Digital Alloys Research Articles

Monolayer-scale AlN/GaN digital alloys grown by plasma-assisted molecular beam epitaxy

The efficiency of usual AlGaN based deep ultraviolet light-emitting devices is still quite low. The difficulties are basically originated from the fundamental material properties of AlGaN. This work has adopted monolayer-scale (AlN)m/(GaN)n ordered digital alloys (DAs) as alternatives to AlGaN random alloys, m and n are the numbers of monolayers. X-ray diffraction scans have demonstrated clear satellite peaks, verifying good periodicity of AlN/GaN DAs grown by molecular beam epitaxy (MBE), and transmission electron microscopy results have revealed atomically sharp and smooth interfaces and quite precise m:n values agreeing well with designs. The electron densities of Si-doped (AlN)m/(GaN)n DAs with high equivalent Al compositions are significantly higher than those of conventional AlGaN:Si random alloys grown in the same MBE system. Si dopant ionization energies in DAs are only 2–5 meV, much lower than that for usual random alloys. The red shift of the light emission for DAs with thinner AlN barriers has suggested strong coupling between the GaN wells and thus formation of a miniband in a vertical direction. The results have demonstrated the potential of the (AlN)m/(GaN)n DAs as electronically functional alternatives for various device applications.

(Invited) Understanding Plasma-Assisted Atomic Layer Epitaxy of InN and Opportunities for Collaboration through the Office of Naval Research Global

The family of III-V nitride compound semiconductors has provided the foundation to a number of advanced electronic device technologies from light emitting diodes to radio frequency transistors to power switches. Of the family, binary indium nitride (InN) and ternaries/quaternaries containing InN have progressed at a slower pace due the material properties and related growth challenges. Having the weakest bonds in the family, the material has a lower melting point and more easily decomposes and phase separates (in ternaries and quaternaries). As such, a number of research efforts are looking for new growth approaches to overcome these challenges and realize the InN-based materials that will enable devices such has high speed switches and IR emitters and detectors.To this end, plasma-assisted atomic layer epitaxy (PA-ALE) of InN and InN-containing ternaries has been explored in recent years. Much like atomic layer deposition, ALE consists of a sequence of self-limiting surface half-reactions that promotes “layer-by-layer” growth of films. The cyclical growth process starts with exposure of the growth surface first to the group III precursor (trimethylindium, for example), then purging excess precursor and reaction byproducts leaving only the ad-/physi-/chemi-sorbed species on the surface. After this purge, these surfaces are then exposed to a collection of plasma species (ions, radicals, neutrals, etc.) which reacts with the “sorbed” layer. The growth cycle is finished with a purge of the surface reaction by-products and unreacted plasma species before repeating. The potential of the process is based on the premise that by providing energy from plasma species to the growth surface, one can lower the temperature of growth to improve the stability of the material. Further, the reactive neutral plasma species also play a role by lowering activation energies for chemical reactions compared to thermal ALD, without necessarily providing energy to the growth surface.A brief review of previous experimental demonstrations of epitaxial growth of InN and InN-containing ternaries will be presented showing the promise of the growth method to grow quality InN binary films and metastable ternaries containing InN not possible by conventional growth methods. For example, a metastable cubic phase of InN (NaCl) was realized as were ternary stoichiometries for both InAlN and InGaN that lie in the thermodynamic miscibility gap. Further, and in an effort to begin to understand this growth process and the role that plasma might play in growth mechanisms, synchrotron-based studies have been employed during growth of binary InN and digital InAlN and InGaN alloys. The results of these studies will be highlighted and key findings identified. Most recently, coherent x-ray scattering has been employed to provide additional insights including surface diffusion during each step of the cyclical growth process. The cumulative findings of these efforts will be presented, and future directions outlined.As the presenting author is now detailed as a Science Director for Power & Energy and Advanced Materials to the U.S. Office of Naval Research Global in London, the last portion of the talk will be dedicated to introducing ONRG and its mission to engage international researchers through research grants, travel grants and conference support. The hope is to increase awareness of collaborative opportunities to the global research community expected to be in attendance.

