Halogenation of gallium arsenide(100) and (111) surfaces using

J . Phys. Chem. 1992, 96, 2253-2258

2253

for any T site i. This sum is independent of the Si,AI distribution, and then eq A18 reduces to 2m

Vp =

- j/2'Y08~k= I

(A201

(A241 and m

where is a constant for each framework composition. Finally, by using eq A16, we have

FLd = CeP(k,i)

(A29

i= 1

Notice that eq A23 is true due to the relationship 2m

2m

C EkFLd = C E''k Fk

XI

k= I

where 2m

up(ij) =

eP(k,i).eP(kJ)

k= 1

(A22)

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/j100184a042

up(ij) is a 192 X 192 symmetric matrix that contains the information relevant for our problem. Similarly to the Coulomb energy, the change of the polarization energy with respect to a random T-atom distribution, is calculated to be 2m

Vp - q d = -)/2C&~8~[

k= I

-Dx~]

(-423)

where D is a constant defined by

k= I

(A26)

Finally, the relevant parts of the Coulomb and polarization energies can be combined to give

AU = U - u r d = Uo+

Y2a2CCu(ia,ig) i,

(A27)

ig

where Uois a constant term for a given framework composition:

Uo = ~ : 6 ~ ( ' / 2 @

- A)

('428)

and the 192 X 192 symmetric matrix u(ij) = v c ( i j ) - aovP(ij)

(A291 includes both the long-range Coulomb interaction between charge deficits and their interactions via oxygen polarization.

Halogenation of GaAs (100) and (111) Surfaces Using Atomic Beams Andrew Freedman* and Charter D. Sthespring? Aerodyne Research, Inc., 45 Manning Road, Billerica, Massachusetts 01821 (Received: July 25, 1991; In Final Form: Nouember 12, 1991)

GaAs (100) and (1 11) gallium-rich surfaces (Ga/As N 1.4) have been halogenated using beams of atomic chlorine and fluorine under ultrahigh-vacuum conditions. XPS and LEED analyses of the resulting thin films indicate that chlorine atoms produce a disordered highly arsenic-deficient GaCl, reaction product layer whose thickness decreases as temperature increases from 130 to 273 K. At 323 K and above, a stable reaction layer is not observed but an ordered, stoichiometric to As-rich GaAs surface is formed; a residual submonolayer of chlorine is present at temperatures up to 573 K. More limited studies on arsenic-rich GaAs surfaces (Ga/As = 0.85) indicate identical behavior at temperatures of 173 K and above. At 140 K, though, preferential etching does not occur and equal amounts of AsCl, and GaCl, are observed. Fluorination of Ga-rich GaAs (100) surfaces at 305 K produces a disordered GaF, layer which is thermally stable (no loss of fluorine) up to a temperature of 573 K. These results indicate that any realistic model of chemical etching of GaAs should admit the possibility (if not likelihood) of the formation of a three-dimensional reaction product layer, whose thickness and chemical composition are a function of halogen atom flux and substrate temperature.

Introduction As the chemical processing of electronic materials has improved to meet higher specifications for device fabrication, increased attention has been placed on halogen atoms, especially with respect to 111-V semiconductor materials.'V2 Due to their ease of production (thermally, chemically, and in discharges) and their high reactivity (both in the gas phase and on surfaces), halogen atoms play key roles in both etching and deposition system^.^-^ This paper is one of a series from this laboratory detailing the interactions of both fluorine and chlorine atoms with a number of electronic materials, including silicon,* diamond?JO and gallium arsenide." In this particular study, we have halogenated GaAs (100) and (1 11) surfaces using very low flux beams of atomic chlorine and fluorine and used in situ X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) to analyze the resulting surfaces. Important questions to be *To whom correspondence should be addressed. 'Permanent address: Department of Chemical Engineering, West Virginia University, Morgantown, WV 26506.

