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Defect states in amorphous InP
Journal of Non-CrystallineSolids97&98 (1987) 1107-1110 North-Holland, Amsterdam
1107
DEFECT STATES IN AMORPHOUSInP M.L. THEYE+, A. GHEORGHIU+, D. UDRON+, C. SENEMAUD*, E. BELIN*, J. VON BARDELEBENx, S. SQUELARDx and J. DUPIN. +Laboratoire d'Optique des Solides, UA CNRS 781, Universit~ P. et M. Curie, 4 place Jussieu, 75252 Paris C@dex05, France *Laboratoire de Chimie Physique, LA CNRS 176, Universit~ P. et M. Curie, 11 rue P. et M. Curie, 75231 Paris C~dex 05, France XGroupe de Physique des Solides de l'Ecole Normale Sup~rieure, LA CNRS 17, Universit~ Paris VII, 2 place Jussieu, 75251 Paris C6dex 05, France .D~partement de Physique des Mat@riaux, LA CNRS 172, Universit~ Claude Bernard, Lyon I, 69621 Villeurbanne, France The electronic density of states of flash-evaporated amorphous Inl_xPx films, with x > 0.5, is investigated in detail by X-ray photoelectron spectroscopy and by combined optical and photothermal deflection measurements. The results are analyzed in terms of structural and chemical defects, in relation with the predictions of recent theoretical work. I. INTRODUCTION Previous studies on flash-evaporated amorphous (a-) InP have shown that this IIIV compound is highly disordered, both structurallyI and chemically 2. Our interest is focused here on the various types of defects present in this material, which we try to characterize by more sensitive and specific experiments. The results on the electronic density of states obtained by X-ray photoelectron spectroscopy (XPS) and photothermal deflection spectroscopy (PDS) are discussed as a function of composition, in relation with recent theoretical predictions 3. Preliminary electron spin resonance (ESR) results are also reported. 2. EXPERIMENT The samples were I000 to 5000 A thick films deposited by flash-evaporation of crystalline InP powder under high-vacuum (10-8 Torr range). Glass substrates were used for optical, PDS and ESR measurements, Me substrates for XPS experiments. The film composition was measured with 5 % accuracy by t~-particle back-scattering. When the substrates are maintained at low or room temperature, the flash-evaporation technique tends to give films containing an excess of P with respect to stoichiometry4. The P excess could be reduced by increasing the substrate temperature, but films deposited at 100 o C are already partially crystallized. Films deposited at 200 o C are polycrystaUine and stoichiometric. However, films deposited at room temperature remain amorphous upon annealing up to 250 ° C, without any composition change. We therefore studied a-lnl_xP x samples, with x ranging from 0.54 to 0.65.
0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M.L. Theye et aL / Defect states in amorphous InP
1108
The PDS measurements were performed both with a standard experimental set-up with a monochromatic exciting beam, and with a set-up combining mirage detection with Fourier transform spectroscopy5, down to 0.5 eV. The spectra were calibrated by fitting to the film absorptance values deduced from optical measurements in the 104 cm-I range, and the absorption coefficient 04- was then determined by using appropriate thin film formulae6. The XPS photoelectron spectra were induced by monochromatized AI K¢~ radiation. The total instrumental broadening was 0.5 eV for the In 4d and P 2p core levels, 0.8 eV for the valence band. The binding energies were evaluated for each sample by referring to the C Is level, taken at 285.0 eV. The ESR experiments were performed with an X-band spectrometer as a function of temperature, on films peeled off from large area substrates with collodion.
