Surface Morphology Of GaN Research Articles

Impacts of off-angle and off-direction on surface morphology of GaN grown by metalorganic vapor phase epitaxy on (0001) GaN substrate

Impacts of off-angle and off-direction on surface morphology of GaN grown by metalorganic vapor phase epitaxy on (0001) GaN substrate

Impact of etching process on Al2O3/GaN interface for MOSc-HEMT devices combining ToF-SIMS, HAXPES and AFM

In this work we present the effect of inductively coupled plasma reactive ion etching (ICP-RIE) combined with atomic layer etching (ALE) on the Al2O3/GaN interface for MOSc-HEMT devices. Time of flight secondary ion mass spectrometry (ToF-SIMS) and hard x-ray photoelectron spectroscopy (HAXPES) highlight an increase of the N/Ga ratio near the interface after etching. ToF-SIMS profiles also show the presence of impurities (H, C, B) at this interface. Atomic force microscopy (AFM) also illustrates a change of the GaN surface morphology for the etched sample.

MOVPE studies of zincblende GaN on 3C-SiC/Si(0 0 1)

Cubic zincblende GaN films were grown by metalorganic vapour-phase epitaxy on 3C-SiC/Si (001) templates and characterized using Nomarski optical microscopy, atomic force microscopy, X-ray diffraction, and transmission-electron microscopy. In particular, structural properties were investigated of films where the growth temperature of a GaN epilayer varied in the range of 830 °C to 910 °C and the gas-phase V/III-ratio varied from 15 to 1200 at a constant reactor pressure of 300 Torr. It was observed that with increasing epi temperature at a constant V/III-ratio of 76, the film surface consisted of micrometer-sized elongated features aligned along [1–10] up to a temperature of 880 °C. The zincblende phase purity of such samples was generally high with a wurtzite fraction of less than 1%. When grown above 880 °C the GaN surface morphology degraded and the zincblende phase purity reduced as a result of inclusions with the wurtzite phase. A progressive narrowing of the 002 reflection with increasing epi growth temperature suggested an improvement of the film mosaicity. With increasing V/III-ratio at a constant growth temperature of 880 °C, the film surface formed elongated features aligned along [1–10] at V/III values between 38 and 300 but the morphology became granular at both lower and higher V/III values. The zincblende phase purity is high at V/III values below 300. A slight broadening of the 002 X-ray diffraction reflection with increasing V/III-ratio indicated a small degradation of mosaicity. Scanning electron diffraction analyses of cross-sectional transmission-electron micrographs taken of a selection of samples illustrated the spatial distribution, quantity and structure of wurtzite inclusions within the zincblende GaN matrix. Within the limits of this study, the optimum epilayer growth conditions at a constant pressure of 300 Torr were identified to be at a temperature around 860 °C to 880 °C and a V/III-ratio in the range of 23 to 76, resulting in relatively smooth, zincblende GaN films without significant wurtzite contamination.

Effect of H2 addition on growth rate and surface morphology of GaN(0001) grown by halide-vapor-phase epitaxy using GaCl3

We investigated the effect of H2 addition in halide-vapor-phase epitaxy of GaN on Ga-polar GaN(0001) using an external GaCl3 supply method. To overcome the problem of the very low growth rate on GaN(0001) using GaCl3, we intentionally added H2 to convert GaCl3 to GaCl in the reactor. Using this approach, we successfully increased the growth rate, and also improved the surface morphology of the grown layer.

