JP2701569B2 - Method for manufacturing 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 semiconductor laser alone, a semiconductor laser and a semiconductor optical modulator, or an optical semiconductor device in which an optical waveguide is integrated on the same semiconductor substrate, which is used for optical communication and optical information processing. It relates to a manufacturing method.

[0002]

2. Description of the Related Art Semiconductor lasers used for optical communication and optical information processing are required to have higher performance. On the other hand, in order to cope with an application requiring a low price, such as for a subscriber optical communication, it is necessary to manufacture devices with a high yield using a large-area wafer. In order to satisfy such demands, it is necessary to perform crystal growth by a vapor phase growth method such as metal organic vapor phase epitaxy (MOVPE) capable of high-area uniform growth. Also, by using the vapor phase growth method, it is possible to manufacture a semiconductor laser having a quantum well structure having various features such as a low threshold high efficiency operation and a narrow spectral line width operation.

FIG. 7 shows a typical method of manufacturing a semiconductor laser for optical communication using MOVPE. Here, it is a distributed feedback (DFB) laser that operates in a single mode, and current is confined by a buried ridge structure. First n-
After forming the grating on the InP substrate 1, the n-I
nGaAsP guide layer 8, InGaAsP active layer 3, p
-InP clad layer 4 is laminated (FIG. 7A),
The iO ₂ film 21 is formed in a stripe shape having a width of 2 μm (FIG. 7).
(B)), mesa etching is performed until the substrate 1 is reached (FIG. 7 (c)). Thereafter, the p-InP layer 5 and p ⁺
An InGaAsP cap layer 7 is grown (FIG. 7D),
The current is narrowed by forming a high-resistance region 31 into which protons are implanted around the active layer (FIG. 7E).

In a semiconductor light emitting device, a semiconductor optical integrated device having characteristics that cannot be obtained by a semiconductor laser alone, for example, an integrated device of a DFB semiconductor laser and a semiconductor optical modulator having a small spectrum spread at the time of high-speed modulation, a wavelength tunable There is also an increasing demand for devices such as distributed reflection (DBR) semiconductor lasers having functions. In these integrated devices, further improvement in characteristics is necessary, and improvement in uniformity and yield is indispensable in consideration of arraying. As a conventional example of such an integrated device, a structure in which two semiconductor laser devices, a multiplexer, and an optical waveguide are integrated is shown in FIG.
Shown in FIG. 8A is a plan view schematically showing an element.
(B) is a perspective view showing the structure of the element. The active layer 3 exists only in the laser region, and the guide layer 8 exists over the entire laser region and the waveguide region. For example, when InGaAsP having a wavelength of 1.55 μm is used for the active layer 3, InGaAsP having a wavelength of 1.3 μm is used for the guide layer 8. In order to allow a current to flow only in the laser region and to provide electrical insulation between the two laser elements, a high resistance buried structure buried with high resistance InP13 is used, and mesa etching is used.

[0005] The fabrication process of this device will be described. MOVPE is generally used for crystal growth. First, nI
A grating is formed only on the laser region on the nP substrate 1, and the n-InGaAsP guide layer 8 and the InGaAs
A P active layer 3 and a p-InP cladding layer 4 are grown. Next, using the SiO ₂ film as a selection mask, the p-InP
The cladding layer 4 and the InGaAsP active layer 3 are removed, and In
A GaAsP waveguide layer and a p-InP cladding layer (not shown in the figure) are selectively grown. Next, mesa etching is performed using the SiO ₂ film as a mask, and Fe-doped high-resistance In
The P layer 13 is buried and grown. After removing the SiO ₂ film, the p-InP cladding layer 5 and p ⁺ −
The InGaAs cap layer 7 is grown. After forming an insulating groove between the laser region and the waveguide region and between the two laser elements by etching, an SiO ₂ film 21 is deposited on the entire surface, and the upper portion of the laser portion is opened to open the p-side. And the n-side electrode 33 on the substrate side is completed. In this example, a multi-wavelength light source can be obtained by changing the pitch of the grating in the two laser regions or by using a multi-electrode structure.

