1.
Introduction
Group IIIA nitrides such as gallium nitride (GaN), and aluminum nitride (AlN) are used in many devices; they have an importance for electronic and optoelectronic applications.
GaN has demonstrated its superiority to achieve high ultraviolet light-emitting diodes (LEDs) performance; it admits a direct and wide band gap, great chemical and thermal stabilities[1]. UV LEDs have attracted considerable interest due to a wide range of applications[2–4] such as water treatment, high density optical data storage, sterilization of medical equipment, and biological imaging[5, 6]. Each of these applications requires light sources emitting at a different UV wavelength[6] where the electromagnetic spectrum is divided into regions based on the wavelength of the radiation, with the UV region defined as the wavelength range from 10 to 400 nm[7] In order to achieve the desired purpose of each application with high performance and optimal reliability such as cost-efficiency and energy-efficiency, many parameters should be controlled, for example the wavelength, the output power, the high densities and so. In this context recently various studies have been realized such as the using of the low-temperature barriers to improve strain relaxation and high performance GaN based LEDs where the authors success to increase the strain-relaxed mechanism of low-temperature barriers and address this problem[8–12]. Maximum output power and external quantum efficiency were obtained by employing 222–282 nm AlGaN and InAlGaN-based deep UV LEDs fabricated on low threading dislocation density AlN template[13], and different growth methods were proposed to obtain AlGaN/AlN templates to obtain low threading dislocation density (TDD)[12–14, ].
With a methodology based on stress management on underlying GaN-based epitaxial films, the authors demonstrated numerically and experimentally that the optimal tensile stress can increase by over 100% in comparison to the LEDs with compressive stresses[15]. Involving a simple high-quality AlN/AlGaN buffer template on sapphire substrate which allows a remarkable improvement in the output power of 340 nm-band quaternary InAlGaN-based UV-LEDs, however a maximum output power of 7.1 mW was reached and the authors found that the crystalline quality of the AlN/AlGaN templates strongly affects the output power of UV-LEDs[14].
These materials acquired many attention towards Solid-State lighting due to their continuous improvement in efficiency and longer life time when compared to other lighting systems including Compact Fluorescent Lamp (CFL) bulbs, filament bulbs etc[16].
Recently, boron nitride (BN) has been introduced in electronic and optoelectronic components. Some of its properties have certain advantages such as electrical conductivity, high mechanical and electrical resistances, chemical stability, and optical transparency in a wide spectral range. It can be distinguished from other elements to give better performance for components; many alloys of the type B(Al, Ga, In)N have been recently proposed to take advantage of these properties[17].
Boron nitride (BN) has some interests, particularly for its hexagonal (h-BN)[18–22], rhombohedral (r-BN)[19–22], and turbostratic (t-BN)[20–23] structures. It can occur in several polymorphs, including cubic zinc blende (c-BN)[21, 22, 23]. In its cubic structure[24], (c-BN) has a band gap of 6.4 eV[23].
Although the BN does not occur naturally in the wurtzite structure, one can combine the BN with AlN or GaN in order to obtain wider band intervals in this structure[25].
The BN can serve as a natural extension of the family of III-nitrides such as GaN and AlGaN[26] with low concentration of B, to obtain BGaN, and BAlN alloys[22].
2.
Proposed structure and simulation model
Several works exist on the AlxGa1–xN/GaN LED structures but very little on the AlxGa1–xN/BGaN LEDs. The purpose of our work is therefore to make a comparison between these two structures, using the ATLAS simulation tool. ATLAS enables device technology engineers to simulate the electrical, optical, and thermal behavior of semiconductor devices. The structures proposed with GaN and BGaN single quantum well (SQW) are shown in Fig. 1.
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Figure1.
(Color online) Schematic structure of UV LEDs with GaN and BGaN single quantum well.
