(Sc2O3)x(Ga2O3)1−x was grown by molecular beam epitaxy at low temperatures (100 °C) using a variety of growth sequences to avoid surface segregation of Ga. Continuous and digital growth techniques always produced Ga segregation. This surface segregation was attributed to the stronger bond between the Sc and O compared to the Ga and O. A digital growth technique (alternate opening of Sc and Ga shutters with the O shutter open continuously during the growth) was unsuccessful in eliminating this effect. The segregation was eliminated using a growth technique in which the Ga shutter was closed for a set amount of time toward the end of the growth while the O and Sc shutters remained open. Characterization with reflection high energy electron diffraction, x-ray diffraction, and transmission electron microscopy revealed the growth of a fine-grained polycrystalline film under these conditions. A third growth technique was used that involved closing the Ga shutter for a set amount of time toward the end of the growth while the O and Sc shutters were open continuously. This technique was successful in depositing a uniform film. However, the breakdown field was only 1.40 MV/cm (at 1 mA/cm2). The addition of Ga to Sc2O3 diminished the insulating properties of the film. These initial experiments indicate that phase segregation is likely to be a major issue with most growth techniques and that alloying Ga2O3 with elements other than Sc, such as Gd or Al, might be a more successful approach.

The (SmxGa1−x)2O3 alloy system is a potential new dielectric for compound semiconductors such as GaAs. Using molecular beam epitaxy under metal-modulated growth conditions, we grew the binary oxide, Sm2O3, at two substrate temperatures (100 and 500 °C) and optimized the structural, morphological, and electrical properties of the films. Decreasing the Sm cell temperature suppressed the formation of the monoclinic phase and promoted the growth of the cubic phase. Next, the ternary oxide, (SmxGa1−x)2O3, was deposited to investigate the effects of Ga incorporation. Optimization experiments were used to determine the effects of substrate temperature and samarium cell temperature (i.e., growth rate) on film stoichiometry, phase distribution, and microstructure in these films. Films grown at 500 °C showed significant surface roughness and the presence of multiple crystalline phases. Since all of the Sm-based oxides (i.e., samarium oxide with and without gallium) were found to have unbonded Sm metal, annealing experiments were carried out in oxygen and forming gas to determine the effects of annealing on film stoichiometry. The motivation behind annealing in forming gas was to see whether this commonly used technique for reducing interface densities could improve the film quality. GaAs metal-oxide-semiconductor diodes with (SmxGa1−x)2O3 showed breakdown fields at 1 mA/cm2 of 4.35 MV/cm, which decreased with increasing Sm unbonded metal content in the films.

The role of additive noble gases He, Ar and Xe to Cl2-based Inductively Coupled Plasmas for etching of GaN, AlN and InN were examined. The etch rates were a strong function of chlorine concentration, rf chuck power and ICP source power. The highest etch rates for InN were obtained with Cl2/Xe, while the highest rates for AlN and GaN were obtained with Cl2/He. Efficient breaking of the III-nitrogen bond is crucial for attaining high etch rates. The InN etching was dominated by physical sputtering, in contrast to GaN and AlN. In the latter cases, the etch rates were limited by initial breaking of the III-nitrogen bond. Maximum selectivities of ∼ 80 for InN to GaN and InN to AlN were obtained.

AlGaN/GaN high electron mobility transistors with 0.125 mu m gate length were rf stressed at 10 GHz. Depth resolved cathodoluminescence (CL) was employed to investigate the role of defects in devices that exhibited typical mean time to failure (MTTF) compared to those that suffered from infant mortality. The CL spectra exhibited both laterally and vertically localized defect emission within the channel of devices that failed early, indicating the presence of an increased concentration of V-Ga-based defects within the device. Additionally, residual compressive stress was observed in all devices after electrical stress. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.002306ssl] All rights reserved.

Sub-micron AlGaN/GaN high electron mobility transistors were RF stressed at various drain bias conditions at 10 GHz under 3 dB and 3.7 dB compression. Rapid degradation was observed above a drain bias of 20 V, with significant degradation of the Schottky contact. Additionally, electroluminescence and cathodoluminescence was performed on stressed devices. Localization of 2.2 eV defect emission was observed on a device suffering from infant mortality.

