GB2029083A

GB2029083A - Semiconductor waveguide devices

Info

Publication number: GB2029083A
Application number: GB7833856A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1978-08-18
Filing date: 1978-08-18
Publication date: 1980-03-12
Also published as: NZ191335A; GB2029083B; AU4977879A; DE2933149A1

Abstract

A conventional planar heterostructure III-V semiconductive device provides optical waveguiding in a direction normal to the plane of the layers, but not in the lateral direction. A stripe of material deposited on the surface of the semiconductive device is arranged to produce a strain pattern in the underlying semiconductive material thereby producing lateral waveguidance by virtue of the photo-elastic effect. Alternatively the strain pattern is produced by a stripe-shaped window opened in a layer of material deposited upon the semiconductive material. A particular application is in the provision of laser devices.

Description

SPECIFICATION Semiconductor waveguide devices This invention relates to the provision of optical waveguiding structures in Ill-V semiconductive material, and finds particular but not exclusive application in the provision of semiconductive laser structures.

In a conventional design of double heterostructure laser there is a higher refractive index semiconductor layer sandwiched between underlying lower refractive index material and an upper layer of lower refractive index material. The resulting two interfaces between higher and lower refractive index material provide optical waveguiding in a direction normal to the plane of the layers, but not any lateral waveguiding in the plane of the layers. Indeed, if the injection of minority carriers into the higher refractive index material is confined to a narrow stripe, the minority carriers are liable to produce a region of slightly reduced refractive index, with the result that in the lateral direction the structure is actually antiwaveguiding at low current drive levels around the lasing threshold.

The present invention discloses how photoelastic effects in the semiconductor material may be used in a double heterostructure device to provide optical waveguiding properties in the lateral direction.

According to the present invention there is provided a Ill-V semiconductor device incorporat ing at least one optical waveguide, which device has a higher refractive index layer sandwiched between underlying lower refractive index material and an overlaying layer of lower refractive material, for which waveguide a waveguiding effect in the direction normal to the layers is provided by the interfaces between higher and lower refractive index material, and a waveguid ing effect in a lateral direction in the plane of the layers is provided by a strain pattern, penetrating the higher refractive index layer, produced by material deposited upon the surface of the semiconductor device in such a way as to be under stress at ambient temperatures.

In the following description of how suitable stress patterns may be generated, and of embodiments of the present invention, reference will be made to the accompanying drawings in which:~ Figure 1 is a graph relating film thickness and compressive stress for an insulating film deposited upon a gallium arsenide wafer by r.f. plasma deposition.

Figure 2 depicts a section through the wafer showing the configuration of a stripe window in the deposited film.

Figure 3 depicts the strain pattern for the configuration of Fig. 2.

Figure 4 is a graph of the stress in the plane of the film at different depths beneath the film, plotted as a function of position across the stripe width.

Figure 5 is a graph of the stress normal to the plane of the film at different depths beneath the film, plotted as a function of position across the stripe width.

Figure 6 depicts the relationship between the crystal axes of the semiconductor wafer and the coordinate axes used for calculating the stresses and strains.

Figure 7 is a graph of the resulting stress induced dielectric constant profile for different strip widths, plotted as a function of position across the stripe width.

Figure 8 is a graph of the resulting stress induced dielectric constant profile at different depths beneath the film, plotted as a function of position across the stripe width.

Figure 9 depicts four basic types of slot or strip configuration providing lateral waveguiding properties in the underlying semiconductive material, and Figures 10, ii, 12 and 13 depict perspective schematic views of four lasers each utilising the photo-elastic effect to provide lateral waveguiding action.

In the structure of laser previously referred to, current is confined to a narrow stripe by opening a stripe-shaped window in an electrically insulating film deposited on the surface of the semiconductive material. Depending upon the material used for this film, and its method of deposition, there may be quite considerable stress in the film at ambient temperature. This may arise from thermal expansion mismatch, or because the method of deposition induces an intrinsic stress in the film as it is deposited, or it may arise from a combination of both these factors.

