Strong Carrier Localization Effect Research Articles

(Invited) Recent Progress on Molecular Beam Epitaxy of AlGaN Nanowires for Deep Ultraviolet Light Emitting Devices

Aluminum gallium nitride (AlGaN) ultrawide bandgap semiconductors have been attractive for their applications to disinfection, material identification, and ultraviolet (UV) curing, and for their potential to replace conventional mercury lamp based light sources. Remarkable progress has been made in the past decades from understanding material fundamentals to device performance improvement. Nonetheless, the enduring challenges related to high dislocation densities and poor p-type doping, among others, make AlGaN nanowires an alternative appealing platform. In this talk, I will discuss our recent development on the molecular beam epitaxy of AlGaN nanowires for deep UV light emitting devices.First, I will show that by exploiting the compositional fluctuations in high Al content AlGaN nanowires in nitrogen-rich environment, electroluminescence (EL) at 207 nm can be obtained. Such EL emission is measured from the device top surface. As today it has remained challenging to obtain EL emission from the surface around this wavelength (i.e., near vacuum UV), such nanowires could offer an interesting approach to obtain surface emitting at near vacuum UV. I will further show that, by using graphene contact, the light output of surface emitting AlGaN nanowire deep UV LEDs can be improved. It is found that comparing to conventional metal contact, by using graphene contact the light output can be improved by a factor of two and the peak external quantum efficiency can be improved by a factor of four. Lastly, I will show that, by using nanowire template on Si substrate, AlN epilayers with a relatively smooth surface can be obtained, which further enables the formation of AlGaN epilayers with various Al contents on top. Such AlGaN epilayers are found to have high internal quantum efficiency, presumably due to a strong carrier localization effect. Current progress on device development using such an AlN on Si template will also be discussed.

Enhancement of carrier localization effect and internal quantum efficiency through In-rich InGaN quantum dots

InGaN-based light emitting diodes have a problem known as green gap. InGaN quantum dots have been proven to be a kind of promising structure to solve this problem due to its strong carrier localization effect. In this study we fabricated InGaN/GaN quantum dots and conventional InGaN/GaN multiple quantum wells by tuning growth mode and compared their quantum confined stark effect and internal quantum efficiency by means of intensity-dependent as well as temperature-dependent photoluminescence measurements. It was found that the surface morphology transited from two dimensional step flow to three dimensional quantum dots with increasing the well thickness. In addition the quantum confined stark effect was weakened as a result of releasing the compressive strain. Furthermore, the internal quantum efficiency of the quantum-dot structures was four times higher than that of the conventional multiple quantum wells due to the enhancement of carrier localization effect.

Electroluminescence properties of InGaN/GaN multiple quantum well-based LEDs with different indium contents and different well widths

Two InGaN/GaN multiple quantum well (MQW)-based blue light emitting diodes (LEDs) emitting photons at approximately the same wavelength, with different indium contents and well widths, are prepared, and the temperature-dependences of their electroluminescence (EL) spectra at different fixed injection currents are investigated. The results show that, compared with sample B with its lower indium content and larger well width, sample A with its higher indium content and smaller well width, has a stronger carrier localization effect and higher external quantum efficiency (EQE) at the lower fixed currents; however, upon increasing the injection current, both the localization effect and EQE for sample A decrease at a faster rate. The former is mainly attributed to the deeper potential levels due to the larger indium fluctuations originating from the higher indium content, and to the smaller well width-induced stronger carrier quantum-confine effect (QCE); the latter is mainly attributed to the more significant growing in the electron leakage and/or electron overflow originating from the smaller well width and larger lattice mismatch-induced stronger piezoelectric field, and to the more significant reduction in carrier localization effect originating from the smaller well width-induced smaller density of high-energy localized states.

Strong carrier localization effect in carrier dynamics of 585 nm InGaN amber light-emitting diodes

Temperature dependence and time-resolved photoluminescence (TRPL) have been carried out to study carrier dynamics for 585 nm InGaN amber light-emitting diodes (LEDs). It is found that in InGaN amber LEDs, peak emission energy only shows a slight blueshift from 588 to 575 nm, as temperature increased from 10 K to 300 K. Moreover, radiative recombination lifetime has demonstrated independent of temperature based TRPL results. These two features indicate that a strong carrier localization effect plays a dominant role in carrier dynamics for InGaN amber LEDs. Also, activation energy of 40.3 meV is obtained through Arrhenius plot of PL intensity versus temperature.

