Localized deep levels in Al_xGa_(1-x)N epitaxial films with various Al compositions

By using high-temperature deep-level transient spectroscopy （HT-DLTS） and other electrical measurement techniques, localized deep levels in n-type AlxGal xN epitaxial films with various A1 compositions （x = 0, 0.14, 0.24, 0.33, and 0.43） have been investigated. It is found that there are three distinct deep levels in AlxGal-xN films, whose level position with respect to the conduction band increases as AI composition increases. The dominant defect level with the activation energy deeper than 1.0 eV below the conduction band closely follows the Fermi level stabilization energy, indicating that its origin may be related to the defect complex, including the anti-site defects and divacancies in AlxGa1-xN films.

Deep level transient spectroscopy investigation of deep levels in CdS/CdTe thin film solar cells with Te:Cu back contact

Deep levels in Cds/CdTe thin film solar cells have a potent influence on the electrical property of these devices. As an essential layer in the solar cell device structure, back contact is believed to induce some deep defects in the CdTe thin film. With the help of deep level transient spectroscopy （DLTS）, we study the deep levels in CdS/CdTe thin film solar cells with Te：Cu back contact. One hole trap and one electron trap are observed. The hole trap H1, localized at Ev＋0.128~eV, originates from the vacancy of Cd （VCd. The electron trap E1, found at Ec-0.178~eV, is considered to be correlated with the interstitial Cui= in CdTe.

Deep Levels in Undoped Semi-insulating Liquid Encapsulated Czochralski GaAs Detected by Photocurrent Measurement

Deep levels in undoped semi insulating (SI) liquid encapsulated czochralski (LEC) GaAs were investigated through measuring extrinsic photocurrent spectra at 300 K. Two broad photoresponse bands M 1 and M 2 were observed in the range of 0.40 ～ 0.70 eV and the range from about 0.80 eV to the bandgap energy, respectively. It was shown that M 2 band is related to the photoionization of the deep defect EL2. M 1 band, which is reported for the first time, reveals several characteristic structures including five peaks at 0.46, 0.49, 0.56, 0.65 and 0.69 eV, respectively. The relative magnitudes of these peaks vary from sample to sample and are related to the thermal histories of the samples. These peaks are likely due to different deep levels.

Emission and capture characteristics of deep hole trap in n-GaN by optical deep level transient spectroscopy

Emission and capture characteristics of a deep hole trap(H1)in n-GaN Schottky barrier diodes(SBDs)have been investigated by optical deep level transient spectroscopy(ODLTS).Activation energy(Eemi)and capture cross-section(σ_(p))of H1 are determined to be 0.75 eV and 4.67×10^(−15)cm^(2),respectively.Distribution of apparent trap concentration in space charge region is demonstrated.Temperature-enhanced emission process is revealed by decrease of emission time constant.Electricfield-boosted trap emission kinetics are analyzed by the Poole−Frenkel emission(PFE)model.In addition,H1 shows point defect capture properties and temperature-enhanced capture kinetics.Taking both hole capture and emission processes into account during laser beam incidence,H1 features a trap concentration of 2.67×10^(15)cm^(−3).The method and obtained results may facilitate understanding of minority carrier trap properties in wide bandgap semiconductor material and can be applied for device reliability assessment.

Electronic properties and deep level transient spectroscopy of CdS/CdTe thin film solar cells

It is well known that preparing temperatures and defects are highly related to deep-level impurities. In our studies, the CdTe polycrystalline films have been prepared at various temperatures by close spaced sublimation （CSS）. The different preparing temperature effects on CdS/CdTe solar cells and deep-level impurities have been investigated by I-V and C-V measurements and deep level transient spectroscopy （DLTS）. By comparison, less dark saturated current density, higher carrier concentration, and better photovoltaic performance are demonstrated in a 580℃sample. Also there is less deep-level impurity recombination, because the lower hole trap concentration is present in this sample. In addition, three deep levels, Ev ＋ 0.341 eV（H4）, E, ＋ 0.226 eV（HS） and Ec - 0.147 eV（E3）, are found in the 580℃sample, and the possible source of deep levels is analysed and discussed.

