- Email: [email protected]
X-ray photoelectron spectroscopy of etched ZnSe
371
Applied Surface Science 35 (1988-89) North-Holland. Amsterdam
371-379
X-RAY PHOTOELECTRON
SPECTROSCOPY
T.F. MCGEE III and H.J. CORNELISSEN Philips Laboratories, NY 19510, USA Received
OF ETCHED
ZnSe
*
North American Philips Corporation, 345 Scarborough Road, Briarcliff, Manor,
29 June 1988; accepted
for publication
1 September
1988
To produce longitudinal ZnSe laser structures the GaAs substrate must be removed from the MBE grown layers. When an aqueous solution of sulfuric acid and hydrogen peroxide was used for this purpose, a red film was left on the surface of the Z&e. X-ray photoelectron spectroscopy was used to examine the wafers before etching, after etching and after etching followed by heating in vacuum or in an argon atmosphere. The as-polished surface contained Se in two different chemical states: ZnSe and Se oxide. The etch predominately removed the Zn leaving a red Se-rich surface with mostly Se-Se bonding. The Zn Auger parameter decreased toward that of the oxide. The etched surface could be restored to ZnSe by heating in vacuum but not in an argon atmosphere. The one heated in the argon showed the Se Auger parameter approaching that of ZnSe while the Zn Auger parameter increased past that of ZnSe toward metallic Zn.
1. Introduction
ZnSe is a II-VI compound semiconductor which produces short wavelength visible light upon carrier recombination. Because of this property there has been much interest in ZnSe for production of electron beam pumped blue lasers. Transverse lasers have the drawback of a large divergence angle and not being able to be scanned in two directions. To produce longitudinal lasers it is necessary to produce high quality ZnSe material on transparent substrates. To meet these two requirements the substrate must be lattice matched, single crystalline and twin-free over a large area, as well as have a bandgap larger than that of ZnSe. ZnSe or ZnS,Se, _-xcrystals of high structural quality over large areas are difficult to obtain; however, GaAs can be used as a substrate to grow high quality ZnSe by molecular beam epitaxy. Although the GaAs lattice constant closely matches that of ZnSe it is not transparent to blue light. One possibility to solve this problem is to etch away the GaAs substrate after growth of the ZnSe film, thereby removing the absorbing layer. This can be done using a solution of 3 H,SO, : 2 H,O, : 40 H,O [l]. When this was done the surface of the remaining ZnSe layer had a reddish appearance which could be removed by heating. * Present address: Netherlands.
Philips
Research
Laboratories,
P.O. Box 80000,
0169-4332/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
5600 JA Eindhoven,
The
372
T. F. McGee III, H. J. Cornelissen / XPS of etched ZnSe
Spectra obtained using X-ray photoelectron spectroscopy (XPS) contain information relating photoelectrons and Auger electrons. It has been shown that, for certain elements, the Auger electrons are more sensitive to the chemical environment than the photoelectrons. Zn is such an element, exhibiting a large shift in the position of the Auger electrons between the chemical state of Zn metal and ZnO [2,3] while a relatively small shift in the photoelectron spectra is observed. In addition, a change in the shape of the distribution of electrons from the Auger transitions occurs between Zn and ZnO. Charge build-up on a sample, due to the loss of electrons, can affect the absolute position of the photoelectron and Auger electron peaks. The Auger parameter (a) is the difference in energy between the photoelectrons and the Auger electrons from the same element on the same sample. Measuring the Auger parameter has the advantage that sample charging effects can be neglected and the chemical state can be identified if the differences between compounds are large enough, as it is for Zn. It was the purpose of this study to use X-ray photoelectron spectroscopy to study the red film produced by etching away the GaAs layer as well as the effects on the ZnSe substrate of the heat treatments used to remove this red film.
