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

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Wet oxidation for optoelectronic and MIS GaAs devices

oxidation rates are achievable with 98% Al compositions while

remaining within a virtually linear growth region.

The transition from reaction-limited (linear) to diffusion-limited

(parabolic) behaviour is often difﬁcult to detect for relatively short

oxidation depths. A comparison of the oxidation behaviour of 94%

Al and 98% Al layers at 400 and 440

◦

C with 80

◦

C water and

an N

ﬂow rate of 3.0 slm in a 4



diameter three-zone furnace

(FIGURE 10.6) reveals that the slower-oxidising material remains

in the linear regime at both temperatures, while a shift from linear

towards parabolic is seen for the faster-reaction 98% Al mater-

ial at the higher temperature [2]. However, for shorter reaction

times (shallower oxidation depths), the parabolic dependence is

well approximated by a straight line and the assumption of linear

time dependence would be adequate for selecting oxidation times

for depths less than 15 μm. The speciﬁc times and temperatures

at which the linear-to-parabolic transition will be observed will

depend on the water vapour supply, which is controlled by the

bubbler temperature and gas ﬂow rate, and can vary depending on

speciﬁc operating conditions.

From these studies, it is clear that faster oxidation rates, whether

due to higher Al mole fraction or higher oxidation temperatures,

will shift the balance from linear towards parabolic time depend-

ences. It appears in general that for oxidation rates of AlGaAs

below 0.2 μm/min, the process appears reaction-rate limited while

initial rates above 1.3 μm/min appear diffusion limited at longer

times. The transition region lies between these rates. The slower

reaction rate obtained with higher Ga mole fractions enables the

loss of As, forming the porous oxide, to keep pace with the advan-

cing dense oxidation front. This allows such materials to remain

oxidised depth (mm)

time (min)

0 10203040506070

400°C

98%

94%

0 5 10 15 20 25 30

440°C

time (min)

98%

94%

(a) (b)

FIGURE 10.6 Change between linear and parabolic oxidation versus

composition and temperature from Ashby et al. [2].

312

Wet oxidation for optoelectronic and MIS GaAs devices

in the linear regime at higher temperatures and for longer times

than their higher-Al analogues.

It is clear in FIGURE 10.6 that some time may pass between

insertion into the furnace at t = 0 and the onset of oxidation.

In these cases, t = 0 was deﬁned as the time at which the wafer-

loaded boat was returned to the central zone of a three-zone tube

furnace through which water vapour was ﬂowing entrained in N

The lag in the apparent onset of oxidation may be attributed to

the much slower rates while the wafer and boat were reheating

to the furnace reaction temperature. Thermal equilibration times

will vary between different reactors and may be signiﬁcant or

negligible. This will be discussed further in Section 10.4.

10.3.2 Layer thickness effects

Above a certain layer thickness (about 100 nm), the oxidation rate

for a particular AlGaAs composition is not appreciably inﬂuenced

by the layer thickness. However, for thicknesses below 80 nm, a

very strong dependence on layer thickness appears, as illustrated in

FIGURES 10.7 and 10.8. This effect is probably related to the ease

of diffusive transport of either reactants or oxidation products from

the advancing oxidation front. A similar retardation of etching

reactions is seen when trying to selectively etch a thin layer, as in

selective channel recessing, discussed in Section 4.5.

The rate of diffusion-controlled reactions can be limited by the

diffusion of either reactants or products. A study of the time

FIGURE 10.7 TEM of oxidation front region showing layer thickness effect.

313

Wet oxidation for optoelectronic and MIS GaAs devices

AlGaAs layer thickness (nm)

0 50 100 150 200

oxidation rate (mm/min)

0.0

0.1

0.2

0.3

0.4

0.5

425ºC

0.94

0.04

0.98

0.02

FIGURE 10.8 Effect of layer thickness on oxidation rate.

dependence as a function of layer thickness could distinguish

between the two in this case, since a transition from linear to

parabolic behaviour would signal the formation of a progressively

thicker interfacial front if the formation and removal of products

failed to keep pace in the thinner channels.