Fine modulation of the energy band strategy to control the carrier confinement capability of digital alloys

In this paper, a strategy to finely modulate the energy band structure to control the carrier confinement capability of digital alloys (DA) is proposed. Strain analysis shows that As and Sb atoms are exchanged within the AlAsSb DA. The bottom of the corrected potential well is low on the left and high on the right in the growth direction, resulting in a higher band offset of the AlSb potential barrier layer on the left side of the potential well than on the right side. The modulation of the band leads to a higher probability of electron tunneling in DA under the action of an electric field opposite to the growth direction. Conversely, it is difficult for the electrons to tunnel into the lower energy level potential wells. The I–V curve of DA shows that the current value under positive bias is significantly smaller than the value under negative bias when the voltage is higher. The measured results correspond perfectly with the modified energy band model, which verifies the feasibility of energy band modulation. This is important for the structural design of DA and the reduction of dark current in optoelectronic devices.

Device-Level Impact of Highly Anisotropic Thermal Conductivity of AlN/GaN Digital Alloys

AlN/GaN digital alloys (i.e., short period superlattices) have the advantage of being an ultra-wide bandgap due to the quantum confinement effect, preservation of high mobility (no strong alloy scattering), and higher thermal conductivity than AlGaN random alloys.Herein, we investigate the influence of AlN/GaN thickness combinations on digital alloy (DA) thermal conductivity. Using time-domain thermoreflectance and steady-state thermoreflectance, we measure the anisotropic thermal conductivity of AlN/GaN DA’s, with thickness combinations: (2 nm/1 nm), (4 nm/1 nm), (6 nm/1 nm), (11 nm/1 nm). DA's with thick AlN layers exhibit highly anisotropic thermal conductivity (i.e., 4x difference between cross-plane and in-plane).Using finite-element modeling, we thermally simulate AlGaN and AlN/GaN DA devices. Improved lateral heat spreading in DA devices results in lower hot-spot temperatures compared to AlGaN devices. As the first thermal measurements of AlN/GaN DA’s, this study provides guidance for future device design.

Ultrathin GaN film and AlGaN/GaN heterostructure grown on thick AlN buffer by MOCVD

High-quality AlGaN/GaN/AlN heterostructures with thin GaN channel and thick AlN buffer layer were grown by metal organic chemical vapor deposition (MOCVD) on SiC substrate. By analyzing growth modes of GaN films on AlN buffer layers with different thicknesses, it is revealed that film-forming point of GaN grown on AlN buffer increases with the increase in AlN buffer thickness. Accordingly, new growth model of GaN on AlN buffer was proposed, which shows that there is an optimal matching value between Ga source flow rate and AlN thickness when GaN is grown on AlN buffer of different thicknesses. Under optimal conditions, AlGaN/GaN/AlN heterostructures with 120 nm thin GaN channel layer and thick AlN buffer show excellent carrier-limited domain, high crystalline quality, and good transport properties. Results in this work would be useful for preparing high-quality heterostructures on AlN buffer and high electron mobility transistor (HEMT) devices. Moreover, these findings can also be applied to the growth of other hyperfine structures (quantum wells, superlattices, and digital alloys) in the future.

Increasing the mobility and power-electronics figure of merit of AlGaN with atomically thin AlN/GaN digital-alloy superlattices

Alloy scattering in random AlGaN alloys drastically reduces the electron mobility and, therefore, the power-electronics figure of merit. As a result, Al compositions greater than 75% are required to obtain even a twofold increase in the Baliga figure of merit compared to GaN. However, beyond approximately 80% Al composition, donors in AlGaN undergo the DX transition, which makes impurity doping increasingly more difficult. Moreover, the contact resistance increases exponentially with the increase in Al content, and integration with dielectrics becomes difficult due to the upward shift of the conduction band. Atomically thin superlattices of AlN and GaN, also known as digital alloys, are known to grow experimentally under appropriate growth conditions. These chemically ordered nanostructures could offer significantly enhanced figure of merit compared to their random alloy counterparts due to the absence of alloy scattering, as well as better integration with contact metals and dielectrics. In this work, we investigate the electronic structure and phonon-limited electron mobility of atomically thin AlN/GaN digital-alloy superlattices using first-principles calculations based on density-functional and many-body perturbation theory. The bandgap of the atomically thin superlattices reaches 4.8 eV, and the in-plane (out-of-plane) mobility is 369 (452) cm2 V−1 s−1. Using the modified Baliga figure of merit that accounts for the dopant ionization energy, we demonstrate that atomically thin AlN/GaN superlattices with a monolayer sublattice periodicity have the highest modified Baliga figure of merit among several technologically relevant ultra-wide bandgap materials, including random AlGaN, β-Ga2O3, cBN, and diamond.

