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Large negative persistent photoconductivity of bulk GaAs1-xPx (x=0.02−0.03 ) single crystals
Materials Science and Engineering, B2I (1993) 325-328
325
Large negative persistent photoconductivity of bulk GaAs I _ xPx (x = 0.02-0.03) single crystals T. Slupifiski, G. Nowak, R. Bogek, A. W y s m c r l e k a n d M. Baj Institute of Experimental Physics, Warsaw University, Hoga 69, 00-681 Warsaw (Poland)
Abstract It is known that n-type GaAs under hydrostatic pressure at low temperature exhibits negative persistent photoconductivity (NPPC) related to the EL2 defect. In this paper we present the observation of NPPC in GaAs~ _~P.~bulk mixed crystals with x = 0.02-0.03 at ambient pressure. The growth of single crystals by the liquid encapsulated Czochralski method is described and discussed. Selected samples of GaAs~ .,P, with energy gap higher than that of GaAs by about 20-30 meV were investigated by optical absorption and photo-Hall measurements in the temperature range 10-60 K at ambient pressure.
1. Introduction The reported negative persistent photoconductivity (NPPC) is an inherent property of n-type GaAs material at low temperature and with the energy gap increased by high hydrostatic pressure [1]. It can be observed if a sufficiently high concentration of EL2 native deep donor centres is present in GaAs, whose [EL2] °/+ electronic level is responsible for the high resistivity of semi-insulating undoped GaAs, and which is also present in slightly doped n-type GaAs. Such unusual behaviour of semiconducting material, i.e. an increase in resistivity during illumination with an appropriate light, is a consequence of the existence of the acceptor state of the metastable configuration, EL2*, of the EL2 defect. In n-type GaAs at low temperatures (below about 45K), illumination with 1.0-1.3 eV photons causes a transition from the normal to the metastable configuration of EL2, manifesting itself in the disappearance of the absorption spectrum characteristic of the EL2 ° state. Simultaneously, a new acceptor-like [EL2*] -/° level appears within the conduction band at an energy of about E c + 0.02 eV. A high hydrostatic pressure applied to a sample increases the energy gap and moves this level into the gap. Free electrons are then captured onto this level. Consequently, in n-type low-resistivity material this leads to NPPC, which lasts as long as the EL2 is not transferred to its normal configuration either by heating the sample above 45 K or illuminating with light of energy about 1.5 eV. We propose that observation of NPPC should be possible in mixed crystals, where the increase in energy gap is caused by an addi-
tion of GaP to GaAs, and not by a high pressure applied during measurements. In the present paper we report the growth and investigations of bulk mixed crystals of GaAs~ _xPx specially designed for observation of the [EL2*] -/° level. We describe the NPPC effect and finally we draw some conclusions concerning the homogeneity of our crystals. Thin GaAsl _xPx layers with x = 0.3-0.8 have been widely grown for LED applications for many years. However, growth of bulk GaAsl_xP x crystals is a serious problem owing to the different compositions of solid and liquid phases at equilibrium in the GaAs-GaP high temperature mixture, and consequently segregation phenomena during crystal growth. Nevertheless, several attempts have been described in the literature. Cerrina et al. [2] grew GaAsl _xPx polycrystals with x=0.1-0.4 by pulling from a Ga-rich solution. Saito and Seki [3] obtained GaAs~_xp, , x=0.1-0.8 polycrystals by using a "synthesis solute diffusion"-like method also from Ga solution. Ga-rich conditions, usually utilized in solution growth, prevent a high EL2 defect concentration in GaAs crystals, since the EL2 defect is associated with excess arsenic in the crystal. Also, near-stoichiometric methods have been used to grow bulk mixed crystals. For example Bachmann et al. [4] grew mixed single crystals of Ga~Inj _xP~.Asl ~ with x = 0-0.07 and y = 0-0.8 by a gradient freezing method. Kimura et al. [5] used phosphorus as a complementary impurity to indium to reduce the dislocation density in GaAs liquid encapsulated Czochralski (LEC) crystals grown from melt alloyed with In and Elsevier Sequoia
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"ESlupihski et al.
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Large negative persistent photoconductivi O'
InP. The concentration of P in their crystals was 0.1 - 1 mol.% and that of indium about 1%. Recently Jordan et al. [6] have grown GaAsl _~P~ crystals with x = 0 . 0 1 (single crystal) and x = 0.13 (polycrystal) by the horizontal Bridgman method with addition of GaP to the melt, aimed at basic investigations of antisite-related defects.
o o~
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ca_
2
......
