JP2708045B2 - Light emitting diode array - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a light emitting diode array for an electrostatic printer light source having excellent luminance uniformity. "Prior art" An electrostatic printer, that is, forming an electrostatic latent image of a character or a figure on a photosensitive surface on which a layer made of an electrostatic photosensitive material is formed,
A printer of a type in which toner is sprinkled and developed, and then transferred to paper or the like is widely used as a printer such as a computer and a word processor because of high speed and low noise. Conventionally, gas lasers,
Semiconductor lasers and the like have been used. However, gas lasers are large, which hinders miniaturization of electrostatic printers.Although semiconductor lasers are small, the emission wavelength is limited by the sensitivity of commonly used electrostatic photosensitive materials. Therefore, there is a problem that high efficiency cannot be obtained because the wavelength does not coincide with the wavelength region of In other words, photosensitive materials such as selenium and cadmium sulfide, which are inexpensive and can easily produce a photosensitive surface, show practical sensitivity at wavelengths shorter than 700 nm. The shortest wavelength is 780 nm. Therefore, in an electrostatic printer using a semiconductor laser, a photosensitive surface using a special photosensitive material in order to improve sensitivity in a long wavelength region, for example, a tellurium layer / a selenium layer / tellurium layer added with tellurium, and three layers, A photosensitive surface laminated on a substrate was used (for example,
23, No. 2, pp. 47-55 (1984)). On the other hand, a light emitting diode (hereinafter, referred to as “LED”)
Has a wide range of emission wavelengths from the infrared region to the green light, so that any wavelength can be selected. As a result, when an LED is used as a light source, wavelength matching with a photosensitive material can be easily performed. Further, LEDs have features that they have a simple structure and a long service life, and thus have attracted attention as light sources for electrostatic printers. However, LEDs have lower brightness than lasers. Therefore, when used as a light source for an electrostatic printer,
LE with LEDs formed on a straight line at a rate of 16 to 32 LEDs per mm
A D array is provided at a position close to the photosensitive surface. In addition, the brightness of each LED included in such an LED array needs to be uniform. That is, each LED
Is required to be within ± 20% of the average value of the brightness of all the LEDs included in the array. This is because uneven brightness causes unevenness in print density. "Problems to be Solved by the Invention" As described above, the production yield of LED arrays having uniform luminance characteristics has conventionally been extremely low. "Means for Solving the Problems" The present inventors have made intensive studies for the purpose of solving the problems, and as a result, have reached the present invention. The above object of the present invention has a plurality of light emitting portions along the longitudinal direction of one rod-shaped epitaxial wafer composed of a gallium arsenide single crystal substrate and a gallium arsenide phosphide mixed crystal layer formed on the substrate. In the LED array, the gallium arsenide single crystal substrate has an etch pit density on the side of the gallium arsenide phosphide mixed crystal layer of 1 × 10 ^{4 to} 5 × 10 ⁵ cm ⁻² . The gallium arsenide single crystal substrate has an etch pit density of 1 ×
10 ^{4 to} 5 × 10 ⁵ cm ⁻² , preferably 2 × 10 ⁴ to 1 × 10 ⁵ cm ⁻²
Is appropriate. If the etch pit density is less than 1 × 10 ⁴ cm ⁻² , the brightness of the obtained LED array will not be uniform, and 5 × 1
If it exceeds 0 ⁵ cm ^-2 , the brightness of the LED will decrease, and both are not preferable. In the present specification, the etch pit density, after etching the substrate with molten potassium hydroxide at 400 ° C. for 15 minutes,
It refers to the average of the values counted under a microscope. The substrate usually has a thickness of 0.2 to 0.5 mm. It is appropriate to cut out a desired surface from a gallium arsenide single crystal and use it. The surface is mirror-finished by mechanical and / or chemical polishing. The surface of the substrate is 0.5 ~ 1.0 from {100} plane or {100} plane.
