US20180122986A1

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Publication number: US20180122986A1
Application number: US15/857,306
Authority: US
Inventors: Tien-Chang Lu; Chiao-Yun Chang; Heng Li
Original assignee: Epistar Corp
Current assignee: Epistar Corp
Priority date: 2014-09-03
Filing date: 2017-12-28
Publication date: 2018-05-03
Also published as: US9887317B2; TWI612686B; TW201611324A; US20160064596A1

Abstract

A light-emitting device including a substrate; a first conductivity semiconductor layer on the substrate; a first barrier on the first conductivity semiconductor layer; a well on the first barrier and including a first region having a first energy gap and a second region having a second energy gap and closer to the semiconductor layer than the first region; a second barrier on the well; and a second conductivity semiconductor layer on the second barrier; wherein the first energy gap decreases along a stacking direction of the light-emitting device and has a first gradient, the second energy gap increases along the stacking direction and has a second gradient, and an absolute value of the first gradient is smaller than an absolute value of the second gradient.

Description

REFERENCE TO RELATED APPLICATION

This present application is a continuation application of U.S. patent application Ser. No. 14/636,939 filed on Mar. 3, 2015, entitled as “LIGHT-EMITTING DEVICE AND MANUFACTURING METODE THEREOF”, which claims priority to Taiwan Application Serial No. 103130523, filed on Sep. 3, 2014, and the content of which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD REFERENCE TO RELATED APPLICATION

The disclosure is related to a light-emitting device, and more particularly, a light-emitting device with a quantum well structure.

DESCRIPTION OF THE RELATED ART

In comparison with conventional light sources, the light-emitting diode with longer service life, smaller volume, lighter weight, and higher efficiency is widely adopted in optical display devices, traffic lights, information storage apparatuses, communication apparatuses, lighting apparatuses, and medical appliances. A light-emitting diode can be used solely or connected to other devices for forming a light-emitting device. For example, a light-emitting diode can be disposed on a substrate and then connected to a side of a carrier or soldered/glued on a carrier for forming a light-emitting device. Additionally, the carrier further includes an electrode which is electrically connected to the light-emitting device.

Generally, a light-emitting diode may include an n-type semiconductor layer, an active layer, and a p-type semiconductor layer. In order to enhance the light efficiency of light-emitting devices, a multi-quantum well structure is formed in the active layer. How to enhance the light efficiency by a quantum well structure becomes a major topic for improving the performance of light-emitting diodes.

SUMMARY OF THE DISCLOSURE

The disclosure is relative to a light-emitting device including a substrate; a first conductivity semiconductor layer on the substrate; a first barrier on the first conductivity semiconductor layer; a well on the first barrier and including a first region having a first energy gap and a second region having a second energy gap and closer to the semiconductor layer than the first region; a second barrier on the well; and a second conductivity semiconductor layer on the second barrier; wherein the first energy gap decreases along a stacking direction of the light-emitting device and has a first gradient, the second energy gap increases along the stacking direction and has a second gradient, and an absolute value of the first gradient is smaller than an absolute value of the second gradient.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is included to provide easy understanding of the present application, and is incorporated herein and constitutes a part of this specification. The drawing illustrates the embodiment of the present application and, together with the description, serves to illustrate the principles of the present application.

FIG. 1A shows a cross section of a light-emitting device in accordance with a first embodiment of the present application.

FIG. 1B shows a detailed view ofFIG. 1A.

FIG. 1C show a detailed alignment view ofFIG. 1B.

FIG. 2A shows flow rates as functions of time while forming a well and a barrier in accordance with the first embodiment of the present application.

FIG. 2B shows a diagram of the well and the barrier in accordance with the first embodiment of the present application.

FIG. 2C shows the operational temperature as a function of time while forming the well and the barrier in accordance with the first embodiment of the present application.

FIG. 2D shows energy bands and structures of the well and the barrier in accordance with the first embodiment of the present application.

FIG. 3A shows flow rates as functions of time while forming a well and a barrier in accordance with a second embodiment of the present application.

FIG. 3B shows a diagram of the well and the barrier in accordance with the second embodiment of the present application.

