Nonpolarizing radiation detectors based on wide-gap semiconductor crystals

Wide gap, insulating semiconductor crystals offer great promise in photoelectric devices, especially as detectors of electromagnetic and nuclear radiation, but are not widely used because they become polarized during operation. During operation of detectors using these crystals with a high concentration of deep impurity levels, electrical charges build up and produce a change, over time, in the electric field within the crystal and in the magnitude of the detector photoresponse. Since it is impossible to avoid impurity centers in these crystals at this time, we propose new approaches to creating radiation detectors-dosimeters which do not become polarized over time, but rely on productive use of the polarization charges that accumulate in them. Fiz. Tekh. Poluprovodn. 33, 1475–1478 (December 1999)     

Pyroelectric properties of the wide-gap CdSe semiconductor in the low-temperature region

In the temperature range T = 4.2–300 K, the spontaneous polarization of nonstoichiometric CdSe samples grown by sublimation at 1423 K and subjected to annealing in selenium vapors is measured. From these data, the temperature dependences of the pyroelectric coefficient in CdSe are calculated. Anomalies are found in the ranges T < 10 K and T > 210 K. A change in the charge state of the defect subsystem of the samples caused by a weak electric field results in strengthening of the anomalies and in the occurrence of a new anomaly at T = 236 K. As a rule, the anomalies at T < 10 K and T = 236 K depend on the polarity of the external action. The action of shunting by the intrinsic conductivity of the sample manifests itself only in the region above 270 K. Investigations of the nonpolar-cut sample leads to somewhat unexpected results: the anomalies are retained, but their values considerably decrease. Within the framework of the crystallophysical approach, attempts are made to explain the anomalies observed below 270 K related both to associates with a dipole moment and to their transformation with the possible participation of an uncontrollable oxygen impurity in macroformations of the cluster type, the orientation of the total dipole moment of which does not coincide with the polar direction of an ideal crystal.     

Magnetic and photomagnetic properties of polycrystalline wide-gap semiconductor Cd1-xMnxTe thin films

The magnetic and photomagnetic properties of polycrystalline Cd_1-xMn_xTe thin films prepared by radio frequency (rf) sputtering have been investigated. Magnetization measurements were carried out using a rf superconducting quantum interference device magnetometer in the temperature range of 1.8∼300K at various magnetic fields up to 5.5 T. For temperatures above 40K, Curie-Weiss behavior was observed with a negative Weiss constant indicating predominantly antiferromagnetic interactions. Spin-glass transitions occurred at low temperatures. Photoinduced magnetization has been observed when the sample was illuminated by unpolarized light from an optical fiber.     

Silicon epitaxial layer recombination and generation lifetime characterization

We have made recombination and generation lifetime measurements on silicon p-epitaxial layers on p/sup +/ and on p-substrates. The recombination lifetimes are dominated by surface/interface recombination for layers only a few microns thick. By coupling measurements of p/p with those of p/p/sup +/ samples, it is possible to extract the epi-layer lifetime. For p/p/sup +/ samples, recombination lifetimes are poorly suited to characterize epi-layers. Generation lifetime measurements are eminently suitable for epi-layer characterization, since carrier generation occurs in the space-charge region confined to the epitaxial layer, and when coupled with corona charge/Kelvin probe, allow contact-less measurements.     

Si-gate CMOS devices on a Si lateral solid-phase epitaxial layer

Si-gate CMOS devices fabricated on a lateral solid-phase epitaxial Si layer grown from vacuum-deposited amorphous Si over SiO₂ patterns are discussed. Electrical characteristics are examined and correlated with microstructural characteristics of the layer by performing transmission electron microscopy on actual transistors. The layer can be divided into three regions. Carrier mobilities obtained from each region are discussed in terms of the crystalline quality. The maximum obtained field-effect mobilities are 570 cm²/V-s and 160 cm²/V-s for n-channel and p-channel transistors, respectively. The SMOS inverter chain with 100 stages and a channel length of 1.5 μm has a delay time of 310 ps per gate. These results indicate that the lateral solid-phase epitaxy has potential for the fabrication of high-speed silicon-on-insulator devices     

Raman scattering study of a GaAsN epitaxial layer

Raman scattering study of a dilute GaAsN epitaxy layer was carried out at variable temperature and pressure. The localization due to the presence of the N atoms is responsible for the small correlation length in the GaAsN alloy, which is also evident from the broadening and asymmetry of the LO mode of the GaAs-like Raman band. The temperature dependence of the correlation length was analyzed. Nitrogen-induced localization also has a strong influence on the pressure dependence of the Born's effective dynamic charge e*.     

