Piezoelectric Spectroscopy of the Defects States on the Surfaces of Semiconducting Samples M. Maliński 1, J. Zakrzewski 2, K. Strzałkowski 2, F. Firszt.

Piezoelectric Spectroscopy of the Defects States on the Surfaces of

Semiconducting Samples

M. Maliński1, J. Zakrzewski2, K. Strzałkowski2, F. Firszt2

1 Department of Electronics and Computer Science, Technical

University of Koszalin,

2 Śniadeckich St, 75–328 Koszalin, Poland

2 Instytut Fizyki, Uniwersytet Mikołaja Kopernika, ul.Grudziadzka 5/7,

87–100 Torun, Poland

ABSTRACT

This presentation shows both theoretical and experimental aspects connected with piezoelectric detection of defects’ states located on surfaces of semiconducting samples. This kind of states can provide absorption bands visible in the energy gap region of semiconductors. Theoretical considerations presented in this paper comprise computations of the piezoelectric spectra for defects’ states located on different surfaces. Experimental part of the paper comprises numerical analysis of several experimental amplitude and phase piezoelectric spectra of a group of Zn1-x-yBeyMnxSe mixed crystals for different x, y compositional parameters i.e. y=0.05, x=0.05, 0.10, 0.15, 0.20 after different surface treatment.

SAMPLE PREPARATION & THE MEASURING METHOD

Zn1-x-yBexMnySe (x=0.05, y= 0.05, 0.10, 0.15 and 0.20) samples were grown

from the high purity powder with the high pressure Bridgman method. The crystal rod was cut into about 1mm thick samples which were first grinded, then polished with diamond paste and finally chemically etched. Solution of H2SO4 (96%), K2Cr2O7 and water was used for etching the samples. After

etching, the samples were rinsed in distilled water and then put for a few seconds in boiling NaOH. Then the samples were rinsed again in cold and next in boiling distilled water and finally in ethyl alcohol.

In the piezoelectric photothermal experiment samples were illuminated with the intensity modulated beam of light of a xenon lamp after passing through the prism monochromator. The piezoelectric signal was detected with a lock-in phase selective amplifier. The characteristics were measured at room temperature in the rear experimental configurations.

INTRODUCTION

Typical optical transmission measurements do not bring information on the spatial locations of the defects responsible for the absorption bands. Another basic problem of this type of measurements is the lack of the possibility of identification of the type of the optical absorption i.e. a bulk or surface one.

Piezoelectric phase measurements bring information on the spatial location of the absorbing centers!

Numerical analysis of the PZE spectra can also bring information on the type of absorption: a volume or a surface one.

THEORY

Let’s consider the surface type optical absorption coefficient spectra connected with the presence of surface defects in the form of the Gaussian distribution

2

2/1

2/1exp2/1)(2/1

EEAE

THEORY

The bulk type optical absorption coefficient spectra in the Urbach edge energy region, below the energy gap value, and in the band to band absorption region, above the energy gap value, are given by the formulae:

025.03exp3)(3

gEEAE

)(34)(4 EggEEAE

PIEZOELECTRIC SIGNAL

Piezoelectric signal S is computed according to Jackson& Amer formula.

T(x) is the temperature distribution in the sample

l ldxxTx

l

ldxxT

lS

0 0)(

226

)(1

),(),(),()( 32211 xTxTxTxT

DIAGRAM OF A SAMPLE

0 l

I 0

S 1 S 2

d d

I 0

T 1 (x,)

))))

T 2 (x,)

)

TEMPERATURE DISTRIBUTIONSSURFACE & VOLUME ABSORPTION

)2exp(1

)2exp()exp()1exp(10),(1 l

xlxdsIxT

)2exp(1

)(2exp())(exp()2exp(10),(2 l

xllxldsIxT

l

xNxMIxT

2exp12

,,0),(

lxlxllxlxlxxxM

exp2expexp2expexpexp,

llxxlxxlxlxxN

expexpexp2exp2expexp,

THEORY-surface absorption

Parameters of the surface absorption band: Eg=2.81 eV,

A3=80 cm-1,=1, E1=2.18 eV, 1=0.13 eV, A1=290 cm-1.

1.5 2 2.5 30

500

1000

ENERGY [eV]

OP

TIC

AL

AB

S.

CO

EF

F

[cm

-1]

THEORY- PZE SPECTRA

Piezoelectric amplitude and phase spectra computed for the parameters: =0.03 cm2/s, f=36 Hz and l=0.1 cm & the rear experimental configuration.

