High-Brightness Light-Emitting Diode with Surface Microstructures

Abstract

A high-brightness light-emitting diode with surface microstructure and preparation and screening methods thereof are provided. The ratio of total roughened surface area of light transmission surface of a light emitting diode to vertically projected area is greater than 1.5, and the peak density of light transmission surface is not less than 0.3/um.sup.2. The higher the ratio of total roughened surface area of an epitaxial wafer to vertically projected area and the higher the number of peak over the critical height within a unit area, the more beneficial to improve light extraction efficiency of the epitaxial wafer. As a result, light extraction efficiency of the epitaxial wafer is greatly improved.

Claims

1. A high-brightness light-emitting diode (LED) having a light-emitting surface, wherein: the light-emitting surface has a surface microstructure; and a ratio of total roughened surface area of the light-emitting surface to a vertically-projected area is not less 1.5.

2. The LED of claim 1, wherein a peak density of the light-emitting surface is not less than 0.3/um.sup.2.

3. The LED of claim 2, wherein: the light-emitting surface is of a layer that is Al doped GaInP layer or Mg doped GaP layer, and in the Al doped GaInP layer, a mol ratio of Al to GaInP is 0.67-1.5.

4. The LED of claim 1, wherein: the light-emitting surface is of a layer that is Al doped GaInP layer or Mg doped GaP layer, and in the Al doped GaInP layer, a mol ratio of Al to GaInP is 0.67-1.5.

5. A method of making a high-brightness light-emitting diode (LED), the method comprising: (1) exposing a light-emitting surface of an epitaxial wafer of the LED to be roughened, and protecting the light-emitting surface not-to-be roughened with a photoresist or metal; (2) preparing a roughening solution based on dose proportions in weight percentages as: TABLE-US-00011 CH.sub.3COOH 30~50% HF 40~60% H.sub.2SO.sub.4 0~2% HNO.sub.3 0~20% H3PO4 0~10% (3) dipping the epitaxial wafer processed by Step (1) in the roughening solution prepared by Step (2) for 1-8 min; wherein for the resulting LED: the light-emitting surface has a surface microstructure; and a ratio of total roughened surface area of the light-emitting surface to a vertically-projected area is not less 1.5.

6. The method of 5, further comprising: drying the epitaxial wafer for subsequent preparation of the LED.

7. The method of claim 5, wherein: the epitaxial wafer processed by Step (1) comprises a high reflection metal surface with electric contact and mirror system; and a bonding metal is evaporated over the mirror system and achieves bonding effect by applying a high temperature and a high pressure.

8. The method of claim 7, wherein: the high temperature is 250-350, and the high pressure is 60-200 KPa.

9. A method of selecting high-brightness light-emitting diodes (LEDs), the method comprising: (1) selecting an epitaxial wafer with a surface roughened; (2) measuring a total roughened surface area of a light-emitting surface on the epitaxial wafer selected in Step (1); (3) calculating a ratio of the total roughened surface area to a vertically projected area; (4) selecting epitaxial wafers with the ratio of not less than 1.5; and (5) fabricating LEDs with the epitaxial wafers selected in Step (4); wherein for the LEDs: the light-emitting surface has a surface microstructure; and a ratio of total roughened surface area of the light-emitting surface to a vertically-projected area is not less 1.5.

10. The method of claim 9, wherein the fabricating comprises: (a) exposing a light-emitting surface of an epitaxial wafer of the LED to be roughened, and protecting the light-emitting surface not-to-be roughened with a photoresist or metal; (b) preparing a roughening solution based on dose proportions in weight percentages as: TABLE-US-00012 CH.sub.3COOH 30~50% HF 40~60% H.sub.2SO.sub.4 0~2% HNO.sub.3 0~20% H3PO4 0~10% (c) dipping the epitaxial wafer processed by Step (1) in the roughening solution prepared by Step (2) for 1-8 min.

11. The method of claim 10, wherein the fabricating further comprises: drying the epitaxial wafer for subsequent preparation of the LED.

12. The method of claim 10, wherein: the epitaxial wafer processed by Step (a) comprises a high reflection metal surface with electric contact and mirror system; and a bonding metal is evaporated over the mirror system and achieves bonding effect by applying a high temperature and a high pressure.

