Chapter 2 Experimental Instruments and Methods
2.2 Dry Etching Process and E-gun Evaporation
2.2 Dry Etching Process and E-gun Evaporation
Dry Etching Process
Dry etching process is critical for scientists to fabricate the device according to their plan, and there are two types of etching processes: wet etching and dry etching. Dry etching process uses plasma to etch the semiconductor material and it is a kind of anisotropic etching process. The linewidth of dry etching process is smaller than wet etching, therefore, dry etching process gradually replace wet etching process after 1980.
The mechanism of dry etching process is as follow: first, the etching gas has been
diffuse to chamber under ultra-low pressure. Second, when the pressure is stable, plasma is produced by RF frequency. Third, the radicals produced by bombardment of high speed electron would diffuse to the wafer and attach to its surface. Fourth, with the help of ion bombardment, these radicals would react with the atoms on the surface and form by-product as gas. At the end, these volatile by-products would then leave the surface of wafer and discharge from chamber. Fig. 2.2.1 shows the inductively coupled plasma and reactive ion etching (ICP-RIE) system used to etch Si3N4 layer to transfer the patter from PMMA layer, and Fig. 2.2.2 shows the ICP-RIE equipment used to etch GaN layer.
E-gun Evaporation
E-gun evaporation, also called as E-beam evaporation, is one kind of physical vapor deposition (PVD). The difference of E-gun evaporation is that it uses electron beam to heat up the source material. The advantage of E-gun evaporation is that it could heat only one small part of the surface of the source material. This will reduce the energy consumption used by PVD. Therefore, it is a common way to use E-gun evaporation to deposit metal on to the device. For an E-gun evaporation system, it consists of an electron beam evaporation gun, a
2.3 Micro-Photoluminescence (μ-PL) System
Micro-Photoluminescence is an advanced Photoluminescence system to measure sample in micrometer even nanometer scale. The spot size of the light source for μ-PL system has shrunk to micrometer scale to observe the optical properties of sample. In our μ-PL system, we use Nd:YVO4 laser with lasing wavelength 355nm as a pumping source, and the
spot size is about 50μm, frequency of the laser is 1kHz, and the pulse width is about 500ps.
The schematic diagram of μ-PL system is shown in Fig. 2.3.
2.4 Fabrication Process of Metal- coated GaN Grating Structures
We use the 2 μm thick undoped GaN as the gain medium of grating structure laser, which is grown on a c-plane (0001) sapphire substrate by metal-organic chemical vapor deposition (MOCVD) system (EMCORE D-75). We use Trimethygallium (TMGa) and Ammonia (NH3) as the gallium and nitride sources respectively. A thermal cleaning process is carried out at 1080℃ for 10 minutes in a stream of hydrogen ambient before the growth of epitaxial layers to clean the sample surface. Then, the 30nm thick GaN nucleation layer is first grown on the sapphire substrate at 530℃, and at the end the 2μm thick undoped GaN layer is grown on it at 1040℃.
After the undoped sample is prepared, the 200 nm thick Si3N4 is deposited on the planed GaN layer as an etching mask by plasma-enhanced chemical vapor deposition
(PECVD). After that, we coat the 250nm polymethylmethacrylate (PMMA) on Si3N4 by spin-coating method. We define the grating pattern on the PMMA layer by E-beam lithography, then using reactive ion etching (RIE) with CHF3/O2 mixture to etch down to the Si3N4 layer. Next, we transfer the grating pattern from Si3N4 layer to the undoped GaN layer to form the grating structure with about 200nm depth by inductively coupled plasma reactive ion etching (ICP-RIE) with Cl2 /Ar mixture. The Si3N4 mask layers are removed by wet etching after all above processes. Before we begin to do the next step, we clean our sample by wet etching methods, washing away the particles created in the previous dry etching processes to promote the performance of our device. To improve the quality factor of the device, we deposit 30nm Si3N4 layer on the patterned GaN layer. After that, a 50 nm aluminum layer is coated on the device by E-gun evaporation to form the grating structure of metal-coated GaN laser.
