Band-Bending of Ga-Polar GaN Interfaced with Al2O3 through

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Band-Bending of Ga-Polar GaN Interfaced with AlO through Ultraviolet/Ozone Treatment 2

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Kwangeun Kim, Jae Ha Ryu, Jisoo Kim, Sang June Cho, Dong Liu, Jeongpil Park, In-Kyu Lee, Baxter Moody, Weidong Zhou, John Albrecht, and Zhenqiang Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 27 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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ACS Applied Materials & Interfaces

Band-Bending of Ga-Polar GaN Interfaced with Al2O3 through Ultraviolet/Ozone Treatment

Kwangeun Kim1, Jae Ha Ryu1, Jisoo Kim1, Sang June Cho1, Dong Liu1, Jeongpil Park1, In-Kyu Lee1, Baxter Moody2, Weidong Zhou3, John Albrecht4, and Zhenqiang Ma1*

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Department of Electrical and Computer Engineering, University of Wisconsin-Madison, 1415

Engineering Drive, Madison, WI 53706, USA 2

HexaTech, Inc., 991 Aviation Parkway, Morrisville, NC 27560, USA

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Department of Electrical Engineering, University of Texas at Arlington, 701 S. Nedderman Drive,

Arlington, TX 76019, USA 4

Department of Electrical and Computer Engineering, Michigan State University, 428 S. Shaw

Lane, East Lansing, MI 48824, USA

*Corresponding Author: Prof. Zhenqiang Ma, E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract: Understanding the band bending at the interface of GaN/dielectric under different surface treatment conditions is critically important for device design, device performance, and device reliability. The effects of ultraviolet/ozone (UV/O3) treatment of the GaN surface on the energy band bending of atomic-layer-deposition (ALD) Al2O3 coated Ga-polar GaN were studied. The UV/O3 treatment and post-ALD anneal can be used to effectively vary the band bending, the valence band offset, conduction band offset, and the interface dipole at the Al2O3/GaN interfaces. The UV/O3 treatment increases the surface energy of the Ga-polar GaN, improves the uniformity of Al2O3 deposition, and changes the amount of trapped charges in the ALD layer. The positively charged surface states formed by the UV/O3 treatment-induced surface factors externally screen the effect of polarization charges in the GaN, in effect, determining the eventual energy band bending at the Al2O3/GaN interfaces. An optimal UV/O3 treatment condition also exists for realizing the “best” interface conditions. The study of UV/O3 treatment effect on the band alignments at the dielectric/III-nitride interfaces will be valuable for applications of transistors, light-emitting diodes, and photovoltaics.

Keywords: X-ray photoelectron spectroscopy (XPS), band alignment, polarization, GaN, Al2O3, atomic-layer-deposition (ALD), trapped charge density


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INTRODUCTION The physical and chemical interactions at the surface of III-nitrides are important to the surface and interface charge behaviors and to device performance and stability.1-24 Among various III‒nitrides, GaN has been prevailingly used in high-power and high-frequency transistors and optoelectronic devices owing to its direct wide band gap, high saturation velocity, and high breakdown electric field.3,15-18 Ga-polar GaN has naturally occurring polarization bound charges on its surface due to spontaneous polarization. The surface states that screen the polarization charges behave as pinning energy levels or trapping centers, which lead to current collapse, leakage current, and thermionic emission.15,17 Previous studies revealed the effects of surface and interface states on the performance of GaN-based electronic devices such as high-electron-mobility transistors (HEMTs),20,21 light-emitting diodes (LEDs),7,11,22 photovoltaics,23,24 and metal-oxidesemiconductor (MOS) capacitors.3,12-14 For the devices structured with bipolar type GaN, e.g. diodes, the change of surface band bending associated with the surface and interface states is very important. Surface band bending affects the device performance by changing the junction barrier height of charge carriers for conduction and increasing/decreasing the GaN internal polarization field, and thereby electron-hole pair recombination rate. The p-GaN layer of the bipolar devices is located at the center or top of the bipolar layer stacks and critically affects the whole device performance due to its natural polarization charges in addition to the low hole mobility (~200 cm2V-1s-1) compared to the electron mobility of n-GaN (~1000 cm2V-1s-1).25 However, most studies of surface and interface treatments were performed for HEMTs with unintentionally doped GaN, but studies on the bipolar type GaN-based devices with the p-GaN has not yet been reported.

