CA1073998A

CA1073998A - Double heterostructure laser for direct coupling to an optical fibre

Info

Publication number: CA1073998A
Application number: CA281,897A
Authority: CA
Inventors: Anthony J. Springthorpe; John C. Dyment
Original assignee: Northern Telecom Ltd
Current assignee: Nortel Networks Ltd
Priority date: 1977-07-04
Filing date: 1977-07-04
Publication date: 1980-03-18
Also published as: GB1568465A; DE2820646A1; NL7806634A; JPS5414693A

Abstract

A DOUBLE HETEROSTRUCTURE LASER FOR
DIRECT COUPLING TO AN OPTICAL FIBRE
Abstract of the Disclosure A double heterostructure laser has a first confining layer on the substrate, an active layer on the first confining layer and a second confining layer on the active layer. A groove is etched through the second confining layer and active layer to form a reflecting surface at approximately 45° to the plane of the active layer. The resonant cavity is formed between a cleaned end surface and a surface at the substrate. A hole through the substrate to the first confining layer is aligned with the reflecting surface for emission therethrough.

Description

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This invention relates to lasers, and in particular to double heterostructure lasers, for direct coupling to an optical fibre.
Conventional stripe geometry CW lasers cannot readily be butted directly to optical fibres because the light emitting stripe is not at a convenient distance from the heat sink. Thus, typically, the light emitting stripe is about 10-12~m away from the heat sink. The axis of the fibre mounted on the heat sink would be at a greater distance than this and therefore coupling is not effective. Coupling can be achieved by mounting the laser on a raised heat sink, with suitable lensing between laser and the core of the fibre, but this is complex and expensive.
U.S. patents 3,996,492 and 3,996,528, both issued on December 7, 1976, disclose folded cavity lasers in which the lasing light is reflected twice by mirror surfaces etched into the p-n junction. In U.S. patent 3,996,492, a full mirror surface is formed on the back surface of a substrate at one position, aligned with one of the mirror surfaces, and a partially transmissive mirror is formed on the back surface of the substrate and aligned with the other mirror surface. The laser cavity extends from the full mirror - up to the first mirror surface across to the other mirror surface and down to the partially transmissive mirror. Thus the lasing light is turned twice. In U.S. patent 3,996,528 light can issue at two spaced apart positions on the back surface of the substrate as partially transmissive mirrors are formed at both positions, substantially aligned with each mirror surface. A diffraction grating is formed at each partially transmitting mirror.
The present invention provides a new structure ', , ~
, - 1 -,' ~

- for a laser. In the present invention the resonant cavity of the laser is formed between an end surface of the active layer and a surface associated with the substrate surface, the associated surface being generally parallel to the active layer, with rotation of the laser beam through about 90 by means of a mirror at about 45 to the substrate surface.
The invention will be readily understood by the following description of certain embodiments, by way of example, in conjunction with the accompanying diagrammatic drawings, in which:-Figure 1 is a cross-section through a convent-ional stripe laser and an optical fibre, illustrating coupling difficulties;
Figure 2 is a cross-section through one form of device in accordance with the present invention, illustrating the basic principle of the invention;
; Figure 3 is a cross-section similar to that of Figure 2, illustrating a typical structure for a device as in Figure 2;
Figure 4 illustrates a modification of the structure of Figure 2;
Figures 5 to 13 illustrate the various steps in the production of a device in accordance with the present invention.
In figure 1, a conventional laser chip 10 is mounted on a heat sink 11. An optical fibre 12 is shown butted to the laser chip and it will be seen that coupling is ineffective in that little of the light emitted from the laser enters the fibre core 13.
The broad general principle of the present invention is illustrated in figure 2. This is basically a

