integrated optical detectors

8/2/2019 Integrated Optical Detectors

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Integrated Optical Detectors

Detectors for use in integrated-optic applications must have highsensitivity, short

response time, large quantum efficiency and low power consumption

[1]. In this

chapter, a number of diffierent detector structures having theseperformance characteristics

are discussed.

17.1 Depletion Layer Photodiodes

The most common type of semiconductor optical detector, used inboth integrated

optic and discrete device applications, is the depletion-layer photo-

diode. Thedepletion-layer photodiode is essentially a reverse-biased

semiconductor diode in

which reverse current is modulated by the electron-hole pairs

produced in or near

the depletion layer by the absorption of photons of light. The diode isgenerally

operated in thephotodiode mode, with relatively large bias voltage,

rather than inphotovoltaic mode, in which the diode itself is the electrical generator

and no bias

voltage is applied [2].

17.1.1 Conventional Discrete Photodiodes

The simplest type of depletion layer photodiode is the p-n junction

diode. The

energy band diagram for such a device, with reverse bias voltage Va

applied is shownin Fig. 17.1. The total current of the depletion layer photodiode

consists of two components:

a drift component originating from carriers generated in region (b)and a

diffusion component originating in regions (a) and (c). Holes and

electrons generated

in region (b) are separated by the reverse bias field, with holes being

sweptinto the p-region (c) and electrons being swept into the n-region (a).

Holes generated

in the n-region or electrons generated in the p-region have a certain

probabilityof diffusion to the edge of the depeletion region (b), at which point

they are swept

across by the field. Majority carriers, electrons in (a) or holes in (c) are

held in theirrespective regions by the reverse bias voltage, and are not swept

across the depletion

layer.

Fig. 17.1 Energy band

diagram for a p-n junctiondiode under application of a

reverse bias voltage Va

In order to minimize series resistance in a practical photodiode while

still maintaining

maximum depletion width, usually one region is much more heavilydoped

than the other. In that case, the depletion layer forms almost entirelyon the more

lightly doped side of the junction, as shown in Fig. 17.2. Such a device

is called ahigh-low abrupt junction. In GaAs and its ternary and quaternary

alloys, electron

mobility is generally much larger than hole mobility. Thus, the p-

region is usually

made thinner and much more heavily doped than the n-region, so that

the device


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will be formed mostly in n-type material, and the p-region then serves

essentiallyjust as a contact layer.

For a device with the high-low junction geometry indicated in Fig.

17.2, it can

be shown that the total current densityJtot is given by [3]Jtot = q0

_

1 eW

_1 + L p__

+ qpn0

DpL p

(17.1)

where 0 is the total photon flux in photons/cm2s, Wis the width of

the depletion

layer, q is the magnitude of the electronic charge, is the opticalinterband absorption

coefficient, Lp is the diffusion length for holes, Dp is the diffusion

constant forholes, and Pn0 is the equilibrium hole density. The last term of (17.1)

represents the

reverse leakage current (or darkcurrent), which results from

thermally generated

holes in the n-material. This explains why that term is not

proportional to the photon

flux 0. The first term of (17.1.1) gives the photocurrent, which is

proportionalFig. 17.2 Energy band

diagram for a p+-n (high-low)

junction diode underapplication of a reverse bias

voltage Va

to 0, and includes current from both the drift of carriers generated

within the depletion

layer and the diffusion and drift of holes generated within a diffusion

length Lp

of the depletion layer edge. The quantum efficiency q of the detector,

or the number

of carriers generated per incident photon, is given by

q = 1 eW_1 + L p_

, (17.2)

which can have any value from zero to one. It should be noted that(17.1) and (17.2)

are based on the tacit assumption that scattering loss and free carrier

absorption are

negligibly small. The effect of these loss mechanisms on the quantum

efficiency,when they are not negligible. is discussed in Section 17.1.3.

It can be seen from (17.2) that, in order to maximize q. it is desirable

to make

the products Wand Lp as large as possible. When Wand Lp are

large enoughso thatq is approximately equal to one, the diode current is then

essentially proportional

to 0, because the dark current is usually negligibly small.

