DX Centers Research Articles

Low-frequency noise and defects in AlGaAs/InGaAs/GaAs pseudomorphic high-electron mobility transistors

Current–voltage characteristics and low-frequency (LF) noise of industrial-quality AlGaAs/InGaAs/GaAs pseudomorphic high-electron-mobility transistors are evaluated as a function of bias stress and temperature. A small positive shift of threshold voltage Vth and negligible degradation in peak transconductance GM are observed under ON-state bias conditions at elevated temperatures. The Vth measurements suggest activation of an acceptor-like defect or impurity center. The GM measurements demonstrate that newly activated defects are not located close enough to the two-dimensional electron gas to scatter carriers strongly. First-principles calculations and comparisons with previous work suggest that OAs impurity centers, other oxygen-related defects, isolated AsGa antisites, and dopant-based DX centers may contribute significantly to low-frequency (LF) noise in as-processed devices. LF noise is relatively unaffected by voltage stress at elevated temperatures, consistent with the small changes in Vth and peak GM.

Chemical Trend of Nonradiative Recombination in Cu(In,Ga)Se2 Alloys

Understanding nonradiative recombination is important for improving semiconductor devices. For Cu(In,Ga)Se${}_{2}$ (CIGS) solar cells, antisite defects have long been considered the main recombination centers, yet the underlying mechanism has remained elusive. Here first-principles calculations show that these ``killer centers'' themselves cannot capture holes efficiently for effective recombination. However, internal conversion to the distorted neutral DX center does open an efficient hole-capture pathway, and DX's stability in CIGS increases with Ga concentration, which resolves the longstanding issue of why the efficiency of CIGS solar cells decreases at high Ga concentration.

Anomalous Screening Effect of Superlattice-Doped GaAs / (Al,Ga)As Heterostructures under Illumination

The $\mathrm{Ga}\mathrm{As}$/$(\text{Al,Ga})\mathrm{As}$ heterostructure with short-period superlattice (SPSL) doping possesses ultrahigh mobility of its two-dimensional electron gas by placing donors within the remote $\mathrm{Ga}\mathrm{As}$ layers. Here, we investigate its magnetotransport property under a heavily doped situation. After long enough illumination at cryogenic temperature, the change of the electron concentration inside the quantum well (QW) is only $5.9\mathrm{%}$. Meanwhile, the quantum lifetime ${\ensuremath{\tau}}_{q,\text{QW}}$ of the electron shows an anomalous behavior. It increases slightly and then exhibits an exponential decay until saturation. This is different from the monotonic increase of ${\ensuremath{\tau}}_{q,\text{QW}}$ under illumination for the conventional doping situation. The increase of ${\ensuremath{\tau}}_{q,\text{QW}}$ originates from the larger donor filling-fraction-enhanced screening effect. Meanwhile, the decrease of ${\ensuremath{\tau}}_{q,\text{QW}}$ may be caused by stronger scattering of ionized ${d}^{+}$ states evolved from DX centers. The transfer of excess electrons between the $\mathrm{Al}\mathrm{As}$ layers can also cause the decrease of ${\ensuremath{\tau}}_{q,\text{QW}}$. This work provides an insight into the mechanism of DX centers on the quantum transport properties of SPSL-doped heterostructures.

