Patent GB-1419695-A (GB)
IPC: H01S5/40
Filed: 1973-03-12
Abstract: 1419695 Electroluminescence HITACHI Ltd 12 March 1973 [13 March 1972] 11840/73 Heading C4S [Also in Division H1] A heterojunction laser has a stripe shaped mesa above its active region so as to prevent the laser current diverging before it reaches the active region. By preventing divergence the threshold current is kept low; because the active region itself is not included in the mesa heat dissipation is easier, and also there are no large refractive index discontinuities in the plane of the active region so that single mode output is obtained with a polarization which is current independent. The basic construction of a double heterojunction laser of the invention is shown in Fig. 1, where the active region is a layer 3 of GaAs sandwiched between GaAlAs layers 2 and 4. A mesa is formed above layer 3 by the layer 2, a GaAs layer 1, insulator 7 and electrode 6. Regions 8 are spaces produced during etching, the block 5 is a GaAs substrate and layer 9 is the second electrode. If desired the layer 2 may be very thin and left unetched during manufacture as in Fig. 4. The laser may be provided with further non-light emitting mesas to ease mounting requirements, Fig. 6 (not shown), or since heat dissipation and current confinement are good several independent lasers having a common electrode may be formed on a single substrate, Fig. 13. The mesa may also be used with single heterojunction devices. Because slight refractive index changes occur between the parts of layer 3 which do and do not carry current there is a degree of lateral light confinement, and a curved mesa may be used as a result. Materials.-In Fig. 1 layer 1 is P-type Zn doped GaAs; layer 2 is P-type Zn doped Ga 0 . 7 Al 0 . 3 As; layer 3 is P-type Si doped GaAs; layer 4 is N-type Te doped Ga 0 . 7 Al 0 . 3 AS; layer 5 is N-type GaAs; electrode 6 is Cr with Au; insulator 7 is a phosphosilicate glass; regions 8 are apertures; electrode 9 is Au with Ge and Ni. Manufacture.-Zn is initially diffused into the upper surface of a multi-layer crystal formed by liquid phase epitaxial growth of materials 1-5 above. This forms a P + -type layer on the surface. A glass stripe shape mask is deposited on the surface, then all of layer 1 except the regions under the mask are etched away with a 4 : 1 : 1 mixture of H 2 SO 4 +H 2 O 2 +H 2 O. Further etching with a 1 : 1 mixture of HF+H 2 O removes all of layer 2 except that portion under the mesa, Fig. 3c, and also removes the glass mask. The portion of layer 2 remaining is slightly narrower than layer 1, forming apertures 8 of Fig. 1. This structure is now coated with glass layer 7 which is etched away above the mesa so that the conductor 6 can be evaporated on to the mesa. Evaporation of conductor 9 on to the other face of the crystal and clearing completes the manufacture of the laser pellet, which is then mounted on a stem to provide a laser diode. In practice the H 2 SO 4  etchant removes some of layer 2 and may also attack layer 3; a 1 : 40 : 40 mixture of HF+H 2 O 2 +H 2 O does not attack layer 2 and is to be preferred. If this latter etchant is used the whole of layer 2 may be preserved so that the structure of Fig. 4 can be made. It is also possible to form the mesa by a known sputtering technique.