PHOTODIODE FOR COHERENT DETECTION : MODELING AND EXPERIMENTAL RESULTS

HAL Id: jpa-00227965

https://hal.archives-ouvertes.fr/jpa-00227965

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

PHOTODIODE FOR COHERENT DETECTION : MODELING AND EXPERIMENTAL RESULTS

J. Viallet, S. Mottet, L. Le Huerou, C. Boisrobert

To cite this version:

J. Viallet, S. Mottet, L. Le Huerou, C. Boisrobert. PHOTODIODE FOR COHERENT DETECTION :

MODELING AND EXPERIMENTAL RESULTS. Journal de Physique Colloques, 1988, 49 (C4),

pp.C4-321-C4-324. �10.1051/jphyscol:1988467�. �jpa-00227965�

PHOTODIODE FOR COHERENT DETECTION : MODELING AND EXPERIMENTAL RESULTS

J.E. VIALLET, S. MOTTET, L. LE FJEROU and C. BOISROBERT

Centre National d'Etudes des TBlBcomunications, F-22300 Lannion, France

Resume

-

En detection coherente, la puissance optique de l'oscillateur local peut conduire le photodetecteur en regime de forte injection. Des degradations de performances du dispositif peuvent alors &re constatees A partir de simulations numitriques et de mesures.

Abstract

-

In coherent detection, the optical power of the local oscillator can lead the photodetector in the high injection regime. Degradation of the device performance can then be observed from numerical simulations and experiments.

1

-

INTRODUCTION

Coherent detection can subtantially improves optical communication system performances [I]

.

Wavelength stabilized semiconductor lasers provide for the necessary local oscillator.

Optical power of the local oscillator greater than the radiant power corresponding to the optical signal, yields better signal to noise ratio.

Photodetectors design is a compromise between different parameters such as sensitivity.

response time, capacitance, gain, noise. The intrinsic material width of a PIN photodiode must be such that sufficient light is absorbed (sensitivity), that the free carriers thus generated are quickly collected (response time), driven at their top velocity by an uniform electric field, and so that the junction capacitance is low to limit RC time constant.

At high level of optical illumination, free carrier densities can be such that the electric field in PIN photodiodes is no longer uniform and exibit low and high values areas. This behaviour is expected to affect the response time of the device.

The goal of this study is to determine the influence of the optical power on the photodiode performances, in coherent detection, when a high radiant optical power shines on the photodetector, mixed along with the transmitted low level signal from the fibe?

output. The operation of Si, Ge, GaInAs/InP photodiodes under low and high optical power are described in terms of sensitivity, linearity, response time and diode capacitance as predicted by numerical simulation and confirmed by measurements.

2

-

MODELING AND NUMERICAL METHODS

Unidimensional numerical simulation of PIN photodiodes are performed, using finite difference methods, solving the following set of equation that describes the behaviour of semiconductor devices. This set includes Poisson equation, electron and hole continuity equations and current formulation. Thermal generation recombination term as well as optical generation are taken into account [I].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988467

JOURNAL

DE

PHYSIQUE

ELECTRIC FIELD IN INTRINSIC LRIER k V l c m FLUX o f 1E17 to IE23 P h o t o n s / a l c m Z

Fig.1

-

E l e c t r i c f i e l d d i s t r i b u t i o n i n G e PIN photodiode, with high and low f i e l d value.

div E

-.

.grad Y ) = q-(n

-

^p

-

^{C ) x =} JA=*a(*) .em[- a(*)

.XI

6 an

-

-.div 1 (n %grad EFn

-

)

-

^{USER + Go,, and}

-

^{n - P}

^-

^{n, -P,}

( p wpgrad EFp)

-

^{USER + G o p t 'SRR = rn ( p + p I ) +}

b

(n + n,

With n , p; T

,,

rp; pn, pp; EFn, EFp a r e respectively e l e c t r o n and hole c a r r i e r d e n s i t i e s , c a r r i e r l i f e t i m e s , mobilities and Fermi l e v e l s . The e l e c t r o s t a t i c p o t e n t i a l is cp. The d i e l e c t r i c constant; is E and t h e fixed charge density is C

.

Thermal generation recombination is described by t h e Schockley-Hall-Read formulation USER and the e x t e r n a l o p t i c a l generation GOp,(x) takes i n t o account the a absorption value a t position x. as a function of wavelength A.

The s e t of equation i s l i n e a r i z e d and solved within an uncoupled scheme of resolution, using a v a r i a b l e i m p l i c i t method t o solve the t r a n s i e n t case.

Fig.1

-

E l e c t r i c f i e l d d i s t r i b u t i o n i n Ge PIN photodiode, with high and low f i e l d value.

3 -

NUMERICAL SIWiTION RESULTS

A t high o p t i c a l powc?r, t h e e l e c t r i c f i e l d i n t h e i n t r i n s i c l a y e r of t h e PIN photodiodes is no longuer uniform (Fig 1). A s t h e o p t i c a l power increases, l a r g e r p o t e n t i a l drops arise a t the P'v and v N + junctions and increasingly wider areas of low e l e c t r i c f i e l d spread i n t h e i n t r i n s i c material. The peak value of t h e e l e c t r i c f i e l d does not modify carrier v e l o c i t i e s and is unsufficient t o enhances avalanche. With e l e c t r i c f i e l d as low as a few kV/cm, c a r r i e r v e l o c i t i e s can d r a s t i c a l l y decrease and lead t o l a r g e transit t i m e of t h e c a r r i e r s across the i n t r i n s i c layer. With slow d r i f t , i n poor q u a l i t y material, t h e c a r r i e r s can s u f f e r s from recombination before being c o l l e c t e d and thus s e n s i t i v i t y lowers. The obtained e l e c t r i c f i e l d d i s t r i b u t i o n is equivalent t o t h a t of two junctions.

