Brian Hertog

Very high channel conductivity in low-defect AlN/GaN high electron mobility transistor structures

Low defect AlN/GaN high electron mobility transistor (HEMT) structures, with very high values of electron mobility (>1800u2002cm2/Vu2009s) and sheet charge density (>3×1013u2002cm−2), were grown by rf plasma-assisted molecular beam epitaxy (MBE) on sapphire and SiC, resulting in sheet resistivity values down to ∼100u2002Ω/◻ at room temperature. Fabricated 1.2u2002μm gate devices showed excellent current-voltage characteristics, including a zero gate saturation current density of ∼1.3u2002A/mm and a peak transconductance of ∼260u2002mS/mm. Here, an all MBE growth of optimized AlN/GaN HEMT structures plus the results of thin-film characterizations and device measurements are presented.

Site-selective spectroscopy of Er in GaN

We investigated different Er3+ defect sites found in Er-doped GaN layers by site-selective combined excitation-emission spectroscopy and studied the role of these sites in different direct and multistep excitation schemes. The layers were grown by molecular beam epitaxy and were 200 nm thick. Two majority sites were found along with several minority sites. The sites strongly differ in excitation and energy transfer efficiencies as well as branching ratios during relaxation. For this reason, relative emission intensities from these sites depend strongly on emission and excitation. The sites were identified for several transitions and a comprehensive list of energy levels has been compiled. One of the minority sites appears strongly under ultraviolet excitation above the GaN band gap suggesting that this site is an excellent trap for excitation energy of electron-hole pairs.

Hybrid CdZnO/GaN quantum-well light emitting diodes

We report on the demonstration of light emission from hybrid CdZnO quantum-well light emitting diodes. A one-dimensional drift-diffusion method was used to model the expected band structure and carrier injection in the device, demonstrating the potential for 90% internal quantum efficiency when a CdZnO quantum well is used. Fabricated devices produced visible electroluminescence that was found to redshift from 3.32 to 3.15 eV as the forward current was increased from 20 to 40 mA. A further increase in the forward current to 50 mA resulted in a saturation of the redshift.

Minority carrier transport in p-type Zn0.9Mg0.1O doped with phosphorus

Minority carrier diffusion length and lifetime in p‐Zn0.9Mg0.1O doped with phosphorus were obtained from local electron beam irradiation measurements. The irradiation resulted in an increase of up to 25% in minority electron diffusion length from the initial value of ∼2.12μm and in a simultaneous decrease of the peak near-bandedge cathodoluminescence intensity. The observed phenomena are attributed to charging of phosphorus-related deep acceptor level(s), which is consistent with the activation energy of 256±20meV found for the effect of electron injection in Zn0.9Mg0.1O.

Yellow luminescence in ZnO layers grown on sapphire

We conducted a detailed study of the yellow luminescence (YL) band that has a maximum of 2.19eV at 10K in undoped and N-doped ZnO layers grown on sapphire substrates. Important characteristics of this band and the related defect are established. The YL band is attributed to a transition between a shallow donor and an acceptor with an energy level ∼0.4eV above the valence band. Quenching of the YL intensity with activation energies of 85meV and 0.4eV is observed at temperatures above 100 and 320K, respectively. The YL band is possibly due to a defect complex that may include a Zn vacancy.

Thermal stability of CdZnO∕ZnO multi-quantum-wells

The thermal stability of CdZnO∕ZnO multi-quantum-well (MQW) structures was studied using rapid thermal annealing in nitrogen from 300to750°C. Photoluminescence (PL) emission from the MQWs was studied while varying the annealing temperature and time. For 15min annealings, the PL center wavelength showed a 7nm reduction for temperatures up to 650°C. Above 650°C, the wavelength changed rapidly, with a 50nm reduction at 750°C. Annealing at 700°C for up to 20min produced a systematic reduction in PL wavelength up to 39nm. The data suggest that CdZnO∕ZnO MQWs are relatively stable for nitrogen annealing below 650°C for times up to 15min.

