PDA and surface The increased
 polarityGa-O/Ga-N
 formation.ratioThe
with the PDA
increased at 500 ◦ Cratio
 Ga-O/Ga-N andwith700 ◦theC isPDA
 attributed
 at 500 °C to the
 anddiffusion
 700 °C isof O atomsto
 attributed ofthe
 Al2diffusion
 O3 into GaN. of O While,
 atoms
the reduction in Ga-O bonds of D3 annealed at 900 ◦ C with respect to that of C3 at 700 ◦ C could be
of Al2O3 into GaN. While, the reduction in Ga-O bonds of D3 annealed at 900 °C with respect to that
ascribed
of C3 at to 700clean up effect
 °C could of PDA totopassivate
 be ascribed clean upGa-O effectbonds,
 of PDA based on the difference
 to passivate Ga-O bonds, in thebased
 magnitudes
 on the
of negativeinGibb’s
difference free energyof
 the magnitudes ofnegative
 Al2 O3 (−1582.3
 Gibb’s free kJ/mol)
 energyand of Ga-O
 Al2O (−998.3
 3 (–1582.3kJ/mol)
 kJ/mol)[34–36]. The net
 and Ga-O (–
area
998.3charge
 kJ/mol) densities
 [34–36].(σThe net ) at
 netthearea O3 /GaN
 Al2charge interfaces
 densities (σnetare
 ) atcalculated
 the Al2O3by /GaNthe interfaces
 following are Equation [8]:
 calculated
by the following Equation [8]: r   ϕ 
 dϕs ε 2e
 
 s
 σnet = ε × =± × NC × Vt × exp − + ND × ϕ s + D (2)
 d 
 dx e 2 ε V t
 = × =± × × × exp − + × + (2)
 d 
where ε is permittivity of GaN, ϕs is surface potential at GaN conduction band edge with respect
to Fermi
where ε islevel, x is distance
 permittivity of GaN,fromφsGaN surface
 is surface into inner
 potential at GaN e is electronic
 bulk,conduction band charge,
 edge withNC isrespect
 effectiveto
density of states in GaN conduction band, V is
Fermi level, x is distance from GaN surface tinto inner bulk, e is electronic thermal voltage, N D is doping concentration,
 charge, NC is effective and
D is integration
density of states constant (−5.7 × 109band,
 in GaN conduction eV/Fcm 2 From the calculation based on the measured values
 Vt is).thermal voltage, ND is doping concentration, and D is
using XPS, the
integration net charge
 constant (−5.7 ×densities
 109 eV/Fcm at the 1 nmthe
 2). From andcalculation
 3 nm Al2 Obased 3 /GaN oninterfaces
 the measured with values
 no PDAusing (A1
 to be −1.19 × 10 12 −2 12 −2
and
XPS,A3) thearenetestimated
 charge densities at the 1 nmcm and and3 nm−1.28
 Al2O×3/GaN 10 cm , respectively,
 interfaces with no and PDAcorresponded
 (A1 and A3)
 13 cm−2 and 1.68 × 1013 cm−2 . These results
interface
are estimated charge to densities
 be −1.19 ×are 10determined
 12 cm−2 and –1.28 to be ×1.691012×cm 10−2 , respectively, and corresponded interface
are comparable value of 1.7to×be 13 cm−2 reported
charge densitiestoare thedetermined 101.69 × 1013 cm−2 and from
 1.68another
 × 1013 cm research
 −2. These group
 results (Table 2) [8,30,31].
 are comparable
The
to theinterface
 value ofcharge1.7 × 10densities
 13 cm reported
 −2 at the Al from /GaN interfaces
 2 O3another research group with PDA (Table are2) also determined
 [8,30,31]. to be
 The interface
1.73 × 10 13 cm−2 , 2.73 × 1014 cm−2 , and 3.19 × 1013 cm−2 for the samples B1, C1, and D1, individually,
charge densities at the Al2O3/GaN interfaces with PDA are also determined to be 1.73 × 1013 cm−2, 2.73
 13 −2 10−214for −2 , and 4.81 × 1013 cm−2 for the samples B3, C3, and D3,13separately.
and
× 1014 cm×
 1.71 −2,10andcm 1013 ×cm
 3.19 ,×8.38 cmthe samples B1, C1, and D1, individually, and 1.71 × 10 cm−2, 8.38
The ◦ C, compared to that
× 1014decrease
 cm−2, andin4.81 the×interface
 1013 cm−2 for charge density B3,
 the samples of D1
 C3, and D3, D3 separately.
 annealed atThe 900decrease in the interface
obtained at 700 ◦ C, is possibly due to clean up effect of PDA to suppress the formation of Ga-O
charge density of D1 and D3 annealed at 900 °C, compared to that obtained at 700 °C, is possibly due
to clean up effect of PDA to suppress the formation of Ga-O bonds [34–36]. In case of the reference
GaN with no Al2O3, the interface charge density is determined to be 1.72 × 1013 cm−2 according to the

