JPS63136580A - Optical bistable element - Google Patents

Abstract

PURPOSE:To shorten the relaxation time of free carriers largely, and to increase the working speed of an etalon type optical bistable element sharply by providing a compound semiconductor superlattice, in which two kinds of compound semiconductor thin-films having different band gaps are laminated alternately, reflecting films formed onto both surfaces of the compound semiconductor superlattice and a means through which an electric field is applied between both reflecting films. CONSTITUTION:An AlGaAs film 102 and a GaAs/AlGaAs superlattice 103 are laminated onto a GaAs substrate 101, a window is bored to the substrate 101, and metallic reflecting films 106a and 106b having reflectivity of approximately 95% are shaped onto a window section and the GaAs/AlGaAs superlattice. When an electric field is applied to the GaAs/AlGaAs superlattice 103, free carriers being generated are tunneled to barrier layers in the mutually opposite directions by electrons and holes. The relaxation time of free carriers is determined by a tunnel time constant, but the value is faster than the recombination time of free carriers, thus largely increasing the working speed of an element.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to an optical bistable device that has functions such as optical switching and optical storage, and will be an important element in future optical communication systems and optical information processing systems. This invention relates to compound semiconductor nonlinear etalon-type optical bistable devices that can perform high-speed, low-incident-energy switching and optical storage operations and can be highly integrated.

(Prior Art) Research on optical bistable elements, which are important elements in future optical communication systems and optical information processing systems, has become active in recent years. Considering the above-mentioned fields of application, optical bistable devices are required to operate at high speed, operate at low energy, facilitate high integration, and match the operating wavelength with other optical devices and electronic devices. Optical bistable devices based on various operating mechanisms have been proposed and fabricated so far, but devices that utilize the large optical nonlinearity of compound semiconductor thin films are small, suitable for high integration, and have low cost. There is a possibility of energy operation, and it is compatible with current optical devices and electronic devices, so it is promising for future quantities.In particular, it is important to understand the nonlinearity of the refractive index change associated with exciton absorption saturation in compound semiconductors. The etalon-type optical bistable device used has excellent features such as low-energy operation and the possibility of forming a two-dimensional array.

FIG. 4 schematically shows the structure of the etalon type optical bistable element. A nonlinear refractive index medium 4010 whose refractive index nonlinearly changes depending on the intensity of incident light is sandwiched between reflective films 204a and 204b on both sides. Due to the nonlinear response of the refractive index to the intensity of incident light by the nonlinear refractive index medium 401 and the positive feedback to the nonlinear response by the reflective films 204a and 204b on both sides, optical bistableness is created between υ incident light and transmitted light. Characteristics arise. GaAs/AtGaAs has excitons stably existing even at room temperature as a nonlinear refractive index medium
H-M-Gib has shown that using superlattices, it is possible to create optically bistable devices that can operate at room temperature.
bg) Rayo Shea Bride Fixtures Letters (A
pPlied Ph7sica Letters) Magazine 1
August 1, 982 issue (Volume 41) pages 221-22
It is reported on page 2. GaAs/AtGa
Etalon-type optical bistable devices using As superlattices are able to reach exciton absorption saturation even at room temperature, take advantage of the large nonlinearity of the refractive index, and can be highly integrated, and their operating wavelengths are similar to current GaAs-based semiconductor lasers. It is a very promising optical bistable device because of its good matching.

(Problems to be Solved by the Invention) However, the above-mentioned GaAs/AtGaAs -r-
Talon-type bistable devices have a problem in that they switch on/off quickly, but switch off slowly. This is the exciton energy relaxation time that determines the switch-on time (the time during which an exciton is formed by incident light, and then the exciton is decomposed into electrons and holes by phonons). is very short (less than picoseconds), but this is because the relaxation time of free carriers generated by exciton decomposition, which determines the switch-off time, is very slow at about 30 nanoseconds in the case of spontaneous recombination.

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

(Means for solving the problem) In order to solve the above-mentioned problem, the optical bistable device provided by the first invention of the present application is made by alternately laminating two types of compound semiconductor thin films with different band gaps. It is characterized by comprising a compound semiconductor superlattice, a reflective film formed on both sides of the compound semiconductor superlattice, and means for applying an electric field between both of these reflective films.

In addition, in order to solve the above-mentioned problems, the optical bistable device provided by the second invention of the present application has two devices having different band gaps.
Reflective films are formed on both sides of a compound semiconductor pin diode (i-layer is a compound semiconductor superlattice made by alternately laminating different types of compound semiconductor thin films), and in the p-layer and n-layer compound semiconductor of the pin diode, It is characterized by the introduction of impurities that form deep levels.

