MOCVD-GROWN WIDER-BANDGAP CAPPING LAYERS IN HG1-XCDXTE LONG-WAVELENGTH INFRARED PHOTOCONDUCTORS

The use of MOCVD-grown wider-bandgap Hg1-xCdxTe as a capping layer for long-wavelength infrared (LWIR) Hg1-xCdxTe photoconductors has been s tudied using both theoretical and experimental results. A device model is derived which shows that in the presence of a suitable energy barr ier between the Hg1-xCdxTe infrared absorbing layer and the overlaying passivation layer, the high surface recombination rate which is usual ly present at the semiconductor/passivant interface is prevented from having a significant effect on device performance. The energy barrier, which repels photogenerated minority carriers from the semiconductor surface, is introduced by employing an n-type Hg1-xCdxTe wafer which c onsists of a wider-bandgap capping layer that is grown in situ by MOCV D on an LWIR absorbing layer. The derived model allows the responsivit y to be calculated by taking into account surface recombination at bot h the front and back interfaces, thickness of capping and absorbing la yers, recombination at the heterointerface, and variations in equilibr ium electron concentration. Calculations show that for an x = 0.22 Hg( 1-x)C(d)xTe absorbing layer, the optimum capping layer consists of x g reater than or equal to 0.25 and a thickness of the order of 0.1 to 0. 2 mu m. Experimental results are presented for x = 0.22 n-type Hg1-xCd xTe conventional single-layer LWIR photoconductors, and for heterostru cture photoconductors consisting of an LWIR absorbing layer of x = 0.2 2 capped by an n-type layer of x = 0.31. The model is used to extract the recombination velocities at the heterointerface and the semiconduc tor/substrate interface, which are determined to be 250 cm s(-1) and 1 00 cm s(-1) respectively. The experimental data clearly indicate that the use of a heterostructure barrier between the overlaying passivatio n layer and the underlying LWIR absorbing layer produces detectors tha t exhibit much higher performance and are insensitive to the condition of the semiconductor/passivant interface.