Reliability of InGaAsInP based separate absorption grading multiplication avalanche photodiodes

(~)

Microelectron. Reliab., Vol. 36, No. 7/8, pp. 973-1000, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0026-2714/96 $15.00+.00

Pergamon

PII :S0026-2714(96)00026-1

REVIEW PAPER RELIABILITY OF InGaAs/InP BASED SEPARATE ABSORPTION GRADING MULTIPLICATION AVALANCHE PHOTODIODES P. MONTANGERO, G.A. AZZINI, H.C. NEITZERT, G. RICCI AND L. SERRA Centro Studi e Laboratori Telecomunicazioni (CSELT), via (3. Reiss Romoli, 274, 10148 ~ Torino, Italy

Abstract An overview over typical defects and reliability aspects regarding the operation of InGaAs/InP avalanche photodiodes for the use in fiberoptic communication systems is given. Measurements

regarding the performance

types of top

illuminated

and stability of different

InGaAs/InP Separate

Absorption

Grading

Multiplication avalanche photodiodes have been compared with special emphasis on the homogeneity of the photoresponse. High resolution scanning optical microscopy measurements allowed

to

misalignment

identify

specific

resulting

in

at different wavelengths

technological

increased

problems.

responsivity

of

Guard the

ring

junction

perimeter have been found on devices from one manufacturer. The responsivity images of the active area of some of the investigated photodiodes showed sharp peaks due to microplasma formation. The impact of these microplasmas on dark current and photocurrent of the devicea has been investigated in detail. A step-stress test performed at successively increasing temperatures resulted in the catastrophic failure of the tested devices from one family. An analysis of the dark current-voltage characteristics before and after degradation leads to the conclusion that a modification of the semiconductor-insulator

interface

properties

caused

a

dramatic

increase in the surface leakage currents. Copyright © 1996 Elsevier Science Ltd

Introduction

Avalanche photodiodes (APDs) are widely used as receivers in long distance fiber-optic communication systems. APDs have, compared to 973

974

P. Montangero et al.

the

conventional

pin

photodiodes,

the

advantage

of

inherent

photocurrent gain in the multiplication regime of the current-voltage characteristics

and

better

noise

performance

bandwidths. The wavelengths around wavelengths

of

communication.

highest

interest

A wavelength

at

high

modulation

1300rim and 1550nm are the for

around

long

distance

1300nm

fiber-optic

corresponds

to

the

absolute dispersion minimum and a relative absorption minimum of a standard

silica fiber,

while the wavelength

region

around

1550nm

corresponds to the absolute absorption minimum of a standard fiber and in the case of a dispersion-shiftet fiber - also to the dispersion minimum. For this wavelength range photodiodes are most commonly made from Ge or from

InP based ternary (InGaAs) and quaternary

(InGaAsP) compounds. It should be mentioned that also HgCdTe has been investigated for this wavelength range [1]. The

InGaAs/InP

material

system

has

for

photodetector

applications several advantages in the long wavelength range compared to Ge, like a better noise figure [2], lower dark current [3] and a high quantum efficiency in a broad wavelength range (1.0-1.6~m) [4]. It has also a better performance than Ge regarding the temperature stability of the dark current and the noise figure [5], making it very attractive for

operation

in

uncontrolled

environments

where

the

ambient

temperature is varying in a wide range. While it was initially thought to be difficult to obtain

high multiplication factors

in the

InGaAs/InP

system [6], state-of-the-art devices achieve rather high photocurrent multiplication values [7]. For the

prediction

of the

reliability of optical

communication

systems, the investigation of the degradation mechanisms of avalanche photodetectors as one of the key components is crucial. The main device parameter change observed during accelerated stress tests at elevated operating

temperatures

performed

on

InGaAs

pin

diodes

without

multiplication was a substantial increase in the dark current [8,9]. The dark current increase has been reported to be induced either by the degradation of the surface passivation layer [6] or by the formation of premature breakdown regions at the perimeter of the active area of the photodiode

[10].