Reduction of Deep Level Defects in Unintentionally Doped 4H-SiC Homo-epilayers by Ion Implantation

In order to reduce deep level defects, the theory and process design of 4H-SiC homoepitaxial layer implanted by carbon ion are studied. With the Monte Carlo simulator TRIM, the ion implantation range, location of peak concentration and longitudinal straggling of carbon are calculated. The process for improving deep energy level in undoped 4H-SiC homoepitaxial layer by three times carbon ion-implantation is proposed, including implantation energy, dose, the SiO2 resist mask, annealing temperature, annealing time and annealing protection. The deep energy level in 4H-SiC material can be significantly improved by implantation of carbon atoms into a shallow surface layer. The damage of crystal lattice can be repaired well, and the carbon ions are effectively activated after 1 600 ℃ annealing, meanwhile, deep level defects are decreased.

Ionization behavior of deep level donors in passive film formed on surface of stainless steel in 5% salt solution

Deep level donor＇s ionization behavior of passive film formed on the surface of stainless steel was investigated by Mott-Schottky plots. It is indicated that transformation process of deep level donors＇ ionization behavior of passive film on surface of stainless steel can be divided into 4 stages with rising immersion time. At the initial immersion stage （10 min）, Fe（II） located in the octahedral sites of the unit cell is not ionized and the deep level does not appear in Mott-Schottky plots. At the second stage （9-38 h）, Fe（II） located in the octahedral sites starts to be ionized, which results in deep level donors＇ generation and density of deep level donors almost is constant with augmenting immersion time but the thickness of space charge layer is more and more thicker with rising immersion time. At the third stage （48 h-12 d）, density of deep level donors rises with increasing immersion time and the thickness of passive films space charge layer decreases. At last stage （above 23 d）, both the space charge layer＇s thickness and density of deep level donors are no longer changed with increasing immersion time. In the overall immersion stage, the shallow level donors＇ density is invariable all the time. The mechanism of deep level donor＇s ionization can be the generation of metal vacancies, which results in crystal lattice＇s aberration and the aberration energy urges the ionization of Fe（ II ） in octahedral sites.

Charge-sensitive deep level transient spectroscopy of helium-ion-irradiated silicon,as-irradiated and after thermal annealing

Electrically active defects in the phosphor-doped single-crystal silicon, induced by helium-ion irradiation under thermal annealing, have been investigated. Isothermal charge-sensitive deep-level transient spectroscopy was employed to study the activation energy and capture cross-section of helium-induced defects in silicon samples. It was shown that the activation energy levels produced by helium-ion irradiation first increased with increasing annealing temperature, with the maximum value of the activation energy occurring at 873 K, and reduced with further increase of the annealing temperature. The energy levels of defects in the samples annealed at 873 and 1073 K are found to be located near the mid-forbidden energy gap level so that they can act as thermally stable carrier recombination centres.

Design of deep-water omnidirectional spirit level

Attitude adjustment is a key link in the installation process of underwater facilities in deep water.To solve this problem,an omnidirectional spirit level for deep water was developed.The sealing principle of the spirit level and the principle of deep-water pressure resistance are analyzed,and the threaded connection strength is checked.The mechanical simulation verifies that the spirit level can withstand the pressure of 2000 m water depth,and the water pressure test is carried out for 30 min in a 20 MPa hyperbaric chamber.After the experiment is completed,the appearance of the spirit level is intact and there is no leakage.The experiment results show that the deep-water omnidirectional spirit level can be used in the deep sea within 2000 m.

Defects evolution in n-type 4H-SiC induced by electron irradiation and annealing

Radiation damage produced in 4H-SiC by electrons of different doses is presented by using multiple characterization techniques. Raman spectra results indicate that SiC crystal structures are essentially impervious to 10 Me V electron irradiation with doses up to 3000 kGy. However, irradiation indeed leads to the generation of various defects, which are evaluated through photoluminescence(PL) and deep level transient spectroscopy(DLTS). The PL spectra feature a prominent broad band centered at 500 nm, accompanied by several smaller peaks ranging from 660 to 808 nm. The intensity of each PL peak demonstrates a linear correlation with the irradiation dose, indicating a proportional increase in defect concentration during irradiation. The DLTS spectra reveal several thermally unstable and stable defects that exhibit similarities at low irradiation doses.Notably, after irradiating at the higher dose of 1000 kGy, a new stable defect labeled as R_(2)(Ec-0.51 eV) appeared after annealing at 800 K. Furthermore, the impact of irradiation-induced defects on SiC junction barrier Schottky diodes is discussed. It is observed that high-dose electron irradiation converts SiC n-epilayers to semi-insulating layers. However, subjecting the samples to a temperature of only 800 K results in a significant reduction in resistance due to the annealing out of unstable defects.