FIG. 3C shows the operational temperature as a function of time while forming the well and the barrier in accordance with the second embodiment of the present application.

FIG. 3D shows energy bands and structures of the well and the barrier in accordance with the second embodiment of the present application.

FIG. 4 shows energy bands of the wells and the barriers of light-emitting devices in accordance with the first embodiment and the second embodiment of the present application and the conventional art.

FIG. 5 shows internal quantum efficiency as functions of power for the light-emitting devices in accordance with the first embodiment and the second embodiment of the present application and the conventional art.

FIG. 6A shows output power as functions of current density for the light-emitting devices in accordance with the first embodiment and the second embodiment of the present application and the conventional art.

FIG. 6B shows normalized efficiency as functions of current density for the light-emitting devices in accordance with the first embodiment and the second embodiment of the present application and the conventional art.

FIG. 7A shows concentration of carriers as functions of position and energy band as functions of position for the well and the barrier for the light-emitting device in accordance with the conventional art.

FIG. 7B shows energy bands of the well and the barrier and Femi energies of electrons and holes for the light-emitting device in accordance with the conventional art.

FIG. 8A shows concentration of carriers as functions of position and energy band as functions of position for the well and the barrier in accordance with the first embodiment of the present application.

FIG. 8B shows energy bands of the well and the barrier and Femi energies of electrons and holes in accordance with the first embodiment of the present application.

FIG. 9A shows concentration of carriers as functions of position and energy band as functions of position for the well and the barrier in accordance with the second embodiment of the present application.

FIG. 9B shows energy bands of the well and the barrier and Femi energies of electrons and holes in accordance with the second embodiment of the present application.

FIG. 10 shows a simulation of the recombination rate as functions of position for the light-emitting devices in accordance with the first embodiment and the second embodiment of the present application and the conventional art.

FIG. 11 shows a simulation of the normalized efficiency as functions of current density for the light-emitting devices in accordance with the first embodiment of the present application and the conventional art.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To better and concisely explain the present application, the same name or the same reference number given or appeared in different paragraphs or figures along the specification should has the same or equivalent meanings while it is once defined anywhere of the present application.

The following shows the description of embodiments of the present application in accordance with the drawing.

FIG. 1A shows a cross section of a light-emitting device in accordance with an embodiment of the present application. A light-emittingdevice1 includes asubstrate10, anucleation layer20, abuffer layer30, a firstconductivity semiconductor layer40, astrain releasing stack50, anactive layer60, a secondconductivity semiconductor layer70, afirst electrode80, and asecond electrode90. In the embodiment, the abovementioned layers are epitaxially grown on thesubstrate10 by approaches such as metal organic chemical vapor deposition (MOCVD) or molecular-beam epitaxy (MBE) and the growth direction is indicated by an arrow C_NThe substrate can be a single crystal substrate, an electrically conductive substrate, or an insulating substrate. The electrically conductive substrate can be a silicon substrate, a gallium nitride substrate, or a silicon carbide substrate. The insulating substrate can be a sapphire substrate. In the embodiment, each layer is epitaxially grown on the C plane of the sapphire substrate by MOCVD and thesubstrate10 optionally has a patterned surface by etching for enhancing light extraction efficiency. Additionally, for epitaxially growing the layers, trimethylgallium (TMGa), triethylgallium (TEGa), trimethylaluminum (TMAl), and trimethylindium (TMIn) can be used as group IIIA sources; ammonia (NH₃) can be used as a group VA source; silane (SiH₄), and bis-cyclopentadienyl magnesium (Cp₂Mg) can be used as dopant sources.

In order to reduce lattice mismatch between thesubstrate10 and the firstconductivity semiconductor layer40, thenucleation layer20 and thebuffer layer30 can be sequentially formed between thesubstrate10 and the firstconductivity semiconductor layer40. The thicknesses of thenucleation layer20 and thebuffer layer30 can be tens of nanometers (for example, 30 nm) and several micrometers, (for example, 3 μm) respectively. Materials of the nucleation layer and buffer layer can be group IIIA-VA materials including but not limited to gallium nitride or aluminum nitride.

