Patent ID: 12218486

DESCRIPTION OF PREFERRED EMBODIMENTS

The following will describe embodiments of the present invention in detail. In the following embodiments, a case where the present invention is embodied as a surface emitting laser (semiconductor laser) will be described. However, the present invention is not limited to the surface emitting laser but applicable to various kinds of vertical cavity surface emitting devices, such as a vertical cavity surface emitting diode.

Embodiment 1

FIG.1is a cross-sectional view of a Vertical Cavity Surface Emitting Laser (hereinafter referred to as a surface emitting laser: VCSEL) according to Embodiment 1.FIG.2is a schematic top view of a surface emitting laser10.FIG.1is a cross-sectional view taken along the line V-V inFIG.2. The configuration of the surface emitting laser10will be described with reference toFIG.1andFIG.2.

The surface emitting laser10includes a substrate11and a first multilayer film reflecting mirror (hereinafter simply referred to as a first reflecting mirror)12formed on the substrate11. In this embodiment, the first reflecting mirror12is formed on the substrate11and has a structure in which first semiconductor films (hereinafter referred to as high refractive index semiconductor films) H1and second semiconductor films (hereinafter referred to as low refractive index semiconductor films) L1having a refractive index lower than that of the high refractive index semiconductor film H1are stacked in alternation.

That is, in this embodiment, the first reflecting mirror12is a semiconductor multilayer film reflecting mirror constituting a Distributed Bragg Reflector (DBR) made of a semiconductor material.

In this embodiment, the substrate11has a composition of GaN. The substrate11is a substrate for growth used for crystal growth of the first reflecting mirror12. The high refractive index semiconductor layer H1in the first reflecting mirror12has a composition of GaN, and the low refractive index semiconductor layer L1has a composition of AInN. In this embodiment, between the substrate11and the first reflecting mirror12, a buffer layer (not illustrated) having a composition of GaN is disposed.

The surface emitting laser10includes a light-emitting structure layer EM formed on the first reflecting mirror12and including a light-emitting layer14. In this embodiment, the light-emitting structure layer EM includes a plurality of semiconductor layers made of a nitride-based semiconductor. The light-emitting structure layer EM includes a n-type semiconductor layer (first semiconductor layer)13formed on the first reflecting mirror12, the light-emitting layer (active layer)14formed on the n-type semiconductor layer13, and a p-type semiconductor layer (second semiconductor layer)15formed on the light-emitting layer14.

In this embodiment, the n-type semiconductor layer13has a composition of GaN and contains Si as n-type impurities. The light-emitting layer14has a quantum well structure that includes a well layer having a composition of InGaN and a barrier layer having a composition of GaN. A p-type semiconductor layer15has a GaN-based composition and contains Mg as p-type impurities.

The configuration of the light-emitting structure layer EM is not limited to this. For example, the n-type semiconductor layer13may include a plurality of n-type semiconductor layers having mutually different compositions. The p-type semiconductor layer15may include a plurality of p-type semiconductor layers having mutually different compositions.

For example, the p-type semiconductor layer15may include, for example, an AlGaN layer as an electron-blocking layer (not illustrated) that reduces an overflow of electrons injected into the light-emitting layer14to the p-type semiconductor layer15at the interface with the light-emitting layer14. The p-type semiconductor layer15may include a contact layer (not illustrated) to form an ohmic contact with an electrode. In this case, for example, the p-type semiconductor layer15only needs to include a GaN layer as a cladding layer between the electron-blocking layer and the contact layer.

In this embodiment, the p-type semiconductor layer15includes a first region15A and a second region15B in an upper surface15U. The second region15B is disposed outside the first region15A and depressed from the first region15A toward the light-emitting layer14. The second region15B includes an inactivated region15C where p-type impurities are inactivated. As illustrated inFIG.2, in this embodiment, the first region15A projects from the second region15B in a columnar shape.

For example, the first region15A can be formed by performing dry etching on the upper surface15U of the p-type semiconductor layer15with a circular region left. A surface of the semiconductor, such as the p-type semiconductor layer15, containing impurities is roughened by dry etching. This inactivates the p-type impurities in the etched part, thus forming the inactivated region15C.

