Geetak Gupta

Influence of AlN interlayer on the anisotropic electron mobility and the device characteristics of N-polar AlGaN/GaN metal-insulator-semiconductor-high electron mobility transistors grown on vicinal substrates

This paper presents an experimental investigation of the influence of an AlN interlayer on the electron mobility and the device characteristics of N-polar AlGaN/GaN metal-insulator-semiconductor-high electron mobility transistors grown by metal-organic chemical vapor deposition on miscut sapphire substrates. Use of miscut substrates leads to the formation of multiatomic steps at the AlGaN/GaN interface and anisotropy in electron transport properties. A combination of van der Pauw Hall, gated transfer length measurements, and capacitance-voltage measurements has been used to study the desired properties in directions parallel and perpendicular to the multiatomic steps and qualitative explanations were provided for the observed trends. Similar to the Ga-polar devices, the introduction of AlN interlayer improved the device performance by increasing both the electron mobility and the two-dimensional electron gas charge density in the devices. Orienting the devices such that the conduction occurred parallel to...

Very high channel conductivity in ultra-thin channel N-polar GaN/(AlN, InAlN, AlGaN) high electron mobility hetero-junctions grown by metalorganic chemical vapor deposition

Different back barrier designs comprising of AlN, AlGaN, and InAlN layers are investigated for ultra-thin GaN channel N-polar high-electron-mobility-transistors grown by metalorganic chemical vapor deposition. A combinational back barrier with both AlGaN and InAlN materials is proposed. The dependence of channel conductivity on channel thickness is investigated for different back barrier designs. The study demonstrated that the back barrier design of AlN/InAlN/AlGaN is capable of retaining high channel conductivity for ultra-scaled channel thicknesses. For devices with 5-nm-thick channel, a sheet resistance of ∼230 Ω/◻ and mobility ∼1400 cm2/V-s are achieved when measured parallel to the multi-step direction of the epi-surface.

Barrier height inhomogeneity and its impact on (Al,In,Ga)N Schottky diodes

III-N materials, especially ternary and quaternary alloys, are profoundly affected by barrier height inhomogeneity as evidenced by great variability in reported barrier height and Richardson constant values for Schottky diode samples involving epilayers with identical material composition. Research into AlInGaN-based devices is gaining traction due to its usefulness for strain engineering, polarization engineering, and vertical device design. Thus it is important to characterize the Schottky barrier height between AlInGaN and technologically relevant metals like nickel. It is proposed that alloy composition fluctuations inherent to low-temperature III-N alloys result in a Schottky barrier height inhomogeneity, and that the Schottky barrier height follows a Gaussian distribution. Current vs voltage data as a function of temperature was measured for three AlInGaN samples of varying composition. Utilizing a model tailored to thermionic emission over a Gaussian distribution of barriers, both the average barri...

Measurement of the hot electron mean free path and the momentum relaxation rate in GaN

We present a method for measuring the mean free path and extracting the momentum relaxation time of hot electrons in GaN using the hot electron transistor (HET). In this device, electrons are injected over a high energy emitter barrier into the base where they experience quasi-ballistic transport well above the conduction band edge. After traversing the base, high energy electrons either surmount the base-collector barrier and become collector current or reflect off the barrier and become base current. We fabricate HETs with various base thicknesses and measure the common emitter transfer ratio (α) for each device. The mean free path is extracted by fitting α to a decaying exponential as a function of base width and the relaxation time is computed using a suitable injection velocity. For devices with an injection energy of ∼1 eV, we measure a hot electron mean free path of 14 nm and calculate a momentum relaxation time of 16 fs. These values are in agreement with theoretical calculations where longitudinal optical phonon scattering is the dominant momentum relaxation mechanism.

Estimation of Hot Electron Relaxation Time in GaN Using Hot Electron Transistors

In this paper, we report for the first time an estimation of hot electron relaxation time in GaN using electrical measurements. Hot electron transistors (HETs) with GaN as the base layer and different base-emitter barrier-height configurations and base thicknesses were fabricated. Common-base measurements were performed to extract the differential transfer ratio, and an exponential decay of the transfer ratio with increasing base thickness was observed. A hot electron mean free path was extracted from the corresponding exponential fitting and a relaxation time was computed, which, for low energy injection, matched well with theoretically predicted relaxation times based on longitudinal optical (LO) phonon scattering.

