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The consequences of ammonia flow rate on the impurity incorporation and material characteristics of Si-doped GaN films grown by reactive molecular beam epitaxy (RMBE) procedure were studied. The ammonia flow rate apparently has a marginal effect on the introduction of impurities into the Si-doped GaN films except that there was a little decrease in O and Si with increasing ammonia flow rate when the Si concentration in the film is larger than 10(super 18) cm(super -3).

Researchers conducted a range of tests to study the effects of high-temperature annealing at 1100 degrees centigrade on undoped semi-insulating GaAs. The high-temperature annealing introduces a number of effects not caused by standard 1000 degrees centigrade annealing. Theeffects are higher 4.2-K PL emission band intensities at 0.8 eV together with decreased PL intensity of the transition, and higher concentrations of both NTSC traps and acceptor centres. Slow cooling causes some reduction in the additional centres.

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.

The deep trap energy (E(T)) and deep trap concentration (N(T)) in GaAs are determined by a new method, which does not require the emission-rate data. The method involves measurement of capacitance at different forward bias voltages. The concentration of the various deep trap centers such as EL6, EL3 and EL2 are determined by considering the lambda effect. N(EL2) has a value of 1.14 times 10 to the 16th power per cubic centimeter and E(EL2) has a value of 0.71 electron volts. N(EL6) has a value of 8 times 10 to the 15th power per cubic centimeter and E(EL6) has a value of 0.42 electron volts.

Deep-level transient spectroscopic studies of molecular-beam epitaxially-grown, Si-doped GaAs layers reveals an 0. 65 eV electron trap associated with As(Ga) lambda effects. The overlapping peaks are studied numerically. The electron trap is also related to the EB4 trap in electron irradiated GaAs layers.

The photoquenching and subsequent thermal recovery of a prominent thermally stimulated current peak observed in semi-insulating GaAs was investigated. The results showed that the current peak photoquenched after exposure to infrared illumination and recovered thermally at the rate 2. 0x10(super 8) exp(-0.26 eV/kT). It was proposed that the the current peak and the midgap donor level EL2 were microscopically similar and that the current peak was actually an electron trap.

Depth-resolved cathodoluminescence spectroscopy (DRCLS) reveals the evolution of surface and near surface defects at polar surfaces with remote oxygen plasma (ROP) treatment. Furthermore, this evolution exhibits significant differences that depend on surface polarity. ROP decreased the predominant 2.5 eV defect emission related to oxygen vacancies on the O face, while creating a new 2.1 eV defect emission on the Zn face that increases with ROP time. The surface-located 2.1 eV emission correlates with carrier profiles from capacitance-voltage measurements and a shift of the E3 trap to higher binding energy from deep level transient spectroscopy (DLTS). This result suggests that ROP generates Zn vacancies on the Zn face which act as compensating acceptors at the surface and in the near surface region. Secondary ion mass spectrometry (SIMS) shows no polarity dependence due to impurities. We conclude that the near-surface deep level optical emissions and free carrier densities of ZnO depend strongly on the ROP modulation of native defects related to Zn or O vacancies. [ABSTRACT FROM AUTHOR]

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed DLTS measurements on the two SBDs show that (i) different trap species are seen in the two SBDs: electron traps Ax (0.9 eV), A1 (0.99 eV), and A2 (1.2 eV), and a holelike trap H1 (1.24 eV) in the low-[C] SBD; and electron traps A1, A2, and A3 (∼1.3 eV), and a holelike trap H2 (>1.3 eV) in the high-[C] SBD; (ii) for both SDBs, in the region close to GaN buffer layer, only electron traps can be detected, while in the AlGaN/GaN interface region, significant holelike traps appear; and iii) all of the deep traps show a strong dependence of the DLTS signal on filling pulse width, which indicates they are associated with extended defects, such as threading dislocations. However, the overall density of electron traps is lower in the low-[C] SBD than in the high-[C] SBD. The different traps observed in the two SBDs are thought to be mainly related to differences in microstructure (grain size and threading dislocation density) of GaN buffer layers grown at different pressures. [ABSTRACT FROM AUTHOR]

