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Cathodoluminescence dependence on electron beam diameter
Materials Science and Engineering, B24 (1994) 141-143
141
Cathodoluminescence dependence on electron beam diameter S. Achour, M. T. Benlahrache, A. H a r a b i a n d N. T a b e t Research Unit in Materials Physics and Applications, University of Constantine, 25000 Constantine (Algeria)
Abstract An interesting behaviour of the cathodoluminescence (CL) intensity as a function of electron beam spot size was found, i.e. the relative CL intensity of both edge and defect emissions in all semiconducting materials studied here depends strongly upon the electron beam diameter; while the defect emission increases, the edge emission always decreases with increasing spot size.
1. Introduction
2. Experimentaldetails
It is usual to divide cathodoluminescence (CL) emission spectra into (i) intrinsic or edge emission and (ii) extrinsic or defect emission. The edge CL arises from transitions from conduction band to valence band and it is an intrinsic property of the material, while the defect emission arises from transitions across impurity or defect levels introduced in the energy gap. The shape of emission spectra (peak position, half-width and intensity) is known to depend on beam parmeters [1-5]. Nevertheless, the dependence on beam diameter has not been systematically studied. The variation in the beam diameter leads to the change in the generation volume and the current density as well as in the excited surface. The CL intensity is proportional to/a m where i a is the absorbed current [1]. Under the condition of low injection, i.e. when the excess carrier density (Ap for n type semiconductor) is much less than the doping concentration no, theory predicts a linear relationship ( r e = l ) between the radiative recombination rate (for the intrinsic recombination) and the injection level. This linearity no longer holds in the presence of significant trapping of the minority carriers and/or for high injection levels (Ap > no). For a fixed beam diameter, both intrinsic and extrinsic emissions increase with increasing beam current. However, for a fixed beam current, we have observed that the extrinsic emission decreases with decreasing beam diameter (with increasing beam current density or injection level). Therefore a systematic but nonexhaustive study of the CL change with beam diameter is reported here.
2.1. Materials In this work, different materials have been investigated in order to obtain a general idea on this dependence: (a) undoped CdS single crystals; (b) CdS sintered pellets; (c) CdS layers prepared by thermal evaporation; (d) Ag-diffused CdS layers. (e) ZnO ceramic which was pressed and then sintered at 1300 °C for 2 h in air; (f) Te-doped GaAs(111) single crystals. In order to obtain different amounts of surface detects, all these samples were mechanically polished, cleaved or chemically etched. The influence of different kinds of surface preparation will be reported later. CL was obtained at room temperature using an electron probe microanalyser. The CL signal was detected with an S20R photomultiplier combined with a light-dispersive monochromator. The excitation was made at different electron energies (10-35 keV) and beam currents (10-100 nA). In order to minimize the effect of irradiation time, a special experimental arrangement was used to record the CL intensity rapidly as a function of beam diameter.
0921-5107/94/$7.00 SSD1 0921-5107(93)00505-S
2.2. Automatic variation of beam diameter As shown in Fig. 1, a d.c. motor with a variable speed drives a 10 turn precision potentiometer which acts on the objective lens excitation circuit. The potentiometer can be driven in two directions, leading to either excitation or de-excitation of the objective lens, which in turn commands the electron beam focusing and defocusing. Consequently, a continuous change in © 1994 - Elsevier Sequoia. All rights reserved
142
s. A c h o u r et aL
/
('L dependence on electron beam diamewr
the CL intensity can be synchronously recorded as a function of spot size (beam diameter). For example, the beam diameter can change from about 1 to 200 /~m with increasing values when the objective lens is excited whereas during de-excitation the beam diameter changes from higher to lower values (200 to about 1/~m). Therefore, in order to distinguish between these two modes of variation, negative values are given on the axis of the figures representing the CL intensity vs. beam diameter for the de-excitation operation. The beam diameter d resulting from defocusing was directly measured in the optical microscope attached to the apparatus. The luminescent area, which is the intersection of the beam with the sample surface, looks like a perfect homogeneous circle. As a result, the electron density in the vicinity of the vertical incidence does not differ very much from those in the other directions. Thus the situation corresponding to the focusing and defocusing can be illustrated by the very simplified scheme in Fig. 2, where d is the beam diameter (1-200/~m) and h is the penetration depth which, in the case of the experimental conditions used, varied between 1 and 2 ktm.
