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Auger line shape analysis of Zn3P2
51
Applied Surface Science 26 (1986) 51-60 North-Holland, Amsterdam
AUGER LINE SHAPE ANALYSIS OF Zn,P, Stephen M. THURGATE School of Mathematical
and Ganesh N. RAIKAR
and Physical Sciences, Murdoch Universiiy, Murdoch,
Received 30 April 1985; accepted for publication
WA 6150, Australia
6 February 1986
The technique of Auger line shape analysis is applied to Zn,P, single crystal. ZnsP, is a semiconductor that has some potential as a material suitable for solar energy conversion. The measured line shapes are band-like and so are amenable to analysis and comparison with theory. The line shapes are recovered from the spectra by background subtraction followed by the Van Cittert deconvolution. This technique is first applied to the LsW line of copper and the results checked against those already in the literature. The line shapes for Zn,P, are compared with band structure calculations. In general, there is good agreement between the theory and our measured valence band structure.
1. Introduction Zn,P, is a II-V semiconductor with a band gap = 1.5 eV. As such it may have a future role in solar energy applications [l]. While no theoretical computations of the valence band density of states (DOS) has been made (to the knowledge of the authors), such information is of importance in considering particular applications. To this end, the Auger line shape of several Auger lines in the energy loss spectrum of Zn,P, have been analysed. The results of this analysis have been compared with pseudopotential computations of the band structure. There has been increasing evidence that Auger line shapes may be useful in interpreting the chemical environment of atoms on solid surfaces ]31. The process of extracting DOS information from Auger line shapes is complicated by a number of effects. In this case we selected the Lzs3W lines of Zn and P to analyse. If the hole-hole interaction energy, U,, of the final state holes in the valence band is sufficiently strong (greater than twice the valence band width W [2]) then the resulting line will exhibit “atomic-like” features. In our case, the valence band is s-like and the line shape is “band-like”, implying that TJ, =z2~. In such cases the Auger line can be considered to be a linear combination of the weighted partial DOS. The weighting is by the appropriate 0169-4332/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
52
S. M. Thurgate, G. N. Raikar / Auger line shape analysis
of Zn3Pz
transition matrix element for the particular part of the valence band under consideration. Before the DOS information can be extracted, it is necessary to deconvolute the line shape from the measured signal [3]. This is a two-stage process involving the removal of the rapidly varying cascade background followed by deconvolution of the signal with a measured instrument response function. We used a background correction technique described by Sickafus. We then used the Van Cittert iterative method to perform the deconvolution. We then tested our instrument and deconvolution techniques by measuring the L,W 920 V peak of Cu(OO1). The results were compared with data previously published.
2. Experimental All measurements were made in a Varian FC-12E UHV chamber previously described [4]. A four-grid LEED/AUGER retarding field analyser was used to measure the Auger spectra. This was interfaced to an LSI-11/23 computing system. It controlled the retarding potential of the grids and collected the output from the lock-in amplifier. The experimental set-up is shown in fig. 1. It differs from many other systems in that it used a neutralization scheme to cancel the effect of the capacitive coupling between the modulating grid and the collector (screen). This was achieved by phase shifting and mixing part of the oscillator output with the output of the preamplifier. The phase and amplitude of the oscillator output was adjusted until no signal could be detected with the electron -gun switched off. When the electron beam was switched on, the phase of the modulated electrons could easily be found. The computer was programmed to permit repeated scans so that the signal-to-noise ratio.could be improved. Generally 16 scans were averaged to produce the final spectrum. The Cu(OO1) crystal was cut with a low speed diamond saw after alignment by Laue backscatter X-ray analysis. The sample was then mechanically polished followed by electropolishing before being put into vacuum. The surface was repeatedly ion bombarded with 400 eV Ar ions followed by annealing at = 6oO’C. This procedure was repeated until a bright LEED pattern and clean Auger spectrum could be obtained from the surface. The Zn,P, crystals were obtained from the Institute of Energy Conversion at Delaware University [5]. They came as oriented single crystal slabs, 10 mm diameter by 1.0 mm thickness. They were prepared for vacuum by etching in a 2% bromine in ethanol solution for 10 min. The surface was prepared in vacuum by Ar ion bombardment followed by annealing at = 3OO’C. A faint LEED pattern could be made out with square geometry.