answered include the following: Do F and C1 atoms react differently with GaAs as is the case with Si? Is a reaction product layer formed, and if so, what is its composition as a function of E. Pure Appl. Chem. 1988, 60, 703. (2) Flamm, D. L. In Plasma Etching, Manos, D. M., Flamm, D. L. Eds.; Academic Press: San Diego, 1989; Chapter 2. (3) Seward, K. L.; Moll, N. J.; Coulman, D. J.; Stickle, W. F. J . Appl. Phys. 1987, 61, 2358. (4) Lishan, D. G.;Hu, E. L. Appl. Phys. Leu.1990, 456, 1667. ( 5 ) Meguro, T.; Hamagaki, M.; Modaressi, S.; Hara, T.; Aoyagi, Y.; Ishii, M.: Yamamoto. Y. A D D /Phvs. . Lett. 1990. 56. 1552. (6) Pearton, S. J.; Crhakraiarti, K.; Hobson, W. S i Kinsella, A. P. J. Vac. Sei. Technol. 1990, 38, 607. (7) d'Agostino, R. In Plasma-Surface Interactions and Processing of Materials; Aucillo, O., et al., Eds.; Kluwer Academic; Amsterdam, 1990; pp 425-456 .-- .- - . (8) Stinespring, C. D.; Freedman, A. Appl. Phys. Lett. 1986, 48, 718. (9) Freedman, A.; Stinespring, C. D. Appl. Phys. Lett. 1990, 57, 1194. (10) Freedman, A.; Stinespring, C. D. Mater. Res. Sac. Symp. Proc. 1991, (1) Ibbotson, D.

204, 571. (1 1) Freedman, A.; Stinespring, C. D. Mater. Res. Sac. Symp. Proc. 1990, 158, 389.

