JP3311275B2 - Nitride based semiconductor light emitting 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 nitride semiconductor light emitting device used in technical fields such as optical information processing, optical communication, and optical measurement.
H: a nitride-based semiconductor light-emitting device having a Separate Confinement Heterostructure.

[0002]

2. Description of the Related Art In recent years, demand for short-wavelength light emitting devices has increased,
Research and development of short-wavelength light emitting devices using ZnSe-based or GaN-based materials have been actively conducted. In a ZnSe-based material, continuous oscillation at room temperature of a short-wavelength semiconductor laser having an oscillation wavelength of about 500 nm has been achieved. However, device deterioration due to growth of crystal defects has become a problem, and a longer lifetime of the device has not been achieved.
It has not been put to practical use.

On the other hand, in recent years, a blue light emitting diode (LED) has been put to practical use as a GaN-based material, and research and development of a GaN-based blue semiconductor laser are currently being vigorously conducted. Recently, continuous oscillation at room temperature has also been achieved in a GaN-based semiconductor laser. However, in this material system, there are still many unknown parts regarding physical properties, and there are many problems to be solved in practical use. The main problem is that the threshold current is high.

In a GaN-based blue semiconductor laser, conventionally, p
Between an AlGaN cladding layer having n-type and n-type conduction,
An SCH structure having an InGaN-based active layer having a multiple quantum well structure (MQW) is often used. The reason why the threshold current in this case is high is that electrons and holes are not effectively confined in the multiple quantum well and overflow, and the electron field is generated due to an internal electric field generated when a voltage is applied. And holes are localized in different well layers, and the probability of recombination decreases.

[0005]

As described above, the conventional SC
In a nitride semiconductor light emitting device employing an H structure,
Leading to carrier overflow and lower recombination probability,
This has been a factor that hinders a decrease in the threshold current.

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to suppress carrier overflow and a decrease in recombination probability when an SCH structure is employed. An object of the present invention is to provide a nitride-based semiconductor light emitting device capable of reducing a threshold current.

[0007]

[Means for Solving the Problems]

(Constitution) In order to solve the above problems, the present invention provides a nitride semiconductor light emitting device having a hetero structure in which an active layer having a multiple quantum well structure is sandwiched between a pair of p-type and n-type cladding layers on a substrate. A part or a plurality of well layers of the multiple quantum well structure forming the active layer, starting from the one closer to the p-type cladding layer;
It is characterized in that only the layer is a semiconductor layer to which an n-type impurity is added.

The present invention also provides a nitride semiconductor light emitting device having a hetero structure in which an active layer having a multiple quantum well structure is sandwiched between a pair of p-type and n-type cladding layers on a substrate.
A part of the well layer of the multiple quantum well structure that constitutes the active layer is changed by one or more layers from the side closer to the n-type cladding layer.
And a semiconductor layer to which a type impurity is added.

The present invention also provides a nitride semiconductor light emitting device having a hetero structure in which an active layer having a multiple quantum well structure is sandwiched between a pair of p-type and n-type cladding layers on a substrate.
A part of the well layer of the multiple quantum well structure constituting the active layer is formed by n or more layers from the side closer to the p-type cladding layer.
A semiconductor layer to which a p-type impurity is added, and a semiconductor layer to which only one or a plurality of p-type impurities are added from the side closer to the n-type cladding layer.

Here, preferred embodiments of the present invention include the following. (1) The active layer having a multiple quantum well structure is sandwiched between a pair of optical confinement layers inside the cladding layer. (2) The multiple quantum well structure constituting the active layer is made of InGaN
Be a system material. (3) The light confinement layer is GaN. (4) The cladding layer is made of AlGaN. (5) The substrate is sapphire or SiC.

