Summary

In conventional GaAs substrate RCLEDs device summary, AlGaInP-based RCLEDs with high power and high speed were demonstrated. In this investigation, the RCLEDs were designed with different light-output aperture sizes 84, 60, and 40 µm, for different applications of plastic optical fibers. The 40 µm aperture RCLEDs could be achieved that the small-signal modulation bandwidths as high as 310 MHz at a forward current of 20 mA and the output power as high as 1.5 mW. The devices with 84 µm apertures had an output power of more than 3.5 mW at a driving current of 20 mA and a maximum efficiency of over 12 % with an epoxy-encapsulated package. The devices with 40 µm apertures satisfied with the IEEE 1394b s400 standard. Furthermore, the devices showed stable coupling efficiency for various currents and output powers at different ambience temperatures. In addition, the lifetime of devices is over 1300 h, and the power decay is less than 0.5 dB at 85°C for a 40 mA driving current.

Furthermore, we had developed the novel MBRCLEDs devices, having high performances and replacing conventional GaAs substrate with a high thermal conductivity of Si substrate. In 650 nm MBRCLEDs summary, MBRCLEDs has high performances, high temperature insensitively, and high reliability have been successfully fabricated on Si substrates by twice wafer bonding techniques. Although the RCLEDs epi-structure was optimum designed for detuning wavelength, devices are still insufficient for high current injection or high temperature operating. However, MBRCLEDs were based on this structure and using metal bonding technique could improve devices performances especially in high temperature ambiance. The junction temperature variations of the MBRCLEDs were relatively smaller as compared with the RCLEDs. The MBRCLEDs with 84 µm apertures provided high wall-plug efficiency of 13.7 % and a smaller power drooped of -0.31dB from RT to 100ºC due to the better heat dispersion substrate. Furthermore, the stable beam profile,

high reliability over 1000 hours and clearly eye diagram in high temperature operation. These excellent performances of the MBRCLEDs devices should be suitable for high temperature ambiance, high current injection and high data communication applications.

REFERENCE

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Figure 2.1: Attenuation of a PMMA step-index plastic optical fiber.

Figure 2.2: Intensity of the optical mode at the top mirror for an m=1 cavity. While the reflectivity of the bottom DBRs was r22

=1, the reflectivity r1 of the top DBRs was varied.

Figure 2.3: Intensity of the optical mode at the top mirror for an m=1 cavity for various detuning d. The reflectivity of the top DBRs was r₁²=0.6 and the bottom DBRs was r22

=1.

Figure 2.4: The 650nm AlGaInP-based RCLEDs epi-structure.

n-GaAs Sub <111> 3 inch n-GaAs buffer

n- DBRs 35 pairs of Al

0.5

Ga

0.5

As/Al

0.92

Ga

0.08

As Si-doped and doping concentration

is 3×10-18 cm-3 (R>98%)

Active layer is three +1% strain un-doping Ga

0.5

In

0.5

P quantum wells of 1-λ cavity thickness and

(Al

0.7

Ga

0.3

)

0.51

In

0.49

P barrier layer

p- DBRs 6 pairs of Al

0.5

Ga

0.5

As/Al

0.92

Ga

0.08

As C-doped

Oxidation Layer Al

_0.98

Ga

_0.02

As p+-GaAs contact layer of highly-doped

(over 5×10-19 cm-3)

p- Cladding Layer Al

0.9

Ga

0.1

As

n- Cladding Layer Al

_0.9

Ga

_0.1

As

p-DBRs

Figure 2.5: Schematic fabrication processes of RCLEDs devices: (1) The Ti/Pt for p-ohmic contact ring were deposited on p⁺-GaAs cap. (2) A thick SiNx about 1 µm was grown on wafer surface for mesa dry etching and protection steam during oxidation process. (3) The circular mesa size of 130 µm was defined using standard photolithography and a chemical etchant. (4) Using ICP etcher for dry-etching until the active layer was exposed. (5) A high resistivity region of AlO_x material was formed through the oxidation process. (6) The thick polyimide was coated on surface for planarization device structure. (7) The polyimide final pattern after curing process. (8) SiNx film was removed for metal line connection.

