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Resonant and deep impurity levels under hydrostatic pressure in pure n-type InAs
Physica 139 & 140B (1986) 426-428 North-Holland, Amsterdam
RESONANT AND DEEP IMPURITY LEVELS UNDER HYDROSTATIC PRESSURE IN PURE n-TYPE InAs
A. KADRI, R.L. AULOMBARD, K. ZITOUNI and L. KONCZEWICZ* Groupe d'Etude des Semiconducteurs (CNRS-L.A. 357), Universitd des Sciences et Techniques du Languedoc, 34060 Montpellier Cedex, France
Hall coefficient (RH) and electrical resistivity (Oo) were m e a s u r e d as a function of hydrostatic pressure up to 18 kbar, in the 4.2 K - 1 2 0 K temperature range, on nominally undopted n-type InAs with free carrier concentration - 2 x 10 ~6cm-3. In the 4.2-30 K range, R H and P0 versus pressure variations indicate the deionization of impurity states which are resonant in the FI~ band at normal pressure. The position and the pressure variation of the resonant impurity level are discussed. At T > 30 K, evidence is made for the existence of a donor-like impurity level lying ~ 1 0 meV below the Ftc band m i n i m u m and moving with pressure at the rate of - 1 . 8 meV/kbar with respect to this band.
1. Introduction
Possible application of indium arsenide alloy systems in microwave [1] and multiquantum well laser [2] devices has stimulated great interest in the pure InAs compound. Though many works have been devoted to the study of its properties, the origin of the always present residual im16 -3, • • • purities (N 0 - 2 x 10 cm ) in this material is still unknown. Because of its strong influence on the energy of band states and impurity states, the hydrostatic pressure can induce the redistribution of the electrons amongst these states, and allows direct investigation of their properties. We have recently investigated the electrical properties of n-type InAs under high magnetic field and hydrostatic pressure in the 2.7-8 K range [3]. We have shown that at high pressure, the magnetic freeze-out takes place into both hydrogenic and resonant impurity states. In this work, we investigate the hydrostatic pressure dependence of the low magnetic field electrical transport properties (R H and P0) of similar samples of InAs as in our previous work [3] in the 4.2-120K temperature range. The results show that the conduction is dominated in the 4.2-30 K range by scattering impurity states * High Pressure Poland.
Research
Center
(P.A.S.),
Warsaw,
resonant in the F~c band. These resonant states exhibit variations versus pressure typical of deep levels [4]. At T > 30 K, a donor-like impurity level lying in the gap becomes active.
2. Experimental methods
Two specimens for this study were cut from unintentionally doped n-type InAs provided by Sumitomo Electric Industries. At P = 0 kbar and T = 4.2 K, the free electron concentrations (n t and the Hall mobilities (/xa) were 2.1 × 1016 cmand 3.1 x 10 4 c m 2 V -1 s - t for sample 1 and 1.8 × 1016 cm-3 and 1.9 × 104 cm 2 V-~ s- 1for sample 2. In order to perform hydrostatic pressure experiments, the samples were put inside a Cu-Be clamp cell [3] where a mixture of light hydrocarbons was used as the pressure transmitting medium. The pressure cell was closed and pressurized at room temperature and then immersed in a liquid H2 cryostat placed between the pole pieces of an electromagnet. It was shown [5] that the pressure remains fully hydrostatic when the transmitting medium freezes at low temperatures. In the whole temperature range investigated, the pressure was measured by means of a calibrated InSb gauge [6]. The temperature was measured by means of a calibrated carbon resistor and a copper-constantan thermocouple.