Lifetime prediction for device operation has usually relied on accelerated testing at elevated temperature and then extrapolation back to room temperature operation. This technique frequently fails for scaled, high current density devices found in modern technologies. Device failure is driven by electric field or current mechanisms or low activation energy processes that are masked by other mechanisms at high temperature. Device degradation can be driven by failure in either active structures or passivation layers. We have seen that many issues have an affect on compound semiconductor performance and reliability, including the material quality, strain state, surface cleaning process, and the actual voltage and current conditions during aging. We have conducted comprehensive plan of reliability engineering for III-V device structures. This includes materials and electrical characterization and reliability testing. These techniques were utilized to develop new simulation technologies for device operation and reliability. This allows accurate prediction not only of reliability, but the ability to design structures specifically for improved reliability of operation. Our intensely integrated approach of utilizing new characterization methods, device simulations and realistic device stressing and aging provided new insights into device failure mechanisms. DISTRIBUTION A: Distribution approved for public release. 1. Reliability Test Station In order to accomplish the goal of creating new methods for optically and electrically characterizing and stressing semiconductor devices, an in-house design from readily available, off-the shelf parts such as power supplies and data-acquisition and control equipment was created. The system was designed for maximum flexibility in order to provide long term voltage/current/temperature stress testing, characteristic IV plots, gate pulse testing and also incorporated laser and shutter control for optical pumping from different laser sources. DCstress test measurements are taken every second and stored at a user-defined interval (minimum of a second) in a SQL database for future analysis, whereas characterization tests require data points to be measured and stored. An intuitive and accessible user interface is necessary for any level of flexibility. I-V plots, shown in Figure 1, and DC stress results, shown in Figure 2, are displayed graphically and in real-time. Details of this system design can be found in [Cheney 2012] (available on-line at http://ufdc.ufl.edu/UFE0044885/00001) . High-speed, high-voltage configuration The high-speed, high-voltage variation shown in Figure 3 displays the hardware, comprised of two power supplies, a pico-ammeter, and a USB oscilloscope. This combination of the different commercially available off-the-shelf instruments creates a robust system with accurate measurement, large bias-voltages, and high-speed measurement. All voltage measurements are made directly by the USB oscilloscope and can handle voltages as high as ±80 volts. During long term DC stress tests, the system samples every second so it takes advantage of the higher precision 16-bit sampling. This wider sampling precision is also applied during characterization measurements since the sample speed is dependent on how quickly the power supplies can sweep voltages and not on sample-rate of the measurement. When performing gate-lag tests with a narrow gate pulse (1 μs), the USB scope samples at the maximum speed (up to 50MHz) using the reduced sample precision to 12 bits. The 50MHz sample rate enables the gate pulse widths to decrease by orders of magnitude into the nanosecond pulse width range. Long-term test configuration The long-term test set can stress four devices at a time. This setup does not have all the capabilities of the high-speed, high-voltage set, lacking high-voltage gate control with a ±10 volt range and also the data acquisition is limited to 50k samples per second. Figure 4 shows the hardware, comprised of an Instek DC power supply for the drain bias, a National Instruments (NI) analog output module serving as the gate bias supply, an NI high-voltage input module (±60 volt) that measures the drain voltage, and an NI input module to read gate voltages, gate current and the drain current measurement. Optical pumping configuration The optical pumping station, shown in Figure 5, has a unique requirement to stream continuously at high speeds. This requirement precludes the data acquisition equipment in the long-term station because the TDM sampling is too slow. The USB oscilloscope in the highspeed, high-voltage station is not able to stream continuously and sacrifices too much precision to sample fast enough. The compromise is the NI USB-6366 that has 16-bit simultaneous DISTRIBUTION A: Distribution approved for public release. sampling up to 2 MHz. It uses the Instek power supply as the drain bias source. The NI USB6366 has digital output controls that are used to switch shutters on the different lasers. Probe station configuration The probe station, Figure 6, is a version of the high-speed, high-voltage configuration without the gate pulsing capabilities, using a USB oscilloscope for data acquisition. The voltage inputs VD and VG are connected to probes on a probe station, allowing unpackaged parts to be tested on the system. Figure 1 Software user interface showing I-V plots DISTRIBUTION A: Distribution approved for public release. Figure 2. Software GUI showing DC stress DISTRIBUTION A: Distribution approved for public release. Figure 3. High-speed, high-voltage test station Figure 4. Long term test station DISTRIBUTION A: Distribution approved for public release. Figure 5. Optical pumping configuration Figure 6. Probe station configuration DISTRIBUTION A: Distribution approved for public release. Section 2. Device Testing – bias and temperature stressing Devices from two different vendors were used in this study. Eighteen HEMTs and six transmission line modules (TLM) from the Air Force Research Labs (AFRL) underwent accelerated aging. In addition, there were two devices from an undisclosed vendor, called vendor A. AFRL Devices The HEMT layer structures used in these experiments were grown by metal-organic chemical vapor deposition (MOCVD) on 6H SiC semi-insulating substrates with an unintentionally doped 3nm GaN cap, 15nm Al0.28Ga0.72N barrier and 2.25μm Fe-doped GaN buffer. Plasma enhanced chemically vapor deposited SiN was used for device passivation. The HEMTs had dual submicron Ni/Au gates with dimensions of 0.125, 0.14 or 0.17 x 150 μm. The HEMTs employed a Ti/Al/Ni/Au Ohmic metallization with a gate periphery of 300 μm, sourceto-gate and gate-to-drain distances of 2 μm with SiNx passivation. The devices were packaged and wire-bonded for testing. The initial quality of the devices varied widely, with less than 20% actually being suitable for stressing post-packaging. Pre-stress device screening removed parts whose gate leakage was above 0.5 μA or with drain leakage more than 500 μA. The devices underwent accelerated aging by DC-stressing at 1500C at three different bias points along the one-watt load curve, shown in Figure 7: VG = 0 (on-state), VG = -2 (semi-on), VG < VTH (off-state). The bias points were chosen with the expectation of devices stressed in the onstate mostly experiencing stress on the channel, while gate defects would manifest themselves as VG approaches pinch-off and VDG approaches the device’s critical voltage [del Alamo 2009]. Devices that are biased between the on-state and off-state experience stress on both the channel and the gate. Since a gate bias of VG = -2 is closer to the operating point of a transistor in a circuit, and because this bias point has the potential to show more interesting results, most of the devices are stressed under this bias condition. To maintain a constant power-dissipation of one watt in the channel, VD was adjusted based on the measured ID, except in the off-state case, where VD was set to constant voltage The stress tests were interrupted when either a 10% change in the drain current or an order of magnitude change in gate current were observed. The devices tended to fail in one of two ways, either an abrupt sharp drop in drain current (ΔID > 10%) or a gradual decline in this current. Figure 8 is representative of both types of degradation over time. Abrupt failures Table 1 lists the devices that failed abruptly. Figure 9 shows the typical characteristic results of a device that failed abruptly. The dc characteristics show little to no change as a result of the DC-stress test. This suggests the degradation observed was possibly due to contact degradation. Since contact failure has been shown to be temperature dependent [Meneghesso 2008] [Douglas 2011] and to further support the hypothesis that this is contact failure, the fluctuations in current reduced dramatically as the test base-plate temperature was lowered. At a lower base-plate temperature, the devices appear to behave normally, however, they do experience a second abrupt drop in channel current once stress continues. DISTRIBUTION A: Distribution approved for public release. Gradual failures Table 2 lists the conditions and devices that displayed a gradual failure mechanism. Most all of the devices that did not exhibit the abrupt drop in current drop, tended to have a similar degradation pattern to that shown in Figure 8-B, with an initial drop in drain current for the first 10 minutes. This has been suggested as resulting from the device reaching a new steady-state as trapping and de-trapping reach equilibrium [del Alamo 2009]. In several of the devices, the current increased before gradually declining to the 10% threshold where the test stopped. Figure 10 shows the typical pre and post electrical characteristic I-V plots, which are indicative of permanent degradation. It is interesting to note that the gate pulse transconductance plot shows no change in the

