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The electrical properties of planar n+−p junctions in InAs produced by S+ ion implantation
Nuclear Instruments North-Holland
and Methods
in Physics Research
187
B58 (1991) 187-190
The electrical properties of planar n+-p junctions in InAs produced by St ion implantation N.N. Gerasimenko, G.L. Kuryshev, and G.S. Khryashchev
A.M. Myasnikov,
V.I. Obodnikov,
L.N. Safronov,
Institute of Semiconductor Physics, Academy of Sciences of the USSR, Siberian Branch, 630090 Novosibirsk 90, USSR Received
6 August
1990 and in revised form 6 December
1990
The electrical properties of n-type conduction layers obtained in InAs by S+ and Ar+ ion implantation were compared. The electrical activity of sulphur implanted with a dose D > 5 x 10“’ cmp2 was found to peak at 350°C annealing. The activation efficiency, which was determined by the difference of electron concentration in S+ and Ar+ implanted layers, changed from 0.3 to 0.04 with implantation doses from 5 X 1O’4 cm-* to lot6 cmm2, respectively. Planar n+ -p diodes were formed in InAs by sulphur ion implantation. The electrical properties of these diodes vs temperature were investigated for 77-200 K. It was found that the forward current of n+-p junctions has diffusion and recombination components. The values of zero bias resistance of these diodes were about lo9 Q cm-2 at 77 K. The zero bias dependence on the temperature has been described within the framework of a generation-recombination current model.
1. Introduction In the formation of doped regions by ion implantation an implant annealing is necessary. This annealing is needed in the first place for dopant activation and in the second place for the removal of radiation defects. Sometimes annealing is used for drive-in diffusion of implanted impurity and for the production of a p-n junction deeper than the region of stopping where the radiation defect concentration is high. It is known that the parameters of ion implantation and implant annealing have to be optimized to obtain the maximum efficiency of the implanted impurity. For narrow gap compound semiconductors the activation of the implanted impurity is complicated due to the electrical activity of the radiation defects induced by ion implantation. Thus Ar + ion implantation in p-type InAs results in the conversion to n-type conduction. The charge carrier concentration is above lo** cmd3 (at 77 K), remains n-type during annealing up to 700 o C, and correlates with the electrical activity of radiation defects in InAs [l]. In papers reporting on n+-p junctions produced by S+ ion implantation in InAs it is suggested that just the chemical doping of a semiconductor occurs during implant annealing [2,3]. The diode formed has rectifying 1-V characteristics that cannot be proof of chemical doping of InAs in as far as the rectifying takes place on 0168-583X/91/$03.50
Q 1991 - Elsevier Science Publishers
the barrier between the crystal and the layer disordered by ion implantation [I]. In this work a comparison of the electrical properties for InAs samples irradiated by S’ and Art ions which have almost the same masses was carried out to select the contribution of radiation defects and doping impurity in the formation of an n-type conduction layer.
2. Experimental Mn-doped p-InAs wafers having a carrier concentration of about 2 X lOI cmp3 at 77 K were divided into two groups for implantation; one group was implanted with Art ions, while the other was doped with S’ ions. Implantation was carried out at 250 keV with the average current density up to 0.15 uA,/cm2. Dose was varied from 10’s to lOI cm-‘. The wafers implanted at room temperature with Arf and S+ ions had an 800 A thick protective SiO, layer. Samples were then cut from the wafers and annealed at temperatures from 200° C to 650° C in a nitrogen ambient for 30 min. The type of conduction and the sheet carrier concentration n, were measured at 77 K as in ref. [l] by the van der Pauw technique. For the fabrication of the planar n+-p junctions by sulphur ion implantation we have used a dose of 5 X lOI cm-*. Sf ion implantation was carried out at an energy
B.V. (North-Holland)
N.N. Gerasimenko et al. / S + implantation of planar junctions in In
188
Table 1 4 3
I
The difference N, between the sheet carrier concentration in layers implanted sulphur and argon and sulphur efficiency K for different
doses. T,,, = 400 o C.
