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P-type InP grown by liquid phase epitaxy with rare earths: not intentional Ge acceptor doping
Materials Science and Engineering B80 (2001) 10 – 13 www.elsevier.com/locate/mseb
P-type InP grown by liquid phase epitaxy with rare earths: not intentional Ge acceptor doping K. Zda´nsky´ *, J. Zavadil, O. Procha´zkova´, P. Gladkov Institute of Radio Engineering and Electronics AV CR, Chaberska´ 57, Prague 8, 18251, Czech Republic
Abstract InP single crystal layers were grown by liquid phase epitaxy (LPE) on semi-insulating InP:Fe substrates with praseodymium added to the melt. Room temperature Hall effect measurements revealed p-type conductivity of the layers with the hole concentration 6 ×1014 cm − 3 and mobility 150 cm2 V − 1 s − 1. By measuring temperature dependence of the hole concentration the binding energy of the dominant acceptor was determined as 223 meV. A photoluminescence line was found at 1.195 eV, close to the previously estimated no-phonon line of Ge acceptor transitions in Ge doped n-type InP. It was concluded that Ge acceptors cause the p-type conductivity of the grown layers. © 2001 Elsevier Science B.V. All rights reserved. Keywords: InP; Ge; Rare earth; Hall effect; Photoluminescence
1. Introduction High purity and controlled doping of indium phosphide single crystals are essential for successful device fabrication with this material. Doping of InP with germanium usually results in n-type conductivity [1]. It has been used in preparation of doped liquid-encapsulated Czochralski (LEC), solution grown, liquid-phaseepitaxial and vapour-phase-epitaxial materials [2–5]. However, some experiments show on amphoteric behaviour of Ge in InP; it can either substitute the group III or V sublattice and become donor or acceptor, respectively. Photoluminescence spectroscopy indicates lines attributed to recombination of excitons bound to Ge acceptors [5,6]. Low electron mobility of Ge-doped InP was explained by auto-compensation by Ge acceptors [7,8]. Recently a number of studies has been carried out to investigate the amphoteric behaviour of Ge in InP [9–13]. It has been shown that an enhancement of group IV dopant substitution into a specific sublattice in a III –V semiconductor, and the resulting changes in the electrical behaviour, can be achieved by changing the local stoichiometry of the semiconductor [14 –17]. Thus an improved n-type Ge activity in InP * Corresponding author. Tel.: +420-2-6881804; fax: + 420-26880222. E-mail address: [email protected] (K. Zda´nsky´).
has been observed [17,18] in samples co-implanted with Ge and P. Much improved p-type conductivity was found in GaAs by co-implanting Ge with C [14]. However, to the best of our knowledge no method for improving p-type activity of Ge in InP has been reported, yet. We are going to demonstrate in this paper that the p-type Ge activity in InP can be improved by adding the rare earth element Pr into the molten solution for growing InP by liquid phase epitaxy (LPE). It was already recognised a long time ago that admixture of rare earths (RE) into the molten solution for growing InP and GaAs layers can reduce the background donor concentration by several orders of magnitude [19]. This is because RE form stable compounds with residual impurities, which are insoluble in the indium melt, thus preventing their incorporation into the grown layers. Donor impurities preferentially are gettered, acceptor impurities are also gettered but to a lesser extent. When a smaller amount of RE is added to the melt, a high purity n-type InP layer can be grown while with a larger amount of RE the conductivity of InP can be converted into p-type [20]. It will be shown that RE doping of the melt containing Ge amphoteric impurity could favour the density of Ge acceptors in the grown InP layer while suppressing the density of Ge donors. In this way it could be possible to grow p-type InP with holes activated from Ge acceptors.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 5 7 4 - 2
K. Zda´nsky´ et al. / Materials Science and Engineering B80 (2001) 10–13