Epitaxial growth of the dilute magnetic semiconductors GaMnP and GaMnN has been investigated by Gas Source Molecular Beam Epitaxy (GSMBE). GaMnP films grown with < 4.5% Mn show the preferential formation of the second phases MnP and Mn 5.64 P 3 , resulting in only a slight deviation from purely diamagnetic behavior. GaMnN films grown on both Al 2 O 3 and Metal-Organic Chemical Vapor Deposition (MOCVD) derived GaN surfaces show strong ferromagnetism when grown with either C codoping or at elevated temperatures to raise the concentration of n-type carriers. Comparable GaMnN films grown under conditions which produce highly resistive material show only paramagnetism, indicating the importance of carrier concentration on the resulting magnetic behavior. The formation of second phases was not observed in the GaMnN material for Mn concentrations less than 9%.

Etch rates up to 7,000 {angstrom}/min. for GaN are obtained in Cl{sub 2}/H{sub 2}/Ar or BCl{sub 3}/Ar ECR discharges at 1--3mTorr and moderate dc biases. Typical rates with HI/H{sub 2} are about a factor of three lower under the same conditions, while CH{sub 4}/H{sub 2} produces maximum rates of only {approximately}2,000 {angstrom}/min. The role of additives such as SF{sub 6}, N{sub 2}, H{sub 2} or Ar to the basic chlorine, bromine, iodine or methane-hydrogen plasma chemistries are discussed. Their effect can be either chemical (in forming volatile products with N) or physical (in breaking bonds or enhancing desorption of the etch products). The nitrides differ from conventional III-V`s in that bond-breaking to allow formation of the etch products is a critical factor. Threshold ion energies for the onset of etching of GaN, InGaN and InAlN are {ge} 75 eV.