Biaxial strain modulated valence-band engineering in III-V digital alloys

Some III-V digital alloy avalanche photodiodes exhibit low excess noise. These alloys have low hole ionization coefficients due to presence of small 'minigaps', enhanced effective mass and large separation between light-hole and split-off bands in the valence band. In this letter, an explanation for the formation of the minigaps using a tight binding picture is provided. Furthermore, we demonstrate that decreasing substrate lattice constant can increase the minigap size and mass in the transport direction. This leads to reduced quantum tunneling and phonon scattering of the holes. Finally, we illustrate the band structure modification with substrate lattice constant for other III-V digital alloys.

Digital alloy contact layers for perovskite solar cells

Thin film optoelectronic devices commonly require an individual material to perform multiple functions, but the physical properties of single-composition materials often cannot be tuned to optimize these diverse requirements. The electron selective contact layer in perovskite solar cells is a prime example. The material must simultaneously have optimal conduction band alignment, facilitate carrier extraction, prevent recombination, and provide a chemically stable interface with the notoriously volatile perovskite semiconductor. The pulse-by-pulse nature of the thin-film deposition method pulsed laser deposition (PLD) provides an opportunity to form material alloys where the chemical composition is controlled at the nanometer scale. These digital alloys may prove to be a powerful materials class to meet some of the multifunctional needs of thin film devices. Using PLD to make electron transport layers from ZnO and MgZnO targets, we demonstrate that digital alloy gradients can be tuned to significantly outperform either of the parent materials in perovskite solar cells.

Comparison of Different Period Digital Alloy Al${}_{\text{0.7}}$InAsSb Avalanche Photodiodes

We report Al0.7InAsSb avalanche photodiodes grow as ternary–binary and binary–binary digital alloys. Their characteristics of ideality factor, activation energy, temperature-dependent excess noise, temperature stability, and impact ionization coefficients are compared.

Tuning of energy dispersion properties in InAlAs digital alloys

InAlAs digital alloy avalanche photodiodes (APDs) exhibit lower noise than their random alloy counterparts. The electronic dispersion properties of digital alloy materials are unique, creating minigaps in their valence band structure. In this paper, we use computational models with environment-dependent tight-binding parameters to calculate the electronic dispersion properties of InAlAs digital alloys with various stacking directions, stacking modes, and periods, which can provide guidance for optimizing the structure of InAlAs digital alloy APDs.

2.1 μm InGaSb quantum well lasers exhibiting the maximum conversion efficiency of 27.5% with digitally grown AlGaAsSb barriers and gradient layers

Abstract The 2.1-μm-wavelength InGaSb/AlGaAsSb/GaSb double quantum well lasers, exhibiting a high performance with the peak output power of 1.62 W and the maximum conversion efficiency of 27.5%, had been grown by molecular-beam-epitaxy (MBE). Digitally grown AlxGa1-xAsSb barriers and grading layers were employed in the devices to increase the valence-band offset and to improve the simplicity and controllability of MBE growth. The quaternary digital alloys were grown with short-period superlattices (SPS) of AlSb, AlAs and GaSb and varieties of growth conditions such as growth temperature, interface control and growth rate were considered to get the high-quality crystal films. The digitally grown film was parametrically characterized by atomic force microscopy, high resolution X-ray and transmission electron microscope, showing a smooth surface, clear satellite peaks and clear growth cross sections, respectively. Besides the good power performance of the devices, a low threshold current density of 60 A/cm2 was also achieved and the calculated differential resistance was 0.17 Ω.

