Chapter 2 Annealing effect on Nitrogen-Polar InGaN/GaN MQWs grown
2.4 Material Characteristics of Nitrogen-Polar InGaN/GaN MQWs
2.4.3 Atomic Force Microscopy (AFM) Study
@
1 2
1 t
t sample are larger than that of the N-polar t11@t25 one.
2.4.3 Atomic Force Microscopy (AFM) Study
In order to study the surface morphology of the N-polar samples, AFM measurement was conducted. Figures 2.13 (a) and (b) show the AFM images of the N-polar InGaN/GaN SQW and MQW samples, respectively. The InGaN mounds affect the surface roughness. The surface roughness in 5 μm × 5 μm area are 7.327 and 24.985 nm for the N-polar InGaN/GaN SQW and MQW samples, respectively. As the growth number of QW increases, a larger surface roughness is observed.
34
Figures 2.13 (c) and (d) show the AFM images of the N-polar t11@t25 and t11@t22 samples, respectively. The InGaN mounds affect the surface roughness. The surface roughness in 5 μm × 5 μm area are 27.422 and 28.958 nm for the N-polar t11@t25 and
2
@
1 2
1 t
t samples, respectively. Although the growth time (t2) for the two samples is different, the surface roughness is almost similar.
2.5 Temperature-dependent PL Studies of Nitrogen-Polar InGaN/GaN MQWs
Figures 2.14 (a) and (b) show the PL spectra of the N-polar InGaN/GaN SQW and MQW samples as a function of temperature, respectively. The PL spectra of the two samples are categorized into two parts. One spectral range is from 350 nm to 370 nm, and the other one is from 370 nm to 500 nm. The PL intensity decays with increasing temperature.
The PL peak positions as a function of temperature for the two samples are shown in Figure 2.15 (a). The N-polar InGaN/GaN SQW and MQW samples have three peaks: ~360 nm (3.444 eV), ~379 nm (3.272 eV), and ~390nm (3.179 eV). The dominate peak is around
~379 nm (3.272 eV). The emission peak positions of ~360 nm (3.444 eV) are related to GaN, while those of ~379 nm (3.272 eV) and ~390nm (3.179 eV) are related to InGaN. The peak positions of the SQW and MQW samples are nearly the same. With increasing temperature form 20 to 300 K, PL positions of the two samples are nearly temperature-independent. In addition, the normalized integral PL intensities as a function temperature of the two samples are shown in Figure 2.15 (b). The integral PL intensities of the N-polar SQW sample decay faster than those of the N-polar MQW one after 80K.
Figure 2.16 (a) and (b) show the PL spectra of the N-polar t11@t25 and t11@t22 samples as a function of temperature, respectively. The PL spectra of the two samples are categorized into two parts. One spectral range is from 350 nm to 430 nm, and the other one is from 430 nm to 500 nm. The PL intensity decays with increasing temperature.
35
The PL peak positions as a function of temperature for the two samples are shown in Figure 2.17 (a). The N-polar t11@t25 sample has three peaks: ~379 nm (3.272 eV),
~390nm (3.179 eV), and ~450 nm (2.756 eV). The N-polar t11@t22 sample also has three peaks: ~379 nm (3.272 eV), ~390nm (3.179 eV), and ~470 nm (2.638 eV). The dominate peaks of the two samples are around ~379 nm (3.272 eV). The three emission peak positions of the two samples are related to InGaN with different Indium contents. The peak positions around ~379 nm (3.272 eV) and ~390 nm (3.179 eV) for the two samples are nearly the same.
With increasing temperature form 20 to 300 K, PL peak positions of the two samples are nearly temperature-independent. In addition, the normalized integral PL intensities as a function temperature of the two samples are shown in Fig 2.17 (b). The integral PL intensity of the N-polar t11@t25 sample decays faster than those of the N-polar t11@t22 one.
2.6 Material Characteristics of annealed Nitrogen-Polar InGaN/GaN MQWs
Thermal annealing with 900°C for 60 seconds in Argon environment was conducted for the N-polar t11@t25 and t11@t22 InGaN/GaN MQW samples.
2.6.1 Scanning Electron Microscope (SEM) and Cathodoluminescence (CL) Studies
Figure 2.18 shows the SEM [(a) and (b)], panchromatic CL [(c) and (d)], and monochromatic CL [(e) and (f)] images for the corresponding SEM regions using the 11kV excitation electron voltage for the annealed N-polar t11@t25 and t11@t22 samples, respectively. The total area ratio of mounds of the annealed N-polar t11@t22 sample is larger than that of the annealed N-polar t11@t25 one. Monochromatic CL mapping was collected at a wavelength of 450 nm and 460 nm for the annealed N-polar t11@t25 and
36 those of the as-grown ones
Figure 2.19 shows the CL spectra of the two annealed samples using excitation voltages of 5, 7, 9, and 11 kV at RT. The CL spectra of the annealed N-polar t11@t25 and t11@t22 InGaN/GaN MQW samples show two CL peaks: one in ~360 nm and the other one in ~450 nm and ~460 nm, respectively. The CL peak positions of the annealed samples are the same as those of as-grown samples in Figure 2.11, but the CL intensities of the annealed N-polar
5
@
1 2
1 t
t and t11@t22 samples are weaker than those of the as-grown ones. After annealing, the relative intensities of two CL peaks are different.
