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The generation of point defects in GaAs by electron-hole recombination at dislocations
Sol&Bav Eltcrmnrcs Vol. 21. PP. 1413-1417 0 Pcrpamon Press Ltd.. 1978 Printed m Great Britain
THE GENE-RATION OF POINT DEFECTS IN GaAs BY ELECTRON-HOLE RECOMBINATION AT DISLOCATIONS P. W. HUTCHINSON Department
of Physical Metallurgy.
and P. S. DOBSON Birmingham University,
England
and B. WAKEFIELD and S. O’HARA PORLI, Martlesham Heath, Suffolk. England Abstract-The degradation of GaAs heterojunction lasers results in the formation of long dislocation dipoles which grow by a climb process involving point defect concentrations of the order of lOI9 cm-‘. The driving force for this climb process is not understood and it has been suggested that the material contains a supersaturation of native interstitials which condense on the dislocation during device operation. An alternative model proposed that the driving force is related to the energy released by electron-hole recombination on the dislocation which is partially dissipated by the dislocation emitting vacancies into the surrounding lattice. Gallium arsenide substrates containing greater than 2 x IO’* tellurium atoms cm-’ contain interstitial concentrations of the order of IO” cm-’ which condense out to form small dislocation loops during an anneal at 880°C. The presence of these loops indicates that the annealed material does not contain excess interstitials in solution. This annealed material was optically pumped and examined by transmission electron microscopy. It was found that the loops developed into dipoles typical of degraded lasers. The number of point defects involved in this climb process increased with increasing pumping power and time. These results are discussed in terms of the two mechanisms listed above and it is concluded that the energy released by electron-hole recombination at the dislocation provides the driving force for the climb process.
INTRODUCTION
The
degradation
of
gallium arsenide heterojunction lasers during device operation or optical pumping can result in the formation of dark line defects which are associated with long dislocation dipoles lying within the vicinity of the active layer [ 1.2). The dipoles originate at threading dislocations or other small dislocation type defects and grow by a climb processI3.41. Since the dipoles are of interstitial character[3], this climb process must involve either the absorption of gallium and arsenic interstitials or the emission of vacancies from the dislocation. The driving force for this climb process is at present uncertain and two models have been suggested to account for the observations. It was originally suggested[5] that the device contains excess native point defects which migrate during operation as a consequence of recombination enhanced diffusion and condense out on the dislocation to produce the observed climb. It was later argued[6] that a supersaturation of just one interstitial species, i.e. gallium, would enable climb to occur by the dislocation effectively converting one gallium interstitial into one arsenic vacancy and undergoing one unit of climb in the process. From observations of the size of the dipoles it is clear that very large interstitial concentrations (- lOI cmm3)would be required. Since there is no confirmatory evidence that laser material contains such large concentrations of native interstitials an alternative model was proposed [7]. This model considers that the energy relased by electron-hole recombination at the dislocation causes the dislocation to emit vacancies into the surrounding lattice. In this mechanism it is envisaged that an atom in the lattice
adjacent to the dislocation core jumps onto the extra half plane of the dipole, leaving behind a vacant site in the lattice, which then migrates away from the dipole. If the material contained large concentrations of excess interstitials then it would be expected that they would condense out to form small dislocation loops during a suitable high temperature anneal. This does not occur in laser material. It has been shown however[8] that excess interstitials are present in tellurium or selenium doped n+ gallium arsenide containing doping concentrations in excess of 2 x lOI cmV3. When this n+ material is annealed at temperatures greater than 4OOYJ, the excess interstitials condense out of solution to form small dislocation loops which can be observed by transmission electron microscopy. From measurements of loop size and density it was calculated that the interstitial concentration in the n+ material is of the order of lO”~rn-~. An anneal of 30min at 800°C is sufficient to cause all the excess interstitiais to condense out since no increase in total loop area per unit volume occurred after further annealing. We regard the evidence of dislocation loops in annealed n’ material and their stability on further annealing as evidence that all the excess interstitials have condensed out and that the annealed material does not contain any significant concentration of native interstitials in solution at 800°C. If native interstitials are responsible for the observed climb during laser operation then it would be expected that no climb would occur during the optical pumping of annealed n+ material. If however the climb occurs by vacancy emission from the dislocation, then it would be expected that optical pumping of the annealed n+ material would result in the loops developing into large
1413
1414
P. W.
HUTCHINSONet al.
