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Cathodoluminescence dependence upon irradiation time
MATERlAM SCIEMCE & ENGINEERIWG ELSEVIER
B
Materials Science and Engineering B42 (1996) 289-292
Cathodoluminescence
dependence upon irradiation
S. A&our*, Ceramics
Laboratory,
Reseasch
Unit of Materials
Physics
A. Harabi, and Applications,
time
N. Tabetl University
of Constantine,
25000
Constantine,
Algeria
Abstract
The cathodoluminescence (CL) intensity varies with beam exposure time. In this work, the CL change as a function of irradiation time has been studied using various semiconducting materials: CdS single crystal, CdS evaporated thin films, ZnO ceramics and GaAs single crystal. A current density as low as 60 ym/cm’ was used in an electron microprobe analyser. In the case of low excitation level, two stages of the CL variation have been generally observed, i.e. increasing and decreasing parts. In the case of a relatively high excitation, only a decreasing stage can be observed. It is believed that the CL time dependence is closely related to the adsorption-desorptionprocessand the surface contamination which are stimulated by the electron beam excitation. Keywords:
Cathodoluminescence; CdS single crystal; Surface contamination
1. Introduction The cathodoluminescence (CL) kinetics of semiconducting materials, when investigated at 300 K, have more or lessa fast initial period of increase of the l!ght emission, followed by a relatively slow decreasing stage. This variation is usually interpreted as a result of a stimulated interaction between the irradiated surface and the residual vacuum, as well as different induced processeswithin the sample. This is an embarrassing problem during CL intensity measurement. Therefore, an understanding of the CL intensity change with the time is necessary to obtain a correct information about the CL mechanisms. The present paper deals with the CL intensity change as a function of time, taking into account the accelerating voltage, the beam diameter, the sample nature and the surface treatment.
2. Experimental Various semiconducting materials namely, CdS single crystals, CdS evaporated films, ZnO ceramics and GaAs single crystal were used. The above materials were studied in various forms: as prepared, cleaved, vacuum annealed, mechanically polished and chemi* Corresponding author. ’ Present address: KFUPM, Arabia.
Physics Department, Dhahran, Saudi
0921-5107/96/$15.00 0 1996 - Elsevier Science S.A. All rights reserved PII SO921-5107(96)01723-O
tally polished. A current density as low as 60 PA/cm2 was used in a microprobe analyser. The CL detection system which has been used (type S20R) covers the wavelength range between 0.2 and 0.9 pm. The accelerating voltage was taken between 10 and 35 keV. Both the edge and the defect CL emission were recorded as a function of the irradiation time at room temperature.
3. Results We have investigated in a systematic way the effect of the observation conditions (energy and size of the electron
beam)
as well
as that
of the surface
preparation
mode on the CL kinetics in different materials. 3.1. Effect of the size and electron beam energy Figs. 1 and 2 display the variation of the CL intensity versus time which has been obtained in CdS single crystals for different beam sizes and two beam energies (10 and 35 keV). Two stages can be distinguished. In the very first stage, the CL intensity increases,reaches a maximum value and then, rapidly decreasesduring the second stage. It can be noticed that the maximum is reached faster as the beam diameter decreases, i.e. as the current density increases. In addition, for a given electron beam size, d = 40 pm for instance, the maximum CL value is reached much faster at low beam energy (after 1 min for 10 keV, and 3 min for 3.5keV).
290
S. Achow
3.2. Comparison
et al. / Materials
Science and Engineering
of diffeerent materials
342
(1996)
289-292
E= 35 keV
We have reported in Fig. 3 the CL kinetics that we have observed in different materials under the same excitation conditions. The CL intensity of both the defect and the edge luminescence increases at the beginning of irradiation, even when the selected wavelength is changed over the whole spectrum. The maximum value can be reached rapidly or slowly, depending on the beam generation conditions and the material structures. For example, the defect emission in as prepared ZnO ceramics reaches its maximum value rapidly, even under very low beam current density, i.e. 60 uA/cm2. Similar observations have been reported for ZnO in Ref. [2]. 3.3. Effect of the surface preparation
i a= 75 I-IA
mode
Furthermore, surface defects were found to enhance the rate of the CL variation with the irradiation time as shown in Fig. 4. In fact, the change is faster in mechanically polished than in the case of cleaved, chemically etched CdS single crystals or evaporated CdS layers.
