Interest in Be as a shallow acceptor in III-Nitrides is growing. Recently, we reported the first systematic study of MOCVD growth of GaN:Be. It is well known that impurities tend to segregate at threading dislocations in GaN. Despite the relatively high quality of MOCVD-grown GaN, lattice mismatch with foreign substrates such as sapphire usually results in dislocation density on the order of 108 cm-2. In this study, we investigate the impact of substrate quality on the GaN:Be. The effects of the substrate lattice mismatch and dislocation density on total Be incorporation, optical characteristics, and Be activation efficiency are discussed.
We present the most recent results of photoluminescence (PL) studies, classification of defects in GaN and their properties. In particular, the yellow luminescence band (labeled YL1) with a maximum at 2.17 eV in undoped GaN grown by most common techniques is unambiguously attributed to the isolated CN acceptor. From the Zero-Phonon Line (ZPL) at 2.59 eV, the −/0 level of this acceptor is found at 0.916 eV above the valence band. The PL also reveals the 0/+ level of the CN at 0.33 eV above the valence band, which is responsible for the blue band (BLC), with the ZPL at 3.17 eV. Another yellow band (YL2) with a maximum at 2.3 eV, observed only in GaN grown by the ammonothermal method, is attributed to the VGa3H complex. The nitrogen vacancy (VN) causes the green luminescence (GL2) band. The VN also forms complexes with acceptors such as Mg, Be, and Ca. These complexes are responsible for the red luminescence bands (the RL2 family) in high-resistivity GaN. The results from PL studies are compared with theoretical predictions. Uncertainties in the parameters of defects are discussed.
P-type doping in III-Nitrides has long presented a challenge in the development of wide bandgap optoelectronic devices. To date, magnesium is the only commercially viable acceptor in III-Nitrides. Beryllium has been considered a potential alternative to magnesium, and initial theoretical calculations as well as photoluminescence studies suggested that it is shallower than magnesium in GaN. However, to date, there have been no reliable or repeatable examples of p-type GaN:Be in literature. Here, we present a systematic study of MOCVD-grown GaN:Be with varied doping conditions. All samples show prominent UV and yellow luminescence, characteristic of beryllium acceptor in GaN.
We have investigated point defects in GaN grown by HVPE by using steady-state and time-resolved photoluminescence (PL). Among the most common PL bands in this material are the red luminescence band with a maximum at 1.8 eV and a zero-phonon line (ZPL) at 2.36 eV (attributed to an unknown acceptor having an energy level 1.130 eV above the valence band), the blue luminescence band with a maximum at 2.9 eV (attributed to ZnGa), and the ultraviolet luminescence band with the main peak at 3.27 eV (related to an unknown shallow acceptor). In GaN with the highest quality, the dominant defect-related PL band at high excitation intensity is the green luminescence band with a maximum at about 2.4 eV. We attribute this band to transitions of electrons from the conduction band to the 0/+ level of the isolated CN defect. The yellow luminescence (YL) band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV. Another yellow luminescence band, which has similar shape but peaks at about 2.2 eV, is observed in less pure GaN samples and is attributed to the CNON complex. In semi-insulating GaN, the GL2 band with a maximum at 2.35 eV (attributed to VN) and the BL2 band with a maximum at 3.0 eV and the ZPL at 3.33 eV (attributed to a defect complex involving hydrogen) are observed. We also conclude that the gallium vacancy-related defects act as centers of nonradiative recombination.
N-type GaN exhibits upward, near-surface band bending that can be decreased by generating a surface photovoltage
(SPV). Fitting SPV measurements with a thermionic model based on the emission of charge carriers over the nearsurface
barrier provides information about the band bending in dark. We have studied the temperature dependent SPV
behavior from a Si-doped, n-type GaN sample grown by hydride vapor phase epitaxy in order to determine how the
magnitude of band bending changes at higher temperatures. We have measured the effect of temperature and oxygen on
the steady-state SPV behavior, where oxygen is photo-adsorbed on the surface under band-to-band illumination in an
air/oxygen ambient more efficiently at higher temperatures. As predicted, the intensity-dependent SPV measurements
performed at temperatures between 295 and 500 K exhibit a decrease in the maximum SPV with increasing temperature.
