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Observation of lifetime controlling recombination centres in silicon power devices
Solid-Yorr fierrmnir~ Vol 23. pp. 129-132 Pcrgamon Pren Ltd.. 1980. Printed in Great Britain
OBSERVATION OF LIFETIME CONTROLLING RECOMBINATION CENTRES IN SILICON POWER DEVICES D. H. PAXMAN and K. R. WHIGHT Philips Research Laboratories, (Received
19 April
Redhill. Surrey. England
1979: in revised form
16 July 1979)
Abstract-A survey has been made by D.L.T.S. of the recombination centres present in n and p base silicon power devices. By comparing this data with high injection lifetime measurements, it has been possible to show that two of the levels observed have high capture cross sections and can control the lifetimes in a processed device. lNTRODUCTlON
In the manufacture of silicon power devices, e.g. power diodes, triacs and thyristors the high voltage require. ments demand the use of lightly doped base regions, so that most devices of this type contain a lOO-3OOpm thick base with dopings in the range 10’3-10’4cm-3. In forward conduction the carrier density introduced into this base is controlled by injection and recombination, and thus the carrier lifetime in the base region (7) has an important effect on both the forward I/V characteristic and the switching properties of the device. In the normal production processing of a power device the required lifetime for an unkilled structure is l-20 ps, and this is achieved by careful control of the processing procedure. However little is known as to the exact nature of the recombination centres which are active in typical devices, whether all processing yields the same type of centres, and even more critically what occurs when the processing fails to achieve the desired electrical result. This work was therefore aimed at examining the recombination centres present in a variety of conventionally diffused power devices, and then by a combination of this data and lifetime measurements to attempt to establish the degree of lifetime control exercised by particular centres. Such information leads to a better understanding of the role of the processing and enables a closer control to be obtained of device production
ristors in which the base region was identical n type material. The power transistors were n+lp(nln+ structures either triple diffused on (Ill) 40 51 cm n base material to give collectors -80 pm thick or using a thick = 100km epitaxial n layer on an n* substrate. 3. MEASUREMENTS
Two basic measurements were made on the devices. The recombination centre content (NT) of the lightly doped n or p bases was obtained using the conventional Deep Level Transient Spectroscopy (DLTS)[l] technique with a temperature scan from 77 to 3OOK.The equipment was capable of detecting
2. DEMCES All devices used for measurements were obtained from normal production processing either as fully encapsulated devices, or in the pre-encapsulation wafer form. Typical structures studied were simple p+(n(n+ and p’JpJn’ power devices made by diffusions into (111) float zone 10’3-10’4cm-3 doped n and p type bulk silicon. The p+ and n+ diffusions were from C.V.D. 1 I@- -erf deposited layers of phosphorus and boron doped glasses Q I + (J&J [“Silox”] driven in at 1260°Cfor > 25 hrs to yield diffused p+ and n+ layers 50pm deep. Base widths from 90 to where T,,,/Q is the ratio of the steady current phase 25Opm were obtained by suitably dimensioning the during switching to TVthe base lifetime and J, and J, are starting * slice thickness. These prolonged high tem- the forward and reverse currents, without corrections as perature diffusion treatments were common to the thy- the dimensions of the diode were large with respect to
J( >
D. H. PAXMAN and K. R. WHIGHT
130
diffusionlengths. In the case of particularlysimplespectra it was possibleto examinefor correlationsbetweenthe number and type of trap present and the corresponding lifetime.If a strong correlation exists then it would seem probable that the lifetime was being controlled by the centre beingobserved,or an associatedlevel inthecaseof multiplechargestate centres. e.g. Au.If traps were present havingcharacteristiclevelsin the minorityhalf of the band gap thus not accessible to the conventional DLTS technique and not associatedwith the observed centre such a correlation would be unlikelyto occur.
4.0
4.rWut.n
6.0
IO 0
60
Fig. 2. Emission rate v. 1000/T plots.
