NZ203381A - Etching of channel in substrate for inverted rib waveguide of a laser - Google Patents

Description

2033 Priority Date(s): Complete Specification Filed: $4/ Class: M . H.Q/A^JO.O. f 6 DEC 1985 Publication Date: !,??.

P.O. Journal, No: Mz NEW ZEALAND THE PATENTS ACT, 1953 COMPLETE SPECIFICATION "INJECTION LASER MANUFACTURE" WE. INTERNATIONAL STANDARD ELECTRIC CORPORATION, a - 0 - ■ - 7 Corporation of the State of Delaware, United States of America, of 320 Park Avenue, New York 22, New York, United States of America, hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 2 033 8 1 This invention relates to injection laser manufacture and in particular to the manufacture of an (ln,Ga)(As,P) laser having the structure referred to sometimes as an Inverted Rib Waveguide (IRW) laser, and also referred to as a Piano-Convex Waveguide (PCW) laser. An IRW laser is characterised by having, between the active layer and the substrate an intermediate layer which provides a measure of dielectric waveguiding effect in the lateral direction by virtue of a rib formed in its surface facing the substrate, this rib extending into material of lower refractive index. In the case of (In,Ga)(As,P) lasers grown upon In P substrates, the low refractive index of In P relative to that of (ln,Ga)(As,P) makes it possible to adopt a relatively simple structure in which the intermediate layer is grown directly onto the surface of the substrate so that its rib extends into the substrate material.

The manufacture of (In,Ga)(As,P) IRW lasers has been described by M. Ueno et al in IEEE Journal of Quantum Electronics Vol. QE-17 No. 9 pp 1930-40 (September,1981), by K.

Sakai et al in the same journal Vol. QE-17 No. 7 pp 1245-50 (July, 1981), and by Y Noda et al in Electronics Letters Vol. 17 No. 6 pp 226-7 (March 1981) .

In the papers referred to it has not been specified whether the orientation is such that the rib extends in a (Oil) or a (oil) direction along the (lOO)plane. We have found that there is a distinct advantage in using the (Oil) 20338 1 direction rather than the (Oil) . A channel extending in the (Oil) direction is readily etched to a reproducable shape having A-plane side walls, whereas when etching a channel extending in the (Oil) direction the side walls may be A-plane, B-plane, (Oil) and (Oil) planes, or some mixture of these, and it depends i^pon the nature of the etchant and the nature of the masking material used to delineate the channel. The difference between {111}A and {111}B faces affects the growth characteristics. Liquid phase epitaxy of InP and its related alloys is normally based on solutions in liquid In metal, so that the concentration of In in the system is much higher than that of P. The attachment of a new atom to both types of {111} surface is relatively weak, since it consists of a single bon^. The high concentration of In in the liquid would be expected to increase the probability of attachment of In atoms to a {111}B surface, whereas the probability of attaching P atoms to produce nucleation on a {111}A surface is much lower. For this reason growth on {111}A faces does not occur readily.

Growth on {011} surfaces can be initiated by the attachment of either In or P atoms and thus growth occurs readily.

The slow growth of material on A-planes means that a channel with A-plane sides is not filled.by subsequent liquid phase epitaxial growth nearly as quickly as when the sides are not formed by A-planes. This in turn means that adequate filling of the channel to produce a substantially plane surface upon which to grow the active layer requires the use of a thicker intermediate layer when filling channels with A-plane sides than when filling channels with differently oriented sides. This problem of growing sufficient material to provide a plane surface upon which to grow the active layer is expressly referred to in the paper by Ueno et al to which previous reference has been made, and the authors of that paper stated that they found it necessary to grow a layer whose thickness beyond the confines of the channel was greater than the depth of the channel formed in the substrate. We have found that this restriction can be avoided by using channels which do not have A-plane sides. Remembering that the laser must be designed so that the optical field extends deep enough for the rib to have the requisite lateral waveguiding effect, a consequence of being able to use a thinner intermediate layer is that the rib can be closer to the active layer, thereby relaxing a design constraint by making possible the use of designs in which the optical field does not have to extend so far from the active layer.

According to the present invention there is provided a method of making by liquid phase epitaxy upon the (100) surface of an InP substrate an (In,Ga)(As,P) IRW laser having, between the (In,Ga)(As,P) active layer of the laser and the substrate, an (In,Ga)(As,P) intermediate layer of larger band gap material than that of the active layer, which intermediate layer provides 2 033 8 1 a measure of dielectric waveguiding effect for the laser in the lateral direction by virtue of a rib formed in its surface filling a channel formed in the surface of direction the substrate, wherein the channel extends in the (Oil) direction and is formed by an etching process that produces side walls to the channel that are not A-planes.

There follows a description of the manufacture of an IRW laser embodying the invention in a preferred form. The description refers to the accompanying drawing depicting a schematic cross-section of the laser.

