[doi 10.1007%2f978-1-4020-2103-9_6] shur, michael s.; Žukauskas, artūras -- uv solid-state light...

8/17/2019 [doi 10.1007%2F978-1-4020-2103-9_6] Shur, Michael S.; Žukauskas, Artūras -- UV Solid-State Light Emitters and …

1/16

UV MET L SEMICONDUCTOR MET L

DETECTORS

A robust choice or Al, Ga)N based detectors

J-L. REVERCHON \ M. MOSCA

1

N. GRANDJEAN

2

F. OMNES

2

F. SEMOND

2

, J-Y.

DUBOZ

2

,

and

L

HIRSCH

3

1

Thales Research Technology, 91404 Orsay Cedex, France

2

CRHEA-CNRS, rue Bernard Gregory, Sophia Antipolis, 06560 Valbonne, France

3

IXL-CNRS-ENSEIRB, University o Bordeaux I, 33405 Talence, France

Abstract:

UV

detection is interesting for combustion optimization, air contamination

control, fire and solar blind rocket launching detection. Most o these applica

tions require that UV detectors have a huge dynamic response between

UV

and the visible, and a very low dark current in the range o the

UV

flux meas

ured. (Al,Ga)N alloys present a large direct bandgap in this range and there

fore can be used as an active region in such detectors. To take advantage o the

large Schottky barrier, the good alloy quality, and to avoid any doping prob

lems, we have developed MSM photodetectors. High quality material has been

grown with MOCVD and MBE on sapphire substrates. Stress management is

employed for aluminum contents up to 65 to reduce crack density. This is

correlated with non-ideal features like dark current, sub-bandgap response and

non-linearity between photocurrent and optical flux. The spectral selectivity

between UV and visible reaches five orders o magnitude. A geometry o in

ter-digitized fingers is optimized in regards to the peak response. The Schottky

barrier and a dielectric passivation result in dark currents lower than 1

fA

up to

30 V for a 100 x 100 1m

2

pixel. Consequently, detectivity is mainly limited by

shot noise and corresponds to a noise o 500 photons per second and per pixel.

Key words: UV solar blind detectors, Metal-Semiconductor-Metal detectors, stress man

agement in (Al,Ga)N, IBICC.

77

M.S. Shur and A. Zukauskas eds.), UV Solid-State Light Emitters and Detectors, 77-92.

© 2004 Kluwer Academic Publishers.


2/16

8 J

L

everchon et l

1

INTEREST AND SPECIFICATIONS FOR Al,Ga)N

S N

ACTIVE LAYER FOR UV DETECTION

1 1 Al,Ga)N for

UV

Detection

Due to their use for blue LEDs and lasers, GaN based materials are gaining

more and more importance and a great effort has been undertaken in order to

improve their quality. As a result, nitrides can now be considered for many

other applications such

as

high power-high frequency electronics and ultra

violet UV) detection [1,2]. As a direct band gap III-V semiconductor,

Al,Ga)N is well suited for detecting light at energies higher than its band

gap energy and providing a large rejection at lower energies. The band gap

energy varying from 3.43 eV GaN) to 6.2 eV AlN), makes it possible to

adjust the wavelength of absorption from 360 nm to 195 nm. In particular,

we will focus on wavelengths of about 280 nm for which sunlight is ab

sorbed by the ozone layer and never reaches the surface

of

the earth. Conse

quently, detectors sensitive in this range see only UV sources coming from

the earth and are said to be solar blind.

Fundamentally Al,Ga)N based devices suffer from difficulties such as a

large activation energy required not only for magnesium

p

doping but also

for n doping in high aluminum content alloys. For the same reasons, ohmic

contacts are also difficult to achieve. On the contrary, this large barrier gives

the opportunity to achieve high barrier Schottky contacts. This

is

a great ad

vantage for obtaining

of

low dark current in Schottky based detectors. Fi

nally the main difficulties come from the quasi absence

of

GaN or AlN sub

strates. Nitrides are traditionally grown on sapphire or SiC with a lattice and

thermal expansion mismatch inducing strain, dislocations and cracks. In Sec

tion 2 we will discuss how to avoid cracks and to reduce the non-ideal fea

tures attributed to related electrical defects.