.-
x)
3. Experimental results To evaluate the total EL2 defect concentration in the crystal optical absorption measurements were
~ ........ C - - f .
\,,
i
2. Crystal growth Our idea was to grow GaAs~ -~Px crystal by the conventional LEC method used for GaAs growth, but with InP added to the melt before the process. In this case the large difference between segregation coefficients of phosphorus (k0 = 3) and indium (k 0 = 0.12)[5, 6] leads to a much higher concentration of P than In in the seed part of the crystal. Since the concentration of phosphorus in a crystal must drop rapidly as the growth proceeds, an almost constant phosphorus concentration can be obtained if only a small fraction of liquid is solidified. The input Ga + As + InP charge of about 1 kg contained 0.82 mol.% InP, corresponding to the expected 2.5 mol.% P and 0.1% In at the beginning of the crystal. To reduce the gradient of phosphorus concentration we kept the diameter of the crystal within 1-1.5 cm until the fraction of liquid solidified g reached 0.02. (100)-oriented GaAs seed and B203 encapsulating layer were used. Nitrogen pressures of 60 bar during in situ synthesis and melting, about 3 bar during melt homogenization and about 10 bar during growth was applied. The pulling speed of about 8 mm h- ~ did not cause any problems with polycrystallization. To reach a high EL2 defect concentration, a slightly As-rich melt was used with molar ratio [As]/ [Ga] = 1.06 in the initial charge. The crystal was doped with tellurium to obtain a free electron concentration of a few times 1016 cm - 3. Figure 1 shows the crystal shape and phosphorus contents both expected and obtained. The composition was measured by near band gap photoluminescence (PL). The results are 2.2 + 0.2 mol.% P for fraction of liquid solidified g = 0 . 0 1 and 1.9_+0.2% for g=0.02, assuming that the PL lines shift to higher energies at a rate d E J d x = 12 meV per 1% [8]. The phosphorus composition x along the diameter of the initial part of the crystal was constant to within 0.4%. Concluding, the addition of 2-3 mol.% P to the crystal enlarged the energy gap by about 2 0 - 3 0 meV relative to GaAs.
,\
8
0
!
0
2
" " : - -: X
4 crgst@ length [cm]
6
8
Fig. 1. Crystal radius ( ), fraction of liquid solidified multiplied by 10 (10g ......), and expected phosphorus concentration ( - - - ) vs. crystal length. The points indicate the phosphorus content measured by near-band-gap photoluminescence.
8
% ~d 6
a /
C/,
® o v 4 F~ o_ 0 o 2
.-
105
~ /
/is
-
120 1.35 photon energy [eV]
1.50
Fig. 2. Absorption spectra of GaAsa _xP, at 10 K after cooling in the dark (curve a), after illumination with 1.05 ktm (1.18 eV) light (curve b) and after subsequent illumination with 0.825 Hm (1.5 eV)light (curve c).
taken in the photon energy range 0.7-1.55 eV at 10 K on a sample cut from the part corresponding to the fraction of liquid solidified g=0.01. The spectrum obtained after cooling the sample down in the dark (see Fig. 2, curve a) is typical of GaAs containing a neutral EL2 defect [9]. If practically all the absorption around 1.18 eV energy originates from neutral EL2, the concentration of the defect should be equal to about 2 × 1016 cm -3 [10]. Subsequent strong illumination with 1.05 /zm light (1.18 eV) only partially quenched the EL2 absorption related to the transition of EL2 to its metastable EL2* configuration, see curve b. This is very similar to the case of n-type GaAs under hydrostatic pressure [11]. Since at high pressure the EL2 defect in GaAs recovers very rapidly from its metastable configuration under near-band-gap illumination, we attempted to check whether this phenomenon occurs at ambient pressure in GaAs~-,Px. In the sample previously illuminated with 1.5 eV photons only a few per cent absorption in the 1.18 eV region was restored (see Fig. 2, curve c), much less than in GaAs under pressure.
