Olof Engström

Interface charge control of directly bonded silicon structures

The influence of interface states and charges on the properties of Si/Si and Si/SiO2 interfaces prepared by wafer bonding, using the direct bonding technique, has been investigated. Surface potentials of Si/Si interfaces with all combinations of doping type (n‐n,p‐p,p‐n) are dependent on surface and heat treatments in the bonding procedure and on wafer dopant concentration. In earlier reported works, hydrophilic wafer surface properties have been reported as necessary for a good mechanical bonding. We find that wafer treatment in HF giving hydrophobic surfaces not only gives good mechanical properties, but also better electronic properties as well. For all combinations of doping type, lower magnitudes in surface potential were measured in samples prepared from wafers pretreated in HF in order to etch off the native oxide layer, normally present on silicon surfaces. If a native oxide is present when the bonded interface is prepared, the current through the interface will be influenced by an energy barrier ...

Thermodynamical analysis of optimal recombination centers in thyristors

Abstract The main criterion for an optimal recombination center in thyristors has been reformulated, starting from a basic thermodynamical level. From an extended grand canonical ensemble, statistical expressions for the thermal emission rates of an impurity atom are obtained in terms of changes in enthalpy, electronic and vibrational entropy. The ratio between lifetimes at high and low injection levels is evaluated for single and double level recombination centers and calculated for different enthalpy positions and entropy changes of the center and for different resistivities of the thyristor middle region. It is shown that the expected changes in entropy, when a charge carrier is emitted or captured by the center influences the optimal enthalpy position in silicon by as much as 0.1 eV. Also, a complete change in the influence of injection level on lifetime at standard resistivities and temperature for high power devices is noted. The optimization criterion is compared with experimental data pertinent to thyristor optimization. The paper demonstrates the necessity of making a profound thermodynamical analysis of the charge carrier traffic at a recombination center when using experimental data for deep impurities in the optimization of thyristor lifetime ratings.

Scanned light pulse technique for the investigation of insulator–semiconductor interfaces

A new measurement method, scanned light pulse technique, for the investigation of the interface properties of metal–insulator–semiconductor (MIS) structures is introduced. The measured signal is the charge contents of current transients induced by light pulses applied to the investigated sample. This signal depends on surface potential, interface state density and voltage applied to the MIS structure. By analyzing the results of such measurements it is quantitatively demonstrated how insulator charge and interface state distribution influence the voltage dependence. By scanning a light spot across the surface of an MIS structure with transparent metal electrode it is further shown that pictures of the spatial variation of the surface potential and the interface state density can be obtained.

Energy concepts of insulator–semiconductor interface traps

A general statistical language developed for impurities and defects in semiconductors has been applied to the interpretation of experimental results from different methods of investigations of insulator–semiconductor interface traps. Interface state energy distributions obtained from traditional capacitance–voltage methods or ‘‘Scanned Light Pulse Technique’’ are interpreted in a Gibbs free energy scale. On the other hand, distributions obtained from Deep Level Transient Spectroscopy (DLTS) are put on an enthalpy scale. The difference between the two parameters depends on the entropy properties of the trap centers. Carrier capture data taken from DLTS are demonstrated to be influenced by the ‘‘entropy factor’’ of the interface trap resulting in discrepancies of capture cross sections of up to two orders of magnitude. It is further pointed out that the energy resolution of experimental energy state distributions is fairly rough for different methods ranging up to 0.1 eV for CV technique at room temperature.

Time‐dependent positive charge generation in very thin silicon oxide dielectrics

We have studied the rate at which positive charge is generated starting near the oxide‐silicon interface when electrons are injected from the gate through the very thin oxide layer in metal‐oxide‐(p)silicon tunnel diodes. By varying the oxide thickness, we find that the charging rate is not strongly controlled by the flux of tunneling electrons over a five order of magnitude range in current density. This implies that if the tunneling electrons do participate, then the charge generation in these oxides is at least a two‐step process. A comparison of charge generation in aluminum and polycrystalline silicon gate devices suggests that the process does not involve aluminum‐related defects. Measurements of the charging rate versus temperature, T, show that it is weakly dependent on T below 150–200 K and apparently thermally activated above this temperature range.