1 2

Specifications for UV Detection

UV

is

in the range of energy involved in chemical bonding. Thus, UV detec

tion presents a great interest for combustion optimization, air contamination

control, UV A/UVB medical control, and fire/flame detection and in particu

lar solar blind detection. Most

of

these applications require stringent specifi

cations because of the low fluxes to be measured. Indeed UV radiation is

diffused by the Rayleigh mechanism especially when UV sources are far

away in the atmosphere. As a consequence, dark current must be as small as

possible in comparison to photocurrent. Moreover, as far as noise

is

con-


3/16

UV

Metal Semiconductor Metal etectors

79

cerned, a low current would diminish the shot noise and the lifnoise, which

are respectively proportional to current and current squared.

In this paper, we present Al,Ga)N based detectors which are in competi

tion with photomultipliers PM) and silicon based charge coupled devices

CCD). PMs are not available in large array configuration, are fragile, and

use a high bias. Large array CCDs are available with a huge detectivity and

even with photon counting mode but only when cooled to reduce dark cur

rent. One

o

the advantages

o

Al,Ga)N based detectors versus PM and

ceo would be the intrinsic spectral selectivity between uv and visible.

t

prevents use

o

interference filters whose sensitivity to non-normal incidence

is a drawback. In the case

o

Al,Ga)N based detectors, such interference

filters may be added to the intrinsic selectivity to obtain even larger rejec

tion.

Obviously, we require from photodetectors a responsivity as large

as

pos

sible. t means that gain the ratio between electron pairs created per photon

absorbed) may be close to one in the case

o

photovoltaic detectors and as

large

as

possible in the case

o

photoconductor or phototransistor structures.

Moreover, the proportionality between photocurrent and incident power

linearity), must be preserved. Concerning the response time, a short one

may be expected due to the low capacitance and transit time

o

device [3].

Capacitance can be estimated to be lower than

0 1

pF for 100 x 100

f m

de

vices. Nevertheless, because

o

the need for large detectivity in imaging with

low fluxes, a long integration time is necessary. Thus the time response is

not so important and needs only be reasonably fast for imaging at several

hundreds

o

hertz.

1 3 The Choice o Metal Semiconductor Metal Detectors

A first kind

o

semiconductor detectors is the photoconductor that may show

a high internal gain defined

as

the ratio

o

lifetime

o

carriers to transit time

between electrodes. As we will see in Section 1.5, the lifetime depends on

density and occupancy

o

deep levels, so that non-ideal and uncontrolled

behaviors may appear like, e.g. a non-linear dependency o the photocurrent

on the incident power. Moreover, photoconductors present an intrinsic high

dark current leading to lifnoise. Therefore, photoconductors are not suitable

as flame detectors in terms

o

dark current, noise and detectivity.

Al,Ga)N

p i n

photodiodes do not exhibit the above-mentioned draw

backs. The gain is limited to one, the current

is

low and the responsivity is

linear. But p-type doping with high Al content is difficult to obtain. Some

attempts to use p-GaN show that long wavelength contribution could be lim

ited by convenient band diagram design [4]. Another difficulty is to make

good ohmic contacts on such wide bandgap semiconductors even i they are


4/16

80

J- L Reverchon et a

highly doped. Generally, a high-temperature annealing cycle (700-900 °C)

is necessary to make the contact ohmic. For this reason, together with the

need for a good

n-type

conducting layer, good Schottky photodiodes are not

so easy to achieve at high aluminum content.

