Semi-insulating Layer Research Articles

InGaN–GaN MQW Metal–Semiconductor–Metal Photodiodes With Semi-Insulating Mg-Doped GaN Cap Layers

InGaN-GaN multiple-quantum-well metal-semiconductor-metal photodiodes (PDs) with in situ grown 40-nm-thick unactivated semi-insulating Mg-doped GaN cap layer were successfully fabricated. The dark leakage current of this PD was comparably much smaller than that of conventional PD without the semi-insulating layer, because of a thicker and higher potential barrier of semi-insulating cap layer, and also a smaller number of surface states involved. For the PDs with the semi-insulating Mg-doped GaN cap layers, the responsivity at 380nm was 0.372A/W when biasing at 5 V. In short, incorporating a semi-insulating Mg-doped GaN cap layer into the PDs beneficially leads to the suppression of dark current and a corresponding improvement in the ultraviolet-to-visible rejection ratio

Analysis of buffer‐related lag phenomena and current collapse in GaN FETs

Drain-current responses of GaN MESFETs with a semi-insulating buffer layer are calculated when the drain voltage and/or the gate voltage are changed abruptly, and pulsed I-V curves are derived from them. It is shown that so-called current collapse or current slump is not so dependent on the gate length LG (0.3 μm – 1 μm), and this is essentially determined by a deep-acceptor density NDA in the buffer layer. LG and NDA dependence of gate lag is also studied, indicating that the gate lag is weaker for longer LG and the lag rate becomes high with NDA, although it may show a saturation behaviour. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical and optical characteristics of vanadium in 4H-SiC

A semi-insulating layer is obtained in N-type 4H-SiC by vanadium-ion implantation. A little higher resistivity is obtained by increasing the annealing temperature from 1450 to 1650 °C. The resistivity at room temperature is as high as 7.6 × 106 Ω.cm. Significant redistribution of vanadium is not observed even after 1650 °C annealing. Temperature-dependent resistivity and optical absorption of V-implanted samples are measured. The activation energy of vanadium acceptor level is observed to be at about EC- 1.1 eV.

Bismuth surfactant effects for GaAsN and beryllium doping of GaAsN and GaInAsN grown by molecular beam epitaxy

Bi was investigated as a possible surfactant for growth of GaAs 1− x N x layers on (1 0 0) GaAs substrates by molecular beam epitaxy (MBE) using a radio frequency (RF) plasma nitrogen source. Importantly, Bi extends the useable growth conditions producing smoother surfaces to a significantly higher group V fractional N content than without Bi, enhancing possibilities for growth of structures requiring a larger nitrogen content. The conductivity of Be-doped GaAsN and GaInAsN decreased significantly with increasing N concentration. Temperature-dependent Hall measurement suggests possible compensation and increased activation energy. SIMS and Raman measurements indicate that the N composition increased with introducing Be, and for low [N], with the presence of Bi. The addition of Bi during growth of Be-doped GaAsN only produced semi-insulating layers at all concentrations investigated suggesting it enhances the formation of compensating defects.

Integrated nitride-based acoustic wave devices and methods of fabricating integrated nitride-based acoustic wave devices

A monolithic electronic device includes a substrate, a semi-insulating, piezoelectric Group III-nitride epitaxial layer formed on the substrate, a pair of input and output interdigital transducers forming a surface acoustic wave device on the epitaxial layer and at least one electronic device (such as a HEMT, MESFET, JFET, MOSFET, photodiode, LED or the like) formed on the substrate. Isolation means are disclosed to electrically and acoustically isolate the electronic device from the SAW device and vice versa. In some embodiments, a trench is formed between the SAW device and the electronic device. Ion implantation is also disclosed to form a semi-insulating Group III-nitride epitaxial layer on which the SAW device may be fabricated. Absorbing and/or reflecting elements adjacent the interdigital transducers reduce unwanted reflections that may interfere with the operation of the SAW device.

