Baca A.G., Ashby C.I.H. Fabrication of GaAs Devices

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Heterojunction bipolar transistors

in the high-bandgap emitter. The larger bandgap of the emitter

leads to more energy associated with REDR and can negatively

impact reliability. Also, most AlGaAs is initially more defective

than InGaP making the incidence of REDR more likely. Today,

GaAs HBT reliability is an evolving story. However, the evidence

indicates that more progress is being made at this time in raising

the maximum reliable current density using InGaP emitters with

ledge passivation. Veriﬁcation that the REDR mechanism is the

main limiting factor may suggest that the base doping levels need

to be considered and possibly lowered to provide the lower initial

recombination levels.

Another example of gradual degradation is dopant diffusion.

Beryllium diffusion into the emitter will extend the base-emitter

junction from the heterointerface into the wide bandgap material

and cause V

shifts. Beryllium was the predominant early prefer-

ence for MBE doping of the base, but has largely been supplanted

by C-doping to avoid this problematic diffusion of the base dopant.

Nevertheless, Be-doped HBTs were designed to be reliable at cur-

rent densities of 25 kA/cm

by optimising the structure and growth

conditions, and by being conservative in rating the operating cur-

rent density [19]. Zinc doping in MOCVD has similar diffusion

concerns and has been supplanted by carbon doping.

Although we have focused thus far on gradual degradation of

HBT electrical parameters, some devices fail suddenly, as in

the case for LEDs and LDs. Failure modes include dislocation

propagation, dislocation loops and dark line defects. These defects

can be observed by electroluminescence and, with considerable

patience, TEM. The dislocations may have propagated from the

substrate or be the result of mechanical stress in the device. Low-

dislocation substrates are one means of minimising the former

type of propagation. Mechanical damage can result from stresses

arising during sawing, scribing or bonding the chip. They can also

result from process steps such as high-stress metals or other steps

that may induce strain. Studies of process-induced defects are little

more than exploratory case studies at this time. Relatively little is

known about the true origin of these types of defects.

After long-term bias-temperature stress, carbon precipitates

are sometimes observed in GaAs HBTs with a C-doped base.

For example, both AlGaAs/GaAs and InGaP/GaAs HBTs with

4 × 10

−3

C-doping of the base were biased at 6 × 10

A/cm

at junction temperature (T

) of 150–260

◦

C [20]. After an ini-

tial burn-in, a stage of gradual β degradation resulted, followed

by an abrupt failure. InGaP/GaAs HBTs displayed both slower

gradual degradation and longer times until abrupt failure than

AlGaAs/GaAs HBTs. TEM analysis with EDX was performed

on some of the failed samples. Both C-precipitates and occasional

302

Heterojunction bipolar transistors

migration of metallic impurities were imaged by TEM/EDX for

the InGaP/GaAs HBTs. The C-precipitates were 10–15 nm in

diameter.

Another reported degradation mechanism of GaAs-based HBTs

is a microtwin-like defect [21]. It has been suggested that these

defects are similar to typical micro defects in hydrogenated Si, but

it is not presently known if they are H-related in GaAs.

Reliability studies of GaAs-based HBTs have provided useful

information that can guide certain technology choices. They have

provided useful guides to operating current-density ranges. Best

practice for the choice of ohmic contacts, base dopant, emitter

material, and ledge-passivation schemes is also indicated from

reliability studies. Information is readily available for minimising

H-passivation effects. Companies involved in high-volume pro-

duction run reliability evaluations on multiple lots every month

and unquestionably will have access to information about many

process choices that are not publicly available. Most reliability

issues, however, are poorly understood, including the origin of

various types of extended defects, the best methods of dielectric

passivation and the best ways of avoiding process-induced stress.

In addition, most device engineers are not satisﬁed with the limit-

ations on HBT current density and would agree that much remains

to be done.

