JP2500529B2 - Flat optical semiconductor device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface type optical semiconductor device such as a cylindrical vertical cavity surface emitting semiconductor laser used for optical transmission or optical information processing.

[0002]

2. Description of the Related Art A surface type optical semiconductor device such as a surface emitting semiconductor laser which oscillates in a direction perpendicular to a semiconductor substrate is a key device indispensable for data transmission between computers and optical computing. The surface emitting semiconductor laser is a conventional semiconductor laser that horizontally oscillates on the substrate and has an end face of 45 °.
Although there is one that forms a mirror and bends the oscillation light in the vertical direction to emit the light, the surface emitting semiconductor laser mentioned here is a laser that oscillates by oscillating light back and forth in a direction that is really perpendicular to the substrate.

As such a conventional surface emitting semiconductor laser, for example, Electronic Letters (Electron.
tt.), Vol. 25, No. 20, 1377-137, 1989.
The contents are detailed on page 8. This surface emitting semiconductor laser has a cylindrical vertical cavity in which an active layer is sandwiched by upper and lower semiconductor multilayer films. FIG. 3 shows a cross-sectional structure of a conventional surface emitting semiconductor laser having such a cylindrical vertical cavity. In the figure, 100 Å thick InGaAs (λ≈980 n
m) A p-type semiconductor multilayer film 325 and an n-type semiconductor multilayer film 327 are formed above and below the active layer 326 composed of a strained quantum well, and these serve as a reflection mirror for laser oscillation to cause laser oscillation. In FIG. 3, light is emitted downward through the n-type semiconductor substrate 328 made of GaAs. The p-type semiconductor multilayer film 325 has a thickness of λ / 4 (where λ is the wavelength in the medium) p-A
1As 317, 315, 313, ... And p-GaAs 316 and 314 also having a thickness of λ / 4 are alternately laminated in a pair of 15.5. n-type semiconductor multilayer film 3
27 is n-AlAs 318, 32 of λ / 4 thickness
0,322 and n-GaAs3 also having a thickness of λ / 4
19, 321 are formed by alternately stacking 24.5 pairs. 324 is a p-type electrode made of CrlAu,
We do not usually alloy to make contacts.
The reason is that the number of layers of the p-type semiconductor multilayer film can be reduced by depositing Au having a reflectance of nearly 98% on the p-type semiconductor multilayer film 325. In a vertical cavity surface emitting semiconductor laser having a short cavity length, it is necessary to increase the reflectance of the upper and lower reflecting mirrors to near 99.9%, or the threshold current will increase. If an attempt is made to form a mirror with high reflectance using only a semiconductor multi-layer film, the overall layer thickness becomes thicker, and a step is formed vertically, making the process difficult. If non-alloy Au is attached to the upper portion as shown in FIG. 3, the number of layers of the p-type semiconductor multilayer film 325 can be reduced, and the overall height can be suppressed to a low level. 330
Is a semiconductor layer made of p-GaAs for phase compensation.

[0004]

The problem with the conventional example of FIG. 3 is that the absorption of light in the semiconductor multilayer film acting as a reflecting mirror affects the oscillation threshold value and raises the threshold value. there were. In FIG. 3, the wavelength of the laser light is 980 nm,
Since the active layer 326, the surrounding clad layer and the semiconductor multilayer film are made of a material having a bandgap wavelength larger than that of the active layer 326, the light absorption there seems not to be a problem, but it actually works. The cause of light absorption there is mainly due to free carrier absorption. Free carrier absorption α _fc (cm ⁻¹ ) is expressed by α _fc ≈3 × 10 ⁻¹⁸ n + 7 × 10 ⁻¹⁸ p (1). In the equation (1), n and p are electron and hole densities expressed in cm ⁻³ unit. In FIG. 3, the n-type semiconductor multilayer film 327 is 24.5 pairs, and the p-type semiconductor multilayer film 3 is
25 are formed in 15.5 pairs and have thicknesses of about 3.69 μm and 2.26 μm, respectively. The carrier concentration is AlAs, G in the p-type semiconductor multilayer film 325.
Both aAs are 2 × 10 ¹⁸ cm ⁻³ . The n-type semiconductor multilayer film 327 is also 2 × 10 ¹⁸ cm ⁻³ , but
The carrier density that is actually activated and effectively works in the layer is below that. The laser oscillation threshold Ith is determined by the sum of optical absorption loss and mirror loss, and the differential quantum efficiency η _D
Is determined by their ratio. In FIG. 3, the optical absorption loss and the mirror loss are designed to have the same value, but η
This is necessary in order to obtain a practically necessary value of _D of several 10%.

Therefore, if the light absorption loss in the semiconductor multilayer film can be reduced, the reflectance can be increased (that is, the number of layers can be increased), whereby Ith can be lowered while maintaining η _D. However, in the semiconductor multi-layered film, when the number of layers is increased to increase the reflectance, there is a problem that the light absorption loss there increases in proportion to this.

[0006]

A surface-type optical semiconductor device of the present invention has a semiconductor multilayer film reflecting mirror formed on a semiconductor substrate in a lower portion and a second conductivity type multilayer film reflecting mirror in an upper portion. Then
In a surface-type optical semiconductor device having a semiconductor active layer for laser oscillation between the reflecting mirrors, the side closer to the semiconductor active layer for laser oscillation in the semiconductor multilayer film reflecting mirror is the second
It is characterized in that it is of a first conductivity type opposite to the conductivity type, and the rest is non-doped, and an electrode for conduction is formed in a portion of the semiconductor multilayer film of the first conductivity type.

