JPH0451997B2 - - Google Patents

Description

[Detailed description of the invention] The present invention integrates a semiconductor laser and a modulator,
The present invention relates to a semiconductor light emitting device that can externally modulate output light from a semiconductor laser at high speed by varying the absorption peak wavelength of a modulator.

Semiconductor lasers that can be modulated at high speeds are considered important in the field of optical communications as they enable high-speed, large-capacity transmission of information, but they are also used in other fields.
For example, in the fields of optical computation and optical information processing, their importance will tend to increase in the future.

Conventionally, a direct modulation method has been used to modulate semiconductor lasers. This method directly modulates the current flowing through the laser and is simple, but the upper limit of the modulation speed is determined by the resonant frequency, which is practically about several GHz. Furthermore, in the direct modulation method, the laser has the characteristic of oscillating in multi-axis modes during modulation, so the spectrum is broadened, and when used as a light source for optical communications, the problem arises that the transmission band is degraded by the dispersion effect of the fiber. On the other hand, such a problem does not occur with the external modulation method. Conventional modulators for external modulation of semiconductor lasers
Experiments have been carried out using the properties of ferroelectric materials such as LiNbO ₃ and LiTaO ₃ , mainly their electro-optic effects. However, in order to obtain sufficient modulation, the modulator needs to be several centimeters long, so it has a large capacitance, and a high applied voltage of several tens of volts is also required.Furthermore, in order to integrate it with the laser into a device, the optical axis must be aligned. Such complicated adjustments were necessary.

The present invention has been made in view of the above-mentioned drawbacks, and it is an object of the present invention to provide a semiconductor light emitting element that is small and capable of high-speed modulation by monolithically forming a light emitting section and a light modulating section on a semiconductor substrate.

The semiconductor light emitting device of the present invention has a semiconductor layer including a p-type semiconductor, a multilayer thin film, and an n-type semiconductor in this order, and the multilayer thin film has a thickness of about the de Broglie wavelength of electrons and holes or less. It has a structure in which p-type semiconductor thin film layers and n-type semiconductor thin film layers are alternately laminated, and a light emitting part is provided with an electrode for applying a voltage in the forward direction to a part of the semiconductor layer, and It is characterized in that it has a light modulating section provided with an electrode for modulating the intensity of light from the light emitting section by an applied voltage in the opposite direction in another section.

Embodiments of the present invention will be specifically described below with reference to the drawings.

FIG. 1 is a band diagram of a light emitting section and a light modulating section showing the operating principle of a semiconductor light emitting device according to the present invention.
In the figure, 1a, 1b, 1c and 1d are p-type semiconductor layers, and 2a, 2b, 2c and 2d are n-type semiconductor layers. Black circles indicate electrons, and white circles indicate holes. The thickness of each semiconductor layer is approximately the de Broglie wavelength of electrons and holes, or less than that.
Here, the de Broglie wavelength λd is defined as λd = h/p using Planck's constant h and carrier momentum P. For example, in InP, it is about λd300 Å, which is a small value compared to the carrier diffusion length. be. In such a thin semiconductor layer, the kinetic energy of carriers in the thickness direction is quantized. 11,
12 and 13 schematically show discrete energy levels of electrons appearing in the conduction band. Further, 21 and 22 are the energy levels of holes in the valence band. The spacing between energy levels directly depends on the thickness of the semiconductor layer, and increases as the thickness decreases. The p-type semiconductor layer 1a is n in the multilayer thin film shown in FIG.
Assume that a negative voltage is applied to the type semiconductor layer 2d. In such a state, the p-type semiconductor layer 1b and the n-type semiconductor layer 2a are in a forward bias state. On the other hand, the adjacent n-type semiconductor layer 2a
The p-type semiconductor layer 1a and p-type semiconductor layer 1a are in a reverse bias state, and the thickness of the depletion layer formed at the junction thereof increases. As a result, the effective thickness felt by electrons existing inside the n-type semiconductor layer 2a decreases, and the spacing between the energy levels of the conductor increases. The same thing happened to other n
It is also understood that this occurs for the conduction band energy levels of the p-type semiconductor layers 2b, 2c, and 2d, and that the interval between the valence band energy levels of the p-type semiconductor layers 1a, 1b, 1c, and 1d widens. It will be. Consider the case where light is incident on a semiconductor layer having the above band structure. The absorption coefficient of light depends on the spacing between the energy levels of the semiconductor layer, so applying an externally applied voltage in the opposite direction shortens the absorption edge wavelength and changes the amount of light absorbed. Become. In other words, the optical modulator of the present application absorbs light close to the absorption edge wavelength when no voltage is applied, and becomes transparent to the light when a voltage is applied, making it possible to intensity-modulate the input light.