9.8 CONCLUSIONS

GaAs HBTs are greatly affected by the unintentional introduction

of defects in certain key parts of their structure. These process-

sensitive parts of the HBT structure need to be understood and

controlled by process engineers. HBT operation, testing and per-

formance characterisation supply the basic information that mater-

ials, processing and device engineers need to solve and understand

the often complex material and process interactions. Reliability

characterisation is an equally important guide for HBT engineers

to the extent that such information is available. Many HBT process

choices are available and some of the main ones were reviewed

in this chapter. As in most facets of engineering, knowledge and

information are a starting point for a job that needs to be done

better than is now possible.

REFERENCES

[1] W. Liu [Handbook of III-V Heterojunction Bipolar Transistors (John

Wiley and Sons, New York, 1998)]

303

Heterojunction bipolar transistors

[2] W. Lui [Fundamentals of III-V Devices: HBTs, MESFETs, and

HFETs/HEMTs (John Wiley and Sons, New York, 1999)]

[3] N. Pan et al. [IEEE Electron Device Lett. (USA) vol.19 (1998) p.115]

[4] M.J. Cich, J.A. Johnson, G.M. Peake, O.B. Spahn [Appl. Phys. Lett. (USA)

vol.82 (2003) p.651]

[5] E. Neubauer, M. Mai, J. Wurﬂ, W. Osterle [Semicond. Sci. Technol. (UK)

vol.15 (2000) p.818]

[6] F. Ren, S.J. Pearton, W.S. Hobson, T.R. Fullowan, J. Lothian, A.W. Yanof

[Appl. Phys. Lett. (USA) vol.56 (1990) p.860]

[7] S.J. Pearton [Int. J. Mod. Phys. B (Singapore) vol.30 (1994) p.1247]

[8] A.G. Baca, C. Monier, P.C. Chang, R.D. Briggs, M.G. Armendariz,

S.J. Pearton [Solid-State Electron. (UK) vol.46 (2002) p.797]

[9] T. Nittono, K. Nagata, O. Nakajima, T. Ishibashi [IEEE Electron Device

Lett. (USA) vol.10 (1989) p.506]

[10] H. Shimawaki, Y. Amamiya, N. Furuhata, K. Honjo [IEEE Trans. Electron

Devices (USA) vol.42 (1995) p.1735]

[11] T. Oka, K. Hirata, K. Ouchi, H. Uchiyama, K. Mochizuki, T. Nakamura

[IEEE Trans. Electron Devices (USA) vol.45 (1998) p.2276]

[12] Y.-F. Yang, C.-C. Hsu, E.S. Yang, H.-J. Ou [IEEE Electron Device Lett.

(USA) vol.17 (1996) p.531]

[13] M.-C. Ho, R.A. Johnson, W.J. Ho, M.F. Chang, P.M. Asbeck [IEEE

Electron Device Lett. (USA) vol.16 (1995) p.512]

[14] W.L. Chen, H.F. Chau, M. Tutt, M.C. Ho, T.S. Kim, T. Henderson [IEEE

Electron Device Lett. (USA) vol.18 (1997) p.355]

[15] F. Ren, C.R. Abernathy, S.N.G. Chu, J.R. Lothian, S.J. Pearton [Solid-

State Electron. (UK) vol.38 (1995) p.1137]

[16] J.Y. Chi, K. Lu [IEEE Electron Device Lett. (USA) vol.19 (1998) p.408]

[17] T. Henderson [Microelectron. Reliab. (UK) vol.39 (1999) p.1033–42

[18] O. Ueda [J. Electrochem. Soc. (USA) vol.135 (1988) p.11C]

[19] D.C. Streit et al. [J. Vac. Sci. Technol. B (USA) vol.14 (1996) p.2216]

[20] O. Ueda [Microelectron. Reliab. (UK) vol.39 (1999) p.1839]

[21] H. Suguhara, J. Nakano, T. Nittono, K. Ogawa [GaAs IC Symp. Technol.

