28 research outputs found
Effects of dislocation density on injection and temperature sensitivity of InGaN LED emission spectra: a combined experimental and simulation approach
The aim of this paper is to describe a combined simulation and characterization activity carried out on blue LEDs grown on templates with different threading dislocation densities (TDDs)
Correlating electroluminescence characterization and physics-based models of InGaN/GaN LEDs: Pitfalls and open issues
Electroluminescence (EL) characterization of InGaN/GaN light-emitting diodes (LEDs), coupled with numerical device models of different sophistication, is routinely adopted not only to establish correlations between device efficiency and structural features, but also to make inferences about the loss mechanisms responsible for LED efficiency droop at high driving currents. The limits of this investigative approach are discussed here in a case study based on a comprehensive set of current- and temperature-dependent EL data from blue LEDs with low and high densities of threading dislocations (TDs). First, the effects limiting the applicability of simpler (closed-form and/or one-dimensional) classes of models are addressed, like lateral current crowding, vertical carrier distribution nonuniformity, and interband transition broadening. Then, the major sources of uncertainty affecting state-of-the-art numerical device simulation are reviewed and discussed, including (i) the approximations in the transport description through the multi-quantum-well active region, (ii) the alternative valence band parametrizations proposed to calculate the spontaneous emission rate, (iii) the difficulties in defining the Auger coefficients due to inadequacies in the microscopic quantum well description and the possible presence of extra, non-Auger high-current-density recombination mechanisms and/or Auger-induced leakage. In the case of the present LED structures, the application of three-dimensional numerical-simulation-based analysis to the EL data leads to an explanation of efficiency droop in terms of TD-related and Auger-like nonradiative losses, with a C coefficient in the 10−30 cm6/s range at room temperature, close to the larger theoretical calculations reported so far. However, a study of the combined effects of structural and model uncertainties suggests that the C values thus determined could be overestimated by about an order of magnitude. This preliminary attempt at uncertainty quantification confirms, beyond the present case, the need for an improved description of carrier transport and microscopic radiative and nonradiative recombination mechanisms in device-level LED numerical models
Design of high-efficiency photonic devices based on wide gap semiconductors
The General Assembly of the United Nations has proclaimed 2015 the International Year
of Light and Light-based technologies. If a few years ago lighting was synonym of incandescent
lamp, now it is undeniable that we have entered a light-emitting diode (LED)
era. From automotive to streetlights, from lights in our houses to the displays of TVs
and smartphones, LED-based technology is making its way in the market. This proliferation
would have been impossible without GaN-based LEDs, whose invention by Isamu
Akasaki, Hiroshi Amano and Shuji Nakamura has been rewarded with the 2014 Nobel
Prize in Physics. The fact that products based on LED technology are widely commercialized
does not mean that there aren’t still gray areas in the understanding of their operating
principles. Not surprisingly, one of the main targets of the EU Horizon 2020 research program
is to achieve a 20% increase in energy efficiency. A critical contribution to research
is given by computer-aided design, modeling, and simulation. My research activity was
mainly focused on the numerical analysis of efficiency-limiting mechanisms in wide gap
optoelectronic devices and, in particular, in GaN-based LEDs.
The thesis is divided in two parts. In the first part, a short theoretical introduction
on optoelectronic devices is presented, to provide an historical perspective. Chapter 1
illustrates LEDs physical properties while in Chapter 2 are discussed the peculiarities of
vertical-cavity surface-emitting lasers (VCSELs). The simulation analysis is based on
two commercial simulation tools: APSYS from Crosslight Software Inc. and TCAD
Sentaurus from Synopsys Inc. Chapter 3 illustrates the physical models implemented in
APSYS.
In the second part, the research activity is discussed. LEDs intended to operate in the
ultraviolet (UV) spectral range are of great interest for several applications. One material
system with the potential of leading to efficient UV LEDs is zinc oxide and its ternary
compounds (MgZnO and BeZnO). Two-dimensional numerical simulation, employed to
assess a number of possible design approaches aimed at optimizing the internal quantum
efficiency (IQE) of BeZnO-based light-emitting diodes is presented in Chapter 4.
The realization of gallium-nitride GaN-based VCSELs emitting blue light has encountered
some challenges associated with the difficulties of obtaining current transport close
to the optical axis in the active region and of realize efficient distributed Bragg-reflector
(DBR) mirrors in nitride-based materials. Therefore, a number of cavity structures have
been realized or suggested, by different research groups, where a current-confining aperture,
usually referred as current aperture, is used, on top of which a dielectric DBR mirror
is deposited as the top mirror of the VCSEL. Chapter 5 shows numerically that many of
these GaN-based VCSEL cavities balance dangerously close to the border between the
guided and antiguided regime.
In Chapter 6 GaN-based LEDs with color-coded QWs has been used to analyze the
carrier injection and recombination phenomena in MQW-based LEDs. These devices
have a MQW structure where each QW has a different indium content, corresponding to a
different emission wavelength. Colorcoded devices have been used to study asymmetries
in carrier distribution, i.e., to understand if in a MQW structure all the QWs contribute
to light emission, to evaluate the relative contribution of each well to the overall light
emission, and to understand how carriers are injected and recombine within the LED
structure.