10.3.3 Proximity enhancement effect

The close proximity of a rapidly oxidising layer can accelerate the

oxidation of a lower Al-content layer relative to that obtained for an

isolated layer. A possible cause of the enhancement is the injection

of defects from adjacent oxidised regions. The enhancement can

be modelled as the combination of the regular lateral oxidation of

an isolated layer and a contribution from a species diffusing from

nearby oxidised zones [5]. The activation energy for this species

is consistent with the diffusion of column III vacancies, which are

known to enhance oxidation and whose diffusion is enhanced by

strain, which is present in the layers adjacent to the oxidised layer.

For example, lattice expansion is observed in thin (55 nm) GaAs

layers in oxidised superlattices so adjacent layers are strained.

From a practical perspective, this enhancement must be con-

sidered when proﬁle control is important. Compositional steps

are used to fabricate tapered-oxide current apertures for optimum

VCSEL performance (Section 10.5) so the observation of proﬁles

that show appreciably deeper oxidation than expected from com-

positional effects alone is important. The enhancement resulting

from adjacent regions with faster oxidation rates actually simpliﬁes

314

Wet oxidation for optoelectronic and MIS GaAs devices

(c)

(a)

(b)

FIGURE 10.9 Enhancement effects from adjacent oxidised layers:

(a) continuous compositional grade from 98 to 90% Al in AlGaAs, (b) step

grade of 0.002% Al from 98 to 90% Al with 5 nm GaAs barriers between

AlGaAs layers and (c) schematic of oxidised step-graded structure.

the grown-in structure for a sharp taper. A sharp, smooth taper can

be obtained with only three layers and only two compositions, as

shown in FIGURE 10.14.

Enhancement is most pronounced when a continuous gradation

of Al percentage is employed in the structure (FIGURE 10.9(a)).

Where the compositional grading was expected to produce a

convex oxidation front replicating the Al-composition proﬁle (as in

FIGURE 10.9(b)), a concave oxidation proﬁle is obtained due to

enhancement from adjacent higher-Al-content regions.

The enhancement can be mitigated by insertion of thin (5 nm)

layers of GaAs between layers of a step-graded structure that

covers the same composition range (FIGURES 10.9(b) and

10.9(c)). The GaAs layers servetoimpedetheoxidation-enhancing

diffusion from the oxidised layers and the ﬁnal oxidation proﬁle

is closer to that expected based on composition alone.

10.3.4 Wet oxidation of other materials

While most wet oxidation work has focused on AlGaAs materials,

other III–V semiconductors with high Al content also have the

potential to undergo rapid wet oxidation.

315

Wet oxidation for optoelectronic and MIS GaAs devices

As mole fraction

0.0 0.2 0.4 0.6 0.8 1.0

100

120

oxidised depth (mm)

25 min at 380°C

FIGURE 10.10 Wet oxidation of AlAsSb as a function of As mole fraction.

10.3.4.1 Sb-containing materials

Another III–V material family that might be amenable to wet

oxidation is the AlAsSb family of compounds. Among the

thermodynamically favoured products are As, which can mostly

escape as in the AlGaAs case, and Sb, which will remain behind.

Oxidation of these materials proceeds at temperatures lower than

those required for AlGaAs (325–380

◦

C.)

In the absence of As, wet oxidation produces a non-porous ﬁlm

and the oxidation proceeds at a slow, diffusion-controlled rate. The

introduction of a small amount of As is sufﬁcient to accelerate the

reaction by increasing the porosity (FIGURE 10.10). The fall-off

towards higher As mole fractions reﬂects the slower oxidation rates

for AlAs at these relatively low temperatures.

