Polarization-Charge Inversion at Al 2 O 3 / GaN Interfaces through Post-Deposition Annealing

: The e ﬀ ects of post-deposition annealing (PDA) on the formation of polarization-charge inversion at ultrathin Al 2 O 3 / 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 Al 2 O 3 / GaN interfaces with polarization-charge inversion, which is analyzed using X-ray photoelectron spectroscopy. Net charge and interface charge densities at the Al 2 O 3 / GaN interfaces are estimated after PDA at 500 ◦ C, 700 ◦ C, and 900 ◦ C. The PDA temperatures a ﬀ ect 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 Al 2 O 3 layers exhibit the passivation ability of the Al 2 O 3 surface layer and the e ﬀ ects 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. properties obtained from the 3 nm Al 2 O 3 / GaN structure can be interpreted for the surface property of GaN at the Al 2 O 3 / GaN interface.


Introduction
Chemical and physical properties at GaN surfaces and interfaces have a remarkable influence on the electronic device performance and stability [1][2][3][4][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][7][8][9][10][11][12][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][15][16][17][18][19][20][21][22][23]. Deposition of a high-k dielectric passivation layer can counteract the effects of surface states on the conduction. Al 2 O 3 is a prevalent dielectric material for surface passivation owing to large bandgap, high permittivity, and high breakdown field [24][25][26][27].
Post-deposition annealing (PDA) is a subsequent step for atomic-layer deposition (ALD) Al 2 O 3 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][7][8][9][10][24][25][26][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 Al 2 O 3 passivation. Up to now, the analysis on the energy band bending and surface potential at the Al 2 O 3 /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][7][8]. The correlation between the PDA-induced surface states and device performance

Materials and Methods
Wurtzite Ga-polar GaN was grown by metal organic chemical vapor deposition with a Si doping concentration of 5 × 10 17 cm −3 from the top surface to 400 nm depth for a Schottky contact region, and 5 × 10 18 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, H 2 SO 4 :H 2 O 2 = 3:1) and an ammonium hydroxide solution (NH 4 OH:H 2 O 2 :DI H 2 O = 1:1:5) for organic debris removal, then a hydrofluoric acid solution (HF:DI H 2 O = 1:50) for surface oxide removal, and a hydrochloric acid solution (HCl:H 2 O 2 :DI H 2 O = 1:1:5) for ionic debris removal (as-cleaned GaN). Al 2 O 3 was deposited on the as-cleaned GaN by ALD. The GaN samples were loaded into an ex-situ ALD chamber, where an H 2 O source was applied as an oxidant at a stage temperature of 250 • C, followed by ten-and thirty-cycles deposition of Al(CH 3 ) 3 and H 2 O precursors in alternative pulses. The growth rate of ALD layer is 1.0 Å/cycle. The Al 2 O 3 -deposited GaN then went through PDA in a N 2 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 Al 2 O 3 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 Al 2 O 3 /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 2p 3/2 , Ag 3d 5/2 , and Au 4f 7/2 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,

Results and Discussion
XPS is utilized to identify the surface polarity of GaN by scanning a near-valence band (VB) region ( Figure 1b). There are two well-defined peaks (P1 and P2) emerged from the atomic p-and sorbital states of GaN [28,29]. The P1 and P2 peaks at the binding energy (BE) of 4.00 eV and 8.66 eV, individually, are associated with the Ga 4p-N 2p hybridization and Ga 4s-N 2p hybridization states, respectively. Here, the intensity of P1 peak is stronger than that of the P2 peak, with the 1.14 intensity ratio of P1 to P2 peaks, which verifies Ga-face polarity of GaN [28]. The BE difference obtained from the peak shift in Ga 3d core levels can represent the surface band bending of GaN (Figure 1c). The Ga 3d peak BEs of as-prepared and as-cleaned GaN are 19.82 and 19.68 eV, respectively, which indicates a demonstration of 0.14 eV upward band bending on the GaN surface after cleaning steps. An internal spontaneous polarization (Psp = −0.033 C·m −2 ) of Ga-faced GaN heading from surface to bulk forms negative bound sheet charges (1.81 × 10 13 cm −2 ) nearby the surface band therefore induces upward band bending [8,30,31]. Figure 2 exhibits XPS spectra of Ga 3d, Al 2p, O 1s, and N 1s core levels at 1 nm Al2O3/GaN interfaces with no PDA (sample A1) and with PDA at 500 °C (sample B1), 700 °C (sample C1), and 900 °C (sample D1) for 3 min. The peak BE of Ga 3d core level of 1 nm Al2O3 as-deposited GaN (A1) is located at 19.55 eV and then the peak BE shifts to 19.70, 20.16 and 20.01 eV with the 3 min PDA at 500 °C, 700 °C and 900 °C, individually ( Figure 2a). The increase in the peak BE of Ga 3d core levels upon PDA indicates the increase in Ga-O bonding at the Al2O3/GaN interfaces in which O atoms diffuse from Al2O3 to GaN through PDA [32,33]. The behavior of the peak shift in Al 2p core level according to the PDA conditions is similar with that of Ga 3d core level. The peak BE of Al 2p core level at the 1 nm Al2O3/GaN interface is located at 75.60 eV, then shifts to 75.74, 76.22 and 76.11 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.

