Dominant Residual Impurity Research Articles

Residual impurities in a process chamber on the characteristics of a-Si:H solar cells

Hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells were grown in a single, non-load-locked chamber by using 13.56MHz plasma. Residual air impurities were controlled with high vacuum pumping, or not controlled, and the concentrations were measured at the onset, before p-layer deposition. The low pressures (approximately 10−7–10−8Torr) of N2 and O2 near the lowest vacuum pressure of the chamber, incorporated base contamination levels of nitrogen and oxygen (approximately 1018–1019atoms/cm3). High pressures of H2O (10−5–10−6Torr) resulted in extremely high oxygen contamination levels (approximately 1021atoms/cm3), which was the dominant residual impurity. High concentration un-stabilized H2O showed a rapidly decreasing rate which induced non-uniform oxygen doping and resulted in a non-uniform distribution of the internal electric field in the i-layer. The net loss (ΔQE(0,V)) of quantum efficiency (QE) of the cells between zero and a forward bias showed the increase of QE loss in the long-wavelength region for increasing forward bias resulted in poor performance of the low fill factor and energy conversion efficiency, and a high series resistance of the cells. The removal of residual water vapor from the chamber is a key factor in improving the performance of a-Si:H solar cells.

Preparation and photoluminescence study of high purity CdTe single crystals

CdTe single crystals were grown by vertical Bridgman (VB) and physical vapor transport (PVT) methods. The photoluminescence was measured at 4.2 K to evaluate the crystals and to examine the behavior of residual impurities and the effect of Cd purity. The segregation of dominant residual impurities was confirmed in VB as-grown crystals. The purification effect of PVT was also confirmed. The dominant emission line (A0,X) at 1.5896 eV in the PL spectrum, related to CuCd or VCd, has been found to have a doublet structure shared by two origins, though a complete understanding is still lacking.

Growth and Characterization of GaAs Epitaxial Layers by MOCVD

Abstracttechnique, on both semi-insulating and semi-conducting CraAs substrates with (100) orientation, offset by 2° towards (110) direction. Systematic variation of As/Ga was performed to gain an understanding of growth process, type of formation and other related physical properties. The films were characterized by using the variety of techniques, such as SEM, EDAX, HRTEM, XRD, and PL. Optical and electrical properties of undoped CyaAs epilayers are presented with reference to the growth conditions and AsH3/TMGa ratio. Photoluminescence measurements of GaAs epilayers were recorded at 4.2K and shows the emission of free exciton and confirmed their high purity. The dominant residual impurities in GaAs are presented by using PL. Finally, electrochemical depth profiling exhibited almost homogeneous background carrier distribution and excellent abruptness between the thin GaAs epilayer and substrate.

Electrical transport analysis of ultra-pure silicon

Silicon with ever increasing purity and perfection is demanded for devices with smaller feature sizes and for specialized applications such as infrared detectors. Achievement of high purity Si has necessitated development of measurement procedures and data analysis techniques that are quantitatively sensitive to very low concentrations (<10 ppta) of electrically active impurities. Described are results from a research program that has improved all aspects of the temperature dependent Hall effect experiment while emphasizing p-type Si with very low concentrations of residual impurities. The improvements cover sample preparation and contacting, precise voltage and current measurements over a broad temperature range (4.2–400 K), sensitive temperature control, and calculations of the effective mass, m *( T), and the Hall scattering factor, r( T), that incorporate the valence bands' non-parabolicities and anisotropies. This is a quantitative technique that can be used as a standard for other analytical methods such as photoluminescence and photoabsorption. Two classes of high purity Si have been studied, namely undoped intrinsic material with boron as the dominant residual impurity at concentrations below 10 12 cm -3 (20 ppta) and material doped (generally with Group III elements) to concentrations in excess of 10 16 cm -3 in the presence of residual impurity concentrations near or below 10 12 cm -3. Results from the measurement of samples with a variety of doping and impurity concentrations are shown. Comparisons between the mobility calculations and the measured mobilities are discussed as well as direct measurements of r( T) made on intrinsic Si at low temperatures. Significant improvements in the ability to measure high purity samples have been realized and in some cases the accuracy has been increased by over 50% in the measured dopant and impurity concentration values.

Growth and characterization of high quality LPEE GaAs bulk crystals

High purity GaAs bulk crystals were grown by liquid phase electroepitaxy (LPEE). One inch diameter more than 4 mm thick ingots were free of polycrystalline edges and Ga inclusions. Typical free carrier concentration was in the 10 14 cm -3 range. Low temperature photoluminescence (PL) measurements identified the Si acceptor as the dominant residual impurity which was distributed very uniformly throughout the ingots. In addition, a correlation was established between residual impurity concentration and key growth parameters, such as temperature, baking time of the Ga-As solution, and the solution volume.

