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, Redhill. Surrey. England

(Received 19 April 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 con- ventionally diffused power devices, and then by a com- bination of this data and lifetime measurements to attempt to establish the degree of lifetime control exer- cised 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

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 sili- con. The p+ and n+ diffusions were from C.V.D. deposited layers of phosphorus and boron doped glasses [“Silox”] driven in at 1260°C for > 25 hrs to yield diffused p+ and n+ layers 50pm deep. Base widths from 90 to 25Opm were obtained by suitably dimensioning the starting * slice thickness. These prolonged high tem- perature diffusion treatments were common to the thy-

ristors in which the base region was identical n type material. The power transistors were n+lp(nln+ struc- tures either triple diffused on (Ill) 40 51 cm n base material to give collectors -80 pm thick or using a thick = 100 km 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] tech- nique with a temperature scan from 77 to 3OOK. The equipment was capable of detecting<O.l% traps cor- responding to c 10” cme3 traps in the devices studied. For comparison purposes all the spectra were taken at a constant reference time 7,r of 30 ms, and for simplicity all spectral lines were labelled by the temperature of the peak at this reference time constant. In order to deter- mine the emission characteristics the time constant could be varied from 5 to 999 ms.

A constant capacitance system was used in the DLTS equipment and the amplitude of the spectral lines relative to the initial reverse bias (- 1OV) was used to determine the trap concentration relative to the doping level.

Lifetimes (high injection lifetimes) were measured by the Open Circuit Voltage Decay (OCVD)[2] method. This technique allows the carrier density decay to be followed over several decay time constants. By examin- ing the (N/at) characteristic, and taking lifetimes from the latter end of the decay phase it was oossible to avoid the initial rapid transient decays caused by end region effects, and diffusion. For fast diodes 7 ~0.5 ps the Diode Recovery[3] method was used with low current densities to minimise end region effects but maintain high injection conditions. The lifetime was derived from the switching curve using

J( > I@- 1 erf --

Q I + (J&J

where T,,,/Q is the ratio of the steady current phase during switching to TV the base lifetime and J, and J, are the forward and reverse currents, without corrections as the dimensions of the diode were large with respect to

130 D. H. PAXMAN and K. R. WHIGHT

diffusion lengths. In the case of particularly simple spec- tra it was possible to examine for correlations between the 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 being observed, or an associated level in the case of multiple charge state centres. e.g. Au. If traps were present having characteristic levels in the minority half of the band gap thus not accessible to the conventional DLTS tech- nique and not associated with the observed centre such a correlation would be unlikely to occur.

4.rWut.n (i) Recombination ctwttr spectm

The spectra obtained in n base devices were usually rather simple showing one or two lines. Two particularly complex spectra are illustrated in Fig. I. These two devices show a total of 7 diierent lines corresponding to traps lying in the upper half of the band gap labelkd with the temperature of the line peak at 7nf = Mms. These spectral line positions are reproducible to within 2 1°K. It is interesting to note that though each of these devices has similar OCVD lifetimes, 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 emission rate and T the absolute temperature. In addition to the 7 lines observed in Fig. 1 there is a complex series of lines in the region of “164°K line. The line at “175°K is resolvable but the line be- tween 175 and 1’64K is difficult to resolve unam- biguously. In normally processed devices these centres occur with concentrations between 10”-lO”cm-“, and with the exception of the 260 and 156K tines (see below) each centre can occur as an individual species.

Tronsirtor basr collector

p/n/n+ diode

1 116K lS6K

l06K

77K Tomrmmtura o< 326K

Fig. 1. Spectra in n base devices. (TV, = 30 mS)

4.0 6.0 60 IO 0

Fig. 2. Emission rate v. 1000/T plots.

By comparing Fig. 2 with published data and examin- ing “Au killed” p and n base devices the 260K line was identified as the acceptor level of Au at E-561 meV. The 156K line seen only in devices having the 260K line 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 de- pletion on the p side of the test diode. The 175K line lies close to a process induced quenched-in recombination centre reported by Sah and Yau [S] and Barbolla ei al. [5]. The two lines at 165 and 164K are difficult to resolve over the experimentally available time constant range (5-999 mS) and were normally treated as a single line for comparison with lifetime data.

The other lines observed were not associated with other simple metallic contaminants Pt, Rh and Pd examined by Pals[6].

Table 1 summarises the properties of the centres observed in n base devices with the data presented in the form

Emission time constant r = $expg .

The recombination centre spectrum in the p base devices studied showed only one line that had significant

Table 1.

Line designation kT Comment

260

204

175

164

156 145

116

106

3.3 10-a 0.561 x

T=- exp-z- T2 Au acceptor

6.5 IO-’ 0.460 x r=-expkT TL

1.63 lo-’ 0.264 x ‘=T2expkT

4.47 10-e 0.276 x T=-expCT Tz

1.2 IO-* 0.332 x

r=~--yIrT 0.356 A” donor

7= T* exPkT 9.6 IO-’ 0.221 x

7= T* expkT 7 = 2.726 1O-9 0.236 x

T’ expkT

Observation of lifetime controlling recombination centres 131

effect on lifetime at 165K shown in Fig. 5 for which

3.2 x lo-’ 0.31 r=-expkT T

(ii) Lifetime t$ects n base devices. In order to determine the effects of a

particular recombination centre on lifetime a large num- ber 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

I

77K 300K

Fig. 3. Simple Au and process induced line spectrum. (Tag = mS)

ffect of Au

Id’ IO" IO" No. of recembinat~on cenlres km-‘)

IO-

Fig. 4. Effect of 164K line on lifetime with gee defined from T = I/(A$uefl) V, with Y = 10’ cmsec-‘.

at long lifetimes. The points for Nr(164) at 5 x lOI cmm3 represent devices which had been deliberately rapidly cooled from 1OOOYJ which was shown to intro- duce 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 in- dicated a control of the lifetime by this centre with

L 77K I

273K Tmperature (K)

Fig. 5. Spectrum in p base device. (r,,, = 30 mS)

Effect of Au

\

2 IO"

No. of recombination centres hi’)

Fig. 6. Effect of 165K line on lifetime.

132 D. H. PAXMAN and K. R. WHICHT

T = (3.2 x lo’)/& with (T.= = 3.1 x 1O-‘5 cm* (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 sam- ples and the 164K lines seen in n type samples showed that these are not associated levels of the same recom- bination centre for in such a case the effective cross sections would be identical.

5. DISCUSSION AND CONCLUSIONS

It has been shown that a limited number of recom- bination 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 recom- bination 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 im- portant effects on controlling the lifetime in unkilled device structures. These centres were observed in con- centrations from 5 x 10” to 5 x 10” cm-’ in high tem- perature processed devices examined and determined the

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

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