Polycrystals growth on dielectric masks during InP/GaAs selective MOVPE

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Journal of Crystal Growth 261 (2004) 411–418

*Corresp

ing, Univer

8656, Japa

E-mail

(M. Sugiya

0022-0248/

doi:10.101

Polycrystals growth on dielectric masks during InP/GaAsselective MOVPE

Masakazu Sugiyamaa,d,*, Ho-jin Ohb,d, Yoshiaki Nakanoc,d,Yukihiro Shimogakib,d

aDepartment of Electronic Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JapanbDepartment of Materials Engineering, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

cResearch Center for Advanced Science and Technology, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, JapandCore Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation, Japan

Abstract

In selective-area metalorganic vapor-phase epitaxy, polycrystals are generated on masks of large area. The generation

of such crystals should be avoided because they act as uncontrollable sink of film precursors. Although generation of

polycrystals is sometimes associated with surface migration of a precursor on masks, our observation suggested that

polycrystals are generated when the gas-phase concentration of a film precursor just above masks exceeds a critical

value. Reducing either total pressure or partial pressure of the source gas suppressed generation of polycrystals, which is

consistent with the gas-phase mechanism. The threshold of gas-phase precursor concentration for generation of

polycrystals was investigated as a function of growth temperature and V/III ratio for both GaAs and InP. The

threshold increased with temperature, that is to say, fewer polycrystals were observed at higher temperature. The

threshold for InP was significantly larger than that for GaAs. This information is useful for designing mask patterns

which do not generate polycrystals, and it helps to provide a detailed understanding of nucleation phenomena.

r 2003 Elsevier B.V. All rights reserved.

Keywords: A1. Growth models; A1. Surface processes; A3. Metalorganic vapor phase epitaxy; B2. Semiconducting gallium arsenide;

B2. Semiconducting indium phosphides

1. Introduction

Selective-area growth (SAG) of InGaAsP usingmetalorganic vapor-phase epitaxy (MOVPE) is apromising technology for monolithic integrationof optoelectronic integrated circuits [1]. With asingle growth on properly designed mask patterns

onding author. Department of Electronic Engineer-

sity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-

n. Tel.: +81-3-5841-2336; fax: +82-3-5841-6724.

address: [email protected]

ma).

$ - see front matter r 2003 Elsevier B.V. All rights reserve

6/j.jcrysgro.2003.11.036

using a dielectric thin film, it is possible to formpassive and active devices simultaneously. Thisreduces the cost for fabrication and enhancesproduction yield. However, there are some bottle-necks in SAG, including polycrystalline growth onmasks. If polycrystals are generated over a mask,they act as a sink of a film-forming intermediate,and the surface area of the polycrystals cannot becontrolled. Therefore, once polycrystals are gen-erated, controlling the growth rate and composi-tion in the growth area between masks becomesimpossible.

d.

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M. Sugiyama et al. / Journal of Crystal Growth 261 (2004) 411–418412

Theoretical design of the mask patterns andgrowth conditions for SAG is under development.The major factor determining the growth rate in apatterned substrate is the gas-phase concentrationgradient just above the pattern over a distancewhich is roughly the same as typical width ofthe mask [1,2]. This gradient is determined by theratio of gas-phase diffusion coefficient (D) tosurface reaction rate constant (ks) of the inter-mediate. This ratio can be obtained by experi-ments, or estimated based on measured rateconstants [2]. Therefore, distribution of growthrate and composition in a patterned substrate canbe simulated by solving a simple diffusionequation. However, once polycrystals are formedon masks, such a prediction fails. Therefore, weneed the criteria for avoiding the generation of thepolycrystals.In previous works [3,4], generation of polycrys-

tals was related to the surface diffusion length ofthe intermediate on a mask. The intermediate,which adsorbed on the mask, cannot migrate tothe growth area if the distance from the mask edgeis larger than the effective surface migrationlength. Therefore, the precursor accumulates onthe mask, except in the region within the surfacemigration length from the mask edge, and, as aresult, its surface concentration exceeds the criticalvalue for nucleation. Based on this scenario, thesurface migration length on the mask was esti-mated.On the other hand, in a preceding letter [5], we

