Self-Assembled Monolayers of Alkylphosphonic Acid on GaN

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Langmuir 2008, 24, 6630-6635

Self-Assembled Monolayers of Alkylphosphonic Acid on GaN Substrates Takashi Ito,*,† Sarah M. Forman,† Chundi Cao,‡ Feng Li,† Charles R. Eddy, Jr.,§ Michael A. Mastro,§ Ronald T. Holm,§ Richard L. Henry,§ Keith L. Hohn,‡ and J. H. Edgar‡ Department of Chemistry and Department of Chemical Engineering, Kansas State UniVersity, Manhattan, Kansas 66506, and Code 6880, U.S. NaVal Research Laboratory, 4555 OVerlook AVenue Southwest, Washington, D.C. 20375 ReceiVed March 6, 2008. ReVised Manuscript ReceiVed April 20, 2008 In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (CnH2n+1X; n ) 16-18; X ) -COOH, -NH2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 °C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.

1. Introduction In this paper we report on the formation, characterization, and stability of self-assembled monolayers of octadecylphosphonic acid (ODPA) on GaN substrates. Water contact angles were used to assess the adsorption of primarily substituted hydrocarbons with different terminal functional groups on the surfaces of GaN (0001) films of different carrier concentrations and polarities. The stability of ODPA monolayers in acidic, neutral, and basic aqueous solutions was investigated with and without heating of the monolayers at 160 °C in a vacuum. The surfaces of the GaN substrates with and without ODPA monolayers were characterized using spectroscopic ellipsometry and X-ray photoelectron spectroscopy (XPS) to explain the findings on the adsorption behaviors of the organic molecules. In addition to their applications for short-wavelength optoelectronic and high-power electronics, group III nitrides (AlN, GaN, and InN) have been employed as components of chemical and biological sensors for gas and solution samples.1–3 The adsorption of species on the nitride’s surface significantly changes its electrical properties, due to its high spontaneous polarization and piezoelectric constants. Thus, sensitive chemical sensors are possible if the surface of the group III nitride interacts * To whom correspondence should be addressed. E-mail: [email protected]. Phone: (785) 532-1451. Fax: (785) 532-6666. † Department of Chemistry, Kansas State University. ‡ Department of Chemical Engineering, Kansas State University. § U.S. Naval Research Laboratory. (1) Pearton, S. J.; Kang, B. S.; Kim, S.; Ren, F.; Gila, B. P.; Abernathy, C. R.; Lin, J.; Chu, S. N. G. J. Phys.: Condens. Matter 2004, 16, R961–R994. (2) Kang, B. S.; Wang, H.-T.; Tien, L.-C.; Ren, F.; Gila, B. P.; Norton, D. P.; Abernathy, C. R.; Lin, J.; Pearton, S. J. Sensors 2006, 6, 643–666. (3) Steinhoff, G.; Purrucker, O.; Tanaka, M.; Stutzmann, M.; Eickhoff, M. AdV. Funct. Mater. 2003, 13, 841–846.

with specific species. Because of their excellent chemical and physical stability, chemical sensors based on group III nitrides can be operated under severe conditions such as high temperature, power, and flux/energy radiation. Recent studies focused mainly on developing chemical and biological sensors using AlxGa1xN/GaN high electron mobility transistors (AlGaN/GaNHEMTs)1,2 and AlxGa1-xN/GaN field effect transistors (GaNFETs).1,3 These devices can be miniaturized to develop portable chemical sensors, similar to silicon-based FETs (Si-FETs),4 and will be more robust chemically and physically as compared with Si-based devices. AlGaN/GaN-HEMTs and GaN-FETs are sensitive signal transducers for chemical sensors,1,2 but they can currently detect only a limited number of chemical species because of their generally low chemical selectivity; the low chemical reactivity of the AlxGa1-xN-based gate surface makes functionalization difficult. For example, although vapor-phase adsorption of anilines and thiols onto GaN was studied in an ultrahigh vacuum, the saturation density of the molecules on GaN was low (1 h) between the sample preparation and the sample loading into the XPS instrument. XPS spectra were collected using a Perkin-Elmer PHI 5400 with an achromatic Al KR X-ray source (1486.6 eV) radiation operating at 240 W (12 kV and 20 mA). The base pressure of the chamber was about 2 × 10-9 Torr. The analyzer pass energy was set to 17.9 eV, and the contact time was 50 ms. Before the samples were tested, the spectrometer was calibrated by setting the binding energies of Au4f7/2 and Cu2p3/2 to 84.0 and 932.7 eV, respectively. All spectra were taken at a 90° takeoff angle between the sample surface and the direction of the photoelectron detected by the analyzer. For high-resolution measurements of Ga3d, O1s, N1s, P2s, P2p, and C1s spectra, peak binding energies were referenced to the Ga2p3/2 peak, which was first normalized to the C1s peak (285.7 eV) on the HCl-treated GaN,31,32 because the shapes of the Ga and N peaks were almost identical for all the samples. This procedure gave binding energy values similar to those of previous reports.31,32 Data analysis was performed using the CasaXPS software package. The XPS spectra were fit by allowing CasaXPS to vary the binding energy and intensity of the three or four peaks until the difference between the experimental data and the fit was minimized. The values reported were the averages and standard deviations of XPS data obtained from four (HCl-treated GaN) or three (UV/O3treated and ODPA-coated GaN) separate samples.

