JP2003092455A - Semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser in which a wide gap semiconductor is used as a laser by a current injection, and an operation in practical efficiency can be expected. SOLUTION: Electrodes 2, 6 are disposed on both planes of a laser device which is joined by PIN-injection, and a voltage is applied thereto. Defects or impurities having non-uniformity are put in a laser medium. In portions which intersect on both end planes shown by an oblique line, a reflectivity is increased for structuring a laser resonator. This imperfect area 4 is formed as a waveguide. The imperfect area 4 is close to an I-type crystal as a boundary between a P-type crystal 3 and an N-type crystal 5, but as a collective excitation phase moves comparatively freely, the phase may exist anywhere in the P-type crystal 3 and the N-type crystal 5. However, it is necessary that the collective excitation phase is within a range where the phase can move.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser made of a wide-gap semiconductor having an oscillation wavelength in a blue region or a wavelength region shorter than that.

[0002]

2. Description of the Related Art Nowadays, development of a short-wavelength laser which is an alternative to an excimer laser is required due to its application to a lithograph and a high-density optical recording device. As the short-wavelength laser, a semiconductor laser using a GaN wide-gap direct transition type semiconductor is mainly used. However, there is a need for techniques to achieve even shorter UV to violet wavelengths.

As a medium of a laser that oscillates a short wavelength,
There are wide-gap indirect transition semiconductors such as diamond. In particular, for diamond, a laser oscillation technique by electronically excited exciton emission and a laser oscillation technique by current injection are known as conventional techniques. Efficient oscillation by current injection is important for practical use, but in reality, there is no example of oscillation by current injection so efficient that it can be put to practical use.

[0004]

In order to realize a semiconductor ultraviolet laser, a technique using diamond as an indirect transition type semiconductor as a gain medium is known, but a practical example of laser oscillation by current injection is shown. There is no such situation.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a semiconductor laser which can be expected to operate with a practical efficiency as a laser using a wide gap semiconductor by current injection. Especially.

[0006]

In order to achieve such an object, the present invention provides a semiconductor laser using a wide-gap semiconductor having a PN junction or a PIN junction as an anode. Laser oscillation light is emitted from the end face of the wide-gap semiconductor, and a nonuniform structure is introduced into the laser medium along the optical path in the laser medium of the wide-gap semiconductor. To do. Note that the non-uniform structure means a non-uniform structure that includes defects, strains, composition fluctuations, and the like, and causes spatial condensation of carriers due to defects or strains along the optical path in the medium. .

The invention described in claim 2 is the same as claim 1
In the invention described in (3), the end face from which the laser oscillation light is emitted is surface-treated so as to be a mirror having a high reflectance at the oscillation wavelength.

The invention described in claim 3 is the same as claim 1
Alternatively, the nonuniform structure introduced into the laser medium of the wide-gap semiconductor is a defect or a strain introduced into the laser medium by an optical pulse.

The invention according to claim 4 is the same as claim 1
Alternatively, the nonuniform structure introduced into the laser medium of the wide gap semiconductor is a defect mechanically introduced into the laser medium by a pressing member. The pressing member is preferably a sharp mechanical pressing member such as a blade.

The invention described in claim 5 is the same as claim 1
Alternatively, in the invention described in (2), the non-uniform structure is introduced into the laser medium of the wide gap semiconductor, and the strain is introduced due to mechanical pressure applied to the laser medium.

The invention according to claim 6 is the same as claim 1.
Alternatively, the nonuniform structure introduced into the laser medium of the wide-gap semiconductor is strain generated by introducing impurity atoms into the laser medium or different lattice constants.

The invention described in claim 7 is the same as claim 1.
Alternatively, the nonuniform structure introduced into the laser medium of the wide-gap semiconductor may be a composition fluctuation structure of a semiconductor crystal of the laser medium.

