Journal of Crystal Growth 66 (1984) 327—332 North-Holland, Amsterdam
GROWTH AND PROPERTIES OF InP SINGLE CRYSTALS FANG Dun-fu, WANG Xiang-xi, XU Yong-quan and TAN Li-tong Shanghai Institute of Metallurgy, Chinese Academy of Sciences, 865 Chang Ning Road, Shanghai 200050, People’s Rep. of China Received 16 September 1983; manuscript received in final form 21 November 1983
lnP single crystals with various dopants including S. Sn, Zn and Fe have been grown successfully by the Czochralski method under high pressure with liquid encapsulation. It is found that by carefully adjusting the thermal symmetry of the heating field and by further improving the quality of the polycrystals and by dehydrating B 203. twin-free lnP crystals can be obtained even with a shoulder angle of up to 54°,and defects caused by thermal decomposition appear on the surface of the crystals during pulling. Furthermore, a comparison of the crystal perfection and uniformity between S-doped and Sn-doped InP crystals shows that the quality of the former is better than that of the latter. Dislocation-free Zn-doped 5 cm3 p-InP and the single diameter crystals lesswithout than 30precipitates mm. By controlling have also been the iron easily content, obtained semi-insulating when the carrier thermally concentration stable single is greater crystalsthan of InP 2X10’ doped with ~ 0.03 wt% of Fe without precipitates and with a homogeneous resistivity can be produced.
dopants, such as In
Indium phosphide is a substrate material of high potential for long-wave opto-electronic and high speed microwave devices. The growth of homogeneous InP single crystals with good thermal stability and crystal perfection has become a very important semiconductor technology [1,2]. In this paper the growth and physical properties of InP single crystals doped with various dopants including S, Sn, Zn and Fe are discussed. The crystals were grown by the Czochralski method under high pressure, using liquid encapsulation.
2. Experimental 2.1. Crystal growth
The polycrystalline InP was prepared by direct synthesis under high pressure with a conventional gradient freeze technique using elemental indium and red phosphorus of 6N grade as the raw
2S3, ZnP2 and Fe2P, were synthesized and purified in our laboratory. The InP single crystals were grown in the GYL-1 type high pressure puller. This apparatus, which incorporated a graphite resistance heater, was designed and manufactured by our institute. A fused silica crucible with a hemispherical bottom of 73 mm dia. and 60 mm depth was used. Typically, 300 g of raw material and 70 g of B203 (5N purity) were used. The encapsulant produced an encapsulating layer between 8 and 10 mm. In order to ensure the successful growth of single crystals, it is very important to dehydrate the B203. The in situ dehydration of B203 was carried out under a vacuum (— 5 x iO~ Torr) before crystal pulling. The final temperature of dehydration was about 1100°C. Half an hour after the bubbling in the B203 layer ceased, the pressure of 2 the dry to 40thekg/cm and thenAr2 thewas rawgradually material increased was put into crucible by a charging hopper. The melting of the InP must be carried out as quickly as possible under the B 203 encapsulant. In this work, the seed end in contact with the melt was (111)B. The seed rotation and rate the waspulling 7 rpm, rate the 18—20 crucible rotation rate 10 rpm mm/h.
materials .The electrical properties of this polycrystalline InP are typically: ND NA = (0.5—1.5) 3 and ~t 2/V s. The x 1016 cm 300 4000 cm 0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) —
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Growth and propcruei of ml’ W7~’/C ri
The electrical properties were measured by the Van der Pauw method. Etch pits were revealed on the (100) wafer by immersing in a solution of H3P04 and HBr 141. The compensation ratio and carrier concentration of Sn- and S-doped specimens were determined by the infrared absorption method  using a Perkin-Elmer 577 spectrophotometer. The precipitates in Fe- and Zn-doped specimens were observed by A JEM-6A type TEM.
3. Experimental results Using the high pressure LEC technique described above, both doped and undoped twin-free InP single crystals were grown reproducibly with diameters between 25 and 45 mm. and weight of 200—450 g, as shown in fig. 1. The yield of undoped and Sn-doped InP twin-free crystals is about 90%, while that of Fe-doped, S-doped and 80%. Zn3) is about doped InP (n orp ~ lx 10~cm The heavy S-doping or Zn-doping (n or p ~ 3 x 1018 cm3) also gives twin-free ingots with 50% yield if the greater longitudinal temperature gradient during pulling is adopted. Their yields were not influenced by the magnitude of total shoulder angle or the conditions which gave thermal decomposition at the surface. The electrical properties at
lie. 1. Seseral t’ain-free lnP crs~.tak gro’.An technique.