[0006]

In order to manufacture a large number of semiconductor lasers and various integrated optical devices, it is necessary to use a large-area wafer and precisely control the layer structure. is important. The layer thickness can be sufficiently controlled by using a vapor phase growth method such as MOVPE.
Conventionally, the waveguide width is controlled by mesa etching using SiO ₂ or the like as a mask, and there is a problem that sufficient controllability cannot be obtained by side etching or the like. For example, in the mesa etching shown in FIG. 7C, even if the width of the SiO ₂ film 21 is exactly 2 μm, the width of the active layer varies due to variations in the mesa structure and side etching during the active layer etching. . In particular, in a process using a large-diameter wafer such as a 2-inch substrate, the variation in the wafer surface becomes considerably large. Variations in active layer and waveguide width affect device characteristics such as threshold current, oscillation wavelength, and beam pattern, which not only reduce device yield but also make it difficult to operate as designed. There was a need for improvement. Also, when manufacturing an optical integrated device, it is necessary to fabricate the active layer and the waveguide layer by abutting each other, but the etching and burying growth process is complicated and lacks uniformity and reproducibility. There was also a problem in terms of improving the yield.

[0007]

A method for manufacturing an optical semiconductor device according to the present invention for solving the above problems is as follows.

(1) A step of forming two opposing dielectric thin film stripes on a semiconductor substrate with an optical waveguide forming region interposed therebetween, and forming an active layer on the semiconductor substrate other than the dielectric thin film stripes A selective crystal growth step of laminating a semiconductor multilayer film including the step of: after the selective crystal growth step, partially removing the opposing inner side edge of the dielectric thin film stripe, A method of exposing a part of the semiconductor substrate, and a step of selectively growing a semiconductor cladding layer covering the selectively grown semiconductor multilayer film following this step. .

(2) forming a multi-layer semiconductor substrate including a semiconductor guide layer on a crystal substrate divided into a first region having a diffraction grating formed on the surface and a second region having a flat surface; Forming two opposing dielectric thin film stripes on the multilayer semiconductor substrate with an optical waveguide formation region therebetween, and including an active layer on the multilayer semiconductor substrate other than the dielectric thin film stripes; A selective crystal growth step of laminating a semiconductor multilayer film, wherein the width of the optical waveguide forming region sandwiched between the two dielectric thin film stripes is constant, but the width of the dielectric thin film stripe is the first region. Wherein the semiconductor laser is formed in the first region and a semiconductor optical modulator is formed in the second region. Manufacturing of Law.

(3) a first region and a second region having a flat surface;
And a step of forming a multilayer semiconductor substrate including a semiconductor guide layer on a crystal substrate arranged in this order, divided into a third region having a diffraction grating formed on the surface thereof; and Forming two opposing dielectric thin film stripes with an optical waveguide forming region therebetween, and selectively crystallizing a semiconductor multi-layer film including an active layer on the multi-layer semiconductor substrate other than the dielectric thin film stripes And a width of the optical waveguide forming region sandwiched between the two dielectric thin film stripes is the same in the first region, the second region, and the third region, and the width of the dielectric thin film stripe is The second region and the third region are the same, but the stripe width in the first region is formed wider than the second region and the third region.
A method for manufacturing an optical semiconductor device, wherein a phase control unit is formed in a region, and a variable wavelength Bragg reflector is formed in a third region.

[0011]

The [011] of the surface of the (100) -oriented semiconductor substrate
A thin film of dielectric thin film such as a SiO ₂ film is formed in _two parallel directions to form a double hetero (DH) structure in MOVPE.
After selective growth by the method, the portion sandwiched between the stripes is a (100) plane with a flat surface and a (111) B with a smooth side surface.
Since the active layer is grown in a ridge shape as a surface, the width of the active layer can be determined only by patterning the SiO ₂ film without using a method lacking in uniformity such as mesa etching. In this selective growth, the side surface of the active layer can be covered with the upper cladding layer during the growth. For this reason, it has become possible to manufacture a semiconductor laser having excellent controllability and reproducibility and having good characteristics with little interfacial recombination component.

