Journal of Crystal Growth 203 (1999) 25}30

Interface supersaturation in microchannel epitaxy of InP

Zheng Yan*,1, Shigeya Naritsuka, Tatau Nishinaga

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

Received 30 October 1998; accepted 15 December 1998Communicated by K.W. Benz

Abstract

By employing microchannel epitaxy (MCE), it became possible to reduce the dislocation density in liquid phaseepitaxy (LPE) of InP so that steps supplied from only one screw dislocation can cover the whole surface of MCE island.This enabled us to measure interstep distance by AFM and to calculate interface supersaturation even in metallicsolution with the help of Cabrera and Levine formula by assuming appropriate value of interface free energy.

The interface supersaturation was found being ranged from 0.02 to 0.05 upon various experimental conditions.A minimum interface supersaturation was realized when the growth temperature and cooling rate were chosen as 5003Cand 0.053C/min, respectively. The ratio of width to thickness of the grown MCE layer, which is de"ned as=/¹ ratio, wasfound to increase rapidly with the decrease of the interface supersaturation. A=/¹ ratio as high as 20 was accomplishedunder the growth condition where minimum interface supersaturation can be realized. ( 1999 Elsevier Science B.V. Allrights reserved.

PACS: 78.66.Fd; 81.15.Lm

Keywords: Interface supersaturation;=/¹ ratio; Spiral steps; Microchannel epitaxy

1. Introduction

Optical interconnection is a key technology toachieve the task of fast data transmission with ultralarge scale [1]. To realize the optical interconnec-tion, the technique of manufacturing optical devi-ces of compound semiconductors on Si is required.The heteroepitaxy of III}V compound semiconduc-

*Corresponding author.E-mail address: yan@ee.t.u-tokyo.ac.jp.

1Present address: Semiconductor & Integrated Circuits Divi-sion, Hitachi Ltd., Japan.

tors on Si has been found as one of such prospectivetechniques. In aim of practicing long wavelengthoperation, the growth of InP on Si with hetero-epitaxial technique has been studied intensively.However due to the large mis"ts of lattice andthermal expansion coe$cient between InP and Si,the dislocation density in heteroepitaxial InP on Sigrown by conventional method is over 107 cm~2,which prevents the laser device from high leveloperations of performance and reliability.

Increased research e!orts have been devoted tothe reduction of dislocation in heteroepitaxy. Forexample, by introducing a bu!er layer prepared at

0022-0248/99/$ } see front matter ( 1999 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 0 5 0 - 0

Fig. 1. Schematic illustration of the MCE mechanism. The ratioof the width to the thickness of the grown layer, (¸!d)/¹, isde"ned as =/¹ ratio.

low temperature between heteroepitaxial layer andSi substrate, one can improve the crystallinity ofepitaxial layer grown at high temperature [2].Besides the new growth methods, the post-growth-thermal-annealing [3] also proved useful to de-crease the dislocation density. Although a greatdeal of work in employing these techniques havebeen done to reduce the dislocation density, theepitaxial layer on Si still contains a large quantityof dislocations which prevents it from fabricatinglaser diodes with comparable properties to conven-tional ones. In the previous research, we have pro-posed microchannel epitaxy (MCE) which consistsof the growth through a microchannel cut in SiO

2"lm on substrate and the epitaxial lateral over-growth (ELO) [4]. This technique was found to beone of the most promising techniques to reduce thedislocation density in GaAs grown on Si [5,6]. TheMCE has also been applied to grow InP on Sisubstrates to obtain the layer with low dislocationdensity [7].

Fig. 1 shows schematically the cross section of anepitaxial island grown through a microchannel bythe MCE. As the propagation of dislocations fromthe substrate is stopped by SiO

2mask, the laterally

grown area becomes dislocation-free [4]. To obtainwide dislocation-free area, thin and wide MCElayer should be grown. The ratio of width to thick-ness of MCE layer is de"ned as =/¹ ratio. Large=/¹ ratio gives high reduction e$ciency of dislo-cation density. In aim of achieving large=/¹ ratioin MCE, faster growth rate in the lateral directionthan the vertical one should be realized. By aligning

the orientation of microchannels in o!-directionsto any low index orientations, one can make sidesurfaces of grown layers through the microchannelatomically rough. Hence, by using the substratewith low index plane, it is possible to obtain fastergrowth in the lateral direction compared with thevertical direction. To enhance anisotropy of thegrowth rates between the lateral and the verticaldirections, the study of interface supersaturationdependence of the growth anisotropy has to becarried out. However, because interface super-saturation is di!erent from bulk supersaturation,few studies have reported its dependence on growthconditions in liquid phase epitaxy (LPE) ofsemiconductors. In our previous study of LPE-MCE of InP with the substrates of low index ori-entations, the vertical growth of MCE was founddepending on the steps supplied from spirals.Because the dislocation density has been drasticallydecreased in MCE, the number of active spirals isfrequently only one on a single island [8]. There-fore, by measuring the interstep distance of thespiral steps and using the equation given byCabrera and Levine [9], the interface supersatura-tion can be determined [10].

