Search

We investigated the defect states and optoelectrical properties of methylammonium lead iodide (MAPbI3) perovskite films synthesized via a two-step dry process. To synthesize MAPbI3 thin films, PbI2 thin films were deposited via rf magnetron sputtering and were subsequently exposed to methylammonium iodide (MAI) vapor to synthesize MAPbI3 thin films. By using deep level transient spectroscopy and space-charge-limited current methods, the defect density of the perovskite films was proven to be reduced by about 6 times as the reaction temperature increased from 90 to 110 °C . At a high reaction temperature of 130 °C , the deformation process could occur, resulting in a rising trap density within the perovskite film. Along with a significant decline in defect density, the photoresponsivity of films fabricated at 110 °C were higher by about 2.05 and 18.76 times more than those of samples prepared at 100 and 130 °C , respectively. An excellent detectivity of 1.17 × 1013 Jones was observed from the sample synthesized at 110 °C , demonstrating the superiority of a two-step dry process.

Potential strategies such as surface passivation and perovskite material halide mixing may protect material surfaces, improve luminescence, and reduce charge traps for device stability. In this study, we used deep level transient spectroscopy to investigate the effect of CdSe/ZnS core-shell quantum dots (QDs) on defect states and carrier transport in methylammonium (MA) lead halide perovskites (CH3NH3PbX3 where X = I, Br). In MAPbI3 and MAPbI2Br films with CdSe/ZnS QDs, the density of hole traps located at Ev + 0.37 eV and Ev + 0.56 eV was reduced dramatically. Deep traps at Ev + 0.78 eV and Ev + 1.08 eV were removed, and one broad electron trap signal dominated. Film photoresponsivity under 600-nm wavelength light and a bias voltage of −0.7 V was 10 and 18 mA/W, which is 100 and 27 times larger than the 0.1 and 0.67 mA/W of bare perovskites (PS), respectively. This demonstrates that carrier transport was enhanced due to defect suppression. Our findings on defect suppression and photoresponsivity enhancement provide an important direction for optimizing high-performance PS device fabrication.

The influence of oxygen partial pressure and annealing on the properties of thin films of β-Ga2O3 grown by pulsed laser deposition were studied. The Ga2O3 samples were deposited at a substrate temperature of 250 °C at an oxygen pressure of 0–50 mTorr and then annealed at a temperature of 600 °C. We observed the crystallinity of Ga2O3 enhanced with annealing and with increasing oxygen pressure. The full width at half maximum of annealed β -Ga2O3 ( 4 ¯ 01 ) peaks decreased, corresponding to the grain size increasing from 6.76 nm to 11.25 nm. The conductivity of the obtained, as-grown Ga2O3 films increased with oxygen pressure from 2.1 to 7.9 mScm−1. As a result, the conductance and the energy band gap of β-Ga2O3 without annealing were controlled by the oxygen partial pressure. This was attributed to the oxygen vacancies, based on the composition ratio between O and Ga ions. These results clearly showed that the energy band gap and conductance of β-Ga2O3 thin films could be controlled in such a way that could be utilized for high-performance photo-electronic devices.

In this study, the charge transport mechanism of Pd/Si-based FS-GaN Schottky diodes was investigated. A temperature-dependent current&ndash, voltage analysis revealed that the I-V characteristics of the diodes show a good rectifying behavior with a large ratio of 103&ndash, 105 at the forward to reverse current at &plusmn, 1 V. The interface states and non-interacting point defect complex between the Pd metal and FS-GaN crystals induced the inhomogeneity of the barrier height and large ideality factors. Furthermore, we revealed that the electronic conduction of the devices prefers the thermionic field emission (TFE) transport, not the thermionic emission (TE) model, over the entire measurement conditions. The investigation on deep level transient spectroscopy (DLTS) suggests that non-interacting point-defect-driven tunneling influences the charge transport. This investigation about charge transport paves the way to achieving next-generation optoelectronic applications using Si-based FS-GaN Schottky diodes.

We studied Si-doped Ga2O3 photodetectors with a metal-semiconductor-metal structure that were fabricated using pulsed laser deposition and a post-annealing process under an oxygen atmosphere in a temperature range from 500 °C to 800 °C. After post-annealing, the Si-doped Ga2O3 photodetectors with 500 °C was showed the highest photoresponsivity (0.34 A/W) and external quantum efficiency (166.23%), while the device with 800 °C was exhibited the fastest switching speeds (0.88 s/0.18 s) for the on/off switching. These results are due to changes in the defect densities from the chemical bond formation between Si and O atoms. Our results show that post thermal annealing of Si-doped β-Ga2O3 layers can modify the photodetector performance depending on applications for high photoresponsivity or fast response time.

Because the power conversion efficiency (PCE) of hybrid halide perovskite solar cells (PSCs) could exceed 24%, extensive research has been focused on improving their long-term stability for commercialization in the near future. In a previous study, we reported that the addition of a number of ionized iodide (triiodide: I3-) ions during perovskite film formation significantly improved the efficiency of PSCs by reducing deep-level defects in the perovskite layer. Understanding the relationship between the concentration of these defects and the long-term chemical aging of PSCs is important not only for obtaining fundamental insight into the perovskite materials but also for studying the long-term chemical stability of PSCs. Herein we aim to identify the origin of the natural decay in PCE during long-term chemical aging of PSCs in the dark based on formamidinium lead triiodide by comparing the performance of control and low-defect (LD) devices. After aging for 200 days, the change in the PCE of the LD devices (1.3%) was found to be half that of the control devices (2.6%). We investigated this difference using grazing incidence wide-angle X-ray scattering, deep-level transient spectroscopy, scanning photoelectron microscopy, and high-resolution photoemission spectroscopy. The addition of I3- was found to reduce the amounts of hydroxide and Ox in the halide perovskites (HPs), affecting the migration of defects and the structural transformation of the HPs.