observed the trend that favored an alternativemechanism: formation of polycrystals when thegas-phase concentration of a precursor just abovea mask exceeds a critical value. This mechanismwas supported by the fact that reducing totalpressure suppressed formation of polycrystals, andalso by corresponding simulations of the gas-phaseconcentration of a precursor based on the mea-sured values of D=ks:In this work, we extended the study on the

generation of polycrystals according to the gas-phase scenario above. The threshold concentrationof a film-forming intermediate will be estimatedfor each growth condition. Its dependenceon growth temperature and on the partialpressure of group V precursor will be discussed.

Differences between the growth of GaAs and InPwill also be discussed. These results are indis-pensable for designing mask patterns for poly-crystal-free SAG.

2. Experimental procedure and numerical analysis

The masks for observing polycrystals wereeither 1 or 2mm in width, and 7mm in length.They were made of a 200-nm thick SiO2 filmformed by magnetron sputtering, and were definedon the surface of GaAs or InP substrate usingphotolithography and wet etching. The substratealso contained a pattern for measuring the value ofD=ks: 380-mm-wide growth area between 50-mm-wide masks. By fitting the profile of film thicknessin this growth area with the result of thesimulation of gas-phase concentration profile, thevalue of D=ks was estimated [2].Using those patterned substrates, GaAs or

InP was grown using a horizontal MOVPEreactor. For GaAs growth, trimethylgallium(TMGa) and tertiarybutylarsine (TBAs) wereused with a hydrogen carrier. In some experiments,arsine (AsH3) was used instead of TBAs.For InP growth, trimethylindium (TMIn) andeither phosphine (PH3) or tertiarybutylphosphine(TBP) were used. The growth period was adjustedso that the film thickness without a mask wasB200 nm.Polycrystals generated on a mask were observed

with a Nomarski optical microscope. In order toobserve the position and the size of polycrystalsmore precisely, the height of the mask surfacewas scanned by a stylus profilometor (TENCORP10). As mentioned below, a polycrystal wasdetected as a spike in the height profile, butpolycrystals seemed merged in the profile wheretheir concentration was high because the size ofthe stylus was larger than the distance betweenpolycrystals.In order to calculate the concentration profile of

a film-forming intermediate above the masks, wenumerically solved the diffusion equation in thegas-phase [2]

D DC ¼ 0 ð1Þ

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50 mbar

00.10.20.30.40.50.60.7

0 500 1000 1500 2000M

MG

a pr

essu

re [P

a]

00H

eigh

t [m

icro

met

er]

Posiiton [micrometer]

Fig. 2. Upper graph: Calculated concentration profile of a film-

forming intermediate for the total pressure of 1� 104 (—) and

5� 103 Pa (- - -). The growth conditions are the same as in Fig.

1. Both ends of the horizontal axis correspond to the mask

edges. The intermediate is assumed to be MMGa but its nature

has no effect on the profiles. Lower graph: Height profile of the

2-mm mask after the growth of GaAs with TBAs at the total

pressure of 1� 104 (—) and 5� 103 Pa (- - -). These correspond

to the right-side photographs in Fig. 1. The spikes in the profile

for 1� 104 Pa indicate polycrystals formed on the mask. Near

the center of the mask, polycrystals appeared merged in the

profile because of the size of the stylus was larger than the

distance between polycrystals. Dashed lines connecting both

graphs indicate the boundary of the polycrystals-containing

region and the critical concentration for generation of

polycrystals.

M. Sugiyama et al. / Journal of Crystal Growth 261 (2004) 411–418 413

with the boundary conditions

ð�DrCÞ �~nn ¼ ksC ðon the growth areaÞ; ð2Þ

ð�DrCÞ �~nn ¼ 0 ðon the maskÞ; ð3Þ

where ~nn is the vector normal to the substrate. Thecalculation used the commercial software FEM-LABt which solves partial differential equationsusing a three-dimensional grid with the finite-element method. The value of D=ks was measuredfor each growth as described above.