When illumination ceases, the band bending then begins to restore to its dark value with a rate proportional to the sample
temperature, which also fits a thermionic model.
We have studied the surface photovoltage (SPV) for band-to-band illumination on a variety of p-type (Mg-doped) GaN
samples. In particular, differences in the steady-state and transient SPV have been investigated in air and vacuum for
samples grown by hydride vapor phase epitaxy (HVPE) or metal-organic chemical vapor deposition (MOCVD). The
SPV spectra for both samples behave in a similar manner, but larger SPVs are generated for HVPE vs. MOCVD samples
under identical illumination conditions. Interestingly, we have found that p-type GaN can be sensitive to the illumination
geometry, where illumination of the electrical contacts results in an anomalous "offset" of the SPV signal. Regardless of
illumination geometry, such offsets always appear in the case of MOCVD samples, whereas they do not appear for
HVPE samples when the contacts are not illuminated. Since we have never observed such behavior for n-type samples, it
appears that the stability of p-type samples under illumination may be an issue.
We report on the effects of substrate temperature and surface morphology of p-GaN templates on the
properties of ZnO:Ga (GZO) layers grown by plasma-assisted molecular beam epitaxy. Substrate temperature varying
from 200 °C to 450°C was found to have only a moderate effect on the electrical properties of GZO films but it greatly
affects the surface morphology of the GZO films. The surface morphology and growth mode of GZO were also found to
be considerably affected by the surface morphology of underlying p-GaN templates. On p-GaN templates with a smooth
surface (RMS = 0.4 nm) featured by clear atomic steps, GZO layers grew in 2D growth mode and exhibited smooth
surfaces with RMS roughness of 2 nm. In contrast, on p-GaN without clear atomic steps but having comparable surface
roughness of 0.6 nm, GZO layers grew in 3D growth mode and exhibited rough surface (RMS roughness of ~17.0-20.0
nm). The results of surface roughness are consistent with those from TEM measurements. The lowest resistivity of
~2.3×10-4 Ω·cm for as-grown GZO layers has been achieved at substrate temperature of 350°C, while the data for 2D
GZO layers was affected by a parallel conduction channel from underneath GaN and require further studies. Although
the differences in electrical properties and surface morphology existed, the GZO layers grown on different p-GaN
templates showed optical transparency higher than 90% in the visible spectral range. The performance of 3D GZO layers
as p-electrode was tested in InGaN light emitting diodes.
The surface charge behavior on n-type GaN was investigated as a function of UV exposure in ambient. It was first
noticed that the surface photovoltage (SPV), or change in surface contact potential under UV illumination, could
slowly decrease during illumination. The SPV for as-grown samples is ~0.6 eV and can drop by up to 0.3 eV over
1 h of UV exposure in air ambient. We believe that this slow decrease is due to the photo-induced chemisorption of
oxygen species. Interestingly, samples exhibit a smaller drop in SPV during continuous UV illumination after many
hours of accumulated UV exposure. This can be explained by the UV-induced growth of a thicker surface oxide
layer which inhibits electron transfer from the bulk to the surface. The original SPV behavior can be restored by
chemical etching to remove the additional surface oxide. We have also investigated the surface behavior by locally
charging the surface before and after UV-induced oxide growth. Measurements of the surface contact potential using
scanning Kelvin probe microscopy (SKPM) demonstrate that negatively charged regions initially spread laterally on
the UV-exposed surface, but not on the clean surface. The UV-induced oxide therefore appears to be relatively
conductive and enhance the lateral motion of injected electrons. The discharge of both the negatively and positively
charged regions under dark conditions is similar for both the initial and UV-exposed surfaces and shows a
logarithmic time dependence.