ОСОБЛИВОСТІ ДІЇ УЛЬТРАЗВУКУ НА ЕЛЕКТРОННИЙ ТРАНСПОРТ 2DEG ТА 3DEG НОСІЇВ ЗАРЯДУ В ГЕТЕРОСТРУКТУРАХ GaN/AlGaN/GaN/AlN

We have conducted the temperature and amplitude dependencies of the charge carriers concentration and its mobility analysis in GaN/Al0,2Ga0,8N/GaN/AlN on sapphire structures growing MOCVD method under the ultrasonic loading (longitudinal waves, fUS = 9 MHz). We found out the temperature peculiarities of electronic characteristics changes, it is allowed us to consider our system as complex with parallel conduction channels. In order to study the effect of ultrasound loading separately on both the flow of 3DEG and 2DEG charge carriers, a mathematical operation of separating these flows was carried out. It is shown, that at high temperatures (Т ˃ 200 K), 3DEG conductivity prevails, which has a thermal activation character and is described by the acoustically induced transformation of the metastable DX center. At temperatures (T ˂ 150K), 2DEG conductivity prevails. Thus, AlGaN/GaN and GaN/AlGaN boundaries can exhibit 2DEG conductivity (σ2D), while film layers (GaN buffer and AlGaN barrier layer are 3DEG conductivity (σ3D)). In the AlxGa1-xN structures containing DX centers, under US loading, there is a periodic change in the distance between the positions of the donor atom. Here, the determining mechanism is tunneling, the nature of which is related with dislocations. The horizontal sections observed in the experiment arise as a result of the tunnel emission of charge carriers through the quasi-resonant level. It was established that 3DEG and 2DEG charge carriers differ in the nature of temperature changes in concentration (with increasing temperature, there is an increase in 3DEG concentration and independence from temperature in 2DEG concentration) and mobility (decrease in 3DEG, but increase in 2DEG). It is shown that the influence of ultrasound in both cases, both for 3DEG and for 2DEG, of charge carriers is qualitatively similar - there is an increase in the 3DEG and 2DEG carriers concentration and a decrease in the absolute values of 3DEG and 2DEG mobility. Acoustically induced changes in electrical parameters most likely occur near dislocations that actively interact with ultrasonic waves.

Electrical and optical properties and defects of (100)- and (001)-oriented V-doped β-Ga2O3 crystals grown by EFG

Altering the n-type conductivity and optical properties of ultra-wide bandgap β-Ga2O3 by impurity doping has been a topic of research interest in the semiconductor field. Simultaneously, β-Ga2O3 with a monoclinic structure has exhibited interesting anisotropy. In this study, (100) and (001) V-doped β-Ga2O3 crystals were synthesized by the edge-defined film-fed growth (EFG) method, and their structural, electrical and optical properties, and defects were systematically investigated. V-doped β-Ga2O3 crystals with a (100) plane have a better crystalline quality and flatter surface than those with a (001) plane. Compared with undoped crystals (∼1016 cm−3), the carrier concentration of the (100) and (001) V-doped β-Ga2O3 substrates only increased to 4.30 × 1017 cm−3 and 1.92 × 1017 cm−3, respectively, which was related to the VxOy clusters formed by excess V as trapping traps and the carbon DX center as an electron capture trap formed by the relaxation of C atoms in the direction opposite to that of the C–O bond. The generation of absorption peaks and flat regions in the transmittance spectra was attributed to the introduction of V doping. The etch pit morphology and Raman peak intensity on the surfaces of the (100) and (001) V-doped β-Ga2O3 substrates were different owing to the anisotropy of the monoclinic lattice structure. These results contribute to the understanding of the influence of V and anisotropy on β-Ga2O3 crystals and expand their applications in electrical and optical fields.

Realization of homojunction PN AlN diodes

Aluminum nitride (AlN) is an insulator that has shown little promise to be converted to a semiconductor via impurity doping. Some of the historic challenges for successfully doping AlN include a reconfigurable defect formation known as a DX center and subsequent compensation that causes an increase in dopant activation energy resulting in very few carriers of electricity, electrons, or holes, rendering doping inefficient. Using crystal synthesis methods that generate less compensating impurities and less lattice expansion, thus impeding the reconfiguration of dopants, and using new dopants, we demonstrate: (a) well behaved bulk semiconducting functionality in AlN, the largest direct bandgap semiconductor known with (b) substantial bulk p-type conduction (holes = 3.1 × 1018 cm−3, as recently reported in our prior work), (c) dramatic improvement in n-type bulk conduction (electrons = 6 × 1018 cm−3, nearly 6000 times the prior state-of-the-art), and (d) a PN AlN diode with a nearly ideal turn-on voltage of ∼6 V for a 6.1 eV bandgap semiconductor. A wide variety of AlN-based applications are enabled that will impact deep ultraviolet light-based viral and bacterial sterilization, polymer curing, lithography, laser machining, high-temperature, high-voltage, and high-power electronics.

DX center formation in highly Si doped AlN nanowires revealed by trap assisted space-charge limited current

Electrical properties of silicon doped AlN nanowires grown by plasma assisted molecular beam epitaxy were investigated by means of temperature dependent current–voltage measurements. Following an Ohmic regime for bias lower than 0.1 V, a transition to a space-charge limited regime occurred for higher bias. This transition appears to change with the doping level and is studied within the framework of the simplified theory of space-charge limited current assisted by traps. For the least doped samples, a single, doping independent trapping behavior is observed. For the most doped samples, an electron trap with an energy level around 150 meV below the conduction band is identified. The density of these traps increases with a Si doping level, consistent with a self-compensation mechanism reported in the literature. The results are in accordance with the presence of Si atoms that have three different configurations: one shallow state and two DX centers.

