JPH11284223A - Superluminascent diode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To hold a hole injection efficiency so as to obtain a high luminous efficiency and high polarization quench ratio, by specifying the thickness of a barrier layer and no. of well layers in a multiple quantum well structure of an active layer. SOLUTION: A barrier layer 5 of a superluminescent diode SLD is set to 4 nm or less, without forming a strain in a well layer 4. If about 4 nm or less, the wave functions of the carriers esp. holes begin to couple with each other to thereby improve the uniformity of the carriers in the quantum well layer. The densities of electrons and holes in the SLD are higher than those of a semiconductor laser LD and hence either the no. or thickness of the well layers 4 must be increased. If the barrier layer 5 is 4 nm or less thick, increase of the layer no. never deteriorates the emission efficiency and hence the barrier layer 5 is set to 4 nm or less thick and the no. of the well layers 5 is set to 5 or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superluminescent diode (hereinafter referred to as "SLD") used for optical measurement of a fiber gyro or the like.

[0002]

2. Description of the Related Art An SLD is characterized by emitting light with a narrow emission angle and intensity comparable to a semiconductor laser (hereinafter referred to as an LD) while having a broad spectrum close to that of a light emitting diode. This SLD has been put into practical use as a light source for a fiber gyro, and has a high resolution OTDR (Op
physical Time Domain Reflect
Application to fields such as ommetry) and engine combustion monitoring is also being studied.

A conventional SLD will be described with reference to FIG. FIG. 4 is a diagram showing an example of a conventional SLD using an InGaAsP / InP-based material. In order to obtain this SLD, the first growth is performed on the n-type InP substrate 21 by liquid phase epitaxy (LPE), vapor phase epitaxy (VPE, MO-CVD) or molecular beam epitaxy (MBE). n
InGaAsP light guide layer (λ: 1.3 μm composition) 2
2. Non-doped InGaAsP active layer (λ: 1.5 μm
Composition) 23, p-type InP cladding layer 24, p-type InGa
An AsP electrode layer (λ: 1.1 μm composition) 25 is grown.

Next, a thin film of SiO ₂ or SiN is formed by p-type InGaAsP by RF sputtering or CVD.
The electrode layer 25 is formed on the entire surface of the layer. Thereafter, in order to bury the active layer by the photo-etching technique, the current injection region 29 is linearly formed in a stripe shape along the <110> direction with a width of 4 to 5 μm, a length of 400 μm, and a non-current injection region 30.
Similarly, after forming the same width as the stripe width of the current injection region 29 so that the length of the non-current injection region 30 becomes 200 μm, this SiO ₂ stripe thin film or SiN
Using the striped thin film as a mask, each layer of 25, 24, 23, and 22 is etched with a 4% solution of bromomethanol until it reaches the substrate 21 to form an inverted mesa-shaped laminate. Next, as a second growth, a current confinement buried growth of the p-type InP layer 26 and the n-type InP layer 27 is performed by LPE on the portions removed by etching.

[0005] The upper surface of the wafer thus obtained is Au-Zn.
And a p-type ohmic electrode 28 is formed only in the current injection region 29 using a photo-etching technique. An SiO ₂ or SiN film is formed thereon again, and a window for forming the groove 33 in the non-current injection region 30 is formed by a photo etching technique. Using this as a mask, each layer of the wafer is etched with a 4% solution of bromomethanol to form a groove 33. The depth of the groove 33 is set so as to exceed the position of the active layer 23 from the upper end. Further, the angle of the wall of the groove 33 is formed so that the active layer 23 is cut obliquely in FIG. 3A and also obliquely in the depth direction. On the substrate 21 side, Au-Ge-Ni is vapor-deposited after polishing until the entire thickness becomes about 80 μm, and an n-type ohmic electrode 32 is formed on the entire surface. The structure of each layer is as follows, and each crystal layer matches the lattice constant of InP.

