coalescence in microchannel epitaxy of inp

*Corresponding author. Present address: 1st SH Microcom-puter Development Dept., Semiconductor & Integrated CircuitsDiv., Hitachi Ltd. 5-20-1, Josuihon-cho, Kodaira-shi, Tokyo187-8588, Japan.

E-mail address: [email protected] (Z. Yan).

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0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 0 3 1 - 2

Journal of Crystal Growth 212 (2000) 1}10

Coalescence in microchannel epitaxy of InP

Zheng Yan*, Yoshiyuki Hamaoka, Shigeya Naritsuka, Tatau Nishinaga

Department of Electronic Engineering, The Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113, Japan

Received 8 October 1999; accepted 12 January 2000Communicated by K.W. Benz

Abstract

We have studied the lateral coalescence in microchannel epitaxy (MCE) of InP by LPE. In the present study, wesuggest that there are two modes of the coalescence, òne-zippera and `two-zippera. New patterns of microchannels weredesigned to realize these two coalescence modes. The coalesced MCE islands were chemically etched and it was foundthat dislocations were usually generated in the coalesced region when the growth was carried out in the `two-zipperamode; however, when the growth occurred in the òne-zipperamode, dislocations were not found in the coalesced region.It is concluded that by conducting the growth in the òne-zippera mode, the formation of dislocations in the coalescedregion can be avoided. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 78.66.Fd; 81.15.Lm

Keywords: Coalescence; Microchannel Epitaxy; One-zipper; Dislocation

1. Introduction

Increased research e!orts have been devoted tothe reduction of dislocation density in highly lat-tice-mismatched heteroepitaxy. The technique ofmicrochannel epitaxy (MCE) had been proposed toreduce dislocations in the epitaxial layer by ourgroup a decade ago. We have reported its success inthe growths of GaAs on GaAs [1], GaAs on Si [2]and InP on Si [3]. Recently, we also studied the

growth mechanism of the MCE intensively, andreported the results of the step sources [4], theinterface supersaturation dependence of the growth[5] and the step velocity [6] in the growth of theMCE.

By employing the low-dislocation density MCElayers for device fabrications, the performances ofthose devices can be signi"cantly improved. How-ever, in the device fabrication, sometimes a largesize of the area is required. Coalescence betweenMCE islands is one of the techniques to increasethe area. However, without proper control, disloca-tion will be generated in the coalesced region. Thus,an understanding of the lateral coalescence processin the MCE becomes important. Nagel et al. andBanhart et al. reported [7,8] the experimental re-sults of the lateral coalescence and the evaluation of

the dislocations at the coalesced area in the lateralgrowth of Si by LPE. They found that by changingthe pattern of line seed and con"ning the lateralgrowth in certain directions, the coalescence couldbe realized more e$ciently. In addition, they alsonoticed that under the growth condition with lowsupersaturation, the formation of dislocation at thecoalescence could be avoided.

A large reduction of dislocation density in GaNheteroepitaxy was made recently by introducingthe MCE technique to grow GaN on sapphire[9,10] and SiC [11] substrates. Blue laser diodesfabricated on the MCE GaN have exhibited highoutput power and long lifetime [10,12]. However,due to the formation of dislocations, the laserdiodes fabricated right on the coalesced area be-tween MCE islands were found to have a poorperformance. Detailed studies on how to reduce thedislocation formation in the coalesced areas havenot been reported so far.

In this paper, we will describe our study of thecoalescence in the MCE. In the "rst part, we willsuggest two growth modes } one leading to theformation of dislocations in the coalesced region,and the other one resulting in a dislocation-freecoalescence. In the second part, we will describe theseed patterns employed for MCE. Several new pat-terns of microchannel have been designed tochange the mode of the lateral coalescence, and thegrowth on those new patterns will be described. Inthe last part, we will describe the distribution ofdislocations in the coalesced regions. Based on thepresent experiments, we suggest that, when the co-alescence of MCE starts from more than twopoints, dislocations are usually introduced in thecoalesced region; however, when the coalescencestarts from only one point, the coalesced region canbe dislocation-free.

