WO2008071038A1

WO2008071038A1 - Gan led chip and the method of the same

Info

Publication number: WO2008071038A1
Application number: PCT/CN2006/003421
Authority: WO
Inventors: Philip Chan; Raymond Wang
Original assignee: Podium Photonics (Guangzhou) Ltd.
Priority date: 2006-12-15
Filing date: 2006-12-15
Publication date: 2008-06-19
Also published as: TW200834957A

Abstract

A GaN LED chip and the method of the same are provided to solve the problem in the prior art that the transparent conductive layer made of Ni/Au emits light poorly. The LED chip comprises a n-type GaN layer (1), a p-type GaN layer (2) and transparent conductive layer (3), wherein the ZnO transparent conductive layer formed on the surface of the GaN layer (2), the ZnO layer is doped with III group metal and has a thickness of 5nm-1000nm. The LED chip increase the light extraction rate to more than 90%, the light extraction rate is 1.7 times compare to the Ni/Au severed as transparent conductive layer. The performance of high current diffusionand perfect heat stability are provided. The LED chip can be used widely in the filed of communication, traffic, display, backlight source and so on.

Description

Technical field

The present invention relates to a gallium nitride light emitting diode, and more particularly to a high brightness gallium nitride light emitting diode chip and a method of fabricating the same. Background technique

GaN LEDs are ultra-high brightness LEDs, which not only save energy, but also have the characteristics of no pollution, long life, small size and vivid colors, so they are widely used in communication, transportation, display, backlight. Source and other fields. In recent years, people have done a lot of research work in developing and improving the P-type ohmic contact of LEDs, and have developed many suitable structures, which basically meet the requirements of practical devices. A conventional GaN LED chip includes an N-type GaN layer, a P-type GaN layer, and a transparent conductive layer, and a P-type ohmic contact of the LED chip, particularly a transparent conductive layer, usually deposited on the surface of the epitaxial chip by electron beam evaporation equipment or A multi-layer high-work function metal, such as Ni metal, is deposited on the front side of the electron beam, and then a layer of a conductive metal such as gold (Au) is vapor-deposited in the same manner, and finally the alloy is treated to reduce the contact resistance, but due to Ni /Au has a large absorption of light in the visible light band, so that the light emission rate of the LED is only 50-70%. Summary of the invention

The main object of the present invention is to provide a high-brightness gallium nitride light-emitting diode chip and a manufacturing method thereof, which can effectively improve the luminous efficiency of a GaN LED chip, and prepare a low-attenuation, high-brightness LED light-emitting chip with good thermal stability.

In order to solve the technical problem, the technical solution of the present invention is:

The high-brightness gallium nitride light emitting diode chip comprises an N-type GaN layer, a P-type GaN layer and a transparent conductive layer, and is characterized in that: a transparent conductive ZnO is grown on the surface of the P-type GaN material The electric layer, the ZnO transparent conductive layer is grown or deposited on the surface of the P-type GaN by an organometallic chemical vapor deposition method, molecular beam epitaxy, electron beam evaporation, or the like, as a current diffusion and light-emitting layer, and the element for the ZnO transparent conductive layer The first and third groups of metals in the periodic table are doped, and the film thickness of the transparent conductive layer is controlled between 5 nm and 100 Onm. The film is a transparent conductive layer having a high current conductivity and having positive ion characteristics or negative ion characteristics.

The above first and third group metals are usually Be, Al, In.

The method of manufacturing the above high brightness gallium nitride light emitting diode chip comprises the following steps:

1) Etching, cleaning, and heat-treating the surface of the P-type GaN layer to reduce the contact resistance with the ZnO conductive layer. The etching or cleaning method is a commonly used method.

2) growing a ZnO transparent conductive layer on the surface of the GaN material. The ZnO material is grown or deposited on the surface of the P-type GaN by organometallic chemical vapor deposition, molecular beam epitaxy, and electron beam evaporation, as a current diffusion and light source extraction layer.

The above method also includes the following steps:

3) annealing the ZnO transparent conductive layer, the chip is gradually heated in a low pressure chamber at a rate of 0.1 ° C - 10 CTC per minute in a hydrogen or inert atmosphere, at 250 ° C - 1000 ° C Annealing under temperature conditions for 30 seconds - 45 minutes.

