High quality SiO2/Si interfaces of poly-crystallinesilicon thin film transistors by annealing in wet atmosphere

A new post-metallization annealing technique was developed to improve the quality of metal-oxide-semiconductor (MOS) devices using SiO ₂ films formed by a parallel-plate remote plasma chemical vapor deposition as gate insulators. The quality of the interface between SiO₂ and crystalline Si was investigated by capacitance-voltage (C-V) measurements. An H₂O vapor annealing at 270°C for 30 min efficiently decreased the interface trap density to 2.0×10¹⁰ cm^-2 eV^-1, and the effective oxide charge density from 1×10 ¹² to 5×10⁹ cm^-2. This annealing process was also applied to the fabrication of Al-gate polycrystalline silicon thin film transistors (poly-Si TFT's) at 270°C. In p-channel poly-Si TFT's, the carrier mobility increased from 60-400 cm² V^-1 s^-1 and the threshold voltage decreased from -5.5 to -1.7 V     

A study of frequency response in silicon heterojunction bipolartransistors with amorphous silicon emitters

A detailed physical model of amorphous silicon (a-Si:H) is incorporated into a two-dimensional device simulator to examine the frequency response limits of silicon heterojunction bipolar transistors (HBT's) with a-Si:H emitters. The cutoff frequency is severely limited by the transit time in the emitter space charge region, due to the low electron drift mobility in a-Si:H, to 98 MHz which compares poorly with the 37 GHz obtained for a silicon homojunction bipolar transistor with the same device structure. The effects of the amorphous heteroemitter material parameters (doping, electron drift mobility, defect density and interface state density) on frequency response are then examined to find the requirements for an amorphous heteroemitter material such that the HBT has better frequency response than the equivalent homojunction bipolar transistor, We find that an electron drift mobility of at least 100 cm² V^-1 s^-1 is required in the amorphous heteroemitter and at a heteroemitter drift mobility of 350 cm ² V^-1 s^-1 and heteroemitter doping of 5×10¹⁷ cm^-3, a maximum cutoff frequency of 52 GHz can be expected     

Rapid isothermal annealing of high- and low-energy ion-implantedInP and In0.53Ga0.47As

Rapid isothermal annealing (RIA) was performed on 0.5-16-MeV Si ⁺, 1-MeV Be⁺, and 150-keV Ge⁺ implanted InP:Fe and 380-keV Fe⁺ implanted InGaAs. Annealings were performed in the temperature range 800-925°C using an InP proximity wafer in addition to the Si₃N₄ dielectric cap. Dopant activations close to 100% were obtained for 3×10¹⁴ cm^-2 Si⁺ and 2×10¹⁴ cm^-2 Be⁺ implants in InP:Fe. For the elevated temperature (200°C) 1×10¹⁴ cm^-2 Ge⁺ implant, a maximum of 50% activation was obtained. No redistribution of dopant was observed for Si and Ge implants due to annealing. However, redistribution of dopant was seen for Be and Fe implants due to annealing. Phosphorous coimplantation has helped to eliminate the Be in-diffusion problem in InP, but did not help to reduce Fe in-diffusion and redistribution in InGaAs. Using an RIA cycle with low temperature and short duration is the only solution to minimize Fe redistribution in InGaAs     

Bulk and surface properties of amorphous hydrogenated fluorinatedsilicon grown from SiF4 and H2

A detailed study of the growth of amorphous hydrogenated fluorinated silicon (a-Si:H, F) from a DC glow discharge in SiF₄ and H₂ is discussed. The electrical properties of the films can be varied over a very wide range. The bulk properties of the best films that were measured included an Urbach energy E_u =43 meV, a deep-level defect density N_s=1.5×10¹⁵ cm^-3, and a hole drift mobility of 8×10^-3 cm² V^-1 s^-1, which reflects a characteristic valence band energy of 36 meV. It was found that E_u, N _s, and the density of surface states N_ss are related to each other. Under the deposition condition of the films with the best bulk properties, N_ss reaches its highest value of 1×10¹⁴ cm^-2. It is suggested that in growth from SiF₄/H₂, the density of dangling bonds at the growing surface is very sensitive to the deposition conditions     