The dry etching processes may cause the surface roughness of the device, but the Si3N4 would passive this roughness when we coat the dielectric layer on the grating structure. The surface roughness will generate the nonradiative center on the surface of grating structure reducing the performance of device. The Si3N4 layer between the metal and GaN will reduce
and the process flow chart of metal-coated GaN grating structure is shown in Fig. 2.4
ICP-RIE (SAMCO RIE-101PH)
GaN film etching：
Cl2：25sccm Ar：10sccm ICP power：200W Bias power：200W Pressure：0.33Pa
Etching rate：545nm/min
E-beam Lithography System (JEOL JSM-6500)
Spin coating use PMMA (A5)
Spinning rate in first step：1000 rpm (10sec)
Spinning rate in second step：3500 rpm (25sec)
Hard bake：
Temperature：180℃ (90sec)
Exposure：
Beam voltage：50KeV
Development：
MIBK：IPA (1:3) ：70sec
Fixing：
IPA：40sec
E-gun Evaporation System (ULVAC EBX-8C)
Source：Aluminum
Pressure：3×10 -6 Torr
Current：170mA for first 5nm 190mA for the rest 45nm
Fig. 2.1.1 Schematic Diagram of E-beam Lithography System.
Fig. 2.1.3 JSM-7000F SEM System.
Fig. 2.2.1 ICP-RIE System (Oxford Plasmalab System 100).
Fig. 2.2.2 ICP-RIE System (SAMCO RIE-101PH).
Fig. 2.3 Schematic Diagram of Micro-Photoluminescence System.
Fig. 2.4 Process flow chart of metal-coated GaN grating structure.
References
[1] A. Mizrahi, V. Lomakin, B. A. Slutsky, M. P. Nezhad, L. Feng, and Y. Fainman, “Low threshold gain metal coated laser nanoresonators,” Opt. Lett. 33, 1261 (2008).
Chapter 3 Band Edge Mode Lasing in Metal-coated GaN Grating Structure at Room Temperature
3.1 Design and Optimize the Device
Photonic crystals are regarded much attention owing to their photonic band structure recently [1]. Light at the band edge of one dimensional photonic bandgap has nearly zero group velocity and forms a standing wave which makes the laser action possible. The uniformed grating structure would form the plasmonic bandgap and we can observe the band edge mode from it theoretically. By combining the advantages of the metal-coated cavities and the structure of one dimensional photonic crystal like grating structure, we can obtain single-mode output and high quality factor laser possible. A grating structure with uniformed periodicity can be used to couple light to the SPPs. The SPPs exist at a specific condition
which is needed to satisfy the following equation [2]:
𝑘𝑠𝑝𝑝 = 𝑛𝑒𝑓𝑓𝑘0 = 𝑘0 sin(𝜃) ± m2𝜋𝑎 (1)
The peak of emission wavelength of GaN is about 365nm which can be speculated from the bandgap of GaN (3.4eV). The Fig. 3.1.1 shows the Photoluminescence (PL) spectrum of undoped GaN layer and we can find that the emission wavelength range of GaN is from 360nm to 380nm. Therefore, we use the FEM to design the metal-coated GaN grating structure with the high quality factor at about 370nm.
undoped GaN and sapphire. The refractive indices of aluminum and undoped GaN layers are established by Palik [3] and Peng et al [4]. We also set a perfectly matched layer at the bottom of GaN to absorb redundant signal which might reflect back to the metal-coated grating structure and use the 50nm metal layer and 30nm dielectric layer which is referred from report [5] to simulate the electric-field mode profile in it. Finally, we find a band edge mode in the designed structure. Fig. 3.1.2 shows the simulated electric-field intensity plot through the metal-coated GaN grating structure. By using FEM, we can see that the SPPs clearly exist in the metal-coated GaN grating structure from Fig. 3.1.2 with wavelength of 369.5 nm and the quality factor of it is estimated about 200. Then, we fix the period of this structure which is 840nm and further optimize the height and width of it to get the higher quality factor laser.
The higher quality factor represented the longer average lifetime of resonant photons in the cavity and result in better performance of laser. Fig. 3.1.3 (a) shows schematic diagram of the model used to optimize the metal-coated GaN grating structure. Fig. 3.1.3 (b) is the quality factor versus width of grating structure plot, and we find the best width of it is about 220nm.
Afterwards, we fix the period and width of the grating structure and tune the height of it to achieve the best performance in our device. We find the best height of this structure is about
will find that the wavelength of Fig. 3.1.3 (b) and Fig. 3.1.4 (a) increase with increasing the width or height of grating structure.