The above method also includes the following steps:

4) Wet etching the ZnO transparent conductive layer. The realization of the ZnO transparent conductive layer light-emitting region electrode pattern needs to protect the ZnO layer that does not need to be etched by a non-metal oxide or a photoresist, and then etched by various acid or alkali dilutions, such as using hydrochloric acid, phosphoric acid, Acetate, BOE, NaOE KOH, etc. are wet etched. Wherein BOE is a mixture of HF and NH ₄ F, and the ratio of HF:NH ₄ F may be 1:6, 1:7 or 1:9.

The above method also includes the following steps:

5) cleaning the GaN LED chip with pure water, 6) Bake the chip treated in step 5) for 5-20 minutes at 60 ° C - 100 ° C. Unlike the transparent conductive layer of the conventional GaN LED chip, the hexagonal wurtzite structure of ZnO easily absorbs water vapor in the air and affects the electrical characteristics, so the wafer needs to be baked after the pure water cleaning process.

The above method also includes the following steps:

7) Dry etching the GaN N electrode. The dry etching must use a protective film which can be removed by a general organic solution as a material for protecting the P-type light-emitting layer. The present invention uses a photoresist as a protective layer of the P-type ZnO light-emitting layer, and is etched by an inductively coupled ion etching machine. - An organic solution such as methyltetrahydropyrrole (NMP) which dissolves the photoresist without reacting with ZnO.

The wire electrode material which forms an ohmic contact with the transparent conductive layer of ZnO uses at least two layers of high work function metals. Commonly used high work function metals are Cr/Pt/Au, Cr/Pd/Au, Cr/Al/Cr/Au, Cr/Al/Pt/Au, Cr/Al/Pd/Au, Cr/Al/Ti/Au, Cr/Al/Co/Au, Cr/Al/Ni/Au, Pd/Al/Ti/Au and Pd/Al/Pt/Au. The wire electrode is deposited by electron beam or sputtering equipment and a metal stripping technique is used to form a symmetrical electrode shape for subsequent packaging 'J test and wiring.

The above chip is annealed in an inert gas atmosphere at a temperature of 200 ° C to 30 ° C for 5 to 30 minutes to achieve optimum thermal stability and minimum contact resistance of the wire electrode and the light-emitting layer and GaN.

After the electrode process is completed, the refractive index difference between ZnO and air is large, and the light extraction efficiency is weak. To increase the light extraction efficiency, the present invention grows a surface of the ZnO transparent conductive layer with a refractive index close to that of ZnO. The silicon material (Si ₃ N ₄ ) increases the light extraction rate by about 15% or more than the conventional Si0 ₂ as a protective layer.

Due to the above technical solution, the high-brightness gallium nitride light-emitting diode chip of the present invention utilizes ZnO material and GaN as compared with the conventional gallium nitride light-emitting diode chip using Ni/Au as the P-type ohmic contact light-emitting layer material. The material's low refractive index difference and good lattice matching characteristics can control the light intensity attenuation of GaN LED within 10 hours within 10%, and have good thermal stability. Sexuality, whose characteristic contact resistance is reduced to nE-5 Ω-cm ² (where n is any number between 1-10), is a stable and reliable core component of semiconductor solid-state lighting.

The method for manufacturing a high-brightness gallium nitride light-emitting diode chip according to the present invention optimizes the conductivity and luminous efficiency of the chip through a targeted chip fabrication process, and improves the diffusion efficiency of the current while reducing the contact resistance of the P-type light-emitting region. Finally, the balance between contact resistance and luminous efficiency is achieved, thereby increasing the life of the chip. The high-brightness gallium nitride light-emitting diode chip produced by the process has good electrical conductivity and over 90% light penetration performance (450-540 nm band). ), the luminous intensity is about 1.7 times higher than that of the conventional Ni/Au material as the transparent conductive layer, so that it can well meet the market demand for preparing high-brightness LED chips, and other photoelectric properties are in line with backlight, display and Application standards such as lighting. DRAWINGS

1 is a process flow diagram of one embodiment of a method for fabricating a high-brightness gallium nitride light-emitting diode chip according to the present invention;

2 is a process flow diagram of a second embodiment of a method for fabricating a high-brightness gallium nitride light-emitting diode chip according to the present invention;

