LEC- and VGF-growth of SI GaAs single crystals—recent developments and current issues

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Journal of Crystal Growth 275 (2005) 283–291 www.elsevier.com/locate/jcrysgro

LEC- and VGF-growth of SI GaAs single crystals—recent developments and current issues M. Jurisch, F. Bo¨rner, Th. Bu¨nger, St. Eichler, T. Flade, U. Kretzer, A. Ko¨hler, J. Stenzenberger, B. Weinert Freiberger Compound Materials GmbH, Am Junger Lo¨we Schacht 5, 09599 Freiberg, Germany Available online 8 December 2004

Abstract The paper reviews the progress made in crystal growth of semi-insulating GaAs by liquid encapsulation Czochralski and vertical gradient freeze techniques during the last few years under the continuous need for cost reduction of the production process. r 2004 Elsevier B.V. All rights reserved. PACS: 81; 81.05.Ea, 81.10.Fq, 81.30.Fb Keywords: A1. Computer simulation; A1. Heat transfer; A1. Segregation; A2. Gradient freeze technique; A2. Liquid encapsulated Czochralski method; B2. Semi-insulating gallium arsenide

1. Introduction Producers of both microelectronic and optoelectronic devices using semi-insulating (SI) and semiconducting (SC) GaAs substrates, respectively, typically look for the lowest cost substrate that will meet their needs. On the other hand, the device must work properly and reliably, not deteriorating with age and, therefore, requiring uniform, high-quality substrates with tailored Corresponding author. Tel.: +49 3731 280 212;

fax: +49 3731 106. E-mail address: [email protected] (M. Jurisch).

device-specific properties, but without any substrate-related problems that could adversely affect the performance or lifetime of the device. It is the permanent challenge for the crystal grower and substrate manufacturer to balance these two needs: to supply reliably customer specific, highquality substrates at the lowest possible cost. In addition, this is a prerequisite to stay competitive with respect to Si and other alternative materials. To reduce costs of the products two main strategies are followed: the increase of yield, i.e. the fraction of crystals/wafers which meet customer’s specification, and the reduction of process costs. This means growing single crystals with the

0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.10.092

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required diameter, an optimum, not necessarily maximum length and with predetermined structural and electrical properties homogeneously distributed across and along the parallel body of the crystal with highest productivity, reproducibility and reliability. Therefore, each step of the production chain has to be analyzed and optimized continuously. Due to differences between the competitors concerning equipment, technologies and economical ‘‘boundary conditions’’ different solutions have been and will be found the details of which belong to the technical/technological know-how of the producers and are scarcely published. This paper will illustrate with examples the impact of such cost reduction efforts on synthesis, crystal growth and properties of SI GaAs produced by liquid encapsulation Czochralski (LEC) and vertical gradient freeze (VGF) technique during the last years with main emphasis on the results obtained in the authors company caused by the reasons mentioned above.

2. Synthesis For LEC growth, synthesis from high-purity components is performed either in the puller before crystal growth [1,2] or in a separate highpressure equipment (E100 bar) as described, e.g. in Ref. [3] and shown in Fig. 1 in the present-day version for up to 50 kg charges. The necessity of additional pBN crucibles and boron oxide charges for the ex situ method is contrary to cost reduction, but this disadvantage is more than compensated by lower puller costs due to the smaller pressure range necessary for the growth process and the related lower maintenance costs and, more conclusively, by the possibility to roughly specify the boron, carbon and oxygen content in the required field applying relationships derived from a thermochemical consideration of the reaction system outlined in Section 3.3 [4]. To give an example, oxygen chemical potential in the working gas can be influenced by freezing-out Ga2O formed at the interface between the GaAs melt and the boron oxide encapsulant and transported through the boron oxide to the gas

Fig. 1. Equipment for ex situ direct synthesis.

phase. Furthermore, the purification ability of the boron-oxide can be used to further improve purity. For VGF and related growth techniques synthesis in a separate equipment is mandatory. The equipment shown in Fig. 1 houses a graphiteheater structure surrounded by heat insulation in a pressure vessel (100 bar max) with water-cooled walls and is equipped with a fully computerized process control system. It can be charged with one 1600 crucible or several smaller crucibles using appropriate handling systems. Charge preparation from the components is performed in a nitrogen flow box under permanent control of oxygen fugacity. As-losses during the process which sensitively depend on the size distribution of the incoming As-charge are compensated by a corresponding, empirically determined surplus of As. Ar as working gas is preferred if a low oxygen level is required for the ingots. After reaction and homogenization the melt is directionally solidified. Average cycle time is (12–18) h. Near-net shape ingots fitted to the LEC or VGF growth crucibles are produced in pretreated pBN crucibles. The ingot extraction procedure from the crucible determines crucible life time and is an important issue which is under current consideration. There is no information whether or not lowpressure synthesis by the injection or evaporation method, which would potentially allow for a stochiometry control by the As-fugacity, is commercially used for compounding SI GaAs.

