Nuclear Instruments and Methods in Physics Research B62 (1992) 361-363 North-Holland

Nuclear Instruments &Methods

in Physics Research section 8

The effect of high energy boron co-implantation on the activation of silicon implants in GaAs

R.J. Wilson, B.J. Sealy and R.M. Gwilliam Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrev. CU2 SXH, UK

The sheet carrier concentration of 200 keV %if implants at doses of 4x 10” and 10’” cm-’ has been investigated as a function of 500 keV B co-implantation dose in the range 5 X 10” to lOI cmF2 and annealing conditions. The activation behaviour of the Si implant was found to depend strongly on the B dose, the anneal temperature and whether rapid thermal or furnace annealing was used. Increased or decreased activation of the silicon implant was found to be possible depending on the implant and annealing schedules used.

1. Introduction

The dopant profile in the channel region of GaAs MESFETs will affect their operationai behaviour [l]. For example tailoring the channel profile to give an abrupt tait has been demonstrated to give improved low noise operation [2]. The work reported here is a continuation of previous work [3j which used 200 keV B co-implantation in the tail region, to modify the activation of Si layers in semi-insulating GaAs, and proposed self-compensation and damage complexes as the cause of the effects. This work has been extended by looking at the effect of higher energy (500 keV) B implants at a range of doses (5 X 1012 to 1 X 1O’4) on the activation of 4 x lOI2 and 1 X 1013 cm-’ silicon implants after both RTA and furnace annealing at a range of temperatures.

2. Experimental details

Undoped semi-insulating (100) GaAs substrates were implanted with a dose of either 4 X 1012 cm-* or 1013 cm-2 ZYSi+ ions at an energy of 200 keV either before or after receiving implants of 5 X lo”, 1 X 1013, 5 x 1Or3 or 1 X 1014 cm-’ of “B+ at an energy of 500 keV. Control substrates received no B implantation. All implants were in a nonchannelling direction using a substrate tilt angle of 7 o and rotation of 15 O. The substrates were then capped using PECVD SiN and cleaved into samples for annealing at various tempera- tures. Sampies were either furnace annealed at tem- peratures between 700 D C and 900 * C in flowing N, for 15 min or rapid thermai annealed (RTA) at tem- peratures between 750 o C and 950 o C in a N, ambient for 5 s. Foliowing annealing Hall samples were pre-

pared to enable the sheet carrier concentration and mobility to be measured.

3. Results

Figs. 1 and 2 show the atomic carrier concentration and damage distribution respectively for a series of silicon and B implantations. The range of the Si im- plantation is about 2000 A whilst the range of the B

Gon~ntration /cc 1 .OOOE+19

1 .oOOE+l8

i.OOOE+17

l.O00E+15

0 2 4 6 8 10 12 14 16 18 20

Depth in Angstroms 1 E3

- Dose4E12Si2OOkeV -#- DoseiEl3 SimeV

* tkxelE14BfiOOkeV --A-- ooseiE138 5ocicev

Fig. 1. Calculated concentration profiles for 200 keV Si and 500 keV B implants at various doses.

0168-583X/92/$05.00 Q 1992 - Elsevier Science Publishers B.V. All rights reserved II. PR~FILES/REDISTRIB~ION

362 R.J. Wilson et al. / The effect of B co-implantation on the actbation of silicon implants in GaAs

5 Damage %

0 2 4 6 8 10 12 14 16 18 20

Depth in Angstroms 1 E3

4 85OOkeviE14 + Si2awNiE13

-% Bmlkev1E13

Fig. 2. Calculated damage profiles for 200 keV Si and 500 keV B implants at various doses.

implantation is 9300 A. Little damage from nuclear stopping occurs at B doses of 5 X 10”’ cm -’ and IOr cme2. At a B dose of lOI cm-’ the damage due to the B is stiil less than the silicon up to around 2000 A and so the B implant is responsible for damage in the tail of the silicon profile.

Fig. 3 shows the variation of sheet carrier concen- tration with furnace anneal temperature and B dose for 4 X 10” cm-’ 29Si implants. The graph shows that, for the Si implant only, carrier concentration increases smoothly with increasing temperature. Implantation of I3 at low (5 x 10’2 or 1 x 1013) doses produces a reduc- tion in the carrier concentration at each anneal tem- perature except at 900°C for a dose of 5 x 101* B where there is an increased activation compared to the Si only implant. For both doses the sheet carrier con- centration increases smoothly with increased anneal temperature. For higher B doses (5 X lo’“, I X 10r4) there is a large reduction in the sheet carrier concen- tration between 700 and 8OO”C, where a minimum is present, and a iess severe reduction at higher anneal- ing temperatures. A comparison is shown for the lOI cm-* B implant where the Si implant was performed before @i/B) or after (B/Si) implant. The variation of sheet carrier concentration is similar in both cases. Fig. 4 shows the variation of sheet carrier concentration for a 1013 Si implant with and without a subsequent lOI B implant after RTA and furnace annealing at a range of temperatures. A similar trend is seen as for the 4 X IO’* implant (fig. 31, with a large reduction and a plateau in

Sheet Carrier Concentration 1 El 2 /cm”2 -.

