Rationally controlled growth of inorganic semiconductor nanowires is important for their applica- tions in nanoscale electronics and photonics. In this article, we discuss the rational growth, physical proper- ties, and integration of nanowires based on the results from the authors× laboratory. The composition, diameter, growth position, and orientation of the nanowires are controlled based on the vapor ± solid ± liquid (VLS) crystal growth mechanism. The thermal stability and optical properties of these semiconductor nanowires are investigated. Particularly, ZnO nanowires with well- defined end surfaces can function as room-temperature ultraviolet nanolasers. In addition, a novel microfluidic- assisted nanowire integration (MANI) process was de- veloped for the hierarchical assembly of nanowire build- ing blocks into functional devices and systems.

/pdf/inorganic-semiconductor-nanowires-rational-growth-assembly-405kkhocl1.pdf

Inorganic Semiconductor Nanowires: Rational Growth, Assembly, and Novel Properties

In order to realize 250–350-nm-band high-efficiency deep ultraviolet (UV) emitting devices using group-III-nitride materials, it is necessary to obtain high-efficiency UV emission from wide-band-gap (In)AlGaN. The use of the In-segregation effect, which has already been used for InGaN blue emitting devices, is quite effective for achieving high-efficiency deep UV emission. We have demonstrated high-efficiency UV emission from quaternary InAlGaN-based quantum wells in the wavelength range between 290 and 375 nm at room temperature (RT) using the In-segregation effect. Emission fluctuations in the submicron region due to In segregation were clearly observed for quaternary InAlGaN epitaxial layers. An internal quantum efficiency as high as 15% was estimated for a quaternary InAlGaN-based single quantum well at RT. Such high-efficiency UV emission can even be obtained on high threading-dislocation density buffer layers. A comparison of electroluminescence is made between light-emitting diodes (LEDs) with InAlGaN, AlGaN, and GaN active regions fabricated on SiC substrates with emission wavelengths between 340 and 360 nm. The emission intensity from the quaternary InAlGaN UV-LED was more than one order of magnitude higher than that from the AlGaN or GaN UV-LEDs under RT cw operation. We therefore fabricated 310–350-nm-band deep UV-LEDs with quaternary InAlGaN active regions. We achieved submilliwatt output power under RT pulsed operation for 308–314-nm LEDs. We also demonstrated a high output power of 7.4 mW from a 352-nm quaternary InAlGaN-based LED fabricated on a GaN substrate under RT cw operation. The maximum external quantum efficiency (EQE) of the 352-nm InAlGaN-based LED was higher than that obtained for an AlGaN-based LED with the same geometry. From these results, the advantages of the use of quaternary InAlGaN in 350-nm-band UV emitters were revealed.

Quaternary InAlGaN-based high-efficiency ultraviolet light-emitting diodes

It is impossible to imagine now modern solid-state physics without semiconductor heterostructures. Semiconductor heterostructures and, particularly, double heterostructures, including quantum wells, wires, and dots, are today the subject of research of two-thirds of the semiconductor physics community. The ability to control the type of conductivity of a semiconductor material by doping with various impurities and the idea of injecting nonequilibrium charge carriers could be said to be the seeds from which semiconductor electronics developed. Heterostructures developed from these beginnings, making it possible to solve the considerably more general problem of controlling the fundamental parameters inside the semiconductor crystals and devices: band gaps, effective masses of the charge carriers and the mobilities, refractive indices, electron energy spectrum, etc. Development of the physics and technology of semiconductor heterostructures has resulted in remarkable changes in our everyday life. Heterostructure electronics are widely used in many areas of human civilization. It is hardly possible to imagine our recent life without double heterostructure (DHS) laser-based telecommunication systems, heterostructure-based light-emitting diodes (LED’s), heterostructure bipolar transistors, or lownoise high-electron-mobility transistors for highfrequency applications including, for example, satellite television. Double-heterostructure lasers now enter practically every house with CD players. Heterostructure solar cells have been widely used for space and terrestrial applications. Our interest in semiconductor heterostructures was not occasional. Systematic studies of semiconductors were started in the early 1930s at the Physico-Technical Institute under the direct leadership of its founder, Abraham Ioffe. V. P. Zhuze and B. V. Kurchatov studied the intrinsic and impurity conductivity of semiconductors in 1932, and the same year Ioffe and Ya. I. Frenkel created a theory of rectification in a metalsemiconductor contact based on the tunneling phenom-

Nobel Lecture: The double heterostructure concept and its applications in physics, electronics, and technology

Theoretical analysis of the gain and threshold current of a semiconductor quantum dot (QD) laser is given which takes account of the line broadening caused by fluctuations in quantum dot sizes. The following processes are taken into consideration together with the main process of radiative recombination of carriers in QDs: band-to-band radiative recombination of carriers in the waveguide region, carrier capture into QDs and thermally excited escape from QDs, photoexcitation of carriers from QDs to continuous-spectrum states. For an arbitrary QD size distribution, expressions for the threshold current density as a function of the root mean square of relative QD size fluctuations, total losses in the waveguide region, surface density of QDs and thickness of the waveguide region have been obtained in an explicit form. The minimum threshold current density and optimum parameters of the structure (surface density of QDs and thickness of the waveguide region) are calculated as universal functions of the main dimensionless parameter of the theory developed. This parameter is the ratio of the stimulated transition rate in QDs at the lasing threshold to the spontaneous transition rate in the waveguide region at the transparency threshold. Theoretical estimations presented in the paper confirm the possibility of a significant reduction of the threshold currents of QD lasers as compared with the conventional quantum well lasers.

