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3. Il'inskij A. V., Ivanova N. E., Shadrin E. B, Jutkina N. L. Patent № MPK 7G 01 N 27/02, A 61 V 5/05 «Ustrojstvo dlja opredelenija jelektricheskih parametrov tverdyh i zhidkih sred».
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7. Barman S. R., Shanthi N., Shukla A. K., Sama D. D. Order-disorder and electronic transitions in Ag2+aS single crystals, studied by photoemission spectroscopy // Phys. Rev. B. 1995. V. 53. № 7. P. 3746-3751.
8. Phillips J. C. The microdomain hypothesis and dual phases in solid electrolytes // Electrochomica Acta. 1977. V. 22. P. 709-712.
9. Kobayashi M. Electronic structure of superionic conductors // Physics of Solid State Ionics. 2006. P. 1-15.
P. Seregin, A. Dashina, A. Nikolaeva, T.Rabchanova
A STUDY OF IRON IMPURITY ATO MS IN GAAS AND GAP APPLYING THE METHOD OF EMISSION MOSSBAUER SPECTROSCOPY
Mossbauer emission spectroscopy of the 57Co(57mFe) isotope is used to find the dependence of the charge state of Fe impurity atoms in GaAs and GaP on the Fermi level position in the band gap. The iron atoms substitute gallium atoms and have a tetrahedral surrounding. The electron configuration for a neutral state of these centres Fe0 is 3d54s°'52pL56 (in GaAs) or 3d54s°'79p2'37 (in GaP), while in the ionized state the population of the 3d shell of iron is increased (3d6). A value EFe - ECo = -(0,016 ± 0,003) eV has been obtained for GaAs and EFe -Eco = -(0,035 ± 0,003) eVfor GaP. The process of fast electron exchange via the valence band between the Fe0 and Fe centres at 295 К is observed in slightly overcompensated GaAs and GaP.
Keywords: impurity atoms, electron exchange, Mossbauer emission spectroscopy.
П. П. Серегин, А. Ю. Дашина, А. В. Николаева, Т. Ю. Рабчанова
ИССЛЕДОВАНИЕ ПРИМЕСНЫХ АТОМОВ ЖЕЛЕЗА В GaAs И GaP
МЕТОДОМ ЭМИССИОННОЙ МЁССБАУЭРОВСКОЙ СПЕКТРОСКОПИИ
Эмиссионная мессбауэровская спектроскопия использована для обнаружения зависимости зарядового состояния примесных атомов Fe в GaAs и GaP от положения уровня Ферми в запрещенной зоне. Атомы железа замещают атомы галлия и имеют тетреаэдри-ческое окружение. Электронная конфигурация нейтрального состояния центров Fe0 3d54s0'52p156 (в GaAs) или 3d54s0'79p2'37 (в GaP), тогда как в ионизованном состоянии засе-
ленность 3d оболочки железа возрастает (3d6). Значение EFe - ECo = -(0.016 ± 0.003) эВ получено для GaAs и EFe - ECo = -(0.035 ± 0.003) эВ для GaP. Процесс быстрого электронного обмена через валентную зону между центрами Fe0 и Fe при 295 К наблюдался для слабо перекомпенсированных GaAs и GaP.
Ключевые слова: примесные атомы, электронный обмен, эмиссионная мессбау-эровская спектроскопия.
1. Introduction
The Mossbauer effect provides a perspective method for the study of the state of impurity atoms in solids [1]. The parameters of the Mossbauer spectra make it possible to define the charge state of impurity atoms, their electronic structure, their position in the lattice, the symmetry of the local surroundings, and the formation of various associates of the impurity atoms with lattice defects.
In this study, the potentials of the emission Mossbauer spectroscopy of the 57Co(57mFe) isotope are used to identify neutral and ionized states of Fe impurity atoms in the bulk regions of GaAs and GaP, as well as to observe fast electron exchange between neutral and ionized Fe centers in the bulk region.