We have investigated the thermal stability of high resistivity regions introduced by ion bombardment of GaAs, AlGaAs, InP and InGaAs. For low doses in which the ion species density is below that of the doping density in the target material, we obtain the usual damage-related compensation in which deep levels created by the bombardment trap the charge carriers. By this method one creates material with resistivities around 108 Ω/□ (n- or p-type GaAs and AlGaAs, p-type InP), around 106 Ω/□ (n-type InP) or around 105 Ω/□ (n-type InGaAs or p-type InGaAs), with a return of the initial resistivity after elevated temperature annealing (∼600°C for GaAs and AIGaAs, ∼500°C for InP and InGaAs). The more interesting case is the use of higher dose implants of species which create chemical deep levels. This occurs for O in n-type AlGaAs where O creates a deep acceptor (Ec-0.49 eV), and Fe in n-type InP and InGaAs, where it is also a deep acceptor. When the concentration of these species exceeds the doping density in the material, the bombarded regions retain their high resistivity even after high temperature annealing (> 1000°C for GaAs and AIGaAs, >850°C for InP and InGaAs). The case of O in GaAs appears to represent a third mechanism; it creates thermally stable material only in the case of Be-doped GaAs, suggesting an ion-pairing reaction.

ABSTRACT The wide band gap group-III nitride materials continue to generate interest in the semiconductor community with the fabrication of green, blue, and ultraviolet light emitting diodes (LEDs), blue lasers, and high temperature transistors. Realization of more advanced devices requires pattern transfer processes which are well controlled, smooth, highly anisotropic and have etch rates exceeding 0.5 μm/min. The utilization of high-density chlorine-based plasmas including electron cyclotron resonance (ECR) and inductively coupled plasma (ICP) systems has resulted in improved etch quality of the group-III nitrides over more conventional reactive ion etch (RIE) systems.

The surface stoichiometry, surface morphology, and electrical conductivity of AlN, GaN, InN, InGaN, and InAlN were examined at rapid thermal annealing temperatures up to 1150 °C. The sheet resistance of the AlN dropped steadily with annealing, but the surface showed signs of roughening only above 1000 °C. Auger electron spectroscopy (AES) analysis showed little change in the surface stoichiometry even at 1150 °C. GaN root mean square (rms) surface roughness showed an overall improvement with annealing, but the surface became pitted at 1000 °C, at which point the sheet resistance also dropped by several orders of magnitude, and AES confirmed a loss of N from the surface. The InN surface had roughened considerably even at 650 °C, and scanning electron microscopy showed significant degradation. In contrast to the binary nitrides, the sheet resistance of InAlN was found to increase by ∼102 from the as grown value (3.2×10−3 Ω cm) after annealing at 800 °C and then remain constant up to 1000 °C, while that of InGaN increased by two orders of magnitude between 700 and 900 °C. The rms roughness increased above 800 and 700 °C, respectively, for InAlN and InGaN samples. In droplets began to form on the surface at 900 °C for InAlN and at 800 °C for InGaN, and then evaporate at 1000 °C, leaving pits. AES analysis showed a decrease in the N concentration in the top 500 Å of the sample for annealing ≥800 °C in both materials.

Films of MgO and MgCaO ternaries were grown at low temperature as dielectrics on 6H-SiC by gas-source MBE. MgO grown at 300 and 400°C revealed crystallites textured toward the (111) orientation on SiC (0001). A solid-solution Mg .75 Ca .25 O ternary was grown having a minimal lattice mismatch and low root mean square (RMS) roughness of 0.5 nm. SiC pretreatments in UV-ozone reduced carbon contaminants on the surface of SiC, but resulted in the increase of fixed oxide charge in the oxide/SiC interface. Electrical breakdown fields >3.5 MV cm −1 and low density of interface states on the order of 10 11 cm −2 eV −1 were achieved for a Mg .75 Ca .25 O ternary grown at 300°C. These oxides are presented for the first time as low temperature alternatives to SiO 2 gate dielectrics for SiC MOS applications.

Hydrogen is found to readily diffuse into InGaN, InAIN and InGaAIN epitaxial layers during plasma exposures at 170 – 250 °C for 40 sec - 30 min. The diffusivity of hydrogen is > 10−11 cm2-s−1 at 170 °C, and the native donor species are passivated by association with the hydrogen. Reactivation of these species occurs at 450–500°C, but the hydrogen remains in the material until ≥ 800 °C.

