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The changes of electrical properties and deep trap spectra induced in n-type β-Ga2O3 by 1 GeV protons with a fluence of 4 × 1013 cm−2 were studied. The carrier removal rates were ∼100 cm−1 at this energy. For comparison, for 20 MeV proton irradiation at comparable fluences (5–10 × 1014 cm−2), the removal rate was ∼400 cm−1 for donor concentrations of 3 × 1016 cm−3 and ∼100 cm−1 for concentrations of <1016 cm−3. These removal rates were in stark contrast with modeling results that predicted the introduction rates of vacancies to be two orders of magnitude higher for 20 MeV protons. Measurements of deep electron and hole traps densities by deep level transient spectroscopy with electrical or optical injection (DLTS or ODLTS), and capacitance–voltage profiling under monochromatic light illumination showed that the 1 GeV proton irradiation resulted in the introduction of deep donors E2*(Ec-0.75 eV) and E3 (Ec-1 eV) and deep acceptors with optical ionization threshold near 2.3 eV producing a feature near 250 K in ODLTS and 3.1 eV with related ODLTS feature near 450 K. The total concentration of all deep traps was much lower than that necessary to explain the observed decrease in net donor density upon irradiation. The donor densities showed a nonuniform distribution in proton irradiated films with the concentration greatly decreased toward the surface. Possible reasons for the observed performance are discussed. [ABSTRACT FROM AUTHOR]

The effects of 20 MeV proton irradiation with fluences of 5 × 1014 and 1015 p/cm2 on electrical properties of lightly Sn doped n-type (net donor concentration 3 × 1017 cm−3) bulk β-Ga2O3 samples with (010) and (−201) orientation were studied. Proton irradiation decreases the net donor density with a removal rate close to 200 cm−1 for both orientations and similar to the electron removal rates in lightly Si doped β-Ga2O3 epilayers. The main deep electron traps introduced in the β-Ga2O3 crystals of both orientations are near Ec−0.45 eV, while in Si doped films, the dominant centers were the so-called E2* (Ec−0.75 eV) and E3 (Ec−0.1 eV) traps. Deep acceptor spectra in our bulk –Ga2O3(Sn) crystals were dominated by the well-known centers with an optical ionization energy of near 2.3 eV, often attributed to split Ga vacancies. These deep acceptors are present in a higher concentration and are introduced by protons at a higher rate for the (010) orientation. Another important difference between the two orientations is the introduction in the surface region (∼0.1 μm from the surface) of the (010) of a very high density of deep acceptors with a level near Ec−0.27 eV, not observed in high densities in the (−201) orientation or in Si doped epitaxial layers. The presence of these traps gives rise to a very pronounced hysteresis in the low temperature forward current–voltage characteristics of the (010) samples. These results are yet another indication of a significant impact of the orientation of the β-Ga2O3 crystals on their properties, in this case, after proton irradiation. [ABSTRACT FROM AUTHOR]

Bulk n-type β-Ga2O3 samples with orientation (−201) and (010) were exposed to a high density hydrogen plasma at 330 °C for 0.5 h. The effects were radically different for the two orientations. For the (−201) sample, H plasma exposure increased the net surface concentration of shallow donors from 2.7 × 1017 cm−3 to 2.6 × 1018 cm−3, with the shallow donors having an ionization energy close to 20 meV as deduced from the temperature dependence of the series resistance of Ni Schottky diodes. By sharp contrast, H plasma exposure of the (010) sample led to a strong decrease in the net shallow donor density from 3.2 × 1017 cm−3 to below 1015 cm−3 in the top 0.9 μm of the sample and to 3.2 × 1016 cm−3 near the edge of the space charge region at 0 V, with the total width of the region affected by plasma treatment being close to 1.1 μm. For both orientations, we observed a major decrease in the concentration of the dominant E2 traps near Ec-0.82 eV related to Fe acceptors. The deep trap spectra in hydrogenated samples were dominated by the E2* traps commonly ascribed to native defects in β-Ga2O3. The peak of these traps with a level near Ec-0.74 eV was masked in the starting samples by the peak of the E2 Fe acceptors present in high concentration, so that E2* only broadened the Fe peak on the low temperature side, but could be revealed by the modeling of the spectra. The concentration of the E2* center was not strongly affected in the hydrogen-treated samples with orientation (010), but in the (−201) samples, the concentration of the E2* peak was greatly enhanced. The results are discussed in conjunction with previous reports on hydrogen plasma treatment of β-Ga2O3 and on obtaining p-type conductivity in the surface layers of β-Ga2O3 crystals annealed in molecular hydrogen at high temperatures [Islam et al., Sci. Rep. 10, 6134 (2020)]. [ABSTRACT FROM AUTHOR]

