JPH0545003B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical modulator capable of high-speed modulation and capable of obtaining a high extinction ratio with a low driving voltage.

(Prior Art and its Problems) When rapidly changing the output intensity and phase of a semiconductor laser used as a light source in optical communications and the like, there are broadly two types of methods. There are two methods: one is to directly change the current that drives the semiconductor laser, and the other is to modulate the optical output from the light source by passing it through an optical modulator, which is a passive element. Both have their own advantages and disadvantages. The former does not use an optical modulator, so there is no insertion loss caused by the optical modulator. However, during high-speed modulation of several hundred megahertz or higher, distortion of the modulation waveform due to the gentle vibration of the carrier in the semiconductor laser, and temporal changes in the oscillation wavelength ( chirping) occurs, making it difficult to detect the signal light. Further, this modulation speed is limited by the carrier life, and direct modulation of 4 gigabits per second or more is theoretically difficult.

On the other hand, the latter allows high-speed modulation of about 10 gigabits per second and has little chirping even during high-speed modulation, but ordinary optical modulators have large insertion loss, which is particularly disadvantageous for long-distance transmission. Furthermore, in order to obtain modulation with a high extinction ratio, it is necessary to drive at a high voltage.

Therefore, an optical modulator of the latter type using a multilayer thin film semiconductor has been proposed, which is capable of high-speed modulation with low loss. One example is the Japanese Journal of Applied Physics, by Mr. Yamanishi et al.
As described in Volume 22, L22, 1983, by applying an electric field to a multilayer thin film semiconductor, the absorption edge is shifted to longer wavelengths, but this simultaneously shifts electrons and holes spatially. This has the disadvantage that the probability of absorption is small. Furthermore, the driving voltage required to obtain modulation with a high extinction ratio is still relatively high in practice.

(Means for Solving the Problems) The optical modulator according to the present invention has one layer or multiple layers.
In an optical modulator that has a semiconductor thin film whose film thickness is equal to or less than the mean free path of electrons and has means for applying an electric field to the semiconductor thin film in the stacking direction, the narrow forbidden band width in the semiconductor thin film is It is characterized in that the forbidden band width has a maximum value near the center in the stacking direction and decreases as it approaches both ends.

(Operation/Principle of the Invention) The operation/principle of the present invention will be explained below with reference to the drawings. First, the band structure of the semiconductor thin film structure of the optical modulator according to the present invention is schematically shown in FIG. 1a when no electric field is applied, and in FIG. 1b when an electric field is applied in the stacking direction. . Here, the electron wave function 11 and the hole wave function 12
Consider deformation due to electric field. In the case of no electric field, that is, in the case shown in Figure 1a, both wave functions exist near the heterointerface on both sides of the narrow band gap layer (hereinafter referred to as quantum well layer), and have almost symmetrical shapes. I can see that you are doing this. Therefore, the value of the overlap integral of the two wave functions is approximately 1. However, when the band structure in the quantum well layer is deformed by the application of an electric field, the wave function is very localized near the hetero-interface where electrons and holes are on opposite sides, as shown in Figure 1b, according to the structure according to the present invention. Therefore, the value of the overlap integral of the two wave functions becomes a value close to 0 because they overlap only in the region where the wave functions decrease exponentially. The value of this multiline integral is approximately proportional to the absorption coefficient, and the absorption edge energy 15 determined by the difference between the electron energy level 13 and the hole energy level 14 by applying an electric field (Fig. ) decreases, so
The change in the absorption coefficient spectrum due to the application of this electric field is as shown in FIG. 1c. The solid line is the absorption coefficient spectrum 16 when no electric field is applied, and the broken line is the absorption coefficient spectrum 17 when an electric field is applied.

Therefore, if we consider light with energy 18 that is slightly larger than the absorption edge in the absence of an electric field, the absorption coefficient is large in the absence of an electric field, so it is greatly absorbed by this quantum well layer, but when an electric field is applied, At times, the absorption coefficient takes a value close to 0, so it can be seen that almost no absorption occurs in this quantum well layer. Therefore, it can be seen that the intensity modulation of the light having energy 18 can be performed with a high extinction ratio by turning on and off the application of the electric field.

In addition, the size of the electric field required at this time can be extremely small with the structure according to the present invention, and since the essence of high-speed modulation is to change the shape of the wave function by the electric field, in principle it can reach several tens of GHz or more. modulation is possible.

