DE2449931C2 - Method for producing a red glowing gallium phosphide light-emitting diode - Google Patents

Description

The invention relates to a method for producing a red-lucent gallium phosphide light-emitting diode, in which an N-conductive gallium phosphide layer is first formed on an N-conductive gallium phosphide substrate by epitaxial growth and then a P-conductive gallium phosphide layer is formed by epitaxial growth from a liquid phase the substrate with tellurium, sulfur or selenium in a concentration of 1 ... 3 χ 10 ' ⁷ Cm- ³ and the N-conductive layer with tellurium, sulfur or selenium with a concentration No of 2 ... 5, 5 χ 10 "cm- ³ are doped in the vicinity of the PN junction and the P-conductive layer is doped with zinc and oxygen in such a way that the zinc concentration N _A in the vicinity of the PN junction ... 3 χ 3O ' ⁷ cm- 'is.

A method of the type mentioned above, wherein the substrate χ in a concentration of 2 10 "cm- ^J, the N-type layer having a concentration N _D of 3 χ 10" to 20 χ 10 "cm- ³ and the P A conductive layer is doped with zinc in a concentration N _A in the vicinity of the PN junction of 3 χ ΙΟ ¹⁷ to 10 χ 10 "cm ^-3 , is known from US Pat. No. 3,690,964.

In this known method, an increase in the quantum yield is achieved through a high dopant concentration, especially in the P-conductive layer, aimed at increasing the concentration of the Zn-0 pairs. This view that an increase in the quantum yield is already possible by increasing the dopant concentration, especially in the P-type layer, but leaves the influence of the crystal structure the P-conductive layer and the influence of the number of radiationless reaction centers in this layer is not taken into account. High Doticrstoffkorizentrationen disrupt the crystal structure and lead to a higher number of radiationless recombination centers, resulting in hai. that through higher doping alone only a limited increase in the quantum yield is possible.

It is well known that the cooling rate in the epitaxial growth of the P-type ends Layer from the liquid phase closely related to the resulting oxygen concentration stands

From "Fujitsu Scientific and Technical Journal", September 1973, pages 173 to 195, it is known in the First manufacture a red glowing gallium phosphide diode on an N-conductive substrate N-conductive, tellurium-doped gallium phosphide layer ίο and on top of it a P-conductive, zinc-doped gallium phosphide layer from the liquid phase, the cooling rate being 1 ° C / min.

The object on which the invention is based is to develop the known method of the above-mentioned> called Art so that with him a red-luminescent gallium phosphide light-emitting diode with a Quantum yield of more than 4% can be produced.

According to the invention, this object is achieved in that the epitaxial fluid is cooled at a rate of 02 ... 3 ° C./min when the P-conductive layer is formed.

According to the invention, the doping of the individual layers and the cooling rate are therefore determined the epitaxial fluid selected in the formation of the P-conductive layer so that the radiating areas in the vicinity of the PN junction the best possible crystallinity and thus the lowest possible Have number of radiationless recombination centers.

In the method according to the invention, the following preferably applies to the concentration: No = \ $ .. AN _A.

In the following the invention is based on the drawing explained in more detail It shows

J5 F i g. 1 in a schematic cross-sectional view the Main components of an apparatus for forming a gallium phosphide layer by epitaxial growth from the liquid phase,

F i g. 2 the structure of a glowing red gallium phosphide diode,

F i g. 3 in a graphical representation the relationship between the donor concentration No of the N-conducting gallium phosphide layer and the external quantum yield η of the corresponding gallium phosphide diode,

F i g. 4 in a graphical representation the relationship between the acceptor concentration N _A in the P-conductive gallium phosphide layer and the external quantum yield // of the corresponding gallium phosphide diode,

F i g. 5 shows the relationship between the donor concentration in the N-type gallium phosphide substrate and the external quantum yield η of the corresponding gallium phosphide diode, and

F i g. 6 in a graphical representation the relationship between the speed with which the epitaxial fluid cooled for the epitaxial growth of the P-conductive gallium phosphide layer from the liquid phase becomes, and the external quantum yield /; the bo corresponding gallium phosephid diode.

