GaN Templates Research Articles

High-resistivity nitrogen-polar GaN for GaN/AlGaN high electron mobility transistors by metalorganic chemical vapor deposition

Nitrogen-polar (N-polar) III-nitride has natural advantages in the fabrication of electronic devices. In this work, unintentionally doped N-polar GaN films were grown on sapphire substrates by metalorganic chemical vapor deposition. By optimizing the growth parameters including growth temperature, pressure, and V/III ratio, N-polar GaN with a relatively high sheet resistivity of 3.6 × 105 Ω/sq was achieved. The secondary ion mass spectroscopy and x-ray diffraction measurement results demonstrate that the increased carbon impurity concentration is primarily responsible for the high resistivity. Moreover, N-polar GaN/Al0.25Ga0.75N heterojunctions were deposited on the high-resistivity N-polar GaN template. An extremely high-density two-dimensional electron gas of up to 1.12 × 1013 cm−2 was realized at the interface between the Al0.25Ga0.75N and the GaN. It is reasonably believed that the experimental results obtained here are beneficial for the development of high-performance N-polar GaN-based electronic devices.

Growth of highly relaxed InGaN pseudo-substrates over full 2-in. wafers

A highly relaxed InGaN buffer layer was demonstrated over a full two-inch c-plane sapphire substrate by metalorganic chemical vapor deposition. The InGaN buffer layer was grown on a 100 nm GaN decomposition stop layer with a 3 nm thick high indium composition InGaN underlayer. After thermal decomposition of the underlayer at 1000 °C, a 200 nm thick In0.04Ga0.96N buffer showed 85% biaxial relaxation measured by a high resolution x-ray diffraction reciprocal space map. When used as a pseudo-substrate for the regrowth of InGaN/InGaN multi-quantum wells, the sample showed a 75 nm red-shift in room temperature photoluminescence when compared to a co-loaded GaN template reference. The longer emission wavelength is associated with higher indium incorporation in the InGaN layers from the lessening of the compositional pulling effect caused by compressive strain. Using this technique, a simple red light emitting diode was demonstrated with an active layer growth temperature of 825 °C and a peak wavelength of 622 nm at a current density of 20 A cm−2. This work represents a unique method to relax a III-nitride based layer over a full substrate.

Influence of lattice misfit on crack formation during the epitaxy of InyAl1-yN on GaN

InyAl1−yN epifilms with alloy compositions (y) ranging from 0.13 to 0.47 were grown on a GaN template using a low-temperature molecular-beam-epitaxy technique. Because of the larger lattice constant of high-indium-content InyAl1−yN epifilm, the compressive stress accumulated during epitaxy. When the compressive stress exceeded the elastic limit of the InyAl1−yN epifilms, a “concave-valley” crack with an equivalent width (~25 nm) was generated to form a prismatic domain. Our calculation and fitting results showed that no cracking occurred when the lattice mismatch was<1% because the stress was within the elastic limit. By contrast, the cracks appeared when the lattice mismatch was>2.4% because the stress exceeded the elastic limit.

Growth of bulk single crystal ScAlMgO4 boules and GaN films on ScAlMgO4 substrates for GaN-based optical devices, high-power and high-frequency transistors

Single crystal growth of ScAlMgO4 boules with 10 mm up to 4 in. (c.a. 101.6 mm) in diameter by Czochralski technique was demonstrated. Some high quality ScAlMgO4 single crystal boules with 2-inches (c.a. 50.8 mm) in diameter were successfully grown without dislocation, which was revealed by X-ray rocking curve and X-ray topography measurements for the c-plane wafers. After a GaN template was formed on a substrate cut out from a grown ScAlMgO4 boule by using metalorganic vapor phase epitaxy, a thick GaN film was epitaxially grown on the template using hydride vapor phase epitaxy (HVPE). At the cooling stage after the growth in the HVPE process, the thick grown GaN film was separated by itself from the ScAlMgO4 substrate due to high cleavability of ScAlMgO4, which makes it possible the reuse of the substrate. The free-standing GaN grown on the reused ScAlMgO4 substrate showed the same quality as a GaN fabricated on a fresh ScAlMgO4 substrate. The reuse of ScAlMgO4 substrates is beneficial for saving the GaN films fabrication cost. The fabrication of a free-standing GaN substrate with 2 in. in diameter and 1 mm in thickness on ScAlMgO4 substrates was demonstrated to be more high-yield than the fabrication on a sapphire substrate. The growth method shown in this study is very promising, and it opens an efficient way to obtain GaN free-standing wafers and to reduce the production cost of GaN wafers.

