A. M. Wowchak

Very high channel conductivity in low-defect AlN/GaN high electron mobility transistor structures

Low defect AlN/GaN high electron mobility transistor (HEMT) structures, with very high values of electron mobility (>1800 cm2/V s) and sheet charge density (>3×1013 cm−2), were grown by rf plasma-assisted molecular beam epitaxy (MBE) on sapphire and SiC, resulting in sheet resistivity values down to ∼100 Ω/◻ at room temperature. Fabricated 1.2 μm gate devices showed excellent current-voltage characteristics, including a zero gate saturation current density of ∼1.3 A/mm and a peak transconductance of ∼260 mS/mm. Here, an all MBE growth of optimized AlN/GaN HEMT structures plus the results of thin-film characterizations and device measurements are presented.

Development of enhancement mode AlN/GaN high electron mobility transistors

Enhancement mode AlN/GaN high electron mobility transistors (HEMTs) were fabricated from originally depletion-mode structures using oxygen plasma treatment on the gate area prior to the gate metallization. Starting with a depletion mode AlN/GaN HEMT, the threshold voltage of the HEMT could be shifted from −3.2 to 1 V depending on the oxygen plasma treatment time to partially convert the AlN barrier layer into Al oxide. The gate current was reduced and the current-voltage curves show metal-oxide semiconductor diodelike characteristics after oxygen plasma treatment.

Deep traps responsible for hysteresis in capacitance-voltage characteristics of AlGaN∕GaN heterostructure transistors

The origin of hysteresis in capacitance-voltage (C-V) characteristics was studied for Schottky diodes prepared on AlGaN∕GaN transistor structures with GaN (Fe) buffers. The application of reverse bias leads to a shift of C-V curves toward higher positive voltages. The magnitude of the effect is shown to increase for lower temperatures. The phenomenon is attributed to tunneling of electrons from the Schottky gate to localized states in the structure. A technique labeled “reverse” deep level transient spectroscopy was used to show that the deep traps responsible for the hysteresis have activation energies of 0.25, 0.6, and 0.9eV. Comparison with deep trap spectra of GaN buffers and Si doped n-GaN films prepared on GaN buffers suggests that the traps in question are located in the buffer layer.

GaN growth by a controllable RF-excited nitrogen source

Abstract The group III nitrides have attracted increasing interest because of their significant potential in optoelectronics such as blue emitter and high temperature electronics. In order to take advantage of bandgap engineering of heterostructures such as GaN/AlN, a controllable growth technique is highly desired. The growth of group III nitrides by MBE (molecular beam epitaxy) requires using energetic nitrogen species which can be generated by techniques such as ion, RF (radio-frequency) and ECR (electron cyclotron resonance) sources. We report nitride results using an RF source to achieve epitaxy on sapphire. The nitrogen flux has been carefully characterized and related to the GaN quality. Schottky diode and ultra-violet (UV) photo-response are also reported.

Pyrometric interferometry for real time molecular beam epitaxy process monitoring

Pyrometric interferometry (PI) has recently been demonstrated for simultaneous real time wafer temperature and thickness measurement during the molecular beam epitaxy process. Both parameters of the thin film layer can be determined from the changing interference conditions in the layer. We used a reflection assisted version of PI to follow the thermal history of the wafer under different conditions and were able to resolve temperature to less than 1 °C. For thickness measurements, a parabolic fitting algorithm was used to accurately determine the endpoints of the GaAs/AlAs quarter wave stacks. Compared to other noncontact methods this technique can be used for very thick layers and is unaffected by the layer absorption and optical effects.

Electrical and structural properties of AlN/GaN and AlGaN/GaN heterojunctions

The electrical and structural properties of AlN/GaN heterostructures grown by molecular beam epitaxy on sapphire are compared with those of AlGaN/GaN heterostructures. The structural characteristics as assessed by x-ray diffraction show little difference but the electron density in the two-dimensional electron gas is about twice higher for AlN/GaN structures with only slightly lower mobility than in AlGaN/GaN. By proper choice of the Fe doping in GaN(Fe) and the thickness of unintentionally doped GaN layers, the composite buffer of the structure can be made semi-insulating. The current through the AlN/GaN structures is determined by tunneling through the AlN barrier and is much higher than that for AlGaN/GaN films due to the lower thickness of AlN compared to AlGaN. Increasing the thickness of AlN from 3 to 4 nm decreases the leakage current by about an order of magnitude.

