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Journal of Crystal Growth 248 (2003) 108–113

MOVPE homoepitaxial growth used to study the effect of

aging and chemical treatment on GaAs substrates

Dan Allwooda, Nigel Masona, Andrew Mowbrayb, Ruth Palmerb,*aClarendon Laboratory, Department of Physics, University of Oxford, Oxford OX1 3PU, UK

bWafer Technology Ltd, 34 Maryland Road, Tongwell, Milton Keynes MK15 8HJ, UK

Abstract

Epiready GaAs substrates have been studied to establish the effect of aging and chemical treatment on the wafer

surface. A 6-month aging trial was carried out by growing homoepitaxial layers on the substrates by MOVPE and

analysing the morphology of the layers by optical microscopy and AFM. The study has shown that the chemical nature

of the native oxide changes over time leading to degradation in the quality of homoepitaxial growth. As the on-axis

substrates age, growth quality can deteriorate after 4 weeks or 12 weeks, depending on the growth conditions used;

growth quality on vicinal wafers was good throughout the 24-week period. The aging effect can be reduced by careful

choice of growth conditions, substrate orientation and chemical treatments. Epiready wafers have been compared to

wafers prepared using two different chemical treatments: HCl/H2O and NH4OH/H2O2/H2O. It is recommended that

wafers are stored in their original packaging until just prior to use and storage at reduced temperature will prolong the

lifetime of an epiready wafer.r 2002 Elsevier Science B.V. All rights reserved.

PACS: 78.66.Fd; 81.15.Gh; 81.40.Cd; 81.65.b

Keywords: A1. Atomic force microscopy; A1. Aging; A1. Surface treatments; A3. Metalorganic vapor phase epitaxy; B1. Oxides;

B2. Semiconducting gallium arsenide

1. Introduction

Successful epitaxial growth of high qualitysemiconductor layers demands that the substrates

used provide a perfect template for epitaxy. A

GaAs wafer has a thin oxide layer present on the

surface covering the underlying bulk GaAs [1,2].

This oxide must be of controlled composition and

thickness in order for it to be removed easily

before growth takes place, leaving the underlying

surface flat and free from contamination. Wafers

provided by the manufacturer in this state areknown as epiready [3], intended for use straight

from the packaging with no need for additional

surface preparation for most applications.

Previous research by the MOVPE group at

Oxford University and Wafer Technology [4] has

indicated that the age of a GaAs substrate relates

directly to the quality of epitaxial layers that can

be grown upon it. Some wafer manufacturers only

guarantee their products as being epiready for 3

months. The epireadiness of InP with age has been

*Corresponding author. Tel.: +44-1908-210444; fax: +44-

1908-210443.

E-mail address: [email protected] (R. Palmer).

0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 2 0 4 3 - 2


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studied [5], but no equivalent study of GaAs

substrates exists.

There is currently an array of different chemical

etches [6,7] and treatments used to pretreat even socalled epiready wafers prior to growth. These

treatments may be used because the growth

process is not tuned for the use of the epiready

substrate or just to follow a historic recipe and

may in some cases do more harm than good. In

this study MOVPE growth quality on substrates

prepared with two different chemical treatments

has been compared with growth on epiready

wafers.

The aim of this paper is to assess the effect of

wafer aging on epitaxial growth over a 6-month

period. The role of the reactor ambient conditions

in the pregrowth stages is unknown but is expected

to affect the quality of grown layers. The selection

of substrate orientation and the effect of com-

monly used wet chemical treatments on the wafer

surface are considered. Recommendations for

prolonging the lifetime of epiready wafers by

considering the storage conditions used are also

presented.

2. Experimental procedure

The substrates used in the aging trial were 2-in

diameter VGF grown GaAs, doped with silicon.

Wafers were supplied by Wafer Technology,

350mm thick and single side polished. Half were

(100)70.11 orientation (termed on-axis) and

half were cut 21 from (1 0 0) towards ½ %110 (termed

vicinal). Chemical treatments used Ashland

cleanroom grade solutions: HCl (36%), NH4OH

(29%) and H2O2 (30%) and 18 MO cm deionised

water.

