Active control of the electrical properties of semi-insulating GaAs

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CRYSTAL G R O W T H

Journal of Crystal Growth 174 (1997) 208 212

ELSEVIER

Active control of the electrical properties of semi-insulating GaAs W.M. Higgins*, R.M. Ware, M.S. Tiernan, K.J. O'Hearn, D.J. Carlson M/A-COM, 1CBU, GaAs Materials, Lowell, Massachusetts 01853, USA

Abstract The resistivity of semi-insulating gallium arsenide has been controlled over the range 2 x l0 T to 5 × l0 s f~.cm by the carbon monoxide concentration in the puller atmosphere. To achieve this range of resistivity the CO levels were varied from 0.01% to 32%. The relationship between CO concentration, carbon in the crystal and resistivity was found to hold over the whole of this range, until the carbon content exceeded 3 × 1016 at cm -3 and the material became P-type. Very low CO concentrations were achieved by gettering with hot titanium, and higher CO concentrations by the use of argon and CO2 mixtures. A flow-through gas system was used to maintain the concentration ratios of the various species at the desired level. The concentrations of At, CO, CO2, H20 and H2 were monitored throughout the growth runs with a residual gas analyzer. Keywords. GaAs; Electrical properties; Carbon; Carbon monoxide; Resistivity

1. Introduction With increasing sophistication of device fabrication there has arisen a need to supply material to tight resistivity specifications within the range of 2 x 107 to 5 x i0 s f~-cm while maintaining high electron mobilities. For practical purposes, the electrical properties of semi-insulating gallium arsenide can be explained by the simple three level model of Kirkpatrick et al. [1] in which n(H) oc N E L 2 / ( E N s a -- ENsd),

* Corresponding author. Fax: + 1 508 656 2800.

where n(H) is the Hall concentration of electrons and NEL2, ENsa, and ~Nsd are the concentrations of the deep donor EL2, shallow acceptors and shallow donors, respectively. To obtain semi-insulating behavior, the following criterion must be satisfied: NEL2 > ZN~a > ENsd. In annealed LEC GaAs the concentration of EL2 is in the range 1-3 x 1016 cm -3. With modern high purity raw materials and handling techniques it is straightforward to reduce the total shallow impurity concentration to less than 5 x 1014 at cm -3. It is therefore practical to control the resistivity by controlling the dominant shallow acceptor, carbon. Pearah et al. [2] clearly demonstrate the

0022-0248/97/$17.00 Copyright ~i;' 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)01 106-2

~ M . Higgins et al. / Journal o[ Cr)'stal Growth 174 (1997) 208-212

dependence of resistivity, carrier concentration and electron mobility on carbon concentration. There are two main sources of carbon: (1) Carbon is present as an impurity in the raw materials, especially the arsenic. The so-called 'Seven Nines' arsenic typically contains 1 2 ppm carbon. Since the distribution coefficient of carbon is greater than unity [3], one would expect at least 4 x 1016 at cm 3 carbon in the crystal. However, LEC crystal growth is not a simple segregation process. Rumsby and Ware [4] showed that the water content of the boric oxide encapsulant affected the levels of several impurities and the melt stoichiometry. Hunter et al. [5] related the residual water content of the oxide to carbon content in the crystal. Presumably, the carbon is removed from the melt by a reaction with hydroxyl ions in the boric oxide encapsulant, such as 2C,,,~l, + 2(-OH)o~ia~--~2CO + H2.

(1)

(2) The second source of carbon is carbon monoxide generated within the puller. This results from the reaction of the hot graphite furnace components with residual atmospheric water vapor and oxygen [6 8] and with oxygen from the decomposition of B203 [3]. The CO present in the growth atmosphere dissolves in the boric oxide and is incorporated into the melt by reaction with the gallium or arsenic at the oxide/melt interface. COo,,ide + 2Gam~l,+--+Ga2Ooxid~ + Creel,,

12)

3COoxid~, + 2Asmeld-+As203 oxide + 3Cmelt.

(3)

Carbon monoxide in the puller atmosphere has the potential to reverse the process of removal of carbon from the melt, and after the initial reduction by reaction (1), the carbon concentration in the melt, and hence in the crystal becomes a function of the concentration of CO in the atmosphere, and in the boric oxide. Doering et al. [9] reported that in their system the carbon in the crystals was a function of thickness of the boric oxide. This suggests that diffusion through the oxide is a rate-limiting step. Before the CO concentration can be used as a control parameter, the background level must be reduced by a combination of vacuum baking

209

[6,9], purging [6, 9] and for very low levels, gettering [9].

