C. F. Zhu

Study of low-frequency excess noise in GaN thin films deposited by RF-MBE on intermediate-temperature buffer layers

Low-frequency excess noise was measured in a series of GaN epitaxial films deposited by RF-plasma assisted molecular beam epitaxy (MBE). The GaN epitaxial layers were grown on double buffer layers, each consisting of an intermediate-temperature buffer layer (ITBL) deposited at 690/spl deg/C and a conventional low-temperature buffer layer grown at 500/spl deg/C. The Hooge parameters for the as-grown films were found to depend on the thickness of ITBL with a minimum value of 7.34/spl times/10/sup -2/ for an optimal ITBL thickness of 800 nm. The observed improvements in the noise properties are attributed to the relaxation of residual strain within the material, leading to a corresponding reduction in crystalline defects.

High-mobility GaN epilayer grown by RF plasma-assisted molecular beam epitaxy on intermediate-temperature GaN buffer layer

Abstract High-mobility GaN thin films were grown by RF plasma-assisted molecular beam epitaxy on (0xa00xa00xa01) sapphire. A conventional low-temperature buffer layer and an intermediate-temperature buffer layer (ITBL) were first deposited before the growth of the epitaxial layer. Electron mobility is found to vary strongly with the ITBL thickness with value as high as 460xa0cm2xa0V−1xa0s−1 obtained from the sample grown on a 800xa0nm ITBL on top of a low-temperature buffer layer. A systematic shift in the photoluminescence peak position, following the same trend as the mobility, suggests the relaxation of residual strain in the top GaN epitaxial layer by utilizing an ITBL. Detailed characterizations of G-R noise indicate reduction of deep levels by over an order of magnitude for the sample with 800xa0nm ITBL compared to the control sample which has the same total thickness but with only the low-temperature buffer layer.

Effects of rapid thermal annealing on the structural properties of GaN thin films

Effects of rapid thermal annealing (RTA) on the structural properties were investigated in undoped GaN film grown by rf-plasma-assisted molecular beam epitaxy (MBE), Detailed characterizations of the photoluminescence (PL), high-resolution X-ray diffraction and low-frequency noise were conducted on both the as-grown and annealed films, PL and X-ray diffraction measurements showed that the crystallinity of the films improved with RTA at 800/spl deg/C with significant reduction in the yellow emission. Annealing at 900/spl deg/C and 1000/spl deg/C resulted in an increase in the FWHHM of the X-ray diffraction, indicative of thermal decomposition of the materials, The results are in excellent agreement with our study of low-frequency noise, which demonstrates similar trends in the magnitudes of the Hooge parameters as a function of the annealing temperature. The temperature dependence of the voltage noise power spectra S/sub u/(f) was examined from 400 K to 80 K in the frequency range between 30 Hz and 100 kHz. At the low-frequency range the fluctuation is dominated by 1/f/sup /spl gamma// noise, and for f>1 kHz the noise is dominated by G-R noise processes. Our experimental results show that 800/spl deg/C is the optimal temperature for RTA, which results in substantial improvements in both the optical, structural and noise properties for the material,,whereas annealing at 1000/spl deg/C is found to result in significant material degradation.

Characterizations of GaN films grown with indium surfactant by RF-plasma assisted molecular beam epitaxy

Abstract A small indium flux was used as a surfactant during the growth of gallium nitride by rf-plasma assisted molecular beam epitaxy. The effects of the In surfactant on the optical and structural properties of undoped GaN were studied by photoluminescence (PL), X-ray diffraction, atomic force microscopy (AFM), and Rutherford backscattering spectrometry (RBS). PL studies show that the use of In surfactant is beneficial to the reduction of deep-level defects. 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 surface roughness for films grown with and without In surfactant are 5.86 and 6.99 nm respectively indicating significant improvement in surface morphology. The improved surface morphology is attributed to the enhanced 2-dimensional growth promoted by the application of In surfactant. RBS studies show that the χ min values along [0xa00xa00xa01] direction are 2.06% and 2.16% for the samples grown with and without In surfactant respectively. Off-normal ion channeling studies were performed to further investigate the effects of In surfactant on the crystallinity. It is found that the number density of stacking faults is smaller for the sample grown with In surfactant compared to the one grown without In surfactant. However, defect analysis shows that dislocations are found in the sample grown with In surfactant in contrary to the one grown without In surfactant. We speculate that there is a thickness limit of GaN grown with In surfactant and the thickness of our samples exceed this limit, leading to the presence of dislocation.

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 690u200a°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.

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.

Degradation of low-frequency noise in AlGaN/GaN HEMTS due to hot-electron stressing

We report on the degradation of low-frequency excess noise in recessed gate AlGaN/GaN HEMTs due to hot-electron stressing. The I-V characteristics and the low-frequency noise power spectral densities, SV(f), of the open circuit voltage fluctuations across the drain source terminal were characterized with the stress time. Based on these results, we observed that the overall low-frequency noise degradation process can be identified to occur in two distinct phases. In the first phase, devices initially show fluctuations in the noise properties around a relatively constant average value. Detailed characterizations of the gate-source bias, VGS, dependence of SV(f) at cryogenic temperatures indicate signature-patterns in the variations of SV(f) as a function of VGS. This is shown to arise from the modulation of the percolation paths of the carriers in the two-dimensional electron gas (2DEG). The onset of the second phase of degradation arises from the irreversible generation of interface states at the AlGaN/GaN hetero-interface.

Low‐frequency Noise Characterization of Hot‐electron Degradation in GaN‐based HEMTs

Hot‐electron degradation in MOCVD‐grown GaN‐based HEMTs, with different gate recess depths, was monitored by flicker noise measurement. Drastic changes were observed in the flicker noise power spectral density, SV(f) and I‐V characteristics when the devices were subjected to voltage stress at VD=10V for a short stress time of tS=1 minute. The degradations can be partially reverted by annealing the devices at 100°C for 20 minutes. Further stressing of the devices were performed with VG=−1.5V and VD=10V, which results in irreversible degradation in the SV(f). Detailed analyses of the data suggest that the stressing of the devices, with short tS, results in the generation of H+ at the AlGaN/GaN interface leading to the observed increase in ID, and SV(f). This can be easily annealed as the H+ has relatively low formation energy. The experimental results suggest that the H+ accumulated at the AlGaN/GaN interface may result in a network of percolation paths formed by the depression of the surface potential at t...

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