O. Katz

Gain mechanism in GaN Schottky ultraviolet detectors

Schottky barrier GaN ultraviolet detectors, both in vertical and in lateral configuration, as well as in a metal–semiconductor–metal geometry were implemented. All devices exhibit a high gain at both reverse and forward bias. The photoresponse in the forward bias is in the positive current direction. We attribute the gain to trapping of minority carriers at the semiconductor–metal interface. The excellent agreement between the calculated responsivity and the experiment indicates that the model is valid for all device structures under study, and represents a unified description of gain mechanism in GaN Schottky detectors.

Dependence of the refractive index of AlxGa1−xN on temperature and composition at elevated temperatures

The refractive index of hexagonal AlxGa1−xN at room temperature and its temperature dependence at elevated temperatures have been determined with high accuracy by spectroscopic ellipsometry. Measurements have been conducted on samples with aluminum molar fractions ranging from 0% to 65% and at temperatures between 290 and 580 K. The refractive index in the transparent spectral region has been determined as a function of photon energy, using the Kramers–Kronig relations with suitable approximations, and applying a multilayer model. An analytical expression for the composition and temperature dependent refractive index in the transparent region, above room temperature, has been obtained. The refractive index has been found to increase with increasing temperature. The shift of the refractive index is strongest for GaN and decreases for AlGaN with increasing aluminum molar fraction. The impact on the properties of GaN based waveguides is illustrated by a slab waveguide calculation.

Persistent photocurrent and surface trapping in GaN Schottky ultraviolet detectors

GaN-based Schottky detectors were implemented and their photoresponse as a function of the incident power and time was measured. The measured photoresponse shows gain saturation and persistent photoconductivity behavior. These effects are shown here to be related to each other, arising from a nonideal semiconductor surface. A microscopic model of the gain mechanism to explain these observations is presented. Trap density at the semiconductor metal interface, characteristic lifetime, and carrier capture coefficient are extracted from our measurements.

Electron mobility in an AlGaN/GaN two-dimensional electron gas. I. Carrier concentration dependent mobility

The transport properties of two-dimensional electron gas (2-DEG) at the AlGaN/GaN interface were studied by characterizing the 2-DEG mobility dependence on carrier concentration, n/sub s/, and temperature. High-quality AlGaN/GaN heterostructures were grown, and heterostructure field effect transistors (HFETs) using a Fat FET geometry were fabricated. Measurements of 2-DEG mobility were performed by magnetoresistance and capacitance-conductance. In order to understand the dominant transport factors, the mobility was modeled using different scattering mechanisms and compared to our results. It is found that mobility dependence on n/sub s/ shows a bell-shape behavior over the whole temperature range. For low n/sub s/ the mobility is dominated by Coulomb interaction from interface charge, and at high n/sub s/ the mobility is dominated by interface roughness. Using previously reported experimental values of interface charge and interface roughness in our modeling, we show good agreement with mobility measurement results. Scattering from interface states in AlGaN/GaN heterostructures, seems to be related to the high polarization field in the heterointerface. At temperatures higher than 200K polar optical phonon scattering dominates the transport, yet both interface charge and roughness affect the mobility at the low and high n/sub s/, respectively.

Anisotropy in detectivity of GaN Schottky ultraviolet detectors: Comparing lateral and vertical geometry

Vertical and lateral geometry GaN-based Schottky barrier photodetectors have been implemented, using similar quality material and the same fabrication process. The vertical detector exhibits two orders of magnitude higher responsivity. This is attributed to improved ohmic backcontacts, due to the highly doped buried layer. The vertical detectors exhibits also lower 1/f noise level, which is attributed to the reduced effect of dislocations on the carrier transport, resulting in lower mobility fluctuations. The vertical detector normalized detectivity is four orders of magnitude higher.

