CA1088658A - Stripe lasers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An injection laser in which the current flow across the active layer of the device is restricted by confining the current flow across an adjacent interface to a pair of closely spaced parallel stripes. The width and spacing of these stripes is chosen to promote the waveguiding of the zero order mode of the laser in preference to the waveguiding of the first order mode and to waveguiding by the so called "gain guiding" mechanism.

Description

~ 1~8865~
,, : .;` i This invention relates to stripe lasers.
According to the present invention there is provided an injection laser comprising: a first metal contact layer; a first semiconductor layer disposed on and coextensive with said first con~act layer; a second semi-conductor layer disposed on and coextensive with said first semiconductor layer; an active semiconductor layer disposed on and coextensive with said ':`.`
~ second semiconductor layer; a third semiconductor layer disposed on and co-- extensive with said active layer; a fourth semiconductor layer disposed on and coextensive with said third semiconductor layer; an insulating layer disposed on said fourth semiconductor layer to provide a pair of closely spaced parallel stripes on a surface of said fourth semiconductor layer, each of said stripes having a given width and adjacent edges of said stripes being separated from each other by an amount greater than 2.5 ~m and not greater than 5.5 ~m to promote real dielectric waveguiding of the zero order mode of said laser in preference to either gainguiding or to waveguiding of the first order mode of said laser; and a second metal contact layer ,;,.~
disposed on and coextensive with said insulating layer and said stripes.
This twin stripe arrangement produces a carrier density distribu-tion that is more suited to optical guiding than that produced by the single strip of a conventional stripe laser. This, in turn, reduces the recurrent ~' problem in conventional stripe lasers of optical instability which causes non-linear light/current characteristics and excess noise.
~ In the following description a heterostructure ;' .'~ , .

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C.H.L. Goodman-P.A. Kirkby 9-3 (CAP)

