JP7319618B2 - semiconductor light emitting device - Google Patents

Description

Embodiments relate to semiconductor light emitting devices.

Semiconductor light emitting devices such as light emitting diodes are required to have high luminous efficiency.

Japanese Unexamined Patent Application Publication No. 2007-201040

Embodiments provide a semiconductor light emitting device with high luminous efficiency.

A semiconductor light emitting device according to an embodiment includes: a first n -type semiconductor layer; a second p -type semiconductor layer provided on the first semiconductor layer; and a light-emitting layer disposed therebetween. The light emitting layer includes a plurality of quantum well layers and barrier layers adjacent to each of the plurality of quantum well layers. The plurality of quantum well layers include a material having a lattice constant larger than that of the material of the first semiconductor layer, are arranged in a direction from the first semiconductor layer toward the second semiconductor layer, and are arranged in a first quantum well layer. A well layer and a second quantum well layer adjacent to the first quantum well layer on the second semiconductor layer side . The barrier layer is provided between the first quantum well layer and the second quantum well layer and includes a first region and a p-type second region connected to the first region , A second region is located between the first quantum well layer and the first region, is connected to the first quantum well layer, and is provided between the first region and the second quantum well layer. can't The first region of the barrier layer comprises a material having a wider bandgap than the material of the quantum well layer and a lattice constant smaller than that of the material of the first semiconductor layer. The second region of the barrier layer comprises a material with a wider bandgap than the material of the first region and has a thickness of 10 nanometers or less in the direction from the first semiconductor layer to the second semiconductor layer. and contains carbon at a concentration of 2×10 ¹⁷ cm ⁻³ or less.

1 is a schematic cross-sectional view showing a semiconductor light emitting device according to an embodiment; FIG. FIG. 2 is a schematic diagram showing a material of a semiconductor light emitting device according to an embodiment, showing a mixed crystal with a direct transition band structure having physical properties close to those of GaAs. 1 is a schematic diagram showing a band structure of a light-emitting layer of a semiconductor light-emitting device according to an embodiment; FIG. 4 is a graph showing characteristics of the semiconductor light emitting device according to the embodiment; 5 is another graph showing the characteristics of the semiconductor light emitting device according to the embodiment;

Hereinafter, embodiments will be described with reference to the drawings. The same parts in the drawings are given the same numbers, and detailed descriptions thereof are omitted as appropriate, and different parts will be described. Note that the drawings are schematic or conceptual, and the relationship between the thickness and width of each portion, the size ratio between portions, and the like are not necessarily the same as the actual ones. Also, even when the same parts are shown, the dimensions and ratios may be different depending on the drawing.

1A and 1B are schematic cross-sectional views showing a semiconductor light emitting device 1 according to an embodiment. FIG. 1A is a cross-sectional view showing the structure of a semiconductor light emitting device 1. FIG. FIG. 1B is a cross-sectional view showing the structure of the light-emitting layer of the semiconductor light-emitting device 1. FIG. The semiconductor light emitting device 1 is, for example, a light emitting diode (LED).

As shown in FIG. 1A, the semiconductor light emitting device 1 includes a semiconductor section 100, a first electrode 110, and a second electrode 120. As shown in FIG. The semiconductor section 100 is provided between the first electrode 110 and the second electrode 120 .

The semiconductor portion 100 includes a first conductivity type semiconductor substrate 10, a first conductivity type first semiconductor layer 20, a light emitting layer 30, a second conductivity type second semiconductor layer 40, and a second conductivity type semiconductor layer 40. 3 semiconductor layers 50; The first semiconductor layer 20 , the light emitting layer 30 , the second semiconductor layer 40 and the third semiconductor layer 50 are laminated in order on the semiconductor substrate 10 . The semiconductor substrate 10 is, for example, an n-type GaAs substrate.

The first semiconductor layer 20 is, for example, an n-type AlGaAs layer. A first semiconductor layer 20 is formed on the semiconductor substrate 10 . The light emitting layer 30 and the second semiconductor layer 40 are laminated on the first semiconductor layer 20 . The light emitting layer 30 is provided between the first semiconductor layer 20 and the second semiconductor layer 40 . Emissive layer 30 includes at least one quantum well. The second semiconductor layer 40 is, for example, a p-type AlGaAs layer.