Various methods were introduced to improve the surface and interface states, such as wet cleaning,3,17,18 interface passivation layers,26,27 and surface plasma treatment.28-31 Among them,


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ultraviolet/ozone (UV/O3) treatment is important due to its ability to internally/externally screen polarization bound charges of GaN. As of today, the UV/O3 treatment was mainly used for cleaning. A study of the UV/O3 treatment effect on surface and interface states of GaN with the change of internal polarization charges has not yet been explored. Passivating p-GaN with a high‒ k dielectric layer can also change the surface and interface states and thus the polarization-related band bending. Al2O3 realized via atomic layer deposition (ALD) is a promising passivation dielectric due to its large band gap (7.00 eV), high dielectric constant (8.9), high breakdown electric field (10 MV/cm), and high Gibbs free energy (‒1582 kJ·mol-1).2,15,18,32 Here, we look into the band bending changes of p-GaN interfaced with the ALD Al2O3 layer under optimum UV/O3 treatment conditions. In turn, the approach for the band bending and surface treatment can be utilized for device performance improvements.

In this work, the effects of UV/O3 treatment on the band bending of ALD Al2O3-coated Ga-polar p-GaN were explored in detail from X-ray photoelectron spectroscopy (XPS) analysis and metal-oxide-semiconductor capacitor (MOSCAP) characterization. The band alignments of ALD Al2O3/GaN exhibited variations in the surface potential (Ψs), valence band offset (VBO), conduction band offset (CBO), and interface dipole, depending on the UV/O3 treatment conditions on the GaN surface. The UV/O3 treatment altered the band bending at the ALD Al2O3/GaN interface through modifying the charged surface states and thereby compensating for the effects of GaN polarization charges on band bending.

EXPERIMENTAL SECTION GaN preparation. The p-type Wurtzite Ga-polar GaN was grown on c-plane sapphire substrates (0001) by metal organic chemical vapor deposition (MOCVD). A 25 nm-thick GaN


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nucleation layer was first grown on the sapphire substrate at 530°C, and then a 1.2 µm-thick undoped GaN buffer layer and a 50 nm-thick Mg-doped GaN layer (NA = 5×1019 cm-3) were grown at 1040°C. The p-GaN-on-sapphire (GaN, ibid.) went through a wet chemical cleaning process, which included sonicating in acetone and isopropyl alcohol (IPA) individually for 20 min at room temperature (RT), dipping in Piranha solution (H2SO4 : H2O2 = 3 : 1) for 20 min at 100°C, and RCA cleaning steps in an ammonium hydroxide solution (29% NH4OH : 30% H2O2 : deionized water (DI H2O) = 1 : 1 : 5) for 15 min at at 75°C to remove organic debris, a hydrochloric acid solution (37% HCl : 30% H2O2 : DI H2O = 1 : 1 : 5) for 15 min at 75°C to remove ionic debris, and lastly a hydrofluoric acid solution (49% HF : DI H2O2 = 1 : 50) for 1 min at RT to remove surface oxides. UV/O3 treatment. Directly following the completion of the wet chemical cleaning process, the GaN wafers were stored in the N2 ambient carrier and transferred into the UV/O3 treatment system. The atmospheric exposure between the cleaning and the N2 carrier, and between the N2 carrier and UV/O3 treatment system were less than 1 sec. The GaN was exposed to a hot cathode and low-pressure mercury vapor lamp with process active wavelengths of 85% 254 nm and 15% 185 nm. The UV/O3 treatment times for the GaN are 1, 2, 3, and 4 min, separately, with an O3 dose rate of 7 mg/l. Right after the UV/O3 treatments, the GaN wafers were stored in the N2 ambient carrier and directly transferred inside the ALD vacuum chamber. The interval is less than 30 sec. ALD Al2O3 deposition. The 8 nm Al2O3 was deposited on the as-cleaned (no treatment: NT) and UV/O3-treated (1, 2, 3 and 4 min) GaN using ex-situ Ultratech/Cambridge Nanotech Savannah S200 ALD system. The ALD process began with 2 cycles of 0.015 sec H2O pretreatment at 0.37 Torr. Then 80 cycles of 0.015 sec trimethylaluminium (Al(CH3)3, TMA) precursor pulse at 0.37 Torr, 5 sec N2 purging at 0.19 Torr, 0.015 sec H2O precursor pulse at 0.37 Torr, and 5 sec