Digest (IEEE, Piscataway, NJ, USA, 1993) p.115]

304

Chapter 10

Wet oxidation for optoelectronic and MIS

GaAs devices

Chapter scope p.305

Mechanism of wet oxidation

processes p.305

Chemistry of wet and dry oxidation

of AlGaAs p.306

Electronic consequences of

oxidation processes p.307

Rates and proﬁle evolution p.308

Al-mole-fraction effects p.310

Layer thickness effects p.313

Proximity enhancement effect p.314

Wet oxidation of other materials p.315

Miscellaneous observations p.317

Practical wet oxidation p.319

Applications in optoelectronic

devices p.321

Structural issues for oxide

VCSELs p.321

Defect-related issues for

optoelectronic devices p.324

Applications in electronic GaAs

devices p.325

Problems with wet and dry

oxidation for MIS devices p.325

GaAs-on-insulator applications p.326

Conclusion p.326

References p.327

10.1 CHAPTER SCOPE

Most of the post-growth fabrication issues of importance in making

electronic devices are equally applicable in making optoelectronic

devices. Many of the performance characteristics are grown in

by the speciﬁc design of mirror stacks, quantum-well numbers

and thicknesses, dopant proﬁle distributions, etc. Post-growth pro-

cessing, with the exception of wet oxidation, is generally the same

as that of electronic devices. The previous chapters in this book

dealing with cleaning, passivation, etching and contact forma-

tion address the main requirements for fabricating optoelectronic

devices. The details of the technique of regrowth, which involves

etching a structure into a wafer and introducing it again into a

growth chamber for deposition of additional semiconductor layers

on the textured substrate, are beyond the scope of this book. To fab-

ricate top-surface gratings for a DFB (distributed feedback) laser,

proper mask alignment relative to the crystal axes and crystallo-

graphic etching can be used to form the grating, as discussed in

Section 4.3.2. Crystallographic etching can also be used to form

smooth, vertical mirrors with proper alignment of the mask relative

to a wafer’s crystal planes.

Because the principal additional fabrication technique for opto-

electronic devices is the wet oxidation of buried Al-containing

layers to make current apertures in VCSELs (vertical cavity sur-

face emitting lasers), this chapter will focus on that process. Both

fundamental issues and practical considerations for using a wet

oxidation process are presented here. The chapter concludes with

a discussion of attempts to make electronic devices, such as GaAs-

on-insulator (GOI) MESFETs and metal-insulator-semiconductor

(MIS) devices, using wet oxidation.

10.2 MECHANISM OF WET OXIDATION PROCESSES

The oxidation of Si has been the topic of exhaustive research

because of its importance in forming the Si/SiO

interface, the

305

Wet oxidation for optoelectronic and MIS GaAs devices

basis of CMOS technology. By comparison, the oxidation of GaAs

and AlAs is not as well understood. This section addresses some of

the fundamental chemical and physical properties that profoundly

affect the application of wet oxidation technology in GaAs devices.

10.2.1 Chemistry of wet and dry oxidation of AlGaAs

Dry oxidation of AlAs (and similarly of GaAs) can be described

relatively simply by

2AlAs + 3O

= Al

+ As

(10.1)

which presumably started with Al-O and As-O surface species.

In contrast, wet oxidation of AlAs or AlGaAs is much more

complicated.

Water can dissociatively adsorb on AlAs to give Al-O, Al-OH

and As-H. Some of the intermediate species in the wet oxidation

process and their evolution with oxidation time have been identi-

ﬁed using Raman spectroscopy [1] and the following suite of reac-

tions [2] are consistent with the observedproducts of As and As

and the surface species formed by water adsorption. (Molecular

analogues, such as AsH

and AlO(OH), are used here to represent

surface species, such as As-H and Al-OH, so the thermodynamic

calculations are approximations to the actual situation.)

