High-temperature GaN Growth Research Articles

GaN growth on ScAlMgO4 substrates via thermally-dewetted thin Al films

In this note, we demonstrate the high-temperature growth of GaN on ScAlMgO4 substrates by metalorganic vapor phase epitaxy when a thin Al film is deposited ex situ on the ScAlMgO4 surface, prior to GaN growth. Mirror-like high-quality GaN epitaxial layers were obtained when N2 was used as carrier gas during the reactor temperature ramp-up preceding GaN growth, leading to a higher GaN quality compared to direct growth on ScAlMgO4 using a trimethylaluminium preflow. This opens a pathway for high-temperature GaN growth on ScAlMgO4 when an Al precursor line is not present.

Role of V-defect density on the performance of III-nitride green LEDs on sapphire substrates

In this study, we experimentally investigated the role of V-defect density on the performance of green III-nitride LEDs grown on sapphire substrates by metalorganic chemical vapor deposition. We systematically varied the threading dislocation (TD) density from 4 × 108 to 1 × 109 cm−2 by changing the V/III ratio during initial high temperature GaN growth. A 30-period InGaN/GaN superlattice promoted V-defect formation and growth at TDs, where the density of V-defects was correlated to the TD density. By interrupting the LED growth and examining the surface of the active region, we quantified the average size and density of V-defects. In a series of LEDs, we measured a systematic decrease in forward voltage (VF) with V-defect density. At a V-defect density of 5.0 × 108 cm−2 and TD density of 1 × 109 cm−2, green LED devices were demonstrated with λ = 523 nm and VF = 2.94 V at 20 A cm−2. These results highlight the potential of using V-defect engineering to achieve low VF long wavelength LEDs on sapphire substrates, where opening of remaining threading dislocations into V-defects presents an opportunity for further VF reduction.

2 in. Free-standing GaN grown by HVPE with sputtered AlN buffer on GaAs substrate

A magnetron sputtered AlN buffer layer is applied for preparing 2 in. free-standing (FS) GaN on the GaAs substrate by hydride vapor phase epitaxy (HVPE). The AlN buffer layer with excellent compactness can prevent As atoms from diffusing to the GaN growth surface. Thus it serves as an effective protective layer for GaAs substrate and also a seed layer for the deposition of GaN in the early stage of the high-temperature GaN growth. Benefiting from the decomposition property of GaAs at temperature higher than 900 °C, thick GaN film self-separated completely from GaAs substrate during growth. By this way, a 2 in. FS-GaN layer with the thickness of 1.3 mm is finally obtained after the long growth time about 20 h.

Microstructural Evolution of MOVPE Grown GaN by the Carrier Gas

We report the effect of total carrier gas flow of GaN during both GaN nucleation layer and high temperature GaN growth steps on structural, optical, electrical and morphological properties. The formation of dislocations in GaN layer and their effects were investigated in detail as a function of carrier gas flow. It has been found that the more carrier gas requires longer recovery time for transition from 3D (3 dimensional) to 2D growth and results in smaller edge-type dislocation density. The images obtained from the AFM measurements have shown terraces widths varying between 60–150 nm depending on the hydrogen flow rate.

(Invited) Monolithic Integration of Si-CMOS and III-V-on-Si through Direct Wafer Bonding Process

Integration of Si-CMOS and III-V compound semiconductors (with device structures of either InGaAs HEMT, AlGaInP LED, GaN HEMT or InGaN LED) on a common Si substrate is demonstrated. The Si-CMOS layer is temporarily bonded on a Si handle wafer. Another III-V/Si substrate is then bonded to the Si-CMOS containing handle wafer. Finally, the handle wafer is released to realize the Si-CMOS on III-V/Si substrate. For GaN HEMT or LED on Si substrate, additional wafer bonding step is required to replace the fragile Si (111) substrate after high temperature GaN growth with a new Si (001) wafer to improve the robustness of the GaN/Si wafers. Through this substrate replacement step, the bonded wafer pair can survive the subsequent processing steps. The monolithic integration of Si-CMOS + III-V devices on a common Si platform enables new generation of systems with more functionality, better energy efficiency, and smaller form factor.