Growth of unintentionally doped (UID) semi-insulating GaN on SiC and highly resistive GaN on sapphire using the ammonia molecular-beam epitaxy technique is reported. The semi-insulating UID GaN on SiC shows room temperature (RT) resistivity of 1011 Ω cm and well defined activation energy of 1.0 eV. The balance of compensation of unintentional donors and acceptors is such that the Fermi level is lowered to midgap, and controlled by a 1.0 eV deep level defect, which is thought to be related to the nitrogen antisite NGa, similar to the “EL2” center (arsenic antisite) in unintentionally doped semi-insulating GaAs. The highly resistive GaN on sapphire shows RT resistivity in range of 106–109 Ω cm and activation energy varying from 0.25 to 0.9 eV. In this case, the compensation of shallow donors is incomplete, and the Fermi level is controlled by levels shallower than the 1.0 eV deep centers. The growth mechanisms for the resistive UID GaN materials were investigated by experimental studies of the surface kinetics during growth. The required growth regime involves a moderate growth temperature range of 740–780 °C, and a high ammonia flux (beam equivalent pressure of 1×10-4 Torr), which ensures supersaturated coverage of surface adsorption sites with NHx radicals. Such highly nitrogen rich growth conditions lead to two-dimensional layer by layer growth and reduced oxygen incorporation. [ABSTRACT FROM AUTHOR]

Effects of 1 MeV electron-irradiation at room temperature on the electrical properties of AlGaN/GaN heterostructures, including leakage currents, threshold voltages, and electron traps, have been investigated using Schottky barrier diodes (SBDs) fabricated on the AlGaN. The SBDs, before and after the irradiation with a dose of 5×1015 cm-2, were characterized by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy. It is found that the irradiation causes (i) significant increase in leakage currents, dominated by tunneling conduction, at both reverse and low-forward biases; (ii) a clear negative shift in threshold voltage in the pinch-off region; and (iii) creation of traps Ae(∼1.1 eV) or A2(1.2 eV) and Ee(0.09 eV) in the GaN buffer and AlGaN regions. The irradiation-induced traps can be used to account for the increase in leakage currents and shift in threshold voltage. However, as compared to traps A2(1.2 eV) and E(0.13 eV) induced in thick GaN layers by electron-irradiation, the irradiation-induced traps in the AlGaN/GaN heterostructures show some changes in activation energy and electron capture behavior. [ABSTRACT FROM AUTHOR]

Deep level transient spectroscopy, current-voltage, and capacitance-voltage measurements are used to study interface traps in metal-on-bulk-ZnO Schottky barrier diodes (SBDs). c-axis-oriented ZnO samples were cut from two different vapor-phase-grown crystals, and Au- and Pd-SBDs were formed on their (0001) surfaces after remote oxygen-plasma treatment. As compared to Au-SBDs, the Pd-SBDs demonstrated higher reverse-bias leakage current and forward-bias current evidently due to higher carrier concentrations, which might have been caused by hydrogen in-diffusion through the thin Pd metal. The dominant traps included the well-known bulk traps E3 (0.27 eV) and E4 (0.49 eV). In addition, a surface-related trap, Es (0.49 eV), is observed but only in the Pd-SBDs, not in the Au-SBDs. Trap Es is located at depths less than about 95 nm and shows an electron capture behavior indicative of extended defects. A possible correspondence between trap Es and the well-known 2.45 eV green band is suggested by depth-resolved cathodoluminescence spectroscopy on the same samples, which reveals an increase in the intensity of this band within ∼100 nm of the Pd/ZnO interface. [ABSTRACT FROM AUTHOR]

Thermally stimulated current (TSC) spectroscopy and temperature–dependent dark current (DC) measurements have been applied to study traps and photoinduced persistent surface conduction in two hydrothermally grown bulk ZnO samples, as-grown, and annealed at 600 °C in N2 ambient for 30 min, respectively. The as-grown sample had a room-temperature (RT) resistivity of 1.6×103 Ω cm, mobility of 2.1×102 cm2/V s, and carrier concentration of 1.8×1013 cm-3, while the annealed sample was highly resistive, with RT resistivity of 3.6×106 Ω cm, mobility of 4.4 cm2/V s, and carrier concentration of 3.9×1011 cm-3. The as-grown sample showed strong conduction at low temperatures, which has been shown to be due to near-surface carriers in other studies. The annealed sample did not demonstrate this phenomenon. The dominant trap in the as-grown sample had an activation energy of 0.16 eV, was strongest near the surface, and is possibly related to VZn. In the annealed sample, however, the dominant trap had an activation energy of 0.22 eV, was of bulk nature, and is tentatively assigned to LiZn. After several routine TSC measurements, the DC for the as-grown sample increased by more than one order of magnitude at low temperatures (T<180 K), while for the annealed sample, the DC increased by a factor of 2 at high temperatures (T>200 K). These effects are generated by the TSC trap-filling illumination and can persist for many days under vacuum. At RT, the DC in the annealed sample returns to its equilibrium state if the sample is vented to air. [ABSTRACT FROM AUTHOR]