E= 10 keV TIK
( HlNUTE
)
Fig. 2. CL time dependence of the edge emission in single crystal CdS with chemical etching in HCL (10 s), for 35 keV and different beam diameters d (pm).
4. Discussion
I
012
3456 TIHE
78 ~WIJTE
)
Fig. 1. CL time dependence of the edge emission in single crystal CdS with chemical etching in HCL (10 s), for 10 keV and different beam diameters d (pm).
The CL kinetics in alkali halides which have been previously reported in the literature, were attributed to the non-radiative recombination at defects which are created by the electronic irradiation [l]. The authors of Ref. [2] have observed the CL kinetics of CdS, ZnO and GaN:Zn crystals, using electron current density of more than 100 A/cm2, under continuous excitation at room temperature. They found no variation in ZnO CL intensity; in GaN:Zn they observed a maximum, while for CdS, the CL intensity decreased continuously with time. Among the generation of new radiative and nonradiative centres in the volume, a possible degradation under a local temperature action has been proposed. Using a 2 keV and 10 mA/cm’ electron irradiation, the authors of Refs. [3,4] have observed a continuous quenching with time of the edge CL-intensity in CdS single crystal. They found that this effect is related to the presence of water vapour. In any case, such quenching must be due to an electron beam induced increase in the non-radiative surface recombination velocity, for both the edge and the defect emission.
S. Achour et al. / Materials Science and Engineering B42 (1996) 289-292
Our results lead to the conclusion that the injection level play a crucial role in the on going processes under the beam excitation. The possible local temperature effect which has been suggested in Ref. [2] cannot be taken into consideration in the present work, since the experiments have been carried out in the frame of the non-heating regime (low current density). In addition, the sensitivity of the CL kinetics to the surface preparation mode supports an interpretation which relates the observed effects to the modification of the surface under the beam excitation. Following a suggestion of the authors of Ref. [S], the availability of holes in n-type CdS crystal is the limiting step for oxygen adsorption to occur during band gap illumination [5]. Based on this assumption, the CL variation dependence on beam voltage could thus be explained as follows. At low voltages, the availability of holes resulting from pair generation is considerable in the surface region. Therefore, the electron stimulated adsorptiondesorption is more important, leading to the rapid CL variation which is observed. At high accelerating voltages, the electron-hole pairs are generated deeply in the bulk. Accordingly, the adsorption-desorption rate must decrease. This results in a slow CL-intensity variation with time
'2
35
keV
d=
20
pT4
ia=
15 nA
Fig. 3. CL time dependence of different materials for edge and defect emission. (a) Chemical polishing in 49:11H,PO,:HNO, at 60°C of (11 l)Te-doped GaAs, (b) undoped ZnO ceramic, (c) evaporated CdS film and (d, e) mechanically polished CdS single crystal.
291
E= 10
keV
d=
40
typ
15
nA
ia=
1
. I c
1 I
0
1
234567
(MIHl)TE ) TM Fig. 4. Edge emission intensity vs. irradiation time for different surface treatments of samples: (a) chemical etching in HCL (10 s), (b) evaporated layer and (c) mechanical polishing.