2. Modes of lateral coalescence

In this section, we will describe two growthmodes: one leads to the formation; while the otherone does not lead to the formation of dislocationsin the coalesced regions. Fig. 1 shows schematicallythe possible processes when two islands coalescewith each other, where (a) and (b) illustrate the

coalescence processes of MCE that start fromone and two points, respectively.

If coalescence starts from one point, it proceedsfrom this point outwards as shown by the arrows inFig. 1a. Therefore, it can be expected that the openregion between the two islands will be closed bya zipper. Under this mode of the growth, no dislo-cations are likely to be introduced into the coales-ced regions. We have named this growth mode asòne-zippera growth. On the other hand, when thecoalescence starts from two points, a closed openarea is expected to appear. In this case, the lateralgrowth occurs inwardly as shown in Fig. 1b by thearrows, consequently, the zipper is closed from twoopposite directions. In Fig. 1b, n and m denote thenumber of lattice points between the two connect-ing positions. Because the substrate is not an idealcrystal, dislocations may exist in the substrate re-gion beneath the closed open area, which indicatesthat nOm. Therefore, mis"ts must appear some-where when the open area is closed by the lateralgrowth. Another scenario is that, when the twocoalescing islands do not grow in exactly the sameplane, a dislocation with screw component will alsobe generated. We have named this growth modewhere coalescence starts from over two points as`two-zippera growth.

In the following, we will describe the microchan-nels designed to realize the one- and two-zippermodes in the growth. Fig. 2 shows the patternsdesigned for the òne-zippera growth. Facets ofM1 1 1N and M1 0 0N are known to be stable in LPE.The pattern shown in (a) has three edges aligned inthe M1 0 0N direction, from which lateral growthcannot occur, so that the solute is expected to bedirected to the other edges that are atomicallyrough. In addition, the two rough edges form a V-shape with an angle of 303, which makes anunequal distance of coalescence along S0 1 0T.When growth is carried out on this pattern, thecoalescence is expected to start from the corner ofthe V-shape towards the open area, consequentlythe òne-zippera growth is realized. However, thispattern has a large area of seed opening, whichmakes the lateral growth slow. We modi"ed thispattern by decreasing the seed area. The new pat-tern is shown in (b). Since the pattern has a smallerarea, a high interface supersaturation is expected to

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2 Z. Yan et al. / Journal of Crystal Growth 212 (2000) 1}10

Fig. 1. Schematic drawings of the `one-zippera and `two-zipperamodes of growth. (a) `One-zipperamode, where the coalescence startsfrom one point, (b) `two-zippera mode where the coalescence starts from two points. Arrows show the directions of the lateral growth.n and m denote the number of lattice points between the two connecting positions of the coalescence.

be induced to increase the lateral growth, especiallyin the initial stage. By connecting 15 sets of thepattern in (b), we have got another new pattern asshown in (c). This new pattern not only retains thecharacters of that shown in (b), but also extends thesymmetry of the seed patterning, which inducesa more uniform interface supersaturation aroundthe pattern to avoid the formation of M1 1 1N facets.

Fig. 3 shows the pattern of the microchannelsdesigned for the `two-zippera growth. In the areabetween the two-triangle window, supersaturationbecomes high at both ends due to the higher supplyof the solute, so that the coalescence is expectedto start from both ends, which realizes the growthin the `two-zippera mode. In addition to thispattern, a conventional linear microchannel hasalso been used to realize the `two-zippera growth.The width and the separation of the linear

microchannels employed were 5 and 50lm, respec-tively.