The firstconductivity semiconductor layer40, for example, an n-type semiconductor layer, is formed on thesubstrate10, thenucleation layer20, and thebuffer layer30. In the embodiment, the thickness of the first conductivity semiconductor layer is several micrometers (for example, 2.5 μm) and the material of the firstconductivity semiconductor layer40 can be gallium nitride. A ratio of group VA source (for example, ammonia) to the group IIIA source (for example, trimethylgallium) can be 1000 for forming the firstconductivity semiconductor layer40. Additionally, by introducing silane as a doping source, GaN layer with silicon dopants can be formed and functions as the firstconductivity semiconductor layer40. The material of the firstconductivity semiconductor layer40 is not limited hereto and can be other group IIIA-VA material.

Theactive layer60 is formed after thestrain releasing stack50 is formed. Please refer toFIGS. 1B and 1C.FIG. 1B shows a detailed view ofFIG. 1A andFIG. 1C show a detailed alignment view ofFIG. 1B. In the embodiment, theactive layer60 includes a multi-quantum well structure but is not limited to it. In other embodiment, the active layer can include a single quantum well structure and is formed by alternately stacking a plurality ofwells601 andbarriers603. In the embodiment, onebarrier603 is firstly formed on thestrain releasing stack50, one well601 is formed onsuch barrier603, and anotherbarrier603 and another well601 are alternately formed on such well601 repeatedly while the last one isbarrier603 or well601. The steps of forming the abovementioned multi-quantum well structure of theactive layer60 can include forming the well601 first, then forming thebarrier603, and alternately forming the well601 and thebarrier603 repeatedly. The thickness of each of thewells601 is several nanometers (for example, 2 nm˜3 nm), and the well601 can include three regions designated as region I6010, region II6012, and region III6014. In the embodiment, the region I6010 is closer to the firstconductivity semiconductor layer40 and thestrain releasing stack50. The region II6012 is disposed between the region I6010 and region III6014, and theregion III6014 is away from the firstconductivity semiconductor layer40 and thestrain releasing stack50. The material of thebarrier603 can include a group IIIA-VA material, for example, gallium nitride or aluminum nitride. The material of the well601 can include a group IIIA-VA material, for example, In_xGa_(1-x)N, Al_xGa_(1-x)N, Al_xIn_yGa_(1-x-y)N, Al_xIn_1-xN or combinations thereof, wherein 0≤x, y<1. In the embodiment, the material of thebarrier603 is gallium nitride and the material of the well601 is gallium indium nitride. A ratio of the group VA source (for example, ammonia) to the group IIIA source (for example, trimethylindium) can be 18000 for forming the well601 of theactive layer60; a ratio of the group VA source (for example, ammonia) to the group IIIA source (for example, triethylgallium) can be 2000 for forming thebarrier603 of theactive layer60, but the present application is not limited hereto.

The secondconductivity semiconductor layer70 is formed on theactive layer60. In the embodiment, the secondconductivity semiconductor layer70 can be a p-type conductivity semiconductor layer, for example, a gallium nitride layer doped with magnesium dopants. but the present application is not limited hereto. The material of the secondconductivity semiconductor layer70 can be other group IIIA-VA material. In the embodiment, a ratio of the group VA source (for example, ammonia) to the group IIIA source (for example, trimethylgallium) can be 5000 for forming the secondconductivity semiconductor layer70 and bis-cyclopentadienyl magnesium can be used as a magnesium dopant source. After the secondconductivity semiconductor layer70 is formed, thefirst electrode80 and thesecond electrode90 are manufactured by processes, such as lithography, etching, and metal deposition for completing the light-emittingdevice1. The abovementioned firstconductivity semiconductor layer40 and the secondconductivity semiconductor layer70 can be single layer or multilayers. Additionally, an undoped semiconductor layer can be disposed on the firstconductivity semiconductor layer40 or the secondconductivity semiconductor layer70.