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There are presently two main techniques commercially used to grow SI single-crystalline GaAs with a diameter larger than 300 . The vertical boat techniques which include both VGF and vertical Bridgman (VB) and the LEC method. Both methods have been developed to an equivalent technological level at FREIBERGER during the last years. There are some differences in the two types of crystal growth and also in the resulting GaAs material. Under economical considerations, in the LEC process the temperature gradients inside the furnace are quite large. This allows heat to be removed very efficiently, i.e. the crystals can be grown fairly rapidly, typically (7–10) mm/h. For VGF and VB furnaces, the temperature gradients are considerably smaller. As a result, the crystal growth speed is also considerably slower at about 3 mm/h. So it is obvious that the productivity of LEC crystal growth is somewhat higher than for VGF/VB crystal growth. However, LEC furnaces are more expensive than VGF furnaces, so capital cost and depreciation are higher. In addition, the VGF/VB processes tend to be more highly automated than LEC, leading to lower labor costs. Summing up, roughly similar production costs can be stated. More scientifically, smaller non-linearities of the temperature field and the related thermal stress in the as-solidified crystal for boat methods compared to meniscus-controlled LEC result in a better structural perfection characterized by an EPD which is lower by at least one order of magnitude, larger size of dislocation cells and lower FWHM of rocking curves of the former. This is related to some minor changes of the physical properties of the crystals like reduced residual strain, shift of the size distribution of Asprecipitates and related COP’s to higher bins, reduced mesoscopic homogeneity, lower average EL2-concentration, etc. The vapor pressure controlled Czochralski (VCz) method [5] which was favorite in the mid1990s as the most appropriate method for SI GaAs single crystals with 150 mm in diameter and larger according to an evaluation of equipment invest-

ment, productivity, carbon control, dislocation density, etc. [6], is presently not of commercial relevance due to its higher equipment costs. 3.1. Crystal length As the length of the parallel section of GaAs single crystals significantly influences economics melt size and crystal diameter for both LEC (Fig. 2) and VB/VGF growth were incrementally increased over the time. Development/optimization of graphite setups and especially design of radiation/convection screens and of the growth technologies have been supported by computer simulations using advanced 2D global and local models which apart from heat transfer by conduction and radiation also account for convection in the fluid phases [7–9] and allow for to estimate thermal stress during growth as a basis for further improvement of structural perfection of the crystals. With increasing melt size and correspondingly higher Rayleigh numbers in LEC growth unstable and turbulent melt and gas flows and the breakdown of axisymmetry have to be considered. The necessary 3D simulations are presently being developed [10]. The FCM state-of-the-art high pressure LECpuller [11] is designed for crucibles up to 1600 in diameter und charges up to 50 kg with a heater setup consisting of three independently controllable graphite heaters. The central main heater is used for diameter control. The lower sub-heater controls the axial temperature gradient in the

10000 FCM 2000 200 mm

Wafer area / cm2

3. Crystal growth

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1000 Westinghouse 1984 100 mm

100 10

RSRE 1964 First LEC-grown GaAs-crystal

1960

Litton Airtron1991 150 mm Bell Laboraties 1971 2”

1970

1980 1990 Year

2000

2010

Fig. 2. Diameter of SI GaAs single crystals grown by the LEC method.

