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P-type InP grown by liquid phase epitaxy from melts with rare earth admixtures
Materials Science and Engineering B91– 92 (2002) 38 – 42 www.elsevier.com/locate/mseb
P-type InP grown by liquid phase epitaxy from melts with rare earth admixtures K. Zdansky *, O. Prochazkova, J. Zavadil, J. Novotny Institute of Radio Engineering and Electronics, ASCR, Chaberska 57, 18251 Prague 8, Czech Republic
Abstract InP single crystal layers were grown by liquid phase epitaxy (LPE) on semi-insulating InP with various rare earth elements added to the melt. The layers were characterized by temperature dependent Hall measurements and low temperature photo-luminescence spectroscopy. The work is focused on studying p-type InP grown with Tb and Yb admixtures. The dominant acceptor in the case of Tb was identified as Mn on the In site. In the case of Yb the dominant acceptor was identified as isoelectronic Yb on the In site subjected to a strong electron-lattice interaction. © 2002 Elsevier Science B.V. All rights reserved. Keywords: InP; Rare earth; Hall effect; Luminescence
1. Introduction Indium phosphide is a semiconductor of a wide interest due to its potentiality for fabrication of highspeed electronic devices. Indium phosphide doped with rare earth (RE) elements is a promising material for optical communications due to its potential for producing temperature stable light sources. Another significant property of RE elements is their high reactivity, which is helpful in the preparation of indium phosphide of higher purity by small additions of RE elements in the growth processes. It was recognized a long time ago that admixture of RE elements into the molten solution for growing InP by liquid phase epitaxy (LPE) can reduce the background donor concentration by several orders of magnitude [1]. This is because RE form stable compounds with residual impurities that are insoluble in the indium melt thus preventing their incorporation into the grown layers. Donor impurities are preferentially gettered, acceptor impurities are also gettered but to a lesser extent. It was found that in the case of some RE elements, a high purity n-type InP can be grown when a smaller amount of the RE was added to the melt, while with a larger amount of the RE the conductivity of InP can be converted into p-type. This be* Corresponding author. Tel.: + 420-2-688-1804; fax: +420-2-6880222. E-mail address: [email protected] (K. Zdansky).
haviour has been reported for the case of Pr [2,3], Dy [4] and Yb [5,6]. Ytterbium is probably the only RE element which can be introduced into the InP lattice during the LPE growth. P-type InP layers doped with Yb were obtained which lead the authors to the suggestion that Yb might form the acceptor explaining the p-type behaviour [5]. Furthermore, theoretical calculations [7] have shown that Yb may act as a shallow or moderately deep acceptor in InP. However, there was no other experimental verification of this apart from the fact that crystals grown by LPE with Yb admixture in the melt were usually p-type. It was shown later that in p-type MOCVD grown InP [8] and in synthesis grown InP [9], the Yb introduced an acceptor-like level at 30 meV below the conduction band edge. In order to explain the origin of this level, it was proposed that Yb behaves like an isovalent trap [8]. According to this hypothesis Yb can also create a hole trap in p-type InP which was confirmed experimentally [10]. In this paper we report on preparation and characterization of InP crystal layers grown by LPE with various RE elements added to the melt. It will be shown that some RE elements give always n-type InP and others give n-type InP when a smaller content of RE is put to the melt and p-type InP with a larger content. Different purification efficiency of the grown InP from donors and acceptors caused by different RE elements will be documented even when no RE is introduced into the
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 9 6 2 - X
K. Zdansky et al. / Materials Science and Engineering B91–92 (2002) 38–42
InP lattice. When a RE element is introduced into the lattice, it can contribute directly as a shallow donor or a shallow acceptor to the conductivity of the grown InP. This paper is focused on investigation of p-type InP obtained by growing with some RE elements, and in particular on the mechanism leading to the change from n-type to p-type InP due to RE admixtures.
