Silicon nano-particles in SiO2 sol–gel film for nano-crystal memory device applications

A novel 2-bit nano-silicon based non-volatile memory is proposed to double memory density. The thin film structure exhibits two conduction states (ON and OFF) at different voltages and has a cost-effective structure. The structure utilizes the good electrical properties of fluorinated SiO₂ thin films, together with the bi-stable properties conferred by the nano-silicon particles therein embedded. A polymeric layer of 8-hydroxyquinoline aluminum salt (Alq3) further deposited on the top of the nano-particle layer through chemical evaporation and a silver paste contact determines the final structure. The positive 0–15 V scan reveals two discontinuities with an ON/OFF ratio of 10⁴–10⁵ (2–4 V) and OFF/ON of 10³ (12.5–13.0 V). The reverse scan displays again two distinct thresholds, range of 10.5–11.0 V (ON/OFF ratio 10⁻³), respectively, 0.5 V (OFF/ON ratio 10⁻⁵–10⁻⁴).     

Simulation and analysis of amorphous silicon image sensor having a p−i−n structure

The current-voltage characteristics of a p−i−n a-Si : H contact image sensor under dark and illuminated conditions have been simulated by solving the Poisson's equation and the continuity equations, and the results are correlated with the experiments. The dependence of the dark and photo-currents on the parameters such as the density of states in the gap, intrinsic layer width, dopant concentrations of p⁺ layer and n⁺ layer are discussed.     

Lateral tunneling transistors fabricated by plane-dependent Si-doping in nonplanar epitaxy on GaAs (311)A and (411)A substrates

We have demonstrated the lateral tunneling transistors on GaAs (311)A and (411)A patterned substrates by using the plane-dependent Si-doping technique. Lateral p⁺-n⁺ tunneling junctions are formed by growing heavily Si-doped layers on patterned substrates. Current—voltage curves for both transistors show gate-controlled negative differential resistance characteristics. Furthermore, the peak current density of the lateral tunneling diodes fabricated on the (311)A patterned substrates increases as buffer layer thickness is increased, and a typical peak current density of 58 A/cm² for p = 6 × 10¹⁹ cm⁻³ and n = 7 × 10¹⁸ cm⁻³ is obtained when the buffer layer thickness is 1.2 μm. This study shows that plane-dependent Si-doping in non-planar epitaxy is a promising technique for fabricating tunneling transistors.     

Reproducibility of growing AlGaN/GaN high-electron-mobility-transistor heterostructures by molecular-beam epitaxy

High-quality GaN/AlGaN high-electron-mobility transistors (HEMT) characterized by room temperature mobilities of 1000 cm² V⁻¹ s⁻¹ and sheet electron densities in the range of 3×10¹²–2×10¹³ cm⁻² have been grown by reactive molecular-beam epitaxy on insulating C-doped GaN template layers. Growth data and mobility values resulting from over 50 HEMT growth experiments on 2 in. diameter sapphire wafers are presented to show the remarkable overall high yield and reproducibility of the HEMT structures grown by this method. The use of insulating C-doped GaN buffer layers has greatly increased reproducibility of the device structures by ensuring device isolation through controlled carbon doping. Moreover, an undoped GaN channel layer of remarkably low defect density and high mobility can be grown on the C-doped GaN template with high reproducibility. Precise control of the growth temperature was key to achieving the high quality and reproducibility of the structures.     

n-Channel AlSb/GaSb modulation-doped field-effect transistors

We report the first fabrication of a GaSb n-channel modulation-doped field-effect transistor (MODFET) grown by molecular beam epitaxy. The modulation-doped structure exhibits a room temperature Hall mobility of 3140 cm² V⁻¹ s⁻¹ and 77 K value of 16000 cm² V⁻¹ s⁻¹, with corresponding sheet carrier densities of 1.3 × 10¹² cm⁻² and 1.2 × 10¹² cm⁻². Devices with 1 μm gate length yield transconductances of 180 mS mm⁻¹ and output of 5 mS mm⁻¹ at 85 K. The device characteristics indicate that electron transport in the channel occurs primarily via the L-valley of GaSb above 85 K. The effective electron saturation velocity is estimated to be 0.9 × 10⁷ cm s⁻¹. Calculations show that a complementary circuit consisting of GaSb n- and p-channel MODFETs can provide at least two times improvement in performance over AlGaAs/GaAs complementary circuits.     