One form that this film can take is that of a radio frequency (r.f.) plasma deposited film of silica/silicon nitride. Such a film may be deposited using 4% silane in ammonia and nitrous oxide. The semiconductor wafer is supported on a graphite susceptor, and the reagents maintained at a pressure of about 0.5 torr. Typically, using a 500 watt generator at 1 MHz, the power level is adjusted to run the susceptor at a temperature of about 525 C. The properties of films deposited in this way vary depending on the deposition conditions but the film is always under considerable compression on cooling to room temperature. This produces a bending effect on the wafer in just the same manner as mismatched (GaAl)As epitaxial layers on a GaAs substrate.When windows are opened in the film, the edge of the window exerts a force on the GaAs, parallel to the surface of the GaAs and perpendicular to the edge of the window. The force acts in a direction away from the window edge. This produces a complex stress field in the wafer beneath, which builds up to very high stress values immediately beneath the edge of the film.

Calculations of the resulting elastic stresses and strains are particularly complicated because GaAs has an anisotropic elastic compliance tensor. In certain simple cases this is easy to allow for, but the calculation of the stress field beneath a stripe window is extremely complicated. The treatment now to be given therefore uses an isotropic average value for Young's modulus 'E' and Poissons ratio v for GaAs.

The stress in the film can be deduced from measurement of the radius of the curvature 'p' of the substrate. If the film has a thickness 't' and is under a compressive stress vox then, following Jaccodine and Schlegel (Jnl.Appl.Phys. 37 No.6 pp 2429-2434),

where E 1 - =~= 1.23 x 1012 dynes/cm2 in the (1 00) plane (GaAs) 1-y S d = thickness of substrate = Poisson's ratio (GaAs) A plot of the value of a0xt as a function of the film thickness is shown in Fig. 1. This was obtained by measurement of the radius of curvature of a polished GaAs substrate 200 jum thick.

The radius of curvature was measured using a sodium interference microscope objective. The two sets of data are for two typical sets of deposition conditions and correspond to values a0X of - 5 X 109 dynes/cm2 and - 10'0 dynes/cm2 respectively. The chemical etch rate of the two types of film also differs, and is probably a result of different SiO2 - Si3N4 ratio in the film. This stress compares with values of approximately - 3 X 109 dynes/cm2 for thermally grown oxide films on silicon.According to Reinhart and Logan (J.Appl.Phys. 44 No.7 pp 3171-3174, July 1973) the stress in the active layer of a double heterostructure wafer immediately beneath such; a uniform film is given by

This in itself can produce significant tensions, for instance 2000 A of the higher stress film deposited on a wafer 100 ym thick produces a tension of 0.4 X 108 dynes/cm2. This is the range of stress produced in the actice layer of a conventional double heterostructure laser wafer by the heterostructure interface mismatch (0 - 0.77 X 108 dynes/cm2).

Referring to Fig. 2, when a stripe window is opened in the film 20 a force per unit length S = (rOxt (dynes/cm) is exerted on the GaAs substrate 21 by the edge of the oxide. When the force S is is applied, the coordinates of point P change from (x, z) to (x + u, z + w). Blech and Meieran (J.Appl.Phys. Vol. 38 No. 7 pp 2913-2919, June 1967) have calculated the displacement u in the x direction beneath an infinitely long stripe film in an isotropic substrate.

The displacement u is calculated by integrating the effect of an element of force S.dy along the entire length of the stripe (y = - oo to y = + cho). A similar integration can be performed to calculate the displacement w in the z direction. Because of the symmetry of the configuration there is of course no displacement in the y direction.

The displacements are given by

where x1, x2, r1 and r2 are as defined in Fig. 2 A=(1 + #)/2#E = 1.63X 10-13cm2/dyne B=(3 - 4#)=2.08 C = (1 + #)(1 - 2#)/2#E = 8.81 X 10-14cm2/dyne The values of A, B and C depend on Young's modulus E and Poisson's ratio v.The three independent elastic constants of GaAs are given by C11 = 11.88 x 1011.C12 = 5.38 X 1011 and C44 = 5.94 X 1011 dynes/cm2 (Williadson and Beer "Semiconductors and Semimetals" Vol. 12, Academic Press, New York 1966 p.110.). The Voight average shear modulus and Lamé constant # are given by Hirthe and Lothe "Theory of disclocations" New York McGraw Hill 1968, H =C44- --=4.86X 1011 dynes/cm2 5 H #=C12- --=4.30X 1011 dynes/cm2 5 where H=2C44+C12-C11 (5) and (3# + 2 ) E= -------- = 1.2 X 1012 dynes/cm2 +# and # #= ----- = 0.23 (6) 2( +#) These averaged elastic constants can be used in equations (3) and (4).

