Deep Donor EL2 Research Articles

Влияние барьерных контактов на транспорт носителей заряда в однородных структурах из GaAs, легированных глубокими центрами Cr и EL2

The results of studying the transport of charge carriers in GaAs structures doped with deep donor EL2 centers and acceptor levels of Cr for detectors of ionizing radiation and ultrafast photoelectric switches are presented. Three configurations of structures are investigated: p-i-n, n-i-n and p-i-p- types. The system of differential equations for the temperature of charge carriers, Poisson's equations and continuity was solved using a commercial software. It was found that the choice of the type of the barrier layer makes it possible to control the uniformity of the electric field strength in the structures. It is shown that p-i-p- type structures have the best uniformity of the electric field strength.

Nonlinearity of Volt–Ampere Characteristics of Homogeneous Compensated Detector GaAs Structures

The results of a study of carrier transfer and recharging of deep levels in semiconductor structures for ionizing radiation detectors are presented. Resistive-type arsenide–gallium structures with Schottky barriers and uniform distribution of deep chromium acceptors and donor EL2 centers have been studied. Solving the continuity and Poisson equations using the commercial design package, the effect of the volume depletion of detector structures by electrons with an increase in the applied voltage has been observed. It is established that the nonlinearity of the volt-ampere characteristics of structures occurs due to a change in the type of conductivity upon transition from an equilibrium to a nonequilibrium state. In this case, structures with initial (equilibrium) hole-type conductivity have close to linear volt–ampere characteristics.

Photoresistances of semi-insulating GaAs photoconductive switch illuminated by 1.064 μm laser pulse

The Shockley–Read–Hall model (SRHM) and its simplified model (SSRHM) were used to describe the characteristics of a photoconductive semiconductor switch (PCSS) made from a semi-insulating (SI) gallium arsenide (GaAs) chip, biased at low voltage, and illuminated by a 1.064 μm laser pulse. These characteristics include the free carrier densities, dynamic photoresistance, and time evolution of output pulses of the PCSS. The deep donor EL2 centers in SI GaAs play a dominant role in both the SRHM and SSRHM as electrons at EL2 unionized centers are strongly excited by the subband-gap photons at the wavelength of 1.064 μm. Theoretical modeling on the evolution of the experimental measured output pulses led to a two-step micromechanism of electron excitation process within the GaAs chip. The minimum photoresistances predicted by the SSRHM are in good agreement with experimental measurements, which confirms the dominant role of EL2 in the generation of electric pulses from a SI GaAs photoconductivity switch on which the 1064 nm laser pulse is illuminated.

Experimental study of the admixture of continuum states to the discrete excited state of EL2 in GaAs

High-resolution absorption spectroscopy of the zero-phonon line (ZPL) of the deep donor EL2 in GaAs was performed with a narrow-band laser. Measurements of the temperature broadening of the ZPL and frequency-selective bleaching (‘spectral-hole burning’) experiments led to the conclusion that the transition is essentially homogeneously broadened. The observed asymmetry of the line shape is caused by a Fano resonance of the 1T 2 excited state with continuum states of the conduction band. The line width of 1.0 meV at 12 K implies that autoionization of the excited state occurs after a mean lifetime of 0.66 ps. The detailed study of the ZPL offered an experimental access to the question of relative participation of T 2 and a 1 symmetry states to the optical transition from the EL2 ° ground state to the conduction-band continuum. In summary, the EL2 turns out to be an interesting case study of degeneracy between a discrete excited state and a continuum in a solid.

Effect of the carbon acceptor concentration on the photoquenching and following enhancement of the photoacoustic signals of semi-insulating GaAs