(Invited) Monolithic Integration of Si-CMOS and III-V-on-Si through Direct Wafer Bonding Process

Integration of Si-CMOS and III-V compound semiconductors (with device structures of either InGaAs HEMT, InGaP LED, GaAs HBT, GaN HEMT or InGaN LED) on a common Si substrate is demonstrated. The Si-CMOS layer is temporarily bonded on a Si handle wafer. Another III-V/Si substrate is then bonded to the Si-CMOS containing handle wafer. Finally, the handle wafer is released to realize the Si-CMOS on III-V/Si substrate. For GaN LED or HEMT on Si substrate, additional wafer bonding step is required to replace the fragile Si (111) substrate after high temperature GaN growth with a new Si (001) wafer to improve the robustness of the GaN/Si wafers. Through this substrate replacement step, the bonded wafer pair can survive the subsequent processing steps. The monolithic integration of Si-CMOS + III-V devices on a common Si platform enables new generation of systems with more functionality, better energy efficiency, and smaller form factor.

Monolithic Integration of Si-CMOS and III-V-on-Si Through Direct Wafer Bonding Process

Integration of silicon-complementary metal oxide-semiconductor (Si-CMOS) and III-V compound semiconductors (with device structures of either InGaAs HEMT, AlGaInP LED, GaN HEMT, or InGaN LED) on a common Si substrate is demonstrated. The Si-CMOS layer is temporarily bonded on a Si handle wafer. Another III-V/Si substrate is then bonded to the Si-CMOS containing handle wafer. Finally, the handle wafer is released to realize the Si-CMOS on III-V/Si substrate. For GaN HEMT or LED on Si substrate, additional wafer bonding step is required to replace the fragile Si (111) substrate after high temperature GaN growth with a new Si (001) wafer to improve the robustness of the GaN/Si wafers. Through this substrate replacement step, the bonded wafer pair can survive the subsequent processing steps. The monolithic integration of Si-CMOS + III-V devices on a common Si platform enables new generation of systems with more functionality, better energy efficiency, and smaller form factor.

Site-controlled GaN nanocolumns with InGaN insertions grown by MBE

The site-controlled plasma-assisted molecular beam epitaxy (PA MBE) has been developed to fabricate the regular array of GaN nanocolumns (NCs) with InGaN insertions on micro-cone patterned sapphire substrates (μ-CPSSs). Two-stage growth of GaN NCs, including a nucleation layer grown at metal-rich conditions and high temperature GaN growth in strong N-rich condition, has been developed to achieve the selective growth of the NCs. Microcathodoluminescence measurements have demonstrated pronounced emission from the InGaN insertions in 450-600 nm spectral range. The optically isolated NCs can be used as effective nano-emitters operating in the visible range.

Fabrication of free-standing GaN by using thermal decomposition of GaN

Free-standing GaN wafers were fabricated by hydride vapor phase epitaxy (HVPE) using an in-situ self-separation technique. Separation of GaN happened during the high-temperature GaN growth through thermal decomposition of the decomposable buffer layer (DBL). We optimized the growth condition of DBL, which affects not only the separation property but also the crystallinity of GaN. Free standing GaN revealed negligible residual stress, narrow (0002) omega rocking curve linewidth (67ʺ), and low etch pit density (6×106cm−2) as well. The carrier concentration of 4×1018cm−3 and mobility of 150cm2/Vs was observed from Hall effect measurement. These results showed the feasibility of the in-situ self-separation method to obtain high quality free-standing GaN wafer.

