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Conversion of conduction in p-InAs by Ar+ ion implantation
480
Nuclear
CONVERSION
OF CONDUCTION
and Methods
in Physics
Research B39 (1989) 480-482 North-Holland, Amsterdam
IN p-InAs BY Ar + ION IMPLANTATION
N.N. GERASIMENKO, A.M. MYASNIKOV, and G.S. KHRIASCHEV Institute
Instruments
A.A. NESTEROV,
V.I. OBODNIKOV,
of Semiconductor Physics, Academy of Sciences of the USSR, pr. Ak. Lavrentyeva,
Novosibirsk,
L.N. SAFRONOV
630090, USSR
The electrical properties of n-layers induced by Ar+ ion implantation of p-InAs with E = 250 keV at Timp = 20 o C (D = lOI -1016 cm-‘) and T+ = 350 o C (D = 1014-3 x lOI cm-‘) were investigated. The layers were found to remain n-type during annealing up to 650°C. The charge carrier concentration was above lOI cmm3 m any case. Diodes were formed by a masked Ar+ ion implantation and had zero bias resistance lo6 D mm’. A model explaining the observed behavior is presented.
1. Introduction
3. Results
can be used to form n-type layers in InAs and to fabricate diodes with satisfactory electrical and optoelectrical properties [1,2]. P-type doping of InAs by ion implantation is very difficult due to the influence of donor centers induced by the ion implantation [2,3]. It seems reasonable to suppose that the donor centers of radiation nature may be essential to, and define the relative simplicity of, n-p junction formation by sulphur ion implantation. The purpose of our work is to measure donor center concentrations in the inert-ion implanted layers of InAs, to estimate the temperature stability of these implants and to study the possibility of n-p junction formation. Ion
implantation
of
sulphur
2. Experimental In this work Ar+ ions were implanted into (111) oriented Mn-doped p-type InAs having a carrier concentration of about 2 X 1016 cm-3 at 77 K. Ion implantation was carried out at 250 keV with doses from lOi to lOI6 cm-* at room temperature and with doses from 1014 to 3 X lOi cmm2 at 350 o C. The sample which was implanted at room temperature by Ar+ ions had an 800 A thick protective layer of Si02. The high temperature implantations were carried out without protection. The average current density was up to 0.15 PA/cm2 at 20” C and up to 15 l.tAA/cm2at 350” C. The samples implanted at room temperature were annealed at different temperatures in a nitrogen ambient for 30 min. The hot implants received no anneals. Conductivity type and the sheet carrier concentration n, were measured at 77 K by the van der Pauw technique. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Samples implanted at 20 o C had a weak dependence upon the implanted dose. For samples implanted at room temperature, n s varied from 4 x lOI cm-* to 10” cm-* by changing the dose from lOI cm-* to lOI cm-*. Fig. 1 shows the dependence of sheet electron concentration vs annealing temperautre for a dose of D = 5 X lOI4 cm-*. The same temperature dependence was observed for the other doses. Samples remained n-type for all temperatures studied. The sheet carrier concentration reached a maximum value of (2-5) x 1014 cm-* at a temperature of 35O’C for all doses. The layers formed by implantation at 350 o C had approximately the same value of n,. The results of our investigation showed that the donor concentration (N, > lOi cmp3) remained high up to 650 “C. Our data were in accordance with the results of other authors [3], who used high doses and low energies of cadmium ions in order to reveal the
1015
3
Ar:
InAs @II?
I
I
200
Fig. 1. Effect of annealing
:
Ll=5x10’tm-2
p s lo’6cm-3
1o’3oloo
E ~250 neV
300
400
500
I
600 T “c onn4
on the sheet electron concentration at 77 K for a dose of D = 5 x lOI cmm2.
N.N. Gerasimenko
et al. / Ar + implantation
Si,N,
481
into p-InAs
leakage resistance of about 70-100 MQ. These diodes have electrical properties similar to the properties of diodes fabricated by sulphur ion implantation [2].
