AlN Substrates Research Articles

Impact of metalorganic vapor phase epitaxy growth conditions on compressive strain relaxation in polar III-nitride heterostructures

A novel approach to estimating the critical thicknesses (CTs) of compressively strained III-nitride layers is suggested, based on a quasi-thermodynamic growth model and accounted for the effect of material decomposition during dislocation half-loop formation on the CT value. The approach provides good quantitative agreement with available data on CTs of MOVPE-grown InGaN/GaN and AlGaN/AlN epilayers. The extremely large CTs observed for high Al-content AlGaN alloys grown on bulk AlN substrates may be attributed, in particular, to the dominant AlGaN decomposition mechanism, producing group-III metallic liquid and gaseous nitrogen. The suggested approach may also be helpful for analysis of threading dislocation inclination in compressively strained layers and applicable to studying point defect formation in semiconductors and its dependence on growth conditions.

Theoretical investigations on the growth mode of GaN thin films on an AlN(0001) substrate

The growth modes of GaN thin films on an AlN(0001) substrate are systematically investigated using our macroscopic theory with the aid of empirical interatomic potential and ab initio calculations. Using empirical interatomic potential calculations, we demonstrate that a GaN/AlN(0001) system with misfit dislocation (MD) is stabilized compared with a coherently grown system when the GaN film thickness exceeds six monolayers. According to the calculated results including the surface energy of GaN, the macroscopic free energy calculations for the growth mode imply that the growth of GaN on AlN(0001) proceeds along the lower-energy path from two-dimensional coherent (2D-coherent) to 2D-MD under Ga-rich conditions but from 2D-coherent to three-dimensional coherent with truncated hexagonal pyramid islands consisting of {} facets under N-rich conditions owing to surface energy anisotropy, which depends strongly on the growth conditions. These results suggest that surface energy anisotropy is crucial for the growth of GaN on an AlN(0001) substrate.

Study on thermal field coupling simulation of methane sensor based on ceramic micro-hot plate

In view of the lack of high concentration online gas detection sensors in the current mine safety monitoring system to cope with gas emission and gas outburst, a design method of full-range gas sensor based on ceramic micro-hot plate is presented in this paper. COMSOL finite element simulation is used to study the thermal field coupling of sensor array units. Based on the electro-thermal coupling model, the thermal field was est ablished, and the influence of isolation groove, base material, working temperature and platinum film thickness on the thermal interference between the sensor units was studied. The results show that the heat loss is reduced by 2.57 times and the heating efficiency is improved. Compared with AlN and SiC ceramic substrates, Al2O3 has the least thermal disturbance and the best thermal performance. The average working temperature at 1.3v reaches 587.77k. The temperature difference between the units is directly proportional to the unit operating temperature. Increasing the thickness of platinum film can improve the working temperature and heating efficiency. The research provides a t heoretical basis for the design of the integrated MEMS array gas sensor.

Self-oriented growth of (020) MgSiO3-orthopyroxene and (002) α-Mg2SiO4 films using metal-organic chemical vapor deposition

Abstract We demonstrated metal-organic chemical vapor deposition of single-phase MgSiO3-orthopyroxene (-opx) and α-Mg2SiO4 films, and α-Mg2SiO4–MgO composite films with self-oriented growth on a polycrystalline AlN substrate using Mg acetylacetonate and tetraethyl orthosilicate precursors. We obtained (020)-oriented MgSiO3-opx films, (002)-oriented α-Mg2SiO4 films, and (021)-oriented α-Mg2SiO4–(200) MgO composite films at 22, 46, and 81 mol%MgO, respectively, at a deposition temperature of 1200 K and a total chamber pressure of 0.8 kPa. Maximum deposition rates of 42 and 131 μm h−1 were attained for the (020)-oriented MgSiO3-opx and (002)-oriented α-Mg2SiO4 films, respectively.

Highly radiative nature of ultra-thin c-plane Al-rich AlGaN/AlN quantum wells for deep ultraviolet emitters

