JP2010141039A - Super-luminescent diode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a super-luminescent diode capable of easily reducing the valleys between peaks in spectrum waveform, as compared with a conventional manner, while maintaining a wide band of the spectrum waveform of the output light, and enhancing the symmetrical properties of the spectrum waveform. <P>SOLUTION: The super-luminescent diode 1 is a super-luminescent diode made of a GaAs material. An active layer 30 contains a multiple quantum well structure and includes at least one out of an In and an Al as a composition. The number of layers of barrier layers 32 in the active layer 30 is four or less, and the thickness range of the barrier layers 32 is 1.5-3.0 nm, respectively. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a superluminescent diode.

  A super luminescent diode (hereinafter referred to as SLD) has a relatively broad spectrum distribution like a light emitting diode and can obtain a relatively high output like a semiconductor laser, and therefore requires high resolution. It attracts attention as a light source used in the medical field and the measurement field. For example, it has been devised to use an SLD as a light source for OCT (Optical Coherence Tomography) used in ophthalmic examination.

  The SLD described in Non-Patent Document 1 is an active layer having a single quantum well (SQW) structure capable of obtaining two energy levels in order to broaden the spectrum distribution of output light. Is provided. This SLD realizes a wide full width at half maximum (FWHM) by using superposition of emission spectra from two energy levels. However, the SLD described in Non-Patent Document 1 has a problem that two peaks occur in the spectrum waveform of the output light, and a dent occurs between these peaks. Further, the SLD described in Non-Patent Document 1 has a problem that the spectrum waveform of the output light becomes asymmetrical.

  However, for example, in an OCT light source for ophthalmologic examination, it is desired that the dent between two peaks in the spectrum waveform is small, and that the spectral waveform has good left-right symmetry.

  On the other hand, the SLD described in Patent Document 1 includes an active layer having a structure with different gain wavelengths along the light guiding direction in order to broaden the spectrum distribution of output light. In addition, this SLD divides at least one of the upper and lower electrode layers of the laminate into two or more electrodes separated from each other in the waveguide direction, and changes the amount of injected current and its ratio for each electrode. Thus, the asymmetric irregularities of the spectral distribution of the output light are controlled to an arbitrary shape, for example, a Gaussian distribution.

The SLD described in Patent Document 1 controls the spectral distribution of output light to a Gaussian distribution, thereby causing the above-described problems of the SLD described in Non-Patent Document 1, the depression between peaks in the spectral waveform of output light, and It is considered that the left-right asymmetry of the spectrum waveform can be improved.
JP 2007-184557 A ATSemenov and athers, “Spectral control in multisection AlGaAs SQW superluminescent siodes at 800nm”, Electronics Letters, Vol.32, No.3, 1stFebruary 1996

  However, in the SLD described in Patent Document 1, it is necessary to provide a control circuit for controlling the amount of injected current and the ratio of each electrode.

  Therefore, the present invention can reduce the indentation between the peaks in the spectrum waveform more easily than the conventional one while maintaining the broad band of the spectrum waveform of the output light, and the symmetry of the spectrum waveform. An object of the present invention is to provide a super luminescent diode capable of increasing the brightness.

  As a result of intensive studies, the inventors of the present application have made the active layer a multiple quantum well (MQW) structure in the SLD, and the thickness of the barrier layer in the multiple quantum well structure is 1.5 nm or more 3 By setting the wavelength to 0.0 nm or less, the superposition of the emission spectra from two or more energy levels in the multiple quantum well is used, and the broadening of the spectrum waveform of the output light is maintained, while the peaks in the spectrum waveform are reduced. It was found that the indentation can be reduced and the left-right symmetry of the spectrum shape can be enhanced.

  Therefore, the superluminescent diode of the present invention is a superluminescent diode made of a GaAs-based material, the active layer has a multiple quantum well structure, and at least one of In and Al is used as a composition. In addition, the number of barrier layers in the active layer is 4 or less, and the thicknesses of the barrier layers are 1.5 nm or more and 3.0 nm or less, respectively.

  According to this superluminescent diode, since the active layer has a multiple quantum well structure and the thickness of the barrier layer in the active layer is not less than 1.5 nm and not more than 3.0 nm, two or more energies in the multiple quantum well can be obtained. By using the superposition of emission spectra from levels, it is possible to reduce the indentation between peaks in the spectrum waveform while maintaining a broad spectrum waveform of the output light, and to improve the symmetry of the spectrum shape. Can be increased. Further, in this super luminescent diode, since it is not necessary to provide a control circuit for controlling the amount of injected current and the ratio of each electrode as in the prior art, drive control is simple.