Investigation of Traps in AlGaN/GaN Heterostructures by Ultrasonic Vibrations

A method of dynamic deformations has been proposed as a useful informative tool in the characterization of transportation properties of a two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. It is found that the exposing of a sample to ultrasonic vibrations results in the persistent acousto-conductivity (PAC) which was observed up to room temperatures. The PAC behaves itself like persistent photoconductivity (PPC), and the carrier density in the 2DEG channel is primarily contributed by the transfer of electrons excited from traps (like DX centers) as a result of their reconstruction under the ultrasonic loading.

Значення DX–ЦЕНТРІВ для АКУСТОІНДУКОВАНИХ процесів перебудови дефектів В GaN/AlGaN

The experimental results of amplitude effects are compared (from an ultrasonic wave deformation amplitude – a tension τUS) for electron concentration and changes of the lattice parameter on the same sample GaN/Al0.2Ga0.8N/GaN/AlN. It has been experimentally established that at ultrasonic loading (frequency 5–10 MHz, amplitude – towards 2·104 W/m2) there is a nonlinear increase in the effective electron concentration and an increase in the lattice parameter; at the same time, the mobility of electrons decreases and μН(τUS) ~ |τUS|. The energy parameters of the acoustic activation charge carriers process are calculated from the approximation of experimental amplitude changes – Еа ≈ 50 meV and γn(300 K) ≈ 2,5·10-27 m3. The amplitude dependences (increase) of the relative lattice parameter change (ΔС/С) from the tension τUS have been investigated experimentally at different frequencies. The energy of DX-center transition UDX ≈ 108 meV and the activation volume of this transition γDX ≈ 6,6·10-27 m3 are calculated from the approximation of the experimental amplitude changes. The revealed correlation of the magnitude of acoustic induced effects in different experiments allows to build a quantitative energy model of the acoustic action process based on the properties of metastable DX centers. It is shown that the acoustic induced process occurs due to the dimensional displacement of the DX-center atom (a background impurity of silicon atoms) from the non-central position to the centrally symmetric one; herewith DX-center is ionized, one goes into the d0-state. It is believed that the changes are most likely to occur near penetrating dislocations in the barrier layer Al0.2Ga0.8N – acoustic modulated oscillations of the distance between the possible positions of the donor atom lead to a decrease in the barrier to the displacement of the defect.

Growth of highly conductive Al-rich AlGaN:Si with low group-III vacancy concentration

The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 mΩ cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition.

Distinguishing persistent effects in an undoped GaAs/AlGaAs quantum well by top-gate-dependent illumination

Persistent photoconductivity of GaAs/AlGaAs heterostructures has hampered the measurement of charge- and spin-related quantum effects in gate-defined quantum devices and integrated charge sensors due to Si-dopant-related deep donor levels (DX centers). In this study, this effect is overcome by using an undoped GaAs/AlGaAs heterostructure for photonic purposes. We also measure the electron transport before and after LED illumination at low temperatures. In addition to a regular rapid saturation showing the increased carrier density, a slow accumulation of illumination effects appeared when different top-gate voltages were applied during illumination, which indicated the redistribution of charge at the oxide–GaAs interface. This study provides interesting insights into the development of optically stable devices for efficient semiconductor quantum interfaces.

Solubility Limit of Ge Dopants in AlGaN: A Chemical and Microstructural Investigation Down to the Nanoscale.