T. Slupifiski et al.
/
Large negative persistent photoconductivi O,
327
15
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I
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1.18eV
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/
~5
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/
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2x10~5 0
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photon energy [eV] 0
20
40
60
Tempemlzur~[~ Fig. 3. Hall electron concentration measured as a function of temperature in GaAst_xPx, x=0.02. The arrows indicate the direction of the temperature sequence during experiment.Illumination with 1.18 eV light at 10 K causes a persistent increase in resistivity,thus showingthe NPPC effect.
To check the existence of NPPC we took resistivity and Hall effect measurements on a sample cut from the initial part of the crystal. Typical results are presented in Fig. 3. First we measured the Hall coefficient while cooling the sample to 10 K (the increase in Hall concentration observed at T< 15 K probably results from a two-band conduction mechanism in conduction and shallow impurity bands [ 12]). The sample was then illuminated at 10 K with 1.1 #m wavelength fight (1.13 eV), for 10 min, with a halogen lamp through an interference filter. The resistivity increased and the Hall electron concentration decreased by much more than one order of magnitude, thus showing a large NPPC effect. During subsequent heating of the sample the Hall concentration remained small up to about 40 K, when it returned to its original value before illumination. Successive cooling did not reveal any changes in comparison with the initial curve.
4. Discussion
The observed phenomena can be explained as follows. After cooling, illumination of the sample transformed the EL2 defect from its normal to the metastable configuration EL2*. Since the energy gap of GaAsl xP~ for x = 0 . 0 2 is higher than that of GaAs and since the acceptor level [EL2*]-/0 is already in the forbidden gap, it captures free electrons, thus decreasing the measured free electron concentration. After heating the sample over 40-45 K the EL2* defect returns to its normal coniguration, releasing previously captured electrons.
Fig. 4. An example of photoluminescencespectra observed for samples with fractions of liquid solidified of g= 0.01 (curve a) and g=0.02 (curve b). The shift in line energy between the curves indicateschangesin the energygap. The double structure in the 1.54 eV energyrange of curve b is discussed in the text.
A similar large EL2 defect related NPPC effect was discovered earlier in GaAs under pressure P> 0.3 GPa [1]. We observe some differences between the properties of GaAs under high pressure and GaAsl xPx at ambient pressure. Although incomplete photoquenching of EL2 (curves a and b in Fig. 2) is characteristic of n-GaAs under pressure, only partial photorecovery of the EL2 defect (curves b and c) is not consistent with the whole picture. There are at least two possible explanations for the above discrepancy: on an atomic scale nearby phosphorus atoms can modify some subtle properties of EL2 defects (e.g. photorecovery from metastable to normal state) or some large-scale fluctuations in P content can alter inhomogeneously the energy gap or can introduced mechanical stresses. Such inhomogeneities in our crystal were clearly seen from PL measurements as a broadening of some lines in the spectrum. As shown in Fig. 4, curve b, around 1.54 eV, a double structure is present. This results from simultaneous observation of the PL signal from at least two domains with different phosphorus contents, and thus with different energy gaps. The difference in energy of these lines corresponds to about 0.4% difference in P content between the domains. The existence of such domains explains why for a given fraction of liquid solidified g we have various P contents, evaluated from PL lines measured for different laser spot positions, seen in Fig. 1. The average size of such domains should be of the order of the laser light spot (a few hundred micrometres). The reason for the non-homogeneous phosphorus composition can be understood by considering the segregation of phosphorus (or rather GaP in GaAs + GaP liquid mixture) during growth. A segregation coefficient not equal to unity results in continuous
328
7~Stupifiski et al.
/
Large negative persistent photoconductivity
transport of excess dopant to (k 0 > 1 ) or from (k 0 < 1 ) the crystal-liquid interface. Various instabilities in such solute migration can cause irregularities of growth. Recently such behaviour was presented by Elliot et al. [13] for GaAs grown from As-rich melt. In our case the growth velocity was higher than that usually used for mixed material (equal to that for GaAs growth), thus amplifying the instabilities. A lower growth velocity should be helpful in decreasing the variations in phosphorus concentration. We also tried to grow crystals of GaAs~ xPx with higher x by the L E C method. Starting from the melt with 3 mol.% GaP we grew mixed crystals with initial composition x = 0.09. Unfortunately, only polycrystals were obtained. The probable reason for polycrystalline growth immediately after seeding is a rise in the crystallization temperature caused by the presence of G a E and thus a high initial supercooling necessary for GaAs seed. Successive utilization of GaAs~ _xP, seeds with higher and higher phosphorus concentrations could be tried to obtain single-crystal growth, as has been done for Ga~ _~In~As growth [14]. Initially, in our work we used InP as a source material for phosphorus. Later, GaP was added equally well to the charge, though to melt it completely (it could be easily seen on the melt surface), overheating of the melt by about 150 degrees was necessary.
