Optoelectronic Modulation Spectroscopy Research Articles

A physical surface-potential-based drain current model for polysilicon thin-film transistors

A physical drain current model of polysilicon thin-film transistors based on the charge-sheet model, the density of trap states and surface potential is proposed. The model uses non-iterative calculations, which are single-piece and valid in all operation regions above flat-band voltage. The distribution of the trap states, including both Gaussian deep-level states and exponential band-tail states, is also taken into account, and parameter extraction of trap state distribution is developed by the optoelectronic modulation spectroscopy measurement method. Comparisons with the available experimental data are accomplished, and good agreements are obtained.

Comparisons using optoelectronic modulation spectroscopy of n-type GaAs epitaxial layers formed on buffer layers prepared at normal and low temperatures

Optoelectronic modulation spectroscopy (OEMS) has been used to examine defect-related states in GaAs metal-semiconductor, field-effect transistor (MESFET) structures prepared by molecular-beam epitaxy (MBE) on buffer layers formed at normal and low temperatures. The technique was used to simultaneously observe the spectra of defect states near to the interface with the buffer layer and in the active layer. There were few responses that were common to the structures. The most prominent was an electron trap with a depth of 0.92 eV. This was present throughout both structures and may be from a native defect in GaAs. With few exceptions the states seen in a particular structure were present in the active layer as well as near to the interface with the buffer layer suggesting that, in each case, the defects originated from the active/buffer layer interface or from the buffer layer. The most significant difference was that states in the low-temperature (LT)-based structure generally exhibited replicating responses with specific energy periodicities. A model describes this in terms of optical-absorption transitions to the local vibrational states of defects. Five different replica series were observed. The vibrational characteristics of the defects found in the LT material suggest that they are different in character and extent to those defects found in material formed on a normally prepared buffer layer. Their Frank-Condon energies ranged from 9–332 meV.

The GaN yellow luminescence centre observed using optoelectronic modulation spectroscopy

Optoelectronic modulation spectroscopy (OEMS) has been used to explore the sub band-gap responses of deep energy states in GaN prepared by MOVPE. Modulation of the channel current of an AlGaN HEMT revealed a replica structure centred on 2.16 eV which cannot be attributed to Fabry-Perot oscillations. This was superimposed on a broad background with an anti-phase response, suggesting a continuous spectrum of states to which electrons were excited from the valence band.The phase of the replica responses confirmed that each peak was due to a transition to the conduction band. The average peak separation was 89 meV, close to the LO phonon energy in GaN, 92 meV, suggesting that charge in the state at 2.16 eV was strongly coupled to the lattice. This appears to be the first report of the phonon replica structure in an absorption spectrum. The observations are compared with a previously published luminescence replica spectrum. It is demonstrated using configuration coordinate diagrams that there should be a coincidence of lines, and this was confirmed experimentally. These observations provide confirmation that the electron state concerned is located 2.16 eV below the conduction band.The geometry of the configuration coordinate diagram leads to the conclusion that the Frank-Condon energy of this defect was between 0.40 and 0.55 eV with a corresponding Stokes shift between 0.80 and 1.10 eV.

Observations of deep levels in 4H-SiC using optoelectronic modulation spectroscopy

Optoelectronic modulation spectroscopy (OEMS) has been used to reveal defect states in 4H-SiC. Pairs of magnitude and phase spectra have been used to infer whether they were electron or hole traps. Eleven discrete trap responses have been observed, eight were assigned as electron traps and three as hole traps. Five of these had been observed previously using optical admittance spectroscopy (OAS). An electron trap at 1.20 eV gave the most prominent response with a distinctive signature indicating that these traps were spatially delocalized with an extent of at least 1.35 nm, possibly associated with an extended defect structure. An unresolved continuum of be the superimposed response of an electron and hole trap at closely similar energies. Cosistency has been demonstrated with previous work using DLOS and OAS.