2. Experimental Procedure
The Mossbauer spectra were recorded at 80 and 295 К. As a conventional absorbent, we
2 57
used K4Fe(CN)6.3H2O with a surface density of 0.05 mg/sm with respect to the Fe isotope. The apparatus width of the spectral line was G = (0.26 ± 0.02) mm/s. The isomer shifts 5 are given relative to K4Fe(CN)6.3H2O. The error of the isomer shift measurement was ± 0.01 mm/s, that of the linewidth G measurement ± 0.02 mm/s.
The starting materials were GaAs 8p = 2.2 x 1018 sm 3, n = 9.0 x 1017 sm 3, n = 5.0 x 1016 sm-3 at 295 K) and GaP (p = 1.0 x 101 cm 3, n = 9.0 x 1017 sm 3 and n = 5.0 x 1016 sm 3 at 295 K) single crystals. Zinc was the acceptor impurity and tellurium was the donor one.
Cobalt was electrolytically deposited on the sample surface from an ammonium solution of
57
the noncarrying preparation CoCl2. The diffusion process was held in evacuated quartz ampoules at 1050°C (GaAs) and 1150°C (GaP). After the diffusion process the samples were treated
57
with a mixture of HNO3 and HCl (to remove the remaining Co that had not diffused in) and then ~ 40 p,m thick surface layer was removed. The samples prepared in this way were used as the Mossbauer sources.
3. Experimental Results and Discussion
3.1. The electron configuration of the Fe centre
In the emission variant of Mossbauer spectroscopy isotope 57Co is introduced into the sample, the radioactive transformation of 57Co (electron capture) results in the Mossbauer level 57mFe. Cobalt and iron create deep acceptor levels in GaAs and GaP [2].
When analyzing experimental results, one should bear in mind that NCo >> NFe (Nc0 and NFe are the concentrations of Co and Fe impurity, respectively); hence, the position of the Fermi level in the band gap is governed by the concentrations of Co impurity atoms and dopant atoms, as well as by the type of their electric activity (Zn and Te are used as dopants to fabricate p- and n-type GaAs and GaP, respectively).
Let the starting sample be Te-doped. Then, if NTe > NCo (where NTe is the tellurium concentration) the Fermi level is at the bottom of the conduction band (at 295 to 80 K) and the iron centres are ionized Fe-. If the starting sample is zinc-doped, the Fermi level will be at the top of the valence band (at 295 to 80 K) and the iron centres will be not ionized in this case Fe0. In principle, the Mossbauer spectra for the two states Fe- and Fe0 must have different isomer shifts.
1.5 -1.0 -0.5 0 0.5
Velocity, mm/s
Fig. 1. Emission Mossbauer spectra of 57Co(57mFe) in GaAs and GaP at 295 K
In order to prove this, Mossbauer spectra of 57mFe in GaAs were recorded for two differ-
17 -3
ent samples with NCo =1.0 x 10 sm , NTe =
9.0 x 1017 sm-3 and NCo = 1.0 x 1017 sm-3, NZn 18 —3
= 2.2 x 10 sm . For this purpose the diffusion was carried out for 24 h. The Mossbauer spectra at 295 K are shown in fig. 1, and the results of their treatment are given in table 1. As it is seen, both, the «-type and the p-type spectra consist of single rather broadened lines (G =
0.50 mm/s). The isomer shift values for n- and p-type samples are different. It is evident that the isomer shift of the 57mFe Mossbauer spectra in p-type GaAs corresponds to the non-ionized centres of iron Fe0, while for the n-type samples it corresponds to the ionized Fe- centres. The absence of the quadrupole splitting in the
Mossbauer spectra suggests cubic symmetry of the local surroundings of the iron atoms.
