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Single-crystalline Zn3N2 thin films have been grown on MgO (100) and YSZ (100) substrates by plasma-assisted molecular beam epitaxy. Depending on growth conditions, the film orientation can be tuned from (100) to (111). For each orientation, x-ray diffraction and reflection high-energy electron diffraction are used to determine the epitaxial relationships and to quantify the structural quality. Using high-temperature x-ray diffraction, the Zn3N2 linear thermal expansion coefficient is measured with an average of (1.5 ± 0.1) × 10−5 K−1 in the range of 300–700 K. The Zn3N2 films are found to be systematically n-type and degenerate, with carrier concentrations of 1019–1021 cm−3 and electron mobilities ranging from 4 to 388 cm2 V−1 s−1. Low-temperature Hall effect measurements show that ionized impurity scattering is the main mechanism limiting the mobility. The large carrier densities lead to measured optical bandgaps in the range of 1.05–1.37 eV due to Moss–Burstein band filling, with an extrapolated value of 0.99 eV for actual bandgap energy. [ABSTRACT FROM AUTHOR]

This paper discusses the results of high temperature resistivity and Hall effect studies of Mg-doped GaN and AlxGa1−xN epilayers (0.05 < x < 0.23). The studied samples were grown by molecular beam epitaxy on low temperature buffers of GaN and AlN deposited on a sapphire substrate. The experiments were carried out at temperatures ranging from 300 up to 1000 K. Up to a certain critical temperature TC (around 800 K), a typical increase of the conduction processes due to the excitation of impurity states has been observed with an activation energy of about EA = 200 meV. However, at this critical temperature TC, an annealing effect was observed in all the investigated samples. At this critical temperature, the increase in the free carrier concentration as a function of time leads to an irreversible decrease in the samples' resistivity of more than 60%. The observed temperature dependences of the electrical transport properties are analyzed in the frame of an impurity model including shallow donors and Mg-related acceptors (with EA ∼ 200 meV). In some cases, an additional conduction channel not related to free carriers in the valence band must be taken into account. This can lead to an incorrect determination of hole concentration in the valence band, an important parameter in the process of radiative recombination. [ABSTRACT FROM AUTHOR]

AlyGa1−yN quantum dots (QDs) have been grown by molecular beam epitaxy on AlxGa1−xN (0001) using a 2-dimensional–3-dimensional growth mode transition that leads to the formation of QDs. QDs have been grown for Al compositions y varying between 10% and 40%. The influence of the active region design [composition y, QD height, and bandgap difference (ΔEg) between the AlxGa1−xN cladding layer and the AlyGa1−yN QDs] is discussed based on microscopy, continuous wave photoluminescence (PL), and time-resolved PL (TRPL) measurements. In particular, increasing y leads to a shift of the QD emission toward shorter wavelengths, allowing covering a spectral range in the UV from 332 nm (UVA) to 276 nm (UVC) at room temperature (RT). The low-temperature (LT) internal quantum efficiency of the QD ensembles was estimated from TRPL experiments at 8 K and values between 11% and 66% were deduced. The highest internal quantum efficiency (IQE)-LT is found for the QDs with higher Al content y. Then, the PL spectrally integrated intensity ratios between RT and LT were measured to estimate the IQE of the samples at RT. The PL ratio is higher for larger ΔEg, for QDs with y of 0.1 or 0.2, and high PL intensity ratios up to 30% were also measured for QDs with larger y of 0.3 and 0.4. RT IQE values between 5% and 20% are deduced for AlyGa1−yN QDs emitting in the 276–308 nm range. [ABSTRACT FROM AUTHOR]

We have studied Be-acceptors in a p-type GaInNAs epilayer using magneto-luminescence spectroscopy. The band edge photoluminescence (PL) spectra at T=7K contain two features: the first is associated with the free exciton while the second with the conduction band to acceptor (CB → A) transition. The intensity of the latter decreases with increasing temperature while the excitonic feature survives up to T=250 K. From the energies of the two PL features, as well as the exciton binding energy in GaInNAs, we determined the Be-acceptor binding energy to be equal to 42 meV. The energy of the CB → A feature varies linearly with magnetic field B and has a slope of 5:5 × 10-4 eV/T. [ABSTRACT FROM AUTHOR]

