Optoelectronics (e.g., light-emitting diodes, photodetectors) is one of the most widely used fields nowadays. But it is still necessary to improve their characteristics for using in general lighting. In this chapter, the heterostructure conductivity type, impurity and indium atoms influence on the LEDs and phototransistor characteristics are investigated by computer simulation. It was found that current-voltage characteristic and quantum efficiency depend on impurity and indium atoms change a lot. By varying impurity and indium atom concentration, controlling their distribution in InGaN and AlGaP heterostructure LEDs and photodetector characteristics can be improved.
- light-emitting diode
The ancients said: “Per crucem ad lucem” (“Through the cross to the light”). Another field of modern science and technology could be hardly remembered, which has influenced economics and science so greatly as semiconductor devices, especially optoelectronics. Such devices have a very fascinating history. The optoelectronics started at the beginning of the twentieth century, and its progress was so dynamic that it can be compared with the modern scientific and technological revolution.
Below, valuable steps of optoelectronics development should be briefly pointed out. In 1907, a captain Henry Joseph Round (Figures 1 and 2), personal assistant to Guglielmo Marconi, took a piece of carborundum and saw the yellow light by applying voltage to this material, but he in the paper described only experiment without any physical explanation of this phenomenon [1–4].
Oleg Vladimirovich Losev’s first papers were at the beginning of the twentieth century.
O.V. Losev investigated and carried out physical explanation for injectional and prebreakdown luminescence effects in details [2–6]. Even more, O.V. Losev received first patent for presample of light-emitting diode (LED) in 1927 (Figures 3 and 4).
As it was written by Egon E. Loebner upto 50th year, such effects were called “Losev light” . First contemporary explanation of p-n junction lighting effect was proposed by Kurt Lehovec et al. from Signal Corps Engineering Laboratories (New Jersey) in 1951. In the 1960s, the first GaAs-based near-infrared semiconductor lasers and red-orange light-emitting diodes were introduced by Nick Holonyak and Mary George Craford. In parallel to that development, photodetectors based on III–V semiconductors were developed [4, 7]. In 1963, Zhores Ivanovich Alferov proposed the idea of using nanoheterojunctions (NH) in emitters (Figure 5).
Under Alferov leadership, GaAs-AlGaAs heterojunction investigations were carried out [5, 8, 9]. In 1966, for the first time, effective radiative recombination at p-n junction of four-component solid solution As
Quantum-sized semiconductors used in photonics and optoelectronics (e.g., light-emitting diodes, lasers, photodetectors, etc.) are AIIIBV and AIIBVI N. Their solutions are very interesting due to their unique properties—the wide band gap, strong bonds, and high thermal conductivity. The main outstanding properties of nitride heterostructures are forbidden gap energy that depends on the indium concentration and could change in the range 1.95–6.3 eV. AlGaInN has very bright future in various applications fields—short-wavelength electroluminescence and high-power/temperature/frequency electronic devices. Now, problem of limited color range and lack of high-power white LEDs that previously prevented LED usage for general lighting have been solved. Unfortunately, there are still several problems need to be solved, e.g., LED degradation, efficiency droop nature understanding, quantum efficiency (QE) increase, obtaining optimal quantum size area structure, photodetectors efficiency increase, and developing the method for quick NH and device investigation.
For complex materials and optoelectronic devices, the basic factors that determine their quality, such as current-voltage characteristics (I-V) and the efficiency, can be investigated by computer simulations and include taking into account major structural and physical NH and device parameters [15–24].
2. Simulation basis
For complex materials and optoelectronic devices, the basic factors that determine their quality, such as I-V characteristics and the efficiency, can be investigated by computer simulations that include taking into account major structural and physical NH and device parameters [14–23]. The last decade proved an increased usage of the software for simulation semiconductor devices. Device simulations play an important role in their research. The formulas for devices are complicated. The growth process was simulated, and light propagation and extraction, and the possibility of the external efficiency increase of AlGaInN LED were studied by simulation; explanation of the electroluminescence efficiency degradation at increasing current was suggested.
