JP2006245441A - Semiconductor light emitting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor light emitting element whose response characteristics can be improved without lowering light emission efficiency. <P>SOLUTION: Grated layers 5 and 7 nearby an active layer 6 suppress absorption of emitted light due to a free carrier, so the carrier density is set low (non-dope). Further, grated layers 4 and 8 nearby clad layers 3 and 9 continuously increase in carrier density with the distance from the active layer 6. The graded layers 4 and 8 continuously increase in carrier density nearby the clad layers 3 and 9, so an energy band gap is deterred from being notched on their interfaces and a carrier can smoothly be injected into the active layer 6. The semiconductor light emitting element, therefore, has its response characteristics improved without having a decrease in light emission efficiency. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a semiconductor light emitting device.

  A semiconductor light emitting element such as a light emitting diode includes an active layer between a P-type semiconductor and an N-type semiconductor. When a forward bias voltage is applied to the light emitting diode, the energy barrier between the PN junctions is lowered. The electrons transferred from the N-type semiconductor to the P-type semiconductor transition from the high energy level conduction band to the low energy level valence band and recombine with holes. The energy lost with this transition is emitted to the outside as light. The larger the energy band gap, the shorter the emission wavelength.

  Conventionally, in order to improve the carrier recombination efficiency and the light emission efficiency in the active layer, the active layer is sandwiched between clad layers having an energy band gap larger than that of the active layer. When a multi-quantum well (MQW) structure is used for the active layer, the recombination probability is improved due to the localization of carriers for recombination to the well layer, and the light emission efficiency can be improved.

In addition, there has also been proposed a method in which a graded layer whose composition is continuously changed is provided between the active layer and the clad layer without direct contact therebetween. 1, Patent Document 2, Patent Document 3, and Patent Document 4.
Japanese Patent Laid-Open No. 11-121796 JP 2004-297060 A JP 2001-156329 A JP 2004-111648 A

  However, when the graded layer interposed between the active layer and the cladding layer is formed non-doped, the Fermi level of the graded layer exists in the middle of the forbidden band. The Fermi level of the high impurity concentration P-type cladding layer exists near the valence band. The Fermi level of the N-type cladding layer having a high impurity concentration exists near the conduction band.

  That is, at the boundary between the non-doped graded layer and the high-concentration cladding layer adjacent to each other, when these Fermi levels are matched, a notch in the energy band is generated. In the case of the P-type cladding layer, the notch becomes a barrier against holes, and in the case of the N-type cladding layer, the notch becomes a barrier against electrons, and in any case, smooth injection of carriers into the active layer is hindered. As a result, the response characteristics are degraded.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a semiconductor light emitting device capable of improving response characteristics without reducing light emission efficiency.

  In order to solve the above-mentioned problem, a semiconductor light emitting device according to the present invention includes a first cladding layer, a first graded layer formed on the first cladding layer, and a first graded layer. In a semiconductor light emitting device comprising: an active layer formed; a second graded layer formed on the active layer; and a second cladding layer formed on the second graded layer. The second cladding layer has a high carrier concentration, and the first and / or second graded layers are continuous in the vicinity of the boundary with the adjacent cladding layer as the carrier concentration moves toward the cladding layer. It is characterized by increasing.

The “active layer” in the semiconductor light emitting device is a semiconductor layer in which carrier recombination occurs, and the “cladding layer” is a semiconductor layer having a larger energy band gap than the “active layer”. A “graded layer” is a semiconductor layer having an energy band gap between the “active layer” and the “cladding layer”, and the energy band gap is continuously changed. The “high carrier concentration” means a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ or more.

  The “cladding layer” is a semiconductor layer having a low refractive index and a large energy band gap, and has a blocking function (function for blocking electrons and holes) and a window function (a P-type semiconductor layer or an N-type semiconductor layer is a kind of window). It has a function of having a function of taking out light efficiently with little self-absorption of light generated in the active layer, and is not necessarily required to be adjacent to the active layer.