The summ of t h e corresponding space charge regions is then smaller than the i n t r i n s i c l a y e r width and it can be expected t h a t the o v e r a l l capacity increases.

Accurate computation of t h e capacity of the device is performed taking i n t o account t h e l o c a l v a r i a t i o n of the e l e c t r i c charge and of the displacement vector between two c l o s e b i a s [2]. Evolution with o p t i c a l power i s given i n Fig 2.

Numerical simulations show t h a t f o r o p t i c a l power l a r g e r than 10" photons/s/ cm2 t h e behaviour of t h e photodiodes can be f a r d i f f e r e n t than t h e low o p t i c a l regime such photodiodes had been designed f o r . An o p t i c a l power of

5

mW shining over 100

w2

(surface of the core of a monomode f i b e r ) is equivalent t o some 5 . 1 0 ' ~ photons/s/cm2.

4

-

MEASUREMENTS AND IMPROVED EQUIVALENT C I R C U I T

YI m

"

²⁰

>

2

1s

1

n

.G 10 E 3

m

N 5 X

The complex impedance of t h e photodiodes has been measured on a 50 R load, using a network analyser, through SI1 parameter. a f t e r proper c a l i b r a t i o n , i n the .1

-

^{2 GHz} band width.

Experimental d a t a ( + ) and impedance f i t (continuous l i n e ) .

Fig.3

-

Improved equivalent c i r c u i t f o r Ge photodiode and value e x t r a c t i o n method provide good impedance f i t i n the .l-2 GHz range.

5'

I I I

P

- I ⁰

I

,d -

i ;

-

P/

^d'

B- ---4' a

f----

u

- - - - - - - -

-0'

The d i f f e r e n t elements of the equivalent c i r c u i t of t h e photodiode ( j u n c t i o n and package capacitances, junction and s e r i a l r e s i s t a n c e s , w i r e inductances) a r e obtained through an

YI

-

₀

>

4 m _-

m E 3

m

-

It

ln

. . .

..'

. . .

^J ₀ ^{u al}

19 2 0 2 I 2 2 2 3

log( f l u x of p h o t o n s )

o S i PIN photodiode X = .85 w 0 G e PIN photodiode X = 1.5 im

Fig.2

-

Increase of PIN photodiodes c a p a c i t i e s , with increase of t h e l o c a l o s c i l l a t o r radiant power, a s obtained by numerical simulation.

JOURNAL

DE

PHYSIQUE

improved parameter extraction method. This method allows t o determine t h e b e s t f i t values of the d i f f e r e n t elementls of the equivalent c i r c u i t described by the operator without any p r i o r knowledge of the values o r ranges. The equivalent c i r c u i t of the photodiodes had t o be completed by elements such as w i r e inductance o r chip t o package capacitance t o obtain correct f i t a s shown i n figure 3.

The variation of the junction capacitance and s e r i a l r e s i s t a n c e of t h e photodiode have thus been extracted f o r o p t i c a l power a t 1 . 3 p m ranging from obscurity t o 4.4 mW f o r the GaInAs/InP diode (3.5 mA photocurrent, 7.5 1020photons/s/cm2 ) and 9 mW f o r the Ge diode ( 7 mA. 1.5

lo2 '

^{ph/s/cmZ )}

.

PHOTOCURRENT ( m R )

A GaInAs/InP PIN diode Ge PIN diode

Fig.4

-

Relative junction capacitance increase versus diode photocurrent, as 1 . 3 ~ l o c a l o s c i l l a t o r r a d i a n t power increases. The junction c a p a c i t i e s a r e obtained from microwave measurements and improved equivalent c i r c u i t s o f photodiodes.

A s it can be seen i n f i g u r e 4. the junction capacity of the Ge and GaInAs/InP photodiodes continuously increase with increasing o p t i c a l power. The t i m e constant (junction capacitance

-

s e r i a l resistance product) continuously increases but f o r such o p t i c a l power i s s t i l l below t r a n s i t time.

5

-

CONCLUSION

Numerical simulations of photodiodes p r e d i c t t h a t above an o p t i c a l power corresponding t o flux g r e a t e r than 1 0 ~ ~ ~ h o t o n s / s / c m ~ , t h e response time and the junction capacitance w i l l continuously rise. Fkperimental measures confirm t h i s increase of the capacitance and R;C t i m e constant. For standard diode a c t i v e diameter (70

w ) ,

t h e o p t i c a l power delivered by a conventionnal semiconductor l a s e r ( a few mW) is beneath the l i m i t f o r which photodiode performances would shrink.

6

-

REFERENCES

[ I ] Mears, C.L. and Batchman, T.E., IEEE vol LT 5, N'6, June 87, pp 827-837

121

V i a l l e t . J.E. and Mottet. S.. Nasecode I V Conference. Dublin 1985

-

^{PP 530-541}

C31 Mottet, S. and V i a l l e t , J.E., Nasecode V Conference, Dublin 1987, PP 289-294