JP2001060716A - Light emitting thyristor and self-scanning light emitting device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light emitting thyristor whose outside light emitting efficiency is satisfactory. SOLUTION: A (p) type AlGaAs layer 12, an (n) type AlGaAs layer 14, a (p) type AlGaAs layer 16, and an (n) type AlGaAs layer 18 are successively laminated on a (p) type GaAs substrate, and an uppermost layer 30 made of materials selected from a group constituted of InGaP, InGaAsP, and AlGaInP is laminated on the (n) type AlGaAs layer 18 so that an ohmic contact with a cathode electrode 20 can b e realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting thyristor having an improved light emission amount and a self-scanning light emitting device using such a light emitting thyristor.

[0002]

2. Description of the Related Art A self-scanning light-emitting device using a surface-emitting light-emitting thyristor is disclosed in Japanese Patent Application Laid-Open No. 2-14584.
FIG. 1 shows a basic structure of a surface-emitting thyristor.

In FIG. 1, reference numeral 10 denotes a p-type GaAs substrate, on which a p-type AlGaAs layer 12, an n-type AlGaAs layer 14, a p-type AlGaAs layer 16, and an n-type A
The lGaAs layers 18 are sequentially stacked, and an n-type GaAs layer 22 is formed on the n-type AlGaAs layer 18 to make ohmic contact with the cathode electrode 20. In the figure, 2
4 is a gate electrode provided on the p-type AlGaAs layer 16, and 26 is an anode electrode provided on the lower surface of the GaAs substrate 10.

In this example, although a p-type layer, an n-type layer, a p-type layer, and an n-type layer are laminated on a p-type GaAs substrate in this order,
When an n-type layer, a p-type layer, an n-type layer, and a p-type layer are stacked in this order on an n-type GaAs substrate, the uppermost layer electrode is an anode electrode, and the lowermost electrode is a cathode electrode.

In such a light emitting thyristor, light emitted from the gate layer is emitted through the uppermost layer.

The present inventors have realized that the self-scanning function of the emitted light can be realized by arranging the light emitting thyristors having such a structure in an array and giving appropriate interaction between the light emitting thyristor arrays. The above publication discloses that the light source for an optical printer can be easily mounted, the arrangement pitch of the light emitting elements can be reduced, and a compact self-scanning light emitting device can be manufactured.

[0007]

In the conventional light emitting thyristor, GaAs is used as the uppermost layer material in order to facilitate ohmic contact with the electrode and to simplify the material system. Since the emission wavelength of the light emitting thyristor is about 780 nm, when the GaAs layer is used as the uppermost layer, the absorption edge wavelength becomes about 860 nm.
Therefore, the emitted light is absorbed while passing through the uppermost layer, and the light amount is reduced.

In order to reduce the amount of light absorbed by the GaAs layer, the thickness of the GaAs layer may be reduced. However, if the thickness of the GaAs layer is small, the following problem occurs.

That is, in order to form an ohmic electrode, it is necessary to alloy the electrode material with GaAs. However, the distance of movement of atoms by this heat treatment is large, and the electrode material is extended to the AlGaAs layer which is the lower layer of the GaAs layer. The alloying zone is reached. As a result, the crystallinity of the AlGaAs layer is disturbed,
It causes light scattering and the like.

FIG. 2 is a graph showing the absorption coefficient of n-type GaAs at 297K. The vertical axis indicates the absorption coefficient α, and the horizontal axis indicates light energy. The amount of light absorbed is

[0011]

(Equation 1)

## EQU1 ## From this graph, 780 nm
It can be seen that the absorption coefficient for light having the wavelength of is about 1.5 × 10 ⁴ . Assuming that the film thickness t is 0.02 μm, if the absorption amount is simply calculated from the above equation, the decrease in the light amount is 3 to 4%.
It can be seen that it is. Further fluctuations in the atomic arrangement due to fluctuations in the film thickness or alloying, changes in the composition, etc., cause further reduction.

An object of the present invention is to provide a light emitting thyristor which has solved the above-mentioned problems.

Another object of the present invention is to provide a self-scanning light-emitting device using such a light-emitting thyristor.

[0015]

According to the present invention, the material of the uppermost layer is made of a material whose absorption edge wavelength is shorter than 780 nm, for example, InGaP, InGaAsP or AlGaInP.
By doing so, light absorption in the uppermost layer can be eliminated. It is desirable that this material is lattice-matched to the GaAs substrate. However, there is no problem even if the material does not match the lattice as long as the film has a sufficient lattice relaxation.