SO,
4. Discussion The origin of p-n conversion in damaged layers of InAs is now discussed. The conversion of a p-type semiconductor to an n-type one by ion implantation is typical for narrow-gap materials such as InSb [4] and HgCdTe [5]. Usually these phenomena are explained by either preferential formation of donor defects in implanted layers or by reactions of defects with amphoteric impurities resulting in a change in the sublattice occupation [6]. An additional model for conduction conversion is proposed, the essential feature of which is a comparable concentration of donor and acceptor states induced by irradiation in the forbidden gap. In this case it is known [7] that spatial fluctuations of the potential arise and create deep (- E,) tails to the state density for electron and holes. When the effective masses of the charge carriers are comparable and the deep tails of the state density are localized (e.g. in amorphous silicon), then a semiconductor becomes an insulator. If me < M h, the holes are localized at deep states. Due to their small effective masses electrons are bound more weakly than holes and can migrate under the action of external fields inducing n-type conduction in a compensated material (fig. 4). Local disordered nonstoichiometric regions may also be the reason for fluctuations of the potential. An argument in favour of p-n conversion being due to large fluctuations of the potential but not the preferential generation of donor defects is the absence of clearly expressed stages on the curve of the isochronous annealing (fig. 1). An additional argument in favour of the proposed model comes from the results of optical measurements close to the fundamental absorption edge of InAs at hv < _Q,. These measurements show [2] the existence of
p-hAs
ti Fig. 2. Cross-section
of diode structure.
acceptor properties of cadmium in n-InAs and to promote a high level of volume impurity concentration for compensation of donor centers induced by implantation. N-layer formation by Ar+ ion implantation was used for the fabrication of the planar diode structures (fig. 2). We used the same semiconductor material as before. Selective ion implantation was carried out at an energy of 250 keV and a dose of 5 X lOI cm-’ using a layer of aluminium as a mask. The implanted area was 10P4 cm2. After implantation and removal of the aluminium mask, annealing was performed at a temperature of 350 o C for 30 min. A sandwich structure of SiO,-Si,N, was used as a cap insulator. The fabricated planar diodes had gate electrodes for parameter optimisation. Fig. 3 shows the voltage-current characteristics for optimized value of the gate voltage (VP = - 4 V). The forward current at U < 0.3 V is described by the expression I = I, exp( qU/nkT), where I, = 2 x lo-l2 A and n = 2.8. The reverse current at low voltages originates from the
1.p
InAs Ar+, E=ZSOtteV,
l-
5~lO~~ctri~
1 -
us= ov
2 - lJ,=-4v
T,,,, = 350 “C
E-I
5 = 10-Lmll~
i
ll
T=77K
Hole n-Type
Ar’
Level
Bombarded Layer
4, -Ee
Fig. 3. Characteristics of diode structure with (1) and without (2) the gate voltage (VI = - 4 v) on gate electrode
Electmn Level
at 77 K.
Fig. 4. Energy
scheme of the contact and crystal.
between
V. SEMICONDUCTORS:
disordered
GaAs.
layer
FIB
.
482
N. N. Gerasimenko
et al. / Ar + implantation
tails in the state density, which is typical of a disordered crystal [7], while a shift of the absorption edge into the short-wave band according to the Moss-Burstein effect is observed for heavy doped semiconductors. Thus, in our work, the contact between disordered layer and crystal of InAs may be interpreted as a semimetal-semiconductor barrier but not a p-n junction (fig. 4).
References [l] P.J. McNally, Radiat. Eff. 6 (1970) 149. [2] I.P. Akimchenko, E.G. Panshina, O.V. Tikhonova and E.A. Frimer, Fizika i Tekhnika Poluprovodnikov 13 (1979) 2210.
into p-InAs
E.G. Panshina, O.V. Tikhonova and E.A. [31 I.P. Akimchenko, Frimer, Kratkie Soobschenia po Fizike 7 (1980) 3. and G.A. Kachurin, Fizika i Tekhnika [41V.A. Bogatyrev Poluprovodnikov 11 (1977) 1360. Nucl. lnstr. and Meth. 209/210 (1983) [51G.L. Destefanis, 567 (Proc. 3rd lnt. Conf. Ion Beam Modification of Materials). N.N. Gerasimenko, L.V. Lezhejko, [61A.N. Blaut-Blachev, E.V. Lubopitova and V.I. Obodnikov, Fizika i Tekhnika Poluprovodnikov 14 (1980) 306. and A.L. Efros, Elektronnye Svoistva [71B.I. Shklovsky Legirovannykh Poluprovodnikov (Nauka, Moscow) in Russian.
