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Spin dependent exciton recombination at nitrogen isoelectronic traps in GaP
Solid State Communications, Vol. 23, pp. 71—74, 1977.
Pergamon Press.
Printed in Great Britain
SPIN DEPENDENT EXCITON RECOMBINATION AT NITROGEN ISOELECTRONIC TRAPS IN GaP B.C. Cavenett, R.F.Brunwin and J.E. Nicholls Department of Physics, University of Hull, Hull, U.K. (Received 24 January 1977 by C. W. McCombie) An optically detected resonance at the conduction band g-valuehas been observed by monitoringthe emission of excitons bound at nitrogen isoelectronic traps in GaP. The observed decrease in emission intensity is explained in terms of a spin dependent formation of weakly radiating exciton states at resonance. THE OPTICAL SPECTRA due to electron—hole recombination in GaP containing nitrogen have been investigated in detail and, in particular, Zeeman measurements [1—3] on the sharp zero-phonon lines have confirmed that they are produced by the annihilation of excitons bound at isoelectronic traps. Excitonic molecules bound to the nitrogen traps have been investigated by Merz et a!. [4] and, in GaP with higher concentrations of nitrogen, pair spectra [3] and energy transfer processes between the pairs have been studied [5]. The purpose of this investigation is to characterize the details of the bound exciton complex by observing the exciton resonances using optical magnetic resonance techniques. These methods have been successfully applied to the investigation of recombination radiation in semiconductors, details of which can be found in the reviews by Lanipel [6] and Cavenett [71.In particular, here in this laboratory, we have used microwave optically detected magnetic resonance techniques to investigate donor— acceptor recombination in ZnS [8,9] and CdS [10].We note that recombination processes in GaP : N are particularly attractive for an investigation of bound excitons since the emission spectrum is due entirely to the one process of exciton annihilation at the nitrogen isoelectronic traps. In the present paper we report a microwave induced change of the bound exciton emission at the conduction band g-value which is interpreted in terms of a spin dependent exciton recombination process. The Gap: N samples were obtained from MCP and excitation with 488 nm radiation from an argon ion laser produced an emission spectrum similar to that reported by Thomas and Hopfield [3Jand, at the highest energy, structure was observed as shown in Fig. 1 (upper curve). The spectrum was recorded using a Spex double monochromator with an RCA C3 1034 photomultiplier tube and Brookdeal photon counter. The *
spectral resolution is 0.5 x i0~eV. The sharp lines labelled A and B result from the coupling of an S = conduction electron with a / = 3/2 hole from the valence band giving two exciton states! = 1 and! = 2 separated by 0.8 x i0~eV. Transitions to the ground state from these exciton levels give the Aand B lines respectively. The B line transitions are electric-dipole forbidden and so the B line is only observed at low temperatures where the! = 2 state is preferentially occupied. Observation of the microwave induced change in the luminescence was carried out at 8.9 GHz using 1.5W ofmicrowave power, chopped at 1 kHz. The sample was placed in a rectangular cavity with an optical window such that a polished (110) face was perpendicular to the magnetic field. The sample, cavity and superconducting magnet were immersed in liquid helium at 2 K. Luminescence, excited by 488 nm laser radiation with power up to 500 mW, was observed in a direction parallelto the magnetic field and the intensity of the total emission was monitored by an EM! 9558QB photomultiplier and a phase sensitive detector which was sensitive to 1mmnescence changes at the microwave chopping frequency. A polaroid circular polarizer was used to observe changes inI,+ and Ii,... Changes in I,~were also measured using a similar experimental arrangement but with a conventional magnet instead of the split-coil superconducting magnet. When the magnetic field was swept a strong resonance at g = 1.996 ±0.002 was observed as a decrease of 0.1% in the light emitted parallel to the magnetic field and this is shown in Fig. 2. Each component, I~,I~,_and I,~was found to decrease at resonance. The line width of the resonance is 0.01 T and the g-value corresponds to that measured by Title [111and Mehran eta!. [12] for donors in GaP. In order to determine whether all of the emission contributed to the resonance a spectral dependence of the resonance was obtained by passing the luminescence through the Spex monochromator and, with the magnetic field set at the resonance value, the
Present address: Department of Electrical Engineering and Electronics, U.M.I.S.T., Manchester, U.K. 71
72
NITROGEN ISOELECTRIC TRAPS IN GaP
I
GaP:N
Vol. 23, No. 1
Al
Fig. 2. Optically detected conduction band resonance in
GaP: N. The bound exciton emission decreases at resonance by a maximum value of ~J= 0.1%; I
I
2314
2316
2318
ENERGY (eVi Fig. 1. The upper diagram shows the bound exciton emission in GaP : N in the region of the zero phonon lines labelled A and B. The lower diagram shows the spectral dependence of the conduction band resonance of Fig. 2. The excitation, 500mW 3eV, at are488 the nm, sameand forthe both spectral resolution, 0.5 x l0 spectra.
