Effect of surface modification on the radiation stability of diamond ohmic contacts

The ohmic contact interface between diamond and metal is essential for the application of diamond detectors.Surface modification can significantly affect the contact performance and eliminate the interface polarization effect.However,the radiation stability of a diamond detector is also sensitive to surface modification.In this work,the influence of surface modification technology on a diamond ohmic contact under high-energy radiation was investigated.Before radiation,the specific contact resistivities(ρc)between Ti/Pt/Au-hydrogen-terminated diamond(H-diamond)and Ti/Pt/Au-oxygenterminated diamond(O-diamond)were 2.0×10^(-4)W·cm^(2) and 4.3×10^(-3)Wcm^(2),respectively.After 10 MeV electron radiation,the ρc of Ti/Pt/Au H-diamond and Ti/Pt/Au O-diamond were 5.3×10^(-3)W·cm^(2)and 9.1×10^(-3)W·cm^(2),respectively.The rates of change of ρc of H-diamond and O-diamond after radiation were 2550%and 112%,respectively.The electron radiation promotes bond reconstruction of the diamond surface,resulting in an increase in ρc.

Physical mechanism of oxygen diffusion in the formation of Ga_(2)O_(3) Ohmic contacts

The formation of low-resistance Ohmic contacts in Ga_(2)O_(3) is crucial for high-performance electronic devices. Conventionally, a titanium/gold(Ti/Au) electrode is rapidly annealed to achieve Ohmic contacts, resulting in mutual diffusion of atoms at the interface. However, the specific role of diffusing elements in Ohmic contact formation remains unclear.In this work, we investigate the contribution of oxygen atom diffusion to the formation of Ohmic contacts in Ga_(2)O_(3). We prepare a Ti/Au electrode on a single crystal substrate and conduct a series of electrical and structural characterizations.Using density functional theory, we construct a model of the interface and calculate the charge density, partial density of states, planar electrostatic potential energy, and I–V characteristics. Our results demonstrate that the oxygen atom diffusion effectively reduces the interface barrier, leading to low-resistance Ohmic contacts in Ga_(2)O_(3). These findings provide valuable insights into the underlying mechanisms of Ohmic contact formation and highlight the importance of considering the oxygen atom diffusion in the design of Ga_(2)O_(3)-based electronic devices.

Back interface passivation for ultrathin Cu(In,Ga)Se_(2) solar cells with Schottky back contact: A trade-off of electrical effects

Back interface passivation reduces the back recombination of photogenerated electrons, whereas aggravates the blocking of hole transport towards back contact, which complicate the back interface engineering for ultrathin CIGSe solar cells with a Schottky back contact. In this work, theoretical explorations were conducted to study how the two contradictory electrical effects impact cell performance. For ultrathin CIGSe solar cells with a pronounced Schottky potential barrier(E_(h)> 0.2 eV), back interface passivation produces diverse performance evolution trends, which are highly dependent on cell structures and properties. Since a back Ga grading can screen the effect of reduced recombination of photogenerated electrons from back interface passivation, the hole blocking effect predominates and back interface passivation is not desirable. However, when the back Schottky diode merges with the main pn junction due to a reduced absorber thickness,the back potential barrier and the hole blocking effect is much reduced on this occasion. Consequently, cells exhibit the same efficiency evolution trend as ones with an Ohmic contact, where back interface passivation is always advantageous.The discoveries imply the complexity of back interface passivation and provide guidance to manipulate back interface for ultrathin CIGSe solar on TCOs with a pronounced Schottky back contact.

Low-resistance ohmic contacts on InAlN/GaN heterostructures with MOCVD-regrown n+-InGaN and mask-free regrowth process

This paper studied the low-resistance ohmic contacts on InAlN/GaN with metal–organic chemical vapor deposition(MOCVD)regrowth technique.The 150-nm regrown n-InGaN exhibits a low sheet resistance of 31Ω/□,resulting in an extremely low contact resistance of 0.102Ω·mm between n^(+)-InGaN and InAlN/GaN channels.Mask-free regrowth process was also used to significantly improve the sheet resistance of InAlN/GaN with MOCVD regrown ohmic contacts.Then,the diffusion mechanism between n^(+)-InGaN and InAlN during regrowth process was investigated with electrical and structural characterizations,which could benefit the further process optimization.

