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Boron implantation effects on Au: GaAs Schottky barrier
Vacuum/volume Printed
in Great
37/numbers Britain
5/6/pages
415 to 417/l
Boron implantation barrier A Pbrez, P Roura, Facultat
J Esteve,
de Fisica, Universitat
H Altelarrea, de Barcelona,
0042-207X/87$3.00+ Pergamon Journals
987
effects J A Anth, Diagonal
.OO Ltd
on Au: GaAs Schottky A Cornet
645, Barcelona
and J R Morante, 08028,
Catedra
d’Electrdnica,
Spain
In this work, we analyse the use of boron implantation in order to change the barrier height of GaAs Schottky contacts. The dependence on the annealing temperature and implantation dose of the barrier height variation, well as of the diode quality factor are also reported. In both cases, the observed behaviour is related to the presence of defects created by implantation in the surface layer, and their annealing kinetics.
as
1. Introduction
3. Results and discussion
The shallow (lOCr500 A) implantation of ions of the same or different type than the basic dopant allows us, respectively, to obtain Schottky contacts with higher or lower barrier height than standard value. In the same way, the modification of the semiconductor surface by etching techniques changes the expected values of the Schottky barrier height. In the first case, the variation is attributed to the modification of the impurity profile in the region near the metal-semiconductor interface, while in the second case it is attributed to the increase of surface states due to damage. Both cases, are interesting methods to obtain, in the same chip, Schottky contacts with different barrier height values. In this work, we present the analysis of the results obtained for the modification of the barrier height in an Au:GaAs Schottky contact by boron ion-implantation.
In the GaAs lattice, boron is not electrically active, but its implantation produces lattice damage which creates defects and defect complexes. These defects are responsible for the compensation obtained in n-type substrates. The results of the C-V characteristics and luminescence spectra have been interpreted in order to study the change of the defect concentration with the distance from the surface. In the latter case, a chemical method was used to remove layers of different depths. The luminescence of different layers was compared in order to obtain information about defect profiles. The presence of peaks on these spectra, recently attributed to the double acceptor levels associated with B,, and GaAs (refs l-3) allows us to study the evolution of the defects created by the boron implantation. The qualitative behaviour of the peak heights as a function of etching depth (shown in Figure l), agrees with the
2. Technological
process and experimental details
We have used, as starting material, Bridgman n-GaAs with a free electron concentration of about 8 x 1016 cme3. In order to decrease the native electron trap concentration, the polished surface was capped with a 1000 A CVD Si,N, layer and then annealed at 870°C for 15 min. As a consequence of this treatment, the concentration of any native electron trap is reduced in a 1 pm thick layer below the surface. The sample was implanted with boron after removing the S&N, capping layer. The dose was varied from 10” to 1013 ions cm-’ having a kinetic energy of 100 keV. The annealing treatments were carried out by means of an open furnace using the close-contact technique under H, ion flux during 15 min at a temperature range between 300 and 800°C. Finally, the Schottky diodes were obtained by evaporating gold electrodes on the surface. The analysis of the implantation effects has been realised by using electrical techniques: Z-V and C-V characteristics, deep level transient spectroscopy (DLTS), isothermal transient spectroscopy (ITS); as well as using photoluminescence before the contacts were made.
LOO
*--+
IB-12
zi d
1L90eV 1511ev ILL9eV
b__ w----m=
P
“L
0
1000
2000
3000
LOO0
ETCHING
5OCO
DEPTH
6000
(A,
Figure 1. Evolution of the intensity of the maxima of the photoluminescence peaks as a function of the depth. 415
A P&ez et al: Boron implantation effects on Au: GaAs Schottky barrier implantation profile deduced from ellipsometric spectrometry results4. We can detect a high variation in the layer near the surface. The use of DLTS’ and ITS6 allowed us to obtain the spectroscopic analysis of deep levels caused by implantation. The DLTS spectra have confirmed the presence of three known levels, EL3, EL6 and EL2, and a broad peak (U-band) which does not correspond to a single level. This band has recently been attributed to the existence of interaction between EL2 and EL6 defects’. Figure 2 shows the evolution of these spectra with annealing temperature which agrees with the results found by other authors’. In addition, when the annealing temperature is higher than about 500°C the analysis at several applied biases indicates the existence of an exo-diffusion process of these defects’.