In the method of the present invention, the p-InP cladding layer and the p ⁺ InGaAs cap layer are also formed by selective growth. Therefore, the device fabrication process is
_It is constituted only by patterning and selective growth of a dielectric thin film such as _2, and there is no need to use any semiconductor etching which is a source of various problems. In this way, elements can be manufactured by batch growth / process excellent in uniformity and reproducibility using a large-area wafer, and the advantage of forming the active layer by selective growth can be maximized.

Furthermore, in this MOVPE selective growth, Si
The feature is that the growth rate increases as the O ₂ stripe width increases, and the group III composition also changes during mixed crystal growth. FIG. 5A shows InP and In grown selectively.
Relationship between stripe width and growth rate of GaAs mixed crystal, FIG.
(B) In _x Ga _1-x As and In _x Ga selectively grown
An example of the result of measuring the relationship between the stripe width of _1-x As _0.6 P _0.4 mixed crystal (1.3 μm wavelength composition) and the In composition x is shown. The growth rate increases as the stripe width increases.
This is because the amount of grown species that diffuse laterally from the SiO ₂ film and reach the semiconductor surface increases. The increase in the In composition is considered to be due to the fact that the In source species is more likely to diffuse in the lateral direction than the Ga source species. From this, InG
In the selective growth of a quantum well structure using aAs or InGaAsP for a well, if the stripe width is increased, the well layer thickness is increased, and lattice strain (compressive stress) is applied so that the In composition of the well is increased. Reduces the transition energy of the quantum well structure.

FIG. 6 shows Inx Ga _{1 -x} As well and Ix Ga _{1 -x} As well.
nx Ga 1 _{over x} As _y P 1 _{over y} of the barrier multi-quantum well (MQW) when the structure is selectively grown, showing the measurement results of the stripe width dependence of the emission wavelength from the selective growth layer. The wavelength becomes longer as the stripe width becomes wider, and the wavelength becomes about 1.4 μm when the width is about 4 μm and about 1.55 μm when the width is 10 μm. Therefore, in an integrated device such as a semiconductor laser and an optical waveguide, by increasing the stripe width of the laser region as compared with the waveguide region, a semiconductor laser having a wavelength of 1.55 μm band and a waveguide layer having a wavelength of 1.3 μm band are obtained. Can also be formed collectively. That is, by one crystal growth step, a laser oscillation region and a waveguide region transparent to this light can be formed in the waveguide direction, and can be applied to optical element manufacturing methods of various structures. In the production of the optical integrated device of the present invention, by selectively growing the active layer after growing the guide layer over the entire surface, the layer thickness of the selectively grown layer does not increase, and lattice strain generated as the growth proceeds is reduced. Can be minimized. I of the quantum well active layer
Since the nGaAs well has a layer thickness of about several nm, even if lattice distortion occurs, the strained quantum well structure is relaxed in the well, so that crystal defects do not occur. In InGaAsP used for the barrier, there is no problem because the composition variation due to the stripe width is not so large as that of InGaAs and the composition variation due to the growth is not so large.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a description will be given of the result of fabricating a buried ridge structure semiconductor laser using the invention of claim 1 of the present invention. FIG. 1 shows the manufacturing method. An SiO ₂ film 21 (about 2000 A (angstrom) in thickness) is deposited on the surface of the (100) -oriented n-InP substrate 1 by the CVD method, and has a width of 10 μm and an interval of 1.8 μm by the photolithography method. Two stripes were formed (FIG. 1).
(A)). Then, the Si-doped n-InP cladding layer 2 (layer thickness: 1000 A, carrier concentration: 1 × 10 ¹⁸ cm ⁻³ ), the InGaAsP active layer 3 are formed by the reduced pressure MOVPE method.
(1.55 μm composition, layer thickness 800 A), Zn-doped p-
InP cladding layer 4 (layer thickness 500A, carrier concentration 5 ×
10 ¹⁷ cm ^-3 ) was selectively grown (FIG. 1B). The layer thickness was a value in the active region sandwiched between the SiO ₂ films 21, and the layer thickness was constant in this region. Next, the SiO ₂ film 21 is removed in a stripe shape having a width of 10 μm around the active region (FIG. 1C), and the remaining SiO ₂ film 21 is used.
p-InP cladding layer 6 (layer thickness 1.5 μm, carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) and p ⁺ -InGaAs cap layer 7 (layer thickness 0.3 μm, carrier concentration 1 × 10 ¹⁹ cm)
^-3 ) is selectively grown (FIG. 1 (d)), and only the upper part of the active region of the SiO ₂ film 21 formed again on the entire surface is removed in a stripe shape having a width of about 7 μm (FIG. 1 (e)). The laser was completed by forming the electrode 32 and the n-side electrode 33 (FIG. 1).
(F)). When this laser was evaluated at a cavity length of 300 μm, the threshold current was 10.2 mA on average, the standard deviation was 0.2 mA, the slope efficiency was 0.23 W / A on average, and the standard deviation was 0.04 W / A. The active layer width is 1.52 on average
μm, standard deviation 0.12 μm. This result was improved as compared with the result of the conventional example, and it was confirmed that the device characteristics were improved by using the present invention. By using MOVPE growth capable of uniform growth over a large area in this manner, a low-priced semiconductor laser with high characteristic yield can be manufactured. In this example, bulk InGaAsP was used for the active layer, but the quantum well structure (MQ
By using W), the characteristics can be further improved. Although the n-type substrate is used for the substrate 1, a p-type substrate may be used. In this case, the conductivity type of the growth layer is opposite.