dipoles. This paper reports observations of the dislocation structures in annealed rrc gallium arsenide after optical pumping in an attempt to distinguish between the two models. MPERlMMTAL. Gallium arsenide slices containing 4.4 x IO” Te atoms cm-’ were annealed in a closed ampoule for 5 hr at 880°C. The surface of the annealed slice was passivated by using liquid phase epitaxy to deposit a lprn thick layer of Gao.,Al,,.~As and the sample was optically pumped using a Kr ion laser at 751 nm with a ISprn diameter beam. Several areas were pumped using a power of I40 or IOOmW for times varying from I to 80 min. After optical pumping the beam was defocussed and the photoluminescent output from the GaAlAs surwas face observed. The intensity of the photoluminescent output gave an indication of the relative rates of radiative recombination in the pumped and unpumed areas. In many cases, the pumped region gave rise to dark spots in the photoluminescent output which were an aid in locating the areas to be examined in the electron microscope. The samples were chemically thinned from the substrate surface down to approximately 3p.m and part of the GaAlAs layer was removed by ion beam thinning. These samples were then examined using 800 kV electrons in an EM7 high voltage electron microscope. RESULTS
Examination of the annealed R+ substrate confirmed that the material contained perfect dislocation loops of about 4000 A diameter (Fig. 1) and the equivalent interstitial concentration was -2 x 10” cme3. The sample also contained some faulted dislocation loops which exhibit fringe contrast and which are believed to be GazTez precipitatesI91. Examination of the interfacial regions of the substrate after liquid phase epitaxy showed that the loop distribution was unaffected by this process. The GaAIAs epitaxial layer itself was defect free apart from a few defects which had grown into the layer from loops which intersected the original substrate surface. Part of the slice was then annealed for 3hr at 850°C and no changes were observed in the loop distribution. A dark spot was observed in the photoluminescent output from each region which had been pumped at I40 mW for times between I and 80 min. the intensity of the spot increasing with pumping time. Dark spots only became visible after pumping for longer than 5 min at 100mW. Figure 2 shows a transmission electron micrograph, taken at low magnification, of a part of the sample which contains two regions which had been optically pumped. Region A was pumped for 10min at 140mW and region B had been pumped for 80 min at 140mW. It can be seen that each regicn contains a dense tangle of dislocations over a circular area of about 10pm dia and stereo microscr?y showed that they are contained within the top Irm of the substrate. The surrounding regions which had not been optically pumped contain small dislocation loops typical of the annealed material. Examination of
the
pumped regions at higher magnification showed that the dislocation tangles consist of a number of dipoles which have developed from the perfect loops. The faulted loops have been virtually unaffected by the optical pumping. An analysis of the dislocation dipoles confirmed that they had the same structural and morphological characteristics as those previously observed in degraded lasers. The effect of different optical pumping conditions can be seen in Figs. 3(a)-(c) which shows electron micrographs from three regions which had been pumped at 14OmW for 2min and 1OOmW for 3min and 2min respectively. The two regions pumped at 100mW did not show visible dark spots in the photoluminescent output but the electron micrographs demonstrate that dislocation climb has occurred. It is clear from this figure that the amount of climb increases with pumping power and time. An approximate calculation of the number of point defects involved in the climb process can be made from the size and number of the dislocation dipoles and the equivalent point defect concentration can be determined if it is assumed that climb is confined to a circle of 15pm diameter (i.e. the beam diameter) and a depth of Ipm. Since the dipoles occupy a circular area of somewhat smaller diameter (8-I lpm), presumably because of radial variations in the optical power density of the pumping beam, this calculation tends to underestimate the point defect concentration. The calculated point defect concentrations for the structures shown in Figs. 3(a)-(c) are 3x 10”. I X 10” and 4 X IO” cm+ respectively. It is more difficult to make this calculation for the more heavily pumped regions because of the tangled nature of the dense array of dipoles but it is clear that point defect concentrations in excess of lOI cmb3 are associated with the structures shown in Fig. 2. DISCUSSION
The observations of the defect structure in the annealed nc gallium arsenide substrate are identical to those described earlier[8,91 and show that the substrate contains perfect dislocation loops together with a few faulted loops which are believed to be GazTeo precipitates. The excess interstitial concentration which has condensed out to form these perfect loops is of the order of 2 x 10“cm-‘. No change of the defect structure was observed after epitaxial growth of the GaAIAs passive layer and after further annealing at 850°C. This indicates that the material is in thermal equilibrium with respect to the interstitial concentration in solution. Optical pumping of this material caused the loops to undergo further considerable climb and to develop into long dislocation dipoles. For the more heavily pumped regions (140 mW) this optical degradation was apparent from dark spots in the photoluminescent output whereas no dark spots were detected in those regions pumped at 1OOmW for less than 5 min. However examination by transmission electron microscopy showed that dislacation climb had occurred in all cases and the extent of the climb increased with pumping power and time. The concentration of point defects involved in this climb
The generation of point defects in GaAs by electron-hole
Fig. I. Dislocations loops in a GaAs substrate doped with 4.4 X IO’* Te atoms cme3 after annealing for 5 hr at 880°C. The marker bar represents Ipm.
