Abstract:
In a method of forming and passivating device regions in III-V semiconductor substrates, a substrate surface is pretreated in a halogen-carbon plasma prior to depositing of insulating or passivating layers. Devices produced by pretreating the substrate surface have considerably better electrical values than devices fabricated without this pretreatment. In particular, devices fabricated with this pretreatment have a low reverse current (dark current).

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for fabricating and passivating semiconductor devices and in particular for fabricating passivating layers on semiconductor devices yielding improved electrical properties. 
     It has been known for a fairly long time that in the case of III-V compound semiconductors a native oxide layer forming on the semiconductor surface during the fabrication process is the reason why the electrical properties of devices made from such semiconductors are not as good as they should be in theory or as is desirable. 
     For example, as stated in an article by M. Taillepied and S. Goussier in &#34;Appl. Phys. Lett.&#34; 48 (15), 1986, pages 978 to 980, the drain current of field-effect transistors fabricated from III-V compound-semiconductor material is not stable. Also, in photodiodes fabricated from such semiconductor material, too high a dark current is measured, which results in an undesirable noise source in optoelectronic applications. Reduction of this dark current is the subject matter of the published European Patent Application 128 724. 
     Various authors have proposed methods for suppressing the undesired effect of the native oxide layer during the fabrication of III-V compound-semiconductor devices. This is usually done by preventing or at least reducing the formation of the native oxide layer or by selectively removing the native oxide layer prior to the deposition of an insulating or passivating layer. 
     In a paper by P. Boher et al from the INFOS 1987 conference at Leuven, Mar. 12 through 14, 1987, and subsequently also appeared in print and comes closest to the present invention, the removal of the native oxide layer using a hydrogen plasma is described. However, even though the substrate surface is coated with silicon nitride (Si 3  N 4 ) immediately after removal of the native oxide layer, formation of a thinner native oxide layer prior to the coating with Si 3  N 4  cannot be prevented. 
     The last-mentioned method, as expected, results in an improvement in the electrical values of the semiconductor devices, but this improvement still appears to be insufficient. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method whereby the diffusion-masking and passivating layers needed to fabricate the semiconductor devices can be deposited without affecting the semiconductor surface, that is, in a manner which degrades the electrical properties of the devices. 
     The above and other objects, advantages and features are accomplished in accordance with the invention by the provision of a method of forming and passivating device regions on a compound-semiconductor substrate after deposition and photolithographic patterning of a diffusion-masking layer, impurity atoms are diffused to form doped regions whose interfaces to the substrate form pn junctions, and wherein prior to the deposition of the diffusion-masking layer or a passivating layer, the substrate surface is pretreated by exposing it to a plasma. The plasma is excited in an atmosphere containing at least one halogen-carbon compound. 
     The pretreatment of the semiconductor surface by means of a halogen-carbon compound under the action of a plasma results in a considerable reduction of the reverse pn-junction currents. For example, pin diodes with surfaces pretreated in accordance with the invention have very low dark currents. Edge breakdowns in avalanche photodiodes (APDs) due to surface degradation no longer occur. 
     Unlike the prior art, however, these advantages are attributable not to a removal of the native oxide layer--which is essentially preserved--, but to the formation of a thin layer (possibly a polymer layer) which protects the semiconductor and its native oxide layer during the subsequent deposition of layers against direct action of the plasma. 
     In a preferred embodiment of the method according to the invention, CF 4  is used as the halogen-carbon compound. This compound has produced good results and has the advantage of containing the same halogen as many hydrofluoric-acid-containing etchants used for surface pretreatment. 
     Further embodiments of the invention relate to the production of the plasma--preferably, an ordinary parallel-plate reactor is employed, and to semiconductor-insulating-layer combinations especially suited for use in the method according to the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1C show a conventional planar InGaAs-InP pin photodiode in different phases of its fabrication. 
     FIG. 2 is a graph of dark-current characteristics of planar pin photodiodes fabricated on InGaAs (II) and on InP (I). 