For a comprehensive

study

of the

dark

current

mechanisms in pin-diodes the reader is referred to Ref. [11]. In addition to the former mentioned degradation mechanism, which

Multiplication avalanche photodiodes

975

is common for pin-diodes with and without photocarrier multiplication, the performance of avalanche photodiodes is additionally limited by the existence of small spots within the active region of the device, where the breakdown voltage is significantly lowered in comparison to the surrounding area [12,13]. This type of defect, which has been referred to as "microplasma," is already described in the very early literature about silicon pn-diodes [14,15,16]. Besides degrading the homogeneity of

the

photoresponse

of

the

avalanche

photodiode,

microplasma

formation is as well one of the reasons for an increase in the electrical noise of the APDs [15,17]. It corresponds to a local increase in the electric field

in the

multiplication

layer of the avalanche diode. In

particular the correlation between the microplasma density and the density

of

dislocations

in

the

semiconductor

layers

has

been

investigated. For InGaAs APDs some authors find approximately the same microplasma and dislocation density [18,19]. Others report for GaAs on Si [20], Si [21] and InGaAs [22] APDs that the microplasma density is orders of magnitude lower than the dislocation density. In t h e latter case it has been argued that only the decorated dislocations create

electrically

active

defect

sites

or

that

other

crystal

imperfections may be responsible for the microplasma formation [23]. It has been proposed that the dislocations cause a local enhancement of the

dopant

concentration

which

increases

the

electric

field

and

therefore leads to a local decrease of the avalanche breakdown voltage [18]. Local changes in the electric field strength in the multiplication layer can also be due to variations in the layer thicknesses or doping instabilities

[23],

The degradation of the metallic contacts [24] is another problem, which limits the long term stability of InGaAs avalanche photodiodes. Similar problems are already known for Ge APDs, where for example fast AI diffusion has been observed [25]. Here we will focus more on problems that are relevant in the normal temperature operating range of the avalanche photodetectors. Recently in some laboratories other types of InGaAs/InP APDs, which are - to our knowledge - not yet commercially available, have been realized. For example a charge layer has been inserted into the

976

P. Montangero

et al.

SAGM structure and the resulting SAGCM InGaAs high modulation 100GHz

based APDs exhibited

speed with a gain-bandwidth product higher than

[26] and low noise operation [5]. The substitution of the

homogeneous

multiplication

region by superlattice

[27] and multi-

quantum-well [28] structures enables the operation of these detectors in

the

single-carrier-initiated,

single-carrier-multiplication

mode.

This results in very low excess noise and allows for the operation of the device in the regime with high photocarrier multiplication [29], resulting

in elevated gain-bandwidth products.

Staircase APDs, as

proposed for operation at rather low applied voltages [30] have recently been realized using the GaAs/AIGaAs [31] and the InAIGaAs material systems [32]. To our knowledge reliability data on this kind of advanced structures is not yet available. In the present study we will give a short introduction to the electro-optical

characteristics

of

InGaAs/InP

Separate

Absorption

Grading Multiplication (SAGM)APDs, showing the results obtained on devices of four different manufacturers. Then we will demonstrate the existence of some of the above mentioned defects and more in detail study the microplasma formation and other inhomogeneities in the photoresponse. Additionally the degradation of these devices during a bias step-stress test at increasing temperatures has been monitored and the performances before and after degradation will be discussed. The physical mechanisms leading to the failure of some devices during this step-stress test will be discussed.

Experimental A)

Device Structures

In this work we report measurements families

of

commercially

available

performed avalanche

on four different photodiodes

from

different manufacturers. All the four APD families (labelled A,B,C,D), whose

schematical

cross

sections

are

given

in

Fig.l,

are

top

illuminated Separate Absorption Grading Multiplication (SAGM) APDs. They are specified to conform with the requirements of fiber-optic communication systems. I-or a description of the different types of InP based APDs the reader is referred to [33].

#C

n - kiGa,~P

n - kiP

n" - InP

#A

Fig.1

_~/tl

SHALLOW - O R

SI Nx (PASSIVATION)

/ IJi

#D

GRADED LAYER /

p - ELECTRODE (TI/Pt/Au)

SiN

k

.

n - Inp

n" InGmAi

I CONTACT

Separate Absorption Grading Multiplication InGaAs/InP APDs

PASSIVATICN

n " - kip

Si IMPLANT

n+ kiP (SUBSTRATE)

CONTACT

n - ELECTRODE (Au/Ge)

1

InP Substrale

n - InGa~

n - InGaAsp

% _ J ~ L _ ~ n'-,,

G U A R D RING

Schematic cross section of the four different families of

I

n - ELECTRODE

n + - ~ P SUB.

