Low-temperature Buffer Research Articles

The influence of V/III ratio in the initial growth stage on the properties of GaN epilayer deposited on low temperature AlN buffer layer

The V/III ratio in the initial growth stage of metalorganic chemical vapor deposition has an important influence on the quality of a GaN epilayer grown on a low-temperature AlN buffer layer and c-plane sapphire substrate. A weaker yellow luminescence, a narrower half-width of the X-ray diffraction peak, and a higher electron mobility result when a lower V/III ratio is taken. The intensity of in situ optical reflectivity measurements indicates that the film surface is rougher at the beginning of GaN growth, and a longer time is needed for the islands to coalesce and for a quasi-two dimensional mode growth to start. A comparison of front- and back-illuminated photoluminescence spectra confirms that many threading dislocations are bent during the initial stage, leading to a better structural quality of the GaN layer.

The effects of LT AlN buffer thickness on the optical properties of AlGaN grown by MOCVD and Al composition inhomogeneity analysis

We have investigated the growth of AlGaN epilayers on a sapphire substrate by metalorganic chemical vapour deposition using various low-temperature (LT) AlN buffer thicknesses. Combined scanning electron microscopy and cathodoluminescence investigations reveal the correlation between the surface morphology and optical properties of AlGaN films in a microscopic scale. It is found that the suitable thickness of the LT AlN buffer for high quality AlGaN growth is around 20 nm. The Al compositional inhomogeneity of the AlGaN epilayer is attributed to the low lateral mobility of Al adatoms on the growing surface.

Post-annealing Induced Formation of ZnO Nanowires on the ZnO Films in the N2O Ambient

AbstractZnO films were grown on (0001) sapphire substrates by atomic layer deposition (ALD) using diethylzinc (DeZn) and nitrous oxide (N2O) in an inductively heated reactor operated at atmospheric pressure. Low-temperature (LT) ZnO buffer layers having various thicknesses were deposited at 400¢J followed by subsequent growth of ZnO films at 600¢J. Some of the ZnO films were then post-annealed at 1000¢J in the N2O flow. Under certain growth conditions, ZnO nanowires were formed on the post-annealed ZnO samples. Room temperature (RT) photoluminescence (PL) spectra of the ZnO nanowires show strong ultraviolet (UV) near band edge emissions at 3.27 eV with a typical full width at half-maximum ( FWHM ) of ~130 meV and quenched defect luminescence at 2.8 eV. 10 K PL spectra of the post-annealed ZnO all exhibit sharp excitonic emissions with the dominant emission being located at 3.36 eV having a FWHM of 4.6 meV.

Effect of low-temperature GaN buffer layer on the crystalline quality of subsequent GaN layers grown by MOVPE

We have investigated the correlation between the formation of single-crystal GaN islands in low-temperature (LT)-GaN buffer layer and the crystalline quality of subsequently grown high-temperature (HT)-GaN layer. X-ray diffraction (XRD) analysis showed that the LT-GaN layer was partially crystallized even without an annealing process. The annealing process transformed the nanometer-sized LT-GaN grains into the larger-sized islands whose size and density could be controlled by changing the growth temperature of LT-GaN layer. Atomic force microscopy (AFM) and XRD analyses indicated that the crystalline quality of HT-GaN layer depended strongly on the size and density of the islands in the LT-GaN layer. The full-width at half-maximum (FWHM) values of XRD rocking curves of GaN(0 0 0 2) and ( 1 0 1 ¯ 2 ) reflections were 198 and 232 arcsec, respectively, when the LT-GaN layer was grown at 475 °C.

Sb as surfactant at plastic relaxation of GeSi/Si(0 0 1) films grown by molecular-beam epitaxy: Reduction of surface roughness value

Plastically relaxed GeSi films with a fraction of Ge ranging from 0.19 to 0.29 are grown on Si(0 0 1) substrates with the use of a low-temperature (350 °C) buffer layer of Si. Their root mean square (RMS) surface roughness is in the 1.7–2.7 nm range. Addition of Sb as a surfactant smoothing the surface of the stressed film during its growth is shown to reduce the surface roughness of the plastically relaxed heterostructure. The RMS roughness lower than 1 nm is reached for a film with a 0.29 fraction of Ge and a threading dislocation density close to 10 6 cm −2.