D (cm-*)
N, (cm-*)
K = NJD
5x10’4 3x1o’5 lOI
1.4x10’4 3 x1014 3.8 x lOI
0.3 0.1 0.04
4-l
Fig. 1. The cross-section of diode structures. 1 - p-type wafer, 2 - S+ ion implanted region, 3 - anode oxide, 4 - SiO,, 5 gate electrode, 6 - metallization.
of E = 250 keV through 80 x 110 urn* windows using a layer of aluminum as a mask. After implantation and removal of the aluminum mask, annealing was performed at a temperature of 450°C for 30 min. At higher temperatures arsenic was evaporated from InAs [4], which could have an effect on the properties of an insulator-semiconductor interface and on the performance of the planar n+-p junctions. Fig. 1 shows the cross-section of the fabricated diodes.
3. Results and discussion 3.1. The electrical properties of layers implanted with S + and Ar + ions The results of our investigation showed that Ar+ and S+ ion implantation led to conversion of a p-type semiconductor to an n-type one. The sheet carrier concentration in these implanted layers had no dependence on the chemical origin of ions and was in the range of
Fig. 2. The sheet carrier
concentration in layer of InAs irradia10”s as a function of annealing -*, E = 250 keV, T,,,, = 77 K.
5 X 1013-5 X 1Ol4 cm-* for all doses. At a temperature of annealing > 200 o C there took place the transformation of a set of defects induced by irradiation and a change in the sheet carrier concentration (fig. 2). At a temperature of annealing about 300” C there was a stage that was characterized by an increase of the sheet carrier concentration. This stage occurred for both implanted ions. Some defects were likely to dissociate at 400°C annealing and the chemical donors (sulphur) were shown at this stage. The sheet carrier concentration in InAs layers doped with sulphur was higher than the one in layers irradiated with Ar + ions which contained the radiation defects only. The difference of the sheet carrier concentrations reached a value of about 1.4 x lOI cm-* (at a dose of 5 X lOI cm-*) after 350°C annealing and did not change significantly at higher temperatures. It is logical to connect this difference with the dopant activation. An increase of the dose up to 10”’ cm-* (by more than an order) has resulted in slight increase of the difference between the sheet carrier concentrations in layers up to (3-4) X lOI cm-*, and the sulphur efficiency has been reduced from 0.3 at a dose of 5 x lOi cm-’ to 0.04 at a dose of 1016 cm-* (see table 1). It should be noted that the contribution of the electrically active sulphur was separated only at doses more than 5 x lOI cm-*. Therefore we cannot declare that sulphur efficiency tends to unity for lower doses, even though the high limit of the sulphur solubility in InAs (> lOI cmm3) [5] suggests that this might be so. A comparison of the profile of the charge carrier concentration and the profile of implanted sulphur showed (61 that the sulphur efficiency did not exceed - 0.1 at a dose of 5 X lOi cmm2. In this case the efficiency was estimated without subtracting the contribution from the radiation defects. Our data shows that the highest contribution of the electrically active sulphur occurs at a dose of 5 X lOI cm-*. At the lower doses conduction was caused by radiation disorder of the lattice, but at the higher doses the activation efficiency has decreased and a larger fraction of sulphur has formed defect complexes.
N.N. Gerasimenko et al. / S + implantation
ofplanarjunctions
in
In
189
Table 2 The n+-p junctions parameters vs temperature dependence 18 .
105 150 185
1.7 1.3 1.3
4.2x lo-l2 7.2~10-‘~ 4.3x10-s
1o-b-4 i
lo-e 3.2. The Z-V characteristics
of n +-p-junction
The Z-V characteristics were measured at variable voltage on the gate electrode that was shifted by a negative potential (Us) relative to the wafer. The measurements showed that the forward and reverse currents decreased with an increase of U. and saturated at Us = - 15 V within the temperature range from 105 to 185 K. For different temperatures the Z-V characteristics obtained at Us = - 15 V were presented in figs. 3 and 4. The forward regions of the Z-V characteristics on the linear stage were described by the equation: Z=Z,(exp(qU/DkT)
- I>,
(1)
where q is the charge of an electron, k Boltzmann’s constant, T the temperature. Values of B and I, vs temperature are presented in table 2. The values obtained for B showed that the forward current has diffusion and recombination components [7]. With an increase of temperature there was a rise of the diffusion component relative to recombination (/3 decreased and tended to unity). This behavior is natural for an n+-p junction with mixed conduction, as the diffusion component of I, is proportional to nf, and the recombination one is proportional to fr, (ni is intrinsic concentra-
lo-lo -
d2
-
Fig. 4. The reverse I-V characteristics of diode structure. T,,,_: (1) - 105 K, (2) - 150 K, (3) - 185 K; Us = - 15 V.