(9) Deposited Ti/ Pt /Au for p-electrode pad and Au/ Ge for n-ohmic contact.

p-DBRs

Figure 2.6: Schematic diagram of MBRCLEDs fabrication: (1) The conventional RCLEDs structure. (2) The epi-wafer was temporarily bonded to a sapphire substrate using wax in vacuum ambient. (3) The GaAs substrate and etching stop layer was removed by chemical etching. (4) Bonding metal was deposited on Si and epi-layer surface. (5) The MBRCLEDs process was completed.

etching stop layer

Figure 2.7: The OM figures of three different light emission window sizes of 84, 60 and 40 µm.

Figure 2.8: The typical light intensity-current-voltage (L-I-V) characteristics of 84, 60, and 40 µm aperture size RCLEDs.

Figure 2.9: The wall-plug efficiency and output power characteristics of 84 µm devices with epoxy encapsulated.

Power (mW)

Wall-plug Efficiency (%)

Current (mA)

Figure 2.10: The output power variation versus detuning wavelength.

Figure 2.11: The coupling efficiency versus wide driving current into 0.5NA POF.

Figure 2.12: The cut-off frequency -3dB (f_-3dB) of these three different devices operated at 20

Figure 2.13: The eye diagram of 40 µm RCLEDs could achieve 622 Mbit/s.

Figure 2.14: The eye diagram for the transceiver using RCLEDs devices with single QW transmitting 500 Mbit/s data rate through graded-index POF over 50 m.

Figure 2.15: (1) The SEM figure of the MBRCLEDs chip profile. (2) The MBRCLEDs device EL state and light intensity distribution under 20 mA current injection.

(1)

(2)

Figure 2.16: The typical intensity-current (L-I) characteristics of the MBRCLEDs and the RCLEDs versus temperature variations.

0 1 2 3 4 5 6 7 8

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

MBRCLEDs_RT RCLEDs_RT

MBRCLEDs_60°C RCLEDs_60°C

MBRCLEDs_100°C RCLEDs_100°C

P o w e r (m W )

Current (mA)

Figure 2.17: The wall plug efficiency as a function of injection current for the MBRCLEDs and the RCLEDs at RT.

0 2 4 6 8 10 12 14 16

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 MBRCLEDs

RCLEDs

∆ efficiency: 1.2 %

∆ efficiency: 1.9 %

Current (mA)

W al l- p lu g E ff ic ie n cy ( % )

Figure 2.18: The Junction temperature increase (∆Tj) and forward voltage (Vf) as a function

Figure 2.19: The far-field patterns of the MBRCLEDs devices under 20 mA current injection versus ambiance temperature of (1) 25°C, and (2) 85°C.

100 75 50 25% 0% 25 50 75% 100

% 60

90°

- 30° 30

0°

- 60°

-

90 30

100 75 50 25 0% 25 50 75 100 60

-

0°

- 30°

-

(1) 25°C

(2) 80°C

MBRCLED-1 MBRCLED-2 MBRCLED-3

RCLED-1 RCLED-2 RCLED-3

0

Time (hour)

1000 800

600 200 400

20 40 60 100

80 120

P o w e r D e c a y R a ti o ( % )

Figure 2.20: Reliability results of the MBRCLEDs and the RCLEDs under a condition of 85°C and 20 mA driving current for life test.

Figure 2.21: (1) 25 °C and (2) 80 °C eye diagram of the MBRCLEDs under 20 mA driving current. The scale in the figure is 2.5 ns/div.

(1) RT

2.5ns/div

2.5ns/div

85 °°°° C

(2)

CHAPTER 3

Light Extraction Enhancement of AlGaInP-Based Light-Emitting Diodes in Wafer Bonding Technique

3.1 Introduction

The III-phosphide material system, (AlxGa1-x)1-yInyP, is lattice matched to GaAs substrate for y=0.48. The (Al_xGa_1-x)_1-yIn_yP-based alloy material with wide commercial availability of high quality GaAs substrate allows for relatively straightforward epitaxial growth of optoelectric devices in this material system, with high quality epi-wafer was demonstrated by metal-organic chemical vapour deposition (MOCVD) as well as molecular-beam epitxay (MBE) method (MBE). The (AlxGa1-x)1-yInyP-based alloy materials with the light output spectrum from the red to yellow-green visible part were made available. Direct bandgap emission is available from x=0 (InGaP) with a ~1.9 eV bandgap and ~650 nm (deep red) to x=0.53 with a ~2.2 eV and ~560 nm (yllow-green). Recently, the high performance

