Hydride vapor phase epitaxy for gallium nitride substrate

Due to the remarkable growth rate compared to another growth methods for gallium nitride(GaN)growth,hydride vapor phase epitaxy(HVPE)is now the only method for mass product GaN substrates.In this review,commercial HVPE systems and the GaN crystals grown by them are demonstrated.This article also illustrates some innovative attempts to develop homebuilt HVPE systems.Finally,the prospects for the further development of HVPE for GaN crystal growth in the future are also discussed.     

Doping of gallium nitride using disilane

The silicon doping of n-type GaN using disilane has been demonstrated for films grown on sapphire substrates by low pressure organometallic vapor phase epitaxy. The binding energy of an exciton bound to a neutral Si donor has been determined from low temperature (6K) photoluminescence spectra to be 8.6 meV. Nearly complete activation of the Si impurity atom in the GaN lattice has been observed.     

Flow modulation epitaxy of indium gallium nitride

InGaN layers were grown on GaN films by flow modulation epitaxy (FME) using the precursors trimethylgallium, trimethylindium, and ammonia. The indium composition of the FME grown layers was generally lower than of films grown under the same conditions in the continuous growth mode, but which had been of poor optical quality. The indium incorporation efficiency increased with decreasing ammonia flush time, increasing ammonia flow during group-III injection, and increasing group-III precursor injection time. Films grown under optimized conditions showed intense band edge related luminescence at room temperature up to a wavelength of 465 nm. Atomic force microscopy investigations revealed a strong dependence of the surface morphology of the InGaN films on the growth mode.     

Pendeo-epitaxial growth of gallium nitride on silicon substrates

Pendeo-epitaxy (PE)¹ from raised, [0001] oriented GaN stripes covered with silicon nitride masks has been employed for the growth of coalesced films of GaN(0001) with markedly reduced densities of line and planar defects on Si(111)-based substrates. Each substrate contained previously deposited 3C-SiC(111) and AlN(0001) transition layers and a GaN seed layer from which the stripes were etched. The 3C-SiC transition layer eliminated chemical reactions between the Si and the NH₃ and the Ga metal from the decomposition of triethylgallium. The 3C-SiC and the GaN seed layers, each 0.5 μm thick, were also used to minimize the cracking and warping of the GaN/SiC/silicon assembly caused primarily by the stresses generated on cooling due to the mismatches in the coefficients of thermal expansion. Tilting in the coalesced GaN epilayers of 0.2° was confined to areas of lateral overgrowth over the masks; no tilting was observed in the material suspended above the trenches. The strong, low-temperature PL band-edge peak at 3.456 eV with a FWHM of 17 meV was comparable to that observed in PE GaN films grown on AlN/6H-SiC(0001) substrates.     

Chemically assisted ion beam etching of gallium nitride

Chemically assisted ion beam etching of gallium nitride (GaN) grown by metalorganic chemical vapor deposition has been characterized using an Ar ion beam and Cl₂gas. The etch rate of GaN was found to increase linearly with Ar ion beam current density, increase linearly then saturate with Ar ion beam energy, vary slightly with Cl₂ flow rate, and lastly, increase moderately with substrate temperature. Etch rates as high as 330 nm/min were obtained at high beam energies and 210 nm/min at a more nominal level of 500 eV. The anisotropy of etched profiles improved in the presence of Cl₂ in comparison to those etched by Ar ion milling only. Elevated substrate temperatures further enhanced the anisotropy to obtain near-vertical profiles for fairly deep-etched structures. Auger electron spectroscopy was used to investigate etch-induced surface changes. Oxygen contamination was observed on the as-etched surface but a dilute HC1 treatment restored the stoichiometry of the material to its unetched state.     

MOCVD of group III chalcogenides

A review is presented of recent advances in the metal organic chemical vapour deposition (MOCVD) of thin films of group III chalcogenides, including their application for the passivation of GaAs surfaces. The majority of studies involve the deposition of thermodynamic phases of composition ME and M₂E₃ (M?Ga, In; E?S, Se, Te), however, MOCVD allows for the growth of either high-pressure (tetragonal InS) or non-thermodynamic phases (metastable cubic phases of GaS and InSe). Based on the results to date, a series of goals for molecular control over the structure of deposited films is discussed.     