3. RESULTS AND DISCUSSION The existence of partial chemical disorder in a-InP has been verified by comparative XPS studies of the In and P core levels in amorphous and crystalline samples. The energy separation of the P 2p and In 4d levels is larger by about I eV in a-lnP, confirming without ambiguity that each level has shifted towards its energy in the pure element. This shift, which increases with x, is shown in figure I for the P 2p level ; it is accompanied by a significant broadening. The tailing of the spectra towards higher binding energies (i.e. the energy in pure P) is slightly more pronounced for the largest x value. Since the oxide contribution is about the same in both cases, this effect is consistent with the presence of an increasing proportion of P clusters. Figure 2 compares the valence band density of states,
LP2p,ro i n
as deduced f r o m XPS
measurements after background substraction, for c-lnP and for two a-lnP films with the same overall composition (x = 0.59) but deposited under different evaporation
,j 132
130
12R
126
BINDING ENERGY (eV)
FIGURE 1 P 2p core level spectra for c-InP ( - - ) and a-lnP with X = 0.59 (I) and 0.65 (2)
|0
~
EF
BiNDiNG ENERGY(eV)
FIGURE 2 Valence band p h o t o e l e c t r o n s p e c t r a for c-lnP (- -) and for t w o d i f f e r e n t a-lnP fi l ms w i t h x = 0.59 ( - - )
M.L. Theye et al. / Defect states in amorphous lnP
1109
conditions. One first notices that the upper peak I, which corresponds to p-like In-P bonding states, remains centered at the same energy as in c-lnP, but is broadened on both sides. In particular, the upper edge is shifted as a whole to lower binding energies. These results are in agreement with those of previous X-ray K~ emission experiments2. The shift of the edge, also observed in the a-GaAs case7, can be ascribed to structural disorder, in particular to bond distortion8. There may be some additional contribution from P dangling bonds3. The intermediate structure II, composed of mixed In s and P p states, is washed out for film (a), as usually observed for a-III-V compounds9. The filling up of the valley between peaks I and II could be due to additional states introduced by P-P wrong bonds3. For film (b) on the contrary, a structure can be seen in this energy range. This discrepancy between two films with the same overall composition could be due to a larger amount of chemical disorder in film (b). This interpretation is supported by the simultaneous broadening of region II towards high binding energies, which could be related to In-ln wrong bond states. Perhaps the most striking difference between c- and a-lnP is however the splitting of the bottom peak Ill, which corresponds to slike P states. The sub-peaks are more intense in film (b) but they are centered at about the same energies. Such an effect has already been observed in As-rich a-GaAs films7. It is considered as the signature of V-V wrong bonds in a-III-Vcompounds3, 8 The location and energy separation of the two maxima suggest that, contrary to the a-GaAs case, there is a significant proportion of P clusters in addition to isolated P-P wrong bonds. Figure 3 presents the optical absorption edge, as deduced from combined optical and PDS measurements from 1.5 down to 0.5 eV, for a-lnP with the same composition (x = 0.59), as-deposited and annealed at increasing temperature TA. The increase of the optical gap is similar to that already reported 4. For as-deposited films, the edge is roughly exponential down to 0.5 eV, with an inverse slope Ee of the order of 140 meV.
i / / ~ I~
J
0.5
,
i
i
i
1.1
0
i
i
i
i
1.5
Enero¥ (eV) FIGURE 3 Optical absorption spectra for a-lnP films with x = 0.59 as-deposited (1) and annealed at TA = I00 (2), 150 (3), 200(4)°C.
1 1 lO
M.L Theye et al. / Defect states in amorphous InP
Upon annealing, one observes a slight gradual sharpening of the edge : Ee = 115 meV for TA = 200° C. But the most striking effect is the extra-absorption which appears more and more clearly at low .energies as the gap opens up and the edge sharpens. These results indicate first that a-lnP presents an important band tailing, somewhat reduced by annealing, which must primarily be due to its high structural disorder but may also include defect states close to the band edges, for example from P dangling bonds, like in a-GaAsI0. They also reveal the presence of a significant density of defect states deeper in the pseudo-gap, which seems not to be affected by annealing. This is consistent with theoretical computations 3, which locate the cation (In-In) wrong bond states in the phosphides closer to mid-gap because of the larger anion electronegativity. The contribution of states introduced by complex P-P wrong bond defects has also to be considered. It must however be noticed that the density of states at the center of the gap, where the Fermi level
is locatedp remains small, which explains that a T-I/4
behaviour of the conductivity at low temperatures is not observed in these samples4. Preliminary ESR experiments on the same samples were unable to detect any signal similar to that reported for sputtered a-lnP11. They revealed instead a thermally activated signal 12 which compares well with that observed in pure amorphous p13. More work is needed to decide whether
this signal is related to the sample
off-stoichiometry or is intrinsic to a-lnP. ACKNOWLEDGEMENT The authors are indebted to S. Fisson for his valuable help in sample preparation, and to C. Boccara for his participation in PDS experiments.
