B.H. Leung

Low-frequency noise in GaN thin films deposited by rf-plasma assisted molecular-beam epitaxy

We report detailed investigations of low-frequency excess noise in GaN thin-film cross-bridge structures deposited by rf-plasma assisted molecular-beam epitaxy on top of an intermediate-temperature buffer layer (ITBL) grown at 690 °C. The experimental data indicates strong dependence of the voltage noise power spectra on the thickness of the ITBL with an optimal thickness of 800 nm. A model has been presented to account for the observed noise, which stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers. The process results in the correlated fluctuations in both the carrier number and the Coulombic scattering rate. Detailed computation shows that number fluctuation dominates in our samples. Our numerical evaluation indicates a reduction in the trap density by over an order of magnitude with the use of an ITBL in the growth of GaN thin films.

Study of the effects of rapid thermal annealing in generation–recombination noise in MBE grown GaN thin films

Abstract Low-frequency excess noise was measured from GaN thin films deposited by plasma assisted molecular beam epitaxy (MBE). The noise power spectra is dominated by 1/ f noise at low frequency and by Lorentzians at frequencies beyond 3 kHz. The temperature dependencies of the Lorentzians were examined from room temperature to about 90 K. From the Arrhenius plot of the time constants, the thermal activation energy of the fluctuation time constant was found to be around 30 meV. From the temperature dependencies of voltage noise power spectra, we estimated the magnitudes of the capture and emission activation energies. Based on the results, we have formulated a model, which stipulates that the generation–recombination (G–R) noise arises from the capture and emission of carriers by localized states in the bulk of the film. The process leads to fluctuations in the carrier mobility due to the modulation of the Coulombic scattering rate. We next conducted a systematic investigation on the effects of rapid thermal annealing on G–R noise in GaN thin films. Experimental results showed that annealing at 900°C resulted in the minimum FWHM in the rocking curve. Furthermore, we observed a substantial reduction in the noise level, indicating that rapid thermal annealing can be used as an effective means for noise reduction in GaN based devices.

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.

Characterization of low-frequency noise in molecular beam epitaxy-grown GaN epilayers deposited on double buffer layers

We report the growth of high-mobility Si-doped GaN epilayers utilizing unique double buffer layer (DBL) structures, which consist of a thin buffer layer and a thick GaN intermediate-temperature buffer layer (ITBL). In this study, three types of DBL were investigated: (i) thin GaN low-temperature buffer layer /GaN ITBL (type I); (ii) nitridated Ga metal film/GaN ITBL (type II); and (iii) thin AlN high-temperature buffer layer /GaN ITBL (type III). Systematic measurements were conducted on the electron mobilities and the low-frequency noise over a wide range of temperatures. It is found that the electron mobilities of the GaN films are substantially improved with the use of DBLs, with the sample using type III DBL which exhibits the highest low-temperature mobility. Furthermore, the same sample also demonstrates the elimination of deep levels at 91 and 255 meV below the conduction band. This is believed to result from the relaxation of tensile stress during growth with the use of type III DBLs.

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.

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.

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.

Study of GaN thin films grown on intermediate-temperature buffer layers by molecular beam epitaxy

A detailed characterisation study of GaN thin films grown by rf-plasma molecular beam epitaxy on intermediate-temperature buffer layers (ITBL) was carried out with Hall, photoluminescence (PL) and deep-level transient Fourier spectroscopy (DLTFS) techniques. The unique feature of our GaN thin films is that the GaN epitaxial layers are grown on top of a double layer that consists of an ITBL, which is grown at 690 degreesC, and a conventional low-temperature buffer layer deposited at 500 degreesC. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm(2)V(-1)S(-1) for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. The DLTFS results suggest a three-order-of-magnitude reduction in the deep level at E-c-0.40 eV in the device fabricated with the GaN films grown on an ITBL thickness of 1.25 mum in comparison with the control device without an ITBL. Our analyses indicate that the utilization of an ITBL in addition to the conventional low-temperature buffer layer leads to the relaxation of residual strain within the material, resulting in an improvement in the optoelectronic properties of the films

Low-Frequency Noise Characterization in AlGaN/GaN HEMTs with Varying Gate Recess Depths

Low-frequency noise measurements were performed on a number of AlGaN/GaN HEMTs with different gate recess depths, which were formed by dry etching. Detailed characterizations of the low-frequency noise properties were performed on the devices as a function of as a function of hot-electron stressing conducted at V D = 10 V and V G = -1.5 V. The room temperature voltage noise power spectral density, S V (ƒ), of the devices were found to show 1/ƒ dependence. A comparison of S V (ƒ) measured from different devices clearly indicate increase in the noise levels for the devices with large recess depths, reflecting the degradation caused by ion-impact induced damage during recess formation. Furthermore, the results of low-frequency noise measurements showed fast degradations for the devices with larger gate recess depths. Our experimental data clearly show that the dry etching process has induced damages in gates.

Electrical properties of high quality MBE-grown GaN thin films on intermediate-temperature buffer layers

High quality GaN thin films were grown by RF-plasma assisted molecular beam epitaxy on an intermediate-temperature buffer layer (ITBL). The electrical properties of the GaN films were systematically characterized by Hall coefficients and low-frequency noise measurements. The results show a systematic change in the electrical properties, which vary as a function of the ITBL thickness. Room temperature Hall mobility increases steadily from 87 cm/sup 2/ V/sup -1/ s/sup -1/, for GaN films grown on a conventional low-temperature buffer layer (LTBL), to 390 cm/sup 2/ V/sup -1/ s/sup -1/, for a sample grown with an 800 nm-thick ITBL inserted between the LTBL and the top epitaxial layer. The Hooge parameter reaches a minimum value of 7.34/spl times/10/sup -2/ for an optimal ITBL thickness of 800 nm. The observed improvements in the electrical properties are attributed to the relaxation of residual strain in the overgrown GaN by use of the ITBL.