Sputtering and Etching of GaN Surfaces - American Chemical Society

Sputtering and Etching of GaN Surfaces. Ying-Huang Lai and Chuin-Tih Yeh. Department of Chemistry, National Tsing-Hua UniVersity, Hsin-Chu 300, Taiwan...
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J. Phys. Chem. B 2001, 105, 10029-10036

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Sputtering and Etching of GaN Surfaces Ying-Huang Lai and Chuin-Tih Yeh Department of Chemistry, National Tsing-Hua UniVersity, Hsin-Chu 300, Taiwan

Jung-Min Hwang and Huey-Liang Hwang Department of Electrical Engineering, National Tsing-Hua UniVersity, Hsin-Chu 300, Taiwan

Chien-Te Chen and Wei-Hsiu Hung* Synchrotron Radiation Research Center, Hsin-Chu 300, Taiwan ReceiVed: May 7, 2001

Sputtering of the GaN(0001) surface by Ar+ and N2+ ion beams is investigated using synchrotron-radiation photoemission spectroscopy. For Ar+ sputtering, the N atom is preferentially removed and a Ga-enriched GaN surface is produced. The excess Ga atoms on the Ar+-sputtered surface aggregate to form metallic Ga clusters at temperatures above 623 K. A better-ordered GaN(0001)-1 × 1 surface can be obtained by N2+ sputtering, instead of Ar+ sputtering. In addition to acting as a sputtering particle, the N2+ ion also serves as a reactant which compensates for the preferential loss of the N atom caused by physical ion bombardment. During chlorination of GaN, chlorine preferentially reacts with surface Ga atoms to form Ga chlorides. Although Ga monochloride (GaCl) is the major product formed on the N2+-sputtered surface, however, volatile chlorides (GaCl2 and GaCl3) are mainly produced on the Ar+-sputtered surface. The formation of volatile products on the Ar+-sputtered GaN surface may result in higher etching rates and lower etching temperatures for ionassisted chemical etching.

Introduction GaN and related nitride materials are recognized primarily for their applications in the fields of optoelectronics and hightemperature devices. Fabrication of microelectronic devices often involves plasma etching or implantation on their surfaces. However, the ion bombardment process may change the stoichiometry and chemical bond of the surface region to a depth of several atomic layers and thereby affect the structural and the electronic properties of the interface or surface.1-5 For instance, surface stoichiometry can affect the resistance of a metal contact (i.e., Ohmic contact or Schottky barrier) which is subsequently deposited on the pre-etched GaN surface.6,7 Hence, the study of ion-induced damage to nitrides is a necessary step in optimizing processes for device fabrication. To that aim, an essential stage is the characterization of stoichiometry and structure of sputtered surfaces. There are discrepancies between the previously reported results of electron affinity, banding bending, and band offset for GaN, which has been attributed to residual contamination and nonstoichiometry of the prepared surface.8 The Ar+ ion beam is extensively used for surface cleaning and ion-assisted dry etching of semiconductors.9,10 Pearton et al. studied the effect of Ar+ ion milling with respect to different ion energies and angles of incidence on groups III-V nitrides, concluding that Auger electron spectroscopy showed no preferential sputtering of nitrogen.11 However, other results indicated that Ar+ sputtering produced a nitrogen-deficient GaN surface.4 Two methods have been used to prepare an atomically clean and ordered GaN surface, i.e., N2+ ion sputtering followed by annealing, and in * Corresponding author. Fax:+886-3-578892. E-mail:[email protected].

