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

Подождите немного. Документ загружается.

Wet oxidation for optoelectronic and MIS GaAs devices

92% Al

98% Al

100 nm

tapered oxide

FIGURE 10.14 Tapered oxide aperture from Choquette et al. [8].

the aperture oxide is grown deep into the structure, there is also

a lesser depth of oxidation in the lower Al-content layers of the

DBR. While general VCSEL design issues will not be discussed

here, we will discuss some factorsthat are speciﬁcally related to the

as-grown structure that is to be oxidised and the effect of oxidation

on some key optical properties.

While it is possible to form an aperture by oxidising an AlGaAs

layer of uniform composition, VCSEL performance is improved

when the aperture oxide is tapered rather than blunt at its terminus.

FIGURE 10.14 [8] shows a tapered aperture that takes advantage

of the proximity enhancement effect (Section 10.3.3) to achieve

a relatively sharp tip proﬁle with only three AlGaAs layers and

only two different Al mole fractions.

The location of the point of the taper within the AlGaAs

layer has a strong inﬂuence on VCSEL properties, as does the

diameter of the aperture [9]. For a given aperture diameter, mov-

ing the position of the oxide aperture within a λ/4 layer in the

DBR shifts the resonance wavelength; moving from ﬁeld min-

imum (node) to maximum (antinode) decreases the resonance

wavelength. Moving the oxide towards the node also results in loss

of conﬁnement and a rise in the threshold material gain. The loss

of conﬁnement is more severe for the higher modes, so the modal

stability of the fundamental mode is enhanced with the oxide near

the node.

With the oxide layer ﬁxed at the antinode position, decreas-

ing the aperture size between 8 and 1 μm decreases the lasing

wavelength. The wavelength difference between the fundamental

and ﬁrst-order modes depends on aperture geometry (diameter,

thickness, position) and increases as the aperture size decreases.

Reducing the aperture also results in loss of conﬁnement and higher

thresholds. Reducing the aperture size below about 9 μm also

reduces the carrier lifetime, as measured by turn-on delay.

322

Wet oxidation for optoelectronic and MIS GaAs devices

GaAs

AlGaAs

oxide

aperture

p-DBR

n-DBR

optical cavity

implant

FIGURE 10.15 Double aperture VCSEL: oxide and implanted apertures.

Improved beam quality can be attained with a double-aperture

VCSEL, schematically shown in FIGURE 10.15. The combination

of both implanted and oxide aperture provides better mode

control than either aperture alone and improves single-mode output

[10]. In such hybrid structures, the implanted aperture is situated

above the oxide aperture. Latticedisorderproducedbyion implant-

ation produces current conﬁnement into the unimplanted area.

When the implanted aperture is slightly smaller than the oxide

aperture, the conﬁned current ﬂow is directed into the centre of

the optical cavity deﬁned by the oxide aperture. This preferen-

tially pumps the fundamental mode over the higher-order modes.

A thermal lens can also form in the central region due to the

implanted aperture. This changes the local refractive index and

further enhances the fundamental mode. The ﬁnal modal distribu-

tion in such devices is a result of optical conﬁnement, localised

carrier injection, thermal lensingand enhanced loss of higher-order

modes through absorption in the implanted region.

The relative sizes of the two apertures determines whether the

hybrid device operates more like an implanted VCSEL or an oxide

VCSEL. When the apertures are the same size or the oxide aper-

ture is smaller, the wavelength splitting between the fundamental

and ﬁrst-order modes is determined by the oxide aperture. When

the oxide aperture becomes largerthan the implanted one, the beha-

viour begins shifting towards that of an implanted VCSEL. When

the oxide aperture is much larger, the implant aperture dominates

modal behaviour. By combining a smaller implanted aperture with

a slightly larger oxide aperture, both optical and electrical conﬁne-

ment can be combined to direct a majority of the power into the

323

Wet oxidation for optoelectronic and MIS GaAs devices

fundamental mode. Excellent single-mode performance has been

obtained by combining oxide and implanted apertures of 8 and

6 μm, respectively.

10.5.2 Defect-related issues for optoelectronic devices

Because wet oxidation liberates elemental As, the effect of

As-generated defects on the optical properties of a device should

be considered. A general discussion of defects resulting from wet

oxidation has been presented in Section 10.2.2. Two different

problems are paramount in optical devices: non-radiative recom-

bination at defects and vacancy-driven intermixing at quantum

well interfaces.

