Joseph H. Chen

HIGH PERFORMANCE GLOW DISCHARGE A-SI1-XGEX:H OF LARGE X

Radio frequency glow discharge chemical vapor deposition has been used to deposit thin films of a-Si1−xGex:H which possess optoelectronic properties that are greatly improved over any yet reported in the range of x⩾0.6. These films were deposited on the cathode (cathodic deposition) of an rf discharge. Their properties are assessed using a large variety of measurements and by comparison to the properties of alloys conventionally prepared on the anode (anodic deposition). Steady state photoconductivity measurements yield a quantum-efficiency-mobility-lifetime product, ημτ, of (1–3)×10−7 cm2 V−1 for 1.00⩾x⩾0.75 and (6–10)×10−8 cm2 V−1 for 0.75⩾x⩾0.50, and photocarrier grating measurements yield ambipolar diffusion lengths several times greater than previously obtained for alloys of large x. It is confirmed that the improvements in phototransport are not due to a shift in the Fermi level. In fact, results of recent measurements on lightly doped samples strongly suggest that for these cathodic alloys neither ...

Studies of defects in n‐type CdTe by charge transient spectroscopy

Charge transient measurements have been used to study deep levels in n‐type CdTe single crystals. The samples, including In‐doped, undoped, and Sn‐doped single crystals, were grown by the Bridgman or the travelling heater method (THM). The temperature dependence of the capture cross section was investigated under the partial filling conditions. The activation energies of capture cross section were determined. For In‐doped samples, the results indicate both levels Ec −0.66 eV (Bridgman samples) and Ec −0.68 eV (THM samples) are associated with the same indium‐defect complex, but levels Ec −0.34 eV (Bridgman) and Ec −0.32 eV (THM) exhibited different capture mechanisms. The unusual capture properties of the trap at Ec −0.61 eV in the undoped sample can be attributed to a doubly charged Cd interstitial. For Sn‐doped samples, four levels were observed, and levels Ec −0.89 eV and Ec −0.43 eV can be related to the presence of tin.

The mott lecture. Structural and electronic properties of amorphous SiGe:H alloys

It is universally found that the photoelectronic properties of a-Ge:H and a-Si 1-x G x :H alloys are much poorer than those of a-Si:H. For example, the efficiency-mobility-lifetime products from steady-state photoconductivity measurements and the mobility-lifetime products from time-of-flight data decrease by about a factor of 100 between a-Si:H and a 50% Ge alloy. Estimates of the gap density-of-states from spin resonance or optical absorption, however, indicate increases of only an order of magnitude, and measurements of the Urbach tail parameter show essentially no change throughout the alloy series. There would appear to be a growing consensus that these results are reconciled by a model of a rapidly decreasing electron mobility with alloying

Visible room-temperature photoluminescence from oxidized germanium

Abstract Strong visible photoluminescence (PL) has been observed at room temperature from oxidized amorphous hydrogenated germanium. Excitation by the 488 nm line from an argon ion laser yields a broad PL peak at 2.2 eV with a full-width-half-maximum (FWHM) of 0.5 eV. The luminescence intensity depends on the morphology of the untreated a-Ge:H film produced by plasma enhanced chemical vapor deposition (PECVD). Films ranging from ones with coarse columnar microstructure and poor photoelectronic properties to ones with microstructure similar to that of device-quality a-Si:H were intentionally chosen. After oxidation in air at 500°C for one hour, the films with coarse microstructure yield the highest luminescence intensity, comparable to that from porous silicon, while the best films with minimum microstructure yield orders of magnitude lower intensity. Removing the germanium oxide from the film reduces the luminescence intensity. Powder-like Ge was also produced by thermal evaporation in 0.3 Torr of argon. After annealing in air at 500°C for two hours strong PL is observed with peak position and spectral width similar to those from oxidized poor PECVD a-Ge:H and commercial GeO2.

Effects of impurities on the annealability and resistivity of oxygen-bearing copper

The effects of selenium, sulfur, lead, and silver on the conductivity and annealability of oxygen-bearing copper were studied. High purity materials were used to cast ingots containing ETP levels of oxygen. The alloys were homogenized, then equilibrated at 675°C, 735°C, 800°C, and 900°C. Resistivity ratios were measured. Then the specimens were drawn to wire and their half-hard temperatures were measured. The first few ppm of an element had a larger effect on annealability than subsequent equal additions. For less than 5 ppmw, sulfur had a larger effect, on the basis of dissolved ppmw than Se (12.4°C/ppmwvs 11°C/ppmw), but its total effect was limited by low solubility. There is evidence supporting the existence of a ternary eutectic in the Cu-O-Se and Cu-O-S systems below 900°C. Lead undergoes a reaction with oxygen between 735°C and 800°C. Silver has little effect on annealability. The resistivity measurements support the findings based on annealability, and provide new data on solubilities in the systems containing sulfur, selenium, lead and oxygen.

