JPH0332916B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to light emitting diodes, and in particular to diffused type light emitting diodes.
This invention relates to a method for manufacturing a ZnSe blue light emitting diode. ZnSe is a direct transition type and has a forbidden band at room temperature.
This crystal is expected to emit light at 2.67eV, which corresponds to the blue region (450nm to 490nm). However, in crystals obtained by the conventional melt growth method, many Se vacancies with high vapor pressure occur, and because these serve as donors, only n-type high-resistance crystals are usually obtained. However, a practical p-type crystal has not been obtained, and therefore a pn junction has not been formed. In other words, when the vacancy of this group element combines with the impurity, it acts as a non-luminescent center or forms a deep level, so even if a p-n junction can be formed, the luminous efficiency will be extremely low or there will be no light emission from the deep level. Only those in which luminescence is dominant can be produced. Therefore, there is a strong desire for a pn junction formed from a highly perfect crystal that does not contain deep levels. In order to achieve this objective, it is important to suppress deviations from the stoichiometric composition in the heat treatment process during diode fabrication to prevent light emission from deep levels and defects that serve as non-light-emitting centers. Therefore, it is essential to use the vapor pressure controlled temperature difference method invented by the inventor of the present application when growing the p-type crystal that will become the base material of the diode. This method was developed by the inventor of the present invention in Patent No. 1345720 (Japanese Patent Publication No. 10439/1983), which describes a method for growing crystals of - group compounds in which an element with a high vapor pressure is used as a solvent to control the vapor pressure of a component element with a low vapor pressure. providing. Focusing only on ZnSe, in the conventional growth method, the vapor pressure of Se is higher than that of Zn, so crystals grown at high temperatures have a large deviation from the stoichiometry with a lack of Se. Nakatsuta. Therefore, by using Se as a solvent, it is possible to compensate for the lack of Se and obtain ZnSe crystals with less deviation from stoichiometry, and by optimally controlling the Se pressure, it is possible to obtain a high impurity density that has not been previously achieved. It also becomes possible to create p-type crystals. The problems when using Se as a solvent are:
Although the growth temperature can be lowered, it is necessary to have a high vapor pressure and to select an appropriate temperature difference. To solve this problem, you need to perform the following operations. Generally, a quartz tube with an inner diameter of about 10 mmφ is processed and the first
When a quartz crucible 4 as shown in the figure is manufactured, it can sufficiently withstand up to about 8 to 10 atmospheres even if the wall thickness is about mm. However, if the pressure exceeds 10 atmospheres, there is a high risk of explosion if the crucible is distorted or has thin parts. It is necessary to reduce the effective pressure applied to the quartz crucible 4 by applying pressure with a gas such as air, Ar, or _N2 . When pressure is applied to the outside of the quartz crucible, the thermal conductivity of the gas increases in proportion to the pressure, so setting the temperature difference between the crystal precipitation area and the source crystal area is important. In this case, there is an optimal value for the growth conditions to obtain good crystallinity, and this is determined by the growth temperature - vapor pressure -
Conditions such as temperature difference and pressure around the crucible are interrelated. Generally, it is appropriate for the temperature difference between the source crystal part and the crystal precipitation part to be about 10 to 50°C. There are no strong limitations on the amounts of the solvent and source crystals in the crucible, but a preferred range is about 2 to 20 g of source crystals per 10 g of solvent. There are various ways to set up the source crystal, but it is preferable to design it based on the relationship between the specific gravity of the solvent and the crystal to be grown.If the specific gravity of the solvent is large, the source crystal can be set by floating it on top of the solution. It is relatively easy to set. However, if the relationship is reversed, the source crystal will sink before being dissolved in the solvent, so the purpose can be achieved by creating a horizontal structure between the two chambers. The general structure of the ampoule shape is shown in FIGS. 1 and 2. However, if high vapor pressure components of the constituent elements are simply used as a solvent, it is possible to compensate for the high vapor pressure components of the grown solution, but conversely, the lack of low vapor pressure components or the high vapor pressure It is clear that crystals with excessive amounts of components will grow, and it will not be possible to obtain defect-free crystals with controlled stoichiometric composition in the strict sense. As a means to overcome this drawback, it is possible to control the stoichiometric composition by applying the vapor pressure of a low vapor pressure component element during crystal growth to compensate for the lack of low vapor pressure components. Become. but,
If you simply put a low vapor pressure component into a high vapor pressure component element, which is a solvent, the reaction between the two will proceed and form a crystal, so the vapor pressure of the low vapor pressure component element will be the same as at the start of growth. At the end, the stoichiometry of the grown crystal will show different values.
