Selective area growth on planar masked InP substrates by metal organic vapour phase epitaxy (MOVPE)

Prog. Crystal Growth and CharacL

( ~

Vol.35, Nos.2-4. pp. 263-288,1997

O 1998 Elsevier Science Ltd. All rights reserved Printed in Great Bdtain 0960-8974/98 $19.00 + 0.00

Pergamon

PII: S 0 9 6 0 - 8 9 7 4 ( 9 8 ) 0 0 0 0 3 - 5

SELECTIVE AREA GROWTH ON PLANAR MASKED InP SUBSTRATES BY METAL ORGANIC VAPOUR PHASE EPITAXY (MOVPE) Tom Van Caenegem, Ingrid Moerman and Piet Demeester Department of InformationTechnology(INTEC), Universityof Gent-IMEC, Sint-Pietersnieuwstraat 41, B-g000 Gent, Belgium Introduction The increasing demand for monolithic integration of electrical and optical devices has led to the development of advanced growth techniques which facilitates to restrict growth to laterally defined regions, thereby avoiding complicated etching and regrowth steps. Lateral control of thickness and composition during Metal Organic Vapour Phase Epitaxy (MOVPE) growth can be achieved using a variety of techniques : shadow masked growth (SMG) [1,2], laser assisted selective growth [3,4], growth on pre-patterned substrates by wet or dry etching [5], selective area Atomic Layer Epitaxy (ALE) [6], selective growth and phase selective growth (PSG) on ion implanted substrates [7] and selective area growth (SAG). SAG, which restricts growth in windows etched in dielectric masks, is now believed to be the most promising candidate for the mass production of InP based Photonic Integrated Circuits (PICs) for long haul optical fibre communication. In this article we will focus on the MOVPE SAG on InP substrates, restricting the discussion to growth of InxGa ~.xAs~.yPymaterial. The different aspects of SAG by MOVPE will be outlined : first the principle of SAG will be given, followed by a discussion about selectivity. If relevant we will refer throughout this article to the SAG characteristics encountered with the Metal Organic Molecular Beam Epitaxy (MOMBE) growth technique. The mechanism behind the MOVPE SAG will be explained, including the presentation of a simple mathematical model. We will also focus in separate sections on growth rate enhancements, compositional changes and the geometry of selectively deposited structures. In the last section several applications of SAG in the photonic domain are given, with the good performances of the devices proving the excellent quality of selectively grown material.

Principle of masked non-planar Selective Area Growth Prior to SAG, for most applications, pairs of dielectric (SiO2, SIN,, ..) stripes are defined on the InP substrate surface by means of a conventional lithographic process. The nucleation properties for the growth species on the dielectric mask and semiconductor surface are very much different, favouring the selective growth in the open areas between the dielectric stripes. In addition to the concentration gradient of the growth species normal to the sample surface that is present in the diffusion controlled growth regime of MOVPE, there is also a lateral concentration gradient in the vapour phase which is due to the incorporation of the reagents exclusively in the mask opening (Fig. 1). This causes an enhanced growth rate in the mask opening which becomes even more pronounced by increasing the width of the dielectric mask or by decreasing the opening stripe width.

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Furthermore, in the case o f an alloy (InGaAs or InGaAsP) a compositional change is observed as compared to growth in a region far from any masks. The compositional change can be explained by a difference in the effective diffusion lengths for the different group III metal organic growth species. Surface diffusion of the growth species over the mask and semiconductor surfaces is now believed to play a minor role in the SAG induced changes to the growth rate and composition. However, the semiconductor surface diffusion (taking place over only several microns) does significantly influence the cross-sectional shape of the selectively grown structures. They can be bounded, depending upon the stripe orientation, by well defined crystallographic planes, as schematically depicted in Fig. 1.

[111 Dielectric Mask Fig. 1 Principle of Selective Area Growth

Selectivity

Whenever the SAG technique is applied, growth on the dielectric mask, which occurs as observed as polycrystalline particles [8], should always be avoided. It can hinder not only postprocessings, but will also lead to a low reproducibility and controllability. To obtain complete selectivity (this means no growth takes place on the dielectric mask), the conditions have to be selected such that the growth process is determined by the nature of the surface.

To analyse the mechanisms which influence selectivity, and more generally SAG itself, comparison of the MOVPE growth process with the other two major growth techniques, MOMBE and molecular beam epitaxy (MBE), is required. For a distinct understanding, the three techniques will be very briefly described here. For a more general treatment cf. [9,10].

Molecular Beam Epitaxy (MBE) During MBE beams of evaporated group lIl atoms (Ga and In) impinge directly on the substrate in an ultra high vacuum (UHV) environment and stick to the surface while the group V dimers or tetramers are adsorbed onto the substrate, migrate on the surface and eventually dissociate, thus forming V/III compounds. Phosphorous containing materials are in general not grown by MBE due to technical problems associated with the high vapour pressure of the phosphorous source.

Selective Area Growth on Planar M a s k e d InP Substrates

Metal Organic VapourPhase Epitaxy (MOVPE) The group III component is transported to the substrate surface by means of an organic precursor (e.g. TMI (Trimethylindium), TMG (TrimethylgaUium),..). The carner for the group V component is a hydride (e.g. PH3, AsH~ ). The compounds are well mixed when they enter the reactor. Usually a VfllI ratio >> 1 is used. The growth precursors can reach the substrate after passing through a gas boundary layer. Near the heated substrate the precursors are pyrolysed. Choice of a growth temperature in the region 550°-751Y' C is mandatory such that the growth is controlled by mass transport of the group III precursors.

Metal Organic Molecular Beam Epitaxy (MOMBE) OrganometaUie group III elements and elemental group are injected into a UHV system. Because of the low background pressure there is no boundary layer or collisions as in MOVPE. After reaching the heated substrate the group III alkyl dissociates to leave a free group III (metal) atom on the surface. Group V elements can then react readily with the surface group III atoms.