tion of charge carriers that rises with temperature and is proportional to T3/* exp( - E,/kT)) [7]. Here it is necessary to note that the value of p at 185 K was overestimated by the resistance of the wafer and the contact resistance. As seen from fig. 3 there was a series resistance in the forward Z-V characteristics at 185 K over a range of measurement. The n+-p junction breakdown was at the reverse voltage U > 2.5 V (fig. 4). Fig. 5 shows a zero bias resistance ( Rd) vs temperature dependence. Measurements were performed at bias voltage in the range from -10 to +lO mV and at a value of Ug to obtain a maximum value of R,. Zero bias resistance vs temperature has been described within
8
2 10" 10‘O lo9 lo8 10'
Fig. 3. Tbe forward I-V characteristics of diode structure. T,,,_: (1) - 105 K, (2) - 150 K, (3) - 185 K; Us = - 15 V.
5
6
7 8
9 19
lOOO/T.K-'
Fig. 5. Zero bias resistance vs temperature dependence.
N.N. Gerasimenko
190
the framework model: R,(T)
of the generation-recombination
=2kT/ql,(7-)=2ki-/q’Wn,(Z-),
et al. / S + implantation
current
(2)
where W is the width of the space charge region, r is the minority carrier lifetime. Eq. (2) was reduced from (1) by expansion in term of U at Z,, = qWni/T and Z3= 2. It is supposed for the calculation of W that the n+-p junction is sharp, the electron concentration in the ion implanted layer is significantly higher than the hole concentration (n > 1018 cmm3), and there is no profile spreading due to the small diffusivity of sulphur in InAs (lo-l4 cm2/s) [5]. By varying 7 it is established that the calculation was in agreement with experimental R, vs temperature at r - lo-’ s. It is worth noting that a description of R, vs temperature in terms of the generation-recombination current model did not conflict with a claim that the forward current had both diffusion and recombination components. At small voltage the forward current was described by a recombination component, but diffusion had an effect on current at higher voltage on n+-p junction and increased with U as fast as a recombination one ( Zdir - exp( au), Z, - exp( aU/2) [7]).
In this work a comparison is made between the electrical properties of the n-type layers formed by Art and St ion implantation in InAs. The contribution of electrically active sulphur to the n-region formation has been separated out. It is found that the electrical activ-
in In
ity of sulphur peaks at 350 o C annealing. Planar n+-p junctions were produced by sulphur ion implantation. Zero bias resistance of these diodes was about 5-7 x 10” s2 (- lo9 Q cm2) at liquid nitrogen temperature, which was of the order of R, for the p-n junction obtained in ref. [7] by a diffusion process.
Acknowledgements The authors wish to thank Mr. S.A. Sukhikh for ion implantation of wafers and Mr. A.Yu. Surtaev and Mr. V.L. Plotnikov for help in measurements.
References [l] N.N. Gerasimenko, A.M. Myasnikov,
[2] [3] [4]
4. Conclusions
of planar junctions
[5] (61
[7]
[8]
A.A. Nesterov, V.I. Obodnikov, L.N. Safronov and G.S. Khryashchev, Nucl. Instr. and Meth. B39 (1989) 480, P.J. McNally, Radiat. Eff. 6 (1970) 149. I.P. Akimchenko, E.G. Panshina, O.V. Tikhonova and E.A. Frimer, Fizika i Tekhnika Poluprovodnikov 13 (1979) 2210. R. Veresegyhazy, B. Pecz and I. Mojzcs, Phys. Status Solidi 94a (1986) Kll. E. Schillmann, Z. Naturforsch. lla (1956) 463. G.V. Kohsov, Y.V. Krutenyuk, S.V. Bologov, L.V. Boriskina, A.N. Simonov and E.A. Frimer, Poverkhnost 8 (1983) 118. S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1981) vol. 1. M.E. Greiner and C.J. Martin, Proc. SPIE 686 (1986) 34.