  The “near boundary” of the graded layer with respect to the cladding layer means a position relatively closer to the boundary with the active layer.

  Further, the energy band gap of the active layer when the active layer of the semiconductor light emitting device has a multiple quantum well structure is defined as the energy band gap of the well layer.

  In the semiconductor light emitting device described above, since the cladding layer has a high carrier concentration, the overflow of carriers from the active layer is suppressed. In the vicinity of the active layer, the carrier concentration is set low in order to suppress absorption of light emission caused by free carriers. The graded layer interposed between the active layer and the clad layer relaxes the rapid composition change between them, and has the effect of efficiently guiding carriers to the active layer, thereby improving the light emission efficiency.

  Here, since the carrier concentration continuously increases in the vicinity of the clad layer of the graded layer, the occurrence of notches in the energy band gap is suppressed, and the carrier is smoothly injected into the active layer. Therefore, in this semiconductor light emitting device, the response characteristics can be improved without reducing the light emission efficiency. The term “response characteristics” as used herein means to consider smooth carrier injection when driving a semiconductor light-emitting element, in addition to the response speed from when current is injected until it takes effect. .

  In addition, when the first cladding layer, the first graded layer, the second graded layer, and the second cladding layer are each made of AlGaInP, a response speed improvement of about 10% was obtained.

  The active layer preferably has a multiple quantum well structure in which GaInP and AlGaInP are alternately stacked. In this case, the barrier layer (AlGaInP) is easily lattice-matched with the cladding layer or graded layer. The luminous efficiency does not decrease.

  In addition, the first and / or second graded layer can perform smooth carrier injection when the carrier concentration and the composition are set to be equal to those of the cladding layer at the interface with the adjacent cladding layer.

  According to the semiconductor light emitting device of the present invention, the response characteristics can be improved without lowering the light emission efficiency.

  Hereinafter, a semiconductor light emitting device (light emitting diode) according to an embodiment will be described. In addition, the same code | symbol shall be used for the same element and the overlapping description is abbreviate | omitted.

  FIG. 1 is a plan view of a semiconductor light emitting device according to the embodiment, and FIG. 2 is a cross-sectional view taken along the line II-II of the semiconductor light emitting device shown in FIG.

  The semiconductor light emitting device 100 includes a DBR layer 2, a cladding layer 3, a graded layer 4, a graded layer 5, an active layer 6, a graded layer 7, a graded layer 8, a clad layer 9, a DBR on the contact layer 1. A layer 10 and a contact layer 11 are sequentially stacked. A cathode electrode E1 is provided on the lower surface of the lower contact layer 1, and an anode electrode E2 having an open central portion is provided on the upper surface of the upper contact layer 11. An insulating layer 20 is formed on the contact layer 11.

  The anode electrode E2 includes an annular portion E2a formed on the insulating layer 20 and having an open center portion, an electrode pad portion E2b formed on the insulating layer 20 and electrically connected to the annular portion E2a, and an annular portion A plurality of contact portions E2c extending so as to converge inward from a plurality of positions on E2a are provided. Openings are formed between the contact portions E2c.

The material, composition, conductivity type, carrier concentration (cm ⁻³ ), and thickness (nm) of each of the semiconductor layers 1 to 11 are as follows. In the table, the value when it is close to the active layer 6 is described on the left side of “˜”, the value when it is far from the active layer 6 is described on the right side, and the left and right of “/” are alternately stacked. Two semiconductor layers will be described.

  More specifically, the active layer 6 is sandwiched between two lower graded layers 5 and 4 and two upper graded layers 7 and 8.

The lower graded layer 5 changes from Al _0.3 Ga _0.2 In _0.5 P to Al _0.325 Ga _0.175 In _0.5 P in the thickness direction as the distance from the active layer 6 increases. The dead layer 4 changes from Al _0.325 Ga _0.175 In _0.5 P to Al _0.35 Ga _0.15 In _0.5 P in the thickness direction, and the cladding layer 3 (Al _0.35 Ga _{0. 15} In _0.5 P).

