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The Department of Defense (DoD) is investing in the development of Silicon Carbide (SiC) for a wide range of applications. Over the past year, SiC technology has demonstrated excellent device performance results for power devices, high temperature electronic devices and microwave devices. The materials growth and processing technology for SiC is now at a level of sufficient maturity to support substantial device development efforts. While there is still considerable materials and device research required for SiC to achieve it's full potential, the fundamental technology has been proven for several critical applications. A perspective on some Air Force device performance requirements will be presented. The status of SiC materials development, material limits to advances in device performance and issues relating to supporting technology will also be discussed.
The present status of SiC high-voltage power switching devices is reviewed. The figures of merits that have been used for unipolar and bipolar devices to quantify the intrinsic performance improvement over silicon are presented. Analytical and numerical modeling and simulations to estimate the BV and device choice are described. The active area and termination design of trenched-gate MOS power transistors, together with an integrated process for their fabrication, is presented. The progress in high-voltage power device experimental demonstration is described. The material and process technology issues that need to be addressed for SiC device commercialization are discussed. Finally, the impact of SiC power devices on motor drive systems is estimated.
Using atmospheric pressure MOCVD we have obtained high quality InGaN/GaN and AlGaN/GaN heterostructure materials and devices. For nominally undoped 4 μm thick GaN films, we obtained 300 K mobilities of 780 cm2/Vs and an unintentional background impurity level of n300K = 6*1016 cm−3. For InGaN/GaN heterostructures we have obtained direct band-edge transitions with FWHM as narrow as 7.9 nm (59 meV) for 50Å thick In0.16Ga0.84N quantum wells at 300K, which is the among the best reported values. The quantum wells display energy shifts towards shorter wavelength with decreasing well thickness, and the shift agrees with predicted quantum effects. These materials have been incorporated into InGaN single quantum well LEDs that emit at 450 nm. In addition AlGaN/GaN heterostructure materials have been incorporated into HFETs and MODFETs. Gate-drain breakdown voltage well exceeding 100 V, and extrinsic transconductance gm of up to 140 mS/mm were realized in the MODFET.
The properties and characteristics of vacuum microtriodes based on NEA diamond surfaces were modelled. Specifically, an NEA diamond vacuum microtriode array was investigated using electrical measurements, electron optics software, and microwave circuit simulation. Data for emission current versus applied voltage for various anode-to-cathode distances for diamond NEA surfaces was analyzed and various parameters were extracted. Electron optics software was used to determine Fowler-Nordheim and space-charge-limited DC I-V characteristics for each microtriode. Microwave circuit simulation was done to determine the behavior of arrays of these vacuum microtriodes in an RF amplifier circuit.
This paper describes the design and fabrication of a carbon based thin film transistor (TFT). The active layer is formed from a novel form of amorphous carbon (a-C) known as tetrahedrally bonded amorphous carbon (ta-C) which can be deposited at room temperature using a filtered cathodic vacuum arc (FCVA) technique. In its ‘as grown’ condition, ta-C is p-type and the devices described here, produced using undoped material, exhibit p-channel operation.
In this paper, we present the first calculations of the electron and hole initiated interband impact ionization rate in zinc blende phase GaN as a function of the applied electric field strength. The calculations are performed using an ensemble Monte Carlo simulator including the full details of the conduction and valence bands along with a numerically determined, wave-vector dependent interband ionization transition rate determined from an empirical pseudopotential calculation. The first four conduction bands and first three valence bands, which fully comprise the energy range of interest for device simulation, are included in the analysis. It is found that the electron and hole ionization rates are comparable over the full range of applied electric field strengths examined. Based on these calculations an avalanche photodiode, APD, made from bulk zinc blende GaN then would exhibit poor noise and bandwidth performance. It should be noted however, that the accuracy of the band structure employed and the scattering rates is presently unknown since little experimental information is available for comparison. Therefore, due to these uncertainties, it is difficult to unequivocally conclude that the ionization rates are comparable.
The OBIC (Optical Beam Induced Current) technique is a powerful method to investigate the electric field distribution of p-n junctions in SiC. In a previous work we found strong indications for the presence of a high density of negative surface charge in n-type SiC. In order to study samples of both conductivity types under similar conditions we prepared Schottky contacts on ntype and p-type 6H-SiC CVD epitaxial layers.
OBIC measurements show an extension of the depletion region of several hundreds of microns from the edge of the contact on n- and p-type samples, thus interconnecting diodes on an area up to several mm2. Our results imply that there is no fixed surface charge but a high density of both acceptor- and donorlike surface states leading to a dependence of the net surface charge on the Fermi energy, in which case the sign of the surface charge reverses from negative on ntype material to positive on p-type 6H-SiC.