The oxidation of AlAsSb lattice matched to InP has been stud-

ied [6]. The structure shown in FIGURE 10.11 consisted of a

109 nm cap of In

0.53

0.47

As, 288 nm layer of AlAs

0.56

0.44

109 nm layer of In

0.53

0.47

As and an InP substrate. While surface

oxidation of AlSb, GaSb and InSb produces a layer of Sb which

moves ahead of the advancing oxidation front, in lateral oxidation

this Sb layer segregates to one of the oxide/semiconductor lateral

interfaces, typically the top one as shown in FIGURE 10.11, where

deformation can accommodate the volumetric changes caused by

oxidation. There is signiﬁcant deformation of the surface layer

as the front advances, as can be seen in FIGURE 10.11(a), and

a dip where the fronts meet if the layer is completely oxidised

(FIGURE 10.11(b)).

316

Wet oxidation for optoelectronic and MIS GaAs devices

InGaAs

InSbO

InGaAs

InSbO

(a)

(b)

100 nm

FIGURE 10.11 Sb segregation at interfaces in InAsSb wet oxidation from

Blum et al. [6].

10.3.4.2 P-based materials

Wet oxidation of P-based materials has also been the subject

of study. These materials differ appreciably in their principal

chemistry from As-based ones. While post-oxidation materials

analysis shows that most of the As is lost from the oxide, most

of the P is retained. Thermodynamic calculations suggest that the

principal products of this oxidation are very different for AlAs

and AlP. While the formation of As, Al

and AlO(OH) are most

favoured for AlAs, the change in Gibbs free energy is twice as great

for the formation of AlPO

than for P and Al

or AlO(OH).

Rates for phosphide materials are slower, and temperatures in

excess of 500

◦

C are employed.

The oxidation of In

0.5

P has been compared with that of

0.5

As. While InAlP oxidises more slowly than AlGaAs, it

has a lower activation energy. With oxidation at 500

◦

C, In is seen

to build up at the oxide/semiconductor interface. At 550

◦

C, the In

is distributed uniformly throughout the ﬁlm and a parabolic time

dependence prevails. At 650

◦

C, In is depleted at the interface [7].

An interesting and potentially useful property of GaP is the

ability of a very thin layer (two monolayers) to serve as a diffusion

barrier to the movement of As away from the oxidised layer and

into adjacent semiconductor layers. The importance of this will be

discussed in Section 10.6.

10.3.5 Miscellaneous observations

The observed change in layer volume upon oxidation of AlAs and

AlGaAs is not as expected if calculated based upon theoretical

317

Wet oxidation for optoelectronic and MIS GaAs devices

molar volumes of the starting semiconductor and a dense, crys-

talline oxide. AlAs shrinks by about 13% while Al

0.98

0.02

shrinks only 6.7% upon oxidation. Analysis of oxidised ﬁlms

has revealed the presence of appreciable quantities of hydrogen,

suggesting the presence of a considerable amount of hydrous

species such as AlO(OH) in the oxidised layer. This is consist-

ent with thermodynamic predictions. Since the relative amounts of

AlO(OH) and Al

are not well deﬁned and may change depend-

ing on the speciﬁc process conditions, a priori prediction of ﬁlm

volume is not strictly possible at this time. Similar behaviour is

expected for AlGaAs materials.

A serious mechanical problem can exist when oxidation is taken

to completion, i.e. when the oxidation fronts from opposing mesa

walls coalesce. The oxide/semiconductor interface is weak, and

the upper layers of the structure will delaminate from the oxide

unless a portion of unoxidised structure is present to hold the device

together. This is naturally the case for oxide-aperture VCSELs, but

use of wet oxidation for GOI (GaAs-on-insulator) devices will face

serious problems unless provision is made for holding the device

together. This problem is most pronounced for AlAs and is less

problematic with AlGaAs alloys.

Changes in composition mean changes in dimension, producing

stresses, especially in overlying layers. These can deform if they

are not too physically constrained by a thick semiconductor over-

layer. As seen in FIGURE 10.2, the oxidation-front proﬁle tends to

be slightly deeper at the top interface than at the bottom interface.

This may be related to stress, although the origin of the effect is

not deﬁnitively established and the effect is rather small.