The (100) face of an n+-type wafer of InP, which is to form the substrate 1 of the laser, is polished and then etched in a bromine-methanol etch to remove approximately 10 microns of material that may have suffered polish damage. A silica masking layer (not shown) is then deposited pyrolitically from a mixture of oxygen and silane in nitrogen at a temperature of typically 400-450°C. Windows extending in the (Oil) direction are etched through the mask with buffered hydrofluoric acid etch. Typically these windows are 2.75 microns wide. Then the etch is changed for a 1:1 hydrobromic: phosphoric acid etch (50% and 85% concentrated respectively) to form channels 2, in the substrate 1 extending in the (Oil) direction. This etch undercuts the mask slightly and leaves {111}B channels walls 3. (Each laser requires only one channel but in accordance with conventional semiconductor device manufacturing 203381 processing many devices are made at the same time on a single wafer which is subsequently divided up to give the individual devices.) The ability of this etchant to undercut the mask appears to be important to achieve the production of the (111)b sides, for if (In,Ga)(As,P) deposited by liquid phase epitaxy is used as the masking material, little if any undercutting occurs, and (111)a walls are produced instead of (111)b ones. In view of the propensity of the etch to produce etch pits at dislocations it is desirable to choose a substrate with a low dislocation density. This is usually achieved by using a highly sulphur doped substrate. The etch time is usually quite short (typically between 5 and 10 seconds) to produce a channel typically about 0.5 microns deep. After this the silica mask is removed with buffered hydrofluoric acid and then the substrate is given a short etch (typically 20 seconds) in 0.1% bromine by volume in methanol to remove damage that may have occurred during the deposition of the mask. It is believed that this final etch also improves growth quality. It results in a slight rounding of channel shoulders which may ease LPE growth at the sides of the channel.

At this stage the channelled substrate wafer is ready for the growth of its epitaxial layers and is mounted in a liquid phase epitaxy reactor. The first layer to be grown is a guide layer 4 of lattice matched n-type (ln,Ga)(As,P) having a composition whose luminescence peak is centred at about 1.05 203381 microns. This layer is typically grown to a depth of about 0.25 microns in regions remote from a groove, and by virtue of the fact that the channel walls 3 are not A-plane walls this depth is sufficient to provide a substantially plane upper surface to the layer notwithstanding the fact that this thickness is not as great as the depth of the grooves 2. The next layer to be grown is an active layer 5, typically about 0.24 microns thick of lattice matched (In,Ga)(As,P) typically having a composition whose luminescence peak is centred at about 1.3 microns. This active layer is in turn covered by p-type passive and capping layers 6 and 7. The passive layer is a low index layer made of indium phosphide as is typically 1.5 microns thick while the capping layer is made of lattice matched (In,Ga)(As,P), typically having a composition whose luminescence peak is centred at 1.2 microns, or of (In,Ga)As. In either instance the capping layer is typically 0.3 microns thick.

The next stage of manufacture concerns depositing an electrically insulating silica mask layer 8 and opening up windows 9 in that layer to register with the grooves 2. The silica of layer 8 is deposited by a plasma deposition process. The windows 9 are opened up with buffered hydrofluoric acid using conventional photolithography.

If the growth of the epitaxial layers 4, 5, 6 and 7 has been prevented from extending right to the edge of the wafer, 203381 for instance by arranging for the wafer to extend a few millimetres under the walls of the sliding boat containing the melts, then the requisite registry of the windows 9 with the grooves 2 can be obtained simply by visually aligning the mask with the grooves where they lie exposed to view at the periphery of the wafer.

If however the epitaxy has been allowed to proceed up to the edge of the wafer, a preliminary processing stage is required to remove a portion of these layers at the periphery so as to expose the location of the grooves 2. This can be done by etching the p-type capping layer with a potassium iodide/iodine etch, etching the p-type passive layer with a hydrochloric/phosphoric acid etch, etching the active layer with a nitric acid etch, and finally removing the guide layer by etching it alternately with bromine methanol and with hydrochloric/phosphoric acid etches. The etching of the guide layer involves more complicated processing than the corresponding etching of the capping layer because the guide layer composition is too close to indium phosphide to be selectively etched with a potassium iodide/iodine etch. The bromine methanol etch is a non-selective etch that will etch both the material of the guide layer and the material of the underlying substrate. On the ohter hand the hydrochloric/phosphoric acid etch-will only etch the substrate material. Therefore this etch is used to test whether or not the previous etching, the bromine methanol, 203381 etch, has proceeded deep enough to expose any substrate material. Once the guide layer has been breached, the hydrochloric/ phosphoric acid etch serves to reveal the channels in reverse relief.

Once the windows 9 have been opened in the silica layer 8, a short zinc diffusion is performed to produce a p+ region immediately under the window for facilitating the making of a good electrical connection with the capping layer. The device is then thinned to reduce the substrate thickness to approximately 80 microns before the application of metal contact layers 10 and 11, which are evaporated and alloyed contacts respectively.

In a modification of the above described method of manufacture a 4:1 phosphoric : hydrochloric acid etch is substituted for the 1:1 phosphoric : hydrobromic acid etch when etching the grooves 2 in the indium phosphide substrate 1.

This produces (Oil) and (Oil) side walls to the channels instead of the B plane walls.

It is also possible to redesign the structure so as to be able to use a p-type substrate. This entails growing each of the epitaxial layers in material of the opposite conductivity type to that used with the n-type substrate.

Claims (5)

203381 What we claim is:-

1. A method of making by liquid phase epitaxy upon the (100) surface of an InP substrate an (In,Ga)(As,P) IRW laser having, between the lasers (In,Ga)(As,P) active layer and the substrate, an (In,Ga)(As,P) intermediate layer of larger band gap material than that of the active layer, which intermediate layer provides a measure of dielectric waveguiding effect for the laser in the lateral direction by virtue of an inverted rib formed in its surface filling a channel formed in the surface of the substrate, wherein the channel extends in the (Oil) direction and is formed by an etching process that produces side walls to the channel that are non A-planes.

2. A method as claimed in claim 1, wherein the etch used to form the channel is an etch that produces side walls to the channel that are predominantly (111)b planes.

3. A method as claimed in claim 1, wherein the etch used to form the channel is an etch that produces side walls to the channel that are (Oil) planes.

4. A method of making an IRW laser substantially as hereinbefore described with reference to the accompanying drawings.

5. An IRW laser made by the method claimed in any preceding claim. INTERNATIONAL STANDARD ELECTRIC CORPORATION