Measurement of low densities of surface states at the Si---SiO2-interface

By a careful process Si---SiO₂-interfaces can be made with a low oxide charge Q_ox and with a low surface states density N_ss. For dry oxides on (100) N_ss-values as low as a few 10⁹ cm⁻² eV⁻¹ are found on samples with an oxide charge density of 3·0 × 10¹⁰ cm⁻². Only the Nicollian-Goetzberger conductance method is proved to give reasonable results on these structures. The quasi-static low frequency C-V-technique is in good agreement with the conductance technique for samples with N_ss-values higher than 1·0 × 10¹⁰ cm⁻² eV⁻¹. The spatial fluctuation of surface potential, mainly caused by oxide charge fluctuations, is an important parameter when studying the high or low frequency C-V-characteristics. Some irregularities in the experimental N_ss-φ_s-curves are explained.     

Process and device characterization of high voltage gallium arsenide P-i-N layers grown by an improved liquid phase epitaxy method

GaAs P-i-N layers with an i-region net doping of less than 10¹² cm⁻³ were grown on P⁺ and N⁺ substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P⁺-P-i-N-N⁺ diode with a 25 μm-wide i-region exhibits a breakdown voltage of 1000 V, a t_rr of 50 ns, and reverse current densities (at V_R = 800 V) of − 3 × 10⁻⁶ A/cm² at 25°C and 10⁻² A/cm² at 260° C.     

Evolution of GaAs quantum well excitons with excess electron density and magnetic field

We present an experimental study of excitonic optical transitions in GaAs quantum wells (QWs) as a function of their excess electron density and an applied magnetic field. There is a dramatic weakening of the neutral excitonic transitions (X) upon adding 10¹⁰ cm⁻² excess electrons to the QW, accompanied by strengthening of the transition due to the negatively-charged exciton (X⁻) to lower energy. Increasing the density further causes X to be completely quenched from the spectra, while X⁻ evolves smoothly into the Fermi-edge singularity. A qualitatively different evolution with an applied magnetic field is observed for X and X⁻, the former showing similar behaviour to that in undoped QWs. Assignment of the X⁻ transition is confirmed by its partial circular light polarization in excitation spectra taken at field, caused by the higher population of excess electrons in the lower energy spin state at low temperature. There is a large enhancement of the second electron binding energy with magnetic field, which stabilizes excited (spin-triplet) X⁻ states.     

Estimation for the capture cross-sections of surface state on p-Si surface by photovoltaic method

A new method which can nondestructively measure the surface-state density (SSD) D_s and estimate the capture cross-sections (CCS) of surface state σ⁰_n and σ⁻_p on surface of p-type semiconductor crystals is proposed. This method is based on the photovoltage measurements at various temperatures. The photovoltage experiment was carried out with a (1 1 1) p-type Si single crystal (N_A=4.8×10¹⁴ cm ⁻³). Owing to that the surface barrier height φ_BP=0.6421 V and the surface-recombination velocity s_n=9.6×10³ cm s⁻¹ of this sample can be determined, the SSD D_s=1.2×10¹¹ cm⁻² eV⁻¹ can therefore be obtained, furthermore CCS σ⁰_n≈5×10⁻¹⁴ cm² and σ⁻_p≈2×10⁻¹⁰ cm² can also be estimated. These results are consistent with that of related reports obtained by other methods.     

A study of noise in surface and buried channel SiGe MOSFETs with gate oxide grown by low temperature plasma anodization

A study is made of noise in p- and n-channel transistors incorporating SiGe surface and buried channels, over the frequency range f=1 Hz–100 kHz. The gate oxide is grown by low temperature plasma oxidation. Surface n-channel devices are found to exhibit two noise components namely 1/f and generation–recombination (GR) noise. It is shown that the 1/f noise component is due to fluctuations of charge in slow oxide traps whilst bulk centers located in a thin layer of the semiconductor close to the channel, give rise to the GR noise component. The analysis of the noise data gives values for the density D_ot of the oxide traps in the SiGe and Si nMOSFETs of the order 1.8×10¹² and 2.5×10¹⁰ cm⁻² (eV)⁻¹, respectively. The density D_GR of the bulk GR centres is equal to 3×10¹⁰ cm⁻² in both the SiGe and Si devices. The electron and hole capture cross-sections for these centres as well as their energy level and their depth below the oxide/semiconductor interface are also the same in the devices of both types. This suggests that those GR centers are of the same nature in all devices studied. p-Channel devices show different behaviour with only a 1/f noise component apparent in the data over the same frequency range. Buried SiGe channel and Si control devices exhibit quite low and similar slow state densities of the order low to mid 10¹⁰ cm⁻² (eV)⁻¹ whereas surface p-channel devices show even higher slow state densities than n-channel counterparts. The Hooge noise characterized by the Hooge coefficient _H=2×10⁻⁵ is also detected in some buried p-channel SiGe devices.     