Results and Discussion
XPS is utilized to identify the surface polarity of GaN by scanning a near-valence band (VB) region ( Figure 1b). There are two well-defined peaks (P 1 and P 2 ) emerged from the atomic p-and s-orbital states of GaN [28,29]. The P 1 and P 2 peaks at the binding energy (BE) of 4.00 eV and 8.66 eV, individually, are associated with the Ga 4p-N 2p hybridization and Ga 4s-N 2p hybridization states, respectively. Here, the intensity of P 1 peak is stronger than that of the P 2 peak, with the 1.14 intensity ratio of P 1 to P 2 peaks, which verifies Ga-face polarity of GaN [28]. The BE difference obtained from the peak shift in Ga 3d core levels can represent the surface band bending of GaN ( Figure 1c). The Ga 3d peak BEs of as-prepared and as-cleaned GaN are 19.82 and 19.68 eV, respectively, which indicates a demonstration of 0.14 eV upward band bending on the GaN surface after cleaning steps. An internal spontaneous polarization (P sp = −0.033 C·m −2 ) of Ga-faced GaN heading from surface to bulk forms negative bound sheet charges (1.81 × 10 13 cm −2 ) nearby the surface band therefore induces upward band bending [8,30,31].   Table 1.    Table 1.   Table 1.   The energy band bending at the 1 nm where 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 surface after the deposition of 1 nm Al2O3. The Ψs decreases to (-) 0.10 eV, 0.34 eV, and increases back to 0.19 eV for the samples A1, B1, C1, and D1. The valence band offset (VBO) at the Al2O3/GaN interfaces are determined by the equation [6,24,25]: where ECL are the BEs of atomic core levels, EV are the BEs of VBMs, and subscripts b and i stand for the bulk GaN and Al2O3/GaN interface, respectively. The second term for bulk Al2O3 is estimated to be 71.   Some reports show that the density of positive charges at GaN surface increased owing to the formation of Ga-O bonds [8,30]. From the comparison of Ψs values at the Al2O3/GaN interfaces with different thickness of Al2O3 but same PDA temperature (A1 and A3, B1 and B3, C1 and C3, and D1 and D3), it is noticed that the direction of band bending at the 1 nm and 3 nm Al2O3 interfaces are same and the magnitude of Ψs at the 3 nm Al2O3 interface is larger than that of the 1 nm Al2O3 interface. Considering the measurement principle of XPS where X-ray penetrates top surface up to few nanometers, the values measured from the GaN interfaced with 3 nm Al2O3 top layer can represent the Ψs relatively close to outer surface side, compared to the values from the GaN with 1 nm Al2O3 layer, which shows the Ψs relatively at inner bulk side. Therefore, the physical and chemical properties obtained from the 3 nm Al2O3/GaN structure can be interpreted for the surface property of GaN at the Al2O3/GaN interface.
