The Characteristics Of GaN Films Deposited On ITO Substrates At Low Temperature

Gallium nitride (GaN) is known as the typical material of third generation semiconductor after Silicon (Si), Gallium Arsenide (GaAs), attributed to its appealing physical properties such as direct wide band gap (3.39eV), high saturated electron velocity (1.5×107cm/s) and high breakdown voltage. At present, Gallium nitride (GaN) and related group III-V nitrides have been widely applied in optoelectronic devices such as laser diodes (LDs), light-emitting diodes (LEDs), ultraviolet detector, and it has immense potential in the application of high-power devices such as solid-state lighting. Conventional GaN films are hetero-epitaxially grown on lattice-mismatched sapphire substrates, leading to a high dislocation density and stacking faults in GaN layers. And the poor electrical conductivity of sapphire leads to the current crowding effect, causing the degradation of lifetime and efficiency of the devices. In contrast, Tin-doped indium oxide (ITO, cubic bixbyite structure, aITO=1.0118nm) coated glass substrates have a large area and they are inexpensive. Meanwhile, low electrical resistivity (<5×10-4Ωcm) ITO is an n-type semiconductor with bandgap～3.5-4.3eV, which is close to the bandgap of GaN (3.39eV). This will be imperative for improving the current spread capability of high power GaN devices. In addition, its merits of excellent adherence, hardness, chemical inertness also make ITO coated glass a good candidate to be used as the substrate for GaN films.Energy-activated gas such as ammonia was commonly used in high temperature MOCVD process. But the high deposition temperature that is necessary for decomposition of ammonia may cause decomposition of the ITO coated glass substrate, which will deteriorate the qualities of the GaN films. In contrast, Electron Cyclotron Resonance-Plasma Enhanced Metal Organic Chemical Vapor Deposition (ECR-PEMOCVD) with N2as N precursor can provide a feasible method to deposit GaN films at a low temperature, because the ECR system is a feasible method to remarkably activate reactive energy of N2and hence GaN film and relative nitride films can be grown by ECR-PEMOCVD at an extremely low temperature.In this paper, highly c-axis preferred orientation GaN films were deposited on ITO coated glass substrates by ECR-PEMOCVD. The TMGa and N2are applied as precursors of Ga and N, respectively. High purity H2were employed as carry gases. X-ray diffraction analysis (XRD), reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), room temperature photoluminescence (PL) and Ⅰ-Ⅴ measurement were used to systematically investigate the crystalline quality, morphological, photoluminescence and electrical properties of the as-grown GaN films when different deposition temperature and N2flues carried out. The results indicated that these parameters had great influences on the the qualities of GaN films. The dense and uniformed GaN films with highly c-axis preferred orientation were obtained under optimized deposition temperature of430℃and the N2flues in the range of90to100sccm with other deposition parameters unchanged. And in this condition, PL spectrum shows a intense NBE emission. Finally, I-V measurement shows the Ohmic contact characteristics or Schottky contact characteristics between GaN films and ITO coated glass substrates due to the different deposition parameters.