Patent GB-1248288-A (GB)
Title: Apparatus for producing electromagnetic radiation
IPC: H01J29/18
Filed: 1969-03-18
Abstract: 1,248,288. Lasers; semiconductor devices. INTERNATIONAL BUSINESS MACHINES CORP. 18 March, 1969 [18 July, 1968], No. 14008/69. Headings H1C and H1K. In a semi-conductor luminescent device e.g. a laser the composition of the material of the device is such that when an electron or photon beam is directed on to one surface thereof recombination radiation is generated in the material of the device adjacent the surface, which radiation is not significantly absorbed in the remainder of the material. Semi-conductor element 14 is supported on a cooling plate 11- 7A in a vacuum chamber 11-1A and a beam of electrons from source 10 is directed on to the front surface 24 of layer 18 the thickness of which is comparable with the active layer in which the electron-hole pairs are produced. Radiation 16 generated in layer 18 is transmitted through inactive layer 20 of element 14 and reflected by totally reflecting mirror 24A and partially reflecting mirror 22A through which the output 16A is obtained. In order to suppress the usual laser mode parallel to the active layer the sides of the element 14 are roughened, made non parallel or provided with non-reflecting coatings 26, 28. In accordance with the invention the composition of the layers 18, 20 is such that the emission spectrum of the active layer 18 is well outside the absorption spectrum of the inactive layer 20 and is achieved by suitable choice of energy gaps and distribution of impurity levels. Thus the first layer 18 of a CaAs crystal may be appropriately doped by diffusion with P-type material e.g. zinc and the substrate 20 may be N-type material. In this case the recombination'radiation generated in the P-type layer is at a longer wavelength than the absorption spectrum of the N-type layer. Alternatively the front layer 18 can be formed by solution growth e.g. a GaAs layer may be grown on to a Ga 1-x Al x As layer or both the front layer 18 and the substrate may be Ga 1-x Al x As but the front layer has a lower Al mole fraction. The variation of Al mole may be achieved by variation in the cooling rate during growth, Figs. 7, 8, 9 (not shown), or by the addition of oxygen during growth. In the arrangement of Fig. 1B (not shown) the semiconductor element is positioned on a massive heat sink in a heat bath and the surface in contact therewith is made totally reflecting and the top surface to which the electron beam is applied at an angle is made partially reflective for output energy extraction. In Fig. 2 (not shown) the semi-conductor element 14 is in the form of a block of inactive semi-conductor material into the top surface of which is diffused a chequer board pattern of zinc P-type dopant. The front active layer 18 of Fig. 1 may be replaced by two separate active layers the composition of the layers being such that the effective energy gaps of the layers increases towards the substrate. In Fig. 6A (not shown) the front active layer has a continuous graduation of composition so that the energy gap is continuously increasing with distance from the front surface. This structure is achieved by using liquid phase epitaxy of Ga 1-x Al x As, the substrate layer being grown first at a slow cooling rate and the active layer with variable composition at an increasing cooling rate. Suitable materials are III-V or IV-VI compound semi-conductors. If the composition variation is achieved with a ternary alloy then ternary alloys in the III-V, II-VI or IV-VI compounds may be used. The semi-conductor element may be curved, laser action being achieved if the front and back surfaces are locally parallel.