13. The method of claim 12, wherein: the high temperature is 250-350, and the high pressure is 60-200 KPa.

14. The method of claim 9, wherein a peak density of the light-emitting surface is not less than 0.3/um.sup.2.

15. The method of claim 14, wherein: the light-emitting surface is of a layer that is Al doped GaInP layer or Mg doped GaP layer, and in the Al doped GaInP layer, a mol ratio of Al to GaInP is 0.67-1.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1 is a scatter plot of data related to the change of roughening ratio of samples 1-4 with that of the ratio of specific surface area to vertically projected area;

[0044] FIG. 2 is a scatter plot of data related to the change of brightness after roughening of samples 5-8 with that of the ratio of specific surface area to vertically projected area;

[0045] FIG. 3 is a scatter plot of data related to the change of roughening ratio of samples 9-12 with that of the dosage of Al doped; and

[0046] FIG. 4 is a scatter plot of data related to the change of brightness after roughening of samples 13-16 with that of the ratio of specific surface area to vertically projected area.

DETAILED DESCRIPTION

[0047] In some embodiments, the roughened surface of the epitaxial wafer after roughening is just the surface with microstructure, and the total roughened surface area of the epitaxial wafer is also called specific surface area. In some the embodiments of the disclosure, the equipment used for testing the light intensity of samples is BRUKER AFM, and the single-point probe test is performed at 20-350 mA.

[0048] The pretreatment of the epitaxial wafer prior to roughening comprises the following steps according to some embodiments:

[0049] (1) fabricate a high reflection metal surface with electric contact and mirror system on the p-electrical cladding of the epitaxial wafer, and a dielectric layer can be added to prepare an omnibearing reflective mirror system to improve reflectivity;

[0050] (2) evaporate bonding metal over the mirror system and have the bonding metal fit with a conductive substrate and then high temperature and high pressure are applied to realize bonding effect;

[0051] (3) following the bonding, remove the epitaxial substrate by etching till the n-electrical cladding is exposed;

[0052] (4) fabricate N electrode on the exposed epitaxial layer and define a main light emitting area; a wafer ready for roughening is finally formed.

Embodiment 1

[0053] Four samples are prepared for Embodiment 1, and all are the light emitting diode with surface microstructure and with n-cladding on the top and share the basically same structure, and the same growth method of the epitaxial wafer: depositing a buffer layer and a n-electrical cladding over the epitaxial substrate and then a n-cladding, further depositing a multilayer quantum well (MQW) structure above the n-cladding, followed by a p-cladding deposited on the MQW structure and finally an electrical cladding, thereby completing the growth of the epitaxial layer structure; the difference between the samples 1-4 is that each respective functional layer is made of different materials, so that samples 1-4 emits light with different wavelengths, and the components of specific functional layers of samples 1-4 are provided in Table 1 below:

TABLE-US-00002 TABLE 1 Components of functional layers of samples 1-4 Layer Material N-elec- Sub- Buffer trical n- p- No. strate layer cladding cladding MQW cladding Sam- GaAs GaAs n-GaAs n-AlGaInP InGaP/ p-GaP ple 1 AlGaInP Sam- GaAs GaAs n-GaAs n-AlGaInP AlGaInP/ p-GaP ple 2 AlGaInP Sam- GaAs GaAs n-GaAs n-AlGaAs AlGaAs/ p-GaP ple 3 AlGaAs Sam- Al.sub.2O.sub.3 AlN n-GaN InGaN/ p-GaN ple 4 GaN

[0054] After completion of the growth of the epitaxial wafer of samples 1-4, the epitaxial wafer can be prepared into the wafer ready for roughening and finally perform surface roughening to form samples 1-4 with surface microstructure.

[0055] The roughening of samples 1-4 is basically the same, all comprising the following steps:

[0056] (1) have the light transmission surface of samples 1-4 requiring roughening exposed, and protect the light transmission surface requiring no roughening with photoresist or metal;

[0057] (2) prepare the first roughening solution in weight percentage and with the following components:

TABLE-US-00003 CH.sub.3COOH 30% HF 60% H.sub.2SO.sub.4 2% HNO.sub.3 5% H.sub.3PO.sub.4 3%

[0058] Prepare the second roughening solution with a concentration of 40% NaOH.