Freeware computer program SimWindows was used in our investigation . The specific features of this program are: (1) the electrical, optical, and thermal device properties for simulation based on system of exact physical equations, (2) the simulation possibilities with different approximations for two-lead devices and (3) the quantum-sized device simulation. The software extends a lot of traditional electrical models by adding effects such as quantum confinement, tunneling current, and complete Fermi-Dirac statistics. The optical model includes computing electromagnetic field reflections at interfaces. The software is very flexible for semiconductor device simulation. Exact solutions of electrical, optical, and heat transport phenomena in 1D situation are included. For example, drift-diffusion currents, thermoionic and tunneling currents for electrons and holes are taken into account, and in recombination of charge carriers, radiating and nonradiating are included. For band diagram calculations of devices, Fermi-Dirac or Boltzmann statistics and full version of Poisson differential equation have been used. During our investigation, special files for NH, photodetectors, and LED simulations were created. Of course, piezoelectric and spontaneous effects were additionally taken into consideration by including piezo and elastic matrix coefficients. The main parameters for NH, photodetector, and LED type were included into the individual, relating each device and material, special files for simulation. In those files, parameters such as geometric sizes for emitters, QW, and barriers; the QW and barriers quantity; the solid solution content; the conductivity type; and doping concentrations were included. In the materials file for heterostructures, more than 25 parameters such as the band gap, refractive index, optical absorption, thermal conductivity, mobility, and lifetime of charge carriers, and the coefficients of radiative and nonradiative recombination were included. Initial data-preparing files were based on Refs. [24–30].
3. Light-emitting diode improvements
One of the most important parameters that describe LED is the QE (
Understanding the QE dependence according to different influence is very useful for predicting the NH and LED reliability.
Blue InGaN NH contained, n-GaN-emitter (
In Figure 6, it could be seen as main characteristics that could be done during simulation, e.g., InGaN heterostructure. The dependence for main parameters vs. voltage is shown below.
With applying no voltage, electrons and holes are mainly concentrated in n- and p-type areas, respectively, and after applying voltage, they are redistributed into the middle QWs (Figure 6). Basic dependence trend of simulation results blue/green LED (Figure 7). Based on the results, it can be seen that the most economically sound and effective (based upon internal quantum efficiency) production is LEDs with 4 QWs (3.5 nm width). It is due to LED with 4 QWs, the internal quantum efficiency is maximum, and at the same time, resistance is not at its maximum value.
The maximum QE was in the central QWs. It is detected that the active region should contain 4 QWs (central ones are according to recombination and edge concentrate carrier currents for a higher recombination). In red and yellow NH (Figure 7, right), the QE increase at different QW width is observed. Maximum QE occurs at QW width—50 nm. For yellow NH, the QE was three times less than in the red one. The difference is because of reducing the radiative recombination efficiency and small energy gap between the G- and X-minimum in the conduction band.
Now, let us discuss on the investigation including green and blue InGaN LEDs simulation with different indium atom concentration for blue
where NanoLED—LED p-n junction area,
It was supposed that, for blue LED, indium concentration varies
The current density-voltage dependences on p-n junctions having different indium content
Spectrum is with asymmetric shape. Simulation proves the “blue shift” vs. current increase even at quantum Stark-Keldysh effect neglecting is proved by the simulation (Figure 11).
It was detected that by In concentration changes I-V or spectrum curves could be shifted. Doping into the p-emitter of NH (e.g., Al0.2Ga0.8N) was suggested to eliminate electron injection from the active region, which is especially important in device simulation with a low content of In (
Next, on the basis of the optimized structure of the NH, the effect of the impurity and In atoms doped into the barriers between QWs in the NH active region was studied; this effect is shown by the nonideality coefficient dependence, as shown in Figure 13.