The three different processes are schematically depicted in Fig. 2.

e e e o o e o e e e o o o e

•o *vo ovo ovo ovolo ovovo ov v v v v v Group III atomic beam

MBE

~IvvvvVVVvVVvV

UItV i

MOMBE

/

l

l

Subs~'ate

~'¢ ~ y ~ , X ~(A~ ~V Group III MO beam

Subst~te

MOVPE

Partially pyrolised group III MO's Gas boundary layer Substrate

Fig.2 Schematic representations of growth kinetics of MBE, MOMBE and MOVPE (D. Wood [10])

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Selectivity is very hard to obtain with MBE, which means that metal atoms are equally well adsorbed on dielectric as on semiconductor substrate surfaces. There is, however, a substantial difference concerning the nature of dielectric and substrate surfaces in the case of MOVPE, where we do not deal so much with metal atoms, but rather with metalorganics. The semiconductor surface acts here as a catalytic element in the pyrolysis of the metalorganic precursors whereas the dielectric surface remains rather inert. Therefore, it should be clear that the arrival o f elemental group III species (Indium and Gallinm) at the surface has to be avoided as much as possible to achieve full selective growth by MOVPE [11] (for MOMBE this condition is automatically fulfilled due to the nature of the process). So the pyrolysis of the organometallic group III species has to take place as much as possible near or on the substrate surface, to reduce the number of elemental group IIl species reaching the dielectric surface. This means that gas phase reactions should be minimised. To achieve this, several actions can be taken :

The most straight forward method for limiting the gas phase reactions is lowering the pressure and/or choosing a higher gas velocity which shortens the transit time of the growth species in the reactor [12]. In addition, a lower flux of group III precursors and hence a lower growth rate will also induce a higher selectivity [ 11 ].

Another way to prevent gas phase reactions is by choosing a more stable metalorganic precursor. In [13] Eckel compares the selective area growth of lnGaAs where the In-precursor was either the common TMI or DADI (DimethylAminopropylDymethyllndium). The nucleation density on the dielectric mask for the DADI-grown InGaAs stripes was lower than the density for the TMI-grown stripes. This was attributed to a faster decomposition of the less stable TMI and its stronger tendency tO undergo parasitic gas phase reactions. Similar tendencies have been reported when growing InP with either TEI (TriEthyllndium) or TMI [14]. Enhanced selectivity was obtained with the more stable TMI.

Furthermore choice of a precursor that leads to a decomposition product that provides a suitable back-reaction to the deposition reaction on the mask surface might enhance selectivity. In the case of the growth o f GaAs such a precursor has been found in DiEthylGaUiumChloride [15], since the GaCI has a high volatility, reasonable stability and a high adsorption probability on the exposed substrate surface. Due to its high volatility, the GaC1 exhibits a low probability of adsorption on the mask, and as a result a supersaturation sufficient to nucleate polycrystalline GaAs can never be achieved. The presence of HC1 which forms as a growth reaction by-product, would additionally serve to etch the GaAs nuclei from the mask. Polycrystalline growth on the mask can also be inhibited by inserting an extra chlorine compound to the reactor chamber besides the growth precursors. Carbon tetrachloride (CC14) is such a compound. Perfect selectivity for InP growth was obtained using CC14even on wafers with a mask ,:overage factor of 96 %. [16,17]

The presence of elemental group V species can also eliminate the selectivity as they are presumed to enhance the deposition of In and Ga containing species [3,11]. Since elemental group V species are created only at the substrate surfaces in conventional MOVPE systems [18] there is no effect of the Will ratio on the selectivity.

Selective Area Growth on Planar M a s k e d InP Substrates

For MOMBE, however, where group V dimers or tetramers are cracked in a pyrolysis furnace before they enter the reactor the V/III ratio has a significant influence on the selectivity and has therefore to be kept below a critical value depending upon other reactor conditions.

A higher temperature will also enlarge the selectivity as more adsorbed particles on the mask will desorb with increasing temperature.

The mask pattern itself can also significantly influence the selectivity [3,19]. One aspect of the mask pattern is the filling factor which is defined as the ratio of the unmasked area and the total area ~. The reagent partial pressure (or reactant concentration) above the mask surface increases with decreasing filling factor. Deposition on the mask surface will occur once the partial pressure -corresponding to a certain mask width- reaches the value needed to form stable nuclei for the polycrystalline deposition. However, when the dielectric stripes are sufficiently small, the lateral concentration gradients will prevent any nuclei from becoming stable even though the partial pressure above part of the mask exceeds the value needed to form stable nuclei.

Selective Growth Mechanism

In all MOVPE SAG experiments an enhanced growth rate is observed for the material deposited between or near the dielectric masks when compared to the growth in a region far from any mask. This growth rate increases with increasing width of the mask. This enhanced thickness is predominantly caused by lateral vapour phase diffusion. It is now believed that surface migration of growth species over the dielectric masks does not take place and hence does not contribute to the growth enhancement.

A lot of experiments seem to conf'm'n this latter statement. Gibbon et al. [20] performed a growth experh-nent on wafers m which deep trenches (width 4 ~rn, depth 55 ttm) were etched in the vicinity of pairs of dielecu-ic stripes. In the situation where lateral diffusion takes place predominantly in the gas above the wafer, the trench should only have a small effect on the profile of excess thickness. In the case where surface migration dominates however, the precursors would have to migrate over an extra 120 ~tm of semiconductor material. This would severely disturb the growth profile as measured compared to a profile with no groove present because of the depletion of the precursors by deposition on the groove walls. No significant changes were observed in the profile as can be seen in Fig. 3. A similar experiment was carried out by Sasaki et al. [21] with the same conclusion.

IA more detaileddescriptionof the filling factorcan be found in [31]

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Oxide Masks

Subs~te

curve B

'~/~ 55~nI Deep Trench

Fig. 3 Trench experiment of Gibbon et al. : curve A : vapour phase diffusion dominates curve B : surface diffusion dominates

Further indication that the enhanced growth rate (EGR) in SAG is due to lateral vapour phase diffusion is given by SAG experiments performed by MOMBE where vapour phase diffusion reactions are absent. No enhanced growth rates on partially masked substrates are observed, so that one can conclude that reactants adsorbed on the mask apparently leave the mask by reevaporation instead of migrating towards the open areas [11,22]. Note however that MOMBE growth takes place at a significantly lower growth temperature (100-150 °C lower) when compared to MOVPE.