AlGaInP-based LEDs were widely used for many applications, such as optical communications, automobile tail light, traffic light, full-color display, interior and exterior display [1], [2]. The AlGaInP-based materials have been continuously improved for many years since the first practical LED was developed in 1962 made of the GaAsP material [3], and this allow material system becomes the most major material for high brightness and high efficiency light-emitting diodes (LEDs) applications. In recent years, the internal quantum efficiency IQE (ηi) AlGaInP-LEDs has closely approached 100% due to the excellent epitaxy technique [4], [5]. Although AlGaInP LEDs has higher internal quantum efficiency than conventional GaN-based LEDs grown on c-plane sapphire substrates, the light extraction efficiency is estimated to be less than the GaN-based LEDs due to the large refractive index

law, the critical angle (θ_c) is approximately 18.2˚ and the most of the generated photons will be trapped in the semiconductor due to the total internal reflection (TIR) effect. In addition, the conventional AlGaInP-LEDs have an absorbing substrate and a planar light emitting surface. These effects will serious limit the external quantum efficiency especially in AlGaInP-based material. Therefore, several methods to solve the light extraction of conventional LEDs structure were invented, such as an ultra-thick GaP window layer was grown on the surface [6], replaced the GaAs absorbing substrate (AS) with a GaP transparent substrate (TS) through a directly wafer bonding technique [7], the truncated-inverted-pyramid (TIP) geometry LEDs method to enhance the lateral light output [8], a surface-textured by natural lithography method to increase the critical angle and the probability of emitted light escape from air-semiconductor interface [9], a triangle-like morphology roughness was applied on the surface [10], a surface was deposited a transparent film of CTO (cadmium-tin oxide) or ITO (indium-tin oxide) for enhancing the critical angle and as a function of a current spreading layer [11], a transparent, conductive and lower refractive index film was deposited on device surface [12], a novel technique of omni-directional reflector (ODR) structure could achieve high light extraction efficiency [13], [14], and the absorbing GaAs substrate was replaced with a transparent sapphire substrate which has a geometric shaping sidewall [15], [16]. In this chapter, we presented several methods for enhancing light extraction. Firstly, we used glued bonding to replace the absorbing GaAs substrate with a transparent sapphire substrate, which has a geometric sapphire substrate, for enhancing total output power from devices sidewall. And then, we produced the flip-chip form under this devices structure for solving the poor thermal dispersion sapphire substrate. Finally, we produced surface roughness using micro- and nano-scale surface textured for enhancing light extraction efficiency.

3.2 Enhancing Light Extraction Efficiency of AlGaInP-Based LEDs in Glue Bonding 3.2.1 AlGaInP-Based LEDs with a Geometric Sapphire Substrate

In this study, the AlGaInP-based LEDs with a dominant wavelength (λd) at 585 nm were grown on 2-inch GaAs substrates by a low pressure metal–organic chemical vapor deposition (MOCVD) system. The epi-wafer structure consisted of a 0.08 µm-thick n-Ga0.5In0.5P etching stop layer grown on a GaAs buffer layer, a 1 µm-thick Si doped n-(Al_0.7Ga_0.3)_0.5In_0.5P, a 0.7 µm-thick Si doped n-Al0.5In0.5P cladding layer, a 0.5 µm-thick unintentionally doped active layer with 20 periods (Al0.7Ga0.3)0.5In0.5P/ (AlxGa1-x)0.5In0.5P multiple quantum wells (MQWs), a 0.7 µm-thick Mg doped p-Al0.5In0.5P cladding layer, a 5 µm-thick Mg doped p-GaP window layer. Finally, a 5 nm-thick heavily doped (p = 1×10^-19 cm^-3) p⁺-GaP contact layer was grown on the epi-layer surface for improving the ohmic contact.