In Figures 46, we show the evolution, with annealing temperature, of the barrier height extrapolated at V= 0 V and of the quality factor of the diodes obtained from the samples implanted with boron at doses of lOto, 10” and lOi ions cm-*. We do not show the values of the 10” ions cm-’ because they are very similar to those of the 10” ions cm-‘. The higher the annealing temperature is, the better the diode quality factor. At the same time, 4, goes to 0.9 eV” (standard value for an Au:GaAs Schottky). We think that this behaviour can be explained by taking into account the annealing kinetics of the
AC/C
-I
h I
300
400
500
600
700
BOO’C
Figure 4. Evolution of the barrier height and quality factor with the annealing temperature in a sample implanted with a dose of lOLo ions cm-*.
Figure 2. Evolution of the intensity of the peaks present in the DLTS spectra as a function of the annealing temperature.
Z-V characteristics of Schottky barriers realized in the same samples, have been measured in the temperature range between 40 and 380 K. The theoretical fitting of these characteristics has been carried out assuming a dependence of the apparent barrier height, $a, with the applied bias voltage. This dependence has been found to be linear for low forward bias, and allowed us to extrapolate the values for zero bias, &a (V=O). These values increase linearly with the temperature, with a proportionality factor of about 2 x lo- 3 eV Km 1 (Figure 3). This behaviour is similar to that found by Hattori et aI”. This fact corroborates the existence of an interfacial layer of several A of thickness, which modifies the transport mechanism through the contact.
2
i I
I
400
300
I
500
105
I
600
700
BOO’C
TA
Figure 5. Evolution of the barrier height and quality factor with the
annealing temperature 10” ions cm-‘.
n
9,,(V=O)
- 06
in a sample
implanted
with
a dose
of
IB 13 7
10
IB
13/400
08-
5
2
I
4KJ
T/K
Figure 3. Barrier height at 0 V, #~a,vs measurement temperature.
416
500
600
700
Figure 6. Evolution of the barrier height and quality factor with the
annealing temperature lOI ions cm-*.
in a sample
implanted
with
a dose
of
A PBrez et a/: Boron
implantation
effects
on Au: GaAs Schottky
barrier
levels introduced by implantation as shown in Figure 2. Nevertheless, for annealing temperatures higher than about 500°C and low implantation doses (less than 1013 ions cm-‘), we observe a reduction of the apparent barrier height and a worsening of the quality factor, due to an exo-diffusion process of the EL2 defect and to the recuperation of the free carrier concentration with the annealing treatment 4,7-9. For higher implantation doses this recuperation does not take place and, consequently, the apparent barrier height does not diminish. On the other hand, the optical ITS also shows the existence of a level located at E,- 1 .l eV, whose concentration increases with the annealing temperatures, giving rise to a barrier height diminution again. The variation of the quality factor of the as-implanted samples vs the implantation dose is shown in Figure 7. It is in agreement with the theoretical model reported by Wu et a1”.
tion to become negligible. Therefore, there is a reduction of the positive charge which leads to a recuperation of the standard value. However, at temperatures higher than about 500°C a degradation takes place again. This degradation is due to the EL2 exo-diffusion mechanism, as well as to the activation of the deep levels located in the lower half of the forbidden band.
4. Conclusion The possibility of modifying Schottky barriers on n-GaAs by ion implantation is an interesting result, when viewed in the light of its application in technological processes. The dependence of the barrier height variation and of the diode quality factor on the annealing temperature and implanted dose is related to the presence of defects created by implantation in the surface layer.