2. Experimental The XPS analysis was performed using a VG ESCALAB MK II equipped with a triple detector at a pressure < 5 X lo-” Torr. Each of the three input slits was 15 X 6 mm and the three output slits were 10 X 6 mm each. Survey scans were taken from O-1400 eV binding energy at 0.25 eV steps and 300 ms/channel using an Al source at 400 W. The pass energy was set at 50 eV for these scans. Higher resolution scans were obtained at 10 eV pass energy, 0.05 eV steps, and 3000 ms/channel for the range from 0 to 1070 eV divided into 10 regions to include all the Zn, Se and 0 peaks in this regime. The high resolution scans resulted in a FWHM of 1.05 eV for the Ag3d,,, peak. Etching of both MBE-grown ZnSe layers deposited on GaAs substrates and polished slices obtained from bulk ZnSe resulted in a reddish film and a Se-rich surface. Therefore, the polished slices obtained from bulk ZnSe were examined. ZnSe slices were cut parallel to the twin planes (111) from a boule obtained by zone melting followed by recrystallization (ZMRC) [4]. The surface of the slices were then chemo-mechanically polished using a bromine-methanol solution followed by cleaning with xylene and then isopropan01 solvent cleaning. The slices were - 1 cm2 containing several large grains. These slices were then mounted on a Cu stub and their surface covered by an Al mask leaving a 0.5 cm diameter hole in the center for analysis. Although several orientations were exposed to the etch, the mounting proce-
T.F. McGee III, H.J. Comelissen
/ XPS of etched ZnSe
373
dure insured the analysis was performed exactly on the same area each time, for a given slice. One slice was analyzed after polishing and solvent cleaning only. This slice was then removed, the surface solvent cleaned (trichloroethylene, acetone and methanol) followed by a deionized H,O rinse then etched in a H,SO, (95%98%) H,O, (30%) and H,O solution at volume ratios of 3 : 2 : 40, respectively, and a second deionized H,O rinse. The sample was blown dry using N,. This sample was remounted so the analysis was performed on the same area. After analysis the sample was heated in the ESCALAB to 400 o C for 15 min and analyzed again. A second slice was then given the same cleaning and etching treatment and then analyzed. At this point the sample was removed from the system and heated in pure flowing argon to 400 o C for 15 min and analyzed again. Several slices were analyzed using each procedure described above. Standards of Se, SeO,, Zn, ZnO and ZnSe were also analyzed using the same conditions. The SeO, and ZnO were powders. Se pellets were crushed to obtain a powder. The ZnSe was obtained by crushing in air part of an ingot of a boule of ZnSe into a powder. The powders of freshly crushed Se and ZnSe were used to minimize oxygen contamination. No oxygen was detected on the Se or on the ZnSe powders. Pellets of polycrystalline Zn metal were flattened and in-situ argon ion sputtered clean.
3. Results and discussions 3.1. Polished sample Table 1 is a summary of the peak positions and the Auger parameters for Zn and Se obtained from the samples and standards. The spectra, including the Se3d and Zn3p, for the sample which was polished and solvent cleaned only are shown in fig. 1. The two peaks located between 50 and 70 eV correspond to the Se3d from different chemical states. The lower binding energy peak was found to be at 58.55 eV. The C peak measured on the same sample was found to be at 288.8 eV. After adjusting the C peak, to account for charging, to the value used by Langer and Vesely [3] of 283.8 eV, a value of 53.55 eV is obtained for the Se3d. This is slightly higher than their value of 53.3 eV. However, using the value of Shenasa et al. [5] of at 284.6 eV a value of 54.35 eV is obtained which is slightly lower than their value of 54.7 eV for Se in the same chemical state. The value we obtain for Se3d is in between these two values. The second Se3d peak is located at 4.6 eV higher binding energy. This is between the values used by Weser et al. [6] (4.3 eV) and Shenasa et al. [5] (5.2 eV) for Se in SeO,.
/ XPS of eiched ZnSe
T. F. McGee III, H.J. Come&en
314
Table 1 Zn and Se peak positions and Auger parameter (x Sample
Zn
Se 3d
2P,,, (ev)
Pen
(ev)
Wf&f.x, (eV)
58.55 63.15 61.35 59.2 61.05 62.2 66.95 59.25
184.15 189.8 185.95 184.15 186.85 186.3 193.55 185.1
125.6 126.65 124.6 124.95 125.8 124.1 126.6 125.85
_
_
Polished
1029.6
504.4
525.2
Etched Ar heated Vat. heated Se SeO, ZnSe Zn ZnO
1029.15 1027.05 1029.05
505.5 501.35 503.8 _
523.65 525.7 525.25 _
1027.1 1021.5 1021.9
501.8 494.2 498.4
525.3 527.3 523.5
Fig. 2a shows the region containing the SeL,M,,,M,,, Auger transitions for this sample while figs. 3a, 3e and 3f show the Auger transitions for Se, ZnSe and SeO, powders, respectively. The energy scales for Figs. 2-4 are 20 eV wide; however, the absolute positions of the peaks have been shifted to align the peak maxima for shape comparisons. Notice the shape of the Auger peak is different between the standards. In table 1, two Se Auger parameters are shown for the polished sample. The value of 125.6 eV was obtained by measuring between the lower binding energy Se XPS peak, shown in fig. 1 and the larger Auger transition shown in fig. 2 (marked Al). This value is reasonably close to that of the ZnSe powder (125.85 eV). Measuring between the higher binding energy Se XPS peak shown 3.10. 2.79'. 2.48.. 2.17 " 1.86 ,. 1.55" 1.24 '. 0.93" 0.62"
BINDING
ENERGY
(eV)
Fig. 1. Se 3d and Zn 3p region for the polished sample.