The invention described in claim 8 is the same as claim 1.
Alternatively, in the invention described in 2, the nonuniform structure introduced into the laser medium of the wide-gap semiconductor is periodic along a laser optical path traveling in the laser medium,
The period is an integral multiple of half of the wavelength of the laser oscillation light in the laser medium.

The invention described in claim 9 is the same as claim 1.
Alternatively, in the invention described in 2, the wide-gap semiconductor is GaN, ZnO, SiC, CuCl, CuBr,
AlN, diamond, c-BN, BCN, SiO ₂ ,
It is characterized by being any one of BP, MgF ₂ and CaF ₂ .

To oscillate a laser beam having a short wavelength
Requires a wide bandgap semiconductor. band
G is well known as a semiconductor with a wide gap
aN, ZnO, diamond, c-BN, BCN, Si
O_Two, BP, etc. Structure of valence band and conduction band of semiconductor
Classified into direct transition semiconductors and indirect transition semiconductors by
Be done. GaN and ZnO are direct transition type diamond, c
-BN, BCN, SiO _Two, BP is an indirect transition type.
Normally, the indirect transition type semiconductor emits light, but the direct transition type semiconductor
Since it has a slightly more complicated mechanism than semiconductors,
It is said that the academic gain is small and it is not suitable for laser oscillation.
In the present invention, as a result of examining the gain and loss of laser oscillation,
Since laser oscillation is possible even with indirect transition type semiconductors,
There is no need to exclude indirect transition semiconductors from the invention.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. First, the basic conditions of laser oscillation according to the present invention will be described below. [Gain Mechanism] In the basic principle of laser oscillation, the emitted light is used as a seed in the laser amplification medium, and the light is amplified to obtain a laser light having a uniform phase. Therefore, in order to amplify the light, an optical gain by high density free carriers is essential. In general, the process of emitting light is the reverse process of the process of absorbing light, and their probabilities are related.

Therefore, the direct type semiconductor having a large light absorption has a large optical gain, and conversely, the indirect type semiconductor having a small light absorption has a small optical gain. One of the conditions for laser oscillation is that there is a net optical gain obtained by subtracting loss from this gain. In the conventional direct-transition type visible / infrared semiconductor laser, the optical gain due to the high-density free carriers injected with current greatly exceeds the loss due to absorption.

By the way, when an electric current is applied to a wide-gap semiconductor having a PIN junction, the holes in the P-type semiconductor and the electrons in the N-type semiconductor meet at the insulating layer portion of the PIN junction to form an electron-hole pair. When there are a large number of Pd, they interact with each other to form a spatially condensed electron-hole phase.

For example, in Si or Ge, electron-hole droplets that are spatially condensed at low temperature self-form. The free carriers and electron-hole pairs in this state do not make independent movements but move collectively as a group, and are therefore called a collective excitation phase. The electrons and holes in the collective excited state emit light and return to the ground state. Since the collective excited phases spatially condensed by current injection have an energy level lower than the conduction band, the light emitted from these phases has a valence band −
Since the light is hardly absorbed by the interband transition of the conduction band, the loss is small.

On the other hand, the high-density free carriers required for obtaining the optical gain, on the contrary, produce the loss appearing in the low energy region. This is a factor that particularly hinders the laser action of the indirect transition semiconductor. In the case of a wide-gap semiconductor, a net gain exists because absorption of free carriers generated on the low energy side is small near the band edge where the gain occurs. That is, since there is a net gain in a wide-gap semiconductor regardless of whether it is a direct transition semiconductor or an indirect transition semiconductor, laser oscillation is possible.

[Formation of Collective Excitation Phase] In the conventional semiconductor laser, current confinement is performed, and a large current density is obtained by passing a current through a small volume, thereby realizing a high laser gain. However, in the first place, free carriers, which are the gain of laser oscillation, are in a state where the carrier density in the crystal is non-uniformly distributed due to the imperfections of the lattice caused by the crystal growth.
For example, it is known that they are generated as islands that are not connected.

