JP2586671B2 - Semiconductor multilayer film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a semiconductor multilayer film required for a surface emitting semiconductor laser or a semiconductor functional element.

(Prior Art) A surface emitting semiconductor laser is capable of generating oscillation light perpendicular to a semiconductor substrate, and is an indispensable key device for realizing a super-parallel optical communication system and two-dimensional information processing using light.

Figure 4 shows the 7th Integrated Conference held in Japan in 1989.
Optics and Optical Fiber Communication (IOC'89)
AT & TB in the Post Deadline Papers (18B2-6, PD)
1 shows a cross-sectional structure diagram of a surface emitting semiconductor laser published by JLJewell of ell Lab. P-type semiconductor multilayer film 42 and n
The type semiconductor multilayer film 41 is formed above and below the InGaAs active layer 43 in which In is added and the band gap wavelength is shifted to the longer wavelength side than GaAs, and each functions as a reflector.

The cavity length L of a surface emitting laser is usually on the order of μm.
When L is shortened, the threshold current density J _th (A / cm ² ) increases when the reflectivity R is the same level as that of a laser (L 300 μm) that oscillates in the normal lateral direction (L = 0.3 μm). Therefore, in a surface emitting laser, it is necessary to set R to an extremely high value.
Further, a reflecting mirror having such a high reflectance must be formed. In the conventional example shown in FIG. 4, a reflecting mirror having an R of 99% or more is made of a semiconductor multilayer film having a thickness of λ / 4. Here, λ is the wavelength in the semiconductor multilayer film.

On the other hand, the differential quantum efficiency η _d of the surface emitting semiconductor laser is Given by Where η _i : internal differential quantum efficiency d: Active layer thickness L: Resonator length α _ac : Light loss in active layer (cm ⁻¹ ) α _ex : Light loss in other than active layer (cm ⁻¹ ) α _s : Optical scattering loss (cm ⁻¹⁾ ).

It is important in application that η _d is high. For this purpose, dα _ac + (L−d) α _ex + Lα _s (2) must be reduced in equation (1). In the conventional example shown in FIG. 4, an oscillation characteristic at a low threshold of the order of mA is obtained in the SQW structure in which the thickness d of the active layer is 100 °. L is 5 to 6 μm, which is larger than d. Assuming that the optical scattering loss α _s can be reduced, it is necessary to reduce the light loss α _ex in areas other than the active layer in order to reduce the value of equation (2). (L-d) α _ex further _{(L-d) α ex Lα} ex = L 1 α ex (1) + L 2 α ex (2) + L 3 α ex (3) + L
₄ α _ex ⁽⁴⁾ …… (3) Here, L ₁ , α _ex ⁽¹⁾ : n-type semiconductor multilayer film thickness and light absorption loss L ₂ , α _ex ⁽²⁾ : n-type cladding layer thickness and light absorption loss L ₃ , α _ex ⁽³⁾ : P-type cladding layer thickness and light absorption loss L ₄ , α _ex ⁽⁴⁾ : Thickness and light absorption loss of P-type semiconductor multilayer film. In the conventional example of FIG. 4, the n-type semiconductor multilayer film 41 has a wavelength of λ / 4.
It is made by continuous growth of thick AlAs and GaAs for 231/2 cycles. Since the refractive indices n of AlAs and GaAs are estimated to be about n3.18 and 3.62, respectively, for light having a wavelength of λ ₀ = 980 nm, the thickness of the n-type semiconductor multilayer film is considered to be up to 3.4 μm. Further, since the p-type semiconductor multilayer film is formed for 15 periods, the thickness is considered to be up to 2.2 μm. Since the distance between the n-type multilayer film and the p-type multilayer film is set to one wavelength, L ₂ + L ₃ = λ
_{0 /} n It seems to be 0.29 μm.

Accordingly, since the thickness of the n-type and p-type semiconductor multilayer films occupies a large proportion of the cavity length, the light absorption loss in the semiconductor multilayer film is reduced to reduce the value of the expression (3). It is very important to do.

In the AlGaAs system, AlAs has the lowest refractive index, GaAs has the lowest,
This is the best combination as a combination having a high refractive index and increasing the reflectance of the semiconductor multilayer film. And
In FIG. 4, they are alternately laminated with a thickness of λ / 4.