- 2 -1~7;~

"Burrus" type light emitting diode structure 15 modified to form the laser cavity between the natural (110) cleavage face 16 and the interface 17 between the substrate 18 and first confining layer 19. Rotation of the beam is performed by a mirror 20 at 45 to the substrate surface, or interface 17.
Coupling of an optical fibre is readily accomplished by inserting the fibre 12 in a hole 21 etched through the substrate 18. The mirror 20 is formed by, for example, etching through second confining layer 22 and active layer 23, to the first confining layer 19.
The device as illustrated in figure 2 relies on the natural (110) cleavage face 16 for one of the reflecting surfaces of the laser. If the emission from this face is observed then it will appear as a conventional CW stripe laser.
To utilize the energy that would be lost through this face a totally reflecting coating is applied to ensure that all the available energy exits via the interface 17.
For efficient operation, two basic requirements exist, (a) the interface between substrate and first confining layer should be perfectly plane, and (b) the mirror should be at 45. However, as will be described, it is possible to accommodate, and provide for, some slight departures from such requirements. Producing the mirrors is possible by various methods. Etching can be used, although this can create problems due to the lack of a natural etching plane at 45 to the (100) surface of the substrate. Ion milling is another technique.
Figure 3 illustrates one particular structure for a device as in figure 2. On n-type GaAs substrate 18 is formed the first confining layer 19 of n-type Gal xAlxAs, followed by active layer 23 of n or p type Gal yAlyAs/ suitably ~3~

doped to produce a p-n layer at one of the two surfaces of the active layer, (y<<x). Second confining layer 22 is of p-type Gal xAlxAs. The mirror 20 is formed, as by etching, and deposition of a layer of SiO2 25. A heat sink layer 26 typically of gold is formed over the second confining layer and the SiO2 layer.
The two confining layers 19 and 22 and active layer 23 are conveniently formed by conventional epitaxial growth on the substrate 18. The SiO2 layer is deposited, either through a mask, or by photolithographically etching.
The gold heat sink is formed by plating in a conventional manner. The hole 21 is etched through the substrate after masking of the substrate. Electrical contact is made to the substrate and to the gold layer with the substrate negatively biased to the gold layer, as indicated at 28 and 29.
As previously stated, the interface between substrate and first confining layer should be perfectly plane. Generally, with present day techniques, the surface of the substrate on which is produced the first confining layer is usually satisfactorily smooth and flat by normal production processes. It is usually possible to produce a suitable surface on the substrate by mechanical polishing, the confining layer being formed on the polished surface.
However, if the surface is rough it is possible to grow a thin layer of n-GaAs on the substrate. This layer grows to give a flat, smooth surface.
While the mirrors should be at 45 to substrate surface some departure can be accommodated. It is possible to provide some correction at, for example, the interface bet~een substrate and confining surface.

