In this study, we report a novel monolithically integrated GaN-based light-emitting diode (LED) with metal-oxide-semiconductor field-effect transistor (MOSFET). Without additionally introducing complicated epitaxial structures for transistors, the MOSFET is directly fabricated on the exposed n-type GaN layer of the LED after dry etching, and serially connected to the LED through standard semiconductor-manufacturing technologies. Such monolithically integrated LED/MOSFET device is able to circumvent undesirable issues that might be faced by other kinds of integration schemes by growing a transistor on an LED or vice versa. For the performances of resulting device, our monolithically integrated LED/MOSFET device exhibits good characteristics in the modulation of gate voltage and good capability of driving injected current, which are essential for the important applications such as smart lighting, interconnection, and optical communication.
© 2014 Optical Society of America
The demand of monolithic integration of light-emitting diodes (LEDs) with filed-effect transistors (FETs) is increased for the recent surge in smart-lighting applications, which generally involves sophisticated electronic circuit designs for the AC-DC power convertor, current source, and photo-detectors . Through the monolithic integration that shares the same material platform, the fabrication cost and dimension of lighting systems can be considerably reduced, offering a great functionality and stability for a broad range of applications. Among previous successful demonstrations of monolithic integration of optoelectronic devices on various material systems [2–12], the monolithic integration of GaN-based LEDs and FETs is of research interest due to their unique material properties such as large breakdown voltage [13, 14], high operating frequency [15, 16], and high temperature duration , and that makes it preferable for the emerging markets of high power-high voltage lighting systems and visible light communication [18, 19]. However, there have been only few reports on the monolithic integration of GaN-based LEDs and FETs, due mainly to the restriction of realizing complicated epitaxial structures. For example, Li et al. reported a monolithic integration of GaN-based LED and high-electron-mobility transistor (HEMT) structure . Although such LED-on-HEMT is easy to make on-chip interconnection, it would be difficult to grow high performance LED on the top of HEMT with AlGaN epitaxial layer instead of sapphire substrates due to the lattice mismatch issue . Very recently, Liu et al. reported a monolithic integration of GaN-based HEMT-on-LED structure, but to avoid damaging multiple quantum wells (MQWs) of LEDs underneath, a conflicting choice of low growth temperature for HEMT might degrade its crystalline quality and result in low mobility of the transistor [11, 12]. In this study, we demonstrate a monolithic integration of GaN-based LED and metal-oxide-semiconductor (MOS) FETs. As the MOSFET is directly fabricated on the exposed n-type GaN layer of the LED after dry etching process, no additional growth of HEMT architecture is required, and that eliminates as-mentioned issues which might be arisen during epitaxial procedures. In short, our monolithically integrated LED/MOSFET device is able to circumvent undesirable issues that might be faced by other kinds of integration schemes by growing a transistor on an LED or vice versa. For the performances of resulting device, the fabricated monolithically integrated LED/MOSFET device exhibits a maximum output current of IDS = 1050 mA/mm and a peak transconductance of Gm = 368 mS/mm. The LED exhibits a well rectifying behavior with a slightly high turn-on voltage of 5.32V. Its light out power is about 4.86 mW at I = 20mA, which corresponds to a peak EQE value of 7.52%. Those output characteristics measured on our monolithically integrated LED/MOSFET device are compactable to that of previously reported GaN-based monolithically integrated schemes [10–12]. Additionally, such design provides a large flexibility and feasibility for on-chip interconnection and optical modulation since the cathode of the LED and the drain of the MOSFET are electrically connected. As a result, we believe the proposed integration structure is viable and highly promising for smart-lighting applications.