recombination
at dislocations
Fig. 2. Dislocation dipoles in annealed n+ GaAs after optical pumping at 14OmW. Regions A and B were pumped for 10 and 60 min respectively. The marker bar represents IOpm.
(a)
Fig. 3. Dislocation
1415
dipoles in annealed n + GaAs after optical pumping. (a) 140 mW for 2 min (b) 100 mW for 3 min and (c) 100 mW for 2 min. The marker bars represent Iem.
The generation of point defects in GaAs by electron-hole tgcombination at dislocations ranged from 4~ 10’7cm-3 for the most lightly pumped case (100mW. 2 min) to > IO” crnm3for the most heavily
pumped case (140mw. 80 min). These results demonstrate that extensive climb has occurred during optical pumping in samples which contain a very low concentration of excess interstitials in solution. We regard these experiments as confirming that the climb occurs by vacancy emission from the dislocation as a consequence of the energy released by electron-hole recombination. This process creates large vacancy concentrations in the lattice during degradation which will diffuse away from the climbing dipole. If it is assumed that this vacancy migration involves recombination enhanced diffusion then the vacancies cannot escape in significant numbers from the recombination region. Although there is some evidence&] of small vacancy type loops lying outside a climbed dipole, the majority of these vacancies have not been detected. The reasons for this are not clear but it is well established from studies of ionimplantation damage[lOl. where equal numbers of vacancies and interstitials are created, that the interstitials condense out to form observable defects whereas the vacancies remain undetected by transmission electron microscopy. It has been argued[l I] that this is because the vacancies agglomerate into a large density of
1417
submicroscopic clusters. If significant numbers of the emitted vacancies remain in solution then these will give rise to a chemical stress on the climbing dipole and climb will become progressively more difficult as degradation proceeds. Acknowledgements-Two of us (P.W.H. and P.S.D.) acknowledge financial support from the science Research Council. REFERENCES
1. P. Petroff and R. L. Hartman, Appl. Phys. Letr. 23, 469 (1973). 2. P. W. Hutchinson, P. S. Dobson, S. O’Hara and D. H. Newman, Appl. Phys. Len. 26,250 (1975). 3. P. W. Hutchinson and P. S. Dobson, Phil. Mug. 32, 745 (1975). 4. H. Saito and T. Kawakami, IEEE J. Quantum Electron QE13, 564 (1977). 5. P. Petroff and R. L. Hartman, 1. Appl. Phys. 45, 3899 (1974). 6. P. M. Petrol7and L. C. Kimerling,Appl. Phys. Letf. 29. 461 (1976). 7. S. O’Hara, P. W. Hutchinson and P. S. Dobson. Appl. Phys. I_.-&30. 368 (1977). 8. P. W. Hutchinson and P. S. Dobson, 1. Mater. Sci. 10, 1636 (1975). 9. P. W. Hutchinson and P. S. Dobson, Phil. Mag. Jo.65 (1974). 10. R. S. Nelson, Radiation Damage and Defects in Semiconductors, p. 140.Institute of Physics Conf. Ser. No. 16 (1972). 11. B. L. Eyre, J. Phys. F. 3,422 (1973).