almost linearly with increasing d until it reaches a saturation value at high d, whereas the edge CL decreases continuously and tends also to a saturation value. The saturation tendency at higher beam diameters may be ascribed to the fact that the optical collection system used in this experiment is out of the field. The same phenomenon was observed for other fixed beam voltages (10-35 kV) and beam currents (10-100
hA). The increase in beam diameter (during defocusing) increases both the irradiated surface and the generation volume as illustrated by the scheme in Fig. 2. This means that the number of excited surface defects increases with increasing beam diameter, since the irradiated region of the surface grows larger and larger. Radiative surface defects are believed to be predominant even though radiative defects exist in the bulk and increase also with increasing generation volume.
~!~____.~1 nc i den t e l e c t r o n s ~ . . . . . . . . . . . .d
I
3. Results and discussion
7\
h
/ \
.. Surface ~
(a)
lena
-
'//', beam ......
Sample--c=..
tr
, l
', h
,,
, l
Fig. 2. A simplified diagram of electron trajectories of (a) a focused and (b) a defocused beam, showing the increase in the excited surface region and generation volume.
OBJECTIVE LENS IX----TI IX---'ZI EXCITATION ~ X l ~ IX I CIRCUIT CONTROL - v "~ ///\~1~v xI
Electro,,
I
r,r"
(b)
Objective
-
, ! ,
Bu,~--~.-
I __L_Y.__
The variations in both edge and defect emission intensities, for CdS and GaAs single crystals, as a function of beam diameter d are shown in Fig. 3. Those for polycrystalline CdS and ZnO have a similar behaviour. In all cases, the defect CL intensity increases
10 T U R N PRECISION POTENTIOMETER
Region ~ .............
/
-_J-
/
L'o"t~ # /
",
.......
1
CL COLLECTION DETECTION A N D AMPLIFICATION
Fig. 1. Schematic diagram of the automatic variation in electron beam spot size.
J-
S. Achour et aL
/
CL dependence on electron beam diameter
Such defects introduced levels into the band gap on which transitions take place [6]. The surface recombination velocity as well as radiative lifetime of the defect emission which can vary with local injection level could have an important effect on the CL variation with beam diameter. For example, in silicon, it has been shown that the surface recombination rate increased with decreasing injction level [7, 8]. This corroborates the behaviour of defect emission as a function of beam diameter, i.e. the continuous increase in radiative emission through surface and bulk defect states with increasing d (decreasing injection level). The same behaviour was also observed for the emission defect associated with Ag (or their complexes) introduced in the band gap of CdS materials [9]. It must be emphasized that the defect emission increases or
A
=
//
.> ,=, r~ !
t
-160
-80
/
143
decreases as the beam current does only if the beam diameter is fixed. Otherwise, this emission varies in an opposite way to the current density. Therefore the beam diameter variation defect cannot be attributed only to the variation in injection level. The change in surface defects and radiative lifetimes with the change in generation volume should more probably be considered. At low temperatures, the green edge emission for CdS is believed to be due to donor-acceptor recombination, i.e. to defect states (instead of the band-toband transition at room temperature) [10]. Therefore this emission would behave as defect emission does. This is really what was observed during experiments conducted at liquid-nitrogen temperature (Fig. 4). Accordingly, it seems that it is not only the emission through surface defects which increases with increasing beam diameter but also all emissions through any defect introducing levels into the band gap. This may be used to distinguish defect emissions from those occurring across the band gap. 4. Conclusion
1
BEAN DIANEIER
80
160
(,tim)
Fig. 3. CL intensity as a function of beam diameter (E = 35 keV; ia = 15 nA): curve a, edge emission of CdS single crystal; curve b, defect emission of CdS single crystal; curve c, edge emission of GaAs.