  According to the present invention, it is possible to reduce the indentation between the peaks in the spectrum waveform more easily than in the past while maintaining the broadband of the spectrum waveform of the output light of the superluminescent diode, and The left-right symmetry of the spectrum waveform can be enhanced.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.

  FIG. 1 is a plan view of a superluminescent diode according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view taken along line II-II shown in FIG. FIG. 3 is a diagram showing an energy band of the active layer shown in FIG.

  The super luminescent diode (hereinafter referred to as SLD) 1 is configured by laminating a plurality of semiconductor layers on an n-type GaAs substrate 10. An n-type AlGaAs cladding layer 20 is provided on the n-type GaAs substrate 10, a GaAs active layer 30 containing GaAs as a main component is provided on the n-type cladding layer 20, and a p-type AlGaAs cladding is provided on the active layer 30. A layer 40 is provided, and a p-type GaAs cap layer 50 is provided on the p-type cladding layer 40.

  Thus, the SLD 1 has a double hetero structure by the active layer 30 and the n-type cladding layer 20 and the p-type cladding layer 40 sandwiching the active layer 30. The n-type cladding layer 20 and the p-type cladding layer 40 are made of a material having a refractive index smaller than that of the material of the active layer 30, and thus act to confine light in the active layer 30.

  As shown in FIG. 3, the active layer 30 has a multiple quantum well (MQW) structure including a well layer 31 and a barrier layer 32. The active layer 30 has a separate confinement heterostructure (SCH) including the light guide layer 33. The light guide layer 33 sandwiches the well layer 31 and the barrier layer 32 and acts to confine carriers (electrons and holes) in the well layer 31. In the well layer 31, these electrons and holes are recombined to generate light.

  On the opposite side of the n-type substrate 10 from the n-type cladding layer 20 and on the opposite side of the p-type cap layer 50 from the p-type cladding layer 40, there are electrode layers for injecting current from the outside. Formed (not shown).

  The p-type cladding layer 40 and the p-type cap layer 50 have a ridge shape, and the electrode layer on the p-type cap layer 50 is in electrical contact only with the ridge portion 2. Thereby, the current injected from the outside, that is, electrons and holes, is efficiently guided only to the active region 3 of the active layer 30 corresponding to the ridge portion 2. Due to the difference in refractive index between the active region 3 and its peripheral portion, an effective refractive index difference occurs in the active layer 30, so that the active layer 30 corresponds to the active region 3, that is, according to the ridge portion 2. A refractive index type optical waveguide is generated.

  As described above, by adopting the ridge configuration, the current injection region and the optical waveguide region are limited, and the spatial transverse basic mode operation is realized.

  Further, as shown in FIG. 1, the exit surface 5 and the reflection surface 6 in the SLD 1 are each coated with a low reflection film, and the active region 3 corresponding to the ridge 2, that is, the optical waveguide is formed by these exit surfaces. 5 and the reflecting surface 6 are inclined. As a result, the reflectivity at the exit surface 5 and the reflection surface 6 viewed from the waveguide is extremely reduced, so that laser oscillation can be suppressed, and a relatively broad spectrum distribution like a light-emitting diode and a semiconductor are provided. An SLD capable of obtaining a relatively high output like a laser is realized.

  As shown in FIG. 3, the active layer 30 includes, for example, three well layers 31 and two barrier layers 32 that separate the well layers 31, and the thickness of the barrier layers 32. The length (width) Wb is 1.5 nm or more and 3.0 nm or less. On the other hand, the thickness (width) of these well layers 31 has two or more energy levels in all three layers, and the full width at half maximum (Full width) of the spectrum waveform obtained by superimposing emission spectra from these energy levels. (Width at Half Maximum: FWHM) is set in advance so as to be a desired value or more, for example, 50 nm or more.

  The number of barrier layers 32 is preferably 4 or less. According to this, an SLD that can suppress laser oscillation, has a relatively broad spectrum distribution like a light emitting diode, and can obtain a relatively high output like a semiconductor laser is realized.