Improved a-Si1 − xGex:H of large x deposited by PECVD

Abstract By plasma enhanced chemical vapor deposition a-Si 1 − x Ge x :H thin films of large x have been prepared which possess optical, electrical and structural properties that are greatly improved over any yet reported. This work extends our previous work on improving the properties of a-Ge:H [W.A. Turner et al., J. Appl. Phys. 67 (1990) 7430]. Steady-state photoconductivity measurements yield an ημτ of (1 to 3) × 10 −7 cm 2 V −1 for 1.00 ≥ x ≥ 0.75 and (6 to 10) × 10 −8 cm 2 V −1 for 0.75 ≥ x ≥ 0.50. Photocarrier grating measurements yield an ambipolar diffusion length much greater than previously obtained for alloys of large x . The electronic state defect density, as determined by drive level capacitance measurements, decreases from 5.3 × 10 16 cm −3 for x = 1.00 to 6.5 × 10 15 cm −3 for x = 0.57. The Urbach parameter, E 0 , was found to be 41 ± 2 meV for a-Ge:H and 45 ± 2 meV for the alloys. Small angle X-ray scattering measurements reveal a structure that is nearly as homogeneous as device quality a-Si:H. Much of the improvement in electronic and optical properties is associated with the reduction of heterogeneities in the structure. The elimination of columnar structure is attributed to increased ion bombardment during growth and conditions which yield a high electron temperature in the discharge plasma, resulting in favorable discharge chemistry.

Examination of (crystallized a-Ge:H)/a-SiNx:H multilayers which display photoluminescence

A study has been conducted of (crystallized a-Ge :H)/a-SiN x :H multilayers which display room temperature photoluminescence in the visible range. The a-Ge :H/a-SiN x :H multilayers were prepared by glow discharge chemical vapor deposition by changing gas flows in a continuous deposition using a computer. Crystallization of the a-Ge :H layers was achieved by either thermal annealing in vacuum or exposure to a scanning laser light. The multilayer structure was confirmed by glancing angle X-ray diffraction. Crystallization was confirmed by Raman scattering. When crystallization of the a-Ge :H is achieved using the laser-scanning technique, strong visible photoluminescence with a peak centered around 625 nm, independent of the a-Ge :H layer thickness, is observed. The crystallization is accompanied by severe cracking and pitting of the film. A study of several a-Ge :H/a-SiN x :H layered structures reveals that this laser-induced crystallization only occurs when the a-Ge :H is in a state of high stress. Further study reveals that a-SiN x :H also photoluminesces with a similarly broad peak in the same wavelength range as observed from the crystallized multilayers. Using slow thermal annealing, it was possible to crystallize the a-Ge :H with minimal physical damage to the film. Photoluminescence measurements of these quantum-well structures yield a low intensity broad peak in the visible range before crystallization which does not change after crystallization. One is led to conclude that the photoluminescence observed in the laser-crystallized a-Ge :H/a-SiN x :H is from the a-SiN x :H and cannot be attributed to effects of quantum confinement.

Growth mechanism of amorphous silicon by evaporation containing hydrogen from radio‐frequency discharge

Amorphous silicon containing high concentrations of H was produced by Si evaporation and the simultaneous addition of H from rf discharge. The H content in the films is approximately 60–70% of that found in glow‐discharge produced amorphous Si containing H. The optical gap was found to be 1.7 eV. Effectiveness in the reduction of the dangling bond density has been determined by electron spin resonance. It was found that the Si–H bond was primarily formed in the gaseous phase and subsequently condenses into the solid phase. Thermal annealing and infrared spectra indicate that films produced in this manner result in improved thermal stability when compared to samples produced by glow discharge.

Electrical properties of Se1−xTex crystals

Single crystals of Se1−xTex (with x varying from 0.1 to 0.9) were successfully grown by the Bridgman method. D.C. Hall effect and temperature dependence of π of these crystals were measured. π at 300°K is found to decrease and the carrier concentration to increase approximately exponentially with x. The temperature dependence of π shows that the conduction is thermally activated. At low temperature (< 300 °K) extrinsic conduction is dominant with the value of the activation energy decreasing from 0.065 to 0.006 eV as x increases from 0.1 to 0.9. At higher temperature, intrinsic conduction is observed for alloys with x = 0.4, 0.5, 0.6 and 0.8 yielding the values of intrinsic band gaps (Eg). The values of Eg are found to decrease with x.

Ambipolar Phototransport (μτ e = μτ h ) Observed as an Intrinsic Property of a-SiGe:H

A study is presented of a series of high quality PECVD a-Si 0.33 Ge 0.67 films, produced by cathodic deposition, in which small concentrations of PH 3 , B 2 H 6 or air impurities were added during deposition. The quantum efficiency-mobility-lifetime product (ημτ) increases, and the ambipolar diffusion length (L amb ) decreases monotonically with dopant concentration for both PH 3 and B 2 H 6 . This result is strong evidence that for these films neither photocarrier is dominant (μτ e = μτ h ) at zero doping. This result is very different from what has been typically observed by other researchers, that the electron is the dominant photocarrier for undoped a-SiGe:H. Drive level capacitance (DLC) measurements of these alloys show an unusual behavior of being temperature-independent, and the dark conductivity activation energy is maximum for zero doping. It is proposed that all of these unusual properties are due to the unusually low impurity concentration of these films, and that these properties are, in fact, the intrinsic properties for a-SiGe:H alloys. To verify this, films were prepared with a calibrated and controlled air leak introduced during deposition. As the air leak was increased, the film properties changed to typical behavior. Even for air concentrations as low as 2 ppm (gas phase), the transport measurements showed changes consistent with a shift in the Fermi level toward the conduction band.