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Photoelectronic properties of the GaAs:Si epitaxial layers on the GaAs substrate
46/number 5161pages 489 to 491/1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207x795 $9.50+.00
Vacuum/volume
Pergamon 0042-207x(94)001
13-8
Photoelectronic properties of the GaAs : Si epitaxial layers on the GaAs substrate 2 Czekala-Mukalled, S Kuimiriski and M TJaczaIa*, hstitute of Physics, Wroclaw, Poland *Institute of Electron Technology, Wyspianskiego 27,50-370 Wroclaw, Poland
Technica! University of Wroctaw, Technical University of Wroclaw, Wybrzeze
Surface photovoltage spectroscopy (SPS) with a modified Kelvin method and photoconductivity (PC) measurements have been carried out for epitaxial GaAs- GaAs: Si layers deposited by a MOCVD method on the semi-insulated GaAs substrate compensated with chromium. The substrate orientation was (IOU). The silicon concentration in the doped epitaxial layer amounted to 5.4 x 1016 cmp3. The measurements were carried out at a temperature of 83 K. The existence of three types of doping levels in the bulk with energies E , = 0.83 eV, E2 = 1.17 eV and E, = 1.43 eV below the bottom of the conduction band, and two types of surface leveis with energies Ef4 =0.05 eV and E, = 0.19 eV, also below the bottom of the conduction band, have been stated. The value of the surface voltage barrier V: = 0.22 eV at 83 K has been measured and the influence of the photoadsorption of the rest gases in the measuring chamber on its height has been investigated. The energy of the band gap at 83 K, equal to 1.51 eV, and its temperature coefficient M.= 3.3 x low4 eV/K, have been determined.
1. Introduction Surface photovoltage spectroscopy (SPS) is a method which enables us to obtain information on the surface states and also energies of the bulk states’. This method gives good results, especially in the case of the wide band gap semiconductor when other measuring methods fail, e.g. the field effect method. A modified SPS method’ makes it possible to investigate the temperature dependency with an accuracy of 0.1 K.
2. Experimental Investigations have been carried out for the following sample: semi-insulating GaAs/GaAs/GaAs : Si with the n-type silicon doped layer. The thickness of the intrinsic layer amounted to 0.19 pm and of the doped one 0.38 pm. The defect concentration in the substrate was less than 5 x i04 cm’. The layer was produced by the epitaxial method from metallorganic compounds in the Institute of Electron Technology, Technical University of Wroclaw. A semi-insulating GaAs single crystal compensated with chromium, with a surface orientation of (100) was used as a substrate. The growth of the epitaxial layer was carried out at atmospheric pressure. Before the beginning of the deposition, the substrates were etched in a solution with chemical composition H,SO, : H,02 : Hz0 in proportions 12 : 1 : 1 at temperatures between 40 and 60°C for 2 min. Then they were rinsed in deionized water and dried. The organic impurities were removed by rinsing in trichlorethylene and acetone. The n-type sample with a surface area of 8 x 5 mm and a thickness of 411.52 pm (41 I pm substrate, 0.14 pm buffer, 0.38
pm epitaxial layer) was placed on the cold finger, inside the measuring chamber. The SPS measurements were carried out at a temperature of 83 K, and at a pressure of lo-” Pa, using a modified Kelvin method with constant illumination and reference electrode vibration frequency of 561 f 1 Hz. A steel, gold coated strip with dimensions of 80 x 1 x 0.05 mm was used as the reference electrode, placed at a distance of about 0.2 mm from the surface of the sample being investigated. The measuring set automatically ensures a constant vibration frequency of the reference electrode with an accuracy of 1 Hz, and a constant vibration amplitude, independent of the pressure and temperature inside the measuring chamber. For the investigations, a G-60 prism and a halogen lamp as the light source were used. This is an optimal measuring set ensuring a good dispersion and a suitable spectral distribution of the light source. A typical SPS experimental curve, reduced to the characteristic of the light source obtained at 83 K is presented in Figure 1. We can observe six distinct effects on this curve. Four of them are connected with the increase in photovoltage AV, and two relate to inversion of the photovoltage. In Figure 2 are presented the curves of the photoconductivity (PC) measurements at a temperature of 83 K obtained using a Si-68 prism and a halogen lamp as the light source. A value of the surface voltage barrier equal to 0.22 eV at a temperature of 83 K was determined from the surface photovoltage saturation after illumination of the sample with the white light. Before the measurements the sample was kept in the dark for 24 h at a pressure of 10e4 Pa. In Figure 3 are presented the changes in the voltage barrier e at a temperature of 83 K as 489
Z Czekala-Mukalled
et al: Photoelectronic
properties
of GaAs
: Si 3. Discussion
Figure 1. Spectral distribution of the surface photovoltage for SiGaAs/GaAs/GaAs : Si at 83 K.
