Cuimei Wang

Simulation of In0.65Ga0.35N single-junction solar cell

The performances of In0.65Ga0.35N single-junction solar cells with different structures, including various doping densities and thicknesses of each layer, have been simulated. It is found that the optimum efficiency of a In0.65Ga0.35N solar cell is 20.284% with 5 × 1017 cm−3 carrier concentration of the front and basic regions, a 130 nm thick p-layer and a 270 nm thick n-layer.

Influence of AlN buffer layer thickness on the properties of GaN epilayer on Si(111) by MOCVD

The effect of thickness of the high-temperature (HT) AlN buffer layer on the properties of GaN grown on Si(111) has been investigated. Optical microscopy (OM), atomic force microscopy (AFM) and X-ray diffraction (XRD) are employed to characterize these samples grown by metal-organic chemical vapor deposition (MOCVD). The results demonstrate that the morphology and crystalline properties of the GaN epilayer strongly depend on the thickness of HT AlN buffer layer, and the optimized thickness of the HT AlN buffer layer is about 110nm. Together with the low-temperature (LT) AlN interlayer, high-quality GaN epilayer with low crack density can be obtained.

Theoretical design and performance of InxGa1?xN two-junction solar cells

The efficiencies of InxGa1-xN two-junction solar cells are calculated with various bandgap combinations of subcells under AM1.5 global, AM1.5 direct and AM0 spectra. The influence of top-cell thickness on efficiency has been studied and the performance of InxGa1-xN cells for the maximum light concentration of various spectra has been evaluated. Under one-sun irradiance, the optimum efficiency is 35.1% for the AM1.5 global spectrum, with a bandgap combination of top/bottom cells as 1.74 eV/1.15 eV. And the limiting efficiency is 40.9% for the highest light concentration of the AM1.5 global spectrum, with the top/bottom cell bandgap as 1.72 eV/1.12 eV.

Hydrogen sensors based on AlGaN/AlN/GaN HEMT

Pt/AlGaN/AlN/GaN high electron mobility transistors (HEMT) were fabricated and characterized for hydrogen sensing. Pt and Ti/Al/Ni/Au metals were evaporated to form the Schottky contact and the ohmic contact, respectively. The sensors can be operated in either the field effect transistor (FET) mode or the Schottky diode mode. Current changes and time dependence of the sensors under the FET and diode modes were compared. When the sensor was operated in the FET mode, the sensor can have larger current change of 8mA, but its sensitivity is only about 0.2. In the diode mode, the current change was very small under the reverse bias but it increased greatly and gradually saturated at 0.8mA under the forward bias. The sensor had much higher sensitivity when operated in the diode mode than in the FET mode. The oxygen in the air could accelerate the desorption of the hydrogen and the recovery of the sensor.

An investigation on InxGa1−xN/GaN multiple quantum well solar cells

The conversion efficiency of InxGa1−xN/GaN multiple quantum well solar cells is originally investigated in theory based on the ideal diode model and the ideal unity quantum well model. The results reveal that the conversion efficiency partially depends on the width of the quantum well and the thickness of the barrier region but is dominated by the number of quantum wells and indium content of InxGa1−xN. The calculated results are found to be basically trustworthy by comparing with reported experimental results. An In0.15Ga0.85N/GaN multiple quantum well solar cell is successfully fabricated with a conversion efficiency of 0.2%. The main discrepancy between calculated and experimental results is the material quality and manufacturing technology which need to be improved.