(Operation) As described above, in the nitride semiconductor light emitting device having the SCH structure, when carriers are electrically injected, for example, a sufficient inversion layer necessary for laser oscillation is formed in the quantum well. To do so requires a fairly high injection current (threshold current). In order to lower the threshold current, it is considered effective to increase the energy depth of the injection well layer to increase the carrier density in the well, but for that purpose, the In composition of the InGaN well layer must be increased. However, when the In composition is 20 to 30
% Or more is very difficult because it causes spinodal decomposition of InN and deterioration of morphology.

Therefore, in the present invention, high concentration n-type doping is performed on a well layer near a p-type cladding layer, and high concentration p-type doping is performed on a well layer near an n-type cladding layer.
Alternatively, by adopting both of them, the depth of the quantum well is effectively increased, and the level density of energy higher than the energy of electrons or holes in the quantum well is increased at the end of the cladding layer, so that the carrier of the carrier is increased. Overflow (carrier flowing over the cladding layer and flowing out of the active layer) is less likely to occur, thereby making it possible to lower the threshold current.

Hereinafter, the principle will be described by taking as an example a case where a layer near the p-type cladding layer is heavily doped with n-type doping. First, consider the case where the simplest well layer closest to the p-type cladding layer is doped with only one high-concentration n-type layer.

FIG. 1 shows a band energy diagram of carriers. This figure shows a state in which a voltage is applied to the element to emit light. It should be noted that in a semiconductor laser using a GaN-based thin film, the applied voltage required for oscillation is generally large, and the conduction electron energy of the n-type cladding layer is larger as shown in the figure.

FIG. 1A corresponds to a conventional structure in which a well layer is not doped with an impurity, and FIG. 1B corresponds to a structure of the present invention in which only one of the well layers is doped with an n-type impurity. . In FIG. 1B, since one of the well layers becomes n-type, an internal electric field is generated, and the band is significantly bent.
In addition, the electron energy at the conduction band edge of the well layer and the barrier layer in that region decreases. And, for the following two reasons, bending of this band leads to lowering of the oscillation threshold.

The first reason is that the well layer is effectively deepened as shown in FIG. 1 (b), so that the carrier level in the well layer increases and the carrier once trapped in the well layer becomes outward. This is because the loss is reduced.

Next, the second reason will be described. As shown in FIG. 1B, the difference between the conduction electron energy of the well layer or the barrier layer and the conduction electron energy of the cladding layer is increased, and the boundary between the cladding layer and the active layer of the quantum well structure is effectively reduced. The density of states of electrons increases. As a result, when electrons existing in the well layer recombine and disappear, electrons existing in this portion fall to the well layer and contribute to the next recombination. Therefore, if the density at the boundary between the cladding layer and the active layer of the quantum well structure increases, even if the injection current is not increased,
Since the light emission probability increases (the laser oscillation inversion layer is easily formed), the threshold current density decreases as a result.

It should be noted that the edge of the active layer is more Al than the clad layer.
This is a structure in which AlGaN having a large composition is provided thinly, and an effect of lowering the threshold value can be obtained for the same reason as the second reason described above. However, there is a problem that a high-quality AlGaN layer cannot be grown unless the temperature is higher than 1100 ° C.
In the heating process after growing the GaN-based active layer, the active layer
Exposure to a high temperature of 00 ° C. or more causes re-evaporation of the active layer and a significant decrease in quality. Also, AlG with high Al composition
If the aN layer is formed adjacent to the active layer, it is not desirable because the active layer is greatly strained and leads to a reduction in device reliability.

According to the present invention, the above-mentioned problem does not occur, and the effect can be obtained by a simpler method. In the case of the second reason, there is an effect of increasing the light emission probability particularly in the well layer near the clad layer.

When the number of well layers to be heavily doped is increased, the spatial change at the conduction band edge is moderated, but there is an effect similar to the doping of the single layer. Also,
As shown in FIG. 2A, when a well layer close to the n-type cladding layer is doped with p-type, the same effect is obtained for holes, and as a result, the threshold value decreases. Further, as shown in FIG. 2B, the well layer near the p-type cladding layer is doped with n-type and the well layer near the n-type cladding layer is doped with p-type, so that the threshold value can be further reduced. It is also possible to measure.