In-doping collaboratively controlling back interface and bulk defects to achieve efficient flexible CZTSSe solar cells

Focusing on the low open circuit voltage(V_(OC))and fill factor(FF)in flexible Cu_(2)ZnSn(S,Se)_(4)(CZTSSe)solar cells,indium(In)ions are introduced into the CZTSSe absorbers near Mo foils to modify the back interface and passivate deep level defects in CZTSSe bulk concurrently for improving the performance of flexible device.The results show that In doping effectively inhibits the formation of secondary phase(Cu(S,Se)_(2))and VSndefects.Further studies demonstrate that the barrier height at the back interface is decreased and the deep level defects(Cu_(Sn)defects)in CZTSSe bulk are passivated.Moreover,the carrier concentration is increased and the V_(OC) deficit(V_(OC,def))is decreased significantly due to In doping.Finally,the flexible CZTSSe solar cell with 10.01%power conversion efficiency(PCE)has been obtained.The synergistic strategy of interface modification and bulk defects passivation through In incorporation provides a new thought for the fabrication of efficient flexible kesterite-based solar cells.

Peculiar Photoconduction in Semi-Insulating GaAs Photoconductive Switch Triggered by 1064nm Laser Pulse

The peculiar photoconduction in semi insulating GaAs photoconductive switch being triggered by 1064nm laser pulse is reported.The gap between two electrodes of the switch is 4mm.When it is triggered by laser pulse with energy of 0 8mJ and the pulse width of 5ns,and operated at biased electric field of 2 0 and 6 0kV/cm,both linear and nonlinear modes of the switch are observed respectively.Whereas the biased electric field adds to 9 5kV/cm,and the triggered laser is in range of 0 5～1 0mJ,the peculiar performed characteristic is observed:the switch gives a linear waveform firstly,and then after a delay time of about 20～250ns,it outputs a nonlinear waveform again.The physical mechanism of this specific phenomenon is associated with the anti site defects of semi insulating GaAs and two step single photon absorption.The delay time between linear waveform and nonlinear waveform is calculated,and the result matches the experiments.

Kink effect in current–voltage characteristics of a GaN-based high electron mobility transistor with an AlGaN back barrier

The kink effect in current-voltage （IV） characteristic s seriously deteriorates the performance of a GaN-based HEMT. Based on a series of direct current （DC） IV measurements in a GaN-based HEMT with an AlGaN back barrier, a possible mechanism with electron-trapping and detrapping processes is proposed. Kink-related deep levels are activated by a high drain source voltage （Vds） and located in a GaN channel layer. Both electron trapping and detrapping processes are accomplished with the help of hot electrons from the channel by impact ionization. Moreover, the mechanism is verified by two other DC IV measurements and a model with an expression of the kink current.

Investigation of GaAs Photoconductive Switch Irradiated by 1553nm Laser Pulse

Gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS's) with a 1.55mm gap spacing trigged by 1553nm femtosecond fiber laser pulse is presented.The switches are biased with 3.33～10.3kV/cm and irradiated by femtosecond fiber laser operated at a wavelength of 1553nm with pulse width of 200fs and pulse energy of 0.2nJ.The experiments show that,even if the semi-insulating GaAs photoconductive switch operates under the electrical field of 10.3kV/cm,it will be still linear response,and a clear corresponding output electric pulse with the peak voltage of 0.8mV is captured.From the weak photoconductivity on laser intensity,photoabsorption mediated by EL2 deep level defects is suggested,as the primary process for the photoconductivity.

Electric properties of Ge quantum dot embedded in Si matrix

The electric characteristics of Ge quantum dot grown by molecular beam epitaxy in Si matrix were investigated by admittance spectroscopy and deep level transient spectroscopy. The admittance spectroscopy measurements show that the activation energy of 0.341eV can be considered as the emitting energy of hole from the ground state of the quantum dot. And the capacitance variation with temperature of the sample shows a platform at various frequencies with reverse bias (0.5 V,) which indicates that the boundary of space charge region is located at the quantum dot layer where the large confined hole concentration blocks the further extension of space charge region. When the temperature increases from 120K to 200K, the holes in the dot emit out completely. The position of the platform shifting with the increase of the applied frequency shows the frequency effects of the charges in the quantum dot. The deep level transient spectroscopy results show that the charge concentration in the Ge quantum dot is a function of the pulse duration and the reverse bias voltage, the activation energy and capture cross-section of hole decrease with the increase of pulse duration due to the Coulomb charging effect. The valence-band offsets of hole in Ge dot obtained by admittance spectroscopy and deep level transient spectroscopy are 0.341 and 0.338eV, respectively.