(compare Figs. 1 and 2). This is also supported by the observed variation as a function of beam diameter (Figs. 1 and 2). On the other hand, we have found that if the electron beam is turned off before the CL variation beian to decrease and thereafter turned on after some resting time (15 min or more), the CL intensity will tend to recover its initial value before irradiation (Fig. 5). These observations are in accordance with a desorption effect during the first stage of CL change. Nevertheless, once the CL intensity began to decrease, the beam turning on and off did not remarkably lead to the above reversible phenomenon. The emission persisted in decreasing from its value or less (perhaps, because of the well known memory effect in the adsorption theory) suggesting a stimulated adsorption of the residual gas during this variation stage. Thus, the process can be explained as follows. At the beginning of the electron irradiation, a stimulated desorption (or cleaning) of the surface occurs. This is manifested by the enhancement of the integrated intensity. After that desorption, many active adsorbed centres would be released and more stable adsorbent species began to accumulate on the irradiated surface: carbon contamination due to the re-
292
S. A&our
et al. 1 Materials
Science
E= 15 keV
and Engineering
342 (1996)
289-292
5. Conclusion
The major finding of this work is the general increase of the CL intensity at the beginning of irradiation when low current densities are used. The CL time dependence is closely related to surface defects and surface contaminants as well as to the beam generation conditions. The increasing part of the’ CL variation with time can be more probably attributed to the contaminant desorption, while the decreasing part may be mainly due to the residual gas adsorption and/or carbon contamination. The general observed temporary low excitation electron induced desorption can find a practical application in-situ device processing. This can be done by cleaning the surface during the appropriate time, which was just necessary to reach the CL maximum change.
Fig. 5. Behaviour of the edge emission as a function of irradiation time in chemically etched CdS single crystai: (b) response from the same area corresponding to the (a) curve, but after the beam was off during 1 h.
maining vacuum can appear clearly as dark spots in the optical microscope if a long exposure time and a high current density are used.
[I] A. Nouailhat, G. Guillot and P. Pinard, J. Lumb~ex., 5 (1972) 218. [2] G.V. Saparin, S.K. Obyden, M.V. Chukikev and S.I. Popov, J. Luminesc.,
31-32
(1972)
684.
[3] R.P. Holmstrom, J. Lagowski and H.C. Gates, Sur$ Sci., 100 (1972) L467. [4] M. Lichtensteeiger, C. Webb and J. Lagowski, Surj Sci., 97 (1980) L375. [5] M. Lichtensreeiger and C. Webb, Srrrf, Sci., 154 (1985) 455.
B
Materials Science and Engineering B42 (1996) 289-292
Cathodoluminescence
dependence upon irradiation
S. A&our*, Ceramics
Laboratory,
Reseasch
Unit of Materials
Physics
A. Harabi, and Applications,
time
N. Tabetl University
of Constantine,
25000
Constantine,
Algeria
Abstract
The cathodoluminescence (CL) intensity varies with beam exposure time. In this work, the CL change as a function of irradiation time has been studied using various semiconducting materials: CdS single crystal, CdS evaporated thin films, ZnO ceramics and GaAs single crystal. A current density as low as 60 ym/cm’ was used in an electron microprobe analyser. In the case of low excitation level, two stages of the CL variation have been generally observed, i.e. increasing and decreasing parts. In the case of a relatively high excitation, only a decreasing stage can be observed. It is believed that the CL time dependence is closely related to the adsorption-desorptionprocessand the surface contamination which are stimulated by the electron beam excitation. Keywords:
Cathodoluminescence; CdS single crystal; Surface contamination
1. Introduction The cathodoluminescence (CL) kinetics of semiconducting materials, when investigated at 300 K, have more or lessa fast initial period of increase of the l!ght emission, followed by a relatively slow decreasing stage. This variation is usually interpreted as a result of a stimulated interaction between the irradiated surface and the residual vacuum, as well as different induced processeswithin the sample. This is an embarrassing problem during CL intensity measurement. Therefore, an understanding of the CL intensity change with the time is necessary to obtain a correct information about the CL mechanisms. The present paper deals with the CL intensity change as a function of time, taking into account the accelerating voltage, the beam diameter, the sample nature and the surface treatment.
2. Experimental Various semiconducting materials namely, CdS single crystals, CdS evaporated films, ZnO ceramics and GaAs single crystal were used. The above materials were studied in various forms: as prepared, cleaved, vacuum annealed, mechanically polished and chemi* Corresponding author. ’ Present address: KFUPM, Arabia.