Attaining low-resistivity AlxGa1-xN layers is one keystone to improve the efficiency of light-emitting devices in the ultraviolet spectral range. Here, we present a microstructural analysis of AlxGa1-xN/Ge samples with 0 ≤ x ≤ 1, and a nominal doping level in the range of 1020 cm-3, together with the measurement of Ge concentration and its spatial distribution down to the nanometer scale. AlxGa1-xN/Ge samples with x ≤ 0.2 do not present any sign of inhomogeneity. However, samples with x > 0.4 display μm-size Ge crystallites at the surface. Ge segregation is not restricted to the surface: Ge-rich regions with a size of tens of nanometers are observed inside the AlxGa1-xN/Ge layers, generally associated with Ga-rich regions around structural defects. With these local exceptions, the AlxGa1-xN/Ge matrix presents a homogeneous Ge composition which can be significantly lower than the nominal doping level. Precise measurements of Ge in the matrix provide a view of the solubility diagram of Ge in AlxGa1-xN as a function of the Al mole fraction. The solubility of Ge in AlN is extremely low. Between AlN and GaN, the solubility increases linearly with the Ga mole fraction in the ternary alloy, which suggests that the Ge incorporation takes place by substitution of Ga atoms only. The maximum percentage of Ga sites occupied by Ge saturates around 1%. The solubility issues and Ge segregation phenomena at different length scales likely play a role in the efficiency of Ge as an n-type AlGaN dopant, even at Al concentrations where Ge DX centers are not expected to manifest. Therefore, this information can have direct impact on the performance of Ge-doped AlGaN light-emitting diodes, particularly in the spectral range for disinfection (≈260 nm), which requires heavily doped alloys with a high Al mole fraction.

Особливості акустоіндукованих змін електрофізичних характеристик у гетероструктурах GaN/Al0,2Ga0,8N/GaN/AlN

We studied temperature, amplitude and time dependencies of electrophysical parameters in GaN/Al0,2Ga0,8N/GaN/AlN structures when the ultrasound (US) was switched on/off (fUS = 9 MHz). We found out the charge carriers concentration n(Т) increases and the mobility μH(Т) decreases under the ultrasonic loading. With decreasing the temperature, the effect of acoustic induced changes increases. When US switches on/off, long-term (up to ~ 500 s) relaxation of the acoustic conductivity US(t) is observed, with increasing the amplitude of the ultrasound at low temperatures the acoustic conductivity increases exponentially, and at high ones decreases exponentially. It is established that the main mechanisms of charge carrier scattering at low temperatures (T ≤ 150 K) are ionized centers scattering and dislocations scattering; at high temperatures (T > 200 K) the charge carriers mobility is limited by polar optical phonons scattering. An acoustic deformation mechanism of charge carrier redistribution as a result of acoustic lattice deformation and corresponding additional structure piezopolarization is proposed. In our opinion, the determining factor that contributes to these effects, is the high density of boundary dislocations, as well as the change in the linear charge density on the dislocations in the process of their forced oscillations in the field of an external ultrasonic deformation. We have also considered an alternative mechanism that related with an acoustic induced (AI) transformation of metastable DX centers and can occur simultaneously. The mechanism of AI concentration n(Т) increase in this model is associated with a decrease of the barrier for an electron capture in DX0-state as a result of a periodic change distance between possible positions of the donor atoms (at the lattice node and non-central DX¯-state). To study the effect allows to obtain information both about such defect structure of the material and about the nature of changes in its macroscopic characteristics under the ultrasonic loading influence. From a practical point of view, it leads to the search for new opportunities for using of ultrasound for control the physical parameters kinetics of semiconductor structures.

Chemical trends of deep levels in van der Waals semiconductors

Properties of semiconductors are largely defined by crystal imperfections including native defects. Van der Waals (vdW) semiconductors, a newly emerged class of materials, are no exception: defects exist even in the purest materials and strongly affect their electrical, optical, magnetic, catalytic and sensing properties. However, unlike conventional semiconductors where energy levels of defects are well documented, they are experimentally unknown in even the best studied vdW semiconductors, impeding the understanding and utilization of these materials. Here, we directly evaluate deep levels and their chemical trends in the bandgap of MoS2, WS2 and their alloys by transient spectroscopic study. One of the deep levels is found to follow the conduction band minimum of each host, attributed to the native sulfur vacancy. A switchable, DX center - like deep level has also been identified, whose energy lines up instead on a fixed level across different hosts, explaining a persistent photoconductivity above 400 K.

Capacitance Spectroscopy of Heteroepitaxial AlGaAs/GaAs p–i–n Structures

The temperature dependences of the capacitance-voltage (C–V) characteristics and deep-level spectra of a graded high-voltage AlxGa1 – xAs p0–i–n0 junction grown by liquid-phase epitaxy via autodoping with background impurities are studied. Changes in the C–V characteristics at varied measurement temperature and optical illumination demonstrate that the p0-, i-, and n0-type layers in AlxGa1 – xAs under study contain bistable DX centers. Spectra furnished by deep-level transient spectroscopy (DLTS), measured at various bias voltages Vr and filling-pulse voltages Vf, show a positive DLTS peak for the n0-type layer, with a thermal activation energy of Et = 280 meV and electron-capture cross section of σn = 3.17 × 10–14 cm2, which is unusual for a majority-charge-carrier trap. This peak is attributed to the negatively charged state of the Se/Te donor impurity, which is a bistable DX center with negative correlation energy U.

