2003/5/12 國立彰化師範大學 - 屠嫚琳 1 The Blue Laser Diode 屠嫚琳 Man-Lin Tu

2003/5/12 國立彰化師範大學 - 屠嫚琳 2 Content Introduction Biography of Nakamura Applications Background Growth methods for crystalline GaN The steps of grow crystalline GaN Highly p-type Mg-doped GaN Comparison of the GaN and AlN buffer layers

2003/5/12 國立彰化師範大學 - 屠嫚琳 3 Introduction Using shorter wavelength blue lasers would decrease the spot size on the disk, creating a four-fold increase in data storage capacity on conventional disks. Much research has been done on high- brightness blue LEDs and LDs for use in full- color displays and full-color indicators with high efficiency, high reliability and high speed.

2003/5/12 國立彰化師範大學 - 屠嫚琳 4 Pioneer of the blue LDs- Nakamura 1989 ： Started III-V nitride research ： Develops new ‘two-flow’ MOCVD equipment for growth of high quality single crystal GaN layers ： begins to grow InGaN single crystal layers for the production of double heterostructures ： Succeeded in developing a blue LED with a luminous intensity as high as 2cd using III-V nitride materials ： Developed high-brightness SQW structure blue/green LEDs with a luminous intensity of 2 cd and 10 cd, and developed a violet laser diode using III-V nitride materials for the first time ： The first current infection III-V nitride based LDs were fabricated ： Announces the first CW blue GaN based injection laser at room temperature.

2003/5/12 國立彰化師範大學 - 屠嫚琳 5 Applications Large scale displays (large outdoor television screens) Smaller full-color flat panel display screens (inside trains or subway stations) Full-color scanners Full-color photocopying machines Full-color FAX machines Traffic lights LED white lamps

2003/5/12 國立彰化師範大學 - 屠嫚琳 6 Background SiC is another wide band gap material for blue LEDs. The brightness of SiC blue LEDs is only between 10mcd and 20mcd because of the indirect band gap of this material. GaN and related materials such as AlGaInN are III-V nitride semiconductors with the wurtzite crystal structure and a direct energy band gap.The energy band gap of AlGaInN varies between 6.2eV and 1.95eV. High-brightness blue LEDs have been fabricated on the basis of these results, and luminous intensities over 1cd have been achieved.

2003/5/12 國立彰化師範大學 - 屠嫚琳 7 Comparison of light bulbs and LEDs

2003/5/12 國立彰化師範大學 - 屠嫚琳 8 Growth methods for crystalline GaN Halide vapor phase epitaxy (HVPE) method using an equilibrium mixture of nitrogen and Ga-containing gas. It is achieved by depositing GaN on a sapphire crystal at 1000 o C using a mixture of GaCl 3 and ammonia as the Ga and nitrogen source gases, respectively.However the growth speed using this method is too high(several  m/min) to control the thickness of thin epitaxial layers with precision.

2003/5/12 國立彰化師範大學 - 屠嫚琳 9 New two-flow MOCVD system for GaN growth It has two different gas flows.One is the main flow which carries the reactant gas parallel to the substrate with a high velocity through the quartz nozzle. Another flow is the subflow which transports the inactive gas perpendicular to the substrate for the purpose of changing the direction of the main flow to bring the reactant gas into contact with the substrate.

2003/5/12 國立彰化師範大學 - 屠嫚琳 10 New two-flow MOCVD system

2003/5/12 國立彰化師範大學 - 屠嫚琳 11 Growth of GaN films with AlN buffer layer Usually, sapphire is used as a substrate to grow GaN.Due to the large lattice mismatch and the large difference in the thermal expansion coefficients between GaN and sapphire,it used to be fairly difficult to grow high-quality epitaxial GaN film with a flat surface free from cracks. Amano et al. and Akasaki et al. have overcome these problems by prior deposition of a thin AlN layer as a buffer layer before the growth of GaN by means of the MOCVD method.