Temperature-dependent photoluminescence (PL) lifetimes were measured for a series of ultra-thin c-plane Al0.61Ga0.39N/AlN multiple quantum wells (QWs) on bulk AlN substrates with the well thickness varying from 0.6 to 2 nm. At temperatures below 75 K, estimates of the internal quantum efficiency indicate that the recombination is primarily radiative, with a lifetime of ∼160 ps for the 0.6 nm QWs, comparable to the low temperature PL lifetime observed in bulk AlGaN films of a similar Al content. This short lifetime is observed despite the presence of layer thickness fluctuations and the quantum-confined Stark effect associated with the large polarization field in the heterostructures, which tend to increase the radiative lifetime. This behavior is explained using many-body calculations of radiative recombination rates that extend beyond the conventional ABC rate equation model by accounting for both excitons and free carriers within a nonequilibrium Green's function formalism. The results indicate that the combination of the large wave function overlap integral (∼0.65) and exciton binding energy (1.82 times the 3D Rydberg) for the 0.6 nm QWs leads to an ∼20-fold increase in the radiative recombination rate relative to that obtained for the 2 nm QWs. This greater radiative recombination rate competes favorably with trapping at interface fluctuations and defect-induced nonradiative recombination that dominates recombination at higher temperatures.

Strain study of epitaxial Al1−xGaxN based on first-principles theory

The mechanical properties of Al1−xGaxN epitaxial layers grown on GaN and AlN substrates were studied by using density functional theory and anharmonic elastic calculation method. Accurate theoretical calculation of nonlinear elastic responses is a basic and important scientific question, which is the key to understand and further explore the potential synthesis and design of new solids. In this paper, detailed numerical calculations were carried out for the nonlinear mechanical properties of würtzite GaN and AlN compounds. The variation of strain energy and stress of disordered Al1−xGaxN epitaxial layer with gallium composition was analyzed. We found that in the epitaxial structure of AlN substrate, with the increase of gallium content, the influence of nonlinear effect on internal stress reached 29.4%, which proved the great influence of nonlinear mechanical response on semiconductor alloy material and heterostructure.

Monolithic integrated 4[formula omitted]25 Gb/s transmitter optical subassembly at 1.55 [formula omitted

We report a monolithic integrated 4×25 Gb/s transmitter optical subassembly (TOSA) module at 1.55 μm. The module consists of 4 directly modulated lasers (DMLs) and an arrayed waveguide grating (AWG) with 400 GHz channel spacing. The DMLs and AWG are highly integrated on a 4 mm×6 mm InP chip which is mounted on an AlN substrate and butterfly packaged. The characteristic of the TOSA module is measured in terms of optical spectra, crosstalk and small-signal frequency response. Data transmission propagating over 3.1 km single mode fiber (SMF) at 25 Gb/s with non-return to zero on–off keying (NRZ-OOK) modulation is explored experimentally. The results show an excellent uniformity of the 4 DMLs, which all reaches 25 Gb/s. The transmission rate up to 100 Gb/s for back-to-back (BTB) and 3.1 km fiber transmission with an error rate of 10−9 is reached. Of great importance, the packaged 4×25 Gb/s TOSA module meets the ITU standards very well, showing a promising extension on more channels and higher transmission speed

The role of transient surface morphology on composition control in AlGaN layers and wells

The mechanisms governing “compositional pulling” during the growth of AlxGa1−xN wells are investigated. Gallium-rich AlxGa1−xN wells grown on high dislocation density AlN/sapphire templates exhibit asymmetric and diffuse composition profiles, while those grown on low dislocation density native AlN substrates do not. Furthermore, strain in all AlxGa1−xN wells is found to be pseudomorphic, ruling it out as the dominating driving force. Rather, the high threading dislocation density of the AlN template is considered to play the defining role. We propose that a transient surface morphology is introduced during dislocation mediated spiral growth, which, in conjunction with process supersaturation, determines the Ga incorporation. These findings provide insights into compositional pulling in high Ga content AlxGa1−xN grown on AlN and provide a route to grow thicker wells with very abrupt interfaces on native AlN substrates.

Theoretical study of optical properties of non-polar BAlGaN/AlN quantum wells lattice-matched to AlN

Light emission characteristics of non-polar ultraviolet (UV) BxAlyGa1−x−yN/AlN quantum well (QW) structures lattice-matched to AlN substrate were theoretically investigated using the multiband effective-mass theory. The peak intensity of the non-polar BAlGaN/AlN QW structure lattice-matched to AlN is found to be much larger than that of the c-plane BAlGaN/BAlGaN QW structure lattice-matched to AlN. Also, the lattice-matched BAlGaN/AlN QW structure shows a large polarization ratio and its value changes from 0.7 to 0.9 in an investigated range of carrier density. We expect that non-polar BAlGaN/BAlGaN QW structures lattice-matched to AlN could be used as a UV light source with a higher crystal quality.

