Sumitomo develops VCZ crystal for 3-inch InP crystals

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nP wafers are widely used as substrates for optoelectronic devices such as LDs and PDs and for electronic devices such as HEMTs and HBTs. As this industry advances it will require 3-inch rather than 2-inch diameter lowdislocation density (LDD) wafers. There is a strong demand for low defect density substrates and development of t e c h n i q u e s for g r o w i n g L D D crystals has been underway for some years. The technique which has been developed at Sumitomo is based on the conventional liquid encapsulated Czochralski (LEC) process in which a p h o s p h o r u s atmosphere is controlled making it possible to grow the crystal in a low temperature gradient and produce L D D substrates. This method is called Vapour Pressure Controlled Czochralski, VCZ for short see Figure 1.

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Crystal for 3-,nch Redgcttlynducethe InP Crystals Some time ago we reported[I], Sumitomo's success in developing the VCZ process for the manufacture of 2-inch diameter SI InP wafers. In response to the industry demand for better device performance and higher yield from electronic and optoelectronic devices, this process has been developed further to enable the production of 3-inch wafers.

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Techniques for LDD G r o w t h In general there are two ways to grow L D D crystals. One is the reduction of the thermal stress which causes the generation o f dislocations during growth. This stress can be decreased by reducing the temperature gradient along the growth axis and in the radial direction. However, in the conventional LEC method, reducing the temperature gradient causes the dissociation of InP at the crystal surface where it is not coated with the liquid encapsulant, due to the rise in the temperature. This dissociation of phosphorus seriously degrades the quality of the crystal. The m e t h o d also has some other problems such as the difficulty of controlling the shape and the occurrence of twinning.

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Some methods which have a low temperature gradient are: • FEC - the fully encapsulated Czochralski; V B - vertical Bridgman; • VCZ - vapour pressure controlled Czochralski. The second way to grow L D D crystals is by making the crystal resistant to ther-

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dislocation density, a reduction of temperature gradient a r o u n d the solidliquid interface is necessary. In the case of IiI-V compound crystals, a low temperature gradient causes a dissociation of the Group V element from the crystal surface, w h i c h degrades the crystal quality. In the VCZ m e t h o d which we have developed to suppress p h o s p h o r u s dissociation at high temperature, a t e m p e r a t u r e gradient of 30-50"C, cm in t h e B203 layer can be achieved. This is much lower than the 100-130~C cm of the conventional LEC method. We have already demonstrated that the EPDs of 2" diameter lnP crystals can be greatly reduced using the VCZ method [2], [3]. As we will describe in this article, a similar success has been obtained for 3-inch diameter InP crystals by using the VCZ method.

mal stress. The crystals are usually doped using electrically inert elements as the impurity. For InP, L D D crystal is obtained by doping with G r o u p III o r Group V elements: As, Ga or Sb. This method has the drawback that thermal stability or homogeneities in the physical properties may

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Basically the same VCZ method as described before[l] has been applied to growing the 3-inch crystals. In addition, in order to grow long crystals with low dislocation density, we have increased the amount of starting material, ie we have increased the charge weight, and also optimized such growth conditions as B203 thickness and the temperature gradient. The starting material used was presynthesized InP polycrystal of 1.5-4kg and the crystal growth was performed under a phos-

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phorus partial pressure higher than 1 atm. A vertical m a g n e t i c field o f 0-2,400 Gauss was applied to the melt during growth to stabilize the melt convection. The growth rate was 2-6mm/Hr.

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Figure 2 shows the depend e n c e o f E P D on the carrier concentration in 3" diameter InP. It can clearly be seen that the impurity hardening effect of S-doping and Zn-doping in the VCZ m e t h o d occurs at much lower cartier concentration in comparison with the c o n v e n t i o n a l L E C method. As a result, both Zn-doped and S-doped 3" diameter InP crystals grown by VCZ have low EPD at low cartier concentration. In the case of S-doped InP, the average EPD of VCZ-grown crystal is 4.8 x 102cm-2 at the carrier con-

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The E P D maps of 3" diameter Z n - d o p e d crystals grown by the VCZ and the conventional LEC methods are shown in Figure 3. The cartier concentrations of both crystals are

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the same. The average EPD of the VCZ crystal is about two orders of magnitude less than that of the conventional LEC crystal. The low EPD area (less than 500cm -2) of the VCZ crystal

