JPS62188392A - Low current semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser of low oscillation threshold current by a method wherein the device is made to perform laser action making electron density and hole density in an active region to be differed remarkably as they are. CONSTITUTION:Electron density on the conduction band side and hole density on the valence band side in an active region are increased holding both in condition being differed remarkably as they are, beam absorption to be generated according to electron transfer to the conduction band side from the valence band side is reduced, moreover at the same time, probability of spontaneous emission is reduced also, and an oscillation threshold current is made to be reduced. For example, an active region layer 1 is constructed of a P-type semiconductor having the smallest energy gap Eg1, and P-type semiconductor layers 9, 10 of two pieces having large energy gaps are put on both the sides thereof. Moreover on the outsides thereof, a P-type electromagnetic field trapping layer 7 and an N-type similar layer 8 having band gaps Eg7, Eg8 to vary in the form of a parabolic function are attached. Finally, a P-type layer 2 and an N-type layer 3 having the largest band gaps are attached to the outsides of the layers 7, 8 respectively.

Description

[Detailed Description of the Invention] Technical Field The present invention relates to a semiconductor laser used in the field of optoelectronics, in which the oscillation threshold current is particularly reduced and the conversion efficiency from current to optical output is improved. be.

Prior Art Figure 1 shows the configuration of a conventional double heterojunction semiconductor laser. The active region N1, the outer region layer 2, and the same layer 3 are laminated and crystal grown, and an electrode 4 is formed on the upper part and an electrode 5 is formed on the lower part.
Attach and cleave in the X-Y cross section perpendicular to the axial direction Z,
Laser oscillation is performed using the cleavage plane as a reflecting mirror surface.

In conventional semiconductor lasers, a double heterojunction as shown in FIGS. 1 and 2, or a quantum well structure as shown in FIG. 3 has been used as the laminated structure in the thickness direction Y.

In other words, in a double heterojunction, as shown in FIG. 2 (2L), a semiconductor with a relatively small bandgap Eg+ is used as the active region layer l, and a semiconductor with a larger bandgap Eo2 is used as the active region layer l.
The structure is sandwiched between a type semiconductor 2 and an n-type semiconductor 3 with a band gap of E, 3, and the semiconductor in the active region N1 is type 1 (intrinsic).
, or may be p-type or p-type. When a voltage is applied to the electrodes 4 and 5 of the laser, the Fermi level on the n-type side increases and the Fermi level on the p-type side decreases, as shown in Figure 2(b), and electrons are transferred from the n-type side to the p-type side. Holes are injected into the active region. The laser injection current value is
It is determined by the spontaneous release probability in the active region.

In addition, in the conventional quantum well structure, the width of the active region layer 1 is set to 100 nm or less, and as shown in Fig. 3(a), the band gap Eg6 is grown alternately with thin N6 layers whose band gap is slightly larger than E and I. There is also a structure in which the band gaps El and Egs of the outer N7 and N8 are gradually increased as shown in FIG. 3(b). Even in the quantum well structure, the outer layer 2 is p-type and the layer 3 is n-type. Layer 1, N6, N7, N8, etc. exist in any case of 1 type, n type, or p type, but they are p-n or p- type as in the case of double hetero in Fig. 2.
Electrons and holes are injected into the active region layer 1 through the 1-n junction.

In the lasers of FIGS. 2 and 3, the electron density and hole density within the active region layer 1 are automatically determined to maintain electrical neutrality conditions between these charges and impurity ions, During laser operation, the electron density and hole density are approximately the same value. The general relationship between injection current and laser gain in conventional lasers is as shown in FIG. That is, when the current I is 1. When it is smaller, the laser gain g is zero, light is absorbed within the laser, electrons in the valence band transition to the conduction band, and no laser oscillation occurs. When the current becomes larger than 1 g, the laser gain g becomes positive, electrons on the conduction band side transition to the valence band side, and stimulated emission begins.

Then, when the laser gain reaches a certain value gth, the laser enters the oscillation state. This gih is called an oscillation threshold gain, and the corresponding current level lth is called an oscillation threshold current. In conventional semiconductor lasers, since the current value of 1° at which the laser gain becomes positive has a finite value, there is a limit to the reduction of the oscillation threshold current lih. Therefore, there was a drawback that the conversion efficiency from the injected current to the output light was not very high.

OBJECTS OF THE INVENTION In order to eliminate the above-mentioned conventional drawbacks, an object of the present invention is to realize a method of operating a laser while the electron density and hole density in the active region are significantly different, and to achieve a low oscillation threshold. The object of the present invention is to provide a semiconductor laser with a low current.

Principle of the Invention This invention newly discovered the principle that the oscillation threshold becomes extremely small when operating with either the hole density or the electron density being excessive, and developed a method to achieve this state. This is a new laser structure devised.

If the hole density in the active region is P and the electron density is N, then the relationship between the charge density difference P-'N and the laser gain g when the injection current is constant is as shown in Figure 5. something was discovered. That is, near the point where the difference in charge density becomes zero, the laser gain decreases by 4-. This phenomenon occurs because the physical mechanism that determines the injection current and the physical mechanism that determines the laser gain are different.

The injection current is determined by the spontaneous emission probability, and the laser gain is determined by the difference between the stimulated emission probability and the absorption probability. If the difference in charge density is close to zero, the spontaneous emission probability will increase, but if the difference in charge density is large, the spontaneous emission probability will decrease, and the light absorption effect will also decrease. Therefore, if we make a difference in charge density as shown in Figure 6, the current value 1 at which the laser gain becomes positive is
g becomes 0, and a positive laser gain can be obtained from I=O.