In the light emitting section, the semiconductor layer is used as an active layer, and by applying a voltage in the forward direction and injecting current, it becomes a light emitting element, and if a resonator is further formed, it becomes a semiconductor laser. Recombination of electrons and holes occurs at the junction between the forward biased semiconductor layers, thereby allowing lasing to occur. Since each semiconductor layer is as thin as the de Broglie wavelength, carriers injected from one end can tunnel through the potential barrier created in the part of the semiconductor layer that is in a reverse bias state. Therefore, the carrier can reach the semiconductor layer at the other end,
It is possible to inject and supply carriers throughout the multilayer thin film. As a result, the laser gain becomes larger than when the n-type semiconductor layer and the p-type semiconductor layer each consist of a single layer.

FIG. 2 is a cross-sectional view of one embodiment of the present invention in a direction perpendicular to the optical axis, and FIG. 3 is a cross-sectional view in the optical axis direction. On a semiconductor substrate 31 made of n-type InP made of InGaAsP mixed crystal, Sn doped with a layer thickness of 1 μm,
A cladding layer 32 made of n-type InP with n=1×10 ¹⁸ /cm ³ is formed. On this layer 32, a layer thickness of 100 Å is applied.
Be-doped, p-type In _0.47 Ga _0.53 with p=1×10 ₁₇ /cm ³
A p-type semiconductor layer made of As and a layer thickness of 100 Å.
Sn-doped, n-type In _0.47 Ga _0.53 with n=1×10 ₁₇ /cm ³
The n-type semiconductor layers made of As are alternately
A multilayer thin film is formed by laminating layers one by one. 3a, 3
b, 3c and 3d are p-type semiconductor layers, 4a, 4b,
4c and 4d are n-type semiconductor layers. Molecular beam epitaxial growth is used to form each layer. 33
and 34 are Be-doped with a layer thickness of 1 μm, p=1×10 ¹⁸ /
cladding layer consisting of p-type InP of cm ³ and p-type In _0.53
Contact layers 35, 36 and 37 made of Ga _0.47 As are electrodes. The multilayer thin film is separated into two regions, left and right, by a groove 38. Groove 38 in Figure 3
The right side 39 is a light emitting section that operates as a semiconductor laser, and the left side 40 is an optical modulation section. Light emitting part 3
The lengths of the grooves 9, the grooves 38, and the optical modulator 40 in the optical axis direction are 300 μm, 20 μm, and 50 μm, respectively. The end face of the light emitting part 39 facing the groove 38 is formed perpendicular to the optical axis by reactive ion etching. The opposite end surface is a cleavage plane. In FIG. 3, Po indicates light that passes through the light modulation section 40 and is modulated there and output. In order to prevent the light emitted from the light emitting section 39 from being reflected by both end surfaces of the light modulating section 40 and returning to the light emitting section 39 and destabilizing the oscillation, both end surfaces of the light modulating section 40 are arranged in a direction perpendicular to the optical axis. It is formed at a slight angle from the top. The wavelength of the laser light is approximately 1.5 μm. Therefore, according to this embodiment, the absorption edge wavelength of the optical modulator 40 can be changed in the range of 1.5 μm to 1.4 μm by simply changing the voltage applied to the electrode 37 by a few volts, and the extinction ratio of 100% is 1 GHz.
The speed can be modulated.

Further, in the above embodiment, since laser light is emitted from the light emitting section 39 in the opposite direction to the light modulating section 40, if a light modulating section is provided symmetrically with the same structure on the opposite side, each can be modulated independently. Two signal lights can be sent, resulting in an economical light source.

[Brief explanation of drawings]

FIG. 1 is a band diagram of a light emitting section and a light modulating section showing the operating principle of a semiconductor light emitting device according to the present invention, and FIG. 2 is a cross-sectional view in a direction perpendicular to the optical axis of an embodiment according to the present invention. FIG. 3 is a cross-sectional view along the optical axis direction. 1a, 1b, 1c, 1d, 3a, 3b, 3c and 3d are p-type semiconductor layers, 2a, 2b, 2c, 2
d, 4a, 4b, 4c and 4d are n-type semiconductor layers,
11, 12, 13, 21 and 22 are energy levels, 31 is a semiconductor substrate, 32 and 33 are clad layers, 34 is a contact layer, 35, 36 and 37 are electrodes, 38 is a groove, 39 is a light emitting part, and 40 is a This is a light modulation section.

Claims (1)

[Claims] 1. A semiconductor layer including a p-type semiconductor, a multilayer thin film, and an n-type semiconductor in this order, and the multilayer thin film has a thickness of approximately the de Broglie wavelength of electrons and holes or less. It has a structure in which p-type semiconductor thin film layers and n-type semiconductor thin film layers are alternately laminated, and includes a light emitting part provided with an electrode for applying a voltage in the forward direction to a part of the semiconductor layer, and a part of the semiconductor layer other than the semiconductor layer. 1. A semiconductor light-emitting device, comprising: a light modulation section provided with an electrode for modulating the intensity of light from the light-emitting section by an applied voltage in a reverse direction.