Ohmic and Schottky contacts of hydrogenated and oxygenated boron-doped single-crystal diamond with hill-like polycrystalline grains

Hill-like polycrystalline diamond grains(HPDGs)randomly emerged on a heavy boron-doped p+single-crystal diamond(SCD)film by prolonging the growth duration of the chemical vapor deposition process.The Raman spectral results confirm that a relatively higher boron concentration(~1.1×10^(21) cm^(-3))is detected on the HPDG with respect to the SCD region(~5.4×10^(20) cm^(-3)).It demonstrates that the Au/SCD interface can be modulated from ohmic to Schottky contact by varying the surface from hydrogen to oxygen termination.The current-voltage curve between two HPDGs is nearly linear with either oxygen or hydrogen termination,which means that the HPDGs provide a leakage path to form an ohmic contact.There are obvious rectification characteristics between oxygen-terminated HPDGs and SCD based on the difference in boron doping levels in those regions.The results reveal that the highly boron-doped HPDGs grown in SCD can be adopted as ohmic electrodes for Hall measurement and electronic devices.

High energy electron radiation effect on Ni/4H-SiC SBD and Ohmic contact

The Ni/4H-SiC Schottky barrier diodes （SBDs） and transfer length method （TLM） test patterns of Ni/4H-SiC Ohmic contacts were fabricated, and irradiated with i MeV electrons up to a dose of 3.43 × 10^14 e/cm^-2. After radiation, the forward currents of the SBDs at 2 V decreased by about 50%, and the reverse currents at -200 V increased by less than 30%. Schottky barrier height （ФB） of the Ni/4H-SiC SBD increased from 1.20 eV to 1.21 eV under 0 V irradiation bias, and decreased from 1.25 eV to 1.19 eV under -30 V irradiation bias. The degradation of ФB could be explained by the variation of interface states of Schottky contacts. The on-state resistance （Rs） and the reverse current increased with the dose, which can be ascribed to the radiation defects in bulk material. The specific contact resistance （ρc） of the Ni/SiC Ohmic contact increased from 5.11× 10^-5Ω· cm^2 to 2.97× 10^-4Ω· cm^2.

Ti-Al based ohmic contacts to n-type 6H-SiC with ion implantation

The Ti-Al ohmic contact to n-type 6H-SiC has been fabricated. An array of TLM （transfer length method） test patterns with Au/Ti/A1/Ti/SiC structure is formed on N-wells created by P^＋ ion implantation into Si-faced p-type 6H-SiC epilayer. The specific contact resistance pc as low as 8.64×10-6Ω·cm^2 is achieved after annealing in N2 at 900℃ for 5min. The sheet resistance Rsh of the implanted layers is 975Ω. X-ray diffraction （XRD） analysis shows the formation of Ti3SiC2 at the metal/n-SiC interface after thermal annealing, which is responsible for the low resistance contact.

The fabrication of nickel silicide ohmic contacts to n-type 6H-silicon carbide

This paper reports that the nickel silicide ohmic contacts to n-type 6H-SiC have been fabricated. Transfer length method test patterns with NiSi/SiC and NiSi2/SiC structure axe formed on N-wells created by N^＋ ion implantation into Si-faced p-type 6H-SiC epilayer respectively. NiSi and NiSi2 films are prepared by annealing the Ni and Si films separately deposited. A two-step annealing technology is performed for decreasing of oxidation problems occurred during high temperature processes. The specific contact resistance Pc of NiSi contact to n-type 6H-SiC as low as 1.78× 10^-6Ωcm^2 is achieved after a two-step annealing at 350 ℃for 20 min and 950℃ for 3 min in N2. And 3.84×10-6Ωcm^2 for NiSi2 contact is achieved. The result for sheet resistance Rsh of the N＋ implanted layers is about 1210Ω/□. X-ray diffraction analysis shows the formation of nickel silicide phases at the metal/n-SiC interface after thermal annealing. The surfaces of the nickel silicide after thermal annealing are analysed by scanning electron microscope.

Annealing temperature influence on the degree of inhomogeneity of the Schottky barrier in Ti/4H–SiC contacts

Tung＇s model was used to analyze anomalies observed in Ti/Si C Schottky contacts. The degree of the inhomogeneous Schottky barrier after annealing at different temperatures is characterized by the ‘T0anomaly＇ and the difference（△Φ）between the uniformly high barrier height（Φ0B） and the effective barrier height（Φeff B）. Those two parameters of Ti Schottky contacts on 4H–Si C were deduced from I–V measurements in the temperature range of 298 K–503 K. The increase in Schottky barrier（SB） height（ΦB） and decrease in the ideality factor（n） with an increase measurement temperature indicate the presence of an inhomogeneous SB. The degree of inhomogeneity of the Schottky barrier depends on the annealing temperature, and it is at its lowest for 500-°C thermal treatment. The degree of inhomogeneity of the SB could reveal effects of thermal treatments on Schottky contacts in other aspects.