2. Experimental The single crystal layers of InP were grown on (100) oriented semi-insulating (SI) substrates of InP:Fe by the LPE technique [3]. Growth melts consisted of 6 N purity indium and n-type InP crystals of electron concentration 1016 cm − 3. Various RE elements were added to the melts for the purpose to grow InP layers of higher purity. Purifying effects of Er, Ho, Nd, Yb, Tb and Pr were studied. The concentration of the RE addition was systematically varied from zero to 0.3 wt.%. With greater RE admixtures layers with bad morphology were obtained. The Hall coefficient and resistivity measurements were made over a wide temperature range. A computer controlled apparatus with high-impedance inputs and a switchbox for van der Pauw measurements was designed to allow the current source and current sink to be individually applied to any sample contact. The error voltages are eliminated by taking eight dc measurements of the Hall voltage at each temperature with two directions of the magnetic field. The computer control is designed around the standard IEEE-488 interface bus. The set-up is equipped with a close-cycle helium cryogenic system that enables to work in the temperature range 6.5– 320 K. To fit between magnet poles the head of the cryogenic system has been adapted with a protrusive copper sample holder closely
Fig. 1. Hole concentration and mobility of two p-type InP layers grown by LPE with Tb admixture (triangles — sample TbI5M; circles — sample TbI10) and of InP doped with Mn (stars).
39
surrounded by a screening and an outside cover. The holder can accommodate plate samples up to the size of 11× 11 mm2. Two separate silicon diodes are used as thermometers for the temperature control and for the temperature measurement. The control thermometer is a part of the original commercial cryogenic system and is kept outside of the magnetic field. The measurement thermometer has been arranged in the copper holder close to the sample and is read only when the magnetic field is off. For measurements at higher temperatures (up to 450 K) a separate temperature controlled system was used. Electric contacts on p-type InP were prepared at room temperature by rubbing liquid gallium–indium alloy with a zinc rod followed by soldering indium metal and applying an electric spark to break the potential barrier. The ohmic nature of the contacts was tested by measuring current–voltage characteristics. The size of the contacts was less than 0.5 mm in diameter, on a typical sample of 10× 10 mm2 in size. Photoluminescence spectra were taken at various temperatures. The spectrometer consists of an optical cryostat, a monochromator and a detector part. The optical cryostat with a closed cycle helium refrigeration and an automatic temperature controller is used. It enables measurements in the temperature range 3.6– 300 K. The 1-m focal length monochromator with the cooled Ge detector or cooled S1 photo-multiplier enables sensitive and high resolution measurements in the spectral range 400–1800 nm, by using the lock-in technique and the computer controlled data collection. Excitation was provided by a He–Ne or an Ar ion laser and the level of excitation was changed by using neutral density filters.
3. Results and discussion Results of the Hall measurements of p-type InP layers grown with Tb admixture in the melt are shown in Fig. 1 that shows curves of the hole concentration p and the hole mobility vp as a function of the reciprocal temperature 1/T. The logarithmic plots of p show straight lines in the range of several decades of the concentration. The lines are nearly identical for the two samples taken from two different growth runs of InP layers with Tb admixture in the melt. The binding energy of the dominant acceptor determined from the slope of the straight lines and corrected by the T 3/2 term is equal to 0.22 eV. This value is close to the binding energy of Ge acceptor, 2109 20 meV [11] and to the binding energy of Mn acceptor, 0.23 eV [12]. Both these elements are possible candidates for being the dominant acceptor in the measured samples and they can be hardly distinguished on the basis of temperature dependent Hall measurements alone.
40
K. Zdansky et al. / Materials Science and Engineering B91–92 (2002) 38–42
Fig. 2. PL spectra of p-type InP grown by LPE with Tb admixture showing a characteristic group of three peaks due to Mn doping with the zero phonon line at 1.184 eV (curve marked by Tb) and of p-type InP grown by LPE with Pr admixture with a peak at 1.1952 due to Ge doping (curve marked by Pr).
Fig. 3. Hole concentration and mobility of p-type InP grown by LPE with Yb admixture.
Fig. 4. PL spectrum of p-type InP grown by LPE with Yb admixture showing characteristic lines of Yb inner 4f shell transitions.