3. Experimental procedure

InP wafers with nominal orientation of (1 0 0)were used as the substrates. Prior to LPE growth,a SiO

2layer with a thickness of about 100nm was

deposited onto the substrate by spinning anorganic solution (OCD, Tokyo Ouka). The de-signed microchannels were opened in the SiO

2mask by employing a conventional photolitho-graphic technique. The LPE was carried out withan indium (7N) solution using a conventional hori-zontal sliding boat system. The growth temper-ature, the cooling rate and the growth time werevaried from 450 to 6003C, from 0.05 to 0.33C/min

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Z. Yan et al. / Journal of Crystal Growth 212 (2000) 1}10 3

Fig. 2. Microchannels designed for the `one-zippera growth. The closed areas in the "gures indicate the window openings. Lateralgrowth is expected only from the rough surfaces, and coalescence will start from the intersection point of the rough surfaces.

Fig. 3. A microchannel, designed for the `two-zippera growth.The closed areas in the "gures indicate the window openings.Since the two rough surfaces are aligned parallel to each other,the coalescence is expected to occur from multiple points.

and from 0.1 to 3h, respectively. Dislocation den-sity of MCE islands was measured from the num-ber of etch-pits. The etchant employed hada composition of HBr : H

3PO

4("1 : 2 in volume).

The etching was carried out at room temperaturefor 10}30 s.

4. Results and discussion

4.1. Coalescence in `one-zippera growth mode

Fig. 4 shows the coalesced islands grown on themicrochannel pattern of Fig. 2a. The growth tem-perature and the cooling rate were 5503C and0.13C/min, respectively. In the photographs, littlegrowth from three of the M1 0 0N facets has beenobserved. Lateral growth has occurred only on thetwo rough edges, and the MCE islands have coales-ced along the S0 1 0T direction as expected. Fig. 4ashows the result where the coalescence is not fullycomplete. With further growth, the coalescencecontinues and a grown layer as shown in Fig. 4b isobtained. This coalesced island has three M1 0 0Nand two M1 1 1N sidewalls, which are illustratedschematically in Fig. 4c. In our experiment, most of

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Fig. 4. Photographs of the MCE islands grown on the micro-channel pattern of Fig. 2a. (a) An uncompleted coalescence, (b)a completed coalescence with M1 11N sidewalls, and (c) a sche-matic illustrating the crystalline orientations of the MCE islandin (b). The dotted lines indicate the position of the originalmicrochannel edges.

Fig. 5. Photograph of a rectangular island grown on the micro-channel pattern of Fig. 2a. The dotted lines show the originalposition of the microchannel edges.

the MCE islands grown on this pattern have theshape shown in Fig. 4b.

In the experiment, we noted that a rectangularMCE island could also be obtained. Fig. 5 shows

an example. The growth temperature, the coolingrate and the growth time were 5503C, 0.23C/minand 20 min, respectively. The island is surroundedby four M1 0 0N facets while the M1 1 1N facets seen inFig. 4 have not appeared. It was found that therectangular islands always appeared near the edgesof the substrate, where locally high supersaturationwas induced. Two possible scenarios can be at-tributed to the formation of the rectangular MCEislands. The "rst one is that a fast and uniformlateral growth occurs from the rough side surfaces,so that M1 1 1N facets do not appear in the growth.The second is that although M1 1 1N facets appear,growth cannot be stopped by them. This is due tothe existence of a re-entrant corner formed by twoM1 1 1N facets. Growth steps are created continuous-ly from this corner, resulting in a much fastergrowth rate on M1 1 1N facets than that on the (0 0 1)top surface.

The rectangular island has a larger lateral grownarea that can be used for device fabrication. More-over, the M1 0 0N sidewalls may be used directly asmirrors in laser devices. Thus, we believe that thecoalesced rectangular island is a perspective candi-date for the techniques to fabricate lasers inmicrometer size.

