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

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.

Thermally-Induced Change of Conductivity and Defect Density in Amorphous Silicon-Germanium Alloys

1990 ◽  
Vol 192 ◽  
Author(s):  
Tatsuo Shimizu ◽  
Xixiang Xu ◽  
Hiroyuki Sasaki ◽  
Hui Yan ◽  
Akiharu Morimoto ◽  
...  
Keyword(s):  
Amorphous Silicon ◽  
Silicon Germanium ◽  
Defect Density ◽  
Equilibrium Temperature ◽  
Exponential Form ◽  
Thermally Induced ◽  
Neutral Defect ◽  
Fast Cooling ◽  
Amorphous Silicon Germanium ◽  
Silicon Germanium Alloys

ABSTRACTThermally-induced metastable phenomena in amorphous silicon-germanium alloys were studied by conductivity and ESR measurements. Fast cooling from 250 °C reduced both dark- and photo-conductivities by a factor of 3–4 while the neutral defect density remained unchanged. Thermally-induced change in conductivity relaxed towards equilibrium with a stretched exponential form. The thermal equilibrium temperature was found to be roughly proportional to the optical gap for a–Si:H, a–Sii−xCx:H, a–Si1−xNx:H and a–Si1−xGex:H:F.

Simulation of Hydrogenated Amorphous Silicon Germanium Alloys for Bandgap Grading

1999 ◽  
Vol 557 ◽  
Author(s):  
E. Schroten ◽  
M. Zeman ◽  
R. A. C. M. M. van Swaaij ◽  
L. L. A. Vosteen ◽  
J. W. Metselaar
Keyword(s):  
Amorphous Silicon ◽  
Material Properties ◽  
Silicon Germanium ◽  
Hydrogenated Amorphous Silicon ◽  
Model Parameters ◽  
Current Voltage ◽  
Current Voltage Characteristics ◽  
Amorphous Silicon Germanium ◽  
Parameter Values ◽  
Hydrogenated Amorphous

AbstractComputer simulations are reported of hydrogenated amorphous silicon germanium (a-SiGe:H) layers that make up the graded part of the intrinsic layer near the interfaces of a-SiGe:H solar cells. Therefore the graded part is approached with a ‘staircase’ bandgap profile, consisting of three layers within which the material properties are constant. Calibrated model parameters are obtained by matching simulation results of material properties of intrinsic a-SiGe:H single layers to measurements. Using the obtained model parameter sets subsequent simulations of p-i-n devices with intrinsic material similar to the single layers are matched to measured current-voltage characteristics. The changes in parameter values are evaluated as a function of optical gap.

Thermal Equilibration Between Band Tail and Near Surface Defect States in Hydrogenated Amorphous Silicon and Silicon-Germanium Alloys

1990 ◽  
Vol 192 ◽  
Author(s):  
Samer Aljishi ◽  
Shu Jin ◽  
Lothar Ley ◽  
Sigurd Wagner
Keyword(s):  
Valence Band ◽  
Photoelectron Spectroscopy ◽  
Silicon Germanium ◽  
Total Yield ◽  
State Density ◽  
Defect States ◽  
Band Tail ◽  
Characteristic Energy ◽  
Near Surface ◽  
Average Value

ABSTRACTWe employ total yield photoelectron spectroscopy to measure the density of occupied states at the clean a-SixGe1_x:H alloy surface. The near surface defect states are observed to lie at 0.57 eV above the valence band edge with a density of 4×l017 cm−3, independent of Ge content. The valence band tail characteristic energy is also measured to be independent of alloy composition with an average value of 54 meV. We demonstrate that thermodynamic equilibrium at the surface between weak bonds (forming the valence band tail) and the dangling bonds provides an excellent description of the experimental data and explains why the surface state density in a-Si:H cannot be lowered below the 1011 to 1012 cm−2 range.

Determination of the Trap State Density Differences in Hydrogenated Amorphous Silicon-Germanium Alloys

2005 ◽  
Vol 20 (1) ◽  
pp. 48-53
Author(s):  
M. Boshta ◽  
K. Bärner ◽  
R. Braunstein ◽  
B. Alavi ◽  
B. Nelson
Keyword(s):  
Silicon Germanium ◽  
State Density ◽  
Trap States ◽  
Trap State ◽  
Thermoelectric Effects ◽  
Time Resolved ◽  
Amorphous Silicon Germanium ◽  
First Time ◽  
Hydrogenated Amorphous

Time-resolved photo- and thermoelectric effects (TTE) were used to determine simultaneously trap levels and trap state density differences in amorphous (a-SiGe:H) samples. In particular, the trap state density differences are obtained from the decay of the ambipolar charge distribution (i.e., stage II of the TTE transients). This type of spectroscopy has been applied for the first time to a-SiGe:H samples, and indeed trap states that seem to relate to concentration fluctuations, that is, Si(Ge) and Ge(Si) clusters, are observed.

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

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.

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

1993 ◽  
Vol 297 ◽  
Author(s):  
Fan Zhong ◽  
J.David Cohen
Keyword(s):  
Silicon Germanium ◽  
Thermal Emission ◽  
Relaxation Processes ◽  
Defect States ◽  
Energy Position ◽  
Deep Defect ◽  
Large Phase ◽  
Photocurrent Measurement ◽  
Amorphous Silicon Germanium ◽  
Defect Relaxation

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.

Metastable Defects in the Amorphous Silicon-Germanium Alloys

2003 ◽  
Vol 762 ◽  
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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