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Abstract
A double-sided mask process is used to define the micro actuators on a silicon-on-insulator (SOI) wafer. Then, an InGaN/GaN multilayer film was deposited on the surface of the above patterned SOI substrate by employed MBE technology. Thus, the final device consisting of comb-drive actuators, springs and micro-mirror is implemented to obtain the two-dimensional (2D) tuning effect of the mirror. The displacement response is characterized by applying voltage to the micro actuators. It shows an approximation linear relationship between the displacement and the square of applied voltage. And the influences of the cross-axis coupling effect are also measured and evaluated. Moreover, the luminous performances of the InGaN/GaN multiquantum well films were also studied by analyzing temperature-dependent and laser intensity-dependent PL spectra. The authors observed the localized state effect and/or band-gap narrowing effect which results in an S-shaped behavior with increasing temperature. The PL intensity and wavelength are also significantly affected by the excitation energy of laser.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Micro-electro-mechanical system (MEMS) is one of the most advanced technology developments of the recent trend. It is the integration of mechanical, electrical, magnetic or other (thermal, fluidic, etc.) elements on a common silicon or silicon-on-insulator (SOI) substrate by using micro fabrication technology. MEMS is used in a wide range of applications including energy harvesting systems, radio frequency (RF) devices, biomedical devices, projection systems, and data storage structures [1–4]. In the optical regime, MEMS technology has also been an enabling tool for numerous cutting-edge devices for telecommunication, displays, sensing, and imaging systems [5–8]. Cornerstones for the development of the micro-opto-electro-mechanical systems (MOEMS) technology include actuator technology, optics design, and the progress of tunable micromechanical elements, and the combinations of these elements enable MOEMS devices to perform desired functions [8]. The refractive (micro-lens, micro-mirrors) [9, 10] and diffractive (gratings, beam splitters) [11, 12] elements are the important movable optical parts of the MOEMS devices. In particularly, a lens or a mirror actuator is a basic and indispensable optical component for the applications of new-generation optical data storage systems and tunable optical devices.
To drive the lens or mirror, various kinds of mainstream actuation techniques can be used in the field of MOEMS. These actuation mechanisms include magnetic [13], thermal [14], piezoelectric [15], and electrostatic [10]. The piezoelectric scheme is not often used to drive silicon-based actuators due to the limited material selection (compatible problem) and the high level of the thin film technology. Though the magnetic actuation, as well as the thermal actuation, is capable for the low voltage operation, the power consumption is high [16]. The electrostatic mechanism has been commonly adopted due to mass production with high reliability as well as its extremely low power consumption [17]. However, it also faces many challenging issues such as low mechanical stability and a very high operation voltage for the electrostatic comb-drive.
Similar to the Si devices, III-V semiconductors also have attracted many attentions for their potential applications. Especially, monolithic integration of a III-V semiconductor and a micro-mechanical silicon structure is useful for functional optical light source devices. For example, an optical switch was fabricated from a GaAs optical waveguide combined with a GaAs comb-drive actuator [18]. AlN with piezoelectric properties was useful for high frequency acoustic filters [19]. There were also some simple micro structures such as bridges, cantilevers and diaphragms fabricated on a Si substrate for III-nitride MEMS [20–22]. One of the most important advantages of fabricating GaN-MOEMS is monolithic integration of a GaN-based light source and a micro mechanical Si structure [23]. A GaN-LED was incorporated in a micro optical system on a Si substrate for directional illumination and in a micro total analysis system, such as endoscope, and photonic crystal slab [24–26]. Therefore, monolithic integration using a GaN-based optical semiconductor is useful for functional optical light source devices. However, few reports on GaN-MOEMS, freestanding structures such as tunable gating [23] and micro mirror [27] have been fabricated and studied recently. The deposition of the GaN-based films is carried out before the etching process in these works. In other words, the GaN-based films must be etched in the fabrication process. It is difficult to etch GaN directly with conventional wet etching techniques due to its stable chemical properties (GaN is a tough material). On the other hand, the etching process will produce a higher residual stress which might cause cracking or deformation for the fabricated GaN micro-actuators. In recent years, Tang et al [28] reported the selective growth of GaN on a patterned SiC substrate by MBE. Yang et al [29] reported the fabrication of GaN micro-beams by a GaN-on-patterned-silicon technique and the stress decreased 47%. It is a feasible scheme to fabricate the suspended GaN microstructures by means of the growth of GaN on patterned Si substrates to avoid the manufacturing of GaN.
