Optical DLTS

By optically creating carriers with a pulse of above band-gap illumination, then monitoring the subsequent current transient due to thermal detrapping, Hurtes et al (1978) and Fairman et al (1979) were able to apply the DLTS method to bulk, high-resistivity materials. This method, however, is unable to distinguish between electron and hole traps, and the calculation of trap densities is difficult. 

Fig. 4. Energy below the conduction band of levels reported in the literature for GaP. States are arranged from top to bottom chronologically, then by author. At the left is an indication of the method of sample growth or preparation liquid phase epitaxy (LPE), liquid encapsulated Czochralski (LEC), irradiated with 1-MeV electrons (1-MeV e), and vapor phase epitaxy (VPE). Next to this the experimental method is listed photoluminescence (PL), photoluminescence decay time (PLD), junction photocurrent (PCUR), photocapacitance (PCAP), transient capacitance (TCAP), thermally stimulated current (TSC), transient junction dark current (TC), deep level transient spectroscopy (DLTS), photoconductivity (PC), and optical absorption (OA).

Fig. 1. Scheme of the capture of electrons in a polar matrix, (a) Orientation of solvent dipole molecules around an electron (b) potential well for et (du and dlt the ground and the excited levels of an electron in a trap). The arrows indicate the optical transitions of the trapped electron. 

Capacitance transient spectroscopy encompasses a powerful set of techniques to detect and characterise deep levels in semiconductors. The list of techniques applied for III-V nitrides includes deep level transient spectroscopy (DLTS) [1,2], double correlation DLTS (DDLTS) [3], isothermal capacitance transient spectroscopy (ICTS) [2], photoemission capacitance transient spectroscopy (ODLTS) [4] and optical ICTS (OICTS) [5], This Datareview presents the current status of deep level studies by capacitance transient techniques for III-V nitrides. A brief introduction to the techniques is given, followed by an example that demonstrates the application of DLTS and DDLTS for Si-doped... 

For wide bandgap semiconductors, such as III-V nitrides, the thermal techniques (DLTS, DDLTS, ICTS) can only detect deep levels which are energetically located within 1 eV of either bandedge for practical measurement conditions. To access deep levels near midgap optical excitation of carriers (ODLTS, OICTS) is necessary. 

For p-type Mg-doped GaN, DLTS measurements [15,19] have detected three deep levels with activation energies for hole emission to the valence band of -0.21 (DLPi), -0.32 (DLP2) and -0.42 eV (DLP3). The concentrations ranged from -4 x 1014 to -9 x 1014 cm 3. For the same Mg-doped GaN sample the ODLTS spectrum exhibited a dominant deep level near midgap (OLPt) with an optical threshold energy for hole emission to the valence band of -1.8 eV (180 K) its concentration was estimated to be -2.4 x 1015 cm 3. 

The DLTS measurements so far describe only electronic states in the upper half of the band gap. The lower half of the gap is harder to study because of the difficulty of making a stable junction to p type a-Si H. One technique uses light illumination of n-type samples to probe the lower half of the gap (Lang et al. 1984). The optical absorption populates the defect states with holes which are removed by thermal excitation to the valence band. This experiment measures the thermal emission energy because the information comes from the thermal emission step rather than from the initial optical excitation. The data for the lower half of the gap in Fig. 4.17 are derived from this type of experiment. The results are consistent with the usual DLTS where the two results overlap, but there are various new peaks seen in the spectra, with no obvious correlation with the sample growth properties. The addition of the illumination makes the analysis much harder and it is difficult to judge whether all the extra structure is real. 

The energies of thermal emission, observed by DLTS and by optical absorption and luminescence agree to within 0.1 eV. The Stokes shift is therefore very small and unable to account for the capture cross-section within the multiphonon model. The mechanism of non-radiative capture at defects remains puzzling. 

There are several pieces of backup hardware that are currently available. You can back up your information to magnetic tape, Digital Audio Tape (DAT), Digital Linear Tape (DLT), optical disk, removable hard disk, and many other removable media. The key here is that all of these media can be removed from the drive and stored in a safe place. 

Figure 12.18. Probe head based on a cube about 5 cm on a side and incorporating filters, beamsplitter, and focusing optics. The probe is designed to accept standard fiber-optic cables terminated with. SMA connectors. Designed by DLT, Inc. (23) and used to acquire Figure 12.14C.

Optical techniques like photoluminescence (10) and infrared photothermal spectroscopy (11.) work well for the characterization of shallow level impurities, while electrical techniques work well for deep level impurities. There are a number of methods that have been used for electrical characterization. I will only discuss deep level transient spectroscopy (DLTS), however, because it has become the most popular and gives a fairly complete characterization. 

It should be noted that a-Si H exhibits a density of states that is neither constant nor discrete. Deep-level transient spectroscopy (DLTS) (Lang et al, 1982), optical absorption (Jackson and Amer, 1982), and photoconductivity (Jackson et al, 1983b) measurements have indicated that a rather broad defect band lies in the gap and is centered below the Fermi energy. This means that for cases in which the built-in potential is less than the width of... 

Another TL application is the analysis of the electronics states in materials, which are defined by both material and technology of its production. For example, for PS different contact methods to define the electronics states in metal/PS stmctures have been aheady applied thermally stimulated depolarization currents (TSDC) and thermally stimulated current (TSC) (Ciurea et al. 1998 Anastasiadis and Triantis 2000 Brodovoy et al. 2002), optical charging spectroscopy (OCS) (Ciurea et al. 2000), and deep-level transient spectroscopy (DLTS) (Pincik et al. 1999 Tretyak et al. 2003). The mentioned methods determine the parameters of traps related with both PS material and metal/PS interface and often differ from results obtained from TL experiments. Two advantages of the TL method are that it is contactless and it can reveal the energy distribution of both bulk and/or surface states. The obvious drawback of TL is that it can only be applied to luminescent materials. 

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