More information can be obtained by photoluminescence (PL) spectroscopy. The characteristic PL band of the Mn acceptor is the group of three peaks at 1.184
eV, 1.145 and 1.107 eV, which were interpreted as zero phonon line and one- and two-phonon replicas [12]. The Ge acceptor can be characterized by a peak at 1.1952 eV, which was observed in this laboratory on InP layers grown by LPE with Pr admixture [3]. This Ge related peak is not present in the PL spectrum of InP grown with Tb admixture while the Mn related PL band with three distinct peaks has been clearly observed, as shown in Fig. 2. The PL spectrum with the Ge related peak measured on InP grown with Pr admixture is also plotted in Fig. 2, to show the difference from the spectrum with the Mn related band measured in InP grown with Tb admixture. On the basis of this finding, we believe that the dominant acceptor in InP grown with Tb admixture can be identified as Mn while the one in InP grown with Pr as Ge in agreement with our previous conclusion [3]. Because the PL spectrum of Mn has been observed also in InP layers grown without any RE, the Mn in InP grown with Tb comes most likely from In used for preparing the growth melt. We believe that this is also the source of Ge found in InP grown with Pr. Temperature dependent Hall measurements of p type InP layers grown with Yb admixture have been performed in the range from room temperature to about 35 K. Below this temperature the conductivity of the sample changes very slowly so that the temperature equilibrium cannot be reached in a time of hours. A slow decay of the conductivity starts to be observed at about 60 K. When the temperature of the sample, measured by thermometers in the cryostat, is stabilized within 0.05 K the decay of conductivity lasting from minutes at 60 K to an hour at 35 K is monitored. The InP samples approach a metastable state at lower temperatures below 35 K and results of Hall measurements are not reliable any more. Typical temperature dependence of the hole concentration p and of the hole mobility is shown in Fig. 3. The curve for p reveals a straight line at low temperatures with the data giving the binding energy of the dominant acceptor equal to 40 meV, after the T 3/2 correction. We believe that the dominant acceptor is the Yb3 + ion incorporated into the InP lattice for the following reasons: (i) Inner 4f shell radiation transitions of Yb3 + have been observed by PL spectroscopy in InP grown with Yb admixture. This is a direct proof of Yb ions being incorporated into the host lattice. The typical measured PL spectrum is shown in Fig. 4. The spectrum is identical with the one measured on Yb implanted InP [13], which has been interpreted as due to Yb in In substitutional site with cubic symmetry. (ii) The concentration of the dominant acceptors in p-type InP increases with the increase of the Yb content in the melt. This has been monitored by measurements of capacitance-voltage characteris-
K. Zdansky et al. / Materials Science and Engineering B91–92 (2002) 38–42
tics of mercury Schottky diodes. The dependence of the acceptor concentration as a function of the Yb content is shown in Fig. 5. (iii) Slow decay of conductivity when the sample is cooled to temperatures below 60 K shows a small and temperature dependent capture rate of free holes on the dominant acceptors. This can be explained by a potential barrier to be overcome by a hole in the valence band edge to get to the acceptor energy level. Such potential barrier can be formed by a strong electron– lattice interaction. Any strong electron– lattice interaction cannot appear at shallow acceptors with electronic states split of the valence band edge. This excludes atoms of the II and IV group of the periodic table like Mg, Zn or C as possible dominant acceptors. On the other hand the Yb can be subjected to a strong electron– lattice interaction because its atomic radius is much larger than the atomic radius of In in the host lattice. Such difference of atomic radii will cause local lattice strain leading to a strong electron–lattice interaction. (iv) The hole trap attributed to Yb with the activation energy 50 meV has been observed by admittance spectroscopy [10]. To explain its origin, it was proposed that Yb acts as an isoelectronic trap in InP causing a large local lattice deformation in the InP lattice due to the large size of Yb or due to the large difference between electro-negativities of Yb and In. The activation energy of this hole trap is close to the binding energy of the dominant acceptor in our InP samples grown with Yb admixture. In fact, the height of the potential barrier is given by the difference between the activation energy of the Yb hole trap (50 meV) and its binding energy (40 meV) and is equal to
41
10 meV. This value is in a good agreement with the observed long time decay of conductivity at temperatures below 60 K.
4. Conclusions InP layers have been grown with RE content in the melt systematically varied from zero to 0.3 wt.% and the purifying effect of various RE has been studied, in particular of Er, Ho, Nd, Yb, Tb and Pr. N-type InP layers with a reduced concentration of electrons have been grown when a small admixture of RE has been used, below 0.1 wt.%. This is the case for all kinds of RE named above. However, various RE give various results when higher admixtures are applied; some of them give always n-type InP, in particular Er, Ho and Nd, and the others give p-type InP, in particular Yb, Tb and Pr. This results in different purification efficiency of the grown InP from donors and acceptors caused by different RE. Thus, Er, Ho and Nd purify InP from both donors and acceptors to the same extent. On the other hand Tb and Pr purify InP more significantly from donors than from acceptors. With the help of temperature dependent Hall measurements and PL spectroscopy the nature of the dominant acceptor in p-type InP has been identified. Dominant acceptors differ in p-InP layers purified by Pr or Tb. When Pr was used, the dominant acceptor was identified as Ge [3]. When Tb was been used the acceptor was identified as Mn in the In site. In the case of Yb admixture, the Yb is introduced into the InP lattice in the form of isoelectronic impurity in the In site and acts as the dominant acceptor in the grown p-InP. This acceptor is subjected to a strong electron-lattice interaction and causes a metastable conductivity state of the InP layer at temperatures below 35 K.