The third semiconductor layer 50 is provided on the second semiconductor layer 40 . The third semiconductor layer 50 is, for example, a p-type GaAs layer.

The first electrode 110 is provided on the back surface of the semiconductor substrate 10 . The first electrode 110 is electrically connected to the semiconductor substrate 10 . The first electrode 110 is, for example, a metal layer containing germanium (Ge), nickel (Ni), gold (Au), or the like.

A second electrode 120 is provided on the third semiconductor layer 50 . The second electrode 120 is electrically connected to the third semiconductor layer 50 . The second electrode 120 is, for example, a metal layer containing chromium (Cr), gold (Au), or the like.

The semiconductor light emitting device 1 is a diode in which forward current flows from the second semiconductor layer 40 to the first semiconductor layer 20 . In the semiconductor light emitting device 1, for example, forward current is passed between the second electrode 120 and the first electrode 110 to inject electrons and holes into the light emitting layer 30, and emit light generated by their recombination. Discharge to the outside.

As shown in FIG. 1B, the light emitting layer 30 is provided between the first semiconductor layer 20 and the second semiconductor layer 40 . The light emitting layer 30 includes, for example, quantum well layers 33 and barrier layers 35 . A barrier layer 35 is provided adjacent to the quantum well layer 33 . Emissive layer 30 includes at least one quantum well layer 33 . In this example, light emitting layer 30 includes four quantum well layers 33 . The quantum well layer 33 contains, for example, a ternary compound semiconductor InGaAs.

The barrier layer 35 includes, for example, a first region 35a and a second region 35b. The second region 35b is provided between the quantum well layer 33 and the first region 35a. The second region 35b is, for example, one of the two first regions 35a located on both sides of the quantum well layer 33 in the stacking direction from the first semiconductor layer 20 to the second semiconductor layer 40, and the quantum well layer 33 provided between

For example, in the stacking direction, the width of the first region 35a is wider than the width of the second region 35b. The first region 35a contains, for example, the ternary compound semiconductor GaAsP, and the second region 35b contains, for example, the quaternary compound semiconductor AlGaAsP.

FIG. 2 is a schematic diagram showing materials of the semiconductor light emitting device 1 according to the embodiment. FIG. 2 shows the lattice constant and bandgap energy (hereafter, bandgap ) is a diagram showing the relationship between The compound semiconductor material shown in FIG. 2 has a direct transition band structure. The horizontal axis is the lattice constant (Å) and the vertical axis is the bandgap (eV).

GaAs is shown in the center of FIG. A line indicating In _x Ga _1-x As is drawn in the direction of increasing lattice constant from GaAs. As the In composition ratio “x” increases, the lattice constant of In _x Ga _1-x As increases and the bandgap narrows.

Also, a line indicating GaAs _1-y P _y is drawn in the direction from GaAs in which the lattice constant decreases. As the composition ratio “y” of P increases, the lattice constant of GaAs _1- yP _y decreases and the bandgap widens.

Further, a line indicating Al _z Ga _1-z As is shown in the direction from GaAs to the widening of the bandgap. As the Al composition ratio “z” increases, the bandgap of Al _z Ga _1-z As widens. The lattice constant of Al _z Ga _1-z As increases as the Al composition ratio “z” increases, but the amount of change is small.

The quantum well layer 33 contains, for example, In _x Ga _1-x As with an In composition ratio x=0.2. The bandgap of In _0.2 Ga _0.8 As is approximately 1.2 eV, and the quantum well layer 33 emits light in the wavelength band to which silicon photodiodes are sensitive. The lattice constant of In _0.2 Ga _0.8 As is about 5.73 Å, which is larger than that of GaAs.

The first region 35a of the barrier layer 35 contains, for example, GaAs _1-y P _y with a P composition ratio y=0.05. The bandgap of GaAs _0.95 P _0.05 is about 1.5 eV and the lattice constant is about 5.64 Å. The lattice constant of GaAs _0.95 P _0.05 is smaller than that of GaAs and In _0.2 Ga _0.8 As.

