Development of technology for improved nitride-based optoelectronic devices

Abstract

The main objective of this thesis is the development of the device fabrication technology for nitrides, towards establishing a technology platform, which can serve as a benchmark for the realization of Nitride-based optoelectronic devices and beyond. In this light, research was focused on basic fabrication procedures, which are not yet detrimental for nitrides and restrain realization of novel optoelectronic devices that are costly and reliable as well. Such procedures are dry-etching, ohmic contact formation to p-GaN and optimization of active region of laser structures. The formation of optical cavity is one of the main issues in fabricating a semiconductor laser and it can be achieved, either by cleaving the semiconductor or by etching its surface in order to form mesas with vertical and smooth sidewalls. Our study resulted in obtaining mesas with mirror-like facets, having a slope of 90o and a roughness of less than 100 nm, using a conventional Reactive Ion Etching (RIE) reactor with chlorine-based chemistry and photoresist AZ 5214 as the etch mask. These results were obtained after thorough optimization of lithography and RIE processes. For enhanced material protection during plasma etching, we have developed an alternative lithography procedure with a double photoresist layer, which ensures that photoresist etch-mask is sufficiently thick throughout the etching process. Next, a study aiming in forming low resistance ohmic contacts to p-GaN followed. The influence of several pre-deposition surface treatments and different contact metals to the electrical properties of metal/p-GaN contacts was studied. A low resistance as-deposited ohmic contact was achieved, with the use of boiling aqua regia (AR) as the surface treatment, and Cr/Au as the contact scheme. The ohmic resistance of contacts with 50 μm interspacing was found to be 50 5. The results of X-ray Photoelectron Spectroscopy (XPS) characterization of p-GaN might be able to explain formation of this low resistance contact scheme. Towards this end, a formation mechanism is proposed, based on the appropriate preparation of p-GaN surface by boiling AR, and on covalenttype of bonding between Cr and semiconductor atoms, as evidenced by the short-range epitaxial character of Cr layer on p-GaN, ascertained by HRTEM and the Ga-Cr intermetallic phase formed at the interface without heat treating the contacts, ascertained by XPS. Concerning active region of laser structures, a series of InAlGaN (QN) thin films and InAlGaN/GaN QWs of various In and Al compositions were grown and characterized. For the In0.08Al0.28Ga0.64N/GaN QWs, a reduction of internal-fields by a factor of 4 as compared to equivalent GaN/AlGaN QWs was estimated after fitting of our experimental results with theoretically calculated curves indicating that an internal-field of 0.265 MV/cm exists inside QN QWs. Following the same procedure, the corresponding internal-field value for In0.14Al0.28Ga0.58N/GaN QWs was estimated to be essentially zero as expected from theoretical calculations, which predict zero internal-field in case of In/Al composition ratio close to ½. The reduced internal-fields inside the QN QWs are also indicated by temperature-, power-dependent and time-resolved PL characterization, with the latter yielding radiative recombination times of few hundreds of ps. Moreover, optical pumping experiments on laser structures composed of QN QWs in the active region show reduced power threshold values as compared to equivalent GaN/AlGaN structures. Finally, by employing the optimized dry-etching and the novel, as deposited p-GaN contact scheme, violet EL was readily observed. The latter suggests that the as deposited Cr/Au contacts are good candidates for serving as the p-electrode in nitride-based optoelectronics. Furthermore, a novel nitride-based sensor, monolithically integrating an LED and a HEMT, is designed and realized. A study concerning process flow optimization resulted in achieving operation of both LED and HEMT parts of the devices in a satisfying level, with the first showing good performance and intense light emission, while the second needs further optimization. Considering the complexity of the structure and the multi-step nature of process flow, the fact that both parts of the devices satisfyingly operate and can be individually controlled, is very inspiring and opens the way for our novel, integrated sensor device to be tested in actual sensing experiments and, depending on the results, to be patented and commercialized.