Status on long-wavelength InP waveguide heterojunction phototransistors

Status on long-wavelength InP waveguide heterojunction

phototransistors

Samuel Dupont, Vincent Magnin, Manuel Fendler, Filippe Jorge, Sophie Maricot, Jean-Pierre Vilcot, Joseph Harari, Didier Decoster

Institut d'Electronique de Microélectronique et de Nanotechnologie, UMR CNRS 8520

Université des Sciences et Technologies de Lille59 652 Villeneuve d’Ascq France

• IntroductionGeneralities about HPTWhy lateral illumination ?

• Scholar study of a side illuminated HPTOptical modeling Alignment tolerance Optical structure optimization

• 3T waveguide HPT making ofStructureCharacterization

• HPTs state of artWhat performances could we expect?

Vc

Vb

Ib

Iph

Outline

InP substrate

air

InP substrate

air

GHz

µm²

• Offers gain compared to PIN-diode or UTC

• Good noise performances (/ APD)

• Compatible with HBT fabrication process

• Several configurations:– Top / substrate / side illumination– 2T / 3T

• Specific applications: injection locked oscillators, clock recovery setups…

Heterostructure phototransistor

Heterojunction bipolar PhotoTransistor principle

InGaAsCollector

InGaAsBase

InP Emitter

Vc

Vb

Ib

Iph

holes frombase current

photocreated holes

HPT: electrical modelling

Electric field Carrier densities

Conditions:

-3-T HPT, Ib = 50 µA

- Vce = 1.5 V

- darkness (—)

- 3 mW optical input ()

Conditions:

- 3-T HPT, Ib = 10 µA

- Vce = 1.5 V

- 0, 1, 5, 10 mW optical input

Popt

NPN HPT behavior under optical illumination

Popt

Why lateral illumination?

More flexibility for the design :Optimisation of device in terms of electronic behaviour + optimisation of device to improve optoelectronic efficiency.

To decorrelate light absorption and carrier transport directionsSo, in a first approach, to allow the separate optimisation ofoptoelectronic (efficiency,…) and electronic (bandwidth,…) properties

h e- ; h+

• Introduction

• Scholar studyOptical modeling

Optical structure optimization

• 3T HPT making ofStructure

Characterization

• HPT state of art

• Side illumination requires optical guiding properties of the device

• Need a specific design to optimize the coupling efficiency

BPM simulations of the structure

InP Emitter 0.3 µm

InGaAs Base 0.1 µm

InGaAs Collector 0.28 µm

InP Collector 0.2 µm

InGaAs Sub-collector 0.2 µm

Inspired of: H. Kamitsuna, Y. Matsuoka, N. Shigekawa, “Ultrahigh-speed InP/InGaAsP DHPTs for OEMMICs”, IEEE Trans. Microwave Theory Tech., vol. 49, no. 10, (2001), pp. 1921-1925.

Lensed fibre

Optical study

Absorbing layers

Example of a phototransistor: top illuminated HPT structure

Device size: 6x4 µm²Spot size: m

… but study in the case of lateral illumination !

InP substrate

air

Internal responsivitySide illumination: 0.52 A/W TE

0.64 A/W TM(Top illumination: 0.37 A/W)

Device size: 6x4 µm²

N InP (emitter 0.3 µm)P+ InGaAs (base 0.1 µm)InGaAs (collector 0.28 µm)N InP (collector 0.2 µm)N+ InGaAs (sub-collector 0.2 µm)

2D BPM modeling of side illuminated HPT

Simulation of light propagating inside the device

= 1.55 µmspot : 2.4 µm

air

substrate

Carriers generation rate

0.00E+00

2.00E+23

4.00E+23

6.00E+23

8.00E+23

1.00E+24

1.20E+24

1.40E+24

0 1 2 3 4 5 6 7

Light penetration (µm)

(cm

-3.s

-1)

