GB2138999A - Semiconductor lasers - Google Patents

Abstract

In a distributed feedback semiconductor laser it is the upper face (10) of the upper confinement layer (1) which has the distributed feedback corrugations. This face is electrically isolated, for example by means of a dielectric layer (6). Contacting means (4, 5) are provided whereby a current can be made to pass across a p-n junction, for example the interface between the active layer (2) and the lower confinement layer (3), so as to cause lasing in the active layer. This structure permits a sequence of fabrication steps in which there is no need to overgrow corrugations with semiconductor. This avoids the possible risk of deformation of the corrugations. The laser is suitable for use in optical communications. <IMAGE>

Description

SPECIFICATION Semiconductor lasers The present invention relates to semiconductor lasers, particularly to lasers having distributed feedback, especially for use in communication along optical fibres.

Semiconductor laser structures include a p-n junction across which current flows (the conventional current from p to n) and an "active layer" in which electrons and holes combine with the production of photons by stimulated emission. The 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 active layer and in contact with the opposite faces of the active layer are known as "confinement layers".

Silica optical fibres have loss minima at 1.3 and 1.55 sm, the minimum at 1.55 am being the deeper one, and therefore lasers operating near these wavelengths are especially preferred. Semiconductor lasers operating in this region of the infrared usually comprise regions of indium phosphide and of quaternary materials (In,Gai.,As,P1,). By suitable choices of x andy 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.

Semiconductor lasers comprising regions of gallium aluminium arsenide and gallium arsenide are also used for communications purposes. These operate near to 0.9 ijm.

Both for telecommunications and other purposes it is often desirable that the laser power should be concentrated into a very narrow frequency range. In the case of telecommunications systems with silica fibres, this is especially important for operation near 1.55 sm where the dispersion in the fibre is usually much greater than near 1.3 Fm.

In one of the simplest semiconductor laser designs (the Fabry-Perot type), the laser usually operates, undesirablyforsuch purposes, in several longitudinal modes of differing wavelength. In addition, Fabry Perot lasers are difficult to incorporate in integrated optics structures.

Longitudinal mode control can be achieved by means of a diffraction grating. One laser structure incorporating a diffraction grating is known as the distributed feedback (DFB) laser (see G.H.B. Thompson, Semiconductor Lasers, Wiley, 1980). A DFB laser operating at 1.53 Am has been described by K.

Sakai, K, Utaka, S. Akiba, and Y. Matsushima (IEEE J.

Quantum Electronics, QE-18, no 8, pages 1272-1278, August 1982). In constructing their laser they made a first order diffraction grating with a period of 2365 (0.2365 sm) on the surface of doped InP by holog raphictechniques and chemical etching.The corrugation depth thus achieved was typically 1000 A (0.1 Ism), but the subsequent growth of a doped quaternary layer (ie a layer containing Ga, In, As, and P plus dopant) reduced the corrugation depth to 200-500 A (0.02-0.05 Fm). They attributed this to dissolution of the grating in the melt used for growing the quaternary layer. (This technique of growth is known as liquid phase epitaxy or LPE.) The final depth of the DFB corrugations is one of the most important parameters of the device. The reduction of the corrugation depth in LPE has an adverse effect on laser efficiency and in particular raises the threshold current of the laser. High threshold currents make for heating of the laser in use and consequent control difficulties and for low upper working temperatures.

The problem of deformation during semiconductor overgrowth of DFB corrugations can be overcome by the carefully-controlled use of the metal organic chemical vapour deposition method (see L.D. Westbrook, A.W. Nelson, and C. Dix, Electronics Letters, volume 19, pages 423-424(1983)). However, this is not a convenient approach to the problem in all cases.

The present invention provides a semiconductor laser which comprises (i) a semiconductor upper confinement layer, (ii) a semiconductor active layer, and (iii) a semiconductor lower confinement layer in sequential face contact, at least a part of the upper face of the upper confinement layer having distributed feedback corrugations and being electrically isolated and a p-n junction being present beneath the corrugations, and (iv) contacting means for making current flow across the p-n junction so as to cause lasing in the active layer beneath the corrugations.