Electric field effects on adsorption/desorption of proteins and colloidal particles on a gold film observed using surface plasmon resonance

We present results for the enhancement or retardation of the attachment of the protein soybean peroxidase to a gold substrate using electric fields. We detect the influence of the electric fields on the proteins using the surface plasmon resonance phenomenon. The gold surface on which the surface plasmon resonance is generated also acts as one of the electrodes required to apply the electric field. The second electrode has a semi-insulating layer that limits current flow, and limits electrolysis effects. The results show that at pH 7, when the soybean peroxidase is negatively charged, a greatly enhanced deposition of protein is obtained when a positive potential is applied to the gold electrode. A negative potential can inhibit protein attachment, or reduce the amount of protein attached; however, after repeated applications of a positive potential, a negative potential has little effect. Results are presented of experiments using colloidal silica that has negatively charged particles of similar size to soybean peroxidase, showing the attraction and repulsion of negatively charged particles by the gold surface.

Photoelectric properties of highly excited GaN:Fe epilayers, grown by modulation- and continuous-doping technique s

Investigation of photoelectrical properties of iron modulation-doped (MD) and continuously doped (CD) GaN layers has been carried out by transient photo-Hall, photo-conductivity, and time-resolved picosecond four-wave-mixing (FWM) techniques. The MD semi-insulating (SI) layers exhibited prolonged photocurrent relaxation time and the presence of deep defects with thermal activation energy of 217 meV. Low electrical activity of threading dislocations (TD) in the upper part of the MD layers as well as high carrier mobility at low temperature was confirmed by FWM measurements and pointed out to vanishing dislocation-related heterogeneous barriers due to Fe doping. In contrast, shorter carrier lifetimes and low mobility in CD-layers were attributed to Fe-related defects, more “detrimental” centers than dislocations.

Homoepitaxial Growth of Iron-Doped 4H-SiC Using BTMSM and t-Butylferrocene Precursors for Semi-Insulating Property

In this paper, we attempted to grow semi-insulating SiC epitaxial layer by in-situ iron doping. Homoepitaxial growth of iron-doped 4H-SiC layer was performed by MOCVD using organo-silicon precursor, bis-trimethylsilylmethane (BTMSM, [C7H20Si2]) and metal organic precursor, t-butylferrocene ([C14H17Fe]). Doping-induced crystallinity degradation showed different tendency depending on conducting type of substrate. The crystal quality of epilayer grown on n-type substrate was not degraded significantly despite of the Fe doping but in case of semi-insulating substrate, crystallinity was remarkably degraded as increasing iron contents. For measurement of resistivity of highly resistive iron-doped 4H-SiC epilayer, we used the on-resistance technique which is firstly attempted for measuring resistivity of epilayer. From on-resistance of epilayer measured by I-V, it is shown that the residual donor concentration of epilayer was decreased as increasing partial pressure of t-butylferrocene. The resistivity of iron-doped 4H-SiC epilayer was about 107 Ωcm. From this result, it is concluded that Fe could effectively act as a compensation center in the iron-doped 4H-SiC.

Nitride-Based MIS-Like Photodiodes With Semiinsulating Mg-Doped GaN Cap Layers

Nitride-based metal-insulator-semiconductor (MIS)-like photodiodes (PDs) with in situ grown 30-nm-thick unactivated semiinsulating Mg-doped GaN cap layers were fabricated. The authors found that the reverse leakage current of the aforementioned PD was comparably much smaller than that of conventional PD without the semiinsulating layer due to the facts that inserting a semiinsulating layer would result in a thicker and higher potential barrier, and also less amounts of interface states introduced. To sum up, it was determined that the benefits of incorporating a semiinsulating Mg-doped cap layer into the PD would encompass a larger photocurrent-to-dark-current contrast ratio and larger ultraviolet-to-visible rejection ratio

Nitride-based MIS-like diodes with semi-insulating Mg-doped GaN cap layers

Nitride-based diodes with an in situ grown, 30 nm thick unactivated Mg-doped GaN cap layer were fabricated. It was found that we could significantly reduce leakage current by using the semi-insulating Mg-doped GaN cap layer. This is due to the thicker and higher potential barrier and the effective passivation of surface states when the semi-insulating Mg-doped GaN cap layer was inserted.