The non zero elastic strains are given by du dw exx = -- and ezz = -- (7) dx dz and the shear strain by

The stesses are given by #xx=(#+2 )exx + #ezz #yy = #exx + #ezz #zz = #exx + (# + exx (9) And the shear stress (10) According to equation (7) the differentiation of equations (3) and (4) with respect ot x and z respectively gives explicit expressions for the strain in the x and z directions. Thus:~

The strain field is best visualised by plotting contour lines of equal strain beneath the stripe window. Such plots are shown in Figs. 3(a) and (b).Fig. 3(a) is the grain field e,, beneath a stripe window 20 m wide. The contour lines are marked in units of strain of 10-4 for a film edge stress S = C'oxt = - 3 X 10 dynes/cm. (3000 A of the higher stress film.) Negative values correspond to compression. The strain builds up rapidly as the edge of the film is approached, and the contour lines crowd very closely together. For clarity therefore the contour lines have been omitted in regions of extremely high stress and the sign of the strain, i.e.

whether it is tensile or compressive, indicated by the symbols T or C. As would be expected there is high compressive strain just inside the edge of the window and high tensile strain just outside. In theory the strains (and stresses) tend to infinite as the edge of the film is approached. In practice such factors as the finite thickness of the film and plastic deformation will limit the strains to finite values. At the usual depth of the active region of 2 ym the greatest strain occurs 5 ym in from the edge of the film and has a value of - 3 X 10-4.

Fig. 3(b) shows the strain field beneath a stripe only 2.5 ym wide. All other parameters are identical. Note that the distance scale of the plot is doubled. The shape of the contour pattern is independent of stripe width, it is only the scale which is changed. For instance the - 4 X 10-4 contour line of Fig. 3(b) is identical in shape with the - 0.5 X 10-4 contour line of Fig. 3(a).

The strain is 8 times larger for the narrow stripe because its width is 8 times smaller. The greatest strain at a depth of 2 jum is, as in the 20 ym case, approximately - 3 X 10-4, and it is true in general that the greatest strain at a particular depth is strongly dependent on the value of the depth, increasing rapidly as the depth reduces, but only weakly dependent on the stripe width. One noteworthy point concerning this plot is that a region of tensile strain Qx is indicated 3 to 4 Clm beneath the centre of the stripe.

These predictions can be qualitatively confirmed using a model of the mechanical situation constructed in for instance 6 mm thick poly methylmethacrylate plastics sheet. Two inward forces corresponding to the forces S may be applied by means of a toolmakers clamp to the edges of a slightly raised region on the edge of the plastics sheet. The strain field can then be observed by illuminating the plastics sheet from behind with light polarised at 45 to the horizontal and viewing through a crossed polariser. The agreement between the observed pattern and that calculated is quite good. Even the region of tension Exx beneath the centre of the stripe is revealed. Strictly, however, this arrangement measures the difference between the strain e,, and the strain e through the strain induced birefringence in the perspex.However the strain ezz is in general opposite in sign to eXx at any particular point and so the observed pattern is expected to be similar to the strain field exx.

The values of the stresses in the GaAs have been calculated for the case of the 20 itm wide stripe using equations (3) and (4) in conjunction with (11) and (12). The stress aXx at depths if 1, 2 and 4 ym beneath the stripe window are plotted as a function of distance x in Fig. 4. The values of S = Qxt = 2.5 X 105 dynes/cm corresponds to 2500 of the higher stress film. The stresses are strongly dependent on active layer depth. At the depth of 2 ym the greatest stress is ~3.5 - 3.5 X 108 dynes/cm2, and it occurs roughly 4 ,um in from the edge of the stripe window.At a depth of 1 ym the maximum stress is considerably larger reaching - 6 X 108 dynes/cm2 (compression) just inside the edge of the window and more than 4 X 108 dynes/cm2 (tension) just outside. This further illustrates that the active layer depth is the most important parameter in determining the stresses caused by a particular film.

The stress in the vertical direction razz has a slightly smaller maximum value than c'xx and is sharply peaked just inside and outside the film edge. It is compressive inside and tensile outside the film edge and results from the twisting moment of the film edge force S. It is plotted in Fig.