MOCVD of AlGaAs/GaAs with novel group III compounds

The MOCVD of AlGaAs and GaAs from coordinatively saturated group III source materialsi.e. 1–3-dimethyl-aminopropyl-l-galla-cyclohexane ((C₅H₁₀)Ga(C₂N(CH_{3 2}) and the corresponding Al compound) was investigated. It was demonstrated that these precursors, which are inherently free of alkoxy contamination, are suitable for epitaxial growth of GaAs layers and structures of GaAs/AlGaAs. For comparison, data achieved with TEA (Al(C₂H₅)₃) or TⁱBA (Alⁱ(C₄H₉)₃) and TEG (Ga(C₂H₅)₃) are presented. A basic finding of this study is that due to the low thermal stability of TEA, TⁱBA and TEG the layers grown from these compounds suffer from insufficient homogeneity of layer thickness and composition. In contrast, the coordinatively saturated compounds show a reactivity suitable for large area growth. Additionally, intrinsic impurity (N, C) uptake appears to be low and electrical as well as PL data show the satisfactory quality of GaAs and AlGaAs layers grown from this new type of precursors. Specifically, a reduction of oxygen incorporation compared to growth from the standard trialkyls is indicated by PL measurements on layers grown at different temperatures.     

应用原子层外延技术分析Turbo-Disk MOCVD 外延生长模式

应用原子层外延与分子层外延的理论研究了Turbo-Disk MOCVD外延生长过程,发现生长主要发生在衬底表面的台阶处,当通过控制生长参数达到优质外延时,实际上是一种亚原子外延过程．优化调整反应参数实现了优质外延。     

Optical third-harmonic investigations of gallium nitride nucleation layers on sapphire

The magnitude of the χ _xxxx ⁽³⁾ element of the third-order optical susceptibility was measured in a series of wurtzite phase GaN nucleation layers (～450Å) deposited on (00.1) sapphire at 540°C and annealed to various temperatures up to 1050°C. The nonlinear optical response exhibited a significant increase in films that were annealed to temperatures in the range of 1015 to 1050°C. In addition, the correlation between the magnitude of χ _xxxx ⁽³⁾ with both the maximum value of the linear absorbance gradient and the residual homogeneous strain in the overlayer suggests that variations in the crystalline content of the film and the bonding distance between the Ga and N atoms are primary factors in determining the third-order nonlinearity in GaN.     

Growth of InGaN HBTs by MOCVD

The design and growth of GaN/InGaN heterojunction bipolar transistors (HBTs) by metalorganic chemical vapor deposition (MOCVD) are studied. Atomic-force microscopy (AFM) images of p⁺InGaN base layers (∼100 nm) deposited under various growth conditions indicate that the optimal growth temperature is limited to the range between 810 and 830°C due to a trade-off between surface roughness and indium incorporation. At these temperatures, the growth pressure must be kept above 300 Torr in order to keep surface pit density under control. An InGaN graded-composition emitter is adopted in order to reduce the number of V-shaped defects, which appear at the interface between GaN emitter and InGaN base and render an abrupt emitter-base heterojunction nearly impossible. However, the device performance is severely limited by the high p-type base contact resistance due to surface etching damage, which resulted from the emitter mesa etch.     

Basic studies of gallium nitride growth on sapphire by metalorganic chemical vapor deposition and optical properties of deposited layers

We have studied the growth of gallium nitride on c-plane sapphire substrates. The layers were grown in a horizontal metalorganic chemical vapor deposition reactor at atmospheric pressure using trimethylgallium (TMG) and ammonia (NH₃). Variation of the V/III ratio (150–2500) shows a distinct effect on the growth rate. With decreasing V/III ratio, we find an increasing growth rate. Variation of the growth temperature (700–1000°C) shows a weak increase in growth rate with temperature. Furthermore, we performed secondary ion mass spectroscopy measurements and find an increasing carbon incorporation in the GaN films with decreasing ammonia partial pressure and a growing accumulation of carbon at the substrate interface. Photoluminescence measurements show that samples with high carbon content show a strong yellow luminescence peaking at 2.2 eV and a near band gap emission at 3.31 eV. With increasing carbon content, the intensity of the 3.31 eV line increases suggesting that a carbon related center is involved.     