In the following, we will describe the results ofthe growth on microchannel patterns of Figs. 2band c. Fig. 6 shows the photograph of the MCEisland grown on the pattern of Fig. 2b. It was foundthat the MCE islands had M1 1 1N and M1 0 0N facets

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Fig. 7. Photographs of the MCE island grown on the microchannel pattern of Fig. 2c. (a) An island whose coalescence along S0 10T isnot completed. The growth was carried out under small interface supersaturation, (b) an island whose coalescence is completed. Thegrowth was carried out under high interface supersaturation.

Fig. 6. Photograph of the coalesced island grown on the micro-channel pattern of Fig. 2b. The solid line indicates the originalposition of the microchannels.

as their sidewalls. The coalescence occurred alongS0 1 0T in a length of approximately 60lm.

Fig. 7 shows the photographs of the MCE is-lands grown on the microchannel pattern of Fig. 2c.In Fig. 7a, we can see an MCE island where the

lateral coalescence was not conducted homogene-ously along the longitudinal direction. At the twoends, the coalescence was complete, however, in themiddle region, the coalescence was delayed by theappearance of M1 1 1N facets. This non-uniformcoalescence is due to the inhomogeneity of theinterface supersaturation along the longitudinal di-rection during the growth. Because of the edgee!ect in bulk di!usion, high interface supersatura-tion appears at both ends of the pattern. Wechanged the growth condition and conducted thegrowth with higher interface supersaturation. Theresult is shown in Fig. 7b. By increasing interfacesupersaturation, the lateral coalescence was com-pleted in the whole region of the pattern.

Fig. 8 shows a photograph of the (1 1 0) crosssection of the coalesced region of Fig. 7b. Due tothe direction of cleavage, three microchannel re-gions appear in the cross section. It is con"rmedthat there is no void in the lateral overgrowthregion and a complete coalescence can be seen inthe entire vertical direction of the growth even inthe center between the microchannels.

With the microchannel pattern of Fig. 2c, thelateral coalescence has been successfully conducted.

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Fig. 8. Cross-sectional photograph of the coalesced islandshown in Fig. 7b. Due to the cleave direction of S1 1 0T, threemicrochannels can be seen in the photograph.

Fig. 9. The etched MCE island grown by the òne-zipperamode. (a) The MCE island shown in Fig. 4b. (b) The MCE islandshown in Fig. 5. The dotted lines indicate the original position ofthe microchannel edges.

However, we have found that complete coalescencewas blocked by the appearance of M1 1 1N facets inmany cases. By applying high interface super-saturation in a local region, the lateral coalescencehas been enhanced and completed. The locally highinterface supersaturation can be realized by chang-ing the microchannel arrangement.

In the following, we will describe the dislocationdensity of the MCE islands. Fig. 9 shows the photo-graphs of the etched MCE islands grown from thepattern of Fig. 2a. In both photographs, etch pitsare found only in the regions above the microchan-nel as indicated by arrows. It was con"rmed that nodislocations existed in the lateral grown region aswell as in the coalesced areas. A line pattern seenin the coalesced region in Fig. 9a suggestsnonuniformity in impurity doping in the coales-cence process. The coalesced islands grown on themicrochannel patterns of Figs. 2b and c after etch-ing are shown in Fig. 10. Etch-pits have been foundonly above the microchannels and the lateralgrown and the coalesced regions are both disloca-tion-free.

As a conclusion, the òne-zippera growth hasbeen realized in the growth on microchannel pat-terns of Fig. 2. Based on the etch-pit measurement,it has been con"rmed that dislocations are notintroduced into the coalesced regions in the òne-zippera growth mode.

4.2. Coalescence in `two-zippera growth mode

Fig. 11 shows the etched MCE island grown onthe microchannel pattern of Fig. 3, in which solidlines indicate the edges of the original microchan-nel. The lateral growth from the two separate tri-angular microchannels coalesced with each otherin the region between them, and a rectangular is-land was formed. Because the two rough edges ofthe triangular microchannel are parallel to eachother, the lateral coalescence is expected to occur atboth ends of the gap between the triangles. In the

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Fig. 10. Photographs of the etched MCE islands grown from the microchannels designed for `one-zippera mode. (a) and (b) show theislands grown on the microchannel pattern of Fig. 2b. (c) shows that of Fig. 2c. The solid lines indicate the original position of themicrochannels.

