Light Emitting Diodes (LED’s) are solid state devices that are used for their light emission. LED’s work on the principle called electroluminescence. Electroluminescence is the process of electron and hole recombination through the application of an electric fields.

When forward biasing a LED using a battery (or any other direct current source), electrons and holes are injected into the system from n-type and p-type layer, respectively. The electrons and holes recombine radiatively in the active layer, thereby emitting photons. The resulting photon has an energy approximately equal to the band gap of the material. Modifying the band gap of the material creates photons of different energies.[1]

Fig.1 Illustration for a basic p-n junction LED [1]
Fig.2: A schematic structure of a p-n junction LED. [2]

BlueLED’s: Requirements & Challenges

In order to get good quality blue LED’s for applications it is essential to grow high-quality single crystals, for blue light we need materials that has band gap higher than the materials we found in nature and successfully produce p-n junctions. And we have 2 requirements for a successful blue LED. [2]

Requirement 1: it is essential to use semiconductors with an Eg of approximately 2.6 eV or larger, equivalent to a wavelength of 480 nm or shorter (blue light). Semiconductors that have such a large Eg are referred to as “wide-band gap semiconductors.” In contrast, the Eg for the most commonly used semiconductor, silicon, is 1.1 eV.

Requirement 2: it is advantageous to use direct band gap semiconductors in which the momentum of electrons at the bottom of the conduction band is almost equal to that of holes at the top of the ground state valance band, as shown in Fig. 1, yielding a high radiative recombination probability. In contrast, indirect band gap semiconductors exhibit a lower radiative recombination probability, because the momentum of these electrons and holes is different.

Fig.3: Band structures of GaN (direct transition type) and Si (indirect transition type). [2]

Candidate Materials: ZnSe vs GaN:

Fig.4: Table for comparison between ZnSe and GaN [2]

When we compare the candidate materials ZnSe can be seen as a obvious choice. ZnSe emits bright light under excitation by an electron beam. Good-quality single-crystal ZnSe film can be grown using vapor-phase epitaxial growth on GaAs single-crystal substrates because the lattice constants are very similar. Thus, many researchers had been working on ZnSe, aiming to develop blue-light-emitting devices. But Akasaki worried about the instability of ZnSe due to its low cohesive energy (bonding energy), and its poor crystallinity because of the low growth temperature required and start to work on GaN based devices.[2]

GaN Growth on Sapphire and The First Blue/UV LED

Isamu Akasaki and his student Hiroshi Amano were the very few researcher who saw the potential of GaN. Main challenge to achieve blue LED with GaN is to make clean and reproducible crystal growths. Back in 1980’s there are 3 main techniques: 1) Molecular Beam Epitaxy, 2) Hydride Vapor Phase Epitaxy, 3) Metal Organic Phase Epitaxy. They are all techniques for the layer by layer controlled growth of materials. Main chemical reaction and disadvantages can be seen in the Fig. 5.

Fig. 5: Main growth methods for GaN.[2]

Akasaki and Amano choose to work with MOVPE technique and made modifications on the reactor. They growth GaN with inclined substrates and they tune the gas flow rate to the reactor. Amano invented a Low Tempurature Buffer which enabling the homogenous growths (Figures 6-7)

  Fig.6: Schematic drawings of the reactor part of the MOVPE system before and after the reactor design was changed.[2]
 Fig.7: Scanning electron microscopic images of GaN on a sapphire (0001) substrate (a) without and (b)with a low-temperature- deposited AlN buffer layer. [3]

Using the high quality GaN crystals Akasaki and Amano managed to create first p-n junction blue/Uv LED.

Fig. 8:  (a) GaN p-n junction Blue/UV LEDs (black dots). An electric current is only being passed through the centered LED that is emitting blue light. (b) I-V characteristics of p-n junction LED (left) and MIS LED (right). (c) EL spectra of GaN p-n junction blue/UV LED.[2]

Growth of InGaN and Double Heterostructure (DH) BLUE LED’s

From the work of Akasaki and Amano we got our first blue LED but it is not actually open to industrial usage since it’s not so efficient and not only emits light  in blue color. Therefore we need to way to increase efficiency and tuning the color. Work of Shuji Nakamura in the beginning of 1990’s provide all the improvements we need for the commercial blue LED’s.