G

(kJ/mol

at 425

◦

2AlAs + 3H

(g)

= Al

+ 2AsH

−451 (10.2)

2AlAs + 4H

(g)

= 2AlO(OH) + 2AsH

−404 (10.3)

2AsH

= 2As + 3H

−153 (10.4)

2AsH

+ 3H

O = As

3(l)

+ 6H

−22 (10.5)

3(l)

+ 3H

= 2As + 3H

(g)

−131 (10.6)

2AlAs + 3H

(g)

= Al

+ 2As + 3H

−604 (10.7)

2AlAs + 4H

(g)

= 2AlO(OH) + 2As + 3H

−557 (10.8)

The Gibbs free energy for these reactions depends on the temper-

ature; at 425

◦

C, all have a negative change in free energy, G,

which means that the reaction should proceed spontaneously if

not kinetically constrained. A general caveat should always be

remembered when discussing reactions in terms of their thermody-

namics. While thermodynamic calculations predict what reactions

will have negative changes in Gibbs free energy (G) and should,

306

Wet oxidation for optoelectronic and MIS GaAs devices

therefore, proceed spontaneously, the kinetics of the reactions

often determine what actually happens. If there is a high activation

energy (large E

∗

= large kinetic barrier) for a reaction, it may

proceed only very slowly, if at all, despite having a large −G.

The H

from water serves as the oxidising agent for the

low-oxidation-state Al in AlAs. As

is observed by Raman

spectroscopy as an intermediate in the reaction, but the elemental

H resulting from the reduction of the H

of water in the forma-

tion of Al

or AlO(OH) can react readily with As

to form

As. An analogous set of equations can be formulated for GaAs,

with AlGaAs being a composite of the two. Changes in the rel-

ative rates of formation and loss of these species can explain

most of the observations regarding composition dependence, time

dependence, layer shrinkage upon oxidation, changes in elec-

tronic properties in adjacent layers and the inability to form a

good GaAs/oxide interface for MIS devices. This will be discussed

further in Section 10.3.

The formation of elemental As is the most fundamental differ-

ence between wet and dry oxidation. It can readily volatilise as

elemental As or react with H to form AsH

, a gas under these

reaction conditions. Signiﬁcant quantities of AsH

are not detec-

ted in standard wet oxidation processes, so formation of volatile

elemental As is apparently strongly favoured. Elemental analysis

of completely oxidised AlAs or AlGaAs shows that very little As

remains after oxidation. This loss of As favours formation of a

relatively porous oxide material that facilitates transport of react-

ants and products. This porosity enables a linear dependence on

time as the oxidation proceeds over relatively long distances, as

will be discussed in more detail in Section 10.3.

Wet oxidation of AlGaAs can be suppressed by addition of

even small amounts of oxygen to the gas. This may be due to

the preferential consumption of the reduced H-species by oxygen,

preventing formation of signiﬁcant amounts of As. In the presence

of oxygen, the process proceeds like a dry oxidation even when

plenty of water vapour is present.

+Ga

GaAs + V

FIGURE 10.1 Defects generated

by free As atoms.

10.2.2 Electronic consequences of oxidation processes

While the generation of elemental As is highly desirable to permit

rapid, deep oxidation of an AlGaAs layer, the presence of free As

atoms is problematic for the electronic properties and crystallo-

graphic perfection of the semiconductor adjacent to an oxidised

layer. Some of the possible reactions with adjacent GaAs are illus-

trated in FIGURE 10.1. Arsenic atoms can react with the surface

Ga atoms, causing them to move forward onto a new lattice site

and leaving a Ga vacancy (V

) behind. Column III vacancies

307

Wet oxidation for optoelectronic and MIS GaAs devices

play an important role in enhancing interdiffusion, which can

degrade heterojunction interfaces. An As atom can diffuse into

the semiconductor as an interstitial As (As

). The As

can move

deeper into the semiconductor quite rapidly and undergo reaction

quite distant from the initial point of generation. The interstitial

can form anti-site defects (As

) by binding at vacant Ga sites or

by displacing a Ga from its site, forming a Ga interstitial (Ga

These defects quench luminescence. Arsenic precipitates result-

ing from the diffusion of interstitials have been observed at remote

interfaces (tens of nm away from the oxide) and even at the GaAs

surface of a wet-oxidised sample.