Other evidence for vapour phase diffusion being the predominant mechanism can also be found in modelling calculations. Since the reactions and gas flows inside a MOVPE reactor are highly complicated and even still not fully understood, it is difficult to develop a mathematical model that forms a complete description of the SAG process. In several papers however, experimental data are compared to mathematical solutions obtained by solving a simple two dimensional gas phase diffusion model. Ida et al. [23] made use of this model and the agreement between experiment and calculated values was fairly good. Also Gibbon et al. [20] found agreement between experiment and theory to within the error of measurement for plots of normalised thicknesses and compositional changes for InGaAsP material as a function of mask and gap width.

We give here a description of the gas phase diffusion model as it is found in [20,23,24] because it can offer some additional insight into the SAG mechanism. Inside the boundary layer of the gas flow above the substrate, a window of width w and height h in which the significant transport processes take place, is defined as depicted in Fig. 4. The actual value of the height h is not important as long as a change of this value does not influence the solutions. Also we limit the calculations to two dimensions what implies that the stripes should be infinitely long. The profiles of the concentrations for the In and Ga species above the wafer are solved independently from each other using Laplace's equation:

Df d 2C+ d 2CI ~dx 2 dy~J =0

Selective Area Growth on Planar M a s k e d InP Substrates with D being the vapour phase diffusion coefficient.

The following boundary conditions apply :

•

above the boundary layer we have the carrier gas stream acting as a material reservoir, such that a constant concentration at the upper border is maintained :

c = co •

the vertical boundaries (x = O, x = w) are a centre of symmetry, such that no lateral diffusion takes place:

OC

- - = 0

Fx

s

at the lower border of the window, the boundary condition over the dielectric mask is given by (using Fick's law and the fact that there is no deposition on the mask) :

3C Oy

s

=0

the boundary condition on the semiconductor surface reads (assuming Langmuir absorption [9]):

D OC =kC Oy

with D being the diffusion coefficient and k the absorption rate.

C=C o 14/

oLo,~+o~):o (0 2C 0 2C

O o c •kC

ay

h

._

r

o c =o Ox Centre of symmetry

~Y I

X ~

oc Ox =0 Centre of symmetry

Fig.4 Schematic representation of the simple 2D vapour phase diffusion model

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The equations can be solved numerically using the f'mite difference method. Since we are only interested in normalised growth rates (the growth rate being proportional to D. 6C/5y), the value of COis adjusted to give a growth rate of unity in the unperturbated region far from the mask. The ratio of the diffusion coefficient to the absorption rate D/k is then used as a fitting parameter. In Fig. 5 solutions o f the 2D gas phase model fitted with different D/k values are presented. As one can see, the curves with the lowest D/k values show the highest growth rate enhancements but at the same time they also exhibit the largest decay length. With the fitting parameters D/k for the In and Ga precursors, growth rate and composition can be predicted for different mask dimensions (see next sections). Normalised excess

thickness

1i 0.1

~ ....... D~=20 gm 0.01

Mask

L 50

]. . . . . . . . . . . . . . . . . 100 Distance from centre (/.tin)

Fig. 5 Computed profiles of normalised excess thickness for different values of D/k (h and w (Fig. 4) were chosen 300 mm in the simulation model) (after Gibbon et al. [20])

This simple 2D diffusion model makes several assumptions [20], i.e. the fact that there are no forced convection effects and that the diffusion coefficient has a constant value in the whole window (which implies a constant temperature in the boundary layer). Analytical solutions of this model were presented by Zybura et al. [25] and Korgel and Hicks [26].

Fujii et al. [27,28] developed an equation simulating the SAG including the growth pressure dependence. The model revealed that three parameters dominate the growth rate distribution : diffusion length on the epilayer, diffusion length on the mask and the lifetime ratio for the mask and the epilayer regarding desorption and solidification. The lateral diffusion constants def'med at the epilayer and mask surfaces were found experimentally to be inversely proportional to the growth pressure, which is considered as a direct evidence that vapour phase diffusion dominates selective area MOVPE. They further showed that the growth rate enhancement induced by mask patterning increased and saturated as the growth pressure increased. This was also predicted by their theoretical model ifa growing probability on the mask surface different from zero was assumed. This should be interpreted as a chemical reaction taking place at the mask surface that produces non-

Selective Area Growth on Planar Masked InP Substrates

reactive species. Furthermore, the stighfly different sticking coefficient at the mask surface (0M) for the In and Ga source materials participating in this reaction, together with a larger escape probability of desorbed Ga source materials from the mask surface to the bulk of the gas flow, explained quantitatively the compositional variation of InGaAs in selective area MOVPE. A schematic representation of the chemical reaction taking place at the mask surface yielding the finite 0 M,as proposed by Fujii, is depicted in Fig. 6.

Other models were worked out by Coronell and Jensen [29] (this model takes into account also surface migration) and YU.N.Makarov et al [30].

*

( :' L;

Hydride

~) ~" )

Organic )1) I~Metal

'() ""

tD

~

"\ (,

;~

"•

T

]

Fig. 6 Chemical reaction model showing that decomposed metalorganic source materials react with hydride group V species terminating the mask surface (introducing a finite sticking coefficient on the mask surface) forming non-reactive species (after Fujii et al [27,28])

Although it is now clear that vapour phase diffusion is the main mechanism behind the observed growth rate changes for SAG, surface diffusion of growth species on the facets that bound the selectively deposited structures, also takes place and can even have a significant effect on growth rate, composition and overall geometry of small (< 3 gm) SAG structures. This will be discussed in more detail in the section geometrical

considerations.