Growth of high crystalline quality semi-insulating GaN layers for high electron mobility transistor applications

Semi-insulating character (sheet resistivity of 3.26×1011Ω∕sq) of thick GaN layers was developed for AlGaN∕GaN high electron mobility transistor (HEMT) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN∕GaN HEMT structures on sapphire substrates.

Carrier transport and capture in GaN single crystals and radiation detectors and effect of the neutron irradiation

Carrier transport and trapping were investigated in GaN single crystals and semi-insulating epitaxial MOCVD layers by thermally stimulated current (TSC) and thermally stimulated depolarization (TSD) spectroscopy. Effect of irradiation by 100 keV reactor neutrons with fluencies of up to 10 16 n/cm 2 was identified. We demonstrate that in the unirradiated samples TSC spectra might be caused not by carrier generation from traps, but it rather originates from the thermal mobility variation. The numerical analysis revealed that mobility limited by ionized impurities varies as ∼ T 2.8 and lattice scattering causes the dependence ∼ T −3.5. The highest mobility values were up to 1550 cm 2/Vs at 148–153 K, indicating relatively high quality of the unirradiated samples. The irradiation, depending on the doses, induced complex changes of the carrier transport. The growth of the sample resistivity took place because of the more intense carrier trapping and scattering accompanied by appearance of the percolation transport phenomena. The whole ensemble of the defect traps was identified by the TSC in multiple heating regime and by measurements at different applied biases. In the samples irradiated by 10 16 n/cm 2 the following activation energy values were found: 0.16–0.2, 0.27–0.32, 0.36–0.45 and 0.73–0.74 eV. Effects associated with the percolation transport in a disordered media were observed in the samples irradiated by 5×10 14–10 16 n/cm 2, e.g., current instabilities and decrease of the effective thermal activation energy values by heating in the temperature region below 150 K. A model of carrier percolation transport is presented, in which effect of potential fluctuations due to crystal inhomogeneities is involved. The TSD measurements had confirmed the intensifying effect of inhomogeneities upon irradiation.

Simulation of slow current transients and current collapse in GaN FETs

Two-dimensional transient simulations of GaN MESFETs are performed in which a three-level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor and a deep acceptor are considered. When the drain voltage V D is raised abruptly (while keeping the gate voltage V G constant), the drain current I D overshoots the steady-state value, and when V D is lowered abruptly, I D remains a low value for some periods, showing drain-lag behavior. These are explained by the deep donor’s electron capturing and electron emission processes. We also calculate a case when both V D and V G are changed abruptly from an off point, and quasi-pulsed I-V curves are derived from the transient characteristics. It is shown that the drain currents in the pulsed I-V curves are rather lower than those in the steady state, indicating that so-called current collapse could occur due to deep levels in the semi-insulating buffer layer. It is also shown that the current collapse is more pronounced when V D is lowered from a higher voltage during turn-on, because the trapping effects become more significant.

On Counter-Charges in Development Rollers for Electrophotography

The roles of counter-charges in toner charging and toner deposition processes for single-component development are studied by quantitative analyses of charge injection into and transport through the development rollers, taking into consideration the non-Ohmic nature of the semi-insulating overcoat layer of the rollers. The electrical requirements for the roller coatings, especially as a consequence of increased printing speed, are elucidated. Based on the findings, an ideal evaluation technique for development roller performance is suggested.

Physics-based simulation of buffer-trapping effects on slow current transients and current collapse in GaN field effect transistors

Two-dimensional transient analyses of GaN metal-semiconductor field effect transistors (MESFETs) are performed in which a three level compensation model is adopted for a semi-insulating buffer layer, where a shallow donor, a deep donor, and a deep acceptor are included. Quasipulsed current-voltage (I-V) curves are derived from the transient characteristics and are compared with steady-state I-V curves. It is shown that when the drain voltage VD is raised abruptly, the drain current ID overshoots the steady-state value, and when VD is lowered abruptly, ID remains at a low value for some periods, showing drain-lag behavior. These are explained by the deep donor’s electron capturing and electron emission processes quantitatively. The drain lag could be a major cause of current collapse, although some gate lag is also seen due to the buffer layer. The current collapse is shown to be more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because the change of ionized deep-donor density becomes larger and hence the trapping effects become more significant. It is suggested that to minimize the current collapse in GaN-based FETs, an acceptor density in a semi-insulating layer should be made low, although the current cutoff behavior may be degraded.