(i) Recombination ctwttr spectm The spectra obtained in n base devices were usually
rather simpleshowingone or two lines. Two particularly complex spectra are illustrated in Fig. I. These two devices show a total of 7 diierent lines correspondingto traps lyingin the upper half of the band gap labelkd with the temperature of the line peak at 7nf = Mms. These spectral line positionsare reproducibleto within2 1°K.It is interesting to note that though each of these devices has similarOCVDlifetimes,and overall trap density, the nature of the centres involved is completely different. A summary of all the lines seen in n base devices is presented in the form of loge,, u(1tWT) plot in Fii. 2 where e, is the electron emissionrate and T the absolute temperature. In additionto the 7 lines observed in Fig. 1 there is a complex series of linesin the region of “164°K line. The line at “175°K is resolvable but the line between 175 and 1’64Kis difficult to resolve unambiguously. In normally processed devices these centres occur with concentrations between 10”-lO”cm-“, and with the exception of the 260and 156Ktines (see below) each centre can occur as an individualspecies.
By comparingFig. 2 with publisheddata and examinand n base devices the 260K line was identified as the acceptor level of Au at E-561 meV. The 156Kline seen only in devices havingthe 260Kline present, which also contain rather lightly doped p’ layers is attriiuted to the Au donor level at Ev+ 332meV being observed due to small amounts of depletion on the p side of the test diode. The 175Kline lies close to a process induced quenched-in recombination centre reported by Sah and Yau[S]and Barbollaei al.[5]. The two lines at 165 and 164K are difficult to resolve over the experimentally available time constant range (5-999mS) and were normallytreated as a singleline for comparisonwith lifetime data. The other lines observed were not associated with other simple metallic contaminants Pt, Rh and Pd examinedby Pals[6]. Table 1 summarises the properties of the centres observed in n base devices with the data presented in the form ing“Au killed” p
Emissiontime constant r = $expg Tronsirtor basr collector p/n/n+
.
The recombination centre spectrum in the p base devices studied showed only one line that had significant
diode
Table 1.
Linedesignation
kT
Comment
116K 1 lS6K
260
3.3 x 10-a 0.561 T2 exp-z6.5 x IO-’ 0.460 r=-expkTTL 1.63 x lo-’ 0.264 ‘=T2expkT 4.47 x 10-e 0.276 T=-expCTTz x 1.2 IO-* 0.332 r=~--yIrT 0.356 7=
T=204 175
l06K
164
156 145 116 77K
Tomrmmtura o<
326K
Fig. 1. Spectra in n base devices. (TV, = 30 mS)
106
T*
7=
9.6 x IO-’
T*
exPkT 0.221
expkT
7 = 2.726 x 1O-9 0.236 expkT T’
Au acceptor
A” donor
Observation
of lifetime controlling recombination
effect on lifetime at 165K shown in Fig. 5 for which 3.2 x lo-’ r=-expkT T
0.31
(ii) Lifetime t$ects n base devices. In order to determine the effects of a particular recombination centre on lifetime a large number of devices having particularly simple spectra were examined. Figure 3 shows a simple spectrum indicating the presence of 4 x IO” cm-” 164K line centres and 5 x IO” cme3 Au centres. The lifetime contribution due to the Au was known from experiments on heavily doped Au samples[7]. In this case as Au is a relatively inefficient recombination centre the lifetime associated with its presence is = 300~s. The high level OCVD lifetime of the diode was 9.7 I.IS and this was therefore attributed entirely due to the presence of the 164K centres. As the diodes examined showed concentrations of the l64K line in the range 10”-10’“cm-3 it was possible to determine a lifetime D.concentration plot for this centre as shown in Fig. 4. This plot also includes data for Au as a comparison. The spread of data at NT(M) < lOI cme2 is probably due to the influence of other recombination effects, e.g. surface recombination
centres
131