Fabrication of ZnO-based metal–insulator–semiconductor diodes by ion implantation

A ZnO-based metal–insulator–semiconductor junction has been fabricated using an isolation layer fabricated by N⁺ ion implantation. I–V dependences show a good rectifying diode-like behavior with a low leakage current of 10⁻⁶ A and a threshold voltage of about 3 V. Ultraviolet light emission under forward bias exhibits a wavelength maximum of 388 nm and a full width at half maximum of 128 meV at room temperature.     

The physics and fabrication of in situ back-gated (311)A hole gas heterojunctions

This paper reports the fabrication of an in situ back-gated hole gas on the (311)A surface of GaAs. The hole density can be varied from fully depleted to p_s = 2.1 × 10¹¹ cm⁻² with mobilities of up to μ = 1.1 × 10⁶ cm²V⁻¹ s⁻¹. It is seen that for carrier densities down to p_s = 4 × 10¹⁰ cm⁻² the mobility in the [ ] direction is greater than that in the [ ] direction. Using a combination of front- and back-gates we are able to keep the carrier density constant and deform the hole gas wavefunction such that the holes are pushed up against or moved further away from the heterointerface. Thus we are able to separately investigate the various scattering mechanisms that determine the mobility, and compare the experimental data with theoretical calculations based on the shape of the wavefunction.     

Selenium doped high-index GaAs epilayers grown by molecular beam epitaxy

Using elementary Se we grew Se-doped GaAs films on GaAs (111), (411), (711) and (100) substrates by molecular beam epitaxy. The films grown on all the high-index substrates showed n-type conduction and the maximum carrier concentration reached 2.1 × 10¹⁹ cm⁻³ for the film grown on the (411)B substrate. The carrier concentration began to saturate at a Se concentration near 10¹⁹ cm⁻³ but continued to increase up to a Se concentration of 2 × 10²⁰ cm⁻³. Above 2 × 10²⁰ cm⁻³ Se concentration, slow reduction of the carrier concentration was observed. We obtained excellent surface morphology when n-type GaAs films were grown on (411)A and (711)B substrates even at a Se concentration of 7 × 10²⁰ cm⁻³.     

Carrier recombination in single crystal PbSe

The photoconductivity decay curves after illumination of single crystal n- and p-type PbSe were analysed assuming recombination through different localized impurity levels in conjunction with direct recombination. The lifetimes deduced for direct (Auger and radiative) recombination below 250 K were in agreement with the calculated values for carrier concentrations 2·10¹⁷ cm⁻³. Furthermore, the existence of up to three impurity levels was concluded from the longer lifetime-components present in the decay curves. Appropriate approximations of the general recombination theory yielded energies separated between 20 and 50 meV from the nearer band edge and minority carrier cross sections 10⁻¹⁷−4·10⁻¹⁹ cm² in the temperature range 250-100 K, and majority carrier cross sections 10⁻¹⁹−10⁻²⁰ cm² at T < 100 K for these levels.     

Optical and electrical characterization of GaN layers grown on silicon and sapphire substrates

Characterization of the structural, optical and electrical properties of GaN layers grown by two epitaxial techniques (ECR-MBE and MOCVD) using different substrates (vicinal Si111 and sapphire) has been performed. The quality of the samples grown by MOCVD seems to be influenced by the nitrogen source used for the growth. Unintentionally doped MBE samples with n-type concentrations around 10¹⁸ cm⁻³ and Hall mobility of 15 cm² (V s)⁻¹ were studied. GaN films doped with Mg and grown using AlN buffer layers have also been analyzed to study the influence of the thickness of the buffer layer on the optical properties of the GaN epilayer. In the samples with low Mg doping, a thin AlN buffer layer improved the optical quality of the film. In general, all the MBE samples doped with Mg were highly resistive, probably due to a low activation or high ionization energy of the Mg acceptors. Technological issues related to the formation of ohmic contacts on GaN layers are also presented.     