It is preferable to use a plane inclined by about で because an abnormal growth product is not generated on the surface of the epitaxial wafer and a flat surface can be obtained. The gallium arsenide phosphide mixed crystal layer is preferably grown by a vapor phase epitaxial growth method. This is because it is easy to change the mixed crystal ratio. Mixed crystal ratio of potassium arsenide mixed crystal (in this specification, “mixed crystal ratio” means gallium arsenide GaAs ₁ -xPx
Means the value of x when expressed as ) Is first formed on the substrate in a region where the value continuously changes from 0 to a desired value. This is to alleviate the strain caused by the difference in lattice constant between the substrate and the mixed crystal layer. After forming the region where the mixed crystal ratio changes, a region where the mixed crystal ratio is constant at a desired value is formed. This area is the LED pn
This is a region where a junction is formed and radiative recombination occurs. Therefore, the mixed crystal ratio in this region determines the emission wavelength of the LED. The mixed crystal ratio in the region where the mixed crystal ratio is constant is 0.35 to 0.45
Is preferable. This is because the light-sensitive material emits light in the region of 640 to 680 nm showing practical sensitivity. In particular, when the mixed crystal ratio is 0.4, cadmium sulfide is preferable because the peak emission wavelength is 660 nm at which the maximum sensitivity is obtained. The thickness of the region where the mixed crystal ratio changes is preferably 5 to 200 μm, more preferably 20 to 100 μm. Further, the thickness of the region where the mixed crystal ratio is constant is preferably in the range of 10 to 200 μm, more preferably 20 to 150 μm. It is preferable that the conductivity type of the single crystal substrate and the mixed crystal layer be n-type. This is because the diffusion of the p-type impurity is easier than the diffusion of the n-type impurity, so that the formation of the pn junction is easier. When growing an n-type mixed crystal layer,
It is obtained by mixing and growing a desired amount of a Group VIb element such as sulfur or tellurium or a compound thereof, for example, hydrogen sulfide or tellurium hydride. Vapor gas _{composition, Ga-HCl-AsH 3 -PH} 3 -H 2 system,
GaR ₃ —AsH ₃ —PH ₃ —H ₂ (R = CH ₃ or C ₂ H ₅ ) or the like is used. Other conditions for the vapor phase epitaxial growth may be conditions that are usually used (for example, those described in “Electronic Technology Complete Book [6], Light-Emitting Diodes”, published by the Industrial Research Institute, Ltd., 1977). In this way, the obtained epitaxial wafer
An impurity of a conductivity type different from the conductivity type of the mixed crystal layer is selectively diffused according to a desired pattern to form a light emitting portion, that is, an LED. That is, when the mixed crystal layer is n-type, p-type impurities are diffused. Specifically, as the p-type impurity, a Group IIb element such as zinc or cadmium or a compound thereof is used.
It is necessary to select a pattern in which the LEDs to be formed are arranged in a straight line. The density of the LED is usually about 9 to 32 per 1 mm. The size of one LED is usually 20 to 80 μm square. The LED array is obtained by dividing the epitaxial wafer on which the LEDs are formed into a rectangular shape, and it is appropriate that the LED array has a plurality of LEDs on its longitudinal center line. The length of the LED array is usually about 2 to 6 mm. The LED array is formed by a conventional method, for example, as described in JP-A-61-40067
Can be produced by the method described in Japanese Patent Application Publication No. A large number of such LED arrays are arranged so as to have a length corresponding to the printing width of the electrostatic printer, and a printing LED head is configured. “Effect of the Invention” The LED array of the present invention has the following remarkable effects, and therefore has a great industrial value. (1) The LED array of the present invention has excellent brightness uniformity, and the production yield of the LED array is dramatically improved. (2) No special measures need to be taken during vapor phase epitaxial growth. "Examples" The present invention will be specifically described based on examples and comparative examples. Example As a single crystal substrate, a circular 50 mm diameter, 350μ thick
g and a single crystal substrate of gallium arsenide having an etch pit density of 5 × 10 ⁴ cm ⁻² . The surface of this substrate was polished with a mirror surface, and the surface was inclined 2.0 ° from the (001) plane in the <110> direction. This GaAs single crystal substrate is doped with silicon and has an n-type carrier concentration of 7.0 × 1
0 ¹⁷ cm ^-3 . The single crystal substrate was placed in a horizontal epitaxial reactor made of quartz. Subsequently, a quartz boat containing metal gallium was installed in the reactor. After flushing the reactor with argon and replacing the air,
Stop supplying argon and dispense high-purity hydrogen gas 2500 ml
The temperature was raised while flowing through the reactor at a flow rate of / min. The temperature of the gallium-containing quartz boat installation section is 830 ° C,
After the temperature of the substrate mounting section reached 750 ° C., while maintaining the temperature, hydrogen chloride gas was supplied to the reactor from the downstream side of the gallium-containing quartz boat at a flow rate of 90 ml / min for 2 minutes. Then, the surface of the GaAs single crystal substrate was etched. After the supply of the hydrogen chloride gas was stopped, a hydrogen gas containing 10 vol ppm of diethyl tellurium was supplied to the reactor at a flow rate of 10 ml / min. Subsequently, hydrogen chloride gas was blown into the reactor at a flow rate of 18 ml / min so as to touch the surface of gallium in the gallium-containing quartz boat in the reactor. Subsequently, arsine (AsH ₃ ) and phosphine (PH ₃ ) were supplied as follows to form a region where the mixed crystal ratio continuously changed. That is, hydrogen gas containing 10% by volume of arsine was supplied to the reactor at a flow rate of 336 ml / min, and the flow rate was gradually reduced to 252 ml / min for 60 minutes. At the same time, hydrogen gas containing 10% by volume of phosphine was added from 0 ml / min.