To investigate the effects of PDA on the formation of Ga-O bonds and associated positive charge densities at the Al2O3/GaN interfaces, the Ga-O/Ga-N ratio (%) are obtained from the XPS Ga 3d spectra of samples A3, B3, C3, and D3. The Ga-O area ratio increases from 19.45% to 21.01% and 22.24% by the application of PDA at 500 °C and 700 °C, respectively, then decreases to 21.79% with the PDA at 900 °C. Here, the trend of changes in the Ga-O/Ga-N ratio and Ψs at the interface are identical, which reveals the strong correlation between the PDA and surface polarity formation. The increased Ga-O/Ga-N ratio with the PDA at 500 °C and 700 °C is attributed to the diffusion of O atoms of Al2O3 into GaN. While, the reduction in Ga-O bonds of D3 annealed at 900 °C with respect to that of C3 at 700 °C could be ascribed to clean up effect of PDA to passivate Ga-O bonds, based on the difference in the magnitudes of negative Gibb's free energy of Al2O3 (-1582.3 kJ/mol) and Ga-O (-998.3 kJ/mol) [34][35][36]. The net area charge densities (σnet) at the Al2O3/GaN interfaces are calculated by the following Equation [8]: where ε is permittivity of GaN, φs is surface potential at GaN conduction band edge with respect to Fermi level, x is distance from GaN surface into inner bulk, e is electronic charge, NC is effective density of states in GaN conduction band, Vt is thermal voltage, ND is doping concentration, and D is integration constant (−5.7 × 10 9 eV/Fcm 2 ). From the calculation based on the measured values using XPS, the net charge densities at the 1 nm and 3 nm Al2O3/GaN interfaces with no PDA (A1 and A3) are estimated to be −1.19 × 10 12 cm −2 and -1.28 × 10 12 cm −2 , respectively, and corresponded interface charge densities are determined to be 1.69 × 10 13 cm −2 and 1.68 × 10 13 cm −2 . These results are comparable to the value of 1.7 × 10 13 cm −2 reported from another research group (Table 2) [8,30,31]. The interface charge densities at the Al2O3/GaN interfaces with PDA are also determined to be 1.73 × 10 13 cm −2 , 2.73 × 10 14 cm −2 , and 3.19 × 10 13 cm −2 for the samples B1, C1, and D1, individually, and 1.71 × 10 13 cm −2 , 8.38 × 10 14 cm −2 , and 4.81 × 10 13 cm −2 for the samples B3, C3, and D3, separately. The decrease in the interface 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][35][36]. In case of the reference GaN with no Al2O3, the interface charge density is determined to be 1.72 × 10 13 cm −2 according to the  [34][35][36]. The net area charge densities (σ net ) at the Al 2 O 3 /GaN interfaces are calculated by the following Equation [8]: where ε is permittivity of GaN, ϕ s is surface potential at GaN conduction band edge with respect to Fermi level, x is distance from GaN surface into inner bulk, e is electronic charge, N C is effective density of states in GaN conduction band, V t is thermal voltage, N D is doping concentration, and D is integration constant (−5.7 × 10 9 eV/Fcm 2 ). From the calculation based on the measured values using XPS, the net charge densities at the 1 nm and 3 nm Al 2 O 3 /GaN interfaces with no PDA (A1 and A3) are estimated to be −1.19 × 10 12 cm −2 and −1.28 × 10 12 cm −2 , respectively, and corresponded interface charge densities are determined to be 1.69 × 10 13 cm −2 and 1.68 × 10 13 cm −2 . These results are comparable to the value of 1.7 × 10 13 cm −2 reported from another research group (  [34][35][36]. In case of the reference GaN with no Al 2 O 3 , the interface charge density is determined to be 1.72 × 10 13 cm −2 according to the 0.14 eV peak BE shift as shown in Figure 1c. The results of positive interface charge density formed at the Al 2 O 3 /GaN interfaces through PDA indeed support the trend in energy band bending. The current density-voltage (J-V) measurements are performed to assess the effects of PDA on the electrical features of GaN SDs interfaced with ultrathin Al 2 O 3 layers ( Figure 6). J-V characteristics of GaN SDs made of reference, A1, A3, B1, B3, C1, C3, D1, and D3 exhibit that the conduction properties improved through PDA, in terms of leakage current, on/off ratio, and ideality factor. The Ga-O/Ga-N ratio in Schottky contact area (gate area) shows no change after PDA and subsequent removal of Al 