0022-365419212096-2253%03.00/0 0 1992 American Chemical Society

2254 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992

Freedman and Stinespring

CL ATOM SOURCE temperature? And, what ramifications can be drawn from these data for modeling these systems? Almost all of the previous work aimed at understanding the C12/AR GAS INLET microchemistry of halogen-GaAs systems has involved the use of molecular chlorine, either by itself’2J3J6-19~23 or in conjunction EVENSON. TYPE CAVITY with ion,13-16320322 electron,21or photon beam^.'^,^^ These include, QUARTZ TUBE for example, mass spectrometric analysis of gas-phase products ULTRA. TORR UNION caused by beam-induced etching,12-18various forms of surface analysis of postetched substrates,’2~13~19-21~21~23~25 photon-induced dissociation of adsorbed chlorine,19 and etch rate measurements aimed at showing crystallographic orientation preference^.^^-^^ TO ROTARY PUMP Among some simple conclusions that can be drawn from this work regarding chemical etching are the following: molecular chlorine dissociatively chemisorbs on GaAs surfaces with high probability; the onset of steady-state chemical etching occurs at -325 K; the gas-phase etch products include gallium and arsenic chloride molecules and molecular arsenic species whose identity and temperature dependence are still a matter of dispute; and chemical etching produces a stoichiometric to As-rich surface with a residual 25cm chlorine coverage. It is important to note that, under typical plasma etching conditions, though, atomic rather than molecular chlorine is thought to be the dominant chemical etchant s p e ~ i e s . ~ , ~ ~ Figure 1. Schematic of chlorine atom beam source. We present XPS data for CI atoms that indicate the formation cell (ultimate vacuum 3 X Torr) interfaced to an ion/subof a 3-dimensional reaction product layer whose thickness and limation pumped analysis chamber. The diagnostics available in composition are a function of initial substrate composition and the analysis chamber are X-ray photoelectron spectroscopy (XPS) temperature. This stable reaction product layer persists (with and low-energy electron diffraction (LEED). The sample is decreasing thickness) up to 273 K. At 323 K (the temperature transferred between chambers using a linear motion feedthrough where gas-phase products are first measured) and above, a stable with sample heating (1200 K) and (1 30 K) capabilities. reaction product layer is not observed and an approximately The XPS analyses were performed using PHI 15-keV Mg Ka stoichiometric GaAs surface obtains. A residual submonolayer and Al K a X-ray sources and a PHI doublepass cylindrical mirror coverage of C1 atoms persists up to 573 K. There is no appreciable electron energy analyzer operated a t a bandpass energy of 25 eV. difference in reactivity of the (100) and (1 11) surfaces. The analyzer was calibrated using the Au 4f, peak at 83.8 eV GaAs( 100) has also been fluorinated at 305 K using an atomic and is accurate to f0.2 eV. All spectra are reierenced to the Ga beam. As in the case of chlorine, arsenic is depleted, presumably 3d peak (of annealed GaAs) at 18.8 eV.32 At low temperatures, due to the relatively high volatility of the formed arsenic halide there is a noticeable drift to higher binding energies for all peaks product, leaving a GaF, reaction layer whose thickness reaches as halogenation increases, presumably due to the formation of a -40 A after 3 W M L exposure (ML = monolayer). This reaction reaction layer. As this does not occur for higher temperatures, layer is stable to 573 K above this temperature, the fluoride layer all spectra have corrected by assigning the “bulk” Ga 3d peak a desorbs leaving behind a slightly gallium-rich surface. value of 18.8 eV. The LEED device is of a reverse-view variety (Princeton Instruments). Experimental Section The GaAs samples used in these studies were p-type electronic grade single-crystal substrates. In situ sample cleaning involved The experimental apparatus used in these studies comprises a Ar+ etching at 1 keV until no trace of oxygen or carbon conturbomolecular pumped, liquid nitrogen trapped ultrahigh-vacuum tamination could be observed. This was followed by annealing at 850 K to restore surface order (as observed by LEED patterns as low as 33 eV). Correction of XPS intensities for relative (12) DeLouise, L. A. J . Chem. Phys. 1991, 94, 1528; J. Vac. Sci. Technol. photoelectron production cross sections indicates that the surface 1991, A9, 1332. region (50-100 A deep) is substantially Ga-rich (Ga/As = 1.4). (13) DeLouise, L. A. J . Appl. Phys. 1991, 70, 1718. Photoemission studies on the (100) surface indicate that measured (14) McNevin, S. C.; Becker, G. E. J . Appl. Phys. 1985, 58, 4670. ratios of this magnitude result from a surface that is totally Ga (15) O’Brien, W. L.; Paulsen-Boaz, C. M.; Rhcdin, T. N. J . Appl. Phys. 1981, 64, 6523. terminated.33 The fuzzy 1 X 1 LEED pattern obtained on the (16) Balooch, M.; Olander, D. R.; Siekhaus, W. J. J . Vac. Sci. Technol. (100) surface is consistent with this picture.34 The (1 11) surface 1986, 84, 794. produced similarly streaky hexagonal patterns with the same (17) Hou, H.; Zhang, Z.; Chen, S.; Su,C.; Yan, W.; Vernon, M. Appl. measured Ga/As ratio. As described later, slightly As-rich Phys. Letf. 1989, 55, 801. (18) Hou, H.; Su,C.; Vernon, M. Private communication. surfaces (Ga/As = 0.85) were prepared by Ar+ etching, followed (19) Herman, V. L.; Haase, G.; Osgood, R. M. Chem. Phys. Lett. 1991, by etching with C12at 473 K. The LEED patterns obtained here 176, 319. were much more complicated and difficult to interpret, but the (20) Ameen, M. S.; Mayer, T. M. J. Appl. Phys. 1986.59, 967; 1988,63, aforementioned photoemission studies indicate a surface that is 1152. (21) Mokler, S.M.; Watson, P. R.; Ungier, L.; Arthur, J. F. J . Vac. Sci. from 50 to 65% As terminated. It is interesting to note that Technol. 1990, B8, 1109. virtually identical Ga/As ratios for both Ar+ etched and annealed (22) Davis, R. J.; Wolf, E. D. J . Vac. Sci. Technol. 1990, B8, 1798. and chlorinated GaAs (1 10) surfaces have been measured by (23) Troost, D.; Koenders, L.; Fan, L. Y.; Monch, W. J . Vac. Sci. Technol. DeLouise. I 1987, BS, 11 19. The fluorine atom source has been described in detail else(24) Ha, J. H.; Ogryzlo, E. A.; Polyhronopoulos, S . J . Chem. Phys. 1988, 89, 2844. where.3s Briefly, it consists of a miniature fast-flow tube whose (25) Faruhata, N.; Miyamoto, H.; Okamoto, A.; Ohata, K. J . Electron. output is sampled by a small aperture (40 pm), which produces Mater. 1990, 19, 201.

-----A

+


- -

(26) Li, J. Z.; Adesida, 1.; Wolf, E. D. Appl. Phys. Lett. 1984, 45, 897. (27) Ibbotson, D. E.; Flamm, D. L.; Donnelly, V. M. J . Appl. Phys. 1973, 54, 5974. (28) Sugata, S.; Asakawa, K. J. Vac. Sci. Technol. 1987, B5, 894. (29) Ashby, C. I. H. Appl. Phys. Lett. 1984, 45, 892. (30) Ha, J. H.; Ogryzlo, E. A. Plasma Chem. Plasma Proc. 1991.11, 317. (31) Donnelley, V. M.; Flamm, D. L.; Tu, C. W.; Ibbotson, D. E. J . Electrochem. SOC.1982, 129, 2533.