3. Results and discussions

3.1. Effect of total pressure and mask width

The photographs of Fig. 1 are the top views ofmasks after GaAs growth at 873K. The totalpressure was 1� 104 and 5� 103 Pa while thepartial pressure of TMGa was kept constant.TBAs was used as a group V source. Polycrystalswere only observed on the 2-mm mask at1� 104 Pa. If we neglect the effect of total pressure,the result can be interpreted to be the effectivemigration length of a film-forming intermediatewas larger than 0.5mm (half of the mask width,1mm, on which no polycrystal was observed) andwas smaller than 1mm (half of the mask width onwhich polycrystals were observed). However, theeffect of total pressure suggests, according to thisscenario, that the migration length on the maskdrastically changed with total pressure.According to the alternative gas-phase mechan-

ism, the effect of total pressure can be explained

50 mbar

100 mbar

50 mbar

100 mbar

Polycrystals

50 mbar

100 mbar

50 mbar

100 mbar

Polycrystals

mask (1mm width) mask (2mm width)

Fig. 1. Top views of the masks after GaAs growth at 873K

(Nomarski microscopy). Total pressure was 1� 104 and

5� 103 Pa while the inlet partial pressure of TMGa was kept

at 0.44Pa. TBAs was used as a group V source with the inlet

partial pressure of 6.0 Pa. Polycrystals were observed in the

darker area.

more naturally. The upper graph in Fig. 2 showsthe calculated concentration profiles of a film-forming intermediate for the total pressures of1� 104 and 5� 103 Pa on the 2-mm-wide mask.The growth conditions are the same as in Fig. 1.The intermediate is assumed to be monomethyl-gallium (MMGa), but its nature has no effect onthe profiles because D=ks; the only parameterdetermining the concentration profile above amask pattern, was measured experimentally usingthe profile of growth rate in the 380-mm-widegrowth area between 50-mm-wide SiO2 masks. Asan approximation, the relative concentrationobtained by solving the diffusion equation wasconverted to the absolute value of MMGa

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(a)

(b)

0

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0 500 1000 1500 2000

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TMGa 0.07 Pa

0

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0 500 1000 1500 2000

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ght[

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r]

TMGa 0.28 MPa

Fig. 3. Height profile of the 2-mm mask after the growth of

GaAs with AsH3 at 873K. The total pressure was 1� 104 Pa

and TMGa inlet partial pressure was 0.07 Pa (a) and 0.28 Pa (b).

The inlet partial pressure of AsH3 was 7.2 Pa. Growth period

was 4 times longer for (a) than for (b) so that the film thickness

of GaAs without mask would be the same.


concentration using the inlet partial pressure ofTMGa (0.44 Pa), assuming that TMGa completelydecomposed to MMGa at the wafer position. Theconcentration at 50mbar is lower because fastergas-phase diffusion of MMGa enhances its lateraldiffusion from above the mask to the growth area.The height profile of the mask shown in Fig. 2clearly shows the boundary of the region in whichpolycrystals were generated. At 1� 104 Pa, theconcentration of MMGa at this boundary position(2.6 Pa) is assumed to be the critical value, abovewhich polycrystals were generated. At 5� 103 Pa,the MMGa concentration on the mask did notexceed this critical value, which is why polycrystalswere not generated at this total pressure. Similarly,on the 1-mm-wide mask, the calculated concentra-tion of MMGa did not exceed the critical value,which is consistent with the fact that no poly-crystals were observed on this mask.

3.2. Effect of partial pressure and group V source

According to the gas-phase mechanism above,reducing the partial pressure of TMGa shouldsuppress the formation of polycrystals, because theconcentration of MMGa above the mask de-creases proportionally with the concentration ofTMGa. This trend was certainly observed asshown in Fig. 3. The growth used AsH3 and thetotal pressure was 1� 104 Pa. For a TMGa partialpressure of 0.07 Pa, polycrystal-free regions wereobserved near the mask edges. Whereas, for0.28 Pa, polycrystals cover the whole mask andthey are denser than the 0.07 Pa case. A compar-ison between the height profiles in Figs. 2 and 3(b)reveals the difference between the growth usingTBAs (Fig. 2) and AsH3 (Fig. 3(b)). Although thepartial pressure of TMGa is smaller in the growthwith AsH3 (0.28 Pa) than with TBAs (0.44 Pa),more polycrystals were observed with AsH3. Thisdifference suggests that the growth using AsH3