5 again for a stripe 20 ym wide. Its magnitude is also strongly dependent on the active layer depth, reaching maximum values of 1.5, 2.5 and 5.2 X 108 dynes/cm2 for depths of 4, 2 and 1 jum #m respectively.

The dielectric constant (and hence refractive index) of a crystal is in general dependent on the stress (or strain) in the crystal. This effect is called the photo elastic effect; it is similar to the electro-optic effect which is the effect of applied electric field on the dielectric constant of a crystal. The standard treatment of the photo elastic effect is given by Nye in "Physical properties of crystals" Clarendon Press, Oxford 1957. The photo elastic coefficients, Pirs which form a fourth rank tensor relate the strain tensor ers to the ABij tensor. ABi represents the small change in the relative dielectric impermeability tensor Bij caused by the effect of the strain.

Relative dielectric impermeability is the inverse of dielectric constant e.

Thus the dielectric constant profile caused by the effect of the film window strain field in the active layer of a stripe laser can be calculated by multiplying the strain field of the stripe by the photo elastic tensor ABi = Pijrsers or (ijrs = x, y, z) (13) Because many of the terms in this matrix are identical the expression can be abbreviated using matrix notation (per Nye) to: AB#=prn5e5(m,n=1,2...6) (14) At this point a complication arises because the values of the photo elastic coefficient Pmn in GaAs are given in the literature only with respect to the primary crystal axes x' y' z', not the coordinates used in the stress field calculation x y z. Fig. 6 shows the relationship between the axes.The axis z is coincident with z' but the x and y axes are rotated through 45 about the z axis with respect to the x' and y' axes. It is therefore necessary to transform the axes of the film strain field to the primary crystal axes, multiply by the photo elastic tensor and transform the resulting ABq back to the x y z axes.

The strain field in the x y z coordinates which has no component in the y direction can therefore be represented by:

The direction cosine matrix which rotates the axes of this tensor in the desired manner is:-

The stress field e1 is then given with respect to the primary axes x' y' z' by:

For a crystal of the class 4 3m such as GaAs there are only 3 independent photo elastic constants, Pq1, p12 and p44 in abbreviated notation. The #B'ij matrix can now be found using equation (14) taking care to note the factors of 2 required in the non diagonal terms for the abbreviated notation (per Nye).

This can now be transformed back to the original x, y, z axes ABij = ajkaj,B kl

This ABi tensor allows the dielectric constant change as a result of the strain ers to be calculated. The light wave of a stripe geometry laser travels beneath the stripe parallel to the y axis. It can be polarised with its electric vector either in the x direction (transverse electric T.E.) or in the z direction (transverse magnetic T.M.).Because the change in the relative dielectric impermeability AB is small compared to B it can readily be shown by series expansion that to a first approximation the change in dielectric constant for the T.E. wave ##xx and the T.M. wave A# is given by:

If the values of the strain field beneath the stripe window e55 and ezz (equations 11 and 12) are substituted into equations 20 and 21 the strain induced dielectric constant profile for the T.E. and T.M. laser modes respectively, can be plotted. The numerical values of the three independent photo elastic coefficients are given by Dixon1 (Jnl. Appl. Phys. 33 No.13 Dec.

1967 pp 5149-5153). They are p11 = -0.165, p12 = -0.140, p44 -0.072. These values were determined by diffraction from microwave acoustic waves in GaAs at a wavelength of 1.1 5 pm. It seems likely that these values of the photo elastic coefficients are not strictly accurate at the lasing wavelength, which is well into the absorption edge of the active layer. For instance enhanced elasto optic coefficients in the neighbourhood of the band gap have been found in ZnP and CdS. Unfortunately no information on the photo elastic coefficients of GaAs at shorter wavelengths than 1.15 ym has been found.The coefficients would presumably become very difficult to measure as the band edges approached due to the rapidly increasing absorption. In the absence of any other information the values of the photo elastic coefficients are therefore taken as quoted above. This uncertainty must however be remembered.

Using these values of the photo elastic coefficients, the stress induced dielectric constant profile for the T.E. and T.M. waves propagating beneath stripes of various widths are plotted in Fig. 7. As usual the film edge stress S = a05t =~2.5 X 105 dynes/cm and the active layer depth is taken at 2 #m in all cases.