Low and small resistance hole-injection barrier for NPB realized by wide-gap p-type degenerate semiconductor,LaCuOSe:Mg

LaCuOSe:Mg is a wide-gap p-type semiconductor with a high conductivity and a large work function. Potential of LaCuOSe:Mg as a transparent hole-injection electrode of organic light-emitting diodes (OLEDs) was examined by employing N,N′-diphenyl-N,N′-bis (1,1′-biphenyl)-4,4′-diamine (NPB) for a hole transport layer. Photoemission spectroscopy revealed that an oxygen plasma treated surface of LaCuOSe:Mg formed a hole-injection barrier as low as 0.3 eV, which is approximately a half of a conventional ITO/NPB interface. Hole-only devices composed of a LaCuOSe:Mg/NPB/Al structure showed a low threshold voltage ∼0.2 V and high-density current drivability of 250 mA cm⁻² at 2 V, which is larger by two orders of magnitude than that of ITO/NPB/Al devices. These results demonstrate that LaCuOSe:Mg has great potential as an efficient transparent anode for OLEDs and other organic electronic devices.     

Evaluation of the nanoindentation behaviors of SiGe epitaxial layer on Si substrate

In this paper, ultra-high vacuum chemical vapor deposition (UHV/CVD) was employed to synthesize silicon-germanium (SiGe), and sequence to endure annealing treatment. Morphological characterization, roughness, and microstructural morphology were observed by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The elements distribution, crystallographic, and nanomechanical behavior were carried out using energy-dispersive X-ray spectroscopy (EDS) mapping technique, X-ray diffraction (XRD), and nanoindentation technique.The annealing treated SiGe leads to the 2D germanium segregation on the surface. The phenomenon is interpreted in terms of dislocation-induced structural changes in annealing treatment. Thus, the dislocation propagation in the microstructure was observed. Subsequently hardness and elastic modulus were increased because of a comparatively unstable microstructure after annealing treatment.     

Investigation of the degradation of smooth SiGe epitaxial layer on Si substrate

The degradation of smooth SiGe epitaxial layer was investigated by transmission electron microscopy (TEM), X-ray reflectivity (XRR) and atomic force microscopy (AFM). It was shown from AFM results that the crosshatch was formed with increasing annealing temperature, which indicated the degradation of smooth surface. The surface degradation was caused by the internal dislocations, which were observed by plan-view TEM (PTEM) and cross-sectional TEM (XTEM). From XTEM, the sharp interface between SiGe top layer and Si substrate was broadened and there were a lot of 60° dislocations formed in SiGe top layer, which resulted in the crosshatch on the surface. The crosshatch was also verified by PTEM.     

Effects of layer thickness variations on vertical-cavitysurface-emitting DBR semiconductor lasers

A theoretical analysis of the influence of layer thickness variation in vertical-cavity surface-emitting lasers with distributed Bragg reflectors (DBRs) on lasing wavelength is presented. It is shown that changing the active region length of one of the layers in the DBR mirror by only one unit cell (0.56 nm) is sufficient to produce shifts in the lasing wavelength up to 0.12 nm (for an AlGaAs laser). This could limit the precision with which a desired wavelength, its reproducibility, and its uniformity across a large wafer can be obtained. Possible influences on the linewidth of broad area devices are also discussed     

Influence of lattice polarity of nitrogen and aluminum doping on 4H-SiC epitaxial layer

We have reinvestigated the characteristics of N and Al doping on 4H-SiC epitaxial layer. It was found that thermally activated process and desorption process are aspects of the mechanism of the N incorporation to which attention should be paid. The process strongly depends on the lattice polarity and C/Si ratio. Thermally activated process dominates the N incorporation of the Si-face under the high C/Si ratio. Under the low C/Si condition, the desorption process dominates the N incorporation of the Si-face. For C-face, the desorption process dominates the N incorporation regardless of the lattice polarity and the C/Si ratio. Concerning Al doping, the desorption process dominates the Al incorporation without reference to the lattice polarity and the C/Si ratio.     

Thermodynamic stability of bulk and epitaxial Ge1−x Snx semiconductor alloys

The total energy for an unconstrained and biaxially confined model of Ge_1?x Sn_x alloys was calculated using molecular-dynamics simulation. This made it possible to study the thermodynamic stability of both disordered and ordered phases of the semiconductor alloys. A remarkable suppression of the phase separation in Ge_1?x Sn_x due to biaxial strain was found.     