(Problem to be Solved by the Invention) In the structure of FIG. 4, a differential quantum efficiency of about 10% is achieved, but it is necessary to increase the differential quantum efficiency in order to improve the laser characteristics.

(Means for Solving the Problems) In a semiconductor multilayer film according to the present invention, a first semiconductor film having a small light absorption loss for a predetermined light and a second semiconductor film having a large light absorption loss for the predetermined light are alternately arranged. In the semiconductor multilayer film formed by laminating, the wavelength of the predetermined light in the semiconductor multilayer film is λ, the thickness of the first semiconductor film is greater than λ / 4, and the second semiconductor film is The thickness of
/ 4, and the rate of change of each film thickness from λ / 4 is substantially the same. Here, the predetermined light is, for example, oscillation light from an active layer in a surface emitting laser, and is external light when the present invention is used as a reflecting mirror.

(Effect) Even if the thickness of the semiconductor layers having a high refractive index and a low refractive index is slightly shifted from the λ / 4 thickness, the overall reflectance does not decrease sharply. Even if the thickness of one semiconductor is increased, the manner of decreasing the reflectance can be reduced by reducing the thickness of the other semiconductor at the same rate. Therefore, the thickness of the semiconductor having a larger light absorption loss is relatively reduced with respect to the thickness of the semiconductor having a smaller light absorption loss. Then, while maintaining high reflectance, light absorption loss in the semiconductor multilayer film can be reduced,
Thereby, a highly efficient surface emitting semiconductor laser can be realized.

GaAs in Figure 1, AlAs is increased AlAs thickness d ₁ from the optimum thickness of lambda / 4 multilayer film consisting of 20 periods, the reflectivity R of the multilayer film when having reduced GaAs thickness d ₂ at the same rate The results of calculations show how they change. In FIG. 1, the rate of change (deviation) is δd ₁ / d ₁ ,
a case as .delta.d ₂ / d ₂ of _{δd 1 / d 1 = -δd 2} / d 2. λ / 4
R = 0.99924 at the time of thickness changes and decreases and decreases to R = 0.99922 and 0.99916 with 5% and 10% film thickness changes, respectively, but the change is gradual. FIG. 1 shows the case where the rate of change (shift) from the thickness of λ / 4 in the same direction, ie,
The change in R when both GaAs and AlAs are increased (or when both are decreased) is shown at the same time. In this case, the reflectance R sharply decreases with respect to the considered wavelength. Therefore, it is necessary to shift the displacement in the opposite direction (increase one and decrease the other) as in the present invention.

On the other hand, the light absorption of the semiconductor film consists of inter-band absorption, intra-band absorption, and valence band absorption as shown in FIG. In the example shown in FIG. 4, the semiconductor multilayer film is set so as to be in a transparent wavelength range with respect to the laser light, so that the light absorption loss becomes a problem. Is called free carrier absorption, and in GaAs, α (cm ⁻¹ ) 3 × 10 ⁻¹⁸ n + 7 × 10 ⁻¹⁸ p (4) at room temperature. However, n and p are expressed in cm ^-3 units. Electron and hole density. Assuming that the carrier concentration of each of the p-type and n-type multilayer films is 2 × 10 ¹⁸ cm ⁻³ , the absorption in the band is 3 × 10 ⁻³ and 2 × 10 ⁻³ in the example of FIG.

On the other hand, the valence band absorption can be expressed as α (cm ⁻¹ ) = K ₀ N + α ₂ (5). K ₀ is a proportionality constant that depends on temperature, and α ₂ is a loss coefficient that does not depend on N. The main factor is considered to be a transition from the split-off band to the acceptor level. The absorption loss between the valence bands depends on the structure of the valence band. Generally, it can be considered that the smaller the band gap energy is, the larger the band gap energy is.
For m-band GaInAsP-based lasers, about 100 cm ^-1
It is about several tens cm- ¹ . In the case of a GaAs / GaAlAs-based laser, it is known to be several to several tens cm- ¹ . The expression (5) does not clearly include the hole density of the valence band, but is obviously influenced by the size. Considering the structure of FIG. 4, if it is assumed to be 10 cm ⁻¹ , the light absorption due to the valence band absorption is almost equal to that in the band. Further, it is expected that light absorption due to valence band absorption will be further increased in the InGaAsP system.