300 K and the etch pit densities of various lnP single crystals with diameters of about 30 mm are summarized in table 1. 4. Discussion 4.1. Twinning in the InP crystal growth process
Twinning is a very troublesome problem in the InP crystal growth process. Various explanations
Table I Electrical properties at 300 K and EPD of various lnP single crystals Crystal No.
Resistivity (12 cm)
2390 1210 1470 2370
3.88 x 104 7.3 xl0 1.58>< 1O~ 3.I2x1O-~
3.6 4.1 xxl0~ l0~ 4 1.d2>< 101 l.74x10
7.47x 10 ~
5.9 ~ i0~
IP-1-l0 IP-T-44 iP-S-18 IP-T-37 1P-S-9 IP-S-17 IP-S-6 IP-Z-3 lP-Z-26 IP-Z-21 lP-Z-8 lP-F-8 IP-F-5
Sn In Sn ln2S1 ln 2S3 2S3 ln2S3 ZnP2 ZnP2 ZnP2 ZnP2 Fe2P Fe2P
1.06x lOis 6.37>< 10n7 7.083<1018 18 7.5 >< 10~ 7 1.4 X i0
105 79.3 67.3
54.3 2400 2100
1.4 >< 10 6.7 ><102 3.7 x102
2.26X10_2 8.7 3<108 2.2 x iO~
2.58 x 10~ 0
0 —0 2.4 ~ 6.0 x 10~ —
Fang Dun -fu et a!. / Growth and properties of InP single crystals
flat solid—liquid interface and (c) a clean melt without scum and bubbles, as well as a transparent B203 layer. It has been reported by Bonner  that
Fig. 2. The surface of a twin-free lnP crystal showing residual indium solid solution caused by thermal decomposition.
have been suggested by several authors [6—9].It is our opinion that the occurrence of twinning is related to the arrangement of the temperature field distribution, the stoichiometry of the polycrystalline raw materials, the presence of any residual oxide and the H20 content of the B203 . It was found that the necessary preconditions for avoiding twinning are to establish: (a) an appropriate temperature field distribution in order to keep a suitable longitudinal temperature gradient; (b) a
the shoulder angle can influence the yield of twin free crystals However twin free crystals can often be obtained even when the rate of diameter en largement is such that the angle which the growing crystal makes with the (111)i growth direction is up to 20°—27°as shown in fig 1 if the above mentioned preconditions were adopted strictly The defects caused by thermal decomposition or sometimes even by the residual indium solid solution which often appears on the surface of the crystals (see fig. 2) caused no twinning, as has been pointed out by Shinoyama et al. . .
4.2. Comparison of properties between S-doped and Sn-doped InP crystals
Sulphur significantly reduces the dislocation density in heavily doped InP crystals by virtue of the impurity effect , whereas Sn does not. The effective distribution coefficient of S was determined to be 0.68 which is 30 times larger than that of Sn . Therefore, S-doped InP crystals
5n doped Sdoped un—doped
-. ._ —
N3 . NA (cr11 N~— NA and compensation ratio Fig. 3. The relationship between carrier concentration
Fang Dun .fu ci al.
Growth and properties of InP single crystals
exhibit a superior homogeneity in the longitudinal direction of the ingot. For example, in a single crystal with a weight of 300 g and diameter of about 30-35 mm, the carrier concentration dif-
Sn-doped InP crystal it is a factor of about 1—L5. S-doped and Sn-doped wafers with  orientation have been used to study the homogeneity based on the compensation ratio and carrier concentration distributions which were determined point by point across the surface. The variation in the N1) NA distribution is 0% and 20% for S-
4.3. Properties of Zn-doped p-type JnP
Recently p-type InP has been used as substrate materials for opto-electronic devices . The most effective dopant Zn can reduce the dislocation density in InP by virtue of the impurity effect [10,111 as shown in table 1. Dislocation-free p-lnP single crystals without precipitates were easily obtamed, if the carrier was than greater 3 andconcentration the diameter less 30 than The 2 x 1018 cm distribution coefficient of Zn in mm. effective lnP was calculated to be 1.4, which is greater than in the published data . This discrepancy may. in our opinion, be due to the physical behaviour of the dopant and its doping method. It is evident that ZnP 2 is a relatively ideal dopant with a melting point of 1040°C, close to that of InP. The carrier concentration along the InP crystal from seed to tail also showed that the effective distribution coefficient of Zn is obviously greater than one. It must be pointed out that the absorption coefficient of Zn-doped InP is rather high. The absorption efficiency measurements were carried
doped and Sn-doped InP. respectively. In this case N[) varies by 9—18% and 7—29% for S-doped and other InP crystals, while the variation in NA is much larger. In addition, it can he seen from fig. 3 that in some concentration ranges the compensation ratio for S-doped samples is lower than that for Sndoped ones. Therefore, S-doped n-InP is superior not only in perfection and homogeneity but also in compensation ratio,
Fig. 4. Carrier concentration NA — N1, versus absorption coefficient (a) for various wavelengths.
