Comparison between Experiment and Simulation Results…

Chapter 4 Optical pumping GaN-based 2D SEPC DFB Laser

4.4 Comparison between Experiment and Simulation Results…

Figure 4.6 shows normalized frequency as a function of the lasing wavelength.

We find that the normalized frequency has some groups that are marked by pink area.

We are able to classify the normalized frequency into few groups. The reason why the normalized frequency of our laser was classified into few groups and what causes this kind of phenomenon can be explained by the band diagram and Bragg diffraction.

In section 4.1, the band diagram of PC was calculated as shown in Fig. 4.4. To compare the results of experiment and simulation, we put Fig.4.4 and Fig 4.6 on the same place with same scale as shown is Fig.4.15. It can be seen that different groups of the normalized frequency occur at different points of Brillouin-zone boundary such as Γ、M、K points. All points of normalized frequency can exactly correspond to points of Brillouin-zone boundary. In the other word, the stimulated emission only occurs at Γ or M or K point because the Bragg condition only satisfy at these points.

Chapter 5 Conclusion

The fabrication and characteristics of GaN-based 2D surface-emitting photonic crystal distributed-feedback (SEPC DFB) laser were investigated in this thesis. We also simulated the band diagram of the PC structure to realize the lasing mechanism by using plane-wave expansion method. The laser action of photonic crystal devices was achieved under the optical pumping at room temperature. The lasing characteristics in the devices with different lattice constants (a=190-300nm) emitting different lasing wavelengths were measured and realized using the format of normalized frequency as a function of the lasing wavelengths. All these devices show a similar threshold pumping energy densities to be about 3.5mJ/cm². The GaN-based 2D SEPC DFB laser emits violet wavelength (from 395nm to 425nm) with a linewidth of about 1.1Å. The degree of polarization and divergence angle of the laser emission is measured to be about 53% and smaller than 10∘. The emission images of the laser indicate that the stimulated emission occurs over a large area, and the lasing spectrum shows the laser is a single mode laser. Finally, we compare experiment results with simulation results. We find that normalized frequency of each laser emission from photonic crystal devices can exactly correspond to the points of Brillouin-zone boundary, Γ、M、K points. That is because the Bragg conditions only satisfy at these points. This also further confirms the laser action in our devices comes from the band edge of photonic crystal band we designed. All the experiment results provide a strong evidence that SEPC DFB laser could have highly potential in the application of large area and single-mode vertical emitting lasers.

source type source size brightness Table 3.1 Properties of the electron sources

Step Process Conditions

1 Soft mask

(1) I.C.

) (2) Deposition of 200nm SiN by PECVD.

) (3) Spinning of 150nm PMMA by spinner

(4) Definition of pattern of nano-cavity laser by EBL.

(5) Development.

(6) Hard bake

2 Hard mask

(1) I.C.

(2) Dry etching by ICP-RIE (Oxford Plasmalab system 100) to form the hard mask.

(2) Dry etching by ICP-RIE ((SAMCO RIE-101PH) to transfer the hard mask to GaN.

(3) Remove hard mask by BOE.

(4) Hard back.

Table 3.2 Process flowchart

Figure 1.1 Schematic diagram of topical 2D PC

Figure 1.2 Spectrum of the laser. The spontaneous emission is shown in the

Figure 1.3 L-L curve showing the power at the laser wavelength versus the incident pump power.

Figure 1.4 Schematic diagram of electrically driven single-cell photonic crystal

` Figure 1.5 L-I curve of the laser. The lasing spectrum is shown in the

Figure 1.6 Schematic diagram of PC membrane nano-cavity

Figure 1.7 Resonance modes of nano-cavities and r /a =0.262±0.004, 0.254±0.004, and 0.246±0.004 from top to bottom

Figure 1.8 Schematic structure of the surface-emitting laser with 2D triangular-lattice structure. The inset shows the SEM photograph of the triangular-lattice structure.

Figure 1.9 L-I characteristic of the 2D PC laser at RT under pulsed condition (repetition period: 1ms, pulse width: 500 ns).