generates polycrystals more easily than the growthwith TBAs. We should consider the difference inthe decomposition yield between AsH3 and TBAs,though their partial pressures were similar: 7.2 Pafor AsH3 and 6.0 Pa for TBAs. The effect of groupV sources will be discussed further in Section 3.5.

3.3. Effect of narrow growth area on the mask

By forming a narrow growth area in the centerof a mask, generation of polycrystals can besuppressed because the film grown in this areaabsorbs film-forming intermediates and thus pre-vents their accumulation on the mask. The lowergraph in Fig. 4 shows the distribution of InPpolycrystals on a 2-mm-wide mask with a 20-mm-wide growth area in its center. The growth of InPused TBP under 1� 104 Pa total pressure and aheater temperature of 873K. Polycrystal-free areasare observed at both mask edges and around thegrowth area. The width in which almost nopolycrystals are observed is B350 mm from bothsides of the mask. The width of polycrystal-freearea against both sides of the narrow growth areais B250 mm.According to the surface migration mechanism,

the width of polycrystal-free area corresponds tothe effective surface migration length of theintermediate and should be the same from everymask edge. However, the widths were distinctively

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ght[

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-]

Fig. 4. Upper graph: Calculated profile of the relative

concentration of a film-forming intermediate at the growth

condition. Both ends of the horizontal axis correspond to the

mask edges. The solid line is the profile assuming that the

growth area in the middle of the mask is flat. The dotted line, on

the other hand, is the profile taking account of the topology of

the film grown in this growth area, which corresponds to about

3 times larger surface area than the flat surface. Lower graph:

Height profile of 2-mm-wide mask with a 20-mm-wide growtharea in its center, after the growth of InP with TBP at 873K.

The total pressure was 1� 104 Pa, the inlet partial pressure of

TMIn was 0.59 Pa, and that of TBP was 18Pa. The center

growth area is from 990 to 1010mm. In this growth area, B1.2-

mm-thick InP film was grown. The thickness at both edges of

the center area was much larger than in the center, causing what

appears to be spikes in the figure.


different between the area near the mask sides andthe area around the center growth area.The gas-phase concentration mechanism, on the

other hand, can explain this difference. The uppergraph in Fig. 4 is the calculated concentrationprofile of the intermediate just above the substrate.The solid line is the result assuming that thegrowth area in the middle of the mask is flat.However, the film in this growth area had athickness of B1.2 mm, and a significantly thickerfilm was grown at both sides of this area.Considering this topology, the surface area of thefilm in the growth area was about 3 times largerthan the flat surface. The dotted line is theconcentration profile taking this surface area intoaccount. Since the surface area of grown filmincreased with time, the gas-phase concentration

profile at the instance of nucleation of polycrystalswas between those two curves. Using the gas-phaseconcentration profile indicated by the dotted line,the region in which the gas-phase concentrationexceeded about 11 agreed well with the region ofpolycrystals formation. The agreement may not beso good considering that the gas-phase concentra-tion around the center of the mask may have beenlarger than that indicated by the dotted line.However, judging from the trend above, theassumption that gas-phase concentration abovethe masks determines the generation of polycrys-tals seems more reasonable than the assumptionrelying on a surface diffusion length.