Growth rate changes

Under 'standard' MOVPE conditions [9] the deposition process for InP (lnGaAsP) is determined by the transport of the group III precursors to the surface. This also holds for the selective area growth as indicated by the linear dependence of the growth rate oflnP on the TMI pressure [12]. However, the growth rate for selective growth can be expected to be higher when compared to the planar growth on unpattemed samples taking place under identical growth conditions (growth pressure, growth temperature, group III partial pressure(s), V/Ill ratio), due to the additional lateral supply from regions above the masks. This additional supply can be significantly altered by changing the mask geometry. The growth rate enhancement, defmed as the ratio of the thickness of the selectively grown layer measured in the centre of the stripes and the layer thickness in the unperturbated area, over a wide pressure range increases

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T. V a n G a e n e g e m e t a / . linearly with increasing mask width as observed by a variety of research groups. The growth rates also generally increase if the gap width decreases, since the laterally supplied material is distributed over a smaller area where epitaxial growth can take place (Fig. 7). As already discussed in section Selective Growth Mechanism the fitting parameters D/k for the In and Ga precursors obtained in the 2D diffusion model can serve to predict the growth rate enhancement for a given compound : for any mask geometry. Galeuchet [31,32] derived for the growth rate enhancement versus filling factor F, defined as the percentage of the unmasked area with respect to the total area, an upper limit of IO0/F which represents the relative growth rate enhancement assuming (i) an identical sticking coefficient of the growth species on the mask and in the open stripe and (ii) that all adsorbed species on the mask are transported to the stripe. He found that for F < 15 % a strong deviation from the 100/F curve is observed and the relative growth rate enhancement eventually saturates for F< 10 %. He concluded that for such low filling factors (narrow growth stripes, large mask surfaces) the supply of growth species to the open stripe is so strong that the reaction kinetics on the (001) surfaces start to become dominant. This reduces the lateral concentration gradients of the unreacted species and enhances the local supersaturation of growth species on the dielectric masks eventually leading to the spontaneous formation of crystaUites. The growth rate enhancement is also related to the material composition and decreases with increasing Ga content. [20,33]. This might be related to the lower decomposition temperature of TMI that is commonly used as Indium precursor, as suggested by Caneau [34]. Consequently, due to the higher diffusion coefficient of decomposed fragments, more In than Ga species can reach the mask opening (see also section compositional changes). Another parameter affecting the selective growth rates is the reactor pressure : decreasing the pressure yields a lower growth rate enhancement. In addition, when the growth pressure reaches values below 5 Tort, no enhanced growth rates can be observed any more. This means that the mask dimensions do not affect anymore the growth pattern, which is precisely observed with MOMBE selective growth [11]. This is explained by the increased vapour phase diffusion length with decreasing reactor pressure resulting in the distribution o f excess material over a much larger area which enhances the chance that growth species reevaporated from the mask are leaving the stagnant layer [35]. As can be expected from this explanation, the planar growth rate on unpattemed substrates is not affected by pressure changes.

1."

A30mtm~mpp mls m ~ t p

0.

1° 0.

Mask wtdth (microR)

Fig. 7 Growth rate enhancements of SAG of InP for various mask and gap widths measured in the centre of the gap (growth pressure 150 Torr)(after Gibbon et al.) 2 the D/k values for the differentgroup Ill precursorschangewith changingalloy compositionas is explainedin sectioncompositional changes

Selective Area Growth on Planar Masked InP Substrates

Compositionalchanges Depending upon the application of the SAG a relatively large compositional change in the selectively deposited layers might be required (e.g, active-passive integration) or on the contrary has to be avoided, because this compositional variation can create non-homogeneous material properties and in this way decrease the efficiency or performance of the final device. All SAG experiments revealed an In enrichment (and hence compressive strain) for selectively grown InGaAs and InGaAsP layers. This In enrichment disappears gradually when moving away from the masks towards the open unmasked regions. This can again be attributed to vapour phase effects - and more specifically the different behaviour of the dissimilar group III precursors in the gas phase - since no compositional change is observed in selectively grown layers by MOMBE.

To explain the In enrichment for selectively grown InGaAs and InGaP layers, Caneau et a1.[34] first studied the selective growth of the different binaries : InP, GaAs, InAs and GaP. She found the following order for their growth rate enhancements : R(InAs) > R(InP) > R(GaAs) > R(GaP). She explained this by taking the decomposition temperatures of the various precursors that were used in the growth experiments into account : In [9] it can be found that the 50 % decomposition temperature in a H2 ambient for TMI is around 320 °C and approximately 450 °C for TMG. The consequences of the decomposition of the TMI molecule at a lower temperature compared to TMG are twofold : it leads to a greater proportion of decomposed species, which are smaller in size than the parent, and hence to a higher 'effective' diffusion coefficient of the species 3. It also allows the decomposition of molecules at a greater (vertical) distance from the mask, where a lower temperature exists than close to the mask. To explain the difference in growth rate enhancement between InAs and InP, and between GaAs and GaP, she referred to the decomposition temperatures for the two hydrides, the decomposition temperature for AsH 3 being lower than the one for PH3 (50 % decomposition temperature in H2 ambient for PH 3 amounts 850 °C, for AsH 3 600 °C [9] ). She assumed a larger number of reactive species (H, AsH~) in the case of AsH~ decomposition, enhancing the decomposition of both TMI and TMG as compared to PH 3. For growth on unpattemed substrates thickness and composition of both InGaAs and InGaP from the data of the binaries can easily be calculated. Selective epitaxy of InGaAs and InGaP could however not simply be related to the studies done for the corresponding binaries. Therefore, she postulated that in the high temperature region above the mask, decomposition of TMI is the strongest driving force for the decomposition of AsH3, and these decomposition products in turn trigger the decomposition of TMG ( there is no direct interaction between metal alkyl species [9] ). This is the reason for instance why the growth rate (as a function of distance to the oxide mask) of the InAs fraction of selectively grown InGaAs (InAst,c,~,) is smaller than the growth rate of binary InAs. TMI, whose 3 The diffusioncoefficientfor bimolcculargas phase diffusionis inverselyproportionalto the squareof the mean collisiondiameterof the two molecules

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T. V a n C a e n e g e r n e t al. decomposition starts at the lowest temperature of the three reagents present, yields reactive species which play an important role in the decomposition of AsH~, whose reactive decomposition fragments in turn can interact with, and provoke the decomposition of TMG and TMI molecule.,;. Since, in the case of selectively grown InGaAs, the AsH3 fragments will interact with both TMG and TMI, the latter will decompose slightly less than in the case when it is the only metal alkyl present in the gas phase with AsH~ (as is the case with InAs).