Hydrogen in-situ etching of GaN surface to reduce non-radiative recombination centers in 510-nm GaInN/GaN quantum-wells

This study aims to improve the internal quantum efficiency (IQE) of GaInN/GaN 510-nm green quantum-wells (QWs) by reducing the surface defects (SDs) at the n-type GaN surface using in-situ etching. For this purpose, we examined hydrogen (H2) and a mixture of H2 and ammonia (NH3) as the etching gases for surface etching of n-type GaN at various reactor temperatures and pressures. The results show that the in-situ etching performed effectively for the SD reduction, while the surface morphology of n-type GaN slightly deteriorated. Next, we examined the effect of in-situ etching on the IQE of QWs via the growth of 510-nm GaInN/GaN QWs on the surface-etched n-type GaN. The results show that the IQE at 300 K improved by a factor of 2.1 through the H2 + NH3 etching and 3.0 through the H2 etching, which is attributable to the fact that the SDs were more sufficiently removed using H2 as an etching gas, despite the slightly undesired shape of QW because they were grown on the roughed n-type GaN surface. Meanwhile, some SDs remained when H2 + NH3 was used as an etching gas.

Modification of the surface energy and morphology of GaN monolayers on the AlN surface in an ammonia flow

Based on theoretical predictions, it has been demonstrated that it is possible to purposefully change the elemental composition and position of the adsorbed particles on the GaN surface, thereby controlling the surface energy and morphology of GaN. Comparison of experimental data obtained by reflected high-energy electron diffraction and the calculated concentration of ammonia fragments on the GaN surface, and surface energy showed that the movement of adsorbed ammonia fragments into strongly bound states is an effective mechanism to control the GaN morphology. The minimum value of equivalent NH3 beam pressure at different temperatures to prevent the conversion of the two-dimensional (2D) GaN layer to three-dimensional (3D) islands has been established. It was shown that the boundary between the 2D and 3D states on the surface is defined by the elemental composition of adsorbed particles on the surface and the temperature dependence of the surface energy of the facets of islands.

H3PO4-based wet chemical etching for recovery of dry-etched GaN surfaces

The impact of several wet etchants commonly encountered in the microelectronic industry on the surface chemistry of GaN on silicon was explored. In order to get closer to fully recessed gate HEMT fabrication processes, we investigated different kinds of GaN surfaces. This study was conducted on as-grown GaN and dry etched GaN, with etching consisting of inductive coupled plasma reactive ion etching (ICP-RIE), followed by atomic layer etching (ALE) and O2 plasma stripping. The impact of each wet treatment was evaluated by parallel Angle Resolved X-ray Photoelectron Spectroscopy (pAR-XPS). Treatment with phosphoric acid (H3PO4) showed a significant modification of the surface and further studies were performed using this treatment. The impact of H3PO4 on GaN surface chemistry and morphology was assessed by pAR-XPS and atomic force microscopy (AFM) respectively. A delayed effect was observed for dry etched samples compared to as-grown samples, with a successful recovery of the surface after 60 min of treatment. We also proposed a mechanism explaining the progressive formation on steps on the surface over time. Further research was performed on dry etched samples without ALE which also modified the delay time of the H3PO4 treatment, but still enabled a recovery of the surface morphology. In contrast to other studies, we showed that, with the appropriate choice of parameters for the H3PO4 treatment, it was possible to successfully recover the GaN surface after dry etching without significantly opening dislocation holes. This is therefore a promising treatment to be used during GaN HEMT processing to recover good quality surfaces after etching.

Effect of annealing time and pressure on electrical activation and surface morphology of Mg-implanted GaN annealed at 1300 °C in ultra-high-pressure nitrogen ambient

We performed an isothermal annealing study on Mg-implanted GaN at 1300 °C in an ultra-high-pressure (1 GPa) nitrogen ambient. Annealing for more than 30 min resulted in a high acceptor activation ratio and a low compensation ratio that were comparable to those obtained with annealing at 1400 °C for 5 min. We also performed annealing at 1300 °C in a reduced nitrogen pressure of 300 MPa which makes us possible to expand the inner diameter of annealing equipment in the future. High electrical activation, similar to one obtained by annealing at 1 GPa, was successfully obtained.

