JP2542588B2 - Optical bistable element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention has functions such as optical switching and optical storage,
Regarding optical bistable elements that will be important elements in future optical communication systems and optical information processing systems, especially high speed,
The present invention relates to a compound semiconductor non-linear etalon type optical bistable device which has a possibility of operating switching and optical storage with low incident energy and can be highly integrated.

(Prior Art) In recent years, research on optical bistable elements, which are important elements in future optical communication systems and optical information processing systems, has been activated. Considering the above-mentioned application fields, the optical bistable element is required to have high-speed operation, low-energy operation, easy integration, and an operating wavelength matching with other optical devices and electronic devices. Optical bistable devices have been proposed and manufactured based on various operating mechanisms, but devices that utilize the large optical nonlinearity of compound semiconductor thin films are suitable for small size and high integration. It is most promising in the future because it has potential for energy operation and has good compatibility with current optical and electronic devices. In particular, the etalon-type optical bistable device utilizing the nonlinearity of the refractive index change due to the absorption saturation of excitons of the compound semiconductor has an excellent feature that it can be operated at low energy and can be formed into a two-dimensional array.

FIG. 4 schematically shows the structure of the etalon type optical bistable device. The non-linear refractive index medium 401 whose refractive index changes non-linearly according to the intensity of incident light is provided with reflective films 204a and 204b on both sides. The non-linear response of the refractive index to the incident light intensity by the non-linear refractive index medium 401 and the positive feedback to the non-linear response by the reflecting films 204a and 204b on both sides provide an optical bistable characteristic between the incident light and the transmitted light. Occurs. It is possible to realize an optical bistable device capable of operating at room temperature by using a GaAs / AlGaAs superlattice in which excitons exist stably even at room temperature as a nonlinear refractive index medium. HM Gibbs et al. By Applied Physics Letters Magazine, 1982 8
It is reported on pages 221 to 222 of the monthly issue (Vol. 41). The etalon type optical bistable device using GaAs / AlGaAs superlattice can utilize the nonlinearity of large refractive index due to exciton absorption saturation even at room temperature and can be highly integrated. It is a very promising optical bistable device because it matches well with the laser.

(Problems to be Solved by the Invention) However, the above-mentioned GaAs / AlGaAs etalon type bistable element has a problem that the switch-on is performed at high speed but the switch-off is performed at low speed. This is because the energy relaxation time of excitons that determine the switch-on time (the time during which light is incident and the excitons are decomposed into pairs of electrons and holes by phonons) is very short (picoseconds or less).
This is because the relaxation time of free carriers generated by exciton decomposition that determines the switch-off time is as slow as about 30 nanoseconds in the case of spontaneous recombination.

It is an object of the present invention to reduce the relaxation time of a free carrier that determines the switch-off time of an etalon type optical bistable device using a compound semiconductor superlattice to speed up the device.

(Means for Solving the Problems) An optical bistable device provided by the first invention of the present application in order to solve the above problems is a compound in which two kinds of compound semiconductor thin films having different band gaps are alternately laminated. It is characterized by comprising a semiconductor superlattice, reflective films formed on both surfaces of the compound semiconductor superlattice, and means for applying an electric field between the reflective films.

In order to solve the above-mentioned problems, the optical bistable device provided by the second invention of the present application uses a compound semiconductor superlattice in which two kinds of compound semiconductor thin films having different band gaps are alternately laminated as an i layer. Reflective films are formed on both sides of the compound semiconductor pin diode, and impurities forming a deep level are introduced into the p layer and n layer compound semiconductors of the pin diode.

Further, in order to solve the above-mentioned problems, the optical bistable device provided by the third invention of the present application uses a compound semiconductor superlattice in which two kinds of compound semiconductor thin films having different band gaps are alternately laminated as an i layer. A reflection film is formed on both sides of the compound semiconductor pin diode, and an impurity forming a deep level is introduced into the compound semiconductor having a larger bandgap in the thin film forming the compound semiconductor superlattice. Is characterized by.

(Operation) In each invention of the present application, the superlattice is applied by applying an electric field from the outside to the compound semiconductor superlattice or by applying an electric field to the compound semiconductor superlattice by using a built-in potention having a pin structure. Tilts the energy band of the free carrier, and tunnels the free carrier accumulated in the well layer of the superlattice toward the barrier layer. At this time, the relaxation time of the carrier is determined by the tunnel time constant, which is about several hundred picoseconds, and therefore the operation speed of the optical bistable element is sufficiently faster than that in the case of natural recombination. In the case of a pin structure element, electrons and holes tunnel in opposite directions and p
If holes are accumulated on the layer side and electrons are accumulated on the n layer side, the inclination of the band disappears. Therefore, in order to prevent this, the present invention is provided with a layer into which an impurity serving as a recombination center is introduced. I am letting you.