As described above, by using a material having an absorption edge in a wavelength region shorter than the emission wavelength of the light emitting thyristor for the uppermost layer, light absorption of the emitted light in the uppermost layer can be eliminated, and external quantum efficiency can be increased. .

According to the present invention, a self-scanning light emitting device having the following structure can be realized by using a light emitting thyristor as a light emitting element.

In the first structure, a plurality of light emitting thyristors are arranged, and a gate electrode of each light emitting thyristor is connected to a gate electrode of at least one light emitting thyristor located near the light emitting thyristor.
This is a self-scanning light-emitting device in which a plurality of wirings to which a voltage is applied from the outside are connected to an anode electrode of each light-emitting thyristor, which is connected through an electric element having electric resistance or an electric unidirectionality.

In the second structure, a plurality of thyristors are arranged, and a gate electrode of each thyristor is connected to at least one of the thyristor gate electrodes located near the thyristor by an electric element having electric resistance or electric unidirectionality. A self-scanning switch element array formed by connecting a power supply line to the gate electrode of each thyristor using electrical means, and connecting a clock line to the anode electrode of each thyristor, and a light emitting thyristor A plurality of light-emitting element arrays arranged, a plurality of light-emitting thyristors constituting the light-emitting element array, each gate electrode is connected to the gate electrode of the thyristor constituting the self-scanning switch element array by electrical means, Self-scanning light emitting device provided with a line for applying a current for light emission to the anode electrode of each light emitting thyristor It is.

According to the self-scanning light-emitting device having such a structure, it is possible to realize a light-emitting device having good external luminous efficiency, high definition, compactness, and low cost.

[0021]

FIG. 3 is a schematic sectional view of a light emitting thyristor according to one embodiment of the present invention. The structure is the same as that of the conventional example of FIG. 1, except that the uppermost GaAs layer is replaced by a layer 30 of InGaP lattice-matched to the GaAs substrate.

In the case of In _1-x Ga _x P, lattice matching with GaAs occurs when the composition ratio x is about 0.5. InG
MOCVD was used for aP growth. Tri-methyl indium (TMI) was used as the In raw material, trimethyl gallium (TMG) was used as the Ga raw material, and phosphine was used as the P raw material. Since growth conditions depend on the structure of the reactor used, conditions must be set in order to obtain a desired composition ratio x = 0.5. As a general condition, a reduced pressure growth method was used, and the growth temperature was 600 to 700 ° C. The group III raw material supply molar ratio (TMG / TMI) is determined by the mixed crystal composition ratio (x / 1−
x) was determined as proportional. The proportional coefficient at this time cannot be uniquely determined because it slightly varies depending on the growth temperature, but was in the range of about 0.8 to 1.0. Selenium was used as a dopant for obtaining n-type InGaP, and hydrogen selenide was used as a raw material.

For evaluation of optical characteristics, a single layer of InGaP was grown on a GaAs substrate to obtain a measurement sample. FIG. 4 shows the photoluminescence of In _0.5 Ga _0.5 P measured at room temperature. The emission center wavelength is about 660 nm.
FIG. 5 shows a light absorption spectrum of the same In _0.5 Ga _0.5 P layer in comparison with GaAs (FIG. 2). In _0.5
The absorption edge wavelength of Ga _0.5 P is about 650 nm (0.9 eV)
At an emission wavelength of 780 nm from the AlGaAs layer, the absorption coefficient is 10 cm ⁻¹ or less, which is 1.5 × of GaAs.
A value significantly smaller than 10 ⁴ cm ^-1 was obtained.

A light-emitting thyristor having such a structure was manufactured using the uppermost cathode layer as the InGaP layer. I
The method of growing the nGaP layer is as described above, and the other manufacturing processes are the same as in the case of using the disclosed GaAs layer. However, an HCl solution was used as an etchant for removing the cathode layer and exposing the gate layer. AuGeNi was used to obtain ohmic contact with the cathode electrode. The light emitting thyristor fabricated on the wafer was brought into contact with the cathode electrode and the gate electrode for each element with a probe of a manual prober, and the anode electrode was taken out by bringing a metal plate into contact with the back surface of the substrate.

In order to measure the light output, the light emitting thyristors were connected as shown in FIG. The gate electrode 42 of the light emitting thyristor 40 is connected to an anode electrode 46 via a resistor 44 of 30 kΩ.
And a constant current source 48 was connected between the anode electrode 46 and the cathode electrode 50, and the light output under a constant cathode current (for example, 10 mA) was measured by a Si photodiode. Since the absolute value of the light output changes depending on the area of the light emitting region and the degree of light blocking by the cathode electrode, the absolute values must be compared under the same structure and the same driving current.

