J. Dashdorj

Interface recombination velocity measurement by a contactless microwave technique

The interface or surface recombination velocity (SRV) is a critical and important parameter in many applications. In this work, we have developed and applied a contactless microwave technique, which in combination with a continuously tunable pulsed light source, is able to probe the excess carrier lifetime in the surface and bulk regions of a semiconductor wafer. The technique is called resonant coupled photoconductive decay and has been described by the authors in the literature. For strongly absorbed light, the initial (t=0) decay time is a strong function of the absorption coefficient α, as well as the bulk lifetime. The effective bulk lifetime is measured by using very weakly absorbed light, or by measuring the asymptotic decay rate of strongly absorbed light. For asymmetric surfaces (such as a wafer polished on one surface only), the measurement with strongly absorbed light is made at both wafer surfaces. We have developed a method to measure SRV independently of bulk lifetime by plotting the surface lifetime data versus the absorption coefficient of the incident light pulse. A number of measurements of silicon wafers, with a variety of surface conditions, will be described.

Charge transfer in semi-insulating Fe-doped GaN

Charge transfer kinetics is studied in free-standing Fe-doped GaN using photo-induced electron paramagnetic resonance (EPR). Samples with Fe concentrations of 1017 cm−3 reveal an increase in Fe3+ during exposure with photon energy greater than 0.8 eV, while samples with higher Fe concentrations exhibit a decrease in the Fe3+ under the same conditions. Steady-state photo-EPR measurements of the most lightly doped sample imply the existence of an Fe2+/3+ defect level within 0.8 eV of the conduction band edge consistent with earlier work, but time-dependent measurements of more heavily doped crystals indicate a multi-step charge transfer process. Analysis of time-dependent photo-EPR data reveals that charge exchange may be separated into two processes, one that is temperature independent and one that depends monotonically on temperature. While a physical model for the charge transfer is not apparent, likely scenarios involve charge trapping at extended defects and phonon interactions.

Passivation and activation of Mg acceptors in heavily doped GaN

Electron paramagnetic resonance measurements are used to monitor the passivation and activation of the Mg-related acceptor in GaN doped with different concentrations of Mg, up to 2 × 1020 cm−3. Samples were annealed in either forming gas (H2:N2) or pure N2 between 200 and 900 °C. As expected, the Mg-related EPR signal is reduced by at least a factor of ten during the forming gas treatment; while the pure N2 environment revives the signal. However, the study also shows that reactions between Mg and hydrogen occur at a temperature as low as 525 °C in the 1020 cm−3 Mg doped samples; while in more lightly doped samples, temperatures greater than 700 °C are required to observe changes in the Mg signal intensity. While the observations support the model in which a hydrogen atom ionizes at the Mg impurity and the remaining proton bonds at a near neighbor, the different temperature dependence suggests that hydrogen diffusion is affected by the increased Mg concentration.

Measurements of optical cross sections of the carbon vacancy in 4H-SiC by time-dependent photoelectron paramagnetic resonance

Time-dependent photoinduced electron paramagnetic resonance measurements have been made on high purity semi-insulating 4H-SiC to develop a more complete understanding of the optical transitions of the positively charged carbon vacancy VC+. The single defect model originally proposed is given validity by demonstrating that the time dependence of the photoinduced changes in VC+ may be fitted by a first order kinetic process. In addition, the photon energy dependence of the optical cross sections for capture and emission of electrons from VC+ is extracted by incorporating both processes into one expression for charge transfer. The data are interpreted by considering the role of the electronic density of states as well as participation of phonons. Analysis assuming only phonon participation yields thermal and optical energies of 1.6 and 2.15 eV, respectively, for charge transitions between VC+ and one of the band edges. Charge transfer between VC+ and the opposite band edge is associated with a thermal and an...

Iron-related defect levels in SrTiO3 measured by photoelectron paramagnetic resonance spectroscopy

The optical absorption of defect centers in SrTiO3 substrates grown by the Vernuil method are investigated using photoinduced electron paramagnetic resonance (photo-EPR) at room temperature. As received samples revealed EPR signatures for Fe3+, Cr3+, Mn4+, and an iron-oxygen vacancy complex, Fe3+VO. An anneal in dry oxygen at 800 °C decreased the intensity of all of the centers except, Mn4+. More significantly, the anneal induced a new charge state of Fe, Fe5+. Photo-EPR data of the annealed samples were interpreted in terms of two defect levels, the 4+ to 5+ transition of Fe (Fe4+/5+) at 2.5 eV below the conduction band edge and the 3+ to 4+ transition of the iron-oxygen vacancy complex (Fe3+/4+VO) at 1.2 eV above the valence band edge. The former is consistent with reported defect levels and the latter provides a modified value based on measurements over a larger portion of the band gap than had been reported previously.