Forward and reverse current-voltage (I-V-T) measurements of MOCVD grown 4H-SiC p+/n diodes are compared to classical recombination-generation theory over the temperature range of 100 to 750 K. The forward bias data indicate that the I-V characteristics of the wellbehaved devices follow a classical recombination dominant transport mechanism. Ideality factors were determined to be in the range of 1.85 to 2.09, and the forward activation energy found to be EA = l.56 eV compared to a nearly ideal value of 1.6 eV. A majority of the devices tested under forward bias conditions were, however, found to exhibit significant leakage current components due to tunneling at forward biases of up to 2.2 V for turn-on voltages in the 2.5 to 3.0 range. Deep level transient spectroscopy (DLTS) was also performed on the diode structures over the same wide temperature range, and the results were correlated to those obtained from reverse I-V-T and C-V-T characterization. Deep level defects at energies between 200 and 856 meV were identified from the DLTS data, and these levels are believed to be responsible for the tunneling dominant current conduction. Intrinsic deep levels, common to all devices tested, are emphasized and suggested as possible reverse bias tunneling paths for breakdown to explain the lack of an avalanche mechanism in all of the 4H-SiC diodes tested.
A new bias-enhanced nucleation method based on an AC-bias step to form highly oriented diamond (HOD) nuclei on silicon substrates is presented. The uniformity of the nucleated film and the bias time strongly depended on the substrate temperature and the substrate holder. In our case the shortest bias time and highest nucleation densities were achieved at ∼ 850°C while using a graphite susceptor. Following this nucleation enhancement step the diamond films were grown out using conditions employing an α-parameter slightly greater than 2. This ultimately leads to extremely smooth and well-faceted (100) textured HOD films which could be used as substrates for the fabrication of electronic devices.
Schottky diodes with high rectification ratios and high breakdown voltages have been fabricated for the first time via selective growth of the active boron doped diamond layers on these HOD films. Results of the growth procedure and diode performance will be given.
The radiative recombination rates have been calculated for the first time in the wide band gap wurtzite semiconductors GaN, InN and AIN and their solid solutions GaxAl1−xN and InxAl1−xN on the base of existing data on the energy band structure and optical absorption in these materials. We calculated the interband matrix elements for the direct optical transitions between the conductivity band and the valence one using the experimental photon energy dependence of the absorption coefficient near the band edge. In our calculations we assumed that the material parameters of the solid solutions (the interband matrix element, carrier effective masses and so on) could be obtained by a linear interpolation between their values in the alloy components. The temperature dependence of the energy gap was taken in the form proposed by Varshni. The calculations of the radiative recombination rates were performed in the wide range of temperature and alloy compositions.
We report on the measurements of the pyroeffect in wurtzite n-type GaN films deposited over basal plane sapphire substrates. The voltage drop between the contacts was measured while the sample was subjected to uniform heating. Our results show that the pyroelectric effect in GaN can be partially attributed to the secondary pyroelectricity, caused by the development of strain in the material due to thermal expansion.
Miniature metal-diamond wire detectors have been fabricated by Hot Filament Chemical Vapor Deposition (HFCVD) method and characterized as x-ray dosimeters in the energy range 50 keV–250 keV. Linearity of the response and the sensitivity as a function of energy have been accurately determined in a range routinely used for x-ray radiotherapy. With a Mo wire substrate, a 10 μm thick CVD diamond film can reach sensitivities of the order of 4×10−8 A/Gy/min, more than one order of magnitude larger than for a 6 cm3 standard ionization chamber.
The long-term reliability of gate insulator under high field stress of either polarity presents a constraint on the highest electric field that can be tolerated in a 4H-SiC UMOSFET under on or off condition. A realistic performance projection of 41H-SiC UMOSFET structures based on electric field in the gate insulator (1.5 MV/cm under on-condition and 3 MV/cm under offcondition) consistent with long-term reliability of insulator is provided for the breakdown voltage in the range of 600 to 1500 V. The use of P+ polysilicon gate allows us to use a higher field of 3 MV/cm in the insulator under off-condition and leads to a higher breakdown voltage as the Fowler Nordheim (FN) injection from the gate electrode is reduced. FN injection data is presented for p type 4H-SiC MOS capacitor under inversion at room temperature and at 325°C. It is concluded that the insulator reliability, and not the SiC, is the limiting factor and therefore the high temperature operation of these devices may not be practical.
A 280 V 6H-SiC thyristor has been fabricated and characterized. The switching characteristics of the SiC thyristor were investigated over a temperature range from 23 °C to 400 °C, with a switched current density of 4900 A/cm2 being observed under pulse bias condition. The thyristor has shown a dV/dt of 400 V/ms. Both the turn-on time and turn-off time increase significantly at 400 °C. The thyristor forward breakover voltage decreases by only 5% when the operating temperature is increased from 23 °C to 400 °C.