[0007]

Function: The electric current is made to flow without penetrating the entire semiconductor multilayer film. According to the present invention, the portion that becomes the current path is doped to have the first conductivity type, but the rest can be undoped. As a result, light absorption in the semiconductor multilayer film with respect to light traveling up and down can be reduced, and a lower threshold value can be achieved.

[0008]

1 is a sectional view showing a first embodiment of the present invention. Reference numeral 126 is an active layer made of InGaAs (λ = 0.98 μm), which is a 15.5 pair p-type semiconductor multilayer film 125 made of p-AlAs 113 and p-GaAs 114 having a thickness of λ / 4, and a thickness of λ / 4. n-AlAs 117 and n-
It consists of repeating 2 pairs of GaAs118, and further below it, non-doped AlAs119 and non-doped GaAs1.
It is sandwiched by an n-type semiconductor multilayer film 127 which is formed by repeating 20 22.5 pairs. p-AlAs113 and p-
GaAs 114 is Be-doped and the doping concentration is 2 × 10 ^18.
cm ^-3 . n-AlAs 117 and n-GaAs 118
Is Si-doped, and the doping concentration is 2 × 10 ¹⁸ cm ⁻³ .
The non-doped region 116 is p- and the carrier concentration is 1 × 10.
^{It is 15} cm ^-3 . The semiconductor layer is epitaxially grown by the MBE method, but the non-doped layer becomes p-. 124 is C
A p-type electrode made of r / Au, and p-GaAs 130 is a semiconductor layer inserted for phase compensation. The n-type electrodes 123 and 129 are made of AuGeNi / Au, and in FIG. 1, two n-type electrodes 123 and 129 are drawn on both sides of the mesa.
mφ) shape. In addition, the p-type semiconductor multilayer film 125
The distance between the n-type semiconductor multilayer film 127 and the n-type semiconductor multilayer film 127 needs to be an integral multiple of the in-medium wavelength λ. Etching was stopped on the surface of n-GaAs 118 as shown in FIG. 1 using an etching solution based on HCl and H ₂ O ₂ with a mesa diameter of 30 μmφ. AlAs and GaAs can be selected for etching, and ALAs is etched much faster than GaAs. If the change in the color of the wafer is observed, it is very easy to stop the etching at the place as shown in FIG. In the embodiment shown in FIG. 1, the current flows from the p-type electrode 124 to n through the n-AlAs 117 and n-GaAs 118.
It flows to the mold electrodes 123 and 129. On the other hand, light sees the entire n-type semiconductor multilayer film 127, but 22.5 pairs out of 24.5 pairs are non-doped regions 116, so the light absorption loss is greatly reduced. It has become possible to reduce the threshold current.

FIG. 2 is a sectional view showing a second embodiment of the present invention. It is an element called vertical resonator type VSTEP, and p
^{The n pn} structure is formed in the vertical direction, and two stable states, an OFF state with high impedance and an ON state with low impedance, appear in the current-voltage characteristics. This element is switched by light incident from below, and when turned from OFF to ON, laser oscillation occurs and light is emitted downward. An active layer 205 made of In _0.2 Ga _0.8 As is formed inside in order to cause laser oscillation. The p-type semiconductor multilayer film 208 and the n-type semiconductor multilayer film 202 act as a reflecting mirror for forming a resonator in the vertical direction. The p-type semiconductor multilayer film 208 is 15.5 pairs, and the n-type semiconductor multilayer film 202 is
24.5 pairs are formed. n-type semiconductor multilayer film 20
Of the two, two pairs are n-doped regions 220 (ST, 2 × 10
¹⁸ cm ⁻³ ), and the remaining lower 22.5 pairs are undoped regions 221. The diameter of the mesa including the InGaAs active layer 205 is 30 μmφ and the p-type electrode 21
5 is Cr / Au, and the n-type electrodes 216 and 217 are A
It consists of uGeNi / Au. The n-type electrodes 216 and 217 are actually ring-shaped as in the case of FIG. 1 and have an inner diameter of 40 μmφ, which makes it possible to greatly reduce the oscillation threshold current in the ON state.

[0010]

As described above, according to the present invention, it is possible to reduce the optical absorption loss in the semiconductor multilayer film and reduce the threshold current of laser oscillation in the surface direction. Can be realized.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of the present invention.

FIG. 2 is a sectional view showing another embodiment of the present invention.

FIG. 3 is a cross-sectional view showing a conventional surface-type optical semiconductor device.

[Explanation of symbols]

128,328 n-type semiconductor substrate 127,202,327 n-type semiconductor multilayer film 123,129,216,217,323,329
n-type electrode 126,326 active layer 125,208,325 p-type semiconductor multilayer film 130,214,330 p-GaAs 124,215,324 p-type electrode 113,218,313,315,317 p-Al
As 114, 219, 314, 316 p-GaAs 117, 220, 322, 320, 318 n-Al
As 118, 221, 321, 319 n-GaAs 115, 220 n-doped region 116, 221 non-doped region 119, 222 non-doped AlAs 120, 223 non-doped GaAs 224 n-GaAs substrate 203 p-GaAs 204, 206 i-GaAs 205 InGaAs active Layer 207 n-GaAs

Claims (1)

(57) [Claims] 1. A semiconductor active layer for laser oscillation, which has a semiconductor multi-layered film reflective mirror formed on a semiconductor substrate in a lower part and a second conductive type multi-layered film reflective mirror in an upper part, between the reflective mirrors. In a surface-type optical semiconductor device having a layer, in the semiconductor multilayer film reflecting mirror, a side close to the semiconductor active layer for laser oscillation is a first conductivity type opposite to the second conductivity type, and the rest are non-conductive. A surface-type optical semiconductor element, which is doped and has an electrode for current conduction formed in a portion of the semiconductor multilayer film which is of the first conductivity type.