Nuclear
CONVERSION
OF CONDUCTION
and Methods
in Physics
Research B39 (1989) 480-482 North-Holland, Amsterdam
IN p-InAs BY Ar + ION IMPLANTATION
N.N. GERASIMENKO, A.M. MYASNIKOV, and G.S. KHRIASCHEV Institute
Instruments
A.A. NESTEROV,
V.I. OBODNIKOV,
of Semiconductor Physics, Academy of Sciences of the USSR, pr. Ak. Lavrentyeva,
Novosibirsk,
L.N. SAFRONOV
630090, USSR
The electrical properties of n-layers induced by Ar+ ion implantation of p-InAs with E = 250 keV at Timp = 20 o C (D = lOI -1016 cm-‘) and T+ = 350 o C (D = 1014-3 x lOI cm-‘) were investigated. The layers were found to remain n-type during annealing up to 650°C. The charge carrier concentration was above lOI cmm3 m any case. Diodes were formed by a masked Ar+ ion implantation and had zero bias resistance lo6 D mm’. A model explaining the observed behavior is presented.
1. Introduction
3. Results
can be used to form n-type layers in InAs and to fabricate diodes with satisfactory electrical and optoelectrical properties [1,2]. P-type doping of InAs by ion implantation is very difficult due to the influence of donor centers induced by the ion implantation [2,3]. It seems reasonable to suppose that the donor centers of radiation nature may be essential to, and define the relative simplicity of, n-p junction formation by sulphur ion implantation. The purpose of our work is to measure donor center concentrations in the inert-ion implanted layers of InAs, to estimate the temperature stability of these implants and to study the possibility of n-p junction formation. Ion
implantation
of
sulphur
2. Experimental In this work Ar+ ions were implanted into (111) oriented Mn-doped p-type InAs having a carrier concentration of about 2 X 1016 cm-3 at 77 K. Ion implantation was carried out at 250 keV with doses from lOi to lOI6 cm-* at room temperature and with doses from 1014 to 3 X lOi cmm2 at 350 o C. The sample which was implanted at room temperature by Ar+ ions had an 800 A thick protective layer of Si02. The high temperature implantations were carried out without protection. The average current density was up to 0.15 PA/cm2 at 20” C and up to 15 l.tAA/cm2at 350” C. The samples implanted at room temperature were annealed at different temperatures in a nitrogen ambient for 30 min. The hot implants received no anneals. Conductivity type and the sheet carrier concentration n, were measured at 77 K by the van der Pauw technique. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
Samples implanted at 20 o C had a weak dependence upon the implanted dose. For samples implanted at room temperature, n s varied from 4 x lOI cm-* to 10” cm-* by changing the dose from lOI cm-* to lOI cm-*. Fig. 1 shows the dependence of sheet electron concentration vs annealing temperautre for a dose of D = 5 X lOI4 cm-*. The same temperature dependence was observed for the other doses. Samples remained n-type for all temperatures studied. The sheet carrier concentration reached a maximum value of (2-5) x 1014 cm-* at a temperature of 35O’C for all doses. The layers formed by implantation at 350 o C had approximately the same value of n,. The results of our investigation showed that the donor concentration (N, > lOi cmp3) remained high up to 650 “C. Our data were in accordance with the results of other authors [3], who used high doses and low energies of cadmium ions in order to reveal the
1015
3
Ar:
InAs @II?
I
I
200
Fig. 1. Effect of annealing
:
Ll=5x10’tm-2
p s lo’6cm-3
1o’3oloo
E ~250 neV
300
400
500
I
600 T “c onn4
on the sheet electron concentration at 77 K for a dose of D = 5 x lOI cmm2.
N.N. Gerasimenko
et al. / Ar + implantation
Si,N,
481
into p-InAs
leakage resistance of about 70-100 MQ. These diodes have electrical properties similar to the properties of diodes fabricated by sulphur ion implantation [2].