In addition to the above, the structure of the present invention additionally provides the following effects. Although different from the state shown in FIG. 1, when the applied voltage is small and the conduction electron energy of the n-type cladding layer is lower than the conduction electron energy of the p-type cladding layer (normal band in a semiconductor laser of another material type). Structure), since the internal electric field generated when a voltage is applied acts in the direction in which the doping of the well layer decreases, the effect of localizing holes and electrons in different wells is reduced, and carriers are evenly distributed in each well layer. Will be present. The effect of reducing the internal electric field is usually obtained by doping the layer outside the well layer (such as a guide layer) with a high concentration. However, the effect can be realized by doping the well layer as in the present invention. it can.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments. (Embodiment 1) FIG. 3 is a sectional view of an element structure showing a nitride-based short wavelength semiconductor laser according to a first embodiment of the present invention.

The semiconductor laser of this embodiment has an SCH structure. The multilayer film for the semiconductor laser was formed by a well-known metal organic chemical vapor deposition (MOCVD) method. As an organic metal raw material, trimethylgallium (TMG), trimethylaluminum (TMA), trimethylindium (TMI), and biscyclopentadienyl magnesium (Cp ₂ Mg) were used. Ammonia (NH ₃ ) and silane (SiH ₄ ) were used as gas raw materials. In addition, hydrogen and nitrogen were used as carrier gases.

The semiconductor laser of this embodiment is manufactured as follows. First, an undoped GaN underlayer 102, an n-type GaN contact layer 103, and a thickness of 0.2 are formed on a sapphire substrate 101 via a buffer layer (not shown).
An n-type Al _0.18 Ga _0.82 N cladding layer 104 of 5 μm is formed sequentially.

Next, a 4 nm-thick GaN guide layer 105 is formed as a light confinement layer, and a 2 nm-thick In _0.15
Ga _0.85 N well layer 111 and 4 nm thick In _0.05 Ga
InGaN-based active layer 11 having a multiple quantum well structure (MQW) in which each of _0.95 N barrier layers 112 includes four layers
0 is formed. At this time, the uppermost In _0.15 Ga _0.85
When growing the N well layer 111, high concentration silicon doping (n-type) is performed. A GaN light guide layer 121 as a p-side light confinement layer and a p-type Al _0.18 Ga _0.82
An N cladding layer 122 and a p-type GaN contact layer 123 are formed in this order.

A part of the multilayer structure was removed to the n-type GaN contact layer 103 by dry etching, and an n-side electrode 125 made of Ti / Al was formed on the exposed contact layer 103. Further, a p-side electrode 126 was formed on the p-type GaN contact layer 123.

Next, a semiconductor mirror was formed by cleaving the wafer on which the semiconductor multilayer film having the above-mentioned electrodes was formed into a size of 350 μm × 500 μm to form a resonator mirror. The band structure when a voltage is applied to this element will be described.

The conduction band edge of the doped well layer 111 is lowered, the depth of the well is effectively increased, and the confinement of electrons is enhanced. At the same time, a dent at the conduction band end is formed at the end of the cladding layer (on the active layer side), so that the average density of electrons existing in the vicinity thereof increases. Furthermore, in an element having a low operating voltage, the localization of electrons and holes can be reduced by weakening the band bending due to the electric field inside the active layer.
Therefore, the probability of recombination of electrons and holes in the quantum well structure increases, and an inversion layer required for oscillation can be obtained at a low injection current density.

When a current was injected into the semiconductor laser of this embodiment, the semiconductor laser oscillated continuously at room temperature at a wavelength of 417 nm. The threshold current density was about half that of a laser having a structure in which the well layer was not doped.