Depth defined optoelectronic modulation spectroscopy

It is shown that if the gate depletion region of a MESFET is maintained with a constant width, channel current optoelectronic modulation spectroscopy reveals a spectrum of responses arising predominantly from charge variations in the depletion region at the interface between the active layer and the substrate. Optically induced charge variations as small as 2 × 109 electrons/cm2/eV have been detected. A refinement of the basic method is described which should allow the responses of electron states in a region of selected depth within the active layer states to be seen. The method will be of particular value in observing states that have a role in back-gating.

Depletion mode optoelectronic modulation spectroscopy

The depletion mode of opto-electronic modulation spectroscopy (OEMS), spectroscopically senses the electrical response to wavelength modulated monochromatic illumination as trap states in the depletion region of a semiconductor device are cyclically excited. The method is demonstrated using metal–Si3N4–GaAs field-effect transistors in which the charge in the gate depletion region is detected through its effect on the channel current. The optical transition energies of trap levels were revealed and the charge exchange mechanisms identified unambiguously by examining the phase of the detected signal with reference to the phase of the photon energy variation. In-phase responses originate from electron trap states while responses of opposite sign derive from hole traps. Many of the states corresponded closely in energy with ones previously reported in vapor-phase-epitaxy GaAs materials. Charge exchange with energetically discrete and continuously distributed traps is theoretically described when they are excited by photons having periodic energy modulation. The results indicate that the magnitude and phase of the OEMS response spectra are determined by the relative thermal and optical emission rates of trapped charges. Deep level transient spectroscopy (DLTS) measurements made on the same samples could not reveal the responses of bulk levels that the OEMS technique showed were present; this was because the DLTS spectra were dominated by the interface continuum response. This illustrates clearly an important advantage of the technique. The method is equally applicable to other devices in which a space-charge region regulates the channel current.

Optoelectronic modulation spectroscopy applied to the characterization of field effect transistors

Optoelectronic modulation spectroscopy has been applied to field effect transistors. Spectra include responses from bulk levels as well as from the continuum of states at the insulator-semiconductor interface. States that are energetically deep in a continuum can be probed but would not be accessible to the Fermi level in an electrical measurement.

Optoelectronic modulation spectroscopy (OEMS)—a new tool for device investigations

The principle of optoelectronic modulation spectroscopy (OEMS) is explained. Examples of its use in three different measurement modes are described and it is shown that the energy locations of mid-gap bulk and interface states can be ascertained when the sub-band-gap admittance and depletion modes are used together; the band with easiest electrical communication is also revealed for each state. Using super-band-gap radiation allows spectral features associated with resonant electron states to be observed directly without the need to distort the band structure.

A Direct Observation of the DX Center in GaAs Using Optoelectronic Modulation Spectroscopy Above the Bandedge

Optoelectronic modulation spectroscopy has been used to examine sulfur‐doped VPE and tin‐doped MBE using photon energies in excess of the fundamental energy gap. A peak in the modulated electrical admittance at 1.56 eV is attributed to the DX level and locates the level 1.56 eV above the valence band maximum, resonant with the conduction band. Transitions to it from the valence band seem to be indirect, and the level appears to have an optical cross section which increases with photon energy. This is the first direct measurement of its energy. The method avoids the necessity for high doping and the use of hydrostatic pressure to move the level into the energy gap.

Opto-electronic modulation spectroscopy (OEMS)

A technique is described which combines wavelength modulated monochromatic illumination with impedance measurements to discriminate the responses of bulk and surface states in the depletion regions of electronic devices. The method reveals the inherent ambiguity in conventional capacitance vs voltage and conductance vs voltage (C-V and G-V) measurements of state densities. The opto-electronic modulation spectroscopy (OEMS) spectra peak energies provide the state to band transition energies directly and these can be indexed onto an energy diagram to give the spatial positions of the responding states as well as the band with which they communicate. Measurements made on an SiO2/n-GaAs device show an absence of surface state related peaks and suggest that surface potential pinning under positive bias may be related to bulk states emerging at the surface.