21: Sn-doped n-type InP substrate, thickness 8
0 μm, carrier density 3 × 10 ¹⁸ cm ⁻³ 22: n-type InGaAsP light guide layer, thickness 0.2 μm
m, Sn-doped, carrier density 3 × 10 ¹⁷ cm ⁻³ 23: n-type InGaAsP active layer, thickness 0.2 to 0.3
μm, non-doped 24: p-type InP crystal layer, thickness 1.5 μm, Zn-doped, carrier density 5 × 10 ¹⁷ cm ⁻³ 25: p-type InGaAsP electrode layer, thickness 0.7 μm, Z
n-doped, carrier density 5 × 10 ¹⁸ cm ⁻³ 26: p-type InP current blocking layer, thickness 1.5 μm, Zn-doped, carrier density 1 × 10 ¹⁷ cm ⁻³ 27: p-type InP current blocking layer, thickness 1. 5 μm, Zn-doped, carrier density 1 × 10 ¹⁷ cm ⁻³ This element was divided into fixed pellets having an element length of 600 μm and a width of 400 μm, and mounted on a heat sink with AuSn solder, at a current, wavelength and 1.55 μm. The light output characteristic increases without oscillating the light output according to the current injection in the continuous operation at 25 ° C., and an incoherent light output of 3 mW at 200 mA can be obtained. Groove 33 formed in non-current injection region 30
The light reflected at the end face of the first layer is not coupled to the active layer 23 forming the current injection region 29 again, and the FP mode is sufficiently suppressed without lengthening the non-current injection region 30.
This is to obtain SLD oscillation.

[0007] As described above, the SLD is structurally similar to the LD, but differs from the LD in that it is devised to suppress end face reflectance and prevent oscillation. Like the LD, there are two types of active regions: a bulk structure and a multiple quantum well (hereinafter referred to as MQW) structure. Since the SLD is operated without oscillation, the injected carrier density is much higher than that of the LD. In order to avoid overflow of carriers from the hetero interface, the active layer was designed to be thicker or wider. In the MQW structure, the number of well layers or the thickness of well layers is set to be large.

In the MQW structure, the well layer is usually 4 to 12 nm.
To the extent that the wave functions of the confined electrons do not overlap with the wave functions of the electrons in the adjacent well layers.
It is usual to have a thickness of 2 nm.

The number of quantum well layers of a high-output semiconductor laser in a long wavelength band is usually about four. This is because the use in a low current region is emphasized in order to lower the threshold value, and is different from the SLD in which the injected carrier density and the internal gain are increased.

[0010]

In the bulk structure, since the density of states is of a quadratic function type, the light output is easily increased, but the half-width of the spectrum is reduced. On the other hand, while the MQW structure has a wide spectrum width, it has been difficult to increase the optical output with a structure similar to the LD. Therefore, the SLD is designed to increase the light output by increasing the number of layers and the layer thickness as described above. In the MQW structure, if the wave functions overlap with each other by reducing the thickness of the barrier layer, quantum levels are split by the number of wells, which adversely affects the device performance as an LD, such as a decrease in differential gain. It is difficult to do. In an SLD in which the total active layer thickness is increased, injected carriers, particularly holes, are not uniformly injected into each well layer. As a result, there is a problem that the luminous efficiency is reduced.

[0011]

A super luminescent diode according to the present invention comprises a first conductive type semiconductor substrate 1, a first conductive type cladding layer 2, and an active layer including a multiple quantum well structure. 3 and a cladding layer 7 of the second conductivity type in this order, wherein the multiple quantum well structure of the active layer has a barrier layer 5 having a layer thickness of 4 nm or less, and The number of the layers 4 is five or more.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to solve the above-mentioned problems, the present invention is characterized in that the thickness of a barrier layer of an MQW structure is reduced. Thereby, the number of well layers can be increased without increasing the thickness of the entire active region. Moreover, since the wave functions are coupled to each other, even when the number of quantum well layers is increased, the uniformity of carriers in the active layer does not deteriorate. Therefore, the number of well layers can be increased as compared with the conventional quantum well structure. Since the carrier density of the SLD is much higher than that of the LD, there are few problems such as a decrease in the differential gain and the state density near the threshold current as in the LD. In the active layer having the bulk structure, the polarization extinction ratio of the light output is 5 to 7 dB.
It is. By increasing the number of layers, it is possible to realize an optical output higher than that of the bulk active layer and a high polarization extinction ratio, which is an advantage of the MQW structure, without reducing the spectral half width. It should be noted that the IEICE technical technique 0QE93-90, pp51-
Light emission due to a higher degree of coincidence between the electron and hole envelope functions by reducing the thickness of the barrier layer, as calculated in 56 “Design of a Multiple Quantum Well LD Optimizing the Envelope Function of Electrons” There is also an advantage that efficiency can be improved.