In this paper, in order to integrate a GaN-base light source and an actuator, the design and fabrication of electrostatic comb-drive actuators from an InGaN/GaN multilayer grown on a patterned SOI substrate are described. A double-side process for SOI substrate is used to define two kinds of integrated actuators (horizontal and vertical comb actuators) to achieve two dimension (2D) movements of the micro-mirror. Then the InGaN/GaN epitaxial structures fabricated by molecular beam epitaxy (MBE) technology depend on the pattern of the processed SOI substrate. The performances of the combined actuators are examined, and the optical characteristics of the InGaN/GaN film on patterned SOI substrate are also investigated. The combination of the GaN-on-patterned-SOI epitaxial growth technique and Si micromachining technology is an important step for the further integration of the MOEMS devices.
2. Design and fabrication
Figure 1(a) shows a schematic diagram of the proposed micro-mirror actuator (without InGaN/GaN layer). The whole actuators are symmetric with respect to the centerline crossing the centre of the micro-mirror. The device consists of a micro-mirror, two folded-springs, shared movable combs, and fixed combs of the horizontal comb actuators (HCA) and vertical comb actuators (VCA). The gap between the movable comb fingers and fixed comb fingers is 5μm. The design parameters of the device are summarized in Table 1.
Fig. 1 (a) Schematic structure of the micro-mirror actuator; (b) Layered structure of the micro-mirror actuator; (c) Magnified view of the integrated comb-drive actuator.
Table 1. The design parameters of the device
We can found in Fig. 1(c) for a magnified view of the integrated comb-drive area, only the fixed combs of the VCA are realized on the Si handling layer and oxide layer, while the others are all defined on the top Si layer of the SOI wafer. The micro-mirror is anchored to the wafer through the two symmetrical springs. The displacement of the movable combs connected to the springs results in a 2D movement of the micro-mirror. The fixed combs of the HCA are divided into two symmetric parts, thus the bi-direction movements of the micro-mirror in horizontal direction (in two opposite directions) are achieved to increase the motion range. The detailed patterns of each layer for the SOI substrate are shown in the Fig. 1(b). When the fabrication of the patterned SOI substrate is finished, an InGaN/GaN multilayer film is deposited on the surface of the patterned SOI substrate by MBE finally, and then the micro actuators for 2D tuning of the InGaN/GaN micro-mirror used in optical pickup are fabricated.
A SOI-base process is employed to define the proposed integrated device on SOI substrate. Figure 2 illustrates schematically the fabrication process. The starting substrate is 2 cm × 2 cm SOI cut wafer which consists of a 50 μm top silicon layer, a 2 μm buried oxide layer, and a 150 μm silicon handling layer (steps a). The top silicon layer is photo-lithographically patterned and subsequently etched down to the silicon oxide layer by deep reactive ion etching (DRIE) where the silicon oxide layer acts as an etching stop layer. So the microstructures including the Si micro-mirror, springs, HCA and movable combs of the VCA are created (steps b and c). Then these components on the top silicon layer are protected by thick photoresist (step d). The fixed combs of the VCA are obtained on the silicon handling layer by backside alignment technology and subsequently etched down to the silicon oxide layer from the backside by DRIE (steps e and f). The silicon oxide layer is removed in the buffered HF (BHF) solution and then the patterned SOI substrate is achieved (step g). Finally, the InGaN/GaN multilayer film is deposited on the surface by using MBE technology and the GaN/Si-MOEMS actuator for 2D tuning of the micro-mirror is fabricated (step h).