Therefore, the second region15B functions as a high resistance region HR having an electrical resistance higher than that of the first region15A. Meanwhile, the region where etching is not performed, namely, the first region15A, which is the region where the inactivated region15C is not disposed, functions as a low resistance region LR. In the second region15B, the p-type semiconductor layer15is partially removed by dry etching. Therefore, the second region15B is depressed toward the light-emitting layer14side from the first region15A.

In other words, the p-type semiconductor layer15includes the low resistance region LR and the high resistance region HR in the upper surface15U. The high resistance region HR is depressed from the low resistance region LR toward the light-emitting layer14outside the low resistance region LR. The p-type impurities are inactivated in the high resistance region HR such that the high resistance region HR has the electrical resistance higher than that of the low resistance region LR.

The p-type semiconductor layer15functions as a current confinement layer that confines a current path from which a current is injected to the light-emitting structure layer EM. The first region15A of the p-type semiconductor layer15functions as a current injected region from which a current is injected into the light-emitting layer14. On the other hand, the second region15B of the p-type semiconductor layer15functions as a non-current injected region from which the injection of a current into the light-emitting layer14is suppressed.

The surface emitting laser10includes a light-transmitting electrode layer16in contact with the first and the second regions15A and15B of the p-type semiconductor layer15and formed on the upper surface15U of the p-type semiconductor layer15. The light-transmitting electrode layer16is a conductive film having translucency to light emitted from the light-emitting layer14. For example, the light-transmitting electrode layer16is made of a metal oxide film, such as ITO or IZO.

The surface emitting laser10includes an insulating layer17formed on the light-transmitting electrode layer16. For example, the insulating layer17is made of a metal oxide, such as Ta2O5, Nb2O5, ZrO2, TiO2, and HfO2. The insulating layer17has translucency to the light emitted from the light-emitting layer14.

The surface emitting laser10includes a second multilayer film reflecting mirror (hereinafter simply referred to as a second reflecting mirror)18formed on the insulating layer17. The second reflecting mirror18is disposed at a position facing the first reflecting film12via the light-emitting structure layer EM. A resonator OC having a direction perpendicular to the light-emitting structure layer EM (a direction perpendicular to the substrate11) as a resonator length direction is constituted between the second reflecting mirror18and the first reflecting mirror12.

In this embodiment, as illustrated inFIG.2, the second reflecting mirror18has a column shape. Therefore, in this embodiment, the surface emitting laser10includes the column-shaped resonator OC.

In this embodiment, the second reflecting mirror18has a structure in which first dielectric films (hereinafter referred to as high refractive index dielectric films) H2and second dielectric films (hereinafter referred to as low refractive index dielectric films) L2having a refractive index lower than that of the high refractive index dielectric films H2are stacked in alternation.

That is, in this embodiment, the second reflecting mirror18is a dielectric multilayer film reflecting mirror constituting a Distributed Bragg Reflector (DBR) made of a dielectric material. In this embodiment, the high refractive index dielectric film H2is formed of a Ta2O5layer, and the low refractive index dielectric film L2is made of an Al2O3layer.

The low resistance region LR and the high resistance region HR of the p-type semiconductor layer15in the light-emitting structure layer EM are disposed in the region between the first reflecting mirror12and the second reflecting mirror18. That is, in this embodiment, the resonator OC includes: a central region R1corresponding to the low resistance region LR of the p-type semiconductor layer15and extending between the first and the second reflecting mirrors12and18, and an outer region R2disposed corresponding to the high resistance region HR outside the central region R1.

In this embodiment, a layer thickness of the p-type semiconductor layer15in the first region15A (low resistance region LR) is thicker (larger) than a layer thickness of the p-type semiconductor layer15in the second region15B (high resistance region HR). Therefore, the outer region R2in the resonator OC has an equivalent refractive index lower than that of the central region R1. That is, the central region R1functions as a high refractive index region, and the outer region R2functions as a low refractive index region having a refractive index lower than that of the central region R1. In this embodiment, the central region R1has a column shape, and the outer region R2has a cylindrical shape.

The surface emitting laser10includes first and second electrodes E1and E2that apply a current to the light-emitting structure layer EM. The first electrode E1is formed on the n-type semiconductor layer13. The second electrode E2is formed on the light-transmitting electrode layer16.

The application of a voltage between the first and the second electrodes E1and E2emits the light from the light-emitting layer14in the light-emitting structure layer EM. The light emitted from the light-emitting layer14repeats reflection between the first and the second reflecting mirrors12and18, thus entering a resonance state (performing laser oscillation).