Analysis of V-Shaped Pits Originated from Threading Dislocation in III-Nitrides Compound for Light Emitting Diodes

Light emitting devices (LEDs) made with III-nitrides compound on c-plane sapphire substrate using metal organic chemical vapor deposition (MOCVD) have so many threading dislocation defects due to the lattice mismatch. [1, 2] Threading dislocations were caused some problems such as poor luminescence and leakage current. In order to reduce the problems from threading dislocation defects, v-shaped pit initiated from threading dislocations used to construct in III-nitrides compound of LEDs. [3] V-shaped pits were made using different growth rate of etch lattice plane at specific growth temperature. (called mid-temp GaN in Figure 1) Furthermore v-shaped pits grown this method were served as insulator due to a lack of magnesium doping density in pGaN layer. Therefore leakage current through the threading dislocation defects can be prevented by characteristic of v-shaped pits. However some v-shaped pits still have a problem of leakage current. In this paper, dimension and electrical properties of v-shaped pits were separated for identify the causes of leakage current. Figure 1 shows a diagram of v-shaped pit originated from threading dislocation which has an open hexagonal inverted pyramid. The ‘g’ samples show lower current at reverse bias voltage than that of ‘ng’ samples which shown in Figure 2. It means the ‘ng’ samples are acting like leakage source at reverse voltage. Although the ‘g’ samples go to saturation around 5 reverse voltage, the ‘ng’ samples cannot be saturated even at 15 reverse voltage. (Figure 3) It means the ‘ng’ samples have higher carrier concentrations than ‘g’ samples. SEM images are shown in Figure 4(a)-4(d). The mean diameter and depth of the ‘g’ samples are 330.4nm and 306.7nm, respectively. (Figure 4(a), 4(c)) On the other hand, the mean diameter and depth of ‘ng’ samples are 417.7nm and 387.1nm, respectively. (Figure 4(b), 4(d)) As can be seen, the pits of ‘g’ samples are smaller than that of ‘ng’ samples. It means etch samples had a different starting point to making the v-shaped pits. The ‘g’ samples were started to make the v-shaped pits during the mid-temperature GaN growth. However the v-shaped pits of the ‘ng’ sample were made as soon as began of the mid-temperature GaN growth. Some of threading dislocation defects were subjected to tensile stress due to heat budget during high-temperature GaN growth, after that v-shaped pits began to form immediately due to the lower growth temperature. As a result, leakage current was expected to occur the v-shaped pits enlarged by the tunneling effect. In order to solve this problem, evaluation was conducted the step by step decreasing the growth temperature during high-temperature GaN growth. Eventually, a advanced result was obtained to control the dimension of v-shaped pits.

A Study on the In-situ Separation of GaN by Using Thermal Decomposition

The free-standing GaN (FS-GaN) substrate was fabricated by using an in-situ self-separation technique that was accomplished by using the thermal decomposition of GaN. For the self-separation of GaN, a decomposable buffer layer (DBL) was introduced between the sapphire substrate and the overgrown thick-GaN, and self-separation of GaN happened during the high-temperature GaN growth. The thick GaN was almost unaffected by the thermal stress during the separation from sapphire substrate, so we could fabricate low-defect and high-quality FS-GaN. In this study, the DBL growth temperature (Tg: 700 ◦C) and the thermal annealing temperature (T ∼ 900 ◦C) were optimized. The reduced thermal stress was confirmed by using cathodoluminescence. Also, the values of the full widths at half maximum for the (002) and the (102) ω-rocking curves were 67 and 96 arcsec, respectively, and a low etch pit density (6 × 10 cm−2) was observed, both of which indicate the feasibility of this method for fabricating FS-GaN.

High quality N-polar GaN two-dimensional growth on c-plane sapphire by metalorganic vapor phase epitaxy

We report the growth of atomically smooth N-polar GaN on c-plane sapphire by metalorganic vapor phase epitaxy. A two-step growth technique was adopted; low-temperature growth of GaN buffer before high-temperature GaN growth. The complete two-dimensional N-polar GaN growth process was recorded by in situ reflectance. The phase composition of the low-temperature GaN was examined by X-ray diffraction pole figure measurements. The thickness of the low-temperature GaN buffer dramatically affected the crystalline and electronic properties of the N-polar GaN. A very small full width at half maximum for the (0002) X-ray rocking curve, 51arcs, was obtained for 700-nm-thick N-polarity GaN by optimizing the buffer thickness.