situ deposition and thermal desorption of Ga metal.12,13 It was claimed that there was no significant difference between the surfaces prepared by these methods. However, care must be taken to completely desorb the metallic Ga in the latter case.14 The reconstructions of GaN(0001) surfaces with different Ga/N stoichiometric ratios have been studied experientially and theoretically.15 On the Ga-rich surface, the surface Ga exhibits metallic character involving significant overlap between Ga valence band electrons. Nitrogen gas has been used in ion-assisted etching of groups III-V nitrides.16,17 Sputtering with nitrogen ions may significantly reduce surface nitrogen depletion and thus produce an ordered GaN(0001)-1 × 1 surface with nearly stoichiometric composition.11,16,18 In this article, we report the effects of Ar+ and N2+ ion sputtering, and of subsequent thermal annealing on the GaN(0001) surface. The variation of surface composition and stoichiometry due to ion sputtering is characterized by photoemission spectrometry using synchrotron radiation as the photon source. The addition of ion bombardment to the dry etching process has been demonstrated to promote the etching rate and lower the desorption temperatures of generated products.19 Energetic ions transfer kinetic energy to the surface mainly via nuclear collisions and induce the damage of the surface structure, resulting in the change of surface reactivity. The role of energetic ions in etching reactions on the Si surface have been extensively discussed and reviewed by Winters and Coburn.19-21 Ionassisted chemical etching involves the local introduction of a reactive gas around a sample with simultaneous bombardment with an inert gas ion beam. This etching configuration may allow for the physical and chemical etching components to be

10.1021/jp011728k CCC: $20.00 © 2001 American Chemical Society Published on Web 09/26/2001

10030 J. Phys. Chem. B, Vol. 105, No. 41, 2001 controlled separatively.22,23 The operating parameters of the ion beam (e.g., beam energy, incident angle, and sputtering gas) can be thus adjusted to obtain high etching rates for various materials. For binary semiconductors such as GaN, sputtering ions with different chemical properties may cause the surfaces to have different stoichiometry, resulting in a variation of etching reactions.24 Thus, the selection of sputtering gas for compound semiconductors becomes an important factor for attaining better etching efficiency. Chlorine-based etchants are very effective for etching groups III-V compounds due to the high volatility of the group III and group V chlorides generated.25 However, because of the chemically stable Ga-N bond, Cl2 gas is unable to etch GaN at temperatures below 200 °C without energetic-ion sputtering.26 It has been shown that the addition of noble gases in the inductively coupled plasma enhances the etching by initial breaking of the Ga-N bond.9 The etching rate of GaN by inductively coupled Cl2/Ar plasmas was also studied as a function of Cl ion/radical density and ion energy using mass plasma spectroscopy.10 Under sufficient ion bombardment, GaN etching was more affected by the chemical reaction between Cl and GaN than by physical sputtering itself. This suggests that the reactivity and stoichiometry of the surface during ion sputtering are critical to obtain higher etching rates. However, a comprehensive understanding of the mechanisms (physical bombardment and chemical reaction) is required to further develop effective processes of GaN etching. In this article, soft X-ray photoelectron spectroscopy is used to study how the etching reaction is affected by the stoichiometry of the surfaces sputtered by Ar+ or N2+ ions upon exposing the GaN to chlorine. Experimental Section Experiments were performed in a UHV chamber equipped with a quadrupole mass spectrometer, a LEED, and a hemispherical electron energy analyzer (VSW HA100). The XPS measurements were carried out at the HSGM and the U5 undulator beamlines of the Synchrotron Radiation Research Center in Taiwan. The incident angle of the photon beam was 55° from the surface normal. The photoelectrons were collected with the electron energy analyzer normal to the sample surface. In this article, all XPS data were presented after Shirley background subtraction with a third-order polynomial to each side of the peak in all fits.27,28 To identify the chemical species on the sample surface, the XPS spectra were fitted numerically with Gaussian-broadened Lorentzian functions. Photoemission onset from an Au foil attached to the sample holder was referred to as the Fermi level. The GaN film grown on a sapphire (0001) substrate was purchased from CREE Research, and was unintentionally doped n-type with a carrier concentration of ∼1 × 1017/cm3. Prior to use, the sample was cleaned in NH4OH/H2O solution and rinsed in deionized water. The GaN sample was clamped to a 0.3 mmthick Si wafer (0.01∼0.001 Ω cm) by two Ta foils. The sample was heated by resistive heating of the Si wafer and the temperature was monitored by a K-type thermocouple spotwelded on a Ta foil which was inserted between the GaN sample and the Si wafer. An AG5000 cold cathode ion gun (VG Microtech) was used to produce the energetic ions for GaN sputtering. The gas (Ar or N2) was directly fed into the ion gun via a leak valve, providing an ion current density of 2 µA/cm2 on the sample at the pressure of ∼5 × 10-6 Torr. N2+ was the major ion (80-85%) generated by the ion gun when N2 was used as the sputtering gas. The GaN surface was sputtered by the ion beam at a 45° incident angle.

