GaN Based Micro-Led-Fabrication, Advancements, and Applications

Brandon Mai Nguyen Undergrauduate Student

University of California, Irvine Brandmn1@uci.edu

Abstract—In this paper, we will be discussing the structure, fabrication process, advancements, and applications of GaN based - µLEDs.

Keywords—GaN, µLEDs, Diodes, fabrication, structure

I. INTRODUCTION In todays advancements, Gallium Nitride (GaN) - based

micro-light-emmiting diode (µLED) is a new emerging technology that will replace conventional LEDs and its applications. µLED was initially invented in 2000 by Professor Jiang and Prof. Jingyu Lin at Kansas State University. They continued their research in this field at Texas Tech University [5]. Following the release of the results of their research, many other groups decided to get involved this µLED advancement. Unlike conventional LEDs, µLED have a diameter of 12µm and are mainly fabricated from InGaN/HaN quantum wells. In comparison to LEDs, these µLEDs have proven to have lower latency, higher light ouput power, lower voltage usage, and is smaller in size. These benefits enables µLEDs to be used in fields such as optoelectronics, imaging, and spectroscopic applications. In this paper, we will first discuss the fundamental structure and fabrication of conventional LEDs. This will help enable us to have a better fundamental understanding of µLEDs: structure, fabrication process (both structural and array), experimental results, advancements, and its applications.

II. LIGHT-EMITTING-DIODES Structure, Fabrication, and the use of GaN in conventional

LEDs are discussed.

A. What is a LED? A Light-Emitting-Diode (LED) is a semiconductor

diode with a PN Junction. The p-side contains holes while the n-side contains electrons. When a voltage source is applied to the diode, the electrons from the n-side moves towards the p-side and the holes from the p-side moves towards the n-side. This process occurs near the junction. As holes and electrons collide with each other (recombine), it releases energy (photons) in the form of light that is emitted by the LED as seen in Figure 1 [16]. In most LEDs, it is most common to use Aluminum Gallium Indium Phosphide (AlGaInP or AlInGaP) and Indium gallium Nitride (InGaN).

Figure 1: Recombination occurring at the PN Junction The semiconducting material and the energy band-gap of the respective element determines the color of the LED [16]. Different materials have distinct band-gap energies that correlate with their respective wavelength and emission color. This correlation can be seen in the equation below as well as in Figure 2:

!"#$%&#' = ℎ+,

E: Energy (bandgap) h: Planck’s Constant c: Speed of Light ,: Wavelength

Figure 2: The difference in the emission color by the semiconductor.

In Figure 2, AlGaAs band-gap energy creates a red emission color. AlGaInP and InGaP can also be an alternative material for red, and can also be used to create orange, and yellow LEDs. InGaN is used for green, blue, and white LEDs. The result of green and blue is determined by the ratio of In/Ga. Slight changes in the composition of these alloys’ ratios in relation to its band-gap, lattice constant, and wavelength can change the color of the emitted light [16]. These ratios can be determined in Figure 3.

Figure 3: Band-gap energy and corresponding wavelength versus lattice constant of (AlxGal-x)yln1-yP at 300 K. The dashed vertical line shows (AlxGa1-x)0.5ln0.5P lattice matched to GaAs

B. Basic Structure of LEDs In general, the structure of an LED comprised of a

compound semiconductor epitaxial film grown on top of a suitable substrate. The epitaxial structure can either be grown on a n-type GaAs substrate for Red LEDs or Al2O3 (sapphire) for Blue/Green LEDs. The basic structure of a LED consists of the substrate, followed by a n-type cladding, active, p-type cladding, Silicon Oxide (SiO2) layer, and lastly a metal contact finish as shown in Figure 4. The material in the active layer determines the color. Conventional LED structures differ in the materials used in each of the layers, but fundamentally have the same structure and organizations the for any LED color.

Figure 4: LED Device Structure of InGaP Red and InGaN Blue/Green LEDs

C. Fabrication Process of LEDs There are multiple fabrication techniques for LEDs. Here are a few common fabrication methods:

• Metal Organic Chemical Vapor Deposition (MOCVD) – (vapor transport) is a technique for depositing thin layers of atoms onto a semiconductor wafer. It uses atoms of the respective compound in combination with complex organic gas molecules [12]. Ultra pure gases are injected into a reactor and are finely dosed onto a semiconductor wafer. This technique would be applied in fabricating the epitaxial structure with a precise controlled layer thickness. With this level of capability, devices are able to achieve specific optical and electrical characteristics [12].