Enhancement of Crystalline Quality of Strained InAs／InP Quantum Well Structures by Rapid Thermal Annealing

The effect of rapid thermal annealing on the optical properties of astrained InAs/InP single quantum well structrure has been investigated in this paper.The luminescence intensity of the quantum well at 8 K was increased by a factor of 4 and 1.55 meV blue shift of the quantum well photoluminescence peak was observed after annealing at the optimal condition of 700℃ for 5 s. Furthermore,we found that the luminescence efficiency of the deep radiative levels in the samples was also affected by rapid thermal annealing.Our experimental results have demonstrated that Rapid thermal annealing significantly improves the crystalline quality of strained quantum well structures after growth and is an important way for enhancement of the performance of the laser device.

Deep level defects in unintentionally doped 4H-SiC homoepitaxial layer

Unintentionally doped 4H-SiC homoepitaxial layers grown by hot-wall chemical vapor deposition （HWCVD） have been studied using photoluminescence （PL） technique in the temperature range of 10 to 240 K. A broadband green luminescence has been observed. Vacancies of carbon （Vc） are revealed by electron spin resonance （ESR） technique at 110 K. The results strongly suggest that the green band luminescence, as shallow donor-deep accepter emission, is attributed to the vacancies of C and the extended defects. The broadband green luminescence spectrum can be fitted by the two Gauss-type spectra using nonlinear optimization technique. It shows that the broad-band green luminescence originates from the combination of two independent radiative transitions. The centers of two energy levels are located 2.378 and 2.130 eV below the conduction band, respectively, and the ends of two energy levels are expanded and superimposed each other.

Semiconductor steady state defect effective Fermi level and deep level transient spectroscopy depth profiling

The widely used deep level transient spectroscopy（DLTS） theory and data analysis usually assume that the defect level distribution is uniform through the depth of the depletion region of the n–p junction. In this work we introduce the concept of effective Fermi level of the steady state of semiconductor, by using which deep level transient spectroscopy depth profiling（DLTSDP） is proposed. Based on the relationship of its transition free energy level（TFEL） and the effective Fermi level, the rules of detectivity of the defect levels are listed. Computer simulation of DLTSDP is presented and compared with experimental data. The experimental DLTS data are compared with what the DLTSDP selection rules predicted. The agreement is satisfactory.

THEORY AND PRACTICE ON DETERMINATION OF DEEP LEVEL PROFILES IN MULTIHIGH-DENSITY-LEVEL CASE

This work is an improvement of the theory proposed by Qin Guogang and C. T. Sah for determination of deep level profiles in the multi-level ease. The previous theory cannot be applied to the ease when a level whose density is comparable to the carrier density exists between the Fermi level and the deep level under study or when the deep level under study locates near the middle of the forbidden gap. The present work has overcome those restrictions so that it is applicable to more general cases. For the proton-implanted CZ-Si sample, the density profile of E(0.22), second acceptor level of divacancies, has been calculated in the presence of highly concentrated oxygen-vacancy level E(0.15) and has been compared with the profile of the same level E(0.22) calculated without considering the existence of E(0.15).

Deep level transient spectroscopy:Tracing interface and bulk trap‐induced degradation in AlGaN/GaN‐heterostructure based devices

The exceptional physical properties of gallium nitride(GaN)position GaNbased power devices as leading candidates for next‐generation high‐efficiency smart power conversion systems.However,GaN's multi‐component nature results in a high density of epitaxial defects,whereas the introduction of dielectric layers further contributes to severe interface states and dielectric traps.These factors collectively impair reliability,manifesting as threshold voltage instability and current collapse,which pose significant barriers to the advancement of GaN‐based electronics.Establishing the intrinsic relationship between device reliability and defects is crucial for understanding and addressing reliability degradation issue.Deep level transient spectroscopy(DLTS)offers valuable insights by revealing defect‐induced changes in electrical parameters during the capture and emission processes under varying biases,thereby elucidating the influence of defects from GaN buffer layers,AlGaN barriers,dielectric layer,and even at dielectric/(Al)GaN interfaces.This research aims to provide a foundational understanding of reliability degradation whereas further enabling enhancements in device performance from the perspectives of epitaxial growth and process preparation,ultimately striving to improve the reliability of GaN‐based devices and unlock their full potential for practical applications.