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Figure 1. Ga 3d and N 1s photoelectron spectra for the GaN(0001) surface sputtered by 1 keV Ar+ for 10 min at room temperature and then annealed to 623, 723, and 823 K, respectively. The dots represent the collected data after background subtraction, the solid lines are the curve fits to the data, and the various components are shown in dashed lines.

For the chlorination of GaN, Cl2 gas (99.99% min., Matheson) was dynamically introduced onto the surface through a 1/4 in. stainless tube with a pinhole (dia. 250 µm). During Cl2 dosing, the GaN surface was placed at ∼1 cm in front of the pinhole with the Cl2 partial pressure of 2 × 10-9 Torr in the chamber. This procedure can minimize the contamination and the corrosion damage to the UHV chamber and other equipment, which are possibly caused by Cl2. Results and Discussion A. Sputtering of GaN(0001) by Ar+ and N2+ Ions. Figure 1 shows the Ga 3d and N 1s core-level spectra for the GaN(0001) surface which is sputtered by the 1 keV Ar+ ion beam at a current density of 2 µA/cm2 for 10 min and then annealed to indicated temperatures. As determined from the fitting process, there are two Ga chemical states with Ga 3d5/2 at 20.4 and 19.5 eV for the sputtered GaN surface. The peak at 20.4 eV is attributed to the bulk GaN.14 The peak at 19.5 eV is lower than that of the bulk GaN by 0.9 eV and is assigned to the surface Ga atom and the Ga atom with the dangling bonds at the subsurface. Bombardment of energetic Ar+ ions can cause the breaking of Ga-N bonds at the subsurface region and lead to the formation of Ga atoms with dangling bonds. These Ga atoms bond to fewer electronegative N atoms than those in the bulk GaN, and thus have a lower Ga 3d binding energy. On the other hand, Ar+ sputtering also results in the formation of N atoms with dangling bonds, which bond to fewer electropositive Ga atoms than those in the bulk GaN. As shown in Figure 1, a shoulder N 1s peak at 398.9 eV is assigned to the N atoms with dangling bonds at the subsurface, which is higher

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Figure 3. LEED patterns for the GaN(0001) surfaces sputtered by (a) 1 keV Ar+ and (b) N2+ sputtering at 300 K for 10 min and then annealed to 823 K for 1 min. The electron beam energy used for the LEED measurement is ∼115 eV.

Figure 2. Valence band spectra of the GaN(0001) surfaces sputtered by (a) 1 keV Ar+ and (b) 1 keV N2+ for 10 min at room temperature. The sputtered GaN samples are subsequently annealed to 623, 723, and 823 K, respectively.

in binding energy than the main peak at 397.4 eV assigned to the bulk GaN. By annealing the sample above 623 K, an asymmetric Ga 3d5/2 peak appears at 18.9 eV, whereas the peak intensity at 19.5 eV gradually decreases as the temperature increases up to 823 K. The full width at half-maximum (fwhm) of the Ga 3d5/2 at 18.9 eV is ∼0.5 eV, which is much smaller than the values observed for the bulk GaN (1.2 eV) and the surface Ga atom (1.0 eV). The asymmetric Ga 3d5/2 peak with a small fwhm is indicative of the presence of metallic Ga on the GaN surface.24 This suggests that the Ar+ ion preferentially sputters nitrogen atoms, resulting in a Ga-enriched GaN surface. The excess Ga atoms begin to aggregate to form metallic Ga on the surface when annealing the sample above 623 K. It is also noticeable from Figure 1 that the intensities of Ga 3d (19.5 eV) and N 1s (398.9 eV) gradually decrease after annealing the sample to higher temperatures. Evidently, the Ga and N atoms with dangling bonds produced by ion bombardment can rearrange and reform the Ga-N bonds at high temperatures (>623 K). The formation of metallic Ga is also revealed by the valenceband spectra shown in Figure 2a, where a shoulder band appears near the Fermi level upon annealing the sample above 623 K. This valence band is attributed to the 4s and 4p states of metallic Ga. Annealing the Ga-rich GaN surface induces the formation of metallic Ga atoms with a Ga-Ga separation which is small enough to have a significant overlap between Ga valence electrons.15,29 Thus, the formation of metallic Ga produces highly dispersive bands and lowers the total surface energy. Furthermore, Figure 1 shows that upon annealing the Ar+-sputtered surface from 623 to 823 K, the binding energy of Ga 3d5/2 (18.9 eV) assigned to metallic Ga does not significantly change at different annealing temperatures. This is a characteristic of the metal, in which the binding energy is not affected by the average