Most of the light is absorbed along the 1st 5 µm

0

0.2

0.4

0.6

0.8

1

-4 -3 -2 -1 0 1 2 3 4

Injection offset (µm)

Res

pons

ivity

(A/W

) TE

TM

Tolerance to the fibre position

emitterbase

collectorsub-collector

air substrate

0.62 A/W

= 1.55 µmspot : 2.4 µm

air

substrate

• Optimal injection is centered on the base layer

• Misalignment tolerance +/- 0.5 µm (10% of the maximum)

Considering a typical HPT structure:

Changing from top illumination to side illumination can result in:• 0.52 A/W TE

0.62 A/W TM

0.37 A/W top• +/- 0.5 µm alignment tolerance

Optical guiding properties of the device are not optimized

• Introduction




Characterization


• Introduction


Optical structure optimizations


Characterizations


• What do we want ?A more efficient light collection

• How to get it ?Get a better light confinement

Add a confinement layer

InP Emitter 0.3 µm

InGaAs Base 0.1 µm


InP Collector 0.2 µm

InGaAs Sub-collector 0.2 µm

Spot size


InGaAsP confinement w

Insertion of an InGaAsPOptical confinement layer

Optimization parameter: Its thickness w

Modified structure

To get better guiding properties:

N InP (emitter 0.3 µm)P+ InGaAs (base 0.1 µm)InGaAs (collector 0.28 µm)Q-1.3 0.5 µmN InP (collector 0.2 µm)N+ InGaAs (sub-collector 0.2 µm)

2D-BPM modeling

Modified structure:

Better absorption

Lower losses

Without

With

Comparison: with and without confinement layer:

2D-BPM modeling

More efficient light collection increased response

Find the optimal confinement layer width

Device optimisation

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0 0.2 0.4 0.6 0.8 1 1.2

Confinment layer width (µm)

Re

sp

on

siv

ity

(A

/W)

TMTE

• Optimal confinement layer width: w = 0.5 µm

• Increase / saturation / decrease of R with w

W

R

= 1.55 µm

0

0.2

0.4

0.6

0.8

1

-4 -3 -2 -1 0 1 2 3 4

Injection offset (µm)

Res

po

nsiv

ity (

A/W

)

TMTE

0.74 A/W

• Optimal injection is centered on the absorbing region

• Misalignment tolerance +/- 0.65 µm (10% of the maximum)

emitterbase

collectorconfinement sub-collector

air substrate

Tolerance to the fibre position

w = 0.5 µm = 1.55 µmspot : 2.4 µm

air

substrate

Side illumination:

up to 0.74 A/W @ 1.55 µm

Internal responsivity increase: 16% more compared to non optimized structureup to 2x better than vertical illumination

up to 6 dB more microwave power

0.37

0.52

0.620.62

0.74

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Vertical Side Side optimized

TE

TE

TM

TM

Internal responsivity (A/W)

Comparison: responsivities

• Introduction


Alignment tolerance



Characterization


• Side illumination is more efficient

• Optimized structure gives about twice the responsivity(with the same absorbing layers)

6 dB more microwave power

• Introduction


Alignment tolerance



Characterization


• Light collection: Can we find more efficient structures ?

Thicker absorbing layer Thicker confinement layer

(not without consequences on bandwidth!)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 0.2 0.4 0.6 0.8 1 1.2 1.4

InGaAsP thickness (µm)

Inte

rnal

res

pons

ivity

(A

/W)Absorption and confinement

layers widths optimization:

-Two polarizations-Two 1.55 µm ; 1.3 µm- Two fibres cleaved ; lensed

Trade off between several optimal values

0.5 µm < w < 0.8 µm

= 1.55 µm

= 1.3 µm

Device definition

w InGaAs = 0.49 µm

E

B B

InGaAs (p++)

InGaAsP (n+)

Substrate InP (I.)

