Microcavities can confine light on the sub-wavelength scale resulted in dramatically enhanced light field intensity and prolonged photon lifetime of microdisks and microring as a platform for light and matter interaction，such as laser devices^［1-3］，active gyroscopes^［4-7］and frequency combs^［8-10］. As an outstanding representative of 3rd generation semiconductor materials，gallium nitride（GaN）demonstrates superior optical and electrical properties ^［11-14］. Therefore，GaN microcavity based Light-Emitting Diodes（LED）^［15-18］ and Laser Diodes（LD）^［19-24］ offer significant advantages in illumination，laser printing，full-color displays，high-density optical disk storage，and many other fields. The reported electrically pumped lasing devices could be classified as Vertical Cavity Surface-Emitting Lasers（VCSELs）^［25-28］ and waveguide laser according to the type of structures. The GaN VCSELs are widely reported due to its excellent performance and matured processing technology. Ref.［29］introduced grating into GaN surface-emission structures to realize a laser with a controllable polarization direction. Ref.［30］fabricated an electrically injected GaN-on-Si LD with ‘sandwich-like’ waveguide architecture，two AlGaN cladding layers were introduced for the optical field confining. Compared with the normal one which usually need to be excited by external light source，light source integrated devices are more convenient. In this case，the electrically pumped microdisk or microring light source，which may have high potential application in active gyroscope and frequency combs，are rarely reported. The reason behinds lie at the limit in material growth and fabrication process. On the other hand，it is also a challenge about reducing optical loss and improving optical gain of the device. The key point to overcome this problem is finding suitable p/n-type electrodes configuration to make the luminescent region as effective microcavity region.

In this study，three well-designed electrically pumped InGaN/GaN microdisk devices were fabricated via photolithography，Inductively Coupled Plasma（ICP）dry etching，and wet isotropic etching technology. Electroluminescence（EL）properties were studied systematically by optimizing the optical loss，the gain region and the position of electrodes. The floating InGaN/GaN quantum well LED based on floating microdisk cavity（Type II）with cylindrical p-type GaN area dramatically reduces the optical loss in the vertical direction of microcavity. The electrode design guarantees that the luminous region and microcavity region can fix together. It is in favor of the microdisk device's optical gain. Finally，resonant mode appears in the device. Taking the resonant spectra for consideration，the optimized device realizes the EL emission with peak wavelength center of 408.2 nm and Full Width At Half Maximum（FWHM）of 2.62 nm under a small injection current about 0.7 mA. The novel structure design of floating electrically pumped InGaN/GaN QW microdisk device is of key significance for electrically pumped microcavity lasers.

Commercially available silicon wafer InGaN/GaN QW epitaxial layer was used to prepare microdisks by using semiconductor micro-nano processing technology. Take the fabrication process of device Ⅱ（Fig. 1）as an example and the others are fabrication via changing the pattern of lithography. Microdisk structures were patterned on the photoresist layer via photolithography（Fig. 1（a））. An epitaxial layer with 2.5 μm thickness is etched onto the n-GaN layer via ICP etching with the photoresist layer as a mask. Then，the residual photoresist was removed by using an acetone solution（Fig. 1（b））. The circular or ring pattern was defined by photolithography（Fig. 1（c））. The sample was etched onto the silicon substrate layer，and the residual photoresist was removed（Fig. 1（d））. The photoresist layer was patterned by photolithography（Fig. 1（e））. The p-electrode and n-electrode were prepared on the p-GaN and n-GaN layers by implementing magnetron sputtering and removing the residual photoresist by using acetone solution. The p/n electrode was composed of nickel（20 nm thick）and gold（100 nm thick）（Fig. 1（f））. Finally，the suspended InGaN/GaN QW microdisk devices were obtained by isotropic wet etching of Si with HF and HNO₃ mixing solution（HF∶HNO₃=1∶1）（Fig. 1（g））. The silicon substrate under InGaN microdisk was etched into the columns to support the InGaN microdisk devices. The structure of fabricated devices Ⅰ-Ⅲ was depicted in box of the right corner of Fig. 1.

The morphological properties of the samples were characterized by field emission scanning electron microscopy（SEM，Carl Zeiss Ultra-Plus）. EL properties and the Ⅳ curves of the device were measured by confocal micro photoluminescence（μ-PL）setup（Olympus BX35）with integrated probe system and alternating current source（Keysight B2901）at room temperature.