and the statistical weighting of the basis states. In the first instance we also assume that no thermal redistribution occurs within the exciton states and that tran-
sitions from the 12, ±2) states to the ground state are forbidden. The principal features of our model can best be seen if we consider the limiting case at absolute zero when electrons will occupy only the I~, ~) state and —
holes only theformed 13/2, 3/2) which can be are state. Il, 1) The and only 12, 1) exciton of the Astates and B excitons respectively. At resonance and assuming microwave saturation, the electron states l~, ~>and I~, will be equally occupied. Now excitons can be formed in the 12, 2) state but, whereas both of the states before resonance gave rise to luminescence, the 12, 2) state has a highly forbidden transition to the ground state and so the total emission from the A and B lines falls by 50%. Thus at temperatures above T = 0 we assume that when resonance occurs in the electron states the rate of formation of exciton states which will emit decreases and the formation of non.emitting states 12, 2) and 12,— 2) increases. A detailed calculation in which we assume that the rate of formation of a state such as Ii, 1) is proportional to ~n 2n3+ *n1n4, etc, confirms that all of the components I~+,4,.. and I~,decrease at resonance in agreement with experimental results. For observation parallel to the magnetic field the calculated emission decrease is 0.6% for microwave saturation of the electron states in comparison with the observed 0.1% change. As there was no evidence of saturation of the resonance the experimental change should be smaller than the calculated value. Since several assumptions have been made in the development of the model it is important to consider the possible effects that other factors may have on the interpretation of the results. Firstly, for example, no account has been taken of thermalization within the exciton states whereas it is known that thermalization does occur both between the J = I and J = 2 states [131 and, in the presence of a magnetic field, within the —
change in luminescent intensity was measured as a function of wavelength. Alow resolution spectral dependence measurement was made across the whole of the emission and showed that all of the recombination radiation contributed to the resonance. Using 500mW of laser power a high resolution measurement was made in the region of the zero-phonon lines and this is shown in the lower part of Fig. 1. Since the resonance is detected by monitoring the exciton emission, the observation of a signal at the conduction band g.value is unexpected. In order to explain the observed results we outline below a model involving a spin dependent exciton recombination process which involves the conduction electron g.value. The energy levels before and after the exciton formation in the presence of a magnetic field are shown in Fig. 3. The two electron states, I~,+ ~) and l~,—~),and the four hole states, 13/2, 3/2), j3/2, ~), 13/2, — ~) and 13/2, — 3/2), are shown on the left of the diagram. On the right hand side of the diagram the exciton states are shown and labelled by the appropriate combinations of electron and hole states. The diagram also shows the allowed transitions as observed, for example, by Yafet and Thomas [21. The transitions from the 12 ±2) states are dashed because they are observed to be weak. We assume a Boltzmann distribution in the electron and hole states and that the rate of formation ofthe exciton in the IJ,Mj) state is proportional to the populations
Vol. 23, No. 1
NITROGEN ISOELECTRIC TRAPS IN GaP
11,1> 11,0)
A ELECTRON
fl
1
1112.1/2)
STATES
n3
I1/2rV2)
fl4
n6
13/2.3/2) 13/2,1/2, 13/2.-in) I3/2,-3/2)
—1/21/2,1/2>13/2.1/2>... 11/2, —1/2)13/2,3/2> I//!l 1/2,—l/2)13,2.1/2)/112 —1//711/2,1/2)13/2—1/2)
~,-1> 112IV2,-1/2)1312,-1/2>
EXCITON STATES
12,2> 12,1) 12,0)
B
HOLE STATES
73
-/~I1/2.1/2>I3/2,—3/2>
I1/2,112)I3/2. 312)
11211IZ4/2)13/2.3/2>+ ñ/2I 1/2.1/2)13/2,1/2) 11,1711,2,-1/2)1312,1,2),1Ifll 1/2.1/2)13/2, —1/2)
12,-I) = V2I1/2,112)13/2.-3/2+,ff/2I1/2,_1/2)13/2,_1n) 12-2): I1/2,-112)13/2.-3/2)