2. Experimental procedure The H P L E C growth runs were performed in a Cambridge Instruments 358 puller. This production puller was routinely used for the H P L E C growth of SI GaAs. The 8 k growth charges were made from 7N gallium and 7N arsenic using in-situ synthesis. A pyrolytic boron nitride crucible was used to hold the charge. The boric oxide encapsulant (1200ppm H20) was from Johnson-Matthey. The chamber atmosphere was either ultra high purity argon or a pre-mixed gravimetric master Ar and CO, gas mixture from Scott Specialty Gases and the pressure was kept at 200 psi during the growth of the crystal. The CO2 in the gas reacts with the hot graphite in the puller to form CO. The use of a hot Ti getter was evaluated for the selective removal of atmospheric gases. The crystal and crucible were counter-rotated at 6 and 30 rpm, respectively. The pull rate was 9 mm per hour. The puller atmosphere was measured with a Leybold Inficon Transpector CIS Gas Analysis System through a pressure reduction manifold. A two-stage pressure regulator reduced the puller pressure from 200 psi down to 10 psi and a small orifice in the manifold was differentially pumped to reduce the sampling pressure from 10 psi down to 3.5 × 10 5 Tort. The RGA samples and measures this low-pressure gas. The use of capillary tubing connections results in short gas transit times between the puller and the RGA. A Nicolet 20DXC FTIR spectrometer was used to analyze the carbon content of the GaAs wafers. The 1988 Japan Electronic Industry Development Association calibration factor of 11.8×1015 was used [10]. The electrical resistivities of the GaAs wafers were measured by the van der Pauw technique.

3. Generation and control of CO and t12 In this section we describe the puller atmosphere during crystal growth runs under three sets of conditions. In the first, the CO and Hz present are

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I'EM. Higgins et aL /Journal of C~stal Growth 174 (1997) 208-212

formed from residual and melt/oxide reactions. In the second the CO and H2 are reduced to very low levels by titanium gettering. In the third, the CO concentration is increased to very high levels by using an argon and CO2 mixture. In each case the graphite had a 1460°C vacuum bake (heater temperature) before loading and a 140°C vacuum bake after loading. The puller atmosphere was purged after synthesis and melting, and before growth started, by lowering the pressure from 800 to 60 psi and then backfilling to 200 psi. Fig. 1 shows the CO concentration in the atmosphere of the puller during the three types of flowthrough runs. The RGA record begins on filling the puller with argon after the 140°C bake. Once the graphite gets hot the concentration of water vapor falls as CO and hydrogen are generated by the following reactions: Cgraphite + H 2 0 ~ - r C O + H2,

(4)

2Cgraphit e q- O2,--~2CO.

(5)

Note that these reactions should produce a CO/He ratio greater than or equal to unity, whereas it is consistently observed in pure argon runs where no extra CO is added that the H2 concentration significantly exceeds the CO. Nishio and Fujita [11] suggested that this is due to the CO remaining in solution in the boric oxide, but

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H 2 may also be generated by reaction between the water in the B203 and the gallium.

4Gamezt + 2(-OH)oxid~*--~2Ga2Ooxiae+ H2.

(6)

This interpretation is supported by Rumsby and Ware's observation [4] that increased water concentration in the encapsulant led to increased gallium in the oxide and on the puller wall after the run. In a pure argon run, Ar, CO, and H2 were the dominant gases in the system. A flow of approximately 250 ml/min (at STP) of argon was maintained throughout the run. The CO changed from 170 to 580ppm. The carbon concentration was 5.5 x 1014 at cm -3 at the seed end of the parallel section of the crystal and 5.1 x 1014 at cm -3 at the tail end. At low CO and carbon concentrations we are also seeing the effect of variations in the residual carbon in the elements and the melt. The effect of CO in the puller atmosphere can be overridden by residual carbon in the melt. The apparent reduction in carbon concentration in the bulk GaAs is within the measurement error of the IR spectrometer at these low carbon concentrations. Since this is an open flow-through system and there is a continual addition and removal of carbon from the melt, segregation coefficient is not the overriding influence on carbon concentration in the melt. Fig. 1 shows the CO seen when a hot Ti getter is used in the growth system during an otherwise standard run. Note the constant reduction in CO. It is being gettered by the hot Ti. The CO concentration was reduced from 160 ppm at the seed end to less then 60 ppm at the tail. Carbon content in the crystal is reduced below the detection limit of the FTIR. The resistivity of the crystal was 2 x l07 ~ . cm at the seed end and 4.9 x 105 ~ - cm at the tail. The loss of resistivity is a result of reducing the carbon concentration below that of the shallow donors so the condition for semi-insulating properties is no longer met. Fig. 1 shows the CO seen during a controlled atmosphere run. Above a heater temperature of 780°C there is a rapid conversion of CO2 to CO. As expected the CO concentration is nearly twice as large as the initial COz concentration, indicating almost complete conversion of the CO2 to CO by reaction with graphite. Continuous flow of Ar and

