D. Pang

Structural, optical, and electrical characterization of improved amorphous hydrogenated germanium

High‐quality amorphous hydrogenated germanium has been deposited using the diode rf glow discharge method out of a gas plasma of GeH4 and H2. The optical, electrical, and structural properties of this material have been extensively characterized. The optical and electrical properties are all consistent with material containing a low density of defect related states in the energy gap. In particular, this material has an ημτ=3.2×10−7 cm2/V, ratio of photocurrent to dark current of 1.3×10−1, and flux dependence of the photocurrent with γ=0.79 at 1.25 eV measured using photoconductivity, a μτ=4×10−8 cm2/V measured using time of flight, an Urbach energy of 51 meV and α at 0.7 eV of 8.3 cm−1 measured using photothermal deflection spectroscopy, a dangling bond spin density of 5×1016 cm−3 measured using electron spin resonance, photoluminescence with a peak energy position of 0.81 eV and full width at half maximum of 0.19 eV, an activation energy of 0.52 eV and σ0 of 6.1×103 (Ω cm)−1 measured using dark conductiv...

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 ...

Stress and thermomechanical properties of amorphous hydrogenated germanium thin films deposited by glow discharge

Stress measurements of a-Ge:H thin films deposited by rf glow discharge using a large variety of deposition conditions are reported. It was observed that the stress of the films is strongly related to their structure. Tensile films are usually porous or have many defects, while compressive films are usually homogeneous. High quality films are always compressive. A strong correlation of the stress in the films with the unbonded hydrogen concentration was observed, which may explain the origin of the compressive stress. There was no systematic or consistent link between the stress and the bonded hydrogen content or the deposition rate. The thermal expansion coefficient and the elastic constant were determined for high quality films.

Room‐temperature 1.5 μm luminescence of co‐deposited erbium and germanium

Nanocrystalline germanium films containing erbium were deposited by thermal evaporation under 0.6 Torr of argon onto crystalline silicon wafer substrates. Weak broad photoluminescence (PL) around 1.5 μm was observed at room temperature. Annealing under 10−7 Torr of vacuum for 3 h at 500 °C produced no change in the PL spectrum. After 1 h oxidation in air at 500 °C the PL intensity increased by an order of magnitude with reduction of the spectral linewidth and appearance of distinct structures, a portion of which is similar to that observed for Er‐implanted Si:O. Subsequent increase in oxidation time reduced the PL intensity slightly with no change in the spectral shape. The PL intensity exhibits a sublinear increase with pump power and approaches saturation at 200 mW. Raman spectra before and after anneal are also presented. Annealing increased the average grain size from 5 to 10 nm. The PL spectrum of erbium metal after oxidation in air at 500 °C is quite different from that of these oxidized Ge:Er films.

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.

Stress measurement of glow discharge produced a-Ge:H thin films and its relation to electronic and structural properties

Stress measurements are reported for thin films of a-Ge:H deposited under a large variety of conditions using glow discharge CVD. The stress is nonuniform with thickness, and, for films exhibiting high tensile stress, changes reversibly due to atmospheric contamination. Strong correlations are observed for stress with hydrogen content, structure, and photoresponse. In particular, higher photoresponse is observed for films with a higher compressive stress.

Plasma deposited non-stoichiometric hydrogenated germanium sulfide a-Ge1−xSx:H (x < 0.3)

Abstract Hydrogenated amorphous germanium-sulfur alloys were prepared by PCVD from H 2 , GeH 4 and SF 6 . Addition of sulfur reduces the hydrogen concentration in the films by a factor of two and decreases the activation energy of the conductivity from 0.5 eV for a-Ge:H to 0.14 eV and is related to n-type doping. Sulfur incorporation significantly improves the quality of films prepared at the unpowered electrode: the ημτ-product increases by three orders of magnitude to 10 −6 cm 2 V −1 , the tensile stress is reduced and there is no significant post-oxidation under ambient pressure. The IR-absorption band around 360 cm −1 related to GeS bond stretching vibrations, increases proportional to sulfur concentration. Radial distribution functions show a continuous reconstruction of the Ge network with sulfur.

The effects of sulphur incorporation on the optoelectronic properties of hydrogenated amorphous germanium

Abstract Hydrogenated amorphous germanium-sulphur (a-G1 −xSx : H) alloys have been prepared by the glow-discharge technique from mixtures of hydrogen, germane and sulphur hexafluoride gases. The sulphur content of the films prepared at the powered and the unpowered electrodes varied between 2 and 30 at.%. Addition of sulphur decreases the activation energy of the conductivity from a typical value of 0.5 eV for hydrogenated amorphous germanium (a-Ge: H) to minima of 0.14 and 0.28 eV for the alloys deposited on the unpowered electrode and powered electrode respectively. Concurrent with this, for the cathode-deposited films, the ημτ product of electrons is increased by one order of magnitude to a maximum of 2 × 10−6 cm2 V −1, and the ambipolar diffusion length decreases from 470 to 350 A. From these trends, and with a consideration of various additional experimental findings, it is inferred that n-type doping occurs when sulphur is added to a-Ge: H. Sulphur incorporation improves the quality of films prepare...

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.

Femtosecond studies of photoinduced bleaching and hot carrier dynamics in a-Si:H and a-Ge:H

The femtosecond pump and probe technique with 2eV and 4eV photons was applied to the study of hot photocarriers with considerable excess energy in extended states of a-Si:H and a-Ge:H. Photoinduced changes in the complex dielectric constant e 1 + ie 2 were calculated from the measured changes in reflectivity ΔR and transmission ΔT. This technique enables us to estimate that the average carrier cooling rates in our a-Si:H and a-Ge:H samples are about 2eV/psec and 1eV/psec, respectively. Photoinduced bleaching associated with the filling of band states dominates over intraband contributions to Δe 1 and Δe 2 for photon energies significantly exceeding the bandgap.