3. Results and Discussion A. Water Contact Angle Measurements of GaN Substrate Surfaces before and after the Adsorption of a Primarily Substituted Hydrocarbon. Table 1 summarizes water contact angle values (θwater) of the surfaces of GaN substrates before and after 17 h of immersion in a toluene solution of a primarily substituted hydrocarbon. Unintentionally doped Ga- and N-polar GaN and Si-doped Ga-polar GaN were used to investigate the effects of the carrier concentration and surface polarity on the adsorption of the organic compounds. In addition, the GaN substrate surfaces were cleaned in the two different ways: HCltreated GaN and UV/O3-treated GaN. Five organic compounds having a long alkyl chain and different terminal functional groups were used to compare the adsorptivities of these functional groups onto GaN substrates. The long alkyl chain will stabilize the resulting monolayers due to the van der Waals interactions (35) Ito, T.; Forman, S. M.; Cao, C.; Eddy, C. R. J.; Mastro, M. A.; Holm, R. T.; Henry, R. L.; Hohn, K.; Edgar, J. H. ECS Trans. 2007, 11, 97–101.

Figure 1. Water contact angle of the surfaces of UV/O3-treated, Gapolar undoped GaN after immersion in a 5 mM toluene solution of ODPA (filled circles) and stearic acid (open circles) for 0, 5, 10, 20, 30, 60, 120, 240, and 1020 min.

between adjacent molecules.14,15,17 Additionally, the alkyl chain makes it possible to assess the monolayer formation by measuring the water contact angle,14 because the adsorption of the compounds through their polar functional groups will change the substrate surface to be hydrophobic as a result of the exposure of the alkyl chains. The observed θwater values are summarized as follows. i. Effects of HCl and UV/O3 Treatments on the θwater of Unmodified GaN Substrates. Unmodified GaN substrates prior to the adsorption of the organic compounds were hydrophilic (θwater < 30°), especially the UV/O3-treated surfaces (θwater ≈ 10°). The smaller θwater upon UV/O3 treatment would mainly reflect the surface exposure of the hydrophilic gallium oxide layer due to the removal of organic adsorbates from the GaN substrates.32,33 On the other hand, the relatively larger θwater upon HCl treatment would reflect the decrease in the exposed hydrophilic surface oxide layer.32,33 These interpretations were verified through XPS data as shown in a later section. ii. Dependence of the Adsorption Time on Monolayer Formation. Immersing the GaN samples in toluene solutions of the organic compounds increased the θwater value as compared with that of the unmodified GaN, suggesting the adsorption of these compounds onto GaN. Figure 1 shows the relationship between the immersion time and θwater value of UV/O3-treated Ga-polar undoped GaN for ODPA (C18H37PO(OH)2) and C17H35COOH. For both molecules, θwater quickly increased and reached a plateau within 20 min. Thus, the θwater data shown in Table 1, which were obtained after 17 h of immersion, reflect equilibrium molecular adsorption values. In addition, for ODPA, the equilibrium θwater value is ca. 105°, suggesting that this compound forms well-packed monolayers.14,36 Hughes et al.24 previously reported a much smaller θwater value for an ODPA monolayer on GaN, probably because they used polar tetrahydrofuran as a solvent, which weakened the interactions between (36) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45–52.