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
A. Kadri et al. / Impurity levels under hydrostatic pressure in pure n-type l n A s
3. Results and discussion
• P= 0 k b a r oP= 10.5 • P= 13 ,P= 15 ~P= 15.4
E Typical variations of R H and P0 versus temperature are shown at different pressures in figs. 1 and 2 respectively. The R H and P0 versus 1/T variations show a strong dependence upon the pressure. Since no intrinsic behavior can be expected in the temperature range under investigation, the features of figs. 1 and 2 must be attributed to the influence of impurities. In agreement with the results that we have previously obtained from magnetic freeze-out experiments [3], the low temperature data versus pressure give strong support to a pressure induced deionization of impurity states which are resonant in the continuum of the F~c band. On the other hand, the increase with pressure the slope of RH(1/T) curves at T > 40 K suggest the existence of impurity states lying below the Fl~ band edge. Assuming an appropriate F~ band [7], and Fermi-Dirac statistics, the rise of R H and P0 beyond 13 kbar at low temperatures can be well explained by the trap-out of carriers to a single impurity level lying at (68 - 1) meV above the Ftc .,,.-....
~U E u mZ
• P= 0 k b a r o P= 10.5 -P=13 o P=15 P=I 5.4
102
, ~ ~
3.5 10 t_a,,.
310
--_.
[]o "~...~
2,
", ~ o o
,,
~
~
~ o ~
426
r 2.5 102/ 0
,
,
,
0.05
0.10
0.15
,
- ~ K -1 )
Fig. 1. Hall coefficient versus reciprocal temperature at different pressures.
U
C
v Q.
210
427
-2
~'~
0 [3
HoDA& A,~: AA A 0 0
O 0
0
Cl
0
0
• 0
• 0
• 0
• 0
• 0
•
• •
•
•
•
0
~.-o o°
-2 10
~
~A o 0
mm m mm
,m"
•
•
,j 4 10 -3
I
0
o.05
I
I
O.lO o.15
I
+(K-1
Fig, 2. Electrical resistivity versus reciprocal temperature at
different pressures. band and moving with pressure at a rate of ( 4 - 0 . 1 ) m e V / k b a r with respect to this band. This is however insufficient to explain the low pressure results ( P < 1 3 k b a r ) since this level would be too far above the Ftc band edge to induce any population changes. Two possible explanations can be considered for the results at P < 13 kbar. (i) By assuming linear pressure variations for the impurity levels, a second impurity level is needed lying at 15 meV above the bottom of the Ftc band and moving with pressure at a rate of - 0 . 2 3 m e V / k b a r with respect to this band. The existence of such an impurity level lying a few meV above the Fermi level can be related to the so-called "shallow" donors which, from magnetic freeze-out experiments, were found by Kaufman et al. [8] and by us [3] to lie in the continuum of the F~c band at low magnetic field strengths and low pressures. (ii) Based on the results of Pitt et al. [9] and EI-Sabbahy et al. [10] who found an even stronger variation at P > 20 kbar for the resonant level, an alternative explanation can be that of a single
428
A. Kadri et al. / Impurity levels under hydrostatic pressure in pure n-type InAs
resonant impurity level moving with pressure with a non-linear pressure coefficient. As shown in fig. 1, by solving the neutrality equation in the whole temperature and pressure ranges, both assumptions (i) and (ii) can give qualitative agreement with the experimental results as far as the overall shape and general features are concerned. In both cases, the rise of R H when the temperature is increased from 4.2 K up to the appearance of the Hall maximum is due to the thermal excitation of carriers from the conduction band into resonant impurity states lying a few meV above the Fermi level. At temperatures above the Hall maximum, the decrease of R H is due to the ionization of impurity states lying 10 meV below the Fie band and moving with respect to this band at a rate of 1.8 meV/ kbar. This level was also observed by Jung et al. [11] from room temperature transport experiments under pressure. Because of their fast shift with pressure with respect to the Flc band, these states are unlikely to be associated with this band, but rather behave as deep impurity states. Though the two above assumptions could describe the temperature variation of R H in the low pressure range, both of them fail to account for the results at P > 15 kbar. As suggested by the results of fig. 1, as the resonant level becomes occupied, the R H vs 1 / T curve tends to flatten out in a larger range of temperature higher than 4.2 K. An attempt to fit this behavior was made by assuming a Lorentzian distribution for the resonant impurities density of states. However, the large resonance width obtained in this way W - 20 meV supposes a strong influence of disorder and then a high degree of compensation. This is in contradiction with the rather small values of compensating acceptor concentrations needed to account for the result both here and in our previous work [3]. It is interesting to note that the same conclusion was obtained by Jung et al. [11].