In this embodiment, the first reflecting mirror12has reflectance slightly lower than that of the second reflecting mirror18. Therefore, a part of the light resonated between the first and the second reflecting mirrors12and18transmits through the first reflecting mirror12and the substrate11and is taken to the outside. Thus, the surface emitting laser10emits the light in the direction perpendicular to the substrate11and the light-emitting structure layer EM.

The first region15A of the p-type semiconductor layer15defines a luminescence center in the light-emitting layer14and defines a center axis CA of the resonator OC. The center axis CA of the resonator OC passes through the center of the first region15A and extends in the direction perpendicular to the p-type semiconductor layer15(light-emitting structure layer EM). In this embodiment, the center of the first region15A of the p-type semiconductor layer15is disposed at a position corresponding to the center of the second region15B.

Here, an exemplary configuration of each layer in the surface emitting laser10will be described. In this embodiment, the first reflecting mirror12is formed of 44 pairs of GaN layers and AlInN layers. The n-type semiconductor layer13has a layer thickness of 650 nm. The light-emitting layer14is formed of an active layer having a multiple quantum well structure in which 4 nm of InGaN layers and 5 nm of GaN layers are stacked three times. The second reflecting mirror18is formed of 10 pairs of Ta2O5layers and Al2O3layers.

The p-type semiconductor layer15has a layer thickness of 50 nm in the first region15A. The p-type semiconductor layer15has a layer thickness of 40 nm in the second region15B. The first region15A has a width (outer diameter) of 6 μm.

FIG.3is a drawing schematically illustrating an optical property of the resonator OC in the surface emitting laser10. AlthoughFIG.3is a cross-sectional view similar toFIG.1,FIG.3omits hatchings. In this embodiment, as described above, the layer thickness of the p-type semiconductor layer15in the first region15A (low resistance region LR) is larger than the layer thickness of the p-type semiconductor layer15in the second region15B (high resistance region HR). The layer thicknesses of the other layers between the first and the second reflecting mirrors12and18are each constant.

Therefore, an equivalent refractive index n1 of the central region R1in the resonator OC is higher than an equivalent refractive index n2 of the outer region R2. Additionally, an optical distance OL1between the first and the second reflecting mirrors12and18in the central region R1is larger than an optical distance OL2in the outer region R2. That is, an equivalent resonator length in the central region R1is longer than an equivalent resonator length in the outer region R2.

FIG.4is a drawing schematically illustrating an electrical property in the resonator OC (in the light-emitting structure layer EM) of the surface emitting laser10.FIG.4is a drawing schematically illustrating paths of currents CR flowing through the inside of the light-emitting structure layer EM. AlthoughFIG.4is a cross-sectional view similar toFIG.1,FIG.4omits hatchings. In this embodiment, the central region R1corresponding to the first region15A functions as the low resistance region LR, and the outer region R2corresponding to the second region15B functions as the high resistance region HR.

Therefore, as illustrated inFIG.4, the current CR is injected into the light-emitting layer14only in the central region R1, and the current is hardly injected into the light-emitting layer14in the outer region R2. That is, while light is generated (a gain is generated) in the central region R1, light is not generated in the outer region R2.

FIG.5is a drawing schematically illustrating light emitted from the surface emitting laser10. In this embodiment, a standing wave in the surface emitting laser10is taken to the outside from the first reflecting mirror12. Here, as illustrated inFIG.5, light resonated in the surface emitting laser10is taken to the outside while being converged at the central region R1.FIG.5schematically illustrates a beam outer edge of a laser beam LB emitted from the surface emitting laser10by the dashed line.

Specifically, first, in this embodiment the equivalent refractive index n2 of the resonator OC (laser medium) in the outer region R2is smaller than the equivalent refractive index n1 of the resonator OC in the central region R1.

This suppresses an optical loss due to divergence (emission) of the standing wave in the resonator OC from the central region R1to the outside. That is, a large amount of light remains in the central region R1, and the laser beam LB is taken to the outside in the state. Accordingly, a large amount of light concentrates on the proximity of the center axis CA of the resonator OC, thereby ensuring generating and emitting the laser beam LB with high output power.