SAMs of Alkylphosphonic Acid on GaN Substrates

polar functional groups on the substrate surface and ODPA. In addition, their θwater value (36°) for unmodified GaN may indicate that the GaN substrate was covered with organic adsorbates. In contrast, the equilibrium θwater values for C17H35COOH (ca. 76°) were much smaller than that of ODPA, reflecting weak adsorption of the molecule to the substrate surface, as discussed in section iv. iii. Molecular Adsorption onto Different GaN Substrates. The HCl and UV/O3 treatments affected the surface hydrophilicity as discussed in section i and also the affinity of the surfaces to organic molecules. As shown in Table 1, upon the adsorption of primarily substituted hydrocarbons, UV/O3-treated surfaces gave larger θwater values than the corresponding HCl-treated surfaces. These observations indicate that the surface-exposed oxide layer plays an essential role in the adsorption of the organic compounds. On the two types of Ga-polar GaN, very large θwater values were observed only for ODPA (Table 1). The low affinity of the Ga-polar GaN to the alkanoic acid and alkylamine indicates the low basicity and acidity of the gallium oxide layer as compared with other metal oxide surfaces that can form well-packed monolayers of these molecules: on SiO2,37 Al2O3,36,38 and ZrO239 for the former and on YBa2Cu3O7 for the latter.40 On the other hand, the Si-doped GaN offered slightly larger θwater values than the undoped GaN upon molecular adsorption, which is perhaps related to the formation of a thicker surface oxide layer resulting from its accelerated growth in the presence of dopants. ODPA monolayers on N-polar GaN gave a highly hydrophobic surface (θwater ≈ 104°) without the UV/O3 treatment, which probably reflected its higher reactivity compared to Ga-polar GaN.41 The UV/O3 treatment of N-polar GaN increased the affinity of all the organic compounds examined. In particular, in addition to ODPA (θwater ≈ 109°), C17H35COOH formed densely packed SAMs on the GaN substrate, as suggested by the θwater values (ca. 104°). The very high affinity to -COOH groups might be due to the electron-donating properties of the surfaceexposed nitrogens in GaN.1 iV. θwater of GaN Substrates Coated with Primarily Substituted Hydrocarbons HaVing Different Terminal Functional Groups. Among the primarily substituted hydrocarbons examined, ODPA offered hydrophobic surfaces on the three types of GaN, especially on the UV/O3-treated ones (θwater ≈ 103-109°), indicating that the molecules can form well-packed monolayers on these surfaces. In contrast, the other molecules cannot offer such a hydrophobic surface except C17H35COOH on UV/O3-treated N-polar GaN (ca. 104°) and thus cannot form well-packed monolayers on GaN substrates. Regardless of the types of GaN, the order of θwater was approximately as follows: C18H37OH < C16H33SH < C18H37NH2 e C17H35COOH < C18H37PO(OH)2. This order indicates that functional groups having a high acidity and/or basicity adsorb to the GaN substrate more strongly. According to Abraham et al.,42 -COOH has a higher hydrogen bond acidity (sA ) 0.60) than -NH2 (sA ) 0.16), -OH (sA ) 0.37), and -SH (sA ) 0.00). In addition, -NH2 has a stronger hydrogen bond basicity (sB ) 0.61) as compared with -COOH (sB ) 0.45), -OH (sB (37) Whidden, T. K.; Yang, S.-J.; Jenkins-Gray, A.; Pan, M.; Kozicki, M. N. J. Electrochem. Soc. 1997, 144, 605–616. (38) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52–66. (39) Pawsey, S.; Yach, K.; Halla, J.; Reven, L. Langmuir 2000, 16, 3294– 3303. (40) Ritchie, J. E.; Murray, W. R.; Kershan, K.; Diaz, V.; Tran, L.; McDevitt, J. T. J. Am. Chem. Soc. 1999, 121, 7447–7448. (41) Sumiya, M.; Yoshimura, K.; Ohtsuka, K.; Fuke, S. Appl. Phys. Lett. 2000, 76, 2098–2100. (42) Abraham, M. H.; Platts, J. A. J. Org. Chem. 2001, 66, 3484–3491.

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Figure 2. Water contact angle of the surfaces of ODPA-coated undoped Ga-polar GaN after immersion in (a) 0.1 M HCl, (b) water, or (c) 0.1 M NaOH for 0, 4, 24, and 48 h. Open and filled circles represent data obtained for ODPA monolayers with and without the heating treatment at 160 °C for 24 h under a vacuum (0.6 Torr).