The GaN-LED structure consists of sapphire substrate, 5 nm GaN buffer layer, 500 nm n-doped GaN cladding layer with a concentration of 1 × 1018 cm?3, 60 nm n-doped Al0.2-Ga0.8N cladding layer with a concentration of 1 × 1019 cm?3, undoped GaN QW, 60 nm p-doped Al0.2Ga0.8N cladding layer with a concentration of 1 × 1018 cm?3, and 120 nm p-doped GaN contact layer with a concentration of 1 × 1018 cm?3.
In the BGaN-LED, GaN QW is replaced by an undoped BGaN with a boron concentration fixed to only 1%. The band gaps of GaN, BGaN, and AlGaN are then 3.42, 3.32, and 3.8 eV, respectively.
We consider different thicknesses for the QW: 7, 10, 15, and 20 nm.
For the binary materials (GaN, AlN, and BN), the properties are taken from the ATLAS user’s manual and from references[27, 28]. For GaN, effective density of states in the conduction band (Nc) is 1.2 × 1018 cm?3, effective density of states in the valence band (Nv) is 4.2 × 1019 cm?3, electron mobility (μn) is 1000 cm2/V s, and hole mobility (μp) is 350 cm2/V·s.
For AlxGa1-xN and BxGa1-xN, the parameters are calculated using the Vegard law[29]. For carrier recombination, both Shockley-Read-Hall (SRH) and Auger recombination are defined by formulas[30, 15]; the SRH and Auger recombination coefficients are 1 × 1017 s?1 and 1 × 10?34 cm6/s, respectively[15]; effective densities of states in the conduction band Nc are 3 × 1018 cm?3 and 2.35 × 1018 cm?3, respectively; effective densities of states in the valence band Nv are 1.4 × 1019 and 4.6 × 1019 cm?3, respectively. Carrier mobilities for AlGaN are μn = 860 cm2/(V·s), and μp = 283 cm2/(V·s), respectively; for BGaN μn = 992 cm2/(V·s), and μp = 351 cm2/Vs, respectively. Saturation velocity (Vsat) is 2.5 × 107 cm/s for AlGaN, and 2.3 × 107 cm/s for BGaN. The band offset between AlGaN and BGaN is 0.48 eV and between AlGaN and GaN is 0.38 eV.
The origin of the piezoelectric polarization is the crystalline structure of gallium nitride. The two structures of GaN, hexagonal and cubic, are non Centro-symmetric. This mismatch between the layers induces a kind of mechanical stress, forming an interface bias charge, and thus creating what is called the piezoelectric field.
The spontaneous polarization (polarization at zero strain) is very large in wurtzite group-III nitrides resulting in a further increase of polarization induced electric fields and interface charges[31].
A layer of gallium nitride can present a macroscopic polarization not null. There is an electric field because the crystal has an asymmetry between the barycenters of the electron charges with those of the holes. Also a link between the two atoms that is not purely covalent is a factor that can cause the creation of this electric field.
To obtain good precision, we refine the meshes in the QW layer, under the electrodes, and in the current path from the anode to the cathode.
3.
Results and discussion
The Injection current versus Forward voltage is simulated for both LEDs, and for various thicknesses of the QW layer; results are illustrated by Fig. 2. In GaN-LED, the anode current decreases slightly when the QW thickness increases; the best one is of 0.52 mA (a current density of 26 kA/cm2) for 7 nm QW thickness. In BGaN-LED, the Injection current increases more slightly with the QW thickness; the best one is of 0.27 mA (a current density of 13.5 kA/cm2) for 20 nm QW thickness. We obtain a greater anode current in GaN-LED than that in BGaN-LED.
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Figure2.
(Color online) Injection current versus forward voltage for GaN-LED and BGaN-LED.
These results are logical because the BGaN compound is more resistive than GaN, in other words it contains fewer free electrons; in addition the charge carriers have better mobility in GaN than in BGaN.