Implementation Example A configuration example for realizing the above principle is described below.
The present invention will be explained in more detail.

FIG. 7 shows an example of the configuration. FIG. 7(a) shows a semiconductor band structure in a state where changes in charge distribution due to junctions are not considered. That is, the active region N1 is a p-type semiconductor with the smallest energy gap Eg+ and has a thickness of 20
It is 0 Å or less. Two p-type semiconductors N9 having a large energy change and a layer 10 are sandwiched from both sides thereof. The thickness of this N9 and FTIO is also less than 200 Å. Furthermore, on the outside,
A layer 8, which is the same as the electromagnetic field confinement layer 7, whose band gap E97 and Ege change in a parabolic manner is provided. Layer 7 is p
However, N8 is an n-type. The thickness of layer 7 and layer 8 is
About 1,000 people ate it up. Finally, n-type layer 2 and n-type layer 3 having the largest band gap are formed on the outside of layer 7 and layer 8, respectively. Figure 7 shows the band structure taking into account the energy change due to bonding. Since the active region N1 is sufficiently thin and is sandwiched from both sides by p-type semiconductors with a large band gap, the Fermini level is lowered deep into the valence band, and the hole density P becomes significantly larger than the electron density N. . Holes necessary for laser transition can be directly injected from the p-type side, and electrons can be injected into the active region by tunneling from N8. Laser gain is obtained by electrons and holes injected into the active region.

As described above, the present invention is structurally different from conventional semiconductor lasers in that the outside of the active region layer 1 is formed by p-type N9 and M
The p-n junction plane is not in contact with the active region, but exists at the boundary between layer 10 and layer 8, and layer 9 and layer 10 are located at the boundary between the Fermi level in the active region. This is a potential control layer inserted to penetrate deep into the band.

The principle of the present invention can also be realized by various configurations as shown in FIG. That is, in FIG. 8(a), the energy gap between the electromagnetic field confinement N7 and the same layer 8 is kept constant, and the operations of layer 9, layer 1, and layer 10 are
Same as the figure.

In FIG. 8(b), the number of active regions is two, and there is a layer 11 between them.
is inserted. Like layer 9 and N1°, Hll is for lowering the Fermini level into the valence band. The principle is the same even in a multilayer structure in which the number of active regions is increased to two or more layers, and is included in the present invention. Figure 8 (c
) has an active region of l type (intrinsic). FIG. 8(d) shows an example in which the active region is sandwiched between n-type layers so that the electron density is significantly higher than the hole density in the active region, contrary to the configuration examples described above. The principle of the present invention is to utilize a state in which the electron density and hole density are significantly different, and both cases where the electron density is high and hole density is high are included in the present invention. Furthermore, a multilayer structure as shown in FIG. 8(b) based on n-type as shown in FIG. 8(d), and a layer of N9 and 10
is n-type and corresponds to FIG. 8(C) in which only the active region is i-type, and the energy gap between electromagnetic field confinement layers 7 and 8 in FIGS. The present invention also includes continuous changes as shown in Figure (a).

Effects As explained above, the semiconductor laser according to the present invention has an oscillation threshold current of 115 to 1 compared to conventional semiconductor lasers.
Since it can be reduced to /10 or less, the conversion efficiency from current to optical output is significantly improved, excessive heat generation due to reactive power can be prevented, and it can be used as a light source for optoelectronics with low power consumption.

[Brief explanation of drawings]

FIG. 1 is a perspective view schematically showing the structure of a conventional double heterojunction structure semiconductor laser. Figure 2 is a band structure diagram of a conventional double heterojunction structure semiconductor, in which (a) shows the state before current injection, and (b
) indicates the state during current injection. FIGS. 3(a) and 3(b) are band structure diagrams of a conventional quantum well structure semiconductor laser. FIG. 4 is a general characteristic diagram of current versus optical output in a conventional semiconductor laser. FIG. 5 is a characteristic diagram of laser gain with respect to the difference between electron density and hole density, which is the principle of the present invention. FIG. 6 is a characteristic diagram of laser gain with respect to injection current. FIGS. 7(a) and 7(b) are band structure diagrams of the present invention. FIG. 8 is a band structure diagram showing another configuration example of the present invention. 1.1', 1''...active region, 2...n-type outer layer, 3...p-type outer layer, 4...
・Positive electrode, 5... Negative electrode, 6... Intermediate layer, 7.8... Electromagnetic field confinement layer, 9.10... Potential control layer, (p)... P-type semiconductor , (j)...i-type (intrinsic) semiconductor, (n)...n-type semiconductor, E, I~Eg+1+...band gap, g...
・Laser gain, gth...oscillation threshold gain, ■
... Injection current, lih... Laser threshold current, 1g... Current value at which the laser gain becomes positive, N... Electron density, P... Hole density, Figure 1 (a) b) (c) (d)

Claims (1)

[Claims] In the active region, the electron density on the conduction band side and the hole density on the valence band side are increased while remaining significantly different, and the electron density is increased by electron transition from the valence band side to the conduction band side. A semiconductor laser device designed to reduce light absorption and at the same time reduce the probability of spontaneous emission, thereby reducing the oscillation threshold current.