Next, as an embodiment of the second aspect of the present invention, M
The result of fabricating a device in which a DFB semiconductor laser having an active layer of a QW structure and a semiconductor optical modulator utilizing the quantum confined Stark effect are manufactured will be described. 2 and 3 show the manufacturing method. A grating (diffraction grating) 11 is formed only in the laser region of the (100) n-InP substrate 1 in the <011> direction (FIG. 2A), and n-InGa is formed on the entire surface.
AsP guide layer 8 (wavelength 1.3 μm composition, carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 1000 A), n-InP spacer layer 9 (carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 500)
A) was grown (FIG. 2 (b)). Then, two SiO
_{The two} films 21 are parallel straight lines (interval of 2 μm) on the sides facing each other and have a stripe width of 10 in the laser region.
μm and 6 μm in the modulator region (FIG. 2C). The transition region length of the stripe width is 20
μm. Next, the n-InP cladding layer 2 (carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 500A), the MQW active layer 3 and the p-InP cladding layer 4 (carrier concentration 5 × 10 ¹⁷
(cm ^-3 , layer thickness 500A) was selectively grown (Fig. 3).
(A)). MQW has 4 wells and the wells are InGaAs
s and the barrier were InGaAsP. In the active region, the well and the barrier are lattice-matched to the InP substrate 1,
The growth conditions were set so that the well thickness was 75 A and the barrier thickness was 150 A. As a result, the emission wavelength in the active region is 1.5
6 μm, and about 1.45 μm in the modulator region. Next, the SiO ₂ films 21 on both sides adjacent to the waveguide region are respectively removed over a width of 2 μm (FIG. 3B).
InP clad layer 6 (carrier concentration 5 × 10 ¹⁷ c
m ^-3 , layer thickness 1.5 μm) and the p ⁺ -InGaAs cap layer 7 (layer thickness 0.3 μm, carrier concentration 1 × 10 ¹⁹ c)
m ^-3 ) was selectively grown (FIG. 3 (c)). Finally, a window was formed on the entire surface of the SiO ₂ film between the laser and modulator regions each having a length of 20 μm, and the p ⁺ -InGaAs cap layer 7 was removed by etching to achieve electrical insulation between the regions. Then, a p-side electrode was formed in a pad shape, and an n-side electrode was also formed on the substrate 1 side to make an element. The cleaved laser region length was 500 μm, and the modulator region length was 200 μm.

The oscillation threshold current of a typical device is 20 m
In A, the maximum CW light output from the modulator side was 30 mW. The oscillation wavelength is 1.55 μm, and -5 V
The extinction ratio when applied was 20 dB. Further, the coupling efficiency estimated from the quenching characteristic was as high as 98%. The separation resistance between the regions was 10 kΩ. In all of the 20 randomly selected devices, an extinction ratio of 15 dB or more when -5 V was applied was obtained. As described above, by the technique of simultaneously growing the active layer and the absorbing layer by the selective growth of the present invention, a good coupling waveguide structure can be easily manufactured,
It was confirmed that the integrated device can realize good device characteristics and uniformity.