Cascaded Ni hard mask to create chlorine-based ICP dry etched deep mesas for high-power devices

A highly selective cascaded Ni hard mask without stress and film damage issues suitable for the formation of deep vertical mesas for high-power devices is presented. A Cl2/BCl3/Ar inductively coupled plasma dry etch was used to investigate the effect of photoresist, and patterned Ni hard masks applied via four different methods on the etch rate of GaN, and mask selectivity. Strain in the e-beam only deposited Ni films produced low yield due to poor adhesion. Electroplated Ni showed rough mask morphology. Sputtered Ni left damage/chemically reacted layers on the GaN under the mask. A cascaded 50 nm e-beam evaporated Ni followed by 2 µm of sputtered Ni mask method improved the yield to 100%, achieved high selectivity and retained an undamaged GaN surface morphology. The cascaded Ni hard mask successfully achieved GaN mesas up to 13 µm. The dry etching rates of the photoresist, GaN and Ni masks were 250, 275 and 13 nm min−1, respectively giving an etch selectivity of the GaN/Ni hard mask of ∼20, much higher than the etch selectivity of 1.1 for the photoresist. Furthermore, yield of the cascaded Ni hard mask was 100% making it suitable for commercial high-power electronics.

Effects of the pretreatment of Si substrate before the pre-deposition of Al on GaN-on-Si

In this work, we report a method that the Si substrate is pre-treated in the atmosphere of high temperature AlN before the depositon of Al to improve the quality of GaN-on-Si, which is studied by few people. Compared to the untreated sample, the pre-treated one exhibits smoother surface morphology with a root mean square roughness (RMS) value of 0.580 nm and the full width at half maximum (FWHM) of GaN (102) plane also decreases from 1484 arcsec to 1303 arcsec. Moreover, we give an explanation for the extra shoulder peaks in the photoluminescence (PL) spectrum. This enhancement of quality and surface morphology of GaN can be attributed to the improvement of surface flatness of Si substrate, which contributes to the conversion of AlN to two-dimensional growth mode, leading to a smoother surface of GaN. And the reason for the improvement of Si substrate is also investigated in this work. • GaN with higher quality and lower dislocations was obtained on the Si(111) substrate with pretreatment. • Surface morphology of AlN grown on the Si substrate with pretreatment is improved greatly compared to the untreated one. • The cause of the shoulder peak in the PL spectrum of GaN is explained. • Reason for the improved surface morphology of Si with pretreatment is explained through the results of XPS spectra.

Effect of ammonia/gallium ratio and growth temperature towards the surface morphology of semi-polar GaN grown on m-plane sapphire via MOCVD

A single-crystalline semi-polar gallium nitride (11-22) was grown on m-plane (10-10) sapphire substrate by metal organic chemical vapor deposition. Three-step approach was introduced to investigate the grain size evolution for semi-polar (11-22) GaN. Such approach was achieved due to the optimized gallium to ammonia ratio and temperature variations, which led to high quality (11-22) oriented gallium nitride epilayers. The full width at half maximum values along (-1-123) and (1-100) planes for the overgrowth temperature of 1080°C were found to be as low as 0.37° and 0.49°, respectively. This was an indication of the enhanced coalescence and reduction in root mean square roughness as seen by atomic force microscopy. Surface analysis via atomic force microscopy indicated the orientation towards semi-polar plane. Field emission scanning electron microscopy analysis further indicates that higher temperature of 1080°C during the deposition of the overgrowth promoted closely packed surface coalescence. Room temperature Raman revealed that the overgrowth temperature of 1080°C portrayed compressive strain free as compared to other overgrowth temperature. Based on these results, the promising overgrowth temperature of 1080°C can be further utilized in future work for optoelectronics devices.

Growth and UV detector of serrated GaN nanowires by chemical vapor deposition

Serrated GaN nanowires were synthesized on sapphire substrate by chemical vapor deposition. Copper nanowires were synthesized by liquid-phase reduction method. The surface morphology of GaN and copper nanowires was observed by scanning electron microscopy. An UV detector based on GaN nanowires was prepared with copper nanowires as electrode. The results show that the device is a photoconductive detector. The detector has different response under different wavelength light illumination and has the maximum response under 365nm ultraviolet light. Photocurrent–time characteristics show the detector has good stability over time.