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Ga(l x)Al(x)As (x ~ o) stripe contact laser constructed in accordance with the present invention is contrasted with a . conventional stripe contact laser.
Figure 1 depicts a schematic perspective view of a stripe contact laser constructed in accordance with the present invention;
Figure 2 depicts carrier density and light output profiles for a conventional stripe contact laser;
Figures 3a and 3b depict the light current and noise characteristics of, respectively, fairly linear and - very non-linear oxide insulated conventional stripe lasers;
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Figures 4 and 5 depict the carrier density distribution of the laser of Figure 1 for various widths ` and separations of the stripes; and :, ; 15 Figure 6 depicts the carrier density, refrac-; tive index, and optical intensity profiles of the laser of Figure 1 at a particular drive level.
, Referring to Figure 1, a heterostructure Ga(l x)AlxAs (x ~ o) laser consists of a substrate 10 of GaAs upon which a set of layers 11 to 14 are epitaxially grown to provide a conventional heterostructure laser structure. The particular structure is a double hetero-,:
, structure laser in which the active layer, layer 12, is ... .
sandwiched between two layers 11 and 13 of higher band-gap, lower refractive index material to provide optical and , carrier confinement in a direction normal to the plane of ~ the layers. Layers 11 and 13 are of opposite conductivity ; type, and layer 13 is covered with a further layer, layer 14, made of GaAs in orde~ to facilitate making electrical ;.
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~886~8 C.~.L. GOOD~A~, ET AL. 9-3 contact with the top surface of the structure.
Thus far the structure is the same as for a ;
conventional double heterostructure stripe contact laser and may, if desired, be replaced with a single heterostructure device or a multi-layer heterostructure device having a local-ized gain region as described, for instance in Great Britain Patent No. 1,263,835 of G.H.B. Thompson or the corresponding ` U.S. Patent No. 3,911,376, issued October 7, 1975.
` Next the structure is provided with an oxide insulating layer 15 covering the whole surface of layer 14 except for two closely spaced parallel stripes 16. This oxide layer is covered with a metal contact layer 17 which forms the top contact by making electrical contact with the underlying semiconductive material along the two exposed stripes. A metal contact layer 18 on the under surface of the substrate 10 provides the counter-electrode contact.
The light output is in the direction indicated generally .;, l by the arrow 19. A conventional double heterostructure laser typically has the same structure with the difference that there is only one stripe along which the top contact makes contact with the underlying semiconductive material.
Typically this single stripe is about 20Jum wide. The : carrier density distribution of such a laser is found experimentally to be approximately as illustrated in Figure 2. At or below the lasing threshold the distribution is as shown in curve _ with the highest carrier density beneath the centre of the stripe. The optical distribution above `~ threshold is less than 10~m wide. As the light intensity increases carriers recombine relatively more rapidly in the regions of highest optical intensity thus tending to clamp ~, the carrier density in the reginn beneath the center of the ;~
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stripe. The carrier density, however, continues to rise outside the lasing region producing carrier profiles as shown in curve b at 7.5mw and curve c at 15mW.
This rapidly changing carrier density distri-,' 5 bution is undesirable because it is this carrier density :, distribution which controls the transverse optical power distribution and the losses of the optical wave as it propagates beneath the stripe. The optical distribution "~ is determined by the real and imaginary parts of the ,, 10 dielectricconstant (refractive index -2-~ . The imaginarY
part of the dielectric constant describes th,e gain or loss of the medium, and the optical gain of course increases as ' the carrier density increases, Over a small range of carrier density the gaincan be expressed as g= ~ (n-nO), 15 where ~ and nO are parameters which depend on the wave-, length concerned and the temperature. A negative value of g corresponds to loss.
~,; ' The gain profile, therefore, has approximately ,,, e the same shape as the carrier density profile with the zero 20 gain axis offset to the carrier density nO. The real part of the dielectric constant is also dependent on the carrier density with the dielectric constant reduc~ as the :,., ,," carrier density increases. Again for small variation ',- of n the relationship is given by E= El - a n, where the El is the effective dielectric constant of the active layer ^~, waveguide (about 13.8~ and ~ is a proprotionality constant ,'' again dependent on wavelength. a is very small, with ' ''- Yariations of dielectric constant of only 5- 25 x 10 3 being observed across the optical output region of typical lasers. This corresponds to carrier variations in the , --5 ,:
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C.~.L. Goodman- P.A. Kirkby 9-3 (CAP) ~ 86~8 range 2- 5 x 10 cm . These small variations in dielectric constant are, however, very important in determining the optical distribution.
At threshold, therefore, although the gain is ` 5 greatest beneath the center of the stripe, the dielectric constant is lowest in this region. This concave dielec-tric constant profile produces an anti-wavegui~ngeffect which is only overcome when the gain is sufficiently high to produce the phenomenon known as 'gain guiding'.
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In this 'gain guiding' situation, light ~; propagating under the stripe is divergent, but the beam .i~.
~ width is limited by the absorbtion of the regions flanking :- .
; that beneath the stripe. In the direction of the stripe the loss due to beam divergence is offset by the gain of the region under the stripe so that light is able to propagate under the stripe in the direction of the stripe with a net gain. Then, as the current drive increases well above threshold, the optical intensity in the 'gain ..
guiding~ region increases to the extent that the associated carrier depletion produces a dip at the middle of the con-vex carrier profile thereby producing an inverted 'W' ` shaped dielectric constant profile which is convex in the middle. Over this convex central region real dielectric waveguiding occurs with the light constantly being focus-sed along the axis of the stripe. The gain is of course : higher at the edges of the optical distribution than in the middle. At sufficiently high output such a wide and ; deep dielectric profile is produced that the first order mode d is guided and comes into operation competing with the zero order mode e. (Figure 2).
Thesechanges inthe opticalwaveguideparameters, 'B -6-. . .
' :1~13865~3 ~-~ C.H.L. Goodman-P.A. Kirkby 9-3 (CAP) -particularly the changes from the lossy 'gain guiding' situation to the self focussed real dielectric waveguide situation, change the operating characteristics of the laser, and are believed responsible for producing the non-linearities in the light current characteristics, such as illustrated in the light/current curves shown in Figs. 3a and 3b, which relate, respectively, to a fairly , . .
linear and a very non-linear oxide insulated conventional stripe laser. The non-linearity of the light/current ; 10 characteristic above threshold can be seen in both lasersJ
, although the laser of Figure 3b, shows much larger non-linearities than that of Figure 3a. The linearity varies greatly from laser to laser, and from wafer to wafer, probably because of the extreme sensitivity of the 'gain guided' wave to small undulations in active layer thick-''ness. We have found thatthe noise, which is the unwanted ; ' random a.c. component in the light output, appears to be .~
closely related to the non-linearity of the laser, with high levels of excess noise associated with the unstable '~ 20 regions of anomalously high slope efficiency. In extreme cases self pulsing occurs. This is the cause of the very high a.c. output that occurs at high output level in the laser illustrated in Figure 3b.
These non-linearities and the associated instability and noise are a significant deficiency in the operating characteristics of c.w. lasers. Measurements on . lasers with deliverate dielectric waveguiding in the plane of the junction show that improved linearity of output characteristics can be obtained. For example, the chan-nelled substrate laser, described in Great Britain Patent ' Specification No. 1,530,323 of P.A. Kirkby,published ; -7-. ~D