and Methods
in Physics Research
187
B58 (1991) 187-190
The electrical properties of planar n+-p junctions in InAs produced by St ion implantation N.N. Gerasimenko, G.L. Kuryshev, and G.S. Khryashchev
A.M. Myasnikov,
V.I. Obodnikov,
L.N. Safronov,
Institute of Semiconductor Physics, Academy of Sciences of the USSR, Siberian Branch, 630090 Novosibirsk 90, USSR Received
6 August
1990 and in revised form 6 December
1990
The electrical properties of n-type conduction layers obtained in InAs by S+ and Ar+ ion implantation were compared. The electrical activity of sulphur implanted with a dose D > 5 x 10“’ cmp2 was found to peak at 350°C annealing. The activation efficiency, which was determined by the difference of electron concentration in S+ and Ar+ implanted layers, changed from 0.3 to 0.04 with implantation doses from 5 X 1O’4 cm-* to lot6 cmm2, respectively. Planar n+ -p diodes were formed in InAs by sulphur ion implantation. The electrical properties of these diodes vs temperature were investigated for 77-200 K. It was found that the forward current of n+-p junctions has diffusion and recombination components. The values of zero bias resistance of these diodes were about lo9 Q cm-2 at 77 K. The zero bias dependence on the temperature has been described within the framework of a generation-recombination current model.
1. Introduction In the formation of doped regions by ion implantation an implant annealing is necessary. This annealing is needed in the first place for dopant activation and in the second place for the removal of radiation defects. Sometimes annealing is used for drive-in diffusion of implanted impurity and for the production of a p-n junction deeper than the region of stopping where the radiation defect concentration is high. It is known that the parameters of ion implantation and implant annealing have to be optimized to obtain the maximum efficiency of the implanted impurity. For narrow gap compound semiconductors the activation of the implanted impurity is complicated due to the electrical activity of the radiation defects induced by ion implantation. Thus Ar + ion implantation in p-type InAs results in the conversion to n-type conduction. The charge carrier concentration is above lo** cmd3 (at 77 K), remains n-type during annealing up to 700 o C, and correlates with the electrical activity of radiation defects in InAs [l]. In papers reporting on n+-p junctions produced by S+ ion implantation in InAs it is suggested that just the chemical doping of a semiconductor occurs during implant annealing [2,3]. The diode formed has rectifying 1-V characteristics that cannot be proof of chemical doping of InAs in as far as the rectifying takes place on 0168-583X/91/$03.50
Q 1991 - Elsevier Science Publishers
the barrier between the crystal and the layer disordered by ion implantation [I]. In this work a comparison of the electrical properties for InAs samples irradiated by S’ and Art ions which have almost the same masses was carried out to select the contribution of radiation defects and doping impurity in the formation of an n-type conduction layer.
2. Experimental Mn-doped p-InAs wafers having a carrier concentration of about 2 X lOI cmp3 at 77 K were divided into two groups for implantation; one group was implanted with Art ions, while the other was doped with S’ ions. Implantation was carried out at 250 keV with the average current density up to 0.15 uA,/cm2. Dose was varied from 10’s to lOI cm-‘. The wafers implanted at room temperature with Arf and S+ ions had an 800 A thick protective SiO, layer. Samples were then cut from the wafers and annealed at temperatures from 200° C to 650° C in a nitrogen ambient for 30 min. The type of conduction and the sheet carrier concentration n, were measured at 77 K as in ref. [l] by the van der Pauw technique. For the fabrication of the planar n+-p junctions by sulphur ion implantation we have used a dose of 5 X lOI cm-*. Sf ion implantation was carried out at an energy
B.V. (North-Holland)
N.N. Gerasimenko et al. / S + implantation of planar junctions in In
188
Table 1 4 3
I
The difference N, between the sheet carrier concentration in layers implanted sulphur and argon and sulphur efficiency K for different
doses. T,,, = 400 o C.
