Figure 4.2 the SEM image of the GaN micro-lens array Figure 4.2 the SEM image of the GaN micro-lens array
CCD
Figure4-3 The probe-station system is used for electronically characteristic measurements
0
Figure4-4 Curves of current-voltage (I-V) of VLEDs with micro lens array and a conventional VLED fabricated from the same wafer.
0 5 10 15 20 25 30 35 40 45
0
Figure4-5 Light output power of VLEDs with micro lens array and a conventional LED as functions of injected current density. The insert shows the enhancement-lens size curves.
Figure4-6 the VLEDs with 3um lens array have injected 0.5 mA and 1.0 mA .
0.5mA 3um Lens 1mA 3um Lens
0
Figure 4-7 The same coverage for the different size lens. Light output power of VLEDs with micro lens array and a conventional LED as functions of injected current density. The insert shows the enhancement-lens size curves.
(units:um)
pad
Boning metal (80% reflection)
MQW
Boning metal (80% reflection)
MQW
Figure 4-8 an irradiance map of GaN VLEDs with micro lens array of 300×300 µm2 simulated from the Trace Pro program
Experiment
Conventional 5um lens
Conventional 5um lens
Conventional 5um lens
Figure 4-9 Light intensity distributed from the surface of detector box that is most next to the top surface of device by Trace Pro.
Figure4-10 (a) the experiment that fixed the space with different size of micro lens compared with simulation results. (b) the experiment that same coverage of micro lens compared with simulation results.
0 5 10 15 20 sample A (2000/45)
Lens size(um) sample A (2000/45)
Lens size(um)
pad
MQW
P-GaN
N-GaN 1.85
0.25 0.05
Units :um)
Figure 4-11 the light cone for photon escaping outsider is the largest as the curvature center of micro –lens is located at MQW
H
Lens height (after etching) 1.61 1.99/ 3.36/ 3.81/
ens height (PR)/
L Lens height (after etching) /0.71 1.16 1.92 2.12 /2.38
Table 4-1 The height of different size of micro lens
Table 4-2 Parameters for the light-tracing simulation of VLED with micro lens array
Chapter5
Fabrication and characteristic of GaN VLEDs with surface roughening
In this chapter, I demonstrate the fabrication of GaN VLEDs with surface roughening, and we will discuss the scanning electron micrographs (SEM) image, the atomic force microscopy (AFM) image, the transmission electron microscopy (TEM) image of N-GaN surface roughening and performances of devices.
5-1 The Fabrication of GaN VLEDs with surface roughening
Figure 5-1 shows the process of the GaN vertical light emitting diodes with surface roughening which began 2-4 “the structure of vertical light emitting diodes”.
At first, because of some treatments such as dry etching for roughening surfaces might cause electrical deterioration , we protected the surface of pad and passivation, the sample was deposited a 0.5um SiNx thin film layer on GaN-based VLED film and followed by photolithography patterning. For n-GaN surface roughening formation, a dry etching method by inductively coupled plasma (ICP) self-aligned dry etching (Samco ICP-RIE). The dry etching was performed in a gas mixture of Cl2/Ar = 30 /5 sccm with an ICP source power of 500 W, a bias power of 0 W, and a chamber pressure of 6 mTorr. The roughenss could be controlled by the etching time. In our experiment, we have three etching times: 1min, 3.5min and 5min for different devices.
The result of etching described detailedly in chapter3.
Then, the sample was deposited a 2um SiO2 film as etching mask and followed by photolithography patterning to define the mesa region of device, and dry etched the mesa of device (300μm × 300μm), Finally, the trilayers of Ti/Pt/Au (20 nm/20 nm/100 nm) for n-type ohmic contact were deposited by an e-gum. The diameter for circle n-type ohmic contact is 100μm, the n-GaN film is 3.85um and MQW film is 0.05um
in our device. The conventional VLEDs with the same mesa size (300 µm × 300 µm) were also fabricated from the same wafer for comparison. Figure 5-2(a) shows plan-view microphotographs of an VLED bonded on a Si substrate before roughening the surface and after roughening the surface is shown in Figure 5-2(b).
The electrical characteristics of VLEDs with surface roughening and conventional VLEDs were both measured by probe station system and evaluated by injecting different current density. The device is driven by Keithley 238CW current source, and its light output from top view could be observed by CCD. Current-voltage (I-V) measurements were performed using the probe station and the data could be fee-backed to the computer from these facilities by a GPIB card. The light output power was measured using a calibrated power meter with a large Si detector (detector area 10×10 mm2) approximate 5 mm above the device, collecting the light emitted in the forward direction.
5-2 Characteristics of GaN VLEDs with surface roughening
Figure 5-3 plots the current-voltage (I-V) characteristics with different etching times for the surface roughening LLO GaN VLED devices. All I-V curves showed linear characteristics up to 50mA. Because of the higher thermal conductivity of Si compared sapphire, these devices are advantageous for high-power operation. The forward voltage at 20mA, VF = 4.44, 4.43, 4.53 and 4.55V for conventional VLED, nano-roughened with1-min-etched, 3.5-min-etched and 5-min-etched, respectively. In this data, we could understand roughening surfaces might cause electrical deterioration in our case.
As shown in Figure 5-4, the output power from the surface of the chip versus dc injection current (L-I) characteristics for the VLEDs with different etching times. All L-I curves showed linear characteristics up to 50mA. The output power at a given
current increased with increasing etching time. As compared with the output power for a flat-surface VLED and the 10-min-etched surface VLED, the roughening treatment resulted in an increase of output power by a factor of 2.1 from the top surface. From other measurements on different etching time devices, the power also showed to enhance 30% and 95%. This output power measurement was performed from the upper side of the chip using a Si photodiode. It means that the power measurement is a relative output measurement from the top surface of the chip.
Figure5-5 shows the VLEDs with surface roughening and conventional VLEDs at the same injected current 1mA, we could observe the relative intensity about output power.
In summary of the chapter, an etching method has been applied to a GaN-based VLED for the purpose of increasing extraction efficiency. VLED output test results have indicated that, presumably due to the decrease in light propagation in GaN film, there is a relationship between a roughened appearance and extraction efficiency.
Although total integrated optical power has not been measured, the extraction efficiency from a top surface was increased 30% to 110% compared to that of a conventional VLED and the electrical properties of devices were similar. It is notable that the technique mentioned in this chapter is simple and does not require complicated processes, implying that it will be suitable for manufacturing of GaN-based VLEDs with surface roughening.
Units :um
Figure 5-1 the processing of the GaN vertical light emitting diodes with surface roughening
Figure 5-1 the processing of the GaN vertical light emitting diodes with surface roughening