Nitrogen-polar GaN growth evolution on c-plane sapphire

This letter presents a study on N-polar GaN growth evolution on sapphire using a low-temperature GaN buffer, which is distinctly different from the two-step growth of Ga-polar GaN according to both in situ reflectance and ex situ microscopy. Annealed N-polar GaN buffer exhibits densely packed tiny grains, serving as a template for the subsequent high-temperature GaN growth, which starts in a quasi-two-dimensional mode without any roughening-recovery process. Atomically smooth N-polar GaN has been achieved with no stacking fault or inversion domain observed. The mosaic microstructure, electrical, and optical properties of N-polar GaN are compared with those of Ga-polar GaN.

Influence of growth rate in the early stage of high temperature GaN layer growth on quality of GaN films

The effect of growth rate in the early stage of high temperature (HT) GaN layer growth on the quality of GaN by MOCVD has been investigated. The properties of GaN layers were measured by double crystal X-ray diffraction (DCXRD), Hall method, photoluminescence, and atomic force microscopy, respectively. The results reveal that the quality of GaN layers is influenced by the growth rate of HT GaN growth in the early stage. The properties of GaN layers are obviously improved by decreasing the growth rate in the early stage of HT GaN growth properly. However, when the growth rate decreases to some extent, the quality of GaN layer deteriorates instead. The reasons behind this phenomenon were discussed.

Self-separated freestanding GaN using a NH4Cl interlayer

Thick GaN films were grown on void buffer layer by hydride vapor phase epitaxy. The void-buffer layers were consisted of NH4Cl, GaN dots, and low-temperature (LT) GaN buffer layer. Instead of GaN, NH4Cl was easily synthesized in NH3 and HCl atmospheres by simply lowering the growth temperature to 500°C, and LT GaN buffer growth was followed during increasing substrate temperature to 600°C. The LT GaN buffer acted as a protecting layer against evaporation of the NH4Cl and a seeding layer for the high temperature (HT) GaN. The NH4Cl layer between a sapphire substrate with GaN dots and the LT GaN buffer were fully evaporated during the HT GaN growth at 1040°C. Many voids were formed at interface caused by evaporation of the NH4Cl layer, which strongly assisted self-separation of thick HT GaN during cooldown after the growth resulting in a 200μm thick freestanding (FS) GaN. The FS GaN showed smooth surface morphology and absence of any crack. The a-axis and c-axis lattice constants of FS GaN were 3.189 and 5.185Å, respectively, which well agrees with those of strain-free bulk GaN. The observed donor-bound exciton emission peak at 3.4718eV agreed with the peak position of bulk GaN. All these features indicate that the obtained FS GaN through the self-separation process is nearly strain-free.

Quantitative simulation of in situ reflectance data from metal organic vapour phase epitaxy of GaN on sapphire

Multi-wavelength in situ reflectometry at normal incidence has been applied to monitoring metal organic vapour phase epitaxy of GaN films on sapphire (0 0 0 1) substrates. A new quantitative analysis model has been developed to incorporate time-dependent light scattering by a rough surface, and a time-dependent vertical growth rate during growth on a rough surface, into the virtual interface model that has previously been applied to multilayer structures with optically smooth surfaces and interfaces. It is shown that the vertical growth rate increases as the surface roughness decreases in the early stage of high-temperature GaN growth, reaching a limiting value when the surface becomes optically smooth. The time dependence of growth rate is correlated with microscopic crystal growth mechanisms on the rough surface, which involve mass transport on the facets and/or mass exchange between the growing surface and gas-phase ambient. Our optical modelling is supported by direct morphological investigations of films from growths terminated at various stages, using atomic force microscopy. High-temperature optical constants of GaN layers extracted from the simulations are well matched to literature values.

