S.T. Liu

Performance of InGaN based green laser diodes improved by using an asymmetric InGaN/InGaN multi-quantum well active region

Series of green laser diodes (LDs) with different (In)GaN barrier layers are investigated. It is found that the optical confinement factor of multi-quantum well (MQW) always increases with increasing indium content of InGaN barrier layer, which results in a decrease of threshold current when indium content of InGaN barrier layer increases from 0 to 5%. However, when a high In content InGaN barrier is used (> 5%), both threshold current and slop efficiency of LDs deteriorate. It may be attributed to the waste of carriers in the potential well at the interface between the last barrier (LB) and the upper waveguide (UWG) layers, which is induced by the piezoelectric polarization effect in high In content InGaN LB layer. Therefore, a new LD structure using a thin thickness of the LB layer to reduce the effect of polarization shows a low threshold current and a high output power even when the In content of barrier layers is as large as 7%.

Effects of Si-doping on field emission characteristics of AlN films grown on n-type 6H-SiC by MOCVD

Abstract The field-emission (FE) characteristics of 200 nm-thick Si-doped AlN films with different concentration of Si-dopant is investigated. It is found that the AlN film with lower concentration of Si-dopant has a smaller turn-on voltage, larger maximum current density and more stable FE current. Accompanying with atomic force surface micro-images, it is suggested that the stable current is attributed to a uniform surface, which is related a uniform local field enhancement factors and thus a stable FE current without discontinuous fluctuations. Furthermore, the analysis of scanning electron microscopy indicates that a smaller turn-on voltage and larger maximum current density may result from the electron transport channel of V-defects.

Theoretical investigations on the N-polar GaN/AlxGa1-xN/GaN heterostructures: Considering the existence of both two-dimensional hole and electron gases

In this paper, a theoretical study of N-polar GaN/AlxGa1-xN/GaN heterostructures is conducted systematically. The dependence of two-dimensional hole gas (2DHG, at the bottom AlxGa1-xN/GaN interface) and electron gas (2DEG, at the upper GaN/AlxGa1-xN interface) sheet densities on variables, such as GaN top layer thickness, AlxGa1-xN back barrier thickness, and Al content, is investigated. The effect of n-type doping in AlxGa1-xN and δ-doping concentrations in the GaN buffer on 2DHG and 2DEG sheet densities is also presented. For the unintentionally doped structure, the 2DHG-induced electric field E2DHG provides the only driving force in the 2DEG formation. Thus, in order to get high dense of 2DEG in such heterostructure, 2DEG and 2DHG shall coexist through proper AlxGa1-xN back barrier design. While for intentionally doped structure (with the n-type doped AlxGa1-xN back barrier layer or δ-doped GaN buffer layer), ionized donor-induced electric field Edonor is another driving force for 2DEG. The obtained in...

Resistivity reduction of low temperature grown p-Al0.09Ga0.91N by suppressing the incorporation of carbon impurity

Reducing the resistivity of low temperature grown p-Al0.09Ga0.91N layers is crucial to improving the performance of GaN-based laser diodes. In this study, growth conditions of low temperature grown p-Al0.09Ga0.91N layers are monitored and the role of C impurity is investigated systematically. On the basis of the dependence of resistivity on C concentration and the photoluminescence analysis, it is found that C impurities act as donors in p-Al0.09Ga0.91N layer, and reducing the C concentration can reduce its compensation effect on Mg acceptor. Finally, a low resistivity of 4.2 Ω·cm is achieved for the low temperature grown p-Al0.09Ga0.91N.Reducing the resistivity of low temperature grown p-Al0.09Ga0.91N layers is crucial to improving the performance of GaN-based laser diodes. In this study, growth conditions of low temperature grown p-Al0.09Ga0.91N layers are monitored and the role of C impurity is investigated systematically. On the basis of the dependence of resistivity on C concentration and the photoluminescence analysis, it is found that C impurities act as donors in p-Al0.09Ga0.91N layer, and reducing the C concentration can reduce its compensation effect on Mg acceptor. Finally, a low resistivity of 4.2 Ω·cm is achieved for the low temperature grown p-Al0.09Ga0.91N.

Deep levels induced optical memory effect in thin InGaN film

An optical memory effect is found in a 20 nm InGaN film. With increasing illumination time, photoluminescence (PL) intensity of InGaN rises at first and then falls. We present that this effect is caused by carriers capture in deep levels near interfaces between GaN and InGaN. Firstly, carriers captured by deep levels near the interfaces reduces the band inclination in InGaN. This cause the rise of PL intensity. Secondly, more and more captured carriers may form anti-shielding, which enhances band inclination and results in the decrease of PL intensity. Carriers captured in previous illumination can remain for a long time after illumination is blocked, which make InGaN show an optical memory effect.An optical memory effect is found in a 20 nm InGaN film. With increasing illumination time, photoluminescence (PL) intensity of InGaN rises at first and then falls. We present that this effect is caused by carriers capture in deep levels near interfaces between GaN and InGaN. Firstly, carriers captured by deep levels near the interfaces reduces the band inclination in InGaN. This cause the rise of PL intensity. Secondly, more and more captured carriers may form anti-shielding, which enhances band inclination and results in the decrease of PL intensity. Carriers captured in previous illumination can remain for a long time after illumination is blocked, which make InGaN show an optical memory effect.

Evolution of differential efficiency in blue InGaN laser diodes before and after a lasing threshold

The optical power emitting from the cavity facet of blue InGaN-based laser diodes (LDs) is measured to investigate the efficiency droop. The efficiency droop behavior of blue InGaN-based LDs near the threshold is confirmed in our experiments. From measurements of optical power at different wavelengths, it is analyzed that the droop behavior of LDs can be ascribed to the efficiency reduction of longer wavelengths. The efficiency of longer wavelengths is subject to the carrier occupation process in quantum levels. In addition, it is found that the droop behavior may be largely affected by the relatively large threshold current of InGaN-based LDs and the screening effect of polarization, and it can be suppressed by stimulated emission.