Structural characteristics of m-plane AlN substrates and homoepitaxial films

Homoepitaxial non-polar AlN films were realized on m-plane (101-0)-oriented AlN single crystals by metalorganic chemical vapor deposition (MOCVD). The microstructural properties of m-plane AlN substrates and homoepitaxial films were assessed by means of atomic force microscopy and high resolution x-ray diffraction characterization. Results indicated that both m-plane AlN substrates and films possessed exceptional structural quality, with some anisotropic mosaic distributions due to the quasi-bulk nature of the non-polar single crystals. An increase in the MOCVD growth temperature was noted to minimize the degree of inherited mosaic anisotropy without altering the m-plane AlN film growth rate, indicating that high temperature growth is critical to produce optimal film crystallinity. A dramatic change in the film surface morphology from heavily faceted “slate-like” features to monolayer steps was observed as the growth temperature was increased. The “slate-like” surface morphology produced low intensity cross-streaks in symmetric (101-0) reciprocal space maps, tilted about 18° away from the (101-0) crystal truncation rod. The orientation of these diffuse streaks corresponds to the physical alignment of the slates with respect to the substrate surface normal. X-ray line scans and defect-selective reciprocal space mapping confirmed that these low intensity streaks are solely dependent on this peculiar surface structure produced at low MOCVD growth temperatures and unrelated to basal plane stacking faults or other extended defects. All observations confirm that high quality III-nitride epitaxial structures on m-plane AlN substrates are attainable with controllable MOCVD growth processes, as demanded for future high performing AlN-based non-polar devices.

AlxGa1−xN‐Based Quantum Wells Fabricated on Macrosteps Effectively Suppressing Nonradiative Recombination

AbstractAlxGa1−xN‐based quantum wells (QWs) are fabricated on AlN with macrosteps, which are formed by using vicinal sapphire and AlN (0001) substrates. The QWs on macrosteps (MS‐QWs) show photoluminescence lifetimes of nearly 2 ns at an emission wavelength of 242 nm at room temperature (RT). This is the longest lifetime so far reported for AlxGa1−xN‐based QWs emitting below 270 nm, indicating the suppression of nonradiative recombination, the dominant recombination process at RT. Compared with planar QWs without macrosteps, the emission internal quantum efficiency estimated by photoluminescence spectroscopy is improved approximately by two orders of magnitude under a weak excitation condition. Consequently, AlxGa1−xN‐based MS‐QWs are promising structures for highly efficient ultraviolet emitters.

Crystal defect analysis in AlN layers grown by MOVPE on bulk AlN

MOVPE growth of AlN layers on bulk AlN substrates with low threading dislocation density (<105 cm−2) can result in enhanced defect formation. Chemo-mechanical polishing (CMP) of bulk AlN using colloidal silica sol can produce locally disturbed surface with SiOx residuals. These surface disturbances lead to generation of threading dislocations (∼108 cm−2) in homoepitaxially grown AlN layers. These dislocations show a tendency to form nano- and even micropipes, which may be associated with oxygen accumulation along the dislocation lines. As result, the subsequently grown AlGaN-based layer structures exhibit a high number of v-pits and micropipes. Inductively coupled plasma etching of AlN substrate surface prior to MOVPE growth results in clean AlN surfaces and improves the AlGaN layer quality.

Investigation towards the optimum of power capability, ageing stability and costs effectiveness on thick film resistor pastes for AlN ceramics

Abstract The focus of this work was the optimization of a 10 Ω/□ thick film resistor (TFR) paste composition to obtain increased power capability, aging stability and minimum use of ruthenium oxide for cost savings without changing the defined narrow sheet resistance (R□) and temperature coefficient of resistance (TCR) specifications. In times of highly fluctuating precious metal costs, the use of a minimum of the precious metal ruthenium respectively ruthenium dioxide is one essential part for cost-effectiveness. The thick film paste formulation consists of the electrically conducting phase ruthenium dioxide, a lead-free glass phase and two inorganic additives for tuning thermo-mechanical and electrical properties of the formed films. A phthalate free organic vehicle with ethyl cellulose polymer was used to formulate a screen printable ceramic thick film paste. For this paper, RuO2 powders with various specific surface area values (BET) were prepared by thermal annealing of a precipitated fine ruthenium dioxide powder. All other solid and liquid components of the paste were the same as used for IKTS 10 Ω/□ TFR paste FK9611 for AlN substrates. Furthermore, the content of ruthenium dioxide in the paste compositions was changed systematically around an assumed target content to achieve the desired sheet resistivity. Concurrent to the variation of the ruthenium dioxide content the inorganic additives had to be adapted too. The influence of the variations of raw material and paste composition on the film properties were investigated by screen printing 24 resistors of 2 mm × 1 mm dimension on an 1” × 1” AlN substrate, firing at 850 °C for 10 minutes in air atmosphere and subsequently measuring R□, TCR, the stability of resistance ΔR/R0 effected by artificial aging of the resistors (stored 100 up to 1000 hours @ 200°C) and the maximum rated power dissipation (MRPD) as well as short term overload voltage (STOL). The results are discussed in regard to find an optimum between all demands of the most important electrical film properties.