electrical field felt by the Ga and N atom in the bulk.30 However, the components of Ga 3d (20.4 eV) and N 1s (397.4 eV) assigned to the bulk GaN shift to binding energy higher by ∼0.2 eV, when the sample is annealed from 300 to 823 K. This binding energy shift is due to the surface reconstruction (Ga aggregation) induced by sample annealing, which results in a change in surface band bending. As shown in Figure 3a, a diffuse 1 × 1 LEED pattern is observed on the Ar+-sputtered and annealed GaN(0001) surface. This means that the GaN surface is not perfectly ordered due to the presence of metallic Ga. Metallic Ga may exist on the surface in the form of clusters, rather than a continuous film, and cover only a small fraction of surface area. The (1 × 1) domain may be close to or less than the coherence width (100 Å) of the electron beam source used in the LEED experiment. The metallic Ga cluster may be somewhat randomly dispersed on the surface at a distance which, on average, is less than the coherence width. Most of the GaN surface area is covered by the surface Ga atoms which are associated with a 3d5/2 peak at 19.5 eV, as shown in Figure 1. This is consistent with the previous observations that the stable growth of GaN occurs under a metal-rich or near metal-rich condition, and that the GaN surface is stabilized by Ga atoms.15,29 On the other hand, a similar situation was recently reported, where metallic In clusters existed on the Ar+-sputtered and annealed InP(100) surface.24 In an attempt to reduce the deficiency of nitrogen, N2 was used as the sputtering gas. Figure 4 shows the Ga 3d and N 1s spectra for the N2+-sputtered and annealed GaN surface. The GaN surface is sputtered by the 1 keV N2+ ion beam at a current density of 2 µA/cm2 for 10 min. Parallel to the spectra observed on the Ar+-sputtered surface, the N 1s features at 397.5 and 399.2 eV are assigned to the bulk GaN and the N atom with dangling bonds produced by ion bombardment, respectively. An additional shoulder peak at 400.6 eV is attributed to the N atom embedded in the near-surface of GaN during high-energy N2+ ion bombardment. The embedded N atom is intercalated in the disordered and damaged structure of the subsurface region. In addition, the embedded N atom is weakly interacting with its neighboring atoms (Ga or N), and is less positively charged than those N components at 397.5 and 399.2 eV. The N 1s component at 400.6 eV disappears at 623 K, at which point the embedded N may either diffuse and desorb from the surface in the form of molecular nitrogen, or react with neighboring Ga atoms to reform Ga-N bonds. Upon annealing the sample to 823 K, the Ga 3d5/2 component at 19.6 eV assigned to the surface Ga atom and the Ga atom with dangling bonds decreases in intensity. The N 1s peak at 399.2 eV due to the nitrogen

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Figure 5. Ga 3d photoelectron spectra for the GaN(0001) surfaces sputtered by 1 keV (a) Ar+ and (b) N2+ for 10 min at room temperature and then annealed to 823 K. Figure 4. Ga 3d and N 1s photoelectron spectra for the GaN(0001) surface sputtered by 1 keV N2+ for 10 min at room temperature and then annealed to the indicated temperatures.