Picosecond transient thermoreflectance for thermal conductivity characterization

ABSTRACTWe developed a picosecond transient thermoreflectance (ps-TTR) system for thermal property characterization, using a low-repetition-rate picosecond pulsed laser (1064 nm) as the heating source and a 532 nm CW laser as the probe. Low-repetition-rate pump eliminates the complication from thermal accumulation effect. Without the need of a mechanical delay stage, this ps-TTR system can measure the thermal decay curve from 500 ps up to 1 ms. Three groups of samples are tested: bulk crystals (glass, Si, GaAs, and sapphire); MoS2 thin films (157 ~ 900 nm thickness); InGaAs random alloy and GaAs/InAs digital alloy (short period superlattices). Analysis of the thermoreflectance signals shows that this ps-TTR system is able to measure both thermal conductivity and interface conductance in nanostructures. The measured thermal conductivity values in bulk crystals, MoS2 thin films, and InGaAs random alloy are all consistent with literature values. Cross-plane thermal conductivity in MoS2 thin films does not show obvious thickness dependence. Thermal conductivities of GaAs/InAs digital alloys are smaller than InGaAs random alloy, due to the efficient scattering at interfaces. We also discuss the advantages and disadvantages of this newly developed ps-TTR system comparing with the popular time-domain thermoreflectance system.

Temperature dependence of the ionization coefficients of InAlAs and AlGaAs digital alloy: erratum

Digital alloy In0.52Al0.48As avalanche photodiodes exhibit lower excess noise than those fabricated from random alloys. This paper compares the temperature dependence, from 203 to 323 K, of the impact ionization characteristics of In0.52Al0.48As and Al0.74Ga0.26As digital and random alloys. These results provide insight into the low excess noise exhibited by some digital alloy materials, and these materials can even obtain lower excess noise at low temperature.

Integral Monolayer-Scale Featured Digital-Alloyed AlN/GaN Superlattices Using Hierarchical Growth Units

Acquiring AlN/GaN digital alloys with matching coherent lattices, atomically sharp interfaces, and negligible compositional fluctuations remains a challenge. In this work, the nature and formation mechanism of the constituent elements of AlN and GaN atomic layers growth was examined by first-principle calculations and experimental demonstration. On the basis of the calculated formation enthalpies, we developed a hierarchical growth method wherein AlN and GaN growth units are digitally stacked layer by layer through metal organic vapor-phase epitaxy, which involves the growth sequence instantaneously to control chemical potentials of the hierarchical growth units under different atmospheres. High-resolution X-ray diffraction and transmission electron microscopy confirmed that the hierarchical GaN and AlN growth units of digital-alloyed AlN/GaN structures had coherent lattices, abrupt interfaces, and integral monolayers at the atomic scale. The cathodoluminescence properties featured with single emission, c...

Strain effect on band structure of InAlAs digital alloy

Recently, InAlAs digital alloys have been shown to exhibit unique electronic dispersion properties, which can be used to make low-noise avalanche photodiodes. In this paper, the strain effect is analyzed for its impact on the band structure of the InAlAs digital alloy. Simulation using a tight binding model that includes the strain effect yields bandgap energies that are consistent with experimental results. The bandgap would be larger without strain. In addition, a positive relationship has been found between minigaps of the InAlAs digital alloy and the band offset between bulk InAs and AlAs at the same position in k-space.