Furthermore, in order to solve the above-mentioned problems, the optical bistable device provided by the third invention of the present application includes a single layer of a compound semiconductor superlattice in which two types of compound semiconductor thin films with different band gaps are alternately laminated. Reflective films are formed on both sides of a compound semiconductor pin diode that forms a compound semiconductor superlattice, and impurities that form a deep level are introduced into the compound semiconductor that has a larger breadth gap among the thin films that form the compound semiconductor superlattice. It is characterized by

(Function) In each invention of the present application, an electric field is applied to the compound semiconductor superlattice from the outside, or an electric field is applied to the compound semiconductor superlattice by using the built-in potential of the pin structure. By tilting the energy band of the superlattice, free carriers accumulated in the well layer of the superlattice are tunneled toward the barrier layer. At this time, the relaxation time of carriers is determined by the tunneling time constant, which is approximately several hundred picoseconds, so the operating speed of the optical bistable element is sufficiently faster than in the case of natural recombination. In the case of an element with a pin structure, if electrons and holes tunnel in opposite directions and accumulate electrons in the p-layer side and holes in the n-layer side, the band slope disappears. ,
In order to prevent this, a layer into which impurities that become recombination centers are introduced is provided.

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

FIG. 1 shows GaAs/AtGaA according to the first invention of the present application.
1(a) shows the device structure, and FIG. 1(b) shows the energy bands. FIG.

In the embodiment shown in FIG.
A tGaAs film 102 and a GaAs/AtGaAs superlattice 103 are laminated, a window is formed in the GaAs substrate 101, and metal reflective films 106a and 10 with a reflectance of about 95% are formed on the window and on the GaAs/AtGaAs superlattice.
6b are formed respectively. This metal reflective film 106
a and 106b have GaAs/AtGaAs on the outside.
An external power source 107 is connected to apply an electric field to the superlattice. With the above structure F) GaAs/AtGaAs
The superlattice 103 is a nonlinear refractive index medium, and the metal reflective film 10
An etalon type optical bistable element is constructed in which 6 a and 106 b serve as reflecting mirrors.

A method for manufacturing the GaAs/AtGaAg nonlinear Kotalon type optical bistable device shown in FIG. 1(a) will be briefly described below. First, a layer of At with a thickness of 1 μm or less is deposited on the GaAs substrate 101 to serve as an etching stop layer for the GaAs substrate.
A GaAs film 102 is formed.

On the AtGaAs film 102, there is a GaAs film with a thickness of about 300λ.
GaAs/At film layered alternately by MBE method etc.
A GaAs superlattice is formed with about 60 periods. Then GaA
s substrate 101 with AtGaAs etching stop layer 1
Etching is performed until 02 is exposed and a window is opened, and finally As/
High reflectance metal reflective film 1 on AtGaAs superlattice 103
06a and 106b, respectively.

The principle by which the optical bistable device having the structure shown in FIG. 1(a)K, which has been successfully obtained above, operates at high speed will be explained.

GaA@/
When an electric field is applied to the AtGaAs superlattice 103, a tilt occurs as shown in FIG. 1(b), and the free carriers generated due to the decomposition of excitons form electrons and holes in the barrier layer in opposite directions. Tunnel to. At this time, the relaxation time of free carriers is determined by the tunneling time constant, and Masumoto et al. have measured the free carrier tunneling time constant in a GaAs/AtGaAs superlattice. Physical review shows that the tunnel time constant is about 430 ps (
PHYSICAL REVIEW) B magazine, April 15, 1986 issue (Volume 33), pages 5961 to 5964. This value is approximately 100 times faster than the free carrier recombination time (about 30n5ec), and when an electric field is applied perpendicular to the layer in the GaAs/AtGaAs superlattice, the device's speed is significantly higher. is possible. The problem in this case is that since the electric field is constantly applied, exciton absorption saturation may be suppressed when light is input and the switch is turned on. The process in which free carriers generated by decomposition of excitons (by phonons) frees the Coulomb force and promotes the decomposition of excitons takes place at sub-picosecond speeds, which is sufficiently short compared to the relaxation time of free carriers. Therefore, exciton absorption saturation occurs even if an electric field is constantly applied.

Therefore, according to the present invention, in which the reflective film is made of a metal film and an external power supply for applying an electric field is added, it is possible to significantly increase the speed of the optical bistable element.

FIG. 2 shows an embodiment of the optical bistable device according to the second invention of the present application. Also in this example, GaAs/juGaAs
By applying an electric field to the superlattice layer, we aim to increase the speed of the optical bistable device. Tilting the bands of a superlattice layer without applying an external electric field.

FIG. 2(a) shows the device structure of the embodiment according to the second invention of the present application. GaAs/AtGaAs superlattice layer 2
03 as one layer, and the upper and lower layers are P-GaAs201 and n
- It has a pin diode structure sandwiched between GaAs202. And the p-GaAs5201 side and the n-
High reflectance reflective film 204a on the GaAs 202 side surface
and 204b are formed, respectively. In this example, since the device has a pin structure, the band of the GaAs/AtGaAs superlattice layer 203 can be tilted by utilizing the building potential as shown in FIG. 2(b). I can do it. Therefore, the relaxation time of free carriers can be significantly reduced using the same principle as in the embodiment of FIG. 1 without applying an external electric field. However, in this case, the electrons and holes transferred by tunneling are
1 and n-GaAs 202, the band slope disappears. Therefore, in this example,
Impurities, such as Cr, form deep levels in the p-GaAs layer 201 and n-GaAs layer 202 and become recombination centers.
etc. will be introduced. By doing this, the p-GaAs layer 2
Since the carriers that have migrated into the 01 and n-GaAs layers 202 by tunneling recombine at high speed, the band slope does not disappear, and the high speed performance of the optical bistable device is not impaired.

