D. R. Hang

AlxGa1−xN/GaN band offsets determined by deep-level emission

We present studies of the compositional dependence of the optical properties of AlxGa1−xN(0<x<0.22) alloys by modulation spectroscopy and photoluminescence. The yellow luminescence, which is well known in GaN and is generally assigned to shallow donor–deep acceptor pair recombination has also been observed in AlxGa1−xN. As aluminum concentration increases, the color of the band changes from yellow (2.2 eV) to blue (2.6 eV). The shift was less than that of the band gap. Together with previously published studies, it implies that the deep acceptor level is pinned to a common reference level to both materials, thus the deep level responsible for the yellow emission is used as a common reference level to determine the band alignment in AlxGa1−xN/GaN heterojunctions. Combining with the near-band-edge modulation spectra, the estimated ratio of conduction-to-valence band discontinuity is 65:35. Our results are close to the values obtained from PL measurements on Al0.14Ga0.86N/GaN quantum wells and those calculat...

Crystal growth and properties of LiAlO2 and nonpolar GaN on LiAlO2 substrate

In this study, the growth and properties of LiAlO2 material and a nonpolar GaN-based light-emitting-diode (LED) structure on LiAlO2 have been investigated. The LiAlO2 material is grown by the Czochralski pulling technique and is used as a substrate for nonpolar nitride growth. An improved surface roughness can be obtained by a four-step polishing process. With subsequent nitridation treatment, a pure M-plane (1010) GaN can be obtained. An electron microscope shows an abundance of cracks that are oriented parallel to the (001) and (100) planes of the LiAlO2 substrate on the rear surface of GaN. The absence of the polarization-induced electric field of a GaN-based LED structure on LiAlO2 was shown by using photoluminescence measurements. Therefore, this approach is promising to further increase the luminescence performance of GaN-based LEDs.

Transport in a gated Al0.18Ga0.82N/GaN electron system

We have investigated the low-temperature transport properties of front-gated Al0.18Ga0.82N/GaN heterostructures. At zero gate voltage, the Hall mobility increases with decreasing temperature (20 K⩽T⩽190 K) due to a reduction in phonon scattering. For T⩽20 K, the mobility decreases with decreasing temperature. This is due to weak localization in a weakly disordered two-dimensional system. By changing the applied gate voltage, we can vary the carrier density n from 3.11×1012 to 6.95×1012 cm−2 in our system. The carrier density shows a linear dependence on the applied gate voltage, consistent with a simple parallel-plate capacitor model. The average distance between the GaN electron system and the AlGaN/GaN interface is estimated to be 240 A. At high carrier densities (n>4.65×1012 cm−2), the measured mobility (μ) is found to be a decreasing function of carrier density as μ∼n−0.31. Loss of mobility with increasing carrier density is dominated by interface roughness scattering. At low carrier densities (n 4.65×1012 cm−2), the measured mobility (μ) is found to be a decreasing function of carrier density as μ∼n−0.31. Loss of mobility with increasing carrier density is dominated by interface roughness scattering. At low carrier densities (n<4.24...

Electrically detected and microwave-modulated Shubnikov–de Haas oscillations in an Al0.4Ga0.6N/GaN heterostructure

We report the drastic enhancement pattern of Shubnikov–de Haas (SdH) oscillations observed in an AlGaN/GaN heterostructure by microwave modulation. The dependence of the SdH pattern on microwave power and temperature is investigated. The underlying mechanism is attributed to the effect of carrier heating. This technique helps study the transport properties of two-dimensional electrons in many wide-band-gap heterostructures, in which moderate mobilities and heavier electron effective mass (rapidly damping SdH amplitudes) are frequently encountered. In addition, this method has the advantage of keeping the carrier concentration fixed and not requiring expensive high-energy laser facilities compared with carrier-modulated SdH measurements.

Effective mass of two-dimensional electron gas in an Al0.2Ga0.8N/GaN heterojunction

We have performed a magnetotransport study on an AlGaN/GaN heterostructure at low temperatures. The effective-mass values have been evaluated by analyzing the exact form of the temperature-dependent Shubnikov–de Haas oscillation function. The values obtained increase with the magnetic field. This mass enhancement is attributed to conduction-band nonparabolicity. The effective-mass variation with the magnetic field was extrapolated to zero field, together with further correction due to the triangular confinement of the carriers, yielding an effective mass of 0.185±0.005 of the free-electron mass. Our result is in excellent agreement with the results obtained by first-principle calculations and the tight-binding method, and suggest the significance of magnetic-field-induced nonparabolicity in transport measurements.