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C.H.L. Goodman-P.A. Kirkby 9-3 (CAP) ~8865B

October 25, 1978, which has a strong waveguide in the plane of the junction, has linear light current characteristics and low noise. The twin stripe laser of the present invention is an alternative method for pro-. . .
- 5 ducing transverse dielectric waveguides in the plane of the junction. It has the advantage that the perturbations in dielectric constant are very small so that a wide zero order mode distribution can be guided.
Calculated carrier density profiles beneath the contacts of a variety of twin stripe lasers are ; illustrated in Figures 4 and 5. Figure 4 shown the :
profile beneath a twin stripe contact of total width 15~m for the separation between the stripes of 1, 2.5 and 5~m. Figure S shows the profile for twin stripes each 3~m wide separated by l, 2 and 3~m. These curves are all calculated on the basis of the effective lateral diffusion length being 4~m. The actual value will vary with such parameters as active layer doping level, thick-ness, etc, However, it can be seen that by controlling `~ the width separating the stripes a dip can be formed in '' the carrier density profile. The depth and width of this :.
~ dip can be controlled over a wide range by controlling ; the stripe separation (and the total width). The dip .~ .
in the carrier density will produce a real dielectric waveguide at all currents both below and above threshold.

This will eliminate the deleterious change from 'gain guiding' to dielectric waveguiding, which occurs in the 20~m wide single stripes of conventional stripe contact lasers, improving the stabiiity and linearity.
In practice, the total width of the stripes, and the width separating them, will have to be optimized to produce the correct strength of waveguide to guide the : B
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C.H.L. Goodman- P.A. Kirkby 9-3 (CAP) ~886~8 ;: 9 zero order mode rather than the first order mode, not only at threshold, but up to high output power. Referring to ; Figure 6 the strength of the waveguide is given by D=~o (~) / W/2, where ~O is the free space propagation ....
constant, ~E iS the difference in the real part of the dielectric constant between the center and the edges of ` the waveguide and W is the width of the waveguide. For a parabolic profile similar to the plotted profiles, the zero order mode is completely guided when D= 1, and the first .
~ 10 order mode comes into operation when D= 3. So D can vary ;-over a range of 3:1 with satisfactory waveguiding. The strength of the waveguide is proportional to the width of ~,, ~ the waveguide, but only to the square root of the differ-r`. ~
~-` ence in dielectric constant ~. Calculations of the shape : ~
;~ 15 of the profile as the light intensity increases above thres-1'.'`.
~Idshow that there is a considerable advantage in reducing ~; the total width of the stripes well below 20~m if the onset ~; of the first order mode is to be delayed to as high an out-put power as possible. On a naive theoretical basis the ~ 20 optimum structure would be two stripes of infinitesimal - width separated by the required distance t~ 2 - 6~m). In this structure the width of the waveguide is independent of light output intensity with the peaks in the carrier density profile always occurring beneath the stripes. For stripes . . .
~ 25 of finite width both the width and the depth of the wave-i :
guide increase as the light output increases. In practice, optimization has to take account of the contact resistance of the laser which obviously increases as the total stripe area reduces. A typical structure lies in the rangecovered _9_ B