atom with dangling bonds completely disappears. This suggests that reformation of GaN from Ga atoms and N atoms with dangling bonds occurs on the N2+-sputtered GaN, as observed on the Ar+-sputtered GaN. However, there is only a small amount of metallic Ga on the N2+-sputtered GaN surface, unlike the case of Ar+ ion bombardment. The valence band spectra of the N2+-sputtered GaN surface are shown in Figure 2b. There is no apparent change in the photoemission onsets of valence band spectra after annealing the sputtered sample from 300 to 823 K. The valence band spectra exhibit an emission tail extending from the valence band maximum (VBM) into the Fermi level because there is still a small amount of metallic Ga on the surface. As a result, this causes some difficulty and ambiguity in determining the VBM of GaN. As shown in Figure 3b, a somewhat better 1 × 1 LEED pattern can be obtained for the N2+-sputtered and annealed GaN(0001) surface, compared to the Ar+-sputtered and annealed surface. However, when the sample is annealed to ∼970 K, the GaN surface begins to decompose at the near-surface region and desorbs molecular nitrogen under ultrahigh vacuum condition. An intensive and narrow peak of 3d5/2 at 18.9 eV, indicative of metallic Ga, can thus be seen in Figure 4. The decomposition temperature of GaN obtained in our study is 50 °C lower than the value measured by laser reflectivity.31 This difference may originate from the fact that the XPS spectrometry is a surfacesensitive technique and can accurately measure the threshold temperature of GaN decomposition at the near-surface region. Comparing the stoichiometry of the GaN surfaces sputtered by Ar+ and N2+ ions, the XPS spectra consisting of N 1s and Ga 3d core-levels are shown in Figure 5. The peak ratio of N 1s to Ga 3d (or 3p) is clearly higher on the N2+-sputtered surface than on the Ar+-sputtered surface. This suggests that the nitrogen ion (N2+ or N+) acts as an energetic particle for removing Ga and N atoms from the GaN surface and also reacts with the

excess Ga atoms on the surface to form GaN. It is also a possibility that N2 in the background adsorbs on the surface during sputtering and is energetically activated to react with surface Ga by ion bombardment. These reactions could compensate for the preferential loss of nitrogen induced by physical (nonreactive) ion bombardment. Accordingly, the stoichiometric ratio of N/Ga on the N2+-sputtered GaN surface can be closer to unity, and result in a better ordered-1 × 1 surface upon annealing to 823 K, compared to the Ar+-sputtered surface. It is expected that the impinging ion with a larger atomic mass can transfer more kinetic energy onto the surface and thereby induce more severe damage on the surface structure. Thus, an Ar+ ion will generate more surface damage than a N2+ ion at the same sputtering condition (e.g., beam energy), because of its larger atomic mass. The observed difference between the Ar+- and the N2+-sputtered GaN surfaces in stoichiometry may be ascribed to the extent of momentum transfer during ion bombardment. To understand the effect of momentum transfer on the stoichiometry of the sputtered surface, Ne gas was used to produce ions. Ne+ has a smaller atomic mass and thereby transfers less kinetic energy to the surface than Ar+ and N2+. Figure 6 shows a comparison of the GaN surfaces sputtered by Ar+, Ne+, and N2+ ions and subsequently annealed to 823 K. It can be seen that Ne+ sputtering also produces a Ga-enriched surface as obtained by Ar+ sputtering, except that there is a slightly lower intensity of metallic Ga 3d5/2 at 18.9 eV. This indicates that the physical bombardment of both inert Ar+ and Ne+ ions preferentially sputters the N atom from the GaN surface. Therefore, it is concluded that the stoichiometry of the sputtered surface is mainly determined by the chemical properties of the incident ions, not by the extent of structural damage caused by physical bombardment at the ion beam energy below 1 keV. B. Chlorination and Etching of GaN(0001). As mentioned above, it is important to understand the reaction mechanism involved in chemical etching of the processing technology applied to GaN. Chlorine-based etchants are widely used in etching groups III-V compounds because of their high reactivity and the high volatility of generated products. The adsorption

Sputtering and Etching of GaN Surfaces

Figure 6. Comparison of photoelectron spectra for the GaN(0001) surfaces sputtered by 1 keV (a) Ar+, (b) Ne+, and (c) N2+ for 10 min at room temperature and then annealed to 823 K. The photon energy used in the XPS measurement is 500 eV.