Referring toFIGS. 2A to 2D for further understanding the formation of theactive layer601.FIG. 2A shows flow rates as functions of time while forming the well601 and thebarrier603 in accordance with the first embodiment of the present application.FIG. 2B shows a diagram of the well601 and thebarrier603 in accordance with the first embodiment of the present application.FIG. 2C shows operational temperature as a function of time while forming the well601 and thebarrier603 in accordance with the first embodiment of the present application.FIG. 2D shows energy bands and structures of the well601 and thebarrier603 in accordance with the first embodiment of the present application. As above mentioned, theactive layer60 is formed by alternately stacking the plurality ofwells601 andbarriers603. As shown inFIGS. 2A to 2D, the well601 is between twobarriers603. While forming thebarrier603, a gallium based gas such as triethylgallium (TEGa), an indium based gas such as trimethylindium (TMIn) and a nitrogen based gas such as ammonia (NH₃) are introduced. In the present embodiment, a flow rate of the gallium based gas FR1, a flow rate of the indium based gas FR2, and a flow rate of the nitrogen based gas FR3 are constants. The operational temperature for forming thebarrier603 can maintain at a first predetermined value T₁, for example, 870 degrees Celsius. A thickness of thebarrier603 is several nanometers to tens of nanometers (e.g., 12 nm).

While forming the well601, in an interval between t₁and t₂(about 160 seconds), the flow rate of the gallium based gas FR1, the flow rate of the indium gas FR2, and the flow rate of nitrogen based gas FR3 can be maintained at fixed values, respectively, and the operational temperature is decreased from the first predetermined value T₁(e.g., 870 degrees Celsius) to a second predetermined value T₂, (e.g., 755 degrees Celsius) for forming the region I6010 of thewell601. The operational temperature can be decreased linearly, stepwise or in other ways. Generally speaking, while epitaxially growing layers by MOCVD, the indium content of the layer is increased as the operational temperature is decreased. In other words, the indium content of the layer is decreased as the operational temperature is increased. By the abovementioned approaches which adjust the operational temperature, the indium content of the region I6010 is modulated to be increased along the stacking direction (indicated by arrow C_N) of the light-emittingdevice1. A composition of the region I6010 can range from GaN to In_{0 25}Ga_{0 75}N, but the present application is not limited hereto. The rise of the indium content of the region I6010 can be linear, stepwise or in other ways.

After the region I6010 is formed, the region II6012 of the well601 is formed in an interval between t₂and t₃(about 60 seconds) and the operational temperature is maintained at the second predetermined value T₂. Additionally, the flow rate of the gallium based gas FR1, the flow rate of the indium based gas FR2, and the flow rate of the nitrogen based gas FR3 are maintained at predetermined values. In the interval between t₂and t₃, the operational temperature is maintained at the second predetermined value T₂and thus the indium content of the region II6012 is substantially constant (a composition of the region III can be maintained at In_{0 25}Ga_{0 75}N).

After the region I6010 and theregion6012 are formed, the region III6014 of the well601 is formed in an interval between t₃and t₄. In the interval between t₃and t₄, the flow rate of the gallium based gas FR1, the indium based gas FR2, the nitrogen based gas FR3 are maintained at the abovementioned values, and the operational temperature is increased linearly/stepwise to a third predetermined value T₃so that the indium content of theregion III6014 is decreased along the stacking direction. The indium content of the region III6014 can be decreased linearly or stepwise. Additionally, for theindividual well601, theregion III6014 is closer to the secondconductivity semiconductor layer70 than the region I6010, and the region II6012 is formed between the region I6010 and theregion III6014. In other words, the region I6010, the region II6012, and the region III6014 are formed in sequence. In other embodiment, the sequence can be changed.

After the well601 is formed, anotherbarrier603 is formed thereon the gallium based gas such as TEGa, the indium based gas such as TMIn, and the nitrogen based gas such as NH₃are introduced at the same flow rate used in forming the above-mentioned well601 and thebarrier603, and the operational temperature is maintained at the third predetermined value T₃.

Additionally, the region I6010 has an energy gap EI (not shown in figures). The energy gap EI is decreased linearly, stepwise or in other ways along the stacking direction (indicted by the arrow C_N) of the light-emittingdevice1 and has a first gradient. In the embodiment, the first gradient ΔEI/ΔDIis defined as an energy gap difference per unit thickness in the region I6010 while the thickness is defined along the stacking direction C_N. In the embodiment, indium gallium nitride (In_xGa_1-xN) functions as the well. The energy gap EI is decreased (x becomes bigger) since the operational temperature is decreased along the stacking direction while forming the region I6010 so that the indium content is increased.

