GB2147223A - Semiconductor substrates - Google Patents

Abstract

Single crystal compound semiconductor substrate material of constant composition is grown from the melt 2 by a method in which compositional control is effected by adding to the melt source material comprising two or more different materials 5, 6 at a rate matched with the rate of extraction of material 4 from the melt by the crystal growth. <IMAGE>

Description

SPECIFICATION Semiconductor substrates This invention relates to the manufacture of bulk semiconductive material for semiconductor substrates. The construction of most types of single crystal semiconductor devices involve epitaxial growth upon a semiconductor substrate, and, in order to achieve good crystal quality in the epitaxially grown material, it is normally necessary to have a good match between its lattice constant and that of the substrate upon which it is grown.

For this reason quaternary (In,Ga)(As,P) material devices have hitherto always been made in epitaxial layers lattice matched with InP and grown upon InP substrates, or in epitaxial layers lattice matched with GaAs grown upon GaAs substrates. In the former instance this allows quaternary material to be grown with a band gap lying in the range 0.72 eV to 1.35 eV (the band gap of InP), while for the latter instance the range is from 1.42 eV (the band gap of GaAs) to 1.91 eV. In each instance a particular band gap is satisfied by a specific composition, whereas in general there exists a continunum of compositions within this semiconductor system that have any chosen value of band gap.Furthermore it will be noticed that in neither instance is it possible to grow lattice matched material having a band gap in the range from 1.35 eV to 1.42 eV,-nor is it possible to grow material in the range 1.91 eV to 2.26 eV (the band gap of GaP) or in the range 0.35 eV (the band gap of InAs) to 0.72.

These types of restriction are avoided according to the present invention by the use of substrate material that is not necessarily binary.

According to the present invention there is provided a method of preparing bulk single crystal compound semiconductor material, wherein said material is grown from a melt whose volume and composition are maintained constant by continuous addition to the melt of source material comprising two or more different materials at a rate matched with the rate of extraction of material from the melt by said growth.

It is believed that one of the factors militating against previous use of 'multinary' composition material (ternary, quaternary or higher) for semiconductor device substrates has been the different segregation coefficients of the different elements of a multinary system. This means that material crystallising from a melt does not have the same composition as the melt. A result of this is that if nothing is added to the melt it becomes progressively depleted of the element or elements of the mixture that preferentially freeze. The consequent progressive change in melt composition produces, in turn, a progressive change in the composition of the growing crystal.

The continuous replenishment of the melt afforded by the method of the present invention ensures that the melt volume remains constant.

Under these circumstances, if the rate at which any constituent element of the melt is added to the melt from the source material is greater than the rate at which it is withdrawn in the growing crystal, then the abundance of that element in the melt tends to increase. This increases the proportion of that element in the freezing material until eventually the two rates match. Similarly, if the rate of supply of an element is less than the rate of its extraction, then the abundance of that element in the melt decreases until the rate of supply matches the.rate of extraction. Thus, if the melt composition is not initially quite appropriate for growing the material whose composition is determined by the relative rates of source material supply to the melt, it will nevertheless become stabilised at the appropriate composition.

There follows a description of bulk semiconductor growing embodying the invention in a preferred form. This description refers to the accompanying drawings in which Figure 1 schematically depicts the apparatus employed, and Figure 2 is a diagram depicting changes in band gap and lattice parameter within the (Ga,ln)(As,P) system.

Referring to Figure 1, a crucible 1 containing a semiconductor melt 2 is located in a furnace here depicted schematically by the coils 3 of an induction heater. Ihe crucible is supported upon a transducer 4 which provides a continuous read out of its mass. (In an alternative arrangement, that is not shown, the crucible is instead suspended from such a transducer). A rod 4 of single crystal material is grown from a single crystal seed by withdrawing the rod 4 at a controlled rate vertically from the surface of the melt 2. At the same time the melt is replenished by lowering into the melt rods 5 and 6 of source material. This replenishment is also at a controlled rate, and is adjusted to keep the melt volume constant. Stirring means, not shown, is also provided to ensure proper mixing of the source material as it goes into solution in the melt.As an alternative to continuous measurement of the mass of the crucible and its contents, it may instead be preferred to measure the mass of the rod 4, and to balance its increase against the decreasing mass of undissolved source material.