Dispersion engineering of GaN based coupled waveguides for blue frequency combs

We propose a design platform to obtain large anomalous dispersion at short wavelengths (in vicinity of 450 nm) in vertically coupled GaN/AlN/GaN waveguides realizable on commercially available GaN template wafer technology. We present finite-difference time-domain simulation based results on dispersion engineering by tuning the waveguide geometry, to obtain anomalous dispersion as large as ps (nm-km)−1. Numerical simulations based on the Lugiato–Lefever model show efficient Kerr frequency comb generation in blue spectral range. We present results based on tailoring the geometric parameters of the waveguides to modify the dispersive wave that induces asymmetrical spectral profile in the comb on account of higher-order dispersion.

β-Ga2O3 epitaxial growth on Fe-GaN template by non-vacuum mist CVD and its application in Schottky barrier diodes

In this work, we report on demonstrating lateral β-Ga2O3 Schottky barrier diodes (SBDs) fabricated on Fe-GaN/sapphire (0001) substrates by using the non-vacuum, low-cost mist chemical vapor deposition (mist CVD) method for the first time. The x-ray diffraction scanning pattern identifies that β-Ga2O3 layers are grown with (−201) planes parallel to the (0001) plane of the GaN template, and the transmission electron microscopy shows that the β-Ga2O3 lattice is regularly and neatly arranged, indicating good crystal quality. β-Ga2O3 based SBDs with 4 and 20 µm anode–cathode lengths (LAC) exhibit the specific on-resistance (Ron,sp) of 1.58 and 39.8 Ω cm2 and breakdown voltage (Vbr) of 580 and 2400 V, respectively. The present results show the great potential of the non-vacuum and cost-effective mist CVD method as the epitaxial growth technique employed in β-Ga2O3 devices.

Fully epitaxial ferroelectric ScAlN grown by molecular beam epitaxy

We report on the demonstration of ferroelectricity in ScxAl1-xN grown by molecular beam epitaxy on GaN templates. Distinct polarization switching is unambiguously observed for ScxAl1-xN films with Sc contents in the range of 0.14–0.36. Sc0.20Al0.80N, which is nearly lattice-matched with GaN, exhibiting a coercive field of ∼ 4.2 MV/cm at 10 kHz and a remnant polarization of ∼135 μC/cm2. After electrical poling, Sc0.20Al0.80N presents a polarization retention time beyond 105 s. No obvious fatigue behavior can be found with up to 3 × 105 switching cycles. The work reported here is more than a technical achievement. The realization of ferroelectric single-crystalline III–V semiconductors by molecular beam epitaxy promises a thickness scaling into the nanometer regime and makes it possible to integrate high-performance ferroelectric functionality with well-established semiconductor platforms for a broad range of electronic, optoelectronic, and photonic device applications.

Origins of nanoscale emission inhomogeneities of high content red emitting InGaN/InGaN quantum wells

The origin of the nanoscale emission inhomogeneities of red emitting InGaN/InGaN quantum wells (QWs) grown directly on a GaN template and on an InGaN on sapphire (InGaNOS) substrate is investigated. InGaNOS is a partly relaxed InGaN pseudo-substrate fabricated by Soitec. As the latter approach provides an interesting optical internal quantum efficiency of 6.5% at 624 nm at 290 K, a deeper study, at the microstructure level, was conducted. The emission inhomogeneities on InGaNOS were highlighted by cathodoluminescence wavelength mappings where three areas were chosen: one emitting at a shorter wavelength, i.e., 588 nm, and two at a longer wavelength, i.e., 607 and 611 nm. Specimens from these zones were extracted by focused ion beam milling to perform cross-sectional characterization techniques. High-angle annular dark field scanning transmission electron microscopy images demonstrated that, while red emitting areas present homogeneous QWs, shorter wavelength areas exhibit non-uniform QWs, in terms of thickness and In composition. Complementary deformation mappings in the growth direction obtained by geometrical phase analysis show that longer emission wavelengths are originating from homogeneous QWs with an InN mole fraction evaluated at 39.0 ± 1.5%. This result demonstrates the possibility of achieving red emission with a coherent (In,Ga)N alloy when using an adapted substrate. A comparison of identical QWs grown on a GaN template is also given.