If the interband absorption coefficientis too small compared toWand

Lp. many

of the incident photons will pass completely through the active layersof the diode

into the substrate, as shown in Fig. 17.3. Only those photons absorbed

within the

depletion layer, of thickness W, have maximum effectiveness in carrier

generation.


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Photons absorbed at depths up to a diffusion length Lp from the

depletion layeredge are somewhat effective in generating photo-carriers, in that holes

can diffuse

into the depletion layer. Photons that penetrate to as depth greater

than (W+ Lp)before being absorbed are essentially lost to the photo-generation

process because

they have such a very low statistical probability of producing a hole

that can reachthe depletion layer and be swept across. Within the semiconductor,

the photon flux(x) falls off exponentially with increasing depthxfrom the surface, asshown in

Fig. 17.4. Thus, ifis not large enough, many photons will penetrate

too deeply

before being absorbed, thus producing carriers that (on average) will

recombinebefore diffusing far enough to reach the depletion layer.

Interband absorption is a strong function of wavelength in a

semiconductor. Theabsorption coefficientusually exhibits a special response curve that

rises sharply at

the absorption-edge (band-edge)wavelength and then saturates at

awavelength that is

Fig. 17.3 Diagram of a

conventional mesa-geometry

photodiode with p+-n doping

profile showing photonpenetration

Fig. 17.4 Optical absorption

versus depth from the surface

in a conventional mesa

photodiodeslightly shorter than the bandgap wavelength, increasing slowly for

yet shorter wavelengths.

Thus, it is impossible to design a diode with an ideal Wfor all

wavelengths.For wavelengths near the absorption edge, the long-wavelength

response of a diode

is limited by excess penetration of photons into the substrate, as

shown in Figs. 17.3and 17.4; its short wavelength response can be limited by too strong

an absorption of

photons in the p+ layer near the surface, where recombinationprobability is large.

Aside from the reduction of quantum efficiency that results from poor

matching

of, Wand Lp, there are some other limitations to depletion layer

photodiode performancethat are also important. Since Wis usually relatively small (in the

range

from 0.1 to 1.0 m), junction capacitance can limit high-frequencyresponse through

the familiar R-C time constant. Also, the time required for carriers to

diffuse from

depths between Wand (W+ Lp) can limit the high frequency response

of a conventional

photodiode. The waveguide depletion layer photodiode, which is

discussed in

the next section, significantly mitigates many of these problems of theconventional

photodiode.

17.1.2 Waveguide PhotodiodesIf the basic depletion layer photodiode is incorporated into a

waveguide structure, as


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shown in Fig. 17.5, a number of improvements in performance are

realized. In thiscase, the light is incident transversely on the active volume of the

detector, rather than

being normal to the junction plane. The diode photocurrent density is

then given byJ= q0 _1 eL _, (17.3)

where L is the length of the detector in the direction of light

propagation. Since W

and L are two independent parameters, the carrier concentrationwithin the detector

volume and the bias voltage Va can be chosen so that the depletion

layer thickness Wis equal to the thickness of the waveguide, while L can be made as long

as necessary

to make L>>1. Thus 100% quantum efficiency can be obtained for

any value of,

by merely adjusting the length L. For example, for a material with therelatively small

value of= 30 cm1, a length ofL = 3 mm would give q = 0.99988.

(Again, ithas been tacitly assumed in (17.3) that scattering loss andfree-carrier absorption are

negligible.)

Because a waveguide detector can be formed in a narrow channel

waveguide, the

capacitance can be very small. even ifL is relatively large. For example,

for a material

with a relative dielectric constant = 12, such as GaAs, a 3 mm long

detectorformed in a 3m wide channel waveguide has a capacitance of only

0.32 pF. This

capacitance is about a factor of ten less than that of a typicalconventional mesa

photodiode. Hence, the high frequency response can be expected to be

correspondingly

improved. Experimentally demonstrated bandwidth of 5 GHz and

quantumefficiency of 83% have been obtained with waveguide detectors on

GaAs substrate

material [4], and InGaAs waveguide photodetectors on InP substrates

also haveexhibited a 5 GHz bandwidth for light in the wavelength range of 1.3

1.6m [5].