Consequently, the situation is far more convenient for (Schottky)

Metal-

Semiconductor-(Schottky) Metal (MSM) detectors that

don t

need either

doping or ohmic contacts. The only but important difficulty is a mismatch

between the substrates and (Al,Ga)N layers. A MSM consists only

of a

photoabsorbing epitaxial layer with two interdigitated Schottky metal con

tacts deposited on the semiconductor surface. In this paper, we will focus on

this planar technology whose simplicity contributes to robustness.

1 4 Electrical and Optical Characterization Tools

In most detectors, dark current is measured with a picoammeter ( 485, Keith

ley), but when necessary, dark current is measured with a source/meter

(6430, Keithley) in the fA range taking care

of

the connections (Guarded

Tri-axial Cable). For the photoresponse measurement, we use a Xenon lamp

filtered by a monochromator and the light is focused on the back side of the

detector for samples grown on

Ab0

3

,

and on the front side for samples

grown on

Si(lll

. The incident power is measured by a calibrated pyrome

ter. The detectors are biased with a voltage source and connected in series

with a transimpedance amplifier. The photocurrent is measured both in AC

conditions with a chopper and a lock-in amplifier (7220, EG G Instru

ments) and in DC conditions with a picoammeter (485, Keithley).

There-

sponsivity is calculated as the ratio of the photocurrent to the power incident

on the detector. All measurements are made at room temperature.

1 5 Non Ideal Features

n

MSM

Due to dislocations or cracks, some layers may present defects that are elec

trically active and lead to traps or recombination centers. For MSM based on

such material, the high quantity of defects and deep levels gives poor rectify

ing contacts. These levels give both channels across the junction and a bow

ing

of

the conduction band that diminish the depletion thickness at the

Schottky barrier. Finally, this injection via trap-assisted tunneling corre

sponds to a photoconductive behavior. But, in photoconductors, responsivity

depends on the lifetime

of

carriers. This lifetime has been linked in many

ways to traps or deep levels [5,6,7,8].

t

results in a strong nonlinearity

of

photoresponse versus absorbed optical flux. These spectra also present a sub

band-gap absorption and a reduced dynamics depending strongly on fre

quency when spectra are acquired with a chopped flux.


5/16

VMetal Semiconductor Metal etectors

8

2 MATERIAL GROWTH

We present here the structure used for back illuminated samples and the

conditions

o

growth by MOVPE and MBE. The choice

o

nucleation layer

and the efforts to eliminate cracks will be particularly stressed.

2 1 Sample Structures for UV Detection

One o the main difficulties encountered in the growth

o

nitride materials

has been the absence

o a lattice matched substrate. In the case o large array

detectors, we have also to take into account that a Readout Integrated Circuit

ROIC) on the front side obliges us to use a substrate transparent to UV.

Thus, even

i

GaN, AlN substrates or pseudo substrates have been improved

during the last few years, GaN ELOG Epitaxial Lateral OverGrowth)

[9]

or

f. ELOG [10] and bulk GaN [11] cannot be used. On the other hand HYPE

[12] or bulk AIN [13] substrate may be adapted to UV detection i a good

transparency to UV is guaranteed. Up to now, sapphire is still the substrate

o choice. The choice o the nucleation layer must provide good optical and

electrical qualities for Al,Ga)N. As far as optical properties are concerned,

an AlN buffer layer is the only solution to provide transparency at 280 nm.

GaN buffer layer can be used only

i

its thickness is sufficiently low to guar-

anty transparency to UV. After the buffer growth, cracking may arise from

the lattice mismatch between Al,Ga)N and sapphire and also between

Al,Ga)N layers with very different aluminum contents. Thus, one o the

greatest challenges is to manage this mismatch whereas we have to use lay-

ers as thick as possible to minimize dislocations. In our case, we use a thick

window layer o 1

f. m

transparent to UV to improve materials quality. Then

the active layer is grown with a thickness o 0.4

f. m

, sufficient to easily col-

lect carriers.