In this paper we report on the preparation of high purity p-type InP single crystal layers unintentionally doped with Ge acceptors grown by LPE with praseodymium admixture in the melt. Temperature dependence of resistivity and Hall coefficient, and low temperature photoluminescence spectra were measured to characterise the layers. 2. Experimental The semiconductor epitaxial layers for the measurement of the Hall coefficient and resistivity must be grown on substrates of sufficiently high resistivity not to cause shunting effect on the layer. Semi-insulating (SI) single crystals of InP doped with iron for preparing substrates were grown by Czochralski (LEC) method [21]. The substrates had resistivity larger than 107 V cm at room temperature, which permitted electrical measurements of high purity InP layers in a wide temperature range without any influence of the substrate on the result. The layers of InP were grown on (100) oriented SI substrates of InP:Fe by LPE technique. The process was performed in a horizontal quartz reactor tube with a graphite boat in Pd-purified hydrogen ambient. The In solvent was first baked at 800°C for 24 h to reduce the residual impurities. Then a cleaned InP substrate and polycrystalline InP were loaded with a small amount of praseodymium (0.15 wt.%) into the boat to complete the growth solution. This was first annealed for 1 h at 700°C and then the growth was initiated at 640°C using super-cooling technique with 10°C supersaturation. The cooling rate was 0.8°C min − 1. An optical microscope was used to determine the thickness of the grown layers after etching a cleaved wafer. The typical thickness of the layers was 6 mm. The details of the growing technology were described in a recent paper [22]. The Hall coefficient and resistivity measurements were made over a wide temperature range. The set-up is equipped with a close-cycle helium cryogenic system that enables to work in the temperature range 8–320 K 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 holder close to the sample and is read only when the magnetic field is off. Ohmic contacts on p-type InP were prepared at room temperature by rubbing liquid gallium – indium alloy with a zinc point and conditioning with high electric voltage to break the potential barrier. The ohmic nature of the contacts was always tested by reversing the current and varying its magnitude. The size of the contacts was less than 0.5 mm in diameter, on a typical sample of 10× 10 mm in size.
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The parameters determined from the experimental measurements are the Hall coefficient RH and the resistivity z, while the quantities of interest are the hole concentration p and mobility vp. The carrier concentration p is calculated from the RH and z using expressions: p= rH/(q RH)
(1)
vp = RH/(rH z)
(2)
where q is the elemental charge and the Hall coefficient factor rH is a numerical factor close to unity. The rH is in general a function of the used magnetic field. By making measurements in the high magnetic field limit, where the rH is unity, the hole concentration p and mobility vp can be determined directly from the values of the Hall coefficient RH and resistivity z. In our measurements the magnetic field was at least 0.75 T and it was assumed that rH = 1. Photoluminescence spectra were taken at various temperatures and various levels of excitation (6–600 mW cm − 2). The spectrometer consists of an optical cryostat, a monochromator and a detection part. The optical cryostat with the closed cycle helium refrigeration system (Balzers, USA) and the automatic temperature controller enables measurements in the temperature range 3.6/300 K. The 1 m focal length monochromator (Yobin-Yvon THR1000) 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. The excitation was provided by a HeNe or an Ar ion laser and the level of excitation was changed by using neutral density filters.
3. Results and discussion The room temperature Hall coefficient and resistivity measurements revealed the p-type conductivity of the InP layers; the hole concentration and mobility determined by using Eqs. (1) and (2) were: p=5.8×1014 cm − 3 and vp = 144 cm2 V − 1 s − 1. The temperature dependence of the hole concentration and mobility was measured in the range from 120 to 300 K. Logarithmic plots of the hole concentration and hole mobility as a function of the reciprocal temperature 1/T are shown in Fig. 1. The curve of p is linear in the range of several decades of the hole concentration. From its slope, the binding energy of the dominant acceptor impurity has been determined as 223 meV. This value is in accord with the binding energy of Ge acceptor, 210920 meV measured by luminescence by White et al. [6] on Ge doped n-type InP. This accord supports the idea that the dominant acceptor in our samples of p-type InP can be assigned to the acceptor of Ge.
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K. Zda´nsky´ et al. / Materials Science and Engineering B80 (2001) 10–13
Fig. 3. Photo-luminescence spectrum of InP layer measured at 3.6 K with 26 mW cm − 2 of Ar ion laser excitation. Detection in the range 870 }880 nm was provided with cooled S1 photo-multiplier. Fig. 1. Hole concentration and mobility of the InP layer as a function of inverse temperature.