The second region 35b of the barrier layer 35 is, for example, Al _z Ga _1-z As (0<z<1) or Al _z Ga _1-z As _1-y P _y (0<y<1, 0<z< 1). The second region 35b contains a compound semiconductor mixed crystal to which Al is added, and has a bandgap wider than that of the first region 35a.

In the light emitting layer 30, the crystal strain caused by the difference in lattice constant between the material of the semiconductor substrate 10 and the material of the quantum well layer 33 is GaAs _1-y P _y mixed crystal (0<y<1). , Al _z Ga _1-z As mixed crystal (0<z<1) or Al _z Ga _1-z As _1-y P _y mixed crystal (0<y<1, 0<z<1) Compensation is provided by layer 35 .

For example, a light-emitting diode used in an optical coupling device such as a photocoupler preferably emits light having a wavelength in the 1 μm band to which silicon photodiodes are sensitive. A 1 μm band light-emitting diode is manufactured using a relatively inexpensive GaAs substrate. For light emission in the 1 μm band, an MQW (Multi-Quantum Well) having a quantum well layer of an In _x Ga _1-x As mixed crystal having an In composition ratio x of 0.15 to 0.2 is used. However, such an InGaAs mixed crystal has a lattice constant larger than that of GaAs, and lattice strain resulting from the difference therebetween causes a problem of reduced luminous efficiency.

In the light-emitting layer 30 according to the embodiment, a GaAs _1-y P _y mixed crystal having a lattice constant smaller than that of GaAs and an Al _z Ga _1-z As mixed crystal or Al _z Ga _1-z As _1-y By forming the barrier layer 35 using a _Py mixed crystal, it is possible to compensate for such lattice distortion and improve the luminous efficiency.

3A and 3B are schematic diagrams showing the hand structure of the light emitting layer 30 of the semiconductor light emitting device 1 according to the embodiment. FIG. 3A is a schematic diagram showing the band structure of the light-emitting layer 30 according to the embodiment. FIG. 3B is a schematic diagram showing the band structure of the light emitting layer of the semiconductor light emitting device 2 according to the comparative example. The distribution of carriers injected into the light-emitting layer 30 is dominated by electrons with small effective masses and long diffusion lengths among electrons and holes. Therefore, the conduction band Ec is shown in FIGS. 3(a) and 3(b).

As shown in FIG. 3A, the conduction band Ec of the light-emitting layer 30 includes energy band discontinuity due to the difference in bandgap between the quantum well layer 33 and the barrier layer 35, that is, a so-called quantum well. . For example, the energy barrier between the quantum well layer 33 and the second region 35 b of the barrier layer 35 is higher than the energy barrier between the quantum well layer 33 and the first region 35 a of the barrier layer 35 . The difference between the two energy barriers is due to the difference between the InGaAs bandgap of the quantum well layer 33 and the AlGaAs or AlGaAsP mixed crystal bandgap, and the difference between the InGaAs bandgap and the GaAsP mixed crystal bandgap.

For example, when a forward bias is applied to the semiconductor light emitting device 1 , electrons move from the first semiconductor layer 20 toward the second semiconductor layer 40 . Electrons injected from the first semiconductor layer 20 into the light emitting layer 30 are trapped in the quantum well and recombine with holes (not shown) injected from the second semiconductor layer 40 into the light emitting layer 30 . Thereby, light having a predetermined wavelength is emitted from the light emitting layer 30 .

In the semiconductor light emitting device 2 shown in FIG. 3B, the barrier layer 35 in the light emitting layer does not have the second region 35b. Therefore, the energy barrier between the quantum well layer 33 and the first region 35a of the barrier layer 35 in the direction of electron movement is lowered. Therefore, some of the electrons trapped in the quantum well of conduction band Ec are emitted without recombining with holes.