[0059] (3) dip the sample 1-3 processed by Step (1) in the first roughening solution prepared by Step (2) for 2 min., dip sample 4 in the second roughening solution for 2 min. and then take out and dry samples 1-4 respectively for subsequent preparation of the light emitting diode with surface microstructure. Luminous intensity tests have been carried out to samples 1-4 and the testing results are shown in Table 2:

TABLE-US-00004 TABLE 2 Measurements and luminescence property of the surface microstructure of samples 1-4 No. Measurements Sample 1 Sample 2 Sample 3 Sample 4 Specific surface area 175 237 153 202 (m.sup.2) Ratio of specific 1.75 2.37 1.53 2.02 surface area to vertically projected area (100 m.sup.2) Average critical 662 711 452 683 height of roughened surface (nm) No. of peak over 58 62 31 65 average critical height Peak density 0.58 0.62 0.31 0.65 Brightness before 383 mcd 88 mW 108 mW 223 mW roughening Brightness after 678 mcd 217 mW 160 mW 480 mW roughening Roughening ratio = 1.77 2.46 1.48 2.15 mcd before roughening/mcd after roughening

[0060] As influenced by the sensitivity of human eyes towards wavelength, the brightness of the light of different wavelengths also differs, and therefore, the absolute value of the brightness of the light of different wavelengths cannot be directly compared. In this Embodiment 1, the quality of the roughened surface should be judged by the roughening ratios before and after roughening, and as revealed in the above Table 2, the sample with higher roughening ratio has a higher ratio of specific surface area to vertically projected area, that is, the higher the roughening ratio is, the better the roughening effect of the sample is, and in turn, the higher the ratio of specific surface area to vertically projected area is, the higher the ratio of brightness before and after roughening is, and the present disclosure considers that when the ratio of total roughened surface area to vertically projected area of a sample is not less than 1.5, the sample has a good light intensity, and therefore, this method can be applied to screen high brightness light emitting diode.

Embodiment 2

[0061] Four samples are prepared for Embodiment 2, namely, samples 5-8, and share a same preparation method, specifically: depositing a buffer layer (GaAs) and an electrical cladding over the epitaxial substrate and then a n-cladding (n-GaAs), further depositing a multilayer quantum well (MQW) structure (InGaP/AlGaInP) above the n-cladding, followed by a p-cladding (p-GaP) deposited on the MQW structure and finally an electrical cladding, thereby completing the growth of the epitaxial layer structure; after the completion of the growth of the epitaxial wafer of samples 5-8, the epitaxial wafer can be prepared into the wafer ready for roughening and finally perform surface roughening to form samples 5-8 with surface microstructure.

[0062] The roughening of samples 5-8 is basically the same, all comprising the following steps:

[0063] (1) have the light transmission surface of samples 5-8 requiring roughening exposed, and protect the light transmission surface requiring no roughening with photoresist or metal;

[0064] (2) prepare roughening solution as per the following dose proportion in weight percentage and with the following components: 30% CH.sub.3COOH, 60% HF, 2% H.sub.2SO.sub.4, 5% HNO.sub.3 and 3% H.sub.3PO.sub.4;

[0065] (3) dip the samples 5-8 processed by Step (1) in the roughening solution prepared by Step (2) and then take out and dry samples 13-16 for subsequent preparation of the light emitting diode with surface microstructure.

[0066] The difference between the samples 5-8 is that the roughening time of samples 5-8 in Step (3) gradually shortens, 3 min. for sample 5, 2.5 min. for sample 6, 2 min. for sample 7 and 1.5 min. for sample 8.

[0067] Luminous intensity tests have been carried out to samples 5-8 and the testing results are shown in Table 3:

TABLE-US-00005 TABLE 3 Measurements of the surface microstructure of samples 5-8 No. Measurements Sample 5 Sample 6 Sample 7 Sample 8 Specific surface area 188 172.2 156.1 132.8 (m.sup.2) Ratio of specific 1.88 1.722 1.561 1.328 surface area to vertically projected area (100 m.sup.2) Average critical 646 563 463 359 height of roughened surface (nm) No. of peak over 55 60 51 47 average critical height Peak density (m.sup.2) 0.55/um.sup.2 0.6/um.sup.2 0.51/um.sup.2 0.47/um.sup.2 Brightness before 365 365 365 365 roughening (mcd) Brightness after 620.5 613.2 591.3 565.7 roughening (mcd) Roughening ratio = mcd 1.70 1.68 1.62 1.55 before roughening/mcd after roughening

[0068] The data from Tables 2 and 3 show that the light intensity of the epitaxial wafer has little to do with the material of the epitaxial wafer itself, but mainly related to microstructure of the light transmission surface of the epitaxial wafer, and the light intensity of the epitaxial wafer increases with the ratio of specific surface area to vertically projected area of the light transmission area, especially when the ratio is not less than 1.5 as well as that the peak density of the sample is not less than 0.34 m.sup.2, the epitaxial wafer has a high light intensity.