Nakamura realized that in order to increase the efficiency he should use DH LED’s which has a higher internal quantum efficieny than the standard homojunction LED’s. In DH LED’s there is an active layer where emission happens and for blue light  emission Nakamura choose a InGaN. To grow high quality InGaN DH LED’S Nakamura invent a technique, which he called two flow Metal Organic Chemical Vapor Deposition (MOCVD).The main breakthrough of this technique was the introduction of a subflow which gently pushed the carrier gases down to the substrate, thereby also improving the thermal boundary layer,[4]. Hydrogen (H+) is source of passivation of p-type GaN As grown MOCVD GaN contains significant hydrogen concentrations. By thermal Annealing in H+ free environment Nakamura achieved; p-type GaN and industrial process compatible,[5-6].By controlling the ratio of In ın the crystal he could tuned the color of LED,[7]. Following this improvements, in 1994 Nakamura published the first breakthrough DH Blue LED device with InGaN,[8].

 

Fig.9: Structural differences between HJ LED and DH LED,[1]

Fig.10: Two flow MOCVD reactor design,[4]

Fig.11: Resistivity vs temperature plot for first p type GaN with thermal annealing in Hydrogen free environment, [5-6]
Fig.12: Tuning the color with different concentrations of In in a) and b), [8]

Fig.13: Structure and Output power vs Forward current graph of first breakthrough device in 1994,[8]

Timeline for BLUE LED Research

Figure 14-15 shows the timeline and improvements in the efficiency in the LED technologhy.

Fig.15: Improvements in LED technology,[1]

Applications of BLUE LED’s

Most important application of blue LED’s is to make an efficient white light source. Using the white light we use this LED’s in solid state lightning, decorative and indoor lightning, automobile lightning, displays of smart devices and agriculture.

Blue LED’s are also important for the energy saving, according to US department of energy:~ 40 % Electricity Savings (261 TWh) in USA in 2030 due to LEDs Eliminates the need for 30+ 1000 MW Power Plants by 2030 Avoids Generating ~ 185 million tons of CO2’s,[1].

Fig.16: Ways of creating white light,[9]

2014 NOBEL Prize in Physics

The NOBEL Prize in Physics 2014 was awarded jointly to Isamu Akasaki, Hirsoshi Amano and Shuji Nakamura “for invention of efficient blue light-emitting diode which has enabled bright and energy saving white light sources”.

References:

  1. Shuji Nakamura – Nobel Lecture. NobelPrize.org. Nobel Media AB 2019. Tue. 24 Dec 2019. <https://www.nobelprize.org/prizes/physics/2014/nakamura/lecture/&gt;
  2. Isamu Akasaki – Nobel Lecture. NobelPrize.org. Nobel Media AB 2019. Wed. 25 Dec 2019. <https://www.nobelprize.org/prizes/physics/2014/akasaki/lecture/&gt;
  3. Hiroshi Amano – Nobel Lecture. NobelPrize.org. Nobel Media AB 2019. Tue. 24 Dec 2019. <https://www.nobelprize.org/prizes/physics/2014/amano/lecture/&gt;
  4. S. Nakamura et al., Appl. Phys. Lett., 58 (1991) 2021—2023
  5. S. Nakamura et al., Jpn. J. Appl. Phys., 31 (1992) L139—L142
  6. S. Nakamura et al., Jpn. J. Appl. Phys., 31 (1992) 1258—1266
  7. S. Nakamura et al., Jpn. J. Appl. Phys., 31 (1992) L1457—L1459
  8. S. Nakamura et al., Appl. Phys. Lett., 64 (1994) 1687—1689
  9. http://energy.gov/eere/ssl/led-basics

U. Karadeniz

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s