JPH0636435B2 - Light emitting diode - Google Patents

Description

The present invention relates to a semiconductor light emitting diode used as a light source for an optical fiber transmission system or the like. More specifically, it is configured to widen a modulation band without degrading linearity of light output and luminous efficiency. And a double heterojunction light emitting diode for optical communication.

Conventionally, as a light source for optical fiber transmission, a semiconductor laser diode (Laser Dilde, hereinafter abbreviated as LD) and a light emitting diode (Light Emitting Diode, hereinafter abbreviated as LED) have been used. This LED is inferior to the LD in light output and modulation band, but has the advantages that it is excellent in reliability and economy, and there is no operational instability due to reflected light. Therefore, if the modulation band, which has been about several MHz to 100 MHz in the past, is improved to 1,000 MHz or more without deteriorating the linearity of light output or luminous efficiency, the LED is very promising as a light source for analog transmission of multi-channel image signals. Becomes

Broadening of the band of the LED can be performed by two methods: (1) high injection method (2) high doping method.

The method (1) described above shortens the radiative recombination life by injecting excess minority carriers having a majority carrier density (supplied by doping) or more into the light emitting layer. However, this method has a narrow modulation band in the low injection current region and causes nonlinear distortion during large signal modulation.

Further, in the method (2), majority carriers having an injection carrier density or higher are previously doped into the light emitting layer, and the modulation band does not depend on the injection current, and low distortion characteristics can be obtained. However, when the doping amount is increased, crystal defects increase in the crystal, and these crystal defects serve as non-radiative recombination centers, so that there is a drawback that the luminous efficiency decreases.

As described above, it has been difficult with the conventional method to achieve a wide band while maintaining the optical output and linearity required for multi-channel analog transmission.

Therefore, an object of the present invention is to provide a light emitting diode having a wide modulation band characteristic without deteriorating the linearity of light output and the light emission efficiency.

In order to achieve such an object, in the present invention, in a light-emitting diode having a double heterojunction, a plurality of light-emitting layers and barrier layers are alternately provided to form an active layer, and the thickness of the light-emitting layer is determined by electron quantum mechanical The width is almost the same as that of the diffusion, and the barrier layer is highly doped with doped atoms, and the forbidden band of the light emitting layer and the barrier layer are mutually isolated so that the potential of the light emitting layer is lower than the potential of the barrier layer with respect to the doped carriers. By making the quantum well structure different from each other, the doped carriers are accumulated in the light emitting layer, and the non-radiative recombination centers caused by the doped atoms are spatially separated from the doped carriers.

Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a conventional LD or LED double heterojunction structure. Here, (a) is an n-type clad layer, (a)
Is an active layer, (c) is a p-type cladding layer, (d) is a doped atom (ionized at room temperature), and (e) is a doped carrier (generated by ionization of the doped atom). That is, the active layer (a) is sandwiched between n-type and p-type clad layers ((a) and (c)), and the doped atoms (d) and the doped carriers (e) supplied from the doped atoms are both active layers. It is in. In this case, the active layer acts as a light emitting layer, but since non-radiative recombination centers due to the doped atoms are generated in the light emitting layer, the doped and injected carriers are trapped in the light emitting layer, and the luminous efficiency is reduced. The thickness of the active layer is 0.
It is about 1 to 0.2 μm and is of p-type.

FIG. 2 is an energy band diagram showing an embodiment of the double heterojunction structure according to the present invention.

FIG. 3 is an enlarged view showing a partial area A of FIG. 2 in detail.

Here, (d) is a doped atom, (f) is a quantum well type light emitting layer (undoped), (ki) is an undoped region of a barrier layer,
(H) indicates a highly doped region of the barrier layer, and (H) indicates a doped carrier generated by ionization of the doped atoms and accumulated in the quantum well type light emitting layer (F).