The graded layer 7 changes from Al _0.3 Ga _0.2 In _0.5 P to Al _0.325 Ga _0.175 In _0.5 P in the thickness direction as the distance from the active layer 6 increases. Changes from Al _0.325 Ga _0.175 In _0.5 P to Al _0.35 Ga _0.15 In _0.5 P in the thickness direction, and the cladding layer 9 (Al _0.35 Ga _0.15 In _{0 .5} P) is continuous.

Further, DBR (Distributed Bragg Reflector) layers 2 and 10 are multilayer reflective films, which are formed by alternately stacking a high refractive index layer (Al _0.5 GaAs) and a low refractive index layer (Al _0.98 GaAs). It is. Further, from the viewpoint of emission efficiency, it is preferable that the reflectance with respect to the emission wavelength of the DBR layer 2 on the cathode electrode E1 side is higher than the reflectance with respect to the emission wavelength of the DBR layer 10 on the anode electrode E2 side.

  The active layer 6 has a non-doped multiple quantum well (MQW) structure, and the photoluminescence peak wavelength is 650 nm. The thickness and number of the well layers and barrier layers are 5 nm for the well layer, 5 nm for the barrier layer, 3 layers for the well layer, and 2 layers for the barrier layer.

  When a forward bias voltage is applied between the cathode electrode E1 and the anode electrode E2, electrons are injected from the cathode electrode E1 and holes are injected from the anode electrode E2. When these recombine in the active layer 6, light emission occurs. .

Since the clad layers 3 and 9 have an energy band gap larger than that of the active layer 6, the effect of confining carriers is high, but the clad layers 3 and 9 have a high carrier concentration (5 × 10 ¹⁷ cm ⁻³ or more). Therefore, the Fermi level of the cladding layer approaches the valence band or the conduction band, the energy difference between the active layer and the cladding layer increases, and the overflow of carriers from the active layer 6 is suppressed.

  Graded layers 4, 5, 7, and 8 interposed between active layer 6 and cladding layers 3 and 9 alleviate abrupt composition changes between them, and have the effect of efficiently introducing carriers to the active layer. Yes, the luminous efficiency is improved.

  The graded layers 5 and 7 positioned in the vicinity of the active layer 6 have a low carrier concentration (non-doped) in order to suppress absorption of light emission caused by free carriers. Further, the graded layers 4 and 8 located in the vicinity of the clad layers 3 and 9 have the carrier concentration continuously increasing as the distance from the active layer 6 increases. Since the carrier concentration continuously increases in the vicinity of the clad layers 3 and 9 of the graded layers 4 and 8, the occurrence of notches in the energy band gap at these interfaces is suppressed, and carriers to the active layer 6 are suppressed. Can be injected smoothly. Therefore, in this semiconductor light emitting device, the response characteristics can be improved without reducing the light emission efficiency.

The carrier concentrations C ₄ and C ₈ of the graded layers ₄ and ₈ are such that the interface positions between the active layer 6 and the graded layers 5 and 7 are origins O and O ′, and the distance in the thickness direction away from these origins is d and gray. Assuming that the thicknesses of the dead layers 5 and 7 are D, the initial value C ₀ , and the coefficient C, for example, the following is shown.
C ₄ = C ₀ + C × (d−D)

Here, C ₀ is about 0.5 to 1.0 × 10 ¹⁷ cm ⁻³ and the coefficient C is about 0.4 to 1 × 10 ¹⁶ cm ⁻³ / nm. The thickness D is about 25 nm to 70 nm. The carrier concentration of the cladding layer is preferably about 1 to 2 × 10 ¹⁸ cm ⁻³ .

  Note that Mg can be used as a P-type impurity added to the graded layer. Mg has the advantage that the diffusion constant is smaller than that of Zn, and control during addition is easy. Si was used as the N-type impurity.