5. Conclusions Bulk GaAs~ _xPx single crystals with x = 0.02-0.03 can be grown by the LEC technique from GaAs melt with the addition of either InP or GaP. For a high crystallization rate of about 8 mm h-~ some inhomogeneities of phosphorus concentration can occur. Such material enables investigations of the EL2* defect acceptor state without application of high hydrostatic pressure and thus allows, for example, uniaxial stress experiments. The negative persistent photoconductivity is a material property of GaAsl xPx with x > 0.02 at tem-
peratures below 45 K, with the E L 2 defect concentration comparable with or greater than the free electron concentration and with the free electron concentration not higher than a few times 1()16 cm
Acknowledgments We are very much indebted to Dr. M. Sadowski for careful reading of the manuscript. The GaAsl xP, crystals were grown at the Laboratory of Physics of Crystal Growth in Warsaw. This work was supported by KBN (Poland) grants No. 2-0424-91-01 and 2-0179-91-01.
References l M. Baj, P. Dreszer and A. Babifiski, Phys. Rev. B, 42 (1991) 2070. 2 F. Cerrina, D. Margadonna and P. Perfetti, J. C~st. Growth, 18 (1973) 202. 3 T. Saito and Y. Seki, J. Cryst. Growth, 23 ( 1974) 2 t 7. 4 K. J. Bachmann, E A. Thiel and S. Ferris, J. C~st. Growth, 43 (1978)752. 5 H. Kimnra, A. T. Hunter, E.-H. Cirlin and H. M. Olsen, J. Cryst. Growth, 85 (1987) 116. 6 M. Jordan, T. Hangleiter and J.-M. Spaeth, Semicond. Sci. Technol., 7 (1992) 738. 7 M. B. Panish and M. Ilegems, Prog. Solid-State Chem., 7 (1972) 39. 8 D.J. Wolford, J. A. Bradley, K. Frey, J. Thompson and H. E. King, Int. Conf. on Gallium Arsenid and Related Compounds, Inst. Phys. Conf. Serv., Vol. 65, Institute of Physics, Bristol, 1982, p. 477. 9 G.M. Martin, Appl. Phys. Let., 39 ( 1981 ) 747. 10 M. Skowrofiski, J. Lagowski and H. C. Gatos, J. Appl. Phys., 59 (1986) 2451. 11 M. Baj and P. Dreszer, Phys. Rev. B, 39 (1989) 10470. 12 D. C. Look, in Electrical Characterization of GaAs Material and Devices, Wiley, pp. 112-115. 13 A. G. Elliot, A. Flat and D. A. Vanderwater, J. Cryst. Growth, 121 (1992) 349. 14 W. A. Bonner, R. E. Nahory, H. L. Gilchrist and E. Berry, Proc. Conf. on Semi-insulating III-V Materials, 7bronto, 1990, p. 199.
325
Large negative persistent photoconductivity of bulk GaAs I _ xPx (x = 0.02-0.03) single crystals T. Slupifiski, G. Nowak, R. Bogek, A. W y s m c r l e k a n d M. Baj Institute of Experimental Physics, Warsaw University, Hoga 69, 00-681 Warsaw (Poland)
Abstract It is known that n-type GaAs under hydrostatic pressure at low temperature exhibits negative persistent photoconductivity (NPPC) related to the EL2 defect. In this paper we present the observation of NPPC in GaAs~ _~P.~bulk mixed crystals with x = 0.02-0.03 at ambient pressure. The growth of single crystals by the liquid encapsulated Czochralski method is described and discussed. Selected samples of GaAs~ .,P, with energy gap higher than that of GaAs by about 20-30 meV were investigated by optical absorption and photo-Hall measurements in the temperature range 10-60 K at ambient pressure.