High thermal conductive AlN substrate for heat dissipation in high-power LEDs

Heat dissipation is important in high-power LEDs and depends on the thermal conductivity of the substrate. This work investigates the heat dissipation performance of AlN ceramic substrate in high-power LEDs. AlN substrate with a thermal conductivity of 193 W m−1 K−1 and flexural strength of 380 MPa has been prepared by pressureless sintering, and then bonded with a Cu film by direct plating copper (DPC). By using the AlN substrate for heat dissipation, the junction temperature (78 °C) is lowered by 42 °C compared with the case using Al2O3 substrate and well below the upper limit of the operation temperature of the LEDs. From the experimental results, AlN ceramic substrate with a high thermal conductivity is a promising candidate for heat dissipation in high-power LEDs.

Empirical pseudo potential method for the energy gaps and optical properties of XN (X = Al, Ga, B) and their alloys materials Journal of Physics: Conference Series

The pseudo potential method associated with the improved virtual crystal approximation (VCA) is used to calculate symmetric and anti-symmetric form factors for BN, AlN, GaN and to predict the electronic band structure of the zinc-blende BxAlYGa1-x-yN alloys lattice matched to AlN. The optical properties of these alloys are calculated. The range of compositions for which the alloys are lattice-matched to AlN substrate is determined. We find that the addition of boron to the AlGaN alloys decreases the direct gap (EΓΓ) and a direct to indirect transition occurs at a Al concentration of about 0.5 (y = 0.5), which corresponds to a band gap energy of about 4.75 eV.

6 kW/cm2 UVC laser threshold in optically pumped lasers achieved by controlling point defect formation

Optically pumped lasing from AlGaN/AlN multiple quantum wells grown on single-crystalline AlN substrates with lasing thresholds as low as 6 kW/cm2 is demonstrated via the reduction of unintentional point defects in the active region and waveguide, which reduces the non-radiative recombination by 2 orders of magnitude. A higher lasing threshold of 11 kW/cm2 is observed for AlGaN barriers, owing to the reduced localization of electrons and holes in the wells. It is shown that for electrically injected UVC laser diodes, AlGaN barriers are essential.

An ab initio approach to polarity inversion of AlN and GaN films on AlN substrate with Al overlayers: an insight from interface energies

The stability of group-III nitrides on a N-polar AlN substrate with Al overlayers is theoretically investigated using an ab initio approach that yields the energy for individual polar interfaces. We find that the trends in the interface energy depend on the number of Al overlayers m. The interface energies of Al- (Ga-) polar AlN (GaN) with m = 2 and 4 are lower than those of N-polar AlN (GaN). The calculated results suggest that the formation of stable bonds between the topmost layer of Al overlayers and the bottom layer of group-III nitrides is crucial for the polarity inversion on a N-polar AlN substrate.