It is to be understood that the words "upper", "lower", and "beneath" in the above indicate merely a reference direction and not the actual orientation of the laser.

The principal advantage of the laser over the known DFB lasers is that by avoiding the use of semiconductor material directly on top of the DFB corrugations one may avoid the difficulties of fabrication referred to above. The corrugations may be exposed or they may be covered by a dielectric.

In a first advantageous laser in accordance with the present invention, the contacting means comprises a semiconductor upper contacting portion in peripheral contact with either or both (preferably both) of the upper confinement layer and the active layer and a semiconductor lower contacting portion in contact with the lower confinement layer, preferably in contact with the lower face of the lower confinement layer. Most preferably in this case the p-n junction lies in one of the faces of the layers (other than the upper face of the upper confinement layer) or else in a plane substantially parallel to these faces. Especially preferably the p-n junction is the interface between the active layer and the lower confinement layer.

Such a laser is operated by applying a potential between the upper contacting portion and the lower contacting portion in the sense determined by the sense of the p-n junction.

A second advantageous laser in accordance with the present invention has a p-n homojunction which divides the active layer laterally beneath the corrugations.

It will be appreciated by the man skilled in the art that the aforesaid layers and portions may comprise sub-layers or sub-portions of differing composition or else have graded compositions. For example, variations of band gap within the layers may be useful to optimise the distribution of optical power during operation.

In the case where a contacting portion or a layer is substantially uniform in band gap, reference to the following symbols will be made: E1 for the band gap of the upper confinement layer; E2 for that of the active layer; E3 for that of the lower confinement layer; E4 for that of the upper contacting portion (if present); E5 for that of the lower contacting portion (if present) In this case, generally E1 > E2 and E3 > E2.

When contacting portions as aforesaid are present, additionally E4 > E2 preferably E4 E1, more preferably E4 > E1, and especially preferably E4 E5 > E1 = E3 > E2.

It will be appreciated that the current supply to one side of the p-n junction must be lateral to the extent that vertical flow through the electrically isolated corrugations is not possible. If the laser is devised so that the flow on the other side of the junction is also substantially lateral rather than vertical, then it will be possible to mount the laser on a semi-insulating substrate together with other devices to form an integrated optics structure.

The present invention will now be more particular ly described by reference to the accompanying Figures as follows: Figure 1 shows a schematic section (without hatching) of a distributed feedback laser in accord ance with the invention and having upper and lower contacting portions as aforesaid.

Figures 2 to 7 show schematically the stages in the production of the device of Figure 1 (Figure 7 corresponding to Figure 1). Figures 2 and 4 to 7 are sections (without hatching) whereas Figure 3 is a perspective view.

Figure 8 is a schematic perspective view of a laser in accordance with the present invention having a p-n homojunction dividing the active layer laterally beneath the corrugations.

Figures 9 to 11 are perspective views showing schematically the stages in the production of the device of Figure 8.

Figure 12 is a schematic perspective view, metalli sation and any dielectric being omitted for clarity, of a laser in accordance with the present invention having a p-n homojunction dividing the active layer laterally beneath the corrugations, this laser being located on a semi-insulating substrate.

The upper confinement layer in accordance with the invention is 1 in Figure 1, this being a Zn-doped (ie p-type) indium gallium arsenide phosphide of band gap 1.0 eV (equivalent to a wavelength in vacuo of about 1.25 Fm). The upper face 10 of 1 has distributed feedback corrugations (not shown), covered by a layer of insulator 6 such as silica, silicon nitride, or alumina.

The active layer is 2, being a Zn-doped (ie p-type) indium gallium arsenide phosphide of band gap 0.8 eV (equivalent to a wavelength in vacuo of about 1.55 ism).

The lower confinement layer is 3, being a Tedoped (ie n-type) indium gallium arsenide phosphide of band gap 1.0 eV (equivalent to a wavelength in vacuo of about 1.25 clam).

The upper contacting portion is 4, being Zn-doped (p-type) indium phosphide of band gap 1.4 eV. 4 carries a heavily Zn-doped (p-type) layer 7 overlaid with metallisation 11.

The lower contacting portion is 5, being heavily Sn- or S-doped (n-type) indium phosphide of band gap 1.4 eV. The carrier density is about 1 ol3 cm-3.