Kim et al. [33] also refer to the relatively low decomposition temperature of TMl to explain the observed Indium enrichments. The higher number of mono and di methyl In species, as compared to the gallium species, gives rise to a higher effective diffusion coefficient for In species. This difference in diffusion also takes place in the case of planar growth but it is compensated by adjusting the input flows to give layers of a predetermined composition. However, in the case of SAG the partially decomposed molecules spend much more time in the hot zone near the substrate due to the repeating mechanism of impinging on and desorbing from the mask until they strike the open growth areas. It is during this extra amount of time that more decomposition of TMl as compared to TMG could occur than can be accounted for in the case of planar substrates, resulting in a higher average diffusion coefficient for In species.

Other research groups do not agree with the higher diffusion coefficients for In species as an explanation for the In enrichment. Their arguments are based upon the outcome of the two dimensional gas phase diffusion model. Indeed, best fittings to the experimental data are obtained with D/k (ratio of the diffusion coefficient and the incorporation rate) values for the indium and gallium species where D/k ~,a~u,,< D/k ~li~,, [20,23,24].The absolute value of the D/k value depends very much upon the growth conditions so that it makes little sense to compare the different values given in the different papers.

Gibbon et al. [20] assumed equal diffusion coefficients for the In and the Ga species, and hence explained the In enhancement for SAG by different incorporation rates for the diffi~tent group III elements. Best fittings were obtained with DIn=DGa for kGa/kln = O.14 (growth at 650 °C, 150 Torr, with TMG, TMI and ASH3). They found that a ratio less than one is consistent with the fact that the methyl-gallium bond is stronger than the methyl-indium bond [9]. This would indicate that the energy required to break a metal-methyl bond may be the rate limiting step in the adsorption process.

Ida et al. [23] interpreted the D/k value as the horizontal diffusion length of the source molecules since the growth profile versus distance to the mask edge nearly behaves exponentially (see Fig. 4). With the selective growth of InGaAs at 400°C and 550 °C at 30 Torr, using the precnrsors TMI, TEG and AsH~, Ida et al. also found a higher D/k value for TEG than for TMI. TEG being larger in size than TMI, and both molecules having a similar decomposition temperature, they suspected that the smaller k for TEG is the reason for the longer D/k value. The GaAs growth mode at 400 °C, however, was situated in the kinetically limited region, while the IrtAs growth mode was still in the mass transport limited region.

Selective Area Growth on Planar Masked InP Substrates

Based upon the 2D diffusion model, we can further expect that an enhanced composition uniformity can be obtained by choosing precursors for In and Ga which have comparable D/k coefficients. This was experimentally confu-med by Schoiz et al. [13,24] by growing InGaAs (growth temperature 640 *C, growth pressure 20 hPa) with either TEG and TMI or "lEG and DADI (DimethylAminopropylDymethylindium) as precursors. In the latter ease improved composition uniformity was obtained (s¢¢ Fig. 8), as was expected from the relative values of the D/k fitting parameters : (D/k) TMI = 30 i- 5 / a n < (D/k)DAD I = 120 :~ 1 5 / a n < (D/k) TEG = 280 :t 3 0 / a n . The observation that TEG appears to be chemically more stable than DADI could be

due to the fact that the decomposition of TEG yields C~H, [9]. This non-reactive decomposition product is expected not to trigger the decomposition of any other molecules [36].

0,49 0.47 0.45 0.43

• TMI/TEG • DADI/rEG

0.41 0.39 0.37 0.35 0

20

40

60

80

100

Rltlrt~l factor (%)

Fig. 8 Compositional change for InGaAs when using either TMI/TEG or DADI/TEG as group III growth precursors (after Eekel et al. [13])

In a very particular case it is however possible to obtain a Gallium enrichment with MOVPE SAG as demonstrated by Galeuchet [31]. This is the case for the selective deposition of very small structures where the transport of excess growth species is mainly controlled by the surface diffusion on the (I 11) facets forming the boundary of the SAG structure, rather than vapour phase diffusion (see section Geometn'cal considerations). Selective growth on a 2 I~m wide stripe (mask widths approximately 3.3 ~tm) of a 300 nm thick lnGaAs layer sandwiched between InP layers, resulted in an 5.2 % Ga enrichment as compared to the planar grown latticematched composition. This was explained by assuming a higher diffusion coefficient for gallium containing molecules on (111) InGaAs facets than for those containing indium.

No compositional change has yet been observed with respect to the group V components of the selectively deposited alloys. This is probably related to the high V/III ratio used during MOVPE. Since only a small amount

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T. Van C a e n e g e m e t al. of group V species is consumed as compared to the high quantity supplied the different behaviour between the As and the P species is insignificant [33]. Only Silvestre et al. [57] reported an arsenic enrichment for selectively grown quaternary material with an energy bandgap corresponding to a wavelength of 1.3 txm ( Q1.3 material ). However, this was deduced from indirect measurements assuming that the In precursors behave in an identical way during the SAG of lnGaAs and InGaAsP. This though is experimentally contradicted by Cancan [34]. Only in the case of the SAG of InGaAs and In]?, the movement o f l n is similar (also confirmed by Gibbon et al. [20] ).

Geometrical

considerations

The fact that selectively deposited structures are bounded by facets reflects the difference in growth kinetics for the different crystallographic planes involved. Each crystal plane has a different bonding situation and defect structure. The planar growth rate on differently oriented substrates is however identical because different steady state concentrations of the reactants are built up on the different sUbstrate surfaces in the supply limited growth regime. The same applies for selective area growth but the different steady state concentrations that are present on the adjacent facets of sidewalls and top surface will now lead tO the migration of the reactants from planes covered with a higher reactant concentration (slow growing planeS) to the faster growing planes. The migration length of the reactants on the different surfaces, and therefore also the geometry of the selectively grown structures, will thus be determined by the growth conditions [11]. In [37] Sugiura et al proposed a model of dangling surface phosphor atoms to explain the facet growth of InP with MOMBE SAG. By assuming that the growing surface is always covered with group V atoms providing dangling bonds to combine with metalorganic molecules, he assigned a relative dangling bond density per unit area to each substrate orientation (see Fig. 9) : 1.73 for a (11 l)A surface, 1 for (001) and 0.58 for (11 l)B.

