Semi-insulating Layer Research Articles

Fe-doping in hydride vapor-phase epitaxy for semi-insulating gallium nitride

Fe-doping of GaN layers of 3in. in diameter and a thickness of 1mm in a vertical AIX-HVPE reactor is studied. Ferrocen was used as Fe source. It is shown that a sufficient uniformity of growth conditions, a high purity of undoped GaN layers, and a moderate Fe incorporation of 2×1018cm−3 allow for growth of semi-insulating GaN layers with a sufficiently high specific resistivity even at elevated temperature. This makes the material suitable as substrate for electronic power devices at high power or in harsh ambient.

Reduced gate leakage current of AlInN:Mg/GaN high electron mobility transistors

AlInN/GaN high electron mobility transistors fabricated with the semi-insulating barrier layer was investigated to reduce the gate leakage current. The gate leakage current was obtained to be as low as 4.1 × 10−4 mA/mm at the gate bias of −10 V owing to the use of higher and thicker potential barrier of Mg-doped AlInN.

Atomic and electronic structure of amorphous and nanocrystalline layers of semi-insulating silicon produced by chemical-vapor deposition at low pressures

Semi-insulating silicon SIPOS layers are prepared on single-crystal silicon substrates of two orientations Si (111) and Si (100) by chemical-vapor deposition at a reduced pressure in a horizontal hot-wall reactor at a temperature of 638°C and pressure of P = 20 Pa with a silane flow rate of 8 L/h and the addition of nitrous oxide into the reactor environment. Electronic-structure and phase-composition investigations by ultrasoft X-ray emission spectroscopy (USXES) and X-ray diffraction (XRD) methods show that the addition of nitrous-oxide gas into the reactor environment results in the formation of amorphous silicon layers. The layer-by-layer-phase analysis via USXES of the SIPOS samples to a depth of 120 nm without destruction indicates the predominance of oxide phases in the sample surface layers with a thickness of about 10 nm. The general content of unbound and bound oxygen of less than about 10%, below which nanocrystalline silicon is formed with an average particle size of 60–70 nm, can be considered as some conditional crystallization threshold of SIPOS amorphous layers.

Estimation of background carrier concentration in fully depleted GaN films

Buffer leakage is an important parasitic loss mechanism in AlGaN/GaN high electron mobility transistors (HEMTs) and hence various methods are employed to grow semi-insulating buffer layers. Quantification of carrier concentration in such buffers using conventional capacitance based profiling techniques is challenging due to their fully depleted nature even at zero bias voltages. We provide a simple and effective model to extract carrier concentrations in fully depleted GaN films using capacitance–voltage (C–V) measurements. Extensive mercury probe C–V profiling has been performed on GaN films of differing thicknesses and doping levels in order to validate this model. Carrier concentrations as extracted from both the conventional C–V technique for partially depleted films having the same doping concentration, and Hall measurements show excellent agreement with those predicted by the proposed model thus establishing the utility of this technique. This model can be readily extended to estimate background carrier concentrations from the depletion region capacitances of HEMT structures and fully depleted films of any class of semiconductor materials.

A new growth method of semi-insulating GaN layer for HEMT structure by eliminating degenerate layer at GaN/sapphire interface

A new method of growing a semi-insulating GaN layer for high electron mobility transistor (HEMT) structure by eliminating a degenerate layer located at the GaN/sapphire interface is proposed. In the process, during the temperature ramp-up after the growth of a low temperature GaN buffer layer, trimethylgallium (TMGa) was flowed into the reactor together with ammonia, leading to the growth of an additional GaN layer of high carbon concentration. We confirmed that the introduction of TMGa induced high carbon concentration in the degenerate layer. The incorporated carbon formed deep acceptors, CN, compensating donors such as ON, resulting in the elimination of the degenerate layer. A HEMT device made on the sample grown by the new growth process shows a good pinch-off characteristic and high off-state breakdown voltage over 800 V at a gate voltage of −4 V, indicating the new scheme is effective to grow a high quality semi-insulating GaN layer.

Growth and characterization of semi-insulating carbon-doped/undoped GaN multiple-layer buffer

We have proposed a new semi-insulating GaN buffer layer, which consists of multiple carbon-doped and undoped GaN layer. The buffer layer showed sufficiently good semi-insulating characteristics, attributed to the depletion effect between the carbon-doped GaN and the undoped GaN layers, even though the thickness of the carbon-doped GaN layer in the periodic structure was designed to be very thin to minimize the total carbon incorporation into the buffer layer. The AlGaN/AlN/GaN heterostructure grown on the proposed buffer exhibited much better electrical and structural properties than that grown on the conventional thick carbon-doped semi-insulating GaN buffer layer, confirmed by Hall measurement, x-ray diffraction, and secondary ion mass spectrometry. The fabricated device also showed excellent buffer breakdown characteristics.

