PA-MBE GaN-based optoelectronics on silicon substrates
Loading...
Date
2009
Authors
Chuah, Lee Siang
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
In this project, radio-frequency (RF) nitrogen plasma-assisted molecular
beam epitaxy (PA-MBE) technique was used to grow GaN-based layers on Si(111)
substrate using high temperature grown AlN as buffer layer. During growth, doping
was done using high purity Si and Mg as n- and p-type dopants, respectively. A total
of seven techniques were employed to study the properties of the GaN-based films
(unintentionally doped n-type GaN, n- and p-doped GaN, unintentionally doped ntype
Al0.09Ga0.91N, n-type In0.47Ga0.53N/GaN heterostucture, AlN cap layer/GaN).
These were X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX),
scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall
measurements, photoluminescence (PL) and Raman spectroscopy. The films were
evaluated in terms of structural, optical and electrical properties.
Since porous GaN-based materials on silicon substrates are a new type of
material, the properties are hardly found in the literature. Several different
characterization tools have been used to investigate the morphological, structural,
and optical properties of porous GaN produced by Pt assisted electroless etching
methods. Different features metal contacts on GaN materials have been investigated
in this project for the purpose of device fabrication. Nickel was found to have
excellent electrical properties and thermal stability at elevated temperatures among
the metal contacts on n-type GaN. A Ni/Ag bi-layer ohmic contact on p-GaN has
been explored. The specific contact resistivities (SCRs) of this bi-layer scheme were
observed to be sensitive to the change of annealing temperatures and durations.
Other than that, the study of Schottky contacts based on four different metallization
schemes, Ti, Ag, Ti/Ag, and Ag/Ti were performed on p-type GaN, and heat
treatment was found able to improve the electrical properties of Schottky contacts
generally. Before heat treatment, the Schottky barrier heights (SBHs) of Ti, Ag,
Ti/Ag, and Ag/Ti were determined to be 0.58, 0.71, 0.53 and 0.62 eV, respectively.
After annealing, the SBHs of Ti, Ti/Ag, and Ag/Ti were found to be 0.67, 0.69 and
0.66 eV, respectively.
Following the intensive investigations of material quality and metal contacts,
metal-semiconductor-metal (MSM) photodetectors based on porous GaN-based
materials were subsequently fabricated and compared to other non-porous-based
devices so that the potential of porous GaN-based materials could be fully explored.
The study also showed that porous GaN layer was able to enhance the electrical
properties of Ni Schottky contacts on GaN in which the SBH and leakage current
were improved significantly. Photodetector fabricated from porous GaN layer also
showed promising properties in which low dark current and higher photocurrent to
dark current ratio were observed. The characteristics of novel GaN-based ultraviolet
(UV) Schottky barrier photodiodes with AlN cap layer (50 nm) were presented.
Thermal annealing treatment has resulted in improved device characteristics by
enhancement of Schottky barrier height, and suppression of dark current of the
fabricated Schottky photodiodes. For Schottky diodes annealed at 500 ºC, 600 ºC,
and 700 ºC, the dark currents were 3.25 x 10-4, 4.97 x 10-5, and 5.05 x 10-5 A,
respectively, under 10 V applied bias. The p-GaN/n-Si heterojunction photodiode
was fabricated to observe the photoelectric effects.
Description
PhD
Keywords
Chemical science , Optoelectronics , Silicon substrates