Preparation of semiconductor devices

Abstract: It has been found that layers which include arsenic and/or zinc can have an adverse effect upon optoelectronic semiconductor devices such as lasers. This is reduced by treatments in which arsenic and zinc are excluded. Preferably the substrate is cooled from reaction temperature in the presence of a mixture of hydrogen and PH₃ (replacing AsH₃ and/or Zn(CH₃)₂ used to grow the final layer). Alternatively, devices having a contact layer of heavily p-type gallium indium arsenide are improved by the deposition of a protective layer of indium phosphide. This layer is removed immediately before metalisation. Even though the protective layer is not present in the final product it has a beneficial effect.

Description: This invention relates to the preparation of semiconductor devices and particularly to the preparation of devices based on indium phosphide by growing a succession of layers onto a substrate, especially by vapour phase processes such as metal organic vapour phase epitaxy. The use of semiconductor devices based on indium phosphide has become well established during recent years. This is particularly true of optoelectronic devices such as lasers. A publication describing such a device is as follows: "Low Threshold GaInAsP/InP Lasers with Good Temperature Dependence Grown by Low Pressure MOVPE" by J P Hirtz et al, ELECTRONICS LETTERS 17 (3), 5th February 1981 p.113-115. This invention is concerned with devices in which indium phosphide is an important chemical component. In many devices the indium phosphide is an important component of every layer. Other chemical components of the devices include gallium arsenide and dopants, eg. zinc, sulphur and cadmium (either as the element or a compound). During the preparation of laser devices based on indium phosphide, it is often desired to grow a contact layer onto a confining layer which interfaces with the active layer. On its other side the active layer interfaces with a second confining layer which also interfaces with a second contact layer. In the use of the device the lasing action occurs in the active layer. The two confining layers together with the active layer form a wave-guide structure so that the radiation travels in the active layer. One confining layer usually consists of p-form indium phosphide doped with zinc or cadmium at a level of 10¹⁷ to 10¹⁸ atoms per cm³. The adjacent contact layer usually consists of heavily p-type gallium indium arsenide doped with zinc at a level of 10¹⁹ atoms per cm³. In the final device the contact layers are metal coated, eg with gold, tungsten or titanium. It has been noticed that the products sometimes have unsatisfactory properties and it is believed that these are associated with the contact layer. This invention is particularly concerned with processes in which the last epitaxially grown layer of a device contains one or both of zinc and arsenic either as the element or a compound, eg GaInAs layers. The production of these layers usually involves the use of AsH₃ and/or Zn(CH₃)₂. The deposition of such layers as the final step in an MOVPE process, most surprisingly, affects the properties of other layers within the product, eg the deposition of said final layer appears to cause an unacceptable increase in the ohmic resistance of the final product. It will be appreciated that the growing of a contact layer is usually the final stage of MOVPE (because its function requires that the contact layer be external). Difficulties arise when the contact layer contains arsenic and/or zinc and it is a particular object of this invention to reduce these difficulties. It has now, most surprisingly, been discovered that the undesirable effects of the deposition of such last-grown layers are substantially diminished by subsequent processing in the absence of As and Zn compounds, eg by a treatment carried out in the presence of an inert gas, eg hydrogen, preferably containing PH₃. According to the present invention, there is provided a method of preparing a semi-conductor device based on indium phosphide, wherein said device comprises a plurality of semi-conductor layers grown by metal organic vapour phase epitaxy, which method comprises growing said plurality of epitaxial semi-conductor layers onto a substrate, the last-grown epitaxial layer of said plurality containing arsenic, and thereafter subjecting the layers and substrate of the device to an atmosphere in which an inert gas, eg. hydrogen, is present and both arsenic and zinc compounds are absent, at a temperature suitable for metal organic vapour phase epitaxy growth, and cooling the layers and substrate to room temperature in the presence of said atmosphere. The invention also includes depositing a protective layer, eg. an InP layer, onto said last-grown layer. The protective layer is removed at a later stage, eg. by chemical etching. (It is convenient to remove the protective layer immediately before metal coating). According to one embodiment of this invention, the preparation of an optoelectronic device includes the steps of growing, in unbroken sequence, at least two layers, namely a contact layer, eg. heavily p-type gallium indium arsenide, followed by a protective layer, eg. p-type indium phosphide, and at a later stage removing the protective layer, eg. by chemical etching. It is usually convenient to grow at least three layers in unbroken sequence, namely a confining layer, a contact layer and the protective layer. The confining layer may be of p-type indium phosphide doped with zinc or cadmium at a level of 10¹⁷ to 10¹⁸ atoms per cm³. All the layers of the device may be grown in unbroken sequence. The structure (without the protective layer) is a conventional device which is prepared by conventional methods, eg. MOVPE. After the protective layer has been removed a metallic coating, eg. gold, tungsten or titanium, may be applied to the newly exposed surface to provide means for attaching electrical leads. It might be noted that the use of a "dummy" layer is disclosed in "Improvement in Surface Morphology of InGaAsP - InP Wafers by Dummy Layer Technique" by J Kinoshita and Y Uematsu, published in Electronics Letters 17 (6) 19th March 1981. However, the dummy layer here is in the context of a liquid phase epitaxial growth process rather than metal organic vapour phase epitaxy, and for the purpose of improving surface morphology and process reproducibility. The protective layer of embodiments of the present invention, which is preferably of p-type indium phosphide with zinc or cadmium at a level of about 10¹⁸ atoms per cm³, is chemically similar to layers conventional in this art and it is grown by conventional methods, e.g. MOVPE. The reactants used to grow the contact layer may include: (a) trimethyl indium or a reaction equivalent thereof; (b) trimethyl gallium or a reaction equivalent thereof; (c) a volatile arsenic compound capable of reacting with (a) and (b). The reactants may also include PH₃ and/or precursors of dopants. Two specific embodiments will now be described by way of example. A comparative test will also be described. The starting material (for all three preparations) took the form of a substrate which was a partially formed device having an exposed active structure; this could have a shaped surface as in a distributed feedback laser. The substrate was placed into a reaction chamber and the air was purged with inert gas, ie hydrogen. First the confining layer was grown using suitable reactants, i.e. In(CH₃), PH₃ and Zn(CH₃)₂ and/or Cd(CH₃)₂. When this layer had grown the reactants were changed at reaction temperature and without interrupting the flow of hydrogen to In(CH₃)₃; Ga(CH₃)₃; AsH₃ and Zn(CH₃)₂. This grew the contact layer. The completion of the contact layer marked the end of the procedure common to all three sequences (ie two according to the invention and one comparison). This common part was carried out using conventional metal organic vapour phase epitaxy. After the contact layer had grown the three procedures diverged. Examples 1 and 2 describe further treatments in accordance with the invention. Example 1. The mixture used to grow the contact layer was changed at reaction temperature into hydrogen with 5% by volume of PH₃. The substrate was allowed to cool to room temperature in the presence of this mixture and tested as described below. Example 2. The reactants were changed at reaction temperature and without interrupting the flow of hydrogen into In(CH)₃ and PH₃ and a protective layer, 100nm thick, of InP was grown by conventional MOVPE. The metal organic compound was then removed from the gas stream and the substrate was then allowed to cool to room temperature in the presence of PH₃ and hydrogen. In a final product the protective layer is etched off before metal coating. After the protective layer had been removed the product was tested as described below. Comparative Sequence When the contact layer was complete, the metal organics were removed from the gas flow and the substrate was allowed to cool to room temperature in the presence of AsH₃ and hydrogen. The product was tested as described below. Tests The number of active sites (holes) per cm³ were measured using a conventional technique known as electrochemical profile plotting. The products obtained in Examples 1 and 2 gave resuls in the range (1 to 2)x10¹⁸ whereas the comparative preparation gave only 2 x 10¹⁷. Thus the further treatment according to the invention gave active site concentrations, and therefore conductivities, 5 to 10 times higher than the conventional technique of cooling in the presence of AsH₃ and Zn(CH₃)₂. (Note. The higher values, ie the products of the invention, correspond to expectations based on doping calibration experiments). Further experiments have confirmed the adverse effect produced when substrates are heated in the presence of zinc and/or arsenic without further corrective treatment. Thus the conductivity, ie the number of holes, falls if a finished substrate is heated at about 600°C to 700°C in an atmosphere which contains AsH₃ and/or Zn(CH₃)₂. It has also been observed that, where a substrate has been adversely affected, the effect is reversed by re-heating the substrate to about 600°C to 700°C and cooling to room temperature in the presence of hydrogen or a mixture of H₂ and PH₃ which contains 5% of the PH₃. It is well understood that phosphide layers tend to decompose at elevated temperatures, eg above 500° and especially above 600°C. It is common practice to ensure the presence of PH₃ whenever such layers are exposed to temperatures at which decomposition constitutes an unacceptable risk.