n - I n P BUFFER

#B

1__ ,.~..ACE

p - ELECTRODE

GUARD RING

p+ - REGION

n" - InGaAs

DEEP - OR

n - ELECTRODE

I

n + l n P - Sub

n - InP

n" InGaAs

n" ktGaAsP

n - InP

p - ELECTRODE

0 0

g

E

_~. O

F,"

m-

978

P. Montangero et aL

More than 12 APDs from each manufacturer have been characterized and most results reported here give an idea of the typical behaviour of the devices from each family. The insertion of an InGaAsP grading layer between the InGaAs absorption layer and the InP multiplication layer reduces the hole pileup at the valence band discontinuity and enables therefore the operation at higher modulation frequencies [34]. The active area of the device B is about 801.tin wide while for devices A,C and D it is 50gm wide and they are all illuminated from the top. The main differences between the investigated APD families is the definition of the lateral confinement and the guard ring structure. The photodetectors from the family A exhibit a very simple structure with only one guard-ring and with a multiplication layer defined by the p+ diffusion in the n--InP. The guard ring is created by ion implantation. In the family B the multiplication region is defined by a n-lnP mesa buried into the n--InP layers and a guard ring at the junction edges [35]. Devices C have two guard rings with a PLEG structure (Preferential Lateral Extended Guard-ring) [4]. For the

APDs from family D the multiplication area is obtained by

silicon implantation and not by the growth of a suitably doped layer as in the

other

implanted

structures

[36];

the

p+

region is larger than the Si

area in order to prevent edge breakdown at the curved

junction. This solution is alternative to the guard-ring implementation.

B)

Measurements techniaues

Dark current voltage (IV) characteristics of the avalanche photodiodes have

been

measured

different temperatures. illumination intensities

with (about

a

with

a semiconductor

parameter

analyzer at

The photocurrent has been measured laser

diode

at

100nW),

using

a

1550nm lock-in

at

low

technique

under

illumination with

a low

frequency modulation (f=4kHz) of the light source. To avoid casual results due to inhomogeneities of the photoresponse, the laser spot has been defocused to cover about 80% of the active photodiodes area. In the case of the characterization of a microplasma site, the incident light beam has been focused to the relevant part of the APD with a spot diameter of about l~m. In this latter case much lower light intensities (between 0.3nW and 8nW) have been used for characterization.

Multiplication avalanche photodiodes

979

High resolution imaging of the photoresponse of the APDs has been clone at different wavelengths and at different applied bias voltages using a scanning wavelengths

optical microscope

(SOM).

846nm, 880nm and 1320nm

Laser diodes at the

have been used as light

sources. Light with a wavelength of 1320nm is mainly absorbed in the InGaAs layer and probes therefore the photoelectric properties of the APDs under normal operation. Light at wavelengths of 846nm or 880nm is mainly absorbed in the upper InP layer and is used in this study for diagnostic

purposes.

responsivity

The

mapping,

light power

will

be

given

densities in the

(Io), used for the

figure

captions.

The

measurement setup has been described in detail elsewhere [37,38].

Results

A)

and

Discussion

Dark and photocurrent voltage characteristics before dearadation

A comparison

of the

current-voltage

(IV)

characteristics

at room

temperature of the four families of APDs is shown in Fig.2.

All diodes

exhibit at low applied bias voltages rather low dark current (Id) values,

10-4

10"4

,tot

10-6

,'l

#A

p-

I---

z

z

rr"

cC

LU 10-8 rr :::) O

Itot

10-8

10-10

10"1(]

10-12

t

10-6

II

,

,

,

,

1

l

l

1

,

-50 VOLTAGE (V)

10"12

,

- 100

10-4

S rl

0

i

i

i

i

#B

• 1

i

i

i

i

- 100

10-4

Itot

10-6

,,(

#C

I-Z

10-6

i

-50 VOLTAGE (V) f Itot

,* It

D #

F-

Z 10-8 rl"

10-8 rr

O 10-10

,~J/\,a

10-12

Fig.2

J

,

,

,

O

=

,

-50 VOLTAGE (V)

,

,

10-10

10-12

,

-100

-50 VOLTAG E (V)

- 1O0

Reverse IV characteristic, measured at 20°C without and with illumination at 1550nm for the four different APD types

P. Montangero et at.

980

which rapidly increase when the voltage reaches a threshold value. This threshold

corresponds

to the

punch-through

depletion

zone extends down to the

voltage,

InGaAs/InP

at which

interface.

the

Type

B

exhibits additionally a second rise of the dark current at about 23V. Also type D shows a small second increase of the dark current (in this case at about 28V ). The reverse dark current (Id) in this kind of APDs is mainly dominated (g-r)

current,

by two contributions:

which

interface defects,

increases

with

and additionally

the generation-recombination

increasing

by surface

density

of bulk and

leakage currents. The

leakage current at the surface can often be distinguished from bulk current contributions, because it is in general not multiplied at higher bias voltages. For APD families D and B only, the growth of the InP multiplication and

the

layer has been interrupted for the silicon

definition

of the

mesa

structure

respectively.