Physics Department, Dhahran, Saudi
0921-5107/96/$15.00 0 1996 - Elsevier Science S.A. All rights reserved PII SO921-5107(96)01723-O
tally polished. A current density as low as 60 PA/cm2 was used in a microprobe analyser. The CL detection system which has been used (type S20R) covers the wavelength range between 0.2 and 0.9 pm. The accelerating voltage was taken between 10 and 35 keV. Both the edge and the defect CL emission were recorded as a function of the irradiation time at room temperature.
3. Results We have investigated in a systematic way the effect of the observation conditions (energy and size of the electron
beam)
as well
as that
of the surface
preparation
mode on the CL kinetics in different materials. 3.1. Effect of the size and electron beam energy Figs. 1 and 2 display the variation of the CL intensity versus time which has been obtained in CdS single crystals for different beam sizes and two beam energies (10 and 35 keV). Two stages can be distinguished. In the very first stage, the CL intensity increases,reaches a maximum value and then, rapidly decreasesduring the second stage. It can be noticed that the maximum is reached faster as the beam diameter decreases, i.e. as the current density increases. In addition, for a given electron beam size, d = 40 pm for instance, the maximum CL value is reached much faster at low beam energy (after 1 min for 10 keV, and 3 min for 3.5keV).
290
S. Achow
3.2. Comparison
et al. / Materials
Science and Engineering
of diffeerent materials
342
(1996)
289-292
E= 35 keV
We have reported in Fig. 3 the CL kinetics that we have observed in different materials under the same excitation conditions. The CL intensity of both the defect and the edge luminescence increases at the beginning of irradiation, even when the selected wavelength is changed over the whole spectrum. The maximum value can be reached rapidly or slowly, depending on the beam generation conditions and the material structures. For example, the defect emission in as prepared ZnO ceramics reaches its maximum value rapidly, even under very low beam current density, i.e. 60 uA/cm2. Similar observations have been reported for ZnO in Ref. [2]. 3.3. Effect of the surface preparation
i a= 75 I-IA
mode
Furthermore, surface defects were found to enhance the rate of the CL variation with the irradiation time as shown in Fig. 4. In fact, the change is faster in mechanically polished than in the case of cleaved, chemically etched CdS single crystals or evaporated CdS layers.
E= 10 keV TIK
( HlNUTE
)
Fig. 2. CL time dependence of the edge emission in single crystal CdS with chemical etching in HCL (10 s), for 35 keV and different beam diameters d (pm).
4. Discussion
I
012
3456 TIHE
78 ~WIJTE
)
Fig. 1. CL time dependence of the edge emission in single crystal CdS with chemical etching in HCL (10 s), for 10 keV and different beam diameters d (pm).
The CL kinetics in alkali halides which have been previously reported in the literature, were attributed to the non-radiative recombination at defects which are created by the electronic irradiation [l]. The authors of Ref. [2] have observed the CL kinetics of CdS, ZnO and GaN:Zn crystals, using electron current density of more than 100 A/cm2, under continuous excitation at room temperature. They found no variation in ZnO CL intensity; in GaN:Zn they observed a maximum, while for CdS, the CL intensity decreased continuously with time. Among the generation of new radiative and nonradiative centres in the volume, a possible degradation under a local temperature action has been proposed. Using a 2 keV and 10 mA/cm’ electron irradiation, the authors of Refs. [3,4] have observed a continuous quenching with time of the edge CL-intensity in CdS single crystal. They found that this effect is related to the presence of water vapour. In any case, such quenching must be due to an electron beam induced increase in the non-radiative surface recombination velocity, for both the edge and the defect emission.