As described above, by reducing the thickness of the active layer having the MQW structure, the number of well layers can be increased without increasing the thickness of the entire active region. As a result, since the hole injection efficiency does not decrease, a high luminous efficiency and a high polarization extinction ratio can be obtained.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As an embodiment of the present invention, an SLD of an RVN structure embedded type will be described with reference to FIGS.

FIG. 1 is an external perspective view of this embodiment, and FIG. 2 is a schematic diagram for explaining an embodiment of an MQW active layer.

First, MOVPE (metal organic chemical vapor deposition)
Using a crystal growth apparatus such as
The n-InGaAsP light confinement layer 2 having a composition of 8 μm
Then, a non-doped MQW active layer 3 is deposited thereon. The non-doped MQW active layer includes, for example, a barrier layer 5
The deposition of 2 nm at a composition wavelength of 1.08 μm and the deposition of 5 nm at a composition wavelength of 1.35 μm as the well layer 4 are repeated for 15 periods. And, on top of that, pI of 1.08 μm composition
The nGaAsP light confinement layer 6 is laminated with a thickness of 50 nm, the p-InP cladding layer 7 with a thickness of 2 μm, and the p ⁺ -InGaAsP contact layer 8 with a thickness of 0.2 μm.

Next, a SiNx film is deposited on the surface of the epi-wafer by a method such as plasma CVD. Then, it is processed into a 6 μm-wide SiNx film stripe pattern elongated in the (110) direction by photolithography. Using the SiNx film pattern as a mask, mesa etching is performed using an HC1-based etchant and bromomethanol. A current blocking layer of p-InP9 and n-InP10 for current confinement is stacked thereon by a liquid phase growth method.

Next, a groove pattern 11 for suppressing reflection in the MQW active layer 3 is formed in the same manner as when forming the SiNx film stripe pattern. First, a groove 11 of about 10 μm is dug by etching with bromomethanol or the like.

Next, after the substrate 1 side is polished to a thickness of about 100 μm, a photoresist pattern is formed on the epi-plane side by photolithography. Epi surface side P
Side electrode 12 (for example, Zn: 10 nm, Au: 50 n
m), an n-side electrode 13 (for example, an AuGe alloy: 50 nm) is vapor-deposited on the substrate 1 side, and then the wafer is put into Ascenton to perform lift-off. The metal of the electrode and the semiconductor are alloyed by thermal annealing at about 400 ° C. After cleavage, a non-reflective coating of SiOx is deposited on both end surfaces 14. In the SLD configured as described above, when a current is injected from the p-electrode 12 and the n-electrode 13 into the current injection region 15, light emitted in the current injection region is guided while being lost to the non-current injection region 16. However, the light reflected by the end face 14 is again reflected by the current injection region 15.
The groove 11 cuts the optical waveguide with an inclination with respect to the direction in which the optical waveguide extends and also with an inclination with respect to the depth direction so as not to couple with the optical waveguide (the MQW active layer 3) formed at the bottom. And is formed in the non-current injection region to suppress the Fabry-Perot mode oscillation.

In this embodiment, the MQW active layer 3 is formed by repeatedly forming a well with the barrier layer 5 having a thickness of 2 nm and the well layer 4 having a thickness of 5 nm for 15 periods. Well layer 4 without increasing the thickness
Since the number of layers is increased, there is no overflow of injected carriers, particularly holes, and the efficiency of hole injection does not decrease, so that a high luminous efficiency and a high polarization extinction ratio can be obtained.

The thickness of the barrier layer 5 is 8 in the LD example.
In the present SLD, the thickness is set to 4 nm or less so as not to form distortion of the well layer 4. About 4nm
Below this point, the wave functions of carriers, particularly holes, start to couple with each other, so that the uniformity of carriers in the quantum well layer is improved. The number of well layers 4 is 5 or less in LD.
If the number is larger than this, the oscillation threshold becomes high.
Since the density of electrons and holes is higher in SLD than in LD, it is necessary to increase either the number of layers or the layer thickness.
With the conventional barrier layer thickness, when the layer thickness is 5 or more, the luminous efficiency is reduced, so that the effect of improvement is small. About 4 nm barrier layer
In the following, the luminous efficiency does not deteriorate even when the number of layers is increased. Therefore, in the present invention, the thickness of the barrier layer 5 is set to 4 nm or less, and the number of the well layers 4 is set to 5 or more.
It has a characteristic configuration as an LD.