The resulting light output increased on average by about 3% from the typical value when using a GaAs layer. This is GaA
This corresponds to the case where the absorption of the s layer itself disappears, and In _0.5 G
absorption of a _{0. 5} P layer is found to be negligible.

Three basic structures of a self-scanning light emitting device to which the above light emitting thyristor can be applied will be described.

FIG. 7 is an equivalent circuit diagram of a first basic structure of the self-scanning light emitting device. Light emitting thyristors T (−2) to T (+2) are used as light emitting elements,
(−2) to T (+2) have gate electrodes G _{−2 to} G _{+2 respectively.}
Is provided. Each gate electrode has a load resistance R
_A power supply voltage V _GK is applied via _L. Each of the gate electrodes G _{-2 to} G ₊₂ is connected to a resistor R _I to create an interaction.
Are electrically connected via Also, three transfer clock lines (φ1, φ2, φ3) are connected to the anode electrode of each single light emitting thyristor every third element (as if repeated).

The operation will be described. First, the transfer clock φ ₃
Becomes high level, and the light-emitting thyristor T (0) is turned on. At this time, from the characteristics of the three-terminal thyristor,
The gate electrode G ₀ is lowered to zero volts nearby. Assuming that the power supply voltage V _GK is 5 volts, the gate voltage of each light emitting thyristor is determined from the network of the load resistance R _L and the interaction resistance R _I. Then, the light emitting thyristor T (0)
, The gate voltage of the element close to
As the distance from (0) increases, the gate voltage increases. This can be expressed as:

V _G0 <V _G1 = V _G-1 <V _G2 = V _G-2 (1) The difference between these voltages is the load resistance R _L and the interaction resistance R _I
Can be set by appropriately selecting the value of.

It is known that the turn-on voltage V _ON on the anode side of the three-terminal thyristor is higher than the gate voltage by the diffusion potential V _dif .

V _ON ≒ V _G + V _dif (2) Therefore, if the voltage applied to the anode is set higher than the turn-on voltage V _ON , the light emitting thyristor is turned on.

[0033] Now a state where the light-emitting thyristor T (0) is turned on to apply a high-level voltage V _H to the next transfer clock pulse phi _1. This clock pulse φ ₁ is simultaneously applied to the light emitting thyristors T (+1) and T (−2),
When the value of the high level voltage V _H is set in the following range, only the light emitting thyristor T (+1) can be turned on.

V _G−2 + V _dif > V _H > V _{G + 1} + V _dif (3) The light emitting thyristors T (0) and T (+1) are simultaneously turned on. When the cut high-level voltage of the clock pulse phi _3, so that the light-emitting thyristor T (0) is able to transfer it becomes ON state and OFF.

As described above, in the self-scanning light emitting device, the light emitting thyristor can have a transfer function by connecting the gate electrodes of the light emitting thyristors by the resistance network. From the above-described principle, if the high-level voltages of the transfer clocks φ ₁ , φ ₂ , and φ ₃ are set so as to slightly overlap each other in order, the ON state of the light-emitting thyristor is sequentially transferred. That is, the light emitting points are sequentially transferred, and a self-scanning light emitting element array can be realized.

FIG. 8 is an equivalent circuit diagram of the second basic structure of the self-scanning light emitting device. This self-scanning light emitting device
A diode is used as a method of electrically connecting the gate electrodes of the light emitting thyristor. Light emitting thyristor T (-2)
ＴT (+2) are arranged in a line.
G _{-2 to} G ₊₂ are light-emitting thyristors T (−2) to T (+2)
Represents the respective gate electrodes. R _L represents the load resistance of the gate electrode, and D _{−2 to} D ₊₂ represent diodes that perform electrical interaction. V _GK represents a power supply voltage. Two transfer clock lines (φ1, φ2) are connected to the anode electrode of each single light emitting thyristor every other element.

The operation will be described. Transfer clock phi ₂ becomes high level first, and the light-emitting thyristor T (0) is turned on. At this time, due to the characteristics of the three-terminal thyristor, the gate electrode G ₀ is lowered to near zero volt. Assuming that the power supply voltage V _GK is 5 volts, the gate voltage of each light emitting thyristor is determined from the network of the resistor R _L and the diodes D _{-2 to} D ₊₂ . Then, the gate voltage of the element close to the light emitting thyristor T (0) decreases most, and thereafter the gate voltage increases as the distance from T (0) increases.

