Semi-insulating Layer Research Articles

High-performance GaN/InGaN heterostructure FETs on Mg-doped GaN current blocking layers

Nitride-based GaN/In 0.12Ga 0.88N heterostructure field effect transistors (HFETs) with semi-insulating Mg-doped GaN current blocking layers were successfully fabricated. The as-grown layers exhibited a very smooth surface. The DC characteristics of these HFETs were substantially improved over those previously reported. Devices with a 0.75 μm gate length showed a maximum drain current ( I DS) of 421 mA/mm, and a maximum g m of about 85.6 mS/mm at V GS between −0.5 and −3 V. The device which had a very good pinch-off with source–drain (S–D) leakage current was negligibly small, in the range of 10 −1–10 −2 mA/mm. The RF characteristic of devices showed the current gain cutoff frequency, f T, was 7.45 GHz and the maximum oscillation frequency, f MAX, was 12.36 GHz.

Carrier and defect dynamics in photoexcited semi-insulating epitaxial GaN layers

Transients of fast free-carrier recombination and of multitrapping processes, determined by different types of defects, have been traced by photoluminescence (PL) and contact photoconductivity (CPC) in semi-insulating GaN epitaxial layers. To eliminate effects caused by the electrodes, the CPC decays were supplemented with noninvasive microwave absorption transients. The lifetimes of fast recombination and initial free-carrier capture processes were evaluated using ultraviolet (UV) time-resolved photoluminescence transients. The UV PL band peaked at 3.42 eV with contributions from both stimulated and spontaneous emission was attributed to band-to-band recombination. At the highest excitations, the initial PL decay time exhibited a value of 880 ps due to nonradiative free-carrier recombination. The radiative centers were revealed in continuous-wave PL spectra, where the UV band was accompanied with the bands of blue (B) PL, peaked in the range of 2.82–3.10 eV, and yellow (Y) PL, peaked at 2.19 eV, ascribed to dislocations and bulk donor-acceptor recombination, respectively. The time scale of the relaxation rate exhibited a crossover from picoseconds for stimulated emission to hundreds of nanoseconds for multitrapping. In the asymptotic part, a stretched-exponent decay on the millisecond scale was observed with the disorder factor of α=0.7. The asymptotic decay is explained by competition of centers of nonradiative recombination within bulk of the material and trapping attributed to the dislocations. Behavior of the dislocation-attributed capture centers was simulated using a model of capture cross section, which depends on the excess carrier concentration via screening.

High-energy ion implantation: an alternative technology for micromachining three-dimensional GaAs structures

We present an alternative technology to micromachine gallium arsenide (GaAs) using deep ion implantation for MEMS applications. Energetic low dose nitrogen ions were used to implant deeply into n-type GaAs substrate. After annealing at 600 °C the nitrogen implanted n-GaAs top layer was converted to semi-insulating GaAs. A pulsed electrochemical etching process was used to selectively remove n-GaAs and leave the top patterned semi-insulating GaAs layer as mechanical membrane structures. Various GaAs microstructures, such as cross-bridge, coiled, and corrugated membranes have been successfully fabricated using this unique micromachining technology. By increasing the ion doses, the GaAs will be heavily damaged. The damaged GaAs shows much higher etch rate compared to the un-implanted GaAs using proper etch solutions and can be, therefore, used as sacrificial layer. A unique capacitive pressure sensor fabrication technology using AlGaAs/GaAs wafer is proposed, where the air gap between two electrodes is formed by releasing the locally damaged GaAs.

Fe-doped InP semi-insulating buried heterostructure for high speed and high power operations in directly modulated semiconductor laser

A buried heterostructure based on Fe-doped InP semi-insulating layers is optimised for both high output power and large modulation bandwidth operations up to 70/spl deg/C in a 10 Gbit/s directly modulated 1.3 /spl mu/m InGaAsP/InP distributed feedback laser. The slope efficiency of 0.19 W/A and -3 dB bandwidth of 10 GHz at 1.5 times threshold current is demonstrated experimentally.