Table 1
The isomer shifts of the emission Mossbauer spectra of 57Co(57mFe) in GaAs and GaP
Matrix n, p (sm-3) Nco (sm-3) 5 (295 K) (mm/s) 5 (80 K) (mm/s)
GaAs n = 9.0 X 1017 1017 0.632 0.639
GaAs p = 2.2 x 1017 1017 0.381 0.385
GaP n = 9.0 x 1017 1017 0.610 0.615
GaP p = 1.0 X 1018 1017 0.220 0.240
The authors of [3] who have studied the state of iron atoms in GaAs by EPR measurements, concluded that iron substitutes for gallium and its electron configuration in the p-type samples is 3d5. The isomer shift of the Mossbauer spectra of 57mFe inp-GaAs samples differs from that assigned to a pure electron 3d5 configuration (according to [4] 5 = 1.08 mm/s for 3d5). In other words, the electron configuration of the (Fe)0 centre is not 3d5. Probably, substituting for gallium atoms in GaAs, the iron atoms make sp3-hybrid bonds. The presence of the valence 4sp electrons in addition to the 3d5 shell on iron atoms results in the increase of the electron density at the 57Fe nuclei, so that the electron configuration of the Fe0 centre is 3d54sxp3x If one neglects the effect
57
of 4p electrons on the total electron density at Fe nuclei and compares the experimental value 5 for the Fe0 centre in GaAs with that for iron ions having an electron configuration 3d5, one may find the number of 4s electrons. Indeed, the isomer shift of the Mossbauer spectrum of Fe0 relative to that of iron with 3d5 electron configuration may be given as follows:
S=a(|¥(0)f -|¥(0)|2 ) = aA|¥(0)|:
(1)
where a is a term which depends on the nucleus parameters, |^(0)[2 and | |^(0)|2are the total
electron densities at the 57Fe nuclei for the electron configurations 3d5 and Fe0.
I |2
To find A|Y(0)| one must know the calibration constant a. The most reliable value of a is
-3 0
[4]: a = -0.23ao mm/s. Using it we may find the electron configuration of the Fe centre to be
3d54s°'52pL56.
In order to prove this, Mossbauer spectra of 57mFe in GaP were recorded for two different samples with NCo = 1.0 x 1017 sm-3, NTe = 1.0 x 1018 sm-3 and NCo = 1.0 x 1017 sm-3, NZn = 9.0
18 -3
x 10 sm (for this purpose the diffusion was carried out for 24 h). The Mossbauer spectra at 295 K are shown in figs. 2 and 3, and the results of their treatment are given in table 1. For the p-type samples and for the n-type samples the Mossbauer spectra are single lines, the transition from (Fe)0 to (Fe)- is followed by the increase of the isomer shift (see table 1). The electron con-
0 5 0 79 2 37
figuration of the p-type samples (Fe centre) is 3d 4s ' p ' (the calculation procedure is the same as for GaAs).
Fig. 2. Emission Mossbauer spectra of 57Co(57mFe) in GaAs at 295 K
Fig. 3. Emission Mossbauer spectra of 57Co(57mFe) in GaP at 295 K
The decrease of the electron density at 57mFe nuclei after the transition from Fe0 to Fe- in GaAs and GaP are explained by the increase of the iron 3d shell population (3d5 ^ 3d6) following the ionization. However, if one assumes that the population of the iron valence 4sp shells of the Fe- is just the same as that of the Fe0 centre, then the expected isomer shift of the Mossbauer spectra of Fe- in GaAs must be 1.05 mm/s and 0.95 mm/s for GaP, while the experimental values are ~ 0.63 to 0.61 mm/s for the isomer shift of the Fe- centre. To reconcile the calculated and the experimental values of the Mossbauer isomer shifts of Fe- in GaAs and GaP we must assume that the sixth 3d electron on the Fe- centre occupies the atomic orbital 3d with a radius a little bit larger than that observed for a free ion. This results in less value of the isomer shift for the Fe- centre.
Thus, according to the Mossbauer data, the transition from Fe0 to Fe- results in a change in the iron 3d shell population. This is in full agreement with the EPR data of [3], where an EPR spectrum was observed only for the non-ionized centres of iron Fe0 (the electron configuration is 3d5), while for the ionized centres Fe- (the electron configuration is 3d6) no spectrum was observed even at 1.3 K due to the partial removal of the spin degeneracy of the 5D state in a tetrahedral field.