As described above, according to this embodiment, the p-type cladding layer 122 of the well layer 111 of the MQW active layer 110 is formed.
1B is a high-concentration n-type layer, thereby effectively increasing the depth of the well and efficiently confining electrons as shown in FIG. Since a dent at the conduction band edge is formed on the side, the average density of electrons existing near the dent can be increased. Further, by weakening the bending of the band due to the electric field inside the active layer, the localization of electrons and holes can be reduced. Therefore,
When the SCH structure is adopted, it is possible to suppress the overflow of carriers and the reduction of the recombination probability, which is effective in reducing the threshold current. As a result, it is possible to save power and improve the life of the semiconductor laser.

(Embodiment 2) FIG. 4 is a sectional view of a device structure showing a nitride-based short wavelength semiconductor laser according to a second embodiment of the present invention.

The multilayer film for the semiconductor laser was formed by the same MOCVD method as in the first embodiment. In order to manufacture the semiconductor laser of this embodiment, an undoped GaN is formed on a sapphire substrate 201 via a buffer layer (not shown).
Underlayer 202, n-type GaN contact layer 203, n-type Al _0.18 Ga _0.82 N clad layer 20 having a thickness of 0.25 μm
4 are sequentially formed.

Next, a GaN guide layer 205 having a thickness of 5 nm is formed as a light confinement layer, and a 2.5 nm thick GaN guide layer is formed thereon.
InGaN having a multiple quantum well structure (MQW) composed of 5 m In _0.15 Ga _0.85 N well layers 211 and 5 5 nm thick In _0.05 Ga _0.95 N barrier layers 212.
A system active layer 210 is formed. At this time, the lowermost In
When growing the _0.15 Ga _0.85 N well layer 211, magnesium high concentration doping (p-type) is performed and the uppermost In
When growing the _0.15 Ga _0.85 N well layer 211, high concentration silicon doping (n-type) is performed. A p-side optical guide layer 221, a p-type Al _0.18 Ga _0.82 N cladding layer 222, and a p-type GaN contact layer 223 are formed in this order.

Further, part of the multilayer structure was removed to the n-type GaN contact layer 203 by dry etching, and an n-side electrode 225 made of Ti / Al was formed on the exposed contact layer 203. Furthermore, p-type GaN
A p-side electrode 226 was formed on the contact layer 223.

Next, a semiconductor mirror was formed by cleaving the wafer on which the semiconductor multilayer film having the above-mentioned electrodes was formed to a size of 350 μm × 500 μm to form a resonator mirror. The band structure when a voltage is applied to this element will be described.

In the present embodiment, the conduction band edge of the n-type doped well layer 211 adjacent to the p-type cladding layer 222 is reduced, and the well is effectively deepened, so that electron confinement is enhanced. . Further, since the valence band edge of the p-type doped well layer 211 adjacent to the n-type cladding layer 204 is increased, the depth of the well is effectively increased and the confinement of holes is enhanced.

In this case, the position where the density of electrons and holes is high is separated, but the influence of the electric field inside the active layer is weakened by the doping of the well layer 211, so that the influence is small. Further, since a dent at the conduction band end is formed at the end of the p-type cladding layer 222 and a valence band is formed at the end of the n-type cladding layer 204, the average density of electrons existing in the vicinity increases, and carrier overflow occurs. It becomes difficult to do.

When current was injected into the semiconductor laser of this embodiment, continuous oscillation at room temperature with a wavelength of 417 nm was obtained. The threshold current density was about half that of the laser without the well layer.

(Embodiment 3) FIG. 5 is a sectional view of a device structure showing a nitride-based short wavelength semiconductor laser according to a third embodiment of the present invention.

The multi-layer film for the semiconductor laser is formed using the sapphire substrate 301 as in the first embodiment.
This was performed by a CVD method. First, the buffer layer 302 is grown,
Next, the temperature was raised to 1100 ° C., and an n-type GaN contact layer 303 was grown to about 2 μm. In addition, the temperature is 110
The n-type AlGaN cladding layer 304 is kept at 0 ° C.
Was formed to a thickness of about 500 nm, and then a GaN light guide layer 305 was formed to a thickness of about 200 nm.