REFERENCES l) A. Gheorghiu, M. Ouchene, T. Rappeneau and M.L. Theye, J. Non-Cryst. Sol. 59/60 (1983) 621. 2) M. Ouchene, C. S~n~maud, E. Belin, A. Gheorghiu and M.L. Th~ye, J. Non-Cryst. Sol. 59/60 (1983) 625. 3) E.P. O'ReiUy and J. Robertson, Phys. Rev. B34 (1986) 8684. 4) A. Gheorghiu and M.L. Th~ye, in : TetrahedraUy-bonded amorphous semiconductors, eds. D. Adler and H. Fritsche (Plenum, New York, 1985) p. 213. 5) D. Fournier, C. Boccara and J. Badoz, Appl. Opt. 21 (1982) 74. 6) K. Driss-Khodja, A. Gheorghiu and M.L. Th~ye, Optics Comm. 55 (1985) 169. 7) C. S~n~maud, E. Belin, A. Gheorghiu and M.L. Th~ye, J. Non-Cryst. Sol. 77/78 (1985) 1289. 8) J.D. Joannopoulos and M.L. Cohen, Phys. Rev. B10 (1974) 1545. 9) N.J. Shevchik, J. Tejeda and M. Cardona, Phys. Rev. B9 (1974) 2627. I0. M.L. Th~ye, A. Gheorghiu, K. Driss-Khodja and C. Boccara, J. Non-Cryst. Sol. 77/78 (1985) 1293. 11) B. Hoheisel, J. Stuke and M. Stutzmann, in : Proc. 17th Int. Conf. on "Physics of Semiconductors", eds. J. Chadi and W.A. Harrison (Springer, New-York, 1985) p. 877. 12) P.C. Taylor, E.J. Friebele and S.G. Bishop, Sol. St. Commun. 28 (1978) 247. 13) B.V. Shanabrook and P.C. Taylor, Phys. Rev. B28 (1983) 1239.
1107
DEFECT STATES IN AMORPHOUSInP M.L. THEYE+, A. GHEORGHIU+, D. UDRON+, C. SENEMAUD*, E. BELIN*, J. VON BARDELEBENx, S. SQUELARDx and J. DUPIN. +Laboratoire d'Optique des Solides, UA CNRS 781, Universit~ P. et M. Curie, 4 place Jussieu, 75252 Paris C@dex05, France *Laboratoire de Chimie Physique, LA CNRS 176, Universit~ P. et M. Curie, 11 rue P. et M. Curie, 75231 Paris C~dex 05, France XGroupe de Physique des Solides de l'Ecole Normale Sup~rieure, LA CNRS 17, Universit~ Paris VII, 2 place Jussieu, 75251 Paris C6dex 05, France .D~partement de Physique des Mat@riaux, LA CNRS 172, Universit~ Claude Bernard, Lyon I, 69621 Villeurbanne, France The electronic density of states of flash-evaporated amorphous Inl_xPx films, with x > 0.5, is investigated in detail by X-ray photoelectron spectroscopy and by combined optical and photothermal deflection measurements. The results are analyzed in terms of structural and chemical defects, in relation with the predictions of recent theoretical work. I. INTRODUCTION Previous studies on flash-evaporated amorphous (a-) InP have shown that this IIIV compound is highly disordered, both structurallyI and chemically 2. Our interest is focused here on the various types of defects present in this material, which we try to characterize by more sensitive and specific experiments. The results on the electronic density of states obtained by X-ray photoelectron spectroscopy (XPS) and photothermal deflection spectroscopy (PDS) are discussed as a function of composition, in relation with recent theoretical predictions 3. Preliminary electron spin resonance (ESR) results are also reported. 2. EXPERIMENT The samples were I000 to 5000 A thick films deposited by flash-evaporation of crystalline InP powder under high-vacuum (10-8 Torr range). Glass substrates were used for optical, PDS and ESR measurements, Me substrates for XPS experiments. The film composition was measured with 5 % accuracy by t~-particle back-scattering. When the substrates are maintained at low or room temperature, the flash-evaporation technique tends to give films containing an excess of P with respect to stoichiometry4. The P excess could be reduced by increasing the substrate temperature, but films deposited at 100 o C are already partially crystallized. Films deposited at 200 o C are polycrystaUine and stoichiometric. However, films deposited at room temperature remain amorphous upon annealing up to 250 ° C, without any composition change. We therefore studied a-lnl_xP x samples, with x ranging from 0.54 to 0.65.