at long lifetimes. The points for Nr(164) at 5 x lOI cmm3represent devices which had been deliberately rapidly cooled from 1OOOYJ which was shown to introduce the 164K line in our samples. In view of the experimental difficulties in making the measurements, the data shows that it was possible to associate a lifetime effect with the 164K line having a systematic control over the range 5 x 10” - 5 x 10” cme3. With the effective lifetime given by 7 = (4.6 x l@)/Nr corresponding to an effective cross s’ection (a.~), taking v = IO’cmsec-I, of 22 x lo-” cmw2.This is considerably greater than for Au for which the equivalent value is IO-‘-’cm’. p base devices. With the simpler spectra observed in the p base devices showing the 165K line only, similar correlations between lifetime and concentrations were more easily observed. The results shown in Fig. 6 indicated a control of the lifetime by this centre with
I
77K
300K
L
77K
I
Fig. 3. Simple Au and process induced line spectrum. (Tag = mS)
273K
Tmperature (K) Fig. 5. Spectrum in
p base device. (r,,, = 30 mS)
ffect of Au
Effect
of Au
\
Id’
IO"
IO"
IO-
No. of recembinat~on cenlres km-‘)
Fig. 4. Effect of 164K line on lifetime with gee defined from T = I/(A$uefl) V, with Y = 10’ cmsec-‘.
2 No. of recombination
IO"
centres hi’)
Fig. 6. Effect of 165K line on lifetime.
132
D. H. PAXMANand K. R. WHICHT
T = (3.2 x lo’)/& with (T.== 3.1 x 1O-‘5cm* (Y = 10’cm set-‘). Again good correlation was observed and the centre was more effective than Au in controlling lifetime. The difference in effective high injection crdss section between this 165K line observed in p type samples and the 164K lines seen in n type samples showed that these are not associated levels of the same recombination centre for in such a case the effective cross sections would be identical. 5.DISCUSSIONAND CONCLUSIONS shown that a limited number of recombination centre levels were observed in conventionally processed silicon power devices using the DLTS examination technique. It was possible to obtain a good correlation between high level lifetime and recombination centre content for n and p base devices showing simple spectra, and it was demonstrated that the 164K (T,~ = 30 ms) line observed in n type material had a particularly large effective cross section of 22X lo-” cm*. Similarly in p base devices the 165K (calf = 30ms) line correlated well with lifetime giving an effective cross section of 3.1 x IO-” cm’. Both these values were greater than for Au which had an effective cross section of 10-‘5cm2, showing that they had important effects on controlling the lifetime in unkilled device structures. These centres were observed in concentrations from 5 x 10” to 5 x 10” cm-’ in high temperature processed devices examined and determined the It has
been
electrical
properties of these devices. The 164K line observed in n type devices was introduced by deliberate rapid cooling from 1OOO’C. The different effective cross sections for the two levels seen in n and p base devices examined showed that they were not two related levels of the same recombination centre. The possible presence of an independent level in the lower half of the band gap [the 165K p base level] would represent a situation in which lifetime reductions would be observed in n base devices without a corresponding DLTS spectra. Such devices have been observed. Thus a calculation of expected lifetime from an observed spectrum will only give an upper value for the lifetime which could be obtained from such a device. Acknowledgemenls-We would like to thank our colleagues concerned with power device production and development at Mullard Hazel Grove and Phillips Nijmegen for providing devices and helpful discussions. REFERENCES I. G. L. Miller, J. V. Ramirez and D. A. H. Robinson, /. Appl. Phys. 46.2638 ( 1975). 2. H. Schlangenotto-and W. Gerlach, Solid-St. Ekcfron. IS, 393 (1972). 3. D. L. Lewis, Solid-St. Electron. 18, 87 (1975). 4. L. D. Yau and C. T. Sah, Solid-St. Electron. 17, 193 (1974). 5. J. Barbolla, L. Bailon, J. C. Brabant, M. Pugnet and M. Brousseau, Revue de Physique Applique 11.403 (1976). 6. J. Pals, Solid-St. Elecrmn. 17. II39 (19751. 7. S. D. Brotherton, J. Appl. Phys. 49, 66; (1978) (and private communication).