It changes during growth, making it impossible to obtain crystals with good uniformity. Therefore, it is desirable that the vapor pressure of the low vapor pressure component be applied at a nearly constant value during crystal growth, so that the direct reaction between both components is minimized, and the vapor pressure of the low vapor pressure component is maintained at a nearly constant value during crystal growth. It goes without saying that it is necessary to apply a constant value from above the solvent. In order to keep the vapor pressure of Zn constant during growth, it is necessary to use a growth ampoule 4 as shown in FIGS. 1 and 2. That is, in addition to the crystal precipitation part 12 and the source crystal part 13, another chamber 21 is created, and Zn 22 is placed inside this chamber.
Steam pressure control is possible to some extent. however,
If the connecting pipe 23 between the crystal growth section and the Zn chamber is thick, ZnSe will be formed in the Zn chamber 21 due to the gas phase reaction between the high vapor pressure gas phase Se and Zn. , it becomes impossible to control the vapor pressure of Zn. In order to avoid this, it is necessary to
It is effective to make the surface area of 4 as small as possible and to attach a roof 25 to the upper part of the Se on the source crystal setting part and the connecting pipe, and to create a structure in which Zn vapor is applied only from above the crystal precipitation part. is preferred. Furthermore, it is effective to make the passage narrow between the crystal growth section and the Zn chamber for the purpose of thermal isolation. As for the method of thinning, it is desirable to adopt a structure in which a quartz tube or a solid quartz rod having a narrow inner diameter and an outer diameter approximately equal to the inner diameter of the ampoule is inserted into a tube connecting both regions within the ampoule after the raw material is introduced. Furthermore, a heat sink 9 is installed at the tip of the quartz ampoule in order to facilitate the escape of heat from one point in the crystal growth area. An example of the relationship between a specific ampoule shape and the temperature distribution and vapor pressure distribution of each part is shown in FIGS. 1 and 2. It is necessary to keep the temperature of the crystal precipitation part 12 constant at 1050°C and a temperature difference of 5 to 50°C between it and the temperature T2 of the source crystal part ₁₃ to keep the temperature of each part constant during growth. , this temperature difference is one of the factors strongly related to crystallinity. Next, steam pressure control room 2
It is preferable that the temperature of No. 1 can be controlled independently, and growth was performed under each temperature condition using the temperature of the vapor pressure control chamber as a parameter in order to determine the deviation from the stoichiometric composition. The relationship between vapor pressure and temperature of Zn is from RCA Review 1969 June PP.285~
It can be determined from the relational expression log ₁₀ Pzn=-6113/T+2.628 log ₁₀ T obtained by interpolating the data of Honig et al. in PP.305. From this relational expression, for example, Table 1 is obtained.

[Table] Specific examples will be described. Se = 20 g, ZnSe = 5 g, Zn = 4 g of materials were placed in an ampoule of the shape shown in Figure 1, height 3 cm, length 8 cm.
In a total capacity of 10 c.c., Se is shaped so that it connects between the crystal precipitation part and the source crystal part, and it is put into an ampoule, and after the material is put in, it is sufficiently baked in a vacuum, The ampoule was sealed in a vacuum of about 10 ^-6 mmHg, and crystal growth was performed at a temperature of 1050°C for the crystal precipitation part, 1060°C for the source crystal part, and 1100°C for the Zn. A bulk product was obtained. Under the same conditions other than the Zn pressure applied during growth, a crystal grown at a constant temperature with the addition of a group A element such as Li has a p value at any Zn pressure.