Finally, as an embodiment of the DBR semiconductor laser according to the third aspect of the present invention, a result of fabricating a wavelength tunable DBR semiconductor laser array having multiple wavelengths will be described. FIG. 4 shows an element structure in which a waveguide region divided into an active region, a phase adjustment region, and a DBR region is arranged in an array. The manufacturing method is based on FIGS. First, an n-InGaAsP guide layer 8 (wavelength 1.3) is formed on an n-InP substrate 1 having a grating 11 formed only in the DBR region.
μm composition, carrier concentration 1 × 10 ¹⁸ cm ^-3 , layer thickness about 10
00A), n-InP spacer layer 9 (carrier concentration 1 ×
10 ^{18 cm-3,} the growth of the layer thickness of about 500A). continue,
The two SiO ₂ films were patterned so that the side surfaces facing each other were parallel straight lines (interval 2 μm), and the stripe width was 10 μm in the active region and 4 μm in the phase adjustment region and the DBR region. The transition region width of the stripe width was 20 μm. Next, the n-InP cladding layer 2 (carrier concentration 1 × 10 ¹⁸ cm ⁻³ , layer thickness 500 A), the MQW active layer 3 and the p-InP cladding layer 4 (carrier concentration 5 × 1
0 ¹⁷ cm ^-3 , layer thickness 500A) was selectively grown. MQW has 4 wells, the well is InGaAs, the barrier is InG
aAsP. The growth conditions were set so that the well and the barrier were lattice-matched to the InP substrate in the active region, and had a well thickness of 75A and a barrier thickness of 150A. As a result, the emission wavelength in the active region was 1.56 μm, and was approximately 1.35 μm in the phase adjustment region and the DBR region. Next, the SiO ₂ films on both sides adjacent to the waveguide region were respectively removed over a width of 2 μm, and then the p-InP layer cladding layer 6 (carrier concentration 5 × 10 ¹⁷ cm ⁻³ , layer thickness 1.5 μm) and p ^{A +} -InGaAs cap layer 7 (layer thickness 0.3 μm, carrier concentration 1 × 10 ¹⁹ cm ⁻³ ) was selectively grown. Finally, a window was formed in the entire surface of the SiO ₂ film over a region having a width of 20 μm, and the p ⁺ -InGaAs cap layer 7 was removed by etching to achieve electrical insulation between the regions. Then, the p-side electrode 32 in each region was formed in a pad shape, and the n-side electrode 33 was also formed on the substrate side to make an element. The length of the cleaved active region is 500 μm, the length of the phase adjustment region is 150 μm, and D
The BR region length was 300 μm. The waveguide spacing is 60
The thickness was set to 0 μm, and after applying an anti-reflection coating to the emission end face on the active region side, four channels were cut out and evaluated.

The typical oscillation threshold current of this four-channel tunable laser is 18 mA, and the oscillation wavelength of four channels when current is injected only into the active region is 1.553 μm.
m ± 0.003 μm. Single mode operation was confirmed up to an optical output of 30 mW. Further, by injecting current into the phase adjustment region and the DBR region, it was possible to obtain a wavelength tunable range of 5 nm or more in each channel while keeping the optical output at 5 mW.

In the second and third embodiments of the present invention, the current confinement structure using the selective growth according to the first embodiment is employed. However, a conventional method, such as that shown in FIG. The effect of lattice distortion reduction described in the second and third aspects of the present invention can be obtained even if a structure formed by proton implantation is used.

[0021]

As described above, when the method of manufacturing an optical semiconductor device according to the present invention is used, etching of a semiconductor having poor uniformity and reproducibility is not required at all, and a device having a uniform active layer and a uniform waveguide width can be obtained. Can be produced with good controllability. By performing this method by batch growth / process using a large-area wafer, a low-priced semiconductor laser with high characteristics can be manufactured at a high yield. Further, since the composition fluctuation occurring in the growth layer can be suppressed to a level that does not cause any problem, M
The emission wavelength and the effective refractive index can be controlled by changing the mask width while keeping the crystallinity of the QW structure good. As a result, various types of semiconductor optical integrated devices, which conventionally required complicated processes, can be easily manufactured with good uniformity while maintaining good characteristics such as high waveguide coupling efficiency.