THE GENE-RATION OF POINT DEFECTS IN GaAs BY ELECTRON-HOLE RECOMBINATION AT DISLOCATIONS P. W. HUTCHINSON Department
of Physical Metallurgy.
and P. S. DOBSON Birmingham University,
England
and B. WAKEFIELD and S. O’HARA PORLI, Martlesham Heath, Suffolk. England Abstract-The degradation of GaAs heterojunction lasers results in the formation of long dislocation dipoles which grow by a climb process involving point defect concentrations of the order of lOI9 cm-‘. The driving force for this climb process is not understood and it has been suggested that the material contains a supersaturation of native interstitials which condense on the dislocation during device operation. An alternative model proposed that the driving force is related to the energy released by electron-hole recombination on the dislocation which is partially dissipated by the dislocation emitting vacancies into the surrounding lattice. Gallium arsenide substrates containing greater than 2 x IO’* tellurium atoms cm-’ contain interstitial concentrations of the order of IO” cm-’ which condense out to form small dislocation loops during an anneal at 880°C. The presence of these loops indicates that the annealed material does not contain excess interstitials in solution. This annealed material was optically pumped and examined by transmission electron microscopy. It was found that the loops developed into dipoles typical of degraded lasers. The number of point defects involved in this climb process increased with increasing pumping power and time. These results are discussed in terms of the two mechanisms listed above and it is concluded that the energy released by electron-hole recombination at the dislocation provides the driving force for the climb process.
INTRODUCTION
The
degradation
of
gallium arsenide heterojunction lasers during device operation or optical pumping can result in the formation of dark line defects which are associated with long dislocation dipoles lying within the vicinity of the active layer [ 1.2). The dipoles originate at threading dislocations or other small dislocation type defects and grow by a climb processI3.41. Since the dipoles are of interstitial character[3], this climb process must involve either the absorption of gallium and arsenic interstitials or the emission of vacancies from the dislocation. The driving force for this climb process is at present uncertain and two models have been suggested to account for the observations. It was originally suggested[5] that the device contains excess native point defects which migrate during operation as a consequence of recombination enhanced diffusion and condense out on the dislocation to produce the observed climb. It was later argued[6] that a supersaturation of just one interstitial species, i.e. gallium, would enable climb to occur by the dislocation effectively converting one gallium interstitial into one arsenic vacancy and undergoing one unit of climb in the process. From observations of the size of the dipoles it is clear that very large interstitial concentrations (- lOI cmm3)would be required. Since there is no confirmatory evidence that laser material contains such large concentrations of native interstitials an alternative model was proposed [7]. This model considers that the energy relased by electron-hole recombination at the dislocation causes the dislocation to emit vacancies into the surrounding lattice. In this mechanism it is envisaged that an atom in the lattice
adjacent to the dislocation core jumps onto the extra half plane of the dipole, leaving behind a vacant site in the lattice, which then migrates away from the dipole. If the material contained large concentrations of excess interstitials then it would be expected that they would condense out to form small dislocation loops during a suitable high temperature anneal. This does not occur in laser material. It has been shown however[8] that excess interstitials are present in tellurium or selenium doped n+ gallium arsenide containing doping concentrations in excess of 2 x lOI cmV3. When this n+ material is annealed at temperatures greater than 4OOYJ, the excess interstitials condense out of solution to form small dislocation loops which can be observed by transmission electron microscopy. From measurements of loop size and density it was calculated that the interstitial concentration in the n+ material is of the order of lO”~rn-~. An anneal of 30min at 800°C is sufficient to cause all the excess interstitiais to condense out since no increase in total loop area per unit volume occurred after further annealing. We regard the evidence of dislocation loops in annealed n’ material and their stability on further annealing as evidence that all the excess interstitials have condensed out and that the annealed material does not contain any significant concentration of native interstitials in solution at 800°C. If native interstitials are responsible for the observed climb during laser operation then it would be expected that no climb would occur during the optical pumping of annealed n+ material. If however the climb occurs by vacancy emission from the dislocation, then it would be expected that optical pumping of the annealed n+ material would result in the loops developing into large
1413
1414
P. W.
HUTCHINSONet al.
