In praise of the confined vertical growth of semiconductors

Mat. R e s . B u l l . , Vol. 21, p p . 1123-1129, 1986. P r i n t e d in t h e USA. 0025-5408/86 $3.00 + .00 C o p y r i g h t (e) 1986 P e r g a m o n J o u r n a l s L t d .

IN PRAISE OF THE CONFINED VERTICAL GROWTH OF SEMICONDUCTORS A. Horowitz Nuclear Research Center - Negev, P.O.B. 9001 Beer Sheva 84190, Israel and Y.S. Horowitz Physics Department, Ben Gurion University of the Negev Beer Sheva 84105, Israel

( R e c e i v e d J u n e 16, 1986; R e f e r e e d ) ABSTRACT The growth of semiconductor materials via the confined vertical method has been criticized in the literature because of the expansion of the material upon solidifcation. In this paper, the arguments against confined vertical growth are rebutted and the merits of the technique are discussed. If heat flows are controlled to maintain convex solid-melt interface in confined vertical growth, high quality crystals can be grown. In most cases, these will be superior to "state of the art" Czochralski grown crystals. The full potential of the technique lies, to a great extent, in the development of compatible insulating crucibles. MATERIALS INDEX:

gallium arsenide, gallium phosphide, phide, semiconductors

indium phos-

Introduction Advances in modern device technology continue to dictate stricter and stricter demands on the specifications of electronic materials. The requirements on materials such as silicon and germanium, and especially on materials intended for optoelectronic devices, e.g., GaAs, GaP, InP and several II-VI compounds, are difficult to fulfill. These requirements include: high purity, exact stoichiometry and doping levels, high homogeneity and perfect crystalline defects.

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Perhaps the best example of the problematics involved in meeting these stringent requirements is illustrated by GaAs. Single crystals of this mater, ial are currently grown by pulling techniques, generally under B203 encapsulation and high pressures, this to prevent loss of arsenic from the melt and from the grown crystals. In recent years, the problems of GaAs crystal purity have been satisfactorily resolved by the development of PBN crucibles which enabled the growth of undoped semi-insulating single crystals, Two problems, however, remain, which can deteriorate crystal quality: Thermal convections in the melt can cause striations and inhomogeneities, and large thermal gradients on the grown crystals ( facilitated by gas convections ) can cause thermal stresses leading to high dislocation etch pit density ( of the order of 105 cm-2)(I-3). Such imperfections reduce the minority carrier lifetimes and consequently the device performance. In recent years, considerable effort has been invested in attempts to eliminate the defects created by the LEC growth process. Magnetic fields were applied for the reduction of convective flows (4). Impurity hardening was found effective for the reduction of dislocations (2,3,5,6) but introduced other defects (6) and reduced growth efficiency (7). The lowering of thermal gradients via reduction of the ambient temperatures and pressures (2,3,7-9) is an obvious solution, but leads to serious difficulties in preserving shape stability and avoiding a highly concave solid-liquid interface. This latter effect often leads to a sudden termination of the growth (2,3). The Kyropoulos method or full encapsulation of the crystal also reduce thermal gradients and prevents araenic losses from the grown crystal ( a problem with reduced pressures ) (I0,II). But, this prevents in situ observation of the growing crystal, desirable for the diameter-controlled Czochralski method, especially with the shape stability problems encountered when flat crystal melt interfaces are pursued (2,3). In view of these many growth problems, it is somewhat surprizing that the Czochralski technique is still the most commonly used method for the commercial growth of semiconductors. The two main technical problems mentioned above: thermal convecbions and large thermal gradients are greatly reduced and easier to control when a confined vertical growth is employed. In the following we will rebut the various arguments usually employed versus confined vertical growth and point out, what appears to us, its many advantages. Objections to confined vertical srowth of semiconductors: Facts or fiction? Some of the most useful semiconductor materials (eg., Si, Ge, III-V compounds ) expand upon freezing due to changes in both the coordination number and the nature of the chemical bond ( from semiconductor to metallic type(12)). This volume expansion associated with solidification has led to numerous objections in the literature against the growth of such materials in containers (13-16) and to quote a typical statement: " The compounds expand upon freezing and therefore are badly strained when constricted by crucibles. Therefore, the vertical Bridgman or stockbarger technique is not adequate". We believe, however, that this often quoted opinion is not, in fact, accurate. The difficulties involved in confined vertical growth originate not from the expansion of the material upon solidification but rather from their low thermal conductivities and from the fact that at the melting point, the thermal conductivity of the melt is higher than that of the solid (eg., 17-19). These properties lead to the development of concave crystal-melt interfaces which then promote polycrystallinity and stresses in the grown crystals. If convex