OBSERVATION OF LIFETIME CONTROLLING RECOMBINATION CENTRES IN SILICON POWER DEVICES D. H. PAXMAN and K. R. WHIGHT Philips Research Laboratories, (Received
19 April
Redhill. Surrey. England
1979: in revised form
16 July 1979)
Abstract-A survey has been made by D.L.T.S. of the recombination centres present in n and p base silicon power devices. By comparing this data with high injection lifetime measurements, it has been possible to show that two of the levels observed have high capture cross sections and can control the lifetimes in a processed device. lNTRODUCTlON
In the manufacture of silicon power devices, e.g. power diodes, triacs and thyristors the high voltage require. ments demand the use of lightly doped base regions, so that most devices of this type contain a lOO-3OOpm thick base with dopings in the range 10’3-10’4cm-3. In forward conduction the carrier density introduced into this base is controlled by injection and recombination, and thus the carrier lifetime in the base region (7) has an important effect on both the forward I/V characteristic and the switching properties of the device. In the normal production processing of a power device the required lifetime for an unkilled structure is l-20 ps, and this is achieved by careful control of the processing procedure. However little is known as to the exact nature of the recombination centres which are active in typical devices, whether all processing yields the same type of centres, and even more critically what occurs when the processing fails to achieve the desired electrical result. This work was therefore aimed at examining the recombination centres present in a variety of conventionally diffused power devices, and then by a combination of this data and lifetime measurements to attempt to establish the degree of lifetime control exercised by particular centres. Such information leads to a better understanding of the role of the processing and enables a closer control to be obtained of device production
ristors in which the base region was identical n type material. The power transistors were n+lp(nln+ structures either triple diffused on (Ill) 40 51 cm n base material to give collectors -80 pm thick or using a thick = 100km epitaxial n layer on an n* substrate. 3. MEASUREMENTS
Two basic measurements were made on the devices. The recombination centre content (NT) of the lightly doped n or p bases was obtained using the conventional Deep Level Transient Spectroscopy (DLTS)[l] technique with a temperature scan from 77 to 3OOK.The equipment was capable of detecting
2. DEMCES All devices used for measurements were obtained from normal production processing either as fully encapsulated devices, or in the pre-encapsulation wafer form. Typical structures studied were simple p+(n(n+ and p’JpJn’ power devices made by diffusions into (111) float zone 10’3-10’4cm-3 doped n and p type bulk silicon. The p+ and n+ diffusions were from C.V.D. 1 I@- -erf deposited layers of phosphorus and boron doped glasses Q I + (J&J [“Silox”] driven in at 1260°Cfor > 25 hrs to yield diffused p+ and n+ layers 50pm deep. Base widths from 90 to where T,,,/Q is the ratio of the steady current phase 25Opm were obtained by suitably dimensioning the during switching to TVthe base lifetime and J, and J, are starting * slice thickness. These prolonged high tem- the forward and reverse currents, without corrections as perature diffusion treatments were common to the thy- the dimensions of the diode were large with respect to
J( >
D. H. PAXMAN and K. R. WHIGHT
130
diffusionlengths. In the case of particularlysimplespectra it was possibleto examinefor correlationsbetweenthe number and type of trap present and the corresponding lifetime.If a strong correlation exists then it would seem probable that the lifetime was being controlled by the centre beingobserved,or an associatedlevel inthecaseof multiplechargestate centres. e.g. Au.If traps were present havingcharacteristiclevelsin the minorityhalf of the band gap thus not accessible to the conventional DLTS technique and not associatedwith the observed centre such a correlation would be unlikelyto occur.
4.0
4.rWut.n
6.0
IO 0
60
Fig. 2. Emission rate v. 1000/T plots.