https://iopscience.iop.org/article/10.1088/0268-1242/11/4/017/pdf

Inhomogeneous line broadening and the threshold current density of a semiconductor quantum dot laser

The density-matrix theory of semiconductor lasers with relaxation broadening model is finally established by introducing theoretical dipole moment into previously developed treatments. The dipole moment is given theoretically by the k . p method and is calculated for various semiconductor materials. As a result, gain and gain-suppression for a variety of crystals covering wide wavelength region are calculated. It is found that the linear gain is larger for longer wavelength lasers and that the gain-suppression is much larger for longer wavelength lasers, which results in that single-mode operation is more stable in long-wavelength lasers than in shorter-wavelength lasers, in good agreement with the experiments.

Density-matrix theory of semiconductor lasers with relaxation broadening model - Gain and gain-suppression in semiconductor lasers

The laser threshold of three-dimensional GaInAsP/InP quantum-box lasers is analyzed. The optimized quantum-box array laser structure for the lowest threshold current density at room temperature is obtained theoretically, taking into account the effect of carrier leakage. The lowest threshold current densities are 14, 27, and 61 A/cm/sup 2/ for 10, 20, and 40 cm/sup -1/ of cavity loss, respectively. The threshold current density is calculated, taking into account fluctuation in quantum-box size. The ideal structure of the quantum-box laser is discussed. It is pointed out that the modulation-doped structure looks promising for the suppression of carrier leakage. >

Threshold current density of GaInAsP/InP quantum-box lasers

Current injection efficiency, i.e. the proportion of current into the active region to total current, is analyzed for separate confinement heterostructure (SCH) quantum film lasers. It is shown that the current injection efficiency changes stepwise with the active layer thickness. and is larger for lower injected carrier density and deeper quantum well. Comparison is made between step-, parabolic-GRIN-, and linear-GRIN-SCH structures. The efficiency of linear-GRIN-SCH is the highest for the same well depth. Threshold current density is discussed for these SCH structures, taking into account the current injection efficiency and the optical loss due to the carrier leakage to the optical confinement layers. >

Analysis of current injection efficiency of separate-confinement-heterostructure quantum-film lasers

Carrier capture time of the quantum well, which is an important parameter in the laser operation, was estimated for separate-confinement-heterostructure single-quantum-well (SCH-SQW) lasers by measuring the spontaneous emission from the optical confinement layers, which increases with current even above the laser threshold due to finite capture time. By fitting theoretical analysis to the measurement, hole capture time was found to be the dominant factor for the spontaneous emission increase, and was estimated to be 0.2-0.3 ps for GaInAs/GaInAsP/InP step- and graded-refractive-index-(GRIN-) SCH-SQW lasers, independent of the optical confinement structures. The same measurement was done for multiquantum-well lasers, and it was found that transport across the barrier was also responsible for the spontaneous emission increase and inhomogeneous injection into each well. The effect of the hole capture time and the transport time on the threshold current and the quantum efficiency was analyzed for high-power operation, considering the absorption loss by the carriers in the optical confinement layers. GRIN-SCH structure is shown to keep high differential efficiency in high-power operation in comparison with step-SCH structures, because the carrier density in the confinement layer is small and increases very little above threshold. >

Carrier capture time and its effect on the efficiency of quantum-well lasers

We calculated spectral characteristics of the linewidth enhancement factor α for multidimensional quantum-well lasers. By increasing the quantization dimension, the wavelength dependence of α becomes stronger and its value around the maximum gain becomes smaller. For a quantum box, α is almost zero at the gain peak, and becomes negative at a shorter wavelength. The negative α yields the possibility of long-distance high-capacity optical fiber communication.

Spectral Characteristics of Linewidth Enhancement Factor α of Multidimensional Quantum Wells

Carrier capture time was estimated for separate confinement heterostructure single quantum‐well lasers by measuring the spontaneous emission from the optical confinement layers which increases with current even above the laser threshold due to finite carrier capture time of the well. By fitting theoretical analysis to the measurement, hole capture time was found to be the dominant factor for the spontaneous emission increase, and was estimated as 0.2–0.25 ps for GaInAs/GaInAsP/InP quantum well lasers. Additional transport time of 0.1–0.2 ps across the barrier was also obtained for multiquantum well lasers. It appears that the finiteness of carrier capture time affects not only the threshold current but also the differential quantum efficiency in high power operation through the absorption loss by the carriers in the optical confinement layers.

Y. Miyake

Papers

Threshold current density of GaInAsP/InP quantum-box lasers

Analysis of current injection efficiency of separate-confinement-heterostructure quantum-film lasers

Carrier capture time and its effect on the efficiency of quantum-well lasers

Spectral Characteristics of Linewidth Enhancement Factor α of Multidimensional Quantum Wells

Estimation of carrier capture time of quantum‐well lasers by spontaneous emission spectra