In other respect, the region II6012 has an energy gap EII (not shown in figures) Since the operational temperature is maintained at the second predetermined value T₂while forming the region II6012, the indium content of the region II6012 is substantially fixed and the energy gap EII can be regarded as a constant. In other words, the energy gap EII is devoid of gradient variation along the stacking direction.

The region III6014 has an energy gap EIII (not shown in figures) which can be increased linearly, stepwise or in other ways along the stacking direction as above mentioned and has a second gradient ΔEIII/ΔDIII defined as an energy gap difference per unit thickness in theregion III6014. As shown inFIG. 2D, the energy gap EI difference is smaller than the energy gap EIII difference from magnitude point of view. In other words, an absolute value of the first gradient |ΔEI/ΔDI| is smaller than an absolute value of the second gradient |ΔEIII/ΔDIII|. It is because while forming the region I6010, the operational temperature is varied from the first predetermined value T₁to the second predetermined T₂in a longer interval between t₁and t₂, for example, 160 seconds, so that the indium content in region I6010 correspondingly varies from a lower fraction to a higher fraction in such longer interval and the operational temperature varies from the second predetermined value T₂to the third predetermined value T₃in a shorter interval between t₃and t₄, for example, 60 seconds, so that the indium content in region III6014 varies from a higher fraction to a lower fraction in such short interval. Additionally, as shown inFIG. 2D, an average of the energy gap EI and an average of the energy gap EIII are greater than the energy gap EII and the energy gap of thebarrier603 is greater than the energy gap of thewell601.

In the embodiment, although the indium content of each region is adjusted by the operational temperature so that the energy gaps of different regions of the well are varied, the present application is not limited to adjust the operational temperature or the aforementioned gases, and the adjusted content is not limited to indium content. In other embodiment, metal content, for example, aluminum content of the well can be adjusted in other ways so that the energy gap is adjusted and the absolute value of the first gradient |ΔEI/ΔDI| can be smaller or greater than |ΔEIII/ΔDIII|. For example, a material of the barrier can include aluminum nitride (AlN), a material of the well can include aluminum gallium nitride (Al_xGa_(1-x)N; 0≤x≤1), and the introduced gas can include aluminum based gas. Since the energy gap of aluminum nitride is about 6.1 eV, which is greater than that of gallium nitride 3.4 eV, in order to make the energy gap EI of the region I decrease along the stacking direction and make the energy gap EIII of the region III increase along the stacking direction, aluminum content in the region I can be decreased along the stacking direction and aluminum content in region III can be increased along the stacking direction.

As shown inFIGS. 3A to 3D,FIG. 3A shows flow rates as functions of time while forming the well and the barrier in accordance with the second embodiment of the present application,FIG. 3B shows a diagram of the well and the barrier in accordance with the second embodiment of the present application,FIG. 3C shows the operational temperature as a function of time while forming the well and the barrier in accordance with the second embodiment of the present application, andFIG. 3D shows energy bands and structures of the well and the barrier in accordance with the second embodiment of the present application. The second embodiment inFIG. 3A toFIG. 3D is similar to the first embodiment inFIG. 2A toFIG. 2D. One difference is in the structure of the well of the active layer. In the second embodiment, the active layer includes a well601′ and at least twobarriers603′. Similarly, a region I6010′ of the well601′ is formed in the interval between t₁and t₂and a region II6012′ of the well601′ is formed in the interval between t₃and t₄. In the embodiment, a composition of the region I6010′ is ranged from GaN to In_{0 25}Ga_0.75N, a composition of the region II is In_{0 25}Ga_{0 75}N, and a composition of the region III6014′ is ranged from In_{0 25}GaN_{0 75}N to GaN. Another difference between the first embodiment and the second embodiment is the interval between t₁and t₂shown inFIGS. 3A to 3D is shorter than the interval between t₃and t₄and thus an indium content difference per unit time in the interval between t₁and t₂is greater than an indium content difference per unit time in the interval between t₃and t₄. Accordingly, as shown inFIG. 3D, an energy gap EI′ difference of theregion6010′ per unit thickness DI′ is smaller than an energy gap EIII′ difference of theregion6014′ per unit thickness DIII′ from magnitude point of view. In other words, an absolute value of a first gradient |ΔEI′/ΔDI′| in the second embodiment is greater than an absolute value of a second gradient in the second embodiment |ΔEIII′/ΔDIII′|.