out using a (100) oriented InP sample with both sides polished, by a SPECORD 61 NIR photometer. The results are shown in fig. 4. It was found that for Zn-doped InP the absorption coefficient increases with increasing carrier concentration and was higher than that of S-doped InP. which is generally less than 10 cm Because the high absorption coefficient is a negative factor influencing the device light emission efficiency, the proper balance between crystal perfection and light absorption mustconcentration. be considered in selecting the optimum doping .
4.4. The properties of the Fe-doped semi-insulating InP
Fe was used as a dopant to obtain semi-insulating (SI) InP [14—16].Because of its lower distribution coefficient and solubility in InP, Fe influences not only the crystal growth process but also the quality of the crystal . In our experiments. doping with 0.02 to 0.06 wt% of Fe, except for some heavily Fe doped samples. shows no second phase in SI-InP crystals by TEM. Fig. 5 shows the resistivity as a function of mass fraction of crystal for various Fe-doped SI-lnP crystals. Generally, a
Fang Dun -fu et al.
Growth and properties of InP single crystals
5. Conclusions (1) By polycrystalline controlling thermal improving qualityfield and distribution, dehydrating
B203, various types of twin-free InP single crystals were reproducibly produced. 0 0
Fig. 5. Resistivity (p) plotted as a function of mass fraction (g) of crystal solidified from InP melts of three different Fe concentrations,
(2) The homogeneity, crystal perfection and compensation ratio of S-doped InP are superior to those of Sn-doped InP. (3) Doping with Zn, dislocation-free p-type InP crystals without precipitates were easily prepared. (4) By controlling the Fe content, perfect single crystals of InP without precipitates and with homogeneous resistivity and thermal stability can be produced with a view to satisfying the need of epitaxy and ion-implantation applications. Acknowledgements
resistivity as high as 106 ~ cm was obtained. According to the requirements for epitaxy and ion implantation the thermal stability of SI-InP was also studied by capless annealing in a purified H2 ambient and the appearance of wafers after annealing at 700°Cfor 60 mm was intentionally observed by a DX-3 scanning electron microscope. It is evident that no significant degradation occurred on those wafers, except on their edges. After annealing at 500—650°C for about 30—60 mm, the surface leakage current increased from 2 x i0~ to 2 x 10_6 A. The variation in sheet resistance between Fe-doped (100) samples after simulated2,Ne~ ~i = i x roomimplantation temperature)(150 and keV, a 700°CaniO’~cm’for about 15 mm, and the unimplanted nealing samples after 650°Cannealing for 30 mm, is in the range of iO~—1011 Q/D. After that we used three doses of Si~(1 x 1013, 5 x 1013 and 1 x 1014 cm2) and made the implantation at 150 keV, then annealing was carried out at 700°Cfor 15 mm. The measured R~and are in good agreement with those predicted by LSS theory, implying there was little diffusion after annealing. The implanted layer shows n-type characteristics, its carrier concentration is 2 x 1017 cm and its mobility is 2100 cm2/V. s. The homogeneity in resistivity, the crystal perfection and thermal stability make these crystals promising for practical uses.
The authors would like to thank Professor Zou Yuanxi, Mr. Peng Ruiwu and Mr. Zhu Jian for their guidance and continuous encouragement during the whole period of this work. We are also grateful to Mr. Qiao Yong, Mr. Zhang Xikang, Mr. Mou Panjian and Ms. Miao Hanying for their help.
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Growth and properiie.s of InP single crystals
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