Formation of As antisite defects (As

) by displacement of

Ga or reaction with a Ga vacancy decreases luminescence by

facilitating non-radiative recombination. The infamous emission-

quenching EL2 is an antisite defect. Quantum wells near an

oxidised layer may have their photoluminescence decreased by

more than 90%. The decrease is more severe when the quantum

wells are located closer to the oxide layer, but quantum wells tens

of nanometres away are also affected. The effect is greater if the

sample is exposedtowet oxidation conditions for longer times after

the oxidation front has passed that particular part of the quantum

well. The As generated by the oxidation then has a longer time to

diffuse into that part of the quantum well. The time and temperat-

ure dependence of this quenching is consistent with the entry into

the QW of a defect whose diffusivity is comparable to that of As

in GaAs.

Some restoration of luminescence can be achieved by anneal-

ing in a hydrogen atmosphere. Hydrogen is known to passivate

deep-level defects, such as EL2. Extended hydrogen annealing

can restore up to 70% of the pre-oxidation emission intensity, but

H can also passivate dopants in the structure, altering the electrical

characteristics.

Another way to avoid the diffusing defect problem is to grow a

thin barrier layer into the structure. As little as two monolayers of

GaP can stop the diffusion of As through an oxidised amorphous-

AlGaAs/GaAs structure. However, introduction of such layers

is often prohibited by device design constraints for GaAs-

based devices. The possibility of including a thin InP layer in

InGaAs/InAlAs/InP materials might also be considered.

Luminescence intensity is not degraded by the oxidation of

InAlP on GaAs. Since free As is not being generated directly by

oxidation of the As-free layer, only very small quantities of As

that are generated as the oxidation slowly advances into adjacent

324

Wet oxidation for optoelectronic and MIS GaAs devices

GaAs will be present. These quantities appear too small to produce

a signiﬁcant effect.

The generation of column III vacancies can produce wavelength

shifts by changing the effective size of narrow quantum wells

through vacancy-promoted intermixing at the interface. This

problem is compounded by post-oxidation annealing, which

accelerates defect diffusion. This effect can be of the order of

several tens of meV near the oxidised layer but even regions remote

from the oxide can blue-shift of the order of 10–20 meV.

10.6 APPLICATIONS IN ELECTRONIC GaAs DEVICES

There has long been a desire to have the GaAs equivalent of

Si CMOS. However, while the native SiO

/Si interface is almost

“ideal” in terms of possessing a very low density of interface states

within the bandgap, the native oxide/GaAs(100) surface has an

extremely high density of interface states that preclude CMOS

behaviour. Wet oxidation has been explored as a possible route to

a useful interface with <10

states/cm

. In this section, the fail-

ure and probable futility of this approach will be explained. The

application of wet oxidation to GaAs-on-insulator (GOI) devices

will also be discussed.

10.6.1 Problems with wet and dry oxidation for MIS devices

The presence of As at the interface between GaAs and the oxide is

a major source of a high density of interface states. Large quant-

ities of As are generated as a dominant product in wet oxidation,

so it is unlikely that enough of the interfacial As could ever be

removed to reach <10

/cm

densities. However, even dry oxid-

ation, which does not directly generate As, will not be a benign

process. The dominant As-containing product in dry oxidation is

. The stability of this compound with respect to GaAs and

AlAs is the issue for long-term, low interface-state densities.

Both GaAs and AlAs strongly favour reaction with As

form Ga

or Al

and elemental As. Thus, the GaAs/oxide

surface is thermodynamically unstable. While the rate of this

reaction will be determined by kinetics, the ability to maintain

an As-free interface for extended times under device operation,

including resistive heating, is unlikely.

Attempts to oxidise AlAs layers on AlGaAs/GaAs or GaAs have

generally produced interface-state densities in the low 10

range

at best. Slightly higher densities are obtained with wet oxidation

than with dry oxidation. A MOSFET using wet-oxidised AlAs

on AlGaAs/GaAs has been reported that operated in depletion

325

Wet oxidation for optoelectronic and MIS GaAs devices

source drain

gate

3 nm etch-stop AlGaAs

100 nm GaAs channel

10 nm Al

0.3

0.7

As spacer

buffer

Si GaAs

GOI = GaAs on insulator

50 nm oxide

FIGURE 10.16 GaAs-on-insulator (GOI) MESFET.

mode, but the source-to-drain conductance was only one fourth

the expected value, possibly due to spatially localised depletion

regions around As precipitates.