The measured room temperature hole mobility is near the value of 150 cm2 V − 1 s − 1, which is expected for pure InP [23]. The value of vH increases with decreasing temperature and reaches the maximum value of about 850 cm2 V − 1 s − 1 at 130 K. This value is slightly smaller than the average value in low concentration Zn, Cd or Mg doped p-type InP, measured at 77 K [24]. The drop of vH at temperatures below 130 K is probably due to the onset of impurity conduction band [25]. Typical photoluminescence spectrum is shown in Fig. 2. It consists of several bands: exciton recombinations
Fig. 2. Photo-luminescence spectrum of InP layer measured at 4 K with 26 mW cm − 2 of Ar ion laser excitation. Detection in the range 860} 940 nm was provided with cooled S1 photo-multiplier.
around 1.42 eV, shallow acceptors related transitions around 1.375 eV and their phonon replicas around 1.33 eV [26]. As seen in Fig. 3, four peaks can be resolved in the band of exciton recombinations: free exciton (X), shallow donor bound exciton (D°X), shallow acceptor bound exciton (A°X) [18] and an unidentified peak at 1.413 eV. The peak A°X is due to zinc acceptor in the InP layer. The peak at 1.413 eV could not be a phonon replica. Owing to the apparent half-width of the line at 1.413 eV, being of the order of the half-widths of D°X and A°X lines, and a reasonable correspondence with the Haynes rule applied to the Ge acceptor with activation energy 220 meV [6], we attribute tentatively the line at 1.423 eV as due to radiative recombination of excitons bound to Ge acceptors. The Hayes rule EX 0.1EA (that relates the exciton EX and impurity EA binding energies) was derived for Si and the parameter 0.1 is expected to be too high for III –V semiconductors, where the electron effective mass is much larger than the hole effective mass. The intensity of the peak at 1.413 eV is smaller than that of A°X, which can be assigned to smaller luminescence efficiency of excitons bound to Ge than that of excitons bound to Zn. Photoluminescence spectrum in the lower energy region (1.15/1.32) eV is shown in Fig. 4. This spectrum was measured with the cooled Ge detector and, to improve the signal to noise ratio, the high excitation level 600 mW cm − 2 was used. The peak at 1.29 eV (denoted as OC in Fig. 4) has been related in Refs. [27] and [28] to the formation of a point defect –oxygen complex. The peak at 1.1952 eV is close to the estimated position of the no-phonon line (1.215 eV) of the band –Ge acceptor transitions [6]. The presence of this peak in the spectrum further supports the idea that the
K. Zda´nsky´ et al. / Materials Science and Engineering B80 (2001) 10–13
dominant acceptors in our samples of p-type InP can be assigned to the acceptor of Ge. The line-width in our high purity p-type InP is much narrower (15 meV) than the line-width in the Ge-doped n-type InP ( 200 meV) as shown in Ref. [6]. The broadening of the line in the Ge doped n-type InP can be caused by the interaction of the GeP acceptors with GeIn donor impurity band. We attribute the increased p-type activity of Ge in InP, grown by LPE in the presence of RE and especially Pr, to the related change of stoichiometric ratio of In and P at the growing interface so that the generation of higher concentration of phosphorus vacancies is favoured. Alternatively, it can be speculated that not single Ge but Ge – P and Ge – In clusters are incorporated during the growing process and the RE pulls out Ge –P clusters in favour of Ge – In ones, thus increasing the p-type activity of Ge in the grown crystal.
4. Conclusion We conclude that Ge acceptors cause the p-type conductivity in the InP layers grown by LPE with praseodymium melt doping. This conclusion is derived from the results of the temperature dependent Hall coefficient and resistivity measurements and of the low temperature luminescence spectroscopy. Germanium doped p-type InP has been obtained for the first time as far as we know. Our results show that growing crystals by LPE with rare earths in the melt can be used not only for preparation of high purity crystals of III –V compounds but that it also opens
Fig. 4. Photo-luminescence spectrum of InP layer measured at 4 K with 600 mW cm − 2 of Ar ion laser excitation. Detection in the range 950 }1080 nm was provided with cooled Ge detector.