The electroluminescence (EL) characteristics of a set of InAs/GaInNAs quantum dot (QD) light-emitting diodes with varying In and N contents are analyzed. Room-temperature EL around 1.5 μm is obtained with 15% In and 2% N in the QD capping layer. It is shown that the addition of N results in a degradation of the external efficiency, ηext, probably due to an increase in the nonradiative recombination in the QD heterostructure and an increase in the carrier escape from the QD to the capping layer, which yield a degradation of the current injection efficiency into the QD. Nevertheless, ηext can be partially recovered if a postgrowth rapid thermal annealing is performed, although this also results in a blueshift in the EL peak wavelength. The different contributions to the EL spectra are also analyzed and identified by looking at their dependence on injected current and temperature. It is found that N-containing devices show two radiative transitions. The lowest energy transition has been ascribed to the QD ground state recombination, whereas the higher energy transition has been attributed to recombination of carriers confined in the capping layer. Moreover, the carrier loss mechanisms responsible for the quenching of the EL in the dilute nitride-based devices are studied. It is found that the EL thermal quenching has an activation energy which can be ascribed to carrier escape from the QD to the capping layer. [ABSTRACT FROM AUTHOR]

The techniques of micromachining are used for the manufacturing of a thermal inclinometer, which requires no solid proof mass and has a low-cost production. The sensor principle is as follows: a heating resistor creates a symmetrical temperature profile and two thermocouples, placed symmetrically on both sides of the heater, measure a differential temperature. When an acceleration is applied on the sensitive axis x of the sensor, the convection heat transfer and the temperature profile become asymmetric and the differential temperature was shown to be proportional to the acceleration and to the Grashof number. Platinum resistor deposited by electron beam evaporation and flash evaporated Bi0.5Sb1.5Te3(p)/Bi2Se0.3Te2.7(n) thermocouples are used, respectively, as heater and temperature sensors on a polyimide substrate. These ternary materials present higher figures of merit than 10−3 K−1 and the thermocouple sensitivity is 440 μV K−1. Two types of sensors have been manufactured: the first one is deposited on a continuous polyimide film and the second one on a micromachined film, which permits to limit the energy consumption by a factor of 2 and to obtain a heater temperature rise of 3.44 K mW−1 and a sensitivity of 1410 μV g−1 (1g=9.81 m s−2) for a heating power of 100 mW. [Copyright &y& Elsevier]

Epitaxial growth of WTe 2 offers significant advantages, including the production of high-quality films, possible long-range in-plane ordering, and precise control over layer thicknesses. However, the mean island size of WTe 2 grown by molecular beam epitaxy (MBE) in the literature is only a few tens of nanometers, which is not suitable for the implementation of devices at large lateral scales. Here we report the growth of T d -WTe 2 ultrathin films by MBE on monolayer (ML) graphene, reaching a mean flake size of ≃110 nm, which is, on overage, more than three times larger than previous results. WTe 2 films thicker than 5 nm have been successfully synthesized and exhibit the expected Td phase atomic structure. We rationalize the epitaxial growth of T d -WTe 2 and propose a simple model to estimate the mean flake size as a function of growth parameters that can be applied to other transition metal dichalcogenides (TMDCs). Based on nucleation theory and the Kolmogorov-Johnson-Meh-Avrami (KJMA) equation, our analytical model supports experimental data showing a critical coverage of 0.13 ML above which WTe 2 nucleation becomes negligible. The quality of monolayer WTe 2 films is demonstrated by electronic band structure analysis using angle-resolved photoemission spectroscopy (ARPES), which is in agreement with first-principles calculations performed on free-standing WTe 2 and previous reports. We found electron pockets at the Fermi level, indicating a n -type doping of WTe 2 with an electron density of n = 2.0 ± 0.5 × 10 12 cm -2 for each electron pocket.

We build new material descriptors to predict the band gap and the work function of 2D materials by tree-based machine-learning models. The descriptor's construction is based on vectorizing property matrices and on empirical property function, leading to mixing features that require low-resource computations. Combined with database-based features, the mixing features significantly improve the training and prediction of the models. We find R[Formula: see text] greater than 0.9 and mean absolute errors (MAE) smaller than 0.23 eV both for the training and prediction. The highest R[Formula: see text] of 0.95, 0.98 and the smallest MAE of 0.16 eV and 0.10 eV were obtained by using extreme gradient boosting for the bandgap and work-function predictions, respectively. These metrics were greatly improved as compared to those of database features-based predictions. We also find that the hybrid features slightly reduce the overfitting despite a small scale of the dataset. The relevance of the descriptor-based method was assessed by predicting and comparing the electronic properties of several 2D materials belonging to new classes (oxides, nitrides, carbides) with those of conventional computations. Our work provides a guideline to efficiently engineer descriptors by using vectorized property matrices and hybrid features for predicting 2D materials properties via ensemble models., (© 2023. The Author(s).)