Structure and properties of ZnO films grown on Si substrates with low temperature buffer layers

Zinc oxide (ZnO) thin films were grown on Si (1 0 0) substrates by pulsed laser deposition (PLD) using two-step epitaxial growth method. Low temperature buffer layer (LTBL) was initially deposited in order to obtain high quality ZnO thin film; the as-deposited films were then annealed in air at 700 °C. The effects of LTBL and annealing treatment on the structural and luminescent properties of ZnO thin film were investigated. It was found that tensile strain was remarkably relaxed by employing LTBL and the band-gap redshifted, correspondingly. The shift value was larger than that calculated from band-gap theories. After annealing treatment, it was found that the annealing temperature with 700 °C has little influence on strains of ZnO films with LTBLs other than directly deposited film in our experiments. Interestingly, the different behaviors in terms of the shift of ultraviolet (UV) emission after annealing between films with and without buffer were observed, and a tentative explanation was given in this paper.

Growth of Ge Thick Layers on Si(001) Substrates Using Reduced Pressure Chemical Vapor Deposition

Growth of Ge thick layers with and without a low temperature Ge buffer has been investigated using reduced pressure chemical vapor deposition with emphasis on the evolution of surface morphology and its roughness. Growth at 400 °C exhibited a 200 s incubation period for which about 1 equivalent monolayer of Ge was deposited with a surface roughness of around 0.1 nm followed by an abrupt increase due to an island growth. Higher growth temperature resulted in shorter or no observed incubation period and led to rougher surface due to larger size of islands. Ge layers grown at 600 °C on at low temperature Ge buffer first grown at 400 °C exhibited around 1 nm surface roughness independent of the layer thickness, which is much lower than those at 400–600 °C without the low temperature buffer. The Ge thick layer with the low temperature Ge buffer had thinner defective interface of Ge/Si than that without the buffer.

Growth mode and structural characterization of GaSb on Si (001) substrate: A transmission electron microscopy study

Growth mode and structural properties of GaSb layers grown on silicon substrate by molecular beam epitaxy method are investigated by transmission electron microscopy. It is found that the GaSb grows to three-dimensional islands and grains are tilted to reduce a lattice mismatch through twin boundaries when they are directly grown on Si substrate. A low-temperature (LT) AlSb buffer plays a key role in transferring the growth mode from a three-dimensional island to a layer-by-layer structure. When the LT AlSb layer is used as a buffer, 90° misfit dislocations, with the Burgers vector b of 1∕2a⟨110⟩, are observed on the interface.

Development of pit‐defect free smooth a‐plane GaN surfaces on r‐plane sapphire using metalorganic chemical vapor deposition: A growth mechanism study

AbstractWe report on development of smooth a‐plane GaN without the use of lateral epitaxial overgrowth. Effects of GaN buffer growth conditions, namely V/III ratio and temperature, on the quality of high temperature a‐plane GaN layers on r‐plane sapphire substrates have been studied. Optimization of the buffer growth conditions resulted in surfaces of 2 µm thick overgrown a‐plane GaN at high temperature with less pit defects, the presence of which is reported as a common occurrence for a‐plane GaN grown on r‐plane sapphire. The surfaces were smooth with RMS roughness of less than 2 nm for a 5 µm x 5 µm AFM scanning area. We will present the effect of V/III ratio in low temperature buffer growth and qualitative growth mechanism model that has been constructed based on the structural and morphological study of the layers grown at various stages. Layers were characterized using AFM, SEM, and HRXRD. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth of GaN on sapphire substrates using novel buffer layers of ECR-plasma-sputtered Al 2O 3/graded-AlON/AlN/Al 2O 3