Acknowledgements The authors thank V. Majerikova´ , V. Va´ vra and V. Gorodynskyy for technical assistance. The work has been supported by the Grant Agency of the Czech Republic, project no. 102/99/0341 and by the Key Project no. 7 of the Academy of Sciences of the Czech Republic.
References
Fig. 5. Dominant acceptor concentration as a function of Yb content in the melt of the p-type InP grown by LPE with Yb admixture.
[1] K.A. Gatsov, A.T. Gorelenok, S.L. Karpenko, et al., Soviet Phys. Semiconductors 17 (1983) 1373. [2] O. Prochazkova, J. Zavadil, K. Zdansky, et al., J. Electr. Eng. 50 (1999) 20. [3] K. Zdansky, J. Zavadil, O. Prochazkova, P. Gladkov, Mater. Sci. Eng. B80 (2001) 10.
42
K. Zdansky et al. / Materials Science and Engineering B91–92 (2002) 38–42
[4] I.M. Tiginyanu, V.V. Ursaki, B. Podor, L. Csontos, V.P. Shontya, Proceedings of the 8th International Conference on Indium Phospide and Related Materials, Schwabisch-Gmund, Germany, 21 – 25 April 1996, New York, USA, IEEE, 1996, p. 602. [5] W. Korber, J. Weber, A. Hangleiter, K.W. Benz, J. Cryst. Growth 79 (1986) 741. [6] J. Novotny, O. Prochazkova, K. Zdansky, J. Zavadil, F. Srobar, Mater. Sci. Eng. B66 (1999) 58. [7] L.A. Hemstreet, in: J.H. von Bardeleben (Ed.), Material Science Forum, vols. 10 –12, Trans Tech Publication, Aedermannsdorf, Switzerland, 1986, p. 85. [8] P.S. Whitney, K. Uwal, H. Nakagome, K. Takahei, Appl. Phys.
Lett. 53 (1988) 2074. [9] B. Lambert, A. Le Corre, Y. Toudic, C. Lhomer, G. Grandpierre, M. Gauneau, J. Phys. Condens. Matter 2 (1970) 479. [10] D. Seghier, T. Benyattou, G. Bremond, F. Ducroquet, J. Gregoire, G. Guillot, C. Lhomer, B. Lambert, Y. Toudic, A. Le Corre, Appl. Phys. Lett. 60 (1992) 983. [11] A.M. White, P.J. Dean, B. Day, in: F.G. Fumi (Ed.), Proceedings of the 13th International Conference on Phys. Semicond, Typographic Marves, Rome, 1976, p. 1057. [12] L. Eaves, A.W. Smith, M.S. Skolnick, B. Cockayne, J. Appl. Phys. 53 (1982) 4955. [13] H. Ennen, U. Kaufmann, G. Pomreke, J. Schneider, J. Windscheif, A. Axman, J. Cryst. Growth 64 (1983) 165.
P-type InP grown by liquid phase epitaxy from melts with rare earth admixtures K. Zdansky *, O. Prochazkova, J. Zavadil, J. Novotny Institute of Radio Engineering and Electronics, ASCR, Chaberska 57, 18251 Prague 8, Czech Republic
Abstract InP single crystal layers were grown by liquid phase epitaxy (LPE) on semi-insulating InP with various rare earth elements added to the melt. The layers were characterized by temperature dependent Hall measurements and low temperature photo-luminescence spectroscopy. The work is focused on studying p-type InP grown with Tb and Yb admixtures. The dominant acceptor in the case of Tb was identified as Mn on the In site. In the case of Yb the dominant acceptor was identified as isoelectronic Yb on the In site subjected to a strong electron-lattice interaction. © 2002 Elsevier Science B.V. All rights reserved. Keywords: InP; Rare earth; Hall effect; Luminescence
1. Introduction Indium phosphide is a semiconductor of a wide interest due to its potentiality for fabrication of highspeed electronic devices. Indium phosphide doped with rare earth (RE) elements is a promising material for optical communications due to its potential for producing temperature stable light sources. Another significant property of RE elements is their high reactivity, which is helpful in the preparation of indium phosphide of higher purity by small additions of RE elements in the growth processes. It was recognized a long time ago that admixture of RE elements into the molten solution for growing InP by liquid phase epitaxy (LPE) can reduce the background donor concentration by several orders of magnitude [1]. This is because RE form stable compounds with residual impurities that are insoluble in the indium melt thus preventing their incorporation into the grown layers. Donor impurities are preferentially gettered, acceptor impurities are also gettered but to a lesser extent. It was found that in the case of some RE elements, a high purity n-type InP can be grown when a smaller amount of the RE was added to the melt, while with a larger amount of the RE the conductivity of InP can be converted into p-type. This be* Corresponding author. Tel.: + 420-2-688-1804; fax: +420-2-6880222. E-mail address: [email protected] (K. Zdansky).