The behaviour of the CL intensity as a function of beam diameter seems to be a general phenomenon which, if correctly modelled, would give valuable information on surface defects and surface recombination velocity as well as on bulk defects. The band-to-band emission intensity decreases with increasing beam spot size while the intensity of emission through defect levels in the band gap increases with increasing beam spot size. A further investigation on surface treatments and injection level is necessary to clarify the role of surface defects and the injection level. References
A
z d
20
40 60 80 100 1;ZO 140 BEAM DIAMETER ( r.m~ )
Fig. 4. CL intensity of green emission in CdS at low temperatures vs. beam diameter ( E = 1 5 keV; ia= 100 nA; T--93 K): curve a, evaporated layers 45/~m thick; curve b, mechanically polished single crystal.
1 B.G. Yacobi and D. B. Holt, J. Appl. Phys., 59(1986) R1. 2 T. S. Rao-Sahib and D. B. Wittry, J. Appl. Phys., 40 (1969) 3145. 3 S. Hilberandt, J. Schreiber, W. Hergert and V. I. Petrov, Phys. Status Solidi A, 100 (1988) 894. 4 J. Piqueras and E. Kubalek, Solid State Commun., 54 (1985) 745. 5 R. A. Street and R. H. Williams, J. Appl. Phys., 52 (1981) 402. 6 V. A. Zuev, D. V. Korbutyak and V. G. Litovchenko, Surf. Sci., 50 (1975) 215. 7 S. M. Davidson, R. M. Innes and S. M. Lindsay, Solid-State Electron., 25 (1982) 261. 8 S. Mynajlenko, W. Ke and B. Hamilton, J. Appl. Phys., 54 (1983) 866. 9 S. Achour, Philos. Mag. B, 61 (1990) 347. 10 V.V. Dyakin, M. A. Rizakhanov, N. N. Khilimova and M. K. Sheinkman, Sov. Phys.--Semicond., 14 (1980) 407.
141
Cathodoluminescence dependence on electron beam diameter S. Achour, M. T. Benlahrache, A. H a r a b i a n d N. T a b e t Research Unit in Materials Physics and Applications, University of Constantine, 25000 Constantine (Algeria)
Abstract An interesting behaviour of the cathodoluminescence (CL) intensity as a function of electron beam spot size was found, i.e. the relative CL intensity of both edge and defect emissions in all semiconducting materials studied here depends strongly upon the electron beam diameter; while the defect emission increases, the edge emission always decreases with increasing spot size.
1. Introduction
2. Experimentaldetails
It is usual to divide cathodoluminescence (CL) emission spectra into (i) intrinsic or edge emission and (ii) extrinsic or defect emission. The edge CL arises from transitions from conduction band to valence band and it is an intrinsic property of the material, while the defect emission arises from transitions across impurity or defect levels introduced in the energy gap. The shape of emission spectra (peak position, half-width and intensity) is known to depend on beam parmeters [1-5]. Nevertheless, the dependence on beam diameter has not been systematically studied. The variation in the beam diameter leads to the change in the generation volume and the current density as well as in the excited surface. The CL intensity is proportional to/a m where i a is the absorbed current [1]. Under the condition of low injection, i.e. when the excess carrier density (Ap for n type semiconductor) is much less than the doping concentration no, theory predicts a linear relationship ( r e = l ) between the radiative recombination rate (for the intrinsic recombination) and the injection level. This linearity no longer holds in the presence of significant trapping of the minority carriers and/or for high injection levels (Ap > no). For a fixed beam diameter, both intrinsic and extrinsic emissions increase with increasing beam current. However, for a fixed beam current, we have observed that the extrinsic emission decreases with decreasing beam diameter (with increasing beam current density or injection level). Therefore a systematic but nonexhaustive study of the CL change with beam diameter is reported here.