Acknowledgements
6-
This work has been supported by a French-Spanish Cooperation Program, CIRT (Comissi6 Interdepartamental de Recerca i Innovaci6 Tecnolbgica) and CAICYT (Comisi6n Asesora de Investigacibn Cientifica y Tecnolbgica) project No 2340/82.
5-
4-
References
3-
10’0
IO”
IO’2
IO’3
10’4
Dose(cm-‘) Figure 7. Quality factor variation vs implantation dose. (a) Our results. (b) Theoretical model12.
These results indicate the existence of a high concentration of defects near the surface, which have been created by the ion implantation followed by the annealing process. These defects give rise to donor-like states, whose positive charge reduces the barrier height. The annealing process causes the EL6 concentra-
1 P Dansas, J appl Phys, 58, 2212 (1985). ’ S G Bishop, B V Shanabrock and W J Moore, J appl Phys, 56, 1785 (1983). 3 K R Elliot, Appl Phys Lett, 42, 274 (1983). 4 M Erman and J B Theeten, Surface Sci, 135, 353 (1983). ’ D V Lang, J appl Phys, 45, 3014 (1974). 6 J Samitier, J R Morante, L Giraudet and S Gourrier, Appl Phys Lett, 48, (28 April 1986). ’ J Samitier, J R Morante, A Cornet, A PBrez, A Herms, L Giraudet and S Gourrier, IVth Conf on Semi-Insulating Ill&V Materials, Hakone, Japan (May 1986). 8 G M Martin, E Esteve, P Langlade and S Makram-Ebeid, J appl Phys, 56, 2655 (1984). 9 S Makram-Ebeid, G Gautard, P Devillard and G M Martin, Appl Phys Lett, 40, 161 (1982). lo K Hattori, T Yamasaki, Y Uraoka and T Fujii, J appl Phys, 54, 7020 (1983). ” S M Sze, Physics of Semiconductor Deoices, Wiley, New York (1982). I2 C Y Wu, M C Chang and A J Shoy, Solid State Electron, 26,893 (1983).
417
in Great
37/numbers Britain
5/6/pages
415 to 417/l
Boron implantation barrier A Pbrez, P Roura, Facultat
J Esteve,
de Fisica, Universitat
H Altelarrea, de Barcelona,
0042-207X/87$3.00+ Pergamon Journals
987
effects J A Anth, Diagonal
.OO Ltd
on Au: GaAs Schottky A Cornet
645, Barcelona
and J R Morante, 08028,
Catedra
d’Electrdnica,
Spain
In this work, we analyse the use of boron implantation in order to change the barrier height of GaAs Schottky contacts. The dependence on the annealing temperature and implantation dose of the barrier height variation, well as of the diode quality factor are also reported. In both cases, the observed behaviour is related to the presence of defects created by implantation in the surface layer, and their annealing kinetics.
as
1. Introduction
3. Results and discussion
The shallow (lOCr500 A) implantation of ions of the same or different type than the basic dopant allows us, respectively, to obtain Schottky contacts with higher or lower barrier height than standard value. In the same way, the modification of the semiconductor surface by etching techniques changes the expected values of the Schottky barrier height. In the first case, the variation is attributed to the modification of the impurity profile in the region near the metal-semiconductor interface, while in the second case it is attributed to the increase of surface states due to damage. Both cases, are interesting methods to obtain, in the same chip, Schottky contacts with different barrier height values. In this work, we present the analysis of the results obtained for the modification of the barrier height in an Au:GaAs Schottky contact by boron ion-implantation.