Embodiment 3

[0069] Four samples are prepared for Embodiment 3, namely, samples 9-12 and all are of epitaxial wafer in vertical structure, and share a same preparation method, specifically: depositing a buffer layer (GaAs) and an electrical cladding over the epitaxial substrate and then a n-cladding (Al.sub.X(GaInP).sub.X), further depositing a multilayer quantum well (MQW) structure (InGaP/AlGaInP) above the n-cladding, followed by a p-cladding (p-GaP) deposited on the MQW structure and finally an electrical cladding, thereby completing the growth of the epitaxial layer structure; after the completion of the growth of the epitaxial wafer of samples 9-12, the epitaxial wafer can be prepared into the wafer ready for roughening and finally perform surface roughening to form samples 9-12 with surface microstructure.

[0070] The roughening of samples 9-12 is basically the same, all comprising the following steps:

[0071] (1) have the light transmission surface of samples 9-12 requiring roughening exposed, and protect the light transmission surface requiring no roughening with photoresist or metal;

[0072] (2) prepare roughening solution as per the following dose proportion in weight percentage and with the following components: 30% CH.sub.3COOH, 60% HF, 2% H.sub.2SO.sub.4, 5% HNO.sub.3 and 3% H.sub.3PO.sub.4;

[0073] (3) dip the samples 9-12 processed by Step (1) in the roughening solution prepared by Step (2) and then take out and dry samples 13-16 for subsequent preparation of the light emitting diode with surface microstructure.

[0074] The difference between samples 9-12 is that the dose proportion of Al in the epitaxial layer structure is different; the roughened n-cladding layer is made of GaInP, and Al doped GaInP is a material combining III-V elements, and by controlling the proportion of family III Al and family V GaInP during the growth process of the epitaxial structure, samples 9-12 are obtained. Specific contents of Al are shown in Table 4:

[0075] Luminous intensity tests have been carried out to samples 9-12 and the testing results are shown in Table 4:

TABLE-US-00006 TABLE 4 Testing results of components and properties of n-cladding of samples 9-12 Measurements Ratio of luminance Luminance Luminance after n-cladding before after roughening material roughening roughening to before No. component (mcd) (mcd) roughening Sample 9 Al.sub.0.4(GaInP).sub.0.6 271 475 1.75 Sample 10 Al.sub.0.5(GaInP).sub.0.5 280 495 1.77 Sample 11 Al.sub.0.6(GaInP).sub.0.4 285 489 1.72 Sample 12 Al.sub.0.7(GaInP).sub.0.3 262 430 1.64

[0076] As shown in Table 4 above, when the mol ratio of Al to GaInP is between 0.67 and 1.5, the light intensities of the samples are high.

TABLE-US-00007 TABLE 5 Characterization data of roughened surface of samples 9-12 Measurements Ratio of specific surface area to No. of peak Specific vertically over average surface projected area critical Peak No. area (m.sup.2) (100 m.sup.2) height density Sample 9 167 1.67 52 0.52 Sample 10 184 1.84 58 0.58 Sample 11 155 1.55 49 0.49 Sample 12 123 1.23 41 0.41

[0077] The data from Tables 4 and 5 show that light intensities of samples 9-11 are high, especially that of sample 10 is as high as 495mcd, and property data results in Table 4 are consistent with the samples in Table 5 selected by the screening method based on that the ratio of specific surface area to vertically projected area of the light transmission surface is not less than 1.5, indicating that the screening method provided according to some embodiments of the present disclosure is proper.

Embodiment 4

[0078] Four samples are prepared for Embodiment 2, namely, samples 13-16, and share the same component structure and preparation method, specifically: depositing a buffer layer (GaAs) and an electrical cladding over the epitaxial substrate and then a n-cladding (Al.sub.0.5(GaInP).sub.0.5), further depositing a multilayer quantum well (MQW) structure (InGaP/AlGaInP) above the n-cladding, followed by a p-cladding (p-GaP) deposited on the MQW structure and finally a p-electrical cladding, thereby completing the growth of the epitaxial layer structure; after the completion of the growth of the epitaxial wafer of samples 13-16, the epitaxial wafer can be prepared into the wafer ready for roughening and finally perform surface roughening to form samples 13-16 with surface microstructure.

