III-V nitride-based materials have become major semiconductors for optoelectronic devices such as blue-light-emitting diodes (LEDs) and laser diodes (LDs). Gallium nitride (GaN) is a wide-bandgap and high-bond-energy semiconductor material. Because GaN is chemically stable and insoluble in all mineral acids [1Lee H, Oberman DB, James S. Reactive ion etching of GaN using CHF3/Ar and C2ClF5/Ar plasmas. Appl Phys Lett 1995; 67: 1754-6.
[http://dx.doi.org/10.1063/1.115039] -6Xu L, Pan G, Zou C, Shi X, Liu Y. Atomically smooth gallium nitride surfaces generated bychemical mechanical polishing with non-noble metal catalyst(Fe-Nx/C) in acid solution. In: International Conference on Planarization/CMP Technology (ICPT). 2014; pp. 237-41.], performing wet etching during device fabrication is extremely difficult. Therefore, for fabricating blue LEDs or LDs, we used dry etching techniques such as reactive-ion-etching (RIE), inductively coupled plasma (ICP), and electron cyclotron resonance (ECR) reactors. However, bombardment of energetic ions damages the surface critical layer of a GaN device. Heavy ion bombardment of the crystal surface at low pressure produces pits, craters, and hillocks [7Kucheyev SO, Williams JS, Jagadish C, Li G, Pearton SJ. Strong surface disorder and loss of N produced by ion bombardment of GaN. Appl Phys Lett 2000; 76: 3899-901.
[http://dx.doi.org/10.1063/1.126814] -14Velasco AJ, Chacon AL, Serrano WA. Negative ion generation and isotopic effect in electron cyclotron resonance plasma. IEEE Trans Plasma Sci 2015; 1-4.]. The formation of these defects is explained by variations in the sputtering rate across the surface because of irregularities and defects in the surface. We classified these damage defects according to the lattice disorders and point defects.

The layer distribution of a completely formatted structure of GaN-based blue LED is shown in Fig. (1). The epitaxial structure of a GaN-based LED from bottom to top is sequentially presented as follows. An undoped u-GaN buffer layer and u-GaN layer were grown on a sapphire substrate to facilitate the growth of the single crystal material n-GaN. The n-AlGaN and InGaN/GaN multiquantum well (MQW) active layers and the p-AlGaN and p-GaN layers were then grown on the single crystal n-GaN. The MQW active layer, composed of InGaN/GaN, is the primary area that irradiated blue light when a forward-bias voltage was applied between the two electrodes.

Electron-hole recombination occurred at the P/N junction, and released photon energy, which was radiated in a photonic emission. ICP etching was employed to etch back the n-GaN electrode area after the various layers were grown, and coat a transparent conductive layer (TCL). The TCL uniformly spread currents, preventing conduction crowding. Finally, the p- and n-electrode areas of this LED were coated with gold (Au) for wiring and creating contacts. In this study, we used Cl₂/Ar gas sources to etch the n-GaN surface in an ICP system. The plasma reactor is shown in Fig. (2). The main reason for using the Cl₂-based plasma was to form GaCl₃ and NCl₃ [15Gao Z, Romero MF, Calle F. Etching of AIGaN/GaN HEMT structures by Cl2-based ICP. In: Spanish Conference on Electron Devices. 2013; pp. 29-32.]. The surface was expected to be Ga-rich because of the high volatility of the NCl₃. The addition of an Ar source should efficiently remove etching residue products on the surface. However, attention should be focused on the induced damage problem on the GaN surface. Therefore, a thermal-annealing treatment was used for preventing surface damage on the n-GaN material after the dry etching process.

Fig. (1) Cross-section view of a preliminary formation structure of GaN blue LED.

Fig. (2) Schematic illustration of an inductively-coupled-plasma reactor.

The assistance of dc bias voltage is a major factor in the ICP process [16Kim H, Schuette ML, Lu WCl. 2/BCl3/Ar plasma etching and in situ oxygen plasma treatment for leakage current suppression in AlGaN/GaN high-electron mobility transistors. J Vac Sci Technol B 2011; 29: 031204-5. [Microelectron Nanometer Struct].
[http://dx.doi.org/10.1116/1.3581090] ]. The variation of dc bias achieved by changing the coil and platen power in a Cl₂/Ar plasma system is shown in Fig. (3). In general, to increase the dc bias voltage, more energetic ions must be bombarded. Preliminarily, the n-GaN samples used in this study were grown on a sapphire substrate through metal-organic chemical vapor deposition and were approximately 3.5 μm thick. This n-type GaN layer had a carrier concentration of approximately 3 × 10¹⁸ cm⁻³. In the following experimental groups, the device-under-test (DUT) samples were etched using an ICP process by using Cl₂/Ar gas sources in an STS plasma system. A total gas flow rate of 50 sccm and etching duration time of 60 s was chosen as the optimal condition (Fig. 3). In addition, all samples were etched using a platen power of 120 W and a dc bias voltage of −100 V. Table 1 lists the detailed physical parameters of the fabrication process.

For easy implementation, a dry etching process was required for fabricating GaN electronic devices. To improve and recover the electric properties caused by ion-induced damage, the n-GaN samples were treated using a thermal-annealing process in an N₂ ambient after dry etching. In this study, annealing was performed in a flowing N₂ gas atmosphere for 40 min and as a function of ambient temperatures (500, 550, and 600 °C). After the annealing, some of the DUTs of the n-GaN materials were deposited on the Ti/Al metal layer through e-beam evaporation and were lifted-off, then alloyed at 550 °C for 5 min to form satisfactory ohmic contact; however, the remaining DUTs of the complete LED devices were fabricated as described in Section 1 (Introduction).

Fig. (3) Platen power vs. coil power in an ICP system as the DC-bias voltage (negative value) varied.

Table 1

Detail setting parameters of an ICP system.

The defects on the surface of n-GaN materials and GaN LEDs caused by plasma dry etching by using an ICP reactor were investigated in this study. We demonstrated the thermal annealing influence on the emission characteristic of the n-GaN materials and GaN devices. Furthermore, a suitable thermal annealing treatment of the GaN samples might improve the luminescence intensity and emission life time. From the experimental data, the resistance decreased dramatically because of exposure at 550 ˚C for 40 min of annealing in an N₂ ambient after dry etching. In addition, the luminescence intensity and emission life time were improved in most aspects typically at 550 ˚C annealing temperature, which was because of the improvement of crystal quality at the surface and reduction of nonradioactive related centers (called recombination centers) in the material.

The authors confirm that this article content has no conflict of interest.