(111)B

(001)

!

(111)A

Fig.9 Difference in dangling bonds of phosphorus atoms chemically adsorbed on (111)A, (100) and (111)B, explaining the facet growth during SAG (model of Sugiura et al. [37])

Selective A r e a Growth on Planar M a s k e d InP Substrates

277

Group III reactants will tend to migrate from planes with lower dangling bond density (higher steady state concentration of group III reactants) to planes with higher dangling bond density. This is depicted in Fig. 10 for the situation where the mask stripes are oriented along the [110] direction (on (001) substrates). If the migration length along the (111)B sidewalls is long enough (exceeds the length of the sidewall), no growth on the (111 )B planes will occur. With MOVPE this migration length on the (111)B plane can be directly controlled by changing the V/III ratio. Choosing a higher PH 3 concentration suppresses the migration of the reactants and introduces growth on the (111)B planes. Kayser [11] suggests that the higher PH 3 pressure creates an increased concentration of In vacancies - or a higher density of phosphorous dangling bonds if the model of Sugiura also holds for MOVPE conditions - and hence reduces the migration length. Growth on the (11 I)B planes was also observed by Kayser when the growth temperature was decreased since increasing the temperature enhances the reevaporation of adsorbed phosphorous atoms, which in turn decreases the total number of dangling bonds. Kayser found for his growth conditions ( pressure 20 hPa, growth temperature 610 °C, PH~ pressure 60 Pa) a migration length of at least 2 microns for the growth of lnP. However, in the case where the stripe direction is parallel to the [1!0] direction, the facets involved are (111)A planes and a (001) top surface plane, yielding growth on the sidewalls due to the much shorter migration length on a (111 )A plane as compared to the ( 111 )B plane. Galeuchet [31 ] also studied the growth behaviour of (111) facets with SAG MOVPE and concluded that the (11 I)A InP planes seem to be "passive" surfaces which inhibit or suppress the supply of material towards (001), in particular for growth of InGaAs on a InP buffer layer, whereas (111 )B planes appear to be "active" surfaces which supply growth species toward (001). On the other hand, he also observed that (111)A InGaAs facets do supply growth species towards (001).

[OOl]

[ll

MO Molecule decomposed MO Molecule

[llOl

c~ •

c)

c)

/~L

I

101am migration length on (001) surface

Fig. 10 Occurrence o f ( l 11)B facets for SAG stripes parallel to [110]

Growth species migrating along the sidewall create an edge growth spike where the top surface and the sidewalls meet due to the modest value of the migration length on the (001) top surfaces (larger dangling bond density).

T. Van C a e n e g e m et aL

278

These edge spikes can be avoided if the structures (mask openings) are sufficiently small. Sasaki et al. and Kudo et al. studied the flatness for narrow stripe ( 2 om or less) seieetive MOVPE of InP, InGaAsP and also InGaAs. Sasaki [21] observed a larger growth edge spike for InGaAsP than for InP whereas no excess growth was observed for InGaAs. This was attributed to the large binding energy between Ga-P atoms and hence Ga containing species that migrated are thought to combine easily with surface P atoms. They also found that the migration length could be enhanced by decreasing the concentration of the group III species. Kudo et al. [38] were able to stake out an area on the graph of the V/Ill ratio versus growth temperature where flat interfaces could be simultaneously obtained for QI.26 and Q1.46 materials. In the low temperature region no fiat interfaces could be achieved for Q1.26 material (relative high P content). This was attributed to the large binding energy of Ga-P. The catalytic effect of AsH 3 on the decomposition of the group III species [34] and hence the easier incorporation with group V atoms on the surface explained the non-fiat surface in the case of QI.46 material. In addition, a growth temperature that is too high also enhances the decomposition of the group III species and the AsH 3 too much. Consequently, optimum growth conditions could be selected (growth temperature around 635 °C, V/III ratio < 400), resulting in very fiat surfaces for both materials. The surface flatness of small SAG structures was also studied by In Kim et al. [39] using TBA (TertiaryButylArsine) and TBP (TertiaryButylPhosphine) as group V precursors4. They observed that reduction of the growth rate and of the V/Ill ratio led to flatter surfaces of quaternary SAG material. "[he surface flatness of SAG grown quaternary layers was also found to be dependent on the V/Ill ratio at which the underlying InP buffer layer was grown. A relatively high V/Ill ratio had to be used during growth of this buffer, to avoid surface fluctuation o f the overgrown InGaAsP. As already mentioned, no significant growth rate differences can be found for the planar growth oflnP and InAs on differently oriented substrates. GaAs, however, behaves rather differently. The growth rate on (11 I)A substrates is much lower compared to growth on (100) substrates [11]. This difference in reaction rate (inducing a very large migration length on the (111)A planes) will prevent any growth of InGaAs on the InP (111) sidewalls during SAG and this provides a simple way to fabricate buried lnGaAs structures by single step epitaxy [31 ](see section applications).

Material quality and characterisation techniques In order to evaluate the quality of selectively grown layers, one has to be able to assess such parameters as composition, bandgap, thickness,.., m regions as narrow as only a few micrometers. So it is necessary to make use of material characterisation techniques that have sufficient spatial resolution. The different characterisatiun techniques that can be used, are : Surface Profiling, Scanning Electron Microscope (SEM) (growth rate measurements); Spatially Resolved Photoluminescence (SRPL) System, Cathodoluminescence (CL) measurements (optical quality assessments); Secondary Ion Mass Spectroscopy (SIMS), micro-Auger Electron Spectroscopy (p_ -AES), Raman Spectroscopy [41] (characterisation of composition). The increasing popularity 4 TBA and TBP are muchless toxicthan the hydridesAsH3and PH3and also showa higherdecompositionefficiency