The growth and characterization of high quality MOVPE GaAs and GaAlAs

High-purity GaAs and GaAlAs epilayers have only been grown by MOVPE very recently. They are successfully used for the layers of microwave MESFETs and semiconductor lasers. This paper briefly reviews the recent progress in the area of high-quality epitaxial growth of GaAs and GaAlAs. Then, the important problems which remain unsolved are discussed. Electrical and optical properties of epilayers are presented with reference to the starting materials quality, growth conditions, and impurity doping. The dominant residual impurities in GaAs and GaAlAa are discussed. Finally the important role of deep levels on the layer quality is pointed out.

Growth and characterization of the complete Cd1-xZnxTe alloy series

The complete series of Cd1-xZnxTe alloys, from x ȣ 0 to x = 1, have been prepared by a single THM pass. The grown ingots present large pure single crystals, and a longitudinal composition gradient of about 1.25%/mm with a perfect radial homogeneity. Electrical and photoluminescence measurements have been used to characterize the crystals and show that Ag, the dominant residual impurity in the Cd-rich end of an ingot, segregates more than copper, dominant on the Zn-rich side.

High purity GaAs prepared from trimethylgallium and arsine

A study of the sources and control of residual impurities in GaAs grown by metalorganic chemical vapor deposition (MOCVD) is presented. The effects of source purity, growth temperature, and reactor pressure upon residual impurities incorporation are detailed. Far infrared photoconductivity and photoluminescence indicate that C, Si and Zn are the dominant residual impurities in undoped material. It is demonstrated that GaAs with total impurity concentrations as low as 5 × 10 14 cm -3, μ(77 K) = 125,000 cm 2/V · s, can be grown by MOCVD. The conditions for growing such material are detailed.

Effect of H2 on residual impurities in GaAs MBE layers

The introduction of hydrogen during the MBE growth of GaAs is shown to produce a major improvement in the electrical properties of the epitaxial layers. The observed increase in 77 K electron mobilities reflects a significant decrease in total ionized impurity concentration. Evidence is presented that the dominant residual impurities in MBE-grown GaAs are oxygen and carbon.

GaAs LPE growth and its application to FET

Residual impurities and deep levels in the LPE GaAs layers grown by a sliding boat method were studied. Residual impurities were investigated by monitoring oxygen and water vapor contents in the exhaust gas during heat treatment. The results are satisfactorily explained by assuming oxygen as a dominant residual impurity. Electron traps with a density higher than 5 × 10 12 cm -3 were not observed in the LPE layers, whereas in VPE layers, 0.82 eV electron traps were always observed. LPE double layers (high purity buffer layer and active layer) were fabricated into FET's. GaAs FET's with a 1 μm gate showed no hysteresis loops in the I–V characteristics and had fairly good high-frequency characteristic ( f max = 70 GHz, NF = 2.4 dB at 10.4 GHz).

Liquid Phase Epitaxial Growth of High Purity GaAs by Sliding Boat Method

High purity LPE layers are grown by the sliding boat method, varying the experimental conditions, such as the pre-heat temperature, growth temperature and the type of carrier gas. Dominant residual impurity and its purification mechanism are systematically investigated. The results of the present work (residual net donor concentration>1×1014 cm-3) can be satisfactorily explained by assuming oxygen as a dominant residual impurity which behaves as a shallow donor. The purification mechanisms are explained by both deoxidization of Ga2O3 by the hydrogen gas and the desorption of H2O and O2 adsorbed by the porous graphite. It is possible to grow high purity undoped layers reproducibly with carrier concentrations in the (1∼3)×1014 cm-3 range, taking special care not to introduce oxygen in the grown layers.

Microsegregation and the Tendency for Pitting Corrosion in High‐Purity Aluminum

Single crystals of 99.99%, 99.999%, and 99.9999% Al have been grown from the melt under conditions controlled to produce either homogeneous solute distributions, or microsegregation (on the scale ∼100μ) of the dominant residual impurities Fe, Cu, and Si. Exposure of samples to resulted in a fine‐scale pitting of segregated specimens while homogeneous specimens remained free of such pits. Various mechanical and annealing treatments did not disturb this contrast and, in general, the pitting could be quantitatively related to a previous segregation study in these metals. Polarization measurements showed that the main effect of segregation is to produce a decrease in the cathodic overvoltage which shifts the corrosion potential into a range where pitting occurs. Since the two less pure grades exhibited segregation in the same size scale, and were relatively indistinguishable, it seems that the distribution, rather than the total amount of residual impurity, is more important in producing a tendency for pitting.