The deposition of the InGaN/GaN multiple quantum well (MQWs) active layer was carried out by a Riber 32 MBE system with RF nitrogen plasma as the nitrogen source. First the patterned SOI substrate was preheated in a high-vacuum small chamber at a temperature of 200 °C for 12 hours. Then the substrate was transferred into the growth chamber and cleaned at a temperature of 800 °C for 30 min. Nitridation of the surface of SOI substrate was processed after the first-phase preparations finishing, which is a very important step for improving the GaN crystal growth. The surface of SOI substrate was nitrified at the temperature of 650 °C for 30 min with N plasma pressure of 1.4 × 10−4 Torr. Then a 20 nm low-temperature GaN nuclei layer and a 15 nm GaN layer was deposited at a temperature of 300 °C and 500 °C, respectively. Finally the five-period InGaN/GaN layered structure was deposited at a temperature of 580 °C. The intended thickness of the InGaN and GaN layers was 4 nm and 11 nm respectively.
The surface microstructures of the device were observed using scanning electron microscopy (SEM, SU8000, Hitachi). The Temperature-dependent photoluminescence (PL) were measured by using a Laser Roman Spectrometer (Horiba), and with a 325 nm He-Cd laser as the excitation source. The spot size is 10 μm. The minimum testing temperature of 4 K was obtained by using a closed-cycle Helium cooling system (OptistatAC-V, Oxford Instruments)
3. Theories
The in-plane (X-Y) and out-of-plane (X-Z) schematic diagrams of the comb structures at the initial position are shown in Fig. 3(a) and Fig. 3(c), respectively. We take the length direction of the fingers as X axis, the width direction as Y axis, and the thickness direction as Z axis. Then the resulting electrostatic field cause related displacement in X (horizontal) and Z (vertical) direction, which are illustrated in Fig. 3(b) and Fig. 3(d). When the actuation voltages V1 and V2 are applied as shown in Fig. 3(d), the stored energy of the parallel plate capacitances between the overlapped sides for the adjacent comb-fingers can be given by and . And and are the capacitances between the horizontal combs and vertical combs, respectively, where is the permittivity of the air, h, h1 and h2 are the thicknesses of the top Si layer, the Si handling layer and the Si oxide layer, respectively (as shown in Fig. 3(c)), g is the gap width between the adjacent comb-fingers in Y direction (as shown in Fig. 3(a)), Dx and Dz denote the actuation displacements in horizontal direction and vertical direction, respectively (as shown in Fig. 3(d)), l is the length of the combs which is just equal to the overlapping length of the movable combs and the fixed combs of the VCA.
Fig. 3 Schematic diagrams of two-axis motion for the integrated comb structures: (a) initial position at X-Y plane; (b) actuated position at X-Y plane; (a) initial position at X-Z plane; (a) actuated position at X-Z plane.
Because only half of the fixed comb-fingers of the HCA are actuated and all of the fixed comb-fingers of the VCA are actuated at the same time, the total stored energy is the sum of the ones of the individual comb-fingers. It is given by
Where n is the number of the fixed comb-fingers of the HCA. The electrostatic forces generated by the energy fields and balanced by the mechanical restoring forces can be given by . The resulting actuation displacements in horizontal direction and vertical direction can be calculated as follows when combined with Eq. (1),Where and are the stiffness of the spring for supporting the comb actuator in horizontal direction and vertical direction, respectively. For the two-sided folded beam structure, the stiffness of the spring is given by [30]where w and b are the width and the length of the spring beam, respectively, and E is the Young’s modules.The above model is based on the text-book theories of the parallel plate capacitances and it can be easily applied for all kinds of electrostatic comb actuators of MEMS. There are some details worthy of specific note when it is employed here.