2 O 3 by wet process (B1, B3, C1, C3, D1, and D3), while the ratio in the contact area recovers initial ratio before Al 2 O 3 deposition when no PDA is applied (A1 and A3). In this case, interface charge densities at gate contact area of reference and A1 and A3 have no difference and therefore reduced leakage of A1 and A3 SDs with respect to the reference SD indicates the passivation ability of ultrathin Al 2 O 3 layer deposited between Schottky and ground electrodes (surface area). When Al 2 O 3 /GaN substrates undergo PDA at 500 • C (B1 and B3), similar interface charge densities are induced, compared to that of A1 and A3, at both gate and surface areas of SDs, resulting in the J-V curves in Figure 6. When the PDA temperature goes up to 700 • C, the larger interface charge densities are formed at both gate and surface areas, compared to that of B1 and B3, leading to the further reduced leakage current levels. In detail, from the comparison of conduction properties in reference and C3 SD, the leakage current (−0.5 V) decreased by one order of magnitude from 3.51 × 10 −6 A/cm 2 to 1.19 × 10 −7 A/cm 2 and current on/off ratio (±0.5 V) increased by one order of magnitude from 2.16 × 10 4 to 2.15 × 10 5 . The ideality factor decreased from 1.44 to 1.13 as well. When 900 • C PDA is applied (D1 and D3), however, the leakage current levels increase with reduced interface charge densities owing to the micro-crystallization that shapes grain boundaries in Al 2 O 3 layers as high-leakage paths at such a high PDA temperature [32,33]. Whether the sole effect of positive interface charges at the gate or surface areas on the conduction remains unclear at this stage since the interface charges and oxide layers are formed together at both areas through PDA.
Electronics 2020, 9, 1068 7 of 10 0.14 eV peak BE shift as shown in Figure 1c. The results of positive interface charge density formed at the Al2O3/GaN interfaces through PDA indeed support the trend in energy band bending. The current density-voltage (J-V) measurements are performed to assess the effects of PDA on the electrical features of GaN SDs interfaced with ultrathin Al2O3 layers ( Figure 6). J-V characteristics of GaN SDs made of reference, A1, A3, B1, B3, C1, C3, D1, and D3 exhibit that the conduction properties improved through PDA, in terms of leakage current, on/off ratio, and ideality factor. The Ga-O/Ga-N ratio in Schottky contact area (gate area) shows no change after PDA and subsequent removal of Al2O3 by wet process (B1, B3, C1, C3, D1, and D3), while the ratio in the contact area recovers initial ratio before Al2O3 deposition when no PDA is applied (A1 and A3). In this case, interface charge densities at gate contact area of reference and A1 and A3 have no difference and therefore reduced leakage of A1 and A3 SDs with respect to the reference SD indicates the passivation ability of ultrathin Al2O3 layer deposited between Schottky and ground electrodes (surface area). When Al2O3/GaN substrates undergo PDA at 500 °C (B1 and B3), similar interface charge densities are induced, compared to that of A1 and A3, at both gate and surface areas of SDs, resulting in the J-V curves in Figure 6. When the PDA temperature goes up to 700 °C, the larger interface charge densities are formed at both gate and surface areas, compared to that of B1 and B3, leading to the further reduced leakage current levels. In detail, from the comparison of conduction properties in reference and C3 SD, the leakage current (−0.5 V) decreased by one order of magnitude from 3.51 × 10 −6 A/cm 2 to 1.19 × 10 −7 A/cm 2 and current on/off ratio (±0.5 V) increased by one order of magnitude from 2.16 × 10 4 to 2.15 × 10 5 . The ideality factor decreased from 1.44 to 1.13 as well. When 900 °C PDA is applied (D1 and D3), however, the leakage current levels increase with reduced interface charge densities owing to the micro-crystallization that shapes grain boundaries in Al2O3 layers as highleakage paths at such a high PDA temperature [32,33]. Whether the sole effect of positive interface charges at the gate or surface areas on the conduction remains unclear at this stage since the interface charges and oxide layers are formed together at both areas through PDA.