(32) Eastman, D. E.; Chiang, T. C.; Heimann, P.; Himpsel, F. J. Phys. Rev. Lett. 1980, 45, 656. (33) Handbook of Chemistry and Physics, 53rd ed.; CRC Press: Cleveland, 1972; edited by R. C. Weast. (34) Cho, A. Y. J. Appl. Phys. 1976, 47, 2841. (35) Stinespring, C. D.; Freedman, A. J . Vue. Sci. Technol. 1986, A4, 1946.


Halogenation of GaAs (100) and (1 11) Surfaces an atomic or molecular beam. A 5% fluorine in argon gas mixture (2 Torr) flows ( 5 0 0 sccm) through an alumina tube which is surrounded by an Evenson-type microwave cavity and past the aperture and exhausts through a mannular passage. Gas pressure is measured downstream of the aperture using a capacitance manometer. Operating the discharge at 70-W power produces nearly 100% dissociation of the fluorine. The alumina flow tube is readily passivated, and no recombination of F atoms in the gas or on the walls is seen when the beam is sampled by a mass spectrometer. In order to produce atomic chlorine, the source is modified by replacing the alumina tube with a halocarbon wax coated quartz tube. The coating reduces wall-induced recombination. Chlorine atom recombination is further reduced by replacing the flat aperture plate with a skimmer made of nickel (Beam Dynamics) with a 40-pm aperture (see Figure 1). The skimmer wall thickness is only -30 pm at the aperture, “ i z i n g boundary layer effects. Typical dissociation fractions of 50-70% are reached when using 5% Clz in Ar gas mixtures.36 In a typical experiment, the sample is heated to a specified temperature and exposed to the halogen atom source for a given amount of time. The chamber pressure is -lod Torr during exposure (primarily argon). After sample exposure, the beam source is then pumped out and the chamber evacuated to lo“ Torr. The sample is then transferred to the analysis chamber where LEED and XPS spectra are taken. The description of the reaction layers as “ordered” or “disordered” refers to the presence or absence of a LEED pattern. The sample is returned to the reaction chamber for further exposure. Dose levels (total chlorine flux whether atomic or molecular) are calculated assuming effusive and 7 X mL/s for flow. Typical flux levels are 2 X chlorine and fluorine, respectively. Possible errors in this calculation including uncertainties in pressure, aperture size, and aperturesubstrate distance are estimated at approximately 25%.

Results pnd Discussion The chemical composition of the surface layers as a function of depth was measured using X-ray photoelectron spectroscopy (XPS). The sensitivity of XPS to depth is related to the escape depths of the emitted electrons, which in turn is a function of their characteristic kinetic energies. By observing transitions at widely varying binding energies (binding energy = hv - kinetic energy), one can interrogate surface depths which vary by a factor of 3-4. The Ga and As 3d and 2p3,, transitions present just such a case. The 3d transitions (measured using the Mg anode X-ray source) involve small binding energies and thus sample comparatively thick layers on the order of 50-100 A (3 times the estimated electron mean free ~ a t h ) . ~ ’ -The ~ ~ 2p3 transitions, on the other hand, are quite surface sensitive. Proding the Ga 2p3/ztransition using the Mg anode (hv = 1253.6 eV) and the As 2p3,, transition using the AI anode (hv = 1486.6 eV) produces electrons for both elements with almost equal kinetic energies and thus approximately equal mean free paths (-6-8 A). Thus, these transitions can be used to interrogate the first 5-10 layers of the surface (lattice parameter 5.65 A). In addition, the chlorine surface concentration was monitored with the C12p transitions using the A1 anode in order to obviate interference from a gallium Auger peak. C1 Atoms. At temperatures below 323 K,exposure of gallium-rich (Ga/As = 1.4) GaAs (100) and (1 11) surfaces results in the creation of a highly arsenic-deficient disordered GaCl, reaction product layer. This reaction product layer, at 130 K, is estimated to be on the order of 20-30 A thick. As the temperature and concomitantly the volatility of GaCl, increase, the reaction product layer thickness decreases. Virtually identical (36) Stinespring, C. D.; Freedman, A. Atomic Chlorine Source for Semiconductor Etching Studies. NSF Final Report, Aerodyne Research, Inc., Report No. ARI-RR-614, 1987. (37) Seah, M. P. Surf. Interface Anal. 1986, 9, 8. (38) Tanuma, S.;Powell, C. J.; Penn, D. R. J . Vac. Sci. Technol. 1990, A& 2213. (39) Nefcdov, V. I. X-ray Photoelectron Spectroscopy of Solid Surfaces; VSP BV: Utrecht, 1988.