10.6.2 GaAs-on-insulator applications

Wet oxidation has been explored as a method to electrically isolate

a MESFET from the substrate, as illustrated in FIGURE 10.16. As

in other wet-oxidised devices, the generation of free As by the

reaction and the subsequent generation of other defects by the As

must be considered in fabricating the device.

Arsenic precipitates can form in the channel as a result of

As interstitials diffusing through an unoxidised spacer layer of

AlGaAs [11]. The oxidised devices tend to display reduced mobil-

ity and lower sheet charge densities than do unoxidised devices.

The reductions are more severe with higher oxidation temperatures

and longer oxidation times, both of which favour more diffusion

of defects into more distant regions of the device.

10.7 CONCLUSION

While many of the fabrication issues for optoelectronic devices

are the same as those for electronic devices, wet oxidation of

AlGaAs is a process primarily of utility for optoelectronic devices.

By careful design through proper selection of Al mole fractions,

layer thicknesses and layer placement, high-power, single-mode

VCSELs can be made. While this is an established baseline pro-

cess for high-efﬁciency VCSELs, it has several drawbacks that

are associated with the generation of free As. Through its role in

326

Wet oxidation for optoelectronic and MIS GaAs devices

generating point defects, this elemental As can have an import-

ant inﬂuence on defect-related materials properties that affect

optoelectronic device operation. The electronic problems associ-

ated with elemental As severely constrain the use of wet oxidation

for electronic device fabrication.

REFERENCES

[1] C.I.H. Ashby, J.P. Sullivan, P.P. Newcomer, N.A. Missert, H.Q. Hou,

B.E. Hammons, M.J. Haﬁch, A.G. Baca [Appl. Phys. Lett. (USA) vol.79

(1997) p.2443–5]

[2] C.I.H. Ashby, M.M. Bridges, A.A. Allerman, B.E. Hammons, H.Q. Hou

[Appl. Phys. Lett. (USA) vol.75 (1999) p.73–5]

[3] K.D. Choquette, K.M. Geib, C.I.H. Ashby, R.D. Twesten, O. Blum,

H.Q. Hou, D.M. Follstaedt, B.E. Hammons, D. Mathes, R. Hull [IEEE

Sel. Top. Quant. Electron. (USA) vol.3 (1997) p.916–26]

[4] M. Ochiai, G.E. Giudice, H. Temkin, J.W. Scott, T.M. Cockerill [Appl.

Phys. Lett. (USA) vol.68 (1996) p.1898–900]

[5] O. Blum, C.I.H. Ashby, H.Q. Hou [Appl. Phys. Lett. (USA) vol.70 (1997)

p.2870–2]

[6] O. Blum, K.M. Geib, M.J. Haﬁch, J.F. Klem, C.I.H. Ashby [Appl. Phys.

Lett. (USA) vol.68 (1996) p.3129–31]

[7] F.A. Kish et al. [J. Electron. Mater. (USA) vol.21 (1992) p.1133–9]

[8] K.D. Choquette, K.M. Geib, H.Q. Hou, D. Mathes, R. Hull [Proc. 10th

Int. Conf. on Semiconducting and Insulating Materials, Berkeley, CA,

USA, 1–5 June 1998 (IEEE, New Jersey, USA, 1998) p.209–13]

[9] P. Bienstman, R. Baets, J. Vukusic, A. Larsson, M.J. Noble, M. Brunner,

K. Gulden, P. Debernardi, L. Fratta, G.P. Bava, H. Wenzel, B. Klein,

O. Conradi, R. Pregla, S.A. Riyopoulos, J.-F.P. Seurin, and S.L. Chuang,

[IEEE J. Quant. Electron. (USA) vol.37 (2001) p.1618–31]

[10] E.W. Young, K.D. Choquette, J.-F. Seurin, Shun Lien Chuang, K.M. Geib,

A.A. Allerman [IEEE J. Quantum Electron. (USA) vol.39 (2003) p.634–9]

[11] P.A. Parikh, P.M. Chavarkar, U.K. Mishra [IEEE Electron Device Lett.