haviour has been reported for the case of Pr [2,3], Dy [4] and Yb [5,6]. Ytterbium is probably the only RE element which can be introduced into the InP lattice during the LPE growth. P-type InP layers doped with Yb were obtained which lead the authors to the suggestion that Yb might form the acceptor explaining the p-type behaviour [5]. Furthermore, theoretical calculations [7] have shown that Yb may act as a shallow or moderately deep acceptor in InP. However, there was no other experimental verification of this apart from the fact that crystals grown by LPE with Yb admixture in the melt were usually p-type. It was shown later that in p-type MOCVD grown InP [8] and in synthesis grown InP [9], the Yb introduced an acceptor-like level at 30 meV below the conduction band edge. In order to explain the origin of this level, it was proposed that Yb behaves like an isovalent trap [8]. According to this hypothesis Yb can also create a hole trap in p-type InP which was confirmed experimentally [10]. In this paper we report on preparation and characterization of InP crystal layers grown by LPE with various RE elements added to the melt. It will be shown that some RE elements give always n-type InP and others give n-type InP when a smaller content of RE is put to the melt and p-type InP with a larger content. Different purification efficiency of the grown InP from donors and acceptors caused by different RE elements will be documented even when no RE is introduced into the
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 9 6 2 - X
K. Zdansky et al. / Materials Science and Engineering B91–92 (2002) 38–42
InP lattice. When a RE element is introduced into the lattice, it can contribute directly as a shallow donor or a shallow acceptor to the conductivity of the grown InP. This paper is focused on investigation of p-type InP obtained by growing with some RE elements, and in particular on the mechanism leading to the change from n-type to p-type InP due to RE admixtures.
2. Experimental The single crystal layers of InP were grown on (100) oriented semi-insulating (SI) substrates of InP:Fe by the LPE technique [3]. Growth melts consisted of 6 N purity indium and n-type InP crystals of electron concentration 1016 cm − 3. Various RE elements were added to the melts for the purpose to grow InP layers of higher purity. Purifying effects of Er, Ho, Nd, Yb, Tb and Pr were studied. The concentration of the RE addition was systematically varied from zero to 0.3 wt.%. With greater RE admixtures layers with bad morphology were obtained. The Hall coefficient and resistivity measurements were made over a wide temperature range. A computer controlled apparatus with high-impedance inputs and a switchbox for van der Pauw measurements was designed to allow the current source and current sink to be individually applied to any sample contact. The error voltages are eliminated by taking eight dc measurements of the Hall voltage at each temperature with two directions of the magnetic field. The computer control is designed around the standard IEEE-488 interface bus. The set-up is equipped with a close-cycle helium cryogenic system that enables to work in the temperature range 6.5– 320 K. To fit between magnet poles the head of the cryogenic system has been adapted with a protrusive copper sample holder closely
Fig. 1. Hole concentration and mobility of two p-type InP layers grown by LPE with Tb admixture (triangles — sample TbI5M; circles — sample TbI10) and of InP doped with Mn (stars).