Figure 1 Left: cross section o sample structures for UV detection. ROIC is on the front side

and light comes from backside. Right: overview

o

interdigitized fingers

o

a MSM.


6/16

82

J

L

R

everchon

t a

2.

2 Sa

mples Gr

own by L

ow Pres

sure Meta

lorganic

Vapour

Phase Ep

itaxy

So

me sample

s are grow

n by low-p

ressure me

talorganic v

apour-phas

e epi

taxy (LP-M

OVPE) on

c-sapphire

substrates

in an Aix t

ron growth

chamber

AIX200 R

F. Trimeth

ylgallium,

trimethylalu

minum, an

d ammonia

are used

as

precursors.

GaN or AI

N buffer la

yers are 25

-nm and 10

-nm thick a

nd are

gr

own at 525

oc

and 8

90 °C, resp

ectively, in

a pure nit

rogen carri

er gas.

(

Al,Ga)N al

loys are gro

wn at 118

0°C with a

V/III ratio

between 20

00 and

310

0 in a pure

hydrogen c

arrier gas. N

H

3

flux is

2 /min and

the total fl

ux

is

5 1

/min . The g

rowth press

ure is low (

20 mbar) in

order to a

void parasit

ic re

actions between N H

3

and TMAI. Finally, the growth rate is 1

f Lm/h

for th e

window

layer Alo.6sGao

35

N) a

nd

1

.8 flm

/h for the ac

tive layer (A

l

0

.

5

Ga

0

.

5

N)

.

More

details are

given in R

ef.

14

.

We now pay

attention to

layers gro

wn

with

a GaN bu

ffer layer. W

e notice a

strong sub

band gap a

bsorption c

orre

spondin

g to deep l

evels (Fig .

2, left) eve

n if no crac

k networks

are presen

t.

All dev

ices grown

on this la y

er present h

igh dark cu

rrent with n

on-ideal fe

a

tu res of

photoco

nductors a

lready men

tioned. Fo

r example,

we notice

in

Fig. 2 (rig

ht) that the

dynamics c

an be reduc

ed when hi

gh bias is a

pplied and

participa

te to trap-a

ssisted tunn

eling across contacts. The frequency depend

ence als

o shows th

e long tim e

needed to n

eutralize su

b-band-gap

absorption

10

4

.8

i OHz

.

20V

Q

0

.6

/)

.

>.

10

2

· 8 Hz

.

2

V

/

)

E

0.4

·

, · . : · - - 80Hz 2V

/)

\ •

/)

Buffer GaN

c

10°

'

_

c

0

'

··

tl

0.2

I

- Buffer

AIN ..

Buffer GaN

a.

1-

uffer

AIN

/)

. . . ,

Q

1o

2

..

.0

0

:::

200

500

300

400

Waveleng

th (nm)

Waveleng

th (nm)

Figure 2

Left: tra

nsm ission spe

ctra for layers

g rown on a G

aN or AlN bu

ffer. Right: sp

ectral

response de

pending on bi

as and frequen

cy for sample

with GaN bu

ffer layer.

O

n the contr

ary, the bes

t samples h

ave been o

btained wit

h an AlN bu

ffer

lay

er. The tra

nsmission

is good dow

n to 280

nm showin

g the absen

ce of

d

eep levels.

This is con

firmed by t

he dynamic

s independ

ent of bias

and AC

or DC mo

de used for

spectral a

cquisition (

Figure 3).

Then, no d

eep level

con

tribute to s

ub-band-ga

p absorptio

n or trap a

ssisted tun

neling acro

ss the

Sc

hottky barr

ier. Conseq

uently, the

time respon

se is due o

nly to the

transit


7/16

VMetal Semiconductor Metal Detectors

83

time needed for carriers to cross the spacing between electrodes and non

ideal features disappear. We must stress here that some good results have

been observed with the layers grown with a GaN buffer layer in the past

[14]. We don't have any clear explanation for these differences. We can only

mention that materials quality has been shown to depend closely on growth

parameters and that the average aluminum content is closer to AlN than to

GaN in such layers.