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

FIG. 1 is a diagram showing the structure of a GaAs / AlGaAs superlattice etalon type optical bistable device according to the first invention of the present application. FIG. 1 (a) shows the device structure, and FIG. 1 (b) shows energy. Each band is shown.

In the embodiment shown in FIG. 1, AlGaAs is formed on the GaAs substrate 101.
A film 102 and a GaAs / AlGaAs superlattice 103 are laminated to form a GaAs substrate 101.
A window is formed in the window, and metal reflection films 106a and 106b having a reflectance of about 95% are formed on the window portion and the GaAs / AlGaAs superlattice, respectively. The metal reflection films 106a and 106b
An external power supply 107 for applying an electric field from the outside to the GaAs / AlGaAs superlattice is connected to the. With the above structure, GaA
An etalon-type optical bistable element is constructed using the s / AlGaAs superlattice 103 as a nonlinear refractive index medium and the metal reflection films 106a and 106b as reflecting mirrors.

A method of manufacturing the GaAs / AlGaAs nonlinear etalon type optical bistable device shown in FIG. 1 (a) will be briefly described below. First Ga
On the As substrate 101, an AlGaAs film 102 having a thickness of 1 μm or less, which is an etching stop layer of a GaAs substrate, is formed. Approximately 300Å GaAs film is alternately MB on the AlGaAs film 102.
About 60 periods of GaAs / AlGaAs superlattices stacked by the E method are formed. After that, the GaAs substrate 101 is etched to form a window until the AlGaAs etching / stop layer 102 is exposed, and finally the exposed AlGaAs film 102 and GaAs / AlGaAs are exposed.
Highly reflective metal reflective films 106a and 106b are formed on the superlattice 103, respectively.

The principle by which the optical bistable device having the structure shown in FIG. 1 (a) obtained as described above operates at high speed will be described.

GaAs / AlGaA is externally applied to the device having the structure shown in FIG.
s When an electric field is applied to the superlattice 103, an inclination occurs as shown in Fig. 1 (b), and the free carriers generated by the decomposition of excitons tunnel between electrons and holes in the opposite barrier layers. To do. At this time, the relaxation time of free carrier is determined by the tunnel time constant.
The free carrier tunnel time constant in the GaAs / AlGaAs superlattice has been measured, and the electric field strength is 5.6 × 10 ^-3 V.
PHYSICAL REVIEW B magazine, April 1986
It is reported on pages 1561 to 5964 of the 15th of the month (Vol. 33). This value is about 100 times faster than the free carrier recombination time (about 30 nsec), and GaAs / AlGa
By applying an electric field in the direction perpendicular to the layers in the As superlattice, it is possible to realize a significant speedup of the device. The problem here is that the electric field is always applied, so excitonic absorption saturation may be suppressed when light is input and the switch is turned on. The process by which the free carrier formed by the child's decomposition by phonon promotes the decomposition of excitons by screening the Coulomb's force by sub-picoseconds, and it takes a sufficiently short time compared to the relaxation time of the free carrier. Absorption saturation of excitons occurs even when an electric field is constantly applied. Therefore, according to the present invention in which a power supply for applying an electric field is added to the outside by using the reflection film as a metal film, it is possible to significantly speed up the optical bistable device.

FIG. 2 shows an embodiment of an optical bistable element according to the second invention of the present application. In this embodiment also, the speed of the optical bistable element is increased by applying an electric field to the GaAs / AlGaAs superlattice layer, but unlike the case of the embodiment shown in FIG.
The structure is used, and the band of the superlattice layer is tilted by applying its built-in potential without applying an electric field from the outside.

FIG. 2 (a) shows an element structure of the embodiment according to the second invention of the present application. The GaAs / AlGaAs superlattice layer 203 is an i-layer, and the upper and lower parts of the i-layer are sandwiched by p-GaAs201 and n-GaAs202.
It has a pin diode structure. Then, on the surfaces of the p-GaAs 201 side and the n-GaAs 202 side, reflecting films 204a and
04b is formed respectively. In this embodiment, since the element has a pin structure, its built-in potential is used to change the Ga as shown in FIG. 2 (b).
The band of the As / AlGaAs superlattice layer 203 can be tilted. Therefore, the relaxation time of the free carrier can be greatly reduced by the same principle as in the embodiment of FIG. 1 without applying an external electric field. However, in this case, if electrons and holes moved by tunneling are accumulated in the n-GaAs 202 and p-GaAs 201, respectively, the inclination of the band disappears. Therefore, in this embodiment, impurities such as Cr, which form deep levels in the p-GaAs layer 201 and the n-GaAs layer 202 and serve as recombination centers, are introduced. By doing so, the p-GaAs layer 201 and n
Carriers that have moved to the GaAs layer 202 by tunneling are recombined at high speed, so that the disappearance of the band tilt does not occur and the high speed performance of the optical bistable element is not impaired.