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quite new ways for unusual controlled doping by amphoteric impurities. Acknowledgements The authors thank V. Majerı´kova´, V. Va´vra and S. Javorsky´ for technical assistance. The work was 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 [1] P. Kringhoj, V.V. Gribkovskij, A.N. Larsen, Appl. Phys. Lett. 57 (1990) 1514. [2] E.W. Williams, W. Elder, M.G. Astles, M. Weble, J.B. Mullin, B. Strangham, et al., J. Electrochem. Soc. 120 (1973) 1741. [3] M.G. Astles, F.G.H. Smith, E.W. Williams, J. Electrochem. Soc. 120 (1973) 1750. [4] J.B. Mullin, A. Royle, B.W. Strangham, P.J. Tufton, E.W. Williams, J. Cryst. Growth 13/14 (1972) 640. [5] M.S. Skolnick, P.J. Dean, L.L. Taylor, D.A. Anderom, S.P. Najda, C.J. Armistrad, et al., Appl. Phys. Lett. 44 (1984) 881. [6] A.M. White, P.J. Dean, B. Day, in: F.G. Fumi (Ed.), Proceedings of the 13th International Conference on Physics and Semiconductors, Typographic Marves, Rome, 1976, p. 1057. [7] S.W. Sun, B.W. Wessels, J. Appl. Phys. 68 (1990) 606. [8] W. Walukiewicz, J. Lagowski, L. Jastrzebski, P. Rava, M. Lichtensteiger, C.H. Gatos, et al., J. Appl. Phys. 51 (1980) 2659. [9] M.C. Ridgway, C. Jagadish, T.D. Thompson, S.T. Johnson, J. Appl. Phys. 71 (1992) 1708. [10] P. Kringhoj, J.L. Hansen, S.Y. Shiryaev, J. Appl. Phys. 72 (1992) 2249. [11] P. Kringhoj, J. Appl. Phys. 71 (1992) 1748. [12] K.M. Yu, A.J. Moll, W. Walukiewicz, N. Derhacobian, C. Rossington, Appl. Phys. Lett., 64, (1994) 1543. [13] K.M. Yu, A.J. Moll, W. Walukiewicz, J. Appl. Phys. 80 (1996) 4907. [14] A.J. Moll, K.M. Yu, W. Walukiewicz, W.L. Hansen, E.E. Haller, Appl. Phys. Lett. 60 (1992) 2383. [15] S. Honglie, Y. Genquing, Z. Zuyav, Z. Shichang, Semicond. Sci. Technol. 4 (1989) 951. [16] P. Kringhoj, Appl. Phys. Lett., 64, (1994) 351. [17] M.C. Ridgway, P. Kringhoj, C.M. Johnson, Nucl. Instrum. Meth. B96 (1995) 311. [18] K.M. Yu, M.C. Ridgway, Appl. Phys. Lett. 71 (1997) 939. [19] K.A. Gatsov, A.T. Gorelenok, S.L. Karpenko, et al., Soviet Phys.-Semiconductors 17 (1983) 1373. [20] O. Procha´zkova´, J. Zavadil, K. Zda´nsky´, et al., J. Elect. Engng 50 (1999) 20. [21] L. Peka´rek, K. Zda´nsky´, in: M. Koman, D. Miklos (Eds.), Ninth Seminar on Development of Materials Science, Gabcikovo, Slovakia, Chemistry Technological Faculty, State Technical University, Bratislava, Slovakia, 1999, p. 57. [22] O. Procha´zkova, J. Zavadil, Sci. Found. China 7 (2) (1999) 44. [23] D. Kranzer, Phys. Stat. Sol. 26 (1974) 11. [24] E. Kuphal, J. Cryst. Growth 54 (1981) 117. [25] D.N. Nasledov, Y.G. Popov, N.V. Synkaev, S.P. Staroseltseva, Soviet Phys.-Semiconductors 3 (1969) 387. [26] J. Zavadil, K. Zda´nsky´, O. Procha´zkova, Czech. J. Phys. 49 (1999) 765. [27] R.A. Street, R.H. Williams, J. Appl. Phys. 52 (1981) 402. [28] T. Kamijoh, H. Takano, M. Sakuta, J. Appl. Phys. 55 (1984) 3756.