Over 60 minutes, the flow rate was gradually increased to 80 ml / min. 60 minutes after the start of the formation of this region, the flow rate of the hydrogen gas containing arsine is 252 ml / min, the flow rate of the hydrogen gas containing phosphine is 80 ml / min, and the hydrogen gas containing diethyl tellurium Was maintained at 10 ml / min to form a region where the mixed crystal ratio was constant for 60 minutes. Subsequently, the temperature of the reactor was lowered to complete the manufacture of the epitaxial wafer. As a result, a region where the mixed crystal ratio continuously changes from 0 to 0.4 is formed on the GaAs single crystal substrate.
An epitaxial wafer was obtained in which a region having a mixed crystal ratio of 0.4 μm and a region where the mixed crystal ratio was constant and the n-type carrier concentration was 8.0 × 10 ¹⁶ cm ⁻³ and the region where the mixed crystal ratio was constant was 20 μm was formed. Using the obtained epitaxial wafer, zinc is diffused by the selective diffusion method to form a square LED with a side of 40 μm.
Manufactured a bar-shaped LED array having a length of 4 mm and a width of 1 mm formed in a row, 16 pieces per mm. The average peak wavelength of the obtained LED array was 660 nm. In addition, the variation in the brightness of the LEDs constituting the LED array was within ± 20% of the average value in the array, and the array yield was 80%. Comparative Example An epitaxial wafer was manufactured in the same manner as in Example except that a GaAs single crystal substrate having an etch pit density of 5 × 10 ³ cm ⁻² was used. In the obtained epitaxial wafer, a region where the mixed crystal ratio continuously changes from 0 to 0.4 is formed on a GaAs single crystal substrate.
In this region, a region having a mixed crystal ratio of 0.4 and a constant mixed crystal ratio of 21 μm was formed. The n-type carrier concentration in both regions was 8.2 × 10 ¹⁶ cm
^{Was -3} . Using this epitaxial wafer, zinc was diffused by a selective diffusion method to produce a bar-shaped LED array having a length of 4 mm and a width of 1 mm formed in a row with 16 LEDs of 40 μm square at a density of 16 per mm. The average peak emission wavelength of all the obtained LED arrays was 661 nm. In addition, the yield of LED arrays in which the variation in the brightness of each LED within the same array is within ± 20% of the average value is 2%.
It was 0%. As is apparent from the above Examples and Comparative Examples, according to the present invention, when a substrate having an etch pit density within a predetermined range is used, compared with a case using a substrate having an etch pit density outside the range, The yield of LED arrays that can be used as light sources for printers (with a luminance variation of within ± 20%) is greatly improved.

Claims (1)

(57) [Claims] A light-emitting diode array having a plurality of light-emitting portions along the longitudinal direction of one rod-shaped epitaxial wafer comprising a gallium arsenide single crystal substrate and a gallium arsenide phosphide mixed crystal layer formed on the substrate. etch pit density of phosphorus ratio gallium arsenide mixed crystal layer side, 1 × 10 ⁴ ~
5 × 10 ⁵ cm ^-2 light emitting diode
array. 2. 2. The light emitting diode array according to claim 1, wherein the substrate has an etch pit density of 2 * 10 < ⁴ > to 1 * 10 ^{< 5} > cm ^<-2 >.