We have used two treatments to improve the

wafer surface before growth. Wafers were stored influoroware trays inside nitrogen filled bags in

cleanroom conditions at 201C and tested at time

intervals shown in Table 1. Epiready wafers have

been given Wafer Technology’s proprietary polish

and epiready treatment. The chemical treatments

that have been developed were carried out

immediately after polishing. APM etch, NH4OH/

H2O2/H2O of composition 2:1:40 was found to be

a smooth, controllable etch which maintained the

mirror finish of the polished wafers while removing

1 mm of GaAs per minute. An etch time of 30 s was

previously found to be sufficient to remove the

surface anisotropy [8]. The HCl treatment, using

HCl/H2O 1:1 for 1 min, is not an etch as only the

oxides are removed. Clean wafers were achieved

by quenching the HCl with flowing deionised

water, resulting in regrowth of the oxide.

In this work, growth has been carried out under

hydrogen in a horizontal reactor cell, details of the

set-up are provided elsewhere [4]. The flow rate of

hydrogen through the reactor was 16 min1 at

1000 mbar and the precursors used were tBAs,

Mochem, 51C, 60Torr and TMGa, Epichem,91C, 42 Torr. All wafers were tested with two

types of homoepitaxy and two types of deoxida-

tion, the conditions are given in Table 2. These

were used in order to assess the effect on growth

quality of aging and treatments before and after

growth and to establish the role of tBAs or H2 in

the pregrowth stages.

Ex situ analysis was carried out using optical

microscopy and AFM. Samples were photo-

Table 1Summary of samples included in each run at each time interval

Orientation Treatment Growth A Growth B Deox Anneal

On-axis APM 0,1,4,12,24 0,1,4,12,24 0,1,4,12,24 0,1,4,12,24

HCl 0,1,4,12,24 0,1,4,12,24 0,1,4,12,24 0,1,4,12,24

Epiready 0,1,4,12,24 0,1,4,12,24 0,1,4,12,24 0,1,4,12,24

Vicinal APM 0,1,4,12,24 12,24 12,24 12,24

HCl 0,1,4,12,24 12,24 12,24 12,24

Epiready 0,1,4,12,24 12,24 12,24 12,24

The week at which a type of growth run was carried out on a sample is given. All treatments carried out at time zero.

D. Allwood et al. / Journal of Crystal Growth 248 (2003) 108–113 109


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graphed with a Leitz Laborlux 12Me microscope

at 200 magnification and were given a value for

defect level, graded from 1 for no defects to 9

indicating many defects. The Park Scientific

Instruments Thermomicroscope Autoprobe M5

AFM was operated in the non-contact mode.

The roughness of samples was measured from

average Ra (average roughness) values calculated

from 5 lines drawn across the AFM image.

2.1. Aging trial

When ideal growth conditions were used with

tBAs, as in growth A, high quality epitaxial

growth was observed for the first 3 months on all

wafers. tBAs decomposes at above 4101C in the

deoxidation stage, providing both arsenic andatomic hydrogen. This aids oxide desorption [9]

and limits the desorption of arsenic from the bulk,

leaving a smooth surface for subsequent growth.

Layers produced exhibited atomic terracing, a

typical example is shown in Fig. 1(a). Growth A

begins to deteriorate at week 12, Fig. 1(b). Growth

is nucleated at defects on the terraces left after

deoxidation in preference to the step edges,

causing the rough epitaxial layer shown. The

change in growth mode from step flow growth to

island type growth [10] is demonstrated in Table 3where the defect level and Ra roughness increases

with age.

When only molecular hydrogen was present in

the heatup in Growth B, the growth quality had

begun to deteriorate by week 4, Table 3. Heatup in

a molecular hydrogen atmosphere simulates deox-

idation conditions when insufficient cracking of

the group V precursor has taken place. This

suggests aging issues may be more important

when growth conditions are such that the arsenic

precursor has not fully pyrolysed to arsenic and

atomic hydrogen in the pregrowth stage.

The advantages of the presence of tBAs at this

stage are again shown from the Deox results givenin Table 3. Throughout the aging period, a smooth

surface with consistently low Ra values is obtained

after Deox, Fig. 1(c). However, this highlights the

limits of AFM for predicting good growth because

results from Growth A show a degradation in the

surface at week 12, possibly due to organic

contamination.

Annealed samples deoxidised in molecular

hydrogen clearly show that a smooth surface does

not result when tBAs is absent in the heatup.

Fig. 1. AFM images showing: (a) growth A on the APM

treated wafer at week 1 showing atomic terraces (3mm), (b)

growth A on the epiready sample at week 12 (10 mm), (c) deox

on the epiready sample at week 12 (10 mm), and (d) anneal on

the APM etched sample at week 12 (10mm).