In contrast, in the semiconductor light-emitting device 1 according to the embodiment, the energy barrier between the quantum well layer 33 and the barrier layer 35 is higher than that in the light-emitting device 2 by providing the second region 35b of the barrier layer 35. Become. Therefore, emission of electrons from the quantum well layer 33 to the barrier layer 35 is suppressed. As a result, in the light-emitting layer 30 of the semiconductor light-emitting device 1, confinement of injected carriers becomes more effective, and light emission efficiency can be improved.

FIG. 4 is a graph showing characteristics of the semiconductor light emitting device 1 according to the embodiment. FIG. 4 shows the relationship between the width of the quantum well layer 33 in the lamination direction and the emission intensity. The horizontal axis indicates the width and number of layers of the quantum well layer 33 . Here, the total width (number of wells×layer thickness) of the quantum well layer 33 is constant. The vertical axis is the photoluminescence intensity (PL emission intensity).

As shown in FIG. 4, when the width of the quantum well layer 33 is made narrower than 4 nanometers (nm), the PL emission intensity decreases. On the other hand, when the width of the quantum well layer 33 is in the range of 4 nm to 10 nm, the PL emission intensity is constant. That is, the width of the quantum well layer 33 in the stacking direction is preferably 4 nm or more.

FIGS. 5A and 5B are other graphs showing the characteristics of the semiconductor light emitting device 1 according to the embodiment. FIG. 5A is a graph showing the luminous efficiency of the semiconductor light emitting device 1. FIG. FIG. 5(b) is a graph showing the luminous efficiency of the semiconductor light emitting device 2 according to the comparative example. The vertical axis is luminous efficiency, and the horizontal axis is current density.

In the light emitting layer 30 of the semiconductor _light emitting device 1, the quantum well layer 33 has, for example, a thickness of 5 nm in the lamination direction and contains _In0.2Ga0.8As mixed crystal. The light-emitting layer 30 also includes four quantum well layers 33 . The first region 35a of the barrier layer 35 has, for example, a thickness of 13 nm in the lamination direction and contains GaAs _0.95 P _0.05 . The second region 35b has, for example, a thickness of 2 nm in the lamination direction and contains Al _0.1 Ga _0.9 As _0.95 P _0.05 .

In the semiconductor light emitting device 2, the quantum well layer 33 has a _thickness of 5 nm in the stacking direction and contains _In0.2Ga0.8As . Four quantum well layers 33 are provided. The barrier layer 35 does not include the second region 35b, and the first region 35a has a thickness of 15 nm in the lamination direction and includes GaAs _0.95 P _0.05 , for example.

In the semiconductor light-emitting device 1, the conduction band Ec of the light-emitting layer 30 has a relatively high energy barrier (see FIG. 3A). carrier density increases. Furthermore, by providing the second region 35 b of the barrier layer 35 so as to be in contact with the quantum well layer 33 , carriers can be effectively confined in the quantum well layer 33 .

As shown in FIG. 5(a), the semiconductor light emitting device 1 has the maximum luminous efficiency at a current density of 7 A/cm ² . The semiconductor light emitting device 1 has an emission peak wavelength of 950 nm and an operating voltage of 1.32V.

The semiconductor light emitting device 2 shown in FIG. 5(b) has the maximum luminous efficiency at a current density of 30 A/cm ² . The maximum luminous efficiency of the semiconductor light emitting device 2 is lower than the maximum luminous efficiency of the semiconductor light emitting device 1 .

For example, the luminous efficiency of the semiconductor light emitting device 1 is maximized under the operating condition of the driving current of 7 A/cm ² for the light emitting diode used in the photocoupler. Also, the maximum value of the luminous efficiency of the semiconductor light emitting device 1 is larger than that of the semiconductor light emitting device 2 . That is, by providing the barrier layer 35 with the second region 35b, the luminous efficiency can be improved and the power consumption can be reduced.

Although the operating voltage of the semiconductor light emitting device 1 is increased by about 0.02 V compared to the semiconductor light emitting device 2, the increase is small and the luminous efficiency is increased.

Further, as a modification of the present embodiment, Al _z Ga _1-z As can be used for the second region 35 b of the barrier layer 35 instead of Al _z Ga _1-z As _1-y P _y .