Two types of near-UV light-emitting diodes (LEDs) with an InGaN/GaN single quantum well (QW) differing only in the presence or absence of an underlayer (UL) consisting of an InAlN/GaN superlattice (SL) were examined. The InAlN-based ULs were previously shown to dramatically improve internal quantum efficiency of near-UV LEDs, via a decrease in the density of deep traps responsible for nonradiative recombination in the QW region. The main differences between samples with and without UL were (a) a higher compensation of Mg acceptors in the p-GaN:Mg contact layer of the sample without UL, which correlates with the presence of traps with an activation energy of 0.06 eV in the QW region, (b) the presence of deep electron traps with levels 0.6 eV below the conduction band edge (Ec) (ET1) and at Ec 0.77 eV (ET2) in the n-GaN spacer underneath the QW, and the presence of hole traps (HT1) in the QW, 0.73 eV above the valence band edge in the sample without UL (no traps could be detected in the sample with UL), and (c) a high density of deep traps with optical ionization energy close to 1.5 eV for the LEDs without UL. Irradiation with 5 MeV electrons led to a strong decrease in the electroluminescence (EL) intensity in the LEDs without UL, while for the samples with UL, such irradiation had little effect on the EL signal at high driving current, although the level of driving currents necessary to have a measurable EL signal increased by about an order of magnitude. This is despite the 5 times higher starting EL signal of the sample with UL. Irradiation also led to the appearance in the LEDs with UL of the ET1 and HT1 deep traps, but with concentration much lower than without the UL, and to a considerable increase in the Mg compensation ratio. [ABSTRACT FROM AUTHOR]

Some green light emitting diodes (LEDs) based on GaN/InGaN multiquantum-well (MQW) structures exhibit strong frequency and temperature dependence of capacitance and prominent changes in capacitance–voltage profiles with temperature that make it difficult to obtain reliable deep level transient spectroscopy (DLTS) measurements. DLTS performed at low probing signal frequency and with constant capacitance between the measurements by controlling applied bias mitigates these issues. This allows measurement of deep electron and hole traps in specific quantum wells (QWs) in the MQW structure. The dominant electron and hole traps detected have levels near Ec− (0.45–0.5) eV and Ev+ (0.6–0.63) eV. Their density increases significantly after aging for a long period (2120 h) at high driving current and elevated temperature. The reason for the observed anomalies in DLTS spectra of these green LEDs is the high density of states in the QWs with activation energies near 0.08, 0.12–0.14, and 0.3 eV, detected in admittance spectra, and, for the 0.08 eV and 0.3 eV, these states are likely related to defects. [ABSTRACT FROM AUTHOR]

Persistent photocapacitance (PPC) was observed in β-(Al0.14Ga0.84)2O3/n+Ga2O3 heterojunctions at low temperatures. The effect is seen in capacitance-frequency measurements under illumination and in admittance spectra after illumination. In the latter case, the capacitance versus frequency curve after illumination returned to its dark values only at temperatures between 225 and 275 K. Prominent hole-trap-like peaks in optical deep level transient spectroscopy (ODLTS) near 130 K were attributed to quenching with temperature of the PPC effect. The phenomena were assigned to electrons being excited from deep traps in the (Al0.14Ga0.86)2O3 barrier into the two-dimensional electron gas (2DEG) in the Ga2O3 substrate. The reverse process then involves excitation of non-equilibrium electrons in the 2DEG back into the ternary barrier layer and subsequent re-capture by the host deep traps. The effective barrier height for this recovery process was ∼0.2 eV, as estimated from ODLTS measurements. The spectral dependence of the effect suggests the presence of deep traps with optical ionization thresholds near 1 eV and ∼2.3 eV. The shift of the threshold voltage necessary to deplete the 2DEG as a function of the photon energy during illumination at room temperature indicates that the most prominent centers in the barrier have an optical ionization energy near 2.3 eV and a sheet density of ∼1012 cm−2. In microcathodoluminescence (MCL) spectra of the heterojunction, a blue shift by 0.2 eV of the MCL band near 3.1 eV dominant in the substrate and the emergence of a weak MCL peak near 4.8 eV were observed. Electron beam induced current measurements point to the presence of small local areas where the 2DEG formation is handicapped due to Al composition variations or to defects. [ABSTRACT FROM AUTHOR]