High performance poly-Si TFTs fabricated using pulsed laserannealing and remote plasma CVD with low temperature processing

Key technologies for fabricating polycrystalline silicon thin film transistors (poly-Si TFTs) at a low temperature are discussed. Hydrogenated amorphous silicon films were crystallized by irradiation of a 30 ns-pulsed XeCl excimer laser. Crystalline grains were smaller than 100 nm. The density of localized trap states in poly-Si films was reduced to 4×10¹⁶ cm^-3 by plasma hydrogenation only for 30 seconds. Remote plasma chemical vapor deposition (CVD) using mesh electrodes realized a good interface of SiO ₂/Si with the interface trap density of 2.0×10¹⁰ cm^-2 eV^-1 at 270°C. Poly-Si TFTs were fabricated at 270°C using laser crystallization, plasma hydrogenation and remote plasma CVD. The carrier mobility was 640 cm²/Vs for n-channel TFTs and 400 cm²/Vs for p-channel TFTs. The threshold voltage was 0.8 V for n-channel TFTs and -1.5 V for p-channel TFTs. The leakage current of n-channel poly-Si TFTs was reduced from 2×10^-10 A/μm to 3×10^-13 A/μm at the gate voltage of -5 V using an offset gate electrode with an offset length of 1 μm     

Deep-level dominated current-voltage characteristics of buriedimplanted oxide silicon-on-insulator

Current-voltage characteristics of Au contacts formed on buried implanted oxide silicon-on-insulator (SOI) structures are discussed, which indicate that the dominant transport mechanism is space-charge-limited current (SCLC) conduction in the presence of deep-level states. The deep-level parameters, determined using a simple analysis, appear to be sensitive to anneal conditions used and subsequent processing. Silicon implanted with 1.7×10¹⁸ cm^-2 oxygen ions at 150 keV following a 1200°C anneal for 3 h shows deep level 0.37 eV below the conduction band edge with a concentration of unoccupied traps of ~ 2×10¹⁵ cm^-3 . In contrast, arsenic ion implantation, in the 1200°C annealed material with a dose of 1.5×10¹² cm^-2 at 60 keV and activated by rapid thermal annealing (RTA), introduces a deep level 0.25 eV below the conduction band edge with an unoccupied trap concentration of ~6×10¹⁷ cm^-2     

Carrier transport related analysis of high-power AlGaN/GaN HEMTstructures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm²/V/s and a sheet concentration of 9×10¹² cm at 300 K (7900 cm²/V/s and 8×10¹² cm^-2 at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n_SdH=7.8×10¹² cm^-2 and a high quantum scattering lifetime of τ_q=1.5×10^-13 s at 4.2 K compared to n_SdH=8.24×10¹² cm^-2 and τ_q=1.72×10^-13 s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance     

Bottom-gate poly-Si thin film transistors using XeCl excimer laserannealing and ion doping techniques

High mobility bottom-gate poly-Si thin film transistors (TFTs) have been successfully fabricated on a hard glass substrate using XeCl excimer laser annealing and ion doping techniques. The authors used an a-Si:H film which is deposited by a plasma-enhanced chemical vapor deposition (PECVD) as a precursor film, and then they crystallized the a-Si film by XeCl excimer laser annealing. The maximum field effect mobility and grain size obtained were 200 cm²/V-s (n-channel), and 250 nm, respectively. The poly-Si TFTs showed excellent transfer characteristics, and an ON/OFF current ratio of over 10⁶ was obtained. Successful control of the threshold voltage within 4 V using an ion doping technique is also demonstrated     

4H-SiC MOSFETs utilizing the H2 surface cleaningtechnique

The H₂ cleaning technique was examined as the precleaning of the gate oxidation for 4H-SiC MOSFETs. The device had a channel width and length of 150 and 100 μm, fabricated on the p-type epitaxial layer of 3×10¹⁶ cm^-3. The gate oxidation was performed after the conventional RCA cleaning, and H₂ annealing at 1000°C. The obtained channel mobility depends on the pre-cleaning process strongly, and was achieved 20 cm²/N s in the H₂ annealed sample. The effective interface-state density was also measured by the MOS capacitors fabricated on the same chips, resulting 1.8×10¹² cm^-2 from the photo-induced C-V method     