Effects of hydrogen on the optical properties of ZnCdO/ZnO quantum wells grown by molecular beam epitaxy

Temperature-dependent cw- and time-resolved photoluminescence (PL), as well as optically detected magnetic resonance (ODMR) measurements are employed to evaluate effects of deuterium (2H) doping on optical properties of ZnCdO∕ZnO quantum well structures grown by molecular beam epitaxy. It is shown that incorporation of H2 from a remote plasma causes a substantial improvement in radiative efficiency of the investigated structures. Based on transient PL measurements, the observed improvements are attributed to efficient passivation by hydrogen of competing nonradiative recombination centers via defects. This conclusion is confirmed from the ODMR studies.

Migration and luminescence enhancement effects of deuterium in ZnO∕ZnCdO quantum wells

ZnO∕ZnCdO∕ZnO multiple quantum well samples grown on sapphire substrates by molecular beam epitaxy and annealed in situ were exposed to D2 plasmas at 150°C. The deuterium showed migration depths of ∼0.8μm for 30min plasma exposures, with accumulation of H2 in the ZnCdO wells. The photoluminescence (PL) intensity from the samples was increased by factors of 5 at 5K and ∼20 at 300K as a result of the deuteration, most likely due to passivation of competing nonradiative centers. Annealing up to 300°C led to increased migration of H2 toward the substrate but no loss of deuterium from the sample and little change in the PL intensity. The initial PL intensities were restored by annealing at ⩾400°C as H2 was evolved from the sample (∼90% loss by 500°C). By contrast, samples without in situ annealing showed a decrease in PL intensity with deuteration. This suggests that even moderate annealing temperatures lead to degradation of ZnCdO quantum wells.

Vertical solar blind Schottky photodiode based on homoepitaxial Ga2O3 thin film

High quality germanium doped β-Ga2O3 epitaxial film was grown by PMBE technique and fabricated into a vertical type Schottky photodiode with a Pt/nGa2O3/n+Ga2O3(010) structure. The photodiode exhibited excellent rectifying characteristics with a turn on voltage ~ 1V and near zero bias leakage current ~ 100 fA. The photoresponse measurement showed a true solar blind sensitivity with cutoff wavelength ~260 nm and an out of band rejection ratio of ~104. A maximum responsivity of 0.09 A/W at 230 nm was measured at zero bias, corresponding to an external quantum efficiency of ~52 %. The time response of the photovoltaic diode is in the millisecond range and has no long-time decay component which is very common in the MSM photoconductive wide bandgap devices. The photodiode performance remains stable up to 300°C, suggesting its potential use for high temperature applications.

High Mg content wurtzite phase MgxZn1-xO epitaxial film grown via pulsed-metal organic chemical vapor deposition (PMOCVD)

We report on high quality, wurtzite MgxZn1-xO (MgZnO) epitaxial films grown via the PMOCVD method with a record high Mg content up to 51 %. A series of MgZnO films with various Mg content were grown on ZnO (~30 nm)/Al2O3(0001) and ZnO (~30 nm)/AlN (~25 nm)/Al2O3(0001) substrates. The band gap for the films estimated using UV-visible transmission spectroscopy ranges from 3.24 - 4.50 eV, corresponding to the fraction of Mg between x=0.0 to x=0.51, as determined by Rutherford backscattering spectroscopy (RBS). The cathodoluminescence (CL) measurement showed a blue shift in the spectral peak position of MgZnO, indicating an increase in Mg content. No multi-absorption edges and CL band splitting were observed, suggesting the absence of phase segregation in the as grown films. The phase purity and crystal structure of the films were further confirmed by XRD. The absence of phase separation is attributed to the fast periodic transition steps in the PMOCVD, creating a non-equilibrium system where radicals that are formed will have insufficient time to reach their energy minimum. AFM analysis of the films had decreasing surface roughness with increasing Mg content. MSM photodetector was fabricated from the films to characterize the spectral response. The devices exhibit peak response ranging between 276 - 383 nm, covering a large portion of the solar blind spectral window. Moreover, the Schottky barrier was enhanced by treating the MgZnO surface with H2O2, reducing the device’s dark current.