Shows shape conduction. Using this crystal, an n-type layer was epitaxially grown, or an impurity exhibiting n-type conduction was diffused or alloyed to form a pn junction. In this example, in order not to change the properties of the crystal, a diffusion method that allows formation of an n-type layer at a low temperature in a relatively short time was employed to investigate the Zn pressure dependence of the characteristics. For diffusion, an ampoule in which ZnSe crystals were immersed in a Zn solution was sealed in a vacuum and heat treated at 1000° C. for about 30 minutes to form an n-type layer of about several μm. One side of this crystal was lapped, polished and etched to form electrodes for p- and n-type layers, and a light emitting diode was fabricated. Figure 3 shows a general emission spectrum at 77〓, and the peak ( _Edge ) near the forbidden band is usually
A peak ( _Deep ) involving a deep level is observed. The intensities of these two peaks depend on the applied Zn pressure, and Figure 4 plots _{the Deep} / _Edge intensity ratio against the Zn pressure. As is clear from this figure, the graph showing the intensity ratio of both peaks has a minimum value between 7.0 and 7.4 atm.
When this pressure is applied, the deep level density in the crystal grown is at a minimum. This optimal Zn pressure range can be wide, but based on the general interpretation of natural phenomena, the most preferable Zn pressure range is a range where the intensity ratio is 1/e, that is, about ±30%. I can say that. Furthermore, the n-pressure dependence of the emission intensity of _Edge is shown in Figure 5, and the extreme value of this dependence is also in the vicinity of 7.0 to 7.4 atmospheres, similar to Figure 4, and this Zn pressure region is from the deep level. It can be seen that this is effective in suppressing the luminescence of The impurities added to the Se solvent during crystal growth include Li, LiNO ₃ containing Li, LiN ₃
Compounds such as these are preferred, and in order to obtain a practical specific resistance as a light emitting diode, the addition amount is suitably in the range of 1×10 ⁻³ to 5×10 ⁻¹ mol % based on the Se solvent. When the amount is less than this range, the specific resistance is high, resulting in an increase in the voltage required to cause the light emitting diode to emit light, and a disadvantage that it is difficult to obtain resistance contact with the p-type side layer. On the other hand, if the amount exceeds this range, the crystallinity deteriorates and light emission from deep levels becomes dominant, making it impossible to obtain pure blue light emission. The above dependence was obtained by forming an n-type layer and examining the emission spectrum at 77〓 in order to investigate the dependence of the p-type crystal obtained by growth on the Zn pressure. Even in the diode obtained from the charged crystal, the spectrum at room temperature includes emission from deep levels, and the emission color is not pure blue. The reason for this is that even during the formation of the n-type layer, slight defects were generated due to deviations from the stoichiometric composition, and in order to prevent the occurrence of these defects, it was necessary to introduce a vapor pressure control method similar to that used during crystal growth. confirmed to be essential. This example will be described below. The p-type crystal obtained by crystal growth is cut into plate shapes,
After shaping one side by wrapping and polishing, etching is performed with a Br-methanol solution to prepare for diffusion. The specific apparatus used for diffusion is shown in FIG. At the bottom of a quartz tube 31 with an inner diameter of 5 to 7 mmφ and a wall thickness of 1 mm,
A ZnSe single crystal substrate (thickness: 300 to 500 μm) 32 is placed, a quartz back-up spring 33 is placed on top of it to prevent the substrate crystal from floating, and Zn
2g, group Ga35 as an n-type diffusion impurity
is added in an amount of 3 to 10 mol% based on Zn. A Se vapor pressure chamber 36, which is a high vapor pressure component element, is provided in the upper part of the ampoule. A quartz spacer 37 is inserted to make the passage narrower, and the passage is sealed with a degree of vacuum of about 1×10 ^-6 mmHg.
Various experiments were conducted regarding the diffusion temperature, but 700
The diode properties are best when diffused at ~850°C, especially 720~760°C for about 1 hour, and light emission from deep levels is suppressed. Under these conditions, we performed vapor pressure controlled diffusion by varying the Se pressure, and
The shape layer was formed. In the crystal 40 in this state, the entire crystal surface is covered with the n-type diffusion layer 41, as shown in FIG. 7. Make a 0.5 x 0.5 mm square chip as shown in Figure 2. This crystal was treated with an etching solution such as a Br-methanol mixture, Au42 was evaporated as the p-side electrode (Fig. 7-3), In was placed on the n-type layer, and In was heated at 350 to 370°C in an Ar gas atmosphere for 10 minutes. A diode chip was fabricated by sintering for a minute (Fig. 7, 4), and a light emitting diode was fabricated by mounting it on a stem. FIG. 8 shows the Se pressure dependence of the ratio of the blue emission peak intensity and the deep level emission peak intensity of the injection emission spectrum at room temperature. According to this, the emission peak intensity ratio is the smallest near the Se pressure of 150 Torr, and the blue emission peak is the most dominant. In other words, when forming a diode, it is possible to form a diffusion layer with the least deviation from the stoichiometric composition.