The defect located on the illuminated side of the sample

2 2.5 30

5 104

0.001

0.0015

0.002

0.0025

ENERGY [eV]

AM

PL

ITU

DE

[a.

u]

2 2.5 3100

50

0

50

100

ENERGY [eV]

PH

AS

E [

degs

]

THEORY- PZE SPECTRA

The defect located on the dark side of the sample. Parameters: =0.03 cm2/s, f=36 Hz and l=0.1 cm

2 2.5 30

5 104

0.001

0.0015

0.002

0.0025

ENERGY [eV]

AM

PL

ITU

DE

[a.

u]

2 2.5 3100

50

0

50

100

ENERGY [eV]

PH

AS

E [

degs

]

THEORY- PZE SPECTRA

The defect located on the illuminated and dark side of the sample.

Parameters: =0.03 cm2/s, f=36 Hz and l=0.1 cm

2 2.5 30

5 104

0.001

0.0015

0.002

0.0025

ENERGY [eV]

AM

PL

ITU

DE

[a.

u]

2 2.5 3100

50

0

50

100

ENERGY [eV]

PH

AS

E [

degs

]

THEORY- PZE SPECTRA

Piezoelectric spectra - under the assumption of the volume absorption type connected with the defect center at E=2.18 eV .

Parameters: =0.03 cm2/s, f=36 Hz and l=0.1 cm

2 2.5 30

5 104

0.001

0.0015

0.002

0.0025

ENERGY [eV]

AM

PL

ITU

DE

[a.

u]

2 2.5 3100

50

0

50

100

ENERGY [eV]

PH

AS

E [

degs

]

EXPERIMENTAL RESULTS

Piezoelectric amplitude a) and phase b) spectra of the Zn0.75Be0.05Mn0.20Se sample at f=126 Hz in the rear configuration.

Circles are experimental points, solid lines are theoretical curves.

.

2 2.5 30

0.5

1

1.5

ENERGY [eV]

AM

PL

ITU

DE

[a.

u]

2 2.5 3100

50

0

50

100

150

ENERGY [eV]

PH

AS

E [

deg]

EXPERIMENTAL RESULTS

Optical parameters of the center determined: A1=290 cm-1, 1=0.13 eV, E1=2.18 eV, Eg=2.81 eV, 3=1. The center is located on the illuminated side of the sample.

1.5 2 2.5 30

500

1000

ENERGY [eV]

OP

TIC

AL

AB

S.

CO

EF

F

[cm

-1]

EXPERIMENTAL RESULTS

Piezoelectric amplitude a) and phase b) spectra of the annealed Zn0.85Be0.05Mn0.10Se sample at f=76Hz in the rear configuration. Dots &Circles are experimental points, solid lines are theoretical curves.

.

2 2.5 30

10

20

30

40

ENERGY [eV]

AM

PL

ITU

DE

[a.

u]

2 2.5 3

100

50

0

50

100

ENERGY [eV]

PH

AS

E [

deg]

OPTICAL ABS. COEFF. SPECTRUM

Parameters of these surface absorption bands determined: A1 = 350 cm-1, A2 = 260 cm-1, E1 = 2.35 eV, E2 = 2.60 eV, 1 = 0.15 eV, 2 =

0.15 eV. Both defects are located on the same, illuminated, side of the sample.

2 2.2 2.4 2.6 2.8 3 3.20

200

400

600

800

1000

ENERGY [eV]

OP

TIC

AL

AB

S.

CO

EF

F

[cm

-1]

EXPERIMENTAL RESULTS

Experimental and theoretical PZE amplitude a) and phase b) spectra of Zn0.75Be0.05Mn0.20Se sample measured at f=76 Hz

Circles-exp. results, lines- theoretical curves

2 2.5 3 0

10

20

30

ENERGY [eV]

AM

PLIT

UD

E [

a.u]

2 2.5 3200

100

0

100

200

300

ENERGY [eV]P

HA

SE

[de

g]

CONCLUSIONS

Theoretical considerations, presented in the paper, indicate that it is possible to determine the location of the surface defects, visible in the amplitude piezoelectric spectra, on one of the surfaces of the sample from the numerical analysis of the piezoelectric phase spectra.

Theoretical and experimental results analysed in the paper also proved the possibility of distinguishing between the bulk and surface absorption.

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