Effect of temperature on the stimulated emission from GaAs p-n junctions

The temperature dependence of the threshold current density was examined for a series of GaAs injection lasers with different lengths but otherwise identical structure. From a plot of threshold current density as a function of reciprocal length at constant temperature, the gain and loss factor can be calculated. The laser losses exhibit only a small temperature dependence in the range from 4·2 to 300°K. The reciprocal gain factor, however, has the same temperature dependence as the threshold current density j_t. The detailed nature of the temperature dependence of j_t does depend on the length of the laser and on the doping level in the active region. The influence of these two parameters is discussed and compared with theoretical predictions.     

Pseudo two-dimensional simulation of a three heterojunction GaAs/AlGaAs MODFET

A new model is presented for the simulation of the d.c. characteristics of three heterojunctions AlGaAs/GaAs Field Effect Transistors. The dependence of the carrier densities in the GaAs wells and in the doped AlGaAs layers is derived as a function of gate voltage by analytical resolution of Schrödinger equation and numerical resolution of Poisson's equation using Fermi-Dirac statistics. The simulated d.c. characteristics are obtained by numerical integration of the current density from source to drain and include the source and drain access resistances. The comparison between experimental measurements and calculated results for both long (L_g = 20 μm) and short (L_g = 2 μm) gate lengths MODFETs is excellent and demonstrates the validity of the model to optimize the device parameters. The influence of various parameters such as the AlGaAs layer thickness and electron velocity in the inverted well on the device performance is numerically simulated for a 1 μm gate length device. Their effect on the transistor characteristics such as transconductance and maximum drain current is discussed.     

Thin films of CoSi2 on Si1−yCy substrate layers

The reaction of Co with epitaxial Si_1−yC_y(001) films is investigated with regard to dependence on annealing temperature and C concentration y. Resistance measurements and RBS analysis reveal a small increase in the disilicide formation temperature. The electrical properties are very similar for thin CoSi₂ films grown at 650°C on Si_0.999C_0.001 and on Si. Whereas the CoSi₂ is fully polycrystalline on Si(001), partially oriented CoSi₂ has been observed on C-containing substrate layers. An increase of the number of epitaxially grown CoSi₂ crystallites has been observed with increasing C concentration.     

Recursive Halfband-Filters

The paper describes the properties and the design of recursive halfband-filters.The two possibilities of being complementary are introduced. The lowpass with the transfer function H_Lp(z)and the corresponding highpass, described by H_Hp(z) = H_Lp(-z)can either be strictly complementary or power complementary. According to the respective symmetry, the impulse responses, transfer functions and frequency responses possess certain characteristic properties, which are described in section 2. It turns out that these resulting symmetries of the frequency response reduce the number of the choosable design parameters. We can only prescribe the cutoff frequency and the tolerated deviation either for the passband or the stopband.

In the third section we treat the design of halfband-filters with approximately linear phase. By coupling an appropriately designed allpass of even degree n_A with a delay of order m=n_A±1 we obtain the desired solution by solving a corresponding approximation problem for the phase of the allpass. The resulting lowpass and highpass are strictly as well as power complementary!The kind of approximation will be done in the sense of maximal flatness, where a closed form solution exists [8], or in the sense of Chebychev, where the solution is obtained iteratively [13]. The design of systems with minimum phase is presented in section 4. The resulting lowpass and highpass are power complementary. Closed form solutions yield Butterworthand Cauer filters, if a maximal flat or a Chebychev approximation is desired. In all cases a fixed relation exists between the passband frequency Ω_P and the tolerated deviation δ_P in the passband when the degree n has been chosen.     

Extreme field-induced cyclotron mass variations in coupled double quantum wells

We observe deviations in the cyclotron effective mass m_c near the partial energy gap formed in strongly coupled GaAs double quantum wells (QWs) subject to in-plane magnetic fields B. In k-space, B shifts the two QW dispersion curves relative to one another, resulting in an anticrossing and opening the energy gap. This gives rise to large B-tunable distortions in the Fermi surface and density of states. This system is thus unique in that the Fermi surface and energy position of the gap can be controlled by sweeping B. Recently, Lyo has predicted that m_c undergoes large variations as the partial energy gap is moved through the Fermi level by B. By tilting our sample by a small angle θ, we introduce a small perpendicular magnetic field B, in addition to B, and analyze the temperature dependence of the resulting Shubnikov-de Haas oscillations to obtain m_c(B). Due to the strongly distorted dispersion near the gap, m_c is suppressed by more than a factor of 3 near the upper gap edge, and enhanced by 50% near the lower gap edge, in excellent agreement with the theory of Lyo. We also observe the quantum Hall effect in a double QW at a high, constant B.     