P-type InP grown by liquid phase epitaxy with rare earths: not intentional Ge acceptor doping K. Zda´nsky´ *, J. Zavadil, O. Procha´zkova´, P. Gladkov Institute of Radio Engineering and Electronics AV CR, Chaberska´ 57, Prague 8, 18251, Czech Republic
Abstract InP single crystal layers were grown by liquid phase epitaxy (LPE) on semi-insulating InP:Fe substrates with praseodymium added to the melt. Room temperature Hall effect measurements revealed p-type conductivity of the layers with the hole concentration 6 ×1014 cm − 3 and mobility 150 cm2 V − 1 s − 1. By measuring temperature dependence of the hole concentration the binding energy of the dominant acceptor was determined as 223 meV. A photoluminescence line was found at 1.195 eV, close to the previously estimated no-phonon line of Ge acceptor transitions in Ge doped n-type InP. It was concluded that Ge acceptors cause the p-type conductivity of the grown layers. © 2001 Elsevier Science B.V. All rights reserved. Keywords: InP; Ge; Rare earth; Hall effect; Photoluminescence
1. Introduction High purity and controlled doping of indium phosphide single crystals are essential for successful device fabrication with this material. Doping of InP with germanium usually results in n-type conductivity [1]. It has been used in preparation of doped liquid-encapsulated Czochralski (LEC), solution grown, liquid-phaseepitaxial and vapour-phase-epitaxial materials [2–5]. However, some experiments show on amphoteric behaviour of Ge in InP; it can either substitute the group III or V sublattice and become donor or acceptor, respectively. Photoluminescence spectroscopy indicates lines attributed to recombination of excitons bound to Ge acceptors [5,6]. Low electron mobility of Ge-doped InP was explained by auto-compensation by Ge acceptors [7,8]. Recently a number of studies has been carried out to investigate the amphoteric behaviour of Ge in InP [9–13]. It has been shown that an enhancement of group IV dopant substitution into a specific sublattice in a III –V semiconductor, and the resulting changes in the electrical behaviour, can be achieved by changing the local stoichiometry of the semiconductor [14 –17]. Thus an improved n-type Ge activity in InP * Corresponding author. Tel.: +420-2-6881804; fax: + 420-26880222. E-mail address: [email protected] (K. Zda´nsky´).
has been observed [17,18] in samples co-implanted with Ge and P. Much improved p-type conductivity was found in GaAs by co-implanting Ge with C [14]. However, to the best of our knowledge no method for improving p-type activity of Ge in InP has been reported, yet. We are going to demonstrate in this paper that the p-type Ge activity in InP can be improved by adding the rare earth element Pr into the molten solution for growing InP by liquid phase epitaxy (LPE). It was already recognised a long time ago that admixture of rare earths (RE) into the molten solution for growing InP and GaAs layers can reduce the background donor concentration by several orders of magnitude [19]. This is because RE form stable compounds with residual impurities, which are insoluble in the indium melt, thus preventing their incorporation into the grown layers. Donor impurities preferentially are gettered, acceptor impurities are also gettered but to a lesser extent. When a smaller amount of RE is added to the melt, a high purity n-type InP layer can be grown while with a larger amount of RE the conductivity of InP can be converted into p-type [20]. It will be shown that RE doping of the melt containing Ge amphoteric impurity could favour the density of Ge acceptors in the grown InP layer while suppressing the density of Ge donors. In this way it could be possible to grow p-type InP with holes activated from Ge acceptors.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 5 7 4 - 2
K. Zda´nsky´ et al. / Materials Science and Engineering B80 (2001) 10–13
