W.K. Fong

Effects of Ar+ back-surface gettering on the properties of flicker noise in n-channel nitrided MOSFETs

Abstract Flicker noise in back-surface gettered, nitrided n-channel metal-oxide-semiconductor field-effect transistors is characterized over a wide range of temperature and biases. The gettering was performed using a low-energy (550 eV) argon ion beam, and the gettering time ranged from 10 to 40 min. The noise power spectra for devices with different gettering times are compared to the ungettered devices which serve as the control. It is found that flicker noise is reduced by back-surface gettering for short gettering times. However, a rebound in the noise magnitude is observed for long gettering times. Investigation of the temperature dependences of the noise power spectra indicates that the low-frequency noise arises from thermal activation of carriers to traps at the SiSiO 2 interface. Back-surface gettering results in the modification of the energy distribution of the interface traps, probably due to stress relaxation at the SiSiO 2 interface.

Study of low-frequency excess noise in GaN materials

Abstract We report detailed investigations of low-frequency excess noise in GaN-based metal–semiconductor–metal devices fabricated on GaN thin films deposited by RF-plasma assisted molecular beam epitaxy on different types of buffer structures. Our experimental data indicate two orders of magnitude reduction in flicker noise for samples grown on double buffer layers that consist of a GaN intermediate temperature buffer layer on top of a thin AlN high temperature buffer layer. Experimental results on the temperature dependencies of the current noise power spectra stipulate that the noise arises from thermally activated trapping and detrapping of carriers. Based on the thermal activation model for 1/ f noise, we computed the energy distribution of the traps responsible for the observed flicker noise. We also performed systematic studies on the hot-electron degradation of the devices through the application of a large voltage bias. The data demonstrate substantial improvement in the hot-electron hardness for devices fabricated on the double buffer layer structures.

Influence of double buffer layers on properties of Ga-polarity GaN films grown by rf-plasma assisted molecular-beam epitaxy

Abstract Ga-polarity GaN thin films were grown on sapphire (0001) substrates by rf-plasma assisted molecular beam epitaxy (MBE) using a double buffer layer, which consisted of an intermediate-temperature GaN buffer layer (ITBL) grown at 690 °C and a conventional AlN buffer layer deposited at 740 °C. Raman scattering spectra showed that the E 2 (high) mode of GaN film grown on conventional AlN buffer layer is at about 570 cm −1 , and shifts to 568 cm −1 when an ITBL was used. This indicates that the ITBL leads to the relaxation of residual strain in GaN film caused by mismatches in the lattice constants and coefficients of thermal expansion between the GaN epilayer and the sapphire substrate. Compared to the GaN film grown on the conventional AlN buffer layer, the GaN film grown on an ITBL shows higher Hall mobility and substantial reduction in the flicker noise levels with a Hooge parameter of 3.87×10 −4 , which is believed to be, to date, the lowest reported for GaN material. These results imply that the quality of Ga-polarity GaN films grown by MBE can be significantly improved by using an ITBL in addition to the conventional low-temperature AlN buffer layer.

Study of Low-Frequency Excess Noise Transport in Ga-Face and N-Face GaN Thin Films Grown on Intermediate-Temperature Buffer Layer by RF-MBE

We report detailed investigations of low-frequency excess noise in both Ga-faced and N-faced GaN thin films grown by RF-plasma molecular beam epitaxy. The GaN epilayers were grown on double buffer layers, and consisted of a thick intermediate-temperature buffer layer (ITBL) deposited at 690 °C and a conventional thin buffer layer. Deposition of the thin buffer layer is used to control the polarity of the GaN epilayer. Low-frequency excess noise was studied in detail to examine the effects on the ITBL on the noise. The low-frequency noise is attributed to the correlated fluctuations in number and mobility of carriers, arising from the capture and emission by localized states. Our experimental results show that the polarity of the GaN epilayer and the utilization of ITBL have strong influence on the defect density of the GaN material.

Growth of high quality Cu 2 ZnSnS 4 thin films on GaN by co-evaporation

We report on the growth of high quality Cu2ZnSnS4 (CZTS) thin films on GaN-on-sapphire (GOS) substrates by thermal co-evaporation. Structural characterization was performed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The results are compared to films grown by thermal co-evaporation on glass and those synthesized by the sulfurization of stacked precursors on glass. Our results show that single phase epitaxial quality CZTS films with improved crystallinity can be grown on GOS substrates. The optical and electrical properties such as optical transmission, carrier concentration and Hall mobilities of the films are also reported.