Low angle incidence microchannel epitaxy of GaN using graphene as a mask

Graphene mask was tried to use in GaN low-angle incidence microchannel epitaxy by chemical beam epitaxy. Three types of masks were prepared to study; one time and two times transferred graphene masks, which correspond to one and two-times transferred graphenes, and SiO2 mask as a reference. The amount of the lateral growth was found to increase by the use of graphene mask. This is because TMG was quickly decomposed also on the graphene mask by the penetration of the catalytic effect of the underneath GaN template. Growth islands were formed on the one time transferred graphene mask. The aligned triangular shape of the islands suggests that the crystal information of the GaN template was transmitted through the graphene mask. Namely, the growth seemed to proceed in the mode of remote epitaxy. V/III ratio dependence also indicated that the nucleation behavior of the growth islands was mainly controlled by the remote epitaxy not by the pin holes accidentally opened in the mask. On the other hands, the multi-photon photoluminescence measurement indicated that the high-quality lateral growth without dislocations was successfully grown over the graphene mask.

Development of Micron Sized Photonic Devices Based on Deep GaN Etching

In order to design and development efficient III-nitride based optoelectronic devices, technological processes require a major effort. We propose here a detailed review focussing on the etching procedure as a key step for enabling high date rate performances. In our reported research activity, dry etching of an InGaN/GaN heterogeneous structure was investigated by using an inductively coupled plasma reactive ion etching (ICP-RIE). We considered different combinations of etch mask (Ni, SiO2, resist), focussing on the optimization of the deep etching process. A GaN mesa process with an etching depth up to 6 µm was performed in Cl2/Ar-based plasmas using ICP reactors for LEDs dimen sions ranging from 5 to 150 µm². Our strategy was directed toward the mesa formation for vertical-type diode applications, where etch depths are relatively large. Etch characteristics were studied as a function of ICP parameters (RF power, chamber pressure, fixed total flow rate). Surface morphology, etch rates and sidewall profiles observed into InGaN/GaN structures were compared under different types of etching masks. For deep etching up to few microns into the GaN template, we state that a Ni or SiO2 mask is more suitable to obtain a good selectivity and vertical etch profiles. The optimized etch rate was about 200nm/min under moderate ICP conditions. We applied these conditions for the fabrication of micro/nano LEDs dedicated to LiFi applications.

Abnormal increase of 2DEG density in AlGaN/GaN HEMT grown on free-standing GaN substrate

In this study, AlGaN/GaN high-electron-mobility transistors (HEMTs) were grown on a GaN template and GaN substrate under the same growth conditions. It was observed that, in the HEMT structure grown on the GaN substrate, mobility decreased because of an increase in the two-dimensional electron-gas (2DEG) density; the origin of these redundant electrons was studied. The 2DEG density decreased with decreasing temperature, this phenomenon closely related to unintentionally induced shallow donors with ionization energy calculated to be around 67.8 meV. After Ⅴ/Ⅲ ratio regulation, the 2DEG density returned to a normal level; this combined with photoluminescence, confirmed for the first time that the abnormal increase of 2DEG density in HEMT structure grown on the GaN substrate is associated with nitrogen vacancies. Therefore, increasing the Ⅴ/Ⅲ ratio is beneficial for obtaining higher mobility by returning the 2DEG density to a normal level.

Suppression of the regrowth interface leakage current in AlGaN/GaN HEMTs by unactivated Mg doped GaN layer

The leakage current caused by the Si pileup at the regrowth interface of AlGaN/GaN high electron mobility transistors (HEMTs) is significantly suppressed by the semi-insulating Mg-doped GaN layer. Mg is unintentionally doped and can be originated from the graphite susceptor of metal organic vapor phase epitaxy. Before regrowth of the AlGaN/GaN heterostructure, the GaN template is treated with hydrochloric acid (HCl) and hydrogen peroxide/potassium hydroxide (H2O2/KOH) solution to promote the adsorption of Mg on the GaN surface. The Mg-doped GaN channel layer is highly resistive due to the passivation of hydrogen. The p–n junction formed by the weakly p-type Mg-doped GaN layer and the n-type Si pileup layer depletes the excess electrons at the regrowth interface. As a result, the off-state drain leakage current of the HEMT device can be decreased by two orders of magnitude at 40 V.