1. Introduction The reported negative persistent photoconductivity (NPPC) is an inherent property of n-type GaAs material at low temperature and with the energy gap increased by high hydrostatic pressure [1]. It can be observed if a sufficiently high concentration of EL2 native deep donor centres is present in GaAs, whose [EL2] °/+ electronic level is responsible for the high resistivity of semi-insulating undoped GaAs, and which is also present in slightly doped n-type GaAs. Such unusual behaviour of semiconducting material, i.e. an increase in resistivity during illumination with an appropriate light, is a consequence of the existence of the acceptor state of the metastable configuration, EL2*, of the EL2 defect. In n-type GaAs at low temperatures (below about 45K), illumination with 1.0-1.3 eV photons causes a transition from the normal to the metastable configuration of EL2, manifesting itself in the disappearance of the absorption spectrum characteristic of the EL2 ° state. Simultaneously, a new acceptor-like [EL2*] -/° level appears within the conduction band at an energy of about E c + 0.02 eV. A high hydrostatic pressure applied to a sample increases the energy gap and moves this level into the gap. Free electrons are then captured onto this level. Consequently, in n-type low-resistivity material this leads to NPPC, which lasts as long as the EL2 is not transferred to its normal configuration either by heating the sample above 45 K or illuminating with light of energy about 1.5 eV. We propose that observation of NPPC should be possible in mixed crystals, where the increase in energy gap is caused by an addi-
tion of GaP to GaAs, and not by a high pressure applied during measurements. In the present paper we report the growth and investigations of bulk mixed crystals of GaAs~ _xPx specially designed for observation of the [EL2*] -/° level. We describe the NPPC effect and finally we draw some conclusions concerning the homogeneity of our crystals. Thin GaAsl _xPx layers with x = 0.3-0.8 have been widely grown for LED applications for many years. However, growth of bulk GaAsl_xP x crystals is a serious problem owing to the different compositions of solid and liquid phases at equilibrium in the GaAs-GaP high temperature mixture, and consequently segregation phenomena during crystal growth. Nevertheless, several attempts have been described in the literature. Cerrina et al. [2] grew GaAsl _xPx polycrystals with x=0.1-0.4 by pulling from a Ga-rich solution. Saito and Seki [3] obtained GaAs~_xp, , x=0.1-0.8 polycrystals by using a "synthesis solute diffusion"-like method also from Ga solution. Ga-rich conditions, usually utilized in solution growth, prevent a high EL2 defect concentration in GaAs crystals, since the EL2 defect is associated with excess arsenic in the crystal. Also, near-stoichiometric methods have been used to grow bulk mixed crystals. For example Bachmann et al. [4] grew mixed single crystals of Ga~Inj _xP~.Asl ~ with x = 0-0.07 and y = 0-0.8 by a gradient freezing method. Kimura et al. [5] used phosphorus as a complementary impurity to indium to reduce the dislocation density in GaAs liquid encapsulated Czochralski (LEC) crystals grown from melt alloyed with In and Elsevier Sequoia
.;26
"ESlupihski et al.
/
Large negative persistent photoconductivi O'
InP. The concentration of P in their crystals was 0.1 - 1 mol.% and that of indium about 1%. Recently Jordan et al. [6] have grown GaAsl _~P~ crystals with x = 0 . 0 1 (single crystal) and x = 0.13 (polycrystal) by the horizontal Bridgman method with addition of GaP to the melt, aimed at basic investigations of antisite-related defects.
o o~
4 3
~X\\
ca_
2
......
.-
x)
3. Experimental results To evaluate the total EL2 defect concentration in the crystal optical absorption measurements were
~ ........ C - - f .
\,,
i
2. Crystal growth Our idea was to grow GaAs~ -~Px crystal by the conventional LEC method used for GaAs growth, but with InP added to the melt before the process. In this case the large difference between segregation coefficients of phosphorus (k0 = 3) and indium (k 0 = 0.12)[5, 6] leads to a much higher concentration of P than In in the seed part of the crystal. Since the concentration of phosphorus in a crystal must drop rapidly as the growth proceeds, an almost constant phosphorus concentration can be obtained if only a small fraction of liquid is solidified. The input Ga + As + InP charge of about 1 kg contained 0.82 mol.% InP, corresponding to the expected 2.5 mol.% P and 0.1% In at the beginning of the crystal. To reduce the gradient of phosphorus concentration we kept the diameter of the crystal within 1-1.5 cm until the fraction of liquid solidified g reached 0.02. (100)-oriented GaAs seed and B203 encapsulating layer were used. Nitrogen pressures of 60 bar during in situ synthesis and melting, about 3 bar during melt homogenization and about 10 bar during growth was applied. The pulling speed of about 8 mm h- ~ did not cause any problems with polycrystallization. To reach a high EL2 defect concentration, a slightly As-rich melt was used with molar ratio [As]/ [Ga] = 1.06 in the initial charge. The crystal was doped with tellurium to obtain a free electron concentration of a few times 1016 cm - 3. Figure 1 shows the crystal shape and phosphorus contents both expected and obtained. The composition was measured by near band gap photoluminescence (PL). The results are 2.2 + 0.2 mol.% P for fraction of liquid solidified g = 0 . 0 1 and 1.9_+0.2% for g=0.02, assuming that the PL lines shift to higher energies at a rate d E J d x = 12 meV per 1% [8]. The phosphorus composition x along the diameter of the initial part of the crystal was constant to within 0.4%. Concluding, the addition of 2-3 mol.% P to the crystal enlarged the energy gap by about 2 0 - 3 0 meV relative to GaAs.