I~ ii ii ii
II
HIGHLY FORBIDDEN
10.0) O• it O~
Fig. 3. Splitting of electron, hole and exciton states in a magnetic field. The electron and hole states are shown on the left of the diagram with populations labelled n1, n2,. etc. On the right of the diagram the exciton states are shown with the corresponding combinations of the electron—hole basis states. The / = 1 and J = 2 states give rise to the A and B exciton emissions respectively. Transitions from the B exciton states are only weakly allowed and the dashed transitions from 2, ±2) to the 0,0) ground state are highly forbidden. . .
Zeeman energy levels [1]. Thermalization will reduce the maximum predicted value because the exciton will be thermally transferred from the weakly emitting 12, ±2) states to emitting states. We have not attempted to calculate the magnitude of this effect since the spin lattice relaxation times are not known. A second factor which may be important is the mixing of the A and B exciton states by the magnetic field and crystal strain [14].The mixing of the! = 1 states into the J = 2 states increases the emission rates from the (2, ±1) and 12, 0) states but leaves the emission rates from the 12, ±2) states unchanged. Thus the presence of strain would tend to diminish the effects of thermalization on the magnitude of the observed resonance from the B emission. Finally, in arriving at the proposed model it has been assumed that all of the exciton states form with a probability which is only proportional to the populations and statistical weightings of the basis electron and hole states. If this is not the case and the rate of formation depends on whether an A or B exciton is formed, either emission intensity increases or decreases could occur at resonance depending on whether the rate of formation of the A exciton is greater or smaller than the B exciton. The accepted exciton trapping process which occurs in GaP:N is illustrated in Fig. 4(a—c). Fig. 4(a) shows the electron—hole pairs formed by the laser radiation and Fig. 4(b) shows the electron preferentially trapped by the nitrogen centre. Finally, in Fig. 4(c), the hole is
— -
©
N EXCITATION
•
a
(b)
Ic)
Fig. 4. Exciton formation. The accepted exciton binding
process is shown in the sequence (a),(b) and (c). In (a) electrons and holes are formed and in (b) the electron is preferentially trapped at the nitrogen centre. Fmally i~(c) the hole is also trapped to form the bound exciton complex. An alternative binding process involves only (a) and (c). The exciton is formed from the electron— hole pairs and thenthe excitons are trapped at the nitrogen centres. attracted to the centre and the bound exciton complex is formed. Thus on this model the present experimental results would be interpreted as the observation of the change in recombination rates due to resonance of an electron bound at the isoelectronic trap since the bound electron g-value would be expected to be close to the conduction band value. A second possibility exists, however, and this is illustrated by Fig. 4(a) and (c). Excitons are formed from electrons and holes produced by the laser excitation and the excitons are then trapped at the
74
NITROGEN ISOELECTRIC TRAPS IN GaP
isoelectronic centres. We note that our explanation for
the observed spin dependent recombination does not enable us to distinguish between the two processes since only the electron g-value is involved but our more recent optically detected resonance measurements on GaP: N have shown two weak but reproducible resonances in the magnetic field region expected for the B exciton resonance [I]. We believe that the separation between the lines indicates a zero field splitting of an exciton at g 1.35. One of the lines has three components which we interpret as the nitrogen hyperfine mteraction (I = 1) and since the magnitude of this splitting is very much greater than the width of the conduction band resonance line reported in this paper we favour the model involving exciton formation before trappmg at the isoelectronic centre. Details of the new exciton resonances will be published elsewhere.