  In the case where the AlGaInP composition of the graded layer is a composition that absorbs light of the emission wavelength of the active layer 6 at a certain level or more, it is not doped. In the graded layers 4 and 8 from the AlGaInP composition in which the absorption is reduced to the cladding layers 3 and 9 in which the energy band gap is widened, the carrier concentration gradually increases from non-doped. Further, at the interface between the graded layers 4 and 8 and the clad layers 3 and 9, the AlGaInP composition and carrier concentration of these layers coincide, and smooth carrier injection can be performed.

  The part where carrier doping is started is a part having a graded layer composition (energy band gap E) that sufficiently reduces free carrier absorption for the light emission of the active layer 6. Moreover, the site | part which starts dope of a carrier is a site | part (distance D) away from the active layer 6 to such an extent that the diffusion to the active layer 6 of a dopant does not become a problem.

The energy band gap E and the distance D are as follows.
Energy band gap E = 1.89 to 1.92 eV
Distance D = 25 nm to 70 nm

  FIG. 3 is a longitudinal sectional view (a) of the semiconductor light emitting device shown in FIG. 2, an energy band gap graph (b) with respect to the thickness direction, and an impurity concentration (c) graph with respect to the thickness direction. This thickness is defined such that the exposed surface position of the lower contact layer 1 is 0 and the stacking direction is positive. The impurity concentration indicates the carrier concentration at room temperature.

  The thickness of graded layers 5 and 7 continuing from active layer 6 is 40 nm, and the energy band gap of graded layers 5 and 7 gradually increases as the distance from the boundary with active layer 6 increases. In the separated graded layers 4 and 8, the composition is made closer to the cladding layers 3 and 9. That is, in the graded layers 4 and 8, the impurity concentration is gradually changed from non-doped to the impurity concentration of the cladding layers 3 and 9 while continuously changing the energy band gap.

  With this structure, the notch of the energy level generated at the graded layer-cladding layer interface shown in a comparative example described later is suppressed. By adopting this structure, the response speed can be improved while maintaining the light output. Moreover, since the notch is suppressed, the forward voltage can also be reduced.

  In this example, the gradient of the energy band gap of the inner graded layers 5 and 7 (rate of change with respect to the thickness direction) is smaller than the gradient of the energy band gap of the outer graded layers 4 and 8.

  FIG. 4 is a longitudinal sectional view (a) of another semiconductor light emitting device, a graph (b) of the energy band gap in the thickness direction, and a graph of the impurity concentration (c) in the thickness direction.

  In this semiconductor light emitting device, the slope of the energy band gap of the inner graded layers 5 and 7 is equal to the slope of the energy band gap of the outer graded layers 4 and 8, and is constant with respect to the thickness direction. . That is, the thickness of the outer graded layers 4 and 8 is larger than that shown in FIG. 3, and the rate of change of the energy band gap in the thickness direction is made slow.

  The energy band gap of the graded layer may have a structure in which the slope becomes steeper as it gets closer to the cladding layer.

  The energy band gap of the above semiconductor light emitting device will be further considered.

  FIG. 5 is an energy band diagram in the vicinity of the upper graded layers 7 and 8 of the semiconductor light emitting device according to the embodiment.

The inner graded layer 7 and active layer 6 are non-doped. On the other hand, the outer graded layer 8 is graded doped (impurity concentration is changed in the thickness direction). Fermi level E _F is the active layer 6, the graded layer 7,8 are consistent in the cladding layer 9. Bottom Ec of the conduction band is spaced apart from the Fermi level E _F the distance from the active layer 6, a constant in the cladding layer within 9. The upper end Ev of the valence band, separated from the Fermi level E _F the distance from the active layer 6, a slight outside the graded layer 8, close to the Fermi level, the constant in the cladding layer within 9. In this example, Zn is used as the P-type impurity.

  In a comparative example described later, a notch is generated at a position indicated by a dashed line A, but the notch is not observed in the semiconductor light emitting device according to the embodiment.

  Although the interface between the P-type graded layer and the clad layer is described here, the same applies to the interface between the N-type graded layer and the clad layer.

  Further, a MOVPE growth apparatus can be used to grow the AlGaInP layer.