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crystal-melt interfaces are achieved, via better control of heat flows, near perfect semiconductor single crystals can and have been grown via confined vertical growth° These include: GaP (20-22), InSb (23), Ge (24,25), GaAS (22, 26) and InP (22)° Moreover, dislocation etch pit densities of less than 300 cm -2 were achieved, demonstrating the beneficial effects of reduced thermal gradients on the grown crystals (22,25)° In comparison, the same crystal materials grown via the Czochralski method exhibited much higher dislocation etch pit densities ( 103 - 105 cm-2 )o When a crystal with a flat interface is vertically solidified in a crucible, the freezing direction is parallel, not vertical, to the crucible wallso Therefore, no pressure will be exerted on the crystal during Solidification since the volume expansion is directed upwards, towards the residual melto If the crystal-melt interface is concave, rather than planar, the radial crystallization proceeds inwards (figure 1 ). The relatively cooler crucible walls

a

b FIG.

i

The effect of the solid-melt interface on the growth of a single crystal. The arrows represent the direction of the radial heat flows, a) Material solidification outwards, crystal is single and non-strained, b) Materials solidifies inwards, crystal is strained and polygrained.

will promote spurious nucleation and the new nuclei, like other crystal defects, will grow inwards - towards the crystal core. Pressure will thus be exerted on the inner-still crystallizing part of the crystal by the outer, solidified material whenever ds # d~ ( solid and liquid densities, respectively). If ds < d~ , the outer crystallized material will "inhibit" volume expansion of the still crystallizing inner core and compressive pressure will be exerted. In the same manner, the inner parts of the crystal will be subjected to an outward directed strain when ds > d~ ( II-VI compounds ), leading to cracks and inner cavities. It is therefore best to aim for a slightly convex crystal-melt interface (fig. i). This will reduce the tendency for interface concavity to develop during the growth, prevent polycrystallinity and stresses and will force incidental second nucleation or other surface defects to "grow out" of the crystal (13). The so-called "interface effect" (27), i.e., the effect of a local temperature difference between the crystal and the crucible wall is therefore a

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dominant factor, determining the solid liquid interface shape and by thatthe polycrystallinity and quality of the crystal= The interface effect is obviously dependent on the axial and radial heat flows, especially at the crystal-melt-crucible wall boundaryo An important factor affecting these heat flows is the ratio between the thermal conductivities of the phases involved° For many semiconductors, the ratio between the liquid and solid thermal conductivities is ~2 (17-19), it is approximately unity for alkali halides (18) and smaller than unity for metals (17,28,29) and oxides (19)o This high ratio for semiconductors, coupled with their low absolute thermal conductivity creates an "insulator effect" by the growing crystal thus promoting heat conduction through the crucible wall and consequently a concave interface. We conclude from the above considerations that single crystals of semiconductor materials can be grown via the Bridgman or thermal gradient methods, if the heat conduction during the course of the growth is controlled to ensure a slightly convex interface° Under such conditions , the expansion ( or contraction ) of these materials upon solidification does not interfere with a perfect growth course° optimization of the srowth process in confined vertical growth The realization that better crystals can be grown in confined vertical growth (30,31) via the control of the shape of the interface has begun to generate considerable analytical and experimental research, It was shown that lower axial gradients as well as positioning of the interface in the hot zone of the furnace promote convexity (17,30, 32-36) and the connection between high growth rates and concavity was established (15,18). Furnace endeffects were recognized (17,18, 35-37) as well as the importance of the growth crucible as a medium affecting the relevant heat flows (25,27,30). Considerable effort was invested in the design of furnaces that promote axial and inhibit radial heat flows from the growing interface. These include long lower stems or low end cooling (17,34,37,38), adiabatic growth zones (36,38, 39}~ auxilary heating above the interface (17,27,35,40,41), heat pipes (30) and Peltler heat removal (27). Most of these efforts have only been partially successful in arriving at optimal growth conditions of convex interfaces, most probably due to lack of sufficient attention to the relative importance of the thermal conductivity of the crucible. It is also true, of course, that current options in the choice of optimal crucible materials is limited and this subject requires intensive additional research. The importance of the crucible material is best demonstrated by comparing the thermal conductivity (and density) data in Table I to the various details of growth experiments. InP, GaP and GaAs have all been grown in thermally anisotropic PBN crucibles. For GaAs, however, it was more difficult to maintain the appropriate flat interface because of its lower thermal conductivity ( and probably also because of the increase of the PBN thermal conductivity with temperature ) and special precautions had to be taken to prevent concave interfaces (22). Germanium single crystals could not be grown in HPBN crucibles ( thermal conductivity: 0.256 W cm-I K-I (27)) or in graphite crucibles ( thermal conductivity: 0.30o6 W cm-I K-l) (24,25,27,37,45) but were successfully grown in quartz crucibles (24) ( thermal conductivity at 940°C ~ 0.04 W cm-I K-I (28)) and in porous graphite crucibles ( thermal conductivity - one to two orders of magnitude lower than that of regular graphite (23)). Indium antimonide crystals were also grown in quartz crucibles (26) ( thermal conductivity at 530oc ~ 0°02 W cm~l K-I (28)).