Erase/write cycle tests of n-MOSFETs with Si-implantedgate-SiO2

To discuss the applicability of a MOSFET with Si-implanted gate-SiO₂ of 50 nm thickness to a non volatile random access memory (NVRAM) operating more than 3.3×10¹⁵ erase/write (E/W) cycles, E/W-cycle tests were performed up to 10¹¹ cycles by measuring the hysteresis curve observed in a source follower MOSFET in which a sine-wave voltage of 100 kHz was supplied to the gate. Degradations in the threshold-voltage window of 15 V and gain factor were scarcely observed in a MOSFET with Si-implantation at 50 keV/1×10¹⁶ cm^-2 at a gate voltage of ±40 V. Those degradations observed in a MOSFET with 25 keV/3×10¹⁶ cm^-2 were improved by lowering the gate voltage from ±40 V to ±30 V in sacrificing the smaller threshold-voltage window from 20 to 8.5 V     

Electron transport in 0.15-μm gate In0.52Al0.48As/In0.53Ga0.47As HEMT

Magneto-transport and cyclotron resonance measurements were made to determine directly the density, mobility, and the effective mass of the charge carriers in a high-performance 0.15-μm gate In_0.52 Al_0.48As/In_0.53Ga_0.47As high-electron-mobility transistor (HEMT) at low temperatures. At the gate voltage V_G=0 V, the carrier density n _g under the gate is 9×10¹¹ cm^-2, while outside of the gate region n_g=2.1×10¹² cm^-2. The mobility under the gate at 4.2 K is as low as 400 cm²/V-s when V_G<0.1 V and rapidly approaches 11000 cm²/V-s when V_G>0.1 V. The existence of this high mobility threshold is crucial to the operation of the device and sets its high-performance region in V_G>0.1 V     

Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

Electron mobility models for 4H, 6H, and 3C SiC [MESFETs]

Models for the electron mobility in the three most important silicon carbide (SiC) polytypes, namely, 4H, 6H, and 3C SiC are developed. A large number of experimental mobility data and Monte Carlo (MC) results reported in the literature have been evaluated and serve as the basis for the model development. The proposed models describe the dependence of the electron mobility on doping concentration, temperature, and electric field. The low-field mobility in 4H SiC is much higher than in 6H and 3C in the doping range interesting for RF power transistors (10¹⁶ cm^-3 ...10¹⁸ cm ^-3), whereas the saturation velocities in the three polytypes investigated are nearly the same (slightly above 2×10⁷ cm/s at 300 K). The models developed can be easily incorporated into numerical device simulators     

Strong increase of the derivative of the carrier-induced indexchange of semiconductor lasers at low injected carrier density

The carrier-induced index change of a semiconductor laser was measured for injected carrier density ranging from 3 × 10¹⁶ cm^-3 to 2 × 10¹⁸ cm^-3. A strong nonlinear behavior between index change and carrier density is observed. The derivative of the index change versus carrier density at low carrier density can be 15 times larger than the derivative of the index change at high carrier density     

Minority-carrier transport parameters in heavily doped p-typesilicon at 296 and 77 K

Minority-carrier electron lifetime, mobility and diffusion length in heavily doped p-type Si were measured at 296 and 77 K. It was found that a 296 K μ_n (pSi)≈μ_n (nSi) for N _AA≲5×10¹⁸ cm^-3, while μ_n (pSi)/μ_n (nSi)≈1 to 2.7 for higher dopings. The results also show that for N_AA≲3×10¹⁹ cm^-3, D (pSi) at 77 K is smaller than that at 296 K, while for higher dopings D_n (pSi) is larger at 77 K than at 296 K. μ_n (pSi) at 77 K increases with the increasing doping above N_AA>3×10¹⁸ cm^-3, in contrast to the opposite dependence for μ_n (nSi) in n⁺ Si     