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.     

Double-barrier resonant tunneling transport model

A semiquantum transport model for electron transport in the resonant tunneling diode (RTD) is presented. The total electrons tunneling through the RTD are partitioned into two parts. The first is the coherent tunneling electrons, which do not experience any scattering except by the barriers during tunneling. These electrons are described by the damped resonant tunneling model. The second is the incoherent tunneling electrons, which are the electrons scattered in the quantum well by the phonons, impurities, etc. The hot electron distribution, which is characterized by the effective Fermi energy μ_e and electron temperature T_e, is proposed to model the nonequilibrium distribution of the incoherent electrons in the well. The parameters μ_e and T_e can be uniquely determined by applying the energy conservation law and the particle conservation law to the incoherent electrons in the well. The incoherent electrons play a major role in the operation of the RTD. The capacitance of the RTD is investigated, based on the model and Poisson's equation. Extensive numerical results are presented     

Light-scattering determination of electron tunneling gaps in double quantum wells

Resonant inelastic light-scattering techniques have been used to measure directly the single-particle tunneling gap (Δ_SAS) in Al_xGa_1−xAs/GaAs double quantum wells. We have observed a systematic decrease in Δ_SAS with increasing height of the barrier in agreement with the Δ_SAS calculated self-consistently within a Hartree approximation.     

High-speed Si-based metal-semiconductor-metal photodetectors with an additional composition-graded i-a-Si1−xGex:H layer

The response-speed of Si-based metal-semiconductor-metal (MSM) photodetectors was improved by depositing a composition-graded intrinsic hydrogenated amorphous silicon–germanium (i-a-Si_1−xGe_x:H) layer on crystalline silicon (c-Si). In contrast to the non-composition-graded one (using intrinsic hydrogenated amorphous silicon (i-a-Si:H) layer), the full width at half maximum (FWHM) and fall time of the photodetector transient response were improved from 145.2, 404.6 to 107.6, 223.4 ps respectively. The experimental results showed that the device responsivity and quantum efficiency were increased from 0.329 (A/W) and 0.492 to 0.414 and 0.619 respectively by the employed composition-graded technique. We propose that this enhancement is due to a smoother barrier that is formed at the c-Si and i-a-Si_1−xGe_x:H interface. A lower deposition temperature of i-a-Si_1−xGe_x:H layer could be used to further reduce the fall time of the device transient response from 315.6 (250 °C) to 97.6 (180 °C) ps. To improve the contact properties between Cr electrode and i-a-Si_1−xGe_x:H layer, an annealing technique in hydrogen ambient was employed. The device knee voltage, which is the applied voltage at which the device current start to enter the saturation region in its current (log-scale) versus applied voltage characteristics, could be reduced to around 3.5 V after annealing.     

Energy levels and degeneracy ratios for chromium in silicon

The energy levels and the degeneracy ratios for chromium in silicon have been determined by the Hall coefficients which were measured by the van der Pauw method. Using the curve fitting method for carrier concentration based on the charge balance equation with the root mean square deviation, the analysis shows that chromium in silicon gives rise to two donor levels. The energy levels of the upper and lower donors are located at E_c-0.226(±0.010)eV and E_v+0.128(±0.005)eV, and their degeneracy ratios are 1/3 and 1/4, respectivel     

Co-integration of resonant tunneling and double heterojunctionbipolar transistors on InP

