Deep-level States Research Articles

Substrate Biasing Effect on the Physical Properties of Reactive RF-Magnetron-Sputtered Aluminum Oxide Dielectric Films on ITO Glasses

High dielectric constant (high-k) Al2O3 thin films were prepared on ITO glasses by reactive RF-magnetron sputtering at room temperature. The effect of substrate bias on the subband structural, morphological, electrode/Al2O3 interfacial and electrical properties of the Al2O3 films is systematically investigated. An optical method based on spectroscopic ellipsometry measurement and modeling is adopted to probe the subband electronic structure, which facilitates us to vividly understand the band-tail and deep-level (4.8-5.0 eV above the valence band maximum) trap states. Well-selected substrate biases can suppress both the trap states due to promoted migration of sputtered particles, which optimizes the leakage current density, breakdown strength, and quadratic voltage coefficient of capacitance. Moreover, high porosity in the unbiased Al2O3 film is considered to induce the absorption of atmospheric moisture and the consequent occurrence of electrolysis reactions at electrode/Al2O3 interface, as a result ruining the electrical properties.

Electrical characteristics of Mg-doped p-GaN treated with the electrochemical potentiostatic activation method

• The EPA treatment to p-GaN resulted in lower H concentration up to 56% with an increase in hole concentration. • The combined hopping and band conduction was predominant transport mechanism after EPA treatment. • The density of deep-level states (Mg–H complex) reduced from 1.28 × 10 19 to 7.5 × 10 18 cm −3 after EPA treatment. The electrical characteristics and carrier transport mechanism of Mg-doped p-GaN treated with the electrochemical potentiostatic activation (EPA) method was investigated. The EPA method resulted in lower H concentration up to 56% with a subsequent increase in hole concentration and improved ohmic contacts. The temperature-dependent electrical resistivity of the reference p-GaN followed a ∼ T −1/4 dependence, i.e., hopping conduction, while that of the EPA-treated sample began to follow a ∼ T −1 behavior involving band conduction. This was due to the reduced density of deep-level states associated with the Mg–H complex, e.g., from 1.28 × 10 19 to ∼7.50 × 10 18 cm −3 after EPA treatment. The increase in the hole concentration was responsible for the enhanced ohmic contact via tunneling.

Impact of proton irradiation on deep level states in n-GaN

Deep levels in 1.8 MeV proton irradiated n-type GaN were systematically characterized using deep level transient spectroscopies and deep level optical spectroscopies. The impacts of proton irradiation on the introduction and evolution of those deep states were revealed as a function of proton fluences up to 1.1 × 1013 cm−2. The proton irradiation introduced two traps with activation energies of EC - 0.13 eV and 0.16 eV, and a monotonic increase in the concentration for most of the pre-existing traps, though the increase rates were different for each trap, suggesting different physical sources and/or configurations for these states. Through lighted capacitance voltage measurements, the deep levels at EC - 1.25 eV, 2.50 eV, and 3.25 eV were identified as being the source of systematic carrier removal in proton-damaged n-GaN as a function of proton fluence.

Characteristics of an Indium-Rich InGaN p—n Junction Grown on a Strain-Relaxed InGaN Buffer Layer

An indium-rich InGaN p—n junction is grown on a strain-relaxed InGaN buffer layer. The results show that the n-InGaN is grown coherently on the buffer layer but the p-InGaN layer exhibits a partial strain relaxation. The fabricated InGaN p—n junction has a low reverse leakage current density on the order of 10−8 A/cm2 within the measured voltage range, and exhibits a wide spectral response due to the presence of band tail states or deep level states which origin from indium composition fluctuation or various defects. The measured peak responsivity at 438 nm is 31 mA/W at zero bias and reaches 118 mA/W at 3V reverse bias. In addition, the Raman spectra of the p- and n-type InGaN alloys are also analyzed.

Spin-dependent electronic processes and long-lived spin coherence of deep-level trap sites in CdS nanocrystals

Carrier trapping in colloidal nanocrystals represents a major energy loss mechanism for excitonic states crucial to devices. Surprisingly little is known about the influence of the spin degree of freedom on the nature of these intrinsic trap centers or the types of coupling that these states experience. Here, a pulsed microwave optically detected magnetic resonance study is presented that aims to probe the interaction pathways existing between shallow band-edge trap states and the deep-level emissive chemical defect states responsible for the broad, low-energy emission common to CdS nanocrystals. Due to long spin coherence times (${T}_{2}$) of these states, Rabi flopping detected in the luminescence under magnetic resonance provides access to information regarding the modes of coupling of shallow-trapped electron-hole pairs, both of isolated species and of those in proximity to the emissive defect. Corresponding optically detected spin-echo experiments expose an extraordinarily long intrinsic spin coherence time (${T}_{2}\ensuremath{\approx}1.6$ \ensuremath{\mu}s) for colloidal nanocrystals, and an electron spin-echo envelope modulation indicative of local spin interactions. This effect provides opportunities for gaining the detailed chemical and structural information needed in order to eliminate energy loss mechanisms during the synthetic process.

