Xinkui He

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.

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.

High-efficiency supercontinuum generation in solid thin plates at 0.1 TW level

Supercontinuum generation in a solid-state medium was investigated experimentally. A continuum covering 460 to 950 nm was obtained when 0.8 mJ/30 fs Ti:sapphire laser pulses were applied to seven thin fused silica plates at a 1 kHz repetition rate. The primary processes responsible for spectral broadening were self-phase modulation (SPM) and self-steepening, while SPM and self-focusing were balanced to optimize the spectral broadening and suppress the multiphoton process. The output was compressed to a 5.4 fs and a 0.68 mJ pulse, corresponding to two optical cycles and 0.13 TW of peak power.

Differential resistance of GaN-based laser diodes with and without polarization effect.

In this paper, we used numerical calculation and simulation to investigate the differential resistance of GaN-based laser diodes (LDs) with and without polarization effect. We confirmed the existence of a kink at the vicinity of threshold current in the differential resistance curve of GaN-based LDs and found that the kink polarity can be reversed dependent on the polarization effect. The serial parasitic diodes should be included in the theoretical analysis of the equivalent circuit of the LD devices. We determined that the superposition effects of the n-side, active, and p-side regions of the LDs caused the kink and its polarity. We also found that the differential resistance before and after the threshold was dominated by the p-side region and its gradual reduction is related to an electron overflow into p-side. Finally, we studied the effects of cavity facet reflectivity on the kink.

Emission efficiency enhanced by reducing the concentration of residual carbon impurities in InGaN/GaN multiple quantum well light emitting diodes

A series of samples with varying growth pressure are grown and their optical and structural properties are investigated. It is found that the residual carbon concentration decreases when the reactor pressure increases from 80 to 450 Torr during the InGaN/GaN multiple quantum well growth. It results in an enhanced peak intensity of electroluminescence because carbon impurities can induce deep energy levels and act as non-radiative recombination centers in InGaN layers.

Observation of non-odd order harmonics by sub-2-cycle laser pulses

High order harmonics generation from argon gas was comprehensively investigated by using multi-cycle and few-cycle laser pulses. Non-odd order harmonics were observed for sub-5-fs pulses, compare to the normal odd-order harmonics in the multi-cycle case. Theoretic analysis shows that the new spectral structure origins from the asymmetry of laser field in few-cycle pulses. This asymmetry induced both amplitude and phase difference between attosecond pulses from consecutive half-cycle of the laser field, which change the interference property of attosecond pulses and result in complex spectrum.

The thickness design of unintentionally doped GaN interlayer matched with background doping level for InGaN-based laser diodes

In order to reduce the internal optical loss of InGaNlaser diodes, an unintentionally dopedGaN (u-GaN) interlayer is inserted between InGaN/GaN multiple quantum well active region and Al0.2Ga0.8N electron blocking layer. The thickness design of u-GaN interlayer matching up with background doping level for improving laser performance is studied. It is found that a suitably chosen u-GaN interlayer can well modulate the optical absorption loss and optical confinement factor. However, if the value of background doping concentration of u-GaN interlayer is too large, the output light power may decrease. The analysis of energy band diagram of a LD structure with 100 nm u-GaN interlayer shows that the width of n-side depletion region decreases when the background concentration increases, and may become even too small to cover whole MQW, resulting in a serious decrease of the output light power. It means that a suitable interlayer thickness design matching with the background doping level of u-GaN interlayer is significant for InGaN-based laser diodes.

Long-Term Stabilization of Carrier-Envelope Phase for Few Cycles Ti:Sapphire Laser Amplifier

We realize a long-term carrier-envelope phase (CEP) stabilization for a chirped pulse amplified Ti:sapphire laser by locking the oscillator and amplifier independently. Based on the measurement of CEP by employing f-to-2f interference technique between the octave-spanning spectrum which is generated from a rare gas filled hollow fiber, continuous locking time up to 7.2 h with 85 mrad fluctuation is demonstrated. Finely compensating the dispersion by a set of chirped mirrors, quasi-mono cycle pulses as shorter as 3.8 fs are obtained. Further experimental research on high harmonic generation dependence on CEP shown the waveform of laser pulses has been successfully controlled.