Atomic scale control of hexaphenyl molecules manipulation along functionalized ultra-thin insulating layer on the Si(1 0 0) surface at low temperature (9 K)

Ultra-thin CaF2 layers are grown on the Si(1 0 0) surface by using a Knudsen cell evaporator. These epitaxial structures are studied with a low temperature (9 K) scanning tunneling microscope and used to electronically decouple hexaphenyl molecules from the Si surface. We show that the ultra-thin CaF2 layers exhibit stripe structures oriented perpendicularly to the silicon dimer rows and have a surface gap of 3.8 eV. The ultra-thin semi-insulating layers are also shown to be functionalized, since 80% of the hexaphenyl molecules adsorbed on these structures self-orients along the stripes. Numerical simulations using time-dependent density functional theory allow comparison of computed orbitals of the hexaphenyl molecule with experimental data. Finally, we show that the hexaphenyl molecules can be manipulated along or across the stripes, enabling the molecules to be arranged precisely on the insulating surface.

Two-dimensional device simulation of AlGaN/GaN heterojunction FET side-gating effect

Side-gating effects on AlGaN/GaN heterojunction FETs (HFETs) are simulated using a two-dimensional device simulator incorporating the Shockley–Read–Hall (SRH) model for deep traps. With the trap parameters obtained from experiments, the simulation results are in agreement with the experimental results including the “half-recovery” from the side-gating effect and the current reduction in the recovery process from positive side-gate bias application. The simulation indicated that the finite thickness of semi-insulating i-GaN layers suppresses side-gating effects on AlGaN/GaN HFETs with thin i-GaN layers.

Impact of Multi-Layer Carbon-Doped/Undoped GaN Buffer on Suppression of Current Collapse in AlGaN/GaN HFETs

We present a new semi-insulating GaN buffer layer, which consists of multiple carbon-doped and undoped GaN layers, suitable for AlGaN / AlN / GaN heterojunction field effect transistors. The proposed buffer structure was designed to minimize the total carbon incorporation into the buffer layer because carbon atoms in GaN are a possible cause of deterioration in structural quality and device characteristics such as current collapse. With this new buffer structure, current collapse in GaN MISHFET is drastically reduced while maintaining high breakdown characteristics. We argue that electron transfer from the undoped GaN to the carbon-doped GaN layer leads to total depletion of the undoped GaN layer and effectively compensates the deep-acceptor states in the carbon-doped GaN layer. This mechanism results in high-insulating buffer characteristic and opens the avenue for current collapse suppression in AlGaN / AlN / GaN MISHFET.

Backgating effect in III–V MESFET’s: A physical model

The backgating (sidegating) effect in III–V MESFET’s devices is analyzed through the modelisation of a Metal (Schottky barrier)–N (channel)–SI (Semi Insulating)–N+ (back-gate contact) structure. Numerical and analytical results, using the drift–diffusion charge transport model, show that along the applied voltage range associated with backgating: (i) quasi space charge neutrality across most of the bulk SI layer and (ii) quasi Boltzmann equilibrium for the free electron across the reverse biased N (channel)–SI contact prevail for GaAs (SI) or InP (SI). The circumstances under which a negative bias applied on the back-gate (N+) contact will either develop across the reverse biased N–SI contact (strong backgating) or across the SI layer (negligible backgating) are described by means of a simple analytical relation as a function of the deep level parameters values. The electric field dependence of the carrier mobility (Gunn effect) produces a backgating effect with a threshold voltage. The presence of a low lifetime, buffer layer, at the N–SI interface is shown to strongly reduce it.

Reduction in Buffer Leakage Current with Mn-Doped GaN Buffer Layer Grown by Metal Organic Chemical Vapor Deposition

The effect of Mn-doping into a GaN buffer layer grown by metal organic chemical vapor deposition (MOCVD) on the reduction in the leakage current of high-electron-mobility transistors (HEMTs) was investigated. Both the surface morphology and crystallinity maintained their quality even after heavy Mn-doping. The sheet resistance of GaN films increased with increasing amount of Mn-doping. The origin of semi-insulating GaN layer is considered to be electron scattering and the carrier compensation mechanism involving deep levels generated by the Mn impurity. When using the Mn-doped GaN buffer layer for the HEMT structure, the leakage current was reduced to five orders of magnitude lower than that without Mn-doping. Although Mn-doping is an effective technique for reducing the buffer leakage current, it is found that current collapse is emphasized when using the Mn-doped GaN buffer layer. We suggest that Mn atoms, which diffused to the GaN channel layer, induce the current collapse.