Therefore, the thickness is shifted from the λ / 4 thickness as long as the reflectivity of the semiconductor multilayer film does not significantly deteriorate. Then, the thickness of the semiconductor film having a high valence band absorption is reduced, and at the same time, the thickness of the semiconductor film having a relatively low valence band absorption is increased. This makes it possible to form a multilayer film with low light absorption.

(Example) FIG. 3 is a structural sectional view of a surface emitting semiconductor laser using a semiconductor multilayer film of the present invention as a reflection film in one embodiment of the present invention.
The basic structure is the same as the conventional example shown in FIG. AlAs, GaA
Only the portion of the semiconductor multilayer film made of s is shifted from the λ / 4 thickness. AlAs is increased by 10% from λ / 4 thickness, and GaAs is reduced by 10% from λ / 4 thickness. The P-type semiconductor multilayer film 32 is manufactured by doping Be, and the n-type semiconductor multilayer film 31 is manufactured by doping Si. The whole including the semiconductor multilayer film is manufactured by the MBE method. As in FIG. 4, In is added to the active layer 33, and the composition is In _0.2 Ga _0.8 As. The periods of the p-type semiconductor multilayer film 32 and the n-type semiconductor multilayer film 31 are 15 periods and 231/2 periods, respectively. The other thicknesses of the cladding layers are the same as in FIG. In this embodiment, the expression (3) is (Ld) α _ex L ₁ {(in-band absorption) + (valence band absorption)} + L ₂ {(in-band absorption) + (valence band absorption)}.
(6) As the value of the absorption in the band, n = p = 2 × 10 ¹⁸
Equation (4) is used as cm ^-3 . If the magnitudes of valence band absorption of GaAs and AlAs are α _G (cm ⁻¹ ) and α _A (cm ⁻¹ ), respectively, the value of equation (6) becomes: L-d) α _ex 2 × 10 ⁻³ + 1.8 × 10 ⁻⁴ α _A + 1.6 × 10 ⁻⁴ α _G + 3 × 10 ⁻³ + 1.2 × 10 ⁻⁴ α _A + 1.0 × 10 ⁻⁴ α _G
(7) On the other hand, when the AlAs thickness is increased by 10% from the λ / 4 thickness and the GaAs thickness is reduced by 10% as in this embodiment, (6)
The value of the equation is (Ld) α _ex 2 × 10 ⁻³ + 2.0 × 10 ⁻⁴ α _A + 1.4 × 10 ⁻⁴ α _G + 3 × 10 ⁻³ + 1.3 × 10 ⁻⁴ α _A +0. 9 × 10 ^-4 α _G
(8) The size of the alpha _A and alpha _G is distinct values are presumed but not reported in the difference between the number to several 10 cm ^-1, and can be reduced as a result (L-d) α _ex Become. As a result, several tens% improvement in the differential quantum efficiency η _d was observed.

In this embodiment, an example in which the present invention is applied to a surface emitting semiconductor laser of the AlGaAs / GaAs type has been described. However, in the InGaAsP / InP type, light absorption due to valence band absorption is large and works more effectively. In this embodiment, an example in which a semiconductor multilayer film is applied to a surface emitting laser is shown, but the present invention is not limited to this, and can be applied to a device having a reflection film.

(Effects of the Invention) According to the present invention, it is possible to realize a semiconductor multilayer film capable of reducing a light absorption loss while maintaining a high reflectance. This can be applied as a surface emitting laser, a reflection film of an optical function element, or a semiconductor mirror.

[Brief description of the drawings]

FIG. 1 is a view showing the reflectivity of a semiconductor multilayer film when the thickness is changed at the same ratio from λ / 4 thickness. Fig. 2 is III-V
FIG. 2 is a band diagram showing a mechanism of light absorption in a group III semiconductor. FIG. 3 is a structural sectional view of a light emitting device according to one embodiment of the present invention. 4th
The figure is a structural cross-sectional view of a conventional example. In the figure, 31 and 41 are n
Active semiconductor layers 33 and 43 are active layers 32 and 42 are p-type semiconductor multilayer films.

Claims (1)

(57) [Claims] A first light-absorptive first light having a small light absorption loss;
In a semiconductor multilayer film formed by alternately laminating a semiconductor film of the type and a second semiconductor film having a large light absorption loss for the predetermined light, the wavelength of the predetermined light in the semiconductor multilayer film is As λ, the thickness of the first semiconductor film is set to be larger than λ / 4, and the thickness of the second semiconductor film is set to be smaller than λ / 4. Are the same level.