Patent GB-1499967-A (GB)
Title: Injection lasers
IPC: H01S5/185
Filed: 1976-04-07
Abstract: 1499967 Lasers INTERNATIONAL BUSINESS MACHINES CORP 7 April 1976 [28 May 1975] 14070/76 Headings H1C and H1K In an injection laser including a P-N junction and two electrodes arranged so that the flow of current through the electrodes causes light emission in the plane of the junction and perpendicular to the direction of current flow through the laser, there are provided two facing mirror-like surfaces 36, integral with the laser body. Each surface is disposed at an angle of 45 degrees to the plane of the P-N junction so that lasing light is reflected in a direction substantially parallel to the current direction. The laser system array show in Fig. 2 comprises a plurality of such lasers, the thickest portion of the fabricated two dimensional laser array being the first layer 20 of N-type GaAs which is about 100 microns thick. Above the N-type GaAs 20 is grown an N-type layer 22 of Ga 0À7 Al 0À3 As which can be about 4-5 microns thick. Above layer 22 is grown a 0À3 micron layer 24 of P-type GaAs so as to form the P-N junction 25. Two final thin layers are grown successively above layer 24, namely, a 1 micron layer 26 of P-type Ga 0À7 Al 0À3 As and a 1 micron layer 28 of P-type GaAs. In order to attain a cavity feedback effect, photoresist of SiO 2  strips (30), Fig. 3 (not shown), are deposited over the top layer 28, leaving spaces or windows (32) that are 10-12 microns wide. These windows are chemically etched to a depth of 5-6 microns, producing channelled grooves 34, using a solution of H 2 SO 4 -H 2 O 2 -H 2 O. The exact details of a successful etchant for producing a 45 degree angle # shown in Fig. 2 are described in a paper entitled "Selective Etching of Gallium Arsenide Cyrstals in H 2 SO 4 -H 2 O 2 -H 2 O System" by Shina Iida et al, published in the Journal Electrochemical Society; Solid State Science, May 1971, pp. 768-771. Two examples of etchants that will produce a 45 degree angle are solutions of (a) 1H 2 SO 4 -8H 2 O 2 -1H 2 O and (b) 8H 2 SO 4 -1H 2 O 2 -H 2 O. In the examples chosen, the concentration of H 2 SO 4  is a 98% solution by weight and the concentration of H 2 O 2  is a 30% solution by weight whereas the formula concentrations are by volume. In an actual run, the (a) solution noted above, when cooled to 0 C., was able to etch through the GaAs layers at a rate of 3-4 microns per minute. The etch was found to produce a 45 degree angle channel when the etch was performed on the surface oriented in the (001) direction and the mutually rectangular planes are (1#0) and (110) with the etching taking place into the (001) plane. The etching solution is quenched as soon as the desired amount of etching has taken place. Obviously, the rates of etching can be changed by increasing the temperature of the etchant and the etchant is selective according to the crystal orientation of the material. As is disclosed in the above-mentioned Iida et al article, the 45 degree etch produces a mirror-like surface, so that an edge, such as edge 36 of channel 34 serves as a mirror for the light emanating from P-N junction 24. The etching stops along line 37 in layer 22. This laser light is totally reflected so that after reflection it is propagated in a direction perpendicular to the layers. The geometry of etching is such that when a window (32) has a width of 10-12 microns, the depth of etch is 4 to 5 microns and the length of the channel edge 36 varies from 7 to 8 microns, with that edge 36 making an angle of 45 degrees with the P-N junction plane. Such edge 36, when etched, is 100% reflective for light propagating in the P-N junction. To complete the fabrication of the array, the GaAs substrate 20 is etched upwardly from its bottom surface, through a hole in a photoresist mask, using a solution of at 0 C. for a time between 4-5 minutes, after which the etchant is quenched. Such etching removes entirely the layer 20 of GaAs under the hole in the photoresist mask, exposing the planar lower surface of GaAlAs layer 2 so as to produce a window 38. Z 2000-3000  layer of gold or other reflecting material 40 is deposited over layer 22 and such layer 40 can serve as a totally reflecting layer. In the adjacent window 38<SP>1</SP>, no layer is deposited beneath the etched sides 36 because this interface S, per se, serves as a partially transmissive mirror. The mirrors are deposited alternately, in one window 38 but not in its adjacent window 38<SP>1</SP>, so that one is totally reflecting and its adjacent unmirrored one is partially transmissive, for reasons that will be obvious soon hereafter. A thin layer of metal 44 is deposited on the bottom surface of N-type GaAs 20 and an array of conductors 46 are deposited, through appropriate masks (not shown), each of said conductors 46 serving as an electrode for a given injection laser. Suitable leads 48 are attached to each of said electrodes 46 and connect to a source of power 50. When threshold current is sent through a given electrode 46, the light normally being guided by the heterojunction structure 22, 24 and 26 will impinge on facing mirrors 36, causing the emitted light to be reflected downwardly. Mirror 40 and etched surface S serve as reflectors and ends of an optical cavity. One mirror 40 serves as the fully reflecting mirror for two adjacent edges of two separate lasers (the right edge of a first laser and the left edge of a second laser adjacent the first laser). The etched surface S serves as a partially transmitting mirror for two adjacent lasers (the right edge of the second laser and the left edge of a third laser adjacent the second laser). In this way, a planar mirror 40 or surface S serve as part of the optical cavity of two independent adjacent lasers. In some cases it may be desirable to fabricate the array with individual mirrors for each cavity so that no single mirror serves two separate cavities.