Next, a 2 nm thick In _0.15 Ga _0.85 N
The well layer 311 and the In _0.05 Ga _0.95 N barrier layer 312 each having a thickness of 4 nm form an InGaN-based active layer 310 having a multiple quantum well structure (MQW) composed of ten layers. At this time, four In _0.15 Ga
When growing the _0.85 N well layer 311, 1 × 10 ¹⁹ cm ⁻³
High concentration silicon doping (n-type) was performed.

Next, the p-side GaN light guide layer 321 was formed with a thickness of about 200 nm, and the p-type AlGaN cladding layer 322 was formed with a thickness of about 500 nm. In this state, the temperature was lowered to room temperature, taken out of the MOCVD apparatus, and a 20 μm-wide SiO ₂ film (not shown) was formed on the surface in a well-known thermal CVD apparatus.

Next, the wafer was placed in an RIE apparatus, and the opening was removed by etching with a BCl ₃ gas into a mesa structure. The wafer prepared in this manner is again subjected to MOCVD.
It was mounted on a susceptor in the apparatus and heated to 1100 ° C. in nitrogen.

Next, at a temperature of 1100 ° C., hydrogen, nitrogen,
TMG, ammonia, and DMZ (dimethyl zinc) were supplied to bury the structure from the n-type AlGaN current injection layer 304 to the p-type AlGaN current injection layer 322 with the i-type GaN layer 331.

The formation of the i-type GaN layer 331 is as follows.
In this embodiment, it is formed by growth after mesa etching.
It can also be formed by ion implantation of hydrogen, oxygen, or the like without etching away. For example, hydrogen can be realized by implantation of 200 keV and 1 × 10 ¹⁴ cm ⁻² .

Next, while keeping the temperature at 1100 ° C., hydrogen was flowed, and the SiO ₂ film remaining on the p-type AlGaN current injection layer 322 was removed by etching. Then
While maintaining the temperature at 1100 ° C., the main carrier gas was switched from hydrogen to nitrogen, and hydrogen, nitrogen, and ammonia were supplied to form the p-type GaN contact layer 323 to a thickness of about 500 nm.

The laser structure thus produced is represented by M
After being removed from the OCVD apparatus, a part of the multilayer structure is removed by a dry etching method up to the n-type GaN layer contact 303, and the electrode 32
5,326 were formed. That is, Pt, Ni, and Au are formed in this order on the n-type GaN contact layer 303 by using a well-known vacuum deposition method, a sputtering method, or the like, and the ohmic electrode 325 is formed. On the other hand, on the p-type GaN contact layer 323, Pd, Ti, Pt, Au (thickness 2 μm)
m) was formed and heat treatment was performed in nitrogen to form an ohmic electrode 326.

Next, the laser structure was cleaved from the substrate side using a scriber or the like to form a resonator mirror.
The semiconductor laser thus produced has a wavelength of 420 nm.
Oscillated continuously. At this time, the threshold current density was about 1/3 that of a laser having a structure in which the well layer was not doped.
In addition, since some of the well layers are heavily doped, there is an additional effect of reducing the resistance of the entire device.

When the well layer on the n-type cladding layer side is doped with p-type in addition to this structure, the effect of reducing the threshold value is further increased. Further, in this embodiment, the n-type doping concentration of all the four well layers counted from the upper side is the same. However, a structure in which the concentration gradually decreases as the distance from the p-type cladding layer 322 decreases, also reduces the threshold value. It is effective for.

The present invention is not limited to the above embodiments. The number of well layers to be p-type doped near the n-type cladding layer, or the number of well layers to be n-type doped near the p-type cladding layer is not limited to the number described in the embodiment. , Can be appropriately changed according to the specifications. Further, the number of p-type doped well layers near the n-type cladding layer may be different from the number of doped well layers near the p-type cladding layer.
In that case, the doping concentration may be gradually changed as described in the third embodiment.

