Efficiency Improvement of Nitride-Based Solid State Light Emitting Materials -- CRADA Final Report

The development of In{sub x}Ga{sub 1-x} N/GaN thin film growth by Molecular Beam Epitaxy has opened a new route towards energy efficient solid-state lighting. Blue and green LED's became available that can be used to match the whole color spectrum of visible light with the potential to match the eye response curve. Moreover, the efficiency of such devices largely exceeds that of incandescent light sources (tungsten filaments) and even competes favorably with lighting by fluorescent lamps. It is, however, also seen in Figure 1 that it is essential to improve on the luminous performance of green LED's in order to mimic the eye response curve. This lack of sufficiently efficient green LED's relates to particularities of the In{sub x}Ga{sub 1-x}N materials system. This ternary alloy system is polar and large strain is generated during a lattice mismatched thin film growth because of the significantly different lattice parameters between GaN and InN and common substrates such as sapphire. Moreover, it is challenging to incorporate indium into GaN at typical growth temperatures because a miscibility gap exists that can be modified by strain effects. As a result a large parameter space needs exploration to optimize the growth of In{sub x}Ga{sub 1-x}N and to date it is unclear what the detailed physical processes are that affect device efficiencies. In particular, an inhomogeneous distribution indium in GaN modifies the device performance in an unpredictable manner. As a result technology is pushed forward on a trial and error basis in particular in Asian countries such as Japan and Korea, which dominate the market and it is desirable to strengthen the competitiveness of the US industry. This CRADA was initiated to help Lumileds Lighting/USA boosting the performance of their green LED's. The tasks address the distribution of the indium atoms in the active area of their blue and green LED's and its relation to internal and external quantum efficiencies. Procedures to measure the indium distribution with near atomic resolution were developed and applied to test samples and devices that were provided by Lumilids. Further, the optical performance of the device materials was probed by photoluminescence, electroluminescence and time resolved optical measurements. Overall, the programs objective is to provide a physical basis for the development of a simulation program that helps making predictions to improve the growth processes such that the device efficiency can be increased to about 20%. Our study addresses all proposed aspects successfully. Carrier localization, lifetime and recombination as well as the strain-induced generation of electric fields were characterized and modeled. Band gap parameters and their relation to the indium distribution were characterized and modeled. Electron microscopy was developed as a unique tool to measure the formation of indium clusters on a nanometer length scale and it was demonstrated that strain induced atom column displacements can reliably be determined in any materials system with a precision that approaches 2 pm. The relation between the local indium composition x and the strain induced lattice constant c(x) in fully strained In{sub x}Ga{sub 1-x}N quantum wells was found to be: c(x) = 0.5185 + {alpha}x with {alpha} = 0.111 nm. It was concluded that the local indium concentration in the final product can be modulated by growth procedures in a predictable manner to favorably affect external quantum efficiencies that approached target values and that internal quantum efficiencies exceeded them

Subsurface Defects in Silicon Investigated by Modulated Optical Reflectance Measurements

The investigation of defects in silicon by modulated optical reflectance measurements has proven to be a powerful and easy-to-use method of nondestructive materials characterization. This technique has been used to monitor ion implant dose [1] and measure polishing damage [2] in silicon wafers, and to map O2 swirl precipitates in Czochralski-grown silicon [3]. Laser-induced modulated reflectance offers advantages over some related thermal-wave techniques: it is contactless, and because it can be performed at modulation frequencies of several MHz, it offers micron-scale resolution. Its noncontact and nondestructive nature makes this technique attractive for production-line use in the semiconductor industry

Thermal stability of internal gettering of iron in silicon and its impact on optimization of gettering

The redissolution behavior of gettered iron was studied in p-type Czochralski-grown silicon with a doping level of 2.5×10exp14 cm−3 and an oxide precipitate density of 5×10exp9 cm−3. The concentrations of interstitial iron and iron–boron pairs were measured by deep level transient spectroscopy. It was found that the dependence of redissolved iron concentration on annealing time can be fitted by the function C(t)=C_0[1−exp(−t/tau_diss)], and the dissolution rate tau−1diss has an Arrhenius-type temperature dependence of tau−1diss=4.01×10exp4 × exp[−(1.47±0.10) eV/k_BT] s−1. Based on this empirical equation, we predict how stable the gettered iron is during different annealing sequences and discuss implications for optimization of internal gettering.Peer reviewe

GJETC report 2018 : intensified German-Japanese cooperation in energy research ; key results and policy recommendations

The challenges and also potentials of the energy transition are tremendous in Germany, as well as in Japan. Sometimes, structures of the old energy world need "creative destruction" to clear the way for innovations for a decarbonized, low-risk energy system. In these times of disruptive changes, a constructive and sometimes controversial dialog within leading industrial nation as Japan and Germany over the energy transition is even more important. The German-Japanese Energy Transition Council (GJETC) released a summarizing report for the first project phase 2016-2018. It includes jointly formulated recommendations for politics as well as a controversial dialogue part. The Council jointly states and recommends that: Ambitious long-term targets and strategies for a low-carbon energy system must be defined and ambitiously implemented; Germany and Japan as high technology countries need to take the leadership. Both countries will have to restructure their energy systems substantially until 2050 while maintaining their competitiveness and securing energy supply. Highest priority is given to the forced implementation of efficiency technologies and renewable energies, despite different views on nuclear energy. In both countries all relevant stakeholders - but above all the decision-makers on all levels of energy policy - need to increase their efforts for a successful implementation of the energy transition. Design of the electricity market needs more incentives for flexibility options and for the extensive expansion of variable power generation, alongside with strategies for cost reduction for electricity from photovoltaic and wind energy. The implementation gap of the energy efficiency needs to be closed by an innovative energy policy package to promote the principle of "Energy Efficiency First". Synergies and co-benefits of an enhanced energy and resource efficiency policy need to be realized. Co-existence of central infrastructure and the growing diversity of the activities for decentralization (citizens funding, energy cooperatives, establishment of public utility companies) should be supported. Scientific cooperation can be intensified by a joint working group for scenarios and by the establishment of an academic exchange program