In this paper we report on the preparation of high purity p-type InP single crystal layers unintentionally doped with Ge acceptors grown by LPE with praseodymium admixture in the melt. Temperature dependence of resistivity and Hall coefficient, and low temperature photoluminescence spectra were measured to characterise the layers. 2. Experimental The semiconductor epitaxial layers for the measurement of the Hall coefficient and resistivity must be grown on substrates of sufficiently high resistivity not to cause shunting effect on the layer. Semi-insulating (SI) single crystals of InP doped with iron for preparing substrates were grown by Czochralski (LEC) method [21]. The substrates had resistivity larger than 107 V cm at room temperature, which permitted electrical measurements of high purity InP layers in a wide temperature range without any influence of the substrate on the result. The layers of InP were grown on (100) oriented SI substrates of InP:Fe by LPE technique. The process was performed in a horizontal quartz reactor tube with a graphite boat in Pd-purified hydrogen ambient. The In solvent was first baked at 800°C for 24 h to reduce the residual impurities. Then a cleaned InP substrate and polycrystalline InP were loaded with a small amount of praseodymium (0.15 wt.%) into the boat to complete the growth solution. This was first annealed for 1 h at 700°C and then the growth was initiated at 640°C using super-cooling technique with 10°C supersaturation. The cooling rate was 0.8°C min − 1. An optical microscope was used to determine the thickness of the grown layers after etching a cleaved wafer. The typical thickness of the layers was 6 mm. The details of the growing technology were described in a recent paper [22]. The Hall coefficient and resistivity measurements were made over a wide temperature range. The set-up is equipped with a close-cycle helium cryogenic system that enables to work in the temperature range 8–320 K 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 holder close to the sample and is read only when the magnetic field is off. Ohmic contacts on p-type InP were prepared at room temperature by rubbing liquid gallium – indium alloy with a zinc point and conditioning with high electric voltage to break the potential barrier. The ohmic nature of the contacts was always tested by reversing the current and varying its magnitude. The size of the contacts was less than 0.5 mm in diameter, on a typical sample of 10× 10 mm in size.
11
The parameters determined from the experimental measurements are the Hall coefficient RH and the resistivity z, while the quantities of interest are the hole concentration p and mobility vp. The carrier concentration p is calculated from the RH and z using expressions: p= rH/(q RH)
(1)
vp = RH/(rH z)
(2)
where q is the elemental charge and the Hall coefficient factor rH is a numerical factor close to unity. The rH is in general a function of the used magnetic field. By making measurements in the high magnetic field limit, where the rH is unity, the hole concentration p and mobility vp can be determined directly from the values of the Hall coefficient RH and resistivity z. In our measurements the magnetic field was at least 0.75 T and it was assumed that rH = 1. Photoluminescence spectra were taken at various temperatures and various levels of excitation (6–600 mW cm − 2). The spectrometer consists of an optical cryostat, a monochromator and a detection part. The optical cryostat with the closed cycle helium refrigeration system (Balzers, USA) and the automatic temperature controller enables measurements in the temperature range 3.6/300 K. The 1 m focal length monochromator (Yobin-Yvon THR1000) 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. The excitation was provided by a HeNe or an Ar ion laser and the level of excitation was changed by using neutral density filters.
3. Results and discussion The room temperature Hall coefficient and resistivity measurements revealed the p-type conductivity of the InP layers; the hole concentration and mobility determined by using Eqs. (1) and (2) were: p=5.8×1014 cm − 3 and vp = 144 cm2 V − 1 s − 1. The temperature dependence of the hole concentration and mobility was measured in the range from 120 to 300 K. Logarithmic plots of the hole concentration and hole mobility as a function of the reciprocal temperature 1/T are shown in Fig. 1. The curve of p is linear in the range of several decades of the hole concentration. From its slope, the binding energy of the dominant acceptor impurity has been determined as 223 meV. This value is in accord with the binding energy of Ge acceptor, 210920 meV measured by luminescence by White et al. [6] on Ge doped n-type InP. This accord supports the idea that the dominant acceptor in our samples of p-type InP can be assigned to the acceptor of Ge.
12
K. Zda´nsky´ et al. / Materials Science and Engineering B80 (2001) 10–13
Fig. 3. Photo-luminescence spectrum of InP layer measured at 3.6 K with 26 mW cm − 2 of Ar ion laser excitation. Detection in the range 870 }880 nm was provided with cooled S1 photo-multiplier. Fig. 1. Hole concentration and mobility of the InP layer as a function of inverse temperature.