In Fig. 3.1.4 (a), the height of grating structure which we use to simulate by FEM is from 130 to 250nm. If we continually simulate the height of grating structure from 250nm to 1000nm, we will find the highest quality factor which appears when the height of it is about 900nm as shown in Fig. 3.1.4 (b). However, there is dielectric mode dominate when the height of grating structure is about 900nm and the SPPs is even difficult to find in grating structure as shown in Fig. 3.1.5. For further design the defect mode in grating structure which has the advantage of low threshold power density in chapter 4, it should be depended on surface plasmon effect to achieve [6]. We examine the electric-field mode profile in each height of grating structure by FEM, and find the SPPs clearly exist below the height of it which is approximately 250nm. Therefore, we should demonstrate the grating structure which the height of it is below 250nm first and measure the lasing characteristic to confirm the device is functional. Then, we start to demonstrate the grating structure which the period, width and height of it are 840nm, 220nm and 180nm respectively. The width and height of this structure are both below the wavelength of the expected lasing mode. We also design it with the field size of 50μm×50μm, because the diameter of laser spot size used to pump device is approximately 50μm.
Fabrication procedures are detailed shown in chapter 2.4. After the fabrication
processes are finished, we use SEM to check the size of grating structure and make sure the period, width and height of it is what we want. Fig. 3.1.6 and Fig. 3.1.7 show the SEM image of grating structure in top view and angled view without metal-coated respectively. The period, width and height of it are 840nm, 220nm and 150nm respectively. Although, the height of it is not 180nm which has the best quality factor, we can see that grating structure has not bad quality factor when the height of it is 150nm from Fig. 3.1.4 (a). The SEM image of grating structure after deposition of dielectric and metal layer is shown in Fig. 3.1.8 in angled view, and the schematic diagram of metal-coated GaN grating structure is shown in Fig. 3.1.9.
3.2 Lasing Characteristics of Band Edge Mode in Metal-coated GaN Grating Structure
We use the μ-PL system which is mentioned in chapter 2 to measure the characteristics of the device. The grating structure of metal-coated GaN layer was optically pumped by a frequency-tripled Nd:YVO4 355 nm pulsed laser at room temperature. A 15×objective lens is used to collect the lasing signal from the grating structure laser through a multimode fiber, and couple into a spectrometer with the charge-coupled device detectors. We optically pump our device from the metal-coated surface, because the bulk GaN layer beneath the surface of
and above (red) the threshold power density. We can see a clearly single lasing mode at 368nm from Fig. 3.2.1 and observe the difference between the curve of below and above the threshold power density which ensure a lasing action in the grating structure. Fig. 3.2.2 shows the light-in light-out curves of this mode and the linewidth of the lasing mode above the power density is about 0.53nm. When the power density is above 19W/cm2, we observe the linear behavior in Fig. 3.2.2 which indicates the lasing action, and also find that the linewidth is narrowing. The narrowing linewidth shows that the emission mechanism in GaN is from spontaneous emission to stimulated emission. These evidences prove the lasing action in our metal-coated GaN grating structure. The steady blue-shift behavior as pumping power increase in Fig. 3.2.2 indicates that the performance of device is less influenced by heat effect even operating at high power density due to the good thermal dissipation of metal. The threshold power density of the grating structure is about 19W/cm2 and the quality factor estimated by the wavelength to linewidth around the transparency is 570. Next, we measure the degree of polarization (DOP) of metal-coated GaN grating structure. In Fig. 3.2.3, the DOP is about 96.5%, and the strongest intensity of it is occurred when the polarizer is perpendicular to the grating. The high DOP shows the good linear performance of our device.
To ensure that the lasing action is accurately originated from our grating structure, we use a Nd:YVO4 laser to pump the flat and grating structure region of undoped GaN layer and the flat region of metal-coated GaN layer to see the PL spectrum of them. As shown in Fig.
3.2.4, the peak wavelength of the flat and grating structure region of undoped GaN layer are all about 366nm and no lasing signal is observed. However, the spectrum of the flat region of metal-coated GaN layer is totally different compared to undoped GaN layer without metal-coated. The peak wavelength of undoped GaN around 366nm no longer exists because the energy from 360nm to 380nm had been almost absorbed by metal layer. Therefore, we confirm that the lasing action is truly from grating structure.
3.3 Results and Discussion
The lasing wavelength of metal-coated GaN grating structure may cause a little difference between experimental and simulation results. This is attributed to the imprecise size of device observed from SEM, uncertainty in the fabrication process, small error of material indices used in the simulation model and the slight edge roughness of the device. To check the reason why the GaN grating structure without metal-coated is not a functional laser, we use FEM to simulate the optical mode in GaN grating structure without metal-coated. Fig.