Vol. 23, No. 1
In conclusion, we have also observed optically detected resonances from the exciton I~and 12 lines in CdS. At this stage it is not clear whether the resonances are due due to conduction electrons or the bound excitons. These tesults will be published elsewhere [15]. We are grateful to Dr. W.E. Hagston for the valuable discussions on this work and to Dr. U. Davies for pointing out the effect of unequal exciton formation rates. We are also grateful to Dr. P.J. Dean of RSRE for discussions of the work and particularly for pointing out the effects of crystal strains. We are pleased to acknowledge the technical assistance from G. Sowersby, J. Harrison and E. Norman. We wish to thank Dr. A.E. Hughes and AERE Harwell for initial support this work and oneCouncil ofus (RFB) wishes to thank theofScience Research for a Research Fellowship. We are grateful for fmancial support of the work by SRC. Acknowledgements
—
REFERENCES 1. THOMAS D.G., GERSHENZON M. & HOPFIELD JJ.,Thys. Rev. 131,2397(1963). 2. YAFET Y. & THOMAS D.G.,Phys. Rev. 131, 2405 (1963). 3. 4. 5.
6. 7. 8. 9.
10. 11. 12. 13. 14. 15.
THOMAS D.G. & HOPFIELD JJ., Thys~Rev. 150,680(1966). MERZ J.L., FAULKNER R.A. & DEAN PJ.,Phys. Rev. 188, 1228 (1969). WEISNER PJ., STREET R.A. & WOLF H.D., Phys. Rev. Lett. 13, 1366 (1975). LAMPEL G.,F)-oc. XIII. Conf Phys. Semicond. (Edited by PILKUHN M.H.), p. 743. Teubner, Stuttgart (1974). CAVENETT B.C., Luminescence Spectroscopy (Edited by LUMB M.D.), Chap. 6. Academic Press (in press). JAMES J.R., NICHOLLS J.E., CAVENETT B.C., DAVIES JJ. & DUNSTAN DJ., Solid State Commun. 17, 969 (1975). JAMES J.R., CAVENETT B.C., NICHOLLS J.E., DAVIES J.J. & DUNSTAN DJ., Proc. mt. Conf Luminesc. Tokyo (1975);!. Luminesc. 12—13,447 (1976). BRUNWIN R.F., CAVENETT B.C., DAVIES JJ. & NICHOLLS J.E., Solid State Commun. 18, 1283 (1976). TITLE R.S.,Thys, Rev. 154,668 (1967). MEHRAN F., MORGAN T.N., TITLE R.S. & BLUM S.E.,Thys. Rev. B6, 3917 (1972). CUTHBERT J.D. & THOMAS D.G.,Thys. Rev. 154, 763 (1967). MERZ J.L.,Phys. Rev. 176,961(1968). DUNSTAN DJ., CAVENETT B.C., NICHOLLS J.E. & DAWSON P. (to be published).
Pergamon Press.