Problems due to defect-caused luminescence quenching and

intermixing at heterointerfaces in optoelectronic devices will be

discussed in Section 10.5.2. The importance of elemental As at

the oxide/semiconductor interface is especially problematic when

trying to achieve a very low interface-state density, as will be

discussed in Section 10.6.

10.3 RATES AND PROFILE EVOLUTION

The wet oxidation of AlGaAs can be understood in terms of the

classic model of Deal and Grove. In their model, the time depend-

ence of oxidation can be described as the sum of two terms:

diff

+ d/k

rxn

= t where d is the oxidation depth and t is the

time. The linear term dominates when the oxidation is reaction-rate

limited and the parabolic term is dominant under diffusion-limited

conditions. In dry oxidation of AlGaAs, the parabolic, diffusion-

limited term is dominant and rates slow rapidly as the oxidation

front penetrates into an AlGaAs layer. However, it is most desir-

able to operate in the linear regime when trying to oxidise long

distances (tens and even hundreds of nm) very controllably and

in a relatively short time. Wet oxidation conditions can be found

that produce primarily linear, reaction-rate limited behaviour for

high Al-content AlGaAs. The speciﬁc conditions favouring linear

oxidation depend on both wafer temperature and AlGaAs mole

fraction and factors that favour linear rates are discussed below.

GaAs

AlAs

a-Al

50 nm

FIGURE 10.2 Dense amorphous

interfacial layer at oxidation front

(TEM by R.D. Twesten).

The time dependence of the oxidation rate will be determined

by the relationship between the rate of formation of As

and its

conversion to As (EQNS 10.5 and 10.6 in Section 10.2.1) and the

subsequent rate of loss of As from the oxidised layer to leave

behind a relatively porous matrix of AlO

species, such as

and AlO(OH). TEM studies have revealed the presence of

a thin (a few nm), dense amorphous region at the oxidation front

of a layer oxidised at 440

◦

C (FIGURE 10.2). Behind this dense

region lies a less dense region of amorphous AlO

that extends

308

Wet oxidation for optoelectronic and MIS GaAs devices

AlO(OH)

AlGaAs

AlO(OH)

AlGaAs

AlO(OH)

AlGaAs

(a)

(b)

FIGURE 10.3 Evolution of oxide during wet oxidation of a layer: (a) linear:

ﬁxed thickness of interfacial layer, (b) parabolic: increasing thickness of

interfacial layer [2].

back to the exposed mesa edge. The evolution of the thickness of

this dense interfacial zone as the oxidation penetrates deeper into

the layer will determine whether linear, parabolic or intermediate

time dependence is obtained [2], as illustrated in FIGURE 10.3.

There will be a dense layer at the oxidation front com-

posed mainly of AlO

and As

. When the reduction of

to As is sufﬁciently fast to balance the rate of forma-

tion of As

, this interfacial zone does not grow appreciably

in thickness as the oxidation front moves deeper into the layer

(FIGURE 10.3(a)).While this balance is maintained, a linear time

dependence will appear because the diffusional contribution to

the reaction rate does not increase signiﬁcantly over the time

required to oxidise the device. If reaction conditions are changed

to preferentially increase the formation of As

relative to As

formation and loss, a steady increase in the thickness of the inter-

facial zone will occur (FIGURE 10.3(b)) and the diffusion-limited

parabolic time dependence will become dominant. Raman spectro-

scopy has shown a greater amount of As

to be present during

wet oxidation at higher temperatures, consistent with the trans-

ition from linear to parabolic behaviour as oxidation temperature

increases.