To illustrate the detailed composition of each layer constituting the SLD 1, in order to obtain output light with a wavelength of 850 nm band, the n-type cladding layer 20 and the p-type cladding layer 40 have Al _0.40 Ga _0.60 As (thickness 1). 0.5 μm), the light guide layer 33 and the barrier layer 32 in the active layer 30 are made of Al _0.35 Ga _0.55 As, and the well layer 31 is made of a GaAs layer.

In addition, in order to obtain output light of a wavelength of 1060 nm band, the n-type cladding layer 20 and the p-type cladding layer 40 are Al _0.30 Ga _0.70 As (thickness 1.5 μm), and the light in the active layer 30 The guide layer 33 and the barrier layer 32 are made of Al _0.20 Ga _0.80 As, and the well layer 31 is made of a GaInAs layer. In this wavelength band, a GaAsP layer may be provided in a part of the barrier layer 32 or the light guide layer 33 between the well layers 31 in order to relax the strain of the well layer 31 made of GaInAs.

  Here, an InP-based long wavelength band SLD containing InP as a main component may be able to realize a broad band of 50 nm or more with one spectral waveform in the output light. However, in a GaAs-based SLD containing GaAs as a main component, It is difficult to realize a broad band of 50 nm or more with one spectral waveform in the output light. Therefore, the GaAs-based SLD includes the active layer 30 having a quantum well structure capable of obtaining two or more energy levels as described above, and uses superposition of emission spectra from these energy levels. Thus, the spectral waveform of the output light is broadened.

  However, in this type of SLD, two or more peaks occur in the spectral waveform of the output light, and a dent occurs between these peaks. Further, in this type of SLD, the spectrum waveform of the output light becomes asymmetrical.

  By the way, for example, in the light source of OCT for ophthalmologic examination and other applications, it is generally desired that the indentation between peaks in the spectrum waveform is small, and that the spectrum waveform has good left-right symmetry.

  Therefore, the inventors of the present application have made extensive studies, and as a result, in the SLD, the active layer has a multiple quantum well structure, and the thickness of the barrier layer in the multiple quantum well structure is 1.5 nm to 3.0 nm. By using superposition of emission spectra from two or more energy levels in a multiple quantum well, it is possible to reduce indentation between peaks in the spectrum waveform while maintaining a broad spectrum spectrum of the output light. It has been found that the symmetry of the spectral shape can be enhanced. The examination results are shown below.

  First, referring to FIG. 4, terms in the spectrum waveform of output light are defined. Δλ is the full width at half maximum, and ΔP [%] is the ratio of the size of the recess between peaks to the peak value of the spectrum waveform. A is the area of the long-wavelength side spectral waveform with the wavelength at the minimum portion of the recess between peaks as a boundary, and B is the area of the short-wavelength side spectral waveform.

  5 to 7 show experimental results of the SLD 1 having the output light of the wavelength 850 nm band according to the present embodiment. FIG. 5 is a spectrum waveform of output light using the thickness of the barrier layer 32 in the active layer 30 as a parameter. In FIG. 5, waveforms S <b> 2 to S <b> 7 are spectral waveforms when the thickness of the barrier layer 32 in the MQW active layer 30 is 6 nm, 4 nm, 3 nm, 2 nm, 1.5 nm, and 1 nm in order. In FIG. 5, the spectrum waveform of the SLD 1 including an active layer having a single quantum well (SQW) structure is shown as a waveform S1.

  FIG. 6 is a graph in which the indentation size ΔP and the full width at half maximum Δλ of each of the waveforms S1 to S7 in FIG. 5 are plotted against the thickness Wb of the barrier layer 32. In FIG. 6, a curve S8 corresponds to the left vertical axis and is a characteristic of the size ΔP of the depression with respect to the thickness Wb of the barrier layer 32. On the other hand, the curve S9 corresponds to the right vertical axis and is a characteristic of the full width at half maximum Δλ with respect to the thickness Wb of the barrier layer 32. FIG. 7 is a graph in which the area ratio A / B of each of the waveforms S1 to S7 in FIG. 5 is graphed with respect to the thickness Wb of the barrier layer 32.

  5 and 6, it can be seen that the size ΔP of the dent can be reduced if the thickness of the barrier layer 32 is reduced to 3 nm or less. However, if the thickness of the barrier layer 32 is too thin, such as 1 nm, the full width at half maximum Δλ becomes narrow, and for example, the desired full width at half maximum of 50 nm or more cannot be maintained. From this, it can be seen that in the range where the thickness of the barrier layer 32 is not less than 1.5 nm and not more than 3.0 nm, the indentation size ΔP can be made relatively small while keeping the full width at half maximum Δλ at 50 nm or more.