2.6.2 X-ray Diffraction (XRD) Patterns
Figure 2.20 shows the XRD patterns for the two anneals samples. The diffraction peaks corresponding to InGaN and GaN can be identified. The GaN diffraction peak is mainly from the contact and barrier layers. The side shoulder with a broad distribution below the GaN main peak is attributed to InGaN with various indium contents, sizes, and shapes in the quantum wells [10]. The indium contents of the annealed N-polar t11@t22 sample are larger than those of the annealed N-polar t11@t25 one. After annealing, the density of InGaN mounds increases.
2.6.3 Atomic Force Microscopy (AFM) Study
Figures 2.21 (c) and (d) show the AFM images of the annealed N-polar t11@t25 and surface roughness in 5 μm × 5 μm area are 26.694 and 27.295 nm for the annealed N-polar
37
5
@
1 2
1 t
t and t11@t22 samples, respectively. The surface roughness of the annealed samples becomes smaller than that of the as-grown ones.
2.7 Temperature-dependent PL Studies of annealed Nitrogen-Polar InGaN/GaN MQWs
Figures 2.22 (a) and (b) show the PL spectra of the annealed N-polar t11@t25 and 2
@
1 2
1 t
t samples as a function of temperature, respectively. The peak positions of the two samples are categorized into two parts. One spectral range is from 350 nm to 430 nm, and the other one is from 430 nm to 500 nm. The PL intensity decays with increasing temperature.
After annealing, the relative intensities of PL peaks are different.
The PL peak positions as a function of temperature for the two samples are shown in Figure 2.23 (a). The annealed N-polar t11@t25 sample has three peaks: ~379 nm (3.272 eV), ~390nm (3.179 eV), and ~450 nm (2.756 eV). The annealed N-polar t11@t22 sample also has three peaks: ~379 nm (3.272 eV), ~390nm (3.179 eV), and ~470 nm (2.638 eV). The dominate peaks of the two samples around ~379 nm (3.272 eV). The three emission peak positions of the two samples are related to InGaN. The peak positions around ~379 nm (3.272 eV) and ~390nm (3.179 eV) for the annealed N-polar t11@t25 and t11@t22 samples are nearly the same. With increasing temperature form 20 to 300 K, PL positions of the two samples are nearly temperature-independent. In addition, the normalized integral PL intensities as a function temperature of the two annealed samples are shown in Figure 2.23 (b). The integral PL intensity of the annealed N-polar t11@t25 sample decays slower than those of the annealed N-polar t11@t22 one.
2.8 Discussion and Summary
In summary, we have shown the experimental results of SEM, CL, XRD, AFM, PL measurements of N-Polar InGaN/GaN SQW and MQW samples. The surface roughness of
38
the N-polar InGaN/GaN MQW sample is larger than that of the N-polar InGaN/GaN SQW one. The surface roughness of the N-polar t11@t22 sample is larger than that of the N-polar t11@t25 one. The N-polar InGaN/GaN MQW and t11@t22 InGaN/GaN MQW samples show a larger area ratio of InGaN mounds. The indium content of the N-polar InGaN/GaN MQW sample are larger than those of the N-polar InGaN/GaN SQW one, and the indium content of the N-polar t11@t22 InGaN/GaN MQW sample is larger than those of the N-polar t11@t25 one. With increasing temperature from 20 to 300 K, the integral PL intensity of the N-polar t11@t25 InGaN/GaN MQW sample decays faster than those of the N-polar t11@t22 one.
The N-polar t11@t25 and t11@t22 InGaN/GaN MQW samples were annealed. The surface roughness of the two annealed samples becomes smoother. With thermal annealing, the relative intensities of UV and visible peaks in the CL spectra are changed and The InGaN intensity increases in XRD measurement. The mound and light densities of the annealed N-polar t11@t25 and t11@t22 sample are larger than those of the as-grown ones.
According to experimental results, the rapid thermal annealing can affect surface morphology and light intensity.
39
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Figure 2.1 Schematic illustration of the temporal sequences of valve operation [1].
Figure 2.2 SEM images of N-polar MQW samples grown with (a) Continuous-mode, (b) s
t25 , and (c) t2 2s, while keeping t at 3 s. (d) PL spectra of those N-polar samples 1 at the room temperature [1].
Figure 2.3 SEM images of the samples grown with QW numbers of (a) 1, (b) 3, (c) 5, and (d) 10. (e) PL spectra of those N-polar samples at the room temperature [1].