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interval at 0.19 Torr were chronologically proceeded. After the deposition, a 300 sec stabilization step was followed by a 5 sccm H2O flow at 0.092 Torr. The substrate temperature was 200˚C. The 97.05% purity of TMA (Sigma-Aldrich Co.) and high purity H2O with 18MΩ (The Science Company) were used as the ALD precursors. The ALD Al2O3 on the GaN process is depicted in Scheme 1. XPS measurements. XPS was used to evaluate the effects of UV/O3 treatment on the elements’ core energy levels through band alignment changes between Al2O3 and GaN. A monochromatic Al Kα (hν = 1486.60 eV) X-ray source with a 90° take-off angle (normal to surface) was used with a 3 mA emission current, 1.64 A filament current, and 12 kV accelerating voltage. The survey scans were repeated 20 times with 0.01 eV scan steps, 100 µm spot size, 50 eV pass energy, and 50 ms dwell time. The energy scale was calibrated using the standard binding energy peak positions for Cu 2p3/2 at 933.00 eV, Ag 3d5/2 at 368.20 eV, and Au 4f7/2 at 84.00 eV. To investigate the band alignments of ALD Al2O3 on the untreated and UV/O3-treated GaN samples, the regions for the valence band maximum (VBM) and core levels of Ga 3d, Al 2p, C 1s, N 1s, and O 1s were scanned. The C 1s peak was referenced to the binding energy of 284.80 eV to offset the surface charge induced binding energy shift. The uncertainty of core level centers is ± 0.015 eV, considering the precisions of data measurement and fitting. MOSCAP fabrication. Post-deposition-annealing (PDA) was carried out on the NT and 1, 2, 3, and 4 min UV/O3-treated GaN substrates at 600˚C for 5 min in a N2 ambient for MOSCAP fabrication process. MOSCAPs were fabricated on the PDA Al2O3/GaN samples. Ni/Au (5/100 nm) was deposited on the GaN for the ground electrodes and subsequent ohmic annealing was carried out at 500˚C for 30s in N2 ambient. Au (100 nm) was deposited on the Al2O3/GaN to form circular shaped gate electrodes with a diameter of 100 μm. All electrodes were formed by using


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photolithography and e-beam evaporations. Characterizations. The chemical bonding states at the Al2O3/GaN interfaces were analyzed by a Thermo Scientific K-Alpha+ XPS. The capacitance‒voltage (C‒V) characteristics of MOSCAPs were examined by an Agilent precision E4980 LCR meter. The morphology of ALD Al2O3 on GaN was characterized by a Bruker Multimode 8 atomic force microscopy (AFM). The Raman spectra on the GaN samples were obtained by a Horiba Jobin Yvon Labram Aramis Raman. The contact angle was measured by Dataphysics OCA 15 optical contact angle. Current‒voltage (I‒V) characteristics were measured by a Keithley 4200-SCS semiconductor parameter analyzer. C‒V, AFM, Raman spectroscopy, contact angle, and I‒V measurements were conducted in ambient air at RT. The whole process flow for GaN in this study is depicted in Scheme S1 (see Section A in Supporting Information).

RESULTS AND DISCUSSION The characterized material properties of GaN (0001) with UV/O3 treatment are shown in Figure 1. The Ga-polarity of GaN was verified by the near-valence band (VB) scan using XPS as shown in Figure 1a. The VB spectrum exhibits two distinct peaks originating from the p- and sorbital states.33,34 The P1 peak, with binding energy 2.00 ± 0.015 eV, is related to the Ga 4p-N 2p hybridization state and the P2 peak at 6.50 ± 0.015 eV is associated with the Ga 4s-N 2p hybridization state. The P1 peak is dominant in the spectrum and the ratio of the two polarityrelated peaks (i.e., P1 to P2) is 1.28, which validates the Ga-polarity of GaN.33 The spontaneous polarization (Psp = ‒ 0.033 C·m-2) in wurtzite Ga-polar GaN begins at the surface pointing toward bulk and generates negative bound sheet charges at the surface.15,16


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The piezoelectric polarization induced by the longitudinal strain in GaN is represented as Ppe = (e31-e33C13/C33)ɛxx, where e31 (0.17 C·m-2) and e33 (0.29 C·m-2) are the piezoelectric coefficients, C13 (92.0 GPa) and C33 (389.9 GPa) are the elastic coefficients, and ɛxx is the longitudinal strain.35,36 The strain (ɛxx) can be calculated from the equation ɛxx = σxx/[(C11+C12) ‒ 2C132/C33], where C11 (359.4 GPa) and C12 (129.2 GPa) are the elastic coefficients, and σxx is the biaxial stress which has a linear relationship with the E2 phonon frequency shift at 2.9 cm-1·GPa1 37

