H. G. Grimmeiss

Deep level transient spectroscopy evaluation of nonexponential transients in semiconductor alloys

Evaluation of data obtained from deep level transient spectroscopy (DLTS) is often based on the assumption that the transients are exponential. The applicability of DLTS to the study of deep energy levels in semiconductor alloys has therefore been questioned since thermal transients are often nonexponential in these materials. In this paper we present calculated DLTS spectra in a simple model for broadened defect levels. The calculated spectra are compared with experimental data for a deep electron trap in GaAs1−xPx . The main result is that, within the model, DLTS‐deduced activation energies and thermal emission rates are, indeed, relevant even when the transients are strongly nonexponential as a result of alloy broadening. A method of estimating the corrected concentration of deep levels and the distribution in binding energies is also presented.

Spectral distribution of photoionization cross sections by photoconductivity measurements

A new technique for measuring the spectral distribution of photoionization cross sections in photoconductors is presented. The method makes use of the fact that the occupancy of an impurity level is not changed during illumination with photons of different energy if the photocurrent is kept constant. A constant photocurrent is achieved by adjusting the light intensity. The spectral distribution of the photoionization cross section is then given by the inverse of the photon flux as a function of the photon energy. To demonstrate the applicability of this measuring technique, measurements were performed on oxygen‐doped GaAs samples. Three deep energy levels (two 0.46 and 0.79 eV below the conduction band and one 0.48 eV above the valence band) were investigated and the spectral distribution of five photoionization cross sections were measured. All of them are in good agreement with calculated values, and thus an accurate determination of the threshold energies is achieved. For comparison, similar investigat...

Electrical properties of Fe in GaAs

GaAs has been doped with Fe by diffusion and by liquid‐phase epitaxy. The deep level introduced has an optical cross section for excitation of holes to the valence band with one threshold at 0.46 eV and another at about 0.85 eV. By combining those data with previous measurements of internal transitions between the ground state and an excited state, the level position in the gap is established. Optical excitation of electrons to the conduction band is below the limit of detection. The cross section for capture of electrons is nearly temperature independent with a value of about 10−19 cm2 at 200 K, and the thermal activation energy for emission of holes is 0.54 eV after T2 correction.

Deep sulfur‐related centers in silicon

The electronic properties of two dominant sulfur‐related deep donor levels in silicon have been investigated using capacitance and dark current transient techniques. One of the centers, the B center, has an enthalpy ΔHn of 0.32 eV and an electron‐capture cross section σtnB of 2×10−15 cm2 at 100 K. σtnB varies with temperature as T−3.3. A plot of log etnA vs 1/T for the other center, the A center, shows a ’’thermal activation energy’’ of 0.59 eV. σtnA is estimated to be larger than 10−14 cm2. The nature of the A and B centers is discussed.

Electronic properties of selenium‐doped silicon

Two dominant selenium‐related defects in silicon have been studied by transient capacitance and current techniques. For one of the centers, the A center, Arrhenius plots of thermal emission rates for electrons and holes (log et−1/T) give ’’thermal activation energies’’ of EC −ET =0.52 eV and ET −EV =0.62 eV, respectively. Although the electron‐capture cross section of the center is too large to be measured with our equipment, a conservative estimate gives a value larger than 10−14 cm2. The electron‐capture cross section σtnB of the other center, the B center, showed a T−3.2 temperature dependence and has a value of 3×10−15 cm2 at 100 K. From the measured data for σtnB and etnB a value of 0.30 eV for the Gibbs free energy of the B center is obtained. This energy value is constant and equal to the enthalpy in the temperature range investigated. Plotting log T2σtnB versus 1/T, an activation energy of 14 meV is obtained. This is interpreted as the energy separation between the lowest state accessible in a cas...

Electroluminescence at room temperature of a SinGem strained-layer superlattice

We report for the first time on room temperature electroluminescence in the region 1.3–1.7 μm from a strain‐adjusted Si6Ge4 superlattice. These results, together with photoluminescence, short‐circuit photocurrent spectroscopy, and voltage‐intensity and current‐intensity measurements indicate that the observed electroluminescence consists of two emission bands which are believed to be caused by defect and interband recombination processes.

Identification of deep centers in ZnSe

Cu‐doped, Mn‐doped, and nominally undoped ZnSe crystals were prepared by vapor transport or sublimation. The presence of the dopants was established by photoluminescence EPR, atomic absorption, and radiotracer techniques, and their associated energy levels studied by photoluminescence. From measurements of photocurrent and photocapacitance, it was shown that Cu doping introduces an energy level at about 0.68 eV above the valence band. From the good agreement between the optical cross sections obtained from the measurement of photocapacitance and optical quenching of photoluminescence, it was concluded that this center is responsible for the red Cu‐emission in photoluminescence. This conclusion was supported by the observation that the absolute concentration of this level agrees with the reduction of the free‐carrier concentration caused by Cu doping. Owing, presumably, to residual copper contamination, the 0.68‐eV center was sometimes detected in nonintentionally doped and Mn‐doped material. Evidence is g...

Dynamics of capture from free‐carrier tails in depletion regions and its consequences in junction experiments

Exponentially decaying tails of free carriers extend from the neutral material into the depletion region of p‐n junctions or Schottky barriers. In nonequilibrium, deep level impurities in the depletion region may readily capture free carriers from these tails. The strongly nonexponential time dependence of the capture is calculated here and is compared with experimental data. This nonexponential time dependence is particularly important in deep level transient spectroscopy, and it also appears in many other junction measurements with deep level impurities.

Capture from free‐carrier tails in the depletion region of junction barriers

Junction experiments have been proposed in which measurements of depletion capacitance as a function of decreasing reverse bias are used to determine the energy level of a deep trap. Since deep levels in the depletion layer are filled from the free‐carrier tail extending from the bulk, this process is slow to reach equilibrium. Calculation and experiment demonstrate this effect and it is emphasized that its neglect leads to serious misinterpretation.

Electric and optical properties of the ’’Cu‐red’’ center in ZnSe

Emission and capture rates describing the optical and thermal transitions at the ’’Cu‐red’’ center in ZnSe have been investigated using a modified photocapacitance technique. By keeping the depletion region of a Schottky diode constant during the measurements of transients highly compensated samples could be used, resulting in larger signals than previously possible. This permitted optical‐emission rates for both electrons and holes to be measured with higher accuracy and in a broader temperature region than in previous investigations. The capture cross section of electrons σn was determined in the temperature range 77–200 K. σn increases with decreasing temperature and has a value of 1.4×10−19 cm2 at 77 K.Emission and capture rates describing the optical and thermal transitions at the ’’Cu‐red’’ center in ZnSe have been investigated using a modified photocapacitance technique. By keeping the depletion region of a Schottky diode constant during the measurements of transients highly compensated samples could be used, resulting in larger signals than previously possible. This permitted optical‐emission rates for both electrons and holes to be measured with higher accuracy and in a broader temperature region than in previous investigations. The capture cross section of electrons σn was determined in the temperature range 77–200 K. σn increases with decreasing temperature and has a value of 1.4×10−19 cm2 at 77 K.