,\
8
0
!
0
2
" " : - -: X
4 crgst@ length [cm]
6
8
Fig. 1. Crystal radius ( ), fraction of liquid solidified multiplied by 10 (10g ......), and expected phosphorus concentration ( - - - ) vs. crystal length. The points indicate the phosphorus content measured by near-band-gap photoluminescence.
8
% ~d 6
a /
C/,
® o v 4 F~ o_ 0 o 2
.-
105
~ /
/is
-
120 1.35 photon energy [eV]
1.50
Fig. 2. Absorption spectra of GaAsa _xP, at 10 K after cooling in the dark (curve a), after illumination with 1.05 ktm (1.18 eV) light (curve b) and after subsequent illumination with 0.825 Hm (1.5 eV)light (curve c).
taken in the photon energy range 0.7-1.55 eV at 10 K on a sample cut from the part corresponding to the fraction of liquid solidified g=0.01. The spectrum obtained after cooling the sample down in the dark (see Fig. 2, curve a) is typical of GaAs containing a neutral EL2 defect [9]. If practically all the absorption around 1.18 eV energy originates from neutral EL2, the concentration of the defect should be equal to about 2 × 1016 cm -3 [10]. Subsequent strong illumination with 1.05 /zm light (1.18 eV) only partially quenched the EL2 absorption related to the transition of EL2 to its metastable EL2* configuration, see curve b. This is very similar to the case of n-type GaAs under hydrostatic pressure [11]. Since at high pressure the EL2 defect in GaAs recovers very rapidly from its metastable configuration under near-band-gap illumination, we attempted to check whether this phenomenon occurs at ambient pressure in GaAs~-,Px. In the sample previously illuminated with 1.5 eV photons only a few per cent absorption in the 1.18 eV region was restored (see Fig. 2, curve c), much less than in GaAs under pressure.
T. Slupifiski et al.
/
Large negative persistent photoconductivi O,
327
15
8xi015 [
D aJl.0 >, // 2~1 I~
I
hv
1.18eV
4XI015
r
~ ~
I '11
/
~5
' '/
/
_1 D_
2x10~5 0
0 "1-
1.49
151
153
155
157
photon energy [eV] 0
20
40
60
Tempemlzur~[~ Fig. 3. Hall electron concentration measured as a function of temperature in GaAst_xPx, x=0.02. The arrows indicate the direction of the temperature sequence during experiment.Illumination with 1.18 eV light at 10 K causes a persistent increase in resistivity,thus showingthe NPPC effect.
To check the existence of NPPC we took resistivity and Hall effect measurements on a sample cut from the initial part of the crystal. Typical results are presented in Fig. 3. First we measured the Hall coefficient while cooling the sample to 10 K (the increase in Hall concentration observed at T< 15 K probably results from a two-band conduction mechanism in conduction and shallow impurity bands [ 12]). The sample was then illuminated at 10 K with 1.1 #m wavelength fight (1.13 eV), for 10 min, with a halogen lamp through an interference filter. The resistivity increased and the Hall electron concentration decreased by much more than one order of magnitude, thus showing a large NPPC effect. During subsequent heating of the sample the Hall concentration remained small up to about 40 K, when it returned to its original value before illumination. Successive cooling did not reveal any changes in comparison with the initial curve.
