Step sources in microchannel epitaxy of InP

Journal of Crystal Growth 192 (1998) 11—17

Step sources in microchannel epitaxy of InP

Zheng Yan*, Shigeya Naritsuka, Tatau NishinagaDepartment of Electronic Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan

Received 9 February 1998; accepted 23 March 1998

Abstract

In microchannel epitaxy (MCE) of InP by liquid phase epitaxy (LPE) on InP and InP-coated Si substrates, steps arefound as a result of either a misoriented surface or screw dislocations. An atomically flat and step free MCE layer on theInP substrate has been obtained when no screw dislocation exists in the microchannel. However, usually as-grownsurfaces of the MCE layer are covered by monolayer steps as a result of spirals, which have been found frequently locatedat the edges of the MCE layers where locally high interface supersaturation is expected. A single screw dislocationexisting in a central area usually cannot generate spiral steps, but only perturb the local flow of steps. The experimentwith InP-coated Si substrate which has different widths of the microchannels has been carried out. The result shows thatthe thickness of the MCE layer is irrelevant to the width of the microchannel. It is concluded that this happens becausethe growth is conducted by the steps from the single-fold spiral generated by only one screw dislocation usually locatednear the end of the microchannels. ( 1998 Published by Elsevier Science B.V. All rights reserved.

PACS: 81.05.Ea; 81.15.Lm

Keywords: Screw dislocation; Spiral step; Interface supersaturation; Edge effect

1. Introduction

Optical interconnection is a key technology toachieve the task of fast data transmission in theultra large scale [1]. To realize the optical inter-connection, the technique of manufacturing opticaldevices of compound semiconductors on Si isrequired. The heteroepitaxy of III—V compoundsemiconductors on Si has been found as one of such

*Corresponding author.

prospective techniques. With the aim of practicinglong wavelength operation, the growth of InPon Si with heteroepitaxial technique has beenstudied intensively. However, due to the largemisfits of lattice and thermal expansion coefficientbetween InP and Si, the dislocation density inheteroepitaxial InP on Si grown by conventionalmethod is over 107 cm~2, which prevents the laserdevice from high-level operations of performanceand reliability.

Increased research efforts have been devotedto the reduction of dislocation in heteroepitaxy.For examples, the low-temperature growth [2]

0022-0248/98/$19.00 ( 1998 Published by Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 1 9 - 9

decreased residual stress in heteroepitaxial layer,however, resulted in low crystal quality. By intro-ducing a buffer layer prepared at low temperaturein between heteroepitaxial layer and Si substrate,one can improve the crystallinity of epitaxial layergrown at high temperature [3]. Besides the newgrowth methods, the post-growth-thermal-anneal-ing [4] also proved useful to decrease the disloca-tion density. Although a great deal of worksemploying these techniques have been done to re-duce the dislocation density, the epitaxial layer onSi still contains a large quantity of dislocationswhich prevents it from fabricating laser diodes withcomparable properties to conventional ones. In theprevious research, we have proposed microchannelepitaxy (MCE) which consists of growth througha microchannel cut in the SiO

2film on substrate

and the epitaxial lateral overgrowth (ELO) [5].This technique was found to be one of the mostpromising techniques to reduce the dislocationdensity in GaAs grown on Si [6,7]. The MCE hasalso been applied to grow InP on Si substrates toobtain the layer with low dislocation density [8].

In the MCE, as the propagation of dislocationsin the substrate is stopped by SiO

2mask, laterally

grown area becomes dislocation-free. To get widedislocation-free area, thin and wide MCE layershould be grown. By choosing the orientation ofmicrochannels in off-direction to any low indexorientations, one can make the side surface ofgrown layer through the microchannel atomicallyrough. Hence, by using the substrate with a lowindex plane, it is possible to achieve a faster growthin the lateral direction compared to the verticaldirection [5].

In this paper, through the study of atomic stepson MCE layer by atomic force microscope (AFM),the types of step sources for the vertical growthhave been determined and the contribution of eachstep source has been discussed.