implantation The

rapid

increase of Id for a threshold value of the bias voltage above the punchthrough voltage, which is only present for this two families of APDs, may therefore be explained by an increased number of defect states at this internal interface and hence a rapid increase of the g-r current, when the depletion zone extends down to this region. An alternative interpretation of this rapid current increase for devices of family B and D could be the existence of two different punch-through voltages in the center of the device and in the outside region due to lateral dopant variation in these APD families [5]. Some diodes from the APDs of family A show at a reverse bias voltage of around 30V a significative

increase in the slope of the

current-voltage

clearly

characteristics.

More

than

for

the

diode

presented in Fig.2, this is observed in the characteristics of another APD presented

in Fig.14. The effect might be due to an onset of

tunnelling breakdown before the onset of avalanche breakdown at 63V, which degrades the photodetector performance of this device. For the device of the family C a plateau-like feature in the dark current just before the beginning of the avalanche multiplication has been observed. In the following

the avalanche

breakdown

voltage

(Vb)will

be

defined as the voltage where the reverse dark current equals 100~A. This breakdown occurs at room temperature in a range between 60V and 75V for families A,B and C, whereas the type D APDs exhibit higher breakdown voltages between 82V and 88V. The onset of breakdown is

Multiplication

avalanche

981

photodiodes

sharp for family C and more gradual in the case of family A, B and D. A reason for a more gradual increase may be either the early onset of tunneling breakdown or microplasma formation. The latter effect will be

looked

at

in more

detail

by high

resolution

mapping

of the

responsivity of the APDs in the next chapter. The comparison of the IV characteristics of the four families under illumination is also shown in Fig.2. The light intensities have in this case been chosen to give the same diode current of 100nA for all APDs at

the

punch-through

voltage.

In

the

photocurrent-voltage

characteristics of all four APD types one can clearly see the onset of charge

carrier

multiplication.

In Fig.3 the photocurrent (Iph), determined by lock-in technique has been plotted as a function of the applied bias voltage. Iph is almost negligible at low bias voltages and increases sharply at the punch through

voltage.

photocurrent

Increasing

starts

to

the

rise

bias

again.

voltage

This

is

still the

further, onset

the

of

the

multiplication regime, which is the voltage range where the APD is actually operated. All four APD types exhibit appreciable photocurrent gain. Under the assumption that the photocurrent value at the beginning

10-4

1

10-4

#A

t

v 10-6 IZ I.u n-"

,

#B

10-6 uJ

10-8

.5

10-10

10-10

0-

1` 1` t,

10-12 0

10-12

-100

-5O

VOLTAGE (V)

10-4

~" z ILl E

n"

G.

10-8

J~

•••

,

,

i

i

,

,

,

_~#D

#C

I - 10-6 Z LU

i

-50 VOLTAGE (V)

10-4 i i

¢~

,

0

10-6

-100

~ 5 lO-8

10-10 n

1

.5

10-10

1`1` 10-12 0

Fig.3

,

i

i

i i , ~ , -50 VOLTAGE (V)

i

i

10-12 -100

i

i

i

i

i

i

-50 VOLTAGE(V)

=

i

~ i

-I00

Inverse of the multiplication factor and photocurrent as a function of the reverse bias voltage measured at 20°C under illumination at 1550nm for the four different APD types

982

P. Montangeroet

al.

of the plateau equals a photocurrent multiplication factor (M) of 1, in Fig.3 the inverse of the multiplication factor has been plotted as well in function of the applied bias voltage. An alternative definition for the breakdown voltage is given as the voltage where M approaches infinity. This can be obtained by the extrapolation of the dependence of 1/M vs. bias voltage to l/M=0. For device A this gives a breakdown voltage of 65V, for device B of 68.5V and for device C of 66V. An analytical expression, which is in good agreement with the presented results for all four devices, has been given in literature for the voltage dependence of M [39]. M-

I_(V-R,It/p (1)

where p is a constant with typical values between 1 and 2, Vb is the breakdown voltage, Rs the series resistance of the APD and It the total multiplied current

current.