Figure 1(a) shows a schematic configuration of monolithic integration of GaN-based LED and MOSFET. An LED structure was grown onto 2-inch sapphire substrates using a low-pressure metal-organic chemical vapor deposition (Aixtron 2600G) system. The LED layer-structure comprised of a 30-nm-thick GaN nucleation layer grown at 520°C, a 2-μm-thick undoped GaN layer grown at 1050°C, 2-μm-thick Si-doped n-type (n = 5 × 1018 cm−3) GaN cladding layer grown at 1050°C, an unintentionally doped active region of five periods InGaN/GaN MQWs grown at 700 °C with emitting wavelength of λ = 485 nm, and a 200- nm-thick Mg-doped p-type (p = 3 × 1017 cm−3) GaN layer grown at 800°C. The low growth temperature of p-type GaN layer is to roughen its surface for enhanced light extraction efficiency of the LED [21–23]. The LED structure was then selectively removed by dry etching (inductively coupled plasma, ICP) with Ar/Cl2 mixed gases to expose the n-type GaN layer for the subsequent fabrication of MOSFET. A 2-μm-deep trench down to the undoped GaN layer was created to isolate the LED and MOSFET mesa regions by an additional ICP process. A 500-nm-thick SiO2 passivation layer was then deposited on the sidewalls of LED and MOSFET mesas by plasma-enhanced chemical vapor deposition (PECVD). Next, the remaining of n-type GaN layer on top of the MOSFET mesa was patterned by standard photolithography and ICP dry etching that gives a film of 150 nm for the current channel. After that, a 200-nm-thick SiO2 layer was deposited as a gate dielectric by using PECVD again at an operating temperature of 300°C. Ti/Al was deposited by e-beam evaporation and annealed by rapid thermal annealing (RTA) at 600°C for 30sec in N2 ambient for source/drain contact metals of the MOSFET and cathode of the LED. ITO/Ni was deposited by the RF magnetron sputtering, patterned by lift-off process, and annealed at 450°C for 2 minutes in O2 ambient to form p-type ohmic contacts for the LED. Finally, Ni was deposited and patterned as the gate electrode of the MOSFET.
Figure 2(a) shows the top-view scanning electron microscope (SEM) image of our monolithically integrated LED/MOSFET device. Accordingly, as the LED mesa is surrounded by the MOSFET, the electron flow (originated from the source terminal) injecting into the cathode of LED is uniform and well controlled, and switched by biased gate voltage, so is the LED’s light output power. Additionally, such a serially connected integration provides a much compact and promising way for the subsequent fiber coupling for the application of optical communication. Figure 2(b) shows the atomic force microscope (AFM) images of the LED (top, p-GaN) and MOSFET (bottom, Gate) surfaces with a scanned area of 20 × 20 μm2. The root mean square (RMS) estimated for the LED and MOSFET are 67.49 and 8.16 nm, respectively. As mentioned above, we intentionally roughened the LED’s surface by creating inverted hexagonal pits to enhance the guided light extraction efficiency, and that causes its relatively large surface roughness on the p-GaN layer. After dry etching as shown in the bottom of Fig. 2(b), the roughness of the exposed surface of MOSFET device became smoother by an appropriate modification of ICP condition, providing a suitable platform for the subsequent PECVD growth of gate dielectric and is also beneficial to low leakage current characteristics.
3. Results and discussion
We first examine the DC characteristics of the fabricated device. Figure 3(a) shows output ID-VD measurements of the monolithically integrated LED/MOSFET device with the applied gate voltage (VGS) ranging from VGS = −1.5V to 2V in 0.5V interval. The maximum output current of IDS = 1050 mA/mm is achieved under VGS = 2V, and the specific on-resistance (Ron) is estimated to be Ron = 105Ω. The output characteristic of the MOSFET is comparable to that of previously reported monolithically integrated structures based on GaN-based HEMT devices [10–12]. Figure 3(b) shows the transfer characteristics of the MOSFET for the applied source-to-drain voltage of VDS = 2V, 4V, and 6V. The MOSFET exhibits a peak transconductance of Gm = 368 mS/mm at VDS = 6V, and achieves an OFF-state drain leakage current as low as Ioff = 2.47 × 10−5 mA/mm at VDS = 2V. The DC characteristic shown in Fig. 3 suggests that the performance of the MOSFET is acceptable, and hence can be monolithically integrated with the LED device. It shall be addressed again that the MOSFET was fabricated on the exposed n-GaN layer after dry etching, and no additional growth layers of HEMT was employed in this study. Thus such device structure of monolithically integrated LED with MOSFET is more direct and also feasible without introducing other undesirable issues as previously reported in the literatures.