Figure 7. Ga 3d photoelectron spectra collected after exposing the GaN(0001) surface to 2 × 10-9 Torr Cl2 at 100 K for different dosing preiods. The initial GaN surface is prepared by 1 keV Ar+ sputtering for 10 min and subsequent annealing to 823 K.

and thermal reaction of Cl2 on GaN is also investigated in this work. Figure 7 shows the Ga 3d XPS spectra for the GaN(0001) surface exposed to 2 × 10-9 Torr Cl2 at 100 K with different dosing periods. The initial GaN surface is prepared by 1 keV Ar+ sputtering for 10 min and subsequent annealing to 823 K. With the increase of Cl2 exposure, the Ga 3d5/2 components at 19.5 and 18.9 eV due to the surface and metallic Ga atoms decrease in intensity. The chlorine-induced chemical shifts of Ga 3d appear at higher binding energy, indicative of the formation of Ga chlorides (GaClx, x ) 1, 2, and 3). We make no attempt to deconvolute the Ga 3d spectra of chlorinated GaN surfaces because of the complicated surface composition and the overlap of Ga 3d binding energy between bulk GaN and GaClx. However, the N 1s feature remains essentially unchanged after the adsorption of chlorine, except for a decrease

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Figure 8. Cl 2p photoelectron spectra collected from the GaN(0001) surface exposed to 2 × 10-9 Torr Cl2 for 40 s at 100 K and then annealed to the indicated temperatures. The initial GaN surface is prepared by Ar+ sputtering and subsequent annealing to 823 K.

in its intensity. This suggests that chlorine mainly reacts with the surface and metallic Ga atoms to form Ga chlorides. Chlorination of the GaN surface is saturated at an exposure period greater than 30 s. Figure 8 shows the Cl 2p spectrum for the GaN surface exposed to Cl2 for 40 s at 100 K. Two Cl 2p3/2 chemical states are observed at 199.4 and 200.6 eV and attributed to chlorides and adsorbed Cl2 molecules, respectively. Chlorine reacts with the GaN surface to form chlorides at the initial exposure and molecularly adsorbs on the surface after the saturation of chlorination at high exposures. Figures8 and 9 show the Cl 2p, Ga 3d, and N 1s spectra for the Ar+-sputtered and annealed GaN surface which is exposed to Cl2 for 40 s at 100 K and then heated to different temperatures. At 133 K, the Cl 2p3/2 component at 200.6 eV due to the adsorbed Cl2 molecule diminishes and the Ga 3d intensity at the high binding energy side increases. This indicates that upon heating the sample to 133 K, the adsorbed Cl2 molecule may be desorbed or further chlorinate the surface to form more Ga chlorides with higher oxidation states. Annealing the sample above 223 K causes a gradual decrease of Cl 2p3/2 intensity at 199.5 eV, due to the thermal desorption of chlorides. Upon heating to 423 K, over 50% of the Ga chlorides are desorbed from the surface. The Ga 3d5/2 peak at 18.9 eV becomes well-resolved from other Ga components, although its intensity is less than that on the initial surface. At 673 K, most of the surface Ga atoms are etched from the surface via desorption of Ga chlorides, and the intensity of metallic Ga at 18.9 eV is decreased. Annealing the sample to 773 K can completely remove chlorides from the surface. Nevertheless, Figure 9 also shows that the N 1s feature of the chlorinated GaN surface remains unchanged except for an increase in its intensity after annealing. The N atom stays intact during chlorination and annealing because they are located underneath the surface and cluster Ga atoms. The mechanism of chlorination and etching of the GaN surface is depicted in Figure 10. The initial GaN surface obtained after Ar+ sputtering and annealing is covered by surface Ga and cluster (metallic) Ga. The outermost Ga of clusters and the surface Ga are initially chlorinated at 100 K upon exposing the