S. Achour et al. / Materials Science and Engineering B42 (1996) 289-292
Our results lead to the conclusion that the injection level play a crucial role in the on going processes under the beam excitation. The possible local temperature effect which has been suggested in Ref. [2] cannot be taken into consideration in the present work, since the experiments have been carried out in the frame of the non-heating regime (low current density). In addition, the sensitivity of the CL kinetics to the surface preparation mode supports an interpretation which relates the observed effects to the modification of the surface under the beam excitation. Following a suggestion of the authors of Ref. [S], the availability of holes in n-type CdS crystal is the limiting step for oxygen adsorption to occur during band gap illumination [5]. Based on this assumption, the CL variation dependence on beam voltage could thus be explained as follows. At low voltages, the availability of holes resulting from pair generation is considerable in the surface region. Therefore, the electron stimulated adsorptiondesorption is more important, leading to the rapid CL variation which is observed. At high accelerating voltages, the electron-hole pairs are generated deeply in the bulk. Accordingly, the adsorption-desorption rate must decrease. This results in a slow CL-intensity variation with time
'2
35
keV
d=
20
pT4
ia=
15 nA
Fig. 3. CL time dependence of different materials for edge and defect emission. (a) Chemical polishing in 49:11H,PO,:HNO, at 60°C of (11 l)Te-doped GaAs, (b) undoped ZnO ceramic, (c) evaporated CdS film and (d, e) mechanically polished CdS single crystal.
291
E= 10
keV
d=
40
typ
15
nA
ia=
1
. I c
1 I
0
1
234567
(MIHl)TE ) TM Fig. 4. Edge emission intensity vs. irradiation time for different surface treatments of samples: (a) chemical etching in HCL (10 s), (b) evaporated layer and (c) mechanical polishing.
(compare Figs. 1 and 2). This is also supported by the observed variation as a function of beam diameter (Figs. 1 and 2). On the other hand, we have found that if the electron beam is turned off before the CL variation beian to decrease and thereafter turned on after some resting time (15 min or more), the CL intensity will tend to recover its initial value before irradiation (Fig. 5). These observations are in accordance with a desorption effect during the first stage of CL change. Nevertheless, once the CL intensity began to decrease, the beam turning on and off did not remarkably lead to the above reversible phenomenon. The emission persisted in decreasing from its value or less (perhaps, because of the well known memory effect in the adsorption theory) suggesting a stimulated adsorption of the residual gas during this variation stage. Thus, the process can be explained as follows. At the beginning of the electron irradiation, a stimulated desorption (or cleaning) of the surface occurs. This is manifested by the enhancement of the integrated intensity. After that desorption, many active adsorbed centres would be released and more stable adsorbent species began to accumulate on the irradiated surface: carbon contamination due to the re-
292
S. A&our
et al. 1 Materials
Science
E= 15 keV
and Engineering
342 (1996)
289-292
5. Conclusion
The major finding of this work is the general increase of the CL intensity at the beginning of irradiation when low current densities are used. The CL time dependence is closely related to surface defects and surface contaminants as well as to the beam generation conditions. The increasing part of the’ CL variation with time can be more probably attributed to the contaminant desorption, while the decreasing part may be mainly due to the residual gas adsorption and/or carbon contamination. The general observed temporary low excitation electron induced desorption can find a practical application in-situ device processing. This can be done by cleaning the surface during the appropriate time, which was just necessary to reach the CL maximum change.
Fig. 5. Behaviour of the edge emission as a function of irradiation time in chemically etched CdS single crystai: (b) response from the same area corresponding to the (a) curve, but after the beam was off during 1 h.
maining vacuum can appear clearly as dark spots in the optical microscope if a long exposure time and a high current density are used.
[I] A. Nouailhat, G. Guillot and P. Pinard, J. Lumb~ex., 5 (1972) 218. [2] G.V. Saparin, S.K. Obyden, M.V. Chukikev and S.I. Popov, J. Luminesc.,
31-32
(1972)
684.
[3] R.P. Holmstrom, J. Lagowski and H.C. Gates, Sur$ Sci., 100 (1972) L467. [4] M. Lichtensteeiger, C. Webb and J. Lagowski, Surj Sci., 97 (1980) L375. [5] M. Lichtensreeiger and C. Webb, Srrrf, Sci., 154 (1985) 455.