Effect of V/III ratio on the surface morphologies of N-polar GaN films grown on offcut sapphire substrates

We investigated the influence of the V/III ratio on the surface morphologies of N-polar GaN epilayers during the stage of high-temperature (HT) GaN growth. The epilayers were grown on 4°-offcut sapphire substrates by metal-organic vapor phase epitaxy. We observed prominent undulations on the GaN surface when a low V/III ratio was used, and ridged hillocks on the GaN surface when a high V/III ratio was used. With the V/III ratio increased from 128 to 2237, the density of ridged hillocks increased but the surface roughness was significantly reduced from 13.8 nm to 1.7 nm in 25 × 25 μm2 AFM images. The results differ from the existing reports in which a smoother N-polar GaN surface morphology was usually obtained with a lower V/III ratio. Non-uniform island formation in the initial growth of HT-GaN and enhanced step-bunching effect under a low V/III ratio were blamed for the large surface roughness of the N-polar GaN film. The increasing density of ridged hillocks on the N-polar surface grown under a high V/III ratio was attributed to the reduced diffusion rate of Ga adatoms due to excessive N. Completely hillock-free smooth N-polar GaN surfaces were achieved by further increasing the growth temperature of the GaN buffer layer.

Surface Morphology Improvement of Non-Polar a-Plane GaN Using a Low-Temperature GaN Insertion Layer**Supported by the National Natural Science Foundation of China (Grant Nos. 11574362 and 61704008).

We demonstrate that a low-temperature GaN insertion layer could significantly improve the surface morphology of non-polar a-plane GaN.The two key factors in improving the surface morphology of non-polar a-plane GaN are growth temperature and growth time of the GaN insertion layer. The root-mean-square roughness of a-plane GaN is reduced by 75% compared to the sample without the GaN insertion layer. Meanwhile, the GaN insertion layer is also beneficial for improving crystal quality. This work provides a simple and effective method to improve the surface morphology of non-polar a-plane GaN.

Surface morphology of polar, semipolar and nonpolar freestanding GaN after chemical etching

Surface etching methods were investigated for the removal of residues of chemical reagents and contaminants from polar (Ga- and N-plane), semipolar (r-plane) and nonpolar (a- and m-plane) freestanding GaN surfaces. Ga-, N-, r- and m-plane were etched in H3PO4, NaOH and (NH4)2S solutions. A-plane was treated in the solution of 10 wt% KOH in ethylene glycol (EG) at 80 °C. Different surface morphology were obtained and were characterized with atomic force microscope. The typical characteristics for the surface morphologies and interfacial angles in each etched GaN plane may help identify the type of crystal plane conveniently. Ga-polar GaN shows a clear and uniform step structure on surfaces. N-plane presented a step structure with tooth-like terraces. The (202¯1) r-plane, m-plane and a-plane show stripe-like structures, while the (202¯1¯) r-plane is stable and hard to be etched to exhibit step structures. The m-plane facets on a-plane surface caused by the etching method was found and discussed.

Effect of substrate misorientation on the concentration of impurities and surface morphology of an epitaxial GaN layer on N-polar GaN substrate by MOVPE

This study examines the effect of (0 0 0 −1) GaN substrate misorientation on the residual impurities and surface morphology of N-polar GaN grown by metalorganic vapor-phase epitaxy. Carbon, silicon, and oxygen concentrations decreased with increasing GaN substrate misorientation angle, with the lowest impurity concentration achieved for a misorientation angle of 2° toward the m-axis, with 6 × 1015 cm−3 carbon, 6 × 1015 cm−3 silicon, and 4 × 1017 cm−3 oxygen atoms. The oxygen concentration was measured at a depth of 0.5 μm below the wafer surface, and the oxygen concentration decreased with increasing thickness. The incorporation of carbon and oxygen revealed a strong dependence on the misorientation angle. The step distance height of the steps parallel to the [1 1 −2 0] direction (or perpendicular to the [1 −1 0 0] m-direction) was confirmed to be a double-height layer step. This phenomenon indicated that m-direction steps are stable for N-polar growth in GaN. In cases of large misorientation toward the m-axis in of the GaN substrate it was difficult to control the misorientation perpendicular to the nominal direction leading to a-axis direction by wafer bowing at wafer manufacturing. Therefore, step-bunching was generated for each symmetric m-axis due to an increase in the compound's off-angle, thus causing the surface roughness to become large.