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crucible bottom region in such a way that the buoyancy driven convection is rather weak and unfavorable for single crystalline growth concave solid/liquid interface is suppressed. The upper subheater controls mainly the temperature distribution along the crystal above the boron-oxide encapsulant in order to suppress selective arsenic evaporation. The pullers are equipped with a fully computerized process and diameter control system including pressure and CO control (see below) as well as handling devices for loading/unloading and cleaning. Upon request the furnace can be replaced by a VGF core (see below) leaving other components unchanged. SI GaAs single crystals 150 mm + from 30 to 45 kg melt size are commercially grown [11,12] from which up to 300 wafers can be produced. The capability of the LEC pullers for growing 200 mm + GaAs single crystals has been reported recently [13,14]. There are, however, some technical as well as economical limitations for a further enhancement of the crystal length: as can be verified by theoretical considerations [15] the melt height in the crucible should not exceed about 100 mm in GaAs crystal growth in order to prevent uncontrollable temperature fluctuations in the melt due to increasing buoyancy driven turbulence. This turbulent convection can only partly be damped by commonly used crucible/crystal rotation. The application of static or time-dependent magnetic fields to control convective flow in the melt is not yet commercially used for GaAs crystal growth could be, however, an interesting alternative. Therefore, growth of longer crystals would require crucibles with larger diameter which is economically unfavorable. A compromise has to be made. Another limitation is the strength of the GaAs seed crystal at higher temperatures which would require a complicated supporting mechanism. Only little information is available about commercial VB/VGF crystal growth. The TAMMANN-STOEBER type furnace used at FREIBERGER for liquid encapsulated VGF growth of SI GaAs is schematically shown in Fig. 3 [16]. The heater system is similar to that first described by Ramsperger and Melvin [17]. It has

Fig. 3. Schematic representation of the TAMMANN STO¨BER type VGF furnace.

been upscaled from a design described in Ref. [18] and consists of upper and lower flat graphite or CFC-heaters to define mainly axial temperature gradient and jacket heaters. It is designed for pBN crucibles from 300 up to 200 mm in diameter and 400 mm in length without significant changes of hardware components except the crucibles when changing the diameter. The temperature field and its upward motion with a given solid/liquid interface rate are realized by a proper power control based on precise temperature measurements in different parts of the structure. Complemented by heat insulation the furnace is housed in a pressure vessel allowing for up to 10 bar. Again, the entire process including total and partial pressure definition in the vessel is completely computer controlled using growth programs optimized by inverse modeling [19]. Handling robots for loading and unloading the crucibles are available. The temperature gradient at the phase boundary has been adjusted to 3–5 K/cm for all diameters, the growth rate is between 2 und 4 mm/h. Exact or off-oriented o1 0 04 seeds are placed in a well at the bottom of the cone-shape part of the crucible [20]. Melt size is typically around 18 kg for 150 mm diameter crystals. Growth of 200 mm diameter SI GaAs single crystal by the liquid encapsulated VGF growth has been recently demonstrated [16].

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Apart from the open system described above, growth in closed ampoules with and without liquid encapsulation and, therefore, without and with As-vapour pressure control, respectively, may be also commercially used [21,22]. From Hagi et al. [23] it can be guessed that crystals up to 300 mm in length and 150 mm in diameter are grown commercially by the VB-method. Limitations of the VGF method to further increased crystal length are seen in the occurrence of polycrystalline transitions mainly starting at the crucible wall which are not yet fully understood. Another reason is the price of pBN crucibles which overproportionally increases with the length of the crucible. Again, life time of pBN crucibles is a big issue in spite of some progress made in the past by improving the pretreatment procedure and the extraction process as well as crucible quality/ material. 3.2. Copy exactly Under mass production conditions, a greater number of pullers has to be run. Therefore, a further important issue to save time and money is to follow a copy exactly strategy for LEC and VGF growth. This means not only to use an unified furnace structure applicable for the whole diameter range of the crystals to be grown, but also includes all hardware components as well as operations condition. The realization of this concept makes it possible to run unified growth programs, reduces stock-keeping, facilitates all pre- and post-growth processes, avoids tests after maintenance measures, etc. Furthermore, benefits result from a better crystal-to-crystal reproducibility. However, as will be illustrated by the following examples, the realization of this concept requires consideration of details which could appear to be of minor importance at a first glance. The first example is taken from VGF growth (Fig. 4): the cone-shape part of the pBN crucible is placed on a graphite crucible support. Caused by manufacturing tolerances of crucible and crucible support, local regions with an enhanced thermal resistance can exist. This is modelled in Fig. 4 by an azimuthal slit with increasing width filled by

287

Fig. 4. Schematic representation of a disturbed thermal contact between crucible and support.

working gas. The influence of this slit on the temperature field of the charge in the crucible has been analyzed using the software package CRYSVUN [24]. The results are represented in Fig. 5. The temperature profiles over the symmetry axis of the crucible for four different heater powers (1–4) characterizing different growth progress are plotted. The dotted and the full lines are representing the profiles with and without slit, respectively. The change of the interface position as well as of the temperature gradients, due to the decreased heat conduction in the slit case, is obvious. This simplified model shows that the thermal contact between crucible and crucible support is crucial for the copy exactly project. In analogy, the influence of thickness variations of the crucible wall on the T-distribution in the melt and the growing crystal has likewise been recognized to be remarkable. 3.3. Carbon control Except the length of the single crystal with the required structural quality, yield and related production costs are sensitively determined by the accuracy with which the customer-specific electrical parameters including their axial and radial homogeneity can be met without preliminary test runs