SO,
4. Discussion The origin of p-n conversion in damaged layers of InAs is now discussed. The conversion of a p-type semiconductor to an n-type one by ion implantation is typical for narrow-gap materials such as InSb [4] and HgCdTe [5]. Usually these phenomena are explained by either preferential formation of donor defects in implanted layers or by reactions of defects with amphoteric impurities resulting in a change in the sublattice occupation [6]. An additional model for conduction conversion is proposed, the essential feature of which is a comparable concentration of donor and acceptor states induced by irradiation in the forbidden gap. In this case it is known [7] that spatial fluctuations of the potential arise and create deep (- E,) tails to the state density for electron and holes. When the effective masses of the charge carriers are comparable and the deep tails of the state density are localized (e.g. in amorphous silicon), then a semiconductor becomes an insulator. If me < M h, the holes are localized at deep states. Due to their small effective masses electrons are bound more weakly than holes and can migrate under the action of external fields inducing n-type conduction in a compensated material (fig. 4). Local disordered nonstoichiometric regions may also be the reason for fluctuations of the potential. An argument in favour of p-n conversion being due to large fluctuations of the potential but not the preferential generation of donor defects is the absence of clearly expressed stages on the curve of the isochronous annealing (fig. 1). An additional argument in favour of the proposed model comes from the results of optical measurements close to the fundamental absorption edge of InAs at hv < _Q,. These measurements show [2] the existence of
p-hAs
ti Fig. 2. Cross-section
of diode structure.
acceptor properties of cadmium in n-InAs and to promote a high level of volume impurity concentration for compensation of donor centers induced by implantation. N-layer formation by Ar+ ion implantation was used for the fabrication of the planar diode structures (fig. 2). We used the same semiconductor material as before. Selective ion implantation was carried out at an energy of 250 keV and a dose of 5 X lOI cm-’ using a layer of aluminium as a mask. The implanted area was 10P4 cm2. After implantation and removal of the aluminium mask, annealing was performed at a temperature of 350 o C for 30 min. A sandwich structure of SiO,-Si,N, was used as a cap insulator. The fabricated planar diodes had gate electrodes for parameter optimisation. Fig. 3 shows the voltage-current characteristics for optimized value of the gate voltage (VP = - 4 V). The forward current at U < 0.3 V is described by the expression I = I, exp( qU/nkT), where I, = 2 x lo-l2 A and n = 2.8. The reverse current at low voltages originates from the
1.p
InAs Ar+, E=ZSOtteV,
l-
5~lO~~ctri~
1 -
us= ov
2 - lJ,=-4v
T,,,, = 350 “C
E-I
5 = 10-Lmll~
i
ll
T=77K
Hole n-Type
Ar’
Level
Bombarded Layer
4, -Ee
Fig. 3. Characteristics of diode structure with (1) and without (2) the gate voltage (VI = - 4 v) on gate electrode
Electmn Level
at 77 K.
Fig. 4. Energy
scheme of the contact and crystal.
between
V. SEMICONDUCTORS:
disordered
GaAs.
layer
FIB
.
482
N. N. Gerasimenko
et al. / Ar + implantation
tails in the state density, which is typical of a disordered crystal [7], while a shift of the absorption edge into the short-wave band according to the Moss-Burstein effect is observed for heavy doped semiconductors. Thus, in our work, the contact between disordered layer and crystal of InAs may be interpreted as a semimetal-semiconductor barrier but not a p-n junction (fig. 4).
References [l] P.J. McNally, Radiat. Eff. 6 (1970) 149. [2] I.P. Akimchenko, E.G. Panshina, O.V. Tikhonova and E.A. Frimer, Fizika i Tekhnika Poluprovodnikov 13 (1979) 2210.
into p-InAs
E.G. Panshina, O.V. Tikhonova and E.A. [31 I.P. Akimchenko, Frimer, Kratkie Soobschenia po Fizike 7 (1980) 3. and G.A. Kachurin, Fizika i Tekhnika [41V.A. Bogatyrev Poluprovodnikov 11 (1977) 1360. Nucl. lnstr. and Meth. 209/210 (1983) [51G.L. Destefanis, 567 (Proc. 3rd lnt. Conf. Ion Beam Modification of Materials). N.N. Gerasimenko, L.V. Lezhejko, [61A.N. Blaut-Blachev, E.V. Lubopitova and V.I. Obodnikov, Fizika i Tekhnika Poluprovodnikov 14 (1980) 306. and A.L. Efros, Elektronnye Svoistva [71B.I. Shklovsky Legirovannykh Poluprovodnikov (Nauka, Moscow) in Russian.