53
S. M. Thurgare, G. N. Raikar / Auger line shape analysis of 2% j P2
_-----Grid supply & Modulation
H.V.
I
‘CLI I I
I
I
___--_-I
crys
v
200 I---_
ta1
Retarding Field Ana1y3er I
c!zmpre+
_--mm
'Interface
Control 6.
iAnalogue outputs
Ana1ogue Input I
‘U
I
I
)si.gnal Defection system ---
L
JI
i
?unplifier
I
I
1
=
I
I
Computer syste*o
-l
I
I I --------Fig.1. Experimental
apparatus using LEED-AUGER
i
optics.
The Auger spectra were all measured with a modulating voltage of 3-4 VP_P. The instrument response function was measured by recording the elastic peak and nearby features with a primary electron gun energy equal to the .energy of the Auger line under consideration. When Auger spectra were collected, the primary beam energy was 2.6 keV and the beam current was 5 PA.
54
S. M. Thurgate, G. N. Raikar / Auger line shape analysis of Zn 3Pz
3. Method of analysis
The signal produced by the retarding field analyser is the differentiated form of the true line shape convoluted with a broadening function and superimposed on a non-linear background. The potential modulation differentiation scheme, first employed by Harris [6], reduces the high background caused by inelastically scattered electrons. Distortions of the true Auger line shape occur in the measured spectra and these must be removed after the data is acquired. The large secondary electron background was removed using a functional form of the background suggested by Sickafus [7]. The background arising from the cascade of inelastic scattering events in the sample was hypothesized by Sickafus to have the form AE-“, where m is constant for regions of N(E) away from Auger and loss features. He demonstrated that the linearization of the cascade in the log N(E)/log E display mode is consistent with the major features of cascade theory in spite of some limitations [14]. He obtained this power law by assuming that: electrons were incident normally upon a planar surface of homogeneous material, the ionization cross-section was of the Bethe form (E-’ log E), and the electron interactions could be described by s-wave scattering. This last assumption, originally made by Wolff [14], should limit the validity of the power law to energies less than 100 eV. Nevertheless, power law behaviour has been observed experimentally at energies up to 1000 eV and for various angles of incidence [7]. This correction was applied directly to the differentiated (dN( E)/dE) data. After background correction the Auger signal is normally associated with a “step-like” feature on its low energy side. This feature is presumably caused by inelastic losses suffered by some of the Auger electrons while leaving the solid surface. Madden et al. [8] have introduced a scaling factor to alter the height of the backscattered primary peak with respect to the inelastic losses in the electron backscatter spectrum (or instrument response function). This is done to account for the differences in emission geometry between the internal Auger source and the backscattered electrons. This scaling brings the deconvoluted spectrum to zero on its low energy side. Such adjustments have not been made in our work and the step-like feature has been subtracted after performing deconvolution. Deconvolution techniques [lo] were required to remove the effect of instrumental broadening. Mathematically the convolution integral is given by S(E)=/”
A(E) -CO
B(E-E’)dE’=A(E)*B(E),
where S(E) and A(E)
represent the “experimental”
and “true” Auger line
S. M. Thurgate, G. N. Raikar / Auger line shape analysis of Zn 3Pz
55
shapes respectively and B(E) is the system response function or instrument response function. The retrieval of A(E) from S(E) is referred to as “deconvolution”. It was first suggested by Mularie and Peria [9] that the energy loss and analyzer resolution information contained in the nearly elastically scattered primary electron distribution could be satisfactorily used as the system response function needed in deconvolution. Madden and Houston [ll] have successfully developed iterative deconvolution techniques based on the Van Cittert scheme [12]. We have utilized Madden’s code in somewhat modified form to deconvolute our results. Fourier-transform methods of deconvolution were not suitable in our case because of the “step-like” feature, so we resorted to using the Van Cittert iterative deconvolution method which gave very satisfactory results.