Doping-Assisted Defect Control In Compound Semiconductors

PatentThe present invention relates to the production of thin film epilayers of III-V and other compounds with acceptor doping wherein the acceptor thermally stabilizes the epilayer, stabilize the naturally incorporated native defect population and therewith maintain the epilayer's beneficial properties upon annealing among other advantageous effects. In particular, balanced doping in which the acceptor concentration is similar to (but does not exceed) the antisite defects in the as-grown material is shown to be particularly advantageous in providing thermal stability, high resistivity and ultrashort trapping times. In particular, MBE growth of LT-GaAs epilayers with balanced Be doping is described in detail. The growth conditions greatly enhance the materials reproducibility (that is, the yield in processed devices). Such growth techniques can be transferred to other III-V materials if the growth conditions are accurately reproduced. Materials produced herein also demonstrate advantages in reproducibility, reliability and radiation hardening

Efficiency Improvement of Nitride-Based Solid State Light Emitting Materials -- CRADA Final Report

The development of In{sub x}Ga{sub 1-x} N/GaN thin film growth by Molecular Beam Epitaxy has opened a new route towards energy efficient solid-state lighting. Blue and green LED's became available that can be used to match the whole color spectrum of visible light with the potential to match the eye response curve. Moreover, the efficiency of such devices largely exceeds that of incandescent light sources (tungsten filaments) and even competes favorably with lighting by fluorescent lamps. It is, however, also seen in Figure 1 that it is essential to improve on the luminous performance of green LED's in order to mimic the eye response curve. This lack of sufficiently efficient green LED's relates to particularities of the In{sub x}Ga{sub 1-x}N materials system. This ternary alloy system is polar and large strain is generated during a lattice mismatched thin film growth because of the significantly different lattice parameters between GaN and InN and common substrates such as sapphire. Moreover, it is challenging to incorporate indium into GaN at typical growth temperatures because a miscibility gap exists that can be modified by strain effects. As a result a large parameter space needs exploration to optimize the growth of In{sub x}Ga{sub 1-x}N and to date it is unclear what the detailed physical processes are that affect device efficiencies. In particular, an inhomogeneous distribution indium in GaN modifies the device performance in an unpredictable manner. As a result technology is pushed forward on a trial and error basis in particular in Asian countries such as Japan and Korea, which dominate the market and it is desirable to strengthen the competitiveness of the US industry. This CRADA was initiated to help Lumileds Lighting/USA boosting the performance of their green LED's. The tasks address the distribution of the indium atoms in the active area of their blue and green LED's and its relation to internal and external quantum efficiencies. Procedures to measure the indium distribution with near atomic resolution were developed and applied to test samples and devices that were provided by Lumilids. Further, the optical performance of the device materials was probed by photoluminescence, electroluminescence and time resolved optical measurements. Overall, the programs objective is to provide a physical basis for the development of a simulation program that helps making predictions to improve the growth processes such that the device efficiency can be increased to about 20%. Our study addresses all proposed aspects successfully. Carrier localization, lifetime and recombination as well as the strain-induced generation of electric fields were characterized and modeled. Band gap parameters and their relation to the indium distribution were characterized and modeled. Electron microscopy was developed as a unique tool to measure the formation of indium clusters on a nanometer length scale and it was demonstrated that strain induced atom column displacements can reliably be determined in any materials system with a precision that approaches 2 pm. The relation between the local indium composition x and the strain induced lattice constant c(x) in fully strained In{sub x}Ga{sub 1-x}N quantum wells was found to be: c(x) = 0.5185 + {alpha}x with {alpha} = 0.111 nm. It was concluded that the local indium concentration in the final product can be modulated by growth procedures in a predictable manner to favorably affect external quantum efficiencies that approached target values and that internal quantum efficiencies exceeded them

SiC materials and devices

This volume addresses the subject of materials science, specifically the materials aspects, device applications, and fabricating technology of SiC

Advances in photovoltaics

This volume is the third of a set of seven on the topic of photovoltaics. Solar cell-related technologies covered here include: ribbon silicon; heterojunction crystalline silicon; wafer equivalent crystalline silicon; and other advanced silicon solar cell structures and processes. Semiconductors and Semimetals has distinguished itself through the careful selection of well-known authors, editors, and contributors. Originally widely known as the ""Willardson and Beer"" Series, it has succeeded in publishing numerous landmark volumes and chapters. The series publishes timely, highly relevant

Subsurface Defects in Silicon Investigated by Modulated Optical Reflectance Measurements

The investigation of defects in silicon by modulated optical reflectance measurements has proven to be a powerful and easy-to-use method of nondestructive materials characterization. This technique has been used to monitor ion implant dose [1] and measure polishing damage [2] in silicon wafers, and to map O2 swirl precipitates in Czochralski-grown silicon [3]. Laser-induced modulated reflectance offers advantages over some related thermal-wave techniques: it is contactless, and because it can be performed at modulation frequencies of several MHz, it offers micron-scale resolution. Its noncontact and nondestructive nature makes this technique attractive for production-line use in the semiconductor industry.</p