Search

The authors have fabricated p-on-n LWIR HgCdTe diodes by extrinsically doping a double-layer heterostructure grown by metalorganic chemical vapour deposition (MOCVD) using the interdiffused multilayer process (IMP) on GaAs substrates. The p-side of the junction was obtained by a fast As diffusion, from an ion-implanted source, ahead of the heterointerface when the annealing was performed at very low partial pressure of Hg. Examples of photodiodes fabricated by this process are given. Thus, for devices processed by As implant/diffusion, an R0A of 24.2 Omega cm2 for a cut-off wavelength of 11.7 mu m at 77 K and a quantum efficiency of 51% were obtained for a mesa unpassivated structure, and an R0A of 91 Omega cm2 for a cut-off wavelength of 10.3 at 77 K and a quantum efficiency of 50% were obtained for a planar structure. At 77 K the transport mechanisms limiting the device performance near zero bias have appeared to be thermal processes (ideality factor n between 1 and 2). However, other mechanisms evident in reverse bias such as shunt and tunnelling, may also operate.

The critical steps in the development of MOVPE for the fabrication of MCT focal plane arrays (FPAs) are outlined. Improved purity of the source organometallics has enabled background donor concentrations in the mid 1014 cm-3 to be achieved, together with controlled back doping with donors or acceptors. Large area alloy uniformity is possible by the interdiffused multilayer process (IMP). However, large FPAs will need to be grown onto Si substrates to avoid thermal stress on the hybridized structure. Preliminary results are presented on a 256*256 element MWIR array. Results are also presented on optical in situ monitoring that will form the basis of improved epitaxial control in the future.

The authors compare the material properties of conventional- and interdiffused-grown layers of Hg1-xCdxTe/GaAs grown by metalorganic chemical vapour deposition (MOCVD). These results are also compared with those of state-of-the-art LPE HgCdTe/CdZnTe grown from Te-rich solutions. Data are presented on surface morphology, compositional uniformity, double-crystal X-ray diffraction, chemical defect etching, Hall effect, minority carrier lifetime and laser-beam-induced current (LBIC) characterisation on (111)B, (221) and (100) orientations. The compositional uniformity is 3.3% for 3 in diameter HgCdTe/GaAs. The interdiffused growth method results in better compositional uniformity than the conventional method, while the structural quality of layers grown by both techniques is similar. Etch pit densities of (1-2)*106 cm-2 are routinely obtained for (111)B and (221) orientations, while values as low as 5*105 cm-2 have been occasionally observed. Initial data are also presented for HgCdTe layers grown on (100)GaAs/Si substrates.

Low-leakage high-performance photovoltaic detectors were fabricated from long-wavelength infrared (LWIR) HgCdTe epitaxial material grown by metal-organic chemical vapour deposition (MOCVD) on GaAs substrates. Layers were grown by two different MOCVD techniques, a conventional alloy growth and an interdiffused multiple-layer growth. MOCVD HgCdTe layers were characterised by background electron concentrations of (1-3)*1015 cm-3 with electron mobilities up to 200000 cm2 V-1 s-1 at 77 K. Surface and cross-section dislocation densities were 106-107 cm-2 and occasionally (4-5)*105 cm-2. GaAs substrates were (100) and (111B) misoriented towards 110. The layer structure in which the devices were fabricated consisted of an absorbing LWIR HgCdTe layer grown on a buffer of CdTe. A thin layer (1-2 mu m) of wide-band-gap HgCdTe was grown last. Devices were fabricated in a single as well as double layers using a mesa geometry with a native oxide passivation. Junctions were formed at 2-3 mu m depth by a low-energy arsenic implant ( approximately 100 keV) which behaved as a finite diffusion source during the post-implant anneal. The best measured zero-bias resistance-area products (R0A) at 77 K were 1.67 Omega cm2 for the alloy layers for a cut-off wavelength of lambda c=14.94 mu m and 7.22 Omega cm2 for the interdiffused multiple layers for lambda c=15.92 mu m at 77 K. The results indicated a definite device performance improvement in double-layer compared to single-layer structures. Analysis of the transport properties for a device fabricated in a single layer with a cut-off wavelength of lambda c=8.35 mu m at 77 K and 9.1 mu m at 40 K suggested that the deviation from thermal processes occurs at temperatures of approximately 40 K.