For even higher applied

of all four APD types

is again

bias voltages

dominated

the total

by the sharply

increasing dark current. In Fig.4 the temperature dependence of the dark current-voltage curve of a SAGM APD in the temperature range between 20°C and 100°C is shown for a device

of the family

B. Basically two temperature

lO-4

,o-71 I 10-11~ 10-121" 0 Fig.4

~

~

,

~

I , -50 VOLTAGE(V)

~

f

J -100

Reverse dark IV characteristic, measured at different temperatures between 20°C and 100°C (in steps of 10°C) for an APD from family B

Multiplication avalanche photodiodes

983

dependent changes have been observed. The avalanche breakdown voltage and the dark current are both increasing while increasing the operating temperature. The first effect is indeed a check that the breakdown mechanism is due to avalanche multiplication, because the alternative breakdown mechanism - i.e. tunnelling breakdown - shows the inverse temperature dependence [40]. In all four cases, Vb increases linearly with increasing temperature, following Eq.(2):

(2)

Vb(T)=Vb(To)(I+~(T-To))

with

positive temperature

coefficients

(p)

between

1.42.10"3°C "1

(family B) and 2.32.10"3°C "1 (family D). We took 273K as the reference temperature [7].

Also

(To). Similar values of I~ have been reported in literature the

generation-recombination

current

increases

with

increasing temperature. This effect causes the monotonic dark current increase with an increase in measurement temperature

below the

breakdown voltage regime. In general an activation energy (Ea)for the current in this regime can be defined and the change of dark current activation energy during an accelerated life test will be discussed in the last chapter.

B) Imaging of the resDonsivitv

For more than 12 samples each of all four types of APDs, high resolution

responsivity

images

have

been

taken

at

different

wavelengths and with different intensities of the probing light, varying in a wide range the applied bias voltages. Here we show in the beginning first

some

responsivity

images

of

APDs

with

a

homogeneous

photoresponse, followed with images of APDs, which display different kinds

of local

defects that have already been mentioned

in the

introduction. Most of the responsivity images presented here have been taken with applied bias voltages in the operating range of the APD, close to the avalanche breakdown regime. Typical photocurrent multiplication values in this regime range between 20 and 30. Local maxima in the photocurrent gain due to microplasma breakdown have been observed in all

investigated

APD

families,

but

with

very

different

rates

of

984

P. Montangero et al.

occurence. Microplasma breakdown has been observed for only one device out of 14 APDs from family B while almost every device from family D exhibited these type of defect. Also for APDs from family A microplasma breakdown has been frequently observed. APDs

of family

C showed

in general

a very homogeneous

photoresponse at all measured wavelengths. This is for example shown in Fig.5, where the responsivity image of an APD of the family C measured

at

62V

with

an

excitation

wavelength

of

1320nm

is

presented. At this wavelength, which is as well one of the wavelengths of interest for the application of the APD, the light is absorbed in the InGaAs layer and one gets information about the bulk properties of the photodiodes.

In order to get more detailed information

about the

homogeneity of the junction edge and guard ring region we mapped the same device at a wavelength of 880nm. It has been suggested in literature to use this probing wavelength for the investigated type of APDs in order to check for irregularities situated in the InP layer near the surface [41]. The result is shown in Fig.6. At this wavelength the photoresponse from the guard ring region is observed as well, which manifests itself in the ring with increased photoresponse surrounding the active diode region. In this case the photoresponse in the active region and also in the region of the guard ring is homogeneous.

Fig.5

Image of the responsivity of an APD of the family C, measured at 1320nm, Vbias=62V, Io=5W/cm2

Multiplication avalanche photodiodes

Fig.6

985

Image of the responsivity of an APD of the family C, measured at 880nm, Vbias=62V, Io=15W/cm 2

The

characterization

of an APD with a local

photoresponse

irregularity in the guard ring region is shown in Fig.7. In this case an APD of family A has been mapped at 880nm with an applied reverse bias voltage of 40V, well below the avalanche breakdown regime. A sharp

Fig.7

Image of the responsivity of an APD of the family A, measured at 880nm, Vbias=40V, Io=15W/cm2

P. Montangero et al.

986

peak in the photoresponse situated on the guard ring can be observed. Devices with such irregularities have a high failure probability, when operated in the multiplication regime [41]. Another common problem in this

APD

family

was

mask

misalignment

during

processing.

To

illustrate this problem, in Fig.8 photoresponse images of another APD from family A, which have been taken at different wavelengths and bias voltages are shown. In this case the photoresponse maps are displayed in a manner, that bright areas on the plot correspond to areas of high photocurrent. The black areas within the bright lines correspond to the metallization. The photoresponse map taken at 880nm with 0V applied bias (Fig.8 A) reveals the misalignment of the active area of the photodiode

with

respect

to

the

surrounding

metallization.