Figure 1.10 Lasing spectrum under pulsed condition and the injection current was J=1.56 Jth.

Figure 1.11 Schematic diagram of the device structure.

Figure 1.12 Lasing spectrum of the device under RT-CW condition.

Figure 1.13 Light output power-current characteristic.

d A

B

C

θ_i

D

θ_tθ_t

n

₁

n

₂

d A

B

C

θ_i

D

θ_tθ_t

n

₁

n

₂

Figure 2.1 Schematic draw of the light reflected from the top and bottom of the thin film.

Figure 2.3 Structure of 2D triangular lattice in real space 1 2 .. .. .. .. .. .. .. .. .. m

L₁L₂

n_Hn_L

effective reflector L_pen

n_s

substrate

n_o

1 2 .. .. .. .. .. .. .. .. .. m

L₁L₂

n_Hn_L

effective reflector L_pen

n_s

substrate

n_o

Figure 2.2 Schematic of DBRs.

Figure 2.4 Structure of 2D triangular lattice in reciprocal space.

k₁ k₂

K_d=k_i-k₁+k₂

k_i

k_d=k_i+k₁ K_d=k_i-k₁+2k₂

k_d=k_i+k₁+k₂ k_d=k_i+2k₂

X Y

k₁ k₂

K_d=k_i-k₁+k₂

k_i

k_d=k_i+k₁ K_d=k_i-k₁+2k₂

k_d=k_i+k₁+k₂ k_d=k_i+2k₂

X Y

Figure 2.5 Triangular reciprocal lattice diagram of in-plane direction at Г point.

Figure 2.6 Triangular reciprocal lattice diagram of vertical direction at point.

Γ K₁

k_i k_d=k_i+K₁

X Y

k_d=k_i-K₁ Γ K₁

k_i k_d=k_i+K₁

X Y

k_d=k_i-K₁

Figure 2.7 Triangular reciprocal lattice diagram of in-plane direction at M point.

Γ K₂

k_i k_d=k_i+K₂

X Y

K₁

k_d=k_i+K₁ K

K Γ K₂

k_i k_d=k_i+K₂

X Y

K₁

k_d=k_i+K₁ K

Figure 2.8 Triangular reciprocal lattice diagram of in-plane direction at K point.

E_c

Rsp R12 R21 Rnr

Figure 2.9 Electronic transitions between the conduction and valence bands

R_nr

Figure 2.10 Reservoir with continuous supply and leakage as an analog to a DH active region with current injection for carrier generation and radiative and nonradiative recombination.

Figure 2.11 Illustration of output power versus current for a diode laser

Figure 3.1 Cross-section of a typical electron beam block

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

N-GaN ~560nm

10pairs InGaN/GaN (2.5nm/7.5nm) MQWs

P-GaN ~ 200nm

GaN ~ 2μm

Figure 3.2 Schematic diagram of half-structure by MOCVD Computer

control

Stage control

Electron Gun

Beam Blanker

Stage Deflection Coil e

-Vacuum Chamber

360 380 400 420 440 460 480 500 520 0

20 40 60 80 100

Reflectance%

Wavelength(nm)

Figure 3.3 Reflectivity spectrum of the half structure with 35 pairs of GaN/AlN DBR structure measured by N&K ultraviolet-visible spectrometer with normal incident at room temperature.

Figure 3.4 CCD image of GaN-based material with SiN film 200nm

Figure 3.5 SEM image of soft mask pattern

Figure 3.6 SEM image of hard mask pattern

Figure 3.7 Top view SEM image of 2D SE PC DFB laser.