Electronics 2020, 9, 1068 7 of 11

bonds [34–36]. In case of the reference GaN with no Al2 O3 , the interface charge density is determined
Electronics 2020, 9, 1068 7 of 10
to be 1.72 × 1013 cm−2 according to the 0.14 eV peak BE shift as shown in Figure 1c. The results of
positive
0.14 eV peakinterface charge
 BE shift as density
 shown in formed
 Figure at 1c.
 theThe
 Al2 O 3 /GaNof
 results interfaces
 positive through
 interfacePDA charge indeed support
 density formedthe
trend in energy band bending.
at the Al2O3/GaN interfaces through PDA indeed support the trend in energy band bending.
 The
 The current
 current density-voltage
 density-voltage(J-V) (J-V)measurements
 measurementsare areperformed
 performed to to
 assess thethe
 assess effects of PDA
 effects of PDAon theon
electrical
the electricalfeatures
 featuresof GaN
 of GaNSDsSDsinterfaced
 interfacedwithwith
 ultrathin
 ultrathinAl2 OAl32Olayers (Figure
 3 layers 6). 6).
 (Figure J-VJ-V
 characteristics
 characteristics of
GaN SDs made of reference, A1, A3, B1, B3, C1, C3, D1, and D3 exhibit
of GaN SDs made of reference, A1, A3, B1, B3, C1, C3, D1, and D3 exhibit that the conduction that the conduction properties
improved
properties through
 improved PDA, in terms
 through PDA, of in
 leakage
 termscurrent,
 of leakage on/off ratio,on/off
 current, and ideality factor.
 ratio, and The Ga-O/Ga-N
 ideality factor. The
ratio in Schottky contact area (gate area) shows no change after PDA
Ga-O/Ga-N ratio in Schottky contact area (gate area) shows no change after PDA and subsequent and subsequent removal of Al2 O3
by
removal of Al2O3 by wet process (B1, B3, C1, C3, D1, and D3), while the ratio in the contact ratio
 wet process (B1, B3, C1, C3, D1, and D3), while the ratio in the contact area recovers initial area
before
recovers Al2initial
 O3 deposition
 ratio beforewhen Alno
 2O3PDA is applied
 deposition when(A1 noandPDA A3). isInapplied
 this case,(A1interface
 and A3). charge densities
 In this case,
at gate contact
interface chargearea of reference
 densities at gate and A1 and
 contact areaA3of have no difference
 reference and A1and andtherefore
 A3 havereduced leakage
 no difference andof
A1 and A3 SDs with respect to the reference SD indicates the passivation
therefore reduced leakage of A1 and A3 SDs with respect to the reference SD indicates the passivation ability of ultrathin Al O
 2 3
layer
abilitydeposited
 of ultrathinbetween
 Al2O3Schottky and ground
 layer deposited electrodes
 between (surface
 Schottky andarea).
 ground When Al2 O3 /GaN
 electrodes substrates
 (surface area).
undergo ◦ C (B1 and B3), similar interface charge densities are induced, compared to that of
When AlPDA 2O3/GaNat 500substrates undergo PDA at 500 °C (B1 and B3), similar interface charge densities
A1
are and A3, atcompared
 induced, both gate and surface
 to that of A1areas
 and ofA3,SDs, resulting
 at both gate inandthesurface
 J-V curvesareasinofFigure 6. When the
 SDs, resulting PDA
 in the J-
temperature goes up to 700 ◦ C, the larger interface charge densities are formed at both gate and surface
V curves in Figure 6. When the PDA temperature goes up to 700 °C, the larger interface charge
areas,
densities compared
 are formedto thatat of B1 gate
 both and B3, andleading
 surfacetoareas,
 the further
 compared reduced leakage
 to that of B1current
 and B3,levels.
 leading In detail,
 to the
from
further thereduced
 comparison leakage of conduction
 current levels. properties
 In detail,in reference and C3 SD, the
 from the comparison leakage current
 of conduction (−0.5 V)
 properties in
decreased by one order of magnitude from 3.51 × 10 −6 A/cm2 to 1.19 × 10−7 A/cm2 and current on/off
reference and C3 SD, the leakage current (−0.5 V) decreased by one order of magnitude from 3.51 ×
ratio (±0.52 to
10−6 A/cm V) 1.19
 increased by one
 × 10−7 A/cm order
 2 and of magnitude
 current on/off ratio from 2.16
 (±0.5 104 to 2.15
 V)×increased 105 .order
 by×one The ideality factor
 of magnitude
decreased ◦
from 2.16 ×from 104 to1.44
 2.15to×1.13
 105. as
 The well. When
 ideality 900 decreased
 factor C PDA is from applied1.44(D1 and as
 to 1.13 D3), however,
 well. When 900the °C
 leakage
 PDA
current levels increase with reduced interface charge densities owing
is applied (D1 and D3), however, the leakage current levels increase with reduced interface charge to the micro-crystallization that
shapes
densities grain boundaries
 owing in Al2 O3 layers as high-leakage
 to the micro-crystallization that shapes paths
 grain at boundaries
 such a high PDA in Altemperature
 2O3 layers as[32,33].
 high-
Whether the sole effect of positive interface charges at the gate or surface
leakage paths at such a high PDA temperature [32,33]. Whether the sole effect of positive interface areas on the conduction
remains
charges at unclear
 the gate at or
 thissurface
 stage since
 areasthe interface
 on the charges
 conduction and oxide
 remains layers
 unclear are formed
 at this stage sincetogether at both
 the interface
areas
charges through
 and oxidePDA.layers are formed together at both areas through PDA.