Computer simulation of waveguide photodetectors in AlGaInAs

GaInAs, designedfor ultrawide-band operation at 60 and 100 GHz, predict internal

quantum efficiencies

as high as 94% and 75%, respectively, at 1.55m wavelength [6].Because all of the incident photons are absorbed directly within the

depletion

layer of a waveguide photodetector, not only is q improved, but also

the time

delay associated with the diffusion of carriers is eliminated. This resultis a further

improvement in high frequency response.

Due to the many improvements in performance inherent in thetransverse structure

of the waveguide detector, as compared to the axial geometry of the

conventional

mesa photodiode, waveguide detectors should be considered for use in

discrete-device applications, as well as in optical integrated circuits. At

the present

time, waveguide detectors are not commercially available as discrete

devices. However,they can be fabricated with relative ease in many laboratories. Hence

availability

should not long be a problem.17.1.3 Effects of Scattering and Free-Carrier Absorption

The relations given by (17.1), (17.2) and (17.3) neglected the effects of

free-carrier


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absorption and photon scattering on the quantum efficiency of the

detector. Becauseboth of these mechanisms result in the loss ofphotons without the generation of

any new carriers, they tend to reduce quantum efficiency. In many

cases they can

be neglected, and (17.1), (17.2) and (17.3) will give accurate

predictions. However,

when the free carrier absorption coefficientFC and the scattering

loss coefficients

are not negligible as compared to the interband absorption coefficientIB, a more

sophisticated expression for q is required. Such an expression can be

derived asfollows. The photon flux at any point located a distancexfrom the

surface of the

detector on which the photons are first incident is assumed to have the

form given

by(x) = 0ex , (17.4)

where in general the loss coefficientis given by

= IB + FC + S. (17.5)The hole-electron pair generation rate G(x) is given by

G (x) = IB0ex , (17.6)

since only IB results in carrier generation. Thus the photocurrent

density is given

by

J= q L

0

G (x) dx(17.7)or

J= q0

IBIB + FC + S

_1 e(IB+FC+S)L _ . (17.8)

Comparing (17.8) with (17.3), it is obvious that the effect of additional

lossesdue to scattering and free-carrier absorption is to reduce the quantum

efficiency by

a factor ofIB/, even when L is large enough to maximizeq.

Ifs and FC are small compared to IB, as is generally true, (17.8)

reduces to

(17.3). However, if the waveguide is inhomogeneous or is unusually

rough, or if the

detector volume is heavily doped so thats and FC are not negligible,then (17.8)

must be used.

17.2 Specialized Photodiode StructuresThere are two very useful photodiode structures that can be fabricated

in either a

waveguiding or conventional, nonwaveguiding form. These are the

Schottky-barrier

photodiode and the avalanche photodiode.

17.2.1 Schottky-Barrier Photodiode

The Schottky-barrier photodiode is simply a depletion layer

photodiode in which the

p-n junction is replaced by a metal-semiconductor rectifying(blocking) contact. For

example, if the p-type layers in the devices of Figs. 17.3 and 17.5 were

replaced by

a metal that forms a rectifying contact to the semiconductor, Schottky-

barrier photodiodes

would result. The photocurrent would still be given by (17.1) and

(17.3),

and the devices would have essentially the same performance

characteristics as their

p+-n junction counterparts. The energy band diagrams for a Schottky-

barrier diode,


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under zero bias and under reverse bias, are given in Fig. 17.6. It can be

seen thatthe depletion region extends into the n-type material just as in the case

of a p+-

n junction. The barrier heightB depends on the particular metal-

semiconductor

combination that is used. Typical values for B are about 1 V.