That is, as shown in FIG. 1, when an electric current flows through the laser medium, a spatially condensed collective excited phase is self-formed with imperfections such as defects as nuclei. This occurs because it is more stable when localized as a high-density phase than when it is low in density and spatially evenly distributed. Imperfections such as defects have the effect of deforming the shape of the conduction band of the semiconductor crystal that is the laser medium and reducing the size of the band gap. Since the collective excited phase gathers in a place where the band gap is low, it gathers in places where there are imperfections such as defects.

Therefore, in the present invention, a high gain is naturally obtained by intentionally introducing a non-uniform structure such as a defect into the laser medium so that the collective excitation phase gathers at the position of the space corresponding to the optical waveguide. A collective excited phase is formed.

That is, by intentionally introducing a large defect of the laser medium, which has been considered to be a factor that hinders the laser action, the same effect as the current constriction can be achieved. In the present invention, it is not only defects that cause imperfections, but also local distortions that have the same effect.

[Method for Producing Nonuniformity] As a method for producing nonuniformity by locally reducing the size of the band gap, (1) a method in which a force is applied to a laser medium to directly give distortion, (2) a laser A method of creating some kind of defect in the medium, and (3) a method of consciously utilizing composition fluctuation of the laser medium are conceivable.

In the above-mentioned strain method (1), a composition having a lattice constant different from that of an impurity atom or a base material is partially introduced into a semiconductor crystal without directly applying a force.
It is well known that the state of distortion occurs. in this case,
The result of giving a defect becomes a strain in the semiconductor crystal.

The method (2) of giving a defect is not limited to the method of mechanically giving a tool such as a blade, but a method of causing a defect by partially altering the inside of the semiconductor crystal by the light of a pulse laser. Are known.

Further, the method of composition fluctuation of (3) is a method of introducing impurity atoms into the semiconductor crystal at a high concentration to form an impurity band in the band gap of the semiconductor and narrow the band gap slightly.

[Optical Confinement Mechanism] In the conventional semiconductor laser, the light is prevented from escaping due to the structure of confining the light. However, in the present invention, if the collective excitation phase described above causes a positive change in refractive index. , An optical waveguide is formed in a laser medium by the same principle as an optical fiber. In general, this type of index change depends on the wavelength of light, or energy,
The medium, the temperature, where the change in refraction is positive at the wavelength of laser oscillation
It is necessary to select conditions such as charge carrier density.

[Estimation of Gain] Therefore, absorption by free carriers and gain (α) and refractive index change (Δn) will be evaluated as a function of wavelength energy. 2 (a), (b)
Shows the energy dependence of the wavelengths of α and Δn of a direct transition semiconductor (FIG. 2A) and an indirect transition semiconductor (FIG. 2B) having an energy gap of 5.0 eV and a phonon energy of 0.1 eV. It is a figure.

2A and 2B show absorption (α) by free electrons in the direct transition type semiconductor and the indirect type semiconductor, and a change in the refractive index of the collective excitation phase (Δn: difference between the base refractive index). It is a calculation, the broken line is the calculation when there are no free electrons,
The horizontal axis represents the energy of light. Here, in order to simplify the calculation, the calculation is performed when the temperature is T = 0.

In this calculation, the Fermi energies of electrons and holes are each 0.1 eV (charge carrier density is 10 ¹⁹
Band gap energy shift −0.3
The eV and carrier temperature are 0 ° K. Since the refractive index change Δn is positive in all regions, light confinement is established. Regarding the light absorption rate α, the net optical gain is canceled in the positive part, and laser oscillation does not occur,
Both of them have a gain because it is estimated that there is a region where α becomes negative in a region of energy slightly smaller than 5.0 eV.

[Operating Temperature] Since the energy required for spatial condensation must overcome the thermal effect of carriers, it is better to keep the laser medium at a low temperature for laser oscillation.

The basic principle of laser oscillation according to the present invention has been described above. Next, specific examples of the present invention will be described.