We proposed and demonstrated GaN growth on sapphire substrates by metalorganic vapor phase epitaxy using novel buffer layers of Al 2O 3/graded-AlON/AlN/Al 2O 3. The buffer layers were deposited by electron cyclotron resonance plasma sputtering at room temperature. The total thickness of the buffer layers was around 20 nm. The cross-sectional bright-field transmission electron microscope observation showed that the dislocations were bent at the initial stage of the growth, indicating the enhancement of lateral growth. Typical dislocation density in the GaN layer was 6.5×10 8 cm −2. We obtained high electron mobility of 640 cm 2/Vs at the electron concentration of 2.0×10 17 cm −3 for Si-doped GaN. We also obtained high electron mobility of 1760 cm 2/Vs at a sheet electron concentration of 8.3×10 12 cm −2 for an AlGaN/GaN heterostructure. These results indicate that the quality of the epitaxial layer grown on sapphire substrates using the electron cyclotron resonance plasma sputtered buffer is comparable or superior to that of GaN layers using the conventional low-temperature buffer layer.

Growth of GaN films on GaAs substrates in an As-free environment

We investigated growth of GaN films on [001] GaAs substrates by plasma-assisted molecular beam epitaxy in an As-free chamber. The crystalline quality and the surface morphology of the films were studied with x-ray diffraction and transmission electron, scanning electron, and atomic force microscopes. We determined that direct GaN deposition on the thermally desorbed substrate resulted in the growth of a polycrystalline film containing misoriented grains and inclusions. We achieved a significant improvement of the film quality by adopting a procedure consisting of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth.

Low temperature step-graded InAlAs/GaAs metamorphic buffer layers grown by molecular beam epitaxy

Low-temperature step-graded InAlAs metamorphic buffer layers on GaAs substrate grown by molecular beam epitaxy were investigated. The strain relaxation and the composition of the top InAlAs layer were determined by high-resolution triple-axis x-ray diffraction measurements, which show that the top InAlAs layer is nearly fully relaxed. Surface morphology was observed by reflection high-energy electron diffraction pattern and atomic force microscopy. Under a selected range of growth parameters, the root mean square surface roughness of the sample grown at 380 °C is 0.802 nm, which has the smallest value compared with those of other samples.Furthmore, The ω-2θ and ω scans of the triple-axis x-ray diffraction, and photoluminescence show the sample grown at 380 °C has better crystalline quality. With decreasing As overpressure at this growth temperature, crystalline quality became poor and could not maintain two dimensional growth with increasing overpressure. The carrier concentrations and Hall mobilities of the InAlAs/ InGaAs/GaAs MM-HEMT structure on low-temperature step-graded InAlAs metamorphic buffer layers grown in optimized conditions are high enough to make devices.

Pulsed laser deposited epitaxial Sm–Co thin films with uniaxial magnetic texture

Sm–Co thin films have been grown epitaxially with pulsed laser deposition on Cr buffered MgO(110) substrates. The buffer microstructure plays a significant role in controlling the growth and hence the magnetic properties of the Sm–Co film. High deposition temperatures of the Cr buffer result in a rough and discontinuous microstructure, thereby resulting in an x-ray amorphous or nanocrystalline Sm–Co layer, as has been confirmed by transmission electron microscopy studies. By lowering the buffer deposition temperature from 700to300°C, the roughness decreases from 5to0.6nm. Sm–Co films grow epitaxially on these low temperature buffers with the epitaxial relation MgO(110)[001]∥Cr(211)011∥SmCo(100)[001] which implies a single in-plane orientation of the c axis along the MgO[001] direction. The strong in-plane crystallographic texture seen in the pole figure measurements leads to a very pronounced magnetic texture, quantified by a remanence ratio JrMgO[110]∕JrMgO[001] as low as 0.08 and a high remanence of 0.84T along with a high coercivity of 3T when measured along the easy axis.

ZnO growth on Si with low-temperature CdO and ZnO buffer layers by molecular-beam epitaxy

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.

Effect of nitridation on crystallinity of GaN grown on GaAs by MBE

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. Films are evaluated by X-ray diffraction and the dependence of crystalline quality on the nitridation temperature is studied. It is demonstrated that nitridation has to be performed at low-temperature to achieve c-oriented α-GaN. Higher nitridation temperature promotes formation of mis-oriented domains and β-GaN inclusions

Influence of gas atmosphere during growth interruption in the deposition of ZnO films by magnetron sputtering

In the growth of ZnO films using radio-frequency magnetron sputtering, the effects of the injected ambient gas during the temperature elevation and the in-situ thermal annealing after the growth of the low-temperature buffer layers on the optical and structural quality were investigated. The introduction of inert gases such as argon and nitrogen as the ambient gases during the thermal treatments of the buffer layers led simultaneously to the reduction of full-width at half maximum (FWHM) value in the X-ray diffraction (XRD) rocking curve and the improved emission properties, compared to the samples using oxygen gas. Also, the use of inert gases caused a decreased tilting of the grains and the peak shift of the deep-level emission from the yellow region to the green region.