The spatial distribution of electron-hole pair generation in β-Ga2O3 as a function of scanning electron microscope (SEM) beam energy has been calculated by a Monte Carlo method. This spatial distribution is then used to obtain the diffusion length of charge carriers in high-quality epitaxial Ga2O3 films from the dependence of the electron beam induced current (EBIC) collection efficiency on the accelerating voltage of a SEM. The experimental results show, contrary to earlier theory, that holes are mobile in β-Ga2O3 and to a large extent determine the diffusion length of charge carriers. Diffusion lengths in the range 350–400 nm are determined for the as-grown Ga2O3, while processes like exposing the samples to proton irradiation essentially halve this value, showing the role of point defects in controlling minority carrier transport. The pitfalls related to using other popular EBIC-based methods assuming a point-like excitation function are demonstrated. Since the point defect type and the concentration in currently available Ga2O3 are dependent on the growth method and the doping concentration, accurate methods of diffusion length determination are critical to obtain quantitative comparisons of material quality. [ABSTRACT FROM AUTHOR]

The role of Shockley-Read-Hall non-radiative recombination centers on electroluminescence (EL) efficiency in blue multi-quantum-well (MQW) 436 nm GaN/InGaN light emitting diodes (LEDs) was examined by controlled introduction of point defects through 6MeV electron irradiation. The decrease in the EL efficiency in LEDs subjected to irradiation with fluences above 5x1015 cm-2 was closely correlated to the increase in concentration of Ec-0.7 eV electron traps in the active MQW region. This increase in trap density was accompanied by an increase in the both diode series resistance and ideality factor (from 1.4 before irradiation to 2.1 after irradiation), as well as the forward leakage current at low forward voltages that compromise the injection efficiency. Hole traps present in the blue LEDs do not have a significant effect on EL changes with radiation because of their low concentration. [ABSTRACT FROM AUTHOR]

A wide variety of parameters were measured for undoped n-GaN grown by hydride vapor phase epitaxy and compared to n-GaN films grown by conventional and lateral overgrowth metalorganic chemical vapor deposition. The parameters included deep level electron and hole trap spectra, microcathodoluminescence, electron beam induced current, diffusion length, and electron capture cross section from the dependence of the low temperature persistent photocapacitance on forward bias injection pulse duration. The results show a prominent role of electron traps with levels near Ec-0.56 eV in limiting the lifetime and diffusion length values in all these materials. [ABSTRACT FROM AUTHOR]

Electrical and luminescent properties and deep trap spectra of Si doped GaN films grown by maskless epitaxial lateral overgrowth (MELO) are reported. The dislocation density in the wing region of the structure was 106cm–2, while in the seed region it was 108cm–2. The major electron traps present had activation energy of 0.56 eV and concentrations in the high 1015cm–3 range. A comparison of diffusion length values and 0.56 eV trap concentration in MELO GaN and epitaxial lateral overgrowth (ELOG) GaN showed a good correlation, suggesting these traps could be effective in carrier recombination. The doped MELO films were more uniform in their electrical properties than either ELOG films or undoped MELO films. We also discuss the differences in deep trap spectra and luminescence spectra of low-dislocation-density MELO, ELOG, and bulk n-GaN samples grown by hydride vapor phase epitaxy. It is suggested that the observed differences could be caused by the differences in oxygen and carbon contamination levels. [ABSTRACT FROM AUTHOR]

The applicability of using Electron Beam Induced Current (EBIC) measurements on Schottky barriers to obtain the mean electron–hole pair creation energy in β-Ga2O3 is reported. It is shown that, when combined with Monte Carlo simulation, this approach yields for Si, GaN, and 4H–SiC a data set consistent with empirical expressions proposed earlier in the literature for many different semiconductors. The method is then applied to β-Ga2O3, where complications related to hole trapping in the material give rise to a strong gain in EBIC and have to be carefully treated and taken into account. When this is done, the mean electron–hole pair energy formation is found to be 15.6 eV, in reasonable agreement with the values predicted by empirical expressions. [ABSTRACT FROM AUTHOR]