065
067 073 a78063 066095103113
1X
138
155
177
r---l--
spectroscopy
2.08
E
0.6
h tpml
~I
[WI
83K
1.9
1.8 17
1.6 1.5 1.4 13 12
Figure 2. Spectral distribution GaAs : Si at 83 K.
11
1.0
09
0.8
of photoconductivity
0.7
for SiGaAs/GaAs/
T=83~
I
I
I
I
I
I
I
1
4
8
12
16
20
24
28
32
llsl
Figure 3. Time dependence of the surface voltage barrier for semi-insu-
lating GaAs/GaAs/GaAs
: Si at 83 K.
a function of time. The sample was illuminated with white light for 5 s every 4 min. The character of the curve results from the irreversible adsorption and photoadsorption of the rest gases inside the measuring chamber. 490
The changes in photovoltage present on the SPS curves are connected with the electron transitions induced by light of a suitable energy. The values of the energies can be determined from the spectral positions of the maxima of d V,/dJ.. On the SPS curve presented in Figure 1 we observe the following effects : 1. An increase in photovoltage in the range 1.8-1.4 pm connected with the light enhanced electron transitions from the levels E, with energy of 0.83 eV to the conduction band. These levels are probably connected with Cr 3+‘2+. According to the literature3,4 they are situated 0.9 eV below the bottom of the conduction band. 2. An increase in photovoltage in the range 1.1-l .04 pm connected with the light enhanced electron transitions from the levels Ez with energy of 1.17 eV to the conduction band. They are probably connected with Cr“+:‘+ situated4 1.19 eV below the bottom of the conduction band. 3. An increase in photovoltage in the range 0.888086 pm relating to electron generation from the E3 levels with energy 1.43 eV to the conduction band. These levels can be attributed to the Au or Mn states whose energies amounted to EAu = 0.09 eV5 and E,, = 0.11 eV5 above the top of the valence band. 4. A sharp increase in photovoltage in the range 0.85WI.80 pm connected with the electron band-to-band transitions with band gap energy equal to E, = 1.5 1 eV. The process described of the electron generation from the levels Cr.“+ 3+, Cr3f,?C, Au or Mn to the conduction band result in a lowering of the surface voltage barrier. They are also observed on PC curves (Figure 2) and support a concept that they are of bulk nature. Besides the bulk processes also present on the SPS as well as on the PC curves we observe two effects visible on the SPS curve only. Therefore they are of surface character. I. A distinct inversion of the photovoltage in the range 1.04 0.89 pm relating to the electron transitions from the valence band to the levels Et4 with energy of 1.32 eV above the top of the valence band. These levels are situated slightly above (0.03 eV) the Fermi level for the surface voltage barrier e = 0.22 eV at 83 K. A condition for good isolation of the surface from the bulk is fulfilled (qV,/kT = 28) and the Et4 states are predominantly empty. 2. A slight dip at 0.85 pm (E,, = 1.46 eV) which is a typical effect of the photovoltage quenching. This effect is connected with the electron transitions from the valence band to the empty surface states lying considerably above the Fermi level with energy 0.05 eV below the bottom of the conduction band. The energetic model of the surface epitaxial layer of semiinsulating GaAs/GaAS/GaAs: Si is presented in Figure 4. The data concerning the energy levels found in the epitaxial layer are presented in Table 1. The top layer of GaAs : Si was not intentionally doped with chromium, but its diffusion coefficient in GaAs is high and therefore the chromium ions could penetrate through the thin GaAs layer into the GaAs : Si layer. The effects connected with Si doping were not observed on the SPS and PC curves. The energy of the Si donor states is 0.006 eV below the bottom of the conduction band5.6 and the electron transitions to these empty states are screened by a strong photovoltage increase connected with the band-to-band transitions. From the photovoltage measurements, the value of the temperature coefficient of the energy gap c( = 3.3 x 10m4 eV/K has
2 Czekara-Mukalledetal:
Photoelectronicpropertiesof GaAs: Si
Table 1. Characteristic of the energy states in the epitaxial layer
-.