Claims: A method of preparing a semi-conductor device based on indium phosphide, wherein said device comprises a plurality of semi-conductor layers grown by metal organic vapour phase epitaxy, which method comprises growing said plurality of epitaxial semi-conductor layers onto a substrate, the last-grown epitaxial layer of said plurality containing arsenic, and thereafter subjecting the layers and substrate of the device to an atmosphere in which an inert gas, eg hydrogen, is present and both arsenic and zinc compounds are absent, at a temperature suitable for metal organic vapour phase epitaxy growth, and cooling the layers and substrate to room temperature in the presence of said atmosphere. A method according to Claim 1, wherein the reactants used to grow the last-grown semi-conductor layer include: (a) trimethyl indium or a reaction equivalent thereof; (b) trimethyl gallium or a reaction equivalent thereof; (c) a volatile arsenic compound capable of reacting with (a) and (b). A method according to Claim 2, wherein the reactants also include PH₃ and/or precursors of dopants. A method according to Claim 3, wherein the precursors include zinc dimethyl. A method according to any one of the preceding Claims, wherein said atmosphere comprises a mixture of H₂ and PH₃. A method according to any one of the preceding Claims, wherein said subjecting of the device is carried out immediately after the growth of the last-grown semiconductor layer by substituting said atmosphere, at reaction temperature, for the reactants used to grow the last-grown layer. A method according to any one of Claims 1 to 5, wherein a protective layer of indium phosphide grows in the absence of both arsenic and zinc compounds on said last-grown semiconductor layer before said subjecting of the device, and the method comprises removal, at a later stage, of said protective layer. A method according to either one of Claims 7, which method includes growing, in unbroken sequence, the last-grown semiconductor layer of the device followed by the protective layer and, after cooling the device to room temperature, removing the protective layer so that the last-grown layer of the device is exposed to provide an electrical contact layer. A method according to any one of Claims 7 or 8 wherein said semiconductor device is prepared as one of a plurality of such devices sharing a common substrate, and said method further comprises dividing the substrate to provide a plurality of separate devices, removing the protective layer from each of said separate devices and metallising the separate devices to provide electrical contacts.

Inventors: COLE SIMON, HARLOW MICHAEL J, NELSON ANDREW WILLIAM, WONG STANLEY Y K, COLE, SIMON, HARLOW, MICHAEL J., NELSON, ANDREW WILLIAM, WONG, STANLEY Y.K.
Applicants: BRITISH TELECOMM, BRITISH TELECOMMUNICATIONS PUBLIC LIMITED COMPANY
Application year: 1987
Main classification: H01L21
IPC codes: H01S5, H01L21, H01L33