Example 1 FIGS. 3 (a) and 3 (b) are views showing an example in which SiC is used as a wide gap semiconductor crystal. FIG. 3 (a) is a front view and FIG. 3 (b) is It is a side view. In the figure, reference numeral 1 is a substrate, and an electrode 2 is provided on the surface of the substrate 1.
Is formed, and the P-type crystal 3 and the N-type crystal 5 are formed between the other electrode 6. Reference numeral 4 indicates a non-uniform structure region.

Electrodes 2 and 6 are arranged on both sides of the PIN-bonded laser element, and a voltage is applied to the electrodes 2 and 6.
Defects and impurities, which are a kind of non-uniform structure, are put into the laser medium as shown by the broken line. In FIG. 3 (b), the portions intersecting with the both end surfaces shown by hatching have a high reflectance in order to form a laser resonator. This non-uniform structure region 4
Becomes the waveguide.

The heterogeneous structure region 4 is located near the I-type crystal, which is the boundary between the P-type crystal 3 and the N-type crystal 5. However, the collective excitation phase can move relatively freely, so that the P-type crystal 3 or the N-type crystal 3 is formed.
It does not matter where it is in the mold crystal 5. However, it is important that the collective excitation phase can move. In Example 1, the substrate side is a P-type crystal, but it is not necessary to be particular about this and an N-type crystal may be used.

[Embodiment 2] FIG. 4 is a view showing an embodiment regarding defects and impurities introduced into the laser medium, and S in FIG.
It is the figure which looked at iC crystal from right above. In the figure, reference numeral 11 indicates a laser medium, and 12 indicates a nonuniform structure region.

The regions 12 of defects or impurities introduced into the crystal are indicated by diagonal lines. Since this region 12 has a waveguide structure due to the change in the refractive index of the collective excitation phase that has been integrated, a laser resonator is formed by making both end faces of this region have a high reflectance.

In Example 3, which will be described later, impurity atoms or impurity ions that cause strain to be accumulated inside the crystal are introduced into this region. Or a defect that is continuously introduced by an optical pulse having a strength enough to destroy a crystal so that a defect occurs, or a continuous defect based on a scratch directly mechanically created by a hard object such as a blade. It may be. Further, it may be strain due to mismatch of lattice constants due to heterojunction. Alternatively, it may be an impurity atom having a concentration enough to form an impurity band.

[Embodiment 3] FIG. 5 is a diagram showing a semiconductor laser provided with a mechanism for giving strain to a crystal. This Figure 5
It is equipped with a mechanism that locally applies pressure from the outside to generate strain in the SiC crystal. Due to this distortion, the collective excitation phases gather, so that the gain necessary for laser oscillation can be obtained. Reference numeral 21 indicates a support, and 22 indicates a pressing member.

[Embodiment 4] FIG. 6 is a diagram showing a periodic structure for forming a DFB laser. In the figure, reference numeral 31 is a laser element, 32 is an incomplete region, 33 is an incomplete structure, and 34 is intentional. Introduced into the defect, 35 is a collective excitation phase. In addition, T
Indicates the cycle, and a indicates the direction in which light travels.

The non-uniformity is not simply arranged in a straight line, but a discontinuous structure having a period T is used.
The crystal can have the same effect as a waveguide. For example, a waveguide in which lenses are periodically arranged is well known. In the present invention, since the period is set to a lattice shape that is an integral multiple of a half wavelength of light, the laser light is reflected and fed back as in the DFB laser (distributed feedback laser).
Stable laser oscillation can be expected. In this case, if the end surface of the laser medium is subjected to high reflectivity processing like a normal semiconductor laser, the effect of DFB is diminished, so the end surface of the laser medium where light is emitted is not subjected to high reflectivity processing.

In the above-described embodiments, the case where SiC is used as the wide-gap semiconductor has been described, but other wide-gap semiconductors such as GaN, ZnO and C are used.
uCl, CuBr, AlN, diamond, c-BN,
The same effect can be obtained by using BCN, SiO ₂ , BP, MgF ₂ and CaF ₂ . These embodiments may be used alone or a plurality of embodiments may be combined.