Toward deterministic construction of low noise avalanche photodetector materials

Over the past 40+ years, III-V materials have been intensively studied for avalanche photodetectors, driven by applications including optical communications, imaging, quantum information processing, and autonomous vehicle navigation. Unfortunately, impact ionization is a stochastic process that introduces noise, thereby limiting sensitivity and achievable bandwidths, leading to intense effort to mitigate this noise through the identification of different materials and device structures. Exploration of these materials has seen limited success as it has proceeded in a largely ad hoc fashion due to little consensus regarding which fundamental properties are important. Here, we report an exciting step toward deterministic design of low-noise avalanche photodetector materials by alternating the composition at the monolayer scale; this represents a dramatic departure from previous approaches, which have concentrated on either unconventional compounds/alloys or nanoscale band-engineering. In particular, we demonstrate how to substantially improve upon the noise characteristics of the current state-of-the art telecom avalanche multipliers, In0.52Al0.48As grown on InP substrates, by growing the structure as a strain-balanced digital alloy of InAs and AlAs layers, each only a few atomic layers thick. The effective k-factor, which has historically been considered a fundamental material property, was reduced by 6–7× from k = 0.2 for bulk In0.52Al0.48As to k = 0.05 by using the digital alloy technique. We also demonstrate that these “digital alloys” can significantly extend the photodetector cutoff wavelength well beyond those of their random alloy counterparts.

Optical and Structural Analysis of Zinc Oxysulfide Digital Alloys Grown by Atomic Layer Deposition

We investigated the optical and structural properties of ZnOS digital alloy grown on Si by atomic layer deposition. The refractive index and extinction coefficient were evaluated using spectroscopic ellipsometry. ZnOS alloy film shows a strong absorption tail extended to the below band gap regime, which is attributed to a high density of band tail states formed within the band gap owing to the amorphous crystallinity. The ZnOS alloy exhibits a large Urbach energy of 210–540 meV. After high-temperature annealing, the band tail states are reduced with a decrease in lattice disorder, which is confirmed by enhanced X-ray diffraction peaks.

Temperature dependence of the ionization coefficients of InAlAs and AlGaAs digital alloys

Digital alloy In0.52Al0.48As avalanche photodiodes exhibit lower excess noise than those fabricated from random alloys. This paper compares the temperature dependence, from 203 to 323 K, of the impact ionization characteristics of In0.52Al0.48As and Al0.74Ga0.26As digital and random alloys. These results provide insight into the low excess noise exhibited by some digital alloy materials, and these materials can even obtain lower excess noise at low temperature.

Digital Alloy InAlAs Avalanche Photodiodes

InAlAs digital alloy avalanche photodiodes exhibit lower excess noise than those fabricated from conventional random alloy material. Experiment and Monte Carlo simulation both show that relative to the random alloy the ionization probability for electrons is slightly lower while that of holes is greatly suppressed. We propose that the suppression of carrier ionization probability in digital alloys happens because of the creation of minibands that localize carriers. The difference of suppression between conduction bands and valence bands comes from the difference of scattering path.

Defect properties of InGaAsN layers grown as sub-monolayer digital alloys by molecular beam epitaxy

The defect properties of InGaAsN dilute nitrides grown as sub-monolayer digital alloys (SDAs) by molecular beam epitaxy for photovoltaic application were studied by space charge capacitance spectroscopy. Alloys of i-InGaAsN (Eg = 1.03 eV) were lattice-matched grown on GaAs wafers as a superlattice of InAs/GaAsN with one monolayer of InAs (<0.5 nm) between wide GaAsN (7–12 nm) layers as active layers in single-junction solar cells. Low p-type background doping was demonstrated at room temperature in samples with InGaAsN layers 900 nm and 1200 nm thick (less 1 × 1015 cm−3). According to admittance spectroscopy and deep-level transient spectroscopy measurements, the SDA approach leads to defect-free growth up to a thickness of 900 nm. An increase in thickness to 1200 nm leads to the formation of non-radiative recombination centers with an activation energy of 0.5 eV (NT = 8.4 × 1014 cm−3) and a shallow defect level at 0.20 eV. The last one leads to the appearance of additional doping, but its concentration is low (NT = 5 × 1014 cm−3) so it does not affect the photoelectric properties. However, further increase in thickness to 1600 nm, leads to significant growth of its concentration to (3–5) × 1015 cm−3, while the concentration of deep levels becomes 1.3 × 1015 cm−3. Therefore, additional free charge carriers appearing due to ionization of the shallow level change the band diagram from p-i-n to p-n junction at room temperature. It leads to a drop of the external quantum efficiency due to the effect of pulling electric field decrease in the p-n junction and an increased number of non-radiative recombination centers that negatively impact lifetimes in InGaAsN.