Standard GaN Research Articles

Deep centers in GaN layers grown on epitaxial lateral overgrowth templates by metalorganic chemical vapor deposition

ABSTRACTThe dependence of traps and their concentration in GaN on the quality of templates, on which the layers are grown, has been studied by deep-level transient spectroscopy (DLTS). Thin GaN layers studied were grown on GaN templates employing conventional epitaxial lateral overgrowth (ELO) and nano-ELO with SiNx nanonetwork. The concentrations of traps found in these layers were compared with a reference sample grown on a standard GaN template not utilizing ELO. Two traps A (0.55 eV – 0.58 eV) and B (0.20 eV – 0.23 eV) were detected in the temperature range from 80 K to 400 K. A reduction of traps in layers grown on the ELO and nano-ELO templates was noted. We attribute this reduction to the reduction of threading dislocation density and as a result reduced capture of point defects and complexes as part of dislocation core structure and/or reduced formation of defects and complexes in the vicinity of line defects where the formation can be energetically favorable.

Slip systems and misfit dislocations in InGaN epilayers

We have studied the microstructure of InGaN layers grown on two different GaN substrates: a standard GaN film on sapphire and an epitaxial lateral overgrown GaN (ELOG) structure. These two materials exhibit two distinct mechanisms of strain relaxation. InGaN epilayers on GaN are typically pseudomorphic and undergo elastic relaxation by the opening of threading dislocations into pyramidal pits. A different behavior occurs in the case of epitaxy on ELOG where, in the absence of threading dislocations, slip occurs with the formation of periodic arrays of misfit dislocations. Potential slip systems responsible for this behavior have been analyzed using the Matthews-Blakeslee model and taking into account the Peierls forces. This letter presents a comprehensive analysis of slip systems in the wurtzite structure and considers the role of threading dislocations in strain relaxation in InGaN alloys.

Microstructure and electronic properties of InGaN alloys

AbstractThe InxGa1−xN system has electronic band gaps extending from under 1eV to 3.4 eV, and as such they are used as the active layer in commercially available visible‐light emitting devices. There are many interesting features that make these nitride semiconductor alloys especially useful for efficient light emitters. It has been conjectured that the combination of piezoelectric fields and local composition inhomogeneities may be responsible for the observed high emission efficiencies, in spite of their characteristic high dislocation densities. But it is very difficult to grow InxGa1−xN layers with high indium composition. This paper presents an overview of the properties of InxGa1−xN epilayers based on a systematic study of thick layers and of quantum well structures. We find that the microstructure of thick films varies significantly with indium composition. For x < 0.08, the composition is uniform and unperturbed by dislocations. For 0.10 < x < 0.20, secondary phases nucleate at threading dislocations. For x > 0.20, spontaneous phase separation occurs resulting in a polycrystalline, inhomogeneous layer. A correlation between optical properties and microstructure is presented. It is observed that the misfit strain is affected by threading dislocations. Mechanisms of misfit strain relaxation are presented for InxGa1−xN layers grown on standard GaN on sapphire and on epitaxial‐lateral‐overgrowth GaN layers. In addition, we have studied the properties of quantum well structures using several novel techniques. The electrostatic fields across the wells have been profiled using electron holography in the TEM. The effect of well thickness on the strength of the fields is reported. The effects of localization by compositional fluctuations and of internal field screening have been studied using time‐resolved cathodoluminescence spectroscopy. In spite of significant progress that has been made in the last ten years, much work remains ahead in order to master the science and technology of these alloys. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The synthesis of silica nanowire arrays

Highly oriented silica nanowire arrays have been achieved as byproduct in the synthesis of GaN nanowires by the reaction of gallium with ammonium using In 2O 3 as catalyst. Resulting materials were characterized by field-emission scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). SEM images show that there are a few spheres and random nanowires distributed on the surface of the Si substrate. The spheres have a core/shell structure with nanowire arrays as shell layer. The nanowire arrays have uniform diameter about 60 nm with length about 15 μm. EDS data show that the nanowire arrays consist of only Si and O with a ratio of ∼1:1.8, whereas the cores of the spheres consist of Si, Ga and small amount of In. The random nanowires have uniform diameter about 50 nm with length up to 20 μm. EDS data indicate that only Ga and N with a ratio of ∼1:1 exists in the random nanowires, corresponding to the chemical composition of standard GaN.

Investigation of molecular co-doping for low ionization energy p-type centers in (Ga,Al)N

ABSTRACTThis work initiates an investigation of molecular co-doping to produce p-type centers in (Ga,Al)N with ionization energies lower than Mg. Dopant complexes can be formed between a doubly ionized acceptor such as (Cu, Li or Ag) and a singly ionized donor (silicon). Ion implantation of Cu, Li and Ag into silicon doped GaN films grown by Metalorganic Chemical Vapor Deposition (MOCVD) has been performed. Secondary ion mass spectroscopy (SIMS) data confirmed the simulated depth profile. High resolution X-ray diffraction and Raman spectroscopy were used to characterize the crystalline damage and subsequent recovery upon anneal. A complete recovery was observed after high temperature (700–900°C) annealing. Low temperature (6K) photoluminescence (PL) for Cu-implanted GaN showed bands identified with crystalline lattice damage due to the Cu-implantation. The annealed samples showed recovery of standard crystalline GaN features. Additional donor-acceptor pair features are observed below 3.35 eV indicating the existence of an acceptor state.