Metal-germanium Schottky barriers

A systematic study has been made of the electrical characteristics of Schottky barriers fabricated by evaporating various metal films on n-type chemically cleaned germanium substrates. The diodes, with the exception of Al---Ge contacts, exhibit near-ideal electrical characteristics and age only slightly towards lower barrier height values. Al---Ge contacts exhibit very pronounced ageing towards higher barrier height values, due to formation of an extra aluminium oxide interfacial layer. Because of this, the barrier height values of aged Al---Ge contacts derived from I-V and C-V characteristics differ significantly. The dependence of the barrier height, (φ_b) on the metal work function, φ_m, for different metal-germanium contacts shows that surface states play an important role in the formation of the barrier. The density of germanium surface states is estimated to be D_s = 2 × 10¹³ eV⁻¹ cm⁻².     

Optoelectronic properties of Zn0.52Se0.48/Si Schottky diodes

Zn_0.52Se_0.48/Si Schottky diodes are fabricated by depositing zinc selenide (Zn_0.52Se_0.48) thin films onto Si(1 0 0) substrates by vacuum evaporation technique. Rutherford backscattering spectrometry (RBS) analysis shows that the deposited films are nearly stoichiometric in nature. X-ray diffractogram of the films reveals the preferential orientation of the films along (1 1 1) direction. Structural parameters such as crystallite size (D), dislocation density (δ), strain (ε), and the lattice parameter are calculated as 29.13 nm, 1.187 × 10⁻¹⁵ lin/m², 1.354 × 10⁻³ lin⁻² m⁻⁴ and 5.676 × 10⁻¹⁰ m respectively. From the I–V measurements on the Zn_0.52Se_0.48/p-Si Schottky diodes, ideality and diode rectification factors are evaluated, as 1.749 (305 K) and 1.04 × 10⁴ (305 K) respectively. The built-in potential, effective carrier concentration (N_A) and barrier height were also evaluated from C–V measurement, which are found to be 1.02 V, 5.907 × 10¹⁵ cm⁻³ and 1.359 eV respectively.     

Room temperature electroless plating copper seed layer process for damascene interlevel metal structures

Described in this paper is the development of a room temperature electroless copper seed layer deposition process on ultra-thin TiN barrier layers. This novel process is compatible with damascene interlevel metal structures for sub-0.18 micron ULSI processes. An optimum copper layer thickness of 50 nm and a deposition rate of 45 nm min⁻¹ was targeted and obtained. Atomic force microscopy (AFM) reveals that the non-uniformity of the seed layer is less than 10% of the film thickness, while four-point probe measurements indicate that the resistivity of the copper seed layer is less than 6 μΩ cm⁻¹. Secondary ion mass spectroscopy (SIMS) reveals that potential metallic contaminants such as sodium, potassium, calcium and magnesium ions do not penetrate the TiN barrier layer. Rutherford back scattering (RBS) indicates that the palladium concentration in the seed layer is approximately 1%, which is low enough to avoid wafer contamination and increased resistivity in the subsequent electroplated copper layer.     

Interface characteristics of Ge3N4-(n-type) GaAs MIS devices

Passivation of GaAs surfaces was achieved by the deposition of Ge₃N₄ dielectric films at low temperatures. Electrical characteristics of MIS devices were measured to determine the interface parameters. From C-V-f and G-V-f measurements, density of interface states has been obtained as (4–6)×10¹¹ cm⁻² eV⁻¹ at the semiconductor mid-gap. Some inversion charge buildup was seen in the C-V plot although the strong inversion regime is absent. Thermally stimulated current measurements indicate a trap density of 5×10¹⁸−10¹⁹ cm⁻³ in the dielectric film, with their energy level at 0.59 eV.     

Avalanche breakdown voltage of GaAs hyperabrupt junctions

The avalanche breakdown voltage of a GaAs hyperabrupt junction diode is calculated by using unequal ionization rates for electrons and holes, and shown graphically as a function of the parameters which characterize the impurity profile of the diode. The breakdown voltage decreases abruptly at the critical point of the characteristic length L_c which varies in accordance with the impurity concentration N₀ at X = 0. For example, the critical length L_c is 7.7 × 10⁻⁶ cm and 3.3 × 10⁻⁵ cm for N₀ = 1 × 10¹⁸ cm⁻³ and 1 × 10¹⁷ cm⁻³, respectively. The breakdown voltage of a diode with extremely short or long characteristic length can be estimated from the results for corresponding abrupt junctions. The experimental results agree well with the calculated ones.