The influence of a titanium (Ti) adhesion layer on the ductility of thin Au film well bonded with β-Ga2O3 film on a flexible substrate were investigated with the quality of β-Ga2O3 deep-ultraviolet photodetector. The photodetector fabricated with Ti adhesion layer exhibits excellent photoelectric performance and stability. The photocurrent obtained by the Ti/Au bilayer-device (3.1 × 10−8 A) is significantly greater than that of an Au electrode-device (1.7 × 10−8 A) at 10 V under a flat state. After 250 bending cycles, the photoresponsivity of the Au electrode-device declined approximately 20 times, while little photoresponsivity degradation was observed in the Ti/Au bilayer-device. Additionally, more suppression of the deformation and cracks was observed in the Au film with Ti adhesion layer than the Au grown directly on the β-Ga2O3 films. These results reveal the importance of the adhesion layer in promoting the photodetector.

We have optimized the responsivity and response speed of a β-Ga2O3-based photodetector. The β-Ga2O3 thin films were deposited on a glass substrate under various oxygen partial pressures from 0 to 50 mTorr using pulsed laser deposition. Time-response measurements show that the as-grown β-Ga2O3 at an oxygen partial pressure of 50 mTorr has the fastest response speed and decay times of 33 and 100 ms, which are better than those prepared at lower oxygen pressures. This sample also showed a high photoresponsivity of 5 A W-1 and detectivity of 1012 cmHz1/2/W. The high performance of the β-Ga2O3 detector grown at the oxygen partial pressure of 50 mTorr might be due to the reduction of oxygen vacancies caused by the increase in oxygen content during deposition. The results reveal the importance of the oxygen processing gas in promoting photodetector performance.

We investigate the electrical characteristics of Schottky contacts for an Au/hydride vapor phase epitaxy (HVPE) a-plane GaN template grown via in situ GaN nanodot formation. Although the Schottky diodes present excellent rectifying characteristics, their Schottky barrier height and ideality factor are highly dependent upon temperature variation. The relationship between the barrier height, ideality factor, and conventional Richardson plot reveals that the Schottky diodes exhibit an inhomogeneous barrier height, attributed to the interface states between the metal and a-plane GaN film and to point defects within the a-plane GaN layers grown via in situ nanodot formation. Also, we confirm that the current transport mechanism of HVPE a-plane GaN Schottky diodes grown via in situ nanodot formation prefers a thermionic field emission model rather than a thermionic emission (TE) one, implying that Poole&ndash, Frenkel emission dominates the conduction mechanism over the entire range of measured temperatures. The deep-level transient spectroscopy (DLTS) results prove the presence of noninteracting point-defect-assisted tunneling, which plays an important role in the transport mechanism. These electrical characteristics indicate that this method possesses a great throughput advantage for various applications, compared with Schottky contact to a-plane GaN grown using other methods. We expect that HVPE a-plane GaN Schottky diodes supported by in situ nanodot formation will open further opportunities for the development of nonpolar GaN-based high-performance devices.

We report on the defect states incorporated in a-plane GaN crystals grown on r-plane sapphire substrates by hydride vapor phase epitaxy (HVPE), using deep level transient spectroscopy (DLTS). Two defect states were observed at 0.2 eV and 0.55 eV below the conduction band minimum with defect densities of 5 × 1012/cm3 and 4.7 × 1013/cm3, respectively. The size of capture cross section, non-linear relation of trap densities from the depth profile, filling pulse width, and PL measurements indicated that the electronic deep trap levels in a-plane GaN on r-plane sapphire by HVPE originated from non-interacting point defects such as NGa, complex defects involving Si, O, or C, and VGa-related centres. Even though the a-plane GaN templates were grown by HVPE with high growth rates, the electronic deep trap characteristics are comparable to those of a-plane GaN layers of high crystal quality grown by MOCVD. This study prove that the growth of a-plane GaN templates on r-plane sapphire by HVPE is a promising method to obtain a-plane GaN layers efficiently and economically without the degradation of electrical characteristics.

We investigated the defect states of organic lead halide (CH3NH3PbX3: CH3NH3+) methylammonium (MA; X = Br− and I−) by using optical pulse-induced conductance transient spectroscopy. Organic lead halide single crystals having a bandgap ranging from 2.16 to 1.45 eV were grown by inverse temperature crystallization. The shallow trap energy level of MAPbBr2.5I0.5 was Ev + 0.06 eV. The deep level traps of MAPbBr3 and MAPbI3 were Ev + 1.24 and Ev + 0.84 eV, respectively. The deep level traps of organic lead halide may have decreased the reactivity between iodide and bromide ions in the crystal lattice. Varying the ratio of halides in the perovskite can change the defect species in the crystals and can increase the photoconversion efficiency by decreasing the density of deep level traps.