3 is an exploded perspective view showing an N-type GaN layer, a P-type GaN layer, and a ZnO transparent conductive layer of the high-brightness gallium nitride light-emitting diode chip according to the present invention;

4 is a schematic view showing a structure of a ZnO transparent conductive layer of the chip shown in FIG. 1. FIG. 5 is a plan view of a high-brightness gallium nitride light-emitting diode chip according to the present invention. detailed description

Example 1

Referring to FIG. 1, a method for manufacturing a high-brightness gallium nitride light-emitting diode chip according to the present invention includes the following steps:

(1) Surface treatment of a P-type GaN layer using an organometallic chemical vapor deposition method in P A transparent conductive layer of ZnO is grown on the GaN layer, and the ZnO is deposited by an electron beam evaporation (E-BEAM) method, a sputtering (SPUTTER) method, or the like, and is doped with an A1 or In metal cross layer.

(2) protecting the ZnO transparent conductive layer by photoresist with a ZnO layer that does not need to be etched,

(3) Finishing photolithography of the ZnO transparent conductive layer, washing away the photoresist,

(4) Dry the ZnO transparent conductive layer and bake at 65 ° C for 10 minutes.

(5) dry etching the GaNN electrode, using photoresist as a protective layer of the ZnO light-emitting layer, (6) after etching by an inductively coupled ion etching machine, using N-methyltetrahydropyrrole

The (NMP) solution is cleaned to complete the etching of the N electrode on the N-type GaN layer.

(7) using a sputtering device to evaporate the wire electrode metal and form a symmetrical electrode shape by metal stripping technique, and the wire electrode material is Cr/Pt/Au.

(8) A protective layer of silicon nitride (Si ₃ N ₄ ) or silicon dioxide is grown, and the film thickness of the ZnO transparent conductive layer is 300 nm.

Example 2

Referring to FIG. 2, a method for manufacturing a high-brightness gallium nitride light-emitting diode chip according to the present invention includes the following steps:

(1) surface treatment of a P-type GaN layer,

(2) Before dry etching the N electrode of the GaN layer, a layer of silicon dioxide is grown on the surface of the P-type GaN layer not to be etched, protected by a photoresist on the surface of the silicon dioxide, and etched by the solution to be etched. SiO 2, remove the protective photoresist after etching,

(3) etching the N-type GaN layer by inductively coupled plasma etching,

(4) ZnO transparent conductive layer is grown by electron beam evaporation on the surface of P-type GaN,

(5) Photolithography of the ZnO transparent conductive layer, (6) The chip obtained in the above step 5) was annealed at 450 ° C for 10 minutes.

(7) using a sputtering device to vapor-deposit the wire electrode metal, and forming a symmetrical electrode shape by a metal stripping technique, and the wire electrode material is Cr/Pt/Au,

(8) A protective layer of silicon nitride (Si ₃ N ₄ ) or silicon dioxide is grown on the surface of the ZnO transparent conductive layer. The ZnO transparent conductive layer has a film thickness of 1000 nm.

Example 3

The method for manufacturing a high-intensity gallium illuminide LED chip according to the present invention comprises the following steps:

(1) etching and cleaning the surface of the P-type GaN layer, and then heat-treating the GaN LED wafer,

(2) growing a transparent conductive layer of ZnO on the surface of the GaN material,

(3) annealing the ZnO transparent conductive layer, the chip is gradually heated at a rate of 0.2 ° C per minute in a low pressure chamber in a hydrogen or inert gas atmosphere, and annealed at a temperature of 255 ° C. Minutes,

(4) wet etching the ZnO transparent conductive layer,

(5) cleaning the GaN LED chip with pure water,

(6) Baking the chip treated in step 5 under 7 (TC conditions) for 8 minutes, (7) dry etching the GaN N electrode,

(8) vapor deposition wire electrode metal, using QVPd/Au as the wire electrode material,

(9) The chip prepared in the above step 8) is annealed in an inert gas atmosphere at a temperature of 210 ° C for 20 minutes.

(10) A silicon nitride material is grown on the surface of the ZnO transparent conductive layer prepared in the above step 9), and the ZnO transparent conductive layer has a film thickness of 5 nm.