39
surrounded by a screening and an outside cover. The holder can accommodate plate samples up to the size of 11× 11 mm2. Two separate silicon diodes are used as thermometers for the temperature control and for the temperature measurement. The control thermometer is a part of the original commercial cryogenic system and is kept outside of the magnetic field. The measurement thermometer has been arranged in the copper holder close to the sample and is read only when the magnetic field is off. For measurements at higher temperatures (up to 450 K) a separate temperature controlled system was used. Electric contacts on p-type InP were prepared at room temperature by rubbing liquid gallium–indium alloy with a zinc rod followed by soldering indium metal and applying an electric spark to break the potential barrier. The ohmic nature of the contacts was tested by measuring current–voltage characteristics. The size of the contacts was less than 0.5 mm in diameter, on a typical sample of 10× 10 mm2 in size. Photoluminescence spectra were taken at various temperatures. The spectrometer consists of an optical cryostat, a monochromator and a detector part. The optical cryostat with a closed cycle helium refrigeration and an automatic temperature controller is used. It enables measurements in the temperature range 3.6– 300 K. The 1-m focal length monochromator with the cooled Ge detector or cooled S1 photo-multiplier enables sensitive and high resolution measurements in the spectral range 400–1800 nm, by using the lock-in technique and the computer controlled data collection. Excitation was provided by a He–Ne or an Ar ion laser and the level of excitation was changed by using neutral density filters.
3. Results and discussion Results of the Hall measurements of p-type InP layers grown with Tb admixture in the melt are shown in Fig. 1 that shows curves of the hole concentration p and the hole mobility vp as a function of the reciprocal temperature 1/T. The logarithmic plots of p show straight lines in the range of several decades of the concentration. The lines are nearly identical for the two samples taken from two different growth runs of InP layers with Tb admixture in the melt. The binding energy of the dominant acceptor determined from the slope of the straight lines and corrected by the T 3/2 term is equal to 0.22 eV. This value is close to the binding energy of Ge acceptor, 2109 20 meV [11] and to the binding energy of Mn acceptor, 0.23 eV [12]. Both these elements are possible candidates for being the dominant acceptor in the measured samples and they can be hardly distinguished on the basis of temperature dependent Hall measurements alone.
40
K. Zdansky et al. / Materials Science and Engineering B91–92 (2002) 38–42
Fig. 2. PL spectra of p-type InP grown by LPE with Tb admixture showing a characteristic group of three peaks due to Mn doping with the zero phonon line at 1.184 eV (curve marked by Tb) and of p-type InP grown by LPE with Pr admixture with a peak at 1.1952 due to Ge doping (curve marked by Pr).
Fig. 3. Hole concentration and mobility of p-type InP grown by LPE with Yb admixture.
Fig. 4. PL spectrum of p-type InP grown by LPE with Yb admixture showing characteristic lines of Yb inner 4f shell transitions.
More information can be obtained by photoluminescence (PL) spectroscopy. The characteristic PL band of the Mn acceptor is the group of three peaks at 1.184
eV, 1.145 and 1.107 eV, which were interpreted as zero phonon line and one- and two-phonon replicas [12]. The Ge acceptor can be characterized by a peak at 1.1952 eV, which was observed in this laboratory on InP layers grown by LPE with Pr admixture [3]. This Ge related peak is not present in the PL spectrum of InP grown with Tb admixture while the Mn related PL band with three distinct peaks has been clearly observed, as shown in Fig. 2. The PL spectrum with the Ge related peak measured on InP grown with Pr admixture is also plotted in Fig. 2, to show the difference from the spectrum with the Mn related band measured in InP grown with Tb admixture. On the basis of this finding, we believe that the dominant acceptor in InP grown with Tb admixture can be identified as Mn while the one in InP grown with Pr as Ge in agreement with our previous conclusion [3]. Because the PL spectrum of Mn has been observed also in InP layers grown without any RE, the Mn in InP grown with Tb comes most likely from In used for preparing the growth melt. We believe that this is also the source of Ge found in InP grown with Pr. Temperature dependent Hall measurements of p type InP layers grown with Yb admixture have been performed in the range from room temperature to about 35 K. Below this temperature the conductivity of the sample changes very slowly so that the temperature equilibrium cannot be reached in a time of hours. A slow decay of the conductivity starts to be observed at about 60 K. When the temperature of the sample, measured by thermometers in the cryostat, is stabilized within 0.05 K the decay of conductivity lasting from minutes at 60 K to an hour at 35 K is monitored. The InP samples approach a metastable state at lower temperatures below 35 K and results of Hall measurements are not reliable any more. Typical