2.1. Materials In this work, different materials have been investigated in order to obtain a general idea on this dependence: (a) undoped CdS single crystals; (b) CdS sintered pellets; (c) CdS layers prepared by thermal evaporation; (d) Ag-diffused CdS layers. (e) ZnO ceramic which was pressed and then sintered at 1300 °C for 2 h in air; (f) Te-doped GaAs(111) single crystals. In order to obtain different amounts of surface detects, all these samples were mechanically polished, cleaved or chemically etched. The influence of different kinds of surface preparation will be reported later. CL was obtained at room temperature using an electron probe microanalyser. The CL signal was detected with an S20R photomultiplier combined with a light-dispersive monochromator. The excitation was made at different electron energies (10-35 keV) and beam currents (10-100 nA). In order to minimize the effect of irradiation time, a special experimental arrangement was used to record the CL intensity rapidly as a function of beam diameter.
0921-5107/94/$7.00 SSD1 0921-5107(93)00505-S
2.2. Automatic variation of beam diameter As shown in Fig. 1, a d.c. motor with a variable speed drives a 10 turn precision potentiometer which acts on the objective lens excitation circuit. The potentiometer can be driven in two directions, leading to either excitation or de-excitation of the objective lens, which in turn commands the electron beam focusing and defocusing. Consequently, a continuous change in © 1994 - Elsevier Sequoia. All rights reserved
142
s. A c h o u r et aL
/
('L dependence on electron beam diamewr
the CL intensity can be synchronously recorded as a function of spot size (beam diameter). For example, the beam diameter can change from about 1 to 200 /~m with increasing values when the objective lens is excited whereas during de-excitation the beam diameter changes from higher to lower values (200 to about 1/~m). Therefore, in order to distinguish between these two modes of variation, negative values are given on the axis of the figures representing the CL intensity vs. beam diameter for the de-excitation operation. The beam diameter d resulting from defocusing was directly measured in the optical microscope attached to the apparatus. The luminescent area, which is the intersection of the beam with the sample surface, looks like a perfect homogeneous circle. As a result, the electron density in the vicinity of the vertical incidence does not differ very much from those in the other directions. Thus the situation corresponding to the focusing and defocusing can be illustrated by the very simplified scheme in Fig. 2, where d is the beam diameter (1-200/~m) and h is the penetration depth which, in the case of the experimental conditions used, varied between 1 and 2 ktm.
almost linearly with increasing d until it reaches a saturation value at high d, whereas the edge CL decreases continuously and tends also to a saturation value. The saturation tendency at higher beam diameters may be ascribed to the fact that the optical collection system used in this experiment is out of the field. The same phenomenon was observed for other fixed beam voltages (10-35 kV) and beam currents (10-100
hA). The increase in beam diameter (during defocusing) increases both the irradiated surface and the generation volume as illustrated by the scheme in Fig. 2. This means that the number of excited surface defects increases with increasing beam diameter, since the irradiated region of the surface grows larger and larger. Radiative surface defects are believed to be predominant even though radiative defects exist in the bulk and increase also with increasing generation volume.
~!~____.~1 nc i den t e l e c t r o n s ~ . . . . . . . . . . . .d
I
3. Results and discussion
7\
h
/ \
.. Surface ~
(a)
lena
-
'//', beam ......
Sample--c=..
tr
, l
', h
,,
, l
Fig. 2. A simplified diagram of electron trajectories of (a) a focused and (b) a defocused beam, showing the increase in the excited surface region and generation volume.
OBJECTIVE LENS IX----TI IX---'ZI EXCITATION ~ X l ~ IX I CIRCUIT CONTROL - v "~ ///\~1~v xI
Electro,,
I
r,r"
(b)
Objective
-
, ! ,
Bu,~--~.-
I __L_Y.__
The variations in both edge and defect emission intensities, for CdS and GaAs single crystals, as a function of beam diameter d are shown in Fig. 3. Those for polycrystalline CdS and ZnO have a similar behaviour. In all cases, the defect CL intensity increases
10 T U R N PRECISION POTENTIOMETER
Region ~ .............
