Broadband White-Light Emission from Alumina Nitride Bulk Single
Crystals
Ge Liu, Chengyuan Yan, Guigang Zhou, Jiamin Wen, Zuoyan Qin, Qin Zhou, Baikui Li, Ruisheng Zheng,
Honglei Wu,* and Zhenhua Sun*
College of Optoelectronic Engineering, Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and
Guangdong Province, Shenzhen University, Shenzhen, 518000, China
*
S
Supporting Information
ABSTRACT: Alumina nitride bulk single crystals (AlN BSCs)
were grown using a two-heater Physical Vapor Transport (th-
PVT) method. The crystal contains massive lattice defects
including aluminum vacancy (V
Al
) and oxygen substitution
(O
N
). The photoluminescence (PL) spectrum of the crystal
demonstrated a broad emission covering from 250 to 1000 nm.
By study the PL spectrum, abundant midgap states in the wide
band gap of AlN were nailed down. Based on the crystals, metal-
AlN-metal Schottky devices were fabricated. These devices
emitted high quality white light with color rendering index
(CRI) over 90 under a bias of 60 V. Moreover, it was found that the white light emitting property of AlN BSCs was adjustable
through changing impurity density and device structure. This research aims to pave a new way for solid-state white light source.
KEYWORDS: nitride crystals, lattice defects, midgap states, white-light source, defect-induced emissions
S
olid state white light source is superseding incandescent
and fluorescent lamps due to its advantages in cost,
efficiency, and stability.
1−6
There are three mainstr eam
technologies used to realize solid state white lighting. First, a
blue light-emitting diode (LED) is coated with a yellow
phosphor containing rare-earth elements, renderi ng high
luminous efficacy but poor color ren dition. Secon d, an
ultraviolet LED is coated with a mixture of red, green, and
blue (RGB) phosphors. The efficiency herein is limited due to
the self-absorption. Third, RGB LEDs are used directly to form
a white light. Due to the narrow spectrum of a LED device, the
light formed by this way is a pseudo white light with a
discontinuous spectrum.
7−10
Therefore, a high-performance
solid state white light source based on single component
(without phosphor) is of increasing interests recently for its
wide application potential in illumination, display, visible color
communication, as well as full spectrum laser.
1
Apparently, a
monolithic semiconductor with wide emission spectrum is
pivotal in this device.
A series of monolithic semiconductors including CdSe
quantum dots
2
and organic−inorganic hybrids
3−6
have been
investigated in quest of broadband white-light emitting
materials. Their broadband emissions are attributed to the
wide electronic band gap due to the quantum confinement
effect, and the existence of midgap states due to surface sites
and lattice distortion. Nevertheless, these microscale structures
need delicate synthesis pro cess and suffer from inferior
environment stability. Herein, alumina nitride bulk single
crystals (AlN BSCs) with a good broadband white-light
emitting property are presented. These AlN BSCs can be
grown massively and possess high stability. AlN has a wide
bandgap of ∼6 eV. Different lattice defects in the crystal induce
plenty of radiative midgap energy states. Electron relaxations
among the conduction band, valence band, and those midgap
states lead to a wide spectrum photoluminescence from 210 to
1000 nm. What’s more, biased between two electrodes, AlN
BSCs emit white light covering broadband spectrum from 300
to 1000 nm with a good color rendition. These broadband
emission properties make the AlN BSCs a good material
candidate for phosphor-free, electricity-driving, solid state
white light source.
AlN BSCs were grown on tungsten substrates in a Physical
Vapor Transport furnace with two resistive heaters (th-
PVT).
11,12
Crystals with size at millimeter level were obtained.
The insets of Figure 1a are the top view SEM image of an AlN
BSC on substrate and the schematic of wurtzite AlN crystal
lattice structure. The crystal has a regular hexagonal
appearance, which matches well with the wurtzite crystal
lattice structure. The Figure 1a is the X-ray diffraction (XRD)
pattern of the crystal. Only (0002) plane peak could be
perceived in parallel with the substrate. This proves the single
crystal structure and the c-axis growth orientation of the AlN
BSC. The composition of the crystals was characterized by X-
ray photoelectron spectroscopy (XPS), with the result
demonstrated in Figure 1b. The result reveals the existence
of O element in the crystals. Quantitively analysis pointed out
that the atomic amount of O varied between 0.5% and 0.9%
Received: May 7, 2018
Published: June 24, 2018
Letter
Cite This: ACS Photonics 2018, 5, 4009−4013
© 2018 American Chemical Society 4009 DOI: 10.1021/acsphotonics.8b00601
ACS Photonics 2018, 5, 4009−4013
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