) 0.48), and -SH (sB ) 0.24).42 Although not included in their list, -PO(OH)2 would have an even stronger hydrogen bond acidity and basicity, considering the low pKa1 values reported for n-hexylphosphonic acid (pKa1 ) 2.6, pKa2 ) 7.9)43 and the sB value of -PO(OMe)2 (sB ) 1.08).42 In addition, chemical bonds involving its three P-O moieties in a -PO(OH)2 group with surface Ga atoms would strengthen the adsorption.25,26 The significance of the hydrogen bond acidity and basicity suggests that the adsorption of these molecules onto the GaN substrate is based on the hydrogen bond interactions between the gallium oxide layer on the GaN substrate and polar functional groups. In spite of its very low acidity and basicity, C16H33SH adsorbed to the GaN substrates more strongly than C18H37OH maybe due to the involvement of coordination bond formation between the sulfur and Ga, as shown on GaAs surfaces.17,44 Indeed, the adsorption of organic mercaptans from an aqueous solution was electrochemically detected using GaN electrodes.8 B. Ellipsometric Thickness of an ODPA Layer Adsorbed on Ga-Polar GaN. Two different spectroscopic ellipsometers were employed to measure the thickness of an ODPA layer on Ga-polar undoped GaN. The M-2000 spectroscopic ellipsometer gave 2.42 ( 0.01 nm, and the small-spot M-2000 spectroscopic ellipsometer gave 2.46 ( 0.02 nm. The ellipsometric thicknesses of the ODPA layer agreed well with the length of a C18 carbon chain in all-trans configuration,45 indicating that the ODPA molecules formed a densely packed monolayer on the GaN substrate as suggested by the water contact angle data. C. Stability of ODPA Monolayers on UV/O3-Treated GaN Substrates in Aqueous Solutions. The stability of ODPA monolayers on GaN is important for their future applications. However, it was previously reported that ODPA monolayers on Al2O3 desorbed upon immersion in an aqueous solution.27 Here, the stability of ODPA monolayers on Ga-polar undoped GaN in aqueous solutions was assessed using contact angle measurements. The effect of the heating treatment at 160 °C on the stability of ODPA monolayers was also investigated, because it induces the covalent bond formation between the phosphonic acid group and metal oxide surface (TiO2, SiO2).25,28 The data represented by filled circles in Figure 2 show the change in θwater of unheated ODPA-coated GaN substrates at different immersion times (0, 4, 24, and 48 h). For all cases, θwater decreased with increasing immersion time and then reached a plateau, similarly to that of ODPA monolayers on Al2O3.27 In the basic solution, θwater decreased quickly to reach a smaller plateau value, probably reflecting the electrostatic repulsion between the deprotonated ODPA and negatively charged gallium oxide surface46 in addition to the higher solubility of deprotonated ODPA in more basic solution. The difference in θwater at the

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Ito et al.

Figure 3. (A) Gd3d, (B) N1s, (C) O1s, (D) C1s, (E) P2s, and (F) P2p XPS spectra of Ga-polar undoped GaN: (a) HCl-treated GaN, (b) O3-treated GaN, and (c) ODPA-coated GaN without the heating treatment. Table 2. Surface Elemental Compositiona (atom %) of the GaN Samples HCl-treated UV/O3-treated ODPA-coated

Ga (Ga3d)

O (O1s)

N (N1s)b

C (C1s)

P (P2p)

47.6 ( 5.6 49.6 ( 6.1 30.9 ( 1.9

7.3 ( 1.1 13.4 ( 1.5 11.3 ( 1.5

23.1 ( 3.2 22.8 ( 2.1 21.5 ( 2.3 (2.1 ( 0.3 for PdO)c

21.9 ( 8.2 14.2 ( 3.0 34.0 ( 1.2

2.2 ( 0.2

a Average ( standard deviation. b Determined from a peak (398.4-398.7 eV) obtained using the mathematical deconvolution (see Figure 3B). c Determined from a peak (532.0 eV) obtained using the mathematical deconvolution (see Figure 3C(c)).