The

responsivity map of this APD, measured at an applied bias voltage of 62V (Fig.8 B) exhibits clearly an enhanced response at two points near the metallization outside of the active area. To enable an easier identification of this areas of punctual higher photoresponse with respect to the geometry of the device, in Fig.8C both maps at 880nm and 1320nm (Fig.8A and Fig.8B) are superimposed. A sharp local irregularity in the photoresponse due to microplasma breakdown is evidenced in Fig.9 for a device from family A biased at 65V. In this case the probing wavelength was 846nm, which is strongly absorbed in the top InP layers, including the multiplication layer. Also the overall homogeneity of the photoresponse of this device in the other parts of the active region is rather poor. Increasing only slightly the applied bias voltage, the responsivity map changes completely. This is seen in Fig.10, where the same APD as in Fig.9 has been mapped, but applying a bias voltage of 66V. Now the photoresponse is still quite inhomogeneous, microplasma,

but the

evidenced

sharp in

enhancement

Fig.9 cannot

at the

be seen

site

of the

anymore.

This

illustrates an often mentioned property of microplasma breakdown, which is limitation of the photocurrent due to the creation of a space charge induced by the high local current itself [16]. Increasing the bias voltage further to a value slightly below breakdown, even a reduced photoresponse may be observed on the microplasma site.

It may

therefore be possible that the APD presented in Fig.9 and Fig.10 contains other microplasmas, which

become visible at other bias

voltages. Consequently in the presence of microplasmas it is not

Multiplication avalanche photodiodes

997

P. Montangero et al.

988

i Fig.9

Image of the responsivity of an APD of the family A, measured at 846nm, Vbias=65V, Io=20W/cm 2

Fig.10 Responsivity image of an APD of the family A, measured at 846nm, Vbias=66V, Io=20W/cm 2

Fig.8

Image of the responsivity of an APD of the family A, measured at different wavelengths A) 880nm, Vbias=0V, Io=15W/cm2 B)

1320nm, Vbias=62V, Io=5W/cm 2

C) Superimposed image of A) and B) Bright areas correspond to high photocurrents values.

Multiplication avalanche photodiodes

989

Fig.11 Image of the responsivity of an APD of the family D, measured at 1320nm, Vbias=77V, Io--5W/cm2 sufficient to characterize the homogeneity of an APD only at one value of the bias voltage. The responsivity of almost all the APDs from family D suffered from a unique type of inhomogeneity, which is reported in Fig.11. The photoresponse appears as a series of parallel stripes separated by lower responsivity zones. It should be noted that the same stripes have been observed in the surface morphology of the device, taken with the scanning optical microscope in the differential phase contrast mode. For more details the reader is referred to [42]. Most probably growth i n h o m o g e n e i t i e s or problems related to the substrate surface induce this anomalous behaviour. The local behaviour of the photocurrent-voltage characteristics of the above characterized APD from family D is shown in Fig.12 measured in

two

different

points,

at

the

site

of

a

microplasma

comparison, in the center of the active area. The

local

and,

for

breakdown

voltage - estimated by the linear extrapolation of the I/M curve - is about 10V lower at the microplasma site than in the center of the active area. C)

Temperature

ste~)-stress

Two APDs of each family have been stressed at a constant voltage and at successively increasing temperatures. For the applied bias voltage

990

P. Montangero et al. 10-4 10-5

g

lo-6

,,,z

1o-,

rr :D 0

10"8

£O

10-

13.

10.10

!

-J

~"

%...

"

;,

#B

~'~

~1

1

,p,/oeo'erl - - ~ , ~ - ~ - - ~ ' ~

Iph (max)

10-11

-50

-100

VOLTAGE (V) Fig.12 Photocurrent (Iph) and I/M as a function of the reverse bias voltage measured at 20°C under illumination within (max) and outside (center) of the microplasma region for an APD of the family D. The incident light intensities were about 8nW for the measurement in the center and 0.3nW for the measurements on the microplasma site.

during this step-stress test a value of 90% of the breakdown voltage at the respective temperature has been chosen. This is a value close to the operating

range of the APDs. The step-stress has been performed at

temperatures increasing first from 40°C to 80°C and then in steps of 20°C

up to

a temperature

of

200°C.

After

150

hours

at

each

temperature the devices have been first cooled down to 20°C.

