Z. S. Liu

Investigation on the compensation effect of residual carbon impurities in low temperature grown Mg doped GaN films

The influence of unintentionally doped carbon impurities on electrical resistivity and yellow luminescence (YL) of low-temperature (LT) grown Mg doped GaN films is investigated. It is found that the resistivity of Mg doped GaN films are closely related to the residual carbon impurity concentration, which may be attributed to the compensation effect of carbon impurities. The carbon impurity may preferentially form deep donor complex CN-ON resulting from its relatively low formation energy. This complex is an effective compensate center for MgGa acceptors as well as inducing YL in photoluminescence spectra. Thus, the low resistivity LT grown p-type GaN films can be obtained only when the residual carbon impurity concentration is sufficiently low, which can explain why LT P-GaN films with lower resistivity were obtained more easily when relatively higher pressure, temperature, or NH3/TMGa flow rate ratio were used in the LT grown Mg doped GaN films reported in earlier reports.

The investigation on carrier distribution in InGaN/GaN multiple quantum well layers

The carrier distribution and recombination dynamics of InGaN/GaN multiple quantum well (MQW) light-emitting diode structure are investigated. Two emission peaks were observed in the low temperature photoluminescence spectra of an InGaN/GaN MQW structure, but only one peak was observed in the electroluminescence (EL) spectra. Combined with the spatially resolved cathodoluminescence (CL) measurements, it is found that the electrically injected carrier distribution is governed by hole transport and diffusion in InGaN/GaN MQW structure due to the much lower mobility of hole. And the electron and hole recombination of EL occurs predominantly in the QWs that are located closer to the p-GaN layer.

Effects of thin heavily Mg-doped GaN capping layer on ohmic contact formation of p-type GaN

The growth condition of thin heavily Mg-doped GaN capping layer and its effect on ohmic contact formation of p-type GaN were investigated. It is confirmed that the excessive Mg doping can effectively enhance the Ni/Au contact to p-GaN after annealing at 550 degrees C. When the flow rate ratio between Mg and Ga gas sources is 6.4% and the layer width is 25 nm, the capping layer grown at 850 degrees C exhibits the best ohmic contact properties with respect to the specific contact resistivity (rho(c)). This temperature is much lower than the conventional growth temperature of Mg-doped GaN, suggesting that the deep-level-defect induced band may play an important role in the conduction of capping layer.

Suppression of electron leakage by inserting a thin undoped InGaN layer prior to electron blocking layer in InGaN-based blue-violet laser diodes

InGaN-based blue-violet laser diodes (LDs) suffer from electron leakage into the p-type regions, which could be only partially alleviated by employing the electron blocking layer (EBL). Here, a thin undoped InGaN interlayer prior to EBL is proposed to create an additional forbidden energy range above the natural conduction band edge, which further suppresses the electron leakage and thus improve the characteristics of LDs. Numerical device simulations reveal that when the proper composition and thickness of InGaN interlayer are chosen, the electron leakage could be efficiently eliminated without inducing any severe accumulation of electrons at the interlayer, resulting in a maximum output power of the device.

Effect of V-defects on the performance deterioration of InGaN/GaN multiple-quantum-well light-emitting diodes with varying barrier layer thickness

The effect of quantum barrier (QB) thickness on performances of InGaN/GaN multiple-quantum-well light-emitting diodes (MQW LEDs) with relatively large barrier layer thicknesses has been investigated. It is found that the density and averaged size of V-defects increases with QB thickness, resulting in larger reverse- and forward-bias current in LEDs. Electroluminescence measurement shows that LED with thinner QB has higher internal quantum efficiency but lower efficiency droop-onset current density, which should be ascribed to the faster saturation of carrier leakage into V-defects. Correspondingly, above the droop-onset current density, severer Auger recombination and carrier overflow are induced by higher carrier density due to the less V-defect related carrier leakage, leading to the more serious droop phenomenon in LEDs with thinner QB.

Optical and structural characteristics of high indium content InGaN/GaN multi-quantum wells with varying GaN cap layer thickness

The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) with different thicknesses of low temperature grown GaN cap layers are investigated. It is found that the MQW emission energy red-shifts and the peak intensity decreases with increasing GaN cap layer thickness, which may be partly caused by increased floating indium atoms accumulated at quantum well (QW) surface. They will result in the increased interface roughness, higher defect density, and even lead to a thermal degradation of QW layers. An extra growth interruption introduced before the growth of GaN cap layer can help with evaporating the floating indium atoms, and therefore is an effective method to improve the optical properties of high indium content InGaN/GaN MQWs.

The significant effect of the thickness of Ni film on the performance of the Ni/Au Ohmic contact to p-GaN

The significant effect of the thickness of Ni film on the performance of the Ohmic contact of Ni/Au to p-GaN is studied. The Ni/Au metal films with thickness of 15/50 nm on p-GaN led to better electrical characteristics, showing a lower specific contact resistivity after annealing in the presence of oxygen. Both the formation of a NiO layer and the evolution of metal structure on the sample surface and at the interface with p-GaN were checked by transmission electron microscopy and energy-dispersive x-ray spectroscopy. The experimental results indicate that a too thin Ni film cannot form enough NiO to decrease the barrier height and get Ohmic contact to p-GaN, while a too thick Ni film will transform into too thick NiO cover on the sample surface and thus will also deteriorate the electrical conductivity of sample.

A study on the spectral response of back-illuminated p-i-n AlGaN heterojunction ultraviolet photodetector

The spectral responses of back-illuminated p-i-n AlGaN ultraviolet photodetector under different bias voltage are investigated. The device structure is composed of p-GaN/p-Al0.3Ga0.7N/i-Al0.3Ga0.7N/n-Al0.6Ga0.4N and Ohmic contacts, where the p-GaN layer is very thin. There are two peaks in the spectral response of photocurrent located at 350 and 290 nm, respectively. It is found that in some devices the relative intensity and phase of these two peaks may change strongly with applied forward bias. A detailed analysis suggests that the possible Schottky-type-like behavior of metal and p-GaN contact and an unsatisfactory doping of p-GaN and p-AlGaN are responsible for the abnormal phenomenon.

Temperature dependence of photoluminescence spectra for green light emission from InGaN/GaN multiple wells

Three green light emitting InGaN/GaN multiple quantum well (MQW) structures with different In composition grown by metal-organic chemical vapor deposition are investigated by the X-ray diffraction and the temperature-dependent photoluminescence (PL) measurements. It is found that when the In composition increases in the InGaN/GaN MQWs, the PL spectral bandwidth may anomalously decrease with increasing temperature. The reduction of PL spectral bandwidth may be ascribed to the enhanced non-radiative recombination process which may lower the light emission efficiency of the localized luminescent centers with shallow localization energy in the high-In-content InGaN quantum wells and also cause a reduction of integrated PL intensity.

Stable multiplication gain in GaN p?i?n avalanche photodiodes with large device area

Visible-blind p-i-n avalanche photodiodes (APDs) were fabricated with high-quality GaN epilayers deposited on c-plane sapphire substrates by metal-organic chemical vapour deposition. Due to low dislocation density and a sophisticated device fabrication process, the dark current was as small as similar to 0.05 nA under reverse bias up to 20V for devices with a large diameter of 200 mu m, which was among the largest device area for GaN-based p-i-n APDs yet reported. When the reverse bias exceeded 38V the dark current increased sharply, exhibiting a bulk avalanche field-dominated stable breakdown without microplasma formation or sidewall breakdown. With ultraviolet illumination (360 nm) an avalanche multiplication gain of 57 was achieved.