The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2255 CL on G a A s ( 1 1 1 )

T

K

= 130

BINDING ENERGY (eV) Figure 2. XPS spectra of the Ga 2pjI2 transition for a GaAs (1 11) surface at 130 K as a function of exposure to chlorine atoms.

K

CL on G a A s ( 1 1 1 ) T = 130 ,-

19

’

1327

’

1325

’

1323

’

1321

’

I

1319

BINDING ENERGY (eV) Figure 3. XPS spectra of the As 2p3,2 transition for a GaAs (1 11) surface at 130 K as a function of exposure to chlorine atoms.

results are observed on arsenic-rich surfaces (Ga/As = 0.85) except that, at the lowest temperature studied (140 K), equal amounts of AsCl, and GaCl, are observed. At temperatures of 323 K and above, no stable reaction product layer is observed and the surface becomes ordered and stoichiometric in composition. A residual chlorine surface coverage is observed for temperatures up to 573 K. Figures 2 and 3 present XPS spectra of the surface-sensitive Ga and As 2p3/*peaks of a Ga-rich GaAs( 111) substrate as a function of exposure to the chlorine atom source at a substrate temperature of 130 K. Note that the Ga peak, even at the lowest exposures, shows a large shift to higher binding energy as would be expected from a GaC!, species. As the exposure is increased to only 20 ML (1 ML = 7.2 (6.3) X lof4atoms for the 111 (100) surface), the bulk Ga signal is almost entirely attenuated, leaving behind a chlorinated peak at 1.1 eV higher binding energy. The arsenic spectra are dominated by a continuing decrease in intensity levels although a faint AsCl, peak can be observed at 3 eV higher binding energy after 20-ML exposure. This decrease in signal (a factor of 10) is reflected in the spectra (which have been renormalized to equal peak heights for clarity) by the decreasing signal-to-noise levels. This depletion of the arsenic and creation of a GaCl, reaction product layer are also mirrored in the 3d XPS spectra of a (100) surface. The Ga 3d spectrum, shown in Figure 4, contains two peaks; one at 18.8 eV is attributed to bulk Ga and one at 20.0 eV to the GaCl, species. Comparing the integrated signal of the Ga bulk peak with that of the As 3d bulk peak at 40.7 eV indicates that the chemical composition of the bulk layer near the reaction

2256 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 C1 o n GaAs(100)

Freedman and Stinespring -

T = 130 K

C 1 on G a A s ( l l 1 )

Ga/As

=

0.85

Em z

W E-

E

I

23

21

17

19

15

'328

'326

Figure 4. XPS spectra of the G a 3d transition for a GaAs (1 11) surface at 130 K as a function of exposure to chlorine atoms.

'322

'320

Figure 7. XPS spectra of the Ga 2p3,* transition for 20-ML exposure to C1 atoms as a function of surface temperature for a slightly As-rich GaAs( 11 1) substrate.

- ~ -C1 o-n

C1 o n GaAs(100)

1324

B I N D I N G E N E R G Y (eV)

BINDING ENERGY (eV) Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 26, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/j100184a042

I

,

1330

GaAs(ll1)

,

A

I

/$$..-

I

20 Mi

. " " : : ,

44

1121

1119

1117

1115

1113

BINDING ENERGY (eV) Figure 5. XPS spectra of the G a 2p312transition as a function of the GaAs (100) surface temperature for 20-ML exposure to C1 atoms. C1 on GaAs(100)

1

fi 20 ML

223 K

22

20

18

16

14

BINDING ENERGY (eV) Figure 6. XPS spectra of the G a 3d transition as a function of the GaAs (100) surface temperature for 20-ML exposure to C1 atoms.

layer is only slightly Ga-rich. There is no evidence for any marked segregation of species below the reaction product layer. The effect of temperature on this reaction layer for a (100) surface is shown in Figures 5 and 6,which present Ga 2p3/, and 3d spectra for 20-ML exposure to chlorine atoms as a function of substrate temperature. The thickness of the reaction layer is noticeably decreasing, probably due to the increasing volatility

