Characterization of hydrogenated silicon thin films and its alloys by the photoconductivity frequency mixing and transient thermoelectric effects methods
Photoleitungsfrequenzmischung und zeitauflöste thermoelektrische Effects Methoden für Untersuchung die hydrogenated Silizium und dien alloys Dünnen Schichten
by Mostafa Abd El Moemen Hassan Boshta
Date of Examination:2003-11-19
Date of issue:2003-11-25
Advisor:Prof. Dr. Klaus Baerner
Referee:PD Dr. Michael Seibt
Persistent Address:
http://dx.doi.org/10.53846/goediss-2641
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Abstract
English
The charge transport properties of hydrogenated polycrystalline silicon (poly-Si:H) films prepared by the hot-wire chemical vapor deposition (HWCVD) process as a function of deposition rate have been investigated in detail. A large amount of the work was done by employing the photoconductive frequency mixing technique. In particular, strong evidence for the existence of long-range potential fluctuations in poly-Si:H films has been found from the measurements of electric field dependence of the drift mobility. It was found that the charge transport properties peak at a deposition rate of »90 Å/s. On the other hand, the depth and length of the long-range potential fluctuation plus the charged defect density have a minimum value at this deposition rate. The deterioration of the opto-electronic properties at other deposition rates may be due to increasing grain boundary effects. That makes the photomixing method a valuable too for material quality testing.The transient thermoelectric effect measurements were employed with the poly-Si:H films. From the relaxation amplitude and time of the stage 2, the trap state density difference can be calculated. It was found that the trap states density difference also has a minimum value at the deposition rate »90 Å/s. That makes the TTE also a material discriminative tool.The charge transport properties of hydrogenated amorphous silicon germanium alloys (a-SiGe:H) alloys as prepared by the hot-wire chemical vapor deposition (HWCVD) process was investigated by employing the photoconductive frequency mixing technique. Again, evidence for the presence of long-range potential fluctuations in a-SiGe:H alloys is revealed from the measurements of electric field dependence of the drift mobility. The effect of the long-range potential fluctuations is enhanced by increasing Tauc gap. That usually results in the deterioration of the opto-electronic properties. Specifically, through the drift mobility field dependence, the depth and length of the potential fluctuations as a function of Tauc gap are determined, and subsequently the charged defect density.It was also found that the photoresponse decreases monotonically with increasing Tauc gap due to the decreases in the drift mobility and lifetime as a result of an increase in the concentration of charged defects around microvoids. In particular, these charges also lead to the long-range potential fluctuations whose depth and length increases.The trap state density difference of the a-SiGe:H films is obtained from the decay profile of the transient thermoelectric effects measurements. This quantity also increases with increasing Tauc gap. The low pressure reactive ECR plasma mC-SiGe:H materials have a high quality and favorable transport properties as compared to the other techniques. Their electronic transport properties are measured by both the photomixing and transient thermoelectric effects for the first time. The photoresponse has maximum value at the deposition rate»0.8 Å/s. Probably because of the minimum value of the trap state density difference at this deposition rate.
Keywords: hydrogenated silicon; SiGe:H; poy-Si:H; photomixing; charge transport properties; transient thermoelectric effects mecthod; thin film
Other Languages
Der Ladungstransport in wasserstoffbeladenen Silizium (poly-Si:H)- Schichten wurde untersucht. Die Proben wurden durch die Abscheidung von Silan aus der Gasphase in der Umgebung eines heißen Wolfram-drahtes (HWCVD-Methode) hergestellt. Die poly-Si:H Proben wurden zunächst mit der Methode der Photoleitungsfrequenzmischung (PLFM) untersucht. Aus der Messung der Abhängigkeit der Driftbeweglichkeit des elektrischen Feldes konnte auf die Existenz von langreichweitigen Potentialfluktuationen geschlossen werden. Es wurde insbesondere ein Maximum der Beweglichkeit bei der Abscheiderate von 90 Å/s gefunden.Andererseits haben sowohl die Tiefe als auch die Länge der langreichweitigen Potentialfluktuationen ein Minimum bei dieser Abscheiderate. Die Qualitätsverlust bei den anderen Abscheideraten könnte mit einer zunehmenden Dichte von Korngrenzen zusammenhängen. Insgesamt erweist sich die PLFM-Methode als nützlich wenn man die Materialqualität der Schichten prüfen will. Die Methode der transienten thermoelektrischen Effekte (TTE) wurde ebenfalls auf die Proben angewandt. Aus der Amplitude und der Abklingkonstante des Raumladungstransienten konnte die Differenz der Elektron/Loch-Haftstellenzustandsdichten innerhalb der Bandlücke bestimmt werden. Diese Größe hat wie die Beweglichkeit ein Minimum bei der Abscheiderate von 90 Å/s.Dadurch wird auch die TTE-Methode ein Werkzeug für die Materialprüfung. Der Ladungstransport in amorphem wasserstoffbeladenem Silizium-Germanium und mikrokristallinen Silizium-Germanium Legierungen wird ebenfalls vorgestellt.
Schlagwörter: Photoleitungsfrequenzmischung; zeitauflöste thermoelektrische Effects Methoden; hydrogenated Silizium; Dünnen Schichten