2. Experimental procedure

InP wafers with the orientations of only (1 0 0)and 2° off S0 11 1T, and InP-coated Si wafers of(1 0 0) 2° off S0 11 1T were used as the substrates.The InP-coated Si substrates were prepared by

MOCVD [9], which consisted of a 2 lm thickGaAs buffer layer and a 13 lm thick InP layer onSi. Prior to LPE growth, a SiO

2layer with a thick-

ness of about 100 nm was deposited on the sub-strate by using an organic solution (OCD, TokyoOuka). Microchannels for MCE were opened in theSiO

2mask with an angle of 22° off S0 1 1T using

a conventional photolithographic technique. Theshort microchannels with the length and width of700 and 5 lm, respectively, were employed forMCE on InP substrates. In the experiments withInP-coated Si, the long microchannels of 4, 8, 16and 32 lm wide with 300 lm interval were used.LPE was carried out with a conventional horizon-tal sliding boat. The growth temperatures werevaried from 450 to 600°C. Pure In (7 N) was used asthe solution. The surface morphology of the MCElayer were studied by an AFM and a Nomarskidifferential interference contrast microscope (N-DICM).

3. Results and discussion

In the epitaxial growth, steps are supplied frommisoriented substrate, screw dislocations and two-dimensional nucleation. It is known that in thesolution growth, supersaturation is low enough toavoid 2D nucleation, and steps are supplied fromeither screw dislocations or misoriented substrates[10].

3.1. Growth through microchannel without screwdislocations

In the microchannel area 5 lm wide and 700 lmlong on InP substrate with dislocation density of3]105 cm~2, the average number of dislocationsappearing is 10.5. When by chance no screw dislo-cations exist in the microchannel opening, the stepsare supplied only from the misoriented substrate.When all misorientation steps have moved to theedges of the MCE, and if there are no dislocationssupplying spiral steps, the growth stops, as reportedin the growth of Si by LPE [11,12]. Fig. 1a showsan optical photograph of an MCE layer grown onan InP substrate where steps have been suppliedonly from the misoriented substrate. The thickness

12 Z. Yan et al. / Journal of Crystal Growth 192 (1998) 11–17

Fig. 1. Photographs of an MCE layer grown on an 2° off InP substrate at 550°C with the cooling rate of 0.03°C/min and the growthtime of 20 h. (a) A photograph by an optical microscope; (b) a schematic of the MCE layer in Fig. 1a indicating the orientation of themicrochannel and crystallographic orientations of the edges of a completed MCE layer; (c) an AFM image with a scan area of10]10 lm. The thickness of the grown layer at the left and the right edges of the MCE in (a) are 9 and 0 lm, respectively. In (c) no step isobserved, compared with other AFM pictures. The white points indicate the remainings of the solution.

of the MCE layer are 0 and 9 lm at the right andthe left edges, respectively. The thickness differenceexactly coincides with what is expected from thesubstrate misorientation in this case of 2°. Fig. 1b isa schematic of the MCE in Fig. 1a that indicatesthe orientation of the microchannel and the crystal-lographic orientations of the edges of a completedMCE layer. From the AFM observation, it is con-firmed that no steps are remaining, as shown inFig. 1b, where the white spots show the residues ofthe In solution.

In the case of Si LPE, Si substrate is alwaysdislocation-free, so that it is easy to realize this

mode of growth. However, InP substrate usuallycontains dislocations of the order of 105 cm~2.Nevertheless, by employing the MCE, it is shownhere that dislocation and step free growth can alsobe achieved.

3.2. Growth through microchannel with screwdislocations

In many cases, screw dislocations exist in thearea of the microchannel. The spiral steps suppliedfrom the screw dislocations impose or annihilatethe steps from the misoriented substrate. As the

Z. Yan et al. / Journal of Crystal Growth 192 (1998) 11–17 13

density of steps supplied from 2° misorientation isapproximately 102 higher than that from typicalspirals, the growth in the vertical direction is main-ly conducted by the steps supplied from the mis-orientation at the initial stage. However, as thegrowth proceeds, the spirals become the main stepsource in the area where all steps from misorienta-tion have moved by. Thus, in most cases, the verti-cal growth of the MCE layers continues by thesteps from screw dislocations.

Fig. 2a shows an optical microscope photographof an MCE layer on InP substrate grown by spiralstep mechanism. In this case the thickness at loweredge is not zero but 6 lm. Unlike Fig. 1a, the sideof the MCE island is not sharp. This is because thefaster vertical growth made the lateral growth slow,so that the sides had not enough time to be com-pleted. Fig. 2b shows an AFM image of the MCEsurface of the layer given in Fig. 2a. The spiral seenin Fig. 2b is the active step source. The interstepdistance around the center of the spiral is approx-imately 1.4 lm, and all the spiral steps are of mono-layer height. Through the careful investigation ofthe entire surface, it is found that the spiral is theonly one step source active on the whole surface ofthe MCE layer.