A

current-voltage characteristic has been taken at that temperature and successively the temperature has been increased temperature. intended

The

elevated

only to accelerate

temperatures

to the next higher

during

the degradation

step-stress

processes,

which

are are

already present at operating temperatures, and have therefore been limited to 200°C in order to avoid the creation of new degradation mechanisms, like the above mentioned contact degradation. In Fig.13 the current values at 20°C (named 190), determined after each stress temperature for a voltage, corresponding to 90% of the breakdown voltage of the respective APD, have been plotted as function o! the stressing time for two APDs each from the four different families. The two devices from family D failed at 140°C and 180°C respectively,

while the other three APD types showed only minor

Multiplication avalanche pholodiodes

temperature

991

(°C)

40 80 100 l zo 140 160 180 zoo

10 -4

,o

10 "s

=

10 6

•

A1

t.

A2

•

C1 C2

B1

0

DI

B2

•

D2

7^

10"z o

--=

I 0 "B 1 0 "9

10-1o 0

300

600

900

1200

time (h) Fig.13 Monitoring of the dark current, measured at 90% of the breakdown voltage (190) of the four different families of APDs

at 20°C

after different steps of a temperature

step-stress test at increasing temperatures

(40°C-200°C)

variations in the dark current. The curves for the two APD's from family B showed slight variations in the - rather low - dark current, which were very similar for both devices. In Fig.14 the dark current-voltage curves for one

device from each

10-4

10-4

#A1

#B1

A 10-6

.- 10-6

w II n-

=3, (,3

10-8

10-8

~ 10-1C

10"10

10-12 =

10-12

/

-50

-50

- 100

VOLTAGE (V)

10-4

A 10-6

w 10°8

o r¢

10-4

10-12

.

•

#C1 I-Z uJ cE

.--

#D1

10-6

10-8 (3 Y n.-

J

~ 10-10

- 100

V O L T A G E (V)

~ 10-10 .

0

.

.

, -50

.

.

VOLTAG E (V)

.

.

10-12 - 100

-50

-100

V O L T A G E (V)

Fig.14 Reverse dark IV characteristic, measured at 20°C for the four different APD types before and after temperature step-stress test

992

P. Montangero et al.

APD family, measured before and after step-stress test, are shown. The APD A1 shows a slight increase in the dark current in the whole measured voltage range, but no shift of the breakdown voltage. The kink, seen at 30V in the IV curve before aging test, is less pronounced after the step-stress test. The IV-characteristics of device B1 did not show measureable

changes.

For device C1 a slight increase of the dark

current at low bias voltages and a slight decrease near the avalanche breakdown breakdown

are

observed

voltage.

characteristics

in

After

addition the

to

step

a slight

stress

just below the breakdown

increase

test

the

regime (between

of the

dark

IV-

50V and

62V) shows a plateau like behaviour. The most dramatic changes - as already seen in Fig.13 - are found for the APDs of family D. Here an overall increase in the dark current of almost two orders of magnitude and additionally a significant decrease in Vb is found, as displayed for APD

D1 in Fig.14.

The step in the dark current

observed

before

degradation at 12V applied bias voltage cannot be seen anymore after degradation. The parameter changes of the APDs from family D - which failed during temperature step-stress at 140°C (D2), resp. at 180°C (D1) will be discussed more in detail. For this purpose the dark current has been measured as a function of the measurement temperature. By plotting the logarithm of the dark current, at the voltage where M=I and at 90% of the breakdown voltage,

1 2

'

'

I

'

'

'

I

'

'

'

r

'

'

'

I

'

after 10

- -~- ~ ,,

u

'

I

'

'

'

I

'

'

'

degradation I E =0.26eV a

m

"-"

'

-.

4

before

degradation

-o

C

2 0

E,=O.47eV ,

, , ,

30

I,

32

,,

I

34

t

i

i

I i I

36

I I =

I I I

38

'

40

,

,

J'

42

' '

44

1/kT (ev "1) Fig.15 Arrhenius-plot and resulting values of the activation energies of the reverse dark current at 90% of Vb for device D1 measured at 20°C before (open circles) and after (closed circles) step-stress

test

Multiplication avalanche photodiodes

993

Versus 1/kT, a single activation energy (Ea) has been found in the investigated temperature

range before and after step-stress. This is

shown in Fig.15 for the current vs. temperature dependence of device D1 for an applied voltage of 90% of Vb. In order to distinguish between the bulk clark current (Ibulk) and the surface dark current (Isurface), the dark current has been plotted as a function of the multiplication factor M. The result is shown in Fig.16 for the Diode D1 before and after stepstress. Now it has been assumed, following [4,43] that the surface dark current is not multiplied, so that for the total dark current (Id(tot))

results: (3)