42

40

38

36

BINDING ENERGY ( e V ) Figure 8. XPS spectra of the G a 3d transition for 20-ML exposure to CI atoms as a function of surface temperature for a slightly As-rich GaAs(ll1) substrate.

of GaC13. The reaction layer is totally absent for temperatures of 323 K and above, which is consistent with the observed onset of steady-state chemical etching at this temperature.4*I2This onset is accompanied by the observation of an ordered (as observed by streaky LEED patterns at 373 K) approximately stoichiometric to As-rich surface. The stoichiometric surface is maintained even at the annealing temperature of 850 K. The gallium-rich surface can only be reestablished by Ar+ etching and reannealing. This behavior at etching temperatures is also observed by DeLouise'2J3 on the GaAs (1 10) surface. In this case, Ar+ bombardment leads to the creation of a Ga-rich surface. Upon exposure to 1000-ML doses of Cl,, the surface becomes stoichiometric. Ameen and MayerZ0 have also shown on the (100) surface that adsorbed chlorine leads to As enrichment. In order to establish whether the qualitative picture presented here is unduly affected by the initially Ga-rich nature of the substrate, a small number of experiments were carried out on slightly As-rich (Ga/As = 0.85) surfaces prepared by exposing GaAs (1 11) surfaces to C12 a t 473 K. At temperatures of 173 K and above, the results are virtually identical to the Ga-rich surfaces. But at 140 K, the AsCl, species becomes much more stable, dominating the XPS spectra. Figures 7 and 8 present As 2p3/2 and 3d spectra for 20-ML exposure to C1 atoms at various substrate temperatures. Note that both spectra show unambiguous shifts to higher binding energies (-2.6 eV in both cases) at 140 K. We believe that this is the first unambiguous observation of an arsenic chloride surface species in an etching system. In addition, the resulting reaction layer remains approximately stoichiometric, indicating that under these conditions virtually no preferential etching has occurred.

The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2251

Halogenation of GaAs (100) and (1 11) Surfaces F on GaAs1100) T

= 305

__ F on GaAs(100)

K

T

=

305 K I

I

A

1122

1120

1118

1116

1'14


BINDING E N E R G Y ( e V ) Figure 9. XPS spectra of the Ga 2 ~ , ,transition ~ for a GaAs (100) surface at 305 K as a function of exposure to fluorine atoms.

The existence, composition, and temperature stability of the reaction product layers produced by chlorine on GaAs cannot necessarily be predicted by extrapolation of equilibrium vapor pressuresm of possible etch products. At the lowest substrate temperature experiment, 140 K, GaCl, is quite nonvolatile, but ASCI, has a nominal vapor pressure of lo-' Torr. Given that postetch analysis takes place 5-10 min after exposure, one would not expect to observe a stable AsCl, species, much less one whose existence is dependent on the initial sample preparation (gallium-rich or stoichiometric substrates). Furthermore, the continued existence of an arsenic-deficient gallium chloride layer at higher temperatures (up to 273 K) is at odds with extrapolated GaC1, vapor pressure data. At 223 K, the vapor pressure of GaCl, is -2 X lo4 Torr, and at 273 K, it is - 5 X lo-* Torr. This apparent stability of the reaction product layer is far greater than one might expect. Thus, simple inferences from thermodynamic data are not necessarily valid for modeling these etching systems. In all of the above experiments, the C1 atom concentration was monitored using the C12p transitions. The C 1 2 ~binding ~ / ~ energy was measured to be 198.0 f 0.2 eV at all temperatures. This value is consistent with an ionically bound species (in NaC1, the 2p3 binding energy is 198.5 eV)41and has been observed elsewhere!, in comparison, the binding energy of a chlorine atom adsorbed on the basal plane of (HOPG)graphite at 200 K is approximately 200 eV.42 The intensities of the Cl peaks below 323 K increased proportionately with those of the peaks assigned to the GaCl, (and in one case both a GaCl, and AsCl,) moiety. Unfortunately, it is difficult to ascertain the value of x given the large uncertainties of electron escape depths in the reaction layer. At temperatures between 323 and 573 K, where steady-state etching occurs and no stable reaction layer is observed, a residual submonolayer C1 coverage was measured. This adlayer decreased in intensity as the substrate temperature increased. No discernible changes in positions or line widths of the Ga or As peaks were observed within experimental uncertainties. DeLouise,13 studying the (1 10) surface using molecular chlorine, has also observed a -0.5-ML coverage of C1 at 300 K. In these studies, though, small changes in XPS peak positions and line widths indicated that the chlorine was bound to As atoms. In support of this view, Margaritondo et al.,43using SEXAFS, have argued the adsorbed C1 atoms are bound in atop sites to As atoms. On a contradictory note, Mokler et aL2' studying the (100) surface, claim that chlorine adsorbs preferentially t o Ga atoms at defect sites. (40) Bachrach, R. Z.; Bauer, R. S.; Chiaradia, P.; Hansson, G. V. J . Vac. Sei. Technol. 1981, 18, 191. (41) Siegbahn, K.; Gelius, U.; Siegbahn, H.; Olson, E. Phys. Scr. 1970, I

