Intermediate-temperature Buffer Layer Research Articles

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10 7 cm −2) and a higher edge-dislocation density (8.157×10 9 cm −2) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.

Characteristics of MBE-Grown GaN Detectors on Double Buffer Layers Under High-Power Ultraviolet Optical Irradiation

In this paper, present experimental investigations on the radiation hardness of GaN-based Schottky diode photodetectors. High-power ultraviolet (UV) radiation obtained from a Xenon lamp is used as the light source for the optical-stressing experiment. Two types of devices are being investigated. One has a double-buffer-layer structure that consists of a conventional high-temperature AlN buffer layer and an intermediate temperature buffer layer (type I), and the control device was fabricated with only a conventional AlN buffer layer (type II). Detailed current-voltage, capacitance-voltage, flicker noise, and responsivity measurements performed on the detectors show that the degradations of the devices arose from the defects present at the Schottky junctions due to the exposure of the devices to the high-power UV radiation. Both types of devices exhibit degradation in their optoelectronic properties. However, type-I devices, in general, exhibit gradual and slow degradation, whereas type-II devices exhibit catastrophic breakdowns in the device characteristics. The experimental data indicate significant improvement in the radiation hardness for type-I devices

Influence of GaN polarity and intermediate-temperature buffer layers on strain relaxation and defects

The dependence of strain relaxation and consequent generation defects on Gallium nitride (GaN) polarity and intermediate-temperature buffer layer (ITBL) has been observed by Raman scattering, photoluminescence (PL) and monoenergetic positron beam techniques. Raman scattering studies have indicated that tensile stress prefers and compress stress is present in N-polar and Ga-polar films, respectively. Furthermore, ITBL relaxes strains in Ga-polar GaN films more effectively than in N-polar GaN films. PL results show that peak shifts due to the effect of polarity and ITBL. Depth resolved defect-sensitive S parameter measurements, using monoenergetic positron beam, exhibit larger S parameter and shorter positron effective diffusion length in N-polar GaN samples than those in Ga-polar films. When ITBL is added, S parameter decreases and effective diffusion length increases in both groups. Hall mobility and carrier concentration measurement manifest a reduction of dislocation line and electrons trap centers such as V Ga or Ga vacancy clusters.

Deposition of AlGaN films on (111) Si substrates and optimization of GaN growth on Si using intermediate-temperature AlGaN buffer layers

AlxGa1−xN films having various Al-contents were grown on (111) Si substrates over a temperature range of 800∼1000 °C. It was found that crack free AlxGa1−xN films were achieved when the films were grown at 800 °C. High temperature (HT) GaN films were also deposited on (111) Si substrates using 800 °C grown AlxGa1−xN buffer layers with different thickness and composition combinations. The best HT GaN film was achieved on (111) Si substrate by process optimization with an 800 °C grown 180 nm-thick Al0.58Ga0.42N buffer layer. Room temperature photoluminescence (PL) spectrum of the HT GaN film shows a strong near band edge emission having a linewidth of 100 meV and a quenched yellow luminescence. It is believed that the use of intermediate temperature AlxGa1−xN buffer layer is beneficial to accommodate the misfit strain between HT GaN film and (111) Si substrate.

Low‐frequency noise characterizations on Ni/GaN Schottky diodes deposited on intermediate‐temperature buffer layers

Silicon dioxide passivated back-to-back Schottky diodes were fabricated by depositing Ni on rf plasma-assisted molecular beam epitaxy-grown GaN utilizing a unique double buffer layer structure. In this study, both low-frequency noise and deep-level transient Fourier spectroscopy (DLTFS) measurements were conducted to characterize the effects of intermediate-temperature buffer layers (ITBLs) on the hot-electron hardness of GaN Schottky diodes. Device A was fabricated with a double buffer layer consisting of a thin AlN high-temperature buffer layer (HTBL) and a GaN ITBL. Device B consists of a single AlN HTBL. Low-frequency noise results measured from the as-deposited devices show a significant reduction in the noise level, over an order of magnitude, for device A indicative of the substantial reduction in the trap density in the GaN thin films deposited on ITBLs. Hot-electron hardness of the devices was examined through the application of high voltage stress. The increase in the low-frequency noise for device A after voltage stressing is much smaller and no detectable deep-level is observed by both low-frequency noise and DLTFS techniques. However, both characterization techniques indicate a generation of a deep-level at 780 meV below the conduction band edge for device B. Based on detailed optical and electrical characterizations of the samples, the improved device properties for device A are attributed to the relaxation of residue strain in the epilayer during growth due to the utilization of ITBL [W. K. Fong et al., J. Cryst. Growth 233, 431 (2001)]. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal–semiconductor–metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an A1N high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A1N HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process.

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

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 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.

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.