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1′

2′

2 3′

3 4′

1450 1400

dotted lines full lines same colors

4

: with slit : without slit : same heater powers

1350 0.15

0.20

0.25

0.30

axial position / m Fig. 5. Influence of a slit between crucible and crucible support on temperature profiles with heater power as a parameter.

and characterization procedures. For semiinsulating GaAs this means that carbon control for both LEC and VGF is mandatory. The underlying principles and mechanisms will be outlined in the following. Semi-insulating behavior of GaAs is basically connected with the intrinsic mid-gap double donor EL2 being a single arsenic antisite. The Fermi level is pinned at this localized state resulting in SI GaAs if the condition NEL24[C]+(NN )40 is fulfilled with N SA SSD EL2 and [C] being the EL2 content and the concentration of intentionally doped carbon, respectively, and (NSANSSD)E1014 cm3 the concentration of impurities acting as acceptors (A) and donors (SD) shallower than EL2. Keeping residual impurities low and constant, carrier concentration and by this way electrical resistivity of SI GaAs is determined by the EL2- and carbon concentration. An equilibrium thermochemical study neglecting species transport has been performed first to get a better understanding of the complex reaction system which comprises the gas atmosphere, the boron oxide melt, solid and liquid GaAs, crucible and graphite heaters. But instead of trying to consider a complete set of equations describing the chemical reactions and formulating and solving the corresponding set of mass action laws, the total Gibbs Free Energy of the entire system is minimalized to calculate the equilibrium concen-

-3 -4 -5 -6 -7 lg (x ) B

lg (pCO) -3

-2

-1

-8

0

-7

-6 lg (x ) O

-6.8 -6.0 lg (x ) N -4

1 C(s) satd. 0 -1 GaAs (l)

-2

GaAs (l) + B2O3 (l)

-3

-6

-4

-7 -8 -9 -10

-5 -25

-20 -15 pN2 / bar 2 80 lg p(O2), p in bar

-10 -4

lg (xC)

temperature / K

melt

1500 crystal 1

tration of the components and species. For this, the commercially available ChemSage code [25] has been used. Further details are given in Ref. [4]. The results of such calculations are best visualized as a so-called predominance area diagram [26] at the melting temperature of GaAs, i.e. a plot of log aC over log pO2 ; with aC and pO2 being the carbon activity and the oxygen partial pressure, because mainly redox equilibria are concerned (Fig. 6). The coexistence area of liquid GaAs and B2O3 is bound by the dissociation of boron oxide at lower and oxidation of Ga in the GaAs-melt at higher oxygen chemical potential. The upper boundary of the stability region is given by the solubility limit aC ¼ 1 of carbon in liquid GaAs. It is obvious from this diagram that fixing the chemical potential of oxygen, the carbon concentration in the melt can be defined by setting the carbon activity in the gas phase and vice versa. On the other hand, from the dotted line in Fig. 6,

Ga2O3(s) + B2O3 (l)+ GaAs(l)

1550

lg (aC)

288

-3

0

-1 -2

lg (pCO)

Fig. 6. Predominance area diagram for liquid encapsulated growth.

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109

resistivity / Ωcm

108 107 106 LEC

105

VGF 104 103 1013

1014

1015

1016

carbon content (LVM) /cm-3 Fig. 7. Electrical resistivity versus carbon content for LEC and LEVGF grown GaAs. Solid lines were calculated using the compensation model.

