An investigation on the epitaxial growth of GaN film on Si(111) substrate

Abstract

Gallium nitride (GaN) has been considered as a promising material for photoelectronic devices such as blue, near ultraviolet and violet light emitting diodes (LEDs) [1–3] and laser diodes (LDs) [4] since it has a direct band gap of 3.4 eV at room temperature. Unfortunately, it is difficult to grow large bulk single crystals of GaN. Therefore, heteroepitaxy is so far necessary for the growth of single crystalline GaN films on various substrates including Si, SiC, ZnO, GaAs, GaP and sapphire, among which the sapphire substrate is the most extensively used. GaN single crystal films are conventionally obtained using molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) on sapphire. However, since the sapphire substrate is not conductive, the electrodes for the devices are not readily fabricated. α-SiC and ZnO in the wurtzite structure are promising substrates for the GaN epitaxy because they are conductive and relatively matchable with GaN, but they are difficult to obtain with high quality and large sizes at low costs. GaAs and GaP are thermally unstable at the growth temperatures higher than 1000 ◦C during MOCVD. Accordingly, Si is regarded as one of the most promising substrates for the GaN epitaxy, since it is available in large sizes and high quality at a relatively low cost. Furthermore, the GaN epitaxy on Si will facilitate the integration of microelectronics and optoelectronics. However, it is difficult to grow single crystalline GaN films directly on the Si substrate because of the large lattice mismatch (37.77%) and the large difference in the thermal expansion coefficient (41.16%) between GaN (in the wurtzite structure) and Si . Takeuchi et al. and Watanabe et al. have reported the GaN epitaxy on Si(111) by MOCVD using the buffer layers of SiC and AlN, respectively [5, 6]. Guha et al. have also reported the fabrication of GaN LEDs grown by MBE on the Si(111) substrate, which was used as a bottom contact for electron injection into the device structure, through a thin AlN growth initiation layer [7]. Yang et al. have grown cubic GaN on Si(001) by plasma-assisted MBE [8]. Up to date, it is still difficult to prepare single crystalline GaN film on the Si substrate. In this paper, we report the GaN epitaxy on Si(111) by a novel vacuum reaction method which is much simpler compared with other techniques such as MOCVD and MBE. The properties of the deposited GaN films have been evaluated by surface, structural, and optical characterization methods. GaN films were grown by a vacuum reaction method in a vacuum chamber. The system is illustrated as Fig. 1. Elemental gallium (Ga) as the Ga-atom source put in a cell is heated by a tungsten filament in the center of the chamber. NH3 as the N-atom source is introduced near the substrate. The substrate is heated by a coiled tungsten filament. The Ga cell is close to the substrate, and the evaporating Ga atoms from the cell to the substrate are all within the heat field. Therefore, the technique and growth system are different from MBE. The details of the reactor are described in ref. [9]. The Si(111) substrates were cleaned using RCA1 solution (NH4OH : H2O2 : H2O= 1 : 1 : 6), followed by RCA2 solution (HCl : H2O2 : H2O= 1 : 1 : 6), each for 15 min at 85 ◦C. Then they were dipped into 10% HF solution for 10 s to remove native SiO2 on Si surface. The substrate was immediately transferred into the chamber from the solution and the chamber was pumped down. When the base pressure was pumped to 1× 10−5 Pa, the substrate was heated to 950 ◦C and maintained at that temperature for 20 min to remove the passivation layer on the substrate. NH3 was subsequently introduced for in-situ cleaning of the substrate to form a clean surface [10]. Then the substrate temperature was lowered to 800 ◦C to grow a 50 nm GaN buffer layer. Finally the substrate temperature was raised above 1000 ◦C for the GaN epitaxy. The growth pressure in the reactor was 1× 10−2 Pa. The growth rate varied from 0.4 μm/h to 0.2 μm/h depending on the experimental conditions such as the temperature and gas flow.