. To estimate the piezoelectric polarization, the Raman spectrum (Figure 1b) taken under 532

nm Ar laser excitation displays the high E2 mode at 569.0 cm-1 and the A1 longitudinal optical (LO) phonon mode near 742.0 cm-1. The inset in Figure 1b shows the coupling of the A1(LO) phonon mode and sapphire Eg mode at 750.0 cm-1.38 Compared to the E2 mode of unstrained GaN at 567.6 cm-1, the E2 mode shift across the GaN is 1.4 cm-1 and the estimated piezoelectric polarization is 0.0001 C·m-2. Considering the relatively larger value of spontaneous polarization than that of the piezoelectric polarization, it is expected that the surface band bending of GaN is attributed to the spontaneous polarization. Figure 1c shows the XPS spectra of near-valence band area (0‒2 eV) and the Ga 3d core level of as-prepared GaN. A valence band maximum (VBM) of 0.23 eV was determined by a linear extrapolation of valence band edge to zero intensity and the Ga 3d core level of 18.03 ± 0.015 eV was measured from the as-prepared GaN. Since the VBM is at a relative position with respect to the Fermi level (EF) (i.e., 0.00 eV binding energy), the VBM is positioned 0.23 eV below the Fermi level as shown in Figure 1d. Figure 1e shows the change in intensity of XPS C 1s and O 1s peaks from the GaN surface after the cleaning process (see details in Experimental Section). The decrease in intensity of both the carbon (C) and oxygen (O) peaks is remarkable and the weak level for both peaks after cleaning is possibly due to the surface absorption during the sample loading step.


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The shifting of XPS Ga 3d core levels for the as-cleaned and 1, 2, 3, and 4 min UV/O3treated GaN is represented in Figure 1(f). The binding energy of Ga 3d core level of the as-cleaned GaN is 17.93 ± 0.015 eV, with a 0.10 ± 0.03 eV shift after the cleaning step and this core level further shifts to 17.74, 17.58, 17.45, and 17.55 ± 0.015 eV during UV/O3 treatments for 1, 2, 3, and 4 min, respectively, as shown in Table 1. The dashed lines indicate the binding energies of Ga 3d core levels for the as-cleaned (the left line) and the 3 min UV/O3-treated GaN (the right line). The 0.10 ± 0.03 eV surface potential was measured on the GaN surface with an upward band bending after the cleaning process. Then, the surface potential was changed to 0.29, 0.45, 0.58, and 0.48 ± 0.03 eV after the 1, 2, 3, and 4 min UV/O3 treatments, respectively. The increase in the surface potential up to 3 min treatment time is ascribed to the change of external screening that comes from decreased surface positive charges by the UV/O3 treatments.15,16,36 As the UV/O3 treatment time was further increased (to ~ 4 min), the surface potential became lower, presumably due to the increased surface defect states that screened the polarization bound charges.15,16,39 In order to examine the effects of UV/O3 treatment on the surface energy of GaN, the change in surface wettability of the UV/O3-treated GaN was investigated by measuring the contact angle (𝜃c) of water drops on GaN. The contact angles and the surface energy values were calculated and plotted in Figure 2. The contact angle for NT GaN was initially 60.0° and substantially decreased to 30.4°, 13.2°, 5.9°, and 12.9° with the UV/O3 treatment for 1, 2, 3, and 4 min, respectively. The changes in the measured contact angle indicate a transformation to hydrophilicity.40,41 The fluctuation (reduced) of wettability of the 4 min UV/O3 treatment condition with respect to the wettability of the 3 min treatment was possibly related to the mechanism of O defect state formation on GaN surface at higher O concentrations, in which the O impurity substitutes the Ga vacancies and N atoms and leads to point defects on the GaN surface.42 The


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variation in the contact angle implies the reactivity of O3 molecules at the GaN surface. At the beginning stage of the UV/O3 treatment (~ 1 min), the rate at which the surface energy changed was large, indicating that O3 molecules promptly changed the chemical composition of the GaN surface. However, the rate was reduced as the treatment time increased (from 2‒3 min), indicating that the reaction between O3 molecules and the GaN surface had reached the maximum rate (saturation). With further treatment time (~ 4 min), the reactivity of O3 molecules at the GaN surface began to fluctuate, resulting in the increased contact angle. The surface energy (i.e., surface tension) of the UV/O3-treated GaN was calculated using the modified Berthelot’s rule, derived from the solid-liquid interface tension equation (𝛾𝑠𝑙 = 𝛾𝑙𝑣 + 2

𝛾𝑠𝑣 − 2√𝛾𝑙𝑣 𝛾𝑠𝑣 𝑒 −β(𝛾𝑙𝑣− 𝛾𝑠𝑣 ) ) and the Young’s equation (𝛾𝑙𝑣 cos 𝜃𝑐 = 𝛾𝑠𝑣 − 𝛾𝑠𝑙 ), expressed by the equation:43 𝛾𝑠𝑣

cos 𝜃𝑐 = −1 + 2√

𝛾𝑙𝑣

𝑒 −β(𝛾𝑙𝑣− 𝛾𝑠𝑣)