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

FIG. 3(a) shows an element structure of an embodiment according to the third invention of the present application. GaAs layer AtGaAs superlattice layer 303
is an i layer, and the upper and lower layers are p-GaAs201 and n-
It has a pin diode structure sandwiched between GaAs 202 layers. and p-GaAs 201 and n-G
Highly reflective reflective films 204a and 204b are formed on the surface of the aAs 202, respectively. In this embodiment, the built-in potential of the p1n structure is utilized to tilt the band of the GaAs/AtGaAs superlattice layer 303 to reduce the relaxation time of free carriers, which is similar to the embodiment shown in FIG. , the method of recombining the free carriers tunneled by the band inclination is different from that of the embodiment shown in FIG. In this example, the third
As shown in Figure (b), a deep level is formed only in the AtGaAs barrier layer of the GaAs layer AtGaAs superlattice 303, and impurities serving as recombination centers are selectively doped. The free-kary clear generated in the pGaAs well layer due to the decomposition of excitons is caused by the built-in potential of the p1n structure.
The bands of the As/AtGaAs superlattice 303 are shown in FIG.
As shown in b'), it is sloping, so make a tunnel and go to A.
It reaches the tGaAs barrier layer. Here, AtGaAs
Since the barrier layer is doped with the impurity that serves as a recombination center as described above, high-speed recombination occurs via the impurity. Therefore, even in this example, the disappearance of the band slope due to carrier accumulation does not occur, and carrier tunneling and recombination center layer (At
By introducing a GaAs barrier layer, high speed devices can be achieved.

(Effects of the Invention) As described above, if each of the inventions of the present application is used, the relaxation time of free carriers can be reduced by tunneling free carriers (by an external electric field or an internal electric field using built-in potential). The optical bistable element can be operated at a significantly higher speed.

Each invention of the present application is not limited to the above-mentioned examples, and in the above-mentioned examples, Ga A is used as a conductor in the compound.
Although only the s-type is mentioned, it goes without saying that the inventions of the present application can also be applied to the InP-type.

[Brief explanation of the drawing]

FIG. 1(a) is a cross-sectional perspective view schematically showing the structure of an embodiment according to the first invention of the present application, and FIG. 1(b) is a main view (a).
A diagram showing the energy bands of the superlattice in an example of
FIG. 2(a) is a cross-sectional perspective view schematically showing the structure of the embodiment according to the second invention of the present application, and FIG. 2(b) shows the energy band of the superlattice in the embodiment of FIG. 2(a). FIG. 3(a) is a cross-sectional perspective view schematically showing the structure of an embodiment according to the third invention of the present application, and FIG. 3(b) is a diagram showing a superlattice structure in the embodiment of FIG. Diagram showing energy bands, 4th
The figure is a perspective view showing a schematic structure of an etalon type optical bistable element. 110...Incoming light, 111...Outgoing light, 103...
・GaAs/AAGaAs superlattice, 203...i
-GaAs/AtGaAa superlattice, 303...i
-GaAa/AtGaAs superlattice (AtGaAs layer doped with impurities), 101=GaAs substrate, 102-
AtGaAs film, 106a. 106b...Metal reflective film, 107...External power supply, 1
08- GaAs layer, 109- AtGaAs film, 20
1.p-GaAs, 202-n-GaAs,
204a, 204b...Reflection film, 401...Nonlinear refractive index medium. Agent Patent Attorney Shinsuke Honjo 110 Incident light 111 and 1 point (a) i'81 Figure 110λ taken into account 111 tree 1 light (a) (b) Figure 2 110λ fence and 111 output (a) p i n Itomo τbo level (b) 110λJJ:'r light

Claims (3)

[Claims] (1) A compound semiconductor superlattice in which two types of compound semiconductor thin films with different band gaps are alternately laminated, a reflective film formed on both sides of this compound semiconductor superlattice, and an electric field applied between both reflective films. An optical bistable element comprising means. (2) Reflective films are formed on both sides of a compound semiconductor pin diode whose i-layer is a compound semiconductor superlattice in which two types of compound semiconductor thin films with different band gaps are alternately laminated, and the p-layer and An optical bistable device characterized in that an impurity that forms a deep level is introduced into an n-layer compound semiconductor. (3) Reflective films are formed on both sides of a compound semiconductor pin diode whose i-layer is a compound semiconductor superlattice in which two types of compound semiconductor thin films with different band gaps are alternately laminated to form the compound semiconductor superlattice. An optical bistable device characterized in that an impurity that forms a deep level is introduced into a compound semiconductor with a larger band gap in a thin film.