To further explain the process according to the invention, several examples are given below.

b5 example 1

Fig. I shows schematically a cross-sectional view of a Carriage device for the formation of a N-line index

Gailiuin phosphide layer and a P-type GaIIium phosphide layer on an N-denoted gallium phosphide substrate through cpilaxial growth from the liquid phase. In Fig. 1 is the frame 11 of the device shown, which consists of carbon. The frame has a vertically verlaufer.de cylindrical opening 12, with a separate bottom plate 13 made of carbon close to the underside of the frame 11 so is arranged that the diase can slide horizontally to the vertical opening 12 to close. The base plate 13 and the vertical opening 12 together form one Chamber 14, which is opened upwards in the frame 11 of the device. The chamber 14 is with an epitaxial line 15 filled from gallium phosphide containing N or P dopants Part of the surface of the floor slab 13 is provided with a circular recess 16, the diameter of which is approximately equal to that of the vertical Opening 12 is the recess 16 for receiving a substrate 17 made of gallium phosphide If consequently the bottom plate 13 is moved so that the recess 16 is directly below the vertical Opening 12 comes to rest, the upper surface of the gallium phosphide substrate 17 contacts the gallium phosphide epitaxial solution 15th

As an illustration, the following is the method for producing a red-glowing gallium phosphide diode using the carriage device indicated above.

A single crystal of tellurium (Te) -doped N-conducting gallium phosphide with a donor concentration of 2 ^× 10 17 cm ^{-3 was used as the} starting point. A 300 μm thick substrate was cut from this single crystal and placed with the plane (111) S of the K.ristal-Ies facing up into the recess 16 of the base plate 13. The chamber 14 was charged with 40 g of gallium solution containing 5 g of polycrystalline gallium phosphide and 8 mg of tellurium (Te). The device loaded in this way was heated for 30 minutes at 1050 ° C. in an electric furnace in order to homogenize the gallium solution. The bottom plate 13 is horizontally slidable so that the surface of the substrate 17 can be brought into contact with the tellurium-containing gallium solution, as shown in FIG. 1 is shown. The cooling took place at a rate of 2 ° C./min for the growth of an N-conducting GaI-liumphosphid layer on the substrate 17, the thickness of the N-conducting layer being between 60 and 70 μm. The surface of the N-conductive gallium phosphide layer was lapped by 20 μσι and a thin layer of gold was applied by vapor deposition to the surface of the remainder of the N-conductive gallium phosphide layer in order to form a Schottky barrier layer. Measurements were then carried out to determine the electrical capacitance of the Schottky barrier layer. The donor concentration No on the surface of the N-conductive gallium phosphide layer was calculated from the electrical capacitance as 4 × ^{10 17 cm -3.} After removing the gold layer from the surface of the N-type gallium phosphide layer, a P-type gallium phosphide layer was grown on the N-type gallium phosphide layer in the following manner. The chamber 14 was charged with 40 g of gallium solution containing 5 g of polycrystalline gallium phosphide, 0.5 g of gallium oxide (Ga ₂ Oj) and 10 mg of zinc (Zn). The so loaded device vnrde for 30 minutes at 1050 ⁰ C in an electric furnace heated to homogenize the gallium. The bottom plate 13 is horizontally slidable so that the surface of the nitrogen layer grown on the substrate J7 could be brought into contact with the zinc-containing gallium solution. The cooling took place at a rate of 2 ° C./min for the growth of the P-conductive gailium phosphide layer on the N-conductive layer, the thickness of the P-conductive layer being between 60 and 70 μm.
The surface of the P-conductive gallium phosphide layer was lapped to a part with a thickness of 20 μm. A gold film was evaporated onto the lapped surface, whereby a Schottky barrier layer was formed as when the N-conductive GaP layer was grown. Then, the electric capacity of the quays has been measured Schottky barrier, wherein an acceptor _Ν Λ in this layer μπι at a position approximately 20 from the PN junction from 2 χ 10 ¹⁷ cm- ³ yielded. The substrate was lapped in order to reduce the total thickness of the platelet to approximately 200 μm. The substrate side of this chip was provided with an electrode made of indium (In), while the P-type gallium phosphide layer was provided with an electrode made of an indium-zinc alloy (In / Zn). This assembly was gesindert at a temperature of about 500 ⁰ C. The pressure applied to the P-Ieitende Galiiumphosphid-layer gold film was removed prior to formation of the conductive electrode. The plate thus obtained was cut into a number of rectangular parallelepiped-shaped pieces having a square cross-section with a side of 1 mm. Each piece so cut was placed on a TO-18 socket with the P-type gallium phosphide layer on top to form the gallium phosphide diode as shown in FIG. 2 is shown. In Figure 2, the diode plate 21, the conductive electrodes 22 and 23 and a foot 24 are shown. When a current of 2 mA flows through the diode in the forward direction, it lights up red. Without an epoxy resin coating, the gallium phosphide diodes had an external quantum yield of 7% on average. With an epoxy resin coating, the external quantum yield was 10%.
As is known, the distribution of the acceptor concentration in the area of the P-conductive gallium phosphide layer, which was first formed by the epitaxial growth from the liquid phase, in particular in the area that extends to about 20 μm from the PN junction to the outside , a low gradient. Therefore, approximately the same acceptor concentration occurs in the area of the P-conductive gallium phosphide layer which directly adjoins the PN junction as in the area of this gallium phosphide layer which is about 20 μm away from the PN junction.