plateau region might indicate that the desorption of ODPA preferentially occurs at a defect of the monolayer such as the step edges on the GaN substrate because of the higher accessibility of the aqueous solution. The stability of the DOPA monolayers on GaN in the acidic and neutral aqueous solutions was improved by the heating treatment (160 °C under a vacuum for 24 h), as shown in Figure 2 (data shown by open circles). The θwater values of ODPA monolayers on UV/O3-treated undoped Ga-polar GaN were similar before and after the heat treatment (105.2 ( 0.5° and 104.5 ( 0.5° with and without the heat treatment), but the decrease in θwater was slower for the heated ODPA monolayers in the acidic and neutral solutions. The improved monolayer stability is maybe due to the covalent bond formation as a result of the dehydration from the phosphonic acid adsorbed on the surface oxide layer of GaN.25,28 D. XPS Measurements. XPS measurements were performed (i) to compare the surface compositions of Ga-polar undoped GaN substrates before and after the UV/O3 treatment and (ii) to verify ODPA monolayer formation on Ga-polar undoped GaN. Figure 3 shows typical Ga3d, N1s, O1s, C1s, P2s, and P2p spectra of (a) HCl-treated, (b) O3-treated, and (c) ODPA-coated Gapolar GaN. The surface elemental compositions of these samples, which were determined on the basis of the Ga3d, O1s, N1s, C1s, and P2p spectra, are summarized in Table 2. Note that the surface composition of N may not be accurate, because the N1s peak was overlapped with complex Ga Auger peaks and thus the N1s

peak area was estimated using the mathematical deconvolution (Figure 3B).21,32 i. Effect of the UV/O3 Treatment on the GaN Substrates. According to Table 2, the surface oxygen and carbon concentrations were significantly different between the HCl-treated and UV/O3-treated GaN. The UV/O3 treatment degraded the organic adsorbates on the GaN substrates, as suggested by the decrease in the carbon concentration. The presence of the carbon on the UV/O3-treated GaN reflects the incomplete removal of the organic adsorbates or their adsorption during the exposure of the sample to air (see the Experimental Section).31,32 The HCl treatment resulted in a decrease in the surface oxygen concentration, which may reflect the removal of the surface oxide layer.31–33 However, the surface oxygen was present on the HCl-treated GaN. In the Gd3d spectra, a peak around 21.1 eV with an fwhm of 1.5-1.6 eV was observed on each of the substrates (Figure 3A(a,b)). The peak can be assigned to Ga-O rather than Ga-N, which gives a lower binding energy according to the literature.47,48 The peak position and peak shape are very similar for these two (43) Freedman, L. D.; Doak, G. O. Chem. ReV. 1957, 57, 479–523. (44) Nakagawa, O. S.; Ashok, S.; Sheen, C. W.; Martensson, J.; Allara, D. L. Jpn. J. Appl. Phys. 1991, 30, 3759–3762. (45) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321–335. (46) Kosmulski, M. J. Colloid Interface Sci. 2001, 238, 225–227. (47) Shiozaki, N.; Sato, T.; Hashizume, T. Jpn. J. Appl. Phys. 2007, 46, 1471– 1473. ¨ nneby, C.; Mohney, (48) Wolter, S. D.; Luther, B. P.; Waltemyer, D. L.; O S. E.; Molnar, R. J. Appl. Phys. Lett. 1997, 70, 2156–2158.