The prepared InGaN/GaN QW microdisks have a regular shape and surface with good quality. The inner and out radius is 95 μm and 200 μm for devices Ⅰ and Ⅱ（Fig. 2（a），（b））. The upper and lower electrode layers clearly identify the cylindrical p-type GaN region of these devices. The floating structure of device Ⅱ is identified by the side view SEM picture，in which the microdisk is supported by Si cone pillar（Fig. 2（c））. This design may benefit the reducing of the optical loss from GaN to Si and improve the optical gain. Device Ⅲ is designed with opposite electrode structures to device Ⅱ，thus it presents an annular p-type GaN region on the microdisk（Fig. 2（d））. The inner and out radius of device Ⅲ is 65 μm and 95 μm. The inner n-type circular and outer p-type structures are clearly identified with the lower and upper electrode layers（Fig. 2（e），（f））. With similar methods，GaN LED array can be obtained（Fig. 2（g），（h））.

To evaluate the light emission of the fabricated devices，the EL spectra of devices Ⅰ，Ⅱ，and Ⅲ were obtained under different injection currents as depicted in Fig. 3. Peak wavelength are located at about 408.5 nm，408.2 nm and 406.3 nm respectively. The EL intensity of the microdisk device gradually increases with the increase of injection currents. The inset images in Fig. 3（a）~（c）show the corresponding luminous images of three types of devices. The luminous regions of devices Ⅰ and Ⅱ are the inner p-type electrode circle edge on the microdisk，and the luminous region of device Ⅲ is the outer p-type electrode edge on the microdisk. The different electrode positions and shapes determine the devices' position and luminous shape area. For device Ⅱ，interestingly，the EL spectrum appears clear resonant mode. Taking the resonant spectra for consideration，spectra with peak wavelength near 408.2 nm sharp FWHM can be seen.

Ⅰ-Ⅴ curves of devices Ⅰ-Ⅲ are also measured and shown in Fig. 4. Since the Ni/Au electrode is used both in the p-GaN and n-GaN，it is hard to form an ohmic contact in n-GaN side. Turn-on current of plenary device I is about 18 V and increases the turn-on voltage for suspanded devices. The turn-on voltage of device Ⅲ is higher than 21 V. The increasing of turn-on voltage is due to that the wet etching process will cause some damage to the surface electron of the device. Leak current of different devices is low. The EL intensity rapidly increases in Fig 4（b）. With the increasing of current，FWHM of device I and device Ⅲ are in the region of 12~14 nm. This value is in the similar level of commercial LED. However，due to the cavity effect，the resonant mode appears in device Ⅱ. Two different phenomenon can be obtained in Fig.4（c），for the spectra under low driven current，the resonant peaks are unclear，and FWHM in this case is near 12 nm，and for the higher driven current，considering the resonant peaks of each EL spectra and we re-calculated the FWHM of spectra at resonant position，the FWHM decreased to values below 3 nm for device Ⅱ when the injection current was more than 0.7 mA.

The narrow of FWHM and nonlinearly increasing of EL intensity confirm the well EL properties of the devices. It is also related to the electron and photon coupling in spatial. As demonstrated in the inset of Fig. 3，the luminescence region mainly concentrated in the core of the microdisk in device Ⅰ，Ⅱ and the inner and outer sides of the microring in device Ⅲ. In this case，compare with device Ⅲ，the special electrode structure design of inner p and outer n region in device Ⅰ，Ⅱ are more efficient. To make it clear，the simulation result of surface current distribution is performed by CST（CST Studio Suite）. The surface current distribution of device Ⅰ，Ⅱ is concentrated at the p-type column edge located in the inner region of the microdisk determined by n-type and p-type semiconductor electrical potential difference（Fig. 5（a））. Thus the current distribution of device Ⅰ and device Ⅱ resulted in a luminous region mainly on the p-type column edge（Fig. 3（a），（b））. The surface current distribution of device Ⅲ is mainly concentrated at the inner edge of the p-type located on the outer zone of the microdisk（Fig. 5（b））. Thus the current distribution results in a luminous region mainly in the inner part of the microring（Fig. 3（c））. The overlap of the luminous region and the current region in device Ⅰ，Ⅱ is conducive to the efficient drive of the device. The above simulations analysis of surface current distribution strongly supports the experimental results of EL spectra. Comparing the date in Fig. 3（b）and Fig. 3（c）we can see that the electrode design of the device is significant for the overlap of the luminous region and light confirm region，the realization of an electrically pumped resonant mode.

In summary，the floating InGaN/GaN QW microdisk LED with different kinds of p-type GaN regions is fabricated based on the above analysis. The floating microdisk device has better light confinement owing to the reduction of optical loss in the vertical direction. The structure design with cylinder p-type GaN region on the microdisk can ensure that the microcavity resonance region and luminous region overlap owing to the surface current distribution determined by n-type and p-type semiconductor electric potential difference. Thus，high quality LED with FWHM of 2.62 nm and peak wavelength at 408.2 nm is realized. The novel structure design of an electrically pumped InGaN/GaN QW floating microdisk LED is crucial for electrically pumped lasers.