 Figure
 Figure 6. Current density‒voltage
 6. Current density-voltage (J‒V)
 (J-V) characteristics
 characteristics of GaN SDs made from samples
 samples as-cleaned
 (reference), A1, A3, B1, B3, C1, C3, D1, and D3.
 (reference), A1, A3, B1, B3, C1, C3, D1, and D3.

 Table 2. Charge densities at Al2O3/GaN interfaces.

 Interface
 Al2O3 Anneal Net Charge
 GaN Polarity Charge
No Sample Thickness Condition Method Density Ref.
 (ND, cm−3) Density
 (nm) (°C, min) (cm−2)
 (cm−2)
 This
 1 A1 1 - Ga-face (5 × 1017) XPS −1.19 × 1012 1.69 × 1013
 work
 This
 2 B1 1 PDA (500, 3) Ga-face (5 × 1017) XPS −7.79 × 1011 1.73 × 1013
 work
 This
 3 C1 1 PDA (700, 3) Ga-face (5 × 1017) XPS 2.55 × 1014 2.73 × 1014
 work
 This
 4 D1 1 PDA (900, 3) Ga-face (5 × 1017) XPS 1.38 × 1013 3.19 × 1013
 work

Electronics 2020, 9, 1068 8 of 11

 Table 2. Charge densities at Al2 O3 /GaN interfaces.