10'

2

.

,.

.

i

10

3

OHz SV

......

190

Hz

/5V

>

80Hz/5V

·:;;

10

4

- 80Hz/20V

/)

r::

l 4(1.1 N

UA

~ u n

Al

la

oH ~ · m

0

c.

10 '

5

/)

Q)

0::::

10'

6

lluO


8/16

84

J L

Reverchori et

a

expansion consequences.

In

this way,

we

obtain layers without any cracks

and with exceptional electrical and optical properties.

0.8

I

10-

2

0.6

- I

t\

1

_., .;) o

a 1 1m

..

>- 10·3

Ill

::J

\ l

,..,

(fil

lc

N

im

>

/)

iii

0.4

3

i\IN

IUilnm

c

10

4

iii

0

/)

t.l nm

Q

0.2

·

/)

10-

5

::J

\apphin:

Q)

cr:

0.0

250300350400450500

Wavelength nm)

Figure

4 Transmission and response spectra of the layer grown by MBE with a GaN buffer

layer.

3 OPTIMIZATION OF PROCESSING

3 1 Surface Preparation and Metallization

Most MSM detectors were processed for defining interdigitated fingers by

optical lithography. The spacing equals 2 or 5 11m whereas the width varies

from 1 to 10 f.lm. The surface was deoxidized in HCl for one minute and

rinsed in de-ionized water during four minutes just before being introduced

into the Joule evaporation chamber (Plassys chamber MEB550S). The con

tact consists

of

10 nm

of

platinum followed by 100 nm

of

gold. Even

if

we

take care to limit time between cleaning and deposition, we can expect

(Al,Ga)N to be oxidized. Some studies have shown that an oxide could pre

vent leakage via dislocations. For example, some enhancement

of

Schottky

barrier height on (Al,Ga)N/GaN heterostructures by oxidation annealing has

also been reported [ 18]. It may explain the exceptionally low dark current

low obtained with some samples

(1

fA up to 35 V). Even

if

oxide presence

has not been investigated here,

we

have observed that a smooth etching

just

before deposition could increase leakage via induced defects and oxide

elimination. After lift off, annealing at 400 °C during 10 minutes in nitrogen

atmosphere is used only for mechanical requirements. Higher temperatures

would induce leakage currents.


9/16


8

3.2 Contact Passivation and Connection

If

we want to perform a fast evaluation

of

epilayers, we can use a one-step

processing for which contact pads are evaporated at the same time

as

elec

trodes. But in the case

of

a backside-illuminated device, we observed that

both the interdigitated electrodes and the contact pad areas contribute to the

overall photocurrent even if contact pads are placed at several tens

of

mi

crometers from the interdigitated area. In order to avoid the parasitic current

due to the contact pads, we developed another process where the Pt/Au

Schottky contacts are deposited on the (Al,Ga)N surface whereas the contact

pads are sputtered on a dielectric. Several dielectric films were tested for

their electrical passivation capability.

Si

3

N

4

(300 nm) and

Si

2

(300 nm),

were deposited by plasma-enhanced chemical vapor deposition (PECVD) at

300 °C. Benzocyclobutene (BCB) (1500 nm) was deposited by spin coating

and annealed under vacuum at 250 o for 30 min. The first two PECVD ma

terials show good passivation up to

fA

range at several tens

of

volts. Passiva

tion has been particularly efficient in the case

of

layers having developed

microcracks related to excess stress. In that case, we showed that both the

dark current and the responsivity strongly depend on the crack density. By

using our two-level process, we have reduced the parasitic effects

of

cracks

on the dark current.

4 TRANSPORT PROPERTIES

As we can see in Fig. 4, responsivity is limited to 0.04 A/W. This value is

.low compared to the absolute photovoltaic limit

e h v

that would be

0.22A /W at 280 nm. Indeed, MSM detectors have been fabricated by many

groups on GaN [19,20] or on (Al,Ga)N [3,14,21,22] and exhibit good per

formance but with the same limited collection

of

carriers between fingers.

