The contribution from The contribution from photoluminescence (PL)photoluminescence (PL)

Gordon Davies, King’s College LondonGordon Davies, King’s College London

A short introduction to photoluminescence (PL).A short introduction to photoluminescence (PL).

Excite the sample with light (‘photo’); some of the energy Excite the sample with light (‘photo’); some of the energy is emitted as light (‘luminescence’).is emitted as light (‘luminescence’).

Excitation is usually by a laser, for convenience of directed Excitation is usually by a laser, for convenience of directed beam, with beam power of 100’s of mW.beam, with beam power of 100’s of mW.

Green laser light is absorbed by the crystal, exciting an Green laser light is absorbed by the crystal, exciting an electron from the valence band to the conduction band, electron from the valence band to the conduction band, with a penetration depth of 1/e = 1 with a penetration depth of 1/e = 1 m.m.

Electron-hole pairs (excitons) are created with a lifetime of Electron-hole pairs (excitons) are created with a lifetime of 10’s of 10’s of s in pure Si. They are captured by impurities.s in pure Si. They are captured by impurities.

Introduction to PL continued.Introduction to PL continued.

The exciton is captured by the impurity, exciting the The exciton is captured by the impurity, exciting the impurity.impurity.

Excited state

Ground state

Luminescence emittedLuminescence emitted

Introduction to PL continued.Introduction to PL continued.

What can we observe?What can we observe?

Only (usually) Only (usually) neutralneutral centres (not charged). centres (not charged).

Concentrations over 10Concentrations over 101111 cm cm-3-3. Best 10. Best 101414 to 10 to 101616 cm cm-3-3. .

Require:Require:

Samples with transparent surfaces (no contacts). Samples with transparent surfaces (no contacts).

Samples of about 8 x 8 mmSamples of about 8 x 8 mm22..

(It is a contact-free, non-destructive technique!)(It is a contact-free, non-destructive technique!)

Samples at T < 20 K. Liquid helium.Samples at T < 20 K. Liquid helium.

Introduction to PL continued.Introduction to PL continued.

What do we observe?What do we observe?

Very sharp optical transitions: Very sharp optical transitions:

energy resolution typically 0.1 meV at 1000 meV.energy resolution typically 0.1 meV at 1000 meV.

Each sharp line is characteristic of one atomic-sized defect.Each sharp line is characteristic of one atomic-sized defect.

Very high spectral resolution.Very high spectral resolution.

FZ Si with no oxygen and oxygen diffused, 24 GeV protons, FZ Si with no oxygen and oxygen diffused, 24 GeV protons, 10101616 cm cm-2-2..

The spectral lines C, G, W have widths ~0.1 meV.The spectral lines C, G, W have widths ~0.1 meV.

Energy resolution is 1 part in 10,000.Energy resolution is 1 part in 10,000.

Spectral resolution allows effects of isotopes to Spectral resolution allows effects of isotopes to be measured: chemical identification.be measured: chemical identification.

Link to other techniques. (i)Link to other techniques. (i)

We see some of the Local Vibrational Modes of the defects,We see some of the Local Vibrational Modes of the defects,

labelled Llabelled L11 to L to L44 below. below.

They can be seen by Infrared Absorption (Leonid Murin). They can be seen by Infrared Absorption (Leonid Murin).

‘‘G’ = CG’ = Css - C - Cii

Link to other techniques. (ii)Link to other techniques. (ii)

DLTS measures the difference in energy of DLTS measures the difference in energy of oneone of our of our states to the band. We can link DLTS to PL (within the states to the band. We can link DLTS to PL (within the precision of DLTS).precision of DLTS).

Conduction band

Valence bandDLTSDLTS

But…But…

… … PL does not detect some defects PL does not detect some defects

(for example the di-vacancy, or the simple hydrogen centres).(for example the di-vacancy, or the simple hydrogen centres).

It does not usually detect It does not usually detect anyany charged defect. charged defect.

Also, PL is not quantitative.Also, PL is not quantitative.

PL from one species of defect is not usually proportional to the PL from one species of defect is not usually proportional to the concentration of that species.concentration of that species.

We cannot simply say that if a PL signal is large, we have more of We cannot simply say that if a PL signal is large, we have more of that species of optical centre.that species of optical centre.

To understand this we have been looking at the similar problem (?) To understand this we have been looking at the similar problem (?) of ion-implanted silicon.of ion-implanted silicon.

[Work in collaboration with Paul Coleman, Bath University, UK].[Work in collaboration with Paul Coleman, Bath University, UK].

Examples:Examples:

Luminescence is quenched by divacancies. For example in ion-Luminescence is quenched by divacancies. For example in ion-implanted silicon, the mean separation of the divacancies implanted silicon, the mean separation of the divacancies decreases with dose as 1/ (dose)decreases with dose as 1/ (dose)0.250.25. Positron data by Paul . Positron data by Paul Coleman.Coleman.

Crosses show measured intensity of W line. Line is Crosses show measured intensity of W line. Line is calculated for quenching by energy-transfer to divacancies.calculated for quenching by energy-transfer to divacancies.

At lower doses, the PL intensity is proportional to the dose, At lower doses, the PL intensity is proportional to the dose, because there are fewer quenching defects.because there are fewer quenching defects.

2000 40 00 6000 800 0 1 0000 12000

PL

inte

nsity

(ar

b. u

nits

)

Time (s)20 25 30 35 40 45 50 55 60 65

0

500

1000

1500

2000

2500

20

40

60

80

100

120

De

cay

time

@ 1

19

2 n

m (s

)

Temperature K

Slow

De

cay

time

@ 1

19

2 n

m (s

)

Fast

The quenching process can be complicated so that the radiative The quenching process can be complicated so that the radiative decay is decay is notnot exponential in time. For example, in ion-implanted exponential in time. For example, in ion-implanted silicon, PL from the ‘X’ centre (four interstitials?) has a silicon, PL from the ‘X’ centre (four interstitials?) has a complicated decay curve. (In collaboration with Tom complicated decay curve. (In collaboration with Tom Gregorkiewicz, Amsterdam University.)Gregorkiewicz, Amsterdam University.)

Given this lack of knowledge, we need Given this lack of knowledge, we need

a) to understand the quenching process,a) to understand the quenching process,

b) to understand the link between PL intensity and b) to understand the link between PL intensity and concentrations.concentrations.

Three studies in progress:Three studies in progress:

Irradiation at CERN with 24 GeV protons for optical Irradiation at CERN with 24 GeV protons for optical absorption measurements on the same centres that are absorption measurements on the same centres that are observed in PL. (Optical absorption is proportional to the observed in PL. (Optical absorption is proportional to the concentration of the centre).concentration of the centre).

Samples being prepared to measure the radiative lifetime of Samples being prepared to measure the radiative lifetime of the relevant optical centres with different levels of damage. the relevant optical centres with different levels of damage. (Gives the factor linking absorption to concentration, and (Gives the factor linking absorption to concentration, and shows how the PL is affected by damage in the sample).shows how the PL is affected by damage in the sample).

Samples being implanted to study the trapping of the self-Samples being implanted to study the trapping of the self-interstitial by C in very high carbon silicon. (To check the interstitial by C in very high carbon silicon. (To check the linearity of damage production at low doses).linearity of damage production at low doses).