The spectral and the time dependent piezoelectric photoacoustic (PPA) measurements under the continuous light illumination were carried out at 85 K to investigate nonradiative recombination processes involving EL2 defect levels in carbon concentration controlled and not intentionally doped semi-insulating (SI) GaAs. The decrease of the PPA signal due to the photoquenching effect of EL2 is observed for a short period of illumination in the photon energy region from 1.0 to 1.3 eV. Since almost all of the carbon acceptors are compensated by deep donor EL2 in SI GaAs, electron occupancy of EL2 level can be controlled by changing the carbon acceptor concentration. It is found that the photoquenching becomes drastic with increasing the carbon concentration. After fully photoquenching, the PPA signal increases again through a local minimum by the continuous light illumination and finally exceeds the initial value before illumination until the saturation level is reached. The deep donor level EL6 and its metastable state are proposed. EL6 level donates electrons to compensate a part of carbon acceptors after EL20 to EL2* transition is accomplished. The nonradiative recombination through this level generates the PPA signal. Since the PPA measurement can detect lower concentration of EL6 than that of EL2, the higher sensitivity of the PPA measurements than the optical absorption measurements is pointed out. The usefulness of the PPA technique for studying the nonradiative transition through deep levels in semiconductors is also suggested.

High-resolution spectroscopy of the zero-phonon line of the deep donor EL2 in GaAs

We investigated the zero-phonon line (ZPL) of the deep donor EL2 in GaAs by means of high-resolution absorption spectroscopy with a narrow-band laser. Frequency-selective bleaching (``spectral-hole burning'') experiments and the measurement of the temperature broadening of the ZPL prove an essentially homogeneous broadening of the transition. The observed asymmetry of the line shape is interpreted to be caused by a Fano resonance of the $^{1}\mathrm{T}_{2}$ excited state with the conduction band. A splitting of the $^{1}\mathrm{T}_{2}$ state as the reason for the asymmetry seems unrealistic. The homogeneous broadening of the ZPL prevents the use of spectral-hole burning spectroscopy to study the effect of external perturbations on the ZPL of the EL2.

EL2 deep-level transient study in semi-insulating GaAs using positron-lifetime spectroscopy

Positron lifetime measurements performed on Au/GaAs samples at room temperature with an applied square-wave ac bias show a frequency dependent interface related lifetime intensity that peaks around 0.4 Hz. The observation is explained by the ionization of the deep-donor level EL2 to ${\mathrm{EL}2}^{+}$ in the GaAs region adjacent to the Au/GaAs interface, causing a transient electric field to be experienced by positrons drifting towards the interface. Without resorting to temperature scanning or any Arrhenius plot the EL2 donor level is found to be located 0.80\ifmmode\pm\else\textpm\fi{}0.01\ifmmode\pm\else\textpm\fi{}0.05 eV below the conduction-band minimum, where the first error estimate is statistical and the second systematic. The result suggests positron annihilation may, in some instances, act as an alternative to capacitance transient spectroscopies in characterizing deep levels in both semiconductors and semi-insulators.

The investigation of custom grown vertical zone melt semi-insulating bulk gallium arsenide as a radiation spectrometer

Vertical zone melt (VZM) bulk GaAs boules have been zone refined (ZR) and zone leveled (ZL) to reduce EL2 deep donor levels and impurity concentrations with the intent of improving properties for gamma ray detectors. ZR and ZL GaAs boules had background impurity levels and deep donor EL2 concentrations near or below detectable limits. The crystal mosaic of the material at locations near the seed end was slightly superior to commercial liquid encapsulated Czochralski (LEC) material, and nearly equivalent to commercial vertical gradient freeze (VGF) material. The crystal mosaic in ZL material degraded towards the tail end. The homogeneity of the electrical properties for the ZL and ZR VZM material was inferior compared to commercially available bulk GaAs material. Post growth annealing may help to homogenize some electrical properties of the material. The charge collection efficiency of the ZR GaAs detectors was only 30% maximum, and only 25% maximum for the ZL GaAs detectors. Resulting gamma ray spectra was poor from detectors fabricated with the ZL or ZR VZM material. Detectors fabricated from material that was both ZR and ZL did not demonstrate gamma ray resolution, and operated mainly as counters. The poor spectroscopic performance is presently attributed to the inhomogeneity of the electrical properties of the ZR and ZL GaAs materials. Comparisons are made with detectors fabricated from VGF SI bulk GaAs.