The incorporation and optical activation of Er in III-N optically-pumped and electroluminescent structures has been studied. For the first time, strong Er3+-related photoluminescence (PL) was measured at 300K for GaN:Er doped during growth on c-plane Al2O3 and Si. Also, room temperature electroluminescence was observed for simple GaN:Er structures on Si. Experiments to evaluate the effects C and 0 on the optical activity of Er indicated that these impurities enhance Er PL and EL in GaN. GaN films doped with Er to a concentration of 3 ×1018 cm−3 with [0] ~ 1020 cm−3 and [C] ~021 cm−3 luminesce at 1.54 pim with an intensity ~2 orders of magnitude greater than films with oxygen and carbon backgrounds of less than 1019 cm−3. Implantation and activation annealing was also shown to increase Er3+ signal in p(Mg)-i(Er)-n(Si) structures where C and 0 implant profiles were made to coincide with the Er-doped layers. The thermal PL quenching behavior was also markedly different for samples of varying 0 and C content. Er3+ photoluminescence from Gan:Er/Al2O3 samples with high O and C concentrations quenched by only 10% between 15 K and 300 K while the integrated PL signal from samples with lower [O] and [C] quenched ~85% over the same temperature range.

We have investigated the formation and diffusion of delta-doped layers in GaAs employing the acceptors Zn, Cd, and C. Organometallic Vapor Phase Epitaxy was used for the growth of the Zn and Cd δ-doped layers while Metalorganic Molecular Beam Epitaxy was utilized to achieve C δ-doping. The narrowest atomic profiles for Zn had full width at half maxima of 80 Å for peak Zn concentrations of ≤ 3 × 1018 cm-3, as measured by SIMS. An effective diffusion coefficient of ≤ 7 × 1017 cm2/s is estimated for a growth temperature of 625°C, based on multiple Zn δ-doped layers. For carbon, a doping spike of 7 × 1019cm-3 with a full width at half maximum of 50Å, as measured by electrochemical capacitance voltage profiling, was achieved and represents the highest doping level yet reported for planar doping. By contrast, it was difficult to achieve doping levels ≥ 4 × 1017 cm-3 for Cd due to its high vapor pressure.

Substrate preparation of GaN, both in-situ and ex-situ, and the growth of gadolinium oxide, Gd 2O 3, by Gas source molecular beam epitaxy (GSMBE) have been investigated. Ex-situ cleaning techniques included wet chemical etching and UV-ozone treatments to remove surface contaminants and the native oxide. In-situ cleaning consisted of thermal treatment with and without exposure to an electron cyclotron resonance (ECR) oxygen plasma. A GaN (1x3) streaky RHEED pattern was the final product of this surface treatment study. Various growth initiation techniques were explored to produce Gd 2O 3 films with different microstructures as evidenced by RHEED, TEM, and XRD. Gd 2O 3 films planarized the initial GaN surface and stoichiometry was maintained over a range of substrate temperatures (300° to 650°C). Single crystal gadolinium oxide films were grown at substrate temperatures of 600-650°C. These films exhibited a breakdown field strength (E BD) of ∼1MV/cm, and showed high leakage current at high forward bias due to defects within the oxide. Single crystal oxide films were found to be thermally stable at annealing temperatures up to 1000°C. Quasi-amorphous films were grown at a substrate temperature of 100°C. These films exhibited a higher E BD of ∼3MV/cm and an interface state density of 3 × 10 11 cm −2eV −1. However, the quasi-amorphous films were not thermally stable at 1000°C, showing evidence of re-crystallization in x-ray diffraction (XRD) scans.

LiGaO2 and LiAlO2 have similar lattice constants to GaN, and may prove useful as substrates for III-nitride epitaxy. We have found that these materials may be wet chemically etched in a number of acid solutions, including HF, at rates between 150–40,000 Å/min. Dry etching with SF6/Ar plasmas provides faster rates than Cl2/Ar or CH4/H2/Ar under Electron Cyclotron Resonance conditions, indicating the fluoride etch products are more volatile that their chloride or metalorganic/hydride counterparts. Dry etch rates are low (< 2, 000 Å/min), providing high selectivity (>5) over the nitrides. The incorporation of hydrogen in these materials is also of interest because this could provide a reservoir of hydrogen that may passivate dopants in overlying nitride films. In 2H implanted samples, 50 % of the deuterium is lost by evolution from the surface by annealing at 400 °C for 20 min and all of the deuterium is gone at 700°C. The diffusivity of 2H is ≈10−13 cm2/s at 250°C in LiA1O2, approximately two orders of magnitude higher than in LiGaO2.