2

(1)

where 𝛾𝑠𝑣 is the surface energy of solid-vapor, 𝛾𝑙𝑣 is the surface energy of liquid-vapor (72.70 mJ·m-2 for DI H2O), 𝛾𝑠𝑙 is the surface energy of solid-liquid, and β is an experimental coefficient (0.00012 (m2·mJ-1)2). The surface energies (𝛾𝑠𝑣 ) of UV/O3-treated GaN were estimated to be 47.76, 64.19, 70.85, 72.32, and 70.94 mJ·m-2 with the UV/O3 treatment conditions for NT, 1, 2, 3, and 4 min, respectively. The five insets in Figure 2 exhibit the images of H2O droplets on the GaN surfaces under each condition of treatments. From the trend of increasing surface energy with the UV/O3 treatment, we deduced that the UV/O3 treatment can improve the H2O pretreatment step of ALD with an increased number of hydroxyl groups on the GaN surface (Scheme 1). The effect of UV/O3 treatment on the interface oxide formation of ALD Al2O3 on GaN was investigated by exploring the XPS spectra nearby the Ga 3d core levels as shown in Figure 3. The surface escape depth of photoelectrons by XPS is normally up to 10 nm. In our structure of 8 nm


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Al2O3 on GaN, the photoelectrons were emitted from the adjacent interface of Al2O3/GaN, where the surface oxide was formed by the ALD process or UV/O3 treatment.39 The XPS Ga 3d core levels obtained from the ALD Al2O3 on UV/O3-treated GaN substrates are divided into two components: gallium bonded to nitrogen (Ga-N) and gallium bonded to oxygen (Ga-O). The peak intensity ratio of Ga-O/Ga-N were estimated to be 0.35, 0.39, 0.44, 0.45, and 0.83, under NT, 1, 2, 3, and 4 min UV/O3 treatments, respectively (Figure 3a‒e). Since O 2s peak originated from the ALD Al2O3 thin film, the binding energy of the O 2s peaks did not shift regardless of the UV/O3 treatment conditions. The peak intensity ratio between Ga-O and Ga-N components gradually increased up to the 3 min treatment time and the ratio abruptly increased during the 4 min treatment time (Figure 3f). We attribute the different Ga-O/Ga-N ratio to the different mechanism of Ga-O formation on GaN substrate.42 The trend shown in Figure 3f indicates that the interface quality of ALD Al2O3 on GaN associated with surface oxide formation was unchanged up to the 3 min UV/O3 treatment time while improving the surface energy of GaN (Figure 2).

It is known that the initial chemical states of the GaN surface can affect the growth quality of Al2O3.2 The XPS Al 2p core level intensity mapping with area of 1×1 mm2 was performed to inspect the effect of the initial UV/O3 treatment conditions on the deposition quality of ALD Al2O3 on GaN as shown in Figure 4. Here, the peak of the Al 2p core level was only extracted from the ALD Al2O3 layer, so the deposition quality of the Al2O3 layer could be estimated by scanning the intensity and uniformity of Al 2p core level. From the mapping results of the Al 2p core level in Figure 4a-e, the 3 min UV/O3 treatment condition (Figure 4d) exhibits the most improved deposition quality compared to the NT condition in terms of intensity and uniformity. The roughness of the ALD Al2O3 layer on GaN was characterized by AFM to examine the effect of UV/O3 treatment on the surface morphology of the ALD Al2O3 grown on GaN. The AFM images


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of ALD Al2O3 on UV/O3-treated GaN with area of 1×1 µm2 are shown in Figure 5. Root-meansquare roughness (Rrms) of the ALD Al2O3 layer were 0.27, 0.25, 0.25, 0.24, and 0.26 nm under NT, 1, 2, 3, and 4 min UV/O3 treatment conditions. Though there was a variation in the surface roughness, the top surface qualities of the ALD layers were similar.