/
-_J-
/
L'o"t~ # /
",
.......
1
CL COLLECTION DETECTION A N D AMPLIFICATION
Fig. 1. Schematic diagram of the automatic variation in electron beam spot size.
J-
S. Achour et aL
/
CL dependence on electron beam diameter
Such defects introduced levels into the band gap on which transitions take place [6]. The surface recombination velocity as well as radiative lifetime of the defect emission which can vary with local injection level could have an important effect on the CL variation with beam diameter. For example, in silicon, it has been shown that the surface recombination rate increased with decreasing injction level [7, 8]. This corroborates the behaviour of defect emission as a function of beam diameter, i.e. the continuous increase in radiative emission through surface and bulk defect states with increasing d (decreasing injection level). The same behaviour was also observed for the emission defect associated with Ag (or their complexes) introduced in the band gap of CdS materials [9]. It must be emphasized that the defect emission increases or
A
=
//
.> ,=, r~ !
t
-160
-80
/
143
decreases as the beam current does only if the beam diameter is fixed. Otherwise, this emission varies in an opposite way to the current density. Therefore the beam diameter variation defect cannot be attributed only to the variation in injection level. The change in surface defects and radiative lifetimes with the change in generation volume should more probably be considered. At low temperatures, the green edge emission for CdS is believed to be due to donor-acceptor recombination, i.e. to defect states (instead of the band-toband transition at room temperature) [10]. Therefore this emission would behave as defect emission does. This is really what was observed during experiments conducted at liquid-nitrogen temperature (Fig. 4). Accordingly, it seems that it is not only the emission through surface defects which increases with increasing beam diameter but also all emissions through any defect introducing levels into the band gap. This may be used to distinguish defect emissions from those occurring across the band gap. 4. Conclusion
1
BEAN DIANEIER
80
160
(,tim)
Fig. 3. CL intensity as a function of beam diameter (E = 35 keV; ia = 15 nA): curve a, edge emission of CdS single crystal; curve b, defect emission of CdS single crystal; curve c, edge emission of GaAs.
The behaviour of the CL intensity as a function of beam diameter seems to be a general phenomenon which, if correctly modelled, would give valuable information on surface defects and surface recombination velocity as well as on bulk defects. The band-to-band emission intensity decreases with increasing beam spot size while the intensity of emission through defect levels in the band gap increases with increasing beam spot size. A further investigation on surface treatments and injection level is necessary to clarify the role of surface defects and the injection level. References
A
z d
20
40 60 80 100 1;ZO 140 BEAM DIAMETER ( r.m~ )
Fig. 4. CL intensity of green emission in CdS at low temperatures vs. beam diameter ( E = 1 5 keV; ia= 100 nA; T--93 K): curve a, evaporated layers 45/~m thick; curve b, mechanically polished single crystal.
1 B.G. Yacobi and D. B. Holt, J. Appl. Phys., 59(1986) R1. 2 T. S. Rao-Sahib and D. B. Wittry, J. Appl. Phys., 40 (1969) 3145. 3 S. Hilberandt, J. Schreiber, W. Hergert and V. I. Petrov, Phys. Status Solidi A, 100 (1988) 894. 4 J. Piqueras and E. Kubalek, Solid State Commun., 54 (1985) 745. 5 R. A. Street and R. H. Williams, J. Appl. Phys., 52 (1981) 402. 6 V. A. Zuev, D. V. Korbutyak and V. G. Litovchenko, Surf. Sci., 50 (1975) 215. 7 S. M. Davidson, R. M. Innes and S. M. Lindsay, Solid-State Electron., 25 (1982) 261. 8 S. Mynajlenko, W. Ke and B. Hamilton, J. Appl. Phys., 54 (1983) 866. 9 S. Achour, Philos. Mag. B, 61 (1990) 347. 10 V.V. Dyakin, M. A. Rizakhanov, N. N. Khilimova and M. K. Sheinkman, Sov. Phys.--Semicond., 14 (1980) 407.