3.3.1 shows the quality factor versus wavelength of it and we find that the quality factor of all optical modes are very low around the emission wavelength of GaN. Fig. 3.3.2 shows the best
grating structure is very weak from Fig. 3.3.2. The quality factor of the optical mode shown in Fig. 3.3.2 is about 25 which is eight times smaller than the case in Fig. 3.1.2. All these results indicate that the GaN grating structure without metal-coated is barely possible lasing in low pump power density that is why we cannot observe the laser action in GaN grating structure without metal-coated. device burn out. We think the advantage of the high thermal conductivity and high reflectivity of aluminum increase the lasing possibility even at room temperature. Table 3.3 lists a comparison between nanostripe [8] and grating structure. As shown in the Table 3.3, the quality factor has improved significantly which is about four times larger than nanostripe.
Moreover, the threshold power density of grating structure is much lower than nanostripe. We believe that the utilization of SPPs would be the critical reason for these results.
We use the effective refractive index method to calculate the band diagram of grating
structure. In the beginning, we split the period grating structure into two parts: the stripe part and pitch part. This way will put the two dimensional structure into one dimension to simulate which is easy for calculation and also keeps the high accurate result. Then, we plot the band diagram of the grating device to match the FEM simulation result and estimate the effective refractive index of each part as shown in Fig. 3.3.3. A schematic diagram of the model used in effective refractive index method is shown in Fig. 3.3.4. The estimated effective refractive index of grating structure under transverse electric (TE) mode is shown in Fig. 3.3.5. The effective index of stripe part is 2.605 and the pitch part is 2.6117. Because the effective refractive index of both of them are between the air (n=1) and GaN (n=2.71), we can say the simulation result is reasonable and believe that it is the band edge mode lasing in the metal-coated GaN grating structure.
In the FEM simulation result, we observe that the part energy of electric-field would loss at the bottom of GaN. If we can further improve the vertical confinement in our device, the performance of it will have impressive result. Distributed Bragg Reflector (DBR) might be the one way to significantly reduce this problem. In 2010, C. Y. Lu et al. demonstrated metal-coated microrod with an n-type DBR lasing under room temperature CW operation
3.4 Summary
In summary, we successfully demonstrate the band edge laser with the lasing wavelength around 368 nm. The period, width and height of the metal-coated GaN grating structure are 840nm, 220nm and 150nm respectively. The lasing action from a metal-coated GaN grating structure at room temperature with a high quality factor of 570 and the threshold power density of 19W/cm2 is observed. The FEM simulation result showed the importance of aluminum layer and the lasing evidence of the metal-coated GaN grating structure. From the effective refractive index method simulation result, we confirm that the lasing action of grating device is attributed to the band edge mode. From experimental and simulation results, the grating structure without metal-coated is difficult to achieve lasing action at room temperature due to the poor optical confinement and the terrible thermal conductivity by air even make it lasing impossible under low pump power density. The advantage of band edge mode let us observe the single-mode lasing action which has the huge potential for developing high quality nanolaser in the future.
360 370 380 390 400 410 0.0
0.2 0.4 0.6 0.8 1.0
Int ensit y ( a.u.)
Wavelength (nm)
Fig. 3.1.1 PL spectrum of undoped GaN.
Fig. 3.1.2 Intensity of electric-field mode profile through metal-coated GaN grating
200 210 220 230 240 250 260
Fig. 3.1.3 (a) Schematic diagram of the model used to optimize the grating structure (b) Wavelength and quality factor versus width of metal-coated grating structure plot.
120 140 160 180 200 220 240 260
Fig. 3.1.5 Simulated electric-field intensity through the metal-coated GaN grating structure at 395nm with quality factor about 250. The period, width and height of it are
840nm, 220nm and 900nm respectively.
Fig. 3.1.6 The SEM image of grating structure in top view without metal-coated.
Fig. 3.1.7 The SEM image of grating structure in angled view without metal-coated.
Fig. 3.1.9 Schematic diagram of metal-coated GaN grating structure.
360 370 380 390 400 410 0.0
0.2 0.4 0.6 0.8 1.0
10 W/cm
221 W/cm
2Int ensit y ( a.u.)
Wavelength (nm)
Fig. 3.2.1 PL spectrum of the metal-coated GaN grating structure below (black) and above (red) threshold.
0 5 10 15 20 25
Fig. 3.2.2 Light-in light-out curves of band edge lasing mode.
0.0
360 370 380 390 400 410
Fig. 3.2.4 PL Spectrum of the plane region of metal-coated GaN and the GaN grating structure with and without metal and dielectric layers.
365 370 375 380 385 390
Fig. 3.3.1 The quality factor versus wavelength of GaN grating structure without metal-coated plot.
Fig. 3.3.2 Simulated electric-field intensity through the metal-coated GaN grating structure at 384.6nm with quality factor about 25.