4.1. Cu(OOI) The differentiated spectrum of copper LsW line is shown in fig. 2. The signal-to-noise ratio has been improved by averaging 16 scans. The integrated spectrum with the background removed (Sickafus correction) is also shown in fig. 2 together with the deconvoluted spectrum. These curves compare well with the deconvoluted spectra of Cu LsW presented by Madden et al. [8]. Our spectrum shows the same features and spacing though the energy axis was translated by several volts to line up the main feature at 918 eV. This difference in position was attributed to differences in contract potentials between the two systems. We were unable to obtain the same fine structure as Madden because of the limited resolution of the retarding field analyser. As we increased the number of iterations an artifact at = 926 eV became more prominent. This was due to the Van Cittert scheme as similar observation was reported by Madden et al. [8]. The similarity between our results and Madden et al. [3,8] led us to conclude that our spectrometer and analysis techniques are working satisfactorily. 4.2. Zn,P, The Auger Spectrum of Zn,P, exhibits a number of lines. We chose to use the 120 eV L2,sW line of phosphorus and the 991 eV L3W line of zinc for our analysis. The de-convoluted spectrum from the phosphorus line is shown in fig. 3. Two peaks can be clearly seen at 113 and 105 eV. The deconvoluted spectrum from the zinc line is shown in fig. 4. Comparison with fig. 3 indicates that both
-Relative
Intensity -IN/DE
(Arbitrary
units)
I
SM.
Thurgate, G.N. Raikar / Auger line shape analysis of Zn, P2
I
105
Fig. 3. Deconvoluted
phosphorus
I
115
51
1
125
L2,3W line (10 iterations).
the spectra are similar. The width of the larger peak is approximately the same in both spectra, as is the separation between the two peaks.
5. Discussion A pseudopotential calculation of the band structure of Zn,P, has been made by Lin-Chung [13]. Her calculated structure is reproduced in fig. 5. The lowest valence band is a phosphorus s-like level. The centre of these two bands are separated by approximately 7 eV. The two peaks in the zinc and phosphorus lines are separated by = 8 eV. Given the approximations in Lin-Chung’s calculation and the possible distortions due to hole-hole interactions and lack of inclusion of spin-orbit interaction in Lin-Chung’s model, the agreement between theory and experiment seems reasonable. The fact that both bands in the valence band are s-like implies that the variation in transition probability across the bands is not large. Therefore the
S. M. Thurgate, G. N. Raikar / Auger line shape analysis of Zn 3P2
1
985
Energy (ev) Fig. 4. Deconvoluted
zinc L,W
line (20 iterations).
12-
r15 lo-
E"' P,
I: “J, L,
2-
Q,
XI
WI sl
‘, O(0.0.0)
c,:> 4 I4,
Fig. 5. Pseudopotential
U,O.L
(l,O>Ol
band structure of Zn,P,
(OP.01
by Lin-Chung (131.
S.M. Thurgate, G.N. Raikar / Auger line shape analysis of Zn 3P2
59
difference in height of the two peaks most probably reflects a difference in the density of states. This is further borne out by Lin-Chung’s analysis where she shows that the higher valence band is triply-degenerate. The differences between the zinc and phosphorus line shapes probably reflects a difference in the local density of states around the phosphorus atoms. Such differences have been noted by others in Auger studies of compounds. The upper valence band in Lin-Chung’s calculations is 5 eV wide while the lower band is 1.5 eV wide. The upper peaks in both the zinc and the phosphorus lines in the deconvoluted spectra have a FWHM of = 6 eV, however as the measured spectra are from L,,W transitions, each peak is the self-convolution of the true density of states. Hence the measured spectra imply a valence band width of = 1.5 eV in the lower band.
6. Conclusion Zn,P, appears to be well-suited to Auger line shape analysis. Its spectrum is band-like and the variation in matrix elements effects seem small. The measured density of states appears to agree well with the existing pseudo-potential calculations of the band structure.