(Embodiment) FIG. 2a shows a perspective view of an optical modulator according to the first embodiment of the present invention, and FIG. 2b shows its band diagram. This example was manufactured using the molecular beam epitaxy (MBE) method. This is first a Si-doped n-type
Si-doped n-type with a thickness of 1.0 μm on a GaAs substrate 21
A GaAs doped layer 22 and a Si-doped Al _0.4 Ga _0.6 As cladding layer 23 having a thickness of 2.0 μm were laminated. next
_{Non -} doped Al
Alternating thin-doped Al _0.4 Ga _0.6 As barrier layers 25
A thin film structure was formed by laminating 30 cycles. Thickness on top of this
2.0μm Be-doped p-type Al _0.4 Ga _0.6 As cladding layer 2
6. A multilayer structure was fabricated by growing a Be-doped P-type GaAs contact layer 27 with a thickness of 0.5 μm. Next, make this into a size of about 5 x 5 mm, and after making electrodes 28 on the top and bottom surfaces, form a circular shape on the top and bottom surfaces.
Even the GaAs layer was selectively removed by etching.

Light enters this circular "window" in the vertical direction,
When voltage was applied between the electrodes and the voltage dependence of the absorption coefficient spectrum was investigated, a tendency as shown in Figure 1c clearly appeared. And light with energy above the absorption edge in the absence of an electric field (wavelength approximately 800n)
The transmittance of m) is approximately 3% when no electric field is applied, and approximately 80% when a reverse bias voltage of 5V is applied, which is approximately the extinction ratio.
A very good value of 14dB was obtained. As for high-speed modulation characteristics, good intensity modulation was achieved up to approximately 300MHz. This upper limit is due to the parasitic capacitance between the electrodes.

Next, a second embodiment of the present invention will be described. FIG. 3 shows a perspective view of this embodiment. In other words, the thin film structure is the same as the first embodiment except that the stacking period of the quantum well layer and the barrier layer is 8th anniversary. Next, electrodes 31 were fabricated on the top and bottom surfaces of the substrate, and a SiO ₂ film was deposited on the top surface of the substrate using the CVD method. After that, the SiO ₂ film was deposited in stripes of 1.5 μm width using normal photolithography. remove the part of
After that, the part where the SiO ₂ film is not attached is made into an n-type
After removing the Al _0.4 Ga _0.6 As cladding layer 23 by etching, the remaining SiO ₂ is removed to form a conductive path structure.

When this conductive path length was set to 200 μm and a laser beam with a wavelength of 800 nm was incident, the transmittance was measured by applying an electric field. When no electric field was applied, the transmittance was approximately 0.5%, and when a reverse bias voltage of IV was applied, it was approximately 40%, which is equivalent to the extinction ratio. Approximately 20dB
A very good value was obtained. As for high-speed modulation characteristics, good intensity modulation characteristics were obtained up to approximately 3 GHz or higher. Moreover, this was determined by the parasitic capacitance of the element.

Two examples have been described above, but
The present invention is characterized in that the forbidden band width of the quantum well layer initially widens in the stacking direction and narrows midway through, and the method of this change, the position of the peak of the change,
It is clear that there are no limitations to the material system, semiconductor growth method, etc. The manner in which the forbidden band width changes may be spatially quadratic as shown in Figure 4a, or may vary in a stepwise manner as shown in Figure 4b, but the essential effect is the same. It is.

(Effects of the Invention) The optical modulator according to the present invention is characterized in that it can obtain a high extinction ratio with a low voltage and can theoretically perform intensity modulation at several tens of GHz.

[Brief explanation of drawings]

Figures 1a and b are the band diagrams of the multilayer thin film structure of the optical modulator according to the present invention when no electric field is applied and when an electric field is applied, respectively, and Figure 1c is the absorption coefficient in these two cases. It is a figure showing a spectrum. Figures 2a and 2b are a perspective view and a band diagram of the first embodiment, respectively. FIG. 3 is a perspective view of the second embodiment. FIGS. 4a and 4b are band diagrams showing modified examples of the band structure within the quantum well. In the figure, 11...electron wave function, 12...
Wave function of a hole, 13...Energy level of an electron, 14...Energy level of a hole, 15...Energy of absorption edge, 16...Absorption coefficient spectrum when no electric field is applied, 17...When an electric field is applied Absorption coefficient spectrum of 18... energy slightly higher than the absorption edge, 21... n-type GaAs substrate, 22... n-type
GaAs buffer layer, 23...n-type Al _0.4 Ga _0.6 As
Clad layer, 24...Non-doped AlxGa _1-x As
(x; 0→0.15→0) quantum well layer, 25...non-doped Al _0.4 Ga _0.6 As barrier layer, 26... p-type Al _0.4
Ga _0.6 As cladding layer, 27... p-type GaAs contact layer, 28... electrode, 31... electrode.

Claims (1)

[Claims] 1. A semiconductor thin film structure comprising one or more laminated semiconductor layers having a film thickness equal to or less than the mean free path of an electron, and means for applying an electric field to the semiconductor thin film structure in the stacking direction, and further comprising the semiconductor thin film structure. 1. An optical modulator characterized in that the forbidden band width of a semiconductor layer having a narrow forbidden band width among the semiconductor layers having a narrow band gap takes a maximum value near the center in the stacking direction and decreases as it approaches both ends.