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 1

The Blue Laser Diode

屠嫚琳 Man-Lin Tu


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


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.


Pioneer of the blue LDs- Nakamura

1989 ： Started III-V nitride research. 1990 ： Develops new ‘two-flow’ MOCVD equipment for growth of

high quality single crystal GaN layers. 1992 ： begins to grow InGaN single crystal layers for the production

of double heterostructures. 1993 ： Succeeded in developing a blue LED with a luminous

intensity as high as 2cd using III-V nitride materials. 1995 ： 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.

1996 ： The first current infection III-V nitride based LDs were fabricated.

1996 ： Announces the first CW blue GaN based injection laser at room temperature.


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


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.


Comparison of light bulbs and LEDs


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 1000oC using a mixture of GaCl3 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.


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.


New two-flow MOCVD system


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.


The steps of grow crystalline GaN

The thickness of the GaN buffer layer was varied between 100Å and 1200Å

The substrate was heated to 1050oC in a stream of hydrogen

The substrate temperature was elevated to between 1000oC and 1030oC

to grow the GaN film.

The substrate temperature was lowered to between 450oC and 600oC to

grow the GaN buffer layer.

The total thickness of the GaN film was about 4 m,and the growth time was 60 min.


Experimental Details Substrate ： Two-inch diameter sapphire with (0001)

orientation(C-face) was used as a substrate. The substrate was heated at 1050oC in a stream of hydrogen.

GaN buffer layer ： Trimethylgallium(TMG) and ammonia(NH3) were used as Ga and N sources, respectively.The flow rates of H2, NH3, 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 1035oC to grow the GaN film.The flow rates of H2, NH3, and TMG were maintained at 2.0 l/min, 4.0 l/min, and 54 mol/min.


Comparison of the thickness of buffer layer

GaN AlN

77K 300K 77K 300K200Å 1200Å 200Å 1200Å 200Å 1200Å 200Å 1200Å

1500

cm2/Vs

900

cm2/Vs

600

cm2/Vs

380

cm2/Vs

500

cm2/Vs

500

cm2/Vs

350~

430

cm2/Vs

350~

430

cm2/Vs

Amano et al. and Akasaki et al.


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


Highly p-type Mg-doped GaN Mg-doped GaN films with GaN buffer layers

resistivity fluctuated between 3.2× 102 Ωcm and 1× 105 Ωcm .

Amano et al. and Akasaki et al., the as-grown Mg-doped GaN films with AlN buffer layers show high resistivity (over 108 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.


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× 104 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× 104 Ωcm.


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.


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.


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.

Thanks for your attention!

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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