Impact of Al Pre-Deposition Layer on Crystalline Quality of GaN Grown on Si(111) Substrates.

The effects of Al metal pre-deposition under different conditions on GaN grown on Si(111) substrates by metal-organic chemical vapor deposition (MOCVD) have been investigated. Al pre-deposition improves surface morphology and crystal quality of GaN grown on Si. The surface morphology of Al pre-deposition layer, AlN, and GaN vary depending on Al pre-deposition temperature. With the increase of Al pre-deposition temperature, Al cluster size is observed to increase in the Al predeposition layer due to increased lateral mobility of Al atoms. The Al pre-deposition carried out at about 750 °C enables to grow pit-free GaN layer on Si(111) substrate.

Effect of growth temperature and V/III-ratio on the surface morphology of MOVPE-grown cubic zincblende GaN

The influence of growth temperature and V/III-ratio on the surface morphology of (001) cubic zincblende GaN epilayers during metal organic vapour phase epitaxy growth has been investigated using atomic force microscopy and transmission electron microscopy. The zincblende phase purity as determined by X-ray diffraction was found to be above 98% for most GaN epilayers studied. As the growth temperature was increased from 850 °C to 910 °C and as the V/III-ratio was separately increased from 38 to 300, surface features were found to be elongated in the [1-10] direction, and the ratio of the length to width of such surface features was found to increase. Faceting was observed at V/III-ratios below 38 and above 300, which in the latter case was accompanied by a reduction of the zincblende phase purity. An explanation for these morphological trends is proposed based on effects such as the reduced symmetry of the top monolayer of the (001)-oriented zincblende GaN lattice, diffusion of Ga and N adatoms on such a surface, and the relative energies of the crystal facets.

Separation of Ga-polar GaN layer from Si substrate by wet chemical etching

In this work the effects of H3PO4:CH3COOH:HNO3:HF etching solution taken with different concentrations to the Ga-polar GaN/SixNy/Si(111) epitaxial structures is investigated. Possibility of at least partial separation of Ga-polar GaN film from the silicon substrate without any GaN surface morphology changes by the selected etchants is demonstrated. The etching process mechanism is shown. Resistivity of the photoresist mask to etchants used in the experiments is found.

Influence of Al Preflow Time on Surface Morphology and Quality of AlN and GaN on Si (111) Grown by MOCVD**Supported by the National Key Research and Development Program of China under Grant No 2016YFB0400200.

We investigate the influence of Al preflow time on surface morphology and quality of AlN and GaN. The AlN and GaN layers are grown on a Si (111) substrate by metal organic chemical vapor deposition. Scanning electron microscopy, atomic force microscopy, x-ray diffraction and optical microscopy are used for analysis. Consequently, we find significant differences in the epitaxial properties of AlN buffer and the GaN layer, which are dependent on the Al preflow time. Al preflow layers act as nucleation sites in the case of AlN growth. Compact and uniform AlN nucleation sites are observed with optimizing Al preflow at an early nucleation stage, which will lead to a smooth AlN surface. Trenches and AlN grain clusters appear on the AlN surface while melt-back etching occurs on the GaN surface with excessive Al preflow. The GaN quality variation keeps a similar trend with the AlN quality, which is influenced by Al preflow. With an optimized duration of Al preflow, crystal quality and surface morphology of AlN and GaN could be improved.