High Brightness Light Emitting Diodes

Chapter 3

Chapter 4

Reporter ：楊勝州

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Chapter 3 ： AlGaAs Red Light Emitting Diodes

1.Introduction

2.Device Design

3.Crystal Growth

4. Electroluminescent Spectra

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1.Introduction Gallium arsenide (GaAs) is a direct-energy-gap semiconductor with an

energy gap(1.424 eV) in the near-infrared (IR) portion of the electromagnetic specrum.

Aluminum arsenide (AlAs) is an indirect-energy-gap semiconductor with an energy gap(2.168 eV) in the yellow-green portion of the electromagnetic specrum.

As the AlAs mole fraction x of AlxGa1-xAs is raised from zero, the energy gap of the resulting compound increases from that of GaAs to that of AlAs.

When the value of x is 0.45,the semiconductor switches from having a direct-energy gap to having an indirect-energy gap. GaAs and AlAs differ in lattice constant by less than 0.2% at 250C,so

this enables the growth of very high quality AlGaAs films on GaAs substrates.

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2.Device Design

n-type and p-type are differences in the deep levels and crystal defects that can change with the type of dopant used, so they can exhibit different luminescence efficiencies.

The device uses heterostructure Design( 異質介面 ) As the direct-energy-gap to indirect-energy-gap transition is

approached, the internal efficiency of AlGaAs LEDs falls rapidly. This occurs because more and more of the injected minority-carrier electrons reside in the indirect minima and thus are unavailable for radiative recombination.

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3.Crystal Growth Molecular beam epitaxy is a crystal-growth technique in which

material is deposited by evaporating individually controlled source materials onto a heated substrate in an ultrahigh vacuum chamber( 真空腔 ).

Molecular beam epitaxy is capable of producing high-quality AlGaAs heterostructure devices with very uniform thickness and doping characteristics, and it is often employed for the growth of AlGaAs laser diodes.

The growth rates in MBE are very slow(0.1 to 2 mm/h) Crystal growth by MOCVD ( Metalorganic Chemical Vapor

Deposition) began in the late 1960s. By using MOCVD, it is easier to scale it up to larger growth

areas than it is to scale up LPE, and the uniformity and surface morphology of MOCVD grown lasers are superior to those attainable with LPE.

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4.Electroluminescent Spectra

Electroluminescent Spectra at a constant bias current density of 27.4 A/cm2 at varioustemperatures between 89 and 390 K are shown.At temperatures below approximately 180 K,distinct band-to-band (BB) and band-to-acceptor (BA) peaks are present in the spectra.At 89.9 K ,the BB and BA peaks are comparable in magnitude, which means that roughly half of the total radiative recombination takes place through BA transitions atthis temperature.

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Chapter 4 ：OMVPE Growth of AlGaInP for High-Efficiency Visible Light-Emitting Diodes

1. Introduction

2. OMVPE (Organometallic Vapor Phase Epitaxy) Gas Delivery System

3. Growth Conditions

4. Substrate choice

5. Oxygen Incorporation

6. Summary

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1.Introduction

The difference in thermodynamic stability of AlP and InP makes compositional control extremely difficult by LPE.

These technical problems had kept AlGaInP materials from being used in making LEDs or laser diodes until the late 1980s.

The method is using OMVPE.

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2.OMVPE Gas Delivery System

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3.Growth Conditions AlGaInP is a quaternary system, careful control is

required to ensure lattice-matching of AlGaInP to the GaAs substrate.

Second,aluminum is very reactive and binds easily to oxygen and the most effective ways to suppress oxygen incorporation is to increase the growth temperature.

However, high growth temperature is not ideal for In-containing alloys, which are typically grown at much lower temperature. In addition, In re-evaporation can be a problem if the growth temperature is too high. And now the AlGaInP growth temperature generally falls between 650 and 8000C .

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4.Substrate choice

The thermal expansion coefficients for the binary compounds

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Growth temperature and room temperature lattice matching

conditions for GaxIn1-xP and AlxIn1-xP on GaAs substrates

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5.Oxygen Incorporation

One of the primary challenges in the growth of high-quality AlGaInP LEDs by OMVPE is control over the incorporation of residual( 剩餘的 ) oxygen.

Oxygen has long been known to bedetrimental( 有害的 ) to PL efficiency and to the performance of AlGaAs-based LEDs and lasers due to the introduction of deep-level states that may act as nonradiative recombination centers.

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6.Summary

The progress in the growth of AlGaInP alloys using OMVPE has made possible the commercialization of new high-brightness visible LEDs.

These AlGaInP green, yellow, orange and red LEDs, with their high luminous performance, provide a viable alternative in applications that have traditionally employed incandescent lamps.