JPH0714078B2 - Semiconductor optical memory - Google Patents

Description

The present invention relates to a semiconductor optical memory required for image processing, an optical computer, and the like.

(Prior art and its problems) The semiconductor optical memory, which has a function to oscillate a laser beam with a slight amount of trigger light and continue to oscillate even after the trigger light is exhausted, is designed for future optical switching and parallel optical information processing systems. It is an indispensable key device for configuration. A bistable semiconductor laser is known as a device having such a function.
Details have been reported. The problem with the bistable semiconductor laser is that it requires a trigger light of a relatively strong intensity of about several hundred μm. Bistable semiconductor laser with trigger light
In order to turn it on, it is necessary to irradiate the laser end face with light through a lens and efficiently narrow it down to the active layer in the non-excited state. The width and thickness of the active layer are each about 1 μm,
It was as narrow as 0.1 μm, and efficient optical coupling was difficult. Therefore, the coupling loss becomes large, and a relatively high trigger strength is required.

FIG. 9 is a sectional view of a conventional example of a semiconductor optical memory described in Journal of Applied Physics, Vol. 59, pages 596-600, 1986.

This conventional example has a pnpn thyristor structure. The anode region 93 and the cathode region 95 are p-AlGaAs and n-, respectively.
It is made of AlGaAs and has a structure sandwiching the n-type base layer 4a formed of n-GaAs having a narrow band gap. When the thyristor is turned on and shifts from the high impedance state to the low impedance state, carriers are injected into the n-type base layer 4a and are confined in this portion, resulting in spontaneous emission light. However, unlike the bistable laser, a high optical output cannot be obtained.

(Problems to be Solved by the Invention) The problems in the conventional example are summarized as follows.
That is, in the bistable semiconductor laser, stimulated emission light is obtained as output light, but there is a problem that optical coupling of trigger light is difficult and coupling loss becomes large. Further, in the pnpn thyristor, since the output light is spontaneous emission light, there is a problem that a high optical output cannot be obtained unlike the bistable semiconductor laser.

(Means for Solving Problems) In order to solve the above problems, a semiconductor optical memory having a pnpn structure provided by the present invention is a base n-type semiconductor in which a first semiconductor layer, a second semiconductor layer and The third semiconductor layer is laminated in order, and the forbidden band width of the p-type semiconductor for anode and the n-type semiconductor for cathode is larger than the forbidden band width of either of the first and third semiconductor layers, The forbidden band width of the semiconductor layer is narrower than the forbidden band widths of the first and third semiconductor layers.

By the semiconductor optical memory described above, for the base, by causing the n-type semiconductor to absorb light, a forward conduction state, that is, an ON state is caused, and electrons and holes are relaxed in the second semiconductor layer, It is possible to cause laser oscillation by the stimulated emission process of the electrons and holes.

(Operation) FIGS. 1 and 2 are band diagrams showing the principle of the present invention, and FIG. 3 is an operation diagram.