[0045]

As described above, according to the present invention, P
A semiconductor laser using an N-junction or PIN-junction wide-gap semiconductor is provided with an anode electrode and a cathode electrode, and laser oscillation light is emitted from an end face of the wide-gap semiconductor along an optical path in the laser medium of the wide-gap semiconductor. Since the non-uniform structure is introduced into the laser medium, it is possible to provide a semiconductor laser that can be expected to operate at a practical efficiency by using a wide gap semiconductor as a laser by current injection.

[Brief description of drawings]

FIG. 1 is a diagram showing a state in which a collective excitation phase having a high gain gathers when a laser medium has a defect.

FIG. 2 is a diagram showing calculation of absorption (α) by free electrons in a direct transition type semiconductor and an indirect type semiconductor and a change in refractive index of a collective excited phase (Δn: difference between underlying refractive index), where (a) is The direct transition type semiconductor and (b) show the indirect type semiconductor.

3A and 3B are views showing an embodiment using SiC as a wide-gap semiconductor crystal, in which FIG. 3A is a front view and FIG. 3B is a side view.

FIG. 4 is a diagram showing an example of defects and impurities introduced into a laser medium.

FIG. 5 is a diagram showing a semiconductor laser provided with a mechanism for giving strain to a crystal.

FIG. 6 shows a periodic structure for making a DFB laser.

[Explanation of symbols]

1 substrate 2,6 electrodes 3 P-type crystal 4 Defect area 5 N-type crystal 11 Laser medium 12 Non-uniform structure area 21 Support 22 Pressing member 31 Laser element 32 Non-uniform structure area 33 Enlarged view of non-uniform structure 34 Defects introduced intentionally 35 Collective excitation phase T cycle Direction of light

Claims (9)

[Claims] 1. A semiconductor laser using a PN-junction or PIN-junction wide-gap semiconductor, comprising anode and cathode electrodes, and lasing light is emitted from an end face of the wide-gap semiconductor, and the wide-gap semiconductor laser is emitted. A semiconductor laser having a non-uniform structure introduced into the laser medium along an optical path in the medium. 2. The end face from which the laser oscillation light is emitted is
The semiconductor laser according to claim 1, which is surface-treated so as to be a mirror having a high reflectance at an oscillation wavelength. 3. The semiconductor laser according to claim 1, wherein the nonuniform structure introduced into the laser medium of the wide gap semiconductor is a defect or a strain introduced into the laser medium by an optical pulse. . 4. The semiconductor according to claim 1, wherein the non-uniform structure introduced into the laser medium of the wide-gap semiconductor is a defect mechanically introduced into the laser medium by a pressing member. laser. 5. The non-uniform structure introduced into the laser medium of the wide-gap semiconductor is strain introduced by mechanically applying pressure to the laser medium. The semiconductor laser described in 1. 6. The non-uniform structure introduced into the laser medium of the wide-gap semiconductor is strain generated by introducing impurity atoms into the laser medium or by a different lattice constant. 2. The semiconductor laser according to item 2. 7. The non-uniform structure introduced into the laser medium of the wide-gap semiconductor is a composition fluctuation structure of a semiconductor crystal of the laser medium.
Or the semiconductor laser described in 2. 8. The non-uniform structure introduced into the laser medium of the wide-gap semiconductor is periodic along a laser optical path traveling in the laser medium, and the period causes laser oscillation in the laser medium. 3. The semiconductor laser according to claim 1, which is an integral multiple of half the wavelength of light. 9. The wide gap semiconductor is GaN,
ZnO, SiC, CuCl, CuBr, AlN, _{diamond, c-BN, BCN, SiO} 2, BP, Mg
The semiconductor laser according to claim 1, wherein the semiconductor laser is one of F ₂ and CaF ₂ .