When using source material of known constant cross-section it is possible to control the rate of feed by reference to its linear movement vertically into the melt, but generally it is preferable to control it by reference to its mass. This also has the advantage that knowiedge of the geometry of the source material is no longer necessary.

A multinary semiconductor system of particular interest for device manufacture in the quaternary system (In,Ga)(As,P), though it will be appreciated that the apparatus is applicable also for growing material in other systems. Figure 2 is a diagram depicting how the band gap and lattice parameter varies within the (in,Ga)(As,P) system. In this figure the four binary compounds GaP, GaAs, InP, and InAs are plotted at the four corners of a square such that indium is progressively replaced by gallium in moving vertically upwards from the InP and InAs corners, while phosphorus is progressively replaced by arsenic in moving horizontally to the right from the GaP and InP corners.The locus of points of equal band gap (measured in electron volts) are depicted by unbroken lines 20, whilst the locus of points of equal lattice parameter (measured in nanometers) are depicted by broken lines 21.

Inspection of Figure 2 reveals that at least in principle it is possible to go from the largest possible lattice constant of this system to the smallest by progressively replacing InAs with GaP, and hence moving up the diagonal line 22. This means that only twd source materials InAs and GaP are in principie required to make a substrate material having a lattice parameter matched with any composition within the entire (Ga,ln)(As,P) system.In practice however, problems associated with phase separation may make certain compositions on the line 22 virtually inaccessibie. in such a case the lattice parameter associated with such a composition may alternatively be arrived at by adding a third component, either GaAs or InP, to the existing two component mix of source materials, or by using an alternative pair of the four compounds for the two components of the source material.

The foregoing discussion has been particularly concerned with the growth of substrate material to provide a lattice parameter that is matched neither with that of GaAs nor with that of InP. Additionally however, there can also be advantages in using the invention to provide quaternary (or ternary) substrate material that does have a lattice parameter matched with that of InP so as to avoid some of the problems associated with using InP. One of the properties of InP that causes problems is its dissociation at high temperatures. The equilibrium pressure of phosphorus above an indium phosphide melt at 1070"C is in the region of 30 atmospheres.

Hence InP substrates are relatively expensive to prepare since generally liquid encapsulated Czo chralski growth is required, using molten boric oxide as a blanket to cover the melt surface so as to be able to use an inert high pressure atmosphere instead of one of phosphorus vapour at high pressure. A further consequence of dissociation is that the exposed surface of an InP substrate is, as a result of incongruent evaporation of phosphorus, liable to degradation prior to epitaxial growth upon it of one or more layers necessary for the construction of a semiconductor device.

On the other hand, for arsenic rich (In,Ga)(As,P) alloys having a lattice parameter matched with that of InP, these dissociation problems are less severe because they generally have lower melting points and lower phosphorus equilibrium vapour pressures. The phosphorus equilibrium vapour pressure varies as the fourth power of the phosphorus content of the melt, and hence it can be seen that it is very much reduced by changing the melt composition to move the growth composition up the constant lattice parameter line 21 of Figure 2 that intersects the InP corner to its intersection with the diagonal 22. This intersection occurs at (In0.68GaO32)(As068P0.32). Alternatively the change in composition may be taken to the far end of the line 21 so as to have no phosphorus, but this produces additional problems of melt back.A compromise can be effected by adopting a three component mixture for the source material, comprising mainly InAs together with smaller quantities of any two out of the other three binary compounds of the system.

Two further possible advantages of moving away from InP are that the additional components should help to reduce the mobility of dislocations and thus ease the problem of growing material with a low dislocation density, and that the reduction in growth melt temperature should facilitate the incorporation of deep level dopants in quantities sufficient to permit the growth of substrates of semi-insulating material.

Reverting attention to the growth of substrate material that is not lattice matched with InP, there is beginning to be an increase of interest in the manufacture of semiconductor lasers with emission wavelengths in the range 2 to 2.5 microns, i.e.

material with a band gap in the range 0.6 to 0.48 eV. Inspection of Figure 2 immediately reveals that it is not possible in the (In,Ga)(As,P) system to grow any such material to have a lattice parameter that would match that of an InP or a GaAs substrate. Inspection of an equivalent diagram for the (In,Ga)(As,Sb) system would appear to show that it would be possible in this system to meet the requirements using a GaSb substrate or an InAs one, but work within the (In,Ga)(As,Sb) system gives rise to a number of additional difficulties for laser manufacture. Thus there is a relatively large miscibility gap making it very awkward to produce materials with a band gap in the region of 0.5 eV.