Note that in most cases the shape of the dielectric constant profile for the T.M. wave, plotted as a dashed line, is similar in shape and magnitude to that of the T.E. wave. Because of small differences in the net gain of the T.E. and T.M. mode, double heterostructure lasers almost invariably operate in the T.E. mode. For all the stripe widths plotted the difference in dielectric constant between the centre and the edges of stripe is greater than - 10-2.In the case of the standard 20 ym laser plotted in Fig. 7(a) the dielectric constant of the T.E. wave is 2.5 X 10-3 higher beneath the axis of the stripe than it is at x = f 7 sszm. In the absence of any other waveguiding effects this forms a weak real dielectric waveguide strong enough to guide the zero mode and nearly strong enough to guide the first order mode at A = 0.85 ym.

For all the stripe widths plotted in Fig. 7 that are narrower than 20 jum, that is for (b) 10 ym, (c) 5 ym and (d) 2.5 lim, there is strong anti-waveguiding because the dielectric constant has a minimum value at the stripe axis and rises rapidly away from the axis. For instance for the 10 itm stripe the dielectric constant us 9 x 10-3 higher at x = + 5 ym than it is at x = 0.

These plots show that the stripe width must be much greater than 10 jum for active layer depths of 2 lim if anti-waveguiding is to be avoided. At 20 jum stripe width a weak waveguide is formed which is desirable. Fig. 8 depicts how, for a 20 ym width stripe window the strength of the stress induced waveguide for the T.E. wave varies with active layer depth z. The dielectric waveguide has disappeared completely at 4 jum active layer depth, in fact there is weak antiwaveguiding.At 1 jum junction depth there is much stronger real dielectric waveguiding than at z = 2 ym, with a difference of +9X 10-3 between x=Oandx= + 8.5 jum. In the absence of other effects this waveguide is capable of guiding up to the 2nd order mode at A = 0.85 m.

Reverting attention in particular to Fig. 7 it has been shown that with a stripe window in a film under compression only certain geometries produce a lateral waveguiding effect, while others produce anti-waveguiding. The same stress pattern can alternatively be produced by replacing the film in compression with a stripe under tension located in the position of the former window. Furthermore the inverse stress pattern (i.e. compression where there was formerly tension, and tension where there was formerly compression) can be obtained by replacing the apertured film in compression with an apertured film under tension, or with with a stripe under compression. With the inverse stress pattern the geometries that were formerly waveguiding become anti-waveguiding, while those that were anti-waveguiding become waveguiding.These four waveguiding geometries are set out at (a) to (d) in Fig. 9.

Fig. 10 depicts a construction of double heterostructure stripe-contact laser which utilises the photo-elastic effect to provide a waveguiding effect in the lateral direction. This laser has a GaAs n-type substrate 1 upon which four layers 2, 3, 4 and 5 are grown by liquid phase epitaxy.

Layers 2 and 4 are respectively n-type and p-type layers of GaO 65AI0 35As, and sandwich the higher index active layer 3, which is made of Gaog5AiOO5As. The uppermost layer, layer 5, is a ptype GaAs layer provided so as to facilitate the provision of a good electrical contact between a metal contact layer and the underlying semiconductive material. The area of contact is limited to a strip 20 ,um wide by an electrically insulating masking film 7. The active layer 3 is situated at a a depth of 2 ym beneath the upper surface of the semiconductive material, and the material, thickness, and method of deposition of the insulating film 7 is such as to provide the film with a compressive edge stress at the semiconductor surface of about 2.5 X 105 dynes/cm.This film may be made for instance of silica, silicon nitride, or alumina. A preferred way of providing the film is to place the semiconductive material on a graphite susceptor in a 15 cm diameter vacuum vessel containing 4% silane in ammonia to a pressure of about 0.3 torr. Nitrous oxide is bled into this to produce a total pressure of about 0.5 torr (as registered by thermocouple vacuum gauge). Then a silica/silicon nitride film is deposited by an r.f. excited plasma using a 500 watt 1 MHz generator adjusted to provide a susceptor temperature of 525 C. Typically with this deposition process the requisite compressive stress is provided with a film thickness in the region of 0.25 jum. The film 7 is deposited as a single layer and then divided into two by the formation of the 20 ym wide stripe. This stripe is made by etching using buffered hydrofluoric acid and conventional photolithographic masking techniques. The top contact is provided by a layer 8 of gold on a film 9 of titanium.