FIG. 1A is a band diagram in which no bias voltage is applied, FIG. 1B is a high impedance state, and FIG. 1C is an ON state band diagram. For simplification, the band discontinuity at the heterojunction and the like, which are not related to the essence of the present invention, are qualitatively approximated. FIG. 1 (a) shows the carrier concentration and the forbidden band energy of each layer. The band gap of p-type semiconductor for anode and n-type semiconductor for cathode is E ₄ and
This is indicated by E ₁ (E ₄ = E _{1 in the} figure). The n-type semiconductor layer for the base is composed of two semiconductors having _a forbidden band width E _g of E ₃ and E _a . Of these, the layer with E _g = E _a becomes the active layer for laser operation. The size of the forbidden band should be E ₄ , E ₁ > E ₃ > E _a . Carrier concentration of the n-type semiconductor layer for the base (n ₂
And n _a ) should be kept low to some extent. Positive to the anode,
When a negative voltage is applied to the cathode, a high impedance state in which almost no current flows is initially obtained (FIG. 1 (a)). Since the n-type semiconductor layer for the base has a low concentration, the applied voltage is almost applied to the pn junction in the base region,
The depletion layer extends into the base n-type semiconductor layer. Holes in the p-type semiconductor for the anode are pn between the base and the anode.
The potential barrier occurring at the junction cannot be exceeded, which results in a high impedance state in which almost no current flows. When the applied voltage is further increased and the breakover voltage (V _BO ) shown in Fig. 3 (a) is exceeded, a current suddenly starts to flow and the device is turned on (Fig. 1 (c). The operation mechanism is the same as that of a normal thyristor.In the ON state, the voltage applied to this element is substantially the same as that of one pn junction.The semiconductor layer having a narrow bandgap in the n-type semiconductor for the base. If (E _g = E _a ) is provided, some electrons and holes will fall into the pit of this potential.If a reflector is placed outside, laser oscillation can be obtained when the gain exceeds the loss. . N-type semiconductor for cathode and p for anode
If the band gap of the n-type semiconductor is made larger than that of the n-type semiconductor for the base, the number of electrons flowing out to the anode and holes flowing out to the cathode in the ON state can be reduced, and carriers in the active layer can be reduced. It becomes easy to relieve. Therefore, the luminous efficiency can be improved. A voltage is set in advance at a point of V = V _X shown in FIG. 3 (a), light of an appropriate amount is made incident, and this is absorbed by the base n-type semiconductor. Third
In FIG. 5A, V _H is the minimum voltage that is barely held in the ON state, which is the holding voltage. Then, holes will be injected into the p-type semiconductor for the base. The injected holes increase the number of electrons passing through this layer due to the transistor effect. These electrons moderate the slope of the potential generated in the base n-type semiconductor. Then, holes injected from the anode to the base increase. This positive feedback effect can shift this device from the high impedance state to the ON state. That is, as shown in FIG. 3 (b),
This element can be laser-oscillated by the trigger light.
The V _X can be enhanced trigger sensitivity as close to the V _BO. If you don't bother to provide _a semiconductor layer with E _g = E _a and make the n-type semiconductor layer for base one layer from the beginning, and make it as thin as the active layer of a normal semiconductor laser (0.1 μm), the forbidden band width will be reduced. It may be considered good, but it is useless. This is because the absorption of the trigger light is small at a thickness of about 0.1 μm, so that the trigger sensitivity is lowered.

FIG. 2 is a band diagram of the improved element performed to enhance the trigger sensitivity. The difference from FIG. 1 is that the layer thickness of the base p-type semiconductor is thin. Make this layer thinner,
For example, it is made thin enough to be depleted when the voltage applied from the outside is zero. Then, the photocurrent gain of the n (cathode) -p (base) -n (base) transistor is further increased,
It is convenient to increase the trigger sensitivity.

Journal of Applied Physics (J.App
1 (Phys. 59 (2), pp. 596-600, 1986), an optical thyristor in which the layer thickness of the p-type base semiconductor is thin as shown in FIG. 2 is reported. According to this paper, it is possible to turn on and pass a current of 100 mA or more with a trigger light of a few μm, and if such a current can efficiently flow into the active layer, sufficient laser oscillation is possible.

FIG. 4 is a design diagram for determining parameters such as carrier concentration and layer thickness of the base region of a normal pnpn thyristor used as an electrical switch. By using FIG. 4, a general guideline of what kind of design should be made in the present invention can be obtained.
The transition from the high impedance state of the thyristor to the low impedance state is the punch-through voltage of the n-type base region.
Determined by V _PT and avalanche breakdown voltage V _B1 . When the n-type base region is made of a single semiconductor layer, the on-voltage is determined by using the step junction approximation in punch-through control. In the case of avalanche yield limitation, V _BO = V _B (1-α ₁ −α ₂ ) ^{1 / n} …… (3) V _B 60 (E _g /1.1) ^3/2 (N _D / 10 ¹⁶ ) ^{-3 / 4} …… (4) can be written. In equations (2) to (4), N _D is the carrier concentration of the base layer, d is the base layer thickness, ε _S is the dielectric constant, α ₁ and α ₂ are the current gains of the npn and pnp transistors, and n is a constant, E _g
Is the forbidden band energy. In Eq. (4), the units of E _g and N _D are eV and cm ^-3 , respectively. FIG. 4 shows the relationship between the on-voltage and the carrier concentration when the base is GaAs. In the present invention, the ON voltage is slightly shifted to the high voltage side from the value shown in FIG.