Optimization of [formula omitted] a-plane GaN growth by MOCVD on [formula omitted] r-plane sapphire

A-plane GaN ( 1 1 2 ¯ 0 ) epilayers have been grown on r-plane ( 1 1 ¯ 0 2 ) sapphire by MOCVD, and investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and atomic force microscopy (AFM). This particular orientation is non-polar, as opposed to the c-direction, and avoids polarization charge and the associated screening charge, and the consequent band bending. Our results showed that low pressure, low ammonia flow rate (namely low V/III ratio), high-temperature conditions lead to fully coalesced and relatively high crystalline-quality a-plane GaN films. Both low-temperature GaN buffer and high-temperature AlN buffer were used for a-plane GaN growth on r-plane sapphire, and produced similar crystalline quality and surface morphology. Surface morphological evolution during early stages of a-plane GaN growth revealed behavior different from that of c-plane GaN growth. The possible reasons for striped features and surface undulations of a-plane GaN are discussed.

Structural and optical properties of GaN films grown on GaAs substrates by molecular beam epitaxy

GaN films are grown on [0 0 1] GaAs substrates by plasma-assisted molecular beam epitaxy using a three-step process that consists of a substrate nitridation, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. X-ray diffraction and transmission electron microscopy indicate that this method promotes prismatic growth of c-oriented α-GaN. Photoluminescence studies show that the emission from cubic β-GaN inclusions dominates the spectrum.

Time Evolution of a High-temperature GaN Epilayer Grown on a Low-temperature GaN Buffer Layer using a Low-pressure MOCVD

In this paper, the time evolution of undoped GaN epilayers on a low-temperature GaN buffer layer grown on c-plane sapphire at a low pressure of 300 Torr was studied via a two-step growth condition in a horizontal MOCVD reactor. As a function of the growth time at a high-temperature, the surface morphology, structural quality, and optical and electrical properties were investigated using atomic force microscopy, high-resolution x-ray diffraction, photoluminescence, and Hall effect measurement, respectively. The root-mean-square roughness showed a drastic decrease after a certain period of surface roughening probably due to the initial island growth. The surface morphology also showed the island coalescence and the subsequent suppression of three-dimensional island nucleation. The structural quality of the GaN epilayer was improved with increasing growth time considering the symmetrical (002) and asymmetrical (102) rocking curves. The variations of room-temperature photoluminescence, background carrier concentration, and Hall mobility were measured and discussed.

Growth of InN films by RF plasma-assisted MBE and cluster beam epitaxy

This paper describes the growth, structure, transport and optical properties of InN films grown by MBE using either nitrogen radicals (N 2 *) produced by a RF plasma source or clusters containing on the average 2000 nitrogen molecules (N 2) 2000. The InN films were grown at temperatures between 300 and 600 °C on (0 0 0 1) sapphire substrates using either an InN buffer or a GaN template. It was found that the conversion of the surface of the sapphire from Al 2O 3 to AlN, by exposing it to active nitrogen, is essential for the growth of single-crystalline InN films. Thick films (2–3 μm), produced by the plasma source, tend to delaminate from the substrate presumably due to extreme compressive stresses. On the other hand, films adhere better if nitridation of the sapphire substrate and the low-temperature InN buffer are grown by the cluster source. All films are auto-doped n-type with carrier concentration higher than 3×10 18 cm −3 and best room temperature mobility 1130 cm 2/V s. The energy gap of InN was determined to be 0.77 eV. The plasma frequency was measured by infrared reflectivity and the data were used to determine the electron effective mass (0.09 m 0). We found that the measured optical gap and the electron effective mass are in qualitative agreement with the predictions of the k· p method for direct semiconductors.