Example 4

The method for manufacturing a high-brightness gallium nitride light-emitting diode chip according to the present invention comprises the following steps Step:

(2) growing a transparent conductive layer of ZnO on the surface of the P-type GaN material,

(3) annealing the ZnO transparent conductive layer, the chip is gradually heated at a rate of 90 ° C per minute in a low-pressure chamber in a hydrogen or inert gas atmosphere, and annealed at a temperature of 900 45 for 45 seconds.

(4) wet etching the ZnO transparent conductive layer,

(5) cleaning the GaN LED chip with pure water,

(6) baking the chip treated in step 5) for 10 minutes at 70 ° C,

(7) dry etching the GaN electrode,

(8) Evaporating the wire electrode metal, using Pd/Al/Ti/Au as the wire electrode material,

(9) The chip prepared in the above step 8) is grown on the surface of the ZnO transparent conductive layer at a temperature of 300 ° C (10) in an inert gas atmosphere, and the ZnO transparent conductive layer has a thickness of 100 nm. .

Example 5

The method for manufacturing a high-brightness gallium nitride light-emitting diode chip according to the present invention comprises the following steps:

(3) annealing the ZnO transparent conductive layer, the chip is gradually heated in a low-pressure chamber at a rate of 50 Å per minute in a hydrogen atmosphere, and annealed at a temperature of 600 ° C for 7 minutes. (4) wet etching the ZnO transparent conductive layer,

(5) cleaning the GaN LED chip with pure water,

(6) baking the chip treated in the step 5) for 15 minutes at 60 ° C, (7) dry etching the GaNN electrode,

(8) vapor-depositing the wire electrode metal, using at least two layers of high work function metal as the wire electrode material,

(9) The chip prepared in the above step 8) is annealed in an inert gas atmosphere at a temperature of 250 ° C for 10 minutes.

(10) A layer of vaporized silicon material is grown on the surface of the above ZnO transparent conductive layer, and the ZnO transparent conductive layer has a film thickness of 380 nm.

Example 6

(3) annealing the ZnO transparent conductive layer, the chip is gradually heated at a rate of 3.5 ° C per minute in a low pressure chamber in a hydrogen or inert gas atmosphere, and annealed at a temperature of 550 ° C. 5 minutes.

(4) wet etching the ZnO transparent conductive layer,

(5) cleaning the GaN LED chip with pure water,

(6) baking the chip treated in the step 5) at 70 ° C for 10 minutes,

(7) dry etching the GaNN electrode,

(8) vapor deposition wire electrode metal, using Pd/Al/Ti/Au as the wire electrode material, (9) the chip prepared in the above step 8) is in an inert gas atmosphere at a temperature of 300 ° C Annealing for 5 minutes,

(10) growing a layer of silicon nitride material on the surface of the ZnO transparent conductive layer, and ZnO is transparent.

The conductive layer of the conductive layer has a thickness of 580 nm. Example 7

The method for manufacturing a high-brightness gallium nitride light-emitting diode chip according to the present invention comprises the following steps

Step:

Then, the chip prepared by the step 2) is completed by a conventional subsequent processing step.

Oo. \

Referring to FIG. 3, the high-brightness gallium nitride light-emitting diode t-tube chip of the present invention includes a sapphire

Stone substrate and N-type GaN layer 1, P-type GaN layer 2 and ZnO transparent conductive layer 3, in GaN

After the surface of the LED epitaxial layer is grown with a highly doped ZnO layer, it is wet etched.

Method, a transparent conductive layer pattern of ZnO material is formed on the surface of the P-type GaN, as shown in FIG.

As shown in FIG. 5, after the chip is manufactured, the P electrode 4 and the P-type GaN layer are connected.

N-electrode 5 to which the N-type GaN layer is connected.

The above chips were tested. The test results are shown in Table 1, Table 2 and Table 3. Table 1 shows the chip of Ni/Au material as transparent conductive layer and the chip using ZnO material as transparent conductive layer.

Comparison of light intensity; Tables 2 and 3 are reliability data. Test internal compilation comprehensive analysis

PAD ESD

§20mA Vf (V synthesis

Ir (uA) Iv(mcd) Wd(nm) Vf Ir Iv Wd

PWZ10602) Yield

21104 80.99 93.39 93.39

14mil ZnO 4.29 0.09 245.18 500.29 98.35%

% % %

§20mA

91.74

PWZ10602 14mil Ni/Au 3.53 0.01 143.17 500.89 80.79%

%

21104 The data shown in Table 1 is the result of testing with a probe tester using the same standard.

Using the same chip size, the same piece of LED epitaxial wafer is used for fabrication, divided into two halves. Two materials are used as the chip transparent conductive layer, driven by 20mA current.