Collection efficiency in MSM detectors

is

studied here with submicronic

lithography, the ion beam induced charge collection method (IBICC), and

numerical 2-dimensional calculations of the electric field distribution.

4.1 Submicronic MSM byE-beam Lithography

A way to improve collection

of

carriers

is

to reduce spacing and trapping

between the electrodes. Here, we will study the effects

of

spacing on both

the spectral response and the absolute value

of

the photoresponse. We com

pare sub-micron devices obtained by electron beam lithography (the width

equals 1 lm, and the spacing

is

0.6 lm) to interdigitized fingers defined by

optical lithography (the finger width and spacing equal to 2 lm) in terms

of


10/16

86 J L

Reverchon

eta

responsivity and spectral selectivity. Exceptionally, the (Al,Ga)N layers are

grown on a Si lll) substrate. Pt/Au Schottky contacts are evaporated and

lifted off. The dark currents are in the pA range for biases up to

10

V and 50

V for 0.6 f.lm and

2 11m

spacing, respectively.

The spectral responses of two different detectors are shown in Fig. 5

(left). The cut-off wavelength is 280 nm, with a 3 decades rejection ratio

between 280 and 300 nm for both spacings. For the

2 f..lm

MSM, the re

sponse presents a plateau from 300 to 365 nm corresponding to the GaN

layer that is grown underneath the active (Al,Ga)N region [21]. This compo

nent, not present in de measurements is due to a capacitive coupling between

the 2-D electron gas at the AlN/GaN interface and the electrodes. At a posi

tive bias

of

40 V, the responsivity is 0.044 A/W corresponding to a 20

quantum efficiency. As far as the

0.6 f.lm

MSM is concerned, the response

decreases more regularly, without any plateau, and shows an overall better

rejection of near-UV light. Thus, the parasitic response in the underlying

GaN layer is largely reduced for the applied de field and for the ac photo

voltage. This is due to a reduced coupling between the GaN layer and the

electrodes when the finger spacing is reduced.

\ I

3 ~

.i: 0.20

I :2

• 0 10 • .

' .

0

..

0:: 0 20 40 60

Bias )

280 320 360 400

Lamnda (nm)

Figure

5 Left: response spectra with 2

J.tm

and 0.6 J.tm spacing MSM; responsivity versus

bias is given in the inset. Right: contours of equi-values

of

electric field found from the 2

MSM geometries.

The variation

of

responses with bias is shown in the inset

of

Fig. 5 (left).

We verify that the dark current at a given bias generally varies as the inverse

of

the finger spacing, although deviations from this law can be seen. The

responsivity increases first sub-linearly ( ·

7

)

and then linearly with bias.

The knee at about 40 V for the 2 11m MSM and

10

V for the

0.6 f.lm

MSM

corresponds to a transition from photovoltaic to photoconductive behavior

for which the contacts start to inject current. We note that the responsivity is


11/16

VMetal Semiconductor Metal etectors

87

much larger in the 0.6-flm MSM than in the 2- lm one at the same bias, or

reaches a given responsivity value at a much lower bias.

