Y. Dikme

On the Effect of Step-Doped Quantum Barriers in InGaN/GaN Light Emitting Diodes

InGaN/GaN light-emitting diodes (LEDs) make an important class of optoelectronic devices, increasingly used in lighting and displays. Conventional InGaN/GaN LEDs of c-orientation exhibit strong internal polarization fields and suffer from significantly reduced radiative recombination rates. A reduced polarization within the device can improve the optical matrix element, thereby enhancing the optical output power and efficiency. Here, we have demonstrated computationally that the step-doping in the quantum barriers is effective in reducing the polarization-induced fields and lowering the energy barrier for hole transport. Also, we have proven experimentally that such InGaN/GaN LEDs with Si step-doped quantum barriers indeed outperform LEDs with wholly Si-doped barriers and those without doped barriers in terms of output power and external quantum efficiency. The consistency of our numerical simulation and experimental results indicate the effects of Si step-doping in suppressing quantum-confined stark effect and enhancing the hole injection, and is promising in improving the InGaN/GaN LED performance.

On the origin of the redshift in the emission wavelength of InGaN/GaN blue light emitting diodes grown with a higher temperature interlayer

A redshift of the peak emission wavelength was observed in the blue light emitting diodes of InGaN/GaN grown with a higher temperature interlayer that was sandwiched between the low-temperature buffer layer and high-temperature unintentionally doped GaN layer. The effect of interlayer growth temperature on the emission wavelength was probed and studied by optical, structural, and electrical properties. Numerical studies on the effect of indium composition and quantum confinement Stark effect were also carried out to verify the experimental data. The results suggest that the redshift of the peak emission wavelength is originated from the enhanced indium incorporation, which results from the reduced strain during the growth of quantum wells.

Crystal growth and properties of LiAlO2 and nonpolar GaN on LiAlO2 substrate

In this study, the growth and properties of LiAlO2 material and a nonpolar GaN-based light-emitting-diode (LED) structure on LiAlO2 have been investigated. The LiAlO2 material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure M-plane (1010) GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the LiAlO2 substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LED structure on LiAlO2 was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.

AlGaN-GaN HEMTs on patterned silicon (111) substrate

We report the AlGaN-GaN high-electron mobility transistors (HEMTs) grown and fabricated on patterned silicon (111) substrates. A crack-free AlGaN-GaN HEMT heterostructure was grown on top of rectangular silicon ridges patterned on the silicon substrate. Fabrication of HEMT on the ridges was demonstrated using a polyimide planarization process. Maximum drain current density of 1.05 A/mm and peak transconductance of 150 mS/mm were achieved with 1.0 /spl mu/m gate-length. The current gain cutoff frequency and maximum frequency of oscillation were 9.7 and 20.5 GHz, respectively, for the 1 /spl mu/m /spl times/ 300 /spl mu/m devices.

Luminescence and Stimulated Emission from GaN on Silicon Substrates Heterostructures

Photoluminescence (PL) and stimulated emission of GaN/Si layers grown by MOVPE with AIN and AlGaN buffers have been investigated. It has been found that thermal annealing of samples in nitrogen gas flow leads to an increase of the PL efficiency of up to 20 times, with a thermal activation energy of this process of 3.1-3.4 eV. The annealing promotes a significant decrease of the non-radiative center concentration and an increase of the number of shallow states. The laser action at λ = 377 nm under pulsed optical excitation was achieved at room temperature evidencing a high quality of the samples.

Growth and characterisation of AlGaN/GaN HEMT on silicon substrates

In order to analyse and to compare the properties of AlGaN/GaN HEMT on silicon and on sapphire substrates, studies on both layers and device types have been performed. Besides the substantially lower substrate costs compared to SiC, the use of silicon as substrate provides the advantage of a higher thermal conductivity compared to sapphire allowing a more efficient heat removal from the device and thus higher RF power densities. On silicon, up to 900 nm of GaN as well as HEMT structures have been deposited and characterised regarding their structural, optical and electrical properties. HEMT devices with various gate lengths were processed and measured on-wafer under continuous and pulsed operation conditions. The properties of the layers and devices on silicon substrates are developing to become comparable to those based on sapphire and silicon carbide.

InGaN/GaN based LEDs with electroluminescence in violet, blue, and green tuned by epitaxial growth temperature

In this work, we present a full set of InGaN LEDs based on a single optimal InGaN/GaN quantum design with emission wavelengths spanning from green to blue to violet by tuning the active layer growth temperature to precisely control InN incorporation into the quantum structures

High temperature operation of optically pumped InGaN/GaN MQW heterostructures lasers grown on Si substrates

In this report InGaN/GaN multiple quantum well (MQW) heterostructures were grown in AIXTRON MOVPE reactors on (111)-oriented Si substrates. The temperature behaviour of the laser threshold is defined mainly by spontaneous emission characteristics of excitation level equal to the laser threshold such as spontaneous recombination efficiency, full width at half maximum and position of the spectrum. Rates of spontaneous and nonradiative recombination, the value of internal quantum efficiency were estimated.

Backgating, high-current and breakdown characterisation of AlGaN/GaN HEMTs on silicon substrates

The backgating effect, as well as breakdown and high-current performance, of AlGaN/GaN HEMTs on silicon substrates are studied. The material structure of the investigated devices differs in the thickness of the stress-relaxing intermediate layer sequence (/spl sim/1 /spl mu/m and /spl sim/2.5 /spl mu/m thick). It is shown that the transistor backgating effect is reduced for the thicker sequence. Similarly, the reverse gate current is two orders of magnitude lower and the gate-drain breakdown voltage increases substantially in devices with the thicker sequence. An increase from /spl sim/40 V to /spl sim/160 V of the HEMT blocking capability, measured under electrostatic discharge-like conditions, is also observed.

Investigation of GaN on Si(111) for optoelectronic applications

GaN-based optoelectronics allow covering the spectral range from green to UV. Silicon (Si) is an alternative substrate to the commonly used sapphire and silicon carbide (SiC) but requires sophisticated buffer structures. In this work, two high-temperature (HT) layer stacks and two low-temperature (LT) AlN layers were used for the growth of GaN buffers for optoelectronic devices on (111)-oriented Si substrates using AIXTRON metalorganic vapor phase epitaxy (MOVPE) reactors. AlN, AlGaN and GaN were grown as HT layer stack to form stress reduction layers. GaInN MQW (multi quantum wells), electroluminescence test structures (ELT) and AlN/GaN DBR (distributed Bragg reflectors) were deposited on these buffer structures on Si. The growth process was monitored by in-situ reflectivity measurements. Photoluminescence (PL), electroluminescence and the luminescence under high optical excitation of the samples on Si have been studied. Laser action at optical excitation was obtained in the MQW with a room temperature (RT) laser threshold of Ithr = 40 - 80 kW/cm2. Laser action was achieved up to 350°C. Electroluminescence emission from the ELT InGaN/GaN heterostructures was observed and measured under a minimal DC voltage of about 4 V. AlN/GaN DBR with ten periods showed reflectivities of 60% for wavelengths of 436 nm and 537 nm, respectively.