D (cm-*)
N, (cm-*)
K = NJD
5x10’4 3x1o’5 lOI
1.4x10’4 3 x1014 3.8 x lOI
0.3 0.1 0.04
4-l
Fig. 1. The cross-section of diode structures. 1 - p-type wafer, 2 - S+ ion implanted region, 3 - anode oxide, 4 - SiO,, 5 gate electrode, 6 - metallization.
of E = 250 keV through 80 x 110 urn* windows using a layer of aluminum as a mask. After implantation and removal of the aluminum mask, annealing was performed at a temperature of 450°C for 30 min. At higher temperatures arsenic was evaporated from InAs [4], which could have an effect on the properties of an insulator-semiconductor interface and on the performance of the planar n+-p junctions. Fig. 1 shows the cross-section of the fabricated diodes.
3. Results and discussion 3.1. The electrical properties of layers implanted with S + and Ar + ions The results of our investigation showed that Ar+ and S+ ion implantation led to conversion of a p-type semiconductor to an n-type one. The sheet carrier concentration in these implanted layers had no dependence on the chemical origin of ions and was in the range of
Fig. 2. The sheet carrier
concentration in layer of InAs irradia10”s as a function of annealing -*, E = 250 keV, T,,,, = 77 K.
5 X 1013-5 X 1Ol4 cm-* for all doses. At a temperature of annealing > 200 o C there took place the transformation of a set of defects induced by irradiation and a change in the sheet carrier concentration (fig. 2). At a temperature of annealing about 300” C there was a stage that was characterized by an increase of the sheet carrier concentration. This stage occurred for both implanted ions. Some defects were likely to dissociate at 400°C annealing and the chemical donors (sulphur) were shown at this stage. The sheet carrier concentration in InAs layers doped with sulphur was higher than the one in layers irradiated with Ar + ions which contained the radiation defects only. The difference of the sheet carrier concentrations reached a value of about 1.4 x lOI cm-* (at a dose of 5 X lOI cm-*) after 350°C annealing and did not change significantly at higher temperatures. It is logical to connect this difference with the dopant activation. An increase of the dose up to 10”’ cm-* (by more than an order) has resulted in slight increase of the difference between the sheet carrier concentrations in layers up to (3-4) X lOI cm-*, and the sulphur efficiency has been reduced from 0.3 at a dose of 5 x lOi cm-’ to 0.04 at a dose of 1016 cm-* (see table 1). It should be noted that the contribution of the electrically active sulphur was separated only at doses more than 5 x lOI cm-*. Therefore we cannot declare that sulphur efficiency tends to unity for lower doses, even though the high limit of the sulphur solubility in InAs (> lOI cmm3) [5] suggests that this might be so. A comparison of the profile of the charge carrier concentration and the profile of implanted sulphur showed (61 that the sulphur efficiency did not exceed - 0.1 at a dose of 5 X lOi cmm2. In this case the efficiency was estimated without subtracting the contribution from the radiation defects. Our data shows that the highest contribution of the electrically active sulphur occurs at a dose of 5 X lOI cm-*. At the lower doses conduction was caused by radiation disorder of the lattice, but at the higher doses the activation efficiency has decreased and a larger fraction of sulphur has formed defect complexes.
N.N. Gerasimenko et al. / S + implantation
ofplanarjunctions
in
In
189
Table 2 The n+-p junctions parameters vs temperature dependence 18 .
105 150 185
1.7 1.3 1.3
4.2x lo-l2 7.2~10-‘~ 4.3x10-s
1o-b-4 i
lo-e 3.2. The Z-V characteristics
of n +-p-junction
The Z-V characteristics were measured at variable voltage on the gate electrode that was shifted by a negative potential (Us) relative to the wafer. The measurements showed that the forward and reverse currents decreased with an increase of U. and saturated at Us = - 15 V within the temperature range from 105 to 185 K. For different temperatures the Z-V characteristics obtained at Us = - 15 V were presented in figs. 3 and 4. The forward regions of the Z-V characteristics on the linear stage were described by the equation: Z=Z,(exp(qU/DkT)
- I>,
(1)
where q is the charge of an electron, k Boltzmann’s constant, T the temperature. Values of B and I, vs temperature are presented in table 2. The values obtained for B showed that the forward current has diffusion and recombination components [7]. With an increase of temperature there was a rise of the diffusion component relative to recombination (/3 decreased and tended to unity). This behavior is natural for an n+-p junction with mixed conduction, as the diffusion component of I, is proportional to nf, and the recombination one is proportional to fr, (ni is intrinsic concentra-
lo-lo -
d2
-
Fig. 4. The reverse I-V characteristics of diode structure. T,,,_: (1) - 105 K, (2) - 150 K, (3) - 185 K; Us = - 15 V.