FIG. 3 shows an embodiment of an optical bistable element according to the third invention of the present application. Also in this embodiment, the device has a pin structure,
The built-in potential is used to incline the band and tunnel the free carrier to reduce the relaxation time and speed up the device.

FIG. 3 (a) shows an element structure of the embodiment according to the third invention of the present application. The GaAs / AlGaAs superlattice layer 303 is an i-layer,
The upper and lower sides of the p-GaAs 201 and n-GaAs 202 form a pin diode structure. And p-GaAs201
And reflective films 204a and 204 of high reflectivity on the surface of n-GaAs 202
b is formed respectively. In this embodiment, a GaAs / AlGaA is used by utilizing the built-in potential of the pin structure.
s The tilting of the band of the superlattice layer 303 to reduce the relaxation time of the free carrier is similar to the embodiment of FIG. 2, but the second method is to recombine the free carrier tunneled by the tilt of the band. This is different from the case of the illustrated embodiment. In this embodiment, as shown in FIG.
Impurities serving as recombination centers are selectively doped by forming a deep level only in the AlGaAs barrier layer of the GaAs / AlGaAs superlattice 303. Free carriers generated in the GaAs well layer due to exciton decomposition are generated by the pin structure built-in potentiometer.
Since the band of the AlGaAs superlattice 303 is inclined as shown in FIG. 3B, it tunnels and reaches the AlGaAs barrier layer. Here, since the AlGaAs barrier layer is doped with the impurity serving as the recombination center as described above, high-speed recombination is performed through the impurity. Therefore, in the present embodiment as well, the disappearance of the band inclination due to carrier accumulation does not occur, and the tunneling of carriers by the internal electric field and the introduction of the recombination center layer (AlGaAs barrier layer) achieve the speedup of the device.

(Effects of the Invention) As described above, when the invention of the present application is used, the relaxation time of the free carrier is significantly increased as compared with the conventional one by tunneling the free carrier by the electric field from the outside or the internal electric field using the built-in potential. Therefore, the operating speed of the etalon type optical bistable device can be significantly increased.

The inventions of the present application are not limited to the above-described embodiments. In the above-mentioned embodiments, only the GaAs-based compound semiconductor is taken as an example, but it goes without saying that the inventions of the present application can be applied to InP-based semiconductors.

[Brief description of drawings]

FIG. 1 (a) is a sectional perspective view schematically showing the structure of an embodiment according to the first invention of the present application, and FIG. 1 (b) shows the energy band of the superlattice in the embodiment of this FIG. Fig. 2 (a) is a sectional perspective view schematically showing the structure of an embodiment according to the second invention of the present application, and Fig. 2 (b) is the energy of the superlattice in the embodiment of this Fig. (A).・ Figure showing band, No. 3
FIG. 3A is a sectional perspective view schematically showing the structure of an embodiment according to the third invention of the present application, and FIG. 3B is a view showing the energy band of the superlattice in the embodiment of FIG. FIG. 4 is a perspective view showing a schematic structure of an etalon type optical bistable element. 110 …… Incoming light, 111 …… Outgoing light, 103 …… GaAs / AlGaAs superlattice, 203 …… i-GaAs / AlGaAs superlattice, 303 …… i-GaA
s / AlGaAs superlattice (AlGaAs layer doped with impurities), 101 ... G
aAs substrate, 102 ... AlGaAs film, 106a, 106b ... Metal reflective film, 107 ... External power supply, 108 ... GaAs film, 109 ... AlGaAs
Membrane, 201 ... p-GaAs, 202 ... n-GaAs, 204a, 204b ...
… Reflective film, 401 …… Nonlinear refractive index medium.

Claims (3)

(57) [Claims] 1. A compound semiconductor superlattice in which two kinds of compound semiconductor thin films having different band gaps are alternately laminated, a reflective film formed on both surfaces of the compound semiconductor superlattice, and an electric field is applied between these reflective films. An optical bistable element, characterized by comprising: 2. A reflection layer is formed on both sides of a compound semiconductor pin diode having a compound semiconductor superlattice in which two types of compound semiconductor thin films having different band gaps are alternately laminated as an i layer, and a p layer of the pin diode is formed. An optical bistable device characterized in that an impurity forming a deep level is introduced into the n-layer compound semiconductor. 3. A compound semiconductor pin diode having an i layer of a compound semiconductor superlattice in which two kinds of compound semiconductor thin films having different band gaps are alternately laminated, and a reflective film is formed on both surfaces of the compound semiconductor pin diode. An optical bistable device characterized in that a compound semiconductor having a larger bandgap in the thin film to be formed contains impurities that form a deep level.