Influence of selective GaN islands on optical properties of GaN films grown on sapphire substrate

We report on the growth of GaN films on sapphire substrates by low-pressure metalorganic vapor phase epitaxy using selective GaN islands as buffer layer. These islands are produced by a silane (SiH 4) treatment of the sapphire surface at high temperature, followed by a low temperature GaN buffer deposition. A photoluminescence (PL) study demonstrates that this growth process significantly enhances the luminescence emission of the donor bound exciton (D°X) recombination and leads to a very narrow peak with a full width at half maximum (FWHM) of 4 meV. This width is about 30% smaller as compared with a standard process using a low temperature buffer layer. Changes in PL peak energies due to the residual strain were linked to the growth mode of the GaN epilayers. Photo-electrochemical (PEC) etching in aqueous solutions of KOH was applied to the GaN epilayers. ‘Whisker-like’ structures were observed on the surface of etched GaN samples by scanning electron microscope (SEM). These structures result from distribution of dislocations in GaN films. Their density was reduced from 6×10 9 cm −2 in standard GaN films to 8×10 8 cm −2 in the GaN layers grown on the selective islands.

Improvement of the optical properties of metalorganic chemical vapor deposition grown GaN on sapphire by an in situ SiN treatment

We have studied the effect of an in situ SiN treatment of sapphire substrates on the optical properties of GaN films grown by metalorganic chemical vapor deposition. The SiN deposition, partially covering the substrate, forms a mask for the formation of nanoscale auto-organized GaN islands. These islands formed upon an increase of the temperature after deposition of a GaN buffer layer on this mask. A photoluminescence study of the GaN epilayers obtained by lateral overgrowth of these islands shows significant enhancement of the luminescence emission intensity of the near band edge peaks and a reduction of the full width at half maximum of the donor bound exciton (D0X) peak by 32% down to 4 meV compared to in our standard process. The GaN films grown using SiN treatment are highly stressed as evidenced by a blueshift of 10 meV in the D0X peak energies. Photoelectrochemical etching in aqueous solution of KOH was applied to reveal the dislocation density. The density of “whisker-like” etch features, which form due to the presence of dislocations, was reduced from 6×109 cm−2 in standard GaN films to 8×108 cm−2 in the GaN layers grown with the optimized SiN treatment.

Reduction in crystallographic tilting of lateral epitaxial overgrown GaN by removal of oxide mask

The lateral overgrowth of GaN was carried out by low-pressure metalorganic chemical vapor deposition. SiO2 mask was removed just before coalescence and a subsequent lateral overgrowth was carried out to complete the fabrication of a SiO2-removed lateral epitaxial overgrown (LEO) GaN layer. The crystallographic tilting of (0002) plane, that was apparent in our standard LEO GaN layers, was absent in SiO2-removed LEO layer and x-ray diffraction measurement indicated a superior crystallinity for the SiO2-removed LEO layer. These results are attributed to the elimination of the interface between oxide mask and laterally grown GaN layer. The reduced crystallographic tilting in SiO2-removed LEO GaN layer also enhanced the quality of the coalesced fronts, as determined from cathodoluminescence images.

Epitaxial Lateral Overgrowth of GaN

Since there is no GaN bulk single crystal available, the whole technological development of GaN based devices relies on heteroepitaxy. Numerous defects are generated in the heteroepitaxy of GaN on sapphire or 6H-SiC, mainly threading dislocations (TDs). Three types of TDs are currently observed, a type (with Burgers vector 1/3〈〉); c type (with 〈0001〉) and mixed a+c (1/3〈〉). The Epitaxial Lateral Overgrowth (ELO) technology produces high quality GaN with TD densities in the mid 106 cm—2, linewidth of the low-temperature photoluminescence (PL) near-bandgap recombination peaks <1 meV and deep electron traps reduced below 1014 cm—3 (compared to mid 1015 cm—3 in standard GaN). Numerous modifications of the ELO process have been proposed in order either to avoid technological steps (mask-less ELO) or to improve it (pendeo-epitaxy). Basically developed on either sapphire or 6H-SiC, the ELO technology is also achievable on (111)Si or (111)3C-SiC/Si provided that an appropriate buffer layer is grown to avoid cracks. More sophisticated technologies have been implemented to further increase the useable part of the ELO GaN surface (two technological steps, three-step ELO). Unfortunately, in-depth understanding of the basic ELO process is still missing, i.e. of the growth anisotropy and bending of dislocations.