T. F. McGee III, H.J. Cornelissen / XPS of etched ZnSe
ENERGY
375
(eV)
Fig. 2. SeL3M,,,M4,, Auger transition for (a) polished sample and (b) spectra obtained by the addition of the ZnSe and SeOl standards (spectra have been shifted to align peak maxima for shape comparison).
in fig. 1 and the smaller higher binding energy Auger peak marked A2 results in a value of 126.65 eV which is excellent agreement to that obtained on the SeO, (126.6 eV). The shape of the Se Auger transition from the polished sample (fig. 2a) can be approximated by adding the Se Auger transitions from ZnSe and SeO, powders as shown in fig. 2b. The Auger parameter for the ZnSe powder was added to the lower binding energy Se 3d peak while the SeO, Auger parameter was added to the higher binding energy peak to obtain their relative position. Next the SeO, Auger peak intensity was made to be about 15% of the ZnSe Auger peak and they were then added. Fig. 2b appears to be in good agreement (although not a perfect fit) with that of the polished sample (fig. 2a). This same method was tried for combining the SeO, and the Se Auger peaks and also for Se and ZnSe. Neither of these combinations gave reasonable agreement. The Auger parameters measured between the ZnL,M,,,M,,, and the Zn2p,,, peaks, from table 1, are 525.2 and 525.3 eV for the polished sample and the ZnSe powder, respectively. The Zn LjM4,5M4,5 peak is displayed in fig. 4c for this sample. Figs. 4b, 4d and 4e are the same regions for ZnSe, ZnO and Zn, respectively. Although the Auger parameter matches that of ZnSe, the shoulder peak in the polished sample (fig 4c) is not as well resolved as it is for the ZnSe standard (fig. 4b). 0 was also found to be present on the polished sample and not on the ZnSe powder. This combined with the presence of the two states of Se and the slight difference in the shape of the Zn Auger transitions leads to the conclusion that Zn and Se are bonded to 0 at the
316
T.F. McGee III, H.J. Cornelissen
ENERGY
/ XPS of etched .&Se
(eV)
Fig. 3. SeL,M4,,M,,, Auger transition for (a) Se metal standard, (b) etched ZnSe, (c) heated argon, (d) heated in vacuum, (e) ZnSe standard, (f) SeO, standard (spectra have been shifted align peak maxima for shape comparison).
in to
surface as well as to each other. It is not clear whether they are present as individual oxides or as a complex oxide as suggested by Ludeke [7]. 3.2. Etched sample After the sample was solvent cleaned and chemically etched, the ratio of the Zn 3d/Se 3d peak areas was found to decrease by an order of magnitude. Also,
T. F. McGee III, H.J. Comelissen
ENERGY
/ XPS of etched ZnSe
371
kV)
Fig. 4. ZnL,M4,,M4,, Auger transitions for (a) heated in vacuum, (b) ZnSe standard, (c) polished sample, (d) ZnO standard, (e) Zn metal standard (spectra have been shifted to align peak maxima for shape comparison).