In the present application, the intervals between t₁and t₂, t₂and t₃, and t₃and t₄are not limited to 160 seconds, 60 seconds, and 60 seconds or 60 seconds, 60 seconds, and 160 seconds. In other embodiment, in order to vary indium contents in different ways in different intervals, the intervals can correspond to different durations. For example, the interval between t₁and t₂can be 2 to 3 times of the interval between t₃and t₄, the interval between t₁and t₂is shorter than the interval between t₃and t₄, or the interval between t₃and t₄and the interval between t₂and t₃are longer than the interval between t₁and t₂. But the present application is not limited hereto. As long as the operational temperature difference (absolute value) per unit time in the interval between t₁and t₂is different from the operational temperature difference (absolute value) per unit time in the interval between t₃and t₄, absolute values of the indium contents per unit thickness (gradient) in the region I and region III are different from each other. Additionally, the first predetermined value, the second predetermined value, and the third predetermined are not limited to 870 degrees Celsius, 755 degrees Celsius, and 875 degrees Celsius. The first predetermined value and the third predetermined value can be greater than the second predetermined value. In other embodiment, the first predetermined value and the third predetermined can be about 900 degrees Celsius and the second predetermined value can be smaller than 900 degrees Celsius. Moreover, in other embodiment, the first predetermined can be between 870 degrees Celsius and 900 degrees Celsius, the second predetermined value can be between 750 degrees Celsius and 780 degrees Celsius, and the third predetermined value can be between 870 degrees Celsius and 900 degrees Celsius.

FIG. 4 shows energy bands of wells and barriers of light-emitting devices in accordance with the first embodiment and the second embodiment of the present application and the conventional art. InFIG. 4, S represents the conventional light-emitting device, G represents the light-emitting device of the first embodiment, and N represents the light-emitting device of the second embodiment. InFIG. 4, materials of a well S01 and a barrier S03 of the conventional light-emitting device can include indium gallium nitride (In_0.25Ga_0.75N) and gallium nitride GaN, respectively. The energy gap of the well S01 is fixed and is not varied with its thickness.

FIG. 5 shows internal quantum efficiency as functions of power for the light-emitting devices in accordance with the first embodiment and the second embodiment of the present application and the conventional art. As shown inFIG. 5, S represents the conventional light-emitting device inFIG. 4 and A represents both of the light-emitting devices of the first embodiment and the second embodiment. InFIG. 5, it shows the internal quantum efficiency of the light-emitting devices of the first embodiment and the second embodiment is higher than the internal quantum efficiency of the light-emitting device of the conventional art at the same power value.

In the embodiment, the external quantum efficiency (EQE) at a current density of 13.8 A/cm²for the light-emitting devices of the conventional art, the first embodiment, and the second embodiment are approximately 59.6%, 68.3%, and 66.5%, respectively. The output power values of the light-emitting device of the first embodiment and the second embodiment are increased by 11.7% and 5.8%, respectively, compared with the light-emitting device of the conventional art. As above mentioned, at a current density of 13.8 A/cm²or 69 A/cm², the output power and the efficiency of the light-emitting devices of the first embodiment and the second embodiment are higher than that of the light-emitting device of the conventional art.