We then come to the conclusion that the broadened density of states is unlikely to account for the observed results. This behavior is more probably related to the strongly localized nature of the resonant states as suggested by the fast shift with pressure that they exhibit at P > 13 kbar with respect to the Fie band from which they evolve. Further experiments to higher pressures are needed to make the point clear.
Acknowledgements The authors are grateful to Sumitomo Electric Industries for providing the samples studied in this work.
References [1] T.O. Pearsall, IEEE J. Quant. Electron. OE 16 (1980) 709. [2] Y. Suematsu, K. Iga and K. Kishina, in: GaInAsP Alloy Semiconductors, ed. T.P. Pearsall (J. Wiley, New York, 1982) ch. 14. [3] A. Kadri, R.L. Aulombard, K. Zitouni, M. Baj and L. Konczewicz, Phys. Rev. B31 (1985). [4] W. Jantsch, K. Wunstel, O. Kamagai and P. Vogl, Phys. Rev. B25 (1982) 5515. [5] M. Konczykowski, M. Baj, E. Szafarkiewicz, L. Konczewicz and S. Porowski, Proc. of the Conf. Low-Temp. High Pres. Phys. Cleveland (1977) p. 523. [6] A.M. Davidson, P. Knowles, P. Makado, R.A. Stradling, S. Porowski and Z. Wasilewski, Phys. High Magn. Field (Springer, Berlin, 1981) p. 84. [7] E.O. Kane, J. Phys. Chem. Solids 1 (1957) 249. [8] L.A. Kaufman and L.J. Neuringer, Phys. Rev. B2 (1970) 1840. [9] G.D. Pitt and M.K.R. Vyas, J. Phys. C: Solid St. Phys. 6 (1973) 274, [10] A.N. EI-Sabbahy and A.R. Adams, Int. Phys. Conf. Ser. 43 (1979) 589. [111 Y.J. Jung, B.H. Kim, H.J. Lee and J.C. Woolley, Phys. Rev. B26 (1982) 3151.
RESONANT AND DEEP IMPURITY LEVELS UNDER HYDROSTATIC PRESSURE IN PURE n-TYPE InAs
A. KADRI, R.L. AULOMBARD, K. ZITOUNI and L. KONCZEWICZ* Groupe d'Etude des Semiconducteurs (CNRS-L.A. 357), Universitd des Sciences et Techniques du Languedoc, 34060 Montpellier Cedex, France
Hall coefficient (RH) and electrical resistivity (Oo) were m e a s u r e d as a function of hydrostatic pressure up to 18 kbar, in the 4.2 K - 1 2 0 K temperature range, on nominally undopted n-type InAs with free carrier concentration - 2 x 10 ~6cm-3. In the 4.2-30 K range, R H and P0 versus pressure variations indicate the deionization of impurity states which are resonant in the FI~ band at normal pressure. The position and the pressure variation of the resonant impurity level are discussed. At T > 30 K, evidence is made for the existence of a donor-like impurity level lying ~ 1 0 meV below the Ftc band m i n i m u m and moving with pressure at the rate of - 1 . 8 meV/kbar with respect to this band.
1. Introduction
Possible application of indium arsenide alloy systems in microwave [1] and multiquantum well laser [2] devices has stimulated great interest in the pure InAs compound. Though many works have been devoted to the study of its properties, the origin of the always present residual im16 -3, • • • purities (N 0 - 2 x 10 cm ) in this material is still unknown. Because of its strong influence on the energy of band states and impurity states, the hydrostatic pressure can induce the redistribution of the electrons amongst these states, and allows direct investigation of their properties. We have recently investigated the electrical properties of n-type InAs under high magnetic field and hydrostatic pressure in the 2.7-8 K range [3]. We have shown that at high pressure, the magnetic freeze-out takes place into both hydrogenic and resonant impurity states. In this work, we investigate the hydrostatic pressure dependence of the low magnetic field electrical transport properties (R H and P0) of similar samples of InAs as in our previous work [3] in the 4.2-120K temperature range. The results show that the conduction is dominated in the 4.2-30 K range by scattering impurity states * High Pressure Poland.