Acknowledgements
The helpful discussions of this project with Andris Stelbovics and Philip Jennings is gratefully acknowledged. We would also like to thank Dr. H.H. Madden of Sandia National Laboratories, Albuquerque, USA, for allowing us to use his deconvolution program, and Dr. J. Meakin of the University of Delaware for supplying us with the Zn,P, single crystals.
References [l] M. Bhushan, Appl. Phys. Letters 40 (1982) 51. [2] R. Weissmann and K. Miter, Surface Sci. Rept. 1 (1981) 251; M. Cini, Solid State Commun. 20 (1976) 605; G.A. Sawatzky, Phys. Rev. Letters 39 (1977) 504. [3] H.H. Madden, Surface Sci. 126 (1983) 80. [4] SM. Thurgate and P.J. Jennings, Surface Sci. 114 (1982) 395. [5] E.A. Fagen, J. Appl. Phys. 50 (1979) 6505. [6] L.A. Harris, J. Appl. Phys. 39 (1968) 1419.
60
S. M. Thurgaie, G. N. Raikar / Auger line shape analysis of Zn 3 Pr
[7] E.N. Sickafus, Phys. Rev. B16 (1977) 1436, 1448; E.N. Sickafus, Rev. Sci. Instr. 42 (1971) 933; E.N. Sickafus and C. Kukla, Phys. Rev. B19 (1979) 4056. [8] H.H. Madden, D.M. Zehner and J.R. Noonan, Phys. Rev. B17 (1978) 3074. [9] W.M. Mularie and W.T. Peria, Surface Sci. 26 (1971) 125. [lo] A.F. Carley and R.W. Joyner, J. Electron Spectrosc. Related Phenomena 16 (1979) 1. [ll] H.H. Madden and J.E. Houston, J. Appl. Phys. 47 (1976) 3071; H.H. Madden and J.E. Houston, Advan. X-Ray Anal. 19 (1976) 657. [12] P.H. Van Cittert, Z. Phys. 69 (1931) 298. [13] P.J. Lin-Chung, Phys. Status Solidi (b) 47 (1971) 33. [14] P.A. Wolff, Phys. Rev. 95 (1954) 56.
Applied Surface Science 26 (1986) 51-60 North-Holland, Amsterdam
AUGER LINE SHAPE ANALYSIS OF Zn,P, Stephen M. THURGATE School of Mathematical
and Ganesh N. RAIKAR
and Physical Sciences, Murdoch Universiiy, Murdoch,
Received 30 April 1985; accepted for publication
WA 6150, Australia
6 February 1986
The technique of Auger line shape analysis is applied to Zn,P, single crystal. ZnsP, is a semiconductor that has some potential as a material suitable for solar energy conversion. The measured line shapes are band-like and so are amenable to analysis and comparison with theory. The line shapes are recovered from the spectra by background subtraction followed by the Van Cittert deconvolution. This technique is first applied to the LsW line of copper and the results checked against those already in the literature. The line shapes for Zn,P, are compared with band structure calculations. In general, there is good agreement between the theory and our measured valence band structure.
1. Introduction Zn,P, is a II-V semiconductor with a band gap = 1.5 eV. As such it may have a future role in solar energy applications [l]. While no theoretical computations of the valence band density of states (DOS) has been made (to the knowledge of the authors), such information is of importance in considering particular applications. To this end, the Auger line shape of several Auger lines in the energy loss spectrum of Zn,P, have been analysed. The results of this analysis have been compared with pseudopotential computations of the band structure. There has been increasing evidence that Auger line shapes may be useful in interpreting the chemical environment of atoms on solid surfaces ]31. The process of extracting DOS information from Auger line shapes is complicated by a number of effects. In this case we selected the Lzs3W lines of Zn and P to analyse. If the hole-hole interaction energy, U,, of the final state holes in the valence band is sufficiently strong (greater than twice the valence band width W [2]) then the resulting line will exhibit “atomic-like” features. In our case, the valence band is s-like and the line shape is “band-like”, implying that TJ, =z2~. In such cases the Auger line can be considered to be a linear combination of the weighted partial DOS. The weighting is by the appropriate 0169-4332/86/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
52
S. M. Thurgate, G. N. Raikar / Auger line shape analysis
of Zn3Pz
transition matrix element for the particular part of the valence band under consideration. Before the DOS information can be extracted, it is necessary to deconvolute the line shape from the measured signal [3]. This is a two-stage process involving the removal of the rapidly varying cascade background followed by deconvolution of the signal with a measured instrument response function. We used a background correction technique described by Sickafus. We then used the Van Cittert iterative method to perform the deconvolution. We then tested our instrument and deconvolution techniques by measuring the L,W 920 V peak of Cu(OO1). The results were compared with data previously published.
