DX Centres
Non-conventional Materials for Photoresponsive Devices

DX centres are donors which form very deep, tightly bound states and are very efficient traps of free carriers. For this reason they have been regarded as undesirable defects by the semiconductor industry. We have used the DX centres for the optical writing of persistent but erasable metallic features in insulating materials, by using the persistent photoconductivity (PPC) associated with the DX centers. The photocarriers move freely in the conduction band but are confined to the exposed regions, by Coulomb interaction with the positively charged impurity ions. Erasure is achieved by thermal annealing. In principle features can be written with 100nm resolution. This effect is potentially applicable in the fabrication of submicron devices, in optical switching and holography, and in high-density data storage (~60Gbits/in2).

• "Writing Erasable (Submicron) Metallic Patterns in Insulating AlGaAs:DX"
  T. Thio et al., Appl. Phys. Lett. 65, 1802 (1994).
• "Diffraction from Optically Written Persistent Plasma Gratings in Doped Compound Semiconductors"
  R.A. Linke et al., Appl. Phys. Lett. 65, 16 (1994).
• "Thick Plasma Gratings Using a Local Photorefractive Effect in CdZnTe:In",
  R.L. MacDonald et al., Optics Letters 16, 2131 (1994).
• "Holographic storage media based on optically active bistable defects", 41. R.A. Linke, I. Redmond, T. Thio, D.J. Chadi, J. Appl. Phys. 83, 661 (1998).

We have explored II-VI semiconductors for DX centres with PPC persisting to higher temperatures. In CdZnTe:Cl the Cl ions form several DX centers; the PPC evinces two metastable states, the highest annealing temperature observed is T_a=190K. In ZnSe:Ga the Ga are DX centers with T_a=100K; however, when ZnSe:Ga is grown on a GaAs layer, the photocarriers are trapped at the heterojunction; this boosts the annealing temperature to T_a=350K, making it a very attractive route to future room-temperature devices.

• "Persistent Photoconductivity and DX Centres in CdZnTe:Cl"
  J.W. Bennett et al., J. Appl. Phys. 78, 5827 (1995).
• "DX Centers in II-VI Semiconductors and Heterojunctions", T.Thio et al., paper presented at EMC95 Charlotteville VA (6/95), J. Electronic Matls. 25, 229 (1996).
• "DX Centres in CdZnTe:Cl and Their Applications", T. Thio et al., paper presented at Int'l II-VI Semiconductor Conference, Edinburgh UK (8/95), J. Cryst. Growth.159, 345 (1996).
• "Deep Donors in CdZnTe:Cl", T. Thio, J.W. Bennett, P. Becla; Phys. Rev. B 54, 1754 (1996).
• "Microscopic structure of DX centers in Cd0.8Zn0.2Te:Cl" 40. Y.Y. Shan, K.G. Lynn, Cs. Szeles, P. Asoka-Kumar, T. Thio, J. W. Bennett, C.B. Beling, S. Fung, P. Becla, Phys. Rev. Lett. 79, 4473 (1997).

In collaboration with Chih-Yung Chen of Harvey Mudd College, and K.L. Wang, K.W. Alt and P.C. Sharma of UCLA, we have studied the PPC in pure GaAs, delta-doped with Si. The PPC shows two distinct transition temperatures, at T=50K and at T=250K. The photocarrier density is not correlated with the density of Si dopant ions, suggesting that they are related to a completely different deep centre, such as might be formed by a Ga or As vacancy.

• "Persistent Photoconductivity in Si delta-doped GaAs", C.Y. Chen, Tineke Thio, K.L. Wang, K.W. Alt, P.C. Sharma, Appl. Phys. Lett. 73, 3235 (1998).