- (1) In our present case, this theoretical analysis is based on a few justifiable assumptions. First, a simplified single-layer device structure is assumed during the calculation of the driving displacements. This assumption ignores the effect of the deposited InGaN/GaN layer (the total thickness is about 110 nm) which is much thinner than the top Si layer (50 μm). Second, the thickness of the InGaN/GaN layer and the effect of the interface are ignored during the determination of the spring constants. Third, the gravity-induced deflection in the vertical direction is not taken into account.
- (2) The voltage of V1 is applied to the left comb set as shown in Fig. 3(d), and it also can be applied to the right comb set. Thus the horizontal displacement with respect to the HCA occuring in two opposite directions is . We also must attention that the range for the horizontal and vertical displacements is and , respectively.
- (3) From Eq. (2), the parameters of the comb structures should be optimized to obtain the maximum motion range and the minimum coupling influence in the desired driving voltage region. However, it is difficult to control quantitatively the displacement due to the complex formula (Eq. (2)). The approximate method without the coupling effect developed by Komvopoulos [31] can be used to simplify the actuation. And the calculated displacements with this method are also similar to our results.
4. Results and discussion
SEM images of the fabricated device structure are presented in Fig. 4. The integrated micro-mirror actuator structure is shown in Fig. 4(a), and no fracture-related issues are found. Figure 4(b) shows the detail structures of the micro-mirror having a diameter of 1 mm, which possesses cracks free, dense and uniform surface microstructure. The front side and back side comb structures are clearly shown in Fig. 4(c) and Fig. 4(d), respectively. The movable comb fingers located on both sides of the middle beam are shared by the HCA and VCA. The fixed combs are located on the left and right sides of the pictures, from Fig. 4(d), only the fixed combs of the VCA can be found. But in Fig. 4(c), we can observe two kinds of the fixed combs (HCA and VCA) insulated by SiO2 layer.
Fig. 4 SEM image of the fabricated the InGaN/GaN multilayer film covered micro-mirror actuator: (a) overall structure of the device; (b) micro-mirror; (c) front side of the comb-drive structures; (d) back side of the comb-drive structures.
The simulated and measured displacements of the movable comb as a function of the applied voltage are shown in Fig. 5. As shown in Fig. 5(a) and 5(c), in the horizontal direction, the displacement is mainly determined by the value of V1. And it shows a linear relationship between the simulated displacement of the HCA and the square of the applied voltage V1 when the value of V2 is invariable. But the result in Fig. 5(c) implies clearly that the coupling effect may not be ignored in the 2D actuator. The horizontal displacement is slightly affected by a change of the voltage applied in the vertical direction. The simulated and measured displacement is 22.36 μm and 17.97 μm at no applied voltage of V2, respectively. However, when V2 is applied, the values begin slightly decreasing under the same driving voltage (V1 = 300 V). Specifically, it is 20.79 μm and 15.82 μm at V2 = 200V, respectively. Similarly, as shown in Fig. 5(b) and 5(d), the vertical displacement is dominated by the value of V2, and the influence of V1 shown here is pretty weak. It also has an approximate relationship of . On the other hand, the differences between the experimental and simulated results in Fig. 5(c) and 5(d) may be induced by the fabrication deviations and the reality deviating from the ideal element in structures [10]. Under the circumstance of fulfilling the spring and the practical requirement of micro-mirror, structural parameters (gap width, number of comb and etc.) and material system can be optimized to decrease the power consumption. The displacement and driving voltage also can be improved by controlling the thickness and location of the comb electrodes [32].
Fig. 5 Simulated and measured displacements of (a) (c) the horizontal comb actuator and (b) (d) the vertical comb actuator, the superscripts m and s denote the measurement and simulation, respectively.
