Z-Q. Fang

Deep centers in n-GaN grown by reactive molecular beam epitaxy

Deep centers in Si-doped n-GaN layers grown by reactive molecular beam epitaxy have been studied by deep-level transient spectroscopy as a function of growth conditions. Si-doped GaN samples grown on a Si-doped n+-GaN contact layer at 800 °C show a dominant trap C1 with activation energy ET=0.44 eV and capture cross-section σT=1.3×10−15 cm−2, while samples grown at 750 °C on an undoped semi-insulating GaN buffer show prominent traps D1 and E1, with ET=0.20 eV and σT=8.4×10−17 cm2, and ET=0.21 eV and σT=1.6×10−14 cm2, respectively. Trap E1 is believed to be related to a N-vacancy defect, since the Arrhenius signature for E1 is very similar to the previously reported trap E, which is produced by 1-MeV electron irradiation in GaN materials grown by both metalorganic chemical-vapor deposition and hydride vapor-phase epitaxy.

Electron-Irradiation-Induced Deep Level in n -Type GaN

Deep-level transient spectroscopy measurements of n-type GaN epitaxial layers irradiated with 1-MeV electrons reveal an irradiation-induced electron trap at EC−0.18 eV. The production rate is approximately 0.2 cm−1, lower than the rate of 1 cm−1 found for the N vacancy by Hall-effect studies. The defect trap cannot be firmly identified at this time.

Traps in AlGaN/GaN/SiC heterostructures studied by deep level transient spectroscopy

AlGaN∕GaN∕SiC Schottky barrier diodes (SBDs), with and without Si3N4 passivation, have been characterized by temperature-dependent current-voltage and capacitance-voltage measurements, and deep level transient spectroscopy (DLTS). A dominant trap A1, with activation energy of 1.0 eV and apparent capture cross section of 2×10−12cm2, has been observed in both unpassivated and passivated SBDs. Based on the well-known logarithmic dependence of DLTS peak height with filling pulse width for a line-defect related trap, A1, which is commonly observed in thin GaN layers grown by various techniques, is believed to be associated with threading dislocations. At high temperatures, the DLTS signal sometimes becomes negative, likely due to an artificial surface-state effect.

On the main irradiation-induced defect in GaN

We show that the usual Arrhenius analysis of the main electron-irradiation-induced defect trap in n-type GaN, observed by deep-level transient spectroscopy (DLTS), is not sufficiently accurate. Instead, an exact fitting of the DLTS spectrum for this trap reveals two components, each of which has a thermal energy near 60 meV, not the apparent 140–200 meV, as given in other DLTS studies. This result resolves the discrepancy between Hall-effect and DLTS determinations of the thermal energy of this defect center.

Evolution of deep centers in GaN grown by hydride vapor phase epitaxy

Evolution of deep centers in GaN grown by hydride vapor phase epitaxy Z-Q. Fang and D.C. Look Semiconductor Research Center, Wright State University, Dayton, Ohio 45435 J. Jasinski, M. Benamara, and Z Liliental-Weber Lawrence Berkeley National Laboratory, Berkeley, California 94720 R.J. Molnar Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, Massachusetts Abstract Deep centers and dislocation densities in undoped n GaN, grown by hydride vapor phase epitaxy (HVPE), were characterized as a function of the layer thickness by deep level transient spectroscopy and transmission electron microscopy, respectively. As the layer thickness decreases, the variety and concentration of deep centers increase, in conjunction with the increase of dislocation density. Based on comparison with electron- irradiation induced centers, some dominant centers in HVPE GaN are identified as possible point defects.

Deep traps in AlGaN/GaN heterostructures studied by deep level transient spectroscopy: Effect of carbon concentration in GaN buffer layers

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...

Thermally stimulated current trap in GaN

A thermally stimulated current peak, occurring at 100 K for a heating rate of 0.4 K/s, has been found in semi‐insulating GaN grown by molecular beam epitaxy. This peak has contributions from two traps, with the main trap described by the following parameters: emission thermal activation energy E≂90±2 meV, effective capture cross‐section σ≂3±1×10−22 cm−2, and Nμτ≂3±1 × 1014 cm−1 V −1, where N is the trap concentration, μ the mobility, and τ the free‐carrier lifetime. This trap is much deeper than the typical shallow donors in conducting GaN, but shallower than any of the centers reported in recent deep level transient spectroscopy measurements.

Nonalloyed Ohmic Contacts on Low-Temperature Molecular-Beam Epitaxial GaAs: Influence of Deep Donor Band

The Ohmic nature of the nonalloyed metal contact on molecular beam epitaxial GaAs grown at 200 °C was studied. The specific contact resistances at room temperature and 120 K were 1.5×10−3 and 7.0×10−1 Ω cm2, respectively. These values are anomalously low considering that the conduction‐band electron concentration in this material is less than 1011 cm−3 at room temperature. The experimental results indicate that the carrier transport at the metal/semiconductor interface is dominated by a dense (∼3×1019 cm−3) EL2‐like deep donor band, rather than the usual conduction band.

Plasma-Etching-Enhanced Deep Centers in n-GaN Grown by Metalorganic Chemical-Vapor Deposition

By using deep-level transient spectroscopy (DLTS), deep centers have been characterized in unintentionally doped n-GaN samples grown by metalorganic chemical-vapor deposition and subjected to inductively coupled plasma reactive ion etching. At least six DLTS traps exist in the control sample: A1 (∼0.90 eV), Ax (∼0.72 eV), B (0.61 eV), C1 (0.44 eV), D (0.25 eV), and E1 (0.17 eV), with B dominant. Then, as the etching bias-voltage increases from −50 to −150 V, trap D increases strongly and becomes dominant, while traps A1, C (0.34 eV), and E1 increase at a slower rate. Trap B, on the other hand, is nearly unchanged. Previous electron-irradiation studies are consistent with the E1 traps being N-vacancy related. It is likely that the D traps are also, except that they are in the regions of dislocations.

Zn- and O-face polarity effects at ZnO surfaces and metal interfaces

Depth-resolved cathodoluminescence spectroscopy, current-voltage, capacitance-voltage, and deep level transient spectroscopy of ZnO (0001) Zn- and (0001¯) O-polar surfaces and metal interfaces show systematically higher Zn-face near band edge emission and lower near-surface defect emission. Even with remote plasma decreases of the 2.5 eV near-surface defect emission, (0001)-Zn face emission quality still exceeds that of (0001¯)-O face. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O2/He plasma-cleaned surfaces display a strong polarity dependence that correlates with defect emissions, traps, and interface chemistry. A comprehensive model accounts for the polarity-dependent transport properties and their correlations with carrier concentration profiles.Depth-resolved cathodoluminescence spectroscopy, current-voltage, capacitance-voltage, and deep level transient spectroscopy of ZnO (0001) Zn- and (0001¯) O-polar surfaces and metal interfaces show systematically higher Zn-face near band edge emission and lower near-surface defect emission. Even with remote plasma decreases of the 2.5 eV near-surface defect emission, (0001)-Zn face emission quality still exceeds that of (0001¯)-O face. Ultrahigh vacuum-deposited Au and Pd diodes on as-received and O2/He plasma-cleaned surfaces display a strong polarity dependence that correlates with defect emissions, traps, and interface chemistry. A comprehensive model accounts for the polarity-dependent transport properties and their correlations with carrier concentration profiles.