SALIENT FEATURES OF SHALLOW DONOR INTERACTIONS IN PROTON-IRRADIATED

SILICON

V.V. Emtsev and G.A. Oganesyan

Ioffe Physicotechnical Institute

Russian Academy of Sciences

194021 St.Petersburg, Russia

V.V. Kozlovskii

St. Petersburg State Polytechnical University

195251 St. Petersburg, Russia

OUTLINE

• Introductory remarks

what we have learnt from literature• Aims of the present work

defect formation kinetics, interactions with P, annealing • Experimental details

irradiation and annealing conditions, electrical measurements etc

• Results

close attention to defect interactions with P• Conclusions

Introduction

• Defect production due to proton irradiation of Si at energies of a few MeV is generally considered to be very similar to electron irradiation at a few MeV but more effective.

V.S. Vavilov, N.U. Isaev, B.N. Mukashev, A.V. Spitsyn. Sov. Phys. Semicond. (AIP), vol. 6 (6) (1972). L.C. Kimerling, P. Blood, W.M. Gibson. Defects and Radiation Effects in Semiconductors, 1978. Conf.

Ser. No 46 The Institute of Physics, Bristol and London, 1979) pp. 273-280.

• Experiments on proton-irradiated Si are usually carried out at low doses of 1011 to 1012 cm2 . Heavier doses at about 1014 cm 2 are mostly used in optical and positron annihilation measurements.

• Very interesting data on CZ-Si irradiated with protons at 24 GeV/c have been obtained recently by Davies et al. The production rates of isolated vacancies and self-interstitials were reliably estimated to be close to 1 cm1 , whereas the production rate of divacancies turned out to be one half of that value.

Cf the production rates being about 200 cm1 for proton irradiation at 10 MeV and 0.1 cm1 for electron irradiation at 1 MeV.

G. Davies, S. Hayama, L. Murin, R. Krause-Rehberg, V. Bondarenko, A. Sengupta, C. Davia, A. Karpenko. Phys. Rev. B 73,165202 (2006).

AIMS

• To investigate production and annealing processes of point defects in Si subjected to proton irradiation at 15 MeV.

• Paying special attention to interactions of group-V impurities with intrinsic point defects.

Thick samples of 0.5 to 0.9 mm to eliminate surface effects.

Irradiation through the samples to suppress hydrogen-related defect formation.

Samples cut from moderately doped FZ-Si crystals to suppress side reactions with oxygen and carbon.

Experimental

Starting material

Wafers of FZ-Si doped with P at (5-7)1015 cm3 .

The compensation ratio K= NA / ND is very low, 0.01.

Irradiation and annealing conditions Protons at 15 MeV, irradiation at room temperature. Annealing steps of T=20C and t= 10 min. The annealing

interval was from T=60C to 680C. Reference temperature after each annealing step, T=300 K.

Electrical measurements

• The concentration and mobility of charge carriers in the FZ-Si vs temperature were measured with the aid of the Van der Pauw technique over a temperature range of T= 25 K to 300 K.

• Analysis of n(1/T) curves was performed of the basis of the relevant equation of charge balance adopted for non-degenerate Si of n-type.

Equation of charge balance in n-Si over a temperature range of ionisation of shallow

donors

• n is the electron concentration in the conduction band; ND is the total

concentration of shallow donor states; NA is the total concentration of

compensating acceptors; NC is the effective density-of -states in the

conduction band; ED is the ionisation energy of the singlet ground

states of shallow donors; is the splitting of the ground states of shallow donor states; other symbols have their usual meanings.

kT

kTE

TNnNN

NnnD

CAD

A

exp102

exp2/3

Electrical data on initial and proton-irradiated FZ-Si

• Electron concentration against reciprocal temperature for the CZ-Si before irradiation (blue) and after the proton irradiation (red). Points, experimental; curves, calculated.

• Ionisation energy of shallow donors of phosphorus impurity atoms is given.

Results

• We have first checked the defect formation in this material under fast electron irradiation at 1 MeV. As expected for a strongly dominant formation of E-centers, i e pairing of P atoms with vacancies, a 1:1 correspondence between ND and NA was observed, in accordance with the Watkins’ model.

In the case of the proton irradiation ND was found to be much larger than NA . In other words, the loss of shallow donor states of P due to interactions with intrinsic point defects exceeds substantially the concentration of radiation-produced acceptors.

Results (continued)

• As a result of the proton irradiation of the FZ-Si the

loss of shallow donor states of P is equal to ND = 3.31015 cm-3 , whereas the concentration of

radiation-produced acceptors was found to be NA = 1.51015 cm-3 .

• The removal rate of charge carriers in the FZ-Si irradiated with protons at 15 MeV was found to be

= (12010) cm-1. Interactions of P atoms with intrinsic point defects turned out to be the main way leading to the decreasing concentration of charge carriers under proton irradiation.

Results (continued)

• Taking into account a sizeable contribution of

divacancies V2= / V2

to NA = 1.51015 cm-3 one can conclude that the E-centers play a subsidiary role in the defect formation.

• A large fraction of the lost shallow donors, greater

than ND = 21015 cm-3 , appears as radiation-produced deep donors being electrically neutral in n-FZ-Si.

• This way of arguing is in good agreement with the results obtained in annealing experiments.

Defect annealing

• The annealing stages of E-centers and V2

= / V2 are

clearly seen.

• Their contributions to NA = 1.51015 cm-3 are evident.

• Just above T=300C a strange stage of direct and reverse annealing is observed.

• The complete annealing takes place at about T=680 C.

• In the heavily irradiated n-FZ-Si this stage at T350C is practically lacking, though some weak signs of its presence are still observable.

• The similarities of the two annealing curves at T400C suggest a saturation plateau in the formation kinetics of defects stable at high temperatures.

Defect annealing (continued)

Conclusions

• Changes in the concentration of charge carriers in FZ-Si decreasing due to proton irradiation at 15 MeV are mainly caused by losses of shallow donor states of group-V impurity atoms.

• At the same time the E-centers are present in relatively small concentrations. Therefore, there are other complex defects containing group-V impurity atoms.

Conclusions

• First, these complex defects appear to be deep donors. Second, they are thermally stable up to T=300C, i e much more stable than the E-centers. Together with this, their annealing was found to be very complicated.

• The complete annealing of all radiation-induced defects in FZ-Si after proton irradiation at 15 MeV is observed around T=700C.

Thank you for your attention !