Bias-assisted photoelectrochemical etching of p-GaN at 300 K

Photoelectrochemical (PEC) etching of p-type GaN has been realized in room temperature, 0.1 M KOH solutions. PEC etching of GaN was achieved by applying a positive bias to the surface of the p-GaN layer through a deposited titanium mask. The applied bias reduces the field at the semiconductor surface, which induced the dissolution of the GaN. The effect of bias on etch rate and morphology was examined. It was found that insulating the Ti mask from the KOH solution with Si3N4 significantly increases the etch rate. The rms roughness of the etched region decreased as the bias voltage increased. Etch rates as high as 4.4 nm/min were recorded for films etched at 2 V.

GaN PN junction issues and developments

Critical nitride-based p-n junction issues relating to wide bandgap bipolar device performance include minority carrier lifetime, defect related current characteristics and ohmic contact properties. Recent developments in p-GaN deposition processes resulted in GaN p-i-n UV photodetectors with improved deep UV responsivity, visible light rejection and shunt resistance characteristics. From the device data, the electron diffusion length in p-GaN doped at 1·1018 cm−3 was estimated to be 790 A, and the minority carrier lifetime in the p-GaN was estimated to be 24 ps to 0.24 ns. Improved junction electrical characteristics were achieved using MBE deposition on GaN buffers grown by MOCVD. NiAu ohmic contacts were also made to p-GaN with specific contact resistances less than 10−4 Ω·cm2.

Ordinary optical dielectric functions of anisotropic hexagonal GaN film determined by variable angle spectroscopic ellipsometry

Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angle of incidence. Thus the ordinary optical dielectric functions (E⊥〈c〉) are precisely determined by the isotropic mode VASE measurements at angles of incidence between 20° and 40° in the range of 0.75–6.5 eV. The VASE data were analyzed by a model dielectric function based on the GaN critical point structure, which allows for a nonzero extinction coefficient k below the band gap. The thicknesses of these GaN films are accurately determined via the analysis as well.Standard variable angle spectroscopic ellipsometry (VASE) has been employed to study the ordinary optical dielectric response of hexagonal gallium nitride (GaN) thin films—an important material for blue and ultraviolet light emitting device applications. The GaN films were grown by molecular beam epitaxy on c-plane sapphire substrates (α-Al2O3). Room temperature isotropic and anisotropic mode VASE measurements were made at angles of incidence between of 20° and 80°. Evidence of anisotropy was observed from the anisotropic mode measurements, reflecting the nature of wurtzite crystal structure of GaN. The sizable off-diagonal elements (Aps and Asp) of the Jones matrix indicate that the optical axis 〈c〉 of the c-plane sample are slightly off from the surface normal due to a small miscut of substrates. VASE data simulations by isotropic and anisotropic models indicate that the anisotropic effect on both diagonal and off-diagonal elements of the Jones matrix can be minimized to a negligible level at small angl...

Conductivity and Hall effect characterization of highly resistive molecular-beam epitaxial GaN layers

Highly resistive molecular beam epitaxial GaN layers are characterized by temperature dependent conductivity and Hall effect measurements. Seven n-type GaN samples with room temperature layer resistivity ranging between 8 and 4.2×106 Ω cm are used in this study. The experimental data are analyzed by considering various transport models such as band and hopping conduction, scattering on charged dislocations and grain boundaries controlled transport. The same defect level of 0.23 eV, attributed to nitrogen vacancy, is found for layers with ρ300⩽3.7×103 Ω cm. The Hall mobility for two lower resistivity layers is influenced mainly by phonon scattering (μH∼Tx, x=−1.4). However, higher resistivity layers show positive mobility power, x=0.5–0.9, which can be explained by dominating scattering on charged dislocations. Properties of layers with the highest resistivity (1×105 and 4.2×106 Ω cm) and extremely low Hall mobility (6 and <0.1 cm2 V−1 s−1) are consistent with grain boundary controlled transport. The barri...