Printed in Great Britain
SPIN DEPENDENT EXCITON RECOMBINATION AT NITROGEN ISOELECTRONIC TRAPS IN GaP B.C. Cavenett, R.F.Brunwin and J.E. Nicholls Department of Physics, University of Hull, Hull, U.K. (Received 24 January 1977 by C. W. McCombie) An optically detected resonance at the conduction band g-valuehas been observed by monitoringthe emission of excitons bound at nitrogen isoelectronic traps in GaP. The observed decrease in emission intensity is explained in terms of a spin dependent formation of weakly radiating exciton states at resonance. THE OPTICAL SPECTRA due to electron—hole recombination in GaP containing nitrogen have been investigated in detail and, in particular, Zeeman measurements [1—3] on the sharp zero-phonon lines have confirmed that they are produced by the annihilation of excitons bound at isoelectronic traps. Excitonic molecules bound to the nitrogen traps have been investigated by Merz et a!. [4] and, in GaP with higher concentrations of nitrogen, pair spectra [3] and energy transfer processes between the pairs have been studied [5]. The purpose of this investigation is to characterize the details of the bound exciton complex by observing the exciton resonances using optical magnetic resonance techniques. These methods have been successfully applied to the investigation of recombination radiation in semiconductors, details of which can be found in the reviews by Lanipel [6] and Cavenett [71.In particular, here in this laboratory, we have used microwave optically detected magnetic resonance techniques to investigate donor— acceptor recombination in ZnS [8,9] and CdS [10].We note that recombination processes in GaP : N are particularly attractive for an investigation of bound excitons since the emission spectrum is due entirely to the one process of exciton annihilation at the nitrogen isoelectronic traps. In the present paper we report a microwave induced change of the bound exciton emission at the conduction band g-value which is interpreted in terms of a spin dependent exciton recombination process. The Gap: N samples were obtained from MCP and excitation with 488 nm radiation from an argon ion laser produced an emission spectrum similar to that reported by Thomas and Hopfield [3Jand, at the highest energy, structure was observed as shown in Fig. 1 (upper curve). The spectrum was recorded using a Spex double monochromator with an RCA C3 1034 photomultiplier tube and Brookdeal photon counter. The *
spectral resolution is 0.5 x i0~eV. The sharp lines labelled A and B result from the coupling of an S = conduction electron with a / = 3/2 hole from the valence band giving two exciton states! = 1 and! = 2 separated by 0.8 x i0~eV. Transitions to the ground state from these exciton levels give the Aand B lines respectively. The B line transitions are electric-dipole forbidden and so the B line is only observed at low temperatures where the! = 2 state is preferentially occupied. Observation of the microwave induced change in the luminescence was carried out at 8.9 GHz using 1.5W ofmicrowave power, chopped at 1 kHz. The sample was placed in a rectangular cavity with an optical window such that a polished (110) face was perpendicular to the magnetic field. The sample, cavity and superconducting magnet were immersed in liquid helium at 2 K. Luminescence, excited by 488 nm laser radiation with power up to 500 mW, was observed in a direction parallelto the magnetic field and the intensity of the total emission was monitored by an EM! 9558QB photomultiplier and a phase sensitive detector which was sensitive to 1mmnescence changes at the microwave chopping frequency. A polaroid circular polarizer was used to observe changes inI,+ and Ii,... Changes in I,~were also measured using a similar experimental arrangement but with a conventional magnet instead of the split-coil superconducting magnet. When the magnetic field was swept a strong resonance at g = 1.996 ±0.002 was observed as a decrease of 0.1% in the light emitted parallel to the magnetic field and this is shown in Fig. 2. Each component, I~,I~,_and I,~was found to decrease at resonance. The line width of the resonance is 0.01 T and the g-value corresponds to that measured by Title [111and Mehran eta!. [12] for donors in GaP. In order to determine whether all of the emission contributed to the resonance a spectral dependence of the resonance was obtained by passing the luminescence through the Spex monochromator and, with the magnetic field set at the resonance value, the
Present address: Department of Electrical Engineering and Electronics, U.M.I.S.T., Manchester, U.K. 71
72
NITROGEN ISOELECTRIC TRAPS IN GaP
I
GaP:N
Vol. 23, No. 1
Al
Fig. 2. Optically detected conduction band resonance in
GaP: N. The bound exciton emission decreases at resonance by a maximum value of ~J= 0.1%; I
I
2314
2316
2318
ENERGY (eVi Fig. 1. The upper diagram shows the bound exciton emission in GaP : N in the region of the zero phonon lines labelled A and B. The lower diagram shows the spectral dependence of the conduction band resonance of Fig. 2. The excitation, 500mW 3eV, at are488 the nm, sameand forthe both spectral resolution, 0.5 x l0 spectra.