Figure 4(a) shows measured current versus voltage (I-V) curves of the LED in both linear (red line) and semi-log (black line) scales. An optical image of the fabricated device with lighted up LED at an injection current of 20mA is also inserted in the figure. Because of the absence of current spreading layer on the top of LED, a current crowding effect was clearly observed on the optical image. Nevertheless, the LED’s I-V curve still exhibits a well rectifying behavior with a turn-on voltage of 5.32V, slightly higher than that of typical GaN-based blue-light LEDs due mainly to the un-optimized condition of p-contact metals (ITO/Ni) used in this study, and that could be further improved by substituting it to more generally adopted ohmic contacts metals such as Ni/Au films . For the same reason, the series resistance of the LED estimated to be RS~60.85 Ω is also higher than the typical value. The leakage current in the order of 2.05 × 10−6 A at −5 V is measured, which corresponds to a LED’s shunt resistance of RP~5.27 MΩ, which is acceptable for the LED with a chip size of 300 μm in diameter. Figure 4(b) plots the light output power (LOP) and external quantum efficiency (EQE) versus the injected current of the LED. The emission spectrum under the injection current of 20 mA is also inserted in the figure. The LED’s light output power increases rapidly at low injected current (I < 20mA), and becomes saturated at higher injected currents (I > 90mA). The LED’s EQE therefore achieves its highest value at low injection current, and then decreases pronouncedly with a further increasing of injection current. The possible reason responsible for the EQE droop in the high current could be attributed to the current crowding effect as observed on the inset of Fig. 4(a). The current crowding effect of the LED can be alleviated by the employment of the current spreading layer such as ITO films . It shall be addressed here that as compared to that of the commercial available LED device, the relatively lower LOP and EQE values measured on our sample are mainly due to the un-optimized condition of p-contact metals and the absent of current spreading layer deposited on the LED’s top surfaces. Nevertheless, it will not affect the emission modulation capability of our monolithically integrated LED/MOSFET device much. Further optimization of metal contact and employment of current spreading layer to alleviate the EQE droop and to enhance the LED’s I-V characteristics will be performed.
Next, we are able to test the performance of the monolithically integrated LED/MOSFET device. Figure 5(a) shows LED’s current (same as the source-to-drain current, IDS) and light output power (LOP) versus supply voltage across the entire device (VDD) for different gate voltages ranging from VGS = −1V to 1V in 0.5V interval. Due to the serially connected configuration between the LED and MOSFET devices (as the equivalent electrical circuit plotted in the inset of Fig. 1), IDS is therefore restricted by the LED before its turn-on, and becomes saturated and dominated by the channel current of MOSFET while further increasing VDD. Obviously, by a simple control of the MOSFET’s VGS, we are able to modulate the injected current and light output power of the LED with good linearity. As shown in the inset of Fig. 5(a), we demonstrate a LED light-switch (red line) with an ON/OFF frequency of 10kHz by directly modulating the VGS of the MOSFET (black line) and the IDS injected into the fabricated device (blue line). Similarly, Fig. 5(b) shows IDS and LOP versus VGS of the monolithically integrated LED/MOSFET device for the supply voltage of VDD = 7V and VDD = 15V. Again, the saturation of IDS and LOP observed on the operating condition of VDD = 7V is mainly due to the limited current within the MOSFET channel. By increasing the supply voltage to a larger value of VDD = 15V, both IDS and LOP increases linearly with the increasing of VGS. Observations in Fig. 5 indicates that our monolithically integrated LED/MOSFET device exhibits good characteristics of gate controllability and current driving capability, which are essential for various applications such as smart lighting, interconnection, and optical communication.
In conclusion, we have demonstrated a monolithic integration of optoelectronic (LED) and electronic (MOSFET) devices in the GaN-based platform by using standard semiconductor-manufacturing technologies. The fabricated monolithically integrated LED/MOSFET device exhibits a maximum output current of IDS = 1050 mA/mm and a peak transconductance of Gm = 368 mS/mm. The LED exhibits a well rectifying behavior with a slightly high turn-on voltage of 5.32V, mainly due to the un-optimized condition of p-contact metal. Most importantly, the monolithically integrated LED/MOSFET device exhibits good gate controllability in the LED’s light output power, and hence is viable and highly promising for a broad range of applications.
The authors gratefully acknowledge financial support from the Ministry of Science and Technology in Taiwan (contracts No. MOST 103–2112–M–003–008–MY3 and NSC 102–2218–E–009–017), and from the Bureau of Energy, Ministry of Economic Affairs in Taiwan.
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