Improved Ohmic contacts to plasma etched n-Al0.5Ga0.5N by annealing under nitrogen ambient before metal deposition

The contact characteristics of Ti/Al and Ti/Al/Ni/Au contacts to as-grown, plasma etched and plasma etched + annealed in N2 n-Al0.5Ga0.5N epilayers were compared. After a rapid thermal annealing, both Ti/Al and Ti/Al/Ni/Au contacts to as-grown and plasma etched + annealed in N2 n-Al0.5Ga0.5N became truly Ohmic, whereas the contacts to plasma etched samples still remained rectifying. Surface atomic concentration analysis indicates the N vacancies resulting from plasma treatment act more as deep-level states rather than shallow donors. However, these deep-level states could be effectively removed by annealing the plasma etched n-Al0.5Ga0.5N under N2 ambient, and as a result, the Fermi level was elevated toward conduction band edge, facilitating the formation of Ohmic contacts.

AlGaN-GaN Metal-Insulator-Semiconductor High-Electron-Mobility Transistors With Very High- $k$ Oxynitride $\hbox{TaO}_{x} \hbox{N}_{y}$ Gate Dielectric

We investigated AlGaN-GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) with high-permittivity (high-k) oxynitride TaOxNy gate dielectric (k ~24) obtained by sputtering deposition. The MIS-HEMTs show excellent pulsed current-voltage characteristics, small off-leakage currents ( 1010), and favorable threshold voltage stabilities. From comparisons with MIS-HEMTs with TaOx gate dielectric, we conclude that N incorporation in gate dielectric can suppress current collapse and deep-level states.

Broad visible emission from GaN nanowires grown on n-Si (1 1 1) substrate by PVD for solar cell application

Nanostructured gallium nitrides (GaNs) were grown on a catalyst–free Si (111) substrates using physical vapor deposition via thermal evaporation of GaN powder at 1150°C in the absence of NH3 gas for different deposition time. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometer (EDX) results indicated that the growth of GaN nanostructure varies with deposition time. Both X-ray diffraction (XRD) patterns and Raman spectra reveals a hexagonal GaN with wurtzite structure. Photoluminescence (PL) showed that the UV emission was suppressed, and the visible band emission was enhanced with increasing deposition time. Enhancement of visible band emission from the GaN NWs is due to the increasement of deep level states, which was resulted from growth process. Current–voltage (IV) characteristics of GaN/Si heterostructure were measured and good rectifying behavior was observed for this photodiode (PD). The forward current under illumination was almost three times than that in the dark current at +5V. Responsivity of the photodetector was 10.5A/W at range from 350nm to 500nm, which rapidly increased to 13.6A/W at 700nm. We found that the fabricated photodiode PD has an infra-red (IR) photoresponse behavior. The analysis of optical and electrical properties indications that the grown GaN in the absent of NH3 is a promising optical material and has potential applications in photo voltage solar cell.

Capacitance characteristics of atomic layer deposited Al2O3/n-GaN MOS structure

The Ni/Au/Al2O3/n-GaN metal-oxide-semiconductor structure with circular transparent electrode has been fabricated by using atomic layer deposition technique. Effects of ultra-violet (UV) light illumination on the capacitance characteristics and deep interface states are analyzed. Physical origin of bias-induced capacitance drop in the accumulation region of some non-ideal devices is explored. Due to the extremely long electron emission time and the extremely slow minority carrier generation rate, a typical deep depletion behavior can be observed in the dark room-temperature capacitance-voltage sweep curve, and the deep-level interface state occupancy above the electron quasi-Fermi level remains unchanged. Under the UV illumination, photo-induced holes will empty the deep interface traps above the electron quasi-Fermi level, and also de-charge the deep donor-like traps in the oxide layer. The anomalous capacitance drop in the accumulation region is attributed to the bias-dependent excessive leakage conductance across the dielectric layer, which might be induced by a charge-to-breakdown process related to electrical traps in the oxide and the inferior interface quality.