Research
Center
(P.A.S.),
Warsaw,
resonant in the F~c band. These resonant states exhibit variations versus pressure typical of deep levels [4]. At T > 30 K, a donor-like impurity level lying in the gap becomes active.
2. Experimental methods
Two specimens for this study were cut from unintentionally doped n-type InAs provided by Sumitomo Electric Industries. At P = 0 kbar and T = 4.2 K, the free electron concentrations (n t and the Hall mobilities (/xa) were 2.1 × 1016 cmand 3.1 x 10 4 c m 2 V -1 s - t for sample 1 and 1.8 × 1016 cm-3 and 1.9 × 104 cm 2 V-~ s- 1for sample 2. In order to perform hydrostatic pressure experiments, the samples were put inside a Cu-Be clamp cell [3] where a mixture of light hydrocarbons was used as the pressure transmitting medium. The pressure cell was closed and pressurized at room temperature and then immersed in a liquid H2 cryostat placed between the pole pieces of an electromagnet. It was shown [5] that the pressure remains fully hydrostatic when the transmitting medium freezes at low temperatures. In the whole temperature range investigated, the pressure was measured by means of a calibrated InSb gauge [6]. The temperature was measured by means of a calibrated carbon resistor and a copper-constantan thermocouple.
0378-4363/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
A. Kadri et al. / Impurity levels under hydrostatic pressure in pure n-type l n A s
3. Results and discussion
• P= 0 k b a r oP= 10.5 • P= 13 ,P= 15 ~P= 15.4
E Typical variations of R H and P0 versus temperature are shown at different pressures in figs. 1 and 2 respectively. The R H and P0 versus 1/T variations show a strong dependence upon the pressure. Since no intrinsic behavior can be expected in the temperature range under investigation, the features of figs. 1 and 2 must be attributed to the influence of impurities. In agreement with the results that we have previously obtained from magnetic freeze-out experiments [3], the low temperature data versus pressure give strong support to a pressure induced deionization of impurity states which are resonant in the continuum of the F~c band. On the other hand, the increase with pressure the slope of RH(1/T) curves at T > 40 K suggest the existence of impurity states lying below the Fl~ band edge. Assuming an appropriate F~ band [7], and Fermi-Dirac statistics, the rise of R H and P0 beyond 13 kbar at low temperatures can be well explained by the trap-out of carriers to a single impurity level lying at (68 - 1) meV above the Ftc .,,.-....
~U E u mZ
• P= 0 k b a r o P= 10.5 -P=13 o P=15 P=I 5.4
102
, ~ ~
3.5 10 t_a,,.
310
--_.
[]o "~...~
2,
", ~ o o
,,
~
~
~ o ~
426
r 2.5 102/ 0
,
,
,
0.05
0.10
0.15
,
- ~ K -1 )
Fig. 1. Hall coefficient versus reciprocal temperature at different pressures.
U
C
v Q.
