E. I. Belyakova

Direct bonding of silicon carbide wafers with a regular relief at the interface

The direct bonding of two oxide-free 6H-SiC(0001) silicon carbide single crystal wafers, one smooth and another bearing an artificial microscopic relief, has been studied. According to the X-ray topography data, the bonded surface fraction reaches 85% of the total area. The pattern of stress distribution at the interface is aperiodic, which is indicative of an inhomogeneous microroughness of the surface of bonded wafers.

Current-voltage characteristics of isotype SiC-SiC junctions fabricated by direct wafer bonding

Results obtained in a study of current-voltage characteristics of isotype SiC-SiC structures fabricated by direct bonding of single-crystal n-type 6H-SiC wafers with a donor concentration of ∼1016 cm−3 are presented. The initial wafer bonding was done in deionized water. To enhance the adhesion, the structure was thermally annealed at 1250°C. All the features of the current-voltage characteristics measured are consistently explained in terms of the hypothesis that the SiC-SiC interface is a variable-thickness channel filled with a native SiOx oxide of thickness 10–100 nm. The minimum experimentally measured differential resistance of the structure (6 Θ cm2) is limited by the current transport in the oxide layer, which occurs by the mechanism of space-charge-limited currents.

Current-voltage characteristics of Si/Si1 − xGex heterodiodes fabricated by direct bonding

We have studied the current-voltage (I–U) characteristics of Si/Si1 − xGex (0.02 < x < 0.15) heterodiodes fabricated by direct bonding of (111)-oriented n-type single crystal silicon wafers with p-type Si1 − xGex wafers of the same orientation containing 2–15 at % Ge. An increase in the germanium concentration NGe in Si1 − xGex crystals is accompanied by a growth in the density of crystal lattice defects, which leads to a decrease in the minority carrier lifetime in the base of the heterodiode and an increase in the recombination component of the forward current and in the differential resistance (slope) of the I–U curve. However, for all samples with NGe ≤ 15 at %, the I–U curves of Si/Si1 − xGex heterodiodes are satisfactory in the entire range of current densities (1 mA/cm2–200 A/cm2). This result shows good prospects for using direct bonding technology in the fabrication of Si/Si1 − xGex heterostructures.

Structural and electrical properties of the GexSi1−x/Si heterojunctions obtained by the method of direct bonding

The results of studying the structural and electrical properties of structures produced by the method of direct bonding of GexSi1−x and Si wafers are reported. The wafers were cut from the crystals grown by the Czochralski method. Continuity of the interface and the crystal-lattice defects were studied by X-ray methods using synchrotron radiation and by scanning electron microscopy. Measurements of the forward and reverse current-voltage characteristics of the p-GexSi1−x/n-Si diodes made it possible to assess the effect of the crystallattice defects on the electrical properties of heterojunctions. Satisfactory electrical parameters suggest that the technology of direct bonding is promising for the fabrication of large-area GexSi1−x/Si heterojunctions.

Reverse recovery of Si/Si1 − xGex heterodiodes fabricated by direct bonding

We have studied the process of reverse recovery of Si/Si1 − xGex heterodiodes fabricated by direct bonding of (111)-oriented n-type single crystal silicon wafers with p-type Si1 − xGex wafers of the same orientation containing 4–8 at. % Ge. An increase in the germanium concentration NGe in p-Si1 − xGex layer is accompanied by a decrease in the reverse recovery time of heterodiodes. The presence of a sharp drop in the reverse current on the diode recovery characteristic can be explained by the existence of a narrow region with decreased minority carrier lifetime at the bonding interface (compared to carrier lifetime in the bulk), which is caused by the accumulation of misfit dislocations (generated by bonding (in this region). The results demonstrate the principal possibility of creating fast-recovery diodes based on the Si/Si1 − xGex heterosystem for high-power semiconductor devices manufactured using the direct bonding technology.

Structural and electrical properties of SiGe-on-insulator substrates fabricated by direct bonding

A new method for fabricating SiGe-on-insulator substrates, i.e., direct bonding of thermally oxidized Si wafers with Si1 − xGex wafers cut from Czochralski-grown crystals, is suggested. Si1 − xGex layers no larger than 10 μm thick in SiGe/SiO2/Si compositions were fabricated by chemical mechanical polishing. To increase the Ge content in the Si1 − xGex layer, thermal oxidation was used. It was shown that the increase in the Ge content and heat treatment procedures at 1250°C are not accompanied by degradation of structural and electrical characteristics of Si1 − xGex layers.

Regular relief on a silicon surface as a structural defect getter

An investigation was made to determine how a regular relief on the silicon surface influences gettering in silicon-silicon-dioxide structures. The regular relief was created by a photolithographic technique before oxidation and comprised an orthogonal network of overlapping bands. The gettering was determined from the isothermal relaxation of the capacitance of a silicon-silicon-dioxide structure after switching from strong inversion to even stronger inversion. It is shown that a regular relief at the silicon-silicon-dioxide interface is an effective getter at a depth of several hundred micron.

The formation of shallow-donor distribution profiles in proton irradiation of silicon

A study of the formation of shallow hydrogen-containing donors (hydrogen-related shallow thermal donors, STD(H)) in silicon under proton irradiation followed by annealing in a temperature range of 300–500°C is reported. The effect of postimplantation annealing regimes on the concentration distribution of shallow donors at various proton energies and fluences is examined. It is shown that the shape of the concentration profiles strongly varies with temperature and annealing duration when a fixed concentration of radiation defects is introduced and equally with energy and dose at a given annealing temperature. It is also shown that the process in which hydrogen-containing shallow donors are formed is accompanied by the appearance in n-type silicon of H-induced buried n′-layers, the formation of which near the pn junction in the high-resistivity n-base of diode structures allows the breakdown voltage of high-voltage pn junctions to be controlled. In the general case, this makes it possible to improve the characteristics of power silicon devices of various purposes.

Analysis of charges and surface states at the interfaces of semiconductor-insulator-semiconductor structures

A method for determining the energy spectrum of charges and surface-state densities at the interfaces of semiconductor-insulator-semiconductor structures was developed; the method is based on the analysis of capacitance-voltage characteristics. The method was experimentally tested with Si-SiO2-Si structures prepared by direct bonding of both mirror-polished smooth wafers and wafers with a regular mesoscopic relief pattern at the inner surface of the wafers to be bonded. The density of surface states is lower at the surfaces with a regular relief pattern than that at the surfaces without the surface relief.