The authors report the first co-integration of resonant tunneling and heterojunction bipolar transistors. Both transistors are produced from a single epitaxial growth by metalorganic molecular beam epitaxy, on InP substrates. The fabrication process yields 9-μm²-emitter resonant tunneling bipolar transistors (RTBTs) operating at room temperature with peak-to-valley current ratios (PVRs) in the common-emitter transistor configuration, exceeding 70, at a resonant peak current density of 10 kA/cm², and a differential current gain at resonance of 19. The breakdown voltage of the In_0.53Ga_0.47As-InP base/collector junction, V_CBO, is 4.2 V, which is sufficient for logic function demonstrations. Co-integrated 9-μm²-emitter double heterojunction bipolar transistors (DHBTs) with low collector/emitter offset voltage, 200 mV, and DC current gain as high as 32 are also obtained. On-wafer S-parameter measurements of the current gain cutoff frequency (f_T) and the maximum frequency of oscillation (f_max) yielded f _T and f_max values of 11 and 21 GHz for the RTBT and 59 and 43 GHz for the HBT, respectively     

Magneto-optical studies of the type I/type II crossover and band offset in ZnTe/Zn1−xMnxTe superlattices in magnetic fields up to 45 T

We report measurements of magneto-reflectivity in ZnTe/Zn_1−xMn_xTe superlattices in magnetic fields up to 45 Tesla. From an analysis of the Zeeman splitting, we investigate the change of band alignment with field and the band offset ratio. A crossing of the 1 s exciton transitions from the ZnTe buffer layer and the 1 s heavy hole σ₊ exciton of the superlattice is observed, providing unambiguous evidence of a band alignment change from type I to type II. The excitonic energy levels for both type I and type II band structure are calculated using a variational method. This model fits the experimental data very well at high field for both σ₊ and σ₋ transitions. A conduction band offset ratio of ΔE_c/ΔE_s=0.72±0.04 is deduced.     

SILK compatibility with the IMD process using copper metallization

This work presents the results of SILK compatibility with the materials used in the damascene structure with copper metallization. Firstly, the thermal stability of the material was carefully evaluated; excellent stability at 450°C was confirmed. Moreover, 450°C is a good curing temperature for obtaining a low dielectric constant (2.7). The conventional PECVD hard masks, SiO₂ (from SiH₄ or TEOS precursors) and Si_xN_y do not affect the SILK properties. Finally, it was verified that an OMCVD TiN barrier is efficient in preventing copper diffusion. It was demonstrated that SILK should reach the performance requested for IMD materials in the damascene structure with Cu metallization.     

GaAs surface plasma treatments for Schottky contacts

The properties of different rectifying metallizations (Al, Ti/Pt, WN_x) on GaAs have been investigated for various surface preparation procedures. In particular, in situ hydrogen plasma treatments were used to remove residual surface contamination (mainly O and C) and a nitrogen plasma to grow a thin mixed nitride layer. Al and Ti/Pt Schottky diodes with an ideality factor very close to 1, but with reduced barrier height, were found after the H₂ plasma as a consequence of H diffusion into GaAs. The Schottky barrier height was further reduced if a H₂ + N₂ plasma was performed. The N content in the sputtering environment during the WN_x deposition affects the diode properties of plasma-treated WN_x contacts. By increasing the N₂ partial pressure, the diode barrier height is reduced, probably due to nitridization of the GaAs surface. Such differences disappear after annealing the diodes in arsine overpressure at 800°C. WN_x contacts deposited under different conditions on H₂ plasma treated substrates also show a similar Schottky barrier height after such annealing.     

Determination of the LO-phonon and Γ→L intervalley scattering time in GaAs from hot electron luminescence spectroscopy

Both the LO-phonon scattering time and the Γ→L intervalley scattering time for electrons in the conduction band of GaAs are of fundamental importance, and they are needed for the modelling of devices. We measure the steady state distribution of hot electrons in lightly p-doped bulk GaAs under carrier densities of 10¹³–10¹⁴cm⁻³, which is orders of magnitude lower than in pulsed laser experiments. Using a 16×16 k.p Hamiltonian and taking into account the transition matrix elements in a dipole model, we determine the hot electron lifetime from comparison with the experimentally found lifetime broadening. For electrons with a kinetic energy of 100meV to 300meV we obtain τ_LO=(132±10)fs. We also determine the Γ→L intervalley separation as E_ΓL=(300±10)meV. We find Γ→L scattering times around 150fs to 200fs, corresponding to a value for the associated deformation potential of D_ΓL=(9.4±1.5)×10⁸eV/cm.     