0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M.L. Theye et aL / Defect states in amorphous InP
1108
The PDS measurements were performed both with a standard experimental set-up with a monochromatic exciting beam, and with a set-up combining mirage detection with Fourier transform spectroscopy5, down to 0.5 eV. The spectra were calibrated by fitting to the film absorptance values deduced from optical measurements in the 104 cm-I range, and the absorption coefficient 04- was then determined by using appropriate thin film formulae6. The XPS photoelectron spectra were induced by monochromatized AI K¢~ radiation. The total instrumental broadening was 0.5 eV for the In 4d and P 2p core levels, 0.8 eV for the valence band. The binding energies were evaluated for each sample by referring to the C Is level, taken at 285.0 eV. The ESR experiments were performed with an X-band spectrometer as a function of temperature, on films peeled off from large area substrates with collodion.
3. RESULTS AND DISCUSSION The existence of partial chemical disorder in a-InP has been verified by comparative XPS studies of the In and P core levels in amorphous and crystalline samples. The energy separation of the P 2p and In 4d levels is larger by about I eV in a-lnP, confirming without ambiguity that each level has shifted towards its energy in the pure element. This shift, which increases with x, is shown in figure I for the P 2p level ; it is accompanied by a significant broadening. The tailing of the spectra towards higher binding energies (i.e. the energy in pure P) is slightly more pronounced for the largest x value. Since the oxide contribution is about the same in both cases, this effect is consistent with the presence of an increasing proportion of P clusters. Figure 2 compares the valence band density of states,
LP2p,ro i n
as deduced f r o m XPS
measurements after background substraction, for c-lnP and for two a-lnP films with the same overall composition (x = 0.59) but deposited under different evaporation
,j 132
130
12R
126
BINDING ENERGY (eV)
FIGURE 1 P 2p core level spectra for c-InP ( - - ) and a-lnP with X = 0.59 (I) and 0.65 (2)
|0
~
EF
BiNDiNG ENERGY(eV)
FIGURE 2 Valence band p h o t o e l e c t r o n s p e c t r a for c-lnP (- -) and for t w o d i f f e r e n t a-lnP fi l ms w i t h x = 0.59 ( - - )
M.L. Theye et al. / Defect states in amorphous lnP
1109
conditions. One first notices that the upper peak I, which corresponds to p-like In-P bonding states, remains centered at the same energy as in c-lnP, but is broadened on both sides. In particular, the upper edge is shifted as a whole to lower binding energies. These results are in agreement with those of previous X-ray K~ emission experiments2. The shift of the edge, also observed in the a-GaAs case7, can be ascribed to structural disorder, in particular to bond distortion8. There may be some additional contribution from P dangling bonds3. The intermediate structure II, composed of mixed In s and P p states, is washed out for film (a), as usually observed for a-III-V compounds9. The filling up of the valley between peaks I and II could be due to additional states introduced by P-P wrong bonds3. For film (b) on the contrary, a structure can be seen in this energy range. This discrepancy between two films with the same overall composition could be due to a larger amount of chemical disorder in film (b). This interpretation is supported by the simultaneous broadening of region II towards high binding energies, which could be related to In-ln wrong bond states. Perhaps the most striking difference between c- and a-lnP is however the splitting of the bottom peak Ill, which corresponds to slike P states. The sub-peaks are more intense in film (b) but they are centered at about the same energies. Such an effect has already been observed in As-rich a-GaAs films7. It is considered as the signature of V-V wrong bonds in a-III-Vcompounds3, 8 The location and energy separation of the two maxima suggest that, contrary to the a-GaAs case, there is a significant proportion of P clusters in addition to isolated P-P wrong bonds. Figure 3 presents the optical absorption edge, as deduced from combined optical and PDS measurements from 1.5 down to 0.5 eV, for a-lnP with the same composition (x = 0.59), as-deposited and annealed at increasing temperature TA. The increase of the optical gap is similar to that already reported 4. For as-deposited films, the edge is roughly exponential down to 0.5 eV, with an inverse slope Ee of the order of 140 meV.
