Patent GB-1342767-A (GB)
Title: Light emitting semiconductor devices
IPC: H01S5/30
Filed: 1971-04-27
Abstract: 1342767 Electroluminescence WESTERN ELECTRIC CO Inc 27 April 1971 [1 May 1970] 11536/71 Heading C4S [Also in Division H1] A light emitting semi-conductor diode upon which the invention seeks to improve by adding a second hetero-junction comprises two portions of semi-conductor material 12, 14 of different band gaps containing a common conductivity type heterojunction 24 and a P-N junction 25, these junctions being separated by less than the minority carrier diffusion length. The structure thus has two outer regions of different band gaps and different conductivity types, and an intermediate narrower band gap region between the junctions. Forward bias is applied to electrodes 20, 22 by a current source 18, and the diode has a relatively low stimulated light emission threshold due to confinement of injected minority carriers into the intermediate region by the step in band gap. A further reduction in the threshold current may be effected as in Specification 1,288,082 by the creation of deep band tails or impurity states near the valence and conduction bands in the intermediate region. In the invention the confinement effect is enhanced by adding a further outer layer of wide band gap material 215, Fig. 3, to form a second hetero-junction 226 on the side of the P-N junction away from the first hetero-junction, the spacing of the P-N and second heterojunctions again being less than the minority carrier diffusion length. In this double heterojunction (DH) structure both holes and electrons are confined between the two heterojunctions provided the spacing conditions above are observed, and provided the minimum spacing between the two hetero-junctions is greater than #/2, # being the emission wavelength. The P-N junction may be coincident with the second hetero-junction as in Fig. 4 (not shown). Laser diodes.-A non-inventive single heterojunction (SH) device is shown in Fig. 1 mounted on a heat sink 16, emission being in the plane of the intermediate region 24. The end surfaces 28 and 30 are polished or cleaned, while surfaces 32 and 34 may be roughened. Reflective coatings may be added. The intermediate region is about 2 Á thick, and while the region 14 is typically 5-6 mils thick this may be reduced for heat removal. A DH device according to the invention is shown in Fig. 3 which has a resonator (not shown) as for Fig. 1; as noted above the DH structure exhibits a greater confinement effect, and its intermediate region thickness should be in the range 0À125-1À0 Á. The DH structure also produces optical confinement since the two heterojunctions form a waveguide, and this also lowers the laser threshold.  Electroluminescent diodes.-The above diodes function as electroluminescent diodes if the resonators are omitted. Another diode is shown in Fig. 2, and is an SH device having a P-N homo-junction 125 in narrow band gap material 114 and a P-P hetero-junction 123. Contacts 122, 120 are deposited on the device for current supply connection, emission being from the intermediate region via the dome shaped wide band gap portion. The mesa and dome shapes improve the diode efficiency, and the wider band gap material has low optical absorption. Substrate 114 may be n GaAs doped with Sn or Si at 2-4 Î 10<SP>18</SP> atoms /c.c. with a layer of P Ga x Al 1-x As where x = 0À3-0À5. The mesa top is some 500 Á in diameter, smaller values raising the efficiency. Similar considerations apply to the DH device to which the invention is restricted. Manufacture.-Details are given of the production of a DH structure as in Fig. 4 (not shown) and Fig. 3. Four solutions of Ga and GaAs are placed in different wells of a solution holder with appropriate dopants, and are heated in a furnace. A seed holder is passed over the solutions in turn and while the solutions are cooled at a predetermined rate the seed is immersed in each solution and vibrated. Thus four layers of differently doped solutions are grown on the seed. The seed is Si doped GaAs and forms the substrate 214 of Fig. 3; the first solution, which forms layer 215, is Ga+GaAs+Al+Sn. The second solution is Ga+GaAs+Si; the third is Ga + GaAs + Al + Zn and the fourth, which forms layer 217 of Fig. 3, is Ga+GaAs+Ge. The immersion occurs as the solutions are cooled from 870 C. After cooling the seed and the layers grown on it to room temperature electrical contacts are formed by heavily doping the surfaces with Zn and evaporating Cr and Au on to the surfaces. The seed is then cut and cleaved to give individual diodes. Layer thicknesses and dopant concentrations are given. At room temperature the laser threshold for pulse operation is 3900 A/cm.<SP>2</SP> for an intermediate region thickness of 1À5 Á, which is reduced to 3000 A/cm.<SP>2</SP> for thickness of 1 Á. The addition of reflectors lowered the threshold to 2300-2800 A/cm.<SP>2</SP>.