4.2 IBICC Measurements

We now present experiments based on IBICC measurements on MSM fabri

cated on the same layer on Si lll). IBICC measurements consist offocusing

a 2 MeV

4

He+ microbeam down to a 1 11m

2

spot size with a low flux of less

than 400 ions per second. Ions are absorbed in the crystal and create about

10

5

electron-hole pairs per ion. One electrode (called anode) is grounded

while a negative bias is applied on the other electrode (cathode). For each

incident ion, a signal was obtained, with the pulse height proportional to the

number

of

collected charges. More details of the experimental procedure can

be found in Ref. 23. Figure 6 shows maps

of

collected charges at 75 V. In

Fig. 6 (left), we have selected the events that give rise to a small charge per

ion. We observe that these events are located at the edges

of

the anode. In

Fig. 6 (middle), we have selected the events that give rise to a large charge

per ion. These events are now located close to the cathode in the Schottky

depletion region. Regions in between fingers give rise to a moderate collec

tion. The collection efficiency is given

as

a function

of

position for different

voltages from 0 to 75 V in Fig. 6 (right). On the anode edges, the collection

efficiency increases rapidly with bias up to 30 V, and then remains almost

independent of bias. On the cathode the collection efficiency is increasing

with bias, and is almost flat below the electrode. As far as the region be

tween the electrodes

is

concerned, the decrease of the current when moving

away from the cathode presents an attenuation length of 5 f tm It is a typical

length for minority carriers already found on EBIC measurements [24] .

20

r IS

:

u 10

a Ill IS 20 B

O

d p

Figure

6

IBICC response at anode (left) and cathode (middle). Response is plotted versus

bias on the right-hand side.


12/16

88

J

L

Reverchon et al

Some precautions have to be taken before extending these IBICC results to

UV MSM detectors. For instance, the detector

is

uniformly illuminated by

photons whereas the beam is focused in 1 Jlm

• Nevertheless, we can think

that below the cathode, carriers are created in the depletion region so that

holes are easily collected even at low bias. Electrons drift towards the anode

where they are collected after the screening

of

the build-up field

of

the

Schottky diode.

4 3 Electric Field Calculation

In order to explain these results, we performed a 2-dimensional calculation

of the applied electric field in the structure using a commercial 2-D solver

Atlas-Silvaco . Parameters used for this calculation are described elsewhere

[21]. Figure 5 right) shows the distribution ofth electric field in the direc

tion perpendicular to fingers in the 2-Jlm and 0.6-Jlm MSMs for a bias

of

15

V. The comparison clearly shows that the high field region extends through

the whole spacing between fingers in the 0.6-Jlm MSM whereas it remains

confined to the electrode edge in the 2-Jlm MSM. t also shows that the ver

tical extension

of

the high field region is reduced when the spacing between

fingers is reduced. The calculated field distribution thus explains the larger

response and the reduced coupling to the GaN layer when the spacing is lim

ited.

In order to calculate the response value from the field distribution, we

made the assumption that electron-hole pairs are collected in high-field re

gions only. The high-field criterion was the following: Al,Ga)N alloys show

some localization with a typical energy

of

50 me V on a spatial scale

of

50

nm [21, 25]. Then, a field higher than

10

kV/cm is needed to collect carriers.

For a front side illumination, photons above band edge are absorbed in the

first 0.2

Jlm,

and the volume

of

the high-field region is just proportional to

the lateral extension

of

the high-field region beside fingers that are not trans

parent to UV. Because

of

a slight dependence on the structure parameters

such

as

doping or finger spacing, or on the field value used to define the high

field region, the response depends on bias

as

v with yin the range

of

0.65 to

0.72. The inset of Fig. 5 shows the calculated response as a function of bias

for both MSM. t varies as ·

7

and the absolute value is close to the meas

ured one up to biases where internal gain starts to appear. This variation is

intermediate between the extension

of

the depletion region in a vertical

Schottky diode r= 0.5) and the linear response y= 1 of a photoconductor

with an uniform field assumed.

We describe IBICC results with the same kind of hypotheses and simula

tions

as

those previously used. Incident ions are absorbed in the Al,Ga)N


13/16

V

Metal Semiconductor Metal etectors

89

layer on a scale that is larger than the layer thickness, so that we can con

sider that the electron-hole pair generation is uniform in the vertical direc

tion. Electrons and holes are efficiently separated where the field is high

enough to overcome localization [21,23]. At the cathode, the high field sepa

rates carriers, and holes are all the more easily collected since the distance to

travel is small. Electrons are swept towards the anode, once the build-in field

of

the Schottky diode is screened. When the bias increases, the high-field

region extends below the cathode and separates more and more electron

hole pairs.