the shape of the Se Auger peak for the etched slice (fig. 3b) is similar to that of metallic Se (fig. 3a) and the Se Auger parameter was found to decrease to 124.6 eV, toward the value found for pure Se (124.1 eV). This is probably due to the increase in the number of Se-Se bonds. The Zn Auger peak intensities were very weak on this sample; therefore, we could not make any comparisons of the shape. However, we could obtain the peak position by differentiation of the spectra and comparing to the differentiated spectra from the polished sample. When this was done the Zn Auger parameter was found to decrease to 523.65 eV. Although the Zn Auger parameter after etching was close to that found for the ZnO, it is probably not the oxide. ZnO has a low solubility in water [S] and would not dissolve into
378
T.F. McGee III, H.J. Cornelissen / XPS of etched ZnSe
solution leaving the surface Se rich. Since the etch appears to predominately etch the Zn from the surface it is possible the etching solution forms a zinc sulfate, zinc hydroxide or some intermediate zinc hydroxide with sulphur which dissolves into solution and the Se is oxidized to the metallic Se from the - 2 state in ZnSe. 3.3. Heated samples When the sample was heated in vacuum to 400°C both the Auger parameter for the Se (125.8 eV) and Zn (525.25 eV) changed to the value for the bulk ZnSe powder. The shape of the Se Auger peak (fig. 3d) is once again similar to that of the Se peak in ZnSe powder (fig. 3e) and the Zn Auger peak (fig. 4a) even more closely resembles that of the powder (fig. 4b) than the Zn Auger peak for the polished sample (fig. 4~). This indicates that the Se-rich surface transformed into ZnSe upon heating in vacuum due to Se evaporation from the surface [9]. The sample heated in the argon atmosphere to 400 a C for 15 min did not show the same results as above. The area ratio of the Zn3d/Se3d peaks did increase but the surface still had a factor of 2 more Se than ZnSe and the Se Auger parameter showed only a slight increase to 124.95 eV. The shape of the Se Auger peak (fig. 3c) started to resemble that of the ZnSe standard (fig. 3e), however the shoulder was not as sharply defined. It appears that in an argon atmosphere, using the same temperature and time as in vacuum, all of the excess Se does not evaporate from the surface. The Zn Auger peak was still too noisy, due to decreased signal intensity, to say much about its shape. The Zn Auger parameter increased past the value for ZnSe towards the metal. This is not understood; however, it may be that heating in argon increases the Zn-Zn bonding as well as increasing the Zn-Se bonding. Although some oxygen was detected on this sample, no changes in the chemical state of Se or Zn could be correlated with its presence. It is possible the oxygen is adsorbed on the surface from the atmosphere during handling.
4. Conclusions XPS was used to study the effects of an aqueous solution containing sulfuric acid and hydrogen peroxide on ZnSe wafers. The wafers were studied prior to etching, after etching and after etching followed by heating in either a vacuum or an argon atmosphere. XPS performed on the ZnSe wafers that were polished and solvent cleaned showed Se in two chemical states. One of the chemical states was attributed to Se in ZnSe while the other was thought to be an oxide of Se. The Zn Auger parameter matched that of the ZnSe, however
T. F. McGee III, H.J. Cornelissen / XPS of etched ZnSe
379
the shape of the Auger peak indicated the presence of some oxide. It was not clear whether the oxide was individual Zn and Se oxides or a mixed oxide. After the wafers were etched the surface appeared red. The etch was found to predominately remove the Zn leaving the surface Se-rich. The Se Auger parameter indicated an increase in the number of Se-Se bonds. It was shown that by heating in vacuum to 400 o C for 15 min the Se-rich surface could be transformed back to ZnSe, as determined by the shape of both the Zn and Se Auger transitions and their measured Auger parameters. Heating in an argon atmosphere at the same temperature and time did not have the same effect. The surface remained Se-rich and the Se Auger parameter increased slightly toward ZnSe while the Zn Auger parameter increased past that of ZnSe toward that of metallic Zn. Although this is not well understood it may be that heating in an argon atmosphere increases the Zn-Zn bonds as well as the ZnSe bonds.
Acknowledgements The authors would like to thank J. van de Ven for his enthusiastic etching efforts, M. Athanas and K. Salotti for the sample preparation and B. Fitzpatrick for supplying the samples. We would also like to thank G. Loiacono for the helpful discussions.
References [l] [2] [3] [4] [5] [6] (71 [8] [9]
J. van de Ven, private communication. G. Schon, J. Electron Spectrosc. Related Phenomena 2 (1973) 75. D.W. Langer and C.J. Vesely, Phys. Rev. B 2 (1970) 4885. B.J. Fitzpatrick, T.F. McGee III and P.M. Hamack, J. Crystal Growth 78 (1986) 242. M. Shenasa, S. Sainkar and D. Lichtman, J. Electron Spectrosc. Related Phenomena 40 (1986) 329. U. Weser, G. Sokolowski and W. Pilz, J. Electron Spectrosc. Related Phenomena 10 (1977) 429. R. Ludeke, Solid State Commun. 24 (1977) 725. R.C. Weast, Ed., Handbook of Chemistry and Physics (The Chemical Rubber Company, Boca Raton, FL, 1970) B-154. H.J. Cornelissen, D.A. Cammack and R.J. Dalby, J. Vacuum Sci. Technol. B 6 (1988) 769.