The measured room temperature hole mobility is near the value of 150 cm2 V − 1 s − 1, which is expected for pure InP [23]. The value of vH increases with decreasing temperature and reaches the maximum value of about 850 cm2 V − 1 s − 1 at 130 K. This value is slightly smaller than the average value in low concentration Zn, Cd or Mg doped p-type InP, measured at 77 K [24]. The drop of vH at temperatures below 130 K is probably due to the onset of impurity conduction band [25]. Typical photoluminescence spectrum is shown in Fig. 2. It consists of several bands: exciton recombinations
Fig. 2. Photo-luminescence spectrum of InP layer measured at 4 K with 26 mW cm − 2 of Ar ion laser excitation. Detection in the range 860} 940 nm was provided with cooled S1 photo-multiplier.
around 1.42 eV, shallow acceptors related transitions around 1.375 eV and their phonon replicas around 1.33 eV [26]. As seen in Fig. 3, four peaks can be resolved in the band of exciton recombinations: free exciton (X), shallow donor bound exciton (D°X), shallow acceptor bound exciton (A°X) [18] and an unidentified peak at 1.413 eV. The peak A°X is due to zinc acceptor in the InP layer. The peak at 1.413 eV could not be a phonon replica. Owing to the apparent half-width of the line at 1.413 eV, being of the order of the half-widths of D°X and A°X lines, and a reasonable correspondence with the Haynes rule applied to the Ge acceptor with activation energy 220 meV [6], we attribute tentatively the line at 1.423 eV as due to radiative recombination of excitons bound to Ge acceptors. The Hayes rule EX 0.1EA (that relates the exciton EX and impurity EA binding energies) was derived for Si and the parameter 0.1 is expected to be too high for III –V semiconductors, where the electron effective mass is much larger than the hole effective mass. The intensity of the peak at 1.413 eV is smaller than that of A°X, which can be assigned to smaller luminescence efficiency of excitons bound to Ge than that of excitons bound to Zn. Photoluminescence spectrum in the lower energy region (1.15/1.32) eV is shown in Fig. 4. This spectrum was measured with the cooled Ge detector and, to improve the signal to noise ratio, the high excitation level 600 mW cm − 2 was used. The peak at 1.29 eV (denoted as OC in Fig. 4) has been related in Refs. [27] and [28] to the formation of a point defect –oxygen complex. The peak at 1.1952 eV is close to the estimated position of the no-phonon line (1.215 eV) of the band –Ge acceptor transitions [6]. The presence of this peak in the spectrum further supports the idea that the
K. Zda´nsky´ et al. / Materials Science and Engineering B80 (2001) 10–13
dominant acceptors in our samples of p-type InP can be assigned to the acceptor of Ge. The line-width in our high purity p-type InP is much narrower (15 meV) than the line-width in the Ge-doped n-type InP ( 200 meV) as shown in Ref. [6]. The broadening of the line in the Ge doped n-type InP can be caused by the interaction of the GeP acceptors with GeIn donor impurity band. We attribute the increased p-type activity of Ge in InP, grown by LPE in the presence of RE and especially Pr, to the related change of stoichiometric ratio of In and P at the growing interface so that the generation of higher concentration of phosphorus vacancies is favoured. Alternatively, it can be speculated that not single Ge but Ge – P and Ge – In clusters are incorporated during the growing process and the RE pulls out Ge –P clusters in favour of Ge – In ones, thus increasing the p-type activity of Ge in the grown crystal.
4. Conclusion We conclude that Ge acceptors cause the p-type conductivity in the InP layers grown by LPE with praseodymium melt doping. This conclusion is derived from the results of the temperature dependent Hall coefficient and resistivity measurements and of the low temperature luminescence spectroscopy. Germanium doped p-type InP has been obtained for the first time as far as we know. Our results show that growing crystals by LPE with rare earths in the melt can be used not only for preparation of high purity crystals of III –V compounds but that it also opens
Fig. 4. Photo-luminescence spectrum of InP layer measured at 4 K with 600 mW cm − 2 of Ar ion laser excitation. Detection in the range 950 }1080 nm was provided with cooled Ge detector.
