A known family of opto-electronic devices has the following central structure : a substrate of semiconductor material having a mesa thereon, with burying layers on either side of the mesa. Such a device is described by O. Mikami et al in "1.5 .mu.m GaInAsP/InP Buried Heterostructure Lasers Fabricated by Hybrid Combination of Liquid--and Vapour-Phase Epitaxy", Electronics Letters, 18 (5) (4.3.82) pages 237-239. The word "mesa" in this context is used to describe an upstanding stripe having steep sides and a flat top.
The devices of the family include a p-n junction across which current flows (the conventional current from p to n) and a waveguide region to which light is confined. The waveguide region may comprise an "active layer" in which electrons and holes combine with the production of photons by radiative recombination. Such an active layer has to relate suitably in band gap and refractive index to the other semiconductor regions of the structure in order to achieve a suitable degree of "confinement" of these processes to the active layer. The layers of material to either side of the waveguide region and in contact with the opposite faces of the waveguide region are known as "confinement layers".
A major field of application of semiconductor optical devices is in optical fibre communications systems. In general, the devices are constructed out of materials whose elemental components are selected from Groups III and V of the Periodic Table. Silica optical fibres as produced in recent years have loss minima at 1.3 .mu.m and 1.55 .mu.m approximately, the latter minimum being the lower. Accordingly, there is an especial need for devices operating in the range from 1.1 to 1.65 .mu.m, especially from 1.3 to 1.6 .mu.m. (These wavelengths, like all the wavelengths herein except where the context indicates otherwise, are in vacuo wavelengths). Semiconductor lasers operating in this region of the infrared usually comprise regions of indium phosphide, InP, and of quaternary materials indium gallium arsenide phosphides, In.sub.x Ga.sub.1-x As.sub.y P.sub.1-y. By suitable choices of x and y it is possible to lattice-match the various regions while varying the band gaps of the materials. (Band gaps can be determined experimentally by, for example, photoluminescence). Additionally, both indium phosphide and the quaternary materials can be doped to be p--or n-type as desired.
Describing a selected device of the known family, a semiconductor laser, with its mesa uppermost, it has an active layer within the mesa. Electrical contacts are provided to the mesa and on the furthermost face of the substrate from the mesa. The "confinement" required is provided optically in a vertical direction, by changes in refractive index of the semiconductor material, and both optically and electrically in a horizontal direction by the burying layers. The burying layers act to cause any current flowing between the contacts to flow preferentially through the mesa and therefore through the active layer. In one form, the burying layers may present non-conducting semiconductor junctions to current flow between the contacts in use of the device. Good electrical confinement is provided if the semiconductor layers between the contacts constitute a p-n junction and the burying layers in combination with the substrate constitute a n-p-n junction when taken in the same direction. In use the burying layers and substrate then comprise a reverse biased semiconductor junction in both directions. Alternatively the burying layers and substrate could present multiple reverse biased semiconductor junctions in one or both directions.
In another form, the burying layers may comprise "semi-insulating" materials such as Fe doped InP. These materials have a relatively high resistivity compared to for instance undoped InP. Burying layers in this form have advantages in that they substantially completely oppose current flow and show low capacitance effects. Low capacitance effects tend to increase device speed.
In the past devices of this type have been fabricated by means of liquid phase epitaxy (LPE). However there are problems associated with these techniques such as solutal convection within the melts, inaccurate thickness control of the burying layers and meltback of the mesas during fabrication. These have led in particular to lack of uniformity over large areas and the techniques do not lend themselves easily to large scale production.
In the disclosure by O Mikami et al, a method for manufacturing a device with resistive burying layers is described. The layers which will constitute the mesa, comprising InP and quaternary layers, are grown on a semiconductor wafer by LPE, the mesa being produced by chemical etching on either side of a Si.sub.3 N.sub.4 masking stripe. The burying layers, of high-resistivity InP, are then grown by vapour phase epitaxy (VPE) to either side of the mesa. The Si.sub.3 N.sub.4 stripe remains during the VPE growth stage, preventing growth on the upper surface of the mesa itself, and is only subsequently removed.