Epitaxial Lateral Overgrowth of GaN

Since there is no GaN bulk single crystal available, the whole technological development of GaN based devices relies on heteroepitaxy. Numerous defects are generated in the heteroepitaxy of GaN on sapphire or 6H-SiC, mainly threading dislocations (TDs). Three types of TDs are currently observed, a type (with Burgers vector 1/3〈〉); c type (with 〈0001〉) and mixed a+c (1/3〈〉). The Epitaxial Lateral Overgrowth (ELO) technology produces high quality GaN with TD densities in the mid 106 cm—2, linewidth of the low-temperature photoluminescence (PL) near-bandgap recombination peaks <1 meV and deep electron traps reduced below 1014 cm—3 (compared to mid 1015 cm—3 in standard GaN). Numerous modifications of the ELO process have been proposed in order either to avoid technological steps (mask-less ELO) or to improve it (pendeo-epitaxy). Basically developed on either sapphire or 6H-SiC, the ELO technology is also achievable on (111)Si or (111)3C-SiC/Si provided that an appropriate buffer layer is grown to avoid cracks. More sophisticated technologies have been implemented to further increase the useable part of the ELO GaN surface (two technological steps, three-step ELO). Unfortunately, in-depth understanding of the basic ELO process is still missing, i.e. of the growth anisotropy and bending of dislocations.

Study of high quality GaN grown by OMVPE using an intermediate layer

The crystalline quality of heteroepitaxial GaN can be significantly improved if the threading dislocations originating from the interface are not allowed to reach the layer surface where they can propagate to the active device areas and act as harmful defects. Incorporation of an intermediate layer grown at low temperature has been shown to limit this defect propagation. This method has been proved to be effective but several interlayers have been required in order to reach dislocation density lower than 10 9/cm 2. In this priority communication, we report on high-quality GaN layers grown with the use of only one intermediate layer. The defect analysis shows that the density of dislocation is only 8×10 7/cm 2, compared to over 10 10/cm 2 for layers grown without the intermediate layer. Electron microscopy on cross-section samples shows that deposition under certain low-temperature conditions directly benefits the quality of the subsequently deposited GaN layer. The growth of the GaN top layer appears to be similar to growth observed for lateral epitaxial overgrowth layers. This observation opens the possibility for using standard GaN growth methods to achieve a dislocation density comparable to that achieved with lateral overgrowth epitaxy.

Optimization of the GaN epilayer quality using in-situ reflectance measurements

ABSTRACTAlthough a tremendous amount of work has been done these last years on the nitride semiconductor system, a lot is still to be understood regarding the growth mechanisms of GaN. The standard GaN MOCVD growth process includes the low temperature deposition of a nucleation layer, followed by an anneal at high temperature, and the GaN layer is then deposited. The number of process parameters which can be used to tune the growth is very large (temperatures, times, thicknesses, molar flow rates and ratios …) and, due to the coupling between them, the role of each one is not clearly understood. In this paper, we present systematic series of growth experiments, where in-situ reflectance monitoring was used and correlated to ex-situ optical characterization of the samples by photoluminescence at low temperature (2K). Here, we demonstrate that the nucleation layer and its annealing have a determining effect. The nucleation layer growth temperature was not found to be a very sensitive parameter, while the amount of re-crystallization is. Surprisingly, the amount of ammonia present in the gas phase has a determining effect on the recrystallization behavior of the nucleation layer. Another interesting point is the sensitivity versus growth temperature for the main GaN layer, which was found to affect the initial stages of the growth in a drastic manner when changed by only 5°C. In-situ reflectance allowed us to tune precisely our process and to obtain GaN layers with 500 cm2/Vs electron mobility at room temperature and photoluminescence fwhm of 1.7 meV at 2K for the donor-bound exciton.

Schottky Barrier Ultraviolet Photodetectors on Epitaxial Lateral Overgrown GaN

Schottky barrier photovoltaic detectors have been fabricated on epitaxial lateral overgrown GaN layers. The devices show a responsivity of 130 mA/W, independent of optical power and diode size. Due to the lower defect concentration, an improvement of one order of magnitude in the visible rejection ratio has been observed, in comparison with Schottky barrier detectors on standard GaN layers on sapphire. The device time response is RC limited, and the low capacitance of these devices results in an increased bandwidth (<30 MHz in devices with a diameter of 200 μm). A lower noise equivalent power is also observed, due to the reduced leakage current. Detectivities of 5 × 109 Hz1/2 mW—1 were obtained in devices with a diameter of 400 μm, working at —3.4 V bias.

High UV/visible contrast photodiodes based on epitaxiallateral overgrown GaN layers

Schottky photodiodes have been fabricated on epitaxial lateral overgrown GaN layers, showing a responsivity of 130 mA/W. An improvement of one order of magnitude in the UV/visible contrast has been observed, in comparison with devices on standard GaN on sapphire. The significantly lower residual doping concentration reduces markedly the leakage current, and increases the detector bandwidth (&gt; 30 MHz in devices with a diameter phis = 200 µm). Detectivities as high as 5 × 109 Hz1/2mW–1 were obtained in photodiodes with phis = 400 µm, working at –3.4 V bias.