Applied Surface Science 35 (1988-89) North-Holland. Amsterdam
371-379
X-RAY PHOTOELECTRON
SPECTROSCOPY
T.F. MCGEE III and H.J. CORNELISSEN Philips Laboratories, NY 19510, USA Received
OF ETCHED
ZnSe
*
North American Philips Corporation, 345 Scarborough Road, Briarcliff, Manor,
29 June 1988; accepted
for publication
1 September
1988
To produce longitudinal ZnSe laser structures the GaAs substrate must be removed from the MBE grown layers. When an aqueous solution of sulfuric acid and hydrogen peroxide was used for this purpose, a red film was left on the surface of the Z&e. X-ray photoelectron spectroscopy was used to examine the wafers before etching, after etching and after etching followed by heating in vacuum or in an argon atmosphere. The as-polished surface contained Se in two different chemical states: ZnSe and Se oxide. The etch predominately removed the Zn leaving a red Se-rich surface with mostly Se-Se bonding. The Zn Auger parameter decreased toward that of the oxide. The etched surface could be restored to ZnSe by heating in vacuum but not in an argon atmosphere. The one heated in the argon showed the Se Auger parameter approaching that of ZnSe while the Zn Auger parameter increased past that of ZnSe toward metallic Zn.
1. Introduction
ZnSe is a II-VI compound semiconductor which produces short wavelength visible light upon carrier recombination. Because of this property there has been much interest in ZnSe for production of electron beam pumped blue lasers. Transverse lasers have the drawback of a large divergence angle and not being able to be scanned in two directions. To produce longitudinal lasers it is necessary to produce high quality ZnSe material on transparent substrates. To meet these two requirements the substrate must be lattice matched, single crystalline and twin-free over a large area, as well as have a bandgap larger than that of ZnSe. ZnSe or ZnS,Se, _-xcrystals of high structural quality over large areas are difficult to obtain; however, GaAs can be used as a substrate to grow high quality ZnSe by molecular beam epitaxy. Although the GaAs lattice constant closely matches that of ZnSe it is not transparent to blue light. One possibility to solve this problem is to etch away the GaAs substrate after growth of the ZnSe film, thereby removing the absorbing layer. This can be done using a solution of 3 H,SO, : 2 H,O, : 40 H,O [l]. When this was done the surface of the remaining ZnSe layer had a reddish appearance which could be removed by heating. * Present address: Netherlands.
Philips
Research
Laboratories,
P.O. Box 80000,
0169-4332/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
5600 JA Eindhoven,
The
372
T. F. McGee III, H. J. Cornelissen / XPS of etched ZnSe
Spectra obtained using X-ray photoelectron spectroscopy (XPS) contain information relating photoelectrons and Auger electrons. It has been shown that, for certain elements, the Auger electrons are more sensitive to the chemical environment than the photoelectrons. Zn is such an element, exhibiting a large shift in the position of the Auger electrons between the chemical state of Zn metal and ZnO [2,3] while a relatively small shift in the photoelectron spectra is observed. In addition, a change in the shape of the distribution of electrons from the Auger transitions occurs between Zn and ZnO. Charge build-up on a sample, due to the loss of electrons, can affect the absolute position of the photoelectron and Auger electron peaks. The Auger parameter (a) is the difference in energy between the photoelectrons and the Auger electrons from the same element on the same sample. Measuring the Auger parameter has the advantage that sample charging effects can be neglected and the chemical state can be identified if the differences between compounds are large enough, as it is for Zn. It was the purpose of this study to use X-ray photoelectron spectroscopy to study the red film produced by etching away the GaAs layer as well as the effects on the ZnSe substrate of the heat treatments used to remove this red film.