C.H.L. Goodman - P.A. Kirkby 9-3 (CAP) 88Q55~
.. --10--by two 3~m wide strips separated by 2 - 5~m and two stripes of total width 15~m ~7ith a separation of 1 - 5~m.
The approximate carrier density profile, refractive index profile and optical intensity distribution for twin stripes 3~m wide separated by 3~m are illustrated in Figure 6.
' Although the above described example has employed an oxide layer to restrict the current flow across the interface between a top electrical contact and the under-lying semiconductive material, it is to be understood that this delineation can be achieved by other means. For ~ .
instance~ it can be achieved by proton-bombardment arranged to render certain areas of the underlying semiconductive ` material semi-insulating. An alternative method involves the use of zinc diffusion to establish current carrying ; paths through an n-type isolating layer to p-type material ~ overlying this active layer.
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It is also to be understood that instead of arranging for the current confinement to take place at an interface overlying the active layer, it can be confined at an interface underlying the layer. One way of achieving this is to produce a striped substrate type laser. In the case of one example of such a laser grown on an n-type substrate, the substrate has a p-type dopant diffused into its top surface prior to the growth of an n-type expitaxial layer underlying the active layer. This p-type dopant converts the whole of the top of the substrate into a p-type material, except for two closely spaced parallel stripes, which are masked from the dopant, and hence remain n-type. Once the mask has been removed, and the epitaxial layer grown, the ~' .
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C.H.L. Goodman - P.A. Kirkby 9-3 (CAP) 386S~3 parts of the substrate which have been converted to p-type material operate to confine the current flow between the substrate and the epitaxial layer to the region of the two stripes.
The optimum spacing of the two stripes is dependent upon the amount of current spreading occurring between the active layer and the interface where the current is confined. In the case of a laser of the structure of ....
Figure 1, it is the resistivity and thickness of the layers 12, 13 and 14 that are important. The effect of current spreading can be characterized in terms of an effective .
lateral dîffusion length. The following details of measure-..
ments concerning the effects of varying the stripe separa-tion relate to a specific double heterostructure construc-tion developed in the first instance for conventional single stripe lasers. In this construction the GaAs substrate 10 (Figure 1) is lOO~m thick and is doped with silicon to make it n-type with a nominal carrier concentration of 1013 carriers cm 3. The GaAlAs layer 11 is 3.2~m thick, has the composition GaO 65Alo 35As, and is doped with tin to make it n-type with a nominal carrier concentration of 5 x 1017 carriers cm 3. The active layer 12 is 0.2~m thick, has the compositiOn GaO 95A10.O5As, and is doped to give a net nominal carrier concentration of 3 x 1017 carriers cm 3. The layer 13 is l.O~m thick, has the composition GaO 62Alo 38As and is doped with germanium to make it p-type with a nominal carrier concentration of 4 x 1017 carriers cm .~ Finally the GaAs layer 14 is 1.2~m thick and is also doped with germanium to make it p-type with a nominal carrier concentration of 4 x 1017 carriers cm 3.
To investigate the effects of varying the stripe separation ' ~