Tunnel electron induced charge generation in very thin silicon oxide dielectrics

Positive and negative charging effects are described for small area (0.008–20 μm2), very thin dielectric (∼2.5 nm), metal‐oxide‐silicon diodes in which electrons can tunnel directly between the electrodes. These effects are similar to those seen in conventional, thicker oxide devices in which electrons are injected into the oxide conduction band. We show that at least in the thin oxides, charge generation is possible at a total electron energy level which is well below those suggested in a number of models for damage in the thicker oxides.

Interface Defects in HfO2, LaSiOx, and Gd2O3 High-k/Metal-Gate Structures on Silicon

In this work, we present experimental results examining the energy distribution of the relatively high (> 1 X 10(11) cm(-2)) electrically active interface defects which are commonly observed in high-dielectric-constant (high-k) metal-insulator-silicon systems during high-k process development. This paper extends previous studies on the Si(100)/SiOx/HfO2 system to include a comparative analysis of the density and energy distribution of interface defects for HfO2, lanthanum silicate (LaSiOx), and Gd2O3 thin films on (100) orientation silicon formed by a range of deposition techniques. The analysis of the interface defect density across the energy gap, for samples which experience no H-2/N-2 annealing following the gate stack formation, reveals a peak density (similar to 2 X 10(12) cm(-2) eV(-1) to similar to 1 X 10(13) cm(-2) eV(-1)) at 0.83-0.92 eV above the silicon valence bandedge for the HfO2, LaSiOx, and Gd2O3 thin films on Si (100). The characteristic peak in the interface state density (0.83-0.92 eV) is obtained for samples where no interface silicon oxide layer is observed from transmission electron microscopy. Analysis suggests silicon dangling bond (P-bo) centers as the common origin for the dominant interface defects for the various Si(100)/SiOx/high-k/metal gate systems. The results of forming gas (H-2/N-2) annealing over the temperature range 350-555 degrees C are presented and indicate interface state density reduction, as expected for silicon dangling bond centers. The technological relevance of the results is discussed. (c) 2007 The Electrochemical Society.

Thermal emission of electrons from selected s-shell configurations in InAs/GaAs quantum dots

The thermal emission of electrons from self-assembled InAs/GaAs quantum dots, prepared by molecular-beam epitaxy, with an average base/height size of 20 nm/11 nm in Schottky diodes has been investigated using deep level transient spectroscopy (DLTS). By applying an appropriate set of voltage pulses across the Schottky diode, the two different s-electron configurations have been investigated separately. This avoids the problem of interference between overlapping peaks in DLTS data. We find that a difference in activation energy for the thermal electron emission between the two configurations agrees with expected variation in electron energy levels due to the size distribution of the quantum dots.

The bonded unipolar silicon-silicon junction

The electrical and optical properties of wafer bonded unipolar silicon‐silicon junctions were investigated. The interfaces, both n‐n type and p‐p type, were prepared using wafers with hydrophilic surfaces. The current versus voltage characteristics, the current transients following stepwise changes in the applied bias, and the capacitance versus voltage characteristics as well as the temperature dependence of the current and capacitance were experimentally obtained and theoretically modeled. The proposed model assumes two distributions of interface states, one of acceptors and one of donors, causing a potential barrier at the bonded interface. It is argued that the origins of the interface states are impurities and crystallographic defects in the interfacial region. The capacitance of the bonded structures includes contributions from the depletion regions as well as from minority carriers. When bonded n‐n type samples were illuminated with light of photon energies larger than the silicon band gap the curr...

Discrete conductance fluctuations in silicon emitter junctions due to defect clustering and evidence for structural changes by high‐energy electron irradiation and annealing

Observations of discrete conductance fluctuations are reported at voltages well below the breakdown voltage in selected reverse‐biased p+‐n++ base‐emitter junctions originating from gate turn‐off thyristors. The occurrence of the phenomenon is attributed to the presence of defect clusters at the p‐n junctions. The defect clusters introduce field confinements which activate tunneling processes that would not otherwise be present in these nonabrupt p‐n junctions. The fluctuating reverse current was only observed in voltage and temperature regions where the total reverse current was influenced by tunneling‐related conduction mechanisms. The experimental observations concerning the voltage and temperature dependences of the fluctuation amplitude and rate deviate from earlier reports on decisive points. Both the amplitude and the switching rate of the observed fluctuations were unstable in time and influenced by the measurement procedure itself. This instability is attributed to small structural changes of the...