Observation of Deep Defect Relaxation Processes in Hydrogenated Amorphous Silicon-Germanium Alloy

Modulated photocurrent (MPC) spectroscopy has been used to investigate the energy distribution of deep defect states in photo-CVD grown a-Si1 -xGex:H alloys. We observe a distinct electron trapping feature at a thermal energy of 0.55eV below Ec for higher Ge concentration (x = 0.62) alloys. For those samples with Ge concentration between 30-50 at. %, an anomalously large phase shift was observed within a temperature window between 150K to 240K. The modulated photocurrent exhibits strong quenching at the edges of this window. We suggest that these MPC spectra are controlled by deep defect relaxation processes in this temperature region, which causes electron thermal emission processes to be greatly suppressed. This effect can be reduced by increasing the reverse bias. Under such conditions, the energy position of these defect states can be roughly estimated to lie near Ec - 0.33eV. This result supports the recent result obtained from transient photocapacitance and photocurrent measurement.

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Investigations of Tail and Defect States in a-Si0.6Ge0.4:H Alloys by PDS and ESR - Implications for the Interaction of Weak and Dangling Bonds

MRS Proceedings ◽

10.1557/proc-377-571 ◽

1995 ◽

Vol 377 ◽

Author(s):

Tilo P. Drüsedau ◽

Andreas N. Panckow ◽

Bernd Schröder

Keyword(s):

Silicon Germanium ◽

Defect Density ◽

State Density ◽

Model Parameters ◽

Defect States ◽

Conversion Model ◽

Order Of Magnitude ◽

Underlying Mechanisms ◽

Amorphous Silicon Germanium ◽

Excess Absorption

ABSTRACTInvestigations on the gap state density were performed on a variety of samples of hydrogenated amorphous silicon germanium alloys (Ge fraction around 40 at%) containing different amounts of hydrogen. From subgap absorption measurements the values of the “integrated excess absorption” and the “defect absorption” were determined. Using a calibration constant, which is well established for the determination of the defect density from the integrated excess absorption of a-Si:H and a-Ge:H, it was found that the defect density is underestimated by nearly one order of magnitude. The underlying mechanisms for this discrepancy are discussed. The calibration constants for the present alloys are determined to 8.3×1016 eV−1 cnr2 and 1.7×1016 cm−2 for the excess and defect absorption, respectively. The defect density of the films was found to depend on the Urbach energy according to the law derived from Stutzmann's dangling bond - weak bond conversion model for a-Si:H. However, the model parameters - the density of states at the onset of the exponential tails N*=27×1020 eV−1 cm−3 and the position of the demarcation energy Edb-E*=0.1 eV are considerably smaller than in a-Si:H.

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Observation of Configurational Switching of Deep Defects in a-Si:H Using Thermal Step Insertion During Capacitance Transient Measurements

MRS Proceedings ◽

10.1557/proc-420-709 ◽

1996 ◽

Vol 420 ◽

Author(s):

Adam D. Gardner ◽

J. David Cohen

Keyword(s):

Thermal Emission ◽

Full Range ◽

Type A ◽

Defect States ◽

The Many ◽

Capacitance Transient ◽

Defect Relaxation ◽

Transient Measurements ◽

Temperature Switch ◽

Emission Phase

AbstractIn standard, isothermal capacitance transient measurements, configurational changes in defect states are normally very difficult to identify because, depending on the relative rates of thermal emission to the configurational relaxation, charge will be emitted predominantly from only one configuration. We have found, however, that employing a thermal step during the emission phase of the transient enhances the effect of defect relaxation; one can then observe the resultant switch between distinct configurations. We have applied this method quite successfully to lightly doped n-type a-Si:H samples by varying the overall temperature (between 270K and 350K) and the magnitude of the temperature switch (from 20K to 35K). For the smallest temperature steps, the resultant transients suggest two distinct configurations that, we believe, reflect only a fraction of the many latent configurations that account for the full range of relaxation possible.

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Exploration of Defect Relaxation Dynamics in Hydrogenated Amorphous Silicon using Temperature Switching Experiments

MRS Proceedings ◽

10.1557/proc-336-207 ◽

1994 ◽

Vol 336 ◽

Cited By ~ 1

Author(s):

Adam Gardner ◽

J. David Cohen

Keyword(s):

Density Of States ◽

Thermal Emission ◽

Hydrogenated Amorphous Silicon ◽

Relaxation Processes ◽

Relaxation Dynamics ◽

Temperature Step ◽

Thermally Activated ◽

Hydrogenated Amorphous ◽

Defect Relaxation ◽

Transient Measurements

ABSTRACTWe have carried out a series of charge transient measurements on a-Si:H in which we insert a double temperature step during the period when electrons are being emitted from deep defects. The behavior of this emitted defect charge is completely inconsistent with any density of states that remains static during the emission; that is, defect relaxation must be invoked. Such measurements allow us to separate the temperature dependence of relaxation from that of thermal emission. In particular, we demonstrate that the emission itself exhibits thermally activated behavior in spite of the ongoing relaxation processes.