-. 1
Et4
1.46
-. !
I
b\
i
EC Ef
'\'.._ E ,- 0.83 Et, ~-______~___________-__-_---_
l.32L ‘\
‘\.--____ ‘\ ‘.
____.____
~.__--_-_r_____
-______
E 2 -1.17 ------------
!
El E,-1.51
E ,- 1.43 ____---___-___---__
E, EZ E, E14 -55
WV)
Type
Literature data
0.83*
@+I*+ Cr4+‘3+
4
1.17* 1.43*
1.46** 1.32**
Au or Mn surface states surface states
3,4 (0.90 eV)* (1.19eV)* 6 (0.09 eV, 0. 13eV)**
* Energy below the bottom of the conduction band. ** Energy above the top of the valence band.
-Ev
References Figure 4. Energetic scheme of the surface layer at 83 K
also been determined. Based on the results of the surface photovoltage barrier at different temperatures it has been stated that beginning at 160 K, with lowering of the temperature there can be observed a strong influence of the adsorption and photoadsorption of the rest gases (especially oxygen), present in the measuring chamber, on the height of the voltage barrier.
‘H C Gatos and J Lagowski, J Vat Sci Technol, 10, 130 (1973). *S Kuiminski and A T Szaynok, Phys Stat Sol (a), 89, 623 (1985). ’ G H Stauss and J J Krebs, Inst Phys ConJ'Ser No 33a, Chapter 2,84 (1977). 4G Bremond, G Gullot, B Lambert and Y Toudic. Semi-insulating III-V Materials. Malmo (1988). ’ A G Milnes, Deep Impurities in Semiconductors. John Wiley, New York, London, Sydney, Toronto (1973). b0 Madelumg, Semiconductors Group IV Elements and III-V Compounds. Berlin, Springer Verlag (1991).
491
Vacuum/volume
Pergamon 0042-207x(94)001
13-8
Photoelectronic properties of the GaAs : Si epitaxial layers on the GaAs substrate 2 Czekala-Mukalled, S Kuimiriski and M TJaczaIa*, hstitute of Physics, Wroclaw, Poland *Institute of Electron Technology, Wyspianskiego 27,50-370 Wroclaw, Poland
Technica! University of Wroctaw, Technical University of Wroclaw, Wybrzeze
Surface photovoltage spectroscopy (SPS) with a modified Kelvin method and photoconductivity (PC) measurements have been carried out for epitaxial GaAs- GaAs: Si layers deposited by a MOCVD method on the semi-insulated GaAs substrate compensated with chromium. The substrate orientation was (IOU). The silicon concentration in the doped epitaxial layer amounted to 5.4 x 1016 cmp3. The measurements were carried out at a temperature of 83 K. The existence of three types of doping levels in the bulk with energies E , = 0.83 eV, E2 = 1.17 eV and E, = 1.43 eV below the bottom of the conduction band, and two types of surface leveis with energies Ef4 =0.05 eV and E, = 0.19 eV, also below the bottom of the conduction band, have been stated. The value of the surface voltage barrier V: = 0.22 eV at 83 K has been measured and the influence of the photoadsorption of the rest gases in the measuring chamber on its height has been investigated. The energy of the band gap at 83 K, equal to 1.51 eV, and its temperature coefficient M.= 3.3 x low4 eV/K, have been determined.
1. Introduction Surface photovoltage spectroscopy (SPS) is a method which enables us to obtain information on the surface states and also energies of the bulk states’. This method gives good results, especially in the case of the wide band gap semiconductor when other measuring methods fail, e.g. the field effect method. A modified SPS method’ makes it possible to investigate the temperature dependency with an accuracy of 0.1 K.