The characteristics of two lasers produced by O Mikami et al are claimed to be as follows:
1 (i) active layer thickness 0.2 .mu.m PA1 (ii) active layer width 4 .mu.m PA1 (iii) pulsed threshold current about 85 mA PA1 (ii) active layer width 10 .mu.m PA1 (iii) pulsed threshold current about 800 mA to 500 mA PA1 (iv) single longitidunal mode output at wavelength 1.525 .mu.m PA1 (i) depositing a layer of metal organic vapour phase growth suppressing material on a semiconductor wafer having an InP uppermost layer; PA1 (ii) selectively etching the growth suppressing material to form a stripe of said material extending in the &lt;110&gt;crystallographic direction of the wafer; PA1 (iii) creating the mesa under the stripe, the mesa having substantially non-reentrant lateral surfaces; PA1 (iv) growing burying layers by metal organic vapour phase epitaxy to bury the lateral surfaces of the mesa; and PA1 (v) removing the stripe of growth suppressing material. PA1 By making an initial semiconductor structure as above; the stripe of growth suppressing material can be produced so as to be self aligned, and centrally placed on the mesa. PA1 (a) forming a mask of resist material on the layer of growth-suppressing material; PA1 (b) etching the layer of growth-suppressing material, using the resist mask, such that the mask is undercut; PA1 (c) reflowing the resist material of the mask so that the portions of the mask which are undercut drop into contact with the semiconductor wafer; PA1 (d) etching the semiconductor wafer using the resist mask to create the mesa; and PA1 (e) removing the mask of resist material. A second method comprises: PA1 (f) forming a mask of resist material on the layer of growth-suppressing material; PA1 (g) selectively etching the layer of growth suppressing material by means of the resist mask to form a double layered mask; PA1 (h) etching the semiconductor wafer using the double layered mask to create the mesa; and PA1 (i) removing the mask of resist material,
2 (i) active layer thickness 0.2 .mu.m
The Mikami LPE-VPE hybrid technique solves the problem of meltback of the mesas during fabrication but still suffers from disadvantages. It retains an LPE growth step and therefore still does not lend itself to large scale production, a significant leakage current has been observed in the burying layers and the Si.sub.3 N.sub.4 stripe shows a tendency to be bridged by the burying layers when the width of the stripe is reduced.
An alternative growth technique, metal organic vapour phase epitaxy (MOVPE), has been found promising for large scale device production. It offers a highly desirable combination of features: atomic scale interface abruptness, precise compositional control, and uniformity of thickness and composition over a large area.
Unfortunately it has not been found possible merely to replace the LPE and VPE growth steps of the Mikami technique with MOVPE growth steps. If this were possible, a structure having the better current confinement of reverse biased junction burying layers would be produced, with a concomitant, higher potential output power. Although MOVPE could be used to grow the layers of the mesa, the successful growth of the burying layers in the Mikami technique depends on the growth characteristics of VPE. The chemical etching step of the Mikami technique produces a mesa of a characteristic cross section. If MOPE is used to try and grow burying layers on such a mesa, instead of laterally extending burying layers, upstanding "ears" of InP have been found to develop, the lateral surfaces of the mesa not being continuously covered.
A method of fabricating buried mesa structure lasers using only low pressure "metalorganic chemical vapour deposition" (LP-MOCVD) epitaxial growth steps is outlined in the following paper: "Very Low Threshold Buried Ridge Structure Lasers Emitting at 1.3 .mu.m Grown by Low Pressure Metalorganic Chemical Vapour Deposition" by M Razeghi et al, Applied Physics Letters, 46 (2) (15.1.85) pages 131-133. (MOCVD is an alternative term for MOVPE.) The method comprises the steps of growing onto an InP substrate, an n-doped InP confinement layer, an undoped GaInAsP active layer, and a p-doped InP layer for avoiding the formation of defects near the active layer during etching. Etching using a mask, the active layer is reduced to a mesa. After removing the mask, the mesa is covered by a p-doped InP layer and a p-doped GaInAs cap layer.