On the underside of 5 (not shown) there is metallisation.

8 comprises Te- or Sn-doped (n-type) indium phosphide and 9 comprises Zn-doped (p-type) indium phosphide. These layers function as current blocking layers.

All of the quaternary materials are lattice matched to indium phosphide, and all of the semiconductor materials are epitaxial, the upper face of 5 being the (100) face. The direction of the DFB grooves in 10 are from left to right in Figure 1, in the < 110 > direction and have a spacing of 0.46 am.

In order to operate the laser the conventional current is passed from metallisation 11 to the metallisation on the base of 5 via 7,4, 1 and 2,3, and 5. The current crosses from p-type to n-type material at the interface between 2 and 3. Lasing occurs in 2, being the layer of lowest band gap and the emission is at 1.55 m (the DFB grating operating in its second order mode).

In order to produce the laser just described, a suitable series of processing steps begins (as shown in Figure 2) with the deposition on the (100) face of an indium phosphide substrate 5 of layers 3', 2', and 1' corresponding in their quaternary composition and thickness to layers 3,2, and 1 in the final laser. A suitable technique consists of etching of the surface of 5 and then growing layers 3', 2', and 1' by liquid phase epitaxy (LPE). Then the upper surface 10' of layer 3' is corrugated, for example by the technique described by L.D. Westbrook, A.W. Nelson, and C.

Dix, Electronics Letters, volume 18, pages 863-865 (1982).

After application of a dielectric layer 6 (Figure 4), layers 3', 2', and 1' are etched away in a self-limiting manner to produce the mesa shown in Figure 5 in which the layers 1 2', and 3' now attain the lateral limits desired in the final laser for layers 1,2, and 3.

To achieve this, the etchant is chosen so as to attack the quaternary materials of 1 2', and 3' in prefer ence to the indium phosphide substrate with the mesa walls being (111)A planes.

Then, further LPE growth provides 9,8,4, and 7 as shown in Figure 6. In respect of this LPE growth it is to be noted that the (111 )A planes of quaternary material offer a substantially higher barrier to regrowth than the (100) plane of the indium phosphide substrate portion 5 or the (100) faces of the materials grown thereon.

Finally metallisation 11 is applied to layer 7 (see Figure 7) and to the underside of the substrate. The product is then cleved, sawn, or scribed into individual DFB laser chips having the same section as Figure 7.

In Figure 8, the upper confinement, active, and lower confinement layers are again labelled 1 to 3 respectively, and the layer of dielectric, in this case covering only a part of the upper confinement layer, is labelled again 6 (and shaded black in this Figure).

The grooves are not shown but run from left to right in the Figure from the end face at 12 but terminate at the edge of mesa 13.

The confinement layers 1 and 3 comprise gallium aluminium arsenide having a band gap higher than that of the gallium arsenide which constitutes the active layer 2. Both the mesa 13 and the base 14 comprise gallium arsenide. The mesa is p-type, as are the parts of the layers 1 to 3 and enclosed by the broken line 17. P-type regions are shown hatched.

The rest of the structure is n-type. The dielectric is conveniently silica or silicon nitride.

The laser is operated by the application of a positive potential between metallisation 15 and metallisation 16, and the active layer emits radiation of a wavelength near to 0.9 Fm.

The laser of Figure 8 is fabricated with intermediate process steps as shown in Figures 9 to 11. First, layers 3,2, and 1, and portion 13' are grown on substrate 14 as shown in Figure 8. These correspond in composition respectively to 3,2, 1, and 13 in the final laser, but are n-type. Then a mask 18 is laid down on the right hand portion of layer 13', and a p-type dopant such as Zn is made to diffuse into the structure from above. This establishes the p-n boundary shown by dotted line 17'.

The next stage, as shown in Figure 10, is to produce mesa 13 from layer 13' by photolithographic selective etching, the distance from the mesa edge 19 to the p-n junction 20 on the surface of layer 1 being several micrometres.

DFB corrugations 21 are then formed in the part of layer 1 not covered by the mesa 13 as shown in Figure 11, and the dielectric layer and metallisation layers are applied to give the device shown in Figure 1. (In practice, of course, a number of lasers would be made together and the last step would be a cleaving, sawing, or scribing step).