Photocurrent transients in semi-insulating GaAs, effects of EL2 and other defects

Photocurrent transient effects in undoped semi-insulating gallium arsenide have elicited conflicting explanations. The photocurrent quenching, enhanced photocurrent, and persistent photocurrent effects are reviewed and new results on these effects are reported. A comparison of the photocurrent transients with optical absorption quenching along with other experiments shows that photocurrent quenching is due to the metastable transformation of the deep donor EL2 and that the enhanced and persistent photocurrent effects are due to a seperate effect that is most likely a metastable transformation of another defect. Thermal recovery experiments show that the activated state responsible for the enhanced photocurrent recovers prior to the recovery of the metastable state of EL2. Furnace annealing experiments are presented showing that the enhanced and persistent photocurrent effects are significantly reduced after anneals at 500 °C, well below the annealing temperature of quenchable EL2. After reviewing existing models we suggest that the absence of a persistent photocurrent immediately after quenching is due to compensation of residual impurities by the second level of EL2. A complex defect, perhaps including EL6 as an intermediary, is proposed for the defect responsible for the charge transfer resulting in the enhanced and persistent photocurrent effects.

Optical Nonlinearities and Carrier Transport in GaAs: EL2 at High Excitation Levels

AbstractLaser induced picosecond transient gratings are used to study carrier transport via free carrier and photorefractive nonlinearities in semi‐insulating undoped GaAs bulk crystals. Carrier lifetime (τR = 1.5 ns), hole and ambipolar mobilities (μh = 410 cm2 V−1 s−1, μa = 760 cm2 V−1 s−1) are measured directly and an electron mobility equal to 5200 cm2 V−1 s−1 is calculated. The optical properties of highly excited GaAs reveal deep donor EL2 transformation and intracenter absorption under picosecond excitation. Rotation of the diffracted beam polarization is observed for the first time in orientation of the grating vector along the crystallographic axis [001]. This signal is attributed to a strain field at dislocations in LEC‐grown GaAs wafers.

A correlation between optical backgating and photogeneration mechanism in GaAs MESFETs

The optical backgating effect in GaAs MESFETs was correlated to the photogeneration mechanism in the device semi-insulating substrate. The generation of excess electrons and holes in the device substrate under illumination with photon energies lower than the band-gap was modeled taking into account the contribution of the two most significant levels, namely the deep acceptor HL1(Cr) end and native deep donor EL2. The model was compared to experimental results obtained from photo-Hall measurements performed on the device substrate.

Compensation mechanism in undoped, semi-insulating GaAs

Concentrations of neutral and ionized deep donor EL2 in undoped, semi-insulating GaAs crystals were obtained using the near-infrared photoabsorption method. By varying the carbon content in the crystal in a range of 1015–1016 cm−3, it was verified that only 30% of the total carbon acceptors take part in compensation of EL2, i.e., [EL2+]=0.3[C]. This contradicts with either the conventional two-level model which demands [EL2+]=[C] or the recent findings of electron paramagnetic resonance measurements indicating [EL2+]≫[C]. A model is presented to moderate these contradictions, which includes presence of indirectly carbon-related donor and electrically active and inactive two types of EL2. An evidence to support the model is given by a recent nuclear-magnetic resonance measurement by the same author [Appl. Phys. Lett. 57, 398 (1990)].

Temperature dependence of the persistent photocurrent in Czochralski gallium arsenide.

The persistent-photocurrent effect in bulk, semi-insulating GaAs has been studied as a function of temperature. The persistent photocurrent is activated by illumination with 1.13-eV light and is preceded by an enhanced photocurrent during long-term illumination after the initial photocurrent has decayed in a quenching process similar to that of the deep donor EL2. Both the magnitude of the persistent photocurrent, and its decay rate are strongly temperature dependent and show significant reduction above 40 K. These results indicate that while the metastable transformation of EL2 is required for the activation of the persistent photocurrent, another unidentified metastable process after this is also required.

The distribution of the deep donor EL2 and the net acceptor concentration in semi-insulating GaAs

In semi-insulating GaAs the deep double donor EL2 compensates the net acceptor concentration N A - N D. Inhomogeneities in the electrical properties of wafers could in principle be due to fluctuations of the total EL2 concentration, or the net acceptor concentration, or both. The situation can be clarified by mapping the distribution of both charge states (EL2 0 and EL2 +) of the EL2. Two methods are presented: magnetic circular dichroism and near infrared absorption at different wavelengths. Results show that, contrary to statements found in the literature, the total EL2 concentration shows the familiar fourfold symmetry on 100 wafers, and that it accumulates in the cell walls of the dislocation network.