Multiple-frequency current oscillations in GaAs-AlGaAs quantum wells containing a thin semi-insulating layer

We have investigated non-equilibrium electron transport properties of a quantum well with an inserted thin semi-insulating potential barrier layer in nonlinear bias using a time-dependent simulation technique. We find that the charge redistribution with time in the whole structure has an important effect on the final current-voltage (I-V) curves. The results show that there are two evident current hysteresis phenomena in the negative differential conductance regions and the inserted semi-insulating potential barrier layer induces the formation of multiple emitter quantum wells, which leads to high-frequency terahertz current oscillations with multiple-frequency relations around the valley of current.

Characterization of as-grown and heavily irradiated GaN epitaxial structures by photoconductivity and photoluminescence

The influence of radiation defects on photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN/sapphire substrates. Defects induced by 10-keV X-ray irradiation with a dose of 600 Mrad and 100-keV neutrons with fluences of 5×10 14 and 10 16 cm −2 have been revealed through contact photoconductivity and microwave absorption transients. The amplitude of the initial photoconductivity decay is significantly reduced by the radiation defect density. A simultaneous decrease with radiation-induced defect density is also observed in the steady-state PL intensity of yellow, blue and ultraviolet bands peaked at 2.18, 2.85, and 3.42 eV, respectively. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess carrier multi-trapping. The decay can be described by the stretched exponential approximation exp[ - ( t / τ ) α ] with different values of α in as-grown material ( α ≈ 0.7 ) and irradiated samples ( α ≈ 0.3 ). The value of the fracton dimension d s of the disordered structure, evaluated as d s = 2 α / ( 1 - α ) , changes from 4.7 to 0.86 for as-grown and irradiated material, respectively, implying percolative carrier motion on an infinite cluster of dislocations net in the as-grown material and cluster fragmentation into finite fractons after irradiation.

Role of radiation defects in photoconductivity transients and photoluminescence spectra of epitaxial GaN layers

Effect of radiation defects on photoconductivity transients and photoluminescence (PL) spectra has been examined in semi-insulating GaN epitaxial layers. Manifestation of defects induced by x-ray irradiation with the dose of 600 Mrad and neutrons with the fluence of 5 × 1014 cm–2 has been revealed through steady-state and pulsed PL as well as through contact photoconductivity (CPC) and microwave absorption (MWA) transients. Synchronous decrease in the PL intensity of yellow (Y), blue (B) and ultraviolet (UV) bands peaked at 2.19 eV, 2.85 eV and 3.42 eV, respectively, with increasing the density of radiation-induced defects was observed. The observed effect of radiation on the PL characteristics has been explained by interaction of the radiation defects with the native traps, responsible for Y-band PL or by structural modification of the “yellow” centers. The decrease in the PL intensity with concentration of radiation-induced defects is accompanied by an increase of the asymptotic decay lifetimes in time-stretched multi-trapping processes. The CPC and MWA decays, reflecting these processes, fit well a stretched-exponent approximation exp[–(t/τ)α] with the time-stretching factor α that transforms from α = 0.7 for as-grown material to α = 0.3 for irradiated one. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Buffer‐trapping effects on drain lag and power compression in GaN FET

Buffer-trapping effects in a GaN MESFET are studied by two-dimensional transient simulation. A three-level compensation model is adopted for a semi-insulating buffer layer where a shallow donor, a deep donor and a deep acceptor are considered. It is shown that when the drain voltage VD is raised, the drain current overshoots the steady-state value, and when VD is lowered, the drain current remains at a low value for some periods, showing drain lag behavior. This drain lag is shown to become a cause of so-called power compression in the GaN MESFET. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

On Roller Charging of Photoreceptors for Electrophotography

The process of charging photoreceptors with a biased roller is analyzed with a charge transport model that takes into consideration the space charge effects in the semi-insulating overcoat layer of the roller and the non-Ohmic charge injection at layer boundaries. The effects of these transport parameters on the charging performance are examined. Based on the results, an ideal evaluation technique for charging roller performance is suggested.