All the growth steps of the above method are performed by low pressure MOVPE and hence large scale production should be facilitated. Further it is particularly convenient since only one growth technique must be employed. However, the devices produced either rely on a built-in potential differences between the large area p-n homo-junctions to each side of the active region and the p-n heterojunction through the relatively small area active region active or involve a more complicated fabrication process. Although a lowest measured threshold current of 11 mA using continuous wave operation has been quoted, the values given for measurements relating to 269 devices were variable in the range from 17.9 mA to 50.0 mA inclusive. 44.6% of these devices had a threshold current of more tha 45 mA. Further, an optical power emission of only up to 15 mW is quoted.
It is an object of the present invention to provide an improved semiconductor structure and methods of manufacture thereof.
Hereinafter, structures and devices may be described in terms, such as "uppermost", which imply a specific orientation of the structure or device. The use of such terms is for convenience of description only and should not be taken to limit any structure or device to a specific orientation.
According to a first aspect of the present invention there is provided a semiconductor structure comprising a substrate having a mesa thereon, the uppermost layer of the mesa being provided by InP, and the lateral surfaces of the mesa being substantially non-reentrant, said lateral surfaces being buried by burying layers whose uppermost surfaces extend upwards and away from the uppermost surface of the mesa to form a flared groove.
Semiconductor structures according to embodiments of the present invention have the advantage that they can be fabricated using MOVPE for all growth steps while having InP as the material of the uppermost part of the mesa. InP offers particularly good optical and electrical confinement characteristics.
It is important for growth of the burying layers by MOVPE that the lateral surfaces of the mesa are not substantally re-entrant. In particular it is preferable that the uppermost layer of the mesa should not contribute (III)A planes to the lateral surfaces. There should not be an overhang of any part of either of the lateral surfaces equal to more than 10% of the width of the uppermost surface of the mesa. More preferably any overhang should not be equal to more than 5% of the width of the uppermost surface of the mesa or there should be no overhang, the mesa being vertically sided or even tapered towards its uppermost surface.
According to a second aspect of the present invention there is provided method of making a semiconductor structure, which method comprises the steps of:
The growth suppressing material may for instance comprise silica.
Preferably the stripe of growth suppressing material is substantially equal in width to the width of the uppermost surface of the mesa. This facilitates growth of the burying layers. For instance, the width of the stripe should not be more than 0.4 .mu.m greater or less than the with of the uppermost surface of the mesa.
In certain cases it can be difficult to achieve a stripe of growth suppressing material. It has been found that a method of enhancing the growth suppressing properties of the surface of certain materials, such as silica, is to subject that surface to a temperature which lies in the range from 600.degree. C. to 700.degree. C. inclusive for a period of at least one minute in the present of PH.sub.3. It may be found preferable to increase that period to more than three or even more than five minutes.
There is more than one method of carrying out steps (ii) and (iii) above. A first method comprises:
wherein step (i) is preceded by oxidation of the uppermost surface of the wafer so as to degrade the integrity of the subsequent interface between said uppermost surface and the layer of metal organic vapour phase growth-suppressing material.
The oxidation may be carried out by flowing an oxidising gas or gas mixture, such as a nitrogen and oxygen mixture, over the uppermost surface of the wafer.
A particularly important application of the present invention is in the production of lasers which will operate in a single transverse mode. These lasers are of major importance in optical communication systems. Such lasers can be produced from semiconductor structures made by the method of the present invention in which the uppermost surface of the mesa is restricted to being not more than 5 .mu.m wide.