"gure, etch-pits indicating the existence of disloca-tions are observed. Among them, above the coales-ced region, a large etch-pit can be seen. Therelatively large size of the etch-pit may be a result ofclustered dislocations formed in the `two-zipperagrowth.

Fig. 12 shows the MCE islands grown on linemicrochannel after etching, in which the solid lineshows the original position of the line microchan-nels. It can be observed that the coalescence occur-red primarily at the regions close to the end of themicrochannels. Etch-pits appeared on these MCE

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Fig. 11. Photograph of the etched islands grown on the micro-channel pattern of Fig. 3, which is designed for `two-zipperamode. The etch pits are indicated by arrows and the solid linesshow the original position of the microchannel edges.

Fig. 12. Photograph of the etched MCE islands grown on theline microchannels, from which `two-zippera growth mode isexpected. The solid lines show the original position of themicrochannels. The etch pits appearing in the coalesced regionare indicated by arrows. The growth temperature, cooling rateand growth time were 5503C, 0.33C/min and 1 h, respectively.

islands. Some of the etch-pits located above themicrochannel indicate the dislocations propagatedfrom the substrate through the microchannelopening; while others located between the linemicrochannels show the dislocations formed in thecoalescence. In the "gure, bunched spiral stepshaving its center at an etch-pit can be observed.Similar to the result of Fig. 11, when dislocationsare introduced into the grown layer in the coales-cence, they tend to cluster. The clustered disloca-tion, which has a large Burgur's vector, acts asa strong source of spiral steps in the vertical growth[4]. As a conclusion, we have con"rmed that dislo-

cations are introduced into the grown layer whenthe coalescence occurs in the `two-zippera growthmode.

5. Conclusions

In this paper, we described the coalescence in theMCE, especially, the dislocation formation in thecoalescence process. First, we described two pos-sible modes of lateral coalescence in the MCE, theone- and two-zipper growth modes in which coales-cence occurs from one and two starting points,respectively.

Second, in order to realize the two growthmodes, new microchannel patterns were designedto control the modes of lateral growth. Two groupsof microchannel patterns, on which the lateralgrowth occurs in di!erent directions, were desig-ned to realize the òne-zippera and `two-zipperagrowth.

Then, we described the growth experiments withthe new microchannels. In the growth on themicrochannel patterns for òne-zippera and `two-zippera modes, the lateral coalescence was success-fully carried out from one as well as two startingpoints, respectively. It was found that by increasinginterface supersaturation, complete coalescencecould be realized. From the results of the etch-pitmeasurement, it was con"rmed that dislocationswere not introduced into the coalesced regionswhen the growth occurred in the òne-zipperamode. However, dislocations were found in thecoalesced regions when the growth occurred in the`two-zippera mode.

Finally, it is concluded that in order to realizea dislocation-free coalescence, growth should becarried out in the òne-zippera mode.

Acknowledgements

This work was supported by JSPS research forthe future program in the area of atomic-scale sur-face and interface dynamics under the project of`Self-assembling of nanostructure and its controlsaand Scienti"c Research (B) `Growth of Disloca-tion-Free GaAs on Si by Microchannel Epitaxy

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and Fabrication of Laser Diodea No. 10555119from the Ministry of Education, Science, Sportsand Culture of Japan. The authors would like tothank Dr. M. Tanaka for the discussion. One of theauthors (Z. Yan) acknowledges the support of theResearch Fellowship from the Japan Society forthe Promotion of Science for Young Scientists.

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coalescence in microchannel epitaxy of inp

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