Elsevier

Thin Solid Films

Volume 317, Issues 1–2, 1 April 1998, Pages 425-428
Thin Solid Films

PbSe photodetector arrays for IR sensors

https://doi.org/10.1016/S0040-6090(97)00576-2Get rights and content

Abstract

PbSe Linear arrays have been fabricated to be used as multiple IR gas sensors at room temperature. PbSe layers were obtained by thermal evaporation in vacuum on SiO2/Si wafers. Samples became photoconductive after a sensitization process that includes thermal treatments in an iodine-enriched atmosphere. Both morphological features and chemical composition of the layers were studied along the different steps of the sensitization process with a Scanning Electron Microscope. Photoelectrical measurements were also carried out. By comparison between microscopic and photoelectrical measurements, some relations between changes in the morphology of the PbSe layers and changes in their photoconductive behavior were established. The dispersion observed in the photoconductive characteristics of the array elements is associated to the appearance of inhomogeneities during the sensitization process. Whereas the presence of iodine in the sensitization process accelerates the recrystallization rate of PbSe, the presence of oxygen could be responsible for the photoconductivity.

Introduction

Lead chalcogenide IR detectors present high detectivities at room temperature in the 3–5 μm atmospheric window. Recent works have demonstrated technological compatibility between standard processes used in microelectronics and manufacturing processes of lead chalcogenide detectors 1, 2. The possibility of achieving monolithic integration between the IR sensors and their Si read-out electronics opens new perspectives for low cost mass production in civil market applications. One of the most promising applications is the fabrication of PbSe arrays to be used as multiple IR gas sensors at room temperature [3].

Although lead selenide detectors have been investigated for a long time, the mechanisms involved in the photoconductivity have not yet been well established. The multiple empirical photosensitization methods used and the wide dispersion in the electro–optical properties reported have contributed to create confusion regarding this subject. It is widely accepted that oxygen plays an important role during the sensitization process [4]. It is also known that halogen-based sensitization methods give excellent final results [5]. However, few works have tried to relate physical and chemical properties to the photoconducting response of the material. In this work we have studied the morphological and compositional characteristics of our detectors in each step of the sensitization process. In particular, we have studied the evolution in the behavior of the detectors of a linear array along the fabrication process. This process has been developed at the C.I.D.A. laboratories [6]. Electrical properties like resistance and detectivity have also been measured. Some evidences that relate changes in the PbSe layers with changes in their photoconductive behavior have been found.

Section snippets

Experimental details

The PbSe layers were obtained by thermal evaporation in vacuum on SiO2/Si wafers. The Au contacts were defined with a standard photolithographic method. Once evaporated, PbSe layers were sensitized following a three step process. The first step is an annealing at 493 K in an iodine-enriched atmosphere. During the second step the sample is annealed at 723 K in air and, finally, the sample is baked again at 523 K in an iodine-enriched atmosphere. The composition and morphology of the

Structure observation

Electron micrographs of a PbSe layer surface after deposition and after each step in the photosensitization process are shown in Fig. 1. The as-grown layer is polycrystalline with crystallites randomly scattered on the surface. The size of the crystallites is around 0.4 μm and the intercrystallite distance is also around 0.4 μm. In the first treatment the film drastically changes. Now the layer presents a granular aspect with grain sizes ranging from 0.5 μm to 1 μm. At this stage, the film is

Discussion

SEM studies and the resistivity measurements show that the as-grown arrays exhibit a high degree of homogeneity. It is during the photosensitization process that inhomogeneities appear. According to the electron micrographs, the formation of bubbles and cracking lines in the second thermal treatment is one of the most important morphological features observed. The bubbles formation could be due to a lack of adherence of the layer to the substrate and to the formation of vapour phases. The

Conclusion

The study of the evolution of the PbSe layers forming a linear array has established that inhomogeneities appear during the photosensitization process. These inhomogeneities are related with structural and compositional variations along the PbSe layers. By comparing electrical measurements with results obtained from SEM and EDS studies, it is concluded that the PbSe crystallization rate is increased by the presence of iodine. Changes in the resistivity during the thermal treatments have been

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    In recent years, semiconductor chalcogenide has drawn considerable research interests, among which lead selenide (PbSe) has been found widespread applications as photosensors [1], photodetectors [2], photoelectronic devices [3], and more recently, as photoelectric conversion coatings [4].

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