i / / ~ I~
J
0.5
,
i
i
i
1.1
0
i
i
i
i
1.5
Enero¥ (eV) FIGURE 3 Optical absorption spectra for a-lnP films with x = 0.59 as-deposited (1) and annealed at TA = I00 (2), 150 (3), 200(4)°C.
1 1 lO
M.L Theye et al. / Defect states in amorphous InP
Upon annealing, one observes a slight gradual sharpening of the edge : Ee = 115 meV for TA = 200° C. But the most striking effect is the extra-absorption which appears more and more clearly at low .energies as the gap opens up and the edge sharpens. These results indicate first that a-lnP presents an important band tailing, somewhat reduced by annealing, which must primarily be due to its high structural disorder but may also include defect states close to the band edges, for example from P dangling bonds, like in a-GaAsI0. They also reveal the presence of a significant density of defect states deeper in the pseudo-gap, which seems not to be affected by annealing. This is consistent with theoretical computations 3, which locate the cation (In-In) wrong bond states in the phosphides closer to mid-gap because of the larger anion electronegativity. The contribution of states introduced by complex P-P wrong bond defects has also to be considered. It must however be noticed that the density of states at the center of the gap, where the Fermi level
is locatedp remains small, which explains that a T-I/4
behaviour of the conductivity at low temperatures is not observed in these samples4. Preliminary ESR experiments on the same samples were unable to detect any signal similar to that reported for sputtered a-lnP11. They revealed instead a thermally activated signal 12 which compares well with that observed in pure amorphous p13. More work is needed to decide whether
this signal is related to the sample
off-stoichiometry or is intrinsic to a-lnP. ACKNOWLEDGEMENT The authors are indebted to S. Fisson for his valuable help in sample preparation, and to C. Boccara for his participation in PDS experiments.
REFERENCES l) A. Gheorghiu, M. Ouchene, T. Rappeneau and M.L. Theye, J. Non-Cryst. Sol. 59/60 (1983) 621. 2) M. Ouchene, C. S~n~maud, E. Belin, A. Gheorghiu and M.L. Th~ye, J. Non-Cryst. Sol. 59/60 (1983) 625. 3) E.P. O'ReiUy and J. Robertson, Phys. Rev. B34 (1986) 8684. 4) A. Gheorghiu and M.L. Th~ye, in : TetrahedraUy-bonded amorphous semiconductors, eds. D. Adler and H. Fritsche (Plenum, New York, 1985) p. 213. 5) D. Fournier, C. Boccara and J. Badoz, Appl. Opt. 21 (1982) 74. 6) K. Driss-Khodja, A. Gheorghiu and M.L. Th~ye, Optics Comm. 55 (1985) 169. 7) C. S~n~maud, E. Belin, A. Gheorghiu and M.L. Th~ye, J. Non-Cryst. Sol. 77/78 (1985) 1289. 8) J.D. Joannopoulos and M.L. Cohen, Phys. Rev. B10 (1974) 1545. 9) N.J. Shevchik, J. Tejeda and M. Cardona, Phys. Rev. B9 (1974) 2627. I0. M.L. Th~ye, A. Gheorghiu, K. Driss-Khodja and C. Boccara, J. Non-Cryst. Sol. 77/78 (1985) 1293. 11) B. Hoheisel, J. Stuke and M. Stutzmann, in : Proc. 17th Int. Conf. on "Physics of Semiconductors", eds. J. Chadi and W.A. Harrison (Springer, New-York, 1985) p. 877. 12) P.C. Taylor, E.J. Friebele and S.G. Bishop, Sol. St. Commun. 28 (1978) 247. 13) B.V. Shanabrook and P.C. Taylor, Phys. Rev. B28 (1983) 1239.