2003/5/12 國立彰化師範大學 - 屠嫚琳 12 The steps of grow crystalline GaN The thickness of the GaN buffer layer was varied between 100Å and 1200Å The substrate was heated to 1050 o C in a stream of hydrogen The substrate temperature was elevated to between 1000 o C and 1030 o C to grow the GaN film. The substrate temperature was lowered to between 450 o C and 600 o C to grow the GaN buffer layer. The total thickness of the GaN film was about 4  m,and the growth time was 60 min.

2003/5/12 國立彰化師範大學 - 屠嫚琳 13 Experimental Details Substrate ： Two-inch diameter sapphire with (0001) orientation(C-face) was used as a substrate. The substrate was heated at 1050 o C in a stream of hydrogen. GaN buffer layer ： Trimethylgallium(TMG) and ammonia(NH 3 ) were used as Ga and N sources, respectively.The flow rates of H 2, NH 3, and TMG were maintained at 2.0 l/min, and 4.0 l/min, and 27  mol/min,respectively. GaN film :The temperature was elevated to around 1035 o C to grow the GaN film.The flow rates of H 2, NH 3, and TMG were maintained at 2.0 l/min, 4.0 l/min, and 54  mol/min.

2003/5/12 國立彰化師範大學 - 屠嫚琳 14 Comparison of the thickness of buffer layer GaNAlN 77K300K77K300K 200Å1200Å200Å1200Å200Å1200Å200Å1200Å 1500 cm 2 /Vs 900 cm 2 /Vs 600 cm 2 /Vs 380 cm 2 /Vs 500 cm 2 /Vs 500 cm 2 /Vs 350~ 430 cm 2 /Vs 350~ 430 cm 2 /Vs Amano et al. and Akasaki et al.

2003/5/12 國立彰化師範大學 - 屠嫚琳 15 The Hall mobility was measured at 77K and 300K as a function of the thickness of GaN buffer layer The value of the FWHM is almost constant between 200Å and 1200Å thickness.The optimum thickness of the GaN buffer layer was around 200Å

2003/5/12 國立彰化師範大學 - 屠嫚琳 16 Highly p-type Mg-doped GaN Mg-doped GaN films with GaN buffer layers resistivity fluctuated between 3.2× 10 2 Ωcm and 1× 10 5 Ωcm. Amano et al. and Akasaki et al., the as-grown Mg- doped GaN films with AlN buffer layers show high resistivity (over 10 8 Ωcm ). The as-grown Mg-doped GaN films grown with GaN buffer layers are superior to those grown with AlN buffer layers in terms of their conductivity control.

2003/5/12 國立彰化師範大學 - 屠嫚琳 17 Resistivity change of a Mg-doped GaN film of etching depth from the surface Without low-energy electron beam irradiation (LEEBI) treatment, the resistivity of this sample was 4× 10 4 Ωcm. After LEEBI treatment, the resistivity was 3 Ωcm. After 0.2  m etching, the resistivity of this sample is still low. After 0.5  m etching, the resistivity becomes as high as 4× 10 4 Ωcm.

2003/5/12 國立彰化師範大學 - 屠嫚琳 18 Comparison of forward voltage The forward voltage of using GaN buffer layers is 4 V at 20 mA.Amano et al. and Amano and Akasaki produced p-n junction LEDs using AlN buffer layer, the forward voltage was 6V at 2 mA. The forward voltage of LEDs fabricated with GaN films grown with GaN buffer layers is lower than that with AlN buffer layers, and the value of the forward voltage is low enough to be applied to any electronic circuit.

2003/5/12 國立彰化師範大學 - 屠嫚琳 19 Comparison of radiative recombination centers The number of radiative recombination centers of blue emission in the p-GaN layer with GaN buffer layers is much larger than that with AlN buffer layers because the intensity of the blue electro- luminescence(EL) is much stronger than that of UV EL in GaN LEDs with GaN buffer layers. Blue emission centers are related to the energy level introduced by Mg doping in the energy gap of GaN, and the blue emission in the photoluminescence(PL) measurement of p-GaN layers becomes strong when the hole concentration becomes high.

2003/5/12 國立彰化師範大學 - 屠嫚琳 20 The output power of the p-n junction GaN LED compared to a conventional 8 mcd SiC LED The output power of the p-n junction GaN LED compared to a conventional 8 mcd SiC LED as a function of the forward current.

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