In chip process, a c-plane sapphire substrate was lapped and polished from 450 µm thickness to 220 µm before glue bonding process. A 2 µm-thick SiO2 was deposited onto the front side and backside sapphire substrate via plasma enhance chemical vapor deposition (PECVD), as shown in figure 3.1 (1) and (2) shows that the backside SiO2 film was defined pattern with 1000 µm×1000 µm using a standard photolithography as a function of the wet-etching hard mask. The patterned sapphire substrate was then immersed into a 3H2SO4:1H3PO4 solution at an etching temperature of 340°C for 40 min, and then a patterned array sapphire substrate, which has an oblique sidewall was formed, as shown in figure 3.1 (3).

The etching rate of the sapphire substrate is closely achieved 1.4 µm per minute in this study.

The etching rate depended on the H₃PO₄ concentration and the temperature of etching solution. A 280 nm-thick ITO film as a function of ohmic contact layer was deposited on p⁺-GaP surface by e-beam evaporation. Figure 3.1 (4) shows that the epi-wafer was flipped and bonded to a sapphire substrate using commercially available epoxy glue, and the wafer

bonding process, the absorbing GaAs substrate and the etching stop layer were removed by chemical etching solution of NH4OH based and H3PO4: HCl respectively, as shown in figure 3.1 (5). In surface roughed texture processes, thin metal layers of Au/ AuGe (300Å/ 200Å) were deposited on n-AlGaInP layer surface firstly as shown in figure 3.1 (6). The wafer was alloyed by RTA (rapid thermal annealing) at 450°C for 1 min in nitrogen ambiance. Figure 3.1 (7) shows that the densely, strongly and naturally particles with nano-scale were clustered on surface to serve as the wet etching mask. The wafer was immerged into chemical solutions of KI and H3PO4 respectively. After etching processes, the n-side up surface with a nano-roughed texture was formed shows in figure 3.1 (8). Then, a regular array of Au/ AuGe n-contact micro-dots shape metal was deposited on the roughed surface. A 280 nm-thick ITO was deposited on the surface as functions of current spreading, transparent conductive layer and lower reflective index window layer. The GSS-LEDs with a chip size of 1000 µm×1000 µm were fabricated using standard photolithography processes which were aligned with backside shaping pattern of sapphire substrate. The devices mesa etching using an etcher of inductively coupled plasma (ICP) until p-side ITO layer was exposed for p-electrode formation, as shows in figure 3.1 (9). Figure 3.1 (10) shows the Ti/ Pt/ Au (100 nm/50 nm/2500 nm) metals were then deposited for the p- and n-electrode pads. Finally, the geometric sapphire shaping wafer was subjected to laser scribed and broken into 1000 µm×1000 µm chips size in this study, as shown in figure 3.1 (11). The chip was bonded on ceramic of PLCC 5050 package model for electrical and optical properties measurements by CAS140CT-152 array spectra-meter system.

A schematic diagram of the AlGaInP-based GSS-LEDs structure with an oblique sidewall substrate and a sandwich transparent conductive ITO layer was shown in figure 3.2 (1). In this study, the oblique sidewall angle of sapphire substrate was depends on the sapphire crystallography. In figure 3.2 (2) illustrated that the different extracted light path of the

GSS-LEDs and the conventional glue-bonding LEDs (GB-LEDs). As this schematic diagram results, the GSS-LEDs with more opportunities of output light escaped from the oblique sidewall of sapphire substrate. Figure 3.3 shows the scanning electron micrograph (SEM) images of sapphire shaping structure (1) cross-section and (2) top views. The oblique sidewall with a 100 µm etching depth could be obviously observed. The crystallography facets were (1102) R-plane, (1010) M-plane, and (1120) A-plane against the (0001) c-axis and their angles against the (0001) c-axis are about 60◦, 50◦ and 29◦, respectively. In this study, the sapphire was etched for 70 min via the etching rate of about 1.4 µm/ min and the etching depth was about 100µm. Furthermore, the etching structures are all V-grooves. Figure 3.3 (3) is a sample schematic illustration of the phenomenon mentioned above. According to this figure, the total area of (0001) C-plane decreases as the etching time increases due to its relatively fast etching rate. On the other hand, the oblique surface of the (1102) crystallography etched facet increases with the etching time resulting in the formation of a V-shape groove. Figure 3.5 (1) and (2) shows the photomicrographs of the GB-LEDs and the GSS-LEDs, respectively. These two kinds device structures were replaced the conventional GaAs substrate with a transparent sapphire substrate. The radiate output light is not only surface emitter but a volume emitter.