3.2. The electron exchange between Fe- and Fe
We have considered two extreme cases of the Fermi level position - one at the bottom of the conduction band and the other at the top of the valence band. Fe- and Fe0 correspond to these Fermi level positions. But of most interest is the case when the Fermi level is near the iron level and a partial iron centre ionization must be observed. There are two possible cases.
1. An average state will be observed in the Mossbauer spectrum, corresponding to fast electron exchange between Fe0 and Fe-.
2. The Mossbauer spectrum will correspond to the two iron states Fe0 and Fe-.
Let us discuss in details the conditions for\of a Mossbauer study of the charge exchange of the iron impurity centres. Evidently, for the Fermi level passing over the iron level at the quite low temperature (~ 295 K) it is necessary to obtain a slightly overcompensated semiconductor (NCo > NTe), so that the Fermi level appears to be bound to the cobalt level ECo:
Ц = ECo - kT ln
Pc
NCo - NTe - P + П ' NTe + P - П
iECo - kT ln
R Nco - NTe
NTe
(2)
where pCo is the degeneracy factor of the cobalt level.
Equation (2) is valid if the concentration of free carriers can be neglected both compared with the difference NCo - NTe and with 2 NTe. If the Fermi level is defined in this way, the equilibrium concentrations of the ionized Fe- and non-ionized Fe0 iron centres are defined as follows:
Nf- = -
Fe
Nf.
1 + ^
PFe
Pco NCo - NTe - P + П
NTe + P - П
exp
EFe - ECo
kT
(3)
NCo - NTe - P + П exp | EFe - ECo
exp
N = N eFe NTe + P - n V kT J (4)
Fe” Fe Pco Nco - NTe - P + n( EFe - Ec
1 Co----------------Te r-exp
PFe NTe + P - n I kT
where EFe is the iron level energy and pFe its degeneracy factor.
Equations (3) and (4) are valid if the iron concentration is low enough not to affect the Fermi level position (NFe << NCo, NTe, NCo - NTe) which is the case when the Mossbauer spectra are observed.
From (3) and (4) one can easily obtain
P = NFe0 = pco NCo - NTe - p + n exp ( EFe - ECo ^Fe- PFe NTe ■ P
NCo - NTe - P + n ^ NCo - N
NFe- PFe NTe + P - П V kT / (5)
where S =
NTe + P - n N.
Te
P
Co
It is readily seen from this equation that in order to observe partial ionization of the iron centres in the Mossbauer spectra one must change the NCo and Nxe ratio to fit S ~ 1. The pattern of the Mossbauer spectrum with S ~ 1 will depend on the relation between the time of Fe- □ Fe0
electron exchange and the lifetime of the Mossbauer level of 57mFe (t0 ~ 10-7 s). Two extreme cases are conceivable:
1. The time of the electron exchange is much smaller than to. In this case the Mossbauer spectrum will be a single line with an isomer shift given by
5„- + P5.
5 =
Fe- Fe0
P + 1 (6)
with SFe0 and Spe- being the isomer shifts of the Mossbauer spectra corresponding to Fe0 and Fe-.
2. The time of the electron exchange is much larger than t0. As far as we use the emission variant of the spectroscopy, the electron capture results at first in the creation of multiply charged iron ions which turn to the Fe0 state in a time much less than t0 and then slowly, during a time interval characteristic of the electron exchange, an equilibrium between Fe0 and Fe- is established. At the moment t0 the equilibrium is not yet established and the Mossbauer spectrum must correspond to the state Fe0 only.
