J.F. Siliquini

Improved device technology for epitaxial Hg1-xCdxTe infrared photoconductor arrays

The performance of Hg1-xCdxTe infrared photoconductors is strongly dependent on the semiconductor surface conditions and, in particular, the degree to which the surface contributes to recombination of photogenerated excess carriers. Although published photoconductor fabrication processes based on bulk Hg1-xCdxTe address this issue by fully passivating both major surfaces (i.e. front and back) with anodically grown native oxide, passivation of the sidewalls is neglected. In this paper it is shown both theoretically and experimentally that leaving the sidewalls unpassivated can result in approximately a factor of two reduction in responsivity for long-wavelength infrared (LWIR) detectors used in high-resolution thermal imaging systems. Detector arrays are typically fabricated on x=0.23 Hg1-xCdxTe representing a cut-off wavelength of 9.4 mu m and use individual element sizes of approximately 50*50 mu m2. We describe in detail for the first time a device technology which enables the fabrication of Hg1-xCdxTe photoconductor arrays such that the entire surface of the semiconductor is effectively passivated, including the sidewalls. Of particular interest is the fact that this improved device technology is compatible with present-day Hg1-xCdxTe epitaxial growth processes. This is in contrast to current photoconductor technology which is primarily based on bulk Hg1-xCdxTe. Experimental results are presented which compare device performance of LWIR detectors fabricated using the improved photoconductor technology with current published photoconductor technology. These results clearly indicate that detectors fabricated on liquid phase epitaxially (LPE) grown x=0.23 Hg1-xCdxTe material using the improved photoconductor device technology achieve much higher responsivities and detectivities. Furthermore, it is shown that only a fully passivated device structure is capable of exploiting any future improvements in bulk minority carrier lifetime as it approaches the Auger recombination limit.

Scanning laser microscopy of reactive ion etching induced n-type conversion in vacancy-doped p-type HgCdTe

Laser-beam-induced-current measurements have been used to characterize the extent of reactive ion etching (RIE) induced type conversion in vacancy-doped p-type Hg0.69Cd0.31Te. The technique allows the spatial extent of RIE induced type conversion to be determined and the donor level concentration profile within the n-type converted region to be estimated. For the RIE processing conditions used (410 mT, CH4/H2, 0.4 W/cm2) and an etch depth of 0.2 μm, n-type conversion extending ∼1.5 μm into the semiconductor was observed. The simple and powerful approach developed in this work is of general application to the study of semiconductor junctions, and can be applied to a range of processing techniques used in the formation of p-n junctions in HgCdTe (e.g., epitaxially grown heterojunctions, ion implantation, ion milling and Hg in-diffusion).

Characterisation of reactive-ion-etching-induced type-conversion in p-type HgCdTe using scanning laser microscopy

Abstract In this work we characterise the n-type converted region occurring in both vacancy and extrinsically doped p-type Hg 1− x Cd x Te ( x ≈ 0.3) after standard reactive-ion-etch (RIE) process. The laser beam induced current (LBIC) technique is used to characterise parameters such as lateral and vertical conversion depth. Furthermore, by fitting a theoretically determined LBIC signature to the measured LBIC over the temperature range 80–300 K, it is possible to estimate the donor level density of the n-type converted region.

Mercury annealing of reactive ion etching induced p- to n-type conversion in extrinsically doped p-type HgCdTe

Mercury annealing of reactive ion etching (RIE) induced p- to n-type conversion in extrinsically doped p-type epitaxial layers of HgCdTe (x=0.31) has been used to reconvert n-type regions created during RIE processing. For the RIE processing conditions used (400 mT, CH4/H2, 90 W), p- to n-type conversion was observed using laser beam induced current (LBIC) measurements. After a sealed tube mercury anneal at 200 °C for 17 h, LBIC measurements clearly indicated that no n-type converted region remained. Subsequent Hall measurements confirmed that the material consisted of a uniform p-type layer, with electrical properties equivalent to that of the initial as-grown wafer (NA−ND=2×1016 cm−3, μ=350 cm2 V−1 s−1).