In order to calculate band bending of ALD Al2O3 on UV/O3-treated GaN, the binding energy shift of Ga 3d and Al 2p core levels were measured by XPS. In addition, to examine how the UV/O3 treatment works on PDA condition of the ALD Al2O3/GaN, which is an essential step for GaN-based device fabrication owing to its high ohmic anneal temperature, the ALD Al2O3/GaN samples were post-deposition annealed and the Ga 3d and Al 2p core levels were measured. The results of Ga 3d and Al 2p core levels shift with and without PDA are summarized in Table 2. The surface potential change induced by the PDA at the Al2O3/GaN interface is shown in Table S1 (see Section B in Supporting Information), exhibiting the similar surface potential is induced by the PDA while maintaining the surface potential formed by the initial UV/O3 treatment. The band bending results of the PDA Al2O3/GaN with UV/O3 treatment are represented in Figure 6. Figure 6a shows the XPS binding energy shift of Ga 3d and Al 2p core levels of the PDA Al2O3/GaN with UV/O3 treatment. The dashed lines indicate the binding energies of PDA Ga 3d and Al 2p core levels with the NT and 3 min-treated samples, respectively. Figure 6b‒f depict the energy band alignments at the PDA Al2O3/GaN interfaces, exhibiting surface potential (Ψs), VBO, CBO, and interface dipole (Δ). Under the different UV/O3 treatment conditions, NT, 1, 2, 3, and 4 min, the downward band bending (i.e. surface potential) values of GaN were estimated to be 0.86, 0.82, 0.81, 0.65, and 0.78 ± 0.03 eV, respectively. The equation for determining the VBO (ΔEV) is defined as15,16,45 𝐴𝑙 𝑂3

GaN Δ𝐸𝑉 = [𝐸𝐶𝐿 − 𝐸𝑉GaN ]𝑏 − [𝐸𝐶𝐿2

𝐴𝑙2 𝑂3

− 𝐸𝑉

𝐴𝑙 𝑂

GaN ]𝑏 − [𝐸𝐶𝐿 − 𝐸𝐶𝐿2 3 ]𝑖

(2)


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where 𝐸𝐶𝐿 is the binding energy of the core level, 𝐸𝑉 is the binding energy of VBM, subscript b indicates the bulk, and subscript i denotes the Al2O3/GaN interface. The differences in the binding energy of Ga 3d and Al 2p core levels from the interface of 𝐴𝑙 𝑂

GaN ALD Al2O3 on GaN, [𝐸𝐶𝐿 − 𝐸𝐶𝐿2 3 ]𝑖 were measured to be 55.92, 56.14, 56.19, 56.20, and 56.14

± 0.03 eV under NT, 1, 2, 3, and 4 min UV/O3 treatments, respectively. The difference in the 𝐴𝑙 𝑂3

binding energy of the Al 2p core level and VBM of bulk ALD Al2O3, [𝐸𝐶𝐿2

𝐴𝑙2 𝑂3

− 𝐸𝑉

]𝑏 , was

estimated to be 71.60 eV (not shown). Therefore, the VBO values between ALD Al2O3 and GaN was calculated as 2.12, 2.34, 2.39, 2.40, and 2.34 eV, and the coinciding CBO values were estimated to be 1.48, 1.26, 1.21, 1.20, and 1.26 eV, under NT, 1, 2, 3, and 4 min UV/O3 treatments, respectively. The interface dipoles (Δ) formed by the charge transfer between the interface states of Al2O3/GaN, were calculated to be 0.38, 0.16, 0.11, 0.10, and 0.16 eV, under NT, 1, 2, 3, and 4 min UV/O3 treatments, respectively.46 The measured data from the band alignments of the PDA Al2O3/GaN are summarized in Table 3 and S2 (see Section C in Supporting Information). To investigate the effects of UV/O3 treatment on the oxide growth quality and further the interaction between the oxide quality and band bending, the PDA Al2O3/GaN MOSCAPs were fabricated and the trapped charge density in the Al2O3 layer was estimated from C–V measurements. The absolute flatband capacitance (CFB) from a high frequency C–V curve is determined by the equation47,48 𝐶𝐹𝐵 =

1+

𝐶𝑚𝑎𝑥 (𝐶𝑚𝑎𝑥 /𝐶𝑚𝑖𝑛 )−1

(3)

2√ln(|𝑁𝐴 −𝑁𝐷 |/𝑛𝑖

where Cmax is the maximum capacitance, Cmin is the minimum capacitance, NA is the acceptor concentration (=5×1019 cm-3), ND is the donor concentration (assumed to be zero), and ni is the intrinsic carrier concentration (=1.9×1010 cm-3). The C–V curves of MOSCAPs with UV/O3