SEMICONDUCTORS

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TABLE I Properties of Semiconductor Materials Upon Melting Material

Melting Point oc

ds

d~ g cm-3

5.765(23) 5.76 ( 4 2 )

6.47(23) 6.55(23) 6.48(42)

contraction

Ks

Ks

%

W cm -I K -I

11-13.7

0.0474(3) 0.123(3) 0.0923(23) 0.188(23)

InSb

530-536 (12,23)

InP

1062(3)

GaP

~662(22)

GaAs

1238(12)

5.16 ( 1 2 )

5.71 (12,42)

10.7

Ge

937 (43) 5.26 ( 1 2 )

5.51 (12,44)

~5

0.094 (3) 0.228(3) 0.09 (22)

0.0712(3)

0.178(3)

0.18 (27) 0.39 (27) 0.173 (44) O. 174 (29)

Other Considerations in favour of confined vertical srowth Aside from the striations caused by convections, and stresses (dislocations ) caused by thermal gradients, both associated with Czochralski growth, there are additional superior features of confined vertical growth° In Czochralski growth, thermal and forced convections create complex hydrodynamic flows that are difficult to predict or control. Moreover, these flows change continuously with the progress of the growth° To ensure the growth of a good quality crystal, a planar or very slightly curved interface must be achieved via a unique coupling between the rotation rates and the thermal field. Unfortunately, because of the ever-changing thermal field, many adjustments are necessary during the growth process° The optimal growth conditions are, therefore, not only difficult to predict and control but also are difficult to extrapolate from one growth geometry to another ( scale-up ). In addition, and because of the diameter control, the control system is intentionally designed to be highly sensitive to small changes at the crystal-melt interface. This renders the system very susceptible to electronic and mechanical noise. In confined vertical growth, on the other hand, diameter control is not necessary, the crystal-melt interface is well insulated and less affected by small fluctuations in the outer thermal field° A yet additional advantage of confined vertical growth is that floating impurity particles are physically removed from the growing interface and cannot be incorporated in the crystal. In addition to the above factors relating to crystal quality, there are other economical advantages to confined vertical growth° First, with no need for diameter control and high quality rotation units and verticality, the systems are a-priori relatively inexpensive° Since less on-line supervision is required, operation costs are also lower than those with the Czochralski systemso Second, crystals grown via confined vertical growth techniques are not limited to cylindrical geometry. Rectangular geometries are easily grown (eg., for good packing of solar cell arrays ) and even more complex geometries can he designed ( for optical components )o This versatility in growth

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geometries also enables considerably less material losses during component fabrication° We do not mean to imply that confined vertical growth of semiconductors is necessarily a straightforward process° The main remaining problem lies in the identification of a crucible material compatible with the grown crystal material= In the Czochralski process, it is sufficient that the crucible not introduce unwanted impurities into the melto With confined vertical growth, however, two additional requirements arise: i.e., that the melt will not adhere to the crucible wall and that the crucible wall will be a better thermal insulator than the solid crystal. Obviously, high purity graphites, developed for the Czochralski growth of germanium crystals, have too high a thermal conductivity for the vertical confined growth of semiconductors. Porous graphite products, which are excellent insulators, have not, as yet, been developed with adequate purity (25). Quartz is limited to low melting point materials because of its softness and good thermal conductivity at high temperatures. An additional problem can arise due to the material or its residual oxide sticking to the crucible wallo PBN seems to be a good candidate for the growth of several semiconducting materials, however, its overall thermal conductivity is borderline for the lower thermal conductivity materials. Summary The problematics of the growth of semiconductor materials via confined vertical growth arise from the materials' low thermal conductivity, and from the relatively higher thermal conductivity of the materials' melts and of the crucible materials. These thermal conductivities promote radial heat flows leading to concave interfaces, and by that, to polycrystallinity, stresses and high dislocation densities° High quality crystals with low defect densities and high homogeneity can, however, be grown via the confined vertical method, if convex growth interfaces are maintained during the course of the growth, via the promotion of axial heat flows° Although considerable attention has been paid to the factors affecting the direction of heat flows during confined vertical growth, more research is required into the develop, ment of optimal ( more insulating ) crucible materials. Such a development will even further enhance the renewed interest in the confined vertical growth of semiconductors and the possibilities of its commercial exploitation. Re fe rences Io 2. 3. 4. 5. 6. 7. 8. 9.

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i0. 11,

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17o 18o 19o 20° 21o 22° 230 24°

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