The luminescence intensity of the Ni/Au material as the transparent conductive layer was 143.17 mcd, and the luminescence intensity of the chip using the ZnO material as the transparent conductive layer was 245.18 mcd, which was found to be about 1.7 times higher than that of the Μ/Αι material as the transparent conductive layer.

The reliability data of the chip is shown in Table 2 and Table 3.

The ZnO chip is packaged into a flat-head LED tube with a diameter of 5mni, and then passed through 20mA

Or 30mA, 40mA current, continuously illuminate in normal temperature or high temperature environment for 1000 hours or

0 72 hours to do the burning test, the attenuation ratio of the light intensity before and after the burning was obtained.

Accelerated aging life test report

No.: P19~QE- RF- 010 Version: 02

I test conditions: ~ ~ high temperature (55 ° C) constant bright 72H ± 2H, forward current (If) = 30mA.

(See Reliability Test Standard: P19- QE- IC- 001)

P Z1060 test sheet

Lot No. Wafer NO. QR-C-T-015

221104

Manufacturer Sample Quantity 17 Effective Quantity 17

Before the experiment, Vf changes Wd after the experiment.

NO.

Vf Ir Iv Wd Vf Ir Iv Wd value reduction value

1 4. 26 0 857. 1 502. 1 4. 14 0. 04 1063. 1 501. 9 -0. 12 -24% -0. 2 0K

2 4. 28 0 1067. 7 502. 3 4. 15 0. 02 1104. 4 501. 7 - 0. 13 -3% -0. 6 0K

3 4. 23 0 1065. 6 501. 3 4. 13 0. 01 966, 6 501 -0. 10 9% - 0. 3 0K

4 4. 28 0. 03 851. 1 - 502. 5 4. 16 0. 15 841. 3 501. 9 -0. 12 1% -0. 6 0K

5 4. 26 0 841. 6 501. 4 4. 15 0. 01 931. 1 501. 2 - 0. 11 -11% -0. 2 0

6 4. 25 0. 05 905. 2 501. 4 4. 14 0. 03 1134. 1 501 - 0. 11 -25% - 0. 4 0

7 4. 26 0. 08 889. 5 501. 3 4. 14 0. 02 873. 8 501. 7 -0. 12 2% 0. 4 0K

8 4. 28 0 809. 1 502. 3 4. 15 1. 16 898. 1 501. 6 -0. 13 -11% 0. 7 0K

9 4. 27 0 963. 7 502. 1 4. 16 0. 03 970. 2 500. 9 - 0. 11 -1% - 1. 2 0K

10 4. 24 0 973. 5 501. 5 4. 14 0. 01 1049. 3 501. 3 -0. 10 -8% - 0. 2 0K

11 4. 27 0. 01 908. 1 502. 7 4. 15 0. 05 879. 2 501. 9 -0. 12 3% -0. 8 0K

12 4. 28 0. 01 849. 1 502. 5 4. 16 0. 03 1004. 6 501. 8 -0. 12 -18% -0. 7 0K 13 4.27 0 956.5 502.4 4.15 0.03 975.3 500.7 -0.12 -2% - 1.7 OK

14 4.23 0 975.3 501.1 4.12 0.05 896 501.9 -0.11 8% 0.8 OK

15 4.3 0 913.8 502.5 4.16 0.03 502 -0.14 - 8% -0.5 OK

16 4.29 0 898.3 501.9 4.17 0.02 501.4 -0.12 4% - 0.5 OK

17 4.28 0.04 1007.3 502.5 4.15 0.32 501.8 -0.13 4% -0.7 OK

18

19

20

MIN 4.23 0 809.1 501.1 4.12 0.01 841.3 500.7 -0.14 -25% -1.7

AVG 4.2665 0.0129 925.44 501.99 0, 1182 964.97 501.51 -0.118 -5% -0.476

MAX 4.3 0.08 1067.7 502.7 4.17 1.16 1134.1 502 -0.1 9% 0.8

Experimental conclusion: i qualified door failed

Remarks:

Verify the reliability of the ZnO experimental chip 30mA

Table 2

Aging life test report

No.: P19- QE- RF- 010 Version: 02 Test conditions: Normal temperature constant brightness, 1000 hours, If-20mA.