[0079] The roughening of samples 13-16 is basically the same, all comprising the following steps:

[0080] (1) have the light transmission surface of samples 13-16 requiring roughening exposed, and protect the light transmission surface requiring no roughening with photoresist or metal;

[0081] (2) prepare roughening solution as per the following dose proportion in weight percentage and with the following components:

TABLE-US-00008 CH.sub.3COOH 30~50% HF 40~60% H.sub.2SO.sub.4 0~2% HNO.sub.3 0~20% H.sub.3PO.sub.4 0~10%

[0082] (3) dip the samples 13-16 processed by Step (1) in the roughening solution prepared by Step (2) and then take out and dry samples 13-16 for subsequent preparation of the light emitting diode with surface microstructure.

[0083] The difference between samples 13-16 is that the dose proportion of roughening solutions for processing the samples by the preparation method for the surface microstructure varies. See Table 6 for the specific dose proportion:

TABLE-US-00009 TABLE 6 Dose proportion of roughening solution for samples 13-16 Solution dose proportion No. CH.sub.3COOH HF H.sub.2SO.sub.4 HNO.sub.3 H.sub.3PO.sub.4 Sample 13 50% 50% 0% 0% 0% Sample 14 40% 60% 0% 0% 0% Sample 15 30% 60% 2% 5% 3% Sample 16 40% 40% 2% 10% 8%

[0084] Luminous intensity tests have been carried out to samples 13-16 and the testing results are shown in Table 7:

TABLE-US-00010 TABLE 7 Measurements of samples 13-16 Meas. Roughening ratio = Ratio of specific No. of Luminance Luminance mcd before Spec. surface area peak over before after roughening/ surface to vertically average Peak roughening roughening mcd after area projected area critical density No. (mcd) (mcd) roughening (m.sup.2) (100 m.sup.2) height (m.sup.2) Sam- 375 628 1.68 169.6 1.696 55 0.55 ple 13 Sam- 377 630 1.68 172.6 1.726 53 0.53 ple 14 Sam- 372 615 1.65 153.3 1.533 58 0.58 ple 15 Sam- 373 606 1.63 142.2 1.422 52 0.52 ple 16

[0085] As revealed by Table 7, n-cladding layer made of Al (GaInP) can obtain good roughening effect with roughening solutions with different dose proportions, improving the quality of the epitaxial wafer.

[0086] A method to screen the high brightness light emitting diode, comprising the following steps:

[0087] (1) select epitaxial wafer with surface roughened;

[0088] (2) measure the total roughened surface area of the light transmission surface on the epitaxial wafer chosen in Step (1);

[0089] (3) calculate the ratio of total roughened surface area to vertically projected area;

[0090] (4) screen the epitaxial wafers with the ratio of total roughened surface area to vertically projected area not less than 1.5;

[0091] (5) prepare light emitting diodes with the epitaxial wafers screened by Step (4).

[0092] Samples selected among the 16 samples in the above embodiments 1-4 by the above screening method are: sample 1, sample 2, sample 3, sample 4, sample 5, sample 6, sample 7, sample 9, sample 10, sample 11, sample 12, sample 13, sample 14 and sample 15, consistent with the results of luminance testing, that is, all the epitaxial wafers with the ratio of total roughened surface area to vertically projected area not less than 1.5 have higher light intensity under the same embodiment; further, the screening method based on that the peak density of the light transmission surface is not less than 0.3/um.sup.2 can also be employed, and the screening result is that: sample 1, sample 2, sample 3, sample 4, sample 5, sample 6, sample 7, sample 9, sample 10, sample 11, sample 12, sample 13, sample 14 and sample 15, consistent with the results by the screening method based on that the ratio of total roughened surface area to vertically projected area is not less than 1.5, indicating that both the two screening methods are accurate, and can be employed separately for screening high brightness light emitting diode.

[0093] Although specific embodiments have been described above in detail, the description is merely for purposes of illustration. It should be appreciated, therefore, that many aspects described above are not intended as required or essential elements unless explicitly stated otherwise. Various modifications of, and equivalent acts corresponding to, the disclosed aspects of the exemplary embodiments, in addition to those described above, can be made by a person of ordinary skill in the art, having the benefit of the present disclosure, without departing from the spirit and scope of the disclosure defined in the following claims, the scope of which is to be accorded the broadest interpretation so as to encompass such modifications and equivalent structures.