41
Figure 2.4 In situ optical reflectance of N-face GaN growth for samples with nitridition temperatures (a) 1130°C, (b) 1030°C, (c) 980°C, and (d) 950°C [6].
Figure 2.5 Nomarski optical microscope images of samples with nitridition temperatures (a) 1130°C, (b) 1030°C, (c) 980°C, and (d) 950°C [6].
42
Figure 2.6 Experimental flow chart of this chapter.
Nitrogen-Polar InGaN Multiple Quantum Wells Grown by Pulsed Metalorganic
Chemical Vapor Deposition
Sample preparation
Scanning Electron Microscope (SEM) and Cathodoluminescence (CL) Measurements
X-ray Diffraction (XRD) Measurement
Atomic Force Microscopy (AFM) Measurement
Temperature-dependent Photoluminescence (PL) Measurement
Annealing effect of samples
To investigate microstructures and nanophotonics of samples
To measure the surface morphology of samples
To investigate optical properties of samples
To investigate annealing effect of samples
To investigate the crystal structure characteristic of samples
43
(a) (b)
(c) (d)
Figure 2.7 Sample structures of the N-Polar (a) 131210A(SQW), (b) 131210B(MQW), (c) 140206A(t11@t25), and (d) 140206B(t11@t22) samples.
44
Figure 2.8 SEM [(a) and (b)] and panchromatic CL [(c) and (d)] images for the corresponding SEM regions using the 11kV excitation electron voltage for the N-polar InGaN/GaN SQW and MQW samples, respectively.
Figure 2.9 SEM [(a) and (b)], panchromatic CL [(c) and (d)], and monochromatic CL [(e) and (f)] images for the corresponding SEM regions using the 11kV excitation electron voltage for the N-polar t11@t25 and t11@t22 InGaN/GaN MQW samples, respectively.
45
Figure 2.10 CL spectra of the N-polar InGaN/GaN (a) SQW and (b) MQW samples with the excitations of 5, 7, 9, and 11kV electron voltages at room temperature.
Figure 2.11 CL spectra of the N-polar (a) t11@t25 and (b) t11@t22 InGaN/GaN MQW samples with the excitations of 5, 7, 9, and 11kV electron voltages at room temperature.
0
46
Figure 2.12 XRD patterns for the N-polar (a) InGaN/GaN SQW and MQW and (b) t11@t25 and t11@t22 InGaN/GaN MQW samples.
Figure 2.13 AFM images (5 × 5 μm2) of the N-polar InGaN/GaN (a) SQW (Rq：7.327 nm) and (b) MQW (Rq：24.985 nm) samples and the N-polar (c) t11@t25 (Rq：27.422 nm) and (d) t11@t22 (Rq：28.958 nm) InGaN/GaN MQW samples. Surface roughness of each sample, Rq, is shown in the parentheses.
0.0 0.2 0.4 0.6 0.8 1.0
31 32 33 34 35 36
0.0 0.2 0.4 0.6 0.8 1.0
Intensity(arb. unit)
SQW MQW
(b) (a)
(degrees) t1=1, t2=5
t1=1, t2=2
47
Figure 2.14 PL spectra as a function of temperature for the N-polar InGaN/GaN (a) SQW and (b) MQW samples.
Figure 2.15 (a) PL peak position and (b) normalized PL integral intensity in the 370-500 nm spectral range as a function of temperature for the N-polar InGaN/GaN SQW and MQW
48
Figure 2.17 (a) PL peak position and (b) normalized PL integral intensity in the 370-430 nm and 430-500 nm spectral ranges as a function of temperature for the N-polar t11@t25 and
49
Figure 2.18 SEM [(a) and (b)], panchromatic CL [(c) and (d)], and monochromatic CL [(e) and (f)] images for the corresponding SEM regions using the 11kV excitation electron voltage for the annealed N-polar t11@t25 and t11@t22 InGaN/GaN MQW samples, respectively.
Figure 2.19 CL spectra of the annealed N-polar (a) t11@t25 and (b) t11@t22 InGaN/GaN MQW samples with the excitations of 5, 7, 9, and 11kV electron voltages at room temperature.
0 70000 140000 210000 280000 350000
350 400 450 500 550
0 70000 140000 210000 280000 350000
(b) t
1=1, t
2=2 (a) t1=1, t2=5
Intensity(arb. unit)
5kV 7kV 9kV 11kV
Wavelength (nm)
50
28.958 nm)] and annealed [(c) (Rq：26.694 nm) and (d) (Rq：27.295 nm)] for the N-polar 5
@
1 2
1 t
t and t11@t22 InGaN/GaN MQW samples, respectively. Surface roughness of each sample, Rq, is shown in the parentheses.
0.0
51
Figure 2.22 PL spectra as a function of temperature for the annealed N-polar (a) t11@t25 and (b) t11@t22 InGaN/GaN MQW samples.
Figure 2.23 (a) PL peak position and (b) normalized PL integral intensity in the 370-430 nm and 430-500 nm spectral ranges as a function of temperature for the annealed N-polar