Selective Area Growth on Planar Masked InP Substrates

279

of SAG even led to the development of a novel in situ optical monitoring method with which it is possible to characterise the growth rate as a function of time [40]. The compositional variations and growth rate enhancements during SAG can seriously affect the quality of the selectively grown material. For large ratios of mask width and open stripe width (large filling factors) serious lattice mismatch might develop resulting in relaxed layers. Also growth spikes could occur at the mask edges as previously described, making further photolithography difficult. These thickness and composition variations can be foreseen and corrected by choosing the appropriate growth conditions and mask dimensions. Even when masked areas of different dimensions are present on the substrate, the quality of the SAG material can still be guaranteed, as demonstrated by Suzuki et al., who extensively studied the optical quality of selectively grown MQW (Multi Quantum Well) material [42]. They measured the growth rates (Rg) and compressive strain (c) of selectively grown InGaAsfin_P MQW layers (measurements performed in the centre of the open stripes) as a function of mask width. A linear dependence for both parameters was found and hence they introduced the index

dc/ARg which expresses the degree of SAG induced strain caused by growth rate

enhancement. Ac/dRg was clearly found to be dependent on the open stripe width (Wg) : a value

ofAe/dRg =

0.37 % for Wg = 2 ~m arose whereas a value of 0.25 % was obtained for W"B= 10 ~tm. The (better) lower value

of Ae/dRg for the wider open stripe was explained by the fact that vapour phase diffusion was in this case the dominant mechanism ruling the SAG whilst for a narrow stripe surface diffusion effects start to play a dominant role. The high ds/dRg for narrow open stripes was reflected in the PL measurements. The MQW luminescence efficiency (PL intensity) for Wg = 2 ~tm reduced drastically with increase of mask width (or dEg). For the optimum Wg = 10 pm, a dEg of 253 meV was attained with high luminescence efficiency throughout the tuning range and with a low FWHM of 27-28 meV at room temperature. Besides the potential optical quality degradation due to the SAG, evidence was also found by Thompson et al. that the deposition of SiO: masks can result in low levels of oxygen contamination of the underlying material. It was also shown that the mask itself(e.g, thermal stresses induced by the comer of the mask during the growth process) can induce dislocations in the growing material which are driven back into the material underlying the mask [43]. In the same publication it was also reported that optical loss measurements obtained from selectively grown InP/InGaAsP (Q 1.15)/InP gave results (<3 dB/cm) similar to those obtained for planar grown material. The electrical quality of selectively grown InP was assessed by Kayser. Relatively high mobilities were obtained for intentionally doped selectively grown In-P structures, showing that the selectively grown material is basically of good electrical quality [ 12]. The excellent performances of complex integrated devices realised by means of the MOVPE SAG technology are, nevertheless, the most clear indication of the good material quality of SAG grown structures (see section

Applications).

280

T. Van C a e n e g e m et al.

Applications The greatest advantage offered by MOVPE selective area growth is the in-plane bandgap energy control. This is reflected in the increasing number of publications on active-passive integrated devices. Indeed, the ability to create enhanced growth rates on locally defined areas in combination with the use of quantum wells makes selective area growth by MOVPE an excellent tool to fabricate photonic integrated devices. The quantum wells are grown thicker in the masked region as compared to the unmasked area. This introduces a lower quantum confined energy bandgap Eg in the masked region. The additional compositional change obtained by MOVPE SAG even increases this bandgap shift to values high enough to make the non-selectively grown (passive) material transparent for (laser) light created and amplified in the selectively grown region. Joyner et al. [44] reported bandgap shifts of 136 meV for lnGaAs/lnP quantum welts grown at atmospheric pressure between 12 ~m wide masks (mask gap was 3 gm). The photoluminescence (PL) peak wavelength shifted fromI440 to 1720 n m Sasaki et al. [21] induced a 200 nm PL wavelength shift for InGaAs/InGaAsP (Q1.3) MQW structures thereby still maintaining flat interfaces and a flat top surface. The growth pressure was only 150 Torr and the mask width and open stripe width were 30 gm and 2 gm wide, respectively. From measurements on an identical MQW structure grown at 76 Ton- and using smaller mask widths, he estimated that the contribution of the compositional change to the total wavelength shift was about 30 %. Suzuki et al [42] reported a record bandgap shift AEg of 253 meV (PL peak wavelength varies from 1239 nm to 1658 nm), with equally high PL signal levels throughout the whole wavelenglh range (InGaAs/InP MQW grown at low pressure with a fixed open stripe width of 10 gm and mask width ranging from 0 to 300 gm). The transition between selectively and the non-selectively grown regions in the longitudinal direction happens in a very smooth way due to the nature of the diffusion process. Based upon Scanning Electron Microscope images of transition regions of SAG MQW structures - the wafer was cleaved along the middle of the gap parallel to the long direction of the open stripe - Joyner et al. found a transition length (defmed as 10%-90% of total growth rate enhancement) of 1.5 times the gap width (under the condition that mask width and gap are within a factor of three of each other and the overall mask area to open area ratio is small (<1%))[45]. Itagaki et al. [46] found experimentally that both the mask width and its spacing determine the transition length of the selectively grown MQW structure. The transition length decreases with decreasing mask width and mask opening distance. For an open stripe width of 12 pm and a mask width of 30 ~m they found an E~ transition length (defined as 10%-90% of total PL peak wavelength shift) of 60 gin, whereas for the same mask width and a open stripe width of 70 ~m this value shifts to approximately 110 gm. (growth performed at 150 Ton-, at 650 °C, with source precursors TMI, TEG, 10% AsH3/H2, 100% PH3). For active-passive integrated devices no reflections will take place in the transition region ensuring a higher performance of the integrated device. For many applications, however, this region should be as short as possible since the gradual transition of the bandgap could introduce additional absorption losses. M. Ekawa et al. [47] were able to make the transition length substantially shorter by simply modifying the mask design. If pairs of rectangular mask stripes are provided with a window as depicted in the inset of Fig. 11, both, the compositional and thickness uniformity parallel to the stripe direction in the selectively grown region could be significantly improved (Fig. 11).

S e l e c t i v e A r e a G r o w t h on P l a n a r M a s k e d InP S u b s t r a t e s

A

w

nni

5

280 I.tm

[#}

o

w

¢' W = 80 micron !