In the GaAs lattice, boron is not electrically active, but its implantation produces lattice damage which creates defects and defect complexes. These defects are responsible for the compensation obtained in n-type substrates. The results of the C-V characteristics and luminescence spectra have been interpreted in order to study the change of the defect concentration with the distance from the surface. In the latter case, a chemical method was used to remove layers of different depths. The luminescence of different layers was compared in order to obtain information about defect profiles. The presence of peaks on these spectra, recently attributed to the double acceptor levels associated with B,, and GaAs (refs l-3) allows us to study the evolution of the defects created by the boron implantation. The qualitative behaviour of the peak heights as a function of etching depth (shown in Figure l), agrees with the
2. Technological
process and experimental details
We have used, as starting material, Bridgman n-GaAs with a free electron concentration of about 8 x 1016 cme3. In order to decrease the native electron trap concentration, the polished surface was capped with a 1000 A CVD Si,N, layer and then annealed at 870°C for 15 min. As a consequence of this treatment, the concentration of any native electron trap is reduced in a 1 pm thick layer below the surface. The sample was implanted with boron after removing the S&N, capping layer. The dose was varied from 10” to 1013 ions cm-’ having a kinetic energy of 100 keV. The annealing treatments were carried out by means of an open furnace using the close-contact technique under H, ion flux during 15 min at a temperature range between 300 and 800°C. Finally, the Schottky diodes were obtained by evaporating gold electrodes on the surface. The analysis of the implantation effects has been realised by using electrical techniques: Z-V and C-V characteristics, deep level transient spectroscopy (DLTS), isothermal transient spectroscopy (ITS); as well as using photoluminescence before the contacts were made.
LOO
*--+
IB-12
zi d
1L90eV 1511ev ILL9eV
b__ w----m=
P
“L
0
1000
2000
3000
LOO0
ETCHING
5OCO
DEPTH
6000
(A,
Figure 1. Evolution of the intensity of the maxima of the photoluminescence peaks as a function of the depth. 415
A P&ez et al: Boron implantation effects on Au: GaAs Schottky barrier implantation profile deduced from ellipsometric spectrometry results4. We can detect a high variation in the layer near the surface. The use of DLTS’ and ITS6 allowed us to obtain the spectroscopic analysis of deep levels caused by implantation. The DLTS spectra have confirmed the presence of three known levels, EL3, EL6 and EL2, and a broad peak (U-band) which does not correspond to a single level. This band has recently been attributed to the existence of interaction between EL2 and EL6 defects’. Figure 2 shows the evolution of these spectra with annealing temperature which agrees with the results found by other authors’. In addition, when the annealing temperature is higher than about 500°C the analysis at several applied biases indicates the existence of an exo-diffusion process of these defects’.
In Figures 46, we show the evolution, with annealing temperature, of the barrier height extrapolated at V= 0 V and of the quality factor of the diodes obtained from the samples implanted with boron at doses of lOto, 10” and lOi ions cm-*. We do not show the values of the 10” ions cm-’ because they are very similar to those of the 10” ions cm-‘. The higher the annealing temperature is, the better the diode quality factor. At the same time, 4, goes to 0.9 eV” (standard value for an Au:GaAs Schottky). We think that this behaviour can be explained by taking into account the annealing kinetics of the
AC/C
-I
h I
300
400
500
600
700
BOO’C
Figure 4. Evolution of the barrier height and quality factor with the annealing temperature in a sample implanted with a dose of lOLo ions cm-*.
Figure 2. Evolution of the intensity of the peaks present in the DLTS spectra as a function of the annealing temperature.
Z-V characteristics of Schottky barriers realized in the same samples, have been measured in the temperature range between 40 and 380 K. The theoretical fitting of these characteristics has been carried out assuming a dependence of the apparent barrier height, $a, with the applied bias voltage. This dependence has been found to be linear for low forward bias, and allowed us to extrapolate the values for zero bias, &a (V=O). These values increase linearly with the temperature, with a proportionality factor of about 2 x lo- 3 eV Km 1 (Figure 3). This behaviour is similar to that found by Hattori et aI”. This fact corroborates the existence of an interfacial layer of several A of thickness, which modifies the transport mechanism through the contact.