dipoles. This paper reports observations of the dislocation structures in annealed rrc gallium arsenide after optical pumping in an attempt to distinguish between the two models. MPERlMMTAL. Gallium arsenide slices containing 4.4 x IO” Te atoms cm-’ were annealed in a closed ampoule for 5 hr at 880°C. The surface of the annealed slice was passivated by using liquid phase epitaxy to deposit a lprn thick layer of Gao.,Al,,.~As and the sample was optically pumped using a Kr ion laser at 751 nm with a ISprn diameter beam. Several areas were pumped using a power of I40 or IOOmW for times varying from I to 80 min. After optical pumping the beam was defocussed and the photoluminescent output from the GaAlAs surwas face observed. The intensity of the photoluminescent output gave an indication of the relative rates of radiative recombination in the pumped and unpumed areas. In many cases, the pumped region gave rise to dark spots in the photoluminescent output which were an aid in locating the areas to be examined in the electron microscope. The samples were chemically thinned from the substrate surface down to approximately 3p.m and part of the GaAlAs layer was removed by ion beam thinning. These samples were then examined using 800 kV electrons in an EM7 high voltage electron microscope. RESULTS
Examination of the annealed R+ substrate confirmed that the material contained perfect dislocation loops of about 4000 A diameter (Fig. 1) and the equivalent interstitial concentration was -2 x 10” cme3. The sample also contained some faulted dislocation loops which exhibit fringe contrast and which are believed to be GazTez precipitatesI91. Examination of the interfacial regions of the substrate after liquid phase epitaxy showed that the loop distribution was unaffected by this process. The GaAIAs epitaxial layer itself was defect free apart from a few defects which had grown into the layer from loops which intersected the original substrate surface. Part of the slice was then annealed for 3hr at 850°C and no changes were observed in the loop distribution. A dark spot was observed in the photoluminescent output from each region which had been pumped at I40 mW for times between I and 80 min. the intensity of the spot increasing with pumping time. Dark spots only became visible after pumping for longer than 5 min at 100mW. Figure 2 shows a transmission electron micrograph, taken at low magnification, of a part of the sample which contains two regions which had been optically pumped. Region A was pumped for 10min at 140mW and region B had been pumped for 80 min at 140mW. It can be seen that each regicn contains a dense tangle of dislocations over a circular area of about 10pm dia and stereo microscr?y showed that they are contained within the top Irm of the substrate. The surrounding regions which had not been optically pumped contain small dislocation loops typical of the annealed material. Examination of
the
pumped regions at higher magnification showed that the dislocation tangles consist of a number of dipoles which have developed from the perfect loops. The faulted loops have been virtually unaffected by the optical pumping. An analysis of the dislocation dipoles confirmed that they had the same structural and morphological characteristics as those previously observed in degraded lasers. The effect of different optical pumping conditions can be seen in Figs. 3(a)-(c) which shows electron micrographs from three regions which had been pumped at 14OmW for 2min and 1OOmW for 3min and 2min respectively. The two regions pumped at 100mW did not show visible dark spots in the photoluminescent output but the electron micrographs demonstrate that dislocation climb has occurred. It is clear from this figure that the amount of climb increases with pumping power and time. An approximate calculation of the number of point defects involved in the climb process can be made from the size and number of the dislocation dipoles and the equivalent point defect concentration can be determined if it is assumed that climb is confined to a circle of 15pm diameter (i.e. the beam diameter) and a depth of Ipm. Since the dipoles occupy a circular area of somewhat smaller diameter (8-I lpm), presumably because of radial variations in the optical power density of the pumping beam, this calculation tends to underestimate the point defect concentration. The calculated point defect concentrations for the structures shown in Figs. 3(a)-(c) are 3x 10”. I X 10” and 4 X IO” cm+ respectively. It is more difficult to make this calculation for the more heavily pumped regions because of the tangled nature of the dense array of dipoles but it is clear that point defect concentrations in excess of lOI cmb3 are associated with the structures shown in Fig. 2. DISCUSSION
The observations of the defect structure in the annealed nc gallium arsenide substrate are identical to those described earlier[8,91 and show that the substrate contains perfect dislocation loops together with a few faulted loops which are believed to be GazTeo precipitates. The excess interstitial concentration which has condensed out to form these perfect loops is of the order of 2 x 10“cm-‘. No change of the defect structure was observed after epitaxial growth of the GaAIAs passive layer and after further annealing at 850°C. This indicates that the material is in thermal equilibrium with respect to the interstitial concentration in solution. Optical pumping of this material caused the loops to undergo further considerable climb and to develop into long dislocation dipoles. For the more heavily pumped regions (140 mW) this optical degradation was apparent from dark spots in the photoluminescent output whereas no dark spots were detected in those regions pumped at 1OOmW for less than 5 min. However examination by transmission electron microscopy showed that dislacation climb had occurred in all cases and the extent of the climb increased with pumping power and time. The concentration of point defects involved in this climb
The generation of point defects in GaAs by electron-hole
Fig. I. Dislocations loops in a GaAs substrate doped with 4.4 X IO’* Te atoms cme3 after annealing for 5 hr at 880°C. The marker bar represents Ipm.