In this embodiment, by providing the difference in equivalent refractive index, an optical confinement structure in the resonator OC is formed. Therefore, almost all light serves as the laser beams LB without causing deterioration of intensity. This allows highly efficiently generating and emitting the laser beam LB with high output power.

Next, in this embodiment, the low resistance region LR, that is, the current injected region to the light-emitting layer14is restricted to only the central region R1. That is, the current is not injected into the outer region R2, but the current injected region is disposed surrounding the non-current injected region. This allows stabilizing a transverse mode of the laser beam LB. For example, this allows emitting the unimodal laser beam LB.

FIG.6is a cross-sectional view of a surface emitting laser100according to Comparative Example 1. Except that the surface emitting laser100includes an insulating layer101on the second region15B, the surface emitting laser100has a configuration similar to that of the surface emitting laser10. In the surface emitting laser100, the p-type semiconductor layer15contacts the insulating layer101in the second region15B. The p-type semiconductor layer15contacts the light-transmitting electrode layer16in the first region15A. The insulating layer101is made of SiO2.

FIG.7is a drawing illustrating a relationship between etching depths at the upper surfaces15U of the p-type semiconductor layers15in the surface emitting laser10and the surface emitting laser100, that is, distances between the first and the second regions15A and15B in a direction perpendicular to the p-type semiconductor layers15(correspond to a depth D1in the surface emitting laser10and a depth D2in the surface emitting laser100, seeFIG.1andFIG.6, respectively), and refractive index differences Δn, which are values found by dividing the differences in the equivalent refractive indexes between the central regions R1and the outer regions R2of both by the equivalent refractive indexes of the central regions R1(the values correspond to (n1−n2)/n1 in the surface emitting laser10and (n11−n21)/n11 in the surface emitting laser100, seeFIG.1andFIG.6, respectively).

FIG.7shows simulation results in a case where resonator lengths (corresponding to the optical distances OL1and OL11, respectively, seeFIG.1andFIG.6) in both of the surface emitting lasers10and100are set to be five times or 10 times of a wavelength of the light (that is, the laser beam LB) emitted from the light-emitting layer14.

As illustrated inFIG.7, for example, when the refractive index difference Δn of 3×10−3is attempted to be obtained, in the surface emitting laser100, the p-type semiconductor layer15needs to be etched such that the second region15B becomes lower than the first region15A by the depth D2, which is about 22 nm. On the other hand, it is seen that, in the surface emitting laser10, the refractive index difference Δn of 3×10−3can be obtained by etching of only by the depth D1, which is about 5 nm. It is seen that the sufficient refractive index difference can also be formed by decreasing the etching depth D1in all the other range in the surface emitting laser10.

That is, in the surface emitting laser10, by only removing the slight p-type semiconductor layer15, the sufficient refractive index difference can be formed in the resonator OC compared with a case where, for example, the p-type semiconductor layer15is partially removed and then the insulating layer101is formed to form the refractive index difference in the resonator OC as in the surface emitting laser100.

First, this eliminates the need for a process of forming the insulating layer101, and therefore the manufacturing process of the surface emitting laser10is simplified. Next, an amount of the removed p-type semiconductor layer15(for example, an amount of etching and an etching period) is substantially reduced. This shortens the manufacturing period of the surface emitting laser10.

Note that since the light-transmitting electrode layer16is formed on the first and the second regions15A and15B, for example, a step difference occurs in each layer from the light-transmitting electrode layer16to the second reflecting mirror18at the boundary between the central region R1and the outer region R2. However, as described above, the step difference is slight (for example, 10 nm or less). Therefore, a scattering loss of the laser beam LB that possibly occurs due to the step difference in each layer is almost negligible.

Specifically, for example, when the wavelength of the laser beam LB is 445 nm and the equivalent refractive index n1 of the resonator OC in the central region R1is 2.43, the step height (etching depth) D1between the first and the second regions15A and15B is preferably, for example, 9.2 nm or less, and 4.7 nm or less is further preferred. This is because the step difference in the height range is insensitive to the laser beam LB. This embodiment eliminates the need for the insulating layer101, thereby allowing forming a desired refractive index difference while achieving the range of the etching depth D1.