2. Experimental Investigations have been carried out for the following sample: semi-insulating GaAs/GaAs/GaAs : Si with the n-type silicon doped layer. The thickness of the intrinsic layer amounted to 0.19 pm and of the doped one 0.38 pm. The defect concentration in the substrate was less than 5 x i04 cm’. The layer was produced by the epitaxial method from metallorganic compounds in the Institute of Electron Technology, Technical University of Wroclaw. A semi-insulating GaAs single crystal compensated with chromium, with a surface orientation of (100) was used as a substrate. The growth of the epitaxial layer was carried out at atmospheric pressure. Before the beginning of the deposition, the substrates were etched in a solution with chemical composition H,SO, : H,02 : Hz0 in proportions 12 : 1 : 1 at temperatures between 40 and 60°C for 2 min. Then they were rinsed in deionized water and dried. The organic impurities were removed by rinsing in trichlorethylene and acetone. The n-type sample with a surface area of 8 x 5 mm and a thickness of 411.52 pm (41 I pm substrate, 0.14 pm buffer, 0.38
pm epitaxial layer) was placed on the cold finger, inside the measuring chamber. The SPS measurements were carried out at a temperature of 83 K, and at a pressure of lo-” Pa, using a modified Kelvin method with constant illumination and reference electrode vibration frequency of 561 f 1 Hz. A steel, gold coated strip with dimensions of 80 x 1 x 0.05 mm was used as the reference electrode, placed at a distance of about 0.2 mm from the surface of the sample being investigated. The measuring set automatically ensures a constant vibration frequency of the reference electrode with an accuracy of 1 Hz, and a constant vibration amplitude, independent of the pressure and temperature inside the measuring chamber. For the investigations, a G-60 prism and a halogen lamp as the light source were used. This is an optimal measuring set ensuring a good dispersion and a suitable spectral distribution of the light source. A typical SPS experimental curve, reduced to the characteristic of the light source obtained at 83 K is presented in Figure 1. We can observe six distinct effects on this curve. Four of them are connected with the increase in photovoltage AV, and two relate to inversion of the photovoltage. In Figure 2 are presented the curves of the photoconductivity (PC) measurements at a temperature of 83 K obtained using a Si-68 prism and a halogen lamp as the light source. A value of the surface voltage barrier equal to 0.22 eV at a temperature of 83 K was determined from the surface photovoltage saturation after illumination of the sample with the white light. Before the measurements the sample was kept in the dark for 24 h at a pressure of 10e4 Pa. In Figure 3 are presented the changes in the voltage barrier e at a temperature of 83 K as 489
Z Czekala-Mukalled
et al: Photoelectronic
properties
of GaAs
: Si 3. Discussion
Figure 1. Spectral distribution of the surface photovoltage for SiGaAs/GaAs/GaAs : Si at 83 K.
065
067 073 a78063 066095103113
1X
138
155
177
r---l--
spectroscopy
2.08
E
0.6
h tpml
~I
[WI
83K
1.9
1.8 17
1.6 1.5 1.4 13 12
Figure 2. Spectral distribution GaAs : Si at 83 K.
11
1.0
09
0.8
of photoconductivity
0.7
for SiGaAs/GaAs/
T=83~
I
I
I
I
I
I
I
1
4
8
12
16
20
24
28
32
llsl
Figure 3. Time dependence of the surface voltage barrier for semi-insu-
lating GaAs/GaAs/GaAs
: Si at 83 K.
