136 PHILIPS TECHNICAL REVIEW VOLUME 26 Gallium phosphide light sources and photocells H. G. Grimmeiss, W. Kischio and H. Scholz 621.383 :546.681 '183 Preparation and doping of GaP The methods of analysis used by us have failed to Amongst semiconductors with a relatively large detect the presence of carbon, which interferes with energy (band) gap, gallium phosphide has aroused luminescence in GaP, and this implies that the samples interest for various reasons. For one thing, by reason must have a e concentration of less than 5 X 10-4 %. of its 2.25 eV band gap, it is suitable for making P-N In the undoped state these crystals only exhibit very diodes which in some cases emit light in the visible weak luminescence. range of the spectrum. Our first task was to prepare gallium phosphide of high purity since, as will be made clear below, the efficiency of GaP light sources is very much dependent on the purity of the starting material. GaP is prepared by allowing gallium to react with phosphine; an ample supply of the latter gas, in a very high state of purity, can be obtained by decomposition of aluminium phos- phide with water. The aluminium phosphide is pre- pared by reacting aluminium with phosphorus. A mixture of pure aluminium and red phosphorus in an atomic ratio of 1: 1.1 is placed in an iron crucible and ignited. The reaction is fairly violent: some of the phosphorus evaporates and escapes into the atmo- sphere, where it burns spontaneously (fig. J). The aluminium phosphide yielded by the reaction is a porous sintered substance, yellow in colour. The GaP is synthesized in the apparatus sketched in fig· 2. The round-bottomed flask is charged with alu- minium phosphide under a nitrogen atmosphere. The moist gas that evolves when the compound is decom- posed by water is first passed through a cooling jacket. This condenses most of its water vapour. Thereafter the gas passes through two traps cooled down to -78 DC by a mixture of acetone and dry ice. Finally it enters the furnace containing a charge of gallium. The metal has first been heated up to 800 DC in a stream of hydrogen; once this temperature is attained the hydrogen flow is cut off and the phosphine fed in. Tbe temperature is then raised to 1200 DC and held there for two hours. Fig. I. Reaction between powdered aluminium and red phos- This reaction converts one-fifth of the gallium into phorus, resulting in the formation of aluminium phosphide. GaP, yielding orange-tinted flakes having dimensions of roughly 4 X 4 X 0.2 mm. Most of the flakes are single To produce GaP that will luminesce satisfactorily crystals, the large faces being { I IJ} planes. However, the crystals must be doped, and both zinc and zinc crystals twinned on a { Ill} plane are not uncommon. plus oxygen have shown themselves effective doping The overall impurity concentration is less than 10-3 %. agents. Zinc gives rise to the emission of green light, zinc plus oxygen to the emission of red light with a peak at 700 nm. As will later appear, it is the red- Dr. H. G. Grimmeiss, Dr. W. Kischio and Dr. H. Sc/wlz are research workers at the Aachen laborat ory of' Philips Zen/ra/- luminescing crystals that are mainly of interest. The laboratorium GmbH. zinc and oxygen can be introduced in the form of ZnO 1965, No. 4/5/6 GaP LIGHT SOURCES AND PHOTOCELLS 137 or of Zn3(P04)2, but the use of one particular com- Optical and electrical properties of P-N junctions pound fixes the atomic ratio between the two elements. As already noted, GaP has the relatively large This can be varied at wiII if Ga203 or GaP04 is taken energy gap of 2.25 eV and because ofthis it is a highly as a source of oxygen and employed in conjunction interesting medium to investigate the electrical and opti- with metaIlic zinc or ZnO. cal properties of P-N junctions. It will also be recaIled Doping is carried out inthe foIlowing way. A quartz that particularly pure starting material is required container is filled with GaP, metaIlic gallium and the where luminescence is to be studied. Therefore we doping agents, heated up to 400°C in a vacuum and used GaP prepared in the manner described above for then sealed off. It is then heated up to 1230°C in a making experimental diodes. Î T T Fig. 2. Apparatus for preparing gallium phosphide. Aluminium phosphide in round-bottomed flask K breaks döwn under the action of water and gives off phosphoretted hydrogen which, after being dried and cleaned, is passed over gallium in a furnace o at 1200 "C, . furnace. The GaP forms a homogeneous solution Of the various methods for producing P-N