The current vs voltage characteristics of 4H-SiC MOS capacitors under deep depletion are obtained by a fast ramp response technique so as to obtain the maximum field that can be applied to the MOS structure without the failure of either the semiconductor or the oxide layer. The experiments on n-type 4H SiC wafers having a 5 μm thick epilayer of 1015 – 1016 cm−3 doping concentration and an oxide layer 1200Å – 1500Å thick, indicate the significant influence of the oxide quality and defects in the semiconductor on the nature of the current response during accumulation and deep depletion measurements. The effect of the conductivity of the oxide layer is reflected clearly in the current response, even though classical C-V measurements do not indicate any abnormality. Apart from obtaining the maximum breakdown fields of the semiconductor and the oxide, the fast-ramp response technique provides useful information about the generation processes associated with defects in the MOS structure.
Discrete, buried-gate 4H-SiC JFETs (W/L = 1 mm/5 μm) were packaged and characterized at temperatures ranging from 290 K to 773 K for use in a hybrid, 4H-SiC analog amplifier. A contaminated passivation oxide was found to limit high-temperature operation initially, but upon removal of the passivation layer the devices demonstrated stable operation to 773 K with adequate amplification (Av greater than 200 V/V) up to 673 K. From the 16 devices tested, a peak extrinsic saturated transconductance (gmsa) of 27.1 mS/mm was measured at 308 K, corresponding to a channel mobility of 400 cm2/V.s, excluding significant series resistance effects.
We have characterized the high electric field breakdown process of several epitaxial 4H-SiC p-n structures with oxide passivation. The breakdown voltage was found to be dependent on the size of the diode structures as well as their proximity to any structural defects. The time dependence of the breakdown process was also measured to determine the characteristics of the breakdown mechanism. This time dependence measurement provides an indication of the quality of the diode structures. Both soft and abrupt breakdown mechanisms were observed showing the influence of defects on the high field behavior of the diode structures. Measurements done with and without the use of Fluorinert fluid did not show any difference in the breakdown voltage indicating that surface flashover breakdown mechanism did not play a major role in the avalanche breakdown process.
Results are reported for ohmic contacts formed on n-type 4H and 6H-SiC using nichrome (80/20 weight percent Ni/Cr). In comparison to contacts formed on 6H-SiC using pure Ni, the electrical characteristics of these NiCr contacts are similar (∼ 1E-5 Ω-cm2 for moderately doped material), and composite Au/NiCr contacts exhibit good stability during long-term anneals (∼ 2500 hr) at 300 C without the requirement of a diffusion barrier layer between the ohmic contact layer and the Au cap layer. The use of NiCr also results in success rates near 100% for direct wire bonding to the Au cap layers.
Ni-Ti alloy is a promising candidate for high-temperature contact metallization for SiC electronic devices. In the present study Ni-Ti alloy thin films (100 nm) of two different compositions (Ni90Ti10 and Ni50Ti50) were coevaporated on 6H-SiC substrate. Interfacial reactions, microstructure, compositional changes, and phase formation were investigated as functions of heat-treatments in the range of 400–800 °C. The study was carried out using Auger electron spectroscopy, x-ray diffraction, and analytical transmission electron microscopy. In the case of the Ni90Ti10 alloy the interaction was found to begin at 450 °C. Ni and C are the dominant diffusing species. The reaction zone is divided into three layers. In the first layer, adjacent to the SiC substrate, the presence of a Ni-rich silicide, Ni2Si, and C precipitates, was observed. The second layer is composed mainly of TiC, while the third - of Ni2 Si. In the case of the Ni50 Ti50 alloy the interaction began at 800 °C. Carbon is the dominant diffusant. The reaction zone is divided into two layers. The first, next to the substrate layer is composed of epitaxially grown TiC and the second - of Ni3Ti2Si compound. A thin (∼5 nm) amorphous discontinuous layer was found at the TiC/SiC interface. Factors controlling phase formation in the Ni-Ti/SiC system are discussed.
Ohmic contacts on p-type GaN have been investigated. High quality GaN epilayers on cplane sapphire were prepared using plasma-assisted molecular beam epitaxy that utilized an inductively coupled rf nitrogen plasma source and solid source beams. The resulting film thickness and the doping concentration of the grown samples were in the range of 0.7–1.35 μm and 1018 – 1020/cm3, respectively. The metallization consisted of high work function metal bilayers which included a combinations of 25 nm-thick Ni, Ti, Pt and/or Cr and 200 nm-thick Au on the highly p-doped GaN in a transmission line model pattern. Ohmic contacts were formed by alloying the bi-layers using rapid thermal annealing (RTA) at temperatures in the range of 300–700 °C for 1 min under nitrogen ambient. Current-voltage measurements showed that the specific contact resistance was as low as 1.2 × 10 −4 Ω–cm2 for the sample having 1.4 × 1020/cm3p-type doping concentration with a Cr/Au contact annealed at 500 °C for 1 min by RTA. Judging from the scanning Auger microscopy results and the glancing angle x-ray diffraction analysis, this resistance is attributed to Cr diffusion into the GaN layer.