(Invited) Material Advances Toward Vertically-Conducting AlGaN Power Transistors

Due to their large bandgaps (≥ 3.4 eV) compared to Si and SiC, AlGaN alloys have superior material properties that are appealing for the next generation of high-voltage power devices. Most notable is that the critical electrical field scales as the bandgap to the 2.0-2.5 power. Thus, diodes employing wide bandgap semiconductors are expected to operate at higher breakdown voltages with thinner and more heavily-doped drift regions, leading to lower resistive losses compared to Si-based devices. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer design options not possible for devices based on materials such as SiC and diamond. Noteworthy demonstrations of high voltage (> 3 kV) PN and Schottky barrier diodes in both GaN and AlGaN alloys confirm the potential of III-Nitride materials for power devices. However, full exploitation of the efficiency gains promised by III-Nitrides for power management systems also requires vertically-conducting transistors for current switching, in addition to diodes. Analogous to power diodes, vertically-conducting power transistors such as D-MOSFETs employ a thick n-type drift layer to stand-off high reverse voltage, but unlike simple PN diodes, have the p-type material limited to spatially defined regions over the surface of the device. “Selective area p-doping” is typically accomplished in SiC and Si devices by implantation and activation of p-type dopants. However, implantation processing in III-Nitride structures has to date only yielded PN junctions that exhibit significantly higher reverse current leakage than continuously-grown PN junctions. Selective-area epitaxial regrowth of p-type material in regions etched into the drift layer offers an alternative to implantation. However, regrown pn junctions on plasma-etched III-Nitrides with low reverse leakage current have not been demonstrated. In this presentation we will discuss the advances in epitaxial p-regrowth for forming pn junctions on inductively-coupled plasma (ICP) etched n-type GaN and Al0.3Ga0.7N drift layers. The role and sources of unintentionally-introduced Si at the regrown interface will be presented. Additionally, approaches to ameliorate damage and defect states introduced by ICP etching through wet-chemical and in-situ reactor thermal treatments prior to p-regrowth will be related to current-voltage characteristics of regrown diodes. While high voltage GaN-based diodes have been fabricated on vertically-conducting GaN substrates, AlGaN-based high voltage diodes with thick drift layers (≥ 5 μm) have been fabricated on AlN and sapphire substrates. The insulating nature of these substrates necessitates planar device geometries that result in high ohmic losses, heating, and degraded device performance. However, the thick drift layer and the smaller lattice constant of AlGaN alloys present significant challenges for realizing vertically-conducting AlGaN power devices on GaN substrates. Here we report a method for fabricating vertically-conducting AlxGa1-xN PN junctions grown on conducting GaN substrates. An n-type GaN substrate is patterned with parallel, sub-micron-wide mesas separated by trenches etched > 0.5 μm deep prior to AlxGa1-xN overgrowth using metal-organic vapor phase epitaxy. We find that growth temperature, V/III ratio, and group-III molar flux can be used to manipulate the crystalline facets formed along the etched pattern prior to AlxGa1-xN coalescence. Using the described process, coalesced planar AlxGa1-xN epitaxial layers were achieved with less than 6 μm of overgrowth for Al mole fractions up to 0.45, which is the highest Al mole fraction reported to date. Cathodoluminescence measurements of an overgrown Al0.3Ga0.7N epilayer indicate that the density of threading dislocations is mid x 108 cm-2 which is similar to that measured for Al0.3Ga0.7N epilayers overgrown on patterned Al0.3Ga0.7N on sapphire. The current-voltage characteristics of vertically-conducting AlGaN PN diodes grown on GaN substrates will be presented. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program managed by Dr. Isik Kizilyalli. This work was also supported, in part, by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525

Effect of Substrate Surface on Deposition of AlGaN: A Molecular Dynamics Simulation

The growth of AlGaN has been extensively studied, but corresponding research related to the effect of AlN substrate surface has rarely been reported in literature. In this article, the effects of AlN substrate surface on deposition of AlGaN films were investigated by molecular dynamics (MD) simulations. (0001) Al-terminated and (0001¯) N-terminated AlN were considered as substrates. The quality of surface morphology and atomic scale structure of deposited AlGaN film are discussed in detail. The results show that the surface morphology and crystal quality of AlGaN film grown on (0001) Al-terminated AlN surface are better than for that grown on (0001¯) N-terminated AlN surface under various growing temperatures and Al/Ga injection ratios between Al and Ga. This can be attributed to the higher mobility of Al and Ga adatoms on the (0001) Al-terminated AlN surface. These findings can provide guidance for the preparation of high-quality AlGaN thin films on AlN substrate.

Comparative Thermal and Structural Characterization of Sintered Nano-Silver and High-Lead Solder Die Attachments During Power Cycling

13.5 mm × 13.5 mm sintered nano-silver attachments for power devices onto AlN substrates were prepared at 250 °C and a pressure of 10 MPa for 5 min and compared with Pb5Sn solder joint die attachments under constant current power cycling with an initial temperature swing of 50 °C-175 °C. Both effective thermal resistance and microstructural evolution of the samples were monitored using transient thermal impedance measurement and nondestructive X-ray computed tomography at regular power cycling intervals. The results showed a gradual increase in the effective thermal resistance of the Pb5Sn solder joints from 0.047 to 0.133 K/W from zero to 41k power cycles, followed by a rapid escalation to 0.5018 K/W at 52k cycles. This was accompanied with the formation and development of oblique cracks within the Pb5Sn die attachments until delamination occurred at the solder/device and solder/substrate interfaces. By contrast, the effective thermal resistance of the sintered Ag joints remained almost constant at 0.040 K/W up to 116k power cycles. This was explained in terms of thermally induced continuation of densification of the sintered structure and the formation and development of networked vertical cracks within the sintered Ag die attachments, some of which further extended into the Cu tracks of the AlN substrate.