(USA) vol.18 (1997) p.111–13]

327

GLOSSARY

AES Auger electron spectroscopy; a surface analysis

technique that probes the chemistry of a surface

by measuring the energy of electrons emitted

from that surface when it is irradiated with

electrons of energy in the range 2–50 keV

AFM atomic-force microscopy, a surface-analysis

technique that measures the position of atoms on

the surface using a ceramic or semiconductor tip

at the end of a cantilever. As the tip moves over a

surface, it contacts the surface continuously or

periodically and the cantilever bends as the tip is

repelled or attracted to the surface

acceptor an atom or defect that provides a “hole”, e.g. Zn

on Ga site in GaAs

activation

energy

the energy of a potential barrier that must be

crossed for a reaction to proceed

airbridge a metal interconnect that spans between two

contacts through the air rather than crossing over

a dielectric

alkane hydrocarbons with a formula C

2n+2

. Examples

are methane and ethane

alloy to heat in order to promote a solid-state reaction

such as that needed to form many kinds of

ohmic contacts

analogue referring to a circuit or application using an

architecture with continuously variable voltages

and currents

antisite

defect

a type of defect where an atom of one element of

a compound is located on a lattice site normally

occupied by an atom of a different element of

the compound

329

Glossary

AR coating anti-reﬂection coating, a thin layer of material

that changes the reﬂectivity of a surface;

minimum reﬂection occurs with ﬁlm thicknesses

that are a multiple of 1/4 times the wavelength of

light whose reﬂectance is to be minimised

asher an etch system used to remove hydrocarbon ﬁlms

from a sample, usually with an oxygen plasma

atomic

displacement

threshhold

energy required to move an atom off its normal

lattice site; about 40 eV for GaAs

β the DC current gain of a bipolar transistor deﬁned

by the ratio of the collector current to the base

current

band

bending

refers to the curvature of an energy band in

proximity to a surface (interface) due to charges

at the surface (interface)

bandgap the energy separation between the valence and

conduction bands

bandstructure the energy states of an ordered solid which are

ordered into bands grouped in loose relation to

the energy levels of the atoms they were derived

from

base the control electrode of a bipolar transistor; the

current from the emitter to the collector is

modulated by an applied voltage to the

base-emitter junction

BJT a bipolar junction transistor

blisters pockets of N

gas trapped in photoresist that

crosslinked too quickly

blueshift shift to higher energy

breakdown describes the condition of a device in a high

electric ﬁeld where the current enters a runaway

condition because impact ionisation generates

sufﬁcient carriers to be self-sustaining

breakdown

walkout

an increase in breakdown voltage during the

initial operation of an FET

buffer a region of semiconductor grown by epitaxy as a

transition between the substrate and the active

layers of a device

burn-in an initial operation of a device to stabilise its

electrical characteristics and to screen out weak

devices

burn-out the abrupt failure of a device in operation

330

Glossary

CAIBE chemically assisted ion-beam etching

CBM conduction-band minimum = E

collector the destination of electrons (holes) in a

bipolar transistor

conduction band the energy band of a solid starting with the

lowest unoccupied electron state

conduction band

offset, E

the abrupt energy difference in the

conduction band at a heterointerface

critical dimension smallest feature size

crystallographic

etchant

etchant that etches different crystal facets

at different rates; also called

reaction-rate-controlled or

reaction-rate-limited etchant

C–V capacitance-voltage

CVD chemical vapour deposition

DBR distributed Bragg reﬂector

DC direct current. The current in a device or circuit

that results from applied voltages that do not

vary with time

deep-level

traps

carrier traps with energies located near the centre

of the bandgap

density of

states

the number of energy states per unit energy

depletion

region

region depleted of free carriers

descum remove organic residues by reaction with a

low-energy oxygen plasma or ozone

developer a chemical solution that dissolves a

lithographically exposed (by light or electrons)

polymer

diffusion carrier movement in a semiconductor in response

to concentration gradients

digital referring to an integrated circuit or application

using a binary logic architecture

dilute acid commonly, an acid diluted to a concentration of

6N (6 normal)

dislocation an imperfection in a crystal structure resulting

from the absence of one or multiple atoms in one

or more layers of a crystal

DLTS deep level transient spectroscopy, a method of

probing traps with transient I–V or C–V

techniques

331