2. Experimental All measurements were made in a Varian FC-12E UHV chamber previously described [4]. A four-grid LEED/AUGER retarding field analyser was used to measure the Auger spectra. This was interfaced to an LSI-11/23 computing system. It controlled the retarding potential of the grids and collected the output from the lock-in amplifier. The experimental set-up is shown in fig. 1. It differs from many other systems in that it used a neutralization scheme to cancel the effect of the capacitive coupling between the modulating grid and the collector (screen). This was achieved by phase shifting and mixing part of the oscillator output with the output of the preamplifier. The phase and amplitude of the oscillator output was adjusted until no signal could be detected with the electron -gun switched off. When the electron beam was switched on, the phase of the modulated electrons could easily be found. The computer was programmed to permit repeated scans so that the signal-to-noise ratio.could be improved. Generally 16 scans were averaged to produce the final spectrum. The Cu(OO1) crystal was cut with a low speed diamond saw after alignment by Laue backscatter X-ray analysis. The sample was then mechanically polished followed by electropolishing before being put into vacuum. The surface was repeatedly ion bombarded with 400 eV Ar ions followed by annealing at = 6oO’C. This procedure was repeated until a bright LEED pattern and clean Auger spectrum could be obtained from the surface. The Zn,P, crystals were obtained from the Institute of Energy Conversion at Delaware University [5]. They came as oriented single crystal slabs, 10 mm diameter by 1.0 mm thickness. They were prepared for vacuum by etching in a 2% bromine in ethanol solution for 10 min. The surface was prepared in vacuum by Ar ion bombardment followed by annealing at = 3OO’C. A faint LEED pattern could be made out with square geometry.
53
S. M. Thurgare, G. N. Raikar / Auger line shape analysis of 2% j P2
_-----Grid supply & Modulation
H.V.
I
‘CLI I I
I
I
___--_-I
crys
v
200 I---_
ta1
Retarding Field Ana1y3er I
c!zmpre+
_--mm
'Interface
Control 6.
iAnalogue outputs
Ana1ogue Input I
‘U
I
I
)si.gnal Defection system ---
L
JI
i
?unplifier
I
I
1
=
I
I
Computer syste*o
-l
I
I I --------Fig.1. Experimental
apparatus using LEED-AUGER
i
optics.
The Auger spectra were all measured with a modulating voltage of 3-4 VP_P. The instrument response function was measured by recording the elastic peak and nearby features with a primary electron gun energy equal to the .energy of the Auger line under consideration. When Auger spectra were collected, the primary beam energy was 2.6 keV and the beam current was 5 PA.