Estimation of doping density in HgCdTe p-n junctions using scanning laser microscopy

Quantitative assessment of p- to n- type conversion due to reactive ion etching (RIE) of p-type Hg0.71Cd0.29Te is presented using laser-beam-induced-current (LBIC) measurements. For the RIE processing conditions used (390 mT, CH4/H2, 0.4 W/cm2), n-type conversion was observed in extrinsic arsenic-doped p-type Hg0.71Cd0.29Te which had previously undergone a Hg anneal to eliminate Hg vacancies. Effective doping density of the n-type converted region is determined by fitting a theoretically determined LBIC signature to the measured LBIC signal over a temperature range 80–300 K. Effective n-type doping density is the only fitting parameter used in the simulation, which was carried out using a commercial semiconductor device modeling package (SEMICAD™ DEVICE). This noncontact experimental technique promises to be a useful tool in the characterization of p-n junction diodes in HgCdTe, and for studying the precise nature of p to n conversion in p-type HgCdTe.

MOCVD-grown wider-bandgap capping layers in long-wavelength infrared photoconductors

The use of MOCVD-grown wider-bandgap as a capping layer for long-wavelength infrared (LWIR) photoconductors has been studied using both theoretical and experimental results. A device model is derived which shows that in the presence of a suitable energy barrier between the infrared absorbing layer and the overlaying passivation layer, the high surface recombination rate which is usually 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 wafer which consists of a wider-bandgap capping layer that is grown in situ by MOCVD on an LWIR absorbing layer. The derived model allows the responsivity to be calculated by taking into account surface recombination at both the front and back interfaces, thickness of capping and absorbing layers, recombination at the heterointerface, and variations in equilibrium electron concentration. Calculations show that for an absorbing layer, the optimum capping layer consists of and a thickness of the order of 0.1 to 0.2 . Experimental results are presented for x = 0.22 n-type conventional single-layer LWIR photoconductors, and for heterostructure photoconductors consisting of an LWIR absorbing layer of x = 0.22 capped by an n-type layer of x = 0.31. The model is used to extract the recombination velocities at the heterointerface and the semiconductor/substrate interface, which are determined to be and respectively. The experimental data clearly indicate that the use of a heterostructure barrier between the overlaying passivation layer and the underlying LWIR absorbing layer produces detectors that exhibit much higher performance and are insensitive to the condition of the semiconductor/passivant interface.

A gate-controlled HgCdTe long-wavelength infrared photoconductive detector

A gated photoconductor structure is proposed for the dual purpose of enhancing the performance of and investigating surface effects in HgCdTe photoconductive infrared detectors. It is verified both theoretically and experimentally that the passivating native oxide which accumulates the HgCdTe surface, thereby quenching surface recombination, also causes excessive surface shunting and an overall reduction in device responsivity. It is found that for photoconductive detectors passivated with native oxide/ZnS the optimum surface conditions prevail at a gate bias corresponding to a semiconductor surface potential of 50 mV. Experimentally, it is found that operation at this optimum condition yields approximately a 70% increase in responsivity in comparison to floating gate conditions, which corresponds to a surface potential of 72 mV for the as-grown native oxide/HgCdTe interface. The gated photoconductor is shown to be a powerful diagnostic device structure that can be used to evaluate and optimize surface passivation layers for HgCdTe infrared detectors.