Fig. 11 depicts a construction of double heterostructure stripe contact laser like that of Fig.

10, but with the difference that the stripe width is about 4 lim instead of 20 ym. This is too narrow in relation to the active layer depth of about 2,um for lateral photo-elastic waveguiding to be produced by a stripe window in a compressively stressed film. In this instance the lateral photo-elastic waveguiding is provided by a stripe window in a film 10 under tension. This film 10 is an evaporated layer of nickel chromium. Since this is electrically conductive, an electrically insulating masking layer is required to limit the current flow to the stripe, and hence the silica/silicon nitride film 7 is still employed in this construction.However, since the film 7 is under compression, it tends to counteract the tension provided by the nickel chromium layer 10, and hence the film 7 is made thinner than before, being typically about 0.1 pem thick. When this is covered with a 0.4 lim thick layer of 20% chromium 80% nickel evaporated under vacuum from a tungsten filament on to the substrate maintained at 500 C, the combination provides a tensile edge stress at the semiconductor surface of about 2.5 X 105 dynes/cm.

In a conventional stripe contact double heterostructure laser the stripe extends the whole length of the laser optical cavity. A disadvantage of this is that there is a relatively high current density in the immediate vicinity of the laser reflecting facets. It has been found that particularly with narrow stripe contact lasers the extinction of light in unpumped regions is not as rapid as predicted by the simple theory which asserts that the laser photon energy is just greater than the band gap and hence strongly absorbed. It has been postulated that this is because the high current density slightly reduces the band gap in the pumped regions. This phenomenon in principle allows the stripe contact to be terminated short of the laser ends.However in a conventional narrow stripe laser without photo-elastic waveguiding it is found that the lateral divergence of the laser beam as it is launched from the pumped region into the unpumped region seriously reduces the feedback obtained by reflection at the output facet. Fig. 12 shows a construction of stripe contact double heterostructure laser where this lateral beam divergence in the unpumped regions is 'focuced' by photoelastic waveguiding. The basic construction of this laser is the same as that of the Fig. 10 laser, with the difference that the etching of the 20 jum wide stripe, indicated by reference numeral 11, is terminated before the underlying semiconductive material is exposed.Typically the insulating film 7 is 0.27 jtm thick, and the etching is halted when the insulator thickness at the bottom of the trough 12 is about 0.07 ym. The device is then remasked, and a window 13, typically 4 ym wide, is etched in the centre of the trough 12. For a typical device between 150 and 200 #m long, this window terminates between 25 and 60 ym short of the reflecting facets. (For clarity the metallisation layers 8 and 9 have been depicted with broken lines.) The lateral photo-elastic waveguiding effect provided by the trough, may, in the pumped region, be swamped at certain drive levels by the anti-waveguiding effect produced by the minority carrier distribution produced by a single stripe contact.In the specification of our earlier patent application Serial No 1557072 42611/76 (identified by us as C.H.L. Goodman-P.A.

Kirkby 9-3) there is disclosed a way of obtaining a more favourable minority carrier distribution by means of a double stripe contact. That principle can also be applied in the present instance.

One way of putting this into practice is illustrated in Fig. 13. This relies upon the tensile stress of a nickel chromium film. The semiconductive layers are of the same construction as those described previously with reference to Figs. 10, 11 and 12. The topmost semiconductor layer (layer 5) is covered with a thin insulating film 7 in the same manner, and for the same purpose, the equivalent film 7 of the Fig. 11 constuction. Similarly this film is covered with an equivalent nickel chromium film 10. Prior to the deposition of this film 10, a pair of windows 14 is etched through the underlying insulating film 7. These are typically each 3 lim wide, are separated by 3 ym, and terminate typically between 20 and 50 itm from the laser reflecting facets.

Subsequently two slots 15 are etched through the thickness of film 10 to register with the ends of the strip 16 of insulating film 7 lying between the two windows. (For clarity film 10 has been depicted with broken lines.) In this instance film 10 also serves as the metallisation.