For example, considering the wavelength of the laser beam LB, the preferred ranges of the resonator length of the central region R1and the refractive index difference between the central region R1and the outer region R2formed in the resonator OC may be determined, and the depth D1of the second region15B may be adjusted to meet the ranges. For example, considering obtaining the stable, unimodal far-field pattern for the laser beam LB at the wavelength of 445 nm, the refractive index difference is preferably provided between the central region R1and the outer region R2in a range of 1×10−3to 4×10−3. In this case, for example, the depth D1of the first region15A is preferably in a range of from 1.5 to 12 nm. That is, the high resistance region HR is preferably depressed from the low resistance region LR toward the light-emitting layer14at the depth in of range of from 1.5 to 12 nm.

FIG.8is a drawing illustrating the far-field patterns and their properties of the laser beam LB emitted from the surface emitting laser10when respective driving conditions are adjusted.FIG.8shows measurement results when the width W1(the width of the low resistance region LR and corresponding to the inner diameter in this embodiment, seeFIG.1) of the second region15B is set to 6 μm, the resonator length OL1is set to 10 times of a peak wavelength λ (445 nm in this embodiment) of the laser beam LB, and the refractive index difference Δn between the central region R1and the outer region R2is set to 1.5×10−3.

As illustrated inFIG.8, even when the driving was performed to obtain the optical output exceeding 5 mW, the unimodal far-field pattern was able to be obtained and its half-value angle was 6° or less. That is, it is seen that, in the operation under various driving conditions as well, the stable, unimodal laser beam LB can be emitted. Thus, it is seen that the surface emitting laser10can emit the laser beam LB in the stable transverse mode.

FIG.9is a drawing illustrating a relationship between the driving period and the optical output in the surface emitting lasers10and100.FIG.9is a drawing illustrating the relationship between the driving period and the optical output of the surface emitting lasers10and100with the optical output at the start of the driving set to 1.

As illustrated inFIG.9, it is seen that the reduction in optical output is suppressed in the surface emitting laser10compared with the surface emitting laser100. That is, it is seen that the surface emitting laser10features the stable output characteristics and high quality compared with the surface emitting laser100.

It is considered that this is caused by the surface emitting laser10not including the insulating layer101, which is disposed in the surface emitting laser100. Specifically, to form the insulating layer101, the p-type semiconductor layer15(semiconductor wafer) is irradiated with plasma and stress is applied to the p-type semiconductor layer15. Meanwhile, when the insulating layer101is not formed, damage given to the p-type semiconductor layer15is eliminated. Accordingly, it is considered that the surface emitting laser10maintaining the p-type semiconductor layer15in the high quality state can be obtained and the output characteristics are stabilized.

Thus, in this embodiment, the inactivated region15C is disposed on the upper surface15U of the p-type semiconductor layer15in the surface emitting laser10, and thus the second region15B that functions as the high resistance region HR is provided. Therefore, the laser beam LB in the high quality, stable transverse mode with high output power can be emitted.

In this embodiment, the case where the high resistance region HR and the low resistance region LR are disposed in the p-type semiconductor layer15has been described. However, the high resistance region HR and the low resistance region LR may be disposed in the n-type semiconductor layer13.

In this embodiment, the case where the high resistance region HR is formed by dry etching has been described. However, the method for forming the high resistance region HR is not limited to dry etching. For example, the second region15B in the p-type semiconductor layer15may be formed by slightly removing the surface of the p-type semiconductor layer15and ion implantation is performed to form the inactivated region15C. Alternatively, the inactivated region15C may be formed by ashing process.

FIG.10is a cross-sectional view of a surface emitting laser10A according to modification of this embodiment. Except that the surface emitting laser10A includes an insulating layer19on an outer peripheral portion of the first region15A of the p-type semiconductor layer15between the p-type semiconductor layer15and the light-transmitting electrode layer16, the surface emitting laser10A has a configuration similar to that of the surface emitting laser10.

In this modification, for example, the insulating layer19is formed on the outer peripheral portion of the second region15B of the p-type semiconductor layer15having a region immediately below the second electrode E2, and the light-transmitting electrode layer16is formed so as to embed the insulating layer19.

In this modification, partially forming the insulating layer19allows reliable insulation between the second region15B and the light-transmitting electrode layer16while minimally suppressing damage to the p-type semiconductor layer15. This allows reliably setting the second region15B in the high resistance. Accordingly, a decrease in gain due to, for example, a leakage of a current in the second region15B can be suppressed. Thus, the surface emitting laser10A that emits the laser beam LB in that stable, high quality transverse mode with high output power is obtained.