W.M. Higgins et al. / Journal q/C~stal Growth 174 (1997) 208 212

C O 2 during the course of the run at a fixed concen-

tration resulted in a constant amount of C O in the system (29 259 p p m at the beginning of growth and 28 765 p p m at the tail). The carbon concentration of the crystal was 3.9 x 101~ at cm -3 at the seed end and 4.2 x 10 t6 at cm -3 at the tail. The corresponding values of resistivity were 1.7x 10 s and 1.5 x 10 8 f~' cm.

the correlation between C O concentration in the atmosphere and electrical resistivity of the crystals. We have achieved with our carbon control process excellent uniformity of electrical resistivity in GaAs

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4. Correlations between C O , C and electrical resistivity

By selecting the appropriate process conditions, it is possible to grow semi-insulating gallium arsenide over a wide range of carbon concentrations and resistivities. We have grown material in atmospheres with concentrations of C O varied from less than 60 p p m to 32%. The CO concentration, carbon concentration and resistivity are related across this range as is shown in Figs. 2-4. Fig. 2 shows the relationship of CO concentration in the puller atmosphere to carbon concentration in the crystals. Fig. 3 shows the relationship between carbon concentration and electrical resistivity for the GaAs ingots, and Fig. 4

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212

W.M. Higgins et al. /Journal of Crystal Growth 174 (1997) 208-212

over the length of a number of ingots grown in puller atmospheres with different concentrations of CO.

5. Discussion and conclusions We have demonstrated a clear correlation between the c a r b o n m o n o x i d e concentration in the puller atmosphere, the c a r b o n concentration in the G a A s crystals and their electrical resistivity that results when the following conditions are met: (1) the graphite "furniture" in the puller is v a c u u m baked to minimize C O generation and backfilled with argon to reduce absorption of oxygen and water vapor when the puller is opened for loading; (2) after melting the charge, the encapsulant is purged of C O by a pressure reduction and refill cycle; and (3) a flow-through system of argon and CO2 is employed to control the C O concentration during the run, except when a very low concentration is required in which case a hot titanium getter can be employed. By following these conditions we have grown G a A s with electrical properties ranging from Ntype because the ZNsa < ~Nsd to P-type because the [ C ] > [EL2]. The correlation between C O concentration in the puller atmosphere and electrical resistivity of the G a A s holds over the whole of this range.

Acknowledgements The authors gratefully acknowledge C. Hines, T. Chanh, J. C h u m , J. Fox, R. Giacalone, J. Hines,

K. Kuth, S. N o r n g , D, Wong, A. Wetherald, and C. Wetherald for their assistance in crystal growth and processing. This work was supported by the Title III p r o g r a m office under contract # F33733-94-C1019.

References [1] C.G. Kirkpatrick, R.T. Chen, D.E. Holmes, P.M. Asbeck, R.D. Fairman and J.D. Oliver, in: Semiconductors and Semimetals, Vol. 20, Eds. R.K. Willardson and A.C. Beer (Academic Press, New York, 1984) p. 206-208. [2] P.J. Pearah, R. Tobin, J.P. Tower and R.M. Ware, in: Semi-insulating |II V Materials, Malmo, Eds. G. Grossman and L. Ledebo (Hilger, Bristol, 1988) p. 195. [3] M. Sato, M. Kakimoto and Y. Kadota, in: Semi-insulating III V Materials, Toronto, Canada (Hilger, Bristol, 1990) p. 211. [4] D.H. Rumsby and R.M. Ware, Inst. Phys. Conf. Ser. No. 63 (1981) 573. [5] A.T. Hunter, H. Kimura, J.P. Baukus, H.V. Winston and O.J. Marsh, Appl. Phys. Len. 44 (1984). [6] K. Tereshima, J. Nishio, S. Washizuka and M. Watanabe, J. Crystal Growth 84 (1987) 251. [7] Y. Itoh, Y. Kadota, T. Nozaki, H. Fukushima and K. Takeda, Jpn. J. Appl. Phys. 28 (1989) 210. [8] R.M. Ware, P.J. Doering, B. Freidenreich, R.T. Koegl and T. Collins, Semicond. Sci. Technol. 7 (1992) A224 A228. [9] P.J. Doering, B. Freidenreich, R.J. Tobin, P.J. Pearah, J.P. Tower and R.M. Ware. in: Semi-insulating III V Materials, Toronto, Canada (Hilger, Bristol. 1990) p. 173. [10] T. Arai, T. Nozaki, J, Osaka and M. Tajima, in: Semiinsulating III-V Materials, Malmo, Eds. G. Grossman and L. Ledebo (Hilger, Bristol, 1988) p. 201. [11] J. Nishio and H. Fujita, J. Crystal Growth 134 (1993) 97.