and the statistical weighting of the basis states. In the first instance we also assume that no thermal redistribution occurs within the exciton states and that tran-
sitions from the 12, ±2) states to the ground state are forbidden. The principal features of our model can best be seen if we consider the limiting case at absolute zero when electrons will occupy only the I~, ~) state and —
holes only theformed 13/2, 3/2) which can be are state. Il, 1) The and only 12, 1) exciton of the Astates and B excitons respectively. At resonance and assuming microwave saturation, the electron states l~, ~>and I~, will be equally occupied. Now excitons can be formed in the 12, 2) state but, whereas both of the states before resonance gave rise to luminescence, the 12, 2) state has a highly forbidden transition to the ground state and so the total emission from the A and B lines falls by 50%. Thus at temperatures above T = 0 we assume that when resonance occurs in the electron states the rate of formation of exciton states which will emit decreases and the formation of non.emitting states 12, 2) and 12,— 2) increases. A detailed calculation in which we assume that the rate of formation of a state such as Ii, 1) is proportional to ~n 2n3+ *n1n4, etc, confirms that all of the components I~+,4,.. and I~,decrease at resonance in agreement with experimental results. For observation parallel to the magnetic field the calculated emission decrease is 0.6% for microwave saturation of the electron states in comparison with the observed 0.1% change. As there was no evidence of saturation of the resonance the experimental change should be smaller than the calculated value. Since several assumptions have been made in the development of the model it is important to consider the possible effects that other factors may have on the interpretation of the results. Firstly, for example, no account has been taken of thermalization within the exciton states whereas it is known that thermalization does occur both between the J = I and J = 2 states [131 and, in the presence of a magnetic field, within the —
change in luminescent intensity was measured as a function of wavelength. Alow resolution spectral dependence measurement was made across the whole of the emission and showed that all of the recombination radiation contributed to the resonance. Using 500mW of laser power a high resolution measurement was made in the region of the zero-phonon lines and this is shown in the lower part of Fig. 1. Since the resonance is detected by monitoring the exciton emission, the observation of a signal at the conduction band g.value is unexpected. In order to explain the observed results we outline below a model involving a spin dependent exciton recombination process which involves the conduction electron g.value. The energy levels before and after the exciton formation in the presence of a magnetic field are shown in Fig. 3. The two electron states, I~,+ ~) and l~,—~),and the four hole states, 13/2, 3/2), j3/2, ~), 13/2, — ~) and 13/2, — 3/2), are shown on the left of the diagram. On the right hand side of the diagram the exciton states are shown and labelled by the appropriate combinations of electron and hole states. The diagram also shows the allowed transitions as observed, for example, by Yafet and Thomas [21. The transitions from the 12 ±2) states are dashed because they are observed to be weak. We assume a Boltzmann distribution in the electron and hole states and that the rate of formation ofthe exciton in the IJ,Mj) state is proportional to the populations
Vol. 23, No. 1
NITROGEN ISOELECTRIC TRAPS IN GaP
11,1> 11,0)
A ELECTRON
fl
1
1112.1/2)
STATES
n3
I1/2rV2)
fl4
n6
13/2.3/2) 13/2,1/2, 13/2.-in) I3/2,-3/2)
—1/21/2,1/2>13/2.1/2>... 11/2, —1/2)13/2,3/2> I//!l 1/2,—l/2)13,2.1/2)/112 —1//711/2,1/2)13/2—1/2)
~,-1> 112IV2,-1/2)1312,-1/2>
EXCITON STATES
12,2> 12,1) 12,0)
B
HOLE STATES
73
-/~I1/2.1/2>I3/2,—3/2>
I1/2,112)I3/2. 312)
11211IZ4/2)13/2.3/2>+ ñ/2I 1/2.1/2)13/2,1/2) 11,1711,2,-1/2)1312,1,2),1Ifll 1/2.1/2)13/2, —1/2)
12,-I) = V2I1/2,112)13/2.-3/2+,ff/2I1/2,_1/2)13/2,_1n) 12-2): I1/2,-112)13/2.-3/2)
I~ ii ii ii
II
HIGHLY FORBIDDEN
10.0) O• it O~
Fig. 3. Splitting of electron, hole and exciton states in a magnetic field. The electron and hole states are shown on the left of the diagram with populations labelled n1, n2,. etc. On the right of the diagram the exciton states are shown with the corresponding combinations of the electron—hole basis states. The / = 1 and J = 2 states give rise to the A and B exciton emissions respectively. Transitions from the B exciton states are only weakly allowed and the dashed transitions from 2, ±2) to the 0,0) ground state are highly forbidden. . .
