In situ etching-back processes for a sharper top interface in boron delta-doped diamond structures☆
Highlights
► Challenge of delta-doping of diamond is in reducing width and optimizing interfaces. ► Smart gas switching and in situ O2 and H2 plasma steps make sharp and thin interfaces. ► Multilayer structures were grown without plasma interruption during the process.
Introduction
Because of the difficulty to obtain n-type doping in diamond and of the depth of the phosphorus donor energy level, most of the effort toward diamond-based devices has been devoted to unipolar devices. Among these, delta-doped structures have long been considered promising for field-effect diamond transistors (FET). Following the initial proposal to transpose such architecture from GaAs or Si to diamond [1], various attempts have been made in the 90s, first in Japan [2], then in Germany [3], [4], [5] and Israel [6]. More recently, further developments have emerged in Germany [7], [8], [9], [10], [11] and in the UK [11], [12]. Significant progress has been made in the design and performance of workable FET demonstration devices. In diamond, such a delta-doped structure would require [13] a metallic boron-doped p++ layer (concentration nB ≥ 5 × 1020 at.cm− 3) less than 3 nm-thick intercalated between two non-intentionally doped (NID) layers suited to high mobility transport [14] (nB < 2 × 1017 at.cm− 3).
To our knowledge, no clear evidence has been so far given that the mechanisms expected to govern the behavior of a delta-doped FET were indeed present in any of the devices reported to date: neither the confinement of carriers in the delta doped epilayer, nor the delocalization of holes in a high mobility region. Actually, if one introduces into modern simulation tools [13] one of the few boron concentration profiles published [10] with a dynamical concentration range encompassing the four relevant orders of magnitude, it is clear that a strongly scattered or hopping hole transport along the interface region is expected to contribute significantly and in a detrimental manner to the ON transconductance of the FET, and that delocalization of carriers, if any, occurs only on the steeper top (surface) side of the delta-doped layer.
In order to detect the confinement effects and enhanced mobility expected for an ideal structure, we propose here to test the limitations of a classical MPCVD approach for getting sharp interfaces between thin heavily-doped and thicker non-intentionally doped epilayers. In contrast to most of previously published works, the processes to be investigated in the following avoid plasma interruption and/or exposure of interfaces to air.
The boron concentration profiles of typical multilayered stacks of NID and p++ layers will be discussed. We discarded (111)-oriented growth because of the poor quality of the substrate surfaces, and we reduced the growth rate compared to our previous studies [15]. Because slowing down the growth process by reducing the methane to hydrogen ratio is known to decrease significantly the solid state incorporation efficiency [15] of boron, we had to retort to additional etch-back strategies which will be discussed in some detail below.
Section snippets
Experimental details
All diamond films were homoepitaxially grown from a gas phase by Microwave Plasma-enhanced Chemical Vapor Deposition (MPCVD) on type Ib 100-oriented 3 × 3 mm diamond substrates (purchased from Sumitomo Electric). None of the recently developed smoothing processes [16] were applied to the substrate surface, prior to its introduction into the load–lock chamber of a modified vertical silica tube NIRIM-type reactor with a base pressure below 10− 7 Torr.
The gas introduction system was modified in order
Summary
Our modifications of vertical silica tube NIRIM-type reactor and special process gas management allows slow growth, heavy boron doping and in-situ etching of diamond. Multilayers and single delta-layers have been grown without hydrogen and helium plasma interruptions. The addition of oxygen is necessary during the NID growth to keep a low boron concentration. Also, the presence of O2 in H2 and He methane-free and diborane-free plasmas provided an efficient in-situ etching of p++ epilayers,
Acknowledgments
The financial support of Agence Nationale de la Recherche under contract ANR08-BLAN-0195 is gratefully acknowledged. Gauthier Chicot held a doctoral fellowship from la Région Rhône-Alpes.
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Presented at NDNC 2011, the 5th International Conference on New Diamond and Nano Carbons, Suzhou, China.