(Embodiment) Embodiment 1 FIG. 5 is a perspective view showing an embodiment of the present invention. AlGaAs
An optical memory for the 0.8 μm band using / GaAs-based semiconductors.
On the n-GaAs substrate 41, buffer n-GaAs (thickness d = 2 μ
m, n = 2 × 10 ¹⁸ cm ⁻³ ) and then the cathode side, n-Al _0.4 Ga _0.6 As (d = 2 μm, n = 5 × 10 ¹⁷ cm ⁻³ ) 4
2, p-Al _0.25 Ga _0.75 As (d
= 50Å, n = 1 × 10 ¹⁹ cm ^-3 ) 43, undoped n-Al _0.25 Ga _0.75 As (d = 0.3 μm, which is an n-type semiconductor for the base)
n = 1 × 10 ¹⁵ cm ⁻³ ) 44, undoped n-GaAs (d = 0.
1 μm, n = 1 × 10 ¹⁵ cm ^-3 ) 45, n-Al _0.25 Ga _0.75 As
(D = 0.8 μm, n = 1 × 10 ¹⁷ cm ⁻³ ) 46 is grown, and p-Al _0.4 Ga _0.6 As (d = 1 μm, n =
5 × 10 ¹⁸ cm ^-3 ) 47 and p-GsAs (d = 0.2) for the cap layer
μm, n = 2 × 10 ¹⁹ cm ⁻³ ) 48. Layers 44, 4
Reference numerals 5 and 46 correspond to n ₂ , n _a and n ₂ in FIG. 1 (a), respectively. The n-GaAs 45 serves as an active layer. The p-type dope is Be,
The n-type doping was performed with Si. The growth was carried out by the molecular beam epitaxy (MBE) method. The band diagram is almost the same as in FIG. 1. Layers 44, 45 and 46 correspond to n ₂ , n _a and n ₂ in FIG. 1 (a), respectively. n-GaAs 45 is the active layer n-Al _0.25 Ga _{0.75 As} 44 and 46
Is the main light absorbing layer. The layers 44 and 45 are low-concentration layers in order to prevent photo carriers generated in the light absorption layer from being trapped by n-GaAs45 which is an active layer having a narrow band gap when the trigger light is irradiated. is there. That is, when the layers 44 and 45 are made to have a low concentration, the layers 44 and 45 can be depleted when an appropriate bias voltage is applied in the OFF state. When depleted, photocarriers pass through layer 45, ie, the active layer, because electrolysis is present. Then, the electrons can be accumulated in n-Al _0.25 Ga _{0.75 As} 46, and the holes can be accumulated in p-Al _0.25 Ga _{0.75 As} 43, and the OFF state can be turned on by the trigger light. If the concentration of the layers 44 and 45 is not lowered, the breakover voltage becomes very high, which makes it very difficult to use. In this embodiment, the breakover voltage, that is, the voltage at which the OFF-to-ON state shifts is approximately 4V.
The trigger light sensitivity could be lowered to 1 _pJ . As shown in FIG. 5, etching is performed with a width of 1.5 μm, and the upper portion is formed into a stripe shape. Both sides thereof are covered with polyimide 51, and the transverse mode is controlled by reducing the difference in refractive index between the active layer and both sides thereof. The polyimide coat also serves as passivation. A cleavage plane forms a resonance surface. The resonator length is 100 μm. Electrode 4
AuZn / Cr / Au and AuGe-Ni / Cr / Au were used for 9 and 50. By adjusting the bias voltage, laser oscillation was caused by trigger light of several tens of μm, and an output of several tens of mW could be obtained. The external differential quantum efficiency at that time was 23% on one side. As shown in FIG. 6, the oscillation wavelength was λ870 μm, and stimulated emission light corresponding to the band gap of GaAs was obtained. In addition, before and after oscillation, n-Al _0.25 Ga _0.75 As
No light emission was observed at the wavelengths corresponding to the band gaps of 44 and 46, and it was found that the injected carriers were effectively confined in n-GaAs 45 in the ON state.

Embodiment 2 FIG. 7 is a perspective view showing a second embodiment of the present invention. InP
It is an optical memory for the 1 μm band using a system semiconductor. n-In
N-InP on the cathode side of the P substrate 71 (thickness d = 2 μm,
n = 2 × 10 ¹⁸ cm ⁻³ ) 72, p− which is a p-type semiconductor for base
InGaAsP (λ _g = 1.3 μm, d = 30 A, n = 2 × 10 ¹⁸ cm ^-3 )
73, undoped n-InGaAs as n-type semiconductor for base
P (λ _g = 1.3 μm, d = 10 ¹⁵ cm ^-3 ) 74, undoped n
-In _0.53 Ga _0.47 As (λ _g = 1.55 μm, d = 0.1 μm, n =
5 × 10 ¹⁵ cm ⁻³ ) 75, undoped n-InGaAsP (λ _g = 1.
3 μm, d = 0.3 μm, n = 5 × 10 ¹⁵ cm ⁻³ ) 76 was grown, and p-InP (d = 0.5 μm, n = 2) that became an anode was grown.
× 10 ¹⁸ cm ^-3 ) 77 and p-InGaAsP for the cap layer (λ _g = 1.
15 μm, d = 0.5 μm, n = 0.5 μm, n = 2 × 10 ¹⁹ c
m ^-3 ) 78 and grow. Layers 74, 75 and 76 are shown in FIG. 1 (a).
N ₂ , n _a , and n ₂ , respectively. The n-InGaAs75 becomes the active layer.