The temperature-dependent PL of the InGaN/GaN multilayer structure on the SOI substrate was also carried out from 10 to 300 K and the results are summarized in Fig. 6. As temperature is increased, the peak wavelength position of the PL emission exhibits an S-shaped behavior (redshift - buleshift - redshift). This S-shaped variation is particularly marked for the InGaN/GaN system and has been attributed to the carrier dynamics associated with carrier localization in potential minimums [33]. The InGaN is easy to separate into a high In concentration part and a low In concentration matrix, which results in In-rich clusters and phase separation, and then the exciton localization is caused by the inhomogeneous potential fluctuations. For 10 K<T<50 K, since the radiative recombination process is dominant, the excitons were given more opportunity to relax down into low-energy levels in strongly localized states before recombining. It produces a redshift in the peak wavelength position with increasing temperature as a result of the reducing of the higher energy side emission. For 50 K<T<150 K, since the effect of localized states decrease gradually with the temperature increasing, the carriers will absorb more thermal energy and transfer to the high energy levels in strongly localized states. This behavior leads to a blueshift in the peak wavelength position. When temperature increases up to 150 K, the nonradiative recombination processes become dominant, and the blueshift behavior is smaller than band-gap shrinkage induced by the temperature. Thus, the peak position exhibits a redshift behavior again.
Fig. 6 Temperature-dependent PL from the InGaN/GaN multiquantum well structure on the patterned SOI substrate; the inset shows the laser intensity dependent PL from the InGaN/GaN multiquantum well structure.
In addition, we also performed laser intensity dependent PL measurement at 20 K. The results of such a measurement are shown in the inset in Fig. 6, exhibiting that the stronger the intensity of the incident laser, the higher the peak intensity of the PL spectra. The number of photons emitted by the excitonic transition is increased with the laser power increasing. Moreover, the figure also shows that the emission wavelength shifts to shorter wavelength (blueshift) with increasing laser power. Due to the quantum-confined Stark effect (QCSE), the energy levels of InGaN/GaN quantum wells were shifted to lower energies. However, In the PL experiment (the sample is irradiated by an excitation source), electron–hole pairs are generated by the photon excitation and separated by the piezoelectric field, which results in another electric field with the direction opposite to the initial piezoelectric field and thus the effective electric field is weakened. In other words, the piezoelectric field is screened by the photogenerated carriers, thus weakening the QCSE. Increasing the excitation power density generates more free carriers to screen the piezoelectric field and thus further weakens the QCSE, resulting in blueshifts of the PL peak wavelength [34, 35]. This explains why PL peaks of our InGaN/GaN QWs show blueshifts with increasing excitation intensity. Furthermore, at the higher excitation power density, the band filling effect becomes prominent [35, 36], which also results in a blueshift of the PL peak position with an increase in the excitation power density.
5. Summary and conclusion
The design, fabrication, theoretical and experimental results of the InGaN/GaN multilayer film covered micro-mirror actuator are presented and discussed. A double-sided mask process and molecular beam epitaxial deposition technology were used to define the device. When the voltage is applied on the micro actuator, the consequent electrostatic force generates a motion of the micro-mirror in the corresponding direction. The results show approximately a linear relationship between the displacement and the square of applied voltage. Specially, the measured displacement is ± 17.97 μm and 5.98 μm at the applied voltage of 300 V in horizontal and vertical direction, respectively. And the influences of the cross-axis coupling effect are pretty weak. Moreover, the luminous performance of the InGaN/GaN Quantum well film was investigated with photoluminescence. The peak wavelength position exhibits a “redshift - buleshift – redshift” S-shaped behavior with the increasing of the temperature (0 - 300 K). It is mainly affected by the localized state effect and band-gap narrowing effect. Meanwhile, we also found that the photoluminescence intensity and wavelength are significantly related to the excitation energy of laser.
Funding
National Natural Science Foundation of China (NSFC) (Grant Nos. 61274121, 61574080, 51602160, and 61605086) Natural Science Foundation of Jiangsu Province (Grant Nos. BK2012829, BK20150842, and BK20150850). Talent Project of Nanjing University of Posts and Telecommunications (NUPTSF) (Grant Nos. NY214161, NY214011, and NY215087).
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