Example 2

Diodes were used in the same manner as in Example 1 and the donor concentration

bo Np to 1 χ 10 »em- ^J , 2 χ 10» cm * - ³ , 3 χ 1O ¹⁷ cm- ³ , 5 χ 10 ¹⁷ cm- ^J or 6 χ 1O ^l7 cm ~ ³ , which is selected by a corresponding Change in the content of tellurium (Te) in the epitaxial solution was achieved. The various gallium phosphide o'odes which were raised in this way were supplied with a current of 2 mA in the forward direction in order to measure the electroluminescence yield of these diodes. As shown in Fig. 3, different values occur from external

Quantum yields // without epoxy resin coating. which are 2.3%, 4.3%, 6%, 6.6% and 3.8%, respectively, of the various tellurium donor concentrations Nu listed above. The circles in the curve of FIG. 3 denote the mean values of the external quantum yields η of the sample diodes. The upper and lower ends of the vertical lines, which run parallel to the ordinate and intersect the circles, represent the maximum and minimum values of the external quantum efficiency of various diodes with the same M). 3 shows the value η = 7%, which corresponds to N _n = 4 × 10 "cm- ^J , the result that was achieved in Example I. As can be seen from FIG. 3, there was a higher external quantum yield η was then achieved as 4% without epoxy resin coating when the donor concentration Np in the N-conductive layer was approximately 2 χ 10 ¹⁷ cm ^ 'to 5.5 χ 10' ⁷ cm-'. The reason that the outer Qüufiicfiausbeuie η was below 4% fäiit when the donor concentration Np over 5 χ 5 tO ¹⁷ cm-'ansteigt.liegt presumably that the increased donor concentration results in a less complete crystal formation of the N-type gallium phosphide layer itself. the occurrence of a greater number of non-radiative recombination centers in the N -Galium phosphide conductive layer reduces the injection of electrons from the N-type gallium phosphide layer into the P-type gallium phosphide layer, resulting in less complete crystallization of the P-type gallium phosphide sphid layer. which has grown on the N-type gallium phosphide layer increases the likelihood of the occurrence of lattice defects which form radiationless recombination centers in the P-type gallium phosphide layer. The reason why the external quantum efficiency η also drops when the donor concentration Np in the N-type Gaüiürnphosphid layer falls

2 χ ΙΟ ¹⁷ cm ^-3 is presumably that there is a donor concentration in the N-conductive gallium phosphide layer. which is less than the acceptor concentration in the P-type gallium phosphide layer, limits the injection of electrons from the N-type into the P-type gallium phosphide layer.

Although setting the acceptor concentration in the P-conductive gallium phosphide layer to a lower value of, for example, about 5 10 ' ⁶ Cm-Mn would also have to be considered if the donor concentration in the N-conductive gallium phosphide layer is less than 1 χ 10 ^l7 cm- ^J is. So that a larger amount of electrons can be conducted from the N-conductive to the P-conductive gallium phosphide layer, such a procedure is not recommended, since the reduced acceptor concentration in the P-conductive gallium phosphide layer leads to a lower concentration of ZnO pairs leads in this P-conducting gallium phosphide layer, which form luminescence centers, so that there is a decrease in the external quantum yield // of the red-luminous gallium phosphide diode.

Example 3

These diodes were made in the same way as in the example! produced and thereby the acceptor concentration Na varies. The results obtained when measuring the diodes of Example 3 are shown in FIG. 4, from which it can be seen that an external quantum yield higher than 4% can then be achieved. if the acceptor concentration Na in the P-type layer of gallium phosphide is in the range of 1 χ 1O ^{17 Cm-3} to 3 χ IO ^l7 cm- ^J. If the acceptor concentration in the P-type gallium phosphide layer is reduced to less than 1 ^× 10 17 cm −3, it becomes difficult to obtain a conductive electrode on the P-type gallium phosphide layer. Although this electrode can be made conductive, the resistance in the junction between the P-type galium phosphide layer and the electrode increases so that it is necessary to apply a higher operating voltage, so that there is a decrease in the external quantum yield. The reason that the external quantum yield drops below 4% when the acceptor concentration in the P-type gallium phosphide layer ^{exceeds 3 χ 10 17} cm- ^J is that a higher acceptor concentration results in less complete crystal formation of the P-type gallium phosphide -Layer itself leads and thus a larger number of radiationless recombination centers occurs.