/ 3.5 +

/’ ‘....

3

2.5 -

2-

l.S-

I-

0.5 -

0 L-J-.-... t / 1 J

650 700 150 806 850 900 950

TemperatureC

* OS 8lSi t 5ElZS 3lSi m lE138 E&i

u 5E13B 66 X lE148 S/Si 0 lE146 Si/E

Fig. 3. The variation of the sheet carrier concentration of 200 keV 4 x lOI Si with 500 keV B dose (a/b indicates implant

order) for furnace annealed samples.

Sheet Carrier Concentration 1 El 2 I cm”2

I’ I

* _ ._ _. .__. . .

1 - .‘.. :/. ‘. ““’ “‘.

0 / I 8 I I

850 700 750 800 850 900 950

Temperature’C

. OB FA + lE14BFA

* OBFWA D lE146 RTA

Fig. 4. The variation of sheet carrier concentration of 200 keV 1 x IO’” Si, with 500 keV 1 x lOI 3 dose for furnace (FA) and

rapid thermal (RTA) anneals.

R.L Wilson et ai. / The effect of B co-implantation on the actiuation of silicon implants in &As 363

Sheet Carrier Concentration 1 El 2 / cmA2

2.5 “’ 1

A--- / / / .L..

700 750 800 850 900 950 1000

Temperature%

06 + lE138

Fig. 5. The variation of sheet carrier concentration for 4~ 1012 200 keV Si with 500 keV B dose after rapid thermal anneal-

ing.

the sheet carrier concentration between 750 and 850 ’ C for the furnace anneal, which recovers to almost the Si only value at 900” C. The samples which underwent RTA show a lower activation than for the furnace annealed samples for the case of the Si only implant, while at a dose of 1 X lOI cms2 B the activation is larger at up to 8.50 o C and lower thereafter, with no recovery to near Si only sheet carrier concentration up to 950 o C. Fig. 5 shows the sheet carrier concentration for 4 X 10” Si implants with and without 10’” B im- plants at a range of RTA temperatures. The activation of the Si only implant increases smoothly with tcmper- ature. The sheet carrier concentration is however in- creased by the co-implantation of 1013 cm-* of B at all temperatures investigated except 800 ’ C. The point at 800°C may be a spurious or real effect and further data would be necessary to clarify this.

4. Discussion of results

The activation of silicon as a function of silicon dose, B co-implantation dose and annealing conditions using RTA or furnace annealing, has demonstrated that possibilities occur for both increased and de- creased activation of the silicon implant. This would tend to suggest that more than one mechanism was in operation over the range of conditions investigated

[3-51. The increased activation of the Si implant is most likely to be a result of gallium vacancies formed by the B implantation. The change of sheet carrier concentration with temperature at higher B doses, where there is a large reduction in sheet carrier con- centration and then a recovery at higher temperatures would possibly point to a complex defect involving boron being present. For lower B doses where a small reduction in carrier concentration is seen whilst the carrier concentration increases with tempe~ture fairly smoothly would possibly indicate a com~nsat~on mechanism whereby B occupies Ga sites. Further anal- ysis of the results of mobility measurements and a detailed investigation of the carrier profile using C-V’ measurements are under way to further clarify mecha- nisms. Preliminary C-V results tend to indicate that a wide range of profile shapes may be achieved using the implant and anneal schedules outlined. It is hoped to use the results of the study to modify the eiectrical behaviour of MESFETs.

5. Conclusion

The activation of 200 keV 4 x 1012 or 1 X 10’” sili- con implants in (100) GaAs has beer) shown to depend strongly on the dose of co-implanted B, and the anneal conditions and technique. Both enhanced or reduced activation can be achieved. Further work is under way to measure the carrier profiles, but preliminary results indicate that changes in the carrier profile are occur- ring. Several mechanisms would seem to be required to be able to explain the effects observed. Future work will involve MESFET fabrication to correlate electrical characteristics with profile shape.

The work reported in this paper is funded by the EC Science Programme. The authors would like to acknowledge the assistance of Mrs. K. Arthur in the preparation of this manuscript.

References

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I21

t31

[41 IS1

P.H. Ladbrooke, Gallium Arsenide for Devices and Inte- grated Circuits, eds. H. Thomas et al. (IEEE, London, 1986) chap. 10. P.J. McNally, IEEE Electron Device Lett. EDL-5 (1984) 126. R.M. Gwilliam et al., Electr~hemical Sot. Proc. 90-13 (1989) 121. G.M. Martin et al., J. Appl. Phys. 53 (1982) 8706. J. Osaka, Appi. Phys. Lett. 50 (1987) 191.

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