Photoluminescence (PL) and surface photovoltage (SPV) of GaN layers were studied in vacuum and air ambient. SPV transients were measured with two set-ups: traditional Kelvin probe attached to an optical cryostat and atomic force microscope in contact potential mode. It is found that upward band bending in GaN decreases from its dark value of about 0.9 eV to about 0.3 eV under intense UV light and then gradually increases in air ambient (due to photoadsorption of oxygen) and decreases in vacuum (due to photodesorption of oxygen). Manifestations of such changes were observed as changes in PL intensity and in SPV (both increased in vacuum and decreased in air). The effects were sample-dependent.
We have studied the effects of thermal annealing in air on photoluminescence of bulk ZnO crystals grown by
hydrothermal technique and nominally undoped ZnO layers grown by molecular beam epitaxy on sapphire. Annealing of
the samples in air at temperatures above 600°C resulted in a dramatic enhancement of the Cu-related green luminescence
(GL) band peaking at 2.45 eV and having characteristic fine structure. The GL band quenched at temperatures above 300
K due to escape of holes from the excited state of the CuZn acceptor to the valence band. SIMS profiles revealed
moderate increase of Al concentration and significant increase of Cu concentration in annealed samples. Exciton bound
to hydrogen-related donor (the 3.363 eV line) quenched after annealing the sample at temperatures above 750ºC.
Charge trapping resulting in localized band bending on MBE-grown GaN films was investigated using a new
combination of conducting atomic force microscopy (CAFM) and scanning Kelvin probe microscopy (SKPM). CAFM
was first used to locally inject charge at the surface oxide/semiconductor interface, and then SKPM was performed to
monitor the evolution of the resulting surface potential. In a dark environment, the additionally charged interface states
due to CAFM charge injection resulted in an induced additional band bending that persisted for hours. The induced band
bending is nominal (<0.5 eV) for CAFM voltages less than 8 V, and reaches a saturation value of ~3 eV for voltages
greater than 10 V. The saturation band bending corresponds to a total density of charged interface states (2×1012 cm-2)
that is double the value observed for the intrinsic surface. Induced band bending could still be observed up to 4 h after
charge injection, indicating that charge trapping is relatively stable in a dark environment. However, charged interface
states could be rapidly neutralized by illumination with UV light. A phenomenological model based on a tunneling
mechanism was used to successfully describe the CAFM charge injection, where electrons travel from the tip through an
oxide barrier and become trapped at oxide/GaN interface states. Saturation occurs due to the existence of a finite density
of chargeable states at the interface. After charge injection, the decrease in induced band bending with time was found to
be consistent with a thermionic model of charge transfer from the interface to the bulk.
The effect of plasma-induced ion damage on the optical properties of ZnO films grown by plasma-assisted molecular
beam epitaxy on a-sapphire substrates and GaN(0001)/c-sapphire templates prepared has been studied using steady-state
and time-resolved photoluminescence. We observed that the deflecting the ions produced by the RF oxygen plasma
away from substrate results in improved excitonic emission and modification of the defect-related PL spectrum. The
intensity of the near-band-edge lines in the photoluminescence spectra from the layers grown with the ion deflection was
found to increase by factors 7 to 20 for the layers grown on GaN(0001)/c-sapphire at a plasma power of 350 W and by 3
to 4 times for ZnO grown on a-sapphire substrates at a plasma power of 265 W as compared to the controls grown
without the ion deflection. The yellow-green spectral range was dominated by different defect bands in the films grown
with and without ion deflection. The effect of RF power on peak positions of the defect band was studied for the films
grown without ion deflection. For the ZnO films grown on a-plane sapphire substrates, time-resolved photoluminescence
showed a significant increase in luminescence decay times both at RT and 89 K. However, for ZnO on GaN(0001)/csapphire
substrates, virtually no improvement in decay time was found at 89 K with only a moderate increase in decay
constant at room temperature.
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