Surface Recombination Measurements in HgCdTe by Optical Modulation Frequency Response
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SURFACE RECOMBINATION MEASUREMENTS IN HgCdTe BY OPTICAL MODULATION FREQUENCY RESPONSE*
J.A. MROCZKOWSKI, E. LESONDAK AND D. RESLER Honeywell Electro-Optics Division, Lexington, Massachusetts 02173 ABSTRACT The frequency response in the modulation of the excess electron concentration, produced by a modulated photogenerating pump beam, is used to determine bulk lifetime and the surface or interface recombination velocity. The depth-wise integrated excess electron concentration is contactlessly monitored by the proportional absorption/ transmission modulation of a second probe beam. Using this approach over the 20 kHz to 3 MHz frequency range recombination velocities up to 104 cm/sec have been measured in n-type epitaxial HgCdTe films. INTRODUCTION The measurement of excess carrier recombination parameters is most frequently performed by monitoring some form of electrical transient induced by the carriers such as photoconductivity. An alternate approach, which is particularly suited to narrow bandgap HgCdTe with low electron mass, is to monitor the change in the absorbtance of the material. The absorbtance changes respond immediately to excess carrier modulations as do resistivity changes in photoconductivity measurements. The absorption changes can be monitored as transmission variations in a probe beam. This paper reports the use of the Optically Modulated Absorbtion (OMA) technique, previously reported for bulk or effective lifetime measurements [ 1,21 to surface or interface recombination
measurements. RESPONSE MODEL The response model is based on the configuration shown in Figure l(a). Figures l(b), (c) illustrate the temporal behavior of the incident pump and probe beams, and Figure 1(d) shows the impact of excess carriers on the transmission of the probe beam. The OMA approach adopted in this work used a probe beam with energy slightly greater than the bandgap. Photogenerated excess electrons in the conduction band reduce the density of final states available for direct gap transitions causing the transmission just above the bandgap energy to increase significantly. Frequency Response In principle, the recombination parameters can be determined from the temporal transmission relaxation response as shown in Figure l(d). The instantaneous modulated transmission AI(t) at time t is proportional to the total number of excess electrons A n (t) the probe beam encounters in passing through the epitaxial film. Instead of sampling AI(t) at different times the transmission modulation frequency response AI(w) was meaThis work was supported under NVEOC contract DAAK-83-C-0184. The funding organization was AMMRC, Watertown, Massachusetts.
Mat. ies. Soc. Symp. Proc. Vol. 90. 1987 Materials Research Society
304
(a)
BROAD1 PUMP BEAM
(b)
HgCdTe SUBSTRATE
PUMP INTENSITY
PROBE BEAM "THANSM,,EO
iNIETPOLLAM-
TIME
-
(d)
(c)
TRANSMITTED PROBE INTENSITY
INTENSITY
TIME -
Figures l(a) - (d). Schematic Illustration of Experimental Approach sured. In this case AI(w) is proportional to the peak amplitude of the fundament
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