However, due to the unidirectionality and asymmetry of the diode characteristics, the effect of lowering the voltage works only to the right of T (0). That is, the gate electrode G ₁ is set to a voltage higher than G ₀ by the forward rise voltage V _{dif of the} diode, and the gate electrode G ₂ is set to a voltage higher than G _{1 by} the forward rise voltage V _{dif of the} diode. Is done. On the other hand, the gate electrode G on the left side of T (0)
_{At -1,} no current flows because the diode D _-1 is reverse-biased, and therefore has the same potential as the power supply voltage V _GK .

The next transfer clock pulse φ ₁ is transmitted to the nearest light-emitting thyristors T (1), T (−1) and T (3).
And T (−3), among which:
The element with the lowest turn-on voltage is T (1),
Turn-on voltage of about G ₁ of the gate voltage of the T (1) + V
_dif , which is about twice V _dif . The element with the next lowest turn voltage is T (3), which is about four times V _dif . The ON voltage of T (-1) and T (-3) is about V _GK +
V _dif .

[0040] From the above, by setting the high-level voltage of the transfer clock pulses between about 2 times the V _dif of approximately 4 times the V _dif, it is possible to turn on only the light-emitting thyristor T (1), A transfer operation can be performed.

FIG. 9 is an equivalent circuit diagram of the third basic structure of the self-scanning light emitting device. This self-scanning light emitting device
It comprises switch elements T (-1) to T (2) and write light emitting elements L (-1) to L (2). The configuration of the switch element portion shows an example using diode connection. The gate electrodes G _{−1 to} G ₁ of the switch element are also connected to the gate of the light emitting element for writing. A write signal S _in is applied to the anode of the light emitting element for writing.

The operation of the self-scanning light emitting device will be described below. Now, assuming that the transfer element T (0) is in the ON state, the voltage of the gate electrode G ₀ falls below V _GK (here, 5 volts) and becomes almost zero volt. Therefore, when the voltage of the write signal S _in is equal to or higher than the diffusion potential of the pn junction (about 1 volt), the light emitting element L (0) can be made to emit light.

[0043] In contrast, the gate electrode G _-1 is about 5 volts, the gate electrode G ₁ is about 1 volt. Therefore, the writing voltage of the light emitting element L (-1) is about 6 volts,
The write voltage of the light emitting element L (1) is about 2 volts.
Now, the voltage of the write signal S _in which can write only in the light emitting element L (0) is a range of about 1 to 2 volts. When the light emitting element L (0) is turned on, that is, when the light emitting element enters a light emitting state, the voltage of the write signal S _in line is fixed at about 1 volt, so that an error that another light emitting element is selected can be prevented. it can.

The light emission intensity is determined by the amount of current flowing _{in the} write signal _Sin, and an image can be written at an arbitrary intensity. Further, in order to transfer the light emitting state to the next element, it is necessary to once lower the voltage of the write signal S _in line to zero volt and turn off the light emitting element once.

For such a self-scanning light-emitting device, an element having an In _0.5 Ga _0.5 P layer as the uppermost layer was applied. The device manufacturing method may be the same as the conventional one. The characteristics of the manufactured self-scanning light-emitting device directly reflected the improvement of the light output of the single light-emitting thyristor. When this self-scanning type light emitting device is used for an optical print head, high quality printing can be realized because the external light emitting efficiency of each light emitting element is improved.

As the material of the uppermost layer, InGaAs is used.
In the case of using P, when the composition of Ga is x and the composition of P is y (that is, In _1−x Ga _x As _1−y P _y ), by using the composition on the side where the absorption edge energy is large, the absorption is improved. Coefficient can be reduced.

FIG. 10 shows In to explain this.
An energy state diagram of _{1-x Ga x As 1-} y P y. This state diagram further shows lattice constants. According to this state diagram, the line 32 having an absorption edge energy of 1.6 eV corresponds to an emission wavelength of about 780 nm. The lattice constant 5.65 ° corresponds to the lattice constant of GaAs.
Therefore, from this state diagram, it can be seen that the absorption coefficient can be reduced by using the composition on the high energy side from the point indicated by the mark 34 in the composition equal to the lattice constant of GaAs.

Further, as the material of the uppermost layer, AlGaI
When nP is used, the composition of Al is x, the composition of Ga is y,
When the composition of In is (1-xy) (that is, Al
_{x Ga y In 1-xy P} ), it is necessary to select the respective compositions as lattice matched to GaAs.