Radiation-defect-dependent photoconductivity transients and photoluminescence in semi-insulating GaN

Effect of radiation defects on the photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN∕sapphire substrates. Manifestation of defects induced by 10keV x-ray irradiation with the dose of 600Mrad and 100keV neutrons with the fluence of 5×1014cm−2 have been revealed through steady-state and pulsed PL as well as through contact photoconductivity and microwave absorption transients. Synchronous decrease of the PL intensity of yellow, blue, and ultraviolet bands peaked at 2.19, 2.85, and 3.42eV, respectively, with density of radiation-induced defects is observed. 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 fits the stretched exponent approximation exp[−(t∕τ)α] with the different factors α in as-grown material (α≈0.7) and irradiated samples (α≈0.3).

New optoelectronic integrated device for optically controlled microwave oscillators

A new optoelectronic integrated device for the design of optically tuned microwave oscillators is presented. The device consists of an n-GaAs optically controlled field effect transistor (OPFET) and a p-AlGaAs/p-GaAs/n+-AlGaAs surface emitting double heterostructure light emitting diode (DH-LED) which are developed vertically on a p+-GaAs subtrate and separated by a semi-insulating (SI) GaAs layer. This new device is referred to as a light source integrated OPFET (LSI-OPFET). The proposed device structure is such that optical radiation generated by the LED is fed back into the OPFET as back illumination. The level of back illumination to the OPFET may be controlled by simply changing the bias current of the LED. The back illumination is used as the control signal to the OPFET to change certain parameters of the device such as the transconductance, gate–source and gate–drain capacitances etc. This makes the LSI-OPFET device suitable for the design of optically controlled microwave amplifiers and oscillator circuits. The proposed device is demonstrated in optically tuned common-source and common-drain microwave oscillators in which the frequency of oscillation may be controlled by the level of back illumination from the LED into the OPFET.

InGaN–GaN MQW LEDs With Current Blocking Layer Formed by Selective Activation

Selective activation technique was used to define a semi-insulating current-blocking layer underneath the p-pad of InGaN-GaN multiple-quantum-well light-emitting diodes (LEDs). The output power of the LEDs at 20 mA was increased 10% because less current was injected underneath the opaque p-pad.

Nitride-based HFETs with carrier confinement layers

Nitride-based Al 0.24Ga 0.76N/GaN heterostructure field effect transistors (HFETs) with carrier confinement layers were successfully fabricated. It was found that the source–drain (S–D) leakage currents of the HFETs with carrier confinement layers were much smaller. It was also found that we could enhance the two dimensional electron gas (2DEG) carrier mobility from 1070 to 1180 cm 2/V/s by the insertion of a Mg-doped semi-insulating carrier confinement layer with a Cp 2Mg flow rate of 2.36×10 −8 mole/min. At the same time, we could also achieve a smoother sample surface. The dc and rf characteristics of these HFETs were also found to be good.

Two-dimensional electron gases induced by polarization charges in AlN/GaN heterostructure grown by plasma-assisted molecular-beam epitaxy

We discuss the growth and transport properties of two-dimensional electron gas confined at the AlN/GaN heterointerface grown by plasma-assisted molecular-beam epitaxy. The sheet carrier density was found to be highly dependent on the barrier thickness of AlN grown on a doped or undoped GaN layer. The carrier sheet density monotonously increased from 0.8×1012 to 1.1×1013 cm−2 as the AlN barrier thickness on a semi-insulating GaN layer increased from 15 to 25 Å due to spontaneous and piezoelectric polarization. An AlN barrier of 35 Å in thickness grown on n-GaN gave the highest sheet carrier density, which was 4.3×1013 cm−2. In thin AlN barrier layers, the sheet carrier density was low due to surface depletion.

Highly reliable high performance waveguide-integrated InP/InGaAs pin photodiodes for 40 Gbit/s fibre-optical communication application

Waveguide-integrated pin photodiodes (WGpinPDs) planar-embedded by a semi-insulating (SI) InP buried layer exhibit very stable dark current characteristics after aging tests for over 8000 h at 175°C with a bias voltage of −10 V. A wear-out failure rate of much less than 1 failure in time (FIT) after 25 years of operation at 40°C was primarily estimated. Stable operation for over 5000 h were also confirmed under high power optical input aging test at an atmosphere temperature of 85°C. For the optical receiver module using the WGpinPD and a GaAs pHEMT travelling wave amplifier (TWA), a recorded minimum received power of −11.2 dBm at 40 Gbit/s was obtained for the first time.