For example, a GaAs substrate is set face down on a susceptor, TMI (trimethyllinium) as a source of In, TMG (trimethylgallium) as a source of Ga, TMA (trimethylaluminum) as a source of Al, PH ₃ (phosphine) as a source of P, Si ₂ H ₆ (disilanes) can be used as the N-type dopant, and DEZ (diethylzinc) for Zn or Cp ₂ Mg ((bis) cyclopentadienyl magnesium) for Mg can be used as the P-type dopant. AlGaInP can be typically manufactured at a growth temperature of 650 ° C. and a growth pressure of 50 Torr (6.7 × 10 ³ Pa).
(Comparative Example 1)

  Next, Comparative Example 1 will be described.

  6 is an energy band diagram in the vicinity of the upper graded layers 7 and 8 of the semiconductor light emitting device according to Comparative Example 1. FIG. 7 is a longitudinal sectional view (a) of the semiconductor light emitting device according to Comparative Example 1, a graph (b) of the energy band gap with respect to the thickness direction, and a graph of the impurity concentration (c) with respect to the thickness direction.

  The semiconductor light emitting device according to Comparative Example 1 is different from the semiconductor light emitting device according to the above-described embodiment only in that the outer graded layers 4 and 8 are non-doped, but will be briefly described below.

  Also in Comparative Example 1, the vicinity of the active layer 6 is formed undoped because light emission from the active layer 6 is easily absorbed by free carrier absorption when high concentration doping is performed. Further, the cladding layer 9 is heavily doped in order to suppress the overflow of carriers from the active layer 6. Graded layers 7 and 8 extending from the active layer 6 to the P-type cladding layer 9 are formed non-doped.

In this case, the Fermi level E _F is graded layer 7 and 8 is present in the middle of the forbidden band, the heavily doped to P-type and the cladding layer 9, the Fermi the upper end Ev side of the valence band A level _EF is formed. For this reason, an energy band notch is generated at the boundary between the graded layer 8 and the clad layer 9 as shown by a one-dot chain line A, which becomes a barrier against holes and inhibits smooth carrier injection.

  That is, in the semiconductor light emitting device according to Comparative Example 1, the active layer 6, the inner graded layer 7, and the outer graded layer 8 are all non-doped, and a notch is generated, preventing a smooth flow of carriers, and response characteristics. Decrease and forward voltage increase.

Here, the vicinity of the interface between the graded layer and the P-type cladding layer has been described, but a notch is similarly generated at the interface between the graded layer and the N-type cladding layer. That is, in the case of the P-type cladding layer, the notch becomes a barrier against holes, and in the case of the N-type cladding layer, the notch becomes a barrier against electrons, and in any case, smooth injection of carriers into the active layer is hindered. .
(Comparative Example 2)

FIG. 8 is a longitudinal sectional view (a) of the semiconductor light emitting device according to Comparative Example 2, a graph (b) of the energy band gap with respect to the thickness direction, and a graph of the impurity concentration (c) with respect to the thickness direction. In this example, non-doped layers (Al _0.3 Ga _0.2 In _0.5 P) 5 ′ and 7 ′ are provided in the vicinity of the active layer 6 instead of the graded layers 5 and 7. 8 is different from the embodiment in that P-type and N-type intermediate layers (Al _0.325 Ga _0.175 In _0.5 P) 4 ′ and 8 ′ are provided on the outer side thereof. The energy band gaps of the intermediate layers 4 ′ and 8 ′ are intermediate energy band gaps between the cladding layers 3 and 9 and the non-doped layers 5 ′ and 7 ′, and are constant in the thickness direction. Even in this case, the notch is not eliminated and the response characteristic is deteriorated and the forward voltage is increased.

On the other hand, the semiconductor light emitting device according to the embodiment shown in FIG. 2 has the following effects.
(1) Although the cutoff frequency of the response characteristic of the semiconductor light emitting device of Comparative Example 1 was about 90 MHz, the cutoff frequency of the response characteristic of the semiconductor light emitting device of the embodiment (3 dB emission attenuates from the frequency at which there is no change in emission). (Frequency) was 100 MHz, and the response speed was improved by about 10%.
(2) Almost no decrease in light output of the semiconductor light emitting device of the embodiment was observed.
(3) Although the forward voltage required for light emission was 2.07 V in Comparative Example 1, when the semiconductor light emitting device of the embodiment was used, the forward voltage required for light emission was 2.02 V. 05V also decreased.