Please refer toFIGS. 7A, 8A and 9A.FIGS. 7A to 9A show simulations of concentrations of carriers versus position and energy bands of the well and the barrier versus position under bias.FIG. 7A shows concentration of carriers as functions of position and energy bands as functions of position for the well and the barrier for the light-emitting device in accordance with the conventional art.FIG. 8A shows concentration of carriers as functions of position and energy bands as functions of position for the well601 and thebarrier603 in accordance with the first embodiment of the present application.FIG. 9A shows concentration of carriers as functions of position and energy bands as functions of position for the well601′ and thebarrier603′ in accordance with the second embodiment of the present application. InFIGS. 7A, 8A, and 9A, the structures (i.e. barrier, well, region I, region II, region III) are labeled. As shown inFIGS. 7A, 8A, and 9A, higher concentration of carriers (electrons/holes) presents in the well. In the light-emitting device of the first embodiment or the second embodiment, a position of a peak value of the concentration of electrons s is closer to a position of a peak value of the concentration of holes, compared with the conventional light-emitting device. It means that each wave function distribution of the electrons and the holes of the first embodiment and the second embodiment overlaps more than that of the light-emitting device of the conventional art and the recombination rates in the light-emitting devices of the first embodiment and the second embodiment are greater than the recombination rate of the conventional light-emitting device. As shown inFIG. 8A, the variation of the energy gap in region I6010 of the well is smaller than the variation of the energy gap in theregion III6014. While operating the light-emitting of the first embodiment, the electrons move from the region I6010 to the region III6014 and the holes move from the region III6014 to theregion I6010. With the above structure, the speed of the holes is increased and the movement of electrons is restrained so as to increase the efficiency and decrease electrons overflow.

FIGS. 7B, 8B and 9B show simulations energy bands of the well and the barrier and Femi energies of electrons and holes under bias.FIG. 7B shows energy bands of the well and the barrier and Femi energies of electrons and holes for the light-emitting device in accordance with the conventional art.FIG. 8B shows energy bands of the well and the barrier and Femi energies of electrons and holes in accordance with the first embodiment of the present application.FIG. 9B shows energy bands of the well and the barrier and Femi energies of electrons and holes in accordance with the second embodiment of the present application. There are four lines shown in theFIGS. 7B, 8B, and 9B. An upper line and an upper broken line in figures represent a conduction band profile and Femi energy of electrons, respectively; a lower line and a lower broken line in figures represent a valance band profile and Femi energy of holes, respectively. In comparison withFIG. 7B, the Femi energy of electrons inFIG. 8B is away from the minimum (valley value) of the conduction band. It means the probability that electrons present in the well601 is higher than the probability that electrons present in the well S01. Additionally, the area (as slash lines shown) defined by the Femi energy and the conduction band inFIG. 8B is higher than that of inFIG. 7B. That represents the amount of electrons in the well601 is higher than that of in the well S01.

Please refer toFIG. 10 shows a simulation of the recombination rate as functions of position for the light-emitting devices in accordance with the first embodiment of the present application and the conventional art. S, G, and N represent a conventional light-emitting device, the light-emitting devices of the first embodiment and the second embodiment, respectively. As shown inFIG. 10, recombination rates of the active layers of the light-emitting devices of the first embodiment, the second embodiment are greater than that of the conventional light-emitting device.

Please refer toFIG. 11,FIG. 11 shows a simulation of the normalized efficiency as functions of current density for the light-emitting devices in accordance with the first embodiment of the present application and the conventional art. S and G represent a conventional light-emitting device without a polarization field and the light-emitting device of the first embodiment without a polarization field, respectively. S-P and G-P represent a conventional light-emitting device with a polarization filed (0.7Mvolt·cm⁻¹) and the light-emitting device of the first embodiment of the present application with a polarization filed (0.7Mvolt·cm⁻¹), respectively. As shown inFIG. 11, regardless of a polarization filed, a declining rate of the efficiency of the light-emitting device of the first embodiment of the present application is smaller than that of the conventional light-emitting device.

The principle and the efficiency of the present application illustrated by the embodiments above are not the limitation of the present application. Any person having ordinary skill in the art can modify or change the aforementioned embodiments. Therefore, the protection range of the rights in the present application will be listed as the following claims.

Claims

What is claimed is:

1. A light-emitting device comprising:

a first conductivity semiconductor layer;

a first barrier on the first conductivity semiconductor layer;

a well on the first barrier and comprising:

a first region having a first energy gap; and

a second region having a second energy gap, wherein the first region is closer to the first conductivity semiconductor layer than the second region is;

a second barrier on the well; and

a second conductivity semiconductor layer on the second barrier;

wherein the first energy gap is decreased along a stacking direction of the light-emitting device and has a first gradient, the second energy gap is increased along the stacking direction and has a second gradient, and an absolute value of the first gradient is smaller than an absolute value of the second gradient.