C.~.L. Goodman - P.A. Kirkby 9-3 (CAP) 86~ ~

a number of lasers were made each having the same width of stripe which for the purposes of comparison was set at 2.5~m, A typical example of laser with a separation of 2.5~m between the adjacent edges of the two stripes shows in its far field pattern that the waveguiding of the structure at threshold is insufficient to properly guide the zero order mode. The leakage of this mode is indicated by the excessive width of the far field pattern. At the . lasing threshold the light emission tended to ~snap-on' to about 3mW, but thereafter the light/current character-istic increased relat~vely smoothly up to lOmW light output , (limit of test). Another typical laser with a separation '~ of 4.3~m between adjacent edges of its stripes was found to guide the zero order mode properly at threshold and the first order mode was not generated until the light output reached about 8mW. This laser exhibited a smooth progres-sive light/current characteristic ~ithout any significant 'snap-on r . The best example of laser with a 5~m separation between adjacent edges of lts stripes exhibited the same characterisitcs as the 4.3~m stripe separation laser with the difference that the first order mode was suppressed right up to the limit of testing, namely lOmW light output.
A typical laser with a stripe separation of 5.5~m and a laser length of 250~m had its lasing threshold at a current drive of 75mA. Although Its light~current characteristic was progressive (smooth), the first order mode became ap-parent at a current drive of about 30mA at which time the light output was about 4mW, It should be noted that the above results pertain to particular examples of laser; the characteristics of nominally identical lasers were found to , :.' B
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C.H.L. Goodman - P.A. Kirkby 9-3 (CAP) ~ 3658 ~ differ slightly, probably as the result of minor irregu-: larities in the heterojunction surfaces.
-j~ The effective lateral diffusion length of - current in this structure was measured to be about 3~m.
. The stripe separation would have to be increased for an .~ increased lateral diffusion length.
`~ Although the foregoing specific description ,~ has related exclusively to the Gal xAlxAs semiconductor system it is to be understood that the invention is gener-ally applicable to other semiconductor systems such as, r for instance f to Gal_xInxAsl yPy semiconductor systems.
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Claims (9)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An injection laser comprising:
a first metal contact layer;
a first semiconductor layer disposed on and coextensive with said first contact layer;
a second semiconductor layer disposed on and coextensive with said first semiconductor layer;
an active semiconductor layer disposed on and coextensive with said second semiconductor layer;
a third semiconductor layer disposed on and coextensive with said active layer;
a fourth semiconductor layer disposed on and coextensive with said third semiconductor layer;
an insulating layer disposed on said fourth semiconductor layer to provide a pair of closely spaced parallel stripes on a surface of said fourth semiconductor layer, each of said stripes having a given width and adja-cent edges of said stripes being separated from each other by an amount greater than 2.5 µm and not greater than 5.5 µm to promote real dielectric waveguiding of the zero order mode of said laser in preference to either gain-guiding or to waveguiding of the first order mode of said laser; and a second metal contact layer disposed on and coextensive with said insulating layer and said stripes.

2. A laser according to claim 1 wherein said insulating layer is an oxide layer.

3. A laser according to claim 2, wherein said width is in the order of 2.5 µm.

4. A laser according to claim 1, wherein said given amount is greater than 2.5 µm and not greater than 5.5 µm.

5. A laser according to claim 1, wherein said given width is in the order of 2.5 µm.

6. A laser according to claim 1, wherein said first, second, third and fourth semiconductor layers, said active layer and said insulating layer are composed of Ga1-xAlxAs, where x ? 0.

7. A laser according to claim 1, wherein said first, second, third and fourth semiconductor layers, said active layer and said insulating layer are composed of Ga1-xInxAs1-yPy, where x ? 0 and y ? o.

8. A laser according to claim 2, wherein said first, second, third and fourth semiconductor layers and said active layer are composed of Ga1-xAlxAs, where x ? 0.

9. A laser according to claim 2, wherein said first, second, third and fourth semiconductor layers and said active layer are composed of Ga1-xInxAs1-yPy, where x ? 0 and y ? 0.