JPH0529703A - Semiconductor laser device - Google Patents

Abstract

(57) [Abstract] [Purpose] The present invention aims to provide a pre-formed structure that does not form a mesa or a trench.
(EN) A semiconductor laser device having a dual structure, in which parasitic capacitance can be sufficiently reduced and devices which operate at an ultra-high speed can be obtained with a high yield. [Structure] Diffraction grating 2 on n-InP substrate 1 and n on it
-InGaAs guide layer 3, InGaAsP active layer 4,
Anti-melt back layer 5, p-InP clad layer 6, p
-InGaAsP contact layer 7 is formed. Process them into mesas. Next, a p-InP buried layer 8 was formed around the mesa, and n-InP having a carrier concentration of 3 × 10 ¹⁶ cm ⁻³ was used.
The buried layer 9 and the n-InGaAs cap layer 10 are formed in a planar structure. A striped AuZn layer 11 having a width of 8 μm is formed on the surface of the contact layer. Therefore, the electrode width is 10 μm or less, and the carrier concentration is 5 × 10 ¹⁶ c
It is characterized by having a planar structure of m ^-3 or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser device which operates at an ultrahigh speed of 10 GHz or higher.

[0002]

2. Description of the Related Art A high-speed optical communication system has been actively developed in order to transmit more information. A semiconductor laser operating at high speed is indispensable for such an optical communication system. It has been found that the following three points are important for operating the semiconductor laser at high speed. (1) Reduce the time constant of the element. (2) Increase the relaxation oscillation frequency. (3) Damping is reduced.

Of these, the most important thing is to reduce the time constant of the element (1). (2) and (3) are especially required in a higher frequency band. In the following, the present technology for reducing the time constant of the most important element in the high speed semiconductor laser will be described.

A semiconductor laser for optical communication often adopts a buried structure which can easily obtain a basic lateral mode oscillation. This buried structure has a layer for a current block called a buried layer. There is a large parasitic capacitance in this part. In order to electrically isolate this parasitic capacitance, a method is adopted in which a groove is provided on both sides of the active layer (trench structure) or the buried layer other than the peripheral portion of the active layer is removed (mesa structure). For example, Electronics Letters (ELEC
TRONICS LETTERS), Volume 21, Volume 2
No. 2, pages 1144 to 1145 (trench structure),
Photonics Technology-Letters (IEEE PHO
(TONICS TECHNOLOGYLETTERS)
, Vol. 2, No. 4, pp. 229-230 (Mesa Structure).

The structure of a semiconductor laser having a trench structure is shown in FIG.
Shown in. In was grown on the n-InP substrate 1 by the first crystal growth.
A double heterocrystal composed of GaAs (P) and InP is grown, and this wafer is processed into a mesa stripe shape (1
5). Next, in the second crystal growth, crystals having a band gap larger than that of the active layer, such as p-InP, are formed around the stripe.
The layer 8 and the n-InP layer 9 are grown to form a buried layer. Next, SiO ₂ is formed on the entire surface of the wafer, and striped windows are formed on both sides of the active layer by photolithography. Next, trenches 16 up to the substrate are formed in the buried layers on both sides of the active layer by etching. Next, the SiO ₂ film 12 is formed on the entire surface of the wafer, and a window is opened by photolithography in the surface of the contact layer above the active layer stripe sandwiched by the trenches. Finally, bond pad 1 on the wafer surface
9 and the p-side electrode 17 and the substrate-side n-side electrode 18 are formed,
It is completed by cleaving to a size of 400 μm in width and 300 μm in resonator length.

The structure of a semiconductor laser having a mesa structure is shown in FIG. Similar to the semiconductor laser having the trench structure, the wafer having the buried structure is obtained by performing the crystal growth twice. Next, SiO ₂ is formed on the entire surface of the wafer, and stripe-shaped SiO ₂ is formed only on the peripheral portion of the active layer by photolithography.
Next, by using the SiO ₂ as a mask, the buried layer other than the peripheral portion of the active layer is removed by etching. Next, SiO ₂ is formed on the entire surface of the wafer, and a window is opened on the surface of the contact layer above the active layer stripe by photolithography. Finally, the bonding pad 19, the p-side electrode 17 and the substrate-side n-side electrode 18 are formed on the wafer surface, and the width is 400 μm and the cavity length is 300.
Complete by cleaving μm.

