Localised states in CVD diamond, explored by transient photoconductivity and residual field measurements

• Published in: Diamond and related materials 1999, Vol.8 (12), p.2118-2126
• Main Authors: Marshall, J.M., Walters, A.S.
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; CVD diamond; Electronic structure and electrical properties of surfaces, interfaces, thin films and low-dimensional structures; Electronic transport; Electronic transport phenomena in thin films and low-dimensional structures; Exact sciences and technology; Impurity and defect levels; energy states of adsorbed species; Localised states; Photoconduction and photovoltaic effects; Photoconduction and photovoltaic effects; photodielectric effects; Physics; Space charge; Surface and interface electron states; Transient photoconductivity
• ISSN: 0925-9635; 1879-0062
• DOI: 10.1016/S0925-9635(99)00176-4

Chemical vapour deposited (CVD) diamond is attracting increasing interest as a potentially high performance semiconductor. However, in optimising the material for device applications, detailed information is often required on the properties of localised states within the energy gap. Although data are accumulating concerning mid-gap states, little has been published in respect of the shallower centres which often limit carrier drift mobility and other electronic properties. This paper presents and analyses data from a detailed exploration of the presence and influence of such states, as obtained for the first time using transient photoresponse techniques. It is demonstrated that such procedures constitute a powerful vehicle for the study of localised states in CVD diamond. In particular, measurements show that even in very good quality CVD diamond films, an energetically broad but structured collection of localised states exists. The energy distribution of these has been computed using techniques developed, refined and proven in prior studies of other disordered solids. During the course of our transient photodecay measurements, we have also established that specimens subjected to an applied field can accumulate a considerable space-charge-related residual field. This is obviously of significance to the operation of many potential devices. A new variant of the transient photoresponse system allows the decay of this field to be monitored as a function of time following the removal of an applied voltage. Such data provide further information on the localised states in which the space charge is trapped, and are shown to be at least qualitatively consistent with the localised state distribution computed from our transient photodecay measurements. Moreover, a simple model for the relaxation of the residual field is shown to be in excellent agreement with the experimental data.