Based on our surface studies done with manyMCE samples of both InP/InP and InP/Si, it isfound that only the dislocations located near theedges of MCE layer are able to generate spiralsteps, in spite of the existence of the dislocations inthe central area of microchannels, which is con-firmed by the etch pit measurement.

In the following, we will discuss the reasons whythe dislocations in the area near the end of themicrochannel become active. First, we will showwhy the ends of the microchannel have higherinterface supersaturation. Since LPE is conductedunder nearly isothermal growth conditions, thetemperature distribution can be assumed to be uni-form. On the contrary, due to the nonuniformity ofthe bulk diffusion of P in the In solution, higherinterface supersaturation is expected to appear atthe ends. This has been called the edge effect andhas been evaluated by the computer simulation forInP LPE [13].

Second, we will discuss how the dislocations sup-plying steps with lower density become inactive in

Fig. 2. Photographs of an MCE layer grown on 2° off InPsubstrate with the same growth condition as that in Fig. 1 (a) Aphotograph by optical microscope in which the square showsthe approximate location of where the AFM image in (b) wastaken; (b) an AFM image with a scan area of 10]6 lm. Thethickness of the grown layer at the left and the right edges of themicrochannel are 15 and 6 lm, respectively as shown in (a).A spiral step source has been found as shown in (b). Theinterstep distance is found to be approximately 1.4 lm and all ofthe spiral steps are of monolayer height.

competition with those supplying steps with higherdensity. As shown in Fig. 3a, in the initial stage ofthe growth, the dislocations at the end and thecenter of the microchannel may both generate spi-ral steps. However, because interface supersatura-tion is higher at the ends, the spiral A has higherstep density than the spiral B in the center. As the


Fig. 3. Schematic illustrations of the spiral steps with differentdensities in the initial growth stage of MCE. The arrows indicatethe directions of the step advancing. (a) steps are supplied fromtwo dislocations A and B. Since interface supersaturation ishigher at the ends of the microchannel, spiral A has higherdensity of steps than B. As the growth proceeds, the steps fromspiral A run over the center of spiral B and the growth surface iscovered by the steps from the spiral A as shown in (b).

growth continues, the steps from spiral A andB meet and annihilate each other. Since the stepscoming from the ends have higher density, they winover the steps coming from the center with lowerdensity and finally the steps from the ends covereverywhere on the growth surface as illustrated inFig. 3b. As a result, the spiral A remains to be theactive step source. Thus, the screw dislocations inthe central area of the microchannel do not con-tribute to the step creation.

Fig. 4 shows AFM images of the central area ofa MCE layer, where the inactive dislocations areseen among the steps as indicated by arrows. InFig. 4a, a step is seen terminated by a dislocationwhile in Fig. 4b, an uncompleted spiral and a halfloop of a step, is observed. In both cases, all thesteps are supplied from the spirals in the ends of themicrochannel. The screw dislocations in the centralarea, however, can only be seen as modifyinga regular step flow. The results suggest that not allof the dislocations in the microchannels contributeto the vertical growth.

In the MCE of GaAs on GaAs-coated Si substra-te, it has been found that the change in the width ofthe microchannel gives almost no change in the

Fig. 4. An AFM image of the central area of a MCE layer,where the inactive screw dislocations are observed. The arrowsindicate the positions of the dislocations. (a) A step terminatedat a dislocation; (b) an uncompleted spiral with a half loop ofa step which is generated by a dislocation.

vertical growth rate. This is explained as the in-crease in the number of screw dislocations does notgive rise to the increase of step density [14].

In the present work, we repeated this experimentfor InP on InP-coated Si substrate, which has thedislocation density of 6]106 cm~2, to study theinfluence of the change in the microchannel widthon the vertical growth rate in InP MCE. Fig. 5shows the result where the width was changed


Fig. 5. Dependence of growth thickness on the width of themicrochannels when InP-coated Si was employed as a substrate.The growth was carried out at 550°C, with the cooling rate of0.1°C/min and the growth time of 3 h. In the range of the widthsbetween 4 and 32 lm, the thickness of the MCE layers showalmost no change.

from 4 to 32 lm. It is seen that the growth thicknessof the MCE is found not depending on the widthsof the microchannels, which again shows that thegrowth in the vertical direction is irrelevant to thenumber of dislocations in the microchannels.