PWZ 10602

Batch No. Wafer NO. Test No. QR-C-T-015

21104

The number of samples sampled by the manufacturer 15 Effective quantity 15 Before the experiment Vf change after Iv Id Wd change

N0.

Vf Ir Iv Wd Vf Ir Iv Wd value reduction value

1 4.27 0.02 989.3 501.4 4.13 0.01 890.8 500.9 - 0.14 10% -0.5 0K

2 4.43 0.28 737.8 503.9 4.18 0.02 812.6 503.6 -0.25 -10% -0.3 0K

3 4.55 0.71 941.8 503.8 4.34 1.01 835.1 503.1 -0.21 11% -0.7 0K

4 4.49 0.02 815.9 503.9 4.32 0.04 750 503.2 -0.17 8% - 0.7 0K

5 4.46 0.01 808.6 503.8 4.16 0.1 899.5 503.4 - 0.30 -11% -0.4 0K

6 4.49 0.06 717.2 503.9 4.31 0.26 698.7 503.1 - 0.18 3% -0.8 0K 03421

table 3

Claims

Profit request

A high-brightness gallium nitride light-emitting diode chip comprising an N-type GaN layer, a P-type GaN layer and a transparent conductive layer, wherein: a transparent conductive layer of ZnO is grown on a surface of the P-type GaN material, and the ZnO The transparent conductive layer is doped with the first and third groups of metals in the periodic table, and the transparent conductive layer has a film thickness of between 5 nm and 1000 nm.

2. The high-brightness gallium nitride light-emitting diode chip according to claim 1, wherein the first and third group metals are Be, Al, and In.

3. A method of fabricating a high brightness gallium nitride light emitting diode chip, the high brightness gallium nitride light emitting diode chip comprising an N-type GaN layer, a P-type GaN layer, and a transparent conductive layer grown on a surface of the P-type GaN material The ZnO transparent conductive layer is doped with the first and third metals in the periodic table, and the thickness of the transparent conductive layer is controlled between 5 nm and 1000 nm. The method comprises the following steps:

1) etching and cleaning the surface of the P-type GaN layer, and then heat-treating the GaN LED wafer,

2) growing a ZnO transparent conductive layer on the surface of the GaN material.

4. The method of manufacturing a high brightness gallium nitride light emitting diode chip according to claim 3, wherein the method further comprises the steps of:

3) annealing the ZnO transparent conductive layer, and the light-emitting diode chip obtained in step 2) is heated in a low-pressure chamber at a rate of o.rc - locrc per minute in a hydrogen or inert gas atmosphere, at 250 Ό - 1000 ° Annealing at a temperature of C for 30 seconds - 45 minutes.

5. The method of manufacturing a high brightness gallium nitride light emitting diode chip according to claim 4, wherein: the method further comprises the steps of:

4) Wet etching the ZnO transparent conductive layer.

6. The method of manufacturing a high brightness gallium nitride light emitting diode chip according to claim 5, wherein: the method further comprises the steps of: 5) cleaning the GaN LED chip with pure water,

6) Bake the chip treated in step 5) for 5 - 20 minutes at 60 ° C - 100 ° C.

7. The method of manufacturing a high brightness gallium nitride light emitting diode chip according to claim 6, wherein the method further comprises the steps of:

7) Dry etching the GaN N electrode.

8. A method of fabricating a high brightness gallium nitride light emitting diode chip according to claim 7, wherein: the wire electrode material forming an ohmic contact with the ZnO transparent conductive layer employs at least two layers of high work function metal.

9. The method of manufacturing a high brightness gallium nitride light emitting diode chip according to claim 8, wherein: said chip is annealed in an inert gas atmosphere at a temperature of 200 ° C to 300 ° C for 5 to 30 minutes. .

10. The method of manufacturing a high brightness gallium nitride light emitting diode chip according to claim 9, wherein: a silicon nitride material is grown on the surface of the ZnO transparent conductive layer.

11. The method of manufacturing a high-brightness gallium nitride light-emitting diode chip according to claim 10, wherein in said step 3), the light-emitting diode chip is heated at a rate of 3.5 ° C per minute in the low-pressure chamber. Annealed at 550 ° C for 25 minutes.