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quite new ways for unusual controlled doping by amphoteric impurities. Acknowledgements The authors thank V. Majerı´kova´, V. Va´vra and S. Javorsky´ for technical assistance. The work was 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 [1] P. Kringhoj, V.V. Gribkovskij, A.N. Larsen, Appl. Phys. Lett. 57 (1990) 1514. [2] E.W. Williams, W. Elder, M.G. Astles, M. Weble, J.B. Mullin, B. Strangham, et al., J. Electrochem. Soc. 120 (1973) 1741. [3] M.G. Astles, F.G.H. Smith, E.W. Williams, J. Electrochem. Soc. 120 (1973) 1750. [4] J.B. Mullin, A. Royle, B.W. Strangham, P.J. Tufton, E.W. Williams, J. Cryst. Growth 13/14 (1972) 640. [5] M.S. Skolnick, P.J. Dean, L.L. Taylor, D.A. Anderom, S.P. Najda, C.J. Armistrad, et al., Appl. Phys. Lett. 44 (1984) 881. [6] A.M. White, P.J. Dean, B. Day, in: F.G. Fumi (Ed.), Proceedings of the 13th International Conference on Physics and Semiconductors, Typographic Marves, Rome, 1976, p. 1057. [7] S.W. Sun, B.W. Wessels, J. Appl. Phys. 68 (1990) 606. [8] W. Walukiewicz, J. Lagowski, L. Jastrzebski, P. Rava, M. Lichtensteiger, C.H. Gatos, et al., J. Appl. Phys. 51 (1980) 2659. [9] M.C. Ridgway, C. Jagadish, T.D. Thompson, S.T. Johnson, J. Appl. Phys. 71 (1992) 1708. [10] P. Kringhoj, J.L. Hansen, S.Y. Shiryaev, J. Appl. Phys. 72 (1992) 2249. [11] P. Kringhoj, J. Appl. Phys. 71 (1992) 1748. [12] K.M. Yu, A.J. Moll, W. Walukiewicz, N. Derhacobian, C. Rossington, Appl. Phys. Lett., 64, (1994) 1543. [13] K.M. Yu, A.J. Moll, W. Walukiewicz, J. Appl. Phys. 80 (1996) 4907. [14] A.J. Moll, K.M. Yu, W. Walukiewicz, W.L. Hansen, E.E. Haller, Appl. Phys. Lett. 60 (1992) 2383. [15] S. Honglie, Y. Genquing, Z. Zuyav, Z. Shichang, Semicond. Sci. Technol. 4 (1989) 951. [16] P. Kringhoj, Appl. Phys. Lett., 64, (1994) 351. [17] M.C. Ridgway, P. Kringhoj, C.M. Johnson, Nucl. Instrum. Meth. B96 (1995) 311. [18] K.M. Yu, M.C. Ridgway, Appl. Phys. Lett. 71 (1997) 939. [19] K.A. Gatsov, A.T. Gorelenok, S.L. Karpenko, et al., Soviet Phys.-Semiconductors 17 (1983) 1373. [20] O. Procha´zkova´, J. Zavadil, K. Zda´nsky´, et al., J. Elect. Engng 50 (1999) 20. [21] L. Peka´rek, K. Zda´nsky´, in: M. Koman, D. Miklos (Eds.), Ninth Seminar on Development of Materials Science, Gabcikovo, Slovakia, Chemistry Technological Faculty, State Technical University, Bratislava, Slovakia, 1999, p. 57. [22] O. Procha´zkova, J. Zavadil, Sci. Found. China 7 (2) (1999) 44. [23] D. Kranzer, Phys. Stat. Sol. 26 (1974) 11. [24] E. Kuphal, J. Cryst. Growth 54 (1981) 117. [25] D.N. Nasledov, Y.G. Popov, N.V. Synkaev, S.P. Staroseltseva, Soviet Phys.-Semiconductors 3 (1969) 387. [26] J. Zavadil, K. Zda´nsky´, O. Procha´zkova, Czech. J. Phys. 49 (1999) 765. [27] R.A. Street, R.H. Williams, J. Appl. Phys. 52 (1981) 402. [28] T. Kamijoh, H. Takano, M. Sakuta, J. Appl. Phys. 55 (1984) 3756.