2
i I
I
400
300
I
500
105
I
600
700
BOO’C
TA
Figure 5. Evolution of the barrier height and quality factor with the
annealing temperature 10” ions cm-‘.
n
9,,(V=O)
- 06
in a sample
implanted
with
a dose
of
IB 13 7
10
IB
13/400
08-
5
2
I
4KJ
T/K
Figure 3. Barrier height at 0 V, #~a,vs measurement temperature.
416
500
600
700
Figure 6. Evolution of the barrier height and quality factor with the
annealing temperature lOI ions cm-*.
in a sample
implanted
with
a dose
of
A PBrez et a/: Boron
implantation
effects
on Au: GaAs Schottky
barrier
levels introduced by implantation as shown in Figure 2. Nevertheless, for annealing temperatures higher than about 500°C and low implantation doses (less than 1013 ions cm-‘), we observe a reduction of the apparent barrier height and a worsening of the quality factor, due to an exo-diffusion process of the EL2 defect and to the recuperation of the free carrier concentration with the annealing treatment 4,7-9. For higher implantation doses this recuperation does not take place and, consequently, the apparent barrier height does not diminish. On the other hand, the optical ITS also shows the existence of a level located at E,- 1 .l eV, whose concentration increases with the annealing temperatures, giving rise to a barrier height diminution again. The variation of the quality factor of the as-implanted samples vs the implantation dose is shown in Figure 7. It is in agreement with the theoretical model reported by Wu et a1”.
tion to become negligible. Therefore, there is a reduction of the positive charge which leads to a recuperation of the standard value. However, at temperatures higher than about 500°C a degradation takes place again. This degradation is due to the EL2 exo-diffusion mechanism, as well as to the activation of the deep levels located in the lower half of the forbidden band.
4. Conclusion The possibility of modifying Schottky barriers on n-GaAs by ion implantation is an interesting result, when viewed in the light of its application in technological processes. The dependence of the barrier height variation and of the diode quality factor on the annealing temperature and implanted dose is related to the presence of defects created by implantation in the surface layer.
Acknowledgements
6-
This work has been supported by a French-Spanish Cooperation Program, CIRT (Comissi6 Interdepartamental de Recerca i Innovaci6 Tecnolbgica) and CAICYT (Comisi6n Asesora de Investigacibn Cientifica y Tecnolbgica) project No 2340/82.
5-
4-
References
3-
10’0
IO”
IO’2
IO’3
10’4
Dose(cm-‘) Figure 7. Quality factor variation vs implantation dose. (a) Our results. (b) Theoretical model12.
These results indicate the existence of a high concentration of defects near the surface, which have been created by the ion implantation followed by the annealing process. These defects give rise to donor-like states, whose positive charge reduces the barrier height. The annealing process causes the EL6 concentra-
1 P Dansas, J appl Phys, 58, 2212 (1985). ’ S G Bishop, B V Shanabrock and W J Moore, J appl Phys, 56, 1785 (1983). 3 K R Elliot, Appl Phys Lett, 42, 274 (1983). 4 M Erman and J B Theeten, Surface Sci, 135, 353 (1983). ’ D V Lang, J appl Phys, 45, 3014 (1974). 6 J Samitier, J R Morante, L Giraudet and S Gourrier, Appl Phys Lett, 48, (28 April 1986). ’ J Samitier, J R Morante, A Cornet, A PBrez, A Herms, L Giraudet and S Gourrier, IVth Conf on Semi-Insulating Ill&V Materials, Hakone, Japan (May 1986). 8 G M Martin, E Esteve, P Langlade and S Makram-Ebeid, J appl Phys, 56, 2655 (1984). 9 S Makram-Ebeid, G Gautard, P Devillard and G M Martin, Appl Phys Lett, 40, 161 (1982). lo K Hattori, T Yamasaki, Y Uraoka and T Fujii, J appl Phys, 54, 7020 (1983). ” S M Sze, Physics of Semiconductor Deoices, Wiley, New York (1982). I2 C Y Wu, M C Chang and A J Shoy, Solid State Electron, 26,893 (1983).
417