In conventional mesa devices, a thin, optically transparent Schottky-

barrier contact

is often used (rather than a p+-n junction) to enhance short-wavelength response,

by eliminating the strong absorption of these higher energy photons

that occurs inthe p+ layer. In a waveguide photodiode, a Schottky-barrier contact is

not needed for

improved short-wavelength response because the photons enter the

active volume

transversely. However, case of fabrication often makes the Schottky-barrier photodiode

the best choice in integrated applications. For example, almost

anymetal (exceptfor silver) produces a rectifying Schottky-barrier when evaporated

onto GaAs or

GaAlAs at room temperature. Gold, aluminum or platinum are often

used. Transparent

conductive oxides such as indium Tin oxide (ITO) and cadmium Tin

oxide

(CTO) can also be used to eliminate the photon masking effect of the

contacts andthereby improve the quantum efficiency, as described in Section

17.2.4. Photoresist

masking is adequate to define the lateral dimensions duringevaporation, and no

careful control of time and temperature is required, as in the case of

diffusion of a

shallow p+ layer.

A detailed discussion of the properties of Schottky-barrier diodes isbeyond the

scope of this text so that the interested reader should refer to the

information available

elsewhere [7].

17.2.2 Avalanche Photodiodes

The gain of a depletion layer photodiode (i.e. the quantum efficiency),

of either the

p-n junction or Schottky-barrier type, can be at most equal to unity,under normal

Fig. 17.6

conditions of reverse bias. However, if the device is biased precisely at

the point of

avalanche breakdown, carrier multiplication due to impact ionization

can result in

substantial gain in terms of increase in the carrier to photon ratio. Infact, avalanche

gains as high as 104 are not uncommon. Typical current-voltage

characteristics for

an avalanche photodiode are shown in Fig. 17.7. The upper curve is for

darkenedconditions, while the lower one shows the effects of illummination. For

relatively

low reverse bias voltages, the diode exhibits a saturated dark current

Id0 and a saturated

photocurrentIph0. However, when biased at the point of avalanche

breakdown,

carrier multiplication results in increased dark currentId, as well as

increased photocurrent

Iph. It is possible to define a photomultiplication factor Mph, given by

Mph Iph

Iph0, (17.9)


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and a multiplication factor M, given by

M Iph + Id

Iph0 + Id0

, (17.10)

An exact equation for the current-voltage curve is difficult to obtain in

the region

of bias in which avalanche breakdown occurs. However, Miller [8] has

represented

the functional form of the photomultiplication factor by the expression

Mph = 11 (Va/Vb)n . (17.11)

where Vb is the breakdown voltage, and n is an empirically

determined exponentdepending on the wavelength of light, doping concentration, and, of

course, the

semiconductor material from which the diode is fabricated. For the

case of large

photocurrentIph0 >> Id0 Melchior and Lynch [9] have shown that themultiplication

faSpecialized Photodiode Structures 353

M= 11 _ VaI R

Vb

_n , (17.12)

where Iis the total current, given by

I= Id + Iph, (17.13)

R being the series resistance of the diode (including space-charge

resistance if significant).

The derivation of (17.12) assumes thatIR


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carefully fabricated and are hermetically sealed into adequate

packages, mean timeto failure as high as 105 h at 170 C has been observed [1, p. 80],

which projects to

about 109 h at room temperature.

17.2.3 p-i-n Photodiodes

In Section 17.1.1. it was pointed out that conventional photodiodes

must be designed

so as to have a large Wproduct in order to maximize q; but one

doesnt havecomplete control over either the depletion width W, which depends on

dopant concentrations,

or the absorption coefficient, which depends mostly on the bandgap.In

the p-i-n photodiode, a very lightly doped intrinsic layer is formed

between the p

and n sides of the diode. This layer generally has a carrier

concentration of less thanctor is given by

1014/cm3, but it is compensated by a balance of p- and n-type

dopants rather than

being truly intrinsic. Because of the low carrier concentration, the

depletion layer

in a p-i-n diode extends completely through the i layer so that the total

thickness of

the active layer is the sum of the i-layer thickness Wi and the

deplection width on

the lightly doped (n) side of the junction. Thus the device designer can

adjust thetotal depletion width to produce a large Wproduct by varying the

thickness of the

i-layer. The presence of the relatively thick i-layer also reduces thejunction capacitance

and increases the R-C cutoff frequency of the diode, p-i-n photo-diodes

are

widely used as detectors in optical systems because of their high

quantum efficiency(responsivity) and wide bandwidth. For example, Kato et al. [12] have

reported a

waveguide p-i-n photodiode operating at 1.55m wavelength with a

quantum efficiency

of 50% and a 3 dB bandwidth of 75 GHZ.