• Metal Organic Vapor Phase Epitaxy (MOVPE) – is an epitaxial growth technique. The single crystalline thin film layers are produced by the reaction of molecules in the gas phase on a heated substrate. The growth process is suitable for the deposition of several layers with a thickness of 10µm each [12].

• Organometallic Vapor Phase Epitaxy (OMVPE) – is a technique that involves chemically reacting flow of mixture of organometallic, hydride, and carrier-gas precursors that is delivered via using a rotating disk reactor [14]. As a result, the process creates thin accurate layers in the epitaxial structure. The technique is similar to MOCVD.

• Molecular beam epitaxy (MBE) – (Beam Implantation) is a process for growing thin, epitaxial films from a wide variety of materials [11]. To create these layers, it starts with a substrate base and then fires precise beams of atoms onto the substrate from “guns” called effusion cells. For each different substrate, layer, or molecule, it needs a separate gun for each individual beam [11]. As the molecules land on the surface of the substrate, it condenses then builds up gradually to form ultra-thin layers [11].

D. Importantance of GaN and Blue LEDs and Fabrication Gallium Nitride (GaN) plays a vital role in creating

conventional blue LEDs. It paves a strong foundation and creates the fundamental back bone that is essential in the invention of µLEDs, and its advancements. Common fabrication techniques for blue GaN LEDs are MOVPE and MOCVD [3][7]. Given that the substrate material, GaN, was a difficult task to grow, it took multiple trials and required a new developed epitaxial growth method [15].

The general fabrication process using MOVCVD is as follows: • The epitaxy structure is grown on a sapphire,

transparent (Al203) substrate [4]. • Sapphire is inserted into the chamber and follows

cleaning sanitary process in its preparation for epitaxy growth [4].

• After cleaning process is completed, a GaN buffer layer of about 30 nm is then deposited to the substrate at 525 C [4].

• The temperature is then later elevated 1020 C to grow a 1 micrometer thick GaN layer and then a 1 micrometer thick Si-Doped n-type GaN later [4].

• Temperature then decreases to 715 C to grow the InGaN well layer [4].

• Temperature then increase to 840 degrees to grow the GaN barrier layer [4].

• The InGaN/GaN MQW active region is grown and consists of three pairs of 3nm thick In.04Ga.6N-well layers and 10nm thick GaN barrier layers [4].

• After active region growth, a 30nm GaN cap layer was grown on the multi-quantum well [4]

• Substrate temperature elevates to 727 C to grow a 250 nm thick Mg-doped p-type GaN layer [4].

Figure 5: Structure of fabricated blue LED

The importance of GaN and blue LEDs is that it can

be rendered into white light. This white light can be implicated when a solution of yellow phosphor and epoxy is prepared and then applied on the surface of the blue LED chip. The concentration of the powder phosphor determines the type of white lighting: cool or white light. Other colors can be constructed using different ratios of phosphor [15].

III. GALIUM NITRIDE µ-LIGHT-EMITTING-DIODE (µLED) Through the invention of GaN based blue LEDs, this led to

the replacement of older and inefficient light sources [2]. It has proven to be of importance in chip formats as indicators and in solid-state lighting. These GaN based LEDs can also be fabricated into arrays of µscale elements with lateral

dimensions of less than 100um. Therefore, it has earned the name µLED [3].

IV. THEORETICAL BACKGROUND (HOW µLED WORKS)

A. Single element structure of microled A single µLED elements works the same way as a

convention LED element. A conventional µLED element structure consists of a PN Junction with semiconductor band-gap materials. This PN junction structure is also known as the epitaxy structure. In similarity to conventional LEDs, colored light is created through a forward biased electron flow from the conduction band and recombined with holes from the valence band. This causes an emission of photons at the PN junction. In most cases, these µLED elements are fabricated on a common n-type substrate. The components of these single µLED elements are composed of a buffer layer, a n-type GaN layer, a MQW layer, and a p-type GaN layer [3]. The main factor that sets it apart is its small micro-size. Because of this, these µLED elements, consumes less energy and inherits other beneficial characteristics. A single µLED element is typically 20 µm in diameter with a parabolic shape. This results in high light intensity, a wide wavelength range, temperature stability, and flexibility as seen in figure 6a [6]. The epitaxy structure contains a substrate, buffer layer, n-type GaN layer, MQW layer, and a p-type GaN layer as seen in figure 6b.

