MWIR DLPH HGCDTE PHOTODIODE PERFORMANCE DEPENDENCE ON SUBSTRATE MATERIAL

Mid wavelength infrared p-on-n double layer planar heterostructure (DL PH) photodiodes have been fabricated in HgCdTe double layers grown in situ by liquid phase epitaxy (LPE), on CdZnTe and for the first time o n CdTe/sapphire (PACE-1). Characterization of these devices shed light on the nature of the material limits on device performance for device s performing near theoretical limits. LPE double layers on CdZnTe and on PACE-1 substrates were grown in a horizontal slider furnace. All th e photodiodes are p-on-n heterostructures with indium as the n-type do pant and arsenic the p-type dopant. Incorporation of arsenic is via im plantation followed by an annealing step that was the same for all the devices fabricated. The devices are passivated with MBE CdTe. Photodi odes have been characterized as a function of temperature. R(0)A(imp) values obtained between 300 and 78K are comparable for the two substra tes and are approximately a factor of five below theoretical values ca lculated from measured material parameters. The data, for the PACE-1 s ubstrate, indicates diffusion limited performance down to 110K. Area d ependence gives further indications as to the origin of diffusion curr ents. Comparable R(0)A(imp) for various diode sizes indicates a p-side origin. R(0)A and optical characteristics for the photodiodes grown o n lattice-matched CdZnTe substrates and lattice mismatched PACE-1 are comparable. However, differences were observed in the noise characteri stics of the photodiodes. Noise was measured on 50 x 50 mu m devices h eld under a 100 mV reverse bias. At 110K, noise spectrum for devices f rom the two substrates is in the low 10(-15) A/Hz(1/2) range. This val ue reflects the Johnson noise of the room temperature 10(10) Ohm feedb ack resistor in the current amplifier that limits the minimum measurab le noise. Noise at 1 Hz, -100 mV and 120K for the 4.95 mu m PACE-1 dev ices is in the 1-2 x 10(-14) A/Hz(1/2), a factor of 5-10 lower than pr eviously grown typical PACE-1 n(+)-on-p layers. Noise at 120K for the 4.60 mu m PACE-1 and LPE on CdZnTe was again below the measurement tec hnique limit. Greatest distinction in the noise characteristics for th e different substrates was observed at 163K. No excess low frequency n oise was observed for devices fabricated on layers grown by LPE on lat tice-matched CdZnTe substrates. Photodiode noise measured at 1Hz, -100 mV and 163K in the 4.60 mu m PACE-1 layer is in the 1-2 x 10(-13) A/H z(1/2), again a factor of 5-10 lower than previously grown PACE-1 n(+) -on-p layers. More variation in noise (4 x 10(-13)-2 X 10(-12) A/Hz(1/ 2)) was observed for devices in the 4.95 mu m PACE-1 layer. DLPH devic es fabricated in HgCdTe layers grown by LPE on lattice-matched CdZnTe and on lattice-mismatched PACE-1 have comparable R(0)A and quantum eff iciency values. The distinguishing feature is that the noise is greate r for devices fabricated in the layer grown on lattice mismatched subs trates, suggesting dislocations inherent in lattice mismatched materia l affects excess low frequency noise but not zero bias impedance.