In the embodiment, the semiconductor laser has been described. However, a separated confinement heterostructure in which an active layer having a multiple quantum well structure is sandwiched between a pair of optical confinement layers and further a pair of p-type and n-type cladding layers is sandwiched. If it has, it can be applied to a light emitting diode.

The substrate is not limited to sapphire, but may be SiC, Si, MgAl ₂ O ₄ , GaN or the like. Further, the materials of the active layer, the light confinement layer, and the cladding layer are not limited to the embodiment at all, and can be appropriately changed according to the specifications. In addition, various modifications can be made without departing from the scope of the present invention.

[0053]

As described above, according to the present invention, a part of the well layer of the multiple quantum well structure constituting the active layer is replaced with p-type well layer.
The n-type layer in order from the one closer to the n-type cladding layer or the p-type layer in order from the one near the n-type cladding layer,
In the nitride-based semiconductor light-emitting device employing the H structure, it is possible to suppress the overflow of carriers and the decrease in the probability of recombination, and to reduce the threshold current.

[Brief description of the drawings]

FIG. 1 is a diagram showing a difference between band diagrams of a nitride-based compound semiconductor laser according to a conventional structure and a structure according to the present invention.

FIG. 2 is a diagram showing another example of a band diagram in the structure of the present invention.

FIG. 3 is an element structure sectional view showing a GaN-based semiconductor laser according to the first embodiment.

FIG. 4 is an element structure sectional view showing a GaN-based semiconductor laser according to a second embodiment.

FIG. 5 is an element structure sectional view showing a GaN-based semiconductor laser according to a third embodiment.

[Explanation of symbols]

101, 201, 301 ... sapphire substrate 102,202,303 ... GaN underlying layer (buffer layer) 103, 203, 303 ... n-type GaN contact layer 104, 204, 304 ... n-type Al _0.18 Ga _0.82 N cladding layer 105, 205, 305 ... n-side GaN guide layer (light confinement layer) 110, 210, 310 ... InGaN-based MQW active layer 111,211,311 ... In _0.15 Ga _0.85 n well layers 112,212,312 ... In _0.05 Ga _0.95 n barrier Layers 121, 221 and 321, p-side GaN optical guide layers (optical confinement layers) 122, 222, 322 ... p-type Al _0.18 Ga _0.82 N cladding layers 123, 223, 323 ... p-type GaN contact layers 125, 225, 325 ... n-side electrode 126,225,326 ... p-side electrode

Continuation of the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 H01L 33/00 JICST file (JOIS)

Claims (4)

(57) [Claims] An active layer having a multiple quantum well structure is formed on a substrate by p
In the nitride-based semiconductor light-emitting device sandwiched between a pair of n-type and n-type cladding layers, a part or a plurality of well layers of the multiple quantum well structure constituting the active layer is arranged in the order from the p-type cladding layer. Layer only n
A nitride-based semiconductor light-emitting device comprising a semiconductor layer to which a type impurity is added. 2. An active layer having a multiple quantum well structure is formed on a substrate by p
In the nitride-based semiconductor light emitting device sandwiched between a pair of n-type cladding layers, one or more of the well layers of the multiple quantum well structure constituting the active layer are arranged in order from one closer to the n-type cladding layer. Layer only p
A nitride-based semiconductor light-emitting device comprising a semiconductor layer to which a type impurity is added. 3. An active layer having a multiple quantum well structure is formed on a substrate by p
In the nitride-based semiconductor light-emitting device sandwiched between a pair of n-type and n-type cladding layers, a part of the well layer of the multiple quantum well structure constituting the active layer is formed from the side closer to the p-type cladding layer. One or more layers n
A nitride-based semiconductor light-emitting device comprising: a semiconductor layer to which a p-type impurity is added; and a semiconductor layer to which only one or more p-type impurities are added from the side closer to the n-type cladding layer. 4. The multi-quantum well structure active layer is sandwiched between a pair of optical confinement layers.
4. The nitride-based semiconductor light-emitting device according to any one of claims 1 to 3.