*_I*

1 , LIL.

(42) Freedman, A.; Stinespring, C. D. Proceedings of the 2nd International Symposium on Diamond Materials; Purdes, A. J., et al., Eds.; Electrochemical Society: Pennington, NJ, 1991; Proc. Vol. 91-8, pp 494-501. (43) Margaritondo, G.; Rowe, J.; Bertoni, C. M.; Calandra, C.; Marghi, F. Phys. Rev. 1979, B20, 1538.

24

22

20

18

16

14

BINDING ENERGY ( e V ) Figure 10. XFS spectra of the Cia 3d transition for a GaAs (100) surface at 305 K as a function of exposure to fluorine atoms.

F o n GaAs(100) D O S E = 300 ML

1'22

1120

1118

1116

1114

BINDING ENERGY ( e V ) Figure 11. F 1s XPS spectra for a GaAs (100) surface after exposure to 300 ML of atomic fluorine at various temperatures.

A limited numer of experiments with molecular chlorine performed in this laboratory show that atomic and molecular chlorine are qualitatively similar in their behavior toward GaAs under low-flux conditions. The same GaCl, moiety and temperature dependences are observed, although molecular chlorine requires a higher dose to reach the same chlorination levels. This presumed propensity of molecular chlorine to dissociate upon adsorption, with the resultant atoms penetrating into the bulk, contrasts with the behavior of molecular fluorine on silicon,* where molecular adsorption saturates at approximately a monolayer at low fluxes. F Atoms. In a much less extensive set of studies, GaAs (100) surfaces were exposed to fluorine atom fluxes at a substrate temperature of 305 K. The result is the creation of a disordered GaF, reaction layer, presumably by preferential etching through generation of volatile arsenic fluoride species. This gallium fluoride layer becomes unstable above 573 K. The XPS spectra are consistent with both those obtained by plasma etching with CF4 and by high-pressure exposure to molecular fluorine. Figures 9 and 10 present Ga and 3d spectra, respectively, as a function of GaAs (100) exposure to atomic fluorine at 305 K. Both show the creation of a fairly thick (-30-40 A) GaF, layer as evidenced by the onset of peaks at higher binding energies. This assignment of the 3d peak at 2.4-eV higher binding energy is consistent with spectra of GaF3 single crystals soldered to GaAs substratesu and of GaF, thin films which have been sublimed onto G ~ A S The . ~ ~As 3d and 2p spectra are attenuated by factors of 2 and 5, respectively, compared to a freshly etched and annealed sample, indicating the extensive loss of arsenic. (44) Bernstein, R. W.; Grepstad, J. K. J . Appl. Phys. 1990, 68, 4811. (45) Barritre, A. S.; Couturier, G.; Gevers, G.; Gutgan, H.; Stgueland, T.; Thabati, H. Thin Solid Films 1989, 173, 243.

2258 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 F o n GaAs(100) D o s e = 300 ML 673 K

1

573

l

,

690


, 688

Figure 12. Ga 2p, exposure to 300

/

K

,

I

\

, 686

,

, 684

,

, 682

,

l 680

BINDING ENERGY (eV) is XPS spectra for a GaAs (100) surface after

Ml! of atomic fluorine at various temperatures.