Microstructural characterisation of nanocrystalline GaN prepared by detonations of gallium azides

High quality nanoscale, phase-pure hexagonal gallium nitride (GaN) crystallites have been synthesized by the thermal induced detonation of molecular precursors of the type (R₃N)Ga(N₃)₃ (R=CH₃, C₂H₅, etc.). The method allows the control of the particle size regime from 2 to about 1000 nm. X-ray diffraction and Rietveld simulations revealed an anisotropic platelet-like shape of the particles. The obtained GaN material was as well characterized by transmission electron microscopy and electron diffraction, photoluminescence spectroscopy, SEM, IR, RAMAN, thermal gas effusion/mass spectrometry, thermal analysis, elemental analysis. Gas absorption measurements (BET method) showed a specific surface area of about 90 m² · g⁻¹. © 1998 John Wiley & Sons, Ltd.     

The single-molecule approach to the deposition of compound semiconducting materials by MOCVD and related methods

The last 10 years have seen a number of chemists begin to take a serious interest in the deposition of materials such as compound semiconductors. Chemical deposition routes have a number of potential advantages, many of which arise from the fact that growth can take place well away from equilibrium. Chemists are particularly attracted by the idea that a volatile single molecule can deliver the elements of a compound semiconductor to the reaction site. In the present article recent advances in the deposition of compound semiconductors, principally II/VI (12/16) or III/V (13/15) materials, from single-molecule precursors will be reviewed. The chemistry of these precursors will be discussed in terms of both their synthesis and properties and the effect of the mechanism of their decomposition on the quality of deposited material.     

Metal-organic vapour-phase epitaxy of gallium nitride nanostructures for optoelectronic applications

The two main topics with respect to better gallium nitride (GaN)-based optoelectronic device performance are the improvement of crystal quality and of light extraction. Concerning the first topic, the epitaxial growth of GaN-related materials has to be improved. Since native substrates with good quality are still expensive, foreign substrates like sapphire or silicon which are much cheaper are used, but they introduce a lattice as well as a thermal mismatch leading to detrimental crystal defects. Several approaches to reduce the defect density exist, but in most cases they are coupled with a much higher technological effort.We present an alternative solution to decrease the defect density by the growth of GaN nanostructures, where the small footprint and high aspect ratio leads to a much lower influence of the substrate. Metal-organic vapour-phase epitaxy of GaN nanostructures and InGaN/GaN multi-quantum wells was carried out in a vertical reactor with close-coupled showerhead. Studies on the morphologic properties have been carried out by scanning electron microscopy and energy-dispersive X-ray spectroscopy to reveal the influence of growth parameters and material composition. Cathodoluminescence measurements show the good optical properties of the GaN as well as of the InGaN/GaN structures.     

Trisneopentylgallium as a precursor for atomic layer epitaxy of GaAs

We report the use of a new precursor, trisneopentylgallium (NPG) for the growth of GaAs by atomic layer epitaxy (ALE). In contrast to most other alkyl gallium precursors such as triethylgallium, which decompose via a β-hydride elimination mechanism, this compound undergoes homolysis similar to that of trimethylgallium (TMGa), the normal choice as an ALE precursor. Clear self-limiting growth behavior similar to that of TMGa was observed over a reasonably wide range of growth conditions (430–500°C). Carbon incorporation was not significantly reduced compared with TMGa suggesting that the adsorbed neopentyl radicals undergo decomposition to result in a methyl terminated surface identical to that obtained for growth with TMGa.     

Reactive ion etching of gallium nitride films

Reactive ion etching (RIE) was performed on gallium nitride (GaN) films grown by electron cyclotron resonance (ECR) plasma assisted molecular beam epitaxy (MBE). Etching was carried out using trifluoromethane (CHF₃) and chloropentafluoroethane (C₂ClF₅) plasmas with Ar gas. A conventional rf plasma discharge RIE system without ECR or Ar ion gun was used. The effects of chamber pressure, plasma power, and gas flow rate on the etch rates were investigated. The etch rate increased linearly with the ratio of plasma power to chamber pressure. The etching rate varied between 60 and 500Å/min, with plasma power of 100 to 500W, chamber pressure of 60 to 300 mTorr, and gas flow rate of 20 to 50 seem. Single crystalline GaN films on sapphire showed a slightly lower etch rate than domain-structured GaN films on GaAs. The surface morphology quality after etching was examined by atomic force microscopy and scanning electron microscopy.     