Enhanced performance of InGaN/GaN multiple-quantum-well light-emitting diodes grown on nanoporous GaN layers.

We demonstrate the high efficiency of InGaN/GaN multiple quantum wells (MQWs) light-emitting diode (LED) grown on the electrochemically etched nanoporous (NP) GaN. The photoluminescence (PL) and Raman spectra show that the LEDs with NP GaN have a strong carrier localization effect resulting from the relaxed strain and reduced defect density in MQWs. Also, the finite-difference time-domain (FDTD) simulation shows that the light extraction efficiency (LEE) is increased by light scattering effect by nanopores. The output power of LED with NP GaN is increased up to 123.1% at 20 mA, compared to that of LED without NP GaN. The outstanding performance of LEDs with NP GaN is attributed to the increased internal quantum efficiency (IQE) by the carrier localization in the indium-rich clusters, low defect density in MQWs, and increased LEE owing to the light scattering in NP GaN.

Green GaInN photonic-crystal light-emitting diodes with small surface recombination effect

We have fabricated green GaInN light-emitting diodes (LEDs) containing two-dimensional photonic crystals (PCs). The PC structure is comprised of air holes that penetrate through the active layer. The observed emission intensity at room temperature was enhanced by a factor of ∼3 when a PC was introduced, which is close to the theoretical enhancement calculated without taking into account surface recombination at the air-hole edges. The carrier lifetime changes little when a PC is incorporated due to the low surface recombination. In contrast, the carrier lifetime in blue-emitting LEDs decreased by a factor of 4 when a PC was introduced. The surface-recombination velocity in green-emitting devices is estimated to be 3×102 cm/s, an order of magnitude smaller than in blue-emitting devices. This is due to the strong carrier localization effect in green-emitting GaInN.

Luminescence Dynamics and Structural Investigation of InGaN/GaN Multiple Quantum Well Light Emitting Diodes

Luminescence properties of blue emission InGaN/GaN multiple quantum well (MQW) have been studied by temperature dependent photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL) spectroscopic techniques. Two typical samples are studied, both consisting of five periods of InGaN wells with different indium compositions of 21% and 24%, respectively. According to the PL and PLE measurement results, large values of activation energy and Stokes’ shift are obtained. This indicates that higher Indium composition results in an increase of composition fluctuation in the InGaN MQW region, showing the stronger carrier localization effect. The lifetime at the low-energy side of the InGaN peaks is longer for higher indium composition, as expected from the larger Stokes shift.

Temperature-dependent photoluminescence from bicrystalline ZnS (core/shell) nanocables

The experimental study of the temperature-dependent photoluminescence properties of bicrystalline ZnS core/shell nanocables under 10 K to 300 K was reported. The results show that there are two distinct peaks situated at the UV (about 372.6 nm) and green (about 500 nm) regions, respectively, and one blue-shoulder band (about 465 nm) superimposed between them for all spectra. The UV peak shows an obvious redshift with increasing temperature. The analyses of Gaussian-fitted the blue-shoulder band and the green bands from 10 K to 300 K reveal that all spectra can be well fitted by three Gaussian peaks: blue peak (B, about 2.67 eV), two green peaks (G1, about 2.47 eV and G2, about 2.42 eV). With increasing temperature, the blue band shows blueshift. The green bands show anomalous transition: green band G1 shows blueshift–redshift transition and green band G2 shows redshift–blueshift. The origins of these bands and their temperature-dependent shifts are explained based on defect levels and strong carrier localization effect at the defect levels in addition to band-gap shrinkage.

Ultrahigh photocurrent gain in m-axial GaN nanowires

An ultrahigh photocurrent gain has been found in the ultraviolet-absorbed GaN nanowires with m-directional long axis grown by chemical vapor deposition. The quantitative results have shown the gain values at 5.0×104–1.9×105 of the GaN nanowires with diameters from 40to135nm are near three orders of magnitude higher than the values of 5.2×101–1.6×102 estimated from the thin film counterparts. The intensity-dependent gain study has shown that the gain value is very sensitive to the excitation intensity following an inverse power law and no gain saturation observed in this investigated intensity range from 0.75to250W∕m2. This behavior has strongly suggested a surface-dominant rather than trap-dominant high gain mechanism in this one-dimensional nanostructure. The strong carrier localization effect induced by the surface electric field in the GaN nanowires is also discussed.