This indicates that Se pressure was applied. This is also confirmed in the Zn pressure dependence of the blue emission peak intensity shown in Figure 9, and the optimal value of the applied Se pressure is confirmed.
This shows that the effect of Se pressure application during diffusion is effective. Although there is a slight deviation between the extreme values in Figures 8 and 9, it is 80 to 400 Torr, preferably 80 to 400 Torr.
A range of 250Torr can be considered optimal. This results in
Although it is possible to control Se vacancies generated during diffusion with a Zn solution, the conductivity type was changed by introducing impurities rather than forming an n-type layer by shifting the composition from the stoichiometric composition. By adding several mol% of Ga, which is a group element, as an impurity, by replacing the lattice position of Zn, an n-type diffusion layer is reliably formed, and the stoichiometric composition is changed by the optimum Se pressure. By reducing the deviation of , we were able to obtain a ZnSen layer with good crystallinity. The thickness of the diffusion layer is 5 μm at 470℃ for 1 hour and 18 hours.
20 μm was obtained. In addition to Ga, other group elements such as Al,
In etc. may also be used, and the input amount range is 2~
A range of 15 mol% is preferred. Outside this range, if the amount is too small, the resistance will be high, and if it is too large, the light emission from deep levels will be dominant, making it impossible to obtain blue light emission. Although FIG. 6 shows a specific example in which the ampoule is placed vertically, the ampoule having the same shape can be used in the case of horizontally placing the ampoule, and the same heat diffusion can be performed. As mentioned above, when manufacturing light-emitting diodes using ZnSe, which is a - group compound, defects occur even during a very short heat treatment process, forming deep levels, which inhibits light emission near the bands. It became clear that In order to perform heat treatment without causing this defect, steam pressure control is necessary in each process, and the optimal steam pressure range in each process is extremely narrow as shown in the claims. limited to. A blue light-emitting diode can be obtained only in such a vapor pressure range, and this invention is essential for the manufacturing process of blue light-emitting diodes, and is an invention of extremely high industrial value.

[Brief explanation of the drawing]

Figures 1 and 2 show the shape and temperature distribution of the quartz ampoule used in the present invention, Figure 3 shows the emission spectrum, and Figure 4 shows _{the Deep} /I _Edge versus applied Zn pressure.
Figure 5 shows the ratio of I _Edge to the applied Zn pressure.
Figure 6 shows the structure and temperature distribution of the ampoule used in the present invention, Figure 7 shows the process of manufacturing a ZnSe crystal diode using Se vapor pressure controlled diffusion in a Zn solution, and Figure 8 9 is a diagram showing the Se pressure dependence of the ratio of the blue emission peak intensity of the injection emission spectrum to the emission peak intensity of the deep level at room temperature, and FIG. 9 is a diagram showing the Se pressure dependence of the blue easy peak intensity at room temperature. 12...Crystal precipitation part, 13...Source crystal part, 2
1... Steam pressure control section, 9... Heat sink, 32
... p-type ZnSe substrate, 34 ... Zn solution, 35 ...
Ga, 36...Se vapor pressure chamber.

Claims (1)

[Claims] 1 A constant Zn pressure in the range of 7.2 atm ± 30% was applied from above the Se solvent where a temperature difference was formed, and 1 × 10 ^-3 to 5 × 10 ^-1 mol% of group a elements were added to the solvent. , and a ZnSe source crystal is suspended in the high temperature part of the solvent, and the low temperature part of the solvent is kept at a substantially constant temperature within the range of 1050 ° C ± 5 ° C, and then a p-type ZnSe single crystal is precipitated in the low temperature part of the solvent, and then After that, 2 to 15 mol% of the crystals
placed in a Zn solution containing group elements of 80~
A method for manufacturing a diffused ZnSe blue light emitting diode, characterized in that an n-type region is formed in a p-type crystal by diffusion under a substantially constant Se vapor pressure of 250 Torr.