Thus, in this embodiment, the surface emitting laser10includes the substrate11, the first reflecting mirror12, the n-type semiconductor layer (the first semiconductor layer having a first conductivity type)13, the light-emitting layer14, the p-type semiconductor layer (second semiconductor layer)15, the light-transmitting electrode layer16, and the second reflecting mirror18. The first reflecting mirror12is formed on the substrate11. The n-type semiconductor layer13is formed on the first reflecting mirror12. The light-emitting layer14is formed on the n-type semiconductor layer13. The p-type semiconductor layer15is formed on the light-emitting layer14and has a p-type conductivity type (second conductivity type) opposite to the first conductivity type of the n-type semiconductor layer13. The p-type semiconductor layer15includes the low resistance region LR and the high resistance region HR on the upper surface15U. The high resistance region HR is depressed from the low resistance region LR toward the light-emitting layer14outside the low resistance region LR and p-type impurities are inactivated in the high resistance region HR such that the high resistance region HR has the electrical resistance higher than the electrical resistance of the low resistance region LR. The light-transmitting electrode layer16in contact with the low resistance region LR and the high resistance region HR is formed on the upper surface15U of the p-type semiconductor layer15. The second reflecting mirror18is formed on the light-transmitting electrode layer16. The resonator OC is constituted between the second reflecting mirror18and the first reflecting mirror12. This allows emitting the light in the stable transverse mode and therefore allows providing the high quality surface emitting laser10(vertical cavity surface emitting device) having the simple configuration.

Embodiment 2

FIG.11is a cross-sectional view of a surface emitting laser20according to Embodiment 2.FIG.12is a schematic top view of the surface emitting laser20. Except for a configuration of a light-emitting structure layer EM1, the surface emitting laser20has a configuration similar to that of the surface emitting laser10.

In this embodiment, the light-emitting structure layer EM1includes a p-type semiconductor layer21that includes the high resistance region HR inside the low resistance region LR on an upper surface21U. Specifically, in the p-type semiconductor layer21, the low resistance region LR is disposed in a ring shape and the high resistance regions HR are disposed in both the inside and the outside of the low resistance region LR.

In this embodiment, the p-type semiconductor layer21includes a first region21A disposed in the ring shape on an upper surface21U and functioning as the low resistance region LR, a second region21B1depressed from the first region21A toward the light-emitting layer14at the inside of the first region21A and functioning as the high resistance region HR, and a third region21B2depressed from the first region21A toward the light-emitting layer14at the outside of the first region21A and functioning as the high resistance region HR.

For example, the p-type semiconductor layer21can be formed by performing dry etching on the surface of the p-type semiconductor layer21with the ring-shaped region left. For example, the ring-shaped region as the first region21A has an outer diameter of 10.3 μm and an inner diameter of 3.5 μm.

In this embodiment, a resonator OC1includes a ring-shaped region R11corresponding to the first region21A, an inner region R21corresponding to the second region21B1disposed inside the ring-shaped region R1, and an outer region R22corresponding to the third region21B2disposed outside the ring-shaped region R11. The inner region R21and the outer region R22have equivalent refractive indexes lower than that of the ring-shaped region R11. As illustrated inFIG.12, the inner region R21is formed in a columnar shape, and the ring-shaped region R11and the outer region R22are formed in a cylindrical shape.

In this embodiment, a current flows only through the ring-shaped region R11. The current confinement is performed on the resonator OC1by thus disposing the inner region R21to ensure stabilizing an eigenmode (also referred to as a supermode) of the laser beam LB. Specifically, for example, adjustment of the width (a width of the first region21A corresponding to the width of the current injected region) of the ring-shaped region R11allows stably generating the laser beam LB in various eigenmodes.

Thus, in this embodiment, the p-type semiconductor layer21includes the high resistance region HR in which an inactivated region21C is formed inside the low resistance region LR. This allows emitting the light in the stable transverse mode and therefore allows providing the high quality surface emitting laser20(vertical cavity surface emitting device) having the simple configuration.

DESCRIPTION OF REFERENCE SIGNS

10,10A,20surface emitting laser (vertical cavity surface emitting device)15,21p-type semiconductor layer15C,21C inactivated region