Zeeman energy levels [1]. Thermalization will reduce the maximum predicted value because the exciton will be thermally transferred from the weakly emitting 12, ±2) states to emitting states. We have not attempted to calculate the magnitude of this effect since the spin lattice relaxation times are not known. A second factor which may be important is the mixing of the A and B exciton states by the magnetic field and crystal strain [14].The mixing of the! = 1 states into the J = 2 states increases the emission rates from the (2, ±1) and 12, 0) states but leaves the emission rates from the 12, ±2) states unchanged. Thus the presence of strain would tend to diminish the effects of thermalization on the magnitude of the observed resonance from the B emission. Finally, in arriving at the proposed model it has been assumed that all of the exciton states form with a probability which is only proportional to the populations and statistical weightings of the basis electron and hole states. If this is not the case and the rate of formation depends on whether an A or B exciton is formed, either emission intensity increases or decreases could occur at resonance depending on whether the rate of formation of the A exciton is greater or smaller than the B exciton. The accepted exciton trapping process which occurs in GaP:N is illustrated in Fig. 4(a—c). Fig. 4(a) shows the electron—hole pairs formed by the laser radiation and Fig. 4(b) shows the electron preferentially trapped by the nitrogen centre. Finally, in Fig. 4(c), the hole is
— -
©
N EXCITATION
•
a
(b)
Ic)
Fig. 4. Exciton formation. The accepted exciton binding
process is shown in the sequence (a),(b) and (c). In (a) electrons and holes are formed and in (b) the electron is preferentially trapped at the nitrogen centre. Fmally i~(c) the hole is also trapped to form the bound exciton complex. An alternative binding process involves only (a) and (c). The exciton is formed from the electron— hole pairs and thenthe excitons are trapped at the nitrogen centres. attracted to the centre and the bound exciton complex is formed. Thus on this model the present experimental results would be interpreted as the observation of the change in recombination rates due to resonance of an electron bound at the isoelectronic trap since the bound electron g-value would be expected to be close to the conduction band value. A second possibility exists, however, and this is illustrated by Fig. 4(a) and (c). Excitons are formed from electrons and holes produced by the laser excitation and the excitons are then trapped at the
74
NITROGEN ISOELECTRIC TRAPS IN GaP
isoelectronic centres. We note that our explanation for
the observed spin dependent recombination does not enable us to distinguish between the two processes since only the electron g-value is involved but our more recent optically detected resonance measurements on GaP: N have shown two weak but reproducible resonances in the magnetic field region expected for the B exciton resonance [I]. We believe that the separation between the lines indicates a zero field splitting of an exciton at g 1.35. One of the lines has three components which we interpret as the nitrogen hyperfine mteraction (I = 1) and since the magnitude of this splitting is very much greater than the width of the conduction band resonance line reported in this paper we favour the model involving exciton formation before trappmg at the isoelectronic centre. Details of the new exciton resonances will be published elsewhere.
Vol. 23, No. 1
In conclusion, we have also observed optically detected resonances from the exciton I~and 12 lines in CdS. At this stage it is not clear whether the resonances are due due to conduction electrons or the bound excitons. These tesults will be published elsewhere [15]. We are grateful to Dr. W.E. Hagston for the valuable discussions on this work and to Dr. U. Davies for pointing out the effect of unequal exciton formation rates. We are also grateful to Dr. P.J. Dean of RSRE for discussions of the work and particularly for pointing out the effects of crystal strains. We are pleased to acknowledge the technical assistance from G. Sowersby, J. Harrison and E. Norman. We wish to thank Dr. A.E. Hughes and AERE Harwell for initial support this work and oneCouncil ofus (RFB) wishes to thank theofScience Research for a Research Fellowship. We are grateful for fmancial support of the work by SRC. Acknowledgements
—
REFERENCES 1. THOMAS D.G., GERSHENZON M. & HOPFIELD JJ.,Thys. Rev. 131,2397(1963). 2. YAFET Y. & THOMAS D.G.,Phys. Rev. 131, 2405 (1963). 3. 4. 5.
6. 7. 8. 9.
10. 11. 12. 13. 14. 15.
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