Effect of pressure‐assisted thermal annealing on the optical properties of ZnO thin films

ZnO thin films were prepared by the polymeric precursor method. The films were deposited on silicon substrates using the spin-coating technique, and were annealed at 330 °C for 32 h under pressure-assisted thermal annealing and under ambient pressure. Their structural and optical properties were characterized, and the phases formed were identified by X-ray diffraction. No secondary phase was detected. The ZnO thin films were also characterized by field-emission scanning electron microscopy, Fourier transform infrared spectroscopy, photoluminescence and ultraviolet emission intensity measurements. The effect of pressure on these thin films modifies the active defects that cause the recombination of deep level states located inside the band gap that emit yellow-green (575 nm) and orange (645 nm) photoluminescence.

Predicting Dislocations and Grain Boundaries in Two-Dimensional Metal-Disulfides from the First Principles

Guided by the principles of dislocation theory, we use the first-principles calculations to determine the structure and properties of dislocations and grain boundaries (GB) in single-layer transition metal disulfides MS(2) (M = Mo or W). In sharp contrast to other two-dimensional materials (truly planar graphene and h-BN), here the edge dislocations extend in third dimension, forming concave dreidel-shaped polyhedra. They include different number of homoelemental bonds and, by reacting with vacancies, interstitials, and atom substitutions, yield families of the derivative cores for each Burgers vector. The overall structures of GB are controlled by both local-chemical and far-field mechanical energies and display different combinations of dislocation cores. Further, we find two distinct electronic behaviors of GB. Typically, their localized deep-level states act as sinks for carriers but at large 60°-tilt the GB become metallic. The analysis shows how the versatile GB in MS(2) (if carefully engineered) should enable new developments for electronic and opto-electronic applications.

White electroluminescence from ZnO nanorods/p-GaN heterojunction light-emitting diodes under reverse bias

Heterojunction light-emitting diodes (LEDs) based on arrays of ZnO nanorods were fabricated on p-GaN films by the hydrothermal method. Without any phosphors, white-light electroluminescence (EL) from ZnO nanorods/p-GaN heterojunction LEDs operated at reverse breakdown bias was observed. The EL spectra are composed of an ultraviolet (UV) emission centered at 382 nm, a blue light located at 431 nm and a broadband yellow–green light at around 547 nm, which originated from band-edge emission in ZnO, the Mg acceptor levels in p-GaN and the deep-level states near the ZnO/GaN interface, respectively. The chromaticity coordinates of EL spectrum are very close to the (0.333, 0.333) of standard white light. The origin of these emissions has been discussed and the tunneling effect in the interface is probably the mechanism to explain EL emission.

Transient photoconductivity responses in amorphous In-Ga-Zn-O films

We studied the photoconductivity responses in amorphous In-Ga-Zn-O (a-IGZO) films using a time-resolved microwave photoconductivity decay (μ-PCD) technique. The a-IGZO film characteristics are correlated with three components in the photoconductivity response: the peak value and two decay constants. The peak value originated from the density of the photo-generated free carriers through carrier generation and recombination processes during laser pulse irradiation. Power law characteristics indicated that the peak values are attributed to recombination process related to the exponential distribution of the conduction band tail states. After the laser pulse was turned off, the reflectivity signal decreased rapidly, indicating fast recombination of the photo-generated carriers. This fast decay component is suggested to be related to the recombination processes through the deep level states. Following the fast decay, a slow decay with a decay constant on the order of microseconds appeared. This slow decay was attributed to the reemission of trapped carriers with an activation energy of ∼0.2 eV. In addition, both the fast and slow decays for the wet annealed a-IGZO film were longer than those of the as-deposited a-IGZO film. The decay constants are considered to reflect the density of the subgap states that act as trapping or recombination centers. The μ-PCD method provides a useful estimation of the film quality, such as the density of the defect states, and the physical properties of electronic devices using a-IGZO films.

Effects of a key deep level and interface states on the performance of GaAsN solar cells: a simulation analysis

To explore the origin of low conversion efficiency of GaAsN solar cells, the effects from a key deep level (E1) at about 0.3–0.4 eV below the conduction band and interface states between the GaAsN emitter and GaAs front surface field layer were investigated by numerical simulation. Our results show that a high deep level concentration in GaAsN layers and a large interface recombination velocity are necessary to fit the experimental data. Interface recombination plays an important role in reducing the quantum efficiency in the short-wavelength region. Its effect can be weakened by applying a thin emitter and a high doping level in the GaAs surface field layer. The abundant existence of E1 is responsible for the high contribution of Shockley–Read–Hall recombination from the space-charged region. Its density should be reduced to <1015 cm−3 for higher conversion efficiency.