When the thickness of the light emitting layer is reduced to the quantum mechanical extent of electrons (~ 100Å), the electrons cannot move freely in the layer thickness direction, and the energy seems to have quantized discrete levels. become. Such a thin light emitting layer or an array thereof is called a quantum well structure or a multiple quantum well structure. When the light emitting layer (f) is not doped with dope atoms and only the barrier layer (h) is highly doped (modulation doping method), the doped carriers (ke) supplied from the doped atoms have a lower potential than the barrier layer. Since they are accumulated in the light emitting layer, the non-radiative recombination center caused by the doped atoms and the doped carrier can be spatially separated. Therefore, the doped and injected carriers are not captured by the non-radiative recombination centers, and efficient light emission operation can be performed.

When the active layer (a) shown in FIG. 1 is doped with dope atoms, the n-type cladding layer (a) and the active layer (a) are
Is a depletion layer, the doped carriers cannot move to the n-type clad layer through the depletion layer, and the potential for the doped carrier is higher in the p-type clad layer (c). Doped carriers cannot move through the heterojunction surface between the layer (a) and the p-type cladding layer (c).

Further, as shown in FIG. 3, an undoped region (K) in which the doped atoms are not doped is provided at the boundary between the highly doped barrier layer (K) and the light emitting layer. The inclusion of the undoped region (K) in this way prevents crystal defects in the barrier layer (K) due to high doping, and the non-radiative recombination centers caused by this to reach the interface of the light emitting layer (F). This is to prevent this, and its thickness is preferably around 100Å or less.

Although the case where the doped atom is the acceptor has been described so far, it is obvious that the present invention can be similarly applied to the case where the doped atom is the donor.

FIG. 4 shows a quantum well structure energy band diagram when a donor is used as a doped atom as described above. Here, the respective symbols (f), (ki), (h), ... Shown are as described in FIG.

Next, for the barrier layer, (1) the thickness may be made sufficiently thicker than the spread of the electron wave function to eliminate the coupling between the light emitting layers, or conversely, (2)
The thickness of the barrier layer may be reduced so as to have a bond between the light emitting layers. The structure in this case is called a superlattice structure.

As is apparent from the above, the present invention has been described above.
The method (2), that is, the one in which the modulation band is widened by high doping is attempted.

That is, by carrying out the present invention, it is possible to spatially separate the non-radiative recombination center caused by the doped atoms and the doped carrier. In this case, since only high-density doped carriers are present in the light emitting layer, better linearity (low distortion characteristic) and wider band characteristic can be obtained. On the other hand, since the doped atoms that cause a decrease in light emission efficiency through non-radiative recombination are in the barrier layer, the light emission efficiency in the light emitting layer is not reduced. Further, the emitted light effectively reaches the outside without being internally absorbed by the doped atoms. Therefore, the decrease in emission efficiency, which has been a drawback of high doping in the past, is solved by the spatial separation of the doped carrier and the non-radiative recombination center caused by it and the doped carrier, and a broad band is achieved without reducing the emission efficiency. be able to.

Next, the effect of changing the thickness of the barrier layer will be described with reference to FIGS. 5 and 6. In FIG. 5, the broken line (U) indicates the density of states of free electrons, and (S) indicates the density of electrons of electrons in the quantum well. Moreover, in FIG.
Indicates a subband of carriers formed in a superlattice structure.

As described above, when the thickness of the barrier layer is made sufficiently thicker than the spread of the electron wave function, the density of states of carriers becomes stepwise as shown in FIG. As a result, the emission spectrum width becomes narrow. This has the attendant benefit of easing bandwidth limitations due to optical fiber dispersion.

On the contrary, when the barrier layer is thinned to have a bond between the light emitting layers, a sub band as shown in FIG. 6C is formed by the bonding, and thus the emission spectrum width is as shown in FIG. Although not as narrow as in the case, carriers can move within the sub-band, thus improving the injection efficiency of minority carriers.