Electron and hole traps in N-doped ZnO were investigated using a structure of n+-ZnO:Al/i-ZnO/ZnO:N grown on a p-Si substrate by metalorganic chemical vapor deposition (for growth of the ZnO:N layer) and sputtering deposition (for growth of the i-ZnO and n+-ZnO:Al layers). Current-voltage and capacitance-voltage characteristics measured at temperatures from 200 to 400 K show that the structure is an abrupt n+-p diode with very low leakage currents. By using deep level transient spectroscopy, two hole traps, H3 (0.35 eV) and H4 (0.48 eV), are found in the p-Si substrate, while one electron trap E3 (0.29 eV) and one hole trap H5 (0.9 eV) are observed in the thin ZnO:N layer. Similarities to traps reported in the literature are discussed. [ABSTRACT FROM AUTHOR]

Temperature dependent Hall effect (TDH), low temperature photoluminescence (LTPL), secondary ion mass spectrometry (SIMS), optical admittance spectroscopy (OAS), and thermally stimulated current (TSC) measurements have been made on 6H-SiC grown by the physical vapor transport technique without intentional doping. n- and p-type as well semi-insulating samples were studied to explore the compensation mechanism in semi-insulating high purity SiC. Nitrogen and boron were found from TDH and SIMS measurements to be the dominant impurities that must be compensated to produce semi-insulating properties. The electrical activation energy of the semi-insulating sample determined from the dependence of the resistivity was 1.0 eV. LTPL lines near 1.00 and 1.34 eV, identified with the defects designated as UD-1 and UD-3, were observed in all three samples but the intensity of the UD-1 line was almost a factor of 10 more in the n-type sample than in the the p-type sample with that in the semi-insulating sample being intermediate between those two. OAS and TSC experiments confirmed the high purity of this material. The results suggest that the relative concentrations of a dominant deep level and nitrogen and boron impurities can explain the electrical properties in this material. [ABSTRACT FROM AUTHOR]

Deep-level transient spectroscopy measurements were performed in order to investigate the effects of substrate growth temperature and dopant species on deep levels in Si layers during low-temperature molecular beam epitaxial growth. The structures studied were n[sup +]-p junctions using B doping for the p layer and p[sup +]-n junctions using P doping for the n layer. While the density of hole traps H1 (0.38–0.41 eV) in the B-doped p layers showed a clear increase with decreasing growth temperature from 600 to 370 °C, the electron trap density was relatively constant. Interestingly, the minority carrier electron traps E1 (0.42–0.45 eV) and E2 (0.257 eV), found in the B-doped p layers, are similar to the majority carrier electron traps E11 (0.48 eV) and E22 (0.269 eV) observed in P-doped n layers grown at 600 °C. It is hypothesized that these dominating electron traps are associated with pure divacancy defects and are independent of the dopant species. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Deep level electron and hole traps were studied by means of deep level transient spectroscopy with electrical and optical injection on a freestanding thick n-GaN sample with low dislocation density. It is shown that at both the upper and the lower surface of the sample there exists a thin, ~0.5 μm layer of damaged material with lowered concentration of electrons and enhanced density of deep centers. Deep in the bulk of the film the densities of the majority of the electron and hole traps are shown to be very low, but measurably higher on the lower face (N face), which was originally closer to the Al[SUB2]O[SUB3] substrate. The two faces are also shown to similarly differ in the density of deep hole traps whose concentration is deduced from low-temperature capacitance-voltage measurements in the dark and after illumination (such traps were previously associated with dislocation states). The concentration of persistent photocapacitance centers is shown to be very low on both sides and considerably lower than previously reported for other n-GaN samples with higher dislocation densities. Electron beam induced current measurements on both sides of the sample reveal the presence of dark spots whose density roughly correlates with the density of dislocations at the upper and lower faces. The reverse current at high voltages is shown to be considerably higher on the N face. The possible relationship of the observed phenomena to dislocations is discussed. [ABSTRACT FROM AUTHOR]

Concentrations of deep hole traps were measured in a set of hydride vapor phase epitaxy grown samples with thicknesses varying from 2.6 to 68 µm. Results were obtained from low temperature capacitance-voltage measurements before and after illumination and from deep level transient spectroscopy measurements with optical injection (ODLTS). The former revealed the presence of high densities (∼l0[sup 15] to 10[sup 16] cm[sup -3]) of hole traps whose concentration decreased with sample thickness in a manner similar to that found for the dislocation density. Capacitance versus temperature measurements in the dark and after illumination suggested that these traps form a band of states rather than a single level in the GaN band gap. It is suggested that such states could be associated with dislocations. The main hole traps observed by ODLTS were deep hole traps, of energy near E[sub υ] × 0.9 eV. Their density was also observed to substantially decrease with sample thickness. [ABSTRACT FROM AUTHOR]