210
427
-2
~'~
0 [3
HoDA& A,~: AA A 0 0
O 0
0
Cl
0
0
• 0
• 0
• 0
• 0
• 0
•
• •
•
•
•
0
~.-o o°
-2 10
~
~A o 0
mm m mm
,m"
•
•
,j 4 10 -3
I
0
o.05
I
I
O.lO o.15
I
+(K-1
Fig, 2. Electrical resistivity versus reciprocal temperature at
different pressures. band and moving with pressure at a rate of ( 4 - 0 . 1 ) m e V / k b a r with respect to this band. This is however insufficient to explain the low pressure results ( P < 1 3 k b a r ) since this level would be too far above the Ftc band edge to induce any population changes. Two possible explanations can be considered for the results at P < 13 kbar. (i) By assuming linear pressure variations for the impurity levels, a second impurity level is needed lying at 15 meV above the bottom of the Ftc band and moving with pressure at a rate of - 0 . 2 3 m e V / k b a r with respect to this band. The existence of such an impurity level lying a few meV above the Fermi level can be related to the so-called "shallow" donors which, from magnetic freeze-out experiments, were found by Kaufman et al. [8] and by us [3] to lie in the continuum of the F~c band at low magnetic field strengths and low pressures. (ii) Based on the results of Pitt et al. [9] and EI-Sabbahy et al. [10] who found an even stronger variation at P > 20 kbar for the resonant level, an alternative explanation can be that of a single
428
A. Kadri et al. / Impurity levels under hydrostatic pressure in pure n-type InAs
resonant impurity level moving with pressure with a non-linear pressure coefficient. As shown in fig. 1, by solving the neutrality equation in the whole temperature and pressure ranges, both assumptions (i) and (ii) can give qualitative agreement with the experimental results as far as the overall shape and general features are concerned. In both cases, the rise of R H when the temperature is increased from 4.2 K up to the appearance of the Hall maximum is due to the thermal excitation of carriers from the conduction band into resonant impurity states lying a few meV above the Fermi level. At temperatures above the Hall maximum, the decrease of R H is due to the ionization of impurity states lying 10 meV below the Fie band and moving with respect to this band at a rate of 1.8 meV/ kbar. This level was also observed by Jung et al. [11] from room temperature transport experiments under pressure. Because of their fast shift with pressure with respect to the Flc band, these states are unlikely to be associated with this band, but rather behave as deep impurity states. Though the two above assumptions could describe the temperature variation of R H in the low pressure range, both of them fail to account for the results at P > 15 kbar. As suggested by the results of fig. 1, as the resonant level becomes occupied, the R H vs 1 / T curve tends to flatten out in a larger range of temperature higher than 4.2 K. An attempt to fit this behavior was made by assuming a Lorentzian distribution for the resonant impurities density of states. However, the large resonance width obtained in this way W - 20 meV supposes a strong influence of disorder and then a high degree of compensation. This is in contradiction with the rather small values of compensating acceptor concentrations needed to account for the result both here and in our previous work [3]. It is interesting to note that the same conclusion was obtained by Jung et al. [11].
We then come to the conclusion that the broadened density of states is unlikely to account for the observed results. This behavior is more probably related to the strongly localized nature of the resonant states as suggested by the fast shift with pressure that they exhibit at P > 13 kbar with respect to the Fie band from which they evolve. Further experiments to higher pressures are needed to make the point clear.
Acknowledgements The authors are grateful to Sumitomo Electric Industries for providing the samples studied in this work.
References [1] T.O. Pearsall, IEEE J. Quant. Electron. OE 16 (1980) 709. [2] Y. Suematsu, K. Iga and K. Kishina, in: GaInAsP Alloy Semiconductors, ed. T.P. Pearsall (J. Wiley, New York, 1982) ch. 14. [3] A. Kadri, R.L. Aulombard, K. Zitouni, M. Baj and L. Konczewicz, Phys. Rev. B31 (1985). [4] W. Jantsch, K. Wunstel, O. Kamagai and P. Vogl, Phys. Rev. B25 (1982) 5515. [5] M. Konczykowski, M. Baj, E. Szafarkiewicz, L. Konczewicz and S. Porowski, Proc. of the Conf. Low-Temp. High Pres. Phys. Cleveland (1977) p. 523. [6] A.M. Davidson, P. Knowles, P. Makado, R.A. Stradling, S. Porowski and Z. Wasilewski, Phys. High Magn. Field (Springer, Berlin, 1981) p. 84. [7] E.O. Kane, J. Phys. Chem. Solids 1 (1957) 249. [8] L.A. Kaufman and L.J. Neuringer, Phys. Rev. B2 (1970) 1840. [9] G.D. Pitt and M.K.R. Vyas, J. Phys. C: Solid St. Phys. 6 (1973) 274, [10] A.N. EI-Sabbahy and A.R. Adams, Int. Phys. Conf. Ser. 43 (1979) 589. [111 Y.J. Jung, B.H. Kim, H.J. Lee and J.C. Woolley, Phys. Rev. B26 (1982) 3151.