InP (n+)

InGaAs (n-)

C

InGaAs Cap layer 0.1 µm

InGaAs Base 0.1 µm


InP Substrate

InGaAsP Cladding 0.7 µm

InP Emitter 0.1 µm

Device defined to get an optimum light collection: 90% internal efficiency(8 µm long device ; lensed fibre)

Developed structure

Phototransistor-guide (HPT)

EBC

SC

InP S.I.

- Triple mesa structure- Polymide bridge- Self-aligned base process

Emitter contact

Base contactCollecteur contact

Simple heterostructure

But: - device performance relies on the final cleaving process (couple of microns difference in cleaving decrease either the bandwidth (too long) or the efficiency (too short). - no possible integration with double heterostructure HBT

E

E

C

B C

BCleaving axis

Fabricated device

Device fabricated at IEMN

Optical micrography SEM micrography

Device size after cleaving 4x8 µm² DC currant gain : around 200

• Introduction• Scholar study

Optical modeling

Alignment tolerance



Characterization


• S parameters extraction– ft, fmax

– equivalent model

• Opto-microwave parameters– optical fc

0

5

10

15

20

25

30

35

40

1 10 100

Frequency (GHz)

Gai

n (d

B)

h21

MSG

MSG / MAG

6 dB / octave

3 x 15 µm² HBT dynamic characterisationIc = 13.5 mA

ft = 60 GHzfmax = 42 GHz

Microwave properties

Noise comparison HPT versus PIN + HBT

0

10

20

30

40

50

60

70

80

1 10 100Frequency (GHz)

Equ

ival

ent I

nput

Noi

se (

pA.(

Hz)

-½)

p-i-n/HBT

HPT

Rbb

Cbc

Rb

Cc

Re

Cbe

Ree

Rc Lc

Le

Cpbe CpceRL

RD

CDIph

PIN photodiode

Load50

HBT

Externalbasecicuit

HPT

Load50

RL

RBE

Cpbe Ree

Le

Re

Cpce

LcRc

Cbc

Iph

RbbRb

Cbe

Cc

- Equivalent input noise :

Noise comparison HPT versus PIN + HBT

Advantage can be taken from HPT use


Optical gain cut-off frequency > 45 GHz

Opto-microwave properties


Optical modeling

Alignment tolerance



Characterization


• 3T side illuminated HPT:

– Trade off optimization (1.3 µm, 1.55 µm)

– 8x4 µm² after cleaving

– wInGaAs = 0.5 µm

– wInGaAsP = 0.7 µm

– DC gain: 200

– ft = 60 GHz

– fcopt = 45 GHz


Optical modeling

Alignment tolerance



Characterization


• Electrical DHBT can go up several hundreds of GHz

• BUT:– Optical HPT needs a sufficient

absorbing layer– Side illumination requires a

confinement layer

• However 100 GHz operation has been reported

CNET2002

NTT 2001 DHPT

Cincinnati 98

AT&T 91

ATR 93

IEMN 96BT 93

IEMN 96

NTT 94 CNET 96Naval Research Laboratory 91

Michigan University 93

NTT 95

CNET 99

CNET 97

IEMN 99

0

20

40

60

80

100

0 20 40 60 80 100 120 140 160

Emitter-base junction, µm2

Op

tica

l f t

, G

Hz

Cut-off frequency state of art

Dashed line consistent with emitter-base junction capacity limitation

topside 2Tside 3T

Conclusion

HPT type: Top Side Note:

Fabrication: cleaving

A.R. coating: +

Gain: = =

Fc: ~ Side:

e- transit time <

Alignment: + Side:

Waveguide coupling?

Responsivity: +

S/N: +

Conclusion

• Optimal light collection requires side illuminated structures

• Side illuminated structures can be optimised

• Up to 2x responsivity, 4x microwave power (6dB)

• Best S/N results should be obtained with side illuminated structures

• BUT: increased technological difficulties

Status on long-wavelength InP waveguide heterojunction phototransistors

Documents

Transcript of Status on long-wavelength InP waveguide heterojunction phototransistors