3.4. Effect of growth temperature

Through the discussion above, we have shownthat the gas-phase concentration mechanism canexplain the trend of polycrystals generationreasonably well. We now investigate how thegrowth temperature affects the critical concentra-tion for generation of polycrystals. As shown inFig. 5, with increasing growth temperature, thenumber of GaAs polycrystals observed on themasks decreases. The width of the polycrystal-freearea from both mask edges appears to increasewith temperature, but the trend is subtle.In order to determine the boundary of the

polycrystals-free area clearly, the following rulewas applied: The surface of the mask was scannedfive times at 100 mm intervals. The mask width wasdivided into 50-mm-wide segments. Segments inwhich polycrystals were observed on more than 3scans were regarded to be polycrystals-containingareas. In this way, we can determine the boundaryof the polycrystal-free area, and the concentrationcorresponding to this boundary was regarded asthe critical concentration for the generation ofpolycrystals. This method gave two critical con-centrations corresponding to both sides of themask edges, but the two values were close, within20%, and the average of those two values was usedas the critical concentration.As shown in Fig. 6(a), the threshold concentra-

tion based on the method described aboveincreased with growth temperature, indicating thatpolycrystals are less likely to be formed at higher

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press

[Pa]

973 K



0 500 1000 1500 2000

0 500 1000 1500 2000

(a)

(b)

(c)

Fig. 5. Height profile (left axis) of the 2-mm mask after the

growth of GaAs with AsH3 at growth temperatures of 873K

(a), 923K (b) and 973K (c) The total pressure was 1� 104 Pa,

the inlet partial pressure of TMGa was 0.14 Pa, and that of

AsH3 was 7.2 Pa. The profile in each graph is the superposition

of five scans taken at 100mm intervals. Calculated concentra-

tion profiles (right axis) of a film-forming intermediate are also

shown for comparison. Both ends of the horizontal axis

correspond to the mask edges. The intermediate is assumed to

be MMGa. Dotted lines indicate the boundary of the

polycrystal-containing region and the critical concentration

for generation of polycrystals.

(a)

(b)

0

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860 880 900 920 940 960 980

Thr

esho

ldM

MG

acon

c.[P

a] Integrated height [µm2]

Integrated height [µm2]

Temperature [K]

0

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35

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200

250

300

350

400

Thr

esho

ld M

MIn

con

c.[P

a]

860 880 900 920 940 960 980Temperature [K]

Fig. 6. Threshold concentration (J; left axis) for generation of

polycrystals for the growth of GaAs with AsH3 (a) and InP with

PH3 (b) as a function of growth temperature. The growth

condition for GaAs was the same as in Fig. 5. For the growth of

InP, the total pressure was 1� 104 Pa, the inlet partial pressure

of TMIn was 0.96Pa, and that of PH3 was 96 Pa. Integrated

values of height profiles in Fig. 5 (’; right axis) are also shown,

which roughly reflect the number and the size of polycrystals on

the mask.


growth temperatures. Integrated values of heightprofiles in Fig. 5 are also shown, which roughlyreflect the number and the size of polycrystals onthe mask. This value decreased with temperaturecorresponding to the increase in the criticalconcentration. For the growth of InP (Fig. 6(b)),the same trend was observed. This temperaturedependence seems independent of materials. How-ever, the absolute value of the critical concentra-tion was much larger for InP. The absolute value

of the critical concentration for the generation ofpolycrystals is dependent on materials. It may berelated to their bond strength.Classical theory for nucleation assumes a

saturation concentration. The driving force fornucleation is a supersaturation equivalent to thedifference between the precursor concentrationand the saturation value. Normally, the satura-tion concentration increases with temperature.

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.[P

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Thr

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MM

Inco

nc.

[Pa] Inte

grated

he

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[ µm2]

V/III ratio [-]

(a)

(b)

Fig. 7. Threshold concentration (J; left axis) for generation of

polycrystals for the growth of GaAs with AsH3 (a) and InP with

PH3 (b) as a function of V/III ratio with constant partial

pressures of TMGa (0.14Pa) and TMIn (0.96Pa). The total

pressure was 1� 104 Pa. The heater temperature was 873K.

Integrated values of height profiles (’; right axis) are also

shown which roughly reflect the number and the size of

polycrystals on the mask.


Therefore, with the increase in temperature, if theconcentration of the intermediate is kept the same,the supersaturation becomes smaller. This is apossible reason for the temperature dependence ofthe critical concentration. It is also possible thatthe higher critical concentration for InP is due to ahigher saturation concentration of MMIn, con-sidering that the vapor pressure of In is higherthan that of Ga at the same temperature.