Id(tot)=lsurface + M'lbulk

6 10"3 A


3 10 -3 O

2 10 .3 m "O

1 10.3

/

o o

O

D1

0 100

after

degradation

I

I

I

I

10

20

30

40

I

1

50

M

2 10"4

I

I

1 10 .4 O 0 0= "O

D1 before

0 10o 0

degradation

I

I

I

I

10

20

30

40

50

M

Fig.16 Reverse dark current as a function of M for device D1, measured at 20°C before (closed circles circles) and after step-stress test (open circles). The results of a linear fit to the data for high M values are displayed as well.

994

P. Montangero Table 1

et al.

Bulk and surface dark current values and dark current activation energies (Ea) at the voltage, where M=I and at 90% of the breakdown voltage for the two APDs from family D before and after temperature step-stress test

Isurface (A) Ibu~ (A) Ea (M=I) (eV) Ea (0.9*Vb) (eV)

I D2 (Ere stress)) 9.75 e-8 1.60e- 10 0.49 0.46

The linear extrapolation multiplication

D2 (post stress) 6.27e-6 4.96e-8 0.29 0.28

D1 (pre stress) 8.15e-8 2.20e-9 0.51 0.47

D1 (post stress) 4.33e-6 3.59e-8 0.26 0.26

of the dark current in the regime of high

values to M=0 (Fig.16) gives in this way the Isurface

value and the slope of the linear extrapolation the Ibulk contribution in the voltage regime where M is rapidly increasing. Table 1 summarizes the activation energies and bulk and surface dark current values for the two devices from family D before and after the step-stress test. The bulk dark current as well as the surface dark current contributions of D1 and D2 increase during the bias step-stress by 1-2 orders of magnitude. The activation energies of the dark current at both investigated bias voltages decrease roughly by a factor of two from 0.46-0.52eV before aging to 0.26-0.28eV after aging. Commonly

for

the

generation-recombination

current

a

dark

current activation energy of about half the bandgap is found [37]. This would give a value of about 0.37eV in the InGaAs layer and of about 0.67eV in the InP. The values of Ea before bias step-stress are in between

this two values

expected for the generation-recombination

currents originating from the InGaAs and InP layers. After degradation, however, the activation energy is close to the values of 0.20eV and 0.25eV, which have been reported in literature for the activation energy of surface

dark

currents

dominating

Id

after degradation

of APDs

[32,44]. From the dependence of Id on M we can state that surface as well as bulk contributions increase

during

significant

to the dark current of the investigated

degradation

decrease

in the

of

the

activation

devices energy

from

family

of the

dark

D.

APDs The

current,

however, indicates that the surface current contribution dominates the dark current after degradation. In the literature the most cited reason for the

dark

current

increase

under similar stress conditions

is a

Multiplication avalanche photodiodes

995

degradation of the passivating layer [24,44,45,46]. As possible reasons hot hole injection and trapping [24,45] and mobile charge buildup in the insulator [46] have been proposed. In another work the contribution of avalanche breakdown at the surface of the guard-ring junction formed in the InP cap layer has been found responsible for a surface current increase [41]. It is interesting to note that in this latter case this local enhancement

of the

surface

current

increases

other than

our

assumption made above - with increasing dark current multiplication values. In another study about the degradation of

InGaAs/InP APDs, the

degradation due to premature edge breakdown was reversible and could be baked out at 200°C, while the increase of the surface current due to charge trapping in the passivating layer was permanent [42]. This, however, is known to be partially recovered by a removal of the passivating layer by chemical etching [45].

Conclusions

Four different families of InGaAs/InP Separate Absorption Grading Multiplication avalanche photodiodes have been characterized by high resolution photoresponse imaging with a scanning optical microscope and by dark and photocurrent measurements. Different highly local features in the photoresponse like edge breakdown and microplasma formation have been observed on some devices. Another typical problem for one of the device families was a mask misalignment during processing, resulting also in an anomalous photoresponse at the diode perimeter. During a step-stress test at increasing temperatures up to 200°C the devices from only one APD family failed. The photoresponse of these latter devices, which failed at temperatures between 140°C and 180°C, exhibited already before degradation a highly non-uniform behaviour with microplasma formation

along parallel stripes. After the step-

stress a dramatic increase of the surface dark current of this failed devices has been observed.

996

P. Montangero et al.

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