17.2.4 Metal-Semiconductor-Metal Photodiodes

Metal-semiconductor-Metal (MSM) photodiodes are surface-oriented

devices thatfeature interdigitated, finger-like, Schottky barrier contacts formed on

the surface of

a thin semiconducting layer on a semi-insulating substrate. A typicalMSM photodiode

structure is shown in Fig. 17.8. Carriers generated by the absorption of

photons

in regions of the semiconducting layer between the contacts are swept

by the fringingelectric field and collected by the contacts. Holes are collected by the

cathode

and electrons by the anode. Spacing of the contact fingers must be lessthan the

diffusion length of the carriers in order to produce a high collection

efficiency.

Because the contact fingers are very narrow and closely spaced

(1m) the

capacitance is relatively low and the transit times of carriers are short.

Hence, widebandwidth

operation is possible. Hsiang et al. have made Si MSM diodes with0.2m width and spacing with a full-width-half-maximum pulse

response of 3.7 ps,

corresponding to a 3 dB bandwidth at 110 GHz [13]. The Schottkyelectrodes of the

MSM photodiode are essentially identical to the gate metalization of

field-effect


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transistors, which facilities their monolithic integration with FETs.

For example,Mactaggart et al. [14] have made a fully-integrated 400 Mb/s burst-

mode data

OEIC receiver for application as a phased-array antenna controller.

Approximately

350 source-coupled FET logic gates are present on the GaAs chip,

along with a

780 nm wavelength MSM photodiode. MSM photodiodes have also

been integrated

monolithically with High-Electron-Mobility field effectTransistors

(HEMTs) to

produce OEIC receivers with bandwidths larger than 14 GHz [15, 16].

HEMTs

have also been integrated with p-i-n photodiodes to produce OEIC

receivers with a

bandwidth of 42 GHz [17].The most significant disadvantage of MSM detectors is their inherent

low responsibility

because the metallization for the surface electrodes shadows the

active

light-collecting region. However, this problem can be mitigated by

using a transparent

conducting material for the contact electrodes. Gao et al. [18] have

fabricated

InGaAs MSM photodiodes with transparent Cadmium Tin Oxide (CTO)

electrodes.

The responsivity of these devices to 1.3m light was 0.49 A/W, ascompared to

0.28 A/W for identical control samples with conventional Ti/Au

electrodes.Another approach to improve the overall responsivity of an m-s-m

photodiode is

to monolithically integrate it with an amplifier. For example, Cha et al.

[19] haveintegrated an m-s-m photodiode with a high-electron-mobility

transistor (HEMT)

on an InP substrate, with an InGaAsP buffer layer (g = 1.3 m). They

measured a

responsivity of 0.7 A/W at a wavelength of=1.3 m, and the 1.5100

m2 gate

HEMT hadft andfmax of 18.7 and 47 GHz, respectively.

17.3 Techniques for Modifying Spectral ResponseThe fundamental problem of wavelength incompatibility, which was

encountered

previously in regard to the design and fabrication of monolithiclaser/waveguide

structures in Chapter 14, is also very significant with respect to

waveguide detectors.

An ideal waveguide should have minimal absorption at the wavelength

being used.However a detector depends on interband absorption for carrier

generation. Hence,

if a detector is monolithically coupled to a waveguide, some meansmust be provided

for increasing the absorption of the photons transmitted by the

waveguide within

the detector volume. A number of different techniques have proven

effective in this

regard.

integrated optical detectors

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