Ni/Pd-based ohmic contacts to p-GaN through p-InGaN/p^(+)-GaN contacting layers

Specific contact resistance to p-GaN was measured for various structures of Ni/Pd-based metals and thin(20-30 nm thick)p-InGaN/p^(+)-GaN contacting layers.The effects of surface chemical treatment and annealing temperature were ex-amined.The optimal annealing temperature was determined to be 550°C,above which the sheet resistance of the samples de-graded considerably,suggesting that undesirable alloying had occurred.Pd-containing metal showed~35%lower com-pared to that of single Ni.Very thin(2-3.5 nm thick)p-InGaN contacting layers grown on 20-25 nm thick p^(+)-GaN layers exhib-ited one to two orders of magnitude smaller values of compared to that of p^(+)-GaN without p-InGaN.The current density de-pendence of,which is indicative of nonlinearity in current-voltage relation,was also examined.The lowest achieved through this study was 4.9×10^(-5)Ω·cm^(2)@J=3.4 kA/cm^(2).

Refractory Metal Nitride Schottky Contact on GaN

For thermally stable or high-temperature operating,Schottky contact utilizing refractory metal nitride,TiN,MoN and ZrN,on n-GaN were evaluated.The refractory metal nitride films were formed by reactive sputtering in Ar and N2 ambient.Current-voltage characteristics show that ideality factors of 1.09-1.22 and barrier heights of 0.66-0.75 eV was obtained for the three metal nitrides.For the ZrN contact,the ideality factor and barrier height of became 1.06 and 0.77 eV,respectively,after 800 ℃ annealing.AlGaN/GaN heterostructure FET with TiN gate was also investigated.No obvious degradation was found for the TiN-gate device even after thermal treatment at 850 ℃.This shows that Schottky contact utilizing refractory metal nitride on GaN has the potential for thermal stability or high-temperature operating.

Characteristics of Ni-based ohmic contacts on n-type 4H-SiC using different annealing methods

Nickel is an excellent ohmic-contact metal on 4H-SiC.This paper discusses the formation mechanism of nickel ohmic contact on 4HSiC by assessing the electrical properties and microstructural change.Under high-temperature annealing,the phase of nickel-silicon compound can be observed with X-ray diffraction,and the contact resistance also changes.A comparative experiment was designed to use X-ray diffraction and energy-dispersive spectroscopy to clarify the difference of ohmic-contact material composition and elemental analysis between samples prepared using pulsed laser annealing and rapid thermal annealing.It is found that more Ni2Si and carbon vacancies formed at the interface in the sample prepared using pulsed laser annealing,resulting in a better ohmic-contact characteristic.

Ohmic contact properties of p-type surface conductive layer on H-terminated diamond films prepared by DC arc jet CVD

With the advantages of high deposition rate and large deposition area, polycrystalline diamond films prepared by direct current （DC） arc jet chemical vapor deposition （CVD） are considered to be one of the most promising materials for high-frequency and high-power electronic devices. In this paper, high-quality self-standing polycrystalline diamond films with the diameter of 100 mm were prepared by DC arc jet CVD, and then, the p-type surface conductive layer with the sheet carrier density of 10^11-10^13 cm-2 on the H-terminated diamond film was obtained by micro-wave hydrogen plasma treatment for 40 min. Ti/Au and Au films were deposited on the H-terminated diamond surface as the ohmic contact electrode, respectively, afterwards, they were treated by rapid vacuum annealing at different temperatures. The properties of these two types of ohmic contacts were investigated by measuring the specific contact resistance using the transmission line method （TLM）. Due to the formation of Ti-related carbide at high temperature, the specific contact resistance of Ti/Au contact gradually decreases to 9.95 × 10^-5 Ω-cm2 as the temperature increases to 820℃. However, when the annealing temperature reaches 850℃, the ohmic contact for Ti/Au is degraded significantly due to the strong diffusion and reaction between Ti and Au. As for the as-deposited Au contact, it shows an ohmic contact. After annealing treatment at 550℃, low specific contact resistance was detected for Au contact, which is derived from the enhancement of interdiffusion between Au and diamond films.

Thermal annealing behaviour of Al/Ni/Au multilayer on n-GaN Schottky contacts

Recently GaN-based high electron mobility transistors （HEMTs） have revealed the superior properties of a high breakdown field and high electron saturation velocity. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. This paper reports that an Al layer as thin as 3 nm was inserted between the conventional Ni/Au Schottky contact and n-GaN epilayers, and the Schottky behaviour of Al/Ni/Au contact was investigated under various annealing conditions by current-voltage （I-V） measurements. A non-linear fitting method was used to extract the contact parameters from the I-V characteristic curves. Experimental results indicate that reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing. And high quality Schottky contact with a barrier height of 0.875 eV and the lowest reverse-bias leakage current, respectively, can be obtained under 12 min annealing at 450 ℃ in N2 ambience.