2. The light-emitting device ofclaim 1, wherein the first energy gap and/or the second energy gap are/is varied linearly or stepwise.

3. The light-emitting device ofclaim 1, wherein the well further comprises a third region between the first region and the second region, and the third region has a third energy gap which is a constant or devoid of gradient variation.

4. The light-emitting device ofclaim 3, wherein a material of the first barrier and the second barrier comprises gallium nitride, a material of the well comprises indium gallium nitride, and an energy gap of the first barrier and an energy gap of the second barrier are greater than an energy gap of the well.

5. The light-emitting device ofclaim 4, wherein an indium content of the first region is increased along the stacking direction, an indium content of the second region is decreased along the stacking direction, and an indium content of the third region is a constant.

6. The light-emitting device ofclaim 4, wherein the indium content of the first region and/or the indium content of the second region are/is varied linearly or stepwise.

7. The light-emitting device ofclaim 3, wherein a material of the first barrier and the second barrier comprises aluminum nitride, a material of the well comprises aluminum gallium nitride, an aluminum content of the first region is decreased along the stacking direction, an aluminum content of the second region is increased along the stacking direction, and an aluminum content of the third region is substantially a constant.

8. The light-emitting device ofclaim 1, further comprising a substrate under the first conductivity semiconductor layer, and a buffer layer between the first conductivity semiconductor layer and the substrate.

9. The light-emitting device ofclaim 1, further comprising a strain releasing stack, wherein the strain releasing stack is on the first conductivity semiconductor layer and comprises a superlattice structure.

10. The light-emitting device ofclaim 9, wherein the superlattice structure comprises alternately stacking indium gallium nitride layers and gallium nitride layers.

11. A light-emitting device comprising:

a first conductivity semiconductor layer;

a first barrier on the first conductivity semiconductor layer;

a well on the first barrier and comprising:

a first region having a first energy gap; and

a second barrier on the well; and

a second conductivity semiconductor layer on the second barrier;

wherein the first energy gap is decreased along a stacking direction of the light-emitting device and has a first gradient, the second energy gap is increased along the stacking direction and has a second gradient, and an absolute value of the first gradient is greater than an absolute value of the second gradient.

12. The light-emitting device ofclaim 11, wherein the first energy gap and the second energy gap is varied linearly or stepwise.

13. The light-emitting device ofclaim 11, wherein the well further comprises a third region between the first region and the second region, and the third region has a third energy gap which is a constant or devoid of gradient variation.

14. The light-emitting device ofclaim 13, wherein a material of the first barrier and the second barrier comprises gallium nitride, a material of the well comprises indium gallium nitride, and an energy gap of the first barrier and an energy gap of the second barrier are greater than an energy gap of the well.

15. The light-emitting device ofclaim 14, wherein an indium content of the first region is increased along the stacking direction, an indium content of the second region is decreased along the stacking direction, and an indium content of the third region is a constant.

16. The light-emitting device ofclaim 14, wherein the indium content of the first region and/or the indium content of the second region are/is varied linearly or stepwise.

17. The light-emitting device ofclaim 13, wherein a material of the first barrier and the second barrier comprises aluminum nitride, a material of the well comprises aluminum gallium nitride, an aluminum content of the first region is decreased along the stacking direction, an aluminum content of the second region is increased along the stacking direction, and an aluminum content of the third region is substantially a constant.

18. The light-emitting device ofclaim 11, further comprising a substrate under the first conductivity semiconductor layer, and a buffer layer between the first conductivity semiconductor layer and the substrate.

19. The light-emitting device ofclaim 11, further comprising a strain releasing stack, wherein the strain releasing stack is on the first conductivity semiconductor layer and comprises a superlattice structure.

20. The light-emitting device ofclaim 19, wherein the superlattice structure comprises alternately stacking indium gallium nitride layers and gallium nitride layers.