For extended reaction times at a given temperature, any

deviation from linearity will become increasingly obvious. For

conditions where the thickness is increasing only slowly, the

initial stages of the oxidation will closely approximate a linear

time dependence. This is usually observed with Al

0.98

0.02

under typical processing conditions, e.g. 30 min at 440

◦

for Al

0.98

0.02

As. Conditions producing faster oxidation tend

to tilt the balance in the direction of parabolic dependence.

309

Wet oxidation for optoelectronic and MIS GaAs devices

For oxidation rates of AlGaAs below 0.2 μm/min, the process

appears reaction-rate limited (linear) while initial rates above

1.3 μm/min appear diffusion limited at longer times. Thermody-

namic calculations of the temperature dependence of the important

As-based reactions predict that the formation of As

will

become increasingly favourable versus the reduction to As as tem-

perature increases. This is consistent with the generally observed

shift from linear towards parabolic behaviour with increasing

temperature for both AlAs (FIGURE 10.5) and Al

0.98

0.02

(FIGURE 10.6), as discussed in Section 10.3.1.

It is important to remember that wet oxidation of GaAs also

occurs although at a much lower rate. While the oxidation front is

advancing rapidly into the horizontal AlGaAs layer, the oxidised

layer is also slowly growing vertically into the bounding GaAs or

AlGaAs layers with lower mole fractions. Oxidation will occur

along the entire length of the oxidised channel, and the vertical

oxidation front will have progressed farther near the mesa edge,

where it has been exposed longer to the oxidising atmosphere.

If bounding layers are very thin, it is possible to oxidise completely

through them during long oxidation times.

Several factors strongly affect the rate of wet oxidation: Al mole

fraction, layer thickness and digital versus continuous-grade alloy-

ing. These plus the oxidation of materials other than AlGaAs will

be discussed in more detail.

10.3.1 Al-mole-fraction effects

The addition of even small amounts of Ga to AlAs has a profound

effect on the wet oxidation rate [3], as illustrated in FIGURE 10.4.

Al mole fraction

0.84 0.88 0.92 0.96 1.00

0.0

0.2

0.4

0.6

0.8

1.0

normalised oxidation rate

425°C

FIGURE 10.4 Wet oxidation rate dependence on Al mole fraction.

310

Wet oxidation for optoelectronic and MIS GaAs devices

While Al

0.98

0.02

As has only a 2% differenceinAl content versus

AlAs, a 435% difference in the wet oxidation rate is observed. For-

tunately, it is commonplace to have sufﬁcient control of Al mole

fraction during growth that deviation in oxidation rate due to com-

positional non-uniformity is generally not a problem. The 98%

composition is commonly used because it oxidises rapidly enough

to be convenient for making oxide apertures in VCSELs while

forming an oxide having greater mechanical robustness than that

obtained with pure AlAs. Another practical advantage is the reten-

tion of nearly linear time dependence of oxidation depth (distance

oxidised inward from the exposed mesa edge) for the depths and

processing conditions commonly employed, as will be discussed

below.

The wet oxidation of AlAs clearly displays the transition from

reaction-rate limited (linear versus time) to diffusion-limited rates

(parabolic) as the temperature is increased from 356 to 518

◦

C, as

illustrated in FIGURE 10.5 [4]. The dotted lines indicate the expec-

ted behaviour for a completely linear (d = kt) and completely

parabolic (d

= kt) oxidation. Since relatively minor changes in

temperature can have a pronounced effect on the time dependence

of the oxidised depth, AlAs is less attractive for device applica-

tions where precise control of oxidised depths is essential. Useful

0.1 1 10 100 1000 10000

0.01

0.1

100

518°C

468°C

418°C

356°C

380°C

t/(A

/4B) [1/mm]

/ (A/2) [mm/mm]

=kt

d=kt

AlAs

FIGURE 10.5 Temperature dependence of wet oxidation of AlAs from

Ochiai et al. [4].

311