Figure 11 presents F atom 1s XPS spectra as a function of substrate temperature after the GaAs has been exposed to 300 ML of atomic fluorine at 305 K. There is a slow loss of fluorine up to 573 K whereupon the fluoride layer becomes volatile. The Ga 2pjI2 spectra presented in Figure 12 show similar behavior. These spectra are quite similar to those found in studies by Bernstein and Grepstad, where GaAs( 100) is plasma-etched in CFeu The GaF3 layer created in the present study is somewhat thicker and its apparent sublimation temperature is somewhat higher. (The plasma-etched films volatilize at 473 K.) The trend of thicker films to be more thermally stable is borne out by the studies of Barriere et a1.46,47 In these studies, GaAs(100) structures were fluorinated using molecular fluorine at high pressure (several atmospheres) and high temperature (up to 725 K). Films up to 1 Fm could be formed, although they were somewhat inhomogeneous. Barriere et al. also noted that the arsenic at the fluoridebulk interface appeared to be bound to fluorine; similar results are reported for another 111-V compound, 1np.48

Conclusions We conclude that the interactions of halogen atoms with GaAs and other 111-V materials under most processing conditions are best thought of as proceeding through the creation of a 3-dimensional reaction product layer whose steady-statethickness and chemical composition are functions of halogen flux and substrate temperature. Even at etching temperatures, it is reasonable to surmise that a reaction product layer exists. The presence or absence of such a layer after processing is only dependent on its volatility at the measurement temperature. We agree with F%unm2 that the stability of these thin layers (5-40A) is not necessarily predictable from published vapor pressure data for bulk materials. Both atomic fluorine8 and XeF2,4es2 when etching silicon (1 11) (46) Barribre, A. S.;Desbat, B.; Guigan, H.; Lozano, L.; SCguelong, T.; Tressaud, A.; Alnot, P. Thin Solid Films 1989, 170, 259. (47) Barrisre, A. S.;Couturier, G.; GuCgan, H.; SCguelond, T.; Thabti, A.; Alnot, P.;Chazelas, J. Appl. Surf.Sci. 1989. 41/42, 383. (48) Barribre. A. S.; Couturier, G.;Gevers, G.; GuCgan, H.; Toumay, V.; Bertault, D.; Desbat, B.; Tressaud, A.; Alnot, P. Surf. Sci. 1990, 239, 135.

Freedman and Stinespring surfaces, produce fluorinated reaction product layers at room temperature, even though the etch products (SiF4, SizF6,SisF8, and SiF2)52-s5are quite volatile. In fact, the only group 111-V or IV electronic material which does not appear to undergo this type of etching mechanism with fluorine and chlorine is diamond? It does not seem reasonable that any model, heuristic or not, which depends on the concept of simple halogen adlayer addition or simple surface reconstruction can properly describe the behavior of these systems. It would appear that, in terms of reaction mechanism, the interactions of GaAs surfaces with atomic chlorine and fluorine are quite similar. Both systems exhibit preferential etching mechanisms and evidence for subsurface penetration, albeit in different temperature regimes. The formation of a stable GaF3 reaction layer at comparatively high temperatures appears to be solely due to the low volatility of that species. There is no evidence in our work that GaAs lattice penetration is affected by the size of the etching atom. The difference in etching rates of silicon with atomic chlorine and fluorine has been explained on the basis of an activation energy barrier to lattice penetration caused by the repulsion between the lattice and chlorine electron c l o ~ d s . ~ J ~ Given the similarities in lattice structure and bond distances in GaAs and Si, and the weak polarity of GaAs, is not obvious what is responsible for the efficacy of chlorine lattice penetration of gallium arsenide vis-&vis silicon. In the specific case of chlorine on GaAs, the exact thickness of this layer under steady-state etching conditions is difficult to ascertain. Using separate beams of Clz and rare gas ions, both Balooch et a1.I6 and Ameen and Mayer” have estimated that the reaction layer can reach a depth of up to 40 A. A m e n and Mayer, though, indicate that in the case of pure chemical etching only the top one or two layers are significantly affected at pressures up to 10” Torr. Under reactive ion etching conditions (50mTorr pressure and 100-V bias), Pearton et ala6have found chlorine penetration up to 10 A and an arsenic depletion up to a depth of 50 A; similar results are obtained for GaSb. Unfortunately, any final conclusions await a direct measurement of surface composition during the etching process, a feat which has yet to be accomplished. Acknowledgment. We thank Francis Celii and Glen Westphal at Texas Instruments for providing a GaAs(ll1) wafer. We also express our appreciation to Joda Wormhoudt for his helpful comments. This work was supported by funds from the National Science Foundation and the Strategic Defense Initiative Organization/Office of Innovative Science and Technology (managed by the Office of Naval Research) under the Small Business Innovation Research Program. Registry No. GaAs, 1303-00-0; CI,22537-15-1; F, 14762-94-8.

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Halogenation of gallium arsenide(100) and (111) surfaces using

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