tion of charge carriers that rises with temperature and is proportional to T3/* exp( - E,/kT)) [7]. Here it is necessary to note that the value of p at 185 K was overestimated by the resistance of the wafer and the contact resistance. As seen from fig. 3 there was a series resistance in the forward Z-V characteristics at 185 K over a range of measurement. The n+-p junction breakdown was at the reverse voltage U > 2.5 V (fig. 4). Fig. 5 shows a zero bias resistance ( Rd) vs temperature dependence. Measurements were performed at bias voltage in the range from -10 to +lO mV and at a value of Ug to obtain a maximum value of R,. Zero bias resistance vs temperature has been described within
8
2 10" 10‘O lo9 lo8 10'
Fig. 3. Tbe forward I-V characteristics of diode structure. T,,,_: (1) - 105 K, (2) - 150 K, (3) - 185 K; Us = - 15 V.
5
6
7 8
9 19
lOOO/T.K-'
Fig. 5. Zero bias resistance vs temperature dependence.
N.N. Gerasimenko
190
the framework model: R,(T)
of the generation-recombination
=2kT/ql,(7-)=2ki-/q’Wn,(Z-),
et al. / S + implantation
current
(2)
where W is the width of the space charge region, r is the minority carrier lifetime. Eq. (2) was reduced from (1) by expansion in term of U at Z,, = qWni/T and Z3= 2. It is supposed for the calculation of W that the n+-p junction is sharp, the electron concentration in the ion implanted layer is significantly higher than the hole concentration (n > 1018 cmm3), and there is no profile spreading due to the small diffusivity of sulphur in InAs (lo-l4 cm2/s) [5]. By varying 7 it is established that the calculation was in agreement with experimental R, vs temperature at r - lo-’ s. It is worth noting that a description of R, vs temperature in terms of the generation-recombination current model did not conflict with a claim that the forward current had both diffusion and recombination components. At small voltage the forward current was described by a recombination component, but diffusion had an effect on current at higher voltage on n+-p junction and increased with U as fast as a recombination one ( Zdir - exp( au), Z, - exp( aU/2) [7]).
In this work a comparison is made between the electrical properties of the n-type layers formed by Art and St ion implantation in InAs. The contribution of electrically active sulphur to the n-region formation has been separated out. It is found that the electrical activ-
in In
ity of sulphur peaks at 350 o C annealing. Planar n+-p junctions were produced by sulphur ion implantation. Zero bias resistance of these diodes was about 5-7 x 10” s2 (- lo9 Q cm2) at liquid nitrogen temperature, which was of the order of R, for the p-n junction obtained in ref. [7] by a diffusion process.
Acknowledgements The authors wish to thank Mr. S.A. Sukhikh for ion implantation of wafers and Mr. A.Yu. Surtaev and Mr. V.L. Plotnikov for help in measurements.
References [l] N.N. Gerasimenko, A.M. Myasnikov,
[2] [3] [4]
4. Conclusions
of planar junctions
[5] (61
[7]
[8]
A.A. Nesterov, V.I. Obodnikov, L.N. Safronov and G.S. Khryashchev, Nucl. Instr. and Meth. B39 (1989) 480, P.J. McNally, Radiat. Eff. 6 (1970) 149. I.P. Akimchenko, E.G. Panshina, O.V. Tikhonova and E.A. Frimer, Fizika i Tekhnika Poluprovodnikov 13 (1979) 2210. R. Veresegyhazy, B. Pecz and I. Mojzcs, Phys. Status Solidi 94a (1986) Kll. E. Schillmann, Z. Naturforsch. lla (1956) 463. G.V. Kohsov, Y.V. Krutenyuk, S.V. Bologov, L.V. Boriskina, A.N. Simonov and E.A. Frimer, Poverkhnost 8 (1983) 118. S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, Chichester, Brisbane, Toronto, Singapore, 1981) vol. 1. M.E. Greiner and C.J. Martin, Proc. SPIE 686 (1986) 34.