Zachary Lochner

Ordered Nanowire Array Blue/Near-UV Light Emitting Diodes

[∗] S. Xu , C. Xu , Y. Liu , Y. F. Hu , R. S. Yang , Q. Yang , Prof. Z. L. Wang School of Materials Science and Engineering Georgia Institute of Technology Atlanta, Georgia, 30332–0245 (USA) E-mail: zlwang@gatech.edu J. H. Ryou , H. J. Kim , Z. Lochner , S. Choi , Prof. R. Dupuis School of Electrical and Computer Engineering Georgia Institute of Technology Atlanta, Georgia, 30332–0245 (USA) ZnO-based light emitting diodes (LEDs) have been considered as a potential candidate for the next generation of blue/ near-UV light sources, [ 1 ] due to a direct wide bandgap energy of 3.37 eV, a large exciton binding energy of 60 meV at room temperature, and several other manufacturing advantages of ZnO. [ 2 ] While the pursuit of stable and reproducible p-ZnO is still undergoing, [ 3,4 ] heterojunctions of n-ZnO and p-GaN are employed as an alternative approach in this regard by considering the similar crystallographic and electronic properties of ZnO and GaN. [ 5–7 ] Compared with the thin fi lm/thin fi lm LEDs, [ 5,6 , 8 ] which may suffer from the total internal refl ection, n-ZnO nanowire/p-GaN thin fi lm heterostructures are utilized in order to increase the extraction effi ciency of the LEDs by virtue of the waveguiding properties of the nanowires. [ 9–11 ] But in all of these cases, the n-ZnO nanowires are randomly distributed on the substrate, which largely limits their applications in high performance optoelectronic devices. Here in this work, we demonstrate the capability of controlling the spatial distribution of the blue/near-UV LEDs composed of position controlled arrays of n-ZnO nanowires on a p-GaN thin fi lm substrate. The device was fabricated by a conjunction of low temperature wet chemical methods and electron beam lithography (EBL). The EBL could be replaced by other more convenient patterning techniques, such as photolithography and nanosphere lithography, rendering our technique low cost and capable of scaling up easily. Under forward bias, each single nanowire is a light emitter. By Gaussian deconvolution of the emission spectrum, the origins of the blue/nearUV emission are assigned particularly to three distinct electronhole recombination processes. By virtue of the nanowire/thin fi lm heterostructures, these LEDs give an external quantum effi ciency of 2.5%. This approach has great potential applications in high resolution electronic display, optical interconnect, and high density data storage. The design of the LED is shown in Figure 1a . Ordered ZnO nanowire arrays were grown on p-GaN (Figure 1 b–d), [ 12–14 ]

Control of Quantum-Confined Stark Effect in InGaN-Based Quantum Wells

This paper reviews current technological developments in polarization engineering and the control of the quantum-confined Stark effect (QCSE) for InxGa1- xN-based quantum-well active regions, which are generally employed in visible LEDs for solid-state lighting applications. First, the origin of the QCSE in III-N wurtzite semiconductors is introduced, and polarization-induced internal fields are discussed in order to provide contextual background. Next, the optical and electrical properties of InxGa1- xN-based quantum wells that are affected by the QCSE are described. Finally, several methods for controlling the QCSE of InxGa1- xN-based quantum wells are discussed in the context of performance metrics of visible light emitters, considering both pros and cons. These strategies include doping control, strain/polarization field/electronic band structure control, growth direction control, and crystalline structure control.

Deep-ultraviolet lasing at 243 nm from photo-pumped AlGaN/AlN heterostructure on AlN substrate

Deep-ultraviolet lasing was achieved at 243.5 nm from an AlxGa1−xN-based multi-quantum-well structure using a pulsed excimer laser for optical pumping. The threshold pump power density at room-temperature was 427 kW/cm2 with transverse electric (TE)-polarization-dominant emission. The structure was epitaxially grown by metalorganic chemical vapor deposition on an Al-polar free-standing AlN (0001) substrate. Stimulated emission is achieved by design of the active region, optimizing the growth, and the reduction in defect density afforded by homoepitaxial growth of AlN buffer layers on AlN substrates, demonstrating the feasibility of deep-ultraviolet diode lasers on free-standing AlN.

Bandgap bowing in BGaN thin films

We report on the bandgap variation in thin films of BxGa1−xN grown on AlN/sapphire substrates using metal-organic vapor phase epitaxy. Optical transmission, photoluminescence, and x-ray diffraction were utilized to characterize the materials’ properties of the BxGa1−xN films. In contrast to the common expectation for the bandgap variation, which is based on the linear interpolation between the corresponding GaN and BN values, a significant bowing (C=9.2±0.5 eV) of the bandgap was observed. A decrease in the optical bandgap by 150 meV with respect to that of GaN was measured for the increase in the boron composition from 0% to 1.8%.We report on the bandgap variation in thin films of BxGa1−xN grown on AlN/sapphire substrates using metal-organic vapor phase epitaxy. Optical transmission, photoluminescence, and x-ray diffraction were utilized to characterize the materials’ properties of the BxGa1−xN films. In contrast to the common expectation for the bandgap variation, which is based on the linear interpolation between the corresponding GaN and BN values, a significant bowing (C=9.2±0.5 eV) of the bandgap was observed. A decrease in the optical bandgap by 150 meV with respect to that of GaN was measured for the increase in the boron composition from 0% to 1.8%.