Although the MOVPE method is used for the crystal growth apparatus in the above-described embodiment, the MBE method, the MOBE method,
The LPE method can also be used. Also, in the SLD creation method, the embedded structure is not an RVN structure but a DC-PB
It can also be realized with an H structure or the like. Further, instead of the groove structure, the reflectivity suppressing structure may be configured by a window structure, a diagonal stripe, or the like.

Next, a fiber gyro using the SLD of the present invention as a light source will be described with reference to FIG.

The gyro can be used without any external information.
It is a sensor for detecting the angular velocity and angle of the moving body to which it is attached, and measuring and controlling the position. The fiber gyro is a type in which an optical fiber is formed into a coil shape, coherent light is incident from both directions, and the speed and angle of rotation are detected by using the Sannyak effect. The optical fiber usually uses a polarization maintaining type, and the output from the SLD uses only the light in the polarization direction on one side thereof.

FIG. 3 shows the principle of the fiber gyro.

In the figure, reference numeral 41 denotes an SLD according to the present invention,
42 is a translucent plate for transmitting and reflecting light, 43 is an optical fiber, and 44 is interference output light. The coherent light output from the SLD 41 passes through the translucent plate 42, enters the optical fiber 43 from one end 43a of the optical fiber 43, passes through the optical fiber 43 wound in a coil shape in the direction of arrow a, and becomes translucent. The light transmitted through the plate 42 is output from the output unit 44 as output light a, and a part of the coherent light output from the SLD 41 is reflected by the translucent plate 42 and is incident on the optical fiber 43 from the other end. Through the optical fiber 43 wound in the direction of arrow b,
And is output from the output unit 44 as output light b. The Sagnac effect is a phenomenon in which the light propagating in the opposite direction through the closed optical path changes the propagation time (phase) in proportion to the rotational angular velocity of the closed optical path. The change in phase between the light incident from the other end and the light incident from the other end 43b is detected by interference or the like, and the rotational angular velocity is detected by light interference or the like.

The fiber gyro can detect a slower rotation angle as the area of the closed optical path is larger and the light power of the light source is larger. With the SLD of the present invention, an output higher than the conventional one can be obtained with a large polarization extinction ratio. When the polarization extinction ratio is compared with that of the conventional bulk-structured SLD, it is improved by about 10 dB or more. Even if the same output is converted, the effective output is improved by about 20%, so that the possibility of the fiber gyro can be greatly improved.

[0028]

The superluminescent diode of the present invention comprises a first conductive type cladding layer 2, an active layer 3 having a multiple quantum well structure, and a second conductive type semiconductor substrate 1 on a first conductive type semiconductor substrate 1. A superluminescent diode having a conductive type clad layer 7 stacked in this order, wherein the multiple quantum well structure of the active layer has a barrier 5 layer of 4 nm or less and a number of well layers 4 of 5 Since the number of layers is equal to or greater than the number of layers, in the current injection region 15, the hole injection efficiency can be maintained, and a high luminous efficiency and a high polarization extinction ratio can be obtained. This makes it possible to obtain a high detection resolution in the fiber gyro.

[Brief description of the drawings]

FIG. 1 is a perspective view showing one embodiment of a super luminescent diode of the present invention.

FIG. 2 is a schematic view showing one embodiment of an MQW active layer of the super luminescent diode of the present invention.

FIG. 3 is a diagram illustrating the principle of a fiber gyro.

FIG. 4 is a diagram showing a conventional superluminescent diode.

[Explanation of symbols]

Reference Signs List 1 n-InP substrate (semiconductor substrate) 2 n-InGaAsP light confinement layer 3 MQW active layer 4 well layer 5 barrier layer 6 p-InGaAsP light confinement layer 7 p-InP (cladding layer) 8 p ⁺ -InGaAsP (contact layer) Reference Signs List 9 p-InP (current blocking layer) 10 n-InP (current blocking layer) 11 groove 12 p-electrode (p-side electrode on epi-face side) 13 n-electrode (n-side electrode) 14 anti-reflection coat 15 current injection region 16 Non-current injection area

Claims (1)

[Claims] 1. A semiconductor device of a first conductivity type, comprising:
A super luminescent diode comprising a first conductive type clad layer (2), an active layer (3) having a multiple quantum well structure, and a second conductive type clad layer (7) stacked in this order, A super luminescent diode, wherein the multiple quantum well structure of the active layer has a barrier layer (5) with a layer thickness of 4 nm or less and a well layer (4) with five or more layers.