Design of polarization-dipole-induced isotype heterojunction diodes for use in III–N hot electron transistors

The design of III–nitride-based hot electron transistors (HETs) is investigated using different diode design methodologies. Barrier-limited forward bias current and low reverse leakage current are demonstrated for the emitter-base diode using a barrier formed by a high-Al% AlGaN layer as a polarization-dipole layer. Two different base-collector diode designs are compared, one using 30% AlGaN as the barrier and the other using 10% InGaN as a polarization-dipole barrier. The InGaN polarization-dipole approach is shown to exhibit much lower reverse leakage currents. The impact of threading dislocation density on diode characteristics is also discussed.

Common Emitter Current Gain >1 in III-N Hot Electron Transistors With 7-nm GaN/InGaN Base

Current gain is demonstrated in III-N hot electron transistors (HETs) for the first time using base current controlled common emitter characteristics. The emitter and collector barriers (ØBE and ØBC) are implemented using AlN and In0.1Ga0.9N layers as polarization-dipoles, respectively. The entire structure is grown by plasma-assisted molecular beam epitaxy. Current gain is observed when the base thickness is reduced from 13 to 7 nm. Ohmic contacts to the base 2-D electron gas (2DEG) are achieved using a BCl3/SF6 etch to remove the emitter and selectively stop on the AlN. Subsequent metallization results in a tunnel contact from the metal to the base 2DEG across the thin AlN layer. This dual purpose served by the AlN layer is shown to be critical for achieving scaled base and current gain in III-N HETs.

Extraction of net interfacial polarization charge from Al0.54In0.12Ga0.34N/GaN high electron mobility transistors grown by metalorganic chemical vapor deposition

AlxInyGa(1-x-y)N materials show promise for use in GaN-based heterojunction devices. The growth of these materials has developed to the point where they are beginning to see implementation in high electron mobility transistors (HEMTs) and light emitting diodes. However, the electrical properties of these materials are still poorly understood, especially as related to the net polarization charge at the AlInGaN/GaN interface (Qπ(net)). All theoretical calculations of Qπ(net) share the same weakness: dependence upon polarization bowing parameters, which describe the deviation in Qπ(net) from Vegards law. In this study, direct analysis of Qπ(net) for Al0.54In0.12Ga0.34N/GaN HEMTs is reported as extracted from C-V, I-V, and Hall measurements performed on samples grown by metalorganic chemical vapor deposition. An average value for Qπ(net) is calculated to be 2.015 × 10−6 C/cm2, with just 6.5% variation between measurement techniques.

Optimization of a chlorine-based deep vertical etch of GaN demonstrating low damage and low roughness

The dry etching of GaN to form deep vertical structures is a critical step in many power device processes. To accomplish this, a chlorine and argon etch is investigated in detail to satisfy several criteria simultaneously such as surface roughness, crystal damage, and etch angle. Etch depths from 2 to 3.4 μm are shown in this paper. The authors investigate the formation of etch pits and its contributing factors. In addition, a nickel hard mask process is presented, with an investigation into the causes of micromasking and a pre-etch to prevent it. The authors show the results of optimized etch conditions resulting in a 2 μm deep, 0.831 nm rms roughness etch, with a 7.6° angle from vertical and low surface damage as measured by photoluminescence.

Measuring the signature of bias and temperature-dependent barrier heights in III-N materials using a hot electron transistor

We present a measurement technique to study barrier height inhomogeneity (BHI) in III-N materials. This technique is enabled by a hot electron transistor (HET), a vertical, unipolar device that works by injecting hot electrons over the emitter barrier and into a very thin base layer. After traversing the base, high energy electrons surmount the collector barrier and contribute to the collector current while low energy electrons reflect off the collector barrier and become the base current. The prevailing theory of BHI prescribes the replacement of a constant emitter barrier height with one that depends on both bias and temperature (i.e. ). Because the magnitude of the collector current is a strong function of the emitted electron energy and, therefore, the emitter barrier height, measuring the change in collector current with emitter bias and temperature allows us to determine the dependence of on these quantities. This advance will help provide a more thorough understanding of the physical sources of BHI in the III-Ns and assist in the diagnosis of key device nonidealities.