A high external quantum yield is achieved if the relationship No> Na exists between the donor concentration Np in the N-conducting gallium phosphide layer and the acceptor concentration Na in the P-conducting gallium phosphide layer. In particular with 4 N, \ ä No ä 1.5 Na , a red-glowing gallium phosphide diode with a high electroluminescence yield results. Concentrations of No <Na prevent electron transport from the N-conductive to the P-conductive gallium phosphide layer and even with N _n <1.5 Na this transport is limited, so that no red-glowing gallium phosphide diode with a high external quantum yield can be obtained. The relationship of Np = 1.5 Na , on the other hand, has the effect that a large amount of electrons can be injected from the N-type layer into the P-type layer, whereby the external quantum efficiency can possibly be increased. The relationship Np> 4.0 N, \ leads to a noticeably higher electron flow from the N-conducting to the P-conducting gallium phosphide layer and should naturally increase the external quantum yield; however, in fact, the reverse occurs. This drop in the external quantum yield is presumably due to the fact that a large difference between the donor concentration Np in the N-conductive gallium phosphide layer and the acceptor concentration Na in the P-conductive gallium phosphide layer has disadvantages in adjusting the acceptor concentration of the P-conductive gallium phosphide layer brings with it, which has grown on the N-conductive gallium phosphide layer. This is because, when the P-type gallium phosphide layer is formed on the N-type gallium phosphide layer by epitaxial growth from the liquid phase, the donor contamination of the N-gallium phosphide layer into the P-gallium phosphide layer due to the donors and acceptors are carried which compensate each other in the PN junction, so that it is difficult to adjust the acceptor concentration of the P-type gallium phosphide layer in the vicinity of the PN junction. The above compensation between donors and acceptors in the PN junction has the effect that the acceptor concentration in the vicinity of this junction is lower than the acceptor concentration on the surface of the P-conductive gallium phosphide layer. Layer approximately equal to the donor concentration in the

Area of the layer directly at the PN junction, since the donor concentration as the epitaxial progresses Growth from the liquid phase in the N-conductive galium phosphide layer increases. After, however the P-type gallium phosphide layer is epitaxially grown on the N-type gallium phosphide layer is, the compensation occurs between the donors and the acceptors in the vicinity of the PN junction on, which presumably means that the donor concentration of the N-conductive layer is lower in this area than the donor concentration in the N-conductive galium phosphide layer before the formation of the P-conductive galium phosphide layer is.

Example 4

In this example the procedure was the same as in Example 1, but the gallium phosphide sample diodes with different donor concentrations were formed in the corresponding N-conductive gallium phosphide substrates. The values determined in this example 4 are shown in FIG. 5, from which it can be seen that with a donor concentration in the N-conductive gallium phosphide substrate of 1 × 10 "cm -3 to 3 × 10" cm ^-3 the external quantum yield without epoxy resin layer was over 4%. A donor concentration of less than 1 ^× 10 17 cm ^-3 in the substrate has the disadvantage that the electrodes on this substrate are difficult to keep completely conductive, as is also the case with the P-type galium phosphide layer. Although this electrode can be made conductive, the resistance in the junction between the substrate and the electrode increases, so that it is necessary to apply a higher operating voltage, which overall leads to a lower external quantum yield of the red-luminous GaIium-iiumphosphid-Üiode produced in this way leads. If the donor concentration in the N-conductive substrate ^{exceeds 3 χ 10 l7} cm ^-3 , the crystal formation of this substrate is disturbed. This allows light to be absorbed, which leads to a decrease in the external quantum yield. This disruption of the complete crystal formation of the N-type substrate also affects the crystal formation of the N-type galium phosphide layer which grows on the substrate and that of the P-type galium phosphide layer which is formed on the N-type galium phosphide layer, so that the complete crystal formation of the radiating area in the vicinity of the PN junction and consequently also the external quantum yield is reduced