Next, a comparison between the present invention and the conventional example will be shown by taking a GaAs / AlGaAs multiple quantum well structure as an example. Semi-insulating GaAs (1
00) A GaAs buffer layer having a thickness of 0.47 μm and an Al _0.22 Ga _0.78 As layer having a thickness of 0.10 μm are sequentially formed on a substrate, and a GaAs well layer having a thickness of 100 Å and an Al _0.22 having a thickness of 500 Å are formed thereon.
A multiple quantum well in which Ga _0.78 As barrier layers were laminated for 10 periods was formed, and a GaAs layer having a thickness of 200 Å was further formed on the upper layer. Each layer was formed by the MBE method. Regarding such a multi-quantum well structure, two types of the case where the barrier layer is selectively doped with Si according to the present invention and the case where the light emitting layer (well layer) and the barrier layer are uniformly doped with Si as in the conventional example are provided. Samples were prepared and their carrier density, recombination lifetime and emission intensity were measured as a function of the amount of silicon doping. The results are shown in FIG. In the figure, the horizontal axis represents the doping amount of silicon as the reciprocal of the cell temperature T _si . In the figure, S
The curve indicated by D is the case where only the barrier layer is doped with Si according to the present invention, and the curve indicated by UD is the case where Si is uniformly doped. Compared to the case where the barrier layer and the light emitting layer are uniformly doped with impurities, the carrier concentration (Ns) can be further increased when the barrier layer is doped with impurities only, and the light emission is accordingly increased. This shows that the recombination lifetime (τ) can be shortened and the tendency of the decrease in emission output is small.

As described above, according to the present invention, the quantum well structure or the superlattice structure and the modulation doping method are combined to spatially separate the doped atom and the non-radiative recombination center caused by the doped carrier from each other, and It is possible to obtain low distortion characteristics and wide band characteristics of light emission and at the same time maintain high light emission efficiency. Further, by implementing the present invention, it becomes possible to manufacture a high quality light source for optical fiber transmission and a light source in an optical integrated circuit.

[Brief description of drawings]

FIG. 1 is a diagram showing an energy band (at the time of current injection) and a position of a doped atom in an ordinary double heterojunction structure light emitting device, and FIGS. 2 and 3 are energy band diagrams of a quantum well structure light emitting device according to the present invention. , FIG. 4 is an energy band diagram of a quantum well structure when a doped atom is a donor, FIG. 5 is a diagram showing a step density of states in a quantum well structure, and FIG. 6 is an explanation of formation of a superlattice structure and subbands. FIG. 7 is an energy band diagram showing
It is a characteristic view which shows a recombination lifetime and emission intensity compared with a prior art example. A ... n-type clad layer, a ... active layer, c ... p-type clad layer, d ... doped atom (ionized at room temperature), o ... doped carrier (generated by ionization of doped atom), c ... quantum well type light emitting layer ( Undoped), ki ... undoped region of barrier layer, heavily doped region of barrier layer, doped carrier accumulated in quantum well type light emitting layer by ionization of doped atoms, co ... usually free electrons Density of states, sub-state density of electrons in quantum wells, sub-bands of carriers formed in super-lattice structure.

Continuation of the front page (72) Norio Kobayashi Inventor Norio Nobayashi 1-10-11 Kinuta, Setagaya-ku, Tokyo Inside the Institute of Broadcast Science, Japan Broadcasting Corporation (56) References J. Electro chem. so c. SOLID-STATE SCIEN CE AND TECHNOLOGY. F ebook 1981, P.F. 400-P. 410

Claims (3)

[Claims] 1. In a light emitting diode having a double heterojunction, an active layer is formed by alternately providing a plurality of light emitting layers and barrier layers, and the thickness of the light emitting layer is made approximately the same as the quantum mechanical spread of electrons. In addition, the barrier layer is heavily doped with doped atoms, and the band gaps of the light emitting layer and the barrier layer are mutually isolated so that the potential of the light emitting layer is lower than the potential of the barrier layer with respect to doped carriers. By making the quantum well structure different from each other, the doped carriers are accumulated in the light emitting layer, and the non-radiative recombination centers caused by the doped atoms are spatially separated from the doped carriers. Light emitting diode. 2. The light emitting diode according to claim 1, wherein the active layer has a superlattice structure. 3. A non-doped region for preventing diffusion of a non-radiative recombination center caused by a doped atom into the light emitting layer, is provided at a boundary portion of the barrier layer with the light emitting layer. The light emitting diode according to item 1 or 2.