There are special problems associated with nucleation on GaSb substrates. Semi-insulating GaSb is not currently available, and therefore it is more difficult to characterise epitaxial layers grown on GaSb. Additionally there is the problem that within the (In,Ga)(As,Sb) system an increase in band gap is associated with an increase in refractive index.

Therefore the conventional double heterostructure device, in.which an active layer is sandwiched between two transparent layers of higher band gap material providing confinement of minority carriers, produces an anti-waveguiding optical structure rather than the waveguiding structure provided in the (In,Ga)(As,P) system. Therefore it is preferred to work in the (In,Ga)(As,P) system for laser fabrication. The composition of the active layer of such a laser must lie to the lower right of the 0.6 eV line 20 of Figure 2, and to the upper left of the chain-dotted line 23 at 0.48 eV. However, the full extent of the region between these two lines is not available since it will normally be necessary for the active layer to be bounded by confining layers of lattice parameter matched material of significantly higher band gap. Thus in practice the range of suitable materials is generally confined to the shaded area 24. If the substrate material for such a laser is be made with just two components of source material, then this may be achieved using InAs and GaAs in proportions given by the range of the bold line 25, or in the range defined by bold line 26 when using InAs and GaP, or in the range defined by bold line 27 when using InAs and InP.

As before, in the case of preparing substrate material with a lattice parameter matching that of InP, the disadvantage of extra complexity involved in choosing to use a three component mix of source material may be outweighted by the greater flexibility such a system affords in the choice of composition having regard primarily to its phosphorus content, but also to its band gap and its melting point. In this context it may be noted that the viscosity of boric oxide at the melting point of InP is almost at the upper limit of what is acceptable for satisfactory use of the liquid encapsulated Czo chralski growth method of preparing single crystal material, and hence for some compositions within the (In,Ga)(As,P) system the melting point may be sufficiently depressed below that of InP, while retaining sufficient phosphorus to warrant liquid encapsulation, that a lower melting point medium is required for use in place of boric oxide. Examples of such materials are described in patent specification No. 2118860A (C.H.L. Goodman 13).

Claims (12)

1. A method of preparing bulk single crystal compound semiconductor material, wherein said material is grown from a melt whose volume and composition are maintained constant by continuous addition to the melt of source material comprising two or more different materials at a rate matched with the rate of extraction of material from the melt by said growth.

2. A method as claimed in claim 1, wherein the rate of addition of source material is controlled by reference to the masses of the undissolved source material constituents being fed into the melt.

3. A method as claimed in claim 2, wherein the volume of the melt is maintained constant by monitoring the mass increase af the growing single crystal material and matching this gain with the loss of mass of the undissolved source material.

4. A method as claimed in claim 1, 2 or 3, wherein the source material consists of two components both of which are binary compounds.

5. A method as claimed in claim 1, 2 or 3, wherein the source material consists of three components each of which is a binary compound.

6. A method as claimed in claim 1, 2, 3, 4 or 5, wherein the compound semiconductor material being grown is in the (In,Ga)(As,P) system.

7. A method as claimed in claim 6, wherein the compound semiconductor being grown has a lattice constant matched with InP.

8. A method as claimed in claim 6, wherein the compound semiconductor being grown has a lattice constant matched with material in the (In,Ga)(As,P) system having a band gap in the range 0.6 to 0.48 eV.

9. A method of preparing bulk single crystal compound semiconductor material which method is substantially as hereinbefore described with reference to Figure 1 or Figures 1 and 2 of the accompanying drawings.

10. Single crystal compound semiconductor material prepared by the method claimed in any preceding claim.

11. A semiconductor device fabricated upon a single crystal substrate prepared by the method claimed in any claim of claims 1 to 9.

12. A semiconductor laser fabricated upon a single crystal substrate prepared by the method claimed in any claim of claims 1 to 9.