Table 2

Conditions for the growth and deoxidation runs investigated. Growth rate was approximately 2 mm per hour

Run Heat up Deoxidation Growth Cool down

Growth A To 6001C over 25 min with tBAs and H2 At 6001C for 5 m in At 5801C for 30 m in To 201C over 45min

Growth B To 6001C over 25 min with H2 At 6001C for 5 m in At 5801C for 30 m in To 201C over 45min

Deox To 6001C over 25 min with tBAs and H2 At 6001C for 5 min — To 201C over 45min

Anneal To 6001C over 25 min with H2 At 6001C for 5 min — To 201C over 45min

D. Allwood et al. / Journal of Crystal Growth 248 (2003) 108–113110


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Large droplets of gallium [11] appeared on the

annealed surface after 12 weeks, Fig. 1(d), which

could then act as nucleation sites for island growth

leading to the rough surfaces seen. The droplet

counts in Table 3, which proved a good measure of

aging and growth quality, were calculated by

averaging counts over nine 10 mm AFM images.

Results for the time interval at which poor

growth morphology was first observed are given in

Table 4 for growth A and B and on-axis and

vicinal samples. Growth on the vicinal wafersresulted in smooth layers and few defects through-

out because these substrates have more abundant

nucleation sites for growth at step edges as the

terrace width is much lower than for on-axis

substrates. When vicinal substrates are used, the

optimisation of reactor conditions is less crucial.

At each time interval in the trial, the quality of

growth on the APM treated sample was better

than the HCl treated sample and this was better

then the epiready sample. As an example, week 12

data is shown in Table 5. The APM etch producesthe best growth morphology because the etching

action removes any defects or impurities at the

oxide bulk interface and polishes the surface. The

oxide after HCl treatment is chemically different to

the epiready sample. Treatment removes the

hydroxyl groups present on epiready wafer sur-

faces [12] that can result in an increase in oxidation

rate in the wafer packaging. XPS analysis [13] has

revealed a higher level of arsenic present on HCl

treated wafers.

2.2. Change in chemical composition of oxide with

time

The growth quality on all on-axis wafers

degrades over time due to a change in the chemical

composition of the oxide [11]. This results in

incomplete desorption of the oxide during the

Table 3

Growths A and B and anneal results for on-axis samples treated with APM at time zero

Run Analysis 0 1 4 12 24

Growth A Defect level — 2 3 2 6

Ra roughness — 0.2 0.2 4.0 4.5

Growth B Defect level 2 2 5 5 5

Ra roughness 0.2 0.2 3.3 5.1 4.7

Deox Ra roughness 0.1 0.1 0.2 0.2 0.1

Anneal Droplet count 7 6 9 22 29

Data indicating surface quality are given for runs carried out at intervals over the 6-month storage trial. Defect level is given on a low

(1) to high (9) scale. Average Ra roughness values are in nm. Gallium droplet count is per 10 mm square image. (Conditions were not

optimised for Growth A week 0.)

Table 4

Time interval at which poor morphology is observed in the

aging trial

Orientation Growth A Growth B

On-axis 12 weeks 4 weeks

Vicinal >24 weeks 24 weeks

Table 5

Growth A and B and anneal results for on-axis samples at 12weeks after chemical treatment showing improvement in

growth by chemical treatment

Run Analysis APM HCl Epiready

Growth A Defect level 2 4 6

Ra roughness 4.0 4.1 5.4

Growth B Defect level 5 6 6

Ra roughness 5.1 5.4 6.0

Anneal Droplet count 22 25 32

Defect level is given on a low (1) to high (9) scale. Average Ra

roughness values are in nm. Gallium droplet count is per 10mmsquare image.



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deoxidation stage of growth unless high deoxida-

tion temperatures are used, but this can roughen

the surface due to arsenic loss.

Initially the major component of the native

oxide coating on GaAs is As2O3. This oxide is not

passivating, it is unstable in the presence of GaAs.

As the substrate ages, an internal change in the

chemical nature of the oxide occurs [14]:

2GaAs þ As2O3-4As þ Ga2O3: ð1Þ

This degrades the oxide/bulk interface and

generates gallium oxide which is more difficult to

remove before growth than the arsenic oxide.

If Ga2O3 is not removed before growth it can

be reduced to gallium which coalesces to form the

droplets seen, inducing an island growth mode

leading to a roughened layer.

2.3. Storage trial

Storage of wafers at elevated temperatures

produces droplets of gallium after annealing at a

rate much faster than in ambient storage condi-

tions. Post-anneal gallium droplet counts from

epiready samples stored at room temperature for

periods up to 2 years were compared with samples

stored in a muffle furnace at 2001C for up to 30 h.