However, this is correct only when the source ofmulti-fold spiral steps does not exist in the micro-channels. The multi-fold spiral is generated whenthe distance of the single-fold spirals becomessmaller than the interstep distance of a single-foldspiral (j), or a screw dislocation has a larger Bur-ger’s vector. If one of such multi-fold spirals be-comes to enter the area of microchannel as thewidth is increased, the vertical growth rate shouldincrease. However, the experimental results givenin Fig. 5 shows that there is no such multi-foldspiral active even in the microchannel of 32 lmwide, when the dislocation density is 6]106 cm~2.

In the following, we will estimate the number ofdislocations (N

.) whose distance is smaller than

j in the microchannel of 5 lm wide and 700 lmlong, based on the assumption of binomial distribu-tion of dislocations. Taking j of 2 lm, N

.are about

zero in the case of InP/InP (with the average totalnumber of dislocations in the opening area of asingle microchannel N"10.5) and 3.6 in the case ofInP/Si (N"210), respectively. If the interfacesupersaturation is lowered to have an j of 4 lm,N

.is still about zero in InP/InP, but 41.6 in InP/Si.

On the other hand, with the constant j value of

2 lm, when the dislocation density of the substrateincreases to 3.8]107 cm~2, N

.becomes 652,

which is almost half of the value of N"1330. Thesecalculations show that the multi-fold spiral doesnot exist in InP/InP, but exists in InP/Si. Hence,if the interface supersaturation is small, the multi-fold spiral should become dominant. The multi-fold spiral gives high step density and hence fastgrowth rate in the vertical direction. However,since the interface supersaturation in the presentexperiments is relatively high and the dislocationdensity of the substrate is small, no multi-fold spi-rals have been found to exist in both InP/InP andInP/Si.

The results also suggest that the MCE layer ofthe InP on Si substrate can be grown on a widemicrochannel without the degradation of the crys-tal quality, which can improve its electrical andmechanical characteristics. However, when the dis-location density in the substrate becomes muchhigher, the chances of the multi-fold spiral growthwould rise. In this case, the use of microchannelswith narrower width is the simple and efficient wayto eliminate screw dislocations which supply mul-tiple steps. Therefore, when the dislocation densityof the substrate is extremely high, the use of nar-rower microchannel is preferable to decrease thevertical growth rate.

4. Conclusions

The kinds of step source in LPE microchannelepitaxy (MCE) were investigated by AFM. Thesteps were found being supplied from either mis-oriented substrate or screw dislocations as de-scribed below.

(1) When no screw dislocation exists in themicrochannel, the vertical growth is conductedonly by the steps from the misoriented surface. Thevertical growth stops when all the misorientationsteps have moved to the edges of the MCE layers.Consequently, the grown layer is very thin and thesurface is atomically flat. A step free MCE layerwith large surface area has been obtained.

(2) If a screw dislocation exists in the microchan-nel, the vertical growth is continued by steps gener-ated by screw dislocations when all misorientation


steps have moved by. Although many screw dislo-cations exist in the microchannel, only one of themcan generate growth steps. The active spiral hasbeen found frequently located at the edges of themicrochannel. Other dislocations are found exist-ing in the central region but give little contributionto the vertical growth.

It was found that the thickness of the MCElayers were irrelevant to the widths of the micro-channels. This is explained as even in the widestmicrochannel in the range of the dislocation den-sity of the InP coated Si substrates employed in thepresent experiments, only single-fold spirals can beactive to supply steps.

Acknowledgements

The authors would like to thank Dr. M. Tanakafor his discussion. This work was supported bya Grant-in-Aid (B) “Studies of InP layers grownon Si substrates by epitaxial lateral overgrowthand fabrication of long-wavelength lasers” No.07555107 from the Ministry of Education, Science,Sports and Culture of Japan. One of the authors (Z.Yan) was also supported by the Research Fellow-

ship of the Japan Society for the Promotion ofScience for Young Scientists.

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Step sources in microchannel epitaxy of InP

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