13


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treatment at the measurement frequency of 1 MHz are shown in Figure 7b‒f. During the measurements, the gate bias was first swept from deep depletion (+3 V) to accumulation (–2 V) regions (①), and then swept back to deep depletion (+3 V) from accumulation (–2 V) regions (②). The emergence of deep depletion instead of inversion is due to the low minority carrier generation rate of GaN at RT.12 The empty states of ALD Al2O3 on GaN were filled with positive charges during the first sweep, but the trapped charges were not completely released after the second reverse sweep, bringing about the hysteresis behavior. The flatband voltage shift (ΔVFB) is determined from the difference between gate biases that correspond to flatband capacitance in the first and second sweeps, separately. Under NT, 1, 2, 3, and 4 min UV/O3 treatment conditions of each MOSCAPs, the flatband voltage shifts were estimated to be 0.51, 0.46, 0.38, 0.26, and 0.44 V, respectively. The trapped charge density (QT) in the Al2O3 layer over the GaN bandgap is represented by12,13,49 𝑄𝑇 =

𝐶𝑜𝑥 ∆𝑉𝐹𝐵 𝐸𝑔

(4)

where Cox is the oxide capacitance and Eg is the GaN bandgap. The oxide trapped charge density is lowest (4.70×1011 cm-2·eV-1) for the 3 min UV/O3 treatment and highest (9.22×1011 cm-2·eV-1) for NT, indicating that the initial UV/O3 treatment condition influences the amount of trapped charges in the Al2O3 film and thereby can affect the interface band bending. The flatband voltage shift and trapped charge density of the MOSCAPs are summarized in Table 4 (also see Section D in Supporting Information). Moreover, the optimized interface by the UV/O3 treatment improves the performance of the MOSCAPs (see in Section E in Supporting Information). Figure 8 exhibits an illustration of the band bending mechanism of the Ga-polar GaN interfaced with ALD Al2O3 through UV/O3 treatment. The red dot line indicates the band bending formed by the polarization charges (Qsp) in the GaN and the blue solid line represents the band


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bending formed by the ALD Al2O3 on the GaN with UV/O3 treatment. It is expected that the trapped charges in the Al2O3 thin film nearby the interface of Al2O3/GaN (Qt) induce the positively charged surface states on the GaN. In addition, other factors including surface defects, compositions, and absorbates ideally modulated by the UV/O3 treatment also contribute to the formation of the surface states. The UV/O3 treatment-induced surface factors altogether compensate for the effect of polarization charges in the GaN, determining the eventual band bending at the interfaces.

CONCLUSION The effects of UV/O3 treatment of the GaN surface on the energy band bending of ALD Al2O3 coated Ga-polar GaN were studied. The UV/O3 treatment and post-ALD anneal can be used to effectively vary the band bending, VBO, CBO, and interface dipole at the Al2O3/GaN interfaces. The UV/O3 treatment increased the surface energy of the Ga-polar GaN, improved the uniformity of Al2O3 deposition, and changed the amount of trapped charges in the ALD layer. The positively charged surface states formed by the UV/O3 treatment-induced surface factors externally screen the effect of polarization charges in the GaN, thereby determining the eventual energy band bending at the Al2O3/GaN interfaces. An optimal UV/O3 treatment condition also exists for realizing the “best” interface conditions. The study of UV/O3 treatment effect on the band alignments at the dielectric/III-nitride interfaces will be valuable for applications of transistors, LEDs, and photovoltaics.


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ASSOCIATE CONTENT Supporting information Schematic illustration of process flow in this study, effect of PDA on band bending of ALD Al2O3 on UV/O3-treated GaN, surface potential-related depletion width and density of ionized acceptors in GaN of PDA Al2O3/GaN, comparison of flatband voltage shift and trapped charge density of Al2O3/GaN MOSCAPs, and improved performance of Al2O3/GaN MOSCAPs with UV/O3 treatment.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions K. Kim and Z. Ma designed and performed whole experiments. K. Kim, D. Liu and Z. Ma conducted manuscript writing. J. H. Ryu, S. J. Cho, and D. Liu conducted XPS measurements. J. Kim, J. Park, and I. K. Lee performed MOS fabrication. B. Moody, W. Zhou, and J. Albrecht contributed to material, experiment design and related work discussions. All authors contributed to data analysis and manuscript revisions.

ACKNOWLEDGMENT The work was supported by Defense Advanced Research Projects Agency (DARPA) under grant #HR0011-15-2-0002 (PM: Dr. Daniel Green) and Wisconsin Alumni Research Foundation’s


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(WARF) Accelerator Program. The work was also supported by Office of Naval Research (ONR) under grant #N00014-13-1-0226 (PM: Dr. Paul Maki).