The effect of carrier gas and H(hfac) on MOCVD Cu films using (hfac)Cu(1,5-COD) as a precursor

The deposition characteristics of metalorganic chemical vapor deposition (MOCVD) Cu using (hfac)Cu(1,5-COD)(1,1,1,5,5,5-hexafluro-2,4pentadinato Cu(I) 1,5-cyclooctadiene) as a precursor have been investigated in terms of carrier gas effects and adding H(hfac) to the carrier gas stream. Using hydrogen carrier gas led to a higher MOCVD Cu deposition rate and a lower film resistivity compared to an argon carrier gas system. Improvements in surface roughness of the MOCVD Cu films and a (111) preferred orientation texture were obtained by using hydrogen as a carrier gas. When a ligand such as H(hfac) was added to Ar carrier gas, the deposition rate was significantly enhanced. Moreover, H(hfac) added to both carrier gas streams led, to lower MOCVD Cu film resistivity. However, film adhesion was somewhat weak compared to that observed with the Ar or H₂ carrier gas system, probably due to the larger F content near the interface between the copper and the titanium-nitride film. In conclusion, smooth Cu films with a low resistivity can be obtained by manipulating the deposition conditions, such as carrier gas type and ligand addition. The deposition mechanism of MOCVD Cu is also discussed in the paper.     

Low-temperature metalorganic chemical vapour deposition of AlN for surface passivation of GaAs

Aluminium nitride (AlN) thin films have been grown by low-temperature metalorganic chemical vapour deposition (MOCVD) to passivate GaAs. By utilizing hydrazine (N₂H₄), highly resistive amorphous-like AlN films were obtained at growth temperatures around 400°C. At the AlN-GaAs interface, three deep trap levels were found: 0.6 eV (DL1) and 0.9 eV (DL2) below the conduction band minimum and 0.5 eV (DL3) above the valence band maximum. The number of DL1 levels was reduced by preparing As-dimer-stabilised surfaces of GaAs. The capture cross-sections and time constants of DL1–DL3 suggest that these levels originated from point defects, not from precipitates or disorder. Neither precipitation nor reaction was detectable by Auger electron spectroscopy after annealing at 900°C for 20 min, indicating that the AlN-GaAs interfaces are thermally stable. These results demonstrate that these AlN films are applicable as capping films for processing GaAs as well as passivation films.     

In-situ plasma cleaning of stainless steel III-V MOCVD growth systems

A plasma enhanced, in-situ, dry etching process for the cleaning of stainless steel III-V Metal Organic Chemical Vapor Deposition growth systems was investigated as a function of etchant gas, flow rate, electrode configuration, power density and plasma frequency. The plasma enhanced etching process was investigated using Ar, CH₄ (5% in H₂), CCl₂F₂ (Freon 12)/Ar and Cl₂/Ar plasmas with flows varying from 5 to 25 seem. The plasma was excited using three electrode configurations, and two radio frequency generators (90–460 KHz and 13.56 MHz), singly and in combination. The plasma power was varied over the range from 200 to 700 Watts (∼0.2W/cm² – 0.7W/cm²). The etching rates of GaAs, InP, As, and Mo were measured using a weight difference method. The Cl₂/Ar plasmas exhibited etching rates typically 5 to 10 times greater than that of CCl₂F₂ plasmas, which in turn is several times greater than that of the other etchant gases investigated. At 400 W, elemental As etch rates, as high as ∼180μm/hr and ∼20μm/hr were achieved using Cl₂ and CCl₂F₂ plasmas, respectively. InP/GaAs etch rates using Cl₂ were ∼30μm/hr and using CCl₂F₂ were ∼7μm/hr. Plasma characteristics and etch rate measurements are reported. The in-situ process investigated is a safe, cost effective and an efficient method for increasing reactor uptime.