1016

[CAs] (LVM) / cm-3

which represents a constant CO partial pressure in the system, follows that a CO control alone is not sufficient to control the carbon content of the melt. The oxygen chemical potential has to be independently set, for example by the N2-partial pressure. The ‘‘working area’’ inside of which the C- and Ocontrol of the GaAs melt is accessible by CO-/N2partial pressure control is marked in Fig. 6. Diagram Fig. 6 further contains scales for oxygen, nitrogen and boron in the GaAs-melt the concentration of which is determined by the oxygen chemical potential alone. The right-hand scale gives the atomic fraction of C estimated from aC using the experimentally determined activity coefficient [27]. In the framework of the equilibrium approach there are no differences in the predominance area diagrams of LEC and liquid encapsulated VGF growth of GaAs. Therefore, similar procedures can be used for carbon control in both methods. Their practicability is obvious from Fig. 7 representing electrical resistivity versus carbon content for state-of-the-art C-controlled LEC and liquid encapsulated VGF GaAs single crystals grown by FREIBERGER. But as can be seen from Fig. 8 the equilibrium approach outlined above is insufficient in establishing a control procedure for a constant carbon concentration along the crystal. The axial Cconcentration is given for a growth experiment with an initially constant and then gradually increased CO partial pressure in the working gas. Macro segregation with decreasing C concentration (kC E 2) is followed by a transient region, where carbon content increases without reaching a new equilibrium state. This clearly indicates that transport of reaction products in the fluid phases and/or reaction kinetics at phase boundaries cannot be neglected. Therefore, a transport model has been developed [28] which includes carbon and oxygen as control species according to the thermochemical model and system-releated parameters like area and thickness of the boron oxide layer acting as a diffusion barrier and its transport properties. Neglecting concentration inhomogeneities in the GaAs melt and assuming a planar solid/liquid interface, the balance of carbon and oxygen in

289

1015

1013 0.0

0.2

0.4 0.6 solidified fraction g

0.8

1.0

Fig. 8. Axial C-concentration at constant and step-like increased chemical potential of carbon.

liquid GaAs can be expressed by expanding the well known SCHEIL equation as follows: dN C dN C dN C dN C B 2 O3 m s R ¼ þ  ; dt dt dt dt dN O dN O dN O dN O dN O B2 O 3 m s R res ¼   þ : dt dt dt dt dt The first term on the right-hand side of these equations describes the incorporation of carbon and oxygen into the growing crystal (usual SCHEIL equation), the second a transport of carbon- (e.g.

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1016 [C] / cm-3

CO, CO2) and oxygen- (Ga2O) containing species through boron oxide including reaction kinetics at the phase boundaries, the third a decrease of carbon and oxygen in the melt due to a chemical reaction. The last term in the balance equation of oxygen takes into consideration that boron oxide can serve as a reservoir for oxygen due to the heterogeneous reaction of nitrogen with boron oxide. Further details can be found in Ref. [28]. The system of non-linear differential equations given above has been numerically solved and applied to analyze measured axial C-distributions in 150 mm crystals grown in 1100 crucibles and a charge mass of 27 kg under various CO partial pressures held constant during growth (Fig. 9). The solid lines in Fig. 9 were calculated with a single set of model parameters and fitting the initial concentrations of carbon and oxygen in the melt. In analogy, the axial distribution of oxygen in an oxygen-doped crystal could be described by the same model. The agreement between experimental data (dots in Fig. 9) and calculated results (lines) is satisfactory. Based on this model a control procedure has been developed using a predetermined function pCO ¼ f ðgÞ to ensure a constant axial carbon concentration for LEC and liquid encapsulated VGF in the open systems applied by FREIBERGER [29].

1015

1014 140 mbar CO 47 mbar CO 7.7 mbar CO Step ca.10 mbar CO, O-doped

1013 [OGes] / cm-3

290

1014

0.0

0.2

0.4 0.6 solidified fraction g

0.8

Fig. 9. Comparison of measured (dots) and calculated (solid lines) axial carbon distribution in LEC crystals grown under different conditions.

Furthermore, the basic technology for growth of 200 mm diameter SI GaAs single crystals and wafers by both the LEC and the liquid encapsulated VGF method have been developed. FREIBERGER is the unique producer applying both methods in parallel to provide substrates according to the requirements of the customers.

4. Conclusions Acknowledgements During the last few years the developments in SI GaAs single crystal growth and manufacturing were characterized by efforts to make the production more efficient in order to meet the requirement of the device manufacturers with regards to price and quality of the substrates. Significant cost reductions could be reached by increasing charge size and yield of crystal growth and improving the reliability with which the customer-specific properties of the wafers can be obtained. For this, carbon and oxygen control is now reproducibly possible for both the LEC and the liquid encapsulated VGF growth of SI GaAs single crystals. Both methods are commercially used for the production of 200 up to 150 mm SI GaAs crystals and wafers.

The authors wish to thank Dr. K. Romanek for the critical reading of the manuscript and the Bundesministerium fuer Bildung und Forschung (01BM154) and the Saechsische Wirtschaftsministerium (SAB 6891, SAB 8372) for financial support. References [1] T. Inada, et al., GaAs MANTECH Conference, Vancouver, Canada, April 19–22, 1999, p. 205. [2] R.M.Ware, et al., GaAs IC Symposium, IEEE, New York, 1996, p. 54. [3] H. Immenroth, et al., J. Crystal Growth 142 (1994) 37.

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