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Figure Captions: Scheme 1. Schematic illustration of atomic-layer-deposition (ALD) Al2O3 on GaN with UV/O3 plasma treatment. (a) UV/O3 plasma treatment, 2 cycles of (b) H2O pretreatment, and 80 cycles of (c) trimethylaluminium (TMA) and (d) H2O precursors in alternative pulse forms with N2 purging. Figure 1. Material properties of Ga-polar GaN. (a) XPS spectrum of Ga-polar GaN valence band (VB) with P1 and P2 peaks. (b) Raman scattering of GaN grown on sapphire with high E2 and A1(LO) phonon modes. Inset shows coupling of A1(LO) phonon and sapphire Eg modes. (c) XPS spectra of valence band maximum (VBM) and Ga 3d core level of as-prepared GaN. (d) Energy band of as-prepared GaN. Eg is the band gap of the GaN. (e) Intensity reduction of carbon (C 1s) and oxygen (O 1s) peaks by the cleaning process. (f) Evolution of Ga 3d binding energy from GaN with UV/O3 treatment. NT denotes no treatment. Figure 2. Contact angle (𝜃c) and surface energy of GaN as a function of UV/O3 treatment time. Images exhibit H2O droplets on the NT and UV/O3-treated GaN surfaces. Figure 3. The effect of UV/O3 treatment on the interface quality of ALD Al2O3 on GaN associated with surface oxide formation. XPS Ga 3d core levels of ALD Al2O3 on GaN with the UV/O3 treatment conditions: (a) NT, (b) 1 min, (c) 2 min, (d) 3 min, and (e) 4 min. (f) The trend of peak intensity ratio of Ga-O/Ga-N according to the UV/O3 treatment conditions. A trend line is for data view guiding. Figure 4. XPS Al 2p core level intensity mapping for the effect of UV/O3 treatment on the interface quality of ALD Al2O3 on GaN in terms of uniformity and intensity. XPS intensity mapping (1×1 mm2) for Al 2p core level of ALD Al2O3 on GaN with the UV/O3 treatment conditions: (a) NT, (b) 1 min, (c) 2 min, (d) 3 min, and (e) 4 min.


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Figure 5. AFM topographic images (1×1 µm2) of ALD Al2O3 on GaN with the UV/O3 treatment conditions: (a) NT, (b) 1 min, (c) 2 min, (d) 3 min, and (e) 4 min. Root-mean-square roughness (Rrms) of the Al2O3 thin film are presented below the AFM images. Figure 6. Band bending of post-deposition annealed (PDA) Al2O3/GaN with UV/O3 treatment. (a) XPS binding energy shifting in Ga 3d and Al 2p core levels of PDA Al2O3 on GaN with the UV/O3 treatment. Band alignments of PDA Al2O3 on GaN exhibiting band bending (Ψs), valence band offset (VBO), conduction band offset (CBO), and interface dipole (Δ) under the UV/O3 treatment conditions: (b) NT, (c) 1 min, (d) 2 min, (e) 3 min, and (f) 4 min. Figure 7. (a) Process steps for PDA Al2O3/GaN metal-oxide-semiconductor capacitors (MOSCAPs), scale bar = 50 µm. Capacitance-voltage (C‒V) characteristics with flatband voltage shifts (ΔVFB) of the MOSCAPs under the UV/O3 treatment conditions: (b) NT, (c) 1 min, (d) 2 min, (e) 3 min, and (f) 4 min. Figure 8. Illustration of band bending mechanism of the Ga-polar GaN interfaced with ALD Al2O3 through UV/O3 treatment. The red dotted line indicates the band bending formed by the polarization charges (Qsp) in GaN and the blue solid line represents the band bending formed by the ALD Al2O3 on the GaN with UV/O3 treatment. The UV/O3 treatment-induced positive surface states altogether compensate for the effect of polarization charges in the GaN, determining the eventual band bending at the interfaces. – and + denote the negative and positive charges, respectively.


25


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Table 1. XPS binding energy of Ga 3d core level and surface potential of GaN with UV/O3 treatment. Table 2. XPS binding energy of Ga 3d and Al 2p core levels from ALD Al2O3 on UV/O3-treated GaN before and after PDA. Table 3. Summary of band bending (Ψs), VBO, CBO, and interface dipole (Δ) from band alignment of PDA Al2O3 on GaN with UV/O3 treatment. Table 4. Flatband voltage shift (ΔVFB) and trapped charge density (QT) of Au/Al2O3/GaN MOSCAP with UV/O3 treatment.


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TOC graphic:


27


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Scheme 1.


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Figure 1.



Figure 2.


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Figure 3.



)LJXUH


Page 32 of 40

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Figure 5.



Figure 6.


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Figure 7.



Figure 8.


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Table 1.



7DEOH


Page 38 of 40

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Table 3.



Table 4.


Page 40 of 40

Band-Bending of Ga-Polar GaN Interfaced with Al2O3 through

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