FIG. 11 is a graph showing the relationship between the lattice constant of AlGaInP and the energy gap. The vertical axis indicates the lattice constant a, and the horizontal axis indicates the energy gap Eg.

[0050] In the figure, hatched portions 36 are Al _x Ga _y In
_The composition range of _1-xy P is GaAs.
The composition lattice-matched with s is the composition shown by the solid line 38.

In this range, the energy gap is 780
Since the value is sufficiently large with respect to the wavelength of nm, it can be estimated that the absorption coefficient is sufficiently smaller than that of GaAs.

Although a material system lattice-matching with the GaAs substrate has been described above, other material systems may be used as long as ohmic contact is possible, as long as the material can sufficiently relax the lattice from its film thickness.

[0053]

According to the present invention, since the absorption edge wavelength of the material of the uppermost layer is short, the emitted light has no interaction with the material of the uppermost layer and is transmitted in its entirety. Therefore, since the uppermost layer is transparent to the emission wavelength, there is no decrease in the amount of light when passing through the cathode (uppermost) layer.

Further, according to the present invention, since the material of the uppermost layer is transparent, it is possible to increase the film thickness. Therefore, the thickness of the uppermost layer can be ensured, the diffusion of atoms to the lower layer due to the heat treatment for ohmic contact can be prevented, and a decrease in the amount of light due to the disorder of the atoms can be prevented.

Further, according to the present invention, it is possible to provide a self-scanning light-emitting device in which the light-emitting elements are arrayed and a self-scanning function is added, thereby increasing the external light-emitting efficiency.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a conventional light emitting thyristor.

FIG. 2 is a graph showing an absorption coefficient of n-type GaAs.

FIG. 3 is a schematic sectional view of a light emitting thyristor according to one embodiment of the present invention.

FIG. 4 is a graph showing photoluminescence of InGaP.

FIG. 5 shows a light absorption spectrum of an In _0.5 Ga _0.5 P layer.
It is a figure shown in comparison with GaAs.

FIG. 6 is a diagram showing a light output measuring circuit of the light emitting thyristor.

FIG. 7 is an equivalent circuit diagram of a first basic structure of the self-scanning light emitting device.

FIG. 8 is an equivalent circuit diagram of a second basic structure of the self-scanning light emitting device.

FIG. 9 is an equivalent circuit diagram of a third basic structure of the self-scanning light emitting device.

FIG. 10 is an energy phase diagram of InGaAsP.

FIG. 11 is a graph showing a relationship between a lattice constant of AlGaInP and an energy gap.

[Explanation of symbols]

 Reference Signs List 10 p-type GaAs substrate 12 p-type AlGaAs layer 14 n-type AlGaAs layer 16 p-type AlGaAs layer 18 n-type AlGaAs layer 20 cathode electrode 22 n-type GaAs layer 24 gate electrode 26 anode electrode 30 top layer

Claims (5)

[Claims] An uppermost layer from which light is emitted is InGaP, InGaP.
A light-emitting thyristor comprising a material selected from the group consisting of GaAsP and AlGaInP. 2. The light emitting thyristor according to claim 1, wherein the composition of the selected material is selected so as to lattice match with the material of the substrate of the light emitting thyristor. 3. The light emitting thyristor according to claim 2, wherein the material of said substrate is GaAs. 4. A plurality of light-emitting thyristors are arranged, and a gate electrode of each light-emitting thyristor is connected to a gate electrode of at least one light-emitting thyristor located in the vicinity of the light-emitting thyristor via an electric element having electric resistance or electric unidirectionality. A self-scanning light-emitting device in which a plurality of wirings for externally applying a voltage are connected to the anode electrode of each light-emitting thyristor, wherein the light-emitting thyristor is described in any one of claims 1 to 3. A self-scanning light emitting device, characterized in that it is a light emitting thyristor. 5. A plurality of thyristors are arranged, and a gate electrode of each thyristor is connected to a gate electrode of at least one thyristor located near the thyristor via an electric element having electric resistance or electric unidirectionality. A self-scanning switch element array formed by connecting a power supply line to the gate electrode of each thyristor by electrical means and connecting a clock line to the anode electrode of each thyristor, and a plurality of light emitting thyristors are arranged. A light-emitting element array, wherein each gate electrode of the light-emitting thyristor constituting the light-emitting element array is electrically connected to a gate electrode of a thyristor constituting the self-scanning switch element array, and an anode of each light-emitting thyristor is formed. In a self-scanning light emitting device provided with a line for applying a current for light emission to the electrode, Optical thyristor is self-scanning light-emitting device which is a light-emitting thyristor as described in claim 1.