Study on Mg memory effect in npn type AlGaN/GaN HBT structures grown by MOCVD

AlGaN/GaN npn heterojunction bipolar transistor structures were grown by low-pressure MOCVD. Secondary ion mass spectroscopy (SIMS) measurements were carried out to study the Mg memory effect and redistribution in the emitter-base junction. The results indicated that there is a Mg-rich film formed in the ongrowing layer after the Cp2Mg source is switched off. The Mg-rich film can be confined in the base section by switching off the Cp2Mg source for appropriate time before the end of base growth. Low temperature growth of the undoped GaN spacer suppresses the Mg redistribution from Mg rich film. The delay rate of the Mg profile in sample C with spacer growing in low temperature is about 56 nm/decade, which becomes sharper than 80 nm/decade of the samples A and B without low temperature spacer

Two-dimensional electron and hole gases in InxGa1−xN/AlyGa1−yN/GaN heterostructure for enhancement mode operation

In this paper, a numerical study of In x Ga1 − x N/Al y Ga1 − y N/GaN heterostructure is presented. The dependence of two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) sheet densities on variables, such as In x Ga1 − x N layer thickness and In content, and Al y Ga1 − y N barrier layer thickness and Al content, are systematically investigated. The effect of P-type doping in In x Ga1 − x N on 2DEG and 2DHG sheet densities in this heterostructure is also studied. It is shown that the strong reverse electric field in In x Ga1 − x N cap layer contributes to the depletion of 2DEG at the Al y Ga1- y N/GaN interface. When In x Ga1 − x N layer thickness and In content increases, 2DEG sheet density decreases significantly. P-type doping shows less influence on 2DEG compared to the polarization electric field in In x Ga1 − x N layer. In addition, there exist critical values for all the variables beyond which 2DHG appears at the interface of In x Ga1 − x N/Al y Ga1 − y N. Once 2DHG appears, it will prevent 2DEG from being further depleted. With proper design of Al y Ga1 − y N layer, the coexistence of 2DEG and 2DHG in In x Ga1 − x N/Al y Ga1 − y N/GaN structure can be avoided, showing that this structure has great potential in the fabrication of enhancement mode (E-mode) high electron mobility transistors.

Enhanced performance of InGaN/GaN multiple quantum well solar cells with patterned sapphire substrate

In this paper, the enhanced performance of InGaN/GaN multiple quantum well solar cells grown on patterned sapphire substrates (PSS) was demonstrated. The short-circuit current (Jsc) density of the solar cell grown on PSS showed an improvement of 60%, compared to that of solar cells grown on conventional sapphire substrate. The improved performance is primarily due to the reduction of edge dislocations and the increased light absorption path by the scattering from the textured surface of the PSS. It shows that the patterned sapphire technology can effectively alleviate the problem of high-density dislocations and low Jsc caused by thinner absorption layers of the InGaN based solar cell, and it is promising to improve the efficiency of the solar cell.

Growth of 2 μm Crack-Free GaN on Si(111) Substrates by Metal Organic Chemical Vapor Deposition

A 2 μm high quality crack-free GaN film was successfully grown on 2-inch Si(111) substrates by metal organic chemical vapor deposition with a high temperature AlN/graded-AlGaN multibuffer and an AlN/GaN superlattice interlayer. It is found that the structures, as well as the thicknesses of the multibuffer and interlayer, are crucial for the growth of a crack-free GaN epilayer. The GaN(0002) XRD FWHM of the crack-free sample is 479.8 arcsec, indicating good crystal quality. An AlGaN/GaN heterostructure was grown and tested by Van der Pauw Hall measurement. The electron mobility of two-dimensional electron gas increases from 1928 cm2/V-s to 12277 cm2/V-s when the test-temperature decreases from room temperature to liquid nitrogen temperature. The electron mobility is comparable to that of AlGaN/GaN heterostructures grown on sapphire, and the largest value is obtained for an AlGaN/GaN/Si(111) heterostructure grown by metal organic chemical vapor deposition.

Influence of electric field on persistent photoconductivity in unintentionally doped n-type GaN

The influence of electric field on persistent photoconductivity in unintentionally doped n-GaN is investigated. It was found that under higher electric field the build-up course was slowed down while the decay course was accelerated. After a higher-voltage pulse, it was observed that the current dropped to a value lower than the dark current, and a current increase that lasted for thousands of seconds was observed. It is suggested that the above phenomena should be caused by the increase in capture rate of electron traps with electric field and are related to the Coulomb-repulsive characteristic of defects related to persistent photoconductivity.