Formation of Semi-Insulating Layers on Semiconducting β-Ga2O3 Single Crystals by Thermal Oxidation

Semi-insulating layers (SIL) were formed on the surfaces of nominally undoped β-Ga2O3 (010) single crystals by thermal oxidation. Capacitance–voltage measurement with double Schottky configuration was performed to evaluate the increase in the thickness of the SIL as a function of annealing temperature and time. A SiO2 layer prepared on the surface prevented the extension of the SIL, indicating that oxygen incorporation from air and successive bulk diffusion dominated the carrier compensation process. The activation energy of oxygen diffusion coefficient was estimated to be 4.1 eV.

Contact vs bulk effects in N-semi-insulating-N and P-semi-insulating-P diodes

We analyse the charge transport mechanism in semi-insulating (SI) materials for N-SI-N and P-SI-P structures. The SI layers are large band gap semiconductors obtained by deep levels compensation of residual shallow donors or acceptors. A preceding theory is extended to the case of two, donor or acceptor, deep compensating levels [1]. The conduction mechanism is complex: ambipolar transport, heavy recombination, space charge effect and we show that the current is controlled by both bulk and interface effects at the reverse biased N-SI or P-SI junction. We develop a simple model, without the usual assumption of space charge neutrality, valid up to the beginning of one carrier space charge current. We show that a linear relation exists between the bulk excess free carrier densities and that depending on the value of a quantity M, given as a function of the deep levels electrical parameters, energy position, concentration and capture cross sections, as well as the dopants concentrations, the conduction mechanism is either contact controlled showing a pronounced current saturation effect or bulk controlled with one and for long sample, two, quasi linear J–Va relationship. Numerical modelisations of the drift–diffusion transport model confirm these analytical results for the case of one or two compensating deep levels. GaAs (SI) or InP (SI) layers are used for their high resistivity and insulating properties in FET technology, in buried heterostructures, diode laser and in radiation detectors technologies. These results are of importance for the interpretation of conductivity and Hall Effect measurements and explain the parasitic side-gating effect in GaAs or InP MESFET’s.

Use of Vanadium Doping for Compensated and Semi-Insulating SiC Epitaxial Layers for SiC Device Applications

Vanadium doping from SiCl4 source during epitaxial growth with chlorinated C and Si precursors was investigated as a mean of achieving compensated and semi-insulating epitaxial 4H-SiC layers for device applications. Thin epilayers were grown at 1450°C with a growth rate of ~6 μm/h. Experiments at 1600°C resulted in the growth rates ranging from 60 to 90 µm/h producing epilayers with thickness above 30 µm. V concentrations up to about 1017cm-3 were found safe for achieving defect-free epilayer surface morphology, however certain degradation of the crystalline quality was detected by XRD at V concentrations as low as 3-5x1015 cm-3. Controllable compensation of nitrogen donors with V acceptors provided low-doped and semi-insulating epitaxial layers. Mesa isolated PiN diodes with V-acceptor-compensated n- epilayers used as drift regions showed qualitatively normal forward- and reverse-bias behavior.

Temperature dependence of current conduction in semi-insulating 4H-SiC epitaxial layer

We have investigated temperature dependence (94 K–650 K) of current conduction in semi-insulating 4H-SiC epitaxial layer. The epitaxial layer was grown on highly doped n-type (0001) 4H-SiC substrate using chemical vapor deposition with dichlorosilane precursor. The current—voltage (I-V) characteristics exhibited steps at ∼1 V and ∼70 V that were attributed to the filling of deep level centers by injected electrons. Correlation of the I-V characteristics with the results of thermally stimulated current measurements showed that deep centers peaked at 242 K, 285 K, and 500 K, were responsible for the steps in the I-V characteristics. Slow processes of the injected carrier capture on traps resulted in the I-V characteristic with negative differential resistance.

Characterization of deep levels in n-type and semi-insulating 4H-SiC epitaxial layers by thermally stimulated current spectroscopy

We have investigated deep level centers in n-type and semi-insulating (SI) 4H-SiC epitaxial layers by thermally stimulated current (TSC) spectroscopy. The epitaxial layers were grown using chemical vapor deposition utilizing a dichlorosilane precursor. Both epitaxial layers exhibited relatively shallow levels related to Al, B, L- and D-centers. A deep level center with an activation energy of 1.1 eV, peaked at ∼400 K, was detected in the n-type epitaxial layer and correlated with the IL2 level and the 1.1 eV center in a high purity bulk SI 4H-SiC. The TSC spectra of the SI epitaxial layer was dominated by the peaks at 525–585 K that we attributed to intrinsic defects and their complexes with energy levels close to the middle of the bandgap. The TSC spectra of SI epitaxial layer exhibited peaks with different current polarity which is explained by thermoelectric effect and the built-in electric field reversal. The results of the transfer length method measurements of the SI epitaxial layer and the room temperature current-voltage (I-V) characteristics of both epitaxial layers are also reported.