Although the foregoing specific examples have all been lasers, it will be evident that the photo-elastic waveguide effect operates in passive as well as active devices. The invention is therefore applicable not only to laser per se, but also to double heterostructure semiconductor integrated optics devices, whether or not such devices include lasers. Such a device may incorporate more than one waveguide with photo-elastic lateral waveguiding, and one or more of these may incorporate bends, branches or other features common to integrated optics layouts.

The electro optic effect may be used in conjunction with photo-elastic waveguides to fabricate waveguide switches or modulators. Similarly it will be apparent that this invention is not limited to its application solely to devices constructed in gallium arsenide, but is applicable also to devices constructed in other Ill-V semiconductor materials, such as gallium arsenide/gallium indium arsenide phosphide.

A number of gallium aluminium arsenide double heterostructure stripe contact lasers have been sold that have a construction in which current flow is restricted to a single stripe by a stripe window in an r.f. deposited silica and/or silicon nitride electrically insulating layer. The window extends the full length of the laser. By chance in some instances the thickness and manner of deposition of this insulating film have been such as to provide a lateral photo-elestic waveguiding effect for the laser.

Accordingly we make no claim to a gallium aluminium arsenide, or gallium arsenide-gallium aluminium arsenide, double heterostructure single stripe contact laser in which a lateral photoelastic waveguiding effect for the laser is provided by a window in a layer of silica and/or silicon nitride, which window extends the full length of the laser.

CLAIMS Subject to the foregoing disclaimer 1. A Ill-V semiconductor device incorporating at least one optical waveguide, which device has a higher refractive index layer sandwiched between underlying lower refractive index material and an overlying layer of lower refractive material, for which waveguide a waveguiding effect in the direction normal to the layers is provided by the interfaces between higher and lower refractive index material, and a waveguiding effect in a lateral direction in the plane of the layers is provided by a strain pattern, penetrating the higher refractive index layer, produced by material deposited upon the surface of the semiconductor device in such a way as to be under stress at ambient temperatures.

2. A semiconductor device as claimed in claim 1 wherein the strain pattern producing said lateral waveguiding effect is provided at least in part by one or more apertures in said deposited material, which material is under compressive stress.

3. A semiconductor device as claimed in claim 1 wherein the strain pattern producing said lateral waveguiding effect is provided at least in part by one or more apertures in said deposited material, which material is under tensile stress.

4. A semiconductor device as claimed in claim 1 wherein the strain pattern producing said lateral waveguiding effect is provided at least in part by one or more strips of said deposited material, which material is under tensile stress.

5. A Ill-V semiconductor device as claimed in claim 1 wherein the strain pattern producing said lateral waveguiding effect is provided at least in part by one or more strips of said deposited material, which material is under compressive stress.

6. A semiconductor device as claimed in any preceding claim wherein the device is a stripe contact laser.

7. A laser as claimed in claim 6 wherein the laser is a single stripe contact laser.

8. A laser as claimed in claim 6 wherein the laser is a double stripe contact laser.

9. A laser as claimed in claim 6, 7, or 8 wherein the stripe contact or contacts terminate short of the reflecting facets of the laser.

10. A semiconductor device as claimed in any preceding claim wherein the semiconductive material is constructed in gallium arsenide/gallium aluminium arsenide.

11. A semiconductor device as claimed in any claim of claims 1 to 9 wherein the semiconductive material is constructed in gallium arsenide/gallium indium arsenide phosphide.

12. A Ill-V semiconductor device substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. waveguiding effect for the laser. Accordingly we make no claim to a gallium aluminium arsenide, or gallium arsenide-gallium aluminium arsenide, double heterostructure single stripe contact laser in which a lateral photoelastic waveguiding effect for the laser is provided by a window in a layer of silica and/or silicon nitride, which window extends the full length of the laser. CLAIMS Subject to the foregoing disclaimer

1. A Ill-V semiconductor device incorporating at least one optical waveguide, which device has a higher refractive index layer sandwiched between underlying lower refractive index material and an overlying layer of lower refractive material, for which waveguide a waveguiding effect in the direction normal to the layers is provided by the interfaces between higher and lower refractive index material, and a waveguiding effect in a lateral direction in the plane of the layers is provided by a strain pattern, penetrating the higher refractive index layer, produced by material deposited upon the surface of the semiconductor device in such a way as to be under stress at ambient temperatures.