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Effect of Thermal Equilibration on Dark- and Photo-Conductivities in Undoped Amorphous Silicon-Germanium Alloys

MRS Proceedings ◽

10.1557/proc-192-701 ◽

1990 ◽

Vol 192 ◽

Author(s):

J. Z. Liu ◽

D. S. Shen ◽

P. Roca i Cabarrocas ◽

H. Park ◽

S. Wagner

Keyword(s):

Silicon Germanium ◽

Lower Energy ◽

Annealing Temperature ◽

Equilibrium Configuration ◽

Dark Conductivity ◽

Defect States ◽

Optical Gap ◽

Thermal Equilibration ◽

Amorphous Silicon Germanium ◽

Subsequent Quenching

ABSTRACTWe report the effect of thermal equilibration on the dark- and photo- conductivities of an un-doped a-Si,Ge:H,F with optical gap of 1.47 eV. Annealing at high temperature and subsequent quenching can freeze in the equilibrium configuration at the annealing temperature. The characteristic glass-like transition behavior of the conductivities was observed and used to estimate a freeze-in temperature of about 140°C. As the annealing temperature increases above the freeze-in temperature, the frozen-in dark- and photo- conductivities decrease, the photo- to dark- conductivity ratio increases, and the photoconductivity-generation rate exponent increases. These changes in conductivities are explained by a model calculation, which assumes that the quenching introduces new defect states to die lower energy flanks of the Gaussian defect distributions.

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Measured and calculated distributions of deep defect states in hydrogenated amorphous silicon: Verification of deep defect relaxation dynamics

Physical Review Letters ◽

10.1103/physrevlett.71.597 ◽

1993 ◽

Vol 71 (4) ◽

pp. 597-600 ◽

Cited By ~ 34

Author(s):

Fan Zhong ◽

J. David Cohen

Keyword(s):

Amorphous Silicon ◽

Hydrogenated Amorphous Silicon ◽

Relaxation Dynamics ◽

Defect States ◽

Deep Defect ◽

Hydrogenated Amorphous ◽

Silicon Verification ◽

Defect Relaxation

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Metastable Defects in the Amorphous Silicon-Germanium Alloys

MRS Proceedings ◽

10.1557/proc-762-a2.1 ◽

2003 ◽

Vol 762 ◽

Cited By ~ 1

Author(s):

J. David Cohen

Keyword(s):

Amorphous Silicon ◽

Silicon Germanium ◽

Dangling Bonds ◽

Annealing Behavior ◽

Fundamental Properties ◽

Silicon Materials ◽

Salient Features ◽

The Individual ◽

Amorphous Silicon Germanium ◽

Silicon Germanium Alloys

AbstractThis paper first briefly reviews a few of the early studies that established some of the salient features of light-induced degradation in a-Si,Ge:H. In particular, I discuss the fact that both Si and Ge metastable dangling bonds are involved. I then review some of the recent studies carried out by members of my laboratory concerning the details of degradation in the low Ge fraction alloys utilizing the modulated photocurrent method to monitor the individual changes in the Si and Ge deep defects. By relating the metastable creation and annealing behavior of these two types of defects, new insights into the fundamental properties of metastable defects have been obtained for amorphous silicon materials in general. I will conclude with a brief discussion of the microscopic mechanisms that may be responsible.

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Fabrication of high quality, low bandgap amorphous Silicon & amorphous Silicon Germanium alloy solar cell by Chemical Annealing

10.31274/etd-180810-942 ◽

2011 ◽

Cited By ~ 1

Author(s):

Ashutosh Shyam

Keyword(s):

Solar Cell ◽

Amorphous Silicon ◽

Silicon Germanium ◽

High Quality ◽

Low Bandgap ◽

Chemical Annealing ◽

Germanium Alloy ◽

Amorphous Silicon Germanium

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Properties of Catalytic-Cvd Amorphous Silicon-Germanium (a-SiGe:H)

MRS Proceedings ◽

10.1557/proc-192-499 ◽

1990 ◽

Vol 192 ◽

Cited By ~ 1

Author(s):

Hideki Matsumura ◽

Masaaki Yamaguchi ◽

Kazuo Morigaki

Keyword(s):

Amorphous Silicon ◽

Silicon Germanium ◽

Hydrogenated Amorphous Silicon ◽

Chemical Vapor ◽

Characteristic Energy ◽

Optical Band Gaps ◽

Spin Resonance ◽

Amorphous Silicon Germanium ◽

Conductive Properties ◽

Hydrogenated Amorphous

ABSTRACTHydrogenated amorphous silicon-germanium (a-SiGe:H) films are prepared by the catalytic chemical vapor deposition (Cat-CVD) method using a SiH4, GeH4 and H4 gas mixture. Properties of the films are investigated by the photo-thermal deflection spectroscopy (PDS) and electron spin resonance (ESR) measurements, in addition to the photo-conductive and structural studies. It is found that the characteristic energy of Urbach tail, ESR spin density and other photo-conductive properties of Cat-CVD a-SiGe:H films with optical band gaps around 1.45 eV are almost equivalent to those of the device quality glow discharge hydrogenated amorphous silicon (a-Si:H).

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