4. Discussion
The observed phenomena can be explained as follows. After cooling, illumination of the sample transformed the EL2 defect from its normal to the metastable configuration EL2*. Since the energy gap of GaAsl xP~ for x = 0 . 0 2 is higher than that of GaAs and since the acceptor level [EL2*]-/0 is already in the forbidden gap, it captures free electrons, thus decreasing the measured free electron concentration. After heating the sample over 40-45 K the EL2* defect returns to its normal coniguration, releasing previously captured electrons.
Fig. 4. An example of photoluminescencespectra observed for samples with fractions of liquid solidified of g= 0.01 (curve a) and g=0.02 (curve b). The shift in line energy between the curves indicateschangesin the energygap. The double structure in the 1.54 eV energyrange of curve b is discussed in the text.
A similar large EL2 defect related NPPC effect was discovered earlier in GaAs under pressure P> 0.3 GPa [1]. We observe some differences between the properties of GaAs under high pressure and GaAsl xPx at ambient pressure. Although incomplete photoquenching of EL2 (curves a and b in Fig. 2) is characteristic of n-GaAs under pressure, only partial photorecovery of the EL2 defect (curves b and c) is not consistent with the whole picture. There are at least two possible explanations for the above discrepancy: on an atomic scale nearby phosphorus atoms can modify some subtle properties of EL2 defects (e.g. photorecovery from metastable to normal state) or some large-scale fluctuations in P content can alter inhomogeneously the energy gap or can introduced mechanical stresses. Such inhomogeneities in our crystal were clearly seen from PL measurements as a broadening of some lines in the spectrum. As shown in Fig. 4, curve b, around 1.54 eV, a double structure is present. This results from simultaneous observation of the PL signal from at least two domains with different phosphorus contents, and thus with different energy gaps. The difference in energy of these lines corresponds to about 0.4% difference in P content between the domains. The existence of such domains explains why for a given fraction of liquid solidified g we have various P contents, evaluated from PL lines measured for different laser spot positions, seen in Fig. 1. The average size of such domains should be of the order of the laser light spot (a few hundred micrometres). The reason for the non-homogeneous phosphorus composition can be understood by considering the segregation of phosphorus (or rather GaP in GaAs + GaP liquid mixture) during growth. A segregation coefficient not equal to unity results in continuous
328
7~Stupifiski et al.
/
Large negative persistent photoconductivity
transport of excess dopant to (k 0 > 1 ) or from (k 0 < 1 ) the crystal-liquid interface. Various instabilities in such solute migration can cause irregularities of growth. Recently such behaviour was presented by Elliot et al. [13] for GaAs grown from As-rich melt. In our case the growth velocity was higher than that usually used for mixed material (equal to that for GaAs growth), thus amplifying the instabilities. A lower growth velocity should be helpful in decreasing the variations in phosphorus concentration. We also tried to grow crystals of GaAs~ xPx with higher x by the L E C method. Starting from the melt with 3 mol.% GaP we grew mixed crystals with initial composition x = 0.09. Unfortunately, only polycrystals were obtained. The probable reason for polycrystalline growth immediately after seeding is a rise in the crystallization temperature caused by the presence of G a E and thus a high initial supercooling necessary for GaAs seed. Successive utilization of GaAs~ _xP, seeds with higher and higher phosphorus concentrations could be tried to obtain single-crystal growth, as has been done for Ga~ _~In~As growth [14]. Initially, in our work we used InP as a source material for phosphorus. Later, GaP was added equally well to the charge, though to melt it completely (it could be easily seen on the melt surface), overheating of the melt by about 150 degrees was necessary.
5. Conclusions Bulk GaAs~ _xPx single crystals with x = 0.02-0.03 can be grown by the LEC technique from GaAs melt with the addition of either InP or GaP. For a high crystallization rate of about 8 mm h-~ some inhomogeneities of phosphorus concentration can occur. Such material enables investigations of the EL2* defect acceptor state without application of high hydrostatic pressure and thus allows, for example, uniaxial stress experiments. The negative persistent photoconductivity is a material property of GaAsl xPx with x > 0.02 at tem-
peratures below 45 K, with the E L 2 defect concentration comparable with or greater than the free electron concentration and with the free electron concentration not higher than a few times 1()16 cm
Acknowledgments We are very much indebted to Dr. M. Sadowski for careful reading of the manuscript. The GaAsl xP, crystals were grown at the Laboratory of Physics of Crystal Growth in Warsaw. This work was supported by KBN (Poland) grants No. 2-0424-91-01 and 2-0179-91-01.
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