The essential feature is that a resonant cavity or laser path be provided. While, for convenience, one reflecting surface of the cavity or path is made the interface between substrate and first confining layer, this is not essential, and the reflecting surface can be positioned from the substrate surface, as for example, by an intervening layer between substrate and confining layer.
It is also possible to modify the emitting surface to improve emission efficiency and output. Thus the emitting surface can be in the form of a lens structure. Figure 4 illustrates a modification of the structure of Figure 3 in which a lens formation 35 is formed at the base of the hole 21.
The lens formation may be made integral with the first confining layer 19, or as a separate entity positioned in the base of the hole before positioning a fibre therein. The profile of the lens surface 36 will depend upon circumstances. Thus if the lens is integral with the first confining layer then the surface 36 will become the reflecting surface for the laser path. The profile will then need to be such that light rays reflected back from the surface 36 will be reflected correctly by the mirror 20. It is possible also to provide a grating structure either at the flat emission surface of Figure 3 or on the lens surface of Figure 4. A grating applied to the lens surface will give wavelength and mode control.
- A particular method of preparing 45 mirror lasers by chemical etching is as follows. The method is valid for the crystallographic orientation specified and would need modifying for other orientations. An alternative method of ion milling would generally be applicable for all orientations.
A polished (100) orientation GaAs substrate slice with (110) cleaved sides, has a 3 (or 4) layer double heterostructure grown on the surface in a conventional manner resulting in a structure as in figure 5. The important layers are the N-Gal_xAlxAs and p-Gal_xAlxAs confining layers 19 and 22 and the sandwiched p- or n-Gal Al As active layer 23. In general x 3 0.20 (0.40 preferred), y = 0 - 0.1 (x-y 3 0.1), and the active layer can be p-type, n-type or undoped.
Additionally a buffer layer 40 of n-GaAs can be grown before growing the n-GaAlAs layer 19 to help eliminate problems associated with surface damage-in the substrate (caused by polishing and pre-growth thermal etching). This also ensures that the interface between substrate 18 and the N-GaAlAs confining layer 19 is planar, which is not always the case when growing directly into the substrate. Also a layer 41 of p-GaAs can be grown on top of the p-GaAlAs confining layer 22 which assists in making low resistance ohmic contacts. These extra layers of n-GaAs and p-GaAs do not affect the basic physics of the device and will not be referred to in the remainder of this description.
Following the epitaxial growth of the various layers the slices are cleaned and coated with a layer of insulating dielectric, e.g.' chemically-vapour deposited silicon dioxide. (1000-2000A is suitable). In order to form the reflecting mirror in the right direction it is necessary to identify the different [110] cleavage directions. This can be accomplished by etching a pin-hole 42 (1-2 mil diameter) in the oxide using suitable masking techniques. A selective etch, such a 1~ Br2 in CH30H, applied for ~60 secs. will attack the surface and yield a characteristically shaped pit as shown by dotted outline 43 in figure 6.
The long axis of the pit is an (011) direction as in figure 8. From the cross-sections of the etch pit, ~37~

figures 7 and 8, it can be seen that in order to obtain the 45 mirror so that it reflects the light towards the substrate, the laser must be aligned in the (011) direction. If however the laser is aligned along the (011) direction then the light will be reflected upwards, through the p-GaAlAs layer.
Having established the correct desired direction, in the present example [011], for alignment of the laser stripe, the 45 mirrors can now be etched. Pairs of rectangular slots 44 75 llm long x 12.5 llm wide separated by 500 llm and repeated 10 on 12501lm centers in X and 500 llm in Y are opened in the oxide using conventional masking and etching techniques. The long axis of the slot is in [Oll],as illustrated in figure 9.
The slots are then etched out using a preferential etch such as lH202:lH3P04:6CH30H to a depth greater than the active layer. At 18C this etch attacks GaAs and GaAlAs at ~0.511m/min. so that a total etch depth of 31lm can be readily controlled. This etch, and most other preferential etches, eventually expose (111) planes which are the walls of the trough that is formed. Although the angle 2~ between (111) and (100) is 54 44', figure 10, for shallow etch depths it has been found that the angle is closer to 45, i.e., the (111) plane is not fully developed and the exposed face can be used as a reflecting mirror.
Following the mirror etching step the slices are cleaned and then recoated with oxide so that the holes are completely coated, insulating the exposed p-n junction. A
laser stripe 45 is then etched in the oxide with the stripe aligned along [011], as illustrated in figure 11. The stripe ~ -is discontinuous and does not overlap the mirror slots. The 30 surface of the crystal, exposed in the stripe, is then zinc-diffused to form a thin (~0.2~1m) conducting layer; e.g. ZnAs2 source, at 600C for 15 mins. in a semi-sealed tube.
After diffusion the striped surface is O O
metallized with 200A chromium followed by 2000A of gold. Gold heat sinks are then applied to the metallized surface using photolithographic techniques and electroplating from solution to a thickness of ~lO~m. A typical heat sink pattern is illustrated in figure 12, the heat sinks indicated at 46. The slice is then reduced in thickness on the n-side (opposite from the heat sinks) by lapping, polishing and etching to ~lOO~m. The n-surface is metallized with 200A gold/
germanium eutetic followed by 4000A of gold. 175~m diameter windows are opened in the n-side metallization, (aligned with the mirror slots on the p-surface) using KI-I2 etch. Following this the slice is heat treated at 400C for 2 minutes to allow the n-side contact. The holes in the n-side can then be etched out, using alkaline peroxide etch in a recirculatory etcher, down to the interface between GaAs and N-GaAlAs, where etching stops. A typical process using an alkaline peroxide selective etch is described in copending Canadian application serial number 279,249, filed May 26, 1977, and assigned to Northern Telecom Limited. Other etchants can be used.
The slice can then be cleaved into chips as indicated in figure 13, to yield two kinds of etched lasers, either along lines indlcated at 47 for single lasers, or along lines 48 for double lasers.