Role of initial nucleation in molecular‐beam epitaxy of GaN on lattice‐matched ZrB 2 substrates

Nitrogen-polar GaN was grown on lattice-matched ZrB2 substrates by a two-step rf-MBE process with conventional low-temperature GaN (LT-GaN) used as the nucleation layer. For nucleation in slightly Ga-rich conditions, a streaky RHEED pattern was maintained from the very beginning and persisted throughout subsequent high-temperature growth. However, despite the streaky RHEED pattern the initial nucleation was evidently not 2D as inferred from AFM and x-ray results. For nucleation in slightly N-rich conditions, spots appeared in the RHEED in the early stages of growth but a streak pattern was restored with high-temperature GaN growth. Tradeoffs were identified in optimizing the LT-GaN nucleation process. Slightly Ga-rich nucleation conditions offered the best x-ray characteristics with ω-scan FWHM values of 400-500 and 720-800 arcsec for the (0002) and (104) reflections, respectively. Short-length atomic steps (up to ∼200 nm) were present on the surfaces of such layers, but the morphology showed a sub-grain structure. Slightly N-rich nucleation conditions yielded remarkably smooth films surfaces (rms 0.5 nm over 10 µm2) with uninterrupted atomic steps up to several µm long, but inferior x-ray ω-scans (∼50% larger FWHM values). (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Correlation of in-situ reflectance spectra and resistivity of GaN/Al2O3 interfacial layer in metalorganic chemical vapor deposition

The correlation between the resistivity of an undoped GaN/Al2O3 interfacial layer and in-situ reflectance spectrum in metalorganic chemical vapor deposition and the mechanism of this correlation were investigated. The first minimum reflectance during the initial high-temperature GaN growth was found to be a good indicator of the resistivity of the GaN buffer. The background electron concentration and mobility were both higher in the samples with higher indicative reflectance at that point. The resistivity of the GaN buffer layer was predominantly determined by an ∼0.25-µm-thick layer near the GaN/Al2O3 interface. Atomic force microscope (AFM) and high-resolution x-ray diffraction (HRXRD) results showed that the samples with higher indicative reflectance had smaller sized but higher density nuclei before the high-temperature GaN growth and lower screw threading dislocation (TD) density in the initially grown GaN. The difference in the background electron concentration and mobility of the interfacial layer was related to the relatively higher concentration of the O and Al diffused from Al2O3, which is also dependent on the size and density of the nuclei. These differences were found not to affect the structural and electrical properties or the surface morphology of AlGaN/GaN high electron-mobility transistors (HEMTs, except for the buffer conduction) when the GaN buffer is thick enough (e.g., ∼2.5 µm).

Two-step growth of high quality GaN using V/III ratio variation in the initial growth stage

The effect of V/III ratio in the initial stage of high temperature GaN growth using a metalorganic chemical vapor deposition system was investigated. Nucleation layer growth and coalescence process in the initial growth stage of GaN were in situ characterized by normal incidence reflectance measurement and surface morphology was ex situ characterized by atomic force microscope (AFM). Structural and optical properties were also characterized by a high-resolution X-ray diffraction and photoluminescence, respectively and correlated with AFM images. As the initial V/III ratios at high temperature GaN growth were lowered, in situ reflectance measurements showed low and long reflectance peaks. It is attributed to the enhanced three-dimensional growth mode, which result in rough surface morphology and larger island size in the initial stage of GaN growth. The larger island size means that the density of nuclei is reduced and the mixed or edge dislocations are suppressed during coalescence process. After the initial growth of high temperature GaN at a low V/III ratio, GaN layers were grown at a high V/III ratio to enhance the lateral growth. Such a two-step growth method of high temperature GaN using a V/III ratio variation led to high-quality GaN films with well-aligned growth steps, low dislocation density and intense band edge emission.

Effects of reactor pressure on GaN nucleation layers and subsequent GaN epilayers grown on sapphire substrate

The influence of reactor pressure on GaN nucleation layer (NL) and the quality of subsequent GaN on sapphire is studied. The layers were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrates and investigated by in situ laser reflectometry, atomic force microscope, scanning electron microscope, X-ray diffraction and photoluminescence. With the increase of reactor pressure prior to high-temperature GaN growth, the size of GaN nuclei formed after annealing decreases, the spacing between nucleation sites increases and the coalescence of GaN nuclei is deferred. The optical and crystalline qualities of GaN epilayer were improved when NLs were deposited at high pressure. The elongated lateral overgrowth of GaN islands is responsible for the quality improvement.