2. Experimental The XPS analysis was performed using a VG ESCALAB MK II equipped with a triple detector at a pressure < 5 X lo-” Torr. Each of the three input slits was 15 X 6 mm and the three output slits were 10 X 6 mm each. Survey scans were taken from O-1400 eV binding energy at 0.25 eV steps and 300 ms/channel using an Al source at 400 W. The pass energy was set at 50 eV for these scans. Higher resolution scans were obtained at 10 eV pass energy, 0.05 eV steps, and 3000 ms/channel for the range from 0 to 1070 eV divided into 10 regions to include all the Zn, Se and 0 peaks in this regime. The high resolution scans resulted in a FWHM of 1.05 eV for the Ag3d,,, peak. Etching of both MBE-grown ZnSe layers deposited on GaAs substrates and polished slices obtained from bulk ZnSe resulted in a reddish film and a Se-rich surface. Therefore, the polished slices obtained from bulk ZnSe were examined. ZnSe slices were cut parallel to the twin planes (111) from a boule obtained by zone melting followed by recrystallization (ZMRC) [4]. The surface of the slices were then chemo-mechanically polished using a bromine-methanol solution followed by cleaning with xylene and then isopropan01 solvent cleaning. The slices were - 1 cm2 containing several large grains. These slices were then mounted on a Cu stub and their surface covered by an Al mask leaving a 0.5 cm diameter hole in the center for analysis. Although several orientations were exposed to the etch, the mounting proce-
T.F. McGee III, H.J. Comelissen
/ XPS of etched ZnSe
373
dure insured the analysis was performed exactly on the same area each time, for a given slice. One slice was analyzed after polishing and solvent cleaning only. This slice was then removed, the surface solvent cleaned (trichloroethylene, acetone and methanol) followed by a deionized H,O rinse then etched in a H,SO, (95%98%) H,O, (30%) and H,O solution at volume ratios of 3 : 2 : 40, respectively, and a second deionized H,O rinse. The sample was blown dry using N,. This sample was remounted so the analysis was performed on the same area. After analysis the sample was heated in the ESCALAB to 400 o C for 15 min and analyzed again. A second slice was then given the same cleaning and etching treatment and then analyzed. At this point the sample was removed from the system and heated in pure flowing argon to 400 o C for 15 min and analyzed again. Several slices were analyzed using each procedure described above. Standards of Se, SeO,, Zn, ZnO and ZnSe were also analyzed using the same conditions. The SeO, and ZnO were powders. Se pellets were crushed to obtain a powder. The ZnSe was obtained by crushing in air part of an ingot of a boule of ZnSe into a powder. The powders of freshly crushed Se and ZnSe were used to minimize oxygen contamination. No oxygen was detected on the Se or on the ZnSe powders. Pellets of polycrystalline Zn metal were flattened and in-situ argon ion sputtered clean.
3. Results and discussions 3.1. Polished sample Table 1 is a summary of the peak positions and the Auger parameters for Zn and Se obtained from the samples and standards. The spectra, including the Se3d and Zn3p, for the sample which was polished and solvent cleaned only are shown in fig. 1. The two peaks located between 50 and 70 eV correspond to the Se3d from different chemical states. The lower binding energy peak was found to be at 58.55 eV. The C peak measured on the same sample was found to be at 288.8 eV. After adjusting the C peak, to account for charging, to the value used by Langer and Vesely [3] of 283.8 eV, a value of 53.55 eV is obtained for the Se3d. This is slightly higher than their value of 53.3 eV. However, using the value of Shenasa et al. [5] of at 284.6 eV a value of 54.35 eV is obtained which is slightly lower than their value of 54.7 eV for Se in the same chemical state. The value we obtain for Se3d is in between these two values. The second Se3d peak is located at 4.6 eV higher binding energy. This is between the values used by Weser et al. [6] (4.3 eV) and Shenasa et al. [5] (5.2 eV) for Se in SeO,.
/ XPS of eiched ZnSe
T. F. McGee III, H.J. Come&en
314
Table 1 Zn and Se peak positions and Auger parameter (x Sample
Zn
Se 3d
2P,,, (ev)
Pen
(ev)
Wf&f.x, (eV)
58.55 63.15 61.35 59.2 61.05 62.2 66.95 59.25
184.15 189.8 185.95 184.15 186.85 186.3 193.55 185.1
125.6 126.65 124.6 124.95 125.8 124.1 126.6 125.85
_
_
Polished
1029.6
504.4
525.2
Etched Ar heated Vat. heated Se SeO, ZnSe Zn ZnO
1029.15 1027.05 1029.05
505.5 501.35 503.8 _
523.65 525.7 525.25 _
1027.1 1021.5 1021.9
501.8 494.2 498.4
525.3 527.3 523.5
Fig. 2a shows the region containing the SeL,M,,,M,,, Auger transitions for this sample while figs. 3a, 3e and 3f show the Auger transitions for Se, ZnSe and SeO, powders, respectively. The energy scales for Figs. 2-4 are 20 eV wide; however, the absolute positions of the peaks have been shifted to align the peak maxima for shape comparisons. Notice the shape of the Auger peak is different between the standards. In table 1, two Se Auger parameters are shown for the polished sample. The value of 125.6 eV was obtained by measuring between the lower binding energy Se XPS peak, shown in fig. 1 and the larger Auger transition shown in fig. 2 (marked Al). This value is reasonably close to that of the ZnSe powder (125.85 eV). Measuring between the higher binding energy Se XPS peak shown 3.10. 2.79'. 2.48.. 2.17 " 1.86 ,. 1.55" 1.24 '. 0.93" 0.62"
BINDING
ENERGY
(eV)
Fig. 1. Se 3d and Zn 3p region for the polished sample.
