IMPROVED ION-IMPLANTED GaAs DEVICES

Abstract: The invention provides a means of improving the threshold voltage uniformity of ion-implanted GaAs MESFETs by using co-implantation of two dopant species rather than the normal situation where only one dopant species is used. The two dopant species are chosen so that the effects on activation of stoichiometric changes in the substrate are in opposite senses - thus stoichiometric effects on total activation (and thus on threshold voltage of devices) are reduced or eliminated.

Description: TMPROVF.T) IQN-TMPLANTED GaAs DEVICES This invention relates to Gallium Arsenide (GaAs) devices such as MESFETs, which form part of an integrated circuit. An integrated circuit comprising a plurality of GaAs devices suffers from a disuniformity of characteristics such as threshold voltage from device to device upon the same GaAs substrate. It is believed that this disuniformity is due to crystal lattice dislocations in the substrate. In any event, there is a tendency to localised disuniformity of doping level after ion implantation of a dopant. This gives rise to the above mentioned disuniformity of characteristics of the devices. It is an object of the present invention to provide GaAs devices wherein the aforesaid disuniformity of device characteristics is minimised. According to the present invention, a Gallium Arsenide substrate, whereon devices are to be formed, is co-implanted with both Group IV and Group VI dopants in a ratio dependant upon desired activation of the implanted ions. The invention also provides a method of making Gallium Arsenide devices comprising the steps of ion implanting areas of a Gallium Arsenide substrate with a Group IV dopant, ion implanting the areas of the Gallium Arsenide substrate with a Group VI dopant, and adjusting the ratio of the dopants in dependance upon the required activation of the areas. Conveniently, the ion implantation energies of the dopants are arranged to produce similar implantation profiles. The invention will be described further, by way of example, with reference to the accompanying drawing wherein the single figure is a diagrammatic cross-section of part of a semi-conductor device formed on a Gallium Arsenide substrate. One of the major problems in producing GaAs digital integrated circuits based on MESFET devices is that the uniformity of threshold voltage of the MESFETs across the circuit is not as good as for, say, the silicon MOS devices used in some silicon integrated circuits. Currently, the best uniformity in GaAs devices is obtained using ion implantation and annealing to fabricate the active layers. However, even in this case, there is extensive evidence that the local device threshold voltage depends on the proximity of the device to dislocation structures in the underlaying GaAs substrate. Considerable effort has been spent in attempts to establish the cause of this variation and the overall conclusion drawn is that it is due to local changes in stoichiometry of the substrate around groups of dislocations which cause small changes in the activation of implanted dopants, thus altering the threshold voltage. Such stoichiometrically induced variations in activation occur for virtually all implanted dopants in GaAs - the direction of the change in activation will depend upon both the change in stoichiometry (i.e. whether the region is Ga-rich or As-rich) and the site occupancy of the dopant. Thus a local excess of Ga will result in a decrease in activation of a group IV dopant (such as Si) which acts as a donor when on the Ga lattice, whilst a local excess of As will increase the activation of the same dopant. In contrast the opposite change in activation will occur for the group VI dopant (such as S or Se) which acts as a donor when on the As lattice. Because of these stoichiometrically induced alterations in dopant activation, it will be difficult to produce uniform activation (which would result in uniform threshold voltage devices) from single implants of any dopant species into GaAs substrates containing dislocations. However, if the active layer over the whole substrate is made by a co-implant of suitable group IV and group VI dopants (with implant energies adjusted to produce approximately similar profiles for each species), the stoichiometrically induced variation in activation of the group IV dopant would be partially compensated by an opposite change in activation of the group VI dopant thus resulting in a reduced overall change in total activation (and thus in threshold voltage). Correct choice of the dose ratio between the group IV and group VI dopants should result in near-zero influence of stoichiometry on device threshold voltages. Irrespective of where the device is formed on the substrate, the activation of the active layer is thereby equalised and the devices have more consistent threshold voltages. Such as approach would have the advantage of giving improved uniformity and reproducibility of device threshold voltages for implants into any GaAs substrate, independent of the dislocation density or of overall stoichiometric differences in substrates, whilst still using readily available GaAs substrates and standard implantation, activation, and processing techniques. As shown in the drawing, a Gallium Arsenide crystal lattice substrate 10 has an active layer 11 thereon. The layer 11 is formed by ion implantation of the substrate, in the particular region, of a dopant from Group IV of the periodic table of elements, i.e. a donor implant, in dependance upon the desired extent of activation required to produce a semi-conductor device or part of a device for incorporation into an integrated circuit on the substrate 10. In accordance with the present invention, the active layer 11 is formed by implanting not only ions of a Group IV element but also ions of a Group VI element. Thus, localised increases, or decreases, in activation of the Group IV dopant are partially compensated by localised decreases, or increases, in activation of the Group VI dopant. The layer 11 thus provides a donor region whose uniformity of activation is determined by the dose ratio of the implanted Group IV and Group VI ions, and whose activation level is determined by the total implanted dose Alternative techniques which have been shown to give improved uniformity have involved the use of so-called "ingot annealed" GaAs substrates (where long term annealing has reduced local stoichiometric variations in the substrate) and the use of "low dislocation" or "dislocation free" wafers (where there are fewer dislocations to cause stoichiometric variations). The use of the proposed method of the present invention would improve device uniformities in the case of ingot annealed wafers and may also result in improvements in "low dislocation" wafers. The effect on true "dislocation free" wafers will be minimal, although there is a possibility that the variations already encountered in this material are due to localised stoichiometry changes resulting from causes other than dislocations. These variations also could be minimised by the dual implant procedure.

Claims: CLAIMS 1. A Gallium Arsenide substrate, whereon a semi-conductor device is to be formed, comprising an active layer ion implanted with both Group IV and Group VI dopants, in a ratio dependant upon the desired resultant activation. 2. A substrate as claimed in claim 1 wherein the Group IV dopant is silicon. * 3. A substrate as claimed in claim 1 or 2 wherein the Group VI dopant is selenium. 4. A substrate as claimed in claim 1, 2 or 3 wherein the Group IV dopant implantation profile is substantially the same as the Group VI dopant implantation profile. 5. A substrate as claimed in any preceding claim whereon a plurality of semi-conductor devices are to be formed, wherein the active layer formed by the co-implantation of the ions, extends over the surface of the substrate prior to formation of the individual semi-conductor devices. 6. An integrated circuit comprising a plurality of interconnected semi-conductor devices formed on a Gallium Arsenide substrate as claimed in any preceding claim. 7. A Gallium Arsenide substrate substantially as hereinbefore described with reference to the accompanying drawing. 8. A method of manufacturing a Gallium Arsenide semi-conductor device comprising the steps of ion implanting an area of a Gallium Arsenide substrate with a Group IV dopant, ion implanting the area of the Gallium Arsenide substrate with a Group VI dopant, and adjusting the ratio of the dopants in depcndance upon the required activation of the area. 9. A method as claimed in claim 8 wherein the implantation energies of the dopants are controlled so as to produce similar implantation profiles. 10. A method of manufacturing a Gallium Arsenide semi-conductor device substantially as hereinbefore described.

Inventors: BLUNT ROY TREVOR, STIRLAND DEREK JOHN, BLUNT, ROY, TREVOR, STIRLAND, DEREK, JOHN
Applicants: PLESSEY OVERSEAS, PLESSEY OVERSEAS LIMITED
Application year: 1988
Main classification: H01L21
IPC codes: H01L21