In this paper, we will report the growth conditiondependency of interface supersaturation in theMCE. First, the growth temperature and the coolingrate dependencies of the interface supersaturationwere studied, and the reasons for the dependenciesare discussed. Then, the dependence of=/¹ ratioon the interface supersaturation was studied. Fi-nally, from an optimum growth condition, a large=/¹ ratio has been successfully achieved.

2. Experimental procedure

InP wafers with the orientations of (1 0 0) wereused as substrates. Prior to LPE growth, a SiO

2layer with a thickness of about 100 nm was depos-ited on the substrate by spinning of organic solu-tion (OCD, Tokyo Ouka) and baking at 4503C.The linear microchannels were opened in the SiO

2"lm with an angle of 223 o! from S0 1 1T usingconventional photolithographic and wet etchingtechniques. The length and the width of the micro-channels were 700 and 5 lm, respectively. The LPE

26 Z. Yan et al. / Journal of Crystal Growth 203 (1999) 25}30

was carried out with In (7 N) melt using conven-tional horizontal sliding boat system. The growthtemperature, the cooling rate and the growth timewere varied from 4503C to 6003C, from 0.01 to0.33C/min and from 0.5 to 15 h, respectively. Theinterstep distance of the spiral steps on the surfacesof the MCE layers was measured by AFM.

3. Results and discussion

3.1. Determination of interface supersaturation

The determination of interface supersaturation isthe core of this work. By employing the equationgiven by Cabrera and Levine [9], as shown below,we determined the value of interface supersatura-tion,

p"19ack¹j

, (1)

when the spiral is single folded and the spiral stepshave monolayer height. In Eq. (1), a, c, k, ¹ andj denote lattice constant, step free energy, Boltz-mann constant, growth temperature and interstepdistance, respectively. The step free energy was de-rived tentatively from the latent heat of InP basedon a bond-breaking model, which gives the value ofc"0.12 J/m2. According to Eq. (1), the value ofinterface supersaturation can be calculated fromthe measured interstep distance after con"rming byAFM that the spiral is single folded and the spiralsteps are monolayer high.

Fig. 2 shows a typical as-grown surface of InPMCE observed by AFM. Steps with almost equalseparation disperse the whole surface and all stepswere monolayer high. At the center of the up edgeof the photograph, the step source, a single foldedspiral, can clearly be seen. The interstep distance ofthis spiral was 0.93 lm and the interface super-saturation was calculated to be 0.05.

3.2. Growth conditions and interface supersaturation

As the result of the solute consumption in thegrowth process at interface, the relation betweengrowth conditions and interface supersaturationwas not clear so far. In the following, we will study

Fig. 2. AFM image of the as-grown surface of InP MCE layer.The observed area is 12.5 lm]12.5 lm. Steps from one spiralcover the whole surface and the step source, a single-foldedspiral, can be seen at the center of the up edge of the photograph.The interstep distance near the center of the spiral is 0.93 lm. Allof the steps are monolayer high. The white spots are the residuesof In solution and the curved lines where the white spots line upare meniscus lines, which were generated by the removal of theIn solution at the end of the growth.

Fig. 3. Cooling rate dependence of interface supersaturation,where the growth starting and ending temperatures were kept atthe same values of 5003C and 4873C, respectively. The values ofp are calculated by Eq. (1) from the interstep distance j of thespiral steps measured by AFM.

the e!ects of the changes in cooling rate and growthtemperature on interface supersaturation.

Fig. 3 shows the cooling rate dependence of in-terface supersaturation, where the growth starting

Z. Yan et al. / Journal of Crystal Growth 203 (1999) 25}30 27

Fig. 4. Growth temperature dependence of interface super-saturation, where the cooling rate was kept constant at0.053C/min.

temperatures were kept at 5003C. It was foundthat interface supersaturation increases with theincrease of the cooling rate. This increase is a re#ec-tion of the larger bulk supersaturation when thecooling rate is increased. However, when very slowcooling rate, such as 0.013C/min was employed, nogrowth occurred in the microchannels. This mayhappen because supersaturation was so small thatsteps from a spiral dislocation cannot move prob-ably due to impurity pinning e!ect.

When the growth starting temperature is de-creased, the interface supersaturation decreases butbecomes to increase in low temperature region asshown in Fig. 4. Here the cooling rate was kept ata constant rate of 0.053C/min. The interface super-saturation was found to have a minimum value at5003C. This tendency is interpreted as follows.

Interface supersaturation is de"ned as

p"C

4!C

%C

%

"

C4

C%

!1, (2)

where C4and C

%denote the phosphorus concentra-

tion at the interface and that of equilibrium inindium solution, respectively. From Fig. 4, it is seenthat the ratio C

4/C

%tends to take a larger value

both at higher and lower temperatures, which areattributed to the higher solubility in the solutionand the slower incorporation of phosphorus at thegrowing surface, respectively.