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Figure 11. (a) Ga 3d photoelectron spectra for the GaN(0001) surface exposed to 2 × 10-9 Torr Cl2 for 40 s at 100 K and then annealed to the indicated temperatures. (b) N 1s photoelectron spectra collected before (b) and after (O) exposing the GaN(0001) surface to 2 × 10-9 Torr Cl2 for 40 s at 100 K. The initial GaN surface is prepared by 1 keV N2+ sputtering for 10 min and annealing to 823 K. Figure 9. Ga 3d and N 1s photoelectron spectra for the GaN(0001) surface exposed to 2 × 10-9 Torr Cl2 for 40 s at 100 K and then annealed to the indicated temperatures. The initial GaN surface is prepared by Ar+ sputtering and subsequent annealing to 823 K.

Figure 10. Schematic diagram showing the proposed mechanism of thermal reaction of Cl2 on the Ar+-sputtered and annealed GaN surface which is covered by surface and metallic cluster Ga.

surface to Cl2. The surface is subsequently etched via thermal desorption of Ga chlorides at temperatures higher than 223 K. When the surface is further heated to higher temperatures (>673 K), the Ga clusters remaining after Cl2 etching begin to

decompose and diffuse onto the surface to reform a Gaterminated GaN surface which is more stable than the Nterminated surface.15 However, the surface Ga atoms can be further etched by the cycles of chlorination and annealing. The resulting GaN surface exhibits an ordered LEED pattern, as obtained on the N-sputtered and annealed surface. It has been found that surface stoichiometry is one of the crucial factors in determining whether a surface will be passivated or etched.32 To understand the effect of surface stoichiometry on the chemical reactivity with chlorine, this study also investigates chlorination of the GaN surface prepared by N2+ sputterring and subsequent annealing to 823 K. Figure 11a shows the Ga 3d XPS spectra for the N2+-sputtered and annealed GaN surface which is exposed to 2 × 10-9 Torr Cl2 for 40 s and then annealed to different temperatures. Adsorption of chlorine shifts the main peak of Ga 3d core level to higher binding energy by ∼0.3 eV. In addition, a chlorine-induced N 1s chemical shift is observed at the high binding energy side, as shown in Figure 11b. This is associated with the formation of NClx. Unlike the adsorption of Cl2 on the Ar+-sputtered surface, chlorine can bond to the surface N atom to form NClx on the N2+-sputtered surface. Figure 12 shows the Cl 2p XPS spectra, in which there are two Cl chemical states with 2p3/2 at 199.4 and 200.8 eV. As noted previously, these peaks are attributed to chlorides and adsorbed Cl2 molecules, respectively. It can be seen that most of chlorine molecularly adsorbs on the N2+-sputtered surface at 100 K. After heating the sample to 133 K, the adsorbed Cl2 molecule may be desorbed or dissociate to further chlorinate the surface. The latter reaction shifts the whole Ga 3d peak to higher binding energy. However, the chlorine-induced N 1s feature due to NClx disappears, as shown in Figure 11b. This indicates the weak interaction between chlorine and surface nitrogen. The preferential adsorption/reaction of halogen with the group III atoms is generally observed on the surfaces of groups III-V compounds.33 It is attributed to the interaction of closed-shell halogen molecular orbitals and empty dangling bond orbitals of group

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Figure 13. Relative Cl 2p intensity as a function of annealing temperature for the GaN(0001) surfaces initially exposed to 2 × 10-9 Torr Cl2 for 40 s at 100 K. The initial GaN surfaces are prepared by (O) N2+ sputtering and (b) Ar+ sputtering with subsequent annealing to 823 K, and (4) Ar+ sputtering without subsequent annealing, respectively. Figure 12. Cl 2p photoelectron spectra collected from the GaN(0001) surface exposed to 2 × 10-9 Torr Cl2 for 40 s at 100 K and then annealed to the indicated temperatures. The initial GaN surface is prepared by N2+ sputtering and subsequent annealing to 823 K.