Figure 6a: The structure of a single µLED element

Figure 6b: Epi-structure µLED elements

B. Conventional Array Structure for µLEDs In a standard µLEDs array structure, the µLED epitaxy

structure shares a common n-electrode with individual addressable p-electrode as seen in figure 7 [5].

Figure 7. µLED structure with a common n-electrode with addressable p-electrodes. The n-type GaN layer acts as a shared conductive path for all µLED elements in the array [1]. The array contains a insulating substrate, a buffer layer, a n-type layer, and the µLED’s epitaxy structure. The µLED array matrix is organized by rows and columns. Similar to LED arrays, they can have either a common row anode or a common row cathode as shown in Figure 8. Each element would have a unique address. These elements light up when a control device addresses these specific elements in the matrix [2]. The method is similar to conventional LED matrix but in a much smaller aspect.

Figure 8: An example of a simple 4 x 4 array matrix. Left – common row anode. Right – common row cathode.

C. Driving Schemee of a Micro-Led Array. In a µLED display, there are arrays that contains hundreds

and thousands of µLED elements. It would be unconventional to have individual controls signals for each of these elements. To resolve this issue, these µLED elements are linked together by rows and columns. This can either be in a passive or active matrix mode as seen in Figure 9 [6].

Figure 9: Active and Passive matrix schemes.

An active matrix driving scheme is used to achieve high resolution and performance [9]. The most commonly used active matrix driving circuit consist of two transistors and one capacitor as shown in figure 10 [6]. An NMOS active matrix has a common p-electrode and individual n-electrodes. In an active matrix driving scheme, it is more beneficial to use an NMOS active matrix circuit for it’s performance in high speed applications [6].

Figure 10: Active matrix pixel driver in Left -PMOS and Right - NMOS configurations

Figure 11: Driving principle of µLED arrays Depending on what type of scheme is used, the respective combination logics would be used to control each of these elements. This is seen in figure 11 [2].

V. ADVANCEMENTS IN MICROLED ARRAY STRUCTURE

A. Improvements and Advances in MicroLed Array structure

using addressable n-electrodes and shared p-electrodes.

The configuration of general µLED arrays has a common n-electrode with individually addressable p-electrodes for each µLED element. This is seen in Figure 7. The relevant LED drivers are necessarily based on a PMOS transistors due to this configuration. This configuration is prone to have lower operating speed, larger size, and larger capacitance than their n-type equivalents [1]. Due to the shared n-electrode layer, the different distances between each µLED elements has different series resistance [1]. This causes a non-uniform operating current when the same voltage is applied to each element.

To resolve the drawbacks of conventional µLED configurations, a research was done by the University of Strathclyde, Glasgow, U.K that proposes an array to have addressable n-electrodes with share a common p-electrode as seen in figure 12 [1]. In comparison to conventional µLED, this configuration would minimize the series resistance difference from conductive paths while also offering compatibility with NMOS based LED drivers [1].

Figure 12: Shared p-electrodes with addressable n-

electrodes.

B. Fabrication process of addressable n-electrodes The µLED arrays developed in this research were fabricated

from a commercial blue GaN-based LED wafer on a c-plane (0001) sapphire substrate with periodically patterned surface [1]. In this research, the LED epitaxial structure has a:

• 3.4 µm thick undoped GaN buffer layer • 2.6 µm thick n-type GaN layer • eleven periods of InGaN (2.8 nm)/GaN (13.5 nm)

quantum wells emitting at 450 nm • 30 nm thick p-type AlGaN electron blocking layer • 160 nm thick p-type GaN topmost layer.

Each element on the shared p-electrode µLED 6x6 array has a diameter of 24µm on a 300µm centre-to-centre pitch [1]. To have individually addressed n-electrodes, each µLED needs to be fully isolated from both the p and n type GaN layers. This array configuration with individually addressed n-electrodes is achieved by two steps of Cl2-based plasma etching. This fabrication process is shown in figure 13 [1].