Deep-defect-induced quenching effects in semi-insulating GaN layers detected by photoelectrical spectroscopic techniques

Quenching effects induced by additional below-bandgap illumination in undoped semi-insulating GaN were investigated using optical admittance spectroscopy (OAS) and photocurrent (PC) spectroscopy as well as optically excited, thermally stimulated currents (TSC). In OAS and PC, a decrease of defect-related signals due to the quenching light was observed. The thermal quenching of the defect band between 2.7 and 3.3 eV shows a good agreement with thermal emissions as measured by TSC, indicating the same defects cause the optical transitions in OAS/PC and the thermal transitions in TSC. The thermal emission in the temperature region between 250 and 300 K, which is responsible for the thermal quenching of the blue band (BB) in OAS, also shows an optical quenching under below-bandgap excitation.

AlGaN/GaN Modulation-Doped Field-Effect Transistors with An Mg-doped Carrier Confinement Layer

AlGaN/GaN modulation-doped field-effect transistors (MODFETs) with and without the Mg-doped semi-insulating carrier confinement layer were prepared and a detailed study on the electrical properties of these samples was performed. The addition of the Mg-doped semi-insulating carrier confinement layer resulted in the degradation of crystal quality. Thus, higher values of gm, IDS and the gate voltage swing are achieved by conventional MODFETs. The source-to-drain (S–D) leakage current of conventional MODFETs was also higher, since carriers could flow through the thick unintentionally doped GaN layer. However, the S–D leakage current was reduced from 50 mA/mm to 6 mA/mm with the insertion of the Mg-doped semi-insulating carrier confinement layer.

Role of Potential Barriers in Epitaxial Layers of Semi-Insulating GaN Layers

The electrical properties of semi-insulating GaN were investigated by dc and microwave techniques. The different epitaxial GaN layers were grown by Metal Organic Chemical Vapour Deposition on a Al2O3 (0001) substrate. Space charge effects along and perpendicular to the direction of layer growth were proposed to explain the observed peculiarities of the dark current and photocurrent. From the temporal and temperature dependences of characteristic time constants in the photoconductivity decay, the details of non-equilibrium states were analysed in the semi-insulating GaN. The well-resolved alpha-particle spectra obtained demonstrates the promise of semi-insulating GaN for ionising radiation detection.

Contributions from gallium vacancies and carbon-related defects to the “yellow luminescence” in GaN

Carbon-doped GaN layers grown by molecular-beam epitaxy are studied with photoluminescence and positron annihilation spectroscopy. Semi-insulating layers doped with >1018 cm−3 carbon show a strong luminescence band centered at ∼2.2 eV (yellow luminescence). The absolute intensity of the 2.2 eV band is compared with the gallium vacancy concentration determined by positron annihilation spectroscopy. The results indicate that a high concentration of gallium vacancies is not necessary for yellow luminescence and that there is in fact a causal relationship between carbon and the 2.2 eV band. Markedly different deep-level ionization energies are found for the high-temperature quenching of the 2.2 eV photoluminescence in carbon-doped and reference samples. We propose that while the model of Neugebauer and Van de Walle [Appl. Phys. Lett. 69, 503 (1996)] applies for GaN of low carbon concentration, a different yellow luminescence mechanism is involved when the interstitial carbon concentration is comparable to or exceeds the gallium vacancy concentration.

Semi-insulating silicon carbide layers obtained by diffusion of vanadium into porous 4H-SiC

Semi-insulating silicon carbide layers have been obtained by diffusion of vanadium into porous 4H-SiC. The diffusion was performed from a film deposited by cosputtering of silicon and vanadium, with the content of the latter equal to 20%. The diffusion profile of vanadium in porous silicon carbide has a complex structure with a fast diffusion coefficient of 7×10−15 cm2/s. The activation energy of the resistivity of vanadium-diffusion-doped porous SiC layers is 1.45 eV. The resistivity of vanadium-doped semi-insulating layers is 5×1011 Ω cm at 500 K, which exceeds the resistivity of undoped porous SiC by two orders of magnitude. The results obtained indicate that porous SiC is a promising material for semi-insulating substrates in device structures based on wide-bandgap semiconductors.