For example, as the quantum well layer 33, an In _0.2 Ga _0.8 As mixed crystal with a thickness of 5 nm in the stacking direction is used, and the number of layers is four. Further, as the first region 35a of the barrier layer 35, for example, GaAs _0.95 P _0.05 mixed crystal with a thickness of 13 nm in the stacking direction is used, and as the second region 35b, for example, Al with a thickness of 2 nm in the stacking direction is used. _{A 0.1} Ga _0.9 As mixed crystal is used. This makes it possible to obtain a semiconductor light emitting device having a maximum luminous efficiency at a current density of several A/cm ² .

As described above, in the semiconductor light emitting device 1 according to the embodiment, part of the barrier layer 35 using a GaAsP mixed crystal is replaced with a mixed crystal containing Al to widen the bandgap and improve the carrier confinement effect. Let At this time, the strain compensation effect of the GaAsP mixed crystal is maintained.

The above-mentioned compound semiconductor mixed crystals are produced by metal-organic chemical vapor deposition using raw materials such as trimethylindium (TMI), trimethylgallium (TMG), trimethylaluminum (TMA), arsine (AsH ₃ ), and phosphine (PH ₃ ). (MOCVD method).

In the MOCVD method, for example, a desired mixed crystal can be grown by supplying a raw material gas onto a heated GaAs substrate and thermally decomposing it. For example, when an InGaAs mixed crystal is grown on a GaAs substrate, a crystal containing lattice distortion due to lattice mismatch between the two is deposited. As the thickness of the InGaAs mixed crystal increases, this lattice strain increases, and when the critical point of elastic deformation is exceeded, strain relaxation occurs and crystal defects occur. Such crystal defects deteriorate, for example, the light emission characteristics of light emitting diodes.

In this embodiment, a laminated structure is used in which a GaAsP mixed crystal is formed on a GaAs substrate to produce a lattice distortion opposite to that of an InGaAs mixed crystal, thereby compensating for the lattice distortion of the two. For example, when an InGaAs mixed crystal is used as the quantum well layer 33 of the MQW structure, the GaAsP mixed crystal is used as the barrier layer 35 because it has a wider bandgap than the InGaAs mixed crystal. Thereby, the lattice distortion of the light emitting layer 30 including the MQW structure can be suppressed, and crystal defects can be reduced.

Furthermore, by adding an Al-based mixed crystal having a wider bandgap than the GaAsP mixed crystal as part of the barrier layer 35, the carrier confinement effect can be improved, and the luminous efficiency can be further improved.

However, in the process of growing an Al-based mixed crystal such as Al _z Ga _1-z As _1-y P _y or Al _z Ga _1-z As using the MOCVD method, the carbon atoms C contained in the raw material are There is a problem that aluminum atoms are combined with Al and incorporated into the crystal (Reference: Van Deelen et al., "Parameter study of intrinsic carbon doping of AlxGa 1-xAs by OCVD". Journal of Crystal Growth, 271 (3- 4), pp. 376-384 (2004)).

For example, the carbon atom C can act as an acceptor in the crystal and change the energy band structure of the MQW. When the Al _z Ga _1-z As _1-y P _y crystal was analyzed using SIMS (Secondary Ion Mass Spectroscopy), the concentration of carbon atoms C was about 2×10 ¹⁷ cm ⁻³ . This concentration is sufficient to make the crystal conductivity type p-type and may form multiple pn junctions in the MQW. Therefore, the recombination probability of electrons in the barrier layer near the pn junction increases, and the number of carriers contributing to radiative recombination in the quantum well is reduced. As a result, there is concern that the luminous efficiency of the light-emitting layer will decrease and that the operating voltage of the semiconductor light-emitting device will increase. Carbon atom C may also be an interband level that promotes non-radiative recombination.

In the embodiment, in order to avoid these disadvantages, the thickness of the Al-based mixed crystal in the stacking direction is reduced to reduce the carbon atom C content. For example, in order to suppress the increase in operating voltage to 0.10 V or less, it is preferable to set the thickness of the Al-based mixed crystal to 10 nm or less. Moreover, in order to suppress the influence on the luminous efficiency, it is more preferable to set the thickness of the Al-based mixed crystal to 5 nm or less.