In the following, we will discuss the possibleorigins of the observed dependencies of interfacesupersaturation on cooling rate and growth

temperature. The interface supersaturation de-pends on the concentration of phosphorus at theinterface and re#ects the relative magnitude of thesupply and the consumption that are determinedby the processes of the bulk di!usion and the incor-poration of the solute, respectively. When coolingrate is changed, the main change in the growth isthe bulk supersaturation. Therefore, the coolingrate dependence of interface supersaturation shownin Fig. 3 re#ects the tracing of the interface super-saturation on the bulk supersaturation. However,interface supersaturation becomes to deviate frombulk supersaturation when the growth startingtemperature is changed largely. At low growth tem-perature, the kinetic coe$cient of the incorporationat the interface becomes smaller, which givesa higher interface supersaturation as shown inFig. 4.

We have conducted a separate experiment inwhich a di!erent growth time has been employed,but the di!erence between the starting and theending temperature was kept to be lower than153C. This experiment showed that p does notdepend on the growth time. Therefore, we believethat the growth rate also does not depend on thegrowth time if the di!erence in the temperaturedecrease is small. This means that the p obtainedfrom the surface spiral can represent that of theentire growth.

3.3. Interface supersaturation vs. =/¹ ratio

The e!ect of interface supersaturation on =/¹ratio has been studied through the samples grownunder various growth conditions, which gave di!er-ent values of interface supersaturation. Fig. 5 showsthe result. The =/¹ ratio was found to increaserapidly with the decrease of the interface super-saturation. This is attributed to the di!erence in theinterface supersaturation dependence of the growthrates on the rough side and the smooth top surfa-ces. When the interface supersaturation is relativelysmall, the growth rate on the rough side surface ismuch higher than that on the smooth top surface,because the growth rate on the smooth surfaceshows super-linear dependency with the super-saturation and it becomes very small when thesupersaturation becomes small. On the other hand,

28 Z. Yan et al. / Journal of Crystal Growth 203 (1999) 25}30

Fig. 5. Interface supersaturation dependence of=/¹ ratio. The=/¹ ratios are calculated from the measured values by opticalmicroscope.

the growth rate on the rough surface increaseslinearly with the supersaturation.

3.4. Growth under optimum condition

In Fig. 4, it is shown that the minimum interfacesupersaturation has been realized when the growthtemperature and the cooling rate were 5003C and0.053C/min, respectively. By employing this growthcondition, the MCE island with large =/¹ ratiohas been obtained. Fig. 6 shows an SEM photo-graph of the cross section of the MCE layer grownfor 15 h. The width and thickness were 225 and10.8 lm, respectively, which gave a =/¹ ratio ashigh as 20.4. The large laterally grown area withzero dislocations can be used as a perfect substratefor the fabrications of optical and electronic devices.

Usually, growth in the mismatched systemresults in the large density of dislocations and highvalue of stress in the grown layer. In the presentexperiment of InP MCE, the employed InP sub-strate has a dislocation density as high as3]105 cm~2, which is close to the dislocation den-sity (6]106 cm~2) when Si substrate is employed.Furthermore, since the grown layer has an islandstructure and the growth temperature was as low as5503C, the stress in the MCE islands should be verysmall. We have reported in our previous paper [8]that in the MCE of InP there was no big di!erencein growth behaviors between homo- and hetero-epitaxy. Therefore, as described in the present

Fig. 6. SEM cross-sectional photographs of the MCE layergrown on InP substrate. (a) a whole view, (b) a magni"ed pic-ture of the area near the microchannel. The growth temperature,the cooling rate and the growth time were 5003C, 0.053C/minand 15h, respectively. The width and the thickness are 225 and10.8 lm, respectively, which give a =/¹ ratio of 20.4.

paper, we suggest it is possible to apply theknowledge obtained from the homoepitaxy ofMCE to the heteroepitaxy MCE of InP on Si.

4. Conclusions

In this paper, the interface supersaturation andthe conditions to grow a wide layer of InP by MCEwere discussed. Interface supersaturation was de-termined by measuring the interstep distance of thespiral steps by AFM and by using the Cabrera}Levine equation. By changing the growth temper-ature, it was found that the interface supersatura-tion takes the minimum value. It was also foundthat smaller interface supersaturation gives largerMCE layer =/¹ ratio. With a growth conditionwhich gives the minimum interface supersatura-tion, we could obtain an MCE layer with a =/¹ratio as larger as 20.

Z. Yan et al. / Journal of Crystal Growth 203 (1999) 25}30 29

Acknowledgements

The authors would like to thank Dr. M. Tanakafor his discussion. This work was supportedby JSPS Research for the Future Program inthe Area of Atomic-Scale Surface and InterfaceDynamics under the project of &Self-assemblingof Nanostructures and Its Control' and Scienti"cResearch (B) &Growth of dislocation free GaAson Si by Microchannel Epitaxy and Fabricationof Laser Diode' No. 10555119 from the Ministryof Education, Science, Sports and Culture ofJapan. One of the authors (Z. Yan) was also sup-ported by the Research Fellowship of the JapanSociety for the Promotion of Science for YoungScientists.

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