The ongoing debate on efficiency droop in GaN-based LEDs attests the present limitations
of both characterization techniques and numerical models of III-nitride active
optoelectronic devices. Chapter 7 illustrates the main issues faced when performing a
combined experimental and simulation-based analysis of InGaN/GaN blue LEDs with
an example carried out on two sets of devices grown, characterized and simulated by
the authors. In order to help discriminate the recombination coefficients, structures with
different levels of threading dislocation densities (TDDs) have been considered.
Recent experiments of electron emission spectroscopy (EES) on III-nitride LEDs have
shown a correlation between droop onset and hot electron emission at the cesiated surface
of the LED p-cap. The observed hot electrons have been interpreted as a direct signature
of Auger recombination in the LED active region, as highly energetic Auger-excited electrons
would be collected in long-lived satellite valleys of the conduction band so that they
would not decay on their journey to the surface across the highly-doped p-contact layer.
Chapter 8 discusses this interpretation by using a full-band Monte Carlo model based on
first-principles electronic structure and lattice dynamics calculations. The results of our
analysis suggest that Auger-excited electrons cannot be unambiguously detected in the
LED structures used in the EES experiments
Design of high-efficiency photonic devices based on wide gap semiconductors
The General Assembly of the United Nations has proclaimed 2015 the International Year
of Light and Light-based technologies. If a few years ago lighting was synonym of incandescent
lamp, now it is undeniable that we have entered a light-emitting diode (LED)
era. From automotive to streetlights, from lights in our houses to the displays of TVs
and smartphones, LED-based technology is making its way in the market. This proliferation
would have been impossible without GaN-based LEDs, whose invention by Isamu
Akasaki, Hiroshi Amano and Shuji Nakamura has been rewarded with the 2014 Nobel
Prize in Physics. The fact that products based on LED technology are widely commercialized
does not mean that there aren’t still gray areas in the understanding of their operating
principles. Not surprisingly, one of the main targets of the EU Horizon 2020 research program
is to achieve a 20% increase in energy efficiency. A critical contribution to research
is given by computer-aided design, modeling, and simulation. My research activity was
mainly focused on the numerical analysis of efficiency-limiting mechanisms in wide gap
optoelectronic devices and, in particular, in GaN-based LEDs.
The thesis is divided in two parts. In the first part, a short theoretical introduction
on optoelectronic devices is presented, to provide an historical perspective. Chapter 1
illustrates LEDs physical properties while in Chapter 2 are discussed the peculiarities of
vertical-cavity surface-emitting lasers (VCSELs). The simulation analysis is based on
two commercial simulation tools: APSYS from Crosslight Software Inc. and TCAD
Sentaurus from Synopsys Inc. Chapter 3 illustrates the physical models implemented in
APSYS.
In the second part, the research activity is discussed. LEDs intended to operate in the
ultraviolet (UV) spectral range are of great interest for several applications. One material
system with the potential of leading to efficient UV LEDs is zinc oxide and its ternary
compounds (MgZnO and BeZnO). Two-dimensional numerical simulation, employed to
assess a number of possible design approaches aimed at optimizing the internal quantum
efficiency (IQE) of BeZnO-based light-emitting diodes is presented in Chapter 4.
The realization of gallium-nitride GaN-based VCSELs emitting blue light has encountered
some challenges associated with the difficulties of obtaining current transport close
to the optical axis in the active region and of realize efficient distributed Bragg-reflector
(DBR) mirrors in nitride-based materials. Therefore, a number of cavity structures have
been realized or suggested, by different research groups, where a current-confining aperture,
usually referred as current aperture, is used, on top of which a dielectric DBR mirror
is deposited as the top mirror of the VCSEL. Chapter 5 shows numerically that many of
these GaN-based VCSEL cavities balance dangerously close to the border between the
guided and antiguided regime.
In Chapter 6 GaN-based LEDs with color-coded QWs has been used to analyze the
carrier injection and recombination phenomena in MQW-based LEDs. These devices
have a MQW structure where each QW has a different indium content, corresponding to a
different emission wavelength. Colorcoded devices have been used to study asymmetries
in carrier distribution, i.e., to understand if in a MQW structure all the QWs contribute
to light emission, to evaluate the relative contribution of each well to the overall light
emission, and to understand how carriers are injected and recombine within the LED
structure.
The ongoing debate on efficiency droop in GaN-based LEDs attests the present limitations
of both characterization techniques and numerical models of III-nitride active
optoelectronic devices. Chapter 7 illustrates the main issues faced when performing a
combined experimental and simulation-based analysis of InGaN/GaN blue LEDs with
an example carried out on two sets of devices grown, characterized and simulated by
the authors. In order to help discriminate the recombination coefficients, structures with
different levels of threading dislocation densities (TDDs) have been considered.
Recent experiments of electron emission spectroscopy (EES) on III-nitride LEDs have
shown a correlation between droop onset and hot electron emission at the cesiated surface
of the LED p-cap. The observed hot electrons have been interpreted as a direct signature
of Auger recombination in the LED active region, as highly energetic Auger-excited electrons
would be collected in long-lived satellite valleys of the conduction band so that they
would not decay on their journey to the surface across the highly-doped p-contact layer.
Chapter 8 discusses this interpretation by using a full-band Monte Carlo model based on
first-principles electronic structure and lattice dynamics calculations. The results of our
analysis suggest that Auger-excited electrons cannot be unambiguously detected in the
LED structures used in the EES experiments