Characteristics of In/sub x/Al/sub 1-x/N-GaN high-electron mobility field-effect transistor

GaN-based field effect transistors commonly include an Al/sub x/Ga/sub 1-x/N barrier layer for confinement of a two-dimensional electron gas (2DEG) in the barrier/GaN interface. Some of the limitations of the Al/sub x/Ga/sub 1-x/N-GaN heterostructure can be, in principle, avoided by the use of In/sub x/Al/sub 1-x/N as an alternative barrier, which adds flexibility to the engineering of the polarization-induced charges by using tensile or compressive strain through varying the value of x. Here, the implementation and electrical characterization of an In/sub x/Al/sub 1-x/-GaN high electron mobility transistor with Indium content ranging from x=0.04 to x=0.15 is described. The measured 2DEG carrier concentration in the In/sub 0.04/Al/sub 0.96/N-GaN heterostructure reach 4/spl times/10/sup 13/ cm/sup -2/ at room temperature, and Hall mobility is 480 and 750 cm/sup 2//V /spl middot/ s at 300 and 10 K, respectively. The increase of Indium content in the barrier results in a shift of the transistor threshold voltage and of the peak transconductance toward positive gate values, as well as a decrease in the drain current. This is consistent with the reduction in polarization difference between GaN and In/sub x/Al/sub 1-x/N. Devices with a gate length of 0.7 /spl mu/m exhibit f/sub t/ and f/sub max/ values of 13 and 11 GHz, respectively.

GaN layer growth optimization for high power devices

Abstract In this work, a novel method for GaN layer optimization — statistical multi-parameter design of experiments (DOE) — is presented. According to the statistical model obtained, increasing the buffer layer V/III ratio is beneficial for minimizing the full width at half maximum (FWHM) of the X-ray diffraction rocking curve for the (002) reflection. Statistical models were also obtained for background electron concentration and room temperature Hall mobility, but further data analysis and electrical measurements lead us to the conclusion that those models are disturbed by the presence of a highly conductive layer near the GaN/sapphire interface. Inclusion of an Al x Ga (1− x ) N isolation layer results in a reduction of two orders of magnitude in the measured background concentration, as well as a significant increase in Hall mobility, without degradation of the crystalline quality.

Charge carrier mobility in field effect transistors: analysis of capacitance-conductance measurements

We present a formalism to extract carrier mobility and its dependence on charge concentration and electric field, from field effect transistors (FETs) transfer characteristics. In this formalism, we consider the spatial non-uniformity of carrier mobility across the channel. We show that the mobility is extracted accurately by the new method, even when it is strongly dependent on carrier concentration and electric field. Typical examples of carrier mobility extraction in silicon, conjugated polymers, and in AlGaN/GaN heterostructure based FETs are demonstrated.

Electrical characteristics of proton irradiated AlGaN devices

The growth of high quality GaN films offers the possibility of making this wide band gap material available for the fabrication of novel microelectronic devices with unique properties. However, growth defects that arise from the growth process result in high leakage currents that may be detrimental for some practical applications. It has been shown that these leakage currents can be compensated by the deliberate introduction of new defects by ion beam irradiation. The phenomenon has been investigated by use of a focused ion beam in a nuclear microprobe. AlGaN/GaN films were deposited on sapphire and then an interdigitated electrode pattern was deposited on the surface. Irradiation of the film through the electrodes allowed the resistivity to be monitored on-line. Irradiated areas displayed greatly improved signal to noise ratio when employed as a UV detector.

Surface states and persistent photocurrent in a GaN heterostructure field effect transistor

AlGaN/GaN heterostructure field effect transistors subject to UV illumination exhibit persistent photoconductivity both in the gate leakage and in the drain current. Time decay modelling of the gate photocurrent enables us to extract the surface potential transient. Simultaneous measurements of drain and gate currents indicate that (i) the gate leakage current negatively charges surface states, (ii) the large change in surface potential induced by UV illumination is responsible for the change in drain current in these devices. We present a detailed analytical model describing the gate and drain current decay transients, following UV illumination, in terms of surface potential modulation due to charging and discharging of surface traps. It is also found that for constant illumination and drain bias voltage conditions the change in the two-dimensional electron gas density is nearly constant independent of the gate bias. Using a previously measured mobility–carrier density relation we determine quantitatively the drain current transient.