Daizong Li

Determination of twist angle of in-plane mosaic spread of GaN films by high-resolution X-ray diffraction

X-ray diffraction (XRD) measurements of Phi scan in different chi angle in skew geometry for GaN films have shown that the peak widths of Phi scan decrease with the increment of angle chi. The FWHM of omega scan also increases with the inclination angle and reaches to be equal until the reflection plane is perpendicular to the surface of the sample. Based on these measured results, we developed a method to determine directly the twist angle of un-doped and Si-doped GaN films grown on c-sapphire substrates by MOCVD

Dependence of leakage current on dislocations in GaN-based light-emitting diodes

The reverse bias current-voltage (I–V) characteristics of GaN-based light-emitting diodes (LEDs) were investigated. The leakage current exhibits exponential dependence on the bias voltage with different exponents for various voltage ranges. The leakage current is closely related to the density of dislocations. The number of dislocations in GaN was determined by atomic force microscopy combined with hot H3PO4 etching. Dislocations with a screw component in the GaN films were found to have a strong influence on the reverse leakage current of LEDs. The dislocation electrical activity in GaN grown on c-plane sapphire is different from that in GaN grown on a-plane sapphire.

Influence of the deposition time of barrier layers on optical and structural properties of high-efficiency green-light-emitting InGaN∕GaN multiple quantum wells

We reported the effect of the deposition time of barrier layers on optical and structural properties of high-efficiency green-light-emitting InGaN∕GaN multiple quantum wells (MQWs) by photoluminescence (PL) and high-resolution x-ray diffraction techniques. The MQW samples on (0001)-plane sapphire substrates were prepared with a ramping method by metalorganic chemical deposition. It was found that the structural or interface quality of the MQW system improved as the deposition time of barrier layers increased from 10 to 14min, but lattice relaxation was still observed. The relaxation degree decreases from 33% to 6% as the deposition time increases. Temperature-dependent PL measurements from 12 to 300K indicated that the integrated PL intensities start to decay rapidly as temperature rises above 50K for the sample with the shorter deposition time, and above 100K for the sample with the longer deposition time. The luminescence thermal quenching of the two samples suggests the two nonradiative recombination c...

Effect of low-temperature SiGe interlayer on the growth of relaxed SiGe

A low-temperature Si0.8Ge0.2 (LT-Si0.8Ge0.2) interlayer was grown at 500 degrees C to improve the relaxed Si0.8Ge0.2 surface and reduce the dislocation density in it, which was confirmed by the change of reflective high-energy electron diffraction (RHEED) pattern from spotty to streaky and etch pits counts. For the same extent of strain; the threading dislocation density was reduced from 8 x 10(7) cm(-2) in the latter to 2 x 10(6) cm(-2) in the former

Effects of TMIn flow rate of barrier layer on the optical and structural properties of InxGa1-xN/InyGa1-yN multiple quantum wells

We reported the effect of trimethylindium (TMIn) flow rate in the barrier layer on the structural and optical properties of InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs) grown on (0001)-oriented sapphire substrates by low-pressure metalorganic chemical deposition (LP-MOCVD). It was found that the change of TMIn flow rate dramatically influences the interface quality and optical properties. Temperature-dependent photoluminescence (PL) and high-resolution X-ray diffraction (HRXRD) measurements provide proof evidence that the increment of TMIn flow rate deteriorates the interfacial abruptness, decreases the emission wavelength (blue shift) and the efficiency of nonradiative recombination process

Effects of ammonia flow at growth temperature ramping process on optical properties of InGaN/GaN multiple quantum wells

Different ammonia flow rates were used during temperature ramping up to 1050°C after InGaN/GaN multiple quantum wells (MQWs) were grown at 820°C. Temperature-dependent photoluminescence (PL) as well as time-resolved PL (TRPL) spectra were measured and higher optical efficiency was obtained when higher ammonia flow rates were used. The effects of ammonia flow on the optical properties of InGaN/GaN multiple quantum wells were attributed to different indium content redistribution. At lower ammonia flow, In-rich regions became more uniform while the number of the In-rich regions decreased as a result of the disappearance of some In-rich regions with less In-rich regions. We ascribed enhancement of desorption of indium atoms to the lower ammonia flow.

In-situ annealing during the growth of relaxed SiGe

In this paper, a graded Si1-xGex buffer and thereafter the Si0.8Ge0.2 uniform layer were grown at a little lower temperature to keep the surface smooth, which will provide the gliding dislocations a wider channel and less low energy nucleation sites on the surface. Therefore, the dislocation density may be reduced. However, the motion of the existing threading dislocations cannot retain equilibrium at lower temperature, strain will accumulate and be in favor of the nucleation of dislocation. In situ annealing was used to reduce the residual strain in the sample during the low-temperature growth of SiGe. A fully relaxed Si0.8Ge0.2 layer was obtained with the surface dislocation dnesity of 3x105cm-2.

Preparation and photoluminescence of nc-Si/SiO2 MQW

The deposition rate and refractive index for a-Si(amorphous silicon) and SiO2 grown by PECVD were studied under different pressure, power and proportion of reactant source gases. a-Si/SiO2 MQW(multi-quantum well) with high quality was deposited under suitable conditions, in which the thickness of the a-Si layers is several nanometers. The sample of a-Si/SiO2 MQW was crystallized by laser annealing. Because of the confinement of the SiO2 layers, crystalline grains were formed during the a-Si layers were being crystallized. The size of the crystalline grains were not more than the thickness of the a-Si layers. The a-Si layers were crystallized to be nanometer crystalline silicon (nc-Si), therefore, nc-Si/SiO2 MQW was formed. For the a-Si/SiO2 MQW with 4.0 nm a-Si wells separated by 5 nm SiO2 barriers, most of the a-Si were crystallized to silicon grains after laser annealing, and the size of the grains is 3.8 nm. Strong photoluminescence with three peaks from the nc-Si/SiO2 MQW was detected at 10 K. The wavelength of the peaks were 810 nm, 825 nm and 845 nm, respectively.

Optical characterization of the Ge/Si (001) islands in multilayer structure

We show that the observed temperature dependence of the photoluminescence(PL) features can be consistently explained in terms of thermally activated carrier transfer processes in a multilayer structure of the self-organized Ge/Si(001) islands. The type II (electron confinement in Si) behavior of the Ge/Si islands is verified. With elevated temperature, the thermally activated electrons and holes enter the Ge islands from the Si and from the wetting layer (WL), respectively. An involvement of the type I (spatially direct) into type II (spatially indirect) recombination transition takes place at a high temperature.

The SPER and characteristics of Si1-yCy alloys

Si1-yCy alloys with carbon composition of 0.5 at% were successfully grown on n-Si(100) substrate by solid phase epitaxy recrystallization. The result was presented in this paper. With the help of the SiO2 capping layer, rather uniform carbon profile in amorphous Si layer was obtained by dual-energy implantation. Since ion-flow was small and implantation time was long enough, the emergency of (beta) -SiC was avoided and the dynamic annealing effect was depressed. The pre-amorphization of the Si substrate increased the fraction of the substitutions carbon and the two-step annealing reduced point defects. As a result, Si1-yCy alloys with high quality was recrystallized on Si substrate.