Illumination effects on electrical characteristics of GaN/AlGaN/GaN heterostructures and heterostructure field effect transistors and their elimination by proper surface passivation

The effect of ambient illumination is investigated for differently processed GaN/AlGaN/GaN heterostructure materials. For samples of the same material with different passivation, the difference in sheet resistance of illuminated and non-illuminated material can be as large as 130% (for annealed heterostructure without passivation) and as small as 3% (for heterostructure passivated with low pressure chemical vapor deposition (LPCVD) silicon nitride). The time constant for the decay of the persistent photoconductance (PPC) is also very different for the differently processed samples. The majority of the effect on the conductance is from photons with energies between 3.1 and 3.7 eV. The investigation indicates that delayed recombination of electrons emitted from surface states and from deep level states in the AlGaN layer dominates the PPC. A theory is formulated by which the difference in illumination sensitivity for the differently passivated materials can be explained by different distributions of electrons between the channel two dimensional electron gas and an accumulation layer formed in the cap layer. For practical heterostructure field effect transistor (HFET) measurements, the illumination sensitivity is generally lower than that of the Hall measurements. Furthermore, HFETs fabricated with the LPCVD silicon nitride passivation are practically illumination invariant.

Surface Fermi level pinning and carrier transport of indium-tin-oxide Ohmic contact to p-type GaN

The carrier transport mechanism of indium-tin-oxide (ITO) Ohmic contact to p-type GaN was investigated. The thermally annealed ITO contact to p-GaN produced a low specific contact resistance of 8.1×10−3Ωcm2, due to the low effective barrier height of 0.11eV, for which the carriers must overcome to flow from p-GaN to the metals via hopping conduction through deep-level defect states. The surface Fermi level of a highly Mg-doped p-GaN surface was also found to pin near the midgap states, i.e., ∼0.6 of bandgap above valence band, which are caused by deep-level defect states.

Analysis of the reverse leakage current in AlGaN/GaN Schottky barrier diodes treated with fluorine plasma

The carrier transport mechanism of CF4 plasma-treated AlGaN/GaN Schottky barrier diodes (SBDs) under reverse bias is investigated. The reverse leakage current is reduced by ∼2 orders of magnitude after the CF4 plasma treatment, but increases exponentially with increasing temperature, indicating that a thermally activated transport mechanism is involved. Based on the activation energy estimated from temperature-dependent current-voltage characteristics and the emission barrier height extracted from Frenkel-Poole emission model, it is suggested that the dominant carrier transport mechanism in the CF4 plasma treated SBDs is the Frenkel-Poole emission from fluorine-related deep-level states into the continuum states of dislocations.

Trapped carrier relaxation in deep level states in GaInNAs: studied with picosecond pump-probe measurements

A pump-probe study of carrier dynamics in GaInNAs was carried out.Induced absorption by trapped electrons in deep level states becomes important as inferred from the sign of the transmittance change.A simplified differential equation model has been utilized to simulate photo-generated carrier dynamics in GaInNAs and to fit differential transmission results.The model can interpret the temporal evolution of the trapped carrier density and extract the time constants of the carrier relaxation processes.

Influence of oxygen vacancy on the electronic structure of CaCu3Ti4O12 and its deep-level vacancy trap states by first-principle calculation

The electronic structure, formation energy, and transition energy levels of intrinsic defects have been studied using the density-functional method within the generalized gradient approximation for neutral and charged oxygen vacancy in CaCu3Ti4O12 (CCTO). It is found that oxygen vacancies with different charge states can be formed in CCTO under both oxygen-rich and poor conditions for nonequilibrium and higher-energy sintering processes; especially, a lower formation energy is obtained for poor oxygen environment. The charge transition level (0/1+) of the oxygen vacancy in CCTO is located at 0.53 eV below the conduction-band edge. The (1+/2+) transition occurs at 1.06 eV below the conduction-band edge. Oxygen vacancies of Vo1+ and Vo2+ are positive stable charge states in most gap regions and can act as a moderately deep donor for Vo1+ and a borderline deep for Vo2+, respectively. The polarization and dielectric constant are considerably enhanced by oxygen vacancy dipoles, due to the off-center Ti and Cu ions in CCTO.

A physical surface-potential-based drain current model for polysilicon thin-film transistors

A physical drain current model of polysilicon thin-film transistors based on the charge-sheet model, the density of trap states and surface potential is proposed. The model uses non-iterative calculations, which are single-piece and valid in all operation regions above flat-band voltage. The distribution of the trap states, including both Gaussian deep-level states and exponential band-tail states, is also taken into account, and parameter extraction of trap state distribution is developed by the optoelectronic modulation spectroscopy measurement method. Comparisons with the available experimental data are accomplished, and good agreements are obtained.