Generation of light in LEDs is mainly due to spontaneous emission of photons, the magnitude measured by a spectrometer is the power spectral density, and the magnitude that determines the efficiency of the device is the Light output power.
Spontaneous emission, power spectral density, and light output power of GaN-LED and BGaN-LED are simulated; results are given by Figs. 3, 4, and 5, respectively. Summarized values of spontaneous emission and power spectral density are given by Tables 1 and 2, respectively.
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Figure3.
(Color online) Spontaneous emission of GaN-LED and BGaN-LED.
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Figure4.
(Color online) Power spectral density of GaN-LED and BGaN-LED.
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Figure5.
(Color online) Light output power of GaN-LED and BGaN-LED.
| QW thickness (nm) | 7 | 10 | 15 | 20 | |
| GaN-LED | Peak energy (eV) | 3.43 | 3.43 | 3.44 | 3.45 |
| λ (nm) | 361.5 | 361.5 | 360.5 | 359.4 | |
| Spontaneous emission (1028 s?1 cm?3 eV?1) | 1.15 | 1.53 | 2.36 | 3.34 | |
| BGaN-LED | Peak energy (eV) | 3.43 | 3.36 | 3.36 | 3.36 |
| λ (nm) | 361.5 | 369 | 369 | 369 | |
| Spontaneous emission (1028 s?1 cm?3 eV?1) | 2.04 | 3.43 | 2.53 | 2.08 | |
| Ratio of spontaneous emissions | 1.77 | 2.24 | 1.07 | 0.62 | |
| Difference between spontaneous emissions (1028 s?1 cm?3 eV?1) | 0.89 | 1.9 | 0.17 | ?1.26 | |
Table1.
Summarized values of spontaneous emission for GaN-LED and BGaN-LED.
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| QW thickness (nm) | 7 | 10 | 15 | 20 | |
| GaN-LED | Peak energy (eV) | 3.43 | 3.43 | 3.44 | 3.45 |
| λ (nm) | 361.5 | 361.5 | 360.5 | 359.4 | |
| Spontaneous emission (1028 s?1 cm?3 eV?1) | 1.15 | 1.53 | 2.36 | 3.34 | |
| BGaN-LED | Peak energy (eV) | 3.43 | 3.36 | 3.36 | 3.36 |
| λ (nm) | 361.5 | 369 | 369 | 369 | |
| Spontaneous emission (1028 s?1 cm?3 eV?1) | 2.04 | 3.43 | 2.53 | 2.08 | |
| Ratio of spontaneous emissions | 1.77 | 2.24 | 1.07 | 0.62 | |
| Difference between spontaneous emissions (1028 s?1 cm?3 eV?1) | 0.89 | 1.9 | 0.17 | ?1.26 | |
| QW thickness (nm) | 7 | 10 | 15 | 20 | |
| GaN-LED | Peak energy (eV) | 3.46 | 3.45 | 3.45 | 3.45 |
| λ (nm) | 358.4 | 359.4 | 359.4 | 359.4 | |
| Power spectral density (W cm?1 eV?1) | 1.61 | 1.81 | 1.84 | 1.93 | |
| BGaN-LED | Peak energy (eV) | 3.35 | 3.31 | 3.31 | 3.31 |
| λ (nm) | 370.1 | 374.6 | 374.6 | 374.6 | |
| Power spectral density (W cm?1 eV?1) | 2.4 | 5.6 | 6.1 | 6.7 | |
| Ratio of power spectral densities | 1.49 | 3.09 | 3.31 | 3.47 | |
| Difference between power spectral densities (W cm?1 eV?1) | 0.79 | 3.79 | 4.26 | 4.77 | |
Table2.
Summarized values of power spectral density for GaN-LED and BGaN-LED.