Transient charging of copper phthalocyanine: model and experiment

Thin films of copper phthalocyanine (CuPc) were deposited on indium tin oxide (ITO) substrates in ultra-high vacuum. Without breaking the vacuum, Ag dots were evaporated through a shadow mask. The present contribution is aimed at comparing experimental and simulated data on feedback-charge capacitance/voltage ( C/ V) and isothermal charge deep-level transient spectroscopy (Q-DLTS) of Ag/CuPc/ITO devices kept at ambient temperature. Two types of charge relaxation of different origins in response to a bias step (pulse) Δ U were resolved: (a) dielectric relaxation within the bulk of the CuPc semi-insulating layer; (b) emission from traps of charges injected and captured at the traps during the pulse. The dielectric relaxation is characterized by a well-defined time constant τ D ≈10 −4 s and is observable for both polarities of Δ U. The onset bias for observing the emission from traps corresponds roughly to one of the forward current. A positive Δ U is to be applied to ITO (hole injecting anode) in order to fill at least a fraction of the traps. Realizing that the traps are spatially distributed, the model previously applied to C/ V of a Si:H based devices has been updated to obtain at least a qualitative picture about the spatial distribution of the density of traps N T ( x) in CuPc. N T ( x) seems to rise when approaching the CuPc/ITO interface. These states are classified as acceptor-like ones, a situation analogous to that reported earlier for poly-( p-phenylene-vinylene). Protonation of the CuPc/ITO interface is discussed as the source of the acceptor-like states.

Correlation between Fe–Zn Interdiffusion Observed by Scanning Capacitance Microscopy and Device Characteristics of Electro-Absorption Modulators

The advantages of scanning capacitance microscopy (SCM) in observing Fe–Zn interdiffusion of an electro-absorption (EA) modulator and the relationship between the interdiffusion and device characteristics are discussed. SCM images show that there is a Zn diffusion region, in which the semi-insulating region is converted into p-type due to Zn diffusion, on both sides of the mesa and the Zn diffusion region becomes smaller as the Fe doping concentration is reduced. By comparison, scanning electron microscopy (SEM) images captured after stain etching of EA modulators did not clearly delineate the Zn diffusion front. The influence of a ruthenium (Ru)-doped InP burying layer on the interdiffusion has also been investigated by SCM. These results indicate that in order to improve the performance of EA modulators, it is important to prevent Zn diffusion into the semi-insulating layers.

Electrical barrier properties of meso-porous silicon

We present a contribution dealing with the study of the barrier properties of meso-porous silicon (PS) with metals and p +-Si crystalline silicon. Metal/PS/p +-Si and p +-Si/PS/p +-Si structures with different thickness (1–10 μm) of PS are investigated by means of current–voltage and capacitance–voltage characteristics combined with thermal stimulation in the 150–350 K temperature range. These experiments allowed a clear separation of bulk and contacts contributions to the electrical impedance. The barrier properties (nature and heights) of metal/PS contacts and PS/Si interfaces are then evaluated. The p +-Si/PS/ p +-Si structures exhibit ohmic contacts allowing space charge limited current (SCLC) ensured by carriers injection in the PS layers. From this analysis, the electrical behavior of the metal/PS/p +-Si structures is interpreted in terms of a Schottky barrier biased through a semi-insulating PS layer.

Growth of Semi-insulating GaN Layer by Controlling Size of Nucleation Sites for SAW Device Applications

Semi-insulating undoped GaN films were grown based on controlling the size of the nucleation sites through a special two-step growth method: First, 16 nm LT-GaN was annealed at 950 ° with a ramping time of 4 min, then the GaN was grown at this temperature for 1 min. Second, the growth temperature was increased to 1020° with a ramping time of 2 min and the GaN layer finally grown at 1020 ° for 40 min. The film grown by this sequence exhibited sheet resistance of up to 109 Ω/sq with mirror-like surface morphology. By slow ramping to 950° in the initial phase of growth, smaller grain sizes and higher nuclei densities were formed and the columnar growth mode along the c direction was dominant. The observation of higher resistance in two-step growth is believed due to the increased misorientation of nuclei when the growth proceeds during temperature ramping to 1020°. The fabricated saw filter on semi-insulating GaN exhibited a high velocity of 5342 m/s at center frequencies of 133.57 MHz and an electromechanical coupling coefficient(k2) of about 0.763 %, which was enhanced due to the improvement of surface morphology with high sheet resistance by the two- step ramping technique.

Semi-insulating layers in 4H and 6H SiC by Si and C ion implantation

Semi-insulating regions have been obtained in 6H and 4H silicon carbide using ion implantation. The silicon carbide samples were implanted with either carbon or silicon ions followed by isochronal heat treatments. This leads to compensation, which is achieved by the lattice damage and by the thermal redistribution of atoms.