junctions, with the gaIlium. Cooling must take place slowly if aIloying seemed the best suited to our purpose. But large, well-shaped crystals are to form out of the melt. one has to ensure that during the alloying process For that reason the container is lowered slowly through some gaIlium phosphide is taken up into the molten the floor of the vertical furnace chamber. The zinc aIloying metal, with the result that a greater or lesser segregation coefficient between the Ga melt and the amount of phosphorus is lost, depending on the reac- GaP is approximately unity. At the same time, recrys- tion time. This phosphorus ceases to be available for tallization is accompanied by a sharing of impurities recrystallization as GaP during the subsequent cooling between the solid and liquid phases. The process thus stages; the longer the alloying time, the thinner is the involves further purification of the GaP. recrystallization layer. Since the phosphorus escapes Doping with copper can be carried out by a partic- very quickly, we should have to use an aIloying method ularly efficient two-stage process. GaP and copper that would ensure rapid heating and cooling of the I are brought into contact at 400°C under an air pres- samples. We therefore decided to heat up the GaP sure of 0.5 torr, with the result that copper diffuses electrically, by passing a heavy current through an into the surface of the GaP crystals, which take on a iridium strip. In this way we could easily cut down black coloration: The tinted top layer is about 1 [.Lm the aIloying time to a second or so. In the method deep. It is essential that the GaP should be in intimate adopted by us, the ohmic contact is made of a gold- contact with the copper during this pretreatment. zinc alloy and the P-N junction is formed by alloying The blackened crystals are then annealed for 24 the GaP with tin. hours at 900 °C in an evacuated container. In the The P-N junctions thus produced run more or less course of the annealing the blackening disappears and paraIIel with the surface of the crystal. Fig. 3 shows the material recovers the appearance it had in the clearly that despite the short alloying time, only a undoped state. It is possible in this manner to convert very thin crystaIlization layer is formed. As me~sure- N-type GaP into P-type or, alternatively, by modifying ments of junction capacitance have established, the the pretreatment conditions, to retain the. original junctions obtained in this way are step junctions. Where N-type properties of the material. this is so, the inverse square of the junction capacitance 138 PHILlPS TECHNICAL REVIEW VOLUME 26 shows a linear relationship [1] to applied voltage; fig. 4 is a plot of this kind for P-N junctions made by us. The electrical properties of the diodes are governed above all by the relatively large energy gap of GaP [2]. Because it has a very low intrinsic conductivity, GaP can be used to make diodes which sometimes do not pass more than 10-11 A under a reverse bias of up to about 10 V; some have leakage currents of less than 10-13 A. For the same rea- sons the current in the for- ward direction shows a Fig. 3. Photograph of a P-N junction in a gallium phosphide diode fitted with a tin contact. logarithmic dependence on The P-type GaP is in the lower part of the print; part of the copper wire which carried the diode current can be seen at top right. applied voltage over sev- eralorders of magnitude (fig. 5). The result is that at 1.4 V, for example the rity levels one can arrange for this energy to be ra- ratio of forward to reverse current is about 1011 : 1. diated outwards, not as heat, but as light. The effect It is known that when a P-N junction is biased in is known as P-N or injection luminescence. the forward direction, minority charge-carriers are injected into both the P-region and N-region, where they are able to recombine with majority carriers al- ready present. In general, recombination may take v- 2 place by direct transitions from band to band or by / way of intermediate levels within the forbidden gap. In GaP however, for various reasons, of which the .. I compound's band structure is not the least important, 1/ recombination takes place mainly via impurity levels. 6 The energy released by recombination inside diodes is la normally transferred to the crystal lattice in the form of heat. However, by incorporation of suitable impu- 11 la 8 0 / »: / 2 } ./ V .../ V Ik • Jv / / V 20°C / 6 V 0,4 0,8 1,2 1,6 2 2,4V 1/ -Ud -2 o 2 4 6 8 la 12V -u o 2 4 6 12V Fig.
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