(i) Recombination ctwttr spectm The spectra obtained in n base devices were usually
rather simpleshowingone or two lines. Two particularly complex spectra are illustrated in Fig. I. These two devices show a total of 7 diierent lines correspondingto traps lyingin the upper half of the band gap labelkd with the temperature of the line peak at 7nf = Mms. These spectral line positionsare reproducibleto within2 1°K.It is interesting to note that though each of these devices has similarOCVDlifetimes,and overall trap density, the nature of the centres involved is completely different. A summary of all the lines seen in n base devices is presented in the form of loge,, u(1tWT) plot in Fii. 2 where e, is the electron emissionrate and T the absolute temperature. In additionto the 7 lines observed in Fig. 1 there is a complex series of linesin the region of “164°K line. The line at “175°K is resolvable but the line between 175 and 1’64Kis difficult to resolve unambiguously. In normally processed devices these centres occur with concentrations between 10”-lO”cm-“, and with the exception of the 260and 156Ktines (see below) each centre can occur as an individualspecies.
By comparingFig. 2 with publisheddata and examinand n base devices the 260K line was identified as the acceptor level of Au at E-561 meV. The 156Kline seen only in devices havingthe 260Kline present, which also contain rather lightly doped p’ layers is attriiuted to the Au donor level at Ev+ 332meV being observed due to small amounts of depletion on the p side of the test diode. The 175Kline lies close to a process induced quenched-in recombination centre reported by Sah and Yau[S]and Barbollaei al.[5]. The two lines at 165 and 164K are difficult to resolve over the experimentally available time constant range (5-999mS) and were normallytreated as a singleline for comparisonwith lifetime data. The other lines observed were not associated with other simple metallic contaminants Pt, Rh and Pd examinedby Pals[6]. Table 1 summarises the properties of the centres observed in n base devices with the data presented in the form ing“Au killed” p
Emissiontime constant r = $expg Tronsirtor basr collector p/n/n+
.
The recombination centre spectrum in the p base devices studied showed only one line that had significant
diode
Table 1.
Linedesignation
kT
Comment
116K 1 lS6K
260
3.3 x 10-a 0.561 T2 exp-z6.5 x IO-’ 0.460 r=-expkTTL 1.63 x lo-’ 0.264 ‘=T2expkT 4.47 x 10-e 0.276 T=-expCTTz x 1.2 IO-* 0.332 r=~--yIrT 0.356 7=
T=204 175
l06K
164
156 145 116 77K
Tomrmmtura o<
326K
Fig. 1. Spectra in n base devices. (TV, = 30 mS)
106
T*
7=
9.6 x IO-’
T*
exPkT 0.221
expkT
7 = 2.726 x 1O-9 0.236 expkT T’
Au acceptor
A” donor
Observation
of lifetime controlling recombination
effect on lifetime at 165K shown in Fig. 5 for which 3.2 x lo-’ r=-expkT T
0.31
(ii) Lifetime t$ects n base devices. In order to determine the effects of a particular recombination centre on lifetime a large number of devices having particularly simple spectra were examined. Figure 3 shows a simple spectrum indicating the presence of 4 x IO” cm-” 164K line centres and 5 x IO” cme3 Au centres. The lifetime contribution due to the Au was known from experiments on heavily doped Au samples[7]. In this case as Au is a relatively inefficient recombination centre the lifetime associated with its presence is = 300~s. The high level OCVD lifetime of the diode was 9.7 I.IS and this was therefore attributed entirely due to the presence of the 164K centres. As the diodes examined showed concentrations of the l64K line in the range 10”-10’“cm-3 it was possible to determine a lifetime D.concentration plot for this centre as shown in Fig. 4. This plot also includes data for Au as a comparison. The spread of data at NT(M) < lOI cme2 is probably due to the influence of other recombination effects, e.g. surface recombination
centres
131
at long lifetimes. The points for Nr(164) at 5 x lOI cmm3represent devices which had been deliberately rapidly cooled from 1OOOYJ which was shown to introduce the 164K line in our samples. In view of the experimental difficulties in making the measurements, the data shows that it was possible to associate a lifetime effect with the 164K line having a systematic control over the range 5 x 10” - 5 x 10” cme3. With the effective lifetime given by 7 = (4.6 x l@)/Nr corresponding to an effective cross s’ection (a.~), taking v = IO’cmsec-I, of 22 x lo-” cmw2.This is considerably greater than for Au for which the equivalent value is IO-‘-’cm’. p base devices. With the simpler spectra observed in the p base devices showing the 165K line only, similar correlations between lifetime and concentrations were more easily observed. The results shown in Fig. 6 indicated a control of the lifetime by this centre with
I
77K
300K
L
77K
I
Fig. 3. Simple Au and process induced line spectrum. (Tag = mS)
273K
Tmperature (K) Fig. 5. Spectrum in
p base device. (r,,, = 30 mS)
ffect of Au
Effect
of Au
\
Id’
IO"
IO"
IO-
No. of recembinat~on cenlres km-‘)
Fig. 4. Effect of 164K line on lifetime with gee defined from T = I/(A$uefl) V, with Y = 10’ cmsec-‘.