  As described above, the semiconductor light emitting device according to the embodiment eliminates the energy band gap notch generated at the boundary between the cladding layer and the graded layer, and reduces the response characteristics without reducing the light emission efficiency. Can be improved. That is, in the above example, the (first) cladding layer 3, the (first) graded layer 4, the (second) graded layer 8, and the (second) clad layer 9 are each made of AlGaInP. Response speed improvement of about 10% is obtained. The active layer 6 has an MQW structure in which GaInP and AlGaInP are alternately stacked, and the barrier layer (AlGaInP) is easily lattice-matched with the cladding layer or graded layer, so that the light emission efficiency is improved. . Many other materials for semiconductor light emitting devices are also known.

  The present invention can be used for a semiconductor light emitting device.

1 is a plan view of a semiconductor light emitting element according to an embodiment. It is the II-II arrow sectional view of the semiconductor light-emitting device shown in FIG. 3 is a longitudinal sectional view (a) of the semiconductor light emitting device shown in FIG. 2, a graph (b) of an energy band gap with respect to a thickness direction, and a graph of an impurity concentration (c) with respect to the thickness direction. It is the longitudinal cross-sectional view (a) of another semiconductor light-emitting device, the graph (b) of the energy band gap of the thickness direction, and the graph of the impurity concentration (c) of the thickness direction. It is an energy band figure in the vicinity of the graded layers 7 and 8 on the upper side of the semiconductor light emitting device according to the embodiment. 6 is an energy band diagram in the vicinity of the upper graded layers 7 and 8 of the semiconductor light emitting device according to Comparative Example 1. FIG. It is the longitudinal cross-sectional view (a) of the semiconductor light-emitting device which concerns on the comparative example 1, the graph (b) of the energy band gap with respect to the thickness direction, and the graph of the impurity concentration (c) with respect to the thickness direction. It is the longitudinal cross-sectional view (a) of the semiconductor light-emitting device concerning the comparative example 2, the graph (b) of the energy band gap with respect to the thickness direction, and the graph of the impurity concentration (c) with respect to the thickness direction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Contact layer, 2 ... DBR layer 3 ... Cladding layer, 4 ... Graded layer, 4 '... Intermediate layer, 5 ... Graded layer, 5' ... Non-doped layer, 6 ... Active layer, 7 ... Graded layer, 8 ... Graded layer, 9 ... Cladding layer, 10 ... DBR layer, 11 ... Contact layer, 20 ... Insulating layer, 100 ... Semiconductor light emitting element, A ... Notch, E1 ... Cathode electrode, E2 ... Anode electrode, E2a ... Ring part , E2b ... electrode pad part, E2c ... contact part.

Claims (4)

A first cladding layer;
A first graded layer formed on the first cladding layer;
An active layer formed on the first graded layer;
A second graded layer formed on the active layer;
A second cladding layer formed on the second graded layer;
In a semiconductor light emitting device comprising:
The first and second cladding layers have a high carrier concentration;
The first and / or second graded layer has a semiconductor light emission characterized in that the carrier concentration continuously increases toward the cladding layer in the vicinity of the boundary between the adjacent cladding layers. element.   2. The semiconductor light emitting element according to claim 1, wherein each of the first cladding layer, the first graded layer, the second graded layer, and the second cladding layer is made of AlGaInP.   3. The semiconductor light emitting device according to claim 2, wherein the active layer has a multiple quantum well structure in which GaInP and AlGaInP are alternately stacked. The carrier concentration and the composition of the first and / or second graded layer are set to be equal to the cladding layer at an interface with the cladding layer adjacent to the first and / or second graded layer, respectively. Semiconductor light emitting device.

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