Full-wafer mapping of total and ionized EL2 concentration in semi-insulating GaAs using infrared absorption

By combining infrared absorption data taken at two different wavelengths in the spectral region in which the native deep donor EL2 absorbs in GaAs, it is possible to determine the distribution of the total EL2 concentration and its filled fraction across a semi-insulating GaAs substrate. The distributions of net shallow acceptor concentration and free-electron concentration can then be deduced. Full-wafer maps of these quantities for a semi-insulating GaAs wafer are displayed along with Hall effect measurements which confirm the patterns observed optically.

Persistent photocurrent in semi-insulating GaAs and its relationship to the deep donor EL2

Abstract A strong persistent photocurrent (PPC) has been observed in some undoped semi-insulating Czochralski grown GaAs samples after illumination near 10 K with 1.1 eV light. The temperature dependence of the PPC at the lowest temperatures indicates thermal activation from shallow acceptors. Analysis of the PPC above 40 K indicates that annealing of the effect commences at this temperature. Infrared absorption studies show that EL2, which is quenched during activation of the PPC does not recover from its metastable state until 120 K or higher. We suggest that the PPC is produced by an interaction between EL2 in its metastable state and some other unidentified defect.

Optical techniques for characterizing SI GaAs

A number of optical techniques that can be used for the nondestructive characterization of semi-insulating GaAs are described. These techniques include regular photoluminescence, photoluminescence excitation spectroscopy, selective donor–acceptorpair luminescence, electronic Raman scattering, and local vibrational mode absorption for the identification and concentration determination of acceptors. For donors, magneto-photoluminescence can determine the chemical identity and relative concentration of the shallow donors, while for the important deep donor EL2, absorption and bleaching experiments determine the neutral EL2 concentration. The utility of these techniques for routine and reliable characterization of production and developmental GaAs wafers is discussed. The possible extension of these techniques to spatially resolved 2D mapping of GaAs wafers is also discussed.

Mechanism of EL2 effects on GaAs field-effect transistor threshold voltages

Depending on the magnitude of the out-diffusion of the deep-donor EL2 during the post-implant anneal, the substrate barrier height increases. This in turn affects the depletion of donors on the channel side of the substrate junction, so that the voltage that is needed to pinch off the channel from the Schottky barrier is reduced. Threshold-voltage shifts of the order of less than 13 mV for devices with threshold voltages of −200 mV are predicted.

Effects of macroscopic inhomogeneities on electron mobility in semi-insulating GaAs

We show that defect inhomogeneities of sizes larger than the electron mean free path are responsible for the low values and anomalous temperature dependence of the electron mobility in semi-insulating (SI) GaAs. The room-temperature electron mobility values below about 6000 cm2/V s cannot be uniquely used for the determination of the concentration of ionized defects, since the contribution from inhomogeneities usually exceeds that from scattering by ionized impurities. The effects of the macroscopically inhomogeneous distribution of residual acceptors and the major deep donor EL2 diminish at elevated temperatures between 600 and 900 K, which offers a means for identification of inhomogeneities, and furthermore explains recently reported steplike mobility versus temperature behavior in SI-GaAs.

Pressure effect on the luminescence from the deep level in gallium arsenide.

The luminescence band with a peak at 0.68 eV associated with the main deep donor EL2 in undoped GaAs, is studied at 4.2 and 77 K under quasihydrostatic high pressure up to 2.4 GPa. The peak of the EL2-related band shifts with pressure at a rate of 0.015 eV/GPa, which is only 14% of the band-edge shift. We suggest that the luminescence is due to the transition between the conduction band and the EL2 level and that the electronic state of the EL2 level originates mainly from the \ensuremath{\Gamma} minimim of the conduction band. The Franck-Condon shift \ensuremath{\Delta} is optically determined to be 0.12\ifmmode\pm\else\textpm\fi{}0.01 eV using pressure tuning monitored by the luminescence intensity with the below-band-gap excitation.