Effects of collector epitaxial layer on the switching speed of high-performance bipolar transistors

The importance of the epitaxial layer in designing the scaled-down high-performance bipolar transistors is well accepted, but its effect on the performance of the conventional switching transistors for logic application is not quite observable, because it is usually overwhelmed by the large parasitics of these devices. For self-aligned transistors having very small parasitics, its effect becomes so significant that it is observable in the power-delay characteristics of the switching circuits. We built self-aligned transistors on epitaxial layers of different thicknesses. The measured switching speed of transistors with thicker epitaxial-collector layers is faster at lower currents due to the lower collector capacitance; but it is slower at higher currents due to the base stretching. The present experimental results provide direct confirmation of the design theory [1].     

Electric-field profile and current control of a long epitaxial GaAs n layer

Control of the cathode current is used to achieve an electric-field bias above the negative-resistance threshold field on an extended length of GaAs epitaxial layer grown on a semi-insulating substrate. The cathode current is controlled with a Schottky-barrier contact strip parallel to the cathode.     

一种测量外延层厚度及掺杂浓度的改进方法

在双台面SiGeHBT加工工艺过程中,采用RIE工艺刻蚀发射极台面时,为了避免等离子轰击对外基区表面造成损伤,同时为了防止过刻到基区,必须严格控制发射极台面的高度,从而必须准确知道未刻蚀前的厚度和刻蚀后的厚度。现有的许多对材料厚度及掺杂浓度的分析方法,具有各自的优缺点。本文提出了一种可以同时检测外延层的厚度及掺杂浓度分布的方法,这种方法具有简单、高效、低成本的优点。     

Optical properties of bulk and epitaxial unordered GaxIn1−x P semiconductor alloys

The electron band structure of Ga_xIn_1?x P bulk solid solutions was calculated by the local model pseudopotential method taking into account lattice mismatch. The resulting local strain of the lattice was taken into account in calculations of effective mass and deformation potential. The main optical characteristics of Ga_xIn_1?x P alloys can be explained by the presence of internal local strains and antisite defects. In comparison with bulk samples, the concentration dependences of the spectral peaks E ₁ and E ₁+Δ₁ in pseudomorphic thin films were found to be more sensitive than the fundamental absorption edge E ₀ to deformations caused by the substrate.     

Measuring and calculating the Hall mobility of semiconductor epitaxial layers by a contactless method

A new probe was made from microwave dielectric waveguides. Using this probe, we can measure the magnetoresistance of Al_xGa_1−xAs, Hg_1−xCd_xTe and other epitaxial layers easily. It is a contactless and non-destructive method. During the test, we do not need to specially treat the surface and the shape of the samples. The measuring area is about 4 × 4 mm². This article also presents a method of calculating the mobility of these samples from the magnetoresistance data for different semiconductor materials.     

Determination of the matrix element of the quasi-momentum operator in the zero-gap semiconductor HgSe by the field-effect method in electrolyte

The field-effect method in electrolyte was used to study the zero-gap semiconductor HgSe-electrolyte (saturated solution of KCl) system by measuring the capacitance-voltage and current-voltage characteristics. A technique for the estimation of the matrix element P of the quasi-momentum operator from capacitance-voltage characteristics was proposed, and the value of P for HgSe was determined at T=295 K.     

Photoelasticity and quadratic permittivity of wide-gap semiconductors

Expressions for the photoelastic constants are obtained on the basis of an analogy between the photoelastic and elastic properties of semiconductor crystals. Calculations were performed for wide-gap semiconductors (silicon carbide and boron, aluminum, and gallium nitrides) with sphalerite and wurtzite structures. The quadratic permittivity is also calculated for hexagonal structures. Fiz. Tekh. Poluprovodn. 31, 823–824 (July 1997)     

On the specific features of the density of states of epitaxial graphene formed on metal and semiconductor substrates

Analytical expressions for the local densities of states of epitaxial graphene formed on metal and semiconductor substrates are derived on unified grounds. The conditions of strong and weak graphene-substrate coupling are considered. It is shown that, in the case of strong coupling (the interaction of carbon atoms of graphene with the substrate is much stronger than that of carbon atoms with each other), the local density of states of graphene is close to the density of states of an individual carbon adatom on both metal and semiconductor substrates. In the opposite case of weak graphene-(semiconductor substrate) coupling (the interaction of carbon atoms of graphene with the substrate is much weaker than that of carbon atoms with each other), there is no gap in the local density of states of graphene, and the Dirac point is in the band gap of the semiconductor substrate and coincides in energy with the local level of the separated (individual) carbon adatom. Graphene formed on a metal substrate also exhibits a zero-gap density of states. The problem of the band gap induced in graphene by a semiconductor substrate is considered in the general case. It is shown that, depending on the relation between the parameters of the problem, either one or two band gaps overlapping in energy with the band gap of the substrate can exist in the spectrum of graphene. The dependence of the band gaps on the strength of the graphene-substrate interaction is constructed. Numerical estimations are performed for epitaxial graphene formed on 6H-SiC {0001} faces.