 Al2 O3 Thickness Anneal Condition GaN Polarity Net Charge Density Interface Charge
 No Sample Method Ref.
 (nm) (◦ C, min) (ND , cm−3 ) (cm−2 ) Density (cm−2 )
 1 A1 1 - Ga-face (5 × 1017 ) XPS −1.19 × 1012 1.69 × 1013 This work
 2 B1 1 PDA (500, 3) Ga-face (5 × 1017 ) XPS −7.79 × 1011 1.73 × 1013 This work
 3 C1 1 PDA (700, 3) Ga-face (5 × 1017 ) XPS 2.55 × 1014 2.73 × 1014 This work
 4 D1 1 PDA (900, 3) Ga-face (5 × 1017 ) XPS 1.38 × 1013 3.19 × 1013 This work
 5 A3 3 - Ga-face (5 × 1017 ) XPS −1.28 × 1012 1.68 × 1013 This work
 6 B3 3 PDA (500, 3) Ga-face (5 × 1017 ) XPS −9.98 × 1011 1.71 × 1013 This work
 7 C3 3 PDA (700, 3) Ga-face (5 × 1017 ) XPS 8.20 × 1014 8.38 × 1014 This work
 8 D3 3 PDA (900, 3) Ga-face (5 × 1017 ) XPS 3.00 × 1013 4.81 × 1013 This work
 9 S1 - - Ga-face (4 × 1017 ) XPS 2.09 × 1012 2.01 × 1013 Ref. [8]
 10 S2 - - Ga-face (4 × 1017 ) XPS −1.0 × 1012 1.7 × 1013 Ref. [8]
 PDA (600, 3),
 11 - 6, 12, 18 Ga-face (1 × 1018 ) C-V 4.60 × 1012 2.27 × 1013 Ref. [26]
 PMA (400, 5)
 12 - 6, 12, 18 PDA (700, 1) Ga-face (1 × 1018 ) C-V 9.5 × 1012 2.7 × 1013 Ref. [27]
 PDA (700, 1),
 13 - 6, 12, 18 Ga-face (1 × 1018 ) C-V 4.1 × 1012 2.2 × 1013 (estimated) Ref. [27]
 PMA (400, 5)
 PDA (700, 1),
 14 - 6, 12, 18 Ga-face (1 × 1018 ) C-V 2.2 × 1012 2.0 × 1013 (estimated) Ref. [27]
 PMA (450, 5)
 PDA (700, 1),
 15 - 6, 12, 18 Ga-face (1 × 1018 ) C-V 1.1 × 1012 1.9 × 1013 Ref. [27]
 PMA (500, 5)
 PDA (700, 1),
 16 - 6, 12, 18 N-face (1 × 1018 ) C-V 9.2 × 1012 2.7 × 1013 (estimated) Ref. [27]
 PMA (400, 5)
 PDA (700, 1),
 17 - 6, 12, 18 N-face (1 × 1018 ) C-V 5.9 × 1012 2.4 × 1013 (estimated) Ref. [27]
 PMA (450, 5)
 PDA (700, 1),
 18 - 6, 12, 18 N-face (1 × 1018 ) C-V 2.8 × 1012 2.0 × 1013 (estimated) Ref. [27]
 PMA (500, 5)
 PDA (700, 1),
 19 - 6, 12, 18 N-face (1 × 1018 ) C-V 9.0 × 1011 1.9 × 1013 (estimated) Ref. [27]
 PMA (550, 5)
 20 - 6, 12, 18 PDA (700, 1) m-plane (6 × 1016 ) C-V 7.8 × 1012 2.5 × 1013 (estimated) Ref. [27]
 PDA (700, 1),
 21 - 6, 12, 18 m-plane (6 × 1016 ) C-V 2.5 × 1012 2.0 × 1013 (estimated) Ref. [27]
 PMA (400, 5)

Electronics 2020, 9, 1068 9 of 11

4. Conclusions
 The effects of PDA on the polarization-charge inversion at the Al2 O3 /GaN interfaces and conduction
in GaN SDs are investigated by the energy band analysis and electrical characterization. The PDA
induces positive interface charges associated with increased Ga-O ratio by the diffusion of O atoms in
Al2 O3 layers into GaN, which transforms the surface polarity to positive from negative at the interfaces
and thereby forms downward energy band bending. The electrical conduction in GaN SDs improved
with the application of PDA, mainly due to the formation of positive surface polarity at both gate
and surface areas in addition to the passivation ability of Al2 O3 on surface area. This result of PDA
can be applied to the improvement in GaN Schottky-gate devices where interface states and process
conditions are important in device operation efficiency.

Author Contributions: Conceptualization, methodology, formal analysis, investigation, writing—original draft
preparation, writing—review and editing, visualization, K.K. and J.J.; project administration, funding acquisition,
K.K. All authors have read and agreed to the published version of the manuscript.
Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the
Korea government (MSIT) (2019R1G1A1099677 and 2020R1F1A1066175). This work was supported by 2020
Hongik University Research Fund.
Conflicts of Interest: The authors declare no conflict of interest.

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 [CrossRef]

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