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Claims

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

1. A laser having a semiconductor substrate and a double heterostructure on a surface of said substrate, said structure comprising; a first confining layer of semiconductor material on said surface of said substrate, an active layer of semiconductor material on said first confining layer and a second confining layer on said active layer, said first confining layer of the same conductivity type as said substrate and said active layer and said confining layers differentially doped to form a p-n junction between said active layer and one of said confining layers; a reflecting surface extending through said second confining layer and said active layer and inclined relative to the plane of said active layer to form a resonant cavity between an end surface of said active layer and a further surface at said substrate surface; and a hole through said substrate to said first confining layer, said hole aligned with said reflecting surface for emission of light therethrough.

2. A laser as claimed in claim 1, said resonant cavity extending between said end surface of said active layer and the surface of said first confining layer contiguous with said substrate.

3. A laser as claimed in claim 1, said resonant cavity extending between said end surface of said active layer and a surface of an intervening layer between said first confining layer and said substrate.

4. A laser as claimed in claim 1, said reflecting surface inclined at approximately 45° to said plane of said active layer.

5. A laser as claimed in claim 1, including a lens structure on said further surface at said substrate surface.

6. A laser as claimed in claim 1, said substrate of GaAs, said active layer and confining layers of

7. A laser as claimed in claim 1, including a Vee shaped groove etched through said second confining layer and said active layer, and a layer of SiO2 on one of the surfaces of said Vee shaped groove to form said reflecting surface.

8. A laser as claimed in claim 1, including a heat sink layer on said second confining layer.

9. A method of making a semiconductor laser in the form of a double heterostructure on a substrate, comprising:
forming a first confining layer, an active layer and a second confining layer on a semiconductor material substrate wafer;
orientating the wafer to determine the [110]
cleavage directions;

forming a rectangular aperture through the second confining layer and the active layer, the aperture of Vee shaped cross-section with its axis along one of the [110]
cleavage directions, the side walls of the Vee at approximately 45°;

forming an oxide layer on said side walls;
forming a laser stripe on a surface of the second confining layer remote from the interface between said second confining layer and said active layer; and cleaving the wafer at a position intercepting the laser stripe to form an end surface, whereby a resonant cavity is formed between said end surface and a surface of the wafer, a side wall of the aperture forming a reflecting surface in said resonant cavity for rotation of the laser beam in said cavity.

10. A method as claimed in claim 9, said axis of said aperture aligned along the (01?) direction

11. A method as claimed in claim 9, comprising forming an oxide layer on said second confining layer after orientation; photolithographically etching said apertures; cleaning the wafer; reforming an oxide layer on said second confining layer, the oxide layer extending into the apertures to form the said oxide layer on said side walls; etching a laser stripe through said oxide to expose the surface of the second confining layer; zinc diffusing the exposed surface of the second confining layer; metallizing the surface of the second confining layer; and applying a gold heat sink to the metallized layer.

12. A method as claimed in claim 9, including forming a buffer on the substrate prior to forming the first confining layer.

13. A method as claimed in claim 9, including forming a further layer on said second confining layer prior to orientating said wafer.