3

• W = 40 micron i

"~

2

Z

0

AW = 0 micron

0

2oo

400

Position from stripe centre

600

(pzn)

Fig. 11 Improved thickness uniformity along the stripe direction using a modified mask design (after Ekawa et a1.[47])

The gradual thickness reduction in the transition area can also be of advantage for the realisation of tapers. Tapers are generally applied for light emitting or waveguiding devices to reduce the coupling loss when coupling light into fibers. The gradual thickness tapering of the guiding layer towards the output facet will make the optical mode profile of the coupling light better adapted to the typical symmetrical and relative large mode profile of a fibre. This translates for SAG into a tapering in width of the mask stripes as seen in Fig. 12. Takiguchi et al. [48] fabricated this way 1.3 pm laser diodes (LD) with a monolithically integrated waveguide lens. The total cavity length was 600 ~m and the length of the tapered region was 300 ~tm. The tapering of mask width from 100 ~m (in the active region) to zero (near the facet) resulted in a thickness reduction of a factor 4. Good laser performance was achieved - taking the additional composition change into account -and the coupling loss could be reduced by 5 dB. A frequently used application of the combination SAG and QW's, is the modulator/laser integration. Aoki et al. [49,50] reported a high extinction ratio electroabsorptiun (EA) -modulator integrated DFB laser fabricated this way. The specific advantage offered by integration of an external modulator is the reduction of the chirp. The required bandgap shift is only modest (E s is shifted from 1500 nm in the modulator area to 1560 nm in the laser diode region) and hence a large open stripe width of 100 ~tm could be used. The gPL transition region was less than 70 pm long. Excellent device performance was obtained such as low threshold (12 mA), high efficiency laser characteristics (0.1 A/W efficiency), high speed (2.5 Gbit/s penalty-free data transmission over 80 km

281

282

T. Van C a e n e g e m e t al. normal single mode fibre could be achieved), low driving voltage (2Vpp) and low chirp modulation characteristics (linewidth broadening factor ct in the range of 0.3-0.7). A similar device was realised with SAG by Yamazaki et al. [51]. Also a DFB laser integrated with a Mach-Zehnder optical modulator by means of SAG has been reported by Tanbun-Ek et al. [52].

LD active region

WG Lens region 12pro

II

o

,:i

i

0

lC0

200

300

400

500

600

Position (14m)

Fig. 12 Mask design and corresponding thickness elthancement of InP for a LD with integrated WG lens (after Takiguchi et al.)

Another complex device realised with SAG is the multifrequency waveguide grating laser reported by Joyner et al. [45]. The device consists of a number of identical amplifier sections which are all connected to a waveguide grating that serves as an optical filter. The consecutive amplifiers start lasing on slightly different wavelengths thereby maintaining a fixed wavelength (channel) spacing, which makes this device very important for wavelength division multiplexing (WDM) applications. The InGaAsP/InGaAsP QW's for the amplifiers were selectively grown in a 15 ~tm wide gap between 40 ~tm wide SiO~ masks, shilling the PL peak wavelength from 1352 um in the waveguide grating section to 1605 nm in the amplifier section. Laser operation for all channels could be achieved with a side mode suppression ratio (SMSR) greater than 30 dB. The SAG technique is also attractive because waveguides can be realised without any etching process. Indeed the SAG on 1-2 ~tm narrow open stripes can result in perfect waveguides bounded by smooth (111) facets [38] ensuring thus low propagation losses. Since their dimensions are determined by photolithographic dimensions, SAG waveguides are also expected to show more uniform characteristics than wet etched mesas. Sakata et al. applied the SAG technique to fabricate MQW lasers without a semiconductor etching process [53,54]. The different growth steps are depicted in Fig. 13. A very low-threshold current with high uniformity (Ith = 1.78 5: 0.19 mA) was obtained for 20 consecutive LD's. Also Takeuchi et al. made use of the direct wavegnide

Selective Area Growth on Planar Masked InP Substrates

formation for the realisation of a transceiver PIC for bidirectional optical communication [55]. The open stripe width was held constant at 2 pm, whereas the mask width was 30 ~tm for the DFB Laser Diode and the Photo Detector diode and 6 ~tm for the passive waveguide region. The relative growth rate change was 2.3 and the PL peak wavelength shifted from 1190 n m to 1300 nm. A Y-junction was also implemented (see Fig. 14). A fibre output power of more than 1 m W of the DFB-LD was obtained. 1.5 pan

• 8~'~

!

4)"

__n'tn_Psubstrate

j

Mask i !

n-lnP subslrate

j

..........................................

Fig. 13 Device fabrication process for PBH LDs using the all-selective MOVPE technique by Sakata et aL [53,54]

1.3 p.m Laserdiode

1.3 grn Receiver Photodiode Layer structureof Laserdiode and Photodiodc

Layer stractum of passivewaveguide

Fig. 14 Mask design on substratc for the fabricationof a transceiverPIC by means of S A G with no waveguide etching (Takeuchi et ai.[55])

283

T. Van C a e n e g e m et al.

284

As a final application of the SAG technique, we draw our attention to buried low dimensional structures as reported by Galeuchet et al. (see also section geometrical considerations) [56]. He demonslrated that in situ buried InGaAs/InP quantum dot arrays can be fabricated in a single SAG step. The smallest dots were grown in oxide mask windows of 180 nm × 180 nm and were bounded by ( 111)A and (11 I)B facets. Since no lnC_mAsis grown on these facets, the ternary layers are automatically buried as growth proceeds. The red shift in energy due to SAG compared to the planar growth of the lnGaAs QW even dominated the blue shift expected from an additional zero-dimensional carrier quantum confinement. The quantum efficiencies of the buried dots relative to the QW on the unpatterned areas were found to be up to 60% at room temperature and up to 420% at 5K.

Conclusion

In this paper, we have reviewed several publications on MOVPE selective area growth, and it is evident that once the growth related reactor conditions are optimised with regard to the selectivity, very good lateral control of the composition find of the thickness of selectively grown material with MOVPE can be achieved, thereby still maintaining good crystalline quality. A simple vapour phase diffusion model can be applied to predict quantitatively the growth rate and compositional variations for various mask and open stripe widths. Only in the case of narrow openings surface diffusion effects start playing a rote. In conclusion we have shown that the selective area MOV'PE is a powerful tool to fabricate in a reproducx'ble way low-cost high-functionality integrated components.

References

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Selective Area Growth on Planar Masked InP Substrates

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