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| QW thickness (nm) | 7 | 10 | 15 | 20 | |
| GaN-LED | Peak energy (eV) | 3.46 | 3.45 | 3.45 | 3.45 |
| λ (nm) | 358.4 | 359.4 | 359.4 | 359.4 | |
| Power spectral density (W cm?1 eV?1) | 1.61 | 1.81 | 1.84 | 1.93 | |
| BGaN-LED | Peak energy (eV) | 3.35 | 3.31 | 3.31 | 3.31 |
| λ (nm) | 370.1 | 374.6 | 374.6 | 374.6 | |
| Power spectral density (W cm?1 eV?1) | 2.4 | 5.6 | 6.1 | 6.7 | |
| Ratio of power spectral densities | 1.49 | 3.09 | 3.31 | 3.47 | |
| Difference between power spectral densities (W cm?1 eV?1) | 0.79 | 3.79 | 4.26 | 4.77 | |
The basic mechanisms responsible for the generation of photons in the SC are the spontaneous and the stimulated emissions.
The injected minority carriers give a light if their radiative life time is shorter than their non-radiative life time.
Band-to-band transitions can occur for example from collisions with the crystal lattice (photon-phonon interactions), or from cascades of low energy transitions due to the presence of deep electronic levels in the forbidden band.
These deep electronic levels may be due to chemical impurities, defects in the crystal lattice (dislocations) as well as free surfaces and interfaces with other materials. The performance is limited by these nonradiative transitions.
The second and most important non-radiative recombination mechanism that limits LEDs performance is Auger recombination; it is characterized by the Auger carrier lifetimes which are inversely proportional to the square of the concentrations of the corresponding regions.
BGaN compound contains fewer free electrons; the rate of Auger recombination is therefore smaller in BGaN than in GaN. Thereby, in the case of BGaN, we have better spontaneous emission, better power spectral density and better Light output power.
The better value of the spontaneous emission is of 3.34 × 1028 s?1 cm?3 eV?1, for 20 nm of QW thickness in GaN-LED. It is of 3.43 × 1028 s?1 cm?3 eV?1, for 10 nm of QW thickness in BGaN-LED; and it decreases for a greater thickness because the material becomes too resistant.
The better value of the power spectral density in GaN-LED and BGaN-LED is obtained for 20 nm of QW thickness; it is respectively of 1.93 and 6.7 W cm?1 eV?1. We obtain a ratio of 3.47 between the values.
The Light output power increases with the QW thickness; better values are obtained for 20 nm of QW thickness. In GaN-LED, we have 0.61 mW at an injection current of 0.55 nA; and in BGaN-LED, we have 0.89 mW at only 0.1 nA of injection current. For this last current, the Light output power in GaN-LED is only of about 0.25 mW. We can say that the total luminous power is multiplied by 3.56 in the BGaN-LED.
There is a slight difference between the emission wavelengths in both LEDs, but the performance is significantly improved in the second LED, due to the BGaN material properties.
The flux spectral density is represented by Fig. 6, and obtained values are given by Table 3. For GaN-LED, the smallest value is obtained for the quantum well thickness of 7 nm; it is about 2.91 × 1018 s?1 cm?1 eV?1 for peak energy of 3.47 eV. For 10 nm we have a peak energy of 3.46 eV; for 15 and 20 nm, we have practically the same peak energy of 3.45 eV. For BGaN-LED, we obtain a peak energy of 3.47 eV for the quantum well thickness of 7 nm, and 3.31 eV for other QW thicknesses. The flux spectral density is, respectively, 3.25 × 1018, 3.32 × 1018, 3.5 × 1018 s?1 cm?1 eV?1 for GaN-LED; and 4.48 × 1018, 1.03 × 1019, 1.13 × 1019, and 1.25 × 1019 s?1 cm?1 eV?1 for BGaN-LED. The peak of the flux spectral density increases with the QW thickness.
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Figure6.