4 4 Conclusion for the Geometry

o

UV Detectors

From IBICC studies, we have shown that it is interesting to increase the

cathode area. As far

as

the region between the electrodes is concerned, sub

micronic studies have shown that spacing between the fingers must be as

short as possible. For example, we can see in Fig. 7 (left) that the responsiv

ity increases with cathode area for a constant area and spacing between the

fingers. We see also this tendency in Fig. 7 (right) with a larger responsivity

for a lower spacing and larger area.

~ 5

>

u

~ 10

·u

E

w 5

0 10 20 30

Bias voltage (V)

~ 20

[ 15

c

10

E

w 5

0 10 20 30

Bias voltage (V)

Figure 7 Responsivity versus bias for different electrode area and spacing in MOCVD sam

ple (left) and MBE sample (right).

5 PERFORMANCES AND CONCLUSION

MSM detectors benefit from the large band-gap and Schottky barrier

of

high

quality undoped materials. The most impressive performance is the dark cur

rents that are still in the femtoampere range at

35 V

We couldn t measure

noise in the best samples. Thus we estimated shot noise, Johnson noise and

1/fnoise corresponding to this dark current. A conservative assumption for

the constant

of fJP f

noise ( 5 x 1-

5

)

shows that noise is dominated by shot


14/16

90

L everchon et a l

noise in the fA range. Then we obtain a detectivity

of

4xl

14

w

1

with a

frame rate

of

100 Hz. It corresponds to an equivalent power

of

2.5

fW

or 500

photons/second per pixel of 100 x 100

j.lm

2

• In our case, capacity is not

measured but may be estimated to

10

fF for 100 x 100

11m

pixels. MSM are

also well suited to work at high frequency. Furthermore, we can stress the

advantage of Al,Ga)N based devices which is the intrinsic selectivity be

tween UV and visible close to five orders of magnitude. We notice that this

dynamics of UV/visible is obtained without any antireflection coating that

would improve both the peak responsivity and dynamics.

So, we have shown that MSM photodiodes present all of the desirable at

tributes of a flame detector: fabrication simplicity, robustness, large

UV/visible rejection, high sensitivity, high speed, low dark current, low

noise, high detectivity. Theses performances approach the ones of photo

multipliers PM) and the best cooled charge coupled devices CCD). Now a

new challenge is to design a Readout Integrated Circuits capable

of

reading

1

fA

with an optimal collection of carriers at 10

V.

Risks of breakdown

in

circuits designed on a small area are important. In a first time, it may be eas

ier to find circuits for large linear array.

If we compare MSM to Al,Ga)N-based Schottky or p i n photodiodes,

we observe that spectral selectivity of

4 orders

of

magnitude has been

achieved between UV and visible with an excellent detectivity [26,27,28,29].

The latter devices require a low voltage which is an advantage to adapt to

standard ROIC. On the contrary, it is more difficult to achieve dark currents

as low as those of MSM owing to the mesa processing and remaining mate

rial difficulties dark current in the

nA

or pA range are typical). Conse

quently, different detectors may be adapted to different kinds of applications:

MSM for extremely low fluxes for which very low dark current is required

fA), and Schottky or p i n photodiodes for larger ones pA). In all cases,

the key reason for choosing Al,Ga)N-based device would be the spectral

selectivity between UV and visible light.

CKNOWLEDGEMENTS

This work was partially supported by DGA contract N° 00-34-068). One

author MM) wishes to acknowledge financial support from a Curie Re

search Grant G5TR-CT-2001-00064). Thanks are due to

R.

Me Kinnon

NRC) for numerical simulations and ONERA for technical support.


15/16


91

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[doi 10.1007%2f978-1-4020-2103-9_6] shur, michael s.; Žukauskas, artūras -- uv solid-state light...

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