     FIG. 3 shows a planar double heterojunction on which a passivating layer IS of SiN x  can be deposited. 
     FIG. 4 shows a mesa pin diode with a passivating layer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The method according to the invention will now be described in detail with reference to the figures. 
     FIGS. 1A-1C show a conventional planar InGaAs-InP pin photodiode in different phases of its fabrication. 
     FIG. 1A shows a two-layer substrate S of n-type indium phosphide (InP) and n-type indium gallium arsenide (InGaAs) deposited on the indium phosphide. Since Zn or Cd, for example, is to be diffused into this substrate S to form a p-type region, the substrate is covered with an insulating layer IS of SiO x , SiN y  or SiO x  N y  in which an opening L of predetermined shape and area was formed by photolithographic techniques to define this region. The insulating layer thus represents a diffusion mask. 
     FIG. 1B shows the substrate S with the p-type region P formed by diffusion and a pn junction between the p-type region P and the n-type substrate. 
     FIG. 1C shows the finished pin photodiode with deposited metal contacts K, an antireflective layer A on a light entrance surface, and a passivating layer PS which, after removal of the diffusion mask, covers the n-type and p-type regions on the top side of the substrate. 
     The device shown in FIGS. 1A to 1C in different stages of fabrication can be produced in various ways. The individual semiconductor and insulating layers can be deposited, for example, by sputtering or chemical vapor deposition (CVD). The latter process has become known as thermal CVD, plasma-enhanced CVD (PECVD), or photon-assisted CVD, depending on whether the chemical reaction is initiated by heat, a plasma, or short-wavelength light. 
     The conventionally fabricated device of FIGS. 1A to 1C has insufficient electrical properties, particularly, too high a reverse current (dark current). In various investigations, which will not be dealth with here, the cause of this insufficiency was found to be damage to the semiconductor surface in the area of the pn junction (area B in FIG. 1C). This damage is obviously caused by the action of a native oxide layer present on the semiconductor surface during the deposition of diffusion-masking or passivating layers. 
     In the method according to the invention, a plasma-enhanced CVD process, differs from conventional methods of this kind in that prior to the deposition of an insulating layer serving as a diffusion mask or of a passivating layer, the surface to be coated is pretreated in a halogen-carbon plasma. To this end, the substrate surface to be coated, after being cleaned with buffered hydrofluoric acid, is placed in a commercially available PECVD parallel-plate reactor and heated to a temperature of about 200° C. Thereafter, a CF 4  atmosphere with a pressure of 0.8 mb is produced in the reactor via inlet and outlet connections, and a plasma is produced by applying an RF voltage to the electrode plates of the reactor. 
     After the semiconductor surface has been exposed to the CF 4  plasma for about 1 minute, the CF 4  is replaced by other reaction gasses (e.g., SiH 4 , N 2  O, N 2 ) needed to build up the insulating or passivating layer, and the substrate is coated for about 6 to 7 minutes. The layer formed may be of SiO 2 , Si 3  N 4  or SiO x  N y , depending on the composition of the reaction gasses. 
     The electrical values of a photodiode fabricated by this method are greatly improved in comparison with those of such devices made by conventional methods. FIG. 2 is a graph of dark-current characteristics of planar pin photodiodes fabricated on InGaAs (II) and on InP (I) by the method described in the foregoing. 
     As these characteristics show, the method described is suitable for preventing damage to the semiconductor surface during the deposition of insulating or passivating layers on both InGaAs and InP substrates. This permits the fabrication of devices with good and stable electrical values. 
     The method described is not limited to pin photodiodes and planar devices. FIG. 3 shows schematically the structure of a planar double-heterojunction APD (avalanche photodiode) on which the passivating layer IS of SiN x  can be deposited after the substrate surface has been pretreated by the method described. FIG. 4 shows a mesa pin diode with a passivating layer PS deposited by the method described. The Letters n and p indicate the polarities of the dopants in the semiconductor regions designated by the chemical compositions of their materials, and the superscript signs + or - indicate the doping level (+=heavy, -=weak, no sign=normal).