Growth of 300-nm-thick epitaxial AlInN films on a semi-relaxed c-plane GaInN template by metalorganic chemical vapor deposition

Metalorganic chemical vapor deposition of approximately 300-nm thick epitaxial AlInN films with different alloy compositions was performed using a semi-relaxed c-plane GaInN template as an underlying substrate. The GaInN template consisted of a Ga0.98In0.02N film on a facet-structured GaN film formed on a c-plane sapphire substrate by the epitaxial lateral overgrowth technique, and its surface was treated with the chemical-mechanical polishing. It was observed that an Al0.835In0.165N film grown with an in-plane tensile strain exhibited a relatively smooth surface whereas an Al0.781In0.219N film grown with an in-plane compressive strain exhibited a granular morphology owing to a columnar polycrystalline structure. This phenomenon was quite similar to that observed for AlInN films grown non GaN/sapphire templates (GaN templates); therefore, it was speculated that the microstructure variation might have been caused by the in-plane compressive strain generated in AlInN films on the GaInN template in the same way as on GaN templates or FS-GaN substrates.

GaN ultraviolet photodetector with petal-like β-Ga2O3 microcrystalline layer

A GaN ultraviolet photodetector with a petal-like β-Ga2O3 microcrystalline layer was prepared on the GaN template using electrochemical anodizing and annealing processes. The petal-like β-Ga2O3 microcrystalline layer was found to enhance the absorption of ultraviolet light and suppress the dark current, and a high responsivity from 230 nm (responsivity 8.5 A/W) to 400 nm (responsivity 0.1 A/W) was achieved by the photodetector. The rejection ratio of ultraviolet–visible light is greater than three orders of magnitude representing a high selectivity of ultraviolet light detection. The responsivity slopes of the photodetector under different biases were found to be strongly correlated with the wavelength of light, and the responsivity is much higher than that of conventional metal/insulator/metal wavelength identification photodetectors. This effective method of synthesizing β-Ga2O3 crystallites on GaN can be used to enhance the ultraviolet absorption of GaN photodetectors and improve the detection performance.

Carrier dynamics near a crack in GaN microwires with AlGaN multiple quantum wells

Relaxation of tensile strain in AlGaN heterostructures grown on a GaN template can lead to the formation of cracks. These extended defects locally degrade the crystal quality, resulting in a local increase in non-radiative recombinations. The effect of such cracks on the optical and structural properties of core–shell AlGaN/AlGaN multiple quantum wells grown on GaN microwires is comprehensively characterized by means of spectrally and time-correlated cathodoluminescence (CL). We observe that the CL blueshifts near a crack. By performing 6 × 6 k.p simulations in combination with transmission electron microscopy analysis, we ascribe this shift to the strain relaxation by the free surface near cracks. By simultaneously recording the variations of both the CL lifetime and the CL intensity across the crack, we directly assess the carrier dynamics around the defect at T = 5 K. We observe that the CL lifetime is reduced typically from 500 ps to less than 300 ps and the CL intensity increases by about 40% near the crack. The effect of the crack on the optical properties is, therefore, of two natures. First, the presence of this defect locally increases non-radiative recombinations, while at the same time, it locally improves the extraction efficiency. These findings emphasize the need for time-resolved experiments to avoid experimental artifacts related to local changes of light collection.

Role of threading dislocations and point defects in the performance of GaN-based metal-semiconductor-metal ultraviolet photodetectors

The role of threading dislocations and point defects is investigated by comparing the performance of metal-semiconductor-metal ultraviolet photodetectors (PDs) fabricated on GaN epilayers grown by hydride vapour phase epitaxy (HVPE) and metal organic vapour phase epitaxy (MOVPE) techniques. It is found that the density of threading dislocations is higher in HVPE GaN epilayers, however, the devices fabricated on them show a higher photo response, lower leakage current and faster transient response when compared to those fabricated on MOVPE GaN epilayers. It is noticed that a high density of threading dislocations of HVPE grown GaN epilayers doesn't always restrict their usefulness in the development of specific devices. On the other hand, an inferior performance of PDs fabricated on MOVPE GaN epilayers is observed despite their low dislocation density, which is explained by considering the presence of point defects. Further, the room temperature electronic transport is found to be dominated by thermionic emission (thermionic field emission) mechanism in devices fabricated on GaN epilayers grown by HVPE (MOVPE) technique. In case of HVPE based devices, a switching of dominant transport mechanism is seen at ~200 K during cooling down whereas no such behaviour is observed for MOVPE based devices. Key factors affecting the performance of ultraviolet PDs fabricated on GaN epilayers grown by the two techniques and associated charge transport mechanisms are discussed. • Crystalline quality of MOVPE GaN templates is found to be better than HVPE GaN. • HVPE GaN based MSM devices turn out to be better than those fabricated on MOVPE GaN. • A high dislocation density of HVPE GaN doesn't always restrict their usefulness. • Fundamental physical reasons for a better performance of MSM devices are discussed. • Transport mechanism switches at ~200 K for HVPE based GaN MSM devices.