temperature dependence of the hole concentration p and of the hole mobility is shown in Fig. 3. The curve for p reveals a straight line at low temperatures with the data giving the binding energy of the dominant acceptor equal to 40 meV, after the T 3/2 correction. We believe that the dominant acceptor is the Yb3 + ion incorporated into the InP lattice for the following reasons: (i) Inner 4f shell radiation transitions of Yb3 + have been observed by PL spectroscopy in InP grown with Yb admixture. This is a direct proof of Yb ions being incorporated into the host lattice. The typical measured PL spectrum is shown in Fig. 4. The spectrum is identical with the one measured on Yb implanted InP [13], which has been interpreted as due to Yb in In substitutional site with cubic symmetry. (ii) The concentration of the dominant acceptors in p-type InP increases with the increase of the Yb content in the melt. This has been monitored by measurements of capacitance-voltage characteris-
K. Zdansky et al. / Materials Science and Engineering B91–92 (2002) 38–42
tics of mercury Schottky diodes. The dependence of the acceptor concentration as a function of the Yb content is shown in Fig. 5. (iii) Slow decay of conductivity when the sample is cooled to temperatures below 60 K shows a small and temperature dependent capture rate of free holes on the dominant acceptors. This can be explained by a potential barrier to be overcome by a hole in the valence band edge to get to the acceptor energy level. Such potential barrier can be formed by a strong electron– lattice interaction. Any strong electron– lattice interaction cannot appear at shallow acceptors with electronic states split of the valence band edge. This excludes atoms of the II and IV group of the periodic table like Mg, Zn or C as possible dominant acceptors. On the other hand the Yb can be subjected to a strong electron– lattice interaction because its atomic radius is much larger than the atomic radius of In in the host lattice. Such difference of atomic radii will cause local lattice strain leading to a strong electron–lattice interaction. (iv) The hole trap attributed to Yb with the activation energy 50 meV has been observed by admittance spectroscopy [10]. To explain its origin, it was proposed that Yb acts as an isoelectronic trap in InP causing a large local lattice deformation in the InP lattice due to the large size of Yb or due to the large difference between electro-negativities of Yb and In. The activation energy of this hole trap is close to the binding energy of the dominant acceptor in our InP samples grown with Yb admixture. In fact, the height of the potential barrier is given by the difference between the activation energy of the Yb hole trap (50 meV) and its binding energy (40 meV) and is equal to
41
10 meV. This value is in a good agreement with the observed long time decay of conductivity at temperatures below 60 K.
4. Conclusions InP layers have been grown with RE content in the melt systematically varied from zero to 0.3 wt.% and the purifying effect of various RE has been studied, in particular of Er, Ho, Nd, Yb, Tb and Pr. N-type InP layers with a reduced concentration of electrons have been grown when a small admixture of RE has been used, below 0.1 wt.%. This is the case for all kinds of RE named above. However, various RE give various results when higher admixtures are applied; some of them give always n-type InP, in particular Er, Ho and Nd, and the others give p-type InP, in particular Yb, Tb and Pr. This results in different purification efficiency of the grown InP from donors and acceptors caused by different RE. Thus, Er, Ho and Nd purify InP from both donors and acceptors to the same extent. On the other hand Tb and Pr purify InP more significantly from donors than from acceptors. With the help of temperature dependent Hall measurements and PL spectroscopy the nature of the dominant acceptor in p-type InP has been identified. Dominant acceptors differ in p-InP layers purified by Pr or Tb. When Pr was used, the dominant acceptor was identified as Ge [3]. When Tb was been used the acceptor was identified as Mn in the In site. In the case of Yb admixture, the Yb is introduced into the InP lattice in the form of isoelectronic impurity in the In site and acts as the dominant acceptor in the grown p-InP. This acceptor is subjected to a strong electron-lattice interaction and causes a metastable conductivity state of the InP layer at temperatures below 35 K.
Acknowledgements The authors thank V. Majerikova´ , V. Va´ vra and V. Gorodynskyy for technical assistance. The work has been supported by the Grant Agency of the Czech Republic, project no. 102/99/0341 and by the Key Project no. 7 of the Academy of Sciences of the Czech Republic.
References
Fig. 5. Dominant acceptor concentration as a function of Yb content in the melt of the p-type InP grown by LPE with Yb admixture.
[1] K.A. Gatsov, A.T. Gorelenok, S.L. Karpenko, et al., Soviet Phys. Semiconductors 17 (1983) 1373. [2] O. Prochazkova, J. Zavadil, K. Zdansky, et al., J. Electr. Eng. 50 (1999) 20. [3] K. Zdansky, J. Zavadil, O. Prochazkova, P. Gladkov, Mater. Sci. Eng. B80 (2001) 10.
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