T. F. McGee III, H.J. Cornelissen / XPS of etched ZnSe
ENERGY
375
(eV)
Fig. 2. SeL3M,,,M4,, Auger transition for (a) polished sample and (b) spectra obtained by the addition of the ZnSe and SeOl standards (spectra have been shifted to align peak maxima for shape comparison).
in fig. 1 and the smaller higher binding energy Auger peak marked A2 results in a value of 126.65 eV which is excellent agreement to that obtained on the SeO, (126.6 eV). The shape of the Se Auger transition from the polished sample (fig. 2a) can be approximated by adding the Se Auger transitions from ZnSe and SeO, powders as shown in fig. 2b. The Auger parameter for the ZnSe powder was added to the lower binding energy Se 3d peak while the SeO, Auger parameter was added to the higher binding energy peak to obtain their relative position. Next the SeO, Auger peak intensity was made to be about 15% of the ZnSe Auger peak and they were then added. Fig. 2b appears to be in good agreement (although not a perfect fit) with that of the polished sample (fig. 2a). This same method was tried for combining the SeO, and the Se Auger peaks and also for Se and ZnSe. Neither of these combinations gave reasonable agreement. The Auger parameters measured between the ZnL,M,,,M,,, and the Zn2p,,, peaks, from table 1, are 525.2 and 525.3 eV for the polished sample and the ZnSe powder, respectively. The Zn LjM4,5M4,5 peak is displayed in fig. 4c for this sample. Figs. 4b, 4d and 4e are the same regions for ZnSe, ZnO and Zn, respectively. Although the Auger parameter matches that of ZnSe, the shoulder peak in the polished sample (fig 4c) is not as well resolved as it is for the ZnSe standard (fig. 4b). 0 was also found to be present on the polished sample and not on the ZnSe powder. This combined with the presence of the two states of Se and the slight difference in the shape of the Zn Auger transitions leads to the conclusion that Zn and Se are bonded to 0 at the
316
T.F. McGee III, H.J. Cornelissen
ENERGY
/ XPS of etched .&Se
(eV)
Fig. 3. SeL,M4,,M,,, Auger transition for (a) Se metal standard, (b) etched ZnSe, (c) heated argon, (d) heated in vacuum, (e) ZnSe standard, (f) SeO, standard (spectra have been shifted align peak maxima for shape comparison).
in to
surface as well as to each other. It is not clear whether they are present as individual oxides or as a complex oxide as suggested by Ludeke [7]. 3.2. Etched sample After the sample was solvent cleaned and chemically etched, the ratio of the Zn 3d/Se 3d peak areas was found to decrease by an order of magnitude. Also,
T. F. McGee III, H.J. Comelissen
ENERGY
/ XPS of etched ZnSe
371
kV)
Fig. 4. ZnL,M4,,M4,, Auger transitions for (a) heated in vacuum, (b) ZnSe standard, (c) polished sample, (d) ZnO standard, (e) Zn metal standard (spectra have been shifted to align peak maxima for shape comparison).