54
S. M. Thurgate, G. N. Raikar / Auger line shape analysis of Zn 3Pz
3. Method of analysis
The signal produced by the retarding field analyser is the differentiated form of the true line shape convoluted with a broadening function and superimposed on a non-linear background. The potential modulation differentiation scheme, first employed by Harris [6], reduces the high background caused by inelastically scattered electrons. Distortions of the true Auger line shape occur in the measured spectra and these must be removed after the data is acquired. The large secondary electron background was removed using a functional form of the background suggested by Sickafus [7]. The background arising from the cascade of inelastic scattering events in the sample was hypothesized by Sickafus to have the form AE-“, where m is constant for regions of N(E) away from Auger and loss features. He demonstrated that the linearization of the cascade in the log N(E)/log E display mode is consistent with the major features of cascade theory in spite of some limitations [14]. He obtained this power law by assuming that: electrons were incident normally upon a planar surface of homogeneous material, the ionization cross-section was of the Bethe form (E-’ log E), and the electron interactions could be described by s-wave scattering. This last assumption, originally made by Wolff [14], should limit the validity of the power law to energies less than 100 eV. Nevertheless, power law behaviour has been observed experimentally at energies up to 1000 eV and for various angles of incidence [7]. This correction was applied directly to the differentiated (dN( E)/dE) data. After background correction the Auger signal is normally associated with a “step-like” feature on its low energy side. This feature is presumably caused by inelastic losses suffered by some of the Auger electrons while leaving the solid surface. Madden et al. [8] have introduced a scaling factor to alter the height of the backscattered primary peak with respect to the inelastic losses in the electron backscatter spectrum (or instrument response function). This is done to account for the differences in emission geometry between the internal Auger source and the backscattered electrons. This scaling brings the deconvoluted spectrum to zero on its low energy side. Such adjustments have not been made in our work and the step-like feature has been subtracted after performing deconvolution. Deconvolution techniques [lo] were required to remove the effect of instrumental broadening. Mathematically the convolution integral is given by S(E)=/”
A(E) -CO
B(E-E’)dE’=A(E)*B(E),
where S(E) and A(E)
represent the “experimental”
and “true” Auger line
S. M. Thurgate, G. N. Raikar / Auger line shape analysis of Zn 3Pz
55
shapes respectively and B(E) is the system response function or instrument response function. The retrieval of A(E) from S(E) is referred to as “deconvolution”. It was first suggested by Mularie and Peria [9] that the energy loss and analyzer resolution information contained in the nearly elastically scattered primary electron distribution could be satisfactorily used as the system response function needed in deconvolution. Madden and Houston [ll] have successfully developed iterative deconvolution techniques based on the Van Cittert scheme [12]. We have utilized Madden’s code in somewhat modified form to deconvolute our results. Fourier-transform methods of deconvolution were not suitable in our case because of the “step-like” feature, so we resorted to using the Van Cittert iterative deconvolution method which gave very satisfactory results.
4.1. Cu(OOI) The differentiated spectrum of copper LsW line is shown in fig. 2. The signal-to-noise ratio has been improved by averaging 16 scans. The integrated spectrum with the background removed (Sickafus correction) is also shown in fig. 2 together with the deconvoluted spectrum. These curves compare well with the deconvoluted spectra of Cu LsW presented by Madden et al. [8]. Our spectrum shows the same features and spacing though the energy axis was translated by several volts to line up the main feature at 918 eV. This difference in position was attributed to differences in contract potentials between the two systems. We were unable to obtain the same fine structure as Madden because of the limited resolution of the retarding field analyser. As we increased the number of iterations an artifact at = 926 eV became more prominent. This was due to the Van Cittert scheme as similar observation was reported by Madden et al. [8]. The similarity between our results and Madden et al. [3,8] led us to conclude that our spectrometer and analysis techniques are working satisfactorily. 4.2. Zn,P, The Auger Spectrum of Zn,P, exhibits a number of lines. We chose to use the 120 eV L2,sW line of phosphorus and the 991 eV L3W line of zinc for our analysis. The de-convoluted spectrum from the phosphorus line is shown in fig. 3. Two peaks can be clearly seen at 113 and 105 eV. The deconvoluted spectrum from the zinc line is shown in fig. 4. Comparison with fig. 3 indicates that both
-Relative
Intensity -IN/DE
(Arbitrary
units)
I
SM.
Thurgate, G.N. Raikar / Auger line shape analysis of Zn, P2
I
105
Fig. 3. Deconvoluted
phosphorus
I
115
51
1
125
L2,3W line (10 iterations).
the spectra are similar. The width of the larger peak is approximately the same in both spectra, as is the separation between the two peaks.