Effects of thermal annealing on the semi-insulating properties of radio frequency magnetron sputtering-produced germanate thin films

We report the effects of thermal annealing on the semi-insulating properties of germanate thin films produced by a radio frequency magnetron sputtering process. Electrical and physical characterizations are presented. In the case of PbO–GeO2 films (annealed and not annealed), the electron diffraction analysis from transmission electron microscopy has shown that the amorphous structure predominates, whereas the annealed PbO–GeO2–AgNO3 (1wt.%) films presented crystalline nanoparticles in the range of 1 to 9nm composed of Pb, PbO and Ag4GeO4. Also, not annealed and annealed films were used to produce metal-insulator-semiconductor structures. The electrical properties of these structures were analysed from capacitance–voltage and conductance–voltage characteristics. The results showed that the significant leakage current in the accumulation region in the not annealed films can be increased by the annealing process. In addition, stable semi-insulating layers with an almost constant shifting of the capacitance–voltage characteristics are obtained when annealed PbO–GeO2–AgNO3 (1wt.%) films are employed. Based on a proposed model accounting for the leakage process, the influence of this leakage on the accumulation capacitance was also established. In addition, the decrease of the dielectric constant in the presence of crystalline nanoparticles was shown. The results obtained indicate that the germanate thin films are potential passivating materials for power device applications.

1.3-μm InGaAsP planar buried heterostructure laser diodes with AlInAs electron stopper layer

This study reports on the realization of 1.3-μm InGaAsP buried-heterostructure (BH) laser diodes (LDs) via an Fe-doped semi-insulating InP layer and an AlInAs electron stopper layer (ESL). Experimentally, the as-cleaved BH LD with an AlInAs ESL exhibited improved characteristics in terms of threshold current, slope efficiency, and maximum light output power at 90 °C as compared to those of the normal BH LD without an AlInAs ESL. In addition, high internal quantum efficiency or reduced threshold current density was observed, indicating increased modal gain in BH LDs fabricated with an AlInAs epilayer on top of the active region. It was also found that the temperature sensitivity of the BH LDs with an AlInAs ESL is more stable than that of the normal BH LDs. These results could be attributed to the suppression of thermal carrier leakage out of strain-compensated multiple-quantum-well by a large conduction-band offset of the AlInAs/InGaAsP heterojunction. Otherwise, without consideration of damping factor or coupling loss, the 3-dB bandwidth of the proposed BH LDs reaches a high value of 15.3 GHz. Finally, this TO-can packaged BH LD shows an eye-opening feature with the extinction ratio of 7.49 dB while operating at 10 Gbit/s at 50 mA.

Enhancement of channel conductivity in AlGaN/GaN heterostructure field effect transistors by AlGaN:Si back barrier

Heterostructure field effect transistors with three AlGaN/GaN interfaces were designed and investigated. A Si-doped AlGaN back barrier was used to compensate for the reduction of channel conductivity due to a carbon doped semi-insulating GaN buffer layer. Simulation using a one dimensional Poisson-Schrödinger solver showed an enhancement of the charge carrier density which was then confirmed experimentally. Hall and magnetic field dependent mobility measurements proved the Si-doped AlGaN back barrier layer causes neither degradation of electron mobility nor a parasitic conduction channel. Id,max of the transistors was enhanced by a factor of 1.5–2 without decreasing the off-state breakdown voltage.

Formation of a semi-insulating layer in n-type 4H-SiC by electron irradiation

Electron irradiation has been applied to the formation of a semi-insulating 4H-SiC(0001) layer. The resistivity of the semi-insulating layer, which was irradiated with a fluence of 1.9 × 1018 cm−2 at 400 keV, exceeded 1010 Ω cm at room temperature. From capacitance-voltage characteristics of Schottky structure, the depth of the semi-insulating layer was estimated to be 10 μm, indicating that the whole region of lightly-doped n-type epilayer was converted to the semi-insulating layer by electron irradiation. The semi-insulating property can be ascribed to electron trapping at the Z1/2 and EH6/7 centers generated by electron irradiation. The threshold energy for the generation of Z1/2 center was about 100 keV.