SAMs of Alkylphosphonic Acid on GaN Substrates

substrates, suggesting that Ga is primarily present as its oxide form and the surfaces of these GaN substrates are covered with a surface oxide layer. The presence of the surface gallium oxide layer on these surfaces is supported by the O1s spectra (Figure 3C(a,b)). Each of the O1s spectra includes two peaks. The one at the lower binding energy (532.0-532.5 eV) can be assigned to the gallium oxide, whereas that at the higher binding energy (533.3-533.7 eV) is due to the surface -OH and surface adsorbates.31,32 The presence of the former peak around 532 eV on both samples suggests the presence of the surface gallium oxide on these surfaces, which is consistent with the interpretation on the Ga3d spectra. Indeed, the fwhm of the peak was similar for both substrates (2.0 ( 0.3 and 1.9 ( 0.1 eV for HCl-treated and UV/O3-treated GaN, respectively). The fwhm of the latter peak (around 533.5 eV) was wider for the UV/O3-treated GaN (2.2 ( 0.2 eV) than for the HCl-treated GaN (1.7 ( 0.5 eV), suggesting that the peak of the UV/O3-treated surface might originate from multiple surface species such as hydrogen-bonded -OH. A C1s peak was similarly observed for the HCl-treated and UV/O3-treated GaN (Figure 3D(a,b)). The peak position (285.6-285.7 eV) was consistent with the organic adsorbates.31 The peak shape was also very similar, but the surface carbon concentration was significantly smaller for the UV/O3-treated GaN (Table 2), reflecting the removal of the organic adsorbates during the UV/O3 treatment. These XPS results indicate that the UV/O3 treatment degraded the organic adsorbates and exposed the gallum oxide layer with surface -OH groups to the air/substrate interface.31 The surface -OH groups would offer the lower θwater and also involve the higher molecular adsorptivity (Table 1). In contrast, the HCltreated GaN was covered with a thicker organic adsorbate layer, which increased θwater and reduced the affinity to the polar functional groups in the primarily substituted hydrocarbons examined in this study. The HCl treatment might have dissolved the surface gallium oxide, as suggested by the small surface oxygen concentration (Table 2). However, it is likely that the surface was covered with the oxide layer and the organic molecules adsorbed instantly after the sample was exposed to the air. ii. ODPA Monolayer Formation on GaN. The P2s and P2p spectra of the ODPA-coated GaN showed peaks at 192 and 135 eV, respectively, which were not observed for the HCl-treated and O3-treated GaN (Figure 3E,F, respectively). The ratio of the surface carbon concentration to the phosphorus concentration was 15 ( 1, calculated from data in Table 2, and was close to the value, 18, expected from the molecular formula of ODPA (C18H37PO(OH)2). These observations strongly support the presence of ODPA on the surface. These phosphorus peaks were also observed for ODPA-coated GaN after the heat treatment as reported in our previous paper,35 indicating the high thermal stability of the ODPA monolayer.

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Similar to Figure 3C(a,b), the O1s spectrum of the ODPAcoated GaN (Figure 3C(c)) included two peaks. However, the peak at the higher binding energy (533.2 eV) was much larger than that at the lower binding energy (532.0 eV). The fwhm values of these peaks (2.0 ( 0.1 and 2.2 ( 0.1 eV for the peaks at the lower and higher energy regions, respectively) were similar to those of the peaks appearing at similar energy regions in UV/O3-treated GaN (1.9 ( 0.1 and 2.2 ( 0.2 eV, respectively). We however assigned the latter peak in the ODPA-coated GaN to PdO in ODPA adsorbed on metal oxide surfaces,29 because the surface composition of this oxygen almost corresponded to that of phosphorus (Table 2). On the other hand, the ratio of the total surface oxygen concentration to the phosphorus concentration was much larger than 3, suggesting that the peak around 533 eV reflected the surface oxide of the substrate and P-O in ODPA. The similarity of the experimental carbon, PdO oxygen, and phosphorus compositions to those expected from the ODPA molecular formula was consistent with adsorption of ODPA onto the GaN substrates, which offered the hydrophobic surface, as shown in Table 1.

4. Summary and Conclusions In this paper we showed the densely packed SAM formation of ODPA on a GaN substrate covered with a surface oxide layer. The adsorption behaviors of organic molecules were significantly affected by the chemical treatments of GaN substrates as well as the polarity of the GaN, but not by the carrier concentration. These results give fundamental insights into the response mechanisms and selectivities of unmodified GaN-based chemical sensors, including HEMTs,1,2 FETs,1,3 and electrochemical sensors.7–9 In addition, SAMs of organic compounds having phosphonic acid moieties will provide a means for tailoring GaN substrates for optical, electronic, and sensing devices if the stability of the monolayers in aqueous solutions are improved. In particular, the fairly weak adsorption of -COOH, -NH2, -SH, and -OH groups to the O3-treated Ga-polar GaN substrates indicates that phosphonic acid SAMs having these functional groups on the other end will make it possible to immobilize other chemical moieties such as biomolecules onto GaN substrates.30 Our current efforts focus on applying the above findings for chemical sensor applications as well as on improving the stability of the monolayers in an aqueous environment. Acknowledgment. We thank Dr. Tom Tiwald (J. A. Woollam) for his help with the ellipsometric measurements. We gratefully acknowledge financial support from Targeted Excellence Funds of Kansas State University and the Department of Defense DURIP program (Grant W911NF-05-1-0194). Work at the Naval Research Laboratory is supported by the Office of Naval Research. LA800716R