2 No. of recombination
IO"
centres hi’)
Fig. 6. Effect of 165K line on lifetime.
132
D. H. PAXMANand K. R. WHICHT
T = (3.2 x lo’)/& with (T.== 3.1 x 1O-‘5cm* (Y = 10’cm set-‘). Again good correlation was observed and the centre was more effective than Au in controlling lifetime. The difference in effective high injection crdss section between this 165K line observed in p type samples and the 164K lines seen in n type samples showed that these are not associated levels of the same recombination centre for in such a case the effective cross sections would be identical. 5.DISCUSSIONAND CONCLUSIONS shown that a limited number of recombination centre levels were observed in conventionally processed silicon power devices using the DLTS examination technique. It was possible to obtain a good correlation between high level lifetime and recombination centre content for n and p base devices showing simple spectra, and it was demonstrated that the 164K (T,~ = 30 ms) line observed in n type material had a particularly large effective cross section of 22X lo-” cm*. Similarly in p base devices the 165K (calf = 30ms) line correlated well with lifetime giving an effective cross section of 3.1 x IO-” cm’. Both these values were greater than for Au which had an effective cross section of 10-‘5cm2, showing that they had important effects on controlling the lifetime in unkilled device structures. These centres were observed in concentrations from 5 x 10” to 5 x 10” cm-’ in high temperature processed devices examined and determined the It has
been
electrical
properties of these devices. The 164K line observed in n type devices was introduced by deliberate rapid cooling from 1OOO’C. The different effective cross sections for the two levels seen in n and p base devices examined showed that they were not two related levels of the same recombination centre. The possible presence of an independent level in the lower half of the band gap [the 165K p base level] would represent a situation in which lifetime reductions would be observed in n base devices without a corresponding DLTS spectra. Such devices have been observed. Thus a calculation of expected lifetime from an observed spectrum will only give an upper value for the lifetime which could be obtained from such a device. Acknowledgemenls-We would like to thank our colleagues concerned with power device production and development at Mullard Hazel Grove and Phillips Nijmegen for providing devices and helpful discussions. REFERENCES I. G. L. Miller, J. V. Ramirez and D. A. H. Robinson, /. Appl. Phys. 46.2638 ( 1975). 2. H. Schlangenotto-and W. Gerlach, Solid-St. Ekcfron. IS, 393 (1972). 3. D. L. Lewis, Solid-St. Electron. 18, 87 (1975). 4. L. D. Yau and C. T. Sah, Solid-St. Electron. 17, 193 (1974). 5. J. Barbolla, L. Bailon, J. C. Brabant, M. Pugnet and M. Brousseau, Revue de Physique Applique 11.403 (1976). 6. J. Pals, Solid-St. Elecrmn. 17. II39 (19751. 7. S. D. Brotherton, J. Appl. Phys. 49, 66; (1978) (and private communication).