the shape of the Se Auger peak for the etched slice (fig. 3b) is similar to that of metallic Se (fig. 3a) and the Se Auger parameter was found to decrease to 124.6 eV, toward the value found for pure Se (124.1 eV). This is probably due to the increase in the number of Se-Se bonds. The Zn Auger peak intensities were very weak on this sample; therefore, we could not make any comparisons of the shape. However, we could obtain the peak position by differentiation of the spectra and comparing to the differentiated spectra from the polished sample. When this was done the Zn Auger parameter was found to decrease to 523.65 eV. Although the Zn Auger parameter after etching was close to that found for the ZnO, it is probably not the oxide. ZnO has a low solubility in water [S] and would not dissolve into
378
T.F. McGee III, H.J. Cornelissen / XPS of etched ZnSe
solution leaving the surface Se rich. Since the etch appears to predominately etch the Zn from the surface it is possible the etching solution forms a zinc sulfate, zinc hydroxide or some intermediate zinc hydroxide with sulphur which dissolves into solution and the Se is oxidized to the metallic Se from the - 2 state in ZnSe. 3.3. Heated samples When the sample was heated in vacuum to 400°C both the Auger parameter for the Se (125.8 eV) and Zn (525.25 eV) changed to the value for the bulk ZnSe powder. The shape of the Se Auger peak (fig. 3d) is once again similar to that of the Se peak in ZnSe powder (fig. 3e) and the Zn Auger peak (fig. 4a) even more closely resembles that of the powder (fig. 4b) than the Zn Auger peak for the polished sample (fig. 4~). This indicates that the Se-rich surface transformed into ZnSe upon heating in vacuum due to Se evaporation from the surface [9]. The sample heated in the argon atmosphere to 400 a C for 15 min did not show the same results as above. The area ratio of the Zn3d/Se3d peaks did increase but the surface still had a factor of 2 more Se than ZnSe and the Se Auger parameter showed only a slight increase to 124.95 eV. The shape of the Se Auger peak (fig. 3c) started to resemble that of the ZnSe standard (fig. 3e), however the shoulder was not as sharply defined. It appears that in an argon atmosphere, using the same temperature and time as in vacuum, all of the excess Se does not evaporate from the surface. The Zn Auger peak was still too noisy, due to decreased signal intensity, to say much about its shape. The Zn Auger parameter increased past the value for ZnSe towards the metal. This is not understood; however, it may be that heating in argon increases the Zn-Zn bonding as well as increasing the Zn-Se bonding. Although some oxygen was detected on this sample, no changes in the chemical state of Se or Zn could be correlated with its presence. It is possible the oxygen is adsorbed on the surface from the atmosphere during handling.
4. Conclusions XPS was used to study the effects of an aqueous solution containing sulfuric acid and hydrogen peroxide on ZnSe wafers. The wafers were studied prior to etching, after etching and after etching followed by heating in either a vacuum or an argon atmosphere. XPS performed on the ZnSe wafers that were polished and solvent cleaned showed Se in two chemical states. One of the chemical states was attributed to Se in ZnSe while the other was thought to be an oxide of Se. The Zn Auger parameter matched that of the ZnSe, however
T. F. McGee III, H.J. Cornelissen / XPS of etched ZnSe
379
the shape of the Auger peak indicated the presence of some oxide. It was not clear whether the oxide was individual Zn and Se oxides or a mixed oxide. After the wafers were etched the surface appeared red. The etch was found to predominately remove the Zn leaving the surface Se-rich. The Se Auger parameter indicated an increase in the number of Se-Se bonds. It was shown that by heating in vacuum to 400 o C for 15 min the Se-rich surface could be transformed back to ZnSe, as determined by the shape of both the Zn and Se Auger transitions and their measured Auger parameters. Heating in an argon atmosphere at the same temperature and time did not have the same effect. The surface remained Se-rich and the Se Auger parameter increased slightly toward ZnSe while the Zn Auger parameter increased past that of ZnSe toward that of metallic Zn. Although this is not well understood it may be that heating in an argon atmosphere increases the Zn-Zn bonds as well as the ZnSe bonds.
Acknowledgements The authors would like to thank J. van de Ven for his enthusiastic etching efforts, M. Athanas and K. Salotti for the sample preparation and B. Fitzpatrick for supplying the samples. We would also like to thank G. Loiacono for the helpful discussions.
References [l] [2] [3] [4] [5] [6] (71 [8] [9]
J. van de Ven, private communication. G. Schon, J. Electron Spectrosc. Related Phenomena 2 (1973) 75. D.W. Langer and C.J. Vesely, Phys. Rev. B 2 (1970) 4885. B.J. Fitzpatrick, T.F. McGee III and P.M. Hamack, J. Crystal Growth 78 (1986) 242. M. Shenasa, S. Sainkar and D. Lichtman, J. Electron Spectrosc. Related Phenomena 40 (1986) 329. U. Weser, G. Sokolowski and W. Pilz, J. Electron Spectrosc. Related Phenomena 10 (1977) 429. R. Ludeke, Solid State Commun. 24 (1977) 725. R.C. Weast, Ed., Handbook of Chemistry and Physics (The Chemical Rubber Company, Boca Raton, FL, 1970) B-154. H.J. Cornelissen, D.A. Cammack and R.J. Dalby, J. Vacuum Sci. Technol. B 6 (1988) 769.