In order to observe the charge exchange process (i.e. to obtain a sample with S ~ 1) the diffusion was carried out into GaAs and GaP with n = 5 x 1016 sm-3 during 5 min. After the diffusion successive layers were removed from the sample by grinding, the cobalt concentration in the layers was 1.5 x 1017, 8 x x 1016, 5 x 1016 sm-3 for GaAs and 2.0 x 1017, 1.0 x 1017, and 5.0 x 1016 sm-3 for GaP. The Mossbauer spectra at 295 K of all the layers were single rather broadened lines to correspond to centers Fe0 (see fig. 3), their isomer shifts regularly decreased as the ratio (NCo - Nie)/NTe increased (see table 2), i.e. the picture is a characteristic of fast electron exchange between the centres Fe0 and Fe-. Figure 4 shows the calculated dependence P(S),
R ( e - E ^
which corresponds to the value of RcrexP[ F\T c° J = 0.53 ± 0.03 for GaAs and 0.25 ± 0.03 for
PCo
GaP. Assuming that p— ~ 1 (since this value cannot differ much from unity), we obtain EFe -
P Fe
ECo= -(0.016 ± 0.003) eV for GaAs and EFe - ECo = -(0.035 ± 0.003) eV for GaP.
Table 2
The isomer shifts of the emission Mossbauer spectra of 57Co(57mFe) in GaAs and GaP with various cobalt concentrations
Matrix Nre (sm-3) Nco (sm-3) 5 (295 K) (mm/s) 5 (80 K) (mm/s)
GaAs 5.0 x 1016 1.5 x 1017 0.500 0.62
GaAs 5.0 x 1016 8.0 x 1016 0.562 0.63
GaAs 5.0 x 1016 5.0 x 1016 0.630 0.64
GaP 5.0 x 1016 2.0 x 1017 0.440 0.610
GaP 5.0 x 1016 1.0 x 1017 0.515 0.615
GaP 5.0 x 1016 5.0 x 1016 0.610 0.615
Р Г 1
0.8
0,6
0,4 0,2 0
Fig. 4. Dependence of P on S for GaAs and GaP The time of the electron exchange between Fe_ and Fe0 in GaP and GaAs is, according to
_7
Mossbauer data, much less than 10 s. In the view of an acceptor character of the iron centres, the electron exchange process may be treated as a hole capture at the charged centre Fe_ and a subsequent removal of the hole to the valence band. Then the time of the exchange may be evaluated from the hole capture rate
— = paV, (7)
V
wherep is the hole concentration, Vthe thermal velocity, and a their capture cross-section at the Fe centre.
15 3 7
At 295 K p = 10 sm" (according to Hall measurements), V = 10 sm/s, and using the value
a ~ 10_13 sm2 (the lower limit for Coulomb capture centres) we obtain « rFe0 «10-9 5, i.e. in a
time interval to ~ 10 charge exchanges of the iron centre take place and an "average" iron state is observed in the Mossbauer spectrum.
At 80 K the Fermi level shifts (see equation (2)) and all the iron atoms are in an ionized state and a single line is observed in the Mossbauer spectrum, corresponding to Fe_ only (see table 2).
4. Summary
The iron impurity atom state in GaAs and GaP was studied by emission Mossbauer spectroscopy. The isomer shifts of the iron Mossbauer spectra in the p-type samples correspond to neutral centres Fe0 with electron configuration 3d54s052p156 in GaAs and 3d54s0'79p2'37 in GaP. For «-type samples the isomer shifts move towards positive velocities and correspond to ionized centres Fe with six electrons on the 3d shell of iron.
For slightly overcompensated GaP samples at 295 K the process of fast electron exchange via the valence band between the centres Fe0 and Fe_ was observed, the corresponding spectrum is a single line with an isomer shift depending on the Fermi level position. A value EFe _ ECo = _(0.016 ± 0.003) eV has been obtained for GaAs and EFe _ ECo = _(0.035 ± 0.003) eV for GaP.
In principle, the possibility to define the position of the impurity levels in the energy gap of semiconductors is noted.
REFERENCES
1. Bordovsky G Ä., Nemov S. Ä., Marchenko Ä. V, Seregin P P. Semiconductors. 46. 3 (2012).
2. MilnesA. G Deep Impurities in Semiconductors. John Wiley and Sons, New York. 1973.
3. Wit M. de and Estle T. L. Phys. Rev. 132, 195 (1963).
4. Gütlich P., Bill E., Trautwein A. X. Mössbauer Spectroscopy and Transition Metal Chemistry. Fundamentals and Applications. Springer. London, 2011.