  On the other hand, according to FIGS. 5 and 7, the area ratio A / B of the spectrum waveform can be reduced as the thickness of the barrier layer 32 is reduced, that is, the left-right symmetry of the superimposed spectrum waveform is increased. You can see that Accordingly, in the range where the thickness of the barrier layer 32 is not less than 1.5 nm and not more than 3.0 nm, the left / right symmetry of the spectrum waveform can be relatively enhanced while keeping the full width at half maximum Δλ as wide as 50 nm or more.

  8 to 10 show experimental results of the SLD 1 having output light in the wavelength 1060 nm band of the present embodiment. FIG. 8 is a spectrum waveform of output light using the thickness of the barrier layer 32 in the active layer 30 as a parameter. In FIG. 8, waveforms S11 to S13 are spectrum waveforms when the thickness of the barrier layer 32 in the MQW active layer 30 is 5.2 nm, 2.4 nm, and 1.2 nm in this order. In FIG. 8, a waveform S14 is a spectrum waveform when the thickness of the barrier layer 32 in the MQW active layer 30 is approximately 1.2 nm, but the thickness of the barrier layer 32 is smaller than that in the waveform S13. Thus, in the present embodiment, there is a boundary for presence or absence of the effect when the thickness of the barrier layer 32 is around 1.2 nm.

  FIG. 9 is a graph in which the indentation size ΔP and the full width at half maximum Δλ of each of the waveforms S11 to S14 in FIG. 8 are plotted against the thickness Wb of the barrier layer 32. In FIG. 9, a curve S15 corresponds to the left vertical axis, and is a characteristic of the size ΔP of the depression with respect to the thickness Wb of the barrier layer 32. On the other hand, the curve S16 corresponds to the right vertical axis and is a characteristic of the full width at half maximum Δλ with respect to the thickness Wb of the barrier layer 32. FIG. 10 is a graph in which the area ratio A / B of each of the waveforms S11 to S14 in FIG. 8 is plotted against the thickness Wb of the barrier layer 32.

  According to FIG. 8 and FIG. 9, the full width at half maximum Δλ is kept wide as 50 nm or more in the range where the thickness of the barrier layer 32 is 1.5 nm or more and 3.0 nm or less, similar to the SLD 1 having the output light of the wavelength 850 nm band. However, it can be seen that the indentation size ΔP can be made relatively small.

  On the other hand, according to FIG. 8 and FIG. 10, the full width at half maximum Δλ is 50 nm or more in the range where the thickness of the barrier layer 32 is 1.5 nm or more and 3.0 nm or less, similarly to the SLD 1 having the output light in the wavelength band of 850 nm. It can be seen that the left-right symmetry of the spectrum waveform can be relatively enhanced while keeping it wide.

  Next, the above-described experimental results are analyzed using FIGS. 11 and 12. FIG. 11 shows the energy level (a) of the active layer 30 when the thickness of the barrier layer 32 in the active layer 30 is sufficiently larger than 3.0 nm, and the output light of the SLD 1 including the active layer 30. FIG. 12 is a diagram showing the spectrum waveform (b), and FIG. 12 is a diagram showing the energy level (a) of the active layer 30 in the SLD 1 of the present embodiment and the spectrum waveform (b) of the output light of the SLD 1. .

  As shown in FIG. 11 (a), this structure basically uses two light emission hω1 and hω2 of light emission from the quantum level n = 1 and light emission from the quantum level n = 2. When the distance between the well layers 31, that is, the thickness Wb of the barrier layer 32 is sufficiently larger than 3.0 nm, the interaction between the quantum wells is small and the wave functions do not overlap. Separation does not occur. In this case, each quantum level in the well layer 31 becomes a single level, and the emission spectrum width obtained from each level becomes narrow. As a result, as shown in FIG. 11B, in the superimposed emission spectrum waveform S31 having two peaks corresponding to these quantum levels, the dent between the two peaks becomes large.