a function of time. The sample was illuminated with white light for 5 s every 4 min. The character of the curve results from the irreversible adsorption and photoadsorption of the rest gases inside the measuring chamber. 490
The changes in photovoltage present on the SPS curves are connected with the electron transitions induced by light of a suitable energy. The values of the energies can be determined from the spectral positions of the maxima of d V,/dJ.. On the SPS curve presented in Figure 1 we observe the following effects : 1. An increase in photovoltage in the range 1.8-1.4 pm connected with the light enhanced electron transitions from the levels E, with energy of 0.83 eV to the conduction band. These levels are probably connected with Cr 3+‘2+. According to the literature3,4 they are situated 0.9 eV below the bottom of the conduction band. 2. An increase in photovoltage in the range 1.1-l .04 pm connected with the light enhanced electron transitions from the levels Ez with energy of 1.17 eV to the conduction band. They are probably connected with Cr“+:‘+ situated4 1.19 eV below the bottom of the conduction band. 3. An increase in photovoltage in the range 0.888086 pm relating to electron generation from the E3 levels with energy 1.43 eV to the conduction band. These levels can be attributed to the Au or Mn states whose energies amounted to EAu = 0.09 eV5 and E,, = 0.11 eV5 above the top of the valence band. 4. A sharp increase in photovoltage in the range 0.85WI.80 pm connected with the electron band-to-band transitions with band gap energy equal to E, = 1.5 1 eV. The process described of the electron generation from the levels Cr.“+ 3+, Cr3f,?C, Au or Mn to the conduction band result in a lowering of the surface voltage barrier. They are also observed on PC curves (Figure 2) and support a concept that they are of bulk nature. Besides the bulk processes also present on the SPS as well as on the PC curves we observe two effects visible on the SPS curve only. Therefore they are of surface character. I. A distinct inversion of the photovoltage in the range 1.04 0.89 pm relating to the electron transitions from the valence band to the levels Et4 with energy of 1.32 eV above the top of the valence band. These levels are situated slightly above (0.03 eV) the Fermi level for the surface voltage barrier e = 0.22 eV at 83 K. A condition for good isolation of the surface from the bulk is fulfilled (qV,/kT = 28) and the Et4 states are predominantly empty. 2. A slight dip at 0.85 pm (E,, = 1.46 eV) which is a typical effect of the photovoltage quenching. This effect is connected with the electron transitions from the valence band to the empty surface states lying considerably above the Fermi level with energy 0.05 eV below the bottom of the conduction band. The energetic model of the surface epitaxial layer of semiinsulating GaAs/GaAS/GaAs: Si is presented in Figure 4. The data concerning the energy levels found in the epitaxial layer are presented in Table 1. The top layer of GaAs : Si was not intentionally doped with chromium, but its diffusion coefficient in GaAs is high and therefore the chromium ions could penetrate through the thin GaAs layer into the GaAs : Si layer. The effects connected with Si doping were not observed on the SPS and PC curves. The energy of the Si donor states is 0.006 eV below the bottom of the conduction band5.6 and the electron transitions to these empty states are screened by a strong photovoltage increase connected with the band-to-band transitions. From the photovoltage measurements, the value of the temperature coefficient of the energy gap c( = 3.3 x 10m4 eV/K has
2 Czekara-Mukalledetal:
Photoelectronicpropertiesof GaAs: Si
Table 1. Characteristic of the energy states in the epitaxial layer
-.
-. 1
Et4
1.46
-. !
I
b\
i
EC Ef
'\'.._ E ,- 0.83 Et, ~-______~___________-__-_---_
l.32L ‘\
‘\.--____ ‘\ ‘.
____.____
~.__--_-_r_____
-______
E 2 -1.17 ------------
!
El E,-1.51
E ,- 1.43 ____---___-___---__
E, EZ E, E14 -55
WV)
Type
Literature data
0.83*
@+I*+ Cr4+‘3+
4
1.17* 1.43*
1.46** 1.32**
Au or Mn surface states surface states
3,4 (0.90 eV)* (1.19eV)* 6 (0.09 eV, 0. 13eV)**
* Energy below the bottom of the conduction band. ** Energy above the top of the valence band.
-Ev
References Figure 4. Energetic scheme of the surface layer at 83 K
also been determined. Based on the results of the surface photovoltage barrier at different temperatures it has been stated that beginning at 160 K, with lowering of the temperature there can be observed a strong influence of the adsorption and photoadsorption of the rest gases (especially oxygen), present in the measuring chamber, on the height of the voltage barrier.
‘H C Gatos and J Lagowski, J Vat Sci Technol, 10, 130 (1973). *S Kuiminski and A T Szaynok, Phys Stat Sol (a), 89, 623 (1985). ’ G H Stauss and J J Krebs, Inst Phys ConJ'Ser No 33a, Chapter 2,84 (1977). 4G Bremond, G Gullot, B Lambert and Y Toudic. Semi-insulating III-V Materials. Malmo (1988). ’ A G Milnes, Deep Impurities in Semiconductors. John Wiley, New York, London, Sydney, Toronto (1973). b0 Madelumg, Semiconductors Group IV Elements and III-V Compounds. Berlin, Springer Verlag (1991).
491