Deep-level defects in high-voltage AlGaAs p–i–n diodes and the effect of these defects on the temperature dependence of the minority carrier lifetime

The variation of the effective lifetime of minority carrier lifetime τeff with temperature has been studied in the temperature range 300–580 K in AlxGa1−xAs base regions of p+–p0–i–n0–n+ high-voltage GaAs–AlGaAs diodes grown by liquid-phase epitaxy. It was found by using deep level transient spectroscopy that the emission/capture of electrons and holes in AlxGa1−xAs p0–i–n0 epitaxial layers is governed by the DX− states of the DX center formed by Se/Te background impurities. The Arrhenius plots associated with the τeff were used to determine the activation energy of the hole capture to the DX− level to be Ec = 159 meV. It is shown that the thermal capture of holes to the DX− level determines the relaxation time of nonequilibrium carriers in the AlxGa1−xAs base layers as well as its temperature dependence.

Luminescence line shapes of band to deep centre and donor–acceptor transitions in AlN

Energy structure and electron coupling with local lattice vibrations have been investigated for deep centres in AlN using hybrid functional density functional theory. Local phonon energies and Huang–Rhys parameters have been calculated for defects and defect complexes containing most common unintentional impurities of carbon, oxygen and silicon, and for intrinsic vacancies, nitrogen split-interstitial defect, and complexes of Al and N vacancies in AlN. Luminescence line shapes of band to deep centre transitions in AlN have been calculated in dependence on temperature for most abundant defects in AlN. Donor–acceptor luminescence line shapes for shallow donor to deep acceptor and deep donor to deep acceptor transitions have been considered theoretically. Configuration diagrams of oxygen and silicon DX centres have been calculated by density functional theory with hybrid functional, and peak energies of optical transitions of an electron from the DX-centres to deep acceptors have been estimated. Possible assignments of the experimental luminescence bands in AlN based on the calculations have been discussed.

Prospects for n-type doping of (AlxGa1−x)2O3 alloys

We systematically explore the properties of group-IV (C, Si, Ge, and Sn) and transition metal (Hf, Zr, and Ta) dopants substituting on the cation site in (AlxGa1−x)2O3 (AlGO) alloys using first-principles calculations with a hybrid functional. In Ga2O3, each of these dopants acts as a shallow donor. In Al2O3, they are deep defects characterized by the formation of either DX centers or positive-U (+/0) levels. Combining our calculations of dopant charge-state transition levels with information of the AlGO alloy band structure, we estimate the critical Al composition at which each dopant transitions from being a shallow to a deep donor. We identify Si to be the most efficient dopant to achieve n-type conductivity in high Al-content AlGO alloys, acting as a shallow donor over the entire predicted stability range for AlGO solid solution alloys.

Worst-Case Bias for High Voltage, Elevated-Temperature Stress of AlGaN/GaN HEMTs

The effects of high-field stress are evaluated for industrial-quality AlGaN/GaN HEMTs as a function of bias and temperature. Positive and negative threshold voltage shifts are observed, depending on stress conditions, indicating the presence of acceptor-like and donor-like traps in these devices. Worst-case transconductance degradation under rated device operating conditions is observed for devices subjected to high-voltage stress in the ON bias condition at elevated temperature. This contrasts with results on earlier-generation devices, which often show worst-case response under semi-ON bias conditions, emphasizing that each technology requires characterization under multiple bias-stress conditions. Neutral and charged oxygen donor-like DX centers and substitutional acceptor-like $\text{N}_{\mathrm{ Ga}}$ centers are the dominant defects contributing to low-frequency noise in these devices. Dehydrogenation of $\text{O}_{\mathrm{ N}}$ -H complexes during ON-bias stress and the resulting increases in densities of $\text{O}_{\mathrm{ N}}$ -related donor-like defects are evidently the reliability-limiting mechanism in these devices.

Акустоіндуковані температурні особливості електропровідності в CdZnTe:Cl, обумовлені метастабільними DX-центрами

Акустоіндуковані температурні особливості електропровідності в CdZnTe:Cl, обумовлені метастабільними DX-центрами