Drift hole mobility in strained and unstrained doped Si1-xGex alloys

Results of the drift hole mobility in strained and unstrained SiGe alloys are reported for Ge fractions varying from 0 to 30% and doping levels of 10¹⁵-10¹⁹ cm^-3. The mobilities are calculated taking into account acoustic, optical, alloy, and ionized-impurity scattering. The mobilities are then compared with experimental results for a boron doping concentration of 2×10¹⁹ cm^-3. Good agreement between experimental and theoretical values is obtained. The results show an increase in the mobility relative to that of silicon     

Light-soaking effects on a-Si:H photodiodes deposited by thelaminar-flow photo-CVD method

A 2/3-in, 2-Mpixel, STACK-CCD imaging sensor has been developed for HDTV solid-state imagers. A new a-Si:H photo-conversion layer, fabricated by the laminar-flow photo-chemical-vapor-deposition method, is overlaid on the vertical CCD scanning circuitry in the sensor. The photodegradation behavior of a-Si:H photodiodes is investigated in terms of dark-current density, electron μτ product and transient photocurrent. These properties are degraded as a result of light-induced defects in the a-Si:H layer. The Staeblar-Wronski constants, C_sw , are estimated to be 7.5×10^-7 at no voltage and 1.1×10^-7 at a reverse voltage of 6 V applied to the photodiode during light-soaking with an AM-1 lamp. The lifetime of the photodiode is determined by the degradation of the transient photocurrent, and is estimated to be about 2.2×10⁸ h for 1 lx light exposure. The lifetime is considered to be improved compared with that of previous-type photodiode reported before (1.5×10⁷ h for 1.5 lx light exposure) and clearly satisfies the needs for practical use of the device     

Formation of 0.05-μm p+-n and n+-pjunctions by very low (<500 eV) ion implantation

The current-voltage (I-V) characteristics of ultrashallow p⁺ -n and n⁺-p diodes, obtained using very-low-energy (<500-eV) implantation of B and As, are presented. the p⁺-n junctions were formed by implanting B⁺ ions into n-type Si (100) at 200 eV and at a dose of 6×10¹⁴ cm^-2, and n⁺-p junctions were obtained by implanting As⁺ ions into p-type (100) Si at 500 eV and at a dose 4×10¹² cm^-2. A rapid thermal annealing (RTA) of 800°C/10 s was performed before I-V measurements. Using secondary ion mass spectrometry (SIMS) on samples in-situ capped with a 20-nm ²⁸Si isotopic layer grown by a low-energy (40 eV) ion-beam deposition (IBD) technique, the depth profiles of these junctions were estimated to be 40 and 20 nm for p⁺-n and n⁺-p junctions, respectively. These are the shallowest junctions reported in the literature. The results show that these diodes exhibit excellent I-V characteristics, with ideality factor of 1.1 and a reverse bias leakage current at -6 V of 8×10^-12 and 2×10^-11 A for p⁺-n and n⁺-p diodes, respectively, using a junction area of 1.96×10^-3 cm²     

A physically-based model of the effective mobility in heavily-dopedn-MOSFETs

We present a new analytical mobility model for channel electrons in heavily-doped MOSFETs biased from weak to strong inversion suitable for implementation in device simulation codes. The model accounts for the two-dimensionality of the electron gas and for the effect of charge trapping on the measurements and has been validated by comparing the theoretical curves with an extensive set of mobility measurements performed on devices with channel doping ranging from 3.8×10¹⁷ to 1.25×10¹⁸ cm^-3 over a wide bias and temperature range (141-400 K)     

Amorphous-Si emitter heterojunction UHF power transistors for handytransmitter

A UHF silicon heterojunction bipolar power transistor with a heavily doped amorphous-silicon emitter is reported. The fabrication process utilized an improved glow discharge technique. The deposition rate of amorphous silicon is 0.3-0.4 Å/s, which is slower than that of conventional a-Si:H. The average carrier density in the amorphous-silicon film is estimated to be about 1.5×10¹⁹ cm^-3. The present device can deliver 4.0-W output power with 72% collector efficiency and 8.2-dB gain at 470 MHz for 9.0-V low supply voltage. These preliminary results make the use of n⁺ a-Si:H as a wide-bandgap emitter material for high-frequency and high-power heterojunction bipolar transistors (HBTs) very attractive