As the gallium droplet formation is related to the

amount of Ga2O3 on the surface, these results wereused to calculate an estimated activation energy

for the formation of Ga2O3 (Eq. (1)) of

34.7 kJ mol1. High temperature storage may be

used as an accelerated aging method and the

activation energy may be used to calculate

estimated lifetimes of substrates stored at different

temperatures. Storage at reduced temperatures

should slow the aging process.

Wafers stored for 2 years in a freezer at –201C

showed little sign of aging compared to samples of

the same age stored at ambient temperature.

Smoother epitaxial layers were produced by both

Growth A and B on the freezer stored sample.

Table 6 shows droplet counts after anneal for fresh

epiready wafers, freezer stored samples and 2-year-

old epiready samples. Storage in a fridge at 21C

also showed great improvement over the quality of

grown layers on wafers stored at room tempera-

ture. Storage at reduced temperature is recom-

mended to extend the lifetime of epiready wafers.

Results from comparisons between wafers

stored in nitrogen filled bags and those stored in

air (Table 6) shows slower aging when wafers are

stored in inert conditions. This limits the amount

of As2O3 on the wafer surface and therefore

reduces the amount of Ga2O3 that can be

produced through Eq. (1), leading to the recom-

mendation that wafers are stored as received intheir original packaging until just before use.

3. Conclusion

This work has found that when a flow of tBAs

was used in the oxide thermal desorption stage of

growth, good morphology of the grown layer was

maintained for longer than when tBAs was not

present. In a comparison of substrate orientation,

layers grown on vicinal substrates maintained agood morphology throughout the trial, whereas

the quality of the growth upon on-axis substrates

was seen to degrade. At each time interval in the

trial, samples chemically treated at time zero were

found to produce smoother epitaxial growth with

fewer defects than the epiready sample. An

investigation into storage conditions has shown

that at elevated temperatures the aging effects

observed are accelerated and at reduced tempera-

tures they are attenuated.

Table 6

Average gallium droplet counts over a 10 mm scan after annealing for samples from the storage trial. Fig. 1(d) is a typical example of

such a scan

Conditions Ambient Ambient Furnace Furnace Freezer Fridge N2 Air

Time 1 week 2 years 10 h 29 h 2 years 2 years 20 weeks 20 weeks

Droplet count 10 105 50 120 15 20 10 59

D. Allwood et al. / Journal of Crystal Growth 248 (2003) 108–113112


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References

[1] A.J. Springthorpe, S.J. Ingrey, B. Emmerstorfer,

P. Mandeville, W.T. Moore, Appl. Phys. Lett. 50 (1987)

77.

[2] C.M. Demanet, M.A. Marais, Surf. Interface Anal. 7

(1985) 13.

[3] J. Schwar, Mater. Sci. Eng. 9 (1991) 23.

[4] D.A. Allwood, N.J. Mason, P.J. Walker, J. Crystal

Growth 195 (1998) 163.

[5] A. Knauer, E. Richter, M. Weyers, J. Crystal Growth 146

(1995) 549.

[6] ]D.A. Allwood, N.J. Mason, Wet chemical cleaning of

GaAs and InP for MOVPE of III–V semiconductor

growth, in: L. Vescan (Ed.), Handbook of Thin Film

Process Technology, Institute of Physics, Bristol, England,

2000, p. G7:1.

[7] S.D. Mukherjee, D.W. Woodward, In: M.J. Howes, D.V.

Morgan (Eds.), Gallium Arsenide, Wiley, New York, 1985,

p. 119.

[8] D.A. Allwood, I.R. Grant, N.J. Mason, R.A. Palmer, P.J.

Walker, J. Crystal Growth 221 (2000) 160.[9] M. Yamada, Y. Ide, Jpn J. Appl. Phys. 33 (1994) L671.

[10] C. Orme, M.D. Johnson, K.T. Leung, B.G. Orr, P.

Smilauer, D. Vvedensky, J. Crystal Growth 150 (1995)

128.

[11] D.A. Allwood, S. Cox, N.J. Mason, R. Palmer, R. Young,

P.J. Walker, Thin Solid Films 412 (2002) 76.

[12] Y. Backlund, K. Hermansson, L. Smith, J. Electrochem.

Soc. 139 (1992) 2299.

[13] M. Lowe, Private Communication, University of Warwick.

[14] C.D. Thurmond, G.P. Schwartz, G.W. Kammlott, B.

Schwartz, J. Electrochem. Soc.: Solid-state Sci. Technol.

127 (1980) 1366.


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