  However, as shown in FIG. 12A, when the distance between the well layers 31, that is, the thickness Wb of the barrier layer 32 is as small as 3.0 nm or less, the wave functions overlap due to the interaction between the quantum wells. , Quantum level separation occurs. In this case, each quantum level in the well layer 31 becomes a plurality of levels, and the respective emission spectrum widths are broadened. As a result, as shown in FIG. 12B, it is considered that the depression between the two peaks is reduced in the overlapped emission spectrum waveform S32 having two peaks corresponding to these quantum levels.

  As described above, according to the SLD 1 of the present embodiment, the active layer 30 has a multiple quantum well structure, and the thickness Wb of the barrier layer 32 in the active layer 30 is not less than 1.5 nm and not more than 3.0 nm. By using superposition of emission spectra from two or more energy levels in the well, it is possible to reduce the indentation between peaks in the spectrum waveform while maintaining a broad spectrum spectrum of the output light. In particular, the spectrum waveform can be made uniform over a wide band. That is, the spectrum shape can be smoothed. Furthermore, according to the SLD 1 of the present embodiment, the left-right symmetry of the spectrum shape of the output light can be enhanced.

  Further, in the SLD 1 of the present embodiment, unlike in Patent Document 1, it is not necessary to provide a control circuit for controlling the amount of injected current for each electrode and the ratio thereof, so that drive control is simple.

  The present invention is not limited to the above-described embodiment, and various modifications can be made. For example, in the present embodiment, the SLD 1 that obtains output light in the wavelength of 850 nm band and the wavelength of 1060 nm band is illustrated. However, even in the SLD that obtains output light from the 750 nm band to the 1200 nm band, the active layer has a multiple quantum well structure, The same advantage can be obtained by setting the thickness of the substrate to 1.5 nm to 3.0 nm. At this time, any one of AlGaAs, GaAs, InGaAs, AlGaInAs, GaInAsP, etc. may be used as the material of the well layer according to the wavelength band of the output light. Also in this case, since the barrier layer contains at least one of In or Al as a composition, the active layer has at least one of In and Al as a composition.

  In this embodiment, the SLD 1 is formed by stacking a semiconductor layer on the n-type substrate 10, but the SLD may be formed by stacking a semiconductor layer on a p-type substrate.

It is a top view of the super luminescent diode which concerns on embodiment of this invention. It is sectional drawing which follows the II-II line | wire shown in FIG. It is a figure which shows the energy band of the active layer shown in FIG. It is a figure which defines the term in the spectrum waveform of output light. It is a figure which shows the experimental result of SLD which has the output light of wavelength 850nm band of this embodiment, Comprising: It is the spectrum waveform of the output light which used the thickness of the barrier layer in an active layer as a parameter. FIG. 6 is a graph of the indentation size ΔP and full width at half maximum Δλ of each waveform in FIG. 5 versus the thickness Wb of the barrier layer. FIG. 6 is a diagram in which the area ratio A / B of each waveform in FIG. 5 is graphed with respect to the thickness Wb of the barrier layer. It is a figure which shows the experimental result of SLD which has the output light of the wavelength 1060nm band of this embodiment, Comprising: It is the spectrum waveform of the output light which used the thickness of the barrier layer in an active layer as a parameter. FIG. 9 is a graph in which the indentation size ΔP and full width at half maximum Δλ of each waveform in FIG. 8 are plotted against the thickness Wb of the barrier layer. FIG. 9 is a graph in which the area ratio A / B of each waveform in FIG. 8 is graphed with respect to the thickness Wb of the barrier layer. The energy level (a) of the active layer when the thickness of the barrier layer in the active layer is sufficiently larger than 3.0 nm and the spectrum waveform (b) of the output light of the SLD 1 including this active layer are shown. FIG. It is a figure which shows the energy level (a) of the active layer in SLD of this embodiment, and the spectrum waveform (b) of the output light of SLD.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Super luminescent diode, 2 ... Ridge part, 3 ... Active region, 5 ... Output surface, 6 ... Reflecting surface, 10 ... n-type GaAs substrate, 20 ... n-type AlGaAs clad layer, 30 ... GaAs active layer, 31 ... well layer, 32 ... barrier layer, 33 ... light guide layer, 40 ... p-type AlGaAs cladding layer, 50 ... p-type GaAs cap layer.

Claims (1)

A superluminescent diode made of a GaAs-based material,
The active layer has a multiple quantum well structure, includes at least one of In and Al as a composition,
The number of barrier layers in the active layer is 4 or less,
The thicknesses of the barrier layers are 1.5 nm to 3.0 nm, respectively.
Super luminescent diode.

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