[Brief description of the drawings]

FIG. 1 is a sectional view showing one embodiment of a method for manufacturing a semiconductor laser according to claim 1 of the present invention.

FIG. 2 is a view showing one embodiment of a method for manufacturing an integrated device of a distributed feedback semiconductor laser and a semiconductor optical modulator according to the invention of claim 2;

FIG. 3 is a view showing a method of manufacturing an integrated device according to the invention of claim 2, and is a continuation of FIG. 2;

FIG. 4 is a diagram showing a structure of a distributed reflection type semiconductor laser manufactured according to the invention of claim 3;

FIG. 5 is a diagram showing a relationship between a stripe width of a dielectric thin film and a growth rate and a composition of a selectively grown crystal.

FIG. 6 is a diagram illustrating the relationship between the stripe width and the emission wavelength of a quantum well structure that has been selectively grown.

FIG. 7 is a view for explaining a conventional method of manufacturing a semiconductor laser.

FIG. 8 is a diagram showing an optical semiconductor device manufactured by a conventional method.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 n-InP substrate 2 n-InP cladding layer 3 active layer (including quantum well structure) 4 p-InP cladding layer 5 p-InP layer 6 p-InP cladding layer 7 p ⁺ InGaAs cap layer 8 n-InGaAsP guide layer Reference Signs List 9 n-InP spacer layer 11 grating 13 high-resistance InP layer 21 SiO ₂ film 31 proton injection region 32 p-side electrode 33 n-side electrode

Continuation of front page (56) References JP-A-2-62090 (JP, A) JP-A-63-90879 (JP, A) JP-A-1-321677 (JP, A) JP-A-4-100291 (JP) , A) 1991 (Heisei Era) Spring Proceedings of the Society of Applied Chemistry 28p-ZK-4 222 1990 (Heisei 2) Proceedings of the Fall Society of Natural Sciences 26p-R-2 915

Claims (3)

(57) [Claims] A step of forming two opposing dielectric thin film stripes on a semiconductor substrate with an optical waveguide forming region therebetween, and including an active layer on the semiconductor substrate other than the dielectric thin film stripes A selective crystal growth step of laminating a semiconductor multilayer film, wherein the opposing inner side edges of the dielectric thin film stripe are partially removed after the selective crystal growth step, A method for manufacturing an optical semiconductor device, comprising: a step of exposing a part of a semiconductor substrate; and a step of selectively growing a semiconductor cladding layer covering the selectively grown semiconductor multilayer film following this step. 2. A step of forming a multilayer semiconductor substrate including a semiconductor guide layer on a crystal substrate divided into a first region having a diffraction grating formed on a surface thereof and a second region having a flat surface. Forming two opposing dielectric thin film stripes on a multilayer semiconductor substrate with an optical waveguide forming region therebetween, and a semiconductor including an active layer on the multilayer semiconductor substrate other than the dielectric thin film stripes A selective crystal growth step of laminating a multilayer film, wherein the width of the optical waveguide forming region sandwiched between the two dielectric thin film stripes is constant, but the width of the dielectric thin film stripe is in the first region. A semiconductor laser is formed in the first region;
A method for manufacturing an optical semiconductor device, comprising forming a semiconductor optical modulator in a region. A first region having a flat surface, a second region having a flat surface, and a third region having a diffraction grating formed on the surface;
A step of forming a multilayer semiconductor substrate including a semiconductor guide layer on a crystal substrate arranged in this order; and two dielectric thin films opposed to each other with an optical waveguide formation region interposed therebetween on the multilayer semiconductor substrate. Forming a stripe, and selectively growing a semiconductor multilayer film including an active layer on the multilayer semiconductor substrate other than the dielectric thin film stripe, and sandwiching the two dielectric thin film stripes. The width of the optical waveguide forming region is the same in the first region, the second region, and the third region, and the width of the dielectric thin film stripe is the same in the second region and the third region. The stripe width in the region is wider than the second region and the third region. The first region has a light emitting portion, the second region has a phase controller, and the third region has a variable wavelength Bragg reflector. Is formed Method for manufacturing an optical semiconductor element characterized.