III-Nitrides Resonant Cavity Photodetector Devices.

III-nitride resonant cavity-enhanced Schottky barrier photodetectors were fabricated on 2 µm thick GaN templates by radio frequency plasma-assisted molecular beam epitaxy. The optical cavity was formed by a bottom distributed Bragg reflector based on 10 periods of Al0.3Ga0.7N/GaN, an Au-based Schottky contact as top mirror, and an active zone of 40 nm-thick GaN layer. The devices were fabricated with planar geometry. To evaluate the main benefits allowed by the optical cavity, conventional Schottky photodetectors were also processed. The results revealed a planar spectral response for the conventional photodetector, unlike the resonant devices that showed two raised peaks at 330 and 358 nm with responsivities of 0.34 and 0.39 mA/W, respectively. Both values were 80 times higher than the planar response of the conventional device. These results demonstrate the strong effect of the optical cavity to achieve the desired wavelength selectivity and to enhance the optical field thanks to the light resonance into the optical cavity. The research of such a combination of nitride-based Bragg mirror and thin active layer is the kernel of the present paper.

Determination of the internal piezoelectric potentials and indium concentration in InGaN based quantum wells grown on relaxed InGaN pseudo-substrates by off-axis electron holography

Micro light emitting diodes have been grown by metal organic vapor phase epitaxy on standard GaN and partly relaxed InGaNOS substrates with the purpose of incorporating higher concentrations of indium for identical growth conditions. Green emission has been demonstrated at wavelengths of 500 nm for the GaN template and 525 and 549 nm for the InGaNOS substrates, respectively. The structure, deformation, indium concentration and piezoelectric potentials have been measured with nm-scale spatial resolution in the same specimens by transmission electron microscopy. We show by off-axis electron holography that the piezoelectric potential and information about the indium concentration from the mean inner potential are obtained simultaneously. By separating the components using a model, we show that for higher concentrations of indium in the quantum wells (QWs) grown on InGaNOS substrates, the piezoelectric potentials are reduced. The measurements of the indium concentrations by electron holography have been verified by combining energy dispersive x-ray spectrometry, x-ray diffraction and from the tensile deformation made by precession electron diffraction. A discussion of the limitations of these advanced aberration-corrected transmission electron microscopy techniques when applied to nm-scale QW structures is given.

Investigations on epitaxy and lattice distortion of sputter deposited β-Ga2O3 layers on GaN templates

The lattice distortion and epitaxial nature of β-Ga2O3 layers deposited on GaN templates using radio-frequency magnetron sputtering have been evaluated. The determined values of out-of-plane (tensile type) and in-plane (compressive type) strain are found to decrease with increase in deposition temperature, indicating better relaxation of the unit cell lattice at higher temperatures. The obtained value of angle β is invariably higher than its bulk value, suggesting a distortion in the unit cell of β-Ga2O3 layer. Columnar-type growth for β-Ga2O3 layer on GaN template has been observed. Analysis of its epitaxial nature reveals that six domains of grown layer are in-plane rotated by 60° ± δo (δ = 2°–3°) with each other, which is explained by the presence of three non-equivalent oxygen atoms on the (−201) plane of β-Ga2O3. The calculated values of δ nearly match experimentally observed values. However, the smaller difference between the two is related to residual strain in the layer. The out-of-plane and in-plane epitaxial relationship for β-Ga2O3 layer with respect to GaN are (−201)Ga2O3 || (0001)GaN and (010)Ga2O3 || (11–20)GaN, respectively.

560 nm InGaN micro-LEDs on low-defect-density and scalable (20-21) semipolar GaN on patterned sapphire substrates.

We demonstrate InGaN-based semipolar 560 nm micro-light-emitting diodes with 2.5% EQE on high-quality and low-defect-density (20-21) GaN templates grown on scalable and low-cost sapphire substrates. Through transmission electron microscopy observations, we discuss how the management of misfit dislocations and their confinement in areas away from the active light-emitting region is necessary for improving device performance. We also discuss how the patterning of semipolar GaN on sapphire influences material properties in terms of surface roughness and undesired faceting in addition to indium segregation at the proximity of defected areas.