Room-temperature violet luminescence and ultraviolet photodetection of Sb-doped ZnO/Al-doped ZnO homojunction array

A Sb-doped ZnO microrod array was fabricated on an Al-doped ZnO thin film by electrodeposition. Strong violet luminescence, originated from free electron-to-acceptor level transitions, was identified by temperature-dependent photoluminescence measurements. This acceptor-related transition was attributed to substitution of Sb dopants for Zn sites, instead of O sites, to form a complex with two Zn vacancies (VZn), the SbZn-2VZn complex. This SbZn-2VZn complex has a lower formation energy and acts as a shallow acceptor which can induce the observed strong violet luminescence. The photoresponsivity of our ZnO p-n homojunction device under a negative bias demonstrated a nearly 40-fold current gain, illustrating that our device is potentially an excellent candidate for photodetector applications in the ultraviolet wavelength region.

Optical characteristics of nonpolar a-plane ZnO thin film on (010) LiGaO2 substrate

We investigate the optical properties of non-polar a-plane ZnO film grown on (010) LiGaO2 substrate by chemical vapor deposition. X-ray diffraction indicates the a-plane orientation and Raman spectroscopy reveals the phase purity. Four distinct features in the near band-edge range are identified as neutral-donor-bound-exciton (D°X), free-exciton (FX), free-to-bound, and donor-acceptor pair transitions. The thermal activation energy of D°X is 12 meV while an acceptor energy level of 124 meV is estimated. Temperature evolution of photoluminescence (PL) shows that the room-temperature luminescence is a mixture of free-to-bound and FX transitions. The polarization dependence of the near-band-edge transitions were investigated. The largest degree of polarization of 95.5% occurs in FX transition. It can be attributed to anisotropic in-plane strain and the nonpolar a-plane orientation.

Improved quality of nonpolar m-plane GaN [101¯0] on LiAlO2 substrate using a modified chemical vapor deposition

Nonpolar GaN crystal on lattice-matched [100] γ-LiAlO2 substrate was grown by a newly designed chemical vapor deposition (CVD) reactor. Following the CVD growth, x-ray diffraction indicated that the GaN film was oriented in the nonpolar m-plane with [101¯0] orientation. Further structural characterizations and defect analysis of nonpolar GaN material was performed using transmission electron microscope. Low-temperature photoluminescence was dominated by neutral donor bound excitons and the yellow luminescence was negligible. Raman spectroscopy showed that the as-grown GaN {101¯0} epilayer on [100] γ-LiAlO2 substrate were indeed of good quality. Compared to the previous report, nonpolar GaN with an improved quality was demonstrated by modifying the inner structure of the CVD reactor.

Two-subband-populated AlGaN∕GaN heterostructures probed by electrically detected and microwave-modulated magnetotransport measurements

The authors apply the microwave-modulated technique to study the transport properties of two-subband-populated AlGaN∕GaN heterostructures. The microwave modulation enhances the small Shubnikov–de Haas oscillations at low magnetic fields, providing a direct way to compare the mobilities of different subbands from the experimental data. In addition, this technique can help us to determine the subband-energy separation, especially when the population of the second subband is much lower than that of the first one. Variation of subband-energy separation due to different spacer thicknesses is obtained. Therefore, the authors showed a powerful way to probe parameters of two-subband-populated AlGaN∕GaN heterostructures.

Persistent photoconductivity in InxAlyGa1−x−yN quaternary alloys

The optical properties of InxAlyGa1−x−yN quaternary alloys were investigated by photoconductivity (PC), persistent photoconductivity (PPC), photoluminescence (PL), and photoluminescence excitation (PLE) measurements. Quite interestingly, persistent photoconductivity was observed. Through the combination of our optical studies, we show that the PPC effect arises from composition fluctuations in InxAlyGa1−x−yN quaternary alloys. From the analysis of the decay kinetics, the localization depth caused by composition fluctuations was determined. A comparison between the PL, PLE, and PC measurements gives a direct access to the Stokes’ shift. The Stokes’ shift can be explained in terms of localization due to the existence of nanoscale clusters, and it is consistent with the PPC result. The results shown here provide concrete evidence to support our previously proposed model that the existence of InGaN-like clusters is responsible for the strong luminescence in InxAlyGa1−x−yN quaternary alloys.