5. Discussion A pseudopotential calculation of the band structure of Zn,P, has been made by Lin-Chung [13]. Her calculated structure is reproduced in fig. 5. The lowest valence band is a phosphorus s-like level. The centre of these two bands are separated by approximately 7 eV. The two peaks in the zinc and phosphorus lines are separated by = 8 eV. Given the approximations in Lin-Chung’s calculation and the possible distortions due to hole-hole interactions and lack of inclusion of spin-orbit interaction in Lin-Chung’s model, the agreement between theory and experiment seems reasonable. The fact that both bands in the valence band are s-like implies that the variation in transition probability across the bands is not large. Therefore the
S. M. Thurgate, G. N. Raikar / Auger line shape analysis of Zn 3P2
1
985
Energy (ev) Fig. 4. Deconvoluted
zinc L,W
line (20 iterations).
12-
r15 lo-
E"' P,
I: “J, L,
2-
Q,
XI
WI sl
‘, O(0.0.0)
c,:> 4 I4,
Fig. 5. Pseudopotential
U,O.L
(l,O>Ol
band structure of Zn,P,
(OP.01
by Lin-Chung (131.
S.M. Thurgate, G.N. Raikar / Auger line shape analysis of Zn 3P2
59
difference in height of the two peaks most probably reflects a difference in the density of states. This is further borne out by Lin-Chung’s analysis where she shows that the higher valence band is triply-degenerate. The differences between the zinc and phosphorus line shapes probably reflects a difference in the local density of states around the phosphorus atoms. Such differences have been noted by others in Auger studies of compounds. The upper valence band in Lin-Chung’s calculations is 5 eV wide while the lower band is 1.5 eV wide. The upper peaks in both the zinc and the phosphorus lines in the deconvoluted spectra have a FWHM of = 6 eV, however as the measured spectra are from L,,W transitions, each peak is the self-convolution of the true density of states. Hence the measured spectra imply a valence band width of = 1.5 eV in the lower band.
6. Conclusion Zn,P, appears to be well-suited to Auger line shape analysis. Its spectrum is band-like and the variation in matrix elements effects seem small. The measured density of states appears to agree well with the existing pseudo-potential calculations of the band structure.
Acknowledgements
The helpful discussions of this project with Andris Stelbovics and Philip Jennings is gratefully acknowledged. We would also like to thank Dr. H.H. Madden of Sandia National Laboratories, Albuquerque, USA, for allowing us to use his deconvolution program, and Dr. J. Meakin of the University of Delaware for supplying us with the Zn,P, single crystals.
References [l] M. Bhushan, Appl. Phys. Letters 40 (1982) 51. [2] R. Weissmann and K. Miter, Surface Sci. Rept. 1 (1981) 251; M. Cini, Solid State Commun. 20 (1976) 605; G.A. Sawatzky, Phys. Rev. Letters 39 (1977) 504. [3] H.H. Madden, Surface Sci. 126 (1983) 80. [4] SM. Thurgate and P.J. Jennings, Surface Sci. 114 (1982) 395. [5] E.A. Fagen, J. Appl. Phys. 50 (1979) 6505. [6] L.A. Harris, J. Appl. Phys. 39 (1968) 1419.
60
S. M. Thurgaie, G. N. Raikar / Auger line shape analysis of Zn 3 Pr
[7] E.N. Sickafus, Phys. Rev. B16 (1977) 1436, 1448; E.N. Sickafus, Rev. Sci. Instr. 42 (1971) 933; E.N. Sickafus and C. Kukla, Phys. Rev. B19 (1979) 4056. [8] H.H. Madden, D.M. Zehner and J.R. Noonan, Phys. Rev. B17 (1978) 3074. [9] W.M. Mularie and W.T. Peria, Surface Sci. 26 (1971) 125. [lo] A.F. Carley and R.W. Joyner, J. Electron Spectrosc. Related Phenomena 16 (1979) 1. [ll] H.H. Madden and J.E. Houston, J. Appl. Phys. 47 (1976) 3071; H.H. Madden and J.E. Houston, Advan. X-Ray Anal. 19 (1976) 657. [12] P.H. Van Cittert, Z. Phys. 69 (1931) 298. [13] P.J. Lin-Chung, Phys. Status Solidi (b) 47 (1971) 33. [14] P.A. Wolff, Phys. Rev. 95 (1954) 56.