With no QWs in the active area (
Then, the impurity influence on the I-V was studied. The optimum impurity concentration in barriers between QWs was detected at about
This effect is due to potential barrier decrease between QWs and barriers among them, so
In Figure 15, satisfactory agreement between the simulation and experimental results is evident, despite the fact that the simulation results are obtained without any additional (other than those of the base physical models) approximations.
During simulation, efficiency droop was investigated too. It was detected that the injected electrons and holes are irregularly distributed in the QW. Carrier recombination is concentrated in the QW. In the active region, the local electrostatic potential change is due to the spatial of electrons and holes in active region distribution. Simulation was carried out initially at the assumption that there is little difference between the carriers’ lifetime
It was suggested that there is a big difference between
In Figure 17, indium atom concentration distribution over wafer is shown. Impurity-defect cluster creates deep energy levels near the middle of the band gap, and the electron capture will be faster than the holes by the centers, due to the fact that its efficiency droop will be based on injected carrier redistribution between the QWs. For reducing droop, it needs to improve NH growth quality by usage Si (111) or better GaN substrates .
4. Photodetector (phototransistor) improvements
Photodetector efficiency increase for the ultraviolet spectral range is discussed. Such devices can be widely used in various fields of technology, including devices that analyze the composition of gases and liquids, and in the open optical communication lines with increased noise immunity in high solar radiation.
One of the most promising ways to create UV photodiodes is Al
Our investigations based on computer simulation showed that, based on such NH, not only photodiodes but also phototransistors (PT) with a sensitivity of more than 100 A/cm2 can be produced, in this wavelengths area [35–43]. Ultraviolet phototransistor (UV PT) structure has been proposed, based on the capabilities of this technology with GaN and AlGaN collector. Here, the first step of the PT development is investigated, defining the multilayer NH constructor by computer simulation: the structure type (n-p-n or p-n-p), the aluminum content, dopant thickness, and concentration in the layer structure and other parameters.
Photodetectors are characterized by high values of gain for both shift polarities. Effective injection capacity is provided by the fact that the p-GaN base layer has a narrower band gap than the emitter and collector layers. During NH simulation for high efficiency and high sensitive phototransistor, the structure consisted of the p-Al0.3Ga0.7N collector, n-Al0.3Ga0.7N emitter with an aluminum content of 30% and a p-GaN base. The emitter and collector thickness was 0.875 μm, and the base thickness was 0.3 μm. Acceptor concentration in p-Al0.3Ga0.7N collector was 1017 cm−3, donors in n-Al0.3Ga0.7N emitter were 1017 cm−3 and the acceptor concentration in the p-GaN base was 1017 cm−3 or 1018 cm−3. The lifetime values of the nonequilibrium electrons and holes in all areas of PTs were equal to 50 ns. The device files have been created for both the concentration of acceptors in the base.
Method for PT characteristics simulation at two acceptor concentrations in PTs base (Na = 1017 cm−3 or Na = 1018 cm−3) includes steps as it is shown below: (1) for the dark current density value
Dark current density
It is clearly seen that in the PTs at an acceptor concentration in the base
Figure 22 clearly shows that at the acceptor concentration in the base
At the same time, at the acceptor concentration in the base
Conclusion of this dependence is quite obvious—to obtain a high sensitivity at voltage, applied to the PT, it must be in the range of 6–9 V. At the end of the discussion of UV phototransistor characteristics simulating, data of the PTs sensitivity spectral dependence are presented (Figure 24).
It is clearly seen that the PTs sensitivity is very high in the range of photon energies from 3.5 to 4 eV (wavelength range from 354 to 309 nm), which allows to use them as selective photodetectors. PTs selectivity can be increased by reducing the aluminum concentration in the p-Al0.3Ga0.7N collector up to 20%. Sensitivity also can be increased by improving MOCVD technology for more high-quality AlGaN multilayer structures (with minimum defect concentration). If the sapphire substrate could be replaced by a gallium nitride substrate grown on sapphire, the lifetimes of nonradiative recombination in the PTs base will be increased significantly.