Figure 13: Fabrication Process of shared p-electrode and addressable n-electrodes.

a) Sapphire substrate and n-type GaN layer [1]. b) The GaN mesas are etched down to the sapphire

substrate [1]. c) A µLED element is created at the centre of each mesa

which stops at the n-type GaN [1]. d) Annealed Pd metal layer is used as a metal contact to

p-type GaN [1]. e) The metallization on the isolated n-type GaN mesa is

realized by sputtering a Ti/Au metal bilayer [1]. f) After isolating each µLED element by a SiO2 layer,

another Ti/Au metal bilayer is used to interconnect µLED elements forming a shared p-electrode [1].

Compared with conventional µLED array designs, the conductive paths are formed by Ti/Au metal bilayers rather than the n-type GaN layer. As a result, the series resistance differences between each element are reduced, owing to the significant lower sheet resistivity of the metal bilayer [1].

Figure 14. Schematic Layout of µLED array.

C. Results in comparison with conventional µLED arrays

In this addressable n-electrode µLED array, a single µLED element was able to produce a higher current and higher optical power before thermal rollover as seen in figure 15.

Figure 15: I-V and Optical Power Curve

Due to increased current density, differential carrier lifetime

is shortened, thus increasing the modulation bandwidth (the max rate of change in the output frequency) of the single µLED element [1].The modulation bandwidth produced by this µLED element and its array configuration yields about 440MHz with a corresponding current of 60 mA as seen in Figure 16. This is 12 times higher than the modulation bandwidth of a conventional LEDs [1].

Figure 16: Current vs E-O modulation bandwidth

The high current output, optical power output, and modulation bandwidth is a result from the Pd to metal contacts to p-type GaN as well as the array configuration.

VI. ATTRACTIVE FEATURES OF µLED

µLED has attractive features and many application possibilities with the most common being display applications.

A. Attractive Features • GaN based µLED can indicate conventional chip

formats. It can be used in Solid State Lighting [1].

• GaN based µLED can configure into arrays with small lateral distances [5].

• These µLEDs have both higher operating current densities and optical power densities, which can be applied in micro-displays, projection, and optoelectronic [9].

• Higher modulation bandwidth, as a result from µLEDs arrays, can be used in data communication applications [6].

• When operating µLEDs into an array, in a ganged parallel-addressed fashion, it is possible to obtain a high signal-to-noise ratio, thus longer data transmission distance, while retaining a fast data rate. This makes μLED arrays attractive sources for high-speed visible light communications (VLC) in both polymer waveguide and free-space formats [13].

• µLED displays are thinner in size than current OLED displays. Each element is also self-emitting and does not require backlights [6].

• µLEDs consumes less energy and produces more brightness per watt [10].

B. Applications of µLEDs In today’s display applications LEDs are most

commonly used as an illumination source for displays. However, when building larger high performance displays in a smaller form factor, this new µLED technology is needed to meet the new application requirements [7]. The increased luminance, low energy usage, and efficiency would also unlock new possibilities and developments in projection, high dynamic range, micro-displays, optoelectronics, and augmented /mixed reality. µLED plays an essential factor in display devices. With TV displays getting larger with better resolution, conventional LEDs provide a solution to these advancements but only to a certain extent. For example, when manufacturing high-luminance emissive FHD-resolution LED displays smaller than 70 in this cannot be made from conventional packaged LED pixels [7]. The solution to this would be to implement µLED technology because of its small emission area per pixel [7]. Besides displays applications, µLED would play a role in Visible Light Communication (VLC) [6] an alternative form of wireless transmission due to an increase in the radio frequency spectrum. Since µLED has been tested to yield a higher modulation bandwidth in comparison to conventional LEDs, this would enable VLC to link with higher data rates as well as deployed in displays [6].

STATUS/FUTURE OUTLOOK OF µLEDS

With µLEDs still in its infancy state, there have been more research in finding different µLED array and single element configurations that would bring beneficial results such as addressing n-electrodes with a shared common p-electrode.

Fabricating large amounts of µLED arrays are still a ways to go as many companies are seeking fabrication techniques that would lower manufacturing cost. Companies such as Aixtron, Aledia, Crystalwise Technology, and Apple are producing µLEDs. In a commercial aspect, many companies such as Apple and Samsung are starting to implement µLED technology onto their phone displays and as well as TV displays. For example, Apple has been developing µLED displays for its mobile devices secretly after acquiring a µLED startup company called LuxVue Technology. The development of µLED is motivated by a companies interest in getting ahold of the market. We will soon see µLED technology be implemented into our daily lives sooner than we expect.

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