Figure 3.8 SEM image of Cross-section of 2D SE PC DFB laser

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

N-GaN ~560nm

10pairs InGaN/GaN (2.5nm/7.5nm) MQWs

P-GaN ~ 200nm

GaN ~ 2μm

Figure 3.9 Schematic diagram of nitride structure grown by MOCVD

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

N-GaN ~560nm

10pairs InGaN/GaN (2.5nm/7.5nm) MQWs

P-GaN ~ 200nm

GaN ~ 2μm

SiN ~ 200nm

Figure 3.10 1st step of process: deposing SiN film

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

N-GaN ~560nm

10pairs InGaN/GaN (2.5nm/7.5nm) MQWs

P-GaN ~ 200nm

GaN ~ 2μm

SiN ~ 200nm

PMMA ~150nm

Pattern 50μm

Figure 3.11 2nd step of process: soft mask

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

N-GaN ~560nm

10pairs InGaN/GaN (2.5nm/7.5nm) MQWs

P-GaN ~ 200nm

GaN ~ 2μm

SiN ~ 200nm Pattern 50μm

Figure 3.12 3rd step of process: hard mask

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

N-GaN ~560nm

10pairs InGaN/GaN (2.5nm/7.5nm) MQWs

P-GaN ~ 200nm

GaN ~ 2μm

SiN ~ 200nm Pattern 50μm

Figure 3.13 4th step of process: ICP etching

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

N-GaN ~560nm

10pairs InGaN/GaN (2.5nm/7.5nm) MQWs

P-GaN ~ 200nm

GaN ~ 2μm

Figure 3.14 5th step of pro ess: removing hard mask c

He-Cd laser (325nm)

Figure 4.1 Measurement setup of μ-PL system

380 390 400 410 420 430 440 450 460 470

Figure 4.2 PL spectrum of as-grown structure

0.0 0.2 0.4 0.6 0.8 1.0

Optical Field (arb. units)

Γ g = 0.622 n

_eff

= 2.2

35 pairs AlN/GaN DBR stack N-GaN

Optical Field (arb. units)

Γ g = 0.622 n

_eff

= 2.2

35 pairs AlN/GaN DBR stack N-GaN

P-GaN Active layer

PC

Figure 4.3 Distribution of electric field in the vertical direction of the device

Figure 4.4 Calculated band diagram of the 2Dtriangular-lattice structure for transverse-electric (TE) mode

Nd:YVO4 laser (355nm)

Pulse duration : 500 ps Pulse frequency : 1000 Hz

Figure 4.5 Measurement setup of optical pumping system

0.8

390 395 400 405 410 415 420 425 430 0.0

Figure 4.6 Normalized frequency as a function of the lasing wavelength

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 0.0

0.5 1.0 1.5 2.0

Intensity(a.u.)

Energy density (mJ/cm

)

Figure 4.7 Excitation energy – emission intensity curve (L-I)

370 380 390 400 410 420 430 440 450 460

0.0 0.5 1.0 1.5 2.0

FWHM=0.11nm

424.33nm

Intensity(a.u.)

Wavelength(nm)

Figure 4.8 Spectrum of stimulated emission at room temperature

411 414 417 420 423 426 429 432 435 438 1.33E_th

1.17E_th 1.00E_th 0.66E_th

FWHM=0.11nm 424.33nm

0.33E_th

Intensity(a.u.)

Wavelength(nm)

Figure 4.9 Excitation energy intensity VS. emission spectrum

Figure 4.10 (a) Spontaneous emission image at 0.92Eth , (b) stimulated emission image at 1.47Eth

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

35 pairs AlN/GaN DBR

1.75λ

Figure 4.11 Measurement setup for DOP

0

Figure 4.12 Intensity of laser emission as a function of the angle of the polarizer

Spectrograph

fiber

-90° 90°

Sapphire

GaN ~2μm

35 pairs AlN/GaN DBR

1.75λ

35 pairs AlN/GaN DBR

1.75λ

Figure 4.13 Measurement setup for divergence angle

-60 -40 -20 0 20 40 60

igure 4.14 Intensity of laser emission as a function of the angle of the fiber

390 395 400 405 410 415 420 425 430 0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Normalized Frequency(a/λ)

Wavelength(nm) M M₂₂ ΓΓ₁₁

Γ Γ₂₂

ΓΓ₃₃、、KK₃₃

K K₃₃

KK₃₃ M M₃₃

Fig.4.15 Experiment (left) and simulation results (right)

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