III atoms (Ga atoms), rather than filled dangling bond orbitals of group V (N atoms). By further annealing the sample to higher temperatures (>373 K), the Ga 3d core level gradually shifts toward lower binding energy. This is due to thermal desorption of the surface Ga chlorides. The chlorides are nearly depleted from the surface upon heating the sample to 873 K, which is ∼100 °C higher than that observed on the Ar+-sputtered surface. In contrast to chlorination of the Ar+-sputtered surface, chlorine mainly adsorbs on the N2+-sputterred surface in the form of molecular Cl2 at 100 K. This indicates that the N2+sputtered surface is less reactive than the Ar+-sputtered surface. The different reactivity of these surfaces may come from the bonding configuration and chemical states of surface Ga atoms. On the N2+-sputtered and annealed surface, the outermost surface Ga atom may be tri-coordinated to the subsurface N atoms. Because this surface Ga is relatively unreactive, chlorine mainly adsorbs onto it in the molecular form at low temperature (100 K). Upon annealing the Cl2-covered surface, the surface Ga atom preferably forms Ga monochloride without breaking the stable Ga-N bond to form higher chloride (di- or trichlorides), since it only has a single dangling bond. On the other hand, Ar+ sputtering and annealing produces a Ga-enriched surface which is covered by surface and cluster Ga atoms. It has been proposed that the surface Ga atoms may be composed of additional Ga adlayers residing on top of the Ga-terminated GaN surface.15 These Ga adlayers are reactive to chlorine and subject to forming higher chlorides (di- and trichlorides) which have lower desorption temperatures than the monochloride. The importance of chemical reaction on the surface in the etching process can be understood from the desorption of chlorides as a function of sample temperature. Figure 13 shows the relative amount of chlorides as a function of annealing temperature on the N2+-sputtered and the Ar+-sputtered GaN surfaces with subsequent annealing to 823 K, and the Ar+sputtered GaN surface without subsequent annealing. The total amount of chlorides on the GaN surface is assumed to be proportional to the integrated area of the Cl 2p intensity. The relative Cl 2p intensities are referred to that obtained at 170 K,

where all Cl atoms are present on the surface in the form of chlorides without adsorption of molecular Cl2. On the Ar+- or N2+-sputtered GaN surfaces, the chlorides generated by adsorption of Cl2 are gradually desorbed from the GaN surfaces over a wide range of annealing temperatures. It can be seen that the behavior of thermal desorption of chlorides is essentially the same on the Ar+-sputtered GaN surfaces with or without subsequent annealing. The Cl 2p intensity decreases faster on the Ar+-sputtered surface than on the N2+-sputtered surface, as the surface temperature increases. The volatility of chlorination products is certainly related to the surface stoichiometry. The Ga-enriched surface produced by Ar+ sputtering and annealing can react with Cl2 to form higher Ga chlorides which are desorbed at lower desorption temperatures. Therefore, Ar+ ion bombardment not only removes Ga and N from the GaN surface, but also promotes the surface reactivity with Cl2 to form the etching products with higher volatility. Conclusion In summary, an acceptable method for the preparation of a stoichiometric GaN surface is verified using surface-sensitive photoemission spectroscopy. The energetic Ar+ ion preferentially sputters the N atom from the GaN surface, resulting in a Ga-enriched GaN surface. Annealing this surface induces the aggregation of excess Ga atoms to form Ga clusters on the surface. An ordered stoichiometric GaN surface can be attained by using nitrogen as the sputtering gas and annealing the sample to 823 K. During the N2+ ion bombardment, N2+ can act as a reactant to the sputtered surface to compensate the loss of the N atom, as well as an energetic sputtering particle. Our XPS data show that chlorine preferentially reacts with the surface Ga atom during the chlorination of GaN. After the adsorption of chlorine, Ga monochloride is the main product formed on the N2+-sputtered GaN surface. The Ga-enriched surface obtained from Ar+ sputtering is subject to forming volatile higher chlorides (GaCl2 and GaCl3). As a result, the Ar+ ion bombardment can facilitate the etching reaction at lower temperatures and also enhance the etching rate. In chemically assisted ion etching of GaN, the energetic ion beam is required to remove N and Ga atoms from the surface via physical sputtering and also to promote the surface reactivity with etchants (Cl2) via modification of surface stoichiometry. On the

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