In addition, the carbon concentration in the Al-based mixed crystal depends on the ratio of the group 3 raw materials such as TMI, TMG and TMA to the group 5 raw materials such as _AsH3 and _PH3 in the growth source gas. For example, when the proportion of the group V source material in the source gas decreases, the amount of carbon emitted as methane (CH ₄ ) decreases and the carbon concentration in the crystal increases. Therefore, it is preferable that the carbon concentration in the crystal does not exceed 2×10 ¹⁷ cm ⁻³ even if the consumption of group V raw materials in MOCVD is suppressed and the manufacturing cost is reduced.

Embodiments are not limited to the above examples. For example, the composition ratio of P in the GaAs _1-y P _y mixed crystal of the barrier layer 35 can be appropriately adjusted along with the film thickness within a range in which lattice distortion compensation with the InGaAs mixed crystal of the quantum well layer 33 is taken into consideration. . In addition, the Al composition of the Al mixed crystal system can be appropriately adjusted together with the film thickness in consideration of the carrier confinement effect in the quantum well and the operating voltage.

While several embodiments of the invention have been described, these embodiments have been presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and modifications can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the scope of the invention described in the claims and equivalents thereof.

DESCRIPTION OF SYMBOLS 1, 2... Semiconductor light-emitting device 10... Semiconductor substrate 20... First semiconductor layer 30... Light emitting layer 33... Quantum well layer 35... Barrier layer 35a... First region 35b... Second region 40... Second semiconductor layer 50 Third semiconductor layer 100 Semiconductor portion 110 First electrode 120 Second electrode Ec Conduction band

Claims (4)

an n- type first semiconductor layer;
a p -type second semiconductor layer provided on the first semiconductor layer;
a light-emitting layer provided between the first semiconductor layer and the second semiconductor layer and including a plurality of quantum well layers and barrier layers adjacent to each of the plurality of quantum well layers;
with
The plurality of quantum well layers includes a material having a lattice constant larger than the lattice constant of the material of the first semiconductor layer, and is arranged in a direction from the first semiconductor layer toward the second semiconductor layer. and a second quantum well layer adjacent to the first quantum well layer on the second semiconductor layer side,
the barrier layer is provided between the first quantum well layer and the second quantum well layer and includes a first region and a p-type second region connected to the first region ;
The second region is located between the first quantum well layer and the first region, is connected to the first quantum well layer, and is between the first region and the second quantum well layer. not provided,
the first region of the barrier layer includes a material having a wider bandgap than the material of the quantum well layer and a lattice constant smaller than that of the material of the first semiconductor layer;
The second region of the barrier layer comprises a material with a wider bandgap than the material of the first region and has a thickness of 10 nanometers or less in the direction from the first semiconductor layer to the second semiconductor layer. and containing carbon at a concentration of 2×10 ¹⁷ cm ⁻³ or less. The first region has a first width in the direction from the first semiconductor layer toward the second semiconductor layer, the second region has a second width in the direction , and the first 2. The semiconductor light emitting device according to claim 1, wherein the width is wider than said second width. the plurality of quantum well layers include a ternary mixed crystal represented by a composition formula In _x Ga _1-x As (0<x<1);
The first region of the barrier layer includes a ternary mixed crystal represented by a composition formula GaAs _1-y P _y (0<y<1), and the second region includes a composition formula Al _z Ga _1-z 3. The semiconductor light emitting device according to claim 1, comprising a quaternary mixed crystal represented by As 1-y P _y (0<y<1, 0<z _< 1 ). the plurality of quantum well layers include a ternary mixed crystal represented by a composition formula In _x Ga _1-x As (0<x<1);
The first region of the barrier layer includes a ternary mixed crystal represented by a composition formula GaAs _1-y P _y (0<y<1), and the second region includes a composition formula Al _z Ga _1-z 3. The semiconductor light-emitting device according to claim 1 , comprising a ternary mixed crystal represented by As (0<z<1).

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