It is generally not possible to oxidise a certain distance into

a layer, pull the wafer from the furnace and then reinsert the

wafer to oxidise further for a controlled ﬁnal depth. Wet oxida-

tion under these conditions is not a simple time-additive process.

There appears to be a change in the chemical or structural nature of

the oxide-semiconductor reaction front that discourages restarting

the wet oxidation process, perhaps by inhibiting the free access

of water to AlAs or AlGaAs for the generation of the H that is

essential for the rapid removal of As from the oxidising layer.

When inserting a wafer into a furnace with wet gas ﬂow, there

may be a delay in the onset of rapid wet oxidation (FIGURE 10.6).

This is more of a problem for depth control of shallow oxidations,

since the apparent incubation time can be a substantial fraction

of the total time in the furnace. For such processes, it may be

especially important to load into dry gas, thermally equilibrate

to the ﬁnal reaction temperature and then introduce the wet gas

ﬂow. Since the heat capacities of dry nitrogen and water-saturated

nitrogen are different, it may be necessary to usedifferent ﬂowrates

318

Wet oxidation for optoelectronic and MIS GaAs devices

of dry and wet nitrogen to maintain a constant furnace temperature

when switching gases. This will require calibration for a speciﬁc

reactor.

10.4 PRACTICAL WET OXIDATION

The apparatus for performing wet oxidation can be very simple,

consisting of a water bubbler, an inert gas source and a tube fur-

nace. It is best to use a quartz tube to avoid introducing contaminant

species, such as Na, that can be a problem with ordinary glass

at the elevated temperatures required for oxidation (350–525

◦

C).

A general system schematic is shown in FIGURE 10.12. For

0.98

0.02

As layers thicker than 90 nm, oxidation at 400–440

◦

using 80–90

◦

C water and an N

ﬂow rate of 3 slm in a 4



diameter

three-zone tube furnace works well. It is best to work at a temper-

ature at least two degrees below the boiling point of water, which

will depend on the elevation of the laboratory above sea level.

dry N

water

bubbler

boat

3-zone tube furnace

FIGURE 10.12 Diagram of wet

oxidation apparatus.

Water vapour is entrained in inert gas (N

or Ar) that is bubbled

through liquid water. The water temperature will determine the

equilibrium vapour pressure of water in the ﬂowing gas and there-

fore controls the water concentration experienced at the wafer.

Both nitrogen and Ar work well as carrier gases as long as they

are oxygen-free. Even rather small amounts of oxygen in the gas

will stop the wet oxidation process (Section 10.2.1). The bubbler

should be of sufﬁcient size that the water temperature within it

does not change more than a fraction of a degree during the reac-

tion. Heating the tubing between the bubbler and the inlet to the

furnace avoids condensation in transit to the reaction zone.

These reactions are highly temperature dependent, so it is vitally

important to maintain a uniform temperature in the reaction zone

and to avoid temperature-lag effects as much as possible while

loading wafers. It is helpful to use a three-zone furnace for thermal

stability in the oxidation region. Generally, the wafer will be

situated in the central zone. This allows the ﬂowing gas to pass

through a hot zone before reaching the wafer. For AlGaAs, oxid-

ation temperatures are generally between 400 and 475

◦

C. For

P-containing materials, higher temperatures are used (>500

◦

C),

while Sb-containing materials are oxidised at lower temperatures

(325–380

◦

C).

Since reaction initiation can be sensitive to the surface condition,

it is most advantageousto perform the oxidation as soon as possible

after exposing the layer edge with a mesa etch. Minimal formation

of native oxide or its removal (Section 3.2.2) just prior to oxidation

is generally desirable if a sample has been etched a long time

before oxidising. This is especially important for Sb-containing

319

Wet oxidation for optoelectronic and MIS GaAs devices

materials, which can form a number of surface oxides that vary in

their reactive chemistry.