Further, in FIG. 4, the electrode width is used as a parameter,
The relation between the parasitic capacitance of the buried layer and the carrier concentration is shown.
In the conventional example, the carrier concentration of the buried layer is 1 × 10.
¹⁸ cm ^-3 and electrode width (or mesa width) is 5
Since it is 20 μm, the parasitic capacitance of the buried layer is 2 to 8 p.
It becomes F.

[0008]

The trench and mesa structure shown in the conventional example has a complicated process. Further, in order to reduce the parasitic capacitance, it is better to narrow the electrode width, that is, the width of the trench and the mesa. However, the trench and the mesa are formed by etching. Therefore, when narrowing the width,
Since the etching may reach the active layer portion, it is difficult to control the width of the mesa (or the mesa portion sandwiched by the trenches) to be narrow. Therefore, it is difficult to sufficiently reduce the parasitic capacitance, and therefore, there is a drawback that the yield for obtaining a device operating at high speed is not good.

The present invention has been made to solve the above-mentioned drawbacks, and has a planar structure which does not form trenches and mesas and has a simple process, and can sufficiently reduce parasitic capacitance and obtain a device which operates at ultra-high speed with high yield. Another object of the present invention is to provide a semiconductor laser device that can be used.

[0010]

According to the present invention, a first conductive type semiconductor substrate is provided on a first conductive type active layer and a second conductive type clad layer, and a second conductive type active layer. A mesa-shaped semiconductor crystal is formed by sequentially stacking a contact layer of a mold, and the mesa-shaped semiconductor crystal is buried with a buried layer of a second conductivity type and a buried layer of a first conductivity type having a band gap larger than that of the active layer. A semiconductor laser comprising a striped electrode formed on the contact layer,
The stripe-shaped electrode width is 10 μm or less, and the carrier concentration of the first or second conductivity type buried layer is 5 × 10 ¹⁶ cm ⁻³ or less.

[0011]

The parasitic capacitance of the buried layer becomes 1 pF or less by controlling the electrode stripe width to 10 μm or less and controlling the carrier concentration of the buried layer to 5 × 10 ¹⁶ cm ⁻³ or less (FIG. 4). Therefore, the time constant of the element can be significantly reduced. Therefore, it is not necessary to form a mesa or trench to electrically isolate the buried layer. Therefore, it is possible to obtain an element having a planar structure which operates at a very high speed. This is because the electrode stripe width is 10 μm or less and the carrier concentration is 5 × 1.
It can be realized only when the two conditions of 0 ¹⁶ cm ⁻³ or less are satisfied.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.
An example of application to an m-band distributed feedback semiconductor laser will be described.

FIG. 1 is a sectional perspective view of a semiconductor laser device according to the present invention. First, a diffraction grating 2 is formed on an n-InP substrate 1 by an interference exposure method, and an n-thick film having a thickness of 0.1 μm is formed thereon.
-InGaAsP guide layer 3, 0.1 μm thick InG
an aAsP active layer 4, an anti-meltback layer 5 having a thickness of 0.03 μm, a p-InP clad layer 6 having a thickness of 2.5 μm,
The p-InGaAsP contact layer 7 having a thickness of 0.3 μm is continuously grown. Next, the wafer is processed into a reverse mesa shape by photolithography and etching so that the width of the active layer becomes about 1 μm.

Next, by a second crystal growth, a 2 μm thick p-InP buried layer 8 (carrier concentration: 0.9 × 10 ¹⁸ cm ⁻³ ) and a 2 μm thick n-InP layer were formed around the mesa.
A buried layer 9 (carrier concentration: 3 × 10 ¹⁶ cm ⁻³ ) and a 0.5 μm thick n-InGaAsP cap layer 10 are grown.

SiO ₂ having a thickness of 0.5 μm is etched and removed in stripes on the entire surface of the thus-formed wafer. Then, by a lift-off method, an AuZn layer (p-side electrode) 11 having a width of 8 μm is vapor-deposited and alloyed on the surface of the contact layer having a window. Next, the upper part of the buried layer (SiO ₂ 12
A bonding pad 13 made of Ti, Pt, and Au and having a diameter of 100 μm is formed so as to be in electrical contact with the striped AuZn alloy layer 11 (lift-off method).

Next, SiO ₂ in a portion other than the lower portion of the bonding pad is removed by etching. Finally, A on the board side
An electrode (n-side electrode) 14 made of uGe, Ni, Au is formed and cleaved to a width of 400 μm and a cavity length of 300 μm to complete the process.