3.5. Effect of V/III ratio

Generation of polycrystals can be dependent onthe concentration of group V intermediatesbecause polycrystals are generated from bothgroup III and V elements. For example, it ispossible that the generation rate of GaAs poly-crystals is proportional to the product of theconcentrations of MMGa and As-containingintermediates. Therefore, the effect of V/III ratiowas investigated while keeping the partial pres-sures of TMGa and TMIn constant. By changingthe partial pressures of group V source materials,the concentration of their decomposition productsshould change proportionally.For both GaAs and InP, the trend was not

simple as shown in Fig. 7. For high concentrationof group V sources, with increasing V/III ratio, thethreshold concentration decreases and the inte-grated height, an index of the number and the sizeof polycrystals, increases. This means that poly-crystals are easily generated at high concentrationof group V sources and is consistent with theassumption above. However, for low V/III ratio,the trend becomes reversed. These results indicatethat generation of polycrystals can be reduced bysetting a proper V/III ratio. There must be amechanism for enhanced polycrystal generation atlower V/III ratio, but this is not clear at this stage.This trend should be related to the fact that

GaAs polycrystals were more easily formed whenusing AsH3 than TBAs, as mentioned in Section3.2. Since the decomposition yield of TBAs isdistinctly larger than that of AsH3, the partialpressure of As-containing intermediate should belarger in the growth with TBAs. Considering thepossibility that high concentration of group Vintermediate enhances the generation of polycrys-

tals, the use of TBAs should lead to enhancedgeneration of polycrystals, contrary to the ob-served trend. Detailed investigation is necessary toclarify these complexities.

4. Conclusion

The effect of growth conditions on the genera-tion of polycrystals on SiO2 masks in selective-area

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MOVPE favored the following mechanism: poly-crystals are generated when the gas-phase concen-tration of a film-forming intermediate just abovethe mask exceeds a critical value. This mechanismwas consistent with the observed dependence ofpolycrystal generation on the total pressure, andon the partial pressure of the group III sourcematerial. The width of the polycrystal-free areaaround the narrow growth area formed in thecenter of a mask was also consistent with theproposed mechanism.Based on this mechanism, we evaluated the

critical concentrations for generation of polycrys-tals and investigated their dependence on tem-perature and V/III ratio. The dependence ontemperature was qualitatively explained by con-sidering the temperature dependence of the satura-tion concentration of the intermediate in gas-phase. The absolute value of the critical concen-tration depended on the materials. The effect ofthe V/III ratio was not straightforward andrequires further investigation.In order to design a polycrystals-free mask

pattern for a given material in a given growthcondition, we can simulate the gas-phase concen-tration profile and confirm that the concentrationabove the mask is smaller than the critical

concentration. This concentration, however, mustbe measured for each condition using the testpattern used in this study and the method ofanalysis presented above. If we can formulate thedependence of the critical concentration on growthconditions, such as temperature and the partialpressure of group V sources, we will be able toestimate the critical concentration for a givengrowth condition based on the limited number ofexperimental results. This study is the first steptowards the modeling of the generation of poly-crystals and more profound understanding ofMOVPE process.

References

[1] M. Gibbon, J.P. Stagg, C.G. Cureton, E.J. Thrush, C.J.

Jones, R.E. Mallard, R.E. Pritchard, N. Collis, A. Chew,

Semicond. Sci. Technol. 8 (1993) 998.

[2] H.J. Oh, M. Sugiyama, Y. Nakano, Y. Shimogaki, Jpn. J.

Appl. Phys. 42 (2003) 6284.

[3] K. Hiruma, T. Haga, M. Miyazaki, J. Crystal Growth 102

(1990) 717.

[4] K. Yamaguchi, M. Ogasawara, K. Okamoto, J. Appl. Phys.

72 (1992) 5919.

[5] H.J. Oh, M. Sugiyama, Y. Nakano, Y. Shimogaki, Jpn. J.

Appl. Phys. 42 (2003) L359.

Polycrystals growth on dielectric masks during InP/GaAs selective MOVPE

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