While reactant-gas access to the AlGaAs layers is easily

providedby mesa etching, an alternative approach involves etching

trenches into the wafer in the regions where one wishes oxidation

to occur. This has the advantage of providing an essentially

planar surface for subsequent metallisations and may prove highly

advantageous for reliable fabrication of arrays of devices, such as

VCSELs.

Samples are inserted into the reaction zone by sliding in a quartz

“boat” holding the wafers. Usually the boat is pulled quickly from

the furnace, loaded quickly atthe mouth of the furnace and returned

to the central zone of the furnace as rapidly as possible. Temper-

ature equilibration is fairly rapid but not instantaneous; even with

rapid handling it can take a few minutes to achieve thermal sta-

bility. This can manifest itself as a non-zero intercept in depth

versus time plots (FIGURE 10.6). If an initial delay is observed

under a speciﬁc set of reaction conditions, it may be desirable to

employ a bypass ﬂow design for the most uniform and repeatable

process. Dry nitrogen or Ar is ﬂowed through the tube furnace

until the boat and wafers thermally equilibrate. As discussed in

Section 10.3.5, the best dry-gas ﬂow rate to use during equilibra-

tion may differ from the ﬂow rate of the water-saturated gas. When

the desired reaction temperature is attained, the water-saturated gas

is switched to pass through the reaction zone. Likewise, switching

back to dry gas before unloading will remove reaction time uncer-

tainties. This is less of a problem during unloading than during

loading since cooling after unloading is rapid compared to the ini-

tial time needed to thermally equilibrate a boat that has been pulled

brieﬂy from the furnace for loading.

Flow rates and tube dimensions typically used in wet oxida-

tion result in the development of a parabolic ﬂow-velocity proﬁle

across the diameter of the tube. The virtually zero ﬂow rate at

the tube wall and the maximum ﬂow at the tube centre produce

a non-uniform reaction rate across a wafer due to variation in the

effective water concentration. For a particular water temperature,

wafer temperature and tube radius, the rate increases as the sample

becomes more distant from the ﬁrst wall, then plateaus across the

distance where a certain ﬂow rate is exceeded, and drops again

as the ﬂow rate drops while approaching the second wall. This

can give uniform oxidation across the central portion of the boat,

but samples outside the plateau zone will oxidise irregularly. For

example, a wafer of mesa-etched VCSELs that is 1.5



wide can

be uniformly oxidised in a 4



tube furnace.

This problem can be solved by disrupting or “spoiling” the para-

bolic ﬂow proﬁle by inserting a bafﬂe system just upstream from

320

Wet oxidation for optoelectronic and MIS GaAs devices

the position of the boat. This bafﬂe can be a simple honeycomb

structure mounted on the upstream end of the boat. The disruption

of the ﬂow proﬁle produces a uniform ﬂow rate across the entire

width and length of the boat where the samples are located. This is

especially important for trench oxidation; mesa structures produce

some spoiling of the ﬂow without an additional bafﬂe.

10.5 APPLICATIONS IN OPTOELECTRONIC DEVICES

Wet oxidation is a true enabling technology for high efﬁciency

VCSELs, so this will be the primary focus of this section. There

are both structural considerations and electronic consequences due

to defect generation that should be considered by the VCSEL

designer. These are discussed below.

10.5.1 Structural issues for oxide VCSELs

Wet oxidation of AlGaAs can be extremely useful for locally alter-

ing the refractive index of a heterostructure since the oxide has a

much lower refractive index of n = 1.6. The insulating character of

the oxide also makes current channelling within a device possible.

High-efﬁciency vertical-cavity surface-emitting lasers (VCSELs)

with wall-plug efﬁciencies in excess of 50% have been made pos-

sible by wet oxidation processing. They employ oxide apertures

made by incorporating a higher Al-content layer next to the optical

cavity than those used in the distributed Bragg reﬂector (DBR)

mirror stacks, as illustrated in FIGURE 10.13. Note that while

GaAs

AlGaAs

oxide

layers

mesa

optical cavityoxide extent

FIGURE 10.13 Oxide aperture VCSEL.

321