Example 5

An N-type galium phosphide layer was formed by epitaxial growth from the vapor phase with a thickness of 40 μm using the same substrate as in Example 1. This N-type galium phosphide layer contained a donor with a concentration of 4 χ 10 ¹⁷ cm ^-3 . In the epitaxial growth from the vapor phase, hydrogen gas was used as the carrier and tellurium as the donor. At a temperature of 825 ° C., gallium and PClj react with one another to form an N-conducting layer of galium phosphide

A plate which has the above-mentioned N-type gallium phosphide substrate and the N-type gallium phosphide layer was etched for a few seconds using aqua regia. A P-type galium phosphide layer containing an acceptor in a concentration of 2 10 "cm- ^J was formed on this N-conducting galium phosphide layer by cpituxial growth from the liquid phase. In gallium phosphide diodes produced in this way, external quantum yields of an average of 4.5% were measured.

Furthermore, in the manner described above, a plurality of gallium phosphide test diodes were formed, the N-conducting gallium phosphide layer of which contained a donor with a concentration of 6 × 10 "cm- ^J . These gallium phosphide diodes had an external quantum yield of 2% on average on.

Examples

The procedure was the same as in Example I, with only the P-conducting galium phosphide layer formed at different cooling rates. The values obtained thereby are in Fig. 6, from which it can be seen that at a cooling rate of less than 3 ° C / min the gallium phosphide diodes show an external quantum yield of more than 4%. Preferably the cooling rate is in the range of 0.2 to 3 "C / min. A lower cooling rate than 0.2 ° C / min results in a more complete crystal formation of the P-conductive layer, so that the electroluminescence yield increases, adjusting the temperature for the epitaxial growth from the liquid phase however, it involves great effort and is therefore unsuitable for mass production because an unnecessarily long time for the epitaxial growth from the liquid phase of the P-type gallium phosphide layer is needed at such cooling rates. The lower external quantum yield at a higher cooling rate than 3 ° C / min is presumably based on the fact that incomplete crystal formation occurs at a high cooling rate results, so that a considerable number of radiationless recombination centers occurs.

The same procedure as in Example 1 was followed, with the N-type galium phosphide layer being used formed at different cooling rates. Here too you get at a lower cooling rate than 3 ° C / min an external quantum yield of more than 4%.

In all of the preceding examples, the epitaxial growth from the liquid phase occurred over the entire thickness with a cooling rate of less than 3 ^c C / min. However, it is also possible to use at least those areas of the P-conductive and N-conductive layers which contribute to the emission of the light, in particular those layers which extend from the PN junction up to a point about 10 .mu.m away from this junction, with a cooling rate lower than 3 ° C / min, while the remaining areas can be formed with a greater cooling rate than 3 ° C / min.

The concentration values were all using The Schottky barrier determines if both the zinc acceptor and the oxygen donor for example are doped into the P-type galium phosphide layer is im at each doping concentration In connection with the corresponding P-conductive galium phosphide layer, there is a difference between the Amount of zinc acceptor and that of the acid

Substance acceptor especially with a bound acceptor concentration ascertain. Furthermore, the doping concentrations with respect to the P-Icite are the GaI-lium-phosphide layer 1.2 to 1.5 times greater than those using free carrier concentrations the Haii effect can be determined.

While in all of the preceding examples, the N-type substrate and the N-Galliuipphosphid-leitcnde Sehicht gallium phosphide doped with tellurium as a donor, tellurium can be replaced by other donors such as / .., sulfur or selenium.

For this purpose 3 sheets of drawings

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Claims (2)

. Patent claims: 1. Method for producing a red-glowing gallium phosphide light-emitting diode. in the case of an N-conductive gallium phosphide substrate first an N-conductive gallium phosphide layer by epitaxial growth and then a P-conductive gallium phosphide layer by epitaxial growth from the liquid phase, the substrate with tellurium, sulfur or selenium in one Concentration of 1 .. 3 χ 10 ^{! 7} cm- ^J and the N-conductive layer with tellurium, sulfur or selenium with a concentration N _D of 2 ... 5.5 χ 10 ¹⁷ cm- ³ near the PN- Transition are doped and the P-conductive layer is doped with zinc and oxygen in such a way that the zinc concentration N _A in the vicinity of the PN ^{transition is 1 ... 3 χ 10 '7} Cm- ³ , characterized in that at the Digestion of the P-conductive layer the epitaxial fluid is cooled at a rate of 0.2 ... 3 ° C / min. 2. The method according to claim 1, characterized in that the following applies to the concentration: N _D = 1.5 ...