Characterization of deep levels in Pt-GaN Schottky diodes deposited on intermediate-temperature buffer layers

Gallium nitride (GaN)-based Schottky junctions were fabricated by RF-plasma-assisted molecular beam epitaxy (MBE). The GaN epitaxial layers were deposited on novel double buffer layers that consist of a conventional low-temperature buffer layer (LTBL) grown at 500/spl deg/C and an intermediate-temperature buffer layer (ITBL) deposited at 690/spl deg/C. Low-frequency excess noise and deep level transient Fourier spectroscopy (DLTFS) were measured from the devices. The results demonstrate a significant reduction in the density of deep levels in the devices fabricated with the GaN films grown with an ITBL. Compared to the control sample, which was grown with just a conventional LTBL, a three-order-of-magnitude reduction in the deep levels 0.4 eV below the conduction band minimum (E/sub c/) is observed in the bulk of the thin films using DLTFS measurements.

STUDY OF LOW-FREQUENCY EXCESS NOISE IN Ga-POLARITY GaN EPITAXIAL LAYERS

We report detailed investigations of low-frequency excess noise in Ga -polarity GaN thin films deposited by RF-plasma assisted molecular beam epitaxy. The noise properties of the GaN thin films deposited with and without the intermediate-temperature buffer layers (ITBL) are studied in detailed to examine the effects of the ITBL on the noise. Substantial reduction in the flicker noise levels are observed for samples grown on ITBLs with a Hooge parameter of 3×10-4, which is believed to be the lowest, to date, reported for GaN material. At low-temperatures, Lorentzian bumps originating from the generation-recombination processes are observed. Detailed studies of the temperature dependencies of the voltage noise power spectra have led to the formulation of a model for the observed low-frequency fluctuations. The model stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers. The process results in the correlated fluctuations in the carrier number and the Coulombic scattering rate. Quantitative computation shows that number fluctuation dominates in our samples. Numerical evaluation of the deep-levels indicates substantial reduction in the trap density for the Ga -polarity GaN films.

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°C, and a conventional low-temperature buffer layer deposited at 500°C. 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 μm 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.

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

High-mobility GaN thin films were grown by RF plasma-assisted molecular beam epitaxy on (0 0 0 1) 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 460 cm 2 V −1 s −1 obtained from the sample grown on a 800 nm 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 800 nm ITBL compared to the control sample which has the same total thickness but with only the low-temperature buffer layer.

Characterization of high-quality MBE-grown GaN films on intermediate-temperature buffer layers

High-quality GaN thin films are grown by rf-plasma assisted molecular beam epitaxy. The quality of the GaN epitaxial layer is significantly improved by using an intermediate-temperature GaN buffer layer (ITBL) in addition to a conventional 20-nm-thick low-temperature buffer layer. The GaN epitaxial layers demonstrate systematic improvements in the electron mobility increasing from 82 cm2 V-1 s-1, for films grown with just the low-temperature buffer layer, to about 380 cm2 V-1 s-1 for films grown with an ITBL of thickness 800 nm. The photoluminescence also indicates systematic improvements in the intensity and the full-width-half-maximum with the use of ITBL. Photoreflectance spectra are measured from the GaN films. Detailed analyses of the excitonic transition energy demonstrate that the residual strain relaxes rapidly with the use of ITBL, which is attributed to the observed improvements in the mobility and the PL spectra.

Characterization Of G-R Noise In GaN Films Grown By RF-MBE On Intermediate-Temperature Buffer Layers

AbstractWe report detailed investigations of generation-recombination (G-R) noise in GaN films grown by rf-plasma assisted molecular beam epitaxy on intermediate-temperature buffer layer (ITBL) in addition to conventional low-temperature buffer layer. To characterize the film quality affected by the use of ITBL, low-frequency noise measurements were performed. The voltage noise power spectra show a strong dependence on the thickness of the ITBL. A model has been presented to explain the observed G-R noise, which stipulates that the phenomenon arises from the thermally activated trapping and detrapping of carriers by traps. The process leads to the correlated fluctuations in both the carrier number and the coulombic scattering rate. Detailed numerical evaluation shows that number fluctuation dominates in our samples. The calculated trap densities show that the use of ITBL can effectively reduce defect density by over an order of magnitude.

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.

Growth Of High Quality GaN Thin Films By MBE On Intermediate-temperature Buffer Layers

We report the growth of high quality GaN epitaxial layers by rf-plasma MBE. The unique feature of our growth process is that the GaN epitaxial layers are grown on top of a double layer that consists of an intermediate-temperature buffer layer (ITBL), which is grown at 690°C and a conventional low-temperature buffer layer deposited at 500°C. It is observed that the electron mobility increases steadily with the thickness of the ITBL, which peaks at 377 cm2V−1s−1 for an ITBL thickness of 800 nm. The PL also demonstrated systematic improvements with the thickness of the ITBL. Our analyses of the mobility and the photoluminescence characteristics demonstrate 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 improvement in the optoelectronic properties of the films. A maximum electron mobility of 430 cm2V−1s−1 can be obtained using this technique and further optimizing the growth conditions for the low-temperature buffer layer.