Zn and S were used as p-type and n-type dopants, respectively.

As shown in FIG. 7, etching is performed with a width of 1.5 μm so that the upper portion has a stripe shape. Polyimide on both sides
At 81, the transverse mode is controlled by reducing the difference in refractive index between the active layer and both sides of the active layer. The polyimide coat also serves as passivation. A cleavage plane forms a resonance surface. The resonator length is 100 μm. By adjusting the bias voltage, laser oscillation was caused by a trigger light of several tens of μW, and an output of several mW could be obtained.

Example 3 An optical memory having the same structure as that of FIG. 7 described in Example 2 was manufactured by partially changing the thickness of the semiconductor layer and the carrier concentration. That is, the thickness of the base p-type semiconductor p-InGaAs73 is 500Å, the thickness of the base n-type semiconductor n-InGaAsP74 is 0.3 μm, and the thickness of the n-InGaAsP76 is 0.8 μm.
The electron concentration (n) was grown at 1 × 10 ¹⁷ cm ^-3 . All other conditions were the same as described in Example 2. When the produced optical memory was oscillated with a trigger light of several tens of μW, an output of several mW was obtained.

Embodiment 4 FIG. 8 is an application example of the present invention. If a 45 ° mirror 87 is formed on the outside of the semiconductor optical memory 86 according to the present invention by etching and arranged near the resonance surface, light can be extracted in the layer thickness direction. It is useful for application to parallel optical information processing.

In the embodiment, an example in which an optical memory is manufactured using AlGaAs / GaAs and InGaAsP / InP materials is shown, but the present invention is composed of other materials such as AlGaAs / GaAs and InGaAsP / InP. It can also be applied to such mixed crystal materials with intentionally introduced mismatch and visible light materials such as GaP.

(Effects of the Invention) As described above, according to the present invention, since the trigger light is incident on and absorbed from the absorption layer in the direction perpendicular to the absorption layer, an optical memory with high sensitivity to trigger light is obtained and optical coupling is facilitated. .
Further, as a result, stimulated emission can be generated, so that a semiconductor optical memory with high light output can be realized.

[Brief description of drawings]

1 and 2 are band diagrams showing the principle of the present invention, FIG. 3 is an operation diagram, FIG. 4 is a design diagram, FIG. 5 is a perspective view of an embodiment relating to the present invention, and FIG. FIG. 6 is a perspective view of a second embodiment of the invention, FIG. 6 is a spectrum diagram of the oscillation wavelength of the optical memory of the embodiment, FIG. 8 is an application example, and FIG. 9 is a sectional view of a conventional example. 41 is an n-GaAs substrate, 42 is n-Al _0.4 Ga _0.6 As, 43 is p-Al
_0.25 Ga _0.75 As, 44, 46 is n-Al _0.25 Ga _0.75 As, 45 is n-
GaAs, 47 is p-Al _0.4 Ga _0.6 As, 48 is p-GaAs, 49, 50,
79 and 80 are electrodes, 51 and 81 are polyimide, 71 is n-InP substrate, 72 is n-InP, 73 and 78 are p-InGaAsP, and 74 and 76 are n-
InGaAsP, 75 n-InGaAs, 77 p-InP, 85 semiconductor substrate, 86 semiconductor optical memory, 87 45 ° mirror, 91 semiconductor substrate, 92 semiconductor layer, 92 buffer layer, 93 anode region, 95 cathode region, 96 is a cap layer, 97 is a cathode electrode, 98 is an anode electrode, 94a is an n-type base layer, and 94b is a p-type base layer.

Claims (1)

[Claims] 1. In a semiconductor optical memory having a pnpn structure, an n-type semiconductor for a base is formed by sequentially stacking a first semiconductor layer, a second half layer and a third semiconductor layer, and is a p-type for an anode. The forbidden band widths of the semiconductor and the n-type semiconductor for cathode are larger than the forbidden band widths of the first and third semiconductor layers, and the forbidden band width of the second semiconductor layer is the first and third semiconductor layers. A semiconductor optical memory characterized by being narrower than the forbidden band width of.