recombination
at dislocations
Fig. 2. Dislocation dipoles in annealed n+ GaAs after optical pumping at 14OmW. Regions A and B were pumped for 10 and 60 min respectively. The marker bar represents IOpm.
(a)
Fig. 3. Dislocation
1415
dipoles in annealed n + GaAs after optical pumping. (a) 140 mW for 2 min (b) 100 mW for 3 min and (c) 100 mW for 2 min. The marker bars represent Iem.
The generation of point defects in GaAs by electron-hole tgcombination at dislocations ranged from 4~ 10’7cm-3 for the most lightly pumped case (100mW. 2 min) to > IO” crnm3for the most heavily
pumped case (140mw. 80 min). These results demonstrate that extensive climb has occurred during optical pumping in samples which contain a very low concentration of excess interstitials in solution. We regard these experiments as confirming that the climb occurs by vacancy emission from the dislocation as a consequence of the energy released by electron-hole recombination. This process creates large vacancy concentrations in the lattice during degradation which will diffuse away from the climbing dipole. If it is assumed that this vacancy migration involves recombination enhanced diffusion then the vacancies cannot escape in significant numbers from the recombination region. Although there is some evidence&] of small vacancy type loops lying outside a climbed dipole, the majority of these vacancies have not been detected. The reasons for this are not clear but it is well established from studies of ionimplantation damage[lOl. where equal numbers of vacancies and interstitials are created, that the interstitials condense out to form observable defects whereas the vacancies remain undetected by transmission electron microscopy. It has been argued[l I] that this is because the vacancies agglomerate into a large density of
1417
submicroscopic clusters. If significant numbers of the emitted vacancies remain in solution then these will give rise to a chemical stress on the climbing dipole and climb will become progressively more difficult as degradation proceeds. Acknowledgements-Two of us (P.W.H. and P.S.D.) acknowledge financial support from the science Research Council. REFERENCES
1. P. Petroff and R. L. Hartman, Appl. Phys. Letr. 23, 469 (1973). 2. P. W. Hutchinson, P. S. Dobson, S. O’Hara and D. H. Newman, Appl. Phys. Len. 26,250 (1975). 3. P. W. Hutchinson and P. S. Dobson, Phil. Mug. 32, 745 (1975). 4. H. Saito and T. Kawakami, IEEE J. Quantum Electron QE13, 564 (1977). 5. P. Petroff and R. L. Hartman, 1. Appl. Phys. 45, 3899 (1974). 6. P. M. Petrol7and L. C. Kimerling,Appl. Phys. Letf. 29. 461 (1976). 7. S. O’Hara, P. W. Hutchinson and P. S. Dobson. Appl. Phys. I_.-&30. 368 (1977). 8. P. W. Hutchinson and P. S. Dobson, 1. Mater. Sci. 10, 1636 (1975). 9. P. W. Hutchinson and P. S. Dobson, Phil. Mag. Jo.65 (1974). 10. R. S. Nelson, Radiation Damage and Defects in Semiconductors, p. 140.Institute of Physics Conf. Ser. No. 16 (1972). 11. B. L. Eyre, J. Phys. F. 3,422 (1973).