In the semiconductor laser device according to the present invention, the electrode width is 8 μm and the carrier concentration of the buried layer 9 is 3 × 10 ^16.
cm ^-3 . Although the carrier concentration of the buried layer 9 is set to 3 × 10 ¹⁶ cm ⁻³ in the above embodiment,
The carrier concentration may be 3 × 10 ¹⁶ cm ⁻³ . In this case, considering the leakage current, it is not desirable to set the carrier concentration of both buried layers 8 and 9 to 3 × 10 ¹⁶ cm ⁻³ .

By thus defining the electrode width of the semiconductor laser device and the carrier concentration of one of the buried layers, the parasitic capacitance of the buried layer is 1 as shown in FIG.
It can be seen that the capacitance is below pF and the parasitic capacitance is reduced.

FIG. 5 is a comparison of the small signal frequency characteristic of the semiconductor laser device according to the present invention with that of a conventional semiconductor laser device. The bias current was a threshold current +36 mA. In the figure, (A) shows the characteristics of the present invention, that is, the case where the electrode width is 10 μm and the carrier concentration is 3 × 10 ¹⁶ cm ⁻³ .
(B) electrode width 10 μm and carrier concentration 1 × 10 ¹⁸ c
m ^-3 , (C) are electrode width 400 μm and carrier concentration 3 ×
10 ¹⁶ cm ⁻³ , (D) shows the case where the electrode width is 400 μm and the carrier concentration is 1 × 10 ¹⁸ cm ⁻³ . In the case of (A), a cutoff frequency (f _{-3 dB} ) of 18 GHz was obtained. (B),
(C) and (D) are conventional examples, and the cutoff frequency (f _{-3 dB} ) was 0.5 to 13.5 GHz. In these cases, since the time constant is large, the roll-off phenomenon is largely observed, and even in the case of the best characteristic (B), f _-3dB = 13.5GH
It was z.

Further, according to the present invention, it is not necessary to form a mesa or trench to electrically isolate the buried layer.
That is, since the planar structure is simple in process and does not require careful control of the mesa width, it is possible to obtain an element that operates at high speed with good yield over the entire surface of the wafer.

[0021]

According to the present invention, the parasitic capacitance is sufficiently reduced with a simple planar structure without forming mesas and trenches.
It is possible to obtain a semiconductor laser device that operates at an ultrahigh speed with a high yield.

[Brief description of drawings]

FIG. 1 is a perspective view of a semiconductor laser device according to an embodiment of the present invention.

FIG. 2 is a perspective view of a semiconductor laser device having a trench structure.

FIG. 3 is a perspective view of a semiconductor laser device having a mesa structure.

FIG. 4 is a diagram showing a relationship between a buried layer carrier concentration and a parasitic capacitance.

FIG. 5 is a diagram showing small signal frequency characteristics of the semiconductor laser device of the present invention and the conventional semiconductor laser device.

[Explanation of symbols]

1 ... n-InP substrate, 2 ... diffraction grating, 3 ... n-InGa
AsP guide layer, 4 ... InGaAsP active layer, 5 ... In
GaAsP anti-meltback layer, 6 ... p-InP clad layer, 7 ... p-InGaAsP contact layer, 8 ... p
-InP buried layer, 9 ... n-InP buried layer, 10
... n-InGaAsP cap layer, 11 ... AuZn alloy layer (p-side electrode), 12 ... SiO2 film, 13 ... Ti / Pt
/ Au (bonding pad), 14 ... AuGe / Ni
/ Au (n-side electrode), 15 ... Mesa stripe, 16 ... Trench (groove), 17 ... P-side electrode, 18 ... N-side electrode, 19
… Bonding pad.

Claims (1)

Claim: What is claimed is: 1. A semiconductor substrate of a first conductivity type, a clad layer comprising an active layer of the first or second conductivity type and a second conductivity type, and a second conductivity type. A semiconductor crystal in the form of a mesa formed by sequentially stacking a contact layer that is formed on the semiconductor layer, and burying it with a second conductivity type buried layer having a band gap larger than that of the active layer and a first conductivity type buried layer. A semiconductor laser in which a striped electrode is formed on the contact layer, the striped electrode width is 10 μm or less, and the carrier concentration of the first or second conductivity type buried layer is A semiconductor laser device having a size of 5 × 10 ¹⁶ cm ⁻³ or less.