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Epilayers Some specific l n P - b a s e d devices definitely require a crystal having low dislocation density at low carrier concentration. This is in order to reduce the outdiffusion of impurity into epitaxial layers and to decrease the free carrier absorption in the substrate. As we have shown in this report, the VCZ method is the most suitable crystal growth technique for this purpose thus far. The VCZ technique has shown itself to be capable of providing not only 2inch but now also 3-inch diameter crystals in good yield. D e v e l o p m e n t o f longer crystals of 3-inch SI lnP wafers will, we hope, contribute significantly to the progress of the whole family of InP electronic and optoelectronic devices. Masamichi Yokogawa and Yoshihiro Hosokawa, Semiconductor Division, Sumitomo Eh, ctrie Industries. 1-1-1 Koyakita ltami-shi. Hyogo 664. Japan.

References I. "High Quality InP Single Crystal f r o m S u m i t o m o Electric", l l I - V s Review Vol. 3, Nos. 5 & 6, September/November 1990.

NTT Transmits 10Gbls Down Optical Fibre

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Iron Doped InP Figure 4 shows the EPD distribution in a wafer in 3" diameter Fe-doped lnP crystal. Although the socalled 'W-shaped' distribution still remains, the VCZ m e t h o d can reduce the EPD by about one order of magnitude compared to the c o n v e n t i o n a l LEC method. The average EPD 5 3 -~ across was 3._-7.6x10-cmthe whole wafer.

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2. Tada, K., Tatsumi, M., N a k a g a w a , M., Kawase, T., and Akai, S., "Growth of low-dislocation density InP by the modified CZ method in the atmosphere of phosphorus vapour pressure", Int. Symp. G a A s and Related Compounds, Heraklion, Inst. Phys. Conf. Ser. 91:439(1987). 3. Tatsumi, M., Kawase, T., Arai, T., Yamabayashi, N., Iwasaki, T., Miura, S., Tada, K., and Akai, S., " G r o w t h o f low-dislocation d e n s i t y I n P single crystals by the VCZ method", Int. Conf. InP and Related Materials, Oklah o m k a 18(1989). 4. Hosokawa, Y., Kawarabayashi, S., Yabuhara, Y., Morioka, M., Yokogawa, M., and Akai, S., "Development of 3-inch diameter

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lnP single crystals with low dislocation density using VCZ technique", Invited Paper to be presented at Int. Conf. InP and Related Materials, R h o d e Island (1992).

V C Z at Rhode Island The detail of the development of 3" diameter low-dislocation lnP crystals will be presented at the International Conference on InP and Related Materials, Rhode Island.[4]. T h e p a p e r will be p r e s e n t e d at 1330 on W e d n e s d a y April 22nd in the session "Bulk InP Crystal Growth Technology Bulk II" chaired by Roberto Fornari or MASPEC, Parma, Italy.

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NTT has succeeded, with the help of a ne~ highspeed transmission de~icc. in sending a ~t)Gb s t~ptie~t transmission tim)ugh m,~re than 1260kin of opti~a[ fibre cable from loky~ ~,o Hamamatsu. Shizuoka Prclecture. NTT requires large-capacity transmission technoio g y to be a b l e ~o comprehensi~el~ proxldc its VI&P (Visual, Intelligent & Personatl commun i c a t i o n s services ~t affordable prices I-'he 10Gb/s transmission capacity is equivalent to 130,000 telephone channels. Since il can transmit these many channels through a single optical fiber, it will contribute considerably to the c o m m e r c i a l i z ~ t i ~ : ,~f VI&P. The experiment, a global first, proved that 10Gb/s transmissions are possible just by replacing conventional transmission equipment and that usmg an optical fibre amplifier as a repeater without having ;o convert them into electrical signals makes it possible to flexibly handle high-speed transmissions without no. cessitating any changes m the repeaters. To do this NTT developed new technologies, including: 1) the separation of oscillation and modulation functions, usually performed by a single device: 2) the use of LiNbOa (lithium niobate) MachZehnder interference nmdulator; 3) the mounting of a circuit for compensating c h a r a c t e r i s t i c changes caused bv I)C drifts ~o ensure the stable operalion of the modulator: and 4) the use of a very-high-speed optical modulation circuit with stable modulation characteristics at 10Gb:s. C o n t a c t . M r . H. K a n a m a r u , N T T Press Relations Dept.,

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