Experimental Determination of Changes in Conductivity with Electric Field, Using a Stationary High-Field Domain Analysis

2015

Annalen der Physik

When the electron density decreases stronger than linearly with the electric field in photoconductive CdS due to field quenching, high-field domains must occur that remain attached to either the cathode or anode in slit electrode geometry with blocking cathodes. These Böer domains 1 are easily seen by their shift in optical absorption due to the Franz-Keldysh effect and offer unique opportunities to analyze field dependent parameters within the range of constant electron density and electric field, such as the carrier density or mobility as a function of the field, and give information of the light dependent work function. They also provide insight why a 200Å thick cover layer of CdS on top of a CdTe solar cell increases its efficiency from 8 to 16% . The behavior of these Böer domains escapes conventional current voltage analyses except for their visual observation, while other high-field domains with their current fluctuations or oscillations are easily observed and are the subjects of thousands of publications and many books. In this review we will exclude detailed discussion of dynamic domains, but include some new specifics that help to understand the mechanisms of the Böer domains and their applications. Only properties at low optical excitation intensities are discussed that exclude Joules heating. Within the p-type regime of the anode-adjacent domain extremely steep electronic quenching signal becomes visible that could signalize an intrinsic donor level slightly above the middle of the band gap that may be responsible for not allowing CdS to ever become ptype by doping.

Section: Electron Densities As Function Of the Domain Fieldmentioning

confidence: 99%

The importance of gold‐electrode‐adjacent stationary high‐field Böer domains for the photoconductivity of CdS

2015

Annalen der Physik

“…Virtual cathode produced by a shadow band across the CdS crystal, creating a high-field domain adjacent to it [13]. Figure 9 Neutrality electron density as function of the actual field [19] obtained from a cathode-adjacent high-field domain analysis, using a virtual cathode (see text).…”

Section: Figurementioning

confidence: 99%

“…Soon thereafter a large number of publications started to broaden the field with many theoretical and experimental investigations of the high-field domains [9][10][11][12][13][14][15][16], and separately of the Franz-Keldysh effect. Except for the further analysis of the stationary high-field domains by the research team of the author [17][18][19][20], the moving domains were almost exclusively analyzed by many other groups [21][22][23][24]. They are more easily observed by the kinetic behavior of the current either by periodic oscillations or by different forms of non-organized fluctuations [25].…”

mentioning

confidence: 99%

Stationary high‐field domains as tools to measure field‐dependent carrier densities and mobilities in CdS and work functions of blocking contacts

Physica Status Solidi (b)

2012

It is shown that stationary high-field domains that occur in the range of negative differential conductivity, can be used to clearly identify field-quenched states in CdS. These are distinguished as cathode and anode-adjacent domains and permit an unambiguous determination of electron density and mobility as function of the electric field. The anode-adjacent domain permits additional insight into the high-field properties of CdS in a field range that is now stabilized in the prebreakdown range. Here one finds direct evidence, by using the spectral distribution of the photoconductivity within the domain, of inverting the CdS to p-type either by more complete quenching or by hole injection from the anode. Both types of stationary domains are determined by the work function of blocking contacts and thereby permit a closer analysis of the contact/CdS interface by shifting the space charge region away from the cathode to the bulk-side end of the domain. This allows a more precise determination of the dependence of the work function on the photoconductivity of the adjacent CdS. The field-of-direction (phase portrait) analysis of the time-independent transport and Poisson equations allows a simple classification of the two types of stationary high-field domains relating to the two singular points in the decreasing branch of the current-voltage characteristic. This permits a transparent discussion of the field distribution of these domains that can be directly observed by the Franz-Keldysh effect. Herewith the transition between cathode-to anode-adjacent domains as a function of the applied voltage can be directly followed.

“…1 shows the electron density in the shadowed region S-S (see insert in Fig. 1) of the same crystal obtained from the current-voltage characteristic [9]. Curves 3 to 8 were obtained when the shadowed area was additionally illuminated with quenching light (A x 950 nm) of an intensity stepwise increased from curves 3 to 8.…”

Section: Experimental Evidencementioning

confidence: 99%

Field Quenching as Mechanism of Negative Differential Conductivity in Photoconducting CdS

Dussel

Physica Status Solidi (b)

1970

Self Cite

It is shown that the observed steep decrease of the electron density in photoconducting CdS(A1, Ag) with field in the range between 20 and 70 kV/cm is caused by a redistribution of holes from slow to fast recombination centres (field quenching). This redistribution is produced by field-enhanced ionization of holes from Coulomb-attractive slow recombination centers. The abrupt onset of the field quenching occurs because of the slow recombination traffic masking the fast center traffic until i t becomes predominant. Competing infrared quenching reduces the masking effect and uncovers the earlier phases of field quenching :&heady near 1 kV/cm (at 200 OK). Impact ionization and Zener extract,ion of holes from slow centers cannot explain the observed behavior. However, quantitative agreement between experiment and field quenching via field-enhanced ionization can be reached.Es wird gezeigt, dal3 der steile Abfall der Elektronenkonzentration in photoleitendem CdS(A1, Ag) mit dem elektrischen Feld zwischen 20 und 70 kV/cm durch eine Umverteilung \-on Liichern von langsamen zu schnellen Rekombinationszentren verursacht wird (Feldtilgnng). Diese Umverteilung wird durch ,,field-enhanced ionization" von Lochern aus Coulomb-attraktiven langsamen Zentren verursacht. Der abrupte Einsatz der Feldtilgung tritt auf, weil die lsngsame Rekombination eine Rekombination iiber schnelle Zentren vertleckt, bis diese iiberwiegt. Eine konkurrierende Infrarottilgung vermindert den Verdeckungseffekt und macht eine friihere Phase der Feldtilgung bereits in der Nahe von 1 kV/cm sichtbar. Eine Stoljionisation oder Feldemission von Lochern aus langsamen Zentren kann das experimentell beobachtete Verhaken nicht erklaren; jedoch kann eine q uantitat'ive nbereinstimmung zwischen Experiment und Feldtilgung durch ,,field-enhanced ionization" erreicht werden. I n the case of' photoconducting CdS, the mobility [8] and the conductivity [9] ns it function of the electric field have been determined and indicate a strong field quenching of majority carriers 72 as the mechanism for NDC. The low velocity of propagation of moving domains [lo] indicates that the redistribution of space charge is trap controlled. Introduction