Besides, it is significantly that the oblique sidewall of GSS-LEDs (2) appears higher brightness under 70 mA current injections as compared with the GB-LEDs (1). As this result, the light extraction efficiency was enhanced via oblique sapphire geometry. The corresponding luminous intensity-current-voltage (L-I-V) characteristics of GB-LEDs and GSS-LEDs were measured respectively, as shown in figure 3.6. The forward voltage (at 350 mA) of the GB-LEDs and the GSS-LEDs is 3.30 V and 3.28 V, respectively. Both of the electrical behaviors are very closely and normally. In luminous intensity versus current result, it is clearly observed that the luminous intensity of the GSS-LEDs is larger than the GB-LEDs.

approximately 40.8 mW and 51.7 mW, respectively. It is found that the luminous intensity of the GSS-LEDs without an epoxy lens encapsulated could be enhanced about 26.7% under 350 mA current injections as compared with the GB-LEDs. It is indicated that the oblique sidewall could reduces the total internal reflection and improves the light extraction probability of photons escaping from semiconductor to air. Figure 3.7 shows the normalized output beam pattern of the GB-LEDs and the GSS-LEDs sample under 70 mA current injections, respectively. In this compared figure, it is clearly noted that the 50% power angle of the GSS-LEDs is wider approximately 12.5º as compared with the GB-LEDs. It is afresh indicated that the enhancements of the 50% power angle and output power could be attributed to the geometric sapphire shaping LEDs with an oblique sidewall.

In sumary, the GSS-LEDs with an oblique sapphire geometric substrate were fabricated via glue bonding. In this evolutional GSS-LEDs performances, the light output power could be enhanced 26.7% under 350 mA current injectionsas compared with GB-LEDs.

Furthermore, it was demonstrated that the GSS-LEDs structure could not only reduce the TIR effect but increase more probabilities of output light escaping from the transparent substrate with an oblique sidewall.

3.2.2 AlGaInP-Based Flip-Chip LEDs with a Thick Geometric Sapphire Substrate Window Layer

A novel GSS-LEDs which has higher light extraction efficiency than conventional GB-LEDs was demonstrated in last section. However, as we know the sapphire is a poor thermal dispersion material, and it is still not suitable for high current injection or high temperature ambience operating especially in short wavelength of AlGaInP system material.

The high temperature and high current density will cause the carriers overflow in shallow quantum well, and this phenomenon will result in poor electron-hole recombination efficiency.

In this section, a novel flip-chip AlGaInP-LEDs structure which has a thick geometric sapphire substrate (GSSFC-LEDs) window layer was demonstrated using glue bonding and flip-chip bonding techniques. The flip-chip LEDs have stable properties in high temperature operation and high current density injection due to the p-n junction is closely to heat dispersion sub-mount. We continue using the geometric sapphire substrate technique in last section, and then combine geometric sapphire substrate with AlGaInP epi-wafer. This geometric sapphire substrate could be a thin window layer as a function of enhancing light extraction. Besides, the flip-chip LEDs could provide an excellent performance in high temperature ambience and high current injection.

The epi-wafer structure of layer by layer is the same as the GSS-LEDs, which has a dominant wavelength (λ_d) at 585 nm, were grown on 2-inch GaAs substrates by a low pressure metal–organic chemical vapor deposition (MOCVD) system. In figure 3.8 shows the chip processes flowcharts. Firstly, a c-plane sapphire substrate was lapped and polished from 450 µm to 250 µm thickness. After that, a 2.5 µm-thick SiO2 film as a function of the wet-etching hard mask deposited onto the backside of sapphire substrate via plasma enhanced

The epi-wafer structure of layer by layer is the same as the GSS-LEDs, which has a dominant wavelength (λ_d) at 585 nm, were grown on 2-inch GaAs substrates by a low pressure metal–organic chemical vapor deposition (MOCVD) system. In figure 3.8 shows the chip processes flowcharts. Firstly, a c-plane sapphire substrate was lapped and polished from 450 µm to 250 µm thickness. After that, a 2.5 µm-thick SiO2 film as a function of the wet-etching hard mask deposited onto the backside of sapphire substrate via plasma enhanced

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