(Color online) Flux spectral density for GaN-LED and BGaN-LED.
| QW thickness (nm) | 7 | 10 | 15 | 20 | |
| GaN-LED | Peak energy (eV) | 3.47 | 3.46 | 3.45 | 3.45 |
| λ (nm) | 357.3 | 358.4 | 359.4 | 359.4 | |
| Flux spectral density (1018 s?1 cm?1 eV?1) | 2.91 | 3.25 | 3.32 | 3.5 | |
| BGaN-LED | Peak energy (eV) | 3.35 | 3.31 | 3.31 | 3.31 |
| λ (nm) | 370.1 | 374.6 | 374.6 | 374.6 | |
| Flux spectral density (1018 s?1 cm?1 eV?1) | 4.48 | 10.3 | 11.3 | 12.5 | |
| Ratio of flux spectral densities (s?1 cm?1 eV?1) | 1.54 | 3.17 | 3.40 | 3.57 | |
| Difference between flux spectral densities (1018 s?1 cm?1 eV?1) | 1.57 | 7.05 | 7.98 | 9 | |
Table3.
Summarized values of flux spectral density for GaN-LED and BGaN-LED.
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| QW thickness (nm) | 7 | 10 | 15 | 20 | |
| GaN-LED | Peak energy (eV) | 3.47 | 3.46 | 3.45 | 3.45 |
| λ (nm) | 357.3 | 358.4 | 359.4 | 359.4 | |
| Flux spectral density (1018 s?1 cm?1 eV?1) | 2.91 | 3.25 | 3.32 | 3.5 | |
| BGaN-LED | Peak energy (eV) | 3.35 | 3.31 | 3.31 | 3.31 |
| λ (nm) | 370.1 | 374.6 | 374.6 | 374.6 | |
| Flux spectral density (1018 s?1 cm?1 eV?1) | 4.48 | 10.3 | 11.3 | 12.5 | |
| Ratio of flux spectral densities (s?1 cm?1 eV?1) | 1.54 | 3.17 | 3.40 | 3.57 | |
| Difference between flux spectral densities (1018 s?1 cm?1 eV?1) | 1.57 | 7.05 | 7.98 | 9 | |
Gain TE (TE: transverse electric field), and external quantum efficiency are represented by Figs. 7 and 8, respectively.
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Figure7.
(Color online) Gain TE for GaN-LED and BGaN-LED.
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Figure8.
(Color online) External quantum efficiency for GaN-LED and BGaN-LED.
For GaN-LED, the gain begins to decrease slowly from the energy 3.43 eV (λ = 361.5 nm), then it decreases more rapidly from the energy 3.58 eV (λ = 346.3 nm). For BGaN-LED, the gain begins to decrease from the energy of about 3.34 eV (~ λ = 371 nm). With BGaN, we have a slightly higher frequency bandwidth than with GaN as a quantum well.
The maximum external quantum efficiency exceeds 80% for a current of about 13.7 μA in GaN-LED, and it is of about 68% for a current of about 81.7 μA in BGaN-LED. For this second LED, we achieved a value above 100%, something we do not understand.
At a current of 0.4 μA, we obtain external quantum efficiency greater than 50% for the thicknesses of 15 and 20 nm in BGaN-LED, and only 50% in GaN-LED.
4.
Conclusion
AlGaN/GaN/AlGaN and AlGaN/BGaN/AlGaN UV LEDS with SQW were simulated, using TCAD Silvaco software. The boron concentration in BGaN was only 1%, and the thickness of the SQW was varied from 7 to 20 nm. In BGaN-LED; the spontaneous emission was multiplied by 1.77 for 7 nm of QW thickness, and the power spectral density was multiplied by 3.47 for 20 nm of QW thickness. The total Light output power, the frequency bandwidth, and the external quantum efficiency, were significantly improved with BGaN in the quantum well.
Acknowledgments
This work was supported by the University of Abou-BekrBelkaid, Materials and Renewable Energy Research Unit, Tlemcen, Algeria. I am grateful to Prof. Dr. A. Hamdoune for his support during this work.