The vertical photoconductor: A novel device structure suitable for HgCdTe two-dimensional infrared focal plane arrays

Abstract A novel photoconductive device structure is proposed and described that has been designed specifically as a sensing element for high density two-dimensional infrared focal plane array (IRFPA) applications. Although the design concept can be applied to a variety of epitaxially grown HgCdTe material, optimum performance can be achieved using n-type HgCdTe semiconductor material consisting of epitaxially grown heterostructure layers in which a two-dimensional mosaic of vertical design photoconductors are fabricated. The heterostructure layers provide high performance devices at greatly reduced power dissipation levels, while the vertical design allows for the high density integration of photoconductors in a two-dimensional array geometry with high fill factor. The salient feature of the proposed device structure is that the bias field is applied in the vertical direction such that it is parallel to the impinging infrared radiation. A comprehensive one-dimensional model is presented for the vertical design photoconductor, which is subsequently used to determine the optimum design parameters in order to achieve maximum responsivity at the lowest possible power dissipation level. It is found that the proposed device structure has the potential to be used in the fabrication of long wavelength IRFPAs approaching 10 6 pixels using 25 × 25 μm 2 detector elements. Furthermore, this is achieved with individual device detectivities that are background limited and for a total array power dissipation of less than 0.1 W using a pulsed biasing scheme. Performance issues such as response uniformity, pixel yield, fill factor, crosstalk, power dissipation, detector impedance, array architecture, and maximum array size are discussed in relation to the suitability of the proposed vertical photoconductor structure for use in IRFPA modules. When considering IRFPA operability, it is found that in many cases the proposed technology has the potential to deliver significant advantages, such as higher yield, higher fill factor, better uniformity, less crosstalk, and larger potential array size, when compared to photovoltaic technology.

Heterojunction blocking contacts in MOCVD grown Hg/sub 1-x/Cd/sub x/Te long wavelength infrared photoconductors

The theoretical and experimental performance of Hg/sub 1-x/Cd/sub x/Te long wavelength infrared (LWIR) photoconductors fabricated on two-layer heterostructures grown by in situ MOCVD has been studied. It is shown that heterojunction blocking contact (HBC) photoconductors, consisting of wider bandgap Hg/sub 1-x/Cd/sub x/Te on an LWIR absorbing layer, give improved responsivity, particularly at higher applied bias, when compared with two-layer photoconductors incorporating n/sup +//n contacts. An extension to existing device models is presented, which takes into account the recombination rate at the heterointerface and separates it from that occurring at both the contact-metal/semiconductor and passivant/semiconductor interfaces. The model requires a numerical solution to the continuity equation, and allows the device responsivity to be calculated as a function of applied electric field. Model predictions indicate that a change in bandgap across the heterointerface corresponding to a compositional change of /spl Delta/x/spl ges/0.04 essentially eliminates the onset of responsivity saturation due to minority carrier sweepout at high applied bias. Experimental results are presented for frontside-illuminated n-type Hg/sub 1-x/Cd/sub x/Te photoconductive detectors with either n/sup +//n contacts or heterojunction blocking contacts. The devices are fabricated on a two-layer in situ grown MOCVD Hg/sub 1-x/Cd/sub x/Te wafer with a capping layer of x=0.31 and an LWIR absorbing layer of x=0.22. The experimental data clearly demonstrates the difficulty of forming n/sup +//n blocking contacts on LWIR material, and indicates that heterojunctions are the only viable technology for forming effective blocking contacts to narrow bandgap semiconductors.

Performance of optimized Hgl−xCdxTe long wavelength infrared photoconductors

Abstract The effects of minority carrier “sweepout” in Hg l− x Cd x Te long wavelength infrared photoconductive detectors can severely degrade device performance. A photoconductive device fabricated on n-type LPE grown Hg l− x Cd x Te ( x = 0.23) is described which minimizes the effects of sweepout by using a combined overlap geometry/blocking contact device structure. Both theoretical and experimental data is presented which establishes that such a device structure exhibits enhanced responsivity, 1/f noise performance and detectivity, when compared to photoconductors that just have blocking contacts alone. The device technology described for this combined overlap geometry/blocking contact device structure may be readily applied to present day Hg l− x Cd x Te photoconductor technology in order to gain the benefits of reduced minority carrier sweepout without the need for heterojunction formation.