Analysis of the inhomogeneous barrier and phase composition of W/4H-SiC Schottky contacts formed at different annealing temperatures

The electrical characteristics of W/4H-SiC Schottky contacts formed at different annealing temperatures have been measured by using current-voltage-temperatures（I-V -T） and capacitance-voltage-temperatures（C-V -T） techniques in the temperature range of 25℃-175℃. The testing temperature dependence of the barrier height（BH） and ideality factor（n） indicates the presence of inhomogeneous barrier. Tung＇s model has been applied to evaluate the degree of inhomogeneity, and it is found that the 400℃ annealed sample has the lowest T0 of 44.6 K among all the Schottky contacts. The barrier height obtained from C-V -T measurement is independent of the testing temperature, which suggests a uniform BH.The x-ray diffraction（XRD） analysis shows that there are two kinds of space groups of W when it is deposited or annealed at lower temperature（≤500℃）. The phase of W2C appears in the sample annealed at 600℃, which results in the low BH and the high T0. The 500℃ annealed sample has the highest BH at all testing temperatures, indicating an optimal annealing temperature for the W/4H-SiC Schottky rectifier for high-temperature application.

High-Frequency AlGaN/GaN High-Electron-Mobility Transistors with Regrown Ohmic Contacts by Metal-Organic Chemical Vapor Deposition

Nonalloyed ohmic contacts regrown by metal-organic chemical vapor deposition are performed on AlGaN/GaN high-electron-mobility transistors. Low ohmic contact resistance of 0.15Ω.mm is obtained. It is found that the sidewall obliquity near the regrown interface induced by the plasma dry etching has great influence on the total contact resistance. The fabricated device with a 100-nm T-shaped gate demonstrates a maximum drain current density of 0.95 A/mm at Vgs = 1 V and a maximum peak extrinsic transcondutance Gm of 216mS/ram. Moreover, a current gain cut-off frequency fT of 115 GHz and a maximum oscillation frequency fmax of 127 GHz are achieved.

On the reverse leakage current of Schottky contacts on free-standing GaN at high reverse biases

In this work, a dislocation-related tunneling leakage current model is developed to explain the temperature-dependent reverse current–voltage（I–V –T） characteristics of a Schottky barrier diode fabricated on free-standing GaN substrate for reverse-bias voltages up to-150 V. The model suggests that the reverse leakage current is dominated by the direct tunneling of electrons from Schottky contact metal into a continuum of states associated with conductive dislocations in GaN epilayer.A reverse leakage current ideality factor, which originates from the scattering effect at metal/GaN interface, is introduced into the model. Good agreement between the experimental data and the simulated I–V curves is obtained.

Mechanism of Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures via laser annealing

The physical mechanisms of Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures by laser annealing and rapid thermal annealing are systematically investigated. The microstructures indicate that a better surface morphology and an intact contact interface are formed after laser annealing. None of the TiN alloy spikes are formed at the interface of the laser annealing sample. The experimental results show that the current transport mechanism through the ohmic contact after laser annealing is different from the conventional spike mechanism, and it is dominated by thermionic field emission.

Current transport mechanism of Mg/Au ohmic contacts to lightly doped n-type β-Ga_2O_3

The carrier transport mechanism of Mg/Au ohmic contact for lightly doped β-Ga_2O_3 is investigated. An excellent ohmic contact has been achieved when the sample was annealed at 400 °C and the specific contact resistance is 4.3 × 10-4 Ω·cm2. For the annealed sample, the temperature dependence of specific contact resistance is studied in the range from 300 to 375 K. The specific contact resistance is decreased from 4.3 × 10-4 to 1.59 × 10-4 Ω·cm2 with an increase of test temperature. As combination with the judge of E00, the basic mechanism of current transport is dominant by thermionic emission theory. The effective barrier height between Mg/Au and β-Ga_2O_3 is evaluated to be 0.1 eV for annealed sample by fitting experimental data with thermionic emission model.

Analysis of the abnormal resistance in AlGaN/GaN heterostructure's ohmic contact tests

Ti/Al/Ti/Au and Ti/Al/Ni/Au ohmic contacts were fabricated on AlGaN/GaN heterostructure under different temperatures of rapid thermal processing （RTP）. Since abnormal resistance values were observed during the contact resistance testing,the surface morphology and contact borders of the samples were analyzed to determine the physical mechanism. Such abnormal phenomenon is found to originate from cracking of the AlGaN layer during RTP,flowing of Ti/Al metallic liquid along the crevices,and continuous reaction of the metallic liquid with AlGaN/GaN. Such processes result in abnormal conduction channels. The possible mechanism of the crevice formation was discussed,and the possible solutions to avoid the crevices were proposed.