In Figure 12, the significances of reference numerals 1,2,3, 13, and 17 and of the hatching are the same as in Figure 8, and 21 is the DFB grating as in Figure 11. (Metallisation and any dielectric are omitted for clarity.) However, 22 is a semi-insulating substrate spaced from the lower confinement layer 3 by a buffer layer 23, and accordingly it is the n-type mesa 24 of gallium arsenide which is used to withdraw conventional current from the laser. It is envisaged that the laser shown in Figure 12 can be part of an integrated optics structure on a single semi-insulating substrate carrying for example also one or more other lasers operating at the same or different wavelengths, other optical, electrooptic or optoelectronic devices, and/or control electronics.

Claims (18)

1. A semiconductor laser which comprises (i) a semiconductor upper confinement layer, (ii) a semicondictor active layer, and (iii) a semiconductor lower confinement layer in sequential face contact, at least a part of the upper face of the upper confinement layer having distributed feedback corrugations and being electrically isolated and a p-n junction being present beneath the corrugations, and (iv) contacting means for making current flow across the p-n junction so as to cause lasing in the active layer beneath the corrugations.

2. A semicondictor laser according to claim 1, wherein the contacting means comprises a semiconductor upper contacting portion in peripheral contact with either or both of the upper confinement layer and the active layer and a lower contacting portion in contact with the lower confinement layer, the p-n junction being in one of the faces of the layers other than the upper face of the upper confinement layer or in a plane substantially parallel to these faces.

3. A semiconductor laser according to claim 2, wherein the lower contacting portion is in contact with the lower face of the lower confinement layer.

4. A semiconductor laser according to claim 2 or claim 3, wherein the p-n junction is the interface between the active layer and the lower confinement layer.

5. A semiconductor laser according to any of claims 2 to 4, wherein the two contacting portions comprise indium phosphide and the confinement layers and the active layer comprise indium gallium arsenide phosphides.

6. A semiconductor laser according to claim 1, wherein the p-n junction is a homojunction dividing the active layer laterally.

7. A semiconductor laser according to claim 6, wherein the two confinement layers comprise gallium aluminium arsenide and the active layer comprises gallium arsenide.

8. A semiconductor laser according to any preceding claim, wherein the electrical isolation of the upper face of the upper confinement layer is afforded by a layer of dielectric thereon.

9. A distributed feedback semiconductor laser which comprises (a) a semiconductor upper confinement layer, (b) a semiconductor active layer, and (c) a semiconductor lower confinement layer in sequential face contact, at least a part of the upper face of the upper confinement layer having distributed feedback corrugations and being electrically isolated and a p-n junction being present beneath the corrugations, and (d) a semiconductor upper contacting portion in peripheral contact with either or both of the upper confinement layer and the active layer and (e) a semiconductor lower contacting portion in contact with the lower confinement layer.

10. A semiconductor laser according to claim 9, wherein the p-n junction is in one of the faces of the layers other than the upper face of the upper confinement layer or in a plane substantially parallel to these faces.

11. A semiconductor laser according to claim 9 or claim 10 which has the feature specified in any one of claims 3 to 5 and 8, or any two or more of said features.

12. A distributed feedback semiconductor laser which comprises (a) a semiconductor upper confinement layer, (b) a semiconductor active layer, and (c) a semiconductor lower confinement layer in sequential face contact, at least a part of the upper face of the upper confinement layer having distributed feedback corrugations and being electrically isolated and a p-n homojunction being present beneath the corrugations and dividing the active layer laterally.

13. A semiconductor laser according to claim 12 which has the feature specified in either of claims 7 and 8 or both of said features.

14. A semiconductor laser substantially as described herein with reference to the accompanying Figure 1.

15. A semiconductor laser whenever produced substantially as described herein with reference to the accompanying Figures 2 to 7.

16. A semiconductor laser substantially as described herein with reference to the accompanying Figure 8.

17. A semiconductor laser whenever produced substantially as described herein with reference to the accompanying Figures 9 to 11.

18. A semiconductor laser substantially as described herein with reference to the accompanying Figure 12.