Nature of Low-Frequency Excess Noise in n-Type Gallium Nitride

Low-frequency noise is investigated in n-type GaN film grown by rf-plasma assisted molecular beam epitaxy. The temperature dependence of the voltage noise power spectra, S V (f) , was examined from 400K to 80K in the frequency range between 30Hz and 100KHz, which can be modeled as the superposition of 1/f (flicker) noise G-R noise. At f > 500 Hz the noise is dominated by G-R noise with activation energies of 360meV and 65meV from the conduct band. The results clearly demonstrate the trap origin for both the 1/f noise and G-R noise. At the low-frequency range the fluctuation was dominated by 1/f noise. To determine the origin of the noise we considered both the bulk mobility fluctuation and the trap fluctuation models. Our experimental results showed that rapid thermal annealing (RTA) at 800°C resulted in over one order of magnitude decrease in the Hooge parameter. Annealing at temperatures in excess of 1000°C resulted in significant increase in the noise. Photoluminescence and x-ray diffraction measurements also showed that the crystallinity of the films improved with RTA at 800°C with an accompanying reduction in deep levels. Annealing at 900°C and 1000°C resulted in an increase in the FWHM of the x-ray diffraction indicative of thermal decomposition of the materials. The results are in excellent agreement with the trend of Hooge parameters as a function of annealing temperature, strongly indicating trap origin of the observed 1/f noise.

Investigation of low-frequency noise in van der Waals epitaxies

We report the growth of high quality SnS van der Waals epitaxies (vdWEs) on mica by molecular beam epitaxy (MBE). Record low rocking curve FWHM for the SnS (001) and SnSe (001) epilayers were observed. Detailed investigations of the film morphology indicate layer-by-layer growth mode expected for 2D layered structure. The characteristic structural symmetry of the mica surface leads to the nucleation of crystallites with different lateral orientations. This results in high concentration of grain boundaries and high level of low-frequency excess noise. We investigated the growth mechanisms of the vdWEs and suggested a novel growth technique in which a SnSe buffer layer is utilized to reduce the concentration of the grain boundary. The idea is tested experimentally and significant reduction in the low-frequency noise is observed suggesting the use of the buffer layer had resulted in the reduction in the possible growth orientations of the top SnS epilayer and thereby reducing the grain boundary concentration.

Influence of intermediate-temperature buffer layer on flicker noise characteristics of MBE-grown GaN thin films and devices

Abstract Gallium nitride epitaxial layers were grown by rf-plasma MBE on different buffer layer structures. Type I buffer layer consists of a conventional AlN high-temperature buffer layer (HTBL). Type II buffer layer consists of a GaN intermediate-temperature buffer layer (ITBL) grown on top an AlN HTBL. Measurement of flicker noise in metal–semiconductor–metal (MSM) structures on type II buffer layers exhibited close to two orders of magnitude reduction in the noise level compared to those fabricated on type I buffer structures. This shows that GaN thin films grown with the use of ITBL have significantly lower number of interface traps at the metal–semiconductor interface, which is attributed to be the main cause of the observed improvements in the optical properties of the devices. We also performed systematic studies on hot-electron degradation of the devices through the application of a large voltage bias. The data demonstrate substantial improvement in the hot-electron hardness for devices fabricated on type II buffer layer structures.

Effect of indium surfactant on the optical and structural properties of MBE-grown GaN

A small indium flux was used as a surfactant during the growth of gallium nitride films by rf-plasma assisted molecular beam epitaxy. The effects of the indium surfactant on the optical and structural properties of undoped GaN were studied by photoluminescence spectroscopy, high-resolution X-ray diffraction, atomic force microscopy, Rutherford backscattering spectroscopy, and low-frequency noise. Photoluminescence spectra show that the GaN thin films grown in the presence of In surfactant exhibit supressed yellow luminescence compared to films grown under the same experimental conditions but without In surfactant. The X-ray rocking curves demonstrate a 20% decrease in the full width at half maximum value for the films grown with In surfactant. AFM studies show that the root mean squared roughness for films grown with and without In surfactant are 5.86 and 6.99 nm respectively, indicating significant improvement in surface morphology. RBS and ion channeling shows the presence of stacking faults and dislocations and GaN growth with In surfactant and stacking faults in GaN grown without In. For the characterization of defect properties in the films we conducted detailed studies of 1/f and Generation-Recombination (G-R) noise on our samples. About 65% reduction in the Hooge parameter was observed in the film grown with In surfactant, indicative of a corresponding reduction in defect states within the material. For f

Effects of low-energy back surface gettering on the properties of 1/f noise in n-channel nitrided MOSFETs

GTR 500Hz, G-R noise was the dominating fluctuation process. Detailed characterization of the G-R noise over a wide range of temperatures enabled the determination of the energy levels of the traps responsible for the G-R noise. Three different trap levels were observed using noise measurement technique. For films grown without In surfactant, traps with activation energies 806 meV, 241 meV and 100 meV were observed, whereas for samples grown with In surfactant traps were observed at 666 meV, 208 meV and 90 meV. Such systematic reduction in the energy levels of the traps may arise from the relaxation of strains in the material when grown under the application of In surfactant.