Design and Analysis of 250-nm AlInN Laser Diodes on AlN Substrates Using Tapered Electron Blocking Layers

A theoretical investigation into the operation of AlInN ultraviolet laser (UV) diodes on AlN substrates is presented. 2-D optoelectronic simulation of a prototypical design predicts lasing at a target wavelength of 250 nm. Simulations indicate optical gain degradation attributable to a parasitic inversion layer, which forms as a result of polarization charge associated with homogeneous electron blocking layers. Appreciable improvement in optical gain is demonstrated in designs featuring inhomogeneous electron blocking layers, by virtue of a volumetric redistribution of polarization charge. Numerical simulations inspire confidence in AlInN as a viable alternative to AlGaN technologies for UV laser-diode operation.

Effect of Silicon Doping in the Quantum-Well Barriers on the Electrical and Optical Properties of Visible Green Light-Emitting Diodes

The effect of Si doping in the GaN quantum-well (QW) barriers of the InGaN-GaN multiple QW active region in visible green light-emitting diodes (LEDs) was studied. As the doping level of Si increases, the intensity of electroluminescence (EL) decreases, while the forward voltage of the diodes is improved. Degradation of EL is believed to be mainly due to the hole transport blocking effect caused by Si doping in the QW barriers resulting in increased potential barriers. This effect is believed to be more significant in green LEDs than in violet and blue LEDs.

Sub-250 nm low-threshold deep-ultraviolet AlGaN-based heterostructure laser employing HfO2/SiO2 dielectric mirrors

We report a sub-250-nm, optically pumped, deep-ultraviolet laser using an AlxGa1−xN-based multi-quantum-well structure grown on a bulk Al-polar c-plane AlN substrate. TE-polarization-dominant lasing action was observed at room temperature with a threshold pumping power density of 250 kW/cm2. After employing high-reflectivity SiO2/HfO2 dielectric mirrors on both facets, the threshold pumping power density was further reduced to 180 kW/cm2. The internal loss and threshold modal gain can be calculated as 2 cm−1 and 10.9 cm−1, respectively.

Effects of a step-graded AlxGa1−xN electron blocking layer in InGaN-based laser diodes

A step-graded AlxGa1-xN electron blocking layer (EBL) is studied on InGaN-based laser diodes (LDs). Its efficacy on device performance is investigated with respect to stimulated emission properties, internal quantum efficiency, internal loss, and temperature-dependent characteristics. When compared to a LD structure with an abrupt Al0.18Ga0.82N EBL design, the LD with the step-graded AlxGa1-xN EBL design demonstrates lower threshold current density and higher slope efficiency. The threshold current density is reduced from 4.6 kA/cm2 to 2.5 kA/cm2 under pulsed-current operation and the corresponding slope efficiency is increased from 0.72 W/A to 1.03 W/A. The insertion of the step-graded AlxGa1−xN EBL leads to a dramatic enhancement in internal quantum efficiency from 0.60 to 0.92, while internal loss keeps at 9 ∼ 10 cm−1. The temperature-dependent measurement also shows that the step-graded AlxGa1−xN EBL can improve the thermal stability with reduced red-shift from 0.05 nm/K to 0.034 nm/K. This simple yet...

Threshold voltage control of InAlN/GaN heterostructure field-effect transistors for depletion- and enhancement-mode operation

We describe a method to change the threshold voltage of heterostructure field-effect transistors (HFETs) using InxAl1−xN/GaN heterostructures by using polarization and strain modification in the InAlN barrier layer to realize enhancement-mode operation. The threshold voltage and electronic band structure of the heterostructures were calculated for different indium compositions in the InAlN layer. Band structure calculations predict the enhancement-mode operation of compressively strained InAlN/GaN HFETs with an indium composition higher than 0.25. Studies of InAlN/GaN HFETs with different In alloy compositions show that the sheet resistance increases and the carrier concentration decreases for the heterostructures with increasing indium composition due to changes in the compressive strain and polarization in the InAlN barrier layer. Fabricated HFETs show threshold voltages of −2.5, −0.75, and +0.2 V for In∼0.18Al∼0.82N/GaN, In∼0.22Al∼0.78N/GaN, and In∼0.25Al∼0.75N/GaN HFETs, respectively, corresponding to...

High-Current-Gain Direct-Growth GaN/InGaN Double Heterojunction Bipolar Transistors

We report high-current-gain n-p-n GaN/InGaN double-heterojunction bipolar transistors (DHBTs) using a direct-growth fabrication processing approach. The impact of the indium composition in the base layer was studied, and a burn-in effect using a constant-base-current stressing method was observed. We found that DHBTs with higher indium composition in the InGaN base layer may help reduce the base resistance and lower the surface recombination current but may result in higher bulk recombination current. A device burn-in effect was also studied. The postprocessing current stressing step helps increase free-hole concentration in the base, reduce the bulk recombination current, and enhance the current gain. As a result, a direct-growth GaN/In0.03Ga0.97N DHBT with a peak current gain of 105 and a collector current density > 6.5 kA/cm2 was demonstrated on a sapphire substrate.