A model for Be-related photo-absorption in compensated GaN:Be substrates

A photo-induced electron paramagnetic resonance (EPR) attributed to beryllium-related acceptors was identified in GaN:Be substrates grown by the high nitrogen pressure solution technique. The acceptors, initially compensated by shallow O-related donors, were observed after illumination with photon energy greater than 2.7 eV. To adequately fit the time-dependent photo-EPR data over time periods up to 90 min, a two-defect model was developed based on three charge transfer processes: (1) photo-excitation of electrons from compensated acceptors, (2) electron capture by the positively charged donors and neutral acceptors directly from the conduction band, and (3) electron transfer from the donors to acceptors. The analysis of the spectral dependence of the optical cross section leads to the Be-related acceptor level lying 0.7 eV above the valence band maximum, consistent with the role of the acceptor as a compensating center as well as the 2.2 eV luminescence that others observed from these and other GaN:Be sa...

Effect of local fields on the Mg acceptor in GaN films and GaN substrates

The electron paramagnetic resonance (EPR) spectrum of the Mg acceptor is studied in a variety of GaN samples, including mm-thick free-standing substrates and sub-micron heteroepitaxial films. The former allows a view of the acceptor unique for EPR—in an environment with less than 107 cm−3 dislocations and doping densities ranging from 4 × 1016 to 6 × 1018 cm−3. By probing Mg in a broad range of samples in one study a new feature of the acceptor emerges. The EPR data reveal an anisotropic line shape that reflects a change in the crystal field in the vicinity of the Mg acceptor. This feature must be accommodated by any of the models proposed for the Mg acceptor. Here, we show that one such previously proposed model agrees well with the EPR data obtained from the wide variety of samples studied. The work implies that Mg-doped GaN contains a common Mg-related defect which can be affected by local crystal fields established during growth.

Mg-hydrogen interaction in AlGaN alloys

It is well known that hydrogen passivation of Mg in Mg-doped GaN reduces free hole concentrations. While there are numerous studies of passivation of Mg in GaN, little work has been reported concerning passivation rates in AlGaN alloys. We investigated the hydrogen interaction with Mg in nitrides by measuring the intensity of the electron paramagnetic resonance (EPR) signal associated with the acceptor. The samples were isothermally annealed in sequential steps ranging from 5 min - 6.6 h between 300 and 700 oC in H2:N2 (7%: 92%) or pure N2. The signal intensity decreased during the H2N2 anneal and was revived by the N2 anneal as expected; however, the rate at which the intensity changed was shown to depend on Al concentration. In addition, while all signals were quenched at 700 oC in H2:N2, a 750 oC N2 anneal reactivated only about 30% of the Mg in the alloys and 80% of the intensity in the GaN film. These data suggest that the rate of passivation and activation of Mg by hydrogen is dependent on the concentration of Al in the AlxGa-1xN layer. The EPR annealing data could prove to be beneficial in improving p-type optimization in AlGaN alloys.

Accurate gaseous ion mobility measurements

It is shown that theoretical values of the mobility of atomic ions in gases can be used to calibrate a drift-tube mass spectrometer, leading to subsequent measurements that are accurate to 0.6% for He+ in He near room temperature as the ratio of the electrostatic field strength to the gas number density ranges up to 2 × 10−19 Vm2. Values of the ratio of the parallel diffusion coefficient to the mobility are also obtained.

Intrinsic Surface Defects on 4H SiC Substrates

We have investigated a point defect, common to all SiC substrates, that is thought to be a broken carbon bond. Electron paramagnetic resonance spectroscopy performed in combination with three different etching methods using p -type, n -type, and semi-insulating substrates demonstrate that the center lies near the surface of a wafer. The results suggest that on the order of 10 13 cm -2 defects are removed within the first micron of the surface of a wafer.