Avalanche breakdown in silicon diffused junctions

In silicon erfc or gaussian diffused junctions, as well as in linearly graded and step junctions, avalanche breakdown voltage is given approximately by V_B = (5.8 × 10⁴) X_T^0.84 where X_T is total depletion-layer thickness in cm and V_B is breakdown voltage in volts. This expression holds to ±9 per cent for plane junctions in the range 15 V to 1 kV, as indicated in Fig. 6, and should be useful to the practical device designer. The quantity X_T for a diffused junction of the erfc type can be obtained from Fig. 3, which extends the range of previously published curves and is somewhat easier to read as well. This chart and Fig. 4, which gives peak field , can be used to estimate quantitatively the departure of such a diffused junction from pure step or pure graded behavior. The generalized V_B-X_T relationship is based in part on the results of Sze and Gibbons. When their expressions for V_B in step and linearly graded junctions are recast in terms of X_T (instead of doping N_B and gradient a, respectively) these reduce to power-law expressions differing only in numerical coefficient ( 10 per cent difference). The expression's upper range, 300–1000 V, is based upon the recent diffused-junction data of van Overstraeten and de Man, and the lower range, 15–300 V, is also consistent with the experimental data of Miller on step junctions and Carlson on diffused junctions. Carlson's observations were made in about 1959 on large numbers of commercial diodes and have not previously been generally available. These sets of experimental data are compared with the calculated results of the workers mentioned above, plus the diffused-junction results of Kennedy and O'Brien.     

Influence of SiH4 on the WNx-PECVD process

Silane was added to an existing WN_x PECVD process in different flow ratios to the WF₆, to obtain higher thermal stability of the barrier in comparison to the WN_x. The deposition rate rises drastically with increased SiH₄/WF₆ ratios. The ternary compositions were investigated with regard to the sheet resistance and thickness. The X-ray diffraction (XRD) measurements of selected layers with low electrical resistivities in the as-deposited state show a broad amorphous peak like the WN_x barrier, indicating an amorphous structure. After characterising the as-deposited state of these samples, thermal treatments of the layers were performed at temperature of 600 °C for 1 h in vacuum.     

Polarity dependent gate tunneling currents in dual-gate CMOSFETs

Polarity dependence of the gate tunneling current in dual-gate CMOSFETs is studied over a gate oxide range of 2-6 nm. It is shown that, when measured in accumulation, the I_g versus V_g characteristics for the p⁺/pMOSFET are essentially identical to those for the n⁺/nMOSFET; however, when measured in inversion, the p⁺/pMOSFET exhibits much lower gate current for the same |V_g|. This polarity dependence is explained by the difference in the supply of the tunneling electrons. The carrier transport processes in p⁺/pMOSFET biased in inversion are discussed in detail. Three tunneling processes are considered: (1) valence band hole tunneling from the Si substrate; (2) valence band electron tunneling from the p⁺-polysilicon gate; and (3) conduction band electron tunneling from the p⁺-polysilicon gate. The results indicate that all three contribute to the gate tunneling current in an inverted p⁺/pMOSFET, with one of them dominating in a certain voltage range     

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.     

Formation kinetics and structure of Pd2Si films on Si

Backscattering and X-ray techniques have been used to study properties of palladium silicide (Pd₂Si) formed by evaporating thin Pd layers on Si followed by heat treatment. The rate of formation of Pd₂Si in the temperature range of 200–275°C has been measured by 2-MeV ⁴He⁺ ion backscattering. The Pd₂Si layer is found to grow at a rate proportional to the square root of time for thicknesses ranging from approximately 200–4000 Å. The rate of growth is found to be independent of Si substrate orientation or doping type and the rate constant is found to fit a single activation energy of E_a = 1·5±0·1 eV over the temperature range measured. X-ray diffraction indicates the structure to be Pd₂Si with the basal plane roughly parallel to the substrate surface for films formed on 111, 110, 100 and evaporated (amorphous) silicon substrates. The degree of preferred orientation is markedly stronger on [111] Si. Ion channeling measurements confirm that in this case the c-direction of the Pd₂Si is parallel with the [111] direction in the underlying Si.